Физические основы синтеза методом молекулярно-пучковой эпитаксии и свойства III-V гетероструктурных нитевидных нанокристаллов тема диссертации и автореферата по ВАК РФ 00.00.00, доктор наук Резник Родион Романович

• Резник Родион Романович
• доктор наук
• 2026

Оглавление диссертации доктор наук Резник Родион Романович

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Synopsis

Введение

ГЛАВА 1. Формирование нитевидных нанокристаллов на основе соединений III-V: современное состояние, перспективы развития и применения

1.1. Механизмы роста полупроводниковых III-V нитевидных нанокристаллов

1.2. Синтез III-V нитевидных нанокристаллов на кремнии и других рассогласованных поверхностях

1.3. Кристаллографическая структура III-V нитевидных нанокристаллов

1.4. Зонная структура III-V нитевидных нанокристаллов, содержащих сегменты различной кристаллографической фазы

1.5. Нитевидные нанокристаллы в системе материалов AlGaAs

1.6. Нитевидные нанокристаллы развитой морфологии в системе материалов AlGaAs

1.7. Нитевидные нанокристаллы в системе материалов InGaAs

1.8. III-V нитевидные нанокристаллы с квантовыми точками

1.9. Подходы к увеличению однородности морфологических параметров массивов III-V нитевидных нанокристаллов

ГЛАВА 2. Экспериментальные установки и методы

2.1. Ростовые методы и установки

2.1.1.Метод молекулярно-пучковой эпитаксии

2.1.2.Эффузионные источники установки молекулярно-пучковой эпитаксии

2.1.3.Крекерные источники установки молекулярно-пучковой эпитаксии с изменяемой апертурой

2.1.4. Метод дифракции быстрых электронов на отражение

2.1.5.Особенности методики роста III-V нитевидных нанокристаллов на поверхности кремния методом молекулярно-пучковой эпитаксии

2.2. Метод растровой электронной микроскопии

2.3. Метод просвечивающей электронной микроскопии с возможностью элементного анализа

2.4. Методы фотолюминесценции и микрофотолюминесценции

2.5. Метод спектроскопии комбинационного рассеяния

2.6. Метод атомно-силовой микроскопии

ГЛАВА 3. Вюрцитные нитевидные нанокристаллы на основе системы AlGaAs

3.1. Формирование методом МПЭ вюрцитных AlGaAs нитевидных нанокристаллов на кремниевой подложке с использованием золота в качестве катализатора

3.2. Взаимосвязь условий роста, морфологических и структурных свойств вюрцитных AlGaAs нитевидных нанокристаллов на кремниевой подложке

3.3. Взаимосвязь структурных и оптических свойств вюрцитных AlGaAs нитевидных нанокристаллов и зависимость ширины запрещённой зоны от состава твёрдого раствора

3.4. Источники одиночных фотонов на основе различных кристаллографических фаз в AlGaAs ННК на поверхности кремния

3.5. Вюрцитные AlGaAs нитевидные нанокристаллы развитой морфологии на кремниевой подложке

3.6. Выводы к Главе

ГЛАВА 4. III-V нитевидные нанокристаллы c квантовыми точками

4.1. Нитевидные нанокристаллы с квантовыми точками в системе материалов GaAs/AlGaAs на поверхности кремния

4.1.1.Формирование методом МПЭ AlGaAs нитевидных нанокристаллов с GaAs квантовыми точками на кремниевой подложке

4.1.2.Морфологические и структурные свойства AlGaAs нитевидных

нанокристаллов с GaAs квантовыми точками на кремниевой подложке

4.1.3.Оптические свойства AlGaAs нитевидных нанокристаллов с GaAs квантовыми точками на кремниевой подложке

4.1.4.Направленность излучения из GaAs квантовых точек в AlGaAs нитевидных нанокристаллах на кремниевой подложке

4.1.5.Двухфотонное резонансное возбуждение GaAs квантовых точек в AlGaAs нитевидных нанокристаллах

4.2. Нитевидные нанокристаллы с квантовыми точками в системе материалов InGaAs/AlGaAs на поверхности кремния

4.2.1.Формирование методом МПЭ AlGaAs нитевидных нанокристаллов с InGaAs квантовыми точками на кремниевой подложке

4.2.2.Физические свойства AlGaAs нитевидных нанокристаллов с InGaAs квантовыми точками на кремниевой подложке

4.3. Нитевидные нанокристаллы с квантовыми точками в системе материалов InAsP/InP на поверхности кремния

4.3.1. Формирование методом МПЭ и физические свойства InP нитевидных нанокристаллов с InAsP квантовыми точками на кремниевой подложке

4.3.2. Гибридные наноструктуры на основе InP нитевидных нанокристаллов с InAsP квантовыми точками и коллоидных квантовых точек CdSe/ZnS

4.4. Формирование методом МПЭ и физические свойства нитевидных нанокристаллов с квантовыми точками в системе материалов InGaP/AlGaP

4.5. Формирование упорядоченных массивов нитевидных нанокристаллов с квантовыми точками на поверхности кремния

4.6. Выводы к Главе

ГЛАВА 5. Синтез и физические свойства других функциональных наноструктур на основе III-V нитевидных нанокристаллов

5.1. Вюрцитные GaAs нитевидные нанокристаллы для генерации пьезоэлектрического тока

5.2. Низкотемпературное формирование отверстий в естественном слое SiOx на поверхности кремния с помощью наночастиц индия и формирование методом МПЭ GaAs нитевидных нанокристаллов

5.3. Формирование методом МПЭ III-V нитевидных нанокристаллов малого диаметра на подложке SiC/Si(111)

5.4. Формирование методом МПЭ, морфологические и структурные свойства GaP и InP нитевидных нанокристаллов на SiC подложке с плёнкой графена

5.5. Низкотемпературное формирование методом МПЭ InAs нитевидных нанокристаллов на поверхности кремния по механизму «пар-кристалл-кристалл»

5.6. Низкотемпературное формирование методом МПЭ In0;5Gao,5As нитевидных нанокристаллов на поверхности кремния по механизму «пар-кристалл-кристалл»

5.7. Управление проводимостью InxGa1-xAs нитевидных нанокристаллов с

помощью деформации и поверхностных состояний

5.8. Выводы к Главе

Заключение

Благодарности

Список сокращений и условных обозначений

Список используемой литературы

Список публикаций автора по теме диссертации

Приложение: Тексты публикаций автора по теме диссертации

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I. Общая характеристика диссертации

Введение диссертации (часть автореферата) на тему «Физические основы синтеза методом молекулярно-пучковой эпитаксии и свойства III-V гетероструктурных нитевидных нанокристаллов»

Актуальность темы.

Разработка и создание новых полупроводниковых наноматериалов является актуальной задачей для современного мира. В наши дни наноструктуры на основе полупроводниковых соединений Ш-У уже находят применение для создания оптоэлектронных приборов благодаря прямозонной структуре для большинства 111-У материалов и возможности варьирования ширины запрещённой зоны путём изменения состава твёрдого раствора [1,2]. Среди таких материалов особое место занимают нитевидные нанокристаллы (ННК), обладающие уникальными оптическими, транспортными, электрофизическими и другими свойствами [3]. Благодаря своим уникальным свойствам, высокой степени кристаллического совершенства и возможности строгого управления химическим составом и морфологией, ННК находят применение в оптоэлектронике, фотовольтаике, микроэлектронике, квантовых технологиях, медицине и других областях [3].

Актуальность исследований в данной области обусловлена рядом фундаментальных и прикладных преимуществ Ш-У ННК по сравнению с другими платформами. В частности, (квази)одномерная морфология и особенности механизмов роста ННК позволяют минимизировать плотность дислокаций даже при значительном несоответствии параметров кристаллической решетки с подложкой, что важно, например, для интеграции прямозонных Ш-У материалов с кремниевой платформой [4]. Возможность создания аксиальных и радиальных гетероструктур с атомарно резкими границами раздела позволяет формировать высококачественные гибридные наноструктуры на основе ННК с заранее заданной зонной диаграммой [5], что значительно расширяет спектр возможных применений.

Известно, что гексагональная кристаллографическая фаза при обычных условиях не наблюдается для объёмных Ш-У соединений (за исключением нитридов), и остаётся слабо изученной с точки зрения ее зонной структуры.

Однако такие материалы могут быть получены при синтезе ННК [6]. Формирование ННК на основе материалов с ранее не изученными свойствами может привести к созданию новых приборов на их основе, для чего критически важно обладать информацией о ширине запрещённой зоны и расположении энергетических уровней для различных кристаллографических фаз и их комбинаций для известных ранее и новых материалов.

Современные тенденции в материаловедении демонстрируют растущий интерес к наноструктурам сложной морфологии, что обусловлено их уникальными физико-химическими свойствами, недостижимыми для традиционных материалов [7]. Такие иерархически организованные системы являются перспективными для многофункционального применения и создания суперконденсаторов, эффективных энергоносителей, устройств для очистки и разложения воды, доставке лекарств и других приложений. Развитие современных методов синтеза открывает возможности для создания полупроводниковых наногетероструктур развитой морфологии на основе ННК. Многократное увеличение площади поверхности, контроль состава и кристаллографических фаз, а также формирование единой сети наноструктур на основе ННК развитой морфологии открывают перспективы как для увеличения эффективности физических преобразований в массиве ННК, так и создания новых устройств на их основе. Кроме того, наноструктуры развитой морфологии могут служить «скелетом» для формирования активных элементов на их основе с помощью других материалов [7].

Несмотря на достигнутый прогресс в области синтеза ННК, до сих пор остаётся не решённым целый ряд задач в данном направлении. Для наиболее полного их применения в приборных приложениях необходимо как расширять диапазон материалов, так и формировать и исследовать физические свойства новых наноструктур, а также интегрировать их с кремниевой и другими платформами.

Сказанное выше подтверждает необходимость создания воспроизводимых методов синтеза наноструктур и наногетероструктур на основе ННК на

различных поверхностях, в том числе, монокристаллическом кремнии и исследований их физических свойств, что обуславливает актуальность темы диссертации.

Таким образом, целью работы является разработка воспроизводимых методик синтеза, определение механизмов формирования, взаимосвязей условий роста и физических свойств III-V полупроводниковых наноструктур и наногетероструктур на основе ННК на различных подложках, в том числе монокристаллического кремния, для контролируемого получения наноматериалов с заранее заданными морфологическими, структурными, оптическими и электрофизическими свойствами и их последующего применения в функциональных устройствах.

Для достижения указанной цели решался следующий комплекс задач:

1) Разработка методики МПЭ синтеза с использованием золота в качестве катализатора по механизму «пар-жидкость-кристалл» и установление взаимосвязи условий роста (время роста, соотношения потоков Ga/Al и V/III, температура роста) с морфологическими, структурными и оптическими свойствами AlGaAs ННК в вюрцитной кристаллографической фазе на подложке Si(111). Разработка методики МПЭ синтеза с использованием золота в качестве катализатора AlGaAs ННК развитой морфологии и определение их морфологических и структурных свойств;

2) Разработка методики синтеза на поверхности Si(111) методом МПЭ с использованием золота в качестве катализатора структур типа «ННК с узкозонной нанометровой вставкой (КТ)» на основе материалов GaAs/AlGaAs, InGaAs/AlGaAs, InAsP/InP, InGaP/AlGaP. Установление взаимосвязи условий роста (время роста, температура роста, соотношение величин потоков материалов из источников) с морфологическими, структурными и оптическими свойствами синтезированных наноструктур, включая измерение автокорреляционных функций излучения из КТ для характеризации наноструктур как источников одиночных фотонов;

3) Развитие подходов к увеличению эффективности излучательной рекомбинации ННК путём создания гибридных наноструктур на основе ННК и коллоидных КТ, а также к увеличению однородности морфологических параметров ННК с помощью методов наноструктурирования поверхности подложки;

4) Разработка методов и определение процессов низкотемпературного МПЭ роста In(Ga)As ННК по механизму «пар-кристалл-кристалл» с использованием золота в качестве катализатора на поверхности кремния;

5) Определение влияния механической деформации на электрофизические свойства вюрцитных ННК, а также определение возможности МПЭ роста и физических свойств ННК на поверхности графена.

Основные методы исследования

Для решения поставленных задач в процессе исследований использовались:

1) Метод молекулярно-пучковой эпитаксии для синтеза массивов ННК и ННК с КТ, включая рост на паттернированных подожках.

2) Метод растровой электронной микроскопии для исследования морфологических свойств выращенных образцов.

3) Метод просвечивающей электронной микроскопии с возможностью рентгеноструктурного анализа для исследования структурных свойств выращенных образцов.

4) Методы фотолюминесценции, микрофотолюминесценции с возможностью измерений автокорреляционной функции и метод спектроскопии комбинационного рассеяния для исследования оптических свойств выращенных образцов.

5) Метод атомно-силовой микроскопии с возможностью измерения вольт-амперных характеристик для исследований морфологических и электрофизических свойств выращенных образцов.

Положения, выносимые на защиту:

1) Оптимальный выбор и контроль физических параметров (температуры подложки во время роста, величин и соотношений потоков из источников,

условий сверхвысокого вакуума) при синтезе наноструктур методом молекулярно-пучковой эпитаксии обеспечивают формирование на поверхности подложки 81(111) AlGaAs нитевидных нанокристаллов (диаметром от 30 нм) в вюрцитной фазе в широком диапазоне составов (AlxGa1-xAs, х от 0,1 до 0,6) при использовании нанокапель золота в качестве катализатора.

2) Многостадийное осаждение золота на поверхность AlGaAs нитевидных нанокристаллов с последующим каталитическим синтезом AlGaAs методом молекулярно-пучковой эпитаксии без нарушений условий сверхвысокого вакуума приводит к росту наноструктур («веток»), формирующихся на боковых гранях и наследующих кристаллографию первичных нитевидных нанокристаллов.

3) Синтезированные на поверхности подложки Si(111) гетероструктурные AlGaAs нитевидные нанокристаллы с (In,Ga)As квантово-размерными вставками (высотой от 3 до 15 нм) демонстрируют минимум автокорреляционной функции

Л

излучения фотонов g (1) в нулевой момент времени вплоть до значения 0,01 в диапазоне длин волн от 750 до 920 нм при непрерывном оптическом возбуждении, что характеризует их как источники одиночных фотонов.

4) Нанесение коллоидных квантовых точкек CdSe/ZnS с лигандным слоем триоктилфосфиноксида на поверхность гетероструктурных 1пР нитевидных нанокристаллов с InAsP нанометровыми вставками на порядок и более увеличивает интенсивность фотолюминесценции в длинноволновом диапазоне (~1100 нм) от InAsP вставки, что объясняется безызлучательным переносом энергии из квантовых точек CdSe/ZnS в InAsP вставки и пассивацией поверхности нитевидных нанокристаллов лигандом.

5) Автокаталитический рост однородных по размерам массивов GaAs нитевидных нанокристаллов реализуется с помощью формирования наноразмерных капель 1п на поверхности подложки Si( 111), травления каплями отверстий в естественном слое SiOx при строго контролируемой температуре подложки 400°С, последующей кристаллизации 1п капель в InAs и удаления InAs с поверхности при температуре подложки 550°С.

6) Реализация механизма роста «пар-кристалл-кристалл» с использованием наночастиц золота в качестве катализатора при температуре подложки Si(111), меньшей точки эвтектики золото-кремний (в диапазоне от 220°С до 270°С), способствует получению методом молекулярно-пучковой эпитаксии InGaAs нитевидных нанокристаллов в вюрцитной фазе в широком диапазоне составов (InyGa1-yAs, у от 0,5 до 1).

Научная новизна, практическая и теоретическая значимость работы заключается в том, что:

1) Впервые была продемонстрирована возможность синтеза методом МПЭ AlGaAs ННК на поверхности Si(111) с использованием золота в качестве катализатора, была определена взаимосвязь условий роста (с точки зрения температуры и времени синтеза) и морфологических свойств массивов AlGaAs ННК, исследовано влияние соотношения потоков из источников Ga и А1 на морфологические, структурные и оптические свойства ННК. В частности, результаты исследований структурных свойств AlGaAs ННК показали, что они обладают преимущественно вюрцитной кристаллографический фазой.

2) Впервые была получена экспериментальная зависимость ширины запрещённой зоны вюрцитных AlxGa1-xAs ННК от содержания Al в диапазоне значений x от 0,1 до 0,6.

3) Впервые на поверхности кремния с использованием золота в качестве катализатора сформированы AlGaAs ННК в вюрцитной кристаллографической фазе с нанометровыми включениями кубической кристаллографической фазы и показано, что синтезированные наноструктуры могут являться источниками одиночных фотонов.

4) Впервые синтезированы AlGaAs ННК развитой морфологии на поверхности Si(111) с использованием золота в качестве катализатора и исследованы их морфологические и структурные свойства. Показано, что синтезированные наноструктуры обладают преимущественно вюрцитной кристаллографической фазой, а ННК второго поколения («ветви») наследуют кристаллическую структуру области первичных AlGaAs ННК («стволов»)

5) Впервые методом МПЭ были получены AlGaAs ННК с GaAs КТ на поверхности Si( 111) с использованием золота в качестве катализатора. Показано, что, изменяя параметры роста GaAs КТ и AlGaAs ННК, можно создавать гибридные наноструктуры с заранее заданной зонной диаграммой, управлять длиной волны излучения как из GaAs КТ, так и из AlGaAs ННК. Результаты измерений автокорреляционной функции для GaAs КТ в AlGaAs показали, что они могут являться источниками одиночных фотонов.

6) Обнаружена направленность излучения ФЛ из GaAs КТ в AlGaAs ННК и показано, что интенсивность излучения от стоящих на поверхности кремния GaAs КТ в AlGaAs ННК в направлении роста более чем на 2 порядка превышает интенсивность излучения от лежачих на кремнии КТ в ННК в направлении, перпендикулярном росту ННК.

7) Продемонстрирована возможность непрерывного двухфотонного резонансного оптического возбуждения биэкситона в GaAs КТ в AlGaAs ННК и каскадного излучения «биэкситон-экситон».

8) Впервые синтезированы InGaAs КТ в AlGaAs ННК на поверхности Si(111) с использованием золота в качестве катализатора, установлена взаимосвязь условий роста и физических свойств синтезированных наноструктур. Характерный «провал» автокорреляционной функции до значений 0,01 указывает на то, что синтезированные структуры типа «AlGaAs ННК с InGaAs КТ» могут являться источниками одиночных фотонов.

9) На поверхности Si( 111) синтезированы гибридные наноструктуры на основе ННК с КТ в системе материалов InAsP/InP. Спектры ФЛ, соответствующие излучению из InAsP КТ наблюдаются в диапазоне длин волн 1,25-1,5 мкм при комнатной температуре. Впервые показано, что нанесение коллоидных КТ CdSe/ZnS c лигандным слоем триоктилфосфиноксида на поверхность !п? ННК c InAsP КТ приводит к увеличению интенсивности ФЛ InAsP КТ более чем в 10 раз.

10) Разработана методика упорядоченного синтеза ННК с КТ в отверстиях в слое SiOx на поверхности 81(111) с использованием золота в качестве катализатора.

11) Впервые разработан метод низкотемпературного формирования наноразмерных отверстий в слое естественного SiOx на поверхности Si(111) с помощью наночастиц 1п для последующего синтеза GaAs ННК.

12) Синтезированы GaAs ННК в вюрцитной кристаллографической фазе и исследованы их электрофизические свойства при деформации ННК зондом и лазерном освещении. Увеличение деформации ННК привело к увеличению электрического тока через систему «зонд-ННК» с 1 до 9 пА при отсутствии освещения лазером. При этом освещение лазером привело к увеличению тока до 1 нА благодаря пьезо-фототронному эффекту.

13) Разработана методика МПЭ синтеза GaP и 1пР ННК методом МПЭ на подложке SiC с плёнкой графена. Исследования структурных свойств показали, что сформированные 1пР ННК обладают чистой вюрцитной кристаллографической фазой без дефектов упаковки.

14) Методом МПЭ с использованием золота в качестве катализатора по механизму ПКК синтезированы вюрцитные InAs ННК 81(111) при пониженной температуре подложек 81(111) 270°С. Обнаружено, что низкая температура роста (реализация механизма «ПКК») приводит к значительному увеличению скорости роста InAs ННК по сравнению с ростом по механизму «ПЖК». Для объяснения наблюдаемого эффекта использована модель, которая обобщает теорию роста ННК на случай, когда адатомы могут диффундировать по поверхности или включаться в двумерный слой на подложке, но не могут десорбироваться.

15) Разработана методика МПЭ синтеза с использованием золота в качестве катализатора и исследованы процессы роста InxGa1-xAs ННК при номинальном составе х=0,5-0,85 при пониженной температуре подложки 81(111) во время роста (220-270°С) и предложена модель, которая объясняет формирование стержня In0,5Ga0,5As ННК за счет кинетически контролируемого режима роста.

16) Исследовано влияние растягивающей деформации на электрофизические свойства InxGa1-xAs ННК. Показано, что деформация приводит к увеличению проводимости электрического тока через ННК. Для Ino,85Gao,15As и деформации 4% проводимость ННК увеличивается на 3 порядка. Наблюдаемый эффект был объяснён смещением дна зоны проводимости In0,85Ga0д5As ННК ниже фиксированного уровня Ферми при деформации ННК.

Таким образом, в результате исследований было создано новое научное направление - «Гетероструктурные Ш-У нитевидные нанокристаллы», а также были разработаны методики синтеза и исследованы физические свойства целого класса новых материалов. Показано, что МПЭ является универсальным методом для воспроизводимого формирования гетероструктурных 111-У ННК в широком диапазоне составов на поверхности кремния и других подложках. Полученные в рамках проведения исследований результаты открывают новые перспективы для создания полупроводниковых приборов в областях оптоэлектроники, сенсорики, возобновляемых источников энергии, квантовых технологий и других. Направление исследований и полученные результаты соответствуют паспорту специальности 1.3.11 «Физика полупроводников» в пунктах 1, 2, 4, 6-8, 13, 17.

Достоверность научных результатов.

Достоверность полученных результатов подтверждается их воспроизводимостью, использованием современного высокоточного оборудования, соответствием полученных экспериментальных данных с результатами теоретических расчётов и литературными данными, а также публикацией результатов исследований в ведущих рецензируемых журналах и апробацией на всероссийских и международных конференциях.

Внедрение результатов работы.

Результаты диссертационной работы используются в проведении научных исследований СПбАУ РАН им. Ж.И. Алфёрова и Санкт-Петербургского государственного университета. По теме диссертации зарегистрировано 3 результата интеллектуальной деятельности (РИД).

Апробация работы.

Результаты, вошедшие в диссертационную работу, докладывались и обсуждались на более чем 40 российских и международных конференциях и симпозиумах, включая 10 приглашённых докладов:

Международный симпозиум «Нанофизика и наноэлектроника» (Нижний Новгород, 2017-2025), XI Международная научная конференция «Актуальные проблемы физики твердого тела» (Минск, 2025), XVI Российская конференция по физике полупроводников (Санкт-Петербург, 2022, 2024), «Квантовая неделя в СПбГУ» (Санкт-Петербург, 2024), International Conference Laser Optics "ICLO" (Санкт-Петербург, 2016, 2018, 2020, 2022, 2024); 31st international conference Advanced Laser Technologies (Владивосток, 2024), Международная школе -конференция "Saint-Petersburg OPEN" по Оптоэлектронике, Фотонике, Нано- и Нанобиотехнологиям (Санкт-Петербург, 2015-2025), Международная конференция "ФизикА.СПб" (Санкт-Петербург, 2014-2024), International Conference on Photonics Research (Турция, 2018, 2023), 3th International Advances in Applied Physics & Materials Science Congress & Exhibition (Олюдениз, 2023), Всероссийская молодежная конференция по физике полупроводников и наноструктур, полупроводниковой опто- и наноэлектронике (Санкт-Петербург, 2016, 2022), IV Smart NanoMaterials: Advances, Innovation and Applications (Париж, 2021), International workshop and school "Nanostructures for Photonics" (Санкт-Петербург, 2016, 2018, 2021); VI International Conference on Metamaterials and Nanophotonics (Тбилиси, 2021), International Conefernce SPb Photonic, Optoelectronic, & Electronic Materials (Санкт-Петербург, 2020), EMN meeting on nanowires (Прага, 2018); State-of-the-art trends of Scientific Research of Artificial and Natural Nanoobjects, STRANN-2018 (Санкт-Петербург, Москва, 2016, 2018); International School & Conference on the Physics of Semiconductors "Jaszowiec" (Ширк, 2017, 2019); The 20th European Workshop on Molecular Beam Epitaxy "EuroMBE 2019" (Коробицино, 2017); 25й Международный симпозиум "Наноструктуры: Физика и Технология" (Санкт-Петербург, 2017) и другие.

Личный вклад автора.

Представленные в диссертации результаты получены лично автором или при его непосредственном участии. Задачи работы были поставлены автором лично и совместно с научным консультантом Цырлиным Г.Э. Эксперименты по синтезу образцов проводились лично автором или совместно с Хребтовым А.И. и Илькив И.В. Измерения морфологических, структурных, оптоэлектронных и электрофизических свойств образцов проводились при непосредственном участии автора. Автор лично выдвигал гипотезы о механизмах роста синтезированных наноструктур. При непосредственном участии автора совместно с Дубровским В.Г. были разработаны модели роста ННК по механизму ПКК при пониженной температуре роста.

Структура и объем диссертации.

Диссертация состоит из введения, 5 глав, заключения, списка сокращений и условных обозначений, списка литературы и списка публикаций автора по теме диссертации. Объем диссертации составляет 315 страниц и содержит 97 рисунков, 19 формул, список литературы из 293 источников.

Публикации.

Основные результаты по теме диссертации изложены в 45 публикациях, индексируемых в Scopus, Web Of Science, ВАК или РИНЦ, все из которых изданы в изданиях, входящих в перечень журналов ВАК или приравненных к ним.

II. Основное содержание работы

В реферате представлено обоснование актуальности работы, сформулированы цели и научная новизна проведенных исследований, обоснована практическая значимость, приведены основные результаты и перечислены положения, выносимые на защиту.

В первой главе представлен обзор литературы по теме диссертации. В главе обсуждаются механизмы роста полупроводниковых ННК, их кристаллографическая и зонная структуры, а также перспективы применения III-V ННК для создания приборов. Отмечается, что, несмотря на достигнутый

прогресс в области синтеза III-V наноструктур, до сих пор остаётся не решённым целый ряд задач в данном направлении. Для наиболее полного их применения в приборных приложениях необходимо как расширять диапазон материалов, так формировать и исследовать физические свойства новых наноструктур, а также интегрировать их с кремниевой и другими платформами. В последней части главы сформулированы задачи, которые были выполнены в рамках диссертационной работы.

Вторая глава посвящена обзору методик и установок, с помощью которых проводились ростовые эксперименты и исследования свойств III-V ННК. Для проведения ростовых экспериментов методом молекулярно-пучковой эпитаксии использовалась уникальная установка Compact 21 фирмы RIBER, оснащённая дополнительной камерой металлизации для напыления золота на поверхность образца и последующей его передачей в ростовую камеру без нарушения условий сверхвысокого вакуума. Ростовая камера установки оснащена, в том числе, системой дифракции быстрых электронов на отражение, что позволяет непосредственно во время роста наблюдать физические процессы на поверхности подложки.

Для исследований морфологических свойств синтезированных наноструктур использовались методы растровой электронной микроскопии и атомно-силовой микроскопии. Структурные свойства сформированных образцов были изучены с помощью методов просвечивающей электронной микроскопии и комбинационного рассеяния света. Для исследований оптических свойств массивов и одиночных синтезированных наноструктур использовались методы фотолюминесценции и микрофотолюминесценции, соответственно. Электрофизические свойства синтезированных образцов были исследованы с помощью метода атомно-силовой микроскопии с возможностью визуализации поверхности и одновременным измерением вольт-амперных характеристик.

Третья глава посвящена исследованию формирования методом МПЭ вюрцитных AlGaAs ННК на поверхности Si(111) с использованием золота в

качестве катализатора при различных условиях роста и исследованию их физических свойств.

Методика синтеза ННК включала в себя несколько этапов. На предварительном этапе кремниевая подложка подвергалась химической очистке перед загрузкой в установку МПЭ и термической очистке в дополнительной камере металлизации. После этого на поверхность подложки напылялся тонкий слой золота для формирования капель катализатора. На следующем этапе подложка перемещалась в ростовую камеру без нарушений условий сверхвысокого вакуума, где и происходил синтез ЛЮаЛБ ННК при различных температурах подложки (320-570°С) и соотношениях потоков из источников Л1Юа (0,1/0,9-0,6/0,4). Соотношения потоков их источников были подобраны таким образом, чтобы суммарная скорость роста слоя ЛЮаЛБ на поверхности 0аЛБ(100) была постоянной для всех экспериментов и составляла 1 монослой в секунду (МС/с). В результате экспериментов были определены взаимосвязи условий роста и морфологических свойств массивов ЛЮаЛБ ННК.

Результаты исследований структурных свойств выращенных AlGaAs ННК, синтезированных при наиболее оптимальной с точки зрения морфологии ННК температуре подложки 510°С, показали, что ННК сформировались преимущественно в вюрцитной кристаллографической фазе и обладают структурой типа стержень-оболочка (рисунок 1 (а), (б)), причём мольная доля А1 в стержне меньше, чем в оболочке. Рост стержня AlGaAs ННК происходит под золотой каплей по механизму ПЖК. Боковой рост оболочки ННК обусловлен меньшим коэффициентом миграции адатомов А1 по поверхности по сравнению с адатомами Ga, что приводит к увеличению мольной доли А1 на боковых гранях по сравнению со стержнем ННК [8,9]. Эти факторы приводят к самоорганизованному формированию структуры типа стержень-оболочка, состоящей из цилиндрического стержня с постоянным диаметром, соответствующим размеру золотой капли, и конической оболочки. Отдельно стоит отметить, что для образца, синтезированного при соотношении потоков

АЮа, равному 0,1/0,9, не было выявлено значительного отличия в мольной доли А1 в стержне и оболочке.

600 620 640 660 680 700 720

Дшша волны, нм

Рисунок 1 - Структурные и оптические свойства АЮаАБ ННК: (а) Типичное ПЭМ изображение одиночного А10,6Оа0,4А8 ННК; (б) типичное ПЭМ изображение высокого разрешения того же ННК и вставка типичной картины дифракции при ПЭМ измерениях; (в) типичный спектр ФЛ при температуре 2К от массива А10,6Оа0,4АБ ННК.

Для определения зависимости ширины запрещённой от состава А1хОа1-хАБ ННК, сформированных в вюрцитной кристаллографической фазе, были исследованы структурные и оптические свойства образцов с номинальным составом А1 в ННК в диапазоне х=0,1-0,6. При исследовании оптических свойств AlGaAs ННК методом ФЛ было установлено, что структура стержень-оболочка приводит к возникновению спектров излучения с двумя характерными максимумами (рисунок 1 (в)). Коротковолновый и длинноволновый пики спектра ФЛ соответствуют оптическому сигналу от оболочки и стержня, соответственно.

Список литературы диссертационного исследования доктор наук Резник Родион Романович, 2026 год

Литература

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Different III-V semiconductor nanowires with quantum dots on silicon.

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15. Cirlin G.E., Reznik R.R., Samsonenko Yu.B., Khrebtov A.I., Kotlyar K.P., Ilkiv I.V., Soshnikov I.P., Kirilenko D.A., Kryzhanovskaya N.V. Phosphorus-based nanowires grown by molecular-beam epitaxy on silicon. Semiconductors, 2018, vol. 52, no. 11, pp. 1416-1419. https://doi.org/10.1134/S1063782618110258

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quantum dots tuned to atomic resonances // Nano Letters. 2018. V. 18. N 11. P. 7217-7221. https://doi.org/10.1021/acs.nanolett.8b03363

8. Leandro L., Reznik R., Clement J.D., Repän J., Reynolds M., Ubyivovk E.V., Shtrom I.V., Cirlin G., Akopian N. Wurtzite AlGaAs nanowires // Scientific Reports. 2020. V. 10. N 1. P. 735. https://doi. org/10.1038/s41598-020-57563-0

9. Cirlin G.E., Reznik R.R., Shtrom I.V., Khrebtov A.I., Soshnikov I.P., Kukushkin S.A., Leandro L., Kasama T., Akopian N. AlGaAs and AlGaAs/GaAs/AlGaAs nanowires grown by molecular beam epitaxy on silicon substrates // Journal of Physics D: Applied Physics. 2017. V. 50. N 48. P. 484003. https://doi.org/10.1088/1361-6463/aa9169

10. Cirlin G.E., Reznik R.R., Shtrom I.V., Khrebtov A.I., Samsonenko Yu.B., Kukushkin S.A., Kasama T., Akopian N., Leonardo L. Hybrid GaAs/AlGaAs Nanowire-Quantum dot system for single photon sources // Semiconductors. 2018. V. 52. N 4. P. 462464. https://doi.org/10.1134/S1063782618040103

11. Cirlin G.E., Shtrom I.V., Reznik R.R., Samsonenko Yu.B., Khrebtov A.I., Bouravleuv A.D., Soshnikov I.P. Hybrid AlGaAs/ GaAs/AlGaAs nanowires with a quantum dot grown by molecular beam epitaxy on silicon // Semiconductors. 2016. V. 50. N 11. P. 1421-1424. https://doi.org/10.1134/S1063782616110257

12. Reznik R.R., Shtrom I.V., Samsonenko Yu.B., Khrebtov A.I., Soshnikov I.P., Cirlin G.E. The dependence of the wavelength on MBE growth parameters of GaAs quantum dot in AlGaAs NWs on Si (111) substrate // Journal of Physics: Conference Series. 2017. V. 929. N 1. P. 012047. https://doi.org/10.1088/1742-6596/929/1/012047

13. Leandro L., Hastrup J., Reznik R., Cirlin G., Akopian N. Resonant excitation of nanowire quantum dots // npj Quantum Information. 2020. V. 6. N 1. P. 93. https://doi.org/10.1038/s41534-020-00323-9

14. Reznik R.R., Cirlin G.E., Shtrom I.V., Khrebtov A.I., Soshnikov I.P., Kryzhanovskaya N.V., Moiseev E.I., Zhukov A.E. Coherent growth of InP/InAsP/InP nanowires on a Si (111) surface by molecular-beam epitaxy // Technical Physics Letters. 2018. V. 44. N 2. P. 112-114. https://doi.org/10.1134/S1063785018020116

15. Cirlin G.E., Reznik R.R., Samsonenko Yu.B., Khrebtov A.I., Kotlyar K.P., Ilkiv I.V., Soshnikov I.P., Kirilenko D.A., Kryzhanovskaya N.V. Phosphorus-based nanowires grown by molecular-beam epitaxy on silicon // Semiconductors. 2018. V. 52. N 11. P. 1416-1419. https://doi.org/10.1134/S1063782618110258

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Authors

Rodion R. Reznik — PhD, Junior Researcher, ITMO University, Saint Petersburg, 197101, Russian Federation, gg 57035485600, https://orcid. org/0000-0003-1420-7515, [e-mail]

Konstantin P. Kotlyar — Junior Researcher, St. Petersburg State University, Saint Petersburg, 199034, Russian Federation, S 57092676600, https://orcid.org/0000-0002-0305-0156, konstantin21kt@ gmail.com

Igor V. Shtrom — PhD, Senior Researcher, St. Petersburg State University, Saint Petersburg, 199034, Russian Federation, QC 15049790900, https://orcid.org/0000-0001-8912-2570, igorstrohm@ mail.ru

Yuriy B. Samsonenko — Scientific Researcher, Alferov University, Saint Petersburg, 194021, Russian Federation; Researcher, Institute for Analytical Instrumentation of the Russian Academy of Sciences, Saint Petersburg, 190103, Russian Federation, ^ 35411251800, https://orcid. org/0000-0002-7119-3925, [e-mail]

Artem I. Khrebtov — Senior Researcher, Alferov University, Saint Petersburg, 194021, Russian Federation, gg 6602521504, https://orcid. org/0000-0001-5515-323X, [e-mail]

Авторы

Резник Родион Романович — кандидат физико-математических наук, младший научный сотрудник, Университет ИТМО, Санкт-Петербург, 197101, Российская Федерация, 57035485600, https:// orcid.org/0000-0003-1420-7515, [e-mail] Котляр Константин Павлович — младший научный сотрудник, Санкт-Петербургский государственный университет, Санкт-Петербург, 199034, Российская Федерация, 57092676600, https:// orcid.org/0000-0002-0305-0156, [e-mail] Штром Игорь Викторович — кандидат физико-математических наук, старший научный сотрудник, Санкт-Петербургский государственный университет, Санкт-Петербург, 199034, Российская Федерация, 15049790900, https://orcid.org/0000-0001-8912-2570, [e-mail]

Самсоненко Юрий Борисович — научный сотрудник, Санкт-Петербургский национально-исследовательский академический университет имени Ж.И. Алфёрова РАН, Санкт-Петербург, 194021, Российская Федерация; старший научный сотрудник, Институт аналитического приборостроения РАН, Санкт-Петербург, 190103, Российская Федерация, 35411251800, https://orcid.org/0000-0002-7119-3925, [e-mail]

Хребтов Артём Игоревич — старший научный сотрудник, Санкт-Петербургский национально-исследовательский академический университет имени Ж.И. Алфёрова РАН, Санкт-Петербург, 194021, Российская Федерация, 6602521504, https://orcid.org/0000-0001-5515-323Х, [e-mail]

George E. Cirlin — D.Sc., Head of Laboratory, Alferov University, Saint Petersburg, 194021, Russian Federation; Chief Researcher, Institute for Analytical Instrumentation of the Russian Academy of Sciences, Saint Petersburg, 190103, Russian Federation; Leading Researcher, ITMO University, Saint Petersburg, 197101, Russian Federation, 7005945087, https://orcid.org/0000-0003-0476-3630, [e-mail]

Цырлин Георгий Эрнстович — доктор физико-математических наук, заведующий лабораторией, Санкт-Петербургский национально-исследовательский академический университет имени Ж.И. Алфёрова РАН, Санкт-Петербург, 194021, Российская Федерация; главный научный сотрудник, Институт аналитического приборостроения РАН, Санкт-Петербург, 190103, Российская Федерация; ведущий научный сотрудник, Университет ИТМО, Санкт-Петербург, 197101, Российская Федерация, 7005945087, https://orcid.org/0000-0003-0476-3630, [e-mail]

Received 08.09.2021

Approved after reviewing 05.10.2021

Accepted 30.10.2021

Статья поступила в редакцию 08.09.2021 Одобрена после рецензирования 05.10.2021 Принята к печати 30.10.2021

Работа доступна по лицензии Creative Commons «Attribution-NonCommercial»

Features of the MBE growth of nanowires with quantum dots on the silicon surface

R R Reznik1, K P Kotlyar1'2, A I Khrebtov2 and G E Cirlin1-4

1 Saint-Petersburg State University, Universitetskaya Emb. 13B, 199034 St. Petersburg, Russia

2 Alferov University, ul. Khlopina 8/3, 194021 St.-Petersburg, Russia

3 ITMO University, Kronverkskiy pr. 49, 197101, St-Petersburg, Russia

4 Institute for Analytical Instrumentation RAS, Rizhsky 26, 190103, St-Petersburg, Russia

[e-mail]

Abstract. The development of a new semiconductor element base is necessary to create a new generation of applications. At present time, the synthesis of high-quality hybrid nanostructures based on III-V quantum dots in the body of nanowires of a wide range of material systems is an urgent and important task. In work hybrid III-V nanostructures based on QDs in the body of NWs in GaP/GaAs and AlGaP/InGaP material systems were synthesized in on silicon substrates and their physical properties were investigated.

1. Introduction

Nowadays, the development of a new semiconductor element base is necessary to create a new generation of applications. Semiconductor quantum wells (QWs), nanowires (NWs) and quantum dots (QDs) based on III-V compounds are quantum objects and are promising for creating new devices in the field of optoelectronics, photonics and renewable energy sources and e.g. [1]. Moreover, modern synthesis methods make it possible to form nanostructures based on combinations of such materials, for example, QDs in the body of NWs [2]. Compared to QDs synthesized by other methods, it is an easier task to control the geometric parameters and location of such QDs. In turn, the unique morphology of NWs makes it possible to grow such hybrid structures on a silicon platform [3, 4] due to the unique morphological properties of NWs [5-7], which is an important task for most researchers in the field of micro- and nanoelectronics. In previous works, we have demonstrated that it is possible to synthesize by molecular beam epitaxy (MBE) III-V QDs in the body of NWs based on InP/InAsP [8, 9] and AlGaAs/GaAs [10-15] material systems on silicon wafers. The synthesized nanostructures have high crystallographic and optical qualities. The research results have shown that nanostructures based on InP/InAsP material system exhibit photoluminescence (PL) spectra up to room temperature in a wide range of wavelengths, including strategically important 1.3 and 1.5 ^m, and nanostructures based on AlGaAs/GaAs exhibit record-breaking low photoluminescence line width at half maximum (FWHM) and have the properties of single photon sources, which promising for quantum informatics and cryptography. Nevertheless, the synthesis of high-quality hybrid nanostructures based on III-V QDs in the body of NWs in a wide range of material systems remains an urgent and important task.

\ Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1

Within the framework of this work, hybrid III-V nanostructures based on QDs in the body of NWs in GaP/GaAs and AlGaP/InGaP material systems were synthesized in on silicon substrates and their physical properties were investigated.

2. Experimental procedures

Experiments on epitaxial growth of hybrid III-V nanostructures based on QDs in the body of NWs in GaP/GaAs and AlGaP/InGaP material systems were carried out using a unique MBE machine "Compact 21T" from "Riber" company, equipped with effusion sources of III-V semiconductor materials and additional chambers for cleaning and deposition of metals. Monolithic silicon wafers doped with boron atoms and polished on one side were used as substrates for epitaxial synthesis. Growth technology contained a number of fundamental stages. First, the substrate was subjected to chemical cleaning in hydrofluoric acid diluted with water in a ratio of 1 to 4. In addition to cleaning the surface, this procedure allows to partially remove the layer of natural silicon oxide layer on the surface of the substrate. Immediately after chemical cleaning, the substrates were loaded into the lock chamber of the MBE setup, where they were heated to a temperature of 400°C for preliminary thermal cleaning. After 10 minutes of cleaning, the substrate temperature was reduced to 100°C and the substrate was transferred to the metallization chamber. In this chamber, the substrate was heated to 900°C for the purpose of complete thermal cleaning and subsequent metallization of the surface. After 30 minutes of thermal annealing, the substrate temperature decreased to 550°C. After stabilizing the temperature, a few nm thick gold layer was deposited on the substrate surface. Holding at the same temperature for 1 minute after gold deposition allows the formation of homogeneous drops on the substrate surface, which serve as catalysts for the growth of NWs by the vapor-liquid-solid (VLS) mechanism. [1]. At the next stage, the temperature of the substrates was reduced to 100°C and transferred in ultra-high vacuum conditions into the chamber for the growth of nanostructures. Then to grow the GaP NWs the substrate temperature was increased to the 580°C and Ga and P sources were opened for 16 minutes. An the next step Ga and P shutters were close for 1 minute so that phosphorus was pumped out of the chamber and did not participate in the growth of the QDs. After that, Ga and As shutters were pen for 7 second for GaAs QD formation. Finally, after 1 minute pause for pumping of As, Ga and P shutters were again opened for 5 minutes to form final part of GaP NWs. In case of AlGaP NWs with InGaP QDs the growth method was not fundamentally different, but substrate temperature during the growth was set at 530°C due to the specificity of the aluminum and indium atoms participation in the growth.

For determination of the morphological parameters of the synthesized arrays of NWs with QDs, we used the technique of scanning electron microscopy SEM using Supra 25 Zeiss microscope.

3. Results

Figure 1 shows the typical SEM image of GaP NWs array with GaAs QDs inside grown on silicon substrate. The direction of NWs growth <111> indicates epitaxial growth and bond with the substrate. The grown sample demonstrates the surface density of NWs about 3 107cm-2, which corresponds to the density of gold droplets. We note that the height and diameter of the grown nanostructures is highly inhomogeneous over the surface which can lead to the difference in the optical properties. The possible reason may be different sizes of the Au particles formed prior the growth of GaP NWs and to investigate the optical properties of this array needs an additional optimization.

Figure 1. Typical SEM image of GaP NWs with GaAs QDs grown on Si(111) substrate.

Typical SEM images of AlGaP NWs with InGaP QDs are shown in figure 2. Similar to the case of GaP NWs, AlGaP NWs were formed mainly in the <111> direction. It seen that overage high of grown NWs is 900 nm and diameter - 140 nm. It is important to note, that some of the NWs began to bend and grow in a different direction. This is due to the usage of indium as a part of quantum dot material. According to [16] with a sufficient content of indium in the gold catalyst droplet, such a mixed droplet can etch the facets at the top of the NWs, thereby descending to the side of the NWs, and change the direction of NWs growth.

Figure 2. Typical SEM image of AlGaP NWs with InGaP QDs grown on Si(111) substrate.

4. Conclusions

In conclusion, we have demonstrated the principal growth possibility of GaP NWs with GaAs QDs and AlGaP NWs with InGaP QDs on silicon substrates. Morphological parameters of grown arrays of nanostructures were studied using SEM technique, which confirmed their epitaxial formation. Expansion of the range of material systems and the composition of semiconductor III-V QDs in the body of NWs can lead to the creation of new applications based on them. Thus, the research results

open up new opportunities for creating quantum applications in the field of computer science and cryptography on silicon.

Acknowledgments

The samples were grown under support of St. Petersburg State University research grant 75746688. Morphological properties measurements were done under support of Russian Science Foundation (project 19-72-30004).

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13 3802

ISSN 1063-7850, Technical Physics Letters, 2018, Vol. 44, No. 2, pp. 112-114. © Pleiades Publishing, Ltd., 2018.

Original Russian Text © R.R. Reznik, G.E. Cirlin, I.V. Shtrom, A.I. Khrebtov, I.P. Soshnikov, N.V. Kryzhanovskaya, E.I. Moiseev, A.E. Zhukov, 2018, published in Pis'ma v Zhur-nal Tekhnicheskoi Fiziki, 2018, Vol. 44, No. 3, pp. 55-61.

Coherent Growth of InP/InAsP/InP Nanowires on a Si (111) Surface by Molecular-Beam Epitaxy

R. R. Reznika,b, c, d, G. E. Cirlina, bcd*, I. V. Shtrom® b, A. I. Khrebtov®, I. P. Soshnikov® b, e, N. V. Kryzhanovskaya®, E. I. Moiseev®, and A. E. Zhukov®

a St. Petersburg National Research Academic University, Russian Academy of Sciences, St. Petersburg, Russia b Institute of Analytical Instrument Making, Russian Academy of Sciences, St. Petersburg, Russia c St. Petersburg National Research University of Information Technologies, Mechanics, and Optics, St. Petersburg, Russia d Peter the Great St. Petersburg Polytechnic University, St. Petersburg, Russia e Ioffe Physical Technical Institute, Russian Academy of Sciences, St. Petersburg, Russia *e-mail: [e-mail] Received July 28, 2017

Abstract—Results obtained in a study of the growth of InP/InAsP/InP nanowires on the Si (111) surface are presented. Using a special procedure of substrate preparation immediately before the growth made it possible to obtain a nanowire coherency with the substrate of nearly 100%. A high-intensity emission from nanostruc-tures of this kind was observed at a wavelength of ~1.3 |im at room temperature.

DOI: 10.1134/S1063785018020116

Nanowires (NWs) of semiconductor compounds have recently been the obj ect of intense interest in both fundamental and applied respects [1]. Their properties are determined by their unique geometric parameters (commonly, the transverse size of NWs is 10—200 nm and their height substantially exceeds the diameter), as well as by the possibility of growing defect-free hetero-structures, including ones composed of materials with different crystalfographic lattice constants. The reason for this is that the relaxation of elastic stresses appearing due to the difference in the lattice constants occurs at side faces of NWs, without formation of defects [2]. Coherent growth of NWs on foreign substrates, e.g., that of III—V materials on silicon, is also possible [3— 5]. When NWs are grown, drops are first formed on the substrate surface as growth catalysts, just below which the growth of NWs occurs. One possible mechanism of NW growth in the case of molecular-beam epitaxy (MBE) is based on diffusion, in which adatoms are delivered along side faces of NWs to the interface between a drop and the NW apex, this being where the crystallization occurs [6, 7]. By varying the elemental composition of molecular flows directed to the substrate, we can form NWs of various compositions. In addition, it is possible to form a nanostruc-ture insert in an NW with a larger energy-band width. If the characteristic size of such an insert does not exceed the Bohr radius, a system of this kind can be considered a quantum dot (QD) in the NW. GaAs QDs in AlGaAs NWs [8] or InAsP QDs in InP NWs (see, e.g., [9—12]) can serve as examples of such a

kind. However, the growth of structures of this kind on a silicon substrate has not received any considerable attention [13, 14]. In addition, the data reported in these communications point to a characteristic feature: most NWs do not grow perpendicularly to the substrate, which indicates that the NWs are incoherent with respect to the substrate. In the present communication, we report a nearly 100% coherent growth of NWs in the InP/InAsP/InP system on the Si (111) surface, which possess, among other properties, a high optical quality. The key issue providing a considerable improvement of NW characteristics is that the conditions of ultrahigh vacuum are not violated in the stage of substrate transfer to the growth chamber after the deposition of gold, which is the material catalyzing the growth of NWs.

The growth experiments were performed in a Riber Compact 21 MBE machine. The installation includes a separate metallization chamber with an effusion source of gold, from which a sample can be transferred into the growth chamber without violation of vacuum conditions. Boron-doped Si (111) wafers served as substrates for MBE growth. In the preliminary stage, the silicon substrates were subjected to the standard cleaning procedure in an aqueous solution of hydrofluoric acid (1 : 10). A substrate charged into the metallization chamber was heated to 850°C for thermal cleaning. Then, the substrate temperature was lowered to 550°C and a 0.1-nm-thick layer of gold was deposited to form nanometer drops. A wafer with the catalyst drops was transferred to a growth chamber

COHERENT GROWTH OF InP/InAsP/InP NANOWIRES

113

Fig. 1. SEM image of an InP/InAsP/InP ensemble of NWs: (a) lateral view and (b) top view.

equipped with an effusion source of indium and cracker sources of arsenic and phosphorus, and the substrate was heated to 250°C. After the substrate temperature was stabilized shutters of indium and phosphorus were opened and InP NWs started to be synthesized. The flux of In onto the substrate surface corresponded to a growth rate of a 2D layer equal to 0.5 monolayer per second. After 15 min of growth, the arsenic shutter was opened for 20 s to form the InAsP insert. In the final stage of the growth process, the arsenic shutter was closed and the growth of InP continued for 5 min more to form the capping layer. To study the influence exerted by As on the formation of the InAsP insert, two samples were grown (A and B), which differed from each other only in the relative fluxes of arsenic and phosphorus in the formation of the InAsP insert. According to preliminary calibrations with a Bayard—Alpert sensor, the pressure of the P2 flow was 6.0 x 10-6 Torr for both samples, while the pressure of As4 flow was 1.1 x 10-5 Torr in the first case (sample A) and 9.0 x 10-6 Torr in the second (sample B). To study the surface in situ during the growth experiments, we used the method of reflection high-energy electron diffraction (RHEED). The RHEED patterns unambiguously indicated that the NWs are formed in the wurtzite form.

The surface morphology of the samples we obtained was examined by scanning electron microscopy (SEM) on a Zeiss Supra 25 microscope. Figure 1 shows typical SEM images of the NW arrays grown in the study. It follows from the figure that the average height of InP NWs is ~4 ^m, and their diameter was found to be uneven along the height—100 nm at the base and 30 nm at the NW apex—at a surface NW density of 3 x 10-8 cm-2. Based on the fact that the insert was formed after 15 min of NW growth, we can suppose that it is situated at a distance of ~1 ^m from the NW apex. It follows from the condition of a constant NW-growth rate that the height of the insert in an NW is ~60 nm. Presumably, an InAsP insert of this size cannot exhibit quantum-confinement effects (at least in the NW-growth direction) because the Bohr radius for InAs is 34 nm [15], while it is even smaller for InP (15 nm) [16].

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Fig. 2. Room-temperature (RT) PL spectra of InP/InAsP/InP NW ensembles grown with different ratios between the phosphorus and arsenic fluxes for the InAsP insert.

It should be particularly noted that InP NWs were predominantly formed in the (111) direction, with only less than 5% of all the NWs having other growth directions. This is due to a characteristic of the means of deposition of the metallic layer to form drops as growth catalysts: the absence of a stage in which a substrate is transferred to the growth chamber of the MBE machine with violation of the vacuum conditions. Otherwise, the fast formation of a natural oxide layer on the substrate surface and its further removal at a rather high substrate temperature (~950°C) may lead to an uncontrolled etching at the drop-substrate interface and, accordingly, to disruption of the growth coherence. It should also be noted that such a high percentage of coherent NWs is uncharacteristic even of NW ensembles produced by lithographic methods, for which a coherence of 80% is already substantial [17].

The photoluminescence (PL) from InP/InAsP/InP arrays was studied under excitation with a YLF : Nd3+ laser (wavelength 527 nm, power 10-500 mW). The signal was recorded by the lock-in-detection method with a Spectral Products DK480 monochromator and single-channel InGaAs detector (Sciencetech). Figure 2 shows PL spectra of samples A and B formed with As/P flux ratios of 1.8 and 1.5, respectively.

The PL spectra of both samples show a low-intensity band peaked at around 1.38 eV, which corresponds to the emission from InP NWs. In addition, the PL spectra have a broad emission band in the range 0.8-1.2 eV, which is a superposition of two lines: for sample A, these are line A1 (peaked at ~0.91 eV) and line A2 (peaked at ~1.04 eV); for sample B, these are line B1 (peaked at ~0.96 eV) and line B2 (peaked at ~1.09 eV). Two lines of the PL signal in the

7

6

1

0

438 REZNIK et al.

range of 1000—1500 nm (1.2—0.8 eV) correspond to the emission from InAsP inserts in the InP NWs, but are not of the same nature. The longer-wavelength PL lines A1 and B1 correspond to the emission from inserts elongated in the growth direction, i.e., to the energy-gap widths of the bulk material InAs1 _ xPx obtained in InAsP growth with different As/P ratios. Percentage phosphorus content x can be determined by the formula 0.35 + 0.714x + 0.281x2 = 0 (with the difference of the energy-gap width for the wurtzite phase disregarded) [18]. Thus, x = 0.33 for sample B and x = 0.38 in accordance with the higher flux of As for sample A. According to the results reported in previous publications [9, 10, 12], an InAsP quantum well is formed during the InAsP formation around the NW body in addition to the axial insert. We attribute the PL lines A2 (1.04 eV) and B2 (1.09 eV) to the emission from these quantum wells, which have the form of radial layers with a thickness of several nanometers. To evaluate the optical quality of the structures, we compared their integral PL intensities with the intensity of a reference structure for which the optical quantum efficiency was taken to be 100%. The integral PL intensity of structures A and B is 20 and 15%, respectively, which is indicative of their high optical perfection. The observed difference between the intensities may be due to the different surface densities of NW arrays in the parts under study of structures A and B. A detailed study of the structural properties of InP/InAsP/InP will be carried out separately.

Thus, the results we obtained indicate that nearly 100% of coherent NWs can be formed with high optical quality in the InP/InAsP/InP system on a silicon surface. The presence of a band with maximum emission intensity near 1.3 ^m makes it possible to consider the given system promising for further integration of optical elements on a silicon platform with fiber-optic systems.

Acknowledgments. The samples were fabricated with support from the Russian Science Foundation, project no. 14-12-00393. The study was supported in part by the Russian Foundation for Basic Research (project nos. 16-29-03113-ofi and 16-29-03037-ofi) and the Ministry of Education and Science of the Russian Federation (projects nos. 3.9787.2017/8.9 and 16.9791.2017/8.9).

REFERENCES

1. V. G. Dubrovskii, G. E. Cirlin, and V. M. Ustinov, Semiconductors 43, 1539 (2009).

2. F. Glas, Phys. Rev. B 74, 121302 (2006).

3. V. G. Dubrovskii, I. V. Shtrom, R. R. Reznik, Yu. V. Sam-sonenko, A. I. Khrebtov, I. P. Soshnikov, S. Rouvimov, N. Akopian, T. Kasama, and G. E. Cirlin, Cryst. Growth Des. 16, 7251 (2016).

4. G. E. Cirlin, V. G. Dubrovskii, I. P. Soshnikov, N. V. Si-birev, Yu. B. Samsonenko, A. D. Bouravleuv, J. C. Har-mand, and F. Glas, Phys. Status Solidi RRL 3, 112 (2009).

5. G. E. Cirlin, V. G. Dubrovskii, Yu. B. Samsonenko,

A. D. Bouravleuv, K. Durose, Y. Y. Proskuryakov,

B. Mendes, L. Bowen, M. A. Kaliteevski, R. A. Abram, and D. Zeze, Phys. Rev. B 82, 035202 (2010).

6. V. G. Dubrovskii, G. E. Cirlin, I. P. Soshnikov,

A. A. Tonkikh, N. V. Sibirev, Yu. B. Samsonenko, and V. M. Ustinov, Phys. Rev. B 71, 205325 (2005).

7. G. E. Cirlin, V. G. Dubrovski, N. V. Sibirev, I. P. Sosh-nikov, Yu. B. Samsonenko, A. A. Tonkikh, and V. M. Ustinov, Semiconductors 39, 557 (2005).

8. G. E. Cirlin, I. V. Shtrom, R. R. Reznik, Yu. B. Sam-sonenko, A. I. Khrebtov, A. D. Bouravleuv, and I. P. So-shnikov, Semiconductors 50, 1421 (2016).

9. M. Tchernycheva, G. Patriarche, L. Travers, V. Zwiller, U. Perinetti, and J.-C. Harmand, Nano Lett. 7, 1500 (2007).

10. J.-C. Harmand, F. Jabeen, L. Liu, G. Patriarche, K. Gauthron, P. Senellart, D. Elvira, and A. Beveratos, J. Cryst. Growth 378, 519 (2013).

11. E. D. Minot, F. Kelkensberg, M. van Kouwen, J. A. van Dam, L. P. Kouwenhoven, V. Zwiller, M. T. Borgstrom, O. Wunnicke, M. A. Verheijen, and E. P. A. M. Bakkers, Nano Lett. 7, 367 (2007).

12. G. E. Cirlin, M. Tchernycheva, G. Patriarche, and J.-C. Harmand, Semiconductors 46, 175 (2012).

13. H. Khmissi, K. Naji, M. H. Hadj Alouane, N. Chauvin, C. Bru-Chevallier, B. Ilahi, G. Patriarche, and M. Gendry, J. Cryst. Growth 344, 45 (2012).

14. P. Kuyanov and R. R. LaPierre, Nanotecnology 26, 315202 (2015).

15. A. L. Efros and M. Rosen, Ann. Rev. Mater. Sci. 3, 475 (2000).

16. H. J. Byun, J. C. Lee, and H. Yang, J. Colloid Interface Sci. 355, 35 (2011).

17. A. M. Munshi, D. L. Dheeraj, V. T. Fauske, D. C. Kim, J. Huh, J. F. Reinertsen, L. Ahtapodov, K. D. Lee,

B. Heidari, A. T. J. van Helvoort, B. O. Fimland, and H. Weman, Nano Lett. 14, 960 (2014).

18. G. A. Antypas and T. O. Yep, J. Appl. Phys. 42, 3201 (1971).

Translated by M. Tagirdzhanov

ISSN 1063-7826, Semiconductors, 2018, Vol. 52, No. 11, pp. 1416-1419. © Pleiades Publishing, Ltd., 2018.

Original Russian Text © G.E. Cirlin, R.R. Reznik, Yu.B. Samsonenko, A.I. Khrebtov, K.P. Kotlyar, I.V. Ilkiv, I.P. Soshnikov, D.A. Kirilenko, N.V. Kryzhanovskaya, 2018, published in Fizika i Tekhnika Poluprovodnikov, 2018, Vol. 52, No. 11, pp. 1304-1307.

XXII INTERNATIONAL SYMPOSIUM ^^^^^^^^^^ "NANOPHYSICS AND NANOELECTRONICS", NIZHNY NOVGOROD, MARCH 12-15, 2018

Phosphorus-Based Nanowires Grown by Molecular-Beam Epitaxy on Silicon

G. E. Cirlin"4^,*, R. R. Reznikc, Yu. B. Samsonenko" 4, A. I. Khrebtov"c, K. P. Kotlyar", I. V. Ilkiv", I. P. Soshnikov"4*, D. A. Kirilenko'*, and N. V. Kryzhanovskaya"

a St. Petersburg Academic University, Russian Academy of Sciences, St. Petersburg, 194021 Russia b Institute for Analytical Instrumentation, Russian Academy of Sciences, St. Petersburg, 190103 Russia c ITMO University, St. Petersburg, 197101 Russia d Ioffe Institute, St. Petersburg, 194021 Russia *e-mail: [e-mail] Submitted April 25, 2018; accepted for publication May 7, 2018

Abstract—Data on the growth and physical properties of nanostructures of the type "InAsP insert embedded in InP nanowire (NW)" grown on Si (111) surfaces by Au-assisted molecular-beam epitaxy are presented. It is found that nearly 100%-coherent NWs can be grown with a widely varying surface density. A relationship between the optical and structural properties of the NWs is revealed. It is shown that the NWs under study are formed of a purely wurtzite phase. The suggested technology opens up new opportunities for the integration of direct-gap III-V materials and silicon.

DOI: 10.1134/S1063782618110258

1. INTRODUCTION

The topicality of studies of nanowires (NWs) is due to the need to solve an important problem of creating new nonplanar semiconductor nanomaterials and nanosystems with controlled properties. These materials can be and are already being used in the development of devices for microelectronics, optoelectronics, analytical biomedicine, emission cathodes, probes for scanning tunneling microscopy, high-efficiency solar energy converters, etc. [1—3]. Heterostructures based on phosphide compounds are among the most widely studied because the optical range covers both the infrared (IR) spectral range (e.g., InAsP solid solutions) and the UV-Vis (ultraviolet-visible) spectral ranges (GaAsP, AlP, etc.). One of the important advantages of NWs is their possible growth on foreign substrates, with high crystallographic and optical properties retained. The reason is that mechanical stresses which inevitably arise during the growth of lattice-mismatched compounds are effectively relieved on the lateral faces of NWs and thereby the formation of defects of the type of mismatch dislocations is suppressed [4]. In this context, it becomes possible, when the growth of phosphide NWs is considered, both to perform growth on less expensive substrates, compared with those of the phosphide type, and form heterostructures directly within the body of NWs. In addition, one of the properties of III—V NWs is their possible formation in both the cubic and hexagonal

phases. This effect is observed for most III—V NWs (GaAs, InAs, GaP, InP, etc.) for various growth technologies on III—V and Si (111) substrates, both in terms of catalytic growth on Au particles and in the absence of a catalyst. The possible alternation of structures with different crystalline phases results in the formation of novel heterostructures, e.g., a so-called "crystallographic quantum dot." Therefore, controlling the crystalline phase is presently considered one of the principal problems on the way to the practical application of semiconductor NWs. Previously, we studied the growth of InP/InAsP NW heterostructures grown on native substrates [5, 6]. In this work, we report on the coherent growth of NWs in the InP/InAsP/InP system on the surface Si (111) and on a study of their structural and optical properties.

2. EXPERIMENTAL

The growth experiments were performed on a Riber Compact 21 installation for molecular-beam epitaxy (MBE). The installation includes a separate metallization chamber with an effusion source of gold. A sample can be moved from this chamber into the growth chamber without deterioration of vacuum conditions. Si (111) wafers served as substrates for MBE growth. In the preliminary stage, the silicon substrates were subjected to the standard cleaning procedure in an aqueous solution of hydrofluoric acid (1 : 10). A substrate placed in the metallization chamber was

PHOSPHORUS-BASED NANOWIRES GROWN BY MOLECULAR-BEAM EPITAXY

1417

Fig. 1. SEM images of samples of InP/InAsP/InP NW ensembles with different amounts of gold deposited prior to growth: (a) 0.1 nm and (b) 1 nm.

heated to 850°C for thermal cleaning. Then, the substrate temperature was lowered to 550°C and a gold layer with a thickness of 0.1—1 nm was deposited for nanoscale drops to be formed. A wafer with thus formed catalyst drops was placed in the growth chamber equipped with an effusion source of indium and cracker sources of arsenic and phosphorus, and the substrate was heated there to 250°C. After the substrate temperature stabilized, shutters of indium and phosphor were opened and InP NWs started to be synthesized. The flux of In onto the substrate surface corresponded to a 2D-layer growth rate of 0.5 SL/s. After 15 min of growth, the arsenic shutter was opened for some time (20 s) for the InAsP insert to be formed. In the final growth stage, the arsenic shutter was closed and InP continued to grow for 5 min more, to form the capping layer on the NW body. To examine in situ the surface during growth experiments, the reflection high-energy electron diffraction (RHEED) method was used. The RHEED patterns unambiguously indicated that NWs are formed in the wurtzite phase.

The surface morphology was examined by scanning electron microscopy (SEM). The photoluminescence (PL) from InP/InAsP/InP NW arrays was examined under excitation with a YLF:Nd+3 laser (wavelength of 527 nm, power 10—500 mW) The laser-beam area was ~100 |m2. The synchronous-detection method, Spectral Products DK-480 monochromator, and single-channel InGaAs (Sciencetech) detector were used to record the signal.

3. RESULTS AND DISCUSSION

Figure 1 shows typical SEM images of InP/InAsP/InP NW arrays grown at different gold-layer thicknesses (0.1 and 1 nm). It is evident that the surface density is substantially higher in the second case (by more than an order of magnitude), so that the number of NWs is proportional to the density of gold drops, i.e., growth catalysts, formed on the surface.

Such a significant degree of freedom, making it possible to controllably vary the surface density of NWs, is important for studies of the local physical properties of NWs, when it is necessary to measure in an isolated way the properties of only single NWs. At the same time, a denser array of NWs enables analysis of the collective properties of NW ensembles. It follows from Fig. 1 that the average height of NWs (~4 |m) is independent of their surface density, and the diameter of NWs is nonuniform along their height, being 100 nm at the base of a NW and 30 nm at its top.

It should be noted that the InP NWs were formed predominantly in the (111) direction, just less than 5% of all the NWs had other growth directions. This is due to the specific deposition of a metallic layer from which drops, i.e., growth catalysts, are formed: there is no stage in which a substrate is transferred after this layer is deposited into the growth chamber of the MBE installation with deterioration of vacuum conditions. Otherwise, because of the rapid formation of a natural oxide layer on the substrate surface, its further removal would occur at a rather high substrate temperature (~950°C), which can disrupt growth coherence.

Figure 2 shows the room-temperature PL spectra of InP/InAsP/InP/Si(111) NW ensembles. As expected, the integrated PL intensity from a sample with a lower surface density of NWs is substantially weaker than that for a sample with a high NW density. Nevertheless, to assess the optical quality of the structures, we compared their integrated PL intensity with that for a reference structure having nearly 100% optical quantum efficiency. The integrated PL intensity is 1 and 20% for the less and more dense samples, respectively, which is indicative of their high optical perfection. The PL spectra of both samples show a broad emission band at 1000—1500 nm, which is a superposition of several PL lines. However, the PL line for a less dense sample is substantially narrower than that for the denser sample, which points to a higher uniformity of NWs within the region illuminated by the laser.

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Indeed, the total number of excited NWs exceeds 1000 in the case of higher densities, whereas for the less dense sample, there are only tens of these. To reveal the nature of the PL, we carried out an additional study of the NW structure by transmission electron microscopy (TEM).

Figure 3 shows a typical TEM image of an InP/InAsP/InP NW dispersed onto a carbon grid and the arsenic distribution (x) map for various parts of the NW. Apparently, the arsenic distribution is inhomo-geneous. Predominantly two objects are formed in the NW: (i) InAsP inserts proper, with characteristic

dimensions of 8 nm in diameter and 100 nm in length and an average arsenic content of 40% at a distance of 1 ^m from the NW top; and (ii) an InAsP quantum well (QW) with an average arsenic content of 15—25% around the NW body because of simultaneous lateral growth of the material during insert formation. Thus, we attribute the longest wavelength lines in the PL spectra to the emission from InAs1 _ xPx inserts elongated in the growth direction, which corresponds to the band-gap widths of the InAs1 _ xPx bulk material. The shorter wavelength broadened PL lines correspond to emission from the InAsP QWs having the form of radial layers with a thickness of several nanometers and varied arsenic content.

It should be noted that the NWs under study were found to be rather pure as regards their crystallo-graphic phase. As already noted, NWs show for most materials a poorly controllable change in the crystallo-graphic phase within a NW even across small distances [7]. In our case, a purely wurtzite phase is formed without any stacking faults of the type of twins or wurtzite—zinc-blende phase transference, which is an indubitable advantage of the InP/InAsP/InP/Si(111) NW system under consideration. Presumably, this is due to a low growth temperature, which leads to the stable growth of NW layers under a catalyst drop at the "vacuum—drop—solid" triple boundary. This is a necessary condition for NWs to be formed in the wurtzite phase [8]. In addition, using such a low growth temperature (250°C) favors the possible integration of optical elements on a silicon platform with fiber-optic communication systems.

4

2

Fig. 3. TEM images of single InP/InAsP/InP NWs: (a) general view of a single NW and (b) upper part of a NW, with percentage content of arsenic indicated.

PHOSPHORUS-BASED NANOWIRES GROWN BY MOLECULAR-BEAM EPITAXY

1419

4. CONCLUSIONS

Thus, the growth of nanostructures of the type of "InAsP insert introduced into InP nanowires (NWs)" on the Si (111) surface by molecular-beam epitaxy with gold used as a catalyst was considered and their properties were examined. It was found that nearly 100% coherent NWs with widely varied surface density can be obtained and a relationship was established between the structural and optical properties of the NWs. It was shown that the NWs are composed of a purely wurtzite phase.

ACKNOWLEDGMENTS

The study was financially supported by the Russian Science Foundation (grant no. 14-12-00393). R.R. Reznik and I.V. Ilkiv are greatfull to RFBR grant no. 18-32-00768 mol_a.

REFERENCES

1. V. G. Dubrovskii, G. E. Tsyrlin, and V. M. Ustinov, Semiconductors 43, 1539 (2009).

2. J. Ramanujam, A. Verma, B. González-Díaz, R. Guer-rero-Lemus, C. Cañizo, E. Garcáa-Tabarés, I. Rey-Stolle, F. Granek, L. Korte, M. Tucci, J. Rath, U. P. Singh, T. Todorov, O. Gunawan, S. Rubio, J. L. Plaza, E. Diéguez, B. Hoffmann, S. Christiansen, and G. E. Cirlin, Prog. Mater. Sci. 82, 294 (2016).

3. A. V. Senichev, V. G. Talalaev, I. V. Shtrom, H. Blumtritt, G. E. Cirlin, J. Schilling, Ch. Lienau, and P. Werner, ACS Photon. 1, 1099 (2014).

4. F. Glas, Phys. Rev. B 74, 121302 (2006).

5. M. Tchernycheva, G. E. Cirlin, G. Patriarche, L. Travers, V. Zwiller, U. Perinetti, and J.-Ch. Harmand, Nano Lett. 7, 1500 (2007).

6. G. E. Cirlin, M. Tchernycheva, G. Patriarche, and J.-C. Harmand, Semiconductors 46, 175 (2012).

7. I. P. Soshnikov, G. E. Cirlin, A. A. Tonkikh, Yu. B. Samsonenko, V. G. Dubrovskii, V. M. Ustinov, O. M. Gorbenko, D. Litvinov, and D. Gerthsen, Phys. Solid State 47, 2213 (2005).

8. F. Glas, J. C. Harmand, and J. Patriarche, Phys. Rev. Lett. 99, 146101 (2007).

Translated by M. Tagirdzhanov

ISSN 1063-7826, Semiconductors, 2020, Vol. 54, No. 9, pp. 1141-1146. © Pleiades Publishing, Ltd., 2020.

Russian Text © The Author(s), 2020, published in Fizika i Tekhnika Poluprovodnikov, 2020, Vol. 54, No. 9, pp. 952-957.

XXIV INTERNATIONAL SYMPOSIUM "NANOPHYSICS AND NANOELECTRONICS", NIZHNY NOVGOROD, MARCH 10-13, 2020

Luminescence Photodynamics of Hybrid-Structured InP/InAsP/InP

Nanowires Passivated by a Layer of TOPO-CdSe/ZnS Quantum Dots

A. I. Khrebtov"*, A. S. Kulagina", V. V. Danilov4, E. S. Gromova4, I. D. Skurlovc, A. P. Litvinc,

R. R. Reznikc, I. V. Shtrom"d, and G. E. Cirlin",^

a Alferov University, Russian Academy of Sciences, St. Petersburg, 194021 Russia b St. Petersburg State Transport University, St. Petersburg, 191031 Russia c ITMO University, St. Petersburg, 197101 Russia d Institute for Analytical Instrumentation, Russian Academy of Sciences, St. Petersburg, 190103 Russia

*e-mail: [e-mail] Received April 15, 2020; revised April 21, 2020; accepted April 21, 2020

Abstract—The results of studies of the decay photodynamics of excited states in a hybrid semiconductor nano-structure formed as an array of InP nanowires with an InAsP nanoinsert that are passivated with a quasi-Langmuir trioctylphosphine oxide (TOPO) layer containing colloidal CdSe/ZnS quantum dots are presented. The luminescence spectra and kinetics of InAsP nanoinserts in the near-infrared region at temperatures of 80 and 293 K are recorded. The formation of the layer of TOPO-CdSe/ZnS quantum dots at the surface of InP/InAsP/InP nanowires brings about an increase in the duration of radiative recombination and the appearance of its dependence on temperature. It is established that, in the synthesized structure, there is a type-II heterojunction at the interface between the InAsP nanoinsert and the InP bulk. The influence of interphase processes on an increase in the duration of emission is discussed.

Keywords: molecular-beam epitaxy, nanowires, colloidal quantum dots, luminescence DOI: 10.1134/S1063782620090158

1. INTRODUCTION

Hybrid semiconductor nanostructures have been extensively studied over the last few years. Of particular interest is the creation of composites from semiconductor materials with different dimensionalities [1—6] for wide practical application. It is obvious that, in order to use the nanocomposites, it is essential to understand the character of interphase photodynamics and the specific features of interactions that couple exciton and surface states. One of the important mechanisms of interaction between the components of composite structures is nonradiative energy transfer [7—9]. Studies carried out over the last few years show that spectroscopic and kinetic methods have proved themselves as an efficient tool for analyzing the photodynamics of nonradiative processes in semiconductor nanocrystals [10—16]. In this case, of considerable importance is adequate mathematical description of experimental dependences relating to photoluminescence (PL) decay kinetics [14, 17—22]. By the example of a hybrid semiconductor nanostructure formed as an array of InP nanowires (NWs) with an InAsP

nanoinsert (NI) that are grown by molecular beam epitaxy (MBE) on a Si( 111) substrate, it was shown that the best fit of the spectral and kinetic dependences was achieved in correlating the results of mul-tiexponential decomposition with parameters determined by processing the kinetic curves with the stretched exponential function (Kohlrausch function) [23]. This made it possible to confirm the existence of nonradiative energy transfer through the dipole— dipole mechanism (Forster resonance energy transfer (FRET)) in the researched system and to describe the luminescence decay for InAsP NI by the contact quenching mechanism. In addition, we showed that, in such a structure with a layer of (trioctylphosphine oxide (TOPO))—CdSe/ZnS quantum dots (QDs) deposited onto the nanowire surface, there existed nonradiative energy transfer between the QDs and NIs by the dipole—dipole mechanism (FRET) [24]. This study is aimed at investigating the luminescence pho-todynamics in a hybrid InP/InAsP/InP NW structure passivated with a layer of TOPO-CdSe/ZnS QDs.

X, nm

Fig. 1. Normalized luminescence spectra of hybrid nano-structures: (1, 2) InP/InAsP/InP NWs and (3, 4) NWs passivated with a TOPO—CdSe/ZnSe QD layer. The spectra are recorded at (1, 3) 80 and (2, 4) 293 K. Xex = 633 nm.

2. EXPERIMENTAL

The NW crystals were synthesized by MBE using a Compact 21 (Riber) setup. The average InP NW height was 4 ^m, and the diameter was nonuniform along the NW and corresponded to 100 nm near the NW base and 30 nm at the NW top. The surface density of NWs was 3 x 108 cm-2. The InAsP inserts were 60 nm in length and 15 nm in width; the depth of the insert in the NW body was 10 nm. A solution of colloidal QDs in toluene was deposited onto the substrate with NWs; the concentration of QDs in the solution was ~10-5 M. The QDs were formed as a CdSe core (~3 nm in diameter) coated with a ZnS shell and a TOPO ligand layer. The structural properties of the hybrid structure were studied by transmission electron microscopy (TEM). (The TEM images were presented previously [24].) In addition to the conclusions drawn in [24], it should be noted that, as follows from analysis of the TEM images of the TOPO-CdSe/ZnS QD shell, there exist both short- and long-range orders, with the average spacing between QDs of ~5 nm, and there are no conglomerates. The shell is a structure with the average spacing between QDs of ~5 nm. The above-listed features of the shells make them similar to Langmuir films. The hybrid structure fabricated in the study is stable in air and does not change its PL parameters with time.

In spectroscopic studies of the array of NWs with a NI, the excitation source was continuous-wave laser emitting at a wavelength of 633 nm; the emission power was varied in the range 0.5-6 mW. Secondary emission was collected in accordance with the standard 90° scheme; the excitation radiation was cut off with an appropriate optical filter. The experimentally obtained spectra were normalized to the sensitivity of the detector. For the spectroscopic device, a diffraction monochromator was used. The PL spectra in the

near-IR region were recorded with the use of an InGaAs photodiode (Hamamatsu). The luminescence signal was detected in the wavelength range 1.01.7 ^m. In studies of the PL kinetics, for the excitation source, we used a pulsed laser emitting at the wavelength 635 nm (with the pulse-repetition frequency ~2.5 MHz, the pulse energy ~6 nJ, and the pulse duration <100 ps). The kinetic measurements were conducted at temperatures of 80 and 293 K.

3. RESULTS AND DISCUSSION

In the luminescence spectra of the arrays of InP/InAsP/InP NWs, we observe high-intensity bands in the range 1.1-1.5 ^m corresponding to the emission of InAsP NIs in InP NWs. The luminescence band in the range 1.3-1.4 ^m relates to the InAsP NI with the average As content 40%. The short-wavelength band at 1.1-1.2 ^m relates to radial quantum wells (QWs) with the average As content 15-25%. Such QWs are formed upon the deposition of a thin InAsP layer onto the NW surface during the growth of NIs. The results of TEM studies of the structural properties of NWs suggested that the NWs were formed mainly in the wurtzite crystallographic phase [25].

An increase in the temperature from 80 to 293 K is accompanied by a bathochromic shift of both the NI-and QW-related bands (such a shift is typical of semiconductor structures), an increase in the full width at half-maximum (FWHM) of the bands, and a decrease in the luminescence intensity for InAsP NIs. The deposition of colloidal QDs barely influences the position of the NI band at low temperatures, but induces a bathochromic shift at 293 K (Fig. 1) and a noticeable increase in the luminescence intensity [24]. In publications, the passivating effect of TOPO molecules interacting with the surface of InP NWs was noted [26]. However, the spectroscopic and kinetic effects of the deposited TOPO layer containing colloidal CdSe/ZnS QDs on the NWs are considered here for the first time.

In this study, we recorded the decay kinetics of the luminescence of InAsP NIs and QWs at the temperatures 80 and 293 K in the absence and presence of a shell with colloidal CdSe/ZnS QDs. In both cases, the decay kinetics of the luminescence of NIs and QWs can be satisfactorily described by an exponential decay function with two components (Figs. 2a, 2b, green lines). Test measurements performed for systems at room temperature, onto which only a TOPO layer is deposited to a content corresponding to its content in the shell with QDs, show that TOPO as such does not influence the kinetic characteristics of emission.

Table 1 lists the parameters of the luminescence kinetics of the hybrid InP/InAsP/InP nanostructure. The parameters were calculated on the basis of a series of measurements for the same fragment of the sub-

LUMINESCENCE PHOTODYNAMICS

1143

strate. The faster component (t^ corresponds to the direct excitation of InAsP NIs, and the slower component (t2) to the contributions of other channels of excitation-energy exchange in the hybrid structure [23], including the channels brought about by occupation of the excited state through electron—phonon and trap states in the region of the InAsP/InP heterojunction, by the diffusion of electron—hole pairs (excitons) generated upon excitation of the shell of InP NWs, and by the processes at the NW—TOPO—QD interphase boundaries. Our attention is engaged by the long PL decay times for the InAsP NI. Such long times are untypical of radiative recombination in direct-gap semiconductors. One of the possible causes of such long times can be the spatial separation of charge carriers in NIs because of the formation of a type-II heterointerface between the InAsP NI and InP NW bulk. Specifically, in [27], it was concluded that a type-II heterointerface appeared in a nanostructure formed as an array of core—multishell InP/InAs/InP NWs (a QW within the NW body). With the dependence I ~ constx-13 (where I is the luminescence intensity, t is the decay time, P is an exponent depending on the decay mechanism, the distribution of decay rates, etc.), Pal et al. [27] determined the luminescence decay time that was close to the results obtained here in order of magnitude. The type-II heterojunctions possess, among other properties, a shift of the PL maximum to shorter wavelengths (hypsochromic shift), as the excitation power (P) is increased. The shift is found to be proportional to P1/3. Figure 3 shows the luminescence spectra recorded here for arrays of InP/InAsP/InP NWs at different excitation powers at a temperature of 293 K. The inset in Fig. 3 shows the dependence of the position of the center of gravity of the luminescence E(h/2) (in eV) on the cube root of the power P1/3 (in mW1/3). The character of this dependence supports the inference about the existence of a type-II heterojunction between the InAsP NI and InP bulk. In addition, as the excitation power is increased, we observed an increase in the FWHM of the NI-related luminescence band. In the region of powers under consideration, the increase in the FWHM was 0.017 eV at 80 K and 0.032 eV at 293 K. It is interesting that the deposition of a TOPO—QD layer suppresses the effect of a hypsochromic shift under variations in the excitation power and, at the same time, results in a substantial increase in the PL intensity of NIs [24]. Thus, deposition of the TOPO-QD layer noticeably modifies the overall picture of processes in the hybrid nanostructure.

For QWs, the character of the luminescence photo-dynamics is defined by temperature quenching, as the temperature is elevated from 80 to 293 K, and by an increase in the radiative lifetime in QWs upon the deposition of the TOPO—QD layer (see Table 1). For NIs, the photodynamics is somewhat different. Specifically, the luminescence lifetime in NIs without

100

43 10-' c

cti

^ 10-2

10-

100

£ 10-' s

cti

100 200 300 400 t, ns

10-

(b)

îPm f -1

O O O CD W W O «

2 *

4 J—

50

100

150

t, ns

Fig. 2. (a) (symbols) Normalized experimental luminescence kinetics for NIs and (solid lines) its approximation with (green lines) a two-exponential function and (red lines) the function (1) given in the text: (1, 2) NIs at temperatures of 80 and 293 K and (3, 4) NIs with a TOPO—QD layer at temperatures of 80 and 293 K. (b) (symbols) Normalized experimental luminescence kinetics for QWs and (lines) its approximation with a biexponential function: (1, 2) QWs at temperatures of 80 and 293 K and (3, 4) QWs with a TOPO—QD layer at temperatures of 80 and 293 K.

passivation is independent of temperature. One of the causes of temperature stability of the duration of the emission can be energy transfer from QWs and InP arrays to the lower emitting exciton level in NIs without the noticeable involvement of phonon states. Deposition of the TOPO—QD layer onto NWs results in a substantial increase in the duration of both the t1 and t2 components, and this increase is more pronounced at low temperatures. The most probable cause of slowing of the relaxation of excitation in this case are interphase processes at the interface between the NW and the shell with QDs. In general, the multi-exponential approximation provides a means for rather clearly determining the number of the main excitation transponders supplying the emitting state. However, with this approximation only, it is impossible to obtain a detailed description of the physics of

3

0

2

Table 1. Parameters of the PL kinetics of the InP/InAsP/InP hybrid nanostructure with deposited CdSe/ZnS QDs (from calculations for approximation of the kinetics with the function Ilum = ^1exp(-?/x1) + A2exp(-t/t2) at the condition + A2 = 1 and (t) =

Parameters QW QW + QD

of kinetics* 80 K 300 K 80 K 300 K

A1 0.70 0.57 0.57 0.60

T1, ns 3 1 9 4

A2 0.30 0.43 0.43 0.40

T2, ns 35 13 47 27

<t>, ns 30 12 40 23

NI NI + QD

80 K 300 K 80 K 300 K

A1 0.65 0.60 0.65 0.57

T1, ns 6 5 28 14

A2 0.35 0.40 0.35 0.43

t2, ns 21 21 125 78

<t>, ns 16 17 96 65

*The absolute deviations of calculated values for sampling certain regions of experimental data correspond to t1 ± 1 ns, t2 ± 5 ns, A1, A2 ± 0.1.

relaxation processes, and therefore, it is more efficient to use a combination of several functions, including, for example, a specialized Kohlrausch (stretched exponential) function. In [23], to take into account the losses of emission at traps associated with structural

1.0 1.1

E, eV

Fig. 3. Luminescence spectra of arrays of InP/InAsP/InP NWs at the temperature 293 K at different excitation powers: (1) 0.5, (2) 2, (3) 3.5, (4) 4.5, and (5) 5.4 mW. Inset: the position of the center of gravity of the luminescence E(h/2) (eV) versus the cube root of the power P1/3 (mW1/3).

defects in the crystal lattice, the luminescence decay kinetics in NIs was additionally described in the context of the contact quenching model. The calculation showed that such a model of kinetics quantitatively corresponded to the experimental dependences and the duration of the radiative decay to the results obtained with biexponential decomposition. The presence of a passivating shell expands the morphology of trapping states. On the one hand, passivation of the semiconductor surface with such ligands, as TOPO ones, reduces the number of surface defects, and on the other hand, the interaction of the TOPO film with the surface brings about the formation of new hybrid states that are close in energy to the emitting state and capable of trapping electrons or holes [28, 29]. Such a mechanism can be logically proposed also for the case of a deposited TOPO—QD layer. On the formation of the QD—TOPO—NW structure, antibonding orbitals capable of trapping charge carriers can be formed in the band gap. The subsequent delocalization of charge carriers at such states can be responsible for an increase in the duration of radiative recombination [30]. The rate of transfer is defined by the trap lifetime, which can be rather long in this case. This process can be taken into account with a function that describes the so-called reverse transfer of photogenerated charge carriers. In other words, any trapping of charge carriers is considered as quenching of the excited state in the NI, and delocalization of the trap with subsequent radiative recombination as reverse transfer. Traps near emission centers follow the Poisson distribution; at the same time, we can introduce the concept of the average number of traps per emission center. A model including reverse transfer in passivated structures was thoroughly considered in [31] and, as applied to the case under study (to the description of PL decay in NIs), can be represented in the form of a two-level system that includes a "dark" state (accumulating the processes of the trapping of charge carriers, with subsequent reverse transfer) and an emission "bright" state. The "bright" state emits with the average rate t-1. In other words, charge carriers photogenerated in a NW can be trapped by N traps for electrons (or holes) with a finite average trapping-rate constant k1 (the parameter of the model). A trapped electron (or hole) can be transferred to the NW with the average rate constant k2 (k1 > k2). With these assumptions, the function that describes the luminescence decay through the contact mechanism with consideration for reverse transfer is written as [32]

N

I (t ) = X e- N){N- ( A exp (-«1T

N

+ (1 - A) exp ( -a

LUMINESCENCE PHOTODYNAMICS

1145

A =

1 + NK1 - ^ (1 + NK1 + K2) +

i(1 + NK + K2)2 - K2

V4

2^(1 + NK1 + K2)2 - K2

«1,2 = + NK1 + K2) ± ^±(1 + NK1 + K2)2 - K2.

Here, N is the number of traps interacting with each emission center and distributed in accordance with the Poisson distribution, (N) is the average number of traps per emission center, K1 = k1T and K2 = k2T are dimensionless exchange-rate constants, and t is the average duration of the luminescence.

Figure 2a (red lines) show that the curves calculated by the above formula (1) give a good fit to the experimental dependences in the entire measurement region, which counts in favor of the model suggested. The results of fitting show that the duration of radiative decay of the excited state in NIs (at a temperature of 293 k) in the passivated structure (53.5 ns) exceeds the radiative decay time in the absence of reverse transfer (40 ns [23]). In this case, the quenching and reverse transfer rate constants are related as 5:1. This suggests that the average emission time is defined by the trap delocalization rate. As the temperature is elevated from 80 to 293 K, the trap delocalization rate increases more than twofold, resulting in a decrease in the lifetime of the excited state of NIs. This underlines the correspondence between the model and the experiment.

4. CONCLUSIONS

For a hybrid InP/InAsP/InP semiconductor NW structure passivated with a TOPO—CdSe/ZnS QD layer, the luminescence and its kinetics in the near-IR region are studied at 80 and 293 K upon excitation at a wavelength of 633 nm. It is suggested that the result of passivation of the NW surface with the TOPO— CdSe/ZnS QD shell is the formation of hybridized states defined by strong surface—ligand interactions. Such states are relatively close in energy to the fundamental exciton emission state and capable of trapping charge carriers with subsequent delocalization. In other words, the deposition of a TOPO—QD layer onto NWs results in an increase in the luminescence intensity due to passivation of the surface and to an increase in the contribution of reverse excitation transfer from traps to emission centers. The last-mentioned process increases the duration of radiative decay in InAsP NIs. Another cause of long relaxation times is the formation of a type-II heterojunction at the interface between the InAsP NI and the InP bulk. A model of the kinetics of the relaxation process is proposed. In the model, the role of reverse excitation

transfer from traps to the emitting state is taken into consideration.

FUNDING

The part of the study concerned with the synthesis of samples was supported by the Ministry of Education and Science of the Russian Federation, government order. The part of the study concerned with spectroscopic measurements was supported by the Russian Science Foundation, project no. 19-72-30010. The part of the study concerned with recording steady-state PL spectra was supported by the Russian Foundation for Basic Research, project no. 18-3200980 mol a.

CONFLICT OF INTEREST The authors declare that they have no conflict of interest.

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Translated by E. Smorgonskaya

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Optical Materials 127 (2022) 112277

Contents lists available at ScienceDirect

Optical Materials

journal homepage: www.elsevier.com/locate/optmat

Light quenching of photoluminescence in hybrid films of InP/InAsP/InP nanowires and CdSe/ZnS colloidal quantum dots

A.I. Khrebtova'b'**, A.S. Kulaginab, A.S. Dragunovab, R.R. Reznika, G.E. CirlinaAc'd, V.V. Danilove'*

a St. Petersburg State University, 199034 St. Petersburg, Russia b Alferov University, 194021 St. Petersburg, Russia

c Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia

d Institute for Analytical Instrumentation, Russian Academy of Sciences, 198095 St. Petersburg, Russia e Emperor Alexander I St. Petersburg State Transport University, 191031 St. Petersburg, Russia

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ARTICLE INFO

ABSTRACT

Keywords:

Hybrid semiconductor nanostructures

Thin films

Luminescence

Photodynamics

Auger recombination

Light quenching

We investigate the photoluminescence of a film obtained by the uniform deposition of a colloidal solution of CdSe/ZnS quantum dots capped by trioctylphosphine oxide on an array of isolated InP/InAsP/InP nanowires and subsequently removed from the substrate. The photoluminescence spectrum of the film shows emission bands corresponding to InAsP nano-insertions (1.25-1.5 |im) and quantum wells (1.0-1.2 |im). We find that the dependence of the photoluminescence intensity on the excitation intensity has a nonlinear character, which we interpret as a manifestation of the light quenching effect, and that the photodynamics of excitation of the nano-insertions and quantum wells differ from each other. We analyse the light quenching effect taking into account the significant increase in the luminescence intensity observed in such hybrid film as compared to an array of isolated InP/InAsP/InP nanowires. We hypothesize the possible suppression of Auger relaxation due to multistep nonradiative transfer of excitation.

1. Introduction

The quest for highly efficient photocells for optoelectronics has served as a stimulus for research on new semiconductor nanostructured materials and the processes for converting light energy in these materials. Of particular interest is the creation of heterostructures consisting of nano-insertions (NIs) into semiconductor quantum structures such as nanowires (NWs) luminescing in the IR region. Such one-dimensional heterostructures are important, for example, for fiber-optic communication systems [1]. These new materials are characterized by new features in the photodynamics of excitations. Processes such as carrier multiplication [2,3] and hot-electron extraction [4] are being studied with the aim of improving the operational characteristics of the final optoelectronic devices, however other aspects of the photodynamics are insufficiently studied [5].

We should emphasize the significant progress achieved in recent years in such a practically important area as thin-film electronics based on NWs. The mechanical properties of films containing NWs make them

suitable for applications in flexible optoelectronic devices such as pho-todetectors, solar cells, piezoelectric sensors, transistors, and LEDs [6,7], confirming the relevance of this direction of research. Various techniques are being developed for the creation of flexible structures based on NWs [7,8].

In previous works [9,10] it was found that the deposition of a layer of CdSe/ZnS quantum dots (QDs) capped by trioctylphosphine oxide (TOPO) ligands on an array of InP/InAsP/InP NWs leads to a significant increase in the duration and intensity of the photoluminescence (PL) of the InAsP NIs while the steady-state characteristics of the PL of the heterostructure almost do not change. The present work is devoted to the study of the photodynamics in a film of TOPO-capped CdSe/ZnS QDs containing InP/InAsP/InP NWs; in particular, we attempt to determine the mechanism of the photodynamics of excitations with increasing excitation intensity. We have found a nonlinear dependence of the PL intensity on the excitation intensity, which we interpret as a manifestation of the light quenching [11] effect.

* Corresponding author.

** Corresponding author. St. Petersburg Academic University Zh. I. Alferov, Russian Academy of Sciences, 194021, St. Petersburg, Russia. E-mail addresses: [e-mail] (A.I. Khrebtov), [e-mail] (V.V. Danilov).

https://doi.org/10.10Wj.optmat.2022.112277

Received 26 January 2022; Received in revised form 19 March 2022; Accepted 24 March 2022 Available online 28 March 2022 0925-3467/© 2022 Published by Elsevier B.V.

Fig. 1. SEM image of the hybrid film of colloidal CdSe/ZnS QDs and InP/ InAsP/InP NWs.

2. Materials and methods

The InP/InAsP/InP NWs in this work were obtained on a Si(111) substrate by molecular beam epitaxy (the method is described in detail in Refs. [6,7]). The morphology of the NW ensembles and their structural properties were investigated by scanning electron microscopy (SEM) and transmission electron microscopy and reported in Refs. [6,8]. The average height of the NWs was 4 |im and the diameter was 100 nm at the base and 30 nm at the top at a surface density of 3 x 108 cm- 2. The InAsP NIs (with an average As content of 40%) were 60 nm in length and 15 nm in width. In addition to the InAsP NIs, InAsP quantum wells (QWs) (average As content of 15-25%) were formed inside the NWs. These QWs appeared as a result of the lateral growth of the material during the formation of the NIs, forming a radial layer several nanometers thick.

A colloidal solution of TOPO-capped CdSe/ZnS QDs (CdSe core of about 3 nm diameter) in toluene with a concentration of about 10_5 M was drop-cast onto the InP/InAsP/InP NWs on the Si(111) substrate. The PL maximum wavelength of the QDs was 575 nm. The average distance between the QDs in the resulting layer was about 5 nm. After half an hour exposure, the colloidal QDs solution formed a film on the surface of the NWs, which was easily removed mechanically from the substrate together with the array of NWs. From the SEM analysis performed in Ref. [9], it follows that the hybrid film obtained (several microns thick) consists of an array of InP/InAsP/InP NWs embedded in a layer of TOPO-capped CdSe/ZnS QDs (Fig. 1). The hybrid film was stable in air and its PL parameters practically did not change with time.

For the spectroscopic studies, the excitation source was a YLF:Nd3+ cw laser of wavelength Agx = 527 nm and the radiation power was varied in the range 15-100 mW. The laser beam diameter was ~100 im. The signal was detected using the method of synchronous detection, a Spectral Products DK-480 monochromator, and a single-channel InGaAs (Sciencetech) detector. The PL decay kinetics was registered using an InGaAs/InP avalanche photodiode (MicroPhotonDevices, Bolzano, Italy). PL decay kinetics were registered using an InGaAs/InP avalanche photodiode (MicroPhotonDevices, Bolzano, Italy) synchronized with 635 nm pulsed laser PicoQuant PDL 800-B with laser head LDH-P-C-635B (PicoQuant, Berlin, Germany). The excitation pulse energy was about 0.4 nJ, with pulse width <90 ps and pulse repetition rate varied in the range of 1.0 MHz. The temporal resolution of the registration system did not exceed 100 ps. The PL decay kinetics was recorded at the maxima of the PL bands corresponding to the emission of the InAsP NIs and InAsP QWs at temperature 80 and 300 K.

Wavclength (nm)

Fig. 2. PL spectra of an array of InP/InAsP/InP NWs in the hybrid film (NWs embedded in a layer of TOPO-capped CdSe/ZnS QDs) (^ex = 527 nm).

Fig. 3. PL intensity of the InAsP NIs (1) and InAsP QWs (2) on a Si(111) measured at different excitation intensities, I.

3. Results and discussion

The steady-state PL spectrum of the InP/InAsP/InP NWs (Fig. 2) shows a band corresponding to the PL of the InAsP NIs (1.25-1.5 |im) and a band corresponding to the PL of the QWs (1.0-1.2 im). As shown previously [9,10], the steady-state PL spectrum of the InAsP NIs almost does not change with the deposition of a layer of TOPO-capped CdSe/ZnS QDs on the array of the NWs (although a significant increase in the duration and intensity of the PL is observed).

On Figs. 2 and 3 draws, attention to the difference in the photody-namics of QW and QI excitation associated with excitation intensity. It seems important to study the influence of the intensity of the exciting radiation on the photodynamics of the luminescence of a film heterostructure.

3.1. Light quenching

Let us consider the dependence of the PL intensity on the excitation intensity, I (for relatively low values of excitation intensity). If we take into account only one-photon absorption and emission processes in a two-level scheme (we do not take into account two-photon absorption since it requires other intensities [12]), the kinetic equation has the following form: