Quantum Dots. Optical and Photoelectrical properties of QD of III-V Compounds.

Alexander Senichev Physics Faculty

Department of Solid State Physicssenichev_spb@mail.ru

8-921-5769793

Saint-Petersburg State University

Contents

Introduction Technology of Quantum Dots

Formation Dependence of quantum-dots

morphology from growth conditions Optical and photoelectrical properties

of QDs Conclusion

Introduction If the size of semiconductor crystal is reduced to tens or

hundreds of inter-atomic spacing, all major properties of material change because of size quantization effects.

Introduction

Quantum Well

The extreme case of size quantization is realized in semiconductor structures

with confinement of carriers in three directions – they are Quantum Dots.

Quantum Dots

Introduction Generally, electronic spectrum of the ideal quantum

dots is a set of discrete levels.

E

E

E

E

а)

b)

с)

d)

Qualitative behavior of Density of States in:a) Bulk semiconductorb) Quantum Wellsc) Quantum Wiresd) Quantum Dots

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nsity

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Device application of QDs

Lasers with active area based on QDs Light-Emitting Device (LED) based on QDs Quantum Dots Solar Cells

Technology of QDs Formation The base of technologies of QDs formation is self-organizing

phenomenon. There are three types of initial stage of epitaxial growth:

1. 2D growth of material A on surface of substrate B ; (Frank-van der Merve)

2. 3D growth of material A on surface of substrate B ( Volmer-Weber method);

3. Intermediate mode of growth – the Stranski-Krastanow mode.

2D growth 3D growth Stranski-Krastanow

Technology of QDs Formation Molecular Beam Epitaxy (MBE)

MBE may be defined as the deposition of epitaxial films onto single crystal substrates using atomic or molecular beams.

MBE involves elementary processes:1) Adsorption of atoms and molecules;2) Thermal desorption;3) Diffusion of adatoms on surface of substrate;4) Nucleation;

Solid substrate

1 2

34

Technology of QDs Formation Molecular Beam Epitaxy (MBE)

MBE system consist of:• a growth chamber• a vacuum pump• a effusion (Knudsen) cells• a manipulator and substrate

heater• an in-situ characterization

tool – RHEED (reflection high energy electron diffraction)

The typical rate of MBE growth is about 1 ML/s.

Technology of QDs Formation Molecular Beam Epitaxy (MBE) The oscillation of the RHEED signal exactly corresponds to the

time needed to grown a monolayer. The diffraction pattern on the RHEED windows gives direct indication of the state of the surface.

Technology of QDs Formation Metal organic chemical vapor deposition (MOCVD) Metal organic chemical vapor deposition is a technique used to deposit

layers of materials by vapor deposition process.

MOCVD system contains:1. the gas handling

system to meter and mix reagents

2. the reactor3. the pressure control

system4. the exhaust facilities

Technology of QDs Formation Metal organic chemical vapor deposition (MOCVD) The basic chemistry equation of this reaction is as follows:

Group III sources are trimetilgallium (TMGa), TMAl, TMIn. Group V sources are typically hydride gases such as arsine,

phosphine. Growth rate and composition is controlled by partial

pressures of the species and by substrate temperature

3 3 3 4( ) ( ) 3 ( )CH Ga AsH GaAs solid CH methane gas

Dependence of QDs morphology on growth conditions

The basic control parameters in the case of MBE growth:

1. the substrate temperature;2. the growth rate;3. the quantity InAs, ratios of III/V materials;4. Exposure time in As stream;

As research shows, morphology of QDs ensembles strongly depends on temperature of substrate and growth rate.

Dependence of QDs morphology on growth conditions

Optical properties of QDs

Photoluminescence spectra of various ensembles of QDs:

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Optical properties of QDs

The major processes which explain the temperature behavior of QDs PL-spectra:

1. Thermal quenching of photoluminescenceThermal quenching is explained by thermal escape of carriers from QD

into the barrier (or wetting layer) 2. “Red shifting” As experiment shows, at the temperature, when thermal quenching

begins, we can see a following change: the maximum of PL line is shifting in the “red region”. Such behavior of PL spectrum is explained by thermal quenching of carriers and their redistribution between small and large QDs.

Optical properties of QDs

3. Thermal broadening of PL-spectrum.The one of the major factors which defines PL-line width is size dispersion of

QDs, i.e. statistic disregistry in ensembles of QDs. Other process which affects on PL-line width is the electron-phonon interaction.

4. Tunnel processesTunneling of carriers between QDs competes with escape of carriers from

QDs in all temperature range. Probability of tunneling increases with temperature growth. Tunneling processes can affect on high-temperature component of photoluminescence spectrum.

Photoelectrical properties of QDsPhotoluminescence spectra at 10 K as a function of bias excited at (a) 1.959 eV above the GaAs band gap, (b) 1.445 eV resonant with the wetting layer, and (c) 1.303 eV resonant with the second dot excited state. Schematic excitation, carrier loss, and recombination processes are indicated for the three cases.

Photocurrent spectra as a function of bias at 10 K. Quantum-dot features are observed for biases between -3 and -6 V. The inset shows photocurrent from two-dimensional wetting-layer transition, observed to its full intensity at biases of only ~ -0.5 V.

Quantum Dots. Optical and Photoelectrical properties of QD of III-V Compounds.

Alexander Senichev Physics Faculty

Department of Solid State Physicssenichev_spb@mail.ru

Thank you for your attention!