Transcript Optical and photoelectrical properties of QD of III

Saint-Petersburg State University
Quantum Dots. Optical and Photoelectrical
properties of QD of III-V Compounds.
Alexander Senichev
Physics Faculty
Department of Solid State Physics
[email protected]
8-921-5769793
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
Quantum Dots
The
extreme case of size quantization
is realized in semiconductor structures
with confinement of carriers in three
directions – they are Quantum Dots.
Introduction

Generally, electronic spectrum of the ideal quantum
dots is a set of discrete levels.
а)
E
b)
Qualitative behavior of Density of States in:
a) Bulk semiconductor
b) Quantum Wells
c) Quantum Wires
d) Quantum Dots
E
300
с)
250
E
Intensity
200
150
100
d)
50
0
E
1,05
1,10
1,15
1,20
E, eV
1,25
1,30
1,35
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


1.
2.
3.
The base of technologies of QDs formation is self-organizing
phenomenon.
There are three types of initial stage of epitaxial growth:
2D growth of material A on surface of substrate B ; (Frank-van der
Merve)
3D growth of material A on surface of substrate B ( Volmer-Weber
method);
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
4
of substrate;
4) Nucleation;
1
3
Solid substrate
2
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 reactor
3. the pressure control
system
4. the exhaust facilities
Technology of QDs Formation

Metal organic chemical vapor deposition (MOCVD)

The basic chemistry equation of this reaction is as follows:
(CH3 )3 Ga  AsH3  GaAs(solid )  3CH4 (methane gas)



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
Dependence of QDs morphology on
growth conditions

The basic control parameters in the case of MBE
growth:
1.
the substrate temperature;
the growth rate;
the quantity InAs, ratios of III/V materials;
Exposure time in As stream;
2.
3.
4.

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:

300
2000
250
1500
Intensity
Intensity
200
150
100
1000
500
50
0
0
1,05
1,10
1,15
1,20
E, eV
1,25
1,30
1,35
1,00
1,05
1,10
1,15
E, eV
1,20
1,25
1,30
1,35
Optical properties of QDs

1.
The major processes which explain the temperature
behavior of QDs PL-spectra:
Thermal quenching of photoluminescence
Thermal 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 processes
Tunneling 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 hightemperature component of photoluminescence spectrum.
Photoelectrical properties of QDs
Photoluminescence 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 Physics
[email protected]
Thank you for your attention!