NanoVision 2020

1
Phys 2235 Physics of NanoMaterials The University
of Hong Kong S. J. Xu Department of
Physics (Lecture 11)
2
Research Topics

• Contents of Lecture 11
• Growth, Properties and Infrared Photodetectors of
  InGaAs/GaAs Self-Assembled Quantum Dots
• Optical Properties of SiC nanocrystalline Film

3
Self-Assembled Growth
Table I Critical thickness and mismatch strain of
quantum dots self-organization growth
4
Relationship between Critical Thickness and
Lattice Mismatch Strain
Modeled by using Suo and Zhangs model
Z. Suo and Z. Zhang, Phys. Rev. B 58, 5116
(1998).
5
Single Layer QDs, Coherent Multi-Layers QDs, and
Non-Coherent Multi-Layers QDs
6
Effect of Postgrowth Annealing

• Rapid thermal annealing was used to modify the
  structural and optical properties of the InAs
  dots.
• Significant narrowing of the peak width (from
  78.9meV to 20.5meV) of the emissions occurs
  together with about 260meV blue-shift at
  annealing temperature up to 850C.
• TEM observation shows that quantum dots disappear
  due to too high annealing temperature.

QD layer
Reference QW
Xu, et al., APL 72, 3335 (1998).
7
Annealing Effect on Intersubband Energy Spacing
Band filling effect
Wang, et al., JAP 86, 2673 (1999).
8
Measured HRXRD rocking curves of the as-grown and
850 C annealed InAs/GaAs QDs
Annealing temperature dependence of photon
energies of the ground state and the excited
state emissions.
9
Simulation of XRD Rocking Curves of Annealed QDs
Significant In-Ga atom intermixing even in the
as-grown InAs/GaAs dots.!
Xu, et al., APL 77, 2130 (2000).
10
Pressure PL of QDs

• Left figure Room-temperature PL spectra from the
  InAs dots under different pressures.
• Right figure Pressure dependence and
  coefficients of various transitions.

11
PLE Spectra of QDs

• PL and PLE spectra of the InAs QDs at 10 K.

Electronic structures of the InAs/GaAs QDs system
is rather complex.
12
Room temperature infrared absorption of
InGaAs/GaAs dots

• Absorption peak is centered around photon energy
  of 130 meV.
• Strong polarization dependence on incident
  infrared radiation

13
Quantum Dot Infrared Photodetectors
SEM top view of quantum dot infrared
photodetectors
Cross-sectional TEM image of QDs for QDIP device
14
Dark current-voltage characteristic of
InGaAs/GaAs QDIP

• At low temperatures, slow variation of the dark
  current with temperature indicates stronger
  confinement effect of the electron in Quantum
  dots.
• At higher temperatures, a exponential growth of
  the thermionic emission rate of the electrons
  results in rapid increase of the dark current
  with temperature.

15
Photoresponse spectra of the QDIPs

• Responsivity at 60 K is bout 5 mA/W for
  s-polarized incident light.
• Responsivity is about 0.5 mA/W for p-polarized
  incident light.
• Polarization dependence of the responsivity is in
  agreement with that of the infrared absorption.

16
Negative differential phototconductance effect

• A threshold voltage of about 0.6 V exists, which
  is in agreement with available theoretical
  prediction.
• Strong negative differential photoconductance
  phenomenon that was not predicted by the theory
  is found.

17
Applied electric-field dependence of the
photoresponse spectra

• The maximum responsivity of about 80 mA/W was
  measured at 30 K and at 1.46 V.
• Blue-shift of the response peak occurs with
  increasing applied bias.

Xu, et al., APL 73, 3153 (1998).
18
Summary 1

• A series of QDs were grown with MBE. The critical
  thickness at which QDs forms depends inversely on
  the mismatch strain in a law of 4/3.
• Rapid thermal annealing can be used to strongly
  modify structure and optical properties of
  self-organized quantum dots.
• Strong atom interdiffusion exists even in the
  as-grown InAs/GaAs QDs.
• Pressure coefficients of QDs is significantly
  smaller.
• Electronic structures of QD system is rather
  complex.
• Quantum dot photodetectors were demonstrated.

19
Optical Properties of Silicon Carbide Nanocrystall
ine Films

• Silicon Carbide (SiC) is a family of indirect
  wide band gap semiconductors having many unique
  properties and device applications.
• SiC has been known to have more than 200
  prototypes. 3C, 2H, 6H, 4H are commonly seen
  structures.
• In the study, 3C-SiC nanocrystalline films were
  grown on Si substrate with ECR-CVD (electron
  cyclotron resonance chemical vapor deposition)
  method.
• Intense photoluminescence and sensitive
  atmospheric pressure dependence were observed.

20
Crystal Structure of 3C-SiC 
Si
C
The tetragonal bonding of a carbon atom
with the four nearest silicon neighbours. The
distances a and b are approximately 3.08Å and
1.89Å respectively
21
Structure Characterization of SiC nanocrystalline
films
TEM images of SiC nanocry-stalline films
AFM images of SiC nanocrystalline films
22
Steady-state and time-resolved PL spectra of SiC
nanocrystalline films
Time-resolved PL spectra
Steady-state PL spectra
23
Measured PL spectra of sample C under different
air pressures
24
Measured PL spectra of sample D under different
air pressures
25
Measured PL intensity of sample D against
exposure time of UV laser.
26
Measured PL intensity of sample C against
exposure time of UV laser
27
SiC Nanostructures with different cluster size
Si C H
SiC51_H
SiC111_H
SiC81_H
28
Calculated absorption spectra of SiC nanoclusters
with different sizes

• The spectra show a clear blue shift with
  decreasing size of cluster
• Size dependent absorption spectrum as evidence
  for quantum-size effects.

29
SiC nanoclusters terminated by H, OH or NH2
Si C N O H
SiC81_NH2
SiC81_H
SiC81_OH
30
Absorption spectra of SiC-81 clusters were
calculated. The dangling bonds are saturated by
H, OH, NH2, respectively.
31
Close up of the absorption spectra at lower
energy side
32
Summary 2

• Intense light emission covering whole visible
  range was observed in SiC nanocrystalline films.
• PL spectra of SiC clusters under different air
  pressure were measured.
• Absorption spectra of SiC clusters terminated by
  H, OH or NH2 were calculated.