Semiconductor Materials for Optoelectronics
The Optoelectronic Devices and Materials Research Group (ODM) studies the structural, electronic and optical properties of semiconductor materials important for the electronics and communications industries.
Hydrostatic pressure measurements
On your Wavelength!
Applications:
• Near-infrared lasers and detectors are used in optical
fibre communications - the hardware underpinning the
IT revolution.
Experimental
tools:
• Blue light emitters based on GaN are opening up
applications in displays and high-density DVDs
• New materials (e.g. dilute nitrides) and new structures
(e.g. Quantum Cascade lasers) offer improved light
emitters in the mid-infrared, a region of growing
importance for chemical sensing (e.g. pollutants),
process control, etc.
• new TeraHertz sources and detectors in the far-infrared
to millimeter wave range are opening up new imaging
technologies at the optics-radiowave boundary
optical
telecom
displays
imaging
sensing
tunable lasers / OPA
UV
visible
radar
wireless
THz beam
Free Electron
Ultrafast
Laser
electronics
NIR
MIR
FIR
MMW
manganin coil
pressure gauge
Spectral
range:
Materials for
sources:
GaN
InGaAs
InGaAsP
Pb salts
advanced device
fabrication
• close collaboration between experimentalists and theorists within ODM
• Industrial Collaboration
• ODM has research collaborations with many of the major companies in photonics and telecoms
material
characterisation
device
characterisation
• Fundamental physics using advanced real-world devices
basic
theory
• extremely pure, precision-grown materials are also excellent for discovering new physics and new device concepts!
• Methods
• wide range of experimental and theoretical methods for the investigation of structural, electrical and optical
properties of semiconductors and optical microstructures
wide range of standard methods:
optical, electronic, cryogenic
application of hydrostatic pressure to optoelectronic devices and materials
novel modulated reflectance methods
users of FELIX Free Electron Laser
new Femtosecond Laser laboratory
Semiconductor Materials
group
II III IV V VI
period
physical device
concept
new device
design
• bandstructures and transition rates of semiconductor nanostructures
• mechanical-electronic-optical properties of strained semiconductors
• novel ultrafast photon-electron interactions and transport
• Theoretical methods
Common
tetrahedral
(zincblende)
semconductors:
group IV
III-V
II-VI
device
modelling
2
B C N O
3
Al Si P S
4
Zn Ga Ge As Se
5 Cd In Sn Sb Te
test
structure
fabrication
wafer
growth
ODM
industrial
collaborator
Example: AlGaN/GaN wurtzite quantum dots
Structural, electronic and optical properties of quantum dots
• form truncated hexagonal pyramids
• thin layers of semiconductors grown on substrates with different
lattice constant self-organise into small ‘quantum dots’
• calculations using Fourier-domain Green’s
function method
• these quantum dots have desirable properties for lasers due to
their atomic-like electron density of states
• Example: micrograph of stacked InAs
QDs in a GaAs matrix (courtesy of Paul
Koenraad, TU Eindhoven)
Theoretical calculation of QD optical
properties must include:
• shape of self-organised quantum dot
• strain distribution
• piezoelectric effects
• electronic properties
H1
~50nm
• “dilute nitrides” (GaNAs, GaInNAs) are promising for the infrared (large bowing gives small bandgap)
• not only the bandgap, but also energies of ‘critical points’ in the bandstructure (EG, EL, EX) are
important for optoelectronic device performance
lock-in
detector
spectrometer
lamp
rotatable
sample
reference
chopper
laser
H2
H3
H4
• The size and composition can be designed to maximise the overlap.
Photoreflectance spectra, identifying energy of quantum well emission lines
(QW1, QW2) and cavity mode (CM), as function of angle
QW1 QW2 CM
Apparatus for modulated
reflectance spectroscopy
signal
E4
• Strain and piezoelectric effects cause electron and hole
wavefunctions to be non-overlapping for ‘large’ (height>2nm) QDs.
• Example: mapping electronic and optical resonances in resonant cavity light-emitting diodes
detector
E3
• Drastic consequences for light emission!
• non-contact, non-destructive method
• yields information on ground and excited quantum states
• new line-fitting procedure identifies multiple levels
• III-N materials (AlN, GaN, InN) allow blue-green light emitters
E2
Hole wavefunctions
• direct-gap III-V’s are used for light emission and detection in the visible and near-infrared
• GaInAs lattice-matched to InP dominates applications in optical telecoms
Electron wavefunctions
E1
Modulated reflectance spectroscopy
filter
1.3 µm,
1.55µm
telecoms
bands
• this demonstrated the role of the bandstructure in
determining behaviour at high electric fields
Theoretical calculations
• Silicon is ubiquitous in electronics, but interacts relatively weakly with light
Visible
wavelengths:
displays
pre-stressed
double cylinder
conic,
insulated
feedthroughs
device
under test
pressuretransmitting
fluid
O-ring seal
• a simple 15kbar piston-cylinder pressure cell
allows variation of the bandgap by about 10%
• optical and electrical access to the sample
• other systems available in ODM include helium
gas cells and diamond anvil cells, offering wide
pressure range and low temperature operation.
dilute nitrides:
GaInNAs GaInNSb
inter-subband lasers:
Quantum Cascade
Novel materials/
structures:
• different materials are found to exhibit very different
pressure dependence of breakdown voltage (Vb)
phosphorbronze ring
lower piston
Electronics:
Si, SiGe
GaAs
HgCdTe
• Experiment and Theory
• Experimental methods
Al foil
fibre in
epoxy-filled
stub
RF
Modus Operandi
•
•
•
•
•
• high pressure changes the lattice constant
• electronic and vibrational properties change
• the role of bandstructure in optoelectronic
devices can be conveniently investigated
• the effect is similar to a change in composition….
~50nm
• Visible (red) lasers are used in consumer electronics for
optical storage (CDs, DVDs)
optical
storage
Example: avalanche breakdown in semiconductors
Schematic of all-semiconductor resonant
cavity visible light-emitting diode
distributed
Bragg reflector
AlGaAs
GaAs
AlGaInP
GaInP
AlGaInP
GaAs
AlGaAs
optical cavity
- controls optical
resonance
distributed
Bragg reflector
quantum well
- light emission at
electronic resonance
Optoelectronic Devices and Materials Group
University of Surrey
http://www.ph.surrey.ac.uk/odm
photoreflectance signal (arbitrary units)
Materials which emit, detect, transmit, or switch
light at different wavelengths are important for a
range of applications.
LOAD
(120 Ton)
optical
electrical
fibre
connections
upper piston
16.0 (x5)
65o
14.0 (x3)
60o
55o
12.0
50o
10.0
45o
40o
8.0
35o
6.0
30o
4.0
25o
20o
2.0
(x3)
=13o
0.0
Data
angle
Fit
-2.0
1.86 1.88 1.90 1.92 1.94 1.96 1.98 2.00
energy (eV)