on your wavelength!
DESCRIPTION
advanced device fabrication. signal. detector. lock-in detector. 16.0. material character-isation. 65. o. filter. device character-isation. 14.0. 60. o. 55. o. 12.0. basic theory. 50. o. spectrometer. 10.0. test structure fabrication. 45. o. 8.0. 40. o. device - PowerPoint PPT PresentationTRANSCRIPT
On your Wavelength!Materials which emit, detect, transmit, or switch light at different wavelengths are important for a range of applications.
• Near-infrared lasers and detectors are used in optical fibre communications - the hardware underpinning the IT revolution.
• Visible (red) lasers are used in consumer electronics for optical storage (CDs, DVDs)
• 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
UV visible NIR MIR FIR MMW RFSpectral range:
Applications: optical storage
displays
opticaltelecom
sensing
imaging radar
wireless
Novel materials/structures:
dilute nitrides:GaInNAs GaInNSb
inter-subband lasers:Quantum Cascade
GaN InGaAs Electronics:Si, SiGe
GaAs
Pb salts
InGaAsP HgCdTeMaterials for sources:
Experimental tools: Free Electron
LaserUltrafast
electronics
tunable lasers / OPA THz beam
Semiconductor Materials for Optoelectronics
Optoelectronic Devices and Materials Group University of Surreyhttp://www.ph.surrey.ac.uk/odm
Modus Operandi • Experiment and Theory
• close collaboration between experimentalists and theorists within ODM
• Industrial Collaboration
• ODM has research collaborations with many of the major companies in photonics and telecoms
• Fundamental physics using advanced real-world devices• 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
• Experimental methods
• Theoretical methods
wafer growth
teststructure
fabrication
advanced device fabrication
device character-
isation
new device design
device modelling
material character-
isation basictheory
physical device concept
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.
Theoretical calculations Structural, electronic and optical properties of quantum dots
Theoretical calculation of QD optical properties must include:
• shape of self-organised quantum dot • strain distribution • piezoelectric effects• electronic properties
Example: AlGaN/GaN wurtzite quantum dots
• form truncated hexagonal pyramids
• calculations using Fourier-domain Green’s function method
E1 E2 E3 E4
Electron wavefunctions
H1 H2 H3 H4
Hole wavefunctions
• thin layers of semiconductors grown on substrates with different lattice constant self-organise into small ‘quantum dots’
• these quantum dots have desirable properties for lasers due to their atomic-like electron density of states
• Strain and piezoelectric effects cause electron and hole wavefunctions to be non-overlapping for ‘large’ (height>2nm) QDs.
• Drastic consequences for light emission!
• The size and composition can be designed to maximise the overlap.
~50nm
~50
nm
• Example: micrograph of stacked InAs QDs in a GaAs matrix (courtesy of Paul Koenraad, TU Eindhoven)
detector
spectrometer
rotatable sample
lamp
laser
signal
reference
chopper
filter
lock-in detector
1.86 1.88 1.90 1.92 1.94 1.96 1.98 2.00-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
energy (eV)
phot
oref
lect
ance
sig
nal (
arbi
trar
y un
its)
CM
angleData
Fit
=13
20
25
30
35
40
45
50
55
65
60
o
o
o
o
o
o
o
o
o
o
o
QW2QW1
(x5)
(x3)
(x3)
Modulated reflectance spectroscopy • non-contact, non-destructive method• yields information on ground and excited quantum states• new line-fitting procedure identifies multiple levels
• Example: mapping electronic and optical resonances in resonant cavity light-emitting diodes
Apparatus for modulated reflectance spectroscopy
Photoreflectance spectra, identifying energy of quantum well emission lines (QW1, QW2) and cavity mode (CM), as function of angle
quantum well- light emission at
electronic resonance
distributed Bragg reflector
optical cavity - controls optical
resonance
distributed Bragg reflector
GaInP
GaAsAlGaAs
AlGaInP
AlGaInPGaAs
AlGaAs
Schematic of all-semiconductor resonant cavity visible light-emitting diode
pre-stressed double cylinder
upper piston
lower piston
electrical connections
optical fibre
manganin coil pressure gauge
pressure-transmitting
fluid
O-ring seal
phosphor-bronze ring
LOAD(120 Ton)
Al foil
fibre in epoxy-filled
stub
device under test
conic, insulated
feedthroughs
Hydrostatic pressure measurements• 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….
• different materials are found to exhibit very different pressure dependence of breakdown voltage (Vb)
• this demonstrated the role of the bandstructure in determining behaviour at high electric fields
• 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.
Example: avalanche breakdown in semiconductors
B O
In
C
Si
Ge
Sn
N
Al P
Ga As
Sb
Se
S
Zn
TeCd
III IV V VIII
2
3
4
5
per
iod
groupCommon
tetrahedral (zincblende)
semconductors: group IV
III-VII-VI
Semiconductor Materials
1.3 µm, 1.55µm
telecoms bands
Visible wave-
lengths: displays
• Silicon is ubiquitous in electronics, but interacts relatively weakly with light
• 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
• III-N materials (AlN, GaN, InN) allow blue-green light emitters
• “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 (E, EL, EX) are important for optoelectronic device performance
• 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
• bandstructures and transition rates of semiconductor nanostructures• mechanical-electronic-optical properties of strained semiconductors• novel ultrafast photon-electron interactions and transport industrial
collaborator
ODM