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