1. 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 UVvisibleNIRMIRFIRMMW RF Spectral range: Applications: optical storage displays optical telecom sensing imagingradar wireless Novel materials/ structures: dilute nitrides: GaInNAs GaInNSb inter-subband lasers: Quantum Cascade GaNInGaAsElectronics: Si, SiGe GaAs Pb salts InGaAsP HgCdTe Materials for sources: Experimental tools: Free Electron Laser Ultrafast electronics tunable lasers / OPATHz beam Semiconductor Materials for Optoelectronics Optoelectronic Devices and Materials Group University of Surrey http://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 test structure fabrication advanced device fabrication device character- isation new device design device modelling material character- isation basic theory 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 E 1 E 2 E 3 E 4 Electron wavefunctions H 1 H 2 H 3 H 4 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 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 1.861.881.901.921.941.961.982.00 -2.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 energy (eV) photoreflectance signal (arbitrary units) CM angle Data 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) 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 GaAs AlGaAs AlGaInP GaAs 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 (V b ) 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 AlP GaAs Sb Se S Zn TeCd IIIIVVVIII 2 3 4 5 period group Common tetrahedral (zincblende) semconductors: group IV III-V II-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 , E L, E X ) 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