Absorbtion FTIR Fourier transform infrared spectroscopy ATR attenuated

Absorbtion • FTIR: Fourier transform infrared spectroscopy • ATR: attenuated total reflection • MIR: multiple internal reflection (see JAP 94 (2003) 2707) • RAIRS: reflection absorbtion infrared spectroscopy

Luminescence Spectroscopy • Electrons are moved into excited states using … Photons Photoluminescence (PL) Electron Cathodoluminescence (CL) beam (SEM) Injected Electroluminescence (EL) charge (p-n junction diodes) (electric current) PL hn > Eg EC EV

Luminescence Spectroscopy • Electron transitions result in emission of characteristic light Phonon relaxation EC EV EC hn = EC – EV = Eg EV

Luminescence Spectroscopy • Can measure wavelength of light to determine intrinsic (e. g. , bandgap) and extrinsic (e. g. , impurities, defects) material properties CB VB

Types of Transitions • CB-to-VB (e-h) Phonon relaxation CB hn > Eg VB hn = Eg

Types of Transitions • e-h can form a bound pair called an exciton due to Coulomb attraction • Similar to hydrogen atom with ionization energy: Ex = (m*e 4/2 h 2 e 2) n-2 CB n = 1, 2, … n= n=2 n=1 hn = Eg - Ex VB

Types of Transitions • Excitons Ex = (m*e 4/2 h 2 e 2) n-2 n = 1, 2, … m* = reduced mass = (1/me + 1/mh)-1 Ex ~ few me. V Excitons only observed at low temperature

Types of Transitions • Bound Excitons • Excitons may become bound to impurities • Do. X : exciton bound to neutral donor • Ao. X : exciton bound to neutral acceptor From Pankove, Fig. 1 -14, p. 16

Types of Transitions • Shallow Transitions • e-D+ : e- may transition from CB to ionized donor (donor becomes neutral) • h-A- : e- may transition from ionized acceptor to VB (acceptor becomes neutral) • Ei ~ 10 me. V (donors) ~ 30 - 40 me. V (acceptors) From Pankove, Fig. 6 -24, p. 132

Types of Transitions • Deep Transitions • Do-h : e- may transition from neutral donor to VB (donor becomes ionized) • e-Ao : e- may transition from CB to acceptor (acceptor becomes ionized) From Pankove, Fig. 6 -25, p. 133

Types of Transitions • Do-Ao Transitions • Transitions between neutral donors and neutral acceptors • Coulomb attraction between donors and acceptors hn = Eg – Ed – Ea + e 2/4 per r = donor-acceptor separation From Pankove, Fig. 6 -38, p. 143

Types of Transitions • Do-Ao Transitions: • r varies by discrete increments (lattice sites) From Yu & Cardona, Fig. 7. 6, p. 346

Quantum Wells hn = Eg + Ene + Enh - Ex Ene, h = (ħ 2 / 2 me, h*) (np/Lz)2 Eg Lz

Quantum Wells Interface roughness M. A. Herman, D. Bimberg and J. Christen, “Heterointerfaces in Quantum Wells and Epitaxial Growth Processes: Evaluation by Luminescence Techniques”, J. Appl. Phys. 70, R 1 (1991)

Luminescence Spectroscopy • Advantages: • Non-destructive • Sensitive: < 1012 cm-3 impurity detection • Monolayer detection capability in QWs • Disadvantages: • Peak assignment difficult • Difficult to quantify amount of impurity due to competing non-radiative recombination

Luminescence Spectroscopy Technique PL CL EL Probe Size Lateral Spatial Resolution ~ Ld ~ l/2 (carrier diffusion ~ 250 nm length) ~ 1 mm ~ Ld ~ 50 Å (carrier diffusion length) ~ 1 mm Size of contacts or contacts carrier diffusion length