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 EC hn > Eg 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 hn = Eg VB

e-h can form a bound pair called an exciton due to Coulomb attraction 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*e4/2h2e2) n-2 n = 1, 2, … CB n =  n = 2 n = 1 hn = Eg - Ex VB

Types of Transitions Excitons Ex = (m*e4/2h2e2) n-2 n = 1, 2, … m* = reduced mass = (1/me + 1/mh)-1 Ex ~ few meV  Excitons only observed at low temperature

Excitons may become bound to impurities Types of Transitions Bound Excitons Excitons may become bound to impurities DoX : exciton bound to neutral donor AoX : 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 meV (donors) ~ 30 - 40 meV (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

Transitions between neutral donors and neutral acceptors Types of Transitions Do-Ao Transitions Transitions between neutral donors and neutral acceptors Coulomb attraction between donors and acceptors hn = Eg – Ed – Ea + e2/4per 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 = (ħ2 / 2me,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, R1 (1991)

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, R1 (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 Probe Size Lateral Spatial Resolution PL ~ l/2 ~ 250 nm ~ Ld (carrier diffusion length) ~ 1 mm CL ~ 50 Å EL Size of contacts Size of contacts or carrier diffusion length