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

2. 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

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

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

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

6. 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 n = 1, 2, … CB
n = 
n = 2
n = 1
hn = Eg - Ex VB

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

8. 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

9. 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)
~ meV (acceptors)
From Pankove, Fig. 6-24, p. 132

10. 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

11. 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

12. Types of Transitions Do-Ao Transitions:
r varies by discrete increments (lattice sites)
From Yu & Cardona, Fig. 7.6, p. 346

13. Quantum Wells
hn = Eg + Ene + Enh - Ex
Ene,h = (ħ2 / 2me,h*) (np/Lz)2 Eg Lz

14. 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)

15. 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)

16. 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

17. 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