1. II. Basic Concepts of Semiconductor OE Devices
Semiconductors
energy-band, bandgap energy, lattice constant
element & compound semiconductor
intrinsic & extrinsic semiconductor
direct & indirect semiconductor
carrier transport: Drift, Diffusion, Recombination, Generation
junction: p-n junction, M-S junction, Heterojunction, Multi-heterojunction, Super-lattice, Quantum well
Optoelectronics
OE materials: solid crystalline, non-crystalline
Optical spectrum
Luminescence
Electronic transition: radiative and non-radiative transition
Light Absorption and Emission

2. Optoelectronic Devices
Optoelectronics
the technology dealing with information processing with light
Optoelectronic (Photonic) Devices
devices performing conversion between optical energy (photon) and electrical energy
Electronic energy (singal) ⇔ Optical energy (signal or radiation)
OE devices can be divided into three groups:
(1) Electronic energy ⇒ Radiation
LED, Laser Diode (LD), EL devices, Display devices
(2) Optical signal ⇒ Electrical signal
Photodetectors
(3) Optical radiation ⇒ Electrical energy
Solar Cells

3. Applications of OE devices in modern technology

4. Optoelectronic Materials: Solid Crystalline

5. Optoelectronic Materials: Non-Crystalline

6. Challenges in the area of synthesis and characterization of optoelectronic Materials

7. Luminescence Light generation Luminescence
Incandescent: light generation from materials at high temperatures.
Luminescence: light generation by any methods other than temperature of a material
Luminescence
EL (Electro-luminescence): LED, LD, ELD
PL (Photo-luminescence): light → ⇫ → light (e.g. in a PDP application : EL + PL)
CL (Cathode-luminescence): by e-beam or cathode-ray
RL (Radio-luminescence): by other fast particles or high energy radiation
Chemi-luminescence:by chemical reaction (e.g. firefly, rotting wood)

8. Optical Spectrum for some Optical Materials

9. Radiative Transitions
(a) Absorption
photodetectors, solar cell
(b) Spontaneous emission
LED
(c) Stimulated emission
monochromatic (all emitting photons have the same energy)
coherent (all photons emitted are in phase)
LASER

10. Basic Electronic Transitions
(1) Interband transition
(a) intrinsic emission
(b) hot carrier emission or avalanche emission
(2) Transition related to impurity or physical defects
(c) EC ⇝ EA
(d) ED ⇝ EV
(e) ED ⇝ EA : (pair emission)
(f) ED ⇝ defect center ⇝ EA
(3) Intraband transition
deceleration emission

11. Light Absorption in a Semiconductor
(x + x) = [ d  (x)/dx ] x = -  (x) (x)
(x): photon flux at the distance of x
: absorption coefficient (cm-1)
d  (x)/dx = -  (x)
for  (x) =  0e-  x and  (W) =  0e- W
Cutoff wavelength : λ c = 1.24/Eg (μm)

12. Absorption Coefficients for some Semiconductors
For the direct gap materials, the absorption coefficient is very strong once the photon energy exceeds the bandgap.
The absorption coefficient decreases rapidly at the cutoff wavelength λ c
λ c = 1.24/Eg (μ m)
the optical band-to-band absorption becomes negligible for hυ < Eg or λ > λ c
For indirect materials, the absorption coefficient rises much more gradually.

13. Relationship between bandgap and lattice constant
GaAs and InP are commercially available binary substrates.
AlGaAs can be grown on GaAs substrates with little lattice mismatch.
In0.5Ga0.5P can be grwon lattice-matched to a GaAs substrate. (The lattice constant a of a ternary alloy varies approximately linearly with the composition x).
In0.53Ga0.47As (0.75 eV) can be grown lattice-matched on InP (1.35 eV) since the lattice constants are the same.
GaAs0.6P0.4 (1.9 eV) for red LEDs cannot be grown directly on a GaAs or GaP substrate. A graded layer (buffer layer) is used in growing such a crystal hetero-epitaxial layer.
InxGa1-xAsyP1-y can be grown on InP substartes with resulting band gaps ranging fron 0.75 eV to 1.35 eV.

14. Fluorescence and Phosphorescence
Direct recombination
Fast process (mean lifetime ~ 10-8 s)
M  E  G
Phosphorescence
Recombination via trap (meta-stable) levels
Slow process
Several re-trapping process may occur before recombination
M  G
Phosphor
Light emitting materials used in TV screens, cathod ray tubes (CRTs), etc..
The color of light emitted by a phosphor (such as ZnS) depends on the impurities present.