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
Applications of OE devices in modern technology
Optoelectronic Materials: Solid Crystalline
Optoelectronic Materials: Non-Crystalline
Challenges in the area of synthesis and characterization of optoelectronic Materials
Luminescence Light generation –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)
Optical Spectrum for some Optical Materials
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
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) E C ⇝ E A –(d) E D ⇝ E V –(e) E D ⇝ E A : (pair emission) –(f) E D ⇝ defect center ⇝ E A (3) Intraband transition –deceleration emission
(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) = 0 e - x and (W) = 0 e - W Cutoff wavelength : λ c = 1.24/Eg (μm) Light Absorption in a Semiconductor
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/E g ( μ m) the optical band-to-band absorption becomes negligible for hυ λ c For indirect materials, the absorption coefficient rises much more gradually.
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. In 0.5 Ga 0.5 P can be grwon lattice-matched to a GaAs substrate. (The lattice constant a of a ternary alloy varies approximately linearly with the composition x). In 0.53 Ga 0.47 As (0.75 eV) can be grown lattice- matched on InP (1.35 eV) since the lattice constants are the same. GaAs 0.6 P 0.4 (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. In x Ga 1-x As y P 1-y can be grown on InP substartes with resulting band gaps ranging fron 0.75 eV to 1.35 eV.
Fluorescence and Phosphorescence Fluorescence –Direct recombination –Fast process (mean lifetime ~ 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.