2 Optoelectronic Devices Optoelectronicsthe technology dealing with information processing with lightOptoelectronic (Photonic) Devicesdevices performing conversion between optical energy (photon) and electrical energyElectronic energy (singal) ⇔ Optical energy (signal or radiation)OE devices can be divided into three groups:(1) Electronic energy ⇒ RadiationLED, Laser Diode (LD), EL devices, Display devices(2) Optical signal ⇒ Electrical signalPhotodetectors(3) Optical radiation ⇒ Electrical energySolar Cells
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 materialLuminescenceEL (Electro-luminescence): LED, LD, ELDPL (Photo-luminescence): light → ⇫ → light (e.g. in a PDP application : EL + PL)CL (Cathode-luminescence): by e-beam or cathode-rayRL (Radio-luminescence): by other fast particles or high energy radiationChemi-luminescence:by chemical reaction (e.g. firefly, rotting wood)
9 Radiative Transitions (a) Absorptionphotodetectors, solar cell(b) Spontaneous emissionLED(c) Stimulated emissionmonochromatic (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 transitiondeceleration 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- WCutoff 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 cλ c = 1.24/Eg (μ m)the optical band-to-band absorption becomes negligible for hυ < Eg or λ > λ cFor 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 recombinationFast process (mean lifetime ~ 10-8 s)M E GPhosphorescenceRecombination via trap (meta-stable) levelsSlow processSeveral re-trapping process may occur before recombinationM GPhosphorLight 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.