## 9.3 Summary

### 9.3.1 Summary to: 9. Optoelectronics

| Optoelectronics has two basic branches: |  |
| Wavelength (nm) | Typical Semiconductor
Infrared | 880 | GaAlAs/GaAs
Red | 660 - 633 | GaAlAs/GaAs
Orange to Yellow | 612 - 585 | AlGaInP GaAsP/GaP GaAsP/GaP
Green | 555 | GaP
Blue to Ultraviolet | 470 - 395 | GaN/SiC GaN/SiC InGaN/SiC
|  | 1. Light in Þ electrical signal out: |
|  | Optical sensors as single elements "CCD" chips in "megapixel" matrices. |
|  | 2. Electricity in Þ light out; in two paradigmatic versions: |
|  | LED's Laser diodes |
| Here we only look at the second branch. |
|  | The semiconductors of choice are mostly the III-V's, usually in single-crystalline perfect thin films. |
|  | The present day (2008) range of wavelength covers the IR to near UV. |
|  | Indirect semiconductors like GaP can be used too, if some "tricks" are used. |
| The index of refraction n=(e)½ and thus the dielectric constant e become important |  |
Typical Semiconductor | Dielectric constant | Thermal conductivity [W/cm · K]
Si | 11.9 | 1.5
GaAs | 13.1 | 0,45
GaP | 11.1 | 1.1
GaN | 8.9 | 1.3
SiC | 10 | 5
C (Diamond) | 5.8 | 22
|  | Semiconductors have a relatively large index of refraction at photon energies below the bandgap of n » 3 - 4. |
|  | Diamond has the highest n in the visible region |
| The thermal conductivity becomes important because for generating light one needs power (which we avoided as much as possible for signal processing with Si!) |
|  | Again, diamond has the highest thermal conductivity of all known materials - 5 times better than Cu! |
| LED's come as cheap little "indicator" lights and recently also as replacement for "light bulbs". |  |
|  | Intense white light from LED's becomes possible, Advantages: High efficiencies and long life time |
|  | The key was the "taming" of the GaN material system for blue and UV LED's. |
| LED's based on organic semiconductors (OLED) are rapidly appearing in OLED based displays. |
|  | Advantage: High efficiencies because of active light generation. Problem: Product life time; sensitivity to air. |
| Semiconductor "Diode" Lasers are high-power" LED's plus "mirrors" |  |
Enabling technology for CD / DVD / Blue ray / ... memory technologies!
|  | Advantage: Small and cheap. Problems: Low power, "Quality". |

| There are always several recombination channels active in parallel Direct band-band recombination; producing light. Defect recombination; not producing light. Auger recombination; not producing light. "Exotic" mechanisms like exciton recombination; producing light in indirect semiconductors like GaP |  |
|  | High efficiency LED's need optimized recombination. |
| Without "tricks" only a fraction of the light produced gets out of the semiconductor |  |
|  | Index grating is essential |
|  | Avoiding re-absorption is essential |
|  | Defined recombination volumes are important |
| Hetero junctions of the NnP or NpP type are the solution, but create problems of their own |
|  | Hetero-interfaces must be defect free Þ Avoid misfit dislocations! |
| Laser diodes are similar to LED's but need to meet two additional conditions |  |
| 1. The rate of Stimulated emission, a new process predicted by A. Einstein concerning the interaction of light and electrons in the conduction band, must be at least as large as the rate of fundamental absorption |
|  | Stimulated emissionresults in two fully coherent photons for one incoming photon and thus allows optical amplification. |
|  | Strong stimulated emission his requires large non-equilibrium electron concentrations in the conduction band. Þstrong "pumping" is necessary, moving electrons up to the conduction band just as fast as they disappear by recombination. |
|  | In semiconductor junctions pumping can be "easily" achieved by very large injection currents across a forwardly biased (hetero) junction.Þ cooling problem! |
| 2. There must be some feed-back that turns an (optical) amplifier into an oscillator for one frequency |
|  | Feed-back is achieved by partially transparent mirrors. |
|  | Monochromatic output is achieved by the optical resonator forme by two exactly plan-parallel mirrors |
|  | Only wavelengths l=2L/i (i=integer) that "fit" into the cavity will be able to exist. Together with the condition hn=hc/l=Eg the Laser wavelength is given |
| Semiconductor Lasers now span the range from IR to UV; essential materials are all III-V's, in particular the GaN family. |
| Molecular beam epitaxy is the deposition method of choice for epitaxial multilayer structures |  |
Exercise 9.3-1
All Quick Questions to 9.

© H. Föll (Semiconductor Technology - Script)