### 9.2.3 Summary to: 9.2 Optoelectronics - Important Principles and Technologies

| 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 emission results 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 9.2-1
All Quick Questions to 9.

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