9.2.3 Summary to: 9.2 Important Principles and Technologies

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

$Refraction and LED efficiency$

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 = E_g
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.