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".  
LED; development of efficiency
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
Major recombination channels
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  
Stimulated emission and fundamental absorption

Pumping by injection

Laser and mirrors
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.
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© H. Föll (Semiconductor Technology - Script)