Lecture 13 2013_
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This document discusses the fundamentals of laser diodes, including: 1) Laser diodes use direct bandgap semiconductors where electron-hole recombination emits photons of light equal to the bandgap energy. Population inversion, needed for lasing, can be achieved through heavy doping of both p-type and n-type materials near the depletion layer. 2) Early laser diodes used homojunctions but now use double heterojunctions of GaAs for the active region surrounded by higher bandgap AlGaAs for better optical confinement. 3) Double heterojunction lasers have lower lasing thresholds than earlier designs due to reduced optical losses from improved carrier and light confinement, though
Lecture 13 2013_
- 1. Fundamentals of LASER Example: Laser Diode
- 2. Optical emission from semiconductors In direct bandgap semiconductors, electrons and holes on either side of the forbidden energy gap have the same value of crystal momentum. Thus when electron-hole recombination occurs the momentum of the electron remains virtually constant and the energy released, which corresponds to the bandgap energy Eg, may be emitted as light. In indirect bandgap semiconductors, the maximum and minimum energies occur at different values of crystal momentum and the electron-hole recombination only possible with the aid of a third particle, a phonon. Hence the recombination in indirect bandgap semiconductor is relatively slow. Laser Diode
- 3. Optical emission from p-n diodes In an p-n diode, the normally empty electron states in the conduction band of the p-type material and the normally empty hole states in the valence band of the n-type material are populated by injected carriers (forward bias) which recombine across the bandgap. The energy released as a photon is approximately equal to the bandgap energy and the optical wavelength is This spontaneous emission of light from within the diode structure is known as electroluminescence. hc hfEg gE 24.1 Laser Diode
- 4. Stimulated emission in semiconductor diode Population inversion may be obtained at a p-n junction by heavy doping (degenerative doping) of both the p and n type material. Heavy p type doping with acceptor impurities causes a lowering of the Fermi level or boundary between the filled and empty states into the valence band. Similarly, degenerative n type doping causes the Fermi level to enter the conduction band of the material. When a forward bias nearly equal to the bandgap voltage is applied, an active region exists near the depletion layer that contains simultaneously degenerate populations of electron and holes (or double degenerate), where population inversion is occuring. Laser Diode
- 5. The laser commonly takes the form of a rectangular parallelepiped. The junction is a plane within the structure. Two of the sides perpendicular to the junction are purposely roughened so as to reduce their reflectivity. The other two sides are made optically flat and parallel, by either cleaving or polishing. These two surfaces form the mirrors for the laser cavity. The reflectivity of the air-semiconductor interface is high enough that no other mirrors are needed, although sometimes one of the reflecting surfaces may be coated to increase the reflectivity and to enhance laser operation. Diode laser In operation, electrons are injected into the device from the n-type side and is frequently called injection lasers. The semiconductor laser is a current-controlled device, rather than a voltage- controlled device. This will affect the design of the power supply. Laser Diode
- 6. Operational efficiency of the semiconductor laser Differential external quantum efficiency, is the ratio of the increase in photon output rate for a given increase in the number of injected electrons (the slope quantum efficiency) Internal quantum efficiency, i= External quantum efficiency, e = number of photons produced in the laser cavity number of injected electrons total number of output photons total number of injected electrons Laser Diode
- 7. The first major division is into edge-emitting and surface-emitting devices. For edge-emitting structures, the light emerges from the edge of the device, where the junction intersects the surface. The light emerges in the plane of the junction. The configuration is simple and easy to fabricate. Most diode lasers are edge-emitters. But they do suffer from the drawback that the volume of material that can contribute to the laser emission is limited and they are difficult to package as two- dimensional arrays. Classification of major types of semiconductor laser structures The edge-emitting devices may be divided into homojunctions, single heterojunctions, and double heterojunctions. The homojunctions are no longer used and double heterojunctions dominate most applications. For surface-emitting diodes, the light emerges from the surface of the chip rather than from the edge. This feature is attractive because devices could be packed densely on a semiconductor wafer and it would be possible to fabricate two-dimensional arrays easily. Laser Diode
- 8. Homojunction Laser The p-n junction of the basic GaAs laser described previously is called a homojunction because only one type of semiconductor material is used in the junction with different dopants to produce the junction itself. The index of refraction of the material depends upon the impurity used and the doping level. The junction region is actually lightly doped with p-type material and has the highest index of refraction. The n-type material and the more heavily doped p-type material both have lower indices of refraction. This produces a light pipe effect that helps to confine the laser light to the active junction region. In the homojunction, however, this index difference is low and much light is lost. Laser Diode
- 9. Single heterojunction Laser A fraction of the gallium in the p-type layer has been replaced by aluminum. This reduces the index of refraction of this layer and results in better confinement of the laser light to the optical cavity. This in turn leads to lower losses, lower current requirements, reduced damage, and longer lifetime for the diodes. Single heterostructure diode lasers are still in use, but have largely been replaced by double heterostructure devices. Laser Diode
- 10. Double Heterojunction Laser In the double heterojunction laser, only the junction region is composed of GaAs. Both the p and n regions are of AlGaAs. The result is better confinement of the optical standing wave on both sides of the optical cavity. The band-gap discontinuities that exist in DH laser also confined the injected carriers in the GaAs layer and made to recombine in the active region. This confinement greatly reduces the optical loss, but also leads to two additional difficulties. The optical radiation in the cavity is so well confined that the irradiance at the diode optical surfaces may easily reach the damage threshold, increasing the likelihood of catastrophic failure. The tight confinement of the beam also reduces the effective width of the output aperture of the laser. This increases the divergence angle in the direction perpendicular to the junction. Laser Diode
- 11. Some common material systems used in fabrication of heterojunction lasers. Each of the advances described has lowered the operating threshold of GaAs lasers. The typical current densities necessary to achieve the lasing threshold of the various junction types at 300° K. Homojunction 40,000 A/cm2 Single heterojunction 10,000 Double heterojunction 1,300 Double heterojunction, large optical cavity 600 The difficulties in DH laser are overcome by a further development of a large-optical- cavity (LOC) laser and uses regions of AlGaAs of varying composition. Laser Diode