2 Laser 3- Lecture LayoutBy the end of the course you would be able to answer the following questions:What is homojunction laser?What is heterojunction laser?Explain the principles of heterojunction laser.Sketch a typical stripe geometry laser diodes.What is quantum well laser?
3 IntroductionIn the pervious lectures you have been explain about two important conditions for designing laser:Optical GainMedium which possess the desired energy level structure to support laser action in the case of diode laser this will be the active region of the p-n junctionTo establish a population inversion in a laser system the forward bias current supplied to the diode laser.2. Optical FeedbackHomojunction laser with one end cleaved and the other roughned. This is to achieve the optical feedback (optical gain) of the laser system. Such system is often termed Fabry-Perot Cavity.
4 Threshold Current Density Consider a diagram showing the active region and mode volume of a semiconducting laser:Mode volume, thickness, dpActive region, thickness, tnRecall that when forward biased, with eV > Eg of the material, electrons (from degenerately doped n) and holes (from degenerately doped p) will be injected across the junction to create population inversion.The population inversion is created in a region called active region. Radiative transition may occur resulted in stimulated emission when the photon is absorbed by the electrons in the conduction band.The radiation generated will be spread out in the vicinity of the active region and is almost confined in the thin layer shown above (mode volume).
5 Junction (active region and mode volume) Schematic construction of a homojunction GaAs diode laser.Metal contact (+)Cleaved end (110)Natural crystal planes of the junction so that the end faces are parallelp+ GaAsThe laser beam outputn+ GaAsJunction (active region and mode volume)Metal contact (-)Roughened endThe carriers in the active region increases refractive index of GaAsThe refractive index increment is only ~0.02, hence is not a good dielectric waveguideThe beam therefore can be spread out to the surrounding region – mode volumeVigorous pumping is therefore needed to enhance lasingThe threshold current for the pumping action exceeds 400Amm-2
6 Threshold Current Density Definition If the injected carrier concentration become large enough, the stimulated emission can exceed absorption so optical gain can be achieved in the active region. With appropriate configuration to achieve optical feedback, laser oscillation occurs when gain exceeds losses.For significant gain, a high current density is necessary.The onset of lasing is characterised by the a specific injection current known as the Threshold CurrentSince the simple homojunction laser has high threshold current, it is considered not efficient.The onset of laser action at the threshold current density is indicated by an abrupt increase in radiance of the emitting region, leading to marked decrease in spectral width.
7 Threshold Current Density The typical output spectrum Stimulated EmissionOptical powerOptical PowerlaserSpontaneous EmissionIOptical powerLEDJTH
8 In conclusion about the homojunction laser…. The main problem with the homojunction laser diode is that the threshold current density, Jth is far too high for practical applications.JTH increases with temperature, too high at room temperature, not continuous but pulsed laser output.Homojunction laser has:Poor opticalLess carrier confinementIf Jth is low: improve rate of stimulated emission & improve efficiency of optical cavityTo get low Jth:Confined carriers in a narrow region carrier confinementBuild dielectric waveguide around the optical gain region (increase photon concentration hence stimulated emission) photon confinementHow do we achieve that?heterostructured laser diodes
9 The Heterojunction Laser Single & Double Metal contact (+)GaAs sandwiched between the higher band gap AlGaAsn GaAlAsN GaAs1mp GaAsn GaAlAsp GaAlAsp GaAsP GaAlAsMetal contact (-)GaAs sandwiched between the higher band gap AlGaAs. GaAs is the active region where lasing takes placeN-p-PN-n-p-P
11 Carriers & Photons Confinement N-Ga1-xAlxAs|p-GaAs|P-Ga1-xAlxAsN |ACTIVE LAYER|PGaAs and GaAlAs:Have different refractive indexnGaAlAs < nGaAsHave different EgEg (GaAlAs) > Eg(GaAs)Band gap difference forms barriers for e and h to diffuse from GaAs to the sandwich layers of GaAlAs CARRIER CONFINEMENTStep difference in the refractive index waveguide (Optical/Photons Confienment)Eg (GaAlAs) > Eg(GaAs) Photons produced in GaAs will not be absorbed by GaAlAs.
12 Stripe Geometry DHJ Laser Features:Oxide layer or high resistive layer (produced by proton bombardment) between metal contact and the semiconductor.Restrict current along the junction into narrow stripe (few microns)Small JTH with high Power continuous operationUsed largely in Optical Fibre CommunicationThe configuration is shown to you in Wilson page 217 for DHJ with oxide that isolate the metal contact to the GaInAsP (figure 2.17)
14 Materials Criteria & Selection To date GaAs and GaAlAs are largely used.Advantages of AlGaAs/GaAs system is that:GaAs is direct band gap materialGa1-xAlxAs is direct when x < 0.45Lattice match between Ga1-xAlxAs & GaAs is very small (0.1%) therefore epi growth can be achievedThe band gaps of both materials can be manipulated to produce SH or DH junctions lasers for high optical and carrier confinemntsFor optical fibre communication, wavelength of m is preferred.Refer to Wilson page 216 (figure 5.33) or see the next slide
15 Typical Exam Question on GaAs/GaAlAs Eg(x) = x (eV) Empirical relationshipCalculate the band gap if GaAlAs is to be used as emitter for fibre optics communication at wavelength 1.4m.Calculate compositions of the GaAlAs ternary alloy for peak emission at wavelength 1.4m.
16 Band Gap Engineering To answer: What other system can be used?
17 Quantum Well LasersStructure similar to the DH laser except thickness of active layer is very small (10-20nm)E.g. narrow Eg GaAs sandwich between larger band gap GaAlAsWith this configuration, density of states near the bottom of the conduction band and the top of the valance band increased significantly the hence enhance the population inversionBetter population inversion, smaller active layer hence JTh is smaller.BUT, in single quantum well (SQW) extreme narrowness of the active region created poor optical confinement.So… Solve by Multiple Quantum Well Structure (MQW)SQW can be coupled to produce the MQWOverall active region is now thickerCarriers which are not captured in one well can be captured by the second well etc.MQW has JTH higher than SQW (~ 1mA) but the more optical power due to better optical confinement
18 Cladding Layer and Separate Confinement Heterostructure
19 Preparation for Next week (Monday) Test on Laser and LED