Modeling Study And Performance Optimization Of Biaxial Tensile Strain Based Double Heterojunction Edge Emitting Germanium Laser

In recent years,semiconductor laser coupled with the low loss fiber,have impacted the optical fiber communication greatly and brought significant improvement in application.Nowadays commercial lasers usually use ?-? semiconductor materials as the light source.However,the optoelectronic hybrid integration can not be widely accepted because of the two reasons: the fabrication process of the ?-? semiconductor is incompatible with the CMOS large-scale integration process,high cost and low producing also limit the hybrid integration by chip bonding.Fortunately,Germanium,which is also the ? semiconductor material,became the hot topic with the progress of epitaxial growth technology on Si substrate.Compared to the ?-? material semiconductor laser,one of the main problems in the progress of Ge laser is the extremely high threshold current density.The reasons for such high threshold condition in the electrically pumped double heterojuction edge emitting Ge laser fabricated by MIT included high series resistance from the top Poly-Si contacts,large free carrier absorption from heavily N doped Ge and P doped Poly-Si and light leakage due to the unoptimized optical cavity size.Therefore,the research aims of this thesis are the analysis of the Ge laser performance with different cavity size,biaxial tensile strain defect-limited minority carrier lifetime and temperature,the properties of performance such as threshold current density,wall-plug efficiency in different environment are the focal points in the thesis,in order to search the optimized design for the Ge laser performance.Proposing the model of Ge laser reasonably is the base for analyzing and optimizing the Ge laser performance.Theoretically,the light emitting in semiconductor materials is based on the optical gain by direct transition between the electrons in conduction band and holes in valence band,respectively.In this case,the?valley and L valley have to be in the same energy level in order to achieve the direct bandgap population inversion.The combination of biaxial tensile strain and n type doping is an effective way to achieve such condition.Compared to those papers only focuses on the theoretical calculation of Ge properties in the past,in this thesis,based on the Fabry-Perot double heterojuction edge emitting Ge laser fabricated by MIT,using the 2D edge emitting laser simulation software Lastip,a model based on the indi-rect bandgap material light source Ge and biaxial tensile strain was proposed.Split-off Valence bands effect and Bandgap narrowing effect due to biaxial tensile strain and n type doping were also taken into account,respectively.It was the first time to establish a Ge laser model including cavity structure,metal contact and substrate.Simulation results showed the proposed model was able to match the experimental L-I curve quite well.In this thesis,the variations of the threshold condition in Ge laser with the different cavity size were analyzed.Important cavity parameters,such as length,width,thickness and Poly-Si cladding layer thickness,were optimized with the standard of threshold current.Results showed that after the optimization,there was a 17 times reduction in threshold current.5.6 and 9.6 times increasing in internal quantum efficiency and differential quantum efficiency can be also seen,respectively.These proved that Ge laser performance can be improved by optimizing cavity size.Moreover,results showed that threshold condition can be further decrease by improving the Ge qualities and increasing the defect-limited minority carrier lifetime,showing that it is also another way to achieve high efficiency and low threshold.Besides,increasing biaxial tensile strain in Ge is also an effective way to reduce the threshold condition.Larger strain can reduce the energy difference between?and L valley,as well as the loss by free carrier absorption due to n type doping.In this case,the variations of optical gain,threshold current and wall-plug efficiency were discussed with different strain and doping.Optimized doping value in each strain was also obtained so as to provide theoretical suggestion for the Ge laser fabrication with large strain.In practical application,Ge laser will be impacted by temperature and self heating effects.In this thesis,the temperature model for Ge laser was presented by combining the relationships between the parameters prosperities and temperature.Threshold condition decreased with the increasing temperature,which is totally different with temperature effect in ?-? semiconductor materials laser,was discussed.With a higher threshold current density,the carrier density in the direct bandgap transition energy levels will be higher in high temperature due to the large amount of injected carrier.This explained the reason for the abnormal temperature effect.Results showed that optimizing cavity size and increasing strain is one of the effective ways to reduce the threshold and avoid the abnormal temperature effect.