DEPARTMENT OF TECHNOLOGY AND BUILT ENVIRONMENT Designing of High Reflectance Distributed Bragg reflectors (DBRs), mirrors using AlGaInN material system in the UV wavelength range

Gallium Nitride (GaN) and its alloys with aluminum (AlxGa1-xN) and indium (InyGa1-yN & AlzIn1-zN) have gained lots of attention in optoelectronics research community in the last decade. These semiconductors relativity have a wide bandgap and easy to make n-type layers by Si doping and p-type layers by Mg doping. Consequently nitride alloys have proved to be very attractive candidate for the fabrication of Distributed Bragg reflectors (DBRs) for VCSEL including other optoelectronic devices. Distributed Bragg reflectors have prime importance in the performance of VCSEL. DBRs not only provide highest possible reflectivity, usually 99.9% is required, but also conduct electricity, confine current, to the gain region. That’s why accurate modeling and simulation of DBRs have significant importance for the future fabrication of VCSEL. Matrix transmission method is employed to model and simulate the nitride based DBRs. As refractive index and absorption coefficient is key parameters to design optoelectronic devices and necessary to model the DBRs are modeled. Refractive index is modeled in this thesis work by Adachi’s model and Sellmeier model. In first part of thesis, matrix transmission based DBR model is implemented on AlxGa1xN based DBRs. Refractive index is modeled by Adachi’s model and Sellmeier model. As Adachi’s model is widely considered most accurate model so it is used to calculate the refractive index over the entire range mole fraction of Al (Aluminum). Absorption is also modeled. Reflectivity spectrum is plotted by AlGaN/AlGaN, AlGaN/GaN, AlN/GaN and AlGaN/AlN based DBRs. Phase response and transmissivity also are plotted. The bandwidth is an important parameter in nitride alloys. So how bandwidth is influenced by number of parameters like reflectivity, number of periods and Bragg wavelength are analyzed. The bandwidth with and without dispersion is plotted. Two design curves are plotted that are very useful to practical realization of DBRs. These curves give us minimum operating wavelength and minimum Al content in 2 layers over the number of periods. In second part of thesis, InyGa1-yN alloy is investigated and try to find hurdles to calculate accurate optical parameters e.g. energy bandgap, refractive index and absorption coefficient. Stress free InGaN/AlGaN DBRs are also modeled and their reflection spectrum is plotted. In third and last part of thesis, AlzGa1-zN alloy is investigated and discussed obstacles to calculate its accurate optical parameters. AlGaN/GaN lattice matched DBRs are also discussed with details and reflection spectrum is plotted. In conclusion, we demonstrated which nitride alloy is most promising for designing and fabrication of DBRs in this date and which one could be in future.