Crystallographic Orientation-Dependent Performance of InGaN Blue Laser

Abstract

This thesis attempts to address a comprehensive in-depth study on the crystal orientation and strain-dependent optoelectronic performance and frequency response of 445nm InGaN/GaN single quantum well (QW) Blue Laser operating at 300K by solving a six-band k.p Hamiltonian at the Brillouin zone center point using finite difference method. Here 8nm In0.17Ga0.83N is used in the active layer and 10nm GaN is used in the guiding/barrier layer. The well is compressively strained due to lattice mismatch between well and barrier layer. The simulation is carried out in MATLAB/Simulink environment along (0001), (1010), (1012), (1122) and (1011 ) crystal orientations. Tensor rotation scheme is applied to modify the wave vector and Hamiltonian matrix from conventional (0001) crystal orientation. The compositions of well materials are selected as In0.15Ga0.85 N, In0.17Ga0.83 N, In0.19Ga0.81 N and In0.21Ga0.79 N for strain-dependent performance analysis along (1122 ), (1012) and (0001) crystal orientation. In crystal orientation-dependent analysis for 17% Indium composition, it is found that there is a substantial correlation of the energy band dispersion profile, momentum matrix, piezoelectric (PZ) field, peak gain and peak emission wavelength with change of crystal orientation. The PZ field is found to be zero in (1122) and (1010) crystal orientation (growth angle of 58.4° and 90° w.r.t z-axis). The optical gains are inspected as 3845, 4460, 4880, 4750 and 4178 cm-1 corresponding to peak emission wavelength of 447, 455, 446, 441 and 438nm at (0001), ( 1012), (1122), (1011) and (1010) orientations when the injection carrier density is of 3.5×1019 cm-3. Moreover, the optical gain in (1122) orientation is always higher than other orientation in any injection carrier density. During compressive strain-dependent performance evaluation, significant amount of optical gain and emission wavelength variations are found for the value of In composition from 15% to 21%. For example, in (1122)-oriented QW; optical gain changes from 4730 to 5190 cm-1 with a corresponding emission wavelength shifting from 452 to 430 nm for changing the indium composition from 15% to 21%. Output optical power characteristics are analyzed by developing a Simulink model with the help of 2- level rate equations and following the concept of signal-flow diagram. In (1122), (1012) and (0001)-oriented structure, higher optical power is found when the indium composition is 21% in the active region. Among arbitrary crystal orientated structure, maximum optical power of 4.35mW and minimum threshold current of 0.74mA is obtained in (1122) crystal orientation. A state-space model is formed for each (hkil)-orientated crystal structure in order to observe their frequency responses (Bode Plot) close to corner frequencies. The laser system for each orientation and indium composition is found to be stable as positive gain and phase margin is achieved. Highest magnitude (dB) response is obtained in semipolar (1122) crystal orientation for 21% indium composition in well. This numerical result demonstrates that semipolar (1122)crystal orientation is of special interest and can be incorporated into the active layer of blue laser in order to have best performance for high-speed lightwave communication technologies which can be used for medical & industrial purpose, precision measurement, military defence & security, thermal imaging, DVD and Blu-ray players etc.