Numerical Simulation of Dislocation Reduction in InGaN and InGaAs Heteroepitaxy with Step-graded Interlayers

Abstract

In the recent years, the InGaN and lnGaAs heteroepitaxy with low dislocation density have become crucial important for high performance electronic and optoelectron Ic devices, especially for multi junction solar cell. Theoretical efforts on dislocation reduction have become a potential issue to realize the future novel device using these materials. In this dissertation, a numerical simulation has been carried out for the reduction of dislocation density in wuzrite InGaN as well as cubic lnGaAs heteroepitaxy with step-graded interlayers. An energy balance model has been developed for evaluating the misfit dislocation (MD) density in the step-graded structure of these heteroepitaxy. The residual strain from previous interlayer has been taken into account with misfit strain to calculate the MD density in each interlayer. To obtain a detail understanding of dislocation interactions and their propagation through the material, a reaction model has been developed considering the geometrical parameters of the step-graded heteroepitaxy. The reaction equations in each model are developed considering the possible annihilation and fusion reactions between each pair of threading dislocations (TDs) and blocking of TDs by MDs on their gliding paths. The evaluations of TI) densities have been done using the numerical simulation of the reaction model by Euler method. The simulation results confirm a significant improvement of epilayer quality due to the use of step-graded interlayers for the heteroepitaxy. The calculations have been done for 3 step-graded interlayers each containing 10% composition difference. Each interlayer and the total thickness of the film are 0.2 im and 1.5 tm respectively. The edge, screw and mixed type MDs are found to be 1.14x10, l.4x10'° and 9.9x1011 cm-2 respectively on the 1/3<11-23>(1 1-22) slip of the lno 4Ga06N epilayer. The MDs are also estimated in 1/3<1 1-23>(1-101) and 1/3<1 1-20>(0001) slip systems. Significant decreases in MD densities from first interlayer to second interlayer, second interlayer to third interlayer and third interlayer to final epilayer have been evaluated. The edge, screw and mixed MDs are found to be decreased from 1.6x1010 to 1.14x1011 cm-2. 2.0x lO'° to 1.4x I 0 cm-2 and 1.4x 1012 to 9.9x 1011 cm-2 in the 1/3<1 1-23>( 11-22) slip system from first interlayer to final InGaN epilayer. On the other hand, due to use of 3 interlayers for 11104Ga06As the total edge and mixed type MDs are found to be decreased from l.lx10 to 8.8x 108 cm-2 and 9.2x 10'' to 7.6x 109 cm-2 respectively. These step wise decrease in dislocation densities at each interlayer and final epilayer have good agreement with experimentally observed results. The numerical solutions of TD densities also confirm the improved epilayer quality using step-graded interlayer. Due to more relative motion and step inclination at each interlayer a higher rate of reduction with film thickness have been reported for mixed type TDs. The average edge, screw and mixed type TDs densities for the step graded structures are found to be 1.48x 1010, 3.7x1010 and 1.1x 109 cm-2 respectively at the top surface of the In0.4Ga0.6N epilayer. In contrast, these values are 7.6x 10'°, 1.89x10'' and 6.26x109 cn12 respectively for the without graded structure. In the same way, the average edge and mixed type TD densities decreased from 4.9x109 to 2.05x 109 cm-2 and 2.1x 1010 to 2.28x 109 cm-2 respectively for step grading in lnGaAs heteroepitaxy. The above performance analysis of the proposed step-graded technique suggests that it will be very promising and superior for improving the material quality in case of heteroepitaxial film.