Dislocation reduction in heteroepitaxial In x Ga1-xN using step-graded interlayer for future solar cells

Abstract

The efforts on dislocation reduction have become a potential issue to realize the future high-efficiency solar cells using the InGaN materials. In this work, first, a numerical simulation has been carried out for the reduction of dislocation density in wurtzite InGaN heteroepitaxy using step-graded interlayers. An energy balance model has been developed for evaluating the misfit dislocation (MD) density. The residual strain from previous interlayer has been taken into account with misfit strain in each interlayer. A reaction model for threading dislocations (TDs) has also been developed and solved numerically considering the geometrical parameters. The simulation results confirmed a significant improvement of epilayer quality due to the use of step-graded interlayers. The calculations have been done for 1.5 lm InGaN using 3 stepgraded interlayers each containing 10 % composition difference and 0.2 lm thickness. The edge, screw, and mixed type MDs are found to be 4.69 9 10 10 , 4.53 9 10 9 , and 4.03 9 10 11 cm-2 , respectively, on the 1/3 \ 11-23 [ (11-22) slip and similarly evaluated in other possible slips of the In 0.4 Ga 0.6 N. Significant decreases in MD densities have been evaluated for increasing interlayer up to 4 with 8 % In composition difference. The edge, screw, and mixed MDs are found to be decreased from 3.25 9 10 11 to 4.45 9 10 10 , 3.2 9 10 10 to 3.95 9 10 9 and 2.8 9 10 12 to 1.77 9 10 11 cm-2 in 1/3 \ 11-23 [ (11-22) slip system using four interlayers. Besides, a higher rate of reduction has been reported for the step-graded structure especially for mixed type due to more relative motion and step inclination at each interlayer. The average edge, screw, and mixed type TD densities are found to be 1.48 9 10 10 , 3.7 9 10 10 , and 1.1 9 10 9 cm-2 , respectively, at the top surface of the In 0.4 Ga 0.6 N epilayer. Finally, experimental work has been done to realize the strain profile using reciprocal space mapping as well as dislocations considering the same films to compare the results. The outcome of the numerical simulation found good agreement with the experimental works as well as with the published experimental results.

Microstructural Analysis with Graded and Non-Graded Indium in InGaN Solar Cell

“…All types of dislocations are related to the mosa ic crystal sizes, tilt angle and the twist angle. Metzge r et al 26 stated that edge type disloca tions with a mean twist angle Here a ,il, is a tilt ang le for GaN and bscrew = 0.5185 nm.…”