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.
Description:
This thesis is submitted to the Department of Electrical and Electronic Engineering, Khulna University of Engineering & Technology in partial fulfillment of the requirements for the degree of Master of Science in Electrical and Electronic Engineering, July 2012.
Cataloged from PDF Version of Thesis.
Includes bibliographical references.