Strategies of reducing threading dislocation density in heteroepitaxial thin films

• Main Author: Hu, Chimin
• Format: Dissertation
• Language: English
• Subjects: engineering, materials science
• ISBN: 9780591764963; 0591764962

$\rm CdZn\sb{0.04}Te$ is used as a substrate for electro-optical material, $\rm HgCd\sb{0.2}Te.$ To increase the yield, thin $\rm CdZn\sb{0.04}Te$ films are grown on GaAs or Si. The large lattice mismatch between substrates and thin films leads to high threading dislocation densities in $\rm CdZn\sb{0.04}Te$ films. The goal of the present work is to develop an efficient growth method to filter threading dislocations in $\rm CdZn\sb{0.04}Te$ films.
Dislocations need to be mobile in order to annihilate with one another or move out of samples. Both driving and retarding forces on perfect and partial dislocations are analyzed. Peierls force is included in the retarding force for both static and kinetic cases. From the governing equations, the effective force on threading dislocations in thin films can be affected by substrate orientation, misfit strain, thickness and annealing. The current work develops a novel method to filter threading dislocations by considering these effects.
The effective forces on both perfect and partial dislocations versus substrate orientation are calculated. The optimum substrate orientations for containing the greatest number of mobile dislocations and the fewest number of wide stacking faults are determined. The effects of the stacking fault energy and the sign of stress are investigated. The minimum stacking fault energy required to avoid the formation of wide stacking faults versus mismatch and thickness is also obtained.
A novel growth method, step annealed multiple strained layer (MSL), is designed by employing: (1) misfit strain; (2) thickness; (3) multiple interfaces; and (4) annealing after growth of each layer, as strategies to filter threading dislocations. A quantitative model is developed to simulate: (1) the effective force on threading dislocations, (2) the threading dislocation density on the growth surface, and (3) the misfit strain in the film, versus time. The growth method is experimentally investigated in CdZnTe MSL on (001) GaAs and (112) Si. The simulation model successfully predicts the experimental results. The defect structures in CdZnTe thin films are also examined in the present work. An improved growth design for step annealed MSL is proposed to further filter threading dislocations in heteroepitaxial thin films.