Paper EM+AS+NS+SS-WeA4
Lattice Relaxation and Misfit Dislocations in Sublinearly Graded In_xGa_1-xAs/GaAs (001) and GaAs_1-yP_y/GaAs (001) Heterostructures

Wednesday, October 30, 2013, 3:00 pm, Room 102 A

Metamorphic devices such as high electron mobility transistors (HEMTs), heterojunction bipolar transistors (HBTs), light-emitting diodes (LEDs), laser diodes, and solar cells have been fabricated on GaAs substrates using graded buffer layers. These metamorphic buffer layers usually employ linear grading of composition, and materials including In_xGa_1-xAs and InAs_1-yP_y have been used. The most important function of the linearly- graded buffer layer is to minimize the threading defect density by enhancing the mobility and glide velocity of dislocations, thereby promoting long misfit segments with relatively few threading arms. In general, there are three features of graded buffers which reduce the thread density: (i) a misfit dislocation free zone (MDFZ) near the substrate interface which reduces pinning interactions with substrate defects, (ii) a second MDFZ near the surface which reduces pinning interaction near the device layer; and (iii) a large built-in strain in the top MDFZ which enhances glide of dislocations to sweep out threading arms. However, non-linear grading may be beneficial for better control of the widths of the MDFZs and for higher built-in strain in the surface MDFZ. In this work, we present minimum energy calculations for sublinearly graded heterostructures, with logarithmic and power law grading, and compare the cases of cation (Group III) and anion (Group V) grading. We show that differences in the elastic stiffness constants give rise to significantly different behavior in these two commonly-used buffer layer systems. Moreover, the use of different grading profiles for sublinear buffer layers allows for correlation of a sublinearity factor to the width of dislocated region and peak misfit density.