Misfit dislocation density reduction of InP-based pseudomorphic high electron mobility structures
M.Naidenkova, R. Hsing, M.Goorsky, R. Sandhu, P. Chin, M. Wojtowicz, and T.Block
UCLA, Department of Materials Science and Engineering, Los Angeles, CA 90095
TRW Space and Electronics Division, Redondo Beach, CA 90278
Strain-compensating layers are shown to significantly reduce the density of misfit dislocations in the highly strained channels (In0.75Ga0.25As) of (001) InP-based pHEMT structures grown by molecular beam epitaxy. A series of growth experiments comparing lattice matched and tensile strained buffer and/or barrier and also the channel layer grown temperature has led to the development of a model that predicts dislocation formation. PHEMT structures with lattice-matched buffer and barrier InAlAs layers posses measurable quantities of misfit dislocations when the channel thickness is increased beyond 15 nm. We have shown that the introduction of tensile-strained buffer and barrier layers (In0.45Al0.52As) can lead to misfit dislocation-free structures. The tensile strain in these layers compensates for the compressive strain in the channel and reduces the driving force for misfit dislocation in the channel.
Changing the growth temperature of the channel over a range of 40° C did not have a significant effect on the dislocation density. The growth of structures with only lattice-matched buffer layers and strained 20 nm channel demonstrate that misfit dislocation formation did not occur during growth of the channel layer. The only effect of changing the channel growth temperature over this range was a difference in the channel surface roughness. However, during the subsequent growth of the InAlAs barrier layer, which is deposited at a much higher temperature than the channel layer, misfit dislocations are produced. This observation, coupled with the growth and characterization of the structures with tensile strained buffer layers and lattice matched barrier layers, indicates that introducing tensile strain in the buffer is sufficient to stabilize the pHEMT structure against misfit formation in the channel. Introducing tensile strain in only the barrier layer-as described elsewhere in the literature is much less effective at reducing high temperature barrier layer is grown.