|
We present a study of the electrical and structural properties of 1atUcemismatched InGa,.As strained layers grown
on GaAs (100). This strained system has been used in device structures because of the high carrier mobilities which
may be obtained, but the inherent misfit strain is known to produce dislocations above some critical limit. We have
grown, by molecular beam epitaxy (MBE), structures with p-type (Be), 8 x1019 InGa,As layers of 20 nm thickness.
The indium mole fraction, x, was increased in subsequent samples from 0 to 0.50 in steps of 0.05, simulating the
base of a heterojunction bipolar transistor (HBT). We correlate hole mobilities and sheet resistances with the
microstructure of the strained layers as determined by transmission electron microscopy (TEM). It is shown that the
hole mobility drops from 55.3 cm2V-'s-' to 31.7 cm2V-1s-1 x varies from 0 to 0.50, with a corresponding rise in sheet
resistance. For x □0.40, significant reduction in electrically active carrier concentrations is observed, presumably due
to interaction with the extensive defect structure. HBT structures incorporating the base layers described were grown
by MBE and processed into devices. These devices show a maximum gain at x= 0.10, with good I-V characteristics,
and a subsequent sharp fall in gain at higher x, with consequently poorer I-V characteristics. We believe this shows a
higher sensitivity of minority carriers to the dislocation network than majority carriers. No misfit dislocations are
visible in plan-view ThM until x= 0.25-0.30, considerably higher than the Matthews-Blakeslee mechanical
equilibrium prediction of x= 0.10. This confirms the relatively sluggish relaxation of these structures at the growth
temperature of 500°C. Further, we demonstrite that for x□0.4, generation of new misfit dislocation length is not by
deflection of pre-existing threading dislocations, but rather by generation of new misfit dislocation ioops.
|
