O. Ambacher et al., Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures, J APPL PHYS, 87(1), 2000, pp. 334-344
Two dimensional electron gases in AlxGa1-xN/GaN based heterostructures, sui
table for high electron mobility transistors, are induced by strong polariz
ation effects. The sheet carrier concentration and the confinement of the t
wo dimensional electron gases located close to the AlGaN/GaN interface are
sensitive to a large number of different physical properties such as polari
ty, alloy composition, strain, thickness, and doping of the AlGaN barrier.
We have investigated these physical properties for undoped and silicon dope
d transistor structures by a combination of high resolution x-ray diffracti
on, atomic force microscopy, Hall effect, and capacitance-voltage profiling
measurements. The polarization induced sheet charge bound at the AlGaN/GaN
interfaces was calculated from different sets of piezoelectric constants a
vailable in the literature. The sheet carrier concentration induced by pola
rization charges was determined self-consistently from a coupled Schrodinge
r and Poisson equation solver for pseudomorphically and partially relaxed b
arriers with different alloy compositions. By comparison of theoretical and
experimental results, we demonstrate that the formation of two dimensional
electron gases in undoped and doped AlGaN/GaN structures rely both on piez
oelectric and spontaneous polarization induced effects. In addition, mechan
isms reducing the sheet carrier concentrations like nonabrupt interfaces, d
islocations, and the possible influence of surface states on the two dimens
ional electron gases will be discussed briefly. (C) 2000 American Institute
of Physics. [S0021- 8979(00)03401-0].