Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

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].