Two-dimensional electron gases in Ga-face and N-face AlGaN/GaN heterostructures grown by plasma-induced molecular beam epitaxy and metalorganic chemical vapor deposition on sapphire

We report on the growth of nominally undoped GaN/AlxGa1-xN/GaN (x < 0.4) hi gh mobility heterostructures with N-face or Ga-face polarity on sapphire su bstrates by plasma-induced molecular beam epitaxy (PIMBE) and metalorganic chemical vapor deposition in order to study the formation and electrical tr ansport properties of polarization induced two-dimensional electron gases ( 2DEGs). By depositing a thin AlN nucleation layer on the sapphire substrate s before the growth of a GaN buffer layer by PIMBE, we were able to change the polarity of the wurtzite films from N to Ga face. The switch in the pol arity causes a change in the sign of the spontaneous and piezoelectric pola rization directed along the c axis of the strained AlGaN barrier. As a cons equence the polarization induced 2DEG is confined at different interfaces i n heterostructures with different polarities. The transport properties of t he 2DEGs in Ga- and N-face heterostructures were investigated by a combinat ion of capacitance-voltage profiling, Hall effect, and Shubnikov-de Haas me asurements. Dominant electron scattering mechanisms are studied in order to provide the knowledge necessary for further improvements of the electron t ransport properties and performance of AlGaN/GaN based "normal" (based on G a-face heterostructures) and "inverted" (based on N-face heterostructures) high electron mobility transistors. (C) 2000 American Institute of Physics. [S0021-8979(00)03007-3].