The effects of GaN capping layer thickness on two-dimensional electron mobility in GaN/AlGaN/GaN hetero structures

In this paper we present a study of the effect of GaN capping layer thickness on the two-dimensional (2D)-electron mobility and the two-dimensional electron gas (2DEG) sheet density which is formed near the AlGaN barrier/buffer GaN layer. This study is undertaken using a fully numerical calculation for GaN/AlxGa1-xN/GaN hetero structures with different Al mole fraction in the AlxGa1-xN barrier, and for various values of barrier layer thickness. The results of our analysis clearly indicate that increasing the GaN capping layer thickness leads to a decrease in the 2DEG density. Further-more, it is found that the room-temperature 2D-electron mobility reaches a maximum value of approximately 1.8 x 10(3) cm(2)/Vs(-1) for GaN capping layer thickness grater than 100Angstrom with an Al0.32Ga0.68N barrier layer of 200Angstrom thick. In contrast, for same structure, the 2DEG density decreases monotonically with GaN capping layer thickness, and eventually saturates at approximately 6 x 10(12) cm(-2) for capping layer thickness greater than 500Angstrom. A comparison between our calculated results with published experimental data is shown to be in good agreement for GaN capping layers up to 500Angstrom thickness. (C) 2004 Elsevier B.V. All rights reserved.