Influence of relaxation on the two-dimensional electron-gas in AlxGa1-xN/GaN/AlyGa1-yN double heterostructure with compressively strained GaN layer

Influence of relaxation on the two-dimensional electron-gas (2DEG) in Al_xGa_1-xN/GaN/Al_yGa_1-yN double heterostructure (DH) is investigated by self-consistently solving the coupled Schrodinger and Poisson equations. The GaN channel layer is in compressive strain on a relaxed AlGaN lower barrier (LB) so as to improve the Al content in the Al_xGa_1-xN top barrier (TB) to be double that in the LB (y) and hence the 2DEG density is greatly improved. It is found that the 2DEG sheet density increases with increasing Al content in the TB and achieves a maximum N_S=3.88×10¹³ cm^-2 at x=0.88 (with y=0.38), even behind the onset of relaxation at x=0.76, indicating the dominant influence of the polarization effect and the quantum confinement effect. Further increasing x reduces N_S due to significant relaxation. Taking relaxation, R, into account, the 2DEG sheet density increases with increasing Al content in the Al_yGa_1-yN lower barrier (LB), opposite to the case of fully strained.