Influence of strain relaxation of the AlxGa1-xN barrier on transport properties of the two-dimensional electron gas in modulation-doped AlxGa1-xN/GaNheterostructures

Influences of the thickness of the Si-doped n-type Al0.22Ga0.78N barrier an d the thickness of the Al0.22Ga0.78N spacer on mobility and density of the two dimensional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN h eterostructures were investigated. 2DEG mobilities of 1274 cm(2)/V s at 300 K and 4495 cm(2)/V s at 77 K were reached. Both 2DEG mobility and density decrease dramatically when the Al0.22Ga0.78N barrier becomes partially rela xed, indicating that transport properties of the 2DEG are influenced signif icantly by the piezoelectric polarization of the Al0.22Ga0.78N layer. From our results, the critical thickness of an Al0.22Ga0.78N layer on GaN is est imated to be between 65 and 75 nm, which is much higher than that predicted by theoretical calculation. This may be attributed to the interaction of m isfit dislocations and the presence of a high density of extended defects i n the Al0.22Ga0.78N layer. (C) 2000 American Institute of Physics. [S0003-6 951(00)05219-0].