Effects of alloy disorder on the transport properties of Al_xGa_1-xN epilayers probed by persistent photoconductivity

The effects of alloy fluctuations on the transport properties of Al_xGa_1-xN alloys (x∼0.35) have been probed through the use of persistent photoconductivity (PPC). In the PPC state, the electron mobility, μ_e, as a function of electron concentration, n, in a single sample can be obtained under controlled light illumination conditions. It was found that μ_e is a constant when n is below a critical value n_e and it then increases with n at n>n_e. This mobility behavior was attributed to the effects of alloy fluctuations in Al_xGa_1-xN alloys. As a result, the initial PPC buildup kinetics seen in Al_xGa_1-xN alloys was quite different from those in better understood semiconductor alloys, such as AlGaAs and ZnCdSe, and is a direct consequence of the observed unique dependence of μ_e on n. From these measurements, the total density of the tail states below the mobility edge in the conduction band was estimated to be 1.46×10¹⁷cm^-3 in a Al_0.35Ga_0.65N sample. The results were compared with those in II-VI semiconductor alloys and their implications on III-nitride device applications were discussed.