Two-dimensional electron gas and persistent photoconductivity inAlxGa1−xN/GaNheterostructures

We present results of electrical and optical measurements in an ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructure. The presence of a two-dimensional electron gas at the high-quality ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterointerface is confirmed by Shubnikov–de Haas measurement, which shows well-resolved magnetoresistance oscillations starting in fields below 3 T at 1.3 K. From the temperature dependence of the oscillation amplitude, the obtained effective mass $(0.24\mathrm{}\pm {0.02)m}_0$ is in excellent agreement with the value of cyclotron resonance measurements in two-dimensional (2D) systems, but larger than the values of theoretical and experimental results in GaN bulk films. We point out that the effective-mass enhancement in 2D systems is due to the effects of band nonparabolicity and wave-function penetration into the barrier material. The results of photoconductivity measurements reveal that persistent photoconductivity (PPC) does exist in the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructure, and that the PPC behavior of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterojunction is quite different from that of the GaN epitaxial thin films. A possible mechanism is presented to interpret the observed PPC effect.