Conventional and microwave-modulated Shubnikov-de Haas oscillations in GaN electron systems

We report the drastic enhancement of Shubnikov-de Haas (SdH) oscillations observed in a GaN/AlGaN heterostructure by microwave modulation. The dependence of the SdH pattern on microwave power and temperature are investigated. The underlying mechanism is attributed to the effect of carrier heating. This technique has the advantage of keeping the carrier concentration fixed and not requiring expensive high-energy laser facilities compared with carrier-modulated SdH measurements. Moreover, we have investigated the low-temperature transport properties of front-gated Al_0.18Ga _0.82N/GaN heterostructures. By changing the applied gate voltage, we can vary the carrier density in our sample. At high carrier densities (n > 4.65 × 10¹² cm^-2), the measured mobility (μ) is found to be a decreasing function of carrier density as μ∼n ^-0.31. Loss of mobility with increasing carrier density is dominated by interface roughness scattering. At low carrier densities (n < 4.24 × 10¹² cm^-2), the measured mobility is found to be an increasing function of carrier density as μ∼n^0.34. This is consistent with remote ionized impurity scattering, although the measured exponent 0.34 is smaller than the typical value (0.7-1.5) observed in an AlGaN/GaN electron system. A possible reason is that our sample mobility is approximately five times lower than those in other devices for a similar electron density.