Study On The Magnetotransport Properties Of GaN-based Semiconductor Heterostructure

In recent years, AlGaN/GaN heterostructure has been widely studied due to the potential applications in high power and high frequency electronic devices. Theoretically, the electron density can be increased by increasing the Al composition in the AlxGa1-xN barrier layer. However, the higher Al composition will induce strong scattering and thus reduce the Hall mobility. Kuzmík proposed to use the Al0.82In0.18 N as the barrier layer would improve HEMT performance, as it can provide superior polarization-induced charges together with lower strain. In this paper, high quality AlGaN/GaN, AlIn N/AlN/GaN and AlInN/GaN/AlN/GaN heterostructure samples were prepared by the metal organic chemical vapor deposition(MOCVD) method. The transport properties of these three samples were measured systematically.The carrier density and Hall mobility of AlGaN/GaN and Al InN/AlN/GaN samples at different temperatures(2-270 K) were obtained by using the Hall measurement. In the studied temperature range, the carrier density in AlInN/AlN/GaN is higher than that in AlInN/AlN/GaN. However, the sheet resistance of AlGaN/GaN is lower than that of Al InN/AlN/GaN and the AlGaN/GaN possesses higher Hall mobility. The longitudinal magnetoresistance as a function of magnetic field for AlGaN/GaN and Al InN/AlN/GaN at 2-10 K is measured by using the physical property measurement system(PPMS) and well defined Shubnikov-de Haas oscillations(SdH) is observed in both samples. While the SdH in AlGaN/GaN is much stronger than that in AlInN/AlN/GaN, this is due to the larger quantum mobility in AlGaN/GaN. There is only one peak in the FFT spectrums of AlGaN/GaN and AlInN/AlN/GaN samples, indicating that only the lowest subband of 2DEG is occupied in both samples. The carrier density obtained from the FFT spectrum is consistent with the Hall carrier density, which denmonstrates that there is no parallel conduction in both samples. The ratio of the Hall mobility and the quantum mobility are much larger than 1 in both two samples, indicating that the main scattering mechanisms in these two samples are small angle scatterings. The scattering sources may come from the ionized impurity in the barrier layer.Al InN/GaN/AlN/GaN presents a smoother surface than AlInN/AlN/GaN from the AFM images. This indicates that AlInN/GaN interface in AlInN/GaN/AlN/GaN is smoother than Al InN/Al N interface in Al In N/AlN/GaN, which may result from a case that the GaN interlayer mitigates the lattice mismatch between the AlN and the Al InN layers. Meanwhile, the GaN interlayer results in a larger separation of the electron wave function from the AlInN barrier layer, which suppresses the scattering from the Alloy disorder. The suppression of both the alloy disorder and interface roughness scatterings make a larger Hall mobility and a smaller sheet resistance in AlInN/GaN/AlN/GaN. On the other hand, SdH is remarkably enhanced due to the existence of GaN interlayer. The enhacement of the SdH oscillation is related to the larger quantum mobility q owing to the suppression of the interface roughness, alloy disorder, and ionized impurity scatterings by the GaN interlayer.