Temperature-dependent photoluminescence and transport measurements were performed on the In0.13Ga0.87N:Si/GaN:Si multiple-quantum-well (MQW) structures with different doping levels. By fitting the temperature-dependent emission energy of these samples using the band tail model, an obvious localization effect is observed in lightly doped MQW structures. Correspondingly, the electron mobilities in these structures are significantly higher than those of undoped and heavily doped MQW structures. Furthermore, when the localization effect is stronger, the mobility is higher.

High-quality AlGaN/GaN undoped single heterostructures (SH) with different Al contents have been grown on sapphire substrates. The magnetotransport investigation was performed on these samples at a low temperature. The observation of Shubnikov–de Hass oscillations in the magnetic fields below 3 T and the integer quantum Hall effect confirmed the existence of the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. The Al0.18Ga0.82N/GaN SH shows a Hall mobility of 10 300 cm2/V s at a carrier sheet density of 6.19×1012/cm2 measured at 1.5 K. To the best of our knowledge, this is the highest carrier mobility ever measured in GaN-based semiconductors grown on sapphire substrates. The Al composition dependence of the mobility and carrier sheet density were also investigated. Based on the piezoelectric field effect, the Al composition dependence of the 2DEG sheet density was calculated, which agreed well with the experimental result. The negative magnetoresistance with parabolic magnetic-field dependence in the low magnetic field was also observed in the sample with the highest 2DEG sheet density.

The influence of low-temperature buffer layer thickness on the electrical properties of GaN film is investigated, and the surface morphology is also examined by atomic force microscopy. A best surface morphology does not show best electrical properties, which could be attributed to the usual growth mechanism for GaN film on sapphire substrate. The influence of the growth temperature for the final GaN layer is also investigated. When the growth temperature increases to 1100 °C, the mobility is greatly enhanced to 600 cm2/V s with a background carrier density of 3.3×1016/cm3 at room temperature. The emission energy of the near band gap exciton at a low temperature shows a blueshift with increasing growth temperature due to an enhanced thermal stress. The calculation based on a thermal stress model agrees very well with the photoluminescence measurement. This result could partly explain the reason that the previously published values for the near band gap exciton emission energy are scattered.

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