MOCVD growth and transport investigation of two-dimensional electron gas in AlGaN/GaN heterostructures on sapphire substrates

High quality AlGaN/GaN nominally undoped single heterostuctures (SH) with different Al contents have been fabricated on sapphire substrates. Magnetotransport investigations are performed on these samples at low temperature in a magnetic field up to 10 T. The existence of a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is confirmed by the observation of Shubnikov-de Hass oscillations in the magnetic field below 3 T and the integer quantum Hall effect. In particular, the Al_0.18Ga_0.82N/GaN SH shows a Hall mobility of 10300 cm²/Vs at 1.5 K, which to our knowledge is the highest carrier mobility on sapphire substrates grown by MOCVD. The mobility and carrier sheet density are enhanced by increasing Al composition. Based on the piezoelectric field effect, the Al composition dependence of 2DEG sheet density is calculated, which agrees well with the experimental result. This paper also investigates the effect of doping in the AlGaN layer on the properties of 2DEG. When the AlGaN layer is doped, the situation is changed. In the case of lightly doping in the AlGaN layer, the 2DEG cannot be observed. When the doping level in the AlGaN layer is increased, the 2DEG is demonstrated again together with an enhancement in the electron mobility. These behaviors are attributed to the strain-induced piezoelectric effect and silicon doping-induced screening effect. This result should be highly emphasized in designing GaN-based electrical devices.