Improved electrical properties of the two-dimensional electron gas in AlGaN/GaN heterostructures using high temperature AlN interlayers

The electrical properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures using high temperature (HT) AlN interlayers (ITs) grown on c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD) have been investigated. It is found that the electrical properties (electron mobility and sheet carrier density) are improved compared with those in the conventional AlGaN/GaN heterostructures without HT AlN ITs, and the improved 2DEG properties result in the reduction of the sheet resistance. The results from high resolution X-ray diffraction (HRXRD) and Raman spectroscopy measurements show that HT AlN ITs increase the in-plane compressive strain in the upper GaN layer, which enhances the piezoelectric polarization in it and consequently causes increasing of 2DEG density at the AlGaN/GaN interface. Meanwhile, the compressive strain induced by HT AlN ITs leads to a less tensile strain in AlGaN barrier layer and causes positive and negative effects on the sheet carrier density of 2DEG, which counteract each other. The HT AlN ITs reduce the lattice mismatch between the GaN and AlGaN layers and smooth the interface between them, thus increasing the electric mobility of 2DEG by weakening the alloy-related interface roughness and scattering. In addition, the surface morphology of AlGaN/GaN heterostructures is improved by the insertion of HT AlN ITs. The reason for the improved properties is discussed in this paper.