Structural characterization of strained AlGaN layers in different Al content AlGaN/GaN heterostructures and its effect on two-dimensional electron transport properties

Different Al content AlGaN/GaN two-dimensional-electron-gas (2DEG) structures were grown on 100-mm-diam sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). The structural properties of AlGaN layers, such as alloy composition, layer thickness, tensile strain, in-plane stress, crystal quality, and band gap energy, were investigated in detail mainly by x-ray diffraction and spectroscopic ellipsometry. Correspondingly, the electron transport properties of these epilayers were theoretically as well as experimentally studied, taking into account the structural characterization results. Hall effect measurements showed that 2DEG density linearly increases with increasing Al content and that low-temperature 2DEG mobility largely decreases with the increase of Al content. The calculated results demonstrated that interface roughness scattering is largely enhanced with increasing Al content and has a strong impact on low-temperature 2DEG mobilities in high Al content samples. This is because the roughness of the interface between GaN and AlGaN layers increases with Al content. This calculated result is consistent with the experimental result that the surface of MOVPE-grown samples exhibited poor qualities with increasing Al content. The degradation of the surface and/or interface is associated with the increased strain in AlGaN layers.