MOVPE growth of high electron mobility AlGaN/GaN heterostructures

We have fabricated Al_xGa_1-xN/GaN heterostructures with high two-dimensional electron gas (2DEG) mobilities and high sheet carrier densities by metalorganic vapor phase epitaxy (MOVPE). The 2DEG sheet density and mobility exhibit a compositional dependence on the Al fraction of the electron donor layer. The highest mobility (5750 cm²/Vs at 16K) was measured in a sample with x=0.15 that had a sheet carrier density of 8.5×10¹² cm^-2. The undoped Al_xGa_1-xN layers have low background carrier concentrations and can be intentionally doped n-type using SiH₄. The effect of intentional n-type doping of the Al_xGa_1-xN donor layer on the electrical properties of the 2DEG was studied in structures that included an undoped Al_xGa_1-xN spacer layer of varying thickness. Higher 2DEG mobilities were obtained when a 100 angstrom thick undoped layer was included in the structure due to spatial separation of the 2DEG from ionized impurities in the doped Al_xGa_1-xN. These initial results demonstrate that the electrical properties of Al_xGa_1-xN/GaN heterostructures can be controlled by intentional doping and appropriate layer design.