High growth rate magnetron sputter epitaxy of GaN using a solid Ga target

Abstract

Magnetron sputter epitaxy (MSE) is a promising processing route for group-III nitride semiconductors, with the potential to enable high-quality and low cost GaN growth for widespread use. However, fundamental technological hurdles must be overcome to enable the adoption of MSE in industrial production. Here, we present a new UHV-compatible magnetron design with high-performance cooling, enabling high GaN growth rates at high growth temperatures using a solid Ga target. The magnetron is tested with a wide range of process parameters and a stable process is feasible while maintaining the solid state of the Ga target. High GaN growth rates of up to 5 μm/h are achieved at room temperature and a growth rate of 4 μm/h at high temperature, which is one order of magnitude higher compared to MSE with a liquid target. We grow GaN on c-plane sapphire substrates and show the impact of partial pressure ratio and target-to-substrate distance (TSD) on growth rate, film morphology and crystal quality of GaN films with scanning electron microscopy and X-ray diffraction. While the growth rate and film morphology are strongly impacted by the process parameter variation, the crystal quality is further impacted by the overall film thickness. For a 2 μm thick GaN film a full width at half maximum of X-ray rocking curve (ω-FWHM) of GaN 10 1‾ 1 reflection of 0.32° is achieved. We demonstrate a process window for growth of dense and smooth GaN films with high crystal quality using low N2 flow rates and high TSD. By introducing a 20 nm AlN nucleation layer prior to the growth of 390 nm GaN, the ω-FWHM of GaN 0002 reflection of 0.19° is achieved. The epitaxially grown crystalline structure is precisely examined by transmission electron microscopy.