Ultra-Fast High-Temperature Microwave Processing of Silicon Carbide and Gallium Nitride

Abstract

A novel solid-state microwave annealing technique is developed in this work for post-implantation annealing of SiC and GaN, and for the controlled growth of SiC nanowires. This technique is capable of heating SiC samples to temperatures in excess of 2100 ºC, at ultra-fast temperature ramping rates > 600 ºC/s. Microwave annealing of ion-implantation doped (both p-type and n-type) hexagonal SiC was performed in an uncontrolled (air) ambient, as well as a controlled 100% atmosphere of nitrogen, with or without a protective graphite cap. Microwave annealing was performed in the temperature range of 1500 ºC – 2120 ºC, for durations of 5 s – 60 s. Uncontrolled ambient microwave annealing of SiC at temperatures > 1700 ºC resulted in a significant oxidation of the SiC surface, leading to a loss of the implanted layer. Annealing in a 100% nitrogen atmosphere eliminated the oxidation problem. For microwave annealing at temperatures ≥ 1800 ºC, significant SiC sublimation was observed, even for 15 s annealing. Microwave annealing with a photoresist-converted graphite cap solved this surface sublimation problem for annealing temperatures up to 2100 ºC. For the P+ and Al+-implanted SiC, sheet resistances as low as 14 Ω/ and 1.9 kΩ/ and majority carrier mobilities as high as 100 cm2/Vs and 8.3 cm2/Vs, respectively, were obtained. For the Al+ -implanted SiC, sheet resistances as low as 1.9 kΩ/ and hole mobilties as high as 8.3 cm2/Vs were obtained. These values constitute the best ever reported electrical characteristics for ion-implanted SiC. Microwave annealing at temperatures > 1800 ºC not only removed the implantation-induced lattice damage but also the defects introduced during crystal growth. Microwave annealing of in-situ as well as ion-implantation acceptor doped GaN was performed in the temperature range of 1200 ºC – 1600 ºC, for a duration of 5 s, using different protective caps (AlN, MgO, graphite) for protecting GaN surfaces during annealing. Pulsed-laser deposited AlN was found to protect the GaN surface effectively, for microwave annealing at temperatures as high as 1500 °C. The RMS surface roughness (0.6 nm) of the GaN sample annealed at 1500 °C with an AlN cap is similar to the value (0.3 nm) measured on the as-grown sample with a decrease in the compensating deep donor concentration. Cubic 3C-SiC nanowires were grown by a novel Fe, Ni, Pd, and Pt metal catalystassisted sublimation-sandwich (SS) method. The nanowire growth was performed in a nitrogen atmosphere, in the temperature range of 1650 ºC to 1750 ºC for 40 s durations. The nanowires grow by the vapor-liquid-solid (VLS) mechanism facilitated by metal catalyst islands. The nanowires are 10 μm to 30 μm long with about 52% of them having diameters in the range of 15 nm – 150 nm, whereas 14% of the nanowires had diameters in excess of 300 nm.