Wet etching and its application to the fabrication and characterization of AlGaN/GaN HFETs

We have developed a simple and reproducible UV photo-assisted wet etching process which can be carried out near room temperature and which does not appreciably degrade the GaN surface roughness from its original value for layers grown by molecular beam epitaxy on sapphire substrates: for example, AFM measurements indicate that the removal of 1000 Å of GaN only degrades the RMS surface roughness by ⩽10 Å. Etch rates of 20 A/min have been achieved with a UV power density of only 1.0 mW/cm² at λ=365 nm. The etch rates and the good resultant surface quality make the etch technique attractive for gate recess etching and mesa isolation processes. The process is thus well-suited for the fabrication of high-quality AlGaN/GaN HFETs. The suitability of the technique to device fabrication is demonstrated by the implementation of high-performance 0.2 μm gate devices with f_T=43 GHz and f_MAX=97 GHz using MBE-grown layers on sapphire substrates. We also demonstrate the applicability of the method for device fabrication using MOCVD-grown material on both sapphire and 4H-SiC substrates: the method is generally useful and non-specific as to the substrate used or the growth method