| In recent years, ammonia-based molecular beam epitaxy (MBE) growth of III-nitride materials and devices has generated substantial interest. Though less widely studied than plasma-assisted MBE (PA-MBE) and metal-organic chemical vapor deposition (MOCVD), ammonia MBE offers the potential to combine some of the advantages of these two growth techniques, including: N-rich growth, high growth rates, a wide range of growth temperatures, low background impurity levels due to the ultra-high vacuum (UHV) environment, abrupt heterointerfaces, and the ability to use in-situ UHV diagnostic tools, among others. The potential of ammonia MBE is apparent from the excellent device performance reported despite the relative immaturity of the growth technique, including the first demonstration of MBE InGaN quantum well lasers, and AlGaN/GaN high-electron-mobility transistor (HEMT) power performance of up to 7 W/mm at 10 GHz on Si substrates.;In this work, the growth of (Al,Ga)N on GaN and SiC substrates by ammonia MBE was investigated. Fundamental growth processes involved in ammonia MBE of GaN(0001) were studied using homoepitaxy, and growth regimes relevant for electronic devices were identified and summarized on a growth diagram. Reduced threading dislocation density GaN buffer layers were developed on SiC substrates for use in AlGaN/GaN HEMTs, and transistor structures were processed and measured at microwave frequencies. Excellent power performance was achieved: 11.1 W/mm output power and power-added efficiencies of greater than 70% at 10 GHz, as well as 7.9 W/mm output power at 30 GHz. These results represent the highest output power and the first mm-wave power performance yet reported for AlGaN/GaN HEMTs grown by ammonia MBE and demonstrate the potential of this growth technique for electronic devices. |
