(Invited) Recent Progress on Selective Area Regrowth and Doping for Vertical GaN Power Transistors

Abstract

Wide bandgap semiconductor GaN is a very promising candidate for efficient power conversion applications due to its large bandgap energy and high critical electric field. Vertical GaN power devices grown on bulk substrates are enabling the next-generation power electronics with higher efficiency, higher power density and smaller form factor. However, selective area doping in GaN is far from mature, which is limiting the realization of many advanced GaN power devices such as vertical junction effect transistors (VJFETs), junction barrier Schottky (JBS), superjunction, and junction termination extension (JTE). Epitaxial regrowth or selective area regrowth has been regarded as one of the most promising routes to selective area doping.Currently, the regrown p-n junctions suffer from large leakage currents and premature breakdown. To understand this phenomena, the regrowth experiments were carried out by metalorganic chemical vapor deposition via an etching-then-regrowth process. The etching was performed by inductively coupled plasma (ICP) etching. The regrowth dynamics with different trench widths and depths was revealed, where basal plane and lateral growth modes were identified. It was also found that shallow trenches for selective area regrowth was beneficial for reducing the reverse leakage currents of regrown p-n junctions.The regrowth interface plays a critical role in understating the leakage mechanism of regrown p-n junctions. With developed material characterization techniques, the regrown structures and interface were analyzed in detail. Cathodoluminescence (CL) and secondary electrons (SEs) were used to realize both lateral and vertical dopant profiling at a sub-micron scale [1]. The regrowth interface by dry etching accumulated lots of impurities and charges, contributing to the formation of a p+-n+ tunneling junction. This was further confirmed by the electrostatic potential profile at the regrowth interface using electron holography [2]. This tunneling junction is likely to be the major contributor to the large reverse leakage currents and low breakdown voltages in regrown junctions. To improve the regrowth interface, novel etching technologies were demonstrated. It was found that low-power dry etching significantly reduced the interfacial charges and the reverse leakage currents of regrown p-n junctions [3]. Atomic layer etching (ALE) was also successfully developed for GaN, which uses self-limiting chemical processes, thus removing the damaged layers without inducing further etching damage. The tertiarybutylchloride (TBCl) based in situ etching may serve as an alternative etching method to dry etching with reduced etching damage [4]. In terms of devices, regrown p-n junctions with low leakage currents [3] and the first normally-off VJFETs were demonstrated. Further improvements in selective area regrowth and associated devices can be expected via regrowth optimization and regrowth interface engineering via surface treatments and low-damage etching. These results represent an important step towards realizing selective area doping for GaN power transistors. H. Liu et al., Appl. Phys. Lett., 114, 082102 (2019)S. Alugubelli, et al., Appl. Phys. Lett., 115, 201602 (2019)K. Fu et al., IEEE Electron Device Lett. 40, 1728 (2019)B. Li et al., Appl. Phys. Lett., 115, 162101 (2019)