(Invited) Selective Area Growth of p-Type GaN for Gallium Nitride Power Switching Transistors

Abstract

Superior electronic material properties of GaN semiconductors compared to Si will enable the foundational diodes and transistors for the next revolution in power management technology. To date, high-performance GaN diodes have been demonstrated with breakdown voltages near 5 kV and specific on-resistances of less than 5 mΩ-cm2 [1]. However, a viable GaN vertical power switching transistor is the missing element necessary to realize advanced power management systems utilizing GaN. While GaN transistors are now ubiquitous in RF systems, their lateral device architecture inherently limits these devices to niche applications in power systems. Rather, high-voltage systems rely on vertical device architectures where current flow is normal to the semiconductor surface. Vertical power electronics devices such as junction field effect transistors (JFETs) require selective areas of p-type semiconductor surrounded by lightly-doped n-type semiconductor. For Si power devices, p-type regions are implanted in n-type drift regions to create p/n junctions that act as a gate and pinch-off current. Ion implantation presents challenges in GaN and requires specialized equipment and processes, including high-pressure and high-temperature annealing, for dopant activation [2]. Our previous work using metal-organic chemical vapor deposition (MOCVD) where p-type GaN was regrown on a lightly-doped, non-etched n-type drift layer demonstrated planar p/n diodes with the same electrical performance as continuously-grown devices. However, when dry-etching processes are used before the regrowth of the p-type GaN layer, as would be needed to define a regrown area, planar diodes demonstrate higher forward and reverse leakage currents, and lower breakdown voltage. Deep Level Optical Spectroscopy (DLOS) was used to identify the source of the higher forward and reverse leakage currents observed in diodes with p-type anodes re-grown on inductively coupled plasma (ICP) dry-etched n-type drift layers. We will report DLOS studies of various diode structures that show the formation of a mid-gap trap located ~ 1.7 eV below the conduction band in dry-etched, lightly doped (Nd-Na = 2x1016 cm-3) drift layers. We also will show that various wet-chemical treatments of the dry-etched n-type drift layer prior to p-anode regrowth results in diodes with 1 to 2 orders of magnitude lower reverse leakage current. DLOS measurements of regrown diodes showed a 3 times reduction in the density of the mid-gap deep level trap following wet-chemical treatment. These studies establish the connection between high reverse and forward leakage currents with residual damage caused by dry-etching and that this damage can be mitigated by with wet-chemical treatments prior to regrowth of the p-anode. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. H. Ohta et al., J. J. Appl. Phys., 57, 04FG09 (2018).T.J. Anderson et al., Elec. Lett., 50(3), 197 (2014).