Selective Area Regrowth of p-type GaN and AlGaN for Power Diodes

Wide-bandgap (WBG) GaN and ultra-wide-bandgap (UWBG) AlGaN alloys are appealing semiconductor materials for the next-generation of high-voltage power devices due to their superior material properties. Practical power devices such as merged-PiN-Schottky (MPS) diodes and junction field effect transistors (JFETs) require selective areas of p-type semiconductor surrounded by n-type material. This selective area p-type doping is typically achieved using implantation and thermal annealing to form the p-well in Si and SiC based power devices. However, ion implantation presents challenges in GaN and requires specialized equipment for the high-pressure and high temperature annealing for dopant activation while implantation into AlGaN alloys has received limited attention.<br/> <br/>We have investigated selective-area-regrowth (SArG) to epitaxially grow p-type GaN and AlGaN in place of dopant implantation. Successful pn diode formation by SArG requires a process to remove residual crystalline damage resulting from the inductively coupled plasma (ICP) etch typically used to form the p-well and a method to remove the elevated level of Si found at the regrowth interface of a surface that had been previously exposed to air. For the case of SArG of p-GaN on air-exposed, blanket ICP etched n-type GaN, we will present the novel use of a fluorine-based precursor for in-situ etching of GaN in the MOCVD chamber. Unlike chlorine (TBCl and CCl4) and bromine (CBr4) -based precursors we have studied, we obtained a smooth surface following in-situ fluorine etching of air-exposed GaN. Schottky barrier diodes (SBDs) formed by shadow mask evaporation on in-situ fluorine etched n-GaN that had been previously ICP etched showed reverse leakage currents to -40 V that are equal to those of SBDs formed on as-grown n-GaN layers. The in-situ fluorine/ICP etched diodes had reverse leakage currents more than 3 orders of magnitude lower than those formed on n-GaN layers that had only experienced ICP etching. This suggests that the in-situ fluorine etch was effective at removing the residual damage of the ICP etch process. Furthermore, we discuss the use of in-situ fluorine etching to reduce the concentration of Si at a regrown GaN interface.<br/> <br/>Unlike the case for GaN, we find that pn junction formation by SArG of p-Al_0.3Ga_0.7N on air-exposed and ICP etched n-Al_0.3Ga_0.7N drift layers to be tolerant to residual etch damage and elevated concentration of Si at the regrowth interface. We will present pn junction diodes formed by p-Al_0.3Ga_0.7N regrowth on blanket ICP-etched n-Al_0.3Ga_0.7N drift layers with performance which matches that of continuously grown diodes that have achieved a breakdown voltage of 1.5 kV. Furthermore, we will describe a pn junction diode where the anode consisted of p-Al_0.3Ga_0.7N regrown in ICP etched p-wells in a n-Al_0.3Ga_0.7N drift layer with current-voltage characteristics equal to diodes grown without growth interruption that have achieved a breakdown voltage of 1.8 kV. These demonstrations support the promise of AlGaN alloys for realizing practical, kilovolt-class power diodes and transistors for next generation power systems.<br/> <br/><i>This work was supported in part by the Laboratory Directed Research and Development program at Sandia National Laboratories and in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program 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. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government.</i>