(Invited) Prospects for MOCVD Growth of GaN for 10kV Power PN Diodes

Abstract

GaN wide-bandgap semiconductors have superior electronic material properties compared to Si and even SiC that will enable the foundational diodes and transistors for the next revolution in power management technology. To date, GaN pn diodes with 33 μm thick, lightly n-type doped drift layers have been demonstrated with breakdown voltages of 5 kV. [1] However, applications requiring higher blocking voltages could operate with greater efficiency and higher frequency if GaN based diodes and transistors could be proven. Simulations indicate that GaN pn diodes with blocking voltages of 10 kV require drift layers at least 50 μm thick and with carrier concentration less than 3x1015 cm-3. Achieving such demanding such material requirements presents significant challenges for GaN growth processesHydride Vapor Phase Epitaxy (HVPE) is the growth process of choice for very thick GaN layers due to its high growth rate (>50 μm/hr) and is a prevalent method for producing free-standing N+ GaN substrates. This growth technique and its use of carbon free precursors is a natural choice for growth of 50+ μm thick drift layers needed for 10 kV pn diodes. However, Si from the quartz growth chamber used in conventional hot-wall HVPE system leads to difficulty in controllably achieving low carrier concentrations and thus new materials and equipment designs for the growth chamber are being developed. [2] Furthermore, it is challenging to maintain smooth surface morphology of thick HPVE grown GaN epilayers and to grow p-type GaN epilayers that are needed for continuously grown pn diodes. Anode formation by epitaxial regrowth using Metal-Organic Chemical Vapor Deposition (MOCVD) or Mg implantation and annealing have yet to demonstrate multi-kilovolt diodes with reverse leakage currents equal to the best, continuously grown diodes grown by MOCVD.The low growth rate (< ~5 μm/hr) for MOCVD and the use of carbon containing precursors make it a suspect process for producing the > 50 μm thick, lightly doped (< 3x1015 cm-3) drift layers needed for 10 kV diodes. However, large substrate capacity, high up-time MOCVD reactors running highly-efficient, mature manufacturing processes developed for commercial production of visible LEDs could prove attractive for GaN pn diodes when compared to immature, quartz free HVPE reactors that to date have much lower wafer capacity. Further increasing the growth rate of the HPVE process offers diminishing returns in reducing the overall production cycle time, while modest advances in the growth rate of the MOCVD process would significantly reduce in cycle time.Since the lower growth rate of MOCVD might not be disqualifying, the concern shifts to controlling electron concentration at the low levels (< 3x1015 cm-3) in GaN epilayers where the background carbon concentration is at a similar level due to the use of carbon containing precursors. In this presentation we will discuss MOCVD growth and Si doping of GaN epilayers up to 60 μm thick and the properties of pn diodes with 50 μm thick drift layers. We find that carrier concentration is linear with Si flow down to 1x1015 cm-3, the limit of the mercury probe capacitance voltage system, and that the carrier concentration is independent of epilayer thickness from a few microns to 60 microns showing that the MOCVD process is stable for growth of thick drift layers required for 10 kV diodes. We find that controlling the surface morphology is the greatest challenge to producing 10kV-class pn diode devices and discuss how various elements such as surface mis-cut and substrate vendor affect surface morphology. While more work is needed, our studies suggest that MOCVD growth is capable of producing epilayers required for GaN pn power diodes with breakdown voltages of 10 kV.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 and OPEN+ programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multi-mission 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).H. Fujikura et al., Appl Phys. Lett. 117, 012103 (2020).