MOCVD GaN Epitaxy with Fast Growth Rates

Abstract

Abstract Body: Gallium nitride (GaN) has been considered as a promising candidate for power electronic devices due to its wide bandgap (3.4 eV), high critical electric field (3 MV/cm), and high electron mobility (>1000 cm2/V s). As a key parameter to evaluate the performance of power devices, Baliga’s figure of merit (BFOM) of GaN is more than 500X higher than silicon (Si) and 3X higher than silicon carbide (SiC) [1]. Vertical GaN PN diode with 5kV breakdown voltage (VBR) has been demonstrated [2]. Although tremendous progress has been made in the past few years, achieving vertical GaN power devices with VBR of 10-20 kV remains challenging. Epitaxy of high-quality GaN films with a thick drift layer, low controllable doping, and high mobility is required to advance the field. In this work, the effects of increasing GaN growth rate on the impurity incorporation, charge compensation, surface morphology, and transport properties are systematically studied. Under optimized MOCVD GaN growth condition, high-quality GaN films with controllable low net charge concentration (Nd-Na) at 4×1015 cm-3 was demonstrated. Secondary ion mass spectroscopy (SIMS) measured [Si] and Nd-Na as a function of SiH4 flow rate was investigated. The compensation level at (4.8±1.0)×1015 cm-3 was extracted from the intercept of Nd-Na vs. SiH4 flow rate. Based on the capacitance-voltage (CV) and SIMS analysis, the background [C] level was at ~ 7×1015 cm-3 (SIMS detection limit). Deep level transient spectroscopy / deep level optical spectroscopy (DLTS / DLOS) results show total electron trap concentration at ~2.5×1015 cm-3, mainly from carbon-related deep level defects. These high-quality GaN films were grown with a typical MOCVD GaN growth rate of 2 µm/hr. MOCVD GaN epitaxy with fast growth rate at 5.2 µm/hr was achieved via increasing TMGa flow rate. With a controllable Nd-Na at 1.5×1016 cm-3, the SIMS measured [C] was at 2×1016 cm-3, which is significantly increased as compared to the films grown at lower growth rates. Atomic force microscope (AFM) imaging on GaN surface shows smooth morphology with step-flow growth mode and RMS of 0.647 nm (10x10 μm scan area). The Mn incorporation from the semi-insulating ammonothermal GaN substrate into epilayer was observed from SIMS measurement. Controlled experiments show that Mn incorporation rate into epilayer is highly related to the film growth rate, sharing similar characteristics as Fe impurity. Quantitative SIMS revealed that both Mn and Fe incorporation can be suppressed with faster growth rates. Room temperature Hall measurements of GaN films show that electron mobility decreases from 852 cm2/Vs to 604 cm2/Vs (n~1.5x1016 cm-3) as the growth rate increases from 2 µm/hr to 5.2 µm/hr. In summary, systematic MOCVD GaN epitaxy and materials characterization were performed to investigate the effects of GaN growth rate on background impurities incorporation, charge compensation and charge transport properties in Si-doped GaN films. The increase of GaN growth rate leads to higher background C incorporation due to the use of higher TMGa precursor flow rate, as well as the reduction of electron mobility. This is identified as the key challenge to achieving high-quality thick GaN films with low controllable doping at low-mid 1015 cm-3. As a comparison, we will discuss the GaN film properties grown with CO2 laser excitation introduced during the MOCVD epitaxy. Acknowledgment: The authors acknowledge the funding support from Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy (DE-AR0001036), and U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office, FY18/FY19 Lab Call.