MOCVD Development of Thick GaN for Vertical High Power Devices

GaN and its alloys have been widely utilized in optoelectronics, photonics, and electronics. Due to its large band gap (3.4 eV), strong critical electric field (3.4 MV cm^-1) and high electron mobility (~1200 cm² V^-1 s^-1), GaN exhibits great potential for high power electronics. Vertical GaN PN diode with 5kV breakdown voltage has been demonstrated [1]. To achieve vertical GaN power devices with V_BR&gt;10 kV, one requires the epitaxy of high quality GaN films with thick drift layer and low controllable doping with high mobility. Typical MOCVD GaN films with high mobilities were grown with growth rate (GR) of 2-3 µm hr^-1. Thus, it is important to develop and understand MOCVD GaN growth with fast GR.<br/><br/>Our previous work [2] has demonstrated that laser-assisted metalorganic chemical vapor deposition (LA-MOCVD) can effectively facilitate NH₃ decomposition and, thus, to achieve high-quality GaN films with low carbon (C) and fast GR. Low [C] at 5.5E15 cm^-3 was achieved in the LA-MOCVD GaN film grown with the GR of 4 µm hr^-1. LA-MOCVD GaN exhibited lower [C] incorporation at low input V/III ratio and high GRs as compared to the conventional MOCVD GaN growth. High room-temperature mobilities over 1000 cm² V^-1 s^-1 were achieved by LA-MOCVD with a GR of 4.5 µm hr^-1. The LA-MOCVD GaN growth technique provides a promising approach to obtain high-quality, low-C GaN with fast GR for power device applications.<br/>In this work, the LA-MOCVD GaN growth window was further extended to a higher GR (&gt;7 µm hr^-1) regime. The effect of growth conditions on both MOCVD and LA-MOCVD GaN growths at high GR regime was studied. It was found that the process gas (H₂) flow plays an important role on the growth rate, surface morphology and impurity incorporation. The GRs of both MOCVD and LA-MOCVD GaN increase with the reduction of alkyl flows. The growth rate of LA-MOCVD GaN was similar as that of regular MOCVD GaN at high alkyl flow rates. With the same total process flow rate, redistributing more H₂ flow to hydride injection increases the GR of regular MOCVD GaN but results in a significant reduction of LA-MOCVD GaN GR. The [C] incorporation is dependent on the process gas flow/distribution and positively related to growth rates. The corresponding LA-MOCVD growths show about 40% reduction of [C] under all process gas flow/distribution conditions. It shows that higher hydride flow could effectively suppress the [C] incorporation with both regular MOCVD and LA-MOCVD GaN. Under this condition, relatively low [C] was achieved on both regular MOCVD (5.8E16 cm^-3, GR=11 µm hr^-1) and LA-MOCVD GaN (2.1E16 cm^-3, GR=7.5 µm hr^-1), which represent one of the best reported results with the similar growth rates [3].<br/>Using the optimized growth condition, recently we demonstrated the GaN vertical PN diode with V_BR of ~5 kV with low leakage current. The drift layer thickness was ~30 µm grown with a growth rate of 5 µm hr^-1. The doping was controlled at ~ 3E15 cm^-3. Ion-implanted guard ring structure was applied for the field management. The R_on was 0.88 mΩcm². The Baliga’s figure of merit (BFOM) was over 27 GW/cm², which is the highest among the reported values [1].<br/>In summary, we demonstrated LA-MOCVD GaN growth with fast growth rates. The effect of growth conditions on both MOCVD and LA-MOCVD GaN growths at high GR regime was systematically studied. LA-MOCVD provides an enabling route to achieve high-quality GaN epitaxy with low-C and fast GR simultaneously. The demonstrated GaN vertical PN diode has a breakdown voltage close to 5kV, with a record BFOM over 27 GW/cm².<br/>Acknowledgment: ARPA-E (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.<br/><b>References:</b><br/>[1] H. Ohta, et al, Jpn. J. Appl. Phys. 58, SCCD03 (2019).<br/>[2] Y. Zhang, et al., Phys. Status Solidi RRL, 15, 2100202 (2021).<br/>[3] T. Ciarkowski, et al., Materials 12, 2455 (2019).