(Invited) Vertical GaN Devices for High-Power Electronics

Abstract

With the demand of high-power and high-speed electronics, GaN has achieved widespread interest owing to its wide-bandgap (WBG), large breakdown field, and high mobility that can provide superior performance compared to the Si and SiC that are limited by inherent material properties. In power electronics, GaN is a potential candidate due to its high-quality p-n junctions and availability of native substrates to prevent dislocation-assisted leakage, particularly for vertical devices. For high-power electronics, these vertical devices are more advantageous compared to their lateral counterparts due to their high current capability from large-area backside ohmic, high breakdown voltage from confined electric field within WBG material, scaling feasibility, and better thermal management. Moreover, the punch-through vertical device can extract the full potential of the WBG semiconductors by maximizing the average electric field over the fully depleted drift layer.For high-performance vertical device, high-quality drift layers with low controlled background doping and low compensating defects are essential to obtain the expected high breakdown voltage and low on-resistance. Hence, we have focused on both materials and device development for GaN power devices. Toward the first goal of high-quality GaN epitaxy, we optimized the growth condition for controlled low unintentionally doped (UID) and low impurity drift layer from ammonia molecular beam epitaxy (NH3 MBE). For this, we performed a systematic study of growth rate effects on the drift layer doping for a series of growth rates from 0.37 µm/hr to 1.68 µm/hr in both sapphire and free-standing GaN substrates (Fig. 1). A low background UID doping of ~1015 cm-3 was achieved with a growth rate 1.4 µm/hr on the free-standing GaN substrate. This UID background doping is one of the lowest reported for GaN homoepitaxy [1-3]. The potential of NH3 MBE-growth for low-doped epitaxy is recognized from our work, contributed from the cleaner growth environment of MBE, compared to the conventional metal-organic chemical vapor deposition (MOCVD).The next step of our work involved developing high-performance power devices with these high-quality epitaxies. Our vertical p-n GaN diode consisted of a ~4 µm UID GaN drift layer on ~0.25 µm of n+ GaN ([Si]: 1×1019 cm-3) buffer for the n-type region. The p-type region included a 10 nm p++ GaN cap ([Mg]: 3×1020 cm-3) to support the anode ohmic contact followed by a 0.4 µm p+GaN ([Mg]: 3×1019 cm-3) to form the p-n junction.The sample was fabricated with circular diodes of 80-100 µm diameter Pd/Pt Anode and a backside Ti/ Au Cathode. The field management was achieved by a combination of ~1.4 µm dry etch mesa isolation with a sidewall angle of ~55° and field plate (Fig. 2). For the field-plate dielectric, a stack of atomic layer deposition of Al2O3 (26 nm) and plasma-enhanced chemical vapor deposition of Si3N4 (205 nm) at 300 °C was used. Finally, a Ti/Pt metal stack of 15 µm length was patterned to form the field-plate.Capacitance-Voltage characteristics performed in these p-n diodes showed a doping of ~3×1015 cm-3 in the n- drift layer (Fig. 3a). Secondary ion mass spectrometry (SIMS) detected Si~2×1015 cm-3 and O ~2×1016 cm-3 respectively across the n-layer (Fig. 3b), likely the source of UID doping, accounting for their possible partial ionization. The compensating carbon impurities was less than the SIMS detection limit, certifying the high-quality NH3 MBE-growth. The forward J-V characteristics (Fig. 4) revealed the leakage current below the detection limit (~1.6 pA) for V<2 V. At higher forward biases (V=2.87 V) in the diffusion current region, the ideality factor reaches to a minimum of 1.36. The extracted specific on-resistance (Ron,sp) of 0.28 mΩ-cm2 and the minimum ideality factor of 1.36 are among the best achieved in vertical homoepitaxy GaN p-n diodes [1-3]. The highest breakdown voltage of these diodes were > 1 kV (equipment limit 1 kV) whereas some diodes exhibited breakdown at -890 V (Fig. 5). Silvaco simulation showed a punch-through breakdown of the diodes with peak electric field (EC) >2.6 MV/cm. Remarkably, this breakdown performance with NH3 MBE GaN of only ~4 um channel is comparable with the best available MOCVD GaN p-n diodes with a significantly thicker drift layer of ≥ 8 um (Fig. 6). Our work thus demonstrates the potential of high-performance power devices by NH3 MBE, benefitted from high-quality epitaxy, controlled low doping, and fast growth rates, that will be promising for high-voltage devices.