High-Temperature Characteristics of β-Ga₂O₃-Based p-i-n Heterojunction Rectifiers

Electronic device performance and reliability under extreme conditions such as at very high temperatures is essential for several applications in the industrial, energy, and automotive sectors. Wide band gap semiconductor devices using SiC and GaN have demonstrated operation in the 400-600°C temperature range for prolonged periods of time. However, &gt;600°C operation in oxidizing and reducing conditions requires the next generation of chemically robust ultra-wide band gap semiconductor devices where GaN and SiC fail. Wide bandgap β-Ga₂O₃, an important material for high-power device applications, also shows promise for high-temperature electronics due to reduced temperature-activated parasitic leakage and chemical stability. Ga₂O₃ is not p-type dopable therefore currently studied devices use heterojunction architectures, often with p-type NiO. Unfortunately, NiO and Ga₂O₃ are not predicted to be thermodynamically stable at high temperatures. NiGa₂O₄, however, is stable in contact with Ga₂O₃ at high temperatures and is investigated for the first time in this work.<br/>Here, we explore the high-temperature characteristics of vertical heterojunction NiO/<i>β</i>-Ga₂O₃ and NiGa₂O₄/<i>β</i>-Ga₂O₃ p-i-n diodes fabricated using commercial HVPE grown drift layers (Novel Crystal Technology). For both heterojunctions pulsed laser deposition (PLD) was used to grow ~ 200 nm thick p-layer on a 1μm thick HVPE <i>β</i>-Ga₂O₃ (001) (~3.5x10¹⁶cm^-3) layer deposited on an Sn-doped <i>β</i>-Ga₂O₃ wafer (~2x10¹⁸ cm^-3). The device was processed using photolithography and the device area mesa isolation was achieved by argon dry etching¹. Anode top contacts on the p-type layers were formed by depositing 30 nm Ni / 100 nm Au via e-beam evaporation while stable Ohmic back contact to Ga₂O₃ was formed by 5 nm Ti / 100 nm Au annealed under N₂ at 550 °C for 90 seconds². Temperature-dependent current-voltage (<i>I-V</i>) characteristics were measured in vacuum (≤ 10^-4 Torr) for temperature setpoints in the range of RT–800°C using a custom-built high-temperature probe station.<br/>The rectification ratio was found to be 10⁸( 2V) at room temperature (RT) for both the NiO/Ga₂O₃ and NiGa₂O₄/Ga2O3 devices, however, a lower turn on-voltage is observed for NiGa₂O₄ (1.4V compared to 1.8V) and is attributed to a smaller barrier height in NiGa₂O₄/Ga₂O₃. The turn-on voltage for both device stacks decreased monotonously with increasing temperature. However, while the specific on-resistance decreased rapidly with temperature for the NiO/Ga₂O₃ device, the decrease in specific on-resistance in NiGa₂O₄/Ga₂O₃ device was negligible. Thus, while the rectification ratio of the NiO/Ga₂O₃ device was &gt;10³ at 600°C, the NiGa₂O₄/Ga₂O₃ device showed a lower ratio of 10² due to small forward current attributed to low carrier density in NiGa₂O₄ compared to NiO. The measured reverse leakage current increased with temperature due to increasing intrinsic carriers, nonetheless, both devices maintained a leakage current density of &lt;10^-3A/cm²at 600°C.<br/>To explore long-term continuous operation at high temperatures, the device was soaked at 600°C (&gt;100 hours) and subjected to repeated thermal cycling (30 times) between room temperature and 550°C in flowing N₂ and in laboratory ambient air. Both devices show good stability however the NiO-Ga₂O₃ device had better performance compared to the NiGa₂O₄-Ga₂O₃ device due to a higher built-in voltage. These results suggest that irrespective of thermally driven leakage current increase, Ga2O3-based p-i-n diodes hold great potential for high-temperature operation with &lt; 10^-3A/cm² leakage current density at &gt; 600°C operating temperature.<br/><br/>¹Sohel, S. H. <i>et al.</i> Gallium Oxide Heterojunction Diodes for Improved High-Temperature Performance. Preprint at http://arxiv.org/abs/2204.00112 (2022).<br/>²Callahan, W. <i>et al</i>. Ultrathin stable ohmic contacts for high-temperature operation of β-Ga2O3 devices. Preprint at https://arxiv.org/abs/2304.02161 (2023).