Vertical Wide Bandgap β-Ga2O3/GaN p-n Heterojunction with Mesa Based Edge Termination

Abstract

Abstract Body: Newly emerged wide bandgap semiconductor (WBG) β-Ga2O3 has been extensively researched for power electronics, RF electronics and optoelectronics due to its large bandgap of 4.7- 4.9 eV and high breakdown field of ~ 8 MV/cm. Due to the absence of p-type Ga2O3, most of demonstrated β-Ga2O3 devices are unipolar devices such as high electron mobility transistors (HEMTs) and Schottky barrier diodes. To overcome this limitation, other p-type materials, such as GaN and NiO, have been investigated to produce Ga2O3 based p-n heterojunctions. Among these p-type materials, p-GaN has attracted especial interests since GaN is also a WBG semiconductor and has a clear crystallographic relationship to β-Ga2O3 for epitaxial growth. In addition, both β-Ga2O3 and GaN can be produced in the industrial standard tools, i.e., the metalorganic chemical vapor deposition (MOCVD). The growth of high-quality GaN on Ga2O3 by MOCVD has been recently reported. β-Ga2O3/GaN p-n heterojunction via mechanically exfoliation also showed decent rectifying behaviors. However, the breakdown capability of the β-Ga2O3 based p-n heterojunctions is still far from expectations due to the lack of effective edge terminations. Here we design and simulate mesa based edge terminations for the vertical β-Ga2O3/GaN p-n heterojunctions using SILVACO TCAD simulator. The model was first calibrated by the experimental results, including the conduction and valance band offsets of the heterojunction and electrical properties. The device structure consisted of n+-Ga2O3 n-contact layer, 5 µm unintentionally doped (UID) β-Ga2O3 drift layer and 500 nm p-GaN layer. The ideal breakdown voltage (BV) of the heterojunction was 1.37 kV, while the BV of the reference device without effect mesa edge termination dropped dramatically to 300 V due to the electric field crowding at the device edge. Therefore, it is critical to design and optimize the mesa edge termination to improve the BV of the β-Ga2O3/GaN p-n heterojunctions. Three mesa edge termination structures were investigated in this work: beveled mesa, step mesa, and deeply-etched mesa. The beveled mesa angle θ is the key parameter in determining the device BV. When θ was varied from 15° to 90°, the highest BV of 1224 V was obtained for θ =15°. For the step mesa edge termination, the width (W), depth (D), and the number of steps define the mesa structure. It was found that the BV of the devices increased with increasing number of steps. The combination of (W, D) values of (0.5,0.5) and (0.5,1.0) and (1.0,1,0) was investigated, where all the values are in µm. The highest value was obtained for W = 0.5µm, D = 1.0µm and the number of steps = 5. Smaller W and larger D led to better BV. The electric field distributions of devices with different mesa edge terminations were compared at –300V. Without effect mesa edge termination, the peak electric field at the junction edge was ~ 4.2 MV/cm. By incorporating the mesa edge termination techniques, the electric field crowding at the device edge was mitigated, and the peak electric fields were reduced to 1.8 MV/cm and 2.4 MV/cm for the beveled and step mesa edge terminations, respectively. For the deeply-etched mesa structure, it was found that a more uniform electric field distribution was observed when the mesa depth was increased from 0.5,1.0, 2.0, to 3.0 µm, where the peak electric fields were decreased from 2.1, 1.8 ,1.5 to 1.2 MV/cm, respectively. In summary, this work presents a comprehensive design and optimization of mesa edge terminations for the wide bandgap vertical β-Ga2O3/GaN p-n heterojunctions, which is first of its kind. Three mesa structures were explored, including beveled mesa, step mesa and deeply-etched mesa. It was found that these mesa edge terminations can alleviate the electric field crowding at the junction edge and increase the device BV. This work can provide critical guidance for the development of high-performance β-Ga2O3 based bipolar high voltage and high power devices.