(Invited) Progress and Challenges of AlGaN Schottky Diodes Grown on AlN Substrates

Abstract

Lateral and vertical power switching devices being intensively developed in GaN and SiC materials are beginning to reach a state of maturity and to displace traditional Si devices in certain applications. The relative advantage of these wide bandgap materials, which stems from intrinsic materials properties, can be compared in terms of an applicable figure of merit (FOM), indicating theoretical limits [1]. The most commonly discussed FOM for power applications is the unipolar FOM [2], based on reduction of conduction losses in the on-state and high voltage blocking in the off-state of vertical devices, VB 2/Ron,sp = εμnEc 3/4, where VB is the off-state avalanche breakdown voltage, Ron,sp is the specific on-state resistance, ε is the semiconductor permittivity, μn is the electron mobility, and Ec is the critical electric field. The unipolar FOM scales as the cube of Ec , which is itself a strong function of the bandgap [3], leading to the key advantage for wide bandgap materials. Possessing bandgaps wider than GaN (3.4 eV), AlGaN alloys are expected to provide improved performance. We have investigated quasi-vertical Schottky barrier diodes, consisting of nominal Al0.8Ga0.2N drift layers grown on high quality AlN single crystal substrates. HexaTech has developed proprietary seeded AlN crystal growth technology based on physical vapor transport (PVT), allowing iterative expansion of single crystal size, while maintaining low dislocation density (<103 cm-2), as determined by x-ray diffraction and defect-selective etching. AlN boules were oriented, sliced, lapped, and chemo-mechanically polished (CMP) to produce epi-ready c-plane substrates. Due to the highly insulating nature of the Al-face AlN substrates (resistivity >1013 Ωcm at room temperature), quasi-vertical Schottky diode structures were grown by metal-organic chemical vapor deposition (MOCVD). The device structure consisted of an AlN homoepitaxial layer, followed by a 0.6 µm thick pseudomorphic Si-doped Al0.8Ga0.2N contact layer ([Si] ~2x1019 cm-3), a 1 µm thick low-doped Al0.8Ga0.2N drift layer, and a 0.3 µm Mg-doped AlN cap layer ([Mg] ~2x1018 cm-3), serving as a junction termination extension (JTE). Despite the high activation energy of the Mg dopant in AlN, the Mg acceptors are expected to be ionized under high reverse bias, due to the strong band bending. Following epitaxial growth and dopant activation, circular device mesas were defined by dry etching down to the n-AlGaN contact layer, the JTE region atop the mesa was defined into a series of lateral steps and the Schottky contact area was etched down to the drift layer, Ohmic contacts were deposited and annealed, circular Schottky contacts were deposited, and a blanket polyimide passivation layer was deposited and patterned. Schottky diodes with diameters ranging from 0.3 to 1.0 mm were fabricated. Capacitance-voltage measurements were used to determine the free carrier concentration of the drift region, 2.0x1017 cm-3. The forward and reverse current-voltage characteristics were measured as a function of temperature. The specific on-state resistance was 125 mΩcm2 at room temperature and 190 mΩcm2 at 150 °C, which is partly attributed to high contact resistance. Diodes exhibited a current rectification ratio of ~106. The off-state breakdown voltage was up to 500 V. Breakdown tended to be destructive, likely limited by the polyimide passivation, suggesting improved blocking performance could be achieved with a better dielectric and more developed edge termination strategy. Field calculations based on the measured carrier concentration and breakdown voltage indicated a trapezoidal electric field in the drift region with a 7 MV/cm field at the Schottky junction. This talk will review growth and expansion of high quality AlN single crystals by PVT, and fabrication of epi-ready AlN substrates. Results for high Al composition Schottky diodes grown on these substrates will be presented. Challenges for this emerging technology, including doping limitations and compensation in AlGaN alloys, Ohmic contacts, and device termination will be discussed. [1] T. P. Chow, I. Omura, M. Higashiwaki, H. Kawarada, and V. Pala, IEEE Trans. Electron Devices, 64, 856 (2017). [2] K. Shenai, R. S. Scott, and B. J. Baliga, IEEE Trans. Electron Devices, 36, 1811 (1989). [3] J. L. Hudgins, G. S. Simin, E. Santi, and M. A. Khan, IEEE Trans. Power Electron., 18, 907 (2003).