Developing High-Voltage AlN Schottky Barrier Diodes on AlN Substrates for Power Electronic Device Applications

Aluminum Nitride (AlN) is an ultrawide bandgap (UWBG) semiconductor material with a bandgap of 6.1 eV and a high critical field of 12-15 MV/cm, gaining increasing attention as a material suitable for power electronic devices. AlN, in terms of Baliga's Figure of Merit (BFOM), shows the potential to surpass other wide bandgap semiconductors like GaN, SiC, and Ga₂O₃. Despite the potential, devices based on AlN are still in the early phases of development. They encounter several challenges, including the need for high-quality epitaxial growth, establishing proper Ohmic and Schottky contacts, managing high costs, and dealing with a scarcity of conductive substrates. Recently, we have demonstrated 3 kV AlN Schottky Barrier Diodes (SBDs) with the assistance of AlN bulk substrates. Nevertheless, persistent challenges arise from the resistive nature and defects in the material, resulting in a limited understanding of the current transport mechanisms in both forward and reverse bias conditions. Additionally, under high reverse bias, the devices demonstrate elevated leakage currents, necessitating reduction for practical applications in power electronics.<br/>In this research, AlN epilayers were grown using metal-organic chemical vapor deposition (MOCVD) with trimethylaluminum (TMAl) and ammonia (NH₃) as the Al and N sources, and SiH₄ as the <i>n</i>-type dopant Si precursor. The device structure comprised of a 1-µm-thick resistive UID AlN underlayer, a 200 nm Si-doped <i>n</i>-AlN layer, and a 2 nm UID GaN capping layer to prevent the oxidation of underlying AlN epilayers. Subsequently, lateral SBDs were fabricated through conventional photolithography, with varying distances between the anode and cathode (<i>d</i> = 5, 10, 25, and 50 µm). The device configuration included Ti/Al/Ni/Au as Ohmic contacts, and Pt/Au for the Schottky contact, with the Ohmic contacts annealed at 950 °C for 30 s in an N₂ atmosphere. Previous observations revealed that for, <i>d</i> &gt; 50 µm, the devices did not exhibit breakdown up to 3 kV, necessitating control for managing the devices' breakdown and critical electrical field. Testing was conducted using a Keithly 4200 Semiconductor parameter analyzer equipped with a controllable thermal chuck, and high voltage measurements were performed using a Keysight B1505A Power device analyzer. The devices exhibited rectification ratios ranging from 10⁶ to 10⁷ as the distance changed from 50 to 5 µm. Temperature-dependent measurements indicated an increasing on/off ratio from 10⁷ to 10⁹ as the temperature varies from 293 to 623 K. Similarly, reverse leakage current testing at -40 V showed an increase from 10^-12 to 10^-10 A. Breakdown testing indicated breakdown voltages of 0.62, 1.2, 2.6, and &lt; 3 kV for devices with <i>d</i> = 5, 10, 25, and 50 µm, respectively. These breakdown voltages correspond to an average electric field range of 1.1-1.2 MV/cm. While these electric fields are not high compared to other wideband gap semiconductor technologies, they represent a high value compared to the reported AlN devices without any edge termination or passivation. To enhance device breakdown, SiO₂ passivation will be implemented and tested. Additionally, field-plated structures will be explored to mitigate electric field crowding, particularly at the anode edge, and sustain high breakdown voltages. The potential for further performance enhancement through surface passivation by BN will also be investigated. This research serves as a valuable reference for the development of multi-class AlN high-voltage and high-power devices.