Diamond p-Type Lateral Schottky Barrier Diodes with High Breakdown Voltage (4612 V at 0.01 mA/mm)

Diamond p-type lateral Schottky barrier diodes (SBDs) with a 2-μm-thick drift layer are fabricated with and without Al₂O₃ field plates. Schottky contacts composed of Mo (50 nm) / Pt (50 nm) / Au (100 nm) showed a barrier height of 1.02 ± 0.01 eV and ohmic contacts of Ti (30 nm) / Pt (30 nm) / Au (100 nm) achieved a specific ohmic contact resistance of 1.25 ±0.98×10⁻⁴Ω-cm². Their forward and reverse bias characteristics are studied in detail. Both SBDs, with and without Al₂O₃ field plates, exhibit rectifying ratios larger than 10⁷ at room temperature, and a peak current density of 5.39 mA/mm under 40 V forward bias at 200 °C. The leakage current density at room temperature is stable at approximately 0.01 mA/mm for both diodes. The SBD without the Al₂O₃ field plate exhibited a breakdown voltage of 1159 V, while the SBD with the Al₂O₃ field plate is stable under a reverse voltage of 4612 V, which is higher than many diamond SBDs previously reported.<br/>The fabrication of the SBDs starts with a 2 μm p- drift layer ([B] &lt; 8 × 10¹⁵ cm^-3]) that was grown on a 3 × 3 mm² Type Ib (100) high-pressure high temperature (HPHT) diamond substrate. Then, 200 nm p+ diamond ([B] ~ 3 × 10²⁰ cm^-3]) was selectively grown to form the ohmic contact region. Ohmic metal contacts were formed by e-beam evaporation of Ti (30 nm) / Pt (30 nm) / Au (100 nm), followed by thermal annealing at 450 ^oC in an ambient of Ar gas for 50 minutes. The specific contact resistance of ohmic contacts was determined by TLM measurements and measured to be 1.25±0.98 × 10^-4 Ω-cm². Next, a 300 nm Al₂O₃ field plate was deposited by e-beam evaporation, followed by a lift-off process. Al₂O₃ was chosen as the field oxide because of its high dielectric constant relative to diamond (k = 8.63±0.07 for the as-deposited Al₂O₃ ) which reduces the electric field strength and a large band offset for the oxygen-terminated diamond. The exposed diamond surface was ozone-treated at room temperature for 1.5 hours to obtain a stable oxygen termination prior to the Schottky contact deposition. Schottky metal stack of Mo (50 nm) / Pt (50 nm) / Au (100 nm) was deposited by e-beam evaporation. The inner and outer radii of the Al₂O₃ field plate are 40 μm and 80 μm, respectively. The radius of the Schottky contact is 60 μm, and the separation between the ohmic and Schottky contact is 80 μm.<br/>Both diodes exhibit a rectifying ratio of 10⁷ in the range of +/−5 V. The linear current densities at a 40 V forward bias are 0.049 mA/mm and 0.044 mA/mm for the SBD with and without the field plate, respectively. The SBDs were reprocessed several times before high-temperature and breakdown measurements and showed good reproducibility in forward <i>J-V</i> characteristics. The max difference in forward current densities at the 40 V forward bias between four fabrication batches is 15% and 6% for the SBD with and without the FP, respectively. During reverse bias measurement, the diamond wafer was submerged in 3M™Fluorinert™electronic liquids to prevent air breakdown. The lateral SBD without the field plate broke down at 1159 V when the leakage current drastically increased to the compliance limit of 50 μA. After the first breakdown, reverse current density increased at low reverse bias, which confirmed the generation of leakage paths. No physical damage to the Schottky contact was identified post-measurement. However, repeated breakdown measurements showed a decrease in the breakdown voltage. The SBD with the field plate exhibited stable leakage current up to 4612 V, which is the limit of the Fluorinert™electronic liquids. The leakage current density at 4612 V reverse bias is less than 0.01 mA/mm, which is similar to that of the SBD without the field plate prior to breakdown. The relatively high stable leakage current can be attributed to a high surface roughness of the epitaxially grown drift layer (RMS roughness = 7.5 nm), caused by rough polishing and random growth defects.