Effects of metal spikes on leakage current of high-voltage GaN Schottky barrier diode

We have investigated effects of metal spikes on the leakage current of high-voltage GaN Schottky barrier diodes (SBDs) on Si substrate. The metal spikes are formed underneath Ohmic contacts during a thermal annealing. The diffusion of Ti/Al/Mo/Au into GaN is analyzed by measuring Auger electron spectroscopy (AES). Ti/Al/Mo/Au on GaN is stripped by a wet etchant and its surface is observed to verify metal spikes by scanning electron microscope (SEM) and atomic force microscopy (AFM). The annealing temperature of the Ohmic contact is proportional to the diffusion depth of the metal spikes and the leakage current. The reverse current of GaN SBD with an Ohmic alloy at 700 °C is 0.37 A/cm2 at −100 V while that of GaN SBD with the Ohmic alloy at 800 °C is 13.45 A/cm2 at −100 V. The metal spikes in GaN power devices should be suppressed for the low power loss and the high breakdown voltage. The reverse current of GaN SBD is further decreased by a recessed Schottky contact because the Schottky contact is closer to unintentionally-doped (UID) GaN buffer and the depletion is increased. The reverse current of GaN SBD with the recessed Schottky contact is finally decreased to 0.05 A/cm2 at −100 V. When an anode–cathode distance (DAC) is 5 μm, the measured on-resistance, breakdown voltage and figure-of-merit (BV2/Ron,sp) are 3.15 mΩ cm2, 320 V, and 32.5 MW/cm2, respectively. When DAC is increased to 20 μm, fabricated devices show the breakdown voltage of 450 V and good device-to-device uniformity.

► Metal spikes of GaN SBDs are formed during the annealing for Ohmic contacts.
► Annealing temperature of the Ohmic contacts is proportional to the depth of the metal spikes.
► The metal spikes should be suppressed for low-power loss and high-voltage operation of GaN power devices.
► Fabricated devices with the anode–cathode distance of 20 μm show the uniform breakdown voltage of 450 V.