(Invited) Vertical GaN p-i-n Diodes Formed by Mg Ion Implantation

GaN is an ideal material for power applications due to its high breakdown field, high switching speed, and low switching losses. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge. Dopant implantation and activation is crucial for many devices since it is the most straightforward method of selective lateral doping in GaN. Devices such as the current aperture vertical electron transistor (CAVET), Schottky diodes with guard ring termination or grayscale junction termination extension, and vertical p-i-n diodes will be enhanced and enabled by the ability to selectively dope GaN without the need for etching or regrowth. Herein, we report on the first implanted p-i-n diode fabricated on a bulk GaN substrate.

A 8 μm thick unintentionally doped GaN layer was grown by metal organic chemical vapor deposition (MOCVD) on a n⁺ Ga-face c-oriented GaN substrate. The p-GaN anode and termination regions were formed by Mg implantation to a concentration of 2x10¹⁹ cm^-3 following a box profile to a depth of 500nm. The implanted dopants were activated using the multicycle rapid thermal annealing (MRTA) technique described elsewhere [1]. After activation and removal of the protective cap structure [2], a 1 μm SiO₂ field oxide layer was deposited by plasma enhanced chemical vapor deposition (PECVD). Contact windows were opened by reactive ion etching in a CHF₃-based plasma. The anode metal was formed by lift-off of Pd/Au (20/100nm), followed by rapid thermal annealing in a N₂atmosphere, and the cathode metal was a blanket film of e-beam deposited Al on the back side of the sample.

Rectifying behavior was observed, a first for a p-i-n structure formed by Mg ion implantation in GaN. The turn-on voltage is consistent with a p-n junction, and the ideality factor of 2.6 is consistent with a recombination/generation-based transport mechanism, confirmed by temperature-dependent I-V. Additional non-ideal behavior is associated with recombination in the drift layer due to crystalline defects. The devices suffer from high ON-resistance due to high contact resistance and recombination in the drift layer, which limited the forward current to 1 A/cm². Capacitance-voltage measurements indicated the presence of a p-GaN layer, with a depletion layer thickness ~500nm and a doping density of 2x10¹⁸ cm^-3, corresponding to an activation ratio of 10%.

1. B.N. Feigelson, et. al. J. Cryst. Growth. 350, 21-26 (2012)

2. J.D. Greenlee, et. al. Appl. Phys. Express 8 036501 (2015)