C-V characterisation of MOCVD GaN p-n junctions for high-power HFETs

Abstract

In this work, GaN p-n diodes have been designed, processed and electrically analysed to determine the effective doping profile in the p-GaN layer. The work has proven that it is possible to create functioning vertical p-n diodes on basis of the AlGaN/GaN HEMT process for GaN on Si wafers. Although fabrication turned out to be more difficult for these wafers due to the wafer bow and crack density, the production is reproducible for standard vertical diodes. C-V measurement verified the expected capacitance characteristics of the diodes with 200 um and 400 um diameter and Ni/Au contacts. Accurate C-V measurements were required as a small deviation in the capacitance measurement leads to a difference in the profile of several 10 nm. Comparison of the wafers in regard of their different epitaxial fabrication, leads to the result, that activation of charge carriers within the RTA oven is more suitable than in-situ activation. A large amount of charge carriers is located close to the p-n interface. Employment of RTA activated p-doped GaN in AlGaN/GaN HEMTs, for normally-off gates, can lead to a higher peak electric field depleting the transistor channel. Therefore, the achieved threshold voltage for enhancement mode transistors should be larger for RTA activation than reached with in-situ activated p-doped GaN. The longer Mg pre-flux did not exhibit significant improvement on the charge carrier density, neither did the process variation without the shallow ion implantation. P-n diodes with a WSiN Schottky top contact have been successfully fabricated exhibiting a threshold voltage of +8 V, due to the depletion of holes in the p-doped region. Application of p doped gates with a Schottky top contact in the AlGaN/GaN HEMT transistors is likely to result in a normally-off behaviour with a higher threshold voltage, suitable for high voltage operation. Further research is needed to optimise the charge carrier density activated within the RTA. Multi-cycle processes with different temperatures should be investigated, as well as changing the nitrogen atmosphere to an oxygen ambient may improve the results.