The Characterization and Optimization of GaN Cap Layers and SiN Cap Layers on AlGaN/GaN HEMT Structures Grown on 200mm GaN on Silicon

We have studied capping layers for AlGaN-based high electron mobility transistor (HEMT) structures, looking at different thicknesses of GaN, SiN, and GaN+SiN caps. SiN capping has no effect on the sheet resistance of the layers, as expected from a high quality amorphous passivation layer. GaN cap layers increase the sheet resistance whether combined with a SiN cap or not, a consequence of the polarisation charge present at the GaN/AlGaN interface. For both GaN and SiN caps on their own, we get excellent morphology from only 2nm thickness, with no degradation for layers up to 10nm thick. For GaN+SiN caps, we have a speckled morphology with lots of small holes, which transmission electron microscopy (TEM) analysis showed to be linked to holes created in the GaN layers before the deposition of SiN layers. We attribute this to the roughening effect of silane on GaN layers, known in the literature. Upon processing SiN capped and GaN+SiN capped layers into HEMT based heterojunction diode devices, the two have equivalent forward current, but GaN+SiN capping has greatly increased reverse current. We conclude that GaN+SiN is not an appropriate capping layer for AlGaN HEMT based diodes, compared with high quality in situ SiN passivation directly grown on the AlGaN.