Impact of AlN/Si Nucleation Layers Grown Either by NH3-MBE or MOCVD on the Properties of AlGaN/GaN HFETs

Epitaxial growth of group III nitrides on silicon substrates is challenging as large lattice and thermal expansion coefficient mismatches have to be accounted for. It is generally accepted that AlN is the best nucleation layer for GaN-on-Si growth. However, some open questions remain about this AlN nucleation layer. In this study, it is investigated how the choice of deposition method of the AlN nucleation layer affects the material and electrical properties of a full HFET structure. First, electrical characterization is performed on thin AlN layers deposited on two inch p-type silicon substrates grown by NH3-MBE. CV characteristics indicate that electron injection from an inversion-like layer at the AlN/Si-interface is suppressed. Based on previous studies, this leads to the expectation that choosing MBE-AlN may actually result in superior properties at the device level. Finally, the full HFET stack is grown by MOCVD on both types of AlN nucleation layers. Hall measurements on active devices yield comparable mobilities for both nucleation methods. Preliminary data suggest that MBE-AlN does not have any detrimental impact the dynamic properties of the buffer. The vertical breakdown voltage measured at 1Amm(-2) is increased by 30% and the leakage current measured at 600V is reduced by two orders of magnitude.