Crystalline SiNx Ultrathin Films Grown on AlGaN/GaN Using In Situ Metalorganic Chemical Vapor Deposition
Surface passivation by SiNx films is indispensable for high-power operation of AlGaN/GaN heterojunction field-effect transistors (HFETs) since it can effectively suppress collapse in the drain current. So far, the plasma-enhanced chemical vapor deposition technique has been used for the SiNx deposition; however, possible damage induced by the plasma processing may affect direct-current performance or reliability. In this paper, we present subsequent deposition of SiNx ultrathin films on AlGaN/GaN in the same metalorganic chemical vapor deposition reactor. It is experimentally found that this in situ SiNx passivation doubles the sheet carrier density at the AlGaN/GaN interface from that of the unpassivated sample. High-resolution cross-sectional transmission electron microscopy reveals that in situ SiNx is crystallized on the AlGaN layer as island-like structures via the Stranski-Krastanov growth mode. The lattice constants of in situ SiNx are estimated to be a ≈ 3.2 Å and c ≈ 2.4 Å, which are quite different from those of well-known Si3N4 crystal structures. First-principles calculation predicts that the crystal structure of in situ SiNx is the defect wurtzite structure, which well explains the experimental results. The passivation technique using crystalline SiNx films would be promising for high-power and high-frequency applications of AlGaN/GaN HFETs.