Crystalline SiNx Ultrathin Films Grown on AlGaN/GaN Using In Situ Metalorganic Chemical Vapor Deposition

Surface passivation by SiN _x 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 SiN _x deposition; however, possible damage induced by the plasma processing may affect direct-current performance or reliability. In this paper, we present subsequent deposition of SiN _x ultrathin films on AlGaN/GaN in the same metalorganic chemical vapor deposition reactor. It is experimentally found that this in situ SiN _x 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 SiN _x is crystallized on the AlGaN layer as island-like structures via the Stranski-Krastanov growth mode. The lattice constants of in situ SiN _x 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 SiN _x is the defect wurtzite structure, which well explains the experimental results. The passivation technique using crystalline SiN _x films would be promising for high-power and high-frequency applications of AlGaN/GaN HFETs.