| III-nitride materials hold great promise for high performance microelectronic and optoelectronic applications. The AlGaN/GaN high electron mobility transistors (HEMTs) are capable of handling higher current densities than other III-V HEMTs due to the high density two-dimensional electron gas (2DEG) accumulated at the AlGaN/GaN interface. When combined with the large bandgap, high breakdown voltage, and high carrier velocity, GaN-based materials are excellent candidates for high-power, high-temperature and high-frequency electronic devices. Knowledge of the way in which ohmic properties are determined by the interfacial reaction down to atomic scale at the metal/semiconductor (M/S) interfaces are fundamental concerns in understanding the ohmic formation mechanism and in optimizing the device performances. In order to achieve contact metallization schemes with known and predictable characteristics, focuses of this thesis have been put on contact performance enhancement through extensive properties-parametric testing, on systematic characterization of interfacial structure using analytical cross section TEM, and finally, on carrier transport mechanism determination by variable temperature measurements.;Major findings of this work include: (1) Elucidated the interfacial reactions between Al, Ti, AlTi alloy, and Ti/Al/Mo/Au on GaN. (2) Explained the electrical performance of Ti/Al/Mo/Au on AlGaN/GaN based on microstructural investigations. Proposed a "spike" mechanism. (3) Identified the role of other metals, i.e. V, Mo, and Ta, as the first layer for X/Al/Mo/Au contacts. (4) Correlated the surface roughness, lateral encroachment, and contact performance of the Ti/Al/X/Au (X=Ni, Pt, Ir, Mo, Ti and NM) contacts for AlGaN/GaN HEMTs. (5) Tailored the interfacial reaction pathways and electrical performance of Ti/Al/Mo/Au on AlGaN/GaN by incorporating Si into the metallization. Obtained low-resistance contact schemes having wide processing window. (6) Investigated the electrical performance and microstructure of the Ti/Al/Mo/Au contacts on all-binary AlN/GaN HEMTs. Optimized the Ti/Al/Mo/Au contacts on n+-GaN capped AlGaN/AlN/GaN by combined surface treatment and solid state interfacial reaction. (7) Evidenced ultra-shallow Si plasma implantation and dry etch effects of SiCl4 pre-metallization treatment. Demonstrated the best performance by side-way direct contact of the metal with the 2DEG channel. (8) Characterized the carrier transport mechanism by variable temperature measurement between 100 and 375 K. |
