Gallium nitride processing technologies

The objectives of this thesis are to design and develop metal-semiconductor contacts for advanced GaN and AlGaN devices, and to investigate the relevant processing technologies in realizing the design.; There are five parts in this thesis. The first part of this work concentrates on the development of two simple optical methods, Photo-reflectance (PR) and Photoconductance (PC), to estimate the Al mole fraction in the bulk AlGaN and AlGaN/GaN heterostructures.; The second part emphasizes the characterization of Schottky contacts on AlGaN. The dependence of Schottky barrier heights on the work function of metals and the Al mole fraction of AlGaN were studied using I-V, C-V and internal photo-emission methods. It was found that the Schottky barrier height q&phis;_b on AlGaN increases with work function of the contact metal, and the Al mole fraction in AlGaN.; The third part discusses ohmic contacts to AlGaN/GaN heterostructures. The dependence of the specific contact resistivity, ρ_s, on thickness and Al mole fraction of the AlGaN layer, the metal-semiconductor reactions leading to ohmic behavior were studied. Two approaches to reduce contact resistance were used, (i) implantation of Si into the AlGaN layer, and (ii) reduction of the AlGaN barrier layer using the advancing interface metallization. Both the Ti-based Al/Ti bilayer metallization and the Ta-based Al/Ti/Ta tri-layer metallization were introduced to implement the advancing interface ohmic contact. The temperature dependence of contact resistivity of the advancing interface contact on HFET was also studied. It was found that the advancing interface ohmic contact reduces the contact resistivity by reducing the thickness of the AlGaN barrier, thus increasing the tunneling current between the metal and the 2-dimensional electron gas (2DEG) at the AlGaN/GaN interface. Specific contact resistivities < 10−6 Ω·cm2 was readily obtained on the samples with a thin layer of AlGaN remained underneath the metal contact after annealing.; Part 4 studies the ohmic contacts on p-GaN. The ohmic formation mechanism of the Au/Ni ohmic contact on p-GaN was first studied, followed by the optimization of the thickness ratio and the processing conditions. Several other ohmic contacts, such as Au/Pt/Ni and Au/Ni/Pt, were also developed.; Part 5 focuses on the study of the accumulation of crystalline damage during ion implantation and the removal of the damage by thermal annealing. We found that (i) GaN samples showed strong dynamic annealing effect during implantation, (ii) the crystalline damage increases with increasing dose and mass of the implanted ions, (iii) for the case of Ga +-N+ co-implantation the damage can be significantly reduced by thermal annealing at 800°C for 2 hours, as compared to slow reduction of damage induced by single ion implantation.