The physical mechanisms of Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures by laser annealing and rapid thermal annealing are systematically investigated. The microstructures indicate that a better surface morphology and an intact contact interface are formed after laser annealing. None of the TiN alloy spikes are formed at the interface of the laser annealing sample. The experimental results show that the current transport mechanism through the ohmic contact after laser annealing is different from the conventional spike mechanism, and it is dominated by thermionic field emission.
以Ti/Al/Ni/Au作为欧姆接触金属体系,通过电感耦合等离子体(ICP)刻蚀的预处理,在氢化物气相外延法生长的单晶氮化镓(GaN)材料的N面实现了良好的欧姆接触,其比接触电阻率为3.7×10-4Ω·cm^2。通过扫描电子显微镜、原子力显微镜、阴极荧光和光致发光谱对GaN N面的表面、光学特性进行了对比表征。结果表明:未刻蚀GaN衬底的N面表面存在一定的损伤层,导致近表面处含有大量缺陷,不利于欧姆接触的形成;而ICP刻蚀处理有效地去除了损伤层。X射线光电子能谱(XPS)分析显示刻蚀后样品的Ga 3d结合能比未刻蚀样品向高能方向移动了约0.3 e V,其肖特基势垒则相应降低,有利于欧姆接触的形成。同时对Fe掺杂半绝缘GaN的N面也进行了刻蚀处理,同样实现了良好的Ti/Al/Ni/Au欧姆接触,其比接触电阻率为0.12Ω·cm^2。
The Ti/Al/Ni/Au metals were deposited on undoped AlN films by electron beam evaporation. The influence of annealing temperature on the properties of contacts was investigated. When the annealing temperatures were between 800 and 950 ℃, the AlN-Ti/Al/Ni/Au contacts became ohmic contacts and the resistance decreased with the increase of annealing temperature. A lowest specific contacts resistance of 0.379 Ω·cm^2 was obtained for the sample annealed at 950 ℃. In this work, we confirmed that the formation mechanism of ohmic contacts on Al N was due to the formation of Al-Au, Au-Ti and Al-Ni alloys, and reduction of the specific contacts resistance could originate from the formation of Au2Ti and AlAu2 alloys. This result provided a possibility for the preparation of Al N-based high-frequency, high-power devices and deep ultraviolet devices.
The multi-step rapid thermal annealing process of Ti/Al/Ni/Au can make good ohmic contacts with both low contact resistance and smooth surface morphology for AlGaN/GaN HEMTs.In this work,the mechanism of the multi-step annealing process is analyzed in detail by specific experimental methods.The experimental results show that annealing temperature and time are very important parameters when optimizing the Ti/Al layer for lower resistance and the Ni/Au layer for smooth surface morphology.It is very important for good ohmic contacts to balance the rate of various reactions by adjusting the annealing temperature and time.We obtained a minimum specific contact resistance of 3.22×10^（17）Ω·cm^2 on the un-doped AlGaN/GaN structure with an optimized multistep annealing process.
Ohmic contacts of Ti/Al/Ni/Au multi-layer metal on A10.27 Ga0.73N/GaN heterostructures were fabricated. Specific contact resistivities were measured by the linear transmission line method （LTLM） and the circular transmission line method （CTLM） ,respectively. A minimum specific contact resistivity of 1.46× 10 ^-5Ω· cm^2z was obtained by evaporating a Ti（10nm）/Al（100nm）/Ni（40nm）/Au（100nm） multi-layer and annealing for 30s at 650℃ in ultra-high purity N2 ambient. A10.27 Ga0.73 N/GaN photoconductor ultraviolet （UV） photodetectors were prepared. The dark current-voltage （I-V） characteristics of the detectors were measured and the result shows that the I- V curve was linear. Experimental results indicate that good ohmic contact on the Al0.27 Ga0.73 N/GaN heterostructure is obtained and it can be applied in high- performance AlGaN/GaN UV photodetector fabrications.
