晶格匹配InAlN/GaN异质结Ti/Al/Ni/Au欧姆接触的温度依赖特性

Temperature dependent characteristics of Ti/Al/Ni/Au ohmic contact on lattice-matched InAlN/GaN heterostructures

在硅衬底晶格匹配In_(0.17)Al_(0.83)N/GaN异质结外延片上制备了Ti/Al/Ni/Au欧姆接触传输线模型测试结构,通过测试变温电流-电压特性研究方块电阻(R_(sh))和比接触电阻率(ρ_(sc))的温度依赖特性.结果表明:(1)沟道层的R_(sh)对温度呈指数依赖关系,幂指数约-2.61,主要由高温下半导体的晶格散射机制决定;(2)300~523 K的变温范围内,ρ_(sc)随温度上升呈先增大后减小的趋势;当温度低于350 K时,ρ_(sc)的温度依赖关系主要由TiN合金的类金属特性决定;而在更高的温度下,热场发射机制将逐渐占主导.基于以上2种模型的并联形式对实验数据进行了拟合,并分析了提取的重要物理参数.

Due to the excellent physical properties, such as high breakdown field, high electron drift velocity, and good thermal stability, GaN-based high electron mobility transistors（HEMTs） have emerged as the most promising candidates for the high power and high frequency application. However, for conventional AlGaN/GaN devices, significant inverse piezoelectric effect within the barrier layer severely limits the reliability of devices, which could induce rather larger gate leakage current especially when operating in a long-term large signal mode. Recently, one of the most effective solutions is to replace the traditional AlGaN/GaN structure with a lattice matched In_（0.17）Al_（0.83）N/GaN heterojunction, which has a larger two-dimensional electron gas（2DEG） density without piezoelectric effect inside the barrier layer. Besides, in order to obtain high output current density in GaN-based HEMTs, fabricating high-quality ohmic contact as the source and drain electrodes is essential, which can directly affect the overall electrical performance of the device. However, so far the carrier transport mechanisms of the ohmic contact on In AlN/GaN heterostructure remain unclear and should be well understood in physics. In terms of that, in this work, the transmission line model measurement structure of Ti/Al/Ni/Au ohmic contact was fabricated on the lattice-matched In_（0.17）Al_（0.83）N/GaN heterostructure grown on a Si substrate, and the temperature dependent characteristics of the sheet resistance（R_（sh）） and the specific contact resistivity（ρ_（sc）） are investigated by using temperature-variable current-voltage measurements from 300 to 523 K. The lattice-matched In_（0.17）Al_（0.83）N/GaN epi-layers are grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The epi-structure includes a 3 mm i-GaN layer, a 2 nm AlN spacer layer, an 18 nm In0.17Al0.83 N barrier layer, and a 2 nm GaN cap layer. Standard lithography and lift-off techniques are used to define the electrode patterns. The ohmic contact is formed by annealing a Ti/Al/Ni/Au metal stack in N2 environment at 870℃ for about 30 s. The results show that,（1） 1/R_（sh） follows a power law dependence on temperature with a coefficient of about 2.56, which can be attributed to the lattice scattering of the semiconductor at high temperatures;（2） ρ_（sc） exhibits a roll-over behavior in the measured temperature range from 300 to 523 K. At temperatures lower than 350 K, the temperature dependence of ρ_（sc） can be mainly explained by the metal-like effect of the TiN alloy, while at higher temperatures the thermionic field emission mechanism dominates gradually. The experimental data points can be well fitted with a parallel combination of the above two models. It turns out that, the effect electron density N_D is about 3.27×10^（19） cm^（-3）, which is lower than the result of 9.11×10^（19） cm^（-3） obtained by C-V measurements, due to a weaken polarization effect after the rapid thermal annealing（RTA） process, and the effective tunneling barrier height is about 0.86 e V, which is much smaller than the theoretical prediction result, probably due to the generation and accumulation of negatively charged N vacancy near the barrier surface layer during the RTA process.