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

Abstract

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 In0.17Al0.83N/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 InAlN/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 In0.17Al0.83N/GaN heterostructure grown on a Si substrate, and the temperature dependent characteristics of the sheet resistance (Rsh) and the specific contact resistivity (ρsc) are investigated by using temperature-variable current-voltage measurements from 300 to 523 K. The lattice-matched In0.17Al0.83N/GaN epi-layers are grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The epi-structure includes a 3 μm i-GaN layer, a 2 nm AlN spacer layer, an 18 nm In0.17Al0.83N 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/Rsh 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) ρsc 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 ρsc 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 ND is about 3.27×1019 cm-3, which is lower than the result of 9.11×1019 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 eV, 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.