Microscopic formation mechanism of Si/Tl5Al1/TiN ohmic contact on non-recessed i-InAlN/GaN heterostructures with ultra-low resistance

Abstract

In this work, we use Si/Tl5Al1/TiN for a source/drain ohmic contact to demonstrate an ultra-low contact resistance of 0.11 Ω mm (ρc = 2.62 × 10−7 Ω cm2) on non-recessed i-InAlN/GaN heterostructures. The Ti5Al1 alloy was used to suppress the out-diffusion of Al and extract N from the InAlN layer, which aided the formation of ohmic contact by improving the tunneling efficiency of electrons, as we have reported in the past work. A thin Si inter-layer combined with the Ti5Al1 alloy is proposed to further reduce contact resistance. A heavy n-type InAlN layer was obtained through doping with Si atoms to improve the tunneling transport of electrons. Furthermore, the TiN inclusions penetrated into the GaN channel because the in-diffused Si promoted the decomposition of GaN at a high annealing temperature and the in-diffused Ti reacted with GaN. These TiN inclusions provided direct contact with two-dimensional electron gas, offering an additional path for the injection of electrons into the channel. The tunneling and spike mechanism worked alternately to lower the contact resistance at different annealing temperatures (dividing at 900 °C), implying that the joint effect of tunneling and the spike mechanism was initially promoted in the formation of ohmic contact. The mechanism of this Si/Ti5Al1/TiN ohmic contact was fully understood through microscopic and thermodynamic analyses. These results shed light on the mechanism for the formation of ohmic contact in a gold-free metal stack for GaN-based HEMTs.