Role of Ti/Al relative thickness in the formation mechanism of Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures

Abstract

In this work, Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures with different Ti/Al relative thicknesses were fabricated and characterized. A contact with Ti/Al relative thickness of 20/180 nm presented non-linear current–voltage response and bumpy surface morphology, while a contact with Ti/Al relative thickness of 20/120 nm demonstrated low contact resistance and flat surface morphology. We verify the existence of two electron transport mechanisms, namely carrier tunnelling and direct link through a TiN conductive pathway, by scanning transmission electron microscopy and corresponding electron dispersive x-ray spectra. Ti/Al relative thickness exerts a decisive influence on the alloy reaction processing. Superfluous Al consumes Ti, thus impeding the formation of low-resistance TiN and N vacancies. As a result, electron transport is restricted by the low-efficiency tunnelling mechanism. At the same time, the consumption of Ni causes damage to its blocking effect and arouses the free diffusion of metal alloys, which should be responsible for the rough surface.

Improvement of Au‐Free, Ti/Al/W Ohmic Contact on AlGaN/GaN Heterostructure Featuring a Thin‐Ti Layer and Low Temperature Annealing

“…Figure a shows the scanning transmission electron microscope (STEM) cross section of the Ti (2.7 nm)/Al/W stack on AlGaN/GaN heterostructure after annealing at 500 °C for 10 min, and Figure b shows the corresponding energy dispersive X‐ray spectrometry (EDX) line profile of the metal stack. There is no diffusion of metal into heterostructure as observed in the Au‐containing scheme . The W layer is partially disturbed, and a W‐Al alloy is observed between W and Al layer.…”

Au-free low temperature ohmic contacts for AlGaN/GaN power devices on 200 mm Si substrates

“…The CMOS-compatible metal stack used for ohmic contacts is a Ti/Al/TiN-based stack with a fixed thickness of 60 nm for the TiN cap. The importance of the Ti/Al thickness ratio has already been studied [4,5], explaining the role of Al in reducing the aggressive Ti-GaN reaction, while excess of Ti would lead to the formation of voids below TiN [5,6]. Saturation of current through ohmic contacts to AlGaN/GaN HEMT layers has been investigated and different theories have been proposed as explanation such as the self-heating causing velocity saturation [7] or the impact of surface traps acting as a 'virtual gate' depleting locally the 2DEG [8].…”

Ti/Al/Ti/Ni/Au ohmic contacts on AlGaN/GaN high electron mobility transistors with improved surface morphology and low contact resistance

“…[10][11][12] The thicknesses of individual metal layers have significant influence on the interface reaction and surface morphology of the ohmic contacts on AlGaN/GaN HEMT. 13 For instance, the Ti/Al ratio and Ni/Au ratio have been shown to have influence on R c and surface morphology. 12,13 Furthermore, the Ti/Al/Ti/Ni/Au metal scheme has been used to demonstrate more favorable ohmic behavior compared to a Ti/Al/Ni/Au metal scheme after insertion of a 10-nm Ti interlayer.…”

“…13 For instance, the Ti/Al ratio and Ni/Au ratio have been shown to have influence on R c and surface morphology. 12,13 Furthermore, the Ti/Al/Ti/Ni/Au metal scheme has been used to demonstrate more favorable ohmic behavior compared to a Ti/Al/Ni/Au metal scheme after insertion of a 10-nm Ti interlayer. 14 However, there has not been a detailed study of surface morphology and formation mechanism for the Ti/Al/Ti/Ni/Au metal scheme.…”