Au-free ohmic Ti/Al/TiN contacts to UID n-GaN fabricated by sputter deposition

V Garbe,D C Meyer,A Schmid,M Motylenko,B Abendroth,J Weise,D Rafaja,W Münchgesang

doi:10.1063/1.4975485

Abstract

The fabrication and characterization of an Au-free Ti/Al/TiN (20/100/100 nm) contact stack to unintentionally doped n-GaN with TiN serving as the diffusion barrier is presented. Sputter deposition and lift-off in combination with post deposition annealing at 850 °C are used for contact formation. After annealing, contact shows ohmic behavior to n-GaN and a specific contact resistivity of 1.60 × 10−3 Ω cm2. To understand the contact formation on the microscopic scale, the contact was characterized by current–voltage measurements, linear transmission line method, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The results show the formation of Ti-N bonds at the GaN/Ti interface in the as-deposited stack. Annealing leads to diffusion of Ti, Al, Ga, and N, and the remaining metallic Ti is fully consumed by the formation of the intermetallic tetragonal Al3Ti phase. Native oxide from the GaN surface is trapped during annealing and accumulated in the Al interlayer. The TiN capping layer, however, was chemically stable during annealing. It prevented oxidation of the Ti/Al contact bilayer successfully and thus proved to be a well suitable diffusion barrier with ideal compatibility to the Ti/Al contact metallization.

Highlights

The aim of the presented work is to investigate the efficiency of an Au-free single TiN diffusion barrier replacing the metal/Au cladding bilayer from Ti/Al ohmic contacts to n-GaN
The R–V plot shows a resistance that is on the one hand reduced by two thirds compared to the asdeposited contact and on the other hand a constant value of R for all voltages in the measured range, proving an ohmic contact
The relevant parameters describing the electrical characteristics of the contact are extracted according to Schroder35 and compared to Ti/Al/W contacts deposited by electron beam on the same n-GaN template published elsewhere33 and which were RTA annealed at 850 C for 30 s in a N2 environment

Summary

INTRODUCTION

With Ti and N, the TiN capping layer contains only the elements that are essential in the ohmic contact stack. Fabricated on doped substrates in order to match application requirements, this work focusses on contact formation to unintentionally doped (UID) GaN in order to investigate how the contact metallization itself contributes to the doping. Since the contact resistance depends on the doping concentration, the formation of ohmic contacts to UID GaN is most challenging and becomes easier for doped substrates. Ohmic contact formation initiated by high temperature treatment is investigated in terms of electrical and structural characteristics of as-deposited and annealed samples. We will show the relevant diffusion and crystalline phase formation processes, responsible for ohmic contact formation.

EXPERIMENTAL DETAILS

RESULTS

SUMMARY