TEM assessment of GaN/AlGaN/TiAlTiAu and GaN/AlGaN/TiAlPdAu ohmic contacts
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TEM assessment of GaN/AlGaN/TiAlTiAu and GaN/AlGaN/TiAlPdAu ohmic contacts M W Fay, G Moldovan, I Harrison1, J C Birbeck2, B T Hughes2, M J Uren2, T Martin2, P D Brown School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, University Park, Nottingham NG7 2RD, UK 1 School of Electrical and Electronic Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK 2 QinetiQ Ltd, St Andrews Rd, Malvern, Worcs WR14 3PS, UK ABSTRACT TiAlTiAu and TiAlPdAu contacts to GaN/AlGaN, rapid thermal annealed at temperatures ranging from 650°C to 950°C, have been investigated using conventional and chemical TEM analysis. Ohmic behaviour was seen for TiAlTiAu contacts annealed at 750°C or higher, but was not observed in TiAlPdAu contacts annealed at up to 950°C. The effect of annealing temperature on the structural evolution of the contact is explained in terms of different extents of interfacial reaction. In particular, the formation of TiN after anneals at high temperatures is required to activate the contact. At anneals of 950°C, TiAlTiAu samples show a structure of TiN grains within an interfacial band, with TiN inclusions into the AlGaN preceded by an Al-Au diffusion front. Inclusion formation and the effect on the contact electrical performance is described. INTRODUCTION GaN/AlGaN heterostructure field effect transistors (HFETs) are of interest for high power, high frequency applications. These devices need reproducible low resistance, thermally stable ohmic contacts. TiAl is the conventional contact to n-type GaN and AlGaN [1,2]. This is believed to become ohmic by forming TiN at the nitride/contact interface, with the N being depleted from the surface AlGaN, resulting in a highly n-type doped surface layer [3]. However, TiAl oxidises easily, so superior electrical characteristics have been achieved using contact schemes with Au as a surface layer and a barrier layer such as Ni, Pd or Ti between the Au and the TiAl layers. [4,5,6]. In this work, microstructural and chemical analysis is performed to obtain a comparison of Pd and Ti as effective diffusion barriers and to investigate the GaN/AlGaN/contact interfacial structure observed in these samples after rapid thermal annealing. EXPERIMENTAL METHOD The HFET wafer structure consisted of 30nm of Al0.25Ga0.75N on 1.2 µm of GaN, grown on an (0001) oriented sapphire substrate by metal organic chemical vapour deposition (MOCVD). Onto this was deposited the device contact structure consisting of 20nm Ti / 100nm Al / 60nm Ti / 300nm Au, or 10nm Ti / 220nm Al / 200nm Pd / 200nm Au. Samples were rapid thermal annealed in flowing nitrogen using a 5 minute purge at room temperature followed by a 30 second ramp to the annealing temperature, a 30 second hold at the annealing temperature and a free cool to ~ 150°C before removal.
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Low angle argon ion milling was used to prepare samples for conventional and TEM analysis. Elemental profiles were obtained using the energy dispersive x-ray (EDX) and energy filtered TEM (E
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