Ti/Al- GaN Reaction Mechanism Forming Low Contact Resistivity
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Ti/Al- GaN Reaction Mechanism Forming Low Contact Resistivity Yoshimichi Fukasawa, Tomonori Nakamura, Tohru Nakamura College of Engineering, Hosei University Koganei, Tokyo 184-8584 Japan ABSTRACT The mechanism of Ti/Al reaction to n-GaN was studied to form ohmic contacts with low contact resistivity. N-GaN layers with a carrier concentration of 2.17 1018 cm-3 were deposited on sapphire substrates. Ti/Al metals were deposited by conventional electron-beam techniques. Contact resistivity decreased as the Ti thickness increased, and increased as the Al thickness increased. The lowest contact resistivity was measured at 1.20 10-6 cm2 for 80 nm Ti /100 nm Al. After annealing at 900 C, Al/AlTi/TiN layers on GaN were formed and AlTi alloy thickness decreased as Ti thickness increased, from 1.5 MeV Rutherford Backscattering Spectroscopy(RBS) measurement. It was found that the contact resistivity was reduced as alloy metal thickness into GaN was increased.
INTRODUCTION GaN has a wide band gap and its applications in high speed, high temperature and high power electronic devices are expected. In these applications, high quality ohmic contact formation is very important to obtain high performance characteristics. Contacts containing Ti/Al layers have observed the lowest contact resistivity to n-GaN[1-3]. Due to a TiN layer formed by Ti reaction to GaN after annealing, a high concentration of N vacancies could be created near the interface, resulting that ohmic contacts with contact resistivity as low as 8 x 10-6 cm2 have been achieved[2]. A contact resistivity about –10-6 cm2 on GaN using plasma treatment without annealing at a higher temperature has been reported[4]. However, there is few reports on direct measurements of TiN formation after annealing. This paper reports the reaction between Ti/Al and GaN in order to realize ohmic electrodes with low contact resistivity.
Table 1. The thickness of Ti/Al metal electrodes. Sample 1 2 3 4 Ti 20 [nm] 50 [nm] 80 [nm] Al 100 [nm] 50 [nm]
5 50 [nm] 100 [nm]
6 150 [nm]
7 100 [nm] 0
EXPERIMENTAL
N-GaN (n = 2.17 1018 cm-3) layers were grown on (0001) sapphire substrates by the MOCVD. Ti/Al metals were deposited on n-GaN epitaxial layers by conventional electron beam techniques. The thickness of Ti/Al bilayer is shown in Table 1. After deposition, thermal annealing was carried out at 900 C for 3 min in nitrogen atmosphere. 1.5MeV Rutherford Backscattering Spectroscopy (RBS) was used to clarify the composite metal systems.
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Measurements of the contact resistivity were made using the TLM patterns which were defined by photolithography. The thickness of Al/Ti and Al/AlTi/TiN layers on GaN was measured using KLA-Tencor alfa-step AS500. RESULTS AND DISCUSSIONS The contact resistivity of the Ti/Al contact to n-GaN as a function of as-deposited Ti thickness is shown in Fig. 1. The contact resistivity becomes low as Ti thickness increases. Minimum contact resistivity of 1.20 10-6 -cm2 was obtained at Ti(80 nm)/Al(100 nm) system. The contact re
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