Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

1C/2002S2-002

Mechanism of ohmic-contact formation between Ti electrodes and AlxGa1-xN

Takayuki NAONO*, Jun OKABAYASHI, Ken KANAI, Satoshi TOYODA,
Hiroshi FUJIOKA1,2,
3
Masaharu OSHIMA, and Hisayuki MIKI
Department of Applied Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
1
Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-0041, Japan
2
Kanagawa Academy of Science and Technology, Kawasaki, Kanagawa 213-0012, Japan
3Showa Denko K.K., Ichihara, Chiba 290-0067, Japanpromotes the formation of TiN and AlGaN layers with Nvacancies at the interface, resulting in decrease in contactresistivity.

as deposition
500 °C

20Current (mA)

100-10-20-42-20Voltage (V)

4FIG. 1: Current-voltage characteristics of Ti electrodes/n-type
Al0.2Ga0.8N before and after the annealing at 500 °C

Intensity (arb. units)

1. Introduction
Ohmic contacts between metal electrodes and nitride
semiconductors with low resistivities are required to improvethe device performance. For n-type GaN, Ti-based contacts
have been extensively used due to several advantages such asa low work function and cost performance [1-3]. The mostimportant one is that Ti-based contacts decrease theresistance by a chemical reaction and behave as an ohmic
characteristic by the annealing above 500 ˚C [4]. Thisbehavior is explained by the creation of nitrogen vacancies at
an interfacial GaN layer by the formation of metallic TiNcompounds through the annealing [4,5]. Ti-based electrodes
have been also examined for AlxGa1-xN with low Al ratio, andthe annealing process has been also performed after thedeposition of electrodes [6]. To clarify the mechanism of
low-resistance ohmic-contact formation at Ti/Al1-xGaxN
interface, the investigations for energy-band structures are
required. In this study, we have investigated the chemical
reaction at a Ti/n-Al0.2Ga0.8N interface by photoemission
spectroscopy to discuss the mechanism of the ohmic-contact
formation comparing to the results of the Ti/GaN interface.
2. Experimental
Al0.2Ga0.8N and GaN samples used in this study weregrown by metalorganic chemical vapor deposition on c-plain
sapphire substrates. The thicknesses of both n-type GaN andAl0.2Ga0.8N layer were 0.2 µm. To remove the surface
contamination, samples were dipped into a boiled 39% HClsolution for 50 min. After that, the 0.3-nm thick Ti layer wasevaporated in ultrahigh vacuum (UHV). Photoemission
spectroscopy was performed at the beam line BL-1C ofPhoton Factory in KEK.
3. Results and discussion
Figure 1 shows current-voltage characteristics of Ti
electrodes/n-Al0.2Ga0.8N before and after the annealing at 500
˚C for 10 min. The annealing drastically decreased theresistances, which was consistent with the case of Ti/GaN.
The degree of decrease in the resistance of Ti/Al0.2Ga0.8N wassmaller than that of Ti/GaN.
Valence-band spectra for Al0.2Ga0.8N and GaN taken athν=100 eV are shown in Fig. 2. Spectral line shapes changedby annealing after the Ti deposition, suggesting the interfacialreaction. Satellite structures in Ti 2p photoemission spectra at
500 °C indicate the formation of the TiN compound at theinterface. Similar phenomena were observed for Ti/GaNinterface [4]. Furthermore, Ga 3d, N 1s, and Al 2p peaks
shifted to the lower binding energy by annealing. The
amounts of shifts for Ti/Al0.2Ga0.8N were smaller than thosefor Ti/ GaN. These results indicate that the annealing process

Al0.2Ga0.8Nhν=100 eV
700 ºC
500 ºC

Ti/Al0.2Ga0.8N
HCl-etched
Al0.2Ga0.8N

12840Binding Energy (eV)FIG. 2: Annealing-temperature dependence in valence-band
spectra. Arrow shows a valence-band maximum for n-type
Al0.2Ga0.8N
References[1] J. D. Guo et al., Appl. Phys. Lett. 68 (1996) 235.
[2] C. Lu et al., J. Appl. Phys. 91 (2002) 9218.
[3] A. Motayed et al., J. Appl. Phys. 93 (2003)1087.
[4] T. Naono et al., Appl. Surf. Sci. 244 (2005) 277.
[5] K. Kim et al., J. Appl. Phys. 91 (2002) 9214.
[6] M. W. Fay et al., J. Appl. Phys. 96 (2004) 5588.
*

naono@sr.t.u-tokyo.ac.jp

Users' Report69