Y.-A. Chen, D.A. Cohen and S.P. DenBaars An effective surface treatment method has been developed to realise low resistance ohmic contacts on the lapped backsides of m-plane n-GaN substrates with a moderate doping level of 5.5 1017 cm23. Ti/Al/Au contacts with fine polishing and an ICP treatment followed by annealing at 7008C for 5 min yielded a specific contact resistance as low as 3.9 1025 Vcm2. The same low specific contact resistance was also achieved by annealing at 5008C for 5 min on a relatively rough surface. The effective removal of the lapping damage by ICP treatment and the creation of nitrogen vacancies during annealing are believed to improve the contacts.
etch. The inset of Fig. 2 shows the total resistance as a linear function of the gap between contact pads, after annealing at 7008C. The I-V curves show consistent improvement as annealing temperature increases. After annealing at 7008C for 5 min, the ICP-treated contacts showed an ohmic I-V characteristic and the specific contact resistance was dramatically reduced to 3.9 1025 Vcm2. This improvement is tentatively attributed to the effective removal of the lapping damage by the ICP treatment and the creation of nitrogen vacancies after annealing [3]. 0.002
as-supplied as-lapped
0.001 current, A
Low resistance Ti/Al/Au ohmic backside contacts to nonpolar m-plane n-GaN
0
–0.001
Contact formation: The transfer length method (TLM) was used to characterise the electrical contacts, with gaps of 5, 10, 15, 20, 25, and ˚ /300A ˚/ 30 mm between 100 100 mm pads. The Ti/Al/Au (100A ˚ ) pads were patterned by a conventional metal lift-off process, 3000A using only a photoresist mask, and electron-beam evaporation in a background vacuum pressure below 3 1027 Torr. The as-supplied group A and as-lapped group B samples were then annealed at 5008C for 5 min in an N2 ambient. ICP-treated group C samples were annealed at 500, 600, and 7008C for 5 min in an N2 ambient. Current-voltage (I-V ) characteristics were measured after annealing. Because the transfer length method is intended for characterisation of thin films and its accuracy for measurement of bulk substrates is not certain, a two-terminal vertical ˚/ conduction measurement was also made. Circular Ti/Al/Au (100A ˚ /3000A ˚ ) contact pads with 75 mm diameter were deposited 300A using the same lift-off process. Contact to the unlapped top side of the substrate was made by bonding the top side onto an indium coated glass slide. The vertical I-V measurement was then made by probing the circular pad and the indium coated slide. Results and discussion: Fig. 1 shows the I-V characteristics of annealed Ti/Al/Au contacts on as-supplied and as-lapped backside surfaces. The rectifying electrical characteristic of the as-lapped sample suggests that the backside of the lapped GaN substrates suffered from severe damage during the mechanical lapping process. Fig. 2 shows the annealing temperature dependence of samples after fine polishing and ICP
–10
10 0 voltage, V
20
Fig. 1 I-V characteristics of Ti/Al/Au contacts on as-supplied and aslapped backside surfaces after annealing at 5008C for 5 min (TLM gap 10 um) 0.1
anneal 500°C 5min anneal 600°C 5min anneal 700°C 5min
0.05
0
20
Ω
Sample preparation: In this study, m-plane n-type GaN substrates were provided by Mitsubishi Chemical Corporation. The bulk carrier concentration was 5.5 1017cm23. Three groups of samples were made for comparison. The as-supplied substrates, serving as reference samples, were called group A. Other substrates were first thinned by 50 mm using 400 grit silicon carbide sandpaper, then an additional 50 mm was removed with 600 grit sandpaper. At this step, one set of samples, called group B, was left as-lapped without any further treatment. The lapped backsides of the remaining samples were subsequently polished by 3 and 1 mm diamond paper on a rotating plate to remove the deep damage and roughness caused by the sandpaper. The polished backsides of these samples were then etched 1 um by a Cl2-based inductively coupled plasma (ICP) treatment. During the ICP etching, the Cl2 flow rate was 37.5 sccm and N2 flow rate was 12.5 sccm with chamber pressure 9 mTorr, bias power 200 W over a 15 cm diameter platten, and ICP power of 500 W. The etch rate was 0.65 mm/min. The ICP-treated samples were labelled as group C. ICP etched samples were stored in methanol prior to metallisation, to prevent surface oxidation.
–0.002 –20
current, A
Introduction: Polarisation-free nonpolar GaN materials have attracted much attention owing to their potential for high-efficiency optoelectronic devices such as blue/green light emitting diodes and laser diodes [1, 2]. The success of these devices rests on the realisation of low resistance ohmic contacts. Although there have been many contact studies reported for c-plane n-type GaN [3– 5], little work has been reported for m-plane n-type GaN. For cleaved-facet GaN lasers, thinning substrates are required to make cleaved facets. Furthermore, laser structures with backside contacts are preferable for reduced heating and simpler packaging. However, after the thinning process, the backside surface suffers from lapping-induced damage, resulting in a poor contact. In this Letter, an effective approach to realise low resistance ohmic contacts on a lapped m-plane surface is presented.
10
–0.05 0
–0.1 –4
–2
0
0 voltage, V
10 20 30 gap, μm
2
40
4
Fig. 2 I-V characteristics of ICP-treated Ti/Al/Au contacts against annealing temperature (TLM gap 10 um) Inset: Total resistance as linear function of gap between contact pads after annealing at 7008C
For ICP-treated samples after annealing at 7008C for 5 min, the I-V curves of the vertical conduction measurement were also linear. This indicates that the Ti/Al/Au contact is ohmic and the indium contact to the topside of the substrate is also ohmic. For a two-terminal contact structure with a homogeneous semiconductor of resistivity r and thickness t, the total resistance RT ¼ V/I is given by RT ¼ Rc þ Rsp þ Rct þ Rp
ð1Þ
where Rc is the contact resistance of the backside contact, Rsp is the spreading resistance in the semiconductor, Rct is the contact resistance of the topside contact, and Rp is the probe resistance [6]. The topside contact Rct can usually be neglected because of the large contact area of the whole substrate. The probe resistance can be simply measured by probing two probes on the same metal pad. The spreading resistance Rsp of the circular backside contact of radius r on the surface of a semiconductor of resistivity r, thickness t and a large top contact can be approximated by [7] r 2t Rsp ¼ tan1 ð2Þ r 2p r The substrate resistivity was 3.8 1022Vcm measured by a bulk resistivity measurement; this value corresponded to an electron mobility of 300 cm2/Vs. The thickness of these samples was 200 mm. By subtracting Rsp and Rp from the measured total resistance RT and multiplying by the contact area, the specific contact resistance rc of 3.6 1025 Vcm2 was obtained, which is in good agreement with the specific contact resistance obtained by the TLM method. Ohmic contacts with specific contact resistance as low as 3.9 1025 Vcm2 were also achieved once after annealing at only 5008C for 5 min, using an ICP etch of 10 mm, without the diamond polishing steps. Optical microscopy revealed many deep pits remained in the surface, whereas the diamond polished samples appeared featureless. We speculate that the pits exposed other GaN crystal planes to the metal contact,
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allowing formation of ohmic contacts at lower annealing temperature. While we were unable to reproduce the result, this suggests the possibility of achieving ohmic n-contacts at temperatures more compatible with conventional post-growth fabrication processes. Conclusion: An effective surface treatment method has been developed to realise low resistance ohmic contacts on the lapped backsides of m-plane n-GaN substrates. The lowest specific contact resistance of 3.9 1025 Vcm2 was achieved from annealed contacts with fine polishing and an ICP treatment. Acknowledgments: The authors thank Y.-D. Lin for useful discussion. This work was supported by the DARPA Vigil programme, managed by M. Haney, and by the Solid State Lighting and Energy Center at the University of California, Santa Barbara. A portion of this work was done in the UCSB nanofabrication facility, part of the National Nanotechnology Infrastructure Network funded by the National Science Foundation.
References 1 Lin, Y.-D., Chakraborty, A., Brinkley, S., Kuo, H.C., Melo, T., Fujito, K., Speck, J.S., DenBaars, S.P., and Nakamura, S.: ‘Characterization of blue-green m-plane InGaN light emitting diodes’, Appl. Phys. Lett., 2009, 94, p. 261108 2 Okamoto, K., Kashiwagi, J., Tanaka, T., and Kubota, M.: ‘Nonpolar m-plane InGaN multiple quantum well laser diodes with a lasing wavelength of 499.8 nm’, Appl. Phys. Lett., 2009, 94, p. 071105 3 Smith, L.L., Davis, R.F., Liu, R.-J., Kim, M.J., and Carpenter, R.W.: ‘Microstructure, electrical properties, and thermal stability of Ti-based ohmic contacts to n-GaN’, J. Mater. Res., 1999, 14, pp. 1032–1038 4 Mohammad, S.N.: ‘Contact mechanisms and design principles for nonalloyed ohmic contacts to n-GaN’, J. Appl. Phys., 2004, 95, pp. 4856–4865 5 Wang, L., Mohammed, F.M., and Adesida, I.: ‘Characterization of Au and Al segregation layer in post-annealed thin Ti/Al/Mo/Au ohmic contacts to n-GaN’, J. Appl. Phys., 2005, 98, p. 106105 6 Schroder, D.K.: ‘Semiconductor material and device characterization’ (Wiley, New Jersey, 2005), pp. 135– 136 7 Cox, R.H., and Strack, H.: ‘Ohmic contacts for GaAs devices’, SolidState Electron., 1967, 10, pp. 1213–1218
# The Institution of Engineering and Technology 2010 23 October 2009 doi: 10.1049/el.2010.2976 Y.-A. Chen, D.A. Cohen and S.P. DenBaars (Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA) E-mail:
[email protected]
ELECTRONICS LETTERS 21st January 2010 Vol. 46 No. 2
