ZrB2-based Ohmic contacts to p-GaN

ZrB2-based Ohmic contacts to p-GaN

Applied Surface Science 253 (2006) 1934–1938 www.elsevier.com/locate/apsusc ZrB2-based Ohmic contacts to p-GaN Lars Voss a, S.J. Pearton a,*, F. Ren ...

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Applied Surface Science 253 (2006) 1934–1938 www.elsevier.com/locate/apsusc

ZrB2-based Ohmic contacts to p-GaN Lars Voss a, S.J. Pearton a,*, F. Ren b, I.I. Kravchenko c a

Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA b Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA c Department of Physics, University of Florida, Gainesville, FL 32611, USA Received 16 December 2005; accepted 15 March 2006 Available online 27 April 2006

Abstract The annealing temperature dependence of contact resistance and layer stability of ZrB2/Ti/Au and Ni/Au/ZrB2/Ti/Au Ohmic contacts on p-GaN is reported. The as-deposited contacts are rectifying and transition to Ohmic behavior for annealing at 750 8C, a significant improvement in thermal stability compared to the conventional Ni/Au Ohmic contact on p-GaN, which is stable only to <600 8C. A minimum specific contact resistance of 2  10 3 V cm 2 was obtained for the ZrB2/Ti/Au after annealing at 800 8C while for Ni/Au/ZrB2/Ti/Au the minimum value was 10 4 V cm 2 at 900 8C. Auger Electron Spectroscopy profiling showed significant Ti, Ni and Zr out diffusion at 750 8C in the Ni/Au/ZrB2/Ti/Au while the Ti and Zr intermix at 900 8C in the ZrB2/Ti/Au. These boride-based contacts show promise for contacts to p-GaN in high temperature applications. # 2006 Elsevier B.V. All rights reserved. Keywords: ZnO; Ohmic contacts

1. Introduction Thermally stable low resistance Ohmic contacts on p-GaN are desirable for reliable AlGaN/InGaN heterojunction bipolar transistors (HBTs) [1–4], laser diodes and light-emitting diodes (LEDs) [5–11]. If the contact resistance is high, the reliability of the device suffers because of self-heating of the contacts at high current injection levels. In the structures listed above with thin p-layers, this self-heating may produce spiking of the contact metals through the junction. The normal metallizations used for making Ohmic contacts to p-GaN HBTs, lasers and LEDs are based on metals such as Ni, Pd, Cr or Pt with overlayers of Au to reduce sheet resistance. Typical contact resistances of 10 2 to 10 3 V cm2 are obtained by annealing at 450–650 8C to depassivate residual Mg–H complexes and to form low resistance interfacial phases [11–36]. Another issue is the stability of the contacts during device packaging and operation. The Ni/Au metallization has stability problems above 500 8C. Some of the reported candidates for more

thermally stable p-Ohmic metallization have included Rh [23], Pd [25], W [36] or Zn–Ni and Ni–Mg solid solution schemes [20]. In this paper we report on the properties of ZrB2/Ti/Au and Ni/Au/ZrB2/Ti/Au Ohmic contacts on p-GaN. The ZrB2 has a high melting temperature (3200 8C), excellent thermodynamic stability, high thermal conductivity (80 W m 1 K 1) and high work function (4 eV) [37]. Since the electron affinity of GaN is 4.1 eV, we would expect a low intrinsic barrier height for the compound on p-GaN. Boride compounds generally have good corrosion resistance but are reported to be susceptible to oxidation during annealing [37]. One solution to this issue is to deposit an overlayer of a metal such as Au in the same deposition chamber. In our case, we have examined the use of direct ZrB2 contacts to the p-GaN with overlayers of Ti/Au and also the use of ZrB2 as a diffusion barrier in the normal Ni/Au scheme, where we have added Ti/ Au overlayers. 2. Experimental

* Corresponding author. Tel.: +1 35 2846 1086; fax: +1 35 2846 1182. E-mail address: [email protected] (S.J. Pearton). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.03.041

The samples used for the contact studies were 0.3 mm thick Mg-doped GaN layers grown by metal organic chemical vapor

L. Voss et al. / Applied Surface Science 253 (2006) 1934–1938

Fig. 1. Specific contact resistance of Ni/Au/ZrB2/Ti/Au and ZrB2/Ti/Au Ohmic contacts and p-GaN sheet resistance under the contact as a function of annealing temperature.

deposition on 2 mm thick undoped buffers on c-plane Al2O3 substrates. The hole concentration obtained from Hall measurements after acceptor activation annealing was 1017 cm 3. The surfaces were cleaned by sequential rinsing in acetone and isopropanol prior to insertion in the sputtering ˚ )/ chamber. The first metallization scheme was Ni(500 A

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˚ )/Ti(200 A ˚ )/Au(500 A ˚ ). The Au was ˚ )/ZrB2(500 A Au(800 A added to lower the contact sheet resistance, while the ZrB2 and Ti are diffusion barriers, with the former being the most ˚ )/Ti(200 A ˚ )/ stable. The second scheme consisted of ZrB2(500 A ˚ Au(500 A). In this case we are trying to establish the intrinsic contact properties of the ZrB2 but include the overlayers to reduce sheet resistance. All of the metals or compounds were deposited by Ar plasma-assisted rf sputtering at pressures of 15–40 mTorr and rf (13.56 MHz) powers of 200–250 W. The ˚ s 1 for Au, 1.8 A ˚ s 1 for Ni, and 1 A ˚ s 1 sputter rates were 5 A for both ZrB2 and Ti. The contacts were patterned by liftoff of lithographically defined photoresist and separate samples were annealed at temperatures up to 1000 8C for 1 min in a flowing N2 ambient in a RTA furnace. We used a transmission line method (TLM) geometry to obtain the contact resistance, with the contact pads isolated by mesas formed by Cl2/Ar dry etching. The linear TLM patterns consisted of square contact pads of dimension 100 mm separated by different spacing. Current–voltage (I–V) characteristics of the contact pads were measured using a probe station and Agilent 4145B parameter analyzer. Auger Electron Spectroscopy (AES) depth profiling of the as-deposited contacts showed sharp interfaces between the various metals. The AES system was a Physical Electronics

Fig. 2. AES surface scans and depth profiles of Ni/Au/TiB2/Ti/Au Ohmic contacts on p-GaN as a function of anneal temperature.

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660 Scanning Auger Microprobe. The electron beam conditions were 10 keV, 1 mA beam current at 308 from sample normal. Charge correction was performed by using the known position of the C–(C,H) line in the C 1s spectra at 284.8 eV. The AES spectrometer was calibrated using a polycrystalline Au foil. The Au f7/2 peak position was determined to be 84.00  0.02. For depth profiling, the ion beam conditions were 3 keV Ar+, 2.0 mA, (3 mm)2 raster. The quantification of the elements was accomplished by using the elemental sensitivity factors. We also used scanning electron microscopy (SEM) to examine contact morphology as a function of annealing temperature. 3. Results and discussion Fig. 1 shows the specific contact resistance of the ZrB2/Ti/ Au and Ni/Au/ZrB2/Ti/Au/p-GaN structures as a function of annealing temperature, along with the sheet resistance of the pGaN under the contacts, extracted from the TLM measurements. The contacts are rectifying below an anneal temperature of 750 8C but transition to Ohmic behavior for higher temperatures. The Ni/Au/ZrB2/Ti/Au contact resistance improves with higher anneal temperature, at the expense of poorer morphology. The ZrB2/Ti/Au contacts do not show low resistance values until 800 8C and the value degrades at higher temperatures. The associated sheet resistance generally decreases with temperature. The specific contact resistance values obtained with both types of ZrB2-based schemes is comparable to that achieved on the same samples with Ni/Au metallization annealed at 500 8C but the former have much higher thermal stability, as expected from the high melting temperature of the ZrB2. Fig. 2 shows the AES surface scans and depth profiles from the as-deposited Ni/Au/ZrB2/Ti/Au/p-GaN sample (left) and from the sample annealed at 750 8C (right). Ti, Ni and Zr are evident on the surface after annealing at 750 8C and the concentration increased with annealing temperature. The asdeposited sample shows sharp interfaces between the metals and between the Ni and the GaN. Table 1 summarizes the near-surface composition AES data. The increased oxygen concentration evident from this data most likely comes from oxidation of the out-diffused Ti. The carbon signal in all cases comes from adventitious carbon on the surface. After annealing at 750 8C, the Ni shows significant movement through all of the overlying layers and even the Zr diffuses out of the boride later, leaving the boron in its original location. Both Zr and Au diffuse to the interface with GaN.

Fig. 3. SEM images of Ni/Au/TiB2/Ti/Au contact pads on p-GaN as-deposited (top) or after annealing at either 750 (center) or 800 8C (bottom).

Table 1 Concentration of elements detected on the as-received surfaces (in atom%) Sample ID

C(1)

O(1)

Ti(1)

Ni(1)

Zr(2)

Au(3)

Sensitivity factors As-deposited Ni/Au/ZrB2/Ti/Au Annealed 750 8C Annealed 800 8C As-deposited ZrB2/Ti/Au Annealed 800 8C Annealed 1000 8C

0.076 44 45 42 37 43 48

0.212 4 19 22 4 23 19

0.188 nd 2 9 nd nd nd

0.227 nd 1 1 nd nd nd

0.043 nd 14 16 nd 12 14

0.049 52 19 10 59 22 19

L. Voss et al. / Applied Surface Science 253 (2006) 1934–1938

Fig. 4. AES surface scans and depth profiles of ZrB2/Ti/Au Ohmic contacts on p-GaN as a function of anneal temperature.

Fig. 5. Elemental maps obtained from scanning AES of ZrB2/Ti/Au Ohmic contacts pads on p-GaN.

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Omiya et al. [33] have shown from detailed transmission electron microscopy studies that the initial Au/Ni/GaN structure transforms after annealing at >400 8C to Ni/Au/ GaN with an abrupt interface between the Au and the GaN and the formation of a clean interface is critical to the formation of an Ohmic contact. The increased contact intermixing at higher annealing temperatures did roughen the contact morphology as shown in the SEM pictures of the TLM contact pads as a function of annealing temperature in Fig. 3. After 800 8C annealing the morphology is degraded, but is still much smoother than conventional Ni/Au contacts under these conditions. The darker appearance of the contacts after high temperature anneals is mainly a result of the out diffusion of Ti, which then oxidizes. The same basic trends were observed with the ZrB2/Ti/Au structures. Fig. 4 shows the AES surface scans and surface scans of ZrB2/Ti/Au Ohmic contacts on p-GaN as a function of anneal temperature. The as-deposited sample shows abrupt interfaces but after annealing at 900 8C, the Zr shows a very broad distribution (more so than the B). The contacts after annealing showed the presence of reacted islands, as shown in the elemental maps of Fig. 5. The islands contain both Au and Ga and show that the GaN epi layer has begun to dissociate at this temperature, at least under the contact metallurgy. 4. Summary and conclusions The use of ZrB2 layers either as diffusion barriers in Ni/Aubased Ohmic contacts to p-GaN or as direct Ohmic contacts with Ti/Au overlayers improves the thermal stability relative to conventional Ni/Au. We achieved minimum specific contact resistances in the range of 2  10 3 to 10 4 V m2 for annealing temperatures of 800–900 8C. This contact metallurgy looks promising for high temperature applications where improved stability over Ni/Au is required. Acknowledgments The work at UF is partially supported by the Army Research Office under grant no. DAAD19-01-1-0603, NSF (CTS0301178, monitored by Dr. M. Burka and Dr. D. Senich) and the National Science Foundation (DMR 0400416, Dr. L. Hess). References [1] K. Kumakura, T. Makimoto, Appl. Phys. Lett. 86 (2005) 023506. [2] H. Xing, P.M. Chavarkar, S. Keller, S.P. DenBaas, U.K. Mishra, IEEE Electron Device Lett. EDL-24 (2003) 141. [3] T. Makimoto, Y. Yamauchi, K. Kumakura, Appl. Phys. Lett. 84 (2004) 1964. [4] K. Kumakura, T. Makimoto, N. Kobayashi, Appl. Phys. Lett. 80 (2002) 3841.

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