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Au ohmic contacts on n-GaN
Applied Surface Science 253 (2006) 2340–2344 www.elsevier.com/locate/apsusc
Stability of Ti/Al/ZrB2/Ti/Au ohmic contacts on n-GaN R. Khanna 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 24 February 2006; accepted 21 April 2006 Available online 26 May 2006
Abstract Ohmic contacts on n-GaN using a novel Ti/Al/ZrB2/Ti/Au metallization scheme were studied using contact resistance, scanning electron microscopy and Auger electron spectroscopy (AES) measurements. A minimum specific contact resistivity of 3 10 6 V cm2 was achieved at an annealing temperature of 700 8C. The lowest contact resistance was obtained for 60 s anneals. The contact resistance was essentially independent of measurement temperature, indicating that field emission plays a dominant role in the current transport. The Ti and Al in the contact stack began to outdiffuse to the surface at temperatures of 500 8C, while at 1000 8C the B also began to migrate to the surface. By this latter temperature, AES showed almost complete intermixing of the metallization even though the contact morphology was still smooth. The boride appears susceptible to getting of residual water vapor during sputter deposition. # 2006 Elsevier B.V. All rights reserved. Keywords: GaN; Ohmic contacts
1. Introduction One of the major remaining needs for commercialization of AlGaN/GaN high electron mobility transistors (HEMTs) power amplifiers is reliable and thermally stable ohmic contacts for source and drain electrodes [1–33]. The intended applications for these power amplifiers include uncooled radar and communication systems operating over a broad frequency range from S-band to V-band [1–3]. The standard Ohmic metallization for AlGaN/GaN HEMTs is based on Ti/Al. This bilayer must be deposited with over-layers of Ni, Ti or Pt, followed by Au to reduce sheet resistance and decrease oxidation during the high temperature anneal needed to achieve the lowest specific contact resistivity [4–24]. For improving the thermal stability of Ohmic contacts, there is interest in higher melting temperature metals, including W [25,26,28,33], WSiX [25–27], Mo [4], V [27] and Ir [28,30,31]. Another promising metallization system as the diffusion barrier layer is based on borides of Cr, Zr, Hf, Ti or W [34–38]. Stoichiometric diborides have high melting temperatures (e.g. 3200 8C for ZrB2) and
thermodynamic stabilities at least as good as comparable nitrides or silicides [39]. One attractive option is ZrB2, which has a low resistivity in the range 7–10 mV cm. Recently, it was shown that hexagonal ZrB2 (0001) single crystals have an inplane lattice constant close to that of GaN, prompting efforts at GaN heteroepitaxy on ZrB2 or buffer layers on Si substrates. For contacts on n-type GaN, the only related work is the study of ZrN/Zr/n-GaN Ohmic structures [39] in which the Zr/GaN interface was found to have excellent thermal stability. Even though we are not relying on the ZrB2 to make direct ohmic contact to GaN, it is expected that ZrB2 contacts on GaN will have low barrier heights, given the work function of ZrB2 is 3.94 eV and the electron affinity of GaN is 4.1 eV. In this paper, we report on the annealing temperature and time dependence of contact resistance and contact intermixing of Ti/Al/ZrB2/Ti/Au on n-GaN. These contacts show excellent minimum contact resistance of 3 10 6 V cm2 after annealing at 700 8C and retain a good morphology even after annealing at 1000 8C. 2. Experimental
* Corresponding author. Tel.: +3528461086; fax: +3528461182. 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.2006.04.042
The samples used were 3 mm thick Si-doped GaN grown by metal organic chemical vapor deposition on c-plane Al2O3
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substrates. The electron concentration obtained from Hall measurements was 7 1018 cm 3. Mesas 1.8 mm deep were formed by Cl2/Ar inductively coupled plasma etching to provide electrical isolation of the contact pads. A metalliza˚ )/Al(1000 A ˚ )/ZrB2 (500 A ˚ )/ tion scheme of Ti(200 A ˚ ˚ Ti(200 A)/Au(800 A) was used in these experiments. All of the metals 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 contacts were patterned by liftoff and annealed at 500–1000 8C for 0.5–1.5 min in a flowing N2 ambient in a RTA furnace. Auger electron spectroscopy (AES) depth profiling of the as-deposited contacts showed sharp interfaces between the various metals in both types of contacts. The AES system was a Physical Electronics 660 Scanning Auger Microprobe. The electron beam conditions were 10 keV, 1 mA beam current at 308 from sample normal. 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. The contact properties were obtained from linear transmission line method (TLM) measurements on 100 100 mm pads with spacing 5, 10, 20, 40, and 80 mm. The contact resistance RC was obtained from the relation [34] RC = (RT rSd/Z)/2, where RT is the total resistance between two pads, rS is the sheet resistivity of the semiconductor under the contact, d is the pad spacing, and Z is the contact width. The specific contact resistance, rC, is then obtained from rC = RCLTZ, where LT is the transfer length obtained from the intercept of a plot of RT versus d. 3. Results and discussion Fig. 1 (top) shows the contact resistance as a function of annealing temperature, while the associated GaN sheet resistance under the contact is shown at the bottom of the figure. The as-deposited contacts were rectifying, with a transition to ohmic behavior for anneal temperatures 500 8C.The contact resistance decreased up to 700 8C,reaching a minimum value of 3 10 6 V cm2.This same basic trend is seen in most Ti/Al-based contacts [7–10] and is attributed to formation of low resistance phases of TiN at the interface with the GaN. By comparison with the usual Ti/Al/ Pt/Au metal stack, the ZrB2-based contacts show improved edge acuity, an important factor for small gate length HEMTs in order to reduce the possibility of shorting of the Ohmic metal to the gate. The ZrB2-based contacts show a double minimum in contact resistance versus annealing temperature and even at 1000 8C exhibit a contact resistance below 10 5 V cm2. Fig. 2 shows the specific contact resistivity (top) and sheet resistance (bottom) as a function of anneal time at 700 8C for Ti/Al/ZrB2/Ti/Au on n-GaN. The minimum contact resistance is achieved for 60 s anneals. This is also consistent with the need to form a low resistance interfacial phase whose formation kinetics are probably limited by diffusion of Ti to the GaN interface. Fig. 3 shows the measurement temperature dependence of the Ti/Al/ZrB2/Ti/Au on n-GaN annealed at 700 8C. We did not observe any significant temperature dependence,
Fig. 1. Specific contact resistivity (top) and sheet resistance (bottom) vs. anneal temperature for Ti/Al/ZrB2/Ti/Au on n-GaN.
suggesting that at this anneal temperature the dominant current transport mechanism is field emission [40]. Fig. 4 shows scanning electron microscopy (SEM) images of the as-deposited contact morphology and after annealing at 500,700 or 1000 8C. Even at the highest anneal temperature, the morphology remains quite smooth on the scale accessible to the SEM. This is in sharp contrast to the case of Ti/Al/Pt/Au, where significant roughening occurs above 800 8C and this result suggests that the ZrB2 is an effective barrier for reducing intermixing of the contact compared to Pt. Fig. 5 shows the AES surface scans as a function of anneal temperature. As expected, only carbon, oxygen and gold were detected on the as-deposited surface. The carbon is adventitious and the oxygen comes from a thin native oxide on the Au. After annealing at 500 8C, titanium and aluminum was detected on the contact surface and their concentrations increased at higher annealing temperatures. The surface concentration of gold decreased with increasing temperature of the annealing step. After 1000 8C annealing, Boron was also present on the surface, having outdiffused from the buried ZrB2 layer. Table 1
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Fig. 3. Specific contact resistance versus measurement temperature for Ti/Al/ ZrB2/Ti/Au on n-GaN annealed at 800 8C.
Fig. 2. Specific contact resistivity (top) and sheet resistance (bottom) as a function of anneal time at 700 8C for Ti/Al/ZrB2/Ti/Au on n-GaN.
summarizes the near-surface composition data obtained from AES measurements. Fig. 6 shows the AES depth profiles for the as-deposited and annealed samples. The profile obtained from the as-deposited sample was in good agreement with the prescribed metal layer thicknesses. Oxygen was detected in the ZrB2 layer, consistent with past observations that the borides are excellent getters for water vapor during deposition [38]. The profile obtained from the sample annealed at 500 8C shows extensive diffusion of titanium through the gold layer to the surface. The intermixing of the contact metallurgy becomes more pronounced as the annealing temperature is increased. Some of the transport might be attributed to grain-boundary diffusion, as reported pre-
Fig. 4. Secondary electron images of the Ti/Al/ZrB2/Ti/Au on n-GaN as-deposited (top left) and after annealing at 500 (top right), 700 (bottom left) or 1000 8C (bottom right).
R. Khanna et al. / Applied Surface Science 253 (2006) 2340–2344
Fig. 5. AES surface scans of the Ti/Al/ZrB2/Ti/Au on n-GaN as-deposited and after annealing at 500, 700 or 1000 8C.
Fig. 6. AES depth profiles of the Ti/Al/ZrB2/Ti/Au on n-GaN as-deposited and after annealing at 500, 700 or 1000 8C.
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Table 1 Near-surface composition of contact stack determined by AES measurements Sample ID
C(1)
O(1)
Al(1)
S(1)
Ti(2)
B(2)
Au(3)
Sensitivity factors 1. As-deposited 2. Annealed at 500 8C 3. Annealed at 700 8C 4. Annealed at 1000 8C
[0.076] 48 41 36 28
[0.212] 1 22 29 33
[0.000] nd 16 22 18
[0.652] 2 <1 <1 1
[0.000] nd 3 5 4
[0.000] nd nd nd 15
[0.049] 49 17 8 nd
viously for Mo-based contacts to AlGaN/GaN heterostructures [27]. As noted earlier, the contact morphology does not degrade significantly even after 1000 8C annealing. 4. Summary and conclusions A novel Ti/Al/ZrB2/Ti/Au metallization scheme was used to form ohmic contacts to n-type GaN. A minimum specific contact resistivity of 3 10 6 V cm2 was achieved at an annealing temperature of 700 8C, which is comparable to that achieved with conventional Ti/Al/Pt/Au on the same samples. The ZrB2-based contact appears to have greater thermal stability than the conventional metallization and may have application in power amplifiers if the reliability can be proven. Acknowledgments The work at UF is partially supported by AFOSR (F4962002-1-0366, G. Witt), ONR (N00014-98-1-02-04, H. B. Dietrich) and NSF DMR 0101438. References [1] J. Sun, H. Fatima, A. Koudymov, A. Chitnis, X. Hu, H.-M. Wang, J. Zhang, G. Simin, J. Yang, M.A. Khan, IEEE Electron. Device Lett. 24 (2003) 375. [2] A.P. Zhang, L.B. Rowland, E.B. Kaminsky, J.W. Kretchmer, R.A. Beaupre, J.L. Garrett, J.B. Tucker, B.J. Edward, J. Foppes, A.F. Allen, SolidState Electron. 47 (2003) 821. [3] A.P. Zhang, L.B. Rowland, E.B. Kaminsky, V. Tilak, J.C. Grande, J. Teetsov, A. Vertiatchikh, L.F. Eastman, J. Electron. Mater. 32 (2003) 388. [4] V. Kumar, L. Zhou, D. Selvanathan, I. Adesida, J. Appl. Phys. 92 (2002) 1712. [5] Abhishek Motayed, Ravi Bathe, Mark C. Wood, S. Ousmane, R.D. Diouf, S. Vispute, Noor Mohammad, J. Appl. Phys. 93 (2003) 1087. [6] Jinwook Burm, Kenneth Chu, William A. Davis, J. William Schaff, Lester F. Eastman, Tyler J. Eustis, Appl. Phys. Lett. 70 (1997) 464. [7] M.E. Lin, Z. Ma, F.Y. Huang, Z.F. Fan, L.H. Allen, H. Morkoc¸, Appl. Phys. Lett. 64 (1994) 1003. [8] B.P. Luther, S.E. Mohney, T.N. Jackson, M. Asif Khan, Q. Chen, J.W. Yang, Appl. Phys. Lett. 70 (1997) 57. ¨ zgu¨r Aktas, Andrei E. [9] S. Zhifang Fan, Noor Mohammad, Wook Kim, O Botchkarev, Hadis Morkoc¸, Appl. Phys. Lett. 68 (1996) 1672. [10] S. Ruvimov, Z. Liliental-Weber, J. Washburn, D. Qiao, S.S. Lau, Paul K. Chu, Appl. Phys. Lett. 73 (1998) 2582. [11] A.N. Bright, P.J. Thomas, M. Weyland, D.M. Tricker, C.J. Humphreys, R. Davies, J. Appl. Phys. 89 (2001) 3143. [12] N.A. Papanicolaou, M.V. Rao, J. Mittereder, W.T. Anderson, J. Vac. Sci. Technol. B 19 (2001) 261. [13] Q.Z. Liu, S.S. Lau, Solid-State Electron. 42 (1998) 677–691.
[14] R. Li, S.J. Cai, L. Wong, Y. Chen, K.L. Wang, R.P. Smith, S.C. Martin, K.S. Boutros, J.M. Redwing, IEEE Electron. Device Lett. 20 (1999) 323. [15] S.T. Sheppard, K. Doverspike, W.L. Pribble, S.T. Allen, J.W. Palmour, L.T. Kehias, T.J. Jenkins, IEEE Electron. Device Lett. 20 (1999) 161–163. [16] D. Qiao, Z.F. Guan, J. Carlton, S.S. Lau, G.J. Sullivan, Appl. Phys. Lett. 74 (1999) 2652. [17] V. Tilak, R. Dimitov, M. Murphy, B. Green, J. Smart, W.J. Schaft, J.R. Shealy, L.F. Eastman, Mater. Res. Soc. Symp. Proc. 622 (2000), T7.4.1. [18] S.-H. Lim, J. Washburn, Z. Liliental-Weber, D. Qiao, Appl. Phys. Lett. 78 (2001) 3797. [19] M. Asif Khan, M.S. Shur, Q. Chen, Appl. Phys. Lett. 68 (1996) 3022. [20] S. Murai, H. Masuda, Y. Koide, Masanori Murakami, Appl. Phys. Lett. 80 (2002) 2934. [21] Changzhi Lu, Hongnai Chen, Xiaoliang Lv, S. Xuesong Xie, Noor Mohammad, J. Appl. Phys. 91 (2002) 9218. [22] J.S. Foresi, T.D. Moustakus, Appl. Phys. Lett. 62 (1993) 2859. [23] M.W. Fay, G. Moldovan, P.D. Brown, I. Harrison, J.C. Birbeck, B.T. Hughes, M.J. Uren, T. Martin, J. Appl. Phys. 92 (2002) 94. [24] K.O. Schweitz, P.K. Wang, S.E. Mohney, D. Gotthold, Appl. Phys. Lett. 80 (2002) 1954. [25] M.W. Cole, D.W. Eckart, W.Y. Han, R.L. Pfeffer, T. Monahan, F. Ren, C. Yuan, R.A. Stall, S.J. Pearton, Y. Li, Y. Lu, J. Appl. Phys. 80 (1996) 278. [26] A. Zeitouny, M. Eizenberg, S.J. Pearton, F. Ren, J. Appl. Phys. 88 (2000) 2048. [27] D. Selvanathan, F.M. Mohammed, A. Tesfayesus, I. Adesida, J. Vac. Sci. Technol. B22 (2004) 2409. [28] B. Luo, F. Ren, R.C. Fitch, J.K. Gillespie, T. Jenkins, J. Sewell, D. Via, A. Crespo, A.G. Baca, R.D. Briggs, D. Gotthold, R. Birkhahn, B. Peres, S.J. Pearton, Appl. Phys. Lett. 82 (2003) 3910. [29] H.W. Jang, J.-L. Lee, J. Appl. Phys. 93 (2003) 5416. [30] R.C. Fitch, J.K. Gillespie, N. Moser, T. Jenkins, J. Sewell, D. Via, A. Crespo, A.M. Dabiran, P.P. Chow, A. Osinsky, J.R. LaRoche, F. Ren, S.J. Pearton, Appl. Phys. Lett. 84 (2004) 1495. [31] R.C. Fitch, J.K. Gillespie, N. Moser, T. Jenkins, J. Sewell, D. Via, A. Crespo, A.M. Dabiran, P.P. Chow, A. Osinsky, J.R. LaRoche, F. Ren, S.J. Pearton, J. Vac. Sci. Technol. B22 (2004) 619. [32] D. Selvanathan, L. Zhou, V. Kumar, I. Adesida, N. Finnegan, J. Electron. Mater. 32 (2003) 335. [33] X.A. Cao, S.J. Pearton, G. Dang, A.P. Zhang, F. Ren, J.M. Van Hove, Appl. Phys. Lett. 75 (1999) 4130. [34] L. Voss, R. Khanna, S.J. Pearton, F. Ren, I.I. Kravchenko, Appl. Phys. Lett. 88 (2006) 012104. [35] R. Khanna, S.J. Pearton, F. Ren, I. Kravchenko, C.J. Kao, G.C. Chi, Appl. Surf. Sci. 252–255 (2005) 1846. [36] S.J. Rohit Khanna, F. Ren Pearton, I.I. Kravchenko, J. Electrochem. Soc. 11 (2005) G804. [37] R. Khanna, S.J. Pearton, F. Ren, I.I. Kravchenko, C.J. Kao, G.C. Chi, Appl. Phys. Lett. 87 (2005) 052110. [38] W. Zagozdzon-Wosik, C. Darne, D. Radhakrishnan, I. Rusakova, P. Van der Heide, Z.H. Zhang, J. Bennett, L. Trombetta, P. Majhi, D. Matron, Rev. Adv. Mater. Sci. 8 (2004) 185. [39] S.D. Wolter, B.P. Luther, S.E. Mohney, R. Karlicek, R.S. Kern, Electrochem. Solid-State Lett. 2 (1999) 151. [40] F.A. Padovani, R. Stratton, Solid-State Electron. 9 (1966) 695.
Stability of Ti/Al/ZrB2/Ti/Au ohmic contacts on n-GaN R. Khanna 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 24 February 2006; accepted 21 April 2006 Available online 26 May 2006
Abstract Ohmic contacts on n-GaN using a novel Ti/Al/ZrB2/Ti/Au metallization scheme were studied using contact resistance, scanning electron microscopy and Auger electron spectroscopy (AES) measurements. A minimum specific contact resistivity of 3 10 6 V cm2 was achieved at an annealing temperature of 700 8C. The lowest contact resistance was obtained for 60 s anneals. The contact resistance was essentially independent of measurement temperature, indicating that field emission plays a dominant role in the current transport. The Ti and Al in the contact stack began to outdiffuse to the surface at temperatures of 500 8C, while at 1000 8C the B also began to migrate to the surface. By this latter temperature, AES showed almost complete intermixing of the metallization even though the contact morphology was still smooth. The boride appears susceptible to getting of residual water vapor during sputter deposition. # 2006 Elsevier B.V. All rights reserved. Keywords: GaN; Ohmic contacts
1. Introduction One of the major remaining needs for commercialization of AlGaN/GaN high electron mobility transistors (HEMTs) power amplifiers is reliable and thermally stable ohmic contacts for source and drain electrodes [1–33]. The intended applications for these power amplifiers include uncooled radar and communication systems operating over a broad frequency range from S-band to V-band [1–3]. The standard Ohmic metallization for AlGaN/GaN HEMTs is based on Ti/Al. This bilayer must be deposited with over-layers of Ni, Ti or Pt, followed by Au to reduce sheet resistance and decrease oxidation during the high temperature anneal needed to achieve the lowest specific contact resistivity [4–24]. For improving the thermal stability of Ohmic contacts, there is interest in higher melting temperature metals, including W [25,26,28,33], WSiX [25–27], Mo [4], V [27] and Ir [28,30,31]. Another promising metallization system as the diffusion barrier layer is based on borides of Cr, Zr, Hf, Ti or W [34–38]. Stoichiometric diborides have high melting temperatures (e.g. 3200 8C for ZrB2) and
thermodynamic stabilities at least as good as comparable nitrides or silicides [39]. One attractive option is ZrB2, which has a low resistivity in the range 7–10 mV cm. Recently, it was shown that hexagonal ZrB2 (0001) single crystals have an inplane lattice constant close to that of GaN, prompting efforts at GaN heteroepitaxy on ZrB2 or buffer layers on Si substrates. For contacts on n-type GaN, the only related work is the study of ZrN/Zr/n-GaN Ohmic structures [39] in which the Zr/GaN interface was found to have excellent thermal stability. Even though we are not relying on the ZrB2 to make direct ohmic contact to GaN, it is expected that ZrB2 contacts on GaN will have low barrier heights, given the work function of ZrB2 is 3.94 eV and the electron affinity of GaN is 4.1 eV. In this paper, we report on the annealing temperature and time dependence of contact resistance and contact intermixing of Ti/Al/ZrB2/Ti/Au on n-GaN. These contacts show excellent minimum contact resistance of 3 10 6 V cm2 after annealing at 700 8C and retain a good morphology even after annealing at 1000 8C. 2. Experimental
* Corresponding author. Tel.: +3528461086; fax: +3528461182. 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.2006.04.042
The samples used were 3 mm thick Si-doped GaN grown by metal organic chemical vapor deposition on c-plane Al2O3
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substrates. The electron concentration obtained from Hall measurements was 7 1018 cm 3. Mesas 1.8 mm deep were formed by Cl2/Ar inductively coupled plasma etching to provide electrical isolation of the contact pads. A metalliza˚ )/Al(1000 A ˚ )/ZrB2 (500 A ˚ )/ tion scheme of Ti(200 A ˚ ˚ Ti(200 A)/Au(800 A) was used in these experiments. All of the metals 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 contacts were patterned by liftoff and annealed at 500–1000 8C for 0.5–1.5 min in a flowing N2 ambient in a RTA furnace. Auger electron spectroscopy (AES) depth profiling of the as-deposited contacts showed sharp interfaces between the various metals in both types of contacts. The AES system was a Physical Electronics 660 Scanning Auger Microprobe. The electron beam conditions were 10 keV, 1 mA beam current at 308 from sample normal. 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. The contact properties were obtained from linear transmission line method (TLM) measurements on 100 100 mm pads with spacing 5, 10, 20, 40, and 80 mm. The contact resistance RC was obtained from the relation [34] RC = (RT rSd/Z)/2, where RT is the total resistance between two pads, rS is the sheet resistivity of the semiconductor under the contact, d is the pad spacing, and Z is the contact width. The specific contact resistance, rC, is then obtained from rC = RCLTZ, where LT is the transfer length obtained from the intercept of a plot of RT versus d. 3. Results and discussion Fig. 1 (top) shows the contact resistance as a function of annealing temperature, while the associated GaN sheet resistance under the contact is shown at the bottom of the figure. The as-deposited contacts were rectifying, with a transition to ohmic behavior for anneal temperatures 500 8C.The contact resistance decreased up to 700 8C,reaching a minimum value of 3 10 6 V cm2.This same basic trend is seen in most Ti/Al-based contacts [7–10] and is attributed to formation of low resistance phases of TiN at the interface with the GaN. By comparison with the usual Ti/Al/ Pt/Au metal stack, the ZrB2-based contacts show improved edge acuity, an important factor for small gate length HEMTs in order to reduce the possibility of shorting of the Ohmic metal to the gate. The ZrB2-based contacts show a double minimum in contact resistance versus annealing temperature and even at 1000 8C exhibit a contact resistance below 10 5 V cm2. Fig. 2 shows the specific contact resistivity (top) and sheet resistance (bottom) as a function of anneal time at 700 8C for Ti/Al/ZrB2/Ti/Au on n-GaN. The minimum contact resistance is achieved for 60 s anneals. This is also consistent with the need to form a low resistance interfacial phase whose formation kinetics are probably limited by diffusion of Ti to the GaN interface. Fig. 3 shows the measurement temperature dependence of the Ti/Al/ZrB2/Ti/Au on n-GaN annealed at 700 8C. We did not observe any significant temperature dependence,
Fig. 1. Specific contact resistivity (top) and sheet resistance (bottom) vs. anneal temperature for Ti/Al/ZrB2/Ti/Au on n-GaN.
suggesting that at this anneal temperature the dominant current transport mechanism is field emission [40]. Fig. 4 shows scanning electron microscopy (SEM) images of the as-deposited contact morphology and after annealing at 500,700 or 1000 8C. Even at the highest anneal temperature, the morphology remains quite smooth on the scale accessible to the SEM. This is in sharp contrast to the case of Ti/Al/Pt/Au, where significant roughening occurs above 800 8C and this result suggests that the ZrB2 is an effective barrier for reducing intermixing of the contact compared to Pt. Fig. 5 shows the AES surface scans as a function of anneal temperature. As expected, only carbon, oxygen and gold were detected on the as-deposited surface. The carbon is adventitious and the oxygen comes from a thin native oxide on the Au. After annealing at 500 8C, titanium and aluminum was detected on the contact surface and their concentrations increased at higher annealing temperatures. The surface concentration of gold decreased with increasing temperature of the annealing step. After 1000 8C annealing, Boron was also present on the surface, having outdiffused from the buried ZrB2 layer. Table 1
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Fig. 3. Specific contact resistance versus measurement temperature for Ti/Al/ ZrB2/Ti/Au on n-GaN annealed at 800 8C.
Fig. 2. Specific contact resistivity (top) and sheet resistance (bottom) as a function of anneal time at 700 8C for Ti/Al/ZrB2/Ti/Au on n-GaN.
summarizes the near-surface composition data obtained from AES measurements. Fig. 6 shows the AES depth profiles for the as-deposited and annealed samples. The profile obtained from the as-deposited sample was in good agreement with the prescribed metal layer thicknesses. Oxygen was detected in the ZrB2 layer, consistent with past observations that the borides are excellent getters for water vapor during deposition [38]. The profile obtained from the sample annealed at 500 8C shows extensive diffusion of titanium through the gold layer to the surface. The intermixing of the contact metallurgy becomes more pronounced as the annealing temperature is increased. Some of the transport might be attributed to grain-boundary diffusion, as reported pre-
Fig. 4. Secondary electron images of the Ti/Al/ZrB2/Ti/Au on n-GaN as-deposited (top left) and after annealing at 500 (top right), 700 (bottom left) or 1000 8C (bottom right).
R. Khanna et al. / Applied Surface Science 253 (2006) 2340–2344
Fig. 5. AES surface scans of the Ti/Al/ZrB2/Ti/Au on n-GaN as-deposited and after annealing at 500, 700 or 1000 8C.
Fig. 6. AES depth profiles of the Ti/Al/ZrB2/Ti/Au on n-GaN as-deposited and after annealing at 500, 700 or 1000 8C.
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Table 1 Near-surface composition of contact stack determined by AES measurements Sample ID
C(1)
O(1)
Al(1)
S(1)
Ti(2)
B(2)
Au(3)
Sensitivity factors 1. As-deposited 2. Annealed at 500 8C 3. Annealed at 700 8C 4. Annealed at 1000 8C
[0.076] 48 41 36 28
[0.212] 1 22 29 33
[0.000] nd 16 22 18
[0.652] 2 <1 <1 1
[0.000] nd 3 5 4
[0.000] nd nd nd 15
[0.049] 49 17 8 nd
viously for Mo-based contacts to AlGaN/GaN heterostructures [27]. As noted earlier, the contact morphology does not degrade significantly even after 1000 8C annealing. 4. Summary and conclusions A novel Ti/Al/ZrB2/Ti/Au metallization scheme was used to form ohmic contacts to n-type GaN. A minimum specific contact resistivity of 3 10 6 V cm2 was achieved at an annealing temperature of 700 8C, which is comparable to that achieved with conventional Ti/Al/Pt/Au on the same samples. The ZrB2-based contact appears to have greater thermal stability than the conventional metallization and may have application in power amplifiers if the reliability can be proven. Acknowledgments The work at UF is partially supported by AFOSR (F4962002-1-0366, G. Witt), ONR (N00014-98-1-02-04, H. B. Dietrich) and NSF DMR 0101438. References [1] J. Sun, H. Fatima, A. Koudymov, A. Chitnis, X. Hu, H.-M. Wang, J. Zhang, G. Simin, J. Yang, M.A. Khan, IEEE Electron. Device Lett. 24 (2003) 375. [2] A.P. Zhang, L.B. Rowland, E.B. Kaminsky, J.W. Kretchmer, R.A. Beaupre, J.L. Garrett, J.B. Tucker, B.J. Edward, J. Foppes, A.F. Allen, SolidState Electron. 47 (2003) 821. [3] A.P. Zhang, L.B. Rowland, E.B. Kaminsky, V. Tilak, J.C. Grande, J. Teetsov, A. Vertiatchikh, L.F. Eastman, J. Electron. Mater. 32 (2003) 388. [4] V. Kumar, L. Zhou, D. Selvanathan, I. Adesida, J. Appl. Phys. 92 (2002) 1712. [5] Abhishek Motayed, Ravi Bathe, Mark C. Wood, S. Ousmane, R.D. Diouf, S. Vispute, Noor Mohammad, J. Appl. Phys. 93 (2003) 1087. [6] Jinwook Burm, Kenneth Chu, William A. Davis, J. William Schaff, Lester F. Eastman, Tyler J. Eustis, Appl. Phys. Lett. 70 (1997) 464. [7] M.E. Lin, Z. Ma, F.Y. Huang, Z.F. Fan, L.H. Allen, H. Morkoc¸, Appl. Phys. Lett. 64 (1994) 1003. [8] B.P. Luther, S.E. Mohney, T.N. Jackson, M. Asif Khan, Q. Chen, J.W. Yang, Appl. Phys. Lett. 70 (1997) 57. ¨ zgu¨r Aktas, Andrei E. [9] S. Zhifang Fan, Noor Mohammad, Wook Kim, O Botchkarev, Hadis Morkoc¸, Appl. Phys. Lett. 68 (1996) 1672. [10] S. Ruvimov, Z. Liliental-Weber, J. Washburn, D. Qiao, S.S. Lau, Paul K. Chu, Appl. Phys. Lett. 73 (1998) 2582. [11] A.N. Bright, P.J. Thomas, M. Weyland, D.M. Tricker, C.J. Humphreys, R. Davies, J. Appl. Phys. 89 (2001) 3143. [12] N.A. Papanicolaou, M.V. Rao, J. Mittereder, W.T. Anderson, J. Vac. Sci. Technol. B 19 (2001) 261. [13] Q.Z. Liu, S.S. Lau, Solid-State Electron. 42 (1998) 677–691.
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