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Au to n-GaN by two-step annealing method
Materials Science and Engineering B 111 (2004) 36–39
Ohmic contact formation of Ti/Al/Ni/Au to n-GaN by two-step annealing method Z.Z. Chen∗ , Z.X. Qin, C.Y. Hu, X.D. Hu, T.J. Yu, Y.Z. Tong, X.M. Ding, G.Y. Zhang State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China Received 22 May 2003; accepted 11 March 2004
Abstract The thermal annealing effects on Ti/Al/Ni/Au Ohmic contact to n-GaN are investigated by current–voltage (I–V) characteristics and transmission line method (TLM) measurements. The cladding layer of Ni/Au on Ti/Al plays two roles: preventing inter-diffusion of Ti, Al, Au and anti-oxidation of the contacting layer. The specific contact resistance (ρc ) of Ti/Al/Ni/Au to n-GaN increases slightly at first with the increasing annealing temperature (Ta ). When Ta increases above 500 ◦ C, ρc decreases monotonously in the range of 400–900 ◦ C. However, the morphology of the contact degrades gradually when Ta increases above 600 ◦ C. The minimum of ρc is obtained as 9.65 × 10−7 cm2 by two-step annealing method in this work. Finally, the roles of two-step annealing method in the formation mechanism of the Ohmic contact to n-GaN are also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: n-GaN; Ohmic contact; Current–voltage (I–V) characteristics; Transmission line method (TLM); Two-step annealing method
1. Introduction During the past years, great progress has been made in the development of short-wave optoelectronic devices, such as blue/green light emitting diodes (LEDs), laser diodes (LDs), based on group III-nitrides [1–3]. Thermal stability of contact is a crucial issue for high-temperature device performance and feasibility for bonding, which must be primarily taken into account for the practical applications of these devices. In the earlier works, Ohmic contacts to n-GaN were achieved by depositing low work-function (Φm ) metal layers, such as Al single layer [4–6] (Φm = 4.28 eV [7]), Ti single layer [8–10] (Φm = 4.33 eV ) and Ti/Al bilayer [11] on the semiconductor surface. However, these contacts could be oxidized easily. The cladding layer of Au was able to alleviate the effects of oxidation and made the contact have higher conductivity. But Au was likely to penetrate the metal layer and directly contact to n-GaN, which was disadvantageous to the formation of Ohmic contact [11–13]. Recently, much attention was paid to the cladding layer to the contact to n-GaN. The cladding layers of Ti/Au [13,14],
∗
Corresponding author. E-mail address: [email protected] (Z.Z. Chen).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.03.014
Pt/Au [15,16] and Ni/Au [17–22] were extensively investigated. They played two roles in formation of n-GaN Ohmic contact: preventing the diffusion of Au to n-GaN surface and preventing oxidation of Ti/Al contacting layer. The Ni/Au layer was also conventionally used to form p-GaN Ohmic contact. It became the high conductive cermet-like matter after annealed in air [17,23], and the specific contact resistance (ρc ) to p-GaN was as low as 4 × 10−6 cm2 [23]. Accordingly, the cermet-like matter also played important roles in preventing the oxidation of the contact to n-GaN, and in formation of low-resistance Ohmic contact [21]. However, the annealing temperature (Ta ) for Ti/Al/Ni/Au was about 200 ◦ C higher than those for Ti/Al/Ti/Au and Ti/Al/Ti/Al/Au to obtain Ohmic contact to n-GaN [19]. The higher Ta would lead to strong diffusion of Au and Ni to n-GaN, and degradation of the surface morphology of n-contact [19]. In this work, the roles of Ni/Au cladding layer to Ti/Al contacting layer have been studied by depositing the metal layers of Ti/Au, Ti/Al/Au and Ti/Al/Ni/Au on n-GaN by an e-beam evaporation system. Two-step annealing method was also developed to obtain a high quality n-GaN Ohmic contact with both low ρc and good morphology. The minimum of ρc was achieved to 9.65 × 10−7 cm2 for Ti/Al/Ni/Au contact by two-step annealing method.
Z.Z. Chen et al. / Materials Science and Engineering B 111 (2004) 36–39
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2. Experiment The n-GaN layers were grown on (0 0 0 1) sapphire substrates by MOCVD system. An undoped GaN layer with thickness of 0.6 m was grown followed by the growth of 2 m thick Si-doped GaN layer. The resistivity, carrier concentration and Hall mobility were 0.0134 cm, 2.2 × 1018 cm3 and 221 cm2 /V s, respectively. The Ti (20 nm)/Au (200 nm), Ti (20 nm)/Al (20 nm)/Au (200 nm), and Ti (20 nm)/Al (20 nm)/Ni (20 nm)/Au (200 nm) contacts were deposited by an e-beam evaporation system. Before deposition, the surfaces of the samples were treated by the plasma of CHF3 and O2 in a reactive ion etching (RIE) system to remove native oxide layer. Then the as-deposited samples were introduced into a rapidly thermal processing (RTP) system for annealing in N2 ambient. The annealing temperatures were in the range of 400–900 ◦ C. The annealing processes were performed in two ways. One method annealed samples at a constant temperature for 10 min. The other method first annealed samples in a low temperature for 10 min, and then annealed in a higher temperature for a short time as tens of second (two-step annealing). Next, transmission line method (TLM) pattern was formed with standard photolithographic technology. The contacts for TLM measurement were rectangular (250 m ×200 m), separated by 20, 30, 40, 50, 60, 70, 80 and 90 m. The surface morphologies of the annealed n-GaN contacts were observed by a Zeiss microscope, and the images were obtained by a controlled computer.
3. Results and discussion Fig. 1 shows the current–voltage (I–V) curves for the Ti/Au, Ti/Al/Au and Ti/Al/Ni/Au, contacts to n-GaN. The samples were annealed at 600 ◦ C in N2 ambient for 10 min. As shown in Fig. 1, the I–V characteristic of Ti/Al/Ni/Au contacts is linear, and its slope is the steepest. This demonstrates that an Ohmic contact has been obtained. However, the Ti/Au contact exhibits a nonlinear I–V characteristic. Ti/Al/Au shows a better Ohmic contact than Ti/Au. This result indicates that the introduction of Al layer leads to improvement of the contact significantly. As reported by Kaminska et al. [10], the ρc of Ti’s contact to n-GaN was higher than Al, and Ti reacted easier with O2 even at the room temperature. Luther et al. [6] proposed that Al atoms diffused through the Ti layer, resulting in an Ohmic contact when the low work function Al–Ti of the intermetallic phase came into contact with the GaN surface. The bilayer can also play an anti-oxidization role and prevent the Au atoms from penetrating to the surface of n-GaN Au penetration may degrade the contact. In the three contact schemes, Ti/Al/Ni/Au shows the best Ohmic characteristic. This result may be due to the fact that Ti/Al are combined into an alloy and Ni/Au are forming cermet-like matter after annealing, which prevents oxidation of contacting layer and Au diffusion to surface of semiconductor [6,10,23].
Fig. 1. I–V characteristics of Ti/Au, Ti/Al/Au, Ti/Al/Ni/Au contacts to n-GaN annealing in N2 ambient at 600 ◦ C for 10 min.
Fig. 2 shows the dependence of ρc of Ti/Al/Ni/Au to n-GaN on Ta for 10 min. ρc rises slightly with Ta increasing to 500 ◦ C. Papanicalou et al. believed that there was a high potential barrier between TiN and n-GaN, and this might lead to the rectified characteristic of contact [19]. Above 600 ◦ C, ρc decreases rapidly. It is obtained as 3.35 × 10−6 cm2 at 900 ◦ C. This indicates that the contacting layers formed by Ti, Al, N in higher Ta will lower the potential barrier. We have reported that contacts of Ti/Au and Ti/Al/Au degraded when they were annealed in high temperatures, while ρc of
Fig. 2. Dependence of ρc of Ti/Al/Ni/Au to n-GaN on annealing temperature for 10 min.
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Z.Z. Chen et al. / Materials Science and Engineering B 111 (2004) 36–39
Fig. 3. The surface images of 200× microscopy of Ti/Al/Ni/Au annealed at (a) 550 ◦ C, (b) 600 ◦ C, (c) 800 ◦ C, and (d) 900 ◦ C for 10 min in N2 ambient.
Ti/Al/Ni/Au decreased till 900 ◦ C [22]. This shows a better thermal stability of the contact of Ti/Al/Ni/Au if we do not consider the factor of the contact surface morphology. As mentioned above, the surface morphology of the contact may be degraded when annealed at high temperature. Fig. 3 shows the surface images of 200× microscopy of Ti/Al/Ni/Au contact annealed at (a) 550 ◦ C, (b) 600 ◦ C, (c) 800 ◦ C, and (d) 900 ◦ C for 10 min in N2 ambient. The surface of contact is smooth and is free of visible-defects at 550 ◦ C. A few nonmetallic matters are appeared at the edge of the contact at 600 ◦ C, which may be the nitrides of Ni, Al, and Ti. At 800 ◦ C, the nonmetallic matters spread in the whole surface. At 900 ◦ C, the surface becomes crimpy. The severe degradation of morphology will badly influence the evaporation of solder disk, although low ρc can be obtained in this case. Ni/Au as a cladding layer for contact to n-GaN will make the fabrication processes of n- and p-type Ohmic contact compatible [21]. However, the high-temperature annealing time must be as short as possible to prevent the morphology degradation of Ti/Al/Ni/Au contact. The short annealing time will be disadvantageous to obtain low ρc Ohmic contacts, especially for the existence of a contamination and/or oxide layer on GaN surface. We next bring forward the two-step annealing method, in which the contact is firstly annealed at a relatively low-temperature as 600 ◦ C for 10 min, and then annealed at high-temperature above 600 ◦ C for 10 s. We believe that the low-temperature annealing for a long time will eliminate the effects of the contamination and/or oxide layer, and allow Ti, Al, and GaN to preliminarily react. The following high-temperature annealing for a short time will reduce ρc and not degrade the morphology of contact. Fig. 4 shows the dependence of ρc of Ti/Al/Ni/Au to n-GaN on high annealing temperature for 10 s by two-step annealing method, in which the samples are first annealed at a low-temperature of 600 ◦ C for 10 min. It is observed that ρc decreases after high-temperature annealing for a short time. ρc decreases to 3.84 × 10−5 cm2 as the high-temperature increases to 900 ◦ C. Although this result is one order higher than at 900 ◦ C for 10 min, the morphology is still similar to that at 600 ◦ C. This indicates that two-step annealing method is appropriate to obtain both low-resistance and good-morphology contact.
Finally, the dependence of ρc on the high-temperature annealing time is shown in Fig. 5. The annealing time ranges from 10 to 60 s. All the values of ρc are lower than 10−4 cm2 . Thirty seconds annealing results in the lowest ρc of 9.65 × 10−7 cm2 . When the annealing time is less
Fig. 4. Dependence of ρc of Ti/Al/Ni/Au to n-GaN on high-temperature for 10 s by two-step annealing method, in which the samples are first annealing at low-temperature of 600 ◦ C for 10 min.
Fig. 5. Dependence of ρc of Ti/Al/Ni/Au to n-GaN on high-temperature annealing time in two-step annealing method. The high-temperature is 900 ◦ C.
Z.Z. Chen et al. / Materials Science and Engineering B 111 (2004) 36–39
than 20 s, ρc is two orders higher than the lowest values, indicating that the annealing time is not enough to form a good Ohmic contact to n-GaN. When annealing time is more than 40 s, ρc also increases. In the Ti/Al/Ni/Au multi-layer, the thickness of Ti (20 nm), Al layer (20 nm) and Ni (20 nm) may be not enough to prevent Au from diffusing into GaN for the long-time/high-temperature annealing. This condition needs further study of how ρc changes from 1 to 10 min at 900 ◦ C. 4. Conclusions In summary, I–V characteristics, TLM, and surface microscopy measurements have been performed to investigate the formation mechanisms of Ti/Al/Ni/Au Ohmic contact to n-GaN under different thermal annealing conditions. It is found that the I–V characteristics and thermal stability of Ti/Al/Ni/Au are superior to those of Ti/Al/Au and Ti/Au. High-temperature annealing for a long time may make ρc decrease, but the surface morphology is also degraded. This problem can be resolved by two-step annealing method, by which the minimum of ρc is achieved as 9.65 × 10−7 cm2 . References [1] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 34 (1995) L687. [2] S. Nakamura, M. Senoh, S. Naghama, et al., Jpn. J. Appl. Phys. 35 (1996) 74.
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[3] S. Nakamura, M. Senoh, S. Nagahama, et al., Jpn. J. Appl. Phys. 36 (1997) L1568. [4] L.L. Smith, R.F. Davis, M.J. Kim, et al., J. Mater. Res. 11 (1996) 2257. [5] A.T. Ping, M.A. Khan, I. Adersida, J. Electron. Mater. 25 (1996) 819. [6] B.P. Luther, S.E. Mohney, T.N. Jackson, et al., Appl. Phys. Lett. 70 (1997) 57. [7] D.R. Lide, CRC Handbook of Chemistry and Physics, 77th Edition, CRC Press, Boca Raton, New York, 1996, pp. 12–122. [8] Y.F. Wu, W.N. Jiang, B.P. Keller, et al., Solid-State Electron 41 (1997) 165. [9] L.L. Smith, M.D. Bremser, E.P. Carlson, et al., Mater. Res. Soc. Symp. Proc. 395 (1996) 861. [10] E. Kaminska, A. Piotrowska, M. Gaziewicz, et al., Mater. Res. Soc. Symp. Proc. 449 (1997) 1055. [11] M.E. Lin, Z. Ma, F.Y. Huang, et al., Appl. Phys. Lett. 64 (1994) 1003. [12] C.-T. Lee, H.-W. Kao, Appl. Phys. Lett. 76 (2000) 2364. [13] D.-F. Wang, F. Shiwei, C. Lu, et al., J. Appl. Phys. 89 (11) (2001) 6214. [14] Y. Wu, W. Jiang, B. Keller, et al., Topical Workshop on III–V Nitrides, Proceedings, Nagoya, Japan, 1995. [15] S.J. Cai, R. Li, Y.L. Chen, L. Wong, et al., Electron. Lett. 34 (1998) 2354. [16] Z.M. Zhao, R.L. Jiang, P. Chen, et al., Appl. Phys. Lett. 79 (2001) 218. [17] Z. Fan, S. Mohammad, W. Kim, et al., Appl. Phys. Lett. 68 (1996) 1672. [18] S. Ruvimov, Appl. Phys. Lett. 69 (1996) 1556. [19] N.A. Papanicolaou, M.V. Rao, J. Jittereder, J. Vac. Sci. Technol. B 19 (2001) 261. [20] A.N. Bright, P.J. Thomas, M. Weyland, et al., J. Appl. Phys. 89 (2001) 3143. [21] C.M. Pelto, Y.A. Chang, et al., J. Appl. Phys 92 (8) (2002) 4283. [22] Z.X. Qin, Z.Z. Chen, G.Y. Zhang, et al., Appl. Phys. A 78 (5) (2004) 729. [23] J.K. Ho, C.-S. Jong, C.C. Chiu, et al., J. Appl. Phys. 86 (1999) 4491.
Ohmic contact formation of Ti/Al/Ni/Au to n-GaN by two-step annealing method Z.Z. Chen∗ , Z.X. Qin, C.Y. Hu, X.D. Hu, T.J. Yu, Y.Z. Tong, X.M. Ding, G.Y. Zhang State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China Received 22 May 2003; accepted 11 March 2004
Abstract The thermal annealing effects on Ti/Al/Ni/Au Ohmic contact to n-GaN are investigated by current–voltage (I–V) characteristics and transmission line method (TLM) measurements. The cladding layer of Ni/Au on Ti/Al plays two roles: preventing inter-diffusion of Ti, Al, Au and anti-oxidation of the contacting layer. The specific contact resistance (ρc ) of Ti/Al/Ni/Au to n-GaN increases slightly at first with the increasing annealing temperature (Ta ). When Ta increases above 500 ◦ C, ρc decreases monotonously in the range of 400–900 ◦ C. However, the morphology of the contact degrades gradually when Ta increases above 600 ◦ C. The minimum of ρc is obtained as 9.65 × 10−7 cm2 by two-step annealing method in this work. Finally, the roles of two-step annealing method in the formation mechanism of the Ohmic contact to n-GaN are also discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: n-GaN; Ohmic contact; Current–voltage (I–V) characteristics; Transmission line method (TLM); Two-step annealing method
1. Introduction During the past years, great progress has been made in the development of short-wave optoelectronic devices, such as blue/green light emitting diodes (LEDs), laser diodes (LDs), based on group III-nitrides [1–3]. Thermal stability of contact is a crucial issue for high-temperature device performance and feasibility for bonding, which must be primarily taken into account for the practical applications of these devices. In the earlier works, Ohmic contacts to n-GaN were achieved by depositing low work-function (Φm ) metal layers, such as Al single layer [4–6] (Φm = 4.28 eV [7]), Ti single layer [8–10] (Φm = 4.33 eV ) and Ti/Al bilayer [11] on the semiconductor surface. However, these contacts could be oxidized easily. The cladding layer of Au was able to alleviate the effects of oxidation and made the contact have higher conductivity. But Au was likely to penetrate the metal layer and directly contact to n-GaN, which was disadvantageous to the formation of Ohmic contact [11–13]. Recently, much attention was paid to the cladding layer to the contact to n-GaN. The cladding layers of Ti/Au [13,14],
∗
Corresponding author. E-mail address: [email protected] (Z.Z. Chen).
0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.03.014
Pt/Au [15,16] and Ni/Au [17–22] were extensively investigated. They played two roles in formation of n-GaN Ohmic contact: preventing the diffusion of Au to n-GaN surface and preventing oxidation of Ti/Al contacting layer. The Ni/Au layer was also conventionally used to form p-GaN Ohmic contact. It became the high conductive cermet-like matter after annealed in air [17,23], and the specific contact resistance (ρc ) to p-GaN was as low as 4 × 10−6 cm2 [23]. Accordingly, the cermet-like matter also played important roles in preventing the oxidation of the contact to n-GaN, and in formation of low-resistance Ohmic contact [21]. However, the annealing temperature (Ta ) for Ti/Al/Ni/Au was about 200 ◦ C higher than those for Ti/Al/Ti/Au and Ti/Al/Ti/Al/Au to obtain Ohmic contact to n-GaN [19]. The higher Ta would lead to strong diffusion of Au and Ni to n-GaN, and degradation of the surface morphology of n-contact [19]. In this work, the roles of Ni/Au cladding layer to Ti/Al contacting layer have been studied by depositing the metal layers of Ti/Au, Ti/Al/Au and Ti/Al/Ni/Au on n-GaN by an e-beam evaporation system. Two-step annealing method was also developed to obtain a high quality n-GaN Ohmic contact with both low ρc and good morphology. The minimum of ρc was achieved to 9.65 × 10−7 cm2 for Ti/Al/Ni/Au contact by two-step annealing method.
Z.Z. Chen et al. / Materials Science and Engineering B 111 (2004) 36–39
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2. Experiment The n-GaN layers were grown on (0 0 0 1) sapphire substrates by MOCVD system. An undoped GaN layer with thickness of 0.6 m was grown followed by the growth of 2 m thick Si-doped GaN layer. The resistivity, carrier concentration and Hall mobility were 0.0134 cm, 2.2 × 1018 cm3 and 221 cm2 /V s, respectively. The Ti (20 nm)/Au (200 nm), Ti (20 nm)/Al (20 nm)/Au (200 nm), and Ti (20 nm)/Al (20 nm)/Ni (20 nm)/Au (200 nm) contacts were deposited by an e-beam evaporation system. Before deposition, the surfaces of the samples were treated by the plasma of CHF3 and O2 in a reactive ion etching (RIE) system to remove native oxide layer. Then the as-deposited samples were introduced into a rapidly thermal processing (RTP) system for annealing in N2 ambient. The annealing temperatures were in the range of 400–900 ◦ C. The annealing processes were performed in two ways. One method annealed samples at a constant temperature for 10 min. The other method first annealed samples in a low temperature for 10 min, and then annealed in a higher temperature for a short time as tens of second (two-step annealing). Next, transmission line method (TLM) pattern was formed with standard photolithographic technology. The contacts for TLM measurement were rectangular (250 m ×200 m), separated by 20, 30, 40, 50, 60, 70, 80 and 90 m. The surface morphologies of the annealed n-GaN contacts were observed by a Zeiss microscope, and the images were obtained by a controlled computer.
3. Results and discussion Fig. 1 shows the current–voltage (I–V) curves for the Ti/Au, Ti/Al/Au and Ti/Al/Ni/Au, contacts to n-GaN. The samples were annealed at 600 ◦ C in N2 ambient for 10 min. As shown in Fig. 1, the I–V characteristic of Ti/Al/Ni/Au contacts is linear, and its slope is the steepest. This demonstrates that an Ohmic contact has been obtained. However, the Ti/Au contact exhibits a nonlinear I–V characteristic. Ti/Al/Au shows a better Ohmic contact than Ti/Au. This result indicates that the introduction of Al layer leads to improvement of the contact significantly. As reported by Kaminska et al. [10], the ρc of Ti’s contact to n-GaN was higher than Al, and Ti reacted easier with O2 even at the room temperature. Luther et al. [6] proposed that Al atoms diffused through the Ti layer, resulting in an Ohmic contact when the low work function Al–Ti of the intermetallic phase came into contact with the GaN surface. The bilayer can also play an anti-oxidization role and prevent the Au atoms from penetrating to the surface of n-GaN Au penetration may degrade the contact. In the three contact schemes, Ti/Al/Ni/Au shows the best Ohmic characteristic. This result may be due to the fact that Ti/Al are combined into an alloy and Ni/Au are forming cermet-like matter after annealing, which prevents oxidation of contacting layer and Au diffusion to surface of semiconductor [6,10,23].
Fig. 1. I–V characteristics of Ti/Au, Ti/Al/Au, Ti/Al/Ni/Au contacts to n-GaN annealing in N2 ambient at 600 ◦ C for 10 min.
Fig. 2 shows the dependence of ρc of Ti/Al/Ni/Au to n-GaN on Ta for 10 min. ρc rises slightly with Ta increasing to 500 ◦ C. Papanicalou et al. believed that there was a high potential barrier between TiN and n-GaN, and this might lead to the rectified characteristic of contact [19]. Above 600 ◦ C, ρc decreases rapidly. It is obtained as 3.35 × 10−6 cm2 at 900 ◦ C. This indicates that the contacting layers formed by Ti, Al, N in higher Ta will lower the potential barrier. We have reported that contacts of Ti/Au and Ti/Al/Au degraded when they were annealed in high temperatures, while ρc of
Fig. 2. Dependence of ρc of Ti/Al/Ni/Au to n-GaN on annealing temperature for 10 min.
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Z.Z. Chen et al. / Materials Science and Engineering B 111 (2004) 36–39
Fig. 3. The surface images of 200× microscopy of Ti/Al/Ni/Au annealed at (a) 550 ◦ C, (b) 600 ◦ C, (c) 800 ◦ C, and (d) 900 ◦ C for 10 min in N2 ambient.
Ti/Al/Ni/Au decreased till 900 ◦ C [22]. This shows a better thermal stability of the contact of Ti/Al/Ni/Au if we do not consider the factor of the contact surface morphology. As mentioned above, the surface morphology of the contact may be degraded when annealed at high temperature. Fig. 3 shows the surface images of 200× microscopy of Ti/Al/Ni/Au contact annealed at (a) 550 ◦ C, (b) 600 ◦ C, (c) 800 ◦ C, and (d) 900 ◦ C for 10 min in N2 ambient. The surface of contact is smooth and is free of visible-defects at 550 ◦ C. A few nonmetallic matters are appeared at the edge of the contact at 600 ◦ C, which may be the nitrides of Ni, Al, and Ti. At 800 ◦ C, the nonmetallic matters spread in the whole surface. At 900 ◦ C, the surface becomes crimpy. The severe degradation of morphology will badly influence the evaporation of solder disk, although low ρc can be obtained in this case. Ni/Au as a cladding layer for contact to n-GaN will make the fabrication processes of n- and p-type Ohmic contact compatible [21]. However, the high-temperature annealing time must be as short as possible to prevent the morphology degradation of Ti/Al/Ni/Au contact. The short annealing time will be disadvantageous to obtain low ρc Ohmic contacts, especially for the existence of a contamination and/or oxide layer on GaN surface. We next bring forward the two-step annealing method, in which the contact is firstly annealed at a relatively low-temperature as 600 ◦ C for 10 min, and then annealed at high-temperature above 600 ◦ C for 10 s. We believe that the low-temperature annealing for a long time will eliminate the effects of the contamination and/or oxide layer, and allow Ti, Al, and GaN to preliminarily react. The following high-temperature annealing for a short time will reduce ρc and not degrade the morphology of contact. Fig. 4 shows the dependence of ρc of Ti/Al/Ni/Au to n-GaN on high annealing temperature for 10 s by two-step annealing method, in which the samples are first annealed at a low-temperature of 600 ◦ C for 10 min. It is observed that ρc decreases after high-temperature annealing for a short time. ρc decreases to 3.84 × 10−5 cm2 as the high-temperature increases to 900 ◦ C. Although this result is one order higher than at 900 ◦ C for 10 min, the morphology is still similar to that at 600 ◦ C. This indicates that two-step annealing method is appropriate to obtain both low-resistance and good-morphology contact.
Finally, the dependence of ρc on the high-temperature annealing time is shown in Fig. 5. The annealing time ranges from 10 to 60 s. All the values of ρc are lower than 10−4 cm2 . Thirty seconds annealing results in the lowest ρc of 9.65 × 10−7 cm2 . When the annealing time is less
Fig. 4. Dependence of ρc of Ti/Al/Ni/Au to n-GaN on high-temperature for 10 s by two-step annealing method, in which the samples are first annealing at low-temperature of 600 ◦ C for 10 min.
Fig. 5. Dependence of ρc of Ti/Al/Ni/Au to n-GaN on high-temperature annealing time in two-step annealing method. The high-temperature is 900 ◦ C.
Z.Z. Chen et al. / Materials Science and Engineering B 111 (2004) 36–39
than 20 s, ρc is two orders higher than the lowest values, indicating that the annealing time is not enough to form a good Ohmic contact to n-GaN. When annealing time is more than 40 s, ρc also increases. In the Ti/Al/Ni/Au multi-layer, the thickness of Ti (20 nm), Al layer (20 nm) and Ni (20 nm) may be not enough to prevent Au from diffusing into GaN for the long-time/high-temperature annealing. This condition needs further study of how ρc changes from 1 to 10 min at 900 ◦ C. 4. Conclusions In summary, I–V characteristics, TLM, and surface microscopy measurements have been performed to investigate the formation mechanisms of Ti/Al/Ni/Au Ohmic contact to n-GaN under different thermal annealing conditions. It is found that the I–V characteristics and thermal stability of Ti/Al/Ni/Au are superior to those of Ti/Al/Au and Ti/Au. High-temperature annealing for a long time may make ρc decrease, but the surface morphology is also degraded. This problem can be resolved by two-step annealing method, by which the minimum of ρc is achieved as 9.65 × 10−7 cm2 . References [1] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 34 (1995) L687. [2] S. Nakamura, M. Senoh, S. Naghama, et al., Jpn. J. Appl. Phys. 35 (1996) 74.
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