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Electrical characteristics of molybdenum Schottky contacts on n-type GaN
Materials Science & Engineering B 112 (2004) 30–33 www.elsevier.com/locate/mseb
Electrical characteristics of molybdenum Schottky contacts on n-type GaN C.K. Ramesha, V. Rajagopal Reddya,*, Chel-Jong Choib a
Department of Electronics, Post-Graduate Centre, University of Mysore, Hemagangotri, 573220 Hassan, India b Analytical Engineering Center, Samsung Advanced Institute of Technology (SAIT), P.O. Box 111, Suwon 440-600, Korea Received 27 March 2004; accepted 13 May 2004
Abstract The Schottky barrier heights of molybdenum (Mo) on n-GaN were investigated as a function of annealing temperature by current–voltage (I–V) and capacitance–voltage (C–V) techniques. The Schottky barrier height of the as-deposited Mo/n-GaN was found to be 0.81 eV (I–V) and 1.02 eV (C–V), respectively. However, both measurements indicate that the barrier height slightly decreases upon annealing at 400 8C for 1 min in nitrogen ambient. The barrier height of Mo/n-GaN Schottky contacts at 400 8C was determined to be 0.74 and 0.92 eV, respectively. Further, an increase in annealing temperature up to 600 8C, decreased the barrier height to 0.56 and 0.73 eV. The Mo Schottky contact was also shown to be fairly stable during annealing at 400 8C. # 2004 Elsevier B.V. All rights reserved. Keywords: Schottky barrier height; Mo/n-GaN Schottky diode; I–V and C–V techniques
1. Introduction Gallium nitride (GaN) is a thermally stable, wide direct band gap semiconductor. It is an excellent candidate for high temperature, high frequency and high power electronics devices such as light emitting diodes, laser diodes, metal semiconductor field effect transistors and high electron mobility transistors [1–3]. In order to continue to improve these devices, thermally stable ohmic and Schottky contacts are required. Many studies of Schottky contacts on GaN have already been reported, e.g. Au [4,5], Pd [6], Pt [7], Ni [8], Re [9] and Ru [10]. Hacke et al. [4] fabricated Schottky diode using Au on n-GaN and reported Schottky barrier height of 0.84 and 0.94 eV by I–V and C–V measurements, respectively. Wang and He [5] reported the Schottky barrier height of 1.32 eV for low temperature Au/n-GaN Schottky diode by I–V measurement. Weiderman et al. [6] prepared Pd/n-GaN * Corresponding author. Tel.: +91 817 224 0968; fax: +91 817 224 0674. E-mail address: [email protected] (V.R. Reddy). 0921-5107/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.005
Schottky diode in both in situ and ex situ and reported a barrier height of 0.70in situ and 0.90ex situ from I–V measurement. Suzue et al. [7] investigated Pt/GaN Schottky diode, reported a Schottky barrier height of 1.10 eV from I–V and C–V measurements. Guo et al. [8] measured the Schottky barrier height of Ni on n-GaN as 0.56 and 0.66 eV by C–V and J–T methods, respectively. Venugopalan and Mohney [9] showed that the Rhenium Schottky contact was thermally stable on n-GaN and reported a barrier height of 0.82 and 1.06 eV after annealing at 500 8C by I–V and C–V measurements. Auret et al. [10] reported barrier height of 1.08 eV for Ru Schottky contact on n-GaN using I–V measurements. Recently, Hayes et al. [11] investigated TaN/n-GaN Schottky diode and showed that the contacts were stable at the temperature as high as 800 8C. They reported that the Schottky barrier height of 0.7 and 1.2 eV was obtained using the I–V and C–V methods [11]. In the present work, we have fabricated molybdenum (Mo) Schottky contact on n-type GaN (4.07 1018 cm3). The ideality factor and Schottky barrier heights of Mo are determined by I–V and C–V measurements as a function of annealing temperature.
C.K. Ramesh et al. / Materials Science & Engineering B 112 (2004) 30–33
2. Experimental details The GaN layer used in this study was grown by metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrate. An undoped GaN layer with a thickness of 2 mm was grown, followed by the growth of 2 mm thick nGaN: Si (nd = 4.07 1018 cm3) layer. The n-GaN layer was first ultrasonically degreased with warm trichloroethylene, acetone and methanol for 5 min each. This degreased layer was then dipped into boiling aqua regia [HNO3:HCl = 1:3] for 10 min to remove the surface oxides and rinsed in deionized water. The samples were immediately loaded into the electron beam evaporation system, ohmic metals Ti (30 nm)/Al (60 nm) was deposited on a portion of the sample and annealed at 850 8C in a nitrogen ambient for 30 s. Then, a 200 nm thick Mo (99.999%) was deposited through a stainless steel mask using electron beam evaporator under a pressure of 2 106 Torr. The diameter of the Schottky dots was 1 mm. The Mo Schottky diodes were sequentially annealed at various temperatures from 400 to 600 8C for 1 min under nitrogen ambient in rapid thermal annealing (RTA) system. Schottky diode characteristics were measured using I–V and C–V using Keithley Source measure unit (Model No. 230) and a Boonton capacitance meter (Model No. 72 B), respectively.
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diode leakage current values are 1.2 106 and 5.8 103 A at 1 V. The barrier height and ideality factor of a metal–semiconductor contact were determined using thermionic emission theory is given by [12] qðV IRÞ I ¼ Is exp 1 (1) nkT with Is ¼ AA T 2 exp
qfb kT
(2)
Fig. 1 shows the typical forward and reverse I–V characteristics of Mo/n-GaN Schottky contact measured as a function of annealing temperature. It is found that the properties of all Mo/n-GaN Schottky diodes are uniform over different diodes, although it shows fairly large leakage currents. The leakage current value is 6.8 107 A at 1 V for the as-deposited, whereas annealed at 400 8C, 600 8C
where Is is the saturation current, q the electron charge, V the applied voltage, R the series resistance, n ideality factor, A** the effective Richardson constant, and fb the Schottky barrier height. The value of fb can be deduced directly from the I–V curves if the effective Richardson constant, A** is known. The theoretical value of A** is 26.4 A cm2 K2 based on the effective mass (m* = 0.22m0) of n-GaN [13] and was used here to deduce fb. A plot of I/[1 exp(qV/kT)] versus V (Fig. 2) yields ln Is as the intercept. Once Is was determined, the barrier height (fb) was determined. Calculations showed that the SBH is 0.81 eV for the as-deposited contact, 0.74 eV at 400 8C, 0.61 eV at 500 8C and 0.56 eV at 600 8C. The ideality factor is determined from a plot of natural log of current versus forward bias voltage (figure not shown) for small forward currents where the effect of series resistance is small. The ideality factor for the as-deposited Mo Schottky diodes was found to be 1.29. The ideality factor is improved to 1.04 upon annealing at 400 8C for 1 min in nitrogen ambient. Table 1 shows the values of ideality factor and the Schottky barrier height for Mo/n-GaN Schottky diodes. The values of ideality factor, while they are in general in agreement with those reported for Schottky diodes on n-GaN [5,14–18], nevertheless, are indicative
Fig. 1. The typical I–V characteristics for the Mo Schottky contact on ntype GaN as a function of annealing temperature.
Fig. 2. Plot of I/[1 exp(qV/kT)] vs. V for the Mo Schottky contacts annealed at different temperatures.
3. Results and discussion
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C.K. Ramesh et al. / Materials Science & Engineering B 112 (2004) 30–33
Table 1 The Schottky barrier heights and ideality factor of Mo/n-GaN as a function of annealing temperature Sample
Schottky barrier height (SBH), fb (eV) I–V
Norde
C–V
As-deposited 400 8C 500 8C 600 8C
0.81 0.74 0.61 0.56
0.75 0.66 0.55 0.51
1.02 0.92 0.85 0.73
Ideality factor, n
1.29 1.04 1.14 1.03
of nonideal behavior, suggesting that the transport mechanisms, other than just thermoionic, are probably present in these diodes. To compare the effective Schottky barrier height of contacts, the Norde method was also employed [19]. In this method, a function F(V) is plotted against the V. F(V) is given by F(V) = V/2 kT/2 ln[I(V)/AA**T2]. The effective SBH is given by fb = F(Vmin) + Vmin/2 kT/q, where F(Vmin) is the minimum value of F(V) and Vmin is the corresponding voltage. A plot of F(V) versus V for the samples annealed at different temperatures is shown in Fig. 3. The measurements showed that the SBH are 0.75 eV for the as-deposited, 0.66 eV at 400 8C, 0.55 eV at 500 8C and 0.51 eV at 600 8C, respectively. It is noted that these values are lower than those obtained by the I–V method. The values of SBH are given in Table 1. Capacitance and voltage (C–V) characteristics of Mo/nGaN Schottky diode were measured using a Boonton 72B 1 MHz capacitance meter. Fig. 4 shows a plot of 1/C2 as a function of bias voltage for as-deposited and annealed samples. The C–V relationship for Schottky diode is given by [20] 1 ¼ C2
2 es qNA2
Vbi
kT V q
(3)
Fig. 3. Plot of F(V) vs. V for the Mo Schottky contacts annealed at different temperatures.
Fig. 4. Plot of 1/C2 vs. V for the Mo Schottky contacts annealed at different temperatures.
where es is the permittivity of the semiconductor (es = 9.5e0), V is the applied voltage. The x-intercept of the plot of (1/C2) versus V, V0 is related to the built in potential Vbi by the equation Vbi = V0 + kT/q, where T is the absolute temperature. The barrier height is given by the equation fb = V0 + Vn + kT/q, where Vn = (kT/q) ln(Nc/Nd). The density of states in the conduction band edge is given by Nc = 2(2pm*kT/h2)3/2, where m* = 0.22m0 and its value is 2.53 1018 cm3 for GaN [2]. The barrier heights of Mo/n-GaN are 1.02 eV for as-deposited, 0.92 eV for 400 8C contacts, 0.85 eV for 500 8C contacts and 0.73 eV for contact annealed at 600 8C by C–V method. Fig. 5 shows a plot of barrier heights of Mo/n-GaN as a function of annealing temperature. It can be seen that the barrier height of the Mo Schottky diodes decreases upon annealing. For the contact annealed at 400 8C, the barrier height is high compared with that of the contact annealed at
Fig. 5. Plot of barrier heights of Mo/n-GaN as a function of annealing temperatures.
C.K. Ramesh et al. / Materials Science & Engineering B 112 (2004) 30–33
600 8C. It is also observed that the reverse leakage current is increased for the contact annealed at 600 8C compared to the contact annealed at 400 8C. The higher value in barrier height of metal/n-GaN Schottky contacts on annealing at 400 8C, under conditions where the metal is not observed to react with GaN, has also been reported previously [21,22]. According to Duboz et al. [22] the higher value in barrier height on annealing at moderate temperature of 400 8C can be attributed to a reduction in the density of interfacial defects. Also, Duboz et al. [22] observed that the nature of the metal does not play a major role in the barrier height enhancement on annealing. These researchers find that the Fermi level at metal/GaN interfaces is pinned by defects. Further, a modification of the defects density on annealing could change the pinning. This results in a change in the barrier height. It can also be seen from Fig. 5 the barrier heights, fb, obtained from I–V measurements are lower than those obtained from C–V measurements. This discrepancy between fI–V and fC–V for metal/n-GaN Schottky diodes has been attributed to factors such as the presence of a native oxide layer at the metal/GaN interface [4,23]. However, the metal probably comes into intimate contact with the semiconductor upon high temperature annealing. According to Werner and Guttler [24], spatial inhomogeneities at the metal/semiconductor interface of abrupt Schottky contact can also cause such differences in the barrier height determined from I–V and C–V measurements. Another possibility may be the transport mechanism in these diodes which is not purely due to thermionic emission. For these diodes, the fb obtained from I–V method is voltage or electric field sensitive, whereas the fb obtained from C–V is not.
4. Summary We have fabricated Mo Schottky contacts on n-GaN and characterized using I–V and C–V techniques as a function of annealing temperature. The barrier height of the as-deposited Schottky diode was found to be 0.81 and 1.02 eV, respectively. The barrier height slightly decreased to 0.74 and 0.92 eV when the contact annealing at 400 8C for 1 min. Further, increase in annealing temperature up to 600 8C, the barrier height decreased to 0.56 and 0.73 eV. The Mo Schottky contact was also shown to be relatively stable upon prolonged annealing at 400 8C for 30 min.
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Acknowledgement The authors thank the University Grant Commission (UGC), New Delhi for providing the financial assistance (Grant No. F.14-22/2002 (SR-I).
References [1] S. Nakamura, T. Mukai, S. Senoh, Jpn. J. Appl. Phys. 30 (1991) 1998. [2] H. Amano, M. Kito, X. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1998) 2112. [3] H. Morkoc, S. Strite, G.B. Gao, M.E. Lin, B. Serdlov, M. Burns, J. Appl. Phys. 76 (1994) 1363. [4] P. Hacke, T. Detchprohm, K. Hiramatsu, N. Sawaki, Appl. Phys. Lett. 63 (1993) 2676. [5] H.J. Wang, L. He, J. Electron. Mater. 27 (1998) 1272. [6] Weidermann, M. Hermann, G. Steinhoff, H. Wingbrant, L. Spetz, Stutzmann, M. Eickhoff, Appl. Phys. Lett. 82 (2003) 773. [7] K. Suzue, S.N. Mohammad, Z.F. Fan, W. Kim, O. Aktas, A.E. Botchkarev, H. Morkoc, J. Appl. Phys. 80 (1996) 4457. [8] J.D. Guo, F.M. Pan, M.S. Feng, R.J. Guo, P.F. Chou, C.Y. Chang, J. Appl. Phys. 80 (1996) 1623. [9] H.S. Venugopalan, S.E. Mohney, Appl. Phys. Lett. 73 (1998) 1242. [10] F.D. Auret, S.A. Goodman, G. Myburg, F.K. Koschnick, J.M. Spaeth, B. Beaumont, P. Gibart, Physica B 84–87 (1999) 273. [11] J.R. Hayes, D.-W. Kim, H. Meidia, S. Mahajan, Acta Materialia 51 (2003) 653. [12] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981 (p. 262). [13] M. Drechsler, D.M. Hofman, B.K. Meyer, T. Detchprohm, H. Amano, I. Akasaki, Jpn. J. Appl. Phys. 34 (1995) L1178. [14] S.C. Binari, H.B. Dietrich, G. Kelner, L.B. Rowland, K. Doverspike, D.K. Gaskill, Electron. Lett. 30 (1994) 909. [15] J.D. Guo, M.S. Feng, R.J. Guo, F.M. Pan, C.Y. Chang, Appl. Phys. Lett. 67 (1995) 2657. [16] L. Wang, M.I. Nathan, T-H. Lim, M.A. Khan, Q. Chen, Appl. Phys. Lett. 68 (1996) 1267. [17] A.T. Ping, A.C. Schmitz, M.A. Khan, I. Aldesida, Electron. Lett. 32 (1996) 68. [18] E.V. Kalinina, N.J. Kurnetsov, V.A. Dmitriev, K.G. Irvine, C.H. Carter Jr., J. Electron. Mater. 25 (1996) 831. [19] H. Norde, J. Appl. Phys. 50 (1979) 5052. [20] E.H. Rhoderick, T.H. Williams (Eds.), Metal–Semiconductor Contacts, Oxford Science, Oxford, 1988. [21] M.T. Hirsen, K.J. Duxstand, E.E. Haller, Electron. Lett. 33 (1997) 95. [22] J.Y. Duboz, F. Binet, N. Laurent, E. Rosencher, F. Scholz, V. Harle, O. Briot, B. Gil, R.L. Autombard, Mater. Res. Soc. Symp. Proc. 449 (1996) 1085. [23] D.K. Schroder (Ed.), Semiconductor Material and Device Characterization, Wiley, New York, 1990, p. 153. [24] J.H. Werner, H.H. Guttler, J. Appl. Phys. 69 (1991) 1522.
Electrical characteristics of molybdenum Schottky contacts on n-type GaN C.K. Ramesha, V. Rajagopal Reddya,*, Chel-Jong Choib a
Department of Electronics, Post-Graduate Centre, University of Mysore, Hemagangotri, 573220 Hassan, India b Analytical Engineering Center, Samsung Advanced Institute of Technology (SAIT), P.O. Box 111, Suwon 440-600, Korea Received 27 March 2004; accepted 13 May 2004
Abstract The Schottky barrier heights of molybdenum (Mo) on n-GaN were investigated as a function of annealing temperature by current–voltage (I–V) and capacitance–voltage (C–V) techniques. The Schottky barrier height of the as-deposited Mo/n-GaN was found to be 0.81 eV (I–V) and 1.02 eV (C–V), respectively. However, both measurements indicate that the barrier height slightly decreases upon annealing at 400 8C for 1 min in nitrogen ambient. The barrier height of Mo/n-GaN Schottky contacts at 400 8C was determined to be 0.74 and 0.92 eV, respectively. Further, an increase in annealing temperature up to 600 8C, decreased the barrier height to 0.56 and 0.73 eV. The Mo Schottky contact was also shown to be fairly stable during annealing at 400 8C. # 2004 Elsevier B.V. All rights reserved. Keywords: Schottky barrier height; Mo/n-GaN Schottky diode; I–V and C–V techniques
1. Introduction Gallium nitride (GaN) is a thermally stable, wide direct band gap semiconductor. It is an excellent candidate for high temperature, high frequency and high power electronics devices such as light emitting diodes, laser diodes, metal semiconductor field effect transistors and high electron mobility transistors [1–3]. In order to continue to improve these devices, thermally stable ohmic and Schottky contacts are required. Many studies of Schottky contacts on GaN have already been reported, e.g. Au [4,5], Pd [6], Pt [7], Ni [8], Re [9] and Ru [10]. Hacke et al. [4] fabricated Schottky diode using Au on n-GaN and reported Schottky barrier height of 0.84 and 0.94 eV by I–V and C–V measurements, respectively. Wang and He [5] reported the Schottky barrier height of 1.32 eV for low temperature Au/n-GaN Schottky diode by I–V measurement. Weiderman et al. [6] prepared Pd/n-GaN * Corresponding author. Tel.: +91 817 224 0968; fax: +91 817 224 0674. E-mail address: [email protected] (V.R. Reddy). 0921-5107/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.005
Schottky diode in both in situ and ex situ and reported a barrier height of 0.70in situ and 0.90ex situ from I–V measurement. Suzue et al. [7] investigated Pt/GaN Schottky diode, reported a Schottky barrier height of 1.10 eV from I–V and C–V measurements. Guo et al. [8] measured the Schottky barrier height of Ni on n-GaN as 0.56 and 0.66 eV by C–V and J–T methods, respectively. Venugopalan and Mohney [9] showed that the Rhenium Schottky contact was thermally stable on n-GaN and reported a barrier height of 0.82 and 1.06 eV after annealing at 500 8C by I–V and C–V measurements. Auret et al. [10] reported barrier height of 1.08 eV for Ru Schottky contact on n-GaN using I–V measurements. Recently, Hayes et al. [11] investigated TaN/n-GaN Schottky diode and showed that the contacts were stable at the temperature as high as 800 8C. They reported that the Schottky barrier height of 0.7 and 1.2 eV was obtained using the I–V and C–V methods [11]. In the present work, we have fabricated molybdenum (Mo) Schottky contact on n-type GaN (4.07 1018 cm3). The ideality factor and Schottky barrier heights of Mo are determined by I–V and C–V measurements as a function of annealing temperature.
C.K. Ramesh et al. / Materials Science & Engineering B 112 (2004) 30–33
2. Experimental details The GaN layer used in this study was grown by metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrate. An undoped GaN layer with a thickness of 2 mm was grown, followed by the growth of 2 mm thick nGaN: Si (nd = 4.07 1018 cm3) layer. The n-GaN layer was first ultrasonically degreased with warm trichloroethylene, acetone and methanol for 5 min each. This degreased layer was then dipped into boiling aqua regia [HNO3:HCl = 1:3] for 10 min to remove the surface oxides and rinsed in deionized water. The samples were immediately loaded into the electron beam evaporation system, ohmic metals Ti (30 nm)/Al (60 nm) was deposited on a portion of the sample and annealed at 850 8C in a nitrogen ambient for 30 s. Then, a 200 nm thick Mo (99.999%) was deposited through a stainless steel mask using electron beam evaporator under a pressure of 2 106 Torr. The diameter of the Schottky dots was 1 mm. The Mo Schottky diodes were sequentially annealed at various temperatures from 400 to 600 8C for 1 min under nitrogen ambient in rapid thermal annealing (RTA) system. Schottky diode characteristics were measured using I–V and C–V using Keithley Source measure unit (Model No. 230) and a Boonton capacitance meter (Model No. 72 B), respectively.
31
diode leakage current values are 1.2 106 and 5.8 103 A at 1 V. The barrier height and ideality factor of a metal–semiconductor contact were determined using thermionic emission theory is given by [12] qðV IRÞ I ¼ Is exp 1 (1) nkT with Is ¼ AA T 2 exp
qfb kT
(2)
Fig. 1 shows the typical forward and reverse I–V characteristics of Mo/n-GaN Schottky contact measured as a function of annealing temperature. It is found that the properties of all Mo/n-GaN Schottky diodes are uniform over different diodes, although it shows fairly large leakage currents. The leakage current value is 6.8 107 A at 1 V for the as-deposited, whereas annealed at 400 8C, 600 8C
where Is is the saturation current, q the electron charge, V the applied voltage, R the series resistance, n ideality factor, A** the effective Richardson constant, and fb the Schottky barrier height. The value of fb can be deduced directly from the I–V curves if the effective Richardson constant, A** is known. The theoretical value of A** is 26.4 A cm2 K2 based on the effective mass (m* = 0.22m0) of n-GaN [13] and was used here to deduce fb. A plot of I/[1 exp(qV/kT)] versus V (Fig. 2) yields ln Is as the intercept. Once Is was determined, the barrier height (fb) was determined. Calculations showed that the SBH is 0.81 eV for the as-deposited contact, 0.74 eV at 400 8C, 0.61 eV at 500 8C and 0.56 eV at 600 8C. The ideality factor is determined from a plot of natural log of current versus forward bias voltage (figure not shown) for small forward currents where the effect of series resistance is small. The ideality factor for the as-deposited Mo Schottky diodes was found to be 1.29. The ideality factor is improved to 1.04 upon annealing at 400 8C for 1 min in nitrogen ambient. Table 1 shows the values of ideality factor and the Schottky barrier height for Mo/n-GaN Schottky diodes. The values of ideality factor, while they are in general in agreement with those reported for Schottky diodes on n-GaN [5,14–18], nevertheless, are indicative
Fig. 1. The typical I–V characteristics for the Mo Schottky contact on ntype GaN as a function of annealing temperature.
Fig. 2. Plot of I/[1 exp(qV/kT)] vs. V for the Mo Schottky contacts annealed at different temperatures.
3. Results and discussion
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C.K. Ramesh et al. / Materials Science & Engineering B 112 (2004) 30–33
Table 1 The Schottky barrier heights and ideality factor of Mo/n-GaN as a function of annealing temperature Sample
Schottky barrier height (SBH), fb (eV) I–V
Norde
C–V
As-deposited 400 8C 500 8C 600 8C
0.81 0.74 0.61 0.56
0.75 0.66 0.55 0.51
1.02 0.92 0.85 0.73
Ideality factor, n
1.29 1.04 1.14 1.03
of nonideal behavior, suggesting that the transport mechanisms, other than just thermoionic, are probably present in these diodes. To compare the effective Schottky barrier height of contacts, the Norde method was also employed [19]. In this method, a function F(V) is plotted against the V. F(V) is given by F(V) = V/2 kT/2 ln[I(V)/AA**T2]. The effective SBH is given by fb = F(Vmin) + Vmin/2 kT/q, where F(Vmin) is the minimum value of F(V) and Vmin is the corresponding voltage. A plot of F(V) versus V for the samples annealed at different temperatures is shown in Fig. 3. The measurements showed that the SBH are 0.75 eV for the as-deposited, 0.66 eV at 400 8C, 0.55 eV at 500 8C and 0.51 eV at 600 8C, respectively. It is noted that these values are lower than those obtained by the I–V method. The values of SBH are given in Table 1. Capacitance and voltage (C–V) characteristics of Mo/nGaN Schottky diode were measured using a Boonton 72B 1 MHz capacitance meter. Fig. 4 shows a plot of 1/C2 as a function of bias voltage for as-deposited and annealed samples. The C–V relationship for Schottky diode is given by [20] 1 ¼ C2
2 es qNA2
Vbi
kT V q
(3)
Fig. 3. Plot of F(V) vs. V for the Mo Schottky contacts annealed at different temperatures.
Fig. 4. Plot of 1/C2 vs. V for the Mo Schottky contacts annealed at different temperatures.
where es is the permittivity of the semiconductor (es = 9.5e0), V is the applied voltage. The x-intercept of the plot of (1/C2) versus V, V0 is related to the built in potential Vbi by the equation Vbi = V0 + kT/q, where T is the absolute temperature. The barrier height is given by the equation fb = V0 + Vn + kT/q, where Vn = (kT/q) ln(Nc/Nd). The density of states in the conduction band edge is given by Nc = 2(2pm*kT/h2)3/2, where m* = 0.22m0 and its value is 2.53 1018 cm3 for GaN [2]. The barrier heights of Mo/n-GaN are 1.02 eV for as-deposited, 0.92 eV for 400 8C contacts, 0.85 eV for 500 8C contacts and 0.73 eV for contact annealed at 600 8C by C–V method. Fig. 5 shows a plot of barrier heights of Mo/n-GaN as a function of annealing temperature. It can be seen that the barrier height of the Mo Schottky diodes decreases upon annealing. For the contact annealed at 400 8C, the barrier height is high compared with that of the contact annealed at
Fig. 5. Plot of barrier heights of Mo/n-GaN as a function of annealing temperatures.
C.K. Ramesh et al. / Materials Science & Engineering B 112 (2004) 30–33
600 8C. It is also observed that the reverse leakage current is increased for the contact annealed at 600 8C compared to the contact annealed at 400 8C. The higher value in barrier height of metal/n-GaN Schottky contacts on annealing at 400 8C, under conditions where the metal is not observed to react with GaN, has also been reported previously [21,22]. According to Duboz et al. [22] the higher value in barrier height on annealing at moderate temperature of 400 8C can be attributed to a reduction in the density of interfacial defects. Also, Duboz et al. [22] observed that the nature of the metal does not play a major role in the barrier height enhancement on annealing. These researchers find that the Fermi level at metal/GaN interfaces is pinned by defects. Further, a modification of the defects density on annealing could change the pinning. This results in a change in the barrier height. It can also be seen from Fig. 5 the barrier heights, fb, obtained from I–V measurements are lower than those obtained from C–V measurements. This discrepancy between fI–V and fC–V for metal/n-GaN Schottky diodes has been attributed to factors such as the presence of a native oxide layer at the metal/GaN interface [4,23]. However, the metal probably comes into intimate contact with the semiconductor upon high temperature annealing. According to Werner and Guttler [24], spatial inhomogeneities at the metal/semiconductor interface of abrupt Schottky contact can also cause such differences in the barrier height determined from I–V and C–V measurements. Another possibility may be the transport mechanism in these diodes which is not purely due to thermionic emission. For these diodes, the fb obtained from I–V method is voltage or electric field sensitive, whereas the fb obtained from C–V is not.
4. Summary We have fabricated Mo Schottky contacts on n-GaN and characterized using I–V and C–V techniques as a function of annealing temperature. The barrier height of the as-deposited Schottky diode was found to be 0.81 and 1.02 eV, respectively. The barrier height slightly decreased to 0.74 and 0.92 eV when the contact annealing at 400 8C for 1 min. Further, increase in annealing temperature up to 600 8C, the barrier height decreased to 0.56 and 0.73 eV. The Mo Schottky contact was also shown to be relatively stable upon prolonged annealing at 400 8C for 30 min.
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Acknowledgement The authors thank the University Grant Commission (UGC), New Delhi for providing the financial assistance (Grant No. F.14-22/2002 (SR-I).
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