- Email: [email protected]
WSix Schottky contacts on n-type 4H-SiC
Solid-State Electronics 47 (2003) 1345–1350 www.elsevier.com/locate/sse
High temperature thermal stability of Au/Ti/WSix Schottky contacts on n-type 4H-SiC Jihyun Kim a, F. Ren a
a,*
, A.G. Baca b, R.D. Briggs b, S.J. Pearton
c
Department of Chemical Engineering, University of Florida, P.O. Box 116005, Gainesville, FL 32611, USA b Sandia National Laboratories, Albuquerque, NM 87185, USA c Department of Material Science and Engineering, University of Florida, Gainesville, FL 32611, USA Received 2 November 2002; received in revised form 14 January 2003; accepted 17 January 2003
Abstract The thermal stability of Au/Ti/WSix contacts on 4H-SiC was examined by Auger electron spectroscopy and current– voltage measurements. The silicide-based contacts on SiC are found to exhibit improved thermal stability compared to pure W contacts. The Au/Ti/WSix contacts show a maximum Schottky barrier height of 1.15 eV as obtained from current–voltage (I–V ) measurements. After 500 °C anneals, the Ti diffuses to the surface of the contact structure, followed by a Au-rich layer and finally the WSix . After 1000 °C anneals, the Ti and Au showed significant mixing. Particulates formed on the surface in the latter case were Au-rich phases. Ó 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Due to their excellent set of transport and band gap properties, SiC Schottky rectifiers are gaining interest for use in high power, high temperature electronic switching applications and for sensing of combustion gases in industry and other situations such as long-term space flights. The need to understand the long-term aging characteristics of the contacts, particularly at elevated temperatures has stimulated interest in understanding the electrical properties and thermal stability of different metal rectifying contacts on SiC [1–20]. In particular, systems such as WC [8], Ti0:58 W0:42 [10] and Pt/Ti/WSi/Ni [20] show very promising stability on SiC for high temperature applications. The WC showed a relatively low barrier height of 0.79 eV at 300 K in ntype 6H-SiC [8], while Ti0:58 W0:42 showed a higher value of 1.22 eV in n-type 6H-SiC over the temperature range 24–300 °C [10]. These are promising for rectifying contacts, while the Pt/Ti/WSi/Ni annealed at 1000 °C
*
Corresponding author. Tel.: +1-352-392-4757; fax: +1-352392-9513. E-mail address: [email protected]fl.edu (F. Ren).
showed excellent long-term stability as an ohmic contact on n-type 4H-SiC [20]. We have previously found that sputter-deposited WSix -based contacts show maximum barrier heights of 1.15 eV after a 500 °C post-deposition anneal to remove ion-induced damage [21]. The contacts showed reduced forward and reverse currents when measured at elevated temperatures compared to the more common Ni rectifying contacts used in SiC technology. These results suggest that WSix may be a promising candidate as a stable Schottky metallization on n-type SiC, but little work has appeared on this particular aspect. In this paper we report on the thermal stability of the contact against intermixing, as measured by Auger electron spectroscopy.
2. Experimental The starting substrates were nþ (n 1019 cm3 ) 4HSiC. Approximately 10 lm of undoped (n 5 1015 cm3 ) was grown on these substrates by vapor phase epitaxy technique. E-beam evaporated Ni (200 nm) was deposited for full backside area ohmic contacts and was annealed at 970 °C for 3 min to obtain a low resistance.
0038-1101/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00069-8
1346
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350
The samples were cleaned sequentially in acetone, isopropanol and buffered oxide etchant (BOE) prior to of WSi0:45 using an Ar sputter deposition of 700 A plasma and separate W and Si targets. Detailed characterization of these types of films in GaN have been reported previously [21–23]. To reduce the sheet resis of Ti and 700 tance of the metallization, a further 200 A A of Au were deposited on top of the WSix . The 120 lm diameter Schottky diodes were patterned using standard photolithography processing. The thermal stability of the Au/Ti/WSix diodes were tested for anneals up to 1000 °C for 1 min under flowing N2 in a Heatpulse 610T furnace. Current–voltage (I–V ) measurements were performed in the temperature range (25–300 °C) using an HP4156C parameter analyzer. The Auger electron spectroscopy (AES) was performed on a Physical Electronics 660 Scanning Auger Microprobe with a 10 keV, 1 lA beam at 30° for the sample normal. Profiling was achieved by sputtering with a 3 keV Ar ion beam at a current of 2 lA rastered over a 3 mm2 area. The sputter /min, for Ti was 90 A /min and for rate for Au was 200 A
/min. Optical and secondary electron imWSi was 60 A ages of the analysis areas were also obtained.
3. Results and discussion Fig. 1 shows reverse leakage current characteristics from the diodes as a function of post-deposition anneal temperature (top) and as a function of measurement temperature for a post-deposition anneal of 500 °C, which was found to produce the maximum Schottky barrier height of 1.15 eV. More detail on the electrical characteristics of the contacts is given elsewhere [24]. The ideality factors were typically 1.1 after the optimum
1.0x10-2 As deposited 500oC, 1min, N 2
8.0x10-3
700oC, 1min, N 2
6.0x10
900oC, 1min, N 2
-3
1000oC, 1min, N 2
4.0x10-3 2.0x10-3
-10
-8
-6
-4
-2
0
1.5x10-7
WSi/n-SiC WSi(25oC)
1.0x10-7
WSi(100oC) WSi(200oC) WSi(300oC)
5.0x10-8
-30
-20
-10
0
Fig. 1. Reverse I–V characteristics from Au/Ti/WSi contacts on 6H-SiC as a function of post-deposition anneal temperature (top) or as a function of measurement temperature after an optimized post-deposition anneal of 500 °C.
Fig. 2. Secondary electron images of Au/Ti/WSi contacts as-deposited (top) or after 500 °C (center) or 1000 °C (bottom) anneals.
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350 Table 1 Concentration of elements detected on the as-received surfaces (in at.%)a Sample
C
O
Au
Ti
Si
1––as-deposited 2––500 °C 3––1000 °C
54 48 36
1 34 25
45 7 11
nd 7 14
nd 4 13
a AES does not detect hydrogen and helium and all concentrations are normalized to 100%.
Fig. 3. AES survey spectra from Au/Ti/WSi contacts asdeposited (top) or after 500 °C (center) or 1000 °C (bottom) anneals.
1347
anneal, whereas the unannealed (as-deposited) contacts showed ideality factors of 2 and a lower barrier height of 0.97 eV. The barrier height decreased for anneals above about 600 °C, reaching values of 0.99 eV after 700 °C anneals, 0.69 eV after 900 °C and 0.41 eV after 1000 °C. The reverse current densities were slightly lower at elevated measurement temperatures than for Ni Schottky contacts on the same wafers. Fig. 2 shows secondary electron images of the asdeposited contact (top), after 500 °C anneal (center) and after 1000 °C anneal (bottom). The first two show
Fig. 4. AES elemental depth profiles from Au/Ti/WSi contacts as-deposited (top) or after 500 °C (center) or 1000 °C (bottom) anneals.
1348
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350
Fig. 5. Elemental maps of W, Au and Ti from the 1000 °C annealed contact after sputter-depth profiling to near the WSi/SiC interface.
relatively smooth morphology, while the highest temperature anneal produces many small (1–3 lm) particlelike features and some larger particles. These changes in morphology correspond to changes in the near-surface composition, as summarized in Table 1. AES survey spectra from the as-deposited, 500 and 1000 °C annealed samples are shown in Fig. 3 and the corresponding depth profiles are shown in Fig. 4. The as-deposited sample shows abrupt and smooth interfaces between the various layers in the metallization scheme. After 500 °C annealing, the Ti diffuses out into the Au, while the WSi/SiC interface remains abrupt. The movement of Ti towards the surface is accompanied by its oxidation. After the 1000 °C annealing, the WSi/SiC interface still remains fairly abrupt, while the Ti and Au are completely intermixed and there is outdiffusion of C from the substrate into the reacted surface region. Under these conditions, the contact shows a more ohmic-type behavior and the formation of the b-phase W2 N compound is detected by X-ray diffraction experiments. Elemental maps are shown for the 1000 °C annealed sample in Figs. 5 and 6. These were obtained after the sputter-depth profile was complete and show that the particles present on the surface contain Au. The pres-
ence of the other elements is expected because the profile was stopped near the WSi/SiC interface. The AES data basically confirms the electrical results in that the contact shows significant reaction for anneals above 600–700 °C and this will define the upper limit for the stable range of device or sensor operation on which these contacts are employed. Note that it is the presence of the Au and Ti that dominates the stability characteristics and future work should focus on overlayers that provide low sheet resistance without compromising the thermal and electrical stability.
4. Summary and conclusions The thermal stability of Au/Ti/WSi contacts on SiC has been investigated by AES and I–V measurements. Following the sputter deposition, annealing at 500 °C removes ion-induced damage and produces the best electrical properties. The contact metallurgy is stable under these conditions, but the use of higher annealing temperatures leads to significant intermixing of the contact and a degradation of the barrier height and ideality factor.
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350
1349
Fig. 6. Elemental maps of C, O and Si from the 1000 °C annealed contact after sputter-depth profiling to near the WSi/SiC interface.
Acknowledgements The work at UF is particularly supported by NASA (NAG10-316, Dr. William Knott) and the UCF-UF Space research Initiative, and also by NSF DMR0101438. Sandia is a multiprogram laboratory operated by Sandia Corporation for Lockheed–Martin under DOE contract DE-AC-04-85000.
References [1] Crofton J, Porter LM, Williams JR. Phys Stat Solidi B 1997;202:581. [2] Porter LM, Davis RF. Mater Sci Eng B 1995;34:83. [3] Lee S-K, Zetterling C-M, Ostling M. J Electron Mater 2001;30:242. [4] Melloch MR, Cooper Jr JA. MRS Bull 1997;22:42. [5] Waldrop JR, Grant RW, Wang YC, Davis RF. J Appl Phys 1992;72:4757. [6] Oder TN, Williams JR, Bozack MJ, Iyer V, Mohney SE, Crofton J. J Electron Mater 1998;27:324. [7] Luckowski ED, DeLucca JM, Willimas JR, Mohney SE, Bozack MJ, Isaacs-Smith T, et al. J Electron Mater 2001;27:330.
€ stling M, Zetterling C-M, T€agtstr€ [8] Lundberg N, O om P, Jansson U. J Electron Mater 2000;29:372. [9] Skromme BJ, Luckowski E, Moore K, Bhatnagar M, Weitzel CE, Gehoshi T, et al. J Electron Mater 2000;29: 376. € stling M. J Appl Phys 2000;87: [10] Lee S-K, Zetterling C-M, O 8039. € stling M. Process Technology [11] Zetterling C-M, Lee S-K, O for SiC devices. London: INSPEC, IEE; 2002 (Chapter 6). [12] Schoen KP, Woodall JM, Cooper JA, Melloch MR. IEEE Trans Electron Dev 1998;45:1595. [13] Itoh A, Matsunami H. Phys Stat Solidi A 1997;162:389. [14] Mok D, Egloff R, Arnold E. Mater Sci Forum 1998;264– 268:929. [15] Saxena V, Su JN, Steckl AJ. IEEE Trans Electron Dev 1999;46:456. [16] Lundberg N, Lee S-K, Ostling M. J Electrochem Soc 1996;143:662. [17] Kestle A, Wilks SP, Dunstan PR, Pritchard M, Mawby PA. Electron Lett 2000;36:267. [18] Cole MW, Haffard CW, Demaree JD, Hirvonen JK, Ervin MH, Wood MC. Presented at Army Research Conference, Orlando, FL, August 2002. [19] Cole MW, Joshi PC, Hubbard CW, Wood MC, Ervin MH, Geil B, et al. J Appl Phys 2000;88:2652. [20] Oskam G, Searson PC, Cole MW. Appl Phys Lett 2000;76: 1300.
1350
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350
[21] Cole MW, Eckart DW, Han WY, Pfeffer RL, Monahan T, Ren F, et al. J Appl Phys 1996;80:278. [22] Cole MW, Ren F, Pearton SJ. J Electrochem Soc 1997;144:L275.
[23] Zeitouny A, Eizenberg M, Pearton SJ, Ren F. J Appl Phys 2000;88:2048. [24] Kim J, Ren F, Baca AG, Chung GY, Pearton SJ. J Appl Phys (in press).
High temperature thermal stability of Au/Ti/WSix Schottky contacts on n-type 4H-SiC Jihyun Kim a, F. Ren a
a,*
, A.G. Baca b, R.D. Briggs b, S.J. Pearton
c
Department of Chemical Engineering, University of Florida, P.O. Box 116005, Gainesville, FL 32611, USA b Sandia National Laboratories, Albuquerque, NM 87185, USA c Department of Material Science and Engineering, University of Florida, Gainesville, FL 32611, USA Received 2 November 2002; received in revised form 14 January 2003; accepted 17 January 2003
Abstract The thermal stability of Au/Ti/WSix contacts on 4H-SiC was examined by Auger electron spectroscopy and current– voltage measurements. The silicide-based contacts on SiC are found to exhibit improved thermal stability compared to pure W contacts. The Au/Ti/WSix contacts show a maximum Schottky barrier height of 1.15 eV as obtained from current–voltage (I–V ) measurements. After 500 °C anneals, the Ti diffuses to the surface of the contact structure, followed by a Au-rich layer and finally the WSix . After 1000 °C anneals, the Ti and Au showed significant mixing. Particulates formed on the surface in the latter case were Au-rich phases. Ó 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction Due to their excellent set of transport and band gap properties, SiC Schottky rectifiers are gaining interest for use in high power, high temperature electronic switching applications and for sensing of combustion gases in industry and other situations such as long-term space flights. The need to understand the long-term aging characteristics of the contacts, particularly at elevated temperatures has stimulated interest in understanding the electrical properties and thermal stability of different metal rectifying contacts on SiC [1–20]. In particular, systems such as WC [8], Ti0:58 W0:42 [10] and Pt/Ti/WSi/Ni [20] show very promising stability on SiC for high temperature applications. The WC showed a relatively low barrier height of 0.79 eV at 300 K in ntype 6H-SiC [8], while Ti0:58 W0:42 showed a higher value of 1.22 eV in n-type 6H-SiC over the temperature range 24–300 °C [10]. These are promising for rectifying contacts, while the Pt/Ti/WSi/Ni annealed at 1000 °C
*
Corresponding author. Tel.: +1-352-392-4757; fax: +1-352392-9513. E-mail address: [email protected]fl.edu (F. Ren).
showed excellent long-term stability as an ohmic contact on n-type 4H-SiC [20]. We have previously found that sputter-deposited WSix -based contacts show maximum barrier heights of 1.15 eV after a 500 °C post-deposition anneal to remove ion-induced damage [21]. The contacts showed reduced forward and reverse currents when measured at elevated temperatures compared to the more common Ni rectifying contacts used in SiC technology. These results suggest that WSix may be a promising candidate as a stable Schottky metallization on n-type SiC, but little work has appeared on this particular aspect. In this paper we report on the thermal stability of the contact against intermixing, as measured by Auger electron spectroscopy.
2. Experimental The starting substrates were nþ (n 1019 cm3 ) 4HSiC. Approximately 10 lm of undoped (n 5 1015 cm3 ) was grown on these substrates by vapor phase epitaxy technique. E-beam evaporated Ni (200 nm) was deposited for full backside area ohmic contacts and was annealed at 970 °C for 3 min to obtain a low resistance.
0038-1101/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0038-1101(03)00069-8
1346
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350
The samples were cleaned sequentially in acetone, isopropanol and buffered oxide etchant (BOE) prior to of WSi0:45 using an Ar sputter deposition of 700 A plasma and separate W and Si targets. Detailed characterization of these types of films in GaN have been reported previously [21–23]. To reduce the sheet resis of Ti and 700 tance of the metallization, a further 200 A A of Au were deposited on top of the WSix . The 120 lm diameter Schottky diodes were patterned using standard photolithography processing. The thermal stability of the Au/Ti/WSix diodes were tested for anneals up to 1000 °C for 1 min under flowing N2 in a Heatpulse 610T furnace. Current–voltage (I–V ) measurements were performed in the temperature range (25–300 °C) using an HP4156C parameter analyzer. The Auger electron spectroscopy (AES) was performed on a Physical Electronics 660 Scanning Auger Microprobe with a 10 keV, 1 lA beam at 30° for the sample normal. Profiling was achieved by sputtering with a 3 keV Ar ion beam at a current of 2 lA rastered over a 3 mm2 area. The sputter /min, for Ti was 90 A /min and for rate for Au was 200 A
/min. Optical and secondary electron imWSi was 60 A ages of the analysis areas were also obtained.
3. Results and discussion Fig. 1 shows reverse leakage current characteristics from the diodes as a function of post-deposition anneal temperature (top) and as a function of measurement temperature for a post-deposition anneal of 500 °C, which was found to produce the maximum Schottky barrier height of 1.15 eV. More detail on the electrical characteristics of the contacts is given elsewhere [24]. The ideality factors were typically 1.1 after the optimum
1.0x10-2 As deposited 500oC, 1min, N 2
8.0x10-3
700oC, 1min, N 2
6.0x10
900oC, 1min, N 2
-3
1000oC, 1min, N 2
4.0x10-3 2.0x10-3
-10
-8
-6
-4
-2
0
1.5x10-7
WSi/n-SiC WSi(25oC)
1.0x10-7
WSi(100oC) WSi(200oC) WSi(300oC)
5.0x10-8
-30
-20
-10
0
Fig. 1. Reverse I–V characteristics from Au/Ti/WSi contacts on 6H-SiC as a function of post-deposition anneal temperature (top) or as a function of measurement temperature after an optimized post-deposition anneal of 500 °C.
Fig. 2. Secondary electron images of Au/Ti/WSi contacts as-deposited (top) or after 500 °C (center) or 1000 °C (bottom) anneals.
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350 Table 1 Concentration of elements detected on the as-received surfaces (in at.%)a Sample
C
O
Au
Ti
Si
1––as-deposited 2––500 °C 3––1000 °C
54 48 36
1 34 25
45 7 11
nd 7 14
nd 4 13
a AES does not detect hydrogen and helium and all concentrations are normalized to 100%.
Fig. 3. AES survey spectra from Au/Ti/WSi contacts asdeposited (top) or after 500 °C (center) or 1000 °C (bottom) anneals.
1347
anneal, whereas the unannealed (as-deposited) contacts showed ideality factors of 2 and a lower barrier height of 0.97 eV. The barrier height decreased for anneals above about 600 °C, reaching values of 0.99 eV after 700 °C anneals, 0.69 eV after 900 °C and 0.41 eV after 1000 °C. The reverse current densities were slightly lower at elevated measurement temperatures than for Ni Schottky contacts on the same wafers. Fig. 2 shows secondary electron images of the asdeposited contact (top), after 500 °C anneal (center) and after 1000 °C anneal (bottom). The first two show
Fig. 4. AES elemental depth profiles from Au/Ti/WSi contacts as-deposited (top) or after 500 °C (center) or 1000 °C (bottom) anneals.
1348
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350
Fig. 5. Elemental maps of W, Au and Ti from the 1000 °C annealed contact after sputter-depth profiling to near the WSi/SiC interface.
relatively smooth morphology, while the highest temperature anneal produces many small (1–3 lm) particlelike features and some larger particles. These changes in morphology correspond to changes in the near-surface composition, as summarized in Table 1. AES survey spectra from the as-deposited, 500 and 1000 °C annealed samples are shown in Fig. 3 and the corresponding depth profiles are shown in Fig. 4. The as-deposited sample shows abrupt and smooth interfaces between the various layers in the metallization scheme. After 500 °C annealing, the Ti diffuses out into the Au, while the WSi/SiC interface remains abrupt. The movement of Ti towards the surface is accompanied by its oxidation. After the 1000 °C annealing, the WSi/SiC interface still remains fairly abrupt, while the Ti and Au are completely intermixed and there is outdiffusion of C from the substrate into the reacted surface region. Under these conditions, the contact shows a more ohmic-type behavior and the formation of the b-phase W2 N compound is detected by X-ray diffraction experiments. Elemental maps are shown for the 1000 °C annealed sample in Figs. 5 and 6. These were obtained after the sputter-depth profile was complete and show that the particles present on the surface contain Au. The pres-
ence of the other elements is expected because the profile was stopped near the WSi/SiC interface. The AES data basically confirms the electrical results in that the contact shows significant reaction for anneals above 600–700 °C and this will define the upper limit for the stable range of device or sensor operation on which these contacts are employed. Note that it is the presence of the Au and Ti that dominates the stability characteristics and future work should focus on overlayers that provide low sheet resistance without compromising the thermal and electrical stability.
4. Summary and conclusions The thermal stability of Au/Ti/WSi contacts on SiC has been investigated by AES and I–V measurements. Following the sputter deposition, annealing at 500 °C removes ion-induced damage and produces the best electrical properties. The contact metallurgy is stable under these conditions, but the use of higher annealing temperatures leads to significant intermixing of the contact and a degradation of the barrier height and ideality factor.
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350
1349
Fig. 6. Elemental maps of C, O and Si from the 1000 °C annealed contact after sputter-depth profiling to near the WSi/SiC interface.
Acknowledgements The work at UF is particularly supported by NASA (NAG10-316, Dr. William Knott) and the UCF-UF Space research Initiative, and also by NSF DMR0101438. Sandia is a multiprogram laboratory operated by Sandia Corporation for Lockheed–Martin under DOE contract DE-AC-04-85000.
References [1] Crofton J, Porter LM, Williams JR. Phys Stat Solidi B 1997;202:581. [2] Porter LM, Davis RF. Mater Sci Eng B 1995;34:83. [3] Lee S-K, Zetterling C-M, Ostling M. J Electron Mater 2001;30:242. [4] Melloch MR, Cooper Jr JA. MRS Bull 1997;22:42. [5] Waldrop JR, Grant RW, Wang YC, Davis RF. J Appl Phys 1992;72:4757. [6] Oder TN, Williams JR, Bozack MJ, Iyer V, Mohney SE, Crofton J. J Electron Mater 1998;27:324. [7] Luckowski ED, DeLucca JM, Willimas JR, Mohney SE, Bozack MJ, Isaacs-Smith T, et al. J Electron Mater 2001;27:330.
€ stling M, Zetterling C-M, T€agtstr€ [8] Lundberg N, O om P, Jansson U. J Electron Mater 2000;29:372. [9] Skromme BJ, Luckowski E, Moore K, Bhatnagar M, Weitzel CE, Gehoshi T, et al. J Electron Mater 2000;29: 376. € stling M. J Appl Phys 2000;87: [10] Lee S-K, Zetterling C-M, O 8039. € stling M. Process Technology [11] Zetterling C-M, Lee S-K, O for SiC devices. London: INSPEC, IEE; 2002 (Chapter 6). [12] Schoen KP, Woodall JM, Cooper JA, Melloch MR. IEEE Trans Electron Dev 1998;45:1595. [13] Itoh A, Matsunami H. Phys Stat Solidi A 1997;162:389. [14] Mok D, Egloff R, Arnold E. Mater Sci Forum 1998;264– 268:929. [15] Saxena V, Su JN, Steckl AJ. IEEE Trans Electron Dev 1999;46:456. [16] Lundberg N, Lee S-K, Ostling M. J Electrochem Soc 1996;143:662. [17] Kestle A, Wilks SP, Dunstan PR, Pritchard M, Mawby PA. Electron Lett 2000;36:267. [18] Cole MW, Haffard CW, Demaree JD, Hirvonen JK, Ervin MH, Wood MC. Presented at Army Research Conference, Orlando, FL, August 2002. [19] Cole MW, Joshi PC, Hubbard CW, Wood MC, Ervin MH, Geil B, et al. J Appl Phys 2000;88:2652. [20] Oskam G, Searson PC, Cole MW. Appl Phys Lett 2000;76: 1300.
1350
J. Kim et al. / Solid-State Electronics 47 (2003) 1345–1350
[21] Cole MW, Eckart DW, Han WY, Pfeffer RL, Monahan T, Ren F, et al. J Appl Phys 1996;80:278. [22] Cole MW, Ren F, Pearton SJ. J Electrochem Soc 1997;144:L275.
[23] Zeitouny A, Eizenberg M, Pearton SJ, Ren F. J Appl Phys 2000;88:2048. [24] Kim J, Ren F, Baca AG, Chung GY, Pearton SJ. J Appl Phys (in press).