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Structures, varistor properties, and electrical stability of ZnO thin films
Materials Letters 63 (2009) 2321–2323
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Structures, varistor properties, and electrical stability of ZnO thin films Hui Lu ⁎, Yuele Wang, Xian Lin Department of Physics, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 28 February 2009 Accepted 1 August 2009 Available online 7 August 2009 Keywords: ZnO thin film Microstructure Electrical properties Varistors Stability
a b s t r a c t In this letter, we report the structures, varistor properties, and electrical stability of ZnO thin films deposited by the gas discharge activated reaction evaporation (GDARE) technique. The X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements showed that the thin films thus prepared have polycrystalline structures with the preferred orientation along the (002) plane whose surface consists of ZnO aggregates with sizes of 50-200 nm. The ZnO thin films deposited by GDARE and annealed at 250 °C for 2 h have strong nonlinear varistor-type I-V characteristics. The nonlinear coefficient (α) of a single-layered ZnO thin film sample was 33 and that of a triplelayered sample obtained by the many-time deposition was 62. The varistor voltages (V1mA) of the two samples are found rather close each other. Under a DC bias of 0.75 V1mA and a temperature of 150 °C these thin films exhibit good electrical stability with a degradation rate coefficient KT of 0.05 mA/h1/2. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, ZnO materials have attracted much attention due to their unique physicochemical properties which are of wide applications in light-emitting devices, photo detectors, solar cells, piezoelectric transducers, gas sensors, etc [1–4]. ZnO varistors are semiconducting ceramic devices. They exhibit strong nonlinear voltage–current (V–I) characteristics and have been extensively used to protect various semiconductor devices, electronic circuits and electric power systems from being damaged by transient abnormal voltages [5,6]. Most commercial ZnO varistors are prepared by sintering ZnO powder at high temperatures together with small amount of other oxide additives, such as Bi2O3 [7], CoO [8], etc, and their electrical properties are improved by the advanced ceramic processing techniques [9–11]. Owing to the rapid development of LSI and SLSI electronics, there is a need to develop low-voltage, lower power consumption and highly integrated surface-mountable thin layer varistors. The thin film structure of ZnO has great potential advantages [12–14]. So far, ZnO thin films can be prepared by many methods, including chemical vapor deposition (CVD) [15], magnetron sputtering [16], molecular beam epitaxy (MBE) [17], pulsed laser deposition [18], spray pyrolysis [19], and the sol–gel technique [20]. The properties of ZnO thin films are closely related to their microstructures which depend greatly on the preparation methods and conditions. We have recently developed a deposition method called gas discharge activated reaction evaporation (GDARE) to prepare thin films with good crystalline quality at lower temperatures [21]. In GDARE the
⁎ Corresponding author. Tel.: +86 21 64253101; fax: +86 21 64530721. E-mail address: [email protected] (H. Lu). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.08.001
preparation parameters are easy to control, and consequently, thin films with different structures and properties can be obtained by changing the preparation parameters. In this letter, we present our study on ZnO thin films grown by GDARE under certain conditions, which exhibit strong nonlinear varistor-type I–V characteristics. The electrical properties and the stability against a DC accelerated aging stress of the thin films are also investigated.
2. Experimental procedure ZnO thin films were deposited by GDARE on ITO/glass or glass substrates using the following procedures. The substrate temperature was raised to the desired value. Oxygen was then introduced into the vacuum chamber and the flow rate was set to the desired oxygen pressure. A highly negative voltage was applied to the ring electrode between the evaporation source and the substrate. The zinc powder used as a source material was heated to evaporate in a molybdenum boat. The gas discharge produces a plasma region in which the oxygen molecules were activated. Finally, the evaporated zinc atoms entered into the discharge region and reacted with the activated oxygen molecules, and the product was deposited onto the substrate, forming a ZnO film. Table 1 lists the preparation conditions for the ZnO thin film samples. The thickness of a thin film does not continue to increase if the deposition time is over a certain length (about 20 min), and a multilayered ZnO thin film can be obtained through the many-time deposition. The thicknesses of the single-layered and triple-layered ZnO thin films are 100 nm and 600 nm, respectively. The thickness of the investigated thin film samples is varied from 100 nm to 600 nm. The ZnO thin films fabricated by the above method were heat-treated
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H. Lu et al. / Materials Letters 63 (2009) 2321–2323
Table 1 Preparation parameters of ZnO thin film samples. Substrate temperature (°C) Oxygen gas pressure (Pa) Deposition time (min)
50~100 10~13 15
Gas discharge voltage (V) Evaporation current (A) Distance between the substrate and the evaporation source (cm)
800 35 8∼12
at 250 °C in air for 2 h. Their surface morphology was observed with the AJ-III atomic force microscope (AFM) and the crystal structures were examined by X-ray diffraction (XRD, Rigaku DMAX/VB). The I–V characteristics of the thin films were measured with a programmable voltage source which scans the DC voltage at a speed of 1 V/min. The varistor voltages (V1mA) were measured at a current of 1.0 mA and the leakage currents (IL) were observed at 0.75 V1mA. In addition, the nonlinear coefficient (α) was determined by logI2 − logI1 α = logV , where V1 and V2 are voltages corresponding to I1 2 − logV1 and I2, respectively, and are determined in the sharp region of the I–V curve, with I2 = 10I1. The stability tests were performed under a DC bias of 0.75 V1mA and a temperature of 150 °C for 6 h.
Fig. 2. I–V characteristic curves of the ZnO thin films.
Fig. 1 shows the three dimensional AFM image of a ZnO thin film sample fabricated by GDARE. From this image, it can be found that the surface of the thin film consists of massif-like ZnO aggregates with grain sizes of 50 to 200 nm and the sample has lower surface roughness. The XRD (not shown) analysis indicated that the thin film has polycrystalline structures with the preferred orientation along the c-axis and its composition deviates from that of the stoichiometrical ZnO. The I–V characteristic curves of single-layered and triple-layered ZnO thin films are shown in Fig. 2. It is evident that the ZnO thin films deposited using GDARE exhibited high nonlinear I–V properties. The calculated nonlinear coefficient (α) and varistor voltage (V1mA) of the single-layered ZnO thin film sample were 33 and 19.0 V, respectively, whereas the α and V1mA of the triple-layered thin film sample were 62 and 20.0 V, respectively. This indicates that the degree of nonlinearity of the triple-layered ZnO thin film prepared by the many-time deposition was enhanced significantly but the varistor voltage (V1mA) increased slightly. ZnO varistor is a polycrystalline semiconductor containing a large number of grain boundaries. It is widely accepted that the nonlinear I–V behaviors of ZnO varistors were attributed to the double Schottky barriers formed at grain boundaries [22,23]. A higher barrier results in a lower leakage current (IL) and consequently
a larger nonlinear coefficient (α). The varistor voltage (V1mA) is closely related to the number of grain boundaries, the less the number of grain boundaries, the lower the varistor voltage (V1mA). Based on the experimental results in this work, it can be concluded that the ZnO thin films prepared by GDARE contain a large number of electrically active grain boundaries capable of absorbing strongly oxygen which creates many electronic trapping states, leading to the formation of the high Schottky potential barriers at the interfaces. On the other hand, the many-time depositing method in GDARE would be useful to raise the potential barrier at the grain boundary in ZnO thin films. Therefore, the magnitude of the nonlinear coefficient (α) of a multilayered ZnO thin film sample was enhanced significantly. Furthermore, the increases in the grain size and thickness of a ZnO thin film due to many-time deposition lead to a little increase in the number of grain boundaries. As a result, the varistor voltage (V1mA) of a multilayered ZnO thin film sample slightly increased. In practice, a ZnO varistor is always subjected to a continuous electrical stress and the varistor is gradually degraded with time. Therefore, in addition to the nonlinearity, the electrical stability is a very important characteristics of a ZnO varistor [24]. Fig. 3 shows the variations of the nonlinear coefficient (α) and leakage current (IL) of the ZnO single-layered thin film sample with time under a DC bias of 0.75 V1mA and a temperature of 150 °C. From the figure, we can find that the leakage current (IL) increases with time whereas the nonlinear coefficient (α) decreases with time. The stability of a varistor can be estimated by the degradation1 rate coefficient (KT) calculated from the expression IL = ILo + KT t 2 [25], where IL is the
Fig. 1. AFM image of a ZnO thin film sample.
Fig. 3. Variations of the nonlinear coefficient (α) and the leakage current (IL) of the single-layered ZnO thin film sample with time under a DC stress.
3. Results and discussion
H. Lu et al. / Materials Letters 63 (2009) 2321–2323
leakage current at the degradation time (t) and ILo corresponds to IL at t = 0. The lower the KT value, the higher the stability. As shown in Fig. 3, the ZnO varistor thin film has a KT value of 0.05 mA/h1/2 and therefore exhibits a relatively good stability with essentially no thermal runaway. The varistor voltage (V1mA) of the ZnO thin film sample decreases from 19 V to 11 V under a DC bias of 0.75 V1mA and a temperature of 150 °C for 6 h. The leakage current increases slightly and the varistor voltage decreases greatly in the first 3 h. This means that the decrease of the nonlinear coefficient (α) of the ZnO varistor thin film is not only due to the increase of the leakage current. This experimental result is different from those of the bulk ZnO ceramic varistors [10,14]. Furthermore, according to our experimental results up to now, the electrical stabilities of the single-layered and multilayered ZnO varistor thin films are similar. Therefore, we believe that the distribution and shape of the Schottky barriers at the grain boundaries in the thin films are very complicated, and probably there exist also some special transportation processes. Further work is needed in order to elucidate the mechanism about the conduction and degradation of the thin films. 4. Conclusion The ZnO varistor thin films were fabricated by the GDARE depositing method. The microstructures and electrical properties of the thin films were studied. AFM and XRD results showed that the thin films have polycrystalline structures with the c-axis as the preferential orientation whose surface consists of massif-like ZnO aggregates with grain sizes ranging from 50 to 200 nm. The ZnO thin film annealed at 250 °C for 2 h shows highly nonlinear I–V properties and lower breakdown voltage. The nonlinear coefficient (α) of the single-layered thin film sample is 33 and that of the triple-layered thin film sample
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prepared by the many-time deposition is raised to 62. The varistor voltages (V1mA) of these films are similar. Under a DC voltage of 0.75 V1mA and 150 °C, the ZnO varistor thin films exhibit good stability with the degradation rate coefficient (KT) of 0.05 mA/h1/2. Therefore, we believe that the ZnO thin films thus prepared are a promising material for advanced varistors.
References [1] Rao BB. Mater Chem Phys 2000;64:62–5. [2] Bagall DM, Chen YF, Zhu Z, Yao T, Koyama S, Shen MY, et al. Appl Phys Lett 1997;70: 2230–2. [3] Look DC. Mater Sci Eng 2001;B80:383–7. [4] Wang XB, Song C, Li DM. J Appl Surf Sci 2006;253:1639–43. [5] Levinson LM, Philipp HR. Ceram Soc Bull 1986;65:639–46. [6] Gupta TK. J Am Ceram Soc 1990;73:1817–40. [7] Kutty TRN, Ezhilvalavan S. Mater Chem Phys 1994;38:267–76. [8] Nahm CW, Shin BC, Park JA, Yoo DH. Mater Lett 2006;60:164–7. [9] Banerjec A, Ramamohan TR, Patni MJ. Mater Res Bull 2001;36:1259–67. [10] Nahm CW, Kim HS. Mater Lett 2003;57:1544–9. [11] Nahm CW. Mater Lett 2008;62:4440–2. [12] Herio N, Hiramatsu M, Nawate M, Lmaeda K, Torii T. Vacuum 1998;51:719–22. [13] Mista W, Ziaja J, Gubanski A. Vacuum 2004;74:293–6. [14] Jiang SL, Xie TT, Yu H, Huang YQ, Zhang HB. Microelectron Eng 2008;85:371–4. [15] Hu J, Gordon RG. J Appl Phys 1992;72:5381–92. [16] Ellmer K. J Phys D 2000;32:R17–32. [17] Heo YW, Norton DP, Pearton SJ. J Appl Phys 2005;98 073502.1–073502.6. [18] Choi JH, Tabata H, Kawai T. J Cryst Growth 2001;226:493–500. [19] Ortíz A, Falcony C, Hernández JA, Garcia M, Alonso JC. Thin Solid Films 1997;293:103–7. [20] Kim YS, Tai WP, Shu SJ. Thin Solid Films 2005;491:153–60. [21] Lu H, Yin Q, Xia J, Pan X. Chin J Chem Phys 2005;18:1034–8. [22] Bartkowiak M, Mahan GD, Modine FA, Alim MA. J Appl Phys 1996;79:273–81. [23] Magnusson KO, Wiklund S. J Appl Phys 1994;76:7405–9. [24] Nahm CW. Mater Lett 2001;47:182–7. [25] Fan J, Freer R. J Appl Phys 1995;77:4795.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Structures, varistor properties, and electrical stability of ZnO thin films Hui Lu ⁎, Yuele Wang, Xian Lin Department of Physics, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 28 February 2009 Accepted 1 August 2009 Available online 7 August 2009 Keywords: ZnO thin film Microstructure Electrical properties Varistors Stability
a b s t r a c t In this letter, we report the structures, varistor properties, and electrical stability of ZnO thin films deposited by the gas discharge activated reaction evaporation (GDARE) technique. The X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements showed that the thin films thus prepared have polycrystalline structures with the preferred orientation along the (002) plane whose surface consists of ZnO aggregates with sizes of 50-200 nm. The ZnO thin films deposited by GDARE and annealed at 250 °C for 2 h have strong nonlinear varistor-type I-V characteristics. The nonlinear coefficient (α) of a single-layered ZnO thin film sample was 33 and that of a triplelayered sample obtained by the many-time deposition was 62. The varistor voltages (V1mA) of the two samples are found rather close each other. Under a DC bias of 0.75 V1mA and a temperature of 150 °C these thin films exhibit good electrical stability with a degradation rate coefficient KT of 0.05 mA/h1/2. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, ZnO materials have attracted much attention due to their unique physicochemical properties which are of wide applications in light-emitting devices, photo detectors, solar cells, piezoelectric transducers, gas sensors, etc [1–4]. ZnO varistors are semiconducting ceramic devices. They exhibit strong nonlinear voltage–current (V–I) characteristics and have been extensively used to protect various semiconductor devices, electronic circuits and electric power systems from being damaged by transient abnormal voltages [5,6]. Most commercial ZnO varistors are prepared by sintering ZnO powder at high temperatures together with small amount of other oxide additives, such as Bi2O3 [7], CoO [8], etc, and their electrical properties are improved by the advanced ceramic processing techniques [9–11]. Owing to the rapid development of LSI and SLSI electronics, there is a need to develop low-voltage, lower power consumption and highly integrated surface-mountable thin layer varistors. The thin film structure of ZnO has great potential advantages [12–14]. So far, ZnO thin films can be prepared by many methods, including chemical vapor deposition (CVD) [15], magnetron sputtering [16], molecular beam epitaxy (MBE) [17], pulsed laser deposition [18], spray pyrolysis [19], and the sol–gel technique [20]. The properties of ZnO thin films are closely related to their microstructures which depend greatly on the preparation methods and conditions. We have recently developed a deposition method called gas discharge activated reaction evaporation (GDARE) to prepare thin films with good crystalline quality at lower temperatures [21]. In GDARE the
⁎ Corresponding author. Tel.: +86 21 64253101; fax: +86 21 64530721. E-mail address: [email protected] (H. Lu). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.08.001
preparation parameters are easy to control, and consequently, thin films with different structures and properties can be obtained by changing the preparation parameters. In this letter, we present our study on ZnO thin films grown by GDARE under certain conditions, which exhibit strong nonlinear varistor-type I–V characteristics. The electrical properties and the stability against a DC accelerated aging stress of the thin films are also investigated.
2. Experimental procedure ZnO thin films were deposited by GDARE on ITO/glass or glass substrates using the following procedures. The substrate temperature was raised to the desired value. Oxygen was then introduced into the vacuum chamber and the flow rate was set to the desired oxygen pressure. A highly negative voltage was applied to the ring electrode between the evaporation source and the substrate. The zinc powder used as a source material was heated to evaporate in a molybdenum boat. The gas discharge produces a plasma region in which the oxygen molecules were activated. Finally, the evaporated zinc atoms entered into the discharge region and reacted with the activated oxygen molecules, and the product was deposited onto the substrate, forming a ZnO film. Table 1 lists the preparation conditions for the ZnO thin film samples. The thickness of a thin film does not continue to increase if the deposition time is over a certain length (about 20 min), and a multilayered ZnO thin film can be obtained through the many-time deposition. The thicknesses of the single-layered and triple-layered ZnO thin films are 100 nm and 600 nm, respectively. The thickness of the investigated thin film samples is varied from 100 nm to 600 nm. The ZnO thin films fabricated by the above method were heat-treated
2322
H. Lu et al. / Materials Letters 63 (2009) 2321–2323
Table 1 Preparation parameters of ZnO thin film samples. Substrate temperature (°C) Oxygen gas pressure (Pa) Deposition time (min)
50~100 10~13 15
Gas discharge voltage (V) Evaporation current (A) Distance between the substrate and the evaporation source (cm)
800 35 8∼12
at 250 °C in air for 2 h. Their surface morphology was observed with the AJ-III atomic force microscope (AFM) and the crystal structures were examined by X-ray diffraction (XRD, Rigaku DMAX/VB). The I–V characteristics of the thin films were measured with a programmable voltage source which scans the DC voltage at a speed of 1 V/min. The varistor voltages (V1mA) were measured at a current of 1.0 mA and the leakage currents (IL) were observed at 0.75 V1mA. In addition, the nonlinear coefficient (α) was determined by logI2 − logI1 α = logV , where V1 and V2 are voltages corresponding to I1 2 − logV1 and I2, respectively, and are determined in the sharp region of the I–V curve, with I2 = 10I1. The stability tests were performed under a DC bias of 0.75 V1mA and a temperature of 150 °C for 6 h.
Fig. 2. I–V characteristic curves of the ZnO thin films.
Fig. 1 shows the three dimensional AFM image of a ZnO thin film sample fabricated by GDARE. From this image, it can be found that the surface of the thin film consists of massif-like ZnO aggregates with grain sizes of 50 to 200 nm and the sample has lower surface roughness. The XRD (not shown) analysis indicated that the thin film has polycrystalline structures with the preferred orientation along the c-axis and its composition deviates from that of the stoichiometrical ZnO. The I–V characteristic curves of single-layered and triple-layered ZnO thin films are shown in Fig. 2. It is evident that the ZnO thin films deposited using GDARE exhibited high nonlinear I–V properties. The calculated nonlinear coefficient (α) and varistor voltage (V1mA) of the single-layered ZnO thin film sample were 33 and 19.0 V, respectively, whereas the α and V1mA of the triple-layered thin film sample were 62 and 20.0 V, respectively. This indicates that the degree of nonlinearity of the triple-layered ZnO thin film prepared by the many-time deposition was enhanced significantly but the varistor voltage (V1mA) increased slightly. ZnO varistor is a polycrystalline semiconductor containing a large number of grain boundaries. It is widely accepted that the nonlinear I–V behaviors of ZnO varistors were attributed to the double Schottky barriers formed at grain boundaries [22,23]. A higher barrier results in a lower leakage current (IL) and consequently
a larger nonlinear coefficient (α). The varistor voltage (V1mA) is closely related to the number of grain boundaries, the less the number of grain boundaries, the lower the varistor voltage (V1mA). Based on the experimental results in this work, it can be concluded that the ZnO thin films prepared by GDARE contain a large number of electrically active grain boundaries capable of absorbing strongly oxygen which creates many electronic trapping states, leading to the formation of the high Schottky potential barriers at the interfaces. On the other hand, the many-time depositing method in GDARE would be useful to raise the potential barrier at the grain boundary in ZnO thin films. Therefore, the magnitude of the nonlinear coefficient (α) of a multilayered ZnO thin film sample was enhanced significantly. Furthermore, the increases in the grain size and thickness of a ZnO thin film due to many-time deposition lead to a little increase in the number of grain boundaries. As a result, the varistor voltage (V1mA) of a multilayered ZnO thin film sample slightly increased. In practice, a ZnO varistor is always subjected to a continuous electrical stress and the varistor is gradually degraded with time. Therefore, in addition to the nonlinearity, the electrical stability is a very important characteristics of a ZnO varistor [24]. Fig. 3 shows the variations of the nonlinear coefficient (α) and leakage current (IL) of the ZnO single-layered thin film sample with time under a DC bias of 0.75 V1mA and a temperature of 150 °C. From the figure, we can find that the leakage current (IL) increases with time whereas the nonlinear coefficient (α) decreases with time. The stability of a varistor can be estimated by the degradation1 rate coefficient (KT) calculated from the expression IL = ILo + KT t 2 [25], where IL is the
Fig. 1. AFM image of a ZnO thin film sample.
Fig. 3. Variations of the nonlinear coefficient (α) and the leakage current (IL) of the single-layered ZnO thin film sample with time under a DC stress.
3. Results and discussion
H. Lu et al. / Materials Letters 63 (2009) 2321–2323
leakage current at the degradation time (t) and ILo corresponds to IL at t = 0. The lower the KT value, the higher the stability. As shown in Fig. 3, the ZnO varistor thin film has a KT value of 0.05 mA/h1/2 and therefore exhibits a relatively good stability with essentially no thermal runaway. The varistor voltage (V1mA) of the ZnO thin film sample decreases from 19 V to 11 V under a DC bias of 0.75 V1mA and a temperature of 150 °C for 6 h. The leakage current increases slightly and the varistor voltage decreases greatly in the first 3 h. This means that the decrease of the nonlinear coefficient (α) of the ZnO varistor thin film is not only due to the increase of the leakage current. This experimental result is different from those of the bulk ZnO ceramic varistors [10,14]. Furthermore, according to our experimental results up to now, the electrical stabilities of the single-layered and multilayered ZnO varistor thin films are similar. Therefore, we believe that the distribution and shape of the Schottky barriers at the grain boundaries in the thin films are very complicated, and probably there exist also some special transportation processes. Further work is needed in order to elucidate the mechanism about the conduction and degradation of the thin films. 4. Conclusion The ZnO varistor thin films were fabricated by the GDARE depositing method. The microstructures and electrical properties of the thin films were studied. AFM and XRD results showed that the thin films have polycrystalline structures with the c-axis as the preferential orientation whose surface consists of massif-like ZnO aggregates with grain sizes ranging from 50 to 200 nm. The ZnO thin film annealed at 250 °C for 2 h shows highly nonlinear I–V properties and lower breakdown voltage. The nonlinear coefficient (α) of the single-layered thin film sample is 33 and that of the triple-layered thin film sample
2323
prepared by the many-time deposition is raised to 62. The varistor voltages (V1mA) of these films are similar. Under a DC voltage of 0.75 V1mA and 150 °C, the ZnO varistor thin films exhibit good stability with the degradation rate coefficient (KT) of 0.05 mA/h1/2. Therefore, we believe that the ZnO thin films thus prepared are a promising material for advanced varistors.
References [1] Rao BB. Mater Chem Phys 2000;64:62–5. [2] Bagall DM, Chen YF, Zhu Z, Yao T, Koyama S, Shen MY, et al. Appl Phys Lett 1997;70: 2230–2. [3] Look DC. Mater Sci Eng 2001;B80:383–7. [4] Wang XB, Song C, Li DM. J Appl Surf Sci 2006;253:1639–43. [5] Levinson LM, Philipp HR. Ceram Soc Bull 1986;65:639–46. [6] Gupta TK. J Am Ceram Soc 1990;73:1817–40. [7] Kutty TRN, Ezhilvalavan S. Mater Chem Phys 1994;38:267–76. [8] Nahm CW, Shin BC, Park JA, Yoo DH. Mater Lett 2006;60:164–7. [9] Banerjec A, Ramamohan TR, Patni MJ. Mater Res Bull 2001;36:1259–67. [10] Nahm CW, Kim HS. Mater Lett 2003;57:1544–9. [11] Nahm CW. Mater Lett 2008;62:4440–2. [12] Herio N, Hiramatsu M, Nawate M, Lmaeda K, Torii T. Vacuum 1998;51:719–22. [13] Mista W, Ziaja J, Gubanski A. Vacuum 2004;74:293–6. [14] Jiang SL, Xie TT, Yu H, Huang YQ, Zhang HB. Microelectron Eng 2008;85:371–4. [15] Hu J, Gordon RG. J Appl Phys 1992;72:5381–92. [16] Ellmer K. J Phys D 2000;32:R17–32. [17] Heo YW, Norton DP, Pearton SJ. J Appl Phys 2005;98 073502.1–073502.6. [18] Choi JH, Tabata H, Kawai T. J Cryst Growth 2001;226:493–500. [19] Ortíz A, Falcony C, Hernández JA, Garcia M, Alonso JC. Thin Solid Films 1997;293:103–7. [20] Kim YS, Tai WP, Shu SJ. Thin Solid Films 2005;491:153–60. [21] Lu H, Yin Q, Xia J, Pan X. Chin J Chem Phys 2005;18:1034–8. [22] Bartkowiak M, Mahan GD, Modine FA, Alim MA. J Appl Phys 1996;79:273–81. [23] Magnusson KO, Wiklund S. J Appl Phys 1994;76:7405–9. [24] Nahm CW. Mater Lett 2001;47:182–7. [25] Fan J, Freer R. J Appl Phys 1995;77:4795.