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Influences of deposition temperature on characteristics of B-doped ZnO films deposited by metal–organic chemical vapor deposition
Thin Solid Films 559 (2014) 83–87
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Influences of deposition temperature on characteristics of B-doped ZnO films deposited by metal–organic chemical vapor deposition K. Maejima a, T. Koida b, H. Sai b, T. Matsui b, K. Saito c, M. Kondo b, T. Takagawa a a b c
Thin Film Silicon Laboratory, Photovoltaic Power Generation Technology Research Association, Tsukuba, Ibaraki 305-8568, Japan Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan Faculty of Symbiotic Systems Science, Fukushima University, Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e
i n f o
Available online 10 December 2013 Keywords: Boron-doped zinc oxide Metal–organic chemical vapor deposition Transparent conducting oxide Zinc oxide Chemical vapor reactions Deposition mechanism
a b s t r a c t Boron-doped zinc oxide films were fabricated by metal–organic chemical vapor deposition at deposition temperatures (Td) from 150 to 210 °C. The deposition rate increases abruptly and monotonically with increasing Td. The resistivity also varies drastically, and a minimum resistivity of 1.6 × 10−3Ω cm is obtained at Td = 175 °C. The crystal orientation and surface texture show Td dependence. These characteristics correlate with each other. The dependence of these characteristics on Td is caused by the reactivity of the source materials. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a wide-band-gap semiconductor with a band gap energy of about 3.3 eV [1], and is being investigated as a material for light-emitting [2] and light-detecting devices [3]. Impurity doping gives good electrical conductivity to ZnO, and a transparent conducting electrode is one of the applications of ZnO [4]. Owing to its non-toxicity and abundance, the use of ZnO in photovoltaics is being investigated and commercialized. It is commercially important for the application to photovoltaics to be able to deposit ZnO rapidly at a large scale at a relatively low temperature. There are various methods that can be used to deposit ZnO, and metal–organic chemical vapor deposition (MOCVD) is one of the methods satisfying the above demands [5]. Therefore, MOCVD is an attractive fabrication method for transparent conducting ZnO [6–10]. Since photovoltaics also use near-infrared light, a transparent conducting oxide (TCO) for photovoltaics must have transparency to visible and near-infrared light, and the carrier density in TCO must be suppressed to minimize light absorption by free carriers. The ability to fabricate ZnO with low carrier density is also one of the reasons that MOCVD is considered an attractive fabrication method for transparent conducting ZnO [11]. Recently, improved efficiency and reduced cost have been strongly required for photovoltaics. Accordingly, improving the properties of TCO is important to improve photovoltaics, because TCO is an important component of photovoltaics. Although there have been many reports on ZnO deposition by MOCVD, there
E-mail address: [email protected] (K. Maejima). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.11.137
have been few reports on deposition mechanism [12–16]. Therefore, greater understanding of the MOCVD deposition mechanism is required. In this study, boron-doped ZnO (BZO) films were deposited at various temperatures and characterized using several techniques. BZO characteristics were observed to strongly depend on the deposition temperature, and the origin of the dependence is discussed.
2. Experimental procedure BZO was deposited by MOCVD. Diethylzinc (DEZn; Zn(C2H5)2) and water (H2O) were used as zinc and oxygen sources, respectively. Boron doping was carried out by supplying diborane (B2H6). BZO was deposited on non-alkali glass substrates (Corning EAGLE XG) for 30 min in the temperature range from 150 to 210 °C. The pressure in the reactor was maintained at 50 Pa during BZO deposition. The flow rates of DEZn, water and diborane were 100, 200 and 1 μmol/min, respectively. Ar gas was used as the carrier gas for the liquid sources. BZO films were characterized using several techniques. Their thicknesses were measured by spectroscopic ellipsometry, their electrical properties were characterized by Hall measurement with the van der Pauw configuration, and their crystal orientation was measured by X-ray diffraction (XRD; symmetric θ–2θ scan, and in-plane ω–2θ scan). A two-dimensional detector was used for the symmetric θ–2θ scans, and in-plane ω–2θ scans were conducted with a grazing incident X-ray and a grazing diffracted X-ray. The surface texture of the samples was observed by scanning electron microscopy (SEM). SEM observation was carried out along two directions to obtain plan views and crosssectional views.
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3. Results and discussion 3.1. Deposition rate Fig. 1 shows the rate of deposition of BZO by MOCVD. The deposition rate increases monotonically with increasing deposition temperature (Td). For BZO deposited at a temperature below 180 °C, the deposition rate increases abruptly from 800 to 2900 nm/h. The rate of deposition of BZO deposited at 180 °C is 3.6 times higher than that of BZO at 150 °C. The deposition rate when Td N 180 °C changes moderately compared with that when Td b 180 °C. Therefore, higher deposition temperatures promote BZO deposition, and the change in the deposition rate of BZO is marked when Td b 180 °C. Faÿ et al. reported on the deposition rate of undoped ZnO films, which increased markedly with increasing substrate temperature [17]. Their deposition rate was several μm/h, which is several times higher than our value. 3.2. Electrical properties The electrical properties of BZO films, namely, resistivity, carrier density and mobility, are indicated in Fig. 2. When the deposition temperature increases from 150 °C, the resistivity of BZO decreases abruptly, and the lowest value of 1.6 × 10−3Ω cm is obtained at Td = 175 °C. Then, the resistivity increases rapidly. The maximum resistivity is on the order of 10−1Ω cm. Therefore, the resistivity increases by two orders of magnitude when Td is increased by 35 °C. The dependence of the mobility on Td is opposite to that of the resistivity. The highest mobility of 32 cm2/Vs is obtained at Td = 175 °C. Maximum carrier density of 2.0 × 1020 cm− 3 is obtained at Td = 195 °C. The change in carrier density is moderate compared with that in mobility. Therefore, the change in resistivity is mainly affected by the mobility. W.W. Wenas et al. reported the dependence of the electrical properties of BZO on the deposition temperature from 90 to 240 °C. The data by Wenas et al. show similar dependence to our data (Fig. 2). However, the changes of electrical properties near optimum deposition temperature are not clear in Wenas's data because of the shortage of data points. As shown in Fig. 2, the resistivity changes continuously and abruptly around optimum temperature of Td = 175 °C. 3.3. Crystal orientation The crystal orientation of BZO was investigated by XRD. Fig. 3 shows the results of XRD (symmetric θ–2θ scan). The diffraction peaks from various planes, such as (1010), (0002), (1011) and (1120) planes, were detected. The diffraction peak from the (1120) plane is prevailing for almost all BZO samples. For BZO deposited at a lower temperature of around 150 °C, the diffraction peak from the (0002) plane is relatively
Fig. 1. Deposition rate of BZO as a function of deposition temperature Td.
Fig. 2. Electrical properties of BZO deposited at various temperatures (Td).
intense. For BZO deposited at a higher temperature of around 210 °C, the diffraction peak from the (1010) plane is intense. The intensities of the diffraction peaks from the (1010), (0002) and (1120) planes are indicated in Fig. 4(a). Fig. 4(b) shows the intensities of the diffraction peaks from the (1010) and (0002) planes normalized by that of the peak from the (1120) plane. Figs. 3 and 4 indicate that the ratio of the diffraction peaks from different crystal orientations varies with Td. That is, the composition of crystal orientation along the deposition direction depends on Td. Faÿ et al. reported on a similar dependence of the crystal orientation composition of undoped ZnO films [17]. However, their detailed composition of crystal orientation is different from ours. In-plane ω–2θ scans were also carried out. Fig. 5 shows the grain sizes deduced from the full widths at half maximum (FWHMs) of the diffraction peaks from the (1010) and (0002) planes using the Scherrer equation. The grain size of BZO increases when Td exceeds 175 °C.
Fig. 3. XRD (symmetric θ–2θ scan) results of BZO films.
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Fig. 4. (a) Diffraction peak intensities from (1010), (0002) and (1120) planes. (b) Diffraction peak intensities normalized by that of (1120) plane.
3.4. Surface texture Plan and cross-sectional SEM images are indicated in Fig. 6. For BZO deposited at Td ≤ 175 °C, the surface texture consists of a pyramidal structure whose size increases with Td. It is thought that the dependence of the size of the pyramidal structure on Td reflects grain growth under different deposition rates. As mentioned in Subsection 3.1, the deposition rate changes markedly with Td. This change in deposition rate is also confirmed by the cross-sectional SEM images shown in Fig. 6. Furthermore, the dependence of the size of the pyramidal structure on Td affects mobility of BZO deposited at Td ≤ 175 °C. On the other hand, the surface structure becomes rectangular for BZO deposited at Td = 210 °C. For BZO deposited at Td between 175 and 210 °C, the surface structure is a mixture of pyramidal and rectangular structures. The change in surface structure from pyramidal to rectangular is thought to correspond to the change in crystal orientation mentioned in Subsection 3.3. Namely, the crystal orientation along the deposition to [1010] axis leads to the rectangular surface structure. Therefore, the in-plane growth direction is uniaxial for the grains with the rectangular surface structure. From the result of grain size of BZO deposited at Td = 210 °C (Fig. 5 and data not shown), the grain size along the [0001] axis is larger than those along the [1010] and [1120] axes. Consequently, the uniaxial in-plane growth direction is along the [0001] axis. As mentioned above, the surface structure is pyramidal for BZO deposited at Td ≤ 175 °C. As the pyramidal structure is isotropic, its in-plane growth is isotropic. As can be seen in the plan-view SEM images, pyramidal structures cover the entire surface. On the other hand, the rectangular structures exhibit uniaxial growth and are not aligned. That is, the uniaxial growth direction of the rectangular structures is random. The in-plane growth of rectangular grains perpendicular
Fig. 5. Grain sizes deduced from diffraction peaks from (1010) and (0002) planes in in-plane ω–2θ scans.
to their uniaxial growth direction seldom occurs. Therefore, voids are formed at the grain boundaries between adjacent rectangular grains. The deposition at Td N 175 °C leads to the growth of rectangular grains, and the formation of voids at the grain boundaries. This is considered in the origin of the abrupt decrease in mobility at Td N 175 °C. 3.5. Chemical vapor reactions As shown in the above subsections, within a narrow deposition temperature range of 60 °C, the deposition rate, electrical properties and crystal orientation significantly vary. In this subsection, we discuss the causes of these marked changes in BZO characteristics. DEZn, water and diborane were respectively used as the zinc, oxygen and boron sources for BZO deposition in this study. The B supply is much lower than the supply of the host elements Zn and O. Therefore, the chemical reactions between DEZn and water in vapor phase are considered here. DEZn is known as a spontaneously combustible and water-reactive substance. Therefore, its chemical reactions occur easily under the source combination in this study. According to the results obtained by experiment [18,19] and quantum chemical calculations [12,20], the product of DEZn and water is zinc hydroxide (Zn(OH)2). The reaction progresses with substitution from the ethyl group to the hydroxy group: ZnðC2 H5 Þ2 þ H2 O → HOZnC2 H5 þ C2 H6 → ZnðOHÞ2 þ 2C2 H6 :
ð1Þ
Furthermore, according to the literatures [12,21], oligomerization occurs in substances containing the bond between Zn and O: 2HOZnR → ðZnOÞ2 H2 R2 ;
ð2Þ
2ðZnOÞ2 H2 R2 → ðZnOÞ4 H4 R4 :
ð3Þ
Here, R = \OH or \C2H5. In the reference [12], a quantum chemical study was carried out for the above reactions between DEZn and water. The change in the enthalpy of Eq. (1) was found to be exothermic, and the transition state was deduced [12]. Therefore, the progress of Eq. (1) requires excess energy over the activation energy. Eqs. (2) and (3) are barrierless reactions [12], and Eqs. (2) and (3) are thought to progress with the collision of reactants. Consequently, DEZn and water react first, and then the following oligomerization processes occur easily. That is, dimers and tetramers form easily. MOCVD technique is applied for various materials such as silicon and gallium nitride, the reaction pathway of MOCVD is known to be very complicated [22–26]. Therefore, the reactions listed above are thought to be a portion of reactions between DEZn and water during MOCVD deposition. However, the reactions listed above are known as reactions between DEZn and water nowadays. According to above the consideration, the progress of ZnO film formation is affected by the initial chemical vapor reaction between DEZn and water. The marked change in deposition rate (Fig. 1)
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Fig. 6. SEM images of BZO films. The upper images show plan views and the bottom images show cross-sectional views.
depends on the progress of initial chemical vapor reaction between DEZn and water. Further reactions occur particularly at higher temperatures, and dimers and tetramers are formed. The structures of the dimers and tetramers are different from the wurtzite crystal structure of the ZnO films. Therefore, the composition of molecules in vapor phase affects the crystal orientation and deposition rate. Namely, the progress of the reaction in vapor phase changes the characteristics of the ZnO films. As mentioned above, the characteristics of BZO is very sensitive to the deposition temperature. This might indicate the difficulty in industrial production. Because the slight change of deposition temperature leads the degradation of production quality in reproduction and uniformity of large-scale films. Therefore, the precise control of deposition temperature is required for the fabrication of high quality BZO films. If the sensitivity to the deposition temperature becomes lower, the industrial production of BZO is to be easier with low cost. Consequently, it is important to control the chemical reactions in vapor phase to improve the characteristics of transparent conducting ZnO. Since diborane reacts with water easily, the reactions containing diborane must be included for the more detailed consideration of the reaction mechanism of BZO MOCVD deposition. Further consideration is to be future work. 3.6. Solar cell fabrication On the basis of the above results, an amorphous/microcrystalline silicon tandem solar cell with an antireflective coating was fabricated [27,28]. A 1.7 μm-thick BZO film was deposited, and this BZO film was used as the front TCO. An initial conversion efficiency of 12.7% was obtained. The cell using a commercial SnO2:F film instead of BZO had a conversion efficiency of 12.6%. Therefore, our BZO films have satisfactory properties for the application of photovoltaics. 4. Conclusions BZO films were deposited at temperatures of 150 ≤ Td ≤ 210 °C, and various properties of BZO films were characterized. The properties,
namely, deposition rate, surface texture, electrical properties and crystal orientation change abruptly within the Td range of 60 °C. The deposition rate increases rapidly with increasing Td. The surface texture changes from pyramidal to rectangular when Td exceeds 175 °C. The lowest resistivity of 160 × 10 − 3 Ω cm is obtained for BZO deposited at T d = 175 °C. However, the resistivity increases rapidly away from Td = 175 °C. The crystal orientation along the deposition also changes with Td. These properties correlate with each other. The characteristics of BZO depend strongly on the deposition temperature. The origin of this strong dependence on deposition temperature is the reaction mechanism of MOCVD process. These phenomena are caused by the high reactivity between DEZn and water. Acknowledgments This study was supported by The New Energy and Industrial Technology Development Organization (NEDO). References [1] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Do an, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [2] K. Nakahara, S. Akasaka, H. Yuji, K. Tamura, T. Fujii, Y. Nishimoto, D. Takamizu, A. Sasaki, T. Tanabe, H. Takasu, H. Amaike, T. Onuma, S.F. Chichibu, A. Tsukazaki, A. Ohtomo, M. Kawasaki, Appl. Phys. Lett. 97 (2010) 013501. [3] T. Takagi, H. Tanaka, Sz. Fujita, Sg. Fujita, Jpn. J. Appl. Phys. 42 (2003) L401. [4] In: K. Ellmer, A. Klein, B. Rech (Eds.), Transparent Conductive Zinc Oxide, Springer, New York, 2008. [5] G.B. Stringfellow, Organometallic Vapor-Phase Epitaxy: Theory and Practice, Academic Press, San Diego, 1999. [6] W.W. Wenas, A. Yamada, M. Konagai, K. Takahashi, Jpn. J. Appl. Phys. 30 (1991) L441. [7] W.W. Wenas, A. Yamada, K. Takahashi, M. Yoshino, M. Konagai, J. Appl. Phys. 70 (1991) 7119. [8] B. Sang, Y. Nagoya, K. Kushiya, O. Yamase, Sol. Energy Mater. Sol. Cells 75 (2007) 179. [9] A. Hongsingthong, I.A. Yunaz, S. Miyajima, M. Konagai, Sol. Energy Mater. Sol. Cells 95 (2007) 171. [10] S. Faÿ, J. Steinhauser, N. Oliveira, E. Vallat-Sauvain, C. Ballif, Thin Solid Films 515 (2007) 8558. [11] J. Steinhauser, S. Faÿ, N. Oliveira, E. Vallat-Sauvain, C. Ballif, Appl. Phys. Lett. 90 (2007) 142107. [12] S.M. Smith, H.B. Schlegel, Chem. Mater. 15 (2003) 162.
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K. Maejima, Sz. Fujita, J. Cryst. Growth 293 (2006) 305. K. Maejima, Sz. Fujita, Jpn. J. Appl. Phys. 46 (2007) 7885. Y.S. Kim, Y.S. Won, Bull. Korean Chem. Soc. 30 (2009) 1573. S. Nicolay, S. Faÿ, C. Ballif, Cryst. Growth Des. 9 (2009) 4957. S. Faÿ, U. Kroll, C. Bucher, E. Vallat-Sauvain, A. Shah, Sol. Energy Mater. Sol. Cells 86 (2005) 385. [18] G.E. Coates, M.L.H. Green, K. Wada, Organometallic Compounds, vol. 1Methuen, London, 1968. 130. [19] T.R. Crompton, International Series of Monographs in Analytical Chemistry, vol. 31Pergamon, Oxford, 1968. 322.
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B.H. Munk, H.B. Schlegel, Chem. Mater. 18 (2006) 1878. K. Maejima, Sz. Fujita, Phys. Stat. Sol. c 3 (2006) 1022. M.E. Coltrin, R.J. Kee, J.A. Miller, J. Electrochem. Soc. 131 (1984) 425. M.E. Coltrin, R.J. Kee, J.A. Miller, J. Electrochem. Soc. 133 (1986) 1206. M.E. Coltrin, R.J. Kee, G.H. Evans, J. Electrochem. Soc. 136 (1989) 819. D. Sengupta, S. Mazumder, W. Kuykendall, S.A. Lowry, J. Cryst. Growth 279 (2005) 369. [26] A. Hirako, K. Kusakabe, K. Ohkawa, Jpn. J. Appl. Phys. 44 (2005) 874. [27] T. Matsui, H. Sai, K. Saito, M. Kondo, Jpn. J. Appl. Phys. 51 (2012) 10NB04. [28] T. Matsui, H. Sai, K. Saito, M. Kondo, Prog. Photovolt. Res. Appl. 21 (2013) 1363.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Influences of deposition temperature on characteristics of B-doped ZnO films deposited by metal–organic chemical vapor deposition K. Maejima a, T. Koida b, H. Sai b, T. Matsui b, K. Saito c, M. Kondo b, T. Takagawa a a b c
Thin Film Silicon Laboratory, Photovoltaic Power Generation Technology Research Association, Tsukuba, Ibaraki 305-8568, Japan Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan Faculty of Symbiotic Systems Science, Fukushima University, Tsukuba, Ibaraki 305-8568, Japan
a r t i c l e
i n f o
Available online 10 December 2013 Keywords: Boron-doped zinc oxide Metal–organic chemical vapor deposition Transparent conducting oxide Zinc oxide Chemical vapor reactions Deposition mechanism
a b s t r a c t Boron-doped zinc oxide films were fabricated by metal–organic chemical vapor deposition at deposition temperatures (Td) from 150 to 210 °C. The deposition rate increases abruptly and monotonically with increasing Td. The resistivity also varies drastically, and a minimum resistivity of 1.6 × 10−3Ω cm is obtained at Td = 175 °C. The crystal orientation and surface texture show Td dependence. These characteristics correlate with each other. The dependence of these characteristics on Td is caused by the reactivity of the source materials. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a wide-band-gap semiconductor with a band gap energy of about 3.3 eV [1], and is being investigated as a material for light-emitting [2] and light-detecting devices [3]. Impurity doping gives good electrical conductivity to ZnO, and a transparent conducting electrode is one of the applications of ZnO [4]. Owing to its non-toxicity and abundance, the use of ZnO in photovoltaics is being investigated and commercialized. It is commercially important for the application to photovoltaics to be able to deposit ZnO rapidly at a large scale at a relatively low temperature. There are various methods that can be used to deposit ZnO, and metal–organic chemical vapor deposition (MOCVD) is one of the methods satisfying the above demands [5]. Therefore, MOCVD is an attractive fabrication method for transparent conducting ZnO [6–10]. Since photovoltaics also use near-infrared light, a transparent conducting oxide (TCO) for photovoltaics must have transparency to visible and near-infrared light, and the carrier density in TCO must be suppressed to minimize light absorption by free carriers. The ability to fabricate ZnO with low carrier density is also one of the reasons that MOCVD is considered an attractive fabrication method for transparent conducting ZnO [11]. Recently, improved efficiency and reduced cost have been strongly required for photovoltaics. Accordingly, improving the properties of TCO is important to improve photovoltaics, because TCO is an important component of photovoltaics. Although there have been many reports on ZnO deposition by MOCVD, there
E-mail address: [email protected] (K. Maejima). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.11.137
have been few reports on deposition mechanism [12–16]. Therefore, greater understanding of the MOCVD deposition mechanism is required. In this study, boron-doped ZnO (BZO) films were deposited at various temperatures and characterized using several techniques. BZO characteristics were observed to strongly depend on the deposition temperature, and the origin of the dependence is discussed.
2. Experimental procedure BZO was deposited by MOCVD. Diethylzinc (DEZn; Zn(C2H5)2) and water (H2O) were used as zinc and oxygen sources, respectively. Boron doping was carried out by supplying diborane (B2H6). BZO was deposited on non-alkali glass substrates (Corning EAGLE XG) for 30 min in the temperature range from 150 to 210 °C. The pressure in the reactor was maintained at 50 Pa during BZO deposition. The flow rates of DEZn, water and diborane were 100, 200 and 1 μmol/min, respectively. Ar gas was used as the carrier gas for the liquid sources. BZO films were characterized using several techniques. Their thicknesses were measured by spectroscopic ellipsometry, their electrical properties were characterized by Hall measurement with the van der Pauw configuration, and their crystal orientation was measured by X-ray diffraction (XRD; symmetric θ–2θ scan, and in-plane ω–2θ scan). A two-dimensional detector was used for the symmetric θ–2θ scans, and in-plane ω–2θ scans were conducted with a grazing incident X-ray and a grazing diffracted X-ray. The surface texture of the samples was observed by scanning electron microscopy (SEM). SEM observation was carried out along two directions to obtain plan views and crosssectional views.
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3. Results and discussion 3.1. Deposition rate Fig. 1 shows the rate of deposition of BZO by MOCVD. The deposition rate increases monotonically with increasing deposition temperature (Td). For BZO deposited at a temperature below 180 °C, the deposition rate increases abruptly from 800 to 2900 nm/h. The rate of deposition of BZO deposited at 180 °C is 3.6 times higher than that of BZO at 150 °C. The deposition rate when Td N 180 °C changes moderately compared with that when Td b 180 °C. Therefore, higher deposition temperatures promote BZO deposition, and the change in the deposition rate of BZO is marked when Td b 180 °C. Faÿ et al. reported on the deposition rate of undoped ZnO films, which increased markedly with increasing substrate temperature [17]. Their deposition rate was several μm/h, which is several times higher than our value. 3.2. Electrical properties The electrical properties of BZO films, namely, resistivity, carrier density and mobility, are indicated in Fig. 2. When the deposition temperature increases from 150 °C, the resistivity of BZO decreases abruptly, and the lowest value of 1.6 × 10−3Ω cm is obtained at Td = 175 °C. Then, the resistivity increases rapidly. The maximum resistivity is on the order of 10−1Ω cm. Therefore, the resistivity increases by two orders of magnitude when Td is increased by 35 °C. The dependence of the mobility on Td is opposite to that of the resistivity. The highest mobility of 32 cm2/Vs is obtained at Td = 175 °C. Maximum carrier density of 2.0 × 1020 cm− 3 is obtained at Td = 195 °C. The change in carrier density is moderate compared with that in mobility. Therefore, the change in resistivity is mainly affected by the mobility. W.W. Wenas et al. reported the dependence of the electrical properties of BZO on the deposition temperature from 90 to 240 °C. The data by Wenas et al. show similar dependence to our data (Fig. 2). However, the changes of electrical properties near optimum deposition temperature are not clear in Wenas's data because of the shortage of data points. As shown in Fig. 2, the resistivity changes continuously and abruptly around optimum temperature of Td = 175 °C. 3.3. Crystal orientation The crystal orientation of BZO was investigated by XRD. Fig. 3 shows the results of XRD (symmetric θ–2θ scan). The diffraction peaks from various planes, such as (1010), (0002), (1011) and (1120) planes, were detected. The diffraction peak from the (1120) plane is prevailing for almost all BZO samples. For BZO deposited at a lower temperature of around 150 °C, the diffraction peak from the (0002) plane is relatively
Fig. 1. Deposition rate of BZO as a function of deposition temperature Td.
Fig. 2. Electrical properties of BZO deposited at various temperatures (Td).
intense. For BZO deposited at a higher temperature of around 210 °C, the diffraction peak from the (1010) plane is intense. The intensities of the diffraction peaks from the (1010), (0002) and (1120) planes are indicated in Fig. 4(a). Fig. 4(b) shows the intensities of the diffraction peaks from the (1010) and (0002) planes normalized by that of the peak from the (1120) plane. Figs. 3 and 4 indicate that the ratio of the diffraction peaks from different crystal orientations varies with Td. That is, the composition of crystal orientation along the deposition direction depends on Td. Faÿ et al. reported on a similar dependence of the crystal orientation composition of undoped ZnO films [17]. However, their detailed composition of crystal orientation is different from ours. In-plane ω–2θ scans were also carried out. Fig. 5 shows the grain sizes deduced from the full widths at half maximum (FWHMs) of the diffraction peaks from the (1010) and (0002) planes using the Scherrer equation. The grain size of BZO increases when Td exceeds 175 °C.
Fig. 3. XRD (symmetric θ–2θ scan) results of BZO films.
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Fig. 4. (a) Diffraction peak intensities from (1010), (0002) and (1120) planes. (b) Diffraction peak intensities normalized by that of (1120) plane.
3.4. Surface texture Plan and cross-sectional SEM images are indicated in Fig. 6. For BZO deposited at Td ≤ 175 °C, the surface texture consists of a pyramidal structure whose size increases with Td. It is thought that the dependence of the size of the pyramidal structure on Td reflects grain growth under different deposition rates. As mentioned in Subsection 3.1, the deposition rate changes markedly with Td. This change in deposition rate is also confirmed by the cross-sectional SEM images shown in Fig. 6. Furthermore, the dependence of the size of the pyramidal structure on Td affects mobility of BZO deposited at Td ≤ 175 °C. On the other hand, the surface structure becomes rectangular for BZO deposited at Td = 210 °C. For BZO deposited at Td between 175 and 210 °C, the surface structure is a mixture of pyramidal and rectangular structures. The change in surface structure from pyramidal to rectangular is thought to correspond to the change in crystal orientation mentioned in Subsection 3.3. Namely, the crystal orientation along the deposition to [1010] axis leads to the rectangular surface structure. Therefore, the in-plane growth direction is uniaxial for the grains with the rectangular surface structure. From the result of grain size of BZO deposited at Td = 210 °C (Fig. 5 and data not shown), the grain size along the [0001] axis is larger than those along the [1010] and [1120] axes. Consequently, the uniaxial in-plane growth direction is along the [0001] axis. As mentioned above, the surface structure is pyramidal for BZO deposited at Td ≤ 175 °C. As the pyramidal structure is isotropic, its in-plane growth is isotropic. As can be seen in the plan-view SEM images, pyramidal structures cover the entire surface. On the other hand, the rectangular structures exhibit uniaxial growth and are not aligned. That is, the uniaxial growth direction of the rectangular structures is random. The in-plane growth of rectangular grains perpendicular
Fig. 5. Grain sizes deduced from diffraction peaks from (1010) and (0002) planes in in-plane ω–2θ scans.
to their uniaxial growth direction seldom occurs. Therefore, voids are formed at the grain boundaries between adjacent rectangular grains. The deposition at Td N 175 °C leads to the growth of rectangular grains, and the formation of voids at the grain boundaries. This is considered in the origin of the abrupt decrease in mobility at Td N 175 °C. 3.5. Chemical vapor reactions As shown in the above subsections, within a narrow deposition temperature range of 60 °C, the deposition rate, electrical properties and crystal orientation significantly vary. In this subsection, we discuss the causes of these marked changes in BZO characteristics. DEZn, water and diborane were respectively used as the zinc, oxygen and boron sources for BZO deposition in this study. The B supply is much lower than the supply of the host elements Zn and O. Therefore, the chemical reactions between DEZn and water in vapor phase are considered here. DEZn is known as a spontaneously combustible and water-reactive substance. Therefore, its chemical reactions occur easily under the source combination in this study. According to the results obtained by experiment [18,19] and quantum chemical calculations [12,20], the product of DEZn and water is zinc hydroxide (Zn(OH)2). The reaction progresses with substitution from the ethyl group to the hydroxy group: ZnðC2 H5 Þ2 þ H2 O → HOZnC2 H5 þ C2 H6 → ZnðOHÞ2 þ 2C2 H6 :
ð1Þ
Furthermore, according to the literatures [12,21], oligomerization occurs in substances containing the bond between Zn and O: 2HOZnR → ðZnOÞ2 H2 R2 ;
ð2Þ
2ðZnOÞ2 H2 R2 → ðZnOÞ4 H4 R4 :
ð3Þ
Here, R = \OH or \C2H5. In the reference [12], a quantum chemical study was carried out for the above reactions between DEZn and water. The change in the enthalpy of Eq. (1) was found to be exothermic, and the transition state was deduced [12]. Therefore, the progress of Eq. (1) requires excess energy over the activation energy. Eqs. (2) and (3) are barrierless reactions [12], and Eqs. (2) and (3) are thought to progress with the collision of reactants. Consequently, DEZn and water react first, and then the following oligomerization processes occur easily. That is, dimers and tetramers form easily. MOCVD technique is applied for various materials such as silicon and gallium nitride, the reaction pathway of MOCVD is known to be very complicated [22–26]. Therefore, the reactions listed above are thought to be a portion of reactions between DEZn and water during MOCVD deposition. However, the reactions listed above are known as reactions between DEZn and water nowadays. According to above the consideration, the progress of ZnO film formation is affected by the initial chemical vapor reaction between DEZn and water. The marked change in deposition rate (Fig. 1)
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Fig. 6. SEM images of BZO films. The upper images show plan views and the bottom images show cross-sectional views.
depends on the progress of initial chemical vapor reaction between DEZn and water. Further reactions occur particularly at higher temperatures, and dimers and tetramers are formed. The structures of the dimers and tetramers are different from the wurtzite crystal structure of the ZnO films. Therefore, the composition of molecules in vapor phase affects the crystal orientation and deposition rate. Namely, the progress of the reaction in vapor phase changes the characteristics of the ZnO films. As mentioned above, the characteristics of BZO is very sensitive to the deposition temperature. This might indicate the difficulty in industrial production. Because the slight change of deposition temperature leads the degradation of production quality in reproduction and uniformity of large-scale films. Therefore, the precise control of deposition temperature is required for the fabrication of high quality BZO films. If the sensitivity to the deposition temperature becomes lower, the industrial production of BZO is to be easier with low cost. Consequently, it is important to control the chemical reactions in vapor phase to improve the characteristics of transparent conducting ZnO. Since diborane reacts with water easily, the reactions containing diborane must be included for the more detailed consideration of the reaction mechanism of BZO MOCVD deposition. Further consideration is to be future work. 3.6. Solar cell fabrication On the basis of the above results, an amorphous/microcrystalline silicon tandem solar cell with an antireflective coating was fabricated [27,28]. A 1.7 μm-thick BZO film was deposited, and this BZO film was used as the front TCO. An initial conversion efficiency of 12.7% was obtained. The cell using a commercial SnO2:F film instead of BZO had a conversion efficiency of 12.6%. Therefore, our BZO films have satisfactory properties for the application of photovoltaics. 4. Conclusions BZO films were deposited at temperatures of 150 ≤ Td ≤ 210 °C, and various properties of BZO films were characterized. The properties,
namely, deposition rate, surface texture, electrical properties and crystal orientation change abruptly within the Td range of 60 °C. The deposition rate increases rapidly with increasing Td. The surface texture changes from pyramidal to rectangular when Td exceeds 175 °C. The lowest resistivity of 160 × 10 − 3 Ω cm is obtained for BZO deposited at T d = 175 °C. However, the resistivity increases rapidly away from Td = 175 °C. The crystal orientation along the deposition also changes with Td. These properties correlate with each other. The characteristics of BZO depend strongly on the deposition temperature. The origin of this strong dependence on deposition temperature is the reaction mechanism of MOCVD process. These phenomena are caused by the high reactivity between DEZn and water. Acknowledgments This study was supported by The New Energy and Industrial Technology Development Organization (NEDO). References [1] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Do an, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [2] K. Nakahara, S. Akasaka, H. Yuji, K. Tamura, T. Fujii, Y. Nishimoto, D. Takamizu, A. Sasaki, T. Tanabe, H. Takasu, H. Amaike, T. Onuma, S.F. Chichibu, A. Tsukazaki, A. Ohtomo, M. Kawasaki, Appl. Phys. Lett. 97 (2010) 013501. [3] T. Takagi, H. Tanaka, Sz. Fujita, Sg. Fujita, Jpn. J. Appl. Phys. 42 (2003) L401. [4] In: K. Ellmer, A. Klein, B. Rech (Eds.), Transparent Conductive Zinc Oxide, Springer, New York, 2008. [5] G.B. Stringfellow, Organometallic Vapor-Phase Epitaxy: Theory and Practice, Academic Press, San Diego, 1999. [6] W.W. Wenas, A. Yamada, M. Konagai, K. Takahashi, Jpn. J. Appl. Phys. 30 (1991) L441. [7] W.W. Wenas, A. Yamada, K. Takahashi, M. Yoshino, M. Konagai, J. Appl. Phys. 70 (1991) 7119. [8] B. Sang, Y. Nagoya, K. Kushiya, O. Yamase, Sol. Energy Mater. Sol. Cells 75 (2007) 179. [9] A. Hongsingthong, I.A. Yunaz, S. Miyajima, M. Konagai, Sol. Energy Mater. Sol. Cells 95 (2007) 171. [10] S. Faÿ, J. Steinhauser, N. Oliveira, E. Vallat-Sauvain, C. Ballif, Thin Solid Films 515 (2007) 8558. [11] J. Steinhauser, S. Faÿ, N. Oliveira, E. Vallat-Sauvain, C. Ballif, Appl. Phys. Lett. 90 (2007) 142107. [12] S.M. Smith, H.B. Schlegel, Chem. Mater. 15 (2003) 162.
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