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Double layer structures of transparent conductive oxide suitable for solar cells: Ga-doped ZnO on undoped ZnO
Thin Solid Films 526 (2012) 195–200
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Double layer structures of transparent conductive oxide suitable for solar cells: Ga-doped ZnO on undoped ZnO Housei Akazawa ⁎ NTT Microsystem Integration Laboratories, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan
a r t i c l e
i n f o
Article history: Received 31 July 2012 Received in revised form 29 October 2012 Accepted 30 October 2012 Available online 14 November 2012 Keywords: Zinc oxide Ga-doped zinc oxide Transparent conductive oxide Sputtering Infrared transmittance Solar cell
a b s t r a c t We investigated the transparent conductive properties of hetero-double layers: Ga-doped ZnO (GZO) overlaid on undoped ZnO. We prepared five samples for given unit thicknesses h: ZnO and GZO films with a thickness of h to characterize the hybrid structure, ZnO and GZO films with a thickness of 2h, and a GZO/ZnO double layer with a thickness of h for each layer (h =50, 100, 150, and 200 nm). If we assumed that the upper and bottom half of the 2h-thick GZO films as well as those of GZO/ZnO were connected in parallel in terms of the equivalent electric circuit, the calculated sheet resistance of the upper GZO layer scarcely depended on whether the bottom layer was GZO or ZnO. Hence, the bottom layer played the role of providing a crystalline template for the upper layer that actually governed electrical transport. Also, the infrared transmittance of the upper GZO layer was immune to what the bottom layer consisted of. While GZO/ZnO had 1.1–1.5 times higher sheet resistance than 2h-thick GZO, the optical transmittance of GZO/ZnO in the near-infrared region was 20–40% higher, demonstrating that the GZO/ZnO double layer structure is suitable for transparent electrodes in solar cells. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) films are nowadays widely employed in various kinds of electrical and optical devices. The only concern about optical properties regarding their use in flat panel displays is the transmittance level in the visible wavelength range. The requirement for solar cells is somewhat different; transmittance in the infrared range should be high enough as well because the sunlight spectrum has a substantial fraction of photons with wavelengths between 1000 and 2500 nm. If infrared light is efficiently utilized with additional contrivances such as surface-plasmon-resonance-enhanced photo-absorption, the photovoltaic efficiencies will be dramatically improved. Up to now, SnO2 [1], Sb-doped SnO2 [2,3], and F-doped SnO2 [4,5] have been proposed and actually used as electrodes for solar cells. Because even lower resistivities can be achieved by using ZnO-based materials [6], the feasibility of Ga-doped ZnO (GZO) or Al-doped ZnO (AZO) has also been investigated by focusing on solar cells [7–10]. GZO films are generally more conductive than ZnO films but do not transmit light in the infrared range, which renders GZO rather useful for solar blind windows [11]. This is due to their higher plasma frequency resulting from their higher carrier concentration. ZnO films have lower carrier concentration and thus have higher optical transmittance. Boron-doped ZnO, which is characterized by intermediate resistivity and transmittance between ZnO and GZO, has been expected to be a ⁎ Tel.: +81 46 240 2659; fax: +81 46 270 2372. E-mail address: [email protected]. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.10.111
candidate material for solar cells [12–14]. However, typical resistivities of ZnO:B films in the 10−3 Ω·cm range can easily be obtained with undoped ZnO films. There is still a debate as to whether or not doped boron atoms really behave as extrinsic donors. It is well known that when materials from the ZnO family are directly deposited on glass substrates, their crystallinity is the worst at the initial stage of growth. This makes electron mobility near the interface region extremely low. Furthermore, carriers are not generated if crystallinity is so poor that even small crystallites cannot be created. When the crystal quality increasingly improves farther from the interface [15], both carrier concentration and Hall mobility increase [16–19]. Since poor crystallinity in the interfacial region is the primary bottleneck to achieve ultra-thin TCO films, how to secure superior crystallinity at an early stage of growth is a critical issue to achieve high performance of TCO films. Various attempts have been devoted to solving this problem. Although the best crystallinity can be obtained in epitaxial growth on single crystal substrates such as sapphire [20] and quartz [21], this is obviously a high-cost solution and unrealistic for producing devices. Another approach is to insert a nano-meter-scale inorganic crystalline layer, i.e., a nano-sheet, between a TCO film and the glass substrate [22]. However, covering the macroscopic substrate surface while assembling numerous nano-sheets is a hard task. This nanosheet approach still seems to be far away from the production stage. Also, the adhesive strength of TCO films to the substrate is reduced. Another way is to deposit a thin buffer layer prior to growing the film for the TCO body [21]. A polycrystalline ZnON seed layer has been demonstrated to be promising in promoting crystallization of
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the film for the TCO body at the initial stage of growth [23]. However, once a buffer layer is deposited in the presence of O2 (and N2) gases, the ZnO target surface is oxidized (or nitrided) more than the best conditions required to deposit the TCO film. If the buffer layer is deposited in the same deposition chamber, it interferes with the deposition of the TCO film, which should be carried out under a reduced atmosphere. Against this background, we propose that a double layer structure, where an upper GZO or AZO layer is overlaid on an undoped ZnO bottom layer, would be a competitive candidate for transparent electrodes in solar cells. The basic concept is as follows. If a GZO film is divided into two parts, e.g., as bottom and upper layers, the bottom layer is regarded as a buffer layer that leads to better crystallinity for the upper layer. This means that the bottom layer scarcely contributes to electrical conduction and the upper layer, where crystallinity has been sufficiently improved, primarily provides the conduction path. Nevertheless, the bottom half of GZO is more photo-absorptive than ZnO because of its higher carrier concentration. Hence, if we substitute the bottom half of GZO with ZnO, the transmittance of the resulting GZO/ZnO double layer film would be improved in the infrared range while electrical conductivity would be slightly degraded. We demonstrated the feasibility of this GZO/ZnO double layer structure as a transparent electrode for solar cells. 2. Experimental details The details on our dual target sputtering system have been described elsewhere [24]. The base pressure in the deposition chamber was 5 × 10 −5 Pa. ZnO films were deposited by electron cyclotron resonance (ECR) plasma sputtering from a ZnO target [25]. The ECR plasma was generated from Ar gas without introducing O2 gas into the plasma source. The powers of the microwaves and RF applied to the target were both 500 W. GZO films were deposited by ECR plasma sputtering from the ZnO target and simultaneous RF magnetron sputtering from a Ga2O3 target at a fixed RF magnetron power of 20 W [24]. The substrates we used were rectangular glass plates with dimensions of 52 × 76 mm. Although the substrate was not intentionally annealed by using a heater from its rear side, impinging sputtered atoms and an Ar plasma stream warmed it up to 70 °C during deposition. Ga concentration in the deposited films had a continuous distribution over the substrate surface, which enabled combinatorial evaluation of sheet resistance and optical transmittance as a function of Ga2O3 content. Regarding GZO as a ZnO–Ga2O3 composite material, we denoted Ga concentration in a GZO film as the weight of Ga2O3 with respect to the total weight of ZnO and Ga2O3. Ga2O3 content was determined by inductively coupled plasma atomic emission spectroscopy (SPS1700, Seiko Instruments). Emission lines at 213.856 nm (Zn) and 417.206 nm (Ga) were measured at an RF power of 1.3 kW. Sheet resistance was measured by using a fourpoint prober and optical transmittance spectra were measured with a spectrophotometer (UV-3100, Shimadzu). Crystal structures were evaluated with X-ray diffraction (XRD) (RINT1500, Rigaku) using a Cu Kα line with the Bragg–Brentano configuration. We prepared a set of five samples for unit thickness h, as schematically illustrated in Fig. 1(a). They included a ZnO film with a thickness of h (ZnO(h)), a GZO film with a thickness of h (GZO(h)), a ZnO film with a thickness of 2h (ZnO(2h)), a GZO film with a thickness of 2h (GZO(2h)), and a double layer structure consisting of an upper GZO layer (u-GZO) and a bottom ZnO layer with a thickness of h for each layer (GZO(h)/ ZnO(h)). We regarded GZO(2h) as being composed of an upper half of GZO overlaid on a bottom half of GZO, i.e., GZO(2h) ≡ u-GZO(h)/ GZO(h). Four sets of sample groups with h = 50, 100, 150, and 200 nm were produced. We assumed that the upper and bottom layers in double layers were connected in parallel in terms of an equivalent electrical circuit [Fig. 1(b)]. This was rationalized by the fact that electrons travel horizontally and those scattered at a large angle by ionized
Fig. 1. (a) Schematic model of five structures for unit thickness of h along with provisional upper GZO layer without bottom layer, and (b) equivalent model for electric conduction and optical transmission.
impurities and grain boundaries do not actually contribute to electric conduction. We could thus calculate the sheet resistance of u-GZO in GZO(h)/ZnO(h) from the sheet resistance of GZO(h)/ZnO(h) and that of ZnO(h). Similarly, we could calculate the sheet resistance of u-GZO in GZO(2h) from the sheet resistance of GZO(2h) and that of GZO(h). We also assumed that the optical transmittance of the GZO(h)/ZnO(h) doubled layer would simply be given as the product of the transmittance of u-GZO(h) and ZnO(h) by neglecting interference at the interface [Fig. 1(b)]. This approximation is valid in an infrared range where the effect of interference fringes has decayed. We could calculate the transmittance spectra of u-GZO(h) in GZO(h)/ZnO(h) from the optical transmittance of GZO(h)/ZnO(h) and ZnO(h). Similarly, we could calculate the optical transmittance of u-GZO(h) in GZO(2h) from the transmittance of GZO(2h) and GZO(h).
3. Results and discussion Fig. 2 plots the ZnO(002) XRD peak intensity measured in the ω-2θ scan mode as a function of the thickness of ZnO and GZO films. The ZnO(002) intensity can be fitted to a parabolic function with respect to the thickness, which means that the crystallinity improves in an accelerated manner as the film thickens. The coefficient of the parabolic curve for GZO (0.55) being larger than that for ZnO (0.27) indicates superior crystallinity of GZO films because they contain more oxygen atoms supplied from the Ga2O3 target. Fig. 3 compares the calculated sheet resistance of u-GZO(h) in GZO(2h) and u-GZO(h) in GZO(h)/ZnO(h) for unit thicknesses of 50, 100, and 150 nm. The sheet resistances of u-GZO(h) are similar for each unit thickness, irrespective of what the bottom layer consists of. We can also see that thickness is the major parameter reflecting crystallinity. Whether Ga is doped or not in the bottom layer has little effect on the crystallinity of the u-GZO layer or sheet resistance. The bottom layer can be regarded as a crystalline template to facilitate
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Fig. 2. Dependence of ZnO(002) XRD intensities on film thickness of ZnO (open circles) and GZO (closed circles).
the formation of high-quality crystalline domains just from the beginning of the upper layer. Fig. 4 compares the calculated optical transmittance spectra of the u-GZO layer (Ga2O3 content of 3.6 wt.%) in GZO(2h) and the u-GZO layer in GZO(h)/ZnO(h) for unit thicknesses of 50, 100 and 150 nm. The interference fringe is responsible for the unrealistic transmittance exceeding 100% below 1000 nm. The transmittance curves overlap one another in the infrared range, where interference fringes do not appear. This is a confirmation that the optical properties of u-GZO in the infrared region scarcely depend on whether the bottom layer is ZnO or GZO. The measured transmittance spectra of ZnO(h) and GZO(h) films are compared in Fig. 5 for h = 50, 100, and 150 nm
Fig. 4. Comparison of optical transmittance spectra contributed by u-GZO layer in GZO(h)/ZnO(h) (dashed line) and in GZO(2h) (solid line) for unit thicknesses of (a) 50, (b) 100, and (c) 150 nm. Ga 2 O 3 content in GZO is 3.6 wt.%.
with Ga2O3 content of 3.6 wt.%. Here, the two curves for each h value differ considerably. The transmittance of GZO in the infrared range is more suppressed than that of ZnO, whereas GZO is more transparent between 400 and 800 nm. The reduced transmittance of ZnO within the visible wavelength is caused by oxygen deficiencies in ZnO, but they are necessary to make ZnO conductive [26]. Fig. 6 plots the sheet resistance (R) of ZnO(h), ZnO(2h), GZO(2h), GZO(h)/ZnO(h), and u-GZO(h) in GZO(h)/ZnO(h) as a function of the Ga2O3 content for a unit thickness of h = 50 nm. ZnO(50 nm) has the highest sheet resistance for the five samples. The sheet resistance of u-GZO(50 nm), GZO(50 nm)/ZnO(50 nm), and GZO(100 nm) are comparable. The sheet resistance of ZnO(100 nm) is still higher than these three. The sheet resistance at Ga2O3 content lower than 6 wt.% follows the order: RðGZOð2 hÞÞbRðGZOðhÞ=ZnOðhÞÞbRðu−GZOðhÞÞbRðZnOð2 hÞÞbRðZnOðhÞÞ:
We confirmed this order for all h between 50 and 200 nm. The sheet resistance of u-GZO(h) higher than that of GZO(h)/ZnO(h) means that the bottom ZnO(h) contributes somewhat to electric conduction. The sheet resistance of GZO(h)/ZnO(h) higher than that of GZO(2h) means that the bottom part of GZO(2h), i.e., GZO(h), is more conductive than ZnO(h). Substantial fractions of Ga atoms are non-activated at Ga2O3 content above 10 wt.%, and such impurity atoms prevent crystallization, resulting in changes in the crystal structure of GZO [24,27]. We found the sheet resistance at h = 50 and 100 nm followed the order at 19 wt.%: RðZnOð2 hÞÞeRðGZOðhÞ=ZnOðhÞÞbRðGZOð2 hÞÞbRðu−GZOðhÞÞbRðZnOðhÞÞ: Fig. 3. Comparison of sheet resistance of u-GZO layer in GZO(h)/ZnO(h) (open circles) and in GZO(2h) (closed circles) for unit thickness of (a) 50, (b) 100, and (c) 150 nm.
The carrier concentration was high for GZO film with high Ga2O3 content, but Hall mobility was low. However, the carrier concentration
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Fig. 5. Comparison of optical transmittance spectra of ZnO(h) (dashed line) and GZO(h) (solid line) for unit thicknesses of (a) 50, (b) 100, and (c) 150 nm. Ga2O3 content in GZO is 3.6 wt.%.
Fig. 7. Comparison of optical transmittance spectra of GZO(h)/ZnO(h) (dashed line) and GZO(2h) (solid line) for h values of (a) 50, (b) 100, and (c) 200 nm.
of ZnO was low but its Hall mobility was high. As a result of these factors, the sheet resistance of ZnO(2h) took values comparable to those of GZO(h)/ZnO(h). The optical transmittance spectra of GZO(50 nm)/ ZnO(50 nm) and GZO(100 nm) are compared in Fig. 7(a). While their transmittance levels are similar at wavelengths shorter than 1000 nm, the transmittance of GZO/ZnO is higher by 10–15% in an infrared range between 1000 and 3000 nm. Fig. 8 compares the sheet resistance of the five samples with a unit thickness of 100 nm. In this case, the sheet resistance of GZO(200 nm), GZO(100 nm)/ZnO(100 nm), and u-GZO(100 nm) in GZO(100 nm)/ZnO(100 nm) also take similar values. At a Ga2O3
content of 3.6 wt.%, for instance, R(GZO(200 nm))=46 Ω/□, R(GZO (100 nm)/ZnO(100 nm))=52 Ω/□, and R(u-GZO(100 nm))=59 Ω/□. These values indicate that the sheet resistance of the bottom half of GZO(200 nm), i.e., GZO(100 nm), and that in GZO(100 nm)/ZnO (100 nm) are much higher than that of u-GZO(100 nm), and electric conduction mostly relies on u-GZO(100 nm). The optical transmittance of GZO(200 nm) and GZO(100 nm)/ZnO(100 nm) are compared in Fig. 7(b). Their transmittance at wavelengths shorter than 1000 nm are at similar levels, whereas the transmittance of GZO(100 nm)/ ZnO(100 nm) in the infrared range is higher than that of GZO(200 nm) by 20–40% at each wavelength. In summary, using GZO(100 nm)/
Fig. 6. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h) layer in GZO(h)/ZnO(h) for h = 50 nm.
Fig. 8. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h) layer in GZO(h)/ZnO(h) for h = 100 nm.
H. Akazawa / Thin Solid Films 526 (2012) 195–200
Fig. 9. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h) layer in GZO(h)/ZnO(h) for h = 200 nm.
ZnO(100 nm) instead of GZO(200 nm) deteriorates the sheet resistance by 13%, but improves the infrared transmittance by 20–40% at each wavelength. Fig. 9 plots the sheet resistance of the five samples with a unit thickness of 200 nm. The general tendency is the same as that already seen in Figs. 6 and 8. At a Ga2O3 content of 3.6 wt.%, R(GZO(400 nm))=14 Ω/□, R(GZO(200 nm)/ZnO(200 nm))=21 Ω/□, and R(u-GZO(200 nm)) = 25 Ω/□. The optical transmittance spectra of GZO(400 nm) and GZO (200 nm)/ZnO(200 nm) in Fig. 7(c) again reveal that the transmittance in the visible range is similar but differs by 20–40% at wavelengths longer than 1000 nm. Finally, we investigated the best way of sharing the thickness between u-GZO(h1) and ZnO(h2) in the GZO(h1)/ZnO(h2) double layers when the total thickness is fixed, i.e., h1 +h2 =200 nm. We considered three samples: GZO(50 nm)/ZnO(150 nm), GZO(100 nm)/ZnO(100 nm), and GZO(150 nm)/ZnO(50 nm). Fig. 10 plots the sheet resistance of the three GZO/ZnO double layer films and Fig. 11 plots their optical transmittance. The sheet resistance and optical transmittance of GZO(100 nm)/ ZnO(100 nm) were measured but those of GZO(50 nm)/ZnO(150 nm) and GZO(150 nm)/ZnO(50 nm) were calculated. While GZO(150 nm)/ ZnO(50 nm) has the lowest sheet resistance, its transmittance at wavelengths longer than 1200 nm is the lowest. While GZO(50 nm)/ ZnO(150 nm) has the highest transmittance at wavelengths longer
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Fig. 11. Comparison of optical transmittance spectra of u-GZO(50 nm)/ZnO(150 nm) (dashed line), u-GZO(100 nm)/ZnO(100 nm) (dotted line), and u-GZO(150 nm)/ ZnO(50 nm) (solid line).
than 1200 nm, its sheet resistance is the highest. GZO(100 nm)/ ZnO(100 nm) has a medium sheet resistance and intermediate transmittance level above 1200 nm for the other two samples. The h1 and h2 values can be optimized depending on the weighted priority of infrared transmittance and sheet resistance. Apart from the requirement for high transmittance within the available wavelength of sunlight, three other important factors for application to solar cells include efficient confinement of incident light, efficient conversion of photon energies to electric current, and suppressed recombination between photo-generated carriers. Although it is unknown whether the GZO/ZnO double layer structure performs satisfactorily concerning these requirements, its electric conduction parameters and optical transmittance are at least competitive against SnO2-based TCOs. 4. Conclusion We demonstrated the feasibility of a GZO/ZnO double layer structure for application to transparent electrodes in solar cells. Whether the bottom layer is GZO or ZnO did not affect sheet resistance or the infrared transmittance of the upper GZO layer in GZO(h)/ZnO(h) and in GZO(2h). The optical transmittance of ZnO and GZO bottom layers considerably differed in the infrared range. If we replaced GZO(2h) film with GZO(h)/ZnO(h) film that had the same total thickness, the optical transmittance in the infrared region increased by 15–40% at each wavelength, while sheet resistance increased by 1.1–1.5 times. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] Fig. 10. Comparison of sheet resistance of u-GZO(50 nm)/ZnO(150 nm) (open circles), u-GZO(100 nm)/ZnO(100 nm) (open triangles), and u-GZO(150 nm)/ZnO(50 nm) (open squares).
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[13] X.L. Chen, X.H. Geng, J.M. Xue, D.K. Zhang, G.F. Hou, Y. Zhao, J. Cryst. Growth 296 (2006) 43. [14] T. Miyata, Y. Honma, T. Minami, J. Vac. Sci. Technol. A 25 (2007) 1193. [15] Y. Kaziwara, J. Cryst. Growth 289 (2006) 387. [16] T. Minami, H. Sato, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 24 (1985) L781. [17] B.H. Choi, H.B. Im, Thin Solid Films 193 (194) (1990) 712. [18] S. Kishimoto, T. Yamada, K. Ikeda, T. Yamamoto, Surf. Coat.Technol. 201 (2006) 4000. [19] T. Yamada, T. Nebiki, S. Kishimoto, H. Makino, K. Awai, T. Narusawa, T. Yamamoto, Superlattice. Microst. 42 (2007) 68. [20] B.M. Ataev, A.M. Bagamadova, A.M. Djabrailov, V.V. Mamedov, R.A. Rabadanov, Thin Solid Films 260 (1995) 19.
[21] T. Matsuda, M. Furuta, T. Hiramatsu, C. Li, H. Furuta, H. Hokari, T. Hirao, J. Cryst. Growth 310 (2008) 31. [22] N. Yamada, T. Shibata, K. Taira, Y. Hirose, S. Nakao, N.L.H. Hoang, T. Hitosugi, T. Shimada, T. Sasaki, T. Hasegawa, Appl. Phys. Express 4 (2011) 045801. [23] N. Itagaki, K. Kuwahara, K. Nakahara, D. Yamashita, G. Uchida, K. Koga, M. Shiratani, Appl. Phys. Express 4 (2011) 011101. [24] H. Akazawa, J. Vac. Sci. Technol. A 28 (2010) 314. [25] H. Akazawa, Thin Solid Films 518 (2009) 22. [26] M.J. Brett, R.R. Parsons, J. Vac. Sci. Technol. A 4 (1986) 423. [27] O. Nakagawa, Y. Kishimoto, H. Seto, Y. Koshido, Y. Yoshino, T. Makino, Appl. Phys. Lett. 89 (2006) 091904.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Double layer structures of transparent conductive oxide suitable for solar cells: Ga-doped ZnO on undoped ZnO Housei Akazawa ⁎ NTT Microsystem Integration Laboratories, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan
a r t i c l e
i n f o
Article history: Received 31 July 2012 Received in revised form 29 October 2012 Accepted 30 October 2012 Available online 14 November 2012 Keywords: Zinc oxide Ga-doped zinc oxide Transparent conductive oxide Sputtering Infrared transmittance Solar cell
a b s t r a c t We investigated the transparent conductive properties of hetero-double layers: Ga-doped ZnO (GZO) overlaid on undoped ZnO. We prepared five samples for given unit thicknesses h: ZnO and GZO films with a thickness of h to characterize the hybrid structure, ZnO and GZO films with a thickness of 2h, and a GZO/ZnO double layer with a thickness of h for each layer (h =50, 100, 150, and 200 nm). If we assumed that the upper and bottom half of the 2h-thick GZO films as well as those of GZO/ZnO were connected in parallel in terms of the equivalent electric circuit, the calculated sheet resistance of the upper GZO layer scarcely depended on whether the bottom layer was GZO or ZnO. Hence, the bottom layer played the role of providing a crystalline template for the upper layer that actually governed electrical transport. Also, the infrared transmittance of the upper GZO layer was immune to what the bottom layer consisted of. While GZO/ZnO had 1.1–1.5 times higher sheet resistance than 2h-thick GZO, the optical transmittance of GZO/ZnO in the near-infrared region was 20–40% higher, demonstrating that the GZO/ZnO double layer structure is suitable for transparent electrodes in solar cells. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) films are nowadays widely employed in various kinds of electrical and optical devices. The only concern about optical properties regarding their use in flat panel displays is the transmittance level in the visible wavelength range. The requirement for solar cells is somewhat different; transmittance in the infrared range should be high enough as well because the sunlight spectrum has a substantial fraction of photons with wavelengths between 1000 and 2500 nm. If infrared light is efficiently utilized with additional contrivances such as surface-plasmon-resonance-enhanced photo-absorption, the photovoltaic efficiencies will be dramatically improved. Up to now, SnO2 [1], Sb-doped SnO2 [2,3], and F-doped SnO2 [4,5] have been proposed and actually used as electrodes for solar cells. Because even lower resistivities can be achieved by using ZnO-based materials [6], the feasibility of Ga-doped ZnO (GZO) or Al-doped ZnO (AZO) has also been investigated by focusing on solar cells [7–10]. GZO films are generally more conductive than ZnO films but do not transmit light in the infrared range, which renders GZO rather useful for solar blind windows [11]. This is due to their higher plasma frequency resulting from their higher carrier concentration. ZnO films have lower carrier concentration and thus have higher optical transmittance. Boron-doped ZnO, which is characterized by intermediate resistivity and transmittance between ZnO and GZO, has been expected to be a ⁎ Tel.: +81 46 240 2659; fax: +81 46 270 2372. E-mail address: [email protected]. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.10.111
candidate material for solar cells [12–14]. However, typical resistivities of ZnO:B films in the 10−3 Ω·cm range can easily be obtained with undoped ZnO films. There is still a debate as to whether or not doped boron atoms really behave as extrinsic donors. It is well known that when materials from the ZnO family are directly deposited on glass substrates, their crystallinity is the worst at the initial stage of growth. This makes electron mobility near the interface region extremely low. Furthermore, carriers are not generated if crystallinity is so poor that even small crystallites cannot be created. When the crystal quality increasingly improves farther from the interface [15], both carrier concentration and Hall mobility increase [16–19]. Since poor crystallinity in the interfacial region is the primary bottleneck to achieve ultra-thin TCO films, how to secure superior crystallinity at an early stage of growth is a critical issue to achieve high performance of TCO films. Various attempts have been devoted to solving this problem. Although the best crystallinity can be obtained in epitaxial growth on single crystal substrates such as sapphire [20] and quartz [21], this is obviously a high-cost solution and unrealistic for producing devices. Another approach is to insert a nano-meter-scale inorganic crystalline layer, i.e., a nano-sheet, between a TCO film and the glass substrate [22]. However, covering the macroscopic substrate surface while assembling numerous nano-sheets is a hard task. This nanosheet approach still seems to be far away from the production stage. Also, the adhesive strength of TCO films to the substrate is reduced. Another way is to deposit a thin buffer layer prior to growing the film for the TCO body [21]. A polycrystalline ZnON seed layer has been demonstrated to be promising in promoting crystallization of
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the film for the TCO body at the initial stage of growth [23]. However, once a buffer layer is deposited in the presence of O2 (and N2) gases, the ZnO target surface is oxidized (or nitrided) more than the best conditions required to deposit the TCO film. If the buffer layer is deposited in the same deposition chamber, it interferes with the deposition of the TCO film, which should be carried out under a reduced atmosphere. Against this background, we propose that a double layer structure, where an upper GZO or AZO layer is overlaid on an undoped ZnO bottom layer, would be a competitive candidate for transparent electrodes in solar cells. The basic concept is as follows. If a GZO film is divided into two parts, e.g., as bottom and upper layers, the bottom layer is regarded as a buffer layer that leads to better crystallinity for the upper layer. This means that the bottom layer scarcely contributes to electrical conduction and the upper layer, where crystallinity has been sufficiently improved, primarily provides the conduction path. Nevertheless, the bottom half of GZO is more photo-absorptive than ZnO because of its higher carrier concentration. Hence, if we substitute the bottom half of GZO with ZnO, the transmittance of the resulting GZO/ZnO double layer film would be improved in the infrared range while electrical conductivity would be slightly degraded. We demonstrated the feasibility of this GZO/ZnO double layer structure as a transparent electrode for solar cells. 2. Experimental details The details on our dual target sputtering system have been described elsewhere [24]. The base pressure in the deposition chamber was 5 × 10 −5 Pa. ZnO films were deposited by electron cyclotron resonance (ECR) plasma sputtering from a ZnO target [25]. The ECR plasma was generated from Ar gas without introducing O2 gas into the plasma source. The powers of the microwaves and RF applied to the target were both 500 W. GZO films were deposited by ECR plasma sputtering from the ZnO target and simultaneous RF magnetron sputtering from a Ga2O3 target at a fixed RF magnetron power of 20 W [24]. The substrates we used were rectangular glass plates with dimensions of 52 × 76 mm. Although the substrate was not intentionally annealed by using a heater from its rear side, impinging sputtered atoms and an Ar plasma stream warmed it up to 70 °C during deposition. Ga concentration in the deposited films had a continuous distribution over the substrate surface, which enabled combinatorial evaluation of sheet resistance and optical transmittance as a function of Ga2O3 content. Regarding GZO as a ZnO–Ga2O3 composite material, we denoted Ga concentration in a GZO film as the weight of Ga2O3 with respect to the total weight of ZnO and Ga2O3. Ga2O3 content was determined by inductively coupled plasma atomic emission spectroscopy (SPS1700, Seiko Instruments). Emission lines at 213.856 nm (Zn) and 417.206 nm (Ga) were measured at an RF power of 1.3 kW. Sheet resistance was measured by using a fourpoint prober and optical transmittance spectra were measured with a spectrophotometer (UV-3100, Shimadzu). Crystal structures were evaluated with X-ray diffraction (XRD) (RINT1500, Rigaku) using a Cu Kα line with the Bragg–Brentano configuration. We prepared a set of five samples for unit thickness h, as schematically illustrated in Fig. 1(a). They included a ZnO film with a thickness of h (ZnO(h)), a GZO film with a thickness of h (GZO(h)), a ZnO film with a thickness of 2h (ZnO(2h)), a GZO film with a thickness of 2h (GZO(2h)), and a double layer structure consisting of an upper GZO layer (u-GZO) and a bottom ZnO layer with a thickness of h for each layer (GZO(h)/ ZnO(h)). We regarded GZO(2h) as being composed of an upper half of GZO overlaid on a bottom half of GZO, i.e., GZO(2h) ≡ u-GZO(h)/ GZO(h). Four sets of sample groups with h = 50, 100, 150, and 200 nm were produced. We assumed that the upper and bottom layers in double layers were connected in parallel in terms of an equivalent electrical circuit [Fig. 1(b)]. This was rationalized by the fact that electrons travel horizontally and those scattered at a large angle by ionized
Fig. 1. (a) Schematic model of five structures for unit thickness of h along with provisional upper GZO layer without bottom layer, and (b) equivalent model for electric conduction and optical transmission.
impurities and grain boundaries do not actually contribute to electric conduction. We could thus calculate the sheet resistance of u-GZO in GZO(h)/ZnO(h) from the sheet resistance of GZO(h)/ZnO(h) and that of ZnO(h). Similarly, we could calculate the sheet resistance of u-GZO in GZO(2h) from the sheet resistance of GZO(2h) and that of GZO(h). We also assumed that the optical transmittance of the GZO(h)/ZnO(h) doubled layer would simply be given as the product of the transmittance of u-GZO(h) and ZnO(h) by neglecting interference at the interface [Fig. 1(b)]. This approximation is valid in an infrared range where the effect of interference fringes has decayed. We could calculate the transmittance spectra of u-GZO(h) in GZO(h)/ZnO(h) from the optical transmittance of GZO(h)/ZnO(h) and ZnO(h). Similarly, we could calculate the optical transmittance of u-GZO(h) in GZO(2h) from the transmittance of GZO(2h) and GZO(h).
3. Results and discussion Fig. 2 plots the ZnO(002) XRD peak intensity measured in the ω-2θ scan mode as a function of the thickness of ZnO and GZO films. The ZnO(002) intensity can be fitted to a parabolic function with respect to the thickness, which means that the crystallinity improves in an accelerated manner as the film thickens. The coefficient of the parabolic curve for GZO (0.55) being larger than that for ZnO (0.27) indicates superior crystallinity of GZO films because they contain more oxygen atoms supplied from the Ga2O3 target. Fig. 3 compares the calculated sheet resistance of u-GZO(h) in GZO(2h) and u-GZO(h) in GZO(h)/ZnO(h) for unit thicknesses of 50, 100, and 150 nm. The sheet resistances of u-GZO(h) are similar for each unit thickness, irrespective of what the bottom layer consists of. We can also see that thickness is the major parameter reflecting crystallinity. Whether Ga is doped or not in the bottom layer has little effect on the crystallinity of the u-GZO layer or sheet resistance. The bottom layer can be regarded as a crystalline template to facilitate
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Fig. 2. Dependence of ZnO(002) XRD intensities on film thickness of ZnO (open circles) and GZO (closed circles).
the formation of high-quality crystalline domains just from the beginning of the upper layer. Fig. 4 compares the calculated optical transmittance spectra of the u-GZO layer (Ga2O3 content of 3.6 wt.%) in GZO(2h) and the u-GZO layer in GZO(h)/ZnO(h) for unit thicknesses of 50, 100 and 150 nm. The interference fringe is responsible for the unrealistic transmittance exceeding 100% below 1000 nm. The transmittance curves overlap one another in the infrared range, where interference fringes do not appear. This is a confirmation that the optical properties of u-GZO in the infrared region scarcely depend on whether the bottom layer is ZnO or GZO. The measured transmittance spectra of ZnO(h) and GZO(h) films are compared in Fig. 5 for h = 50, 100, and 150 nm
Fig. 4. Comparison of optical transmittance spectra contributed by u-GZO layer in GZO(h)/ZnO(h) (dashed line) and in GZO(2h) (solid line) for unit thicknesses of (a) 50, (b) 100, and (c) 150 nm. Ga 2 O 3 content in GZO is 3.6 wt.%.
with Ga2O3 content of 3.6 wt.%. Here, the two curves for each h value differ considerably. The transmittance of GZO in the infrared range is more suppressed than that of ZnO, whereas GZO is more transparent between 400 and 800 nm. The reduced transmittance of ZnO within the visible wavelength is caused by oxygen deficiencies in ZnO, but they are necessary to make ZnO conductive [26]. Fig. 6 plots the sheet resistance (R) of ZnO(h), ZnO(2h), GZO(2h), GZO(h)/ZnO(h), and u-GZO(h) in GZO(h)/ZnO(h) as a function of the Ga2O3 content for a unit thickness of h = 50 nm. ZnO(50 nm) has the highest sheet resistance for the five samples. The sheet resistance of u-GZO(50 nm), GZO(50 nm)/ZnO(50 nm), and GZO(100 nm) are comparable. The sheet resistance of ZnO(100 nm) is still higher than these three. The sheet resistance at Ga2O3 content lower than 6 wt.% follows the order: RðGZOð2 hÞÞbRðGZOðhÞ=ZnOðhÞÞbRðu−GZOðhÞÞbRðZnOð2 hÞÞbRðZnOðhÞÞ:
We confirmed this order for all h between 50 and 200 nm. The sheet resistance of u-GZO(h) higher than that of GZO(h)/ZnO(h) means that the bottom ZnO(h) contributes somewhat to electric conduction. The sheet resistance of GZO(h)/ZnO(h) higher than that of GZO(2h) means that the bottom part of GZO(2h), i.e., GZO(h), is more conductive than ZnO(h). Substantial fractions of Ga atoms are non-activated at Ga2O3 content above 10 wt.%, and such impurity atoms prevent crystallization, resulting in changes in the crystal structure of GZO [24,27]. We found the sheet resistance at h = 50 and 100 nm followed the order at 19 wt.%: RðZnOð2 hÞÞeRðGZOðhÞ=ZnOðhÞÞbRðGZOð2 hÞÞbRðu−GZOðhÞÞbRðZnOðhÞÞ: Fig. 3. Comparison of sheet resistance of u-GZO layer in GZO(h)/ZnO(h) (open circles) and in GZO(2h) (closed circles) for unit thickness of (a) 50, (b) 100, and (c) 150 nm.
The carrier concentration was high for GZO film with high Ga2O3 content, but Hall mobility was low. However, the carrier concentration
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Fig. 5. Comparison of optical transmittance spectra of ZnO(h) (dashed line) and GZO(h) (solid line) for unit thicknesses of (a) 50, (b) 100, and (c) 150 nm. Ga2O3 content in GZO is 3.6 wt.%.
Fig. 7. Comparison of optical transmittance spectra of GZO(h)/ZnO(h) (dashed line) and GZO(2h) (solid line) for h values of (a) 50, (b) 100, and (c) 200 nm.
of ZnO was low but its Hall mobility was high. As a result of these factors, the sheet resistance of ZnO(2h) took values comparable to those of GZO(h)/ZnO(h). The optical transmittance spectra of GZO(50 nm)/ ZnO(50 nm) and GZO(100 nm) are compared in Fig. 7(a). While their transmittance levels are similar at wavelengths shorter than 1000 nm, the transmittance of GZO/ZnO is higher by 10–15% in an infrared range between 1000 and 3000 nm. Fig. 8 compares the sheet resistance of the five samples with a unit thickness of 100 nm. In this case, the sheet resistance of GZO(200 nm), GZO(100 nm)/ZnO(100 nm), and u-GZO(100 nm) in GZO(100 nm)/ZnO(100 nm) also take similar values. At a Ga2O3
content of 3.6 wt.%, for instance, R(GZO(200 nm))=46 Ω/□, R(GZO (100 nm)/ZnO(100 nm))=52 Ω/□, and R(u-GZO(100 nm))=59 Ω/□. These values indicate that the sheet resistance of the bottom half of GZO(200 nm), i.e., GZO(100 nm), and that in GZO(100 nm)/ZnO (100 nm) are much higher than that of u-GZO(100 nm), and electric conduction mostly relies on u-GZO(100 nm). The optical transmittance of GZO(200 nm) and GZO(100 nm)/ZnO(100 nm) are compared in Fig. 7(b). Their transmittance at wavelengths shorter than 1000 nm are at similar levels, whereas the transmittance of GZO(100 nm)/ ZnO(100 nm) in the infrared range is higher than that of GZO(200 nm) by 20–40% at each wavelength. In summary, using GZO(100 nm)/
Fig. 6. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h) layer in GZO(h)/ZnO(h) for h = 50 nm.
Fig. 8. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h) layer in GZO(h)/ZnO(h) for h = 100 nm.
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Fig. 9. Sheet resistance of ZnO(h), ZnO(2h), GZO(h), GZO(h)/ZnO(h), and u-GZO(h) layer in GZO(h)/ZnO(h) for h = 200 nm.
ZnO(100 nm) instead of GZO(200 nm) deteriorates the sheet resistance by 13%, but improves the infrared transmittance by 20–40% at each wavelength. Fig. 9 plots the sheet resistance of the five samples with a unit thickness of 200 nm. The general tendency is the same as that already seen in Figs. 6 and 8. At a Ga2O3 content of 3.6 wt.%, R(GZO(400 nm))=14 Ω/□, R(GZO(200 nm)/ZnO(200 nm))=21 Ω/□, and R(u-GZO(200 nm)) = 25 Ω/□. The optical transmittance spectra of GZO(400 nm) and GZO (200 nm)/ZnO(200 nm) in Fig. 7(c) again reveal that the transmittance in the visible range is similar but differs by 20–40% at wavelengths longer than 1000 nm. Finally, we investigated the best way of sharing the thickness between u-GZO(h1) and ZnO(h2) in the GZO(h1)/ZnO(h2) double layers when the total thickness is fixed, i.e., h1 +h2 =200 nm. We considered three samples: GZO(50 nm)/ZnO(150 nm), GZO(100 nm)/ZnO(100 nm), and GZO(150 nm)/ZnO(50 nm). Fig. 10 plots the sheet resistance of the three GZO/ZnO double layer films and Fig. 11 plots their optical transmittance. The sheet resistance and optical transmittance of GZO(100 nm)/ ZnO(100 nm) were measured but those of GZO(50 nm)/ZnO(150 nm) and GZO(150 nm)/ZnO(50 nm) were calculated. While GZO(150 nm)/ ZnO(50 nm) has the lowest sheet resistance, its transmittance at wavelengths longer than 1200 nm is the lowest. While GZO(50 nm)/ ZnO(150 nm) has the highest transmittance at wavelengths longer
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Fig. 11. Comparison of optical transmittance spectra of u-GZO(50 nm)/ZnO(150 nm) (dashed line), u-GZO(100 nm)/ZnO(100 nm) (dotted line), and u-GZO(150 nm)/ ZnO(50 nm) (solid line).
than 1200 nm, its sheet resistance is the highest. GZO(100 nm)/ ZnO(100 nm) has a medium sheet resistance and intermediate transmittance level above 1200 nm for the other two samples. The h1 and h2 values can be optimized depending on the weighted priority of infrared transmittance and sheet resistance. Apart from the requirement for high transmittance within the available wavelength of sunlight, three other important factors for application to solar cells include efficient confinement of incident light, efficient conversion of photon energies to electric current, and suppressed recombination between photo-generated carriers. Although it is unknown whether the GZO/ZnO double layer structure performs satisfactorily concerning these requirements, its electric conduction parameters and optical transmittance are at least competitive against SnO2-based TCOs. 4. Conclusion We demonstrated the feasibility of a GZO/ZnO double layer structure for application to transparent electrodes in solar cells. Whether the bottom layer is GZO or ZnO did not affect sheet resistance or the infrared transmittance of the upper GZO layer in GZO(h)/ZnO(h) and in GZO(2h). The optical transmittance of ZnO and GZO bottom layers considerably differed in the infrared range. If we replaced GZO(2h) film with GZO(h)/ZnO(h) film that had the same total thickness, the optical transmittance in the infrared region increased by 15–40% at each wavelength, while sheet resistance increased by 1.1–1.5 times. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] Fig. 10. Comparison of sheet resistance of u-GZO(50 nm)/ZnO(150 nm) (open circles), u-GZO(100 nm)/ZnO(100 nm) (open triangles), and u-GZO(150 nm)/ZnO(50 nm) (open squares).
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