Influence of the Cation Ratio on Optical and Electrical Properties of

Research Article pubs.acs.org/acscombsci

Influence of the Cation Ratio on Optical and Electrical Properties of Amorphous Zinc-Tin-Oxide Thin Films Grown by Pulsed Laser Deposition Sofie Bitter,* Peter Schlupp, Michael Bonholzer, Holger von Wenckstern,* and Marius Grundmann Universität Leipzig Institut für Experimentelle Physik II, Linnéstraße 5, 04103 Leipzig, Germany ABSTRACT: Continuous composition spread (CCS) methods allow fast and economic exploration of composition dependent properties of multielement compounds. Here, a CCS method was applied for room temperature pulsed laser deposition (PLD) of amorphous zinc-tin-oxide to gain detailed insight into the influence of the zinc-to-tin cation ratio on optical and electrical properties of this ternary compound. Our CCS approach for a large-area offset PLD process utilizes a segmented target and thus makes target exchange or movable masks in the PLD chamber obsolete. Cation concentrations of 0.08−0.82 Zn/(Zn + Sn) were achieved across single 50 × 50 mm2 glass substrates. The electrical conductivity increases for increasing tin content, and the absorption edge shifts to lower energies. The free carrier concentration can be tuned from 1020 to 1016 cm−3 by variation of the cation ratio from 0.1 to 0.5 Zn/(Zn + Sn). KEYWORDS: amorphous oxide semiconductors, zinc-tin-oxide, continuous composition spread, pulsed laser deposition, electrical properties

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INTRODUCTION Investigation of the influence of different compositions of multielement compounds on material properties is of high scientific and technological relevance. Pulsed laser deposition (PLD) is an advantageous method for the deposition of high quality oxide thin films with growth rates of several pm per pulse. An auspicious method to obtain different cation compositions of a compound within just one fabrication step is the continuous composition spread (CCS) approach. It enables an expeditious investigation of ternary compounds within a single sample. Different approaches for CCS methods for PLD have been proposed. The first approaches required the synchronized motion of masks and targets or an offset between the substrate and exchanging targets.1,2 Christen et al. proposed a method that requires a synchronized motion of an aperture and a target exchange.3 A rotation of the substrate in combination with several targets is necessary in the approach proposed by Keller et al.4,5 These methods have been successfully applied for the detailed investigation of crystalline material libraries. Drawbacks are reduced growth rates compared to standard PLD, the need for synchronization of the mask movement, and a laser pulse sequence. Substrate heating or postannealing of the samples might also be required to ensure an intermixing of the individual layers. This is not suited for the growth of amorphous semiconductors. In 2013, von Wenckstern et al. proposed a CCS-PLD method that does not require postannealing.6,7 Instead of two or more individual targets, one segmented target is used, which makes additional movable parts in the chamber and a target exchange obsolete. Moreover, the formation of binary layers is prevented due to a © XXXX American Chemical Society

rapid change of source materials. For a twofold segmented target, the ablated segments are changed after every 18 to 54 pulses for repetition frequencies between 5 and 15 Hz. For the PLD system used for this study, less than one monolayer is deposited for repetition frequencies below 8 Hz. To date, this approach has been employed for crystalline materials only.8−11 Amorphous oxide semiconductors (AOSs) are a material group of special interest. Several AOSs have been proposed as candidates for transparent electronic devices, such as pixel drivers.12 One promising material is amorphous zinc-tin-oxide (ZTO), which consists of only naturally abundant and nontoxic elements and combines the stability of SnO2 against acids and bases with the stability of ZnO against activated hydrogen environments.13−17 It belongs to the group of heavy cation compound materials described by Hosono et al. in 1996, which exhibit a comparably high electron mobility even in the amorphous phase.18 This high mobility can be ascribed to the spatial overlap of the large, spherically symmetric metal s orbitals and their insensitivity to variations of the bonding angle that occurs in amorphous materials.18,19 Moreover, they can be deposited at room temperature, which enables cost-efficient deposition and growth on flexible substrates. To date, only selected zinc-to-tin cation ratios of PLD-deposited amorphous ZTO have been investigated in detail. For thin films with zincto-tin ratios of 1:1 and 2:1, Jayaraj et al. reported electron mobilities as high as 12 cm2 V−1 s−1 and an absorption edge Received: November 30, 2015 Revised: February 11, 2016

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DOI: 10.1021/acscombsci.5b00179 ACS Comb. Sci. XXXX, XXX, XXX−XXX

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ACS Combinatorial Science between 2.37 and 2.86 eV.20 For a zinc-to-tin ratio of 1:2, a Hall mobility as high as 12.7 cm2 V−1 s−1 has been reported.21 Allamorphous heterojunction diodes were prepared on these thin films using p-type zinc-cobalt-oxide.22 A more detailed investigation of the influence of the cation ratio on optical and electrical properties of amorphous ZTO has been performed for thin films deposited by cosputtering.23 However, a crystallization for high zinc contents presented a disadvantage of this method.23 The influence of the zinc-to-tin ratio has also been investigated for solution-processed ZTO thin film transistors.24−26 We present amorphous ZTO thin films prepared at room temperature (RT) by a CCS-PLD approach. Cation concentrations from 0.1 to 0.8 Zn/(Zn + Sn) are realized. The electrical and optical properties in dependence of the cation ratio are systematically investigated and compared to reference thin films with a laterally homogeneous Zn/Sn ratio also prepared by PLD. A monotonic increase of the conductivity with decreasing zinc content is observed. The absorption edge exhibits a blue shift with increasing zinc content.

ments were performed using a PerkinElmer Lambda 19 Spectrometer in the spectral range chosen from 200 to 1200 nm. An aperture of 4 mm diameter was used. Hall effect measurements were conducted at RT in the van der Pauw geometry using a magnetic field B = 0.43 T to obtain the resistivity (ρ), free carrier density (n), and Hall mobility (μ).27−29 Thicknesses were determined with a DektakXT from Bruker and from the interferences in the optical transmission data. The amorphous structure was confirmed by X-ray diffraction (XRD) measurements using a Philips X’Pert diffractometer via a 2Θ-ω scan in the range of 10−110° from the absence of Bragg peaks. The morphology of the thin films was investigated by means of a laser scanning microscope (LSM). A Keyence VKX200 K control unit combined with an X210 microscope were utilized. The LSM is equipped with a 100 W halogen lamp as light source and a CCD camera as detector for recording an optical microscopic image. Laser scanning measurements were performed using a violet laser with a wavelength of 408 nm and an outgoing power of 0.95 mW, which is detected by a photomultiplier. A more detailed investigation of the surface was performed by secondary electron microscopy (SEM) using the device Nova NanoLab 200 by FEI company.

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EXPERIMENTAL PROCEDURES To fabricate zinc-tin-oxide thin films having a continuous composition spread, we used a large-area offset PLD2 process and a segmented target.6 CCS thin films were deposited on 50 × 50 mm2 Corning 1737 glass substrates. Reference samples were deposited by standard PLD on 10 × 10 mm2 Corning 1737 glass substrates. The pulse frequency of the KrF excimer laser (Lambda Physik, λ = 248 nm) was chosen between 5 and 15 Hz. The energy of the incident irritation was approximately 2 J cm−2 at the target surface. CCS thin films were deposited with 50k pulses in oxygen atmosphere at pressures of 0.025 mbar and 0.03 mbar entirely at RT. The distance between target and substrate amounts to 10 cm, and the offset is chosen as 2.4 cm. In our case, the maximum gradient in concentration and thickness was achieved along the diagonal of the thin films. This was realized by aligning the interface between the two target segments in parallel to the diagonal of the substrate. The segmented ceramic targets used for the deposition consisted of ZnO and SnO2 powders purchased from Alpha Aesar with purities of 99.9978 and 99.9%, respectively. Three different types of targets were fabricated for the deposition of the ZTO thin films. Highest concentration gradients were achieved using targets with segments consisting of binary ZnO and binary SnO2, whereas a more detailed investigation was performed using targets with segments having a zinc-to-tin ratio of 1:2 and 2:1, respectively. Reference thin films were deposited using standard PLD targets with zinc-to-tin ratios of 1:2 and 2:1. All targets were pressed into pellets and subsequently sintered at 1150 °C for 12 h. One target (target II) consisting of binary ZnO and binary SnO2 was sintered for 24 h. The spatial variation of chemical composition was investigated by energy dispersive X-ray spectroscopy (EDX) using a Nova NanoLab 200 by FEI company. A total of 81 points were measured. Subsequently, the substrate was sawed into 5 mm wide stripes along the concentration gradient and then broken into 5 × 5 mm2 pieces, which were used for further investigations. The concentration for each 5 × 5 mm2 piece was averaged and henceforth treated as constant within each piece. The maximal deviation of the Zn/(Zn + Sn) ratio from the averaged value amounts to 0.025. For all results, the averaged value is given. Transmission and reflection measure-

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RESULTS AND DISCUSSION Target Preparation and Morphology. Three different segmented PLD targets were used to deposit amorphous ZTO thin films with a lateral composition spread. In a first attempt, two standard targets were fabricated and subsequently sawed into semicircular halves. These halves, consisting of binary ZnO and binary SnO2, were joined to form one segmented target (target I).6 Thin films deposited from this target exhibited large concentration gradients of approximately 0.08−0.80 Zn/(Zn + Sn) but also high droplet and pit density (compare Figure 1b). The droplets had a lateral expansion of approximately 0.9−2.5 μm in diameter and a height of approximately 0.8−1.5 μm. The composition of the droplets could not be determined by EDX unambiguously. The depth of one pit was determined by SEM to be approximately 200 nm in an approximately 1 μm thick thin film. After the PLD process, increased ablation of material at the interface between the target halves was found and ascertained as the source of the droplets. Therefore, new targets were produced with the aim of decreasing ablation at the interface between target segments. In a second more sophisticated approach, a target with an inhomogeneous composition (one-half consisting of ZnO the other of SnO2 powder) was sintered simultaneously (target II). It is known that ZnO and SnO2 targets exhibit different shrinkage during sintering, which has to be considered to avoid decomposition of the segmented target. A preceding sintering step of the individual materials eliminates further shrinkage during the second sintering steps. Therefore, the ZnO and SnO2 powders were calcified/presintered as pellets at 1150 °C for 12 h. Afterward, they were ground, and the powders were filled into the press ram on opposing sides. Subsequently, one target consisting of two different halves was produced (Figure 1a). Thin films deposited from this second target showed improved morphology compared to thin films deposited from target I. However, the droplet density was still higher than that of the reference samples (Figure 1c). The droplets had lateral B


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Figure 1. Photographic image in (a) shows a segmented, ceramic target (target II) before (left) and after (right) a PLD process. Increased ablation at the interface between target segments is visible. In (b), the high droplet density detected by LSM measurements for a thin film deposited from target I is visible. A thin film deposited from target II (c) has an improved morphology but still has a high droplet density. A thin film deposited from target III (d) has a much improved morphology. This last thin film is deposited from a segmented target with halves having zinc-to-tin ratios of 1:2 and 2:1. All LSM pictures have dimensions of 90 × 90 μm.

extensions of approximately 1 μm and heights up to 1 μm. The density and size of pits was strongly reduced. Visual inspections again disclosed an increased ablation at the target interface (see Figure 1a). A possible reason for the increased ablation at the interface is tension that forms during the target fabrication. This tension could lead to an increased ablation of droplets from the interface region. For the morphology to be further improved, a third target was fabricated (target III). The third approach was comprised of filling the press ram on one-half with a powder mixture with a zinc-to-tin cation ratio of 1:2 and on the other half with a powder mixture with a ratio of 2:1. Once more, the target was pressed and sintered as one target consisting of two different halves. No presintering was required for the third target approach. The surface morphology of the deposited thin film was further improved (compare Figure 1d). The number and size of droplets was significantly reduced, and no pits were observed. At the same time, the concentration gradient was reduced to a range from 0.35 to 0.55 Zn/(Zn + Sn). Reference thin films, denoted as single composition target (SCT) thin films in the following, were deposited from targets having zinc-to-tin ratios of 1:2 and 2:1. The surface morphology of these thin films was comparable to that of the thin film deposited from target III. The oxygen pressures were chosen according to previous studies on ZTO deposited in the same chamber.21 Composition and Structure. The spatial composition spread of the deposited ZTO thin films was investigated by EDX in a square of 40 mm side length. The false color representation is depicted in Figure 2a for a thin film having a broad composition spread. Minimum and maximum zinc concentrations of 0.08 and 0.78 Zn/(Zn + Sn) were realized. The change of the composition along the diagonal is indicated by the color gradient (compare Figure 2a). The concentration exhibits an S-shape dependence as predicted,6 which is depicted in Figure 2b along with the change of the

Figure 2. (a) False-color representation of the spatial composition variation determined from 81 EDX measurements for a thin film deposited from target I, which consisted of a binary ZnO and a binary SnO2 segment. Measurement points are indicated by black dots. (b) Dependence of composition and growth rate on position (line scan along the diagonal, see dashed line in (a)). The growth rate was determined from transmission measurements. (c) XRD pattern for zinc contents as labeled.

growth rate. This change probably appears because the heavier tin atoms are scattered less effectively by the oxygen molecules of the background gas than the lighter zinc atoms. The thicknesses varied between 0.96 and 1.44 μm for the zinc-poor and -rich sides, respectively. For thin films deposited from target I and target II, concentration gradients of 0.08−0.78 and 0.13−0.82 Zn/(Zn + Sn), respectively, were obtained. For the thin film deposited from target III, a concentration gradient of 0.35−0.54 Zn/(Zn + Sn) was determined, enabling a more detailed investigation of the composition-dependent properties. Thin films deposited from targets II and III had thicknesses of 0.52−0.96 μm and 0.69−0.96 μm, respectively. A higher thickness corresponds to the tin-rich side. For selected zinc-to-tin cation ratios along the concentration gradient, the amorphous structure was confirmed by XRD measurements. The XRD patterns depicted in Figure 2c indicate that the films are X-ray amorphous independent of the cation ratio. Because of the use of thin gold layers at the corners of the samples for Hall effect measurements, reflection of the Au (111) plane was detected. Additionally, a broad peak with a low count is visible between 43° and 47°. This broad peak could not be assigned to zinc oxide, tin oxide, or a mixed phase. Optical Properties. The transparency in the visible part of the spectrum increases with increasing zinc content (Figure 3a,b). Moreover, the absorption edge exhibits a systematic blue shift. From the transmission and reflection data, the absorption coefficient α was calculated and used to determine the C


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Figure 3. Transmission spectra (a) for selected zinc contents of the CCS thin film deposited from target I. The black arrow indicates the direction of decreasing zinc content. Reflection and transmission data (b) for three different zinc contents on a CCS thin film. Calculated absorption edge for two CCS and two SCT thin films (c) deposited at 0.03 mbar O2 and (d) for SCT thin films deposited at different oxygen pressures. The SCT thin films were deposited from a 2:1 Zn:Sn target.

absorption edge from a linear fit of (αhν)1/2 in dependence of hν, because indirect transitions are likely for amorphous materials.30 The calculated values for the CCS thin films exhibit a reasonably good agreement with the values obtained for the SCT thin films (compare Figure 3c). The absorption edge can be controlled between 1.8 eV (Zn/(Zn + Sn) = 0.1) and 3.1 eV (Zn/(Zn + Sn) = 0.8) and is also strongly influenced by the background gas and pressure during the deposition.21 Therefore, a further tuning is possible by varying the deposition pressure (compare Figure 3d). For a Zn/(Zn + Sn) cation ratio of approximately 0.52 ± 0.02, the absorption edge could be varied from 2.3 to 3.2 eV for oxygen pressures between 0.005 and 0.05 mbar. Electrical Properties. The resistivity for CCS and SCT thin films deposited at two different pressures is depicted in Figure 4. A monotonic increase of the resistivity with increasing zinc content can be observed for two thin films. The black squares correspond to the thin film deposited from target I. The bad morphology appears to have a major influence on the charge carrier transport. For the other CCS and SCT thin films, the measured resistivities are in good agreement. The observed difference in the resistivity for the two thin films deposited from targets II and III is derived from the different deposition pressures. The origin of the local maximum for Zn/(Zn + Sn) = 0.46 visible in the resistivity data for the thin film deposited from target III is not yet understood. It may be due to laterally varying oxygen. The resistivity increases with increasing zinc content for the thin films deposited from targets II and III. We conjecture that the larger spatial spread of the s orbitals of the tin atoms leads to more overlap and thus a lower resistivity in the tin-rich regime, whereas the spatial spread of the zinc atoms is smaller and less overlap may be present. The carrier concentration and mobility were determined for thin films having sufficiently low resistivities. For the thin film deposited from target II, the carrier density and mobility are depicted in Figure 5. The carrier density exhibits a decrease with increasing

Figure 4. Resistivity of thin films deposited from different targets and fabricated at different pressures. The black triangles belong to the thin film deposited from target I, and the dark green squares and red circles belong to thin films deposited from targets II and III, respectively. SCT thin films are represented in open forms according to the deposition pressure.

zinc content. The mobility strongly depends on the position on the substrate as well as the zinc content. The lateral position on the substrate determines the energy of the incident particles and likely the oxygen content, too. A further investigation on the influence of oxygen has to be made. The maximum mobility of 11 cm2 V−1 s−1 was obtained for a zinc content of approximately 0.13 Zn/(Zn + Sn). The lowest resistivity of 5.9 × 10−5 Ωm was determined on the tin-rich side at 0.13 Zn/(Zn + Sn) for the CCS thin film deposited at a pressure of 0.025 mbar of O2 from target II. A scaling of the resistivity over 7 orders of magnitude was achieved. The carrier density was calculated to be between 1 × D


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Table 1. Minimum Resistivities Reported for Amorphous Zinc-Tin-Oxide Thin Films Deposited by PLD, Oxygen Plasma-Assisted PLD (PA-PLD), Sputtering, and Solution Processing (SP) deposition method PLD PLD PLD PA−PLD PLD cosputtering cosputtering sputtering SP

Figure 5. Carrier density and mobility of the thin film deposited from target II.

annealing temperature, °C

450 500

Zn:Sn 1:1 1:2 1:6.6 2:1 2:1 1:9 1:9 1:2

minimum resistivity, Ωm 8.3 2 5.9 0.5 4.3 1 1.9 1.6 7.1

× 10−4 × 10−4 × 10−5 × × × × ×

10−4 10−4 10−5 10−2 10−2

ref 20 21 this work 31 31 23 23 32 33

6b). The origin of the high carrier density at this point cannot be explained. Nonetheless, the resistivity has no outlier as this point has a low mobility. A line scan along the gradient of the thin films exhibits an S-shape similarly to the thin films having a larger concentration gradient. The Hall mobility does not exhibit a clear dependence on the zinc content. However, all measured mobilities are larger than 4 cm2 V−1 s−1, and a maximum mobility of 8 cm2 V−1 s−1 was measured for 0.54 Zn/(Zn + Sn) (compare Figure 6d). As discussed above, the position on the substrate has a major influence on the mobility.

1020 and 2 × 1016 cm−3 for cation concentrations of 0.13 and 0.55 Zn/(Zn + Sn), respectively. For the thin film having a smaller concentration gradient (deposited from target III), one half of the 50 × 50 mm2 thin film was investigated electrically. A comparison of the falsecolor resistivity map with the composition map visualizes the increase of the resistivity for increasing zinc content up to Zn/ (Zn + Sn) = 0.46 (Figure 6a,c). A minimum resistivity of 4 × 10-3 Ωm was measured for low zinc contents of approximately 0.35 Zn/(Zn + Sn). For the thin film deposited from target II, the minimum resistivity was 5.9 × 10−5 Ωm at 0.13 Zn/(Zn + Sn). An overview of the lowest achieved resistivities for a-ZTO deposited by different methods is represented in Table 1. Electron densities between 1 × 1019 and 4 × 1016 cm−3 were measured for cation ratios of 0.35 and 0.55 Zn/(Zn + Sn), respectively (Figure 6b). The highest carrier density of 1 × 1019 cm−3 presents an outlier (compare Figure

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CONCLUSIONS Amorphous zinc-tin-oxide thin films with a continuous composition spread were fabricated by PLD at room temperature. By utilizing segmented targets, a wide composition range was covered. The electrical and optical properties of our thin films exhibit a clear dependence on the cation ratio. The highest transparency was achieved for high zinc contents and

Figure 6. False-color representation of the spatial dependence of (a) resistivity, (b) free electron density, and (c) composition and (d) mobility of a thin film deposited from target III. E


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(11) von Wenckstern, H.; Splith, D.; Purfürst, M.; Zhang, Z.; Kranert, C.; Müller, S.; Lorenz, M.; Grundmann, M. Structural and optical properties of (In,Ga)2O3 thin films and characteristics of Schottky contacts thereon. Semicond. Sci. Technol. 2015, 30, 024005. (12) Görrn, P.; Sander, M.; Meyer, J.; Kroger, M.; Becker, E.; Johannes, H. H.; Kowalsky, W.; Riedl, T. Towards see-through displays: Fully transparent thin-film transistors driving transparent organic light-emitting diodes. Adv. Mater. 2006, 18, 738−741. (13) Minami, T. Properties of transparent zinc-stannate conducting films prepared by radio frequency magnetron sputtering. J. Vac. Sci. Technol., A 1995, 13, 1095−1099. (14) Minami, T.; Nanto, H.; Shooji, S.; Takata, S. The stability of zinc oxide transparent electrodes fabricated by R.F. magnetron sputtering. Thin Solid Films 1984, 111, 167−174. (15) Minami, T.; Nanto, H.; Takata, S. Highly conducting and transparent SnO2 thin films prepared by RF magnetron sputtering on low-temperature substrates. Jpn. J. Appl. Phys. 1988, 27, 287−289. (16) Major, S.; Kumar, S.; Bhatnagar, M.; Chopra, K. L. Effect of hydrogen plasma treatment on transparent conducting oxides. Appl. Phys. Lett. 1986, 49, 394−396. (17) Thomas, J. H. Auger electron and x-ray photoelectron spectroscopy analysis of the hydrogenated amorphous silicon-tin oxide interface: Evidence of a plasma-induced reaction. Appl. Phys. Lett. 1983, 43, 101−102. (18) Hosono, H.; Kikuchi, N.; Ueda, N.; Kawazoe, H. Working hypothesis to explore novel wide band gap electrically conducting amorphous oxides and examples. J. Non-Cryst. Solids 1996, 198−200, 165−169. (19) Hosono, H.; Yasukawa, M.; Kawazoe, H. Novel oxide amorphous semiconductors: Transparent conducting amorphous oxides. J. Non-Cryst. Solids 1996, 203, 334−344. (20) Jayaraj, M. K.; Saji, K. J.; Nomura, K.; Kamiya, T.; Hosono, H. Optical and electrical properties of amorphous zinc tin oxide thin films examined for thin film transistor application. J. Vac. Sci. Technol. B 2008, 26, 495−501. (21) Schlupp, P.; von Wenckstern, H.; Grundmann, M. Amorphous zinc-tin oxide thin films fabricated by pulsed laser deposition at room temperature. MRS Online Proc. Libr. 2014, 1633, 101−104. (22) Schlupp, P.; Schein, F.-L.; von Wenckstern, H.; Grundmann, M. All Amorphous Oxide Bipolar Heterojunction Diodes from Abundant Metals. Adv. Electron. Mater. 2015, 1, 1400023. (23) Ko, J.; Kim, I. H.; Kim, D.; Lee, K. S.; Lee, T. S.; Cheong, B.; Kim, W. M. Transparent and conducting Zn-Sn-O thin films prepared by combinatorial approach. Appl. Surf. Sci. 2007, 253, 7398−7403. (24) Kim, Y. J.; Oh, S.; Yang, B. S.; Han, S. J.; Lee, H. W.; Kim, H. J.; Jeong, J. K.; Hwang, C. S.; Kim, H. J. Impact of the Cation Composition on the Electrical Performance of Solution-Processed Zinc Tin Oxide Thin-Film Transistors. ACS Appl. Mater. Interfaces 2014, 6, 14026−14036. (25) Kim, Y. H.; Han, J. I.; Park, S. K. Effect of Zinc/Tin composition ratio on the operational stability of solution-processed Zinc-Tin-Oxide Thin-Film transistors. IEEE Electron Device Lett. 2012, 33, 50−52. (26) Hu, W.; Peterson, R. L. Charge transport in solution-processed zinc tin oxide thin film transistors. J. Mater. Res. 2012, 27, 2286−2292. (27) van der Pauw, L. J. A method of measuring specific resistivity and Hall effect of discs of arbitrary shape. Philips Res. Reports 1958, 13, 1−9. (28) van der Pauw, L. J. A method of measuring the resistivity and Hall coefficient on lamellae of arbitrary shape. Philips Technol. Rev. 1958, 20, 220−224. (29) von Wenckstern, H.; Brandt, M.; Zimmermann, G.; Lenzner, J.; Lorenz, M.; Grundmann, M. Temperature dependent Hall measurements on PLD thin films. MRS Online Proc. Libr. 2007, 957, 1−6. (30) Tauc, J.; Grigorovici, R.; Vancu, A. Optical Properties and Electronic Structure of Amorphous Germanium. Phys. Status Solidi B 1966, 15, 627−637. (31) Riedl, T.; Görrn, P.; Kowalsky, W. Transparent Electronics for See-Through AMOLED Displays. J. Disp. Technol. 2009, 5, 501−508.

high deposition pressures. The highest conductivity was achieved for low zinc contents. The mobility did not exhibit a clear dependence on the zinc content. We conclude that the CCS approach for PLD is a useful method to investigate a wide range of cation ratios in an effective and reproducible manner. Moreover, a comprehensive study of the electrical and optical properties of amorphous, PLD-grown ZTO was presented.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: sofi[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank G. Ramm (Universität Leipzig) for the target fabrication, J. Lenzner (Universität Leipzig) for EDX measurements, and U. Teschner (Universität Leipzig) for transmission measurements. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 644101 and the Deutsche Forschungsgemeinschaft within Schwerpunktprogramm FFlexCom (SPP 1796, Gr 1011/31-1).

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Influence of the Cation Ratio on Optical and Electrical Properties of

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