Article pubs.acs.org/Langmuir
High-Performance Transistors Based on Zinc Tin Oxides by Single Spin-Coating Process Yunlong Zhao, Lian Duan,* Guifang Dong, Deqiang Zhang, Juan Qiao, Liduo Wang, and Yong Qiu* Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *
ABSTRACT: Films of zinc tin oxide (ZTO), grown from solutions with zinc acetate dehydrate and tin(II) 2-ethylhexanoate dissolved in 2-methoxyethanol, have been used to fabricate thin-film transistors in combination with solutionprocessed aluminum oxide as the gate insulator. And the nonhomogeneity of the single-layer ZTO films, caused by both ZTO film−substrate interaction and surface crystallization, has been studied, which is essential to achieve high performance transistors. In the bottom-contact thin-film transistor based on a Sn-rich layer of ZTO, a high mobility of 78.9 cm2 V−1 s−1 in the saturation region has been obtained, with an on-to-off current ratio of 105 and a threshold gate voltage of 1.6 V.
1. INTRODUCTION Compared with hydrogenated amorphous silicon and organic semiconductors, metal-oxide semiconductors have favorable field-effect mobility, high optical transparency, high uniformity in large-scale fabrication, and excellent thermal and environmental stability in the field of thin-film transistor (TFT) circuitry.1−4 Metal oxide semiconductors with excellent quality are usually fabricated through vacuum deposition techniques, which lead to high manufacturing costs and poor large-area compatibility. Therefore, more and more efforts have been devoted to solution-processed metal-oxide semiconductors recently,5−10 with carrier mobilities of 5−6 cm2 V−1 s−1 for zinc oxide (ZnO),11 16 cm2 V−1 s−1 for indium zinc oxide (IZO),5 28−33 cm2 V−1 s−1 for zinc tin oxide (ZTO),12,13 and 7.6 cm2 V−1 s−1 for indium gallium zinc oxide (IGZO).14 In general, the mobility of the metal oxides is related to densely packed films with connected particles or lattices to carry charges over longer distances.15 Impurity scattering, lattice vibration, and grain boundary et al., also greatly affect the charge transport in the metal-oxide films.16 Thus, the structural properties of the oxide films could have immense influence on the quality of solution-processed metal-oxide films, hence, the performance of their devices.11,17−19 ZTO is one of the most studied metal-oxide semiconductors, with field-effect mobilities up to 60 cm2 V−1 s−1.12,13 Its stoichiometry can generally be described as (ZnO)x(SnO2)1−x (0 < x 1), while the concentrations of Zn and Sn were 18.5−21.4% and 22.0−23.7%, respectively, in the Sn-rich layer (Zn/Sn < 1). Here, the plane with a Zn/Sn ratio of 1 was defined as an “equal Zn/Sn ratio plane” (ERP), for easy description and discussion. The Zn-rich layer, which was above the ERP, had a Zn/Sn ratio larger than 1; and the Sn-rich layer, which was below the ERP, had a Zn/Sn ratio smaller than 1. 3.3. Crystalline and Morphology. Figure 2 showed the GIXRD patterns of the ZTO thin films with different heating rates. The ZTO films obtained by heating at 5 and 10 °C/min and heating directly at 450 °C were amorphous, with a single and extremely broad peak at 2θ (equals ∼33.5°), the characteristic of amorphous ZTO films previously reported in the literature.17,20,26,28 The ZTO films obtained by heating at 20, 30, 40, and 80 °C/min were multicrystalline, with XRD profiles observed at 2θ near 20.8°, 26.5°, 31.8°, 33.7°, 35.8°, 51.8°, and 56.1°. By analysis of these diffraction peaks, it was found that there were three crystalline phases in the latter ZTO films.17,29−31 The distinct peaks at 2θ near 20.8°, 31.8°, and 51.8° represented (1 0 1), (1 0 4), and (1 1 −6) crystal planes of perovskite, ZnSnO3‑x, respectively;30 the distinct peaks at 2θ near 33.7°, 35.8°, 51.8°, and 56.1° represented by (3 1 1), (2 2
Figure 2. Grazing incident angle X-ray diffraction (GIAXRD) patterns of the single spin-coated ZTO films with heating rates of 5, 10, 20, 30, 40, and 80 °C/min and heating at 450 °C directly. The ZTO films were deposited on a SiO2 (300 nm) substrate.
2), (4 2 2), and (5 1 1) crystal planes of spinel, Zn2SnO4−x, respectively,31 and the distinct peaks at 2θ near 26.5°, 33.7°, and 51.8° represented (1 1 0), (1 0 1), and (2 1 1) crystal planes of SnO2−x, respectively.29 The surface morphology of the ZTO films was also affected greatly by the heating rates (Figure S2 of the Supporting Information). With the heating rates of 5 and 10 °C/min and heating directly at 450 °C, smooth and dense ZTO films with the root mean square (RMS) values from 0.3 to 0.4 nm were observed. However, when the heating rates are kept at 20, 30, 40, and 80 °C/min, rough ZTO films with RMS values from 3.1 to 3.7 nm were obtained. The roughness and the peak−valley (P−V) values of the latter were both almost ten times larger than the former. From the analysis of element distribution, surface morphology, and film crystalline, it was obvious that the ZTO films, obtained by heating at very low (below 10 °C/min) and extremely high rates, were much different from the ZTO films obtained by heating at moderate heating rates. The AES and GIXRD experimental results confirmed that there were Zn-rich and Sn-rich layerer corresponding to different phases and compositions in the ZTO films. The Zn-rich layer, which was on the surface, was confirmed to be multicrystalline, while the Sn-rich layer, which was at the bottom, should be amorphous, according to the stabilization of the amorphous substrate.3 In the study from Ong’s group,11 it was also observed that the heating profile had a decisive impact on the resulting ZnO crystal orientation, thus a profound influence over its device properties. When heating at 500 °C directly, highly crystalline solution-processed ZnO thin films, with preferred orientation along the (0 0 2) plane, could be obtained. However, when keeping the heating rate as 10 °C/min, it would display a rather random crystal orientation along the (1 0 0), (0 0 2), and (1 0 1) planes. It indicated that the formation of the highly crystalline ZnO with preferred orientation along the (0 0 2) plane was kinetically controlled, while the formation of ZnO with the other crystal orientations was thermodynamically controlled. Therefore, in this work, the heating rate could affect the phases and the crystal orientations of the ZTO films. And the film−substrate interactions would always favor the formation of an amorphous film near the amorphous substrate.3,23 The two opposite-acting forces might be the reason for the involvement of evolution distributions in the film. 153
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Figure 3. (a) Depth analysis of the O 1s XPS curves of the single spin-coated ZTO film, with the heating rate of 30 °C/min. The depths to the surface are 7, 14, 21, and 28 nm. The former two belong to the Zn-rich layer, and the latter two belong to the Sn-rich layer. (b) Semiquantitative analysis of the fraction of hydroxide (M−OH) and oxide lattices with oxygen vacancies (Vo) as a function of the depths to the surface of the ZTO films. All fractions are calculated based on the area integration of each subpeak.
Figure 4. (a) The structure for TFT I with bottom-contact/top-gate type and Al2O3 as the insulating layer. (b) The structure for TFT II with topcontact/bottom-gate type and Al2O3 as the insulating layer. (c) The transfer characteristic of TFT I with the Zn-rich layer as the channel layer and TFT II with the Sn-rich layer as the channel layer. (d) The output characteristic of TFT I.
3.4. Depth Analysis of Structural Properties. With different Zn/Sn ratios, the Zn-rich and Sn-rich layers showed different films characteristics, as well as different device performances.32,33 Figure 3a showed the depth analysis of the O 1s X-ray photoelectron spectroscopy (XPS) curves of the single spin-coated ZTO film, with a heating rate of 30 °C/min. The O 1s curves were divided into three obvious peaks
centered at around 531.9, 531.2, and 530.0 eV, to reflect three different oxygen environments.7 The binding energy peak at 530.0 eV was attributed to O2− ions surrounded by Zn and Sn atoms.3,12,14,34 The binding energy peak at 531.2 eV was associated with O2− ions in the vacancy oxygen,3,7 and the binding energy peak at 531.9 eV was associated with the O2− ions in the hydroxide.3,7,14 According to the critical effects of 154
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TFT II based on the Sn-rich layer. Furthermore, Hosono et al. reported that amorphous oxides composed of heavy-metal cations with (n − 1)d10ns0 (n ≥ 4) electronic configurations could have high electron mobility.1,17 This was because the ns orbitals overlap between adjacent orbitals, and the large diameter and spherical symmetry of the ns orbitals would lead to a high degree of overlap and conduction-band dispersion. In the ZTO films, Sn4+ and Zn2+ are such heavy metal cations. The 5s orbital of Sn4+ was larger than the 4s orbital of Zn2+, which indicated a higher degree of overlap, and the conduction band dispersion could be generated in the ZTO film with a higher content of Sn.33,35 Therefore, a higher content of Sn was another reason for the higher mobility shown in TFT II based on the Sn-rich layer of the ZTO. The TFTs based on ZTO films with similar insulating layers, such as Al2O312 or sodium beta-alumina (SBA),13 showed high mobilities of 28−33 cm2 V−1 s−1. In this work, tin(II) 2ethylhexanoate, with flexible ligands, was used as the Sn precursor. With a heating rate of 30 °C/min, the solutionprocessed ZTO films would be easy to crystallize. Due to the film−substrate interaction and surface crystallization, the nonhomogeneity of ZTO occurred. The bottom of the ZTO film, with a high content of Sn (24%) and a high content of Vo (11.9%),14,40,41 showed a higher mobility of 78.9 cm2 V−1 s−1 in the TFTs. The conduction band minimum in metal-oxide semiconductors was primarily composed of dispersed vacant s states with short intercation distances for efficient carrier transport.1 This could be achieved in ionic oxides but not in hydroxides.14,42 Therefore, the presence of M−OH would decrease the mobility of the ZTO film. There was 9.5% of M− OH present in the Sn-rich layer, which could suppress the carrier transport. Therefore, it indicated a possibility for much higher mobility with the fraction of M−OH decreased by further study. As shown in Figure 4d and Figure S3 of the Supporting Information, the output characteristics of the two TFTs indicated that the transistors were well-modulated depending on the gate voltage and exhibited clear saturation behavior with typical device performance.1 The drain current markedly increased as the gate voltage increased at a positive gate bias, which indicated the channel was n type.1 However, the output characteristics also showed that the leakage current was a little large, which was approximately 10−10 to 10−7 A at 2−5 V. The leakage current was about 1% of the drain current, thus it would not affect the mobility significantly. Improving the quality of the solution-processed Al2O3 or increasing its thickness might lower the leakage current.
the oxide lattices with oxygen vacancies (Vo) and the hydroxide (M−OH) in the oxide semiconductor films on their device performance,14 the fractions of M−OH and Vo were calculated based on the area integration of the corresponding subpeaks (shown in Figure 3b). As the depth to the surface of the ZTO film increased, the fractions of M−OH and Vo both increased. In the Zn-rich layer of the ZTO film, the fractions of M−OH and Vo were about 3.5% and 6.1%, respectively. However, in the Sn-rich layer, the fractions of M−OH and Vo increased to 9.5% and 11.9%, respectively. 3.5. High Performance Transistor-Based ZTO. According to the room temperature Hall measurement of the single spin-coated ZTO fabricated with the heating rate of 30 °C/min, the Hall mobility was evaluated to be 92.3 cm2 V−1 s−1, with an effective carrier concentration of 7.4 × 1020 cm−3. The large Hall mobility of ZTO provided a possibility to realize a high field-effect carrier mobility in the corresponding TFTs. In order to investigate the electrical performance of the Znrich and Sn-rich layers of the single spin-coated ZTO fabricated with the heating rate of 30 °C/min, two types of TFT were designed, as shown in Figure 4 (panels a and b). The thickness of the accumulation layer was supposed to be less than 20 nm, which is similar to ZnO,21 and the mobility of these TFTs were mainly dependent on the part of the ZTO film contacting the semiconductor/insulator interface. Thus, the performance of the two types of TFTs could indicate the electrical performance of the Zn-rich and Sn-rich layers. A solution-processed aluminum oxide (Al2O3),27 with a thickness of 80 nm and a dielectric constant (k) of 7.7, was used as the insulating layer for the advantages of high relative permittivity, low interfacial trap density with oxide semiconductors, a high band gap (∼9 eV), a high breakdown field (5−10 MV/cm), high thermal stability, and low cost manufacturing.12,35,36 Also, it was reported that the high k Al2O3 insulator, with a high capacitance and a low gate voltage, could greatly improve the mobility of the TFT.12,37−39 Figure 4c showed the transfer characteristic of the two TFTs. The field-effect carrier mobility (μFE), on-to-off current ratio (Ion/off), threshold gate voltage (Vth), and subthreshold slope (S) of these TFTs could be calculated from the transfer curves according to the reported equations:3,12,13 μFE
⎛ ∂ IDS = ⎜⎜ ⎝ ∂VG
Vth = VG −
⎞2 2L ⎟⎟ ⎠ WCi
(1)
2IDSL WCμFE
(2)
On the basis of a Sn-rich layer of the ZTO, TFT II showed a high mobility of 78.9 cm2 V−1 s−1, with an Ion/off of 105, a Vth of 1.6 V, and an S of 7.27 V/decade. However, TFT I, which was based on a Zn-rich layer of the ZTO, showed a much lower mobility of 12.5 cm2 V−1 s−1, with an Ion/off of 105, a Vth of 3.8 V, and an S of 1.45 V/decade. As shown in Figure 3b, the fractions of M−OH and Vo are 3.5% and 6.1%, respectively, in the Zn-rich layer, and 9.5% and 11.9%, respectively, in the Snrich layer. The mobility of a metal-oxide semiconductor was strongly dependent on the carrier concentration generated by an oxygen vacancy formation,14 which indicated that the ZTO with a higher fraction of Vo could show a higher mobility. The fraction of Vo in the Sn-rich layer was twice of that in the Znrich layer, which was the main reason for the higher mobility of
4. CONCLUSIONS In conclusion, it was confirmed that there was nonhomogeneity in the single spin-coated ZTO films when its surface was not amorphous. On the basis of the Sn-rich layer (near the substrate) of the ZTO film, a high-performance TFT with a carrier mobility of 78.9 cm2 V−1 s−1 was obtained. This mobility was more than six times higher than that of the TFT based on the Zn-rich layer (away from the substrate). The substrateinduced high-mobility solution-processed ZTO film, with a high Sn content, would provide a new opportunity for low-cost fabrication of high-performance electronic devices. 155
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ASSOCIATED CONTENT
S Supporting Information *
TGA curves of the precursor materials, AFM images of the ZTO thin films, and some additional properties of the ZTO films. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Y.Q.: e-mail,
[email protected]. Tel.: +86 10 62779988. Fax: +86 10 62795137. L.D.: e-mail, duanl@mail. tsinghua.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grants 50990060 and 61177023) and the National Key Basic Research and Development Program of China (Grants 2009CB930602 and 2009CB623604). A portion of this work is based on the data obtained at 1W1A, Beijing Synchrotron Radiation Facility (BSRF). The authors gratefully acknowledge the assistance of the scientists of the Diffuse X-ray Scattering Station during the experiments.
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