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Ao Liu, Guoxia Liu,* Chundan Zhu, Huihui Zhu, Elvira Fortunato, Rodrigo Martins, and Fukai Shan* transistors (TFTs) because of their excellent charge transport characteristics, high transparency, good uniformity, and solution processability.[1–3] In particular, high-performance oxide TFTs are of great interest due to their broad potential applications in wearable, rollable devices, and portable electronic paper.[4] However, most of these TFTs generally operate at high operating voltages (≥30 V) because of the use of conventional SiO2 as dielectric, which limit their applications in low power electronics. To overcome this bottleneck, various approaches have been explored to achieve large arealcapacitance gate insulators, including ultrathin self-assembled-monolayer dielectrics,[5] electrolyte dielectrics,[6] and inorganic high-k dielectrics.[7] Among these, inorganic high-k oxide dielectrics are regarded as a key element of TFTs because of their large permittivity and the formation of excellent heterogeneous interface with oxide semiconductors.[8,9] To date, the research works based on high-k Al2O3,[10,11] Sc2O3,[12] Y2O3,[13,14] ZrO2,[15,16] HfO2,[17,18] their mixtures,[4,19] and phosphates[20] have been demonstrated to be successful and show the huge advantage compared to conventional dielectrics (i.e., SiO2 and SiNx). However, to our best knowledge, the employment of alkali metal oxide as the gate dielectric in transistor has never been explored before. The alkaline-metal lithium (Li) is well-known as the component of light alloy and battery.[21] Meanwhile, Li is widely used as the dopant in inorganic metal oxide materials. The transparency and p-type conductivity of NiO was found to be influenced by Li+ doping concentration.[22] When Li ions are introduced into NiO film as dopant, Li+ goes into the substitutional sites of Ni2+ and generates excess of uncompensated holes. The Li+ doped NiO also finds potential applications in thermoelectric devices, cathodes of molten carbon fuel cells, and sensors for various gases.[23,24] In addition, Li+ doped ZnO has been demonstrated to be an ideal n-type cationic doping source as electron donor with excellent electron mobility up to 50 cm2 V−1 s−1.[25,26] Most recently, Yim et al.[27] proposed a high-throughput ab initio calculation to explore the new high-k materials. Based on the Inorganic Crystal Structure Database, more than 1800 structures of binary and ternary oxides were calculated and a detailed property map was obtained. Among these, lithium oxide (Li2O) is regarded as a candidate for the high-k materials. In this aspect, the achievement of Li2O dielectric films will be
High-k alkaline lithium oxide (LiOx) thin films are fabricated by spin-coating method. The LiOx thin films are annealed at different temperatures and characterized by various techniques. An optimized LiOx dielectric is achieved at an annealing temperature of 300 °C and exhibits wide bandgap of ≈5.5 eV, smooth surface, relatively permittivity of ≈6.7, and low leakage current density. The as-fabricated LiOx thin films are integrated, as gate dielectrics, in both n-channel indium oxide (In2O3) and p-channel cupric oxide (CuO) transistors. The optimized In2O3/LiOx thin-film transistor (TFT) exhibits high performance and high stability, such as Ion/Ioff of 107, electron mobility of 5.69 cm2 V−1 s−1, subthreshold swing of 70 mV dec−1, negligible hysteresis, and threshold voltage shift of 0.1 V under bias stress for 1.5 h. Meanwhile, the p-channel CuO TFT based on LiOx dielectric shows high Ion/Ioff of 105 and hole mobility of 1.72 cm2 V−1 s−1. All the electrical performances are achieved at an ultra-low operating voltage of 2 V. Considering the simple procedure, the moderate annealing temperature, and the low power consumption merits, these outstanding characteristics represent a significant advance toward the development of battery compatible and portable electronics.
1. Introduction Over the past decade, metal–oxide semiconductors have been intensively studied as channel components for thin-film A. Liu, Dr. G. Liu, C. Zhu, H. Zhu, Prof. F. Shan College of Physics Qingdao University Qingdao 266071, China E-mail:
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[email protected] A. Liu, Dr. G. Liu, C. Zhu, H. Zhu, Prof. F. Shan College of Electronic and Information Engineering Qingdao University Qingdao 266071, China A. Liu, Dr. G. Liu, C. Zhu, H. Zhu, Prof. F. Shan Lab of New Fiber Materials and Modern Textile Growing Base for State Key Laboratory Qingdao University Qingdao 266071, China Prof. E. Fortunato, Prof. R. Martins Department of Materials Science/CENIMAT-I3N Faculty of Sciences and Technology New University of Lisbon and CEMOP-UNINOVA Campus de Caparica 2829-516 Caparica, Portugal
DOI: 10.1002/aelm.201600140
Adv. Electron. Mater. 2016, 2, 1600140
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPER
Solution-Processed Alkaline Lithium Oxide Dielectrics for Applications in n- and p-Type Thin-Film Transistors
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extremely interesting for the fabrication of low-voltage and high-performance transistors. To date, inorganic oxide dielectric materials have been processed by various vapor phase techniques such as sputtering,[28] chemical vapor deposition,[29] and pulsed laser deposition.[30] Low-cost and high-throughput chemical-solution-based methods (i.e., spin-coating,[7] inkjet printing,[31] and spray pyrolysis[18]) are also demonstrated with some success. Among these, high-speed coating method is regarded as an attractive route for the fabrication of large-area thin films due to its simplicity and low-energy consumption. Such coating route also allows the smooth and efficient transformation of high-quality thin films from precursor solutions. However, to achieve the ideal dielectric properties, a relatively high temperature above 450 °C is usually required to convert the precursor into oxide thin film, which restricts the substrate selectivity.[19,32,33] In this aspect, the fabrication of high-quality oxide dielectrics by solution process at low temperatures is critical in this emerging field. Particularly, the processing temperature lower than 350 °C is necessary to meet the requirement of conventional manufacturing processes for flat panel displays.[34] Besides, it is noted that the solution-processed channel materials in previous reports are usually composed of n-type metal oxides (e.g., In2O3, InZnO, ZnSnO, InGaZnO, etc.).[35] Despite the tremendous developments, there are only few reports discussing p-type oxide TFTs. Their properties and fabrication techniques are not yet good enough for practical applications, which need further improvements.[36] High-performance p-type oxide TFTs are highly required to build p–n junction devices, low-power complementary circuits, and light-emitting diodes. Herein, we, for the first time, present a simple spin-coating process for the fabrication of high-k LiOx thin films and their application as dielectrics in TFTs was also explored. The effects of annealing temperature (Ta) on the formation and properties of LiOx thin films were intensively studied. To clarify their applications as gate dielectric in complementary metal oxide semiconductor (CMOS) electronics, the electrical properties of both n-type In2O3 and p-type CuO TFTs were measured.
2. Results and Discussion 2.1. Optical Properties of LiOx Thin Films The transmittances of the LiOx thin films on sapphire annealed at various temperatures were investigated using a spectrophotometer. As shown in Figure 1, all the thin films are highly transparent in the visible region, with the transmittances larger than 90%. It is also observed that the transmittance of LiOx thin film was decreased at higher Ta. One possible reason is due to the increased roughness of surface morphology, which can result in increased light scattering and hence to decrease the optical transmittance. The other reason is attributed to the annihilation of oxygen defects (i.e., vacancies and/or hydroxides) at high temperatures.[37] For the LiOx thin film annealed at low temperature, the existence of large amounts of oxygen defects within the bandgap would lead to the light absorption and generate localized states in the forbidden gap, which would result in the decrease of bandgap energy (Eg).[12] As a result, the Eg 1600140 (2 of 8)
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Figure 1. Optical transmittances of LiOx thin films annealed at various temperatures. The inset shows the Eg values of these LiOx films.
of LiOx thin films, calculated using the Tauc plot,[38] increased from 4.8 to 5.7 eV as the Ta increased from 200 to 400 °C.
2.2. Structural Properties and Surface Morphologies of LiOx Thin Films To clarify the microstructure of LiOx thin films, grazing incidence X-ray diffraction (GIXRD) measurements were carried out and the results are shown in Figure S1 (Supporting Information). No crystalline peak is observed in GIXRD spectra, suggesting that LiOx thin films are amorphous over the entire Ta range. In terms of the bottom-gate TFT structure, amorphous dielectric layers are preferred because grain boundaries in polycrystalline materials act as the conduction paths for impurity diffusion and leakage current. In addition, amorphous dielectric films exhibit smoother surface morphologies than polycrystalline ones, which is beneficial to achieve high-quality dielectric/channel interfaces.[39] For high-performance transistors, the smooth interface between channel and dielectric layers is highly desired because carrier transport is generally limited in a narrow region (1–5 nm) of the interface. Figure 2 shows the atomic force microscope (AFM) images of LiOx thin films as a function of Ta. The corresponding root mean square (RMS) values were 0.35, 0.48, 0.56, and 2.12 nm for LiOx-200, LiOx-250, LiOx-300, and LiOx-400, respectively. For the LiOx thin films annealed at low temperatures (≤300 °C), the surface morphologies were found to be quite smooth with small RMS values (
