Atomic-Layer-Deposition of Indium Oxide Nano-films for Thin-Film Transistors
© The Author(s). 2018
Received: 21 September 2017
Accepted: 17 December 2017
Published: 9 January 2018
Atomic-layer-deposition (ALD) of In2O3 nano-films has been investigated using cyclopentadienyl indium (InCp) and hydrogen peroxide (H2O2) as precursors. The In2O3 films can be deposited preferentially at relatively low temperatures of 160–200 °C, exhibiting a stable growth rate of 1.4–1.5 Å/cycle. The surface roughness of the deposited film increases gradually with deposition temperature, which is attributed to the enhanced crystallization of the film at a higher deposition temperature. As the deposition temperature increases from 150 to 200 °C, the optical band gap (Eg) of the deposited film rises from 3.42 to 3.75 eV. In addition, with the increase of deposition temperature, the atomic ratio of In to O in the as-deposited film gradually shifts towards that in the stoichiometric In2O3, and the carbon content also reduces by degrees. For 200 °C deposition temperature, the deposited film exhibits an In:O ratio of 1:1.36 and no carbon incorporation. Further, high-performance In2O3 thin-film transistors with an Al2O3 gate dielectric were achieved by post-annealing in air at 300 °C for appropriate time, demonstrating a field-effect mobility of 7.8 cm2/V⋅s, a subthreshold swing of 0.32 V/dec, and an on/off current ratio of 107. This was ascribed to passivation of oxygen vacancies in the device channel.
Indium oxide (In2O3) is a transparent metal oxide semiconductor, which exhibits a wide band gap of ~3.7 eV at room temperature, a high transparency for visible light, and excellent chemical stability [1, 2]. Therefore, In2O3 has been investigated for various applications such as photovoltaic devices, electrochemical sensors, and flat panel displays [3–5]. So far, several deposition techniques have been developed to prepare In2O3 thin-films, including sputtering [6, 7], sol-gel [8, 9], and chemical vapor deposition (CVD) [10, 11]. However, the techniques of sputtering and sol-gel usually suffer from a poor uniformity across a large area as well as inexact elemental composition; the CVD technique generally requires relatively high deposition temperatures of > 300 °C. These drawbacks make it challenging to achieve a uniform In2O3 film with precise thickness and composition control at a low deposition temperature.
In recent years, atomic-layer-deposition (ALD) has emerged as a promising approach that can yield excellent step coverage, atomic scale thickness controllability, good uniformity, and a relatively low deposition temperature. Accordingly, the growth of In2O3 thin-films has been explored by means of ALD with different precursors, including InCl3-H2O , InCl3-H2O2 , InCp-O3 , InCp-O2-H2O , and In (CH3)3-H2O . In terms of the precursors of InCl3-H2O and InCl3-H2O2, the deposition temperatures for In2O3 films must be increased to ~ 300–500 °C ; meanwhile, the InCl3 container should be heated to 285 °C in order to obtain ample InCl3 vapor . Furthermore, the byproduct of corrosive HCl can damage the ALD equipment and etch the deposited In2O3 film , and the growth rate of In2O3 is as low as 0.25–0.40 Å/cycle. Although other precursors such as TMIn-H2O and TMIn-H2O2 have been adopted for ALD In2O3 films, the deposition temperatures are still high (i.e., 200–450 °C) in spite of relatively large growth rates (1.3–2 Å/cycle) .
In this work, InCp and H2O2 were proposed as the precursors of ALD In2O3 thin-films, thus the In2O3 thin-films were deposited successfully at lower temperatures, exhibiting a satisfactory growth rate. Additionally, the physical and chemical properties of the deposited films were characterized. Further, the In2O3 thin-film transistors (TFTs) with ALD Al2O3 gate dielectrics have been fabricated, demonstrating good electrical performance, such as a field effect mobility of 7.8 cm2 V−1 s−1, and an on/off current ratio of 107 etc.
Si (100) wafers were cleaned using the standard Radio Corporation of America process, serving as the initial substrates. In2O3 thin-films were deposited onto the pre-cleaned Si (100) substrates using the ALD equipment (Wuxi MNT Micro Nanotech Co., LTD, China) at relatively low temperatures of 150–210 °C, where the temperatures of InCp (Fornano Electronic Technology Co., LTD, China, impurity: 99.999%) and H2O2 (30% aqueous solution) precursors were maintained at 130 and 50 °C, respectively. Nitrogen gas was used as a purging gas. To demonstrate the function of the ALD In2O3 thin-film, the In2O3-based channel TFTs were fabricated as the following processes. Firstly, a 38-nm Al2O3 gate dielectric film was grown on a pre-cleaned p-type Si (100) substrate (< 0.0015 Ω·cm) at 200 °C by ALD using trimethylaluminium and H2O, and such low resistivity silicon substrate served as the back gate. Then, a 20-nm In2O3 channel layer was grown on the Al2O3 film at 160 °C. Source/drain contacts of Ti/Au (30 nm/70 nm) stacks were formed in turn by optical lithography, electron beam evaporation and a lift-off process. Finally, the fabricated devices were annealed at 300 °C in air for different times.
The crystallinity, surface morphology, elemental composition, absorption coefficient, and thickness of the In2O3 films were characterized by X-ray diffraction (XRD) (Bruker D8 Discover), atomic force microscopy (AFM) (Bruker Icon), X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra DLD), ultraviolet-visible spectroscopy (UV-VIS), and ellipsometer (Sopra GES-SE, France), respectively. The electrical measurements of the devices were performed using a semiconductor parameter analyzer (B1500A, Agilent Technologies, Japan) with Cascade probe station in ambient air at room temperature.
Results and Discussion
The Elemental Percentages of In2O3 Films Deposited at Different Temperatures
Atomic Ratio of In:O
Characteristics of the ALD In2O3 Films and In2O3 TFTs From Different Groups
Deposition Temperature (°C)
Growth rate (Å/cycle)
Channel Thickness (nm)
The Elemental Percentages of In2O3 Films Annealed at 300 °C in Air for Different Time
The fast ALD growth of the In2O3 films has been achieved at relatively low temperatures (160–200 °C) with the InCp and H2O2 precursors, exhibiting a uniform growth rate of 1.46 Å/cycle. As the deposition temperature increased, the crystallization of the deposited film was enhanced gradually. Meanwhile, both oxygen vacancies and carbon impurities in the deposited films were also reduced significantly. This thus led to an increase in the Eg of In2O3. Further, with the ALD In2O3 channel layer, the TFTs with an ALD Al2O3 dielectric were fabricated. As the post-annealing time in air was lengthened, the electrical performance of the In2O3 TFT was improved distinctly till 10 h annealing. This is mainly due to the passivation of oxygen vacancies located in the bulk channel and/or the interface between the channel and the dielectric after annealing in air. In terms of 10 h annealing, the device exhibited good performance such as a field-effect mobility of 7.8 cm2/V⋅s, a subthreshold swing of 0.32 V/dec, and an on/off current ratio of 107. In terms of 200 °C deposition temperature, the deposited film exhibits an In:O ratio of 1:1.36 without detectable carbon, thus revealing the existence of oxygen vacancies in the as-deposited film.
This work was supported by the National Key Technologies R&D Program of China (2015ZX02102-003) and the National Natural Science Foundation of China (61274088, 61774041). This work was also sponsored by Shanghai Pujiang Program (16PJ1400800), China.
QM carried out the ALD growth of In2O3 nano-films, and the characterization of films and TFTs. SJD and WJL supervised the work and drafted the manuscript. HMZ, YS, BZ, and DWZ helped to analyze the experimental results. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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