Elsevier

Thin Solid Films

Volume 638, 30 September 2017, Pages 89-95
Thin Solid Films

Investigation of atomic-layer-deposited Al-doped ZnO film for AZO/ZnO double-stacked active layer thin-film transistor application

https://doi.org/10.1016/j.tsf.2017.07.034Get rights and content

Highlights

  • AZO thin films with different Al doping concentration were analyzed.

  • Through physical analysis, 2% AZO showed the best characteristics.

  • The use of AZO showed improved stability through gate bias stress measurement.

  • The main transport mechanism was analyzed through temperature dependence of mobility.

  • The main transport mechanism is thermionic and thermal field emission.

Abstract

In this study, Al-doped zinc oxide (AZO) thin films with different Al concentrations fabricated by atomic layer deposition are investigated to determine the Al doping effect for AZO/ZnO double-stacked active layer thin-film transistor (TFT) applications. The AZO films are analyzed by X-ray diffraction, photoluminescence, and X-ray photoelectron spectroscopy, which show that the Al dopants affect the crystallinity, including the crystal direction and grain size, and reduce the deep trap sites such as oxygen vacancies (VO). The optimized Al doping concentration is about 2%. TFTs with an AZO (2%)/ZnO double-stacked active layer are fabricated and shown to exhibit a lower threshold voltage (Vth), subthreshold slope, and Vth shift under a positive gate-bias stress compared to ZnO single-layer devices. In the case of the on-current, however, the AZO stacked devices exhibit a smaller value. These electrical characteristics can be explained by VO suppression and altered crystal properties due to Al doping. For the field-effect mobility, the temperature dependence also reveals that the main transport mechanisms are thermionic and thermal field emission over the grain boundary in the AZO stacked devices. These results indicate that the AZO film properties depend strongly on the Al concentration and hence the ZnO-based devices can be optimized for specific application by Al doping.

Introduction

Zinc oxide (ZnO), which has a wide band gap of 3.37 eV and high field-effect mobility, has been widely used as the transparent active material of thin-film transistors (TFTs), which can be used as the switching device for active matrix liquid crystal displays and driving device for active matrix organic light-emitting diodes [1]. The main advantage of using ZnO is that it is possible to grow high-quality polycrystalline film near room temperature, which is suitable for flexible display devices [2]. For successful applications, however, it is important to further improve the electrical performance and stability of ZnO TFTs. For non-doped ZnO, the mobility reported in the literature is between 0.01 and 7 cm2/V·s with an on/off current ratio between 105 and 107 and threshold voltage (Vth) between − 1 and 15 V [3], [4]. It is well known that the electrical properties of ZnO are related to native point defects such as oxygen vacancies (VO) and zinc interstitials (Zni) [5]. VO and Zni can affect the conductance and act as defect sites that affect the device stability [6], [7]. As an attempt to enhance the electrical properties, the doping of ZnO with other atoms has been widely studied [6], [7], [8]. Typically, In- and Ga-doped ZnO (IGZO) shows improved mobility with uniform amorphous structures and has been used in large displays with high resolutions. However, the use of rare metal indium is expensive and contributes to toxicity and chemical instability problems [9]. Therefore, significant research is underway to find a replacement for IGZO. Among them, Al-doped zinc oxide (AZO) has been studied because of its large carrier concentration, high light transmittance at visible wavelengths, and low-cost fabrication. Because Al has higher valences and smaller ionic sizes than Zn, Al can get substituted into the Zn sites, providing extra electrons and thereby improving the electrical conductivity of ZnO films. Therefore, the main application of AZO has been focused on transparent conducting oxide (TCO) [8], [9], [10]. Recently, AZO was also used as the active layer of TFTs. Jang et al. reported an AZO TFT, which has an on/off current ratio of 104 and field-effect mobility of 0.17 cm2/V·s by lowering the Al concentration to 2 wt% [11]. Several recent studies have demonstrated that the device stability and Vth controllability can be improved in AZO (or AZO/ZnO bilayer) TFTs [12], [13]. Therefore, it is important to determine the role of Al dopants in ZnO films to optimize the device characteristics for successful application of AZO as the active layer of TFTs as well as of TCO. AZO can be deposited by direct-current (DC) sputtering, radio-frequency (RF) sputtering [14], pulsed laser deposition [15], chemical vapor deposition [16], molecular beam epitaxy [17], and sol-gel deposition [18]. However, as the improvement of the device performance often requires the reduction of film thickness, it is more important to attain precise control over the film thickness, conformity, and morphology. A lower deposition temperature is also required for flexible electronics. Considering these requirements, atomic layer deposition (ALD) has been considered as a promising method. ALD is a gas-phase thin-film deposition method and known to be unique because the film growth proceeds through self-limiting surface reactions [19]. Therefore, this method has been widely used for the accurate control of film thickness and composition at low temperatures.

In this study, to investigate the effect of Al doping in ZnO, AZO thin films with different Al doping concentrations are deposited by ALD and analyzed by photoluminescence (PL), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Inspired by the fact that Al doping is effective at reducing the deep-level trap sites by controlling the Al concentration, AZO/ZnO double-stacked TFTs are also fabricated to improve the device stability (without severely degrading the on-current). By comparing the electrical DC characteristics with those of ZnO TFTs, the stability and temperature dependence of the carrier mobility have been analyzed.

Section snippets

Experimental details

In this study, 100-nm-thick AZO film is formed by ALD where trimethylaluminum (TMA, Al(CH3)3) and diethylzinc (DEZ, Zn(C2H5)2) are alternately injected at 100 °C. Deionized H2O and nitrogen gas are used as the purging gas. The pulse time and purging time are 0.5 and 10 s, respectively. The cycle ratio of DEZ and TMA is regulated to control the amount of Al in AZO from 4:1 to 49:1. Fig. 1 shows the ALD pulse configuration for AZO deposition. AZO thin films with different Al doping concentrations

Doping concentration (XPS)

To verify whether the cycle ratio control in the ALD process is suitable to modulate the Al concentration, XPS spectra of the Zn 3p, Al 2p, and O 1s orbitals of the AZO are compared according to the cycle ratio of DEZ and TMA, as shown in Fig. 3 (a). The intensity of Zn 2p decreases, while that of Al 2p increases as the ratio of DEZ cycles to TMA cycles increases, which implies that the larger the cycle ratio, the more Al is doped to ZnO and can lead to either the substitution of Al3 + ions for

Conclusions

In this study, AZO thin films with different Al doping concentrations fabricated by ALD are analyzed to investigate the effect of Al doping in ZnO for AZO/ZnO double-stacked TFT applications by XRD, PL, and XPS. These physical analyses show that the Al concentration can be optimized at 2% with respect to increased grain size and reduced concentration of deep trap sites such as VO. The AZO/ZnO device is also fabricated based on the optimized Al concentration. The AZO (2%)/ZnO device shows a

Acknowledgments

This research was financially supported by the Ministry of Trade, Industry and Energy (MOTIE) and the Korea Evaluation Institute of Industrial Technology (KEIT) through the IT R&D program (10049270, SoC-SW platform for computer-vision based UI/UX on wearable smart devices), and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2014R1A1A3052808).

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