Applied Science and Convergence Technology 2017; 26(3): 43-46
Published online May 31, 2017
Copyright © The Korean Vacuum Society.
Hyun-Woo Park, and Kwun-Bum Chung*
Division of Physics and Semiconductor Science, Dongguk University, Seoul 04620, Korea
Correspondence to:E-mail: email@example.com
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Aluminum-doped ZnO (AZO) thin films were deposited by atomic layer deposition (ALD) with respect to the Al doping concentrations. In order to explain the chemical stability and electrical properties of the AZO thin films after hydrogen peroxide (H2O2) solution immersion treatment at room temperature, we investigated correlations between the electrical resistivity and the electronic structure, such as chemical bonding state, conduction band, band edge state below conduction band, and band alignment. Al-doped at ~ 10 at % showed not only a dramatic improvement of the electrical resistivity but also excellent chemical stability, both of which are strongly associated with changes of chemical bonding states and band edge states below the conduction band.
Keywords: Transparent conducting oxide, Al doped ZnO, Solution immersion treatment, Chemical bonding state, Electronic structure
Transparent conducting oxide (TCO) materials has attracted a great amount of attention for the technological realization of the next generation of see-through transparent electronics such as organic light emitting diodes (OLEDs), solar cells, smart windows, thin film transistors (TFTs), flat panels displays (FPDs) and touch screen panels (TSPs) [1,2,3]. In most cases, indium tin oxide (ITO) thin film has been extensively employed as the TCO layer because of this material’s excellent electrical and optical properties . However, ITO film has some drawbacks, such as the high cost of indium and its lower wet etch rate compared with those of ZnO or indium gallium zinc oxide (IGZO), which can lead to damage in the active layer . Therefore, alternative TCO films with similar or better properties are needed, and studies have been performed in this area. Recently, aluminum-doped zinc oxide (AZO) film has been intensively studied as a candidate for TCO applications because it has many advantages such as high optical transparency of > 80% in the visible region and low electrical resistivity of ~ 10−3 Ω·cm [3,5]. However, the AZO thin film has a small process window because the electrical properties greatly change depending on the doping concentration of Al. Therefore, in order to widen the process window, it is necessary to develop a process capable of decreasing the resistivity of the AZO thin film via a simple post-treatment method. In addition, since most electronic devices have stacked structures, mechanical/chemical stability must be ensured in the process of depositing and etching adjacent layers (e.g. using photolithography or a lift-off process) .
In our study, we provide a post-treatment method that can ensure facile modulation of the electrical properties at room temperature of an AZO thin film according to the Al doping concentration; this method also provides information on the chemical stability of the AZO thin film with H2O2 solution.
Using an ALD technique at the substrate temperature of 250°C, AZO thin films with various Al doping concentrations and with thicknesses of 100 nm were deposited on thermally grown SiO2 layers on a heavily boron-doped
Figure 1(b) shows that the RBS spectra of the AZO thin films containing 1, 3, 5 and 10 at% of Al deposited on a carbon substrate accurately matched the target Al concentrations, which will hereafter be indexed as AZO 1%, AZO 3%, AZO 5% and AZO 10%, respectively. The electrical properties of the AZO thin films with H2O2 solution immersion treatment were investigated by Hall measurement at room temperature. Figure 2 (a) shows the resistivity of the AZO thin films before and after H2O2 solution immersion treatment with respect to the Al doping concentration, while Table 1 summarizes the detailed electrical resistivity values. First, the AZO thin film has an optimum electrical resistivity at an Al doping concentration of 3%, which is reasonable behavior comparing to previous reports [3,5]. After the H2O2 solution immersion treatment, the electrical resistivity of the AZO thin films decreased regardless of the Al doping concentration. In particular, when the Al doping concentration was 10%, it is obviously seen that the resistivity most strongly decreased compared with the other Al doping concentrations. Figure 2(b) shows the electrical properties of the AZO 10% film before and after H2O2 solution treatment. As can be seen in Figure 2 (b), the decrease in the resistivity of AZO 10% is more closely related to the increase in the carrier concentration rather than it is to the electron mobility. Therefore, the remarkable changes in resistivity of AZO thin film can be associated with the carrier generation by defect states such as oxygen vacancies, which are affected by the H2O2 solution treatment. More discussion is provided below to consider the chemical bonding state and the electronic structure. The chemical bonding states were observed by XPS; Figure 2(c) and (d) shows the changes in the O 1
In order to understand the electronic structure of the AZO 10% thin film before and after H2O2 solution treatment, in such aspects as band gap and band edge states, the extinction coefficient spectra were measured by SE, as shown in Figure 3(a). Except for the slight change of the area of the conduction band above the photon energy of 4.5 eV, no significant change of the electronic structure of the AZO 10% thin film was observed to result due to H2O2 solution treatment. However, the band edge state below the conduction band dramatically increased after H2O2 solution treatment, as shown in Figure 3(b). These changes coincided with the increase of the oxygen deficient state (O2) and the Zn(OH)2 state, as determined by XPS analysis; the increase of these two states is related to the increase of the unoccupied states within the band gap and can generate the charge carrier concentration. Figure 3(c) and (d) shows the valence band spectra and the schematic energy-level diagram for the AZO 10% film before and after H2O2 solution treatment. Noticeable changes are the relatively narrow conduction band offset (ΔECB) between the conduction band minimum and the Fermi level, which could be related to an increase in the carrier density according to the following equation:
In summary, we evaluated the electrical properties and chemical stability of AZO thin films before and after H2O2 solution treatment with respect to the Al doping concentration. The resistivity of all AZO thin films was found to decrease after H2O2 solution treatment. Especially, the resistivity of the AZO 10% thin film was found to decrease 10 times from 3.64 × 10−1Ω·cm to 3.39 × 10−2 Ω·cm. Based on chemical bonding state and electronic structure analysis, it was found that with increasing oxygen deficiency state and band edge state the decrease of the resistivity of the AZO 10% thin film is strongly correlated with the carrier concentration after H2O2 treatment. In addition, the higher the Al doping concentration, the more the chemical stability of the AZO thin film increases due to the decrease in the etching effect of ZnO, which has a relatively low chemical stability, and also due to the very high chemical stability of Al2O3.
This research was supported by the Basic Science Research Program and the framework of international cooperation program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03012877 and 2015K2A2A 7056357).
Electrical resistivity of AZO thin films before and after H2O2 immersion treatment with respect to the Al doping concentration
|Al doping concentration (%)||H2O2 immersion treatment (Ω·cm)|
|1||1.79 × 10−3||1.82 × 10−3|
|3||6.10 × 10−4||5.16 × 10−4|
|5||8.91 × 10−4||7.67 × 10−4|
|10||3.64 × 10−1||3.39 × 10−2|