Effect of Ru addition on the properties of Al-doped ZnO thin ﬁlms prepared by radio frequency magnetron sputtering on polyethylene terephthalate substrate
Chi Shiung Hsi a , Boen Houng b,∗ , Bing Yi Hou b , Guo Ju Chen b , Shen Li Fu c
Department of Materials Science and Engineering, National United University, Miao Li City 360, Taiwan bDepartment of Materials Science and Engineering, I-Shou University, Kaoshiung County 840, Taiwan c Department of Electronic Engineering, I-Shou University, Kaoshiung County 840, Taiwan Received 1 September 2007; received in revised form 8 October 2007; accepted 8 October 2007 Available online 16 October 2007
Abstract The addition of ruthenium in aluminum-doped zinc oxide transparent conductingthin ﬁlms was deposited on polyethylene terephthalate at 20 ◦ C by radio frequency magnetron sputtering technique. The structure and electrical properties of the ﬁlms were investigated with respect to variation of Ru concentration. The XRD and FESEM results show that the ﬁlm with 0.5 wt% Ru doping has the best crystallinity and larger pyramid-like grains, therefore the resistivity reached to alowest value of 9.1 × 10−4 cm. The low carrier mobilities of the ﬁlms (3–7.2 cm2 V−1 s−1 ), however, were limited by ionized impurity scattering and grain boundary scattering mechanisms since the carrier concentrations were ranged from 2.2 × 1020 to 9.5 × 1020 cm−3 . The transmittance in the visible is greater than 80% with the optical band gap in the order of 3.352–3.391 eV. © 2007 Elsevier B.V. Allrights reserved.
PACS: 73.61.Le Keywords: ZnO thin ﬁlms; Magnetron sputtering; Carrier mobility; Transmittance
1. Introduction Zinc oxide has attracted extensive interest because of its important role in various applications, for example, gas sensor , varistors , surface acoustic wave devices , optical waveguides  as well as blue/UV light emitting devices . In addition, ZnO hasbeen considered as an excellent candidate to replace indium tin oxide (ITO) and tin oxide (SnO2 ) as transparent conductive electrodes in ﬂat panel display and solar cell devices [6,7]. The advantages of zinc oxide include inexpensiveness and relative ease of lithography. However, the electrical conductivity of un-doped zinc oxide is not high enough for practical application. Further reduction ofresistivity of zinc oxide can be achieved either by doping group III elements such as B, Al, In and Ga to replace zinc atoms  or group IV elements, F, to substitute oxygen atoms .
Electrical conductivity, σ, depends on the carrier concentration (n) and mobility (μ) according to the relationship σ = neμ (1)
Corresponding author. Tel.: +86 76579708; fax: +86 76578444. E-mailaddress: firstname.lastname@example.org (B. Houng).
where, e is the electron charge. Obtaining high conductivity of the ﬁlms is a traded-off between the carrier concentration and mobility because the relationship between the carrier concentration and mobility is governed by a rule of μ ∝ n−2/3 [10,11]. The highest reported room temperature Hall mobilities of heavily doped ZnO ﬁlms are around 39 cm2 V−1 s−1 ,limited by ionized impurity scattering . Further increase of the mobility to 200 cm2 V−1 s−1 can be achieved through improving crystallinity by preparing single crystal or hetero-epitaxial ZnO ﬁlms . Another way to improve the mobility could be modulation doping through multilayers of high- and low-doped semiconducting ﬁlms [14,15]. However, the data on heavily doped single crystal orepitaxial layers are rarely reported and it relies on the advanced deposition techniques and facilities. Therefore most of the efforts have been focused on increasing the effective number of free carriers through impurity doping although there is a
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.034
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