Physical properties of metal-doped zinc oxide films for surface acoustic wave application
© Nam et al; licensee Springer. 2012
Received: 8 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Metal-doped ZnO [MZO] thin films show changes of the following properties by a dopant. First, group III element (Al, In, Ga)-doped ZnO thin films have a high conductivity having an n-type semiconductor characteristic. Second, group I element (Li, Na, K)-doped ZnO thin films have high resistivity due to a dopant that accepts a carrier. The metal-doped ZnO (M = Li, Ag) films were prepared by radio frequency magnetron sputtering on glass substrates with the MZO targets. We investigated on the optical and electrical properties of the as-sputtered MZO films as dependences on the doping contents in the targets. All the MZO films had shown a preferred orientation in the  direction. As the quantity and the variety of metal dopants were changed, the crystallinity and the transmittance, as well as optical band gap were changed. The electrical resistivity was also changed with changing metal doping amounts and kinds of dopants. An epitaxial Li-doped ZnO film has a high resistivity and very smooth surface; it will have the most optimum conditions which can be used for the piezoelectric devices.
KeywordsMZO surface acoustic waves RF magnetron sputter ZnO piezoelectric devices
ZnO is a group II-VI compound semiconductor that has a hexagonal wurtzite structure. ZnO has typical n-type semiconductor properties because of a nonstoichiometric defected structure and electrical properties according to chemical composition changes. ZnO has piezoelectric properties, wide band gap (3.37 eV), etc. Metal-doped ZnO [MZO] thin films show changes of the following properties by a dopant : First, group III element (Al, In, Ga)-doped ZnO thin films have a high conductivity having an n-type semiconductor characteristic [2, 3]. Second, group I element (Li, Na, and K)-doped ZnO thin films have a high resistivity due to a dopant that accepts a carrier . In the case of the group II-VI oxide semiconductor, oxygen defect, chemical composition, and impurities have significant influences on physical properties such as electrical resistivity, piezoelectricity, and structure. The change of property in ZnO had been reported. It was found that Li atoms in Li-doped ZnO films were involved in the substitution for Zn atoms; they acted as acceptors that compensate the excess Zn atoms . The ZnO film is not only a strong c-axis-oriented crystalline structure, but has also a high resistivity of above 10 Ω cm when used for the piezoelectric device applications. We mainly studied on the effects of deposition conditions on the structural and electrical properties of ZnO films that will be applied as piezoelectric devices.
Two kinds sputtering targets were prepared by mixing with zinc oxide and lithium chloride powders, and zinc oxide and silver nitrate powders where the ratio of dopants was increased from 0 to 10 wt.% by an interval of 2 wt.%. MZO films were deposited on glass substrates at room temperature with a radio frequency [RF] power of 150 W at a target-to-substrate distance of 70 mm. The thickness of the MZO films was up to 500 nm on the glass substrates for optical measurements. The crystal structure, microstructure, and the thickness were observed using X-ray diffraction [XRD] and a scanning electron microscope. Atomic force microscopy was used to examine the surface roughness. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy were also utilized to analyze the chemical ratio of the MZO films. The electrical resistivity was measured by a four-point probe method.
Results and discussion
MZO films are required to have a high electrical resistivity and smooth surface morphology for piezoelectric device application. To find film uniformity, we measured the surface root mean square [RMS] roughness. Figures 3b and 4b show the RMS roughness (open circle) and electrical resistivity (solid circle) of the SZO and LZO films, respectively. As the surface roughness increases, electrical resistivity decreases with the addition of silver in the SZO films as shown in Figure 3b. The reason for this result is to aggregate doped silver with increasing Ag amounts. Low resistive SZO films are not appropriate for piezoelectric devices. Figure 4b shows that the surface RMS roughness decreased from 10.86 nm to 2. 743 nm with increasing Li contents from 0 to 4 wt.%. The surface RMS roughness is also retained regularly (about 6.4 nm) in the Li content range of 4 to 10 wt.%. The smoothest film is obtained from a LZO film with 4 wt.% Li content. In Figure 4b, between 0 and 4 wt.% Li contents, the electrical resistivity of the LZO films is increased up to 40 MΩ cm. Generally, ZnO thin films are n-type semiconductive metal-oxide thin films since the excess Zn or defective O atoms perform the part of a donor. These are induced so that the carriers contribute to the conductivity. However, resistivity only in a small amount of doped LZO thin films increases because Li is operated to reduce the carrier through an acceptor . Therefore, we found that the resistivity increased until the formation of a 4 wt.% Li-doped ZnO thin film (107 Ω cm). As Li doping amounts are increased unduly (above 6 wt.%), resistivity increases because the excess Li atoms act as carriers.
MZO films with various metal contents (Ag and Li of 0 to approximately 10 wt.%) were prepared by RF magnetron sputtering with especially designed ZnO targets. The structural, optical, and electrical properties of MZO films depended on the dopant content ratio in the target. The deposited MZO films have a preferred crystalline orientation in the  direction. However, the crystallinity and electrical resistivity of SZO films are not good for piezoelectric device applications. Highly oriented LZO thin films with 2 to 10 wt.% Li contents can be grown, and among them, the most crystalline LZO film with very smooth surface and high resistivity is observed from the 4 wt.% Li-doped LZO target; it will have the most optimum conditions for deposition which can be used for the piezoelectric devices.
This work was supported by the grants NRF-20110027123 (Basic Science Research Program) and NRF-20110000849 (Priority Research Centers Program).
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