Optical and structural parameters of the ZnO thin film grown by pulsed filtered cathodic vacuum arc deposition
Introduction
Transparent conducting oxide (TCO) films have found extensive applications in optoelectronic devices [1] (for example, solar cells [2], liquid crystal displays, heat mirrors and multiplayer photothermal conversion systems [3]). Zinc oxide has attracted attention as a transparent conducting oxide because of its (i) large band gap (3.3 eV) [4], (ii) high conductivity, (iii) ease in doping, (iv) chemical stability in hydrogen plasma [5], (v) thermal stability when doped with III group elements [6], and (vi) abundance in nature and nontoxicity. In addition to potential use as transparent conducting oxide in optoelectric devices, ZnO thin films also find application as gas sensors [6], because of their high electrical resistivity.
Several deposition techniques are used to grow zinc oxide (ZnO) thin films. These include chemical vapor deposition (CVD) [7], [8], magnetron sputtering [9], [10], [11], [12], cathodic vacuum arc deposition (CVAD), spray pyrolysis [13], [14], and pulsed laser deposition (PLD) [15], [16]. The filtered cathodic vacuum arc, another energetic deposition technique, has recently received little attention for the preparation of ZnO films. Naoe and Nakagawa prepared ZnO films at temperature 600 ∘C by a vacuum arc without macroparticle filtering [17]. Takikawa et al. deposited -axis oriented ZnO films by a simple shielded vacuum arc without external heating in the pressure range of 1×10−3–4×10−2 Torr, where the distance between the cathode and substrate was 20 cm [18].
In comparison with other techniques, pulsed filtered cathodic vacuum arc deposition (PFCVAD) provide high density, high adhesion, excellent coating uniformity of thin films at low deposition temperatures [19]. Besides these advantages nearly 100% ionization of the cathode materials in the plasma means that the impact energy of the depositing ions at the growth surface can be readily controlled using electric fields. Their high-energy plasma plume will readily ionize most background gases. These features make the pulsed cathodic vacuum arc an ideal source for the production of metal oxides and nitrides [19]. And also the PFCVAD technique provides thickness control at the atomic scale and there is no need to cool the system.
The envelope method has been developed for transmittance measurements to evaluate the refractive index, extinction coefficient and absorption coefficient [15]. In the optical transmission spectrum multiple coherent reflections are present due to interference effect and the above parameters can be determined from the envelopes, and along the interference maxima and minima.
The Kramers–Kronig [K–K] relations have been widely used in the analysis of reflection spectra, and much of the discussion in the literature about this method has been concerned with the adequacy of various methods proposed for the evaluation of the integrals which in principle run from .
This paper reports the growth of ZnO film on glass substrate by the pulsed filtered cathodic vacuum arc deposition method. Structural and optical properties of PFCVAD ZnO thin film such as x-ray diffraction, transmittance, refractive index and energy band gap are studied. The envelope method was employed to determine refractive index and extinction coefficient as a function of wavelength. Kramers–Kronig and dispersion relations were also used to evaluate refractive index, complex dielectric constant and dispersion energy using reflection spectra.
Section snippets
Sample preparation and characterization
The details of the PFCVAD system have been described elsewhere. The cylindrical vacuum chamber was made of stainless steel (486 mm diameter and 385 mm in length) and evacuated using a primary and a turbomolecular pump (500 l/s) to a base pressure below 1.3×10−8 Torr [20].
In this system, metallic zinc (1 mm in diameter and purity 99.99%) which was held in an alumina ceramic tube was employed as an cathode target, and oxygen (purity 99.9999%) was employed as the reactive gas. Film was deposited
Structural properties of PFCVAD ZnO film
The crystalline quality and orientation of the as-deposited ZnO thin film has been investigated by x-ray diffraction (XRD). A typical XRD spectrum of a sample deposited with oxygen pressure of 5×10−4 Torr is presented in Fig. 1. This film has polycrystalline hexagonal wurtzite structure with dominant -axis orientation.
The peak position and intensity of ZnO thin film with (002) orientation is located at , 2443.33 a.u., respectively. The value of FWHM for the (002) diffraction peak of
Conclusion
In conclusion, the optical and structural properties of the PFCVAD ZnO thin film on glass substrate have been investigated by transmittance, absorption and reflection measurements in the wavelength range of 190–1100 nm. The refractive index , the absorption coefficient and the film thickness were determined from transmission and reflection data. Kramers–Kronig analysis of reflectance spectra has successfully been used to determine the refractive index and extinction coefficient of the ZnO
Acknowledgment
This work was supported by the Research Fund of the University of Cukurova, Adana, Turkey (Project No: 2004K120360-7).
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