Thickness dependence of the dielectric properties of thermally evaporated Sb2Te3 thin films

Sb2Te3 thin films of different thickness (23 350 nm) were prepared by thermal evaporation technique. The thickness dependence of the ac conductivity and dielectric properties of the Sb2Te3 films have been investigated in the frequency range 10 Hz100 kHz and within the temperature range 293373K. Both the dielectric constant ε1 and dielectric loss factor ε2 were found to depend on frequency, temperature and film thickness. The frequency and temperature dependence of ac conductivity (ζac(ω)) has also been determined. The ac conductivity of our samples satisfies the well known ac power law; i.e., ζac(ω) ω s where s<1 and independent of the film thickness. The temperature dependence of ac conductivity and parameter s is reasonably well interpreted by the correlated barrier hopping (CBH) model. The activation energies were evaluated for various thicknesses. The temperature coefficient of the capacitance (TCC) and permitivity (TCP) were determined as a function of the film thickness. The microstructure of the samples were analyzed using X-ray diffraction (XRD). This results are discussed on the base of the differences in their morphologies and thicknesses. The tendency for amorphization of the crystalline phases becomes evident as the film thickness increases. 1.Introduction Antimony Telluride is a semiconducting chalcogenide of the VB and VIB groups of elements. Due to its attractive photo conducting properties and its high Seebeck coefficient, low thermal conductivity, low electrical resistivity, low band gap, and long-term stability Sb2Te3 has wide industrial applications such as target material in television camera, microwave devices, switching devices and optoelectronic devices [1-4]. Sb2Te3 thin films have received considerable attention because of their potential application in the fabrication of integrated thermoelectric devices. To obtain Sb2Te3 thin films, a number of techniques have been used in the literature, such as thermal evaporation[5,6], atomic layer epitaxy(ALE) [7], sputtering [2,8], electrochemical method [9], flash evaporation [10] and metalorganic chemical vapor deposition (MOCVD) [11]. The electrical, thermoelectrical and optical properties of bulk samples Sb2Te3 were studied [12,13]. However, thickness dependence of the dielectric properties of films prepared from that compound were hardly investigated [5]. Therefore, at this study, the dielectric properties of the thermal evaporated Sb2Te3 thin films have been investigated with recpect to frequency, temperature, and especially film thickness in detail, in the frequency range 10 Hz-100 kHz and in the temperature range 293-373 K and 23 350 nm film thickness. 2.Experimental Details The Al |Sb2Te3| Al thin film sandwich structures were prepared by the following sequential evaporation procedure for dielectric measurements. The substrates were cleaned with detergent and flushed with copious amounts of de-ionized water. Finally the substrates were sequentially rinsed with acetone, methanol and isopropyl alcohol. The base and counter Al electrodes were deposited by thermal evaporation. The system was pumped to a base pressure of less than 1,333x10 -3 Pa. First Al was evaporated to serve as the bottom electrode on the Corning glass substrates. Sb2Te3 ingot of high purity (99.99%) were used as the starting material. The crushed ingot were evaporated from molybdenum boat at a vacuum better than 1,333x10 -4 Pa. Al was thermally evaporated to form the top electrode, finally. The evaporation temperature of Sb2Te3 ingot was about 600°C and the evaporation temperature of Al wires was about 1200°C. The resulting Al|Sb2Te3|Al thin film sandwich 15th International Conference on Thin Films (ICTF-15) IOP Publishing Journal of Physics: Conference Series 417 (2013) 012040 doi:10.1088/1742-6596/417/1/012040 Published under licence by IOP Publishing Ltd 1 structures have a capacitive configuration of effective area 6 mm 2 . Films were annelated at 373 K for 1h. A HP 4192A 5Hz-13MHz Lf Impedance Analyzer were used for the capacitance and dissipation factor measurements. The ohmic character of contacts were determined by applying front and reverse biases between bottom and top electrodes. For both of them, the resistance measurements gave same results [2]. These measurements were made in approximately 1,333x10 -3 Pa vacuum and were carried out in the temperature range 293-373 K. Temperatures of the samples during the study were measured with a copper-constantan thermocouple. The thickness of films were measured by Tolansky interferometric method. The film thickness ranged from 23-350 nm. X-ray difraction analysis was used to investigated the structure of the films. The total conductivity was calculated from the equation ζ(ω) = L/ZA, where L is the thickness of the film and A is the cross-sectional area. The dielctric constant was calculated from the equation: ε1 = LC/A ε0, where C is the capacitance of the film, and ε0 is the permitivity of the free space. The dielectric loss was calculated from equation ε2 = ε1 tanδ, where ( δ = 90 Φ ), Φ is the phase angle. The XRD measurements were carried out in a Rigaku DMAX 2200, Japan. X-ray diffractometer using CuK radiation, setting 2 in the range from 10 o to 65 o .