Synthesis and Characterisation of Zinc-doped Antimony Selenide

Antimony selenide substituted with Sb0.4Se0.6 and doped with zinc at three doping ratios (x=0, 0.01 and 0.03) was prepared via the solid state reaction method. The three prepared compositions were reacted thermally at 400 °C for 3 h. The structure of specimens was characterised via X-ray powder diffractometer to obtain the type of crystalline structure and lattice parameters of the prepared specimens, which showed a polycrystalline, orthorhombic structure. Optical characterisation was then achieved via UV-visible spectroscopy to exhibit the transmittance and reflectance spectra and estimate the band gap values of the prepared compositions. The samples showed high absorption spectra at low wavelengths (from 60% to 90%) and low reflectance values (from nearly zero to 17%). The band gap measurement showed an indirect transition, with values ranging from 1.2 eV to 1.23 eV. The electrical characteristics were represented by DC resistivity measurement at low temperature and AC conductivity measurement against frequency. The compositions showed a semiconducting behaviour in DC resistivity and compatible results in AC conductivity.


1-Introduction
Semiconductor selenides are widely used in various industrial applications, such as solar cells, sensors, optical filters, and laser materials [1]. An antimony selenide (Sb 2 Se 3 ) semiconductor is a considerable member of the V 2 VI 3 compound group which has an orthorhombic crystalline structure and layer-structured form. This type of semiconductor material has high thermoelectric power and efficient photovoltaic characteristics, and can potentially be applied into thermoelectric and optical cooling devices [2,3]. Studies on the effects of doping agents or impurities on the physical properties of Sb 2 Se 3 materials are considerably important in basic and practical researches [4][5][6][7][8]. The aim of this study is the synthesis of semiconductor selenides, by doping the enhanced metal through solid state reaction, and studying their structural, optical, and electrical characteristics. For this purpose, Sb 2 Se 3 with Sb 0.4 Se 0.6 was doped in different ratios with Zn 2+ via solid state reaction.

2-Materials and methods
Each of Sb (121.760g/mol) and Se (78.96g/mol) with molar ratios of 0.4 mol% and 0.6 mol%, respectively, were mixed carefully with Zn 2+ in doping ratios of x=0, 0.01wt.% and 0.03 wt%. Initially, these amounts were placed in an agate mortar for mixing. The second step was adding 5 ml of ethanol alcohol, followed by careful mixing for 1 h to homogenise the precursors. The mixture was dried in an oven at a temperature of 80 °C for 1 h. Solid state reaction was achieved by transporting the mixed wet powders into a silica crucible and fired at 400 °C for 3 h to achieve calcination. The calcined powder was crushed in the mortar and compacted into pellets with a diameter of 1 cm by applying a pressure of 250 MPa. These pellets were annealed at 200 °C. The geometrical densities of these prepared pellets were measured after being fired for 1 hour. Finally, the annealed specimens were prepared for electrical measurement represented by DC and AC conductivity via four-point probe technique. This type of measurement was achieved using a Wayne Kerr 6500 LCR meter with a specialised cell under various frequencies, ranging from 10 Hz to 120 MHz, at a constant temperature of 300 K. The optical measurement was represented by UV-visible spectroscopy. The wavelength ranged from 200 nm to 800 nm as in the spectroscopy setting.

3-Results and Discussion 3.1-X-ray diffraction analysis
Due to solid state reaction type (solid + solid =solid) between the reactanting components, this reaction may be produce new compositions. Thus, the x-ray diffraction test was achieved, as shown in Figure 1. The x-ray pattern provided qualitative and quantitative analyses via comparison with the XRD of the original composition file (98-001-6680) from the Inorganic Crystal Structure Database. Slight in-peak positions, which represent the presence of Zn 2+ impurities in the doped samples, were revealed. Accordingly, the XRD pattern showed an orthorhombic structure with a space group (p n m a). XRD results showed that no other phase was formed with the Sb 2 Se 3 phase, which proved the stoichiometry of the reacted precursors and, hence, no chemical bonds were formed [8]. Lattice parameter values were evaluated using the Rietveld refinement FullProf software and are listed in Table 1. Lattice constant values decreased with the increase in Zn 2+ content. The values of unit cell volume were found to be lower than that in the software's standard pattern. The difference in values may refer to the inhomogeneous preparation in comparison to the standard operation conditions. 3.2 Optical characteristics When various prepared specimens were subjected to optical analyses by previous researches, it was found that the band gap of Sb 2 Se 3 has opposite behaviours related to direct and indirect transitions. Several values of band gap were listed in these reports,, with varied ranges of effective energy values. For instance, Rodrigues-Lazcano reported thin films with an indirect band gap of 1-1.2 eV, which can be derived from the equation (1) stated below [9]. αհυ = [հʋ -E g ] r …………(1) where (α) is the absorption coefficient and r refers to the direct and indirect transition. Rajpure et al. presented Sb 2 Se 3 polycrystalline films with a direct band gap of 2.14 eV. [10]. El-Sayed et al. stated indirect and direct energy gaps in magnitudes of 1.15 and 1.5 eV, respectively [11,12]. Information on the optical characteristics of the samples under study was Jumah et al. Iraqi Journal of Science, 2021, Vol. 62, No. 10, pp: 3552-3559 3555 obtained by measuring the absorbance and transmittance spectra, as shown in Figure 2a,b,c [12]. The absorbance values in wavelengths ˂400 nm were high (nearly 90% in x=0.01 composition), whereas transmittance had low values. In Figure 2, the behaviours of the absorbance and transmittance spectra seemed to be the same for all the compositions, but with , ………… (2) These compositions exhibited low reflectance, which ranged between nearly zero to a maximum value of x=0.03, that is, 17%. The optical transmission measurements showed an indirect transition for all the samples (Figures 3-a, b and c).
a. b.
c.  Table 2. Therefore, energy gap values decrease with the increase in doping ratio.

Electrical measurements
The DC resistivity was plotted against temperature, as shown in Figure 4. The low temperature-dependent DC resistivity (ρ DC ) measurement was achieved using a four-point-probe cryogenic system. The temperature ranged from 5 to 300 K. The resistivity behaviour showed a basic characteristic as a semiconductor, in which the values of ρ increased with the decrease in temperature for the prepared samples. These results referred to the principle behaviour of semiconductor where the resistivity is enhanced when the temperature is increased. Moreover, the values of DC resistivity increased with the presence of the doping element (zinc). The D.C. conductivity (б d.c. ) was calculated according to the following equation: Б d.c. L/ R.A = 1/ ρ ………………(3( where б is conductivity, ρ is resistivity, L is distance between electrodes, R is resistance, and A is cross section area. Whereas the A.C. conductivity (бa.c.) was calculated according to the following equation: Б(ꞷ) = G(ꞷ) d /A …………. (4( where G(ꞷ) is conductance, d is thickness, and A is cross section area of the electrode. The AC conductivity (σ AC ) measurements are presented in Figure 5. Frequency values were graphed in a logarithmic form of angular frequency (ω) to eliminate zeros from the frequency scale. The samples under study showed a reduction in the values of AC conductivity when the frequency increased. However, at high frequencies (30-40 MHz.), an increase in σ AC values was observed, which may indicate an electronic or vibrational response [13,14]. Ren et al. reported similar results in some details [15].

4-Conclusions
The prepared compositions retained the same orthorhombic structure but with varied lattice constants and low unit cell volume. The material possessed high absorbance in the ultraviolet region with 1% ratio of Zn 2+ . Doping with zinc reduced the magnitudes of the energy gap. The DC electrical resistivity values showed increased semiconductor behaviour with the increase in Zn 2+ ratio. The AC conductivity values were enhanced directly with highfrequency region (30-40 MHz) for all compound ratios used. Therefore, the results indicated potential advantages for sensor applications.