1. Introduction

The binary silver selenide and its related ternary semiconducting materials are of great interest because of the importance of investigating new materials that should be more efficient, highly reproducible and less toxic. The studies on I–V–VI$_{2}$ chalcopyrites have been done extensively because of their strong absorbing properties that make these materials good for photovoltaic applications^[1]. Silver antimony selenide is a narrow band gap p-type semiconductor. AgSbSe$_{2}$ is one of the semiconducting materials which have a NaCl structure. AgSbSe$_{2}$ is particularly disordered with selenium atoms occupying the chlorine sites, while silver and antimony atoms are arranged randomly on the sodium sites^[2]. The compound thin film exists in two phases-cubic AgSbSe$_{2}$ and orthorhombic Ag$_{5}$SbSe$_{4}$. Different techniques for the preparation of cubic AgSbSe$_{2}$ have been illustrated in the literature including, by fusing the constituent elements in a vacuum sealed quartz tube^[1], vacuum evaporation^[3-5] and chemical deposition^[6]. Abdelghany et al.^[7] carried out measurements on the electrical conductivity and thermoelectric power of the AgSbSe$_{2}$ in the solid and liquid states from 350 to 975 ℃ . Bindu et al.^[8] have reported the possibilities of this material as an absorber layer for the structure SnO$_{2}$–CdS–(i)Sb$_{2}$Se$_{3}$–(p)AgSbSe$_{2}$ and an open circuit voltage of 530 mV has been observed under an illumination of 2 kW/m$^{2}$ using a tungsten-halogen lamp. They have also reported the thermoelectric measurements on AgSbSe$_{2}$ thin film in the temperature range of 250–320 K. Wojciechowski et al.^[9] pointed out the semiconducting with a narrow band or semimetallic properties of this material by studying the electrical conductivity and the Seebeck coefficient (320 $\mu$V/K at room temperature) is measured as a function of temperature in the range from 300–600 K. Schmidt et al.^[10] estimated the value of $E_{\rm g}$ of this material is 0.09 eV, indicating that the semimetallic feature and alloying of AgSbSe$_{2}$ compounds exhibit an apparent semiconducting behaviour. The AgSbSe$_{2}$ have been studied previously, however, the literature survey showed that the low temperature thermoelectric studies on the indirect band gap semiconductor AgSbSe$_{2}$ were not reported. The analysis of thermoelectric power (TEP) as a function of temperature is particularly significant to assess the relative strength of the electron-phonon interactions and all other phonon interactions. In this work, we report the deposition of AgSbSe$_{2}$ thin films without in situ annealing using a reactive evaporation technique^[11]. To evaluate the potentiality of this material for optoelectronic and thermoelectric applications, the structural, electrical, optical and low temperature thermoelectric properties to enlighten the electron transport behavior, are also studied and presented.

2. Experimental details

The AgSbSe$_{2}$ thin films were coated on glass substrates in a vacuum of 10$^{-5}$ Torr. Glass substrates of $34 \times 12 \times 1$ mm$^3$ size were first cleaned using an industrial detergent and then ultrasonically agitated. These substrates were dried with a hot air blower and then loaded on the substrate holder, placed at a distance of 13 cm from the evaporation sources. For the preparation of ternary AgSbSe$_{2}$, two separate glass crucibles kept in molybdenum baskets were used as sources for the evaporation of 99.999% pure elemental Sb and Se. A molybdenum boat was used for the evaporation of 99.999% pure Ag. The system was evacuated to a vacuum of better than 10$^{-5}$ Torr and the substrates were heated to the required temperature of 398 K. Substrates were maintained at that temperature for 15 min for stabilization. The temperature was monitored by DPM connected with a Chromel-Alumel thermocouple mounted on the substrate. First, a selenium atmosphere was made in the vacuum chamber and then silver and antimony were evaporated from separate sources by controlling the current through the source. After deposition, the substrate temperature was slowly reduced to room temperature. The crystallinity and stoichiometry of the compound depend on the incident fluxes and the substrate temperature and are optimized using the reactive evaporation technique.

The optimized preparative parameters for obtaining the stoichiometric silver antimony selenide thin film using this technique are the:

Impingement rate of silver: $\approx$ 3 $\times$ 10$^{15}$ atoms/(cm$^{2}$$\cdot$s)

Impingement rate of antimony: $\approx$ 2 $\times$ 10$^{15}$ atoms/(cm$^{2}$$\cdot$s)

Impingement rate of selenium: $\approx$ 1 $\times$ 10$^{14}$ atoms/(cm$^{2}$$\cdot$s)

Substrate temperature: $=$ 398 $\pm$ 5 K

The XRD measurements were done using a Rigaku D-MaxC diffractometer with Cu-K$\alpha$ ($\lambda$ $=$ 1.5404 {Å }) radiation, operated at 30 kV, 20 mA. Optical absorbance spectra were recorded using a JASCO V570 UV–vis–NIR spectrophotometer with automatic computer data acquisition. SEM analysis to study the morphology of the film was done using a JEOL JSM 6390 scanning electron microscope. The two probe method was used for doing temperature dependent $I$–$V$ measurements. The thickness of the film was measured using the Veeco Dektak 6M Stylus Profiler. The thermoelectric power measurements were done using automated precision measurement setups for electrical resistivity and simultaneously thermoelectric power of different samples in the temperature range 5–325 K; the experimental set up used is described elsewhere^[12].

3. Result and discussion

3.1 Structural studies

The X-ray diffraction pattern of a typical sample is shown in Fig. 1.

The indexing of the pattern is done and the XRD data is compared with standard ASTM card no: 12-379^[13] and is shown in Table 1. The well defined sharp peaks in the pattern suggest that the grains in the sample are randomly oriented along different crystallographic planes, which indicate the polycrystalline nature of the prepared sample. The structural analysis showed the films are single phase with a NaCl structure, which is in agreement with that reported by Soliman et al.^[2]. The prominent peak in the pattern corresponds to the reflection from the (200) plane. The relative intensities of the other peaks decrease since the penetration depth of the X-ray decreases as the angle increases.

The average particle size (D) is calculated using the Scherrer formula^[14] as in Eq. (1).

$\begin{equation} D=\frac{0.9 \lambda}{\beta \cos \theta}, \end{equation}$

(1)

where $\lambda$ is the wavelength of the X-ray used, $\beta$ the full width at half maximum and $\theta$ the glancing angle. The average particle size is found to be 22 nm. The calculated lattice constant is 5.74 Å.

The dislocation density ($\rho$) in the layers is calculated using the relation^[15] as in Eq. (2),

$\begin{equation} \rho=\frac{1}{D^2}, \end{equation}$

(2)

and we obtained it as 2.066 $\times$ 10$^{15}$ lines/m$^{2}$.

The number of crystallites per unit area ($N)$ calculated as 5.635 $\times$ 10$^{15}$ m$^{-2}$ using the relation^[15] as in Eq. (3).

$\begin{equation} N=\frac{t}{D^3}, \end{equation}$

(3)

where $t$ is the thickness of the film. The thickness of the film, measured using the Stylus profiler, was found to be 600 nm.

The strain in the film ($T$) is calculated using the relation^[15] as in Eq. (4):

$\begin{equation} T=\frac{1}{\sin \theta} \left( \frac{\lambda}{D}-\beta \cos \theta \right), \end{equation}$

(4)

and it is estimated to be 2.8 $\times$ 10$^{-3}$.

Figure 2 shows the scanning electron microscopy (SEM) images of AgSbSe$_{2}$ thin film at 10000 magnifications. From the figure it is clear that there are agglomerated clusters with the formation of islands that are well separated. The film shows well defined grains, indicative of the polycrystalline nature of the film. The grain size obtained from SEM is 300 nm, which is much greater than that from XRD; this is expected due to the clustering of the particles.

Figures 3(a) and 3(b) show the two and three dimensional views of the atomic force microscopic (AFM) images of AgSbSe$_{2}$ thin film, measured over an area of 10 $\times$ 10 $\mu$m$^2$. The grain boundaries are clear in the micrograph. The two micrographs show that the surface of the silver antimony selenide thin film consists of well defined grains of different sizes and shapes. The appearance of holes near the boundaries of large grains in the two dimensional view indicates the tendency of the film to agglomerate. During deposition, holes had been generated in the film and they continue to grow through the grain boundaries, leading to agglomeration. The average grain size obtained from AFM is about 350 nm, which is substantiated with the value obtained from SEM. The surface roughness of the as-deposited film is also investigated and it is 121 nm.

3.2 Electrical studies

The variations in the conductivity as a function of temperature are studied in the range 303–423 K and the conductivity measurements were done inside a conductivity cell at a vacuum of 10$^{-2}$ torr using silver paste as the ohmic contact.

The Arrhenious plot of AgSbSe$_{2}$ thin film for the three continuous heating and cooling cycles are shown in Fig. 4. During heating, a variation of ln $I$ versus 1000/$T$ is observed and the cooling curve lies above the heating curve. The samples show a semiconducting nature with a rise in temperature. From the figure it is clear that after the first heating cycle, the conductivity variation with temperature retraces the same path of the first cooling curve, indicating that the prepared film is stabilized by releasing the strains in the sample and the electrical properties remain the same. The activation energy calculated is 0.08 eV, which closely matches the measurements done by Patel et al.^[16] for the AgSbSe$_{2}$ thin films prepared by the single source thermal evaporation method in a vacuum.

3.3 Optical studies

The reflectance and transmittance spectra for AgSbSe$_{2}$ thin films are shown in Figs. 5 and 6. From the figure it is observed that the transmittance of the film in the near IR region increases. In the NIR region, $T+R=$ 1 indicates that there is no absorption in that region. The absorbance versus the wavelength plot is given in Fig. 7 and the absorption coefficient is about 10$^{4}$ cm$^{-1}$. For indirect transitions, the variation of $\alpha$ with photon energy is as in Eq. (5).

$\begin{equation} \alpha h \nu = A(h \nu - E_{\rm g}\pm E_{\rm p})^{n}, \end{equation}$

(5)

where $A$ is a constant depending on the transition probability and $n$ is an index that characterizes the optical absorption process. The value of $n$ is 2 and 3 for indirect allowed and indirect forbidden transitions respectively. $E_{\rm p}$ corresponds to phonon energy.

The plot of $\alpha h \nu$ versus photon energy $h \nu$ is shown in Fig. 8. The band gap estimation of AgSbSe$_{2}$ thin film was done using the Swanepoel method^[17]. The band gap obtained is 0.64 eV and is indirect allowed one. The high value of the absorption coefficient and the optical band gap of 0.64 eV make this material suitable for photosensitive devices.

3.4 Low temperature thermoelectric properties

The thermoelectric power $S$ is generally the sum of the electronic term $S_{\rm e}$, resulting from the spontaneous tendency of the charge carriers to diffuse from hot to cold, and the phonon term $S_{\rm g}$, resulting from the drag on the charge carriers exerted by the phonons streaming from hot to cold by thermal conduction. The phonon drag contribution to thermoelectric power is an effect in which, in the presence of temperature gradient, the scattering of the charge carriers by the lattice vibrations tend to increase the amplitude of the phonon waves in the same direction of carriers and to decrease the amplitude of the waves in the opposite direction^[18]. The phonon drag effect is widely observed in the thermoelectric power of metals and semiconductors as peaks and a faster enhancement or decrement in TEP than that for pure diffusion alone.

The temperature dependence of TEP of the AgSbSe$_{2}$ samples is shown in Fig. 9. From around 9 K to 48 K, TEP rapidly increases with temperature from 15 mV/K to a large positive value of about 30 mV/K, the conduction should occur predominantly due to holes. This is the largest value of thermoelectric power ever reported for this material at 48 K. At low temperature, even though the electron relaxation time is controlled by impurities, from the linear variation of TEP with an increase in temperature suggests the dominance of the phonon drag effect that illustrates the strong interaction between electron–hole system. The $S_{\rm g}$ tends to zero at both low and high temperatures and gives a maximum value at an intermediate temperature of 48 K where the electron–phonon interaction becomes dominant with respect to the other phonon interactions. The high positive value of the Seebeck coefficient of about 30 mV/K at a temperature of 48 K shows the hole conductivity of the sample. As the temperature increases above 48 K, the thermoelectric power sharply drops and then an inversion of sign is observed, indicating the change of the predominant carriers from holes to electrons. The TEP changes its sign since electrons and holes plus a strong electron–phonon interaction contributes to the supperlattice formation during the structural phase transition of NaCl type structures^[19]. The figure shows that the phonon drag peaks of both holes and electrons occur due to the strong phonon electron interaction, which is the typical feature of this sample. In AgSbSe$_{2}$ thin films, the rock salt structure was reported and one can expect an inversion on carrier type during phase transformation; the zero crossing of TEP is clear in the figure. Sergey et al.^[19] reported the phase transitions in ZnTe under pressure as other chalcoginides by the thermoelectric power method, which is sensitive to changes of sign of dominant charge carriers usually occurring under phase transformations. Lakhani and Jandl^[20] reported under superlattice formation that the low temperature TEP converts its sign, indicating that the dominant carrier type passes from p to n. From the above figure, the TEP dip observed at a temperature of about 80 K confirms the larger role of the phonon drag effect than the drift–diffusion contribution. A pronounced negative minimum observed in the transforming samples exhibits the super lattice state and the TEP dip is due to the strong phonon drag effect, which leads to the softening of the associated zone-boundary phonon. The change of sign of the Seebeck coefficient of the AgSbSe$_{2}$ sample in the solid to liquid transition at high temperature was reported by Abdelghany et al.^[7]. The TEP dip temperature $T_{\rm m}$ allows us to calculate the nonnormalised energy of the phonon using the relation^[21] as in Eq. (6).

$\begin{equation} k_{\rm B}T_{\rm m}=h \omega_{\rm m}, \end{equation}$

(6)

where $k_{\rm B}$ is the Boltzman constant, $h$ is the Plank constant and the evaluated phonon frequency is 56 cm$^{-1}$ at a TEP dip temperature of 80 K.

The magnitude of the Seebeck coefficient (220 $\mu$V/K), which is fairly constant near room temperature, indicates that $S$ is independent of temperature; this is consistent with the nondegenerated semiconducting character of the sample.

To study the temperature dependence of the thermoelectric power of the sample in the high temperature region, consider the equation for the Seebeck coefficient^[21] as in Eq. (7).

$\begin{equation} S=\pm \frac{k}{e}\left( A+\frac{E_{\rm F}}{kT}\right), \end{equation}$

(7)

where $k$ is the Boltzmann constant, $e$ is the electronic charge and $A =$ $\left(\frac{5}{2}-S\right)$ is a constant that varies from 0 to 4 depending upon the scattering process. $E_{\rm F}$ is the separation of the Fermi level from the bottom of the conduction band or that from the top of the valence band depending on the type of conductivity. The values of $A$ and $E_{\rm F}$ can be obtained from the straight line plot near the room temperature region. The value of $A$ estimated is 1.45 and the Fermi level obtained is 0.96 meV.

The ability of a material to efficiently produce thermoelectric power is related to its figure of merit, as in Eq. (8).

$\begin{equation} Z=\frac{S^2 \sigma}{K}, \end{equation}$

(8)

where $S$ is the Seebeck coefficient, $\sigma$ is the electrical conductivity and $K$ is the thermal conductivity. Accordingly, a high Seebeck coefficient, a large electrical conductivity and a low thermal conductivity are the prerequisites for the materials having the potential for thermoelectric applications. The disordered character of this compound crystallizes in a cubic NaCl-type structure in which Ag and Sb randomly occupy the same crystallographic position, which is considered to be the main factor accounting for the extremely low thermal conductivity^[22]. This material exhibits an interesting temperature dependence of TEP, which makes the material particularly attractive for device applications at low temperatures because the same base materials can be used for the p and n legs of the thermoelectric converter.

4. Conclusion

The semiconducting silver antimony selenide thin films deposited on glass substrates using the reactive evaporation technique are found to be homogeneous and highly adherent to the substrate. The films deposited at the substrate temperature 398 K are polycrystalline in nature. The average particle size calculated is 22 nm. The lattice constant calculated is 5.74 {Å }. The dislocation density in the layers is 2.066 $\times$ 10$^{15}$ lines/m$^{2}$. The number of crystallites per unit area is 5.635 $\times$ 10$^{15}$ m$^{-2}$ and the strain estimated is 2.8 $\times$ 10$^{-3}$. The surface morphology of AgSbSe$_{2}$ thin film is studied using SEM. The surface roughness of the as-deposited film measured from AFM analysis is 121 nm. The films obtained are semiconducting in nature and have an activation energy of 0.08 eV. The silver antimony selenide thin films have a high absorption coefficient of about 10$^{4}$ cm$^{-1}$. The AgSbSe$_{2}$ thin film has an indirect band gap of 0.64 eV. The film shows much interesting TEP below room temperature. The phonon drag effect in structurally transforming samples is also studied. The analysis of temperature variations of TEP in terms of the phonon-drag effect reveals that in the case of AgSbSe$_{2}$ thin film there exists a strong electron–phonon interaction, which is a basic feature of the structurally transforming samples. The TEP dip at 80 K, as due to the phonon drag effect in the transforming samples, gives the non-normalised phonon frequency as 56 cm$^{-1}$.

Acknowledgements: One of the authors (KSU) would like to thank the University Grants Commission for the financial assistance in the form of a Research Fellowship in Science for Meritorious Students (RFSMS).