A comparative study of the properties of spray-deposited Sb2Se3 thin films prepared from aqueous and nonaqueous media

Materials Research Bulletin, Vol. 34, No. 7, pp. 1079 –1087, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

PII S0025-5408(99)00095-1

A COMPARATIVE STUDY OF THE PROPERTIES OF SPRAY-DEPOSITED Sb2Se3 THIN FILMS PREPARED FROM AQUEOUS AND NONAQUEOUS MEDIA

K.Y. Rajpure, C.D. Lokhande, and C.H. Bhosale* Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India (Refereed) (Received March 19, 1998; Accepted September 11, 1998)

ABSTRACT Semiconducting antimony triselenide thin films were obtained by spraydepositing aqueous and nonaqueous media, at optimized substrate temperature and solution concentration, onto optically flat glass substrates. Film thickness was of the order of 0.5 ␮m. X-ray diffraction studies revealed that the as-deposited films prepared from the aqueous medium were polycrystalline, while those prepared from the nonaqueous medium were amorphous. The optical gap of the Sb2Se3 thin films prepared from the aqueous medium was 1.88 eV, due to direct interband transition. In contrast, the optical gap for the films prepared from the nonaqueous medium was 1.73 eV. Room-temperature electrical resistivity for the films prepared from both media was found to be of approximately the same order, 107 ⍀-cm. Thermoelectric power (TEP) measurement studies revealed that the films prepared from both media showed p-type conductivity, with Seebeck coefficients of 46.2 and 18.3 ␮V/°C for the polycrystalline and amorphous Sb2Se3 thin films, respectively. © 1999 Elsevier Science Ltd

KEYWORDS: A. chalcogenides, A. thin films, C. X-ray diffraction, D. electrical properties, D. optical properties

*To whom correspondence should be addressed. 1079

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INTRODUCTION It is well known that the conditions of thin film preparation may be exploited to influence the transport and optical properties and thereby obtain films with structures ranging from a completely disordered to a highly ordered form [1]. This makes it possible to prepare a new semiconductor with specific properties required for certain applications and, hence, the performance of certain devices that could not be achieved with a simple elemental or compound semiconductor. Antimony triselenide is a layer-structured semiconductor with orthorhombic crystal structure. Sb2Se3 thin films have attracted wide attention, due to their good photovoltaic properties and high thermoelectric power (TEP), which allow possible applications for optical and thermoelectric cooling devices. Shimakawa [2] and Tichy and Triska [3] have proposed an empirical relation to interpret the compositional dependence of the optical gap in the amorphous semiconducting alloys of SbSe. A few studies on the transport and optical properties of SbxSe1⫺x [3,4] and SbSe [5– 8] have been carried out. Voutsas et al. [9] have determined the crystal structure of Sb2Se3 to be orthorhombic, with cell constants a ⫽ 11.7938(9), b ⫽ 3.9858(6), and c ⫽ 11.6478(7) Å. An experimental investigation of the electrical conductivity, thermoelectric power, and magnetic susceptibility of solid and liquid Sb2Se3 over a wide temperature range was conducted by Glazov and Faradzhov [10]. Wood et al. [11] and Shaffer et al. [12] carried out comparative studies of properties of crystalline and amorphous Sb2Se3. Pramanik and Bhattacharya [13] reported a chemical method for the deposition of Sb2Se3 thin films and the specific resistance of amorphous Sb2Se3 of the order of 107 ⍀-cm, with an optical band gap of 1.88 eV. From a survey of the literature, it was seen that no study had been reported that compared the properties of spray-deposited Sb2Se3 thin films prepared from aqueous and nonaqueous media. Therefore, it was thought that it would be of importance to investigate some structural, optical, and electrical properties of spray-deposited Sb2Se3 thin films prepared from aqueous and nonaqueous media. The present paper deals with experimental observations made on spray-deposited Sb2Se3 thin films prepared from aqueous and nonaqueous media. The samples were characterized by X-ray diffraction (XRD), optical absorption, electrical resistivity, and thermoelectric power measurement techniques. The results obtained are discussed and compared. EXPERIMENTAL Preparation of Thin Films from an Aqueous Medium. Sb2Se3 thin films were deposited by spraying a tartaric acid complex of aqueous antimony trichloride [SbCl3] and selenourea [CSe(NH2)2] solutions mixed in appropriate volumes to obtain a Sb:Se ratio of 2:3. The values for concentration of solution, concentration of complexing agent (tartaric acid), and substrate temperature were kept constant at 0.01 M, 0.5 M, and 275°C, respectively. The spray rate was kept constant at 5 cc min⫺1. Ten cc of 0.5 M tartaric acid was mixed with 8 cc of 0.01 M antimony trichloride to retard precipitate formation between antimony trichloride and selenourea. Then 12 cc of 0.01 M selenourea was added to the complexed antimony trichloride, and the resulting solution was immediately sprayed onto hot glass substrates maintained at 275°C. The films obtained were blackish-brown in color, uniform, pinhole free, and well adherent to the glass substrates. Hereafter, the spray-deposited Sb2Se3 films prepared from the aqueous medium are denoted by the letter A.

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Preparation of Thin Films from a Nonaqueous Medium. 0.01 M solutions of SbCl3 and CSe (NH2)2 were prepared in acetic acid (glacial). Equimolar solutions of SbCl3 and CSe (NH2)2 were mixed together in appropriate volumes to obtain a Sb:Se ratio of 2:3. The films were prepared by spraying the mixed solution (28 cc) onto hot glass substrates, at 150°C. The spray rate was maintained at 3 cc min⫺1. The films were brown in color, uniform, pinhole free, and well adherent to the glass substrates. Hereafter, the spray-deposited Sb2Se3 thin films prepared from the nonaqueous medium are denoted by the letters NA. Characterization. Thickness of the as-prepared films was determined using the relation t ⫽ m/␳A

(1)

where m is the mass of the thin film deposited onto the substrate, A is the area of the deposited films, and ␳ is the density of the deposited material. The structural characterization of the Sb2Se3 thin films was carried out by analyzing the XRD patterns obtained with a Philips PW-1710 X-ray diffractometer (␭ ⫽ 0.15405 nm for Cu K␣). Optical absorption studies were carried out, using a Hitachi 330 UV-vis-NIR spectrophotometer in the wavelength range 300 – 850 nm. To study the electrical characteristics of the films, dark conductivity measurements were carried out, using the two-point probe method in the temperature range 300 –500 K. Silver paste was applied to provide ohmic contact with Sb2Se3 thin films. The type of conductivity of the films with variation in temperature was determined by thermoelectric power (TEP) measurements up to 100°C. RESULTS AND DISCUSSION In the proposed spray pyrolysis method, the materials required to form the desired semiconductor are prepared in solution form and sprayed onto preheated substrates maintained at a desired temperature, which results in the formation of thin films on the substrates. The direct mixing of aqueous solutions of SbCl3 and CSe(NH2)2 results in a whitish turbidity that prohibits the spraying process. Tartaric acid, however, forms a strong complex with Sb⫹3 and, therefore, the addition of selenourea does not cause this type of turbidity. The mixed aqueous solution remains stable for about 1 h and then slowly starts to precipitate. When spraying starts, pyrolytic decomposition of the solution occurs, and uniform, well-adherent films of Sb2Se3 are formed on the substrates. Volatile byproducts and excess solvent escape in the form of vapor. The hot substrate provides thermal energy for decomposition and subsequent recombination of the species and the sintering and recrystallization of the crystallites. This is different for the different materials and for the different solvents used in the deposition process. The optimized substrate temperatures for these films are 275°C for aqueous solvent and 150°C for nonaqueous solvents. Film thickness, as measured by a weight difference method using eq. 1, was found to be 0.51 and 0.52 ␮m for the spray-deposited films prepared from aqueous and nonaqueous media, respectively. The Sb2Se3 compound formation was analyzed from XRD patterns (Fig. 1). It was found that the Sb2Se3 films prepared from the aqueous medium were polycrystalline, with an orthorhombic crystal structure, whereas those prepared from the nonaqueous medium were amorphous. Table 1 shows that the standard d values for Sb2Se3 [14] match the observed d values of the films prepared from the aqueous medium, confirming the formation of Sb2Se3 material at 275°C substrate temperature. The calculated lattice constants were found to be

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FIG. 1 XRD patterns of Sb2Se3 thin films deposited using aqueous (A) and nonaqueous (NA) media.

a ⫽ 12.1817, b ⫽ 11.2906, and c ⫽ 4.0384 Å, which are very close to the values reported for single crystals of Sb2Se3 [9,12]. The structural difference between the Sb2Se3 films prepared from the aqueous and nonaqueous media is partly due to (i) the different substrate temperatures for the two media and (ii) the difference in stoichiometric proportions that occurred during pyrolytic decomposition of the Sb2Se3 particles in the two media. The substrate temperature of 275°C for films prepared from the aqueous medium was more suitable for the sintering and subsequent recrystallization of crystallites during deposition than that of 150°C for films prepared from the nonaqueous medium. The nature of the optical transition involved can be determined on the basis of dependence of absorption coefficient, ␣, on the photon energy, h␯. For allowed direct transition, ␣ is given by ␣ ⫽ [B(E ⫺ E g) n]/E

(2)

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TABLE 1 Comparison of the Observed d Values of Sb2Se3 Thin Films with ASTM Data [14] Observed d values (Å) 5.2508 4.1235 3.6730 3.1713 2.5981 2.1500

Standard d values (Å)

Ratio of relative X-ray intensities, I/Imax (%)

(hkl) planes

5.25 4.14 3.682 3.162 2.608 2.164

76.70 60.28 40.93 100.00 100.00 21.13

(120) (220) (310) (211) (420) (520)

FIG. 2 Plot (␣h␯)2 vs. h␯ for Sb2Se3 thin films deposited using aqueous (A) and nonaqueous (NA) media.

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FIG. 3 Variation of log ␳ with 1000/T for Sb2Se3 thin films deposited using aqueous (A) and nonaqueous (NA) media.

where B is the constant related to the effective masses associated with the valence and conduction bands, Eg is the band gap energy, E ⫽ h␯ is the photon energy, and n ⫽ 1/2 or 2, depending on whether the transition is direct or indirect, respectively. The variation of (␣h␯)2 with h␯ for the Sb2Se3 thin films prepared from the aqueous medium is shown in Figure 2 as a straight line, indicating that the direct transition is the dominant transition involved. The energy gap for the films prepared from the aqueous medium was obtained by extrapolating the linear portion of (␣h␯)2 vs. h␯ plot to ␣ ⫽ 0. The optical gap of the Sb2Se3 thin films prepared from the aqueous medium was found to be 1.88 eV, due to direct transition. This value is in good agreement with the value reported in ref. 13. For the amorphous semiconductor, the lack of long-range order produces strong scattering such that k is not a well-defined quantum number to describe the electron quantum state [15], and ␣, near the fundamental edge, follows a power law [5,16] in the form of eq. 2, where n ⫽ 2. From the (␣h␯)1/2 vs. h␯ plot for the Sb2Se3 thin films prepared from the nonaqueous

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FIG. 4 Variation of generated thermo emf with temperature difference for Sb2Se3 thin films deposited using aqueous (A) and nonaqueous (NA) media.

medium, it was not possible to determine the optical gap of the film, because the nature of the plot is not linear. On the other hand, the (␣h␯)2 vs. h␯ plot is linear (Fig. 2). The observed optical gap for the Sb2Se3 thin films prepared from the nonaqueous medium, obtained by extrapolating the straight line portion of the (␣h␯)2 vs. h␯ plot to energy axis, is 1.73 eV. The values of the optical gap for the polycrystalline and amorphous Sb2Se3 are close, considering that the short-range order in the two phases are not identical [17]. The change of optical properties in the amorphous and crystalline phases was observed to be strongly dependent on the nature and the number of electrons in the bonding that existed in the material. It has been observed [18] that the energy bands and optical transitions at various parts of the Brillounin zone are influenced in a different way and to a different degree due to disorder. The two-point dc probe method for measuring dark resistivity showed that the films prepared from the aqueous and nonaqueous media were semiconducting in nature and had dark resistivities of the order of 107 ⍀-cm. These results are similar to those reported in ref. 13. The high resistivity of the films may be due to large grain boundaries and discontinuities in the films. The variation of log ␳ with 1000/T for the polycrystalline (aqueous medium) and amorphous (nonaqueous medium) films is shown in Figure 3. The observed activation

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energies were 0.77 and 0.82 eV for polycrystalline and amphorous films, respectively. The difference in observed activation energies may be due to difference in stoichiometry of the materials. Figure 4 shows that the thermo emf (electromotive force) generated in both films varies linearly with temperature, up to 100°C. TEP, however, does not vary with temperature over the temperature range studied. Positive polarity of the generated thermoelectric voltage was found. This suggests that the conduction in the films is p-type [19]. The observed value of TEP was 46.2 ␮V/°C for sample A and 18.3 ␮V/°C for sample NA. The difference in the TEP values of these samples may be attributed to the fact that the crystallinity of the polycrystalline material was higher than that of the amorphous material.

CONCLUSIONS p-Sb2Se3 thin film deposition by a spray pyrolysis method using aqueous and nonaqueous solvents is possible. The type of solvent used significantly affects the properties of the material. Sb2Se3 films prepared from the aqueous medium were polycrystalline with a direct bandgap energy of 1.88 eV, whereas those prepared from the nonaqueous medium were amorphous with an optical gap of 1.73 eV. The room temperature electrical resistivity was of the order of 107 ⍀-cm.

ACKNOWLEDGMENTS The grant received from the Department of Science and Technology (DST Project no. SP/S2/M-37/94), Government of India, for the research work on spray-deposited “antimony chalcogenides” is gratefully acknowledged. One of the authors (K.Y. Rajpure) is highly thankful to the DST for the Junior Research Fellowship.

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