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INTRODUCTION. Tin sulfide SnS, by virtue of a high optical absorp tion coefficient (α ~ 104–105 cm–1) in the visible and near infrared spectral regions, is of ...
ISSN 10637834, Physics of the Solid State, 2012, Vol. 54, No. 12, pp. 2497–2502. © Pleiades Publishing, Ltd., 2012. Original Russian Text © S.A. Bashkirov, V.F. Gremenok, V.A. Ivanov, V.V. Shevtsova, 2012, published in Fizika Tverdogo Tela, 2012, Vol. 54, No. 12, pp. 2372–2377.

SURFACE PHYSICS AND THIN FILMS

Microstructure and Electrical Properties of SnS Thin Films S. A. Bashkirov*, V. F. Gremenok, V. A. Ivanov, and V. V. Shevtsova ScientificPractical Materials Research Centre, National Academy of Sciences of Belarus, ul. P. Brovki 19, Minsk, 220072 Belarus * email: [email protected]net.by Received April 25, 2012

Abstract—Thin SnS films are of interest for optoelectronics. The influence of the preparation modes on the microstructure and electrical properties of thin SnS films obtained by the hotwall method on substrates made of pure glass and glass with a molybdenum sublayer has been investigated. It has been established that the formation of SnS films with two texture types (111) and (010) is possible on the substrates made of pure glass, depending on the mode. The resistivity and the temperature coefficient of thermoelectric power of the SnS films on glass vary in the range from 12 to 817 Ω cm and from 37 to 597 μV/K, respectively, depending on the preparation modes. The activation energy is 0.11–0.12 eV. DOI: 10.1134/S1063783412120049

1. INTRODUCTION Tin sulfide SnS, by virtue of a high optical absorp tion coefficient (α ~ 104–105 cm–1) in the visible and nearinfrared spectral regions, is of interest for opto electronics. In some publications, the prospects of using SnS as an absorbing element in thinfilm solar cells of a new generation are discussed [1, 2]. SnS belongs to IV–VI semiconductors, has a lay ered structure, and forms two orthorhombic crystal line phases: α phase with the B16 structure at T < 878 K (the GeS structural type, space group Pbnm) and α phase with the B33 structure at T > 878 K (the TlI structural type, space group Cmcm). The second order phase transition with the atomic shift in the plane of macromolecular layers occurs at T = 878 K [3]. Along with SnS, the formation of crystalline phases of the composition SnS2 [4], as well as Sn2S3 and Sn3S4 [5, 6], is possible in the tin–sulfur system. Thin SnS films are obtained by various methods involving sputtering by the electron beam [7], spray pyrolysis [8], chemical and electrochemical deposi tion [9, 10], etc. The methods based on the thermal evaporation of the polycrystalline material [11–14] are of special interest since, being simple and low cost, they allow us to obtain in specific modes the highquality films not contaminated by impurity ele ments and second phases. It was previously reported on the preparation of thin SnS films on glass substrates by the hotwall method, which is the variant of the thermal evapora tion method that differs by the use of the heated tube to transfer the vapors of the deposited material, which makes it possible to perform the deposition in condi tions close to the thermodynamic equilibrium [15, 16]. The use of this method allows us to successfully

obtain singlephase SnS films with the stoichiometric composition and crystal structure corresponding to the standards [16]. The photosensitive surfacebarrier In/SnS structures based on the SnS films were previ ously for the first time obtained by the hotwall method [17]. In this study, we investigate the features of the microstructure and electrical properties of thin SnS films obtained by the hotwall method on the substrates made of pure glass and glass with the preliminarily deposited 0.6μm molybdenum layer. The interest in fabricating and studying the SnS films on the substrates with the molybdenum sublayer is caused by prospects of the further fabrication of photosensitive heterostruc tures on their base, in which the molybdenum layer will be used as the back ohmic contact. 2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE The procedure of the preparation of the SnS targets and films by the hotwall method is described in detail in previous reports [16, 17]. In this study, we investi gated the films obtained in vacuum at a pressure of 5 × 10 ⎯4 Pa, the wall temperature of 550°C, and the dis tance between the evaporated material and the sub strate of 12 cm. The substrate temperature Ts and dep osition time τ were varied in ranges 200–350°C and 10–50 min, respectively. The selection of the wall tem perature is explained by the necessity of the sublima tion evaporation of the material. The selection of the substrate temperature is motivated by the fact that the SnS vapor does not decompose at the mentioned tem peratures and prevalently consists of the diatomic Sn molecules and their dimers Sn2S2 [18], which mini mizes the deviations from the stoichiometry of the

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(080)

10 min, 0.7 μm

50 min, 3 μm

80 (110) (120) (021)

2θ, deg Fig. 1. Typical Xray diffraction pattern of the SnS film obtained at Ts = 300°C on the (a) linear and (b) logarith mic scales.

20

condensed SnS phase. The deposition time was selected starting from the necessity to obtain the films in a wide thickness range. The crystal structure and the phase composition of the materials were investigated by the Xray diffraction using a Siemens D5000 diffractometer with CuKα radiation (λ = 1.5418 Å) with measuring 2θ in a range from 10 to 100°C and a step of 0.01°. The phase com position was analyzed using the Joint Committee on Powder Diffraction Standard (JCPDS) database. The distributions of elements over the thickness were stud ied by the Auger electron spectroscopy (AES) using a Perkin Elmer Physical Electronics 590 spectrometer with the measurement error of 0.1 at %. The film mor phology was investigated by scanning electron micros copy (SEM) using the mentioned electron micro scope. The film thickness was determined using the SEM images of their cross sections. The temperature coefficient of thermoelectric power and the conduc tion type were determined by the thermoprobe method. The resistivity was determined by the van der Pauw method. The temperature dependence of electrical conductivity was studied at a rarefaction of 10–4 Pa. 3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Phase Composition According to the results of the Xray phase analysis, all the obtained films were singlephase and contained only the αSnS phase with the orthorhombic structure (the GeS structural type, space group Pbnm) with the

40

Powder (113) (341) (133) (191)

60

(b)

(270)

(002) (211) (160) 40

Intensity, arb. units

(080)

30 min, 2 μm

(111), (040) (131) (141) (210), (002) (211) (151) (160), (061) (042) (250) (251) (080)

20

(250)

(040) (120)

(020)

Intensity, arb. units

(040)

(a)

60

80

2θ, deg Fig. 2. Xray diffraction patterns of the SnS films obtained at Ts = 220°C and the SnS powder used as the deposited material. The deposition time and film thickness are indi cated in Xray diffraction patterns.

absence of foreign crystalline phases involving Sn, S, Sn2S3, SnS2, SnO2, etc. We previously reported on the preparation of the SnS films on glass substrates with a clearly pro nounced preferential orientation in plane (010) [16]. The Xray diffraction patterns of the films of this type in the linear and logarithmic scales by intensity are presented in Fig. 1. It was established in the course of further studies that this type of preferential orientation is reproduc ible only for the SnS films on glass substrates obtained in the range of substrate temperatures Ts = 230– 350°C while most films obtained at Ts < 230°C have the preferential orientation in the (111) plane. Figure 2 shows the Xray diffraction patterns of the powders obtained at Ts = 220°C as well as the Xray diffraction pattern of the SnS powder used as the evaporated material. The results show that the films obtained at Ts = 200–220°C, along with the distinction in the orienta tion type, have the more clearly pronounced polycrys talline nature compared with the films obtained at Ts = 230–350°C. The degree of the preferential orientation

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(111) (110) Mo (141) (210), (002) (211) (151) (160), (061) (042) (250) (251) (113) (341) (133) (191)

40

60

(a)

S Sn

60 Concentration, at %

20

(131)

(101) (110) (120) (021)

Intensity, arb. units

90

2499

80

30

90

(b)

S Sn Mo

60

2θ, deg 30 Fig. 3. Typical Xray diffraction pattern of the SnS film on glass with the molybdenum sublayer.

0 0

1

2

and peak intensities also increase as the film thickness increases.

3 Depth, μm

4

5

Fig. 4. Profiles of the distributions of elements over the thickness of the SnS films obtained at Ts = 300°C on (a) pure glass and (b) glass with the molybdenum sublayer.

The preferential orientation in plane (111) is typi cal of the SnS films obtained by thermal methods [12, 13, 19, 20]. Devika et al. [20] observed the change of the preferential orientation of thermally deposited SnS films from (111) to (100) upon increasing the film thickness to more than 0.75 μm. However, in this study, the SnS (111) films were observed in a thickness range from 0.7 to 3.0 μm, while the SnS (010) films were observed in a thickness range of 0.5–8.0 μm. Thus, according to the obtained results, the key factor determining the type of the preferential orientation of the SnS films when using the hotwall method for their preparation is the substrate temperature during the deposition and the substrate material rather than the film thickness.

aration modes, which can be attributed to the orient ing effect of the molybdenum sublayer. Lattice parameters of the SnS films on pure glass and glass with the molybdenum sublayer are presented in Table 1. The divergence in the lattice parameters for various samples of the films of each type to not exceed the experimental error. The largest divergence in lat tice parameters between the films on pure glass and on glass with the molybdenum sublayer is explained by the influence of the molybdenum sublayer on the for mation of the crystal structure of the SnS films.

Figure 3 shows a typical Xray diffraction pattern of the SnS film on glass substrate with the molybdenum sublayer. Thin SnS films grown on glass with the Mo sublayer have the additional peak at 2θ = 39.91– 39.98° (Fig. 3), which corresponds to the (110) plane of the cubic Mo phase (space group Im3m, structural type A2 with the lattice parameter a = 39.9139.98 Å). The films on glass with the Mo sublayer, in contrast with the films on the substrates made of pure glass, are oriented in the (111) plane in the entire range of prep

3.2. Distribution of Elements over the Thickness The distribution of elements over the thickness was investigated by the Auger electron spectroscopy (AES). Typical profiles of the films on pure glass and on glass with the Mo sublayer are presented in Fig. 4. Our studies showed that a uniform distribution of the elements over the thickness with the closetostoichi ometric ratio of elements is typical of the SnS films. A small excess of sulfur up to 5 at % was observed for the

Table 1. Lattice parameters of the SnS phase in the films Unit cell parameters, Å Substrate

Texture a

b

c

Glass

(010)

4.300 ± 0.005

11.203 ± 0.005

3.981 ± 0.003

Glass

(111)

4.301 ± 0.005

11.225 ± 0.007

3.996 ± 0.002

Glass–Mo

(111)

4.313 ± 0.005

11.189 ± 0.003

3.985 ± 0.006

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Table 2. Thickness, resistivity, and the temperature coeffi cient of thermoelectric power for the SnS films obtained in various modes (T = 300 K) Ts, °C 220

300

350

τ, min

d, μm

ρ, Ω cm

α, μV K–1

10

0.5–0.7

12

348

30

1.2–1.5

17

480

50

3–4

33

597

10

0.7–1.0

111

70

30

2.0–2.5

102

139

50

5.0–5.5

176

227

10

2.5–3.0

817

57

30

6–7

336

37

50

10–12

618

203

films on pure glass near the surface of the SnS layer (Fig. 3a), which can be attributed to the absorption of sulfur vapors in the end of the deposition process or with the possible diffusion phenomena in the SnS lay ers during their formation due to the temperature dif ference between the substrate and SnS vapors. According to the AES data, diffusion of tin and sul fur in the Mo sublayer is observed for the films on glass with the Mo sublayer. It is shown in Fig. 4b that an abrupt decrease in the Sn and S concentrations over the depth of the Mo layer is stabilized at a level of about 3 at % Sn, 12 at % S, and 85 at % Mo, which is observed for the samples obtained at substrate temper atures lower than 300°C. The samples obtained at sub strate temperatures higher than 300°C exhibit the lin ear decrease in the Sn and S concentrations, which can be associated with thermal activation of diffusion processes in the Mo sublayers. Taking into account the results of the Xray phase analysis, we can conclude that the diffusion of S and Sn atoms in the Mo sublay ers does not lead to the formation of the phases of sep arate chemical compounds with the structures differ ing from the SnS and Mo structures.

erential orientation along the (010) and (111) planes, which agrees well with the results of the Xray analysis. Large crystalline blocks up to 1 μm in size arranged in parallel to the substrate surface are clearly distin guished in the images of the cross section of the film samples oriented in the (010) plane (Fig. 5a). In the case of the films oriented in the (111) plane, the blocks are arranged mainly diagonally or vertically in regards to the substrate (Fig. 5b). The size of crystalline blocks increases as the substrate temperature increases. A clear interface between the SnS and Mo layers is well distinguishable in the images of the cross section of the SnS films on the Mo sublayer, which agrees with the AES data. Morphology of the films obtained on glass with the Mo sublayer (Fig. 5c) is similar to the films obtained on pure glass and oriented in the (111) plane. In both cases, crystalline blocks are aggregated with each other densely, without the gaps, which is important for the fabrication of thinfilm p–n and heterojunctions. By their microstructure and growth character, the SnS films are close to the previously studied Sn1 ⎯ xPbxS films obtained in similar conditions [21]. The main distinction is in the fact that in the case of SnS without the lead addition, no spherical nanocol umns are formed on the film surface, in contrast to the Sn1 – xPbxS films. 3.4. Electrical Properties

3.3. Morphology

Table 2 represents the values of resistivity and tem perature coefficient of thermoelectric power of the SnS films on glass depending on the preparation modes. All the obtained films were of ptype conduc tion, which is characteristic of this material. The results show that the substrate temperature and the deposition time substantially affect the electrical properties of obtained films. For example, an increase in the substrate temperature during the deposition leads to an increase in resistivity of obtained films and a decrease in the temperature coefficient of thermo electric power, while an increase in the deposition time at a fixed value of Ts in most cases, with several exclusions, increases both mentioned characteristics.

Film morphology was investigated by scanning electron microscopy (SEM). The SEM results showed that the morphology and thickness of the SnS films substantially depend on the fabrication conditions. Table 2 shows the dependence of the film thickness on the substrate temperature and deposition time. It is noteworthy that an increase in the substrate tempera ture at a constant deposition time leads to an increase in the film thickness. Figure 5 shows the SEM images of the surface and cross section of the SnS films on pure glass and on glass with the Mo sublayer. Two types of film morphol ogy can be distinguished. They correspond to the pref

Figure 6 represents a typical temperature depen dence of specific conductivity of the SnS film on glass. The activation energy in a temperature range of 220– 420 K is 0.11–0.12 eV regardless the preparation modes of the films. These values of the activation energy agree well with the data for SnS crystals [22] and correspond to the hole conduction, which is prev alent for this material at temperatures up to 400– 420 K being changed for the electron conduction at higher temperatures, which is governed by bandto band transitions [22]. It is noteworthy that the activa tion energy of conductivity for ptype SnS is close by the order of magnitude to the ionization energy of

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σ, (Ω cm)−1

10–1

10–2

1 μm

(а)

10–3 2.0

2.5

3.0

3.5 4.0 103/T, K–1

4.5

5.0

Fig. 6. Typical temperature dependence of the electrical conductivity of the SnS film on glass.

The film thickness is from 0.5 to 12.0 μm depend ing on the preparation modes. These films are single phase being characterized by the closetostoichio metric distribution of elements over the thickness. 3 μm

(b)

It is established that depending on the preparation modes, the formation of the SnS films with two types of the preferential orientation and morphology are possible. The prevalent orientation in the (111) plane and vertical arrangement of crystallites are typical of the films obtained at substrate temperatures lower than 230°C, while the films obtained at higher sub strate temperatures are characterized by the orienta tion in the (010) plane and horizontal arrangement of crystallites. The preferential orientation in the (111) plane is typical of the SnS films on glass with the molybdenum sublayer over the entire range of prepa ration modes.

N54

3 μm

(с)

Fig. 5. (a, b) SEM images of the cleavages of SnS films on pure glass with orientations (010) and (111), respectively; and (c) image of the SnS film on glass with the molybde num sublayer.

2+

doubly charged tin vacancies V Sn , which are the prev alent type of defect in this material according to [23]. 4. CONCLUSIONS In this study, thin SnS films on the substrates made of pure glass and glass with the molybdenum sublayer were obtained by thermal vacuum hotwall method, and their structural and electrical properties were obtained depending on their preparation modes. PHYSICS OF THE SOLID STATE

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Specific conductivity and the temperature coeffi cient of thermoelectric power for the SnS films on glass vary in ranges from 12 to 817 Ω cm and from 37 to 597 μV/K, respectively, and depend substantially on the preparation conditions. The films with the smallest resistance and the largest temperature coefficient of thermoelectric power are obtained at lowest substrate temperatures. The values of activation energy deter mined by the temperature dependences of specific conductivity of the films are 0.11–0.12 eV and agree with the data for the crystals. Our high values of thermoelectric power allow us to speak about the possibility of formation of SnSbased effective thinfilm thermal elements. The results of this study will underlie further improvement of the technology of formation of the SnS films with the required microstructure and electrical properties; they can be also used when designing thinfilm devices based on this material. 2012

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ACKNOWLEDGMENTS This study was supported by the Belarusian Repub lican Foundation for Fundamental Research. REFERENCES 1. K. Reddy, N. Reddy, and R. Miles, Sol. Energy Mater. Sol. Cells 90, 3041 (2006) 2. M. Gunasekaran and M. Ichimura, Sol. Energy Mater. Sol. Cells 91, 774 (2007). 3. T. Chattopadhyay, J. Pannetier, and H. von Schnering, J. Phys. Chem. Solids 47, 879 (1986). 4. W. Albers and K. Schol, Philips Res. Rep. 16, 329 (1961). 5. R. Sharma and Y. Chang, Bull. Alloy Phase Diagrams 7, 269 (1986). 6. R. Kniep, D. Mootz, U. Severin, and H. Wunderlich, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 38, 2022 (1982). 7. A. Tanuševski and D. Poelman, Sol. Energy Mater. Sol. Cells 80, 297 (2003). 8. M. CalixtoRodriguez, H. Martinez, A. SanchezJua rez, J. CamposAlvarez, A. TiburcioSilver, and M. Calixto, Thin Solid Films 517, 2497 (2009). 9. T. Sajeesh, A. Warrier, C. Kartha, and K. Vijayakumar, Thin Solid Films 518, 4370 (2010). 10. F. Kanga and M. Ichimura, Thin Solid Films 519, 725 (2010). 11. M. Devika, N. Reddy, K. Ramesh, V. Ganesan, E. Gopal, and K. Reddy, Appl. Surf. Sci. 253, 1673 (2006).

12. M. Devika, N. Reddy, K. Ramesh, K. Gunasekhar, E. Gopal, and K. Reddy, Semicond. Sci. Technol. 21, 1125 (2006). 13. M. Devika, N. Koteeswara Reddy, D. Sreekantha Reddy, S. Venkatramana Reddy, K. Ramesh, E. S. R. Gopal, K. R. Gunasekhar, V. Ganesan, and Y. B. Hahn, J. Phys.: Condens. Matter. 19, 306003 (2007). 14. R. Miles, O. Ogah, G. Zoppi, and I. Forbes, Thin Solid Films 517, 4702 (2009). 15. A. LopezOtero, Thin Solid Films 49, 3 (1978). 16. S. A. Bashkirov, V. F. Gremenok, and V. A. Ivanov, Semiconductors 45 (6), 749 (2011). 17. V. F. Gremenok, V. Yu. Rud’, Yu. V. Rud’, S. A. Bash kirov, and V. A. Ivanov, Semiconductors 45 (8), 1053 (2011). 18. R. Colin and J. Drowart, J. Chem. Phys. 37, 1120, (1962). 19. M. Devika, K. T. R. Reddy, N. K. Reddy, K. Ramesh, R. Ganesan, E. S. R. Gopal, and K. R. Gunasekhar, J. Appl. Phys. 100, 023518 (2006). 20. M. Devika, N. Koteeswara Reddy, K. Ramesh, K. R. Gunasekhar, E. S. R. Gopal, and K. T. Rama krishna Reddy, J. Electrochem. Soc. 154, H67 (2007). 21. V. V. Lazenka, K. Bente, R. Kaden, V. A. Ivanov, and V. F. Gremenok, J. Adv. Microsc. Res. 6, 53 (2011). 22. J. S. Anderson and M. C. Mortan, Nature (London) 155, 112 (1945). 23. H. Rau, J. Phys. Chem. Solids 27, 761 (1966).

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Translated by N. Korovin

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