ISSN 10274510, Journal of Surface Investigation. Xray, Synchrotron and Neutron Techniques, 2014, Vol. 8, No. 4, pp. 659–665. © Pleiades Publishing, Ltd., 2014. Original Russian Text © V.F. Markov, S.S. Tulenin, L.N. Maskaeva, M.V. Kuznetsov, 2014, published in Poverkhnost’. Rentgenovskie, Sinkhrotronnye i Neitronnye Issledovaniya, 2014, No. 7, pp. 42–48.
Structure and Composition of Chemically Deposited In2S3 Thin Films V. F. Markova, b, S. S. Tulenina, L. N. Maskaevaa, b, and M. V. Kuznetsovc a
b
Ural Federal University, Yekaterinburg, 620002 Russia Ural Institute of State FireFighting Service, Yekaterinburg GSP145, 620002 Russia email:
[email protected];
[email protected] c Institute of SolidState Chemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620990 Russia Received November 15, 2013
Abstract—Nanostructured thin indium(III) sulfide thin films 285–756 nm thick are obtained via chemical deposition from aqueous solutions containing indium chloride, thioacetamide, tartaric acid, and hydroxy lamine hydrochloride at temperatures of 343–368 K. Oxygen and carboncontaining impurities, which are not observed in the film bulk (at a depth of 12 nm), are detected in the surface layers of the films. When the synthesis temperature increases, the layer morphology changes substantially and the crystallite size increases from 70 to 150 nm. Upon annealing at a temperature of 573 K, crystallite aggregates are fused and In2S3 films are enriched with 6 to 10 at % of oxygen. DOI: 10.1134/S1027451014040120
INTRODUCTION Indium(III) chalcogenides have found wide appli cation in opto and microelectronics and solar power engineering as materials with unique properties. Since the band gap (2.03 eV) of indium(III) sulfide (In2S3) [1] is almost coincident with that of cadmium sulfide, this compound is a close analog of CdS used as the buffer layer of thinfilm solar cells. At the same time, In2S3 is the basis of an ecologically conscious material used in helioenergetics, namely, copper and indium disulfide (CuInS2), the efficiency of which is 13% [2]. In2S3 thin films are prepared using different meth ods: spraying of a solution of indium(III) salt thiourea complex with subsequent pyrolysis on a heated sub strate [3], metal layer sulfidization in hydrogen sulfide (H2S) [4], chemical deposition from the vapor phase, and layerbylayer atomic epitaxy [5]. However, the aforementioned synthesis methods have a number of substantial disadvantages, among which are the need to employ vacuum and high temperatures (the melting temperature of In2S3 is 1363 K), the complexity of controlling the process of deposition and condition selection, and, in some cases, the multistage character of the process. The chemical deposition of In2S3 from aqueous solutions, devoid of the disadvantages listed above, is of significant interest [6–8]. Together with simple hardware implementation and affordability, this method enables us to create layers at temperatures of less than 373 K. It is known that the hydrochemical deposition of indium(III) sulfide films has been per formed in only a few studies, e.g., [9, 10], where In2S3
was synthesized in a subacid medium (pH 2.3–4.0) with the use of indium(III) sulfide and thioacetamide (CH3CSNH2). However, in these publications, the reaction mixture composition was not substantiated and issues concerning the interdependence between the conditions of preparation and the film composi tion were not discussed. The goal of this work is to examine the influence of chemical deposition from aqueous solutions on the com position, morphology, and structure of In2S3 films. EXPERIMENTAL AND INVESTIGATION METHODS In2S3 films were deposited on preliminarily degreased ST150 glassceramic substrates in glass reactors under thermostatically controlled conditions. Deposition was carried out from an aqueous solution containing analytically pure indium chloride (InCl3), analytically pure CH3CSNH2, tartaric acid (C4H6О6), and hydroxylamine hydrochloride (NH2OH · HCl) in the temperature range of 343–368 K. Thereafter, the synthesized In2S3 films were annealed in air at 573 K. The layer thickness was estimated using an MII4M microinterferometer. Crystalline structures and phase compositions were investigated at room temperature using a Shimadzu XRD7000 Xray diffractometer with CuKα1, 2 radia tion, angle 2θ varying from 20° to 65°, a step of Δ(2θ) = 0.03°, and an exposure of 20 s at the point. Microstructures, chemical compositions, and grain sizes were examined via scanning electron microscopy (SEM) by means of a Mira 3 LMU microscope
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MARKOV et al. (a) 1
Point
In, at %
S, at %
1
43.13
56.87
2
41.92
58.08
3
41.06
58.94
4
42.21
57.79
5 6
39.82 42.98
60.18 57.02
7
41.61
58.39
Mean
41.82
58.18
2 3 5 6
4
7
6 μm (b) S
Intensity, rel. units
C O In
Cr
In Si
0
0.5
1.0
1.5
S
2.0
2.5
Cr
3.0
3.5 E, eV
4.0
4.5
5.0
5.5
Cr
6.0
6.5
7.0
Fig. 1. (a) SEM image of a freshly deposited In2S3 film obtained at 368 K and (b) EDX spectrum with the characteristic lines of elements at point 2.
equipped with a JED 2300 energydispersive Xray (EDX) analyzer. The chemical states of the elements were analyzed via Xray photoelectron spectroscopy by means of a VG Scientific ESCALAB MK II electron spectrometer with a magnesium cathode employed as a source of nonmonochromatic MgKα Xray radiation (1253.6 eV). In the case of interpretation of element chemical bonds according to a shift in the bands of the X ray photoelectron spectra, the carbon 1s line with a bind ing energy of 284.5 eV was used as the calibration source. RESULTS AND DISCUSSION In [11, 12], the concentration conditions of In2S3 formation were determined during hydrochemical deposition in Trilon B and sodium hydroxide with the use of thiocarbamide (CSN2H4). However, in this case, In2S3 chemical deposition became complicated due to the generation of In(OH)3, which impeded the creation of In(OH)3 impurityfree films with good adhesion to the substrate. In this study, to surmount these difficulties, thioacetamide was chosen as a chal
cogenizer because, in contrast to thiocarbamide, this substance enables metalsulfide deposition in an acid medium. During deposition by thioacetamide, indium(III) sulfide can be created according to the chemical reaction 2In3+ + 3CH3CSNH2 + 6H2O 3CH3COONH4 + In2S3 + 6H+. The experiments performed demonstrated that highly adhesive In2S3 layers can be reliably produced on glassceramic substrates with the help of thioaceta mide. In this case, the reaction mixture was addition ally enriched with tartaric acid which served as a com plexgenerating agent. The In2S3 film thickness varied from 285 to 756 nm. When the temperature increased to between 343 and 368 K, the thickness increased steadily and layer color changed from pale yellow to reddish orange. Figure 1a presents the SEM image of an In2S3 film surface with the enumerated regions showing the local elemental analysis. The indium, sulfur, and oxygen
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1 μm
(a)
(d)
1 μm
(b)
1 μm
(e)
661
1 μm
(c)
1 μm
Fig. 2. SEM images of (a–d) freshly deposited and (e) heattreated (at 573 K) In2S3 films obtained on glassceramics at different synthesis temperatures: (a) 343, (b, e) 353, (c) 363, and (d) 368 T, K.
contents of the samples were analyzed. The analysis data indicate that both individual globules and the entire film surface are comprised of predominantly indium and sulfur whose contents averaged over sev eral measurements are 37.95 and 53.98 at %, respec tively. The average oxygen concentration fluctuates near 8 at %. Their ratio is 2 : 3 with an insignificant excess of indium (if recalculation is performed without regard to oxygen), which agrees closely with published data on the In2S3 composition [13, 14]. In the pre pared film, the deviations from the average indium and sulfur concentrations are less than 2–3 at %. In the energy spectrum shown in Fig. 1b, the characteristic lines of chromium and oxygen are probably caused by the influence of the glassceramic substrate, and the characteristic lines of carbon are associated with the residues of thioacetamide decomposition prod ucts.Electron microscopic investigations of the freshly deposited In2S3 films revealed that the layers have a pronounced finecrystalline structure with average crystallite sizes of 70–120 nm at synthesis tempera tures of 343–353 K (Figs. 2a, 2b). In addition, there are larger aggregates with the same composition on the film surface, which include several tens of microcrys tals. Their size reaches 800 nm. Analogous results were obtained in [11, 15], but the pH value of the reaction mixture was somewhat higher. The subsequent increase in synthesis temperature gives rise to a change
in morphology [16] and leads to emergence of the reticular fractal structure of the film (Figs. 2c, 2d). At a temperature of 368 K, spatial filament aggregates containing several crystallites with average sizes of 90– 150 nm are formed. Figure 2e presents a SEM image of the film depos ited at 353 K and, afterward, heat treated in air at a temperature of 573 K, followed by slow cooling in a furnace for 12 h. It is distinctly seen that its micro structure underwent appreciable changes: crystallites are partially fused. In compliance with data on energy dispersive analysis of the elemental compositions, the film contains up to 6–10 at % of oxygen as a result of oxidation. In this case, the ratio between the concen trations of the basic elements remained unchanged. From the performed Xray studies of the synthe sized In2S3 films, it was found that they possess cubic structure, as is predicted by published data [17, 18]. This is confirmed by the 311, 400, 422, and 440 dif fraction reflections observed in the typical Xray dif fraction pattern of an In2S3 film obtained on a glass ceramic substrate at 353 K (its thickness is 700 nm), as is shown in Fig. 3a. For comparison, the Xray diffrac tion pattern of the substrate is depicted in Fig. 3b. In accordance with the aforementioned analysis, the lat tice constant was 10.734 Å and the spatial group of
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MARKOV et al. a = 10.734 Å
311 (3.237)
In2S3 Glassceramics
440 (1.897)
Intensity, rel. units
400 (2.681) 422 (2.189)
(а)
(b)
20
25
30
35
40 45 2θ, deg
50
55
60
65
Fig. 3. Xray diffraction patterns of (a) the In2S3 film synthesized at 353 K and (b) the glassceramic substrate.
indium sulfide was defined as I41/amd – D419h . It should be noted that the Xray diffraction pattern of the film has no lines with the smallest crystallographic indices due to the disordered structure of the compound and the intense background of the substrate material.
data are summarized in Table 1. The basic elements of the layers are indium and sulfur. As is evident from analysis data, the ratio of their gross concentrations is approximately unity. In the surface layers of all films, the oxygen content is less than 8.35 at %, coinciding well with the EDX analysis data discussed above. It is likely that the appearance of oxygen is associated with carboncontaining compounds that remained on the surface. Film etching with an Ar+ beam made it possi ble to establish that the oxygen and carbon concentra tions approach zero at a depth of 12 nm (Figs. 2a, 2b; Table 1). This is evidence of the relative purity of the obtained layers.
To determine the elemental composition and con firm the Xray phase analysis data, the In2S3 films were investigated via Xray photoelectron spectroscopy. Their survey Xray photoelectron spectra, separate spectral regions with In4d, In3d, S2p, O1s, and C1s lines, and Auger lines of indium were recorded (Fig. 4). Spectral processing carried out with allow ance for photoionization cross sections made it possi ble to calculate the concentrations of the basic ele ments (indium, sulfur, oxygen and carbon). For layers deposited under different conditions, the calculated
The oxygen 1s and carbon 1s spectra have two com ponents among which the component with the smaller binding energy is the dominant (Table 2). This implies
Table 1. Contents of basic elements (in atomic percents) and their ratio in freshly deposited In2S3 films at different con centrations of indium(III) salt in the solution and various synthesis durations Sample
In
S
O
C
O/In
In/S
[InCl3], mol/L
Synthesis duration, h
T, K
56_I 56_I (at a depth of 12 nm) 56_8 56_III
31.88 53.72 33.54 30.99
38.77 46.28 37.88 36.19
7.06 0.00 6.14 8.35
22.30 0.00 22.43 24.47
0.22 0.00 0.18 0.27
0.82 1.16 0.90 0.86
0.02 0.02 0.04 0.04
2.0 2.0 2.0 2.5
353 353 353 353
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663
(d)
(c)
С1s
56_I
520 524 528 532 536 540 Binding energy, eV
275
280 285 290 Binding energy, eV
(a)
295
(b)
0
200 In4d
400 (e)
600 Binding energy, eV
800 (f)
S2p
1000 In3d5/2
(g) In3d3/2
56_I
0
20 40 60 80 Binding energy, eV
56_I
56_I
100 152 156 160 164 168 172 176 440 444 448 452 456 460 Binding energy, eV Binding energy, eV
Fig. 4. Survey Xray photoelectron spectra of the synthesized In2S3 films, which were recorded (a) before and (b) after etching at binding energies of up to 1000 eV and have separated regions of (c) 276–296 (C1s shell), (d) 520–540 (O1s shell), (e) 0–100 (In4d shell), (f) 152–176 (S2p shell), and (g) 440–460 eV (In3d shell).
that these elements are part of two different com pounds. It seems that the second energy state with the higher binding energy of carbon can be attributed to organic compounds which remain after synthesis and the first state (with the lower energy), to carbonates. Oxygen can be included in sulfides, carbonates, and oxides or hydrocompounds generated on the sample surfaces as a result of oxidation during synthesis. From analysis of the Xray photoelectron spectra, it was ascertained that the deposited layers were
strongly charged (up to 6.4 eV) and have high ohmic resistance. The typical survey spectra of the samples (after deduction of their charge energies), which were measured before and after etching with an argon beam, are depicted in Figs. 4a and 4b. It is clearly seen that, after surface etching, the spectra have no C1s and O1s peaks of the energy shells of carbon and oxygen atoms (Table 1). The typical C1slevel spectrum is pre sented in Fig. 4c (the energy of the basic component is 284.5 eV). The energy of the second component is 285.75 eV and, probably, can be attributed to carbon
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Table 2. Contents of oxygen and carbon (in atomic percent) with different energy states and the ratio of their concentra tions in In2S3 films O1s(I)
O1s(II)
O1s(I)/O1s(II)
C1s(I)
C1s(II)
C1s(I)/C1s(II)
56_I
4.25
1.01
4.23
11.15
5.48
2.04
56_8
3.17
1.38
2.30
11.31
5.30
2.13
56_III
5.40
0.73
7.37
11.94
6.02
1.98
Sample
Table 3. Energies of the characteristic lines of different elements in freshly deposited indium(III) sulfide films with allow ance for the accumulated charge (in electronvolts) Sample
In4d
S2p
In3d
O1s(I)
O1s(II)
C1s(I)
C1s(II)
56_I 56_I (at a depth of 12 nm) 56_8 56_III
17.9 17.7 18.1 18.0
161.4 161.5 161.6 161.5
444.9 444.8 445.0 445.0
531.5 – 531.8 532.0
532.5 – 533.4 533.6
284.5 284.5 284.5 284.5
285.7 – 285.7 285.9
entering into the composition of the organic com pounds [19, 20]. Figure 4d depicts the typical oxygen 1s level spec trum in which two components with different binding energies can be distinguished (Table 3). The high energy component can refer to oxygen entering into carbonates, sulfides, and organic compounds [21, 22]. The lowenergy component can be coupled with indium hydroxide or its oxide (In2O3). In the latter case, the binding energy of the O1s level is about 530.4 eV. The indium 4d energy shell has a typical spectral range of 0–90 eV, as is shown in Fig. 4e. There is a clear and well detectable peak. This indium energy level exhibits insignificant splitting (0.85 eV). The level energy is about 18 eV (Table 3) and decreases to 17.66 eV after sample etching. The typical sulfur 2p level spectrum, which also has two components (Table 3), is presented in Fig. 4f. The nature of the lowenergy component (about 161.4 eV) is ambiguous because it is relatively far away from the S2p level with a binding energy of 162. 2 (in our case, 162.4) eV (this value is inherent to indium trisulfide). The In3d shell (Fig. 4g) indicates that indium is in one energy state (Table 3). The energy of the detectable In3d level is close to the theoretical value corresponding to indium trisulfide. This enables us to infer that the Table 4. Energies of indium Auger lines and its α parameter for different indium(III) sulfide films Sample 56_I 56_8 56_III
In Auger line, eV
αparameter, eV
846.18 846.33 846.43
852.29 852.22 852.12
obtained compound is indium sulfide (with a con taminated and oxidated surface). To implement its more reliable identification com pared with indium oxides, the Auger lines of this metal was recorded (Table 4). It is known that the socalled α parameter or the Wagner Auger parameter [23] is used to identify compounds with close atomicshell energies. Its value can be calculated from the formula [24] Eα(X) = EMgK + EX – EAuger, where Eα(X) is the α parameter (in electronvolts), EMgK is the radiation energy of a magnesium cathode (1253.6 eV), EX is the characteristic line energy, and EAuger is the Auger line energy. In accordance with reference and published data, the α parameters are 852.0, 850.8, and 851.4 eV for In2S3, In2O3, and InOх, respectively [14]. The α parameters calculated during the course of our study are considerably closer to indium trisulfide than to its oxides. CONCLUSIONS Indium(III) sulfide films 285–756 thick are syn thesized on glassceramic substrates by means of hydrochemical deposition from a reaction mixture containing indium(III) chloride, thioacetamide, tar taric acid, and hydroxylamine hydrochloride. Their composition corresponding to the formula In2S3 is identified via energydispersive analysis, Xray dif fractometry, and Xray photoelectron spectroscopy of the deposited layers. It is revealed that they belong to the cubic system. Oxygen and carboncontaining impurity compounds are included into the film surface layers, penetrating to a depth of 12 nm. Changes in the morphology of the deposited layers and an increase of crystallite sizes (from 70 to 150 nm) are observed at
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synthesis temperatures of 343–368 K. It is found that microcrystals are fused after heat treatment of the films in air at 573 K. ACKNOWLEDGMENTS We thank senior researcher A.N. Ermakov, Insti tute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences, for the Xray phase analysis and research worker A.A. Pankratov, Institute of High Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences, for assistance in the electron microscopic investigations. This study was supported by the Russian Founda tion for Basic Research, project no. 140300121. REFERENCES 1. A. V. Novoselova, Physicochemical Properties of Semi conductor Substances (Nauka, Moscow, 1979) [in Rus sian]. 2. S. Fiechter, Phys. Status Solidi B 245, 1761 (2008). 3. T. T. John, C. S. Kartha, K. P. Vijayakumar, et al., Appl. Phys. A 82, 703 (2006). 4. R. Yoosuf and M. K. Jayaraj, Solar Energy Mater. Solar Cells 89, 85 (2005). 5. J. Sterner, J. Malmstrom, and L. Stolt, Prog. Photovolt. Res. Appl. 13, 179 (2005). 6. V. F. Markov, L. N. Maskaeva, and P. N. Ivanov, Hydro chemical Deposition of Metal Sulfide Films: Modeling and Experiment (Yekaterinburg) [in Russian]. 7. L. N. Maskaeva, V. F. Markov, and A. I. Gusev, Poverkh nost’, No. 2, 100 (2004). 8. V. V. Safronov, V. I. Strelov, N. V. Krivonogova, et al., J. Surf. Invest.: XRay, Synchrotron Neutron Tech. 6, 985 (2012). 9. S. S. Kale, R. S. Mane, C. D. Lokhande, et al., Mater. Sci. Eng. B 133, 222 (2006).
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10. Satoshi Aoki, FRG Patent No. EP2216824 B1 (Tokyo, 2012). 11. S. S. Tulenin, V. F. Markov, and L. N. Maskaeva, Butler. Soobshch. 29 (3), 79 (2012). 12. S. S. Tulenin, V. F. Markov, L. N. Maskaeva, et al., Butler. Soobshch. 33 (1), 97 (2013). 13. S. S. Korovin, D. V. Drobot, and P. I. Fedorov, Rare and Trace Elements. Chemistry and Technology (MISIS, Moscow, 1999) [in Russian]. 14. I. V. Bondar’, V. A. Polubok, V. Yu. Rud’, et al., Semi conductors 37, 1308 (2003). 15. C. D. Lokhande, A. Ennaoui, P. S. Patil, et al., Thin Solid Films 340, 18 (1999). 16. V. A. Vasil’ev and P. S. Chernov, J. Surf. Invest.: XRay, Synchrotron Neutron Tech. 7, 565 (2013). 17. B. Yahmadi, N. Kamoun, R. Bennaceur, et al., Thin Solid Films 473, 201 (2005). 18. I. V. Bondar’ and V. V. Shatalova, Semiconductors 46, 1122 (2012). 19. K. Otto, A. Katerski, A. Mere, et al., Thin Solid Films 519, 3055 (2011). 20. Yujie Xiong, Yi Xie, Guoan Du, et al., J. Solid State Chem. 166, 336 (2002). 21. K. Yamaguchi, T. Yoshida, and H. Minoura, Thin Solid Films 431–432, 354 (2003). 22. P. N. Krylov, E. A. Romanov, I. V. Fedotova, et al., J. Surf. Invest.: XRay, Synchrotron Neutron Tech. 7, 458 (2013). 23. V. V. Bolotov, E. V. Knyazev, V. S. Kovivchak, et al., J. Surf. Invest.: XRay, Synchrotron Neutron Tech. 7, 62 (2013). 24. Practical Surface Analysis by Auger and Xray Photo electron Spectroscopy, Ed. by D. Briggs and M. Seah (Wiley, New York, 1983).
Translated by S. Rodikov
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