Growth of Crystalline AgIn5S8 Thin Films on Glass Substrates from

Nov 15, 2007 - ... National Chung Cheng University, 168 University Road, Min-Hsiung, Chia-Yi .... Wen-Sheng Chang , Ching-Chen Wu , Jing-Chie Lin , Tz...
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Growth of Crystalline AgIn5S8 Thin Films on Glass Substrates from Aqueous Solutions Li-Hau Lin, Ching-Chen Wu, and Tai-Chou Lee* Department of Chemical Engineering, National Chung Cheng UniVersity, 168 UniVersity Road, Min-Hsiung, Chia-Yi 621, Taiwan

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2725–2732

ReceiVed December 21, 2006; ReVised Manuscript ReceiVed September 3, 2007

ABSTRACT: Thin films of the silver indium sulfide (AgIn5S8) ternary semiconductor were prepared from acidic aqueous solutions containing silver nitrate, indium nitrate, and thioacetamide. Various preparative parameters, such as pH of the precursor solution, silver to indium concentration ratio [Ag]/[In], and postreaction thermal treatment conditions were changed in order to grow uniform and adherent thin films on glass substrates. A series of X-ray diffraction patterns and scanning electron micrographs were used to reveal the growth process over time. It was found that granular Ag2S primary films were first attached to the glass substrate, followed by the indium sulfide deposition. A (1 1 1) preferred oriented AgIn5S8 with cubic spinel structure was obtained from the [Ag]/[In] ) 4 and pH 0.6 precursor solution after 673 K thermal treatment for 1 h in an Ar environment. A two-step deposition mechanism was proposed and discussed in terms of stability constants of metal complexes and classical nucleation theory. In addition, our preliminary study showed that 3-mercaptopropyl-trimethoxysilane (MPS)-modified glass substrates further promoted the homogeneity and adhesion of AgIn5S8 thin films. Introduction In recent years, the I-III-VI ternary semiconductors with chalcopyrite structure have been attracting considerable research and production interests. The band gap energy, Eg, lies between 0.8 and 2 eV, which is suitable for an absorber in thin film solar cells.1 Notably, the efficiency of a photovoltaic device comprising a heterojunction of Cu(In, Ga)Se2 (CIGS) and CdS goes up to 19.3%.2,3 One phase of the ternary I-III-VI semiconductor is an In-rich compound with the general formula of I-III5-VI8, having cubic spinel structure. AgIn5S8 has direct energy band gaps of 1.80 eV at 300 K and 1.90 eV at 96 K and is considered one of the potential candidates for photovoltaic and optical applications.4–9 However, there are not many reports of the thin film formation of AgIn5S8 in the literature. This type of film can be prepared by the sulfurization of an In-rich metallic precursor8,9 or spray pyrolysis of silver ion, indium ion, and thiourea with various silver to indium molar ratios.10,11 However, AgIn5S8 was found to be present as the secondary phase in the sprayed films. Single source precursors can also be utilized to grow AgIn5S8 by chemical vapor deposition.5–7 More interestingly, O’Brien et al.5,6 and Banger et al.7 showed that the spinel phase of silver indium sulfide (AgIn5S8) thin films was obtained from the single source precursor Ag/In with a stoichiometric ratio of 1:1, as opposed to the formation of AgInS2 residue from pyrolysis of the same precursor. The equilibrium between AgInS2 and AgIn5S8 is not clear. In order to obtain large area and uniform I-III-VI semiconductor thin films on glass substrates with a cost-efficient method, a chemical route is particularly attractive. In addition, the deposition process can be carried out at low temperatures. In addition to the aforementioned chemical methods, chemical bath deposition (CBD) is a relatively simple and convenient way to prepare a wide range of metal sulfide thin films.12–24 CBD is based on the controlled hydrolysis of a sulfur precursor into an alkaline or acidic solution in the presence of metal cations complexed with suitable chelating agents, leading to the precipitation of metal sulfide onto substrates. While the mechanism and film properties of binary metal sulfide * To whom correspondence should be addressed. Telephone: +886-52720411-33409. Fax: +886-5-2721206. E-mail: [email protected].

have been extensively studied, ternary systems have not. It has been shown that preparations of ternary metal chalcogenide thin films from CBD18,20,22 or modified-CBD21 are possible, but the detailed growth processes have not been revealed for ternary metal sulfides. In this paper, we report the deposition of polycrystalline AgIn5S8 on clean and 3-mercaptopropyl-trimethoxysilane (MPS)modified glass substrates by a CBD process. The main focus of this work is to elucidate the nucleation on the substrates with two different surface properties and the evolution of the growth process. Apblett et al. demonstrated the ion-by-ion CdS growth mechanism by direct atomic force microscopy (AFM) and scanning electron microscopy (SEM) observation.15 Karabelas et al. showed that for the CdS thin film formation, the theoretical model of constant surface nucleation appeared to be closer to SEM observations.19 A paper by Hoffmann et al.25 illustrated the deposition mechanism of titania/vanadium composite oxide films by detailed investigations of the reaction solution by UV–vis spectroscopy and dynamic light scattering, in addition to the SEM micrographs of thin films. We follow a similar rationale and study the formation process of ternary AgIn5S8 thin films. By using SEM, X-ray diffraction (XRD), and UV–vis spectroscopy, we investigate the nucleation and growth process, as well as the effects of the pH value of the reaction solution, heat treatment temperatures, and silver to indium ratio, [Ag]/ [In], on the crystal structure and morphology of the films. Experimental Section Materials. Analytical grade silver nitrate (AgNO3), indium nitrate (In(NO3)3 · 5H2O), thioacetamide (CH3CSNH2, TAA), trisodium citrate (C6H8O7Na3 · 2H2O), citric acid (C6H11O7), triethanolamine ((HOC2H4)3N, TEA), sulfuric acid (H2SO4), and ammonium nitrate (NH4NO3) were purchased and used without further purification. Aqueous cationic and anionic solutions were prepared separately before deposition. Glass substrates were cut into slides (1 cm × 2 cm) and used immediately after cleaning to prevent contamination on the surface. First, the glass slides were soaked in piranha solution (H2O2/H2SO4 ) 3:7) for 30 min and then rinsed thoroughly with deionized water. Substrates were then immersed in an ultrasonic bath in acetone, deionized water, and subsequently methanol for 30 min each, followed

10.1021/cg060929g CCC: $37.00  2007 American Chemical Society Published on Web 11/15/2007

2726 Crystal Growth & Design, Vol. 7, No. 12, 2007 by being thoroughly rinsed with deionized water and blown with ultrapure nitrogen. Finally, the substrates were dried in an oven. Surface Modification. The 3-mercaptopropyl-trimethoxysilane (MPS) self-assembled monolayer (SAM) was generated on the precleaned glass substrate. A detailed preparation procedure can be found in another report,26 and a brief summary is provided here. The glass substrates were soaked in the piranha solution for 10 min. Then they were rinsed thoroughly with distilled water, and boiled in a solution of 30% H2O2/ NH4OH/DI water, 1:1:5, at 353 K for 30 min. The cleaned glass slides were rinsed with DI water and refluxed in a solution of 5g of 3-mercaptopropyl-trimethoxysilane in 30 mL of 25% isopropyl alcohol at 363 K for 10 min. Finally, the SAM-coated glass substrates were removed from the solution, rinsed with DI water, and dried with a stream of nitrogen. Deposition of AgIn5S8 Thin Films. The AgIn5S8 thin films were prepared by chemical bath deposition. In this study, the pH value of the reaction solution and silver to indium ratio [Ag]/[In] were varied. Specifically, the chemical bath contained 0.78 mL of 7.4 M TEA and 2.88 mL of 0.5 M trisodium citrate mixed with 1 mL of 0.4 M AgNO3 and 2.5 mL of 0.4 M NH4NO3. Various amounts of In(NO3)3 · 5H2O and citric acid were added to the solution while keeping the concentration of silver the same. TEA and citric acid were used as chelating agents for silver and indium, respectively. The acidity of the chemical bath (pH 0.6-3.2) was further adjusted by adding concentrated sulfuric acid. The cation solution was stirred for 30 min, and finally thioacetamide (0.154 M) was added. Precleaned glass substrates were placed vertically into the chemical bath with a separation between each slide of 1 mm. The bath was put on a hot plate with magnetic stirring. The deposition was carried out at 353 K and kept at this temperature throughout. The as-deposited films were thoroughly rinsed with DI water, followed by being blown dry with a stream of nitrogen. Postdeposition thermal treatment was performed in a tube furnace purged with Ar at various annealing temperatures for 1 h right after deposition. The annealed films were then sonicated in DI water for 30 min. Characterization. The crystallographic study was conducted on a Rigaku MiniFlex X-ray diffractometer using Cu KR1 radiation in the 2θ range from 20° to 70°. Scan rate was set at 2°/min in order to increase signal-to-noise ratio. Grazing incidence X-ray diffraction (GIXRD) was done on a Rigaku D/Max 2500 equipped with an 18 kW rotating anode X-ray generator. The incidence beam was at 6° to the sample surface. The system includes focusing and parallel-beam optics, which provides alignment of the sample to the X-ray beams. The microstructure of the samples was studied by using a Hitachi S4800-I field-effect scanning electron microscope (FE-SEM). EDX attached to FE-SEM was employed to analyze the composition of the thin films. Optical transmission spectra were recorded by a Shimadzu UV-2450 UV–vis spectrometer in the wavelength range from 300 to 900 nm.

Results and Discussion Thin Film Deposition on Clean Glass Substrates. The nucleation and growth of I-III-VI ternary compounds are affected by the degree of supersaturation in the chemical bath. In this study, pH of the solution was changed to tune the growth rate. One set of parameters was used to observe the development of the films. Silver to indium concentration ratio by mole, [Ag]/ [In], and annealing temperature were utilized to study the effects on the crystal structures of the film deposited on substrates. The solution was clear and transparent once prepared. This chemical bath showed a change in appearance during the progress of the experiment. Research findings showed a period of time in the beginning during which no film was deposited on the substrate, which is often referred as the incubation time. It was observed that, in our experimental conditions, there was a two-step growth process for AgIn5S8 thin film deposition. At pH 1.6 bath solution and [Ag]/[In] ) 1, during the first incubation time (10 min from the beginning), the color of the solution gradually turned to dark brown, which is close to the color of silver sulfide (Ag2S). There was no apparent film growth on the substrate at this stage. The solution remained dark in color without color change for 20

Lin et al. Table 1. Effects of pH of the Solution on the Deposition Process first primary film second secondary film termination of pH incubationa growth period incubationa growth period the reactiona 3.2 1.6 0.6 a

10 min 10 min 10 min

50 min 20 min 20 min

14 h 90 min 50 min

1h 30 min 10 min

15 h 2h 1h

The time is counted from the start of the reaction.

Figure 1. XRD patterns of ternary Ag-In-S films deposited by chemical bath deposition from [Ag]/[In] ) 1 and pH 1.6 precursor: deposited for (A) 1 h, (B) 1.5 h, or (C) 2 h, and (D) precipitations on the bottom of the reactor.

min, and film began to grow during this period. But when the stirrer was turned off after 30 min from the beginning, particles then started to precipitate to the bottom of the vial and the solution became clear. After the second incubation (90 min from the beginning), the solution became orange. This color is similar to that of indium sulfide. Finally, the solution became clear after two hours, and then the reaction was terminated. The pH values were varied in order to see the changes in the two incubation times. The results are listed in Table 1. The total reaction times were 15, 2, and 1 h for pH 3.2, 1.6, and 0.6 chemical baths, respectively. Note that the first incubation time did not change with respect to pH values. Films deposited from the pH 3.2 chemical bath peeled off rather easily by rinsing with deionized water. On the other hand, adherent films were obtained from pH 1.6 and 0.6 solutions. These two films were sonicated in DI water for 30 min and no degradations were observed. We denote thin film generated after the first incubation as the primary film and after the second incubation as the final film. In order to investigate the crystal structure of the powder, primary film, and final thin film, reactions were stopped at 1, 1.5, and 2 h. Glass substrates of each reaction time as well as powders in the solution of the last batch were collected, annealed at 673 K in an Ar environment, and analyzed with a X-ray diffractometer. Figure 1 shows the XRD patterns of samples prepared in a pH 1.6 chemical bath with [Ag]/[In] )1. Curves A, B, C, and D are samples dipped in the solution for 1, 1.5, and 2 h and powders collected from the bottom of the reaction cell, respectively. The peaks were identified by matching the positions to the MDI Jade 5 database attached to the X-ray diffractometer. The crystal planes of various compounds are marked on the figure. It is seen that curves A (primary film) and B (transient) did not show much crystallinity, and their peaks can hardly be recognized. However, curve C (final film) shows the AgIn5S8 cubic spinel phase and is polycrystalline in nature. In addition, powders collected from the solution phase consist of mainly

Chemical Bath Deposition of AgIn5S8 Thin Films

Figure 2. Grazing incidence X-ray diffraction (GIXRD) pattern of primary films deposited for 1 h from [Ag]/[In] ) 1 and pH 1.6 chemical bath.

Figure 3. Raman spectrum of primary films deposited for 1 h from [Ag]/[In] ) 1 and pH 1.6 chemical bath.

AgInS2 and metallic silver. Ag2S presents as the secondary phase. These observations indicate that silver ions are less stable in solution under our experimental conditions and react with other species to form various compounds more easily, which follows the homogeneous precipitation mechanism.13 The primary film was further examined in detail by grazing incidence X-ray diffraction and Raman spectroscopy, as shown in Figures 2 and 3, respectively. GIXRD pattern is evidence of the presence of crystalline Ag2S primary film on the glass substrate. The broad peak centered around 220 cm-1 in Figure 3 determined by Raman spectroscopy also indicates that Ag2S was deposited after the first incubation.27 Scanning electron micrographs are helpful in elucidating the growth of AgIn5S8 crystals on the glass substrate. In this experiment, a pH 0.6 chemical bath with [Ag]/[In] ) 1 was used to deposit AgIn5S8 thin films, and the total reaction time was reduced to 1 h. Growth evolution was visualized by taking SEM images of the samples removed from the solution bath every 10 min starting from 20 min after the onset of the reaction until it was terminated, without any postreaction heat treatment. During the first incubation (10 min), no apparent film was deposited on the glass substrate. Figure 4a shows the morphology of the primary film, which was deposited for 20 min Aggregates of ca. 100 nm in diameter were deposited on the glass substrate. It exhibits lower nucleation density and larger agglomerates compared with thin films generated on MPS-

Crystal Growth & Design, Vol. 7, No. 12, 2007 2727

modified glass substrate, which will be discussed in the next section. During the course of deposition, particles start to grow gradually (200 nm) during the primary growth period (30 min, Figure 4b), interconnecting structures appear between particles (40 min, Figure 4c), and then the film grows and becomes smooth during the second growth period (50 min, Figure 4d). Finally, uniform fine needle-like structures cover the entire surface (60 min, Figure 4e). Note that the surface morphology of the as-deposited final film is very similar to that of In2S3 reported in the literature.24 EDX analysis shows that the composition ratio of the primary film is [Ag]/[In]/[S] ) 1:0:0.54, nearly corresponding to the stoichiometric ratio of Ag2S. This finding again agrees with XRD analysis. The effects of postreaction thermal treatment were next investigated for samples prepared in pH 0.6 solution with [Ag]/ [In] ) 1. The as-deposited films were placed in the tube furnace and purged with Ar in order to eliminate the formation of oxides during the thermal treatment. Figure 5 shows the XRD patterns of the as-deposited sample (curve A) and samples annealed at 473 K (curve B), 573 K (curve C), and 673 K (curve D). All samples exhibit the polycrystalline nature of the AgIn5S8 cubic spinel structure. The crystallinity increases with increasing annealing temperature. It was found that when the annealing temperature exceeded 773 K, diffraction peaks of AgIn5S8 decreased. On the other hand, the In2O3 crystal phase appeared due to high-temperature transformation.28 EDX analysis of the sample annealed at 673 K shows the composition ratio of [Ag]/ [In]/[S] ) 1:9.6:16.4. Although EDX is not suitable for detailed elemental analysis, it did show the trend of compositional changes of different preparative conditions. Additionally, the phase diagram of the Ag2S-In2S3 system indicates that this ternary compound has a spinel structure with a wide homogeneity region from 81 to 96 mol % In2S3.29 Therefore, this elemental composition obtained from this study could refer to the homogeneous crystal phase, although the changes in cell parameters were not resolved from our X-ray diffraction analysis. The silver to indium ratio in precursor solutions was further studied. Thin films were deposited with different [Ag]/[In] ratios, while maintaining the growth temperature at 353 K for 1 h and adjusting the pH value to 0.6. Figure 6 shows the XRD patterns of films prepared by varying [Ag]/[In] ratios from 1 to 5 as the [S]/[Ag] ratio was kept unchanged. The diffraction peaks disappeared for the sample prepared in the solution with [Ag]/ [In] ) 5. This implied the concentration of indium in the precursor solution was not enough for the formation of indium sulfide on top of the primary Ag2S film. From the XRD patterns, it is seen that crystalline AgIn5S8 films were obtained from the solution with a silver to indium ratio smaller than 5. The diffraction peaks became sharper as [Ag]/[In] increased. Note that the peaks of crystal planes of (2 2 2) and (4 4 4) at 2θ of 28.5° and 59.1° are enhanced considerably when [Ag]/[In] ) 4 (curve B in Figure 6). The degree of crystal orientation, f, can be evaluated by using Lotgering method.30 Diffraction peaks of (2 2 0) ) 23.2°, (3 1 1) ) 27.3°, (2 2 2) ) 28.5°, (4 0 0) ) 33.1°, (5 1 1) ) 43.4°, (4 4 0) ) 47.5°, (5 3 3) ) 55.6°, (6 2 2) ) 56.3°, (4 4 4) ) 59.1°, (6 4 2) ) 64.4°, and (7 3 1) ) 66.3° were taken into account for the calculation. P - P0 1 - P0

(1)

∑ I(111) ∑ I(hkl)

(2)

f)

P)

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Lin et al.

Figure 4. SEM micrographs of films grown on the clean glass substrates from [Ag]/[In] ) 1 and pH 0.6 chemical bath for (a) 20 min, (b) 30 min, (c) 40 min, (d) 50 min, and (e) 60 min.

Figure 5. XRD patterns of ternary Ag-In-S films annealed at different temperatures for 1 h: (A) as-deposited; annealed at (B) 473 K, (C) 573 K, and (D) 673 K. Peaks marked 2 are due to AgIn5S8.

Figure 6. XRD patterns of AgIn5S8 films deposited from different silver to indium concentration ratios [Ag]/[In] in pH 0.6 precursor solutions after postreaction thermal treatment at 673 K: (A) [Ag]/[In] ) 5, (B) [Ag]/[In] ) 4, (C) [Ag]/[In] ) 3, (D) [Ag]/[In] ) 2, and (E) [Ag]/[In] ) 1. Peaks marked 2 are AgIn5S8.

where P is calculated from the sample and P0 is P obtained from the powder diffraction peaks of AgIn5S8 (JCPDS card number 25-1329). In general, f for a randomly oriented sample approaches 0 and is close to 1 for a highly oriented crystal. The calculated values of f are equal to 0.17 and 0.05 for [Ag]/ [In] ) 4 (curve B in Figure 6) and [Ag]/[In] ) 1 (curve E in Figure 6), respectively. This result suggests that the (1 1 1) crystal planes of AgIn5S8 were enhanced by reducing indium concentration in precursor solutions. SEM micrographs of samples prepared from these four [Ag]/ [In] precursor solutions are shown in Figure 7. All samples were annealed at 673 K in an Ar environment for 1 h. These images show various surface morphologies of the samples prepared from different precursor solutions. The topography of the films grown on the glass substrate by the [Ag]/[In] ) 1 precursor consists of loosely packed and plate-like structures (Figure 7a). The aggregates coalesced to form larger plate-like and porous structures when [Ag]/[In] ) 2 (Figure 7b). More densely packed and crystalline polyhedral-shaped aggregates were formed when

[Ag]/[In] ) 3 (Figure 7c), and these polyhedrons were further enhanced when [Ag]/[In] ) 4 (Figure 7d). However, in this experimental condition, the void area of the substrate exposed to the surface was observed, and the film peeled off more easily compared with others. This change of morphologies by different preparation conditions was also observed with aerosol-assisted chemical vapor deposition.6 Compositional analysis of samples was measured by EDX attached to FE-SEM. The weight percentages of silver, indium, and sulfur were converted to atomic percentages and are listed in Table 2, as well as the degree of crystal orientation of samples prepared in various [Ag]/ [In] precursor solutions. Note that the amount of [Ag]/[In] presented in thin films is a function of that in precursor solutions. In-rich samples were obtained for lower [Ag]/[In] in chemical bath; Ag-rich thin films were deposited for [Ag]/[In] ) 4. Nevertheless, nearly stoichiometric AgIn5S8 thin films with the (1 1 1) preferred crystal plane were deposited on the glass substrate in this condition.

Chemical Bath Deposition of AgIn5S8 Thin Films

Crystal Growth & Design, Vol. 7, No. 12, 2007 2729

Figure 7. SEM micrographs of the films prepared from different silver to indium concentration ratios, [Ag]/[In], in pH 0.6 precursor solutions after postreaction thermal treatment at 673 K: (a) [Ag]/[In] ) 1, (b) [Ag]/[In] ) 2, (c) [Ag]/[In] ) 3, and (d) [Ag]/[In] ) 4. Table 2. Degree of Crystal Orientation, f, and EDX Compositional Analysis of Samples Prepared from Various Precursor Solutions [Ag]/[In]a

fb

Ag/In/S

1 2 3 4

0.05 0.058 0.053 0.17

1:9.6:16.4 1:8.7:14.6 1:7.9:13.7 1:4.3:7.9

a Silver to indium concentration ratio in precursor solution. of (1 1 1) crystal orientation.

b

Degree

Figure 9. XRD patterns of ternary Ag-In-S films deposited on MPSmodified glass substrates from [Ag]/[In] )1 and pH 1.6 chemical bath: (A) GIXRD primary films annealed at 673 K, (B) as-deposited final films, and (C) final films annealed at 673 K for 1 h.

Figure 8. Transmission spectra of the films prepared from different silver to indium concentration ratios [Ag]/[In] in pH 0.6 precursor solutions after postreaction thermal treatment at 673 K: (A) [Ag]/[In] ) 1, (B) [Ag]/[In] ) 2, (C) [Ag]/[In] ) 3, and (D) [Ag]/[In] ) 4.

The transmission spectra of thin films prepared by chemical bath are shown in Figure 8. Symbols A, B, C, and D on the graph are heat-treated samples prepared from chemical bath with [Ag]/[In] ) 1, 2, 3, and 4, respectively. All samples show a steady increase in transmission, with longer wavelengths due to scattering. However, a shoulder located at ca. 650 nm on each curve implies the band edge absorption of our semiconductor thin films, corresponding to an energy gap of 1.9 eV.31 This result agrees qualitatively with the values reported in the literature. Note that transmission of 10% at the lower wave-

length of curve D implies the nonuniform coverage of the thin film prepared from a precursor of [Ag]/[In] ) 4. The incidence light was not absorbed completely by the thin film sample and thus a higher transmittance resulted. Thin Film Deposition on MPS-Modified Glass Substrates. A self-assembled monolayer grown on the substrates provides another degree of freedom to engineer surface properties for the preparation of various inorganic thin films. It has been shown that appropriate SAMs can promote heterogeneous nucleation on a solid surface.32 In this study, 3-mercaptopropyltrimethoxysilane (MPS) was used to modify glass substrates. Our preliminary experimental results demonstrate that MPSmodified glass substrates can increase nucleation density and enhance the adhesion of I-III-VI ternary metal sulfide thin films. An FT-IR spectrometer measured the absorption of organic molecules on the glass substrate by taking clean glass slides as a reference. The IR transmittance spectrum shows two strong absorption peaks at positions of ca. 2845 and 2925 cm-1, corresponding to symmetric and asymmetric stretching of CH2,

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Lin et al.

Figure 10. SEM micrographs of (a) primary films deposited on MPS-modified glass substrate from [Ag]/[In] ) 1 and pH 1.6 chemical bath, (b) as-deposited final films, and (c) annealed at 673 K.

Figure 11. Transmission spectra of films deposited on MPS-modified glass substrates: (A) as-deposited and (B) annealed at 673 K.

respectively. These values agree qualitatively with the octadecyltrichlorosilane (OTS) SAMs that have been reported.33 Uniform and adherent thin films were obtained by dipping MPS-modified glass substrate in [Ag]/[In] ) 1 and pH 1.6 chemical bath. The deposited films that were sonicated in DI water for 30 min showed no damage in appearance. Figure 9 shows the XRD patterns of the sample. GIXRD (curve A) measured the sample dipped in the chemical bath for 1 h and annealed at 673 K in an Ar environment for 1 h. The pattern indicates the primary film has crystalline Ag2S, which is similar to the samples deposited on clean glass substrates. Curves B and C, deposited for 2 h, illustrate the crystal structure of asdeposited and annealed thin films. The AgIn5S8 cubic spinel phase with broader peaks was obtained after thermal treatment. The grain size of 9.67 nm was estimated by applying the Scherrer equation. SEM micrographs, shown in Figure 10a-c, show the morphologies of the primary film, as-deposited final film, and annealed thin film, respectively. The primary film consists of more densely packed Ag2S aggregates compared with those on the glass substrate. It is evident that the size and density of these initial nuclei are greatly affected by substrate/deposit interactions, which play an important role in the characteristics of the final AgIn5S8 thin films. The surface morphology of annealed AgIn5S8 exhibits a plate-like and porous structure. The transmission spectra (Figure 11) were obtained in the wavelength range of 300 to 900 nm on as-deposited (curve A) and annealed (curve B) samples prepared on MPS-modified glass substrate. Both curves have relatively higher transmittance (>50%) at longer wavelengths and a sharper absorption edge at 700 nm, which indicate uniform AgIn5S8 thin films were obtained. The energy gap was estimated to be 1.77 eV. Based on experimental observations in this study, we propose a two-step mechanism

for AgIn5S8 ternary thin film deposition from an aqueous solution. The schematic diagram is shown in Figure 12. The glass substrates are cleaned in piranha solution, which creates -OH groups on the surface. First, loosely packed Ag2S particles are attached to the glass substrates following particle attachment mechanism, according to SEM observation.25 This primary layer serves as the secondary nucleation sites. Depending on the silver to indium concentration ratio, indium sulfide fine structures cover the whole surface. In particular, the film grows preferentially to AgIn5S8 (1 1 1) crystal plane after thermal treatment for a lower indium concentration in the precursor solution. On the other hand, MPS-modified glass substrates have thiol functional groups exposed to the substrate surface, and it has been reported in the literature that MPS-modified surfaces enhance the absorption of silver.34 The better chemical affinity between substrate surfaces and silver increases the nucleation probabilities and lowers the interfacial tension, resulting in the higher density of Ag2S primary films, which finally leads to the adherent and uniform AgIn5S8 thin films obtained in this study. Nucleation and Growth Mechanism. Solution chemistry has great influence on the film formation. The nucleation can take place in solution (homogeneous) or on the substrate surface (heterogeneous). Depending on the degree of supersaturation and interfacial energy, one type of nucleation can be favored over the other.32 In addition, the strength of coordination bond of the chelating reagents may affect the deposition mechanism, and in the end, the properties of the final films will be changed. In this section, we will discuss qualitatively the two-step process observed in our experiments by the stability of the metal complex and classical nucleation theory, as influenced by the metal ion concentrations and pH value of the solution. The processes that take place in the CBD involve several steps: (1) slow release of S2- by hydrolysis of thioacetamide; (2) protonation of the ligands with water in the acidic chemical bath; (3) formation of the metal–ligand complex; (4) nucleation and growth of thin films due to supersaturation of metal sulfide. In concentrated sulfuric acid, thioacetamide undergoes a rapid protonation followed by rate-limiting addition of water to yield amide and H2S.35 CH3CSNH2 + H2O T CH3CONH2 + H2S

(3)

36

In an aqueous solution, H2S dissociates to give H2S + H2O T H3O+ + HS-

K ) 10-7

(4)

HS- + H2O T H3O+ + S2-

K ) 10-17

(5)

From the above equations and their equilibrium constants, it is clear that the predominant species in solution will be HS- and the concentration of S2- will be kept low.

Chemical Bath Deposition of AgIn5S8 Thin Films

Crystal Growth & Design, Vol. 7, No. 12, 2007 2731

Figure 12. Schematic diagram of stages of ternary Ag-In-S thin film formation on clean glass substrates and MPS-modified glass substrates: (a) primary film, Ag2S; (b) secondary film In2S3.

TEA is used as a chelating agent for silver and a complex is formed. Ag+ + nTEA T [Ag(TEA)n]+

(6)

Because the pH value of the aqueous solution is lower than 4, indium presents in the solution as free ions In3+. The complex of indium(III) can be formed via the coordination with citric acid, given by37 In3+ + m(citrate)3- T [In(citrate)m]3-3m

(7)

Stability constant, log Kn, is employed as the measure of the strength of the coordination bond and is defined as MLn-1 + L T MLn

(8)

Kn ) [MLn]/[MLn-1][L]

(9)

where M ) Ag and In, and L ) TEA and citric acid in this study. The values of log K1 for Ag(TEA)+ and In(citrate) are 2.49 and 6.18, respectively.37,38 The greater the stability constant, the stronger the metal ion is bond to the chelating agents. In addition, the equilibrium between water and ligand exhibits pH-dependent ionization of the latter. It was reported that stability constants of MHL-type and MH2L-type complexes are about a few orders of magnitude smaller than that of the ML complex.39 Consequently, at lower pH values, the predominant species in solutions are those less stable protonated complexes.39 Solubility products (Ksp) of silver sulfide and indium sulfide are given by14,17 2Ag+ + S2- f Ag2S

Ksp ) 1.6 × 10-49

(10)

2In3+ + 3S2- f In2S3

Ksp ) 5.8 × 10-74

(11)

For simplicity, we follow the procedure and equations reported in ref 14, by assuming only one complex formed between silver ion and TEA (n ) 1 for eq 9). The free [Ag+] in our experimental condition is 1.25 × 10-8 M. We further assume that the concentrations of free silver ions and free indium ions are the same in the solution. We briefly check the sulfur concentration in the solubility boundary. The sulfur concentrations for saturated silver sulfide and indium sulfide in this condition are on the order of 10-33 and 10-19 M, respectively. This indicates that silver sulfide precipitates more easily under our experimental conditions. Furthermore, the less stable silver-TEA complex is more easily dissociated, which leads to a much higher degree of supersaturation of silver sulfide.

From the classical nucleation and growth theory, the incubation time is proportional to exp(γ3/(ln S)2), where γ is solid/ solution interfacial tension and S is the supersaturation.40 An increase in the degree of supersaturation decreases the incubation time. In our systems, a higher degree of silver sulfide supersaturation has shorter incubation time and initiates nucleation in solution (homogeneous). Powdery primary film is formed on the substrate (Figure 4a), followed by the heterogeneous growth from a lower degree of indium sulfide supersaturation (Figure 4e). This can be interpreted as being due to the better chemical affinity between Ag2S and In2S3, which lowers the interfacial energy and promotes heterogeneous nucleation on the substrate surface. When the pH values of the solution decrease from 3.2 to 0.6, although the presence of S2- further decreases, pH-dependent deprotonation of citric acid reduces the stability of complexes more pronouncedly. As a consequence, the incubation time of the second step is shortened at lower pH. Lower amounts of [Ag]/[In] in precursor solution (lower degree of indium sulfide supersaturation) modulate the growth kinetics to a more thermodynamically stable crystalline phase, which leads to (1 1 1) oriented AgIn5S8 thin films. Finally, MPS-modified glass substrate provides a functionally suitable surface for preparation of denser Ag2S primary films. This is attributed to the lower solid/solution interfacial energy for the nucleation on this modified surface, due to the enhancements of silver absorption on SH-terminated SAMs on the substrate. Thus an adherent and uniform thin film can be obtained after thermal treatment. In this preliminary study, we wish to shed light on the conditions and mechanisms of preparation of ternary I-III-VI thin films from one-pot chemistry in a cost-efficient way. Conclusions Polycrystalline AgIn5S8 thin films were successfully prepared by chemical bath deposition. This work focused on elucidation of the growth mechanism rather than optimization of preparative parameters on crystal structure, thickness, and morphologies of thin films. It was demonstrated that the deposition progress can be revealed by XRD and a series of SEM micrographs. Primary films composed of granular Ag2S were first deposited on the substrates, and then indium sulfide covered the entire surface. The silver to indium concentration ratio in the precursor solution was adjusted to 4 and AgIn5S8 thin films with (1 1 1) preferred orientation were obtained on clean glass substrates. MPS selfassembled monolayers were used to modify the surface properties of the glass substrate. It was found that the particle density

2732 Crystal Growth & Design, Vol. 7, No. 12, 2007

of Ag2S primary film was enhanced, and uniform and adherent final thin films were prepared. This can be attributed to the better chemical affinity between SH-terminal groups on the substrate surface and silver cations in the solution. The effects of chelating agents, pH value of the precursor solutions, and [Ag]/[In] concentration ratios were discussed with regard to the stability constants of the metal–ligand complex and classical nucleation theory. Finally, a two-step deposition mechanism was proposed in this study. Acknowledgment. This work was supported by the National Science Council in Taiwan under Grant No. NSC 95-2221-E194-082. We thank the Instrument Center at National Chung Cheng University and National Cheng Kung University for assistance with SEM micrographs and GIXRD analysis, respectively.

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