A Novel Approach for CdS Thin-Film Deposition: Electrochemically

(d) Chu, T. L.; Chu, S. S.; Schultz, N.; Wang, C.; Wu, C. Q. J. Electrochem. ..... Erik D. Spoerke , Matthew T. Lloyd , Yun-ju Lee , Timothy N. Lamber...
1 downloads 0 Views 287KB Size
Download PDF
J. Phys. Chem. B 1998, 102, 9677-9686

9677

A Novel Approach for CdS Thin-Film Deposition: Electrochemically Induced Atom-by-Atom Growth of CdS Thin Films from Acidic Chemical Bath Koichi Yamaguchi, Tsukasa Yoshida,*,† Takashi Sugiura, and Hideki Minoura*,‡ Department of Chemistry, Faculty of Engineering, Gifu UniVersity, Yanagido 1-1, Gifu 501-1193, Japan ReceiVed: June 4, 1998; In Final Form: September 20, 1998

Thin films of cadmium sulfide (CdS) have been electrochemically deposited in acidic chemical baths (pH between 1.4 and 4.6) containing cadmium chloride and thioacetamide (TAA). Although chemical formation of CdS particles continuously occurs in the solution phase, no chemical growth of CdS thin film takes place under the acidic conditions. Electroreduction of protons at conductive substrates at small current densities (several tens of µA/cm2) was found to trigger a growth of highly crystallized hexagonal CdS thin films. The resultant film consists of particles having hexagonal cylindrical shape. The individual particles are made up of single crystals of CdS. The film growth is achieved by the atom-by-atom growth of individual crystallites and not by the agglomeration of fine particles formed in the solution phase. The film could become as thick as 500 nm without suffering from disordering of the film structure. Kinetic analysis was made for the formation of CdS particles in the solution phase, the film growth, and the electroreduction of protons at the substrate. Different activation energies of 75.3, 56.7, and 21.0 kJ/mol were found for these processes, respectively. The deposition mechanism is different from the ordinary electrodeposition process, since the film growth is not under control of the electrode kinetics. It is supposed that the electroreduction of protons imposes a pH gradient at the vicinity of the substrate to reduce the activation barrier for the decomposition of Cd-TAA complex at the surface of the substrate. The electrochemical reaction is therefore used only to maneuver the chemical formation of CdS to take place preferentially at the substrate surface, while prohibiting the contribution of the solution phase reaction to the film growth.

Introduction Thin films of inorganic compound semiconductors are taking vital roles in the advanced technologies of the modern society. To drastically improve the performance of thin-film-based devices, it is necessary to realize a precise control of the film structure at an atomic level in the film processing. However, the techniques for the film processing should be developed also in view of both economy and ecology, to make the technique applicable in answering the increasing demand in the future.1 Methods of the film processing can be divided into two major groups: those carried out in gas phases (dry process) and in liquid phases (wet process). The dry process includes techniques such as vacuum evaporation,2 chemical vapor deposition (CVD),3 molecular beam epitaxy (MBE),4 or sputtering.5 These techniques, without exceptional, require high vacuum and/or temperature because it is necessary to produce gaseous precursor molecules or atoms. Besides the high energy needed for the film processing, emission of gaseous waste materials is another serious problem with these techniques. Because one can simply exclude the unwanted chemical species from the system, the gas-phase techniques have the advantages of the high controllability of the film growth and the feasibility to obtain a pure material. The gas-phase techniques benefit also from many variables tunable to achieve an intended film growth. For example, in the CVD method which is based on heterogeneous chemical reactions of gaseous precursors, the conditions at the * Authors to whom correspondence should be addressed. † E-mail: [email protected]. ‡ Fax: +81-58-293-2587. E-mail: [email protected].

substrate are actively controlled by applying heat3a-c or light3d which, together with the other parameters such as gas pressure, result in a fine control of the growth process. Among the wet processes, the methods employing chemical and electrochemical reactions in water have the highest advantages in suppressing the cost and the stress to the environment caused by film processing. These techniques which are respectively called chemical bath deposition (CBD) and electrodeposition (ED) have been actively studied for the deposition of various kinds of compound semiconductor thin films in the past few decades.6-23 The CBD method can be regarded as an analogue to the CVD method, since they rely on the same principle to utilize chemical reaction of the precursor molecules or ions, except that the whole event takes place in solution in CBD. Ideally, the film growth is expected to occur only by the heterogeneous chemical reactions at the interface between the substrate and the solution, so that thin films with homogeneous structure and composition can be obtained. However, it is usually difficult to avoid unwanted side reactions in the bulk of the solution that make the product random structured and/or nonstoichiometric. Cadmium sulfide (CdS) is one of the most important II-VI semiconductors finding applications in various optoelectronic devices. CBD6-11,22a and ED16,17,23a of CdS thin films in aqueous/nonaqueous systems have been actively studied in recent years. Thin films of CdS have been most frequently prepared by CBD in alkaline solution containing Cd2+ and thiourea (TU),7-11 and by ED in acidic solution containing Cd2+ and thiosulfate (TS).17 In CBD of CdS, several research groups pointed out that the important factor in obtaining well-adherent, specular CdS thin

10.1021/jp9824757 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/10/1998

9678 J. Phys. Chem. B, Vol. 102, No. 48, 1998 film is the existence of Cd(OH)2 adsorbed on the surface of the substrate, but not as bulky precipitates in the solution phase.7 Rieke and Bentjen studied CBD of CdS on Si substrates.7a They found that no CdS film could be deposited at low pH, namely, when no Cd(OH)2 species was present, and powdery pooradherent CdS films were deposited in the presence of bulky Cd(OH)2 suspended in the solution. O’Brien and Saeed claimed that the borderline of pH of the solution between yielding poor and good quality films closely correlates with theoretical limit for Cd(OH)2 precipitation.7b It is supposed that the growth of CdS thin films proceeds by a heterogeneous reaction between a Cd(OH)2-like surface site and TU in an atom-by-atom fashion in these systems.7,8 Lincot et al. found that homogeneously structured CdS films were formed at the early stage of the film growth.9 The technique realized an epitaxial growth of CdS thin film on an InP single crystal, when the film was grown only at a small thickness (below 100 nm). However, it was also found that the adhesion of CdS particles formed in the bulk of the solution paralleled the atom-by-atom growth when the reaction was extended. It resulted in the formation of a randomly structured outer layer. It can be envisaged that the Cd(OH)2 surface sites are formed not only on the growing surface of the CdS films but also on the CdS precipitates in such alkaline baths. The same surface condition should allow the disturbance of the atom-by-atom growth of the CdS film by the solid formation in the solution phase. The ED of CdS has been reported to be based on the electroreduction of Cd2+ together with elemental sulfur generated by the decomposition of TS in acidic media.17 However, it is well-known that colloidal sulfur is also generated in the bulk of solution.24 Furthermore, homogeneous precipitation of CdS occurs in the aqueous mixture of Cd salt and TS,25 although this fact has been neglected in most of previous reports dealing with the ED of CdS in relevant systems.17 Since both CBD and ED processes involve the formation of solid particles (CdS or S) in the solution as described above, it is difficult to obtain a film with an ordered structure by these methods. It is necessary to develop a method to realize the film growth only by the heterogeneous reactions at the film/ solution interface. Two approaches may be considered. One is to prohibit the solid formation in the solution. Nicolau et al. have developed a method called successive ionic layer adsorption and reaction (SILAR) which tries to grow thin films of CdS by repeating a sequential immersion of the substrate in the solutions of Cd salt and Na2S with rinsing between by water.22a Submonolayer growth of CdS is expected per cycle, since the reaction is supposed to take place only with the adsorbed ion. Colletti et al. have studied electrochemical atomic layer epitaxy (ECALE) by alternately depositing atomic monolayers of Cd and S by employing electrochemical under potential deposition (UPD).23a Both of these techniques are based on a simple idea, to change the solution to prohibit formation of the solid in the solution phase. However, these methods are not attractive from practical viewpoints since they are very timeconsuming and produce a large amount of dilute wastewater. Another approach would be to live with the solid formation in the solution but make it ineffective in the film growth. It is known that CBD of CdS is not possible in an acidic bath where the concentration of Cd(OH)2 is negligibly low, even when the solution is supersaturated with CdS.7 In the CBD process, the fate of the film growth is decided simply by the chemical composition of the solution used. Either no film growth takes place in the acidic baths or the film growth suffers from the solution phase reactions in alkaline baths. Another control factor

Yamaguchi et al. has to be introduced in order to maneuver the film growth at the substrate to take place in a desired way. Electrochemical reactions are useful in achieving a local control of pH. If the pH of the solution at the vicinity of the substrate is increased relative to that in the bulk of the solution, it should promote the atom-by-atom growth of CdS at the substrate, while keeping the bulk of the solution still acidic so that the formation of CdS particles in the solution phase does not contribute to the film growth. Based on this idea, we have attempted an electrochemical control of the growth of a CdS thin film in an acidic chemical bath containing Cd salt and thioacetamide (TAA).26 Local pH increase by electrochemical reduction of protons was found to trigger long-range atom-by-atom growth of CdS thin films. Experimental Section All the chemicals used in this study were of commercially available purest grade. Doubly distilled and ion-exchanged pure water was used in all experiments. Cadmium chloride stock solutions were prepared by dissolving CdCl2‚2.5H2O in water or in dilute HCl aqueous solution. The pH of the CdCl2 stock solution was adjusted in the range between 4.4 and 1.3. Thioacetamide (abbreviated as TAA) was recrystallized in benzene and dried in a vacuum before use. An aqueous solution of TAA was freshly prepared prior to each experiment. A solution containing 0.05 M CdCl2 and 0.10 M TAA was obtained by mixing these solutions at the desired ratio. The initial pH of this mixture was varied between 1.4 and 4.6. An indium-tin oxide (ITO)-coated glass (10Ω/sq) was mainly used as the substrate. It was ultrasonically cleaned subsequently in acetone, 2-propanol, and water before use. An SnO2-coated glass was also used in some experiments. To observe the fine structure of the deposits by a transmission electron microscope (TEM), the film was deposited directly on a Mo mesh (200 mesh) to which a Mo wire was connected for the electrical contact. The deposition of CdS films was performed potentiostatically in a three-electrode configuration in a single-compartment cell. A platinized Pt plate and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Unless otherwise noted, the potential values described in this paper are referred to SCE. The deposited films were ultrasonically washed in water for 1 min, dried in air at room temperature, and subjected to further analyses. The electrochemical measurements and the potentiostatic electrolysis were performed on a Nikko Keisoku NPGFZ2501-A potentiostat equipped with a function generator. The cyclic voltammograms were recorded by a Graphtec WX1200 X-Y recorder. The passed charge was monitored by a digital coulomb meter (Nikko Keisoku NDCM-4). X-ray diffraction (XRD) patterns of the films were measured on a Rigaku Rad2R X-ray diffractometer using CuKR radiation. The morphology of the films was observed by a TOPCON ABT-150FS scanning electron microscope (SEM) and a Hitachi H-8100 transmission electron microscope. The film thickness was measured by a Kosaka Lab. Ltd. SE-2300 surface profilometer. Results and Discussion The electrochemical reactions in aqueous solutions containing CdCl2, TAA, or both of them have been studied by means of cyclic voltammetry. The chemical compositions of the solutions containing TAA are expected to continuously change upon heating due to the decomposition of TAA. For equality, the measurements were performed for all solutions 1 h after start

CdS Thin Film Deposition

J. Phys. Chem. B, Vol. 102, No. 48, 1998 9679

Figure 1. Cyclic voltammograms measured at an ITO electrode in the solutions containing 0.05 M CdCl2 (a), 0.10 M TAA + 0.10 M KCl (b), and 0.05 M CdCl2 + 0.10 M TAA (c). The pH of the solutions was adjusted to 2.4 by adding appropriate amounts of HCl. All measurements were performed under an Ar atmosphere at a scan rate of 50 mV/s. The solutions are heated for 1 h at 70 °C prior to the measurements.

heating them up to 70 °C, which is the temperature typically used for the film deposition. The cyclic voltammogram measured in a 0.05 M CdCl2 solution [Figure 1, curve(a)] shows an abrupt increase of a cathodic current from around -0.7 V in the negative-going scan due to the deposition of metallic Cd and an anodic peak at ca. -0.55 V in the reverse scan due to the anodic stripping. In addition to this reversible character, a cathodic current gradually increases from around -0.4 V in the negative-going scan (Figure 1, inset), close to the equilibrium potential for proton reduction at this pH (2.4). The onset potential for this cathodic current shifted positively by ca. 60 mV upon each 10 times increment of proton concentration in the solution. The current at this potential range was also found to increase quantitatively by lowering pH of the electrolyte (see Supporting Information, Figure 1S). Therefore, this cathodic current arises from proton reduction. In the solution of 0.1 M TAA, no feature other than the small cathodic current of proton reduction is seen. Neither TAA nor its decomposition products are electrochemically reactive in this potential region. When the mixture of CdCl2 and TAA is warmed, yellow turbidity develops in the solution due to the formation of CdS particles. The voltammogram measured in this solution [curve(c)] closely resembles that measured in the solution containing only CdCl2 [curve (a)], except the negative shift of the onset of the Cd2+ reduction current by about 30 mV. Since the reduction of Cd2+ to metallic Cd is a two-electron process, the observed negative shift for 30 mV should correspond to a 90% decrease in Cd2+ activity, as expected from the Nernst equation. However, only less than 10% of Cd2+ is consumed by the precipitation of CdS for this solution, as determined by a separate experiment. It is probable that the observed large decrease of Cd2+ activity is caused by a formation of Cd2+-TAA complex. The cathodic current for the proton reduction is seen also in this solution in the same manner as the other two solutions (Figure 1, inset). Thin films of CdS are obtained in the potential region where no Cd deposition takes place but the small cathodic current for the proton reduction flows (vide infra). The deposition of CdS

Figure 2. SEM images of CdS deposits formed on the ITO surface after potentiostatic electrolyses for 3 h at -0.4 V (a), -0.5 V (b), and -0.6 V (vs SCE) (c) in a 0.05 M CdCl2 + 0.1 M TAA aqueous mixture (initial pH ) 2.4) maintained at 70 °C.

thin films thus does not involve reduction of Cd2+ or TAA but requires the reduction of protons at low current density. Potentiostatic electrolyses were conducted at the ITO glass substrate in the potential range between -0.4 and -0.75 V in the baths containing CdCl2 and TAA. The SEM pictures of the deposits obtained after 3 h of electrolyses at -0.40, -0.50, and -0.60 V are shown in Figure 2. The cathodic current at -0.40 V was negligibly small. A few particles of CdS with cauliflower-like morphology are attached to the substrate, although a large part of the ITO surface remains uncovered (Figure 2a). The deposits obtained on the same ITO simply placed in the solution under open circuit condition had exactly the same morphology. These particles were poorly adhered to the substrate and could be removed easily by a wiping paper. It has been evidenced that no chemical growth of CdS films takes place in acidic solutions supersaturated with CdS, in

9680 J. Phys. Chem. B, Vol. 102, No. 48, 1998 accordance with the fact that all the previous studies on CBD of CdS were performed in basic solutions.7-11 As the more negative potentials down to -0.70 V were applied, although still less than a few tens of µA/cm2, the cathodic current increased and thin films of CdS were deposited on the ITO surface. The films were smooth, transparent, and strongly adhered to the substrate. The SEM pictures (Figure 2b,c) shows that the size of CdS particles becomes small and the ITO surface is completely covered, along the negative shift of the applied potential. When a further negative potential (-0.75 V) was applied, the cathodic current during the electrolysis largely increased due to the deposition of metallic Cd. The resultant film was a mixture of CdS and Cd. Although the CdS precipitation in the bulk of the solution occurs regardless of the electrolysis at the ITO substrate, the formation of CdS thin films takes place only when the electrochemical bias is present, namely, when a local pH gradient is built up by the proton reduction at a small current density. The role of the proton reduction in the film growth is to be further discussed later. Change of the film morphology by different reaction times has been studied (Figure 3). Other conditions, the initial pH of the reaction bath, reaction temperature, and electrode potential were fixed at 4.6 (no HCl added), 70 °C, and -0.65 V, respectively. After 2 h of the reaction, the ITO surface was totally covered by fine CdS particles with mean diameter of 22 nm (Figure 3a). The film thickness of 35 nm was reached at this time. Extension of the reaction time to 3 and 4 h under the same conditions increased the film thickness to 76 and 164 nm, respectively. In a separate experiment, the cathodic polarization of the substrate was held out only for 2 h at the beginning of the reaction and the substrate was simply left in the bath under an open circuit condition for another 2 h. In this case, there was no appreciable increase of the film thickness, confirming the necessity of the continuous cathodic electrolysis for the film growth. It is evident that the electrochemical assistance is essential to the growth of the CdS crystals but not to the formation of nuclei. Along with the film growth, each CdS grain simply becomes larger and its hexagonal shape is distinctively seen (Figure 3b,c). Such faceted growth is typical when the growth rate is limited by that of surface reactions.27 It is therefore expected that these CdS particles grow by overlaying atomic layers on them, namely, in a way called “atom-by-atom” growth.8 The individual particles were found to be of single crystals of CdS (vide infra). In contrast to the deposited film, the precipitates of CdS formed in the bath are of spherical particles with a large variation in size (see Supporting Information, Figure 2S). The growth of such round particles is diffusion-limited, namely, by the rate of the transport of CdS clusters which is three-dimensionally equal in the solution phase. No such particles were incorporated into the deposited films. These observations suggest totally different reaction processes for the film growth and the precipitation in the solution phase. Kinetics of these processes is to be analyzed and discussed in detail later in this paper. The mean diameters of the CdS crystals in Figure 3b,c were 40 and 79 nm, respectively. Comparison of the particle size with the film thickness reveals that there are just a few numbers of the grains, along the thickness of the film. Therefore, it is evident that the increase of the film thickness is mainly achieved by the growth of CdS crystallites and not by piling up fine particles of CdS on the substrate. The XRD spectra of these films are shown in Figure 4, together with that of the CdS precipitates formed in the bath after the reaction for 4 h. Aside from the diffraction peaks of

Yamaguchi et al.

Figure 3. SEM images of the CdS thin films deposited in a 0.05 M CdCl2 + 0.1 M TAA aqueous mixture (initial pH ) 4.6) maintained at 70 °C, after potentiostatic electrolyses at -0.65 V for 2 h (a), 3 h (b), and 4 h (c).

the ITO substrate, those assigned to hexagonal R-CdS28a are only seen in all the spectra. Formation of the cubic phase is not likely, at least to a large extent, because the characteristic diffraction peak from the (200) planes of the cubic structure (d ) 2.90 Å)28b were completely absent when the film was deposited on an SnO2-coated glass instead of an ITO, in order not to obscure the peak by the diffraction from ITO, appearing at the same angle (2θ ) 30.6°). The diffraction peaks of CdS and their relative intensity were not affected by changing the substrate to the SnO2-coated glass. The sharpening of the diffraction peaks of CdS with increasing the reaction time indicates the growth of the CdS crystallites (Figure 4). It can also be noticed that the relative intensity of the diffraction peak from the (002) planes of R-CdS increases as the film grows, suggesting that the CdS deposits become preferentially oriented with their c-axis perpendicular to the substrate. Such observa-

CdS Thin Film Deposition

Figure 4. X-ray diffraction patterns of the CdS films same as (a-c) in Figure 3 and the CdS powder formed in the bath after reaction for 4 h (d).

tion is in accordance with the gradual emergence of the hexagonal shape of the deposits in the SEM picture (Figure 3). The size of CdS crystallites has been estimated from the fullwidth at half-maximum (fwhm) of the (002) diffraction peak using Scherrer’s equation. The deposition for 2, 3, and 4 h was found to increase the crystal size as 28, 39, and 81 nm, respectively, which are very close to the size of the hexagonal particles seen in the SEM pictures (Figure 3). In contrast, the relative intensities of the diffraction peaks for the precipitates were not significantly changed by the difference of the reaction time. It can also be noticed that the diffraction peaks of the CdS powder formed in the bath are much broader than the corresponding peaks of the film when those obtained after the same reaction time are compared (Figure 4c,d), indicating that the film is better crystallized than the precipitates. It is noteworthy that the film becomes more and more ordered along the growth when prepared by the present technique, because the ordered growth was seen only for rather thin films by the CBD in alkaline baths. Extended reaction caused disordering of the structure by the adhesion of randomly agglomerated CdS clusters formed in the solution phase.9 Obviously, such incorporation of the particles formed in the solution is prohibited in the acidic bath used in this study, which is in a way reasonable, considering the fact that no chemical growth of CdS films is observed under acidic conditions. The rate of CdS precipitation is increased by raising the reaction temperature. At the same time, the rate of film growth also increases at an elevated temperature (vide infra). The reaction temperature is a useful variable to control the rate of the film growth also in CBD studies.7b,7c,8,11a Because the rate of homogeneous precipitation is also accelerated by lowering the pH of the reaction bath, in the acidic region,29a solutions acidified by HCl have been used to deposit the films to see the effect of pH. The rate of CdS precipitation is increased upon

J. Phys. Chem. B, Vol. 102, No. 48, 1998 9681 lowering the pH. The initial pH of the solutions was varied from 4.6 (no HCl added), to 2.8, 2.4, 2.0, and 1.6, while the reaction temperature, time, and the electrode potential were fixed to 70 °C, 3 h, and -0.65 V, respectively. The SEM pictures of the films obtained from the acidified baths are shown in Figure 5. The lower the pH, the larger the cathodic current of proton reduction became and the more rapidly the film grew. After 3 h of reaction, the film thicknesses of 76, 161, 205, 284, and 503 nm, respectively, with the mean crystal diameters of 40, 99, 129, 182, and 268 nm were reached for the series of the initial pH values of 4.6, 2.8, 2.4, 2.0, and 1.6. It is evident that the acceleration of the film growth does not cause disordering of the film structure, as seen by the simple enlargement of the hexagonal particles. It should be stressed that the film can become as thick as 500 nm, while retaining the ordered structure. Deposition of CdS on a Mo mesh has been carried out under the conditions used for depositing the film shown in Figure 5b, to observe the fine structure of the deposits by TEM. For comparison, CdS has also been deposited on the same Mo mesh by the CBD method under the standard conditions.8a In the former sample, the same hexagonal particles as those deposited on an ITO were found. The TEM picture shown in Figure 6a corresponds to the side view of the hexagonal particle. Such particles were seen all along the edge of the openings of the Mo mesh. The cylindrical shape of the particle is obvious. In a close-up of the particle (Figure 6b) are seen parallel lines with a 3.58 Å interspace corresponding to the lateral projection of the (100) planes of hexagonal CdS. This lattice fringe was seen uniformly throughout the particle. The complementary selected area electron diffraction (SAD) pattern clearly shows several diffraction spots which can be assigned as indicated in the figure taking a [031] zone axis (Figure 6c). From these observations, it appears that each one of the deposited hexagonal particles is made up of a single crystal of CdS. On the other hand, the CdS deposited by the CBD method had a relatively smooth surface. Particles with sharp crystal faces, such as those seen in Figure 6a, were absent (see Supporting Information, Figure 3S). The film was composed of fine crystallites with diameters of 10 to 30 nm. The SAD pattern of the deposits showed rings characteristic of CdS, but suggesting that the film is made of fine crystals with random orientations (see Supporting Information, Figure 3S). Such ring patterns have also been reported by Lincot et al.10a Although an atom-by-atom growth of CdS has been evidenced by the alkaline CBD,9 it does not occur to a very large extent, as seen with our method. It is a remarkable finding that the present technique realizes long-range atom-by-atom growth of CdS, yielding a film composed of highly crystallized CdS. The crystal sizes are as large as 300 nm, unreachable by any of the previously studied CBD or ED methods, in aqueous systems. It has been shown that the precipitates formed in the reaction bath and the electrodeposited film are composed of CdS having totally different qualities. This can be interpreted as consequences of different reaction mechanisms. Kinetic analysis has been carried out for the growth of the CdS films and for the precipitation of CdS particles in the bath used for depositing the films. The precipitation rate was determined by weighing the amount of CdS formed in the bath after several different reaction times. The yields of CdS in the solution are plotted versus reaction time in Figure 7a for a solution with an initial pH of 1.42 maintained at different reaction temperatures of 50, 60, 70, and 80 °C. The reaction yield is defined as the proportion of Cd precipitated as CdS over the total amount of

9682 J. Phys. Chem. B, Vol. 102, No. 48, 1998

Figure 5. SEM images of the CdS thin films deposited in the baths with different initial pH of 1.6 (a), 2.0 (b), 2.4 (c), and 2.8 (d) as prepared by acidifying aqueous solutions of 0.05 M CdCl2 + 0.1 M TAA by adding HCl. The other deposition conditions, reaction temperature, time, and electrode potential were fixed to 70 °C, 3 h, and -0.65 V, respectively. The SEM image of Figure 3(b) can be put in the series as a film deposited in the bath with initial pH of 4.6 under the same conditions.

Yamaguchi et al.

Figure 6. TEM image of the CdS thin film electrochemically deposited on a Mo mesh in a 0.05 M CdCl2 + 0.1 M TAA aqueous mixture (initial pH ) 2.0) at -0.65 V, 70 °C, and for 3 h; showing a side view of one hexagonal cylindrical particle of which the film is composed (a), a high magnification image of (a) showing lattice fringe with 3.58 Å interspace, corresponding to lateral projection of (100) crystal planes of hexagonal CdS (b), and selected area electron diffraction (SAD) pattern complementary to (a) and (b), in which the diffraction spots are indexed taking [031] zone axis (c).

CdS Thin Film Deposition

J. Phys. Chem. B, Vol. 102, No. 48, 1998 9683 ship describe the changes in concentrations of each chemical species.

d[CdS] d[H+] d[TAA] d[Cd2+] ) ))dt dt dt dt

(2)

Swift et al. have reported that the hydrolysis of TAA in acidic media follows a second-order reaction rate law with respect to the concentrations of TAA and protons (eq 3).29a

-

d[TAA] ) k[TAA][H+] dt

(3)

Here, k is the second-order reaction rate constant. The increase of the proton concentration along the CdS precipitation should therefore give an autocatalysis to the hydrolysis of TAA. When the amount of precipitated Cd, and the initial concentrations of TAA and proton in the solution are x, a, and b [mol/ L], respectively, eq 3 can be solved as eq 4 by taking the relationship of eq 2 into account.

kt )

{

}

a(b + x) 1 ln a-b b(a - x)

(4)

Solving this equation for k yields eq 5.

x)

Figure 7. Upper figure (a); Relationship between the reaction yield of CdS precipitation in solution and the reaction time at the bath temperatures of 50 °C (2), 60 °C (b), 70 °C (9), and 80 °C ([) for the mixed solutions containing 0.05 M CdCl2 + 0.1 M TAA (initial pH ) 1.42). In the figure are also shown the lines fitted to the plots according to the autocatalyzed second-order reaction expressed by eq 5 (see text). Lower figure (b); The Arrhenius plot made from the relationship between the reaction temperature and the second-order reaction rate constants (see Supporting Information, Table 1S) derived from the curve fits in (a). Activation energy of 75.3 kJ/mol was found for the precipitation of CdS in the solution phase.

Cd in the bath. CdS precipitation apparently proceeds at a constant rate for a given temperature. The overall reaction for the precipitation of CdS is written as eq 1 in the acidic regime.29b

Cd2+ + CH3CSNH2 + 2H2O f CdS + CH3COOH + NH4+ + H+ (1) Because the solubility product of CdS in water is very low (Ksp ) [Cd2+][S2-] ) 8 × 10-27),30 the amount of decomposed TAA, namely, the amount of generated S2- ion, can be regarded as equal to the amount of precipitated CdS. Since the reaction mixture used in this study is unbuffered, the pH is expected to shift to lower values as the precipitation of CdS proceeds, by the coproduced proton (eq 1). Therefore, the following relation-

ab[exp{(a - b)kt} - 1] a + b exp{(a - b)kt}

(5)

The experimental conditions give the values of x and b as 0.10 and 0.038 [mol/L], respectively. The data points in Figure 7a were fitted to eq 5 by the least-squares method to obtain the rate constants at different reaction temperatures (see Supporting Information, Table 1S). The fitted curves are almost straight lines as shown in Figure 7a. The Arrhenius plot made from the relationship between the reaction temperature and the reaction rate constant also shows a good linearity (Figure 7b). The activation energy for the precipitation of CdS has been calculated to be 75.3 kJ/mol from the slope of the Arrhenius plot, which is reasonable as compared to the values found in the previous studies of relevant systems.29a,c The growth of the CdS thin films has been examined in the same reaction bath as the one for which the rate of CdS precipitation in the bulk of the solution was analyzed above. In Figure 8a is plotted the film thickness versus reaction time for films deposited at -0.65 V under various reaction temperatures. It can be noticed that the film starts to grow after a certain induction time and then the thickness increases linearly with the reaction time at all the reaction temperatures tested. The lower the reaction temperature is, the longer the induction time needed. This is in contrast with the homogeneous precipitation of CdS which starts taking place almost instantaneously after mixing the solutions of CdCl2 and TAA (Figure 7a). Rates of film growth have been determined by fitting straight lines as shown in Figure 8a. They were 57.3, 94.6, 168, and 349 nm/h for the reaction temperatures of 50, 60, 70, and 80 °C, respectively. A linear relationship has been found in the Arrhenius plot drawn from this relationship (Figure 8b). The activation energy for the film growth has been found as 56.7 kJ/mol, which is much smaller than the value for the CdS formation in the homogeneous phase. It is obvious that the activation barrier to the formation of CdS is significantly lowered at the surface of the substrate by applying the electrochemical bias. Ortega-Borges and Lincot have found an activation energy of ca. 85 kJ/mol for the growth of CdS thin

9684 J. Phys. Chem. B, Vol. 102, No. 48, 1998

Yamaguchi et al. the rather small current density during the deposition, it is evident that the reduction of protons promotes the growth of CdS films in a totally different manner than that in the bulk of the solution. When the film thickness was re-plotted against the electrical charge consumed during the film deposition, linear relationships were found at all temperatures (see Supporting Information, Figure 4S). However, the slope became steeper at higher temperatures. Also noticed was that the film growth commences only after charges of ca. 40 mC/cm2 were consumed at all reaction temperatures. The former indicates the increase of apparent Faradaic efficiency at high temperatures. The latter may suggest that a certain amount of electricity has to be consumed for proton reduction, regardless of the reaction temperature, to build up a sufficient pH gradient to bring about the film growth. The induction time for the film growth observed in Figure 8a corresponds to the time needed for creating the surface for the atom-by-atom growth of CdS to take place. During the linear growth of CdS thin films, the reduction of protons compensates for the lowering of pH by the proton generated upon the CdS formation according to eq 1. Therefore, the overall reaction for the film growth should be written as

Cd2+ + CH3CSNH2 + 2H2O + e- f CdS + CH3COOH + NH4+ + 1/2H2 (6) When the Faradaic efficiencies for the film growth were calculated according to eq 6 and assuming the density of the film to be that of bulk CdS, variation from 39% (50 °C) to 70% (80 °C) was found. The reduction of protons occurs at an extent more than sufficient to compensate the lowering of pH by CdS formation. In the alkaline CBD, the atom-by-atom growth of CdS is supposed to proceed by decomposition of adsorbed thiourea-hydroxo-cadmium complex as expressed by eq 7.8 Figure 8. Upper figure (a); Relationship between the thickness of the CdS films and the deposition time (a). The depositions were carried out at 50 °C (2), 60 °C (b), 70 °C (9) and 80 °C ([) in the bath containing 0.05 M CdCl2 + 0.1 M TAA (initial pH ) 1.60) under a potentiostatic (-0.65 V) condition. In the figure are also indicated straight lines fitted to the plots assuming a linear growth of the films. Lower figure (b); The Arrhenius plot made from the relationship between the reaction temperature and the growth rates derived from the linear fittings in (a). Activation energy of 56.7 kJ/mol was found for the film growth.

films by CBD technique.8a It is close to the value for the hydrolysis of thiourea (TU), which was used as a sulfur source in their study. It was therefore supposed that the rates of both the film growth and the precipitation are limited by that of the hydrolysis of TU in the alkaline CBD. The hydroxylation of the CdS surface is expected to provide sites for the reaction with TU to achieve an atom-by-atom growth of CdS thin films in the moderately alkaline (pH between 10 and 12) bath in the CBD studies.7,8 However, the same event takes place also on the surface of the CdS particles formed in the bulk of the alkaline solution to cause a certain disturbance to the film growth. In the present system, the solution is acidic (pH e 4.6) so that no Cd hydroxide species can be present. However, the continuous electroreduction of protons is expected to provide different surface condition at the substrate. Although formation of Cd hydroxide-like surface site as in the alkaline CBD is not likely in our system upon considering the low pH of the solution and

[Cd(OH)2SC(NH2)2]ads f CdS + CN2H2 + 2H2O (7) It should be noticed that this surface complex not only generates CdS but also produces water and not a proton. A proton has to be generated instead, if OH- ion is absent. The availability of OH- as a proton neutralizer right at the reaction site should be the key for bringing about such a surface reaction. In our system, the coproduced proton is scavenged by the electrochemical reduction, so that the similar event as in CBD takes place only at the surface of the substrate. The variation of the Faradaic efficiency has to be further discussed in order to clarify the difference of the growth mechanism in the present system from the previously studied ED of CdS thin films, where CdS is generated by merely electrochemical reactions.16,17 The current during the film deposition was found to be almost constant and dependent on the reaction temperature (see Supporting Information, Figure 5S). If the rate of film growth is determined by that of the electrochemical process, the activation energy for the film growth should correspond to that of the electrodreduction reactions. The Arrhenius plot derived from the dependence of the current density on the reaction temperature also gave a straight line as shown in Figure 9. The activation energy of 21.0 kJ/mol was found for the electrochemical process which is taking place during the film deposition. It is much smaller than the value found for the film growth. It should be noted that this value found in the CdCl2 + TAA mixture closely

CdS Thin Film Deposition

J. Phys. Chem. B, Vol. 102, No. 48, 1998 9685 CdS thin films, and one table containing second-order reaction rate constants for the precipitation of CdS (7 pages). Ordering information is given on any current masthead page. References and Notes

Figure 9. Arrhenius plot made from the temperature dependence of the current density at the ITO substrate during the film deposition under the same conditions as in Figure 8 (see also Supporting Information, Figure 5S for the actual current-time curves). Activation energy of 21.0 kJ/mol was found for the proton reduction during the film growth.

matches the one found for the proton reduction at an ITO electrode (27.7 kJ/mol) measured in the absence of Cd2+ and TAA (see Supporting Information, Figure 6S). It is therefore evident that the film growth in the present system is not limited by the rate of proton reduction, but is under the control of another chemical process which needs a higher activation energy (56.7 kJ/mol). It is likely that the observed activation energy for the film growth corresponds to that needed for a surfacecatalyzed decomposition of Cd-TAA complex. The role of the electrochemical process in the present system should therefore be to reduce the activation barrier for the chemical growth of CdS thin film by generating the surface having a different condition from that at the substrate simply immersed in the acidic bath. In conclusion, we have introduced the electroreduction of the proton to the CBD process to achieve the desired film growth selectively at the surface of the substrate, just as applying heat or light to the substrate in the CVD processing of thin films.3 The present method has allowed us to deposit thin films of highly crystallized CdS whose growth is not disturbed by the reactions in the solution phase. The same approach may be valid also in the deposition of other compound thin films.31 Use of an appropriate substrate will realize an epitaxial growth of CdS thin film in water. Although solution-phase techniques were believed to be cheap but poor methods for thin film processing as compared to the gas-phase techniques, an active use of electrochemistry has been found to improve the CBD process drastically to make it into a cheap and excellent method. Acknowledgment. The authors gratefully acknowledge one of the reviewers for careful revision of the manuscript and for providing very helpful comments and suggestions, which made it possible for us to present a detailed discussion on the deposition mechanism in the revised manuscript. The present work was supported by the Grant-in-Aid for Scientific Research on Priority-Area-Research “Electrochemistry of Ordered Interfaces” from the Ministry of Education, Science, Sports and Culture of Japan (09237105). Supporting Information Available: Six figures including SEM and TEM images, voltammograms, and plots related to

(1) Yoshimura, M. J. Mater. Res. 1998, 13, 796. (2) Fujii, M.; Kawai, T.; Kawai, S. Solar Energy Mater. 1988, 18, 23. (b) Romeo, N.; Sberveglieri, G.; Tarricone, L. Appl. Phys. Lett. 1978, 32, 807. (3) Jones, A. C. Chem. Soc. ReV. 1997, 26, 101. (b) O’Brien, P.; Walsh, J. R.; Watson, I. M.; Hart, L.; Silva, S. R. P. J. Cryst. Growth 1996, 167, 133. (c) Fainer, N. I.; Kosinova, M. L.; Rumyantsev, Y. M.; Salman, E. G.; Kuznetsov, F. A. Thin Solid Films 1996, 280, 16. (d) Cheon, J.; Zink, J. I. J. Am. Chem. Soc. 1997, 119, 3838. (4) Fujita, S.; Kawakami, Y.; Fujita, S J. Cryst. Growth 1996, 164, 196. (5) Fraser, D. B.; Melchior, H. J. Appl. Phys. 1972, 43, 3120. (b) Liu, Z.; Gelinas, M. P.; Coombe, R. D. J. Appl. Phys. 1994, 75, 3098. (c) Meng, L.; Santos, M. P. Vacuum 1995, 46, 1001. (6) Chopra, K. L.; Kainthla, R. C.; Pandya, D. K.; Thakoor, A. P. Phys. Thin Films 1982, 12, 167, and references therein. (b) Hodes, G. In Physical Electrochemistry: Principles, Methods, and Applications; Rubinstein, I., Ed.; Marcel Dekker: New York, 1995; Chapter 12, and references therein. (7) Rieke, P. C.; Bentjen, S. B. Chem. Mater. 1993, 5, 43. (b) O’Brien, P.; Saeed, T. J. Cryst. Growth 1996, 158, 497. (c) Kaur, I.; Pandya, D. K.; Chopra, K. L. J. Electrochem. Soc. 1980, 127, 943. (d) Kitaev, G. A.; Mokrushin, S. G.; Uritskaya, A. A. Kolloidn. Zh. 1965, 27, 51. (8) Ortega-Borges, R.; Lincot, D. J. Electrochem. Soc. 1993, 140, 3464. (b) Froment, M.; Lincot, D. Electrochim. Acta 1995, 40, 1293. (c) Don˜a, J. M.; Herrero, J. J. Electrochem. Soc. 1997, 144, 4081. (9) Lincot, D.; Ortega-Borges, R. J. Electrochem. Soc. 1992, 139, 1880. (10) Lincot, D.; Mokili, B.; Froment, M.; Cortes, R.; Bernard, M. C.; Witz, C.; Lafait, J. J. Phys. Chem. B 1997, 101, 2174. (b) Froment, M.; Bernard, M. C.; Cortes, R.; Mokili, B.; Lincot, D. J. Electrochem. Soc. 1995, 142, 2642. (11) Don˜a, J. M.; Herrero, J. J. Electrochem. Soc. 1992, 139, 2810. (b) Sahu, S. N. J. Mater. Sci. Mater. Electron. 1995, 6, 43. (c) Hu, H.; Nair, P. K. J. Cryst. Growth 1995, 152, 150. (d) Chu, T. L.; Chu, S. S.; Schultz, N.; Wang, C.; Wu, C. Q. J. Electrochem. Soc. 1992, 139, 9. (e) Dhere, N. G.; Waterhouse, D. L.; Sundaram, K. B.; Melendez, O.; Parikh, N. R.; Patnaik, B. J. Mater. Sci. Mater. Electron. 1995, 6, 52. (f) George, P. J.; Sa´nchez, A.; Nair, P. K.; Huang, L. J. Cryst. Growth 1996, 158, 53. (12) Yoshida, T.; Yamaguchi, K.; Toyoda, H.; Akao, K.; Sugiura, T.; Minoura, H.; Nosaka, Y. Proceedings of The 11th International Conference on Photochemical ConVersion and Storage of Solar Energy, 1997; Paris; pp 37-57. (b) Hariskos, D.; Ruckh, M.; Ru¨hle, U.; Walter, T.; Schock, H. W.; Hedstro¨m, J.; Stolt, L. Solar Energy Mater. Sol. Cells 1996, 41/42, 345. (c) Don˜a, J. M.; Herrero, J. J. Electrochem. Soc. 1994, 141, 205. (d) Savadogo, O.; Mandal, K. C. Solar Energy Mater. Sol. Cells 1992, 26, 117. (e) Meherzi-Maghraoui, H.; Dachraoui, M.; Belgacem, S.; Buhre, K. D.; Kunst, R.; Cowache, P.; Lincot, D. Thin Solid Films 1996, 288, 217. (f) Basu, P. K.; Pramanik, P. J. Mater. Sci. Lett. 1986, 5, 1216. (13) Grozdanov, I. Appl. Surf. Sci. 1995, 84, 325. (b) Grozdanov, I.; Najdoski, M. Appl. Surf. Sci. 1995, 84, 325. (14) Gorer, S.; Hodes, G. J. Phys. Chem. 1994, 98, 5338. (b) Hodes, G.; Howell, I. D. J.; Peter, L. M. J. Electrochem. Soc. 1992, 139, 3136. (c) Hodes, G.; Albu-Yaron, A.; Decker, F.; Motisuke, P. Phys. ReV. B 1987, 36, 4215. (d) Kainthla, R. C.; Pandya, D. K.; Chopra, K. L. J. Electrochem. Soc. 1982, 129, 99. (e) Racheva, T. M.; Dragieva, I. D.; Djoglev, D. H.; Dimitrova, P. P. Thin Solid Films 1973, 17, 85. (15) Gorer, S.; Albu-Yaron, A.; Hodes, G. J. Phys. Chem. 1995, 99, 16442. (b) Don˜a, J. M.; Herrero, J. J. Electrochem. Soc. 1995, 142, 764. (c) Estrada, C. A.; Nair, P. K.; Nair, M. T. S.; Zingaro, R. A.; Meyers, E. A. J. Electrochem. Soc. 1994, 141, 802. (16) Baranski, A. S.; Fawcett, W. R.; McDonald, A. C.; Nobriga, R. M.; MacDonald, J. R. J. Electrochem. Soc. 1981, 128, 963. (b) Roe, D. K.; Wenzhao, L.; Gerischer, H. J. Electroanal. Chem. 1982, 136, 323. (c) Fatas, E.; Herrasti, P.; Arjona, F.; Camarero, E. G.; Medina, J. A. Electrochim. Acta 1987, 32, 139. (d) Balakrishnan, K. S.; Rastogi, A. C. Solar Energy Mater. 1990, 20, 417. (e) Preusser, S.; Cocivera, M. Solar Energy Mater. 1987, 15, 175. (f) Lade, S. J.; Lokhande, C. D. Mater. Chem. Phys. 1997, 49, 160. (17) Power, G. P.; Peggs, D. R.; Parker, A. J. Electrochim. Acta 1981, 26, 681. (b) Jackowska, K.; Skompska, M. J. Pol. Chem. 1986, 60, 551. (c) Fatas, E.; Herrasti, P.; Arjona, F.; Camarero, E. G. J. Electrochem. Soc. 1987, 134, 2799. (d) Morris, G. C.; Vanderveen, R. Solar Energy Mater. Sol. Cells 1992, 26, 217. (e) Dennison, S. Electrochim. Acta 1993, 38, 2395. (f) McCandless, B. E.; Mondal, A.; Birkmire, R. W. Solar Energy Mater Sol. Cells 1995, 36, 369. (g) Sasikala, G.; Dhanasekaran, R.; Subramanian, C. Thin Solid Films 1997, 302, 71.

9686 J. Phys. Chem. B, Vol. 102, No. 48, 1998 (18) Peulon, S.; Lincot, D. AdV. Mater. 1996, 8, 166. (b) Izaki, M.; Omi, T. J. Electrochem. Soc. 1996, 143, L53. (c) Kavan, L.; O’Regan B.; Kay, A.; Gra¨tzel, M. J. Electroanal. Chem. 1993, 346, 291. (d) Natarajan, C.; Nogami, G. J. Electrochem. Soc. 1996, 143, 1547. (19) Sanders, B. W.; Kitai, A. H. J. Cryst. Growth 1990, 100, 405. (b) Ponomarev, E. A.; Albu-Yaron, A.; Tenne, R.; Le´vy-Cle´ment, C. J. Electrochem. Soc. 1997, 144, L277. (20) Massaccesi, S.; Sanchez, S.; Vedel, J J. Electroanal Chem. 1996, 412, 95. (b) Kazacos, M. S.; Miller, B. J. Electrochem. Soc. 1980, 127, 869. (21) Cowache, P.; Lincot, D.; Vedel, J. J. Electrochem. Soc. 1989, 136, 1646. (b) Arico`, A. S.; Silvestro, D.; Antonucci, P. L.; Giordano, N.; Antonucci, V. AdV. Perform. Mater. 1997, 4, 115. (22) Nicolau, Y. F. Appl. Surf. Sci. 1985, 22/23, 1061. (b) Nicolau, Y. F.; Dupuy, M.; Brunel, M. J. Electrochem. Soc. 1990, 137, 2915. (c) Lindroos, S.; Kanniainen, T.; Leskela¨, M.; Rauhala, E. Thin Solid Films 1995, 263, 79. (23) Colletti, L. P.; Teklay, D.; Stickney, J. L. J. Electroanal. Chem. 1994, 369, 145. (b) Gregory, B. W.; Stickney, J. L. J. Electroanal. Chem. 1991, 300, 543.

Yamaguchi et al. (24) La Mer, V. K.; Barnes, M. D. J. Colloid Sci. 1946, 1, 71. (25) Andrushko, A. F. Fiz. TVerd. Tela 1962, 4, 582. (b) Hashimoto, K.; Toda, Y.; Kobayashi, M. Nihon Kagaku Kaishi 1977, 330. (26) Minoura, H.; Kajita, T.; Yamaguchi, K.; Takahashi, Y.; Amalnerkar, D. P. Chem. Lett. 1994, 339. (b) Amalnerkar, D. P.; Yamaguchi, K.; Kajita, T.; Minoura, H. Solid State Commun. 1994, 90, 3. (27) Park, J.; Lee, D.; Shin, H.; Lee, B. J. Am. Ceram. Soc. 1996, 79, 1130. (b) Bever, M. B. Encyclopedia of Materials Science and Engineering, Vol. 5; Pergamon Press: U.K., 1986; pp 3354-3355. (28) Joint Committee on Powder Diffraction Standards, Card 6-314. (b) Joint Committee on Powder Diffraction Standards, Card 10-454. (29) Bowersox, D. F.; Swift, E. H. Anal. Chem. 1958, 30, 1288. (b) Rosenthal, D.; Taylor, T. I. J. Am. Chem. Soc. 1960, 82, 4169. (c) Peeters, O. M.; Ranter, C. J. Chem. Soc., Perkin Trans. 2 1974, 1832. (30) Dean, J. A. Lange’s Handbook of Chemistry, 14th ed.; McGrawHill: New York, 1992; Section 8. (31) Kajita, T. Hyoumen Kagaku 1989, 10, 181. (b) Kajita, T. Hyoumen Kagaku 1989, 10, 470.