Cadmium chalcogenide semiconducting thin films prepared by electrodeposition from boiling aqueous electrolytes

Thin Solid Films, 235 (1993) 51-56

51

Cadmium chalcogenide semiconducting thin films prepared by electrodeposition from boiling aqueous electrolytes Z. Loizos, A. Mitsis and N. Spyrellis Laboratory of General Chemistry, National Technical University of Athens, Zographou Campus, 157 73 Athens (Greece)

M. Froment and G. Maurin UPR 15 du CNRS, "Physique des Liquides et Electrochimie", 4 Place Jussieu, 75252 Paris Cbdex 05 (France)

(Received December 22, 1992; revised May 11, 1993; accepted May 26, 1993)

Abstract Thin n-type semiconductive films of CdTe, CdSe and their mixed compounds CdSex Te~_ x, suitable for solar energy conversion, were prepared by cathodic electrodeposition from an aqueous electrolyte onto titanium electrodes: An electrochemical cell was specially designed to perform electrodeposition in a boiling bath. The composition of the layers, their crystal structure, morphology and band-gap width were studied as functions of the electrochemical parameters and bath concentration. It is shown that the use of a high temperature improves the crystal quality of cadmium chalcogenide thin films even with solutions rather concentrated in selenous acid. CdSe or Se-rich CdSexTel-x, layers tend in many cases to crystallize in the hexagonal (wurtzite) structure.

1. Introduction Because of their specific physical properties, especially their band-gap width of between 1.35 and 1.75 eV, the semiconducting compounds belonging to the cadmium chalcogenide family, such as CdTe, CdSe and their mixed compounds CdSexTel_x, can be advantageously used as thin polcrystalline films for various applications, in particular for the conversion of solar energy in photovoltaic or photoelectrochemical devices [1]. Various methods were used to prepare such layers, such as spray pyrolysis [2], vapour deposition [3], screen printing [4], pressure sintering [5] and electrophoresis [6]. Electrodeposition from an aqueous electrolyte is a simple and low cost preparation technique, which has been successfully applied to obtain semiconductive materials [7-14]. It has been shown [8, 15], that smooth and compact electrodeposited polycrystalline CdTe films can be easily developed by careful control of the electrolysis parameters. A similar investigation has been also carried out on mixed CdTe and CdSe compounds [11-13] and, more recently, on pure CdSe [14]. In our previous works [11-14] we investigated the effects of the main experimental parameters (composition, pH, agitation of the electrolyte and potential) on the structural and semiconductive characteristics of asgrown CdTe, CdSe and CdSexTe~_x films electrodeposited from an acid sulphate solution. It was shown that these characteristics are very sensitive to the electrolyte temperature [8, 16]. According to X-ray diffrac-

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tion, the layers are practically amorphous w i t h a strong excess of unalloyed chalcogen for a deposition temperature less than 50 °C. The characteristic X-ray powder diffraction peaks o f the definite compounds began to be visible for a temperature of about 50 °C within a narrow potential range. When the temperature is even more increased, the layers are better crystallized and they exhibit semiconductive properties; in particular, a significant photocurrent can be detected by using the layers as absorbing electrodes in photovoltaic or photoelectrochemical cells. Whereas the stable structure of CdSe is normally the hexagonal wurtzite one, it is worth noting that in most cases pure CdSe deposits as well as Se-rich CdSexTel_x layers presented a cubic (zinc blende) structure with a strong (111) preferred orientation. In one case, Gutierrez and Ortega [17], who used a very acid electrolyte observed a mixture of the two phases. Increasing the temperature presents other advantages: a larger amount of tellurium oxide can be dissolved and the deposition rate is faster (or the time needed to prepare a given thickness is reduced). Conversely, several difficulties occur, such as the fast evaporation of the electrolyte solution, especially when the cell has to be kept open to operate a rotating-disc electrode. Furthermore, the usual Hg/Hg2C12 or Hg/ Hg2SO4 reference electrodes, which were used to monitor the deposition, potential, are generally designed to be employed below 75 °C. For these reasons, most authors carried out a cadmium chalcogenide electro-

© 1993-- Elsevier Sequoia. All rights reserved

52

Z. Loi'zos et al. / Electrodeposition of Cd chalcogenide semiconducting films

deposition at a compromise temperature of 85 °C. In these conditions, we show that the total amount of dissolved chalcogen species has to be kept below a maximum value of about 2 mmol 1-1. The aim of this work is to show that the crystal quality and the semiconducting performance of the electrodeposited cadmium chalcogenide thin films can be significantly improved by extending the deposition temperature range until the boiling point, using a simple semi-close cell equipped with a pure cadmium reference electrode. We were mainly interested in determining the electrochemical conditions to produce CdSe and CdSexTel_~ thin films with a hexagonal structure, that is expected to exhibit better semiconductive properties and especially greater efficiency as regards photoelectrochemical conversion.

2. Experimental details The electrodeposition method, the set-up as well as the characterization techniques of the thin films, which were used in the present work, were derived from those described in previous papers [11-14]. The electrolytic bath was an acidic solution (pH =2.2), containing 0.2 mol 1-1 CdSO4 and various amounts of selenous acid (H2SeO3), its concentration ranging between 0.5 and 5 m m l -~. In the case of CdTe or CdSexTel_x preparation, the bath was saturated with tellurium oxide (TeO2), its concentration being approximately 0.5 mmol 1-1 at 87 °C and 1 mmol 1-1 at 100 °C. In the present study, a new electrochemical cell was specially designed. It is made of 0.6 litre glassware, tightly closed by a cover, to which are fitted three electrodes. A coil glass cooler is also fitted to the cover, the function of which is to condense the vapours produced by the hot electrolyte. The temperature of the cell can be adjusted from room temperature until the boiling point (about 101 °C). The counter-electrode was indifferently a large platinum grid or, more economically, a plastified carbon foil. The working electrode was a 1.6 cm 2 square titanium plate, which was located horizontally on the symmetry axis of the cell, instead of a classical rotating disc electrode that would be difficult to use in a closed cell. The bath was agitated by a magnetic stirrer, the rotation rate of which was precisely controlled (in standard conditions the rate was 400 r.p.m.). The electrochemical potential of the working electrode was monitored by an electronic potentiostat not against a Hg/Hg2SO4 (SSE) reference electrode, as formerly, but against a metallic cadmium piece plunged directly within the electrolyte. This system presents the advantage of being very simple (no liquid junction) and eliminates any risk of contamination. It was verified

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Fig. 1. Current density vs. cathodic potential (without ohmic drop correction) for CdSe and C d - S e - T e codeposition for various selenous acid concentrations. 0, pure Cd; 1, CdSe (Cse = 0.5 mmol 1-1); 2, CdSe (Cse = 1 mmol I-I); 3, CdSe (Cs¢= 2 m m o l l - ] ) ; 4, CdSe (Cse=4mmoll-l); 5, CdSe ( C s e = 5 m m o l l - ] ) ; 6, CdSeTe (Cs~ = 1 mmol 1-1, CTe ~ I mmol 1-1).

that the equilibrium potential E~d of cadmium in contact with a 0.2moll -1 CdSO4 solution is perfectly stable and accurate. In Fig. 1 it can be seen that the curve obtained with a chalcogen free electrolyte is nearly vertical. Under previous conditions a value of Eca= - 1.075 V v s . SSE was obtained at 85 °C. In this paper all potentials will be reported against the pure cadmium electrode. All deposits, typically 1-5 Ixm thick, are examined by scanning electron microscopy (SEM) imaging and Xray diffraction (XRD). Their composition is evaluated by energy-dispersive X-ray (EDX) local analysis. The band-gap width (Eg) is deducted from the limit of light absorption, detected with a spectrophotometer, equipped with an integrated sphere. In order to be directly observed in a 100 keV transmission electron microscope, thin layers of CdSe or CdSexTel_x (about 1000/~ thick) were deposited on titanium foils, which were preliminarily thinned in their centre by electropolishing to be transparent to the electron beam. Few samples were also submitted to an additional ion-milling treatment in order to remove the titanium substrate and to reduce the layer thickness before their investigation in a 200 keV JEOL 2000 FX high resolution transmission electron microscope.

3. Results Several CdSe or CdSexTel_x electrodeposits were prepared with boiling electrolytes. Smooth layers of good quality were obtained in these conditions. However, the formation of gas bubbles within the bulk of the electrolyte induces hydrodynamic turbulences, which result in large fluctuations of the current density. In order to get a precise control of this parameter, we preferred, in the present study, to maintain the dec-

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Fig. 2. Plateau current density as a function of the rotation velocity in a j.-i vs. ~-1/2 plot for CdSe and C d - S e - T e codeposition (Cse = 10 -3 mol 1- i, E = + 0.050 V vs. Cd). [] CdSe, • Cd(Se, Te).

trolyte temperature a few degrees below the boiling point (0 ~ 98 °C). The results can be extrapolated exactly to the boiling conditions. Current density vs. potential polarization curves were drawn in stationary conditions for each bath composition. In all cases, the overall shape of the polarization curves (Fig. 1) are similar to that obtained previously for CdTe, CdSe and CdSexTel_~, on a rotating disc electrode at a lower temperature [10-14]. In particular, in the central part of the curves, the plateau region, the current density is nearly independent of the potential. For an electrochemical process occurring at a rotating disc, the rate of which is at least partially limited by the diffusion of the active species, it is well known that an increase in the current density is proportional to the increase in the square root of the angular velocity. For example, we observed such a linear j - i v s . ~-~-1/2 relationship in the case of CdSe electrodeposition on a rotating disc electrode [ 14]. In Fig. 2, we reported in a similar manner, the variation of the plateau current density as a function of the rotation rate of the magnetic stirrer. The experimental points fit very well with a linear j - ~ vs. f~-v2 relationship. This result proves that the behaviour of this new electrochemical cell is very similar to that of a rotating disc electrode, although the electrolyte flows outwards from the electrode (and not towards, as for a rotating disc electrode). The linear j - ~ vs. f~- 1/2 relation proves that the current is mainly limited by the convective diffusion of the chalcogen ions and that the electrode surface is uniformly accessible to mass transport. However, the actual rotation velocity of the electrolyte flow in the vicinity of the stationary electrode may be less than the rotation velocity of the magnetic stirrer. As for the previous studies, the deposits prepared at a potential preceding the plateau region, especially for the mixed compounds Cd(Se,Te), contain an excess of non-alloyed chalcogen and have generally an irregular morphology and a heterogeneous surface composition.

53

On the other hand, beyond the plateau, towards cathodic potentials, pure metallic cadmium is deposited at a fast rate because of the high exchange rate and the formation of dendritic outgrowths. Finally, cadmium chalcogenide layers are obtained within the plateau region, the limits of which have to be determined precisely. In Fig. 1, curves Nos. 1-5 are related to Cd and Se codeposition for a given agitation rate. In the plateau region, the limiting current density is, to a first approximation, proportional to Cse, i.e. the concentration of the selenous acid in the bath. These values are of the same order of magnitude as our previous results obtained at 85°C on a disc electrode, rotating at 500 r.p.m. It is worth noting that for Cse = 5 mmol 1-1 the diffusion plateau is still well pronounced (more than 0.1 V in width), whereas at 85 °C it was present only for Cs~ ~<2 mmol 1-1. According to X-ray diffraction, the structure as well as the crystal quality of the electrodeposited semiconducting layers depend strongly on the deposition potential (within the plateau region) and also on the total amount of chalcogen species (TeO2 and H2SeO3) dissolved in the electrolyte. The specific effect of the deposition potential on the structure of the CdSe layers is illustrated in Fig. 3. At the beginning of the plateau, the crystal structure belongs to the cubic zinc blende form with a marked (111) preferred orientation (Fig. 3(d)). For a more negative potential (Figs. 3(c) and 3(b)), the crystal quality is increased (higher and thinner cubic diffraction peaks) and the cubic lattice parameter is practically constant (a = 6.08/~). Moreover, for potentials less than 0.1 V vs. Cd, new diffraction peaks begin to appear, which belong to the hexagonal wurtzite crystal structure. In particular, the (10.0) peak is well distinguished from the cubic peak and increases for more negative potentials (Fig. 3(a)), whereas the (111)cub./(00.2)hex" peaks tend to decrease. This evolution corresponds to a progressive change in the preferred orientation. X-ray diffraction patterns of Cd(Se, Te) mixed compounds, prepared under different cathodic potentials from an electrolyte saturated in TeO2 and containing various amount of H2SeO3, exhibit exclusively the characteristic peaks of the cubic phase. It seems that the presence of tellurium not only stabilizes the cubic phase but also improves dramatically the fibre texture. In Table 1 we reported the variation in the intensity of the (111) diffraction peak as a function of the deposition potential. By comparing these sets of diffraction diagrams (Fig. 3 and Table 1) with the ones obtained in our previous studies, performed at 85 °C for CdSe and Cd(Se, Te) electrodeposition [11-14], it appears clearly that the temperature, increasing from 85 to 100 °C, improves

54

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Fig. 3. X-ray powder diffraction diagrams of CdSe electrodeposits (CuK~ source, Cs~= 10 -~ moll -]) prepared at different cathodic potentials: (a) -0.020 V vs. Cd; (b) +0.050 V vs. Cd; (c) +0.100 V vs. Cd and (d) +0.250 V vs. Cd. TABLE 1. Intensities of the (111) diffraction peak (cubic zinc blende structure) for Cd(Se, Te) deposits, prepared at different potentials (Cs~ = 10 -3 tool 1-1, saturated in TeO2) Potential (V vs. Cd electrode)

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the crystallinity of the semiconducting layer. This is illustrated by the sharper and more intense diffraction paths of the cubic phase and essentially by the apparition of the "normal" hexagonal phase. Moreover, a higher temperature permits the use of baths richer in selenium. In Fig. 4 it is proved that well-crystallized layers are obtained even with a selenous acid concentration as rich as 4 mmol 1-1. The limit of the CdSe formation is shifted from Cs~ = 3 mmol 1-1 at 85 °C to Cs~ = 5 mmol 1-1. It is worth noting that these limits

Fig. 4. X-ray powder diffraction diagrams of CdSe electrodeposits (CuK~ source) prepared from electrolytes containing various amounts of H2SeO3: (a) 1 mmol l-l; (b) 2retool l-l; (c) 4mmol 1-s and (d) 5 mmol 1- L

correspond exactly to the disappearance of the plateau on the polarization curves (see Fig. 1). The direct TEM observations of CdSe or Cd(Se, Te) thin layers (,-~ 1000/~ thick), deposited on prepolished titanium foils, gave additional information on their structure. Deposits prepared at 85 °C are made of tiny microcrystals agglomerated in polycrystalline nodules. These nodules look like hillocks on the deposit surface as observed by SEM (see, for example, Fig. 4 in ref. 11). The size of the microcrystals is so small that it is difficult to distinguish structural features, as can be observed in Fig. 5(a), which corresponds to a Cd(Se, Te) deposit. The associated microdiffraction pattern (Fig. 5(b)) is constituted of homogeneous diffraction rings, characteristic of the cubic zinc blende structure, and also some diffraction dots, given by the titanium substrate. In the case of deposits prepared near the boiling poin, TEM images present a different aspect: microcrystals are sufficiently large (about 400/~)

55

Z. Lo[zos et al. / Electrodeposition of Cd chalcogenide semiconducting films

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to be observed individually and crystallographic details can be identified. In particular, most grains contain numerous parallel twin lamellae and stacking faults. The phenomenon is more important for CdSe layers prepared at the negative border of the plateau region (Fig. 6(a)). These defects are correlated with the cubic to hexagonal phase transformations, occurring in these conditions. In addition to the usual rings of the cubic phase on the microdiffraction pattern (Fig. 6(b)), one can observe diffraction dots located on several rings belonging to the hexagonal phase. They are often prolonged by thin diffusion streaks resulting from irregular stacking sequences of (Ill)cub. or (00.2)hex. dense atomic planes. Similar samples were thinned by ion milling and investigated by H R T E M at 200 keV. The crystal quality was sufficiently good to allow the direct observation of pseudo (ll0)cub. or (ll.0)hex lattices o f atomic row projections and dense atomic planes with a 3.51/~ spacing (Fig. 7). The perfect periodicity of these images was maintained on distances as large as several hundred angstroms. Figure 8 presents the variation in the band-gap width (Eg) as deduced from the limit o f light absorption, versus the deposition potential for CdSe compounds prepared from electrolytes with various selenous acid (Cse) concentrations. It is worth noting that Eg is higher than its theoretical value for Cs~ lower than 2 mmol 1-L

4. D i s c u s s i o n and conclusion

From the whole set of experimental results obtained during this preliminary investigation, it appears that the new semi-close electrochemical cell allows the preparation of semiconductive cadmium chalcogenide compounds. Although the working electrode is stationary, the hydrodynamic conditions are sufficiently well defined to permit a precise control of the electrochemical conditions. In particular, it is possible to define the limits of the potential range corresponding to the diffusion plateau domain, where these compounds can be synthesized. Moreover, with this set-up, it is already possible to carry out electrodeposition until ebullition of the electrolyte solution. The use of a higher temperature extends the experimental domain where semiconducting compounds are formed, but the main interest is certainly the improvement of the crystal quality of the layers. In spite of increasing the reaction rate, the limitation by mass transport does not induce any powdery or dendritic growth. The layers are always compact and smooth and, therefore, they can be used as absorbing electrodes in a photoelectrochemical cell. In a previous paper [11], we showed that the electrodeposition process of CdTe, CdSe and Cd(Se, Te) compounds can be systematically divided into three steps. At first, Te TM ÷ or Se TM + ions are reduced to Te ° or Se° atoms, which are absorbed on the cathodic

56

Z. Lo~'zos et al. / Electrodeposition of Cd chalcogenide semiconducting films

surface that is already partially occupied by C d 2+ absorbed ions. In the second step, at the contact with the Te° or Se° adatoms, Cd 2+ cations are underpotentially reduced to give Cd ° adatoms. Finally, the cadmium chalcogen compound is formed by crystallization of Cd ° with Te° or Se° adatoms. However, some chalcogen adatoms, which had not had sufficient time to be associated with cadmium atoms, are incorporated in the growing crystal. In order to avoid an excess of chalcogen and to get a layer composition closer to the stoichiometric ratio, it is necessary to have a large excess of Cd 2÷ ions to reduce as much as possible the surface concentration of adsorbed chalcogen atoms and to increase the rate of the second step. For this purpose, the electrolyte composition and the deposition parameters have to be chosen in such a manner that the current is limited by the diffusion of Te TM ÷ and/or SeTM÷ ions. Therefore, it appears that a high temperature presents beneficial effects on the defined compounds formation. By accelerating the rate of the second step, it increases the probability of reducing one Cd 2 adion for each recently formed chalcogen adatom and, consequently, the layer composition is closer to the ideal value. In addition, during the third step, the crystal growth rate is also increased giving rise to bigger microcrystals with larger coherent domains, as observed by transmission electron microscopy. The experimental finding that CdSe layers prepared at high temperatures have a crystal structure, which is a mixture of the cubic and the hexagonal phase, is certainly an additional encouraging index of the interest of the new electrodeposition method. Whereas in most cases chemically or electrochemically synthesized CdSe films presented, until now, a metastable cubic structure, the formation of the wurtzite form proves that the compounds prepared under the present conditions have better thermodynamic stability. In a recent study [ 18], we submitted to annealing heat treatments CdSe and Cd(Se, Te) electrodeposits, which were prepared by the usual technique at 85 °C. It was necessary to reach at least 460 °C to transform CdSe layers structure into the hexagonal form and even higher temperatures for Serich Cd(Se, Te) mixed compounds. We hope that by performing electrodeposition at the boiling point, it would be possible to avoid any annealing post-treatment, which was previously considered necessary to get

good semicondueting properties and, in ~ i c u l a r , to obtain significant solar energy conversion efficiency in photovoltaic or photoelectroehemical devices, using such electrodeposited thin films as absorbers. In the next step of this research programme, we will focus our interest on the characterization of the semiconductive properties of cadmium chalcogenide layers, synthesized in boiling electrolyte.

Acknowledgment This research was partially supported by the FrancoHellenic cooperation programme "Platon".

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