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Oriented [1 1 1] ZnSe electrodeposits grown on polycrystalline CdSe substrates
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Journal of Crystal Growth 277 (2005) 335–344 www.elsevier.com/locate/jcrysgro
Oriented [1 1 1] ZnSe electrodeposits grown on polycrystalline CdSe substrates M. Bouroushian, T. Kosanovic, N. Spyrellis General Chemistry Laboratory, School of Chemical Engineering, National Technical University of Athens, Zografos Campus, GR 157 80 Athens, Greece Received 5 April 2004; accepted 6 January 2005 Available online 16 February 2005 Communicated by M.S. Goorsky
Abstract Cathodic electrodeposition of cubic blend ZnSe from aqueous, acidic sulfate solutions on polycrystalline, CdSe(1 1 1) substrates, is described. The structural and morphological properties of the deposited films are discussed in terms of electrolytic growth conditions. It is shown that ZnSe crystallites of well-resolved X-ray diffraction (XRD) response, adopting a strict [1 1 1] preferred orientation, can be obtained due to the substrate texture under low supersaturation growth conditions. The (1 1 1)||(1 1 1) orientation relationship is investigated with respect to underlayers of varying crystallinity and various overlayer thicknesses. The nucleation and growth of ZnSe is discussed in terms of XRD and scanning electron microscopy (SEM). r 2005 Elsevier B.V. All rights reserved. PACS: 68.55.a; 81.05.Dz; 81.10.Dn; 81.15.Pq; 82.80.Ej; 73.40.Lq Keywords: A1. X-ray diffraction; A2. II–VI Epitaxial growth; A3. polycrystalline electrodeposition; B1. ZnSe/CdSe heterostructures; B2. semiconducting II–VI materials
1. Introduction Synthesis and characterization of wide gap II–VI semiconductors are of great importance, since these materials can be utilized in a variety of Corresponding author. Tel.:+30 210 7723097;
fax: +30 210 7723088. E-mail address: [email protected] (M. Bouroushian).
advanced technology optical and optoelectronic applications, viz. solar cells, heterojunction LED and LDs, photoconductors, etc. Hence, as having direct band gap energy of 2.7 eV, zinc selenide (ZnSe) turned out recently to be of exceptional interest for optoelectronics, in particular for fabrication of blue light diodes, while it can also be advantageously used as a buffer/window layer in chalcogenide-based thin film solar cells by
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constructing a heterojunction under lattice matching conditions [1–3]. Various approaches have been addressed to II–VI epitaxial growth, focused mainly in epitaxy on III–V or nonpolar substrates (such as Si and Al2O3), due to the relatively poor quality of II–VI substrates [4,5]. The fabrication of heterojunctions has been dominated by the availability of suitable substrates and the ability of growth, on account of the lattice parameter match as well as the match in band gap energy, electron affinity and thermal expansion coefficient. Alloy systems where such properties are similar over the binary composition range are particularly attractive, although heterostructures with relatively large deviations in lattice dimensions have manifested surprisingly good behavior. Thus, ZnTe on (1 0 0) GaAs substrate with 7% mismatch ratio makes little difference to less deviated systems, at least for ZnTe layers greater than the critical pseudomorphic thickness, while a twice as much mismatch of 14% between CdTe and GaAs does not prevent a clear orientation of the overlayer with respect to the substrate [3–5]. We may note also the successful growth of CdTe crystals on layered NbSe2 in spite of a mismatch ratio as large as 33% [6]. In these cases, certain coordinate and binding relations control the lattice engineering. ZnSe single crystals have been produced by normal or seeded melt growth and vapor transport techniques [5]. Nearly exact lattice matching (99.74%) of ZnSe can be achieved by epitaxial growth on GaAs, although deposition methods (mainly MBE) actually resort to ZnS0.06Se0.94 ternary, as the critical thickness for pseudomorphic growth of binary ZnSe on GaAs is limited to 100 nm [4]. Electrochemical methods of ZnSe epitaxial growth on GaAs and InP are reported recently [7]. Investigation on ZnSe/CdSe system modulated as a window/absorber II–VI heterojunction is of particular interest. CdSe is a well-studied II–VI of n-type conductivity with a direct and compatible with solar spectrum energy gap of 1.7 eV, while ZnSe serves as an effective window layer in CdSebased solar cells on account of its wide gap and the fact that, along with CdTe, comprises the only II–VI obtainable in both n- and p-type forms.
Heterostructure ZnSe/CdSe thin film electrodes have been produced, e.g. by ternary elemental vacuum evaporation [8] and galvanostatic electrochemical co-deposition [9] while efforts are focused lately on ZnSe/CdSe superlattice and quantum dot-containing systems (e.g. [10,11]). In ZnSe/CdSe heterojunction, a moderate lattice mismatch ratio of 6.3%—for CdSe and ZnSe zinc blend lattices—is encountered, while valence, chemical and ionicity mismatches are essentially negligible. Still, a rather large thermal expansion coefficient disparity exists (17.3%), however, the relatively low hardness of II–VI semiconductors as compared to III–V conducts an easier strain relief, that is, growth of crack-free mismatched epilayers can be provided. Electron affinities (w) are of the proper order [w(CdSe)4w(ZnSe] for expecting a gain in open-circuit potential (VOC) of CdSe solar cell configuration [3]; ZnSe has been found to act as an effective insulating layer in CdSe MIS solar cells and enhanced VOC was observed already in early works with sputtered mixed ZnSe/CdSe electrode [8]. Note also that CdSe is effectively stabilized in photoelectrochemical cells (PEC) with suitable redox agents, however, when it comes for applied technology, CdSe photoelectrodes are not devoid of electrochemical corrosion. As presenting an acceptable resistance against corrosion, ZnSe acts as a blocking layer in CdSe-based liquid junctions, providing a barrier to hole transport toward the electrolyte [9]. Flat band potentials of polycrystalline CdSe thin film electrodes stay more positive than single crystals limiting the attainable VOC, so a suitable window layer is essential in this case. Both ZnSe and CdSe semiconductors are obtainable by soft growth techniques such as electrodeposition: as for the more difficult to obtain ZnSe, there have been reported few procedures giving actually microcrystalline samples from acidic selenite [9,12–14] and alkaline selenosulfate solutions [7]. In the present work, an electrochemical method for growth of ZnSe crystallites on polycrystalline CdSe substrates with a strong (1 1 1) dominant orientation is described. The deposited thin film structural and morphological properties are discussed in terms of substrate structure as well as of electrolytic growth conditions.
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2. Experimental details Films of CdSe and ZnSe were both electroplated in a thermostat-controlled conventional threeelectrode cell at 85–901 C, by using a potentioscan (Wenking PGS 81R) system. A rotating disc electrode (RDE) was employed in order to control the mass transfer regime in electrolytic solution. The CdSe deposition substrate was commercially pure Ni of 1.13 cm2 area, abraded and polished by 0.3 mm alumina powder. A platinum grid was used as a counter electrode. The working electrode potentials are measured against Hg/Hg2SO4 saturated sulfate reference (SSE). There was no correction for ohmic drop. Water (of 18.3 MO cm), purified by an ultra-pure water system (Easy Pure Barnstead RF), and as received analytical grade selenite (SeO2), 3CdSO4 8H2O and ZnSO4 7H2O reagents were used for the preparation of experimental solutions. The bath pH was adjusted by sulfuric acid at the working temperatures. Thin films were obtained by cathodic electrodeposition under potentiostatic, DC conditions for various plating times attaining 0.5–5 C electrolysis charge. Several samples were submitted to annealing in a furnace controlled by a Shimaden FP21 programmable controller. Heating was carried out at a 10 1C min1 rate up to the final temperature, and maintained there for 1 h, in air or under an argon stream. The structure of the as-prepared samples was examined by X-ray diffraction (XRD) by a Siemens D5000 unit. Scanning electron microscopy (SEM) images were taken by a JEOL JSM 6100 as well as a Leica Stereoscan 440 model apparatus, while compositional data were obtained by energy-dispersive X-ray (EDX) local analysis during electron microscopy.
3. Results and discussion The adopted working conditions have mainly resulted from earlier investigation on polycrystalline CdSe and ZnSe formation. CdSe films, used as substrates for ZnSe growth, were prepared by electrodeposition on Ni discs at a potential of 0.9 V vs. SSE from an aqueous acidic bath, according to the method described in Ref. [15].
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The mechanism of ZnSe/metal (Me) electroforming and the effects of experimental parameters have been outlined in Ref. [14]. 3.1. Voltammetry The electrochemical potential range for simultaneous reduction of Zn2+ and Se(IV) solution species on CdSe cathodic surface can be determined from the electric current–voltage characteristics of CdSe/Ni RDE at acidic baths of varying composition. Voltammograms recorded in 0.5 103 M SeO2, 0.2 M ZnSO4 solutions of pH ¼ 2, 3 and 4, including partial contributions of electroactive species on electric current are shown in Fig. 1 for applied potential variation between 0.9 and 1.6 V. Measuring range is limited by dissolution of CdSe layer at potentials more anodic than about 0.9 V at one side and the abundant deposition of Zn at 1.45 V. The blank acidic solution contribution in current has been removed from a, b, c curves, which represent potential sweeps in selenite-containing, Zn2+-free solutions. With these curves,
Fig. 1. Cathodic polarization of CdSe/Ni RDE at a 5 mV s1 scan rate in 0.5 103 M SeO2 acidic solutions (of pH ¼ 4, 3 and 2 for curves a, b, c, respectively) and 0.5 103 M SeO2, 0.2 M ZnSO4 acidic solutions (pH ¼ 4, 3 and 2; curves d, e, f) at a bath temperature of 85 1C. Each of a, b, c curves has resulted after removing a blank solution component, namely a sulfuric acid solution of similar pH and ionic strength (controlled by K2SO4). The inset cyclic voltammogram regards curve b and represents two cycles (each with an individual electrode), the second cycle reaching more cathodic potentials.
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the observed current at potentials more anodic than about 1.1–1.2 V depending on pH, is basically due to Se(0) and Se(2) formation during cathodic polarization of CdSe, according to Eqs. (1) and (2) [16,17]: SeO2 H2 O þ 4e þ 4Hþ ! Seð0Þ ðsÞ þ 3H2 O;
(1)
SeO2 H2 O þ 6e þ 6Hþ $H2 Seð2Þ ðaqÞ þ 3H2 O: (2) Selenium hydride from Eq. (2) reacts rapidly in acidic pH with Se(IV) in solution to give Se(0). The voltammetric peaks in the range of 1.1–1.2 V are considered to arise from reductive stripping of either Se(0) deposited on the surface or lattice Se(2d) (24d40) in the CdSe solid layer Se þ 2Hþ þ 2e ! H2 Se:
(3)
It is interesting to note that during a reverse scan, specified in Fig. 1 for b curve (pH ¼ 3), the cathode is strongly depolarized, i.e., even larger than forward cathodic currents are observed. This is true in particular for extended forward polarization, indicating that the electrode surface is irreversibly modified (e.g. growing in area) by the reductive deposition and stripping of selenium. Hydrogen cations effectively participate in the relevant actions either homogeneously in solution viz. in various equilibria with soluble Se species [17] or in adsorbed H+ ads form. This is evident in the abrupt drop of curve c (pH ¼ 2), implying that the behavior of the system is not merely a matter of resolution in blank and ‘‘active’’ current components. Note also, that according to blank solution voltammograms (not in figure), CdSe is found to reversibly adsorb H+ at ca. 1.36 V while at higher potentials is subject to appreciable hydrogen evolution. The voltammetric image is drastically changed by addition of Zn2+ ion excess in solution (d, e, f curves). In their presence, the competition for adsorption sites suppresses the cathodic reactions observed with zinc-free selenite solutions. It can be seen, that the overall polarization curves contain a nearly potential-independent region, where the electrolysis current is limited by mass transport,
in particular by diffusion of chalcogen ions towards the cathode. The as-controlled Se(IV) and Zn(II) co-reduction results in the formation of semiconductive ZnSe-containing layers within a potential range of approximately 1.1–1.4 V/ SSE (see also Ref. [14]). 3.2. XRD study The ZnSe crystallites formed on CdSe substrate exhibit a cubic zinc blend structure (ZB) characterized by a more or less pronounced [1 1 1] preferred orientation, under all preparation conditions. This texture is extinct when electrodepositing directly on a metallic substrate like Ni or HF-treated Ti [14], that is ZnSe/Me consists of ZB crystallites of statistical orientation. Representative XRD diagrams for ZnSe/Ti and ZnSe/CdSe are given in Fig. 2. The relative intensities of the ZnSe (1 1 1), (2 2 0) and (3 1 1) reflections in Fig. 2a satisfy the standard ratios, while electrodeposition on CdSe (Fig. 2b) gives rise to strongly intensified ZnSe(1 1 1). In this case, (2 2 0) and (3 1 1) become non significant. A numerical measure of the crystallographic orientation, i.e., the preference in [1 1 1] relative to the other main directions, can
Fig. 2. XRD patterns (CuKa) of equally thick deposits prepared on Ti (a) and CdSe/Ni (b) at a deposition potential of 1.3 V/SSE from a pH ¼ 3 solution. The reflection intensities (J) for ZnSe(1 1 1) are denoted in figure.
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be expressed as Iða; b; gÞ ¼
a=100 . a=100 þ b=70 þ g=44
(4)
Here a; b; g stand for the photon counts of (1 1 1), (2 2 0) and (3 1 1) reflections from experiment, while 100, 70 and 44 are the respective standard relative intensities from JCPDS. A close to one I value indicates that the solid under study has an (1 1 1) dominant orientation. Actually, ZnSe/Ti films give I in the range 0.25–0.6 while CdSe substrate induces a strong orienting effect on ZnSe crystallites, resulting in I ¼ 1: This effect involves an initially epitaxial growth evolving in a strictly conformal manner. Optimal conditions to achieve an oriented ZnSe growth entail a bath composition of 0.2 M ZnSO4, 0.5 103 M SeO2, pH ¼ 3 and a sufficiently high temperature (ca. 90 1C). The deposition potential is chosen at the middle of the chalcogen-diffusioncontrolled plane current region, where the ratio of ZnSe to Se tends to increase—according to XRD and SEM. At potentials lying near the boundary sections of the diffusion plateau, a stronger excess of elemental Se is observed. The XRD measurements with CdSe substrates of varying orientation character manifest that there exists an almost linear dependence of ZnSe(1 1 1) on CdSe(1 1 1) reflection intensity (Fig. 3, also Fig. 4); in particular, this is valid for CdSe(1 1 1) intensities up to about 70 kcps (for our measuring apparatus). The latter is an apparent critical value above which the texture of ZnSe overlayer is not influenced any longer as indicated at the right conclusion of the approximation curve in Fig. 4. Hence, CdSe(1 1 1) reflection intensities higher than about 70 kcps comprise the ‘‘perfect’’ substrate orientation for our experiment. Obviously, increasing the plating charge leads to obtaining thicker films, however, the deposition mechanism changes at a certain point; the charge passed through the cell, at a constant potential, increases almost proportionally with time up to 3 C but further advance of electrolysis brings about a rapid drop in current density and then the time required to attain any intended thickness is exceedingly increased. It is quite interesting to study the evolution of structure with the plating
Fig. 3. (a) XRD patterns within the low-angle region (CuKa) indicating (1 1 1) reflections of ZnSe/CdSe heterostructures with ZnSe overlayers deposited on CdSe of various (1 1 1) orientation intensities, at 1.3V/SSE for equal plating charges. The substrate diffractograms are shown in (b) in full intensity scale.
charge. This study was accomplished in two ways: (a) either by obtaining a series of ZnSe deposits of various thicknesses on structurally similar, different substrate-films of CdSe, or (b) by performing a roughly in situ determination of XRD response for a single-ZnSe/CdSe structure at different stages of deposition, viz. at various film thicknesses as well. Thereafter, ZnSe films of different thickness were obtained firstly on several, similar CdSe electrodes. As illustrated in Fig. 5, the (1 1 1) ZnSe intensity increases proportionally with deposition charge up to a value of about 3 C, while abruptly decreases afterwards with respect to rapid film degeneration. The latter might be attributed to enhanced elemental Se deposition, though this is not evident
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Fig. 4. XRD (1 1 1) intensities (in arbitrary units) of ZnSe electrodeposits as a function of their CdSe substrate orientation intensity.
Fig. 5. XRD (1 1 1) peaks of ZnSe as a function of plating charge for various films deposited on similar CdSe substrates. The main reflection of Se phase is included in figure.
from XRD spectra probably due to the amorphous nature of the solid chalcogen. The degeneration of deposited layers is implied also in the expansion of the obtained diffraction peaks as expressed by their full-width at half-maximum (FWHM). The ZnSe(1 1 1) FWHM value persistently decreases up to a specific thickness thus indicating an uninterrupted growth of crystallites, increasing afterwards as the deposition control is lost.
Fig. 6. XRD intensities of ZnSe(1 1 1) and Se reflections vs. various plating charges for sequential growth on CdSe substrates, at a constant potential (1.3V/SSE); data are given for two individual CdSe substrates; S1 with (1 1 1) orientation intensity equal to 71 kcps, and S2 (102 kcps).
The successive reestablishment of initial growth conditions according to approach (b), above, might lead to rather unsafe results; howbeit this interrupted electrodeposition of successive ZnSe layers on the same RDE reveals a similar to approach (a) behavior, indicating that growth readily recovers initial disturbances. The ZnSe(1 1 1) XRD response measured after growth steps of 0.5 or 1 C plating charge are brought in Fig. 6 which includes two series of experiments, each with a very well-oriented CdSe substrate. Evidently, the overlayer constitution at each point is analogous with the previously described. Note the too slight increase of the hexagonal Se reflection intensity with electrolysis charge. Using the Scherrer formula (Eq. (5)) in reference with XRD data leads to an estimation of the deposits crystallites’ size by determining their vertical dimension (d): d¼
0:9l . B cos yB
(5)
In Eq. (5), l is the X-ray wavelength in angstroms and yB the Bragg angle in radians, for a particular reflection. B represents the extra peak width at half the peak height indicating the line broadening with respect to a suitable standard.
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This is obtained from the Warren formula: B2 ¼ B2M B2S : Here BM is the measured peak width in radians at half peak height (FWHM) and BS the corresponding value for a closely related reflection of a standard material, i.e., a diffraction peak at close angles to the relevant sample peak. The standard may have a particle size greater than about 2000 A˚ and is usually mixed with the measuring sample; since this is not possible with film characterization, the strong reflections of the underlying titanium substrate were used as reference standards, as well as separately prompted commercial ZnSe and CdSe powders of sufficiently large particle size. The CdSe films were thus estimated to be constituted by grains with sizes in the order of 100 nm while ZnSe crystallites were not found to be larger than 25–40 nm; these values representing crystallites’ depths rather than mean diameter. According to the above mentioned, the texture of ZnSe is determined by substrate and not by electrolytic growth conditions. The observed epitaxial effects are mainly due to the relatively small mismatch amidst cubic ZnSe and CdSe lattices. Still, this mismatch is high enough to limit the
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existence of a pseudomorphic region, hence brings on misfit dislocation within the first few deposited monolayers, promoting thus a 3D island growth (below). We may note here that obtaining ZB structure of ZnSe is not self-evident, since our experiments show that various defective layers with an appreciable content of wurtzite (W) ZnSe phase can be obtained by electrodeposition on (polycrystalline) metal substrates. On the other hand, growth of either W or ZB is geometrically possible on a [1 1 1]-oriented ZB substrate such as CdSe without producing miscoordinated atoms [18]. An epitaxial size selectivity effect is encountered here: the phase with the best elastic match with the substrate is dominantly grown. Consequently, the illustrated buffering effect of CdSe(1 1 1) on ZnSe growth manifests that the ZB structure of ZnSe offers the best elastic match. 3.3. Morphology and growth It is interesting now to examine the growth of the overlayer in terms of morphology. The SEM pictures of Fig. 7 show a CdSe RDE subject to
Fig. 7. SEM micrographs of ZnSe/CdSe deposits prepared at 1.3V/SSE. Plating charge is 2 C for (a) and 5 C for (b); average atomic composition from EDX: (a) 4.7% Zn, 25%Cd, 70.3%Se; (b) 6.2%Zn, 13.1%Cd, 80.7%Se. Brighter contrast forms are almost 95% in Se. Surface details are shown in (c) and (d) where EDX gives: (c) on dendrite: almost 95% Se; (d) on rectangle: 21.1% Zn, 78.9%Se.
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electrodeposition from a zinc sulfate solution of selenite for different charges passed (2 and 5 C in a and b, respectively). It can be seen that a dense underlayer is partly covered by an irregular layer of multi-shaped, loosely adhering particles, which proliferate with thickness. The morphology of this overlayer is ill-determined, as consisting of granular mainly, together with dendritic and flake-like forms. It is apparent also that the visible globules comprise cauliflower-like aggregates of many smaller nanocrystallites. According to EDX, all forms are rich in Se, yet there exists a difference in composition between the visible compact underlayer (Fig. 7a) and the accumulating grains. Upon the extreme assumption that no ternary (Cd,Zn)Se at all is formed and samples consist of stoichiometric CdSe, ZnSe and elemental Se, EDX data processing suggests that the underlayer is richer in ZnSe (26% in Fig. 7a sample) as compared to the general layer composition (19% in ZnSe). Dendritic forms such as the one appearing at the micrograph of Fig. 7c are almost 95% in Se while rectangle shaped details (Fig. 7d) contain more Zn. Thereafter, on account also of the well-resolved XRD reflections, hexagonal and amorphous Se grains together with well-developed but tending to be isolated ZnSe crystallites are present on film surface. The electrodeposition procedure is configured to take place under surface kinetics control. Growth controlled by a planar superficial diffusion of reduced ad-atoms is desirable. Under these conditions a nucleation delay, observed also in other akin systems [4], is expected to occur. An initial, strongly bound to CdSe, very thin ZnSe layer occurs during the first instants of deposition. Should the electrolysis time exceeds few minutes, the coherence of the obtained overlayer is already weak. It appears that after the formation of a few successive polar monolayers of Zn and Se atoms that are structurally registered on the crystallites of the substrate, a 3D island growth prevails and proceeds by lateral growth on a uniformly nucleating ZnSe and Se phase surface, until the particles coalesce and cover the cathode. The nucleation rate progressively decelerates as indicated also by the current density drop; though, lateral growth of crystallites is limited also, and an
accumulation of growing islands seems to be necessary in order to allow the electrical contact between the electrolyte and the less resistive substrate. Then, at a critical point, the resistance of the deposited layer becomes too high to allow a fast charge transfer, and the kinetic control is completely lost. Besides, the deposition potential is subject to a significant drop over the layer formed. During this procedure, growth and clustering of the initial 3D nuclei rapidly give rise to a loosely retained overlayer. Annealing in air at 200 1C for 15 min adjusts stoichiometry establishing a virtually 1:1 Zn/Se atom ratio, according to EDX analysis. The excess of Se totally sublimes giving overall grains stoichiometric composition. The electrodeposit layer, though, remains structured as deposited. A less soft thermal treatment, i.e., 350 1C for 1 h modifies morphology resulting in a film homogenizing (Fig. 8), however a small deficiency of lattice Se may be obtained, due to transformation of ZnSe to ZnO (according to XRD) if oxygen traces are present. Further, CdSe is subject to wurtzite phase transition. It is considered that CdSe underlayer takes part in the morphological changes observed in Fig. 8, possibly providing a proper matrix for ZnSe particles merge. Heating, as promoting the diffusion of metal ions, effectively modifies the interface between the two semiconductor layers, thus Cd(1x)ZnxSe ternary is formed. Then, a range of mixed compositions may exist between CdSe and a thin ZnSe outer layer of crystallites. The sintering effect is certainly stronger at elevated temperatures. Besides, an observed decrease of ZnSe(1 1 1) diffraction response after annealing, when not due to ZnO formation, supports this fact. Note that, the full development of CdSe wurtzite phase above 450 1C complicates the image.
4. Conclusions An (1 1 1)||(1 1 1) orientation relationship can be established between the ZB phases of electrodeposited ZnSe and polycrystalline CdSe film substrate. Cathodic electrocrystallization under kinetic control is well-suited for obtaining ZnSe
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Fig. 8. SEM micrographs of deposits prepared on CdSe (a) as-deposited and (b) after heat treatment at 350 1C for 1 h under an argon stream. Atomic composition from EDX after removing stoichiometric Cd and Se signal is: (a) 4.1% Zn, 95.9%Se; (b) 53% Zn, 47% Se.
crystallites grown dominantly in the /111S direction on strongly textured films of (1 1 1)CdSe. The XRD reflection intensity of the ZnSe samples obtained by this single step electrochemical process is the highest observed as far as the authors know. The investigated heterostructure constitutes a bilayer system, which by further elaboration can be suitable for accomplishing both tasks of (a) increased efficiency in solar conversion, (b) enhanced resistance towards electrochemical corrosion in photoelectrochemical cells. The ability to grow crystalline ZnSe layers with well-ordered structure on CdSe is very important in constructing backwall solar cells where ZnSe serves as a window to CdSe absorber. One problem to be solved before the evaluation of an application device is the control of the hexagonal Se phase formed with ZnSe compound. This can be removed by evaporation; however the influence of a thermal treatment on the ZnSe/CdSe interface
microstructure has to be investigated by further studies.
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Journal of Crystal Growth 277 (2005) 335–344 www.elsevier.com/locate/jcrysgro
Oriented [1 1 1] ZnSe electrodeposits grown on polycrystalline CdSe substrates M. Bouroushian, T. Kosanovic, N. Spyrellis General Chemistry Laboratory, School of Chemical Engineering, National Technical University of Athens, Zografos Campus, GR 157 80 Athens, Greece Received 5 April 2004; accepted 6 January 2005 Available online 16 February 2005 Communicated by M.S. Goorsky
Abstract Cathodic electrodeposition of cubic blend ZnSe from aqueous, acidic sulfate solutions on polycrystalline, CdSe(1 1 1) substrates, is described. The structural and morphological properties of the deposited films are discussed in terms of electrolytic growth conditions. It is shown that ZnSe crystallites of well-resolved X-ray diffraction (XRD) response, adopting a strict [1 1 1] preferred orientation, can be obtained due to the substrate texture under low supersaturation growth conditions. The (1 1 1)||(1 1 1) orientation relationship is investigated with respect to underlayers of varying crystallinity and various overlayer thicknesses. The nucleation and growth of ZnSe is discussed in terms of XRD and scanning electron microscopy (SEM). r 2005 Elsevier B.V. All rights reserved. PACS: 68.55.a; 81.05.Dz; 81.10.Dn; 81.15.Pq; 82.80.Ej; 73.40.Lq Keywords: A1. X-ray diffraction; A2. II–VI Epitaxial growth; A3. polycrystalline electrodeposition; B1. ZnSe/CdSe heterostructures; B2. semiconducting II–VI materials
1. Introduction Synthesis and characterization of wide gap II–VI semiconductors are of great importance, since these materials can be utilized in a variety of Corresponding author. Tel.:+30 210 7723097;
fax: +30 210 7723088. E-mail address: [email protected] (M. Bouroushian).
advanced technology optical and optoelectronic applications, viz. solar cells, heterojunction LED and LDs, photoconductors, etc. Hence, as having direct band gap energy of 2.7 eV, zinc selenide (ZnSe) turned out recently to be of exceptional interest for optoelectronics, in particular for fabrication of blue light diodes, while it can also be advantageously used as a buffer/window layer in chalcogenide-based thin film solar cells by
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.01.053
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constructing a heterojunction under lattice matching conditions [1–3]. Various approaches have been addressed to II–VI epitaxial growth, focused mainly in epitaxy on III–V or nonpolar substrates (such as Si and Al2O3), due to the relatively poor quality of II–VI substrates [4,5]. The fabrication of heterojunctions has been dominated by the availability of suitable substrates and the ability of growth, on account of the lattice parameter match as well as the match in band gap energy, electron affinity and thermal expansion coefficient. Alloy systems where such properties are similar over the binary composition range are particularly attractive, although heterostructures with relatively large deviations in lattice dimensions have manifested surprisingly good behavior. Thus, ZnTe on (1 0 0) GaAs substrate with 7% mismatch ratio makes little difference to less deviated systems, at least for ZnTe layers greater than the critical pseudomorphic thickness, while a twice as much mismatch of 14% between CdTe and GaAs does not prevent a clear orientation of the overlayer with respect to the substrate [3–5]. We may note also the successful growth of CdTe crystals on layered NbSe2 in spite of a mismatch ratio as large as 33% [6]. In these cases, certain coordinate and binding relations control the lattice engineering. ZnSe single crystals have been produced by normal or seeded melt growth and vapor transport techniques [5]. Nearly exact lattice matching (99.74%) of ZnSe can be achieved by epitaxial growth on GaAs, although deposition methods (mainly MBE) actually resort to ZnS0.06Se0.94 ternary, as the critical thickness for pseudomorphic growth of binary ZnSe on GaAs is limited to 100 nm [4]. Electrochemical methods of ZnSe epitaxial growth on GaAs and InP are reported recently [7]. Investigation on ZnSe/CdSe system modulated as a window/absorber II–VI heterojunction is of particular interest. CdSe is a well-studied II–VI of n-type conductivity with a direct and compatible with solar spectrum energy gap of 1.7 eV, while ZnSe serves as an effective window layer in CdSebased solar cells on account of its wide gap and the fact that, along with CdTe, comprises the only II–VI obtainable in both n- and p-type forms.
Heterostructure ZnSe/CdSe thin film electrodes have been produced, e.g. by ternary elemental vacuum evaporation [8] and galvanostatic electrochemical co-deposition [9] while efforts are focused lately on ZnSe/CdSe superlattice and quantum dot-containing systems (e.g. [10,11]). In ZnSe/CdSe heterojunction, a moderate lattice mismatch ratio of 6.3%—for CdSe and ZnSe zinc blend lattices—is encountered, while valence, chemical and ionicity mismatches are essentially negligible. Still, a rather large thermal expansion coefficient disparity exists (17.3%), however, the relatively low hardness of II–VI semiconductors as compared to III–V conducts an easier strain relief, that is, growth of crack-free mismatched epilayers can be provided. Electron affinities (w) are of the proper order [w(CdSe)4w(ZnSe] for expecting a gain in open-circuit potential (VOC) of CdSe solar cell configuration [3]; ZnSe has been found to act as an effective insulating layer in CdSe MIS solar cells and enhanced VOC was observed already in early works with sputtered mixed ZnSe/CdSe electrode [8]. Note also that CdSe is effectively stabilized in photoelectrochemical cells (PEC) with suitable redox agents, however, when it comes for applied technology, CdSe photoelectrodes are not devoid of electrochemical corrosion. As presenting an acceptable resistance against corrosion, ZnSe acts as a blocking layer in CdSe-based liquid junctions, providing a barrier to hole transport toward the electrolyte [9]. Flat band potentials of polycrystalline CdSe thin film electrodes stay more positive than single crystals limiting the attainable VOC, so a suitable window layer is essential in this case. Both ZnSe and CdSe semiconductors are obtainable by soft growth techniques such as electrodeposition: as for the more difficult to obtain ZnSe, there have been reported few procedures giving actually microcrystalline samples from acidic selenite [9,12–14] and alkaline selenosulfate solutions [7]. In the present work, an electrochemical method for growth of ZnSe crystallites on polycrystalline CdSe substrates with a strong (1 1 1) dominant orientation is described. The deposited thin film structural and morphological properties are discussed in terms of substrate structure as well as of electrolytic growth conditions.
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2. Experimental details Films of CdSe and ZnSe were both electroplated in a thermostat-controlled conventional threeelectrode cell at 85–901 C, by using a potentioscan (Wenking PGS 81R) system. A rotating disc electrode (RDE) was employed in order to control the mass transfer regime in electrolytic solution. The CdSe deposition substrate was commercially pure Ni of 1.13 cm2 area, abraded and polished by 0.3 mm alumina powder. A platinum grid was used as a counter electrode. The working electrode potentials are measured against Hg/Hg2SO4 saturated sulfate reference (SSE). There was no correction for ohmic drop. Water (of 18.3 MO cm), purified by an ultra-pure water system (Easy Pure Barnstead RF), and as received analytical grade selenite (SeO2), 3CdSO4 8H2O and ZnSO4 7H2O reagents were used for the preparation of experimental solutions. The bath pH was adjusted by sulfuric acid at the working temperatures. Thin films were obtained by cathodic electrodeposition under potentiostatic, DC conditions for various plating times attaining 0.5–5 C electrolysis charge. Several samples were submitted to annealing in a furnace controlled by a Shimaden FP21 programmable controller. Heating was carried out at a 10 1C min1 rate up to the final temperature, and maintained there for 1 h, in air or under an argon stream. The structure of the as-prepared samples was examined by X-ray diffraction (XRD) by a Siemens D5000 unit. Scanning electron microscopy (SEM) images were taken by a JEOL JSM 6100 as well as a Leica Stereoscan 440 model apparatus, while compositional data were obtained by energy-dispersive X-ray (EDX) local analysis during electron microscopy.
3. Results and discussion The adopted working conditions have mainly resulted from earlier investigation on polycrystalline CdSe and ZnSe formation. CdSe films, used as substrates for ZnSe growth, were prepared by electrodeposition on Ni discs at a potential of 0.9 V vs. SSE from an aqueous acidic bath, according to the method described in Ref. [15].
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The mechanism of ZnSe/metal (Me) electroforming and the effects of experimental parameters have been outlined in Ref. [14]. 3.1. Voltammetry The electrochemical potential range for simultaneous reduction of Zn2+ and Se(IV) solution species on CdSe cathodic surface can be determined from the electric current–voltage characteristics of CdSe/Ni RDE at acidic baths of varying composition. Voltammograms recorded in 0.5 103 M SeO2, 0.2 M ZnSO4 solutions of pH ¼ 2, 3 and 4, including partial contributions of electroactive species on electric current are shown in Fig. 1 for applied potential variation between 0.9 and 1.6 V. Measuring range is limited by dissolution of CdSe layer at potentials more anodic than about 0.9 V at one side and the abundant deposition of Zn at 1.45 V. The blank acidic solution contribution in current has been removed from a, b, c curves, which represent potential sweeps in selenite-containing, Zn2+-free solutions. With these curves,
Fig. 1. Cathodic polarization of CdSe/Ni RDE at a 5 mV s1 scan rate in 0.5 103 M SeO2 acidic solutions (of pH ¼ 4, 3 and 2 for curves a, b, c, respectively) and 0.5 103 M SeO2, 0.2 M ZnSO4 acidic solutions (pH ¼ 4, 3 and 2; curves d, e, f) at a bath temperature of 85 1C. Each of a, b, c curves has resulted after removing a blank solution component, namely a sulfuric acid solution of similar pH and ionic strength (controlled by K2SO4). The inset cyclic voltammogram regards curve b and represents two cycles (each with an individual electrode), the second cycle reaching more cathodic potentials.
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the observed current at potentials more anodic than about 1.1–1.2 V depending on pH, is basically due to Se(0) and Se(2) formation during cathodic polarization of CdSe, according to Eqs. (1) and (2) [16,17]: SeO2 H2 O þ 4e þ 4Hþ ! Seð0Þ ðsÞ þ 3H2 O;
(1)
SeO2 H2 O þ 6e þ 6Hþ $H2 Seð2Þ ðaqÞ þ 3H2 O: (2) Selenium hydride from Eq. (2) reacts rapidly in acidic pH with Se(IV) in solution to give Se(0). The voltammetric peaks in the range of 1.1–1.2 V are considered to arise from reductive stripping of either Se(0) deposited on the surface or lattice Se(2d) (24d40) in the CdSe solid layer Se þ 2Hþ þ 2e ! H2 Se:
(3)
It is interesting to note that during a reverse scan, specified in Fig. 1 for b curve (pH ¼ 3), the cathode is strongly depolarized, i.e., even larger than forward cathodic currents are observed. This is true in particular for extended forward polarization, indicating that the electrode surface is irreversibly modified (e.g. growing in area) by the reductive deposition and stripping of selenium. Hydrogen cations effectively participate in the relevant actions either homogeneously in solution viz. in various equilibria with soluble Se species [17] or in adsorbed H+ ads form. This is evident in the abrupt drop of curve c (pH ¼ 2), implying that the behavior of the system is not merely a matter of resolution in blank and ‘‘active’’ current components. Note also, that according to blank solution voltammograms (not in figure), CdSe is found to reversibly adsorb H+ at ca. 1.36 V while at higher potentials is subject to appreciable hydrogen evolution. The voltammetric image is drastically changed by addition of Zn2+ ion excess in solution (d, e, f curves). In their presence, the competition for adsorption sites suppresses the cathodic reactions observed with zinc-free selenite solutions. It can be seen, that the overall polarization curves contain a nearly potential-independent region, where the electrolysis current is limited by mass transport,
in particular by diffusion of chalcogen ions towards the cathode. The as-controlled Se(IV) and Zn(II) co-reduction results in the formation of semiconductive ZnSe-containing layers within a potential range of approximately 1.1–1.4 V/ SSE (see also Ref. [14]). 3.2. XRD study The ZnSe crystallites formed on CdSe substrate exhibit a cubic zinc blend structure (ZB) characterized by a more or less pronounced [1 1 1] preferred orientation, under all preparation conditions. This texture is extinct when electrodepositing directly on a metallic substrate like Ni or HF-treated Ti [14], that is ZnSe/Me consists of ZB crystallites of statistical orientation. Representative XRD diagrams for ZnSe/Ti and ZnSe/CdSe are given in Fig. 2. The relative intensities of the ZnSe (1 1 1), (2 2 0) and (3 1 1) reflections in Fig. 2a satisfy the standard ratios, while electrodeposition on CdSe (Fig. 2b) gives rise to strongly intensified ZnSe(1 1 1). In this case, (2 2 0) and (3 1 1) become non significant. A numerical measure of the crystallographic orientation, i.e., the preference in [1 1 1] relative to the other main directions, can
Fig. 2. XRD patterns (CuKa) of equally thick deposits prepared on Ti (a) and CdSe/Ni (b) at a deposition potential of 1.3 V/SSE from a pH ¼ 3 solution. The reflection intensities (J) for ZnSe(1 1 1) are denoted in figure.
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be expressed as Iða; b; gÞ ¼
a=100 . a=100 þ b=70 þ g=44
(4)
Here a; b; g stand for the photon counts of (1 1 1), (2 2 0) and (3 1 1) reflections from experiment, while 100, 70 and 44 are the respective standard relative intensities from JCPDS. A close to one I value indicates that the solid under study has an (1 1 1) dominant orientation. Actually, ZnSe/Ti films give I in the range 0.25–0.6 while CdSe substrate induces a strong orienting effect on ZnSe crystallites, resulting in I ¼ 1: This effect involves an initially epitaxial growth evolving in a strictly conformal manner. Optimal conditions to achieve an oriented ZnSe growth entail a bath composition of 0.2 M ZnSO4, 0.5 103 M SeO2, pH ¼ 3 and a sufficiently high temperature (ca. 90 1C). The deposition potential is chosen at the middle of the chalcogen-diffusioncontrolled plane current region, where the ratio of ZnSe to Se tends to increase—according to XRD and SEM. At potentials lying near the boundary sections of the diffusion plateau, a stronger excess of elemental Se is observed. The XRD measurements with CdSe substrates of varying orientation character manifest that there exists an almost linear dependence of ZnSe(1 1 1) on CdSe(1 1 1) reflection intensity (Fig. 3, also Fig. 4); in particular, this is valid for CdSe(1 1 1) intensities up to about 70 kcps (for our measuring apparatus). The latter is an apparent critical value above which the texture of ZnSe overlayer is not influenced any longer as indicated at the right conclusion of the approximation curve in Fig. 4. Hence, CdSe(1 1 1) reflection intensities higher than about 70 kcps comprise the ‘‘perfect’’ substrate orientation for our experiment. Obviously, increasing the plating charge leads to obtaining thicker films, however, the deposition mechanism changes at a certain point; the charge passed through the cell, at a constant potential, increases almost proportionally with time up to 3 C but further advance of electrolysis brings about a rapid drop in current density and then the time required to attain any intended thickness is exceedingly increased. It is quite interesting to study the evolution of structure with the plating
Fig. 3. (a) XRD patterns within the low-angle region (CuKa) indicating (1 1 1) reflections of ZnSe/CdSe heterostructures with ZnSe overlayers deposited on CdSe of various (1 1 1) orientation intensities, at 1.3V/SSE for equal plating charges. The substrate diffractograms are shown in (b) in full intensity scale.
charge. This study was accomplished in two ways: (a) either by obtaining a series of ZnSe deposits of various thicknesses on structurally similar, different substrate-films of CdSe, or (b) by performing a roughly in situ determination of XRD response for a single-ZnSe/CdSe structure at different stages of deposition, viz. at various film thicknesses as well. Thereafter, ZnSe films of different thickness were obtained firstly on several, similar CdSe electrodes. As illustrated in Fig. 5, the (1 1 1) ZnSe intensity increases proportionally with deposition charge up to a value of about 3 C, while abruptly decreases afterwards with respect to rapid film degeneration. The latter might be attributed to enhanced elemental Se deposition, though this is not evident
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Fig. 4. XRD (1 1 1) intensities (in arbitrary units) of ZnSe electrodeposits as a function of their CdSe substrate orientation intensity.
Fig. 5. XRD (1 1 1) peaks of ZnSe as a function of plating charge for various films deposited on similar CdSe substrates. The main reflection of Se phase is included in figure.
from XRD spectra probably due to the amorphous nature of the solid chalcogen. The degeneration of deposited layers is implied also in the expansion of the obtained diffraction peaks as expressed by their full-width at half-maximum (FWHM). The ZnSe(1 1 1) FWHM value persistently decreases up to a specific thickness thus indicating an uninterrupted growth of crystallites, increasing afterwards as the deposition control is lost.
Fig. 6. XRD intensities of ZnSe(1 1 1) and Se reflections vs. various plating charges for sequential growth on CdSe substrates, at a constant potential (1.3V/SSE); data are given for two individual CdSe substrates; S1 with (1 1 1) orientation intensity equal to 71 kcps, and S2 (102 kcps).
The successive reestablishment of initial growth conditions according to approach (b), above, might lead to rather unsafe results; howbeit this interrupted electrodeposition of successive ZnSe layers on the same RDE reveals a similar to approach (a) behavior, indicating that growth readily recovers initial disturbances. The ZnSe(1 1 1) XRD response measured after growth steps of 0.5 or 1 C plating charge are brought in Fig. 6 which includes two series of experiments, each with a very well-oriented CdSe substrate. Evidently, the overlayer constitution at each point is analogous with the previously described. Note the too slight increase of the hexagonal Se reflection intensity with electrolysis charge. Using the Scherrer formula (Eq. (5)) in reference with XRD data leads to an estimation of the deposits crystallites’ size by determining their vertical dimension (d): d¼
0:9l . B cos yB
(5)
In Eq. (5), l is the X-ray wavelength in angstroms and yB the Bragg angle in radians, for a particular reflection. B represents the extra peak width at half the peak height indicating the line broadening with respect to a suitable standard.
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This is obtained from the Warren formula: B2 ¼ B2M B2S : Here BM is the measured peak width in radians at half peak height (FWHM) and BS the corresponding value for a closely related reflection of a standard material, i.e., a diffraction peak at close angles to the relevant sample peak. The standard may have a particle size greater than about 2000 A˚ and is usually mixed with the measuring sample; since this is not possible with film characterization, the strong reflections of the underlying titanium substrate were used as reference standards, as well as separately prompted commercial ZnSe and CdSe powders of sufficiently large particle size. The CdSe films were thus estimated to be constituted by grains with sizes in the order of 100 nm while ZnSe crystallites were not found to be larger than 25–40 nm; these values representing crystallites’ depths rather than mean diameter. According to the above mentioned, the texture of ZnSe is determined by substrate and not by electrolytic growth conditions. The observed epitaxial effects are mainly due to the relatively small mismatch amidst cubic ZnSe and CdSe lattices. Still, this mismatch is high enough to limit the
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existence of a pseudomorphic region, hence brings on misfit dislocation within the first few deposited monolayers, promoting thus a 3D island growth (below). We may note here that obtaining ZB structure of ZnSe is not self-evident, since our experiments show that various defective layers with an appreciable content of wurtzite (W) ZnSe phase can be obtained by electrodeposition on (polycrystalline) metal substrates. On the other hand, growth of either W or ZB is geometrically possible on a [1 1 1]-oriented ZB substrate such as CdSe without producing miscoordinated atoms [18]. An epitaxial size selectivity effect is encountered here: the phase with the best elastic match with the substrate is dominantly grown. Consequently, the illustrated buffering effect of CdSe(1 1 1) on ZnSe growth manifests that the ZB structure of ZnSe offers the best elastic match. 3.3. Morphology and growth It is interesting now to examine the growth of the overlayer in terms of morphology. The SEM pictures of Fig. 7 show a CdSe RDE subject to
Fig. 7. SEM micrographs of ZnSe/CdSe deposits prepared at 1.3V/SSE. Plating charge is 2 C for (a) and 5 C for (b); average atomic composition from EDX: (a) 4.7% Zn, 25%Cd, 70.3%Se; (b) 6.2%Zn, 13.1%Cd, 80.7%Se. Brighter contrast forms are almost 95% in Se. Surface details are shown in (c) and (d) where EDX gives: (c) on dendrite: almost 95% Se; (d) on rectangle: 21.1% Zn, 78.9%Se.
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electrodeposition from a zinc sulfate solution of selenite for different charges passed (2 and 5 C in a and b, respectively). It can be seen that a dense underlayer is partly covered by an irregular layer of multi-shaped, loosely adhering particles, which proliferate with thickness. The morphology of this overlayer is ill-determined, as consisting of granular mainly, together with dendritic and flake-like forms. It is apparent also that the visible globules comprise cauliflower-like aggregates of many smaller nanocrystallites. According to EDX, all forms are rich in Se, yet there exists a difference in composition between the visible compact underlayer (Fig. 7a) and the accumulating grains. Upon the extreme assumption that no ternary (Cd,Zn)Se at all is formed and samples consist of stoichiometric CdSe, ZnSe and elemental Se, EDX data processing suggests that the underlayer is richer in ZnSe (26% in Fig. 7a sample) as compared to the general layer composition (19% in ZnSe). Dendritic forms such as the one appearing at the micrograph of Fig. 7c are almost 95% in Se while rectangle shaped details (Fig. 7d) contain more Zn. Thereafter, on account also of the well-resolved XRD reflections, hexagonal and amorphous Se grains together with well-developed but tending to be isolated ZnSe crystallites are present on film surface. The electrodeposition procedure is configured to take place under surface kinetics control. Growth controlled by a planar superficial diffusion of reduced ad-atoms is desirable. Under these conditions a nucleation delay, observed also in other akin systems [4], is expected to occur. An initial, strongly bound to CdSe, very thin ZnSe layer occurs during the first instants of deposition. Should the electrolysis time exceeds few minutes, the coherence of the obtained overlayer is already weak. It appears that after the formation of a few successive polar monolayers of Zn and Se atoms that are structurally registered on the crystallites of the substrate, a 3D island growth prevails and proceeds by lateral growth on a uniformly nucleating ZnSe and Se phase surface, until the particles coalesce and cover the cathode. The nucleation rate progressively decelerates as indicated also by the current density drop; though, lateral growth of crystallites is limited also, and an
accumulation of growing islands seems to be necessary in order to allow the electrical contact between the electrolyte and the less resistive substrate. Then, at a critical point, the resistance of the deposited layer becomes too high to allow a fast charge transfer, and the kinetic control is completely lost. Besides, the deposition potential is subject to a significant drop over the layer formed. During this procedure, growth and clustering of the initial 3D nuclei rapidly give rise to a loosely retained overlayer. Annealing in air at 200 1C for 15 min adjusts stoichiometry establishing a virtually 1:1 Zn/Se atom ratio, according to EDX analysis. The excess of Se totally sublimes giving overall grains stoichiometric composition. The electrodeposit layer, though, remains structured as deposited. A less soft thermal treatment, i.e., 350 1C for 1 h modifies morphology resulting in a film homogenizing (Fig. 8), however a small deficiency of lattice Se may be obtained, due to transformation of ZnSe to ZnO (according to XRD) if oxygen traces are present. Further, CdSe is subject to wurtzite phase transition. It is considered that CdSe underlayer takes part in the morphological changes observed in Fig. 8, possibly providing a proper matrix for ZnSe particles merge. Heating, as promoting the diffusion of metal ions, effectively modifies the interface between the two semiconductor layers, thus Cd(1x)ZnxSe ternary is formed. Then, a range of mixed compositions may exist between CdSe and a thin ZnSe outer layer of crystallites. The sintering effect is certainly stronger at elevated temperatures. Besides, an observed decrease of ZnSe(1 1 1) diffraction response after annealing, when not due to ZnO formation, supports this fact. Note that, the full development of CdSe wurtzite phase above 450 1C complicates the image.
4. Conclusions An (1 1 1)||(1 1 1) orientation relationship can be established between the ZB phases of electrodeposited ZnSe and polycrystalline CdSe film substrate. Cathodic electrocrystallization under kinetic control is well-suited for obtaining ZnSe
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Fig. 8. SEM micrographs of deposits prepared on CdSe (a) as-deposited and (b) after heat treatment at 350 1C for 1 h under an argon stream. Atomic composition from EDX after removing stoichiometric Cd and Se signal is: (a) 4.1% Zn, 95.9%Se; (b) 53% Zn, 47% Se.
crystallites grown dominantly in the /111S direction on strongly textured films of (1 1 1)CdSe. The XRD reflection intensity of the ZnSe samples obtained by this single step electrochemical process is the highest observed as far as the authors know. The investigated heterostructure constitutes a bilayer system, which by further elaboration can be suitable for accomplishing both tasks of (a) increased efficiency in solar conversion, (b) enhanced resistance towards electrochemical corrosion in photoelectrochemical cells. The ability to grow crystalline ZnSe layers with well-ordered structure on CdSe is very important in constructing backwall solar cells where ZnSe serves as a window to CdSe absorber. One problem to be solved before the evaluation of an application device is the control of the hexagonal Se phase formed with ZnSe compound. This can be removed by evaporation; however the influence of a thermal treatment on the ZnSe/CdSe interface
microstructure has to be investigated by further studies.
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