X-ray photoelectron spectroscopy study of the nucleation processes and chemistry of CdS thin films deposited by sublimation on different solar cell substrate materials

X-ray photoelectron spectroscopy study of the nucleation processesand chemistry of CdS thin films deposited by sublimation on differentsolar cell substrate materials

J. P. Espinós,a� A. I. Martín-Concepción, C. Mansilla, F. Yubero, and A. R. González-ElipeInstituto de Ciencia de Materiales de Sevilla (CSIC-U. Sevilla), C/Américo Vespucio s/n,E-41092 Sevilla, Spain

�Received 9 September 2005; accepted 30 March 2006; published 22 May 2006�

Cadmium sulfide has been deposited by evaporation on five different substrates: CdTe, ZnO, Ag,TiO2, and partially reduced titanium oxide �i.e., TiO1.73�. The deposition rate and the evolution ofthe Cd/S ratio on the different substrates have been determined by x-ray photoelectronspectroscopy. The growth mode of the films has been also studied by analyzing the shape of thebackgrounds behind the photoemission peaks �peak shape analysis�. It has been found that, undercompletely equivalent conditions, the deposition efficiency �i.e., sticking coefficient� is large onCdTe and TiO1.73, but very small on ZnO and TiO2. Silver constitutes an intermediate situationcharacterized by a long induction period where the deposition rate is small and a later increase indeposition efficiency comparable to that on CdTe. For the initial stages of deposition, below anequivalent monolayer, it has been also found that the Cd/S ratio is smaller than unity on TiO1.73 andZnO but larger than unity on CdTe and Ag substrates. For sufficiently long deposition times theCd/S ratio on the surface reaches unity. Except for silver substrate, cadmium appears as Cd2+ andsulfur as S−2 species at the initial stages of deposition. On the silver surface, cadmium adsorbs asCd0 at low coverage. Peak shape analysis has shown that cadmium sulfide grows according tolayer-by-layer mechanism �Frank–van de Merwe model� when the substrates are CdTe and TiO1.73,but large particles are formed that do not cover the surface for ZnO and Ag substrates�Volmer-Weber growth model�. These results are consistent with the different chemical affinities ofthe substrate towards the atoms of cadmium and sulfur produced during the evaporation of the
cadmium sulfide. © 2006 American Vacuum Society. �DOI: 10.1116/1.2198868�
I. INTRODUCTION

Cadmium sulfide, a semiconductor with a band gap ofEg=2.42 eV, is an interesting material for optoelectronic de-vice applications. In particular, n-CdS is used to form het-erojunctions with p-CdTe and other chalcopyrite thin films�i.e., CuInSe2� for its implementation in low-cost and high-efficiency solar cells where photovoltaic conversions in ex-cess of 20% have been reported.1,2

The structure of these solar cells is a superstrate-type con-figuration, where several thin layers of semiconductors �CdS,CdTe, CuInSe2, Cu2S, etc.�, transparent and conducting ox-ides �TCO� �SnO2, In2O3, ZnO, etc.�, and metals �Ag, In,Mo, C, Ni, etc.� are deposited on a cheap transparent sub-strate. For this configuration, the cell is operated with theglass/TCO/CdS structure facing the sun. As an example, avery high photovoltaic conversion efficiency was reportedrecently for a solar module made of glass/TCO/n-CdS/ p-CdTe/carbon/Ag–In.3 In this cell, the following interfaces arepresent: CdS/TCO, CdS/CdTe, and CdS/Ag�In�.

For the fabrication of CdTe thin film solar cells, the CdSlayer can be deposited by sublimation, vapor transport depo-sition, magnetron sputtering, or chemical bath deposition.4,5

In this context, the uniformity of thickness and microstruc-

a�Author to whom correspondence should be addressed; electronic mail:
[email protected]
919 J. Vac. Sci. Technol. A 24„4…, Jul/Aug 2006 0734-2101/2006

ture �i.e., porosity, grain structure, columnar character, etc.�of the CdS film strongly influences the final performance ofthe cells.3,6 According to common knowledge on thin filmgrowth, their microstructure is controlled by many factors.Among them, the first nucleation steps are of great impor-tance for the later evolution of the film characteristics.7

In the present work we have studied the initial stages ofgrowth of CdS on oxides �TiO2 and ZnO�, semiconductor�CdTe�, and metal �Ag� substrates by x-ray photoemissionspectroscopy �XPS�. Both the type of chemical interactionsdeveloping at the different interfaces and the morphology ofthe first nuclei have been investigated for CdS deposited bythermal evaporation on the surfaces of these substrates. Forthe analysis of the morphology of the first-deposited nuclei,we have used evaluation procedures based on the methoddeveloped by Tougaard to simulate the shape of backgroundsbehind the photoemission peaks.8,9

From this study, it has been possible to determine that thetype of substrate and its chemical state are of the utmostimportance for the control of the morphology of the CdSnuclei. Additional information gained from this study refersto the changes of sticking coefficients and to the possiblechemical reactions occurring at the interface during deposi-tion depending on the state of each specific substrate.

In relation with the synthesis of thin film cells prepared bysublimation,4 the obtained results have shown that a precise
control of the CdS thin film properties is possible by select-
919/24„4…/919/10/$23.00 ©2006 American Vacuum Society

920 Espinós et al.: XPS study of the nucleation processes and chemistry 920

ing the appropriate conditions for the conditioning of thesubstrates and for the evaporation of cadmium sulfide.

II. EXPERIMENTAL AND METHODS

Cadmium sulfide was evaporated from a homemade free-evaporation cell, consisting of a ceramic tube �Al2O3, 2 mminner diameter, 5 cm long, the bottom end closed, and anorifice of 3.14 mm2� wound with a tungsten heating coil.This ceramic container was loaded with CdS �Aldrich, pow-der, purity of 99.995%�, pressed into pellets of �1 mm2 toavoid splattering. Proper heat shielding of the evaporationsource was obtained by two concentric cylinders �6 and10 mm of diameter, respectively�, made of a tantalum foilrolled and spot welded.

Evaporation was done by resistively heating the cell, at723 K, in the preparation chamber of the spectrometer. Thischamber has a background vacuum better than 1�10−8 mbar, but pressure rose to 4�10−8 mbar during va-porization. The open end of the source was covered with ashutter for 10 min before exposure of the substrates to thegas phase. This time was enough to stabilize the temperatureof the cell. A single load of the alumina tube with CdS wasenough to carry out all the experiments. The evaporation rateof the cell was not determined.

Sublimation was done over square flat pieces �5�5 mm2� of the five substrates selected for the presentstudy: Ag, CdTe, ZnO, TiO2, and TiO1.73. The distance fromthe open end of the evaporation source to the position of thesubstrates was �15 cm. Substrates were held at room tem-perature during CdS deposition and further photoemissioncharacterization, as measured with a Ni–NiCr thermocuplelocated at the front face of the sample holder, a block ofstainless steel. The experiments were carried out twice.

The following materials were used as substrates: poly-crystalline foils of silver �Goodfellow, purity of 99.99%�,mirror-polished lumps of polycrystalline CdTe �Alfa-Aesar,purity of 99.999%�, ZnO films ��1 �m thick� deposited onSi�100� by plasma enhanced chemical vapor deposition�PECVD�,10 and TiO2 films ��1 �m thick� deposited onSi�100� by ion beam induced chemical vapor deposition�IBICVD�.11

Silver and CdTe substrates were cleaned by Ar+ ion bom-bardment �1 keV ions� in the pretreatment chamber of theXPS spectrometer �base pressure of �10−8 Torr�, while ox-ide substrates were cleaned by exposure to a plasma of oxy-gen �P=2�10−2 Torr�, until complete removal of the con-taminating carbon. In the case of titanium oxide, thissubstrate was also subjected to Ar+ bombardment with ionsof 2 keV. As a result of this treatment, the surface of thetitanium oxide was partially reduced to TiO1.73, because ofpreferential removal of oxygen.12 This stoichiometry was de-duced by curve fitting of the Ti 2p peaks �54% Ti+3 and 46%Ti+4�. Experiments on both the partially reduced �TiO1.73�and the completely oxidized �TiO2� surface of titanium oxidewere carried out.

The oxygen plasma used for cleaning was produced in a
quartz tube attached to the preparation chamber. This tube
J. Vac. Sci. Technol. A, Vol. 24, No. 4, Jul/Aug 2006

was inserted in a cavity excited by microwave power�100 W�, produced by a Microtron 200 generator �Electro-Medical Supplies Ltd.�. Ar+ ions were produced by an 8018ion gun �Vacuum Generators�.

X-ray photoemission spectra were recorded with an ES-CALAB 210 spectrometer fitted with a hemispherical energyanalyzer working at a constant pass energy of 50 eV. Thebase pressure of the analysis chamber was in the range of10−10 Torr. Unmonochromatized Mg K� �for CdS/Ag andCdS/ZnO� or Al K� radiation �for CdS/TiOx �x=2 or 1.73�and CdS/CdTe� was used as excitation sources. Binding en-ergy �BE� calibration of the spectra was done by referencingthe recorded peaks to the following main peaks of the sub-strates: Ag 3d5/2 at 368.22 eV, Te 3d5/2 at 572.5 eV, Zn 2p3/2

at 1022.05 eV, and O 1s at 530.65 eV. Intensity ratios forthe different photoemission signals have been calculated byestimating the area of the elastic photoemission peaks and byreferring the obtained values to their relative sensitivityfactors.13 In the case of the CdS/CdTe system, since cad-mium is present in both the adsorbate and the substrate, theamount of deposited CdS was estimated by subtracting theCd linked to Te from the total recorded intensity of this ele-ment. For this purpose, a constant Cd/Te ratio in the sub-strate was assumed throughout the deposition.

For the analysis of the backgrounds behind the photo-emission peaks, which is the basis of Tougaard’s method,8,9

we used the QUASES program.14 This analysis is based on anappropriate description of the electron energy losses duringtransport in the solid. The output of the analysis is an in-depth profile of the emitters of these peaks within the surfaceregion of the sample and the primary excited spectrum �be-fore electron transport�. This profile is described by a surfacecoverage and an average island shape and size. For all sys-tems studied here, orthorhombic particles of CdS, all of themof the same shape and size, are assumed for each growthstage. Consequently, it is possible to determine, for eachdeposition stage, the average height of the adsorbate particlesand the fraction of covered surface of substrate. The extentof the inelastic tails on the low-kinetic-energy side of eachanalysed XPS peak varied from 60 to 100 eV, depending onthe studied case. The following photoemission signals wereconsidered for analysis of each system:

• CdS/Ag: S 2p excited with Mg K�,• CdS/CdTe: Te 3p excited with Al K�,• CdS/ZnO: Zn LMM, excited with Mg K�, and finally,• CdS/TiOx �x=2 or 1.73�: Cd 3d excited with Al K�.

In general, growth modes are classified in three categoriesdepending on the resulting film morphology.15 They are �a�layer-by-layer, �b� three-dimensional �3D� island, and �c� 3Disland-on-wetting-layer growth. In the layer-by-layer growth,the interatomic interactions between film and substrate ma-terials are stronger than those between the different species�atoms, ions, etc.� within the film material, whereas just theopposite is true in 3D growth. The third mode of growth
occurs for interaction strengths somewhere in the middle.


In this work, morphologies of the CdS films on the differ-ent substrates have been determined by the method devel-oped by Tougaard to simulate the shape of backgrounds be-hind the photoemission peaks.8,9,16

The measured spectra J�E�, the primary excitation spectraF�E� �excited spectrum before electron transport�, the in-depth concentration profile f�z� �where z is the depth belowthe surface�, and the inelastic electron scattering cross sec-tion �K�T� for an energy loss T are linked within electrontransport theory, which for the particular case of normalemission reads

F�E� =1

2�� dE�J�E�� ds exp�− is�E − E��

��0

�

dzf�z�exp�−z

��s� , �1a�

where the function ��s� is given by

��s� = 1 − �0

�

dT�K�T�exp�− isT� , �1b�

where s is an integration variable without physicalsignificance.

The first step of the XPS peak shape analysis is to deter-mine the primary excitation spectrum F�E�, which is as-sumed to be independent of the in-depth distribution of at-oms. This can be done by applying Eq. �1� to a referencesample where f�z� is known. Here we determined F�E� byanalysis of the Cd 3d �and S 2p� spectra from a thick layer ofCdS �several hundred nanometers thick�, and of the Zn LMMand Te 3p spectra from the bare substrates before any depo-sition �for which f�z�=1�. The second step of the analysis isto apply Eq. �1� to the spectrum of a sample with unknownin-depth distribution. Note that the primary excited spectrumF�E� is not an unknown function anymore, since it was de-termined from reference samples as described above. A trial-and-error procedure, in which the parameters that describethe surface nanostructure �in our case, c, the surface cover-age and h, the height of particles� are varied, is performeduntil a consistent analysis within Eq. �1� is achieved. �i.e.,good agreement in both shape and intensity between theprimary F�E� for each sample and that obtained from thereference�.

III. RESULTS

A. Relative sticking efficiency of Cd and S on thedifferent substrates

It is well known from literature17,18 that, at the tempera-ture of the evaporation source �723 K�, CdS evaporates con-gruently with dissociation into its component elements, ac-cording to the reaction

CdS�solid� → Cd�gas� + 12S2�gas�.

This means that differences in the sticking coefficients ofCd and S2 on the different substrates might lead to changes
in the deposition rates and/or to the thin film composition
JVST A - Vacuum, Surfaces, and Films

�particularly at the interfaces�. Also, it must be noticed thatthe S2 species must disociate to form CdS, a reaction thatvery likely can be catalyzed by the substrate. To addressthese points, we have looked first at the evolution of the Cd3d5/2 and S 2p peak intensities for successive evaporationsteps. Thus, Fig. 1 shows the evolution of the intensity ratioI�Cd 3d5/2� / I�M�, where M means a peak of the selectedsubstrate �i.e., M =Te 3d, Ti 2p, Zn 2p, and Ag 3d�, as afunction of the sublimation time of CdS. �As mentioned inthe Experiment section, in the case of CdS/CdTe system,only the Cd due to the adsorbate has been taken into accountfor quantification of this intensity ratio.� It is apparent fromthis figure that, while this ratio increases rather sharply onCdTe and TiO1.73, it increases more slowly on Ag during theinitial stages, and is almost negligible for ZnO and TiO2.Except for the case of these two stoichiometrical oxides,where the deposition efficiency is very low even after longperiods of sublimation, in the other three cases the depositionrates change from low values at the beginning of the depo-sition to higher values when the surface is already saturatedin cadmium sulfide. This kind of induction period is veryimportant on silver, where it lasts for 125 min under ourworking conditions. The maximum deposition rate, estimatedon CdTe or TiO1.73, was �2 nm h−1.

Since the electrical power supplied to the evaporation cellwas always the same for all the experiments, and the distanceand the orientation of the substrates with respect to theevaporation source are also fixed, the different evolutions ofthe ratios in Fig. 1 can only be explained by assuming thatthe sticking coefficient of Cd on the five substrates is quitedifferent. The relative magnitudes of the experimentally de-termined sequence of sticking coefficients are ordered as fol-lows: CdTe=TiO1.73Ag ZnOTiO2.

To check whether the incorporation of sulfur onto the sur-face of the substrates follows a similar profile as that of Cd,we have analyzed the evolution of the intensity ratio Cd

FIG. 1. Deposited amount of cadmium sulfide estimated from the intensityratio I�Cd 3d5/2� / I�M�, where M means a peak of an element of the sub-strate �i.e., M =Te 3d, Ti 2p, Zn 2p, and Ag 3d�, as a function of thesublimation time of CdS. The inset shows an enlargement of the regioncorresponding to the initial deposition stages on TiO1.73 and CdTe assubstrates.

3d /S 2p as a function of sublimation time. The results are


depicted in Fig. 2, which shows the evolution of the Cd/Sratio against the sum of these two elements normalized to theintensity of the main photoemission signal from an element�M� in the substrate �again, for the measurements on CdTe,only the Cd 3d intensity pertaining to CdS is considered forthe calculations�. It is apparent from this figure that duringthe first stages of growth, on a ZnO or TiO1.73 substrate, theintensity of Cd is smaller than that of S, thus indicating thatthe sticking coefficient of S2 is larger than that of Cd onthese two materials. The opposite behavior is found on CdTeand Ag where, for a long period of time, the amount ofadsorpted Cd is larger than that of sulfur. It is also interestingthat, in these latter cases, �especially for Ag�, the time neededto equilibrate the adsorbate composition �i.e., the Cd/S ratioequal to 1, as in the material used for evaporation� is muchlonger than for the other two substrates. Finally, the intensi-ties of Cd 3d and S 2p signals were so small on TiO2 that theCd/S ratio could not be reliably evaluated. Consequently,data for this system are not included in Fig. 2.

Equilibration in the composition of the deposit indicatesthat any specific effect of the substrates on the values of thesticking coefficients of Cd and S2 is removed after a certainsurface coverage has been attained. This equilibration of theCd Cd/S ratio is very fast on ZnO and TiO1.73. By contrast,on silver, a steady state is reached after sublimation for amuch longer period of time, probably indicating the diffusionof sulfur into the metal. This is in fact the basis of the wellknown “tarnishing process” that this metal suffers when it isexposed to sulfur.19

B. Chemical state of Cd and S on the differentsubstrates

The chemical state of both sulfur and cadmium can bedetermined for the different stages of the deposition processby looking at the BEs of the Cd 3d and S 2p peaks. For thispurpose, besides an analysis based on BEs, we have alsoexamined the evolution of the kinetic energy �KE� of the CdM4N45N45 Auger peak. The sum of BE and the KE yields the

20,21

FIG. 2. Cd/S atomic ratio obtained for CdS films deposited on the differentsubstrates, as a function of the total deposited amount of Cd and S.

so called Auger parameter ��. As an example, Fig. 3


shows the evolution of the S 2p, Cd 3d, Te 3d, and Cd MNNphotoemission peaks for successive sublimation times ofCdS on CdTe. It is apparent from this figure that the BE ofthe Cd 3d5/2 peak slightly changes �from 405.15+0.1 to 405.3+0.05 eV� as the sublimation time increases,while the kinetic energy of the Cd M4N45N45 peak shiftsclearly on passing from CdTe �382.1+0.1 eV� to CdS�381.1+0.1 eV�. Small shifts are also observed for the S 2ppeak ��161.7+0.1 eV BE�.

The results of an equivalent experiment carried out on asurface of partially reduced titanium oxide are depicted inFigs. 4 and 5. Here, also small shifts in the positions of Cd3d5/2 �from 405.5+0.1 eV at low coverage to 405.3+0.05 eV at high one� and S 2p can be observed. The CdM4N45N45 peak, which is superposed on the Ti L3M23M23

Auger signal from the substrate, is located at 381.3+0.1 eVKE for a thick CdS film �Fig. 4�. By contrast, as shown inFig. 5 �left panel�, this Auger peak appears at around 381.6+0.2 eV for small surface coverages �initial depositionstages�. For the determination of these peak positions, thesignal due to the substrate has been subtracted. Conse-quently, the Cd Auger parameter decreases with sublimationtime from �787.1+0.2 to 786.6+0.1 eV.

Regarding the TiO1.73 substrate, it is interesting to notethat during the initial stages of CdS deposition, the shapes ofboth Ti 2p and Ti L3M23V spectra change, as shown in Fig. 5�right panel�. The observed changes in the Ti signals arecharacterized by a slight loss in intensity on the right side ofboth peaks. This effect clearly indicates that a partial oxida-tion of Ti+3 to Ti+4 ions has taken place during deposition ofcadmium sulfide.22

From the position of the main photoemission �Cd 3d5/2�and Auger �Cd M4N45N45� lines of cadmium, it is possible tocalculate the value of the Auger parameter �� of this ele-ment at each stage of the deposition process on the differentsubstrates. The derived values as well as those of severalreference compounds �Cd, CdO, CdTe, and CdS� have beenincluded in the modified Wagner plot shown in Fig. 6.23 OnTiO1.73 and ZnO substrates, all points are close together andappear in the proximity of bulk CdS. This result indicatesthat, on these two substrates, cadmium ions are in an envi-ronment similar to that existing in CdS, and are very littleaffected by the substrate �note, however, that for initial depo-sitions, the Cd/S stoichiometry is different from that of thebulk compound�. On CdTe, the Auger parameter of Cdclearly shifts from the typical value for this compound�787.25+0.1 eV� to that for CdS �786.7+0.1 eV�, as ex-pected for the progressive attenuation of the substrate and thegrowth in thickness of the deposited layer. Finally, on silver,a net evolution from 788.4+0.1 eV �a position close to theAuger parameter for metallic Cd, at 789.4+0.1 eV� to thevalue of CdS is found. The arrows in Fig. 6 indicate how thesublimation time �film thickness� increases throughout theexperiments. An initial conclusion from the results in Fig. 6
is that, after sublimation of CdS, cadmium is adsorbed as

or Cd


Cd+2 ions on CdTe, ZnO, and TiO1.73 �and also on TiO2,from data not included in Fig. 6�. By contrast, when theevaporation is carried out on silver, cadmium initially ad-sorbs as a metallic species.

An analysis similar to the latter was also carried out bylooking at the S 2p photoemission peak. A conclusion fromthis study is that in all cases the peak shape �an unresolvedsingle doublet� and position �161.7+0.1 eV binding energy�are typical of S−2 species.24 In other words, for all the studiedsubstrates, no traces of S2 species were found at the surface,a fact that indicates that all S2 molecules dissociate into Satoms after adsorption, and they are subsequently reduced toS−2.

C. Growth modes on the different substrates

As an example of the spectrum evolution which is found

FIG. 3. S 2p, Te 3d, Cd 3d, and Cd M4N45N45 spectra f

when increasing amounts of CdS are deposited on the differ-


ent substrates, Fig. 7 shows the selected Te 3p, Cd 3d, S 2p,and Zn LMM spectra corresponding to the deposition of CdSon CdTe, TiO1.73, Ag, and ZnO, respectively. A general be-havior is that, while the intensity of the elastic photoemissionpeaks of the deposited layer increases with the sublimationtime, the intensity of the peaks of the substrate decreases.Meanwhile, the background intensities behind the elasticpeaks of the lines due to the elements of the substratesstrongly increase.

Analysis of these photoemission lines by QUASES �Refs. 8,9, and 14� and the methodology described in the Experimentsection enables an assessment of the average island height�h� of the adsorbate particles and of the surface coverage ofthe substrate �c� for each particular substrate and sublimationtime.

Figure 8 shows an example of the analysis for the Cd 3d

S deposited on CdTe for increasing sublimation times.

peaks shown in Fig. 7. Note the consistency of the analysis:

spect


a unique primary spectrum F�E� results, within electrontransport theory, from analysis of the widely different mea-sured spectra. A similar degree of consistency was achievedfrom analysis of the other sets of samples �not shown�.

The final results of this analysis are depicted in Fig. 9which shows a representation of the values of h and c ob-tained for the four studied systems. To asses the total amountof deposited substance �AOS�, a series of dashed curves,corresponding to the product c�h=AOS, are also shown inFig. 9 for each experimental situation. AOS represents theequivalent thickness of a uniform layer, and how the c vs hrelationship may change for a given amount of depositedsubstance.

From the plots in Fig. 9 it is apparent that the surfaces of

FIG. 4. S 2p, Ti 2p, Cd 3d, and Cd M4N45N45 �on Ti L3M23M23�

CdTe and TiO1.73 are effectively covered by CdS, even after


deposition of very small amounts of this substance. Mean-while, on the two other substrates, large particles are formedand the surface is covered very slowly. These results showthat CdS grows on CdTe and TiO1.73 according to a layer-by-layer mechanism, while three-dimensional islands areformed on the other substrates. Moreover, for every deposi-tion stage, the height of the islands is different according tothe substrate, the highest particles being formed on ZnO.

IV. DISCUSSION

The previous results indicate several key features of theinterface chemistry of cadmium sulfide deposited by subli-mation on different substrates. A first point refers to the abil-

ra for CdS deposited on TiO1.73 for increasing sublimation times.

ity of cadmium sulfide to form a thin film when this material


is evaporated onto a series of substrates. It is found that,under completely equivalent conditions, the depositedamount follows the order: CdTe�TiO1.73�AgZnO�TiO2�.This tendency defines the evolution of the sticking coeffi-cients on the different substrate materials when cadmium sul-fide is sublimated. A direct practical consequence is thatwhile evaporation can be a very suitable method of deposi-tion of CdS on CdTe, it can be rather inefficient on stoichio-metric oxides such as ZnO and TiO2. In the two latter cases,other alternative procedures of deposition of cadmium sul-fide thin films would be advisable �e.g., chemical bath depo-sition �Ref. 4��. An alternative, at least for titanium oxide,that seems highly efficient is to partially reduce the surfaceof the oxide by ion bombardment, a treatment that could becarried out easily if magnetron sputtering is used for thedeposition of CdS �Ref. 25� or the TCO �Ref. 26� films.

Detailed analysis of the deposition rates �cf. Fig. 1� showsthat, even in the two cases where the deposition rate is high�i.e., CdTe and TiO1.73�, there is a certain induction period.This initial period must correspond to deposition on the baresubstrate surfaces. It is necessary to cover the substrate com-pletely to reach the steady deposition rate corresponding tothe sticking of cadmium sulfide on itself. We also found thatchemical interactions between the elemental cadmium andsulfur, produced after sublimation of CdS, are very importantin controlling their chemical state and the global depositionefficiency. In this context, the sticking coefficient for depo-sition on TiO1.73 is larger for sulfur �as S2� than for cadmium�cf. Fig. 2�, a result that suggests that the whole depositionprocess is controlled by the preferential adsorption of sulfur.In a previous publication on the interaction of SO2 with areduced surface of titanium oxide, it was found that sulfidespecies were formed by reaction with Ti3+ ions at thesurface.27 A similar process �i.e., 1 /2S2+2Ti3+→S−2+2Ti4+�

FIG. 5. Evolution of Cd M4N45N45 signal �after subtraction of the Ti L3M23Minitial stages of CdS deposition on TiO1.73.

must occur here when sulfur atoms or molecules arrive at the


partially reduced surface. This has been shown by the slightdecrease in the percentage of Ti3+ species during the firstdeposition stages, up to the approximate completion of anequivalent monolayer, from an assessment of the Ti L3M23Vand Ti 2p3/2 signals, �cf. Fig. 5�b��. Once a certain amount of

FIG. 6. Wagner plot for CdS deposited on CdTe, ZnO, TiO1.73, and Ag, forincreasing sublimation times �the arrows for Ag and CdTe substrates indi-cate the evolution throughout the deposition�. Points corresponding to me-tallic Cd, bulk CdTe, CdS, and CdO have been also included for

eak from the substrate, left panel�, and Ti 2p signal �right panel� during the
23 p
comparison.


sulfide species is formed on the surface, cadmium atoms tendto stick there according to a process that can be schematizedas Cd0+S−2−2Ti4+→Cd2+−S−2+2Ti3+.

On stoichiometric ZnO and TiO2 it is likely that the ab-sence of any redox process enabling the formation of S−2

species, at the initial stages of deposition, is the reason forthe low overall growth rate on these two oxides. On a non-reactive surface, sulfur must become adsorbed as S0 �S2 mol-ecules and S atoms� and cadmium as Cd0, so that both priordissociation of S2 and reaction of two neutral species �S andCd� are required to yield a stable CdS molecule. The prob-ability of such a complex process would be very small be-

FIG. 7. Experimental photoemission signals used in the peak shape analysis bAg, and ZnO, respectively.

cause of the low evaporation rates and substrate temperature


utilized in our experiment. In addition, the desorption of S0

species from the substrate surface cannot be excluded �thevapor pressures of S2 and cadmium at 293 K are �10−6 and�10−11 Torr, respectively28�.

On cadmium telluride, the growth process must occur ac-cording to a different mechanism. The deposition efficiencyis very high on this substrate, although there is a certaininduction period. Another relevant finding is that the CdTesurface becomes enriched in Cd2+ with respect to sulfur dur-ing the initial stages of deposition �cf. Fig. 2�. Both featurespoint to the oxidation of cadmium to Cd2+ as being the con-trolling factor in the deposition efficiency. The semiconduct-

SES: Te 3p, Cd 3d, S 2p, and Zn LMM for CdS deposited on CdTe, TiO1.73,
y QUA
ing nature of CdTe, where electrons can be located in the


conduction band and fill the cationic vacancies of the CdTesurface, could provide a mechanism for the oxidation of theCd adatoms.

In the case of silver, both the long induction period in thedeposition rate �cf. Fig. 1� and the enrichment of cadmium atthe surface during the initial stages of growth �cf. Fig. 2� canbe accounted for by preferential incorporation �diffusion� ofsulfur into the bulk of silver. As a result, cadmium appears inits metallic state at the beginning of the deposition experi-ment. It is likely that the formation of a metallic alloy acts adriving force for the enrichment of the surface in Cd �cf.Fig. 2�.

Our analysis of the growth mechanism on the differentsubstrates agrees with this model of preferential adsorption/reaction of sulfur or cadmium during the initial stages ofdeposition. Thus, the results in Fig. 9 depict three well de-fined situations with respect to the type of particles/layersthat form on the surface of the different substrates. The caseof ZnO, where CdS grows in the form of large particles thatleave a considerable portion of the substrate surface uncov-ered, is consistent with the poor chemical affinity betweensulfur and/or cadmium and the surface of this oxide. By con-trast, TiO1.73 and CdTe show another behavior, characterizedby a high deposition rate and a monolayer-by-monolayergrowth mechanism.15 This is consistent with a reaction ofeither sulfur �on TiO1.73� or cadmium �on CdTe� with thesubstrate surfaces. This process must produce a completelyhomogeneous seed of ions �S−2 or Cd+2�, thus leading to acompletely homogeneous growth of the CdS film on thewhole surface. Finally, the growth mode on silver is an in-termediate situation where initially the surface is coveredrather efficiently up to approximately 50% coverage. Cad-mium sulfide then grows in the form of islands that cover thesurface completely only for heights larger than 9 nm. Al-though at present we cannot provide a precise model to ex-plain this behavior, it is likely that cadmium becomes pref-erentially incorporated into the surface along grainboundaries of the polycrystalline silver �note that the diffu-

FIG. 8. Comparison of the primary excited F�E� spectra obtanied from theexperimental Cd 3d signals shown in Fig. 7.

sion of sulfur into the bulk is also likely to occur through


these boundaries�. This means that the initial nucleation siteswould be heterogeneously distributed over the surface of thesubstrate and that particles of cadmium sulfide would onlygrow on these sites.

V. CONCLUSIONS

In this study we have shown that both the growth rate andthe morphology of CdS thin films prepared by evaporationon different substrates depend on the type of substrate for theinitial stages of growth. Stoichiometric ZnO and TiO2 arerather inefficient to fix cadmium sulfide, which grows ontheir surface in form of big particles. On these surfaces thereare no available any redox/adsorption mechanism to fix theevaporated sulfur or cadmium adatoms. By contrast, on CdTeand TiO1.73 the growth rate is large and cadmium sulfidegrows according to a layer-by-layer mechanism becausethese substrates are chemically active with respect to theseelements. Silver is an intermediate situation where the diffu-sion of sulfur into the bulk controls the growth mechanismand the growing rate.

These findings have interesting consequences for thepractical use of sublimation methods for the preparation ofthin film solar cells and suggest alternative procedures forthe activation of the surfaces to enhance the adsorption effi-ciency �e.g., by partially reducing the surface of stoichio-metric oxides�. An example is the activation of titanium ox-ide by reducing its surface by ion bombardment where theadsorption/reaction of cadmium sulfide is greatly increased.

ACKNOWLEDGMENT

The authors thank the Spanish Ministry of Science andTechnology for financial support �Project Code MAT2000-1505-C02-01�

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2

FIG. 9. Plot of the island heights against the surface coverage for CdSdeposited by sublimation on different substrates. The dashed lines representfixed AOS, as indicated �see text�.

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X-ray photoelectron spectroscopy study of the nucleation processes and chemistry of CdS thin films deposited by sublimation on different solar cell substrate materials