06) 3866–3870www.elsevier.com/locate/matlet

Materials Letters 60 (20

Some physical properties of Sn-doped CdO thin filmsprepared by chemical bath deposition

L.R. de León-Gutiérrez a, J.J. Cayente-Romero a, J.M. Peza-Tapia a, E. Barrera-Calva b,⁎,J.C. Martínez-Flores b, M. Ortega-López a

a Sección de Electrónica del Estado Sólido, Departamento de Ingenierıa Eléctrica, CINVESTAV-IPN, Avenida IPN No. 2508, C. P. 07360, México, D. F., Méxicob Area de Ingeniería en Energía, IPH, CBI, Universidad Autónoma Metropolitana, C. P. 09340, México, D. F., México

Received 10 November 2005; accepted 31 March 2006Available online 27 April 2006

Abstract

Undoped and Sn-doped CdO thin films were prepared by the chemical bath deposition method by means of a procedure that improves thedeposition efficiency. All as-grown films were crystallized in the cubic structure of cadmium peroxide (CdO2) and transformed into CdO with acubic structure after an annealing process. The as-grown films have a high resistivity (N106 Ω cm) and an optical bandgap around 3.6 eV.Undoped CdO displays an optical bandgap around 2.32–2.54 eV and has an electrical conductivity of 8×10−4 Ω cm. The Sn incorporation intoCdO produces a blue shift in the optical bandgap (from 2.55 to 2.84 eV) and a decrease in the electrical conductivity.

The deposition procedure described here gives colloid-free surface thin films as indicated by the surface morphology analysis.© 2006 Elsevier B.V. All rights reserved.

Keywords: Solar cells materials; Chemical bath deposition; Cadmium oxide; Thin films

1. Introduction

Chemical bath deposition (CBD) has proven its usefulness todeposit a great variety of semiconductor thin films for a numberof technological applications [1]. In particular, CBD has been thepreferred method to form the buffer layer of high efficiency thinsolar cells based on CIGS absorbers [2], because it offers theadvantages for covering rough surfaces in a conformal way,easily and at relatively low cost [3].

It has been established elsewhere [1,4] that CBD is based onthe controlled precipitation of the material to be deposited.However, as the precipitation reaction proceeds, a solid phasedevelops in the bulk solution (homogeneous precipitation), andon the surface substrate and the reactor walls (heterogeneousreaction). Therefore an important part of the starting reactants islost through the undesirable homogeneous precipitation and filmdeposition on the reactor walls. These harmful aspects of CBDhave been the aim of intense studies, because they might restrict

⁎ Corresponding author. Tel.: +52 58044645 107.E-mail address: ebc@xanum.uam.mx (E. Barrera-Calva).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.03.131

the extended application of CBD in the manufacture of solarcells [5].

In an earlier work [6], we have reported that the bulkprecipitation and the film deposition on the reactor walls couldbe minimized during the growth of ZnO thin films by CBD,using a procedure in which the substrate was chemically treatedprior to the deposition process. In the present work we havefollowed a similar procedure to deposit a promising transparentelectrode oxide as cadmium oxide (CdO).

In the past years, CdO has received a considerable attention asthe transparent electrode (TCO) of CdTe-based thin film solarcells [7,8] because its electro-optical properties compare withthose of the traditional TCO's, as F:SnO2 and Al:ZnO [9].

Several researchers have studied the optical and electricalproperties of undoped and doped CdO polycrystalline thin films.Ferro and Rodríguez [10] demonstrated that the electrical con-ductivity of undoped CdO thin films prepared by spray pyrolysiscould be increased from 103 to 104 S/cm by doping it with Fluor.Zhao et al. [11] used the MOCVD technique for preparing Sn-doped CdO thin films. They reported that, controlling theSn incorporation into CdO, the optical gap and electrical

Fig. 1. XRD patterns of as-grown thin films: a) undoped and b)–d) Sn-doped.The Sn content of the last ones after being annealed is indicated in thecorresponding diffractograms of Fig. 2.

Fig. 2. XRD patterns of the same samples as in Fig. 1, after being annealed at400 °C for 3 h in air. The Sn/Cd atomic percent ratio in the film, as determinedfrom EPMA data, is: a) 0.0 (undoped), b) 0.007, c) 0.020 and d) 0.035.

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conductivity could be varied in the range of 2.2–3.3 eVand 103–104 S/cm, respectively. On these bases, we conducted a researchto prepare undoped and Sn-doped CdO thin films by CBD withthe appropriate electro-optical properties as TCO for solar cellapplications. The present work reports our early attempts to ac-complish such an aim, besides proposing a simple procedure forimproving the CBD efficiency during the CdO growth. CdO thinfilms have been prepared by diverse methods, including spraypyrolysis [10], sol gel [8], MOCVD [11], reactive magnetronsputtering [12] and chemical bath deposition [13].

2. Experimental details

The films were prepared in a beaker placed upon a heater–stirrer. CdO was prepared starting from aqueous solutionscontaining NH4OH (5.3M), CdCl2 (0.4M), and H2O2 (30%). Toavoid the spontaneous precipitation of any solid phase, NH4OHand CdCl2 were firstly mixed under constant stirring until totaldissolution, and then hydrogen peroxide was added.

For Sn-doping of CdO, the above solution was used but theSn(CO3)y complex as the Sn-source was added. The Sn(CO3)ycomplex was prepared from freshly prepared aqueous solutionsof NaCO3·2H2O and SnCl4·5H2O, which were mixed undervigorous stirring to obtain a precipitate. The precipitate was thenfiltered, washed several times with deionized water, and finallydissolved in deionized water.

The Sn(CO3)y concentration in the chemical bath, expressedin terms of the Sn(CO3)y to CdCl2 molar ratio, was varied from 0to 0.19.

Before the deposition process, the substrates were cleanedwith a mixture of hydrogen peroxide and sulphuric acid (1:1 v/v), rinsed with deionized water, and dried with nitrogen.

In the deposition process, a precleaned substrate was firstlydipped in the solution while raising the bath temperature, thenwas taken out from that solution and maintained for a fewseconds, then again immersed in the bath. In all cases, the final

bath temperature was 45 °C. This simple deposition processallows the deposition of CdO thin films only on the surfacesubstrate.

The undoped and Sn-doped CdO thin films were obtained byannealing the as-deposited material, at a temperature in the 200–400 °C range. In all cases the annealing process was done in anair atmosphere.

The crystal properties of the films were assessed by X-raydiffraction (XRD, Siemens D-5000) and the film compositionwas analyzed by electron-probe microanalysis (EPMA, JEOLJMS-6300 Electron Microprobe). The optical properties wereanalyzed from the film transmittance data, which were recordedusing a UV–VIS Shimadzu 3101-PC spectrometer. The con-duction type was identified by the hot-probe technique. Theelectrical conductivity was determined from the sheet resistancedata, which were measured by the four-probe method. Thesurface morphology was analyzed by atomic force micros-copy (AFM, Thermomicroscopes CP Research AutoprobeInstrument).

3. Results and discussion

3.1. Structural characterisation

The deposition procedure described here leads to well crystallizedcadmium peroxide (CdO2) as an intermediate, from which CdO can beobtained through an annealing process at a temperature of 200 °C orhigher. In order to illustrate our results, we chose a set of studiedsamples that were annealed at 400 °C in air for 3 h. It was noted that thegrowth rate decreases as the Sn(CO)y concentration in the bath solution,so the deposition timewas adjusted to obtain a thickness of the thin filmsat around 200 nm.

Figs. 1 and 2 display the X-ray diffraction patterns of the samplesbefore and after being annealed. Before the thermal treatment (Fig. 1),the samples display some diffraction peaks all of them belonging tocadmium peroxide (CdO2) which crystallizes in the rock salt structure[JCPDS 39-1221]. After the annealing process (Fig. 2) the filmsbecome CdO [JCPDS 05-0640] with the same structure (rock salt) andpreferred (200) orientation as those of CdO2.

Fig. 3. Representative electron-probe microanalysis (EPMA) spectrum of Sn-doped CdO, which corresponds to a film with Sn/Cd = 0.020 (sample (c) in Fig. 2).

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The Sn incorporation into the films was only corroborated in theannealed samples. Fig. 3 shows an EPMA spectrum representative ofthe studied Sn-doped samples. The Sn content, expressed in terms of theSn to Cd atomic percent ratio (Sn/Cd), varies in the 0.007–0.035 range.The Sn/Cd value for each sample has been included in the cor-responding X-ray diffraction pattern of Fig. 2.

The above results differ from those reported by other workers in twoaspects. Firstly, the presence of hydrogen peroxide in the chemical bathleads to polycrystalline CdO2 as an intermediate of CdO instead ofamorphous Cd(OH)2, which was obtained as a product from chemicalbaths without hydrogen peroxide [13]. Second, a thermal treatment at atemperature as low as 200 °C transforms CdO2 into CdO, while pre-serving the crystalline structure and the preferred orientation.

It is worth noting that, in spite of the relatively high Sn content indoped films, no diffraction peak corresponding to crystalline phasesother than CdO2 and CdO is discernible in their diffractograms (Figs. 1and 2). However, the presence of amorphous mixed oxides derivedfrom the Cd–Sn–O system has been revealed from the EPMA spectraof Sn-doped CdO, Fig. 3 and with the optical characterisation (seebelow).

Fig. 4. Transmittance spectra of as-deposited (a) and annealed (b–e) samples: b)Sn/Cd = 0 (undoped), c) Sn/Cd = 0.007, d) Sn/Cd = 0.020, and e) Sn/Cd = 0.035.The thermal process was done at 400 °C for 3 h in air.

3.2. Optical and electrical characterisation

Hot-probe measurements indicated the usual n-type conduction forannealed CdO. Attempts for determining the conductivity type of CdO2

using the hot-probe method were unsuccessful, because the hot-probewarming partially converts CdO2 into CdO. This feature was deducedbecause colourless CdO2 became light yellow in colour after the hot-probe application.

The electrical resistivity of as-deposited films was ∼106 Ω cm anddecreased to 10−2–10−4 Ω cm, after the annealing process. The lowerresistivity was achieved for undoped CdO, being about 8×10−4 Ω cm,which is in the same order of magnitude as that measured on CdO thinfilms prepared by thermal evaporation [14] and CBD [13]. Furthermore,Sn was ineffective for CdO doping, because the film resistivity slightlyincreases as the Sn content in the film does.

Fig. 4 displays the typical transmittance spectra in the UV–Visspectral range (300–1000 nm) of as-deposited (a) and annealed CdO(b) and Sn-doped CdO (c, d, and e) films. As mentioned above, thethermal treatment was done at 400 °C in air for 3 h.

As can be seen in Fig. 4, the sub-bandgap transmittance with a glassof all films is around 80%. It is also seen that the absorption edge ofCdO2 shifts towards longer wavelengths due to its conversion into CdOafter being annealed, but the Sn-doped CdO films appear to comprise

Fig. 5. Derivative absorbance spectra as calculated from transmittance data ofFig. 4.

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several phases because their absorption edge displays a step-likestructure, indicating either an incomplete transformation from CdO2 toCdO or the formation of new phases involving Sn as a constituent.

The optical bandgap of the deposited films was estimated from theirabsorbance derivative spectra. In this analysis method, the bandgapenergy can be estimated by determining the photon energy at which theabsorbance derivative attains its maximum (or maxima, if variousabsorption mechanisms are present) [15]. Fig. 5 shows the absorbancederivative spectra as calculated from transmittance data plotted in Fig. 4.

It was noted that the CdO2 optical properties were greatly notaffected by the Sn doping, because in all cases the optical bandgap andthe transmittance were around 3.6 eV (Fig. 5a) and 80% (Fig. 4a),respectively.

As mentioned above, the post-thermal treatment transforms CdO2

into CdO and the optical bandgap shifts towards lower energies.Undoped CdO exhibited an optical bandgap ranging from 2.32 to2.55 eV, in good agreement with the bandgap energy values reported inthe literature [7–9]. Fig. 5b displays the absorbance derivative spectrumof undoped CdO whose maximum is placed at 2.55 eV.

Fig. 5c–d shows the absorbance derivative spectrum of Sn-dopedsamples. Notice that, the derivative spectrum of Sn-doped CdO differsfrom that of undoped CdO in that the former displays two absorptionbands, whereas the latter only one. The peak position of the lower-energy absorption band obviously indicates the threshold energy for thefundamental gap of CdO, which slightly increases as the Sn content inthe film does. The higher-energy absorption band, on other hand,appears to originate from the light absorption by a solid phase other thanCdO2, since it peaks around 3.84 eV (Fig. 5c–d), whilst that of CdO2

does it at 3.6 eV. This result suggests that Sn-doped CdO consists of atleast of two coexistent phases, cubic CdO being the predominant one.

Fig. 6. Representative AFM images of as-grown (A and C) and annealed (B and D) thiand (D) correspond to Sn-doped ones (Sn/Cd = 0.035). The annealing process was

We believe that the extra phase might be an amorphous admixture ofcadmium oxide and tin oxide, according to X-ray diffraction studies.This amorphous phase could be responsible for the decrease of theconductivity observed in Sn-doped CdO thin films.

3.3. Morphology

As mentioned elsewhere [1,4], CBD is based on the controlledprecipitation of a metal ion in the reaction bath, and the film phaseformation involves a heterogeneous mechanism which competes withhomogeneous precipitation. The former is an ion-by-ion process takingplace at the substrate surface and gives adherent mirror-like compactfilms. Whilst the latter is a cluster-by-cluster deposition process, inwhich the colloids coming from the bulk solution adhere to the surfacesubstrate, producing a powder-like porous film.

Our experimental approach takes into account that nucleation is anessential process for the formation of a solid phase from a sursaturatedsolution [4], and that the participation of homogeneous precipitationcan be minimized through a reduction of the free-ion concentration byproducing complex ions with an appropriate ligand. In our solutions,ammonia (NH3) binds the cadmium ion reducing its concentration as afree-ion. The nuclei formation at the substrate surface, on other hand,probably occurs during the pretreatment described in the Experimentalsection, because during the deposition process we visually observed nodeposition on the reactor wall or colloidal particles in the bulk solution.Furthermore, experiments involving non-pretreated substrates wereunsuccessful for the film deposition.

Typical AFM images for CdO (undoped) and Sn:CdO films, before(a and c) and after (b and d, respectively) being annealed at 400 °C for3 h are shown in Fig. 6. It is seen that films display similar surface

n films. Images (A) and (B) correspond to undoped samples, whereas images (C)carried out 400 °C for 3 h in air.

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morphologies, comprising of sharp three-dimensional structures, whichprovide evidence for three-dimensional growth features [9]. In general,the grain size depends on the chemical composition of the startingsolution and the postannealing process. The grain size of as-grown filmsis around 20 nm, which decreases to 10–15 nm after the annealingprocess, according to an estimation from the XRD data using theScherrer formula. Notice that no colloidal particles can be seen upon thefilm surface, discarding the existence of homogeneous growth in thesolution.

4. Conclusions

In summary, we have presented some results on the structural,optical and electrical characterisation of undoped and Sn-dopedCdO thin films. These films were grown by CBD using aprocedure that includes a substrate pretreatment, which allowsfor the film deposition only on the substrate surface and givesthin films a colloid-free surface. The predominant phases of theas-grown and annealed films were, respectively, cubic CdO2 andCdO. All deposited films exhibited a transmittance around 80%with an optical bandgap that varied in the 2.54–2.82 eV range.The electrical conductivity of annealed films lies in the rangefrom 8×10−4 to 10−2 S/cm, which corresponds to pure CdO andSn-doped CdO, respectively.

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