Journal of Alloys and Compounds 548 (2013) 166–172

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Journal of Alloys and Compounds

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Synthesis of luminescent zinc sulphide thin films by chemical bath deposition

Maria Stefan a,⇑, Elisabeth-Jeanne Popovici b, Ovidiu Pana a, Emil Indrea a

a National Institute for R&D of Isotopic and Molecular Technology, 65-103 Donath Str., 400295 Cluj-Napoca, Romaniab ‘‘Raluca Ripan’’ Institute for Research in Chemistry, ‘‘Babes-Bolyai’’ University, 30 Fantanele Str., 400294 Cluj-Napoca, Romania

a r t i c l e i n f o

Article history:Received 3 August 2012Received in revised form 4 September 2012Accepted 5 September 2012Available online 13 September 2012

Keywords:Thin filmsChemical synthesisImpurities in semiconductorLuminescence

0925-8388/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2012.09.013

⇑ Corresponding author. Tel.: +40 264 584037; fax:E-mail address: maria.stefan@itim-cj.ro (M. Stefan

a b s t r a c t

Luminescent zinc sulphide thin films were obtained by chemical-bath deposition, coupled with an indi-rect doping technique. The technique offers an original way for introducing the doping metal ions into theCBD grown thin films, using a ZnS-based doping mixture. The samples were investigated by photolumi-nescence (PL) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energydispersive X-ray microanalysis (EDX) and X-ray photoelectron spectroscopy (XPS). Luminescent,morpho-structural and compositional properties are strongly influenced by the annealing regime.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Zinc sulphide (ZnS) thin films have attracted considerableattention due to their application as optical coatings in solar celltechnology [1–3], light emitting diode [4–6], chemical/biologicalsensors, photocatalysis and window materials [7–10]. On accountof its wide energy band gap ZnS is a semiconductor suitable foruse as host matrix for a variety of dopants, aiming to obtain mate-rials for display applications [11–13].

In recent years, much effort has been made to control the size,morphology, crystalline and porosity of the nanostructured ZnSthin films with a view to tune their physical properties [14–17].This characteristic is generally dependent on the material effectivemicrostructure and defects, its composition and the nature of theactivator species involved. Furthermore, impurity ions doped intothese nanostructures can influence the electronic structure andtransition probabilities. In that frame, the optimal luminescentmaterial requires a high purity of crystalline phase and a good dop-ant confinement acting as the luminescent centre. The metal iondoping procedure of ZnS thin films is a rather complex processand varies from one situation to another being conditioned bythe deposition technique of ZnS films.

Most of ZnS films with luminescent properties are obtained byphysical deposition processes involving the insertion of dopingions (activators) in the film by complex vacuum evaporation pro-cess requiring high temperatures as well as sophisticated andexpensive equipments. Plasma-assisted molecular beam epitaxy[18], spray pyrolysis [19–20], sputtering [21,22], metal organic

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+40 264 420042.).

vapour-phase epitaxy [23,24] are among the physical depositiontechniques which have been used to obtain luminescent ZnS thinfilms. To our knowledge, there are few reports on luminescenceproperties of chemical bath deposited (CBD) zinc sulphide thinfilms [25]. The preparation of luminescent thin films by chemicalbath deposition method consists into the placement of the activa-tors directly into the chemical bath, activator concentration in thefilm being difficult to control due to the extremely fragile balanceduring the deposition process. Given that the luminescent proper-ties of ZnS thin films depend on the preparation conditions (typeand concentration of reagents, deposition time, pH, temperature)and the subsequent annealing regime, detailed experiments andinvestigations are necessary in order to obtain films with superiorluminescent properties [26,27].

Our preliminary researches show that addition of doping ionsdirectly into the chemical bath does not ensure a good activationof ZnS films, contrary to the indirect doping procedure [28].

The aim of this paper is to illustrate that the indirect dopingprocedure can be successfully used to obtain homogeneous nano-crystalline ZnS films with luminescent properties. This techniqueoffers an original way for doping with metal ions the CBD grownthin films by using a ZnS-based doping mixture. Our work also in-tends to reveal some new physical and chemical assets regardingthis material.

The new assets refer to the influence of both annealing temper-ature and activator type on the luminescent and morphostructuralproperties of ZnS films. It has to be mentioned that excepting ourpreliminary studies, no other investigations referring to thepreparation of chemical bath deposited ZnS thin films withluminescent properties by indirect doping procedure have beenreported so far.

M. Stefan et al. / Journal of Alloys and Compounds 548 (2013) 166–172 167

2. Experimental part

Preparation of zinc sulphide thin films with luminescent properties was per-formed in two stages, namely films deposition and post deposition doping with me-tal ions, as called indirect doping.

2.1. Deposition of ZnS thin films

Deposition of ZnS thin films was carried out by CBD technique on3 � 4.5 � 1 cm optical glass substrates previously ultrasonically cleaned in ace-tone–ethanol mixture. ZnS films were grown from a mixture of zinc acetate(15 � 10�3 M) as the zinc ion source, thiourea (150 � 10�3 M) as the sulphur source,NH3 (300 � 10�3 M) aqueous solution and sodium citrate (7.5 � 10�3 M). Sodiumcitrate was added as a complexing agent and ammonia was used to adjust the pHvalue. The bath temperature during the deposition was maintained in the rangeof 82–86 �C and the pH of the solution within the values of 9.5–10.5. In as men-tioned conditions ZnS/glass/ZnS heterostructures were obtained. In order to getthicker ZnS films, the as prepared ZnS/glass/ZnS heterostructures were reintro-duced in a new chemical bath (multilayer technique). The details of experimentaltechnique were given in our previous works [28,29]. ZnS thin films were obtainedfrom Zn (CH3COO)2 � 2H2O (p.a., Merck), SC(NH2)2 (for synthesis, Merck), sodiumcitrate Na3C6H5O7 � 2H2O (purum, Fluka) and ammonia (p.a, 25%).

2.2. Indirect doping of ZnS thin films

Indirect doping of ZnS thin films with metal ions was performed during the postdeposition annealing stage by placing the ZnS/glass/ZnS heterostructures into aZnS-based doping mixture [30] consisting of ‘‘luminescent-grade’’ zinc sulphidepowders [31] and activators, preferably in chloride form.

The indirect doping technique allows a good control of the amount of the dop-ant introduced in ZnS films and therefore ensures the formation of a material withhigh luminous efficiency [30]. The doping mixture was obtained by mixing 100 g ofhigh purity zinc sulphide powder with 20 ml solution of copper chloride (CuCl2)containing 5 � 10–4 g Cu/ml and/or 20 ml solution of manganese chloride (MnCl2)containing 0, 02 g Mn/ml. The obtained paste was dried at 170 �C for 5 h. Annealingof ZnS/glass/ZnS heterostructures was done in closed system, in special graphitecrucibles, at 400–550 �C for 1–2 h. After the annealing, the samples were waterwashed and dried in air.

2.3. ZnS thin films were characterised by crystalline structure, thickness andmorphology, chemical composition and photoluminescent properties

The crystalline structure was investigated with a standard DRON-3 M diffrac-tometer. The X-ray diffraction (XRD) patterns were collected in a step-scanningmode with steps of D2h = 0.05� using CoK radiation (kCo = 1.789007 Å), Fe filtered.The crystalline phases and crystallographic structures of the ZnS thin films wereidentified using MATCH search computer program [32]. The Warren–Averbachmethod was used to determine the crystallite size and the equivalent microstrainsfor microstructural characterization of ZnS thin films [33]. The Warren–AverbachX-ray profile Fourier analysis of the ZnS (200) wurtzite diffraction peak profileswere processed by the XRLINE computer program [34]. Pure silicon powder stan-dard sample was used to correct the data for instrumental broadening.

The films morphology was evaluated by scanning electron microscopy (SEM),with a JOEL-JSM 5510LV instrument, using Au-coated samples. Chemical composi-tion of ZnS-based films was determined by energy dispersive X-ray analysis (EDX),with an EDX-system (Oxford Instruments), INCA 200 software. EDX spectra wererecorded from areas of about 1 mm2.

X-ray photoelectron spectroscopy (XPS) associated with Ar ions etching wasused for qualitative and quantitative compositional analysis of nanocomposites,using a SPECS custom built system. The excitation was made by using the Mg anodeof the X-ray source (ht = 1253.7 eV).

Photoluminescence emission (PL) and excitation (PLE) spectra were registeredwith Perkin Elmer 204 Fluorescence Spectrophotometer (Xe lamp of 150 W) inter-faced to a computer. Spectra were recorded using UG1 and GG 5 optical glass filters,mounted before and after the sample, respectively. Film thickness was evaluated bythe micro-weighting method [35].

3. Results and discussion

The chemical bath deposition process for the preparation of ZnSthin films with luminescent properties aims to obtain reproduciblesamples in terms of thickness, optical purity and homogeneity. Onthe other hand, the degree of doping and structural organization ofthese films determines their specific, luminescent performances.The influence of both activator type and annealing temperatureon the photoluminescent and morpho-structural properties ofZnS thin films were also investigated.

As revealed in our previous papers, homogeneous ZnS thin filmswith high adherence to support, good optical properties and con-trollable thickness between 40 and 450 nm could be obtained bychemical bath deposition [28,29,35]. ZnS/glass/ZnS heterostruc-tures as they were prepared contain transparent and high specu-larly reflecting zinc sulphide films, with 300 nm thickness andpossesses amorphous structure.

The indirect doping with copper (Cu+) and/or manganese (Mn2+)ions was realized by annealing the ZnS/glass/ZnS heterostructures,after placing them into the ZnS-based doping mixture consisting ofzinc sulphide and activators, preferably in chloride form. Duringthe thermal treatment, copper/manganese ions diffuse from thezinc sulphide–metal chloride mixtures and incorporate into theZnS film, as Cu+ and/or Mn2+ species. The stabilization of Cu+ ionsinto the ZnS matrix is promoted by the monovalent Cl� ion (co-activator). In manganese doped ZnS thin films, Mn is incorporatedas divalent (Mn2+) ion and the formation of luminescence centresdoes not involve the presence of co-activators as charge compensa-tors. The zinc sulphide used to prepare the doping mixture is ahigh-purity powder with uniform particle size distribution(d50% = 5.5 lm). It contains a mixture of cubic-hexagonal structure(79% sphalerite) and shows great ability to develop luminescentproperties [36].

3.1. Photoluminescence properties

Under ultraviolet excitation (365, 254 nm), Cu and/or Mn dopedZnS films show a blue–green luminescence with variable bright-ness due to the presence of cu-emission centres (ZnS:Cu) and yel-low–red luminescence due to the presence of Mn-emission centres(ZnS:Mn).

Fig. 1 shows the photoluminescence (PL) spectra of ZnS/glass/ZnS heterostructures obtained by annealing at different tempera-tures using ZnS-based mixture without activators or with copper,manganese or copper–manganese addition.

ZnS/glass/ZnS heterostructures annealed in pure ZnS (with no-doping ions) show a more or less intense emission band centredbetween 470 and 508 nm (Fig. 1a). For ZnS films annealed at550 �C, the green emission centred at 508 nm suggests the forma-tion of oxygen containing emission centres (ZnS:O films). It seemsthat, in our experimental conditions, ZnS layer is contaminatedwith small amounts of oxygen, taken during the annealing stage.Generally, O-doped ZnS has a maximum emission at about500 nm [37]. The less intense blue emission at �470 nm for ZnSfilms annealed at 400 and 500 �C is attributed to the self-activatedcentres (ZnS:SA films) which are specific for the un-doped zinc sul-phide. In our case, this blue peak is related with native defectsstates in ZnS host lattice i.e. Zn vacancy and a shallow donor asso-ciated with a sulphur vacancy [14,38].

When copper and/or manganese are used as activators, theemission band of the host- lattice is accompanied by new red-shifted bands associated with the doping ions and PL spectra ofcomposite nature are formed.

The shift of the emission to longer wavelength indicates the pres-ence of green Cu centres (ZnS:Cu films) resulting from copper incor-poration into ZnS matrix and accounts for the formation of ZnS(Cu)/glass/ZnS(Cu) heterostructures (Fig. 1b). The emission spectrum ofZnS films annealed in copper doped ZnS-based mixture at 550 �Cexhibits a green emission at about 520 nm. This peak is known tobe due to the recombination of electrons between the sulphur va-cancy-related donor and Cu-related acceptor [39]. The shoulder atabout 470 nm is associated with the recombination of the chargecarriers on the self activated centres formed through the associationbetween Zn vacancies and substitution chloride co-activators [40].ZnS films annealed at 500 �C exhibit strong emission centred atabout 490 nm. The position of this maximum illustrates the

Fig. 1. Emission spectra (kexc = 365 nm) of ZnS/glass/ZnS heterostructures containing un-doped and doped films, annealed at different temperatures: (a) ZnS; (b) ZnS:Cu;(c)ZnS:Mn; (d) ZnS:Cu, Mn.

168 M. Stefan et al. / Journal of Alloys and Compounds 548 (2013) 166–172

composite nature of the spectrum that evidently contains self acti-vated and Cu-emission centres. The annealing at 400 �C does not ex-ert a significant effect on the incorporation of copper in the ZnS hostlattice.

The photoluminescent emission spectra of films annealed inmanganese containing ZnS-based mixture are depicted in Fig. 1c.The PL spectra of ZnS:Mn films present a broadband emission withvariable intensity, depending on the annealing temperature. Thesamples show two specific bands namely at �466 nm, in the blueregion, due to SA centres and another band at about 550–590 nm, in the yellow–orange spectral domain, associated withMn-centres. This characteristic orange emission is associated with4T1 � 6A1 electronic transitions inside the manganese ions [41,42].The manganese centres are well formed at high temperatures, asillustrated by the intense orange emission of sample annealed at550 �C. In fact, the characteristic PL band tends to shift towardlonger wavelengths with annealing temperature. The appearanceof the orange band accounts for the formation of ZnS(Mn)/glass/ZnS(Mn) heterostructures.

Samples prepared by indirect doping using copper–manganesedoped ZnS-based mixture (Fig. 1d), show specific bands locatedthroughout the spectral domain as follows: at 465 nm (blue re-gion), associated with the presence of self-activated or Cu centres,at �537 nm (green region) due to the presence of Cu-centres, andat 578 nm (orange region) associated with the presence of Mn-cen-tres. The positions of these three bands have found to be indepen-dent of the Cu and Mn concentrations [43]. The complex structureof the PL spectra accounts for the formation of ZnS(Cu, Mn)/glass/ZnS(Cu, Mn) heterostructures.

The Mn addition quenches partially the green luminescenceassociated Cu -centres. The explanation of this phenomenon maybe that a part of excitation energy absorbed by Cu is transferrednon- radiative toward Mn. The strong orange luminescence ob-served for the sample obtained at 550 �C could be correlated with

the copper–manganese mutual interaction [43,44]. It appears thatin double doped ZnS:Cu, Mn the manganese activator can gain en-ergy not only from ZnS host, but also from excited co-dopant Cu.The fact demonstrates that the Mn luminescence can be sensitizedby Cu [43]. The excitation (PLE) spectra of ZnS/glass/ZnSheterostructures doped with different activators at 550 �C are com-paratively presented in Fig. 2.

Excitation spectra of ZnS films with different activators have asimilar shape. They consist of two wide bands, more or less intensedepending on the nature of those activators and experimental con-ditions. The high energy band is approximately located at 335–341 nm corresponding to the host absorption. The low energy bandis relatively broad and it is characteristic for luminescent centrescontaining the activator ions. This band is absent in both un-dopedand Mn-doped ZnS films but is evident in Cu-doped ZnS films. Theband is produced by the transition from the ground state of thegreen luminescence centre (corresponding to the Cu+ acceptor le-vel) to the excited state of the centre (corresponding to the Cl� do-nor level) or to the conduction band.

As already mentioned, the luminescent properties of ZnS/glass/ZnS heterostructures i.e. emission colour and brightness are depen-dent on both the doping and annealing conditions.

3.2. Structural properties

The crystalline structure of the un-doped and copper and/ormanganese doped ZnS/glass/ZnS structures annealed at 400–550 �C, in a zinc sulphide powder surrounding was investigatedby X-ray diffraction (Figs. 3 and 4). The influence of both the ther-mal treatment regime and doping on the crystalline organization offilms was monitored.

The XRD patterns of the un-doped ZnS films before and after thethermal treatment at 400 and 550 �C are depicted in Fig. 3. TheXRD investigation of ZnS-films as they were grown did not reveal

Fig. 2. Excitation spectra of un-doped and doped ZnS/glass/ZnS heterostructuresannealed at 550 �C. Intensity was monitored for the emission maxima: ZnS:Cu(kem = 520 nm), ZnS (kem = 470 nm), ZnS:Mn (kem = 588 nm).

Fig. 3. XRD patterns of un-doped ZnS films annealed at different temperatures.

Fig. 4. XRD patterns of doped ZnS films annealed at 550 �C (⁄ ZnO�2ZnSO4).

M. Stefan et al. / Journal of Alloys and Compounds 548 (2013) 166–172 169

any diffraction peak indicating that zinc sulphide layer is disor-dered. The degree of structural organization increases with thethermal treatment. The XRD patterns of the ZnS films annealedat 400 and 550 �C are similar, both showing the image of the struc-tured layer with a local amorphous type organization. The an-nealed films are single phase materials containing the hexagonalZnS wurtzite type crystalline phase (PDF file No. 36–1450). Thetwo broad scattering maxima (2h = 11.8–11.9�; 2h = 25.9–26.2�)indicate the initiation process of the local ordering. The well de-fined maximum at 2h = 33.3� for films thermally treated at550 �C indicates a highly textured layer with [002] direction ofwurtzite ZnS phase.

Fig. 4 shows the XRD patterns of some luminescent ZnS filmsprepared at 550 �C, using different activators. The diffractogramof the Cu-doped film is similar with the XRD pattern of the un-doped film obtained in similar thermal conditions. The well de-fined maximum at 2h = 33.4� indicates a highly textured layer with[002] direction identified as hexagonal ZnS wurtzite type crystal-line phase (PDF file No. 36–1450).

The X-ray diffraction pattern of Mn-doped ZnS film shows theformation of ZnS–MnS solid solution with wurtzite type crystallinestructure. One can be see that the characteristic wurtzite diffrac-tion lines are slightly shifted to lower 2h angles (see PDF file No.36–1450 for hex- ZnS, and PDF file No. 40–1289 for hex-MnS).Moreover, the developing of the (002) plane (2h = 32.5�) seemsto diminish in favor of the (101) plane (2h = 35.6�). Manganesedoping is favorable for the host-lattice crystallization.

The Cu–Mn double doped ZnS films are multiple phase materi-als containing mainly hexagonal ZnS wurtzite type crystallinephase, with preferential orientation alongside the (002) plane(2h = 33.3�; PDF file No. 36–1450) and small quantities of theorthorhombic ZnO�2ZnSO4 crystalline phase (2h = 28.4; 35.1 and41.1�; PDF file No. 32–1476). The zinc oxy-sulphate impuritymay results from the partial oxidation of the ZnS layer in contactwith traces of air ‘‘captured’’ between the ZnS grains and trans-ferred during the annealing process. The XRD patterns confirm thatboth the thermal treatment regime and the manganese dopinghave a significant effect on the structural organization of films.

Due to the presence of a large broadening of X-ray diffractionlines, for the simultaneous determination of crystallite size and lat-tice imperfections, the Warren–Averbach method was adopted[33]. The microstructural data obtained by single X ray profile Fou-rier analysis were the effective crystallite mean size, Deff, and theroot mean square (rms) of the microstrains, h�2i1=2

m [33]. Table 1summarizes the microstructural parameters of the luminescentZnS films as determined using XRLINE computer code [42]. The

table shows that microstrain lattice parameter, h�2i1=2m can be cor-

related with the effective crystallite size, Deff, in the following

Table 1XRD microstructural parameters of luminescent ZnS films.

Heterostructure T(�C)

Effective crystallitemean size Deff (nm)

Root mean square of the

microstrainsh�2i1=2m � 102

ZnS(Cu)/glass/ZnS(Cu)

550 18 0.380

ZnS(Mn)/glass/ZnS(Mn)

550 12 0.628

ZnS(Cu, Mn)/glass/ZnS(Cu,Mn)

550 14 0.523

ZnS/glass/ZnS 550 14 0.458ZnS/glass/ZnS 400 Amorphous –ZnS/glass/ZnS – Amorphous –

170 M. Stefan et al. / Journal of Alloys and Compounds 548 (2013) 166–172

way: the value of the microstrain increases when the effectivecrystallite size decreases due to the fact that many of the crystal-line faults like dislocations, crystallite surface relaxation and crys-talline non-uniform lattice distortions are concentrated in theinter-crystallites boundary regions.

The microstructural parameters seem to be dependent of bothactivator type and annealing temperature. The un-doped ZnS filmannealed at 550 �C possesses wurtzite structure with preferentialorientation on [002] plane. This film is built up from crystalliteof about 14 nm and it shows a lattice distortion parameterh�2i1=2

m = 0.00458. The manganese doped ZnS films contain smallercrystallites (12 nm) and exhibit the highest number of packing de-fects (h�2i1=2

m = 0.00628). The copper doped ZnS films contain largercrystallites (18 nm) and exhibit the lowest number of packing de-fects (h�2i1=2

m = 0.00380). For the Mn–Cu double doped films, inter-mediate values were obtained. The Cu-doping tends to increase theparticle dimensions and to decrease the lattice distortion while theMn-doping shows an opposite effect.

3.3. Morphological and compositional aspects

The morphology of the un-doped and copper and/or manganesedoped ZnS films was investigated using scanning electron micros-copy (Figs. 5 and 6).

The un-doped ZnS films (obtained by annealing in high-purityZnS powder) present uniform and continuous smooth surfaces,whose morphological organization seems to increase with

Fig. 5. SEM images of un-doped ZnS thin films annealed at dif

Fig. 6. SEM images of un-doped and doped ZnS thin films

annealing temperature (Fig. 5). The small and regular cracks or pin-holes observed on the surface of the as-grown films suggest theamorphous character of the ZnS layer. By annealing at 400 �C, theamorphous state is preserved but the ZnS ‘‘islands’’ start to grow.The morpho-structural organisation of the zinc sulphide layer isobvious at relative higher annealing temperature (550 �C). The filmsurface exhibits a uniform, rather compact, continuous andgranular texture, with grains ranging from 30 to 100 nm. Thesemorphological characteristics are in good agreement with XRDdata which were discussed in the previous section, under whichZnS grains are built up of many small dimensions crystallites ofabout 14 nm.

The copper and/or manganese doped ZnS films have a ratherdense surface morphology (Fig. 6).

The copper and/or manganese doped ZnS thin layers consists ofwell-defined grains with different sizes and morphologies depend-ing on the activators type. The diffusion of copper into the ZnS ma-trix induces the formation of an in-homogenous and poroustexture, resulted from irregularly shaped crystals. The Mn-dopedZnS films consist from tightly packed particles with elongatedshape and laminated aspect, oriented in different directions. Thechange in morphology is ascribed to the structural changes inducesby the doping ions used as activators (CuCl2 and/or MnCl2). Inaccordance with the XRD results, they act as fluxes which locallyincrease the tendency of crystallization [31]. The increased poros-ity can be assigned to the degassing process which can occur dur-ing the thermal annealing process.

The chemical composition of the un-doped and copper and/ormanganese doped ZnS/glass/ZnS heterostructures is analyzed byenergy dispersive X-ray spectroscopy (Fig. 7). EDX spectra allowhighlighting the presence of copper and manganese (doping ions)and chloride (co-doping ions) beside zinc and sulphur (host-latticecomponents). Due to the small thickness of ZnS films, other ele-ments from the glass substrate i.e. O, Na, Mg, Al, K, Ca can be iden-tified in the EDX spectra.

The quantitative results obtained through EDX analysis are onlyestimative. The glass supported zinc sulphide films with no activa-tors or doped with copper and manganese i.e. ZnS/glass/ZnS andZnS(Cu, Mn)/glass/ZnS(Cu, Mn) heterostructures contain 1.68 at.%Zn, 3.81 at.% S and 94.51 at.% others and, correspondingly4.54 at.% Zn, 4.45 at.% S, 0.02 at.%Cu, 0.10 at.% Mn, 2.65 at.% Cland 88.24 at.% others.

ferent temperatures: (a) as grown; (b) 400 �C; (c) 550 �C.

annealed at 550 �C: (a) ZnS; (b) ZnS:Cu; (c) ZnS:Mn.

Fig. 7. EDX images of un-doped and doped ZnS films prepared at 550 �C: (a) ZnS; (b) ZnS:Cu; (c) ZnS:Mn; (d) ZnS:Cu, Mn.

Fig. 8. XPS depth profile of Cu 2p 3/2 core-level line as a function of the Ar ionsetching time.

M. Stefan et al. / Journal of Alloys and Compounds 548 (2013) 166–172 171

The homogeneity of dopant distribution is sustained by the al-most identical EDX spectra recorded on different areas of eachsample (examples are shown in Fig. 7). On the other hand, due tothe specific doping procedure in the post deposition stage, the asresulting surface distribution of dopants should be consideredhomogeneous as long as the deposited ZnS films as arehomogeneous.

As it concerns the evolution of dopant distributions through thethickness of films we performed XPS accompanied by Ar ions etch-ing. Thus the depth profile of dopant concentration was recordedfor etching time going up to 180 min. The applied accelerationvoltage was 1500 V at an emission current of 10 mA. As an exam-ple, the depth profile of Cu+ dopant content versus etching time ispresented in Fig. 8. The Cu+ content is expressed in area units of theCu 2p 3/2 core-level XPS peak normalized by the real sensitivity,transmission and electronic mean free path factors as given byCASA software. As seen in Fig. 8, the dopant content increase imme-diately below the film surface and starts to slowly decreasethroughout the film thickness.

Almost 3 h of 1500 V Ar ions etching represents a very long timeand indicates that the dopant is still present at large depths belowthe film surface. A correlation between the etching time and thereal depth through the film thickness since the ground dischargecurrent is strongly reduced by the high resistivity of the material.The ground discharge contact was realized by a small adhesive rib-bon attached on the side to the film surface.

4. Conclusions

Luminescent zinc sulphide thin films were obtained by chemi-cal-bath deposition, coupled with an indirect doping technique.The procedure offers an original way for insertion the doping metalions into the CBD grown thin films in a post deposition annealingstage by using a ZnS-based doping mixture. Luminescent, mor-pho-structural and compositional properties are strongly influ-enced by the annealing regime.

Photoluminescence spectra put in evidence the specific emis-sion bands located throughout the entire spectral domain, thusillustrating the creation of the emission centres generated by the

diffusion of copper and/or manganese ions from the ZnS-baseddoping mixture. The specific bands are centred at 465 nm (blue re-gion), associated with the presence of self-activated (ZnS:SA) orblue-Cu centres, �537 nm (green region) due to the presence ofCu-centres, and 578 nm (orange region) associated with the pres-ence of Mn- centres. Moreover, some the un-doped ZnS/glass/ZnSheterostructures show green oxygen emission (kpk = 508 nm). Thesimultaneous Cu–Mn addition partially quenches the green lumi-nescence associated with Cu -centres. The most spectacular orangeluminescence was observed for double activated ZnS films, due tothe Cu–Mn mutual interaction (energy transfer).

The as-grown ZnS films are non-structured and as a result, showno luminescence under UV excitation. The annealing in the homo-geneous medium of the ZnS-based doping mixture is in the favourof the structural organization of the matrix. Most of the thermallytreated ZnS films are nanostructured, with wurtzite crystallinearrangement. One can also note that the dopant ions are well dis-tributed throughout the thickness of the annealed ZnS films. Themorphological and structural organization of ZnS films is influ-enced by both the presence of activators and annealingtemperature.

172 M. Stefan et al. / Journal of Alloys and Compounds 548 (2013) 166–172

Acknowledgments

This work was supported by the Romanian National Council forScientific Research, Human Resources Program PD 54/2011.

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