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Journal of Luminescence 130 (2010) 1268–1274

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Tel.:

E-m

journal homepage: www.elsevier.com/locate/jlumin

Effects of copper concentration on electro-optical and structural properties ofchemically deposited nanosized (Zn–Cd)S:Cu films

Ayush Khare n

Department of Physics, National Institute of Technology, Raipur 492 010, Chhattisgarh, India

a r t i c l e i n f o

Article history:

Received 26 October 2009

Received in revised form

13 February 2010

Accepted 19 February 2010Available online 1 March 2010

Keywords:

Nanocrystalline materials

X-ray diffraction (XRD)

Electroluminescence

Chemical deposition technique

Trap depth

13/$ - see front matter & 2010 Elsevier B.V. A

016/j.jlumin.2010.02.037

+91 771 2254199; fax: +91 771 2254600.

ail address: akhare.phy@nitrr.ac.in

a b s t r a c t

Nanocrystalline (Zn–Cd)S films have been co-deposited on glass slide substrates by chemical bath

deposition (CBD) technique at 70 1C for 75 min. Electroluminescent (EL), photoluminescent (PL) and

structural characteristics of these films doped with Cu have been investigated. Cu doping has significant

effects on the growth, structural and optical properties of the deposited (Zn–Cd)S films. EL studies show

the essentiality of copper for EL emission. The effect of Cu concentration is examined on XRD, SEM,

UV–vis spectroscopy, etc. The morphology of these films investigated with SEM and XRD is used to

determine crystalline nature of the films. The optical absorption coefficient of the films has been found

to increase with increase in Cu concentration. Voltage and frequency dependence shows the

effectiveness of acceleration–collision mechanism. The trap-depth values are calculated from

temperature dependence of EL brightness.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

ZnS has been used widely as an important phosphor for EL andPL devices due to its better chemical stability compared to otherchalcogenides such as ZnSe. Doping of ZnS nanoparticles bytransition metal ions like Cu+ 2 [1] has been successfully done bymany techniques such as thermal evaporations, sol–gel proces-sing, co-precipitation, microemulsions, etc. These doped ZnSsemiconductors have wide range of applications in EL devices,phosphors, light emitting displays and optical sensors. Emissionin various visible bands using different dopants has been reportedby different workers. However, due to fast degradation andrelatively low brightness of ZnS:Cu alternating-current thin filmelectroluminescent (ACTFEL) devices, these films are still notadequate for practical purposes. A number of research papershave been published exhibiting electro-optical properties of CdSin different forms like powder, crystals, pellets, nanocrystallitesand thin films [2] but the information available on ternaryZnxCd1�xS system is very limited especially in the presence of Cuas dopant. It is well established that ZnxCd1�xS films possessproperties between those of ZnS and CdS [3,4]. Since theiraddition produces a common lattice in which band structurehas a larger band gap than CdS, it makes the material moreattractive for fabricating EL devices. With Cu doping, it can beused as green and blue emitting active layer. The crystalline

ll rights reserved.

structure of the films, concentration of dopant and the effect of Cuincorporation into the lattice of ZnxCd1�xS, influence the operat-ing parameters and degradation stability of ACTFEL devices. Themain object of such mixing has been to facilitate the substitutionof Cu in their presence because ionic radii of Zn+ 2, Cd+ 2 and Cu+2

are 0.74, 0.97 and 0.72 A, respectively. Therefore, Cu can easily besubstituted in place of Cd+2/Zn+ 2 either in substitutional orinterstitial positions [5]. The luminescence emission and effi-ciency of semiconducting device are dependent on the concentra-tions and characteristics of dopant material. Hence, it becomesimperative to investigate the role of dopant on electro-opticalproperties to understand its mechanism. The present paperreports the effects of Cu concentration on the EL and PL propertiesof (Zn–Cd)S nanosized films prepared by CBD technique [6],which is considered to be inexpensive, simple and capable ofdepositing optically smooth, uniform and homogeneous layers.Recently, there is a new interest in the CBD, which is motivated byits successful use in depositing buffer layers of CdS (and similarmaterials) in thin-film photovoltaic cells based on CdTe andCuInSe2 [7].

2. Experimental details

2.1. Preparational techniques

The EL samples are prepared by vertically dipping the cleanedsubstrates of conducting glass plates of dimension 24�75 mm2

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A. Khare / Journal of Luminescence 130 (2010) 1268–1274 1269

(with high transmission coefficient), prepared by depositingSnO2 conducting layer by spray-pyrolysis method [8] usingSnCl2.2H2O as the original chemical and having resistivityE20 O cm. The construction of EL cell is shown in Fig. 1. ForPL studies, the prepared films are used as samples. The

Fig. 1. Fabrication of EL cell.

Fig. 2. X-ray diffractograms of (a) (Zn0.7–C

deposition of film takes place at 70 1C for 75 min on microglassslide substrates cleaned with double distilled water, acetone andusing ultrasonic cleaner. The aqueous solution used for (Zn–Cd)Sdeposition contained 1 M [Zn (CH3COOH)3]+[Cd (CH3COOH)3],thiourea H2NCSNH2, trietholamine (HOCH2CH2)3N and 30%aqueous NH4OH (all analytical reagent grade—99.9% pure;mixture showed pH �11). In addition appropriate amount of0.01 M solution of Cu[(CH3COOH)3] is also mixed in the originalmixture. The deposition of films is based on precipitationfollowed by condensation. In the beginning, when theprecipitation starts, stirring is done. After that, depositions aremade in the static condition and thereafter films are washedwith distilled water and then dried by keeping in openatmosphere under sun light until its moisture content reducescompletely. This helps in achieving adequate operating life of ELcell made with such films. Bhushan and Chandra [9] reportedthat such prepared films last for more than 2 years. The thicknessof the film is measured by mass difference method and is foundto be between 1 and 2 mm. The sample (Zn0.7–Cd0.3)S:Cu is

d0.3)S and (b) (Zn0.7–Cd0.3)S:Cu films.

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A. Khare / Journal of Luminescence 130 (2010) 1268–12741270

selected on the basis of satisfactory high EL brightness andproper band gap value. Here the subscripts to Zn and Cdrepresent the percentage composition in the solution and not

Table 1XRD data of (Zn–Cd)S film (preparation time=75 min; preparation temperature=70 1C

d-Values (A) Relative intensities

Observed values Reported values Observed values Reported

A. (Zn0.7–Cd0.3)S

3.3600 3.360 100 100

2.0518 2.058 35.5 80

1.7506 1.911 28 74

B. (Zn0.7–Cd0.3)S:Cu

3.3914 3.36 100 100

2.5724 2.705 45.8 10

2.0799 2.058 30.5 80

1.9016 1.911 35.6 74

1.8267 1.753 30.5 60

Fig. 3. SEM micrographs of (a) (Zn0.7–Cd0.3)S and (

the final compositions. Also the dipping time is taken as 75 minbecause of high EL intensity, uniformity and thickness ofdeposited films.

; film thickness �1–2 mm).

hkl Lattice constant (A)

values Observed values Reported values

(1 1 1)c-CdS 5.8196 5.818

(2 2 0)c-CdS 5.8033 5.818

(1 1 0)h-a ZnS 3.02 3.82

(1 1 1)c-CdS 5.87 5.818

(2 0 0)c of ZnS 5.14 5.406

(2 2 0)c-CdS 5.82 5.818

(1 1 0)h-a ZnS 3.80 3.820

(3 1 1)c-CdS 6.05 5.818

b) (Zn0.7–Cd0.3)S:Cu films (magnification=5 k).

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A. Khare / Journal of Luminescence 130 (2010) 1268–1274 1271

2.2. Characterization studies

Optical absorption spectra are recorded with a Varian (UV–vis)DMS-100 spectrophotometer in the wavelength range 360–600 nm.The X-ray analysis of the samples has been carried out by acomputerized Shimadzu diffractometer (Model-98XRD) using Cu/Ka radiations. The particle size and morphological investigations offilms are performed with a scanning electron microscope (JEOL-JSM-6400). The PL spectra are recorded by radiations obtained froma high-pressure Hg source (l=365 nm) and a suitable Carl Zeissinterference filter. The EL emission spectra are recorded using aThermo-Jarrel Ash grating monochromator (Model-82415).

3. Results and discussion

3.1. Structural analysis

Fig. 2 (a) and (b) shows the comparison of typical XRD spectra of(Zn0.7–Cd0.3)S and (Zn0.7–Cd0.3)S:Cu films. As presented in curve (a)three major diffraction peaks are seen corresponding to diffractionangles (2y)=26.511 (1 1 1), 44.101 (2 2 0) and 52.241 (1 1 0). Oncomparing present data with standard data (ASTM), both the cubicand hexagonal phases are found to exist. The assignment of thepeaks has been made from the comparison of ASTM data and theevaluated lattice constant values with those of reported values.Compared to undoped sample, Cu doped sample [curve (b)] hasmore and well developed peaks with relatively lower diffractedintensity. As listed in Table 1, XRD analysis shows that there is nocharacteristic peak corresponding to the impurity phases [10]. Theaverage grain size estimated from Debye–Scherrer [11] formula isaround 3.270.2 nm.

Fig. 3 (a) and (b) shows the SEM micrographs of (Zn0.7–Cd0.3)Sand (Zn0.7–Cd0.3)S:Cu films. Spherically shaped grains are seen inthe form of clusters in undoped sample, which are not distributeduniformly and the sample shows amorphous nature. Cu dopedsample shows compact distribution over the surface and goodconnectivity between grains. The observed differences betweenthe microstructures of (a) and (b) can presumably be attributed tothe addition of Cu. On addition of Cu, spherical shape changes toirregular shape. The average grain size estimated from Heyne’s[12] intercept method is around 2.970.2 nm. It should be notedthat (Zn0.7–Cd0.3)S films alone do not show any EL.

Results of optical absorption spectra for (Zn0.7–Cd0.3)S:Cu filmsat varied Cu compositions are presented in Fig. 4. The absorptioncoefficient (a) and the band gap Eg are related by the following

Fig. 4. Optical absorption spectra of different (Zn0.7–Cd0.3)S:Cu, films at varied

concentrations of Cu: [a—3.0 ml; b—4.0 ml; c—5.0 ml].

expression in direct band gap materials [13]

a¼ Cðhn�EgÞ1=2

hnð1Þ

where ‘Eg’ is the optical band gap and ‘C’ the speed of light. Thus,as shown in Fig. 5, the band gap of materials can be determinedfrom the Tauc’s plots. The materials of the present study are ofdirect band gap nature and with the increase in Cu doping, theband gap is also found to increase [14]. The broadening of bandgap due to copper doping may be due to filling up of conductionband edge by the excessive carriers donated by the impurityatoms. This leads to blue shift in optical band to band transitionsby blocking the low energy transitions. These values arecompared in Table 2.

3.2. PL and EL emission spectra

The PL spectra of the prepared films at varied concentrations ofCu acetate are shown in Fig. 6. In the presence of differentvolumes of Cu compounds, the peak is found to shift towardshigher wavelength side with decrease in intensity. At volumeshigher than 0.5 ml of Cu acetate, the PL emission almost seizesand spectrum above this concentration is not recordable. The

Fig. 5. Tauc’s plots of different (Zn0.7–Cd0.3)S:Cu films at varied Concentration of

Cu: [a—5.0 ml; b—4.0 ml; c—3.0 ml].

Table 2Values of band gap energies for different (Zn0.7–Cd0.3)S:Cu films (preparation

time=75 min; preparation temperature=70 1C; film thickness �1–2 mm).

S. no. Systems Band gap energy (eV)(absorption spectra)

1. (Zn0.7–Cd0.3)S 2.50

2. (Zn0.7–Cd0.3)S; Cu (3.0 ml) 2.54

3. (Zn0.7–Cd0.3)S; Cu (4.0 ml) 2.55

4. (Zn0.7–Cd0.3)S; Cu (5.0 ml) 2.56

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Fig. 6. PL emission spectra of different (Zn0.7–Cd0.3)S, Cu films at different

concentrations of Cu acetate: [1—0.1 ml; 2—0.2 ml; 3—0.3 ml; 4—0.4 ml;

5—0.5 ml].

Fig. 7. PL intensities plotted as a function of Cu mole concentration in (Zn0.7–Cd0.3)

S:Cu films.

Fig. 8. EL emission spectra of different (Zn0.7–Cd0.3)S:Cu films at different Cu

concentrations (excitation frequency=50 Hz): [1—2.0 ml; 2—3.0 ml; 3—4.0 ml;

4—5.0 ml].

Table 3Peak positions of EL emission spectra of different (Zn0.7–Cd0.3)S:Cu films at varied

concentrations of Cu acetate (preparation time=75 min; preparation tempera-

ture=70 1C; film thickness �1–2 mm).

S. no. Concentration of Cu acetate (ml) Peak positions (nm)

1. (Zn0.7–Cd0.3)S:Cu (2.0 ml) 483

2. (Zn0.7–Cd0.3)S:Cu (3.0 ml) 480

3. (Zn0.7–Cd0.3)S:Cu (4.0 ml), 478

4. (Zn0.7–Cd0.3)S:Cu (5.0 ml) 472

A. Khare / Journal of Luminescence 130 (2010) 1268–12741272

diminishing of PL emission above this concentration may bebecause of concentration quenching [15]. As shown in Fig. 6, thepeak position of this emission shifts to longer wavelength region(489–526 nm) with increase in Cu concentration from 0.1 to0.5 ml. This red shift with increase in Cu doping concentrationmay also be due to slight change in particle size of nanoparticlesat different Cu concentrations. Since Cu in small amounts does notaffect the band structure, this effect may be due to increase inparticle size with increase in Cu concentration. The variation of PLpeak intensity (415–615 nm) with change in copperconcentration is depicted in Fig. 7 showing that the intensitycontinuously decreases with the increase in Cu concentration. Theefficient samples have shown a maximum PL of about 125 cd/m2.

Fig. 8 shows the EL emission spectra of (Zn0.7–Cd0.3)S films atdifferent volumes of Cu acetate. It is observed that due to increasein concentration of Cu, the emission peak changes (Table 3),which indicates that Cu is mainly responsible for the emission[16]. Cu ions can substitute Zn in the Zn sites and act as acceptors.Cu ions also play the role of a sensitizer and form acceptor levelswhose energy levels are anomalously deep (beyond 2.7 eV above

the top of valence band) and located near the bottom ofconduction band [17]; hence shift towards lower wavelengthside compared to the band gap is expected. Also, Cu can assist inthe formation of conducting layer for realizing the excitation ofthe materials with the field. The investigated EL film exhibitedblue emission and a brightness of 90 cd/m2 under sinusoidalexcitation.

3.3. Brightness studies

3.3.1. Temperature dependence of EL brightness

The temperature dependence of EL brightness of (Zn0.7–Cd0.3)Sfilms at varied concentrations of Cu acetate is shown in Fig. 9. It islearnt that a maximum emission takes place at around 40 1Ctemperature. From this observation, it is clear that traps areinvolved in this process, which after release of electrons at atemperature close to 40 1C, gives rise to free carrier generationresulting in the maximum emission [18]. Fig. 10 depicts the plotsbetween temperature�1 and log I, which are plotted from theinitial part of temperature dependent curves. The trap depthvalues calculated from these curves are shown in Table 4.

3.3.2. Voltage and frequency dependence of EL brightness

The chemically deposited films are smooth, uniform, adherentand light bluish in colour and it increases with further increase inCu concentration. As shown in Fig. 11, the EL brightness is foundto be maximum at a concentration of 4 ml of Cu acetate. Fig. 12shows voltage dependence of EL brightness of (Zn0.7–Cd0.3)S films

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Fig. 9. Variation of EL brightness with temperature for (Zn0.7–Cd0.3)S:Cu films

with varied Cu concentrations: [1—2.0 ml; 2—3.0 ml; 3—4.0 ml; 4—5.0 ml].

Fig. 10. Plots between temperature�1 and log I for (Zn0.7–Cd0.3)S:Cu films with

varied concentrations of Cu acetate [1—2.0 ml; 2—3.0 ml; 3—4.0 ml; 4—5.0 ml].

Table 4Trap depth values of different (Zn0.7–Cd0.3)S:Cu films at varied concentrations of

Cu acetate.

Systems Trap depth (eV)

(Zn0.7–Cd0.3)S:Cu (2.0 ml) 1.54�10�4

(Zn0.7–Cd0.3)S:Cu (3.0 ml) 1.01�10�4

(Zn0.7–Cd0.3)S:Cu (4.0 ml) 1.81�10�4

(Zn0.7–Cd0.3)S:Cu (5.0 ml) 1.017�10�4

Fig. 11. Dependence of EL brightness on different concentrations of Cu acetate for

(Zn0.7–Cd0.3)S:Cu films (excitation frequency=50 Hz).

Fig. 12. Dependence of EL brightness on applied voltage for (Zn0.7–Cd0.3)S:Cu films

at varied Cu concentrations (frequency=10 kHz) [1—2.0 ml; 2—3.0 ml; 3—4.0 ml].

A. Khare / Journal of Luminescence 130 (2010) 1268–1274 1273

at various concentrations of Cu acetate. It is noted that at lowervoltages the increase in brightness is comparatively slow, whichbecomes fast at higher voltages. The light emission starts at athreshold voltage [19], which depends on the frequency of AC andsaturation EL brightness, is high at lower frequencies. Plotsbetween log B vs 100/V1/2 (Fig. 13) show straight lines indicating

that the brightness can be represented by the empirical equation

B¼ B0 expð�C=V0:5Þ ð2Þ

where ‘B0’ and ‘C’ are parameters, which are considered withrespect to temperature and frequency of alternating voltage, thematerial and other details of the construction of EL cell [20]. Thisbehaviour indicates effectiveness of acceleration–collisionmechanism along with formation of a barrier of Mott–Schottkytype [21].

The frequency dependence of EL brightness at different appliedvoltages for (Zn0.7–Cd0.3)S:Cu films is presented in Fig. 14. It isfound that the brightness first increases, which finally tendstowards saturation at higher frequencies. The increase in EL

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Fig. 13. Plots between log B vs V�1/2 for (Zn0.7–Cd0.3)S:Cu films (frequency=10

kHz) [1—2.0 ml; 2—3.0 ml; 3—4.0 ml].

Fig. 14. Frequency dependence of EL brightness for (Zn0.7–Cd0.3)S:Cu, F films

(voltage=300 V) [1—2.0 ml; 2—3.0 ml; 3—4.0 ml].

A. Khare / Journal of Luminescence 130 (2010) 1268–12741274

brightness with frequency can be understood on the basis ofemptying and refilling of EL centers with the increase infrequency, but when the time period of applied AC cyclesapproaches the lifetime of excited electrons, the electron doesnot have sufficient time to emerge out of the trap [22]. Thus, the

time-averaged brightness will not increase linearly with thefrequency.

4. Conclusions

The effects of Cu doping on electro-optical properties ofnanocrystalline (Zn–Cd)S films prepared by a simple, versatileand inexpensive CBD technique have been studied. It promises tobe a simple and suitable method for obtaining smooth, uniform,high reflecting and strong adherent ZnxCd1�x S thin films. Variousresults show strong dependence of EL and PL characteristics on Cuconcentration. XRD studies show prominent lines of basematerials where as SEM studies present irregular distribution ofcircular grains. Further SEM and XRD studies show particle size tolie in nanometer range. The obtained PL spectra are observed inblue–green regions and PL intensity is found to diminish above aparticular concentration of dopant. EL studies exhibit lowerthreshold and higher brightness for smaller particles. The opticalabsorption spectra exhibit increase in band gap and its mechan-ism is based upon direct transition. Linear voltage–currentcharacteristics indicate Ohmic nature and lower impedance forsmaller particles. In conclusion, it can be stated that the influenceof Cu content on the optical properties of (Zn–Cd)S films isnoticeable.

Acknowledgements

Author is thankful to Prof. Shashi Bhushan, Director, SSEC,Bhilai (C.G.) for his valuable guidance and Dr. (Mrs.) ShashiKrishna Pandey, Director, NIT, Raipur (C.G.) for her constantencouragement and cooperation.

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