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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 26 (2014) 259–266

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Blue-light luminescence enhancement and increased band gapfrom calcium-doped zinc oxide nanoparticle films

Anchal Srivastava n, Nishant Kumar, Kamakhya Prakash Misra, Sanjay KhareDepartment of Physics, University of Lucknow, Lucknow 226007, India

a r t i c l e i n f o

Keywords:Thin filmsZnOCa-dopantSol–gelBand gap tuning and blue-luminescence

x.doi.org/10.1016/j.mssp.2014.05.00101/& 2014 Elsevier Ltd. All rights reserved.

esponding author. Tel.: þ91522 2740449, þail address: asrivastava.lu@gmail.com (A. Sri

a b s t r a c t

Four-fold enhancement in emission in the blue region is observed for the first time fromsol–gel deposited calcium doped zinc oxide (Zn1�xCaxO) nanophosphor films having ahexagonal wurtzite structure. A 4.28% increase in the band gap has been obtained byintroducing very small concentration (1.47 at%) of Ca dopant. A blue-shift of 55 meV in PLemission in UV region occurs with increase in dopant concentration. Optical transmission,FTIR spectra and surface structure of the films have been studied. Dopant concentration inthe films is determined by EDX.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Inorganic semiconductor nanostructures are consid-ered promising for future electronics, photonics, biosen-sors and nanodevices [1–5]. ZnO is a II–VI semiconductorwith band gap of 3.37 eV and exciton binding energy of60 meV at room temperature. Such direct wide band-gapsemiconductors are of considerable interest for blue andUV light-emitting devices [6–9]. Thin films of ZnO alsoplay an important role in solid-state display devices[10,11], solar cells [12,13], etc. Dopants may affect theoptical, electrical, sensing and piezoelectric properties[14–25] of ZnO. Band gap engineering of nanocrystallineZnO plays key role for nanodevices [2,26]. For smoothvariation in band gap, the change in cations in ZnO byisoelectronic impurities is important. Substitution of Zn byCd and Mg or Ca tunes the optical band gap of ZnOfilms towards lower and higher energies, respectively[5,14,15,17,20]. ZnO based films can be obtained by che-mical vapor deposition, thermal evaporation, magnetron

919452266404.vastava).

sputtering, pulsed laser deposition, spray pyrolysis, sol–gel, etc. A sol–gel method is simpler besides producinggood quality homogenous films and can be used to prepareZnO based nanophosphors [27,28].

The quality of phosphor material being important forthe brightness of display devices [29] makes the study ofphotoluminescence (PL) properties of the phosphorimperative. Emission in blue region is important forfabrication of white LEDs. In an earlier report [14] on theeffect of Ca doping in ZnO by the authors the film samplesdid not show luminescence under photoexcitation. Molar-ity of those precursor solutions was 0.1 M and dopantconcentration varied between 5 and 15 at%. To the best ofour knowledge there is no report on PL from calciumdoped zinc oxide (Zn1�xCaxO) films prepared by the sol–gel method. In view of the importance of the subject andmanipulation of emission in visible region, present workhas been carried out. PL is known to be a surface effect andin order to obtain it the film surface has been modified byincreasing the molarity two-fold and lowering the dopantconcentration as compared to that in Ref.[14]. Increasedmolarity is expected to lead to denser films with suitablesurface structure. In this paper, PL spectra and band gapenhancement of nanocrystalline Zn1�xCaxO thin films

Fig. 1. XRD spectra for Zn1�xCaxO thin films. Curves a, b, c, and dcorrespond to samples 1 2, 3 and 4 respectively.

Table 1Orientation parameter, lattice constant and bond length determined fromXRD data for Zn1�xCaxO films.

Samples Orientationparameter (γ)

Latticeconstant

c/a Anion–cation bondlength (Å)

(100) (002) (101) a (Å) c(Å)

1 0.378 0.319 0.302 3.246 5.239 1.613 1.9802 0.309 0.317 0.373 3.296 5.269 1.598 2.0043 0.386 0.321 0.292 3.276 5.202 1.587 1.9884 0.416 0.292 0.291 3.266 5.180 1.586 1.981

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prepared by the sol–gel spin coating method is presented.Band gap tuning obtained is larger than those reportedearlier [5,14].

2. Experimental details

The precursor for undoped film was prepared byobtaining 0.2 M solution of zinc acetate dihydrate inethanol and diethanolamine. The mixture was magneti-cally stirred at 60 1C for 30 min to get a homogeneoussolution. To this solution, appropriate volumes of 0.2 Msolution of calcium nitrate tetrahydrate (Ca(NO3)2 �4H2O)in ethanol were added to obtain 1, 2 and 3 at% doping ofCa. Now onwards atomic% is abbreviated as at%. Thesesolutions were again stirred for 30 min. Both undoped anddoped solutions were aged for 5 days and then were spincoated on properly cleaned glass slides. Spinning speedwas kept at 3000 rpm while the spinning time was 30 s.After each coating, the sample was heated from roomtemperature to 400 1C for 60 min, cooling back naturally.The process was repeated 15 times to obtain appreciablethickness. Finally all the films were annealed at 450 1C for4 h. The four samples: undoped ZnO, ZnO:1 at% Ca, ZnO:2at% Ca and ZnO:3 at% Ca for which x¼0, 0.01, 0.02 and 0.03were named as samples 1, 2, 3 and 4 respectively.

Structural characterization was performed using aBruker D8 Advance X-ray diffractometer with CuKα radia-tion (λ¼1.541841 Å) for 2θ values ranging from 26 to 651and the step size was 0.051. The FTIR transmission spectrawere obtained using a Bruker Alpha FTIR spectrometer.Supra 40 VP from Zeiss FESEM and EDX was used to obtainsurface morphology of the films and the amount ofcalcium dopant in them. Surface topology had beeninvestigated using PicoSPM II AFM from Molecular Ima-ging. The optical transmission spectra were recorded usinga UV–vis–NIR spectrophotometer (JASCO, Model-V670) inwavelength range 300–900 nm for normal incidence and0.25 nm step size. PL spectra had been recorded for every0.5 nm using fluorescence spectrometer (Model- LS-55,Perkin-Elmer). The excitation wavelengths were obtainedfrom a 20 kW Xe discharge lamp. All the measurementshad been performed at room temperature.

3. Results and discussion

3.1. XRD and FTIR

The X-ray diffraction (XRD) pattern for all the samplesis shown in Fig. 1. The peaks correspond to the hexagonalwurtzite structure of ZnO showing preferred orientationsalong (100), (002) and (101) planes [5,14,20,23,27,30]. Noother diffraction peak except that for ZnO is detected,indicating that the doping of Ca does not alter the wurtzitestructure of Zn1�xCaxO films. In earlier reports where Cadoped ZnO films were prepared by sol–gel spin coating[14] and PLD [5] no impurity phase was found for dopingup to 15 and 22.4 at% respectively. Cao et al. [5] havedemonstrated that wurtzite structure of ZnO remainsintact with Ca doping up to 22.4 at% and a new phasecorresponding to CaO occurs when dopant increases to27.8 at%. CaO phase is also reported in Ca doped ZnO

nanoparticles obtained by a precipitation method andsubsequent annealing at 650 1C for 4 h in air atmosphere[17]. In the present work the orientation parameterγ(hkl)¼ I(hkl)/(I(100)þ I(002)þ I(101)) [27,31,32] correspondingto different planes varies from 0.291 to 0.416 indicatingrandom orientation; see Table 1. The peak along the c-axisoccurs at 2θ¼34.251, 34.0510, 34.501 and 34.651 for sam-ples 1, 2, 3 and 4 respectively. The diffraction peak shifts tolower value of 2θ for sample 2, resulting in an increase inc-lattice constant; see Table 1. Such an increment can beattributed to increase in interstitial Zn/Ca. For higherconcentration of Ca, angle of diffraction again increasesand the lattice constant decreases since Ca get substitutedat Zn sites instead of increasing the metallic ions ininterstitials. Bond length [33] is calculated to be 1.980,2.004, 1.988 and 1.981 Å. Maximum error in the determi-nation of the lattice constants, bond length and crystallitesize is 70.015 Å, 70.009 Å and 71 nm respectively.

The lower bound of the crystallite size in the sample isestimated using the Debye–Scherrer (DS) formula asfollows [14,34]:

tDS ¼kλ

β cos θð1Þ

where tDS is the particle diameter, and k is the Scherrerconstant taken equal to one. β is full width at half-maximum (FWHM) of X-ray diffraction peaks in radians.

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Crystallite sizes along different crystallographic planes arepresented in Table 2. The deviation from perfect crystal-linity leads to a broadening of the diffraction peaks [35].Earlier works suggest that broadening may be produced bysmall crystallite size as well as lattice strain. Lattice strainis a measure of the distribution of lattice constants arisingfrom crystal imperfections. Among the available methodsto estimate the crystallite size and lattice strain are thepseudo-Voigt function, Rietveld refinement and Warren–Averbach analysis. Williamson–Hall (WH) analysis is asimplified integral breadth method where both size-induced and strain-induced broadening are deconvolutedby considering the peak width as a function of 2θ [35–37]and is represented by

β cos θ¼ Cλ=tWHþ2ε sin θ ð2Þ

where tWH is the particle size, ε is the strain and C is thecorrection factor taken equal to 1. The WH plot has beenpresented here and is shown in Fig. 2. The trend lineequations y¼0.2731x�0.0645, y¼�0.0025xþ0.0160, y¼�0.0020xþ0.0175 and y¼0.0135xþ0.0110 for samples 1to 4 respectively, are compared with Eq. (2) and the strain

Table 2Particle sizes determined by DS formula and WH plot, and strain inZn1�xCaxO films.

Samples tDS (nm) tWH (nm) ε

(100) (002) (101)

1 13.0 9.1 6.6 2.4 1.36�10�1

2 9.2 7.9 9.3 9.6 �1.25�10�3

3 7.5 11.1 7.0 8.8 �1.00�10�3

4 8.7 12.0 8.0 14.0 6.75�10�3

Fig. 2. Williamson–Hall plots for Zn1�xCaxO films where a, b, c

and crystallite size determined are presented in Table 2.The tensile and compressive strains are indicated bypositive and negative signs respectively. Value of strain

, and d correspond to samples 1, 2, 3 and 4 respectively.

Fig. 3. FTIR spectra of Zn1�xCaxO thin films. Curves a, b, c, and dcorrespond to samples 1, 2, 3 and 4 respectively.

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varies from �1.00�10�3 to 1.36�10�1. The crystallitesizes obtained by DS formula and byWH plot nearly matchfor samples 2, 3 and 4. For sample 1 strain is large and thecrystallite sizes obtained by the two methods are quitedifferent indicating that the broadening of FWHM is due tostrain existing in the films and not due to the crystallitesize.

Besides XRD results the FTIR transmission spectra, seeFig. 3, also confirm the formation of zinc oxide as the peakat 510 cm�1 corresponds to Zn–O stretching vibrations[38,39]. This peak shifts to 498, 493 and 513 cm�1 forsamples 2, 3 and 4 as dopant is increased. The shift isrelated to the change in Zn–O bond length as determinedfrom XRD spectra. Bond lengths for samples 2 and 3 arelarger than that for sample 1 and correspondingly theabsorption peaks of Zn–O stretching vibrations occur atlower frequencies. For samples 4 and 1, the bond lengthsare approximately equal and absorption also occurs atnearly equal frequencies. Though the variations in Zn–Obond length and absorption frequency are not monoto-nous but broadly conform to each other. Absence ofmonotonous variation can be attributed to the strainexisting in the films. The peak at 708 cm�1 arises due

Fig. 4. FESEM of Zn1�xCaxO films where a, b, c, and d

to Zn–O–Zn antisymmetric vibration [40] and shifts suc-cessively to 699, 697 and 699 cm�1 as dopant concentra-tion increases.

The peak at 1063 cm�1 may be attributed to C–Ostretching mode of ethanol [41]. This mode does notappear for sample 3. The absorption peak in region1237 cm�1 is related to C–O stretching vibrations of theester group [39] and shifts between 1154 and 1188 cm�1

as dopant is increased. The peak at 1529 cm�1 due to C¼Ostretching vibrations of the acetate group [39] shiftsbetween 1516 and 1540 cm�1 as dopant is varied. Peakat 1748 cm-1 gets shifted to 1744, 1752 and 1765 cm�1 asdopant is successively increased. Patil et al. [40] havereported peak at 1737 cm�1 due to C¼O bond and Duet al. [39] have reported peak at 1726 cm�1 due to C¼Ostretching vibrations of ester group.

3.2. SEM and EDX studies

The surface morphology of Zn1�xCaxO thin filmsobtained using FESEM is shown in Fig. 4. Doping of Cahas influenced the surface structure of the films. UndopedZnO has rod-like structures spread throughout with

correspond to samples 1, 2, 3 and 4 respectively.

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spherical grains in the background. As the dopant Ca isintroduced (sample 2), the rods become clearly definedwhereas the spherical grains in the background startcoalescing. For sample 3 more grains coalesce to result inlarger ones. For sample 4, the rods also coalesce accom-panied with reduction in size and increment in theirnumber. The rods also appear to coalesce with the back-ground grains.

The EDX spectra, see Fig. 5a–d, show the presence of Znand O as the only elementary species in sample 1 i.e.undoped ZnO. In doped samples, Ca is also present besidesZn and O. The concentration of calcium, shown in theinset, is found to be 0.76, 0.88 and 1.47 at% for samples 2, 3and 4 which is smaller than 1, 2 and 3 at% respectively,introduced while preparing the precursor solutions.

3.3. AFM studies

Three dimensional AFM images scanned over 1�1 mm2

area of the samples show nearly uniform grains distrib-uted throughout the surface and they coalesce as Ca isintroduced in increasing concentration as shown in

Fig. 5. EDX of Zn1�xCaxO thin films where a, b, c, and d

Fig. 6a–d. Two and three dimensional images over a largerarea of 5�5 mm2 of sample 2, see insets of Fig. 6b, showclear rod-like structures in conformity with the corre-sponding surface electron micrograph, Fig. 4b. The RMSroughness of these films lies between 28 and 71 nm.

3.4. Optical transmission

The optical transmission spectra are shown in Fig. 7.Both undoped and Ca doped films are highly transparentshowing 80% to 90% transmittance in the visible andinfrared regions. There is a sharp cut-off in the transmis-sion curve near 370 nm which shifts towards shorterwavelengths with increment in doping.

The absorption coefficient α is calculated from Beer'slaw I¼ I0e

�αt, where I is the transmitted intensity, I0 is theincident intensity, and t is the thickness of the film. αreduces by an order of 1 for doped films which results in atypical spectral interference effect [5,20,42]. The absorp-tion coefficient for direct band gap materials relates tooptical band gap energy Eg according to the expression α¼(hν–Eg)1/2, where h is Planck's constant and ν is the

correspond to samples 1, 2, 3 and 4 respectively.

Fig. 7. Transmission spectra for Zn1�xCaxO thin films. Curves a, b, c, and dcorrespond to samples 1, 2, 3 and 4 respectively.

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frequency of incident photon. Intercept on the energy axisobtained by extrapolating the linear portion of the (αhν)2

versus hν plot determines the band gap Eg; see Fig. 8. Theband gap for samples 1, 2, 3 and 4 is obtained as 3.150,3.245, 3.250 and 3.285 eV respectively allowing a band gapenhancement of 4.28% by doping Ca up to 1.47 at%. Thevariation in band gap with dopant concentration x isshown as ‘a’ in the inset of Fig. 8, linear fit to whichfollows the equation:

EgðxÞ ¼ 0:093xþ3:160 ð3Þ

Authors have earlier reported 4.14% enhancement inoptical band gap for 5 at% Ca in ZnO film [14], datareproduced as ‘b’ in the inset of Fig. 8. Cao et al. [5] hadreported 3.35% enhancement in band gap due to 5.3 at% Cadoping in ZnO films deposited by PLD, an advancedtechnique and the data is reproduced as ‘c’ in the insetof Fig. 8. Thus in the present study a much higher rate ofband gap enhancement is obtained. This improvement canbe attributed to a huge reduction in crystallite size.However, the reference band gap i.e. of undoped ZnO iscomparatively wider in Ref. [14] as well as in Ref. [5] whichmay be attributed to their greater thickness, nearly 100and 200 nm respectively. Increment in band gap for filmsof increasing thickness had been reported earlier in case ofα-Fe2O3, SnO2:F and ITO [43–45]. However, in Ref. [44] the

Fig. 6. Three dimensional AFM of Zn1�xCaxO thin films where a, b, c, and d cordimensional AFM images over 5�5 mm2 scanned area of sample 2.

doping of anion and in Ref. [45] substrate temperature hadalso been changed. Band gap has also been studied for Cadoped ZnO nanoparticles [17] and is reported to changefrom 2.71 to 2.86 eV for sample annealed at 650 1C.

3.5. Photoluminescence

The photoluminescence excitation spectra for an emis-sion at 393 nm corresponding to the optical band gap

respond to samples 1, 2, 3 and 4 respectively. Insets show two and three

Fig. 8. Plot of (αhν)2 versus photon energy for Zn1�xCaxO thin films.Curves a, b, c and d correspond to samples 1, 2, 3 and 4 respectively. Theinset shows variation in band gap with Ca concentration where thedotted lines a, b and c correspond to the present work, Refs. [14,5]respectively.

Fig. 9. Photoluminescence spectra of Zn1�xCaxO films. Curves a, b, c andd correspond to samples 1, 2, 3 and 4 respectively. Inset I shows the PLEspectra of sample 1 and inset II shows variation in Eg and NBE emissionenergy with dopant concentration.

Fig. 10. Variation with dopant concentration of PL intensity normalizedwith that for the undoped sample. Curves a and b correspond to UV andvisible emission respectively. The inset shows ratio IV/IU of PL intensitiesfor the visible (blue) and the UV emission.

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(3.150 eV) of ZnO film in the present study, is recorded.Inset I of Fig. 9 shows possibility of excitation at threewavelengths 325, 256 and 239 nm. 325 nm correspondingto strongest peak is used here for excitation of thesamples. PL spectra, see Fig. 9, of undoped ZnO film i.e.sample 1 shows UV emission peak at 399 nm (3.107 eV)having intensity higher than the other two peaks occur-ring in the blue region at 460 and 486 nm. The peak at460 nm is broad as compared to other peaks. Here UVemission energy (3.107 eV) is slightly less than the opticalband gap 3.150 eV of ZnO film and hence it corresponds tothe near band edge (NBE) emission of ZnO. PL at 486 nmhas been earlier reported by Ghosh et al. [29] for Zn–ZnOnanophosphor. Udaybhashkar et al. [17] have reported PLemission at 389, 407, 442 and 512 nm from Ca doped ZnOnanoparticles annealed at 650 1C.

As dopant is incorporated NBE emission peak showsblue-shift to 396, 394 and 392 nm for samples 2, 3 and 4respectively. Thus the UV emission can be tuned over therange of 55 meV by varying the Ca dopant concentrationfrom 0 to 1.47 at% in ZnO. Nature of change in the NBEemission energy with dopant concentration conforms withthat in band gap Eg as seen in inset II of Fig. 9. Peaks in thevisible region do not shift due to doping but the intensityvaries. An increase in luminescence in the visible and UVregions simultaneously, as seen for samples 2 and 3, hasbeen reported earlier also [32,46,47]. This indicates thatthe electrons in conduction band are not directly partici-pating in the PL process otherwise either visible or UVemission should increase at the expense of other. In thepresent work increment in blue- emission (486 nm) ismore as compared to that in UV as shown by the normal-ized emission intensity versus dopant concentrationcurve; see Fig. 10. The PL intensities from the undopedZnO in UV and blue region are taken as the normalizationfactor for the respective spectral region. Intensity of blue-emission is highest for sample 2 i.e. 0.76 at% Ca doping,being four-fold as compared to that for undoped ZnO.Enhancement in the intensity of blue as compared to UVemission is also highest for sample 2; see inset of Fig. 10.The inset shows variation of the ratio, (IV/IU), of PLintensities in visible (blue) and UV with respect to dopantconcentration. Thus Ca doped ZnO films can be goodcandidate for nanophosphors.

n-type semiconducting behavior of ZnO is attributed toZn interstitial (Zni ) and oxygen vacancy (VO). Coexistenceof zinc vacancy (VZn) with Zni is highly probable as alsoshown by Lin et al. [48]. Refs. [48,49] had shown theintrinsic defect levels of ZnO films (of band gap 3.36 eV)using a full-potential linear muffin-tin orbital method. Zni

levels are at 2.90 eV and VZn at 0.30 eV above the valanceband. VO, Oi and OZn are at 1.62, 2.28 and 3.28 eV belowthe conduction band. According to this picture, the blueluminescence at 486 nm (2.551 eV) occurring in ourexperiment can be correlated with the electronic transi-tion from the donor energy level of Zni to the acceptorlevel of VZn. When the dopant Ca is 0.76 at% it incorporates

A. Srivastava et al. / Materials Science in Semiconductor Processing 26 (2014) 259–266266

itself in the film as Ca interstitial (Cai), as indicated by theincreasing c-lattice constant. This enhances the number ofdonor levels thereby enhancing the blue PL emission four-fold which has been observed for the first time. However,further addition of Ca leads to shrinking of c-latticeconstant indicating a predominant substitution of Zn byCa resulting in lesser number of defect states. The blueluminescence drops down but to intensity higher than thatfor undoped ZnO. There can be two reasons; firstly avail-ability of lesser number of donor levels due to lessernumber of Cai and secondly due to quenching of blueluminescence with the increase in Ca concentration asthey interfere with each other's sphere of activity. It isimportant to mention here that the surface morphology inconformity with the AFM shows clear separate rod-likestructure on granular background for sample 2 for whichthe ratio IV/IU is highest. It appears that surface roughnessalso has a role in increase of PL. Films in Ref. [14],significantly smooth with rms roughness about 5 nm, werenot showing PL whereas in the present study films withroughness ranging over 28 to 71 nm exhibit PL.

4. Conclusion

Four-fold enhancement in blue-emission is observedfor the first time from sol–gel deposited Zn1�xCaxO trans-parent nanophosphor films for 0.76 at% doping. Compara-tively intense NBE emission shows a blue-shift of 55 meVas dopant concentration is increased to 1.47 at%Ca. Anenhancement of 4.28% in the band gap has been obtainedand it can be tuned from 3.150 to 3.285 eV by increasingthe Ca concentration from 0 to 1.47 at%. Doping up to 1.47at% does not alter the wurtzite structure of the films. Rod-like structures spread throughout the film become clearerwhereas uniform spherical grains in the background startcoalescing with introduction of calcium. The RMS rough-ness of these films varies from 28 to 71 nm.

Acknowledgment

Financial assistance from Department of Science andTechnology, New Delhi through Project no. SR/S2/CMP-0028/2010 is gratefully acknowledged.

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