Pb dopant induced changes in structural, optical and electrical properties of CdSe thin films

Accepted Manuscript Pb dopant induced changes in structural, optical and electrical properties of CdSe thin films Jagdish Kaur, S.K. Tripathi PII: DOI: Reference:

S0925-8388(14)02658-9 http://dx.doi.org/10.1016/j.jallcom.2014.11.015 JALCOM 32564

To appear in:

Journal of Alloys and Compounds

Received Date: Revised Date: Accepted Date:

10 May 2014 2 October 2014 3 November 2014

Please cite this article as: J. Kaur, S.K. Tripathi, Pb dopant induced changes in structural, optical and electrical properties of CdSe thin films, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom. 2014.11.015

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Pb dopant induced changes in structural, optical and electrical properties of CdSe thin films Jagdish Kaur and S.K. Tripathi* Department of Physics, Panjab University, Chandigarh- 160014, India *Corresponding author: [email protected]; [email protected]; Fax: +91-172-2783336; Tel: +91-172-2534462 Abstract Thin films of Pb doped CdSe at 1% and 5% dopant concentrations have been prepared by thermal evaporation technique using inert gas condensation method on glass substrates. The effect of Pb doping on structural, optical and electrical properties of CdSe thin films has been studied. Elemental composition of the thin films has been analyzed using Energy Dispersive Xray analysis (EDX) spectra. Transmission electron microscope (TEM) images show the spherical nature of the nanoparticles. X- ray diffraction spectra indicate the presence of hexagonal phase of CdSe in undoped and Pb doped CdSe thin films, and formation of cubic phase of PbSe with the increase in amount of Pb dopant. A decrease in the band gap with increase in Pb doping in CdSe lattice has been observed due to the formation of band tails in the band gap and increase in crystallite size after doping. The photoluminescence (PL) spectra of thin films have been studied and enhancement in the PL intensity is observed after Pb doping. The dark conductivity of the prepared thin films has also been studied and two types of conduction mechanisms have been observed. Hall measurements indicate change in conduction mechanism from n-type to p-type after Pb doping in CdSe. Keywords: Thin film, vapor deposition, optical properties, TEM, luminescence

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1. Introduction Semiconducting nanomaterials of II-VI group have been intensely pursued due to their photovoltaic, photodetection and optoelectronic applications [1-3]. Among these materials, CdSe has several unique properties like direct band gap comparable with the solar energy spectrum [4], high absorption coefficient in the visible and infrared region [5] and adjustable n and p-type conductivity by doping [6, 7] which makes it suitable for various optoelectronic and solar cell devices. The semiconducting nanomaterials deposited in the form of thin films have attracted much interest in a variety of applications [8-9]. There are various techniques such as thermal evaporation technique [5], successive ionic layer adsorption and reaction [10], electrodeposition [11] and chemical bath deposition [12] that have been adopted for the preparation of nanomaterials in the form of thin films. Among these methods, thermal evaporation imparts some feasible device based qualities like optimum stoichiometric ratio, morphology and crystalline alignment of the films, which are the important factors for deciding the suitability of the material for device applications. The doping of semiconductors with metal results in new properties that are significantly different from the undoped materials [7, 13-15]. In recent years, major attention has been given to investigate the electrical and optical properties of doped CdSe thin films in order to find new applications. The Pb doped semiconductors can find applications in photovoltaic cells and photo electrochemical solar cells [16, 17]. Kim et al. [18] have investigated the effect of Pb addition on the morphology developments of CdSe nanomaterials prepared by hot injection method and found that branched nanorods having high aspect ratio are formed with increase in amount of Pb dopant. Delekar et al. [19] synthesized Cd0.7Pb0.3 Se thin films for photo electrochemical solar cells by chemical bath deposition method having power conversion efficiency of 1.401 %. Alvi

2

el al. [20] have also studied the effect of change in composition of amorphous (PbSe)100-xCdx thin films prepared by thermal evaporation technique and found blue shift in the photoluminescence with increase in Cd dopant level. Furthermore, improvements in the optical parameters like refractive index and extinction coefficient have also been observed with increase in Cd amount. Hankare et al. [21] have studied the effect of Pb concentration on Cd1-xPbxSe thin films prepared by chemical method and found change in phase from hexagonal to cubic with the increase in amount of Pb concentration. Although there is extensive research on Pb doped chaclogenide alloys but still no reports are available on the effect of Pb doping on the structural, optical and electrical properties of CdSe thin films prepared by thermal evaporation technique. Therefore, in the present work, thin films of Pb doped CdSe are deposited on glass substrates by thermal evaporation technique in Argon gas atmosphere. The effect of Pb dopant on the structural, morphological, optical and electrical properties of CdSe thin films is investigated. The study will be useful to identify the dopant induced changes in the CdSe thin films for technological applications. 2. Experimental Details: The source material of (Cd35Se65)99Pb1 and (Cd35Se65)95Pb5 has been prepared by direct mixture of high purity (99.999%) constituent elements Cd, Se and Pb using melt quenching technique [5]. The desired amount of constituent elements Cd (35 %), Se (65 %) and Pb (1 % and 5%) are weighed according to their atomic weight percentages and sealed in quartz ampoules under vacuum of 2×10-5 mbar. The sealed ampoules are kept in the furnace, where the temperature is raised to 900 ºC at a constant heating rate of 2-3 ºC/min and then maintained at this temperature for 24 h. The ampoules at this temperature are quenched in ice cold water. Thin films of the Pb doped CdSe have been prepared by thermal evaporation technique using inert gas condensation 3

(IGC) method, taking Argon as inert gas, on well degassed corning 7059 glass substrates at room temperature under base pressure of 2×10-5 mbar. The deposition parameters have been kept constant for all films so that comparison of results could be made for thin films. The thickness of thin films has been measured by spectroscopic ellipsometry measurements. The films are kept in the deposition chamber in dark for 24 h before taking any measurements to attain thermodynamic equilibrium. Crystallographic study is carried out on the prepared thin films using a Spinner 3064 XPERT-Pro X-ray diffractometer in the 2θ range from 10º to 80º. Copper target is used as an Xray source with λ=1.54056 Å. The accelerating voltage is set at 45 kV with a current of 40 mA. TEM analysis is made using Hitachi H7500 electron microscope, operating at 100 KV. Films for TEM measurements are prepared by depositing very thin layer of the desired material on carbon coated copper grids by thermal evaporation using the IGC method. EDX analysis is carried out by energy dispersive spectrometer (EDS) coupled with Jeol Scanning Electron Microscope (SEM) (JSM-6610 LV). The normal incidence transmission spectra of the thin films are measured by UV/VIS/NIR computer controlled spectrophotometer Perkin Elmer Lambda 750 at room temperature (300 K). Photoluminescence spectrum is recorded in the visible region from a computer controlled luminescence spectrophotometer LS-50 B with λaccuracy= ±1.0 nm and pulsed Xenon discharge lamp as an excitation source. For dark conductivity measurements, indium electrodes having electrode spacing 0.8 mm are deposited on the thin films by thermal evaporation technique under base pressure of 2×10-5 mbar. Planar geometry of indium electrodes is used, as it provides stable and good ohmic contacts for electrical measurements. Indium electrodes are preferred because it does not diffuse into the thin films. The dark conductivity measurements are taken by mounting the samples in a specially designed metallic sample holder,

4

where a vacuum of 2×10-3 mbar is maintained throughout the measurements. In this sample holder, small heater is fitted below the thin film to measure the temperature dependence of dark conductivity. A dc voltage is applied across the thin films using Keithley Electrometer, Model 6517A and current is noted using pico-ammeter. To check the consistency in results, all measurements on thin films are done twice. 3. Results and Discussion: 3.1. Structural Properties: The X-ray diffraction (XRD) analysis is carried out to study the structural properties of the prepared thin films. The XRD spectrum of undoped CdSe thin film consists of (1 0 0) plane corresponding to the hexagonal phase of CdSe as reported elsewhere [5]. The comparison of XRD spectra of undoped and Pb doped CdSe thin films are shown in Fig. 1. After doping of Pb in CdSe lattice, in addition to (1 0 0) plane, two more planes (1 1 1) and (1 1 0) corresponding to the cubic phase of PbSe and hexagonal phase of CdSe, respectively emerges out [22-23]. Hankare et al. [21] have also observed diffraction peaks corresponding to the cubic structure of PbSe at high level doping of Pb (0.6 ≤ x ≤ 1.0) in Cd1-xPbxSe thin films. The sharp (1 0 0) peak in CdSe:Pb 1% thin film implies preferential growth of CdSe nanomaterials along “a” axis. The appearance of new (1 1 1) plane after doping of Pb suggests that Pb2+ ions start substitute the Cd2+ ions and incorporate in the CdSe lattice. The intensity of peak corresponding to (1 0 0) plane of CdSe decreases, while the intensity of (1 1 1) plane of PbSe increases at 5% Pb dopant level. This suggests that the growth of nanoparticles along (1 1 1) plane is favored at high level of Pb doping. The lattice plane spacing (d) is calculated using Bragg’s diffraction relation, 2dhklSinθhkl=nλ, (where n=1 and θ is the diffraction angle for the diffraction plane (h k l)). The d5

values for undoped and Pb doped CdSe thin film are calculated corresponding to each diffraction plane and given in Table 1. The crystallite size is calculated from the full width at half maximum (FWHM) using Debye Scherrer’s formula [24]: D=

0 .9 λ β cos θ

(i)

where λ is the wavelength of X-rays used, β is the FWHM of the diffraction peak and θ is the Bragg’s angle. The other micro structural parameters like dislocation density (δ) and lattice strain (ε) are also calculated from the crystallite size (D) and FWHM using relations [24]:

δ=

15 β cos θ 4aD

(ii)

β cosθ 4

(iii)

ε=

The calculated values of D, δ, and ε corresponding to each diffraction plane are given in Table 1. The crystallite size is found to increase after Pb doping in CdSe lattice. But at 5% Pb dopant level, the crystallite size decreases along (1 0 0) plane with corresponding decrease in intensity and increases along (1 1 1) cubic phase of PbSe with corresponding increase in intensity. This implies that at high level of Pb dopant, substitution of Pb2+ ions by Cd2+ ions become more efficient and formation of cubic phase of PbSe increases. 3.2. Surface morphology and compositional analysis: Fig. 2 shows the TEM image of undoped and Pb doped CdSe nanomaterials deposited on the carbon coated copper grids. It is clear from TEM images that films consist of nanoparticles of spherical nature. The size distribution of nanoparticles is also presented along with TEM image in Fig. 2. The TEM image of undoped CdSe consists of spherical nanoparticles, having average diameter of nanoparticles lying between 40-45 nm. The particle size is found to lie in the range of 35-40 nm for CdSe:Pb 1% and 45-50 nm for CdSe:Pb 5% thin films, respectively. The surface 6

morphology of Pb doped CdSe thin films are analyzed using a Scanning Electron Microscope (SEM). The SEM images of Pb doped CdSe thin films are shown in Fig. 3. It is clear from SEM images that the thin films surface are smooth and have spherically shaped grains. The elemental composition of the Pb doped CdSe thin films has been analyzed using EDX spectra and shown in Fig. 4. The presence of emission lines of Cd, Se and Pb in the investigated energy range confirm the doping of Pb in CdSe. The relative atomic percentages of CdSe:Pb 1% and CdSe:Pb 5% thin films are given in corresponding tables in Fig. 4. The decrease in the amount of Se in CdSe: Pb 5% is due to the lesser amount of Se required for the preparation of CdSe:Pb 5% as compared to CdSe:Pb 1% thin films. 3.3. Optical Properties: The optical transmittance spectra of the prepared thin films have been measured in the wavelength range of 400-3000 nm as a function of incident photon energy. The absorption coefficient of the thin films is calculated from transmission by using relation [13]:

1 1 α = Ln  d T

(iv)

where d (~ 165 nm) is the thickness of the films. The study of optical absorption phenomena provides a simple method for the study of band structure, energy band gap and other optical parameters. The variation of the optical absorption coefficient as a function of photon energy is given in Fig. 5. It is observed that the value of absorption coefficient increases with increase in amount of Pb dopant. Similar behavior of the increase in absorption coefficient after doping of Cd in PbSe thin films prepared by evaporation technique have also been observed by Alvi et al. [20] and Khan et al. [25]. After the incorporation of Pb dopant in CdSe lattice bond rearrangements take place, consequent change in the local structure of the thin films which results in the shift of absorption edge and change in the absorption coefficient. Moreover, the 7

undoped CdSe thin film consists of a broad band near 1 eV, which is attributed to the discontinuities at the grain boundaries and disorder in the films during the deposition process. In the nano-sized materials, strains are inherent due to lattice mismatch between the substrate and film, and contribute to the structural disorder [26]. The dopants induce a large number of free carrier density in the bands and high density of ionized dopants, which causes formation of band tails [27]. Therefore, formation of band tails in Pb doped CdSe thin films are attributed to the large number of free carriers in the bands. The optical band gap of the thin films is evaluated by using Tauc’s relation: (α h ν ) = A ( hν − E g ) p

(v)

where A is constant, Eg is the band gap of the material, hν is the incident photon energy and p is the exponent which depends on the type of transition. The exponent p can take values ½ and 2 for direct and indirect band gap transitions, respectively. The variation of (αhν)2 with hν follows a linear behavior in the high absorption region indicating direct transition as shown in Fig. 6. The extrapolation of the linear region of the curves at zero absorption gives value of the band gap of the material. The band gap values of undoped and Pb doped CdSe thin films are given in Table 2. The band gap value of undoped CdSe is found to be 2.25 eV. After doping of Pb, the value of band gap decreases from 2.25 eV to 2.20 eV and 2.13 eV for 1% and 5% Pb doping in CdSe, respectively. The decrease in band gap is attributed to the formation of band tails in Pb doped CdSe thin films. Mukherjee et al. have also observed the decrease in the band gap with increase in Pb concentration in PbxCd1–xSe thin films prepared by electrochemical method [28]. As observed from XRD analysis, although there is improvement in the crystallinity along (1 0 0) phase at 1% Pb dopant level, but there is also substitution of Pb2+ ions by Cd2+ ions resulting in the formation of PbSe cubic phase and leaving Cd2+ ions free, which in turn increase

8

the free carrier density. The free carrier introduced tail states in the bands, which are responsible for the decrease in the band gap. Another reason for the decrease in band gap is attributed to the increase in the grain size of the films after doping as observed from Table 1. Alvi et al. [20] have also reported the decrease in band gap due to increase in the density of localized states and shift in the position of Fermi level with increasing dopant (Cd) concentration in (PbSe)100−xCdx thin films. The refractive index dispersion of thin films is very important for both fundamental and technological point of view. Moreover, knowledge of refractive index is necessary for design and modeling of the optical components of nanodevices. The refractive index of the prepared thin films is evaluated from transmittance spectra using Manifacier approach of Swanepoel’s method as described elsewhere [5]. The variation of refractive index (n) with incident photon energy (hν) is shown in Fig. 7. It is observed that the refractive index decreases with increasing wavelength and increase with addition of Pb dopants. The increase in refractive index is due to the increase in absorption coefficient of the thin films after doping. The increase in refractive index can also be explained on the basis of the Lorentz-Lorenz formula [13]: n 2 −1 4π α' N A = ρ n2 + 2 3 M

(vi)

where α ' is the mean polarizability, M is the molecular weight and NA is the Avogadro number. The refractive index is directly related to the density of the films. Since, the density of Pb (11.34 g cm−3) is higher than the density of CdSe (5.83 g cm-3). Consequently the increase in refractive index with increasing Pb content can be attributed to the increase in density of the film with increase in Pb dopant. Photoluminescence (PL) study has been carried out to obtain information regarding the electronic transitions associated with the dopants, impurities and defect states present in the 9

material. The room temperature PL emission spectra of undoped and Pb doped CdSe thin films recorded at an excitation wavelength of 400 nm, are shown in Fig. 8. The PL signal depends on the density of photoexcited electrons, the intensity of the incident beam and also on the excitation wavelength [29]. In general, emission from semiconductor nanomaterials is composed of band edge emission and emission from surface trap states [30]. The PL spectra of the undoped CdSe thin film consists of near band edge emission peak at 530 nm as well as low energy trap state peak at 563 nm which correspond to the electron-hole recombination via surface trap state. The trap states are due to the surface of nanoparticles, as in the nanoparticles most ions at the surface are non-saturated in coordination [31-32]. These trap states act as an electron-hole acceptor and recombine radiatively. The doping of Pb in CdSe introduces new energy levels within the band gap, which are responsible for altering the luminescence properties of Pb doped CdSe thin films. It is observed that Pb doped CdSe thin films consist of sharp peak at 545 nm which correspond to near band edge emission and a broad emission peak at 573 nm due to the trapped luminescence i.e. presence of electron-hole recombination via trap state or imperfection site. In addition, Pb doped CdSe thin films also consist of a low intensity peak at 531 nm which may correspond to interband radiation of electron-hole recombination. The interband combination emits light with energy equal to or greater than the band gap [33]. The high energy emission is likely associated with high energy recombination centers associated with intrinsic deficiencies or dopant induced defect states [33]. The appearance of this emission band can be attributed to the direct transition from the energy states created in the conduction band of CdSe by the dopant to the valence band. The value of band gap for CdSe, CdSe:Pb 1% and CdSe:Pb 5% are found to be 2.25, 2.20 and 2.13 eV, respectively and the emission band values are somewhat blue shifted. It has also been observed

10

that with increase in Pb doping the intensity of emission peaks increases, which is attributed to the increase in radiative recombination of luminescent centers. 3.4. Electrical Properties: The electrical transport property of the material is an important factor

which reveals important information about transport phenomena and other physical properties of the materials. The temperature dependence of dark conductivity (σd) of undoped and Pb doped CdSe thin films is given in Fig. 9. It is evident from the figure that dark conductivity increases exponentially with temperature indicating the semiconducting behavior of the thin films. It has also been observed that the temperature variation of conductivity consists of two distinct linear regions, indicating two different types of conduction mechanisms. The variation of conductivity with temperature can be expressed as:  ∆E ac1   ∆E ac 2  σ d = σ o1 exp −  + σ o 2 exp −  kT  kT   

(vii)

where σo1 and σo2 are the pre-exponential factors, ∆Eac1 and ∆Eac2 are the activation energies in the high and low temperature regions, respectively, k is the Boltzmann constant and T is the temperature. The first term in eq. (vii) corresponds to the conduction in extended states and the second term corresponds to the conduction in localized states, which takes place as a result of hopping of charge carriers from one impurity level to an adjacent one [34]. At low temperature, conduction is due to thermally assisted tunneling of charge carriers in the localized states in the band tails and the carriers move from one impurity level to another with the help of phonons. The comparison of σd at room temperature (300 K) of undoped and Pb doped CdSe thin films is given in Table 2. The value of σd for Pb doped CdSe thin films are found to be less than undoped CdSe thin film. After addition of Pb dopant, Pb2+ ions substitute at the sites of Cd2+ and at the interstitial sites in the lattice and behave as impurity scattering centers. These impurity centers disturb the orientation of the grains and posses distortion in the lattice. The randomly oriented 11

grain boundaries act as trap centers [35]. The mobility of charge carriers gets reduced by trapping of carriers at trap centers and scattering of carriers at the grain boundaries. At 5% Pb doped the value of σd increases as compared to 1% Pb doped CdSe which can be attributed to the increase in substitution of Pb2+ ions at the sites of Cd2+ and improvement in grain size along PbSe phase as confirmed from XRD analysis, thus reducing the grain boundary scattering. To measure the carrier concentration and type of majority charge carriers, Hall measurement is performed on the prepared thin films. We have observed negative sign of the Hall voltage for undoped CdSe and positive sign of the Hall voltage for Pb doped CdSe thin films which indicate change in nature of charge carriers from n-type to p-type. Mukherjee et al. have also observed p-type nature for Pb0.7Cd0.3Se thin film [28]. The value of carrier concentration and mobility of charge carriers for CdSe, CdSe:Pb 1% and CdSe:Pb 5% thin films are given in Table 2. It is observed that the carrier concentration decreases after doping. The decrease in carrier concentration after doping is due to the introduction of trap states and impurity scattering centers in the host CdSe lattice. It has been found that Pb doping leads to significant changes in the transport properties of the thin films. The ability to tune the optical absorption and photoluminescence by doping of Pb is an important step towards device applications of CdSe thin films. The increase in photoluminescence with increase in Pb doping concentration explores potential applications of Pb doped CdSe thin films in light emitting devices. Conclusions: Thin films of undoped and Pb doped CdSe have been prepared by thermal

evaporation technique using the inert gas condensation method. A systematic study of structural, optical and electrical properties of undoped and Pb doped CdSe thin films at 1% and 5% dopant level is carried out. XRD spectra indicates the increase in the grain size from 12 nm for undoped

12

CdSe to 20 nm and 18 nm for CdSe:Pb 1% and CdSe:Pb 5%, respectively. TEM analysis reveals spherical nature of the nanoparticles. The band gap of thin films decreases with increase in amount of Pb dopant due to formation of band tails. The intensity of luminescent peak increases with Pb doping, which is attributed to the radiative recombination of the sluminescent centers. The dark conductivity of Pb doped CdSe thin films is found to decrease as compared to undoped CdSe which is explained on the basis of scattering centers and trapping of charge carriers at trap centers. The Hall measurement reveals the change in type of charge carriers from n-type to ptype after Pb doping in CdSe. Acknowledgement: This work is financially supported by University Grant Commission (UGC)

[F.No. 42-781/2013(SR)], N. Delhi. Ms. Jagdish Kaur is thankful to UGC, N. Delhi for providing the fellowship. References:

[1]

J. Jie, W. Zhang, I. Bello, C.S. Lee, S.T. Lee, Nano Today 5 (2010) 313.

[2]

S.K. Tripathi, J. Mater. Sci. 45 (2010) 5468.

[3]

N. Gopakumar, P.S. Anjana, P.K.V. Pilla, J. Mater. Sci. 45 (2010) 6653.

[4]

P. Mahawela, S. Jeedigunta, S. Vakkalanka, C.S. Ferekides, D.L. Morel, Thin Solid Films 480 (2005) 466.

[5]

K. Sharma, A.S. Al-Kabbi, G.S.S. Saini, S.K. Tripathi, Mater. Res. Bull. 47 (2012) 1400.

[6]

H.M. Ali and H.A. Abd El-Ghanny, J. Phys.: Condens. Matter 20 (2008) 155205.

[7]

A. Sahu, M.S. Kang, A. Kompch, C. Notthoff, A.W. Wills, D. Deng, M. Winterer, C.D. Frisbie, D.J. Norris, Nano Lett. 12 (2012) 2587.

[8]

L. Korala, Z. Wang, Y. Liu, S. Maldonado, S.L. Brock, ACS Nano 7 (2013) 1215.

[9]

M.S. Kang, A. Sahu, D.J. Norris, C.D. Frisbie, Nano Lett. 10 (2010) 3727.

13

[10]

H.M. Pathan, B.R. Sankapal, J.D. Desai, C.D. Lokhande, Mater. Chem. Phys. 78 (2002) 11.

[11]

S. Thanikaikarasan, K. Sundaram, T. Mahalingam, S. Velumani, Jin-Koo Rhee, Mater. Sci. Eng. B 174 (2010) 242.

[12]

Y. Zhao, Z. Yan, J. Liu, A. Wei, Mater. Sci. Semicon. Proc. 16 (2013) 1592.

[13]

K. Sharma, A.S. Al-Kabbi, G.S.S. Saini, S.K. Tripathi, J. Alloy. Compd. 540 (2012) 198.

[14]

A.S. Al-Kabbi, K. Sharma, G.S.S. Saini, S.K. Tripathi, J. Alloy. Compd. 555 (2013) 1.

[15]

S. Kumar, N. Kumari, S. Kumar, S. Jain, N.K. Verma, Appl. Nanosci. 2 (2012) 437.

[16]

M.A. Barote, S.S. Kambleb, L.P. Deshmukh, E.U. Masumdar, Ceram. Int. 39 (2013) 1463.

[17]

M.A. Barote, S.S. Kamble, A.A. Yadav, R.V. Suryavanshi, L.P. Deshmukh, E.U. Masumdar, Mater. Lett. 78 (2012) 113.

[18]

Y.K. Kim, K. Chung, H.M. Lee, C.J. Choi, Nanotechnology 20 (2009) 505603.

[19]

S.D. Delekar, M.K. Patil, B.V. Jadhav, K.R. Sanadi, P.P. Hankare, Solar Energy 84 (2010) 394.

[20]

M.A. Alvi, Z.H. Khan, Nanoscale Res. Lett. 8 (2013) 148.

[21]

P.P. Hankare, S.D. Delekar, P.A. Chate, S.D. Sabane, K.M. Garadkar and V.M. Bhuse, Semicond. Sci. Technol. 20 (2005) 257.

[22]

JCPDS data file no 8-459.

[23]

JCPDS data file no 78-1902.

[24]

M.G.S. Ahamed Basheer, K.S. Rajni, V.S. Vidhya, V. Swaminathan, A. Thayumanavan, K.R. Murali, M. Jayachandran, Cryst. Res. Technol. 46 (2011) 261.

14

[25]

S.A. Khan, Z.H. Khan, A.A. El-Sebaii, F.M. Al-Marzouki, A.A. Al-Ghamdi, Physica B 405 (2010) 3384.

[26]

G. Perna, V. Capozzi, A. Minafra, M. Pallara, and M. Ambrico, Eur. Phys. J. B 32 (2003) 339.

[27]

K. Sharma, A.S. Al-Kabbi, G.S.S. Saini, S.K. Tripathi, J. Alloy. Compd. 564 (2013) 42.

[28]

N. Mukherjee, G.G. Khan, A. Sinha, A. Mondal, Phys. Status Solidi A 207 (2010) 1880.

[29]

T.S. Shyju, S. Anandhi, R. Sivakumar, S.K. Garg, R. Gopalakrishnan, J. Cryst. Growth 353 (2012) 47.

[30]

A.E. Saunders, A. Ghezelbash, P. Sood and B.A. Korgel, Langmuir 24 (2008) 9043.

[31]

G. Jose, G. Jose, V. Thomas, C. Joseph, M.A. Ittyachen, N.V. Unnikrishnan, Mater. Lett. 57 (2003) 1051.

[32]

S.B. Singh, M.V. Limaye, N.P. Lalla, S.K. Kulkarni, J. Lumin. 128 (2008) 1909.

[33]

A.A. Ziabari, F.E. Ghodsi, J. Lumin. 141(2013) 121.

[34]

P.J. Sebastian, V. Sivaramakrishnan, J. Appl. Phys. 67 (1990) 3536.

[35]

H. Xie, C. Tian, W. Li, L. Feng, J. Zhang, L. Wu, Y. Cai, Z. Lei, Y. Yang, Appl. Surf. Sci. 257 (2010) 1623.

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Table captions Table 1: Structural parameters for undoped and Pb doped CdSe thin films Table 2: Optical band gap and electrical parameters of undoped and Pb doped CdSe thin films

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Figure captions: Fig. 1: X-ray diffraction pattern of undoped and Pb doped CdSe thin films Fig. 2: TEM image of (a) CdSe, (b) CdSe:Pb 1% and (b) CdSe:Pb 5% Fig. 3: SEM image of (a) CdSe:Pb 1% and (b) CdSe:Pb 5% Fig. 4: EDX pattern of (a) CdSe:Pb 1% and (b) CdSe:Pb 5% Fig. 5: Absorption coefficient spectra of CdSe, CdSe:Pb 1% and CdSe:Pb 5% thin films Fig. 6: Plot of (αhν) 2 vs hν for CdSe, CdSe: Pb 1% and CdSe: 5% thin films Fig. 7: Refractive index (n) variation with wavelength (λ) for CdSe, CdSe:Pb 1% and CdSe:Pb

5% thin films Fig. 8: Photoluminescence spectra of CdSe, CdSe:Pb 1% and CdSe:Pb 5% thin films Fig. 9: Plot of Ln σd vs. 1000/T for CdSe, CdSe:Pb 1% and CdSe:Pb 5% thin films

17

Table 1

Thin Film

CdSe

2θ (deg.)

Lattice

Lattice

spacing

plane

FWHM ‘β’

‘d’ (Å)

(h k l)

23.45±0.02

3.789

(1 0 0)

23.89±0.01

3.720

25.28±0.02

Crystallite

Dislocation

size

density 2

strain ‘ε’

‘D’ (nm)

‘δ’(lines/m )

0.676

12

8.2×1015

2.9×10-3

(1 0 0)

0.402

20

2.9×1015

1.7×10-3

3.520

(1 1 1)

0.874

09

14×1015

3.7×10-3

42.03±0.04

2.147

(1 1 0)

0.352

24

2.0×1015

1.4×10-3

23.96±0.02

3.709

(1 0 0)

0.450

18

3.6×1015

1.9×10-3

25.39±0.02

3.504

(1 1 1)

0.491

17

4.2×1015

2.1×10-3

42.00±0.03

2.149

(1 1 0)

0.506

17

4.1×1015

2.1×10-3

CdSe:Pb 1%

CdSe:Pb 5%

Lattice

18

Table 2

Thin Film CdSe

Optical

Dark

band gap

conductivity

‘Eg’ (eV)

‘σd’(cm-1Ω-1)

Activation energy

∆Eac1 (eV)

∆Eac2 (eV)

Carrier concentration

Mobility

(cm-3)

(cm2/V)

2.25±0.01 (4.12±0.31)×10-7 1.211±0.034 0.119±0.001 (1.20±0.24)×1011

CdSe:Pb 1% 2.20±0.01 (2.50±0.20)×10-9 0.967±0.017 0.411±0.005

21.46

(2.51±0.41)×109

6.225

CdSe:Pb 5% 2.13±0.01 (2.01±0.27)×10-7 0.426±0.009 0.137±0.002 (8.63±0.22)×1010

15.20

19

Fig. 1

20

Fig. 2

21

Fig. 3

22

(a)

(b)

Fig. 4

23

Element Weight% Atomic% Se

68.01

75.73±0.2

Cds

29.87

23.37±0.6

Pb

2.12

0.90±0.5

Totals

100.00

Element Weight% Atomic% Se

51.89

72.42±0.5

Cd

43.66

23.22±0.1

Pb

4.44

4.36±0.4

Totals

100.00

Fig. 5

24

Fig. 6

25

Fig. 7

26

Fig. 8

27

Fig. 9

28

Highlights 1. Thin films of Pb doped CdSe are prepared by thermal evaporation technique. 2. The effect of Pb doping on structural properties has been investigated. 3. Grain size and PL intensity increases with increases in amount of Pb doping. 4. Optical band gap and electrical conductivity decreases after Pb doping in CdSe.

29