Structural, optical and electrical properties of cerium and gadolinium doped CdO thin films

Applied Surface Science 274 (2013) 365–370

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Structural, optical and electrical properties of cerium and gadolinium doped CdO thin films L.L. Pan, G.Y. Li, J.S. Lian ∗ Key Laboratory of Automobile Materials, College of Materials Science and Engineering, Jilin University, Nanling Campus, Changchun 130025, China

a r t i c l e

i n f o

Article history: Received 17 December 2012 Received in revised form 12 March 2013 Accepted 13 March 2013 Available online 19 March 2013 Keywords: Cadmium oxide Radio frequency magnetron sputtering Transparent conductor oxide Band gap Hall effect

a b s t r a c t Cadmium oxide thin films doped with different concentration of cerium (Ce) and gadolinium (Gd) have been prepared by radio frequency magnetron (RF) sputtering. Thin films are deposited on glass substrates at a substrate temperature of 400 ◦ C and pressure of 0.1 mbar in Ar:O2 = 4:1 atmosphere. The structural, optical and electrical properties of deposited film are studied. X-ray diffraction reveals that Ce and Gd doped CdO films have good crystallinity and are apt to grow on (2 0 0) orientation with increasing Ce and Gd doping concentrations. These films are highly transparent with an average transmittance over 80%. With a moderate doping (0.4 at.% Ce and 0.8 at.% Gd), the optical band gap (Eg ) blue-shift from 2.59 eV to 2.99 eV. The electrical conductivity increases with increasing Ce and Gd doping concentrations, but for higher doping concentration (0.5 at.% Ce and 1.0 at.% Gd), the conductivity decreases. The increase in carrier concentration due to Ce and Gd doping is the main reason responsible for the increase of and conductivity and the blue shift of band gap. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Transparent conductive oxide (TCO) is the favorable material used in flat panel displays, photovoltaic cells, smart windows, surface acoustic wave device, light emitting diodes and optical wave guides [1–3]. CdO is an n-type degenerate semiconductor with a simple cubic rock salt structure and a relatively low electrical resistivity (10−3 to 10−4  cm) due to oxygen vacancies and high carrier concentration contributed by shallow donors resulting from self non-stoichiometry [4,5]. CdO is a group II-VI transparent conductor and has high transparent in VIS-NIR spectral regions with a direct band gap of 2.3–2.7 eV (depending on the structure and fabrication method) and two indirect band gaps at 1.18–1.2 eV and 0.8–1.12 eV, respectively [6], which makes it useful for various applications such as photovoltaic solar cells, transparent electrodes, gas sensors, smart window, photodiodes, phototransistors and optoelectronic devices [7,8]. CdO has broadly dispersed s-like conduction bands and a small carrier effective mass and is considered to be an ideal model material to be used to study the effects of doping elements on TCO band structure, crystal chemistry, and charge transport [9,14].

∗ Corresponding author at: The Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, 142 Renmin Street, Jilin University, Nanling Campus, Changchun 130025, Jilin, China. Tel.: +86 431 85095875; fax: +86 431 85095876. E-mail address: [email protected] (J.S. Lian). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.03.066

Pure and doped CdO thin films have been obtained by different various physical and chemical techniques such as pulse laser depositions (PLD) [10], magnetron sputtering [11], spray pyrolysis technique (SPT) [12], ultrasonic spray [13], metallo-organic chemical vapor deposition (MOCVD) [14], sol–gel [15], chemical bath deposition (CVD) [16], vacuum evaporation [17] and electrospinning [18]. The electrical conductivity and optical band gap of CdO thin films can be increased by doping suitable elements whose ionic radii are smaller and/or equal to that of host lattice atoms [17,19]. It is experimentally established that doping of CdO with metallic ions having a smaller ionic radius than that of Cd2+ , like In, Sn. Al, Sc, and Y can improve its electrical conductivity and optical band gap, which is explained through the Mosse–Burstein (B–M) effect [20]. Yan et al. [21] have obtained Sn doped CdO films which have an increase of bandgap up to 3.87 eV for 6.2% Sn doping. Gupta et al. [22] have deposited In-CdO thin films which show high mobility (155 cm2 /V s), high carrier concentration (1.41 × 1021 cm−3 ) and low resistivity (2.86 × 10−5  cm). Recently, it is reported that the rare earth elements are potential doping candidates to improve the optical and electrical properties of conducting metal oxides because of their high optical band gap [23,24]. Dakhel [5] has reported that rare earth elements Yb and Eu doped CdO films show high mobility, enhanced carrier concentration and evident increased electrical conductivity. It was known that Ce exhibits three valence states: +2, +3 and +4 and the +2 state is rare, and Gd has one valence state, +3.

366

L.L. Pan et al. / Applied Surface Science 274 (2013) 365–370

Additionally, Ce4+ , Ce3+ and Gd3+ ions have standard ionic radii of 0.092 nm, 0.103 nm and 0.094 nm, respectively, which are similar to that of Cd2+ ions, 0.097 nm [27]. Thus, we expect that when Ce4+ or Ce3+ and Gd3+ ions would substitute for Cd2+ ions while doping in CdO crystalline structure, the carrier concentration should be increased, that leads to the increase of electrical conduction. So, in this paper, rare earth elements Ce and Gd were co-doped in CdO film by radio frequency magnetron (RF) sputtering, in order to improve its conductivity and to modulate its optical band gap. The structural, optical and electrical properties of Ce and Gd co-doped films are studied.

2. Experimental procedure The targets for radio-frequency sputtering were prepared by a standard solid-state reaction using high purity of CdO (99.99%), CeO2 (99.99%) and Gd2 O3 (99.99%) powders. According to the previous experimental results, the optimal doping concentrations of Ce and Gd in CdO were 1.3 at.% [28] and 2.0 at.% [29], respectively. The co-doping ratio of Ce:Gd was selected to be 1:2, but less quantities of them was used. So, the powders were prepared with Ce atomic concentration of 0.1%, 0.2%, 0.3% and 0.4%, and the corresponding Gd atomic concentration of 0.2%, 0.4%, 0.6% and 0.8%, respectively. The accurately weighed quantity of powder was mixed thoroughly using an agate mortar and grinded by a jar mill for 12 h. The well-ground mixture was heated in air at 800 ◦ C for 8 h. Then, the mixture was placed in a grooved stainless steel holder (50 mm in diameter) and pressed into a pellet under a hydrostatic pressure of approximately 8 Mpa. The pellets were then sintered at 900 ◦ C for 12 h in air. The base pressure of sputtering was fixed at 5 × 10−6 mbar. The chamber was backfilled with argon and oxygen as ambient gas for sputtering. The flow of Ar and O2 was 40 and 10 sccm, respectively. The sputtering was performed at a pressure of 0.1 mbar. The target–substrate distance was 65 mm. The films were deposited on glass substrates (30 mm × 25 mm × 1 mm) which cleaned ultrasonically in acetone, ethanol and deionized water for 15 min, respectively. The target was pre-sputtered for 15 min to remove surface contamination. The films were deposited with a substrate temperature of 400 ◦ C for 0.5 h. While the effective RF power was maintained at 140 W and the negative DC self-bias voltage was 220 V. The structural characterization of film was performed by X-ray diffraction (XRD, Rigaku Dymax) with a Cu target and a monochronometer at 40 kV and 250 mA. The sample was mounted at 2.5◦ and scanned from 25◦ to 80◦ in steps of 0.02◦ with a scan rate of 1.2◦ min−1 . The optical transmittance measurement carried out using UV–vis spectrophotometer (Ocean Optics HR4000), at room temperature in the wavelength range from 300 to 1000 nm. The surface and cross-sectional morphology of the film was characterized by cold field emission scanning electron microscope (FESEM, JSM6700F). The resistivity () and Hall coefficient (RH ) measurements were carried out by a standard four-probe technique (Lake-shore’s 7704 Hall system). The film resistivity was determined by taking the product of sheet resistance and film thickness. The Hall effect was measured under the magnetic field of 0.6 T, which was applied perpendicular to film surface according to the standard Vander–Pauw configuration [30]. The carrier concentration (Nel ) and the Hall mobility () were calculated from the electrical resistivity () and the Hall coefficient (RH ) [31]. The carrier concentration (Nel ) was derived from the relation n = 1/eRH , where RH was the Hall coefficient and e was the absolute value of the electron charge. The carrier mobility (el ) was determined using the relation m = 1/ne, where  is resistivity. All measurements were done at room temperature.

Fig. 1. XRD spectra of undoped and Ce and Gd doped CdO films with different Ce and Gd concentration.

3. Results and discussion 3.1. Structural characterization The XRD patterns of the undoped CdO film and Ce and Gd doped CdO films are shown in Fig. 1. No extra peaks are observed on the Ce and Gd doped CdO films, which imply that Ce4+ or Ce3+ and Gd3+ ions are doped into CdO lattice. The existence of these oxidation states can be supported by previous XPS results. According to the XPS spectra (917 eV) of Ce 3d core level peaks, Yousefi et al. [25] had confirmed that Ce3+ and Ce4+ ions have coexisted in Ce–ZnO sample. Moreover, Lu et al. [26] had proved that the characteristic peak near 141 eV can be ascribed to the Gd 4d5/2 , indicating that Gdion in the film is trivalent; while the characteristic peak near 882 eV for the Ce 3d5/2 can also be observed in XPS spectra, indicating Ce4+ existed in the Ce–CdO film and Ce4+ can easily trap electron and change into Ce3+ . The structural parameters of the (2 0 0) diffraction line such as Bragg angle (2), the full width at half maximum (FWHM), texture coefficient (TC) and average grain size (D) calculated using the Scherer’s formula are presented in Table 1. It is seen that the grain size is in the range of 30–50 nm, showing a slight increase with increase in the doping concentration. The increase of grain size means improved crystalline quality and the decrease of grain boundary fraction in the films, which can reduce grain boundary scattering and thus result in a decrease of electrical resistivity [32]. However, the relatively small grain size (30–50 nm) means low surface roughness of the film, which can lead to a decrease in the propagation loss of surface acoustic wave (SAW) devices and an increase in the efficiency for photovoltaic solar cells. The high diffraction intensity of the peak at 2 = 38.29◦ indicates that the films have preferential orientation along the (2 0 0) direction. The

Table 1 The Bragg angle (2), lattice parameter (a), the grain size (D), full width at half maximum (B) and the texture coefficient (TC) for the prepared undoped and Ce and Gd doped CdO thin films with different Ce and Gd concentration. Sample No.

2

D (nm)

FWHM

TC

0.0 at.% Ce + 0.0 at.% Gd 0.1 at.% Ce + 0.2 at.% Gd 0.2 at.% Ce + 0.4 at.% Gd 0.3 at.% Ce + 0.6 at.% Gd 0.4 at.% Ce + 0.8 at.% Gd 0.5 at.% Ce + 1.0 at.% Gd

38.29 38.26 38.25 38.24 38.19 38.11

37.4 45.5 48.6 41.6 31.3 41.2

0.225 0.185 0.173 0.202 0.268 0.204

3.25 3.64 3.82 3.88 3.93 2.89

L.L. Pan et al. / Applied Surface Science 274 (2013) 365–370

(2 0 0) preferred orientation growth of CdO structure is characterized through a texture coefficient (TC) [33]. n 

TC(hkl) = [nIr (hkl)/Istd (hkl)]/

k Irk (hkl)/Istd (hkl)]

(1)

k=1

where Ir (hkl) is the relative intensity of reflection from a given (hkl) plane, Istd (hkl) is the relative intensity of the reflection from the same plane as indicated in a standard polycrystalline sample [34], and n is the total number of reflections observed. From this

367

definition, it is clear that the deviation of the texture coefficient of a particular plane from unity implies the preferred orientation along that plane. Table 1 shows that the maximum value of TC (2 0 0) is 3.93 for Cd98.8 Ce0.4 Gd0.8 O film, indicating the high preferred orientation of doped CdO film. It is possible when doped with Ce and Gd in the distorted lattice, the symmetrical (2 0 0) plane has minimum interfacial energy, facilitating the preferred growth in (2 0 0) direction. There is a slight shift in the diffraction peak position of (2 0 0) toward lower Bragg angle (from 38.29◦ for pure CdO to 38.11◦ for Ce and Gd co-doped CdO). This peak shift can be explained by

Fig. 2. FE-SEM images of as-grown CdO films deposited with different Ce and Gd concentration: (a) 0.0 at.% Ce + 0.0 at.% Gd, (b) 0.1 at.% Ce + 0.2 at.% Gd, (c) 0.2 at.% Ce + 0.4 at.% Gd, (d) 0.3 at.% Ce + 0.0 at.% Gd, (e) 0.4 at.% Ce + 0.8 at.% Gd, (f) 0.5 at.% Ce + 1.0 at.% Gd.

368

L.L. Pan et al. / Applied Surface Science 274 (2013) 365–370

Fig. 3. The variation of resistivity, carrier concentration and mobility of the undoped and Ce and Gd doped CdO films.

two reasons. One possible explanation is that the structural strain (εs = − (2 0 0) cot (2 0 0) ) exists in the CdO lattice, which is of 10−3 order relative to CdO crystal cell. The strain should be caused by a tensile stress due to the thermal expansion in high temperature (400 ◦ C) [35]. Another possibility is that the Ce3+ dopant ions, which have a larger ionic radius (0.103 nm) than that of Cd ionic (0.097 nm), substitute for Cd2+ ions in the CdO lattice. According to the Bragg formula:  = 2dsin, where  is the X-ray wave-length (0.1541 nm), d is the crystalline plane distance for indices (hkl) and  is the diffraction angle of the (2 0 0) peak, it can be understood that the observed lattice plane distance d values for doped films are larger than the standard d values of CdO, which would result in the decrease of the diffraction angle. 3.2. Morphological characterization It is known that the optical and electrical properties TCO films can be influenced by their surface properties. The surface morphologies of undoped and Ce and Gd doped CdO thin films are shown in Fig. 2. For undoped CdO film (Fig. 2(a)), a compact and homogeneous surface consists of rounded and randomoriented grains. For Ce and Gd doped thin films (Fig. 2(b)–(e)), the films are more uniform and compact, packed with a polyhedral grained structure, which may result from the textured growth along (2 0 0) plane. Specifically for Cd98.8 Ce0.4 Gd0.8 O film (Fig. 2(e)), parallel oriented grains reunite into many great grains with complanate (2 0 0) plane, presenting more homogeneous film surface. 3.3. Electrical characterization The electrical conduction parameters including electrical resistivity (), hall mobility (el ) and carrier concentration (Nel ), are shown in Fig. 3 and presented in Table 2. The data demonstrate that undoped and all Ce and Gd doped CdO films are n-type semiconductors. It should be emphasized that the undoped CdO film already has a very low electrical resistivity () of 7.47 × 10−4  cm,

this may result from the intrinsic native defect of oxygen vacancies and cadmium interstitials, since the film was prepared under a low pressure and a low ratio oxygen flux. From Fig. 3, it can be observed that the doping of Ce4+ , Ce3+ and Gd3+ ions into CdO lattice induces an increase of carrier concentration (Nel ) and a decrease of electrical resistivity (). The highest carrier concentration of 6.34 × 1020 cm−3 and lowest electrical resistivity of 3.11 × 10−4  cm are observed for Cd98.8 Ce0.4 Gd0.8 O film. The improvement of the electric conductivity is certainly attributed to the increase of carrier concentration. It is due to the Ce4+ or Ce3+ and Gd3+ ions substituting for Cd2+ ions or some of these ions occupying interstitial locations in CdO lattice, which can liberate more conduction electrons in the conduction band. It is seen that the carrier concentration of 0.4 at.% Ce and 0.8 at.% Gd doped CdO increases about 4.74 times (in comparison with that of undoped CdO). In comparison with previous results of single rare earth element doped CdO, Dakhel [28] had demonstrated that 1.3 at.% Ce-doped CdO film has electrical resistivity of 8.86 × 10−4  cm and carrier concentration of 2.54 × 1020 /cm3 ; and Gupta et al. [29] had presented that 2.0 at.% Gd-doped CdO film has the lowest electrical resistivity of 2.7 × 10−5  cm and highest mobility of 258 cm2 /V s. So, the codoping of Ce and Gd in CdO presents good electrical properties similar to the single doping of Ce or Gd. Meanwhile, when Ce4+ , Ce3+ and Gd3+ ions concentration increases, the interstitial Ce and Gd ions may increase (which act as donor centers), the acceptor  vacancies VCd would also increase to keep the charge balance in the system, which is reasonable for the slight decrease in Nel when the Ce and Gd concentration reaches to 1.5 at.% (seen in Fig. 3). However, the increase of doping concentration of Ce and Gd also induces the decrease of carrier mobility (el ). The carrier mobility decreases from 42.62 cm2 /V s of CdO film to 25.09 cm2 /V s for 1.2 at.% (Ce + Gd) doped CdO film and even to 12.21 cm2 /V s for 1.5 at.% (Ce + Gd) doped CdO film. This negative effect limits the possibility of further improving the electric resistivity to the level of 10−5  cm. When the doping concentration is high, the lattice distortion and disordered arrangement appear, which can enhance the scattering effect of grain boundary and ionized impurity and hence cause the mobility decreases. 3.4. Optical properties The optical transmittance of the undoped and doped CdO films is shown in Fig. 4. It can be observed that the films have an average transmittance of 80% in the visible light range of 550–1000 nm. Doping of Ce and Gd has no evident influence on the optical transmittance in this visible and infrared range. A distinct character is that the absorption edge is observed to blue shift with increasing the Ce and Gd doping concentration. The optical band gap (Eg ) of the films is calculated from the transmittance versus wavelength spectra. The absorption coefficient (˛) can be calculated by the equation:

˛ = ln

1 T

(2)

/d

Table 2 The optical and electronic properties for undoped and Ce and Gd doped CdO thin films with different Ce and Gd concentration. Sample No. 0.0 at.% Ce + 0.0 at.% Gd 0.1 at.% Ce + 0.2 at.% Gd 0.2 at.% Ce + 0.4 at.% Gd 0.3 at.% Ce + 0.6 at.% Gd 0.4 at.% Ce + 0.8 at.% Gd 0.5 at.% Ce + 1.0 at.% Gd

Optical bandgap, (eV) 2.59 2.68 2.75 2.79 2.99 2.88

Electrical properties  (×10−4  cm)

Nel (×1020 cm−3)

 (cm2 /V s)

7.47 4.40 4.09 3.96 3.11 8.33

1.96 4.43 5.93 6.28 9.28 8.14

42.62 33.87 31.98 30.56 25.09 12.21

L.L. Pan et al. / Applied Surface Science 274 (2013) 365–370

Fig. 4. Optical transmission spectra of the undoped and as-grown CdO films deposited with different Ce and Gd content.

where T is transmittance, d is film thickness. The absorption coefficient (˛) and the incident photon energy (h ) are related by the equation: n

(˛h ) = A(h − Eg )

(3)

where h is the photoelectron energy. A and n are constants. Eg is the separation between the bottom of conduction band and the top of valence band. The value of n depends on the probability of transition and it takes values as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions, respectively. The direct band gap values of doped CdO thin films are estimated by extrapolating the linear portion of (˛h )2 to the energy axis of ˛ = 0, as shown in Fig. 5. It can be seen that the Eg values increases from 2.59 of undoped CdO to 2.99 eV of 0.4 at.% Ce and 0.8 at.% Gd doped CdO. The widening of band gap can be explained by the Mosse–Burstein (B–M) effect [20], which states that a sufficient number of states introduced by high carrier concentration (Nel ) can shift the Fermi level upward into the conduction band and the optical absorption edge shifts to higher energy. This effect is expressed by the equation [36,37]: 2/3

Eg = Ego + SBGW Nel

(4)

369

2/3

Fig. 6. Variation of Eg vs. Nel

for the undoped and Ce and Gd doped CdO thin films.

where SBGW = (2 /2 me )(32 )2/3 , Ego is the intrinsic band gap,  is the Plank constant and = m∗vc /me is the ratio of reduced effective mess to free electron mass (me ), which is equal to 0.274 for pure CdO [38]. Therefore, SBGW = 1.35 × 10−18 eV m2 . Fig. 6 shows the variation of band gap (Eg ) with two-third power of carrier con2/3

centration (Nel ). It can be seen that the calculated Eg (solid line) at carrier concentration of 1.96 × 1020 /cm3 (CdO film experimental value) is 2.52 eV, which is very similar to the experimental value of 2.59 eV of the undoped CdO film, the result agrees well with the known range of values (2.2–2.6 eV) for pure CdO films prepared by different methods [39–41]. So, the blue shift of the absorption edge can be reasonably explained by the B–M effect.

4. Conclusions CdO thin films with Ce and Gd doping are prepared by RF magnetron sputtering on glass substrates at a fixed temperature of 400 ◦ C and pressure of 0.1 mbar in 4:1 argon + oxygen atmosphere. The structural, optoelectronic properties and electrical properties of CdO films lightly doped with rare-earth element Ce and Gd are studied in the present work.

(1) Ce and Gd doped CdO films show good crystallinity and evident (2 0 0) preferred orientation. Transmittance spectra show that they have an average transmittance of 80% in the visible light range of 500–800 nm. (2) Doping of Ce and Gd ions in CdO induces a decrease of electrical resistivity () (3.11 × 10−4  cm for 0.4 at.% Ce and 0.8 at.% Gd doped CdO film), which is attributed to the increase of carrier concentration (9.28 × 1020 cm−3 for 0.4 at.% Ce and 0.8 at.% Gd doped CdO film). (3) The optical band gap increases from 2.59 eV of CdO film to 2.99 eV of 0.4 at.% Ce and 0.8 at.% Gd doped CdO film. This blue shift of band gap is also explained by the increase of carrier concentration based on the Mosse–Burstein effect.

Fig. 5. The plots of (˛h )2 vs. h for undoped and Ce and Gd doped CdO thin films. Inset shows the variation of Eg values from the plot.

The low electric resistivity, high average optical transmission and the widened solar light wavelength range of transparence enable the CdO films to be a promising candidate for optoelectronic applications.

370

L.L. Pan et al. / Applied Surface Science 274 (2013) 365–370

Acknowledgments This work was supported by the Foundation of National Key Basic Research and Development Program (No. 2010CB631001), the National Nature Science Foundation (Grant No. 31070841) and the Program for Changjiang Scholars and Innovative Research Team in University. References [1] A.V. Moholkara, G.L. Agawanec, K.-U. Sima, Y.-B. Kwona, K.Y. Rajpurec, J.H. Kim, Applied Surface Science 257 (2010) 93. [2] F.F. Yang, L. Fang, S.F. Zhang, J.S. Sun, Applied Surface Science 254 (2008) 5481. [3] G. Murtaza, B. Amin, S. Arif, M. Maqbool, Computational Materials Science 58 (2012) 71. [4] S.K. Vasheghani Farahani, T.D. Veal, P.D.C. King, C.F. McConville1, Journal of Applied Physics 109 (2011) 073712. [5] A.A. Dakhel, Journal of Materials Science 46 (2011) 1455. [6] A.A. Dakhel, Optical Materials 31 (2009) 691. [7] T. Singh, D.K. Pandya, R. Singh, Materials Chemistry and Physics 130 (2011) 1366. [8] W.-M. Choa, G.-R. Heb, T.-H. Sua, Y.-J. Linb, Applied Surface Science 258 (2012) 4632. [9] M.K.R. Khan, M. Azizar Rahman, M. Shahjahan, M. Mozibur Rahman, Current Applied Physics 10 (2010) 790. [10] B.J. Zheng, J.S. Lian, L. Zhao, Q. Jiang, Applied Surface Science 256 (2010) 2910. [11] F.F. Yang, L. Fanga, S.F. Zhang, K.J. Liao, Journal of Crystal Growth 297 (2006) 411. [12] R. Kumaravel, K. Ramamurthi, I. Sulania, K. Asokan, Radiation Physics and Chemistry 80 (2011) 435. [13] S. Kose, F. Atay, V. Bilgin, I. Akyuz, International Journal of Hydrogen Energy 34 (2009) 5260. [14] S. Jin, Y. Yang, J.E. Medvedeva, J.R. Ireland, Journal of the American Chemical Society 126 (2004) 13787. [15] R. Ferro, J.A. Rodríguez, Thin Solid Films 347 (1999) 295. [16] L.R. de León-Gutiérrez, J.J. Cayente-Romero, J.M. Peza-Tapia, E. Barrera-Calva, Materials Letters 60 (2006). [17] A.A. Dakhel, Journal of Alloys and Compounds 475 (2009) 51.

[18] A.M. Bazargan, S.M.A. Fateminia, M. Esmaeilpour Ganji, M.A. Bahrevar, Chemical Engineering Journal 155 (2009) 523. [19] N. Wongcharoen, T. Gaewdang, T. Wongcharoen, Energy Procedia 15 (2012) 361. [20] T.S. Moss, Proceedings of the Physical Society of London B 67 (1954) 775. [21] M. Yan, M. Lane, C.R. Kannewurf, R.P.H. Chang, Applied Physics Letters 78 (2001) 02342. [22] R.K. Gupta, K. Ghosh, R. Patel, S.R. Mishra, P.K. Kahol, Materials Letters 62 (2008) 3373. [23] J.C. Lin, K.C. Peng, C.A. Tseng, S.L. Lee, Surface and Coatings Technology 202 (2008) 5480. [24] H. Huang, Y. Ou, S. Xu, G. Fang, M. Li, X.Z. Zhao, Applied Surface Science 254 (2008) 2013. [25] M. Yousefi, R. Azimirad, M. Amiri, A.Z. Moshfegh, Thin Solid Films 520 (2011) 721. [26] L. Lu, R. Li, T. Peng, Renewable Energy 36 (2011) 3386. [27] K. Barbalace, Periodic Table of Elements – Sorted by Ionic Radius, Environmental Chemistry.com. 1995–2008. [28] A.A. Dakhel, Materials Chemistry and Physics 130 (2011) 398. [29] R.K. Gupta, K. Ghosh, R. Patel, P.K. Kahol, Journal of Alloys and Compounds 509 (2011) 4146. [30] L.J. Van der Pauw, Philips Research Reports 13 (1958) 1. [31] D.K. Schroder, Semiconductor Material and Device Characterization, John Wiley and Sons, New York, 1990. [32] C.S. Barrett, T.B. Massalski, Structure of metals, Pergamon, Oxford, 1980, p 204. [33] E.F. Kaelble (Ed.), Handbook of X-rays for diffraction, emission, absorption, and microscopy, McGraw-Hill, New York, 1967, p5. [34] Powder Diffraction File, Joint Committee for Powder Diffraction Studies (JCPDS), File no. 05-0640. [35] T. Singh, D.K. Pandya, R. Singh, Materials Science Engineering B 176 (2011) 945. [36] T.S. Moss, Proceedings of the Physical Society of London B 67 (1954) 775. [37] K. Kawamura, K. Maekawa, H. Yanagi, M. Hirano, H. Hosono, Thin Solid Films 445 (2003) 182. [38] D. Hahn, O. Jaschhinski, H.H. Wehmann, A. Schlachetzki, Journal of Electronic Materials 24 (1995) 1357. [39] R. Kumaravel, K. Ramamurthi, V. Krishnakumar, Journal of Physics and Chemistry of Solids 71 (2010) 1545. [40] P.K. Ghosh, R. Maity, K.K. Chattopadhyay, Solar Energy Materials and Solar Cells 81 (2004) 279. [41] R. Maity, K.K. Chattopadhyay, Solar Energy Materials and Solar Cells 90 (2006) 597.