- Email: [email protected]
Effect of La doping on the structural, optical and electrical properties of spray pyrolytically deposited CdO thin films
Journal of Alloys and Compounds 708 (2017) 804e812
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of La doping on the structural, optical and electrical properties of spray pyrolytically deposited CdO thin films P. Velusamy a, R. Ramesh Babu a, *, K. Ramamurthi b, E. Elangovan c, J. Viegas c a
Crystal Growth and Thin Film Laboratory, Department of Physics, Bharathidasan University, Tiruchirappalli, 620 024, Tamil Nadu, India Crystal Growth and Thin Film Laboratory, Department of Physics and Nanotechnology, Faculty of Engineering and Technology, SRM University, Kattankulathur, 603 203, Tamil Nadu, India c Nano-Optics and Optoelectronics (NOOR) Research Laboratory, EECS Department, Masdar Institute, 54224, Abu Dhabi, United Arab Emirates b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2016 Received in revised form 13 February 2017 Accepted 4 March 2017 Available online 7 March 2017
Solution based chemical spray pyrolysis technique was employed to deposit lanthanum (La) doped cadmium oxide (CdO) thin films on soda-lime microscope glass slides. X-ray diffraction (XRD) analysis revealed that deposited films are polycrystalline having cubic crystal system. The undoped and 0.25 wt % La doped CdO films show relatively intense (111) plane, which is shifted to (200) plane for doping concentrations greater than 0.50 wt %. The parameters such as lattice constants, crystallite size, microstrain, dislocation density and texture coefficient are extracted from XRD data. Field emission scanning electron microscopy analysis confirmed the smooth and uniform surface. Energy dispersive spectroscopy analysis was used to estimate the preliminary information on elemental composition. The deposited undoped and La doped films possess optical transmittance ranging 78e82% in the visible and near infrared (NIR) region. The estimated direct optical band gap of undoped and La doped CdO thin films is varied between 2.38 eV and 2.46 eV. Hall measurements confirmed the n-type conductivity in the films. The doping-modulated mobility and carrier concentration are ranging 68 cm2/V se78 cm2/V s, and 1.0 1020 cm3e4.06 1020 cm3, respectively. Room temperature micro-Raman studies confirmed the metal oxide (CdeO) bond vibrations in deposited films. A high figure of merit (11.45 103 U1) is obtained from 1.0 wt % La doped CdO film. © 2017 Elsevier B.V. All rights reserved.
Keywords: Thin films Spray pyrolysis coating X-ray diffraction Electrical transport Optical properties Scanning electron microscopy
1. Introduction Transparent conductive oxides (TCOs) such as zinc oxide (ZnO), tin oxide (SnO2), indium oxide (In2O3), cadmium oxide (CdO), Sndoped In2O3 (ITO), F- doped ZnO (FZO), and F- doped SnO2 (FTO) are widely used for optoelectronic applications. This is due to their unique combination of low electrical resistivity and high optical transmittance in the useful region of solar spectrum [1e3]. In comparison with the other host TCOs, undoped and doped CdO films are extensively studied for the applications in solar cells, smart windows, transparent electronic devices, optical communications system, flat-panel displays and phototransistors. CdO also has an exceptional carrier concentration (1.5 1020 cm3), with high electrical conductivity (102e104 S/cm). However, CdO is not a
* Corresponding author. E-mail addresses: [email protected], (R. Ramesh Babu). http://dx.doi.org/10.1016/j.jallcom.2017.03.032 0925-8388/© 2017 Elsevier B.V. All rights reserved.
[email protected]
popular TCO material for some applications due to its narrow optical band gap of 2.2 eV [4e7]. A wide range of literature survey shows that the electrical and optical properties of CdO films can be tuned by optimizing the deposition parameters and the selection of metal ions doping from Al, In, Ga, Eu, Ce, Sm, etc. [1,8e12]. It is also observed that the resistivity of CdO films is modified when the ionic radius of dopants is nearly equal or smaller than that of Cd2þ ions [13]. CdO thin films were deposited by various techniques such as spray pyrolysis [8], vacuum evaporation [10], RF magnetron sputtering [14], chemical vapour [15], chemical bath [16], solegel [17] and pulsed laser [9] deposition methods. Dakhel and AliMohamed [18] and Alahmed et al. [19] have reported the deposition of CdO films with different La doping concentrations by sol-gel technique. However, the recent reports based on thin-film transistors (TFTs) and thin-film solar cells fabricated by spray pyrolysis have triggered a renewed interest in an economic solution process [20,21]. To the best of our knowledge, no report is available on the detailed study of La doped CdO thin films by an economic spray
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
805
pyrolysis technique. Hence, in the present investigation, the effect of La doping on structural, microstructural, optical and electrical properties of CdO thin films deposited by a homemade chemical spray pyrolysis setup is reported. 2. Experimental 2.1. Deposition of CdO films CdO thin films having different La doping concentrations were deposited on the ultrasonically cleaned microscope glass substrates by spray pyrolysis setup. An appropriate quantity of cadmium acetate dihydrate ((CH3 COO)2$Cd$2H2O; from Merck with 98% purity) was used to prepare 50 ml of 0.05 M precursor aqua solution. The pre-estimated amount of lanthanum trichloride heptahydrate (LaCl3$7H2O; from Sigma Aldrich with 99.90% purity), depending on the doping concentrations varying between 0.25 wt % and 1.0 wt %, was added to the precursor solution. The resulting solution was sprayed on to the microscope glass substrates preheated to 300 C. ChromeleAlumel thermocouple, placed at the center of the hot plate, was used to measure and maintain the substrate temperature. The substrate to nozzle distance was 30 cm. The spray gun was kept at an angle of 45 . Compressed and filtered air, at a pressure of 45 kg/cm2, was used as carrier gas. The spray time (1 s) and spray interval (29 s) were kept constant during the deposition of CdO thin films. Several sets of films were deposited from each precursor solution and found that the properties of thin films were highly reproducible. 2.2. Characterization of CdO thin films Various analytical tools were utilized to study the structural, microstructural, electrical and optical properties of undoped and La doped CdO thin films. The X-ray diffraction (XRD) patterns of the films were recorded in Bragg-Brentano geometry (q and 2q coupled) using PANalytical Empyrean X-ray diffractometer equipped with a CuKa1 radiation source (wavelength 1.5406 Å). The scan length for the diffraction angle (2q) is ranging between 30 and 110 . Thickness of the films measured by Filmetrics (Model: F20XT) is around 300 nm (the accuracy is ±10 nm). The oxidation state of deposited films was analyzed by X-ray photoelectron spectroscopy (XPS) studies (ESCA - Shimadzu 3400 electron spectrometer). The optical transmittance was recorded using a Perkin Elmer (Model: Lambda-35) spectrophotometer in the wavelength ranging between 200 nm and 900 nm. Field emission scanning electron microscopy (FEeSEM) from the FEI (NovaNano) was used to analyze the surface microstructures of films. A preliminary analysis of the elemental composition was carried out by energy dispersive spectroscopy (EDS). The electrical parameters such as carrier concentration, mobility and electrical resistivity were measured by the Hall measurement set up (Ecopia e HMS 3000) at room temperature (RT) in van der Pauw configuration with a permanent magnet of 0.570 T. Raman spectrum was recorded using LABRAM-HR spectrometer equipped with He-Ne laser source (wavelength 632.8 nm). 3. Results and discussion 3.1. X-ray diffraction The XRD patterns obtained from the undoped and La doped CdO thin films, shown in Fig. 1, reveal that the films are polycrystalline with a cubic crystal system [4]. The structural parameters such as lattice constant (a), texture coefficient (TC), crystallite size (D), dislocation density (d) and microstrain (ε) calculated from the XRD
Fig. 1. XRD patterns of CdO and La doped CdO thin films.
data are presented in Table 1. Two high intensity XRD peaks of (111) and (200) planes are obtained along with a few low intensity diffraction peaks of (220), (311), (222), (400), (331), (420) and (422). All the diffraction peaks are indexed by matching with the standard data [4]. The obtained lattice constant (a) (Table 1) from (200) plane is in good agreement with the reported value of 0.4695 nm. Fig. 1 indicates that the intensity of (111) plane is decreased for 0.25 wt % La doping. Further increase in La doping concentration has shifted the preferred growth orientation from (111) plane to (200) plane. The preferred growth orientation of CdO films is investigated through TC as defined in Ref. [11].
h
Im ðhklÞ Io ðhklÞ
i
i TC ¼ h P Im ðhklÞ 1 N
(1)
Io ðhklÞ
where, Im (hkl) is the measured relative intensity of reflections from a given (hkl) plane, Io (hkl) is the intensity of reflections from the same plane as indicated in a standard sample of randomly oriented polycrystalline CdO powder. The total number of reflections (N) is 9 in the present work. The calculated TC values are given in Table 1, which clearly indicates that the TC value for (111) plane is gradually decreased; whereas, it is increased for (200) plane. For 0.5 wt % La doped CdO thin films, TC value of (111) and (200) plane is 2.03 and 2.59, respectively (Table 1); this reveals that the excess La doping has presumably shifted the preferred growth orientation from (111) plane to (200) plane. It is noteworthy that the literature survey [5e10,14e19] reveals
806
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
Table 1 Structural parameters extracted from XRD data: Lattice constant (a), crystallite size (D), microstrain (ε), dislocation density (d) and texture coefficient (TC). Diffraction Plane
La doping (wt %)
(a) (Å)
(D) (nm)
(ε) (103)
(d) (1015 lines/m2)
TC
(200)
0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00
4.690 4.690 4.688 4.690 4.691 4.689 4.690 4.688 4.691 4.691
21 21 22 24 25 27 28 28 29 30
1.37 1.60 1.45 1.65 1.68 1.20 1.16 1.22 1.27 1.24
2.50 2.26 2.06 1.73 1.60 1.37 1.28 1.27 1.89 1.11
1.86 2.08 2.59 2.76 2.80 2.62 2.09 2.03 1.88 1.79
(111)
that the CdO films deposited by other deposition techniques have (111) as the energy favorable orientation in general. However, in the present study, it is observed that the preferred orientation is shifted from (111) to (200), which can be related to the increasing La doping concentration. Since the ionic radius of La3þ ions (0.103 nm) is slightly higher than Cd2þ ions (0.097 nm), additional La dopants presumably influence the strain in CdO lattice and thereby triggering a change in nucleation and growth kinetics of CdO thin films. It is well known that the deposition technique like spray pyrolysis has a strong influence on the nucleation and growth kinetics [22]. Probably, this has in turn changed the preferential growth orientation from (111) plane to (200) plane. This can be further substantiated from the SEM microstructures (Fig. 5), where in it can be noticed that the appearance of grains is changing in accordance with the change in preferred orientation. When the films have the preferred orientation along (111) plane (undoped and 0.25 wt % La doped CdO), the grains seem to agglomerate together and form bigger agglomerates. Additionally, because of this agglomeration, the films seem to be loosely packed. However, when the orientation is shifted to (200) plane for the increased La doping concentrations (0.50 wt %), the films do not have any agglomeration and the grains are separately distributed and tightly packed. Further, the grain size and the uniformity in grains distribution are both increased (images shown and explained later). Overall, the surface microstructures confirm that the nucleation and growth kinetics are altered based on the variation in La doping concentrations. Accordingly, the variation in nucleation and growth kinetics has changed the preferred orientation. The authors have earlier observed this kind of change in preferred orientation for the fluorine and antimony doped tin oxide films deposited by spray pyrolysis [23]. The average crystallite size (D) of the films is calculated using Scherrer's formula [24].
D¼
0:9l b cos q
(2)
where, b is the full width at half maximum (FWHM) of the corresponding diffraction peak and l is the X-ray wavelength (1.5406 Å) and q is the diffraction angle. It is found that the size of crystallites increases with the increasing La doping concentrations. The lattice constant and crystallite size estimated from (111) and (200) diffraction peaks are given in Table 1, which indicate that La dopant improves the crystallinity of CdO thin films. The microstrain (ε) and dislocation density (d) values were calculated using the relation ε ¼ (b cosq)/4 and d ¼ 1/D2 [24] and are summarized in Table 1. The relatively less value of d obtained in the present work confirms that spray pyrolysis is an effective deposition technique to obtain quality undoped and La doped CdO thin films. Meanwhile, the microstrain value for (200) plane is initially increased but then decreased for 0.50 wt % La doping concentration. Further increase in La doping concentration has increased the microstrain for (200)
plane. The microstrain value obtained from (111) plane is initially decreased but then increased with the increasing La doping concentration (Table 1). The variation in microstrain can be related to the crystallization process in polycrystalline thin films and the difference in the ionic radius between Cd2þ and La3þ ions, since the ionic radius of La3þ ions is slightly higher than that of Cd2þ. 3.2. XPS analysis A typical wide scan XPS spectrum obtained for 1.0 wt % La doped CdO (Fig. 2a) confirms the presence of La in the CdO lattice. A marked multiple-peak structure was observed in the Cd 3d5/2 core level. The narrow scan spectrum of CdO thin film (Fig. 2b) shows Cd 3d5/2 and Cd 3d3/2 peaks at 406.7 eV and 413.3 eV, respectively. This agrees well with the 6.6 eV spineorbit energy splitting between Cd 3d5/2 and Cd 3d3/2 states, thus confirming the oxidation state of Cd2þ [12]. The O1s binding energy is located at 533 eV, which represents the characteristics of O2 oxides in 1.0 wt % La doped CdO films (Fig. 2c) [12]. A peak at 841 eV (Fig. 2d) indicates the binding energy of La 3d5/2, which refers to the presence of La3þ ion in 1.0 wt % La doped CdO thin films [25]. Thus, the XPS study revealed the ionic state of La3þ and Cd2þ metal ions. 3.3. Micro-Raman studies Micro-Raman spectra of the undoped and La doped CdO thin films are shown in Fig. 3. In the present investigation, four Raman peaks are observed at 288 cm1, 396 cm1, 571 cm1, and 665 cm1. The intense peak at 288 cm1 reveals the combination of the transverse acoustic and optical phonon (TA þ TO) modes of CdO thin films, due to the lattice disturbance of CdO films [26]. The peak located at 396 cm1 represents phonon sum processes or possible resonant mode of CdO vibration. The peak observed at 571 cm1 represents the metal and oxide bond vibration, and it is due to the band crossing between longitudinal and transverse (LO þ TO) optical phonon modes of vibration of CdO [27]. The less intensity peak observed at 665 cm1 represents the 2LO modes of vibration of CdO thin films [12,27]. The intensity of these peaks is varied with the variation in La doping concentration (Fig. 4). For 1.0 wt % La doping, the peak at 288 cm1 gets red shift (284 cm1) due to the structural defects such as oxygen vacancies, cadmium interstitial, and free carrier concentration effect [28]. 3.4. Morphological studies The surface microstructures of undoped and La doped CdO thin films are shown in Fig. 5 as a function of La doping concentration. Fig. 5a shows the surface microstructure of undoped CdO films, which consists of agglomerated spherically shaped grains distributed on the surface of the film together with various sizes of irregularly shaped structures. Fig. 5bee shows the surface
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
807
Fig. 2. A typical X-ray photoelectron spectrum of 1.0 wt % of La doped CdO thin film.
Fig. 3. Micro-Raman spectrum of CdO thin films as a function of La doping concentrations.
Fig. 4. Intensity modification of Raman peak as a function of La doping concentration.
808
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
Fig. 5. FE-SEM image of CdO thin films as a function of La doping concentration (a) 0 wt % (b) 0.25 wt % (c) 0.50 wt % (d) 0.75 wt % (e) 1.0 wt %.
microstructures obtained from the films doped with 0.25 wt %, 0.50 wt %, 0.75 wt %, and 1.0 wt % of La, respectively. The surface of CdO films is effectively modified due to La doping of 0.25 wt % in the precursor solution (Fig. 5b). Agglomerated spherical structures combine to form nearly uniform circular shapes on the surface. Fig. 5cee shows relatively smooth, uniform, compact and densely packed surface microstructures. The grain size and the uniform distribution of grains on the surface are increased with the increasing La doping concentrations. The increased grain size and the uniform spreading are probably representing the improvement in film quality. Nearly similar surface morphologies observed in Fig. 5cee are probably reflecting the similar XRD patterns of these
films (Fig. 1cee). Fig. 6aee shows the histogram of average grain size of the undoped and La doped CdO thin films measured using the FE-SEM microstructures. It can be understood from Fig. 6aee that the average grain size of undoped CdO is ~64 nm, which is gradually increased to 70 nm for 0.25 wt %, 73 nm for 0.50 wt %, 90 nm for 0.75 wt % and 95 nm for 1.0 wt % of La doping concentrations. The grain size calculated from FE-SEM histogram is found to be greater than the crystallite size calculated from the XRD peaks. Generally, XRD gives the information about out-of-plane grain size; whereas, FE-SEM analysis estimates the in-plane grain size. The foregoing discussion clearly indicates that the surface microstructures of CdO films are effectively modified following the
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
809
Fig. 6. Histogram image for measuring average particle size: (a) CdO (b) 0.25 wt % La, (c) 0.50 wt % La, (d) 0.75 wt % La, and (e) 1.0 wt % La doped CdO thin films.
variation in La doping concentrations. Among the doped CdO thin films, 1.0 wt % La doped CdO shows uniformly distributed nearly spherical shaped grains. The features have relatively increased grain size in comparision with that of other La doped films. The elemental composition of CdO and La doped CdO thin films were measured by EDS analysis and measured ratios of Cd, O and La
elements are given in Table 2, which clearly indicate the presence of La in the doped CdO thin films. 3.5. Electrical properties The carrier concentration, mobility, electrical resistivity and
810
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
Table 2 Elemental analyses obtained from EDS. La doping Concentration (wt %)
0.00 0.25 0.50 0.75 1.00
Percentage of Elements (wt %)
Percentage of Elements (at. %)
O (K series)
Cd (L series)
La (L series)
O (K series)
Cd (L series)
La (L series)
24.29 48.85 32.88 36.57 27.12
75.71 51.04 66.84 62.93 72.20
0.00 0.11 0.28 0.50 0.68
53.94 77.03 77.29 79.99 72.16
46.06 22.95 22.47 19.68 27.07
0.00 0.02 0.24 0.33 0.77
conductivity of undoped and La doped CdO thin films were measured using van der Pauw method (Hall measurements) and the results are schematically shown in Fig. 7 and also summarized in Table 3. The carrier concentration (n) is derived from the relation ne ¼ 1/e (RH), [29] where RH is the Hall coefficient and e is the absolute value of the electron charge. The Hall mobility of charge carriers is determined using the relation me ¼ 1/ner, [29] where r is resistivity. The negative sign of Hall coefficient confirmed that the deposited films are n-type conductor. The undoped CdO thin films possess electrical resistivity and mobility of 9.32 104 U-cm and 68 cm2/V,s, respectively. The carrier concentration and conductivity of CdO thin films are increased with the increasing La doping concentration. A high carrier mobility of 78 cm2/V,s is obtained for the 0.25 wt % La doped CdO films. The increase in mobility for the initial La doping is attributed to the fact that La dopant reduces the grain boundary scattering [30]. The resistivity of CdO thin films is decreased with the increasing La doping concentrations. The low resistivity (3.0 104 U-cm) and relatively high carrier concentration (4.0 1020 cm3) are obtained for 1.0 wt % La doped CdO thin films. The increasing carrier concentration may originate from the following mechanism: substitution of La3þ for Cd2þ in CdO lattice donates one excess free electron to the electrical conductivity (ionic radius of Cd2þ 0.097 nm and La3þ e 0.103 nm) [13]. The obtained electrical parameters are compared with literature values in Table 4, which shows that the films doped with 1.0 wt% La have a better combination of lowest resistivity (3.0 104 U-cm) and high carrier concentration (4.06 1020 cm3) in comparison with the undoped CdO thin film. Dakhel and Hamad [31] have reported an increase in the carrier concentration and mobility due to Cr doping in CdO thin
films. Similarly, La doping in CdO has effectively increased the mobility and electrical conductivity. 3.6. Optical properties Optical transmittance spectra of undoped and La doped CdO thin films are shown in Fig. 8. It is observed that the deposited films exhibit high transmittance, ranging 75e90% in the NIR wavelength region. Further, we found that the transmittance of CdO film is increased with the increasing La doping concentration upto 0.50 wt %, but then decreased for 0.75 wt % and 1.0 wt % La doping concentrations. The optical study shows that La doped CdO thin films possess relatively high transmittance when compared to that of undoped CdO films. The absorption coefficient (a) is calculated using the relation a ¼ (1/d) (ln (1/T)) [10] where ‘d’ is the film thickness and ‘T’ is the film transmittance. The relation between the absorption coefficient and the incident photon energy (hn) is derived from the relation (ahn)2 ¼ A (hn - Eg) [17]; where ‘A’ is a constant and ‘Eg’ is optical energy band gap. Eg can be determined by extrapolating the linear region of (ahn)2 to the X-axis (at Y ¼ 0). The plots of (ahn)2 Vs hn of the CdO thin films as a function of La doping concentration are shown in Fig. 9. It is noticed that the band gap is gradually increased with the increasing La dopant concentration until 0.75 wt %, and then decreased. The blue shift of absorption edge is called band gap widening, which can be described by the MosseBurstein effect [11]. The average transmittance (AT) obtained in the 600e900 nm range and optical band gap value of undoped and doped CdO films are summarized in Table 3 as a function of La doping concentration. The results show that La doping at various levels is effectively influenced the optical
Fig. 7. Variation of carrier concentration, mobility, electrical resistivity and conductivity of the CdO thin films as a function of La doping concentrations.
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
811
Table 3 Electrical and Optical properties of La doped CdO thin films. La Doping (wt %)
ne (1020/cm3)
me (cm2/Vs)
r (104 U cm)
s (103/U.cm)
AT (600e900 nm)
Eg (eV)
CdO 0.25 0.50 0.75 1.00
1.0 1.5 1.7 2.6 4.1
68 78 76 60 52
9.3 5.2 4.8 3.9 3.0
1.0 1.8 2.0 2.5 3.4
78 82 81 79 82
2.38 2.41 2.40 2.46 2.40
Table 4 Comparison of electrical properties of different metal doped CdO thin films deposited by different techniques. Doped and undoped CdO films CdO 1.0 wt % La:CdO 0.015% Ga:CdO 6.0 wt % Al:CdO 0.8 mol. % Eu:CdO 3.8 at. % Ce:CdO 0.4 mol % Sm:CdO 1.0 mol % La:CdO 1.5% Cr:CdO
Resistivity (U cm) 4
9.32 10 3.0 104 1.93 104 4.48 104 5.2 104 3.86 104 5.6 104 3.8 102 4.68 104
Carrier concentration (1/cm3) 20
1.0 10 4.1 1020 11.7 1021 3.7 1020 4.86 1020 2.42 1020 2.5 1020 5.7 1019 2.49 1020
Mobility (cm2/V s)
Deposition Technique
Reference
68 52 27 34 24.42 66.71 43.25 2.5 53.4
Spray pyrolysis Spray pyrolysis Spray pyrolysis Spray pyrolysis Vacuum evaporation Vacuum evaporation Vacuum evaporation Sol-Gel Physical vapour deposition
Present work Present work [1] [8] [10] [11] [12] [21] [28]
properties of CdO thin films. 3.7. Figure of merit
Fig. 8. Optical transmittance spectra of the deposited CdO thin films as a function of La doping concentrations.
Figure of merit (FOM) of the TCO thin films plays an important role in optoelectronic applications. Thus, the quality of TCO films can be investigated by figure of merit defined by Haacke [32], FOM ¼ T10/RSh, where T is the transmittance (600e900 nm) and RSh is the sheet resistance. The FOM values are calculated at different wavelengths of 600 nm, 700 nm, 800 nm, and 900 nm. The calculated values of FOM are given in Table 5. In the present work, the sheet resistance is decreased for 1.0 wt %. La doping concentration. The highest FOM value of 36.19 103 U1 is obtained for 1.0 wt % La doped CdO thin film at 900 nm. The La doped CdO thin films prepared by spray pyrolysis method show improved FOM when compared to that of ITO thin films (11.9 103 U1 at 550 nm) deposited by spin coating method [33]. Thus, the result of this work reveals that CdO films doped with 1.0 wt % of La has high optical transmittance, high electrical conductivity, low resistivity and high FOM. 4. Conclusions
Fig. 9. Plot of (ahѵ)2 against hѵ of the CdO thin films as a function of La doping concentrations.
Undoped and La doped CdO thin films were deposited by cost effective chemical spray pyrolysis technique and their properties were studied as a function of La doping concentration. Various doping levels of La has influenced the structural, morphological, optical, and electrical properties of CdO films. The 0.5 wt % La doping has shifted the preferred growth orientation from (111) plane to (200) plane. The crystallite size of the CdO films is increased with the increasing La doping concentration. The oxidation state of Cd2þ, O2 and La3þ was confirmed by XPS analysis. Micro-Raman analysis confirmed the metal oxide bond vibration of CdeO. FE-SEM analysis revealed that the La doping modified the surface morphology of CdO thin films. A minimum resistivity of 3.0 104 U-cm and a maximum carrier concentration of 4.0 1020 cm3 is observed due to 1.0 wt % La doping in CdO thin films. The AT (600e900 nm) is varied between 78% and 82%; and the band gap is varied between 2.38 eV and 2.46 eV. The 1.0 wt % La doping in CdO film gives the high figure of merit (11.45 103 U1). The range of electrical conductivity and optical transmittance obtained in the present work suggests that the spray deposited La
812
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
Table 5 Figure of merit for undoped and La doped CdO thin films at different wavelengths. La Doping (wt %)
0.00 0.25 0.50 0.75 1.00
Figure of Merit (103 U1) FOM for AT (600e900 nm)
600 nm
700 nm
800 nm
900 nm
2.23 6.60 6.33 6.06 11.45
1.15 3.52 2.93 5.30 4.69
3.26 9.40 8.08 6.88 11.45
4.69 13.38 11.52 6.88 23.20
5.28 11.91 11.52 6.88 36.19
doped CdO films will be suitable for optoelectronic device application. Acknowledgements The authors thank Dr. Vasant Sathe, UGC-DAE-CSR, Indore for micro-Raman analysis. The author E. Elangovan thanks Mike Tiner and Mustapha Jouiad of Masdar Institute for the characterization facilities (XRD and SEM). References [1] A.V. Moholkar, G.L. Agawane, K.U. Sim, Y. Kwon, K.Y. Rajpure, J.H. Kim, Appl. Surf. Sci. 257 (2010) 93e101. [2] F.F. Yang, L. Fang, S.F. Zhang, J.S. Sun, Appl. Surf. Sci. 254 (2008) 5481e5486. [3] G. Murtaza, B. Amin, S. Arif, M. Maqbool, Comput. Mater. Sci. 58 (2012) 71e76. [4] Powder Diffraction File, International Centre for Diffraction Data (ICDD), Reference Code: 00-005-0640. [5] S. Salam, M. Islam, A. Akram, Thin Solid Films 529 (2013) 242e247. [6] M. Alam, M. Islam, A. Achour, A. Hayat, B. Ahsan, H. Rasheed, S. Salam, M. Mujahid, Surf. Rev. Lett. 21 (2014) 1450059e1450068. [7] D.A. Cristaldi, S. Millesi, I. Crupi, G. Impellizzeri, F. Priolo, R.M.J. Jacobs, R.G. Egdell, A. Gulino, J. Phys. Chem. C 118 (2014) 15019e15026. [8] R. Kumaravel, S. Menaka, S.R.M. Snega, K. Ramamurthi, K. Jeganathan, Mater. Chem. Phys. 122 (2010) 444e448. [9] R.K. Gupta, K. Ghosh, R. Patel, S.R. Mishra, P.K. Kahol, Mater. Lett. 62 (2008) 3373e3375. [10] A.A. Dakhel, Opt. Mater. 31 (2009) 691e695. [11] E. Burstein, Phys. Rev. 93 (1954) 632e633. [12] P. Velusamy, R. Ramesh Babu, K. Ramamurthi, J. Viegas, E. Elangovan, J. Mater Sci. Mater Electron 26 (2015) 4152e4164. [13] Kenneth Barbalace. Periodic Table of Elements - Sorted by Ionic Radius.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
EnvironmentalChemistry.Com. 1995-2015. Accessed on-line: 11/16/2015 http://EnvironmentalChemistry.com/yogi/periodic/ionicradius.html. G. Anil Kumar, M.V. Ramana Reddy, K. Narasimha Reddy, Mater. Res. Innov. 19 (2015) 204e211. T. Terasako, K. Ohmae, M. Yamane, S. Shirakata, Thin Solid Films 572 (2014) 20e27. H. Khallaf, C.T. Chen, L.B. Chang, O. Lupan, A. Dutta, H. Heinrich, A. Shenouda, L. Chow, Appl. Surf. Sci. 257 (2011) 9237e9242. F. Yakuphanoglu, J. Sol Gel Sci. Technol. 59 (2011) 569e573. A.A. Dakhel, A.Y. Ali-Mohamed, J. Sol Gel Sci. Technol. 44 (2007) 241e247. Z.A. Alahmed, Z. Serbetci, F. Yakuphanoglu, Acta. Phys. Pol. A 124 (2013) 125e127. C. Girotto, D. Moia, B.P. Rand, P. Heremans, Adv. Fun. Mater. 21 (2010) 64e72. G. Adamopoulos, S. Thomas, D.D.C. Bradley, M.A. McLachlan, T.D. Anthopoulos, Appl. Phys. Let. 98 (2011) 123503e123506. J.P. Hirth, G.M. Pound, Condensation and Evaporation-nucleation and Growth Kinetics, McMillan Company, New York, 1963. E. Elangovan, K. Ramamurthi, A study on low cost-high conducting fluorine and antimony-doped tin oxide thin films, Appl. Surf. Sci. 249 (2005) 183e196. B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, third ed., Prentice-Hall Inc, 2001, pp. 167e171. J.T. Stecher, A.B. Rohlfing, M.J. Therien, Nanomaterials 4 (2014) 69e86. S.G. Choi, L.M. Gedvilas, S.Y. Hwang, T.J. Kim, Y.D. Kim, J.Z. Perez, V.M. Sanjose, J. Appl. Phys. 113 (2013) 83515e83519. S.K. Vasheghani Farahani, T.D. Veal, P.D.C. King, J. Zuniga-Perez, V. MunozSanjose, C.F. McConville, J. Appl. Phys. 109 (2011) 73712e73717. G.C. Sylvain, S.G. Rodney, X.M. Jimmy, Appl. Phys. Lett. 87 (2005) 222104e222108. R.K. Gupta, K. Ghosh, R. Patel, P.K. Kahol, J. Optoelectron. Biomed. Mater. 1 (2009) 235e240. A.A. Dakhel, Adv. Optoelectron. (2013) 1e6. A.A. Dakhel, H. Hamad, J. Solid State Chem. 208 (2013) 14e19. G. Haacke, Appl. Phys. 47 (1976) 4086. Z. Chen, W. Li, R. Li, Y. Zhang, G. Xu, H. Cheng, Langmuir 29 (2013) 13836e13842.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of La doping on the structural, optical and electrical properties of spray pyrolytically deposited CdO thin films P. Velusamy a, R. Ramesh Babu a, *, K. Ramamurthi b, E. Elangovan c, J. Viegas c a
Crystal Growth and Thin Film Laboratory, Department of Physics, Bharathidasan University, Tiruchirappalli, 620 024, Tamil Nadu, India Crystal Growth and Thin Film Laboratory, Department of Physics and Nanotechnology, Faculty of Engineering and Technology, SRM University, Kattankulathur, 603 203, Tamil Nadu, India c Nano-Optics and Optoelectronics (NOOR) Research Laboratory, EECS Department, Masdar Institute, 54224, Abu Dhabi, United Arab Emirates b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2016 Received in revised form 13 February 2017 Accepted 4 March 2017 Available online 7 March 2017
Solution based chemical spray pyrolysis technique was employed to deposit lanthanum (La) doped cadmium oxide (CdO) thin films on soda-lime microscope glass slides. X-ray diffraction (XRD) analysis revealed that deposited films are polycrystalline having cubic crystal system. The undoped and 0.25 wt % La doped CdO films show relatively intense (111) plane, which is shifted to (200) plane for doping concentrations greater than 0.50 wt %. The parameters such as lattice constants, crystallite size, microstrain, dislocation density and texture coefficient are extracted from XRD data. Field emission scanning electron microscopy analysis confirmed the smooth and uniform surface. Energy dispersive spectroscopy analysis was used to estimate the preliminary information on elemental composition. The deposited undoped and La doped films possess optical transmittance ranging 78e82% in the visible and near infrared (NIR) region. The estimated direct optical band gap of undoped and La doped CdO thin films is varied between 2.38 eV and 2.46 eV. Hall measurements confirmed the n-type conductivity in the films. The doping-modulated mobility and carrier concentration are ranging 68 cm2/V se78 cm2/V s, and 1.0 1020 cm3e4.06 1020 cm3, respectively. Room temperature micro-Raman studies confirmed the metal oxide (CdeO) bond vibrations in deposited films. A high figure of merit (11.45 103 U1) is obtained from 1.0 wt % La doped CdO film. © 2017 Elsevier B.V. All rights reserved.
Keywords: Thin films Spray pyrolysis coating X-ray diffraction Electrical transport Optical properties Scanning electron microscopy
1. Introduction Transparent conductive oxides (TCOs) such as zinc oxide (ZnO), tin oxide (SnO2), indium oxide (In2O3), cadmium oxide (CdO), Sndoped In2O3 (ITO), F- doped ZnO (FZO), and F- doped SnO2 (FTO) are widely used for optoelectronic applications. This is due to their unique combination of low electrical resistivity and high optical transmittance in the useful region of solar spectrum [1e3]. In comparison with the other host TCOs, undoped and doped CdO films are extensively studied for the applications in solar cells, smart windows, transparent electronic devices, optical communications system, flat-panel displays and phototransistors. CdO also has an exceptional carrier concentration (1.5 1020 cm3), with high electrical conductivity (102e104 S/cm). However, CdO is not a
* Corresponding author. E-mail addresses: [email protected], (R. Ramesh Babu). http://dx.doi.org/10.1016/j.jallcom.2017.03.032 0925-8388/© 2017 Elsevier B.V. All rights reserved.
[email protected]
popular TCO material for some applications due to its narrow optical band gap of 2.2 eV [4e7]. A wide range of literature survey shows that the electrical and optical properties of CdO films can be tuned by optimizing the deposition parameters and the selection of metal ions doping from Al, In, Ga, Eu, Ce, Sm, etc. [1,8e12]. It is also observed that the resistivity of CdO films is modified when the ionic radius of dopants is nearly equal or smaller than that of Cd2þ ions [13]. CdO thin films were deposited by various techniques such as spray pyrolysis [8], vacuum evaporation [10], RF magnetron sputtering [14], chemical vapour [15], chemical bath [16], solegel [17] and pulsed laser [9] deposition methods. Dakhel and AliMohamed [18] and Alahmed et al. [19] have reported the deposition of CdO films with different La doping concentrations by sol-gel technique. However, the recent reports based on thin-film transistors (TFTs) and thin-film solar cells fabricated by spray pyrolysis have triggered a renewed interest in an economic solution process [20,21]. To the best of our knowledge, no report is available on the detailed study of La doped CdO thin films by an economic spray
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
805
pyrolysis technique. Hence, in the present investigation, the effect of La doping on structural, microstructural, optical and electrical properties of CdO thin films deposited by a homemade chemical spray pyrolysis setup is reported. 2. Experimental 2.1. Deposition of CdO films CdO thin films having different La doping concentrations were deposited on the ultrasonically cleaned microscope glass substrates by spray pyrolysis setup. An appropriate quantity of cadmium acetate dihydrate ((CH3 COO)2$Cd$2H2O; from Merck with 98% purity) was used to prepare 50 ml of 0.05 M precursor aqua solution. The pre-estimated amount of lanthanum trichloride heptahydrate (LaCl3$7H2O; from Sigma Aldrich with 99.90% purity), depending on the doping concentrations varying between 0.25 wt % and 1.0 wt %, was added to the precursor solution. The resulting solution was sprayed on to the microscope glass substrates preheated to 300 C. ChromeleAlumel thermocouple, placed at the center of the hot plate, was used to measure and maintain the substrate temperature. The substrate to nozzle distance was 30 cm. The spray gun was kept at an angle of 45 . Compressed and filtered air, at a pressure of 45 kg/cm2, was used as carrier gas. The spray time (1 s) and spray interval (29 s) were kept constant during the deposition of CdO thin films. Several sets of films were deposited from each precursor solution and found that the properties of thin films were highly reproducible. 2.2. Characterization of CdO thin films Various analytical tools were utilized to study the structural, microstructural, electrical and optical properties of undoped and La doped CdO thin films. The X-ray diffraction (XRD) patterns of the films were recorded in Bragg-Brentano geometry (q and 2q coupled) using PANalytical Empyrean X-ray diffractometer equipped with a CuKa1 radiation source (wavelength 1.5406 Å). The scan length for the diffraction angle (2q) is ranging between 30 and 110 . Thickness of the films measured by Filmetrics (Model: F20XT) is around 300 nm (the accuracy is ±10 nm). The oxidation state of deposited films was analyzed by X-ray photoelectron spectroscopy (XPS) studies (ESCA - Shimadzu 3400 electron spectrometer). The optical transmittance was recorded using a Perkin Elmer (Model: Lambda-35) spectrophotometer in the wavelength ranging between 200 nm and 900 nm. Field emission scanning electron microscopy (FEeSEM) from the FEI (NovaNano) was used to analyze the surface microstructures of films. A preliminary analysis of the elemental composition was carried out by energy dispersive spectroscopy (EDS). The electrical parameters such as carrier concentration, mobility and electrical resistivity were measured by the Hall measurement set up (Ecopia e HMS 3000) at room temperature (RT) in van der Pauw configuration with a permanent magnet of 0.570 T. Raman spectrum was recorded using LABRAM-HR spectrometer equipped with He-Ne laser source (wavelength 632.8 nm). 3. Results and discussion 3.1. X-ray diffraction The XRD patterns obtained from the undoped and La doped CdO thin films, shown in Fig. 1, reveal that the films are polycrystalline with a cubic crystal system [4]. The structural parameters such as lattice constant (a), texture coefficient (TC), crystallite size (D), dislocation density (d) and microstrain (ε) calculated from the XRD
Fig. 1. XRD patterns of CdO and La doped CdO thin films.
data are presented in Table 1. Two high intensity XRD peaks of (111) and (200) planes are obtained along with a few low intensity diffraction peaks of (220), (311), (222), (400), (331), (420) and (422). All the diffraction peaks are indexed by matching with the standard data [4]. The obtained lattice constant (a) (Table 1) from (200) plane is in good agreement with the reported value of 0.4695 nm. Fig. 1 indicates that the intensity of (111) plane is decreased for 0.25 wt % La doping. Further increase in La doping concentration has shifted the preferred growth orientation from (111) plane to (200) plane. The preferred growth orientation of CdO films is investigated through TC as defined in Ref. [11].
h
Im ðhklÞ Io ðhklÞ
i
i TC ¼ h P Im ðhklÞ 1 N
(1)
Io ðhklÞ
where, Im (hkl) is the measured relative intensity of reflections from a given (hkl) plane, Io (hkl) is the intensity of reflections from the same plane as indicated in a standard sample of randomly oriented polycrystalline CdO powder. The total number of reflections (N) is 9 in the present work. The calculated TC values are given in Table 1, which clearly indicates that the TC value for (111) plane is gradually decreased; whereas, it is increased for (200) plane. For 0.5 wt % La doped CdO thin films, TC value of (111) and (200) plane is 2.03 and 2.59, respectively (Table 1); this reveals that the excess La doping has presumably shifted the preferred growth orientation from (111) plane to (200) plane. It is noteworthy that the literature survey [5e10,14e19] reveals
806
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
Table 1 Structural parameters extracted from XRD data: Lattice constant (a), crystallite size (D), microstrain (ε), dislocation density (d) and texture coefficient (TC). Diffraction Plane
La doping (wt %)
(a) (Å)
(D) (nm)
(ε) (103)
(d) (1015 lines/m2)
TC
(200)
0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00
4.690 4.690 4.688 4.690 4.691 4.689 4.690 4.688 4.691 4.691
21 21 22 24 25 27 28 28 29 30
1.37 1.60 1.45 1.65 1.68 1.20 1.16 1.22 1.27 1.24
2.50 2.26 2.06 1.73 1.60 1.37 1.28 1.27 1.89 1.11
1.86 2.08 2.59 2.76 2.80 2.62 2.09 2.03 1.88 1.79
(111)
that the CdO films deposited by other deposition techniques have (111) as the energy favorable orientation in general. However, in the present study, it is observed that the preferred orientation is shifted from (111) to (200), which can be related to the increasing La doping concentration. Since the ionic radius of La3þ ions (0.103 nm) is slightly higher than Cd2þ ions (0.097 nm), additional La dopants presumably influence the strain in CdO lattice and thereby triggering a change in nucleation and growth kinetics of CdO thin films. It is well known that the deposition technique like spray pyrolysis has a strong influence on the nucleation and growth kinetics [22]. Probably, this has in turn changed the preferential growth orientation from (111) plane to (200) plane. This can be further substantiated from the SEM microstructures (Fig. 5), where in it can be noticed that the appearance of grains is changing in accordance with the change in preferred orientation. When the films have the preferred orientation along (111) plane (undoped and 0.25 wt % La doped CdO), the grains seem to agglomerate together and form bigger agglomerates. Additionally, because of this agglomeration, the films seem to be loosely packed. However, when the orientation is shifted to (200) plane for the increased La doping concentrations (0.50 wt %), the films do not have any agglomeration and the grains are separately distributed and tightly packed. Further, the grain size and the uniformity in grains distribution are both increased (images shown and explained later). Overall, the surface microstructures confirm that the nucleation and growth kinetics are altered based on the variation in La doping concentrations. Accordingly, the variation in nucleation and growth kinetics has changed the preferred orientation. The authors have earlier observed this kind of change in preferred orientation for the fluorine and antimony doped tin oxide films deposited by spray pyrolysis [23]. The average crystallite size (D) of the films is calculated using Scherrer's formula [24].
D¼
0:9l b cos q
(2)
where, b is the full width at half maximum (FWHM) of the corresponding diffraction peak and l is the X-ray wavelength (1.5406 Å) and q is the diffraction angle. It is found that the size of crystallites increases with the increasing La doping concentrations. The lattice constant and crystallite size estimated from (111) and (200) diffraction peaks are given in Table 1, which indicate that La dopant improves the crystallinity of CdO thin films. The microstrain (ε) and dislocation density (d) values were calculated using the relation ε ¼ (b cosq)/4 and d ¼ 1/D2 [24] and are summarized in Table 1. The relatively less value of d obtained in the present work confirms that spray pyrolysis is an effective deposition technique to obtain quality undoped and La doped CdO thin films. Meanwhile, the microstrain value for (200) plane is initially increased but then decreased for 0.50 wt % La doping concentration. Further increase in La doping concentration has increased the microstrain for (200)
plane. The microstrain value obtained from (111) plane is initially decreased but then increased with the increasing La doping concentration (Table 1). The variation in microstrain can be related to the crystallization process in polycrystalline thin films and the difference in the ionic radius between Cd2þ and La3þ ions, since the ionic radius of La3þ ions is slightly higher than that of Cd2þ. 3.2. XPS analysis A typical wide scan XPS spectrum obtained for 1.0 wt % La doped CdO (Fig. 2a) confirms the presence of La in the CdO lattice. A marked multiple-peak structure was observed in the Cd 3d5/2 core level. The narrow scan spectrum of CdO thin film (Fig. 2b) shows Cd 3d5/2 and Cd 3d3/2 peaks at 406.7 eV and 413.3 eV, respectively. This agrees well with the 6.6 eV spineorbit energy splitting between Cd 3d5/2 and Cd 3d3/2 states, thus confirming the oxidation state of Cd2þ [12]. The O1s binding energy is located at 533 eV, which represents the characteristics of O2 oxides in 1.0 wt % La doped CdO films (Fig. 2c) [12]. A peak at 841 eV (Fig. 2d) indicates the binding energy of La 3d5/2, which refers to the presence of La3þ ion in 1.0 wt % La doped CdO thin films [25]. Thus, the XPS study revealed the ionic state of La3þ and Cd2þ metal ions. 3.3. Micro-Raman studies Micro-Raman spectra of the undoped and La doped CdO thin films are shown in Fig. 3. In the present investigation, four Raman peaks are observed at 288 cm1, 396 cm1, 571 cm1, and 665 cm1. The intense peak at 288 cm1 reveals the combination of the transverse acoustic and optical phonon (TA þ TO) modes of CdO thin films, due to the lattice disturbance of CdO films [26]. The peak located at 396 cm1 represents phonon sum processes or possible resonant mode of CdO vibration. The peak observed at 571 cm1 represents the metal and oxide bond vibration, and it is due to the band crossing between longitudinal and transverse (LO þ TO) optical phonon modes of vibration of CdO [27]. The less intensity peak observed at 665 cm1 represents the 2LO modes of vibration of CdO thin films [12,27]. The intensity of these peaks is varied with the variation in La doping concentration (Fig. 4). For 1.0 wt % La doping, the peak at 288 cm1 gets red shift (284 cm1) due to the structural defects such as oxygen vacancies, cadmium interstitial, and free carrier concentration effect [28]. 3.4. Morphological studies The surface microstructures of undoped and La doped CdO thin films are shown in Fig. 5 as a function of La doping concentration. Fig. 5a shows the surface microstructure of undoped CdO films, which consists of agglomerated spherically shaped grains distributed on the surface of the film together with various sizes of irregularly shaped structures. Fig. 5bee shows the surface
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
807
Fig. 2. A typical X-ray photoelectron spectrum of 1.0 wt % of La doped CdO thin film.
Fig. 3. Micro-Raman spectrum of CdO thin films as a function of La doping concentrations.
Fig. 4. Intensity modification of Raman peak as a function of La doping concentration.
808
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
Fig. 5. FE-SEM image of CdO thin films as a function of La doping concentration (a) 0 wt % (b) 0.25 wt % (c) 0.50 wt % (d) 0.75 wt % (e) 1.0 wt %.
microstructures obtained from the films doped with 0.25 wt %, 0.50 wt %, 0.75 wt %, and 1.0 wt % of La, respectively. The surface of CdO films is effectively modified due to La doping of 0.25 wt % in the precursor solution (Fig. 5b). Agglomerated spherical structures combine to form nearly uniform circular shapes on the surface. Fig. 5cee shows relatively smooth, uniform, compact and densely packed surface microstructures. The grain size and the uniform distribution of grains on the surface are increased with the increasing La doping concentrations. The increased grain size and the uniform spreading are probably representing the improvement in film quality. Nearly similar surface morphologies observed in Fig. 5cee are probably reflecting the similar XRD patterns of these
films (Fig. 1cee). Fig. 6aee shows the histogram of average grain size of the undoped and La doped CdO thin films measured using the FE-SEM microstructures. It can be understood from Fig. 6aee that the average grain size of undoped CdO is ~64 nm, which is gradually increased to 70 nm for 0.25 wt %, 73 nm for 0.50 wt %, 90 nm for 0.75 wt % and 95 nm for 1.0 wt % of La doping concentrations. The grain size calculated from FE-SEM histogram is found to be greater than the crystallite size calculated from the XRD peaks. Generally, XRD gives the information about out-of-plane grain size; whereas, FE-SEM analysis estimates the in-plane grain size. The foregoing discussion clearly indicates that the surface microstructures of CdO films are effectively modified following the
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
809
Fig. 6. Histogram image for measuring average particle size: (a) CdO (b) 0.25 wt % La, (c) 0.50 wt % La, (d) 0.75 wt % La, and (e) 1.0 wt % La doped CdO thin films.
variation in La doping concentrations. Among the doped CdO thin films, 1.0 wt % La doped CdO shows uniformly distributed nearly spherical shaped grains. The features have relatively increased grain size in comparision with that of other La doped films. The elemental composition of CdO and La doped CdO thin films were measured by EDS analysis and measured ratios of Cd, O and La
elements are given in Table 2, which clearly indicate the presence of La in the doped CdO thin films. 3.5. Electrical properties The carrier concentration, mobility, electrical resistivity and
810
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
Table 2 Elemental analyses obtained from EDS. La doping Concentration (wt %)
0.00 0.25 0.50 0.75 1.00
Percentage of Elements (wt %)
Percentage of Elements (at. %)
O (K series)
Cd (L series)
La (L series)
O (K series)
Cd (L series)
La (L series)
24.29 48.85 32.88 36.57 27.12
75.71 51.04 66.84 62.93 72.20
0.00 0.11 0.28 0.50 0.68
53.94 77.03 77.29 79.99 72.16
46.06 22.95 22.47 19.68 27.07
0.00 0.02 0.24 0.33 0.77
conductivity of undoped and La doped CdO thin films were measured using van der Pauw method (Hall measurements) and the results are schematically shown in Fig. 7 and also summarized in Table 3. The carrier concentration (n) is derived from the relation ne ¼ 1/e (RH), [29] where RH is the Hall coefficient and e is the absolute value of the electron charge. The Hall mobility of charge carriers is determined using the relation me ¼ 1/ner, [29] where r is resistivity. The negative sign of Hall coefficient confirmed that the deposited films are n-type conductor. The undoped CdO thin films possess electrical resistivity and mobility of 9.32 104 U-cm and 68 cm2/V,s, respectively. The carrier concentration and conductivity of CdO thin films are increased with the increasing La doping concentration. A high carrier mobility of 78 cm2/V,s is obtained for the 0.25 wt % La doped CdO films. The increase in mobility for the initial La doping is attributed to the fact that La dopant reduces the grain boundary scattering [30]. The resistivity of CdO thin films is decreased with the increasing La doping concentrations. The low resistivity (3.0 104 U-cm) and relatively high carrier concentration (4.0 1020 cm3) are obtained for 1.0 wt % La doped CdO thin films. The increasing carrier concentration may originate from the following mechanism: substitution of La3þ for Cd2þ in CdO lattice donates one excess free electron to the electrical conductivity (ionic radius of Cd2þ 0.097 nm and La3þ e 0.103 nm) [13]. The obtained electrical parameters are compared with literature values in Table 4, which shows that the films doped with 1.0 wt% La have a better combination of lowest resistivity (3.0 104 U-cm) and high carrier concentration (4.06 1020 cm3) in comparison with the undoped CdO thin film. Dakhel and Hamad [31] have reported an increase in the carrier concentration and mobility due to Cr doping in CdO thin
films. Similarly, La doping in CdO has effectively increased the mobility and electrical conductivity. 3.6. Optical properties Optical transmittance spectra of undoped and La doped CdO thin films are shown in Fig. 8. It is observed that the deposited films exhibit high transmittance, ranging 75e90% in the NIR wavelength region. Further, we found that the transmittance of CdO film is increased with the increasing La doping concentration upto 0.50 wt %, but then decreased for 0.75 wt % and 1.0 wt % La doping concentrations. The optical study shows that La doped CdO thin films possess relatively high transmittance when compared to that of undoped CdO films. The absorption coefficient (a) is calculated using the relation a ¼ (1/d) (ln (1/T)) [10] where ‘d’ is the film thickness and ‘T’ is the film transmittance. The relation between the absorption coefficient and the incident photon energy (hn) is derived from the relation (ahn)2 ¼ A (hn - Eg) [17]; where ‘A’ is a constant and ‘Eg’ is optical energy band gap. Eg can be determined by extrapolating the linear region of (ahn)2 to the X-axis (at Y ¼ 0). The plots of (ahn)2 Vs hn of the CdO thin films as a function of La doping concentration are shown in Fig. 9. It is noticed that the band gap is gradually increased with the increasing La dopant concentration until 0.75 wt %, and then decreased. The blue shift of absorption edge is called band gap widening, which can be described by the MosseBurstein effect [11]. The average transmittance (AT) obtained in the 600e900 nm range and optical band gap value of undoped and doped CdO films are summarized in Table 3 as a function of La doping concentration. The results show that La doping at various levels is effectively influenced the optical
Fig. 7. Variation of carrier concentration, mobility, electrical resistivity and conductivity of the CdO thin films as a function of La doping concentrations.
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
811
Table 3 Electrical and Optical properties of La doped CdO thin films. La Doping (wt %)
ne (1020/cm3)
me (cm2/Vs)
r (104 U cm)
s (103/U.cm)
AT (600e900 nm)
Eg (eV)
CdO 0.25 0.50 0.75 1.00
1.0 1.5 1.7 2.6 4.1
68 78 76 60 52
9.3 5.2 4.8 3.9 3.0
1.0 1.8 2.0 2.5 3.4
78 82 81 79 82
2.38 2.41 2.40 2.46 2.40
Table 4 Comparison of electrical properties of different metal doped CdO thin films deposited by different techniques. Doped and undoped CdO films CdO 1.0 wt % La:CdO 0.015% Ga:CdO 6.0 wt % Al:CdO 0.8 mol. % Eu:CdO 3.8 at. % Ce:CdO 0.4 mol % Sm:CdO 1.0 mol % La:CdO 1.5% Cr:CdO
Resistivity (U cm) 4
9.32 10 3.0 104 1.93 104 4.48 104 5.2 104 3.86 104 5.6 104 3.8 102 4.68 104
Carrier concentration (1/cm3) 20
1.0 10 4.1 1020 11.7 1021 3.7 1020 4.86 1020 2.42 1020 2.5 1020 5.7 1019 2.49 1020
Mobility (cm2/V s)
Deposition Technique
Reference
68 52 27 34 24.42 66.71 43.25 2.5 53.4
Spray pyrolysis Spray pyrolysis Spray pyrolysis Spray pyrolysis Vacuum evaporation Vacuum evaporation Vacuum evaporation Sol-Gel Physical vapour deposition
Present work Present work [1] [8] [10] [11] [12] [21] [28]
properties of CdO thin films. 3.7. Figure of merit
Fig. 8. Optical transmittance spectra of the deposited CdO thin films as a function of La doping concentrations.
Figure of merit (FOM) of the TCO thin films plays an important role in optoelectronic applications. Thus, the quality of TCO films can be investigated by figure of merit defined by Haacke [32], FOM ¼ T10/RSh, where T is the transmittance (600e900 nm) and RSh is the sheet resistance. The FOM values are calculated at different wavelengths of 600 nm, 700 nm, 800 nm, and 900 nm. The calculated values of FOM are given in Table 5. In the present work, the sheet resistance is decreased for 1.0 wt %. La doping concentration. The highest FOM value of 36.19 103 U1 is obtained for 1.0 wt % La doped CdO thin film at 900 nm. The La doped CdO thin films prepared by spray pyrolysis method show improved FOM when compared to that of ITO thin films (11.9 103 U1 at 550 nm) deposited by spin coating method [33]. Thus, the result of this work reveals that CdO films doped with 1.0 wt % of La has high optical transmittance, high electrical conductivity, low resistivity and high FOM. 4. Conclusions
Fig. 9. Plot of (ahѵ)2 against hѵ of the CdO thin films as a function of La doping concentrations.
Undoped and La doped CdO thin films were deposited by cost effective chemical spray pyrolysis technique and their properties were studied as a function of La doping concentration. Various doping levels of La has influenced the structural, morphological, optical, and electrical properties of CdO films. The 0.5 wt % La doping has shifted the preferred growth orientation from (111) plane to (200) plane. The crystallite size of the CdO films is increased with the increasing La doping concentration. The oxidation state of Cd2þ, O2 and La3þ was confirmed by XPS analysis. Micro-Raman analysis confirmed the metal oxide bond vibration of CdeO. FE-SEM analysis revealed that the La doping modified the surface morphology of CdO thin films. A minimum resistivity of 3.0 104 U-cm and a maximum carrier concentration of 4.0 1020 cm3 is observed due to 1.0 wt % La doping in CdO thin films. The AT (600e900 nm) is varied between 78% and 82%; and the band gap is varied between 2.38 eV and 2.46 eV. The 1.0 wt % La doping in CdO film gives the high figure of merit (11.45 103 U1). The range of electrical conductivity and optical transmittance obtained in the present work suggests that the spray deposited La
812
P. Velusamy et al. / Journal of Alloys and Compounds 708 (2017) 804e812
Table 5 Figure of merit for undoped and La doped CdO thin films at different wavelengths. La Doping (wt %)
0.00 0.25 0.50 0.75 1.00
Figure of Merit (103 U1) FOM for AT (600e900 nm)
600 nm
700 nm
800 nm
900 nm
2.23 6.60 6.33 6.06 11.45
1.15 3.52 2.93 5.30 4.69
3.26 9.40 8.08 6.88 11.45
4.69 13.38 11.52 6.88 23.20
5.28 11.91 11.52 6.88 36.19
doped CdO films will be suitable for optoelectronic device application. Acknowledgements The authors thank Dr. Vasant Sathe, UGC-DAE-CSR, Indore for micro-Raman analysis. The author E. Elangovan thanks Mike Tiner and Mustapha Jouiad of Masdar Institute for the characterization facilities (XRD and SEM). References [1] A.V. Moholkar, G.L. Agawane, K.U. Sim, Y. Kwon, K.Y. Rajpure, J.H. Kim, Appl. Surf. Sci. 257 (2010) 93e101. [2] F.F. Yang, L. Fang, S.F. Zhang, J.S. Sun, Appl. Surf. Sci. 254 (2008) 5481e5486. [3] G. Murtaza, B. Amin, S. Arif, M. Maqbool, Comput. Mater. Sci. 58 (2012) 71e76. [4] Powder Diffraction File, International Centre for Diffraction Data (ICDD), Reference Code: 00-005-0640. [5] S. Salam, M. Islam, A. Akram, Thin Solid Films 529 (2013) 242e247. [6] M. Alam, M. Islam, A. Achour, A. Hayat, B. Ahsan, H. Rasheed, S. Salam, M. Mujahid, Surf. Rev. Lett. 21 (2014) 1450059e1450068. [7] D.A. Cristaldi, S. Millesi, I. Crupi, G. Impellizzeri, F. Priolo, R.M.J. Jacobs, R.G. Egdell, A. Gulino, J. Phys. Chem. C 118 (2014) 15019e15026. [8] R. Kumaravel, S. Menaka, S.R.M. Snega, K. Ramamurthi, K. Jeganathan, Mater. Chem. Phys. 122 (2010) 444e448. [9] R.K. Gupta, K. Ghosh, R. Patel, S.R. Mishra, P.K. Kahol, Mater. Lett. 62 (2008) 3373e3375. [10] A.A. Dakhel, Opt. Mater. 31 (2009) 691e695. [11] E. Burstein, Phys. Rev. 93 (1954) 632e633. [12] P. Velusamy, R. Ramesh Babu, K. Ramamurthi, J. Viegas, E. Elangovan, J. Mater Sci. Mater Electron 26 (2015) 4152e4164. [13] Kenneth Barbalace. Periodic Table of Elements - Sorted by Ionic Radius.
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
EnvironmentalChemistry.Com. 1995-2015. Accessed on-line: 11/16/2015 http://EnvironmentalChemistry.com/yogi/periodic/ionicradius.html. G. Anil Kumar, M.V. Ramana Reddy, K. Narasimha Reddy, Mater. Res. Innov. 19 (2015) 204e211. T. Terasako, K. Ohmae, M. Yamane, S. Shirakata, Thin Solid Films 572 (2014) 20e27. H. Khallaf, C.T. Chen, L.B. Chang, O. Lupan, A. Dutta, H. Heinrich, A. Shenouda, L. Chow, Appl. Surf. Sci. 257 (2011) 9237e9242. F. Yakuphanoglu, J. Sol Gel Sci. Technol. 59 (2011) 569e573. A.A. Dakhel, A.Y. Ali-Mohamed, J. Sol Gel Sci. Technol. 44 (2007) 241e247. Z.A. Alahmed, Z. Serbetci, F. Yakuphanoglu, Acta. Phys. Pol. A 124 (2013) 125e127. C. Girotto, D. Moia, B.P. Rand, P. Heremans, Adv. Fun. Mater. 21 (2010) 64e72. G. Adamopoulos, S. Thomas, D.D.C. Bradley, M.A. McLachlan, T.D. Anthopoulos, Appl. Phys. Let. 98 (2011) 123503e123506. J.P. Hirth, G.M. Pound, Condensation and Evaporation-nucleation and Growth Kinetics, McMillan Company, New York, 1963. E. Elangovan, K. Ramamurthi, A study on low cost-high conducting fluorine and antimony-doped tin oxide thin films, Appl. Surf. Sci. 249 (2005) 183e196. B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, third ed., Prentice-Hall Inc, 2001, pp. 167e171. J.T. Stecher, A.B. Rohlfing, M.J. Therien, Nanomaterials 4 (2014) 69e86. S.G. Choi, L.M. Gedvilas, S.Y. Hwang, T.J. Kim, Y.D. Kim, J.Z. Perez, V.M. Sanjose, J. Appl. Phys. 113 (2013) 83515e83519. S.K. Vasheghani Farahani, T.D. Veal, P.D.C. King, J. Zuniga-Perez, V. MunozSanjose, C.F. McConville, J. Appl. Phys. 109 (2011) 73712e73717. G.C. Sylvain, S.G. Rodney, X.M. Jimmy, Appl. Phys. Lett. 87 (2005) 222104e222108. R.K. Gupta, K. Ghosh, R. Patel, P.K. Kahol, J. Optoelectron. Biomed. Mater. 1 (2009) 235e240. A.A. Dakhel, Adv. Optoelectron. (2013) 1e6. A.A. Dakhel, H. Hamad, J. Solid State Chem. 208 (2013) 14e19. G. Haacke, Appl. Phys. 47 (1976) 4086. Z. Chen, W. Li, R. Li, Y. Zhang, G. Xu, H. Cheng, Langmuir 29 (2013) 13836e13842.