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Studies on the structure optical and electrical properties of Zn-doped NiO thin films grown by spray pyrolysis
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Contents lists available at ScienceDirect
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Studies on the structure optical and electrical properties of Zn-doped NiO thin films grown by spray pyrolysis
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Ratnesh Sharma a,∗ , A.D. Acharya b , S.B. Shrivastava a , Manju Mishra Patidar c , Mohan Gangrade c , T. Shripathi c , V. Ganesan c a
School of Studies in Physics, Vikram University, Ujjain, MP, India Department of Physics, Lovely Professional University, Phagwara, Punjab, India c UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore, MP, India b
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Article history: Received 26 October 2015 Accepted 5 January 2016 Available online xxx
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Keywords: NiO thin films Zn doped Spray pyrolysis Oxide semiconductors
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1. Introduction
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Thin films of Ni1−x Znx O with different composition (x = 0, 0.01, 0.05 and 0.10) were successfully fabricated on glass substrates using spray pyrolysis technique. The as-prepared films were further annealed at 623 K in room atmosphere for 2 h. The evolution of their structural, morphological, electrical and optical properties with annealing temperature was investigated. The result reveals that the annealing of the films leads to improved surface morphology and better crystallinity. The prepared films displayed increase in conductivity followed by decrease in band gap with increase in doping concentration. However, the effect is more significant in case of annealed films, where the average transparency shows an increase of about 10% over the as prepared thin films. The red-shift of the optical band gap is due to the deep states in the band gap. The increase in density of states has been confirmed by variable range hoping (VRH) mechanism. The activation energy was found to be decreased when Zn concentration increased. Added to this the dielectric function of thin films and their spectral variation has been tentatively discussed. © 2016 Published by Elsevier GmbH.
In recent years, investigation in transparent semiconducting oxide thin films has assumed significance due to their important optical characteristics, high stability and excellent electrical properties. Transparent conducting oxides are basically metal oxide semiconductors classified as either n-type or p-type. Coatings of ntype semiconductors, such as indium tin oxide, zinc oxide etc., are required in many technological applications such as transparent electronics and nanodevices [1,2]. Among various n-type semiconductors, ZnO, a direct band gap (3.37 eV) material, has been widely used in these optoelectronic devices during past decade. However, in recent years, thin films of p-type semiconductor have attracted much attention due to their important applications in optoelectronic devices such as elements for information display light shutter and variable reflectance mirror [3]. However not many studies have been done on P-type large band gap semiconductors. Such p-type semiconductors could allow the use of new classes of organic compounds. Unfortunately, relatively few metal oxides tend to be P-type. NiO is one of the oxide having wide band gap
∗ Corresponding author. E-mail address: [email protected] (R. Sharma).
semiconductor with the absorption edge in the near UV–visible region[4] and it exhibit functional properties and offer promising candidature for many applications such as ultraviolet optoelectronic devices, positive electrode in batteries, solar thermal absorber, fuel cells, catalysts, in gas sensing devices, in electrochromic display devices etc. [5–8] Thus, nickel oxide can be used as candidate material in UV based optoelectronic applications. The properties of nickel oxide depend on its defect structure and can be controlled by the appearance of nickel vacancies and interstitial oxygen in NiO crystallites. Recently, many attempts have been made to modify the properties of NiO thin films by doping with various elements such as K, Li, P, Mg, Co, etc [9–13] Besides the dopants mentioned above, zinc is a promising transition metal element that can be used as a suitable additive for nickel oxide due to close ionic radius parameter. In addition, the detailed knowledge of various physical properties of Zn-doped NiO film is still limited [14,15]. The systematic analysis of optical and electrical properties of Ni1−x Znx O films as a function of Zn content would provide the necessary and relevant information for its various optoelectronic and coating applications. Thus it would be interesting to investigate the effect of Zn doping on structural, optical and electrical properties of Ni1−x Znx O films. The films have been prepared by using spray pyrolysis technique. A variation of the optical
http://dx.doi.org/10.1016/j.ijleo.2016.01.050 0030-4026/© 2016 Published by Elsevier GmbH.
Please cite this article in press as: R. Sharma, et al., Studies on the structure optical and electrical properties of Zn-doped NiO thin films grown by spray pyrolysis, Optik - Int. J. Light Electron Opt. (2015), http://dx.doi.org/10.1016/j.ijleo.2016.01.050
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band-gap with Zn composition was analyzed. Also, a considerable change in the electronic structure by the Zn substitution was found and compared to NiO.
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2. Experimental details
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Experimentally Ni1−x Znx O (x = 0, 0.01, 0.05 and 0.10) thin films were prepared onto the glass substrates using chemical spray pyrolysis technique. The apparatus used for deposition is described and schematized elsewhere [16]. The precursor solution was prepared from a mixture of 0.1 M nickel chloride (NiCl2 ·6H2 O) and de-ionized water. Zn doping was achieved by the addition of zinc nitrate [Zn(NO3 )2 ] to the precursor solution. The doping concentration of films for a particular precursor in grams has been measured by weight percentile method with the help of molarity equation [17]. The solution was stirred for few minutes to yield a clear and homogeneous solution. For uniform deposition of thin films the solution was sprayed through a linearly moving glass nozzle for a fixed period of 15 min. The solution sprayed onto the preheated glass substrate undergoes evaporation, solute precipitation and pyrolytic decomposition, thereby resulting in the formation of nickel oxide thin films according to the following reaction [17,18]: NiCl2 · 6H2 O → NiO + 2HCl ↑ +5H2 O ↑
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2Zn(NO3 )2 · 6H2 O + O2 → 2ZnO + 4NO2 ↑ +2O2 ↑ +12H2 O ↑
(1)
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The nozzle to substrate distance was 27 cm. The spray rate of 1 ml/min was maintained by using air as a carrier gas. The substrate temperature was kept constant at 623 K during deposition of film by means of electronic temperature controller. After spraying the solution few samples were subsequently annealed at 623 K for 2 h in air. The glass substrates were chemically and ultrasonically cleaned before coating. Several initial trials were made to optimize the deposition conditions before real sample preparation. Thickness of prepared film was determined by weight difference method using a sensitive microbalance and found to be ∼280 nm and ∼240 nm for as-prepared and annealed Ni1−x Znx O films respectively [18].
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3. XRD study
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The structural studies of the above films were done by using X-ray diffraction method. The XRD was performed on Bruker D8 ADVANCE X-ray diffractometer with CuK␣ radiation (wave˚ The XRD patterns of as-prepared films of length = 1.5418 A). Ni1−x Znx O (for x = 0, 0.01, 0.05, and 0.10) are shown in Fig. 1a. The as-prepared films exhibit (1 1 1), (2 0 0) and (2 2 0) diffraction peaks showing the formation of cubic NiO structure. No other peak corresponding to Zn or ZnO has been detected. This indicates that Zn is successfully incorporated into NiO lattice. The XRD patterns of Ni1−x Znx O films, annealed at 623 K are shown in Fig. 1b. The patterns also reveal the presence of polycrystalline NiO films with preferential orientation in the plane (1 1 1). All the films became more strongly oriented along (1 1 1) direction after heat treatment. In Fig. 2, we have compared the highly intense (1 1 1) peaks of asprepared films with the corresponding peaks of annealed films for different concentration of zinc (i.e. x). Figure shows that the diffraction intensity increased after annealing the films, it shows that the best crystalline quality of the film is achieved after annealing [19]. In order to explain this, it is noteworthy that the crystallographic structure is found to be similar for both the as-prepared and annealed films. Thus, the degree of crystallization and composition is mainly controlled by the post thermal treatment rather than by heating during deposition. This means that the efficiency of rearrangement of the constituent atoms (Ni, Zn, O) is small for as-prepared films. While, thermal annealing facilitates the crystal growth at energetically favourable site and promotes atoms to
Fig. 1. XRD patterns of (a) as-prepared and (b) annealed Ni1−x Znx O thin films.
Fig. 2. A comparison between highly intense (1 1 1) peaks observed for different t concentration of Zn.
nucleate at proper sites. This results in the improved crystallinity caused by decrease in native defects such as interstitial oxygen and nickel vacancies in the annealed films [20]. On the other hand, as the zinc content (i.e. x) in the film increased, the diffraction intensity decreased with increase in Full width at half-maximum (FWHM) value. Moreover, the Bragg’s angle of diffraction () shifts to small- angle side with increasing zinc content (Fig. 3a). This is evident that the Zn2+ ions (ionic radius ˚ well substitute Ni2+ ((ionic radius of 0.69 A) ˚ sites in the of 0.74 A) present Zn composition range [11,21]. The crystallite size (D) of the annealed films has been calculated by using the formula given below [22] D=
0.9 , ˇ cos
(2)
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displayed in (Fig. 4a–d) respectively. All the films are found to be homogeneous and well-covered by grain like structures. The surface morphology of Ni1−x Znx O films is found to be significantly influenced by thermal annealing (Fig. 4e–h). In the micrographs of annealed Ni1−x Znx O films the presence of uniformly distributed and well developed grains can be noticed. This suggests that the heat treatment facilitates the process of grain-growth and lead to improved crystalline microstructure. The average particle size was obtained by statistical fitting which are given in Table 1. It can be observed that the average particle size decreased from 92 nm to 58 nm for as-prepared films and from 115 nm to 72 nm for annealed films with increase in Zn content from 0 to 0.10. However, the significant increase in the grain size after heat treatment could be noticed. It is clear that the increase in the surface roughness results from an increase in the grain size in Ni1−x Znx O films (see Table 1). 5. Electrical study
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where ˇ, is the broadening of diffraction line measured at half of its ˚ maximum intensity (FWHM), is the X-ray wavelength (1.5418 A) and is the Bragg’s angle of diffraction. It has been observed that the crystallite size of as-prepared Ni1−x Znx O films decreased slightly from 18.75 nm to 18.24 nm with increase in Zn concentration from 0 to 0.10 (see Fig. 3b). In case of annealed films, crystallite size decreased significantly from 20.58 nm to 18.98 nm with increasing zinc content (Table 1). Consequently, the crystallinity of the films might be deteriorate with increasing Zinc content before and after annealing, which indicates that the large amount of Zn doping may creates lattice distortion.
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4. Morphological study
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= 0 exp
E a kb T
,
(3)
where is the resistivity at temperature T, 0 is a constant, Ea is the activation energy and kb is the Boltzmann constant. The values of activation energy so calculated are listed in Table 1. The activation energy is found to decrease with increase in Zn concentration for both i.e. as-prepared and annealed films. This leads to decrease in the band gap upon Zn doping and suggested that band gap could be controlled by changing the Zn concentration. However, probably due to variation in homogeneity and stoichiometry of as-prepared films, the Ea values for as-prepared films are slightly higher than those measured for annealed films. This discrepancy could be associated with the difference of film thickness, which was found to be ∼280 nm and ∼240 nm for as-prepared and annealed Ni1−x Znx O films respectively. Patil et al. [24] have reported the increase in
The surface morphological images of deposited films were recorded over an area of 1 m × 1 m at room temperature using atomic force microscope (AFM) (Digital instruments nanoscope E with Si3 N4 100 m cantilever 0.58 N/m force constant) in contact mode. The representative two dimensional images of as-prepared Ni1−x Znx O films corresponding to x = 0, 0.01, 0.05 and 0.10 are
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The electrical properties of as-prepared and annealed Ni1−x Znx O films was studied in the temperature range 180 K to 330 K using two probe resistivity method. The resistivity measurements were done using Keithley 616 programmable electrometer in association with Lakeshore temperature controller in a home built apparatus [23] with Teflon-insulated shielded coaxial cables. The unit containing the sample was kept in the cryostat. The liquid nitrogen is used as coolant. The results of the above measurements are summarized in Fig. 5a–f. Fig. 5a and b shows the variation of resistance per unit length as a function of temperature for as-prepared and annealed films respectively. It has been found that, in general, resistance per unit length decreases with increase in temperature demonstrating the semiconductor type of behaviour. Also the resistance per unit length decreases with the increase in Zn concentration. Thus, the conductivity was maximum in case of x = 0.1. The resistance per unit length was found to be larger in case of annealed samples. Fig. 5c and d show the plots of ln[R] verses 1000/T for as-prepared and annealed Ni1−x Znx O films respectively. From the curves the thermal activation energy was calculated using Arrhenius equation given below:
Fig. 3. (a) Variation of Bragg’s angle of diffraction corresponding to (1 1 1) plane (b) plots of crystallite size versus Zn concentration for as-prepared and annealed Ni1−x Znx O thin films.
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Table 1 The various structural, electrical and optical parameters observed for as-prepared (As-pre.) and annealed (Ann.) Ni1−x Znx O films. x
Crystallite size (nm)
Average particle size (nm)
RMS roughness (nm)
Activation energy (eV)
[T0 ]1/4 (K1/4 )
As-pre.
Ann.
As-pre.
Ann.
As-pre.
Ann.
As-pre.
Ann.
As-pre.
Ann.
As-Pre.
Ann.
0 0.01 0.05 0.10
18.75 18.66 18.48 18.24
20.58 20.6 19.56 18.98
92 75 64 58
115 108 86 72
2.5 3.26 4.16 4.74
2.12 2.47 4.65 3.04
0.37 0.373 0.35 0.33
0.41 0.35 0.31 0.29
247 252 226 224
278 252 231 217
3.48 3.46 3.43 3.41
3.52 3.49 3.44 3.42
Optical band gap (eV)
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Fig. 4. (a), (b), (c) and (d) shows the AFM images of as-prepared Ni1−x Znx O films for x = 0, 0.01, 0.05 and 0.10 respectively. While (e), (f), (g) and (h) shows the AFM images of annealed Ni1−x Znx O films for x = 0, 0.01, 0.05 and 0.10, respectively.
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activation energy with increasing film thickness. This was explained as varying stoichiometry caused by difference in the experimental conditions viz. amount of spraying solution, spray rate and cooling of the substrates during decomposition. Furthermore, from Fig. 5c and d one can observe that the plots of ln[R] verses 1000/T do not fit linearly at temperatures below 230 K. In order to explain low temperature transport mechanism, we have also plotted the log of normalized resistance i.e. ln[R/R330 ] 1/4 as a function of [1/T ] in Fig. 5e and f for as-prepared and annealed films respectively. Activation energy in the low temperature region can be obtained from the above graphs by means of variable range hoping (VRH) mechanism as suggested by Ambegaokar et al. [25]. The VRH mechanism is governed by the equation [26]
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= 0 exp
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T 1/4 0
T
,
(4)
where 0 and T0 denote material parameters. Here T 0 = 16˛3 /kb N(E), where N(E) is the density of localized electron states at Fermi level and ˛ is the inverse localization length of the localized state. The curves plotted in Fig. 5e and f gives a straight line. From these figures, the value of T0 has been obtained for Ni1−x Znx O films which are listed in Table 1. It has been found that the values of T0 decreases with increase in Zn concentration in both the cases. This suggests that the density of states at Fermi level increases with the increase in zinc concentration. 6. Optical study Optical measurements were done in transmittance mode as well as in reflectance mode using UV–visible spectrophotometer (Perkin Elmer Lambda 950). The variation in transmittances of as-prepared and annealed Ni1−x Znx O films with UV–visible range wavelength
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Fig. 5. (a) and (b) shows the plot of resistance per unit length versus temperature for as-prepared and annealed films respectively. (c) and (d) shows the plot of ln[R] verses 1/4 for as-prepared and 1000/T for as-prepared and annealed films respectively. (e) and (f) shows the plot of log of normalized resistance i.e. ln[R/R330 ] as a function of [1/T ] annealed films respectively.
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is shown in Fig. 6. It is clear from figure that for all the films, the optical transmission is low in UV region which increases abruptly in the visible region. Moreover, the average transmittance in the visible region decreases as the zinc concentration (x) is increased. For instance, the transmittance for as-prepared NiO film with no zinc addition (x = 0) is as high as 75%, which falls to 65% when 10 at.% zinc is added. This is in good agreement with the XRD and AFM measurements where we have observed the deteriorated crystalline microstructure and increased roughness values at higher zinc concentration. The higher surface roughness enhances the optical scattering of incident radiation leading to lower transparency in Zn-doped NiO films. Besides, the films that were annealed at 623 K, show higher transparencies than as-prepared ones. In fact, the average transmittance for annealed films varies between 85 and 80% which shows an increase of about ∼10% over the as-prepared films (Fig. 7). This may be related to reduced absorption from native defects that are removed after annealing of films. The absorption coefficient (˛) has been calculated from transmittance (T) and reflectance (R) data using the relation [27]
˛=
1 (1 − R)2 · ln t 2T
,
(5)
where t is the film thickness. The spectral variation of ˛ have been shown in Fig. 8 for both as-prepared as well as annealed films. In this figure, an absorption edge near (∼370 nm) is observed for asprepared NiO film. As zinc concentration in the film is increased, the absorption edges of Ni1−x Znx O films show a clear shift to longer wavelengths (red-shift). This trend is also observed in case of annealed films. However, for both the as-prepared and annealed films and for all the Zn composition used in this study (i.e. x = 0 to 0.10 at.%), the onset of noteworthy absorption is in the wave length range less than 400 nm ensuring excellent transparency in the visible region. Further, it is interesting to note that, the absorption edge is found to be sharper in case of annealed films in comparison to as-prepared films. Generally, absorption changes as a function of crystalline properties. The defect states inside the band gap make the optical absorption broad, while a rather sharp behaviour is observed for the films which show good crystalline quality. This
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Fig. 6. Spectral variation of transmittance for as-prepared and annealed Ni1−x Znx O films corresponding to different Zn concentrations.
Fig. 9. The plots of (˛h)2 versus incident photon energy (h) for annealed films. Inset shows the Plot of (˛h)2 versus (h) for as-deposited films.
Fig. 7. A comparison between the average transmittance observed for as-prepared and annealed films as a function of Zn concentration.
Fig. 10. (a) Comparison between the optical band gap and activation energy for annealed Ni1−x Znx O films as a function of Zn concentration. (b) Comparison between the optical band gap values of as-prepared and annealed Ni1−x Znx O films as a function of Zn concentration.
(˛h)
Fig. 8. Spectral variation of absorption coefficient for Ni1−x Znx O films corresponding to different concentration of Zn and inset shows the optical reflectance spectra corresponding to different concentration of Zn.
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is indeed true in our case where we have observed improved crystalline structure for annealed films. The optical band gap have been determined using the Tauc’s relation [28] given below:
1/p
= A h − Eg ,
(7)
where ˛ is the absorption coefficient, Eg is the band gap corresponding to a particular transition occurring in the film, A is a constant, is the transition frequency and p can take the values 1/2, 3/2, 2 or 3, for the transition corresponding to direct allowed, direct forbidden, indirect allowed and indirect forbidden respectively. The NiO is considered as material with direct band gap energy and hence the value of p has been taken to be 1/2 for direct allowed transitions. The band gap has been calculated by extrapolating the linear 2 region of the plots (˛h) versus h on the energy axis as shown in Fig. 9. The band gap values so obtained are given in Table 1. It can be noted that as Zn concentration in the film increases from 0 to 10 at.%, the optical band gap for as-prepared films decreased from 3.48 eV to 3.41 eV and for annealed films, it decreased from 3.52 eV to 3.42 eV. These variations are similar to the change in activation energy observed in the conductivity measurement (see Fig. 10). On comparing the band gap values observed before and after annealing (Fig. 10), it can be seen that for lower doping concentration of Zn (i.e. for x < 0.05 including undoped NiO), the band gap values for annealed films are slightly higher than those measured in case of as-prepared films whereas these values become very close at relatively higher Zn doping concentrations. The band gap shift observed with increasing Zn content can be attributed to degradation in crystallinity and to the defect states (oxygen defects or Ni vacancies) induced by the presence of Zn in NiO thin films.
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Fig. 11. (a) and (b) shows the spectral variation of extinction coefficient (k) and refractive index (n), calculated for annealed Ni1−x Znx O films. (c) Effect of zinc concentration on packing density () of annealed films determined for n at 600 nm. (d) The variation of (εr ) and (εi ) with Zn concentration.
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The optical reflectance spectra (R) have been used to determine the refractive index (n) of the film through the relation [13,29]
300
n=
298
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1+R + 1−R
4R (1 − R)2
− k2 ,
(6)
where k = ˛/4, is the extinction coefficient. Fig. 11a and b respectively show the variation of extinction coefficient (k) and refractive index (n) calculated for annealed Ni1−x Znx O films. The spectral dependence of k and n is similar to those reported for NiO thin films [30,31]. For all the films, it can be observed that the value of k remains constant and is very near to zero for wavelengths beyond 400 nm, but in the wavelength range less than 400 nm, it increases significantly. The average value of k in the visible region for annealed Ni1−x Znx O films are found to be of the order of 10−2 and such low value of k is a qualitative indication of homogeneity of annealed films [32]. Further, as Zn concentration in the film increases, extinction coefficient increases which is in good agreement with the measurement of surface roughness. This suggests that the surface roughness may contribute to change in the extinction coefficient of deposited films. On the other hand, the value of refractive index is practically constant beyond 400 nm, whereas near the band edge a peak is observed due to fundamental band gap absorption. Generally, the refractive index of semiconductor is inversely proportional to the energy band gap [33]. This is indeed true in our case where we have observed the lower band gap values and higher refractive indices with increasing Zn concentration. This increased refractive index results in the densification of film at higher Zn concentration. index The packing density () is closely related to the refractive as: [34] = (n2 − 1) × (n2b + 2)/ (n2 + 2) × (n2b − 1) , where nb is the refractive index of un-doped bulk crystal. The value of nb have been taken from literature [35]. The values of determined for n at 600 nm are plotted in Fig. 11c. It can be seen that as the Zn content in NiO film is increased, the packing density increases, leading to better packing of grains with a reduction of voids. This suggested that the change in refractive index and thus packing density is correlated to the change in morphology caused by grain growth during the deposition of film.
The study of dielectric function of thin films is very important for the use of films in optical applications. The dielectric constant is defined as ε = εr + iεi where εr = n2 − k2 is the real part and εi = 2nk is the imaginary part of complex function [36]. These values have been calculated at the wavelength of 600 nm. The variations of εr and εi for Ni1−x Znx O films as a function of Zn concentration are shown in Fig. 11d. From the figure, it can be seen that both real and imaginary parts of dielectric constant increased systematically with the increase in Zn concentration.
7. Conclusions The chemical spray pyrolysis technique has been successfully employed to deposit as-prepared and annealed Ni1−x Znx O thin films having different Zn concentration (x). For all Ni1−x Znx O films, a red-shift in optical band gap is observed with increasing Zn concentration due to the defect states induced by the presence of Zn in NiO thin films. The band gap shift is found to be in total agreement with measurement of activation energy. Evolution of electrical properties revealed the decrease in activation energy and an increase in conductivity and the density of states at Fermi level with increasing Zn concentration. On the other hand, Ni1−x Znx O films, subjected to an annealing treatment at 623 K, display an increase of about ∼10% in transmittance over the as-prepared films. The fundamental effect of annealing is related to an increase in the size of the crystallites and a decrease in the surface roughness and the native defects (such as interstitial oxygen and Ni-vacancies).
Acknowledgement The authors from Ujjain would like to thank the Director and Centre Director of UGC-DAE CSR, Indore for their cooperation to avail the experimental facilities and to Mr. S. Shanmukharao, Mr. D. Venkateshwarlu, Mr. Durgesh Singh for their assistant and help, Dr. Mukul Gupta for XRD and Dr. Uday Deshpande for optical measurements.
Please cite this article in press as: R. Sharma, et al., Studies on the structure optical and electrical properties of Zn-doped NiO thin films grown by spray pyrolysis, Optik - Int. J. Light Electron Opt. (2015), http://dx.doi.org/10.1016/j.ijleo.2016.01.050
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Studies on the structure optical and electrical properties of Zn-doped NiO thin films grown by spray pyrolysis
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Ratnesh Sharma a,∗ , A.D. Acharya b , S.B. Shrivastava a , Manju Mishra Patidar c , Mohan Gangrade c , T. Shripathi c , V. Ganesan c a
School of Studies in Physics, Vikram University, Ujjain, MP, India Department of Physics, Lovely Professional University, Phagwara, Punjab, India c UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore, MP, India b
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Article history: Received 26 October 2015 Accepted 5 January 2016 Available online xxx
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Keywords: NiO thin films Zn doped Spray pyrolysis Oxide semiconductors
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1. Introduction
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Thin films of Ni1−x Znx O with different composition (x = 0, 0.01, 0.05 and 0.10) were successfully fabricated on glass substrates using spray pyrolysis technique. The as-prepared films were further annealed at 623 K in room atmosphere for 2 h. The evolution of their structural, morphological, electrical and optical properties with annealing temperature was investigated. The result reveals that the annealing of the films leads to improved surface morphology and better crystallinity. The prepared films displayed increase in conductivity followed by decrease in band gap with increase in doping concentration. However, the effect is more significant in case of annealed films, where the average transparency shows an increase of about 10% over the as prepared thin films. The red-shift of the optical band gap is due to the deep states in the band gap. The increase in density of states has been confirmed by variable range hoping (VRH) mechanism. The activation energy was found to be decreased when Zn concentration increased. Added to this the dielectric function of thin films and their spectral variation has been tentatively discussed. © 2016 Published by Elsevier GmbH.
In recent years, investigation in transparent semiconducting oxide thin films has assumed significance due to their important optical characteristics, high stability and excellent electrical properties. Transparent conducting oxides are basically metal oxide semiconductors classified as either n-type or p-type. Coatings of ntype semiconductors, such as indium tin oxide, zinc oxide etc., are required in many technological applications such as transparent electronics and nanodevices [1,2]. Among various n-type semiconductors, ZnO, a direct band gap (3.37 eV) material, has been widely used in these optoelectronic devices during past decade. However, in recent years, thin films of p-type semiconductor have attracted much attention due to their important applications in optoelectronic devices such as elements for information display light shutter and variable reflectance mirror [3]. However not many studies have been done on P-type large band gap semiconductors. Such p-type semiconductors could allow the use of new classes of organic compounds. Unfortunately, relatively few metal oxides tend to be P-type. NiO is one of the oxide having wide band gap
∗ Corresponding author. E-mail address: [email protected] (R. Sharma).
semiconductor with the absorption edge in the near UV–visible region[4] and it exhibit functional properties and offer promising candidature for many applications such as ultraviolet optoelectronic devices, positive electrode in batteries, solar thermal absorber, fuel cells, catalysts, in gas sensing devices, in electrochromic display devices etc. [5–8] Thus, nickel oxide can be used as candidate material in UV based optoelectronic applications. The properties of nickel oxide depend on its defect structure and can be controlled by the appearance of nickel vacancies and interstitial oxygen in NiO crystallites. Recently, many attempts have been made to modify the properties of NiO thin films by doping with various elements such as K, Li, P, Mg, Co, etc [9–13] Besides the dopants mentioned above, zinc is a promising transition metal element that can be used as a suitable additive for nickel oxide due to close ionic radius parameter. In addition, the detailed knowledge of various physical properties of Zn-doped NiO film is still limited [14,15]. The systematic analysis of optical and electrical properties of Ni1−x Znx O films as a function of Zn content would provide the necessary and relevant information for its various optoelectronic and coating applications. Thus it would be interesting to investigate the effect of Zn doping on structural, optical and electrical properties of Ni1−x Znx O films. The films have been prepared by using spray pyrolysis technique. A variation of the optical
http://dx.doi.org/10.1016/j.ijleo.2016.01.050 0030-4026/© 2016 Published by Elsevier GmbH.
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band-gap with Zn composition was analyzed. Also, a considerable change in the electronic structure by the Zn substitution was found and compared to NiO.
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2. Experimental details
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Experimentally Ni1−x Znx O (x = 0, 0.01, 0.05 and 0.10) thin films were prepared onto the glass substrates using chemical spray pyrolysis technique. The apparatus used for deposition is described and schematized elsewhere [16]. The precursor solution was prepared from a mixture of 0.1 M nickel chloride (NiCl2 ·6H2 O) and de-ionized water. Zn doping was achieved by the addition of zinc nitrate [Zn(NO3 )2 ] to the precursor solution. The doping concentration of films for a particular precursor in grams has been measured by weight percentile method with the help of molarity equation [17]. The solution was stirred for few minutes to yield a clear and homogeneous solution. For uniform deposition of thin films the solution was sprayed through a linearly moving glass nozzle for a fixed period of 15 min. The solution sprayed onto the preheated glass substrate undergoes evaporation, solute precipitation and pyrolytic decomposition, thereby resulting in the formation of nickel oxide thin films according to the following reaction [17,18]: NiCl2 · 6H2 O → NiO + 2HCl ↑ +5H2 O ↑
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2Zn(NO3 )2 · 6H2 O + O2 → 2ZnO + 4NO2 ↑ +2O2 ↑ +12H2 O ↑
(1)
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The nozzle to substrate distance was 27 cm. The spray rate of 1 ml/min was maintained by using air as a carrier gas. The substrate temperature was kept constant at 623 K during deposition of film by means of electronic temperature controller. After spraying the solution few samples were subsequently annealed at 623 K for 2 h in air. The glass substrates were chemically and ultrasonically cleaned before coating. Several initial trials were made to optimize the deposition conditions before real sample preparation. Thickness of prepared film was determined by weight difference method using a sensitive microbalance and found to be ∼280 nm and ∼240 nm for as-prepared and annealed Ni1−x Znx O films respectively [18].
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3. XRD study
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The structural studies of the above films were done by using X-ray diffraction method. The XRD was performed on Bruker D8 ADVANCE X-ray diffractometer with CuK␣ radiation (wave˚ The XRD patterns of as-prepared films of length = 1.5418 A). Ni1−x Znx O (for x = 0, 0.01, 0.05, and 0.10) are shown in Fig. 1a. The as-prepared films exhibit (1 1 1), (2 0 0) and (2 2 0) diffraction peaks showing the formation of cubic NiO structure. No other peak corresponding to Zn or ZnO has been detected. This indicates that Zn is successfully incorporated into NiO lattice. The XRD patterns of Ni1−x Znx O films, annealed at 623 K are shown in Fig. 1b. The patterns also reveal the presence of polycrystalline NiO films with preferential orientation in the plane (1 1 1). All the films became more strongly oriented along (1 1 1) direction after heat treatment. In Fig. 2, we have compared the highly intense (1 1 1) peaks of asprepared films with the corresponding peaks of annealed films for different concentration of zinc (i.e. x). Figure shows that the diffraction intensity increased after annealing the films, it shows that the best crystalline quality of the film is achieved after annealing [19]. In order to explain this, it is noteworthy that the crystallographic structure is found to be similar for both the as-prepared and annealed films. Thus, the degree of crystallization and composition is mainly controlled by the post thermal treatment rather than by heating during deposition. This means that the efficiency of rearrangement of the constituent atoms (Ni, Zn, O) is small for as-prepared films. While, thermal annealing facilitates the crystal growth at energetically favourable site and promotes atoms to
Fig. 1. XRD patterns of (a) as-prepared and (b) annealed Ni1−x Znx O thin films.
Fig. 2. A comparison between highly intense (1 1 1) peaks observed for different t concentration of Zn.
nucleate at proper sites. This results in the improved crystallinity caused by decrease in native defects such as interstitial oxygen and nickel vacancies in the annealed films [20]. On the other hand, as the zinc content (i.e. x) in the film increased, the diffraction intensity decreased with increase in Full width at half-maximum (FWHM) value. Moreover, the Bragg’s angle of diffraction () shifts to small- angle side with increasing zinc content (Fig. 3a). This is evident that the Zn2+ ions (ionic radius ˚ well substitute Ni2+ ((ionic radius of 0.69 A) ˚ sites in the of 0.74 A) present Zn composition range [11,21]. The crystallite size (D) of the annealed films has been calculated by using the formula given below [22] D=
0.9 , ˇ cos
(2)
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displayed in (Fig. 4a–d) respectively. All the films are found to be homogeneous and well-covered by grain like structures. The surface morphology of Ni1−x Znx O films is found to be significantly influenced by thermal annealing (Fig. 4e–h). In the micrographs of annealed Ni1−x Znx O films the presence of uniformly distributed and well developed grains can be noticed. This suggests that the heat treatment facilitates the process of grain-growth and lead to improved crystalline microstructure. The average particle size was obtained by statistical fitting which are given in Table 1. It can be observed that the average particle size decreased from 92 nm to 58 nm for as-prepared films and from 115 nm to 72 nm for annealed films with increase in Zn content from 0 to 0.10. However, the significant increase in the grain size after heat treatment could be noticed. It is clear that the increase in the surface roughness results from an increase in the grain size in Ni1−x Znx O films (see Table 1). 5. Electrical study
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where ˇ, is the broadening of diffraction line measured at half of its ˚ maximum intensity (FWHM), is the X-ray wavelength (1.5418 A) and is the Bragg’s angle of diffraction. It has been observed that the crystallite size of as-prepared Ni1−x Znx O films decreased slightly from 18.75 nm to 18.24 nm with increase in Zn concentration from 0 to 0.10 (see Fig. 3b). In case of annealed films, crystallite size decreased significantly from 20.58 nm to 18.98 nm with increasing zinc content (Table 1). Consequently, the crystallinity of the films might be deteriorate with increasing Zinc content before and after annealing, which indicates that the large amount of Zn doping may creates lattice distortion.
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4. Morphological study
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= 0 exp
E a kb T
,
(3)
where is the resistivity at temperature T, 0 is a constant, Ea is the activation energy and kb is the Boltzmann constant. The values of activation energy so calculated are listed in Table 1. The activation energy is found to decrease with increase in Zn concentration for both i.e. as-prepared and annealed films. This leads to decrease in the band gap upon Zn doping and suggested that band gap could be controlled by changing the Zn concentration. However, probably due to variation in homogeneity and stoichiometry of as-prepared films, the Ea values for as-prepared films are slightly higher than those measured for annealed films. This discrepancy could be associated with the difference of film thickness, which was found to be ∼280 nm and ∼240 nm for as-prepared and annealed Ni1−x Znx O films respectively. Patil et al. [24] have reported the increase in
The surface morphological images of deposited films were recorded over an area of 1 m × 1 m at room temperature using atomic force microscope (AFM) (Digital instruments nanoscope E with Si3 N4 100 m cantilever 0.58 N/m force constant) in contact mode. The representative two dimensional images of as-prepared Ni1−x Znx O films corresponding to x = 0, 0.01, 0.05 and 0.10 are
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The electrical properties of as-prepared and annealed Ni1−x Znx O films was studied in the temperature range 180 K to 330 K using two probe resistivity method. The resistivity measurements were done using Keithley 616 programmable electrometer in association with Lakeshore temperature controller in a home built apparatus [23] with Teflon-insulated shielded coaxial cables. The unit containing the sample was kept in the cryostat. The liquid nitrogen is used as coolant. The results of the above measurements are summarized in Fig. 5a–f. Fig. 5a and b shows the variation of resistance per unit length as a function of temperature for as-prepared and annealed films respectively. It has been found that, in general, resistance per unit length decreases with increase in temperature demonstrating the semiconductor type of behaviour. Also the resistance per unit length decreases with the increase in Zn concentration. Thus, the conductivity was maximum in case of x = 0.1. The resistance per unit length was found to be larger in case of annealed samples. Fig. 5c and d show the plots of ln[R] verses 1000/T for as-prepared and annealed Ni1−x Znx O films respectively. From the curves the thermal activation energy was calculated using Arrhenius equation given below:
Fig. 3. (a) Variation of Bragg’s angle of diffraction corresponding to (1 1 1) plane (b) plots of crystallite size versus Zn concentration for as-prepared and annealed Ni1−x Znx O thin films.
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Table 1 The various structural, electrical and optical parameters observed for as-prepared (As-pre.) and annealed (Ann.) Ni1−x Znx O films. x
Crystallite size (nm)
Average particle size (nm)
RMS roughness (nm)
Activation energy (eV)
[T0 ]1/4 (K1/4 )
As-pre.
Ann.
As-pre.
Ann.
As-pre.
Ann.
As-pre.
Ann.
As-pre.
Ann.
As-Pre.
Ann.
0 0.01 0.05 0.10
18.75 18.66 18.48 18.24
20.58 20.6 19.56 18.98
92 75 64 58
115 108 86 72
2.5 3.26 4.16 4.74
2.12 2.47 4.65 3.04
0.37 0.373 0.35 0.33
0.41 0.35 0.31 0.29
247 252 226 224
278 252 231 217
3.48 3.46 3.43 3.41
3.52 3.49 3.44 3.42
Optical band gap (eV)
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Fig. 4. (a), (b), (c) and (d) shows the AFM images of as-prepared Ni1−x Znx O films for x = 0, 0.01, 0.05 and 0.10 respectively. While (e), (f), (g) and (h) shows the AFM images of annealed Ni1−x Znx O films for x = 0, 0.01, 0.05 and 0.10, respectively.
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activation energy with increasing film thickness. This was explained as varying stoichiometry caused by difference in the experimental conditions viz. amount of spraying solution, spray rate and cooling of the substrates during decomposition. Furthermore, from Fig. 5c and d one can observe that the plots of ln[R] verses 1000/T do not fit linearly at temperatures below 230 K. In order to explain low temperature transport mechanism, we have also plotted the log of normalized resistance i.e. ln[R/R330 ] 1/4 as a function of [1/T ] in Fig. 5e and f for as-prepared and annealed films respectively. Activation energy in the low temperature region can be obtained from the above graphs by means of variable range hoping (VRH) mechanism as suggested by Ambegaokar et al. [25]. The VRH mechanism is governed by the equation [26]
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= 0 exp
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T 1/4 0
T
,
(4)
where 0 and T0 denote material parameters. Here T 0 = 16˛3 /kb N(E), where N(E) is the density of localized electron states at Fermi level and ˛ is the inverse localization length of the localized state. The curves plotted in Fig. 5e and f gives a straight line. From these figures, the value of T0 has been obtained for Ni1−x Znx O films which are listed in Table 1. It has been found that the values of T0 decreases with increase in Zn concentration in both the cases. This suggests that the density of states at Fermi level increases with the increase in zinc concentration. 6. Optical study Optical measurements were done in transmittance mode as well as in reflectance mode using UV–visible spectrophotometer (Perkin Elmer Lambda 950). The variation in transmittances of as-prepared and annealed Ni1−x Znx O films with UV–visible range wavelength
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Fig. 5. (a) and (b) shows the plot of resistance per unit length versus temperature for as-prepared and annealed films respectively. (c) and (d) shows the plot of ln[R] verses 1/4 for as-prepared and 1000/T for as-prepared and annealed films respectively. (e) and (f) shows the plot of log of normalized resistance i.e. ln[R/R330 ] as a function of [1/T ] annealed films respectively.
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is shown in Fig. 6. It is clear from figure that for all the films, the optical transmission is low in UV region which increases abruptly in the visible region. Moreover, the average transmittance in the visible region decreases as the zinc concentration (x) is increased. For instance, the transmittance for as-prepared NiO film with no zinc addition (x = 0) is as high as 75%, which falls to 65% when 10 at.% zinc is added. This is in good agreement with the XRD and AFM measurements where we have observed the deteriorated crystalline microstructure and increased roughness values at higher zinc concentration. The higher surface roughness enhances the optical scattering of incident radiation leading to lower transparency in Zn-doped NiO films. Besides, the films that were annealed at 623 K, show higher transparencies than as-prepared ones. In fact, the average transmittance for annealed films varies between 85 and 80% which shows an increase of about ∼10% over the as-prepared films (Fig. 7). This may be related to reduced absorption from native defects that are removed after annealing of films. The absorption coefficient (˛) has been calculated from transmittance (T) and reflectance (R) data using the relation [27]
˛=
1 (1 − R)2 · ln t 2T
,
(5)
where t is the film thickness. The spectral variation of ˛ have been shown in Fig. 8 for both as-prepared as well as annealed films. In this figure, an absorption edge near (∼370 nm) is observed for asprepared NiO film. As zinc concentration in the film is increased, the absorption edges of Ni1−x Znx O films show a clear shift to longer wavelengths (red-shift). This trend is also observed in case of annealed films. However, for both the as-prepared and annealed films and for all the Zn composition used in this study (i.e. x = 0 to 0.10 at.%), the onset of noteworthy absorption is in the wave length range less than 400 nm ensuring excellent transparency in the visible region. Further, it is interesting to note that, the absorption edge is found to be sharper in case of annealed films in comparison to as-prepared films. Generally, absorption changes as a function of crystalline properties. The defect states inside the band gap make the optical absorption broad, while a rather sharp behaviour is observed for the films which show good crystalline quality. This
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Fig. 6. Spectral variation of transmittance for as-prepared and annealed Ni1−x Znx O films corresponding to different Zn concentrations.
Fig. 9. The plots of (˛h)2 versus incident photon energy (h) for annealed films. Inset shows the Plot of (˛h)2 versus (h) for as-deposited films.
Fig. 7. A comparison between the average transmittance observed for as-prepared and annealed films as a function of Zn concentration.
Fig. 10. (a) Comparison between the optical band gap and activation energy for annealed Ni1−x Znx O films as a function of Zn concentration. (b) Comparison between the optical band gap values of as-prepared and annealed Ni1−x Znx O films as a function of Zn concentration.
(˛h)
Fig. 8. Spectral variation of absorption coefficient for Ni1−x Znx O films corresponding to different concentration of Zn and inset shows the optical reflectance spectra corresponding to different concentration of Zn.
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is indeed true in our case where we have observed improved crystalline structure for annealed films. The optical band gap have been determined using the Tauc’s relation [28] given below:
1/p
= A h − Eg ,
(7)
where ˛ is the absorption coefficient, Eg is the band gap corresponding to a particular transition occurring in the film, A is a constant, is the transition frequency and p can take the values 1/2, 3/2, 2 or 3, for the transition corresponding to direct allowed, direct forbidden, indirect allowed and indirect forbidden respectively. The NiO is considered as material with direct band gap energy and hence the value of p has been taken to be 1/2 for direct allowed transitions. The band gap has been calculated by extrapolating the linear 2 region of the plots (˛h) versus h on the energy axis as shown in Fig. 9. The band gap values so obtained are given in Table 1. It can be noted that as Zn concentration in the film increases from 0 to 10 at.%, the optical band gap for as-prepared films decreased from 3.48 eV to 3.41 eV and for annealed films, it decreased from 3.52 eV to 3.42 eV. These variations are similar to the change in activation energy observed in the conductivity measurement (see Fig. 10). On comparing the band gap values observed before and after annealing (Fig. 10), it can be seen that for lower doping concentration of Zn (i.e. for x < 0.05 including undoped NiO), the band gap values for annealed films are slightly higher than those measured in case of as-prepared films whereas these values become very close at relatively higher Zn doping concentrations. The band gap shift observed with increasing Zn content can be attributed to degradation in crystallinity and to the defect states (oxygen defects or Ni vacancies) induced by the presence of Zn in NiO thin films.
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Fig. 11. (a) and (b) shows the spectral variation of extinction coefficient (k) and refractive index (n), calculated for annealed Ni1−x Znx O films. (c) Effect of zinc concentration on packing density () of annealed films determined for n at 600 nm. (d) The variation of (εr ) and (εi ) with Zn concentration.
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The optical reflectance spectra (R) have been used to determine the refractive index (n) of the film through the relation [13,29]
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n=
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1+R + 1−R
4R (1 − R)2
− k2 ,
(6)
where k = ˛/4, is the extinction coefficient. Fig. 11a and b respectively show the variation of extinction coefficient (k) and refractive index (n) calculated for annealed Ni1−x Znx O films. The spectral dependence of k and n is similar to those reported for NiO thin films [30,31]. For all the films, it can be observed that the value of k remains constant and is very near to zero for wavelengths beyond 400 nm, but in the wavelength range less than 400 nm, it increases significantly. The average value of k in the visible region for annealed Ni1−x Znx O films are found to be of the order of 10−2 and such low value of k is a qualitative indication of homogeneity of annealed films [32]. Further, as Zn concentration in the film increases, extinction coefficient increases which is in good agreement with the measurement of surface roughness. This suggests that the surface roughness may contribute to change in the extinction coefficient of deposited films. On the other hand, the value of refractive index is practically constant beyond 400 nm, whereas near the band edge a peak is observed due to fundamental band gap absorption. Generally, the refractive index of semiconductor is inversely proportional to the energy band gap [33]. This is indeed true in our case where we have observed the lower band gap values and higher refractive indices with increasing Zn concentration. This increased refractive index results in the densification of film at higher Zn concentration. index The packing density () is closely related to the refractive as: [34] = (n2 − 1) × (n2b + 2)/ (n2 + 2) × (n2b − 1) , where nb is the refractive index of un-doped bulk crystal. The value of nb have been taken from literature [35]. The values of determined for n at 600 nm are plotted in Fig. 11c. It can be seen that as the Zn content in NiO film is increased, the packing density increases, leading to better packing of grains with a reduction of voids. This suggested that the change in refractive index and thus packing density is correlated to the change in morphology caused by grain growth during the deposition of film.
The study of dielectric function of thin films is very important for the use of films in optical applications. The dielectric constant is defined as ε = εr + iεi where εr = n2 − k2 is the real part and εi = 2nk is the imaginary part of complex function [36]. These values have been calculated at the wavelength of 600 nm. The variations of εr and εi for Ni1−x Znx O films as a function of Zn concentration are shown in Fig. 11d. From the figure, it can be seen that both real and imaginary parts of dielectric constant increased systematically with the increase in Zn concentration.
7. Conclusions The chemical spray pyrolysis technique has been successfully employed to deposit as-prepared and annealed Ni1−x Znx O thin films having different Zn concentration (x). For all Ni1−x Znx O films, a red-shift in optical band gap is observed with increasing Zn concentration due to the defect states induced by the presence of Zn in NiO thin films. The band gap shift is found to be in total agreement with measurement of activation energy. Evolution of electrical properties revealed the decrease in activation energy and an increase in conductivity and the density of states at Fermi level with increasing Zn concentration. On the other hand, Ni1−x Znx O films, subjected to an annealing treatment at 623 K, display an increase of about ∼10% in transmittance over the as-prepared films. The fundamental effect of annealing is related to an increase in the size of the crystallites and a decrease in the surface roughness and the native defects (such as interstitial oxygen and Ni-vacancies).
Acknowledgement The authors from Ujjain would like to thank the Director and Centre Director of UGC-DAE CSR, Indore for their cooperation to avail the experimental facilities and to Mr. S. Shanmukharao, Mr. D. Venkateshwarlu, Mr. Durgesh Singh for their assistant and help, Dr. Mukul Gupta for XRD and Dr. Uday Deshpande for optical measurements.
Please cite this article in press as: R. Sharma, et al., Studies on the structure optical and electrical properties of Zn-doped NiO thin films grown by spray pyrolysis, Optik - Int. J. Light Electron Opt. (2015), http://dx.doi.org/10.1016/j.ijleo.2016.01.050
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