- Email: [email protected]
Characteristics of nanostructured Zn1−xVxO thin films with high vanadium content elaborated by rf-magnetron sputtering
Accepted Manuscript Characteristics of nanostructured Zn1-xVxO thin films with high vanadium content elaborated by rf-magnetron sputtering K. Medjnoun, K. Djessas, M.S. Belkaid, S. Grillo, A. Solhy, O. Briot, M. Moret PII: DOI: Reference:
S0749-6036(15)00097-X http://dx.doi.org/10.1016/j.spmi.2015.02.019 YSPMI 3620
To appear in:
Superlattices and Microstructures
Received Date: Accepted Date:
31 January 2015 4 February 2015
Please cite this article as: K. Medjnoun, K. Djessas, M.S. Belkaid, S. Grillo, A. Solhy, O. Briot, M. Moret, Characteristics of nanostructured Zn1-xVxO thin films with high vanadium content elaborated by rf-magnetron sputtering, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi.2015.02.019
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Characteristics of nanostructured Zn1-xVxO thin films with high vanadium content elaborated by rf-magnetron sputtering K. Medjnouna,b,c, K. Djessasa,b,*, M. S. Belkaidc, S. Grilloa,b, A. Solhyd, O. Briote, M. Morete a
Laboratoire Procédés, Matériaux et Energie Solaire (PROMES)-CNRS, Tecnosud, Rambla de la thermodynamique, 66100 Perpignan, France. b
Université de Perpignan Via Domitia, 52 avenue Paul Alduy, 68860, Perpignan Cedex9, France.
c
Laboratoire des Technologies Avancées du Génie Electrique (LATAGE), Université Mouloud Mammeri de TiziOuzou (UMMTO), B.P N° 17 R.P Tizi-Ouzou, Algérie. d
Mohammed VI Polytechnic University. Lot 660 - Hay Moulay Rachid. 43150 Ben Guerir, Morocco.
e
CNRS, Université Montpellier 2, Laboratoire Charles Coulomb UMR 5221, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France. *Corresponding author: K. Djessas. E-mail address: [email protected] Tel: 0033468682268
Abstract Nanostructured Zn1-xVxO (0 ≤ x ≤ 0.50) thin films were synthesized by rf-magnetron sputtering at two different substrate temperatures (room temperature (RT) and 200 °C) and with variable sputtering powers (60, 80 and 100 W). In this method, single targets based on Zn 1-xVxO nanopowders prepared by the sol-gel process were used. Characterization of the Zn1-xVxO nanoparticles showed that they crystallize in the hexagonal wurtzite structure. Their size ranged from 20 to 40 nm. The effect of process parameters on the physical and chemical properties of Zn1-xVxO thin films has been studied. For x ≤ 0.30, the results obtained at 200 °C and 60 W indicate that the films have a high quality of crystallinity. Vegard’s law is respected, indicating that vanadium is incorporated in the ZnO matrix. The chemical compositions of these films were found to be close to the stoichiometry. The films exhibit a columnar structure and a smooth surface. Their average transmission, from the visible to the NIR, was in the range of 75 to 90%. The values of the band gap of the Zn1-xVxO thin films with x ≤ 0.30 and elaborated at 200 °C and 60 W, vary from 3.29 to 3.74 eV. This is consistent with blue shifting of near-band edge cathodoluminescence emission. Under particular growth conditions, the investigation shows that the Zn0.80V0.20O sample presents the best properties for potential use in various optoelectronic applications, namely: a single wurtzite phase, low surface roughness (Ra ~ 0.2 nm), a high transparency of 90% in the UV-Vis-NIR, a wide band gap of 3.74 eV and a resistivity of ~ 5x10+3 Ω.cm. Keywords: Zn1-xVxO; Nanoparticles; Sol-gel; Thin films; Sputtering.
1
1. Introduction Zinc Oxide (ZnO) is a semiconductor material with a wide direct band gap (Eg ≥3.3 eV) at room temperature and an excitonic binding energy (60 meV). It is commercially available because of its advantages, which include its abundance in nature, nontoxicity, easy fabrication and doping and high chemical stability [1-3]. Recently, considerable attention has been focused on the development of this material for various applications, such as electrochromic and optoelectronic devices. As reported by S. Gowrishankar [4], modulation of the band gap is one of the major requirements in designing optoelectronic devices. Reports in the literature have shown that the ZnO band gap can be engineered by alloying this material with three major elements: Mg [4-5], V [5] and Cd [5-6]. Mg and V are known to broaden the band gap, whereas Cd is known to narrow it [5]. Magnetron sputtering has been used by most researchers to grow ZnO thin films alloyed by Mg, V and Cd in order to tailor the band gaps. This is due to the many advantages of this technique, such as its simplicity and low cost. It is considered to be the most favorable deposition method to obtain highly uniform films with high packing density and a strong adhesion at high deposition rate, without needing a high deposition temperature nor toxic gases. On the other hand, it is worth noting that little research on the development and study of the structural and optoelectronic properties of Zn 1-xVxO thin films has been reported in the literature [7-14]. Some of the films with low vanadium content are obtained by magnetron sputtering and find their applications as Transparent Conducting Oxides (TCO) [8,10,12,14]. However, most studies on the fabrication of Zn1-xVxO thin films used a mixture of ZnO with hard vanadium [10,14] or powders of V2O5 [12] as targets. In the present work, we used Zn1-xVxO nanoparticles, synthesized by the sol gel process, with high vanadium content as targets in order to elaborate Zn1-xVxO thin films by rf-magnetron sputtering. To the best of our knowledge, this is the first time that this approach has been developed for the growth of Zn1-xVxO thin films alloys with high vanadium content. A detailed study of the structural, morphological, optical and electrical properties of the Zn1-xVxO thin films was therefore carried out. The experimental growth conditions of Zn1-xVxO thin films were optimized by varying the vanadium concentration from 0 to 50%. It was found that the resistivity, transmission and band-gap of these thin films can be influenced by the vanadium content and the growth conditions.
2. Experimental 2.1. Materials synthesis 2.1.1. Nanoparticles preparation It should be emphasized that the Zn1-xVxO nanoparticles have been prepared, even at the higher vanadium concentrations, by adopting the conventional method described previously [7,15]. All 2
reagents were purchased from Aldrich Chemical Company and were used without further purification. Synthesis of the samples was performed by the sol-gel process using zinc acetate dehydrate [Zn(CH3COO)2.2H2O] and ammonium metavanadate in various V/Zn ratios of 0, 0.10, 0.20, 0.30 and 0.50 with methanol as solvent. These reagents were mixed, treated in the autoclave and dried under supercritical conditions of ethyl alcohol (EtOH). Finally, a post-synthesis treatment was carried out by calcining the sample at 500 °C in air for 2 hours.
2.1.2. Thin films deposition The Zn1-xVxO thin films were deposited on p-type Si (100) and glass substrates by rf-magnetron sputtering (13.56 MHz- Cezar Rf-power Generator). The sputtering targets were prepared from Zn1xVxO
nanoparticles. The distance between the substrate and the targets was fixed at 75 mm. The
sputtering chamber was evacuated to a base pressure of about 10 -5 mbar, before introducing the sputtering argon gas at high purity and with a flow rate of 11 sccm. The sputtering deposition was carried out at a pressure of 10-3 mbar. Two substrate temperatures were used: RT and 200 °C. The rfsputtering power of the Zn1-xVxO targets was varied from 60 to 100 W in steps of 20 W. The deposition time was varied in order to achieve films with a thickness of ~ 200 nm.
2.2. Characterisation methods The crystalline properties of the Zn1-xVxO nanoparticles and the thin films were studied with an X-ray diffractometer (Philips PW 1729), using Cu-Kα radiation (λ = 0.15406 nm) in Bragg-Brentano geometry (θ-2θ) in the range of 10°-60°. The morphology and size of the Zn1-xVxO nanoparticles were inspected with a Transmission Electron Microscope (TEM, JEM-200CX), operating at 100 kV and using C-coated Cu grids. The chemical composition of the nanoparticles and thin films were obtained by Energy Dispersive X-ray Spectroscopy (EDS, JEOL JSM 5310 LV). The surface morphology of the films was studied using an Atomic Force Microscope (AFM, NT-MDT/SMENA) operating in the intermittent contact mode (325 kHz) with Si cantilevers of 46 N/m spring constant and tip radius <10 nm. A Field Emission Scanning Electron Microscope (SEM-FEG, Hitachi S-4500) operating at 10 kV was employed to observe the cross-section of the films and to determine their thickness. The latter measurements were confirmed by profilometry (Decktak XT). The optical transmittance of the films was measured with a UV-Vis-NIR spectrophotometer (Shimadzu-UV3101PC). Room temperature cathodoluminescence (CL) measurements were carried out using Scanning Electron Microscope SEM (JEOL JSM-840A) operating at an accelerating voltage of 30 kV. The CL system consisted of a parabolic mirror directing the emitted photons onto an Oriel cornerstone 130 monochromator followed by PM:180 photomultiplier. The luminescence emissions were detected by a Hamamatsu Monobloc 3
detector in the range of 180-900 nm. Electrical resistivity measurements were carried out using the four-point probe method with LUCAS LABS-302 System.
3. Results and discussion 3.1. Structural properties of the Zn1-xVxO nanoparticles Fig. 1 shows a series of X-ray diffraction (XRD) patterns of Zn1-xVxO nanoparticles synthesized by varying the x values between 0 and 0.50. According to the JCPDS database (card number 36-1451), the peaks corresponding to the (100), (002), (101), (102) and (110) reflexion planes denote a hexagonal wurtzite ZnO structure. When the vanadium content in the ZnO matrix is increased, the degree of crystallinity of the nanoparticles is found to decrease. Indeed, at x = 0.20, other peaks which do not belong to the wurtzite structure begin to appear and their relative intensity increases with vanadium content. These secondary phases can be attributed to the presence of V2O5, VO2 and Zn3(VO4) compounds. This is probably due to the limited solubility of the vanadium atoms substituting the Zn sites of the ZnO matrix. These results are in good agreement with the trends described in references [16-18]. Fig. 2 shows TEM photographs of the Zn1-xVxO nanoparticles. At vanadium contents below 30%, the crystallites exhibit a mixture of shapes (mainly hexagonal and spherical), with a narrow particle size distribution. The size of the Zn1-xVxO nanoparticles was estimated in the range of 20 - 40 nm. From 30% of vanadium content onwards, particles in the shape of sticks can be observed, concomitantly with degradation in the crystallization of the nanoparticles. The sticks may consist of one or more of the secondary phases discussed above. EDS analysis was performed to measure the chemical compositions of Zn 1-xVxO nanoparticles with x varying from 0 to 0.50. The results are shown in Table 1. The elemental compositions values obtained for the Zn1-xVxO nanoparticles with x < 0.20 are quasi-stoichiometric. This confirms the presence of vanadium in the matrix of ZnO. For the higher x values, the stoichiometry of the Zn 1-xVxO nanoparticles was not respected. These results are in agreement with those of XRD presented above.
3.2. Structural properties of Zn1-xVxO thin films The XRD patterns of Zn1-xVxO films deposited by rf-magnetron sputtering on the p-type Si (100) substrate at RT and at 200 °C, at different sputtering powers of 60, 80 and 100 W are presented in Fig. 3a-c. It should be kept in mind that the (002) peaks are convoluted in these images with a peak at 32.90° due to the Si substrate. It can be seen from these results that all the films showed a polycrystalline hexagonal wurtzite structure with a preferential c-axis orientation. In addition, in the 4
films deposited at RT with a vanadium content of 20, 30 and 50%, the orthorhombic V2O5 structure can be observed. This is denoted by the (101) and (121) peak, respectively at 22.4° and 55.85°, as confirmed by the crystallographic data JCPDS 41,1426. An increase in the intensity of the (121) peak with increasing sputtering power can be observed in the sample with 30% of vanadium content. This orientation disappears completely by increasing the substrate temperature to 200 °C for all cases of the films sputtered at 60, 80 and 100 W. The V2O5 phase is totally absent for the films with x ≤ 0.30 obtained at 200 °C and 60 W. This indicates that the crystallinity of Zn 1-xVxO thin films in these conditions is improved. The grains size G in the Zn1-xVxO films can be calculated using the Debye-Scherrer’s formula [19]:
where λ is the X-ray wavelength, θB is the Bragg diffraction angle and B is the full width at half maximum (FWHM) of the diffraction peak, after correction for the instrument broadening. The values of the FWHM of the (002) peak of the Zn1-xVxO films prepared at 200 °C are significantly lower than those of the films grown at RT, implying that at 200 °C the crystal size is greater. The overall observed trend is that the FWHM of the (002) peak increases with the increase in vanadium content and sputtering power. Besides, as the vanadium content increases, the (002) peak slightly shifts to a lower diffraction angle. This shift means that the spacing of the planes becomes larger as the vanadium content increases. The images show that the highest degree of crystallinity of the films with x ≤ 0.30 was obtained at a sputtering power of 60 W and 200 °C. The calculated grain sizes of the films elaborated at 60 W with different vanadium contents are shown in Table 2. As revealed in this table, crystallite sizes of the films deposited at 200 °C are larger than those elaborated at RT. These results confirm the improvement of the crystallinity of the films elaborated at 200 °C. The lattice constant, c, of the wurtzite structure of the films elaborated at 200 °C and 60 W has thus been calculated, and is plotted against the variation of vanadium content from 0 to 0.5 at Fig. 4. The variation of the c-axis length constant with vanadium content is found to be approximately linear as the latter is increased from 0 to 0.3, in agreement with Vegard’s law. Thus the vanadium ions appear to substitute the Zn ions in the film without changing its wurtzite structure. This is consistent with the fact that no secondary phases are observed in this case.
5
3.3. Composition analysis of Zn1-xVxO thin films The chemical compositions of the Zn1-xVxO thin films deposited on silicon substrate at 60 W and at substrate temperatures of RT and 200 °C were analyzed by EDS. The results of the compositional analysis of all the films are summarized in Table 3. One can observe that the quasistoichiometry of the Zn1-xVxO thin films is only respected in the case of those obtained with x < 0.20 and x ≤ 0.30 at RT and 200 °C respectively. These results correlate well with those reported by XRD above. Fig. 5 shows as an example the EDS spectrum of the as deposited Zn 0.80V0.20O thin film at 200 °C (and 60 W). It reveals that no other elements coming from the solvents, the precursors or the substrate - beside Zn, V and O - are present in the films.
3.4. Morphological properties of Zn1-xVxO thin films Fig. 6 shows the 3D-AFM images (1 x 1 µm2 ) of Zn1-xVxO thin films with x = 0, 0.20 and 0.50, deposited onto the silicon substrates at two different substrate temperatures (RT and 200 °C) and 60 W. The deposited films exhibit granular surfaces. The images show that when the vanadium content increases, the films' surface roughness decreases. The films appear to be smooth and continuous without pinholes nor cracks. The low surface roughness could be explained as a consequence of coalescence of the grains of the films. The surface morphology of the films elaborated at 200 °C becomes very uniform and more compact. The volatilization of the Zn 1-xVxO nanoparticles may be enhanced by increasing the substrate temperature. One could say that the sputtered nanoparticles have enough energy to diffuse, resulting in the formation of a more compact film. This hypothesis would imply that the crystallinity of the alloys improves as the substrate temperature is increased. Indeed, this is in agreement with the findings of XRD analysis presented above. The calculated values of the mean roughness (Ra) of the samples, given in Table 2, were in the range between 0.2 and 2 nm. As underlined in the studies by Lotin et al. [20] and Li-Wei et al. [11], a high smoothness of the film surfaces is important in order to obtain a good matching of the crystal lattice parameters and to minimize the recombination probabilities in heterostructures. The cross-sectional SEM images of the Zn1-xVxO thin films are shown in Fig. 7. From these images the films thickness was estimated to be approximately 200 nm. This value was in agreement with profilemeter measurements. The films exhibit a uniform grain size with a typical dense columnar structure and a very smooth surface. By comparison to the XRD spectra, it can be concluded that the Zn1-xVxO films grew along the (002) orientation, which is perpendicular to the substrate surface. Also consistent with the XRD results is the observation that the size of the columnar grains decreased as the concentration of vanadium was increased. This finding was furthermore confirmed by AFM images of the surface morphology. 6
3.5. Optical and electrical properties of Zn1-xVxO thin films The transmittance spectra of the Zn1-xVxO (0 ≤ x ≤ 0.50) films deposited with different vanadium contents, at sputtering power of 60 W and substrate temperatures of RT and 200 °C are shown in Fig. 8a. An interference phenomenon can be clearly observed in the spectra, indicating a smooth and homogenous surface of the film. The average transmittance of all the films in the wavelength range from the absorption edge to the NIR was found to be between 75 and 90%. As can be seen in the inset of Fig. 8a, a blue shift of the absorption edge was observed for all the Zn 1-xVxO films elaborated with different vanadium contents under a sputtering power of 60 W at RT and 200 °C. The maximal absorption blue shift of Zn1-xVxO corresponds to the film elaborated with 20% of vanadium content at 200 °C. This result confirms that the widest band gap amongst the Zn1-xVxO films studied is attributed to the one with 20% of vanadium content, elaborated at a substrate temperature of 200 °C and sputtering power of 60 W. It can also be clearly seen that this film has the highest average transmittance in the NIR region (~ 90%). Fig. 8b shows the optical transmission spectra in the UVVis-NIR regions of Zn0.80V0.20O films elaborated at RT and 200 °C with different sputtering powers of 60, 80 and 100 W. All the films were highly transparent (85 to 90%) from the absorption edge to the NIR region. We also observe a gradual shift of the absorption edge of the films towards shorter wavelengths when the substrate temperature increases from RT to 200 °C. The inset of Fig. 8b clearly reveals the higher transmittance in the visible range with lower absorption loss in the case of the films elaborated at 200 °C with 60 and 80 W of sputtering power. However, the XRD results showed that the best crystallinity of the Zn0.80V0.20O films was obtained at 60 W. We can thus conclude that the structural and transmission properties of the Zn1-xVxO thin films are optimized at 20% of vanadium content, sputtering power of 60 W and 200 °C growth conditions. The energy band gap values of Zn1-xVxO thin films were calculated using the equations given below [21-22]:
1
(h ) C (h E g ) 2
(3)
where α is the absorption coefficient, T and R are the transmittance and reflectance and d is the film thickness. Eg is the direct band gap energy and C is a constant. Fig. 9a and 9b show respectively the plot of (αhν)2 as a function of photon energy (hν) for Zn1xVxO
(0 ≤ x ≤ 0.5) thin films with different vanadium contents deposited at 60 W and for a
Zn0.80V0.20O film elaborated at different sputtering powers (in both cases, the films were grown at RT and 200 °C). The band gap energy values can be determined by plotting the square of the absorption 7
coefficient and then extrapolating to (αhν) 2 = 0 the linear part of the curve. As can be seen in Table 4, the band gap of Zn1-xVxO thin films deposited at RT decreases slightly from 3.44 to 3.39 eV as the vanadium content is increased from x = 0.10 to 0.50. This effect was possibly caused by the secondary phase segregation. However, for the films deposited at 200 °C, the band gap increases up to 3.74 eV at x = 0.20 and then decreases to 3.54 eV at x = 0.50 of vanadium content. The band gap of undoped ZnO thin films is not influenced significantly by the substrate temperature (Eg = 3.27 eV at RT, and 3.29 eV at 200 °C). It can be concluded that the band gap width of the Zn1-xVxO thin films can be modulated by the variation of the vanadium content (0.10 ≤ x ≤ 0.50) and substrate temperatures. Fig. 9b shows the effect of sputtering power on the value of the band gap of the Zn0.80V0.20O film. It confirms that the properties are optimized for the film deposited at 60 W, which possesses the widest band gap of 3.74 eV.
A study of cathodoluminescence measurements can provide valuable information on the quality and purity of the material. The distribution of defects and impurities can be also revealed. It is acknowledged that there exist two main emission bands in ZnO photoluminescence or cathodoluminescence spectra, Near Band Edge (NBE) and Deep-Level (DL) emissions, which appear respectively in the ultraviolet (UV) around 380 nm and in visible region, i.e. blue-green (520 nm) and red (650 nm) [23,24]. (NBE) emission of ZnO is namely related to the recombination of the free excitons [25,26], and (DL) emission has been attributed to several defects in the crystal structure such as O-vacancy (VO), Zn-vacancy (VZn), O-interstitial (Oi) and Zn-interstitial (Zni) [4,27,28,29,30]. A. Singh et al. showed negligible intensity in the green emission for ZnMgO thin films annealed at 300 °C to 500 °C, but for higher annealing temperature the intensity of this luminescence was increased. This phenomenon has been related to the oxygen vacancies [31]. S. S. Kurbanov reported that the high UV/green emissions intensity ratio indicates that the samples are of good quality [32]. For these reasons, the layers with the best structural and morphological properties, elaborated at a sputtering power of 60 W and substrate temperature of 200 °C have been selected for cathodoluminescence studies in order to confirm the results obtained above. Cathodoluminescence (CL) spectra of Zn1-xVxO (x = 0, 0.10, 0.20, 0.30) films deposited under these conditions were recorded in the wavelength range of 300-700 nm. The obtained CL spectra are shown in Fig. 10. In our case, all the films exhibit strong bands located in the UV region, which originate from the NBE emission. The latter is considered to be a dominant emission. A weak Green Band (GB) emission is also observed for x = 0.30 at 2.32 eV. This emission has been tentatively attributed in the literature to singly ionized oxygen vacancies (VO), zinc interstitials (Zni) [4] and zinc vacancies (VZn) [33]. Compared to the NBE emission, the DL emission is only just observed. This is probably due to fewer native defects in Zn1-xVxO films, as mentioned in the literature [23]. The weak DL emission intensity 8
can be interpreted in terms of the V atom being substitutionally incorporated, inducing the 2.6% change of the lattice constant observed in our data for the c parameter in Fig. 4; however higher V content lead to deviations from Vegard's law. This indicates that V atoms are probably not included in the lattice sites by substitution, and results in the appearance of deep levels and deep broad bands in the luminescence. It is also clearly observed that the NBE emission shifted toward lower wavelengths for the samples with increasing vanadium content up to x ≤ 0.20. This can be attributed to the change of the band gap with V substitution, up to a point where further V addition does not lead to substitution on the host lattice and generates deep levels. Another possibility would be that the band gap changes due to the strain induced by V incorporation, as Fig. 4 shows a large shift of the c parameter upon V incorporation. However, the c parameter is increased by V incorporation. This can hardly correspond to compressive strain, the layer being polycrystalline. The strain effect, if not compressive should then decrease the band gap, and redshift the transitions. The Burstein-Moss effect has to be ruled out because the resistivities of the layers increase with increasing V content, thus demonstrating that the free carrier concentration is decreasing with V incorporation. In our study, Zn1-xVxO thin films deposited at a sputtering power of 60 W exhibit the highest resistivities. The resistivity of the films obtained at RT increases from 0.50 to 110 Ω.cm when the vanadium content is increased from x = 0.10 to 0.20. But by increasing the substrate temperature to 200 °C in the same range of vanadium content, we find that the resistivity increases even more, from 5 to 5x103 Ω.cm. The resistivities of Zn0.70V0.30O and Zn0.50V0.50O thin films could not be measured because their values exceed the limit of the measuring system [34]. Gowrishankar et al. [4] have explained in their studies that the increase in the resistivities of Zn1−xCdxO and Zn1−xMgxO thin films with the increase of the Cd/Mg content might be due to the segregation of Cd/Mg atoms in the grain boundaries, which in turn increases the grain boundary barrier. This leads to enhanced scattering of charge carriers at grain boundaries, which could explain the increase in resistivity.
4. Conclusion Nanostructured Zn1-xVxO thin films with high vanadium content have been synthesized by rfmagnetron sputtering at low substrate temperatures. The targets, for all of the vanadium contents tested, were based on Zn1-xVxO nanoparticles prepared via the sol-gel method. The effect of vanadium content, sputtering power and substrate temperature on the crystal structure, morphological, optical and electrical properties of the Zn1-xVxO films have been discussed in detail. It has been demonstrated that for up to 30% of vanadium content, the Zn1-xVxO thin films with single Wurtzite phase are successfully obtained at growth conditions of 60 W and 200 °C, and that Vegard’s law is respected. It should be noted here that all the films were oriented preferentially along the (002) direction. EDS analysis reveals that there are no other elements beside Zn, V and O in the Zn1-xVxO films. The quasi9
stoichiometry of these films was also obtained. Both XRD and AFM results show that there is an increase in the grain size when the substrate temperature is increased from RT to 200 °C. It was also observed that the density of the films increases and their surface becomes smoother at 200 °C. The average transmittance of all Zn1-xVxO thin films, from the visible to the NIR region, is in the range 7590%. By varying the vanadium content and the substrate temperature, the values of the band gap of the thin films increase from 3.39 to 3.74 eV. The cathodoluminescence analysis of the Zn1-xVxO thin films elaborated with x ≤ 0.30 at 60 W and 200 °C show the dominant NBE emission. The presence of the latter is an indicator of the good crystallinity of the films. These results correlate well to the blueshift of transmittance spectra of these layers. In this study we have demonstrated the potential in the association of the sol-gel process and the rf-magnetron sputtering technique in achieving 30% of vanadium content in the ZnO matrix without secondary phases under mild growth conditions (200 °C and 60 W). The best quality of Zn1-xVxO thin films, with the largest optical band gap, the highest transparency and suitable values of resistivity was obtained at a vanadium content of 20% and at low substrate temperature and sputtering power, of respectively 200 °C and 60 W. These Zn1-xVxO thin films may find promising applications in a variety of optoelectronic devices.
Acknowledgements One of the authors (K. Medjnoun) has received financial support from the European Community under the contract 2010-2539/001-001-EMA2, Erasmus Mundus Partnership program, Action 2, Averroes 3. The authors also wish to thank Mr. J. L. GAUFFIER, Mrs. S. REYJAL and Mr. S. CAYEZ from INSA Toulouse (Laboratory of Physics and Chemistry of Nano-objects) as well as E. HERNANDEZ and H. GLENAT from PROMES-CNRS laboratory for their technical assistance.
References [1] Z. Ben Ayadi, L. El Mir, J. El Ghoul, K. Djessas, S. Alaya, Structural and optical properties of calcium doped zinc oxide sputtered from nanopwder target materials, Int. J. Nanoelectronics and Materials 3 (2010) 87-97. [2] F. Wu, L. Fang, K. Zhar, Y.J. Psm, L.P. Peng, Q.L. hauang, X.F. Your, C.Y. Kong, Effect of thickness on the properties of Ga-doped nano-ZnO thin films prepared by RF magnetron sputtering, J. Super. Cond. Nov. Magn. 23 (2010) 905-908. [3] T. Minemoto, T. Negami, S. Nishiwaki, H. Takakura, Y. Hamakawa, Preparation of Zn 1-xMgxO films by radio frequency magnetron sputtering, Thin Solid Films 372 (2000) 173-176. 10
[4] S. Gowrishankar, L. Balakrishnan, N. Gopalakrishnan, Band gap engineering in Zn1−xCdxO and Zn1−xMgxO thin films by RF sputtering, Ceram. Int. 40 (2014) 2135–2142. [5] B. Ogale Satischandra, Thin Films and Heterostructures for Oxide Electronics, Orlando Auciello & Ramamoorthy Ramesh, Springer Science, U.S.A, 2005. [6] Z. Liping, L. Yang, G. Yanmin, Z. Weishun, C. Ling, H. Haiping, Y. Zhizhen, Band gap modulation of ZnCdO alloy thin films with different Cd contents grown by pulsed laser deposition, J. Alloy. Compd. 547 (2013) 59–62. [7] L. El Mir, F. Ghribi, M. Hajiri, Z. Ben Ayadi, K. Djessas, M. Cubukan, H.J. Bardeleben, Multifunctional ZnO:V thin films deposited by rf-magnetron sputtering from aerogel nanopowder target material, Thin Solid Films 519 (2011) 5787-5791. [8] T.A. Gessert, J.N. Duenow, T. Barnes, T.J. Goutts, High quality doped ZnO thin films, Patent Application Publication (2001)10F4 US 2010/017/082A1. [9] L. Wang, L. Meng, V. Teixeira, S. Song, Z. Xu, X. Xu, Structural and optical properties of ZnO:V thin films with different doping concentration, Thin Solid Films 517 (2009) 3721-3725. [10] K. Lovchinov , H. Nicher, O. Agelou, M. Sendova-Vassileva , V. Mikg, D. Dimova-Malinovska, Structural, optical and electrical properties of V doped ZnO thin films deposited by rf-magnetron sputtering, J. Phys. Conf. Ser. 253 (2010) 012030. [11] W. Li-Wei, X. Zheng, M. Li- Jian, V. Teixeira, S. Shi-Gen, X.X. Rong, Influence of concentration of vanadium in zinc oxide on structural and optical properties with lower concentration, Chin. Phys. Lett. 26 (2009) No.7 077801. [12] T. Miyata, S. Suzuki, M. Ishii, T. Minami, New transparent conducting thin films using multicomponent oxides composed of ZnO and V2O5 prepared by magnetron sputtering, Thin Solid Films 411 (2002) 76-81. [13] L. Wang, L. Meng, V. Teixeirer, S. Song, F. Placido, J. Huang, Z. Xu, Study of ZnO:V thin films prepared by dc reactive magnetron sputtering at different pressures, 2
nd
IEEE International
Nanoelectronics Conference 2008. [14] K. Lovchinov, O. Angelov, H. Nivhev, V. Mikli, D. Dimova-Malinovska, Transparent and conductive ZnO thin films doped with V, Energy Procedia. 10 (2011) 282-286. [15] Z. Ben Ayadi, L. El Mir, K. Djessas, S. Alaya, Electrical and optical properties of aluminumdoped zinc oxide sputtered from an aerogel nanopowder target, Nanotechnology 18 (2007) 445702445707.
11
[16] S. Mensiri, C. Massingboon, V. Promarak, S. Seraphin, Synthesis and optical properties of nanocrystalline V-doped ZnO powders, Opt. Mater. 29 (2007) 1700-1705. [17] J. El ghoul, C. Barthou, L. El Mir, Synthesis, structural and optical properties of nanocrystalline vanadium doped zinc oxide aerogel, Physica E 44 (2012) 1910–1915. [18] A. Dhayal Raj, T. Pazhanivel, P. Suresh Kumar, D. Mangalaraj, D. Nataraj, N. Ponpandian, Self assembled V2O5 nanorods for gas sensors, Curr. Appl. Phys. 10 (2010) 531–537. [19] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley Reading, 1978 MA pp102. [20] A.A. Lotin, O.A. Novodvorsky, E.V. Khaydukov, V.N. Khaydukov, V.N. Rocheva, O.D. Khramova, V.Y. Panchenko, C. Wenzel, N. Trumpaicka, K.D. Chtcherachev, Epitaxial growth and properties of MgxZn1–xO films produced by pulsed laser deposition, Semiconductors 44 (2010) 246-250. [21] W. Hörig, A. Frieser, Exper. Techn. Phys. 19 (1971) 337. [22] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1976. [23] W. Yan , J. Tan, W. Zhang, Y. Liang, S. Feng, X. Lei, H.Wang, Spray pyrolysis derived ZnMgO:In thin films: Investigation of the structural, optical and electrical properties, Superlattice Microstruct. 60 (2013) 407–413. [24] A. El Hichou, M. Addou, A. Bougrine, R. Dounia, J. Ebothé, M. Troyon, M. Amrani, Cathodoluminescence properties of undoped and Al-doped ZnO thin films deposited on glass substrate by spray pyrolysis, Mater. Chem. Phys. 83 (2004) 43–47. [25] L. Xu, F. Gu, J. Su, Y. Chen, X. Li, X. Wang, The evolution behavior of structures and photoluminescence of K-doped ZnO thin films under different annealing temperatures, J. Alloy. Compd. 509 (2011) 2942–2947. [26] S. Yu, W. Zhang, L. Li, H. Dong, D. Xu, Y. Jin, Structural, electrical, photoluminescence and optical properties of n–type conducting, phosphorus-doped ZnO thin films prepared by pulsed laser deposition, Appl. Surf. Sci. 298 (2014) 44–49. [27] A.A.M. Farag, M. Cavaş, F. Yakuphanoglu, F.M. Amanullah, Photoluminescence and optical properties of nanostructure Ni doped ZnO thin films prepared by sol–gel spin coating technique, J. Alloy. Compd. 509 (2011) 7900–7908. [28] R. Swapna, M.C. Santhosh Kumar, Growth and characterization of molybdenum doped ZnO thin films by spray pyrolysis, J. Phys. Chem. Solids. 74 (2013) 418–425.
12
[29] P. Chand, A. Gaur, A. Kumar, U. Kumar Gaur, Structural, morphological and optical study of Li doped ZnO thin films on Si (100) substrate deposited by pulsed laser deposition, Ceram. Int. 40 (2014) 11915–11923. [30] Yu-Lin, Chiung-Ching Lin, Jyh-Ming Wu, Uei-Shin Chen, Jiann-Ruey Chen, Han C. Shih, Formation and cathodoluminescence of Al:ZnO nanoscrew clusters, Thin Solid Films 517 (2008) 1225–1229. [31] A. Singh, D. Kumar, P.K. Khanna, A. Kumar, M. Kumar, Anomalous behavior in ZnMgO thin films deposited by sol-gel method, Thin Solid Films 519 (2011) 5826-5830. [32] S. S. Kurbanov, Photo- and Cathodoluminescence Studies of ZnO-Filled Opal Nanocomposites, J. Korean Phys. Soc. 50 (2007) 617-621. [33] S. Sharma, S. Vyas, C. Periasamy, P. Chakrabarti, Structural and optical characterization of ZnO thin films for optoelectronic device applications by RF sputtering technique, Superlattice Microstruct. 75 (2014) 378–389. [34] T. Minemoto, T. Negami, S. Nishiwaki, H. Takakura, Y. Hamakawa, Preparation of Zn1−xMgxO films by radio frequency magnetron sputtering, Thin Solid Films 372 (2000) 173-176.
13
Table caption
Table 1. EDS analysis of Zn1-xVxO nanoparticles. Table 2. The grain size (G), mean roughness (Ra) of Zn1-xVxO thin films with different vanadium contents deposited at RT, 200 °C and 60 W. Table 3. EDS analysis of the Zn1-xVxO thin films deposited at RT, 200 °C and 60 W. Table 4. Optical band gap energy of the Zn1-xVxO thin films deposited at RT, 200 °C and 60 W.
Figure caption
Fig. 1. X-ray diffraction patterns of Zn1-xVxO aerogel nanoparticles for different vanadium contents. Fig. 2. TEM photographs showing the morphology of Zn1-xVxO nanoparticles at different vanadium contents. Fig. 3. X-ray diffraction patterns of Zn1-xVxO thin films with different vanadium contents deposited at RT and 200 °C, at sputtering powers of (a) 60 W, (b) 80 W and (c) 100 W. Fig. 4. Variation of c-axis length of Zn1-xVxO thin films deposited at 200 °C and 60 W as a function of different vanadium contents. Fig. 5. Chemical analysis spectra of Zn0.80V0.20O thin films deposited at 200 °C and 60 W. Fig. 6. 3D-AFM images (1x1µm2) of Zn1-xVxO thin films with different vanadium contents deposited at 60 W, (a, b, c) at RT and (d, e, f) at 200 °C. Fig. 7. SEM cross sectional images of Zn1-xVxO thin film obtained at 60 W and 200 °C. Fig. 8. Optical transmittance spectra in the UV-Vis-NIR regions of (a) Zn1-xVxO thin films with different vanadium contents deposited at 60 W and (b) Zn0.80V0.20O film elaborated at different sputtering powers. In both cases, the films were grown at RT and 200 °C. Fig. 9. Plots of (αhν)2 against photon energy (hν) of (a) Zn1-xVxO thin films with different vanadium contents deposited at 60 W and (b) Zn0.80V0.20O film elaborated at different sputtering powers. In both cases, the films were grown at RT and 200 °C. Fig. 10. Cathodoluminescence spectra of the Zn1-xVxO thin films with (x = 0, 0.10, 0.20, 0.30) grown at 200 °C and 60 W.
14
Table 1. EDS analysis of Zn1-xVxO nanoparticles.
x 0 0.10 0.20 0.30 0.50
Elemental compositions (at.%) Zn V O 49 0 51 46 5 49 42 12 46 38 20 42 28 33 39
Table 2. The grain size (G), mean roughness (Ra) of Zn1-xVxO thin films with different vanadium contents deposited at RT, 200 °C and 60 W.
x 0 0.10 0.20 0.30 0.50
RT 20 6 5 4 4
G (nm) 200 °C 22 10 13 10 8
RT 0.7 2 0.9 0.6 0.9
Ra (nm) 200 °C 1.4 0.5 0.2 0.2 0.2
15
Table 3. EDS analysis of the Zn1-xVxO thin films deposited at RT, 200 °C and 60 W.
x Zn 51 45 44 43 34
0 0.10 0.20 0.30 0.50
Elemental compositions (at.%) RT 200 °C V O Zn V 0 49 49 0 6 49 44 6 11 45 40 10 13 44 36 15 26 40 33 27
O 51 50 50 49 40
Table 4. Optical band gap energy of the Zn1-xVxO thin films deposited at RT, 200 °C and 60 W.
x 0 0.10 0.20 0.30 0.50
Band gap Eg (eV) RT 200 °C 3.27 3.29 3.44 3.50 3.42 3.74 3.40 3.71 3.39 3.54
16
Highlights
High quality of nanostructured Zn1-xVxO thin films obtained with high V content.
High transmittance for all the films from the visible to the NIR region.
The band gap of Zn1-xVxO films can be adjusted by varying the V content.
The cathodoluminescence reveals intense Near Band Edge Emission (NBE).
A promising material for optoelectronic devices.
17
S0749-6036(15)00097-X http://dx.doi.org/10.1016/j.spmi.2015.02.019 YSPMI 3620
To appear in:
Superlattices and Microstructures
Received Date: Accepted Date:
31 January 2015 4 February 2015
Please cite this article as: K. Medjnoun, K. Djessas, M.S. Belkaid, S. Grillo, A. Solhy, O. Briot, M. Moret, Characteristics of nanostructured Zn1-xVxO thin films with high vanadium content elaborated by rf-magnetron sputtering, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi.2015.02.019
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Characteristics of nanostructured Zn1-xVxO thin films with high vanadium content elaborated by rf-magnetron sputtering K. Medjnouna,b,c, K. Djessasa,b,*, M. S. Belkaidc, S. Grilloa,b, A. Solhyd, O. Briote, M. Morete a
Laboratoire Procédés, Matériaux et Energie Solaire (PROMES)-CNRS, Tecnosud, Rambla de la thermodynamique, 66100 Perpignan, France. b
Université de Perpignan Via Domitia, 52 avenue Paul Alduy, 68860, Perpignan Cedex9, France.
c
Laboratoire des Technologies Avancées du Génie Electrique (LATAGE), Université Mouloud Mammeri de TiziOuzou (UMMTO), B.P N° 17 R.P Tizi-Ouzou, Algérie. d
Mohammed VI Polytechnic University. Lot 660 - Hay Moulay Rachid. 43150 Ben Guerir, Morocco.
e
CNRS, Université Montpellier 2, Laboratoire Charles Coulomb UMR 5221, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France. *Corresponding author: K. Djessas. E-mail address: [email protected] Tel: 0033468682268
Abstract Nanostructured Zn1-xVxO (0 ≤ x ≤ 0.50) thin films were synthesized by rf-magnetron sputtering at two different substrate temperatures (room temperature (RT) and 200 °C) and with variable sputtering powers (60, 80 and 100 W). In this method, single targets based on Zn 1-xVxO nanopowders prepared by the sol-gel process were used. Characterization of the Zn1-xVxO nanoparticles showed that they crystallize in the hexagonal wurtzite structure. Their size ranged from 20 to 40 nm. The effect of process parameters on the physical and chemical properties of Zn1-xVxO thin films has been studied. For x ≤ 0.30, the results obtained at 200 °C and 60 W indicate that the films have a high quality of crystallinity. Vegard’s law is respected, indicating that vanadium is incorporated in the ZnO matrix. The chemical compositions of these films were found to be close to the stoichiometry. The films exhibit a columnar structure and a smooth surface. Their average transmission, from the visible to the NIR, was in the range of 75 to 90%. The values of the band gap of the Zn1-xVxO thin films with x ≤ 0.30 and elaborated at 200 °C and 60 W, vary from 3.29 to 3.74 eV. This is consistent with blue shifting of near-band edge cathodoluminescence emission. Under particular growth conditions, the investigation shows that the Zn0.80V0.20O sample presents the best properties for potential use in various optoelectronic applications, namely: a single wurtzite phase, low surface roughness (Ra ~ 0.2 nm), a high transparency of 90% in the UV-Vis-NIR, a wide band gap of 3.74 eV and a resistivity of ~ 5x10+3 Ω.cm. Keywords: Zn1-xVxO; Nanoparticles; Sol-gel; Thin films; Sputtering.
1
1. Introduction Zinc Oxide (ZnO) is a semiconductor material with a wide direct band gap (Eg ≥3.3 eV) at room temperature and an excitonic binding energy (60 meV). It is commercially available because of its advantages, which include its abundance in nature, nontoxicity, easy fabrication and doping and high chemical stability [1-3]. Recently, considerable attention has been focused on the development of this material for various applications, such as electrochromic and optoelectronic devices. As reported by S. Gowrishankar [4], modulation of the band gap is one of the major requirements in designing optoelectronic devices. Reports in the literature have shown that the ZnO band gap can be engineered by alloying this material with three major elements: Mg [4-5], V [5] and Cd [5-6]. Mg and V are known to broaden the band gap, whereas Cd is known to narrow it [5]. Magnetron sputtering has been used by most researchers to grow ZnO thin films alloyed by Mg, V and Cd in order to tailor the band gaps. This is due to the many advantages of this technique, such as its simplicity and low cost. It is considered to be the most favorable deposition method to obtain highly uniform films with high packing density and a strong adhesion at high deposition rate, without needing a high deposition temperature nor toxic gases. On the other hand, it is worth noting that little research on the development and study of the structural and optoelectronic properties of Zn 1-xVxO thin films has been reported in the literature [7-14]. Some of the films with low vanadium content are obtained by magnetron sputtering and find their applications as Transparent Conducting Oxides (TCO) [8,10,12,14]. However, most studies on the fabrication of Zn1-xVxO thin films used a mixture of ZnO with hard vanadium [10,14] or powders of V2O5 [12] as targets. In the present work, we used Zn1-xVxO nanoparticles, synthesized by the sol gel process, with high vanadium content as targets in order to elaborate Zn1-xVxO thin films by rf-magnetron sputtering. To the best of our knowledge, this is the first time that this approach has been developed for the growth of Zn1-xVxO thin films alloys with high vanadium content. A detailed study of the structural, morphological, optical and electrical properties of the Zn1-xVxO thin films was therefore carried out. The experimental growth conditions of Zn1-xVxO thin films were optimized by varying the vanadium concentration from 0 to 50%. It was found that the resistivity, transmission and band-gap of these thin films can be influenced by the vanadium content and the growth conditions.
2. Experimental 2.1. Materials synthesis 2.1.1. Nanoparticles preparation It should be emphasized that the Zn1-xVxO nanoparticles have been prepared, even at the higher vanadium concentrations, by adopting the conventional method described previously [7,15]. All 2
reagents were purchased from Aldrich Chemical Company and were used without further purification. Synthesis of the samples was performed by the sol-gel process using zinc acetate dehydrate [Zn(CH3COO)2.2H2O] and ammonium metavanadate in various V/Zn ratios of 0, 0.10, 0.20, 0.30 and 0.50 with methanol as solvent. These reagents were mixed, treated in the autoclave and dried under supercritical conditions of ethyl alcohol (EtOH). Finally, a post-synthesis treatment was carried out by calcining the sample at 500 °C in air for 2 hours.
2.1.2. Thin films deposition The Zn1-xVxO thin films were deposited on p-type Si (100) and glass substrates by rf-magnetron sputtering (13.56 MHz- Cezar Rf-power Generator). The sputtering targets were prepared from Zn1xVxO
nanoparticles. The distance between the substrate and the targets was fixed at 75 mm. The
sputtering chamber was evacuated to a base pressure of about 10 -5 mbar, before introducing the sputtering argon gas at high purity and with a flow rate of 11 sccm. The sputtering deposition was carried out at a pressure of 10-3 mbar. Two substrate temperatures were used: RT and 200 °C. The rfsputtering power of the Zn1-xVxO targets was varied from 60 to 100 W in steps of 20 W. The deposition time was varied in order to achieve films with a thickness of ~ 200 nm.
2.2. Characterisation methods The crystalline properties of the Zn1-xVxO nanoparticles and the thin films were studied with an X-ray diffractometer (Philips PW 1729), using Cu-Kα radiation (λ = 0.15406 nm) in Bragg-Brentano geometry (θ-2θ) in the range of 10°-60°. The morphology and size of the Zn1-xVxO nanoparticles were inspected with a Transmission Electron Microscope (TEM, JEM-200CX), operating at 100 kV and using C-coated Cu grids. The chemical composition of the nanoparticles and thin films were obtained by Energy Dispersive X-ray Spectroscopy (EDS, JEOL JSM 5310 LV). The surface morphology of the films was studied using an Atomic Force Microscope (AFM, NT-MDT/SMENA) operating in the intermittent contact mode (325 kHz) with Si cantilevers of 46 N/m spring constant and tip radius <10 nm. A Field Emission Scanning Electron Microscope (SEM-FEG, Hitachi S-4500) operating at 10 kV was employed to observe the cross-section of the films and to determine their thickness. The latter measurements were confirmed by profilometry (Decktak XT). The optical transmittance of the films was measured with a UV-Vis-NIR spectrophotometer (Shimadzu-UV3101PC). Room temperature cathodoluminescence (CL) measurements were carried out using Scanning Electron Microscope SEM (JEOL JSM-840A) operating at an accelerating voltage of 30 kV. The CL system consisted of a parabolic mirror directing the emitted photons onto an Oriel cornerstone 130 monochromator followed by PM:180 photomultiplier. The luminescence emissions were detected by a Hamamatsu Monobloc 3
detector in the range of 180-900 nm. Electrical resistivity measurements were carried out using the four-point probe method with LUCAS LABS-302 System.
3. Results and discussion 3.1. Structural properties of the Zn1-xVxO nanoparticles Fig. 1 shows a series of X-ray diffraction (XRD) patterns of Zn1-xVxO nanoparticles synthesized by varying the x values between 0 and 0.50. According to the JCPDS database (card number 36-1451), the peaks corresponding to the (100), (002), (101), (102) and (110) reflexion planes denote a hexagonal wurtzite ZnO structure. When the vanadium content in the ZnO matrix is increased, the degree of crystallinity of the nanoparticles is found to decrease. Indeed, at x = 0.20, other peaks which do not belong to the wurtzite structure begin to appear and their relative intensity increases with vanadium content. These secondary phases can be attributed to the presence of V2O5, VO2 and Zn3(VO4) compounds. This is probably due to the limited solubility of the vanadium atoms substituting the Zn sites of the ZnO matrix. These results are in good agreement with the trends described in references [16-18]. Fig. 2 shows TEM photographs of the Zn1-xVxO nanoparticles. At vanadium contents below 30%, the crystallites exhibit a mixture of shapes (mainly hexagonal and spherical), with a narrow particle size distribution. The size of the Zn1-xVxO nanoparticles was estimated in the range of 20 - 40 nm. From 30% of vanadium content onwards, particles in the shape of sticks can be observed, concomitantly with degradation in the crystallization of the nanoparticles. The sticks may consist of one or more of the secondary phases discussed above. EDS analysis was performed to measure the chemical compositions of Zn 1-xVxO nanoparticles with x varying from 0 to 0.50. The results are shown in Table 1. The elemental compositions values obtained for the Zn1-xVxO nanoparticles with x < 0.20 are quasi-stoichiometric. This confirms the presence of vanadium in the matrix of ZnO. For the higher x values, the stoichiometry of the Zn 1-xVxO nanoparticles was not respected. These results are in agreement with those of XRD presented above.
3.2. Structural properties of Zn1-xVxO thin films The XRD patterns of Zn1-xVxO films deposited by rf-magnetron sputtering on the p-type Si (100) substrate at RT and at 200 °C, at different sputtering powers of 60, 80 and 100 W are presented in Fig. 3a-c. It should be kept in mind that the (002) peaks are convoluted in these images with a peak at 32.90° due to the Si substrate. It can be seen from these results that all the films showed a polycrystalline hexagonal wurtzite structure with a preferential c-axis orientation. In addition, in the 4
films deposited at RT with a vanadium content of 20, 30 and 50%, the orthorhombic V2O5 structure can be observed. This is denoted by the (101) and (121) peak, respectively at 22.4° and 55.85°, as confirmed by the crystallographic data JCPDS 41,1426. An increase in the intensity of the (121) peak with increasing sputtering power can be observed in the sample with 30% of vanadium content. This orientation disappears completely by increasing the substrate temperature to 200 °C for all cases of the films sputtered at 60, 80 and 100 W. The V2O5 phase is totally absent for the films with x ≤ 0.30 obtained at 200 °C and 60 W. This indicates that the crystallinity of Zn 1-xVxO thin films in these conditions is improved. The grains size G in the Zn1-xVxO films can be calculated using the Debye-Scherrer’s formula [19]:
where λ is the X-ray wavelength, θB is the Bragg diffraction angle and B is the full width at half maximum (FWHM) of the diffraction peak, after correction for the instrument broadening. The values of the FWHM of the (002) peak of the Zn1-xVxO films prepared at 200 °C are significantly lower than those of the films grown at RT, implying that at 200 °C the crystal size is greater. The overall observed trend is that the FWHM of the (002) peak increases with the increase in vanadium content and sputtering power. Besides, as the vanadium content increases, the (002) peak slightly shifts to a lower diffraction angle. This shift means that the spacing of the planes becomes larger as the vanadium content increases. The images show that the highest degree of crystallinity of the films with x ≤ 0.30 was obtained at a sputtering power of 60 W and 200 °C. The calculated grain sizes of the films elaborated at 60 W with different vanadium contents are shown in Table 2. As revealed in this table, crystallite sizes of the films deposited at 200 °C are larger than those elaborated at RT. These results confirm the improvement of the crystallinity of the films elaborated at 200 °C. The lattice constant, c, of the wurtzite structure of the films elaborated at 200 °C and 60 W has thus been calculated, and is plotted against the variation of vanadium content from 0 to 0.5 at Fig. 4. The variation of the c-axis length constant with vanadium content is found to be approximately linear as the latter is increased from 0 to 0.3, in agreement with Vegard’s law. Thus the vanadium ions appear to substitute the Zn ions in the film without changing its wurtzite structure. This is consistent with the fact that no secondary phases are observed in this case.
5
3.3. Composition analysis of Zn1-xVxO thin films The chemical compositions of the Zn1-xVxO thin films deposited on silicon substrate at 60 W and at substrate temperatures of RT and 200 °C were analyzed by EDS. The results of the compositional analysis of all the films are summarized in Table 3. One can observe that the quasistoichiometry of the Zn1-xVxO thin films is only respected in the case of those obtained with x < 0.20 and x ≤ 0.30 at RT and 200 °C respectively. These results correlate well with those reported by XRD above. Fig. 5 shows as an example the EDS spectrum of the as deposited Zn 0.80V0.20O thin film at 200 °C (and 60 W). It reveals that no other elements coming from the solvents, the precursors or the substrate - beside Zn, V and O - are present in the films.
3.4. Morphological properties of Zn1-xVxO thin films Fig. 6 shows the 3D-AFM images (1 x 1 µm2 ) of Zn1-xVxO thin films with x = 0, 0.20 and 0.50, deposited onto the silicon substrates at two different substrate temperatures (RT and 200 °C) and 60 W. The deposited films exhibit granular surfaces. The images show that when the vanadium content increases, the films' surface roughness decreases. The films appear to be smooth and continuous without pinholes nor cracks. The low surface roughness could be explained as a consequence of coalescence of the grains of the films. The surface morphology of the films elaborated at 200 °C becomes very uniform and more compact. The volatilization of the Zn 1-xVxO nanoparticles may be enhanced by increasing the substrate temperature. One could say that the sputtered nanoparticles have enough energy to diffuse, resulting in the formation of a more compact film. This hypothesis would imply that the crystallinity of the alloys improves as the substrate temperature is increased. Indeed, this is in agreement with the findings of XRD analysis presented above. The calculated values of the mean roughness (Ra) of the samples, given in Table 2, were in the range between 0.2 and 2 nm. As underlined in the studies by Lotin et al. [20] and Li-Wei et al. [11], a high smoothness of the film surfaces is important in order to obtain a good matching of the crystal lattice parameters and to minimize the recombination probabilities in heterostructures. The cross-sectional SEM images of the Zn1-xVxO thin films are shown in Fig. 7. From these images the films thickness was estimated to be approximately 200 nm. This value was in agreement with profilemeter measurements. The films exhibit a uniform grain size with a typical dense columnar structure and a very smooth surface. By comparison to the XRD spectra, it can be concluded that the Zn1-xVxO films grew along the (002) orientation, which is perpendicular to the substrate surface. Also consistent with the XRD results is the observation that the size of the columnar grains decreased as the concentration of vanadium was increased. This finding was furthermore confirmed by AFM images of the surface morphology. 6
3.5. Optical and electrical properties of Zn1-xVxO thin films The transmittance spectra of the Zn1-xVxO (0 ≤ x ≤ 0.50) films deposited with different vanadium contents, at sputtering power of 60 W and substrate temperatures of RT and 200 °C are shown in Fig. 8a. An interference phenomenon can be clearly observed in the spectra, indicating a smooth and homogenous surface of the film. The average transmittance of all the films in the wavelength range from the absorption edge to the NIR was found to be between 75 and 90%. As can be seen in the inset of Fig. 8a, a blue shift of the absorption edge was observed for all the Zn 1-xVxO films elaborated with different vanadium contents under a sputtering power of 60 W at RT and 200 °C. The maximal absorption blue shift of Zn1-xVxO corresponds to the film elaborated with 20% of vanadium content at 200 °C. This result confirms that the widest band gap amongst the Zn1-xVxO films studied is attributed to the one with 20% of vanadium content, elaborated at a substrate temperature of 200 °C and sputtering power of 60 W. It can also be clearly seen that this film has the highest average transmittance in the NIR region (~ 90%). Fig. 8b shows the optical transmission spectra in the UVVis-NIR regions of Zn0.80V0.20O films elaborated at RT and 200 °C with different sputtering powers of 60, 80 and 100 W. All the films were highly transparent (85 to 90%) from the absorption edge to the NIR region. We also observe a gradual shift of the absorption edge of the films towards shorter wavelengths when the substrate temperature increases from RT to 200 °C. The inset of Fig. 8b clearly reveals the higher transmittance in the visible range with lower absorption loss in the case of the films elaborated at 200 °C with 60 and 80 W of sputtering power. However, the XRD results showed that the best crystallinity of the Zn0.80V0.20O films was obtained at 60 W. We can thus conclude that the structural and transmission properties of the Zn1-xVxO thin films are optimized at 20% of vanadium content, sputtering power of 60 W and 200 °C growth conditions. The energy band gap values of Zn1-xVxO thin films were calculated using the equations given below [21-22]:
1
(h ) C (h E g ) 2
(3)
where α is the absorption coefficient, T and R are the transmittance and reflectance and d is the film thickness. Eg is the direct band gap energy and C is a constant. Fig. 9a and 9b show respectively the plot of (αhν)2 as a function of photon energy (hν) for Zn1xVxO
(0 ≤ x ≤ 0.5) thin films with different vanadium contents deposited at 60 W and for a
Zn0.80V0.20O film elaborated at different sputtering powers (in both cases, the films were grown at RT and 200 °C). The band gap energy values can be determined by plotting the square of the absorption 7
coefficient and then extrapolating to (αhν) 2 = 0 the linear part of the curve. As can be seen in Table 4, the band gap of Zn1-xVxO thin films deposited at RT decreases slightly from 3.44 to 3.39 eV as the vanadium content is increased from x = 0.10 to 0.50. This effect was possibly caused by the secondary phase segregation. However, for the films deposited at 200 °C, the band gap increases up to 3.74 eV at x = 0.20 and then decreases to 3.54 eV at x = 0.50 of vanadium content. The band gap of undoped ZnO thin films is not influenced significantly by the substrate temperature (Eg = 3.27 eV at RT, and 3.29 eV at 200 °C). It can be concluded that the band gap width of the Zn1-xVxO thin films can be modulated by the variation of the vanadium content (0.10 ≤ x ≤ 0.50) and substrate temperatures. Fig. 9b shows the effect of sputtering power on the value of the band gap of the Zn0.80V0.20O film. It confirms that the properties are optimized for the film deposited at 60 W, which possesses the widest band gap of 3.74 eV.
A study of cathodoluminescence measurements can provide valuable information on the quality and purity of the material. The distribution of defects and impurities can be also revealed. It is acknowledged that there exist two main emission bands in ZnO photoluminescence or cathodoluminescence spectra, Near Band Edge (NBE) and Deep-Level (DL) emissions, which appear respectively in the ultraviolet (UV) around 380 nm and in visible region, i.e. blue-green (520 nm) and red (650 nm) [23,24]. (NBE) emission of ZnO is namely related to the recombination of the free excitons [25,26], and (DL) emission has been attributed to several defects in the crystal structure such as O-vacancy (VO), Zn-vacancy (VZn), O-interstitial (Oi) and Zn-interstitial (Zni) [4,27,28,29,30]. A. Singh et al. showed negligible intensity in the green emission for ZnMgO thin films annealed at 300 °C to 500 °C, but for higher annealing temperature the intensity of this luminescence was increased. This phenomenon has been related to the oxygen vacancies [31]. S. S. Kurbanov reported that the high UV/green emissions intensity ratio indicates that the samples are of good quality [32]. For these reasons, the layers with the best structural and morphological properties, elaborated at a sputtering power of 60 W and substrate temperature of 200 °C have been selected for cathodoluminescence studies in order to confirm the results obtained above. Cathodoluminescence (CL) spectra of Zn1-xVxO (x = 0, 0.10, 0.20, 0.30) films deposited under these conditions were recorded in the wavelength range of 300-700 nm. The obtained CL spectra are shown in Fig. 10. In our case, all the films exhibit strong bands located in the UV region, which originate from the NBE emission. The latter is considered to be a dominant emission. A weak Green Band (GB) emission is also observed for x = 0.30 at 2.32 eV. This emission has been tentatively attributed in the literature to singly ionized oxygen vacancies (VO), zinc interstitials (Zni) [4] and zinc vacancies (VZn) [33]. Compared to the NBE emission, the DL emission is only just observed. This is probably due to fewer native defects in Zn1-xVxO films, as mentioned in the literature [23]. The weak DL emission intensity 8
can be interpreted in terms of the V atom being substitutionally incorporated, inducing the 2.6% change of the lattice constant observed in our data for the c parameter in Fig. 4; however higher V content lead to deviations from Vegard's law. This indicates that V atoms are probably not included in the lattice sites by substitution, and results in the appearance of deep levels and deep broad bands in the luminescence. It is also clearly observed that the NBE emission shifted toward lower wavelengths for the samples with increasing vanadium content up to x ≤ 0.20. This can be attributed to the change of the band gap with V substitution, up to a point where further V addition does not lead to substitution on the host lattice and generates deep levels. Another possibility would be that the band gap changes due to the strain induced by V incorporation, as Fig. 4 shows a large shift of the c parameter upon V incorporation. However, the c parameter is increased by V incorporation. This can hardly correspond to compressive strain, the layer being polycrystalline. The strain effect, if not compressive should then decrease the band gap, and redshift the transitions. The Burstein-Moss effect has to be ruled out because the resistivities of the layers increase with increasing V content, thus demonstrating that the free carrier concentration is decreasing with V incorporation. In our study, Zn1-xVxO thin films deposited at a sputtering power of 60 W exhibit the highest resistivities. The resistivity of the films obtained at RT increases from 0.50 to 110 Ω.cm when the vanadium content is increased from x = 0.10 to 0.20. But by increasing the substrate temperature to 200 °C in the same range of vanadium content, we find that the resistivity increases even more, from 5 to 5x103 Ω.cm. The resistivities of Zn0.70V0.30O and Zn0.50V0.50O thin films could not be measured because their values exceed the limit of the measuring system [34]. Gowrishankar et al. [4] have explained in their studies that the increase in the resistivities of Zn1−xCdxO and Zn1−xMgxO thin films with the increase of the Cd/Mg content might be due to the segregation of Cd/Mg atoms in the grain boundaries, which in turn increases the grain boundary barrier. This leads to enhanced scattering of charge carriers at grain boundaries, which could explain the increase in resistivity.
4. Conclusion Nanostructured Zn1-xVxO thin films with high vanadium content have been synthesized by rfmagnetron sputtering at low substrate temperatures. The targets, for all of the vanadium contents tested, were based on Zn1-xVxO nanoparticles prepared via the sol-gel method. The effect of vanadium content, sputtering power and substrate temperature on the crystal structure, morphological, optical and electrical properties of the Zn1-xVxO films have been discussed in detail. It has been demonstrated that for up to 30% of vanadium content, the Zn1-xVxO thin films with single Wurtzite phase are successfully obtained at growth conditions of 60 W and 200 °C, and that Vegard’s law is respected. It should be noted here that all the films were oriented preferentially along the (002) direction. EDS analysis reveals that there are no other elements beside Zn, V and O in the Zn1-xVxO films. The quasi9
stoichiometry of these films was also obtained. Both XRD and AFM results show that there is an increase in the grain size when the substrate temperature is increased from RT to 200 °C. It was also observed that the density of the films increases and their surface becomes smoother at 200 °C. The average transmittance of all Zn1-xVxO thin films, from the visible to the NIR region, is in the range 7590%. By varying the vanadium content and the substrate temperature, the values of the band gap of the thin films increase from 3.39 to 3.74 eV. The cathodoluminescence analysis of the Zn1-xVxO thin films elaborated with x ≤ 0.30 at 60 W and 200 °C show the dominant NBE emission. The presence of the latter is an indicator of the good crystallinity of the films. These results correlate well to the blueshift of transmittance spectra of these layers. In this study we have demonstrated the potential in the association of the sol-gel process and the rf-magnetron sputtering technique in achieving 30% of vanadium content in the ZnO matrix without secondary phases under mild growth conditions (200 °C and 60 W). The best quality of Zn1-xVxO thin films, with the largest optical band gap, the highest transparency and suitable values of resistivity was obtained at a vanadium content of 20% and at low substrate temperature and sputtering power, of respectively 200 °C and 60 W. These Zn1-xVxO thin films may find promising applications in a variety of optoelectronic devices.
Acknowledgements One of the authors (K. Medjnoun) has received financial support from the European Community under the contract 2010-2539/001-001-EMA2, Erasmus Mundus Partnership program, Action 2, Averroes 3. The authors also wish to thank Mr. J. L. GAUFFIER, Mrs. S. REYJAL and Mr. S. CAYEZ from INSA Toulouse (Laboratory of Physics and Chemistry of Nano-objects) as well as E. HERNANDEZ and H. GLENAT from PROMES-CNRS laboratory for their technical assistance.
References [1] Z. Ben Ayadi, L. El Mir, J. El Ghoul, K. Djessas, S. Alaya, Structural and optical properties of calcium doped zinc oxide sputtered from nanopwder target materials, Int. J. Nanoelectronics and Materials 3 (2010) 87-97. [2] F. Wu, L. Fang, K. Zhar, Y.J. Psm, L.P. Peng, Q.L. hauang, X.F. Your, C.Y. Kong, Effect of thickness on the properties of Ga-doped nano-ZnO thin films prepared by RF magnetron sputtering, J. Super. Cond. Nov. Magn. 23 (2010) 905-908. [3] T. Minemoto, T. Negami, S. Nishiwaki, H. Takakura, Y. Hamakawa, Preparation of Zn 1-xMgxO films by radio frequency magnetron sputtering, Thin Solid Films 372 (2000) 173-176. 10
[4] S. Gowrishankar, L. Balakrishnan, N. Gopalakrishnan, Band gap engineering in Zn1−xCdxO and Zn1−xMgxO thin films by RF sputtering, Ceram. Int. 40 (2014) 2135–2142. [5] B. Ogale Satischandra, Thin Films and Heterostructures for Oxide Electronics, Orlando Auciello & Ramamoorthy Ramesh, Springer Science, U.S.A, 2005. [6] Z. Liping, L. Yang, G. Yanmin, Z. Weishun, C. Ling, H. Haiping, Y. Zhizhen, Band gap modulation of ZnCdO alloy thin films with different Cd contents grown by pulsed laser deposition, J. Alloy. Compd. 547 (2013) 59–62. [7] L. El Mir, F. Ghribi, M. Hajiri, Z. Ben Ayadi, K. Djessas, M. Cubukan, H.J. Bardeleben, Multifunctional ZnO:V thin films deposited by rf-magnetron sputtering from aerogel nanopowder target material, Thin Solid Films 519 (2011) 5787-5791. [8] T.A. Gessert, J.N. Duenow, T. Barnes, T.J. Goutts, High quality doped ZnO thin films, Patent Application Publication (2001)10F4 US 2010/017/082A1. [9] L. Wang, L. Meng, V. Teixeira, S. Song, Z. Xu, X. Xu, Structural and optical properties of ZnO:V thin films with different doping concentration, Thin Solid Films 517 (2009) 3721-3725. [10] K. Lovchinov , H. Nicher, O. Agelou, M. Sendova-Vassileva , V. Mikg, D. Dimova-Malinovska, Structural, optical and electrical properties of V doped ZnO thin films deposited by rf-magnetron sputtering, J. Phys. Conf. Ser. 253 (2010) 012030. [11] W. Li-Wei, X. Zheng, M. Li- Jian, V. Teixeira, S. Shi-Gen, X.X. Rong, Influence of concentration of vanadium in zinc oxide on structural and optical properties with lower concentration, Chin. Phys. Lett. 26 (2009) No.7 077801. [12] T. Miyata, S. Suzuki, M. Ishii, T. Minami, New transparent conducting thin films using multicomponent oxides composed of ZnO and V2O5 prepared by magnetron sputtering, Thin Solid Films 411 (2002) 76-81. [13] L. Wang, L. Meng, V. Teixeirer, S. Song, F. Placido, J. Huang, Z. Xu, Study of ZnO:V thin films prepared by dc reactive magnetron sputtering at different pressures, 2
nd
IEEE International
Nanoelectronics Conference 2008. [14] K. Lovchinov, O. Angelov, H. Nivhev, V. Mikli, D. Dimova-Malinovska, Transparent and conductive ZnO thin films doped with V, Energy Procedia. 10 (2011) 282-286. [15] Z. Ben Ayadi, L. El Mir, K. Djessas, S. Alaya, Electrical and optical properties of aluminumdoped zinc oxide sputtered from an aerogel nanopowder target, Nanotechnology 18 (2007) 445702445707.
11
[16] S. Mensiri, C. Massingboon, V. Promarak, S. Seraphin, Synthesis and optical properties of nanocrystalline V-doped ZnO powders, Opt. Mater. 29 (2007) 1700-1705. [17] J. El ghoul, C. Barthou, L. El Mir, Synthesis, structural and optical properties of nanocrystalline vanadium doped zinc oxide aerogel, Physica E 44 (2012) 1910–1915. [18] A. Dhayal Raj, T. Pazhanivel, P. Suresh Kumar, D. Mangalaraj, D. Nataraj, N. Ponpandian, Self assembled V2O5 nanorods for gas sensors, Curr. Appl. Phys. 10 (2010) 531–537. [19] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley Reading, 1978 MA pp102. [20] A.A. Lotin, O.A. Novodvorsky, E.V. Khaydukov, V.N. Khaydukov, V.N. Rocheva, O.D. Khramova, V.Y. Panchenko, C. Wenzel, N. Trumpaicka, K.D. Chtcherachev, Epitaxial growth and properties of MgxZn1–xO films produced by pulsed laser deposition, Semiconductors 44 (2010) 246-250. [21] W. Hörig, A. Frieser, Exper. Techn. Phys. 19 (1971) 337. [22] J.I. Pankove, Optical Processes in Semiconductors, Dover, New York, 1976. [23] W. Yan , J. Tan, W. Zhang, Y. Liang, S. Feng, X. Lei, H.Wang, Spray pyrolysis derived ZnMgO:In thin films: Investigation of the structural, optical and electrical properties, Superlattice Microstruct. 60 (2013) 407–413. [24] A. El Hichou, M. Addou, A. Bougrine, R. Dounia, J. Ebothé, M. Troyon, M. Amrani, Cathodoluminescence properties of undoped and Al-doped ZnO thin films deposited on glass substrate by spray pyrolysis, Mater. Chem. Phys. 83 (2004) 43–47. [25] L. Xu, F. Gu, J. Su, Y. Chen, X. Li, X. Wang, The evolution behavior of structures and photoluminescence of K-doped ZnO thin films under different annealing temperatures, J. Alloy. Compd. 509 (2011) 2942–2947. [26] S. Yu, W. Zhang, L. Li, H. Dong, D. Xu, Y. Jin, Structural, electrical, photoluminescence and optical properties of n–type conducting, phosphorus-doped ZnO thin films prepared by pulsed laser deposition, Appl. Surf. Sci. 298 (2014) 44–49. [27] A.A.M. Farag, M. Cavaş, F. Yakuphanoglu, F.M. Amanullah, Photoluminescence and optical properties of nanostructure Ni doped ZnO thin films prepared by sol–gel spin coating technique, J. Alloy. Compd. 509 (2011) 7900–7908. [28] R. Swapna, M.C. Santhosh Kumar, Growth and characterization of molybdenum doped ZnO thin films by spray pyrolysis, J. Phys. Chem. Solids. 74 (2013) 418–425.
12
[29] P. Chand, A. Gaur, A. Kumar, U. Kumar Gaur, Structural, morphological and optical study of Li doped ZnO thin films on Si (100) substrate deposited by pulsed laser deposition, Ceram. Int. 40 (2014) 11915–11923. [30] Yu-Lin, Chiung-Ching Lin, Jyh-Ming Wu, Uei-Shin Chen, Jiann-Ruey Chen, Han C. Shih, Formation and cathodoluminescence of Al:ZnO nanoscrew clusters, Thin Solid Films 517 (2008) 1225–1229. [31] A. Singh, D. Kumar, P.K. Khanna, A. Kumar, M. Kumar, Anomalous behavior in ZnMgO thin films deposited by sol-gel method, Thin Solid Films 519 (2011) 5826-5830. [32] S. S. Kurbanov, Photo- and Cathodoluminescence Studies of ZnO-Filled Opal Nanocomposites, J. Korean Phys. Soc. 50 (2007) 617-621. [33] S. Sharma, S. Vyas, C. Periasamy, P. Chakrabarti, Structural and optical characterization of ZnO thin films for optoelectronic device applications by RF sputtering technique, Superlattice Microstruct. 75 (2014) 378–389. [34] T. Minemoto, T. Negami, S. Nishiwaki, H. Takakura, Y. Hamakawa, Preparation of Zn1−xMgxO films by radio frequency magnetron sputtering, Thin Solid Films 372 (2000) 173-176.
13
Table caption
Table 1. EDS analysis of Zn1-xVxO nanoparticles. Table 2. The grain size (G), mean roughness (Ra) of Zn1-xVxO thin films with different vanadium contents deposited at RT, 200 °C and 60 W. Table 3. EDS analysis of the Zn1-xVxO thin films deposited at RT, 200 °C and 60 W. Table 4. Optical band gap energy of the Zn1-xVxO thin films deposited at RT, 200 °C and 60 W.
Figure caption
Fig. 1. X-ray diffraction patterns of Zn1-xVxO aerogel nanoparticles for different vanadium contents. Fig. 2. TEM photographs showing the morphology of Zn1-xVxO nanoparticles at different vanadium contents. Fig. 3. X-ray diffraction patterns of Zn1-xVxO thin films with different vanadium contents deposited at RT and 200 °C, at sputtering powers of (a) 60 W, (b) 80 W and (c) 100 W. Fig. 4. Variation of c-axis length of Zn1-xVxO thin films deposited at 200 °C and 60 W as a function of different vanadium contents. Fig. 5. Chemical analysis spectra of Zn0.80V0.20O thin films deposited at 200 °C and 60 W. Fig. 6. 3D-AFM images (1x1µm2) of Zn1-xVxO thin films with different vanadium contents deposited at 60 W, (a, b, c) at RT and (d, e, f) at 200 °C. Fig. 7. SEM cross sectional images of Zn1-xVxO thin film obtained at 60 W and 200 °C. Fig. 8. Optical transmittance spectra in the UV-Vis-NIR regions of (a) Zn1-xVxO thin films with different vanadium contents deposited at 60 W and (b) Zn0.80V0.20O film elaborated at different sputtering powers. In both cases, the films were grown at RT and 200 °C. Fig. 9. Plots of (αhν)2 against photon energy (hν) of (a) Zn1-xVxO thin films with different vanadium contents deposited at 60 W and (b) Zn0.80V0.20O film elaborated at different sputtering powers. In both cases, the films were grown at RT and 200 °C. Fig. 10. Cathodoluminescence spectra of the Zn1-xVxO thin films with (x = 0, 0.10, 0.20, 0.30) grown at 200 °C and 60 W.
14
Table 1. EDS analysis of Zn1-xVxO nanoparticles.
x 0 0.10 0.20 0.30 0.50
Elemental compositions (at.%) Zn V O 49 0 51 46 5 49 42 12 46 38 20 42 28 33 39
Table 2. The grain size (G), mean roughness (Ra) of Zn1-xVxO thin films with different vanadium contents deposited at RT, 200 °C and 60 W.
x 0 0.10 0.20 0.30 0.50
RT 20 6 5 4 4
G (nm) 200 °C 22 10 13 10 8
RT 0.7 2 0.9 0.6 0.9
Ra (nm) 200 °C 1.4 0.5 0.2 0.2 0.2
15
Table 3. EDS analysis of the Zn1-xVxO thin films deposited at RT, 200 °C and 60 W.
x Zn 51 45 44 43 34
0 0.10 0.20 0.30 0.50
Elemental compositions (at.%) RT 200 °C V O Zn V 0 49 49 0 6 49 44 6 11 45 40 10 13 44 36 15 26 40 33 27
O 51 50 50 49 40
Table 4. Optical band gap energy of the Zn1-xVxO thin films deposited at RT, 200 °C and 60 W.
x 0 0.10 0.20 0.30 0.50
Band gap Eg (eV) RT 200 °C 3.27 3.29 3.44 3.50 3.42 3.74 3.40 3.71 3.39 3.54
16
Highlights
High quality of nanostructured Zn1-xVxO thin films obtained with high V content.
High transmittance for all the films from the visible to the NIR region.
The band gap of Zn1-xVxO films can be adjusted by varying the V content.
The cathodoluminescence reveals intense Near Band Edge Emission (NBE).
A promising material for optoelectronic devices.
17