SiO2 substrate and Mo,In codoping effect on

0 downloads 0 Views 500KB Size Report
Jun 23, 2015 - During the last two decades, zinc oxide [ZnO] ... using an aqueous solution of zinc acetate dehydrate (Zn(CH3COO)H2.2H2O) with molarities ..... [13] S. Fridjine, K.B. Ben Mahmoud, M. Amlouk, M. Bouhafs, A study of sulfur/selenium ... [19] M.C. Jun, S.U. Park and J.H. Koh, Comparative studies of Al-doped ...
Accepted Manuscript SiO2 substrate and Mo,In codoping effect on crystalline and vibrational characteristics of ZnO sprayed thin films A. Souissi, R. Mimouni, M. Amlouk, S. Guermazi PII: DOI: Reference:

S0749-6036(15)30069-0 http://dx.doi.org/10.1016/j.spmi.2015.06.036 YSPMI 3846

To appear in:

Superlattices and Microstructures

Received Date: Accepted Date:

23 June 2015 27 June 2015

Please cite this article as: A. Souissi, R. Mimouni, M. Amlouk, S. Guermazi, SiO2 substrate and Mo,In codoping effect on crystalline and vibrational characteristics of ZnO sprayed thin films, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi.2015.06.036

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.

SiO2 substrate and Mo,In codoping effect on crystalline and vibrational characteristics of ZnO sprayed thin films A. Souissia,*, R. Mimounib, M. Amloukb, S. Guermazia a

Unité de recherche de Physique des matériaux Isolants et Semi-Isolants, Faculté des sciences

de Sfax, 3038, Université de Sfax, Tunisia b

Unité de physique des dispositifs à semi-conducteurs, Faculté des sciences de Tunis,

Université de Tunis, El Manar, 2092 Tunis, Tunisia Abstract Undoped ZnO and codoped ZnO:Mo:In thin films were deposited on an amorphous SiO2 substrate at 460 °C using a (Mo/Zn) molar ratio of 1% and (In/Zn) ratios of 1, 2, 3 and 10%. The thin films were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Raman spectroscopy (RS). The results revealed that the average crystallite sizes ranged from 36.2 to 18.97 nm, decreasing uniformly with the increase of co-doping. They were also lower than the grain size values of 48.29, 51.38, 60.59, 36.76, and 54.52 nm and dependent on the evolution of the residual compressive stress values, namely 0.358, 0.314, 0.569, 0.278 and 0.108 GPa, without and with the codoping, respectively. The non-uniformities recorded at In 2% could presumably be attributed to the variable effect of the SiO2 substrate and dopants on the formation of ZnO:Mo:In thin films. Raman spectroscopy confirmed the findings from structural analysis, showing that all samples crystallized following the hexagonal Wurtzite single phase. It highlighted the presence of two dominant bands, 1LO and 2LO, whose ~ 71 and 137 meV energies were comparable and above the ZnO exciton binding energy of 60 meV. The 2LO band showed marked dependencies on the physicochemical parameters mentioned above. The strong bands noted for optimal co-doping at Mo 1% and In 3% can be used (tested) in various electrical and optoelectronic applications. Keywords: ZnO:Mo:In thin film, Crystallite/Grain, interface and surface states, Stresses, Raman LO modes, Parameter dependencies * Corresponding author: Tel : +216 27 45 58 30 Electronic mail : [email protected]

1. Introduction The search for viable strategies to produce efficient transparent conductive oxide (TCO) thin films for the manufacture of low-cost and high-performance optoelectronic devices has attracted growing attention in recent research. During the last two decades, zinc oxide [ZnO] has emerged as one of the promising materials for the development of stable and transparent electrodes of use in the production of thin film solar cells. Several techniques have been proposed for the deposition of ZnO thin films, including sol-gel, vapor deposition, evaporation, sputtering, and spray. The literature indicates that the spray technique offers a particularly successful strategy for the deposition of conductive and transparent ZnO thin films. Considering that ZnO deposition involves several parameters and conditions for the incorporation of several dopants, various attempts have been made to search for viable techniques to enhance the transparence and reduce the resistivity of ZnO thin films. Recent research indicates that stable ZnO thin films of low resistivity can be obtained by various processes, including the codoping of elements belonging to group III. In fact, the thermoelectric power factor of (Al, In) codoped ZnO has been enhanced by co-doping with indium using a dual magnetron sputtering system. In particular, (ZnO) Al.03In.02 was reported to exhibit better thermoelectric properties with an electrical conductivity of the order of 5.88 × 102 S/cm and a Seebeck coefficient of −220 μV/K at 975 K [1]. Transparent, conductive and flexible Mg and Ga co-doped ZnO (MGZO) thin films have also been prepared on poly-ethylene terephthalate (PET) by RF magnetron sputtering technique at room temperature. The as MGZO thin films prepared at 6 mTorr showed a wider optical band gap energy of about 3.91 eV and lower electrical resistivity of 5.3 × 10−3 Ω cm [2]. Likewise, Y. Kim et al [3] have obtained MGZO thin films on glass substrates by RF magnetron sputtering. Those films showed the best electrical characteristics in terms of carrier concentration (3.7 × 1020/cm3), charge carrier mobility (8.39 cm2/Vs), and low resistivity (1.85 × 10− 3 Ω cm). The MGZO thin films also showed a wider optical band gap energy value of 3.75 eV. Moreover, highly transparent and conducting fluorine (F) and tin (Sn) codoped ZnO (FTZO) thin films were deposited on glass substrates by the sol–gel processing. A minimum resistivity of 1 × 10−3 Ω cm was obtained from the FTZO thin film with 3% F doping, and the average optical transmittance in the entire visible wavelength region was higher than 90% [4].

The codoping effect is generally used to obtain the p character of ZnO thin films. M. Joseph et al [5,6] have previously reported that the use of Ga (donor) and N (acceptor) as dopants improves this type of conductivity. J.M. Bian et al [7] have also reached p type ZnO sprayed thin films using N and In dopants. Recently, the combined effects of Ga–F codoping and smaller crack density have been reported to optimize the opto-electric properties of ZnO based thin films [8]. Nevertheless, the codoping inducing n type ZnO thin films has been used to reinforce the gas sensors sensibility and enhance the photocatalysis. Furthermore, pure and doped ZnO thin films have received increasing attention due to their attractive physical properties. Mo doped ZnO thin films have shown better rates of crystallinity, with a preferred orientation of crystallites along the (002) direction, thus enabling the thermal activation of clear transparency and electrical conductivity according to the Arrhenius law [9]. The n type has also been reinforced with Mo doping. Besides, In doped ZnO thin films have shown a high transmittance of over 85% in the visible range independence of the substrate temperature [10], and its ‘n’ type was reported to increase with the increase of In content. Furthermore, for those dopants, the ionic size of: Mo6+=0.59Å and In3+=0.80Å were close to Zn2+=0.74Å, thus indicating better opto-electric properties. Considering the promising opportunities that stable, transparent and low-resistant zinc oxide [ZnO] thin films might offer for the development of low-cost and high-performance optoelectronic devices, the present study aimed to investigate the codoping effects of In and Mo on ZnO thin films. 2. Experimental procedure Un-doped and (Mo,In) codoped ZnO thin films were deposited on glass substrates at 460°C, using an aqueous solution of zinc acetate dehydrate (Zn(CH3COO)H2.2H2O) with molarities of 10-2M. The solution contained a mixture of distilled water and isopropyl alcohol with fraction volumes of 50 cm3 and 150 cm3, respectively. It was acidified with acetic acid (ph=5) [11-13]. Mo and In dopants derived from chemical solutions: ammonium molybdate tetrahydrate ((NH4)6Mo7O24. 4H2O) and indium chloride (InCl3), respectively. While the molar ratio of (Mo/Zn) was fixed at 1%, the (In/Zn) ratio was assayed at 1, 2, 3 and 10%. Nitrogen was used as the gas carrier (pressure at 0.35 bar) through a 0.5 mm-diameter nozzle. As previously reported elsewhere, the nozzle-to-substrate plane distance was fixed at the optimal value of 27 cm [14]. During the deposition process, the precursor mixture flow rate was taken constantly at 4ml/min throughout the thin films deposition. The X-ray diffraction

data of (Mo,In)-codoped ZnO thin films were determined by a copper-source diffractometer (Analytical X Pert PROMPD) with a wavelength of (λ= 1.54056Å). The surface morphology and chemical composition of thin films were estimated by the field emission-scanning electron microscopy, FE-SEM (JSM-7600F-JEOL). Raman spectra were acquired by an HR800 Horiba micro-Raman spectrometer, with laser excitation of wavelength λ = 488 nm at room temperature. 3. Results and discussions 3. 1. Structural characteristics The basic data obtained for the X-ray diffraction patterns of ZnO:Mo:In thin films with 2 theta varying from 5 to 70 degrees and selected from 31 to 37.1 degrees are presented in Fig. 1.

3.1.1. Lattice behavior Three diffraction peaks were related to reflection planes (100), (002) and (101). They showed the hexagonal Wurtzite ZnO structure (JCPDS file no. 36-1451) of the c and a parameters, calculated from the X-ray diffraction peak positions using the typical relations of the Bragg's law:

2d

sin(theta)= nλ,

(1)

where dhkl is the interplanar spacing, which is written for the structure mentioned below [15]:

d

=

+

,

(2)

where h, k and l are the Miller indices, theta is Bragg’s diffraction angle, n=1 refers to the

first order X-ray diffraction, and λ is the wavelength of CuKα radiation (1.5406 Ǻ). Table 1

presents some of the structural parameters of ZnO:Mo:In thin films for the (002) and (101) diffraction peaks.

The c and a calculated parameters ranged from 5.1873 to 5.1976 Å and 3.242 to 3.244 Å, respectively, and were close to the bulk ZnO: c0 = 5.194 Å and a0=3.242 Å (JCPDS-75-0576) [16]. They were slightly lower than the unstrained crystal lattice constants: c 0 = 5.207 Å and a0 = 3.250 Å [17,18] and the solid ZnO: c0 = 5.200 Å according to the American Society for Testing and Materials [19]. They involved a reduction in the unit cell volume (Vu-crystal= 47.63 Ǻ3), presumably due to the substitutional and intertitial incorporation of Mo and In atoms in the ZnO lattice. The literature indicates that the Mo 6 + ionic radius is between 0.055 and 0.062 nm [20], well below the current value of 0.074 nm recorded for Zn2+ [19,21]. The electronegativity of Mo (2.16) is also higher than that of Zn (1.65), which allowed the Mo 6+ ion to attract the Zn2+ electrons and take its place easily while releasing four electrons in the band gap and rendering the Zn2+ intertitial in the donor state. The same effect can occur for the In atom of an ionic radius ranging from 0.076 nm [22] to 0.080 nm (Shannon radii table), which is similar or slightly higher than that of Zn2+. The electronegativity of the In atom is 1.78 higher than that of the Zn atom. The In3+ ion could be substituted by the Zn2+ ion while affecting a free electron into the conduction band [23] that could be inserted into the ZnO lattice or contribute to the formation of amorphous In2O3 along the grain boundaries [18,24]. These observed phenomena exerted local effects on the lattice by disturbing the distribution of the atoms and implying a decrease in the intensity of the reflected diffraction peaks, depending on the increase of co-doping. Those effects were particularly observed for the diffraction peaks related to the (002) hkl plane. For codoped samples at In 3 and 10%, three diffraction peaks (100), (002) and (101) of lower intensities at 10 % than 3 % were observed, with a shift towards the lower 2 theta, considering the increase in the unit cell volume of ZnO (see Table 1). This could be attributed to the differences in the In3+ and Zn2+ ionic radii, concentration of free and donor state carrier in the band gap, and the ascending heterogeneity of anisotropic n-ZnO:Mo:In thin films. Those films kept the hexagonal Wurtzite structure after co-doping. 3.1.2. Crystallite and stress characteristics The effects of codoping and amorphous SiO2 substrate play an important role in the mechanism of grain growth and the formation of ZnO:Mo:In thin films. Their impacts on the variation of crystallite sizes were investigated using the Sherrer typical formula [25]:

D = (0.9 λ)

β

(theta) .

/ cos

(3)

Considering their effects on the evolution of residual stresses (σ) that were based on the biaxial strain model for a hexagonal ZnO structure, σfilm could then be written as follows [19]:

σ

=

(

where λ = 1.5406 Å, β

)

/

×ε ,

(4)

is the full width at half maximum (FWHM) of the (002) diffraction

peak, theta is the Bragg angle, and

=





is the strain following z axis

perpendicular to the substrate plane. Accordingly, the elastic constants for a ZnO single crystal were : c11 = 208.8, c12 = 119.7, c13 = 104.2, and c33 = 213.8 GPa [19], which implied that if the

= −232.4 ×

(GPa). In fact, the stress is considered as positive and compressive

strain is negative [26], and otherwise as negative and tensile (expansive) [15].

The values calculated for crystallite sizes corresponded to the strong (002) diffraction peaks, ranging from 36.2 to 18.97 nm. They were, therefore, comparable to recent reports in the literature [15,16] and noted to involve a greater density of grain boundaries [19], thus increasing the electron mobility through the grain boundaries depending on the increase in codoping. The stress values, which were calculated for the two standard references cited above: c0 = 5.200 Å [19] and c0 = 5.207 Å [17,18], were slightly higher than the cfilm parameters of the ZnO:Mo:In samples under investigaton, thus suggesting that the latter were under compressive stresses along the c axis perpendicular to substrate plane. The strains calculated for the samples under investigation were relatively low and > at -0.24 and at -0.37% for both c0 values, respectively. Except for the thin film at In = 2 %, the stresses related to those strains had a tendency to decrease with codoping of 0.35 to 0.1GPa (c0= 5.200 Å) and of 0.67 to 0.42 GPa (c0 = 5.207 Å). These compressive stress values were significantly lower or comparable (in absolute value) to previous reports in the literature, including the work of M-C. Jun et al [19] on thin films prepared by the sol-gel spin-coating method ZnO:Al (-0.7 to -1 GPa) and ZnO:Ga (- 0.83 to -0.94 GPa) using the same c0 = 5.200 Å, the work of A. Barhoumi et al [15] on ZnO:Al sputtering thin films (-1.34, -0.62 and -0.53 GPa) using c0 = 5.206 Å and as a function of substrate temperature (200, 300 and 400 °C) respectively, and the work of Z. Pan et al [26] on films prepared by the sol-gel dip coating technique ZnO:al (0.45 to 4.84 GPa) and ZnO:Sn (0.45 to 3.05 GPa). The residual stresses of

the thin films [19,15,26] depend on several physicochemical factors, including film/substrate adhesion. The stresses of our samples were compared with those in the literature, and the results indicated the good physical stability displayed by the ZnO:Mo:In thin films of hexagonal Wurtzite structure. Table 2 summarizes the changes in those physico-chemical parameters and the evolution of the average grain sizes. These observations will be elaborated on in the following section.

3.2. Surface morphologies and chemical analysis The images illustrated in Fig. 2(a-e) show that the thin film top surfaces were parallel to the substrate plane, exhibiting a grain growth that most followed the c-axis. The films displayed regular morphologies, depending both on codoping and film/substrate interaction. The grains observed on the surface were of spherical forms for the undoped ZnO thin film and spherical and polygonal for codoped thin films at In 1% and In 3%. The grains of polygonal and prismatic geometries were observed for the ZnO:Mo:In thin films at 2 and 10%. They were formed of compacted aggregates of average sizes ranging from ~ 36.76 to 60.59 nm. They displayed a non-uniform evolution and were greater than the crystallite sizes determined by XRD [27] for all samples. The agglomeration rate [28] was defined as the ratio of the grain average sizes divided by that of crystallites and was of the order of ~1.33 to 2.87 crystallites per grain. These parameters, which depended on several physicochemical phenomena, are summarized in table 2.

Small white spots were observed on the surfaces of ZnO:Mo:In thin films. They could be attributed to oxygen deficiency and/or oxygen adsorption phenomena in the form of molecule and atoms: O , O , et O

[20,29].

The mass analysis of the compounds under investigation by EDS revealed the presence of O, Zn and In chemical elements doping to low intensities (Fig. 2f) and of Ca, Mg, Al, Si cations with the O anion being mainly related to the amorphous SiO2 substrate and low part of the environment. The absence of an MoO3 phase or Mo metal in the lattice ZnO:Mo:In was confirmed by both XRD and EDS techniques. These results are in good agreement with those of H.S. Woo et al [20], indicating no phase of MoO3 and complete doping Mo in the lattices of ZnO:Mo:In thin films. The EDS analysis showed the presence of indium at different binding energies of low intensities, which was not revealed by XRD [30-32]. Indium was

incorporated in the ZnO:Mo:In lattice by insertion and by the substitution of the Zn site. It was also introduced as an In2O3 amorphous form in the defects of the grain boundaries and into the grain boundaries [18].

Substrates with such low content of impurities play an important role in the formation of films [15,16,33,34], given their physicochemical characteristics (electronegativity, ionic radii, etc.) adaptive to ZnO thin film. During physical vapor deposition at 460 °C, these impurities can migrate to the thin film or contribute to the creation of an ultrafine layer between the substrate and the thin film, as schematically presented in Fig. 3.

The influence of these impurities and Mo,In dopants on the vibrational characteristics of ZnO:Mo:In thin films will be studied in the following section.

3.3. Vibrational Study

The structural analysis of ZnO:Mo:In thin films showed a crystallization according to (100), (002) and (101) planes, with the prevalence of the (002) plane for the undoped thin films and the ones codoped with (Mo,In) at In = 1 and 2%. Those planes matched the Wurtzite hexagonal ZnO structure of P

space group whose corresponding vibration modes are:

2E2, 2E1, 2A1, and 2B1 silent modes. Several tests were performed on the confocal micro Raman HR800, and the results led to the selection of blue laser wavelength λ = 488 nm with an exposure time of 300 s on all samples. Fig. 4 shows the spectral responses into vibrational scale from 50 to 1500 cm-1 for the different ZnO:Mo:In thin films:

3.3.1. Results analysis

The results presented in Fig. 4 show the vibrational stability of all the samples. The vibration frequencies of the various modes marked by dashed lines were almost equal to ± 3 cm-1 [35, 36]. Those curves showed two dominant modes, 1LO and 2LO, usually activated by defects (oxygen deficiencies and/or dopants), and a wide band of ~ 60 to ~ 745 cm-1, which included the dominant and classical E

mode. The substrate effect at 460 ℃ on the adhesion and on

the growth mechanism from this interface of the thin film is important because the Ca, Mg

and Al substrate impurities are of the n-type and can be incorporated in the ZnO lattice of thin

films [33,34] (see Fig. 3). They can also affect the crystalline and optoelectronic characteristics of each thin film, which was the case for undoped ZnO, where the presence of 1LO and 2LO bands that were sensitive to defects was observed. The low Raman intensity related to the thin film at In 2% could be in part explained by its low content of impurities and dopant (weight % In = 0.51). The qualitative analysis by EDS gave the weight % In: 0.99, 0.51, 1.38 and 1.33 for the four codoped thin films, respectively. This thin film at In 2% was of low intensity and influenced by the weak diffusion of indium. It displayed an average grain size that was higher than other thin films. The vibrational modes and (002) X-ray diffraction peak exhibited a partial shift towards the high frequencies at ~ +2 cm-1 and towards higher 2 theta at ~ 0.04 degree, respectively. The presence of LVMS was difficult to observe due to Mo/In dopants, compared to undoped ZnO, given the overlap of the modes in the broadband ~ 60 to ~ 745 cm-1 which included the acoustic, optical and combination modes summarized in Table 3.

The observed modes related to group theory were E

B

~100 to 122 cm-1, E

~441 cm-1,

~266 cm-1, the resulting TO Mode at ~390 cm-1, and 1LO mode at ~569 cm-1, which

were close to A1LO than E1LO [35]. This mode propagated along the c-axis or at an

angle

of the c-axis [35] and is known by its replicas in the band gap of ZnO. These modes reflected the presence of a hexagonal Wurtzite structure and are consistent with the ones previously reported in the literature [37,35]. The modes coming from the miltiphonon process indicated the possible presence of a TO-LA additional Raman mode at ~ 157cm-1 in all ZnO thin films. The results also revealed the 2E

and E

−E

modes and the TA+LO splitting

acousto-optic mode at ~ 664 cm-1. After this band, an independent acousto-optic longitudinal phonon LA+LO mode at ~801 cm-1 and 2LO broadband at ~1104 cm-1 were observed [37], which possibly included the spectral responses of the combined TO+LO mode to the frequencies ~956 and 998 cm-1, which were close to the vibrational frequency ~ 990 cm-1 in ZnO reported by J-M. Calleja et al [38]. Those vibrational modes could presumably be related to ETO+LO(Γ) and ETO+LO(Γ±Δk) response types deduced by extension of the TO and LO optical branches of the ZnO classical dispersion curves: E (k) given at room temperature and at 7-10 K [38,39]. In addition to the 2LO band, some other vibrational modes were observed, with very low intensities at spatial frequencies ~ 1308 and 1455 cm-1 for the thin film at In 1% and ~1403 cm-1 for other thin films.

3.3.2. Possible dependence Given the importance of the 2LO band observed in this study, the authors deemed it useful to explore its possible dependencies on the investigated physicochemical characteristics: (002) diffraction peak, crystallite size, grain size and residual stress. Fig. 5 shows the intensity and FWHM variations of the 2LO band (or mode) as a function of the explored parameters and depending on the (Mo/In) co-doping. The results indicated that several correlations were possible.

The results presented in Fig. 5a are in agreement with several previous reports on the sensitivity of the LO phonon mode at defects [40,41], at residual stresses induced in the thin film/substrate interface [40,41], and at the surface states of thin films (roughness, grain characteristics and surface defects) [42, 40, 43]. The irregularity observed at In = 2% could be attributed to the experimental conditions of deposition in ambient atmosphere, which were difficult to respect for all samples, and to the SiO2 substrate surface state (roughness, corrugation), which might have varied from sample to sample [34,44,45]. Recent studies exploring the effects of Graphene/SiO2 interaction [34,44,45] on those compounds have shown some changes in their characteristics due to SiO2 substrates. These phenomena could be partially transposed to our samples.

The findings illustrated in Fig. 5b show the FWHMs of the 2LO mode and (002) peak, which displayed almost the same ascending evolution with the increase of codoping and had comparable dependencies on crystallite sizes (calculated from (002) diffraction peaks). The FWHMs of the high spectral density 2LO mode were 11.75, 12.24, 13.30, 13.55, and 14.48 meV for different ZnO:Mo:In thin films, respectively.

The results from the vibrational study of ZnO:Mo:In thin films confirmed the findings previously obtained from XRD, SEM and EDS analysis, indicating the presence of two dominant modes, 1LO and 2LO, related to the electron-LO-phonon interactions (fundamental and replica) and with energy values of E

~71 meV [42,46] and E

~137 meV,

respectively [42]. Those characteristic modes of high unicity and polarity of ZnO Wurtzite [36,42,46] are highly valued for optoelectronic applications of single or double emission and have been investigated by different techniques in the literature [46-48].

4. Conclusion This study showed that the co-doping and SiO2 substrate play an important role in the adhesion and formation of ZnO:Mo:In thin films. The individual and synergistic effects of codoping, substrate and environment on the physicochemical characteristics of the samples have been investigated through different characterization assays. The results indicated that the lattice parameters and crystallite sizes were particularly sensitive to the effect of co-doping given their regularities. The stresses, which depended on the crystalline characteristics of thin films, were sensitive (responsive) to the effects of co-doping and film/substrate coupling. They were ≤ to the stresses related to ZnO thin films reported in the literature and did not affect the hexagonal Wurtzite structure of the samples. The results from Raman analysis confirmed the findings obtained by X-ray diffraction of ZnO:Mo:In with regard to the stated structure and were consistent with those generated by SEM and EDS analyses. They revealed the presence of two bands, 1LO and 2LO, for all samples.

The most intense band

corresponded to a thin film of optimal codoping Mo 1% and In 3%. Those LO bands were sensitive to co-doping, residual stresses, and crystallinity of the thin films. The bands (or modes) presented in this work could be exploited together or separately in various electrical and optoelectronic applications. Acknowledgements This study has been supported by the Tunisian Ministry of high education and Scientific Research. The authors thank, from the Faculty of Sciences of Sfax, Prof. Hamadi KHEMAKHEM for the vibrational assays and Mr Anouar SMAOUI for his constructive proofreading.

References [1]

S.

Teehan,

H.

Efstathiadis,

P.

Haldar,

Enhanced

power

factor

of Indium codoped ZnO:Al thin films deposited by RF sputtering for high temperature thermoelectrics, J. Alloy Compd., 509 (2011) 1094-1098. [2] S.W. Shin, I.Y. Kim, K.S. Jeon, J.Y. Heo, G.S. Heo, P.S. Patil, J.H. Kim, J.Y. Lee, Wide band gap characteristic of quaternary and flexible Mg and Ga co-doped ZnO transparent conductive thin films, J. Asian Ceram. Soc., 1(2013) 262-266. [3] I.Y. Kim, S.W. Shin, M.G. Gang, S.H. Lee, K.V. Gurav, P.S. Patil, J.H. Yun, J.Y. Lee, J.H.

Kim,

Comparative

study

of

quaternary

Mg

and

Group

III

element co-

doped ZnO thin films with transparent conductive characteristics. Thin Solid Films, 570 ( 2014) 321-325. [4] Z. Pan, P. Zhang, X. Tian, G. Cheng, Y. Xie, H. Zhang, X. Zeng, C. Xiao, G. Hu, Z. Wei, Properties of fluorine and tin co-doped ZnO thin films deposited by sol–gel method, J. Alloy Compd., 576 (2013) 31-37. [5] M. Joseph, H. Tabata and T. Kawai, p-Type Electrical Conduction in ZnO Thin Films by Ga and N Codoping, Jpn. J. Appl. Phys. 38 (1999) L1205-L1207. [6] M. Kumar, TH Kim, SS Kim, BT Lee, Growth of epitaxial p-type ZnO thin films by codoping of Ga and N, Applied physics letters, 89 (2006) 112103. [7] J.M. Bian, X.M. Li, X.D. Gao, W.D. Yu and L.D. Chen, Deposition and electrical properties of N–In codoped p-type ZnO films by ultrasonic spray pyrolysis, Appl. Phys. Lett. 84 (2004) 541. [8] Q. Shi , K. Zhou, M. Dai, S. Lin, H. Hou, C. Wei , F. Hu, Structural and opto-electric properties of Ga and F codoped ZnO thin films, Vacuum 94 (2013) 81-83. [9] A. Boukhachem, B. Ouni, M. Karyaoui, A. Madani, R. Chtourou, M. Amlouk, Structural, opto-thermal and electrical properties of ZnO:Mo sprayed thin films”, Mater. Sci. Semicond. Process. 15 (2012) 282–292. [10] L.P. Peng, L. Fang, X.F. Yang, H.B. Ruan, Y.J. Li, Q.L. Huang, C.Y. Kong, Characteristics of ZnO:In thin films prepared by RF magnetron sputtering,Physica E 41 (2009) 1819–1823. [11] A. Boukhachem, S. Fridjine, A. Amlouk, K. Boubaker, M. Bouhafs, M. Amlouk, Comparative effects of indium/ytterbium doping on, mechanical and gas-sensitivity-related morphological, properties of sprayed ZnO compounds , J. Alloy Compd. 501 (2010) 339–344 [12] A. Amlouk, K. Boubaker, M. Amlouk, Effects of substrate temperature on sprayed ZnO

thin films optical and morphological properties in terms of Amlouk–Boubaker opto-thermal expansivity ψAB , J. Alloy Compd. 482(2009) 164–167. [13] S. Fridjine, K.B. Ben Mahmoud, M. Amlouk, M. Bouhafs, A study of sulfur/selenium substitution effects on physical and mechanical properties of vacuum-grown ZnS1−x Sex compounds using Boubaker polynomials expansion scheme (BPES), J. Alloy Compd. 479 (2009) 457–461. [14] K. Boubaker, A. Chaouachi, M. Amlouk, H. Bouzouita, Enhancement of pyrolysis spray disposal performance using thermal time-response to precursor uniform deposition , Europ. Phys. J. Appl. Phys. 37 (2007) 105–109. [15] A. Barhoumi, G. Leroy, L. Yang, J. Gest, H. Boughzala, B. Duponchel, S. Guermazi, and J-C. Carru, Correlations between 1/f noise and thermal treatment of Al-doped ZnO thin films deposited by direct current sputtering, J. Appl. Phys. 115, 204502 (2014). [16] R. Swapna, M-C-S. Kumar, The role of substrate temperature on the properties of nanocrystalline Mo doped ZnO thin films by spray pyrolysis, Ceramics International 38 (2012) 3875–3883. [17] F. Maldonado, A. Stashans, Al-doped ZnO: Electronic, electrical and structural properties, J. Phys. Chem. Solids, 71 (2010) 784-787. [18] G. Machado, D.N. Guerra, D. Leinen, J.R.R. Barrado, R.E. Marotti, E.A. Dalchiele, Indium doped zinc oxide thin films obtained by electrodeposition, Thin Solid Films 490 (2005) 124–131. [19] M.C. Jun, S.U. Park and J.H. Koh, Comparative studies of Al-doped ZnO and Ga-doped ZnO transparent conducting oxide thin films, Nanoscale res. Let. 7 (2012) 639. [20] H.S. Woo, C.H. Kwak, Il.D. Kim and J.H Lee, Selective, sensitive, and reversible detection of H2S using Mo-doped ZnO nanowire network sensors, J. Mater. Chem. A, 2 (2014) 6412–6418. [21] Z. Pan, X. Tian, S. Wu, C. Xiao, Z. Li, J. Deng, G. Hu, Z. Wei, Effects of Al and Sn dopants on the structural and optical properties of ZnO thin films, Superlattice Microstruct. 54 (2013) 107–117. [22] Y. Caglar, S. Ilican, M. Caglar, F. Yakuphanoglu, Effects of In, Al and Sn dopants on the structural and optical properties of ZnO thin films, Spectrochim. Acta A 67 (2007) 1113– 1119. [23] C.G. Granqvist, A. Hultaker, Transparent and conducting ITO films: new developments and applications, Thin Solid Films 411 (2002) 1-5.

[24] A. Maldonado, M.L. Olvera, S.T. Guerra, R. Asomoza, Indium-doped zinc oxide thin films deposited by chemical spray starting from zinc acetylacetonate: effect of the alcohol and substrate temperature, Sol. Energy Mater. Sol. Cells 82 (2004) 75-84. [25] Y. Wang, Q. Huang, C. Wei, D. Zhang, Y. Zhao, X. Zhang, Improvement of electrical and optical properties of molybdenum doped zinc oxide films by introducing hydrogen, Appl. Surf. Sci. 258 (2012) 8797-8801. [26] Z. Pan, X. Tian, S. Wu, C. Xiao, Z. Li, J. Deng, G. Hua, Z. Wei, Effects of Al and Sn dopants on the structural and optical properties of ZnO thin films, Superlattices Microstruct. 54 (2013) 107–117. [27] G. Srinivasan, J. Kumar, Effect of Mn doping on the microstructures and optical properties of sol–gel derived ZnO thin films, J. Cryst. Growth 310 (2008) 1841–1846. [28] R.G. Pavelko, A.A. Vasiliev, F. Gispert-Guirado, N. Barrabes, J. Llorca, E. Llobet, V.G. Sevastyanov, Crystallite growth kinetics of highly pure nanocrystalline tin dioxide: The effect of palladium doping, Mater. Chem. Phys. 121 (2010) 267–273. [29] N. Barsan and U. Weimar, Conduction Model of Metal Oxide Gas Sensors, J. Electroceram., 7 (2001) 143–167. [30] P. M. R. Kumar, C. S. Kartha, and K. P. Vijayakumar, Doping of spray-pyrolyzed ZnO thin films through direct diffusion of indium: Structural optical and electrical studies, J. Appl. Phys. 98 (2005) 023509. [31] J. Su, H. Li, Y. Huang, X. Xing, J. Zhao and Y. Zhang, Electronic transport properties of In-doped ZnO nanobelts with different concentration, Nanoscale, 3 (2011) 2182. [32] A. Hafdallah, F. Yanine, M.S. Aida, N. Attaf, In doped ZnO thin films, J. Alloy Compd. 509 (2011) 7267–7270. [33] C. Ertler, S. Konschuh, M. Gmitra, and J. Fabian, Electron spin relaxation in graphene: The role of the substrate, Phys. Rev. B, 80 (2009) 041405. [34] S. Ryu, L. Liu, S. Berciaud, Y-J. Yu, H. Liu, Ph. Kim, G.W. Flynn, and L.E. Brus, Atmospheric Oxygen Binding and Hole Doping in Deformed Graphene on a SiO2 Substrate, Nano Lett. 10 (2010) 4944. [35] K.A. Alim, V.A. Fonoberov, M. Shamsa, A.A. Balandina, Micro-Raman investigation of optical phonons in ZnO nanocrystals, J. Appl. Phys. 97 (2005) 124313. [36] J.F. Scott, uv Resonant Raman Scattering in ZnO, Phys. Rev. B2 (1970) 1209-1211. [37] R. Cuscó, E. Alarcón-Lladó, J. Ibánez, L. Artús, Temperature dependence of Raman scattering in ZnO, Phys. Rev. B, 75 (2007) 165202.

[38] J.M. Calleja and M. Cardona, Resonant Raman scattering in ZnO, Phys. Rev. B 16 (1977) 3753-3761. [39] J. Serrano, F.J. Manjón, A.H. Romero, A. Ivanov, M. Cardona, R. Lauck, A. Bosak, and M. Krisch, Phonon dispersion relations of zinc oxide: Inelastic neutron scattering and ab initio calculations, Phys. Rev. B, 81 (2010) 174304. [40] B. Yan, R. Chen, W. Zhou, J. Zhang, H. Sun, H. Gong and T. Yu, Localized suppression of longitudinal-optical-phonon–exciton coupling in bent ZnO nanowires, Nanotechnology 21 (2010) 445706. [41] H.T. Ng, B. Chen, J. Li, J. Han and M. Meyyappan, Optical properties of singlecrystalline ZnO nanowires on m-sapphire, Appl. Phys. Lett., 82 (2003) 2023. [42] W.K. Hong, G. Jo, M. Choe, T. Lee, J.I. Sohn, and M.E. Welland, Influence of surface structure on the phonon-assisted emission process in the ZnO nanowires grown on homoepitaxial films, Appl. Phys. Lett. 94 (2009) 043103. [43] S.C. Ray, Y. Low, H.M. Tsai, C.W. Pao, J.W. Chiou, S.C. Yang, F.Z. Chien, W.F. Pong, M.H. Tsai, K.F. Lin, H.M. Cheng, W.F. Hsieh, J.F. Lee, Size dependence of the electronic structures and electron-phonon coupling in ZnO quantum dots, Appl. Phys. Lett. 91 (2007) 262101. [44] E. Stolyarova, K.T. Rim, S. Ryu, J. Maultzsch, P. Kim, L.E. Brus, T.F. Heinz, M.S. Hybertsen, and G.W. Flynn, High-resolution scanning tunneling microscopy imaging of mesoscopic graphene sheets on an insulating surface, Proc. Nat. Acad. Sci. U.S.A. 104 (2007) 9209-9212. [45] M. Ishigami, J.H. Chen, W.G. Cullen, M.S. Fuhrer, and E.D. Williams, Atomic Structure of Graphene on SiO2, Nano Lett. 7 (2007) 6. [46] W. Shan, W. Walukiewicz, J.W. Ager III, and K.M. Yu, Nature of room-temperature photoluminescence in ZnO, Appl. Phys. Lett. 86 (2005) 19191. [47] T. Michalsky, M. Wille, C.P. Dietrich, R. Roder, C. Ronning, R.S. Grund, and M. Grundmann, Phonon-assisted lasing in ZnO microwires at room temperature, Appl. Phys. Lett. 105 (2014) 211106. [48] J.D. Ye, P. Parkinson, F.F. Ren, S.L. Gu, H.H. Tan, and C. Jagadish, Raman probing of competitive laser heating and local recrystallization effect in ZnO nanocrystals, Opt. Express 20 (2012) 23282.

Figure captions Fig. 1. X-ray diffraction patterns of ZnO and ZnO:Mo:In thin films. Fig. 2. a-e) Top view of the surface morphologies of ZnO:Mo:In thin films obtained by SEM technique, f) chemical analysis of thin film at In = 3%, g) Lorentzian distribution of grain sizes. Fig. 3. Schematic diagram of ZnO:Mo:In thin film growth on SiO2 substrate at 460 °C with the possible migrations of some impurities in thin film or creating a complex ultrafine layer at level of this interaction, the ionic radii as well as the electronegativities of different atoms of ZnO:Mo:In/SiO2 (with impurities at low percentages) are given to clarify their possible integrations into the formation of thin films.

Fig. 4. Backscattering Raman spectra of ZnO and ZnO:Mo:In thin films.

Fig. 5. Possible dependence of 2LO phonon mode with average grain sizes, residual stresses and crystallite sizes for different codoping, a) 2LO intensity with grain sizes and residual stresses, b) 2LO mode FWHM with crystallite sizes and (002) diffraction peak FWHM, (2LO FWHM = 2 x 2LO FWHMcenter-right, see Fig. 4).

Table captions Table 1. Variation of 2 theta, dhkl, c, a, c/a, and unit cell volume versus In %.

Table 2. Crystallite size, strain, stress, grain size and agglomeration rate evolutions of ZnO:Mo:In thin films.

Table 3 Vibrational mode assignments of ZnO:Mo:In thin films based on the literature.

12

Intensity x 100

(002) undoped In 1% In 2% In 3% In 10%

8

4

(101)

(100)

0 31

32

33

34

35

2 theta (degree) Figure 1

36

37

Figure 2

Figure 3

TO+LO

LA+LO

TA+LO

1LO

TO high E2

2TA low B1

low

E2

3

Raman intensity x 10 (a.u.)

30

2LO

In= x% 20

10 3 2 1 2LA

low high

E2 -E2

0

TO-LA

10

undoped ZnO

lLaser=488nm 200

400

600

800

1000 -1

Frequency (cm ) Figure 4

1200

1400

a)

18

16

2LO intensity Grain size (nm) Stress (GPa)

65

1,0

60

0,9

55 14

0,8 0,7

50 0,6

12 45

0,5 10

40

8

35 0

1

2

3

0,4 0,3

4 10

In (%)

b)

2LO FWHM (cm-1) (002) peak FWHM (rad) Crystallite size (nm)

120 115

0,008 36 0,007 32

110 0,006 28

105 0,005 100 0,004

95 90 0

1

2

In (%)

Figure 5

3

10 4

24

20

0,003 16

Table 1

ZnO:Mo:In

dhkl (Ǻ)

2theta (degree)

Lattice parameters

Thin films

Compactness

Cell

Parameter

volume

(002)

(101)

(002)

(101)

c(002)(Ǻ)

a(100)/(101)(Ǻ)

c/a

(Ǻ3)

Pure ZnO

34.522

36.294

2.596

2.470

5.1920

3.2420

1.602

47.201

In

1%

34.515

36.328

2.597

2.471

5.1930

3.2439

1.602

47.324

In

2%

34.553

-

2.594

-

5.1873

-

-

-

In 3%

34.509

36.260

2.597

2.470

5.1938

3.2429

1.599

47.302

In 10%

34.483

36.158

2.599

2.476

5.1976

3.2429

1.594

47.577

Table 2

Strain %

Stress (GPa)

Crystallite

Thin films

size (nm)

Pure ZnO

36.20

-0,153

-0,288

0,358

0,671 48.29

1.33

In 1%

33.97

-0,134

-0,268

0,314

0,626 51.38

1.51

In 2%

29.19

-0,244

-0,378

0,569

0,882 60.59

2.07

In 3%

27.07

-0,119

-0,253

0,278

0,590 36.76

1.35

In 10%

18.97

-0,046

-0,180

0,108

0,420 54.52

2.87

c0=5.200Ǻ c0=5.207Ǻ c0=5.200Ǻ c0=5.207Ǻ

Grain size

Agglomera-

ZnO:Mo:In

(nm)

tion rate

Table 3

Frequencies (cm-1) Attribution

E

Observed

Ref . 37

E

100,122

99, 120

TO-LA

157

2E

197

B

−E

TO

203

266 326

333

390

378-410

E

441

438

2LA

470

483

1LO

569

574

TA+ LO

664

666

LA+ LO

801

812

TO+LO

956

TO+LO

998

2LO

1104

1105

Highlights ·

Undoped ZnO and codoped ZnO:Mo:In sprayed thin films on SiO2 at 460°C

·

XRD, SEM, EDS, and Raman spectroscopy characterizations

·

Variable effect of the SiO2 substrate and dopants on the formation of ZnO:Mo:In thin films

·

Two strong bands, 1LO and 2LO, of high spectral density and of ~ 71 and 137 meV energies

·

Dependence of 2LO phonon mode with average grain sizes, stresses and crystallite sizes