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Pulsed laser deposited Zn1-xTixO (0.000≤x≤0.050) thin films for tunable refractive index and nonlinear optical applications
Accepted Manuscript Pulsed laser deposited Zn1-xTixO (0.000≤x≤0.050) thin films for tunable refractive index and nonlinear optical applications Gyan Prakash Bharti, Partha P. Dey, Alika Khare PII:
S0254-0584(18)30502-9
DOI:
10.1016/j.matchemphys.2018.06.004
Reference:
MAC 20703
To appear in:
Materials Chemistry and Physics
Received Date: 2 February 2018 Accepted Date: 2 June 2018
Please cite this article as: Gyan Prakash Bharti, Partha P. Dey, Alika Khare, Pulsed laser deposited Zn1xTixO (0=x=0.050) thin films for tunable refractive index and nonlinear optical applications, (2018), doi: 10.1016/j.matchemphys.2018.06.004 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.
ACCEPTED MANUSCRIPT
Pulsed laser deposited Zn1-xTixO (0.000≤x≤0.050) thin films for tunable refractive index and nonlinear optical applications
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Gyan Prakash Bharti, Partha P. Dey and Alika Khare* Department of Physics, IIT Guwahati, Guwahati-Assam (India) *Email: [email protected]
Abstract: In the present paper, a systematic study on the structural, linear and nonlinear optical properties of Zn1-xTixO (0≤x≤0.050) thin films is documented. The thin films are grown onto fused silica substrate via pulsed laser deposition technique employing 2nd
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harmonic of a Q-switched Nd:YAG laser (532 nm, 10ns, 10Hz). The crystallinity of the film
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is observed to be increased initially up-to x=0.02 and then decreased for higher values of x. A small variation in band gap energy is observed in the Zn1-xTixO thin films with x which is mainly due to the Burstein Moss effect followed by the weak quantum confinement effect. The third order optical nonlinearities in the films are experimentally recorded using modified z-scan technique under cw He:Ne laser (λ=632.8 nm) illumination. The nonlinear optical
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coefficients; β and n2, are observed to be in the range of 1.1-6.0 cm/W and (1.0-4.1)×10-4 cm2/W respectively. The maximum value of β (6.0 cm/W) and n2 (4.1 cm2/W) are observed
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in the Zn1-xTixO film having x=0.02. These studies suggest that an optimum concentration of Ti content in ZnO is required for the enhancement in nonlinear optical behavior.
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Keywords: pulsed laser deposition, Zn1-xTixO thin film, nonlinear optical property, z-scan
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ACCEPTED MANUSCRIPT 1. Introduction: Zinc oxide (ZnO) based semiconducting materials are appearing to be very promising in comparison to GaN, GaP etc due to its excellent optical and electrical behavior [1]. The most important feature of this material is its direct and high band gap energy which is the basic
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requirement for the optoelectronic based devices especially towards the short wavelength regime. Additionally, the high exciton binding energy of ZnO, makes it stable at room temperature resulting into controlling the spectral broadening. The UV optical emission
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governed by the excitonic recombination results in a narrower spectral width rather than broader spectra which are dominated by free electron and holes recombination within the
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band edge of a semiconductor. Under the influence of external impurities in ZnO, its structural, electrical and optical behaviors are drastically affected [2-6]. The selection of the impurity in ZnO depends on its applications. The suitable electron rich (n-type) impurity in ZnO enhances the free carrier concentration and consequently its electrical conductivity as
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well as nonlinear optical (NLO) properties undergoes a modification [7-8]. Among the various impurities in ZnO [3-9], Ti infused ZnO (TZO) films are finding its wide applications in photovoltaic cell, optoelectronic devices etc.[10-12]. TiO2 possesses higher linear
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refractive index as compared to that of ZnO hence a suitable percentage of Ti impurity in ZnO can lead to a tunable refractive index medium which may find its application in optical
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communication, optical switching and data storage devices etc. [13]. Ti doped ZnO thin films have been reported by several research groups using variety of fabrication techniques, viz; sol-gel, magnetron sputtering, atomic layer deposition, PLD etc. [9, 14-16]. However, there are a few reports which actually deal with a large variation of Ti concentration in ZnO film and consequently its impact on the various physical and optical properties of ZnO film. Among all the techniques, pulsed laser deposition (PLD) can serve as a promising technique for the fabrication of the complex oxide thin films of Zn1-xTixO (0≤x≤0.050). The fine control
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ACCEPTED MANUSCRIPT over the various deposition parameters as well as the stoichiometric transfer of the ablated materials from the target to the substrate makes PLD, a versatile fabrication tool. In the succeeding sections, the fabrication of the Zn1-xTixO (0≤x≤0.050) thin films via PLD technique has been presented. The structural characterization of the films has been carried out
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by using X-Ray Diffraction (XRD) and Raman spectroscopic measurements. The linear optical parameters (absorption coefficients, band gap energies and refractive indices) of the films have been estimated from UV-Vis-NIR spectroscopic analysis. The third order NLO
2. Experimental Details:
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laser irradiation operating at a wavelength of 632.8 nm.
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response of the films is investigated from the modified Z-scan technique under cw He:Ne
Zn1-xTixO (0≤x≤0.050) thin films were fabricated onto polished fused silica substrate (1×1 cm2) via PLD technique. The details of the PLD set-up used in the present work, is documented in our earlier publication [17]. The PLD targets in form of circular disc having
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diameter of 13 mm, were prepared from the pure ZnO (s. d. fine-chem Ltd., 99%) and Pure TiO2 (Ranboxy, 98%) powders, mixed in appropriate proportion via solid state synthesis
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method so as to obtain the hard pellets of Zn1-xTixO (0≤x≤0.050). All the Ti doped ZnO (TZO) targets were sintered at 1150 0C temperature for 6 hrs in an electric furnace. The
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deposition parameters were kept same as in the case of earlier reported Zn1-xAlxO (0≤x≤0.10) thin films [17]. The thickness of the deposited TZO thin films were measured by using Stylus Profilometer (Model: Veeco- Dektak 6M). In order to get information about the phase and crystal structure of the films, XRD spectra for all the samples were recorded using X-Rays Diffractometer (Rigaku, TTRAX III 18kW) operating at a wavelength of 1.5407 Å of Cu-Kα line. The XRD patterns were recorded using GIXRD mode where, the glancing angle (ω) was kept at 1˚. The Laser Micro Raman
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ACCEPTED MANUSCRIPT spectrometer (Model No. LabRam HR800) was used for analyzing the vibrational modes of the films using 488 nm wavelength of Ar-ion laser as an excitation source. The linear optical parameters (absorption coefficient, band gap energy and refractive index)
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of the Zn1-xTixO thin films were evaluated by using UV-Vis-NIR spectrophotometer (Model No. SHIMADZU UV-3101 PC).
The NLO characterization of Zn1-xTixO thin films was carried out by using modified z-scan
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technique under cw He:Ne laser irradiation operating at a wavelength of 632.8 nm [18]. This technique works on the spatial narrowing and broadening of the far field pattern of a focused
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Gaussian beam as a result of intensity dependent nonlinear optical behavior of the sample. The working principle of the experimental setup and the experimental parameters are reported in the literatures [17-18]. The third order optical nonlinearity via z-scan technique is experimentally measured in two configurations; open aperture (OA) and closed aperture
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(CA). The normalized transmittance in OA configuration (Topen) as a function of sample position (z) is given by the following expression [17].
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Topen ( z ) = 1 −
β I 0 Leff 3
2 2 [1 + ( z
z0
------------------- (1) )2 ]
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Where, β represents the nonlinear absorption coefficient, I0 (=1.12 kW/cm2) is the intensity of the incident beam at the focus (z=0), Leff is the effective film thickness defined as Leff=(1-e)/α, α being the linear absorption coefficient and z0 (=2.2 mm) represents the Rayleigh
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length of the beam which is expressed as z0=πω2/λ, where ω (=21 µ) is the beam waist of the focused beam and λ, the wavelength of the laser beam used in the present experiment. The nonlinear refractive index (n2) is evaluated from the CA graphs and using the following equation [19]. 4
ACCEPTED MANUSCRIPT TClosed ( z ) = 1 −
4n2 I 0 Leff ( z [(1 + ( z
z0
z0
)k
) 2 )(9 + ( z
z0
--------------------- (2)
) 2 )]
Where, k is the magnitude of wave vector given by k=2π/λ.
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Results and Discussions 3.1 Structural Characterization
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Figure 1 shows the XRD spectra of the Zn1-xTixO (0≤x≤0.050) thin film deposited onto fused silica substrate. The XRD spectra in the range of 20-60 degree exhibit various diffraction
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peaks (hkl) indexed as (100), (002), (101), (102) and (110) of ZnO. It is noticed that no signature corresponding to individual Ti or TiO2 is observed in the films which gives a good sign of mutual substitution of Zn2+ ions by the Ti4+ ions.
The Zn1-xTixO thin film having x=0.02 composition shows the preferential c-axis
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orientation along 002 plane. Furthermore, it is also observed that the degree of crystallinity in the TZO thin films is improved with the addition of Ti content till x=0.020. At higher Ti doping (x>0.020), the 002 peak is distorted and the other phases; (100) and (101) are
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appearing to be more prominent. The distortion in the 002 peak could be due to the excess free electrons which occupy the vacant space between the lattice positions and void formation
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due to the presence of excessive amount of Ti. Since the substitution of Ti for Zn atom employs a mutual replacement of Zn2+ ions by Ti4+ ions, hence additional 2 free electrons add to the free carrier density. These free carriers are accommodated within the interstitial positions and thus can cause the deterioration in the c-axis oriented 002 peak. Jeng-Lin Chung et. al. [20] has reported that the excess of TiO2 in ZnO makes the ZnO-TiO2 composite, more amorphous in nature due to the segregation of excess Ti atoms at the grain boundaries. For 002 plane, crystallite size (D), the lattice parameters (a and c) were estimated
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ACCEPTED MANUSCRIPT as a function of x and are listed in table 1. There is a marginal change in the (002) peak position with x. The average crystallite size (D) in the 002 plane in the TZO thin films show a large
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variation in the range of 7.8-25.9 nm. From table 1, a gradual decrease of the average crystallite size with the increase in x value is observed till x=0.020 where it attains the minimum value of 7.8 nm. The decrease in the grain size in Ti doped ZnO thin film is mainly due to the mismatch in the ionic radii of the Ti4+ and Zn2+ ions. Since the estimated lattice
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constants (a, c) are observed to be less than that of the stress free lattice constants of pure
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ZnO hence, films undergo a compressive stress which results in the decrease in the grain size. A decrease in average crystalline size in Ti doped ZnO nanoparticles is also reported by Milton et. al [21]. A further increment in x leads to an increase in D values. This could be due to the abnormally large thickness of the samples for x>0.020 as depicted in Table 2. With the increase in the film thickness, the carrier concentration increases [22]. Therefore, the surface
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energy or grain boundary energy will be lowered due to the interaction between dopants and the surface or grain boundary. This will favor the adsorption of the impinging atoms and results in the larger crystallite size. It is observed that the x=0.020 value is a transition point
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where the substitution of Ti4+ ion at Zn2+ ion is most appropriate and productive towards the
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crystalline quality of the film. A significant variation in the lattice parameters (a and c) is observed in the Ti doped ZnO films as shown in table 1. A small deviation of the lattice parameters in the Zn1-xTixO film from the pure ZnO could be due to the difference in ionic radii of the participating species (r(Zn2+)=60 pm, r(Ti4+)=60.5 pm) [23]. Tsay et. al. have also reported quite similar variation in the lattice parameter (c) in sol-gel derived Zn1-xTixO thin films [24]. The ratio of lattice parameters (c/a) is estimated to be ~1.6 which is closed to the hexagonal crystal structure. However, this ratio shows a deviation from the ideal c/a ratio of the wurtzite hexagonal ZnO structure. The estimated lattice constants in the present case are 6
ACCEPTED MANUSCRIPT smaller than that of the lattice constants of stress free bulk ZnO crystal (a0=3.253Å and c0=5.209Å) [JCPDS data files] indicating the compressive stress in the films. Figure 2 shows the Raman spectra of Zn1-xTixO (0≤x≤0.050) thin films deposited onto
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fused silica substrate. The Raman spectra are recorded in backscattering geometry. ZnO belongs to wurtzite hexagonal crystal structure with C46V symmetry group. According to the Group theory, there are 12 phonon modes near the Γ point of brillouin zone center ascribed as
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an A1 branch, a doubly degenerate E1 branch, two doubly degenerate E2 branches and two B1 branches. The Raman modes; A1 and E1 are of polar nature and are split into longitudinal
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optical (LO) and transverse optical (TO) phonon modes and are Raman and IR active. The two fold degenerate E2 branches are Raman active only while the B1 branches are silent and inactive. In the present case, the Raman peaks positioned at 99 cm-1, 438 cm-1 and 577 cm-1 correspond to E2 (low), E2 (high) and A1 (LO) respectively are observed. No signature of
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Raman peaks related to TiO2 is observed in the Zn1-xTixO (0≤x≤0.050) thin films, which is in consistence with the XRD results. This also justifies the uniform substitution and mutual replacement of Ti4+ ions with the Zn2+ ions in the samples.
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The differences in the ionic radii of the two constituents (Ti4+ and Zn2+) are quiet small
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therefore; the substitution of Ti4+ ions with Zn2+ ions doesn’t affect the wurtzite crystal structure. The E2(low), E2 (high) and defects associated peak, A1(LO), are significantly modified. It is noted that the A1(LO) peak is comprehensively reduced in the TZO films which suggests the reduction of defect density in the system. The clear emergence of E2(low) and E2(high) peak in presence of Ti ions indicates the wurtzite crystal structure. This peak is very narrow and its intensity is maximum for x=0.020 which is in consensus with XRD observation. A weak peak at 276 cm-1 is also observed in the film which is due to the silent
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ACCEPTED MANUSCRIPT B1 Raman modes [25]. This peak has also been observed in ZnO doped with the other transition metals (Al, Sn etc) [26-27].
3.2 Optical Characterization of TZO Thin Films 3.2.1 Absorption Spectroscopy
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The optical absorption spectra of TZO thin films were recorded using UV-Vis-NIR spectrometer (M/s SHIMDZU, UV-3101PC) in the spectral range of 200-1000 nm and are shown in Fig. 3. From the absorbance spectrum, absorption coefficient, α, is calculated [17].
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The thickness of the thin film required for the calculation of α was measured via stylus
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profilometer and is listed in table 2. It is observed that with increasing Ti content, the thin film thickness increases despite of keeping all the other experimental conditions same. This increase in thickness is more pronounced for x>0.020. The increase in the film thickness for higher Ti concentration could be due to the reduction of the ablation threshold which results higher amount of material ablated from the target thereby, increasing the deposition rate. The
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inclusion of Ti into ZnO induces the free carrier concentration which results in increasing the absorption of the incident laser energy. Higher laser absorption lowers the ablation threshold resulting in increased ablation rate from the material [28]. Apart from this the composite
C, TiO2mp=1843 0C) which also favors relatively higher ablation rate as compared to that of
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0
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ZnO-TiO2 will be having lower melting point as compared to that of pure ZnO (ZnOmp=1975
pure ZnO [28-30].
The absorption edge is observed to be slightly shifted towards the lower wavelength
with the addition of Ti into the TZO thin films. This blue shift of the absorption edge is continued till x=0.02 and retraced to the red shift for higher values of x (x≥0.030). The band gap energy of the TZO thin films was estimated using Tauc’s plot as shown in Fig. 4 [17]. The band gap estimated from the tauc′s plot is listed in the table 2.
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ACCEPTED MANUSCRIPT A marginal increase in the band gap energy is observed in the TZO thin films with x till x=0.020 while it reduces slightly for the higher Ti content (x>0.020). The increase in the optical band gap energy for the TZO film is due to the quantum confinement effect as the crystallite size of the films is decreased as the Ti content is increased in the TZO samples,
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table 1 [31]. Since the exciton Bohr radius (aB) of ZnO is 2.34 nm and the estimated crystallite sizes for the low Ti concentration doped ZnO film is in the range of 7.8-11.8 nm, therefore, a weak quantum size effect could be possible [32]. At higher Ti concentration
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(x>0.02), the carrier concentration is increased enormously in the conduction and valence
carrier-impurity interaction [33]. 3.3.2 Transmission Spectroscopy
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band which leads to the shrinkage of band gap as a result of enhanced carrier-carrier and
The UV-Vis-NIR transmission spectra of the Zn1-xTixO (0≤x≤0.050) thin film fabricated via PLD technique are shown in Fig 5(a) in the spectral range of 200-2000 nm. It is very obvious
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that the transmittance of the TZO thin films decreases with the increase in the Ti concentration in the film. Furthermore, the interference fringes start diminishing at higher Ti
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concentration and disappears completely beyond x=0.020. The low optical transmittance at higher Ti concentration in ZnO thin film could be due to the strong absorption in thin films
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including linear absorption and free carrier absorption etc. At higher Ti concentration (x>0.020), the complete absence of interference fringes in
the spectrum indicates the deterioration of the film quality which is also indicated by the XRD and Raman measurement. The linear refractive indices (n0) are estimated in the TZO thin films using Swanepoel envelope method [34]. The estimation of linear refractive index (n) is carried out by using the following equations. 1
n = [N + (N 2 − S 2 ) 2 ]
1
2
------------------- (3) 9
ACCEPTED MANUSCRIPT N = 2S
TM − Tm S 2 + 1 + TM Tm 2
------------------- (4)
Where, S (=1.458) is the refractive index of fused silica substrate. The terms; TM and Tm denote the positions of consecutive maxima and minima in the transmission spectra. The
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linear refractive index (n) as a function of wavelength (λ) was extracted from Cauchydispersion relation by plotting a graph of (1/λ2) vs n evaluated at various maxima and minima
given by equation 5. B
λ2
-------------------- (5)
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n (λ ) = A +
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position of the transmission spectra. The expression for the Cauchy dispersion relation is
Where, A and B are the Cauchy parameters. The parameter, A, is independent of λ and is known as static refractive index, n.
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Figure 6 shows the plot of refractive indices vs. wavelength and the variation in the value of n at λ=633 nm with Ti concentration is displayed in the inset. The estimated linear refractive index decreases with the increase in wavelength which signifies the normal dispersion in the
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refractive index.
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The linear refractive indices in Zn1-xTixO thin films increase for the interval 0≤x≤0.02 as shown by the Fig. 6. The sample x=0.005 shows slightly lower value of n as compared to that of pure ZnO, which is within the error bar. The refractive indices for x>0.02 could not be determined due to the absence of interference fringes in the transmission spectra (Fig.5a). The increase in refractive indices with x may be due to the increased carrier concentration and is in accordance with that of documented in literature [35]. The linear refractive index of TiO2 is higher as compared to ZnO hence the refractive index of ZnO may be tuned with the infusion of low concentration of Ti ions.
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ACCEPTED MANUSCRIPT 3.3 Third Order Nonlinear Optical Study via z-scan Technique The third order NLO characterization of the films was carried out by using modified z-scan technique. This technique provides the simultaneous information of sign as well as magnitude of the optical nonlinearity possessed by the sample. Figure 7 shows the OA spectra for the
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Zn1-xTixO thin films along with error bars. All the films exhibit a transmittance minima at focus (z=0), Fig. 7, indicating the reverse saturation absorption (RSA) in all the samples. Nonlinear absorption coefficients (β) were estimated by fitting the OA z-scan data point
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using the equation 1.
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The nonlinear refractive indices (n2) of the TZO films are obtained from the CA data analysis. Figure 8 shows the CA spectra of Zn1-xTixO (0≤x≤0.050) thin films. A pre-focal transmittance maxima followed by a post focal transmittance minima in the CA graphs indicates the negative nonlinear refraction (self-defocusing) in these films.
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Additionally, the peak and valley separation (∆zp-v) in the CA Z-scan graphs turns out to be in the range of (1.69-1.8) × z0 which satisfies the condition for third order NLO process [19].
x=0.020.
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The nonlinear refractive indices were evaluated and were observed to be maximum for
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The real and imaginary parts of the third order nonlinear optical susceptibilities (χ(3)) are related to the n2 and β respectively. In highly absorbing media (linear absorption), the real and imaginary components of 3rd order nonlinear optical susceptibilities (χ(3) ׳and χ(3) )׳׳are expressed as [17, 36].
χ (3) '(esu ) = 10−7
n0 ' c ( n0 ' n2 '− n0 '' n2 '') 12π 2
--------------------- (7)
χ (3) ''(esu ) = 10−7
n0 ' c ( n0 ' n2 ''+ n0 '' n2 ') 12π 2
------------------------ (8)
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ACCEPTED MANUSCRIPT Where, n0=׳n, n0(=׳׳αλ/4π), n2=׳n2 and n2(=׳׳βλ/4π). Table 3 displays the Quantitative estimation of third order nonlinear optical coefficients (β, n2, χ(3) ׳and χ(3) ׳׳as a function of x.
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There are various mechanisms associated with the nonlinear absorption (NLA) in thin films such as multi photon absorption (MPA), free carrier absorption (FCA), exited state absorption (ESA) and nonlinear scattering etc. [37]. In the present z-scan experiment, under
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the cw laser excitation (λ=632.8 nm), the dominant process for the nonlinear absorption may be due to FCA as it depends on the factor λp, where p>1. The typical values of p are
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1.5≤p≤3.5 [38]. However, a contribution from the two photon absorption process (TPA) is vital as the excitation photon energy (hv=1.92 eV) is more than half of the band gap energy of TZO films [39-40].
The value of nonlinear absorption coefficients (β) in the Z1-xTixO thin film is found to
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be maximum for x=0.020 and thereafter it drops down with further increase in the value of x. Since the observed nonlinear absorption in the films is dominated by FCA hence the NLA is
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increased in the TZO thin films as compared to that of pure ZnO film. This is also supported by the fact that the conductivity of TZO thin films is enhanced as a result of large free carrier
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density as compared to that of pure ZnO which in turn enhances the optical nonlinearity [7, 41-42]. The other reason for the enhancement in NLA coefficient in TZO thin films could be due to the enormous photoluminescence signal as a result of fluorescence resonance energy transfer (FRET) effect [43]. This effect is more prominent in a set of materials whose absorption and emission bands are close enough. In this process the photo generated electron hole pair of the donor atom undergo a non-radiative energy transfer to the acceptor atom resulting into the huge enhancement of band edge emission from the acceptor.
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ACCEPTED MANUSCRIPT Figure 9 shows the variation of absorption coefficient (α), nonlinear absorption coefficient (β) and nonlinear refractive index (n2) with Ti concentration, x. The values of NLO coefficients (β and n2) along with the nonlinear optical susceptibilities (3) ׳׳
), Table 3, are large as compared to that of the NLO coefficients observed
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(χ(3) ׳and χ
under short pulsed laser illumination [44-45]. This could be because of the thermally induced NLO coefficients as a result of CW laser illumination. The thermally induced nonlinear
--------------------- (9)
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2 dn α R n2th = dT κ
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refractive index (n2th) relates with α via following expressions.
Where, dn/dT is the variation of refractive index with temperature, α being the linear absorption coefficient, R is the radius of laser beam and κ is the thermal conductivity of the sample.
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It is remarkable to note that the n2th is proportional to α which supports the increasing behavior of n2 with the increasing Ti concentration till x=0.020 since the magnitude of α
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increases with x as shown in Fig. 10. The theoretically estimated value of n2th ~10-3 cm2/W, is found to be close to the experimentally observed n2 values as listed in table 3. This further
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justifies the thermally induced optical nonlinearity in the films. Similar values have also been reported earlier for thermally induced nonlinearity in ZnO and other materials [46-49]. The estimated values of β and n2 do not follow the linear relationship with α for x>0.020. This discrepancy arises due to the deterioration of the crystalline quality of the films for x>0.020 as indicated by XRD and Raman characterization, Fig. 1 and 2. Although in the present case, by the virtue of the measurement, the thermally induced non linearity is dominant yet the various other mechanisms behind it cannot be ruled out. Thus values of n2 and β obtained can be referred as the effective nonlinear coefficients arising due to the several mechanisms. 13
ACCEPTED MANUSCRIPT 4. Conclusion The Zn1-xTixO (0≤x≤0.050) thin films were fabricated via pulsed laser deposition technique. The XRD and Raman studies revealed the polycrystalline phase of the films. Moreover, the growth of the films along 002 plane was observed to be improved in the films upto x=0.020
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which was in accordance to the substitution of the Zn2+ ions by the Ti4+ ions. However the high Ti concentration (x>0.020), resulted into the deterioration of the 002 peak as a result of high free carrier density effect caused by the excess free electrons during the mutual
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replacement of both the ions, Zn2+ and Ti4+. A tunable band gap energies and linear refractive index coefficients were exhibited by the films with infusion of Ti ions in ZnO which can
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serve as an excellent candidate towards shorter wavelength optoelectronics and nonlinear photonic devices. The films showed good nonlinear optical response and the maximum nonlinear optical coefficients (β and n2) were achieved from the film fabricated with x=0.020 composition. The study showed that a suitable selection of Ti concentration in Zn1-xTixO thin
Acknowledgement
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films can find its applications in UV light emitter and nonlinear photonic devices.
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Author would like to acknowledge the central instrument facility (cif), Indian Institute of Technology Guwahati, for providing the necessary analyzing tool for the completion of this
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work.
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[44] Yun-Pei Chan, Ja-Hon Lin, Chih-Chang Hsu, Wen-Feng Hsieh, Near-resonant high order nonlinear absorption of ZnO thin films, Opt. Exp. 16 (2008) 19900-19908. [45] X. J. Zhang, W. Ji, S. H. Tang, Determination of optical nonlinearities and carrier lifetime in
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under cw laser illumination, Opt. Mat. 35 (2013) 431–439. [47] M Abd-Lefdil, A Belayachi, S Pramodini, P Poornesh, A Wojciechowski, A O Fedorchuk, Structural, photoinduced optical effects and third order nonlinear optical studies on Mn doped and Mn-Al codoped ZnO thin films under continuous wave laser irradiation, Laser Phys. 24 (2014) 035404. [48] K Spoorthi, S Pramodini, I V Kityk, M Abd-Lefdil, M Sekkati, A El Fakir, Ashok Rao, Ganesh Sanjeev, P Poornesh, Investigations on nonlinear optical properties of electron beam treated Gd:ZnO thin films for photonic device applications, Laser Phys. 27 (2017) 065403. 18
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optical nonlinearity and optical limiting in symmetric and unsymmetrical phthalocyanines
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studied using Z-scan, Opt. Com. 280 (2007) 206–212.
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D (nm)
a (Å Å)
17.0±1.5 11.9±0.6 11.4±0.7 7.8±0.2 26.0±0.6 25.9±0.7
3.240 3.241 3.243 3.244 3.239 3.239
c (Å Å)
5.204 5.207 5.209 5.214 5.192 5.192
c/a
1.603 1.607 1.606 1.607 1.603 1.602
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0.000 0.005 0.010 0.020 0.030 0.050
002 peak position (degree) 34.46 34.42 34.42 34.37 34.53 34.52
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Sample (x)
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films.
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Table 1: Variation of crystallite size and lattice parameter of Zn1-xTixO (0≤x≤0.050) thin
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Table 2 Thin film thickness (L), linear absorption coefficients (α) and band gap energy (Eg)
Thickness (nm)
Abs. Coeff. (cm-1)×104 at λ=633 nm
0.000
520
0.677
0.005
680
0.640
0.010
690
0.985
0.020
650
1.527
0.030
1000
0.050
1200
Band gap energy (eV)
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3.26 3.29 3.30 3.40
2.247
3.25
2.669
3.12
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Sample (x)
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with Ti concentration (x)
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Table 3 Third order nonlinear optical coefficients (β, n2, χ(3) ׳, χ(3)) ׳׳of Zn1-xTixO
Sample (x)
β (cm/W)
n2 (cm2/W) ×10-4
χ(3) ( ׳esu)
χ(3) ( ׳׳esu)
×10-3
×10-3
5.45±1.04
1.02±0.19
0.000
3.4±0.2
1.0±0.1
2.
0.005
3.1±0.4
0.4±0.1
3.
0.010
4.0±0.3
2.8±0.6
4.
0.020
6.0±0.3
5.
0.030
4.5±0.5
6.
0.050
1.1±0.2
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1.
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Sr. No.
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(0≤x≤0.050) thin films.
2.05±0.44
0.84±0.12
15.16±3.64
1.44±0.13
24.63±3.17
2.62±0.15
1.6±0.3
-------
-------
1.9±0.4
-------
-------
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4.1±0.5
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Figure Captions
1. Fig. 1 XRD spectra of Zn1-xTixO (0≤x≤0.050) thin films
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2. Fig. 2 Raman spectra of Zn1-xTixO (0≤x≤0.050) thin films. 3.
Fig. 3 Absorption spectra of Zn1-xTixO (0≤x≤0.050) thin films
4.
Fig. 4 Tauc’s plot of Zn1-xTixO (0≤x≤0.050) thin films.
5. Fig. 5 (a) Transmittance spectra of Zn1-xTixO (0≤x≤0.050) thin films and (b) Swanepoel envelope fitted for x=0.000 thin film.
nm with Ti concentration (x) (inset)
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6. Fig. 6 Plot of refractive index vs wavelength and variation of refractive index at 633
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7. Fig. 7 Open aperture z-scan graph of Zn1-xTixO thin films (a) x=0.000, (b) x=0.005, (c) x=0.010, (d) x=0.020, (e) x=0.030 and (f) x=0.050
8. Fig. 8 Closed aperture z-scan graph of Zn1-xTixO thin films (a) x=0.000, (b) x=0.005, (c) x=0.010, (d) x=0.020, (e) x=0.030 and (f) x=0.050.
9. Fig. 9 Variation of (a) nonlinear absorption coefficient (β) and (b) nonlinear refractive
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index (n2) with linear absorption coefficient in Zn1-xTixO (0≤x≤0.050) thin films.
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Highlights
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Fabrication of high quality Zn1-xTixO (0≤x≤0.050) thin films via PLD technique. The Ti concentration plays significant role on the crystalline quality of the ZnO films. The tunable refractive index is obtained in the films. The thin films showed reverse saturation absorption. The third order nonlinear optical coefficients (β and n2) increase with Ti concentration.
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1. 2. 3. 4. 5.
S0254-0584(18)30502-9
DOI:
10.1016/j.matchemphys.2018.06.004
Reference:
MAC 20703
To appear in:
Materials Chemistry and Physics
Received Date: 2 February 2018 Accepted Date: 2 June 2018
Please cite this article as: Gyan Prakash Bharti, Partha P. Dey, Alika Khare, Pulsed laser deposited Zn1xTixO (0=x=0.050) thin films for tunable refractive index and nonlinear optical applications, (2018), doi: 10.1016/j.matchemphys.2018.06.004 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.
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Pulsed laser deposited Zn1-xTixO (0.000≤x≤0.050) thin films for tunable refractive index and nonlinear optical applications
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Gyan Prakash Bharti, Partha P. Dey and Alika Khare* Department of Physics, IIT Guwahati, Guwahati-Assam (India) *Email: [email protected]
Abstract: In the present paper, a systematic study on the structural, linear and nonlinear optical properties of Zn1-xTixO (0≤x≤0.050) thin films is documented. The thin films are grown onto fused silica substrate via pulsed laser deposition technique employing 2nd
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harmonic of a Q-switched Nd:YAG laser (532 nm, 10ns, 10Hz). The crystallinity of the film
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is observed to be increased initially up-to x=0.02 and then decreased for higher values of x. A small variation in band gap energy is observed in the Zn1-xTixO thin films with x which is mainly due to the Burstein Moss effect followed by the weak quantum confinement effect. The third order optical nonlinearities in the films are experimentally recorded using modified z-scan technique under cw He:Ne laser (λ=632.8 nm) illumination. The nonlinear optical
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coefficients; β and n2, are observed to be in the range of 1.1-6.0 cm/W and (1.0-4.1)×10-4 cm2/W respectively. The maximum value of β (6.0 cm/W) and n2 (4.1 cm2/W) are observed
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in the Zn1-xTixO film having x=0.02. These studies suggest that an optimum concentration of Ti content in ZnO is required for the enhancement in nonlinear optical behavior.
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Keywords: pulsed laser deposition, Zn1-xTixO thin film, nonlinear optical property, z-scan
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ACCEPTED MANUSCRIPT 1. Introduction: Zinc oxide (ZnO) based semiconducting materials are appearing to be very promising in comparison to GaN, GaP etc due to its excellent optical and electrical behavior [1]. The most important feature of this material is its direct and high band gap energy which is the basic
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requirement for the optoelectronic based devices especially towards the short wavelength regime. Additionally, the high exciton binding energy of ZnO, makes it stable at room temperature resulting into controlling the spectral broadening. The UV optical emission
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governed by the excitonic recombination results in a narrower spectral width rather than broader spectra which are dominated by free electron and holes recombination within the
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band edge of a semiconductor. Under the influence of external impurities in ZnO, its structural, electrical and optical behaviors are drastically affected [2-6]. The selection of the impurity in ZnO depends on its applications. The suitable electron rich (n-type) impurity in ZnO enhances the free carrier concentration and consequently its electrical conductivity as
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well as nonlinear optical (NLO) properties undergoes a modification [7-8]. Among the various impurities in ZnO [3-9], Ti infused ZnO (TZO) films are finding its wide applications in photovoltaic cell, optoelectronic devices etc.[10-12]. TiO2 possesses higher linear
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refractive index as compared to that of ZnO hence a suitable percentage of Ti impurity in ZnO can lead to a tunable refractive index medium which may find its application in optical
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communication, optical switching and data storage devices etc. [13]. Ti doped ZnO thin films have been reported by several research groups using variety of fabrication techniques, viz; sol-gel, magnetron sputtering, atomic layer deposition, PLD etc. [9, 14-16]. However, there are a few reports which actually deal with a large variation of Ti concentration in ZnO film and consequently its impact on the various physical and optical properties of ZnO film. Among all the techniques, pulsed laser deposition (PLD) can serve as a promising technique for the fabrication of the complex oxide thin films of Zn1-xTixO (0≤x≤0.050). The fine control
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ACCEPTED MANUSCRIPT over the various deposition parameters as well as the stoichiometric transfer of the ablated materials from the target to the substrate makes PLD, a versatile fabrication tool. In the succeeding sections, the fabrication of the Zn1-xTixO (0≤x≤0.050) thin films via PLD technique has been presented. The structural characterization of the films has been carried out
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by using X-Ray Diffraction (XRD) and Raman spectroscopic measurements. The linear optical parameters (absorption coefficients, band gap energies and refractive indices) of the films have been estimated from UV-Vis-NIR spectroscopic analysis. The third order NLO
2. Experimental Details:
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laser irradiation operating at a wavelength of 632.8 nm.
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response of the films is investigated from the modified Z-scan technique under cw He:Ne
Zn1-xTixO (0≤x≤0.050) thin films were fabricated onto polished fused silica substrate (1×1 cm2) via PLD technique. The details of the PLD set-up used in the present work, is documented in our earlier publication [17]. The PLD targets in form of circular disc having
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diameter of 13 mm, were prepared from the pure ZnO (s. d. fine-chem Ltd., 99%) and Pure TiO2 (Ranboxy, 98%) powders, mixed in appropriate proportion via solid state synthesis
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method so as to obtain the hard pellets of Zn1-xTixO (0≤x≤0.050). All the Ti doped ZnO (TZO) targets were sintered at 1150 0C temperature for 6 hrs in an electric furnace. The
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deposition parameters were kept same as in the case of earlier reported Zn1-xAlxO (0≤x≤0.10) thin films [17]. The thickness of the deposited TZO thin films were measured by using Stylus Profilometer (Model: Veeco- Dektak 6M). In order to get information about the phase and crystal structure of the films, XRD spectra for all the samples were recorded using X-Rays Diffractometer (Rigaku, TTRAX III 18kW) operating at a wavelength of 1.5407 Å of Cu-Kα line. The XRD patterns were recorded using GIXRD mode where, the glancing angle (ω) was kept at 1˚. The Laser Micro Raman
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of the Zn1-xTixO thin films were evaluated by using UV-Vis-NIR spectrophotometer (Model No. SHIMADZU UV-3101 PC).
The NLO characterization of Zn1-xTixO thin films was carried out by using modified z-scan
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technique under cw He:Ne laser irradiation operating at a wavelength of 632.8 nm [18]. This technique works on the spatial narrowing and broadening of the far field pattern of a focused
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Gaussian beam as a result of intensity dependent nonlinear optical behavior of the sample. The working principle of the experimental setup and the experimental parameters are reported in the literatures [17-18]. The third order optical nonlinearity via z-scan technique is experimentally measured in two configurations; open aperture (OA) and closed aperture
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(CA). The normalized transmittance in OA configuration (Topen) as a function of sample position (z) is given by the following expression [17].
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Topen ( z ) = 1 −
β I 0 Leff 3
2 2 [1 + ( z
z0
------------------- (1) )2 ]
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Where, β represents the nonlinear absorption coefficient, I0 (=1.12 kW/cm2) is the intensity of the incident beam at the focus (z=0), Leff is the effective film thickness defined as Leff=(1-e)/α, α being the linear absorption coefficient and z0 (=2.2 mm) represents the Rayleigh
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length of the beam which is expressed as z0=πω2/λ, where ω (=21 µ) is the beam waist of the focused beam and λ, the wavelength of the laser beam used in the present experiment. The nonlinear refractive index (n2) is evaluated from the CA graphs and using the following equation [19]. 4
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4n2 I 0 Leff ( z [(1 + ( z
z0
z0
)k
) 2 )(9 + ( z
z0
--------------------- (2)
) 2 )]
Where, k is the magnitude of wave vector given by k=2π/λ.
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Results and Discussions 3.1 Structural Characterization
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Figure 1 shows the XRD spectra of the Zn1-xTixO (0≤x≤0.050) thin film deposited onto fused silica substrate. The XRD spectra in the range of 20-60 degree exhibit various diffraction
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peaks (hkl) indexed as (100), (002), (101), (102) and (110) of ZnO. It is noticed that no signature corresponding to individual Ti or TiO2 is observed in the films which gives a good sign of mutual substitution of Zn2+ ions by the Ti4+ ions.
The Zn1-xTixO thin film having x=0.02 composition shows the preferential c-axis
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orientation along 002 plane. Furthermore, it is also observed that the degree of crystallinity in the TZO thin films is improved with the addition of Ti content till x=0.020. At higher Ti doping (x>0.020), the 002 peak is distorted and the other phases; (100) and (101) are
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appearing to be more prominent. The distortion in the 002 peak could be due to the excess free electrons which occupy the vacant space between the lattice positions and void formation
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due to the presence of excessive amount of Ti. Since the substitution of Ti for Zn atom employs a mutual replacement of Zn2+ ions by Ti4+ ions, hence additional 2 free electrons add to the free carrier density. These free carriers are accommodated within the interstitial positions and thus can cause the deterioration in the c-axis oriented 002 peak. Jeng-Lin Chung et. al. [20] has reported that the excess of TiO2 in ZnO makes the ZnO-TiO2 composite, more amorphous in nature due to the segregation of excess Ti atoms at the grain boundaries. For 002 plane, crystallite size (D), the lattice parameters (a and c) were estimated
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variation in the range of 7.8-25.9 nm. From table 1, a gradual decrease of the average crystallite size with the increase in x value is observed till x=0.020 where it attains the minimum value of 7.8 nm. The decrease in the grain size in Ti doped ZnO thin film is mainly due to the mismatch in the ionic radii of the Ti4+ and Zn2+ ions. Since the estimated lattice
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constants (a, c) are observed to be less than that of the stress free lattice constants of pure
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ZnO hence, films undergo a compressive stress which results in the decrease in the grain size. A decrease in average crystalline size in Ti doped ZnO nanoparticles is also reported by Milton et. al [21]. A further increment in x leads to an increase in D values. This could be due to the abnormally large thickness of the samples for x>0.020 as depicted in Table 2. With the increase in the film thickness, the carrier concentration increases [22]. Therefore, the surface
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energy or grain boundary energy will be lowered due to the interaction between dopants and the surface or grain boundary. This will favor the adsorption of the impinging atoms and results in the larger crystallite size. It is observed that the x=0.020 value is a transition point
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where the substitution of Ti4+ ion at Zn2+ ion is most appropriate and productive towards the
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crystalline quality of the film. A significant variation in the lattice parameters (a and c) is observed in the Ti doped ZnO films as shown in table 1. A small deviation of the lattice parameters in the Zn1-xTixO film from the pure ZnO could be due to the difference in ionic radii of the participating species (r(Zn2+)=60 pm, r(Ti4+)=60.5 pm) [23]. Tsay et. al. have also reported quite similar variation in the lattice parameter (c) in sol-gel derived Zn1-xTixO thin films [24]. The ratio of lattice parameters (c/a) is estimated to be ~1.6 which is closed to the hexagonal crystal structure. However, this ratio shows a deviation from the ideal c/a ratio of the wurtzite hexagonal ZnO structure. The estimated lattice constants in the present case are 6
ACCEPTED MANUSCRIPT smaller than that of the lattice constants of stress free bulk ZnO crystal (a0=3.253Å and c0=5.209Å) [JCPDS data files] indicating the compressive stress in the films. Figure 2 shows the Raman spectra of Zn1-xTixO (0≤x≤0.050) thin films deposited onto
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fused silica substrate. The Raman spectra are recorded in backscattering geometry. ZnO belongs to wurtzite hexagonal crystal structure with C46V symmetry group. According to the Group theory, there are 12 phonon modes near the Γ point of brillouin zone center ascribed as
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an A1 branch, a doubly degenerate E1 branch, two doubly degenerate E2 branches and two B1 branches. The Raman modes; A1 and E1 are of polar nature and are split into longitudinal
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optical (LO) and transverse optical (TO) phonon modes and are Raman and IR active. The two fold degenerate E2 branches are Raman active only while the B1 branches are silent and inactive. In the present case, the Raman peaks positioned at 99 cm-1, 438 cm-1 and 577 cm-1 correspond to E2 (low), E2 (high) and A1 (LO) respectively are observed. No signature of
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Raman peaks related to TiO2 is observed in the Zn1-xTixO (0≤x≤0.050) thin films, which is in consistence with the XRD results. This also justifies the uniform substitution and mutual replacement of Ti4+ ions with the Zn2+ ions in the samples.
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The differences in the ionic radii of the two constituents (Ti4+ and Zn2+) are quiet small
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therefore; the substitution of Ti4+ ions with Zn2+ ions doesn’t affect the wurtzite crystal structure. The E2(low), E2 (high) and defects associated peak, A1(LO), are significantly modified. It is noted that the A1(LO) peak is comprehensively reduced in the TZO films which suggests the reduction of defect density in the system. The clear emergence of E2(low) and E2(high) peak in presence of Ti ions indicates the wurtzite crystal structure. This peak is very narrow and its intensity is maximum for x=0.020 which is in consensus with XRD observation. A weak peak at 276 cm-1 is also observed in the film which is due to the silent
7
ACCEPTED MANUSCRIPT B1 Raman modes [25]. This peak has also been observed in ZnO doped with the other transition metals (Al, Sn etc) [26-27].
3.2 Optical Characterization of TZO Thin Films 3.2.1 Absorption Spectroscopy
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The optical absorption spectra of TZO thin films were recorded using UV-Vis-NIR spectrometer (M/s SHIMDZU, UV-3101PC) in the spectral range of 200-1000 nm and are shown in Fig. 3. From the absorbance spectrum, absorption coefficient, α, is calculated [17].
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The thickness of the thin film required for the calculation of α was measured via stylus
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profilometer and is listed in table 2. It is observed that with increasing Ti content, the thin film thickness increases despite of keeping all the other experimental conditions same. This increase in thickness is more pronounced for x>0.020. The increase in the film thickness for higher Ti concentration could be due to the reduction of the ablation threshold which results higher amount of material ablated from the target thereby, increasing the deposition rate. The
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inclusion of Ti into ZnO induces the free carrier concentration which results in increasing the absorption of the incident laser energy. Higher laser absorption lowers the ablation threshold resulting in increased ablation rate from the material [28]. Apart from this the composite
C, TiO2mp=1843 0C) which also favors relatively higher ablation rate as compared to that of
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0
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ZnO-TiO2 will be having lower melting point as compared to that of pure ZnO (ZnOmp=1975
pure ZnO [28-30].
The absorption edge is observed to be slightly shifted towards the lower wavelength
with the addition of Ti into the TZO thin films. This blue shift of the absorption edge is continued till x=0.02 and retraced to the red shift for higher values of x (x≥0.030). The band gap energy of the TZO thin films was estimated using Tauc’s plot as shown in Fig. 4 [17]. The band gap estimated from the tauc′s plot is listed in the table 2.
8
ACCEPTED MANUSCRIPT A marginal increase in the band gap energy is observed in the TZO thin films with x till x=0.020 while it reduces slightly for the higher Ti content (x>0.020). The increase in the optical band gap energy for the TZO film is due to the quantum confinement effect as the crystallite size of the films is decreased as the Ti content is increased in the TZO samples,
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table 1 [31]. Since the exciton Bohr radius (aB) of ZnO is 2.34 nm and the estimated crystallite sizes for the low Ti concentration doped ZnO film is in the range of 7.8-11.8 nm, therefore, a weak quantum size effect could be possible [32]. At higher Ti concentration
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(x>0.02), the carrier concentration is increased enormously in the conduction and valence
carrier-impurity interaction [33]. 3.3.2 Transmission Spectroscopy
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band which leads to the shrinkage of band gap as a result of enhanced carrier-carrier and
The UV-Vis-NIR transmission spectra of the Zn1-xTixO (0≤x≤0.050) thin film fabricated via PLD technique are shown in Fig 5(a) in the spectral range of 200-2000 nm. It is very obvious
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that the transmittance of the TZO thin films decreases with the increase in the Ti concentration in the film. Furthermore, the interference fringes start diminishing at higher Ti
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concentration and disappears completely beyond x=0.020. The low optical transmittance at higher Ti concentration in ZnO thin film could be due to the strong absorption in thin films
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including linear absorption and free carrier absorption etc. At higher Ti concentration (x>0.020), the complete absence of interference fringes in
the spectrum indicates the deterioration of the film quality which is also indicated by the XRD and Raman measurement. The linear refractive indices (n0) are estimated in the TZO thin films using Swanepoel envelope method [34]. The estimation of linear refractive index (n) is carried out by using the following equations. 1
n = [N + (N 2 − S 2 ) 2 ]
1
2
------------------- (3) 9
ACCEPTED MANUSCRIPT N = 2S
TM − Tm S 2 + 1 + TM Tm 2
------------------- (4)
Where, S (=1.458) is the refractive index of fused silica substrate. The terms; TM and Tm denote the positions of consecutive maxima and minima in the transmission spectra. The
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linear refractive index (n) as a function of wavelength (λ) was extracted from Cauchydispersion relation by plotting a graph of (1/λ2) vs n evaluated at various maxima and minima
given by equation 5. B
λ2
-------------------- (5)
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n (λ ) = A +
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position of the transmission spectra. The expression for the Cauchy dispersion relation is
Where, A and B are the Cauchy parameters. The parameter, A, is independent of λ and is known as static refractive index, n.
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Figure 6 shows the plot of refractive indices vs. wavelength and the variation in the value of n at λ=633 nm with Ti concentration is displayed in the inset. The estimated linear refractive index decreases with the increase in wavelength which signifies the normal dispersion in the
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refractive index.
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The linear refractive indices in Zn1-xTixO thin films increase for the interval 0≤x≤0.02 as shown by the Fig. 6. The sample x=0.005 shows slightly lower value of n as compared to that of pure ZnO, which is within the error bar. The refractive indices for x>0.02 could not be determined due to the absence of interference fringes in the transmission spectra (Fig.5a). The increase in refractive indices with x may be due to the increased carrier concentration and is in accordance with that of documented in literature [35]. The linear refractive index of TiO2 is higher as compared to ZnO hence the refractive index of ZnO may be tuned with the infusion of low concentration of Ti ions.
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ACCEPTED MANUSCRIPT 3.3 Third Order Nonlinear Optical Study via z-scan Technique The third order NLO characterization of the films was carried out by using modified z-scan technique. This technique provides the simultaneous information of sign as well as magnitude of the optical nonlinearity possessed by the sample. Figure 7 shows the OA spectra for the
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Zn1-xTixO thin films along with error bars. All the films exhibit a transmittance minima at focus (z=0), Fig. 7, indicating the reverse saturation absorption (RSA) in all the samples. Nonlinear absorption coefficients (β) were estimated by fitting the OA z-scan data point
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using the equation 1.
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The nonlinear refractive indices (n2) of the TZO films are obtained from the CA data analysis. Figure 8 shows the CA spectra of Zn1-xTixO (0≤x≤0.050) thin films. A pre-focal transmittance maxima followed by a post focal transmittance minima in the CA graphs indicates the negative nonlinear refraction (self-defocusing) in these films.
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Additionally, the peak and valley separation (∆zp-v) in the CA Z-scan graphs turns out to be in the range of (1.69-1.8) × z0 which satisfies the condition for third order NLO process [19].
x=0.020.
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The nonlinear refractive indices were evaluated and were observed to be maximum for
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The real and imaginary parts of the third order nonlinear optical susceptibilities (χ(3)) are related to the n2 and β respectively. In highly absorbing media (linear absorption), the real and imaginary components of 3rd order nonlinear optical susceptibilities (χ(3) ׳and χ(3) )׳׳are expressed as [17, 36].
χ (3) '(esu ) = 10−7
n0 ' c ( n0 ' n2 '− n0 '' n2 '') 12π 2
--------------------- (7)
χ (3) ''(esu ) = 10−7
n0 ' c ( n0 ' n2 ''+ n0 '' n2 ') 12π 2
------------------------ (8)
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ACCEPTED MANUSCRIPT Where, n0=׳n, n0(=׳׳αλ/4π), n2=׳n2 and n2(=׳׳βλ/4π). Table 3 displays the Quantitative estimation of third order nonlinear optical coefficients (β, n2, χ(3) ׳and χ(3) ׳׳as a function of x.
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There are various mechanisms associated with the nonlinear absorption (NLA) in thin films such as multi photon absorption (MPA), free carrier absorption (FCA), exited state absorption (ESA) and nonlinear scattering etc. [37]. In the present z-scan experiment, under
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the cw laser excitation (λ=632.8 nm), the dominant process for the nonlinear absorption may be due to FCA as it depends on the factor λp, where p>1. The typical values of p are
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1.5≤p≤3.5 [38]. However, a contribution from the two photon absorption process (TPA) is vital as the excitation photon energy (hv=1.92 eV) is more than half of the band gap energy of TZO films [39-40].
The value of nonlinear absorption coefficients (β) in the Z1-xTixO thin film is found to
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be maximum for x=0.020 and thereafter it drops down with further increase in the value of x. Since the observed nonlinear absorption in the films is dominated by FCA hence the NLA is
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increased in the TZO thin films as compared to that of pure ZnO film. This is also supported by the fact that the conductivity of TZO thin films is enhanced as a result of large free carrier
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density as compared to that of pure ZnO which in turn enhances the optical nonlinearity [7, 41-42]. The other reason for the enhancement in NLA coefficient in TZO thin films could be due to the enormous photoluminescence signal as a result of fluorescence resonance energy transfer (FRET) effect [43]. This effect is more prominent in a set of materials whose absorption and emission bands are close enough. In this process the photo generated electron hole pair of the donor atom undergo a non-radiative energy transfer to the acceptor atom resulting into the huge enhancement of band edge emission from the acceptor.
12
ACCEPTED MANUSCRIPT Figure 9 shows the variation of absorption coefficient (α), nonlinear absorption coefficient (β) and nonlinear refractive index (n2) with Ti concentration, x. The values of NLO coefficients (β and n2) along with the nonlinear optical susceptibilities (3) ׳׳
), Table 3, are large as compared to that of the NLO coefficients observed
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(χ(3) ׳and χ
under short pulsed laser illumination [44-45]. This could be because of the thermally induced NLO coefficients as a result of CW laser illumination. The thermally induced nonlinear
--------------------- (9)
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2 dn α R n2th = dT κ
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refractive index (n2th) relates with α via following expressions.
Where, dn/dT is the variation of refractive index with temperature, α being the linear absorption coefficient, R is the radius of laser beam and κ is the thermal conductivity of the sample.
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It is remarkable to note that the n2th is proportional to α which supports the increasing behavior of n2 with the increasing Ti concentration till x=0.020 since the magnitude of α
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increases with x as shown in Fig. 10. The theoretically estimated value of n2th ~10-3 cm2/W, is found to be close to the experimentally observed n2 values as listed in table 3. This further
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justifies the thermally induced optical nonlinearity in the films. Similar values have also been reported earlier for thermally induced nonlinearity in ZnO and other materials [46-49]. The estimated values of β and n2 do not follow the linear relationship with α for x>0.020. This discrepancy arises due to the deterioration of the crystalline quality of the films for x>0.020 as indicated by XRD and Raman characterization, Fig. 1 and 2. Although in the present case, by the virtue of the measurement, the thermally induced non linearity is dominant yet the various other mechanisms behind it cannot be ruled out. Thus values of n2 and β obtained can be referred as the effective nonlinear coefficients arising due to the several mechanisms. 13
ACCEPTED MANUSCRIPT 4. Conclusion The Zn1-xTixO (0≤x≤0.050) thin films were fabricated via pulsed laser deposition technique. The XRD and Raman studies revealed the polycrystalline phase of the films. Moreover, the growth of the films along 002 plane was observed to be improved in the films upto x=0.020
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which was in accordance to the substitution of the Zn2+ ions by the Ti4+ ions. However the high Ti concentration (x>0.020), resulted into the deterioration of the 002 peak as a result of high free carrier density effect caused by the excess free electrons during the mutual
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replacement of both the ions, Zn2+ and Ti4+. A tunable band gap energies and linear refractive index coefficients were exhibited by the films with infusion of Ti ions in ZnO which can
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serve as an excellent candidate towards shorter wavelength optoelectronics and nonlinear photonic devices. The films showed good nonlinear optical response and the maximum nonlinear optical coefficients (β and n2) were achieved from the film fabricated with x=0.020 composition. The study showed that a suitable selection of Ti concentration in Zn1-xTixO thin
Acknowledgement
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films can find its applications in UV light emitter and nonlinear photonic devices.
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Author would like to acknowledge the central instrument facility (cif), Indian Institute of Technology Guwahati, for providing the necessary analyzing tool for the completion of this
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work.
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D (nm)
a (Å Å)
17.0±1.5 11.9±0.6 11.4±0.7 7.8±0.2 26.0±0.6 25.9±0.7
3.240 3.241 3.243 3.244 3.239 3.239
c (Å Å)
5.204 5.207 5.209 5.214 5.192 5.192
c/a
1.603 1.607 1.606 1.607 1.603 1.602
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0.000 0.005 0.010 0.020 0.030 0.050
002 peak position (degree) 34.46 34.42 34.42 34.37 34.53 34.52
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Sample (x)
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films.
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Table 1: Variation of crystallite size and lattice parameter of Zn1-xTixO (0≤x≤0.050) thin
20
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Table 2 Thin film thickness (L), linear absorption coefficients (α) and band gap energy (Eg)
Thickness (nm)
Abs. Coeff. (cm-1)×104 at λ=633 nm
0.000
520
0.677
0.005
680
0.640
0.010
690
0.985
0.020
650
1.527
0.030
1000
0.050
1200
Band gap energy (eV)
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3.26 3.29 3.30 3.40
2.247
3.25
2.669
3.12
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Sample (x)
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with Ti concentration (x)
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Table 3 Third order nonlinear optical coefficients (β, n2, χ(3) ׳, χ(3)) ׳׳of Zn1-xTixO
Sample (x)
β (cm/W)
n2 (cm2/W) ×10-4
χ(3) ( ׳esu)
χ(3) ( ׳׳esu)
×10-3
×10-3
5.45±1.04
1.02±0.19
0.000
3.4±0.2
1.0±0.1
2.
0.005
3.1±0.4
0.4±0.1
3.
0.010
4.0±0.3
2.8±0.6
4.
0.020
6.0±0.3
5.
0.030
4.5±0.5
6.
0.050
1.1±0.2
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1.
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Sr. No.
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(0≤x≤0.050) thin films.
2.05±0.44
0.84±0.12
15.16±3.64
1.44±0.13
24.63±3.17
2.62±0.15
1.6±0.3
-------
-------
1.9±0.4
-------
-------
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4.1±0.5
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Figure Captions
1. Fig. 1 XRD spectra of Zn1-xTixO (0≤x≤0.050) thin films
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2. Fig. 2 Raman spectra of Zn1-xTixO (0≤x≤0.050) thin films. 3.
Fig. 3 Absorption spectra of Zn1-xTixO (0≤x≤0.050) thin films
4.
Fig. 4 Tauc’s plot of Zn1-xTixO (0≤x≤0.050) thin films.
5. Fig. 5 (a) Transmittance spectra of Zn1-xTixO (0≤x≤0.050) thin films and (b) Swanepoel envelope fitted for x=0.000 thin film.
nm with Ti concentration (x) (inset)
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6. Fig. 6 Plot of refractive index vs wavelength and variation of refractive index at 633
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7. Fig. 7 Open aperture z-scan graph of Zn1-xTixO thin films (a) x=0.000, (b) x=0.005, (c) x=0.010, (d) x=0.020, (e) x=0.030 and (f) x=0.050
8. Fig. 8 Closed aperture z-scan graph of Zn1-xTixO thin films (a) x=0.000, (b) x=0.005, (c) x=0.010, (d) x=0.020, (e) x=0.030 and (f) x=0.050.
9. Fig. 9 Variation of (a) nonlinear absorption coefficient (β) and (b) nonlinear refractive
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index (n2) with linear absorption coefficient in Zn1-xTixO (0≤x≤0.050) thin films.
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Highlights
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Fabrication of high quality Zn1-xTixO (0≤x≤0.050) thin films via PLD technique. The Ti concentration plays significant role on the crystalline quality of the ZnO films. The tunable refractive index is obtained in the films. The thin films showed reverse saturation absorption. The third order nonlinear optical coefficients (β and n2) increase with Ti concentration.
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1. 2. 3. 4. 5.