Microstructural and optical properties of SnO2–ZnSnO3 ceramics

Author’s Accepted Manuscript Microstructural and optical properties of SnO2ZnSnO3 ceramics I. Saafi, R. Dridi, R. Mimouni, A. Amlouk, A. Yumak, K. Boubaker, P. Petkova, M. Amlouk www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)00038-9 http://dx.doi.org/10.1016/j.ceramint.2016.01.010 CERI11986

To appear in: Ceramics International Received date: 19 December 2015 Revised date: 2 January 2016 Accepted date: 3 January 2016 Cite this article as: I. Saafi, R. Dridi, R. Mimouni, A. Amlouk, A. Yumak, K. Boubaker, P. Petkova and M. Amlouk, Microstructural and optical properties of SnO2-ZnSnO3 ceramics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.01.010 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 galley proof before it is published in its final citable 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.

Microstructural and optical properties of SnO2-ZnSnO3 ceramics I. Saafi 1, R. Dridi 1, R. Mimouni 1, A. Amlouk 1, A. Yumak 2, K. Boubaker 1,*, P. Petkova3, M. Amlouk 1. 1

Unité de Physique des dispositifs à Semi-conducteurs UPDS, Faculté des Sciences de Tunis, Tunis El Manar University, TUNISIA.

2

Physics Department, Faculty of Arts and Sciences, Marmara University, 34722 Göztepe, Istanbul, TURKEY. 3

Shumen University “Konstantin Preslavsky”, 115 Universitetska str., 9712 Shumen, BULGARIA

* Corresponding author: E-mail address: [email protected]

Abstract: This work deals with some physical investigation on SnO2-ZnSnO3 ceramics grown on glass substrates at different temperatures (450°C and 500°C). Structural and optical properties were investigated using X-Ray diffraction (XRD), Raman, infrared (IR) absorption (FTIR), UVvisible spectroscopy and Photoluminescence (PL) techniques. XRD results revealed the existence of a mixture of SnO2/ZnSnO3 phases at different annealing temperatures. Structural analysis showed that both phases are polycrystalline. On the other hand, the optical constants (refractive index, extinction coefficient and the dielectric constants) have been obtained by the transmittance and the reflectance data. The optical band gap energy changed from 3.85 eV to 3.68 eV as substrate temperature increased from 450°C to 500°C. Raman, FTIR modes and PL reinforced this finding regarding the existence of biphasic (SnO2 and ZnSnO3) which is detected also by X-Ray diffraction analysis. Finally, the Lattice Compatibility Theory was evoked for explaining the unexpected incorporation of zinc ions in a rhombohedral structure within SnO3 trigonal lattice, rather than the occupation of SnO2 available free loci. All the results have been discussed in terms of annealing temperature.

Keywords: Ceramics; Crystal growth; X-ray diffraction; Crystal structure.

1.

Introduction

TCO materials that are deposited in the largest quantity (by area)

are of the greatest

economic importance are SnO2 and ZnO [1-4]. Both oxides are inexpensive in terms of raw materials and processing protocols, because they can be easily deposited using chemical methods such as spray pyrolysis from chlorides or from organometallic precursors. Such compounds received more attention for gas sensing [5], inert electrodes in electrochemical processes, and thermoelectric materials with high-energy conversion efficiency and solar cells [6-8]. Recent research [9-11] reports that coupled oxide semiconductors, like ZnO/SnO2, could be even more efficient than just the individual binary photo catalysts [12], while ternary forms of these oxides were taken into account as well because there is a general perception that they allow even more flexibility in materials designing for desired application [13], as their physical and chemical properties are more easily tailored compared to the binary oxides. For the ZnO–SnO2 system (zinc tin oxide, i. e., ZTO) can form a stable ZnSnO3 or Zn2SnO4 phase by reacting with each other, which depend on the preparation method and the final calcinations temperature. Zn2SnO4 has a spinel-type structure and is obtained relatively easily by solid-state synthesis from the initial oxides or by decomposition of appropriate double salts. It has a high thermal stability and has been characterized both chemically and structurally. In contrast, data on ZnSnO3 are ambiguous and contradictory. ZnSnO3 phase exhibited optical, electrical and gas-sensing properties and can be used as opto- electrical, gas-sensing, high-frequency dielectric capacitors, lithium ion battery materials. Similar optical, electrical and gas-sensing properties were also found for the Zn2SnO4 phase [14-16]. In this paper, we report a systematic study of phase formation in the SnO2-ZnSnO3 system by the spray pyrolysis. A new protocol is presented for obtaining ternary phase through esterification reaction. The effect of annealing temperature, on the structural and optical property of the SnO2-ZTO thin films grown on glass substrate was investigated. 2.

Samples preparation

Zinc stannic ceramics were deposited on glass substrates at 450° C using appropriate mixture of two alcohol starting solutions (S1 and S2), both containing Zn+2 and Sn+4 ions with 1:2 as molarity ratio (S1: 1 10-1M ; S2: 2 10-1M). In a typical synthesis protocol, 55.42 g of SnCl4 were dissolved in a mixture of methanol and acetic acid to which a solution (S1) of zinc

precursors (Zn(CH3CO2)2) was added. Under magnetic stirring for 15 min, the water for hydrolysis was slowly obtained by esterification of the acetic acid with methanol. Precursor solutions were selected according to protocols which were achieved previously in our laboratory [17, 18]. Nitrogen was used as gas carrier (pressure at 0.35 bar) through a 0.5 mm-diameter nozzle. Precursor mixture flow rate was taken constant at 4 ml/min throughout the thin films deposition. According to monitored parameters, two samples have been prepared corresponding to sprayed thin films grown on glass substrate at 450°C and annealed thin films during 2 h in air at 500°C.

3.

Characterization techniques

X-ray diffraction analysis of all prepared thin films was performed by a copper source diffractometer (Analytical X Pert PROMPD), with the wavelength (λ= 1.54056 A). Besides, the Infrared spectrum was recorded at room temperature on a Nicolet 380 FTIR spectrophotometer as KBr pellets in the 2500–300 cm−1 region. Raman measurements were performed at room temperature using a Renishaw inVia Raman microscope. In addition, the optical measurements, in the UV–visible range were carried out using a Schimadzu UV 3100 double-beam spectrophotometer, within a (250–2500 nm) wavelength range. The transmission data was taken with a glass sample as a reference (i.e., reflection from the substrate was subtracted). Finally, PL measurements were performed at room temperature using a HORIBA Jobin yvon spectrometer with He-Cd laser with 270 nm excitation.

4.

Results and discussion

4.1.

Structural properties

Figure 1, shows the XRD patterns of SnO2-ZTO sprayed ceramics , which were grown on glass substrate at 450°C and annealed one during 2 hours in air at 500°C.

(211)

*

*

(404)

*

(321)

°

(310)

(220)

*

Intensity (a.u)

(301)

*

*

(024)

*

T=450°C T=500°C

*

(101) (200)

(110)

*SnO2 °ZnSnO3

°

* * (210)

*

* ° 20

40

*

2 (°C)

* 60

(202)

*

*

*

*

°

80

100

Fig.1. X- ray diffraction spectra of SnO2 –ZnSnO3 thin films.

The observed indexed peaks in these XRD patterns are fully matched with the corresponding tetragonal structure SnO2 (PDF number: 88-0287) and rhombohedral structure ZnSnO3 (PDF number: 52-1381). It can be observed that crystalline dominating phase is SnO2. The XRD results indicate that all the films have polycrystalline structure. Also, it is shown that the different films with SnO2 have a preferential axis orientation along (211) plane. We note the disappearance of two peaks (210) and (202) of SnO2 for sample corresponding to T=500°C. The intensity of the privileged peak decreases with increasing of temperature. To reach some valuable information regarding the enhancement of SnO2-ZTO thin films structure, some calculations have been done. Indeed, the interplanar spacing dhkl values of SnO2: ZTO thin films were also calculated by using Bragg equation: 2dhkl sin = nλ Both lattice parameters ‘‘a’’ and ‘‘c’’ for the tetragonal phase are calculated using the following relation:

Both lattice parameters a and c for the tetragonal phase are calculated via (200) and (211) orientations. Table 1 summarizes also calculated values of the interplanar spacing dhkl of SnO2-ZTO thin films. First, it can be seen that dhkl values decreases for film prepared. The average crystallite sizes of obtained thin films are calculated peak by using the Debye– Scherrer formula [19].

Where k = 1.5418 A for Cu radiation,

is the diffraction angle, K = 0.9, and √

width at half maximum FWHM with

, where

e

is the full

is measured from the film

and is the full width at half maximum related to the instrument [19,20]. The calculated values of crystallite size are presented in Table 1. We note that the crystallites size decreases with the annealing temperature; the same result is reported elsewhere [21]. The microstrain ε is calculated using the following relation [22]:

This parameter increases by increasing annealing temperature due to the appearance of ZTO secondary phase with no negligible amount.

Interplanar

Lattice

spacing (dhkl)

parameters (Å)

ε (10-3)

D (nm)

(211)

(200)

T=450°C

1.78

2.39

a=4.08 c=4.35

36.68

2.9

T=500°C

1.77

2.38

a=3.94 c=3.66

28.29

4.1

Table 1: Lattice parameters, Grain size and micro strain for different samples.

4.2 Raman study Raman spectroscopy is widely used to study the material's quality, phase purity and also to understand the transport properties and interactions of phonon with free carriers [23]. Raman spectra of SnO2-ZTO sprayed thin film obtained at room temperature are depicted in Fig. 2.

With a rutile (tetragonal) structure, the unit cell of SnO2 contains two Sn and four O ions and belongs to the space group

(P42/mnm). The symmetry of the normal lattice vibration at the

point of the Brillouin zone may be derived by group theory [4]: Grutile = A1g + A2g + A2u + B1g + B2g + 2Bu +Eg+3Eu Where the modes of A1g, B1g, B2gand symmetry are Raman active. Raman peaks at 471.71 cm-1, 629.54 cm-1 and 776.4 cm-1 were attributed to the first order Raman-active modes Eg (translational), A1g (symmetric Sn–O stretching) and B2g (asymmetric Sn–O stretching) vibration modes of SnO2, respectively. These results are in good agreement with results of Scott and other workers [24,4]. The calculated value of the mode B1g at k = 0 is 121 cm-1 [25]. Moreover, the peak at 301cm−1 could be ascribed to vibrational modes of vacancy-related defect. On the other hand, the signal at 556.87 cm-1 is associated to A1g modes of spinel-type ZnSnO3 [26]. Finally, the peak observed at 1094.06 is due to glass substrate [27].

T=450°C T=500°C

629.54

Intensity (a.u)

1094.06 556.87 471.71 121.8 301.06

300

600

776.4

900

1200 -1

Raman Shift (cm ) Fig.2: Raman spectrum of SnO2- ZnSnO3.

1500

4.3. FTIR study FTIR is usually used as an important technique to prove the presence of OH groups as well as other organic and inorganic species. The molecular structure of SnO2-ZTO was characterized by Fourier transform infrared (FTIR) spectroscopy, as shown in Fig. 3. The main IR features of SnO2-ZTO appearing at 464.67 and 599 cm−1, are assigned to O―Sn―O and Sn―O stretching vibrations, respectively [28]. Moreover, peak located at 653.23 cm-1 is attributed to Sn-O-Sn asymmetric stretching mode of surface bridging oxide. On the contrary, peak situated at 742 cm-1 indicates the presence of Sn-O vibrations in ZnSnO3 [29]. Finally, the sharp peak observed at 1228.35 cm-1 is due to the Sn- OH bending [30].

Reflectance

599

1228.35

653.23

964.17

T=450°C T=500°C 500

1000

1500

2000

2500

-1

Wavenumbers (cm )

Fig.3: FTIR spectrum of SnO2: ZTO. However, at high temperature, Zn and Sn spices on the surface appear to be mobile enough to move and diffuse inside the porous body and contribute to the formation of zinc stannate phase.

4.4. Reflectance and transmission spectra Fig. 4 shows the transmission and reflection spectra as a function of the wavelength for the sprayed and annealed at T=500°C of SnO2-ZTO thin films deposited on glass substrate.

1,0

0,8

R/T

0,6

T=450°C T=500°C

0,4

0,2

0,0 500

1000

1500

2000

2500

wavelength (nm)

Fig.4. Reflection and transmission spectra of sprayed and annealed SnO2-ZTO thin films.

Optical transmission seems to be very sharp near to the UV region due to the onset of fundamental absorption. In the visible region, these films show a high transparency within the visible range with an average transmittance lying between 75% and 90%. This phenomenon can be attributed to less scattering effects, structural homogeneity and an improvement of the crystalline state. Besides, the presence of the interference phenomenon indicates a smooth and homogeneous surface of all observed films [31]. Any variation in transmittance caused by annealing may be related with improvement in the structural order, removal of residual stresses and defects formed during film deposition. The reflection spectra R(λ) present an accurate guide for estimating the thickness of each sample.

In fact, the oscillating behavior of the λ-dependent reflection in the visible range means that the reflectance reaches discrete p-indexed maximal values indexed

for some particular p-

max value ones, Fig. 5. [32–35].

0,8

Reflectance

Reflectance

0,6

0,4 

p

m ax



p +1

m ax

w avelen g th (n m )

0,2

0,0 0

500

1000

1500

2000

2500

wavelength (nm)

Fig.5. Schematic principles of maximum reflectance spectra. These p-occurrences are written in the following relation:

√

√ √

Where ns is the substrate optical index (ns≈1.55) and n is the layer λ-dependent optical index deduced from the optical transmission–reflection spectra. The knowledge of two successive couples of maximal values [33–35] all ows us to calculate the film thickness d:

d=

(

)

(5)

The calculated values of the thickness are of the order of 200nm for sprayed film and annealed one.

4.5. Band gap and Urbach energies

The reflectance and transmission spectra present an accurate guide for estimating the absorption coefficient. When R(λ) value is less than 30%, the absorption coefficient is given by [35]: (6)

Also, in the case of direct transmission, the absorption coefficient can be expressed using Tauc relation [35]:

(hν-

(7)

)

Where A is a constant, hν is the photon energy and Eg is the optical band gap energy. Figure 6 shows the plots of (ahν)2 versus the photon energy (hν) which leads to the gap energy value in the sharp absorption edge by a linear fit. Calculated values of optical band gap energy Eg of SnO2-ZTO deposited and annealed thin films are summarized in Table 2.

15

2,0x10

T=450°C T=500°C 15

(h)

2

1,5x10

15

1,0x10

14

5,0x10

0,0 0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

h (eV)

Fig.6. Plots of (αhν)2 versus photonenergyoftheSnO2-ZTO films.

Optical energy band gap of the ceramic compounds was found to decrease from 3.85 to 3.68 eV with increasing temperature as shown in Fig.6. This shift may be due to the influence of several factors such as thickness, grain size, structural parameters and lattice strain, carrier concentration, presence of impurities (or other defects), or even deviation from stoichiometry. An important parameter that characterizes the disorder of the material is energy Urbach tailing. In fact, if one considers only the direct transitions, it should not be under the absorption band gap and absorption edge should lead to a steep edge. Actually, the different types of defects into the semiconductor often reveal the formation of band tailing in the band gap, the interactions with phonons, and the presence of a tail absorption profile which follows the empirical Urbach law. Urbach energy values are calculated through the following equations system [36, 37]:

(8)

α=

(9)

Where α(hν) represents the experimentally values deduced from optical absorption profile and α0 is a constant. The Urbach energy Eu is obtained from the reverse of the slope of Ln (α)) versus hν (Fig. 7).

T=450°C T=500°C

18 17

Ln ()

16 15 14 13 12 0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

h (eV)

Fig.7. Plots of ln(α) versus hν.

4,5

Eg (eV)

Eu (meV)

V=450°C

3.85

23.14

V=500°C

3.68

21.73

Table 2: Calculated values of the optical band gap Eg and the Urbach energy Eu of the sprayed and annealed thin films. It is interesting to note that the disorder decrease with increased the substrate annealing temperature. This feature is in very good agreement with RX results. In fact, in host substrate temperature the formation and separation of binary -ternary phase is more observable.

4.6. Optical refractive index n(λ) and extinction coefficient k(λ).

In the same framework, as-grown compounds refractive index and extinction coefficient (Fig. 8 a and b, respectively) have been obtained by the theoretical method of Swanepoel [38 , 39]. Using this method, the upper TM and lower Tm envelopes of the interference fringes, obtained from the transmission spectrum, allow calculation of the refractive index by the following equations:

n=√

√

(10)

(11)

Here S is the refractive index of the glass substrate, TM and Tm are the values of transmission maximum at the wavelengths of the upper and lower envelopes, respectively. By using Swanepoel's method, we can calculate the refractive indexes of SnO2-ZTO thin films as a function of wavelength. The extinction coefficient k values decrease down to a certain value of wavelength and then increases. There is a little absorption in the wavelength range of 400–2500nm, showing that the all films are transparent in this wavelength region, which is consistent with the results of

transmission spectra shown in Fig.4. The refractive index is fitted by the Cauchy formula: [40, 41]:

(12)

0,8

(a)

2,06

(b) 0,6 Extinction coefficient

2,04 Refractive index

T=450°C T=500°C

T=450°C T=500°C

2,02

2,00

1,98

0,4

0,2

1,96

0,0 0

500

1000

1500

2000

2500

wavelength (nm)

0

500

1000

1500

2000

2500

wavelength (nm)

Fig.8. Dispersion of refractive index (a)and Extinction coefficient (b) as a function of wavelength.

Where A and B are Cauchy's parameters and λ is the wavelength of the light used, implying that the films have normal dispersion for the entire range of wavelength studied. Table 3 listed A and B values determined by the Cauchy formula.

A

B(µm2)

V=450°C

1.97

0.016

V=500°C

1.94

0.012

Table 3: Cauchy distribution parameters values.

4.8. Dielectric constant

In order to understand the optical properties of all samples, the complex dielectric constants [ε(ω)= (ω) i (ω)] are studied. The real and imaginary parts of dielectric constant forum sprayed and annealed films are determined by [42]: (13) (14) (15)

The variation of the real ( ) and imaginary ( ) parts of the dielectric constant of sprayed and annealed thin films is illustrated in Fig. 9 (a) and (b). These figures reveal that the values of the real part are higher than the imaginary one, which shows the transparency of these thin films. For all samples, it is found that in infrared range the dispersion of square of the wavelength

while the absorption

The obtained values of both

and

is linear with

are used to calculate ε ,

is linear function of the , Fig. 10 (a) and (b). and , high frequency

dielectric constant, plasma pulsation and relaxation time, respectively, through the following expressions [43–44]: (16) (17)

Where, ⁄

represents the carriers concentration to effective mass ratio.

The calculated values of the constants are gathered in Table 4.

The extrapolation of the curve of

at low wavelengths provides the dielectric constant 3,5

(b)

(a) 4,2

T=450°C T=500°C

T=450°C T=500°C

3,0 2,5

4,1 2

1

2,0 1,5

4,0 1,0 0,5

3,9 0

500

1000

1500

2000

0,0

2500

0

500

wavelength (nm)

1000

1500

2000

2500

wavelength (nm)

Fig.9. Variation of real part (a) and imaginary part (b) Vs λ of the dielectric constant of SnO2ZTO thin films. 4,25

3,5

(a)

4,20

T=450°C T=500°C

4,10

2,0

4,05

1,5

4,00

1,0

3,95

0,5

3,90

0,0

0

6

1x10

6

2x10

6

6

3x10

4x10 2

(wavelength) (nm)

6

5x10

6

6x10

T=450°C T=500°C

2,5

2

1

4,15

(b)

3,0

6

7x10

9 9 9 9 9 10 10 10 10 -2,0x10 0,0 2,0x104,0x106,0x108,0x101,0x101,2x101,4x101,6x10

2

3

(wavelength) (nm)

3

Fig.10. Variation of real part Vs λ2 (a )and imaginary part (b) Vs λ3 of the dielectric constant of SnO2-ZTO thin films.

(1013rads-1)

(10-16s)

N/m*(1045cm-3g-1)

V=450°C

3.94

7.89

8.97

8.50

V=500°C

3.92

3.29

3.29

1.47

Table 4: Dielectric constants values

4.9. Photoluminescence (PL) Fig. 11 shows the photoluminescence (PL) spectra of SnO2-ZnSnO3 thin films at room temperature. After excitation by 270 nm light, a broad emission band ranging from UV to visible region was observed. To reach the origin of the PL peaks, it is required to realize the de-convoluted following Gaussian profile fitting. We can see five bands in UV and visible region. Among the peaks reported in the spectra’s Gaussian fitting are 360 nm, 376 nm, 402 nm, 446 nm and 487 nm. The peak position at ~360 nm is lower than the band gap energy of SnO2, they cannot be assigned to the direct recombination between electrons in the conduction band and holes in the valence band [45]. The first UV peak corresponds to donor– acceptor recombination (DAP) due to donor levels near the conduction band and acceptor levels near the top of the valence band [46]. The second UV emission peak, centered at 376 nm, is in very good agreement with previously reported structural results. In fact, this feature was assigned to the band-to-band emissions for the band gap of ZnSnO3 (3.3-3.9 eV). Blue luminescence at 402 nm has been attributed to the zinc vacancy [47]. The emission at 446 nm was attributed to the transition between shallow donors (oxygen vacancy) to the valence band [47]. Sefardjella et al. [48] observed a peak at 437 attributed to Sn interstitials. The 486 nm emission was attributed to a transition between the oxygen vacancy and interstitial oxygen and lattice defects related to oxygen and zinc vacancies [48]. In addition, to violet and blue, emission from SnO2-ZnSnO3 present in spectra fitting, green emission (500 nm) in these thin films are also reported. The green emission has been assigned to oxygen vacancy [49]. It is interesting to note that the recombination intensity of conduction band to valence levels increases with increased annealing temperature.

Fig.11. PL spectra of SnO2: ZTO thin films. 4.10. Investigation within Lattice Compatibility Theory

According to structural patterns, photoluminescence measurement and optical band analysis, the existence of a disjoint mixture of SnO2-ZnSnO3 phases at different annealing temperatures has been verified. Crystallization and decrease of tailing band defects at 500°C, along with the recorded direct transition absorption mechanism show particularly that the enhancement of optical and thermal properties was unexpectedly not accompanied by any effective incorporation of Zn atoms within stannic oxide primal lattices.

For supporting these

experimentally recorded results, the recently proposed Lattice Compatibility Theory [50-60] which was implied in similar cases, has been suggested, as long as it explained earlier to a satisfactory extent, some host lattice and doping-element interaction patterns. The first formulations of this Theory [50-60] have been established by Petkova et al. [50] in a study of

Urbach tailing controversial behaviour within

I-III-O2 ternary oxides.

Consecutively, Colantoni et al. [51] Yumak et al. [52], Ben Said et al. [53], Gherouel et al. [69], Haj Lakhdar et al. [54], Boubaker et al. [55-78], Mimouni et al. [59] and Ben and Messaoud et al. [60] used the Lattice Compatibility Theory for addressing similar intrigues. In this study (SnO2-ZnSnO3 disjoint phases), the Lattice Compatibility Theory can give a defendable explanation to the disparity concerning doping element incorporating dynamics starting from intrinsic doping-element lattice properties in comparison to those of the host. Calculation of SnO2 and similarly Sn-O related thin films relevant parameters (Fig. 12), led to

the certitude that the lack of compatibility with ZnO intrinsic würtzite crystalline structure is exclusively favorable to incorporation of zinc ions in a rhombohedral structure with transitional SnO3 trigonal pyramidal geometry shaped lattice, which is concordant with earlier records.

Figure 12: Lattice Compatibility Theory analysis scheme for ZnSnO3 structure formation.

In fact, this particular behavior of zinc atoms within targeted lattices has been evoked in compounds properties studies by Mihaiu et al. [1], Jin et al. [2], Park et al. [3] and Kim et al. [4].

5.

Conclusion

SnO2-ZnSnO3 ceramics were synthesized by the spray pyrolysis method. The binary and ternary phases were obtained by slow hydrolysis of precursors using an esterification reaction. The structural analysis reveals the existence of a mixture of SnO2-ZnSnO3 phases at different annealing temperatures. At 500°C, the structure of obtained ceramics is more crystallized with decrease of tailing band defects and decreased of grain size. The optical band gap of these films is calculated. The optical absorption spectra show that the absorption mechanism is a direct transition. The basic optical properties and optical constants of the ZTO thin films have been investigated by means of transmittance and reflectance spectra. The real (n)

and imaginary (k) parts of the complex refractive index were determinate. Photoluminescence measurement shows a broad emission band in UV-Visible region. Different origins in the recombination mechanisms of this band are discussed. Also, this finding may be of interest for many physical routes especially in sensitivity applications and pave the way to possibly achievements to perform solar cells using this alloy oxide. REFERENCE

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