Effects of silicon porosity on physical properties of ZnO films

Materials Chemistry and Physics 175 (2016) 233e240

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Effects of silicon porosity on physical properties of ZnO films ^a a, b, A. En Naciri b, *, A. Moadhen a, H. Rinnert c, M. Guendouz d, Y. Battie b, M.-B. Bouzoura d A. Chaillou , M.-A. Zaïbi a, e, M. Oueslati a Universit e de Tunis El Manar, Facult e des Sciences de Tunis, Unit e de nanomat eriaux et photonique, 2092 El Manar - Tunis, Tunisia Universit e Lorraine, Institut Jean Barriol, LCP-A2MC, 1 Bd Arago, 57078 Metz, France c Universit e Lorraine, Institut Jean Lamour, 54506 Vanduvre-l es-Nancy, France d Universit e Europ eenne de Bretagne, CNRS FOTON-UMR 6082, 6 rue de K erampont, BP 80518, 22305 Lannion Cedex, France e Universit e de Tunis, Ecole Nationale Sup erieure des Ing enieurs de Tunis, 5 Avenue Taha Hussein, 1008 Tunis, Tunisia a

b

h i g h l i g h t s
ZnO/PS system can be considered as the best substrate for depositing the ZnO film.
ZnO/PS system which exhibits large emission in both UV and visible domains.
Photoluminescence and ellipsometry show good optical properties of ZnO/PS.
ZnO/PS as a potential and attractive candidate for optoelectronic applications.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 July 2015 Received in revised form 5 February 2016 Accepted 19 March 2016 Available online 25 March 2016

We report on structural and optical properties of ZnO thin films deposited on different Si-based substrates presenting different porosities. ZnO layers were prepared by sol gel method and deposited on crystalline silicon (ZnO/Si), mesoporous silicon (ZnO/PSþ) and nanoporous silicon (ZnO/PS) by spin coating. Several techniques such as scanning electron microscope (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence spectroscopy (PL) and spectroscopic ellipsometry (SE) were used to study the influence of the pore size of porous silicon (PS) on physical properties of ZnO films. SEM images revealed the formation of ZnO granular nanoparticles on Si, PS and PSþ substrates. We show by the XRD analysis that hexagonal crystallized (002) ZnO is mainly obtained for ZnO/PS system causing by a strong absorption of the capillary effect and high adhesion to PS surface. An intense PL related to ZnO and PS was demonstrated for ZnO/PS in UV and visible ranges. Optical properties of ZnO were determined and analyzed by SE using Tanguy dispersion model. For each sample, a specific optical model was carried out. SE confirms a good physical properties of ZnO/PS comparing to ZnO/Si and ZnO/PSþ. For example, the good crystallinity is characterized by low damping factor value (G). This value was found by SE to be low (29 meV) for the ZnO/PS, while the damping factors of ZnO/Si and ZnO/PSþ are 47 meV and 70 meV, respectively. The amplitude of dielectric function of ZnO/PS around 3.4 eV reveals an increase of grain size and crystallinity of ZnO layer. © 2016 Elsevier B.V. All rights reserved.

Keywords: Porous silicon ZnO Photoluminescence Ellipsometry Optical properties

1. Introduction Several semiconductor materials are strongly used in manufacturing electronic components. In the past few years, much attention was done to the one-dimensional structure specially nanorods, nanowires and nanotubes using in many areas: sensors,

* Corresponding author. E-mail address: [email protected] (A.E. Naciri). http://dx.doi.org/10.1016/j.matchemphys.2016.03.026 0254-0584/© 2016 Elsevier B.V. All rights reserved.

transducers, field-effect transistors, optical switches and biomedical science [1e5]. On the other hand, zinc oxide (ZnO) was attracted many researchers [6,7] due to its interesting physical properties: wide band gap (~3.37 eV at ambient temperature (Ta)), large excitonic energy (~60 meV at Ta), size and thermal stability. These properties render ZnO suitability for a wide range of applications in optoelectronic. Furthermore, the development of optoelectronic devices required the deposition of ZnO on different substrates such as glass [8], Si [9] and organic compounds [10]. It is well known that Si substrate is the most widely used in integrated

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circuits technology. However, the growth of ZnO on Si substrate induces a large stress between ZnO and Si due to the large lattice mismatch [11]. It was believed that changing the Si surface can reduce this large stress. Some researchers were interested to elaborate ZnO thin films on different substrates in order to form ZnO nanostructures using several techniques such as pulse laser deposition [12], metalorganic chemical vapor deposition [13], RF magnetron sputtering [14], electrodeposition [15], spray pyrolysis [16] and solegel [17,18]. But as it turns out, the solegel is a versatile and cheap method, its advantage is also in bulk production. PS is an open structure, with a large area, a strong absorbability and with optical emission properties in the visible range at room temperature [19]. PS substrate combined to ZnO will surely pave the way of the next generation of photovoltaic and optoelectronic devices [20]. Hong et al. [21] have confirmed that PS is an adequate substrate for ZnO deposition. The authors have shown by scanning electronic microscopy (SEM) and X-ray diffraction (XRD) that ZnO is well deposited in porous layer and is oriented along its c-axis with less stress. Other authors have reported the formation of ZnO nanoparticles on PS layer, and using photoluminescence (PL), where they have found an enhancement of PL intensity and a broadband luminescence in the visible range [22]. Eswar et al. [23] have observed a large luminescence in the visible attributed to ZnO defect and PS response, and a weak UV response corresponding to ZnO. However, no comparative study between ZnO deposited on crystalline silicon (ZnO/Si), mesoporous silicon (ZnO/PSþ) and nanoporous silicon (ZnO/PS) have been published in the literature. In this paper, we propose structural and optical studies of three different systems: ZnO/Si, ZnO/PSþ and ZnO/PS. We have used SEM, XRD, atomic force microscopy (AFM), PL and spectroscopic ellipsometry (SE) to investigate the influence of the pore size of porous silicon (PS) on physical properties of ZnO films. ZnO thin films were prepared by sol gel method and deposited on Si, PS and PSþ by spin coating. PL measurements performed in visible and UV ranges show a large and an intense luminescence for ZnO/PS suggesting the formation of crystalline ZnO layer. Optical properties of ZnO are determined and analyzed by ellipsometry using Tanguy dispersion model. For each sample, a specific optical model is carried out. SE confirms good physical properties of ZnO/PS comparing to ZnO/Si and ZnO/PSþ. Among these three samples, here we unambiguously demonstrate that the ZnO/PS can be considered as a best structural and optical system. 2. Experimental Lightly doped porous Si (PS) and highly doped porous Si (PSþ) was obtained from an electrochemical anodization of silicon wafer. The lightly and highly p type doped Si oriented along the (100) axis have 0.1e2 U/cm and 102-103 U/cm resistivity, respectively. To increase the conductivity between anode and lightly p-doped silicon wafer, the backside of substrate has been coated with aluminum (Al) and annealed at 500  C during 30 min. The anodization is carried out in a mixture of HF(50%)/C2H5OH/H2O (with a ratio of 2/2/1) and a current density of 10 mA/cm2 was applied during 10 min. Hence, the PS samples were flushed in deionized water and then dried by argon gas. ZnO thin films were prepared by solegel method. Zinc acetate dehydrate (ZAD) was dissolved, at ambient temperature, in a mixture solution of 2-methoxyethanol (2-MOE) and monoethanolamine (MEA), which are the precursor, solvent and complexing agent, respectively. The molar ratio was maintained at 1:1 of MEA/Zinc acetate dehydrate and the final concentration of ZAD was fixed at 0.6 mol/L. After stirring for 2 h at 60  C, a clear, homogenous and colorless solution was obtained and was remained

stable for several days. The solution obtained was kept 48 h at ambient temperature and then was deposited by spin coating at 3000 rpm on different substrates: Si, PS and PSþ. After each coating, the films were preheated at 250  C to evaporate the solvent and to remove organic residuals. Experimentally we have found that repeating the process several times promotes the formation of porous silicon and a ZnO layer with large thicknesses. Sufficient thick layer is necessary for a good sensitivity in ellipsometry measurements. Therefore our samples were obtained after eight times. Finally, the films were annealed at 600  C in order to obtain ZnO crystallized in a wurtzite structure [18]. The sample morphology was analyzed by SEM. The measurements were performed on a Zeiss Supra 55VP operating in secondary electron imaging mode with an accelerating voltage of 2 kV and 2.7 mm working distance. The samples were also analyzed by AFM (nanoscope Nano R2 Pacific nanotechnology) in tapping mode. X-ray diffraction (XRD) technique was used to identify and control the crystalline phase of ZnO deposited on Si, PS and PSþ. XRD patterns of the sample were obtained with Bruker diffractometer equipped with Lynxeye accelerator using a Cu wavelength. PL measurements were carried out in the 330e800 nm spectral range, using a monochromator with a 600 grooves/mm grating coupled to a photomultiplier detector. The excitation was obtained using the 325 nm line of a 15 mW HeeCd laser. All analyses are performed in air and at room temperature. The optical response of ZnO/PS has been investigated by a phase modulated (Horiba Jobin Yvon, UVISEL) ellipsometer in the 0.6e4.75 eV spectral range at an incidence angle of 70 . 3. Results and discussion 3.1. SEM and XRD analysis The SEM images of the ZnO deposited on Si, PS and PSþ were presented in Fig. 1. The growth of ZnO on Si, PS and PSþ substrates exhibits granular nanoparticles and reveals uniform distribution on the surfaces. The ZnO nanoparticles on PSþ obtained in the same conditions as Si and PS, are less developed on the surface. The nanoparticles size distributions (Fig. 1) were obtained by counting 100 particles from SEM images. The Gaussian fit of the three histogram gives an estimation of the average ZnO nanoparticle size. These are estimated at 82 ± 40 nm and 90 ± 42 nm for ZnO/Si and ZnO/PS, respectively. However on PSþ, the average ZnO nanoparticles size is 35 ± 21 nm. Even if Si, PS and PSþ substrates are crystallized in faced-centered cubic structure [24], ZnO films formed on these substrates are different due to the surface structure and interfacial tension on theirs surfaces which are different. However this difference is more pronounced for the layer on PSþ as shown by SEM images of Fig. 1(c). Fig. 1 also shows that the porosity of film on PS is slightly higher than ZnO on Si. The XRD analysis of Si, ZnO/Si, ZnO/PS and ZnO/PSþ were reported as in Fig. 2. The ZnO films are crystallized in a hexagonal wurtzite lattice with c-axis (002) perpendicular to the substrate surface in agreement with the JCPDS database (card No. 00-0361451). The c-axis oriented growth is due to low free surface energy on the (002) plane and high density effect on the film's surface [25]. Moreover, the c-axis of ZnO films deposited on Si and PS is preferentially oriented as normal to the substrate. The (002) peak is more pronounced in the ZnO/PS compare to ZnO/Si or ZnO/PSþ. Consequently, this traduces better crystallinity of ZnO formed on PS than on Si or on PSþ. The difference in intensity can be explained by two phenomena: (i) higher absorption rate of the capillary effect presented in ZnO/PS sample and (ii) the high adhesion due to the high specific surface area in the case of PS. Moreover, as reported by Ressine et al. [26] that the pores size in

^a et al. / Materials Chemistry and Physics 175 (2016) 233e240 M.-B. Bouzoura

235

Fig. 1. SEM surface images and size distributions of ZnO deposited on (a) Silicon, on (b) PSand on (c) PSþ.

superhydrophobic behavior. Besides the (002) ZnO peak, two weakly peaks around 33 and 62 which correspond to SiO2 [27,28] were recorded for ZnO/Si and ZnO/PSþ. An intense Si(400) peak is observed on the XRD spectra as reported in the JCPDS database (card No. 002-0561). The average crystallite size of ZnO is deduced by using Scherrer's equation [29]:

D ¼ ðk lÞ=ðb cos qhkl Þ

Fig. 2. XRD spectra of (a) ZnO standard (JCPDS N 00-036-1451), (b) Si, (c) ZnO/Si, (d) ZnO/PSþ and (e) ZnO/PS.

porous silicon determines its wettability. They confirmed that the nanoporous silicon is hydrophilic and that the increase of PS pores size until macropores (with pores size of 2 mme4 mm) induces a

(1)

where k, l, qhkl and b are the dimensionless shape factor, with a value close to 0.9 for the spherical particles [30], the X-ray wavelength (1.54 Å), the Bragg diffraction angle and the full width at half maximum (FWHM) corresponding to the ZnO diffraction peak, respectively. The structural parameters of ZnO are summarized in Table 1. The XRD angle of standard ZnO film is at 34.42 [31]. The XRD parameters given in Table 1 show that peaks of ZnO/Si, ZnO/PSþ and ZnO/ PS are shifted to lower angle. The weak FWHM value of (002) peak is obtained for ZnO/PS indicating a good crystallization of ZnO/ PS, along the (002) axis, comparing to the other samples. The lattice parameter c of the hexagonal structure is given by the Bragg relation and its value is reported in Table 1. The c value of our samples is slightly larger than that of standard ZnO film which is c0 ¼ 5.206 Å. This expansion indicates that the ZnO nanostructured films are affected by the presence of stress. This stress can analytically be calculated [32]:

^a et al. / Materials Chemistry and Physics 175 (2016) 233e240 M.-B. Bouzoura

236

Table 1 Values of XRD parameters from ZnO films: Bragg angle (2qhkl), Full width high maximum (b(002)), crystallite size along (002)D(002)(Å), lattice parameter of the hexagonal (wurtzite) structure c(Å), stress in the plane perpendicular to c-axis(s(GPa)). Sample

2qhkl( )

b(002)( )

D(002)(Å)

c(Å)

s(GPa)

ZnO/PSþ ZnO/Si ZnO/PS

34.31 ± 0.001 34.33 ± 0.001 34.36 ± 0.001

0.22 ± 0.006 0.20 ± 0.004 0.17 ± 0.002

371 ± 20 404 ± 16 476 ± 11

5.221 ± 0.0006 5.218 ± 0.0006 5.213 ± 0.0006

0.76 ± 0.02 0.66 ± 0.02 0.45 ± 0.02

(2)

This stress comes from two contributions: extrinsic stress and intrinsic stress. Extrinsic stress is comes from the lattice parameter mismatch and thermal expansion mismatch between the film and the substrate. On the other hand, intrinsic stress is attributed to the impurities, defects and lattice distortions in the nanostructured film [31]. The s value is negative for a compressive biaxial stress and positive for a tensile biaxial stress. In our case, the stress of ZnO films is a negative compressive stress as shown in Table 1. ZnO/PSþ stress was found as higher than in the ZnO/Si or in ZnO/PS. This variation is attributed to the difference of the lattice parameter mismatch and of the thermal expansion coefficient of Si, PS and PSþ substrates. Uday et al. [32] have also obtained a compressive stress ZnO film on porous Si elaborated by spray method. For the coherence length D(002) in Table 1, its value for ZnO/PSþ is in agreement with the average size of ZnO nanoparticles. However, the average size of ZnO nanoparticles for ZnO/PS and ZnO/Si are two times higher than the D(002) values. Hence, we can assume that the ZnO nanoparticles in ZnO/PS and ZnO/Si are polycrystallized. In other words, the coherence length is limited by the ZnO grain size in ZnO/PS and ZnO/Si.

3.2. PL analysis Fig. 3 shows PL spectra of ZnO grown on Si, PSþ and on PS. All samples show a luminescence in the UV and visible range. In the UV range, a violet peak located at 380 nm (3.27 eV) has been recorded for ZnO/Si and ZnO/PS samples while the ZnO/PSþ PL response presents a broad band centered at 400 nm (3.04 eV). The PL peak intensity located at 3.27 eV in ZnO/Si and ZnO/PS samples are very close to that of pure ZnO [33]. This peak has been attributed to the recombination in the near band edge (NBE) of ZnO suggesting the formation of well crystallized ZnO layer for ZnO/Si and ZnO/PS samples in agreement with XRD analysis. The band at 3.04 eV for ZnO/PSþ could be attributed Zn vacancies defects [34]. Moreover, a week contribution due to NBE cannot be excluded. Fig. 3 shows that the violet peak intensity is higher in ZnO/PS sample than in ZnO/Si or ZnO/PSþ. The good crystallinity and larger amount of ZnO layer in ZnO/PS system can be probably the origin of this high intensity [31]. In the visible range, several peaks can be observed on ZnO/PSþ and ZnO/PS PL spectra. We assigned these peaks to the modulation of PL response by the Fabry-Perot interferences phenomenon as reported by other authors [35,36]. It is known that the interferences are related to thickness and refractive index values of layer. The thickness layer of ZnO on Si substrate measured by ellipsometry is around 180 nm leading to the absence of stronger interference fringes. However, the ZnO is stretched in PS and PSþ pores inducing a large mixed thickness layer. Subsequently, the luminescence of ZnO nanoparticles in the porous layer submits a reflection and transmission at the interfaces and inducing a PL modulated by interferences (Fig. 3). The layer thickness of ZnO/PS and ZnO/PSþ estimated by ellipsometry are 1 mm and 3 mm, respectively. More details are given in ellipsometry analysis section. The PL broad band of ZnO/Si in the visible range is attributed to the

3.04 eV

+

(a)ZnO/PS (b)ZnO/PS (c)ZnO/Si

(a)

Intensity (a.u)

sðGPaÞ ¼ 233  ½ðc  c0 Þ=c0   100%:

x8

(b) 1.92 eV 3.27 eV

(c)

350 400 450 500 550 600 650 700 750 800

Wavelength (nm) Fig. 3. PL spectra of (a) ZnO/PS, (b) ZnO/PSþ and (c) ZnO/Si.

ZnO layer. This ZnO visible PL is generally explained by several deep level emission mechanisms attributed to simple or complex defects [37e42]. These defects can be controlled by annealing process and precursor/stabilizer ratio. For example, Azlinda et al. [43] have measured PL of ZnO/Si as a function of Zn2þ precursor/CH4N2O stabilizer ratio. They have found a stronger PL emission in the UV region than in the visible for 1:1 ratio after annealing at 90  C and 4 h during the ZnO gel formation. In our case the samples are prepared at 60  C during 2 h suggesting the higher PL emission in visible range as shown in Fig. 3. The comparison between PL of ZnO/PSþ and PSþ is shown in top of Fig. 4. A very weak PL emission in the visible is measured for the PSþ layer. Therefore the ZnO/PSþ PL emission in visible range is attributed to ZnO defects. Since the volume fraction of ZnO on PSþ after depositing process is weak as shown by SEM, XRD and ellipsometry, its PL emission is lower than the PL of ZnO/PS. The comparison between PL spectra of ZnO/PS and PS is given in

^a et al. / Materials Chemistry and Physics 175 (2016) 233e240 M.-B. Bouzoura

performed in the photon range of 0.6e4.75 eV at an angle of incidence of 70 . For clarity, only the ellipsometry detail about the ZnO/ PS is given here, but the same method is used for all samples. Fig. 5 shows the measured ellipsometric parameters of ZnO/PS. The observed oscillations in large spectral range are due to the interference phenomena related to the multi-reflection in the sample. In UV range, the oscillations are attenuated until an abrupt absorption at ~3.4 eV corresponding to the optical gap of ZnO. A physical model is necessary to extract optical constants and chemical composition of each layer by analyzing the IS and IC ellipsometric parameters. Experimental ellipsometric spectra are fitted, by minimizing the following mean-square error:

(d)

Intensity (a.u)

+

(c) PS + (d) ZnO/PS

(c)

x50

237

c2 ¼

1.72 eV

N h .   . i X 1 ICiðthÞ  ICiðexpÞ dIc þ ISiðthÞ  ISiðexpÞ dIS ; 2N  M  1 i¼1

(5)

-

(a) PS (b) ZnO/PS

where ICiðthÞ , ISiðthÞ and ICiðexpÞ , ISiðexpÞ are the theoretical and experimental values of IC and IS, respectively. N is the number of data collected, M is the number of unknown parameters of the model, and dIC and dIs are the estimated data errors. The cross sectional and the surface morphology images have been probed by SEM and AFM structural characterization (see Fig. 6), respectively. Cross sectional image presents a mixed layer between silicon and ZnO compositions deposited on Si substrate. AFM images show two and three dimensional pictures of ZnO/PS- sample surface. The top layer is formed of ZnO thin film covered by roughness layer composed of closely packed ZnO particles separated by micro-voids. According

(b) (a)

350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

1.2

Interference fringe

Fig. 4. PL spectra of (a) PS, (b) ZnO/PS, (c) PSþ.and (d) ZnO/PSþ.

High absorption

Fig. 4. It clearly shows that the visible emission of ZnO/PS is the result of two contributions: ZnO defects and PS PL responses. As can be seen in Fig. 4, PL spectrum of PS exhibits a very intense peak at 720 nm which is reproduced for ZnO/PS case. It comes from the formation of nc-Si in the porous Si layer. In our previous study, we have shown that luminescence of PS as a function of annealing is considerably decreased and blue-shifted beyond 450  C [44]. The advantage of ZnO/PS system is its strong and broad emission in the visible even for annealed samples at 600  C. Therefore this system can have potential for optoelectronics applications.

Ic

0.8

0.4

0.0

Experience Model

Interference fringe

0.8

High absorption

3.3. Ellipsometry analysis

rp




rs ¼ tan jeiD

(3)

The spectroscopic measurements are performed in air at room temperature using a photoelastic modulator ellipsometer. This ellipsometer measures the IS and IC parameters related to the ellipsometric angles by Ref. [46]:

Is ¼ sin 2 j sinD and Ic ¼ sin 2 j cosD

(4)

SE measurements on ZnO/Si, ZnO/PS and ZnO/PSþ are

0.4

Is

Ellipsometry measures the changes in the polarization state between incident and reflected light on the samples. The measured values are the ellipsometric anglesJ and D. They are related to the ratio between the reflection coefficients of the sample for p-polarized light rp and s-polarized light rs by Ref. [45]:

0.0

-0.4 Experience Model

-0.8 0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Energy (eV) Fig. 5. Experimental and calculated (Is, Ic) ellipsometric parameters from ZnO/PS.

238

^a et al. / Materials Chemistry and Physics 175 (2016) 233e240 M.-B. Bouzoura

Fig. 6. (a) Cross sectional SEM image, (b) a two-dimensional (4 mm  4 mm) AFM image and a (c) three-dimensional AFM image of ZnO/PS.

Layer 4: ZnO + Void

2.7 2.6

(c)

εr

2.5 2.4

(b)

2.3

(a)

2.2 2.1 0.30 0.24 0.18

(c)

(a) SiO2 (b) SiO2 defected at 4.8eV (c) SiO2 defected at 5.8eV

εi

to the SEM and AFM images (see Fig. 6), the physical model is shown in Fig. 7. The sample is regarded as four sublayers. The first is a thin layer formed by nc-Si surrounded by SiO2. The second composite sublayer consists of nc-Si, SiO2 and ZnO. The third layer is the ZnO film. The last sublayer is formed by the roughness. It is modeled as a mixture of ZnO and void according to Bruggeman effective medium theory [47]. The unknown parameters are the thickness of each layer, the volume fractions in layers 1 and 2, and the optical constants of ZnO. The known parameters are dielectric functions of Si substrate [48] and nc-Si [49]. The conventional optical constants of SiO2 does not permit to obtain a good agreement between experimental and calculated SE data. We have found that the defects induced by chemical process elaboration must be considered to reproduce experimental data from the model and then to obtain the optical responses of ZnO. In the modeling, the oxide defects are taken into account by adding an oscillator to the silica dielectric function with absorption peaks at 4.8 eV [50] and at 5.8 eV [51] in layers 1 and 2, respectively. The new dielectric functions of defected SiO2 used for modeling of Ic and Is data are given in Fig. 8. Unlike the SiO2 dielectric function, the imaginary dielectric part of defected SiO2 exhibits an absorption at 4.8 eV and at 5.8 eV. Although these absorptions are located in the UV range,

0.12

(b)

0.06

(a)

0.00

Layer 3: ZnO

Layer 2: ZnO + nc-Si + SiO2 (SiO2 with defects at 4.8 eV)

Layer 1: nc-Si + SiO2 (SiO2 with defects at 5.8 eV)

Substrate: c-Si

Fig. 7. Physical model used in the ellipsometry data analysis.

1

2

3

4

Energy (eV)

5

6

7

Fig. 8. Real and imaginary parts of dielectric function of (a) SiO2, (b) SiO2 defected at 4.8 eV and at (a) 5.8 eV.

they introduce a modification of optical constant values of SiO2 in a large spectral range. Consequently the dielectric functions in 0.6e4.75 eV are higher comparing to SiO2 without defects. The dielectric function of ZnO layer is obtained using Tanguy model [52]. Once the best fit is obtained using Eq. (5), numerical results can be determined. The thickness values determined by ellipsometry are 41 ± 1 nm, 831 ± 3 nm, 50 ± 3 nm and 30 ± 2 nm respectively, for layer 1 up to layer 4. Layer 1 is combined by 71 ± 1% of nc-Si and 29 ± 1% of SiO2. Layer 2 is composed by 6 ± 1% of ZnO, 75 ± 1% of SiO2 and 19 ± 1% of nc-Si. We have found that the best fit is obtained by considering 10% of SiO2 damaged with 4.8 eV and 5.8 eV defects center as can be seen in Fig. 8.

^a et al. / Materials Chemistry and Physics 175 (2016) 233e240 M.-B. Bouzoura Table 2 Values of Tanguy parameters: ε∞ is the dielectric constant at high energy, A is the amplitude factor of optical transition, Eg is the gap energy, R is the excitonic energy and G is the damping factor. Sample þ

ZnO/PS ZnO/PS ZnO/Si

ε∞

A (eV3/2)

Eg (eV)

R(meV)

G(meV)

0.01±0.07 2.99± 0.10 2.36± 0.02

63± 7 12.45± 1 16.3± 0.5

3.34± 0.03 3.39± 0.02 3.39± 0.01

1± 1 68± 4 66± 4

70±10 29± 15 47± 1

In Table 2 are summarized the values of Tanguy parameters obtained for the three samples. The optical parameter values of ZnO/PS and ZnO/Si are close to the reported values of standard crystallized ZnO [33,53e55]. Concerning the G damping factor, it is known that its value decreases with crystal quality. In Table 2, the lower value of G is obtained for ZnO/PS system shown a good crystallinity of ZnO layer. For the ZnO/PSþ sample, the lower values of ε∞, Eg and R that can be attributed to the very insignificant contribution of the excitonic bound [52]. This demonstrates once again a poor crystallinity of ZnO film deposited on PSþ. The dielectric functions of ZnO layer of three systems obtained by fitting ellipsometric data are shown in Fig. 9. The amplitude of dielectric function of ZnO/Si and ZnO/PS around 3.4 eV reveals an increase of grain size and crystallinity of ZnO layer. Throughout the

6 5

239

measured spectral range, the optical constants of ZnO deposited on Si and PS show a typical behavior of well-known crystallized ZnO. The broadening of the absorption peak of ZnO/PSþ confirms the poor optical quality of this sample. Comparing to both ZnO/Si and ZnO/PS, the absorption tail around the optical gap is larger for ZnO/PSþ which exhibits the amorphous properties [55]. In addition, we note that the imaginary part of dielectric function of ZnO/PS at absorption peak is higher than that of ZnO/PSþ traducing a good structural and optical properties of ZnO/PS. 4. Conclusion By studying physical properties of ZnO layers deposited on crystalline silicon (ZnO/Si), mesoporous silicon (ZnO/PSþ) and nanoporous silicon (ZnO/PS), we have shown that ZnO/PS system can be considered as the best substrate for depositing the ZnO material. The systems are characterized by SEM, XRD, AFM, PL and ellipsometry. High structural and optical properties are obtained for ZnO/PS system which exhibits large emission in both UV and visible domains. Due to the capillary effect and its high specific surface area of PS, absorption and adhesion phenomena are more pronounced for ZnO/PS than for ZnO/PSþ or ZnO/Si. The high optical properties of ZnO/PS sample is also demonstrated by analyzing dielectric function, optical gap, excitonic energy and damping factor values determined by fitting ellipsometric data with Tanguy dispersion model. The damping factor value was found to be low (29 meV) for the ZnO/PS, while the damping factors of ZnO/Si and ZnO/PSþ are 47 meV and 70 meV, respectively. The amplitude of dielectric function of ZnO/PS around 3.4 eV reveals an increase of grain size and crystallinity of ZnO layer. Good structural and optical properties quality makes ZnO/PS a potential and attractive substrate for optoelectronic applications. Acknowledgment

εr

4

(b)

(c)

3

We gratefully acknowledge Jean-Luc Pierrot from LCP-A2MC for SEM and AFM measurements and Habib Boughzela from Laboratory of Materials and Crystallochemistry at Tunisia for XRD measurements.

(a) References

2 +

3

(a)

(a) ZnO/PS (b) ZnO/PS (c) ZnO/Si

2

εi

(c) (b)

1

0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Energy (eV) Fig. 9. Real and imaginary parts of dielectric function of ZnO extracted by Tanguy model from (a) ZnO/PSþ, (b) ZnO/PS and (c) ZnO/Si.

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