Effect of Al2O3 Coatings on Microstructural and Optoelectronic Properties of Porous Si/ SnO2 Composites

In this work, Al2O3 coating effect on morphology, structure and optoelectronic properties of Si/SnO2/Al2O3 porous matrix composites (PMCs) were investigated. A three-staked thin layers deposited on a ⟨100⟩ oriented silicon substrate made these composites. First, porous silicon layers were achieved by electrochemical etching method. Then Al2O3 and SnO2 layers were successively deposited by physical and chemical vapor deposition, respectively. Morphological and micro-structural properties of the as prepared composites were evaluated by Scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffraction. Results proved that Al2O3 concentration alters notably the porosity of the PMCs.Variable angle spectroscopic ellipsometry (SE) revealed a high correlation between the optical constants (n, k) and the PMC microstructure. Impedance spectroscopy revealed a semiconductor-metallic transition at high frequency in the temperature range between 340 to 410°C.

Several compositions and properties of this family have been widely investigated for a wide range of industrial and technological applications [1][2][3][4][5][6][7]. They are used as insulators, semiconductors, conductors and superconductors. These binary oxides are currently being investigated as potential SiO2 substitutes in the next generations of semiconductor devices [1]. Among these materials, Tin oxide (SnO2) thin films have been proved useful for a variety of applications including gas sensors, batteries, fuel cells, photovoltaic cells, photodetectors, transparent electronics, and thin film transistors (TFT) due to their excellent characteristics including chemical stability, high electrical conductivity and optical transparency. The properties of tin dioxide and alumina layers deposited on porous Si (pSi) depend on several factors such as the method of preparation, variety of phase states and degree of dispersity [8][9][10][11][12]. The SnO2 thin films are n-type semiconductors with direct optical band-gap of 3.9 eV [13,14]. Whereas SnO is a metastable material at room temperature and becomes more stable at higher temperatures. Various methods have been used to prepare SnO2 film such as electron beam evaporation, thermal evaporation of oxide powders, magnetron sputtering, spray deposition, chemical vapor deposition (CVD) [15][16][17] and sol-gel. In this work, two deposition techniques, physical vapor deposition (PVD) and CVD, were respectively used to deposit Al2O3 and SnO2 nano-crystals due to many advantages such as large production area and uniform distribution. Both techniques, metal oxide nanocrystals growth in the cavities of porous silicon permits an easy control of material compositions [18]. Appropriate porosity and high specific surface area of oxide supports are required for catalyst or ion exchange applications. In fact, ion exchange in the nanopores of the Al2O3 / SnO2 binary system for the catalytic reaction is not widely recognized. Interestingly, the Alumina Tin oxide catalysts remains an urgent need to increase the catalytic activity and decrease the abrasive properties of the porous matrix composites (PMCs) as the average pore diameter of support materials corresponds to their particle size. Thus, the pore diameter is significant, as catalysts resist sintering highly particularly in the case of a metal catalyst [19]. Such properties encourage the use of Alumina Tin oxide as a tunnel barrier and dielectric gate [20][21][22]. Spectroscopic ellipsometry (SE) [23][24] is highly appropriate to analyze thin layers such as pSi/SnO2/Al2O3 . In this work, pSi/SnO2/Al2O3 samples were modeled as alternately overlapping five layers, each of them having their intrinsic properties. The main objective is to depict the effect of the Al2O3 content on pSi /SnO2 microstructural and opto-electronic properties.

Experimental detail
Electrochemical anodization was used to prepare the porous Si (pSi) layer. A current density of 10 mA/cm 2 was maintained for a silicon substrate immersed in a hydrofluoric acid (HF) Solution. The obtained pSi layer was etched in an acid mixture solution (HNO3: 10%, HF: 20%, H2O2: 70%) followed then rinsed with distilled water and dried under oxygen to yield a pSi model with an ordered pore structure. The CVD and PVD methods served to deposit on the pSi substrate a thin layer of tin dioxide (SnO2) using Snl2 and O2 respectively as reagents. SnO2 in the first place and therefore Al2O3 at different concentrations.
The thin film of SnO2 was grown by CVD on pSi. A vertical CVD configuration was designed to lay the films [25]. Before depositing the film, the chamber was placed under vacuum at 0.1 mbar. The tin precursor was a powdered tin (II) iodide (SnI2, 99%, from Alfa Aesar Company) for the SnO2 deposition. The SnI2 evaporation rate was kept between 0.004 and 0.6 g / h by setting the SnI2 reservoir temperature in the range of 28 to 460 °C. A flow of argon gas is used to carry the SnI2 vapor to the reaction zone in a separate quartz tube. The substrates were sited on a quartz glass sample holder, about 10 cm away from the orifice of this separate tube. The sample holder rotation was set at 19 rpm during deposition to assure uniform deposition. The O2 gas flow rate was varied from 10 to 200 sccm. The carrier gas (Ar) and the reactive gas (O2) flows were monitored by mass flow regulators. The chamber pressure was kept at 46 mbar during the deposition. The deposition temperature was set to 550°C.
The obtained pSi/ SnO2/Al2O3 structure was then thermally treated under oxygen and introduced into an oven under oxygen flow at a temperature equal to 1500°C.

Characterizations
The crystallographic structures of pSi /SnO2 / Al2O3 were determined by X-ray diffraction (XRD) using a Bruker D8 advance X-ray diffractometer equipped with

Energy dispersive X-ray (EDX) analysis
The EDX results of film composition were obtained. Figure

AFM analysis
The surface roughness of the sample and the alteration caused by Al2O3 on the surface morphology of pSi / SnO2 were studied via AFM. According to Figure Table 1 illustrates the particle size, the mean square (RMS) and the roughness as obtained for these films.

X-ray diffraction analysis
Evolution of XRD spectra of pSi/SnO2/Al2O3 samples annealed at 660°C is presented in Fig.5 To calculate the mean crystallite size of Al2O3, we used Debey Scherrer's equation with D is the average size of crystallite, λ is the wavelength of the X -rays, θ is the Bragg diffraction angle, and β is the adjusted FWHM. The Al2O3 crystallites size increases from 2.8 nm to 6.7 nm as the Al2O3 concentration increases. The crystallites size increase was attributed to a diffusion of aluminum crystals from the surface towards the interior of the pSi coated with SnO2, which improved the crystallinity of the structure of the films.

Opto
The physical parameters (n , k) are adequately determined when the model fits the experimental tan ψ and cos Δ data. Fig. 7 illustrates the used multilayer model. Ψ and Δ are the so-called ellipsometric parameters. They respectively stand for the amplitude ratio and phase difference between the p-polarization and s-polarization components of the polarization state of the incident light. N is the number of points, P the number of parameters, m the measured spectra and s the simulated spectra.
Optimization resulted in congruency between the experimental results and theoretical fit. The curves in Fig. 8 exhibit a close agreement throughout the spectral range. This same method is also useful to measure the thickness of the studied layer based on the interference between the reflected rays. Table 1 records the layer thickness and the corresponding RMSE values. The optical constants including the refractive index (n) and the extinction coefficient (k) were extracted and evaluated as a function of the Al2O3 concentration (Fig.8). The refractive index n and the extinction coefficient k variation depend on the concentration of Al2O3, which is attributed to the diffusion of cations along the grain boundaries of Al2O3. An opposite flux of vacancies was thus created towards the surface of the pSi / SnO2 layer. The vacancies can condense and then reach the SnO2/ Al2O3 interface to form cavities under the Al2O3 grain boundaries.

Conductivity measurements
The complex impedance was measured on all the pSi/SnO2/Al2O3 samples within a frequency range between 100 Hz and 13 MHz, and temperature (190-370°C). The corresponding diagrams are shown in the representation of Nyquist Z'' = f (Z') in Fig.9. In practice, several contributions to a dielectric response of an oxidized materials such as grains, grain boundaries, interface are available. The semicircles relative to pSi/SnO2/Al2O3 display different radii (dissimilar to Debye). This effect is attributed a dipolar system involving multi-relaxation processes [33]. We conclude that all samples are semiconductors. In addition, the resistance R0 changed as a function of the deposited alumina concentrations. The peak Z'' intensity variation pleads in favor of a low-capacity semiconductor region assigned to the response of the SnO2 grain embedded in alumina, and the variation of vacuum rate among the samples.

Imaginary part of the impedance
The evolution of the imaginary part Z'' of the pSi/SnO2/Al2O3 sample impedance with frequency at several temperatures is shown in Fig. 10. A maximum of Z'' gives the frequency fmax relaxations as governed by the Arrhenius [34][35][36][37] Table 2 shows that the increase of activation energy with alumina content to 2.31eV, with an Al2O3 concentration equal to 19% and its decrease beyond this rate. This variation is attributed to the incorporation of Al2O3 in pSi/SnO2, and the decrease in the vacuum rate, that facilitates the rate of jump activated thermally. The decrease in Ea (Z'' ) when [Al2O3] is equal to 24% is a possible result of the nucleation of a new surface layer where Al2O3 crystals are very far apart; which ceases the thermally activated jump [38]. This behavior can also be assigned to vacancies in deposited alumina and to interface defects resulting from Tin dioxide vacancies occupied by oxygen atoms as the surface was coated with Al2O3. The values of Ea(Z'' ) for pSi/SnO2/Al2O3 (Fig. 9) reveal two activation energy domains with temperature. in the variation of the electrical properties of the prepared thin film is due to the presence of a new oxide formed in the pores of Si . This phenomenon was favored by the transfer of the oxygen supplied during the external oxide dissociation.
The oxide dissociation is due to the excessive number of cation created at the grain boundaries, thus diffusing through the layer. Conductivity abides by power law with the pulsation given by the equation below [39]: Where ω is the angular frequency, T is the absolute temperature, σdc is the independent frequency conductivity or dc conductivity and σac is the ac conductivity.

Frequency dependence of ac conductivity
The conduction mechanism of pSi/SnO2/Al2O3 dispersions was determined by calculating the conductivity values of dispersions over the frequency range [1 Hz,10 Hz] and at different temperatures as shown in Fig.11. The alternative conductivity σac at different temperatures was modelled by Jonscher's universal power law: A is the constant dependent on temperature, S is the material property which can have any value between 0 and 1, and σac is the ac conductivity. The evolution of S with temperature depends on the conduction mechanism. The exponent S expresses the relative reduction in the size of alumina crystals with frequency and is defined as  According to Arrhenius law, the dc conductivity of the films decreases when temperature increases. This evolution shows the thermal activation mechanism of the electrical conduction, which indicates that the pSi/SnO2/Al2O3 hybrid system has a semiconductor behavior. The activation energy Ea (dc) is extracted from the slope of ln(σdc·T) vs.1000/T. Ea(dc) values are summarized in Table 2. The electrical results prove that, in thermal fluctuation, sufficient energy can be supplied to a dipole hopping across the potential barrier from one position to another equilibrium position [42][43][44].

Conclusion
The opto-electrical properties of a SnO2/Al2O3 deposit in a porous silicon layer were investigated. The structural properties were found to depend significantly on the Al2O3 concentration. XRD spectra showed that the concentration of Al2O3 deposited on pSi/SnO2 was vital to improving crystallinity. After the deposition of Al2O3 under oxygen on pSi/SnO2, the structure tends to crystallize for a temperature that reaches 660°C. The ellipsometric study of pSi/SnO2/Al2O3 shows the improvement of optical properties (refractive index, extinction coefficient) as a function of the Al2O3 concentration. Consequently, the refractive index is raised while the extinction coefficient becomes lower with the concentration of Al2O3. This change is attributed to the progressive pores filling as a function of Al2O3 concentration and structure alteration. Moreover, according to the impedance measurements, the ac conductivity obeys the universal power law. The transport of charge carriers was using the CBH model. these results exhibits a novel semiconducting behavior that is suitable for thin film engineering and functional coating applications               Figure 1 The EDX analysis of pSi/SnO2/Al2O3 thin lms.    Multilayer model used to t the pSi/SnO2/Al2O3 structure with different Al2O3 concentration. Experimental (Symbols) and tted (Red-lines) SE data of pSi/ SnO2 coated with Al2O3 at different concentration.

Figure 8
Effect of Al2O3concentration on the refractive index n and the extinction coe cient k of pSi/ SnO2/ Al2O3 coated with Al2O3 at different concentration.