Sorption and optical properties of sol–gel thin films measured by X-Ray Reflectometry and Ellipsometric Porosimetry
Highlights
►The accessible porosity of sol–gel SiO2 and TiO2 films was determined. ►Porosity values decrease with thermal treatment. ►For sol–gel TiO2 films, wall refractive index increases with thermal treatment. ►For sol–gel SiO2 films, wall refractive index is constant with thermal treatment. ►Thermal treatment was optimized to obtain sol–gel thin films with very low porosity.
Introduction
For devices possessing optical coatings, the control of thickness and refractive indices of the layers is of major importance and determines their optical behavior. Usually, these functional coatings are single or multilayered stacks of dielectric materials; the combination of different oxides brings new properties to the final device.
Several techniques have been used to prepare oxide thin films for optoelectronic applications, such as reactive sputtering [1], chemical vapor deposition [2], atomic layer deposition [3], [4], [5] and electron beam evaporation [6], [7] among others. Compared to these techniques, the sol–gel synthesis is less expensive, requires less specialized equipment and has the particular advantage of the strict control of pore microstructure [8].
There are numerous examples where the sol–gel technique can be used to synthesize pure, mixed and hybrid oxides thin films on different substrates by spin or dip-coating. This technique allows control of not only the layer thickness but also the refractive index of the deposited layers, which has been used to develop optoelectronic devices [9], [10], [11], [12]. Bragg mirrors [13], [14], [15] and antireflective (AR) coatings [16] obtained by bottom-up techniques are two examples of optical devices that include stacks of dielectric layers with very controlled refractive indices and thicknesses between 30 and 200 nm. In other applications, as SiO2 passivating coatings on Si substrates, thinner films are necessary, reaching 10–20 nm [17], [18].
It is well known that sol–gel materials are porous and the amount of porosity depends on the solution composition and the processing conditions [8]. The determination of some properties (thickness, porosity, refractive index, etc.) of very thin films with very low porosity is in general difficult to perform and the results are controversial. The porosity is usually evaluated considering the differences in refractive indices of a sol–gel material and a fully dense material with the same composition [19], [20], [21], [22]; however, with this method the accessible porosity can not be discriminated from the total porosity.
Many efforts have been focused on the characterization of porous thin films. The Environmental Ellipsometric Porosimetry (EEP) technique was specially designed to obtain water adsorption–desorption isotherms for porous thin films, but the ellipsometric results are difficult to model for very thin films with low porosity because thickness and refractive indices are non-independent parameters. This technique gives information about the accessible porosity, calculated at high relative humidity (RH) values, and the porous microstructure, determined from the shape of the isotherms. X-Ray Reflectometry (XRR) is a very useful technique to determine thickness of very homogeneous thin films with low rugosity and can be adapted to determine accessible porosity. However, this technique is not appropriate to obtain a complete isotherm of a porous thin film and only offers an estimation of the accessible porosity with change in humidity.
In this work, SiO2 and TiO2 thin films were produced using two different synthesis methods: the sol–gel technique and high temperature thermal oxidation. The sol–gel technique allows the deposition of different oxides in the desired order to obtain the optimum variation of refractive indices and thicknesses. The thicknesses were controlled in the range of interest for optical devices and their optical properties were determined for different syntheses and post-processing conditions. The thermal oxides bilayered structures (SiO2–TiO2) were fabricated in order to optimize their final properties for applications as AR coating in encapsulated silicon solar cells [23]. In this particular application, the optimum thickness range for rutile TiO2 thermal oxide is 27–45 nm, while a tolerance of about 10–15 nm of SiO2 is accepted. In the case of a SiO2 single layer on Si, the temperature and time of the thermal process in an oxidant environment ensure the thickness of the layer with enough reliability; oxide thicknesses between a few nm to 1 μm can be obtained [24].
The synthesized materials were characterized by several techniques designed for thin films studies, including Spectroscopic Ellipsometry (SE), Grazing Incidence X-Ray Diffraction (GXRD), XRR and EEP. The absolute value of the accessible porosity was determined by XRR and EEP without using standard dense materials as reference. Besides, the effective refractive index of the porous film and the refractive index of the walls (without considering the air inside the pores) were determined independently. The similarity of the thicknesses and porosities obtained using both techniques confirms the validity of the characterization strategies developed in this work.
Section snippets
Materials
HCl(c), tetraethoxysilane (TEOS) and TiCl4 were supplied by Aldrich. All chemicals were used as received. Pure grade ethanol and 18 MΩ.cm deionized water were used as solvents.
Solutions preparation
The titania solution was composed of TiCl4:EtOH:H2O with a molar composition of 1:40:10 and was used immediately after preparation. Initial silica solution was composed of TEOS:EtOH:H2O (0.1 M HCl) mixture, with a molar composition of 1:40:5. The solution was continuously stirred at 60 °C for 1 h prior to use. In order to
Results and discussion
SiO2 and TiO2 thin films on Si substrates were fabricated using two techniques: 1) a high temperature oxidation method; and 2) the sol–gel technique.
In the first case, the TiO2 samples are actually bilayered structures, Si-SiO2-TiO2; the SiO2 layer is formed as a passivating layer from the Si substrate using this synthesis method. In the second case, SiO2 and TiO2 thin films were deposited on silicon or glass using spin-coating. After deposition and processing, the obtained films are
Conclusions
The relationship between sorption and optical properties, thicknesses and thermal treatment of TiO2 and SiO2 thin films (30–100 nm) prepared by sol–gel and thermal oxidation was studied in this work using XRR and EEP. A method to determine the effective refractive index and the wall refractive index in an independent way was developed and applied to evaluate densification or crystallization processes.
The results indicate that there is a residual porosity in low-temperature processed sol–gel
Acknowledgments
The authors acknowledge financial support from ANPCyT (PICT 2007 No. 01143, PICT 2010 No. 00026 and PAE-37063-PME-2006-00038), CONICET (PIP 2009–2012 No. 02318) and LNLS (beamline D10A-XRD2, projects 9118 and 9236). We thank Paula Angelomé, M. Dolores Perez and Cindy Aquino for the detailed reading of this manuscript. MCF, MPB and JP are CONICET researchers.
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