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Abstract
In this work, the fabrication of a porous silicon Bragg reflector and vertical cavity on P+ silicon substrate is investigated for applications in spectroscopic sensing in the mid-infrared (Mid-IR) wavelength range. The complex refractive index of porous silicon layers is measured. Optical vertical devices are then fabricated and characterized by Fourier transform infrared (FTIR) spectrophotometry. This work demonstrates the use of electrochemically prepared Bragg reflectors with reflectance as high as 99% and vertical cavity based on porous silicon layers operating in the mid-IR spectral region (up to 8 µm). Experimental reflectance spectra of the vertical cavity structures are recorded as a function of air exposure duration after thermal annealing under nitrogen flux (N2) and results demonstrate that these structures could be used for spectroscopic sensing applications in the mid-IR (2-8 µm) by grafting specific biomolecules on the porous silicon internal surface.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Highly sensitive optical integrated sensors have received great interest during these last years [1–9]. In particularly, the ability to rapidly detect, identify and monitor chemical or biological species is imperative for environmental, health monitoring and security applications. The detection of traces of (bio)-chemical molecules requires sample preparation procedures combined with sophisticated analytical tools which can detect, within an acceptable time, disease biomarkers, emerging pollutants, chemical warfare agents or toxic industrial chemicals with high sensitivity to really detect low concentrations with high selectivity not to be affected by other factors in the environment.
In parallel, during these last decades, sensors based on porous silicon (PSi) nanostructures have been developed taking benefit from the large internal surface area and widely tuneable refractive indices of this material. Different types of optical devices have been reported such as single layer interferometers, waveguides, Bragg mirrors, microcavities, rugate filters [10–19]. An inherent feature of PSi material is its open pore network. Normally, this can be used to modify the properties of optical devices in-situ, by filling the pores with other molecules. Previously cited PSi photonic structures have shown a very good sensitivity in the label-free sensing by the modification of the effective refractive index of a porous silicon layer. Moreover, PSi is also a biocompatible material and the functionalization of its internal surface by grafting molecules to detect is interesting for surface detection thanks to its large specific surface area which can reach 800 m2/cm3 [20]. This large surface area of PSi and the open pore network allow an enhancement of the interaction between light and potential target molecules to detect in gas or liquid medium. Most optical devices developed as sensors based on PSi are used for wavelengths from visible to near infrared. However, many molecules can be detected in the Mid- Infrared (Mid-IR) because of their characteristic absorption bands, creating unique molecular fingerprint [21–28]. Photonic integrated devices operating in the Mid-IR have been already implemented using bulk material by taking taken advantage of the Mid-IR transparency of semiconductors materials [22,23,29–36].
So, the ability to tailor Porous Silicon (PSi) refractive index and layer thickness by controlling porosity and anodization etching time [20,37,38] makes it particularly attractive and unpublished for Mid-IR optical applications. Optical multilayer devices such as Bragg reflectors or micro-cavities have already been demonstrated from PSi layers notably for Visible and Near Infra-Red (NIR) wavelength ranges [10–19] to detect solvents, pesticide, bacteria, human antibodies…. However, there have not been any report on the use of PSi materials for the implementation of Mid-IR optical devices to detect a wide variety of liquids or of trace gases such as H2O, CO, CO2, NO, NO2, CH4 due to absorption peaks [21–28].
Two constant current densities are used for the High Porosity (HP) and Low Porosity (LP) layers with low and high refractive indices (nHP and nLP) respectively for the fabrication of these optical devices. The etching times are adjusted to produce an optimized Bragg reflector or cavity with the resonant wavelength in the chosen spectral range.
The implementation of a Mid-IR silicon photonics transducer with broad Mid-IR transparency (up to 8 µm by taking into account Si transparency [39]) is a challenge that could find applications in spectroscopic sensing and environmental monitoring. This paper demonstrates the fabrication of vertical PSi multilayer structures on Si substrates and their sensing potential in the Mid-IR wavelength range notably near the cut-off band of Si due to its absorption at 8 µm [39]. Bragg reflector and vertical cavity on P+ silicon substrate for applications in spectroscopic sensing in the Mid-IR wavelength range are fabricated and optically characterized
The objectives of the paper are, on the one hand, to fabricate porous silicon multilayers in the Mid-IR and, on the other hand, to carry out a simple transduction test (without functionalization) by using vertical cavity that enhance the absorption of target molecules physical properties in order to detect the molecules of the air in an absorbing spectral band which does not belong to the two transparent atmospheric windows (3-5 µm and 8-12 µm) . That is why, the resonance wavelength of cavity is targeted to 6.5 µm in order to enhance a specific air absorption signature.
2. Experimental conditions
The PSi layers were obtained by electrochemical anodization of p-type doped (100)-oriented silicon wafer (4-6 mΩ.cm) at room temperature. The electrolyte was composed of HF(50%):ethanol: deionized water (2:2:1). The refractive indices and porosities of PSi single layers were studied by reflectometry in the near- and mid-infrared spectral ranges. Scanning electron microscopy (SEM) was used to measure the thickness of PSi layers. The thickness of each porous layer was controlled by the anodization etching time. Current densities of 20 and 90 mA/cm2 were used for the low porosity (LP) and high porosity (HP), respectively. These current densities were chosen to obtain a high index contrast.
The infrared reflectance spectra were measured by a Perkin Elmer Spectrum 100 FTIR spectrophotometer from 1.5 to 15 µm with a resolution of 4 cm-1. The calibration is performed by a measurement using a gold mirror with a calibrated reflection equal to 97.5% between 1.5 and 25 µm. The calibration protocol of the spectrometer takes into account absorption from the environment, in particular, absorption fringes of CO2, water vapor and methane. From reflectance spectra, the complex refractive index for each of the fabricated single PSi layers was deduced by fitting experimental spectral fringes using a commercial software (Essential Macleod).with the effective medium theory (Bruggeman model) [40]. The measurements of the single layer reflectance spectra were carried out just after fabrication.
Some samples were annealed under N2 for 1 hour at 300°C in order to desorb the specific surface of the porous layers and to remove contaminants.
Knowing the refractive index and the anodization etching rate of the two studied PSi layers, the design of vertical optical devices was performed using the transfer matrix method in order to determine the best combination of parameters for Bragg reflectors and cavities (reflection bandwidth, resonance wavelength and maximum reflectance).
3. Results and discussion
The porosities and associated Mid-IR refractive indices have been determined from reflectivity measurements of two single PSi layers whose fabrication parameters, given above in the experimental part, correspond to those of the multilayer structures. From the thicknesses measured by scanning electron microscopy equal to 2 µm for the two fabricated layers, porosities and then complex refractive indices were determined by the adjustment of the calculated reflectance spectra of each PSi layer with experimental ones using Bruggeman model [41–43].
The experimental reflectance spectra are shown in Fig. 1(a) for the two PSi single layers obtained with the current densities of 20 (LP) and 90 (HP) mA/cm2. The experimental fringes are well visible until 8 µm before the silicon absorption in the Mid-IR range. The adjustment of the experimental spectra was carried out for wavelengths ranging from 1.5 to 8 µm. A good fit between experimental and theoretical spectra was obtained as illustrated in Fig. 1(b) in the case of a high porosity layer. The complex refractive indices and the extinction coefficients for the two PSi porosities are reported in Figs. 2(a) and (b), respectively. The uncertainties Δn and Δk in Figs. 2(a) and (b) are estimated by the method of J. Manifacier [44].
Fig. 1. (a) Experimental Mid-IR reflectance spectra of the PSi layer of high and low porosities and of the Si substrate. (b) Experimental and fitted reflectance spectra of the low porosity PSi layer.
Fig. 2. Spectral dependances of (a) PSi refractive index and (b) PSi extinction coefficient as a function of the wavelength for the high and low porosity layers.
It should be noted that the extinction coefficient of porous layers is low in 1550 - 8000 nm spectral region as expected. The index contrast has been calculated from the values of refractive index in Fig. 2(a) for the two wavelengths by assuming that the refractive index is constant above 5.5 µm for the two layers. The real refractive index contrast between the HP and LP films was found to be 0.88 at 1550 nm and 0.62 at 6.5 µm. The porosity values are respectively for the low and high porosities equal to 33 and 62% and the extinction coefficient k is higher for the layer of lower porosity as indicated in Table 1 due to the contribution, of the Si absorption that is more important for this low porosity value.
Table 1. Physical parameters of the lower and higher PSi layers
In order to avoid the need for thick films in the multilayers devoted to Mid-IR applications, a large index contrast was targeted between the layers forming the pattern. These values are therefore very promising for envisioned applications considering the low absorption of the PSi layers.
Some examples of SEM micrographs of single layers (Fig. 3(a)) and prepared multilayered structures such as Bragg mirror (Fig. 3(b)) or vertical cavity (Figs. 3(c) and 3(d)) are presented in Figs. 3. The columnar structure of the porous layers is clearly observable in Figs. 3(a) and (d). This morphology is characteristic of mesoporous layers that are fabricated from P+ Si substrate [20]. The interfaces between individual layers as well as the surfaces of the multilayered structures were found to be smooth without any defect during the fabrication of thin film stacks (Figs. 3(b), (c) and (d)).
Fig. 3. SEM micrographs (cross sections) of (a) single layer of PSi, (b) Bragg reflector structure and (c,d) vertical micro-cavity at some different scales.
The achieved reflectivity is determined by the number of layer pairs N and by the refractive index contrast Δn between the layers. The Bragg reflector is characterized by its central wavelength λ0 (at normal incidence) and by the reflection bandwidth Δλ which is determined mainly by the index contrast. These two parameters are defined respectively by the relationships.
with and where nHP (LP) and tHP (LP) are the refractive index and thickness of high (low) porosity layers respectively.The thicknesses of pattern (tHP = 1120 nm and tLP = 785 nm) were fixed to obtain Bragg reflector and cavity structures centered at 6500 nm. The experimental responses of the Bragg reflector is reported in Fig. 4(a). The spectrum is centered close to 6415 nm. The experimental maximal reflectance is around 99% whereas the theoretical value is equal to 99.2%. The experimental spectrum is in close agreement with the theoretical spectrum taking the absolute errors into account. The absolute errors of measured refractive index and thickness of layers are 0.05 and 50 nm respectively.
Fig. 4. Experimental reflectance spectra of (a) Bragg reflector and (b) vertical micro-cavity structures.
This work demonstrates that it is possible to fabricate a Bragg reflector with good optical qualities in the Mid-IR range from P+ silicon substrates.
With the same pattern, a vertical cavity was fabricated with a resonance wavelength equal to 6396 nm. The experimental reflectance spectrum of the microcavity is illustrated in Fig. 4(b). The resonance peak is close to the target value. Moreover, the reflectance value of the device of the bandwidth is as high as 99%.
Another merit factor is the quality factor of the structure. In order to study this parameter, we can use the Fabry-Perot classical model [45]. The quality factor is given by the following relationship:
For the elaborated micro-cavity, the theoretical quality factor is equal to 315 and from the experimental spectrum, the quality factor was estimated at around 200.
However, we noted a degradation of the quality factor as a function of time when the sample was exposed to air as shown in Figs. 5.
Fig. 5. (a) Evolution of the experimental reflectance spectra of the same micro-cavity structure as a function of exposure duration to air after annealing at 300°C of the structure under N2. (b) Magnification of spectra around the resonance wavelength.
After thermal annealing under N2 at 300 °C for one hour, the spectrum of initial experimental reflectance was recovered due to the desorption of the specific surface of the sample.
Several experimental reflectance spectra were recorded as a function of exposure duration to air after thermal annealing at 300°C under N2. These spectra are reported in Fig. 5(a). We note that the longer the micro-cavity is exposed to the ambient air, the greater the degradation of the cavity quality factor is. After a new heat treatment under N2 of the cavity exposed to air for 30 minutes, the quality factor is again improved.
This quality factor reduction is better illustrated in Fig. 5(b) by showing the spectra near the resonance wavelength to emphasize the resonance FWHM and reflectance value increase.
The degradation of the quality factor is due to the adsorption on PSi internal surface of molecules contained in the ambient air. The degradation of the quality factor is due to the adsorption on PSi internal surface of water vapor molecules from the ambient air [47]. Indeed, water molecules have strong absorption lines in the studied wavelength range, around 6.5 µm [48]. Methane could also weakly contribute to the absorption at 6.5 µm [48].
A red shift is noted for the resonance peak and the width of the peak increases strongly due to the molecule adsorption with air exposure time. The initial reflectance is almost obtained after a new annealing under N2 of the 30 minutes – air exposed cavity (green curve in Fig. 5(b)). The difference between the two spectra (just annealing and new annealing after 30 minutes exposure) is due to the time interval between the FTIR measurement and the sample exit from the oven.
Reflectance spectrum overlaps absorption bands of air molecules and their detection is enhanced by the cavity [10–19]. The complex refractive index of PSi single layer also changes over time however, the single layer spectra are less sensitive than the spectrum of cavity that enhances these modifications. These changes can appear in the form of peaks contained in the interference fringes corresponding to molecule absorbance lines but notably they will be difficult to observe on the spectra above 6 μm because the interference fringes disappear due to the absorption of the silicon substrate.
Moreover, the functionalization of the surface of the porous silicon would allow the development of a selective sensor in a gaseous or liquid complex medium able to identify a target molecule among many molecules which may display an overlap of their respective absorption bands. So by a specific functionalization of porous silicon [24], these vertical optical structures could be used as transducers to detect low concentrations of target molecules by choosing the good resonance wavelength, in the Mid-IR range, and relative to its absorption band. However, such micro-cavities should be encapsulated in a neutral atmosphere so that a micro-fluidic system only interacts with a fluid containing molecules to be detected.
4. Conclusions
In conclusion, vertical multilayer stackings based on PSi layers were fabricated for Mid-IR wavelength range. From the experimental reflectance spectra of PSi thick layers, real and imaginary parts of the refractive index of these films of different porosities were obtained in the Mid-IR wavelength range. Bragg mirror and vertical microcavity were then fabricated notably near the cut-off band of Si due to its absorption at 8 µm. Multilayer structures based on PSi layers made from P+ Si substrates were then demonstrated. A maximal reflectance of 99% and a quality factor equal to 200 were obtained respectively for Bragg mirror and vertical micro-cavity. These optical devices could be used as highly sensitive transducers thanks to the PSi layers surfaces. The first results obtained from these PSi multilayer structures demonstrate their potential in the Mid-IR range. In future work, these PSi structures will serve as functionalized transducers by grafting biomolecules to develop spectroscopic sensing applications in the Mid-IR (2-8 µm).
Funding
Agence Nationale de la Recherche under MID-VOC ANR project (ANR-17-CE09-0028-01).
Acknowledgments
Equipment funding of Institut Foton were partly provided by the CPER Sophie.
Scanning Electron Microscopy imaging and porous silicon fabrication were performed in the CCLO - Renatech clean room facilities of Institut Foton.
Disclosures
The authors declare no conflicts of interest.
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