Fabrication and optical characterization of Bragg resonance luminescence porous silicon

Since the discovery of visible photo- and electroluminescence from porous silicon (PS) [1], PS has been vigorously investigated for a variety of applications such as chemical [2] and biological sensors [3], medical diagnostics [4], micro chemical reactors [5], and drug delivery [6]. PS is an ideal candidate for gas- or liquid-sensing applications, because of its characteristic of a very large specific surface area in the order of few hundreds m² cm⁻³. The direction of pores and pore diameters is reportedly depended on surface orientations, doping level and type, the temperature, the current density, and the composition of the etching solution [7]. The main methods investigated to achieve the signal transduction are capacitance [8], resistance [9], photoluminescence (PL) [10], and reflectivity [3].

Typically, PS prepared from p-type silicon wafer under dark condition exhibits well defined Fabry–Pérot fringes in the optical reflectivity spectrum. However, PS prepared from n-type silicon wafer under tungsten lamp shows a strong naked-eyed PL when illuminated by a UV light source. When analytes are introduced to p-type, by capillary condensation effect in the pores, it leads to a shift in the Fabry–Pérot fringes by the modification of the refractive index of PS and when introduced to n-type PS, the luminescent PS shows quenching because the quantum dots have a change in their surrounding dielectric environment. Recently, PS samples exhibiting both strong PL and well defined Fabry–Pérot fringes or Bragg reflections were reported. Both PL and reflectivity of PS were used to detect organic vapors [11] and nerve agent simulants [12].

Building on photonic enhancement of luminescence efficiency, PS is an excellent material to produce photonic crystals which are periodic arrays of dielectric materials having photonic band gaps similar to the electronic bands of solid crystals. Both, the structure and the refractive index contrast between the dielectric materials, play a crucial role on the photonic properties. The first Bragg mirror in PS was discovered by Vincent [13], and the microcavities in PS were studied by Pavesi [14]. The transmission modes of emission from Bragg mirror have been analyzed by Squire et al [15, 16]. The Bragg reflector is characterized by its central wavelength λ₀, and by the reflection bandwidth which is determined mainly by the index contrast. In many cases, narrow reflection bandwidth is highly useful for sensing applications. A strong red PL is observed from PS prepared from an electrochemical etching of lightly doped n-type Si wafer, and Bragg reflection is usually observed from PS prepared by an electrochemical etching of highly doped p-type Si wafer. To date, there is no report on the formation of enhanced luminescence PS based on Bragg resonance showing both a strong narrow naked-eyed PL and a strong optical reflectivity in the visible range from the same sample. This can be highly useful for sensing applications, since the signal changes can be measured simultaneously by two different sensing mechanisms, the quenching of PL and shift of the reflectivity, which can give a huge advantage when comes to selectivity. Here, we reported the synthesis and characterization of Bragg resonance luminescence porous silicon (BRL PS) exhibiting both optical reflectivity and strong narrow visible PL prepared from highly doped n-type silicon wafers through the electrochemical etching.

2.1. Preparation of BRL PS

2.2. PL and reflectance measurements

The wave properties of light make perfect reflectance possible in stratified dielectric media over all spectral ranges due to interference phenomena. In a single layer of PS, the phase shifts imparted on propagating waves at dielectric interfaces may result in constructive and destructive interference. The optical spectrum from a single layer PS is governed by the Fabry–Perot relationship. The wavelength of a peak in the reflectivity spectrum is given by equation (1)

$$\begin{eqnarray}&&m\lambda =2nd\;\mathrm{sin}\;\theta ,\end{eqnarray} \tag{ 1 }$$

where m is the spectral order of the optical fringe, λ the wavelength, n the refractive index of the film, and d its thickness.

There has been growing interest in the development of efficient control for the preparation of PS multilayer stacks. As PS porosity is a function of the current density, PS layers having different refractive indexes can be built up, one after another, on a silicon substrate vertically by alternating the applied current densities during the electrochemical etching. Bragg reflector is a structure which consists of an alternating sequence of layers made of two different refractive indices. For repeating layers of alternating thickness and refractive index, many interferences may coherently interact to produce high reflectivity at certain points in the spectrum. When a periodicity is enforced by alternating between two layers of thickness d_i and refractive index n_i such that

$$\begin{eqnarray}&&{n}_{1}{d}_{1}={n}_{2}{d}_{2}=\lambda /4,\end{eqnarray} \tag{ 2 }$$

peak reflectivity increases rapidly with the number of bilayers N and is given by [17]

$$\begin{eqnarray}&&R\simeq {\left(\displaystyle \frac{1-\left({n}_{s}/{n}_{a}\right){\left({n}_{2}/{n}_{1}\right)}^{2N}}{1+\left({n}_{s}/{n}_{a}\right){\left({n}_{2}/{n}_{1}\right)}^{2N}}\right)}^{2},\end{eqnarray} \tag{ 3 }$$

where it is assumed that n₂ > n₁ and the substrate refractive index ns is similar to that of the layers; n_a is the refractive index of the ambient. While this reflectivity is achieved for the design wavelength λ_c, for a sufficient refractive index contrast n₂/n₁ a wide band of perfect reflectivity centered at λ_c may be observed. The spectral width of the photonic band gap is given by

$$\begin{eqnarray}&&\displaystyle \frac{\Delta \lambda }{{\lambda }_{c}}=\displaystyle \frac{4}{\pi }{\sin}^{-1}\displaystyle \frac{{n}_{2}-{n}_{1}}{{n}_{2}+{n}_{1}}.\end{eqnarray} \tag{ 4 }$$

The quality of Bragg reflector PS in photonics can be improved by increasing the refractive index contrast between layers. The bandgap widening of PS due to quantum confinement effects leads to a decrease of the extinction coefficient k, making PS transparent in the infrared region. PS has a low extinction coefficient in near IR range (1000–2500 nm) but a strong extinction coefficient in UV–vis range (300–1000 nm). PS Bragg reflector with a reflection bandwidth of 300 nm in the IR range has been demonstrated with good spectral behavior due to the low absorption of PS [18].

Recently, there are several reports for the generation of Bragg reflector PS to achieve the same degree of performance in the visible range, even though PS has a strong absorption coefficient from the visible to UV region [19]. For sensing application, the Bragg reflector PS with narrow reflection bandwidth obtained by tuning the refractive index and etching time is desirable. The Bragg reflector PS is an excellent medium for the detection of organic vapors [20, 21] and toxic gases [22], and drug delivery materials [23, 24]. Moreover, to specify an analyte with a sensor, multi-transduction mode within the same sample of PS gives an extra advantage. Previously, the Bragg reflector PS exhibiting both strong PL and Fabry–Pérot fringes was reported [11, 12]. However, the red strong PL peak of Bragg PS in the report mentioned above was similarly broad to that of monolayer PS.

BRL PS exhibiting both sharp reflection and PL peaks resulted from Bragg resonance in the visible range was successfully fabricated. BRL PS showing the luminescence at 702 nm with an excitation wavelength of 400 nm was prepared by applying the current of 360 mA cm⁻² for 1.6 s and 75 mA cm⁻² for 3.6 s with 50 repeats in etching solution of 1:1 volume mixture of absolute ethanol and aqueous 48% HF. For comparison, typical monolayer PS showing the luminescence at 655 nm and prepared by applying 200 mA cm⁻² for 300 s. The PL spectra for both monolayer PS and BRL PS were shown in figure 1. The monolayer PS showed a broad PL spectrum with a full width at half maximum (FWHM) of 125 nm. In contrast, BRL PS exhibited sharp PL peak which reached FWHM of 14 nm, arising from broad PL spectrum. This sharp PL of BRL PS presumably originated from the result of Bragg resonance in PS multilayer.

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Cross-sectional FE-SEM images of BRL PS with repeat numbers of 50 and 60 were obtained using cold FE-SEM and shown in figure 2. Both images illustrate that a repeating etching process results in two distinct refractive indices. The each layer depths of BRL PS for both 50 and 60 repeats were about 250 nm. The porosities of high and low refractive layers measured from monolayer PS samples etched at 360 and 75 mA cm⁻² were about 80% and 70%, respectively. The refractive index range of above PS porosity is from 1.3 to 1.5 according to Bruggeman approximation. The above result suggests that the first-order reflectivity peak of BRL PS is expected to be centered at approximately 1400 nm which indicates that the second-order reflectivity peak should appear at about 700 nm. Therefore, we can reasonably conclude that the sharp PL peak at 700 nm of BRL PS is the second-order luminescence reflectivity peak by Bragg resonance phenomenon.

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To investigate the Bragg-reflective PL property, the monolayer PS and BRL PS samples etched at 300 mA cm⁻² for 1.5 s and 30 mA cm⁻² for 4.5 s with 55 repeats, were placed in an exposure chamber fitted with an optical window and then exposed to a vapor flux of acetone (184 mmHg at 20 °C) with a constant argon flow rate of 0.1 L min⁻¹. Optical reflectivity and luminescence spectra as shown in figure 3 were measured simultaneously using a tungsten–halogen lamp and 400 nm UV LED.

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After an exposure of acetone vapor for 6 s, 48% of luminescence of monolayer PS was quenched without a shift of luminescence wavelength maximum (figure 3(A)). In the case of BRL PS, the reflection wavelength shifted by 37 nm to longer wavelengths due to an increase in refractive indices of the porous medium, consistent with the replacement of a significant amount of empty pore volume with organic vapors by adsorption (figure 3(B)). However, 80% of narrow enhanced luminescence based on Bragg resonance in BRL PS quenched dramatically (figure 3(C)). The reflectivity and PL spectra were overlaid to see the alignment on a scale where the details can be seen. The different peak wavelength in reflectivity and PL spectra is about 7 nm which is small enough to neglect (figure 3(D)). The PL might be quenched by additional recombination due to the presence adsorbed molecules. However such a dramatic quenching PL of BRL PS compare to that of the monolayer PS, is probably due to the Bragg resonance effect on luminescence.

The synthesis and characterization of BRL PS exhibiting both Bragg reflectivity and Bragg PL from highly doped n-type silicon wafers through the electrochemical etching were reported. BRL PS showing the luminescence at 702 nm with an excitation wavelength of 400 nm was prepared with two distinct refractive indices by applying alternating current densities. The monolayer PS showed a broad PL spectrum with a FWHM of 125 nm, however, BRL PS exhibited sharp PL peak which reached FWHM of 14 nm. This sharp PL of BRL PS presumably originated from the result of Bragg resonance in PS multilayer. Analysis of cross-sectional FE-SEM images and porosities of BRL PS samples suggests that the sharp PL peak at 700 nm of BRL PS is the second-order luminescence reflectivity peak by Bragg resonance phenomenon. To investigate the Bragg-reflective PL property, the monolayer PS and BRL PS samples were exposed to a vapor flux of acetone. Optical reflectivity and luminescence spectra were measured simultaneously. Analysis on the changes of reflectivity and quenching PL indicated that the larger change in quenching PL of BRL PS compare to that of the monolayer PS, is due to the Bragg resonance effect of luminescence.

This research was supported (in part) by research funds from Chosun University, 2014.