Bonifacio Alvarado T. and V. Agarwal
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AZojomo (ISSN 1833-122X) Volume 4 July 2008
Results and Discussion
To investigate the reflection of light in random dielectric multilayers, we study here the optical properties of porous-silicon-based heterostructures. The multilayered systems (20 and 40 layers) are fabricated by the porous silicon layers having refractive indices of na=1.2 and nb= 2 in such a way that each layer has the optical length of 162 nm. The reflectance from the structures having partially (only 10 layers) and totally randomized sequence are analyzed and compared with the classical periodic structure.
Heterostructures, Porous Silicon, Photonic Bandgap, Dielectric Multilayers.
Porous silicon has attracted attention due to its efficient visible photoluminescence , the capability to infiltrate its matrix due to its large surface area, its compatibility with standard silicon processes for integrated optoelectronics  and as a promising material for photonic applications [3-5]. The fabrication of porous silicon uses electrochemical etching of crystalline silicon (c-Si) with HF which is easy and cheap. Moreover, this method gives us the possibility of having a wide refractive index contrast within the same material avoiding the problem of interdiffusion between the layers. Porous silicon consists of a structure that contains many nanocrystalline silicon branches surrounded by air. In this material, the porosity is a linear function of the current density for a specific HF concentration and anodization time and is known to show a refractive index contrast for different porosites . Based on the fact that HF removes Si atoms of the crystalline structure mainly around the electronic holes, the Si crystallites in the porous silicon structure are mainly intrinsic and hence the reaction between HF and Si takes place only at the tip of the pores. Therefore, a periodic pulse alternating between two different current densities has been a convenient procedure to fabricate multilayer p-Si films [7-8].
In photonics, it is well known that the control parameter in these structures is the optical path and this path is given by the product of the refractive index and the physical thickness d of the layer, (nd). Thus, thickness and refractive index of porous silicon layers controls the behavior of light in the photonic structure, in particular, the peak wavelength in the reflectivity spectrum of a multilayer mirror. Moreover, dielectric mirrors have an advantage over the metallic ones due to the dispersive and absorbing regions in the visible and infrared spectra of metals . These kind of 1-Dimensional photonic bandgap structures have already found many applications like dielectric mirrors, waveguides, sensors and other devices. It is known that the disorder modifies the electronic band structure e.g. amorphous silicon has a wider electronic bandgap than crystalline silicon. Considering photon modes analogous to electronic states, one can modify the reflectance spectrum by randomization of a periodic structure. Random Bragg reflectors have been obtained  by keeping the thickness and porosity of low refractive index layers constant and by changing the thicknesses but not the porosity of the high refractive index layers. Various thicknesses were generated randomly such that their histograms form a Gaussian distribution centered around a mean value. It was found that the effect of increasing the standard deviation in thicknesses was to broaden the photonic bandgap without a loss of reflectivity. In this paper we discuss in detail the experimental implementation of some other ideas dealing with random dielectric multilayers from PS and their optical characterization. In particular, we present a comparison of the reflectivity spectrum of porous silicon dielectric Bragg mirrors made of 20/40 layers with their corresponding partially and completely randomized sequences, keeping the refractive indices and thicknesses constant.
In section II, we explain in detail the experimental setup and procedure in order to fabricate these photonic nanostructures based on porous silicon. Section III shows some of our results where random multilayers have been designed for visible and near infrared range. Since high quality microcavities have a crucial role in most of the photonic crystal based applications. Finally, section IV concludes the article showing relevance of porous silicon random multilayers as promising photonic structures for different applications.
We have used boron doped p++ type crystalline silicon with resistivity 0.001-0.005 ohm-cm, (100) oriented substrates for fabricating our samples. Anodization was performed at room temperature in a solution of 27% volumetric fraction of HF, i.e. the solution with volume ratio of 3:7 HF (48 wt%), ethanol (98 wt%) was taken for electrochemical anodization process. The current density was controlled by the computer. A high porosity H (70%, current density 80 mA/cm2, with effective refractive index of 1.2) and low porosity (35%, current density 5 mA/cm2, with nb = 2.0), were periodically repeated to form the filters, mirrors and coupled microcavity (CMC) structure. The refractive indices of the pSi layers have been estimated using reflectivity spectra of 2 µm thick single layers at 1500 nm. In addition, in order to maintain a constant HF concentration over the interface between Si and pSi under chemical attack, during the etching process a peristaltic pump is used to circulate the electrolyte within the Teflon cell. Anodization begins when a constant current is applied between the silicon wafer and the electrolyte by means of an electronic circuit controlling the anodization process. After every layer formation a pause of 3 seconds was given for regeneration of HF concentration. The optical characterization of porous silicon mirrors was carried out by a Perkin Elmer UV-Vis-NIR spectrophotometer (UV-3101) at 8° incidence. Scanning Electron Microscopy (SEM) was used to examine the structural features of the film.
Results and Discussion
The optical dielectric Bragg mirrors were obtained by using nd = λ/4, for 650 nm of wavelength. Figure 1 shows the measured reflectivity spectrum of a dielectric Bragg mirror (babababa….10/20 times) centered at 650 nm of wavelength for 20 and 40 layered system. Here “b” and “a” represent the low porosity and high porosity layer respectively. The dielectric structure has been made by using 5 and 80 mA/cm2 of current density which corresponds to the refractive index contrast of nb/na = 2.0/1.2. Here nb and na correspond to low porosity and high porosity layer. The photonic bandgap of around 180 nm wide is observed. Figure 2 shows the reflectivity spectrum of one of the randomized sequence having 20 layers with the same thicknesses and refractive indices as periodic structure. The sequence of the random structure shown in Figure 2 is as follows “bbabbbabaaabaabababa”. The reflectance spectrum shows a division of the photonic bandgap into two parts like in any Fabry-Perot interference filter. The minimum is at 720 nm and the apparent Bragg mirrors appearing on both the sides show very high reflectivity, which makes the structure a very good filter for 720 nm, without increasing the overall thickness of the structure. Such structures can be very useful for chemical and biosensors, where the thickness of the structure is an important factor to ensure the infiltration of the bio/chemical species.
Figure 1. Reflectance spectrum of the dielectric Bragg mirror fabricated for 650nm wavelength with a refractive index contrast of 2.0/1.2 (a) 20 layer system (b) 40 layer system.
Figure 2. Reflectance spectrum of the partially random structure bbabbbabaaabaabababa fabricated with the same layers a and b having the refractive index and thicknesses same as that for periodic structure of 20 layers i.e. refractive index contrast of 2.0/1.2
Similarly some other random structures were also found to demonstrate interesting optical properties. Figure 3 shows another possible sequence from 20 layered “ab” system. The sequence is as follows:
The structure shows two microcavities at 708 and 790 nm respectively. The line width of each reflectivity dip is found to be less than 20 nm. In other words, many narrow resonances are found localized inside the sample. The reflectance of the mirrors is found to be almost 100%.
Figure 4 shows the reflectance spectrum of a randomized structures formed with the 40 layers equivalent to Figure 1(b). The figure demonstrates the formation of seven microcavities at 520,588, 640, 724, 814,898 and 1110 nm respectively. The minimum reflectance is observed at 814 nm where the reflectance drops down to 25%. The observed spectrum can be used as a sensor in the visible and near infrared region as well.
Figure 4. Reflectance spectrum of a sequence bbabbbabaaabaababababbabbbabaaabaabababa fabricated with the same layers a and b having the refractive index and thicknesses same as that for periodic structure of 40 layers i.e. refractive index contrast of 2.0/1.2.
The optical properties of heterostructures obtained from the multilayers of porous silicon were studied. The multilayered systems (20 and 40 layers) are fabricated by the porous silicon layers having refractive indices contrast of 2/1.2 in such a way that each layer has the optical length of 162 nm. The reflectance from the structures having partially (only 10 layers) and totally randomized sequence are analyzed and compared with the classical periodic structure. The partially randomized structures were found to have many features (many narrow resonances localized inside the sample) relevant for its application in optical interferometric biosensors and can be concluded as promising photonic structures.
This work has been financially supported by PROMEP (NPTC-52) and CONACyT under project 42939. We acknowledge the discussions made with Dr. M. Eduardo Mora and J. Escorcia-Garcia.
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Bonifacio Alvarado T. and V. Agarwal*
CIICAP- Universidad Autónoma del Estado de México (UAEM)
Av. Universidad 1001, Col. Chamilpa, CP 62209, Cuernavaca, Morelos
E-mail: [email protected]
This paper was also published in “Advances in Technology of Materials and Materials Processing Journal, 9 (2007) 131-134”.