UV Distributed Bragg reflectors were fabricated by a two-step thermal oxidation process over porous silicon multilayers (PS-ML), which were prepared by room-temperature electrochemical anodization of silicon wafers. The optical behavior of the PS-ML before and after oxidation was studied by reflectance measurements. It was observed an UV shift from 430 to 300 nm in the peak of the reflectance spectrum after oxidation of the PS-ML. This was attributed to the presence of silicon oxide over the surface of the silicon filaments. Such oxide also reduced the refractive index of each porous silicon monolayer. The bandgap of the PS-ML was calculated by the Kubelka-Munk approximation, which showed an increase in the bandgap from 3.11 to 4.36 eV after the thermal oxidation process. It was suggested that the observed optical response could opens the possibility of fabrication of UV optoelectronic devices based entirely in the silicon technology.
Entangled Zn-ZnO nanorods and urchin-like microstructures were synthesized by the hot filament chemical vapor deposition technique at 825 and 1015°C, respectively. X-ray diffraction results showed a mixture of ZnO and Zn phases in both nanorods and urchin-like structures. The presence of Zn confirms the chemical dissociation of the ZnO solid source. The Z-ZnO nanorods with diameter of about 100 nm showed dispersed-like morphology. The urchin-like structures with micrometer diameters exhibited porous and rough morphology with epitaxial formation of nanorods.
This work presents a study of the evolution of the reflectance spectra of porous silicon as-growth and with thermal oxidation under controlled conditions. Porous silicon layers were prepared by electrochemical anodization of a silicon substrate p+ in an aqueous solution of hydrofluoric acid and ethanol. Multilayer structures were formed on the basis of porous silicon, two different layers were realized by controlling the refraction index of each one of the layers using the Bruggemann model, in this case for multilayer as-growth, and subsequently a dry oxidation was performed to observe the change in refractive index and reflectance. A model that contains the refractive index of silicon, air and silicon oxide is used for predicting the behavior of the reflectance spectra. With this model is possible to control the width of the reflectance spectrum of the band pass or also called Distributed Bragg Reflector (DBR), DBR were characterized and measured by SEM and UV-VIS spectroscopy. Reflectance spectra were obtained and we can see that the annealing provoked a shifted towards the ultra violet (UV). These changes on reflectance and refractive indexes indicate that oxidation processes can modify the multilayer porous silicon. The optical band gap energy (Eg) was obtained from 4.36 to 3.98 eV to the DBR. These low cost devices open the way to the development of optical sensing in UV entirely based on silicon.
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