Study of the optical response of oxidized porous silicon structures by thermal oxidation in air

Abstract

This work proposes a methodology based on porous silicon (PSi) thermal oxidation in an air atmosphere to reduce its optical losses and change the optical response of one-dimensional photonic structures through the porosity variations, pore filling, and refractive index tuning. First, electrochemical etching was used to fabricate PSi samples at two different anodizing currents and in-situ photoacoustic monitoring was used to guarantee the porous film’s reproducibility. Then, the PSi samples were oxidized in an air atmosphere at temperatures of 600, 800, and 1000 $^{\circ }$C and different sintering times (0 h, 5 h, 10 h, and 20 h). All the samples were characterized by Fourier-transform infrared spectroscopy (FTIR) and scanning electronic microscopy (SEM) to determine the chemical and morphological evolution produced for thermal treatment. In addition, the optical properties were analyzed by UV-Vis spectroscopy before and after the thermal treatment to relate the obtained spectra with the characteristics of the monolayers using the transfer matrix method (TMM), effective medium theory, and genetic algorithms (GA). Finally, we predicted the optical response of oxidized porous silicon one-dimensional photonic crystal for UV-Vis range applications.

Appendices

Appendix

In-situ x-ray diffraction

Figure 10

In-situ XRD patterns for PSi at room temperature (22 $^{\circ }$C) and from 200 to 1000 $^{\circ }$C taken each 50 $^{\circ }$C at 2 $^{\circ }$/min heating rate. Finally, it is presented a pattern taken at 25 $^{\circ }$C after the thermal treatment

The sample was packed in a platinum holder located in a controlled high-temperature device, Ultima IV-Rigaku diffractometer. The sample was analyzed through in-situ HT-XRD measurement at 2 $^{\circ }$C/min with $\hbox {CuK}_{\alpha }$ radiation $\lambda =1.5406$ Å. XRD patterns were obtained every 50 $^{\circ }$C from room temperature (22 $^{\circ }$C) to 1000 $^{\circ }$C during heating. Diffractograms were obtained from 10$^{\circ }$ to 80$^{\circ }$ on a 2$\theta$ scale with a step size of 0.02$^{\circ }$. Finally, the sample was inertially cooled to room temperature, and a diffraction pattern was taken (25$^{\circ }$).

A peak of about 69.17$^{\circ }$ (in 2$\theta$) corresponds to the (400) plane of the silicon substrate, which shifts at smaller angles as temperature increases due to thermal expansion. A peak of about 20.85$^{\circ }$ was observed that corresponds to the (100) plane of quartz. The peak increases in intensity as the temperature increases, which is indicative of an oxide layer growing. Finally, for the sample measured at room temperature once the thermal treatment finishes, the silicon peak reduces in intensity, and the quartz peak looks more intense, which is an indicator of silicon consumption to create silicon oxide.

Raw films Uv-Vis

Figures 11, 12

Figure 11

Reflectance spectra of samples fabricated at 5 mA/$\hbox {cm}^2$ for different temperatures and sintering times

Figure 12

Reflectance spectra of samples fabricated at 10 mA/$\hbox {cm}^2$ for different temperatures and sintering times

Oxidized films UV-Vis

Figure 13

Figure 13

Reflectance spectra of the samples fabricated at 10 mA/$\hbox {cm}^2$ treated at 600 h for different sintering times