The reorganization through high-temperature annealing of closely-packed pore arrays can be exploited to create ultra-thin (<20 µm) monocrystalline silicon layers that can work as cheap and flexible substrates for both the electronic and the photovoltaic industries. By introducing a periodic diameter modulation along deep etched pores, many thin layers can be produced from a single substrate and in a single technological process. Besides the periodicity, the exact shape of the modulation also has a profound impact on the process and subtle profile changes can lead to important differences on the process outcome. In this paper we study both theoretically and experimentally the effect of the initial profile on the pore reorganization dynamics and the morphology of the thin layers obtained through annealing. We show that process reliability, annealing time and final layer characteristics, all can be engineered and optimized by precisely controlling the initial pore profile.
Cardador, D.; Vega, D.; Segura, D.; Trifonov, T.; Rodriguez, A. Photonics and nanostructures. Fundamentals and applications Vol. 25, p. 46-51 DOI: 10.1016/j.photonics.2017.04.005 Data de publicació: 2017-07-01 Article en revista
A macroporous silicon photonic crystal is designed and optimized theoretically for its use in gas sensing applications and IR optical filters. Light impinges perpendicularly onto the sample surface (vertical propagation) so a three-dimensional (3d) structure is used. For gas sensing, a sharp resonance is desired in order to isolate an absorption line of the gas of interest. The high Q-factors needed mandate the use of a plane defect inside the PhC to give rise to a resonant mode inside the bandgap tuned to the gas absorption line. Furthermore to allow gas passage through the device, an open membrane is required. This can affect the mechanical resilience. To improve the strength of the photonic crystal the pores are extended after the “active” 3d part. The number of modulations, and the extension length have been optimized to obtain the largest Q-factor with reasonable transmitted power. These proposed structures have been experimentally performed, probing an enhancement of almost an order of magnitude in the Q-factor in respect with the basic case. Simulations considering CO2 have been performed showing that the proposed structures are promising as precise optical gas sensors.
In this study the modes produced by a defect inserted in a macroporous silicon (MP) photonic crystal (PC) have been studied theoretical and experimentally. In particular, the transmitted and reflected spectra have been analyzed for variations in the defect’s length and width. The performed simulations show that the resonant frequency is more easily adjusted for the fabricated samples by length tuning rather than width. The optimum resonance peak results when centered in the PC bandgap. The changes in the defect geometry result in small variations of the optical response of the PC. The resonance frequency is most sensitive to length variations, while the mode linewidth shows greater change with the defect width variation. Several MPS photonic crystals were fabricated by the electrochemical etching (EE) process with optical response in the range of 5.8 µm to 6.5 µm. Results of the characterization are in good agreement with simulations. Further samples were fabricated consisting of ordered modulated pores with a pitch of 700 nm. This allowed to reduce the vertical periodicity and therefore to have the optical response in the range of 4.4 µm to 4.8 µm. To our knowledge, modes working in this range of wavelengths have not been previously reported in 3-d MPS structures. Experimental results match with simulations, showing a linear relationship between the defect’s length and working frequency inside the bandgap. We demonstrate the possibility of tailoring the resonance peak in both ranges of wavelengths, where the principal absorption lines of different gases in the mid infrared are placed. This makes these structures very promising for their application to compact gas sensors.
In this study, the resonating states produced by a defect inserted in a Photonic Crystal (PC) based in macroporoussilicon have been studied by simulation. In particular, the transmitted spectrum –where the mentioned states exhib-it a peak inside the bandgap– has been analyzed in order to study two main effects. On one hand, the effects thatthe absorption in silicon has in the quality factor –Q-factor, defined as Q=fc/FWHM– of the resonant peak havebeen simulated. On the other hand, the impact in the optical response due to morphological changes in the defecthas been also studied. The theoretical results for the response to silicon absorption have been compared to experi-mental data, concluding that for a quasi-normal incidence of the light, the absorption effects are given by the scat-tering of the pore. Thus, the recombination mechanisms in the bulk silicon can be neglected. Furthermore, thesimulations made with different lengths and widths for the pore show that the central frequency of the resonantstates may be tuned by adjusting properly both dimensions of the defect and that exists an optimal width that max-imizes the peak’s Q-factor.
Vega, D.; Cardador, D.; Trifonov, T.; Garin, M.; Rodriguez, A. Journal of lightwave technology Vol. 34, num. 4, p. 1281-1287 DOI: 10.1109/JLT.2015.2503354 Data de publicació: 2016-02-15 Article en revista
The effect of losses in photonic crystal structures on transmittance and reflectance has been studied by numerical techniques, and compared to fabricated macroporous silicon samples. Material absorption and tolerances of the pores play an essential role determining performance in optical applications. The relevance of the structure geometry has been investigated through the porosity. The considered photonic crystals in this work are 3-d macroporous silicon structures fabricated by electrochemical etching with pore profile modulation, and defect inclusion. The examined structures are intended to work with quasi-normal light incidence as selective filters. The effect of losses on the forbidden band and resonance is studied and reported for both transmittance and reflectance. The reflectance peak is less affected by losses, while transmitted light experiences a noticeable reduction even for small absorption factors. This has critical implications for the performance of these structures as transmission filters. Furthermore, it is shown that an optimum can be found for a given absorption coefficient by choosing the appropriate porosity.
En este trabajo se ha estudiado la relación existente entre la frecuencia de trabajo de un cristal fotónico con defecto y el tamaño de dicho defecto. Las simulaciones indican una dependencia lineal entre ambas, a la vez que se mantiene el factor de calidad de los picos a lo largo de la
The characteristics of reflection and transmission peaks in the spectra of photonic crystals have been studied theoretically and the results compared to measurements performed in fabricated samples. The aim of this work is to investigate the relation between material losses and the effective Q factors that can be obtained in photonic crystals made with it. Photonic crystals have been designed with defects of periodicity to introduce states in the band gap that give place to reflectance and transmittance peaks at adjustable specific wavelengths. The fabricated structures are described together with their reflection and transmission spectra. The influence of losses in the material in these spectra is evaluated.
Garin, M.; Hernandez, D.; Trifonov, T.; Cardador, D.; Alcubilla, R. European Photovoltaic Solar Energy Conference and Exhibition p. 933-936 DOI: 10.4229/28thEUPVSEC2013-2CO.2.5 Data de presentació: 2013-10-01 Presentació treball a congrés
There is a rising interest, from both photovoltaics and microelectronics industry, in wafer thickness
reduction. During the last decade, it has been steadily reduced from 350 µm to 180 µm, but benefits are foreseen
for thicknesses well below these values. The current sawing technology, however, suffers from large kerf losses
and further reductions are increasingly difficult. Several technologies have emerged aiming to produce thin Si
foils from a wafer, such as layer transfer, induced cleaving, or pore reorganization. These methods produce a
single layer by step. In this work we report on a method able to produce many crystalline layers from a single
silicon wafer and in a single fabrication step.