E. Foca

Pore Etching in III-V and II-VI Semiconductor Compounds in Neutral Electrolyte

We propose to use a neutral electrolyte based on an aqueous solution of NaCl instead of commonly used aggressive acids or alkaline electrolytes for the purpose of electrochemical nanostructuring of GaAs and CdSe substrates. It is shown that the process of material porosification can be controlled by the conditions of anodic etching. A photoluminescence analysis of the porous structures obtained and referenced to the as-grown substrate demonstrates that an effective passivation of the surface occurs during anodization in this electrolyte. The results obtained pave the way for the development of optoelectronic devices based on electrochemically nanostructured GaAs and CdSe compounds, particularly for high-efficiency solar cells.

Strongly frequency dependent focusing efficiency of a concave lens based on two-dimensional photonic crystals

Results of an experimental study of a concave lens based on a two-dimensional microwave photonic crystal with neff<1 are shown. We demonstrate that the lens focuses electromagnetic radiation for transverse electric (TE) and transverse magnetic (TM) polarizations. Intensity gains as high as 5.4 for TE polarization and 6.3 for TM polarization were achieved for definite frequencies lying in the explored interval from 6to15GHz, the smallest area of the focal spot being equal to 0.24λ2 and 1.02λ2 for TE and TM polarizations respectively. The proposed lens serves as a model system that can be scaled to THz and optical frequencies.

Smoothening the Pores Walls in Macroporous n-Si

Our work reports on the etching of very smooth pores in n-Si with backside illumination. From the point of view of the so- called current burst model we explain the origin of the roughness on macropore walls. By trying different types of electrolytes at very low or moderate pH, electrolytes with very small dissolution rates of SiO2, or etching at very low temperatures, we show that the roughness of the walls can be controlled. Finally we propose to use a viscous electrolyte which lead to a roughness of only 9 nm on a 800 x 800 nm2 area which is a factor 5 smaller than the roughness observed for aqueous electrolytes.

Self-induced oscillations in Si and other semiconductors

Abstract Some metals share an elusive property with silicon (and other semiconductors): they may exhibit strong self-induced current oscillations during anodic dissolution in electrochemical experiments. While this feature, as well as related features concerning self-organization at reactive solid-liquid interfaces, is still not well understood, the so-called “current-burst model” of the authors succeeded in reproducing many effects quantitatively that have been observed at the Si electrode. The current-burst model assumes that current flow through the electrode on anm scale is inhomogeneous in both time and space; a single current-burst is a stochastic event. Current oscillations in time and space result from interactions in space or time of single current-bursts. The paper outlines the basics of the model and gives results of Monte Carlo simulations concerning stable and damped oscillations for the current and, as a new feature, for the voltage. With the current-burst model a kind of “nano”-electrochemistry is introduced; its strengths, weaknesses, and possible implications for other electrochemical phenomena and for other materials are briefly discussed.

Efficient Focusing with an Ultra-Low Effective-Index Lens Based on Photonic Crystals

This work focuses on photonic crystals (PC) that can be ascribed an effective index of refraction < 1 or even < 0. We investigate the possibility to design optical elements (in this case a lens) based on this type of PC. A new approach for determining the effective refractive index of PCs with unusual index of refraction is used, which is simpler than earlier methods based on analyzing equi-frequency surfaces in k-space. An ultra-low refractive index PC is given a form approximating a concave lens and is proven theoretically and experimentally that it efficiently focuses the electromagnetic radiation in the microwave range. Strong focusing effects are found for both polarizations (TE and TM mode). Intensity gains as large as 35 for TM polarizations and 29 for TE polarizations are found. Measurements are in a good accordance with simulations.

Impedance Spectroscopy as a Powerful Tool for Better Understanding and Controlling the Pore Growth Mechanism in Semiconductors

This work shows new results toward a better understanding of macropore growth in semiconductors by using in situ FFT impedance spectroscopy. A new interpretation of the voltage impedance is proposed. In particular, the pore quality could be quantified for the first time in situ, especially by extracting the valence of the electrochemical process. The study paves the way toward an automatized etching system where the pore etching parameters are adjusted in situ during the pore etching process.

A stochastic model for current and voltage oscillations of the Si electrode

In this work we present a theoretical model that explains the current and voltage oscillations at the Si electrode in HF media. A simulation computer program based on this model is implemented and results are shown. Oscillations of the current and (for the first time) the voltage obtained numerically fit perfectly with experiments. The model allows to obtain more information about the system in the form of maps (accompanied by histograms) of e.g.: oxide thickness distribution, SiO2/HF and Si/SiO2 interfaces morphology, or local voltage losses. Other characteristics of the oxide like capacitance or roughness can also be obtained.