F. Ben-Hander

Lattice-mismatch induced-stress in porous silicon films

Abstract We have studied the stress in porous silicon films with different porosities at the interface with the substrate. Micro-Raman spectra were measured along a cleaved cross-section to sample different layer depths. Each spectrum was fit to the phonon confinement model, with the bulk phonon frequency as a free parameter to remove phonon confinement effects. At the interface this frequency increases sharply, indicating a compressive stress on the porous silicon pillars. The stress is due to the lattice mismatch, measured by X-ray diffraction, between the porous film and the bulk silicon substrate. For porosities between 50 and 85% the stress and the lattice mismatch vary, respectively, between 4 and 10 kbar, and between 2.9×10 −3 and 3.5×10 −3 . Finally, from the dependence of stress on the lattice mismatch we obtain a microscopic Youngs modulus of 156 GPa. This magnitude, mostly dependent on atomic bonding, is close to the bulk silicon value and is much larger than the macroscopic modulus, strongly dependent on the porous structure, reported in the literature.

Anisotropic textured silicon obtained by stain-etching at low etching rates

The structure, luminescence and etching kinetics for porous silicon stain-etched at different temperatures are studied. The results reveal that for temperatures below 10 °C and for short etching times, a novel anisotropic structure based on surface roughness preferentially oriented in the 100 direction is observed. At temperatures higher than 10 °C or large etching times, typical macropores and mesopores with non-preferential pore wall orientation are detected. The luminescence spectra of the samples with preferential surface roughness orientation are red-shifted with respect to the samples with typical isotropic orientation. The results are interpreted in terms of average etching rates and pore growth.

Optical and compositional characterisation of stain-etched porous silicon subjected to anodic oxidation and thermal treatments

Abstract In this work, we study optical and compositional properties of stain-etched porous silicon (PS) passivated by means of anodic oxidation or thermal treatments in a N2 atmosphere. We search a passivation method that allows us to stabilise the surface properties of this material and to make it attractive to its implementation in a low-cost silicon-based solar cell fabrication process. We observe that the thermal treatment maintains the reflectance coefficient at very low values, and the anodic oxidation generates surfaces with a more homogeneous oxide composition than the thermal treatment. These results are appropriate with the requirements for future solar cells that can use these passivated porous layers as antireflection coatings.

Determination of Stress in Porous Silicon by Micro‐Raman Spectroscopy

We have studied the stress in porous silicon films as a function of depth and porosity using micro-Raman spectroscopy. Raman spectra were measured at different points along a cross section cleaved normal to the layer planes. Each spectrum was fitted using the phonon confinement model with the bulk phonon wavenumber as a free parameter. From the variation of this parameter we get the stress using the known dependence of phonon frequency on stress for bulk silicon. We observe a compressive stress at the interface with the substrate due to the lattice mismatch between porous and bulk silicon. The maximum value of the stress increases with porosity. The results obtained by Raman micro-spectroscopy agree well with the lattice mismatch measured by X-ray diffraction reported in the literature.

Lifetime Measurements of Stain Etched and Passivated Porous Silicon

In this work we present the first experimental study of photocarrier lifetimes in p-type and n-type Si substrates in which stain etched porous silicon (PS) has been formed on the surface. The lifetime values have been obtained before and after the surface passivation of the samples. The surface pasivation has been produced by two different techniques: (i) hydrogen passivation by immersion of the samples in a HF solution; and (ii) deposition of SiN x in a plasma enhanced chemical vapour deposition system. The results show a degradation of the photocarrier lifetime when the porous layers are not adequately passivated. This lifetime degradation is mainly associated to a large concentration of rapid recombination centres located at the Si/PS interface. We have also detected a weak influence of the PS outermost dangling bonds to the photocarrier lifetimes.