M Kamenova

Preparation of thin porous silicon layers by stain etching

Abstract A method of preparation of thin (1000 A) porous silicon layers by chemical etching of c-Si in HF:HNO 3 :H 2 O=1:3:5 solution is reported wherein a thin Al film is deposited by evaporation on the silicon surface prior to etching. The method is characterized by no “incubation” period to the onset of the stain etching. The reaction between Al and HNO 3 produces the required holes for the very fast start of the chemical etching of Si. Using Al patterns deposited through a mask or prepared by photolithography it is possible to achieve selective formation of porous silicon. The porous silicon layers exhibit visible photoluminescence and when prepared on p-type c-Si and a shallow p–n c-Si homojunction the resulting heterostructures also display electroluminescence and photovoltaic properties.

High-efficiency solar cell using a thin porous silicon layer

The performance improvement of solar cells due to the formation of a porous silicon layer on deep-junction n+/p silicon structures has been investigated. The photovoltaic properties of two identical sample groups with and without porous silicon layer are compared. More than a 50% improvement in Isc and about a 5% increase in Voc was demonstrated in structures with PS layer. No change in the fill factor was observed. The performance of the porous silicon solar cell has been found to be sensitive to the PS layer thickness. Optimal thickness was determined using the electrical measurement. The main result is that the photovoltaic energy conversion efficiency of solar cells can be increased by the PS layer. This low-cost PS formation process can be applied in the photovoltaic cell technology as a standard procedure.

Correlation between the photoluminescence and chemical bonding in porous silicon

Abstract The changes in the visible photoluminescence and Raman spectra of n-type porous silicon as a result of rapid thermal annealing and chemical treatment have been studied. Modification of the chemical bonds due to these treatments has been observed by X-ray photoemission spectroscopy. Our results show that the change in the luminescence spectra of porous silicon is related to the modification of the chemical bonds and that complexes such as the terminations in siloxene as well as silicate molecules play a role in determining the optical properties of this new photonic material. The surface containing siloxene-like bonding exhibits very intensive photoluminescence. The presence of silicates leads to a decrease in luminescence intensity and to the appearance of a weak high energy band in the photoluminescence spectra.

Photoluminescence and chemical bonding in porous silicon layers — dependence on the P concentration in the Si substrate

Correlation between the photoluminescence (PL), porosity, crystallite size and chemical bonds in porous silicon (PS) prepared by electrochemical etching of Si with different levels of phosphorus doping has been studied. PL is very strongly related to the chemical bonding and depends on the Si, O and C content which is different for PS prepared on Si substrates with different resistivities. The influence of the doping level of Si on the process of electrochemical etching is discussed.

Polycrystalline silicon thin films obtained by metal-induced crystallization

The formation of high-quality polycrystalline silicon (poly-Si) on different substrates has important applications in the development of thin-film transistors, solar cells, image sensors, etc. In this study, we present the results of an investigation of poly-Si films on glass, formed by aluminum-induced crystallization. The process is based on the isothermal annealing for 3 h at 500 °C of co-sputtered Al+Si or sputtered a-Si films on glass, with and without thermally evaporated Al. The poly-Si films were investigated by Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy.

On the origin of the visible luminescence from porous silicon

A correlation between the photoluminescence, chemical bonds and structure of c-Si subjected to electrochemical and laser induced chemical etching has been obtained by X-ray photoelectron spectroscopy, IR spectroscopy and Raman scattering. Photoluminescence is very strongly related to the chemical bonding and depends on the Si, O and C content and the presence of different oxidized states of Si which are different for porous silicon prepared on Si substrates of different electrical resistivity. Porous silicon having Si oxidized states typical for Si suboxides exhibits very intensive photoluminescence. The influence of the doping level of c-Si on the process of electrochemical etching as well as the influence of the laser power on the process of stain etching are discussed.

Depth dependence of photoluminescence and chemical bonding in porous silicon

Porous silicon (PS) is studied by stepwise peeling of the surface layer to clarify the non-uniformity in the photoluminescence (PL) and correlate it with the in-depth chemical bonding and structure of the 30 μm thick layer. The PL intensity grows by an order of magnitude after the peeling off of the first 10 μm and decreases five times in the next 5 μm while the peak maximum position shifts from 730 to 800 nm. X-ray photoelectron spectroscopy (XPS) measurements show that Si–Si and Si–O bonds are present both on the surface and below, and the preferential oxidation state of silicon changes from 3+ and 4+ on the surface to 1+ and 2+ below 10 μm. Using Raman spectroscopy silicon nanocrystals are shown to exist. Their mean size can be estimated at about 3 nm. These results show that the strongest PL comes from a region in the PS layer where silicon nanocrystallites are surrounded by oxides with a low level of oxidation and not from the strongly oxidized surface layer.

Structural Properties of Poly-Si Thin Films on ZnO:Al Coated Glass Substrates Obtained by Aluminiuminduced Crystallization in Different Atmospheres

Poly-Si thin films were fabricated by Aluminium Induced Crystallization (AIC) of a-Si on ZnO:Al/Al coated glass substrates. ZnO:Al, Al and a-Si layers were deposited by rf magnetron sputtering. We have investigated the influence of different annealing atmospheres on the structural properties of the poly-Si films. The resulting polycrystalline Si films were analysed by microprobe Raman spectroscopy, optical microscopy and X-ray diffraction.