Mechanism of silicon epitaxy on porous silicon layers

The mechanism of silicon epitaxy on porous Si(111) layers is investigated by the Monte Carlo method. The Gilmer model of adatom diffusion extended to the case of arbitrary surface morphology is used. Vacancies and pendants of atoms are allowed in the generalized model, the activation energy of a diffusion hop depends on the state of the neighboring positions in the first and second coordination spheres, and neighbors located outside the growing elementary layer are also taken into account. It is shown that in this model epitaxy occurs by the formation of metastable nucleation centers at the edges of pores, followed by growth of the nucleation centers along the perimeter and the formation of a thin, continuous pendant layer. Three-dimensional images of surface layers at different stages of epitaxy were obtained. The dependence of the kinetics of the epitaxy process on the amount of deposited silicon is determined for different substrate porosities. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 512–517 (10 April 1998)     

Porous silicon layers prepared by electrochemical etching for application in silicon thin film solar cells

In this paper, multilayer structures of porous silicon were fabricated by using electrochemical etching and characterized for its optical properties and surface morphology. Samples of monolayer of porous silicon were grown to study the characteristics of porous layer formation with respect to applied current density, etching time and hydrofluoric acid concentrations. Photoluminescence peaks of red emission at wavelength 695 and 650 nm were observed from multilayer porous silicon structures. By atomic force microscopy measurement, hillocks like surface were clearly observed within the host material, which confirmed the formation of pores.     

Comparative investigation of photoluminescence of silicon wire structures and silicon oxide films

Photoluminescence spectra and their dependence on temperature as well as Raman scattering spectra and Atomic Force Microscopy investigations have been used to study the peculiarities of the red photoluminescence band in low-dimensional Si structures, such as porous silicon and silicon oxide films. It has been shown that the red photoluminescence band of porous silicon is complex and can be decomposed into two elementary bands. It was discovered that elementary band intensities depend very much on surface morphology of porous silicon. The same positions of the photoluminescence bands are also observed in silicon oxide films for different oxide composition. Comparative investigation of the PL temperature dependences in porous silicon and silicon oxide films indicates that silicon-oxide defect related mechanisms of some elementary photoluminescence bands are involved.     

Optical waveguides in porous silicon pre-patterned by localised nitrogen implantation.

FIPOS technology forms islands of silicon isolated from a silicon substrate by (oxidised) porous silicon. The larger refractive index of the silicon islands suggests their use as optical waveguides. Sets of these silicon islands have been fabricated and the anticipated waveguiding has been observed at wavelengths of 1.15 and 1.3 μm in the silicon islands. However, the dominant waveguiding in these FIPOS structures is observed in the porous silicon between the silicon islands, close to the sample surface. A simple dynamic model of the anodisation process has been developed to explain the origin of this unexpected waveguiding.     

Thermodynamics of a phase transition of silicon nanoparticles at the annealing and carbonization of porous silicon

The formation of SiC nanocrystals of the cubic modification in the process of high-temperature carbonization of porous silicon has been analyzed. A thermodynamic model has been proposed to describe the experimental data obtained by atomic-force microscopy, Raman scattering, spectral analysis, Auger spectroscopy, and X-ray diffraction spectroscopy. It has been shown that the surface energy of silicon nanoparticles and quantum filaments is released in the process of annealing and carbonization. The Monte Carlo simulation has shown that the released energy makes it possible to overcome the nucleation barrier and to form SiC nanocrystals. The processes of laser annealing and electron irradiation of carbonized porous silicon have been analyzed.     

Photoluminescence of porous silicon coated by SILD method with LaF3 nanolayers

The method of lanthanum fluoride passivating layer synthesis in the matrix of porous silicon by successive ionic layer deposition was elaborated and optimized. Luminescence and FTIR of obtained structures demonstrate the crucial role of the chemical composition of silicon nanocrystallite surface in the formation of radiative recombination channels and in the stability of porous silicon photoluminescence. The combination of high optical transparency of LaF₃ layers and low recombination losses in silicon covered with such layers allows to recommend the lanthanum fluoride film as an effective passivating coating for silicon optoelectronics devices.     

Photoluminescence studies of porous silicon microcavities

Narrow photoluminescence peaks with a full-width at half-maximum of 14–20 nm are obtained from porous silicon microcavities (PSM) fabricated by the electrochemical etching of a Si multilayer grown by molecular beam epitaxy. The microcavity structure contains an active porous silicon layer sandwiched between two distributed porous silicon Bragg reflectors; the latter were fabricated by etching a Si multilayer doped alternatively with high and low boron concentrations. The structural and optical properties of the PSMs are characterised by scanning electron microscopy and photoluminescence (PL). The wavelength of the narrow PL peaks could be tuned in the range of 700–810 nm by altering the optical constants.     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.     

Investigation of silicon structures with periodically varying porosity by x-ray diffractometry methods

The features of formation of silicon structures with a periodically varying porosity are investigated by X-ray diffractometry. It is revealed that the magnitude of anode current density corresponding to the formation of a layer with higher porosity that is part of a multilayer porous structure also affects the porosity of a second, less porous layer. As time elapses, the single-crystalline matrix of which porous layers consist is amorphized and a structure consisting of amorphous layers with a periodically varying density arises.     

Nanosecond pulsed laser ablation of silicon in liquids

Laser fluence and laser shot number are important parameters for pulse laser based micromachining of silicon in liquids. This paper presents laser-induced ablation of silicon in liquids of the dimethyl sulfoxide (DMSO) and the water at different applied laser fluence levels and laser shot numbers. The experimental results are conducted using 15 ns pulsed laser irradiation at 532 nm. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology shows that the explosive melt expulsion could be a dominant process for the laser ablation of silicon in liquids using nanosecond pulsed laser irradiation at 532 nm. Silicon surface’s ablated diameter growth was measured at different applied laser fluences and shot numbers in both liquid interfaces. A theoretical analysis suggested investigating silicon surface etching in liquid by intense multiple nanosecond laser pulses. It has been assumed that the nanosecond pulsed laser-induced silicon surface modification is due to the process of explosive melt expulsion under the action of the confined plasma-induced pressure or shock wave trapped between the silicon target and the overlying liquid. This analysis allows us to determine the effective lateral interaction zone of ablated solid target related to nanosecond pulsed laser illumination. The theoretical analysis is found in excellent agreement with the experimental measurements of silicon ablated diameter growth in the DMSO and the water interfaces. Multiple-shot laser ablation threshold of silicon is determined. Pulsed energy accumulation model is used to obtain the single-shot ablation threshold of silicon. The smaller ablation threshold value is found in the DMSO, and the incubation effect is also found to be absent.     

Photoluminescence studies on porous silicon/polymer heterostructure

Hybrid devices formed by filling porous silicon with MEH-PPV or poly [2-methoxy-5(2-ethylhexyloxy-p-phenylenevinylene)] have been investigated in this work. Analyses of the structures by scanning electron microscopy (SEM) demonstrated that the porous silicon layer was filled by the polymer with no significant change of the structures except that the polymer was infiltrated in the pores. The photoluminescence (PL) of the structures at 300 K showed that the emission intensity was very high as compared with that of the MEH-PPV films on different substrates such as crystalline silicon (c-Si) and indium tin oxide (ITO). The PL peak in the MEH-PPV/porous silicon composite structure is found to be shifted towards higher energy in comparison with porous silicon PL. A number of possibilities are discussed to explain the observations.     

Features and mechanisms of growth of cubic silicon carbide films on silicon

The mechanisms and specific features of the growth of silicon carbide layers through vacuum chemical epitaxy in the range of growth temperatures from 1000 to 700°C have been considered. The structure of the heterojunction formed has been studied using the results of the performed investigations of photoluminescence spectra in the near-infrared wavelength range and the data obtained from the mass spectrometric analysis. It has been found that, in the silicon layer adjacent to the 3C-SiC/Si heterojunction, the concentration of point defects significantly increases and the dislocation structure is not pronounced. According to the morphological examinations of the surface of the growing film, by analogy with the theory of thermal oxidation of silicon, the theory of carbidization of surface silicon layers has been constructed. A distinctive feature of the model under consideration is the inclusion of the counter diffusion fluxes of silicon atoms from the substrate to the surface of the structure. The growth rate of films and the activation energy of diffusion processes have been estimated. The performed experiments in combination with the developed growth model have explained the aggregates of voids observed in practice under the silicon carbide layer formed in the silicon matrix and the possibility of forming a developed surface morphology (the island growth of films) even under conditions using only one flow of hydrocarbons in the reactor.     

Peculiarities of the capacitance-voltage characteristic of a photoelectric solar energy convertor based on a silicon p-n junction with a porous silicon antireflection coating

Experimental results on the high-frequency capacitance-voltage characteristic of a photoelectric solar energy converter based on the n ⁺-p junction with a thin porous silicon film on the frontal surface are considered. It is shown that the capacitance-voltage characteristic is determined by the surface metal-insulator-semiconductor (MIS) structure formed as a result of growing of a porous silicon layer by electrochemical anode etching. The effective thickness of the insulator layer of the MIS structure, the impurity concentration in its semiconductor region, and the density of surface states are determined.     

Optimization of porous silicon preparation technology for SERS applications

A series of porous silicon samples prepared at different etching parameters, namely etchant composition, etching time and current density, was investigated as substrates for surface-enhanced Raman scattering (SERS). Silver nanostructures were deposited on porous silicon by immersion plating method and Rhodamine 6G was used as analyte. The relation between the etching parameters, morphology of porous silicon surface and its SERS efficiency after silver deposition is examined. We show that a high HF content in the etchant allows the formation of a film with close-packed silver nanocrystals, which possess strong surface enhancement properties.     

Fabrication and characterization of porous silicon layers for applications in optoelectronics

In the present paper, several samples of porous silicon monolayers and multilayers were prepared at different anodization conditions with fixed HF concentration. The room temperature photoluminescence wavelength observed to be increased with increased etching time and current density respectively. By Raman measurement it has been observed that as the size of silicon crystallites decreased with increased etching time, the silicon optical phonon line shifted somewhat to lower frequency from 520.5 cm⁻¹ and became broader asymmetrically. The surface roughness and pyramid like hillocks surface was confirmed by AFM measurement. In SEM images, the porous silicon layers were clearly observed by white and black strips. It was also observed that the reflectivity increased as the number of porous silicon layers was increased.     

银纳米颗粒/多孔硅复合材料的制备与气敏性能研究

采用双槽电化学腐蚀法以电阻率为10-15 Ω·cm的p型<100>晶向的单晶硅片制备了孔径约为1.5 μm, 孔深约为15-20 μm的p型多孔硅, 并以此多孔硅作为基底采用无电沉积法通过调控沉积时间在其表面沉积了不同厚度的银纳米颗粒薄膜. 采用扫描电子显微镜和X 射线衍射仪表征了银纳米颗粒/多孔硅复合材料的形貌和微观结构, 结果表明银纳米颗粒较均匀的分布于多孔硅的表面上且沉积时间对产物的形貌有重要影响. 采用静态配气法在室温下研究了银纳米颗粒/多孔硅复合材料对NH₃的气敏性能. 气敏测试结果表明沉积时间对产物的气敏性能影响较大. 当沉积时间较短时, 适量银纳米颗粒掺杂的多孔硅复合材料由于其较高的比表面积以及特殊的形貌和结构, 对NH₃气体表现出较高的灵敏度、优良的响应/恢复性能. 室温下, 其对50 ppm 的NH₃气体的气敏灵敏度可以达到5.8左右.     

Photovoltage and photo-induced charge trapping in porous silicon

Spectral and kinetic dependencies of photovoltaic effects in porous silicon-crystal substrate structures have been studied. Both as-prepared and aged in air samples were used. It is shown for the first time that besides the photovoltage component connected with a depleted region in the silicon substrate at the interface with the porous layer there is a photovoltage due to porous silicon itself. It is established that the electron states with the relaxation time about several minutes are located on the pores surface. The properties of these states changed with the thermal annealing of structures. The superslow hole traps with a relaxation time of about several hours were registered in the aged samples. These traps are located in the oxide on the silicon skeleton surface of the porous silicon. An energy diagram based on the data is suggested that explains the appearance of photovoltaic effects in the investigated structures.     

Electrochemical and hydrothermal deposition of ZnO on silicon: from continuous films to nanocrystals

This article presents the study of the electrochemical deposition of zinc oxide from the non-aqueous solution based on dimethyl sulfoxide and zinc chloride into the porous silicon matrix. The features of the deposition process depending on the thickness of the porous silicon layer are presented. It is shown that after deposition process the porous silicon matrix is filled with zinc oxide nanocrystals with a diameter of 10–50 nm. The electrochemically deposited zinc oxide layers on top of porous silicon are shown to have a crystalline structure. It is also shown that zinc oxide crystals formed by hydrothermal method on the surface of electrochemically deposited zinc oxide film demonstrate ultra-violet luminescence. The effect of the porous silicon layer thickness on the morphology of the zinc oxide is shown. The structures obtained demonstrated two luminescence bands peaking at the 375 and 600 nm wavelengths. Possible applications of ZnO nanostructures, porous and continuous polycrystalline ZnO films such as gas sensors, light-emitting diodes, photovoltaic devices, and nanopiezo energy generators are considered. Aspects of integration with conventional silicon technology are also discussed.     

Instability of the photoluminescence of porous silicon

Results of studies of the photoluminescence of porous silicon with different prehistories have revealed the mechanism and nature of the instability of the luminescence properties of freshly prepared samples. It was established that the initial quenching and subsequent rise of the photoluminescence is attributable to the intermediate formation of silicon monoxide (photoluminescence degradation) and subsequent additional oxidation to form SiO₂ (photoluminescence rise). Ultraviolet laser irradiation accelerates this process by a factor of 200–250 compared with passive storage of the samples in air. Plasma-chemical treatment in an oxygen environment merely results in a subsequent rise in the photoluminescence as a result of the formation of monoxide on the porous silicon surface. A kinetic model is proposed for this process. Zh. Tekh. Fiz. 69, 135–137 (June 1999)     

White light luminescence from annealed thin ZnO deposited porous silicon

In this study, photoluminescence (PL) properties of annealed ZnO thin films deposited onto a porous silicon (PS) surface by rf-sputtering were investigated. A huge blue shift of luminescence from the ZnO deposited onto the PS surface and a broadband luminescence (white luminescence) across most of the visible spectrum were obtained after the heat treatment at 950 °C in air. The results of Fourier Transform Infrared Spectroscopy (FTIR) analysis suggested that the porous silicon surface was oxidized after ZnO deposition and the broadband luminescence was due to the conversion of Si-H bonds to Si-O-Si bonds on the PS surface. The underlying mechanisms of the broadband PL were discussed by using oxygen-bonding model for the PS and native defects model for ZnO. The experimental results suggested that the heat treatment provides a relatively easy way to achieve white luminescence from thin ZnO deposited porous silicon.