Porous Silicon Formation Process Research Articles

Optical properties of porous Si/PECVD SiNX:H reflector on single crystalline Si for solar cells

Abstract The improvement of optical confinement on the back crystalline silicon solar cell is one of the factors leading to its better performance. Porous silicon (PS) layer can be used as a back reflector (BR) in solar cells. In this work, single layers of porous silicon were grown by electrodeposition on a single crystalline silicon substrate. The measurement of the total reflectivity (RT) on Si/PS surface showed a significant improvement in optical confinement compared to that measured on Si/standard Al back surface field (BSF). The internal reflectivity (RB) extracted from total reflectivity measurements achieved 86 % for the optimized single PS layer (92 nm thick layer with 60 % porosity) in the wavelength range between 950 and 1200 nm. This improvement was estimated as more than 17 % compared to that measured on the surface of Si/BSF Al contact. To improve the stability and passivation properties of PS layer BR, silicon nitride layer (SiNx) was deposited by PECVD on a PS layer. The maximum measured total reflectivity for PS/SiNx achieved approximately 56 % corresponding to an improved RB of up to 83 %. The PS formation process in combination with the PECVD SiNx, can be applied in the photovoltaic cell technology and offer a promising technique to produce high-efficiency and low-cost c-Si solar cells.

Improvement of multicrystalline silicon solar cell performance via chemical vapor etching method-based porous silicon nanostructures

In this paper, we report on the effect of chemical vapor etching-based porous silicon (PS) on the performance of multicrystalline silicon solar cells performed via deep n+/p junction-type structures. Chemical vapor etching of silicon leads to the formation of porous silicon (PS) nanostructures that dramatically decrease the surface reflectivity from 30% to about 8%, and increase the minority carrier diffusion lengths from 90μm to 170μm. As a result, the short-circuit current density was improved by more than 20% and the fill factor (FF) by about a 10%. An enhancement of the photovoltaic conversion energy efficiency of the solar cells from 7% to 10% was observed. This low-cost PS formation process can be applied in the photovoltaic cell technology as a standard procedure.

Investigation of MeV-Cu implantation and channeling effects into porous silicon formation

P-type (1 1 1) silicon wafers were implanted by copper ions (2.5 MeV) in channeling and random directions using ion beam accelerator of the Atomic Energy Commission of Syria (AECS). The effect of implantation direction on formation process of porous silicon (PS) using electrochemical etching method has been investigated using scanning electron microscope (SEM) and photoluminescence (PL) techniques. SEM observations revealed that the size, shape and density of the formed pores are highly affected by the direction of beam implantation. This in turn is seen to influence the PL behavior of the PS.

Connections between morphological and mechanical evolution during galvanic corrosion of micromachined polycrystalline and monocrystalline silicon

Many microsystems fabrication technologies currently employ a metallic overlayer, such as gold, in electrical contact with silicon structural layers. During postprocessing in hydrofluoric-based acid solutions, a galvanic cell is created between the silicon and the metallic layer. Micromachined tensile specimens reveal that such etching in the presence of a galvanic cell can cause a catastrophic reduction in the tensile strength and apparent modulus of silicon. Detailed failure analysis was also used to compare fractured corroded Si to otherwise identical reference specimens via surface based (electron and scanning probe) microscopy as well as cross-section based structural- and composition-characterization techniques. For both polycrystalline and single-crystal silicon, galvanic corrosion can result in a thick corroded surface layer created via porous silicon formation, and/or generalized material removal depending on the etch chemistry and conditions. Under certain etching conditions, the porous silicon formation process results in cavity formation as well as preferential grain-boundary attack leading to intergranular fracture. The nature and severity of corrosion damage are shown to be influenced by the surface wetting characteristics of the etch chemistry, with poor wetting resulting in localized attack facilitated by the microstructure and good wetting resulting in generalized attack. The measured stiffness of the tensile specimens can be used to determine the effective modulus and porosity of the corroded surface layer. Extending beyond previous investigations, the present work examines the quantitative connection between the choice of chemical etchant, the corresponding damage morphology, and the resulting degradation in strength and apparent modulus. The present work also uniquely identifies important differences in polycrystalline and single-crystal Si based on their disparate damage evolution and related mechanical performance.

Step voltage with periodic hold-up etching: A novel porous silicon formation

A novel etching method for preparing light-emitting porous silicon (PS) is developed. A gradient steps (staircase) voltage is applied and hold-up for different periods of time between p-type silicon wafers and a graphite electrode in HF based solutions periodically. The single applied staircase voltage (0–30 V) is ramped in equal steps of 0.5 V for 6 s, and hold at 30 V for 30 s at a current of 6 mA. The current during hold-up time (0 V) was less than 10 μA. The room temperature photoluminescence (PL) behavior of the PS samples as a function of etching parameters has been investigated. The intensity of PL peak is initially increased and blue shifted on increasing etching time, but decreased after prolonged time. These are correlated with the study of changes in surface morphology using atomic force microscope (AFM), porosity and electrical conductance measurements. The time of holding-up the applied voltage during the formation process is found to highly affect the PS properties. On increasing the holding-up time, the intensity of PL peak is increased and blue shifted. The contribution of holding-up the applied steps during the formation process of PS is seen to be more or less similar to the post chemical etching process. It is demonstrated that this method can yield a porous silicon layer with stronger photoluminescence intensity and blue shifted than the porous silicon layer prepared by DC etching.

Formation of porous silicon at elevated temperatures

The features of electrochemical formation process of porous silicon (PS) at the temperatures above the room temperature have been studied. It was found that besides electrochemical dissolution, chemical etching takes part in the formation process of PS even for concentrated HF electrolyte. The role of chemical etching increases with temperature causing an increase of the porosity and the crater depth. The temperature dependence of chemical etching rate has been established. Obtained results enable to conclude that OH − ions play a major role in the chemical etching. Electrochemical etching allows to fabricate PS with good surface quality at the temperatures at least below 65 °C provided that HF electrolyte is concentrated.

Structural modification of silicon during the formation process of porous silicon

Direct examination of porous silicon (PS) by the use of high resolution transmission electron microscopy (HRTEM) allowed us to perform a deep insight into the formation mechanisms of this material. In particular, the structure of the PS/Si interface and that of the silicon nanocrystals that compose porous silicon were analyzed in detail. Furthermore, image processing was used to study in detail the structure of PS. The mechanism of PS formation and lattice matching between the PS layer and the Si substrate is analyzed and discussed. Finally, a formation mechanism for PS based on the experimental observations is proposed.

Modelling of porous silicon formation process

There are many applications where porous silicon (PS) has been used with profit. Advances in PS-based technologies depend strongly on possibilities to provide PS layers with specified reproducible parameters. This paper discusses modelling PS layer formation kinetics and porosity distribution through the PS thickness if a local silicon anodisation within mask openings of different geometry is performed. Calculation of the porosity distribution through the PS thickness depending on chemical erosion of PS matrix with the anodising process is shown. Modelling error is shown to be 5–10%. The developed models can be useful in manufacturing PS-based optoelectronic devices such as integrated optical waveguides (WGs).

Fabrication of laterally displaced porous silicon filters

Porous silicon superlattices have been used to manufacture laterally displaced dielectric filters with different optical properties on one substrate. Two different fabrication processes for two-colour microfilter arrays are presented. Both methods overcome the problem of non-uniform optical properties of the well-known procedure where two filter stacks are grown one upon another, with subsequent partial removal of the upper filter by reactive ion etching. The novel methods give uniform optical properties of the two filter areas, profiting from the main property of the formation process of porous silicon: the formation of porous silicon occurs only at the porous silicon–substrate interface.

Deep electrochemical trench etching with organic hydrofluoric electrolytes

Porous silicon layers etched through openings in an inert masking layer have potential applications for producing microstructures in silicon. In order to overcome the isotropic nature of the porous silicon formation process around the mask edges, optimized organic electrolytes for etching macropores on p-type silicon were employed. New kind of pores (dubbed “trenches”) were found, which are sensitive to mechanical stress and may prove useful for microstructuring silicon. Systematic experiments are presented which investigate the stability, the crystal orientation dependence, and limitations of trench formation.

A New Way to Silicon Microstructuring with Electrochemical Etching

Porous silicon layers etched through openings in an inert masking layer have been employed to produce microstructures in silicon. In order to overcome the isotropic nature of the porous silicon formation process around the mask edges, optimized organic electrolytes for etching macropores on p-type silicon were employed. New kinds of pores (dubbed “trenches”) were found, which are sensitive to mechanical stress and may prove useful for microstructuring silicon. Systematic experiments are presented which investigate the stability, the crystal orientation dependence, and limitations of trench formation. A possible mechanism for the trench formation is discussed in terms of the “current burst” model.

Kinetics of porous silicon growth studied using flicker-noise spectroscopy

The mechanism of the formation of porous silicon (PS) is studied using flicker noise spectroscopy (FNS), a new phenomenological method that allows us to analyze the evolution of nonlinear dissipative systems in time, space and energy. FNS is based on the ideas of deterministic chaos in complex macro- and microsystems. It allows us to obtain a set of empiric parameters (“passport data”) which characterize the state of the system and change of its properties due to the evolution in time, energy, and space. The FNS method permits us to get new information about the kinetics of growth of PS and its properties. Thus, the PS formation mechanisms at n-Si and p-Si, as revealed using the FNS, seem to be essentially different. p-Si shows larger “memory” in the sequence of individual events involved in PS growth than n-Si (if anodized without light illumination). The influence of the anodization variables (such as current density, HF concentration, duration of the process, light illumination) onto the “passport data” of PS is envisaged. The increase of the current density increases memory of the PS formation process, when each forthcoming individual event is more correlated with the preceding one. Increasing current density triggers electrochemical reactions that are negligible at lower currents. Light illumination also produces a positive effect onto the “memory” of the system. The FNS makes it possible to distinguish different stages of the continuous anodization process which are apparently associated with increasing pore length. Thus, FNS is a very sensitive tool in analysis of the PS formation and other complex electrochemical systems as well.

Breaking the isotropy of porous silicon formation by means of current focusing

Porous silicon sacrificial layer technologies have been employed to produce microstructures at the surface of silicon wafers consisting entirely of stress-free bulk crystalline silicon. In order to overcome geometrical limitations inherent in the isotropic nature of the porous silicon formation process, electrical current focusing techniques have been employed. These cause etch fronts to move laterally underneath implanted surface layers of n-type silicon. As a demonstration, we present micromachined silicon hotplates for low-power-consumption gas sensors which exhibit lateral dimensions larger than the wafer thickness.

An Extended Quantum Model for Porous Silicon Formation

Porous silicon formed by anodization of a p‐type silicon substrate is characterized by a distribution of crystallites with diameters smaller than about 100 Å. The corresponding size distributions obtained from Raman measurements show distinct peaks which are explained for the first time by the tunneling of holes through crystallite barriers during the formation process of porous silicon. The theoretical description is based on quantum mechanical calculations of the tunneling probability of the holes through small crystallites into the electrolyte. This tunneling probability shows oscillations as a function of crystallite size which are comparable to the structures observed in the size distributions. The calculations presented provide a deeper understanding of these size distributions and of the basic formation mechanism of porous silicon.

Complete dielectric isolation by highly selective and self-stopping formation of oxidized porous silicon

Complete dielectric isolation of silicon regions 45 μm wide for microcircuits on silicon substrates is demonstrated by selective formation of oxidized porous silicon in heavily doped n-type regions. Anodic etching of n-type silicon in a hydrofluoric acid electrolyte exhibits a concentration-dependent voltage threshold. This voltage dependence allows the porous silicon formation process to be selective to heavily doped regions and self-stopping on lightly doped regions. Rapid oxidation of this porous silicon yields an oxide with dielectric properties approaching those of standard thermally grown silicon oxides. This process is an improvement over previously reported processes utilizing porous silicon in that wider regions (up to 300 μm) are dielectrically isolated by an oxide whose thickness is controllable and uniform so that stress and wafer warpage are minimized.