Macroporous Silicon Layers Research Articles

Metal electrode integration on macroporous silicon: pore distribution and morphology

In this work, a new approach for the one-step integration of interdigitated electrodes on macroporous silicon substrates is presented. Titanium/gold interdigitated electrodes are used to pattern p-type silicon substrates prior the anodization in an organic electrolyte. The electrolyte characteristics, conductivity, and pH have been found to affect the adherence of the metal layer on the silicon surface during the electrochemical etching. The impact of the metal pattern on size distribution and morphology of the resulting macroporous silicon layer is analyzed. A formation mechanism supported by finite element simulation is proposed.

Thick macroporous silicon membranes: Influence of the masking layer on the underetching characteristics

Macroporous silicon (MPS) has been demonstrated as an interesting material to be used in different applications including micromachining and chemical sensing. In most of these applications the formation of MPS in patterned areas is required or preferable. In this work we report the results of a study on the selective anodization of patterned p-type silicon substrates to form thick MPS layers. Two challenging issues were identified: (1) overetching at the edge of Si3N4 layers, which causes are principally associated with the electrical properties of the interface between the mask and the substrate and the distribution of carriers across the patterned area; (2) adherence of organic masking layers at the silicon during the anodization. Metallic Au/Ti masking layer was demonstrated to be a valuable solution to both issues. SEM cross-sectional views of macroporous layers formed on silicon substrates patterned using different materials are shown and analyzed. Experimental results are supported by two-dimensional (2D) simulations of the silicon/mask/electrolyte interface.

Point contact openings in surface passivated macroporous silicon layers

In this paper we demonstrate the preparation of point contact openings in surface passivated macroporous silicon layers. In our experiments we control the etching parameters to vary the percentage of these non-passivated local openings from 0% to 1.6%. We investigate the impact of these local openings in the passivating layer on the effective carrier lifetime. These local openings reduce the measured effective carrier lifetime with increasing percentage of the non-passivated areas. We measure effective carrier lifetimes up to 10μs on 29μm-thick fully passivated macroporous silicon samples. We develop and apply a 3-dimensional numerical model to calculate carrier lifetimes as a function of pore morphology, surface recombination, percentage of non-passivated area, and bulk lifetime. The model agrees with the experimental measurements. We find a surface recombination velocity of (Spass=22.8−1.4+1.6)cms−1 for the passivated surfaces and Snp=(2200−1400+1500)cms−1 for the non-passivated surface.

Nano-WO3 film modified macro-porous silicon (MPS) gas sensor

We prepared macro-porous silicon (MPS) by electrochemical corrosion in a double-tank cell on the surface of single-crystalline P-type silicon. Then, nano-WO3 films were deposited on MPS layers by DC facing target reactive magnetron sputtering. The morphologies of the MPS and WO3/MPS samples were investigated by using a field emission scanning electron microscope. The crystallization of WO3 and the valence of the W in the WO3/MPS sample were characterized by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The gas sensing properties of MPS and WO3/MPS gas sensors were thoroughly measured at room temperature. It can be concluded that: the WO3/MPS gas sensor shows the gas sensing properties of a P-type semiconductor gas sensor. The WO3/MPS gas sensor exhibits good recovery characteristics and repeatability to 1 ppm NO2. The addition of WO3 can enhance the sensitivity of MPS to NO2. The long-term stability of a WO3/MPS gas sensor is better than that of an MPS gas sensor. The sensitivity of the WO3/MPS gas sensor to NO2 is higher than that to NH3 and C2H5OH. The selectivity of the MPS to NO2 is modified by deposited nano-WO3 film.

Back Cover: Thin macroporous silicon heterojunction solar cells (Phys. Status Solidi RRL 5/2012)

The crystalline silicon wafer currently accounts for 40% of the photovoltaic module cost. Kerf-less wafering technologies for generating thin wafers could significantly reduce the silicon material consumption. The electrochemical separation of textured thin macroporous silicon layers from crystalline silicon wafers is a candidate for cost-effective thin wafer production. In their Letter on pp. 187–189, Ernst et al. report on the first successful demonstration of a 33 μm thick free-standing macroporous silicon solar cell. This cell achieves an energy conversion efficiency of 7.2%. One key challenge solved in this work is the avoidance of shunting despite the through-going pores.

Investigation of stability of electrophysical parameters of Si-wafers before forming porous layer

The article studies the influence of Si-wafers processing in acid-peroxide and ammonia-peroxide solutions on the temporary stability of the electronic states of the crystal surfaces before formation of the nanostructured and porous silicon layers. A characteristic parameter of stability of the electrical properties of a semiconductor was selected the change in contact potential difference over time, which was measured by Kelvin. After alkaline treatments during the storage of Si-wafers in air, their contact potential difference decreases with time. The change in the contact potential difference for this type of treatments is small and most of them (up to120 hours) does not reach steady-state values in the studied interval of time. This fact indicates that Si surface is unstable after the treatment in ammonia-peroxide solutions and is characterized by long relaxation changes of the contact potential difference. In case of acid treatments or their combinations with alkaline ones, the changes of the contact potential difference over time is characterized by a sharp decrease during 20 ÷ 40 hours and subsequent attainment of steady-state values of the contact potential difference. This indicates more efficient processing of Si-wafers in acid-peroxide solutions than in ammonium peroxide ones. It was shown experimentally that the time necessary to reach the steady-state values ​​of the contact potential difference and the time of electron activity after Si-wafers surface treatment in acid-peroxide solutions is less than in ammonia-peroxide solutions. The results are treated by the formation on Si surface of oxide and hydroxyl layers that leads to the formation of macroporous, and microporous or mesoporous silicon layers

The porous silicon morphology effect on the growth of electrodeposited FeNi alloy

In this work, we report on cathodic deposition of FeNi thin films into porous silicon (PS) formed on n-type Si. Macroporous and mesoporous silicon layers were formed at constant potential or current density. The electrodeposited thin films were characterized by Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD). The magnetic properties of the FeNi layers were investigated by hysteresis loops measurements. SEM images of the FeNi films indicate that tubular and granular forms were obtained depending on the porous silicon surface morphology. Moreover, the FeNi films compositions are found to depend on the porous silicon microstructure. Finally, the XRD spectra of the deposited films show the presence of FeNi (111) and FeNi (220) peaks. The FeNi (111) peak has been shown for all polarization potentials, in agreement with results reported in the literature.

Modeling effective carrier lifetimes of passivated macroporous silicon layers

We derive and apply a model that determines the effective minority carrier lifetime of macroporous crystalline silicon samples as a function of bulk lifetime, surface passivation and pore morphology. Two cases are considered: A layer of periodic macropores at the surface of a silicon wafer and a free standing macroporous silicon layer. We compare the model with experimental lifetime measurements for samples with randomly positioned macropores with a length of 10–40 μm. The pores have an average pore diameter of 2.4 μm and an average pore distance of 5.2 μm. The surface is passivated by thermal oxidation. The model agrees with the measurements if we assume an average surface recombination velocity S=24 cm/s at the pore surface.

The use of porous silicon layers in thin‐film silicon solar cells

AbstractIn the quest for lowering the manufacturing cost of silicon solar cells, imec has been working successfully on two crystalline Si ‘thin‐film’ cell concepts. In a first concept, a 20 µm‐thin Si solar cell is epitaxially grown on top of a porous Si‐based Bragg‐type reflector which is electrochemically etched in a low‐cost UMG Si substrate. Large area solar cells with efficiencies of 15.2% have been made, using (semi‐)industrial processing tools. This clearly demonstrates that this cell concept has almost reached the stage of industrial application. In a second, longer‐term approach, a 1–5 µm‐thin, stand‐alone mono‐crystalline film is created based on the controlled annealing of an ordered macroporous silicon layer (the ‘Epi‐free’ process). With this very thin Si layer, a simple proof‐of‐concept solar cell has been made exhibiting an efficiency of 4%. By optimizing the cell process in terms of light trapping and passivation, efficiencies over 15% can be expected from this technology.

Macroporous silicon: efficient antireflective layer on crystalline silicon

A macroporous silicon layer (ma-PS) electrochemically grown on crystalline silicon surface can be used as an efficient antireflective layer in optical devices as antireflection coating. In this work, we presented the ma-PS layers fabricated on crystalline silicon (c-Si) n-type and p +-type, obtained by electrochemical etching. The morphology, porosity, thickness of ma-PS layer can be adjusted by controlling the electrochemical formation conditions. The optical behaviour of the antireflective coating over the solar spectrum is determined, resulting in very low values of the normalized reflectivity coefficient (below ~1%). The reflectivity measurements were evaluated at 45° in the different samples of the ma-PS/c-Si.

10.1007/s11455-008-2017-3

The surface of an aluminum metallization coating applied on a macroporous silicon layer and annealed for 10–60 min at 550°C has been studied by atomic-force and electron microscopy techniques. It is established that the annealing results in the formation of through (open) macropores with lateral dimensions within 400–1300 nm in the aluminum film and in the growth of large hillocks with heights of up to 1.2μ m on the surface, which are located at the edges of these macropores. The geometric parameters of large hillocks are independent of the duration of annealing.

Broadband electrical characterization of macroporous silicon at microwave frequencies

AbstractMacroporous silicon layers with randomly distributed vertical cylindrical pores of average diameter 120 nm were fabricated on selected areas on a p+ (resistivity 5 mΩcm) silicon substrate by anodization in HFxethanol solution. The thickness of the layers was 50 μm and they are intended for use as RF isolation micro‐plates for on‐chip passive devices operated at microwave frequencies. In this respect, we present in this work results of their electrical characterization in the 40 MHz‐7 GHz frequency range using a macroscopic platform, where the die containing the porous silicon layer on the highly doped Si substrate is inserted directly underneath a microstrip line. The highly doped Si substrate used acts as a lossy metal ground for the microstrip. We measured and simulated the scattering parameters of the system using a finite‐element full‐wave electromagnetic solver. We have chosen a broad frequency band extending from 40 MHz up to 7 GHz, beyond the first harmonic of the transmission line. By identifying measured and simulated S‐parameters over the whole frequency band we extract both geometrical characteristics of the measurement set‐up not accessible to direct measurements and the complex permittivity of porous silicon. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pore formation in aluminum films on macroporous silicon during high-temperature annealing

The surface of an aluminum metallization coating applied on a macroporous silicon layer and annealed for 10–60 min at 550°C has been studied by atomic-force and electron microscopy techniques. It is established that the annealing results in the formation of through (open) macropores with lateral dimensions within 400–1300 nm in the aluminum film and in the growth of large hillocks with heights of up to 1.2μ m on the surface, which are located at the edges of these macropores. The geometric parameters of large hillocks are independent of the duration of annealing.

High Performance Macroporous Silicon Chemical Sensor With Improved Phase Detection Electronics

Abstract In this paper, a high sensitive, stable and reproducible electrical chemical sensor system based on macroporous silicon has been developed. Screen printed metal contacts have been taken from macroporous silicon layer which result in extremely stable, quasi ohmic and reproducible contacts. Such a sensor structure has been characterized for different organic and ionic solutions commonly used for biochemical applications. The sensitivity of the reported sensor at a particular frequency has been found to be almost ten times compared to previous reports of macroporous silicon sensor where contacts are taken from the backside and also in comparison to interdigited electrode array structure. The improvement and variation of sensitivity with frequency for different solutions has been explained taking into account the dependence of double layer impedance with frequency, distributed RC networks of the macroporous silicon structure, ionic conductivity of the solution and effects of ion sizes. However, from the impedance measurement it has been observed that the sensor capacitance with such stable contacts has an ultra low-Q value and to exploit such sensor for portable field use systems, an embedded signal conditioning unit has been realized to display the measured capacitance value after offset compensation and reduction of parasitic effects.

Development of Porous-Silicon-Based Active Microfilters

With a goal of preparing silicon-based interactive filters in the micropore range, we have developed the first single-step etch-liftoff procedure (one-step separation) based on the formation and removal of macroporous silicon layers. With silicon wafers whose resistivities are in the range from , we are able to create, in a surprisingly controlled manner, films whose pore diameters range from and whose thickness ranges from . These silicon-based films (filters), which carry a polarizing negative charge, represent an alternative to porous alumina (films) filters (pore diameter not yet in the range) both in terms of their size range and potential interaction-reaction at elevated temperature. Preliminary experiments demonstrate that platinum and copper can be introduced to these filters using electroless solutions with a goal to creating an effective reductive surface. Using these and alternate material combinations, interactive-reactive filters in the size range that operate at temperatures well in excess of porous polymer films can be realized.

Carbon nanofibers encapsulated in macropores in silicon

AbstractThis work reports on the development of fuel cells electrodes with a porous silicon structure and carbon nanofibers encapsulated in macropores in silicon. It is demonstrated that decomposition of carbon on a Ni catalyst deposited on the pore walls can be used to create a homogeneous carbon nanotube layer that dramatically increases the specific surface area while simultaneously reducing the resistivity of the macroporous silicon layer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Porous silicon microstructure and composition characterization depending on the formation conditions

The structure and chemical composition of porous silicon (PS) have been investigated as the function of hydrofluoric acid (HF) concentration in solution at electrochemical formation by scanning electron microscopy (SEM) and infra-red (IR) spectroscopy. The PS layers were fabricated in concentrated (∼80%) and in diluted (∼20%) HF-solutions whereas current density remains constant (∼30 mA/cm 2). It was found that the polymerized mesoporous silicon films (meso-PS) have been formed at high HF concentration whereas the macroporous silicon layers (macro-PS) were grown at low HF concentration. A FTIR spectrum measured in a wide range (400 ÷ 4000 cm − 1 ) exhibit the polymerized hydride films (SiH) n on the meso-PS surface, while the silicon oxide was defined on macro-PS surface. This is in accordance with secondary ion mass spectrometry (SIMS) studies. The photoluminescence (PL) studies show that PL peak position corresponds to 655 nm in the case of meso-PS. In contrast, PL spectrum of macro-PS was shifted to shorter wavelength (peak around 600 nm) with a weak intensity. Basic parameters of the PS layers have also been found: the mean pore sizes ( d = 2–8 nm for meso-PS from the low-temperature nitrogen adsorption, Brunauer, Emmet, Teller (BET) method and d = 500–3000 nm for the macro-PS from SEM imaging), a porosity ( p = 60% for meso-PS and p = 86% for macro-PS from gravimetric data), the coefficient of refraction ( n = 1.75 for meso-PS from IR spectrum), the PS thickness h = 5 ÷ 20 μm from profilometric measurements and IR spectrum).

Electrical Porous Silicon Microarray for dna Hybridization Detection

ABSTRACTThe sensitivity of Porous Silicon (PSi) to the presence of charged molecules and its large internal surface area represent two important properties that make this material and ideal candidate for electrical biosensor development. We have demonstrated the use of a macroporous silicon electrical sensor for label-free detection of DNA hybridization in real time as well as identification of organic solvents in liquid phase. Binding of DNA inside the PSi matrix induces a change in capacitance and conductance. Having demonstrated the suitability of macroporous silicon layers for real time detection of DNA hybridization on single devices, we have extended our findings to the fabrication of a microarray with individual device electrical addressing capabilities. On a crystalline p-type silicon wafer, process steps such as KOH etching and electrochemical dissolution are employed in selected regions to create a free-standing porous membrane for sensing applications. Individual electrical contacts are made on the front side of the wafer while the infiltration of the probe and target molecules is done from the back avoiding any direct interaction of the molecules with the contact sites. We will report on the design considerations of the electrical porous silicon array and the preliminary results obtained using synthetic DNA as a model molecule.

Porous Silicon Electrical Biosensors

ABSTRACTThe highly sensitive surface of porous silicon (PSi) and the possibility to measure changes in its electrical properties can be used for electrical biosensor applications. The sensing scenario can be extended from the well-characterized organic solvents and pH sensors to detection of biological processes involving the presence of charged molecules. We have investigated the change in electrical properties (capacitance and conductance) of macroporous silicon layers upon exposure to organic solvents and water. Evaluation of our sensor to detect a biological event was done by addressing DNA hybridization. As opposed to the previous work published in this field, in our devices the electrical contact is made on the crystalline silicon (c-Si) substrate. This allows a complete exposure of the surface to the sensing species and reduces the generation of ionic currents through the porous silicon matrix. We will report results of a complete characterization of our device including response speed, selectivity and sensitivity.

P–n Junction formed in structures with macro-porous silicon

Boron doped glasses, obtained from organic solution under low temperature and deposited by spin-on technique, were used to form a highly doped p +–n junction into structures with macro-porous silicon (PS) layer of 0.4–6 μm thickness. The junction was found to lie about 2.5 μm deeper than the bottom of the pores, and almost independent on their thickness. Enhanced boron diffusion was explained by a presence of local electric fields, caused by tensions present at the border between PS layer and crystalline substrate. Reflectivity values of less than 10% were obtained over 450–1050 nm wavelength range for samples with PS layer, confirming the light-trapping effect in electrochemically etched structures.