Silicon Foils Research Articles

Kerfless layer-transfer of thin epitaxial silicon foils using novel multiple layer porous silicon stacks with near 100% detachment yield and large minority carrier diffusion lengths

Two important aspects for the success of the porous silicon-based layer transfer method in producing kerfless thin (<50µm) silicon foils for future silicon solar modules are addressed in this work: achieving high detachment yield and high minority carrier diffusion lengths. The detachment characteristics of the porous silicon-based lift-off process is studied using finite element modeling as well as experiments. It is shown that for easy detachment and high detachment yield, a low density of thin silicon pillars must be attained in the high porosity detachment layer (HP-DL) after high temperature sintering. This is elegantly achieved by increasing the thickness of the low porosity template layer (LP-TL) which acts as the vacancy supply to increase the post-anneal porosity of the HP-DL. In this way, near 100% detachment yield has been achieved. However, a thicker LP-TL results in a poorer quality epitaxial growth surface. To circumvent this trade-off, novel triple and quadruple layer porous silicon stacks are introduced which decouple the function of the LP-TL that acts as both the template for epitaxy and as the vacancy supply for the HP-DL. In these new stacks, a surface zone of very low void size and density (nearly void-free) is created which allows high quality epitaxy on easily-detachable porous silicon stacks. Minority carrier lifetime measurements on epitaxial foils grown on such a triple layer stack has resulted in an effective lifetime of ~350µs at the injection level of 1015cm−3 which corresponds to a minimum minority carrier diffusion length of ~670µm (> 16 times the silicon thickness). With such high quality epitaxial foils combined with high detachment yield, very high efficiency solar devices on thin silicon substrates would be a reality in the near future.

Thin silicon foils produced by epoxy-induced spalling of silicon for high efficiency solar cells

We report on the drastic improvement of the quality of thin silicon foils produced by epoxy-induced spalling. In the past, researchers have proposed to fabricate silicon foils by spalling silicon substrates with different stress-inducing materials to manufacture thin silicon solar cells. However, the reported values of effective minority carrier lifetime of the fabricated foils remained always limited to ∼100 μs or below. In this work, we investigate epoxy-induced exfoliated foils by electron spin resonance to analyze the limiting factors of the minority carrier lifetime. These measurements highlight the presence of disordered dangling bonds and dislocation-like defects generated by the exfoliation process. A solution to remove these defects compatible with the process flow to fabricate solar cells is proposed. After etching off less than 1 μm of material, the lifetime of the foil increases by more than a factor of 4.5, reaching a value of 461 μs. This corresponds to a lower limit of the diffusion length of more than 7 times the foil thickness. Regions with different lifetime correlate well with the roughness of the crack surface which suggests that the lifetime is now limited by the quality of the passivation of rough surfaces. The reported values of the minority carrier lifetime show a potential for high efficiency (&amp;gt;22%) thin silicon solar cells.

Femtosecond Pulsed Laser Deposition of Graphite on Silicon and Copper Foil

We deposited graphite on silicon (111) and copper foil substrate through femtosecond pulsed laser deposition (fs-PLD). A high purity graphite target was placed inside a vacuum chamber at a base pressure of 10-2mbar. The deposition time was varied for 3 hours, 4 hours and 5 hours. XRD spectra showed a (110) peak indicating an oriented growth for samples deposited on silicon (111) and copper foil substrates. AFM topographical images of the samples deposited on silicon (111) showed flake-like structures. However, samples deposited on copper foil showed the presence of defects and lack of deposited particles.

Room temperature kerfless silicon thin foils obtained via a stress inducing epoxy layer

AbstractWe present 5 × 5 cm2 SLIM‐cut foils obtained by cooling form curing temperatures of 150 °C to room temperature using an epoxy stress inducing layer. Numerical simulations were performed to help the definition of an optimum geometry and we demonstrate the capability to obtain several thin foils from the same substrate. The evolution of minority carrier lifetime upon successive exfoliation of the same substrate is presented. Measured lifetimes in these silicon foils increase after etching suggesting that recombination centers are present close to the foil surface. Effective lifetimes of 50 microseconds were obtained in 120 µm thick foils, corresponding to diffusion length much larger than the foil thickness. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

New Stress Activation Method for Kerfless Silicon Wafering Using Ag/Al and Epoxy Stress-Inducing Layers

The SLIM-cut technique provides a way to obtain thin silicon foils without a standard sawing step, thus avoiding kerf losses. This process consists of three steps: depositing a stress-inducing layer on top of the silicon surface; stress activation by heating and cooling, resulting in crack propagation in the silicon and detachment of a thin silicon layer; and a chemical cleaning to remove the stress-inducing layer. This paper describes a new stress activation method using Ag/Al and epoxy stress-inducing layers. The crack propagation is controlled along the sample length in order to avoid unwanted additional crack formation and interaction with other crack fronts. Silicon foils with thickness ranging between 50 and 130 μm were obtained with effective lifetimes between 1 and 81 μs.

Reorganization of Porous Silicon: Effect on Epitaxial Layer Quality and Detachment

Cost reduction is still a main goal in solar cell research and can be achieved by going towards thinner silicon bulk material. One way to avoid kerf loss and to reach thicknesses of less than 50 μm is a lift-off approach using porous silicon and epitaxial thickening for silicon foil fabrication. The porous layer structure requires a reorganization step that was varied in this work to optimize the detachment properties and the crystal quality of the epitaxial Si film. All processes were carried out in a quasi-inline Atmospheric Pressure Chemical Vapour Deposition (APCVD) reactor. Cross–sections were observed to see if the porous layer shows the desired structure. Stacking fault densities in epitaxial layers deposited on porous silicon layers significantly decrease with increasing reorganization time but are at least one order of magnitude higher than in epitaxial layers deposited on polished wafers. Microwave photoconductive decay (MWPCD) measurements and photo luminescence (PL) imaging were carried out to determine the effective carrier lifetimes of the detached foils and to correlate them with stacking faults and cracks. A detached 40 μm thin silicon foil with an averaged effective carrier lifetime of 22 μs is shown which corresponds to a diffusion length of over 200 μm. This investigation shows that silicon foils deposited in a quasi-inline APCVD reactor exhibit good detachment properties and a good crystal quality, which is both needed for high efficiency solar cell processing.

Solar Cell Processing of Foil Produced by Epoxy-induced Spalling of Silicon

To push silicon solar cells up to their theoretical limit and, in the meantime, minimize the amount of active material employed, many researchers have proposed to fabricate silicon solar cells starting from substrates of 100μm or below. A considerable number of methods have been proposed to overcome the limitations of the standard sawing technique but, all of them present some drawbacks. One of these methods, the Slim-cut method, exploits the stress-induced by the deposition of a layer on top of a thick silicon substrate to separate a thin silicon foil. This process however did not solve the issues linked to the processing of free-standing foils.In this work we present a flow to process the Slim-cut foils into solar cells by minimizing the number of process steps performed on free-standing foils to just one step. The possibility to tune the thickness of the obtained silicon foil is investigated both by analytical calculations and experimental results. Moreover, a solution to remove the stress-inducing layer compatible with the flow is reported and lifetime measurements of samples bonded on glass demonstrate that the quality of the samples is high enough for the fabrication of solar cells.

High-quality Exfoliated Crystalline Silicon Foils for Solar Cell Applications

Kerfless wafering techniques offer a significant cost saving potential via the reduction of silicon consumption. In this paper, we examine thin single crystalline Si foils that were fabricated by a novel kerfless thermo-mechanical exfoliation method utilizing evaporated Al with regard to their suitability for solar cell applications. The foils are 50-80μm thick and smooth to visual inspection across the almost entire surface. We measure the effective minority carrier lifetimes of the foils and the remaining parent substrates by quasi-steady-state photoconductance (QSSPC) and spatially resolved by dynamically calibrated steady state infrared carrier lifetime mapping (dynILM). We find lifetimes of above 120μs for kerfless exfoliated 0.5 Ωcm p-type float-zone (FZ) Si layers. With an additional etching step after exfoliation, we obtain effective lifetimes of above 200 μs. The measurements reveal that there is no critical lifetime degradation due to exfoliation-induced surface features and thus the exfoliated layers are well-suited for high-quality solar cells.

Room Temperature Spalling of Thin Silicon Foils Using a Kerfless Technique

An important factor for cost reduction of solar electricity is the reduction of silicon material quantity used for making a solar cell. It is well known in the PV industry that kerf losses associated with wafering are a limiting factor in the efforts to reduce material usage and some techniques have been developed to address this issue. The kerfless wafering technique described in this paper provides both things: a thin silicon layer foil while avoiding the sawing step. This technique consists of three steps: (i) dispensing a stress inducing layer on the silicon surface of a slab; (ii) thermal processing to activate the stress and detach a thin silicon layer of silicon; (iii) a chemical cleaning to obtain a flat thin foil of silicon. This technique was used with an epoxy stress inducing layer to obtain thin silicon foils, demonstrating for the first time the capability to obtain several foils out of the same substrate and using only room temperature cooling step, unlike other authors who used dipping in liquid nitrogen. The relationship between epoxy layer and foil thickness is presented, along with foils with bulk minority carrier lifetimes of 80μs.

Defects in Si foils fabricated by spalling at low temperature: electrical activity and atomic nature

Silicon foils are fabricated using a low-temperature (<150 °C) lift–off process. The electrical activity of traps present in these foils is analysed by means of deep-level transient spectroscopy, carried out in parallel with electron spin resonance (ESR) studies to assess the atomic nature of traps. Two hole traps are detected in the as-cleaved samples, one of which is associated with a D-line defect. After vacuum annealing, these traps and the D-line disappear but at the same time, a new hole trap is detected associated with the ESR-revealed Pb0 recombination centre. After chemical removal of a thin Si layer from the crack surface, the ESR signal disappears. This shows that samples fabricated with this method are largely dislocation-free and no electronic levels in the bandgap remain if a layer of a few µm is removed from the crack surface.

Analysis of stress intensity factors and T-stress to control crack propagation for kerf-less spalling of single crystal silicon foils

Monocrystalline silicon (called mono silicon) is extensively used in the electronic and solar photovoltaic industries. During the last decade, many new manufacturing processes have been developed to improve solar cells’ efficiency while reducing their cost of production. This paper focuses on a kerf-less technique based on the controlled fracture of silicon foils by depositing an adherent stress-inducing layer on {hkl} cleavage plans. A finite element model (FEM) is defined to study the stress intensity factors (SIFs) associated with a pre-crack located at a certain depth from the interface between the silicon substrate and the stress-inducing layer. A parametric study elucidates the dependence of the crack propagation direction on process variables including thickness of the stress-inducing layer, silicon substrate thickness, and pre-crack depth. The use of stress intensity factors and the T-stress characterize the crack propagation. These results are essential for efficient control of this kerf-less spalling process.

Material Properties of Laser-Welded Thin Silicon Foils

An extended monocrystalline silicon base foil offers a great opportunity to combine low-cost production with high efficiency silicon solar cells on a large scale. By overcoming the area restriction of ingot-based monocrystalline silicon wafer production, costs could be decreased to thin film solar cell range. The extended monocrystalline silicon base foil consists of several individual thin silicon wafers which are welded together. A comparison of three different approaches to weld 50 μm thin silicon foils is investigated here: (1) laser spot welding with low constant feed speed, (2) laser line welding, and (3) keyhole welding. Cross-sections are prepared and analyzed by electron backscatter diffraction (EBSD) to reveal changes in the crystal structure at the welding side after laser irradiation. The treatment leads to the appearance of new grains and boundaries. The induced internal stress, using the three different laser welding processes, was investigated by micro-Raman analysis. We conclude that the keyhole welding process is the most favorable to produce thin silicon foils.

Improvement of seed layer smoothness for epitaxial growth on porous silicon

ABSTRACTIn the last decades many techniques have been proposed to manufacture thin (&lt;50µm) silicon solar cells. The main issues in manufacturing thin solar cells are the unavailability of a reliable method to produce thin silicon foils with contained material losses (kerf-losses) and the difficulties in handling and processing such fragile foils. A way to solve both issues is to grow an epitaxial foil on top of a weak sintered porous silicon layer. The porous silicon layer is formed by electrochemical etching on a thick silicon substrate and then annealed to close the top surface. This surface is employed as seed layer for the epitaxial growth of a silicon layer which can be partially processed while attached on the substrate that provides mechanical support. Afterward, the foil can be bonded on glass, detached and further processed at module level. The efficiency of the final solar cell will depend on the quality of the epitaxial layer which, in turn, depends on the seed layer smoothness.Several parameters can be adjusted to change the morphology and, hence, the properties of the porous layer, both in the porous silicon formation and the succeeding thermal treatment. This work focuses on the effect of the parameters that control the porous silicon formation on the structure of the porous silicon layer after annealing and, more specifically, on the roughness of the top surface. The reported analysis shows how the roughness of the seed layer can be reduced to improve the quality of the epitaxial growth.

Traumatic Tympanic Membrane Perforations

To obtain clinical and audiometric findings in traumatic tympanic membrane perforations from a typical patient collective in a Western industrial nation because the appropriate data have an important relevance in medicolegal questions. Retrospective data collection. Germany's largest university clinic for otorhinolaryngology, head and neck surgery. From the medical records of 198 patients with traumatic tympanic membrane perforations, the following data were collected: demographic data, date and mechanism of the trauma, otoscopic findings and collateral injuries, kind of therapy and its results, pure tone audiometry, and statement of tinnitus or vertigo in the course. Most patients were young (mean age, 29.2 yr) and male (62%). Men and women are equally represented in perforations resulting from a physical blow to the ear (44.7% vs 46.7%); a collision was more often the cause in men (23.6% vs 14.7%), whereas an accidental perforation by insertion of a cotton bud was approximately 2 times more common on women (13.8% vs 25.3%). The left ear was more often affected than the right ear (58.5% vs 41.5%). Collateral damage was found in only 1% of the cases. In blows, collisions, barotraumas, and the insertion of sharp objects, the inferior parts of the tympanic membrane were most often affected; the most severe (subtotal) perforations were caused by explosions, weld beads, and insertion of cotton buds. For therapy, myringoplasty had an overall success rate of 88.9%; splinting with silicon foils, 51.6%; and "no therapy," 53.3%. Bone conduction thresholds for the affected ear were higher in low, middle, and high frequencies compared with the contralateral ear by trend, but a statistical difference was only found in the high frequencies. In follow-up examinations, the hearing thresholds in the high frequencies were no longer significantly different. A "c dip" or "fis dip" was found in 18.0% and 9.2%. Tinnitus and vertigo were reported in 30.8 and 8.1%, respectively, but only in 2.0% and 0% during follow-up. High-frequency hearing loss, tinnitus, and vertigo in traumatic tympanic membrane perforations have a good prognosis.

Wyniki leczenia chirurgicznego nabytego zarośnięcia przewodu słuchowego zewnętrznego

Acquired post inflammatory external auditory canal atresia is a rear complication of external ear disease. It is presented as a connective tissue scar in bony part of external auditory canal. The aim of this study is clinical and epidemiological analysis and presentation of diagnostics and treatment results of patients with atresia treated in the Otolaryngology Department of Medical University of Gdansk. Material and methodsIn the period of 3 years (2008–11) 10 patients (8 women and 2 men) aged 30 to 70-years-old (medium 53 years) were treated for acquired atresia. All of these patients had medial closure of EAC with thick connective tissue scar and tympanic membrane involvement. Intrameatal access was performed in 5 patients, intraural access in 3 patients and retroaurical access in 2 patients. In all cases canaloplasty with meato-tympanic angle enlargement was performed, skin defect was replaced with free epidermal flap, silicon foil with fibrinous sponge was used for coverage. ResultsIn all of our patients external auditory canal widening and hearing improvement was achieved. Due to increasing EAC narrowing four patients underwent second surgery with another free epidermal flap grafting. ConclusionThe selection of surgical access in patients with acquired atresia should comply with the etiology of the disease and the shape of EAC. During surgery we aim at maximal broadening of the bony part of EAC. The success of the treatment depends on meato-tympanic angle enlargement and free epidermal flap grafting.

Epoxy-Induced Spalling of Silicon

One of the main driving forces to lower the silicon solar cell modules overall cost is to decrease the thickness of the silicon substrate and, at same time, to reduce the material losses caused by the sawing of the silicon ingot. For this reason, a considerable number of methods have been proposed in the last decade to manufacture thin silicon foils which aim to replace the commonly used sawing technique. One of these methods is based on peeling off a thin layer from a silicon substrate by means of the residual stress induced either through the only deposition of a layer on a silicon substrate or through a succeeding thermal cycle of such a bilayer. Up to now, three different materials have been successfully employed in this technique: Nickel, Nickel/Chromium alloy and a Silver/Aluminium system. In this work we demonstrate the possibility to induce the lift-off of a thin silicon foil by means of curing an epoxy layer on top of a silicon wafer. This new method has the advantages of reducing metal contamination in the silicon and lowers the operating temperatures below 150°C.

Laser process for extended silicon thin film solar cells

We present a large area thin film base substrate for the epitaxy of crystalline silicon. The concept of epitaxial growth of silicon on large area thin film substrates overcomes the area restrictions of an ingot based monocrystalline silicon process. Further it opens the possibility for a roll to roll process for crystalline silicon production. This concept suggests a technical pathway to overcome the limitations of silicon ingot production in terms of costs, throughput and completely prevents any sawing losses. The core idea behind these thin film substrates is a laser welding process of individual, thin silicon wafers. In this manuscript we investigate the properties of laser welded monocrystalline silicon foils (100) by micro-Raman mapping and spectroscopy. It is shown that the laser beam changes the crystalline structure of float zone grown silicon along the welding seam. This is illustrated by Raman mapping which visualizes compressive stress as well as tensile stress in a range of − 147.5 to 32.5 MPa along the welding area.

Revitalization of cortical bone allograft by application of vascularized scaffolds seeded with osteogenic induced adipose tissue derived stem cells in a rabbit model

Adipogenous tissue derived stem cells (ASC) are available in abundance in the human body and can differentiate in the presence of lineage-specific induction factors, for example, in myogenic, adipogenic, chondrogenic and osteogenic cells. The aim of this study was to evaluate the impact of osteogenic induced ASC's (O-ASC) on revascularization and cellular repopulation of avital cortical bone employing a vascularized bovine scaffold. An inguinal arterio-venous bundle was dissected in the groin of female white New Zealand rabbits (n = 6) and placed centrally inside an O-ASC seeded scaffold via a central drill hole. In the same surgical session this construct was placed into a segment of avital cortical bone allograft from a donor rabbit. Unseeded scaffolds that were implanted and treated in the same fashion served as controls (n = 6). In order to prevent external revascularization, all constructs were wrapped in silicon foil and finally implanted in the rabbits' groin. Three months later, the constructs were explanted and investigated for vascularization of (a) the scaffold (b) the surrounding bone allograft. Histological stainings to determine cell growth, cellular repopulation of the scaffold and the cortical bone matrix, as well as inflammatory parameters were carried out. O-ASC seeded scaffolds showed a significant increase in new vessel formation in the scaffold as well as in the bone allograft compared to unseeded scaffolds. Furthermore, new vital osteocytes as a sign of cellular repopulation inside the bone allograft were found only in the treatment group. Vital chondrocytes were only found in the O-ASC seeded scaffolds as well. The presence of O-ASC significantly induce neo-vascularization and osteocytic repopulation of previously avital bone allograft as opposed to unseeded scaffolds in a rabbit model. Hence, this model might be of relevant value for future bone tissue engineering research and for re-vitalizing marginally nourished bone such as in avascular bone necrosis.

Evidence and Characterization of Crystallographic Defect and Material Quality after SLIM-Cut Process

ABSTRACTThe SLIM-Cut process is a kerf-free wafering technique to obtain silicon substrates as thin as 50μm. The quality of the resulting material must be assessed to ensure that this innovative Si-foil approach does not jeopardize the potential efficiency of the final solar cell in terms of electronic activity, defect density and location. For that reason, we performed Microwave-Detected Photoconductance Decay (MW-PCD), Deep-Level Transient Spectroscopy (DLTS) and optical inspections after defect etching of the foils surface. Analyses indicate that SLIM-Cut generates crystallographic defects which create deep level traps that have a negative impact on the lifetime of the silicon foil. Nonetheless, a decrease of the thermal budget will lead to a reduction of plasticity and hence lower the amount of defects and increase the foil quality.

Laser based joining of monocrystalline silicon foils

Monocrystalline silicon is the basic material for many electronic applications such as solar cells or detectors. Joining of silicon foils is a very difficult task due to the brittleness of the monocrystalline silicon. It was shown that it is possible to join silicon substrates with thicknesses in the millimetre range by laser beam welding. To qualify the laser based joining under industrial environment a stable process has to be ensured. In this paper an analysis of the laser based joining process of silicon foils with a thickness of 50 μm is presented. The welding process is analyzed by finite element simulation and experimental studies. The simulation shows that at the end of the welding process a strong deformation of the foils appears. To validate the simulation model experimental investigations were carried out. The simulation and experimental results correspond very well in regard of the time dependent behaviour of the deformation. The deformation behaviour is attributed to the buckling mechanism.