Failure stress of epitaxial silicon thin films

Ultra-thin silicon wafer have to withstand forces and stresses during handling procedures without breakage. Here we investigate the failure stresses of ?30 ?m thick monocrystalline silicon films produced with the porous silicon process by use of a three line bending setup. We use a finite element simulation in order to evaluate the experiments and conclude that the porous silicon layers break at stresses comparable to those of silicon wafers with standard thickness. The edge preparation has a large impact on the failure stress. For samples with manually cleaved edges the failure stress surpasses 600 MPa, which is the largest stress that is accessible with our testing setup

Multilayer etching for kerf-free solar cells from macroporous silicon

Kerf-free techniques for subdividing a single thick crystalline Si wafer into a multitude of thin Si layers have a large potential for cost reductions. In this paper, we explore pore formation in Si for separating many 18 μm-thick surface-textured layers from a thick wafer with a single etching process. We demonstrate the fabrication and separation of four macroporous Si layers in a single etching step. Generating many instead of single macroporous layers per etching step improves the economics of the macroporous Si process. We present our etching process that maintains the pore pattern defined by photolithography even after etching many absorber and separation layers.Federal Ministry for Environment, Nature Conservation, and Nuclear Safety/FKZ 032514

Lift-off of free-standing layers in the kerfless porous silicon process

We discuss the lift-off of free-standing epitaxially grown silicon layers from the porous silicon (PSI) process, which is a kerfless wafering technology. The lift-off is a crucial step in the PSI cycle. A high-porosity layer serves as a mechanically weak layer for lift-off and consists of widely spaced silicon bridges with thicknesses of 40-100 nm. The low width leads to a 33-fold stress enhancement in the bridges, making them break when a force is applied while the epitaxial layer and the substrate remain intact. We perform the free-standing lift-off with a curved vacuum chuck. A vacuum pressure of 0.2 bar is sufficient for controlled peeling off of the 30-50 um thick silicon layers. We simulate the stresses and the displacements of the epitaxial layer in the lift-off process close to the first non-broken bridge. We demonstrate the defect-free lift-off of 8 of 9 of 9 × 9 cm2 layers from 6" substrates.Renewable Energy Corporatio

Lifetime Analysis for Defect Characterization in Kerfless Epitaxial Silicon from the Porous Silicon Process

Kerfless epitaxial silicon from the porous silicon (PSI) process is a promising alternative for standard wafers. They allow the reduction of PV costs by combining high material quality at reduced production costs. We evaluate the minority carrier lifetime of p-type and n-type epitaxial silicon layers fabricated with the PSI process by means of photoconductance decay measurements. For p-type layers we observe a strong injection dependence of the lifetime that we attribute to bulk Shockley-Read-Hall (SRH) recombination. We determine two limiting defects K3.6 and K157 that describe the injection dependence of 9 samples grown in one batch. Defect K3.6 has a symmetry factor of k=3.6 and is similarly concentrated in all 9 investigated samples. Its concentration decreases upon high temperature processing with and without phosphorous diffusion. The defect K157 has a symmetry factor of k=157 and a higher concentration in samples with a higher porosity in the starting layer. As a consequence of the k-factors being larger than unity the identified defects are less detrimental in n-type silicon than p-type silicon. Accordingly, we fabricate n-type epitaxial layers for which we measure effective lifetimes up to 1330±130 μs at Δp = 1015 cm –3

Analysis methods for meso- and macroporous silicon etching baths

Analysis methods for electrochemical etching baths consisting of various concentrations of hydrofluoric acid (HF) and an additional organic surface wetting agent are presented. These electrolytes are used for the formation of meso- and macroporous silicon. Monitoring the etching bath composition requires at least one method each for the determination of the HF concentration and the organic content of the bath. However, it is a precondition that the analysis equipment withstands the aggressive HF. Titration and a fluoride ion-selective electrode are used for the determination of the HF and a cuvette test method for the analysis of the organic content, respectively. The most suitable analysis method is identified depending on the components in the electrolyte with the focus on capability of resistance against the aggressive HF.State of Lower Saxon

Kerfless exfoliated thin crystalline Si wafers with Al metallization layers for solar cells

We report on a kerfless exfoliation approach to further reduce the costs of crystalline silicon photovoltaics making use of evaporated Al as a double functional layer. The Al serves as the stress inducing element to drive the exfoliation process and can be maintained as a rear contacting layer in the solar cell after exfoliation. The 50-70 μm thick exfoliated Si layers show effective minority carrier lifetimes around 180 μs with diffusion lengths of 10 times the layer thickness. We analyze the thermo-mechanical properties of the Al layer by x-ray diffraction analysis and investigate its influence on the exfoliation process. We evaluate the approach for the implementation into solar cell production by determining processing limits and estimating cost advantages of a possible solar cell design route. The Al-Si bilayers are mechanically stable under processing conditions and exhibit a moderate cost savings potential of 3-36% compared to other c-Si cell concepts. © Materials Research Society 2015

Laser-welded interconnection of screen-printed Si solar cells

We demonstrate the laser welding of Al interconnects to the BSF rear-side of screen-printed two-side-contacted solar cells. The Al paste on the rear side of solar cell is laser-welded to an Al foil. This reduces the silver consumption of the solar cells by making silver pads on the rear side obsolete. Our proof-of-concept modules are free of laser damage. A 3-cell-module from 6" solar cells shows no change in fill factor within the statistical measurement uncertainty after artificial aging in 500 humidityfreeze cycles.German Ministry for the Environment, Nature Conservation, and Reactor Safety/0325192State of Lower Saxon

Thin crystalline macroporous silicon solar cells with ion implanted emitter

We separate a (34 ± 2) μm-thick macroporous Si layer from an n-type Si wafer by means of electrochemical etching. The porosity is p = (26.2 ± 2.4)%. We use ion implantation to selectively dope the outer surfaces of the macroporous Si layer. No masking of the surface is required. The pores are open during the implantation process. We fabricate a macroporous Si solar cell with an implanted boron emitter at the front side and an implanted phosphorus region at the rear side. The short-circuit current density is 34.8 mA cm-2 and the open-circuit voltage is 562 mV. With a fill factor of 69.1% the cell achieves an energy-conversion efficiency of 13.5%.Federal Ministry for Environment, Nature Conservation, and Nuclear Safety/FKZ 032514

Thermomechanical Spalling of Epitaxially Grown Silicon from Porosified Substrates

We combine two kerfless approaches to unite advantages of both processes: the epitaxial layer transfer based on porous silicon (PSI process) and the lift-off of a thin silicon layer from a substrate via controlled spalling by a stress-inducing layer. For this, we deposit an Al stressor layer on top of an epitaxially grown silicon layer. A porous double layer underneath the epitaxial layer serves as determined breaking point. We directionally heat this sample stack and cool it afterwards for controlled spalling of the epitaxial layer from the substrate. We achieve a lift-off rate of 34 out of 36 detached samples. The porous silicon layer enables a smooth surface of the detached epitaxial layer and the remaining substrate. Compared to our standard spalling process the thickness variation of the detached layers is significantly reduced from ≤ 25 μm to less than 2 μm. Furthermore we show that the lifetime of the detached epitaxial layers does not suffer from the Al deposition and the lift-off process.Federal Ministry for Environment, Nature Conservation, and Nuclear Safety/FKZ 0325461State of Lower Saxon

Phase-dependent light propagation in atomic vapors

Light propagation in an atomic medium whose coupled electronic levels form a diamond-configuration exhibits a critical dependence on the input conditions. In particular, the relative phase of the input fields gives rise to interference phenomena in the electronic excitation whose interplay with relaxation processes determines the stationary state. We integrate numerically the Maxwell-Bloch equations and observe two metastable behaviors for the relative phase of the propagating fields corresponding to two possible interference phenomena. These phenomena are associated to separate types of response along propagation, minimize dissipation, and are due to atomic coherence. These behaviors could be studied in gases of isotopes of alkali-earth atoms with zero nuclear spin, and offer new perspectives in control techniques in quantum electronics.Comment: 16 pages, 11 figures, v2: typos corrected, v3: final version, to appear in Phys. Rev.