This article presents an overview of the essential aspects in the fabrication of silicon and some silicon/germanium nanostructures by metal-assisted chemical etching. First, the basic process and mechanism of metal-assisted chemical etching is introduced. Then, the various influences of the noble metal, the etchant, temperature, illumination, and intrinsic properties of the silicon substrate (e.g., orientation, doping type, doping level) are presented. The anisotropic and the isotropic etching behaviors of silicon under various conditions are presented. Template-based metal-assisted chemical etching methods are introduced, including templates based on nanosphere lithography, anodic aluminum oxide masks, interference lithography, and block-copolymer masks. The metal-assisted chemical etching of other semiconductors is also introduced. A brief introduction to the application of Si nanostructures obtained by metal-assisted chemical etching is given, demonstrating the promising potential applications of metal-assisted chemical etching. Finally, some open questions in the understanding of metal-assisted chemical etching are compiled.

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Metal-Assisted Chemical Etching of Silicon: A Review

The use of silicon nanostructures in solar cells offers a number of benefits, such as the fact they can be used on flexible substrates. A silicon wire-array structure, containing reflecting nanoparticles for enhanced absorption, is now shown to achieve 96% peak absorption efficiency, capturing 85% of light with only 1% of the silicon used in comparable commercial cells. Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.

Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications

Silicon has long been established as the material of choice for the microelectronics industry. This is not yet true in photonics, where the limited degrees of freedom in material design combined with the indirect bandgap are a major constraint. Recent developments, especially those enabled by nanoscale engineering of the electronic and photonic properties, are starting to change the picture, and some silicon nanostructures now approach or even exceed the performance of equivalent direct-bandgap materials. Focusing on two application areas, namely communications and photovoltaics, we review recent progress in silicon nanocrystals, nanowires and photonic crystals as key examples of functional nanostructures. We assess the state of the art in each field and highlight the challenges that need to be overcome to make silicon a truly high-performing photonic material.

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Silicon nanostructures for photonics and photovoltaics

The nanowire geometry provides potential advantages over planar waferbased or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, improved band gap tuning, facile strain relaxation, and increased defect tolerance. These benefits are not expected to increase the maximum efficiency above standard limits; instead, they reduce the quantity and quality of material necessary to approach those limits, allowing for substantial cost reductions. Additionally, nanowires provide opportunities to fabricate complex single-crystalline semiconductor devices directly on low-cost substrates and electrodes such as aluminum foil, stainless steel, and conductive glass, addressing another major cost in current photovoltaic technology. This review describes nanowire solar cell synthesis and fabrication, important characterization techniques unique to nanowire systems, and advantages of the nanowire geometry.

Nanowire Solar Cells

Semiconductor nanowires are attracting intense interest as a promising material for solar energy conversion for the new-generation photovoltaic (PV) technology. In particular, silicon nanowires (SiNWs) are under active investigation for PV applications because they offer novel approaches for solar-to-electric energy conversion leading to high-efficiency devices via simple manufacturing. This article reviews the recent developments in the utilization of SiNWs for PV applications, the relationship between SiNW-based PV device structure and performance, and the challenges to obtaining high-performance cost-effective solar cells.

Silicon Nanowires for Photovoltaic Solar Energy Conversion

Silicon nanowire (SiNW)-based solar cells on glass substrates have been fabricated by wet electroless chemical etching (using silver nitrate and hydrofluoric acid) of 2.7 μm multicrystalline p+nn+ doped silicon layers thereby creating the nanowire structure. Low reflectance ( 90% at 500 nm) have been measured. The highest open-circuit voltage (Voc) and short-circuit current density (Jsc) for AM1.5 illumination were 450 mV and 40 mA/cm2, respectively at a maximum power conversion efficiency of 4.4%.

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Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters.

Stress engineering in thin film solar cells on foreign substrates represents an important manufacturing issue to prevent cracking or peeling off the film or even substrate warpage resulting in an increased breakage risk and processing difficulties. Moreover, stresses relax by generating recombination active extended defects such as dislocations and grain boundaries that first of all reduce the efficiency and secondly induce very locally stress concentrations that reduce the mechanical stability of the solar cells. A new process based on a combination of laser crystallization of an amorphous silicon seed layer and epitaxial thickening of the seed layer by solid phase crystallization is being explored. Indeed, we measure by micro-Raman spectroscopy local stress concentrations including both compressive and tensile contributions inside grains and at grain boundaries in the laser crystallized seed layer. We attribute these stresses to particular defect configurations/structures since they do not correlate with both the misorientation inside grains and the low angle grain boundaries determined by electron backscatter diffraction. In most cases, the local stress concentrations in the seed layer are found to relax, presumably by structural rearrangements, during the solid phase epitaxy process. The structural evolution with respect to extended defect formation and stress built up during processing will be discussed.

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Evolution of residual stress and its relation to microstructure in multi- crystalline silicon thin film solar cells on glass prepared by combined laser crystallization and solid phase epitaxy

Laser Crystallized Silicon Layers for Multicrystalline Thin-Film Solar Cells

Residual mechanical stress is currently among the most important parameters for improving processing yields and it will become increasingly important in view of the targeted decrease in the wafer or thin film thicknesses to reduce processing costs. A technique that is intensively used for analysing stress states at the micrometer scale in semiconductors is micro-Raman spectroscopy. Several stress-tensor components can be obtained separately using polarized laser light for the μ-Raman measurements or average stresses when using unpolarized light. We have found inhomogeneous stress distributions including local peak compressive and tensile stresses at (sub)grain boundaries and in areas with high concentrations of extended lattice defects. These stresses most probably form during the different heating and cooling processes of the multicrytalline silicon block and ribbon casting or during the laser melting and re-crystallization of amorphous silicon layers on thermally mismatched glass substrates. The possible mechanisms of stress formation and relaxation are discussed.

G. Andrä

Papers

Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters.

Evolution of residual stress and its relation to microstructure in multi- crystalline silicon thin film solar cells on glass prepared by combined laser crystallization and solid phase epitaxy

Laser Crystallized Silicon Layers for Multicrystalline Thin-Film Solar Cells

Residual Stress Measurements in Multicrystalline Silicon Bulk and Thin Film Solar Cells Using Micro-Raman Spectroscopy

Laser Induced Crystallization Processes for Multicrystalline Silicon Thin Film Solar Cells