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Recently-demonstrated high-quality three-dimensional (3D) subsurface laser processing inside crystalline silicon (c-Si) wafers opens a door to a wide range of novel applications in multidisciplinary research areas. Using this technique, a novel maskless micro-pillars with precise control on the surface reflection and coverage are successfully fabricated by etching the laser processed region of c-Si wafer. To achieve this, a particular selective wet chemical etching is developed to follow subsurface laser processing of c-Si to reveal the desired 3D structures with smooth surfaces. Here, we report the development of a novel chromium-free chemical etching recipe based on copper nitrate, which yields substantially smooth surfaces at high etch rate and selectivity on the both laser-processed Si surface and subsurface, i.e., without significant etching of the unmodified Si. Our results show that the etch rate and surface morphology are interrelated and strongly influenced by the composition of the adopted etching solution. After an extensive compositional study performed at room temperature, we identify an etchant with a selectivity of over 1600 times for laser-modified Si with respect to unmodified Si. We also support our findings using density functional theory calculations of HF and Cu adsorption energies, indicating significant diversity on the c-Si and laser-modified surfaces.

High-quality multicrystalline Upgraded Metallurgical Grade Silicon (UMG-Si) offers significant advantages over conventional polysilicon-based PV technology, associated to lower cost, lower energy budget and lower carbon footprint. The aim of this study is twofold: on the one hand, to ascertain the efficiency potential of solar cells based on this material in terms of carrier lifetime; and on the other hand, to explore, as a result of that, the adoption of high-efficiency cell architectures by establishing an effective rear-side passivation scheme for the implementation of passivated emitter rear contact (PERC) devices. The carrier lifetime and the surface passivation efficacy are investigated for different passivating layer configurations after single and double P-diffusion gettering processes. Layer stacks consisting of Al2O3, SiOxNy and a-SiNx:H capping overlayers have been optimized, on industrial size, saw-damage-etched UMG wafers and results compared to those obtained using reference iodine-ethanol (IE) passivation. Diagnosis based on minority carrier lifetime and implied Voc (iVoc) measurements helped monitor the impact of parameter optimization on wafer quality, particularly after firing processes. Carrier lifetimes over 600 us at 10^15 cm-3 injection level as well as up to 790 us locally have been measured in UMG-Si wafers passivated with IE after a Phosphorus Diffusion Gettering (PDG), demonstrating the suitability of the material for high-efficiency cell architectures. Values higher than 300 us have been obtained with Al2O3-based passivation layers for gettered UMG wafers, with implied Voc values up to 710 mV. These record-breaking lifetimes and iVoc figures obtained with p-type multicrystalline UMG-Si material demonstrate a significant upgrading of its electronic quality by means of industry-scalable technical processes., Comment: 31 pages, 6 figures

Nanostructures, especially of silicon, are of paramount importance for new generation solar cell applications. The key material requirements for solar cells are good electrical conductivity, confinement of excitons, and a tunable band-gap that allows for tandem arrangements to absorb a maximally large portion of the solar spectrum. However, to date, existing Si-based nanostructures have addressed these requirements individually. The difficulty is that these requirements are entangled in complex ways, so an effort to improve one can lead to a major trade-offs in the others. In this study, we report on a novel silicon nanostructure, where the key advance is that we have managed to satisfy all the abovementioned requirements simultaneously: We demonstrate that the excitons are confined, the bandgap is tunable, and the electrons can freely travel along the nc-Si network., Comment: 26 pages, 4 figures

Demonstrating the quantum-confined Stark effect (QCSE) in silicon nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other materials. Here, the recent experimental data from ion-implanted Si NCs is unambiguously explained within the context of QCSE using an atomistic pseudopotential theory. This further reveals that the majority of the Stark shift comes from the valence states which undergo a level crossing that leads to a nonmonotonic radiative recombination behavior with respect to the applied field. The polarizability of embedded Si NCs including the excitonic effects is extracted over a diameter range of 2.5--6.5 nm, which displays a cubic scaling, $\alpha=c D^3$, with $c=2.436\times 10^{-11}$ C/(Vm), where $D$ is the NC diameter. Finally, based on intraband electroabsorption analysis, it is predicted that p-doped Si NCs will show substantial voltage tunability, whereas n-doped samples should be almost insensitive. Given the fact that bulk silicon lacks the linear electro-optic effect as being a centrosymmetric crystal, this may offer a viable alternative for electrical modulation using p-doped Si NCs., Comment: Published version, 7 pages, 7 figures

Understanding charging mechanisms and charge retention dynamics of nanocrystal memory devices is important in optimization of device design. Capacitance spectroscopy on PECVD grown germanium nanocrystals embedded in a silicon oxide matrix was performed. Dynamic measurements of discharge dynamics are carried out. Charge decay is modelled by assuming storage of carriers in the ground states of nanocrystals and that the decay is dominated by direct tunnelling. Discharge rates are calculated using the theoretical model for different nanocrystal sizes and densities and are compared with experimental data. Experimental results agree well with the proposed model and suggest that charge is indeed stored in the quantized energy levels of the nanocrystals., Comment: 10 pages, 5 figures