Techniques are presented for making ohmic contacts to nanowires with a thick oxide coating. Although experiments were carried out on Bi nanowires, the techniques described in this paper are generally applicable to other nanowire systems. Metal electrodes are patterned to individual Bi nanowires using electron beam lithography. Imaging the chemical reaction on the atomic scale with in situ high-resolution transmission electron microscopy shows that annealing in H 2 or NH 3 can reduce the nanowires' oxide coating completely. The high temperatures required for this annealing, however, are not compatible with the lithographic techniques. Low-resistance ohmic contacts to individual bismuth nanowires are achieved using a focused ion beam (FIB) to first sputter away the oxide layer and then deposit Pt contacts. By combining electron beam lithography and FIB techniques, ohmic contacts stable from 2 to 400 K are successfully made to the nanowires. A method for preventing the burnout of nanowires from electrostatic discharge is also developed.

Results are reported for experimentally measured and theoretically predicted optical absorption in bismuth nanowires. Five absorption peaks are seen in the experimental spectral range. The energies of these five peaks agree well with the predicted values for intersubband transitions, thereby validating our model of quantum confinement in bismuth nanowires. We also compare the measured absorption in free-standing wires to the absorption of wires inside an alumina template obtained from an analysis of transmission and reflection measurements using a reverse effective-medium theory. Close agreement between the energies of the largest absorption peak is found by the two methods. The weaker absorption peaks were not observed in the spectrum of wires embedded in alumina.

There are a range of different methods to generate a nanostructured surface on silicon (Si) but the most cost effective and optically interesting is the metal assisted wet chemical etching (MACE) (Koynov et al 2006 Appl. Phys. Lett. 88 203107). MACE of Si is a controllable, room-temperature wet-chemical technique that uses a thin layer of metal to etch the surface of Si, leaving behind various nano- and micro-scale surface features or 'black silicon'. MACE-fabricated nanowires (NWs) provide improved antireflection and light trapping functionality (Toor et al 2016 Nanoscale 8 15448-66) compared with the traditional 'iso-texturing' (Campbell and Green 1987 J. Appl. Phys. 62 243-9). The resulting lower reflection and improved light trapping can lead to higher short circuit currents in NW solar cells (Toor et al 2011 Appl. Phys. Lett. 99 103501). In addition, NW cells can have higher fill factors and voltages than traditionally processed cells, thus leading to increased solar cell efficiencies (Cabrera et al 2013 IEEE J. Photovolt. 3 102-7). MACE NW processing also has synergy with next generation Si solar cell designs, such as thin epitaxial-Si and passivated emitter rear contact (Toor et al 2016 Nanoscale 8 15448-66). While several companies have begun manufacturing black Si, and many more are researching these techniques, much of the work has not been published in traditional journals and is publicly available only through conference proceedings and patent publications, which makes learning the field challenging. There have been three specialized review articles published recently on certain aspects of MACE or black Si, but do not present a full review that would benefit the industry (Liu et al 2014 Energy Environ. Sci. 7 3223-63; Yusufoglu et al 2015 IEEE J. Photovolt. 5 320-8; Huang et al 2011 Adv. Mater. 23 285-308). In this feature article, we review the chemistry of MACE and explore how changing parameters in the wet etch process effects the resulting texture on the Si surface. Then we review efforts to increase the uniformity and reproducibility of the MACE process, which is critical for commercializing the black Si technology.

Metal-assisted catalyzed etching (MACE) of silicon (Si) is a controllable, room-temperature wet-chemical technique that uses a thin layer of metal to etch the surface of Si, leaving behind various nano- and micro-scale surface features, including nanowires (NWs), that can be tuned to achieve various useful engineering goals, in particular with respect to Si solar cells. In this review, we introduce the science and technology of MACE from the literature, and provide an in-depth analysis of MACE to enhance Si solar cells, including the outlook for commercial applications of this technology.

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Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon very large scale integrated circuit technology. Polycrystalline silicon (poly-Si), with a high index of refraction compared to SiO2 and air, is an ideal candidate for use in silicon optical interconnect technology. In spite of its advantages, the biggest hurdle to overcome in this technology is that losses of 350 dB/cm have been measured in as-deposited bulk poly-Si structures, as against 1 dB/cm losses measured in waveguides fabricated in crystalline silicon. We report methods for reducing scattering and absorption, which are the main sources of losses in this system. To reduce surface scattering losses we fabricate waveguides in smooth recrystallized amorphous silicon and chemomechanically polished poly-Si, both of which reduce losses by about 40 dB/cm. Atomic force microscopy and spectrophotometry studies are used to monitor surface roughness, which was reduced from an rms value of 19–20 nm down to about 4–6 nm. Bulk absorption/scattering losses can depend on size, structure, and quality of grains and grain boundaries which we investigate by means of transmission electron microscopy. Although the lowest temperature deposition has twice as large a grain size as the highest temperature deposition, the losses appear to not be greatly dependent on grain size in the 0.1–0.4 μm range. Additionally, absorption/scattering at dangling bonds is investigated before and after a low temperature electron-cyclotron resonance hydrogenation step. After hydrogenation, we obtain the lowest reported poly-Si loss values at λ=1.54 μm of about 15 dB/cm.

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Optical absorption associated with the one-dimensional joint density of states of an intersubband transition in bismuth nanowires is reported. The previously observed strong absorption in bismuth nanowires at ϳ1000 cm Ϫ1 is here shown to depend on the wire diameter and on the polarization of the incident light. The absorption line shape, the decreasing frequency with increasing wire diameter, and the polarization dependence of the reflectivity, all indicate that this resonance is due to an intersubband absorption resulting from quantum-confinement effects.