Joleyn Balch

Silicon nanowire solar cells

Over the past decade, silicon nanowire solar cells have been intensively explored as potential platforms for the next-generation photovoltaic (PV) technologies with high power conversion efficiency and low production cost. This chapter discusses the details of the silicon nanowire solar cells in terms of their device structures, fabrication and characterization, electrical and optical properties benefited from the nanowire geometry. These benefits are not only expected to increase the power conversion efficiency, but also considered to reduce the requirement for the material quantity and quality, allowing for potential efficiency improvements and substantial cost reductions.

Strong broadband optical absorption in silicon nanowire films

The broadband optical absorption properties of silicon nanowire (SiNW) films fabricated on glass substrates by wet etching and chemical vapor deposition (CVD) have been measured and found to be higher than solid thin films of equivalent thickness. The observed behavior is adequately explained by light scattering and light trapping though some of the observed absorption is due to a high density of surface states in the nanowires films, as evidenced by the partial reduction in high residual sub-bandgap absorption after hydrogen passivation. Finite difference time domain simulations show strong resonance within and between the nanowires in a vertically oriented array and describe the experimental absorption data well. These structures may be of interest in optical films and optoelectronic device applications.

Generation and assembly of spheroid-like particles.

This paper reports a straightforward approach in generating spheroid-like particles and also the orientational orders observed in the self-assembly of these particles. Nonspherical particles, such as spheroid-like particles, are useful in both fundamental studies and industrial applications due to the geometry impact that they bring to the bulk properties of various material systems. Developing processes to generate nonspherical particles is an ongoing quest to meet the need of using such particles in different applications. The approach reported here takes advantage of a controlled chemical etching process. Exposing the spherical silica particles partially to carbon tetrafluoride in a reactive ion plasma-etching chamber transformed the particles from spherical shape into spheroid-like shape. A simple model is proposed to predict the geometry of the resulting nonspherical particles. The shape and dimension of the nonspherical particles generated through such a process matched well with the prediction of the model. The assembly of these spheroid-like particles showed a unique orientational order associated with the alignment of their axes. This approach will help further studies on the fundamental properties of the nonspherical particles, such as packing, rheology, and optical interaction.

Mo2C nanowires and nanoribbons on Si by two-step vapor-phase growth

Transition-metal carbides in bulk form have historically been of technological interest primarily due to their excellent mechanical and refractory properties. As electronic materials these ceramic compounds are also particularly intriguing in that their electrical resistivity is relatively low compared to other ceramics and shows metallic temperature-dependent behavior. Some compositions also have superconducting transitions temperatures above 10°K. However, the synthesis of such materials in the form of one-dimensional nanostructures, which may be of interest for various nanoelectronic applications, is relatively difficult due to their refractory nature (Tmelt⩾2000°C). Here we report the synthesis of well-defined Mo2C nanowires and ribbons using a two-step approach in which we catalytically grow metal oxide nanostructures followed by in situ carburization. The growth mechanisms, microstructure, and initial electrical property measurements are discussed.

Heterogeneous integration of semiconducting and carbide nanowires on Si substrates

Integration of nanowires onto foreign substrates, and in functional devices, is widely recognized as a significant hurdle to further development of nanosystems based on quasi-one dimensional nanostructures. We describe methods for directly integrating relevant nanostructures on technologically relevant Si substrates using vapor phase synthesis of the nanowires. It is shown that ZnO nanowires may be directly integrated onto Si substrates containing patterned metal lines. Preferential growth from the edge of the metal lines has been achieved. We also show that growth of refractory transition metal carbides is also possible using catalytic growth. The electrical properties of such systems are also discussed. Finally, methods of integrating nanowires vertically on a Si substrate are also described.

Mo2C Nanowires and Ribbons on Si via Two-Step Vapor Phase Growth

Transition metal carbides are an interesting class of electronic materials owing to their high electrical conductivity at room temperature, which is only slightly lower than that of their constituent transition metal elements. For example, the room temperature electrical resistivity of bulk Mo2 C is ∼70 μΩ-cm compared to that of Mo (4.85 μΩ-cm), whereas that of NbC is ∼50 μΩ-cm as compared to 15.2 μΩ-cm for Nb. Indeed, the temperature dependent resistivity of many transition metal carbides suggests metallic-like conduction. Furthermore, certain transition metal carbides are known to become superconducting, with transition temperatures ranging from 1.15 °K for TiC1−x to 14 °K for NbC. [1] They are also able to withstand high temperatures and are chemically stable. Initial synthesis of metal carbide nanorods was demonstrated using the carbon nanotube (CNT) confined reaction mechanism by Lieber and co-workers [2] and subsequent superconducting behavior was shown by Fukunaga et al. [3]. Vapor-liquid-solid growth was employed by Johnson et al. [4] to synthesize micron-sized carbide whiskers. Here, we have successfully synthesized Mo2 C nanorods and ribbons on Si substrates using a novel two-step catalytic approach, which allows for synthesis of such high temperature nanostructures at manufacturable temperatures (≤ 1000 °C) and time scales (≤ 60 min). In the first step we utilize a catalytic vapor phase process to grow Mo and/or molybdenum oxide nanostructures, which are subsequently carburized in situ to form the desired Mo2 C nanostructures. Unlike true VLS growth of carbides, in which high temperature (≤ 1100–1200 °C) is required to adequately dissolve carbon into the catalyst particles, our strategy is to react the nanostructures along their entire length with a carbon vapor source after creating the oxide/metal nanostructures, which for Mo2 C can be achieved at relatively low temperatures. (≤ 1000 °C). The nanorods and ribbons are polycrystalline, with a mean grain size of 20–50 nm and 50–150 nm, respectively. We hypothesize that the growth mechanism is a complex mixture of VLS, VSS, and auto-catalytic growth, in which molten catalyst nanoparticles enter a three phase region once the metal precursor is supplied. The growth then presumably continues via a vapor-solid-solid process and is possible assisted by the presence of various molybdenum oxide species on the surface. Initial single nanowire electrical measurements yield a higher resistivity than in the bulk, which is attributed to the fine grain sizes and/or the presence of an oxide layer. A discussion of the growth mechanism will be presented along with issues relating to single nanowire device fabrication and control of nanowire orientation.Copyright

Diameter distribution of thermally evaporated indium metal islands on silicon substrates

Although many groups have studied the initial growth stages of various metals, including indium, there is little information in literature on diameter distributions of indium in relation to film thickness or annealing conditions. This paper reports island size distributions of thermally evaporated In islands on Si (100) and Si (111) substrates for nominal film thicknesses ranging from 5 to 50 nm. Because indium has a low melting temperature, and therefore a high homologous temperature at room temperature, 3-dimensional islands form during deposition with no subsequent heat treatments needed. Island diameters were calculated using commercial image analysis software in conjunction with SEM images of the samples. It is found that there is a bimodal island diameter distribution for nominal indium thicknesses greater than 5 nm. While the diameters of the larger islands increase exponentially with nominal thickness, those of the smaller islands increase linearly, and therefore more slowly, with nominal thickness. For nominal thickness of 50 nm, the average diameters of the small and large islands differ by almost an order of magnitude. Anneal conditions were studied in an attempt to narrow diameter distributions. Samples of each nominal thickness were annealed at temperatures ranging from 360°C to 550°C and the diameters again measured. The range of island diameters become narrower with 360°C anneal and volume average island diameter increases by ~30-50%. This narrowing of the distribution occurs due to smaller islands being absorbed by the larger in a process akin to Ostwald ripening, which is facilitated by higher surface diffusivities at higher homologous temperatures.

Selective Area Heteroepitaxy of Nano-AlGaN UV Excitation Sources for Biofluorescence Application

We report on the selective area heteroepitaxy and facet evolution of AlGaN nanostructures on GaN/sapphire substrate using various mask materials. We also report on the challenges associated with selection of an appropriate mask material for selective area heteroepitaxy of AlGaN with varying Al composition. The shape and the growth rate of the nanostructures are observed to be greatly affected by the mask material. The evolution of the AlGaN nanostructures and Al incorporation were studied exhaustively as a function of growth parameters; including temperature, pressure, NH3 flow, total alkyl flow and TMAl/(TMAl+TMGa) ratio. The growth rate of nanostructures was reduced drastically when higher Al percentage AlGaN nanostructures were grown. The growth rates were increased for higher Al percentage AlGaN using a surfactant which resulted in a high quality pyramidal structure. As indicated by high resolution x-ray diffraction (XRD) and cathodoluminescence (CL) spectroscopy, composition of Al in the AlGaN nanostructure is significantly different from that of a thin film grown under the same growth conditions.