Visible and near-infrared photoluminescence (PL) at room temperature is reported from Si nanowires (NWs) grown by chemical vapor deposition from TiSi2 catalyst sites. NWs grown with average diameter of 20 nm were etched and oxidized to thin and passivate the wires. The PL emission blue shifted continuously with decreasing nanowire diameter. Slowed oxidation was observed for small nanowire diameters and provides a high degree of control over the emission wavelength. Transmission electron microscopy, PL, and time-resolved PL data are fully consistent with quantum confinement of charge carriers in the Si nanowire core being the source of luminescence. These light emitting nanowires could find application in future CMOS-compatible photonic devices.

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The concept of a nanowire solar cell with photon-harvesting shells is presented. In this architecture, organic molecules which absorb strongly in the near infrared where silicon absorbs weakly are coupled to silicon nanowires ͑SiNWs͒. This enables an array of 7-m-long nanowires with a diameter of 50 nm to absorb over 85% of the photons above the bandgap of silicon. The organic molecules are bonded to the surface of the SiNWs forming a thin shell. They absorb the low-energy photons and subsequently transfer the energy to the SiNWs via Förster resonant energy transfer, creating free electrons and holes within the SiNWs. The carriers are then separated at a radial p-n junction in a nanowire and extracted at the respective electrodes. The shortness of the nanowires is expected to lower the dark current due to the decrease in p-n junction surface area, which scales linearly with wire length. The theoretical power conversion efficiency is 15%. To demonstrate this concept, we measure a 60% increase in photocurrent from a planar silicon-on-insulator diode when a 5 nm layer of poly͓2-methoxy-5-͑2Ј-ethyl-hexyloxy͒-1,4-phenylene vinylene is applied to the surface of the silicon. This increase is in excellent agreement with theoretical predictions.

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Observation of Purcell enhancement, stimulated emission and non-linear optical effects in microresonators depend critically on their quality factors (Q) and mode volumes (V). Ideally, a high value of Q/V ratio is desired. However, in most systems, there is a trade off between achieving a higher Q and reducing the size of the resonator (small V). For resonators containing Si nanocrystals (nc-Si), the Q-limiting optical loss mechanisms are: (a) the radiation loss resulting from low indices of refraction of the host material, (b) the sidewall scattering loss and (c) the material absorption. Based on these considerations, the aim of the present work is to experimentally characterize the quality factors and mode volumes of microdisk resonators containing luminescent silicon nanocrystals. This study helps us estimate the dimensions of optimal structures needed to observe the abovementioned effects (in particular, the Purcell effect) for nc-Si in the visible wavelength range. We report fabrication and characterization of micron sized, nc-Si coated, silicon nitride microdisk resonators with diameters ranging from 2 µm to 10 µm. Consistent with ray-optics and finite element simulations (FEM), small (diameter < 3 µm) microdisks appear to be limited by radiation loss whereas larger microdisks suffer from material absorption/sidewall scattering loss. These two competing mechanisms lead to a parameter window which achieves maximum Q/V for our system. We use these experimentally obtained values to predict the range of maximum Purcell enhancement achievable in our structures.Study of Si nanocrystal photoemission in microcavities is motivated by an effort to understand the behavior of nanostructured Si active medium in realistic laser structures. Such studies allow us to look into mechanisms contributing to the optical loss and thereby enable design of better cavities. In addition, microcavities also alter the spontaneous emission behavior of the embedded quantum dots by changing the local density of optical states -commonly known as Purcell effect [1]. Such an alteration of radiative rate allows for an individual determination of radiative and non-radiative decay rates thereby yielding photoemission quantum efficiency for individual dots [2]. Furthermore, as evidenced recently by time resolved variable stripe length experiments [3], there is a stiff competition between stimulated emission and non radiative Auger recombination on a time scale of tens of nanoseconds. Microcavities have an effect of enhancing stimulated emission rate by increasing the signal photon density in the vicinity of emitters [4]. Embedding Si nanocrystals in microcavities can also provide a clue as to whether it is possible to tilt the optical response in favor of stimulated emission over Auger processes.As a first step toward enabling the abovementioned studies, we report on fabrication and characterization of dielectric microcavities containing nc-Si active medium. We specifically consider the operation of silicon nitride microdisk resonators (coated wi...

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The Ga + Focused Ion Beam (FIB) is a very versatile tool which, although established as the preeminent tool for microcircuit edit, has recently evolved into the preferred TEM sample preparation tool for site specific applications. However, the applications of FIB extend far beyond the tasks for which FIB is most commonly employed. FIB can be utilized for micro and nano structural creation and/or modification. This can be accomplished by taking advantage of FIB's capability to very precisely remove material (via physical sputtering or using beam induced chemistry for enhancing material removal rate), to deposit material (using beam induced chemistry), to provide localized ion implantation, and to locally induce sample structural damage. In addition to the above capabilities, FIB provides the ability to image the sample via secondary electrons or ions before, after and during micromachining. In many cases, the ability to image while removing or depositing material provides invaluable feedback for process evaluation and control.In this presentation, an overview of FIB will be presented with emphasis on non traditional applications. Examples presented will include a wide range of FIB based nano and micro fabrication techniques including stamp/mold fabrication (Fig. 1), shaping of a diamond indenter tool (Fig. 2), AFM tip characterization structure fabrication (Fig. 3), sharpening of an STM tip (Figure 4), MEMS device cross section (Fig. 5), etc.In some instances where micro or nano material removal or deposition is required, it is possible to perform these tasks using an electron rather than an ion beam. This can be especially advantageous when the Ga "stain" left behind during all FIB processes is deleterious to the desired result e.g. during EUV mask repair. The electron beam when use in conjunction with appropriate chemical precursors, can initiate chemical reactions resulting in either material removal or material deposition. Examples will be presented showing the ability to precisely remove and deposit materials using an electron beam.In summary, FIB is a versatile tool that combines both nanofabrication and microscopy capabilities.