Silicon nanowires of different widths were fabricated in silicon on insulator (SOI) material using conventional process technology combined with electron-beam lithography. The aim was to analyze the size dependence of the sensitivity of such nanowires for biomolecule detection and for other sensor applications. Results from electrical characterization of the nanowires show a threshold voltage increasing with decreasing width. When immersed in an acidic buffer solution, smaller nanowires exhibit large conductance changes while larger wires remain unaffected. This behavior is also reflected in detected threshold shifts between buffer solutions of different pH, and we find that nanowires of width >150 nm are virtually insensitive to the buffer pH. The increased sensitivity for smaller sizes is ascribed to the larger surface/volume ratio for smaller wires exposing the channel to a more effective control by the local environment, similar to a surrounded gate transistor structure. Computer simulations confirm this behavior and show that sensing can be extended even down to the single charge level.

Single-dot luminescence spectroscopy was used to study the emission linewidth of individual silicon nanocrystals from low temperatures up to room temperature. The results show a continuous line narrowing towards lower temperatures with a linewidth as sharp as 2 meV at 35 K. This value, clearly below the thermal broadening at this temperature, proves the atomiclike emission from silicon quantum dots subject to quantum confinement. The low temperature measurements further reveal a 6 meV replica, whose origin is discussed. In addition, an 60 meV TO-phonon replica was detected, which is only present in a fraction of the dots. DOI: 10.1103/PhysRevLett.94.087405 PACS numbers: 78.67.Hc, 78.55.Ap Ideal semiconductor quantum dots have specific discrete, atomiclike energy levels yielding sharp emission lines for optical transitions between excited and ground states. As a consequence, the emission linewidth may be much smaller than the thermal energy k B T. This has, indeed, been observed for nanocrystals of direct band gap semiconductor materials of type II-VI [1] and III-V, usually grown epitaxially by self-assembling techniques. The homogeneous linewidth of a quantum dot is one of its most important parameters where the temperature dependence reflects unique dephasing mechanisms of quantum states by phonons [2]. The sharp spectral features of individual quantum dots are, however, normally smeared because of inhomogeneous line broadening resulting from measurements on large ensembles of quantum dots. Here, individual size variations give rise to large variations of emission energies as a direct consequence of quantum confinement. Recent developments in microspectroscopy techniques have, however, enabled studies of individual quantum dots yielding ultranarrow emission linewidths [1].The luminescent properties of silicon nanocrystals were first explored in porous Si [3] and were later studied in Si nanocrystals formed by various techniques, mostly resulting in nanocrystals passivated by silicon dioxide [4]. The observed spectra were usually very broad, a few hundred meV even at low temperatures, which was generally attributed to the inhomogeneous line broadening. Narrower photoluminescence (PL) bands were demonstrated using size selection methods subsequent to the fabrication step [5]. More detailed information could, however, be achieved using selective spectroscopy where resonant excitation deep in the PL band was employed [6] or by spectral hole burning [7]. This revealed relatively narrow emission lines and both no-phonon and phonon-assisted transitions. Still, an ensemble of quantum dots was addressed rather than a single nanocrystal, screening valuable information on its level structure, emission band, and possible individual variations from dot to dot.In our previous work [8] we successfully used single-dot spectroscopy to obtain PL spectra of individual nanocrystals at room temperature. The main difficulties were the low emission rate and adequate separation of the nanocrystals in order to use far-field lumines...

Photoluminescence (PL) from single silicon quantum dots have been recorded and spectrally resolved at room temperature. The Si nanocrystals (NCs) were fabricated using electron-beam lithography and reactive ion etching resulting in Si nanopillars that were subsequently oxidized to produce luminescent silicon cores. The NCs are organized in a regular matrix which enables repeated observation of a specific single NC. By reflection and PL imaging, the emission is shown to originate from the Si nanopillars. The single-NC PL spectrum has a single band with a width of ∼130 meV. The emission is polarized in arbitrary directions suggestive of geometrical differences in the shape of the nanocrystals. The quantum efficiency of the PL has been found to reach as much as 35% for some nanocrystals. Our experiments support the quantum-confinement model for the PL emission of Si nanocrystals and elucidate the critical role of defect passivation.

We demonstrate that electrochemical size reduction can be used for precisely controlled fabrication of silicon nanowires of widths approaching the 10 nm regime. The scheme can, in principle, be applied to wires defined by optical lithography but is here demonstrated for wires of approximately 100-200 nm width, defined by electron beam lithography. As for electrochemical etching of bulk silicon, the etching can be tuned both to the pore formation regime as well as to electropolishing. By in-situ optical and electrical characterization, the process can be halted at a certain nanowire width. Further electrical characterization shows a conductance decreasing faster than dimensional scaling would predict. As an explanation, we propose that charged surface states play a more pronounced role as the nanowire cross-sectional dimensions decrease.

This letter describes the promising technique of micromachining using the properties of electrochemical etching of (100)-oriented n-type silicon in a hydrofluoric acid electrolyte. The technique is based on electropolishing of a wafer except for areas where vertical structures are needed and does not require a periodic pattern. Predefined steps of a few microns depth prior to the electrochemical etching define the shape and position of the structures. The three-dimensional microstructure width can be adjusted with the etching parameters, also enabling the formation of free-standing structures. The feasibility of this technique is demonstrated by forming high aspect ratio microneedles and tubes.