Free-standing Si Research Articles

Photoluminescence and Raman Spectroscopy Study on Oxidized Free-Standing Porous Si

Photoluminescence and Raman scattering spectra were used to study three types of free-standing porous Si: as-grown samples with various porosities referred to as APS samples, these samples oxidized in air at 200 degreesC fur 200 h as OPS samples, and these samples aged further in air at room temperature for 20 months as AOPS samples. The PL peak energies of OPS and AOPS samples have shifted in small energy ranges centered at about 1.61 and 1.59 eV, respectively. Each Raman spectrum was fitted with an amorphous and a crystalline component. The crystalline component was used to determine the sizes of nanometer Si particles (NSPs), which were in ranges of 2.5-2.2 and 3.3-2.3 nm in OPS and AOPS samples, respectively. The reason why OPS or AOPS samples with different NSP sizes have almost the same FL peak wavelength is discussed with a novel multiple mechanism model.

Gas-source MBE growth of freestanding Si nano-wires on Au/Si substrate

Freestanding Si nano-wires were grown by gas source molecular beam epitaxy on Au/Si (100), (111), (110), and (113) substrates. The morphology of Si nano-wires is investigated using scanning electron microscopy. This method provides a possible way to fabricate silicon quantum wires.

Ternary Ta [sbnd]Si [sbnd]N films for sensors and actuators

Ta Si N films having thicknesses up to several micrometres are investigated in view of their use in sensors and actuators. The films are deposited by reactive sputtering in an Ar/N 2 plasma employing a Ta 5Si 4 target. In addition to their significant electrical characteristics, these ternary film materials also exhibit excellent mechanical properties. On X-ray-amorphous layers a surface roughness of less than 2 nm has been measured. A crystallization temperature exceeding 800°C has been observed by X-ray diffraction analysis. Using Al as a sacrificial layer, the fabrication of freestanding Ta Si N surface microstructures is demonstrated and their application in microelectromechanical transducers is discussed.

Writing FIB implantation and subsequent anisotropic wet chemical etching for fabrication of 3D structures in silicon

The further miniaturization of silicon micromechanical structures in combination with the highly developed microelectronic technology at the micrometre and sub-micrometre level will lead to a new generation of microdevices. A modern technique to fabricate three-dimensional micromechanical structures is the combination of high-concentration p′ -doping by writing ion implantation using a focused ion beam (FIB) and subsequent anisotropic and selective wet chemical etching. FIB-patterned and chemically etched 3D Si structures with nanoscale thickness have been fabricated using 35 keV Ga + ion implantation and subsequent anisotropic etching in KOH/H 2O solution. Design and fabrication considerations to achieve freestanding Si structures are discussed and some typical structures are shown.

Femtosecond dephasing in porous silicon

We have performed non-degenerate four-wave mixing on free-standing porous Si samples. Within the framework of an idealized inhomogeneously broadened two-level system, values of 20 fs for the dephasing time are obtained for different porosities. Fast energy relaxation times are found in the range 1.5–3.5 ps for 64–73% porosities, respectively. They are attributed to carrier thermalization within the band tails, originating from porous silicon nanocrystallites and their surface states. © 1997 Elsevier Science S.A.

AFM-based fabrication of free-standing Si nanostructures

AFM-generated surface modifications are used to fabricate free-standing Si nanostructures. We employ the local electric field of a metal-coated AFM tip which is operated in air to selectively oxidize regions of a H-passivated Si surface. The resulting oxide, thick, is used as a mask for deep selective etches of the unoxidized regions of Si. The etched structures reside on a buried oxide layer which is removed to produce free-standing Si wires and cantilevers. Due to the uniformity of the exposure and self-limiting etch processes, these structures are extremely uniform, which is a critical feature for nanometer-scale device applications.

Absorption and luminescence studies of free-standing porous silicon films.

The result of a combined study of the absorption and photoluminescence (PL) from high-optical-quality free-standing porous Si films is presented. These films, which have a scattering loss less than 10%, allow unambiguous, detailed study of the system's absorption edge and, in addition enable us to correlate the absorption and emission behavior on the same sample. H-ion scattering and H forward recoil data are used to accurately determine the bulk equivalent thickness to allow extraction of the absorption coefficient. X-ray diffraction, in conjunction with transmission electron microscopy, are utilized to measure the lattice constant and to confirm the crystalline nature of the structure. The combined photoluminescence and absorption studies are used to examine the relative energy shift of the luminescence peak with respect to that of the absorption edge. It is shown that the absorption begins coincident in energy with the photoluminescence peak, rising smoothly over a range of 1--2 eV. More importantly, the absorption is very weak in the region of the PL in comparison to a direct-gap semiconductor, indicative of the intrinsically weak oscillator strength. An effective-mass-based model for an ensemble of Si nanocrystallites is compared to the experimental results. Phonon-assisted transitions dominate the optical processes. The results support the picture that the absorption derives primarily from the interior states of nanocrystalline Si particles of characteristic dimension below 100 \AA{}. The dynamics of the photoluminescence are reexamined and found to be consistent either with recombination between interior electron and hole states, of the Si nanocrystallites, or involving shallow trap states.

Microstructure and optical properties of free-standing porous silicon films: Size dependence of absorption spectra in Si nanometer-sized crystallites.

We have studied the microstructure and optical properties of free-standing porous Si thin films fabricated by electrochemical anodization. Raman-spectroscopy and transmission-electron-microscopy examinations show that Si crystallite spheres with diameters of several nanometers are dispersed in the amorphous phase. The blueshift of the optical-absorption spectrum is observed for decreasing average diameter of the Si crystallites. However, there is no clear size dependence of the peak energy of the broad photoluminescence (PL) spectrum. Spectroscopic analysis strongly suggests that the photogeneration of carriers occurs in the c-Si core, whose band gap is modified by the quantum-confinement effect, while the strong PL comes from the near-surface region of small crystallites.

Interpretation of photoluminescence excitation spectroscopy of porous Si layers

Photoluminescence excitation (PLE) measurements allow one to obtain direct information on the absorption processes in porous Si leading to the strong visible luminescence which has recently been reported. This technique does not need a free-standing porous Si sample, but the effect of layer thickness should be included in the interpretation of PLE data. In our PLE spectra, two separate absorption edges are observed and the initial one shifts to higher energies with storage time. The trends of these changes are consistent with the quantum wire model.

Optical Properties of Free-Standing Porous Si Films

AbstractWe have studied optical properties of free-standing porous Si thin films fabricated by electrochemical anodization. The average diameter of Si crystallite spheres is evaluated by Raman spectroscopy and transmission electron microscopy. The blueshift of optical absorption spectrum is observed with a decrease in the average diameter of Si crystallites. However, there is no clear size dependence of the peak energy of broad photoluminescence spectrum. Spectroscopic analysis strongly suggests that the photogeneration of carriers occurs in the c-Si core whose band gap is modified by the quantum confinement effect, whereas the radiative recombination of carriers occurs in the near-surface region of small crystallites.

Optical properties of free-standing silicon quantum wires

We present theoretical studies of the optical properties of free-standing Si quantum wires, using an empirical tight-binding model which includes d orbitals and second-neighbor interactions. The excitonic effects are included within the effective-mass approximation. The predicted exciton transition energy for a quantum wire with a width of 27 Å agrees with the observed luminescence for a sample of similar size. We found that the thermally averaged exciton oscillator strengths for quantum wires with widths around 8 Å can become comparable to that of bulk GaAs.

Theory of optical properties of quantum wires in porous silicon.

We present theoretical studies of the electronic and optical properties of free-standing Si quantum wires which exist in porous Si. We use a second-neighbor empirical tight-binding model which includes d orbitals and spin-orbit interaction. The excitonic effects are included within the effective-mass approximation. We found that for narrow quantum wires with widths around 8 A\r{}, the averaged exciton oscillator strength is comparable to that of bulk GaAs. However, the average exciton oscillator strength decreases dramatically (faster than 1/${\mathit{L}}^{5}$) as the quantum-wire width L increases. The radiative lifetimes of excitons in quantum wires are estimated and we find that the lifetime of the shortest-lived exciton ranges from 57 ns to 170 \ensuremath{\mu}s for wire widths from 7.7 to 31 A\r{}. We have also calculated the absorption spectra and found strong polarization dependence.