Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers
TL;DR: In this paper, free standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography using electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers.
Abstract: Indirect evidence is presented that free‐standing Si quantum wires can be fabricated without the use of epitaxial deposition or lithography. The novel approach uses electrochemical and chemical dissolution steps to define networks of isolated wires out of bulk wafers. Mesoporous Si layers of high porosity exhibit visible (red) photoluminescence at room temperature, observable with the naked eye under <1 mW unfocused (<0.1 W cm−2) green or blue laser line excitation. This is attributed to dramatic two‐dimensional quantum size effects which can produce emission far above the band gap of bulk crystalline Si.
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TL;DR: In this paper, the authors discuss the interpretation of delayed electron emission from excited clusters as a statistical process analogous to thermionic emission from a hot filament and argue that transition state theory is not a good theoretical framework for electron emission.
Abstract: We discuss the interpretation of delayed electron emission from excited clusters as a statistical process analogous to thermionic emission from a hot filament. We argue that transition state theory is not a good theoretical framework for electron emission. Instead the calculation of emission rates may be based on detailed balance and theoretical or experimental cross sections for electron capture, but there can be large uncertainties in theoretical estimates of cross sections. We emphasize the conceptual simplicity obtained with the introduction of the microcanonical temperature. In experiments, the energy distribution is often so broad that it is essential to account for its modification by depletion, which for a very broad distribution leads to a decay rate inversely proportional to time. Another complication is photon emission, and we present estimates of the radiation intensity based on a simple model of a cluster as a sphere containing a gas of free electrons. In the analysis of experiments, we first discuss the information about cluster dynamics obtained from studies of photoelectron spectra. However, we focus mainly on a detailed analysis of measurement of the rate of delayed electron emission and its dependence on the cluster excitation. Often the parameters of a statistical description, derived from fits to measurements, have appeared to be inconsistent with estimates from theory or from independent experiments. We analyse a measurement of laser-induced electron emission from small Nb clusters and find that inclusion of anharmonic effects in the heat capacity and, even more important, of the competition by radiative decay leads to more reasonable parameters in the statistical description than obtained from the original analysis. The most detailed studies have been performed for fullerene anions. For most of the measurements, radiative cooling is not significant, but it is important to take into account the finite width of the energy distribution, deriving from the initial heating in an oven. Measured cross sections for electron attachment can be applied in lifetime calculations, and an improved analysis leads to the conclusion that the experiments are consistent with the interpretation of electron emission as thermionic emission.
49 citations
TL;DR: A silicon (Si) quantum dot (QD)-based hybrid inorganic/organic light-emitting diode (LED) was fabricated via solution processing as discussed by the authors, which exhibited white-blue electroluminescence at a low applied voltage of 6 V, with 78% of the effective emission obtained from the Si QDs.
Abstract: A silicon (Si) quantum dot (QD)-based hybrid inorganic/organic light-emitting diode (LED) was fabricated via solution processing This device exhibited white-blue electroluminescence at a low applied voltage of 6 V, with 78% of the effective emission obtained from the Si QDs This hybrid LED produced current and optical power densities 280 and 350 times greater than those previously reported for such device The superior performance of this hybrid device was obtained by both the prepared Si QDs and the optimized layer structure and thereby improving carrier migration through the hybrid LED and carrier recombination in the homogeneous Si QD layer
49 citations
TL;DR: In this article, the formation and evolution of ridge-trench morphology is explained by the presence of two different etch rates, an enhanced etch rate which generates the porous network and a slower etch process that leads to the terraces of the ridge morphology.
Abstract: Porous gallium nitride (PGaN) is produced by Pt-assisted electroless etching of hydride vapor phase epitaxy (HVPE)–GaN. Ultrathin Pt films are sputtered onto the GaN surface, and etching is carried out in a 1:2:1 solution of CH3OH:HF:H2O2. The evolution of the morphology proceeds by first forming a network of small pores, after which a ridge-trench morphology evolves, with ridges separated by a porous network in trenches between the ridges. As the etch progresses further the ridges evolve to a maximum size and then start to disappear. The formation and evolution of the ridge-trench morphology is explained by the presence of two different etch rates, an enhanced etch rate which generates the porous network and a slower etch rate that leads to the terraces of the ridge morphology. The rate at which the morphology evolves depends on the carrier concentration, with more heavily doped samples etching faster. In all cases, the final depth of the trenches between ridges is independent on the thickness of the sta...
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TL;DR: In this article, the conduction properties of porous silicon films were investigated using hard-ultraviolet/X-ray synchrotron radiation, and it was shown that the conductivity of these films is temperature-dependent and that the films become insulating at low temperatures.
Abstract: Silicon shows photo- and electroluminescence at visible wavelengths when chemically etched into a microporous network of ‘wires’ several nanometres thick1. This raises the possibility of a silicon-based optoelectronic technology. The luminescence properties may be understood on the basis of the injection or creation of electrons and holes in the interconnected network of wires which recombine radiatively with high efficiency1,2. Elucidating the electron-transport mechanisms has been held back by several difficulties, particularly that of making stable, high-quality contacts to the porous material. Here we report experiments that probe the conduction process using photoemission stimulated by hard-ultraviolet/X-ray synchrotron radiation, obviating the need for good electrical contacts. We find that the conductivity of porous silicon films is temperature-dependent, and that the films become insulating at low temperatures. We suggest that these results may be understood in terms of a percolation process occurring through sites in the porous network in which conductivity is thermally activated, and we postulate that this activation may be the consequence of a Coulomb blockade effect3,4 in the nanoscale channels of the film. This is consistent with our observation of optical ‘unblocking’ of conducting pathways. These results imply that the size distribution of the nanowires in the silicon backbone plays a key role in determining the conduction properties, and that porous-silicon light-emitting diodes may use only a small (and the least efficient) fraction of the material. Improvements in electroluminescence efficiency may be possible by taking into account the percolative nature of the conduction process.
49 citations
TL;DR: In this article, Raman and photoluminescence measurements in Si/Si1−xGex nanostructures grown by molecular-beam epitaxy under conditions of near Stranski-Krastanov (S-K) growth mode were performed.
Abstract: We report detailed Raman and photoluminescence (PL) measurements in Si/Si1−xGex nanostructures grown by molecular-beam epitaxy under conditions of near Stranski–Krastanov (S-K) growth mode. In a series of samples with x controllably increased from 0.098 to 0.53, we observe that an increase in Raman signal related to Ge–Ge vibrations clearly correlates with (i) a redshift in the PL peak position, (ii) an increase in the activation energy of PL thermal quenching, and (iii) an increase in the PL quantum efficiency. The results indicate that in S-K Si/Si1−xGex nanostructures with x>0.5 Ge atoms form nanometer-sized clusters with a nearly pure Ge core and a SiGe shell.
49 citations
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TL;DR: In this article, the properties of electrolyte-semiconductor barriers are described, with emphasis on germanium, and the use of these barriers in localizing electrolytic etching is discussed.
Abstract: Properties of electrolyte-semiconductor barriers are described, with emphasis on germanium. The use of these barriers in localizing electrolytic etching is discussed. Other localization techniques are mentioned. Electrolytes for etching germanium and silicon are given.
1,039 citations
TL;DR: It is found that a standard, widespread, chemical-preparation method for silicon, oxidation followed by an HF etch, results in a surface which from an electronic point of view is remarkably inactive, which has implications for the ultimate efficiency of silicon solar cells.
Abstract: We have found that a standard, widespread, chemical-preparation method for silicon, oxidation followed by an HF etch, results in a surface which from an electronic point of view is remarkably inactive. With preparation in this manner, the surface-recombination velocity on Si111g is only 0.25 cm/sec, which is the lowest value ever reported for any semiconductor. Multiple-internal-reflection infrared spectroscopy shows that the surface appears to be covered by covalent Si-H bonds, leaving virtually no surface dangling bonds to act as recombinatiuon centers. These results have implications for the ultimate efficiency of silicon solar cells.
910 citations
TL;DR: In this paper, multiple internal infrared reflection spectroscopy has been used to identify the chemical nature of chemically oxidized and subsequently HF stripped silicon surfaces, and these very inert surfaces are found to be almost completely covered by atomic hydrogen.
Abstract: Multiple internal infrared reflection spectroscopy has been used to identify the chemical nature of chemically oxidized and subsequently HF stripped silicon surfaces. These very inert surfaces are found to be almost completely covered by atomic hydrogen. Results using polarized radiation on both flat and stepped Si(111) and Si(100) surfaces reveal the presence of many chemisorption sites (hydrides) that indicate that the surfaces are microscopically rough, although locally ordered. In particular, the HF‐prepared Si(100) surface appears to have little in common with the smooth H‐saturated Si(100) surface prepared in ultrahigh vacuum.
588 citations
TL;DR: In this article, the authors measured hydrogen desorption from monohydride and dihydride species on crystalline-silicon surfaces using transmission Fourier-transform infrared (FTIR) spectroscopy.
Abstract: Hydrogen desorption kinetics from monohydride and dihydride species on crystalline-silicon surfaces were measured using transmission Fourier-transform infrared (FTIR) spectroscopy. The FTIR desorption measurements were performed in situ in an ultrahigh-vacuum chamber using high-surface-area porous-silicon samples. The kinetics for hydrogen desorption from the monohydride and dihydride species was monitored using the SiH stretch mode at 2102 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the ${\mathrm{SiH}}_{2}$ scissors mode at 910 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. Annealing studies revealed that hydrogen from the ${\mathrm{SiH}}_{2}$ species desorbed between 640 and 700 K, whereas hydrogen from the SiH species desorbed between 720 and 800 K. Isothermal studies revealed second-order hydrogen desorption kinetics for both the monohydride and dihydride surface species. Desorption activation barriers of 65 kcal/mol (2.82 eV) and 43 kcal/mol (1.86 eV) were measured for the monohydride and dihydride species, respectively. These desorption activation barriers yield upper limits of 84.6 kcal/mol (3.67 eV) and 73.6 kcal/mol (3.19 eV) for the Si-H chemical bond energies of the SiH and ${\mathrm{SiH}}_{2}$ surface species.
479 citations