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The structure of [(Si) m /(Ge) n ] p superlattice interfaces and the onset of relaxation and interdiffusion initiated by annealing have been investigated with the use of extended x-ray-absorption fine structure and Raman scattering. For the as-grown material, the Ge-Ge bond length (0.2409 nm) was equal to that calculated for a fully strained Ge layer (0.2412 nm) while the Si-Ge bond length was significantly shorter. The results show conclusively that substantial intermixing along with partial relaxation of the Ge-Ge bonds occurs even for the shortest anneal at 700°C

Ge K-edge extended x-ray-absorption fine-structure (EXAFS) spectra of a series of strained-layer [(Si${)}_{\mathit{m}}$/(Ge${)}_{\mathit{n}}$${]}_{\mathit{p}}$ superlattices grown by molecular-beam epitaxy on the 〈100〉 face of single-crystal Si have provided a determination of the degree of intermixing and the extent of relaxation as a function of the thickness of the Ge layer. The results are obtained with use of constrained simultaneous nonlinear least-squares curve fits to multiple data files from different samples. This procedure, along with associated analysis of the fit surface, has improved the reliability of the analysis. We discuss the general applicability of simultaneous analysis procedures to EXAFS analysis of a series of related samples, such as those spanning compositional variation or thermal processing.

The measurement of the strain coefficients for the three longitudinal-optical phonons in strained ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ (0x0.35) molecular-beam-epitaxy-grown epilayers on (100) Si is reported. Strain in the epilayers was varied by annealing metastable pseudomorphically grown epilayers at various temperatures and was measured by double-crystal x-ray diffractometry. The strain-shift coefficients for the phonons were obtained from Raman-scattering measurements of the annealed specimens. For all compositions it was found that the Si-Si phonon frequencies vary linearly with strain. The strain-shift coefficient, however, showed a small composition dependence, varying from about -750 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at x=0.08 to about -950 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ at x=0.35, corresponding to a stress factor \ensuremath{\tau}\ensuremath{\approxeq}0.4+0.57x+0.13${\mathit{x}}^{2}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$/kbar for this vibration. The magnitude of the corresponding coefficients for the Si-Ge and Ge-Ge lines are slightly less than that of the Si-Si line, and vary in a similar way with x.

High-resolution Raman spectroscopy of thick layer Si/Si 1− x Ge x superlattices has revealed the folded acoustic modes expected at frequencies below the Brillouin line, which is near 5 cm −1 in these structures. However, the measurements unexpectedly revealed much more-complicated fine structure, which is thought to arise from structural irregularities.

This work focuses on improving the crystalline property of the LaS thin films by improving the deposition conditions. Many factors can affect the quality of laser deposited films, including vacuum level, laser energy, laser repetition rate, substrate temperature, target to substrate distance, background gas pressure, and substrate bias., 18th International Vacuum Nanoelectronics Conference, 2005

In three-dimensional cluster morphology multilayer Si/SiGe nanostructures, we find an anticorrelation between photoluminescence (PL) originating from SiGe clusters and the PL associated with electron-hole droplets (EHDs) localized within nanometer-thick Si separating layers. We show that Auger processes eject holes from SiGe clusters and facilitate the exciton/EHD phase transition in the Si layers. An unusual $N$-shaped SiGe cluster PL decay curve is attributed to the reverse EHD/electron-hole gas phase transition and carrier redistribution between Si spacer layers and SiGe clusters.

The many and diverse approaches to materials science problems have greatly enhanced our ability in recent times to engineer the physical properties of semiconductors. Silicon, of all semiconductors, underpins nearly all microelectronics today and will continue to do so for some time to come. However, in optoelectronics, the severe disadvantage of an indirect band gap has limited the application of elemental silicon. Here we review a number of diverse approaches to engineering efficient light emission in silicon nanostructures. These different approaches are placed in context and their prospects for application in silicon-based optoelectronics are assessed., Optical Properties of Nanocrystals, 9-11 July 2002, Seattle, Washington, USA, Series: Proceedings of SPIE

Silicon–germanium epitaxially grown on silicon in the form of two-dimensional (quantum wells) and three-dimensional (quantum dots) nanostructures exhibits photoluminescence and electroluminescence in the technologically important spectral range of 1.3–1.6 μm. Until recently, the major roadblocks for practical applications of these devices were strong thermal quenching of the luminescence quantum efficiency, and a long carrier radiative lifetime. This paper summarizes recent progress in the understanding of carrier recombination in Si/SiGe nanostructures and presents a potential new route toward CMOS compatible light emitters for on-chip optical interconnects.

Germanium nanocrystals (NCs) were formed by in-situ thermal annealing of an amorphous Ge layer deposited by molecular beam epitaxy on a thin SiO2 layer on Si(001). The Ge NCs were then capped in situ with a thin layer of amorphous Si to prevent oxidation. For samples with average NC diameters ranging from 2.5 to 60 nm, the NC photoluminescence (PL) appeared primarily as a wide near-infrared band peaked near 800 meV. Using both the k•p and tight binding theoretical models, we have analyzed the PL spectrum in terms of the NC size distribution required to reproduce the observed asymmetric band shape, which includes, for the smaller diameter NCs, a band gap enlargement due to quantum confinement. The peak energy of the PL band reflects the average NC size and its shape depends on the NC size distribution. The observed size distribution determined from transmission electron microscopy analysis allowed the determination of the nonlinear increase in the PL quantum efficiency with decreasing NC diameter. Alternatively, given a good theoretical description of the system, it is thus possible to evaluate the size distribution of semiconductor NCs from their PL energy dependence.

The understanding of the epitaxy of pure Ge layers on Si is an important step towards the synthesis of SimGen (m, n n ([Formula: see text] monolayers) films on (100) Si by molecular beam epitaxy and studied their structural properties by a variety of techniques including Raman scattering spectroscopy, glancing incidence X-ray reflection, Rutherford backscattering, transmission electron microscopy, and extended X-ray absorption fine structure analysis. All these techniques allowed detection of the thin Ge layers and provided information about the thickness, morphology, strain distribution, and interface sharpness of these heterostructures. The Ge„ films with [Formula: see text] had a two-dimensional nature and showed no sign of strain relaxation. Intermixing at the Si–Ge interfaces was present in all these films and estimated to be not more than two monolayers. This smearing at the interfaces may have contributed to the maintenance of that pseudomorphicity. A thicker Ge layer (n = 12) showed evidence of strain relaxation and clustering in three-dimensional islands.

We report on the interface quality and phonon-assisted tunneling in nanocrystalline Si (nc-Si)/amorphous SiO2 (a-SiO2) superlattices (SLs) prepared by magnetron sputtering and thermal crystallization of nanometer-thick a-Si layers. Phonon-assisted tunneling is observed in a bipolar nc-Si based structure, which confirms that the nc-Si/a-SiO2 junction is not only abrupt but also nearly defect free. The conclusion is supported by capacitance–voltage measurements from which the estimated interface defect density is found to be ∼1011 cm−2 for an eight-period SL. Such high quality interfaces hold considerable promise for the development of nc-Si SL quantum devices.

We report exact diagonalization studies of inelastic light scattering in few-electron quantum dots under the strong confinement regime characteristic of self-assembled dots. We apply the orthodox (second-order) theory for scattering due to electronic excitations, leaving for the future the consideration of higher-order effects in the formalism (phonons, for example), which seem relevant in the theoretical description of available experiments. Our numerical results stress the dominance of monopole peaks in Raman spectra and the breakdown of selection rules in open-shell dots. The dependence of these spectra on the number of electrons in the dot and the incident photon energy is explicitly shown. Qualitative comparisons are made with recent experimental results., 11 pages, 11 figures

In opto-electronics and photonics, the severe disadvantage of an indirect band gap has limited the application of elemental silicon. Amongst a number of diverse approaches to engineering efficient light emission in silicon nanostructures, one system that has received considerable attention has been Si/SiO 2 quantum wells. Engineering such structures has not been easy, because to observe the desired quantum confinement effects, the quantum well thickness has to be less than 5 nm. Nevertheless, such nanometer thick structures have now been produced by a variety of techniques. The SiO 2 layers are amorphous, but the silicon layers can range from amorphous through nanocrystalline to single-crystal form. The fundamental band gap of the quantum wells has been measured primarily by optical techniques and strong confinement effects have been observed. A number of theories based primarily on ab initio approaches have been developed to explain these results with varying degrees of success. In this review, a detailed comparison is made between theoretical and experimental determinations of the band gap in Si/SiO 2 quantum wells.

The general problem of the pairing of strongly interacting elementary excitations producing new quasiparticles such as polarons arises in many areas of solid state physics. Recent interest in polaron formation in semiconductor quantum dots has been motivated by the need to understand the physical nature of the carrier relaxation processes and their role in quantum-dot based devices. We report on the direct observation of polarons in InAs/GaAs self-assembled quantum dots populated by few electrons where the polarons are strongly coupled modes of quantum dot phonons and electron intersublevel transitions. The degree of coupling is varied in a systematic way in a set of samples having electron intersublevel spacing changing from larger to smaller than the longitudinal optical phonon energy. The signature of polarons is evidenced clearly by the observation of a large (12–20 meV) anticrossing for both InAs and GaAs-like quantum dot phonons using resonant Raman spectroscopy.

Germanium nanocrystals (NCs) were formed by in-situ thermal annealing of an amorphous Ge layer deposited by molecular beam epitaxy on a thin SiO2 layer on Si(001). The Ge NCs were then capped in situ with a thin layer of amorphous Si to prevent oxidation. For the present range of particle sizes (2.5 to 60 nm), the NC photoluminescence (PL) appeared primarily as a wide near-infrared band peaked near 800 meV. The peak energy of the PL band reflects the average NC size and its shape depends on the NC size distribution. Using both the k·p and tight binding theoretical models, we have analyzed the PL spectrum in terms of the NC size distribution required to reproduce the observed asymmetric band shape, which includes, for the smaller diameter NCs, a band gap enlargement due to quantum confinement. The observed size distribution determined from transmission electron microscopy analysis allowed the determination of the nonlinear increase in the PL quantum efficiency with decreasing NC diameter. This implies that, given a good theoretical description of the system, it is possible to evaluate the size distribution of semiconductor NCs from their PL energy dependence., 2008 MRS Fall Meeting, December 1-5, 2008, Boston, MA, USA, Series: Materials Research Society Symposium Proceedings; no. 1145

Three-dimensional SiGe nanostructures grown on Si using molecular beam epitaxy exhibit photoluminescence (PL) in the important spectral range of 1.3–1.6 μm. At a higher level of photo-excitation, thermal quenching of the PL intensity is suppressed and the previously accepted type II energy band alignment at Si/SiGe cluster hetero-interfaces no longer controls radiative carrier recombination. Instead, a dynamic type I energy band alignment governs the strong decrease in carrier radiative lifetime and further increase in the luminescence quantum efficiency. In contrast to the strongly temperature dependent and slow radiative carrier recombination found in bulk Si, Auger mediated PL emanating from the nanometer-thick Si layers is found to be nearly temperature independent with a radiative lifetime approaching 10−8 s, which is comparable to that found in direct band gap III-V semiconductors. Such nanostructures are thus potentially useful as CMOS compatible light emitters and in optical interconnects.

Lanthanum monosulfide (LaS) films were grown by pulsed laser deposition on the (100) magnesium oxide (MgO) substrates at an elevated substrate temperature and in a background gas of H2S. The thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), and high resolution transmission electron microscopy. The film surface is composed of grainlike features with an average size of approximately 34nm. The root-mean-square variation of the film surface roughness measured over a 2×2μm2 area by AFM was found to be approximately 1.5nm. XRD data indicate that the average size of the nanocrystalline grains in the film is about 26nm, which is about twice the size of the grains found in LaS thin films deposited at room temperature on Si. The field emission (FE) properties of the films have been characterized by scanning anode field emission microscopy and are interpreted in terms of a recently developed patchwork FE model. The FE data indicate that there is roughly a seven times incre...

The Raman spectra of high-temperature phases I and II of Na2SO4 crystals are reported. The phase I spectra indicate considerable disorder of sulfate tetrahedra even at 520 K, which is well below the melting temperature of 1157 K. Although the phase II spectra are complicated by multidomain effects, they show that the crystal c axis is common to phases I, II and III and that the as-yet unknown crystal structure of phase II is more closely related to the phase I hexagonal structure than the phase III orthorhombic structure.

Images of electrically active defects in Si1−xGex strained epilayers onn-type Si(100) have been obtained using charge collection microscopy (CCM) in a scanning electron microscope. Electrically active defects were generated in the defect-free as-grown material by rapid thermal annealing treatments (3 min) over a temperature range of 550 to 850° C. Point-like contrast, attributed to threading dislocations, as well as an additional line contrast due to the formation of an interface misfit dislocation network, were observed as the metastable strained Si1−xGrx layers relaxed upon annealing. Cross-sectional and plan-view transmission electron microscopy were used to examine the Si1−xGex epilayers in finer detail. Double crystal x-ray diffraction and Raman scattering spectra were also obtained and the annealing induced peak shifts investigated.

New measurements of the low-frequency dielectric response ec of multidomain KDP in applied field as low as 35 mV/cm show no sign of the threshold field reported in earlier studies. The dependencies of the initial dielectric constant and domain wall width on temperature are correlated and discussed in terms of thermally activated domain motion. The ec data indicate the existence of two regimes of domain dynamics.

The generation of interface misfit dislocations, and the accompanying strain relaxation, in a molecular‐beam epitaxy grown 0.17 μm thick metastable Si0.82Ge0.18/Si(100) strained epilayer have been studied in detail as a function of rapid thermal annealing treatments over the 500–850 °C temperature range. Charge collection and transmission electron microscopy were used to determine the onset of relaxation by directly imaging misfit dislocations and to investigate the variation in dislocation density with increasing anneal temperature. The strain variation in the epilayer was carefully monitored using double‐crystal x‐ray diffraction and Raman spectroscopy, and the annealing induced changes in strain related to the electron microscopy observed density of interface misfit dislocations. The relative merit of each experimental technique is discussed in the light of these results. The generation of strain relieving dislocations was found to be an activated process, with an activation energy on the order of 1.5 ...

Recently there has been a great deal of interest in the structural, vibrational, and electronic properties of nonperiodic superlattices.1 This work has been stimulated by the discovery of quasicrystals2 and the realization that 1-D analogs of quasicrystals could be created artificially in multilayer systems.3 By far the majority of the work in these systems has concentrated on quasiperiodic Fibonacci superlattices.4 The Fibonacci structure is a particular case of a class of quasiperiodic structures defined by the recursion relation5 $$ {{S}_{j}} = {{({{S}_{{j - 1}}})}^{n}}{{S}_{{j - 2}}} $$ (1) By defining the basic building blocks S1 and S2 in terms of layers of different materials and thicknesses we have attached a basis to the quasiperiodic lattice. Table 1 illustrates how the recursion relation (1) is used to build up the first 5 generations in terms of S1 and S2.

The Raman spectrum of Na 2 SO 4 in its metastable low temperature phase (space group D 2 h 17 ) has been measured at room temperature. All but two of the 18 Raman-active vibrational modes have been identified and assigned to symmetry species. The Raman spectrum of the metastable (phase III) form of Na 2 SO 4 is very different from the corresponding spectrum of the stable (phase V) form consistent with large differences in their respective orthorhombic structures. The b axis of phase V coincides with the c axis of phase III and the a axis is likely to be common to both phases.

A new type of nanoscale field emitter array, consisting of carbon nanonecklaces and nanotentacles, has been produced by a novel multi-level self assembly process employing flexible porous alumina films. The field emission characteristics of the carbon nanostructures were measured using a scanning field emission microscope (SAFEM) and they exhibited strong Fowler-Nordheim emission. This new synthetic approach could find potential applications in flexible and inexpensive arrays of nanoscale cold cathode emitters., Nanomaterials Synthesis, Interfacing, and Integrating in Devices, Circuits, and Systems II, September 9-11, 2007, Boston, Massachusetts, United States, Series: Proceedings of SPIE; no. 6768

We report a new multi-modal self assembly technique in which a rich structural diversity of distinct moieties emerge when thin films of a wide variety of materials are deposited on nanoporous substrates using different techniques. We also report multi-level self assembly where one set of self assembled nanostructures seeds the self assembly of another set. The multi-modal self assembly has been observed using flexible alumina templates containing hexagonal arrays of cylindrical pores that are 50 nm wide and 500 nm in length and also on nanoporous Si templates. The latter were formed by anodization of thin film alumina templates deposited on platinum coated silicon substrates. The deposition parameters leading to the observation of each of the moieties will be discussed in detail and the field emission (FE) properties, measured by scanning anode field emission microscopy (SAFEM), will be reported., 20th International Vacuum Nanoelectronics Conference, July 8-12, 2007, Chicago, IL, USA

Several nanoscale arrays of metallic, semiconductor, and organic carbon compounds (carbon nanopearls) have been fabricated on nanoporous flexible alumina and silicon templates based on a new self-assembly growth mode. They were obtained using pulsed laser deposition, thermal evaporation, e-beam evaporation, or RF magnetron sputtering. The different moieties that were observed include nanodomes and nanodots (gold, nickel, cobalt, and aluminum nitride), nanonecklaces (carbon nanopearl), and nanopinetrees (gold) self assembled on flexible alumina templates. A nanoneedle array was also self assembled by e-beam evaporation of nickel on silicon substrates that were rendered nanoporous by the use of a porous alumina mask. The physical processes underpinning the new self assembly growth mode have been studied based on extensive characterization of the templates prior to and after deposition of the various metallic, semiconductor, and organic compounds. These include atomic force microscopy (AFM), X-ray diffraction (XRD) analysis, Raman spectroscopy and field emission-scanning electron microscopy (FE-SEM). Some of the arrays have been tested as potential candidates for new cold cathode arrays for vacuum electronic applications using the scanning electron field emission microscopy (SAFEM) technique., 7th IEEE Conference on Nanotechnology, IEEE-NANO 2007, August 2-5, 2007, Hong Kong, Series: Proceedings of the ... IEEE Conference on Nanotechnology; no. 154

The concentration dependence of optical phonons in In1−xGaxAs epilayers grown by chemical beam epitaxy and lattice matched to InP(100), where the epilayer strain is zero for x=0.46809, has been characterized for 0.25

Using MEMS technology, an array of cold cathodes was fabricated by pulsed laser deposition (PLD) of chemically and thermally stable lanthanum monosulfide (LaS) thin film anode and cathode contacts. The latter were defined via etching and processing of two different pieces of (100) Si wafers separated via a highly resistive sputter deposited aluminum nitride (AlN) layer whose thickness was used to control the anode to cathode spacing. The top and bottom Si wafers were aligned and glued together using high temperature, vacuum compatible epoxy. Field emission (FE) characteristics were recorded in a vacuum chamber with a base pressure near 10-7 Torr. An average electric field threshold for Fowler-Nordheim (FN) field emission in the range of 100 V/mum was measured. A maximum current density of 8 mA/ cm2 was recorded which is large enough for flat panel display applications. The largest emission current measured was about 5X10-7 A above which thermal runaway occurred leading to a failure of the cathode. The failure mechanism is most likely due to Joule heating in the aluminum (Al) thin film underneath the LaS cathodes.

Three distinct types of nanostructures—nanodomes, nanodots, and nanowires—have been simultaneously self-assembled by pulsed laser deposition of lanthanum monosulfide on anodic alumina films containing hexagonal arrays of pores about 50nm wide and 500nm deep. The nanostructures have been characterized by x-ray diffraction, atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM). Nanodomes preferentially grow on the boundary separating regions (grains) of the alumina template that have near perfect pore ordering, and their density is ∼109∕cm2. The diameter of a nanodome at the base is about 100nm and their aspect ratio (height/diameter at the base) is between 1 and 3. Additionally, nanodots nucleate on top of the alumina walls that separate adjacent pores. They have a diameter of ∼50nm, a density equal to the pore density (1010∕cm2), and an aspect ratio less than 1. Finally, cross sectional FE-SEM images of the templates indicate that LaS nanowires grow inside the pores with ...

The magnons in the canted rutile-structure antiferromagnet NiF 2 have been studied by Raman scattering. Results are presented for the polarization dependences of the one-magnon spectum over a wide range of temperatures below T N and the two-magnon spectrums at low temperatures. A theoretical analysis of the data, including the important role of the spin canting for the one-magnon properties, is shown to provide good agreement with the measurements.

In this work and with a view to optimizing their FE properties, LaS films of increased crystallinity were obtained by PLD on lattice matched MgO substrates, at elevated substrate temperatures and in a background gas of H2S. The thin films have been characterized by high resolution transmission electron microscopy, atomic force microscopy (AFM), X-ray diffraction, ellipsometry and Raman spectroscopy. The FE properties of the films have been characterized by scanning anode field emission microscopy., 20th International Vacuum Nanoelectronics Conference

Pore growth on n-type GaAs (100) can be initiated in 1 M HCl solution by electrochemical polarization of the material anodic to a critical potential value—the pore formation potential (PFP). At surface defects, however, the PFP is significantly lower (shifted cathodically). Focused ion beam, implantation of Si++ was used to create defined patterns in the substrate. At these implant sites, the growth of porous GaAs was selectively achieved by polarization below the overall PFP. From the porous GaAs patterns visible photoluminescence at green-yellow wavelengths can be observed. This technique, thus, allows the production of light emitting porous GaAs micropatterns of arbitrary shape by a direct writing process.

Thin films of lanthanum monosulfide (LaS) have been successfully deposited on Si and InP substrates by pulsed laser deposition. The values of deposition parameters (chamber pressure, substrate temperature, substrate-to-target separation, laser energy, repetition rate and spot size on the target) leading to a successful growth of cubic rocksalt structure films are identified. For the films grown on Si substrates, the root-mean-square (RMS) variation of surface roughness is found to be 1.74 nm by atomic force microscopy. XRD scans suggest the presence of amorphous regions in the film. This is confirmed by cross-section transmission electron microscopy analysis. For films grown on InP substrates, a thin crystalline film about 500 nm thick followed by an abrupt transition to an amorphous layer about 120 nm thick is observed. The work function of the LaS thin films deposited on Si substrates deduced from Fowler-Nordheim plots at room temperature has a mean value of 0.65 eV, when measured using the scanning field emission microscope technique (SAFEM) at very different locations across the sample. A strong dependence of the field emission current with temperature is also observed. At very large external electric field, the field emission increases dramatically and is no longer of the Fowler-Nordheim type., 18th International Vacuum Nanoelectronics Conference, 2005

Successful growth of lanthanum monosulfide (LaS) nanodot and nanowire arrays has been performed by pulsed laser deposition (PLD) using self-assembled nanoporous alumina templates containing pores about 50 nm wide and 200-300 nm deep. The arrays were characterized by X-ray diffraction, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM) and scanning anode field emission microscopy (SAFEM). The array of LaS nanodots grows selectively at boundaries between different regions of the alumina film with local hexagonal symmetry of pores. The density of these dots is about 109 /cm2. Cross sectional FE-SEM showed that LaS nanowires also grow inside the pores with a density of 1010/cm2 , which is equal to the pore density of the templates. The field emission properties of the LaS nanodots grown on top of the alumina templates were measured using SAFEM. A typical current-voltage characteristic is shown. Using the conventional Fowler-Nordheim relation, the work function of the LaS nanodots has been extracted from the slope of the plot ln(J/F2) vs 1/F, where J is the field emission current density and F is the local applied field. This leads to an outstanding, reproducible effective work function value of ~1 eV for the LaS nanodots. The threshold for an emission current density of 1 mA/cm2 occurs around a 150 V/mum which is considerably lower than the 230 V/mum threshold value reported recently for LaS thin films., 19th International Vacuum Nanoelectronics Conference and 50th International Field Emission Symposium, 2006

We report on the direct observation of polarons in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ self-assembled quantum dots (QD) populated by few electrons. The polarons are strongly coupled modes of QD phonons and electron intersublevel transitions. The degree of coupling is varied in a systematic way with a set of samples having electron intersublevel spacing changing from larger to smaller than the longitudinal-optical-phonon energy. The signature of polarons is evidenced clearly by the observation of a large $(12\char21{}20\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ anticrossing for both InAs and GaAs-like QD phonons using resonant Raman spectroscopy.

Quantum squeezing of a collective spin-wave excitation or magnon using femtosecond optical pulses has been used to generate correlated pairs of spins with equal and opposite wave vectors in antiferromagnetic ${\mathrm{MnF}}_{2}$ and ${\mathrm{FeF}}_{2}$. In the squeezed state, the fluctuations of the magnetization of a crystallographic unit cell vary periodically in time and are reduced below that of the ground state quantum noise. The results obtained, including their temperature dependence for ${\mathrm{FeF}}_{2}$, are compared with earlier spontaneous Raman scattering measurements. The squeezing effect is observed to be at least one order of magnitude stronger in ${\mathrm{FeF}}_{2}$ than in ${\mathrm{MnF}}_{2}$.

Brillouin light scattering has been employed to study the magnetic-field dependence of the discrete spin waves in permalloy Ni80Fe20 nanowires. When a small magnetic field is applied transverse to the nanowire, the results reveal a low-frequency mode, which is absent in the longitudinal case. A Hamiltonian-based microscopic theory for nanowires with inhomogeneous magnetization shows that the appearance of this mode, along with its higher-field behavior, is a significant consequence of a small easy-plane single-ion anisotropy at the nanowire surface. The calculations provide a good description of experimental data for both the transverse and longitudinal cases.

The many and diverse approaches to materials science problems have greatly enhanced our ability in recent times to engineer the physical properties of semiconductors. Silicon, of all semiconductors, underpins nearly all microelectronics today and will continue to do so for some time to come. However, in optoelectronics, the severe disadvantage of an indirect band gap has limited the application of elemental silicon. Here we review a number of diverse approaches to engineering efficient light emission in silicon nanostructures. These different approaches are placed in context and their prospects for application in silicon-based optoelectronics are assessed.

Series: Nanostructure Science and Technology

We review recent progress in the growth and characterization of Si1−xGex islands and Ge dots on (001) Si. We discuss the evolution of the island morphology with Si1−xGex coverage, and the effect of growth parameters or post-growth annealing on the shape of islands and dots. We outline some of the structural, vibrational, and optical properties of Si1−xGex islands and review recent advances in the determination of their composition and strain distribution. In particular, we present an analytical electron transmission microscopy study of the Ge spatial distribution in Ge dots and Si /Si1−xGex island superlattices grown by molecular beam epitaxy and ultra-high vacuum chemical vapour deposition. We describe the use of undulated Si1−xGex island superlattices for infrared detection at telecommunication wavelengths. Finally, we discuss various approaches currently being investigated to engineer Si1−xGex quantum dots and, in particular, control their size, density, and spatial distribution. As examples, we show how C pre-deposition on Si(001) can influence nucleation and growth of Ge islands and how low temperature Si homo-epitaxy can lead to a particular surface cusp morphology that may promote dot nucleation.

Reliable fabrication of high-speed, delta-doped transistors and better understanding of two-dimensional metal-insulator transitions can be achieved using silicon molecular beam epitaxy (MBE). However, this fabrication technique should be performed with care, avoiding dopant segregation on epitaxial Si surfaces and improving the doping efficiency. Here we report comprehensive structural and optical investigations of MBE-grown Si/delta-doped Si:B multilayer structures. Measurements of Auger electron spectroscopy, Raman scattering, optical reflection and photoluminescence are performed. Our results indicate nearly metallic conductivity at room temperature with metal-insulator phase transition near T ∼100 K. In contrast to recently reported data, no enhancement of photoluminescence at room temperature is found. Occasionally, a few samples in specific areas exhibit strong photoluminescence at 1.4-1.6 micron attributed to structural defects, most likely due to B segregation.

Thin films of lanthanum monosulfide (LaS) have been deposited on Si (100) substrates by pulsed laser deposition. The films are golden yellow in appearance with a mirrorlike surface morphology and a sheet resistance around 0.1Ω∕◻, as measured using a four-probe measurement technique. The thin films are characterized by atomic force microscopy (AFM), x-ray diffraction (XRD) analysis, high resolution transmission electron microscopy (HRTEM), ellipsometry, and Raman spectroscopy. The root-mean-square variation of (1μm thick) film surface roughness measured over a 1μm2 area by AFM was found to be 1.74nm. XRD analysis of fairly thick films (micrometer size) reveals the growth of the cubic rocksalt structure with a lattice constant of 5.863(7)A, which is close to the bulk LaS value. HRTEM images reveal that the films are comprised of nanocrystals separated by regions of amorphous material. Two beam bright field TEM images show that there is a strain contrast in the Si substrate right under the interface with the...

We examine the nanostructural properties of Si/Si1−xGex island superlattices with 0.37 < x < 0.56 grown at 620-640 °C by molecular beam epitaxy. Analytical transmission electron microscopy (TEM) shows that during growth Ge atoms migrate towards the center of the large islands to maintain epitaxial growth and that the most regular structures are obtained at higher Ge composition when the built-in strain is also higher. High-resolution x-ray reciprocal space mapping shows that these heterostructures remain pseudomorphic and that the undulations are aligned along [010] directions and exhibit a long-range coherence and vertical correlation as revealed by the presence of strong satellites in a wave vector direction parallel to the surface. Raman spectra of these samples exhibit the usual Ge-Ge, Si-Ge and Si-Si vibrational modes. When compared with planar Si/Si1−xGex superlattices the Ge-Ge and Si-Ge Raman peaks are shifted to lower frequencies indicating an average alloy composition that is approximately x = 0.1 less than the nominal values, which is in general agreement with analytical TEM.

The field emission properties of lanthanum monosulfide (LaS) films, deposited on Si substrates by pulsed laser deposition, have been thoroughly analyzed via the scanning anode field emission microscopy technique. Using the conventional Fowler-Nordheim relation, the work function of LaS thin films has been extracted from the slope of the plot ln (J/F2) vs 1/F, where J is the field emission current density and F is the local applied electric field. The threshold for an emission current density of 1 mA/cm2 occurs around a 230 V/μm electric field applied across the vacuum gap. This leads to an outstanding, reproducible effective work function value of ~1 eV across a 1 cm2 sample area.

We find that in SiGe clusters grown on Si using Stranski-Krastanov (S-K) growth mode, (i) photoluminescence (PL) spectra, (ii) PL lifetime and (iii) PL thermal quench activation energies exhibit strong dependence on the excitation intensity. Under PL excitation intensity increasing from 1 to 104 W/cm2, the PL spectra exhibit blue shift from below Ge bandgap up to ∼970 meV. The PL lifetime shows strong dependence on the excitation wavelength, decreasing from 20 μs at ∼0.8 eV to 200 ns at ∼ 0.9 eV. The process of PL thermal quench has two clearly distinguished activation energies. At low temperature, small (∼15 meV) and excitation-independent activation energy is attributed to exciton thermal dissociation. At higher temperature, excitation-dependent PL thermal quench activation energy (increasing from ∼ 120 to 340 meV as excitation intensity increases) is found, and it is attributed to hole redistribution via tunneling and/or thermal ionization over the Si/SiGe valence band energy barrier., Series: MRS Online Proceedings Library

Self-Assembled Si1-x Ge x Dots and Islands.- Synthesis of Titania Nanocrystals: Application for Dye-Sensitized Solar Cells.- Soft Synthesis of Inorganic Nanorods, Nanowires, and Nanotubes.- Assembly of Zeolites and Crystalline Molecular Sieves.- Molecular Imprinting by the Surface Sol-Gel Process: Templated Nanoporous Metal Oxide Thin Films for Molecular Recognition.- Fabrication, Characterization, and Applications of Template-Synthesized Nanotubes and Nanotube Membranes.- Synthesis and Characterization of Core-Shell Structured Metals.- Cobalt Nanocrystals Organized in Mesoscopic Scale.- Synthesis and Applications of Highly Ordered Anodic Porous Alumina.