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We investigate the longitudinal spin relaxation arising due to spin-flip transitions accompanied by phonon emission in quantum dots where the strength of the Rashba spin-orbit coupling is a random function of the lateral (in-plane) coordinate on the spatial nanoscale. In this case the Rashba contribution to the spin-orbit coupling cannot be completely removed by applying a uniform external bias across the quantum dot plane. Due to the remnant random contribution, the spin relaxation rate cannot be decreased by more than two orders of magnitude even when the external bias fully compensates the regular part of the spin-orbit coupling., 13 pages, 4 figures

A theoretical description of electronic Raman scattering from GaAs/AlxGa1 xAs artificial atoms under the influence of an external magnetic field is presented. Raman spectra with laser excitation energy in the interval Egap 30 meV to Egap are computed in the polarized and depolarized geometry. The polarization ratios for the collective and single-particle excitations indicate a breakdown of the Raman polarization selection rules once the magnetic field is switched on. A Raman intensity jump rule at the band gap is predicted in our calculations. This rule can be a useful tool for identifying the physical nature (charge or spin) of the electronic excitations in quantum dots in low magnetic fields. a few-electron CDEs in InGaAs self-assembled quantum dots by means of Raman spectroscopy. In this paper, we compute the near-resonance Raman spectrum of a medium-size disk-shaped quantum dot con- taining 42 electrons in an external magnetic field applied perpendicular to the plane of the dot. The method and approximations used to obtain the many-body wave func- tions and energies of the required states are explained in Ref. (7). Energies and wave-functions of the final ex- cited states are obtained by means of the random phase approximation 8 (RPA). The calculation of the monopole operator matrix elements between each final state and the ground state, allows the identification of the collec- tive modes (CDEs, SDEs) and the SPEs. By computing the Raman polarization ratios we evaluate the extent to which the magnetic field breaks down the polarization se- lection rules for both collective and SPEs. We report the first observation of what we term the Raman intensity jump-rule. This effect, more clearly seen for the collec- tive modes, can be enunciated as follows: a monotonic increase of a Raman peak intensity when the incident laser energy is increased from below up to the effective band-gap of the dot in the polarized (depolarized) geom- etry indicates that the excited state associated with that peak is a charge (spin) excitation. If we look at the same state in its opposite geometry, we find quite small values of the Raman intensities at any incident energy except at the effective band-gap, in which case a sudden jump of the Raman intensity should be detected. From theory 9 , we know that the Raman peak inten- sity is proportional to the transition amplitude squared, |Afi| 2 , where

We present theoretical results concerning inelastic light (Raman) scattering from semiconductor quantum dots. The characteristics of each dot state (whether it is a collective or single-particle excitation, its multipolarity, and its spin) are determined independently of the Raman spectrum, in such a way that common beliefs used for level assignments in experimental spectra can be tested. We explore the usefulness of below band gap excitation and an external magnetic field to identify charge and spin excited states of a collective or single-particle nature., To appear in a special issue of Solid State Communications dedicated to Eli Burstein

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 the islands and dots. We outline some of the structural, and optical properties of Si1-xGex islands and review recent progress in the determination of their composition and strain distribution. We describe the use of undulated Si1-xGex islands superlattices for infrared detection at telecommunication wavelengths. We discuss various approaches currently being investigated to engineer Si1-xGex quantum dots and in particular control their size, density and spatial distribution. For example, we show how C pre-deposition on Si (001) can influence nucleation and growth of Ge islands. We also show how low temperature Si homoepitaxy can lead to a particular surface cusp morphology that may promote dot nucleation., Quantum sensing and nanophotonic devices II, 23-27 January 2005, San Jose, California, USA

Stabilization of electroluminescence from nanocrystalline porous silicon diodes has been achieved by replacing silicon-hydrogen bonds terminating the surface of nanocrystalline silicon with more stable silicon-carbon (Si-C) bonds. Hydrosilylation of the surface of partially and anodically oxidized porous silicon samples was thermally induced at about 90 °C using various different organic molecules. Devices whose surface have been modified with stable covalent bonds shows no degradation in the EL efficiency and EL output intensity under DC operation for several hours. The enhanced stability can be attributed to the high chemical resistance of Si-C bonds against current-induced surface oxidation associated with the generation of nonradiative defects. Although devices treated with 1-decene exhibit reduced EL efficiency and brightness compared to untreatred devices, other molecules, such as ethyl-undecylenate and particularly undecylenic acid provide stable and more efficient visible electroluminescence at room temperature. Undecylenic acid provides EL brightness as high as that of an untreated device. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report a study of the surface morphology and microstructure of Si epitaxial layers grown by MBE on (001) Si at temperatures at which epitaxy breakdown is observed. For films grown in the 400 - 450 °C temperature range the epitaxy breakdown is very sluggish and characterized by a columnar growth and the formation of surface cusps. We have used atomic force microscopy to study the shape, size and distribution of those surface cusps. Surface cusps are of square shape with sides predominantly oriented along directions and are typically of 50 nm size and 5 nm depth. The cusps can be very regular in size and their surface density (typically of 109-1010 cm-2) is dependent on the growth temperature. The epitaxial Si in this temperature regime exhibits a residual strain of the order of -5 × 10−5 in the growth direction. Photoluminescence (PL) from cusped Si films is characterized by a broad PL at low energy possibly due to impurities incorporation at low growth temperatures. We have observed that Ge self-assembled dots can be grown on cusped surfaces. Large area AFM measurements reveal that surface cusps are “decorated” by clusters of large dome-like Ge dots, while a lower density of smaller dome and pyramid shape islands are seen away from the cusps., 2004 MRS Fall Meeting - Symposium F – Group IV Semiconductor Nanostructures, November 29-December 2, 2004, Boston, Massachusetts, USA, Series: Materials Research Society symposia proceedings

Infrared vibrational spectroscopy in an attenuated total reflection geometry has been employed to investigate the presence of organic and inorganic thin layers on Si-wafer surfaces. Three different processes were compared for surface contaminant removal; microwave plasma, UV-ozone, and a piranha solution cleaning. The CH vibrations at 2928 and 2856 cm-1 characteristic of organic contaminants were monitored before and after each cleaning procedure to determine how well it removed surface contaminants. We found that native oxide removal from the Si surface should only be carried out after a cleaning essay. We observed that surface oxide removal exposed a hydrophobic bare Si surface, attracting organic molecules present in solution or the ambient. A large increase of the CH vibrational signature was observed for a Si wafer after an HF dip. A combination of plasma cleaning followed by UV-Ozone treatment was found the most effective one for Si wafer cleaning. We were able to evaluate the effectiveness of the cleaning methods, hydrogen surface passivation and oxide removal/regrowth., Mat Res Soc Sym, Series: Materials Research Society Symposium Proceedings; no. 901

The spin dynamics of high-aspect-ratio nickel nanorings in a longitudinal magnetic field have been investigated by Brillouin spectroscopy and the results are compared with a macroscopic theory and three-dimensional micromagnetic simulations. Good agreement is found between the measured and calculated magnetic field dependence of the spin wave frequency. Simulations show that as the field decreases from saturation, the rings switch from a ``bamboo'' to a novel ``twisted bamboo'' state at a certain critical field, and predict a corresponding dip in the dependence of the spin wave frequency on the magnetic field.

Thin films of lanthanum monosulfide (LaS) have been successfully deposited on Si 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 films in their cubic rocksalt structure are identified. The films are golden yellow in appearance with a mirror-like surface morphology and possess a sheet resistance around 0.1Ω∕square. X-ray diffraction analysis of thick films (several microns) leads to a lattice constant of 5.863(7)Å, which is close to the bulk LaS value. High-resolution transmission electron microscopy reveals the films to be comprised of nanocrystalline regions separated by amorphous ones. The root-mean-square variation of film surface roughness measured over a 1μm×1μm area is found to be 1.74nm by atomic force microscopy. These films have potential for semiconductor, vacuum microelectronics, and optoelectronics applications.

The peak energy of photoluminescence (PL) from an undecylenic acid functionalized porous Si demonstrated a large PL red-shift (∼ 75 nm) during 2.5 hours of protein incubation in our previous work. [1] Here we present a similar in-situ PL study of surface functionalized planar Si (Si:COOH). The PL of Si:COOH exhibited a 5 nm red-shift in its peak frequency and an approximately 10% drop in its intensity after incubation in a protein solution. Vibrational spectroscopic characterization was carried out upon the Si:COOH sample for which we observed the PL red-shift. The infrared absorption spectra showed clear evidence of protein adsorption on Si:COOH. This correlation study between the PL peak energy and the vibrational spectrum provided strong evidence that the observed red-shift was due to the formation of semiconductor-protein (Si:COOH:BSA) complexes.

Strained In1−xGaxAs epilayers over a wide range of x values from 0.252 to 0.762 have been grown by chemical beam epitaxy on heavily doped InP(100) substrates. The infrared p-polarized reflectivity of these samples has been measured and the first derivative of the reflectance has been analyzed using a specially developed fitting technique. Expressions for the variation of the phonon frequencies with Ga fraction for the InAs-like and GaAs-like modes have been obtained. The present results substantially improve on earlier results obtained from epilayers on semi-insulating substrates having a narrower concentration range around x=0.468 where In1−xGaxAs is lattice matched to InP.

The field emission (FE) properties of lanthanum monosulfide (LaS) films, deposited on Si and InP substrates by pulsed laser deposition, have been thoroughly analyzed via the scanning anode field emission microscopy technique (SAFEM, Fig. 1) at different surface locations and at different temperatures. For one location, the full set of measured I-V characteristics (total measured current versus applied voltage) for different values of Z, the distance between the cathode surface and the probe ball, were then analyzed in order to extract the current density J versus actual applied field F (J-F data), within the approximation that the LaS surface is a plane. A characteristic J-F variation is shown in Fig. 2. The work function of the LaS thin film has been extracted from the slope of the plot ln(J/F2) vs 1/F, by using the conventional Fowler-Nordheim relation, leading to a value of ∼0.65 eV which is in agreement with the onset of the electric field needed to observe an emission current density of 1 mA/cm2 at an applied electric field across the vacuum gap around 230 V/μm., 18th International Vacuum Nanoelectronics Conference

Detailed Raman and photoluminescence (PL) measurements are reported for Si/Si 1− x Ge x nanostructures grown by molecular beam epitaxy under near Stranski–Krastanov (S–K) growth mode conditions. In samples with x ranging from 0.096 to 0.53, we observe that an increase in the Raman signal related to Ge–Ge vibrations correlates with (i) a red shift 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 for x >0.5 Ge atoms form nanometer size clusters with a nearly pure Ge core surrounded by a SiGe shell. Time-resolved PL measurements reveal a stretched-exponential long-lived PL component that is associated with compositional and dimensional fluctuations in the SiGe dots.

Superlattices of $\mathrm{Si}/{\mathrm{SiO}}_{2}$ have been grown at room temperature with atomic layer precision using state of the art molecular beam epitaxy and ultraviolet ozone treatment. Photoluminescence was observed at wavelengths across the visible range for Si layer thicknesses $1ldl3\phantom{\rule{0ex}{0ex}}\mathrm{nm}$. The fitted peak emission energy $E(\mathrm{eV}){\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.60+0.72d}^{\ensuremath{-}2}$ is in accordance with effective mass theory for quantum confinement by the wide-gap ${\mathrm{SiO}}_{2}$ barriers and also with the bulk amorphous Si band gap. Measurements of the conduction and valence band shifts by x-ray techniques correlate with $E(d)$, confirming the role of quantum confinement and indicating a direct band-to-band recombination mechanism.

A consistent semiquantitative theoretical analysis of electronic Raman scattering from many-electron quantum dots under resonance excitation conditions has been performed. The theory is based on random-phase-approximation-like wave functions, with the Coulomb interactions treated exactly, and hole valence-band mixing accounted for within the Kohn-Luttinger Hamiltonian framework. The widths of intermediate and final states in the scattering process, although treated phenomenologically, play a significant role in the calculations, particularly for well above band gap excitation. The calculated polarized and unpolarized Raman spectra reveal a great complexity of features and details when the incident light energy is swept from below, through, and above the quantum dot band gap. Incoming and outgoing resonances dramatically modify the Raman intensities of the single particle, charge density, and spin density excitations. The theoretical results are presented in detail and discussed with regard to experimental observations., Comment: Submitted to Phys. Rev. B

The growth, characterization, and field emission properties of pulsed laser deposition of lanthanum sulfide thin films on silicon and indium phosphide substrates were studied. X-ray diffraction analysis of fairly thick films (micrometer size) reveals the successful growth of the cubic rocksalt structure with a lattice constant of 5.863(7) angstroms, which is close to the bulk value. High resolution transmission electron microscope images of the films reveal that they are comprised of nanocrystals separated by regions of amorphous materials. Raman spectra and ellipsometry measurements were also performed. It was found that the work function deduced from Fowler-Nordheim plots at room temperature has a mean value of 0.65 eV. Also, a strong dependence of the field emission current with temperature was observed.

We present a Raman study of pseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y grown on GaAs(100) with y ranging from 0 to 0.06. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes for y = 0.06 are represented by the strong longitudinal optic (LO1) mode at 287.4 cm-1 and the weaker transverse optic (TO1) mode at 269.0 cm-1, while the GaN-like LO2 mode is observed at 475.6 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration in the alloy increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the linearly increasing GaN-like LO2 phonon band deviates from linearity at higher nitrogen concentration (y ≥ 0.03). The reason for the deviation of the LO2 phonon from linearity is discussed.

Pseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y have been grown on GaAs(100) with y ranging from 0 to 0.05. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes are represented at y = 0.05 by the strong longitudinal optic (LO1) mode at 288.5 cm-1 and the weaker transverse optic (TO1) mode at 268.3 cm-1, while the GaN-like LO2 mode is observed at 474.8 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the GaN-like LO2 phonon band displays a sub-linear increase in wavenumber. Raman results for the unstrained quaternary alloy In0.06Ga0.94N0.02As0.98 are compared with those of GaN0.02As0.98., 2003 MRS Fall Meeting - Symposium Y – GaN and Related Alloys, 1-5 December 2003, Boston, Massachusetts, USA, Series: Materials Research Society symposia proceedings

We present a structural and chemical analysis of high-vacuum deposited Si filmsgrown on clean or oxidized Si (001) wafers by low-temperature molecular-beam epitaxy. For growth on clean Si, we observed a limiting thickness for epitaxy that decreases with decreasing temperature with an activation energy of 0.47 eV. The onset of defect formation is correlated to a peak in the H impurity concentration. The transition to an amorphous phase is, however, observed beyond the depth where impurities are first observed pointing to surface disorder/roughening as a source of epitaxy breakdown. The O and C content in these films remains low until the film crystallinity has strongly deteriorated and reaches a saturation concentration of 2–4 at. % in the fully amorphous regions. The impurity profiles in amorphous-Si filmsgrown on oxidized Si are similar to those obtained on clean Si when grown at the same temperature and indicate that the impurity uptake depends primarily on residual gas and surface condition. Raman scattering results show the structural changes and evolution of the Si bond configuration.

The large surface area within porous Si (pSi) and its strong room temperature photoluminescence (PL) make it an ideal host for biological sensors. In particular, the development of pSi-based optical sensors for DNA, enzyme and other biochemical molecules have become of great interest. Here, we demonstrate that the in-situ monitoring of the pSi PL behaviour can be used as a positive identification of bovine serum albumin (BSA) protein adsorption inside the porous matrix. Electrochemically prepared pSi films were first functionalized with undecylenic acid to produce an organic monolayer covalently attached to the porous silicon surfaces. The acid terminal group also provided favourable BSA binding sites on the pSi matrix sidewalls. In-situ PL spectra showed a gradual red shift (up to 12 meV) in the PL peak energy due to the protein incorporation into the porous matrix. The PL then exhibited a continuous blue shift after saturation of the protein molecules in the pores. This blue shift of the PL peak frequency and a steady increase in the PL intensity is evidence of surface oxidation. Comparing the specular reflectance obtained by Fourier transform infrared spectroscopy (FTIR) before and after BSA incubation confirmed the adsorption of protein in the pSi matrix., Photonics North 2004: Photonic Alications in Astronomy, Biomedicine, Imaging, Materials Processing, and Education, 27 September 2004, Ottawa, Canada, Series: Proceedings of SPIE

Hydrogen-terminated porous silicon (pSi-H) films were fabricated through electrochemical anodization of crystalline silicon in hydrofluoric-acid-based solutions. The pSi-H surface was chemically functionalized by thermal reaction with undecylenic acid to produce an organic monolayer covalently attached to the silicon surface through Si—C bonds and bearing an acid terminal group. Bovine serum albumin (BSA) was adsorbed onto such surface-modified pSi structures. The resulting surfaces were characterized using scanning electron microscopy (SEM), reflection FT-IR spectroscopy, and ellipsometry. SEM showed that the porous films were damaged and partially lifted off the silicon substrate after a prolonged BSA adsorption. Ellipsometry analysis revealed that the BSA penetrated ∼1.3 µm into the porous structure. The film damage is likely a result of BSA anchoring itself tightly through strong electrostatic interaction with the acid-covered Si sidewalls. A change in surface tension during BSA film formation then causes the pSi layer to buckle and lift off the underlying Si substrate. FT-IR results from the undecylenic-acid-modified pSi surfaces before and after BSA adsorption showed the presence of strong characteristic amide I, II, and III vibrational bands after BSA adsorption. The surface properties of the pSi matrix and its interactions with BSA are examined in this study.Key words: ellipsometry, porous silicon, protein adsorption, surface passivation.

Nanometer sized pores were formed for the first time on p-type InP surfaces by electrochemical anodic polarization in HF and HBr solutions in the dark. The surface structures were characterized with scanning electron microscopy (SEM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and photo-luminescence (PL) measurements. It is found that the surface morphology strongly depends on the type of halogen acid present in the electrolyte, and that the anodized samples emit photoluminescence in the visible wavelength range.

We review progress in the growth 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 the islands and dots. We outline some of the structural, and optical properties of Si1-xGex islands and review recent progress in the determination of their composition and strain distribution. We discuss various approaches currently being investigated to engineer Si1-xGex quantum dots and in particular control their size, density and spatial distribution. For example, we show how C pre-deposition on Si (001) can influence nucleation and growth of Ge islands., 2004 MRS Fall Meeting - Symposium F – Group IV Semiconductor Nanostructures, November 29-December 2, 2004, Boston, Massachusetts, USA, Series: Materials Research Society Symposia Proceedings; no. 832

PHOTONIC devices are becoming increasingly important in information and communication technologies. But attempts to integrate photonics with silicon-based microelectronics are hampered by the fact that silicon has an indirect band gap, which prevents efficient electron-photon energy conversion. Light-emitting silicon-based materials have been made using band-structure engineering of SiGe and SiC alloys and Si/Ge superlattices, and by exploiting quantum-confinement effects in nanoscale particles and crystallites1–3. The discovery4,5 that silicon can be etched electrochemically into a highly porous form that emits light with a high quantum yield has opened up the latter approach to intensive study6–12. Here we report the fabrication, by molecular-beam epitaxy, of well-defined superlattices of silicon and SiO2, which emit visible light through photoluminescence. We show that this light emission can be explained in terms of quantum confinement of electrons in the two-dimensional silicon layers. These superlattice structures are robust and compatible with standard silicon technology.

At absolute zero temperature, thermal noise vanishes when a physical system is in its ground state, but quantum noise remains as a fundamental limit to the accuracy of experimental measurements. Such a limitation, however, can be mitigated by the formation of squeezed states. Quantum mechanically, a squeezed state is a time-varying superposition of states for which the noise of a particular observable is reduced below that of the ground state at certain times. Quantum squeezing has been achieved for a variety of systems, including the electromagnetic field, atomic vibrations in solids and molecules, and atomic spins, but not so far for magnetic systems. Here we report on an experimental demonstration of spin wave (i.e., magnon) squeezing. Our method uses femtosecond optical pulses to generate correlations involving pairs of magnons in an antiferromagnetic insulator, MnF2. These correlations lead to quantum squeezing in which 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 mechanism responsible for this squeezing is stimulated second order Raman scattering by magnon pairs. Such squeezed states have important ramifications in the emerging fields of spintronics and quantum computing involving magnetic spin states or the spin-orbit coupling mechanism.

Polarized far infrared reflectance has been measured at oblique incidence for 11 ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{P}$ superlattices grown lattice matched on InP(100) wafers by chemical beam epitaxy. Both the alloy layer thickness (0.25--20 nm) and number of periods (10--40) were varied. The spectra contained sharp reflectance peaks at the frequencies of the vertically propagating, zone-center, transverse optical (TO), and longitudinal (LO) optical phonons. The contributions of the individual phonon modes of the alloy layers were resolved using a generalized analysis procedure including GaAs-like, disorder induced, and InAs-like LO and TO phonons. A nonlinear variation of the alloy LO phonon frequencies with layer thickness was observed arising from phonon confinement in the alloy layers. From this result, the LO phonon in bulk InAs is predicted to exhibit anomalous dispersion. On both sides of the alloy layers we observed 0.6-nm-thick interface layers of significantly different alloy composition. Surprisingly for a lattice matched system, the InP layers exhibit a slight strain arising from the interface formation.

Raman spectroscopy has been used to investigate the lattice vibrations of ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ $(0lxl0.02)$ epilayers grown on Si(100) by electron cyclotron resonance plasma-assisted molecular beam epitaxy. Spectral contributions due to first- and second-order longitudinal and transverse optic phonons together with disorder-induced features have been evaluated by a detailed polarization and C concentration dependence analysis. The band parameters exhibit generally a linear dependence on $x.$ A Raman line observed near $630{\mathrm{cm}}^{\ensuremath{-}1},$ which in past work has been associated with C ordering in the Si lattice, is found not to agree with this interpretation. It is associated with interstitial C. The optical absorption coefficient near 458 nm is deduced from the Raman intensity information, and is found to increase significantly with increasing $x.$

Polarized far infrared reflectance was measured at oblique incidence for a large number of strained In/sub 1-x/Ga/sub x/As epilayers grown on InP wafers. From these measurements the concentration dependence of the zone-center optical phonon frequencies has been obtained for strained In/sub 1-x/Ga/sub x/As epilayers grown by chemical beam epitaxy on InP[100]. With the method used the reflectance spectra contain sharp Berreman peaks precisely at the optical phonon frequencies. To resolve the contributions of the individual phonon modes, curve fitting of the measured spectra was used. With this analysis it was observed that the modes included GaAs-like longitudinal and transverse optic (LO and TO), a disorder induced (TO-like), and InAs-like LO and TO phonons. The concentration dependence of these phonon frequencies, determined for 0.31

Intersubband lasing at 12-16 microm based on a CO2 laser pumped stimulated resonant Raman process in GaAs/AlGaAs three-level double-quantum-well structures is reported. The presence, or lack of, lasing action provides evidence for resonantly coupled modes of collective electronic intersubband transitions and longitudinal optical phonons. An anticrossing behavior of these modes is clearly seen when the difference between the pump and lasing energies (i.e., Stokes Raman shift) is compared with the subband separation. This work reveals the significance of the strong coupling between intersubband transitions and phonons and raises a new possibility of realizing a phonon "laser."

Stabilization of electroluminescence (EL) from nanocrystalline porous silicon (PS) diodes has been achieved by replacing silicon–hydrogen bonds terminating the surface of nanocrystalline silicon with more stable silicon–carbon (Si–C) and silicon–oxygen (Si–O–C) bonds without significant effects on the electrical properties. The surface modification is performed by a thermal treatment of partially and anodically oxidized PS sample at about 90 °C with organic molecules: 1-decene, ethyl undecylenate, or n-caprinaldehyde. The porous silicon device whose surface has been modified with stable covalent bonds shows no degradation in the EL efficiency and EL output intensity under dc operation for several hours. The improved stability can be attributed to the high chemical resistance of Si–C and Si–O–C bonds against current-induced surface oxidation associated with the generation of nonradiative defects.

A specially prepared Ga1−xAlxAs sample with a laterally graded alloy composition has allowed a novel investigation of resonance Raman scattering from the optical phonons. Instead of varying the exciting light energy, the resonance is probed by changing the alloy composition at fixed incident energy. Both incoming and outgoing resonances are observed at the direct gap of the alloy, free of the usual overwhelming photoluminescence background.

This chapter is focused on the fabrication and characterization of layered Si-based nanostructures, which have been called nanocrystalline silicon (nc-Si)—silicon dioxide superlattices [1]. Among the many semiconducting materials, silicon is one of the most studied and definitely the most important material for commercial microelectronics. During the last several decades, the exponential growth of electronic chip complexity and drastic decrease of transistor dimensions has highlighted new directions in electronic device evolution and the potential applicability of Si nanocrystals for nanoelectronics and integrated light-emitters. The latter was stimulated by the discovery of efficient light emission in different forms of Si nanostructures [2] and by the demonstration of a Si-based light-emitting device prototype integrated into conventional microelectronic circuitry [3]. Hence, the interest in reliable fabrication of Si based nanostructures with control over the nanocrystal size, shape, and crystallographic orientation has been growing continuously over the last decade. Recently, the application of Si nanocrystals in electronic devices was suggested and proved by the demonstration of a Si nanocrystal non-volatile memory and other devices utilizing the Coulomb blockade effect [4, 5].

The concentration dependence of optical phonons in strained In1-xGaxAs epilayers grown on InP (100) by chemical beam epitaxy has been characterized with oblique angle polarized far-infrared reflectivity measurements. In this powerful method, the reflectance spectra contain sharp Berreman peaks exactly at the optical phonon frequencies. For radiation polarized in the plane of incidence (p-polarized), peaks for both the TO and LO phonons were observed. For s-polarization only the TO modes were observed. For heavily doped substrates the TO film phonons were observed as reflectance minima, whereas for lightly doped substrates they were seen as maxima. The measured spectra were curve resolved to separate the effects of the various modes which included GaAs-like longitudinal and transverse optic (LO and TO), a disorder induced, and InAs-like LO and TO phonons. The dielectric response function and phonon frequency dependences for all modes were obtained versus Ga fraction for x from 0.25 to 0.75 and the latter showed a quadratic dependence on x over this range. The effects of strain on the phonon frequencies could then be evaluated.

We demonstrate squeezing of the lattice magnetization in antiferromagnetic MnF/sub 2/. Our method uses femtosecond optical pulses to create coherences involving magnon pairs.

A green photoluminescent triruthenium cluster (I) has been chemisorbed on highly luminescent porous silicon (PSi) layers, either freshly prepared or chemically modified with 1-decene, ethyl undecylenate, or undecylenic acid, in order to study the influence of the cluster on the photoluminescence (PL) arising from the PSi. Immersing the hydrogen-terminated PSi in a hexane solution of (I) for several days at room temperature led to the quenching of PL arising from PSi; the only PL detected was due to the Ru cluster (I). A complete quenching of the PL due to PSi was also observed when derivatized PSi surfaces were exposed to the same solution of (I); in these cases, the PL of (I) also shifted to lower energies. Both the quenching of the PL arising from the PSi layers and the shift in the PL of the cluster (I) are likely due to the difference in the chemical interaction of the PO moiety and the CO groups of the Ru cluster with the terminal functional group of the organic monolayer.

A brief review of the history of luminescence and display materials is given in conjunction with a summary of the activities of the Luminescence and Display Materials Division in the luminescence field over the first 100 years of the Society together with a compendium of luminescence related papers published in the Journal on luminescence.

Polarized infrared reflectance from 300 to 1200 cm-1 at different incidence angles was measured at room temperature for sapphire and GaN/sapphire samples. A film of GaN was grown approximately 1.5 μm thick on c-plane sapphire by MOCVD with a thin GaN buffer layer. Because of the hexagonal structure of sapphire and GaN, their uniaxial optical properties are anisotropic. In the p-polarized reflective spectrum, we observed the mixed effect between the distinct infrared-active modes with dipole-moment oscillation perpendicular (E-mode) and parallel (A-mode) to the c axis. The contribution of the A-mode increased with increasing incidence angle. Therefore, we were able to obtain simultaneously the infrared dielectric functions parallel and perpendicular to the c axis, by fitting simultaneously three polarized reflectance spectra at three different incident angles with a suitable model. In the procedure, we adopted a new fitting technique, i.e., fitting the first numerical derivative of the polarized reflectance spectra to improve the accuracy of the phonon parameters and to overcome the inconsistency between the model and measurement in the whole frequency range. Excellent agreement has been obtained between the measured and fitted first derivative reflectance spectra for both the sapphire and GaN/sapphire samples. The dielectric information thus obtained for sapphire and GaN is of greater accuracy than those reported previously.

Ultrathin single quantum wells of crystalline silicon (c-Si) confined by SiO2 have been prepared by chemical and thermal processing of silicon-on-insulator wafers. The photoluminescence (PL) produced by these nanometer-thick single wells contains two bands: one exhibits a peak energy of ∼1.8 eV, while the second increases rapidly in peak energy with decreasing c-Si layer thickness. Comparison with theories based on self-consistent first-principles calculations shows that the increase in PL peak energy of the second band is consistent with that predicted for the c-Si energy gap of such wells. It also agrees with the measured band gap variation. The ∼1.8 eV PL band is attributed to the recombination of electron-hole pairs confined at the c-Si/SiO2 interface.

Measurements of the weak one-magnon light scattering in the antiferromagnet MnF2, including Stokes and anti-Stokes scattering, as a function of temperature are reported. From the theoretical analysis of the temperature and polarization dependences of the integrated Raman intensities, values are deduced for the ratio of quadratic and linear magneto-optical coupling coefficients.

We report a structural study of partially relaxed, 0.1‐μm‐thick, In1−xAlxSb (0.15

Measurements of the optical and structural characteristics of various porous Si samples have been correlated. Although the photoluminescence peak wavelength shows no correlation with the Si nanoparticle size, the optical absorption edge exhibits a strong inverse correlation that is in excellent agreement with theoretical predictions for the optical gap in Si spheres or quantum dots. This constitutes direct evidence for quantum confinement effects in porous Si.

Single nanometer-thick layers of crystalline silicon (c-Si) confined by amporphouse SiO 2 have been prepared from silicon-on-insulator wafers. The photoluminescence from these ultra-thin quantum wells shows an increase in peak energy with decreasing c-Si layer thickness. Comparison with experimental results for the c-Si band gap and also with theory shows that the increase in photoluminescence peak energy is not as rapid as the measured or predicted energy gap. This difference is attributed to recombination of confined electron-hole pairs at the c-Si/SiO 2 interface rather than within the quantum well.

Polarized infrared reflectivity measurements have been performed on strained In1−xGaxAs epilayers grown by chemical beam epitaxy on semi-insulating InP(100) with x varied from 0.314 to 0.556. Fitting of the reflectance spectra yielded excellent agreement with three longitudinal and transverse optical phonon pairs, corresponding to GaAs-like, disorder-induced, and InAs-like modes. The dielectric response function and phonon frequency dependences for all modes were obtained versus Ga fraction.

Electrochemical oxidation of porous silicon (PSi) produces a surface that is covered with native silicon–hydrogen (Si–Hx) bonds and regions with oxidized Si–Si back-bonds (OSi–Hx). Such anodically oxidized PSi layers were chemically modified using 1-decene under thermal conditions. The hydrosilylation reaction consumes mainly the nonoxidized Si–Hx bonds and yields a surface with oxidized and alkylated regions that were characterized using transmission IR and Raman spectroscopies. The brightest photoluminescence (PL) was obtained when the PSi sample was anodized in 1 M sulfuric acid (H2SO4) at 3 mA/cm2 for 5 min. The chemical process preserves the PL and the physical properties of the porous layer. The derivatized PSi surfaces are stable in boiling CCl4 and in water.

The dynamical properties of uniform two-dimensional arrays of nickel nanowires have been investigated by inelastic light scattering. Multiple spin waves are observed that are in accordance with dipole-exchange theory predictions for the quantization of bulk spin waves. This first study of the spin-wave dynamics in ferromagnetic nanowire arrays reveals strong mode quantization effects and indications of a subtle magnetic interplay between nanowires. The results show that it is important to take proper account of these effects for the fundamental physics and future technological developments of magnetic nanowires.

The thermal reaction of undecylenic acid with a hydrogen-terminated porous silicon surface takes place at 95°C to yield an organic monolayer covalently attached to the surface through Si-C bonds. The acid terminal group remains intact and is not affected by the chemical process. Under the same conditions, alcohols break the Si-Si back bonds of the PSi matrix. In contrast, the acid function does not react with either the Si-H or the Si-Si bonds of the PSi surface and the reaction takes place at the terminal Formula double bond of the molecule. When the reaction was carried out with decanoic acid, under the same conditions, the reaction was not complete. The functionalized surfaces were characterized using transmission infrared and X-ray photoelectron spectroscopies. The effect of the chemical process on the photoluminescence has been studied, and the stability against corrosion in 100% humidity was verified using chemography. We have demonstrated that the derivatized surface with undecylenic acid can be activated by a simple chemical route using N-hydroxysuccimide in the presence of N-ethyl-N ′-(3-dimethylaminopropyl) carbodiimide hydrochloride.

Raman optical activity (ROA) of magnons and phonons in antiferromagnetic FeF2 (TN=78 K) has been studied as a function of temperature and in applied magnetic field. For exciting light incident along the rutile-structure c-axis, ROA is observed for magnons but not phonons. In zero field, the small anisotropy-induced splitting (0.09 cm−1) of the two acoustic-magnon branches is observed by light scattering for the first time. The splitting in applied magnetic field is found to reduce with increasing temperature in accordance with theory. No ROA was detected for two-magnon excitations.

A systematic Brillouin study has been carried out on acoustic modes in porous silicon films prepared from p-type Si(100). A surface Rayleigh mode and two bulk acoustic modes, transverse and longitudinal, were observed. Results are presented for the dependence of the phase velocities of these three modes on sample porosity and film thickness. The phonon velocities, especially those of the bulk modes, increase sharply with decreasing film thickness below \ensuremath{\sim}10 \ensuremath{\mu}m. This thickness dependence is attributed to the presence of a transition layer between the porous silicon film and crystalline Si substrate. Its influence on the film bulk modes increases with decreasing thickness. Elastic constants and Young's modulus of porous Si are estimated for different porosities from the phase velocities of the two bulk acoustic modes and are compared with the corresponding data obtained by other techniques.