David J. Lockwood

Nanocrystalline-silicon superlattice produced by controlled recrystallization

Nanocrystalline-silicon superlattices are produced by controlled recrystallization of amorphous-Si/SiO2 multilayers. The recrystallization is performed by a two-step procedure: rapid thermal annealing at 600–1000 °C, and furnace annealing at 1050 °C. Transmission electron microscopy, Raman scattering, x-ray and electron diffraction, and photoluminescence spectroscopy show an ordered structure with Si nanocrystals confined between SiO2 layers. The size of the Si nanocrystals is limited by the thickness of the a-Si layer, the shape is nearly spherical, and the orientation is random. The luminescence from the nc-Si superlattices is demonstrated and studied.

Ordering and self-organization in nanocrystalline silicon

The spontaneous formation of organized nanocrystals in semiconductors has been observed during heteroepitaxial growth and chemical synthesis. The ability to fabricate size-controlled silicon nanocrystals encapsulated by insulating SiO2 would be of significant interest to the microelectronics industry. But reproducible manufacture of such crystals is hampered by the amorphous nature of SiO2 and the differing thermal expansion coefficients of the two materials. Previous attempts to fabricate Si nanocrystals failed to achieve control over their shape and crystallographic orientation, the latter property being important in systems such as Si quantum dots. Here we report the self-organization of Si nanocrystals larger than 80 Å into brick-shaped crystallites oriented along the 〈111〉 crystallographic direction. The nanocrystals are formed by the solid-phase crystallization of nanometre-thick layers of amorphous Si confined between SiO2 layers. The shape and orientation of the crystallites results in relatively narrow photoluminescence, whereas isotropic particles produce qualitatively different, broad light emission. Our results should aid the development of maskless, reproducible Si nanofabrication techniques.

Light scattering in semiconductor structures and superlattices

Inelastic Light Scattering from Semiconductors E. Burstein, et al. Optic Phonons: Acoustic, Optic and Interface Phonons: Low Symmetry Superlattices M. Cardona. Raman Scattering in alpha-Sn1-xGex Alloys J. Menendez, et al. Acoustic Phonons: Interaction of Light with Acoustic Waves in Superlattices and Related Devices J. Sapriel, et al. Localized and Extended Acoustic Waves in Superlattices: Light Scattering by Longitudinal Phonons B. Djafari Rouhani, et al. Strain Related Effects: Optical Phonon Raman Scattering as a Local Probe of SiGe Strained Layers J.C. Tsang, et al. Strain Characterization of Semiconductor Structures and Superlattices E. Anastassakis. Micro Raman/Small Structures/Impurities: The Raman Line Shape of Semiconductor Nanocrystals P.M. Fauchet. Raman Scattering of IIIV and IIVI Semiconductor Microstructures M. Watt, et al. Magnetic Superlattices and IIVI Materials: Surface Modes in Magnetic Semiconductor Films and Multilayers M.G. Cottam, et al. Vibrational, Electronic, and Magnetic Excitations in IIVI Quantum Well Structures A.K. Ramdas, et al. Time Resolved Studies: Nonequilibrium Electrons and Phonons in GaAs and Related Materials J.A. Kash. 30 additional articles. Index.

Quantum confinement in Si and Ge nanostructures

We apply perturbative effective mass theory as a broadly applicable theoretical model for quantum confinement (QC) in all Si and Genanostructures including quantum wells(QWs), wires (Q-wires), and dots(QDs). Within the limits of strong, medium, and weak QC, valence and conduction band edge energy levels (VBM and CBM) were calculated as a function of QD diameters, QW thicknesses, and Q-wire diameters. Crystalline and amorphous quantum systems were considered separately. Calculated band edge levels with strong, medium, and weak QC models were compared with experimental VBM and CBM reported from X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS), or photoluminescence(PL). Experimentally, the dimensions of the nanostructures were determined directly, by transmission electron microscopy(TEM), or indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline materials are best described by a medium confinement model, while amorphous materials exhibit strong confinement regardless of the dimensionality of the system. Our results indicate that spatial delocalization of the hole in amorphous versus crystalline nanostructures is the important parameter determining the magnitude of the band gap expansion, or the strength of the quantum confinement. In addition, the effective masses of the electron and hole are discussed as a function of crystallinity and spatial confinement.

Visible photoluminescence from porous GaAs

Porous GaAs was formed electrochemically on n‐type GaAs(100) in a 0.1 M HCl electrolyte. Scanning electron microscopy revealed feature sizes of the porous structure in the micrometer to nanometer range. The optical properties of the porous material were characterized by photoluminescence (PL) measurements at 295 K. Compared with untreated GaAs, a shift down of the ‘‘infrared’’ PL maximum to ∼840 nm can be observed. An additional ‘‘green’’ PL peak occurs at ∼540 nm that in some samples is readily visible to the naked eye. The ‘‘green’’ and the ‘‘infrared’’ PL are ascribed to quantum confinement effects in GaAs nano‐ and microcrystallites, respectively.

Photoluminescence in amorphous Si/SiO2 superlattices fabricated by magnetron sputtering

Amorphous Si/SiO2 superlattices with 100–525 periods and a 2–3 nm periodicity were deposited by radio frequency magnetron sputtering onto silicon and quartz substrates. All samples exhibited visible photoluminescence (PL) at room temperature and the PL peak wavelength shifted towards shorter wavelengths with decreasing Si layer thickness. For 425 and 525 period superlattices there was a strong modulation of the PL intensity and optical transmittance versus wavelength resulting from optical interference within the superlattice. The PL intensity increased dramatically after annealing the superlattices in air at 1100 °C.

Raman and infrared spectroscopy of α and β phases of thin nickel hydroxide films electrochemically formed on nickel.

The present work utilizes Raman and infrared (IR) spectroscopy, supported by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) to re-examine the fine structural details of Ni(OH)(2), which is a key material in many energy-related applications. This work also unifies the large body of literature on the topic. Samples were prepared by the galvanostatic basification of nickel salts and by aging the deposits in hot KOH solutions. A simplified model is presented consisting of two fundamental phases (α and β) of Ni(OH)(2) and a range of possible structural disorder arising from factors such as impurities, hydration, and crystal defects. For the first time, all of the lattice modes of β-Ni(OH)(2) have been identified and assigned using factor group analysis. Ni(OH)(2) films can be rapidly identified in pure and mixed samples using Raman or IR spectroscopy by measuring their strong O-H stretching modes, which act as fingerprints. Thus, this work establishes methods to measure the phase, or phases, and disorder at a Ni(OH)(2) sample surface and to correlate desired chemical properties to their structural origins.

Stabilization of porous silicon electroluminescence by surface passivation with controlled covalent bonds

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.

Thermal Route for Chemical Modification and Photoluminescence Stabilization of Porous Silicon

This paper describes photoluminescence (PL) stabilization through chemical modification of freshly prepared porous silicon (PSi) surfaces. As-anodized PSi surfaces react with 1-alkenes, non-conjugated dienes and aldehydes at elevated temperatures to form organic monolayers covalently bonded to the surface. This thermal route is very general and tolerant of different functional groups. We have characterized these organic monolayers using diffuse reflectance infrared Fourier-transform (DRIFT), Auger and Raman spectroscopies. The PL intensity and peak energy of the as-anodized PSi is not affected by the chemical functionalization. Aging these derivatized PSi samples in ambient air has no effect on the PL. In fact, it is completely preserved even when they are steam treated for six weeks at 70 °C and 100% humidity. This treatment completely destroys the structural integrity of H-terminated PSi.

Quantum confinement in Si and Ge nanostructures: Theory and experiment

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminesc...