Andrew Williamson

Light emission from silicon-rich nitride nanostructures

Light-emitting Si-rich silicon nitride (SRN) films were fabricated by plasma enhanced chemical vapor deposition followed by low temperature (500–900°C) annealing. The optical properties of SRN films were studied by micro-Raman and photoluminescence spectroscopy and indicate the presence of small Si clusters characterized by broad near-infrared emission, large absorption/emission Stokes shift, and nanosecond recombination. Our results are supported by first-principles simulations indicating that N atoms bonded to the surface of nanometer Si clusters play a crucial role in the emission mechanism of SRN films. Light emission from SRN systems can provide alternative routes towards the fabrication of optically active Si devices.

Electron emission from diamondoids : A diffusion quantum Monte Carlo study

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.

Surface control of optical properties in silicon nanoclusters

Density functional and quantum Monte Carlo calculations are employed to determine the effect of surface passivants on the optical gap of silicon nanoclusters. Our results show that quantum confinement is only one mechanism responsible for visible photoluminescence and that the specific surface chemistry must be taken into account in order to interpret experimental results. Significant changes occur in the optical gap of fully hydrogenated silicon nanoclusters when the surface contains passivants that change the bonding network at the surface. In the case of just one double-bonded oxygen atom, the gap reduction computed as a function of the nanocluster size demonstrates that one contaminant can greatly alter the optical gap. A further significant reduction of the gap occurs with multiple double-bonded oxygen contamination, providing a consistent interpretation of several recent experiments. We predict that other passivants that distort the tetrahedral bonding network at the surface, including other double-bonded groups and in some cases bridged oxygen, will also significantly affect the optical gap. Conversely, single-bonded passivants will have a minimal influence on the optical gap. A discussion of the difference in the strength of the optical transitions for clusters with different passivants is presented.

Atomistic design of thermoelectric properties of silicon nanowires.

We present predictions of the thermoelectric figure of merit ( ZT) of Si nanowires with diameter up to 3 nm, based upon the Boltzman transport equation and ab initio electronic structure calculations. We find that ZT depends significantly on the wire growth direction and surface reconstruction, and we discuss how these properties can be tuned to select silicon based nanostructures with combined n-type and p-type optimal ZT. Our calculations show that only by reducing the ionic thermal conductivity by about 2 or 3 orders of magnitudes with respect to bulk values, one may attain ZT larger than 1, for 1 or 3 nm wires, respectively. We also find that ZT of p-doped wires is considerably smaller than that of their n-doped counterparts with the same size and geometry.

DIFFUSION QUANTUM MONTE CARLO CALCULATIONS OF THE EXCITED STATES OF SILICON

The band structure of silicon is calculated at the G, X, andL wave vectors using diffusion quantum Monte Carlo ~DMC! methods. Excited states are formed by promoting an electron from the valence band into the conduction band. We obtain good agreement with experiment for states around the gap region, and demonstrate that the method works equally well for direct and indirect excitations, and that one can calculate many excited states at each wave vector. This work establishes the fixed-node DMC approach as an accurate method for calculating the energies of low-lying excitations in solids. @S0163-1829 ~98!11819-2#

Light-Emitting Silicon Nanocrystals and Photonic Structures in Silicon Nitride

In this paper, we review our main results on the optical and electrical properties of light-emitting silicon nanocrystals (Si-ncs) obtained from the thermally induced nucleation in amorphous silicon-rich nitride (SRN) films deposited either by plasma-enhanced chemical vapor deposition (PE-CVD) or magnetron sputtering. In particular, we discuss the Si-ncs microscopic light emission mechanism combining the optical data with the first-principle calculations of the absorption/emission Stokes shifts and recombination lifetimes. In addition, we report on the electrical injection characteristics of simple p-i-n device structures showing efficient bipolar transport and room temperature electroluminescence, and demonstrate efficient energy sensitization of erbium (Er) ions from the Si-ncs embedded in the SRN matrices. We further show that the light-emitting nanocrystals in SRN can be embedded in aperiodic photonic environments, where the localized optical modes can be used to significantly enhance the Si-ncs emission intensity at different emission wavelengths. These results suggest that the Si-ncs embedded in the SRN matrices have a large potential for the fabrication of optically active photonic devices based on the Si technology

Finite size errors in quantum many-body simulations of extended systems

Further developments are introduced in the theory of finite-size errors in quantum many-body simulations of extended systems using periodic boundary conditions. We show that our recently introduced model periodic Coulomb interaction @A. J. Williamson et al., Phys. Rev. B 55, R4851 ~1997!# can be applied consistently to all Coulomb interactions in the system. The model periodic Coulomb interaction greatly reduces the finite-size errors in quantum many-body simulations. We illustrate the practical application of our techniques with Hartree-Fock and variational and diffusion quantum Monte Carlo calculations for ground- and excited-state calculations. We demonstrate that the finite-size effects in electron promotion and electron addition/subtraction excitation energy calculations are very similar. @S0163-1829~99!07303-8#

Optical properties of passivated silicon nanoclusters: The role of synthesis

The effect of preparation conditions on the structural and optical properties of silicon nanoparticles is investigated. Nanoscale reconstructions, unique to curved nanosurfaces, are presented for silicon nanocrystals and shown to have lower energy and larger optical gaps than bulk-derived structures. We find that high-temperature synthesis processes can produce metastable noncrystalline nanostructures with different core structures than bulk-derived crystalline clusters. The type of core structure that forms from a given synthesis process may depend on the passivation mechanism and time scale. The effect of oxygen on the optical of different types of silicon structures is calculated. In contrast to the behavior of bulklike nanostructures, for noncrystalline and reconstructed crystalline structures surface oxygen atoms do not decrease the gap. In some cases, the presence of oxygen atoms at the nanocluster surface can significantly increase the optical absorption gap, due to decreased angular distortion of the silicon bonds. The relationship between strain and the optical gap in silicon nanoclusters is discussed.

First-principles calculations of the dielectric properties of silicon nanostructures

We have investigated the static dielectric properties of silicon rods and slabs below 10nm, in the long wavelength limit, by using first-principles density functional theory calculations. Surface structure is found to be the most important factor affecting the changes of the dielectric response at the nanoscale, compared to that of bulk Si, with significant differences observed between slabs and finite rods of similar lateral dimensions.

Anomalous Photoluminescence in CdSe Quantum‐Dot Solids at High Pressure Due to Nonuniform Stress

The application of static high pressure provides a means to precisely control and investigate many fundamental and unique properties of nanoparticles. CdSe is a model quantum-dot system, the behavior of which under high pressure has been extensively studied; however, the effect of nonuniform stresses on this system has not been fully appreciated. Photoluminescence data obtained from CdSe quantum-dot solids in different stress environments varying from purely uniform to highly nonuniform are presented. Small deviations from a uniform stress distribution profoundly affect the electronic properties of this system. In nonuniform stress environments, a pronounced flattening of the photoluminescence enegy is observed above 3 GPa. The observations are validated with theoretical calculations obtained using an all-atom semiempirical pseudopotential technique. This effect must be considered when investigating other potentially pressure-mediated phenomena.