Brian S. Dennis

Nitrogen plasma annealing for low temperature Ta2O5 films

A low temperature oxygen/nitrogen plasma process is reported that substantially reduces leakage currents in chemical vapor deposited (CVD) and physical vapor deposited (PVD) films of tantalum oxide. We show that a combination of nitrogen and oxygen in a remote downstream microwave plasma source reduces leakage currents in CVD films of tantalum oxide and also reduces trap densities as measured by charge pumping. The as deposited CVD films show a high level of photoluminescence that is substantially lowered by the plasma anneal due to a reduction in the density of midgap states. For films deposited by PVD in the thickness range of 100 nm we find low leakage currents with a substantial improvement from the introduction of nitrogen into the plasma. However, PVD films in the thickness range of 20 nm show larger relative leakage currents and less of an improvement from the addition of nitrogen. The role of nitrogen in lowering leakage currents and charge trapping is thought to occur from a reduction in the dens...

Metamorphosis of a quantum wire into quantum dots

Bound states of electron–hole pairs (excitons) in semiconductors possess desirable properties — such as an enhanced oscillator strength for radiative recombination — that hold promise for the next generation of optical devices. However, at typical device operating conditions (room temperature and moderate charge densities), excitons dissociate to form an electron–hole plasma. Dissociation may be prevented by confining excitons to lower dimensions, where their binding energy is expected to increase significantly. But such confinement may in turn influence the dynamical properties of the excitons. Here we report spatially resolved photoluminescence images of excitons confined to an isolated gallium arsenide quantum wire. As the temperature of the structure is lowered, we observe a striking transition from broad and fairly continuous photoluminescence to an intense set of emission peaks which are both energetically sharp and spatially localized. Such behaviour indicates that, at sufficiently low temperatures, the quantum wire acts like a sparse set of quantum dots. Furthermore, at the site of an isolated quantum dot, we observe an unusual decrease in the relaxation rate of excitons, such that they radiate (via recombination) from higher energy states before relaxing to their ground state. We argue that this is the manifestation of an exciton relaxation ‘bottleneck’, the existence of which could pose problems for the development of optical devices based on quantum dots.

Large optical singularities of the one-dimensional electron gas in semiconductor quantum wires

Modulation-doped quantum wire structures have been fabricated in the extreme quantum limit in which only the lowest one-dimensional (1D) subband is occupied by electrons. A pronounced Fermi edge singularity is observed for the first time in the absorption and emission spectra of the 1D electron gas. It has a strong temperature dependence determined by the Fermi energy and is much sharper than in two dimensions. The experimental results agree qualitatively with exact diagonalization studies of finite Hubbard chains.

Inhomogeneous CuO6 tilt distribution and charge-spin correlations in La2-x-yNdySrxCuO4 around the commensurate hole concentration

Phononic and magnetic Raman scattering are studied in La2-x-yNdySrxCuO4 with three doping concentrations: x1/8, y = 0; x1/8, y = 0.4; and x = 0.01, y = 0. We observe strong disorder in the tilt pattern of the CuO6 octahedra in both the orthorhombic and tetragonal phases that persist down to 10 K and are coupled to bond disorder in the cation layers around 1/8 doping independent of Nd concentration. The weak magnitude of existing charge-spin modulations in the Nd-doped structure does not allow us to detect the specific Raman signatures of lattice dynamics or two-magnon scattering around 2200 cm–1. ©2003 The American Physical Society

Compact nanomechanical plasmonic phase modulators

Researchers exploit the strong dependence of gap-plasmon phase velocity on gap width to make a compact phase-modulator. An electromechanically variable gap size enables a 23-μm-long non-resonant modulator with moderate losses.

Observation of Leggett's Collective Mode in a Multiband MgB2 Superconductor

We report observation of Leggetts collective mode in a multiband MgB2 superconductor with Tc=39 K arising from the fluctuations in the relative phase between two superconducting condensates. The novel mode is observed by Raman spectroscopy at 9.4 meV in the fully symmetric scattering channel. The observed mode frequency is consistent with theoretical considerations based on first-principles computations.

COLLECTIVE EXCITATIONS IN THE DILUTE 2D ELECTRON SYSTEM

We report inelastic light scattering measurements of dispersive spin and charge density excitations in dilute 2D electron systems reaching densities less than 10^{10} cm^{-2}. In the quantum Hall state at nu=2, roton critical points in the spin inter--Landau level mode show a pronounced softening as r_s is increased. Instead of a soft mode instability predicted by Hartree--Fock calculations for r_s ~ 3.3, we find evidence of multiple rotons in the dispersion of the softening spin excitations. Extrapolation of the data indicates the possibility of an instability for r_s >~ 11.

Evidence of Correlation in Spin Excitations of Few-Electron Quantum Dots

We report inelastic light scattering measurements of spin and charge excitations in nanofabricated AlGaAs/GaAs quantum dots with few electrons. A narrow spin excitation peak is observed and assigned to the intershell triplet-to-singlet monopole mode of dots with four electrons. Configuration-interaction theory provides precise quantitative interpretations that uncover large correlation effects that are comparable to exchange Coulomb interactions.

Point defect populations in amorphous and crystalline silicon

Abstract Crystalline Si ( c -Si) and relaxed amorphous Si ( a -Si) have been ion-bombarded. The kinetics and temperature dependence of the heat released on annealing of these materials are found to be qualitatively similar for temperatures lower than epitaxial crystallization temperatures (550°C). This behavior suggests a close similarity between the mechanisms of structural relaxation in a -Si and defect annihilation in c -Si. The heat release measurements imply that ion bombardment generates a variety of point defects in both a -Si and c -Si.

Raman study of de‐relaxation and defects in amorphous silicon induced by MeV ion beams

Raman spectroscopy is used as a probe of the state of amorphous Si (a‐Si) and damaged crystalline Si. MeV ion beams have been used to irradiate structurally relaxed a‐Si. When the density of Si atoms displaced by nuclear collisions exceeds 5%, the a‐Si is ‘‘de‐relaxed’’, and thus returns to its as‐implanted state. This behavior is an indication that point defect complexes exist in a‐Si and play an important role in the process of structural relaxation.