287 research outputs found
Optical studies of Ge islanding on Si(111)
We report an experimental study of the optical properties of island layers resulting from molecular beam epitaxial deposition of Ge on Si(111) substrates. The combination of electroreflectance spectroscopy of the E1 transition and Raman scattering allows us to separately determine the strain and composition of the islands. For deposition at 500â°C a deposited layer of 1.36 nm of Ge assembles into 80 nm diameter islands 11 nm thick. The average Si impurity content in the islands is 2.5% while the average in-plane strain is 0.5%. Both strain and Si impurity content in islands decrease with increasing Ge depositio
Increase of the mean inner Coulomb potential in Au clusters induced by surface tension and its implication for electron scattering
Electron holography in a transmission electron microscope was applied to
measure the phase shift induced by Au clusters as a function of the cluster
size. Large phase shifts Df observed for small Au clusters cannot be described
by the well-known equation Df=C_E V_0 t (C_E: interaction constant, V_0: mean
inner Coulomb potential (MIP) of bulk gold, t: cluster thickness). The rapid
increase of the Au MIP with decreasing cluster size derived from Df, can be
explained by the compressive strain of surface atoms in the cluster
Multichannel quantum-defect theory for ultracold atom-ion collisions
We develop an analytical model for ultracold atom-ion collisions using the
multichannel quantum-defect formalism. The model is based on the analytical
solutions of the r^-4 long-range potential and on the application of a frame
transformation between asymptotic and molecular bases. This approach allows the
description of the atom-ion interaction in the ultracold domain in terms of
three parameters only: the singlet and triplet scattering lengths, assumed to
be independent of the relative motion angular momentum, and the lead dispersion
coefficient of the asymptotic potential. We also introduce corrections to the
scattering lengths that improve the accuracy of our quantum-defect model for
higher order partial waves, a particularly important result for an accurate
description of shape and Feshbach resonances at finite temperature. The theory
is applied to the system composed of a 40Ca+ ion and a Na atom, and compared to
numerical coupled-channel calculations carried out using ab initio potentials.
For this particular system, we investigate the spectrum of bound states, the
rate of charge-transfer processes, and the collision rates in the presence of
magnetic Feshbach resonances at zero and finite temperature.Comment: 39 pages, 21 figure
Characterization of nanometer scale compositionally inhomogeneous AlGaN active regions on bulk AlN substrates
The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330 ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions
Schottky barrier height measurements of type-A and type-B NiSi2 epilayers on Si
Schottky barrier heights of single-crystal type-A and type-B NiSi2 epilayers on nondegenerate n-(111) Si have been measured by photoresponse and forward IâV methods. High-quality molecular beam epitaxy grown NiSi2 layers of thicknesses ranging from 70 to 600 Ă
on sputter-cleaned, P-doped Si subtrates (~ 1.5 Ă 1015 cm â 3) were studied. The type-A and type-B orientations consistently yield photoresponse barrier heights which differ by greater than 0.1 eV. We observe the value phi Bn=0.62 ± 0.01 eV for all type-A structures from both photoresponse and IâV measurements. However, we obtain a discrepancy between barrier heights measured by IâV (phi Bn=0.69 ± 0.01 eV) and photoresponse (phi Bn=0.77 ± 0.05 eV) methods, and in addition consistently observe an unusual bowing of the type-B photoresponse curves at low photon energies. We show that both the detailed shape of the type-B photoresponse curves and the discrepancy between IâV and photoresponse-measured barrier heights can be accounted for by modeling the type-B barrier as a mixture of high and low barrier regions. Quantitative agreement with experiment is obtained for the values phi hi =0.81 ± 0.01 eV and phi lo 0.64 ± 0.01 eV, with effective fractional area coverages of 91% and 9% for high- and low-barrier regions, respectively
Heteroepitaxy of AlGaN on bulk AlN substrates for deep ultraviolet light emitting diodes
The authors report the growth of AlGaN epilayers and deep ultraviolet (UV) light emitting diodes (LEDs) on bulk AlN substrates. Heteroepitaxial nucleation and strain relaxation are studied through controlled growth interruptions. Due to a low density of preexisting dislocations in bulk AlN, the compressive strain during AlGaN heteroepitaxy cannot be relieved effectively. The built-up of strain energy eventually induces either an elastic surface roughening or plastic deformation via generation and inclination of dislocations, depending on the stressor interlayers and growth parameters used. AlGaN LEDs on bulk AlN exhibit noticeable improvements in performance over those on sapphire, pointing to a promising substrate platform for III-nitride UV optoelectronics.open352
Physics and chemistry of hydrogen in the vacancies of semiconductors
Hydrogen is well known to cause electrical passivation of lattice vacancies in semiconductors. This effect follows from the chemical passivation of the dangling bonds. Recently it was found that H in the carbon vacancy of SiC forms a three-center bond with two silicon neighbors in the vacancy, and gives rise to a new electrically active state. In this paper we examine hydrogen in the anion vacancies of BN, AlN, and GaN. We find that three-center bonding of H is quite common and follows clear trends in terms of the second-neighbor distance in the lattice, the typical (two-center) hydrogen-host-atom bond length, the electronegativity difference between host atoms and hydrogen, as well as the charge state of the vacancy. Three-center bonding limits the number of H atoms a nitrogen vacancy can capture to two, and prevents electric passivation in GaAs as well
Local field distributions in spin glasses
Numerical results for the local field distributions of a family of Ising
spin-glass models are presented. In particular, the Edwards-Anderson model in
dimensions two, three, and four is considered, as well as spin glasses with
long-range power-law-modulated interactions that interpolate between a
nearest-neighbour Edwards-Anderson system in one dimension and the
infinite-range Sherrington-Kirkpatrick model. Remarkably, the local field
distributions only depend weakly on the range of the interactions and the
dimensionality, and show strong similarities except for near zero local field.Comment: 17 pages, 34 eps-figs included, extensive updates and new results, as
to appear in JPA, find related articles at
http://www.physics.emory.edu/faculty/boettche
Near-Band-Edge Photoluminescence of Wurtzite-Type AlN
Temperature-dependentphotoluminescence(PL)measurements were performed for A-plane and C-plane bulk AlN single crystals and epitaxial layers on sapphire. A strong near-band-edge (NBE) emission and deep-level luminescence were observed. At low excitations, the emission spectra are dominated by free and bound excitonic transitions and their LO-phonon replicas. At high excitations, the broadening and redshift of the NBE band is attributed to dense electronâhole plasma formation. The PL spectra differences of bulk single crystals and epilayers is explained by the electronâhole plasma expansion peculiarities
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