381 research outputs found
Molecular ion trap-depletion spectroscopy of BaCl
We demonstrate a simple technique for molecular ion spectroscopy. BaCl
molecular ions are trapped in a linear Paul trap in the presence of a
room-temperature He buffer gas and photodissociated by driving an electronic
transition from the ground X state to the repulsive wall of the
A state. The photodissociation spectrum is recorded by monitoring the
induced trap loss of BaCl ions as a function of excitation wavelength.
Accurate molecular potentials and spectroscopic constants are determined.
Comparison of the theoretical photodissociation cross-sections with the
measurement shows excellent agreement. This study represents the first
spectroscopic data for BaCl and an important step towards the production of
ultracold ground-state molecular ions.Comment: 5 pages, 5 figure
Transmission electron microscopy investigation of segregation and critical floating-layer content of indium for island formation in InGaAs
We have investigated InGaAs layers grown by molecular-beam epitaxy on
GaAs(001) by transmission electron microscopy (TEM) and photoluminescence
spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and
respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 C. The
parameters were chosen to grow layers slightly above and below the transition
between the two- and three-dimensional growth mode. In-concentration profiles
were obtained from high-resolution TEM images by composition evaluation by
lattice fringe analysis. The measured profiles can be well described applying
the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557].
Calculated photoluminescence peak positions on the basis of the measured
concentration profiles are in good agreement with the experimental ones.
Evaluating experimental In-concentration profiles it is found that the
transition from the two-dimensional to the three-dimensional growth mode occurs
if the indium content in the In-floating layer exceeds 1.1+/-0.2 monolayers.
The measured exponential decrease of the In-concentration within the cap layer
on top of the islands reveals that the In-floating layer is not consumed during
island formation. The segregation efficiency above the islands is increased
compared to the quantum wells which is explained tentatively by
strain-dependent lattice-site selection of In. In addition, In0.25Ga0.75As
quantum wells were grown at different temperatures between 500 oC and 550 oC.
The evaluation of concentration profiles shows that the segregation efficiency
increases from R=0.65 to R=0.83.Comment: 16 pages, 6 figures, 1 table, sbmitted in Phys. Rev.
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
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
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
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
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