574 research outputs found
Low Energy Cathodoluminescence Spectroscopy of Semiconductor Interfaces
Low energy cathodoluminescence spectroscopy (CLS) is a powerful new technique for characterizing the electronic structure of buried semiconductor interfaces. This extension of a more conventional electron microscopy technique provides information on localized states, deep level defects, and band structure of new compounds at interfaces below the free solid surface. From the energy dependence of spectral features, one can distinguish interface versus bulk state emission and assess the relative spatial distribution of states below the free surface. Low energy CLS reveals process changes in the electronic structure of semiconductor interfaces due to metallization, laser annealing, and thermal desorption. Spectral features of metal-semiconductor interfaces uncovered by CLS also provide a new perspective on physical mechanisms of Schottky barrier formation
Raman and Infra-red properties and layer dependence of the phonon dispersions in multi-layered graphene
The symmetry group analysis is applied to classify the phonon modes of
-stacked graphene layers (NSGL's) with AB- and AA-stacking, particularly
their infra-red and Raman properties. The dispersions of various phonon modes
are calculated in a multi-layer vibrational model, which is generalized from
the lattice vibrational potentials of graphene to including the inter-layer
interactions in NSGL's. The experimentally reported red shift phenomena in the
layer number dependence of the intra-layer optical C-C stretching mode
frequencies are interpreted. An interesting low frequency inter-layer optical
mode is revealed to be Raman or Infra-red active in even or odd NSGL's
respectively. Its frequency shift is sensitive to the layer number and
saturated at about 10 layers.Comment: enlarged versio
Topological Dirac Semimetal Na3Bi Films in the Ultrathin Limit via Alternating Layer Molecular Beam Epitaxy
Ultrathin films of Na3Bi on insulating substrates are desired for opening a
bulk band gap and generating the quantum spin Hall effect from a topological
Dirac semimetal, though continuous films in the few nanometer regime have been
difficult to realize. Here, we utilize alternating layer molecular beam epitaxy
(MBE) to achieve uniform and continuous single crystal films of Na3Bi(0001) on
insulating Al2O3(0001) substrates and demonstrate electrical transport on films
with 3.8 nm thickness (4 unit cells). The high material quality is confirmed
through in situ reflection high-energy electron diffraction (RHEED), scanning
tunneling microscopy (STM), x-ray diffraction (XRD), and x-ray photoelectron
spectroscopy (XPS). In addition, these films are employed as seed layers for
subsequent growth by codeposition, leading to atomic layer-by-layer growth as
indicated by RHEED intensity oscillations. These material advances facilitate
the pursuit of quantum phenomena in thin films of Dirac semimetals.Comment: 11 pages, 5 figure
Luminescence Spectroscopy of Semiconductor Surfaces and Interfaces
Low energy cathodoluminescence spectroscopy (CLS) employing incident electron energies in the range of a few kV or less enable measurement of electronic structure near semiconductor surfaces and interfaces. Coupled with photoluminescence spectroscopy (PL), the CLS technique has been extended to characterize electronic structure tens of nanometers below the free surface at metal-semiconductor and semiconductor-semiconductor junctions. CLS has revealed discrete, deep electronic states for clean and metallized semiconductor surfaces as a function of atomic ordering as well as vicinal surfaces as a function of misorientation. A combination of CLS and PL reveals deep level features associated with strain relaxation and dislocations at heterojunction interfaces as well as variations in epilayer growth conditions. Such observations demonstrate the existence of discrete, deep levels in the semiconductor band gap and their sensitivity to chemical and atomic structure near surfaces and interfaces. Furthermore, the energies and densities of such deep levels provide a consistent picture of Fermi level stabilization and band bending at semiconductor contacts. Finally, our results indicate that deep level CLS/PL measurements are an effective, in-situ probe of surface and interface quality
Vacancy defect and defect cluster energetics in ion-implanted ZnO
We have used depth-resolved cathodoluminescence, positron annihilation, and surface photovoltage spectroscopies to determine the energy levels of Zn vacancies and vacancy clusters in bulk ZnO crystals. Doppler broadening-measured transformation of Zn vacancies to vacancy clusters with annealing shifts defect energies significantly lower in the ZnO band gap. Zn and corresponding O vacancy-related depth distributions provide a consistent explanation of depth-dependent resistivity and carrier-concentration changes induced by ion implantation.Peer reviewe
Origins of luminescence from nitrogen-ion-implanted epitaxial GaAsGaAs
We have examined the origins of luminescence in N-ion-implanted epitaxial GaAsGaAs, using a combination of cross-sectional transmission electron microscopy and low-energy electron-excited nanoscale-luminescence spectroscopy. A comparison of reference, as-implanted, and implanted-plus-annealed samples reveals a variety of emissions. In all samples, we observe the GaAsGaAs fundamental band-gap emission, as well as several emissions related to GaAsGaAs native defects. In the as-implanted and implanted-plus-annealed samples, an emission related to the implantation-induced defects, is also observed. Interestingly, in the implanted-plus-annealed samples, we identify a near-infrared emission associated with GaAsNGaAsN nanocrystallites.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69942/2/APPLAB-85-14-2774-1.pd
Universal dynamical conductance in graphite
We find experimentally that the optical sheet conductance of graphite per
graphene layer is very close to , which is the theoretically
expected value of dynamical conductance of isolated monolayer graphene. Our
calculations within the Slonczewski-McClure-Weiss model explain well why the
interplane hopping leaves the conductance of graphene sheets in graphite almost
unchanged for photon energies between 0.1 and 0.6 eV, even though it
significantly affects the band structure on the same energy scale. The f-sum
rule analysis shows that the large increase of the Drude spectral weight as a
function of temperature is at the expense of the removed low-energy optical
spectral weight of transitions between hole and electron bands.Comment: 4 pages, 4 figure
Role of surface defects on visible light enabled plasmonic photocatalysis in Au–ZnO nanocatalysts
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