166 research outputs found
Low temperature removal of surface oxides and hydrocarbons from Ge(100) using atomic hydrogen
Germanium is a group IV semiconductor with many current and potential applications in the modern semiconductor industry. Key to expanding the use of Ge is a reliable method for the removal of surface contamination, including oxides which are naturally formed during the exposure of Ge thin films to atmospheric conditions. A process for achieving this task at lower temperatures would be highly advantageous, where the underlying device architecture will not diffuse through the Ge film while also avoiding electronic damage induced by ion irradiation. Atomic hydrogen cleaning (AHC) offers a low temperature, damage-free alternative to the common ion bombardment and annealing (IBA) technique which is widely employed. In this work, we demonstrate with xray photoelectron spectroscopy (XPS) that the AHC method is effective in removing surface oxides and hydrocarbons, yielding an almost completely clean surface when the AHC is conducted at a temperature of 250 ◦C. We compare the post-AHC cleanliness and
(2 × 1) low energy electron diffraction (LEED) pattern to that obtained via IBA, where the sample is annealed at 600 ◦C. We also demonstrate that the combination of a sample temperature of 250 ◦C and atomic H dosing is required to clean the surface. Lower temperatures prove less effective in removal of the oxide layer and hydrocarbons, whilst annealing in ultra-high vacuum conditions only removes weakly bound hydrocarbons. Finally, we examine the subsequent H-termination of an IBA-cleaned sample using XPS, LEED and ultraviolet photoelectron spectroscopy (UPS) in order to examine changes in the work function of Ge(100) upon hydrogenation
van der Waals epitaxy of monolayer hexagonal boron nitride on copper foil : growth, crystallography and electronic band structure
We investigate the growth of hexagonal boron nitride (h-BN) on copper foil by low pressure chemical vapour deposition (LP-CVD). At low pressure, h-BN growth proceeds through the nucleation and growth of triangular islands. Comparison between the orientation of the islands and the local crystallographic orientation of the polycrystalline copper foil reveals an epitaxial relation between the copper and h-BN, even on Cu(100) and Cu(110) regions whose symmetry is not matched to the h-BN. However, the growth rate is faster and the islands more uniformly oriented on Cu(111) grains. Angle resolved photoemission spectroscopy measurements reveal a well-defined band structure for the h-BN, consistent with a band gap of 6 eV, that is decoupled from the copper surface beneath. These results indicate that, despite a weak interaction between h-BN and copper, van der Waals epitaxy defines the long range ordering of h-BN even on polycrystalline copper foils and suggest that large area, single crystal, monolayer h-BN could be readily and cheaply produced
Severity and Treatment of Alcohol Withdrawal in Elderly Versus Younger Patients
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65425/1/j.1530-0277.1994.tb00903.x.pd
Effect of oxygen and nitrogen functionalization on the physical and electronic structure of graphene
Covalent functionalization of graphene offers opportunities for tailoring its properties and is an unavoidable consequence of some graphene synthesis techniques. However, the changes induced by the functionalization are not well understood. By using atomic sources to control the extent of the oxygen and nitrogen functionalization, we studied the evolution in the structure and properties at the atomic scale. Atomic oxygen reversibly introduces epoxide groups whilst, under similar conditions, atomic nitrogen irreversibly creates diverse functionalities including substitutional, pyridinic, and pyrrolic nitrogen. Atomic oxygen leaves the Fermi energy at the Dirac point (i.e., undoped), whilst atomic nitrogen results in a net n-doping; however, the experimental results are consistent with the dominant electronic effect for both being a transition from delocalized to localized states, and hence the loss of the signature electronic structure of graphene
Pinning effect on the band gap modulation of crystalline Be<sub>x</sub>Zn<sub>1</sub>-<sub>x</sub>O alloy films grown on Al<sub>2</sub>O<sub>3</sub>(0001)
Quasar accretion disk sizes from continuum reverberation mapping in the DES standard-star fields
Measurements of the physical properties of accretion disks in active galactic
nuclei are important for better understanding the growth and evolution of
supermassive black holes. We present the accretion disk sizes of 22 quasars
from continuum reverberation mapping with data from the Dark Energy Survey
(DES) standard star fields and the supernova C fields. We construct continuum
lightcurves with the \textit{griz} photometry that span five seasons of DES
observations. These data sample the time variability of the quasars with a
cadence as short as one day, which corresponds to a rest frame cadence that is
a factor of a few higher than most previous work. We derive time lags between
bands with both JAVELIN and the interpolated cross-correlation function method,
and fit for accretion disk sizes using the JAVELIN Thin Disk model. These new
measurements include disks around black holes with masses as small as
, which have equivalent sizes at 2500\AA \, as small as
light days in the rest frame. We find that most objects have
accretion disk sizes consistent with the prediction of the standard thin disk
model when we take disk variability into account. We have also simulated the
expected yield of accretion disk measurements under various observational
scenarios for the Large Synoptic Survey Telescope Deep Drilling Fields. We find
that the number of disk measurements would increase significantly if the
default cadence is changed from three days to two days or one day.Comment: 33 pages, 24 figure
Heteroepitaxial growth of ferromagnetic MnSb(0001) films on Ge/Si(111) virtual substrates
Molecular beam epitaxial growth of ferromagnetic MnSb(0001) has been achieved on high quality, fully relaxed Ge(111)/Si(111) virtual substrates grown by reduced pressure chemical vapor deposition. The epilayers were characterized using reflection high energy electron diffraction, synchrotron hard X-ray diffraction, X-ray photoemission spectroscopy, and magnetometry. The surface reconstructions, magnetic properties, crystalline quality, and strain relaxation behavior of the MnSb films are similar to those of MnSb grown on GaAs(111). In contrast to GaAs substrates, segregation of substrate atoms through the MnSb film does not occur, and alternative polymorphs of MnSb are absent
Large scale physical modeling synthesis, parallel computing, and musical experimentation:The NESS Project
Identification of Lone-Pair Surface States on Indium Oxide
Indium
oxide is widely used as a transparent electrode in optoelectronic
devices and as a photocatalyst with activity for reduction of CO<sub>2</sub>. However, very little is known about the structural and electronic
properties of its surfaces, particularly those prepared under reducing
conditions. In this report, directional “lone-pair”
surface states associated with filled 5s<sup>2</sup> orbitals have
been identified on vacuum-annealed In<sub>2</sub>O<sub>3</sub>(111)
through a combination of hard and soft X-ray photoemission spectroscopy
and density functional theory calculations. The lone pairs reside
on indium ad-atoms in a formal +1 oxidation state, each of which traps
two electrons into a localized hybrid orbital protruding away from
the surface and lying just above the valence band maximum in photoemission
spectra. The third electron associated with the ad-atoms is delocalized
into the conduction band, thus producing the surface electron accumulation
layer identified previously on vacuum-annealed In<sub>2</sub>O<sub>3</sub>(111) (1 × 1) surfaces. The surface structure is further
supported by low-energy electron diffraction, but there is no chemical
shift in indium core level X-ray photoelectron spectra between surface
In(I) ad-atoms and bulk In(III). The 5s<sup>2</sup> lone pairs confer
Lewis basicity on the surface In sites and may have a pronounced impact
on the catalytic or photocatalytic activity of reduced In<sub>2</sub>O<sub>3</sub>
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