315 research outputs found
Femtosecond Electron Transfer from the Excited State of Chemically Anchored Chromophores into the Empty Conduction Band of Nanocrystalline Spong-like TiO<sub>2</sub> Films*
Atomic scale analysis of the GaP Si 100 heterointerface by in situ reflection anisotropy spectroscopy and ab initio density functional theory
A microscopic understanding of the formation of polar on nonpolar interfaces is a prerequisite for well defined heteroepitaxial preparation of III V compounds on 100 silicon for next generation high performance devices. Energetically and kinetically driven Si 100 step formations result in majority domains of monohydride terminated Si dimers oriented either parallel or perpendicular to the step edges. Here, the intentional variation of the Si 100 surface reconstruction controls the sublattice orientation of the heteroepitaxial GaP film, as observed by in situ reflection anisotropy spectroscopy RAS in chemical vapor ambient and confirmed by benchmarking to surface science analytics in ultrahigh vacuum. Ab initio density functional calculations of both abrupt and compensated interfaces are carried out. For P rich chemical potentials at abrupt interfaces, Si P bonds are energetically favored over Si Ga bonds, in agreement with in situ RAS experiments. The energetically most favorable interface is compensated with an intermixed interfacial layer. In situ RAS reveals that the GaP sublattice orientation depends on the P chemical potential during nucleation, which agrees with a kinetically limited formation of abrupt interface
Time resolved in situ spectroscopy during formation of the GaP Si 100 heterointerface
Though III V Si 100 heterointerfaces are essential for future epitaxial high performance devices, their atomic structure is an open historical question. Benchmarking of transient optical in situ spectroscopy during chemical vapor deposition to chemical analysis by X ray photoelectron spectroscopy enables us to distinguish between formation of surfaces and of the heterointerface. A terrace related optical anisotropy signal evolves during pulsed GaP nucleation on single domain Si 100 surfaces. This dielectric anisotropy agrees well with the one calculated for buried GaP Si 100 interfaces from differently thick GaP epilayers. X ray photoelectron spectroscopy reveals a chemically shifted contribution of the P and Si emission lines, which quantitatively corresponds to one monolayer and establishes simultaneously with the nucleation related optical in situ signal. We attribute that contribution to the existence of Si P bonds at the buried heterointerface. During further pulsing and annealing in phosphorus ambient, dielectric anisotropies known from atomically well ordered GaP 100 surfaces super impose the nucleation related optical in situ spectra. Figure Presente
Photoemission study and band alignment of GaN passivation layers on GaInP heterointerface
III-V semiconductor-based photoelectrochemical (PEC) devices show the highest
solar-to-electricity or solar-to-fuel conversion efficiencies. GaInP is a
relevant top photoabsorber layer or a charge-selective contact in PEC for
integrated and direct solar fuel production, due to its tunable lattice
constant, electronic band structure, and favorable optical properties. To
enhance the stability of its surface against chemical corrosion which leads to
decomposition, we deposit a GaN protection and passivation layer. The n-doped
GaInP(100) epitaxial layers were grown by metalorganic chemical vapor
deposition on top of GaAs(100) substrate. Subsequently, thin 1-20 nm GaN films
were grown on top of the oxidized GaInP surfaces by atomic layer deposition. We
studied the band alignment of these multi-junction heterostructures by X-ray
and ultraviolet photoelectron spectroscopy. Due to the limited emission depth
of photoelectrons, we determined the band alignment by a series of separate
measurements in which we either modified the GaInP(100) surface termination or
the film thickness of the grown GaN on GaInP(100) buffer layers. On
n-GaInP(100) surfaces prepared with the well-known phosphorus-rich (2x2)/c(4x2)
reconstruction we found up-ward surface band bending (BB) of 0.34 eV, and Fermi
level pinning due to the present surface states. Upon oxidation, the surface
states are partially passivated resulting in a reduction of BB to 0.12 eV and a
valence band offset (VBO) between GaInP and oxide bands of 2.0 eV. Between the
GaInP(100) buffer layer and the GaN passivation layer, we identified a VBO of
1.8 eV. The corresponding conduction band offset of -0.2 eV is found to be
rather small. Therefore, we evaluate the application of the GaN passivation
layer as a promising technological step not only to reduce surface states but
also to increase the stability of the surfaces of photoelectrochemical devices
Generalized Modeling of Photoluminescence Transients
Time resolved photoluminescence TRPL measurements and the extraction of meaningful parameters involve four key ingredients a suitable sample such as a semiconductor double heterostructure, a state of the art measurement setup, a kinetic model appropriate for the description of the sample behavior, and a general analysis method to extract the model parameters of interest from the measured TRPL transients. Until now, the last ingredient is limited to single curve fits, which are mostly based on simple models and least squares fits. These are often insufficient for the parameter extraction in real world applications. The goal of this article is to give the community a universal method for the analysis of TRPL measurements, which accounts for the Poisson distribution of photon counting events. The method can be used to fit multiple TRPL transients simultaneously using general kinematic models, but should also be used for single transient fits. To demonstrate this approach, multiple TRPL transients of a GaAs AlGaAs heterostructure are fitted simultaneously using coupled rate equations. It is shown that the simultaneous fits of several TRPL traces supplemented by systematic error estimations allow for a more meaningful and more robust parameter determination. The statistical methods also quantify the quality of the description by the underlying physical mode
Experimental constraints on the -nucleus real potential
In a search for mesic states, the production of -mesons in
coincidence with forward going protons has been studied in photon induced
reactions on C for incident photon energies of 1250 - 3100 MeV. The
pairs from decays of bound or quasi-free -mesons have
been measured with the CBELSA/TAPS detector system in coincidence with protons
registered in the MiniTAPS forward array. Structures in the total energy
distribution of the pairs, which would indicate the population
and decay of bound B states, are not observed. The
cross section of 0.3 nb/MeV/sr observed in the bound state energy regime
between -100 and 0 MeV may be accounted for by yield leaking into the bound
state regime because of the large in-medium width of the -meson. A
comparison of the measured total energy distribution with calculations suggests
the real part of the B potential to be small and only
weakly attractive with 35(stat) 20(syst) MeV
in contrast to some theoretical predictions of attractive potentials with a
depth of 100 - 150 MeV.Comment: 13 pages, 8 figure
K0-Sigma+ Photoproduction with SAPHIR
Preliminary results of the analysis of the reaction p(gamma,K0)Sigma+ are
presented. We show the first measurement of the differential cross section and
much improved data for the total cross section than previous data. The data are
compared with model predictions from different isobar and quark models that
give a good description of p(gamma,K+)Lambda and p(gamma,K+)Sigma0 data in the
same energy range. Results of ChPT describe the data adequately at threshold
while isobar models that include hadronic form factors reproduce the data at
intermediate energies.Comment: 4 pages, Latex2e, 4 postscript figures. Talk given at the
International Conference on Hypernuclear and Strange Particle Physics
(HYP97), Brookhaven National Laboratory, USA, October 13-18, 1997. To be
published in Nucl. Phys. A. Revised version due to changes in experimental
dat
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