93 research outputs found
Adsorption and desorption of hydrogen at nonpolar GaN(1-100) surfaces: Kinetics and impact on surface vibrational and electronic properties
The adsorption of hydrogen at nonpolar GaN(1-100) surfaces and its impact on
the electronic and vibrational properties is investigated using surface
electron spectroscopy in combination with density functional theory (DFT)
calculations. For the surface mediated dissociation of H2 and the subsequent
adsorption of H, an energy barrier of 0.55 eV has to be overcome. The
calculated kinetic surface phase diagram indicates that the reaction is
kinetically hindered at low pressures and low temperatures. At higher
temperatures ab-initio thermodynamics show, that the H-free surface is
energetically favored. To validate these theoretical predictions experiments at
room temperature and under ultrahigh vacuum conditions were performed. They
reveal that molecular hydrogen does not dissociatively adsorb at the GaN(1-100)
surface. Only activated atomic hydrogen atoms attach to the surface. At
temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed
hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface
atoms and result in an inversion of the Ga-N surface dimer buckling. The
signatures of the Ga-H and N-H vibrational modes on the H-covered surface have
experimentally been identified and are in good agreement with the DFT
calculations of the surface phonon modes. Both theory and experiment show that
H adsorption results in a removal of occupied and unoccupied intragap electron
states of the clean GaN(1-100) surface and a reduction of the surface upward
band bending by 0.4 eV. The latter mechanism largely reduces surface electron
depletion
Electrical conductivity and gas-sensing properties of Mg-doped and undoped single-crystalline In2O3 thin films: Bulk vs. surface
This study aims to provide a better fundamental understanding of the gas-sensing mechanism of In2O3-based conductometric gas sensors. In contrast to typically used polycrystalline films, we study single crystalline In2O3 thin films grown by molecular beam epitaxy (MBE) as a model system with reduced complexity. Electrical conductance of these films essentially consists of two parallel contributions: the bulk of the film and the surface electron accumulation layer (SEAL). Both these contributions are varied to understand their effect on the sensor response. Conductance changes induced by UV illumination in air, which forces desorption of oxygen adatoms on the surface, give a measure of the sensor response and show that the sensor effect is only due to the SEAL contribution to overall conductance. Therefore, a strong sensitivity increase can be expected by reducing or eliminating the bulk conductivity in single crystalline films or the intra-grain conductivity in polycrystalline films. Gas-response measurements in ozone atmosphere test this approach for the real application
Modeling the light-induced degradation (LID) in silicon due to ASi-Sii-defects
Light-induced degradation (LID) in silicon is one of the major problems that hamper the progress in silicon solar cell technology. We present a method to model the LID kinetics by a differential equation system based on the assumption of charge-state-change-induced configuration changes of the so-called ASi-Sii-defect. Assuming realistic transition rates, we solve this differential equation system under variation of some of the transition rates. It is found that the LID kinetics can in principle be modeled by this approach but care has to be taken if transition rates put into the model are directly extracted from time-dependent carrier lifetime measurements
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Electrical conductivity and gas-sensing properties of Mg-doped and undoped single-crystalline In2O3 thin films: Bulk vs. surface
This study aims to provide a better fundamental understanding of the gas-sensing mechanism of In2O3-based conductometric gas sensors. In contrast to typically used polycrystalline films, we study single crystalline In2O3 thin films grown by molecular beam epitaxy (MBE) as a model system with reduced complexity. Electrical conductance of these films essentially consists of two parallel contributions: the bulk of the film and the surface electron accumulation layer (SEAL). Both these contributions are varied to understand their effect on the sensor response. Conductance changes induced by UV illumination in air, which forces desorption of oxygen adatoms on the surface, give a measure of the sensor response and show that the sensor effect is only due to the SEAL contribution to overall conductance. Therefore, a strong sensitivity increase can be expected by reducing or eliminating the bulk conductivity in single crystalline films or the intra-grain conductivity in polycrystalline films. Gas-response measurements in ozone atmosphere test this approach for the real application
Quasiparticle interfacial level alignment of highly hybridized frontier levels: HO on TiO(110)
Knowledge of the frontier levels' alignment prior to photo-irradiation is
necessary to achieve a complete quantitative description of HO
photocatalysis on TiO(110). Although HO on rutile TiO(110) has been
thoroughly studied both experimentally and theoretically, a quantitative value
for the energy of the highest HO occupied levels is still lacking. For
experiment, this is due to the HO levels being obscured by hybridization
with TiO(110) levels in the difference spectra obtained via ultraviolet
photoemission spectroscopy (UPS). For theory, this is due to inherent
difficulties in properly describing many-body effects at the
HO-TiO(110) interface. Using the projected density of states (DOS) from
state-of-the-art quasiparticle (QP) , we disentangle the adsorbate and
surface contributions to the complex UPS spectra of HO on TiO(110). We
perform this separation as a function of HO coverage and dissociation on
stoichiometric and reduced surfaces. Due to hybridization with the TiO(110)
surface, the HO 3a and 1b levels are broadened into several peaks
between 5 and 1 eV below the TiO(110) valence band maximum (VBM). These
peaks have both intermolecular and interfacial bonding and antibonding
character. We find the highest occupied levels of HO adsorbed intact and
dissociated on stoichiometric TiO(110) are 1.1 and 0.9 eV below the VBM. We
also find a similar energy of 1.1 eV for the highest occupied levels of HO
when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In
both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than
those estimated from UPS difference spectra, which are inconclusive in this
energy region. Finally, we apply self-consistent QP (scQP1) to obtain
the ionization potential of the HO-TiO(110) interface.Comment: 12 pages, 12 figures, 1 tabl
Experimental and theoretical study of electronic and hyperfine properties of hydrogenated anatase (TiO): defects interplay and thermal stability
In this study we report on the results from emission Fe
Mssbauer Spectroscopy experiments, using dilute Mn implantation
into pristine (TiO) and hydrogenated anatase held at temperatures between
300-700 K. Results of the electronic structure and local environment are
complemented with ab-initio calculations. Upon implantation both Fe and
Fe are observed in pristine anatase, where the latter demonstrates the
spin-lattice relaxation. The spectra obtained for hydrogenated anatase show no
Fe contribution, suggesting that hydrogen acts as a donor. Due to the
low threshold, hydrogen diffuses out of the lattice. Thus showing a dynamic
behavior on the time scale of the Fe 14.4 keV state. The surrounding
oxygen vacancies favor the high-spin Fe state. The sample treated at
room temperature shows two distinct processes of hydrogen motion. The motion
commences with the interstitial hydrogen, followed by switching to the
covalently bound state. Hydrogen out-diffusion is hindered by bulk defects,
which could cause both processes to overlap. Supplementary UV-Vis and
electrical conductivity measurements show an improved electrical conductivity
and higher optical absorption after the hydrogenation. X-ray photoelectron
spectroscopy at room temperature reveals that the sample hydrogenated at 573 K
shows presence of both Ti and Ti states. This could imply that a
significant amount of oxygen vacancies and -OH bonds are present in the
samples. Theory suggests that in the anatase sample implanted with Mn(Fe),
probes were located near equatorial vacancies as next-nearest-neighbours,
whilst a metastable hydrogen configuration is responsible for the annealing
behavior
Birefringence and refractive indices of wurtzite GaN in the transparency range
Birefringence and anisotropic refractive indices of wurtzite GaN within the spectral range from 0.58 eV to 3.335 eV were determined combining optical retardation and spectroscopic ellipsometry measurements on a series of undoped m- and c-plane GaN bulk substrates grown by hydride vapor phase epitaxy. It is observable that the birefringence has a maximum close to the absorption edge and a weak broad minimum in near-IR range. A quantitative explanation of the whole data is given in terms of contributions to the optical response of GaN due to discrete excitons, Coulomb enhanced band-to-band optical transitions near the E(0) critical point of the band structure, high-energy optical transitions, and infrared active optical phonon modes which are different for the ordinary and extraordinary waves both in magnitude and in spectral dependence
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