pinecone/core-2020-05-10-deduplication huggingface dataset:
Hole injection contribution to transport mechanisms in metal/p− /p++and metal/oxide/p− /p++ diamond structures

International audienceHeterostructures such as Schottky diodes and metal/oxide/semiconductor structures are the building blocks of diamond electronic devices. They are able to carry large current densities, up to several kA/cm$^2$, if a heavily boron doped layer (p$^{++}$) is included in the semiconducting stack, thus affording a metallic reservoir of mobile holes close to the lightly doped layer (p$^{-}$). In this work, hole injection effects are evidenced experimentally in the two previously mentioned devices and also simulated numerically. Although the potential barrier height at metal/semiconductor interfaces is a fundamental parameter,  a more general approach consists in defining the current density from the product of an effective velocity and carrier concentration at interface. In accordance with experimental results, such a view permits to describe both depletion and accumulation regimes, which indeed can exist at the metallic or oxide interface, and to take into account the increase of the hole concentration above the thermal equilibrium one in the p$^{-}$ layer. The lower the temperature, the larger is this second effect. For sufficiently thin p$^{-}$ layers, typically below 2~$\mu$m, this effect frees device operation from the limitation due to incomplete ionization of acceptors and allows a strong decrease of the specific resistance and forward losses while preserving breakdown voltages in the range 1.4 to 2 kV
Grain boundary networks in nanocrystalline alloys from atom probe tomography quantization and autocorrelation mapping

A local spatial autocorrelation-based modeling method is developed to reconstruct nanoscale grain structures in nanocrystalline materials from atom probe tomography (APT) data, which provide atomic positions and species, with minimal noise. Using a nanocrystalline alloy with an average grain size of 16 nm as a model material, we reconstruct the three-dimensional grain boundary network by carrying out two series of APT data quantization using ellipsoidal binning, the first probing the anisotropy in the apparent local atomic density and the second quantifying the local spatial autocorrelation. This approach enables automatic and efficient quantification and visualization of grain structure in a large volume and at the finest nanoscale grain sizes, and provides a means for correlating local chemistry with grain boundaries or triple junctions in nanocrystalline materials. Nanoscale grain boundary networks are reconstructed from atom probe tomography data, which provide atomic positions and species for a fraction of atoms within a nanocrystalline material with an average grain size of 16 nm, using a quantization and local spatial autocorrelation-based approach.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Grant W911NF-14-1-0539
Beam matching: a method to study phonon transport through interfaces and
  multilayer structures

Structuring materials is one mechanism to influence the thermal conductivity
and thus thermoelectric efficiency. In order to investigate the scattering of
phonons in multilayer structures we developed a beam matching technique, which
is based on the concept of individual phonons and their scattering at
interfaces. One of the major goals is to efficiently determine the complex band
structure of the bulk materials. The complex band structure is determined using
selected k-points on a triangulated grid in the complex plane of wave vectors
perpendicular to the interface. Matching the phonon modes at an interface is
translated to a singular value problem. Its null-vectors provide the coupling
coefficients of the phonon modes across the interface. Besides giving explicit
access to the modes as they scatter at the interface, the technique provides
the transfer matrices, that provide the transmission coefficient of any
multilayer structure. The transmission coefficient, in turn, yields the
phononic thermal conductance for coherent transport. The knowledge of the
matched phonons forms the basis of investigating incoherent transport under the
influence of phonon-phonon or impurity scattering.Comment: 15 pages, 8 figure
InGaN/GaN core/shell nanowires for visible to ultraviolet range photodetection

International audienceWe report on the fabrication and characterization of single nitride nanowire visible-to-ultraviolet p-n photodetec-tors. Nitride nanowires containing 30 InGaN/GaN radial quantum wells with 18% indium fraction were grown by catalyst-free metal-organic vapour phase epitaxy. Single nanowires were contacted using optical lithography. As expected for a radial p-n junction, the current-voltage (I-V) curves of single wire detectors show a rectifying behavior in the dark and a photocurrent under illumination. The detectors present a response in the visible to UV spectral range starting from 2.8 eV. The peak responsivity is 0.17 A/W at 3.36 eV. The on-off switching time under square light pulses is found to be below 0.1 sec
Reliability aspects of hydrogen-doped indium oxide

We demonstrate an alternative route to reliably prepare hydrogen-doped indium oxide (In2O3:H). The common hydrogen source, water vapor, was substituted in our process by hydrogen and oxygen gas. The resulting films showed similar optical and electrical properties. Nevertheless, the process using gaseous hydrogen led to a simplification of the deposition process. By replacing the hydrogen source we increased the reproducibility of the electrical film properties significantly, thus, paving the way for a reliable device implementation of the material. Furthermore, we investigated the degradation behavior of In2O3:H under damp heat conditions as preliminary test for long-term durability in photovoltaic devices. The results revealed a degradation of the electrical properties that differs in detail regarding the amorphous and polycrystalline material. In the amorphous material, the main degradation is caused by loss of charge carriers, whereas in the polycrystalline material a drop of the charge carrier mobility causes a significant rise of the resistivity. However, we show that the relative degradation of the In2O3:H films is similar to other transparent contacts that are implemented in solar cells. Finally, we demonstrate that the degradation of the charge carrier mobility in the polycrystalline In2O3:H films is completely reversible by vacuum annealing at 200 °C
Valence-band electronic structure evolution of graphene oxide upon thermal annealing for optoelectronics

We report valence-band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. The degree of oxygen functionalization was controlled by annealing temperature, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in the density of states around the Fermi level upon thermal annealing at ???600?????C. The result indicates that while there is an apparent bandgap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of bandgap closure was correlated with the electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ???500?????C leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to an as-synthesized counterpart
In situ graphene doping as a route toward efficient perovskite tandem solar cells

Tandem solar cells consisting of perovskite and silicon  absorbers have the potential to outperform respective  state of the art single junction efficiencies. However, their  development requires the gentle deposition of a transparent  electrode onto the hybrid perovskite and its organic layers.  Implementation of large area graphene obtained by chemical  vapor deposition seems to be an excellent solution. In this  paper, we present the impact of graphene on perovskite solar  cells and their organic layers. Direct application of graphene on  CH3NH3PbI3 is limited by a highly defective interface but  insertion of spiro OMeTAD enables a defect free implementation.  Solar cells containing transparent graphene contacts  approach identical electrical performance compared to devices  with standard Au contacts. Hall effect measurements of  graphene on various organic thin films, revealed the importance  of field effect doping. Gained knowledge enabled the  development of a strategy to increase the charge carrier density  in graphene by 60 , while lowering graphene sheet resistance  by 24 . This combined route of spiro OMeTAD and stabilized  adsorbent doping is an important step toward the targeted  application in high performance monolithic perovskite silicon  tandem solar cell
Laser patterning of amorphous silicon thin films deposited on flexible and rigid substrates

The possibility of direct writing thin semiconductive channels and structures on insulating substrates in a clean room-free process is attractive for its simplicity, cost effectiveness, and possibility of a wide choice of substrates. A broad range of applications, such as large-area electronic devices (touch screens, flexible displays), sensors, or optical wave guides could benefit from such a process. In this work, we directly write on doped hydrogenated amorphous silicon (a-Si:H), with thickness in the range 10 nm–1 μm, using a Nd-YAG laser operating at 532 nm that is part of a Witec Raman confocal system. The contrast in conductivity between the exposed and unexposed areas is so high that the a-Si:H matrix needs not to be removed after exposure. B- and P-doped films were deposited on plastic, glass, and oxidized silicon wafers. The laser power threshold for crystallization was studied. The highest conductivity (886 Ω−1 cm−1) was obtained on wafer. On hard substrates, it is possible to tune the mesoscopic electrical conductivity in a very broad range of values (∼10−4–103) by design of the pattern to be transferred. Patterned films are piezoresistive with gauge factors as high as +18 and −29 for p- and n-type patterns, respectively. SEM image of laser written lines on a 10 nm thick a-Si:H film deposited on a Si/SiO2 substrate. Four regions are clearly distinguishable: the metal contact on the top area; the laser eroded area (lines); the crystallized areas adjacent to lines; the amorphous region at the bottom right.CNPqThe authors acknowledge Witec GmbH for collaboration in disclosing the set of instructions needed to communicate with Witec Four software
Long-term influence of chitin concentration on the resistance of cement pastes determined by atomic force microscopy

The presence of sulfates potentialize damage on cementbased materials, leading to structural failures. Therefore, structures must be designed to compensate for this effect. The mechanical properties of cement–chitin mixtures are investigated with different percentages of chitin (0.5, 1.3, and 2.1 wt.%) and aging of composite in a joint nanoscopic- and macroscopic-scale by experimental study. The objective is to increase the durability of concrete elements at coastal aquifers where concrete structures are in constant exposure to sulfate ions, chloride ions among others. Tapping mode AFM was used to characterize the surface structure and roughness of the cement pastes. To verify the chitin addition and the formation of sulfate-based aggregates Raman and IR spectra were recorded and are presented in this work. Then, force spectroscopy was used to obtain the nanomechanical properties at three different\ud
exposure times (1 day, 6 months, and 1 year) into water or a SO4 2 environment. Macroscopic parameters (e.g., compression strength of cylindrical probes) were assessed for comparison following standard guidelines. The results show a decrease of its mechanical properties as a function of the polymer concentration but more importantly, they correlate the elasticity and adhesion at the nanoscale with the behavior of the bulk material
Severe signal loss in diamond beam loss monitors in high particle rate
  environments by charge trapping in radiation-induced defects

The beam condition monitoring leakage (BCML) system is a beam monitoring
device in the compact muon solenoid (CMS) experiment at the large hadron
collider (LHC). As detectors 32 poly-crystalline (pCVD) diamond sensors are
positioned in rings around the beam pipe. Here, high particle rates occur from
the colliding beams scattering particles outside the beam pipe. These particles
cause defects, which act as traps for the ionization, thus reducing the charge
collection efficiency (CCE). However, the loss in CCE was much more severe than
expected from low rate laboratory measurements and simulations, especially in
single-crystalline (sCVD) diamonds, which have a low initial concentration of
defects. The reason why in real experiments the CCE is much worse than in
laboratory experiments is related to the ionization rate. At high particle
rates the trapping rate of the ionization is so high compared with the
detrapping rate, that space charge builds up. This space charge reduces locally
the internal electric field, which in turn increases the trapping rate and
recombination and hence reduces the CCE in a strongly non-linear way. A diamond
irradiation campaign was started to investigate the rate dependent electrical
field deformation with respect to the radiation damage. Besides the electrical
field measurements via the Transient Current Technique (TCT), the CCE was
measured. The experimental results were used to create an effective deep trap
model that takes the radiation damage into account. Using this trap model the
rate dependent electrical field deformation and the CCE were simulated with the
software SILVACO TCAD. The simulation, tuned to rate dependent measurements
from a strong radioactive source, was able to predict the non-linear decrease
of the CCE in the harsh environment of the LHC, where the particle rate was a
factor 30 higher.Comment: 9 pages, 14 figures, Physica Status Solidi A, 201
Object kinetic Monte Carlo study of the effect of grain boundaries in martensitic Fe-Cr-C alloys

Fe-Cr-C alloys with chromium concentrations in the range from similar to 2 to 12 wt.% form ferritic-martensitic structures by rapid cooling from the austenite state already in the presence of relatively low carbon concentrations. In this process it is possible to obtain different ratios of ferrite and martensite, as well as formation of carbides, by varying the thermal treatment. The presence of ferrite or martensite might have an influence on the nanostructural evolution under irradiation of these alloys. Here, considering a tempered martensite reference alloy with 9% Cr, we make use of an already validated object kinetic Monte Carlo (OKMC) model in order to study the possible effect of the formation of martensite laths on the material nanostructural evolution under neutron irradiation, assuming that the relevant boundaries act as sinks for radiation defects. The results show that the reduction of the grain size (including in this definition the average size of prior austenite grains, packets, blocks, and laths) does not play any relevant role until sizes of the order of similar to 0.5 mu m are reached: for smaller grains, the number of defects being absorbed by the boundaries becomes dominant. However, this threshold is lower than the experimentally observed martensite lath dimensions, thereby suggesting that what makes the difference in martensitic Fe-Cr-C alloys with respect to ferrite, concerning events and mechanisms taking place during irradiation, are not the lath boundaries as sinks. Differences between the nanostructural evolution under neutron irradiation in ferrite and martensite should therefore be ascribed to other factors. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Nanoscale fissure formation in AlxGa1–xN/GaN heterostructures and their influence on Ohmic contact formation

Nanoscale surface fissures on AlxGa1–xN/GaN (15 nm/1 µm) heterostructures grown by metalorganic vapour phase epitaxy (MOVPE) were imaged using tapping-mode atomic force microscopy (AFM) and electron channelling contrast imaging (ECCI). Fissure formation was linked to threading dislocations, and was only observed in samples cooled under H2 and NH3, developing with increasing barrier layer Al content. No strain relaxation was detected regardless of fissure formation up to barrier layer Al composition fractions of x = 0.37. A reduction of measured channel carrier density was found in fissured samples at low temperature. This instability is attributed to shallow trap formation associated with fissure boundaries. For Ti/Al/Ni/Au Ohmic contact formation to high Al content barrier layers, fissures were found to offer conduction routes to the 2DEG that allow for low resistance contacts, with fissure-free samples requiring additional optimisation of the metal stack and anneal conditions to achieve contact resistivity of order those measured in fissured samples. In addition, the effects of fissures were found to be detrimental to thermal stability of sheet and contact resistance, suggesting that fissure formation compromises the integrity of the 2DEG
Vibrational band structure of nanoscale phononic crystals

The vibrational properties of two-dimensional phononic crystals are studied
with large-scale molecular dynamics simulations and finite element method
calculation. The vibrational band structure derived from the molecular dynamics
simulations shows the existence of partial acoustic band gaps along the
$\Gamma$-M direction. The band structure is in excellent agreement with the
results from the finite element model, proving that molecular dynamics
simulations can be used to study the vibrational properties of such complex
systems. An analysis of the structure of the vibrational modes reveals how the
acoustic modes deviate from the homogeneous bulk behaviour for shorter
wavelengths and hints towards a decoupling of vibrations in the phononic
crystal.Comment: 9 pages, 6 figures. This article may be used for non-commercial
  purposes in accordance with Wiley Terms and Conditions for Self-Archivin
Potential of epitaxial silicon carbide microbeam resonators for chemical sensing

Epitaxial silicon carbide is promising for chemical resonant sensing applications due to its excellent mechanical, thermal, and biochemical properties. This paper reviews six important aspects of (i) silicon carbide heteroepitaxial growth and residual stress; (ii) silicon carbide beam resonators, resonator types, and fabrication processes; (iii) sensing principles, dynamic sensing mechanical performance, and transduction techniques; (iv) damping parameters; (v) mean stress influence on mass sensitivity of SiC flexural microbridge resonators; and (vi) gradient stress impact on SiC cantilever static behavior. The primary goal is to suggest the means to improve the mass sensitivity parameter and application range of epitaxial silicon carbide microbeam resonators and benchmark it with other relevant materials.Griffith Sciences, School of Natural SciencesFull Tex
Oxygen vacancies effects in a-IGZO: Formation mechanisms, hysteresis, and negative bias stress effects

The amorphous oxide semiconductor Indium-Gallium-Zinc-Oxide (a-IGZO) has gained a large technological relevance as a semiconductor for thin-film transistors in active-matrix displays. Yet, major questions remain unanswered regarding the atomic origin of threshold voltage control, doping level, hysteresis, negative bias stress (NBS), and negative bias illumination stress (NBIS). We undertake a systematic study of the effects of oxygen vacancies on the properties of a-IGZO by relating experimental observations to microscopic insights gained from first-principle simulations. It is found that the amorphous nature of the semiconductor allows unusually large atomic relaxations. In some cases, oxygen vacancies are found to behave as perfect shallow donors without the formation of structural defects. Once structural defects are formed, their transition states can vary upon charge and discharge cycles. We associate this phenomenon to a possible presence of hysteresis in the transfer curve of the devices. Under NBS, the creation of oxygen vacancies becomes energetically very stable, hence thermodynamically very likely. This generation process is correlated with the occurrence of the negative bias stress instabilities observed in a-IGZO transistors. While oxygen vacancies can therefore be related to NBS and hysteresis, it appears unlikely from our results that they are direct causes of NBIS, contrary to common belief.status: publishe
Preferential adsorption of NH3 gas molecules on MWCNT defect sites probed using in situ Raman spectroscopy

The preferential adsorption of NH3 gas molecules on multiwalled carbon nanotubes (MWCNTs) was studied using in situ Raman spectroscopy. It was observed that the full widths at half maximum of the G band and the intensity ratio I2D/IG of the MWCNTs decreased significantly during NH3 gas adsorption at elevated temperatures. These observations were explained in terms of suppressed second-order-defect associated Raman vibrations resulting in a lower disorder Raman band due to ammonia adsorption on the defect sites. Another corresponding effect was a temporary increase in electron doping levels due to ammonia adsorption. This behaviour was accompanied by a drop of ca. 2% in the resistance of the MWCNTs corresponding to the occupancy of most of the defect sites. We suggest preferential adsorption of ammonia gas molecules on the thermally activated defect sites of MWCNTs as an appropriate gas sensing mechanism. This knowledge can be used to design and tune the selectivity of ammonia gas sensor
Description of radiation damage in diamond sensors using an effective
  defect model

The BCML system is a beam monitoring device in the CMS experiment at the LHC.
As detectors poly-crystalline diamond sensors are used. Here high particle
rates occur from the colliding beams scattering particles outside the beam
pipe. These particles cause defects, which act as traps for the ionization,
thus reducing the CCE. However, the loss in CCE was much more severe than
expected. The reason why in real experiments the CCE is so much worse than in
laboratory experiments is related to the rate of incident particles. At high
particle rates the trapping rate of the ionization is so high compared with the
detrapping rate, that space charge builds up. This space charge reduces locally
the internal electric field, which in turn increases the trapping rate and
hence reduces the CCE even further. In order to connect these macroscopic
measurements with the microscopic defects acting as traps for the ionization
charge the TCAD simulation program SILVACO was used. Two effective acceptor and
donor levels were needed to fit the data. Using this effective defect model the
highly non- linear rate dependent diamond polarization as function of the
particle rate environment and the resulting signal loss could be simulated