26 research outputs found
Multiscale modeling of effects due to impurity clustering in semiconductor systems
The properties of materials visible to us at the macroscopic scale are due to the microscopic behavior of the atoms in the material. In many cases, small disturbances from an ideal microstructure can lead to completely different behavior at the macroscale. In order to exploit or prevent such phenomena, it is important to understand both the microscopic interactions governing the individual atoms and the mechanisms which lead to the observed collective behavior of the particles at the macroscale. Computational methods can offer insight to such problems, since in a computational scheme it is possible to manipulate the systems at will and without unwanted disturbances. On the other hand, these methods necessarily contain approximations and care must be taken in choosing and combining the methods used for examining the different physically relevant length and time scales.
This dissertation presents computational simulations of the role of impurities and impurity clusters in two types of semiconductor systems. The influence of the microstructure of Mn dopants in the diluted magnetic semiconductors (Ga,Mn)N and (Ga,Mn)As is examined. Also, the effect of metal impurities, such as Cu and Pb, on the surface morphologies of wet etched Si surfaces is studied. In both cases, density-functional calculations are used for studying the microscopic interactions and models based on Monte Carlo schemes are developed for simulating the macroscopic behavior. In the presented work, a direct correspondence between the microstructures of the magnetic semiconductors and their magnetic properties is found, explaining the experimentally observed variations in Curie temperatures. The dependence of the roughness of etched surfaces on the types of impurities present during etching is also explained by different adsorption and cluster formation tendencies of the various impurities on the Si surface
High Curie temperatures in (Ga,Mn)N from Mn clustering
The effect of microscopic Mn cluster distribution on the Curie temperature
(Tc) is studied using density-functional calculations. We find that the
calculated Tc depends crucially on the microscopic cluster distribution, which
can explain the abnormally large variations in experimental Tc values from a
few K to well above room temperature. The partially dimerized Mn_2-Mn_1
distribution is found to give the highest Tc > 500 K, and in general, the
presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values
close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.Comment: To appear in Applied Phyiscs Letter
Defect mediated manipulation of nanoclusters on an insulator
With modern scanning probe microscopes, it is possible to manipulate surface structures even at the atomic level. However, manipulation of nanoscale objects such as clusters is often more relevant and also more challenging due to the complicated interactions between the surface, cluster and apparatus. We demonstrate the manipulation of nanometer scale gold clusters on the NaCl(001) surface with a non-contact atomic force microscope, and show that the movement of clusters is in certain cases constrained to specific crystallographic directions. First principles calculations explain this kinetic anisotropy as the result of the cluster attaching to surface defects: cation vacancies allow the clusters to bond in such a way that they only move in one direction. Constraining the movement of clusters could be exploited in the construction of nanostructures or nanomechanical devices, and the manipulation signatures may also be used for identifying cluster-defect complexes.Peer reviewe
First-principles investigations of the magnetic phase diagram of GdCaMnO
We studied for the first time the magnetic phase diagram of the rare-earth
manganites series GdCaMnO (GCMO) over the full
concentration range based on density functional theory. GCMO has been shown to
form solid solutions. We take into account this disordered character by
adapting special quasi random structures at different concentration steps. The
magnetic phase diagram is mainly described by means of the magnetic exchange
interactions between the Mn sites and Monte Carlo simulations were performed to
estimate the corresponding transition temperatures. They agree very well with
recent experiments. The hole doped region shows a strong ferromagnetic
ground state, which competes with A-type antiferromagnetism at higher Ca
concentrations .Comment: Submitted to PR
Self-assembled nanorods in YBCO matrix : a computational study of their effects on critical current anisotropy
In order to understand how the doping with self-assembled nanorods of different sizes and concentrations as well as applied magnetic fields affect the critical current anisotropy in YBa2Cu3O7-x (YBCO) thin films close to YBCO c-axis, we present an extensive and systematic computational study done by molecular dynamics simulation. The simulations are also used to understand experimentally measured J(c)(theta) curves for BaHfO3, BaZrO3 and BaSnO3 doped YBCO thin films with the help of nanorod parameters obtained from transmission electron microscopy measurements. Our simulations reveal that the relation between applied and matching field plays a crucial role in the formation of J(c)(theta)-peak around YBCO c-axis (c-peak) due to vortex-vortex interactions. We also find how different concentrations of different size nanorods effect the shape of the c-peak and explain how different features, such as double c-peak structures, arise. In addition to this, we have quantitatively explained that, even in an ideal superconductor, the overdoping of nanorods results in decrease of the critical current. Our results can be widely used to understand and predict the critical current anisotropy of YBCO thin films to improve and develop new pinscapes for various transport applications
Cutting Ice: Nanowire Regelation
Even below its normal melting temperature, ice melts when subjected to high pressure and refreezes once the pressure is lifted. A classic demonstration of this regelation phenomenon is the passing of a thin wire through a block of ice when sufficient force is exerted. Here we present a molecular-dynamics study of a nanowire cutting through ice to unravel the molecular level mechanisms responsible for regelation. In particular, we show that the transition from a stationary to a moving wire due to increased driving force changes from symmetric and continuous to asymmetric and discontinuous as a hydrophilic wire is replaced by a hydrophobic one. This is explained at the molecular level in terms of the wetting properties of the wire.Peer reviewe
Effect of Cu impurities on wet etching of Si(110): formation of trapezoidal hillocks
We simulate the formation of experimentally observed trapezoidal hillocks on etched Si(110) surfaces, describing their generic geometrical shape and analyzing the relative stability and/or reactivity of the key surface sites. In our model, the hillocks are stabilized by Cu impurities in the etchant adsorbing on the surface and acting as pinning agents. A model of random adsorptions will not result in hillock formation since a single impurity is easily removed from the surface. Instead a whole cluster of Cu atoms is needed as a mask to stabilize a hillock. Therefore we propose and analyze mechanisms that drive correlated adsorptions and lead to stable Cu clusters.Peer reviewe
First-principles investigations of the magnetic phase diagram of Gd1-xCaxMnO3
We studied the magnetic phase diagram of the rare-earth manganites series Gd1−xCaxMnO3
(GCMO) over the full concentration range based on density functional theory. GCMO has been shown to form solid solutions. We take into account this disordered character by adapting special quasi-random structures at different concentration steps. The magnetic phase diagram is mainly described by means of the magnetic exchange interactions between the Mn sites, and Monte Carlo simulations were performed to estimate the corresponding transition temperatures. They agree very well with recent experiments. The hole-doped region x<0.5
shows a strong ferromagnetic ground state, which competes with A-type antiferromagnetism at higher Ca concentrations x>0.6
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Self-assembled nanorods in YBCO matrix - a computational study of their effects on critical current anisotropy
In order to understand how the doping with self-assembled nanorods of different sizes and concentrations as well as applied magnetic fields affect the critical current anisotropy in YBa2Cu3O7−x (YBCO) thin films close to YBCO c-axis, we present an extensive and systematic computational study done by molecular dynamics simulation. The simulations are also used to understand experimentally measured Jc(θ) curves for BaHfO3, BaZrO3 and BaSnO3 doped YBCO thin films with the help of nanorod parameters obtained from transmission electron microscopy measurements. Our simulations reveal that the relation between applied and matching field plays a crucial role in the formation of Jc(θ)-peak around YBCO c-axis (c-peak) due to vortex-vortex interactions. We also find how different concentrations of different size nanorods effect the shape of the c-peak and explain how different features, such as double c-peak structures, arise. In addition to this, we have quantitatively explained that, even in an ideal superconductor, the overdoping of nanorods results in decrease of the critical current. Our results can be widely used to understand and predict the critical current anisotropy of YBCO thin films to improve and develop new pinscapes for various transport applications