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Point defect interactions and structural stability of compounds
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Theoretical studies of point defect interactions and structural stability of compounds have been performed using density functional theory. The defect-related properties, such as activation energy of diffusion, electronic and magnetic structure of selected materials have been studied.

The major part of the present work is devoted to a very important material for semiconductor industry, GaAs. The formation energies of intrinsic point defects and the solution energies of 3d transitions in GaAs have been calculated from first principles. Based on the calculated energies, we analysed the site preference of defects in the crystal. The tendency of defects to form clusters has been investigated for the intrinsic defects as well as for impurities in GaAs. The magnetic moment of 3d impurities has been calculated as a function of the chemical environment. The possibility of increasing the Curie temperature in (Ga,Mn)As by co-doping it with Cr impurities has been examined on the basis of calculated total energy difference between the disordered local moment and the ferromagnetically ordered spin configurations. We found that, in order to reach the highest critical temperature, GaAs should be separately doped with either Cr or Mn impurities. Also, we have shown that diffusion barrier of interstitial Mn depends on the charge state of this impurity in (Ga, Mn)As. The formation of defect complexes between interstitial and substitutional Mn atoms, and their influence on the value of diffusion barrier for interstitial Mn, has been studied.

The pair interactions energies between interstitial oxygen atoms in hcp Zr, Hf and Ti have been calculated using first principles. Based on the calculated energies, the oxygen ordering structures in IVB transition metal solid solutions have been explained. A prediction of nitrogen ordering in Hf-N solid solution has been made.

The thermodynamic description of intermetallic compounds in the Zr-Sn binary system has been obtained. The conclusion has been made that Zr substitution on the Sn sites takes place in the Zr4Sn phase, which accounts for the unusual stoichiometry of this Cr3Si structure type compound.

The influence of pressure on the phase stability in the Fe-Si system has been investigated. We have found instability of the hcp Fe0.9Si0.1 random alloy with respect to the decomposition onto the Si-poor hcp Fe alloy and the B2 FeSi under high pressure. The tendency of this decomposition becomes stronger with increasing the applied pressure.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007.
Keyword [en]
first principles, ab initio, density functional theory, point defects interactions, diluted magnetic semiconductors, structural stability, zirconium alloys
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4605ISBN: 978-91-7178-842-9 (print)OAI: oai:DiVA.org:kth-4605DiVA: diva2:13063
Public defence
2008-01-28, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100624Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2012-03-19Bibliographically approved
List of papers
1. Intrinsic defects and transition metal impurities in GaAs
Open this publication in new window or tab >>Intrinsic defects and transition metal impurities in GaAs
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2004 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, Vol. 272, no 3, 1961-1962 p.Article in journal (Refereed) Published
Abstract [en]

Thermodynamics of intrinsic point defects, 3d-transition metal (TM) impurities, and various defect pairs in GaAs have been studied by means of the locally self-consistent Green's function method. Antisite defects, As-Ga, are found to be the most energetically favorable defects in the As-rich GaAs. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The magnetic moments of TM impurities in the GaAs host are calculated under the assumption that orbital moments of TM are quenched and the total magnetization is due to spill ordering of electrons in the crystal. Thermodynamic possibility of formation of complexes between TM-atoms is investigated.

Keyword
defects, magnetism, semiconductors
National Category
Other Materials Engineering Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-7892 (URN)10.1016/j.jmmm.2003.12.1198 (DOI)000222236800139 ()2-s2.0-23044440896 (Scopus ID)
Note

QC 20150728

Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2015-07-28Bibliographically approved
2. Ab Initio Studies of the Energy Characteristics and Magnetic Properties of Point Defects in GaAs
Open this publication in new window or tab >>Ab Initio Studies of the Energy Characteristics and Magnetic Properties of Point Defects in GaAs
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2005 (English)In: Physics of the solid state, ISSN 1063-7834, E-ISSN 1090-6460, Vol. 47, no 10, 1831-1836 p.Article in journal (Refereed) Published
Abstract [en]

The formation energies of intrinsic point defects and solution energies of transition metal impurities in gallium arsenide are determined on the basis of ab initio calculations using the method of a locally self-consistent Green's function, which is a generalization of the coherent potential approximation. Based on the calculated energies, the conclusion is made that the As-Ga antisite defect is the most common intrinsic defect in GaAs. Calculations showed that transition metal impurities, except for Ni, preferentially occupy gallium sites substitutionally. The magnetic moments of impurity atoms are calculated as a function of the chemical environment. It is shown that, in compensated GaAs, Mn atoms tend to form clusters.

National Category
Physical Chemistry Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-7893 (URN)10.1134/1.2087732 (DOI)000232482900006 ()2-s2.0-26644446691 (Scopus ID)
Note

QC 20100624

Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2016-05-09Bibliographically approved
3. Magnetic properties of 3d impurities in GaAs
Open this publication in new window or tab >>Magnetic properties of 3d impurities in GaAs
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2007 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, Vol. 310, no 2, 2120-2122 p.Article in journal (Refereed) Published
Abstract [en]

Electronic structure, thermodynamic, and magnetic properties of 3d-transition metal (TM) impurities in GaAs have been studied from first principles using Green's function approach. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The possibility of raising the Curie temperature TC in (GaMn) As by co-doping it with Cr impurities was examined on the basis of total energy difference between the disordered local moment (DLM) and the ferromagnetically ordered (FM) spin configurations. The calculated Curie temperature and magnetic moment have maxima for GaAs doped with Cr and Mn. The magnetic properties of Mn-doped GaAs are shown to be more sensitive to antisite As defects than those of Cr-doped GaAs. However, the Cr impurities are sensitive to the presence of acceptor defects, such as vacancies on the Ga sublattice. The investigation of the electronic structure of pseudo-ternary alloys (Ga(1-x-y)MnxCry) As has shown a mutual compensation of Mn and Cr impurities. Therefore, in order to reach the highest critical temperature, GaAs has to be separately doped with Cr or Mn impurities. The GaAs doped with Fe is found to be non-ferromagnetic.

Place, publisher, year, edition, pages
Elsevier, 2007
Keyword
spintronic, ab initio calculation, magnetic semiconductor
National Category
Other Materials Engineering Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-7894 (URN)10.1016/j.jmmm.2006.10.791 (DOI)000247720400080 ()2-s2.0-33847670091 (Scopus ID)
Conference
The International Conference on Magnetism
Note

QC 20100624

Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2014-08-29Bibliographically approved
4. Diffusion of Interstitial Mn in the Dilute Magnetic Semiconductor (Ga,Mn)As: The Effect of a Charge State
Open this publication in new window or tab >>Diffusion of Interstitial Mn in the Dilute Magnetic Semiconductor (Ga,Mn)As: The Effect of a Charge State
2008 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 101, no 17, 177204- p.Article in journal (Refereed) Published
Abstract [en]

Migration barriers for diffusion of interstitial Mn in the dilute magnetic semiconductor (Ga,Mn)As are studied using first-principles calculations. The diffusion pathway goes through two types of interstitial sites: As coordinated and Ga coordinated. The energy profile along the path is found to depend on the ratio of concentrations between substitutional and interstitial Mn in GaAs. Two regions of distinctly different behavior, corresponding to n-type and p-type (Ga,Mn)As, are identified. The difference in mobility is a reflection of the change in the charge state of Mn interstitials (double donors) that occurs in the presence of substitutional Mn impurities (acceptors). In addition, substitutional Mn impurities are shown to act as traps for interstitial Mn. The effective migration barrier for the positively doubly charged Mn interstitials in p-type (Ga,Mn)As is estimated to vary from 0.55 to about 0.95 eV.

Keyword
DILUTE MAGNETIC SEMICONDUCTOR, ELECTRONIC STRUCTURE, DEFECTS, DIFFUSION
National Category
Condensed Matter Physics Other Electrical Engineering, Electronic Engineering, Information Engineering Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-7895 (URN)10.1103/PhysRevLett.101.177204 (DOI)000260383600060 ()2-s2.0-55049107277 (Scopus ID)
Note

QC 20100624

Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2016-05-09Bibliographically approved
5. Structural stability of intermetallic phases in the Zr-Sn system
Open this publication in new window or tab >>Structural stability of intermetallic phases in the Zr-Sn system
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2006 (English)In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 55, no 5, 485-488 p.Article in journal (Refereed) Published
Abstract [en]

A thermodynamic description of the intermetallic compounds in the Zr-Sn binary system has been obtained using total energy calculations by means of the Vienna ab initio simulation package. Our calculations show that hexagonal compounds Zr5Sn4 and Zr5Sn3 are the most stable phases in the Zr-Sn binary system. Their high stability is found to be due to hybridization of the Sn 5p with Zr 4d electronic states. Based on the calculated energies, the conclusion is made that Zr substitution on the Sri sites takes place in the Zr4Sn phase, which accounts for the unusual stoichiometry of this Cr3Si structure type compound.

Keyword
zirconium-tin alloys; intermetallic compounds; first-principle electron theory
National Category
Metallurgy and Metallic Materials Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-5594 (URN)10.1016/j.scriptamat.2006.04.047 (DOI)000239132200017 ()2-s2.0-33745209870 (Scopus ID)
Note

QC 20150728

Available from: 2006-04-11 Created: 2006-04-11 Last updated: 2015-07-28Bibliographically approved
6. Ab initio based investigation of the oxygen and nitrogen interstitial ordering in hcp Hf, Zr, and Ti: An ab initio study
Open this publication in new window or tab >>Ab initio based investigation of the oxygen and nitrogen interstitial ordering in hcp Hf, Zr, and Ti: An ab initio study
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2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 13, 134110- p.Article in journal (Refereed) Published
Abstract [en]

We investigate the ordering of oxygen and nitrogen interstitials in hcp Zr, Hf, and Ti using the corresponding oxygen-oxygen and nitrogen-nitrogen interactions obtained in the state-of-the-art first-principles calculations. Two main contributions, chemical and strain induced, to the interstitial-interstitial interactions are obtained by different techniques. We find that there is the strong repulsion between interstitial atoms at the nearest-and next-nearest-neighbor coordination shells, which is solely determined by the chemical interaction determined on a fixed ideal lattice, while both contributions are important for more distant coordination shells. The Monte Carlo simulations reveal the existence of three stoichiometric compositions, MeI1/6, MeI1/3, and MeI1/2, for the ground-state structures of interstitials, having different ordering types. Our results for the structures of oxygen interstitials are in good agreement with existing experimental data for the Ti and Hf alloys. In the case of Zr-O interstitial alloys, we correctly predict the general type of ordering, although the detailed structure is at variance the experimental observations. The ordering transition temperatures in some cases are overestimated by a factor of 2. We also predict the ordering type of nitrogen interstitials in hcp Ti, Zr, and Hf, which are similar to those in the case of oxygen interstitials.

Keyword
O SOLID SOLUTION; HEAT-CAPACITY MEASUREMENT; DISORDER TRANSFORMATION; POTENTIAL MODEL; ALLOYS; TRANSITION; METALS; TITANIUM; ATOMS; APPROXIMATION
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-7897 (URN)10.1103/PhysRevB.82.134110 (DOI)000282937600001 ()2-s2.0-78049404211 (Scopus ID)
Note
QC 20100624 Uppdaterat från manuskript till artikel 20110106Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2012-03-19Bibliographically approved
7. Iron-silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle
Open this publication in new window or tab >>Iron-silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle
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2003 (English)In: Nature, ISSN 0028-0836, Vol. 422, no 6927, 58-61 p.Article in journal (Refereed) Published
Abstract [en]

The boundary between the Earth's metallic core and its silicate mantle is characterized by strong lateral heterogeneity and sharp changes in density, seismic wave velocities, electrical conductivity and chemical composition(1-7). To investigate the composition and properties of the lowermost mantle, an understanding of the chemical reactions that take place between liquid iron and the complex Mg-Fe-Si-Al-oxides of the Earth's lower mantle is first required(8-15). Here we present a study of the interaction between iron and silica (SiO2) in electrically and laser-heated diamond anvil cells. In a multianvil apparatus at pressures up to 140 GPa and temperatures over 3,800 K we simulate conditions down to the core-mantle boundary. At high temperature and pressures below 40 GPa, iron and silica react to form iron oxide and an iron-silicon alloy, with up to 5 wt% silicon. At pressures of 85-140 GPa, however, iron and SiO2 do not react and iron-silicon alloys dissociate into almost pure iron and a CsCl-structured (B2) FeSi compound. Our experiments suggest that a metallic silicon-rich B2 phase, produced at the core-mantle boundary (owing to reactions between iron and silicate(2,9,10,13)), could accumulate at the boundary between the mantle and core and explain the anomalously high electrical conductivity of this region(6).

Keyword
HIGH-PRESSURE; LIQUID-IRON; INNER-CORE; X-RAY; BOUNDARY; TEMPERATURES; METAL; HETEROGENEITY
National Category
Other Engineering and Technologies not elsewhere specified
Identifiers
urn:nbn:se:kth:diva-7898 (URN)10.1038/nature01422 (DOI)000181343100035 ()
Note
QC 20100624Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2010-12-03Bibliographically approved

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