OSCAR RESTREPO - Academia.edu (original) (raw)

Papers by OSCAR RESTREPO

We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires... more We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires in the statistical doping regime. At zero gate voltage transport is well described by the space charge limited current model and Efros-Shklovskii variable range hopping, but positive gate voltage (electron accumulation) reveals a previously unexplored regime of nanowire charge transport that is not well described by existing theory. The ability to continuously tune between these regimes provides guidance for the extension of existing models and directly informs the design of next-generation nanoscale electronic devices.

Applied Physics Letters, 2011

We propose a new method to calculate polarization induced interfacial charges in semiconductor he... more We propose a new method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4-6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local-density approximation and hybrid functionals). We also find the calculated interfacial charge of (6.8 ± 0.4) × 10 13 cm −2 to be in excellent agreement with experiments and the value of 6.58 × 10 13 cm −2 calculated from bulk polarization constants, validating the use of bulk constants even for very thin films.

One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin ... more One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin relaxation times. We have been developing for the first time a parameter-free first-principles method to determine spin relaxation times. Our effort initially concentrates on silicon and diamond. For liquid-nitrogen temperatures and above, spin relaxation in silicon is dominated by the Elliott-Yafet mechanism. The spin relaxation is induced by momentum scattering off impurities or phonons. The development of a basic methodology based on density-functional calculations that can be used to determine momentum scattering lifetimes has been recently completed by one of us [1]. By considering the spin-orbit mixing of the up and down states, the spin-flip matrix elements can be related to the momentum matrix elements. The underlying theory has previously been derived for III-V semiconductors with direct band gap but did previously not exist for indirect-band gap materials such as silicon or diamond. We report results of an accurate formulation to calculate spin relaxation times in silicon and diamond based on first-principles methods, which we find in excellent agreement with experimental relaxation times. Due to the ab-initio nature of our method, it can be directly applied to study other potentially spin-preserving systems.[4pt] [1] O. D. Restrepo et al., Appl. Phys. Lett. 94, 212103 (2009).

We present a generally applicable parameter-free first-principles method to determine electronic ... more We present a generally applicable parameter-free first-principles method to determine electronic spin relaxation times and apply it to the technologically important group-IV materials silicon, diamond and graphite. We concentrate on the Elliott-Yafet mechanism, where spin relaxation is induced by momentum scattering off phonons and impurities. In silicon, we find a simT−3\sim T^{-3}simT−3 temperature dependence of the phonon-limited spin relaxation time T$_1$ and a value of 4.3 ns at room temperature, in agreement with experiments. For the phonon-dominated regime in diamond and graphite, we predict a stronger simT−5\sim T^{-5}simT−5 and simT−4.5\sim T^{-4.5}simT−4.5 dependence that limits T1T_1T1 (300 K) to 180 and 5.8 ns, respectively. A key aspect of this study is that the parameter-free nature of our approach provides a method to study the effect of {\em any} type of impurity or defect on spin-transport. Furthermore we find that the spin-mix amplitude in silicon does not follow the Eg−2E_g^{-2}Eg−2 band gap dependence usually assigned to III-V semiconductors but follows a much weaker and opposite Eg0.67E_g^{0.67}Eg0.67 dependence. This dependence should be taken into account when constructing silicon spin transport models.

Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in... more Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in thin films of Sr1+δTi1-δO3-δ (δ˜0.2) grown on stoichiometric SrTiO3. We envision the creation of regularly spaced anti-phase domains (APD's) separated by anti-phase boundaries (APB's) through a a/2<111> displacement, thus enabling a locally charge-neutral SrO stoichiometry at the APB's, while preserving the SrTiO3 stoichiometry in the interior of the APD's. This allows for an overall charge-neutrality and correct stoichiometry in the non-stoichiometric layer. Preliminary calculations of the dimensions of the APD's are in good agreement with the observed results. We performed molecular dynamics simulations using Buckingham plus Coulomb empirical potentials and their energetics as well as the equilibrium positions of the atoms and resulting lattice constants were determined. Also, the corresponding STEM signals of the structures were modeled, allowing direct comparison to the STEM images of the non-stoichiometric material. To examine the stability of the observed structures in comparison to the constitutional point defects, we calculated the formation energies of single point defects in SrTiO3, which we also used to benchmark the empirical-potential results against first-principles values.

We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires... more We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires in the statistical doping regime. At zero gate voltage transport is well described by the space charge limited current model and Efros-Shklovskii variable range hopping, but positive gate voltage (electron accumulation) reveals a previously unexplored regime of nanowire charge transport that is not well described by existing theory. The ability to continuously tune between these regimes provides guidance for the extension of existing models and directly informs the design of next-generation nanoscale electronic devices.

Applied Physics Letters, 2011

We propose a new method to calculate polarization induced interfacial charges in semiconductor he... more We propose a new method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4-6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local-density approximation and hybrid functionals). We also find the calculated interfacial charge of (6.8 ± 0.4) × 10 13 cm −2 to be in excellent agreement with experiments and the value of 6.58 × 10 13 cm −2 calculated from bulk polarization constants, validating the use of bulk constants even for very thin films.

One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin ... more One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin relaxation times. We have been developing for the first time a parameter-free first-principles method to determine spin relaxation times. Our effort initially concentrates on silicon and diamond. For liquid-nitrogen temperatures and above, spin relaxation in silicon is dominated by the Elliott-Yafet mechanism. The spin relaxation is induced by momentum scattering off impurities or phonons. The development of a basic methodology based on density-functional calculations that can be used to determine momentum scattering lifetimes has been recently completed by one of us [1]. By considering the spin-orbit mixing of the up and down states, the spin-flip matrix elements can be related to the momentum matrix elements. The underlying theory has previously been derived for III-V semiconductors with direct band gap but did previously not exist for indirect-band gap materials such as silicon or diamond. We report results of an accurate formulation to calculate spin relaxation times in silicon and diamond based on first-principles methods, which we find in excellent agreement with experimental relaxation times. Due to the ab-initio nature of our method, it can be directly applied to study other potentially spin-preserving systems.[4pt] [1] O. D. Restrepo et al., Appl. Phys. Lett. 94, 212103 (2009).

We present a generally applicable parameter-free first-principles method to determine electronic ... more We present a generally applicable parameter-free first-principles method to determine electronic spin relaxation times and apply it to the technologically important group-IV materials silicon, diamond and graphite. We concentrate on the Elliott-Yafet mechanism, where spin relaxation is induced by momentum scattering off phonons and impurities. In silicon, we find a simT−3\sim T^{-3}simT−3 temperature dependence of the phonon-limited spin relaxation time T$_1$ and a value of 4.3 ns at room temperature, in agreement with experiments. For the phonon-dominated regime in diamond and graphite, we predict a stronger simT−5\sim T^{-5}simT−5 and simT−4.5\sim T^{-4.5}simT−4.5 dependence that limits T1T_1T1 (300 K) to 180 and 5.8 ns, respectively. A key aspect of this study is that the parameter-free nature of our approach provides a method to study the effect of {\em any} type of impurity or defect on spin-transport. Furthermore we find that the spin-mix amplitude in silicon does not follow the Eg−2E_g^{-2}Eg−2 band gap dependence usually assigned to III-V semiconductors but follows a much weaker and opposite Eg0.67E_g^{0.67}Eg0.67 dependence. This dependence should be taken into account when constructing silicon spin transport models.

Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in... more Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in thin films of Sr1+δTi1-δO3-δ (δ˜0.2) grown on stoichiometric SrTiO3. We envision the creation of regularly spaced anti-phase domains (APD's) separated by anti-phase boundaries (APB's) through a a/2<111> displacement, thus enabling a locally charge-neutral SrO stoichiometry at the APB's, while preserving the SrTiO3 stoichiometry in the interior of the APD's. This allows for an overall charge-neutrality and correct stoichiometry in the non-stoichiometric layer. Preliminary calculations of the dimensions of the APD's are in good agreement with the observed results. We performed molecular dynamics simulations using Buckingham plus Coulomb empirical potentials and their energetics as well as the equilibrium positions of the atoms and resulting lattice constants were determined. Also, the corresponding STEM signals of the structures were modeled, allowing direct comparison to the STEM images of the non-stoichiometric material. To examine the stability of the observed structures in comparison to the constitutional point defects, we calculated the formation energies of single point defects in SrTiO3, which we also used to benchmark the empirical-potential results against first-principles values.

We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires... more We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires in the statistical doping regime. At zero gate voltage transport is well described by the space charge limited current model and Efros-Shklovskii variable range hopping, but positive gate voltage (electron accumulation) reveals a previously unexplored regime of nanowire charge transport that is not well described by existing theory. The ability to continuously tune between these regimes provides guidance for the extension of existing models and directly informs the design of next-generation nanoscale electronic devices.

Applied Physics Letters, 2011

We propose a new method to calculate polarization induced interfacial charges in semiconductor he... more We propose a new method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4-6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local-density approximation and hybrid functionals). We also find the calculated interfacial charge of (6.8 ± 0.4) × 10 13 cm −2 to be in excellent agreement with experiments and the value of 6.58 × 10 13 cm −2 calculated from bulk polarization constants, validating the use of bulk constants even for very thin films.

One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin ... more One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin relaxation times. We have been developing for the first time a parameter-free first-principles method to determine spin relaxation times. Our effort initially concentrates on silicon and diamond. For liquid-nitrogen temperatures and above, spin relaxation in silicon is dominated by the Elliott-Yafet mechanism. The spin relaxation is induced by momentum scattering off impurities or phonons. The development of a basic methodology based on density-functional calculations that can be used to determine momentum scattering lifetimes has been recently completed by one of us [1]. By considering the spin-orbit mixing of the up and down states, the spin-flip matrix elements can be related to the momentum matrix elements. The underlying theory has previously been derived for III-V semiconductors with direct band gap but did previously not exist for indirect-band gap materials such as silicon or diamond. We report results of an accurate formulation to calculate spin relaxation times in silicon and diamond based on first-principles methods, which we find in excellent agreement with experimental relaxation times. Due to the ab-initio nature of our method, it can be directly applied to study other potentially spin-preserving systems.[4pt] [1] O. D. Restrepo et al., Appl. Phys. Lett. 94, 212103 (2009).

We present a generally applicable parameter-free first-principles method to determine electronic ... more We present a generally applicable parameter-free first-principles method to determine electronic spin relaxation times and apply it to the technologically important group-IV materials silicon, diamond and graphite. We concentrate on the Elliott-Yafet mechanism, where spin relaxation is induced by momentum scattering off phonons and impurities. In silicon, we find a simT−3\sim T^{-3}simT−3 temperature dependence of the phonon-limited spin relaxation time T$_1$ and a value of 4.3 ns at room temperature, in agreement with experiments. For the phonon-dominated regime in diamond and graphite, we predict a stronger simT−5\sim T^{-5}simT−5 and simT−4.5\sim T^{-4.5}simT−4.5 dependence that limits T1T_1T1 (300 K) to 180 and 5.8 ns, respectively. A key aspect of this study is that the parameter-free nature of our approach provides a method to study the effect of {\em any} type of impurity or defect on spin-transport. Furthermore we find that the spin-mix amplitude in silicon does not follow the Eg−2E_g^{-2}Eg−2 band gap dependence usually assigned to III-V semiconductors but follows a much weaker and opposite Eg0.67E_g^{0.67}Eg0.67 dependence. This dependence should be taken into account when constructing silicon spin transport models.

Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in... more Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in thin films of Sr1+δTi1-δO3-δ (δ˜0.2) grown on stoichiometric SrTiO3. We envision the creation of regularly spaced anti-phase domains (APD's) separated by anti-phase boundaries (APB's) through a a/2<111> displacement, thus enabling a locally charge-neutral SrO stoichiometry at the APB's, while preserving the SrTiO3 stoichiometry in the interior of the APD's. This allows for an overall charge-neutrality and correct stoichiometry in the non-stoichiometric layer. Preliminary calculations of the dimensions of the APD's are in good agreement with the observed results. We performed molecular dynamics simulations using Buckingham plus Coulomb empirical potentials and their energetics as well as the equilibrium positions of the atoms and resulting lattice constants were determined. Also, the corresponding STEM signals of the structures were modeled, allowing direct comparison to the STEM images of the non-stoichiometric material. To examine the stability of the observed structures in comparison to the constitutional point defects, we calculated the formation energies of single point defects in SrTiO3, which we also used to benchmark the empirical-potential results against first-principles values.

Mobility is a key factor in charge transport since it describes how the motion of an electron is ... more Mobility is a key factor in charge transport since it describes how the motion of an electron is affected by an applied electric field. As such, it is an important element in the design of new devices. Mobilities are generally modeled using methods that suppress atomic-scale detail (effective mass theory or bulk energy bands for electron velocities, empirical deformation potentials, macroscopic roughness, etc). Parameter fitting to experimental data is needed. As new technologies require modeling of transport at the nanoscale and new materials are introduced, predictive parameter-free mobility modeling is needed. The main scattering mechanisms that limit mobilities are due to phonon, ionized impurities, and interface roughness. A first-principles calculation of mobilities limited by atomic scale roughness with atomic-scale detail was reported recently [1]. We report the development of parameter-free quantum-mechanical methods to calculate scattering rates and electron mobilities limited by phonon and ionized-impurity scattering in a self-consistent way. Results for n-doped silicon are in good agreement with experimental data. This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).

Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. The... more Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. These methods do not take into account quantum mechanical effects with atomic-scale structural resolution, which are key elements to describe transport at the nano-scale. We use a novel first-principles approach to calculate mobilities in ultra-thin SOI MOSFETs [1]. For this report, we use newly constructed interface models of Si(100) and amorphous SiO2. Straining the silicon lattice results in significant increases in carrier mobility. We distinguish between the strain enhancement due to the change in velocities and the enhancement coming from the change in scattering potential. We also compare our calculations with experimental values for mobility degradation caused by radiation induced Coulomb scattering centers. We are able to quantify the contribution to the total mobility from various types of scattering centers, namely, from atomic-scale interface roughness (oxide protrusions, suboxide bonds) and scattering from point defects (dangling bonds, hydrogen). This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).

Mobility is a key factor in charge transport since it describes how the motion of an electron is ... more Mobility is a key factor in charge transport since it describes how the motion of an electron is affected by an applied electric field. As such, it is an important element in the design of new devices. Mobilities are generally modeled using methods that suppress atomic-scale detail (effective mass theory or bulk energy bands for electron velocities, empirical deformation potentials, macroscopic roughness, etc). Parameter fitting to experimental data is needed. As new technologies require modeling of transport at the nanoscale and new materials are introduced, predictive parameter-free mobility modeling is needed. The main scattering mechanisms that limit mobilities are due to phonon, ionized impurities, and interface roughness. A first-principles calculation of mobilities limited by atomic scale roughness with atomic-scale detail was reported recently [1]. We report the development of parameter-free quantum-mechanical methods to calculate scattering rates and electron mobilities limited by phonon and ionized-impurity scattering in a self-consistent way. Results for n-doped silicon are in good agreement with experimental data. This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).

Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. The... more Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. These methods do not take into account quantum mechanical effects with atomic-scale structural resolution, which are key elements to describe transport at the nano-scale. We use a novel first-principles approach to calculate mobilities in ultra-thin SOI MOSFETs [1]. For this report, we use newly constructed interface models of Si(100) and amorphous SiO2. Straining the silicon lattice results in significant increases in carrier mobility. We distinguish between the strain enhancement due to the change in velocities and the enhancement coming from the change in scattering potential. We also compare our calculations with experimental values for mobility degradation caused by radiation induced Coulomb scattering centers. We are able to quantify the contribution to the total mobility from various types of scattering centers, namely, from atomic-scale interface roughness (oxide protrusions, suboxide bonds) and scattering from point defects (dangling bonds, hydrogen). This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).

Applied Clay Science, 2008

Ultramarine blue made with Asturian refined hard kaolin from deposits associated with Armorican q... more Ultramarine blue made with Asturian refined hard kaolin from deposits associated with Armorican quartzite, is compared with the pigment made with China kaolin, both produced in the same synthesis conditions. Properties after a deep characterization (XRD, IRFT and Ramman Spectroscopies, Light Reflectivity, Colorimetry and Micro-analysis) showed that the Asturian Teresa mine kaolin is good enough, once washed and classified, to produce a good blue pigment. The pigment is quite comparable in quality to the one produced with China kaolin. Moreover, it can be obtained from a cheap and abundant material which so far had only been used for calcined fire-clay raw materials, used in low alumina refractories and castables production.

We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires... more We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires in the statistical doping regime. At zero gate voltage transport is well described by the space charge limited current model and Efros-Shklovskii variable range hopping, but positive gate voltage (electron accumulation) reveals a previously unexplored regime of nanowire charge transport that is not well described by existing theory. The ability to continuously tune between these regimes provides guidance for the extension of existing models and directly informs the design of next-generation nanoscale electronic devices.

Applied Physics Letters, 2011

We propose a new method to calculate polarization induced interfacial charges in semiconductor he... more We propose a new method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4-6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local-density approximation and hybrid functionals). We also find the calculated interfacial charge of (6.8 ± 0.4) × 10 13 cm −2 to be in excellent agreement with experiments and the value of 6.58 × 10 13 cm −2 calculated from bulk polarization constants, validating the use of bulk constants even for very thin films.

One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin ... more One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin relaxation times. We have been developing for the first time a parameter-free first-principles method to determine spin relaxation times. Our effort initially concentrates on silicon and diamond. For liquid-nitrogen temperatures and above, spin relaxation in silicon is dominated by the Elliott-Yafet mechanism. The spin relaxation is induced by momentum scattering off impurities or phonons. The development of a basic methodology based on density-functional calculations that can be used to determine momentum scattering lifetimes has been recently completed by one of us [1]. By considering the spin-orbit mixing of the up and down states, the spin-flip matrix elements can be related to the momentum matrix elements. The underlying theory has previously been derived for III-V semiconductors with direct band gap but did previously not exist for indirect-band gap materials such as silicon or diamond. We report results of an accurate formulation to calculate spin relaxation times in silicon and diamond based on first-principles methods, which we find in excellent agreement with experimental relaxation times. Due to the ab-initio nature of our method, it can be directly applied to study other potentially spin-preserving systems.[4pt] [1] O. D. Restrepo et al., Appl. Phys. Lett. 94, 212103 (2009).

We present a generally applicable parameter-free first-principles method to determine electronic ... more We present a generally applicable parameter-free first-principles method to determine electronic spin relaxation times and apply it to the technologically important group-IV materials silicon, diamond and graphite. We concentrate on the Elliott-Yafet mechanism, where spin relaxation is induced by momentum scattering off phonons and impurities. In silicon, we find a simT−3\sim T^{-3}simT−3 temperature dependence of the phonon-limited spin relaxation time T$_1$ and a value of 4.3 ns at room temperature, in agreement with experiments. For the phonon-dominated regime in diamond and graphite, we predict a stronger simT−5\sim T^{-5}simT−5 and simT−4.5\sim T^{-4.5}simT−4.5 dependence that limits T1T_1T1 (300 K) to 180 and 5.8 ns, respectively. A key aspect of this study is that the parameter-free nature of our approach provides a method to study the effect of {\em any} type of impurity or defect on spin-transport. Furthermore we find that the spin-mix amplitude in silicon does not follow the Eg−2E_g^{-2}Eg−2 band gap dependence usually assigned to III-V semiconductors but follows a much weaker and opposite Eg0.67E_g^{0.67}Eg0.67 dependence. This dependence should be taken into account when constructing silicon spin transport models.

Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in... more Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in thin films of Sr1+δTi1-δO3-δ (δ˜0.2) grown on stoichiometric SrTiO3. We envision the creation of regularly spaced anti-phase domains (APD's) separated by anti-phase boundaries (APB's) through a a/2<111> displacement, thus enabling a locally charge-neutral SrO stoichiometry at the APB's, while preserving the SrTiO3 stoichiometry in the interior of the APD's. This allows for an overall charge-neutrality and correct stoichiometry in the non-stoichiometric layer. Preliminary calculations of the dimensions of the APD's are in good agreement with the observed results. We performed molecular dynamics simulations using Buckingham plus Coulomb empirical potentials and their energetics as well as the equilibrium positions of the atoms and resulting lattice constants were determined. Also, the corresponding STEM signals of the structures were modeled, allowing direct comparison to the STEM images of the non-stoichiometric material. To examine the stability of the observed structures in comparison to the constitutional point defects, we calculated the formation energies of single point defects in SrTiO3, which we also used to benchmark the empirical-potential results against first-principles values.

We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires... more We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires in the statistical doping regime. At zero gate voltage transport is well described by the space charge limited current model and Efros-Shklovskii variable range hopping, but positive gate voltage (electron accumulation) reveals a previously unexplored regime of nanowire charge transport that is not well described by existing theory. The ability to continuously tune between these regimes provides guidance for the extension of existing models and directly informs the design of next-generation nanoscale electronic devices.

Applied Physics Letters, 2011

We propose a new method to calculate polarization induced interfacial charges in semiconductor he... more We propose a new method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4-6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local-density approximation and hybrid functionals). We also find the calculated interfacial charge of (6.8 ± 0.4) × 10 13 cm −2 to be in excellent agreement with experiments and the value of 6.58 × 10 13 cm −2 calculated from bulk polarization constants, validating the use of bulk constants even for very thin films.

One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin ... more One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin relaxation times. We have been developing for the first time a parameter-free first-principles method to determine spin relaxation times. Our effort initially concentrates on silicon and diamond. For liquid-nitrogen temperatures and above, spin relaxation in silicon is dominated by the Elliott-Yafet mechanism. The spin relaxation is induced by momentum scattering off impurities or phonons. The development of a basic methodology based on density-functional calculations that can be used to determine momentum scattering lifetimes has been recently completed by one of us [1]. By considering the spin-orbit mixing of the up and down states, the spin-flip matrix elements can be related to the momentum matrix elements. The underlying theory has previously been derived for III-V semiconductors with direct band gap but did previously not exist for indirect-band gap materials such as silicon or diamond. We report results of an accurate formulation to calculate spin relaxation times in silicon and diamond based on first-principles methods, which we find in excellent agreement with experimental relaxation times. Due to the ab-initio nature of our method, it can be directly applied to study other potentially spin-preserving systems.[4pt] [1] O. D. Restrepo et al., Appl. Phys. Lett. 94, 212103 (2009).

We present a generally applicable parameter-free first-principles method to determine electronic ... more We present a generally applicable parameter-free first-principles method to determine electronic spin relaxation times and apply it to the technologically important group-IV materials silicon, diamond and graphite. We concentrate on the Elliott-Yafet mechanism, where spin relaxation is induced by momentum scattering off phonons and impurities. In silicon, we find a simT−3\sim T^{-3}simT−3 temperature dependence of the phonon-limited spin relaxation time T$_1$ and a value of 4.3 ns at room temperature, in agreement with experiments. For the phonon-dominated regime in diamond and graphite, we predict a stronger simT−5\sim T^{-5}simT−5 and simT−4.5\sim T^{-4.5}simT−4.5 dependence that limits T1T_1T1 (300 K) to 180 and 5.8 ns, respectively. A key aspect of this study is that the parameter-free nature of our approach provides a method to study the effect of {\em any} type of impurity or defect on spin-transport. Furthermore we find that the spin-mix amplitude in silicon does not follow the Eg−2E_g^{-2}Eg−2 band gap dependence usually assigned to III-V semiconductors but follows a much weaker and opposite Eg0.67E_g^{0.67}Eg0.67 dependence. This dependence should be taken into account when constructing silicon spin transport models.

Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in... more Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in thin films of Sr1+δTi1-δO3-δ (δ˜0.2) grown on stoichiometric SrTiO3. We envision the creation of regularly spaced anti-phase domains (APD's) separated by anti-phase boundaries (APB's) through a a/2<111> displacement, thus enabling a locally charge-neutral SrO stoichiometry at the APB's, while preserving the SrTiO3 stoichiometry in the interior of the APD's. This allows for an overall charge-neutrality and correct stoichiometry in the non-stoichiometric layer. Preliminary calculations of the dimensions of the APD's are in good agreement with the observed results. We performed molecular dynamics simulations using Buckingham plus Coulomb empirical potentials and their energetics as well as the equilibrium positions of the atoms and resulting lattice constants were determined. Also, the corresponding STEM signals of the structures were modeled, allowing direct comparison to the STEM images of the non-stoichiometric material. To examine the stability of the observed structures in comparison to the constitutional point defects, we calculated the formation energies of single point defects in SrTiO3, which we also used to benchmark the empirical-potential results against first-principles values.

We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires... more We present a comprehensive investigation into disorder-mediated charge transport in InP nanowires in the statistical doping regime. At zero gate voltage transport is well described by the space charge limited current model and Efros-Shklovskii variable range hopping, but positive gate voltage (electron accumulation) reveals a previously unexplored regime of nanowire charge transport that is not well described by existing theory. The ability to continuously tune between these regimes provides guidance for the extension of existing models and directly informs the design of next-generation nanoscale electronic devices.

Applied Physics Letters, 2011

We propose a new method to calculate polarization induced interfacial charges in semiconductor he... more We propose a new method to calculate polarization induced interfacial charges in semiconductor heterostructures using classical electrostatics applied to real-space band diagrams from first principles calculations and apply it to GaN/AlN heterostructures with ultrathin AlN layers (4-6 monolayers). We show that the calculated electric fields and interfacial charges are independent of the exchange-correlation functionals used (local-density approximation and hybrid functionals). We also find the calculated interfacial charge of (6.8 ± 0.4) × 10 13 cm −2 to be in excellent agreement with experiments and the value of 6.58 × 10 13 cm −2 calculated from bulk polarization constants, validating the use of bulk constants even for very thin films.

One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin ... more One of the most fundamental questions in spintronics to date concerns the ultimate limit of spin relaxation times. We have been developing for the first time a parameter-free first-principles method to determine spin relaxation times. Our effort initially concentrates on silicon and diamond. For liquid-nitrogen temperatures and above, spin relaxation in silicon is dominated by the Elliott-Yafet mechanism. The spin relaxation is induced by momentum scattering off impurities or phonons. The development of a basic methodology based on density-functional calculations that can be used to determine momentum scattering lifetimes has been recently completed by one of us [1]. By considering the spin-orbit mixing of the up and down states, the spin-flip matrix elements can be related to the momentum matrix elements. The underlying theory has previously been derived for III-V semiconductors with direct band gap but did previously not exist for indirect-band gap materials such as silicon or diamond. We report results of an accurate formulation to calculate spin relaxation times in silicon and diamond based on first-principles methods, which we find in excellent agreement with experimental relaxation times. Due to the ab-initio nature of our method, it can be directly applied to study other potentially spin-preserving systems.[4pt] [1] O. D. Restrepo et al., Appl. Phys. Lett. 94, 212103 (2009).

We present a generally applicable parameter-free first-principles method to determine electronic ... more We present a generally applicable parameter-free first-principles method to determine electronic spin relaxation times and apply it to the technologically important group-IV materials silicon, diamond and graphite. We concentrate on the Elliott-Yafet mechanism, where spin relaxation is induced by momentum scattering off phonons and impurities. In silicon, we find a simT−3\sim T^{-3}simT−3 temperature dependence of the phonon-limited spin relaxation time T$_1$ and a value of 4.3 ns at room temperature, in agreement with experiments. For the phonon-dominated regime in diamond and graphite, we predict a stronger simT−5\sim T^{-5}simT−5 and simT−4.5\sim T^{-4.5}simT−4.5 dependence that limits T1T_1T1 (300 K) to 180 and 5.8 ns, respectively. A key aspect of this study is that the parameter-free nature of our approach provides a method to study the effect of {\em any} type of impurity or defect on spin-transport. Furthermore we find that the spin-mix amplitude in silicon does not follow the Eg−2E_g^{-2}Eg−2 band gap dependence usually assigned to III-V semiconductors but follows a much weaker and opposite Eg0.67E_g^{0.67}Eg0.67 dependence. This dependence should be taken into account when constructing silicon spin transport models.

Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in... more Z-contrast HRSTEM imaging and EELS has yielded information into the nature of defect formation in thin films of Sr1+δTi1-δO3-δ (δ˜0.2) grown on stoichiometric SrTiO3. We envision the creation of regularly spaced anti-phase domains (APD's) separated by anti-phase boundaries (APB's) through a a/2<111> displacement, thus enabling a locally charge-neutral SrO stoichiometry at the APB's, while preserving the SrTiO3 stoichiometry in the interior of the APD's. This allows for an overall charge-neutrality and correct stoichiometry in the non-stoichiometric layer. Preliminary calculations of the dimensions of the APD's are in good agreement with the observed results. We performed molecular dynamics simulations using Buckingham plus Coulomb empirical potentials and their energetics as well as the equilibrium positions of the atoms and resulting lattice constants were determined. Also, the corresponding STEM signals of the structures were modeled, allowing direct comparison to the STEM images of the non-stoichiometric material. To examine the stability of the observed structures in comparison to the constitutional point defects, we calculated the formation energies of single point defects in SrTiO3, which we also used to benchmark the empirical-potential results against first-principles values.

Mobility is a key factor in charge transport since it describes how the motion of an electron is ... more Mobility is a key factor in charge transport since it describes how the motion of an electron is affected by an applied electric field. As such, it is an important element in the design of new devices. Mobilities are generally modeled using methods that suppress atomic-scale detail (effective mass theory or bulk energy bands for electron velocities, empirical deformation potentials, macroscopic roughness, etc). Parameter fitting to experimental data is needed. As new technologies require modeling of transport at the nanoscale and new materials are introduced, predictive parameter-free mobility modeling is needed. The main scattering mechanisms that limit mobilities are due to phonon, ionized impurities, and interface roughness. A first-principles calculation of mobilities limited by atomic scale roughness with atomic-scale detail was reported recently [1]. We report the development of parameter-free quantum-mechanical methods to calculate scattering rates and electron mobilities limited by phonon and ionized-impurity scattering in a self-consistent way. Results for n-doped silicon are in good agreement with experimental data. This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).

Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. The... more Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. These methods do not take into account quantum mechanical effects with atomic-scale structural resolution, which are key elements to describe transport at the nano-scale. We use a novel first-principles approach to calculate mobilities in ultra-thin SOI MOSFETs [1]. For this report, we use newly constructed interface models of Si(100) and amorphous SiO2. Straining the silicon lattice results in significant increases in carrier mobility. We distinguish between the strain enhancement due to the change in velocities and the enhancement coming from the change in scattering potential. We also compare our calculations with experimental values for mobility degradation caused by radiation induced Coulomb scattering centers. We are able to quantify the contribution to the total mobility from various types of scattering centers, namely, from atomic-scale interface roughness (oxide protrusions, suboxide bonds) and scattering from point defects (dangling bonds, hydrogen). This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).

Mobility is a key factor in charge transport since it describes how the motion of an electron is ... more Mobility is a key factor in charge transport since it describes how the motion of an electron is affected by an applied electric field. As such, it is an important element in the design of new devices. Mobilities are generally modeled using methods that suppress atomic-scale detail (effective mass theory or bulk energy bands for electron velocities, empirical deformation potentials, macroscopic roughness, etc). Parameter fitting to experimental data is needed. As new technologies require modeling of transport at the nanoscale and new materials are introduced, predictive parameter-free mobility modeling is needed. The main scattering mechanisms that limit mobilities are due to phonon, ionized impurities, and interface roughness. A first-principles calculation of mobilities limited by atomic scale roughness with atomic-scale detail was reported recently [1]. We report the development of parameter-free quantum-mechanical methods to calculate scattering rates and electron mobilities limited by phonon and ionized-impurity scattering in a self-consistent way. Results for n-doped silicon are in good agreement with experimental data. This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).

Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. The... more Carrier mobilities in MOSFETs are usually simulated by employing empirical scattering models. These methods do not take into account quantum mechanical effects with atomic-scale structural resolution, which are key elements to describe transport at the nano-scale. We use a novel first-principles approach to calculate mobilities in ultra-thin SOI MOSFETs [1]. For this report, we use newly constructed interface models of Si(100) and amorphous SiO2. Straining the silicon lattice results in significant increases in carrier mobility. We distinguish between the strain enhancement due to the change in velocities and the enhancement coming from the change in scattering potential. We also compare our calculations with experimental values for mobility degradation caused by radiation induced Coulomb scattering centers. We are able to quantify the contribution to the total mobility from various types of scattering centers, namely, from atomic-scale interface roughness (oxide protrusions, suboxide bonds) and scattering from point defects (dangling bonds, hydrogen). This work was supported by NSF Grant ECS-0524655. [1] M. H. Evans et al., Phys. Rev. Lett. 95, 106802 (2005).

Applied Clay Science, 2008

Ultramarine blue made with Asturian refined hard kaolin from deposits associated with Armorican q... more Ultramarine blue made with Asturian refined hard kaolin from deposits associated with Armorican quartzite, is compared with the pigment made with China kaolin, both produced in the same synthesis conditions. Properties after a deep characterization (XRD, IRFT and Ramman Spectroscopies, Light Reflectivity, Colorimetry and Micro-analysis) showed that the Asturian Teresa mine kaolin is good enough, once washed and classified, to produce a good blue pigment. The pigment is quite comparable in quality to the one produced with China kaolin. Moreover, it can be obtained from a cheap and abundant material which so far had only been used for calcined fire-clay raw materials, used in low alumina refractories and castables production.