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Search Results: 1 - 10 of 6008 matches for " Fabio Bernardini "
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Non-linear macroscopic polarization in III-V nitride alloys
Fabio Bernardini,Vincenzo Fiorentini
Physics , 2001, DOI: 10.1103/PhysRevB.64.085207
Abstract: We study the dependence of macroscopic polarization on composition and strain in wurtzite III-V nitride ternary alloys using ab initio density-functional techniques. The spontaneous polarization is characterized by a large bowing, strongly dependent on the alloy microscopic structure. The bowing is due to the different response of the bulk binaries to hydrostatic pressure, and to internal strain effects (bond alternation). Disorder effects are instead minor. Deviations from parabolicity (simple bowing) are of order 10 % in the most extreme case of AlInN alloy, much less at all other compositions. Piezoelectric polarization is also strongly non-linear. At variance with the spontaneous component, this behavior is independent of microscopic alloy structure or disorder effects, and due entirely to the non-linear strain dependence of the bulk piezoelectric response. It is thus possible to predict the piezoelectric polarization for any alloy composition using the piezoelectricity of the parent binaries.
Polarization fields in nitride nanostructures: theory and practical implications
Fabio Bernardini,Vincenzo Fiorentini
Physics , 1998,
Abstract: Huge built-in electric fields are predicted to exist in wurtzite III-V nitrides thin films and multilayers. Such fields originate from heterointerface discontinuities of the macroscopic bulk polarization of the nitrides. We discuss the theoretical background and the intriguing practical implications of polarization fields for nitride nanostructures.
Macroscopic polarization and band offsets at nitride heterojunctions
Fabio Bernardini,Vincenzo Fiorentini
Physics , 1997, DOI: 10.1103/PhysRevB.57.R9427
Abstract: Ab initio electronic structure studies of prototypical polar interfaces of wurtzite III-V nitrides show that large uniform electric fields exist in epitaxial nitride overlayers, due to the discontinuity across the interface of the macroscopic polarization of the constituent materials. Polarization fields forbid a standard evaluation of band offsets and formation energies: using new techniques, we find a large forward-backward asymmetry of the offset (0.2 eV for AlN/GaN (0001), 0.85 eV for GaN/AlN (0001)), and tiny interface formation energies.
Electronic dielectric constants of insulators by the polarization method
Fabio Bernardini,Vincenzo Fiorentini
Physics , 1998, DOI: 10.1103/PhysRevB.58.15292
Abstract: We discuss a non-perturbative, technically straightforward, easy-to-use, and computationally affordable method, based on polarization theory, for the calculation of the electronic dielectric constant of insulating solids at the first principles level. We apply the method to GaAs, AlAs, InN, SiC, ZnO, GaN, AlN, BeO, LiF, PbTiO$_3$, and CaTiO$_3$. The predicted $\einf$'s agree well with those given by Density Functional Perturbation Theory (the reference theoretical treatment), and they are generally within less than 10 % of experiment.
Spontaneous vs. piezoelectric polarization in III-V nitrides: conceptual aspects and practical consequences
Fabio Bernardini,Vincenzo Fiorentini
Physics , 1999, DOI: 10.1002/(SICI)1521-3951(199911)216:1<391::AID-PSSB391>3.0.CO;2-K
Abstract: Macroscopic polarization plays a major role in determining the optical and electrical properties of nitride nanostructures via polarization-induced built-in electrostatic fields. While currently fashionable, this field of endeavour is still by far in its early infancy. Here we contribute some clarifications on the conceptual issues involved in determining built-in fields in III-V nitride nanostructures, sorting out in particular the roles of spontaneous and piezoelectric polarization.
Polarization fields in nitride nanostructures: ten points to think about
Fabio Bernardini,Vincenzo Fiorentini
Physics , 1999, DOI: 10.1016/S0169-4332(00)00434-7
Abstract: Macroscopic polarization, both of intrinsic and piezoelectric nature, is unusually strong in III-V nitrides, and the built in electric fields in the layers of nitride-based nanostructures, stemming from polarization changes at heterointerfaces, have a major impact on the properties of single and multiple quantum wells, high mobility transistors, and thin films. The concepts involved in the theory and applications of polarization in nitrides have encountered some resistance in the field. Here we discuss critically ten ``propositions'' aimed at clarifying the main controversial issues.
Extrinsic levels, diffusion, and unusual incorporation mechanism of lithium in GaN
Fabio Bernardini,Vincenzo Fiorentini
Physics , 1999,
Abstract: Results of a first-principles study of the Li impurity in GaN are presented. We find Li is a channel interstitial, with an onset for diffusion at T$\sim$ 600 K. Above this temperature, Li can transform to a Ga-substitutional acceptor by exothermic recombination with Ga vacancies. This process implies capture of at least one electron; therefore Li acts as an electron sink. Li$_{\rm Ga}$ is stable again interstitialcy, and has a shallow first ionization levels of 0.16 eV, and second ionization at 0.63 eV. Lattice locations and their temperature dependence are in close agreement with recent experiments.
Zn-related deep centers in wurtzite GaN
Fabio Bernardini,Vincenzo Fiorentini,R. M. Nieminen
Physics , 1996,
Abstract: Zn in GaN forms an efficient radiative center and acts as a deep acceptor which can make the crystal insulating. Four different Zn-related centers have been by now identified, leading to light emission in the range between 1.8 eV and 2.9 eV. We present a first-principles investigation total energy and electronic structure calculations for Ga-substitutional and hetero-antisite N-substitutional Zn in wurtzite GaN, using ultrasoft pseudopotentials and a conjugate-gradient total energy minimization method. Our results permit the identification of the blue-light emission center as the substitutional acceptor, while contrary to a common belief the Zn_N heteroantisite has a very high formation energy and donor behavior, which seems to exclude it as the origin of the other centers.
Theoretical evidence for the semi-insulating character of AlN
Antonella Fara,Fabio Bernardini,Vincenzo Fiorentini
Physics , 1998, DOI: 10.1063/1.369197
Abstract: We present ab initio density-functional calculations for acceptors, donors, and native defects in aluminum nitride, showing that acceptors are deeper (Be ~ 0.25 eV, Mg_ 0.45 eV) and less soluble than in GaN; at further variance with GaN, both the extrinsic donors Si_Al and C_Al, and the native donor V_N (the anion vacancy) are found to be deep (about 1 to 3 eV below the conduction). We thus predict that doped AlN will generally turn out to be semi-insulating in the normally achieved Al-rich conditions, in agreement with the known doping difficulties of high-x AlGaN alloys.
Offsets and polarization at strained AlN/GaN polar interfaces
Fabio Bernardini,Vincenzo Fiorentin,David Vanderbilt
Physics , 1996,
Abstract: The strain induced by lattice mismatch at the interface is responsible for the different value of the band discontinuities observed recently for the AlN/GaN (AlN on GaN) and the GaN/AlN (GaN on AlN) polar (0001) interface. We present a first-principles calculation of valence band offsets, interface dipoles, strain-induced piezoelectric fields, relaxed geometric structure, and formation energies. Our results confirm the existence of a large forward-backward asymmetry for this interface.
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