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Vincenzo Fiorentini

Publications and source records attributed to Vincenzo Fiorentini.

At least 55 records · Page 3Linked to original sources
arXiv1999

Polarization fields in nitride nanostructures: ten points to think about

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.

cond-mat.mtrl-sci↗
arXiv1999

Extrinsic levels, diffusion, and unusual incorporation mechanism of lithium in GaN

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.

cond-mat.mtrl-sci↗
arXiv1998

Atomistic modelling of large-scale metal film growth fronts

We present simulations of metallization morphologies under ionized sputter deposition conditions, obtained by a new theoretical approach. By means of molecular dynamics simulations using a carefully designed interaction potential, we analyze the surface adsorption, reflection, and etching reactions taking place during Al physical vapor deposition, and calculate their relative probability. These probabilities are then employed in a feature-scale cellular-automaton simulator, which produces calculated film morphologies in excellent agreement with scanning-electron-microscopy data on ionized sputter deposition.

cond-mat.mtrl-sci↗
arXiv1998

Anomalous relaxations and chemical trends at III-V nitride non-polar surfaces

Relaxations at nonpolar surfaces of III-V compounds result from a competition between dehybridization and charge transfer. First principles calculations for the (110) and (10$\bar{1}$0) faces of zincblende and wurtzite AlN, GaN and InN reveal an anomalous behavior as compared with ordinary III-V semiconductors. Additional calculations for GaAs and ZnO suggest close analogies with the latter. We interpret our results in terms of the larger ionicity (charge asymmetry) and bonding strength (cohesive energy) in the nitrides with respect to other III-V compounds, both essentially due to the strong valence potential and absence of $p$ core states in the lighter anion. The same interpretation applies to Zn II-VI compounds.

cond-mat.mtrl-sci↗
arXiv1998

Faceting and stress transfer in the missing-row reconstruction of Ir (110)

We present ab initio total energy and stress calculations for the unreconstructed and (2x1)-missing-row reconstructed Ir (110) and Rh (110) surfaces. We use a model based on ab initio results to show that the (Nx1) reconstruction is a faceting transition to a long-wavelength--corrugated (111)-like surface, and find the 3x1 structure to be the most stable for Ir in accordance with experiment. We then use the stress density to analyze the stress increase upon reconstruction, and find it to be due to a changed balance of tensile and compressive contributions in the near-surface region.

cond-mat.mtrl-sci↗
arXiv1998

Theoretical evidence for the semi-insulating character of AlN

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.

cond-mat.mtrl-sci↗
arXiv1998

Electronic dielectric constants of insulators by the polarization method

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.

cond-mat.mtrl-sci↗
arXiv1998

Polarization fields in nitride nanostructures: theory and practical implications

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.

cond-mat.mtrl-sci↗
arXiv1997

Macroscopic polarization and band offsets at nitride heterojunctions

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.

cond-mat.mtrl-sci↗
arXiv1997

Polarization-based calculation of the dielectric tensor of polar crystals

We present a novel method for the calculation of the static and electronic dielectric tensor of polar insulating crystals based on concepts from the modern theory of dielectric polarization. As an application, we present the first ab initio calculation of the dielectric constants in the wurtzite III-V nitrides AlN, GaN, and InN.

cond-mat.mtrl-sci↗
arXiv1997

Spontaneous polarization and piezoelectric constants of III-V nitrides

The spontaneous polarization, dynamical Born charges, and piezoelectric constants of the III-V nitrides AlN, GaN, and InN are studied ab initio using the Berry phase approach to polarization in solids. The piezoelectric constants are found to be up 10 times larger than in conventional III-V's and II-VI's, and comparable to those of ZnO. Further properties at variance with those of conventional III-V compounds are the sign of the piezoelectric constants (positive as in II-VI's) and the very large spontaneous polarization.

cond-mat.mtrl-sci↗
arXiv1997

Theoretical evidence for efficient p-type doping of GaN using beryllium

Ab initio calculations predict that Be is a shallow acceptor in GaN. Its thermal ionization energy is 0.06 eV in wurtzite GaN; the level is valence resonant in the zincblende phase. Be incorporation is severely limited by the formation of Be_3N_2. We show however that co-incorporation with reactive species can enhance the solubility. H-assisted incorporation should lead to high doping levels in MOCVD growth after post-growth annealing at about 850 K. Be-O co-incorporation produces high Be and O concentrations at MBE growth temperatures.

cond-mat.mtrl-sci↗
arXiv1996

Ab initio core-level shifts in metallic alloys

Core-level shifts and core-hole screening effects in alloy formation are studied ``ab initio'' by constrained-density-functional total-energy calculations. For our case study, the ordered intermetallic alloy MgAu, final-state effects are essential to account for the experimental Mg 1s shift, while they are negligible for Au 4f. We explain the differences in the screening by analyzing the calculated charge density response to the core hole perturbation.

cond-mat.mtrl-sci↗
arXiv1996

Zn-related deep centers in wurtzite GaN

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.

cond-mat.mtrl-sci↗
arXiv1996

Inhibited Al diffusion and growth roughening on Ga-coated Al (100)

Ab initio calculations indicate that the ground state for Ga adsorption on Al (100) is on-surface with local unit coverage. On Ga-coated Al (100), the bridge diffusion barrier for Al is large, but the Al$\rightarrow$Ga {\it exchange barrier is zero}: the ensuing incorporation of randomly deposited Al's into the Ga overlayer realizes a percolation network, efficiently recoated by Ga atoms. Based on calculated energetics, we predict rough surface growth at all temperatures; modeling the growth by a random deposition model with partial relaxation, we find a power-law divergent roughness $w\sim t^{\,0.07\pm0.02}$.

mtrl-th↗
arXiv1995

Formation Energy, Stress, and Relaxations of Low-Index Rhodium Surfaces

Ab initio local-density-functional-theory calculations of formation energies, surface stress, and multilayer relaxations are reported for the (111), (100), and (110) surfaces of Rh. The study is performed using ultrasoft pseudopotentials and plane waves in a parallel implementation. Calculated values are in good agreement with previous studies where they exist. In particular, we confirm recent results on surface stress anisotropy of transition metals.

mtrl-th↗