Rita Magri | Università degli studi di Modena e Reggio Emilia (original) (raw)

Papers by Rita Magri

Different models for the GaSb-(001) surface reconstruction have been proposed on the basis of exp... more Different models for the GaSb-(001) surface reconstruction have been proposed on the basis of experimental observations in typical GaSb growth conditions (V/III flux ratio > 1). We have analyzed their relative stability by means of first-principles pseudopotential plane-wave calculations. We constructed the surface phase diagram as a function of the chemical potential of the elemental constituents to mimic different growth conditions.

Physical Review B, 2001

In this paper we study the electronic and optical properties of (In 0.5 Ga 0.5 As) n /(InP) n sup... more In this paper we study the electronic and optical properties of (In 0.5 Ga 0.5 As) n /(InP) n superlattices, where the Ga 0.5 In 0.5 As alloy is described both through the virtual crystal approximation ͑VCA͒ and through an appropriate ordered ternary structure. By first-principles calculations of the dielectric tensor elements we address the issue of the giant polarization anisotropy of the optical absorption experimentally observed in these superlattices. The magnitude of the anisotropy depends on the splitting between the hole states at the valence band top which is due to the lowering of the overall symmetry to the C 2v point group and it is greatly influenced by strain not only at the interfaces but also in the bulk alloy.

Physica Status Solidi (c), 2010

We suggest a model for the c(2 x 6) phase of the Sbstabilized GaSb(001) surface, whereby Sb atoms... more We suggest a model for the c(2 x 6) phase of the Sbstabilized GaSb(001) surface, whereby Sb atoms in the second layer are partially substituted by Ga. This ‘doping’ effect enables the surface to fulfil the electron counting rule whilst maintaining a reconstruction based on long dimer chains.Total energy calculations verify that the suggested reconstruction is relatively stable, and calculations of the reflectance anisotropy spectra confirm that it comprises a major component of the largely disordered surface (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Iee Proceedings-optoelectronics, 2003

The authors study the effects of interfacial atomic segregation on the electronic and optical pro... more The authors study the effects of interfacial atomic segregation on the electronic and optical properties of InAs/GaSb superlattices. They describe their atomistic empirical pseudopotential method and test its performance against the available experimental data. They show its ability to predict the band structure dependence on the detailed atomic configuration, and thus to properly account for the effects of interfacial atomic segregation and structural disorder. They also show how their method avoids the approximations underlying the pseudopotential method of Dente and Tilton, which gives different results. The application of the proposed method to the InAs/GaSb superlattices allows the explanation of some observed experimental results, such as: the bandgap difference between ðInAsÞ 8 =ðGaSbÞ 8 superlattices with almost pure InSb-like or GaAs-like interfaces; the large blue shift of the bandgap when the growth temperature of the superlattice increases; and the blue shift of the bandgap of ðInAsÞ 8 =ðGaSbÞ n superlattices with increasing GaSb period n. They present a detailed comparison of their predicted blue shift with that obtained by other theories.

Superlattices and Microstructures, 1989

The epitaxial formation energy of a (GaAs)1 (InAs)1 (111) superlattices grown on an InP substrate... more The epitaxial formation energy of a (GaAs)1 (InAs)1 (111) superlattices grown on an InP substrate is calculated by using a self-consistent pseudopotential approach within the density functional theory. Atomic configurations are considered which have two distinct bond distances and correspond to different relaxations along the [111] direction. The minimum energy is found assigning to the superlattice the same bond distances obtained by minimizing the total energy of the two binary systems under the epitaxial constraint. For all the geometries we find that the superlattice is unstable with respect to disproportionation into the constituents.

Physica Status Solidi B-basic Solid State Physics, 2010

We consider the formation of surface antisite defects on a previously proposed model for the GaSb... more We consider the formation of surface antisite defects on a previously proposed model for the GaSb(001)-c(2 × 6) surface. Based on ab initio total energy calculations, we show how these defects stabilize the otherwise metallic surface and how their formation is driven by the excess charge associated with the Sb-rich surface conditions. The surface-sensitive optical technique of reflectance anisotropy spectroscopy is shown to be crucial for detecting the defects, and computation of spectra yields a good agreement with experiment when defects are included in the surface reconstruction.

Physical Review B, 2011

The structure of the technologically important—but still mostly unknown—GaSb(001)-c(2×6) surface ... more The structure of the technologically important—but still mostly unknown—GaSb(001)-c(2×6) surface reconstruction is investigated by means of ab initio simulations of reflectance anisotropy spectroscopy (RAS) and total energy calculations. A large number of reconstruction models for the GaSb(001) surface in the Sb-rich coverage regime are considered. The influence of each single surface structural motif on the RAS spectra is studied in detail, as well as their role in the surface stability with regard to application of the electron counting rule (ECR). We interpret the features of the RAS data measured for this reconstruction and suggest a new model for the c(2×6) phase. In this model a few Sb atoms in the second layer are randomly substituted by Ga, forming surface antisite defects. When used to fulfill the ECR, this “doping” effect considerably lowers the total energy of the long chain c(2×6) reconstruction model, making it competitive with the more stable short-chain (4×3) reconstructions. Formation of the surface antisites occurs spontaneously in the presence of dynamical negative charge fluctuations and is favored by the excellent matching between GaSb(001) and metallic Sb and by the natural softness of the Ga-Sb bonds. Calculations of the reflectance anisotropy spectra confirm that this structure is a major component of a largely disordered surface, where motifs of the stable (4×3) reconstructions are also present.

Using atomistic pseudopotential calculations we study the evolution of: (i) the conduction band e... more Using atomistic pseudopotential calculations we study the evolution of: (i) the conduction band edge Ec; (ii) the valence band edge Ev , and (iii) the fundamental band gap of the quaternary Ga1-yInyAsxSb1-x alloy lattice-matched to InAs as a function of the composition (x, y). We find a negative bowing parameter for Ec of the quaternary alloy with respect to the

Quaternary alloys without a common atom such as (Ga,In)-(As,Sb) pose a difficult combinatorially ... more Quaternary alloys without a common atom such as (Ga,In)-(As,Sb) pose a difficult combinatorially design problem in that there are many different atomic configurations even when the system is constrained to be lattice-matched on a substrate. Using an atomistic pseudopotential approach we have calculated the band edge energies of this quaternary random alloys as a function of Ga/In (x) and As/Sb (y) compositions assuming lattice-matching to either GaSb or InAs. The alloy is represented by a large supercell with random atomic occupations and atomic positions relaxed via the atomistic VFF functional. We find upwards bowing for both the conduction and valence band edge energies. On GaSb, the transition from staggered to broken-gap lineup is found to occur at x = 0.81 and y = 0.92, while on InAs it occurs at x = 0.59 and y = 0.62. We show that at the usual growth temperatures this quaternary alloy is not random but tends to exhibit an increased number of Ga-Sb and In-As bonds and a reduced number of In-Sb and Ga-As bonds. This effect brings the calculated band gaps in better agreement with experimental data. T. Magri, A. Zunger, H. Kroemer, JAP 98, 043701 (2005)

Physical Review B, 2004

We use extended-basis empirical tight-binding calculations and examine the anisotropy of the refr... more We use extended-basis empirical tight-binding calculations and examine the anisotropy of the refractive index in ultrashort-period superlattices of materials sharing no common atom. We find that a strong birefringence can be engineered in these articial semiconductors, allowing phase matching for frequency difference generation. The prominent role of epitaxial constraint and bond-length alternation is evidenced.

Physica Status Solidi (c), 2010

We study the effects of surface reconstruction and step formation on the surface phase stability,... more We study the effects of surface reconstruction and step formation on the surface phase stability, of an InAs wetting layer on GaAs(001). In particular we focus our attention on the α2 and β2 (2 × 4) surface reconstructions. The two investigated reconstructions have been shown to be formed at an high In coverage, at the onset of the 2D→3D transition. The analysis of the connection between the step stability and the strain distribution around the step edges leads to the conclusion that the favoured step geometries are those minimising the strain. Finally, In diffusion on the flat reconstructed wetting layers has been investigated.We find: (i) the elements of the surface reconstructions favouring In diffusion; (ii) that In diffusion on these surfaces is strongly anisotropic, favoring the [-110] direction; (iii) that the As surface dimers introduce additional adsorption sites with high barriers for In escape (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal of Physics-condensed Matter, 2009

By using density functional theory (DFT) calculations of the potential energy surface in conjunct... more By using density functional theory (DFT) calculations of the potential energy surface in conjunction with the analytical solution of the master equation for the time evolution of the adatom site distribution, we study the diffusion properties of an isolated In adatom on InxGa1-xAs wetting layers (WL) deposited on the GaAs(001). The WL reconstructions considered in this study are, listed in the order of increasing In coverage: c(4 × 4), (1 × 3), (2 × 3), α2(2 × 4) and β2(2 × 4). We analyze the dependence of the diffusion properties on WL reconstruction, composition and strain, and find that: (i) diffusion on the (2 × N) reconstructions is strongly anisotropic, owing to the presence of the low barrier potential in-dimer trench, favoring the diffusion along the [\bar 110] direction over that along the [110] direction; (ii) In diffusion at a WL coverage θ = 2/3 monolayers (ML; with composition x = 2/3) is faster than on clean GaAs(001) c(4 × 4), and decreases at θ = 1.75 ML (x = 1; e.g. InAs/GaAs(001)) (iii) diffusion and nucleation on the (2 × 4) WL is affected by the presence of adsorption sites for indium inside the As dimers; (iv) the approximation used for the exchange-correlation potential within DFT has an important effect on the description of the diffusion properties.

We investigate the potential energy surface (PES) and the adsorption properties of an In adatom o... more We investigate the potential energy surface (PES) and the adsorption properties of an In adatom on InAs (2×4) reconstructed wetting layers (WLs) deposited on a GaAs substrate. The results are then used to derive the diffusion properties of a single In adatom on the WLs. We find that: (i) the adsorbate diffusion is highly anisotropic; (ii) the adsorption sites within the As dimers have to be taken into account since they strongly affect diffusion; (iii) the most stable adsorption sites are the ones within the dimers and those located besides the in-dimers.

Physical Review B, 2008

The band edges and band gaps of ͑InAs͒ n / ͑GaSb͒ m ͑n , m =1,20͒ superlattices have been theoret... more The band edges and band gaps of ͑InAs͒ n / ͑GaSb͒ m ͑n , m =1,20͒ superlattices have been theoretically studied through the plane-wave empirical pseudopotential method for different situations: ͑i͒ different substrates, GaSb and InAs; ͑ii͒ different point group symmetries, C 2v and D 2d ; and ͑iii͒ different growth directions, ͑001͒ and ͑110͒. We find that ͑a͒ the band gaps for the ͑001͒ C 2v superlattices on a GaSb substrate exhibit a nonmonotonic behavior as a function of the GaSb barrier thickness when the number of ͑InAs͒ n layers exceed n =5; ͑b͒ substrate effects: compared with the GaSb substrate, the different strain field generated by the InAs substrate leads to a larger variation of the band gaps for the ͑001͒ C 2v superlattices as a function of the InAs well thickness; ͑c͒ effect of the type of interfacial bonds: the In-Sb bonds at the interfaces of the ͑001͒ D 2d superlattices partially pin the band edge states, reducing the influence of the confinement effects on electrons and holes, and lowering the band gaps as compared to the ͑001͒ C 2v case. The valence band maximum of the ͑001͒ D 2d superlattices with Ga-As bonds at the interfaces are shifted down, increasing the band gaps as compared to the ͑001͒ C 2v case; ͑d͒ effect of layer orientation: the presence of In-Sb bonds at both interfaces of the ͑110͒ superlattices pin the band edge states and reduces the band gaps, as compared to the ͑001͒ C 2v case. An anticrossing between the electron and hole levels in the ͑110͒ superlattices, for thin GaSb and thick InAs layers, leads to an increase of the band gaps, as a function of the InAs thickness; ͑e͒ superlattices vs random alloys: the comparison between the band edges and band gaps of the superlattices on a GaSb substrate and those for random alloys, lattice matched to a GaSb substrate, as a function of the In composition, shows that the random alloys present almost always higher band gaps and give a clear indication of the effect of superlattice's ordering and period on the behavior of the band gaps and band edges. Inclusion of interfacial interdiffusion, using the approach of Magri and Zunger ͓Phys. Rev. B 65, 165302 ͑2002͔͒, is shown to significantly increase the band gaps relative to the predictions for abrupt superlattices, bringing the results closer to experiment. It is noteworthy that k · p model fit instead measured gaps corresponding to interdiffused interfaces using a chemically abrupt model.

Solid State Communications, 1992

We show that the energy of substitutional randomization of atoms in zinc-blende compounds is surp... more We show that the energy of substitutional randomization of atoms in zinc-blende compounds is surprisingly small. This suggests the existence of a new class of defects in these materials (“random aggregates”), which consist of regions of ≲ 10 atoms where the sites of the diamond lattice are randomly occupied by A or B atoms in place of the ordered AB crystal occupancy. The structural and electronic properties of these defects are outlined.

Physical Review B, 1991

The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical b... more The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical bowing of the direct band gaps are calculated for three perfectly random semiconductor alloys within the first-principles pseudopotential method using the concept of ''special ...

Doping control at the nanoscale can be used to modify optical and electronic properties thus indu... more Doping control at the nanoscale can be used to modify optical and electronic properties thus inducing interesting effects that cannot be observed in pure systems. By using Density Functional Theory, Silicon Nanocrystals (Si-nc) of different size (diameter ranging from 1.1 nm to 1.8 nm) have been studied localizing impurities at different substitutional sites and calculating the impurity formation energies. Starting from hydrogen terminated silicon Si-nc, we found that codoping is always energetically favored with respect to a single B- or P-doping and that the two impurities tend to occupy nearest neighbor sites near the surface. The formation energy depends on the distance between the two impurities. The codoped Si-nc present band-edge states localized on the impurities which are responsible for a red-shift of the absorption threshold with respect to that of pure undoped Si nanocrystals. Concerning the emission spectra, we find a Stokes shift of the photoluminescence to a lower energy with respect to the absorption edge due to the nanocrystal (nc) structural relaxation after the creation of the electron-hole pair. We have calculated the absorption and emission spectra going beyond a single-particle approach showing the important role played by the many-body effects. The presence of electronic quasi-direct optical transitions between the donor and acceptor states within the band-gap makes it possible to engineer the optical properties of Si-nc.

Different models for the GaSb-(001) surface reconstruction have been proposed on the basis of exp... more Different models for the GaSb-(001) surface reconstruction have been proposed on the basis of experimental observations in typical GaSb growth conditions (V/III flux ratio > 1). We have analyzed their relative stability by means of first-principles pseudopotential plane-wave calculations. We constructed the surface phase diagram as a function of the chemical potential of the elemental constituents to mimic different growth conditions.

Physical Review B, 2001

In this paper we study the electronic and optical properties of (In 0.5 Ga 0.5 As) n /(InP) n sup... more In this paper we study the electronic and optical properties of (In 0.5 Ga 0.5 As) n /(InP) n superlattices, where the Ga 0.5 In 0.5 As alloy is described both through the virtual crystal approximation ͑VCA͒ and through an appropriate ordered ternary structure. By first-principles calculations of the dielectric tensor elements we address the issue of the giant polarization anisotropy of the optical absorption experimentally observed in these superlattices. The magnitude of the anisotropy depends on the splitting between the hole states at the valence band top which is due to the lowering of the overall symmetry to the C 2v point group and it is greatly influenced by strain not only at the interfaces but also in the bulk alloy.

Physica Status Solidi (c), 2010

We suggest a model for the c(2 x 6) phase of the Sbstabilized GaSb(001) surface, whereby Sb atoms... more We suggest a model for the c(2 x 6) phase of the Sbstabilized GaSb(001) surface, whereby Sb atoms in the second layer are partially substituted by Ga. This ‘doping’ effect enables the surface to fulfil the electron counting rule whilst maintaining a reconstruction based on long dimer chains.Total energy calculations verify that the suggested reconstruction is relatively stable, and calculations of the reflectance anisotropy spectra confirm that it comprises a major component of the largely disordered surface (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Iee Proceedings-optoelectronics, 2003

The authors study the effects of interfacial atomic segregation on the electronic and optical pro... more The authors study the effects of interfacial atomic segregation on the electronic and optical properties of InAs/GaSb superlattices. They describe their atomistic empirical pseudopotential method and test its performance against the available experimental data. They show its ability to predict the band structure dependence on the detailed atomic configuration, and thus to properly account for the effects of interfacial atomic segregation and structural disorder. They also show how their method avoids the approximations underlying the pseudopotential method of Dente and Tilton, which gives different results. The application of the proposed method to the InAs/GaSb superlattices allows the explanation of some observed experimental results, such as: the bandgap difference between ðInAsÞ 8 =ðGaSbÞ 8 superlattices with almost pure InSb-like or GaAs-like interfaces; the large blue shift of the bandgap when the growth temperature of the superlattice increases; and the blue shift of the bandgap of ðInAsÞ 8 =ðGaSbÞ n superlattices with increasing GaSb period n. They present a detailed comparison of their predicted blue shift with that obtained by other theories.

Superlattices and Microstructures, 1989

The epitaxial formation energy of a (GaAs)1 (InAs)1 (111) superlattices grown on an InP substrate... more The epitaxial formation energy of a (GaAs)1 (InAs)1 (111) superlattices grown on an InP substrate is calculated by using a self-consistent pseudopotential approach within the density functional theory. Atomic configurations are considered which have two distinct bond distances and correspond to different relaxations along the [111] direction. The minimum energy is found assigning to the superlattice the same bond distances obtained by minimizing the total energy of the two binary systems under the epitaxial constraint. For all the geometries we find that the superlattice is unstable with respect to disproportionation into the constituents.

Physica Status Solidi B-basic Solid State Physics, 2010

We consider the formation of surface antisite defects on a previously proposed model for the GaSb... more We consider the formation of surface antisite defects on a previously proposed model for the GaSb(001)-c(2 × 6) surface. Based on ab initio total energy calculations, we show how these defects stabilize the otherwise metallic surface and how their formation is driven by the excess charge associated with the Sb-rich surface conditions. The surface-sensitive optical technique of reflectance anisotropy spectroscopy is shown to be crucial for detecting the defects, and computation of spectra yields a good agreement with experiment when defects are included in the surface reconstruction.

Physical Review B, 2011

The structure of the technologically important—but still mostly unknown—GaSb(001)-c(2×6) surface ... more The structure of the technologically important—but still mostly unknown—GaSb(001)-c(2×6) surface reconstruction is investigated by means of ab initio simulations of reflectance anisotropy spectroscopy (RAS) and total energy calculations. A large number of reconstruction models for the GaSb(001) surface in the Sb-rich coverage regime are considered. The influence of each single surface structural motif on the RAS spectra is studied in detail, as well as their role in the surface stability with regard to application of the electron counting rule (ECR). We interpret the features of the RAS data measured for this reconstruction and suggest a new model for the c(2×6) phase. In this model a few Sb atoms in the second layer are randomly substituted by Ga, forming surface antisite defects. When used to fulfill the ECR, this “doping” effect considerably lowers the total energy of the long chain c(2×6) reconstruction model, making it competitive with the more stable short-chain (4×3) reconstructions. Formation of the surface antisites occurs spontaneously in the presence of dynamical negative charge fluctuations and is favored by the excellent matching between GaSb(001) and metallic Sb and by the natural softness of the Ga-Sb bonds. Calculations of the reflectance anisotropy spectra confirm that this structure is a major component of a largely disordered surface, where motifs of the stable (4×3) reconstructions are also present.

Using atomistic pseudopotential calculations we study the evolution of: (i) the conduction band e... more Using atomistic pseudopotential calculations we study the evolution of: (i) the conduction band edge Ec; (ii) the valence band edge Ev , and (iii) the fundamental band gap of the quaternary Ga1-yInyAsxSb1-x alloy lattice-matched to InAs as a function of the composition (x, y). We find a negative bowing parameter for Ec of the quaternary alloy with respect to the

Quaternary alloys without a common atom such as (Ga,In)-(As,Sb) pose a difficult combinatorially ... more Quaternary alloys without a common atom such as (Ga,In)-(As,Sb) pose a difficult combinatorially design problem in that there are many different atomic configurations even when the system is constrained to be lattice-matched on a substrate. Using an atomistic pseudopotential approach we have calculated the band edge energies of this quaternary random alloys as a function of Ga/In (x) and As/Sb (y) compositions assuming lattice-matching to either GaSb or InAs. The alloy is represented by a large supercell with random atomic occupations and atomic positions relaxed via the atomistic VFF functional. We find upwards bowing for both the conduction and valence band edge energies. On GaSb, the transition from staggered to broken-gap lineup is found to occur at x = 0.81 and y = 0.92, while on InAs it occurs at x = 0.59 and y = 0.62. We show that at the usual growth temperatures this quaternary alloy is not random but tends to exhibit an increased number of Ga-Sb and In-As bonds and a reduced number of In-Sb and Ga-As bonds. This effect brings the calculated band gaps in better agreement with experimental data. T. Magri, A. Zunger, H. Kroemer, JAP 98, 043701 (2005)

Physical Review B, 2004

We use extended-basis empirical tight-binding calculations and examine the anisotropy of the refr... more We use extended-basis empirical tight-binding calculations and examine the anisotropy of the refractive index in ultrashort-period superlattices of materials sharing no common atom. We find that a strong birefringence can be engineered in these articial semiconductors, allowing phase matching for frequency difference generation. The prominent role of epitaxial constraint and bond-length alternation is evidenced.

Physica Status Solidi (c), 2010

We study the effects of surface reconstruction and step formation on the surface phase stability,... more We study the effects of surface reconstruction and step formation on the surface phase stability, of an InAs wetting layer on GaAs(001). In particular we focus our attention on the α2 and β2 (2 × 4) surface reconstructions. The two investigated reconstructions have been shown to be formed at an high In coverage, at the onset of the 2D→3D transition. The analysis of the connection between the step stability and the strain distribution around the step edges leads to the conclusion that the favoured step geometries are those minimising the strain. Finally, In diffusion on the flat reconstructed wetting layers has been investigated.We find: (i) the elements of the surface reconstructions favouring In diffusion; (ii) that In diffusion on these surfaces is strongly anisotropic, favoring the [-110] direction; (iii) that the As surface dimers introduce additional adsorption sites with high barriers for In escape (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Journal of Physics-condensed Matter, 2009

By using density functional theory (DFT) calculations of the potential energy surface in conjunct... more By using density functional theory (DFT) calculations of the potential energy surface in conjunction with the analytical solution of the master equation for the time evolution of the adatom site distribution, we study the diffusion properties of an isolated In adatom on InxGa1-xAs wetting layers (WL) deposited on the GaAs(001). The WL reconstructions considered in this study are, listed in the order of increasing In coverage: c(4 × 4), (1 × 3), (2 × 3), α2(2 × 4) and β2(2 × 4). We analyze the dependence of the diffusion properties on WL reconstruction, composition and strain, and find that: (i) diffusion on the (2 × N) reconstructions is strongly anisotropic, owing to the presence of the low barrier potential in-dimer trench, favoring the diffusion along the [\bar 110] direction over that along the [110] direction; (ii) In diffusion at a WL coverage θ = 2/3 monolayers (ML; with composition x = 2/3) is faster than on clean GaAs(001) c(4 × 4), and decreases at θ = 1.75 ML (x = 1; e.g. InAs/GaAs(001)) (iii) diffusion and nucleation on the (2 × 4) WL is affected by the presence of adsorption sites for indium inside the As dimers; (iv) the approximation used for the exchange-correlation potential within DFT has an important effect on the description of the diffusion properties.

We investigate the potential energy surface (PES) and the adsorption properties of an In adatom o... more We investigate the potential energy surface (PES) and the adsorption properties of an In adatom on InAs (2×4) reconstructed wetting layers (WLs) deposited on a GaAs substrate. The results are then used to derive the diffusion properties of a single In adatom on the WLs. We find that: (i) the adsorbate diffusion is highly anisotropic; (ii) the adsorption sites within the As dimers have to be taken into account since they strongly affect diffusion; (iii) the most stable adsorption sites are the ones within the dimers and those located besides the in-dimers.

Physical Review B, 2008

The band edges and band gaps of ͑InAs͒ n / ͑GaSb͒ m ͑n , m =1,20͒ superlattices have been theoret... more The band edges and band gaps of ͑InAs͒ n / ͑GaSb͒ m ͑n , m =1,20͒ superlattices have been theoretically studied through the plane-wave empirical pseudopotential method for different situations: ͑i͒ different substrates, GaSb and InAs; ͑ii͒ different point group symmetries, C 2v and D 2d ; and ͑iii͒ different growth directions, ͑001͒ and ͑110͒. We find that ͑a͒ the band gaps for the ͑001͒ C 2v superlattices on a GaSb substrate exhibit a nonmonotonic behavior as a function of the GaSb barrier thickness when the number of ͑InAs͒ n layers exceed n =5; ͑b͒ substrate effects: compared with the GaSb substrate, the different strain field generated by the InAs substrate leads to a larger variation of the band gaps for the ͑001͒ C 2v superlattices as a function of the InAs well thickness; ͑c͒ effect of the type of interfacial bonds: the In-Sb bonds at the interfaces of the ͑001͒ D 2d superlattices partially pin the band edge states, reducing the influence of the confinement effects on electrons and holes, and lowering the band gaps as compared to the ͑001͒ C 2v case. The valence band maximum of the ͑001͒ D 2d superlattices with Ga-As bonds at the interfaces are shifted down, increasing the band gaps as compared to the ͑001͒ C 2v case; ͑d͒ effect of layer orientation: the presence of In-Sb bonds at both interfaces of the ͑110͒ superlattices pin the band edge states and reduces the band gaps, as compared to the ͑001͒ C 2v case. An anticrossing between the electron and hole levels in the ͑110͒ superlattices, for thin GaSb and thick InAs layers, leads to an increase of the band gaps, as a function of the InAs thickness; ͑e͒ superlattices vs random alloys: the comparison between the band edges and band gaps of the superlattices on a GaSb substrate and those for random alloys, lattice matched to a GaSb substrate, as a function of the In composition, shows that the random alloys present almost always higher band gaps and give a clear indication of the effect of superlattice's ordering and period on the behavior of the band gaps and band edges. Inclusion of interfacial interdiffusion, using the approach of Magri and Zunger ͓Phys. Rev. B 65, 165302 ͑2002͔͒, is shown to significantly increase the band gaps relative to the predictions for abrupt superlattices, bringing the results closer to experiment. It is noteworthy that k · p model fit instead measured gaps corresponding to interdiffused interfaces using a chemically abrupt model.

Solid State Communications, 1992

We show that the energy of substitutional randomization of atoms in zinc-blende compounds is surp... more We show that the energy of substitutional randomization of atoms in zinc-blende compounds is surprisingly small. This suggests the existence of a new class of defects in these materials (“random aggregates”), which consist of regions of ≲ 10 atoms where the sites of the diamond lattice are randomly occupied by A or B atoms in place of the ordered AB crystal occupancy. The structural and electronic properties of these defects are outlined.

Physical Review B, 1991

The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical b... more The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical bowing of the direct band gaps are calculated for three perfectly random semiconductor alloys within the first-principles pseudopotential method using the concept of ''special ...

Doping control at the nanoscale can be used to modify optical and electronic properties thus indu... more Doping control at the nanoscale can be used to modify optical and electronic properties thus inducing interesting effects that cannot be observed in pure systems. By using Density Functional Theory, Silicon Nanocrystals (Si-nc) of different size (diameter ranging from 1.1 nm to 1.8 nm) have been studied localizing impurities at different substitutional sites and calculating the impurity formation energies. Starting from hydrogen terminated silicon Si-nc, we found that codoping is always energetically favored with respect to a single B- or P-doping and that the two impurities tend to occupy nearest neighbor sites near the surface. The formation energy depends on the distance between the two impurities. The codoped Si-nc present band-edge states localized on the impurities which are responsible for a red-shift of the absorption threshold with respect to that of pure undoped Si nanocrystals. Concerning the emission spectra, we find a Stokes shift of the photoluminescence to a lower energy with respect to the absorption edge due to the nanocrystal (nc) structural relaxation after the creation of the electron-hole pair. We have calculated the absorption and emission spectra going beyond a single-particle approach showing the important role played by the many-body effects. The presence of electronic quasi-direct optical transitions between the donor and acceptor states within the band-gap makes it possible to engineer the optical properties of Si-nc.