Showing papers by "Alex Zunger published in 2002"
TL;DR: It is found, via first-principles calculations, that the tin interstitial and oxygen vacancy have surprisingly low formation energies and strong mutual attraction, explaining the natural nonstoichiometry of this system.
Abstract: SnO2 is a prototype "transparent conductor," exhibiting the contradictory properties of high metallic conductivity due to massive structural nonstoichiometry with nearly complete, insulator-like transparency in the visible range. We found, via first-principles calculations, that the tin interstitial and oxygen vacancy have surprisingly low formation energies and strong mutual attraction, explaining the natural nonstoichiometry of this system. The stability of these intrinsic defects is traced back to the multivalence of tin. These defects donate electrons to the conduction band without increasing optical interband absorption, explaining coexistence of conductivity with transparency.
808 citations
TL;DR: In this article, the authors generalize this result to other oxides by recognizing that there exist a hydrogen pinning level at about 3.0±0.4 eV below vacuum.
Abstract: First-principles total-energy calculations suggest that interstitial hydrogen impurity forms a shallow donor in SnO2, CdO, and ZnO, but a deep donor in MgO. We generalize this result to other oxides by recognizing that there exist a “hydrogen pinning level” at about 3.0±0.4 eV below vacuum. Materials such as Ag2O, HgO, CuO, PbO, PtO, IrO2, RuO2, PbO2, TiO2, WO3, Bi2O3, Cr2O3, Fe2O3, Sb2O3, Nb2O5, Ta2O5, FeTiO3, and PbTiO3, whose conduction band minimum (CBM) lie below this level (i.e., electron affinity>3.0±0.4 eV) will become conductive once hydrogen is incorporated into the lattice, without reducing the host. Conversely, materials such as BaO, NiO, SrO, HfO2, and Al2O3, whose CBM lie above this level (i.e., electron affinity<3.0±0.4 eV) will remain nonconductive since hydrogen forms a deep impurity.
278 citations
TL;DR: In this paper, the effect of interfacial atomic mixing on the electronic structure of InAs/GaSb superlattices, including electron and hole energies and wave function localization, interband transition energies, and dipole matrix elements, was studied.
Abstract: Abrupt InAs/GaSb superlattices have In-Sb and Ga-As interfacial chemical bonds that are not present in the constituent materials InAs and GaSb. We study the effect of interfacial atomic mixing on the electronic structure of such superlattices, including electron and hole energies and wave function localization, interband transition energies, and dipole matrix elements. We combine an empirical pseudopotential method for describing the electronic structure with two different structural models of interfacial disorder. First, we use the ``single-layer disorder'' model and change in a continous way the composition of the interfacial bonds. Second, we study interfacial atomic segregation using a layer-by-layer kinetic model of molecular beam epitaxy growth, fit to the observed scanning tunneling microscopy segregation profiles. The growth model provides a detailed structural model of segregated InAs/GaSb superlattices with atomic resolution. The application of the empirical pseudopotential method to such structures reveals remarkable electronic consequences of segregation, among them a large blueshift of the band gap. This result explains the surprising gap increase with growth temperature observed for similar structures. In particular we find that (i) superlattices with only In-Sb interfacial bonds have lower band gaps (by 50 meV) than superlattices with only Ga-As interfacial bonds. (ii) Heavy-hole--to--electron transition energies increase with the number of Ga-As interfacial bonds more than light-hole--to--electron transition energies. (iii) The heavy-hole $\mathrm{hh}1$ wave functions show a strong localization on the In-Sb interfacial bonds. The heavy-hole wave functions have very different amplitudes on the Ga-As interface and on the In-Sb interface. (iv) Sb segregates at InAs-on-GaSb growth, whereas As and In segregate at GaSb-on-InAs growth, but Ga does not segregate. (v) The segregation of Sb and In induces a blueshift in the band gap. (vi) There is an in-plane polarization anisotropy due to the low symmetry of the no-common-atom InAs/GaSb superlattice. This anisotropy is reduced by interfacial segregation.
104 citations
TL;DR: In this paper, it was shown that phosphorus impurity in diamond gives rise to a shallower donor level, and has a higher bulk solid solubility than sulphur.
Abstract: Previous calculations on n-type doping of diamond by P and S predicted that S has a shallower level and a higher solubility than P. Our first-principles calculations show that the opposite is true: Phosphorus impurity in diamond gives rise to a shallower donor level, and has a higher bulk solid solubility than sulphur. This agrees with the trends expected from the strength of the atomic pseudopotentials. We predict that coherent epitaxial expansion would substantially increase the solubility of P, and that complex formation of P with H is exothermic, also leading to passivation of the P donor action; removal of H then is needed to achieve good n-type characteristicy.
92 citations
TL;DR: In this paper, the authors show theoretically how ferromagnetism results from the interaction of Mn with hole-producing intrinsic defects in chalcopyrite semiconductors.
Abstract: The chalcopyrite ${\mathrm{CdGeP}}_{2}$ doped with Mn have been recently found to exhibit room-temperature ferromagnetism. Isovalent substitution of the Cd site is expected, however, to create antiferromagnetism, in analogy with the well-known CdTe:Mn $({d}^{5})$ case. However, chalcopyrite semiconductors exhibit low-energy intrinsic defects. We show theoretically how ferromagnetism results from the interaction of Mn with hole-producing intrinsic defects.
79 citations
TL;DR: In this paper, a mixed-basis cluster expansion is proposed to estimate the configurational energy of a binary A1−xBx alloys using a set of ab initio local density approximation calculations.
Abstract: Many measurable properties of crystalline binary A1−xBx alloys, such as phase diagrams and excess thermodynamic functions, could be predicted via lattice statistical mechanics methods if one knew the `configurational energy'. The latter describes the energy at T = 0 for each of the 2N possible occupation patterns of the N lattice sites by an A or a B atom. Traditional approaches described the configurational energy either via empirically fitted, truncated Ising Hamiltonians, or through highly approximated coherent-potential constructs. We illustrate here the alternative approach of `mixed-basis cluster expansion' which extracts from a set of ab initio local density approximation calculations of the total energies of a few ordered A–B compounds a complete configurational energy function. This method includes both pair and multibody terms, whose number and range of interaction are decided by the variational procedure itself, as well as long-range strain terms. In this paper, we describe the computational details of this method, emphasizing methods of construction, interpolations, fits and convergence. This procedure is illustrated for Ni–Pt, Cu–Au and ScS–S (where denotes cation vacancy). The parameters of the final expansions are provided on our webpage (http://www.sst.nrel.gov).
77 citations
TL;DR: In this paper, the empirical pseudopotential method and its recent applications to the III-V nitride alloys GaAsN, GaPN, GaInAsN and GaAsPN were discussed.
Abstract: We review the empirical pseudopotential method and its recent applications to the III–V nitride alloys GaAsN, GaPN, GaInAsN and GaAsPN. We discuss how studies using this method have provided an explanation for many experimentally observed anomalous nitride phenomena, including sharp photoluminescence lines in dilute alloys, high effective masses, Stoke's shift between emission and absorption in higher concentration alloys for GaAsN and GaPN ternaries. We also discuss predictions of unusual effects that remain to be experimentally discovered in GaInAsN quaternaries and complex GaAsPN solid solutions.
75 citations
TL;DR: The use of a small number of bands in conventional k · p treatment of nanostructures leads to hyperopia, whereby the correct, detailed atomistic symmetry is not seen by the model, but only the global landscape symmetry is noted.
Abstract: The use of a small number of bands in conventional k · p treatment of nanostructures leads to “farsightedness” (hyperopia), whereby the correct, detailed atomistic symmetry is not seen by the model, but only the global landscape symmetry is noted. Consequently, the real symmetry is confused with a higher symmetry. As a result, a number of important symmetry-mandated physical couplings are unwittingly set to zero in the k · p approach. These are often introduced, after-the-fact, “by hand”, via an ansatz. Sometimes physical effects (e.g., piezoelectricity) are invoked to fix the otherwise incorrect symmetry. Thus, whereas in atomistic theories of nanostructures (tight-binding, pseudopotentials) the physically correct symmetry is naturally forced upon us by the structure itself, in the standard k · p model it is accommodated ex post facto once it is known from sources outside the model itself.
50 citations
TL;DR: In this paper, an empirical pseudopotential approach, fitted to bulk and interfacial reference systems, provides a unified description of the electronic structure of random alloys (bulk and epitaxial), superlattices and related complex systems.
Abstract: We show how an empirical pseudopotential approach, fitted to bulk and interfacial reference systems, provides a unified description of the electronic structure of random alloys (bulk and epitaxial), superlattices, and related complex systems. We predict the composition and superlattice-period dependence of the band offsets and interband transitions of InAs/GaAs systems on InP and GaAs substrates.
48 citations
TL;DR: Using density-functional calculations, this paper obtained the (001) biaxial strain dependence of the valence and conduction band energies of GaN, GaP, GaAs, InN, InP, and InAs.
Abstract: Using density-functional calculations, we obtain the (001) biaxial strain dependence of the valence and conduction band energies of GaN, GaP, GaAs, InN, InP, and InAs. The results are fit to a convenient-to-use polynomial and the fits provided in tabular form. Using the calculated biaxial deformation potentials in large supercell empirical pseudopotential calculations, we demonstrate that epitaxial strain reduces the InGaN alloy bowing coefficient compared to relaxed bulk alloys.
43 citations
TL;DR: In this article, the authors studied the time evolution of the distribution of precipitate shapes and sizes in Al-Zn alloys via a mixed-space cluster expansion and kinetic Monte-Carlo simulations.
Abstract: The time evolution of the distribution of precipitate shapes and sizes in Al-Zn alloys is studied via a mixed-space cluster expansion and kinetic Monte-Carlo simulations. We find that the growth of precipitates in Al-rich Al-Zn alloys follows classical Ostwald ripening already after ageing times of a few seconds. Moreover, the distribution of the precipitates is temperature-dependent: the higher the ageing temperature, the smaller the distribution width of the precipitate size. We discuss the time evolution of the precipitates in terms of short-range order parameters and compare them with experimental data.
TL;DR: In this paper, the authors used pseudopotential theory to provide the band offsets of strained GaAs and InAs on various substrates and the energies of the valence and conduction bands of InxGa1−xAs alloy, as a function of composition.
Abstract: We use pseudopotential theory to provide (1) the band offsets of strained GaAs and InAs on various substrates and (2) the energies Ev(x) and Ec(x) of the valence and conduction bands of InxGa1−xAs alloy, as a function of composition. Results are presented for both the bulk alloy and for the alloy strained on InP or GaAs. We predict that while Ec(x) bows downward for relaxed bulk alloys, it bows upward for strained epitaxial alloys. The calculated alloy offsets are used to discuss electron and hole localization in this system.
TL;DR: In this article, the authors combine a kinetic model of MBE growth with the empirical pseudopotential band structure method to study the effects of interfacial disorder and segregation on the optical properties of InAs/GaSb superlattices.
Abstract: We combine a kinetic model of MBE growth with the empirical pseudopotential band structure method to study the effects of interfacial disorder and segregation on the optical properties of InAs/GaSb superlattices. We fit the layer-by-layer growth model to the observed STM segregated profiles, extracting surface-to-subsurface atomic exchange energies. These are then used to obtain a detailed simulated model of segregated InAs/GaSb superlattices with atomic resolution. The application of the pseudopotential calculations to such structures reveals remarkable electronic consequences of segregation, including a blue shift of band gap with increasing sample growth temperature.
19 May 2002
TL;DR: In this paper, the basic physical principles underlying the formation energy of various intrinsic defects in common photovoltaic materials are reviewed and used to explain why doping of semiconductors is in general limited and which design principles can be used to circumvent such limits.
Abstract: I will review the basic physical principles underlying the formation energy of various intrinsic defects in common photovoltaic materials. I then use the above principles to explain why doping of semiconductors is in general, limited and which design principles can be used to circumvent such limits. This work can help design strategies of doping absorber materials as well as explain how TCOs work. Recent results on the surprising stability of polar [112] + (1~1~2~) surfaces of CIS will also be described in this context.