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Showing papers by "Alex Zunger published in 2001"


Journal ArticleDOI
TL;DR: In this paper, the authors study the intrinsic defect physics of ZnO and find that ZnOs cannot be doped p type via native defects, despite the fact that they are shallow donors.
Abstract: ZnO typifies a class of materials that can be doped via native defects in only one way: either n type or p type. We explain this asymmetry in ZnO via a study of its intrinsic defect physics, including ${\mathrm{Zn}}_{\mathrm{O}},$ ${\mathrm{Zn}}_{i},$ ${\mathrm{V}}_{\mathrm{O}},$ ${\mathrm{O}}_{i},$ and ${V}_{\mathrm{Zn}}$ and n-type impurity dopants, Al and F. We find that ZnO is n type at Zn-rich conditions. This is because (i) the Zn interstitial, ${\mathrm{Zn}}_{i},$ is a shallow donor, supplying electrons; (ii) its formation enthalpy is low for both Zn-rich and O-rich conditions, so this defect is abundant; and (iii) the native defects that could compensate the n-type doping effect of ${\mathrm{Zn}}_{i}$ (interstitial O, ${\mathrm{O}}_{i},$ and Zn vacancy, ${V}_{\mathrm{Zn}}),$ have high formation enthalpies for Zn-rich conditions, so these ``electron killers'' are not abundant. We find that ZnO cannot be doped p type via native defects $({\mathrm{O}}_{i},{V}_{\mathrm{Zn}})$ despite the fact that they are shallow (i.e., supplying holes at room temperature). This is because at both Zn-rich and O-rich conditions, the defects that could compensate p-type doping ${(V}_{\mathrm{O}}{,\mathrm{}\mathrm{Zn}}_{i},{\mathrm{Zn}}_{\mathrm{O}})$ have low formation enthalpies so these ``hole killers'' form readily. Furthermore, we identify electron-hole radiative recombination at the ${V}_{\mathrm{O}}$ center as the source of the green luminescence. In contrast, a large structural relaxation of the same center upon double hole capture leads to slow electron-hole recombination (either radiative or nonradiative) responsible for the slow decay of photoconductivity.

1,724 citations


Journal ArticleDOI
TL;DR: In this article, the authors studied the evolution of the electronic structure of low-nitrogen-content alloys and proposed a new model for lownitrogen content alloys with perturbed host states.
Abstract: Using the empirical pseudopotential method and large atomistically relaxed supercells, we have systematically studied the evolution of the electronic structure of ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ and ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x},$ from the dilute nitrogen impurity regime to the nascent nitride alloy. We show how substitutional nitrogen forms perturbed host states (PHS) inside the conduction band, whereas small nitrogen aggregates form localized cluster states (CS) in the band gap. By following the evolution of these states and the ``perturbed host states'' with increasing nitrogen composition, we propose a new model for low-nitrogen-content ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ and ${\mathrm{GaP}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ alloys: As the nitrogen composition increases, the energy of the CS is pinned while the energy of the PHS plunges down as the nitrogen composition increases. The impurity limit (PHS above CS) is characterized by strongly localized wave functions, low pressure coefficients, and sharp emission lines from the CS. The amalgamation limit (PHS overtake the CS) is characterized by a coexistence of localized states (leading to high effective mass, exciton localization, Stokes shift in emission versus absorption) overlapping delocalized PHS (leading to asymmetrically broadened states, low temperature coefficeint, delocalized ${E}_{+}$ band at higher energies). The alloy limit (PHS well below CS) may not have been reached experimentally, but is predicted to be characterized by conventional extended states. Our theory shows that these alloy systems require a polymorphous description, permitting the coexistence of many different local environments, rather than an isomorphous model that focuses on few impurity-host motifs.

352 citations


Journal ArticleDOI
TL;DR: This work studies the distribution of bonds using Monte Carlo simulation and finds that the number of In-N and Ga-As bonds increases relative to random alloys, which affects the band structure.
Abstract: In contrast to pseudobinary alloys, the relative number of bonds in quaternary alloys cannot be determined uniquely from the composition. Indeed, we do not know if the ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}{\mathrm{As}}_{0.5}{\mathrm{N}}_{0.5}$ alloy should be thought of as $\mathrm{InAs}+\mathrm{GaN}$ or as $\mathrm{InN}+\mathrm{GaAs}$. We study the distribution of bonds using Monte Carlo simulation and find that the number of In-N and Ga-As bonds increases relative to random alloys. This quaternary-unique short range order affects the band structure: we calculate a blueshift of the band gap and predict the emergence of a broadband tail of localized states around the conduction band minimum.

236 citations


Journal ArticleDOI
TL;DR: A novel mechanism of alloy formation where localized cluster states within the gap are gradually overtaken by a downwards moving conduction band edge, composed of both localized and delocalized states is found.
Abstract: Addition of nitrogen to III-V semiconductor alloys radically changes their electronic properties. We report large-scale electronic structure calculations of GaAsN and GaPN using an approach that allows arbitrary states to emerge, couple, and evolve with composition. We find a novel mechanism of alloy formation where localized cluster states within the gap are gradually overtaken by a downwards moving conduction band edge, composed of both localized and delocalized states. This localized to delocalized transition explains many of the hitherto puzzling experimentally observed anomalies in III-V nitride alloys.

230 citations


Journal ArticleDOI
TL;DR: In this paper, the defect-induced reconstructions make the (112)-cation plus $(1/ifmmode\bar\else\textasciimacron\fi{}1\ifmmodes\bar/else\ textasciima-conc rn\fa{}2\ifmodesode ǫ-bar/ǫǫ n−n−n −n−1.
Abstract: In zinc-blende semiconductors, the nonpolar (110) surface is more stable than all polar surfaces because the formation of the latter requires the creation of charge-neutralizing but energetically costly surface reconstruction. Our first-principles calculations on ${\mathrm{CuInSe}}_{2}$ reveal this in the double-zinc-blende (chalcopyrite) structure, the defect-induced reconstructions make the (112)-cation plus $(1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2\ifmmode\bar\else\textasciimacron\fi{})$-anion polar facets lower in energy than the nonpolar (110) plane, despite the resulting increased surface area. We show that this spontaneous facetting results from the remarkable stability of surface defects (Cu vacancy, Cu-on-In antisite) in chalcopyrites, and explains the hitherto puzzling formation of polar microfacets when one attempts to grow epitaxialloy a nonpolar chalcopyrite surface.

115 citations


Journal ArticleDOI
TL;DR: In this paper, the energy levels of CdSe quantum dots are studied by scanning tunneling spectroscopy, where the tip-dot distance of the quantum dot can be varying to switch from shell-filling to shell-tunneling.
Abstract: The energy levels of CdSe quantum dots are studied by scanning tunneling spectroscopy. By varying the tip-dot distance, we switch from “shell-filling” spectroscopy (where electrons accumulate in the dot and experience mutual repulsion) to “shell-tunneling” spectroscopy (where electrons tunnel, one at a time, through the dot). Shell-tunneling spectroscopy provides the single-particle energy levels of the CdSe quantum dot. The results of both types of tunneling spectroscopy are compared with pseudopotential many-body calculations.

106 citations


Journal ArticleDOI
TL;DR: In this paper, the electronic structure and optical properties of cubic InGaN alloys were investigated using large scale atomistic empirical pseudopotential calculations, and it was shown that strong hole localization exists even in the homogeneous random alloy, with a preferential localization along the [1,1,0] In-N-In-N -In-In -N-N −In chains, consistent with current photoluminescence, microscopy, and Raman data.
Abstract: The electronic structure and optical properties of cubic (nonpiezoelectric) InGaN are investigated using large scale atomistic empirical pseudopotential calculations. We find that (i) strong hole localization exists even in the homogeneous random alloy, with a preferential localization along the [1,1,0] In–N–In–N–In chains, (ii) even modest sized (<50 A) indium rich quantum dots provide substantial quantum confinement and readily reduce emission energies relative to the random alloy by 200–300 meV, depending on size and composition, consistent with current photoluminescence, microscopy, and Raman data. The dual effects of alloy hole localization and localization of electrons and hole at intrinsic quantum dots are responsible for the emission characteristics of current grown cubic InGaN alloys.

94 citations


Journal ArticleDOI
TL;DR: In this paper, the physical content of these energies can be calculated via quantum Monte Carlo (QMC) and configuration interaction (CI) methods, which can be used to resolve the energies of excitons, multiexcitons and charging of semiconductor quantum dots.
Abstract: Single-dot spectroscopy is now able to resolve the energies of excitons, multiexcitons, and charging of semiconductor quantum dots with $\ensuremath{\lesssim}1$ meV resolution. We discuss the physical content of these energies and show how they can be calculated via quantum Monte Carlo (QMC) and configuration interaction (CI) methods. The spectroscopic energies have three pieces: (i) a ``perturbative part'' reflecting carrier-carrier direct and exchange Coulomb energies obtained from fixed single-particle orbitals, (ii) a ``self-consistency correction'' when the single particle orbitals are allowed to adjust to the presence of carrier-carrier interaction, and (iii) a ``correlation correction.'' We first apply the QMC and CI methods to a model single-particle Hamiltonian: a spherical dot with a finite barrier and single-band effective mass. This allows us to test the convergence of the CI and to establish the relative importance of the three terms (i)--(iii) above. Next, we apply the CI method to a realistic single-particle Hamiltonian for a CdSe dot, including via a pseudopotential description the atomistic features, multiband coupling, spin-orbit effects, and surface passivation. We include all bound states (up to 40 000 Slater determinants) in the CI expansion. Our study shows that (1) typical exciton transition energies, which are $\ensuremath{\sim}1$ eV, can be calculated to better than 95% by perturbation theory, with only a $\ensuremath{\sim}2$ meV correlation correction; (2) typical electron addition energies are $\ensuremath{\sim}40$ meV, of which correlation contributes very little $(\ensuremath{\sim}1$ meV); (3) typical biexciton binding energies are positive and $\ensuremath{\sim}10$ meV and almost entirely due to correlation energy, and exciton addition energies are $\ensuremath{\sim}30$ meV with nearly all contribution due to correlation; (4) while QMC is currently limited to a single-band effective-mass Hamiltonian, CI may be used with much more realistic models, which capture the correct symmetries and electronic structure of the dots, leading to qualitatively different predictions from effective-mass models; and (5) CI gives excited state energies necessary to identify some of the peaks that appear in single-dot photoluminescence spectra.

83 citations


Journal ArticleDOI
TL;DR: In this article, a pseudopotential approach to study the electronic structure of semiconductor quantum dots is presented, emphasizing methodology ideas and a survey of recent applications to both free-standing and semiconductor embedded quantum-dot systems.
Abstract: This paper reviews our pseudopotential approach to study the electronic structure of semiconductor quantum dots, emphasizing methodology ideas and a survey of recent applications to both free-standing and semiconductor embedded quantum-dot systems.

79 citations


Journal ArticleDOI
TL;DR: In this article, the shape, size, and composition profile of semiconductor-embedded quantum dots are given, and a model consistent with both the observed material structure and measured electronic/optical properties of a quantum dot sample is established.
Abstract: Provided that the shape, size, and composition profile of semiconductor-embedded quantum dots are given, theory is able to accurately calculate the excitonic transitions, including the effects of inhomogeneous strain, alloy fluctuations, electron-hole binding, and multiband and intervalley coupling. While experiment can accurately provide the spectroscopic signature of the excitonic transitions, accurate determination of the size, shape, and composition profile of such dots is still difficult. We show how one can arrive at a consistent picture of both the material and the electronic structure by interactive iteration between theory and experiment. Using high-resolution transmission electron microscopy, electron-energy-loss spectroscopy, and photoluminescence (PL) spectroscopy in conjunction with atomistic empirical pseudopotential calculations, we establish a model consistent with both the observed material structure and measured electronic/optical properties of a quantum dot sample. The structural model with best agreement between measured and predicted PL is a truncated cone with height 35 {angstrom}, base diameter 200 {angstrom}, and top diameter 160 {angstrom}, having a nonuniform, peaked composition profile with average 60% In content. Next, we use our best structure to study the effect of varying (i) the amount of In in the dots, and (ii) the spatial distribution of In within the dots. We find thatmore » by either increasing the amount of In within the dot or by concentrating a given amount of In near the center of the dot, both electrons and holes become more strongly bound to the dot. A small change of In content from 50 to 60% causes an exciton redshift of about 70 meV. Changing the composition profile from a uniform In distribution to a centrally peaked distribution can redshift the exciton by an additional 20--40 meV.« less

79 citations


Journal ArticleDOI
TL;DR: In this article, the first principles calculated total energy of general Cu-Zn fcc-lattice configurations using a mixed-space cluster expansion were derived. But no 3:1 ordered phase has yet been directly observed even though the negative mixing enthalpy of the disordered alloy suggests ordering tendencies.
Abstract: Alloys of copper and zinc (brass) have been widely used since Neolithic times due to the discovery that unlike regular copper this alloy can be worked ``cold'' around a 3:1 copper-to-zinc ratio. While it is now known that the as-grown system is a disordered fcc solid solution, no 3:1 ordered phase has yet been directly observed even though the negative mixing enthalpy of the disordered alloy suggests ordering tendencies. Moreover, neutron scattering experiments have been deduced that this disordered alloy contains peculiar chains of Zn atoms. We have expressed the first-principles calculated total energy of general Cu-Zn fcc-lattice configurations using a mixed-space cluster expansion. Application of Monte Carlo--simulated annealing to this generalized Ising-like Hamiltonian produces the predicted low-temperature ground state as well as finite-temperature phase diagram and short-range order. We find (i) that at low temperature the disordered fcc alloy will order into the ${\mathrm{DO}}_{23}$ structure, (ii) the high-temperature short-range order in close agreement with experiment, and (iii) chains of Zn atoms in the [001] direction, as seen experimentally. Furthermore, our model allows a detailed study of the influence and importance of strain on the phase stability.

Journal ArticleDOI
TL;DR: An unbiased search of fcc-based Ag(1)-( x)Pd(x) structures consisting of up to many thousand atoms by using a mixed-space cluster expansion finds an unsuspected ground state at 50%-50% composition-the L1(1) structure, currently known in binary metallurgy only for the Cu(0.5)Pt(0.)5) alloy system.
Abstract: The complexity of first-principles total energy calculations limits the pool of structure types considered for a ground-state search for a binary alloy system to a rather small, O(10) , group of ''usual suspects.'' We conducted an unbiased search of fcc-based Ag{sub 1-x}Pd {sub x} structures consisting of up to many thousand atoms by using a mixed-space cluster expansion. We find an unsuspected ground state at 50%-50% composition -- the L1{sub 1} structure, currently known in binary metallurgy only for the Cu{sub 0.5}Pt {sub 0.5} alloy system. We also provide predicted short-range-order profiles and mixing enthalpies for the high temperature, disordered alloy.

Journal ArticleDOI
TL;DR: In this paper, a layer-by-layer growth model was fit to the observed cross-sectional scanning tunneling microscopy (STM) profiles, extracting surface-to-subsurface atomic exchange energies.
Abstract: Largely because of the lack of detailed microscopic information on the interfacial morphology, most electronic structure calculations on superlattices and quantum wells assume abrupt interfaces. Cross-sectional scanning tunneling microscopy (STM) measurements have now resolved atomic features of segregated interfaces. We fit a layer-by-layer growth model to the observed STM 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. Applying pseudopotential calculations to such structures reveals remarkable electronic consequences of segregation, including a blueshift of interband transitions, lowering of polarization anisotropy, and reduction of the amplitude of heavy-hole wave functions at the inverted interface.

Journal ArticleDOI
TL;DR: In this article, a moderately electronegative passivation potential was shown to induce long-lived excitons without appreciable changes to the band gap, and the symmetry of the valence-band maximum was improved.
Abstract: One of the most interesting properties of quantum dots is the possibility to tune the band gap as a function of their size. Here we explore the possibility of changing the lifetime of the lowest-energy excited state by altering the surface passivation. We show that a moderately electronegative passivation potential can induce long-lived excitons without appreciable changes to the band gap. In addition, for such passivation the symmetry of the valence-band maximum is ${\ensuremath{\gamma}}_{{8}_{v}} {(t}_{1}$ derived) instead of the more usual ${\ensuremath{\gamma}}_{8v} {(t}_{2}$ derived). This reverses the effect of the exchange interaction on the bright-dark exciton splitting.

Journal ArticleDOI
TL;DR: By combining first-principles total energy calculations with lattice statistical mechanics, an astronomical number of possible structures are scanned, identifying the stable ground states of nonstoichiometry.
Abstract: Whereas nearly all compounds A(n)B(m) obey Dalton's rule of integer stoichiometry (n: m, both integer), there is a class of systems, exemplified by the rocksalt structure Sc1-x squarexS, that exhibits large deviations from stoichiometry via vacancies, even at low temperatures. By combining first-principles total energy calculations with lattice statistical mechanics, we scan an astronomical number of possible structures, identifying the stable ground states. Surprisingly, all have the same motifs: (111) planes with (112) vacancy rows arranged in (110) columns. Electronic structure calculations of the ground states (identified out of approximately 3x10(6) structures) reveal the remarkable origins of nonstoichiometry.

Journal ArticleDOI
01 Jul 2001-EPL
TL;DR: In this paper, the physical mechanisms governing the observed size and temperature dependence of precipitate shapes in Al-Zn alloys via quantum-mechanical first-principles simulations were elucidated.
Abstract: We have elucidated the physical mechanisms governing the observed size- and temperature dependence of precipitate shapes in Al-Zn alloys via quantum-mechanical first-principles simulations. In remarkable quantitative agreement with alloy aging experiments, we find that with decreasing temperature and increasing average size, the precipitates change from spherical to plate-like shape. Although the precipitates are created by an inherently kinetic heat treatment process, the entire series of their size vs. shape relation can be explained in terms of thermodynamic arguments and understood in terms of strain and chemical energies.

Journal ArticleDOI
TL;DR: In this paper, the electronic and atomic structure of substitutional nitrogen pairs, triplets, and clusters in GaP and GaAs was studied using the multiband empirical pseudopotential method with atomistically relaxed supercells.
Abstract: The electronic and atomic structure of substitutional nitrogen pairs, triplets, and clusters in GaP and GaAs is studied using the multiband empirical pseudopotential method with atomistically relaxed supercells. A single nitrogen impurity creates a localized a1(N) gap state in GaP, but in GaAs, the state is resonant above the conduction-band minimum. We show how the interaction of multiple a1 impurity levels, for more than one nitrogen, results in a nonmonotonic relationship between energy level and impurity separation. We assign the lowest (NN1) line in GaP to a [2,2,0] oriented pair, the second (NN2) line to a triplet of nitrogen atoms, and identify the origin of a deeper observed level as an [1,1,0] oriented triplet. We also demonstrate that small nitrogen clusters readily create very deep levels in both GaP and GaAs.

Journal ArticleDOI
01 Jan 2001-EPL
TL;DR: In this paper, a many-body, atomistic empirical pseudopotential approach was used to predict the multi-exciton emission spectrum of a lens-shaped InAs/GaAs self-assembled quantum dot.
Abstract: We use a many-body, atomistic empirical pseudopotential approach to predict the multi-exciton emission spectrum of a lens-shaped InAs/GaAs self-assembled quantum dot. We discuss the effects of i) the direct Coulomb energies, including the differences of electron and hole wave functions, ii) the exchange Coulomb energies and iii) correlation energies given by a configuration interaction calculation. Emission from the ground state of the N exciton system to the N − 1 exciton system involving e0 → h0 and e1 → h1 recombinations are discussed. A comparison with a simpler single-band, effective mass approach is presented.


Journal ArticleDOI
TL;DR: In this article, first-principles calculations of step-formation energies show that the formation of steps on the (2x1) reconstructed surface requires energy, but that on the 1x1 surface, steps form exothermically.
Abstract: The exposure of the miscut Si(001) surface to H gives rise to a rich sequence of stable step structures as a function of the H chemical potential. First-principles calculations of step-formation energies show that the formation of steps on the (2x1) reconstructed surface requires energy, but that on the (1x1) surface, steps form exothermically. This explains surface roughness at high H chemical potentials.

Journal ArticleDOI
TL;DR: In this paper, a pseudopotential calculation for a dot molecule, coupled with a basic configuration interaction calculation of the exciton energy levels, provides directly the energy required to dissociate an exciton into an electron and a hole localized in different dots.
Abstract: One of the most important parameters that determine the transport properties of a quantum dot array is the exciton dissociation energy, i.e., the energy $\ensuremath{\Delta}E$ required to dissociate an exciton into an electron and a hole localized in different dots. We show that a pseudopotential calculation for a dot molecule, coupled with a basic configuration interaction calculation of the exciton energy levels, provides directly the exciton dissociation energy, including the effects of wave function overlap, screened Coulomb attraction between the electron and the hole in different dots, and polarization effects. We find that $\ensuremath{\Delta}E$ decreases as the interdot distance decreases and as the dielectric constant of the medium increases.

Journal ArticleDOI
TL;DR: In this article, it was shown that the CsCl phase is dynamically unstable with respect to the transverse acoustic TA[ξξ0] phonon, while ionic compounds in the β-Sn structure exhibit phonon instabilities in the longitudinal optical LO[00ξ] branch.
Abstract: Recent high-pressure X-ray experiments show that, contrary to traditional expectations and numerous calculations, the NaCl structure is not present in covalent semiconductors, the diatomic β-Sn structure is absent in all compound semiconductors, and the CsCl structure is not seen in ionic semiconductors. We explain these systematic absences in terms of dynamical phonon instabilities of the NaCl, β-Sn, and CsCl crystal structures. Covalent materials in NaCl structures become dynamically unstable with respect to the transverse acoustic TA[001] phonon, while ionic compounds in the β-Sn structure exhibit phonon instabilities in the longitudinal optical LO[00ξ] branch. The latter lead to predicted new high pressure phases of octet semiconductors. For InSb, we find no phonon instability that could prevent the CsCl phase from forming, but for the more ionic GaP, GaAs, InP, and InAs, we find that the CsCl phase is dynamically unstable at high pressures with respect to TA[ξξ0] phonons. Analysis of the soft normal modes via “isotropy subgroup” suggests two candidate structures that will replace the CsCl structure at high pressure: the tP4 (B10) InBi-type and the oP4 (B19) AuCd-type. Experimental examination of these predictions is called for.

Journal ArticleDOI
TL;DR: In this paper, the authors present a theory of the evolution of the electronic structure of GaAsN alloys, from the dilute impurity limit to the fully formed alloy.
Abstract: We present a theory of the evolution of the electronic structure of GaAsN alloys, from the dilute impurity limit to the fully formed alloy. Using large scale empirical pseudopotential calculations, we show how substitutional nitrogen forms Perturbed Host States (PHS) inside the conduction band whereas small nitrogen aggregates form localized Cluster States (CS) in the band gap. By following the evolution of these states with increasing nitrogen composition we develop a model that explains many of the experimentally observed phenomena, including high effective masses, Stokes shift in emission versus absorption, and anomalous pressure dependence.

Journal ArticleDOI
TL;DR: In this article, the authors show that the theoretically calculated phonon sequence in ordered GaInP 2 does not violate the alternation rule, contrary to the assertion of Alsina et al. in the preceding Comment.
Abstract: We show that, contrary to the assertion of Alsina et al. in the preceding Comment, the theoretically calculated phonon sequence in ordered GaInP 2 does not violate the ‘‘alternation rule.’’ Analysis of the firstprinciples calculated phonon dispersion shows that CuPt-ordered GaInP 2 is a system where anisotropy of short-range forces is of the same magnitude as the electrostatic Coulomb interactions and the associated LO/TO splittings. We show how the phonon modes change with the degree of order, and demonstrate that our results, without revision, successfully account for the experimental infrared data.

Journal ArticleDOI
TL;DR: In this article, it was shown that the strong bowing of the bandgap of GaInN, which is primarily due to bowing on the valence band edge, translates into a strongly composition dependent ratio of the conduction band offset to the Valence band offset with respect to GaN.
Abstract: We show that the strong bowing of the bandgap of GaInN, which is primarily due to bowing of the valence band edge, translates into a strongly composition dependent ratio of the conduction band offset to the valence band offset with respect to GaN. For common In mole fractions of 0–20 % this leads to a reversal of the band offset ratio and to very weak electron confinement. This theoretical picture is verified by comparing results of time-resolved spectroscopy on asymmetric AlGaN/GaInN/GaN and AlGaN/GaN/AlGaN quantum wells. Since electron confinement is much stronger for GaN/AlGaN wells than for GaInN/GaN wells, the effect of asymmetry is very weak for the former and fairly strong for the latter.

Journal ArticleDOI
TL;DR: The current status of our understanding of Quantum Mechanics is that if one specifies the chemical formula of a compound (e.g., CuAu, or GaAs, or NiPt) it is still impossible to predict if this material is a superconductor or not, but it is now possible to predict its crystal structure.
Abstract: The current status of our understanding of Quantum Mechanics is that if one specifies the chemical formula of a compound (e.g., CuAu, or GaAs, or NiPt) it is still impossible to predict if this material is a superconductor or not, but it is now possible to predict its crystal structure. This is a nontrivial accomplishment for there are as many as 2N possible structures for a binary compound. This article reviews this classic question of structural chemistry and condensed matter physics: How can one figure out which of the astronomic number of possible crystal structures is selected by Nature?