Showing papers by "Alex Zunger published in 2008"
TL;DR: In this paper, the authors compared the results of different gap-correction methods and concluded that to date there is no universal scheme for band gap correction in general defect systems, and they turn instead to classification of different types of defect behavior to provide guidelines on how the physically correct situation in an LDA defect calculation can be recovered.
Abstract: Contemporary theories of defects and impurities in semiconductors rely to a large extent on supercell calculations within density-functional theory using the approximate local-density approximation (LDA) or generalized gradient approximation (GGA) functionals. Such calculations are, however, affected by considerable uncertainties associated with: (i) the ``band-gap problem,'' which occurs not only in the Kohn-Sham single-particle energies but also in the quasiparticle gap (LDA or GGA) calculated from total-energy differences, and (ii) supercell finite-size effects. In the case of the oxygen vacancy in ZnO, uncertainties (i) and (ii) have led to a large spread in the theoretical predictions, with some calculations suggesting negligible vacancy concentrations, even under Zn-rich conditions, and others predicting high concentrations. Here, we critically assess (i) the different methodologies to correct the band-gap problem. We discuss approaches based on the extrapolation of perturbations which open the band gap, and the self-consistent band-gap correction employing the $\text{LDA}+U$ method for $d$ and $s$ states simultaneously. From the comparison of the results of different gap-correction, including also recent results from other literature, we conclude that to date there is no universal scheme for band gap correction in general defect systems. Therefore, we turn instead to classification of different types of defect behavior to provide guidelines on how the physically correct situation in an LDA defect calculation can be recovered. (ii) Supercell finite-size effects: We performed test calculations in large supercells of up to 1728 atoms, resolving a long-standing debate pertaining to image charge corrections for charged defects. We show that once finite-size effects not related to electrostatic interactions are eliminated, the analytic form of the image charge correction as proposed by Makov and Payne leads to size-independent defect formation energies, thus allowing the calculation of well-converged energies in fairly small supercells. We find that the delocalized contribution to the defect charge (i.e., the defect-induced change of the charge distribution) is dominated by the dielectric screening response of the host, which leads to an unexpected effective $1/L$ scaling of the image charge energy, despite the nominal $1/{L}^{3}$ scaling of the third-order term. Based on this analysis, we suggest that a simple scaling of the first order term by a constant factor (approximately 2/3) yields a simple but accurate image-charge correction for common supercell geometries. Finally, we discuss the theoretical controversy pertaining to the formation energy of the O vacancy in ZnO in light of the assessment of different methodologies in the present work, and we review the present experimental situation on the topic.
968 citations
TL;DR: It is shown that signatures of oxidation states and multivalence—such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization—that have often been interpreted as literal charge transfer are instead a consequence of the negative-feedback charge regulation.
Abstract: Transition-metal atoms embedded in an ionic or semiconducting crystal can exist in various oxidation states that have distinct signatures in X-ray photoemission spectroscopy and 'ionic radii' which vary with the oxidation state of the atom. These oxidation states are often tacitly associated with a physical ionization of the transition-metal atoms--that is, a literal transfer of charge to or from the atoms. Physical models have been founded on this charge-transfer paradigm, but first-principles quantum mechanical calculations show only negligible changes in the local transition-metal charge as the oxidation state is altered. Here we explain this peculiar tendency of transition-metal atoms to maintain a constant local charge under external perturbations in terms of an inherent, homeostasis-like negative feedback. We show that signatures of oxidation states and multivalence--such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization--that have often been interpreted as literal charge transfer are instead a consequence of the negative-feedback charge regulation.
255 citations
TL;DR: In this paper, a partially occupied intermediate band (IB) between the valence band (VB) and conduction band (CB) of the semiconductor absorber is proposed to enhance the current gain from the solar spectrum whilst maintaining a large open-circuit voltage.
Abstract: The intermediate-band solar cell (IBSC) concept has been recently proposed to enhance the current gain from the solar spectrum whilst maintaining a large open-circuit voltage. Its main idea is to introduce a partially occupied intermediate band (IB) between the valence band (VB) and conduction band (CB) of the semiconductor absorber, thereby increasing the photocurrent by the additional $\text{VB}\ensuremath{\rightarrow}\text{IB}$ and $\text{IB}\ensuremath{\rightarrow}\text{CB}$ absorptions. The confined electron levels of self-assembled quantum dots (QDs) were proposed as potential candidates for the implementation of such an IB. Here we report experimental and theoretical investigations on ${\text{In}}_{y}{\text{Ga}}_{1\ensuremath{-}y}\text{As}$ dots in a ${\text{GaAs}}_{1\ensuremath{-}x}{\text{P}}_{x}$ matrix, examining its suitability for acting as IBSCs. The system has the advantage of allowing strain symmetrization within the structure, thus enabling the growth of a large number of defect-free QD layers, despite the significant size mismatch between the dot material and the surrounding matrix. We examine the various conditions related to the optimum functionality of the IBSC, in particular those connected to the optical and electronic properties of the system. We find that the intensity of absorption between QD-confined electron states and host CB is weak because of their localized-to-delocalized character. Regarding the position of the IB within the matrix band gap, we find that, whereas strain symmetrization can indeed permit growth of multiple dot layers, the current repertoire of ${\text{GaAs}}_{1\ensuremath{-}x}{\text{P}}_{x}$ barrier materials, as well as ${\text{In}}_{y}{\text{Ga}}_{1\ensuremath{-}y}\text{As}$ dot materials, does not satisfy the ideal energetic locations for the IB. We conclude that other QD systems must be considered for QD-IBSC implementations.
220 citations
TL;DR: These behaviors for column V-B dopants in anatase TiO2 are quantified by combining electronic structure calculations, thermodynamic simulations, and percolation theory to identify two general types of electronic behaviors for transition-metal impurities.
Abstract: We identify two general types of electronic behaviors for transition-metal impurities that introduce excess electrons in oxides. (i) The dopants introduce resonant states inside the host conduction band and produce free electrons; (ii) the dopants introduce a deep gap state that carries a magnetic moment. By combining electronic structure calculations, thermodynamic simulations, and percolation theory, we quantify these behaviors for the case of column V-B dopants in anatase ${\mathrm{TiO}}_{2}$. Showing behavior (i), Nb and Ta dopants can convert the insulator ${\mathrm{TiO}}_{2}$ into a transparent conductor. Showing behavior (ii), V dopants could convert nonmagnetic ${\mathrm{TiO}}_{2}$ into a ferromagnet. Whether a dopant shows behavior (i) or (ii) is encoded in its atomic $d$ orbital energy.
158 citations
TL;DR: It is found from first-principles calculations that in ternary I-III-VI2 chalcopyrites like CuInSe2 and CuGaSe2, DX-like centers can develop without the presence of any extrinsic impurities.
Abstract: In III-V and II-VI semiconductors, certain nominally electron-donating impurities do not release electrons but instead form deep electron-traps known as ``$DX$ centers.'' While in these compounds, such traps occur only after the introduction of foreign impurity atoms, we find from first-principles calculations that in ternary $\mathrm{I}\mathrm{\text{\ensuremath{-}}}\mathrm{III}\mathrm{\text{\ensuremath{-}}}{\mathrm{VI}}_{2}$ chalcopyrites like ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$, $DX$-like centers can develop without the presence of any extrinsic impurities. These intrinsic $DX$ centers are suggested as a cause of the difficulties to maintain high efficiencies in ${\mathrm{CuInSe}}_{2}$-based thin-film solar-cells when the band gap is increased by addition of Ga.
127 citations
01 May 2008
TL;DR: In this article, the authors report that intrinsic donor-type defects In{sub Cu, Ga, sub Cu, and V{sub Se} cause metastability, but also act to limit V{ sub OC; and that growth conditions which minimize these defects (Cu-rich/Se-rich) are very different from those currently used.
Abstract: The conclusions of this report are: (1) intrinsic donor-type defects In{sub Cu}, Ga{sub Cu}, and V{sub Se}, and their complexes with V{sub Cu} cause metastability, but also act to limit V{sub OC}; (2) growth conditions which minimize these defects (Cu-rich/Se-rich) are very different from those currently used; and (3) overcoming V{sub OC} limitation requires to address other issues and trade-offs.
120 citations
TL;DR: In this paper, the magnetic coupling of the impurity pairs was determined in the presence of a correctly positioned conduction band (with respect to the $3d$ states) through the addition of empirical nonlocal external potentials to the standard density functional Hamiltonian.
Abstract: The well-known ``band-gap'' problem in approximate density functionals is manifested mainly in an overly low energy of the conduction band (CB). As a consequence, the localized gap states of $3d$ impurities states in wide-gap oxides such as $\mathrm{ZnO}$ occur often incorrectly as resonances inside the CB, leading to a spurious transfer of electrons from the impurity state into the CB of the host, and to a physically misleading description of the magnetic $3d\text{\ensuremath{-}}3d$ interactions. A correct description requires that the magnetic coupling of the impurity pairs be self-consistently determined in the presence of a correctly positioned CB (with respect to the $3d$ states), which we achieve here through the addition of empirical nonlocal external potentials to the standard density functional Hamiltonian. After this correction, both Co and Cr form occupied localized states in the gap and empty resonances low inside the CB. In otherwise undoped $\mathrm{ZnO}$, Co and Cr remain paramagnetic, but electron-doping instigates strong ferromagnetic coupling when the resonant states become partially occupied.
118 citations
TL;DR: In this article, the authors proposed an approach for energy transfer involving semiconductor quantum dots (QDs) in order to exploit their properties, such as high photostability and size-tunable optical properties.
Abstract: Excitation energy transfer involving semiconductor quantum dots (QDs) has received increased attention in recent years because their properties, such as high photostability and size-tunable optical...
114 citations
TL;DR: In this article, the authors introduce a DCM "figure of merit" R2(E) which is proportional to the ratio between the biexciton density of states ρXX and the single-exciton density ρX.
Abstract: Direct carrier multiplication (DCM) occurs when a highly excited electron−hole pair decays by transferring its excess energy to the electrons rather than to the lattice, possibly exciting additional electron−hole pairs. Atomistic electronic structure calculations have shown that DCM can be induced by electron−hole Coulomb interactions, in an impact-ionization-like process whose rate is proportional to the density of biexciton states ρXX. Here we introduce a DCM “figure of merit” R2(E) which is proportional to the ratio between the biexciton density of states ρXX and the single-exciton density of states ρX, restricted to single-exciton and biexciton states that are coupled by Coulomb interactions. Using R2(E), we consider GaAs, InAs, InP, GaSb, InSb, CdSe, Ge, Si, and PbSe nanocrystals of different sizes. Although DCM can be affected by both quantum-confinement effects (reflecting the underly electronic structure of the confined dot-interior states) and surface effects, here we are interested to isolate th...
78 citations
Journal Article•
TL;DR: In this paper, the Coulomb effects in self-assembled quantum dots were analyzed by interpreting experimental spectra with an atomistic calculation, and they were shown to be tunable in situ by controlling the quantum dot charge from +6e to −6e.
Abstract: Coulomb interactions between electrons lead to the observed multiplet structure and breakdown of the Aufbau principle for atomic d and f shells1. Nevertheless, these effects can disappear in extended systems. For instance, the multiplet structure of atomic carbon is not a feature of graphite or diamond. A quantum dot is an extended system containing ∼106 atoms for which electron–electron interactions do survive and the interplay between the Coulomb energy, J, and the quantization energy, ΔE, is crucial to Coulomb blockade2,3,4,5. We have discovered consequences of Coulomb interactions in self-assembled quantum dots by interpreting experimental spectra with an atomistic calculation. The Coulomb effects, evident in the photon emission process, are tunable in situ by controlling the quantum dot charge from +6e to −6e. The same dot shows two regimes: J≤ΔE for electron charging yet J≃ΔE for hole charging. We find a breakdown of the Aufbau principle for holes; clear proof of non-perturbative hole–hole interactions; promotion–demotion processes in the final state of the emission process, effects first predicted a decade ago6; and pronounced configuration hybridizations in the initial state. The level of charge control and the energy scales result in Coulomb effects with no obvious analogues in atomic physics.
77 citations
TL;DR: Using theoretical modeling of the fundamental causes of FSS in QDs, it is predicted that the intrinsic FSS of InAs/InP QDs is an order of magnitude smaller than that of In as/GaAs dots, and, better yet, their excitonic gap matches the 1.55 microm fiber optic wavelength and, therefore, offers efficient on-demand entangled photon emitters for long distance quantum communication.
Abstract: To generate entangled photon pairs via quantum dots (QDs), the exciton fine-structure splitting (FSS) must be comparable to the exciton homogeneous linewidth Yet in the $(\mathrm{In},\mathrm{Ga})\mathrm{As}/\mathrm{GaAs}$ QD, the intrinsic FSS is about a few tens $\ensuremath{\mu}\mathrm{eV}$ To achieve photon entanglement, it is necessary to cherry-pick a sample with extremely small FSS from a large number of samples or to apply a strong in-plane magnetic field Using theoretical modeling of the fundamental causes of FSS in QDs, we predict that the intrinsic FSS of $\mathrm{InAs}/\mathrm{InP}$ QDs is an order of magnitude smaller than that of $\mathrm{InAs}/\mathrm{GaAs}$ dots, and, better yet, their excitonic gap matches the $155\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ fiber optic wavelength and, therefore, offers efficient on-demand entangled photon emitters for long distance quantum communication
TL;DR: In this article, the formation enthalpies of cubic zinc blende (Ga,In)N alloy were calculated under the three forms of thermodynamic states established during growth.
Abstract: The morphology and microstructure of ${A}_{1\ensuremath{-}x}{B}_{x}C$ semiconductor alloys depend on the type of thermodynamic states established during growth We distinguish three main cases: (i) bulk-incoherent structures occur when the alloy grows without being coherent with an underlying substrate and when each of the possible alloy species---phase separated $AC$ and $BC$ constituents, random ${A}_{1\ensuremath{-}x}{B}_{x}C$ alloy, or ordered ${(AC)}_{n}/{(BC)}_{m}$ structures---maintain their own lattice structures and lattice constants, giving up mutual coherence Bulk incoherence is common in thick films with sufficient dislocations For cubic (Ga,In)N, bulk-incoherent structures are found to have a positive excess enthalpy $\ensuremath{\Delta}{H}_{\text{bulk}}^{\text{incoh}}g0$ and, thus, to phase separate (ii) Bulk-coherent structures occur when the alloy grows without being coherent with a substrate, but each of the possible species internal to the alloy film is forced to be coherent with the film matrix Thus, the constituents $AC$-rich and $BC$-rich solid solution phases share the same lattice structure at their interface, leading to internal strain that destabilizes the $AC+BC$ separated constituents This can expose the intermediate ${(AC)}_{n}/{(BC)}_{m}$ ordered phases as stable structures with respect to the strained constituents, ie, $\ensuremath{\Delta}{H}_{\text{bulk}}^{\text{coh}}l0$ Bulk coherence is applicable to growth when the development of dislocations is inhibited, eg, small size precipitates in the alloy matrix For cubic (Ga,In)N alloy, we find that the coherent ground state phases are three ordered superlattice structures: ${(\text{InN})}_{2}/{(\text{GaN})}_{2}$ $(=\text{chacolpyrite})$, ${(\text{InN})}_{3}/{(\text{GaN})}_{1}$, and ${(\text{InN})}_{4}/{(\text{GaN})}_{1}$, along (201) [and its cubic symmetry equivalent, ie, (102), (210), etc] crystal direction (iii) Epitaxially coherent structures occur when the alloy is made coherent with an underlying substrate, eg, in thin film pseudomorphic growth Depending on the substrate, the formation enthalpy $\ensuremath{\Delta}{H}^{\text{epi}}l0$ For cubic (Ga,In)N grown on GaN (001) substrate, we find that the stablest epitaxial phases are chalcopyrite and the ${(\text{InN})}_{4}/{(\text{GaN})}_{1}$ superlattice along the (210) crystal direction Here, we calculate, from first principles, the formation enthalpies of cubic zinc blende (Ga,In)N alloy under the three forms of thermodynamic states indicated above to establish a cluster expansion, from which we calculate the finite-temperature phase diagrams This illustrates how the thermodynamic constraints during growth can radically alter the alloy phase behavior and its microstructures
TL;DR: It is shown from density-functional calculations within a band-gap corrected approach that ferromagnetic Cr-Cr coupling can be switched on and off via electron doping in the wide-gap transparent n-type conductive oxide In2O3.
Abstract: Carrier-induced ferromagnetism in wide-gap transparent conductive oxides has been widely discussed and debated, leading to confusion and skepticism regarding whether dilute magnetic oxides exist at all. We show from density-functional calculations within a band-gap corrected approach that ferromagnetic Cr-Cr coupling can be switched on and off via electron doping in the wide-gap transparent n-type conductive oxide In{sub 2}O{sub 3}. We show that (1) Cr does not produce in In{sub 2}O{sub 3} any free electrons and renders the system an insulating paramagnet. (2) Extrinsic n-type doping of In{sub 2}O{sub 3}:Cr via Sn produces free electrons, whose concentration is controllable via the oxygen partial pressure. Such additional carriers stabilize a strong long-range Cr-Cr ferromagnetic coupling.
TL;DR: The Auger mechanism can explain the experimentally observed P-->S intraband decay time scale without the need to invoke any exotic relaxation mechanisms.
Abstract: In solids the phonon-assisted, nonradiative decay from high-energy electronic excited states to low-energy electronic excited states is picosecond fast. It was hoped that electron and hole relaxation could be slowed down in quantum dots, due to the unavailability of phonons energy matched to the large energy-level spacings ("phonon-bottleneck"). However, excited-state relaxation was observed to be rather fast ( S electron Auger relaxation rate. We find that the Auger mechanism can explain the experimentally observed P-->S intraband decay time scale without the need to invoke any exotic relaxation mechanisms.
TL;DR: It is shown here that using a genetic-algorithm search with a pseudopotential "Inverse-band-structure (IBS) approach it is possible to identify those configurations that are naturally lattice matching and have a specific band gap at more than one composition.
Abstract: Quaternary systems illustrated by (Ga,In)(As,Sb) manifest a huge configurational space, offering in principle the possibility of designing structures that are lattice matched to a given substrate and have given electronic properties (e.g., band gap) at more than one composition. Such specific configurations were however, hitherto, unidentified. We show here that using a genetic-algorithm search with a pseudopotential "Inverse-band-structure (IBS) approach it is possible to identify those configurations that are naturally lattice matching (to GaSb) and have a specific band gap (310 meV) at more than one composition. This is done by deviating from randomness, allowing the IBS to find a partial atomic ordering. This illustrates multitarget design of the electronic structure of multinary systems.
TL;DR: In this paper, the authors examined how the two different mechanisms proposed historically for biological evolution compare for the determination of crystal structures from random initial lattice configurations, and showed that the minimum energy configurations of a binary alloy in the full $0.1$ concentration range can be found much faster if the conventional Darwinian genetic progression (mating configurations and letting the lowest energy (fittest) offspring survive) is allowed to experience Lamarckian-style fitness improvements during its lifetime.
Abstract: We examine how the two different mechanisms proposed historically for biological evolution compare for the determination of crystal structures from random initial lattice configurations. The Darwinian theory of evolution contends that the genetic makeup inherited at birth is the one passed on during mating to new offspring, in which case evolution is a product of environmental pressure and chance. In addition to this mechanism, Lamarck surmised that individuals can also pass on traits acquired during their lifetime. Here we show that the minimum-energy configurations of a binary ${A}_{1\ensuremath{-}x}{B}_{x}$ alloy in the full $0\ensuremath{\le}x\ensuremath{\le}1$ concentration range can be found much faster if the conventional Darwinian genetic progression---mating configurations and letting the lowest-energy (fittest) offspring survive---is allowed to experience Lamarckian-style fitness improvements during its lifetime. Such improvements consist of $A\ensuremath{\leftrightarrow}B$ transmutations of some atomic sites (not just atomic relaxations) guided by ``virtual-atom'' energy gradients. This hybrid evolution is shown to provide an efficient solution to a generalized Ising Hamiltonian, illustrated here by finding the ground states of face-centered-cubic ${\text{Au}}_{1\ensuremath{-}x}{\text{Pd}}_{x}$ using a cluster-expansion functional fitted to first-principles total energies. The statistical rate of success of the search strategies and their practical applicability are rigorously documented in terms of average number of evaluations required to find the solution out of 400 independent evolutionary runs with different random seeds. We show that all exact ground states of a 12-atom supercell (${2}^{12}$ configurations) can be found within 330 total-energy evaluations, whereas a 36-atom supercell (${2}^{36}$ configurations) requires on average $39\text{ }000$ evaluations. Thus, this problem cannot be currently addressed with confidence using costly energy functionals [e.g., density-functional theory (DFT) based] unless it is limited to $\ensuremath{\le}20$ atoms. The computational cost can be reduced at the expense of accuracy: Searching for all approximate-minimum-energy configurations (within 3 meV) of a 12- or 36-atom supercell requires on average 30 or 580 total-energy evaluations, respectively. Thus it could be addressed even by costly energy functionals such as density-functional theory.
TL;DR: In this paper, Magri and Zunger showed that the band edges and band gaps of superlattices on a GaSb substrate exhibit a nonmonotonic behavior as a function of the InAs barrier thickness when the number of InAs layers exceeds 5$.
Abstract: The band edges and band gaps of ${(\mathrm{In}\mathrm{As})}_{n}∕{(\mathrm{Ga}\mathrm{Sb})}_{m}$ $(n,m=1,20)$ superlattices have been theoretically studied through the plane-wave empirical pseudopotential method for different situations: (i) different substrates, GaSb and InAs; (ii) different point group symmetries, ${C}_{2v}$ and ${D}_{2d}$; and (iii) different growth directions, (001) and (110). We find that (a) the band gaps for the (001) ${C}_{2v}$ superlattices on a GaSb substrate exhibit a nonmonotonic behavior as a function of the GaSb barrier thickness when the number of ${(\mathrm{In}\mathrm{As})}_{n}$ layers exceed $n=5$; (b) 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 (001) ${C}_{2v}$ superlattices as a function of the InAs well thickness; (c) effect of the type of interfacial bonds: the In-Sb bonds at the interfaces of the (001) ${D}_{2d}$ 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 (001) ${C}_{2v}$ case. The valence band maximum of the (001) ${D}_{2d}$ superlattices with Ga-As bonds at the interfaces are shifted down, increasing the band gaps as compared to the (001) ${C}_{2v}$ case; (d) effect of layer orientation: the presence of In-Sb bonds at both interfaces of the (110) superlattices pin the band edge states and reduces the band gaps, as compared to the (001) ${C}_{2v}$ case. An anticrossing between the electron and hole levels in the (110) superlattices, for thin GaSb and thick InAs layers, leads to an increase of the band gaps, as a function of the InAs thickness; (e) 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 (2002)], 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 $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model fit instead measured gaps corresponding to interdiffused interfaces using a chemically abrupt model.
TL;DR: In this article, a unique structural anomaly of pure MnN, in which local density calculations fail to predict the experimentally observed distorted rocksalt as the ground-state structure, is resolved under the GGA+U$ and B3LYP formalisms.
Abstract: Pure MnN and (Ga,Mn)N alloys are investigated using the ab initio generalized gradient approximation $+U$ $(\text{GGA}+U)$ or the hybrid-exchange density-functional (B3LYP) methods. These methods are found to predict dramatically different electronic structure, magnetic behavior, and relative stabilities compared to previous density-functional theory (DFT) calculations. A unique structural anomaly of MnN, in which local-density calculations fail to predict the experimentally observed distorted rocksalt as the ground-state structure, is resolved under the $\text{GGA}+U$ and B3LYP formalisms. The magnetic configurations of MnN are studied and the results suggest the magnetic state of zinc-blende MnN might be complex. Epitaxial calculations are used to show that the epitaxial zinc-blende MnN can be stabilized on an InGaN substrate. The structural stability of (Ga,Mn)N alloys was examined and a crossover from the zinc-blende-stable alloy to the rocksalt-stable alloy at an Mn concentration of $\ensuremath{\sim}65%$ was found. The tendency for zinc-blende (Ga,Mn)N alloys to phase separate is described by an asymmetric spinodal phase diagram calculated from a mixed-basis cluster expansion. This predicts that precipitates will consist of Mn concentrations of $\ensuremath{\sim}5$ and $\ensuremath{\sim}50%$ at typical experimental growth temperatures. Thus, pure antiferromagnetic MnN, previously thought to suppress the Curie temperature, will not be formed. The Curie temperature for the $50%$ phase is calculated to be ${T}_{C}=354\text{ }\text{K}$, indicating the possibility of high-temperature ferromagnetism in zinc-blende (Ga,Mn)N alloys due to precipitates.
TL;DR: In this article, the authors discuss the physical origin of the previously predicted quantum-size-induced electronic transitions in spherical GaAs quantum dots and use atomistic pseudopotential calculations to distinguish two types of direct/indirect transitions: (i) in freestanding GaAs dots, the conduction-band minimum changes from $\ensuremath{\Gamma}$-like to $X$ -like as the radius of the dot is reduced below 1.6 nm.
Abstract: We discuss the physical origin of the previously predicted quantum-size-induced electronic transitions in spherical GaAs quantum dots. By using atomistic pseudopotential calculations for freestanding GaAs dots and for GaAs dots embedded in an AlGaAs matrix, we are able to distinguish two types of direct/indirect transitions: (i) in freestanding GaAs dots, the conduction-band minimum changes from $\ensuremath{\Gamma}$-like to $X$-like as the radius of the dot is reduced below 1.6 nm, leading to a direct/indirect transition in reciprocal space. (ii) In GaAs dots embedded in AlAs, the conduction-band minimum changes from dot localized to barrier localized as the radius of the dot is reduced below 4.2 nm, corresponding to a direct-to-indirect transition in real space.
TL;DR: In this paper, a set of crystal structures with randomly selected lattice vectors and site occupations is evolved through a sequence of generations in which a given number of structures of highest LDA energy are replaced by new ones obtained by the generational operations of mutation or mating, each new structure is locally relaxed to the nearest total energy minimum by using the ab initio atomic forces and stresses.
Abstract: Two types of global space-group optimization (GSGO) problems can be recognized in binary metallic alloys AqB1−q: (i) configuration search problems, where the underlying crystal lattice is known and the aim is finding the most favorable decoration of the lattice by A and B atoms and (ii) lattice-type search problems, where neither the lattice type nor the decorations are given and the aim is finding energetically favorable lattice vectors and atomic occupations. Here, we address the second, lattice-type search problem in binary AqB1−q metallic alloys, where the constituent solids A and B have different lattice types. We tackle this GSGO problem using an evolutionary algorithm, where a set of crystal structures with randomly selected lattice vectors and site occupations is evolved through a sequence of generations in which a given number of structures of highest LDA energy are replaced by new ones obtained by the generational operations of mutation or mating. Each new structure is locally relaxed to the nearest total-energy minimum by using the ab initio atomic forces and stresses. We applied this first-principles evolutionary GSGO scheme to metallic alloy systems where the nature of the intermediate A–B compounds is difficult to guess either because pure A and pure B have different lattice types and the (i) intermediate compound has the structure of one end-point (Al3Sc, AlSc3, CdPt3), or (ii) none of them (CuPd, AlSc), or (iii) when the intermediate compound has lattice sites belonging simultaneously to a few types (fcc, bcc) (PdTi3). The method found the correct structures, L12 type for Al3Sc, D019 type for AlSc3, 'CdPt3' type for CdPt3, B2 type for CuPd and AlSc, and A15 type for PdTi3. However, in such stochastic methods, success is not guaranteed, since many independently started evolutionary sequences produce at the end different final structures: one has to select the lowest-energy result from a set of such independently started sequences. Interestingly, we also predict a hitherto unknown (P 2/m) structure of the hard compound IrN2 with energy lower than all previous predictions.
11 May 2008
TL;DR: In this paper, the role of intrinsic point defects in the limitation of the opencircuit voltage (VOC ) in Cu(In,Ga)Se 2 solar cells was investigated using first-principles defect theory.
Abstract: Using first-principles defect theory, we investigate the role of intrinsic point defects in the limitation of the opencircuit voltage (V OC ) in Cu(In,Ga)Se 2 solar cells. We find that the intrinsic donors In Cu (In-on-Cu antisite defect) and V Se (Selenium vacancy) and their defect complexes with V Cu (Cu vacancies) represent two independent mechanisms that are expected to cause saturation of V OC around 1 eV, when the absorber band gap is increased towards Ga-rich compositions. Strategies to avoid these sources of V OC limitation are discussed.
27 Mar 2008
TL;DR: In this article, the authors discuss the physical factors that affect doping limits in semiconductors and demonstrate the dependencies of the defect formation enthalpy on the atomic chemical potentials and on the electron Fermi energy.
Abstract: Using defect thermodynamics, we discuss physical factors that affect doping limits in semiconductors. The dependencies of the defect formation enthalpy on the atomic chemical potentials and on the electron Fermi energy are demonstrated. These dependencies, in particular on the Fermi energy, lead to spontaneous formation of charge-compensating defects that can limit doping. Experimental data compiled for III-V, II-VI, and I-III-VI2 compounds support this view and further provide insight into the connections among different host materials. We argue that what matters is not the magnitude of the band gap that determines the dopability of a material, but rather, the relative position of the conduction-band minimum (in the case of n-doping) and the valence-band maximum (in the case of p-doping) with respect to vacuum.
TL;DR: In this article, the inverse band structure approach was used to search for lattice-matched InAs/GaAs multiple-repeat unit structures with individual layer thicknesses lower than the critical thickness for a 3D morphological transition.
Abstract: Thermophotovoltaic (TPV) devices are intended to absorb photons from hot blackbody radiating objects, often requiring semiconductor absorbers with band gap of $\ensuremath{\simeq}0.6\text{ }\text{eV}$. The random ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}$ alloy lattice matched $({x}_{\text{In}}=0.53)$ to a (001) InP substrate has a low-temperature band gap of 0.8 eV, about 0.2 eV too high for a TPV absorber. Bringing the band gap down by raising the In concentration induces strain with the substrate, leading to a two-dimensional $(2\text{D})\ensuremath{\rightarrow}\text{three}$-dimensional (3D) morphological transition occurring before band gaps suitable for TPV applications are achieved. We use the inverse band structure approach, based on a genetic algorithm and empirical pseudopotential calculations, to search for lattice-matched InAs/GaAs multiple-repeat unit structures with individual layer thicknesses lower than the critical thickness for a $2\text{D}\ensuremath{\rightarrow}3\text{D}$ transition. Despite the fact that quantum confinement usually increases band gaps, we find a quantum superlattice structure with the required reduced gap (and a significant optical transition) that matches all target requirements. This is explained by the predominance of (potential-energy) level anticrossing effects over (kinetic) quantum confinement effects.