Showing papers by "Alex Zunger published in 2012"

Identification of potential photovoltaic absorbers based on first-principles spectroscopic screening of materials.

Abstract: There are numerous inorganic materials that may qualify as good photovoltaic (PV) absorbers, except that the currently available selection principle-focusing on materials with a direct band gap of ∼1.3 eV (the Shockley-Queisser criteria)-does not provide compelling design principles even for the initial material screening. Here we offer a calculable selection metric of "spectroscopic limited maximum efficiency (SLME)" that can be used for initial screening based on intrinsic properties alone. It takes into account the band gap, the shape of absorption spectra, and the material-dependent nonradiative recombination losses. This is illustrated here via high-throughput first-principles quasiparticle calculations of SLME for ∼260 generalized I(p)III(q)VI(r) chalcopyrite materials. It identifies over 20 high-SLME materials, including the best known as well as previously unrecognized PV absorbers.

Correcting density functional theory for accurate predictions of compound enthalpies of formation: Fitted elemental-phase reference energies

Abstract: Despite the great success that theoretical approaches based on density functional theory have in describing properties of solid compounds, accurate predictions of the enthalpies of formation ($\ensuremath{\Delta}{H}_{f}$) of insulating and semiconducting solids still remain a challenge. This is mainly due to incomplete error cancellation when computing the total energy differences between the compound total energy and the total energies of its elemental constituents. In this paper we present an approach based on GGA $+$ $U$ calculations, including the spin-orbit coupling, which involves fitted elemental-phase reference energies (FERE) and which significantly improves the error cancellation resulting in accurate values for the compound enthalpies of formation. We use an extensive set of 252 binary compounds with measured $\ensuremath{\Delta}{H}_{f}$ values (pnictides, chalcogenides, and halides) to obtain FERE energies and show that after the fitting, the 252 enthalpies of formation are reproduced with the mean absolute error $\text{MAE}=0.054$ eV/atom instead of $\text{MAE}\ensuremath{\approx}0.250$ eV/atom resulting from pure GGA calculations. When applied to a set of 55 ternary compounds that were not part of the fitting set the FERE method reproduces their enthalpies of formation with $\text{MAE}=0.048$ eV/atom. Furthermore, we find that contributions to the total energy differences coming from the spin-orbit coupling can be, to a good approximation, separated into purely atomic contributions which do not affect $\ensuremath{\Delta}{H}_{f}$. The FERE method, hence, represents a simple and general approach, as it is computationally equivalent to the cost of pure GGA calculations and applies to virtually all insulating and semiconducting compounds, for predicting compound $\ensuremath{\Delta}{H}_{f}$ values with chemical accuracy. We also show that by providing accurate $\ensuremath{\Delta}{H}_{f}$ the FERE approach can be applied for accurate predictions of the compound thermodynamic stability or for predictions of Li-ion battery voltages.

Band-structure, optical properties, and defect physics of the photovoltaic semiconductor SnS

Abstract: SnS is a potential earth-abundant photovoltaic (PV) material. Employing both theory and experiment to assess the PV relevant properties of SnS, we clarify on whether SnS has an indirect or direct band gap and what is the minority carrier effective mass as a function of the film orientation. SnS has a 1.07 eV indirect band gap with an effective absorption onset located 0.4 eV higher. The effective mass of minority carrier ranges from 0.5 m0 perpendicular to the van der Waals layers to 0.2 m0 into the van der Waals layers. The positive characteristics of SnS feature a desirable p-type carrier concentration due to the easy formation of acceptor-like intrinsic Sn vacancy defects. Potentially detrimental deep levels due to SnS antisite or S vacancy defects can be suppressed by suitable adjustment of the growth condition towards S-rich.

Extracting E Versus K Effective Band Structure from Supercell Calculations on Alloys and Impurities

Abstract: The supercell approach to defects and alloys has circumvented the limitations of those methods that insist on using artificially high symmetry, yet this step usually comes at the cost of abandoning the language of $E$ versus $\stackrel{P\vec}{k}$ band dispersion. Here we describe a computational method that maps the energy eigenvalues obtained from large supercell calculations into an effective band structure (EBS) and recovers an approximate $E(\stackrel{P\vec}{k})$ for alloys. Making use of supercells allows one to model a random alloy A${}_{1\ensuremath{-}x}$B${}_{x}$C by occupying the sites A and B via a coin-toss procedure, affording many different local environments (polymorphic description) to occur. We present the formalism and implementation details of the method and apply it to study the evolution of the impurity band appearing in the dilute GaN:P alloy. We go beyond the perfectly random case, realizing that many alloys may have nonrandom microstructures, and investigate how their formation is reflected in the EBS. It turns out that the EBS is extremely sensitive in determining the critical disorder level for which delocalized states start to appear in the intermediate band. In addition, the EBS allows us to identify the role played by atomic relaxation in the positioning of the impurity levels.

Optical spectroscopy of single quantum dots at tunable positive, neutral, and negative charge states

Abstract: We report on the observation of photoluminescence from positive, neutral, and negative charge states of single semiconductor quantum dots. For this purpose we designed a structure enabling optical injection of a controlled unequal number of negative electrons and positive holes into an isolated InGaAs quantum dot embedded in a GaAs matrix. Thereby, we optically produced the charge states -3, -2, -1, 0, +1, and +2. The injected carriers form confined collective ``artificial atoms and molecules'' states in the quantum dot. We resolve spectrally and temporally the photoluminescence from an optically excited quantum dot and use it to identify collective states, which contain charge of one type, coupled to few charges of the other type. These states can be viewed as the artificial analog of charged atoms such as ${\mathrm{H}}^{\ensuremath{-}},$ ${\mathrm{H}}^{\ensuremath{-}2},$ ${\mathrm{H}}^{\ensuremath{-}3},$ and charged molecules such as ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{H}}_{3}^{+2}.$ Unlike higher dimensionality systems, where negative or positive charging always results in reduction of the emission energy due to electron-hole pair recombination, in our dots, negative charging reduces the emission energy, relative to the charge-neutral case, while positive charging increases it. Pseudopotential model calculations reveal that the enhanced spatial localization of the hole wave function, relative to that of the electron in these dots, is the reason for this effect.

Sorting Stable versus Unstable Hypothetical Compounds: The Case of Multi-Functional ABX Half-Heusler Filled Tetrahedral Structures

Abstract: Electronic structure theory has recently been used to propose hypothetical compounds in presumed crystal structures, seeking new useful functional materials. In some cases, such hypothetical materials are metastable, albeit with technologically useful long lifetimes. Yet, in other cases, suggested hypothetical compounds may be signifi cantly higher in energy than their lowest-energy crystal structures or competing phases, making their synthesis and eventual device-stability questionable. By way of example, the focus here is on the family of 1:1:1 compounds ABX called “fi lled tetrahedral structure” (sometimes called Half-Heusler) in the four groups with octet electron count: I-I-VI (e.g., CuAgSe), I-II-V (e.g., AgMgAs), I-III-IV (e.g., LiAlSi), and II-II-IV (e.g., CaZnSn). First-principles thermodynamics is used to sort the lowestenergy structure and the thermodynamic stability of the 488 unreported hypothetical ABX compounds, many of which were previously proposed to be useful technologically. It is found that as many as 235 of the 488 are unstable with respect to decomposition (hence, are unlikely to be viable technologically), whereas other 235 of the unreported compounds are predicted to be thermodynamically stable (hence, potentially interesting new materials). 18 additional materials are too close to determine. The electronic structures of these predicted stable compounds are evaluated, seeking potential new material functionalities.

Genetic-Algorithm Discovery of a Direct-Gap and Optically Allowed Superstructure from Indirect-Gap Si and Ge Semiconductors

Abstract: Combining two indirect-gap materials-with different electronic and optical gaps-to create a direct gap material represents an ongoing theoretical challenge with potentially rewarding practical implications, such as optoelectronics integration on a single wafer. We provide an unexpected solution to this classic problem, by spatially melding two indirect-gap materials (Si and Ge) into one strongly dipole-allowed direct-gap material. We leverage a combination of genetic algorithms with a pseudopotential Hamiltonian to search through the astronomic number of variants of Si /Ge /.../Si /Ge superstructures grown on (001) Si Ge . The search reveals a robust configurational motif-SiGe Si Ge SiGe on (001) Si Ge substrate (x≤0.4) presenting a direct and dipole-allowed gap resulting from an enhanced Γ-X coupling at the band edges. © 2012 American Physical Society.

Surface Origin of High Conductivities in Undoped In2O3 Thin Films

Abstract: The microscopic cause of conductivity in transparent conducting oxides like ZnO, In{2}O{3}, and SnO{2} is generally considered to be a point defect mechanism in the bulk, involving intrinsic lattice defects, extrinsic dopants, or unintentional impurities like hydrogen. We confirm here that the defect theory for O-vacancies can quantitatively account for the rather moderate conductivity and off-stoichiometry observed in bulk In{2}O{3} samples under high-temperature equilibrium conditions. However, nominally undoped thin-films of In{2}O{3} can exhibit surprisingly high conductivities exceeding by 4-5 orders of magnitude that of bulk samples under identical conditions (temperature and O{2} partial pressure). Employing surface calculations and thickness-dependent Hall measurements, we demonstrate that surface donors rather than bulk defects dominate the conductivity of In{2}O{3} thin films.

Cation off-stoichiometry leads to high p -type conductivity and enhanced transparency in Co 2 ZnO 4 and Co 2 NiO 4 thin films

Abstract: We explore the effects of cation off-stoichiometry on structural, electrical, optical, and electronic properties of Co${}_{2}$ZnO${}_{4}$ normal spinel and Co${}_{2}$NiO${}_{4}$ inverse spinel using theoretic and experimental (combinatorial and conventional) techniques, both at thermodynamic equilibrium and in the metastable regime. Theory predicts that nonequilibrium substitution of divalent Zn on nominally trivalent octahedral sites increases net hole density in Co${}_{2}$ZnO${}_{4}$. Experiment confirms high conductivity and high work function in Co${}_{2}$NiO${}_{4}$ and Zn-rich Co${}_{2}$ZnO${}_{4}$ thin films grown by nonequilibrium physical vapor deposition techniques. High $p$-type conductivities of Co${}_{2}$ZnO${}_{4}$ (up to 5 S/cm) and Co${}_{2}$NiO${}_{4}$ (up to 204 S/cm) are found over a broad compositional range, they are only weakly sensitive to oxygen partial pressure and quite tolerant to a wide range of processing temperatures. In addition, off-stoichiometry caused by nonequilibrium growth decreases the optical absorption of Co${}_{2}$ZnO${}_{4}$ and Co${}_{2}$NiO${}_{4}$ thin films, although the 500-nm thin films still have rather limited transparency. All these properties as well as high work functions make Co${}_{2}$ZnO${}_{4}$ and Co${}_{2}$NiO${}_{4}$ thin films attractive for technological applications, such as hole transport layers in organic photovoltaic devices or $p$-type buffer layers in inorganic solar cells.

Angle-resolved photoemission and quasiparticle calculation of ZnO: The need for d band shift in oxide semiconductors

Abstract: ZnO is a prototypical semiconductor with occupied d 10 bands that interact with the anion p states and is thus challenging for electronic structure theories. Within the context of these theories, incomplete cancellation of the self-interaction energy results in a Zn d band that is too high in energy, resulting in upwards repulsion of the valence band maximum (VBM) states, and an unphysical reduction of the band gap. Methods such as GW should significantly reduce the self-interaction error, and in order to evaluate such calculations, we measured high-resolutionandresonantangle-resolvedphotoemissionspectroscopy(ARPES)andcomparedthesetoseveral electronic structure calculations. We find that, in a standard GW calculation, the d bands remain too high in energy by more than 1 eV irrespective of the Hamiltonian used for generating the input wave functions, causing a slight underestimation of the band gap due to the p-d repulsion. We show that a good agreement with the ARPES data over the full valence band spectrum is obtained, when the Zn-d band energy is shifted down by applying an on-site potential Vd for Zn-d states during the GW calculations to match the measured d band position. The magnitude of the GW quasiparticle energy shift relative to the initial density functional calculation is of importance for the prediction of charged defect formation energies, band-offsets, and ionization potentials.

Correcting Density Functional Theory for Accurate Predictions of Compound Enthalpies of Formation:Fitted elemental-phase Reference Energies (FERE)

Abstract: Despite the great success that theoretical approaches based on density functional theory have in describing properties of solid compounds, accurate predictions of the enthalpies of formation (�H f ) of insulating and semiconducting solids still remain a challenge. This is mainly due to incomplete error cancellation when computing the total energy differences between the compound total energy and the total energies of its elemental constituents. In this paper we present an approach based on GGA + U calculations, including the spin-orbit coupling, which involves fitted elemental-phase reference energies (FERE) and which significantly improves the error cancellation resulting in accurate values for the compound enthalpies of formation. We use an extensive set

Genomic Design of Strong Direct-Gap Optical Transition in Si/Ge Core/Multishell Nanowires

Abstract: Finding a Si-based material with strong optical activity at the band-edge remains a challenge despite decades of research. The interest lies in combining optical and electronic functions on the same wafer, while retaining the extraordinary know-how developed for Si. However, Si is an indirect-gap material. The conservation of crystal momentum mandates that optical activity at the band-edge includes a phonon, on top of an electron-hole pair, and hence photon absorption and emission remain fairly unlikely events requiring optically rather thick samples. A promising avenue to convert Si-based materials to a strong light-absorber/emitter is to combine the effects on the band-structure of both nanostructuring and alloying. The number of possible configurations, however, shows a combinatorial explosion. Furthermore, whereas it is possible to readily identify the configurations that are formally direct in the momentum space (due to band-folding) yet do not have a dipole-allowed transition at threshold, the problem becomes not just calculation of band structure but also calculation of absorption strength. Using a combination of a genetic algorithm and a semiempirical pseudopotential Hamiltonian for describing the electronic structures, we have explored hundreds of thousands of possible coaxial core/multishell Si/Ge nanowires with the orientation of [001], [110], and [111], discovering some "magic sequences" of core followed by specific Si/Ge multishells, which can offer both a direct bandgap and a strong oscillator strength. The search has revealed a few simple design principles: (i) the Ge core is superior to the Si core in producing strong bandgap transition; (ii) [001] and [110] orientations have direct bandgap, whereas the [111] orientation does not; (iii) multishell nanowires can allow for greater optical activity by as much as an order of magnitude over plain nanowires; (iv) the main motif of the winning configurations giving direct allowed transitions involves rather thin Si shell embedded within wide Ge shells. We discuss the physical origin of the enhanced optical activity, as well as the effect of possible experimental structural imperfections on optical activity in our candidate core/multishell nanowires.

Prediction of A 2 B X 4 metal-chalcogenide compounds via first-principles thermodynamics

Abstract: Current compilations of previously documented inorganic compounds reveal a significant number of materials that are not listed. Focusing on the ${A}_{2}B{X}_{4}$ metal-chalcogenide group with $A$ and $B$ atoms being either main group elements or only one of them being a $3d$ transition metal, a total of 255 are reported, whereas 429 chemically reasonable ${A}_{2}B{X}_{4}$ are unreported. We have applied first-principles thermodynamics based on density functional methodology, predicting that about 100 of the 429 unreported ${A}_{2}B{X}_{4}$ metal-chalcogenides are likely to be stable. These include 14 oxides, 34 sulfides, 28 selenides, and 24 tellurides that are predicted here to be energetically stable with respect to decomposition into any combination of elemental, binary, and ternary competing phases. We provide the lowest-energy crystal structures of the predicted ${A}_{2}B{X}_{4}$ compounds, as well as the next few energetically higher metastable structures. Such predictions are carried out via direct first-principles calculations of candidate structure types and confirmed for a few compounds using the global space-group optimization (GSGO) search method. In some cases, uncommon oxidation states and/or coordination environments are found for elements in the stable ${A}_{2}B{X}_{4}$ compounds predicted here. We estimated the growth conditions in terms of temperature and partial pressure of the reactants from extensive thermodynamic stability analysis, and found dozens of compounds that might be grown at normal synthesis conditions. Attempts at synthesis of the stable ${A}_{2}B{X}_{4}$ compounds predicted here are called for.

Two-dimensional polaronic behavior in the binary oxides m-HfO2 and m-ZrO2.

Abstract: We demonstrate that the three-dimensional (3D) binary monoclinic oxides ${\mathrm{HfO}}_{2}$ and ${\mathrm{ZrO}}_{2}$ exhibit quasi-2D polaron localization and conductivity, which results from a small difference in the coordination of two oxygen sublattices in these materials. The transition between a 2D large polaron into a zero-dimensional small polaron state requires overcoming a small energetic barrier. These results demonstrate how a small asymmetry in the lattice structure can determine the qualitative character of polaron localization and significantly broaden the realm of quasi-2D polaron systems.

Band or Polaron: The Hole Conduction Mechanism in the p-Type Spinel Rh2ZnO4

Abstract: Given the emerging role of oxide spinels as hole conductors, we discuss in this article the traditional vs. new methodologies of determining the type of conduction mechanism at play––localized polaronic vs. band-like transport. Applying (i) traditional small polaron analysis to our in-situ high temperature four-point conductivity and thermopower measurements, we previously found an activated mobility, which is indicative of the small polaron mechanism. However, (ii) employing the recent developments in correcting density functional methodologies for hole localization, we predict that the self-trapped hole is unstable and that Rh2ZnO4 is instead a band conductor with a large effective mass. The hole mobility measured by high-field room temperature Hall effect also suggests band rather than polaron conduction. The apparent contradiction between the conclusion of the traditional procedure (i) and first-principles theory (ii) is resolved by taking into account in the previous transport analysis the temperature dependence of the effective density of states, which leads to the result that the mobility is actually temperature-independent in Rh2ZnO4. Our case study on Rh2ZnO4 illustrates the range of experimental and theoretical approaches at hand to determine whether the transport mechanism of a semiconductor is band or small polaron conduction.

Genomic design of strong direct-gap optical transition in Si/Ge core/multishell nanowires

Abstract: Finding a Si-based material with strong optical activity at the band-edge remains a challenge despite decades of research. The interest lies in combining optical and electronic functions on the same wafer, while retaining the extraordinary know-how developed for Si. However, Si is an indirect-gap material. The conservation of crystal momentum mandates that optical activity at the band-edge includes a phonon, on top of an electron-hole pair, and hence photon absorption and emission remain fairly unlikely events requiring optically rather thick samples. A promising avenue to convert Si-based materials to a strong light-absorber/emitter is to combine the effects on the band-structure of both nanostructuring and alloying. The number of possible configurations, however, shows a combinatorial explosion. Furthermore, whereas it is possible to readily identify the configurations that are formally direct in the momentum space (due to band-folding) yet do not have a dipole-allowed transition at threshold, the problem becomes not just calculation of band structure but also calculation of absorption strength. Using a combination of a genetic algorithm and a semiempirical pseudopotential Hamiltonian for describing the electronic structures, we have explored hundreds of thousands of possible coaxial core/multishell Si/Ge nanowires with the orientation of [001], [110], and [111], discovering some "magic sequences" of core followed by specific Si/Ge multishells, which can offer both a direct bandgap and a strong oscillator strength. The search has revealed a few simple design principles: (i) the Ge core is superior to the Si core in producing strong bandgap transition; (ii) [001] and [110] orientations have direct bandgap, whereas the [111] orientation does not; (iii) multishell nanowires can allow for greater optical activity by as much as an order of magnitude over plain nanowires; (iv) the main motif of the winning configurations giving direct allowed transitions involves rather thin Si shell embedded within wide Ge shells. We discuss the physical origin of the enhanced optical activity, as well as the effect of possible experimental structural imperfections on optical activity in our candidate core/multishell nanowires.

Ab initio theory of phase stability and structural selectivity in Fe-Pd alloys

Abstract: In Fe-Pd alloys, the competing geometric (fcc versus bcc) and magnetic tendencies result in rich phase stability and ordering physics. Here, we study these alloys via a first principles mixed-basis cluster expansion (CE) approach. Highly accurate fcc and bcc CEs are iteratively and self-consistently constructed using a genetic algorithm, based on the first principles results for $\ensuremath{\sim}$100 ordered structures. The structural and magnetic ``filters'' are introduced to determine whether a fully relaxed structure is of fcc/bcc and high-/low-spin types. All structures satisfying the Lifshitz condition for stability in extended phase diagram regions are included as inputs to our CEs. We find that in a wide composition range (with more than 1/3 atomic content of Fe), an fcc-constrained alloy has a single stable ordered compound, L${1}_{0}$ FePd. However, L${1}_{0}$ is higher in energy than the phase-separated mixture of bcc Fe and fcc-FePd${}_{2}$ ($\ensuremath{\beta}$2 structure) at low temperatures. In the Pd-rich composition range, we find several fcc $\ensuremath{\beta}2$-like ground states: FePd${}_{2}$ ($\ensuremath{\beta}2$), Fe${}_{3}$Pd${}_{9}$, Fe${}_{2}$Pd${}_{7}$, FePd${}_{5}$, Fe${}_{2}$Pd${}_{13}$, and FePd${}_{8}$, yet we do not find FePd${}_{3}$ with the the experimentally observed L1${}_{2}$ structure. Fcc Monte Carlo simulations show a transformation from any of the attempted $\ensuremath{\beta}2$-like ground states directly into a disordered alloy. We suggest that the phonon and/or spin excitation contributions to the free energy are responsible for the observed stability of L1${}_{2}$ at higher temperatures, and likely lead to a $\ensuremath{\beta}2\phantom{\rule{0.16em}{0ex}}\ensuremath{\leftrightarrow}\phantom{\rule{0.16em}{0ex}}$L${1}_{2}$ transition. Finally, we present here a complete characterization of all the fcc and bcc Lifshitz structures, i.e., the structures with ordering vectors exclusively at high-symmetry $k$ points.

Large Insulating Gap in Topological Insulators Induced by Negative Spin-Orbit Splitting

Abstract: In a cubic topological insulator (TI), there is a band inversion whereby the $s$-like ${\ensuremath{\Gamma}}_{6\mathrm{c}}$ conduction band is below the $p$-like ${\ensuremath{\Gamma}}_{7\mathrm{v}}+{\ensuremath{\Gamma}}_{8\mathrm{v}}$ valence bands by the ``inversion energy'' ${\ensuremath{\Delta}}_{i}l0$. In TIs based on the zinc-blende structure such as HgTe, the Fermi energy intersects the degenerate ${\ensuremath{\Gamma}}_{8\mathrm{v}}$ state so the insulating gap E${}_{g}$ between occupied and unoccupied bands vanishes. To achieve an insulating gap E${}_{g}g0$ critical for TI applications, one often needs to resort to structural manipulations such as structural symmetry lowering (e.g., Bi${}_{2}$Se${}_{3}$), strain, or quantum confinement. However, these methods have thus far opened an insulating gap of only $l$0.1 eV. Here we point out that there is an electronic rather than structural way to affect an insulating gap in a TI: if one can invert the spin-orbit levels and place ${\ensuremath{\Gamma}}_{8\mathrm{v}}$ below ${\ensuremath{\Gamma}}_{7\mathrm{v}}$ (``negative spin-orbit splitting''), one can realize band inversion (${\ensuremath{\Delta}}_{i}l0$) with a large insulating gap (E${}_{g}$ up to 0.5 eV). We outline design principles to create negative spin-orbit splitting: hybridization of $d$ orbitals into $p$-like states. This general principle is illustrated in the ``filled tetrahedral structures'' (FTS) demonstrating via GW and density functional theory (DFT) calculations E${}_{g}g0$ with ${\ensuremath{\Delta}}_{i}l0$, albeit in a metastable form of FTS.

Influence of the atomic-scale structure on the exciton fine-structure splitting in InGaAs and GaAs quantum dots in a vertical electric field

Abstract: We investigate the vertical electric field tuning of the exciton fine-structure splitting (FSS) in several InGaAs and GaAs quantum dots (QDs) using the atomistic empirical pseudopotential approach and configuration interaction. We find that the FSS is surprisingly tunable, with a rate similar to the one reported for lateral electric fields. The minimum FSS for GaAs QDs often lies below the radiative linewidth, which makes them good candidates for the generation of entangled photon pairs. We highlight, however, that random alloy fluctuations affect the minimum FSS by ±1.4 μeV, so that a postselection of QDs may still be beneficial to obtain entangled photon pairs with the highest fidelity. We suggest a simple experimental procedure for this task. The FSS is therefore a rare observable, where the specific decoration of the random alloy lattice matters significantly.

Dissecting Biexciton Wave Functions of Self-Assembled Quantum Dots by Double-Quantum-Coherence Optical Spectroscopy

Abstract: Biexcitons feature prominently in various scenarios for utilization of quantum dots (QDs) for enhancing the efficiencies of solar cells, and for the generation of entangled photon pairs in single QD sources. Two-dimensional double quantum coherence (2D-DQC) nonlinear optical spectra provide novel spectroscopic signatures of such states beyond global intensity and lifetime characteristics which are available by more conventional techniques. We report the simulation of a prototype 2D-DQC optical experiment of a self-assembled InAs/GaAs dot. The simulations consider the QD in different charged states and are based on a state-of-the-art atomistic many-body pseudopotential method for the calculation of the electronic structure and transition dipole matrix elements. Comparison of the spectra of negatively charged, neutral, and positively charged QD reveals optical signatures of their electronic excitations. This technique directly accesses the biexciton ($XX$) energies as well as the projections of their wave functions on the single-exciton manifold. These signals also provide a unique tool for probing the charged state of the QD and thus the occupation of the quantum state. Signatures of Pauli blockade of the creation of certain single and two excitons due to charges on the particles are observed. For all quantum states of the QD, the spectra reveal a strong multiconfiguration character of the biexciton wave functions. Peak intensities can be explained by interference of the contributing Liouville space pathways.

Three-dimensional assemblies of semiconductor quantum dots in a wide-gap matrix providing an intermediate band for absorption

Abstract: We consider a self-assembled quantum dot (QD) system consisting of the QD itself, the wetting layer and the matrix on a substrate. The electronic structure for various III-V material combinations was determined by atomistic empirical pseudopotential calculations. Taking the widely investigated InAs/GaAs/GaAs(001) system as benchmark, we analyze the changes induced in the energy levels and offsets relevant for a QD-based intermediate band solar cell (IBSC). We explore the effects of (i) the dot material, (ii) the matrix material, and (iii) dot-matrix-substrate combinations that may enable strain balanced structures. Using as unique reference criterion the relative position of the intermediate band inside the band gap of the matrix, we suggest the dot/matrix/substrate combinations InAs/(In,Ga)P/GaAs(001), In(As,Sb)/GaAs/InP(001), and InAs/Ga(As,Sb)/InP(001) as promising candidates for QD-IBSCs.

The Nature of the magnetism-promoting hole state in the prototype magnetic semiconductor

Abstract: Recent experiments [1] suggest that the ferromagnetism (FM) in GaAs:Mn is determined by the impurity band rather than holes in the valenceband. We discuss here the physical mechanism of FM mediated by thecarriers in impurity band, where the Mn d-level play a crucial role. Thetheory is based on the first principle approach. The paradigm system that combines ferromagnetism (FM) with semicon-ductivity involves Mn (2+) impurity ions substitution for Ga (3+) atoms in GaAs[2, 3, 4, 5, 6, 7]. Such acceptor substitution creates a hole that interacts withthe local moment of d 5 Mn. This doping-induced magnetism could lead to elec-trical control of FM, to the potential benefit of spin-electronics (spintronics).The nature of the ferromagnetism, including its dependence on the hole concen-tration and on that of the Mn ions depends, however, on the physical natureof the hole state. One view -the ”host like hole” model [2, 3, 4, 5] has beenthat the hole resides inside the GaAs valence band. Such view would permitthe use of the language of GaAs semiconductor physics (s-p bonding, extendedwave functions, RKKY exchange) in analyzing the ensuing magnetism and itsdependence on concentration of the relevant species. This scenario, underlyingmost Model Hamiltonian treatments of the problem [3, 5], was inspired by thepreviously known case of isovalent Mn doping of CdTe, where, on account ofthe host metal atom Cd