Showing papers by "Alex Zunger published in 1999"

Predicted band-gap pressure coefficients of all diamond and zinc-blende semiconductors: Chemical trends

Abstract: We have studied systematically the chemical trends of the band-gap pressure coefficients of all group IV, III-V, and II-VI semiconductors using first-principles band-structure method. We have also calculated the individual ``absolute'' deformation potentials of the valence-band maximum (VBM) and conduction-band minimum (CBM). We find that (1) the volume deformation potentials of the ${\ensuremath{\Gamma}}_{6c}$ CBM are usually large and always negative, while (2) the volume deformation potentials of the ${\ensuremath{\Gamma}}_{8v}$ VBM state are usually small and negative for compounds containing occupied valence d state but positive for compounds without occupied valence d orbitals. Regarding the chemical trends of the band-gap pressure coefficients, we find that (3) ${a}_{p}^{\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Gamma}}$ decreases as the ionicity increases (e.g., from $\mathrm{G}\stackrel{\ensuremath{\rightarrow}}{e}\mathrm{GaA}\stackrel{\ensuremath{\rightarrow}}{s}\mathrm{ZnSe}),$ (4) ${a}_{p}^{\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Gamma}}$ increases significantly as anion atomic number increases (e.g., from $\mathrm{Ga}\stackrel{\ensuremath{\rightarrow}}{N}\mathrm{Ga}\stackrel{\ensuremath{\rightarrow}}{P}\mathrm{GaA}\stackrel{\ensuremath{\rightarrow}}{s}\mathrm{GaSb}),$ (5) ${a}_{p}^{\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Gamma}}$ decreases slightly as cation atomic number increases (e.g., from $\mathrm{AlA}\stackrel{\ensuremath{\rightarrow}}{s}\mathrm{GaA}\stackrel{\ensuremath{\rightarrow}}{s}\mathrm{InAs}),$ (6) the variation of ${a}_{p}^{\ensuremath{\Gamma}\ensuremath{-}L}$ are relatively small and follow similar trends as ${a}_{p}^{\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Gamma}},$ and (7) the magnitude of ${a}_{p}^{\ensuremath{\Gamma}\ensuremath{-}X}$ are small and usually negative, but are sometimes slightly positive for compounds containing first-row elements. Our calculated chemical trends are explained in terms of the energy levels of the atomic valence orbitals and coupling between these orbital. In light of the above, we suggest that ``empirical rule'' of the pressure coefficients should be modified.

Effects of Na on the electrical and structural properties of CuInSe2

Abstract: We found theoretically that Na has three effects on CuInSe2: (1) If available in stoichiometric quantities, Na will replace Cu, forming a more stable NaInSe2 compound having a larger band gap (higher open-circuit voltage) and a (112)tetra morphology. The ensuing alloy NaxCu1−xInSe2 has, however, a positive mixing enthalpy, so NaInSe2 will phase separate, forming precipitates. (2) When available in small quantities, Na will form defect on Cu site and In site. Na on Cu site does not create electric levels in the band gap, while Na on In site creates acceptor levels that are shallower than CuIn. The formation energy of Na(InCu) is very exothermic, therefore, the major effect of Na is the elimination of the InCu defects and the resulting increase of the effective hole densities. The quenching of InCu as well as VCu by Na reduces the stability of the (2VCu−+InCu2+), thus suppressing the formation of the “Ordered Defect Compounds.” (3) Na on the surface of CuInSe2 is known to catalyze the dissociation of O2 int...

Many-body pseudopotential theory of excitons in inp and cdse quantum dots

Abstract: We present a pseudopotential approach to the calculation of the excitonic spectrum of semiconductor quantum dots. Starting from a many-body expansion of the exciton wave functions in terms of single-substitution Slater determinants constructed from pseudopotential single-particle wave functions, our method permits an accurate and detailed treatment of the intraconfiguration electron-hole Coulomb and exchange interactions, while correlation effects can be included in a controlled fashion by allowing interconfiguration coupling. We calculate the exciton fine structure of InP and CdSe nanocrystals in the strong-confinement regime. We find a different size dependence for the electron-hole exchange interaction than previously assumed (i.e., ${R}^{\ensuremath{-}2}$ instead of ${R}^{\ensuremath{-}3})$. Our calculated exciton fine structure is compared with recent experimental results obtained by size-selective optical spectroscopies.

The inverse band-structure problem of finding an atomic configuration with given electronic properties

Abstract: Modern crystal-growth techniques, such as molecular beam epitaxy or metal–organic chemical-vapour deposition, are capable of producing prescribed crystal structures, sometimes even in defiance of equilibrium, bulk thermodynamics. These techniques open up the possibility of exploring different atomic arrangements in search of a configuration that possesses given electronic and optical properties1. Unfortunately, the number of possible combinations is so vast, and the electronic properties are so sensitive to the details of the crystal structure, that simple trial-and-error methods (such as those used in combinatorial synthesis2) are unlikely to be successful. Here we describe a theoretical method that addresses the problem of finding the atomic configuration of a complex, multi-component system having a target electronic-structure property. As an example, we predict that the configuration of an Al0.25Ga0.75As alloy having the largest optical bandgap is a (GaAs)2(AlAs)1(GaAs)4(AlAs)1 superlattice oriented in the [201] direction.

Electronic structures of [110]-faceted self-assembled pyramidal InAs/GaAs quantum dots

Abstract: We calculate the electronic structures of pyramidal quantum dots with supercells containing 250 000 atoms, using spin-orbit-coupled, nonlocal, empirical pseudopotentials. We compare the results with previous theoretical calculations. Our calculation circumvents the approximations underlying the conventional effective-mass approach: we describe the potential, the strain and the wave functions using atomistic rather than continuum models. The potential is given by a superposition of screened atomic pseudopotentials, the strain is obtained from minimizing the atomistic strain energy, and the wave function is expanded using a plane-wave basis set. We find the following. (1) The conduction bands are formed essentially from single envelope functions, so they can be classified according to the nodal structure as $s, p,$ and d. However, due to strong multiband coupling, most notably light hole with heavy hole, the valence states cannot be classified in the language of single-band envelope functions. In fact, the hole states have no nodal planes. (2) There is a strong anisotropy in the polarization of the lowest valence state to conduction state optical transition. This is in contrast to the eight band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model, which finds essentially zero anisotropy. (3) There are at least four bound electron states for a 113-\AA{}-based quantum dot. This number of bound states is larger than that found in eight band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ calculations. (4) Since our atomistic description retains the correct ${C}_{2v}$ symmetry of a square-based pyramid made of zinc-blende solids, we find that the otherwise degenerate p states are split by about 25 meV. This splitting is underestimated in the eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ calculation.

Localization and anticrossing of electron levels in GaAs 1 − x N x alloys

Abstract: The electronic structure in nitrogen-poor ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ alloys is investigated using a plane-wave pseudopotential method and large supercells. Our calculations give a detailed description of the complex perturbation of the lowest conduction band states induced by nitrogen substitution in GaAs. The two principal physical effects are (i) a resonant impurity state ${a}_{1}(N)$ above the ${a}_{1}({\ensuremath{\Gamma}}_{1c})$ conduction band minimum (important at ``impurity'' concentrations, $x\ensuremath{\sim}{10}^{17} {\mathrm{cm}}^{\ensuremath{-}3})$ and (ii) the creation of ${a}_{1}{(L}_{1c}),$ and ${a}_{1}{(X}_{1c})$ states due to the splitting of the degenerate ${L}_{1c}$ and ${X}_{1c}$ GaAs levels (important at alloy concentrations, $x\ensuremath{\sim}1%$ or $\ensuremath{\sim}{10}^{21} {\mathrm{cm}}^{\ensuremath{-}3}).$ We show how the interaction of ${a}_{1}(N),$ ${a}_{1}({\ensuremath{\Gamma}}_{1c}),$ ${a}_{1}{(L}_{1c}),$ and ${a}_{1}{(X}_{1c})$ provides a microscopic explanation for the origin of the experimentally observed anomalous alloy phenomena.

Linear combination of bulk bands method for large-scale electronic structure calculations on strained nanostructures

Abstract: A {open_quotes}strained linear combination of bulk bands{close_quotes} method is introduced for calculating the single-particle electronic states of strained, million-atom nanostructure systems, within an empirical pseudopotential Hamiltonian. This method expands the wave functions of a nanostructure (superlattice, wire, and dot) as linear combinations of bulk Bloch states of the constituent materials, over band indices {ital n} and wave vectors {ital k}. This allows one to use physical intuition in selecting the {ital n} and {ital k} that are most relevant for a given problem. This constitutes a useful approximation over the {open_quotes}direct diagonalization{close_quotes} approach where the basis is complete (individual plane waves) but unintuitive. It also constitutes a dramatic improvement upon the {bold k}{center_dot}{bold p} approach, where the continuum model Hamiltonian is used, losing the atomistic details of the system. For a pyramidal InAs quantum dot embedded in GaAs, we find electronic eigenenergies that are within 20 meV of the exact direct diagonalization calculation, while the speed of the current method is 100{endash}1000 times faster. The sublinear scaling of the current method with the size of the system enables one to calculate the atomistic electronic states of a million-atom system on a personal computer in about 10 h. Sufficient detail ismore » provided in the formalism, so that the method can be promptly implemented. {copyright} {ital 1999} {ital The American Physical Society}« less

Resonant hole localization and anomalous optical bowing in InGaN alloys

Abstract: Using large supercell empirical pseudopotential calculations, we show that alloying of GaN with In induces localization in the hole wave function, resonating within the valence band. This occurs even with perfectly homogeneous In distribution (i.e., no clustering). This unusual effect can explain simultaneously exciton localization and a large, composition-dependent band gap bowing coefficient in InGaN alloys. This is in contrast to conventional alloys such as InGaAs that show a small and nearly composition-independent bowing coefficient. We further predict that (i) the hole wave function localization dramatically affects the photoluminescence intensity in InGaN alloys and (ii) the optical properties of InGaN alloys depend strongly on the microscopic arrangement of In atoms.

InAs quantum dots: Predicted electronic structure of free-standing versus GaAs-embedded structures

Abstract: Using an atomistic pseudopotential approach, we have contrasted the (i) strain profiles, (ii) strain-modified band offsets, (iii) energies of confined electrons and holes, and (iv) wave functions and Coulomb interactions between electrons and holes for three types of InAs quantum dots: (a) a free-standing spherical dot, (b) a GaAs-embedded spherical dot, and (c) a GaAs-embedded pyramidal dot. A comparison of (a) and (b) reveals the effects of strain, while a comparison of (b) and (c) reveals the effects of shape. We find that the larger band offsets in the ``free-standing'' dots (i) produce greater quantum confinement of electrons and holes and (ii) act to confine the wave functions more strongly within the dot, resulting in larger electron-hole Coulomb energies. The lower symmetry of the pyramidal dot produces a richer strain profile than the spherical dots, which splits the degeneracy of the hole states and polarizes the emitted light.

Excitonic exchange splitting in bulk semiconductors

Abstract: We present an approach to calculate the excitonic fine-structure splittings due to electron-hole short-range exchange interactions using the local-density approximation pseudopotential method, and apply it to bulk semiconductors CdSe, InP, GaAs, and InAs. Comparing with previous theoretical results, the current calculated splittings agree well with experiments. Furthermore, we provide an approximate relationship between the short-range exchange splitting and the exciton Bohr radius, which can be used to estimate the exchange splitting for other materials. The current calculation indicates that a commonly used formula for exchange splitting in quantum dot is not valid. Finally, we find a very large pressure dependence of the exchange splitting: a factor of 4.5 increase as the lattice constant changes by 3.5%. This increase is mainly due to the decrease of the Bohr radius via the change of electron effective mass.

Predicted bond length variation in wurtzite and zinc-blende ingan and algan alloys

Abstract: Valence force field simulations utilizing large supercells are used to investigate the bond lengths in wurtzite and zinc-blende InxGa1−xN and AlxGa1−xN random alloys. We find that (i) while the first-neighbor cation–anion shell is split into two distinct values in both wurtzite and zinc-blende alloys (RGa−N1≠RIn−N1), the second-neighbor cation–anion bonds are equal (RGa−N2=RIn−N2). (ii) The second-neighbor cation–anion bonds exhibit a crucial difference between wurtzite and zinc-blende binary structures: in wurtzite we find two bond distances which differ in length by 13% while in the zinc-blende structure there is only one bond length. This splitting is preserved in the alloy, and acts as a fingerprint, distinguishing the wurtzite from the zinc-blende structure. (iii) The small splitting of the first-neighbor cation–anion bonds in the wurtzite structure due to nonideal c/a ratio is preserved in the alloy, but is obscured by the bond length broadening. (iv) The cation–cation bond lengths exhibit three dis...

Overcoming doping bottlenecks in semiconductors and wide-gap materials

Abstract: There often exist strong doping bottlenecks that may severely restrict potential applications of semiconductors, especially in wide-band-gap materials where bipolar doping is impossible. Recent rapid progress in semiconductor research has reached a point where these doping limitations must be overcome in order to tune semiconductors for precisely required properties. Here, we discuss how to find out what causes the doping bottlenecks. We based our discussion on a set of recent, novel developments regarding the doping limitations: the “doping limit rule” distilled from both phenomenological studies and from first-principles calculations. The thermodynamic doping bottlenecks are identified as due mainly to the formation of intrinsic defects whose formation enthalpies depend on the Fermi energy, and always act to negate the effect of doping.

Band structure and stability of zinc-blende-based semiconductor polytypes

Abstract: Using a first-principles generalized one-dimensional Ising model we have studied the band structure and stability of two types of zinc-blende-based polytype series: type-a GaInP{sub 2} and type-b CuInSe{sub 2}. The interaction parameters for the formation energy are found to be short range, while the convergence is slower for the band-gap and conduction-band energies of the type-a GaInP{sub 2} polytypes. We predict that the CuAu-like phase can coexist in nominally chalcopyrite CuInSe{sub 2} and CuInS{sub 2}, while such coexistence is less likely in CuGaSe{sub 2}. We also predict that type-II band alignment can exist between different ordered type-a GaInP{sub 2} polytypes, despite that the band alignment between ordered and disordered GaInP{sub 2} are predicted to be type I. {copyright} {ital 1999} {ital The American Physical Society}

Phonons in GaP quantum dots

Abstract: The phonon structure of GaP quantum dots is studied using an atomistic potential model. The dot eigenmodes are obtained from a direct diagonalization of the dynamical matrix and classified using an efficient dual-space analysis method. Our calculations provide a theoretical explanantion for several experimental observations. (1) Depending on the spatial localization, the phonon modes of dots are either dot-interior (bulklike) or surfacelike. (2) The frequencies of the dot-interior modes can be qualitatively described by the {open_quotes}truncated crystal method{close_quotes} using a single branch and a single wave vector of the bulk-phonon dispersion. In contrast, the surface modes cannot be described by this model. (3) The dot-interior modes have a dominant bulk parentage from a specific part of the Brillouin zone, while the surface modes do not. (4) The frequencies of the bulklike {Gamma}-derived longitudinal optical (LO) and transverse optical (TO) phonon modes are found to decrease with decreasing dot size. This decrease reflects the downward dispersion of the bulk optical-phonon branches away from the {Gamma} point. (5) The surface modes located between the bulk TO- and LO-phonon bands have a significant bulk {Gamma} character, and are thus Raman detectable. (6) The dot-interior modes exhibit only a slight LO/TO mode mixing, whilemore » the surfacelike modes show a strong mode mixing. {copyright} {ital 1999} {ital The American Physical Society}« less

Multiband coupling and electronic structure of ( InAs ) n / ( GaSb ) n superlattices

Abstract: The electronic structure of abrupt $(\mathrm{InAs}{)}_{n}/(\mathrm{GaSb}{)}_{n}$ superlattices is calculated using a plane wave pseudopotential method and the more approximate eight band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ method. The $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ parameters are extracted from the pseudopotential band structures of the zinc-blende constituents near the $\ensuremath{\Gamma}$ point. We find, in general, good agreement between pseudopotential results and $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ results, except as follows. (1) The eight band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ significantly underestimates the electron confinement energies for $nl~20.$ (2) While the pseudopotential calculation exhibits (a) a zone center electron-heavy hole coupling manifested by band anticrossing at $n=28,$ and (b) a light hole--heavy hole coupling and anticrossing around $n=13,$ these features are absent in the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model. (3) As $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ misses atomistic features, it does not distinguish the ${C}_{2v}$ symmetry of a superlattice with no-common-atom such as InAs/GaSb from the ${D}_{2d}$ symmetry of a superlattice that has a common atom, e.g., InAs/GaAs. Consequently, $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ lacks the strong in-plane polarization anisotropy of the interband transition evident in the pseudopotential calculation. Since the pseudopotential band gap is larger than the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ values, and most experimental band gaps are even smaller than the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ band gap, we conclude that to understand the experimental results one must consider physical mechanisms beyond what is included here (e.g., interdiffusing, rough interfaces, and internal electric fields), rather than readjust the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ parameters.

Theory of Systematic Absence of NaCl-Type ( β -Sn–Type) High Pressure Phases in Covalent (Ionic) Semiconductors

Abstract: Recent high pressure x-ray experiments show that, contrary to traditional expectations, the NaCl structure is not present in covalent semiconductors, and the diatomic $\ensuremath{\beta}\ensuremath{-}\mathrm{Sn}$ structure is absent in all compound semiconductors. We explain these systematic absences in terms of dynamical phonon instabilities of the NaCl and $\ensuremath{\beta}\ensuremath{-}\mathrm{Sn}$ crystal structures. Covalent materials in the NaCl structure become dynamically unstable with respect to the transverse acoustic TA[001] phonon, while ionic compounds in the $\ensuremath{\beta}\ensuremath{-}\mathrm{Sn}$ structure exhibit phonon instabilities in the longitudinal optical $\mathrm{LO}[00\ensuremath{\xi}]$ branch. The latter lead to predicted new high pressure phases of octet semiconductors.

Coherent phase stability in Al-Zn and Al-Cu fcc alloys: The role of the instability of fcc Zn

Abstract: The coherent phase stability of fcc-based Al-Zn and Al-Cu alloys is studied theoretically by first-principles total energy calculations, a mixed-space cluster expansion approach, and Monte Carlo thermodynamic simulations. We find that a large portion of the differences between Al-Zn and Al-Cu can be explained by the differences between fcc-Zn and fcc-Cu: While Zn is stable in the hcp structure, fcc-Zn shows an instability when deformed rhombohedrally along (111). In contrast, fcc-Cu is the stable form of Cu and is elastically extremely soft when deformed along (100). These elastically soft directions of the constituents permeate the phase stability of the alloys: (111) superlattices are the lowest energy coherent structures in Al-Zn, while (100) superlattices are stable coherent phases in Al-Cu. The short-range order of both Al-rich solid solutions show clustering tendencies, with the diffuse intensity due to short-range order in Al-Zn and Al-Cu showing streaks along (111) and (100), respectively. The mixing enthalpies and coherent phase boundaries are also calculated and found to be in good agreement with experimental data, where available.

Electronic consequences of lateral composition modulation in semiconductor alloys

Abstract: Lateral composition modulation (CM) is a periodic, position-dependent variation in alloy composition occurring in the substrate plane, perpendicular to the growth direction. It can be induced by growing size-mismatched short-period AC/BC superlattices (SL). Here we study the electronic structure induced by such lateral composition modulation in GaAs/InAs, GaP/InP, and AlP/GaP, in search of optical properties relative to the corresponding random alloys. We investigate in detail the properties of (a) pure CM without any SL, (b) pure SL without any CM, and (c) the combined CM+SL system. The systems are modeled by constructing a large supercell where the cation sublattice sites are randomly occupied in the lateral (vertical) direction according to the composition variation induced by CM (SL). The atomic structure and strain induced by CM and SL are explicitly taken into account using an {ital atomistic} force field. This approach is found to be crucial for an accurate description of the microscopic strain in CM+SL systems. The electronic structure is solved using specially constructed empirical pseudopotentials and plane-wave expansion of the wave functions. We find that (i) CM in GaAs/InAs and GaP/InP systems induces type-I band alignment (electrons and holes localized in the same spatial region), while CM in AlP/GaPmore » is shown as an example exhibiting type-II band alignment. (ii) CM and SL both induce significant contributions (which add up nearly linearly) to band-gap redshift with respect to random alloy. CM in GaP/InP is found to induce larger band-gap redshifts than in GaAs/InAs due to larger band offsets in the former system. (iii) The symmetry of electronic states at the valence band maximum is sensitively affected by CM: the lowest energy optical transitions exhibit strong polarization where transitions polarized perpendicular to the CM are favored, while transitions polarized parallel to the CM are surpressed by being shifted to higher energy. These observations, as well as the magnitude of the predicted band-gap redshift, agree with available experimental data, and suggest that control of composition modulation during growth might be used to tailor band gaps, carrier localization, and transition polarizations relative to random alloys. {copyright} {ital 1999} {ital The American Physical Society}« less

Anomalous Behavior of the Nitride Alloys

Abstract: This paper summarizes the way in which nitride alloys InGaN and GaAsN are anomalous. It predicts wave function localization states inside the band gap, unusual pressure dependence, effects of short-range order and percolation.

Excitonic transitions and exchange splitting in Si quantum dots

Abstract: In a quantum dot made of an indirect gap material such as Si, the electron–hole Coulomb interaction alone can give rise to “dark” excitons even in the absence of exchange interaction. We present the predicted excitonic spectra for hydrogen-passivated Si dots and find very good agreement with the recent experiment of Wolkin, Jorne, Fauchet, Allan, and Delerue [Phys. Rev. Lett. 82, 197 (1999)]. The calculated splitting between dark and bright excitons, arising from Coulomb and exchange interactions, agrees very well with the optical data of Calcott, Nash, Canham, Kane, and Brumhead [J. Phys Condens. Matter 5, L91 (1993)].

Electronic Structure of {open_quotes}Sequence Mutations{close_quotes} in Ordered GaInP{sub 2 } Alloys

Abstract: The electronic consequences of layer thickness fluctuations in CuPt-ordered GaInP{sub 2} (layer sequence Ga-In-Ga-Inthinspthinsp)thinspthinspare investigated We show that the formation of a {open_quotes}sequence mutated{close_quotes} Ga-In-In-Gathinspthinspthinspthinspregion creates a hole state h1 localized in the In-In double layer, while the electron state e1 is localized in the CuPt-ordered region Thus, the system exhibits electron-hole {ital charge separation} in addition to spatial localization This physical picture is preserved when the dimension of the mutated segment is reduced from 2D to 0D, resulting in disklike dot structures Our theory explains the long-standing puzzle of the origin of the peculiar luminescence properties of ordered GaInP{sub 2} {copyright} {ital 1999} {ital The American Physical Society}

Instability of the high-pressure cscl structure in most iii-v semiconductors

Abstract: Using the density-functional linear response method, we study dynamical instabilities of the high-pressure CsCl phase in III-V semiconductors. For InSb, we find no phonon instability that could prevent the CsCl phase from forming, consistent with the experimental observation. In contrast, for the more ionic GaP, GaAs, InP, and InAs, we find that, while statically stable, the CsCl phase is dynamically unstable at high pressures with respect to transverse-acoustic [{xi}{xi}0] phonons. Analysis of the soft normal modes via {open_quotes}isotropy subgroup{close_quotes} suggests two candidate structures that will replace CsCl structure at high pressure: the tP4 (B10) InBi type and the oP4 (B19) AuCd type. Experimental examination of these predictions is called for. {copyright} {ital 1999} {ital The American Physical Society}

Indirect band gaps in quantum dots made from direct-gap bulk materials

Abstract: The conditions under which the band gaps of free standing and embedded semiconductor quantum dots are direct or indirect are discussed. Semiconductor quantum dots are classified into three categories; (i) free standing dots, (ii) dots embedded in a direct gap matrix, and (iii) dots embedded in an indirect gap matrix. For each category, qualitative predictions are first discussed, followed by the results of both recent experiments and state of the art pseudopotential calculations. We show that: Free standing dots of InP, InAs, and CdSe will remain direct for all sizes, while dots made of GaAs and InSb will turn indirect below a critical size. Dots embedded within a direct gap matrix material will either stay direct (InAs/GaAs at zero pressure) or will become indirect at a critical size (InSb/InP). Dots embedded within an indirect gap matrix material will exhibit a transition to indirect gap for sufficiently small dots (GaAs/AlAs and InP/GaP quantum well) or will be always indirect (InP/GaP dots, InAs/GaAs above 43 k bar pressure and GeSi/Si dots).

P-P and As-As isovalent impurity pairs in GaN: Interaction of deep t 2 levels

Abstract: The electronic and atomic structure of substitutional {ital n}th neighbor (1{le}n{le}6) P-P and As-As impurity pairs in zinc blende GaN is investigated using self-consistent plane-wave pseudopotential and empirical pseudopotential methods. A single impurity introduces a deep t{sub 2} gap level; we show that the interaction between the t{sub 2} defect orbitals of the impurity pairs leads to an interesting pattern of single-particle level splitting, being largest for the first (n=1) and fourth (n=4) neighbor pairs, both exhibiting a C{sub 2v} symmetry. The total energy of the {ital n}th order pair {Delta}E{sup (n)} relative to isolated (n{r_arrow}{infinity}) impurities indicates pairing tendency for n=1 and n=2 ({Delta}E{sup (1,2)}{lt}0) while n=4 pairs are unstable ({Delta}E{sup (4)}{gt}0). We explain this behavior of {Delta}E{sup (n)} vs {ital n} as a consequence of the interplay between two effects: an {open_quotes}elastic contribution{close_quotes} representing the interaction between the elastic strain fields of the two impurities and an {open_quotes}electronic contribution{close_quotes} describing the interaction of the defect orbitals of the two impurity atoms. The binding energies of the impurity-pair bound excitons are calculated for the n=1 As-As and P-P pairs and are found to be significantly larger than for the corresponding isolated impurities. The probabilities for electronic transitions between themore » defect levels and conduction band are calculated. The results predict existence of a rich series of spectroscopic features distinct from single impurities. {copyright} {ital 1999} {ital The American Physical Society}« less

Comment on "Quantum Confinement and Optical Gaps in Si Nanocrystals"

Abstract: A Comment on the Letter by Serdar {umlt O}{breve g}{umlt u}t, James R. Chelikowsky, and Steven G. Louie, Phys.thinspthinspRev.thinspthinspLett.thinspthinsp{bold 79}, 1770 (1997). The authors of the Letter offer a Reply. {copyright} {ital 1999} {ital The American Physical Society}

Fitting of accurate interatomic pair potentials for bulk metallic alloys using unrelaxed LDA energies

Abstract: We present a general and simple method for obtaining accurate, local density approximation (LDA-) quality interatomic potentials for a large class of bulk metallic alloys. The method is based on our analysis of atomic relaxation, which reveals that the energy released in the relaxation process can be approximated by calculating the epitaxially constrained energies of the constituents {ital A} and {ital B}. Therefore, the pair potential is fitted to the LDA-calculated epitaxial energies of the constituents (to capture the relaxation energies), and to the unrelaxed energies of ordered A{sub n}B{sub m} compounds (to capture the fixed-lattice {open_quotes}chemical{close_quotes} energy). The usefulness of our approach is demonstrated by carrying out this procedure for the Cu{sub 1{minus}x}Au{sub x} alloy system. The resulting pair potential reproduces the relaxed LDA formation energies of ordered compounds rather accurately, even though we used only unrelaxed energies as input. We also predict phonon spectra of the elements and ordered compounds in very good agreement with the LDA results. From the calculations for {approx}10000 atom supercells representing the random alloy, we obtain the bond lengths and relaxation energies of the random phase that are not accessible to direct LDA calculations. We predict that, while in Cu-rich alloys the Cu-Cumore » bond is shorter than the Cu-Au bond, at higher Au compositions this order is switched. Furthermore, we find that Au-rich Cu{sub 1{minus}x}Au{sub x} alloys have ground states that correspond to (001) superlattices of {ital n} monolayers of fcc Au stacked on {ital m} monolayers of the L1{sub 0} CuAu-I structure. The potential developed in this work is available at the site http://www.sst.nrel.gov/data/download.html for interested users. {copyright} {ital 1999} {ital The American Physical Society}« less

Magnetic destabilization of Ni7Al

Abstract: Previously unknown cubic ordered Ni{sub 7}Al and Cu{sub 7}Pt compounds have recently been theoretically predicted to be stable phases in the Ni-Al and Cu-Pt systems. While Cu{sub 7}Pt was subsequently synthesized and identified, Ni{sub 7}Al remains experimentally unobserved. Using first-principles total energy calculations, we reinvestigate the stability of this Ni{sub 7}Al compound. We find the stability of this compound to be qualitatively effected by spin polarization, ignored in previous calculations. The effect of ferromagnetism is to stabilize the two-phase mixture of Ni+Ni{sub 3}Al relative to the Ni{sub 7}Al compound such that the latter is stable in nonmagnetic calculations, but unstable when spin polarization is taken into account. This reversal of relative stabilities of Ni{sub 7}Al and Ni+Ni{sub 3}Al with magnetism also has a dramatic effect on the calculated Ni{sub 3}Al/Ni interfacial energy {sigma} and spin-polarized calculations lead to a positive value of {sigma}, which is in qualitative agreement with values obtained from precipitation experiments. {copyright} {ital 1999} {ital The American Physical Society}

Dark excitons due to direct Coulomb interactions in silicon quantum dots

Abstract: Electron-hole exchange interactions can lead to spin-forbidden ‘‘dark’’ excitons in direct-gap quantum dots. Here, we explore an alternative mechanism for creating optically forbidden excitons. In a large spherical quantum dot made of a diamond-structure semiconductor, the symmetry of the valence band maximum ~VBM! is t 2. The symmetry of the conduction band minimum ~CBM! in direct-gap material is a1, but for indirect-gap systems the symmetry could be ~depending on size! a1 , e ,o rt 2. In the latter cases, the resulting manifold of excitonic states contains several symmetries derived from the symmetries of the VBM and CBM ~e.g., t 2 3t 25A11E1T11T2 or t 23e5T11T2). Only the T2 exciton is optically active or ‘‘bright,’’ while the others A1 , E, and T1 are ‘‘dark.’’ The question is which is lower in energy, the dark or bright. Using pseudopotential calculations of the single-particle states of Si quantum dots and a direct evaluation of the screened electron-hole Coulomb interaction, we find that, when the CBM symmetry ist 2 , the direct electronhole Coulomb interaction lowers the energy of the dark excitons relative to the bright T2 exciton. Thus, the lowest energy exciton is forbidden, even without an electron-hole exchange interaction. We find that our dark-bright excitonic splitting agrees well with experimental data of Calcott et al., Kovalev et al., and Brongersma et al. Our excitonic transition energies agree well with the recent experiment of Wolkin et al. In addition, and contradicting simplified models, we find that Coulomb correlations are more important for small dots than for intermediate sized ones. We describe the full excitonic spectrum of Si quantum dots by using a many-body expansion that includes both Coulomb and exchange electron hole terms. We present the predicted excitonic spectra.

Band structure and stability of ordered zinc-blende-based semiconductor polytypes

Abstract: Using a first-principles generalized one-dimensional Ising model we have studied the band structure and stability of two types of zinc-blende-based polytype series: type-a GaInP2 and type-b CuInSe2. The interaction parameters for the formation energy are found to be short range, while the convergence is slower for the band-gap and conduction-band energies of the type-a GaInP2 polytypes. We predict that the CuAu-like phase can coexist in nominally chalcopyrite CuInSe2 and CuInS2, while such coexistence is less likely in CuGaSe2. We also predict that type-II band alignment can exist between different ordered type-a GaInP2 polytypes, despite that the band alignment between ordered and disordered GaInP2 are predicted to be type I. @S0163-1829~99!51804-3#