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[en] Valence force field simulations utilizing large supercells are used to investigate the bond lengths in wurtzite and zinc-blende In_xGa_1-xN and Al_xGa_1-xN random alloys. We find that (i) while the first-neighbor cation endash anion shell is split into two distinct values in both wurtzite and zinc-blende alloys (R_Ga-N1≠R_In-N1), the second-neighbor cation endash anion bonds are equal (R_Ga-N2=R_In-N2). (ii) The second-neighbor cation endash 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 endash 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 endash cation bond lengths exhibit three distinct values in the alloy (Ga endash Ga, Ga endash In, and In endash In), while the anion endash anion bonds are split into two values corresponding to N endash Ga endash N and N endash In endash N. (v) The cation endash related splitting of the bonds and alloy broadening are considerably larger in InGaN alloy than in AlGaN alloy due to larger mismatch between the binary compounds. (vi) The calculated first-neighbor cation endash anion and cation endash cation bond lengths in In_xGa_1-xN alloy are in good agreement with the available experimental data. The remaining bond lengths are provided as predictions. In particular, the predicted splitting for the second-neighbor cation endash anion bonds in the wurtzite structure awaits experimental testing. copyright 1999 American Institute of Physics

[en] The electronic consequences of layer thickness fluctuations in CuPt-ordered GaInP₂ (layer sequence Ga-In-Ga-In...)are investigated. We show that the formation of a sequence mutated Ga-In-In-Ga...region 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 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 GaInP₂ . copyright 1999 The American Physical Society

[en] 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 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/GaP 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 1999 The American Physical Society

[en] It has been recently shown that growth of [001]-oriented short period (AC)_n/(BC)_n vertical superlattices (n∼1-2) spontaneously creates a lateral composition modulation in the substrate plane ([110] direction), where wire-like AC-rich and BC-rich domains alternate with a period of ∼100-200 Angstrom. This creates a new type of lattice structure with orthogonal [001] and [110] strain fields and compositional waves. Using a three-dimensional plane-wave pseudopotential approach, we study here the electronic properties of this type of structure in a GaInAs alloy, predicting valence band splittings into distinctly polarized components, a ≤100 meV band-gap reduction and strong, type I electron and hole confinement in the In-rich lateral channels. copyright 1998 American Institute of Physics

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No abstract available

[en] We use molecular dynamics simulations and ab initiocalculations to study the structures and formation probabilities of isolated surface defects produced by ion irradiation of (1000) graphite. We improve the conventionally used Tersoff potential [J. Tersoff, Phys. Rev. Lett. 61, 2879 (1988)] to realistically describe interlayer forces in graphite and high-energy processes in carbon. We identify three defect structures which correspond to experimentally observed hillocks on graphite surfaces, and examine their formation at different implantation energies. copyright 1996 The American Physical Society

[en] The electronic structure of abrupt (InAs)_n/(GaSb)_n superlattices is calculated using a plane wave pseudopotential method and the more approximate eight band k·p method. The k·p parameters are extracted from the pseudopotential band structures of the zinc-blende constituents near the Γ point. We find, in general, good agreement between pseudopotential results and k·p results, except as follows. (1) The eight band k·p significantly underestimates the electron confinement energies for n≤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 k·p model. (3) As k·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, k·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 k·p values, and most experimental band gaps are even smaller than the k·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 k·p parameters. copyright 1999 The American Physical Society

[en] 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. copyright 1999 American Institute of Physics