Optical and electronic properties of (GaAs)n / (InAs)n(001) superlattices: LMTO-ASA approach

The optical and electronic properties of (GaAs)_n/(InAs)_n superlattices are calculated by means of LMTO-ASA method. The too small band gap problem of bulk material and superlattices is corrected by adding to the effective potentials an additional external potential that is sharply peaked at the atomic sites. The results show that the optical properties of GaAs/InAs(001) superlattices are about average of that of two bulks of GaAs and InAs.     

FTIR-spectroscopy of (GaAS)n/(AlAs)m superlattices

The optical properties of (GaAs)_n/(AlAs)_m superlattices in the infra-red spectral region have been studied. The confinement of optical phonons has been observed in both GaAs and AlAs layers of superlattices under investigation. The superlattice modes caused by the coupling between LO phonons and collective intersubband excitations have been found in doped superlattices. Macroscopic and microscopic calculations have been used for the analysis of experimental results. Good agreement with experiment has been obtained.     

Electronic structure of (BP)n/(BAs)n (0 0 1) superlattices

An accurate ab initio full potential linear muffin-tin orbital method has been used to investigate the structural, electronic and optical properties of BP, BAs and their (BP)_n/(BAs)_n superlattices (SLs). The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW). The calculated structural properties of BP and BAs compounds are in good agreement with available experimental and theoretical data. It is found that BP, BAs and their alloys exhibit an indirect fundamental band gap. The fundamental band gap decreases with increasing the number of monolayer n. The optical properties show that the static dielectric constant significantly decreases in superlattices compared to their binary compounds.     

(Si)n/(Ge)n超晶格几何结构

本文用Keating模型计算了Si_1-xGe_x(x=0—1)作衬底、沿(100)方向生长的(Si)_n/(Ge)_n(n=1—6)应力超晶格的几何结构,并讨论了衬底对超晶格生长的影响,计算结果发现对于(Si)_n/(Ge)_n超晶格,用适当的Si_1-xGe_x作衬底有利于超晶格的生长。 关键词：     

Quantum chemical studies on (ZnO)n/(NiO)n heterostructured nanoclusters

The structural stability and electronic properties of (ZnO)_n, (NiO)_n, (ZnO)_n/(NiO)_n for n = (1 to 4) and 3D structures were studied using density functional theory. The geometrical optimisation of clusters implies that when the atoms in the cluster increase it leads to an increase in its stability. The stability drastically increases for the heterostructure of (ZnO)_n/(NiO)_n. The dipole moment of the clusters depends on the geometry of the cluster and it is found to be minimum for heterostructures representing more neutralised clusters. HOMO-LUMO energies, ionisation potential, electron affinity, chemical hardness, binding energies and vibrational analysis of different clusters are calculated and reported. The adsorption of CO on the different sites of nanoclusters are studied and discussed.     

应变超晶格(ZnS)1/(ZnSe)1的光学性质

用linear mufin tin orbital能带方法，计算Si衬底上（ZnS）_n/（ZnSe）_n（001）超晶格的能带结构，计算中采用外加调整势进行带隙修正．在得到较准确的能带结构和波函数的基础上，计算该超晶格系统的光学介电函数虚部ε₂（ω）．结果表明，该超晶格系统的光学性质结合了ZnS和ZnSe体材料光学性质的特点，在相当宽的能量范围内有较好的光谱响应．并且该超晶格系统是直接带隙材料，在光电器件应用中有很大潜力． 关键词：     

Cyclotron masses in InGaAs/GaAs superlattices and InGaAs/AlAs superlattices

Cyclotron resonance (CR) measurements have been carried out to evaluate the effective mass of electron in (InGaAs)_n/(GaAs)_nsuperlattices (SLs) and (InGaAs)_n/(AlAs)_nSLs. To clarify the dependence of cyclotron mass on the monolayer numbern , we measured CR signals using pulsed high-magnetic fields up to 150 T and a far-infrared laser. We found clear cyclotron resonances in the transmission of 10.6 μ m at 75 T at room temperature in (InGaAs)_n/(GaAs)_nSLs and little dependence on the monolayer number n in the SLs. However, for (InGaAs)_n/(AlAs)_nSLs, a large dependence of cyclotron mass on the monolayer number n was observed. We consider that these dependencies are related to the difference between the barrier height in the SLs and the influence of nonparabolicity on the conduction subbands in the SLs.     

Period dependence of coherent oscillations in manganite superlattices

In three-component superlattices of [(NdMnO3)_n/(SrMnO3)_n/(LaMnO3)_n]_m consisting of nonferroelectric and antiferromagnetic layers, the generation of coherent phonons in the system is investigated by the relaxation process of the superlattice using a pump–probe technique. The coherent phonons are related to MnO₆ octahedra in the layers. While the frequency of the phonons is 49 GHz without regard to the period of each layer, the amplitude of the phonons is influenced by the period of the superlattices. The superlattices have the maximum amplitude of the acoustic phonons at a period of (5, 12) together with strong emergent interfacial effects such as ferromagnetism and ferroelectricity, which are not present in bulk materials. This implies a possible correlation of the lattice structure with interface-induced properties.     

Electronic structure of strained Sin/Gen(001) superlattices

Using the empirical tight binding method we have investigated the electronic properties of the Si_n/Ge_n(001) strained superlattices as a function of the superlattice periodicity and the band misfit. For n ≥ 4 we have found that first and second conduction band states are localized in Si. The hole states localized in Ge appear for n ≥ 4. The difference between the direct and indirect band gaps is reduced from 2.01 eV for bulk Si to 0.01 eV for n=6 which can be considered to be quasi-direct. For the cases n=6 and n=8, the band gap might become direct for large values of band misfit.     

The spin effect in zinc-blende CdEuS and CdEuSe: GGA and GGA+U studies

We have investigated the structural, electronic and magnetic properties of substitutional europium rare earth impurity in cubic CdS and CdSe by employing the ab-initio method. Calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the framework of spin-polarized density functional theory (DFT). The electronic exchange-correlation energy is described by generalized gradient approximation GGA and GGA+U (U is the Hubbard correction). The GGA+U method is applied to the rare-earth 4f states. We have calculated the lattice parameters, bulk modulii, the first pressure derivatives of the bulk modulii and the cohesive energies. The calculated densities of states presented in this study identify the metallic behavior of CdEuS and CdEuSe when we use the GGA scheme, whereas when we use the GGA+U, we see that these compounds are half-metallic.     

X-ray diffraction investigation of BaTiO3/(Ba,Sr)TiO3 superlattices

Samples of BaTiO₃/(Ba_{1 ? x} Sr _x )TiO₃ (BT/BST-x) superlattices have been studied using X-ray diffraction analysis. A complete parallel orientation of all the studied films and the substrate has been revealed, and the modulation periods ?? of the superlattices, the parameters of the unit cell averaged over the ?? period, and the parameters of individual components of the superlattices have been determined.     

Electronic structure and conduction properties of copolymers of silole based donor-acceptor polymers: effect of multi-neighbour interaction

Using the ab initio band structure results of two novel silole based donor-acceptor polymers PSICF (A)_x and PSICN (B)_x, the electronic structures and conduction properties of their various quasi-one-dimensional superlattices (copolymers) (A_mB_n)_x, belonging to the class of type-II staggered superlattices, have been investigated using negative factor counting method taking into account multi-neighbour interaction. Both PSICF and PSICN consist of a bicyclopentadisilole unit bridged by an electron-accepting group Y (Y=CCF₂ in PSICF and Y=CC(CN)₂ in PSICN). The trends in the electronic structures and conduction properties of these copolymers (A_mB_n)_x as a function of the (i) block sizes m and n; (ii) composition (m/n) and (iii) arrangement of blocks (periodic or aperiodic) in the copolymer chain are discussed. The results obtained are important guidelines for designing copolymers with tailor-made conduction properties.     

Structural and electronic properties of GaAsBi

The structural and electronic properties of the GaAs_1−xBi_x ternary alloy are investigated by means of two first principles and full potential methods, the linear augmented plane waves (FPLAPW) method and a recent version of the full potential linear muffin-tin orbitals method (FPLMTO) which enables an accurate treatment of the interstitial regions. In particular, we have found that the maximal GaBi mole fraction x for which GaBi_xAs_1−x remains a semiconductor is probably around x=0.5. The electronic properties of (GaAs)_m/(GaBi)_n quantum well superlattices (SLs) have also been calculated and it is found that such SLs are semiconductors when m is larger or equal to n.     

形变超晶格Si/Ge的能带结构

用经验的紧束缚方法对短周期的(Si)_n/(Ge)_m形变超晶格的电子态进行了计算。结果表明,由于布里渊区折迭的要求,只有当n+m=10时超晶格才可能产生直接能隙。对周期为n+m=10的超晶格,Γ,N,△处的导带谷间的相对位置对直接能隙的形成具有决定作用,而n的大小与衬底的组分对此有极大影响。(Si)₆/(Ge)₄和(Si)₈/(Ge)₂超晶格在Si_1-xG 关键词：     

Ground state energy of the electron-hole liquid in type-II (GaAs)m/(AlAs)m quantum wells

The ground state energy of quasi-two-dimensional electron-hole liquid (EHL) at zero temperature is calculated for type-II (GaAs)_m/(AlAs)_m (5≤m≤10) quantum wells (QWs). The correlation effects of Coulomb interaction are taken into account by a random phase approximation of Hubbard. Our EHL ground state energy per electron-hole pair is lower than the exciton energy calculated recently for superlattices, so we expected that EHL is more stable state than excitons at high excitation density. It is also demonstrated that the equilibrium density of EHL in type-II GaAs/AlAs QWs is of one order of magnitude larger than that in type-I GaAs/AlAs QWs.     

Zone edge phonons in CdS1−xSex

Zone edge phonons of mixed CdS_1?xSe_x have been studied by mean of infrared absorption and Raman scattering techniques. In the A point of the Brillouin zone, it has been shown that transverse acoustical phonons have a one mode behaviour, and that optical phonons have a two modes behaviour. CdS and CdSe zone center phonons can combined and give a LO(CdS) + LO(CdSe) Raman peak in addition to the 2LO(CdS) and 2LO(CdSe) peaks; this is not the case for phonons from the edge of the Brillouin-zone where no CdS + CdSe combination can take place.     

Atomic structure of low-index and (11n) surfaces in ordered Cu3Au

By computing the surface energy at T = 0 K, we test the relative stability of various among possible atomic configurations of low-index and (11n) surfaces in the L1₂ compound Cu₃Au. The computations rely on two n-body phenomenological potentials and are performed using a classical energy minimization scheme. The energy of (100) and (110) surfaces is at a minimum for a mixed composition terminal layer, in agreement with available experimental results and previous calculations. For (113) and (115) surfaces the minimum energy is obtained when all (100) terraces are mixed. In this case, the height and width of steps and terraces are twice as large as those of a standard surface configuration.     

Raman scattering studies of (GaP)n/(InP)n (n = 1, 1.7, 2) short‐period superlattice structures

We report Raman scattering from (GaP)_n/(InP)_n (n = 1, 1.7, 2) short‐period superlattice (SPS) structures to study the effect of lateral composition modulation (LCM) on the behavior of optical phonons. Cross‐sectional transmission electron microscope images revealed that LCM was formed with complex pattern in the n = 1.7 and n = 2 samples grown at 490 °C. Interestingly, both the InP‐ and the GaP‐like longitudinal optical (LO) phonon energies increased systematically as the number of monolayers was increased from n = 1 to n = 2. A significant broadening of the phonon line shapes was also observed for the n = 1.7 and n = 2 samples. In contrast, for samples grown at 425 °C, both the increase of the LO phonon energies and the broadening of the phonon line shapes were observed only when n = 1.7. Our results demonstrate that the optical phonons in the (GaP)_n/(InP)_n SPS structures are significantly affected in the occurrence of LCM related to the growth temperature and the number of monolayers.Copyright © 2009 John Wiley & Sons, Ltd.     

Two-step annealed CdS/CdSe co-sensitizers for quantum dot-sensitized solar cells

CdS/CdSe co-sensitizers on TiO₂ films were annealed using a two-step procedure; high temperature (300 °C) annealing of TiO₂/CdS quantum dots (QDs), followed by low temperature (150 °C) annealing after the deposition of CdSe QDs on the TiO₂/CdS. For comparison, two types of films were prepared; CdS/CdSe-assembled TiO₂ films conventionally annealed at a single temperature (150 or 300 °C) and non-annealed films. The 300 °C-annealed TiO₂/CdS/CdSe showed severe coalescence of CdSe QDs, leading to the blocked pores and hindered ion transport. The QD-sensitized solar cell (QD-SSC) with the 150 °C-annealed TiO₂/CdS/CdSe exhibited better overall energy conversion efficiency than that with the non-annealed TiO₂/CdS/CdSe because the CdSe QDs annealed at a suitable temperature (150 °C) provided better light absorption over long wavelengths without the hindered ion transport. The QD-SSC using the two-step annealed TiO₂/CdS/CdSe increased the cell efficiency further, compared to the QD-SSC with the 150 °C-annealed TiO₂/CdS/CdSe. This is because the 300 °C-annealed, highly crystalline CdS in the two-step annealed TiO₂/CdS/CdSe improved electron transport through CdS, leading to a significantly hindered recombination rate.