Ultrathin Superlattices Research Articles

Space group and phonon symmetry of (RE1Ba 2 Cu 3 O 7) m (RE2Ba 2 Cu 3 O 7) n superlattices

It is shown by group theory methods that the symmetry of (RE1Ba 2 Cu 3 O 7) m (RE2Ba 2 Cu 3 O 7) n superlattices grown along [1 0 0], [0 1 0] and [0 0 1] crystallographic directions is described by the same space group Pmmm ( D 2 h 1) as that of the initial bulk crystalline structure. The arrangement of atoms over Wyckoff positions in multiplied primitive cells of the superlattices is obtained for each of growth directions and all ( m, n) combinations. Symmetry classification of phonons at k = 0 is also carried out. Some features of phonon spectra of ultrathin superlattices are briefly discussed.

Optical characterization of coherently strained short-period superlattice (InAs) n(AlAs) n grown by molecular beam epitaxy

Ultra-thin superlattices (UTSL's) (InAs) n (AlAs) n for n = 1,2,3 and 4 have been grown by molecular beam epitaxy in order to obtain the artificial construction of a perfect layered alloy. The optimum MBE growth condition of InAs and AlAs monolayers was obtained through the calibration of RHEED intensity oscillations. It was found that a strong oscillation of the specular beam intensity during the growth of InAs can be succesfully obtained only in the narrow temperature region between 450 and 480°C. The structural property of the UTSL's was investigated by X-ray diffraction, complemented by Raman scattering measurement. Coherently strained superlattices were kept up to n = 3, while in the case of (InAs) 4(AlAs) 4 UTSL superlattices are rather intermixed due to the large excess strain energy. Unlike the (AlAs) n (GaAs) n the photoluminesne peak energy reaches maximum when n = 2, at the energy of about 50 meV below the fundamental gap of In 0.5Al 0.5As. This is the first experimental observation that gives a corresponding result with the tight-binding calculation on the subband of the strained (InAs) n (AlAs) n UTSL's.

Superconducting Multilayer Structures Grown by All-Mbe

ABSTRACTThin films of various Bi-Sr-Ca-Cu-O (2201, 2212,…, 2–2–11–12) compounds have been synthesized using atomic-layer-by-layer molecular beam epitaxy (ALL-MBE). Single-crystal films with excellent transport properties, smooth surfaces and atomically sharp interfaces in multilayer structures have been obtained. That made it possible to deposit virtually perfect ultrathin layers and superlattices, and fabricate hysteretic Josephson junctions from SIS multilayers.

Cross-sectional transmission electron microscopy (TEM) observation of ultrathin YBa 2Cu 2O 7 films and YBa 2Cu 3O 7/PrBa 2Cu 3O 7 superlattices

We have examined the microstructures of ultrathin YBa 2Cu 3O 7(YBCO) films and YBCO/PrBa 2Cu 3O 7(PrBCO) superlattices by cross-sectional transmission electron microscopic(TEM) observation. The ultrathin films and superlattices were prepared by reactive evaporation method with RHEED oscillations monitoring. The cross-sectional TEM observation provided informations relevant to the growth mechanism of 1:2:3 compounds as well as the epitaxial relations between the films and substrates. Sharp interfaces between the 123 compound and substrate were confirmed.

Ultrathin films and superlattices of high Tc superconductors controlled in the unit-cell scale using RHEED oscillations

Isolated ultrathin films of YBa 2Cu 3O 7−x (YBCO) on PrBa 2Cu 3O 7−x (PrBCO) and YBCO/PrBCO superlattices were epitaxially grown on SrTiO 3 by reactive coevaporation with measurement of reflection high energy electron diffraction (RHEED) specular intensity. X-ray diffraction profiles and cross sectional electron microscopic (TEM) observation reveal that the heterostructures are excellent crystallinity with no interface disorder and fit for the ptudy of superconductivity of the 1 unit-cell layer of YBCO. The 1 unit-cell layer of YBCO sandwiched between PrBCO layers exhibits a superconducting transition with the onset temperature at 70K and the zero resistance at 30K. A reduction of the onset temperature is found to be mainly due to a decrease of the hole carriers but not due to the absence of interlayer coupling. It was demonstrated that the 1 unit-cell layer of YBCO needed the overlayer of PrBCO to give rise to superconductivity. The overlayer of PrBCO may have the role of supply of carriers for the underlying YBCO layer. The resistive transition for the ultrathin YBCO layer below 4 unit-cells thick was independent of the magnetic field up to 10T applied parallel to the CuO 2 layer.

Chemical shift and zone-folding effects on the energy gaps of GaAs-AlAs (001) superlattices.

The chemical shift and zone-folding effects obtained from quasiparticle calculations for ultrathin GaAs-AlAs superlattices are incorporated within a Kronig-Penny model for superlattices of the arbitrary lattice period. We determine that superlattices with lattice periods in the range of 3\ifmmode\times\else\texttimes\fi{}3 to 9\ifmmode\times\else\texttimes\fi{}9 have an X-derived pseudodirect gap. This result explains both the results from first-principles calculations for ultrathin superlattices and those from experiments for a broader lattice period.

Low lying conduction states in (GaAs) n(AlAs) n superlattices

Using the self-consistent pseudopotential method and the effective mass theory we have studied the energy dispersion and confinement of low lying conduction band electronic states in (GaAs) n(AlAs) n superlattices along the [001] and [110] directions of growth. It is found that the nature of the lowest conduction band in the ultrathin superlattices is quite different for the two directions of growth. For the (001) superlattices the lowest conduction band is L-like for n=1, X z-like for 2⩽n⩽12, and Γ-like for n>12. For the (110) superlattices the lowest conduction band is L-like for n=1, 3 and 5, Δ-like for n=2, 4 and ⩾6, and Γ-like for n>15.

Superlattices of group IV elements, a new possibility to produce direct band gap material

The diamond lattice type group IV semiconductors C, SiC, Si and Ge exhibit an indirect band gap with the conduction band minimum outside the Brillouin zone center. Ultrathin superlattices are predicted to convert the indirect band gap into a quasi-direct one under certain circumstances.Growth of Si/Ge strained monolayer superlattices (SMS) by molecular beam epitaxy is explained, and experimental results obtained with these structures are discussed.

Direct Imaging of Ordering in Si-Ge Alloys, Ultrathin Superlattices, and Buried Ge Layers

ABSTRACTWe review recent Z-contrast imaging studies of Si-Ge ultrathin superlattices, alloys, and buried Ge layers. It is found that whenever Si is deposited onto a Ge (2 × 1) surface, Ge is pumped into the growing Si layer, and this is accompanied by interfacial ordering. This is explained by a novel Ge atom pump mechanism which occurs during MBE growth. Codeposition and alloy growth results in long range (111) ordering as a consequence of lateral segregation during nonequilibrium growth.

Using Ultrathin-Film, Modulated Composites to Control the Reaction Mechanism of Ternary Compound Formation

ABSTRACTA predictable, controllable approach to the synthesis of ternary compounds through known intermediates is presented. Thin and ultrathin film superlattices were made in the Mo-Se, Cu-Se and Mo-Cu systems. Differential scanning calorimetry, low- and high-angle x-ray diffraction were used to assess the interdiffusion and nucleation reactions between elemental layers in these one-dimensional crystals. The experimental parameter modulation distance was used to influence the interfacial reactions. The results from each binary system were then used to predict the reaction pathway in the synthesis of a ternary compound, Cu2Mo6Se8. Superlattices with two different lengthscales were investigated. In the first, only one intermediate, MoSe2 which typically crystallizes at ∼200'C, is observed prior to the crystallization of Cu2Mo6Se8. In the second, no crystalline intermediates are observed below 6000 C.

Quasiparticle band offset at the (001) interface and band gaps in ultrathin superlattices of GaAs-AlAs heterojunctions.

A newly developed first-principles quasiparticle theory is used to calculate the band offset at the (001) interface and band gaps in 1\ifmmode\times\else\texttimes\fi{}1 and 2\ifmmode\times\else\texttimes\fi{}2 superlattices of GaAs-AlAs heterojunctions. We find a sizable many-body contribution to the valence-band offset which is dominated by the many-body corrections to bulk GaAs and AlAs quasiparticle energies. The resultant offset \ensuremath{\Delta}${\mathit{E}}_{\mathit{v}}$=0.53\ifmmode\pm\else\textpm\fi{}0.05 eV is in good agreement with the recent experimental values of 0.50--0.56 eV. Our calculated direct band gaps for ultrathin superlattices are also in good agreement with experiment. The ${\mathit{X}}_{1\mathit{c}}$-derived state at point \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}, is however, above the ${\mathrm{\ensuremath{\Gamma}}}_{1\mathit{c}}$-derived state for both the 1\ifmmode\times\else\texttimes\fi{}1 and 2\ifmmode\times\else\texttimes\fi{}2 lattices, contrary to results obtained under the virtual-crystal approximation (a limiting case for the Kronig-Penny model) and some previous local-density-approximation (corrected) calculations. The differences are explained in terms of atomic-scale localizations and many-body effects. Oscillator strengths and the effects of disorder on the spectra are discussed.

Raman investigation of strong quasi-direct optical transitions in ultrathin GaAsAIAs superlattices

We investigate optical transitions in ultrathin (GaAs) n (AlAs) n superlattices using resonance Raman scattering. For n = 1, 2, we find that superlattice-induced or “quasidirect” transitions have an oscillator strength comparable to bulk-like direct transitions. Theory, on the other hand, predicts the quasidirect transitions to be orders of magnitude weaker. Our results suggest that the discrepancy is due to Γ-X mixing in the conduction band of the superlattices. For bulk-like transitions, we find that the thickness dependence of energy and wavefunction localization disagrees with predictions based on effective-mass theory.

Ultrathin layers and short-period superlattices in semiconductor structures

During the last decade the application of quantum-size-effect-heterostructures to solid state micro- and optoelectronics has attracted great attention, because it has made possible to improve drastically the parameters of most known devices and to invent new ones. Results are given of the investigation of the photoluminescence (PL), electroluminescence (EL), and galvanomagnetic properties of MBE-grown AIIIBV heterostructures with different sequences of ultrathin layers and short-period superlattices. The influence of introduction of such sequences on device structure characteristics is discussed. [Russian Text Ignored.]

Interpretation of Raman spectra of Ge/Si ultrathin superlattices.

We present a study of Raman scattering by phonons in ${\mathrm{Ge}}_{\mathit{m}}$${\mathrm{Si}}_{\mathit{n}}$-type ultrathin superlattices. We calculate the Raman spectra of ideal unstrained structures and then successively include strain and interface smudging. The calculations reveal interesting systematics about quasiconfinement. It is shown that quasiconfinement, strain, and interface smudging have to be treated concurrently in the interpretation of observed spectra. The nature of the ``Ge-Si''-like mode often seen in experimental spectra is elucidated. Using the information base built up from the study of theoretical spectra, we analyze experimental spectra of superlattices grown on Si(100) and Ge(100) substrates to obtain quantitative information useful for structural characterization of the samples studied.

Interface geometry of (GaAs)n(InAs)n and (GaP)n(InP)n ultra-thin superlattices

We report results of a self-consistent pseudopotential study within the local density formalism of the ultra-thin (GaAs)1(InAs)1 and (GaP)1(InP)1 superlattices. The lattice mismatch effect is observed in the electronic structure and the equilibrium interface geometry is analysed by using the total energy and force methods.

PHASE-LOCKED EPITAXY OF Ge/Si SUPERLATTICES BY USING RHEED INTENSITY OSCILLATIONS

Reflection High Energy Electron Diffraction (RHEED) intensity oscillations during the growth of alternating Ge and Si layers were observed, and the growth behavior during molecular beam epitaxy of Ge/Si ultra-thin multi-layered structures on Si(l00) and Si(lll) substrates has been studied. By using RHEED intensity oscillation, we have fabricated the Ge(2ML)/Si(2ML) and Ge(4ML)/Si(4ML) ultra-thin superlattices by phase-locking method.

High-resolution z-contrast imaging of semiconductor interfaces

Using a high-angle annular detector on a high-resolution STEM it is possible to form incoherent images of a crystal lattice characterized by strong atomic number or Z contrast. Figure 1 shows an epitaxial Ge film on Si(100) grown by oxidation of Ge-implanted Si. The image was obtained using a VG Microscopes' HB501 STEM equipped with an ultrahigh resolution polepiece (Cs ∽1.2 mm, demonstrated probe FWHM intensity ∽0.22 nm). In both crystals the lattice is resolved but that of Ge shows much brighter allowing the interface to be located exactly and interface steps to be resolved (arrowed). The interface was indistinguishable in the phase-contrast STEM image from the same region, and even at higher resolution the location of the interface is complex. Figure 2 shows a thin region of an MBE-grown ultrathin super-lattice (Si8Ge2)100. The expected compositional modulation would show as one bright row of dots from the 2 Ge monolayers separated by 4 rows of lighter Si columns. The image shows clearly that strain-induced interdiffusion has occurred on the monolayer scale.

Studies of novel magnetic systems by electron capture spectroscopy (ECS)

The electron spin polarization (ESP) at surfaces of bulk and artificially structured, thin film magnetic materials is studied using electron capture spectroscopy (ECS) and ion-induced emission of spin-polarized electron. For V and Tb surfaces, it is found that the temperature dependence of the surface magetic order is quite different from that of the bulk. For ultra-thin magnetic superlattices, V(100)/Ag(100) and Tb (0001)/W(110), we find novel magnetic surface anisotropies and critical behavior. At surfaces of 5 monolayers V(100) on Ag(100), the critical behavior of the surface-ESP is found to be identical to that of the two-dimensional Ising ferromagnet. At surfaces of PtMnSb and Fe80B20, we findnonzero ESP due to long-ranged or short-ranged ferromagnetic order. Thesign of the ESP at surfaces of the Heusler alloy PtMnSb is in agreement with recent band structure calculations. At surfaces of the spin glass Fe80B20, we find, in agreement with ECS data, that the ESP ofion-induced emitted spin-polarized electrons is extremely sensitive to the cleanness of thetopmost surface layer.

Raman study of order and disorder in SiGe ultrathin superlattices

We present a Raman scattering study of ultrathin SiGe superlattices. The spectra show evidence for the formation of superlattice structures with disordered interfaces. We explain the Raman peak energies in terms of strain and confinement effects and present a simple model for the Raman spectrum, from which we obtain semiquantitative information on the degree of superlattice ordering.

Substrate and capping layer effects on the phonon spectrum of ultrathin superlattices

GemSin ultrathin superlattices have been grown on (001) Si substrates and partly capped with Si. Raman scattering from acoustic phonons in the long-wavelength region shows unexpectedly intense broad peaks that develop into an increased number of sharper intense peaks on capping. We identify these peaks with resonant phonon modes and use a linear chain model to expose the importance of substrate-superlattice-capping layer interactions in these multilayer structures.