Table of contents

Microstructures and characteristics of nano-size hydrogenated crystalline silicon films (nc-Si:H) have been studied by high-resolution electron microscopy (HREM), X-ray diffraction patterns and Raman spectroscopy. The microcrystalline grains in nc-Si:H films are about 3-5 nm in size and are separated by different characteristic boundaries. The volume fraction of the crystalline component is about 46%. Microdefects in nanocrystalline grains were also found. The electrical conductivities of the films were found by measurement to be about 10^-3-10^-2 ( Omega cm)^-1. Analysis of the experimental results shows that the nano-size hydrogenated crystalline silicon films are in good agreement with the international definition of nanocrystalline material.

By measuring the resistance in situ during the deposition of metals-C₆₀ multilayer thin films on sapphire substrates at room temperature, the authors find that the resistance decreases sharply when adding C₆₀ on some metal layers. There are two possible explanations: one is the formation of a conduction layer of metal- (Sn-, Ba-, Ga-) doped C₆₀; the other is a bonding interfacial interaction between the metal and C₆₀ layers, which may result in a better continuous metal layer.

The carbides Dy₂Fe₁₇C_x with x=1.5-3.0 have been successfully prepared by the melt-spinning method. The X-ray diffraction patterns and the thermomagnetic curves show that they are of single phase in general with Th₂Zn₁₇ structure type when x=1.5, 2.0, 2.5 and 2.8 with a small amount of alpha -Fe as an impurity phase, while for x=3.0 the proportion of the alpha -Fe phase is high. The lattice parameters a and c are both enlarged and increase with the carbon content x. The Curie temperatures T_c are greatly enhanced when x<2.5 and remain almost constant when x=2.5-3.0, but the saturation magnetizations, sigma _s, increase slightly. The Dy₂Fe₁₇C_x ribbons still maintain the rhombohedral Th₂Zn₁₇-type structure after being annealed at 1100 degrees C for 14 h, which indicates the high stability of Dy₂Fe₁₇C_x (x=1.5-3.0) compounds obtained by rapid solidification. The heat-treated ribbon samples can be magnetically oriented and prefer an easy-plane anisotropy at room temperature.

Fe/Ag multilayers were prepared by alternate vapour deposition. The microstructure, periodicity, thickness, chemical composition and the magnetic moment of the films were determined and measured by various methods. The magnetic moment per Fe atom in Fe/Ag multilayers was considerably enhanced with decreasing Fe layer thickness, and it was up to 2.60 mu _B at an Fe layer thickness of 1.2 nm. The enhancement may be attributed to the nonequilibrium Fe atomic configuration at the Fe/Ag interface, i.e. probably the epitaxial growth of the Fe layer on the Ag layer with an orientation relationship of (00l)_Fe//(001)_Ag, (100)_Fe//(110)_Ag.

The authors present experimental results obtained by far-infrared reflection spectroscopy in the investigation of the optical phonons in ultrathin-layer (GaAs)_n(AlAs)_m superlattices (UTSLs). It was found that the frequencies of confined optical phonons in UTSLs are much closer to the frequency of the corresponding bulk phonon at the centre of the Brillouin zone than in SLs with sufficiently thick layers. The influence of the barrier thickness was also studied.

The Green's function method and the chemisorption theory of Einstein and Schrieffer are used to calculate the chemisorption energy of O or CO on a Ni-Cu disordered binary alloy surface using the average T-matrix approximation. The change of surface segregation due to chemisorption is investigated in detail. From the chemisorption point of view, the behaviour of O resembles that of CO to a certain extent. The calculated results show that the chemisorption of O/Ni-Cu and CO/Ni-Cu systems can all constrain surface segregation, resulting in more stable chemisorption.

A LEED, AES and ARUPS study has been made of CdI₂ adsorbed on the clean In-rich InSb(001) c(8*2) reconstructed surface at room temperature. A chemisorbed p(2*1) structure formed which consisted of a double layer of iodine atoms with cadmium atoms between them. It is suggested that this surface consists of linear I-Cd-I molecules aligned perpendicularly to the surface. Continued adsorption caused growth of an orientational relationship epitaxial layer of CdI₂. The epitaxial layer had the basal plane of the layered CdI₂ material parallel to the InSb(001) surface, with the hexagonal mesh of the CdI₂ at an angle of approximately=7 degrees to the square net of the substrate. On heating to approximately=400 K the epitaxial layer desorbed leaving a p(2*1) structure identical to that formed during adsorption. The p(2*1) was stable to 433 K beyond which further loss of Cd and I caused a change to a (1*1) structure. This structure is thought to consist of a (1*1) monolayer of iodine with Cd below the iodine layer, possibly incorporated into the substrate. Heating to 500 K caused complete loss of iodine and cadmium from the surface and the formation of a clean, Sb-rich surface.

Comparison of the linear and nonlinear optical response of mixed liquid crystal/stearic acid molecular monolayers is used to show that significant geometric and electronic structural information can be extracted from the optical second-harmonic (SH) response. The combination of SH intensity and phase measurements provides much more information than intensity studies on their own, and adds an important spectroscopic dimension to SHG from molecular monolayers.

The Ni 2p and O 1s XPS of NiO single crystals were measured using monochromatic Al K alpha radiation. Different treatments of the crystals allow the authors to give a description of the influence of defects on these spectra. The Ni 2p_3/2 spectrum of in-situ-cleaved NiO exhibits clearly visible features at 854.1 ev 855.6 eV and 861 eV. The intensity ratio changes after ion bombardment and an additional peak at 852.2 eV appears. Apart from a slight increase in linewidth the O 1s spectrum remains unchanged. The O 1s spectrum from an in-situ-cleaved NiO single crystal exhibits only a single peak at 529.4 eV which can be fitted by a Gaussian line profile. Further measurements allow the authors to attribute the well known O 1s satellite at 531.2 eV to emission from oxygen-containing species adsorbed at defects.

The interface spin waves at the (001) interface between a ferromagnet and an antiferromagnet, both with body-centred crystal structures, were investigated in the context of the Heisenberg model. The transfer matrix technique was employed to derive the Green functions at the interface layers. Interface spin waves which are localized in the neighbourhood of the interface were found to exist. The spectrum of these bound states was calculated for a number of different sets of parameters. The possible extensions of the present work were discussed.

One to nine monolayer Co films on Cu(100) and one- to five-layer-thick Co/Cu/Co trilayers on Cu(100) were fabricated at room temperature using metal vapour-phase epitaxy techniques under ultrahigh-vacuum conditions. Using electron diffraction and Auger electron spectroscopy, the authors determined that the single layers and multilayer films grow epitaxially and predominantly in a layer-by-layer growth mode. Longitudinal and polar magneto-optic Kerr effect (MOKE) measurements established that the easy axis of magnetization is in the substrate plane and provided the change of total magnetization with layer and multilayer thickness. The data suggest that the interface magnetization is affected by the thickness of the top Co layer.

The ferromagnetic phase transition in ultra-thin films and surfaces is studied by the Green function method based upon the Heisenberg model (S>or=1) with the out-of-plane spin orientation of the atoms in the surfaces. A model calculation shows that the order parameter (spontaneous magnetization) changes with the atomic planes in the ferromagnetic films but the Curie temperatures of the atomic planes are all the same; the Curie temperature of ultra-thin films may be higher or lower than the bulk Curie temperature which depends on the fact that the spin is pinned to some degree at the surfaces. It is impossible for an ordered surface phase to coexist with a disordered bulk phase and it is also impossible for a disordered surface phase to coexist with an ordered bulk phase in homogeneous films or a bulk magnet.

Using the same approach as Camley and Barnas, the author studies theoretically the magnetotransport properties of spin-valve multilayers. He emphasizes that the absolute change in sheet conductance ( Delta G) between parallel and antiparallel alignment of the magnetizations of successive ferromagnetic layers is the most relevant macroscopic quantity to represent and compare the magnetoresistance in these structures. He presents results on the influence of the thicknesses of the ferromagnetic and nonmagnetic layers on the magnetoresistance for the two cases most studied experimentally: sandwiches and multilayers with a large number of periods. He also investigates the influence of the number of periods on the magnetoresistance and discusses the similarities and differences obtained in the respective cases of bulk or interfacial spin-dependent scattering.

X-ray photoelectron spectroscopy (XPS) has been employed to study electronic states in Ce-Rh, C-Pd, and Ce-Ag interfaces. Thin Ce films were deposited onto evaporated substrates of Rh, Pd and Ag, and conversely, thin films of Rh, Pd and Ag were deposited onto evaporated Ce films. Tetravalent Ce emission was observed in the Rh and Pd systems. Substrate core level attenuation curves show a deviation from exponential decay as material is deposited. Substrate core level shifts as well as the appearance of a tetravalent Ce signal are indicative of interface alloy formation; however, clustering of the evaporated species may also take place.

The cumulant expansion of the EXAFS Debye-Waller factor is discussed for the polarized and nonpolarized K EXAFS formulae. The improved formula for the nonpolarized Irs turns out to give a shorter near-neighbour distance than the value obtained by the expansion derived previously. Specifically, in CuBr at around 600 K its difference is about 0.01 AA.

The CuFeTe₂ ternary compound with a layered crystalline structure has been studied. The compound is paramagnetic in the temperature range 1-200 K. Investigation of the mu (T) temperature dependence demonstrates the ferron character of the carrier transport with the Mott diffusional mechanism. The obtained power dependences n(T), R(T) and sigma (T) allow the authors to conclude that CuFeTe₂ belongs to the gapless semiconductors. The temperature behaviour of n(T), R(T) and sigma (T) can be explained by examining features from the electron spectrum of CuFeTe₂ under conditions where the character of the carrier transport is dose to the zone type and the general character of the spectrum is determined by the symmetry group and by the type of irreducible representation of the group.

Interband optical transitions in CeSn₃ and LaSn₃ have been investigated by measuring and calculating from band theory the optical conductivities of both compounds. For both compounds, interband transitions start at low energies (0.3 eV for CeSn₃ and 0.5 eV for LaSn₃) and these structures are mostly due to d to f and f to d transitions between hand-like d and f states originating from Ce or La atoms. The structures at higher energies (greater than 1 eV) strongly involve f to d and d to p transitions with p states originating from Sn atoms. Good qualitative agreement between calculation and experiment has been achieved for these higher-energy structures for both compounds. The larger strength of the structures in CeSn₃ above 1 eV is mainly due to the existence of more occupied band-like f states in CeSn₃ than in LaSn₃.

The theory of AC magnetoresistance in inhomogeneous solids is developed using the effective-medium theory. Calculations are performed for systems which consist of random high- and low-conductivity regions. The weak magnetic field H is directed along the z axis. The effective conductivity changes Delta sigma _m^xx and Delta sigma _m^zz and the magnetoresistance Delta rho _m^xx and Delta rho _m^zz in both the low- and the high-frequency regions are calculated up to the H² approximation. The calculations show that, owing to inhomogeneities, plateaux of the finite values Delta sigma _m^zz and Delta rho _m^zz in the low-frequency region occur. It is possible to have both Delta rho _m^xx> Delta rho _m^zz and Delta rho _m^xx< Delta rho _m^zz. In the high-frequency limit the equalities Delta sigma _m^zz= Delta rho _m^zz=0 hold as in homogeneous solids.

Conductance noise was measured in tunnel junctions with alumina barriers grown by plasma oxidation or thermal oxidation in atmospheres containing different amounts of water. Noise measurements were made in the frequency range 7 Hz-70 kHz, under biases -1 V to +1 V, and at temperatures 1 K to 4.2 K. In junctions with barriers grown under dry conditions, shot noise dominated at all frequencies. In junctions with barriers grown in the presence of water, noise power at low frequencies was four orders of magnitude larger than shot noise; it had an approximately 1/f frequency variation and a strong bias dependence, but was temperature independent below 4.2 K. The authors show that the results arc not quantitatively consistent with models involving modulation of the tunnelling barrier. They develop a new model, based on switching of resonant channels, that is able to explain the levels of noise observed.

A new type of Fibonacci optical superlattice is analysed. Its electro-optic effect is studied. The phase-matching concept is proposed for the first time for the Fibonacci optical superlattice. The transmission spectrum is nonself-similar owing to the dispersive effect of the refractive index. An extinction phenomenon exists provided that the thicknesses of the domains are properly selected.