The principles of scanning tunneling microscopy and its application to study silicon surfaces are briefly reviewed. Scanning tunneling microscopy “topographs” contain both geometric information about the locations of atoms at the surface as well as about the charge densities of surface localized states. We describe procedures by which these two components can be distinguished so as to produce images of the surface electronic states with atomic resolution. This ability to spatially resolve the surface electronic structure provides new information to understand the local structure and nature of bonding, and in some cases can be used as a means to chemically image specific features of the surface.

The design and performance of a scanning tunneling microscope for use in the study of metal surfaces is described. The system was designed for ultra high vacuum together with standard sample cleaning and characterization techniques. The STM provides both topographic and spectroscopie information. Topographic images of well annealed Au(lll) show very smooth planes with single atomic steps. Corrugation on the (111) planes, which is expected from reconstruction models for this surface, is not seen. Other areas show monatomic steps in the form of an ordered array with a period corresponding to that derived from previous studies. A possible alternative reconstruction mechanism is suggested by these STM data. On the same surface are steeper sloped regions with multiple steps of equal height with wide facets. Spectroscopie first derivative data for Au and Pd show peaks which correspond to surface and bulk electronic states, for both filled and unfilled cases. The energy values of these states are compared directly with the results of other experimental methods. The use of combined topographic and spectroscopie mode for metals is anticipated.

A calculation of the effective elastic moduli of a super-lattice, in terms of the elastic moduli of its constituents, is presented. Contrary to what has been found in many real systems, these calculations show that in the long wavelength limit the effective elastic moduli are independent of the modulation wavelength. A discussion of the experimental results on Nb/Cu, Mo/Ni and Au/Cr shows that the elastic anomalies can be understood on the basis of concomitant changes in lattice spacings.

The persistent photoconductivity (PPC) observed between 200 and 400K in npnp doping modulated hydrogenated silicon (a-Si:H) having individual layer thicknesses between 200 and 600Å iS compared with similar effects in compensated a-Si:H and in amorphous silicon/silicon nitride multilayers. The thermal activation barrier against creation of PPC in these materials, the low quantum efficiency, and the very slow recombination at 300K and higher temperatures suggest the presence of special traps that are spatially or energetically removed from the excess majority carriers. The PPC is accompanied by strongly nonohmic effects which exhibit similar metastabilities and long decay times.

We present the results of a study we have done to explore the effects of high levels of Be dopant, >1×1019 /cm3, in GaAs/AlGaAs superlattice structures. The materials were grown using Molecular Beam Epitaxy (MBE) at 680 C using standard growth conditions. The samples were studied using a variety of structural, optical and electrical probes. Structural data obtained from Auger and SIMS profiling is presented as well as optical data from photoluminescence and Raman scattering. We also present the results of electrochemical profiling of the samples. The results of these studies provide evidence of intermixing of the superlattice during growth with sufficiently high Be levels. We have found evidence for a concentration threshold in the diffusivity of Be in these structures. We also find that the dominant mechanism for Be redistribution in these samples appears to be diffusive in nature rather than due to surface riding.

The physical processes occurring during the initial stages of ultrafast laser heating of metals are described. Femtosecond laser irradiation is used to create nonequilibrium heating in metals. In such a nonequilibrium state, the electron temperature can be heated up to a few thousand degrees above the lattice temperature. Electron-lattice relaxation is time-resolved in copper and found to be 1 – 4 ps depending on the laser heating ffuence. The technique of time-resolved electron diffraction (a lattice structural and temperature probe) is described. Utilization of this technique for lattice temperature measurement of thin metal films is demonstrated.

The semiconducting alloy (GaSb)1−x Ge2x is an example of a mixed crystalline alloy that undergoes a phase transition between its constituent crystal types. The order-disorder transition modifies both the long-range and short-range order in the alloy as a function of composition x. By using the Kikuchi approximation, the bond probabilities are calculated and then compared with EXAFS data. These calculations show that the EXAFS data are consistent with the existence in (GaSb)1−x Ge2x of “wrong” bonds of Ga-Ga and Sb-Sb. The theory also predicts a violation of Vegard's Law for the behavior of the mean bond length as a function of composition x.

During the last few years, it became clear that electron interference phenomena strongly modify the conductance of disordered systems. In thin metal films, this interference produces an anomalous low temperature magneto-resistance which enables to obtain detailed information about the inelastic, spin-orbit and magnetic scattering mechanisms. In a multiply-connected geometry, the interference leads to magnetoresistance oscillations. In long metal cylinders, only the oscillations which are not destroyed by impurity averaging, can be observed (flux period h/2e). In single loops, the original Aharonov-Bohm oscillations with flux period h/e are also present. A review will be presented of recent experimental results obtained for quasi two-dimensional plane films, cylinders, rings and arrays.

Since the coherence length is of order 50 Å to 1 μm, it is possible to alternately layer thin films of a superconductor with another material to affect the physical properties of the resulting superlattice. A brief description of our sputtering technique used to prepare superlat t ices consisting of Nb/Cu and Ta/Mo is given. Optical interferometry, x-ray diffraction and ion beam analysis techniques independently confirm that control of ±0.3% is achieved over the amount of material deposited in each layer. Results of a study of Josephson tunneling to Nb/Cu metallic super lattices are reviewed. These measurements enable a determination of the London penetration depth to be made as a function of superlattice layer thickness.

The epitaxial growth of thin films has been studied by molecular-dynamics computer simulation. In these simulations atoms are projected towards a temperature-controlled substrate, and the equations of motion of all atoms are solved for a given interaction potential. The calculations give insight into the microscopic structure of thin films, the dynamics of the adsorption process, and they help answer the way in which substrate temperature, form of the substrate, flux of impinging atoms, and form of the interaction potential, affect epitaxial growth. Simulations were performed for monatomic and binary systems with spherically symmetric atomic interactions, and for systems in which the atoms are interacting via a three-body potential to simulate the epitaxial growth of silicon.

Recent results from EXAFS spectroscopy are discussed, including (i) the determination of the site and local bond lengths of Fe impurities implanted into Si, (ii) the determination of the ferroelectric phase transition in Pb1−x Gex Te as order-disorder rather than displacive, and (iii) the nondestructive measurement of Aℓ diffusion into GaAs at an Aℓ/GaAs Interface.

Recent advances in spin-sensitive spectroscopie techniques combined with the rapidly expanding capabilities to synthesize novel materials utilizing molecular beam epitaxy (MBE) are stimulating new interest in magnetism and magnetic materials. Epitaxial growth offers opportunities to stabilize and study crystal phases of elemental magnetic materials which do not naturally occur in nature, and to vary the crystal parameters and composition of epitaxial films in precisely controlled ways. Multilayer superlattice films can also be fabricated using MBE. These structures offer unprecedented new opportunities to explore the relationship between crystal structure and various magnetic properties such as spontaneous magnetization, the Curie temperature and magnetic anisotropy in well characterized systems. The present paper reviews some of the more recent work on thin film magnetic systems including structure analysis of film quality, new techniques for probing the electronic and magnetic properties of the films and the important role large scale numerical calculations are beginning to play in guiding the choice of magnetic systems for study.

Hot electron relaxation in bulk semiconductors has been studied for several decades, but only through recent advances in crystal growth has it become possible to investigate the ther-malization of hot quasi-two-dimensional carriers in quantum wells. These same advances have opened the possibility of constructing various semiconductor devices which rely on hot electrons for their operation. We discuss experimental results on the energy relaxation of hot electrons in GaAs/AlGaAs quantum wells. The experiments make use of optical spectroscopy for determining the carrier distribution. In particular, steady-state hot photoluminescence measurements have been employed with modulation-doped quantum wells in order to minimally perturb the system by the photoexcited carriers. Both the relaxation of very energetic electrons and the cooling of a hot thermalized carrier distribution are considered. The quantum well results are compared to results from similar experiments with bulk GaAs.

We deliberate on the conditions favorable to the growth of metallic crystalline superlattices (MCS) with (111) f.c.c./(110) b.c.c. interfaces. We use, with some motivation, equilibrium criteria (i) to justify the occurrence of the Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) orientations, and to show with analyses which also allow for elastic relaxation, that only the NW orientation that occurs at n.n. distance ratios in the interval 0.8 ≲ bf.c.c / ab.c.c ≲ 1.0 can yield the regular orientational relationships required for high quality MCS; ahd(ii) to show that, for the acquisition of the required smoothness of the interfaces, which is mainly determined by the growth mode - monolayer-by-monolayer (FM = Frank-van der Merwe) or island nucleation and growth (VW ≊ Volmer-Weber) mode - it is desirable to use material combinations with small surface free energy mismatch. Only then can VW growth (which inevitably occurs in each superlattice period) at relatively high supersaturation be FM-like and with low density of defects.

The angular dependence of the upper critical fields, Hc(θ) for a set of NbTi-Ge superlattices were studied at various temperatures. The behavior of Hc(θ) at lower temperatures deviates from the Tinkham expression which is expected to be valid only in the Ginzberg-Landau regime close to Tc. We examine a model for calculating Hc(θ) involving the lowest eigenvalue of the gauge invariant diffusion equation (subject to boundary conditions appropriate to a slab) in the de Gennes expression for the upper critical field of a dirty superconductor at all temperatures. The disorder related localization and interaction effects as well, as the paramagnetic limiting effect, are also considered.

Grazing-incidence x-ray diffraction, a surface-sensitive technique, has been used to obtain structural details parallel to the interface of an epitaxial system, such as lattice parameters, strain, crystallite size and orientation, on films with thicknesses ranging down to a few mono-atomic layers. Tungsten grows epitaxially on the (1102) plane of sapphire, with the orientation W (001) ∥ Al2O3 (1102) and W [110] ∥ Al2O, [1120]. Sufficient diffraction intensity for characterization could be obtained from ∼30A-thick W films. Layers of GaAs can be grown epitaxially on the basal plane of sapphire with the orientation GaAs(111) ∥ Al2O3(00.1) and GaAs [110] ∥ Al2O3[1120]. Niobium films grow on GaAs (001) and (111) substrates with a (001) plane parallel to the interface, whereas molybdenum films prefer to grow with a (111) plane on both substrates. The best orientation, i. e. the smallest mosaic spread, of the film is obtained when the substrate plane has the same symmetry as the preferred film growth plane. In all these cases with relatively large misfit, the strain observed parallel to the interface is only a small fraction of the theoretical misfit strain, indicating the relief of the misfit strain within the first few atomic layers.

The adsorption of formic acid (HCOOH), acrylic acid (CH2= CHCO2H), propiolic acid (HC=CCO2H), and the corresponding alcohols on the Si(111)(7×7) surface have been investigated by NEXAFS. In each case, well-defined dipole transitions to σ* and π* molecular orbital s were observed above the C and 0 K-edges and used to probe the orientation and chemistry of these molecules on this silicon surface. Monolayer coverages of these molecules on silicon, bond strongly to the silicon surface via the carboxylic acid or alcohol group. In contrast, the C-C double and triple bonds of these molecules do not react initially with the silicon surface. Upon heating, however, the C-C double and triple bonds which are held in proximity to the surface by the carboxylic acid or alcohol group, are lost either by polymerization on the surface or reaction with the silicon substrate. These results illustrate the capabilities of NEXAFS to investigate molecular orientations on surfaces and the electronic structure of polyatomic adsorbates.

We combine the techniques of EXAFS and XANES using synchrotron x-rays, and the corresponding techniques using electron energy loss spectroscopy (EXELFS and ELNES) to characterize and study the local atomic arrangement in thin amorphous films. These probes are complementary in that x-rays are best suited for absorption edges above 3 keV and electron energy loss spectroscopy for low energy edges, and thus this combination is very useful for amorphous materials containing both low and high Z elements. We use in addition transmission electron microscopy, electron diffraction, and electron energy loss spectroscopy in the low energy loss region. Results from amorphous T1O2 and amorphous SnO2 made by reactive sputtering are presented.

EXAFS studies using glancing angle fluorescence on the interface of the Cu-Al thin film system show that the sputtering process generates a different interface structure than for an evaporated one. To obtain quantitative information, the anomalous dispersion effects have to be considered. Simple model calculations are presented for the correction factor which yield good results for Au thin film data. Extension of this model to bilayer systems is discussed.

We are in the process of studying strained-layer growth of two-dimensional Lennard-Jones lattices. To do so, we have developed three techniques, based on the Monte Carlo method and molecular dynamics, of simulating atomistic crystal growth from the vapor phase. The Monte Carlo method efficiently simulates the effects of long time-scale processes on the growth of strained-layer systems, but omits the transient dynamics of particle adsorption. The second technique, using molecular dynamics, gives results suggesting that epitaxial growth of strained-layer systems can occur on the picosecond timescales. However, this technique cannot capture the influence of the long time-scale processes on the growth process. In view of the shortcomings of the previous two techniques, A hybrid technique incorporating both the Monte Carlo method and molecular dynamics, has been developed. In principle, this technique models the transient dynamics of adsorption as well as the long term evolution of the system. This technique, however, is limited by artifacts that may only be eliminated by use of unwarrented amounts of supercomputer time.