The crystalline quality of the transition region between buffer layer and epilayer in MOMBE (metalorganic molecular beam epitaxy) and plasma-MOVPE (metalorganic vapour phase epitaxy) grown GaAs layers on Si(100) is analysed by Raman scattering with variation of the penetration depth and besides bevel polishing or step etching. The region with considerable lattice imperfections is essentially confined to the original buffer layer.

Single crystal undoped germanium has been grown successfully for the first time on (1102) sapphire [1,2]. Germanium was found to grow (110) with an in plane registry of Ge(111)/Al203(1012) [1]. When a 2μm germanium film was grown on (1102) sapphire at 800°C and after a substrate preanneal of 1400°C, the X-ray line width of the Ge(220) reflection was measured to be 230 arcseconds [2]. Strain was observed only in germanium films that were thinner than 40 nm [2].

We report the first HRXTEM study of epitaxial rare earth fluoride/ GaAs(111) interfaces. Such interfaces are of interest because they are the starting point for growth of buried epitaxial rare earth / rare earth fluoride sandwich structures which exhibit interesting and non bulk-like magnetic properties. Also, the optical transitions in ultrathin epitaxial NdF3 films may be influenced by strain and defects in the NdF3 film and the nature of the interface to GaAs.

We find that the rare earth fluoride / GaAs interfaces are semi- coherent but chemically abrupt with the transition taking place within 3Å. However, the interface is physically rough and multiple monolayer steps in the GaAs surface tend to lead to tilt boundaries in the fluoride. The origin of these steps is believed to be thermal etching of the GaAs during the heat-cleaning stage prior to epitaxy. The surface of the fluoride film is much smoother than the initial GaAs surface indicating planarization during epitaxy.

Heteroepitaxial growth of CaxSr1-xF2 layers on Si (100) substrates by molecular beam epitaxy has been investigated for GaAs / fluoride / Si structures. The Si (100) substrates off-oriented toward [011] show no considerable influence on crystallinity of CaF2 and SrF2 epitaxial layers. The off - oriented substrates, however, influence a remarkable effect on CaxSr1-xF2 layers, resulting in poor crystallinity rather than good one. This influence of off-oriented Si (100) substrates on the heteroepitaxy of CaxSr1-xF2 layers was the opposite of that found in GaAs/Si (100) structures.

Epitaxial layers of (111) GaAs of approximately 1 µm thickness were grown on epitaxial CaF2 buffer layers which were either 140 or 380 nm thick on Si(111) substrates. The best nucleation temperature for the GaAs on CaF2/Si(111) we have observed was 620 °C. This resulted in high quality GaAs films which exhibited channeling minimum yields of 4%. The density of threading dislocations in the GaAs layers was observed by TEM to be ~108 cm-2. Double-crystal x-ray diffraction measurements showed that the strain (ε┴.) was less than 2.2×10-4 in both sets of GaAs samples. Ion channeling, however, revealed a large tetragonal strain of 3.5×10-3 (ε┴ = 1.7×10-3) in the thinner (140 nm) CaF2 buffer layers. By doing ion channeling with high energy (2.5 MeV) protons, it was possible to determine strain more accurately. Using this technique, we were able to set an upper limit for the tetragonal strain of 2.5×10-4 in both the GaAs (which implies ε┴ < 8×10-5 and CaF2 (ε┴ < 1.5×10-4) layers for the thicker (380 nm) CaF2 buffer layer structure. These results are in good agreement with the strain predicted from previous strain measurements of CaF2 epitaxial layers on Si.

Si(100) single crystals containing circular gratings etched into the surface using reactive ion etching were analysed using both monochromatic and white beam reflection X-ray topography. Some features of observed X-ray topographic contrast associated with the gratings were modelled using a diffracting zone model, based on a simple radial in-plane strain field. Reasonable agreement was obtained between this model and observations.

We describe an atomic oxygen source based on a D.C. plasma discharge, compatible with cristal growth in a Molecular Beam Epitaxy(M.B.E.) system. The physical characteristics of the oxygen cell are presented. The efficiency of the cell has been proved by direct deposition of CuO at high temperature(500°C). Moreover, we used successfully this cell for direct epitaxial growth of high temperature superconductors, with an ambient pressure as low as 2 10-5 Torr.

We report recent results of studies of the structure and thermal stability of periodic multilayers based on hydrogenated amorphous silicon, hydrogenated amorphous germanium, silicon nitride and silicon oxide. By varying the sublayer thickness from 1 nm to 20 nm it is possible to extract information on the range and magnitude of relaxation and interdiffusion in these metastable materials. It is also possible to gain information on the influence of interfaces on crystallization and relaxation. The principal techniques discussed here are Raman scattering, optical absorption and high resolution x-ray reflectivity.

This paper reviews some of the novel electronic effects observed in amorphous semiconductor superlattices. Quantum size effects have been reported in compositionally modulated amorphous semiconductors based upon optical absorption and tunneling studies. However, the quantum well effects are in direct conflict with transport data estimates of the inelastic scattering length. In doping modulated amorphous semiconductors, a large persistent photoconductivity PPC is observed, which cannot be explained by the field separation model alone, but rather requires the presence of a defect which undergoes a large lattice relaxation upon trapping a charge (similar to the DX center). Studies of npnp amorphous silicon multilayers have enabled a microscopic determination of the process responsible for PPC.

Electrodeposition is a fast and relatively inexpensive method for making metal multi-layered films. We present a dual bath electrodeposition technique capable of producing crystalline Ni/amorphous NiPx multilayers with repeat lengths of 20Å and above, of sufficient quality to give several low-angle x-ray reflections. The compositional and structural modulations of the films are analysed using x-ray diffraction and microscopy, and the effects of deposition parameters, such as the current density, are examined.

X-ray diffraction is one of the main methods of determining the structure of multilayers. Low angle reflectivity measurements are particulary useful for multilayers containing polycrystalline or amorphous constituents, and for obtaining specific structural data. We present a method based on both kinematic and dynamic scattering calculations, and use it to extract specific structural parameters such as the roughness or diffuseness of the external surface, the thickness of the constituent layers, and the roughness or diffuseness of the internal interfaces. Results are given for a sputtered A1/A12O3 multilayer.

Studies have been made of the dependence of the structure on the deposition angle and the substrate temperature of a series of Mo/Si multilyers fabricated in a UHV system by e-beam evaporation. The detailed morphology was determined by cross-sectional high resolution electron microscopy. Columnar growth in the crystalline Mo layers was found to follow the tangent rule. The overall quality of the multilayers was found to depend strongly on the growth conditions.

X-ray diffraction, electrical resistivity and dimensional variation measurements have been performed in order to characterise the structural and transport properties of Fe/Ti multilayers both in the as-prepared state and during thermal annealing. For small periods, the multilayers consist of hep Ti and amorphous iron-rich layers. Increasing the period leads to the crystallisation of the Fe-rich layers, which correlates with a sudden decrease of the electrical resistivity. Depending on the period (that is on the initial structure of the layers), very different types of evolution are observed upon annealing, corresponding to either interdiffusion or preferential diffusion of one of the species.

A relaxation of the magnetization following a rapid change in the magnetic field has been found in a compositionally-modulated Ni/Cu multilayer produced by sputtering. This finding demonstrates that the relaxation previously found in compositionally-modulated alloys produced by electrodeposition is a property of the material and not an artifact of the electrodeposition process.

We have performed extensive structural and electronic transport measurements of Pb/Cu multilayered structures. The high-angle x-ray diffraction spectrum indicates the presence of continuous interfacial disorder. In the low-angle x-ray spectrum, pronounced minima occur at positions given by q = 2π/Λ, where Λ is the bilayer thickness. This is in sharp contrast to the results for Pb/Ge (crystalline/amorphous) multilayers, where minima are observed at positions q = 2π / tPb with tPb the Pb thickness. These low-angle x-ray spectra can be explained by taking into account lateral coherence and continuously distributed thickness fluctuations. The effect of this interfacial disorder on the electronic properties is investigated by measuring the low-temperature resistivity and the superconducting transition temperature of the multilayers. The calculated values for the elastic mean free path in the individual layers confirm the presence of strong interfacial scattering. The thickness dependence of the critical temperature is explained by taking into account an interfacial barrier with penetration probability σ ≃ 0.5.

Laser Raman spectroscopy has been found to be useful for characterizing amorphous semiconductor multilayers, especially the interfaces of multilayers. Recently, we have extended this technique to the characterization of magnetron sputtered multilayers commonly used as reflectors in soft x-ray optics. Unlike the multilayers previously studied which contained only semiconductors and dielectrics, these are generally semiconductor/metal multilayers. We report here on the Raman characterization of the most common class of multilayers used in soft x-ray optics, those that contain a high density metal like tungsten interspersed with layers of carbon. In all of the metal/carbon multilayers the dominate feature in the Raman spectra is due to a-C. The a-C spectra consists of a broad peak at about 1560 cm-1 (G-peak) and a shoulder at about 1400 cm-1 (D-peak). This can be deconvoluted with Gaussian line shapes to yield two peaks (one at about 1560 to 1570 cm-1 and the other at about 1380 to 1420 cm-1). Among the W/C multilayer samples peak positions and relative magnitudes changed little with carbon thickness over the range of 1 to 12 nm. Significant differences are, however, seen as the identity of the metal component is altered or, especially, as the preparations are varied. For example, the intensity ratio of the D-peak to G-peak was much larger for multilayer samples prepared under conditions of good plasma confinement.

The utilization of W/B4C multilayer structures as Bragg diffractors rests in their reflective efficiency and dependability, both of which rely on the stability of the layered structure. The layers within the vapor deposited multilayers, often amorphous, are typically metastable, hence susceptible to thermally induced structural change. In fact, crystallization and compound formation are known to occur in annealed multilayer systems. In this study, multilayers of W/C and W/B4C were vacuum heat treated, then structurally examined. A comparison between the pre- and post- heat treated structures of the two multilayer systems, accomplished using diffraction and electron microscopy, indicate structural stability in the W/B4C multilayers as opposed to structural degradation noted for the W/C system.

A series of periodic multilayer Al/Tb-Fe-Co thin films have been prepared by dc-magnetron sputtering. The films are of the form (xA/yB)n, where x and y are the thicknesses of the A and B layers respectively, and n is the number of bilayer units. Individual layers are from 2-40nm thick. The magnetic properties of the films have been measured as a function of Al and Tb-Fe-Co layer thicknesses by vibrating sample magnetometry (VSM). An homogeneous film of the Tb-Fe-Co alloy was grown under identical conditions and found to have in-plane easy magnetization. Pronounced interfacial and/or reduced dimension effects have been found in the multilayer systems. These include evidence for a “dead layer” in the interface regions (~2nm thick) and reversal of the direction of easy magnetization from in-plane (in the homogeneous thin film samples) to strongly perpendicular in the multilayer configuration.

Experimental and theoretical studies demonstrate that it is possible to redefine the basic character of band-edge electronic wavefunctions in superlattices, the period of which is comparable to the lattice parameter of the constituent materials. In using superlattice periodicity to redefine electronic wavefunctions, strain and crystalline orientation can be used to select which wavefunctions will form the band edge states. These principles can be used to form a direct bandgap semiconductor superlattice from Si and Ge, two indirect bandgap materials.

Modulation spectroscopies, such as electroreflectance and photoreflectance have become popular as room temperature probes of layered structures. Because of their derivative nature, one observes sharp optical features related to critical points in the dielectric function of the material under study. This allows us to obtain information such as alloy concentration, carrier density, well widths of quantum wells and thicknesses of the constituent layers in superlattices. In this paper, we will review the application of modulation spectroscopy to the study of optical and electronic properties of layered structures.