This paper discusses variants that occur in heteroepitaxy when the substrate possesses symmetries that are absent from the epilayer. Variants are enumerated in terms of 2-D symmetries of the substrate and epilayer, as they exist terminated at their interface. In this formulation, all possible cases can be presented in a single table. The role of miscut in controlling otherwise equal variants proportions is discussed with a view to eliminating unwanted variants and thereby improving the translational invariance of the materials. We summarize an experimental study of a (3m) symmetry epilayer growing on a (2mm) substrate, specifically Cu3Au (111) grown by molecular beam epitaxy on Nb (110), for which the two variants predicted from symmetry are stacking twins. Miscuts of about 1° along the indicated azimuth are sufficient to eliminate all except ∼ 0.1% of the less favored variant. This has interesting consequences in the context of the theory. A detailed understanding of the mechanisms for this example requires information about nanostructures that develop on the miscut Nb (011) surface. State-of-the-art information about novel substrate nanostructures, derived from scanning probe and low energy electron microscopies, is presented, and the prospects for a predictive science of variant control in heteroepitaxial growth are assessed.

Metal(100) homoepitaxial systems constitute perhaps the simplest class of systems in which to study thin film growth. Yet, our Variable-Temperature Scanning Tunneling Microscopy (VTSTM) analysis of Ag/Ag(100) homoepitaxy reveals that the variation of roughness with temperature is extraordinarily complex. As the deposition temperature is reduced from 300K to 50K, the roughness of 25 monolayer films first increases, then decreases, and then increases again. Furthermore, a transition from mound formation to self-affine (semi-fractal) growth occurs at around 135K. We postulate that the following the atomistic mechanisms underly this behavior: the existence of a small step-edge barrier inhibiting diffusive downward transport; “downward funneling” of atoms deposited at step edges and microprotrusions towards lower four-fold hollow adsorption sites; and statistically significant deviations from “complete” downward funneling at lower temperatures, where deposited atoms instead become trapped on the sides of (the more prevalent) small steep microprotrusions. To support these postulates, we employ kinetic Monte Carlo simulations to show that atomistic (lattice-gas) models for epitaxial growth, which incorporate these mechanisms, reproduce the experimental data quantitatively.

Despite numerous theoretical and experimental studies of strain relaxation in metal films on silicon substrates, the exact mechanisms by which dislocations mediate plasticity in these structures are not well understood. To elucidate these mechanisms, we present results from in-situ transmission electron microscopy annealing of thin aluminum films grown on Si (100). As a model system, we have chosen to focus on aluminum films which contain two (011) epitaxial variants with respect to the silicon substrate. In this paper we discuss our observations of the glide and climb behavior of dislocations in these structures during thermal cycling. These observations give qualitative insight into the mechanisms by which dislocation motion accommodates thermally induced strains in thin metal films.

Detailed analyses of non-equilibrium lattice-gas models of island nucleation and growth during film deposition (or etching) have been invaluable in elucidating basic issues in nucleation theory, deviations from mean-field predictions, and experimental observations. Particularly interesting and useful is the behavior of spatial correlations in the adlayer which develop during island (or pit) formation. In particular, a strong depletion in the population of island pairs at separations smaller than the average follows from depletion in the density of diffusing adspecies near islands. This feature delays percolation of clusters of coalesced islands. Another recently discovered and more subtle feature is a strong correlation between the width of the island-free region surrounding an island and the size and growth rate of that island. This direct correlation between island sizes and separations controls the shape of the island size distribution. If incorporated into rate-equation descriptions, it recovers the exact form of the scaling function for the island size distribution.

We model the growth of Ag films deposited on Ag(100) below 140K. Our recent Variable-Temperature Scanning Tunneling Microscopy (VTSTM) studies reveal “smooth growth” from 120-140K, consistent with earlier diffraction studies. However, we also find rougher growth for lower temperatures. This unexpected behavior is modeled by describing the deposition dynamics using a “restricted downward funneling” model, wherein deposited atoms get caught on the sides of steep nanoprotrusions (which are prevalent below 120K), rather than always funneling down to lower four-fold hollow adsorption sites. At OK, where no thermal diffusion processes are operative, this leads to the formation of overhangs and internal defects (or voids). Above 40K, low barrier interlayer diffusion processes become operative, producing the observed smooth growth by 120K. We also discuss how the apparent film morphology mapped out by the STM tip “smears” features of the actual film morphology (which are small at low temperature), and also can lead to underestimation of the roughness.

Using both low energy electron diffraction (LEED) and scanning tunneling microscopy (STM), we have made the first observation of a new ordered surface structure of oxygen on W(110). This structure is characterized by the matrix relative to the (1×1) W(110) structure, in which 15 tungsten atoms make up the rectangular unit cell. Based on high resolution STM images, a model for the structure is proposed which includes 6 adsorbed oxygen atoms and has a coverage of 0.40 ML.

We have investigated the initial stages of Cu (001) oxidation in dry and moist oxidizing conditions using in situ ultra-high vacuum (UIHV) transmission electron microscopy (TEM). To investigate the role of moisture in the solid state reactions in Cu oxidation, we have examined the oxidation of Cu (001) with water vapor. Our observation indicate that water vapor causes reduction of Cu2O and retards the oxidation rate if both oxygen gas and water vapor are used simultaneously which contradicts the thermochemical data. We are also modeling the nucleation to coalescence of the oxide scale using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation and have noted a qualitative agreement.

We have developed a novel method for in-situ measurements of the true work function (ø) of metal surfaces by combined ultra-high vacuum compatible Kelvin Probe and photoelectric effect measurements. The work function is an extremely sensitive parameter of surface condition and can be used to study oxidation and thin film growth on metal surfaces. For example, the increase in ø due to oxidation of polycrystalline rhenium is 1.9eV.

The Kelvin Probe measures local work function differences between a conducting sample and a reference tip in a non-contact, truly non-invasive way over a wide temperature range. However, it is an inherently relative technique and does not provide an absolute work function if the work function of the tip (øtip) is unknown.

We present a novel approach to measure øtip with the Kelvin Probe via the photoelectric effect, using a Gd foil as the photoelectron source, hereby combining the advantages of both methods to provide the absolute work function of the sample surface. We demonstrate the application of the technique by in-situ work function measurements during oxidation of polycrystalline rhenium. The extended Kelvin Probe method therefore has potential applications as a characterisation tool for thin film epitaxy and work function engineering of surfaces.

We have prepared (Fe(001)/Pd(001)) multilayers onto Pd-buffered sapphire substrates. Structural investigations utilizing HRTEM, RHEED and XRD show that the films grow epitaxially for up to 32 bilayers. Whereas the Fe lattice is found to assume the lateral (in-plane) lattice spacing of the underlying Pd layers, the out-of-plane lattice parameter along the [001] growth direction of the film is successively reduced with increasing Fe layer thickness. This outof-plane lattice relaxation even survives a subsequent growth of Pd onto the Fe. Thus, by varying the Fe layer thickness we have control of the atomic volume of Fe. Magnetization measurements reveal that the magnetic moment of Fe is as high as 2.7 µB per atom when the atomic volume is larger than Vat(Fe) ≥ 11.7 Å3 and is reduced to 2.2 µB per atom for Vat(Fe) ≤ 11.1 Å3.

Structural and thermodynamic properties of equilibrated ultrathin Pd-Au, Cu-Ag, and Co-Ag alloys on Ru(0001) were studied and are comparatively discussed. These studies provide a framework for a detailed understanding of the energetics of surface alloy formation.

Additionally, the differences between the two bulk immiscible systems (Cu-Ag and Co-Ag) and the bulk miscible Pd-Au system are discussed.

The successful preparation of fully epitaxial spin-valves elaborated on NiO(111) single crystals is reported. The growth conditions of a smooth 2D permalloy (Py = Ni80Fe20) layer have been determined. A very strong magnetic exchange interaction is evidenced on the Py/NiO(111) interface. It strongly modifies the magnetic properties of Py that acquires high coercivity, leaving it as a hard magnetic material. The second Py layer has unchanged magnetic properties and plays the role of the sensing soft magnetic layer. A giant magnetoresistance (GMR) of 3.5% at room temperature was easily obtained without optimizing the thickness of the different layers. Complementarily energy-filtered (EF-) and high-resolution (HR-) transmission electron microscopy (TEM) were used to fully characterize the spin valves. Within the framework of magnetic exchange coupling, our results open the possibility to elaborate model spin valves in which the role of each interface can be investigated and controlled at the atomic level. The detrimental effect of an inter-diffusion at the Py/NiO interface is evidenced.

The formation of epitaxial CoSi2 islands of nanoscopic dimensions is reported using the technique of reactive cluster deposition. Co clusters in the size range 5-50nm were synthesized by sputtering a high purity Co target inside a UHV sputtering chamber. The clusters were then deposited on the reconstructed Si (111) surface. Upon annealing the particles reacted with the Si substrate to form epitaxial CoSi2.

Our observations were made using a JEOL 200CX transmission electron microscope modified for in-situ sputtering and ultrahigh vacuum conditions.

Using a variety of in situ monitors and when possible adsorption-controlled growth conditions, layered oxide heterostructures including new compounds and metastable superlattices have been grown by reactive molecular beam epitaxy (MBE). The heteroepitaxial layers grown include Bi4Ti3 O12—SrTiO3 and Bi4Ti3O12—PbTiO3 Aurivillius phases, Srn+1TinO3n+1 Ruddlesden-Popper phases, and metastable PbTiO3 / SrTiO3 and BaTiO3 / SrTiO3 superlattices. Accurate composition control is key to the controlled growth of such structures, and to this end combinations of reflection high-energy electron diffraction (RHEED), atomic absorption spectroscopy (AA), a quartz crystal microbalance (QCM), and adsorption-controlled growth conditions were employed during growth. The structural perfection of the films has been investigated using in situ RHEED, four-circle x-ray diffraction, atomic force microscopy (AFM), and high-resolution transmission electron microscopy (TEM).

Using an oblique-incidence optical reflectivity difference technique, we investigated kinetic processes in SrTiO3 homoepitaxy on SrTiO3(001) under pulsed laser deposition conditions. Depending upon growth temperature and oxygen ambient pressure, we found that the oxidation of an as-grown SrTiO3 monolayer may take a much longer time to complete than the recrystallization of the monolayer. The oxidation reaction was found to be characterized by an effective activation energy barrier of 1.35 eV and a large pre-exponential factor.

ZnO single crystal thin film is grown on Al2O3(0001) substrate by plasma-assisted molecular beam epitaxy employing a thin MgO buffer layer. Using reflection high-energy electron diffraction (RHEED), we investigated the surface morphology evolution during the buffer and ZnO growth. We found that a few nanometers thick MgO layer deposited on Al2O3(0001) substrate strongly influences the subsequent growth of ZnO by promoting lateral epitaxial growth, which eventually leads to an atomically flat surface. As a result, (3×3) surface reconstruction of ZnO is observed and RHEED intensity oscillations are recorded. Structural investigations indicate that MgO with rock-salt crystallographic structure forms on the Al2O3(0001) surface as a template between the substrate and ZnO epilayer. Above that, the ZnO epilayer grows with little strain. The mosaicity in the ZnO film is suppressed by more than two orders as indicated by both symmetric and asymmetric X-ray rocking curves. The twin defect with a 30° in-plane crystal orientation difference is completely eliminated. Free exciton emissions at 3.3774 eV (XA) and 3.383 eV (XB) are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.

The growth of strongly oriented or epitaxial thin films of metal oxides generally requires relatively high growth temperatures or infusion of energy to the growth surface through means such as ion bombardment. We have grown high quality epitaxial thin films of Co3O4 on different substrates at a temperature as low as 450°C by low-pressure metal-organic chemical vapor deposition (MOCVD) using cobalt(II) acetylacetonate as the precursor. With oxygen as the reactant gas, polycrystalline Co3O4 films are formed on glass and Si(100) in the temperature range 350-550°C. Under similar conditions of growth, highly oriented films of Co3O4 are formed on SrTiO3(100) and LaAlO3(100). The film on LaAlO3(100) grown at 450°C show a rocking curve FWHM of 1.61°, which reduces to 1.32° when it is annealed in oxygen at 725°C. The film on SrTiO3(100) has a FWHM of 0.330 (as deposited) and 0.29° (after annealing at 725°C). The ø-scan analysis shows cube-on-cube epitaxy on both these substrates. The quality of epitaxy on SrTiO3(100) is comparable to the best of the pervoskite-based oxide thin films grown at significantly higher temperatures.

An in situ study of barrier layers using spectroscopic ellipsometry (SE) and Time-of-Flight (ToF) mass spectroscopy of recoiled ions (MSRI) is presented. First the formation of copper silicides has been observed by real-time SE and in situ MSRI in annealed Cu/Si samples. Second TaSiN films as barrier layers for copper interconnects were investigated. Failure of the TaSiN layers in Cu/TaSiN/Si samples was detected by real-time SE during annealing and confirmed by in situ MSRI. The effect of nitrogen concentration on TaSiN film performance as a barrier was also examined. The stability of both TiN and TaSiN films as barriers for electrodes for dynamic random access memory (DRAM) devices has been studied. It is shown that a combination of in situ SE and MSRI can be used to monitor the evolution of barrier layers and detect the failure of barriers in real-time.

The surface termination of c-axis oriented YBa2Cu3O7-δ (YBCO) and the oxygen incorporation mechanism has been investigated using a unique combination of spectroscopic ellipsometry (SE) and time of flight ion scattering and recoil spectrometry (ToF-ISARS). The high surface sensitivity of the ToF-ISARS technique combined with the bulk oxygen sensitivity of SE are shown to yield complimentary information. The SE provided the film orientation and quality, while ToF-ISARS supplied surface compositional and structural information and enabled isotopic 18O tracer studies. It was determined that the 0 content of the film had little effect on the surface termination of the film, indicating a lack of labile Cu(l) sites at the c-axis oriented YBCO surface. Also, strong evidence for a Ba or BaO terminated structure is shown. The data related to the 18O tracer studies indicate that O from the reaction ambient incorporates only into the labile Cu(1) sites during both deposition and annealing, while stable O sites were populated with O from the sputtered target, indicating either the need for sputtered atomic O or sputtered YCuO complexes to occupy the stable Cu(2) sites.

Epitaxial cubic MgO thin films were deposited on single crystal Si (001) substrates by metalorganic molecular beam epitaxy (MOMBE) using the solid precursor magnesium acetylacetonate as the source and an RF excited oxygen plasma as the oxidant. The growth process involved initial formation of an epitaxial β-SiC interlayer followed by direct deposition of a MgO overlayer. The film structure was characterized by X-ray diffraction as well as conventional and high-resolution transmission electron microscopy. Both the MgO overlayer and β-SiC interlayer had an epitaxial relationship such that MgO (001) (or SiC (001)) // Si (001) and MgO [110] (or SiC [110])// Si [110]. No evidence of an amorphous layer was observed at either the MgO/SiC or SiC/Si interface. Dielectric properties of the epitaxial MgO thin films on Si (001) were evaluated from capacitance-voltage (C-V) characteristic of metal-oxide-semiconductor (MOS) structures. The C-V measurements indicated an interface trap density at midgap as low as 1011 to 1012 cm−2 eV−1 and fixed oxide charge of the order of 1011/ cm2, respectively. These results indicate that epitaxial MgO deposited by MOMBE has potential as a gate insulator.

Using molecular beam epitaxy, thin films perovskite-type oxide SrxBa1−xTiO3 (0≤×≤1) has been grown epitaxially on Si(001) substrates. Reflection high energy electron diffraction measurements and X-ray diffraction analysis indicate that high quality heteroepitaxy on Si takes place with SrxBa1−xTiO3(001)//Si(001) and SrxBa1−xTiO3[010]//Si[10]. Extensive atomic simulations have been carried out to understand the initial growth mechanism of the oxide layers on silicon. SrTiO3 layers grown directly on Si have been used as the gate dielectric for the fabrication of MOSFET devices. By varying the growth conditions the thickness of the amorphous interfacial silicon oxide layer formed during the growth of the oxide layers has been engineered to minimize the device short channel effects. An effective oxide thickness <10 Å has been obtained for a 110 Å thick SrTiO3 dielectric film with interface state density around 6.4 × 1010 cm−2 eV-1, and the inversion layer carrier mobilities of 220 cm2 V−1 s−1 and 62 cm 2 V−ls−1 for NMOS and PMOS devices, respectively. The gate leakage in these devices is 2 orders of magnitude smaller than a comparable SiO2 gate dielectric MOSFET.