No abstract

The growth of PbTi03 films by a metalorganic chemical-vapor-deposition technique has resulted in three-dimensionally epitaxial heterostructures on various single-crystal substrates. These heterostructures consist of PbTi03 films on the (001) surface of the single crystals: potassium tantalate (KTa03), strontium titanate (SrTi03), and magnesium oxide (MgO). It was found that the presence of a structural (ferroelectric) phase transition in PbTi03 leads to a "strain-accommodating" mechanism in which a domain pattern forms as the system cools through the Curie temperature and limits the extension of interfacial strain in the heterostructurethus minimizing the total energy of the heterostructure. ForPbTi03/KTa03 (001), the interfacial strain is accommodated by the formation of a periodic domain pattern in the overlayer. In PbTi03/Sr Ti03 (001), which exhibits an excellent lattice match between respective a lattice parameters, the film exists as a single c domain. The PbTi03/MgO(001) system, having a poor lattice match for both the a and c axes, appears to find the energy minimum by locking into domains of two-dimensional superlattices with the greatest atomic coincidences. It is found that the nature of the domain pattern depends very strongly on both the film thickness and measuring temperature.A theoretical model of the domain-pattern formation has been developed by using linear-elasticity theory and a Landau-Ginzburg-Devonshire-type phenomenological theory for the substrate and the overlayer, respectively. The theoretical predictions and the experimental measurements were in good agreement in both the thickness and temperature dependence of the relative domain population and the spontaneous strains.

A total-electron-yield technique is described in which near-surface extended x-ray-absorption 6ne-structure (EXAFS) data are obtained from direct measurements of specimen current. Experiments with several model systemsamorphous germanium and crystalline germanium, nickel, and cobalt; and arsenic ion implanted into silicondemonstrate that this technique can reproduce EXAFS X(k) functions obtained from transmission and fluorescence measurements. Experiments also reveal that EXAFS amplitudes from total-electron-yield data can be 5-10% smaller than those from transmission measurements for samples where the very-near-surface structure, at depths of tens to hundreds of angstroms, divers from the bulk structure. Measurements with buried layers con6rm that the sampling depth for this total-electron-yield technique is determined primarily by the penetration ranges of Auger electrons emitted from the absorbing atoms. For the model systems listed above, LMM Auger electrons have ranges of hundreds of angstroms and ELL Auger electrons have ranges of thousands of' angstroms. Expressions are derived for the sampling depth for total-electron-yield EXAFS experiments. The total-electron-yield technique described here is particularly useful for studying impurities within a few thousand angstroms of the surface of single crystals, where Bragg difFraction complicates the use of fluorescence measurements.sult of contributions of inelastically scattered photoelectrons. Guo and denBoer' and Lytle et al. ' used a gas-Qow electron detector' to measure electron total yield EXAFS signals for Ni, Cu, Fe, and Cr K edges. They observed considerable differences for Ni, Cu"and Fe between the amplitude values obtained from transmission measurements and the corresponding values from a gasflow detector. The range of subsurface sensitivity for total-electron-yield EXAFS has been estimated to be 1000 A from measurements for Cu (E edge), less than 390 A for A1203 (Al K edge), and 700-1000 A for GaAs (E edge). '6The present paper describes a method for totalelectron-yield measurements" ' mhich is particularly mell suited for near-surface EXAFS from single-crystal samples, where Bragg difFraction often makes fluorescence measurements diScult or impossible. The method reported here detects the total yield of electrons from the sample by measuring the sample current. Following descriptions of the technique and its applications, comparisons of total-electron-yield and transmission or fluorescence EXAFS measurements are given, to demonstrate the reliability of total-electron-yield measurements, particularly in light of previously reported di5culties with EXAFS amplitudes from total-electronyield measurements. ' ' ' Measurements and calculations are then given concerning the range of subsurface sensitivity for total-electron-yield EXAFS measurements.Results from the present study are also compared with those of previous investigations. 37 24SO

No abstract