Christopher J. Palmstrom

Interdiffusion and marker movements in concentrated polymer-polymer diffusion couples

Abstract Intrinsic and interdiffusion coefficients of binary polymer-polymer diffusion couples with initially large concentration gradients are markedly dependent on concentration, even when the polymers differ only in molecular weight. Assuming local thermal equilibrium of vacancies (or free volume) within the couple leads to different intrinsic diffusion coefficients for the two polymer species and to net vacancy fluxes. These fluxes should produce movement of inert markers in the couple relative to a point far from the initial interface. That such marker movements and vacancy fluxes actually occur is demonstrated by experiments in which the motion of 20 nm diameter Au islands is monitored by Rutherford backscattering spectrometry. The markers which are initially at the interface between thin films of monodisperse polystyrene (one 2 × 10 7 M w and the other 1.1 × 10 5 M w ) are progressively displaced toward the lower molecular weight side of the couple. As expected the marker displacement is proportional to the square root of time at the diffusion temperature.

Concentration profiles of non-Fickian diffusants in glassy polymers by Rutherford backscattering spectrometry

Rutherford backscattering spectrometry (RBS) is used to determine the concentration profile of 1,1,1 -trichloroethane (TCE) as it diffuses into a crosslinked polymethylmethacrylate (PMMA) glass. The penetration characteristics are those of Case II diffusion, i.e. a diffusion front moves into the glass at a velocity that is roughly constant. Due to the excellent depth resolution (<30 nm) and sensitivity (<500 p.p.m. Cl) of RBS the details of the front can be observed. Behind the front where the concentrationϕ of TCE is high and the PMMA is plasticized to a rubbery state, the concentration gradient of TCE is negligible, indicating that the diffusion coefficient of TCE in this region is greater than 10−9 cm2 sec−1. At the front the concentration of TCE decreases abruptly in less than 100 nm to a lower concentrationϕ0, and subsequently decays exponentially with the distance x from the front. These results are consistent with the Fickian solution to diffusion ahead of a moving boundary, i.e.ϕ(x)=ϕ0exp [− (υx/D)], whereυ is the velocity of the front and D is the (Fickian) diffusivity of the TCE in the glass ahead of the front. These observations are in qualitative agreement with the predictions of a model of Case II diffusion by Thomas and Windle, and a simplified version of their model is proposed.

Misfit Dislocations at Mismatched Epitactic Heterojunctions

The formation and structures of misfit dislocations are significant factors in understanding heteroepitaxy of lattice-mismatched materials. In this study, GaAs/Si, CoGa/GaAs and ErAs/GaAs heterojunctions in materials grown by molecular-beam epitaxy have been characterized using transmission electron microscopy. Different types of misfit dislocations have been generated at these interfaces. The different dislocation configurations are discussed, along with interactions between 60° and 90° dislocations in GaAs/Si heterojunctions; the 60° dislocations might be associated with surface steps or edges of islands. The growth of antiphase boundary structures in the CoGa and ErAs grown on GaAs are proposed.

Characterization of ErAs/GaAs and GaAs/ErAs/GaAs Structures

A series of ErAs/GaAs and GaAs/ErAs/GaAs epilayers have been grown on (100) GaAs substrates by molecular-beam epitaxy. Misfit dislocations at the ErAs/GaAs interface have been analyzed using the weak-beam technique of transmission electron microscopy. The microstructure of GaAs/ErAs/GaAs layers have been characterized using conventional and high-resolution electron microscopy. Twinning inside the upper GaAs layer is the major defect. Although the desired epitactic (100) GaAs on (100) ErAs does dominate, small grains of GaAs with (111) or {122} orientations have been observed at the GaAs/ErAs heterojunction.

Ion Beam Analysis of Diffusion in Polymer Melts

Two ion beam depth profiling methods have been used to measure the diffusion of polymer chains of molecular weight M into a matrix of polymer of molecular weight P. In the first the displacement xm of Au markers at the original interface of a diffusion couple between polystyrene with P=2×10 7 and a thin film of PS with M 0 5 , where D* the tracer diffusion coefficient of the M chains at 174°C, is found to be D*=O.007M −2 cm 2 /sec, in good agreement with the D*=DR expected for the reptation mechanism. Forward recoil spectrometry, a technique in which the energies of recoiling deuterons are detected, is used to obtain concentration profiles, and hence D*, of deuterated PS M-chains diffusing into a hydrogenated PS P-chain matrix. When P>>M, D*=0.008M −2 , in good agreement with the marker data. When P e 2 /(Mp 3 ) describes the diffusion of the M-chain by release of its topological constraints (by diffusion of the surrounding P-chains) and M e is an entanglement molecular weight. D* for self-diffusion (M=P) is dominated by reptation except for Ms close to M e .

RBS and TEM Analysis of Ta Silicides on GaAs

Electron-beam and sputter-deposited Ta silicides on GaAs were annealed in an As 2 overpressure ambient to temperatures as high as 920°C for 20mim. The films were then characterized with RBS, cross-sectional TEM and both electron and x-ray diffraction. The morphology of sputtered TaSi 2 /GaAs interfaces did not change, however, some interaction was detected at electron-beam deposited GaAs/silicide interfaces. Arsenic in-diffusion was detected at temperatures above 800°C and it was found to be dependent on the stoichiometry of the films. Arsenic diffusion into Si-rich electron-beam and sputter deposited films was low, whereas significantly more As diffused into the Ta-rich silicide. Some indium (3×l0 15 atoms/cm 2 ), from the InAs used as the source of As 2 overpressure, was observed to accumulate at all GaAs/silicide interfaces at temperatures above 800°C.

High-Resolution Imaging of Coga/GaAs and Eras/GaAs Interfaces

CoGa/GaAs and ErAs/GaAs grown by molecular-beam epitaxy have been studied using high-resolution transmission electron microscopy (HRTEM). The epitactic interfaces have been shown to be abrupt on the atomic scale. Computer simulations of the HRTEM images have been obtained for different interface structures under various specimen and image conditions. Practical problems in the comparison between the simulated and experimental images are discussed.

Growth of γ-alumina on crystallographically distinct aluminium substrates

The growth ofγ-alumina on crystallographically distinct aluminium substrates has been studied using transmission electron microscopy and diffraction and Rutherford backscattering. Oxides grown thermally on single-crystal substrates showed a preferred epitactic orientation relationship with the substrate, while oxides grown by the same technique on polycrystalline substrates did not exhibit a preferred orientation relationship. The grain size of the oxide was found to be at least one order of magnitude smaller than the initial grain size of the polycrystalline aluminium substrate.