G.B. Stephenson

Nanocrystalline materials by crystallization of metal–metalloid glasses

Abstract The formation of ultrafine microstructures by crystallization of metal-metalloid glasses was investigated by means of electron microscopy as well as in situ time-resolved X-ray diffraction. The results can be understood on the basis of nucleation and growth theories, taking into account the effect of recalescence during massive crystallization and the differences in the mode of crystallization and the diffusivity. In a polymorphic crystallizing Fe66Ni10B24 glass the finest microstructure can be achieved by annealing at temperatures significantly below the “nose” of the TTT diagram; the finest grain size can be calculated and observed to be in the range of about 0.1 μm. In glassy Fe73.4Cu1Nb3.1Si13.4B9.1 (FINEMENT) the combination of a reduced growth rate due to the niobium content as well as with increasing size of the primary crystals and an accelerated nucleation rate due to the copper additions allows the formation of extremely fine-grained microstructures in primary crystallizing metal-metalloid glasses at temperatures above the glass transition.

Instrumentation for millisecond‐resolution scattering studies (invited)

Time‐resolved x‐ray scattering studies of phase transition kinetics have been carried out using the wide‐bandpass monochromator and fast linear position‐sensitive detector system at the IBM/MIT beamline X‐20C at the National Synchrotron Light Source (NSLS). We report here on the instrumentation that has been developed for these studies, and in particular on the methods used to measure, change, and control sample temperature with millisecond resolution.

Optical characterization of surfaces during epitaxial growth using RDS and GIXS

Structures of (001)GaAs surfaces during organometallic vapor phase epitaxy (OMVPE) have been studied by simultaneous measurements using reflectance difference spectroscopy (RDS) and grazing incidence X-ray scattering (GIXS). The data obtained by the two techniques are strongly correlated. We find that RDS, although not a diffraction probe, can distinguish most surface reconstructions of (001)GaAs. The information obtained by these two probes is complementary, so by combining RDS and GIXS data obtained from surfaces under static and dynamic conditions we can gain detailed information about atomic arrangements and growth mechanisms. Under static conditions, we find that RD spectra under OMVPE conditions are similar to those previously observed in ultrahigh vacuum (UHV). By simultaneously monitoring the surface reconstructions using GIXS and RDS, we have established that RDS accurately reflects the local structural configurations but is not uniquely sensitive to changes in long range order or subtle differences in symmetry. Growth oscillations were also measured simultaneously by RDS and GIXS under various conditions. The observed oscillation periods agree with each other, but the details are condition dependent, thus providing important clues concerning the surface processes involved. For example, the one-to-one correlation observed between RDS and GIXS oscillations suggests that the RDS oscillations are related to island formation.

Atomic scale studies of epitaxial growth processes using X-ray techniques

In this work we have applied grazing incidence X-ray scattering to the study of nucleation and growth of GaAs by organometallic vapor phase epitaxy. Using the sensitivity of scattering at the 110 crystal truncation rod (CTR) position to the smoothness of the surface, we are able to monitor the crossover from layer-by-layer to step-flow growth. More detailed information about the nucleation process was determined by using diffuse scattering near the CTR position to measure the spacing of two-dimensional islands during growth as a function of flux and deposition temperature. Application of standard nucleation theory to the analysis of this data suggests that the critical island size during OMVPE growth is likely to be much larger than a single adatom, in contrast with what is usually assumed for semiconductor systems.

Asymmetrically cut crystals as optical elements for highly collimated x‐ray beams

Asymmetrically cut perfect crystals, in both the Laue and Bragg geometries, are examined as single crystal monochromators for x‐ray beams that are collimated to a small fraction of the Darwin width, as is typical in experiments with coherent x rays. Both the Laue and asymmetric Bragg geometries are plagued by an inherent chromatic aberration that increases the beam divergence much beyond that of the symmetric Bragg geometry. Measurements from a recent experiment at the ESRF are presented to compare Si(220) (symmetric Bragg), diamond(111) (asymmetric Laue), and diamond(111) (symmetric Bragg inclined) geometries.

Characterization of vapor phase growth using X-ray techniques

Abstract Chemical vapor deposition environments, while technologically quite important, are difficult to study using traditional analytical probes, such as electron-based techniques and optical tools. In this work, we will describe some of the ways in which X-rays can be applied to understand not only the gas phase composition through fluorescence, but also surface processes such as nucleation and diffusion.

In-situ X-ray studies of organometallic vapor phase epitaxy growth

Abstract In this paper we present an overview of the use of X-ray measurements to determine the atomic mechanisms of organometallic vapor phase epitaxy (OMVPE). Detailed information about OMVPE processes has been difficult to obtain because of the high pressure, chemically reactive environment present during growth. X-ray scattering measurements using very intense synchrotron X-ray sources have been uniquely productive in determining the structure of surfaces during OMVPE growth. For example, surface smoothness, step ordering, growth modes, growth rates and surface reconstructions have been determined. In addition, X-ray spectroscopy measurements have revealed much about the complicated gas phase behavior in the OMVPE reactor. From these scattering and spectroscopy measurements, a more complete model of OMVPE growth is being developed.

Pyrometric temperature controller for million degree per second heating

A high‐power proportional temperature controller, using a fast infrared pyrometer, has been developed to change and control the temperature of metallic ribbon samples with microsecond response. The apparatus provides uniform and controlled heating for time‐resolved x‐ray scattering studies of structural phase transitions. When high‐power pulse heating is used, the system is capable of increasing the sample temperature at rates in excess of 106 K/s, without overshoot and with subsequent control to ±1 K at temperatures as low as 650 K.

A general technique for characterizing x-ray position sensitive arrays

We present a general statistical technique for characterizing x-ray sensitive linear diode arrays and CCD arrays. We apply this technique to characterize the response of a linear diode array, Princeton Instrument model X-PDA, and a virtual phase CCD array, TI 4849, to direct illumination by x-rays. We find that the response of the linear array is linearly proportional to the incident intensity and uniform over its length to within 2 %. Its quantum efficiency is 38 % for Cu K{sub {alpha}} x-rays. The resolution function is evaluated from the spatial autocorrelation function and falls to 10 % of its peak value after one pixel. On the other hand, the response of the CCD detecting system to direct x-ray exposure is non-linear. To properly quantify the scattered x-rays, one must correct for the non- linearity. The resolution is two pixels along the serial transfer direction. We characterize the noise of the CCD and propose a model that takes into account the non-linearity and the resolution function to estimate the quantum efficiency of the detector. The quantum efficiency is 20 %.

Rapid crystallization of amorphous CoZr and FeB close to eutectic compositions

We present in situ time-resolved X-ray scattering data and transmission electron microscopy studies on ribbons of Co 92 Zr 8 , and Fe 81.5 Bi 18.5 . Both systems exhibit a profound change as the crystallization temperature, and hence transformation rate, is raised. At low temperatures (half-time t 1/2 for crystallization of the order of minutes), crystallization occurs by eutectic growth of two crystalline phases; at high rates (t 1/2 of the order of seconds) the mode changes to simultaneous polymorphic crystallization of the two phases, with no eutectic growth. It is suggested that the change of mode occurs when the temperature exceeds the glass transition temperature