G.F. Clark

In situ X-ray topography studies during the molecular beam epitaxy growth of InGaAs on (001) GaAs: effects of substrate dislocation distribution on strain relaxation

We report results from a novel facility constructed to enable in situ X-ray diffraction studies during the molecular beam epitaxy growth of Ill-V strained layer device structures on 50 mm diameter substrates. This new facility, used in conjunction with the Daresbury synchrotron source, permits X-ray topographic imaging of individual misfit dislocations formed during the molecular beam epitaxy growth process. The misfit dislocation growth and interactions can be imaged as a function of layer thickness, strain, growth and post-growth conditions. Our recent results show that the nature and distribution of dislocations threading up from the substrate are crucial in determining the initial pattern of epilayer relaxation. Under certain growth conditions and substrate dislocation distributions, large areas of the epilayer remain free of misfit dislocations at epilayer thicknesses significantly higher than the measured initial critical thickness tc1. We have observed in situ for the first time a second critical thickness tc2 (under certain conditions tc2>2tc1) at which there is a rapid increase in misfit dislocation density as a second misfit dislocation source(s) becomes active.

Evidence for very-large-area magnetic domain walls in haematite (α–Fe2O3)

Abstract Magnetic domain walls up to approximately 20 mm2 in area have been identified in thin basal plane platelets of the weak ferromagnet α-Fe2O3 using synchrotron radiation X-ray topography. The behaviour of these basal plane walls under small magnetic fields was examined in real time using the Daresbury X-ray imaging system. Area contrast is exhibited by the walls in white radiation X-ray topographs.

Low temperature synchrotron X-radiation topography observations of magnetic domains in TbAl2

Abstract Ferromagnetic domain structures have been observed in Czochralski grown single crystals of TbAl 2 using synchrotron radiation for low temperature X-ray topography. The evolution of the domain walls has been studied as external fields of up to 4x10 5 Am -1 have been applied. We discuss the interpretation of the X-ray images in terms of possible domain configurations and use the results to explain the anisotropy of magnetostriction observed previously.

Stroboscopic synchrotron-X-radiation topography and its application to the imaging of travelling surface acoustic waves

Abstract Images of travelling Rayleigh waves on surface acoustic wave (SAW) devices have recently been obtained using high frequency stroboscopic X-ray topography. When the excited SAW was phase locked to the pulsed synchrotron X-radiation emitted from the Daresbury SRS operating in single bench mode, well defined, “time frozen” images of the strains associated with the propagating wavefronts were obtained. The instrumentation and experimental requirements for these and similar stroboscopic experiments using multiple bunches are described. Examples of results of work on LiNbO3 and quartz SAW devices are given and the extension to more general stroboscopic X-ray scattering experiments is discussed.

Domain structures in haematite (α-Fe2O3)

Abstract Magnetic domain structures in basal-plane platelets of α-Fe2O3 have been studied by X-ray topography in fields up to 56kAm−1. Three types of wall were found: ferromagnetic walls perpendicular to the basal plane, in-plane 60° antiferromagnetic Bloch walls and walls inclined to the trigonal axis containing an a direction in the basal plane. Inclined walls showed dynamical diffraction fringes and became invisible when the diffraction vector was perpendicular to the wall trace. The first and third. types of wall were seen only in low fields (less than 1·5 k Am−1) and strong interactions with lattice defects occurred. Both were identified as 180° ferromagnetic boundaries. The Bloch walls also were pinned by inversion twin boundaries, leading to the wall being tilted out of the basal plane. Tilting always occurred in the same direction, irrespective of the sense of the field applied parallel to the twofold axis. Complex interdigital domain structures were observed on the tilted walls which, it is sugge...

Assessment of the crystal lattice perfection of rare earth and intermetallic single crystals by x-ray topography

Abstract Single crystals of a number of rare earths and intermetallics used for bulk magnetic measurements have been examined by X-ray diffraction topography using the Bragg geometry. Experiments were performed using both a conventional X-ray source and synchrotron radiation. Tb grown by solid state electrotransport (SSE) from high purity start material had the highest perfection as well as the highest residual resistance ratio. SSE refined Dy was of somewhat inferior perfection while Ho crystals cut from an ingot not SSE processed were of very much poorer quality. The reduction in lattice strain as cutting damage was removed was monitored. A significant decrease in perfection of the Tb was noted over several months following thermal cycling through the Curie temperature. Increase in surface strains due to oxidation was most marked in Gd and the Gd-Tb alloys. In contrast no polishing problems occurred with RAl 2 crystals grown by the Czochralski method. Clean images, showing little long range strain, were obtained from TbAl 2 , NdAl 2 and GdAl 2 .

X-ray topography of lattice relaxation in strained layer semiconductors: post-growth studies and a new facility for in situ topography during MBE growth

The ability of X-ray topography to image individual dislocations makes it an ideal tool with which to investigate the initial stages of lattice relaxation in strained-layer semiconductors. Topographs provide a useful insight into the origin of the first misfit dislocations, which define the critical thickness, and make possible quantitative analysis of the initial strain relaxation process. Examples are given for the InxGa1-xAs/GaAs and In1-xAlxSb/InSb materials systems. In the second case the rate of increase in misfit dislocation density with layer thickness is substantially lower. This is believed to be due to the different substrate dislocation densities of GaAs (>104 cm-2) and InSb (<102 cm-2). The early success of these experiments has led to the development of an MBE facility which allows X-ray topography during the growth and post-growth processing stages of the materials. The system is described and the potential benefits over ex situ studies are discussed.

Measurement of the spontaneous magnetostriction of terbium by x-ray diffractometry

Abstract The magnetostriction of high purity, SSE refined, single crystal terbium has been measured in zero applied magnetic field by X-ray diffractometry. This technique uses the splitting of the Bragg diffraction peaks due to the presence of ferromagnetic domains to determine the spontaneous strains. The measured gamma-strain is found to be much lower than that reported previously by various techniques but, unlike the high field data, it falls smoothly to zero at the Curie temperature. Reasons for the discrepancy are analysed and it is suggested that the incoherency of the domain boundary walls acts to reduce the strain within each domain. The effects on peak shape and position of slow and rapid cooling through the Curie temperature are discussed. A satisfactory fit to the experimental data is obtained using single ion theory plus a substantial term representing a two-ion interaction. This good fit can only be obtained when zero field magnetization data, derived from neutron diffraction measurements, are used.

Economical on-line image processing of synchrotron x-radiation topographs

Abstract The modestly priced INTELLECT 100 image processing system has been used to enhance white radiation topographs taken with the TV detector at the X-ray topography station at the Daresbury SRS. A very substantial reduction in noise with a corresponding dramatic improvement in image quality was obtained by gamma variation and integration times of less than 0.5s. This compromise enabled many dynamic processes to be followed, effectively in real time with low noise. With such a processor, the spatial resolution of the system was measured to be 17±2 μm. Examples of the evolution of magnetic and ferroelectric domains in applied fields are presented. Changes in micostructure associated with the phase transition in DKDP are briefly described.

Dislocation contrast in X-ray reflection topography of strained heterostructures

Abstract X-ray reflection topography for imaging misfit dislocations in highly strained systems with a small critical thickness is assessed. The samples often contain complex heterostructures such as capping layers, quantum wells or superlattices. For the first time the strain within the heterostructure is taken into account explicitly into simulation of X-ray reflection topographs. A new algorithm to simulate such heterostructures is proposed on the basis of a local kinematical approximation within one step of the numerical integration of the Takagi-Taupin equations. Dramatic effects of the heterostructure configuration on the contrast of dislocation images are predicted by simulation, based on three effects: firstly the depth of the dislocation (referred to the free surface) determines the effect of surface relaxation on the distortion field; secondly the composition around the dislocation determines which part of the dislocation field takes part in image formation; thirdly interference between differen...