Search Results - (Author, Cooperation:I. K. Robinson)

2018-03-07

American Physical Society (APS)

1098-0121

1095-3795

Physics

Structure, structural phase transitions, mechanical properties, defects

2013-05-25

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2015-05-02

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7550

AIP Digital Archive

Physics

The state of relaxation in two different superlattices (SLs) of a system with large lattice mismatch, Ga0.8In0.2As/GaAs grown on GaAs [001] by molecular beam epitaxy, has been investigated by surface-sensitive grazing-incidence diffraction (GID). The SL is squeezed between the substrate and a thick GaAs top layer. The thickness of individual GaInAs layers ta (active layer) is the same in both samples, while the GaAs barrier thickness tb is different. We have studied the influence of the thickness ratio tb/ta on the state of relaxation for different distances from the sample surface. We find that for thick barriers the whole SL remains coherently strained and for the thinner barrier thickness the SL is partially relaxed against the the GaAs top layer. The GID technique was applied for the first time to obtain depth resolution of the lateral lattice parameter in a SL. It is demonstrated to be especially well suited for SL systems with a small difference of the average electron density between the sublayers. The scattering contrast is improved by measuring the intensity as a function of the exit angle ("rod scans'') from the "weak'' (200) Bragg reflection. Comparing computer simulations with the measured variation of the scattering contrast between GaAs and GaInAs layers obtained from different "information depths'' and at different angular positions of the in-plane rocking curves, the state of relaxation can quantitatively be evaluated. On the basis of these results we propose two models for the partial relaxation of the SL into the state of strain-reduced domains. We believe that the partial relaxation is due to the elastic field interaction between the GaInAs layers accross the GaAs barriers, if tb is small.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

X-ray diffraction has been employed to investigate the strain relaxation of both components of a GaN/AlN bilayer on sapphire (0001) as a function of the GaN layer thickness. Below a critical thickness, GaN and AlN both relax with the same in-plane lattice constant, consistent with the energy minimum condition of elasticity theory for a bilayer. Above the critical thickness, however, the strain relaxations in the two layers were different. We can fit this strain relaxation behavior with a free standing bilayer model with an additional term describing the interaction of dislocations. © 1999 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

We present the design and first results of the performance of a new four-circle x-ray diffractometer featuring the less-common "kappa'' geometry originally used on the CAD-4 instrument. This geometry permits access to all reciprocal-space settings while the mechanical supports remain entirely on one side of the beam; this is very useful for split ports on bending-magnet beamlines. Our design is able to carry heavy loads, such as a Displex-type cryostat, and operates at relatively high speeds because it uses direct drive servomotors. Considerable attention has been given to the distribution of loads which is optimized for horizontal-axis operation. A sphere of confusion of 50 μm was achieved fairly easily for the mutual alignment of the axes, and this can probably be improved. Our initial tests show that accurate alignment of crystals can be achieved, and reliable measurements have been made on a number of experimental systems. Rocking curves of silicon have been measured, but these are at the limit of the setting accuracy. Arbitrary settings are achieved in about 1 s, but up to an additional 1 s settling time is needed for high accuracy work. © 1995 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

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. © 1995 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A strained pseudomorphic Si0.7Ge0.3 film grown by gas-source molecular-beam epitaxy on Si(001) was irradiated at room temperature with 25 keV Ga+ ions. The gradual strain relaxation of the metastable Si0.7Ge0.3 film was monitored using in situ x-ray diffraction as a function of dose. Based on a dimensional argument, the ion-induced damage scales as extended defects. The Hendricks-Teller model was successfully applied to explain the shifting and broadening of the additional diffuse scattering. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have undertaken a new set of experiments to investigate the behavior of adsorbed-impurity induced reconstructions at growth interfaces. We have observed a striking difference in the stability of the B(3)1/2×(3)1/2 and Ga(3)1/2×(3)1/2 two-dimensional structures at the interface between Si(111) and a-Si, and in their segregation behavior during molecular beam epitaxy crystal growth. This leads to a new model of dopant behavior in silicon molecular beam epitaxy.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Superstructure reflections of lead magnesium niobate (PMN), Pb(Mg1/3Nb2/3)O3, were measured with synchrotron radiation x-ray diffraction. Our data conclusively demonstrate that the (h+〈fraction SHAPE="CASE"〉12,k+〈fraction SHAPE="CASE"〉12,l+〈fraction SHAPE="CASE"〉12)-type reflections are the result of chemical ordering between the Mg and Nb ions in domains of 50 Å. Structure factor analysis of these peaks reveals an oxygen displacement within the ordered regions of 0.044(3) Å along a 〈100〉 direction towards the Nb ion. Single crystals of La-doped PMN (La–PMN) exhibit much larger chemically ordered regions (up to 900 Å for 10% La–PMN). Structural analysis of the La–PMN suggests that as the ordered region increases in size, the ordering changes from complete Nb:Mg ordering and approaches Nb:(Mg2/3Nb1/3) ordering. Although anticipated from electrostatic energy arguments, our conclusions are based on a clear trend in the bond length of the mixed site as a function of La doping. © 2000 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

Angle calculations for the four-circle diffractometer are usually made by the method of Busing and Levy [Acta Cryst. 22, 457 (1967)]. Here an additional mode of operation is described that allows control of the incident and exit angles at the sample surface. Experiences with this mode for surface crystallographic data collection are described.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Synchrotron x-ray diffraction was employed to measure the lattice constants a and c of GaN films grown with an AlN buffer layer on sapphire (0001) over a thickness range of 50 A(ring) to 1 μm. We used multiple reflections and a least-squares fit method for high reliability. As the thickness increased, the lattice constant a increased from 3.133 A(ring) to 3.196 A(ring) and c decreased from 5.226 A(ring) to 5.183 A(ring). The expected trend was fitted to an equilibrium theory, allowing the critical thickness of GaN on AlN to be estimated at 29 A(ring) ± 4 A(ring) in good agreement with a theoretical prediction. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We demonstrate the utility of resonant x-ray scattering in probing the structure of doping layers at a heterostructure interface. The positions of germanium layers inserted at the interface of a silicon epitaxial film assert a strong influence of the phase of the scattered intensity along the crystal truncation rods. The phase of the scattering, and hence the internal structure of the layers, can be determined conveniently by analyzing its energy dependence in the vicinity of the Germanium absorption edge at 11.103 keV. © 2001 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We present the results of an x-ray diffraction investigation of the formation of oxide on electropolished Cu(111) surfaces, both in situ at pH 4.5 and in air. In both cases the oxide is found to be crystalline cuprite and epitaxially aligned with the substrate, but with two possible epitaxial orientations. We followed the progress of oxidation by monitoring the shapes of the diffraction peaks for the two orientations as a function of time and potential. There is a narrow potential region where the oxide is a single monolayer thick. Beyond that, only one of the two oxide orientations becomes thicker, and does so in an inhomogeneous manner, thickening in narrow regions before it spreads. © 1999 American Institute of Physics.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] The full three-dimensional structure determination of tomato bushy stunt virus has shown how its protein subunit can accommodate to different packing environments in the viral coat and how the size of the particle is nonetheless determined without ambiguity. Reversible, cooperative expansion of the ...

Electronic Resource