G. S. Green

X-ray topography of the coherency breakdown in GexSi1−x/Si(100)

An x‐ray topography study is presented of the coherency breakdown in GexSi1−x/Si(100) strained epilayers. Finite dislocation densities (in excess of 103 cm−2) are observed at compositions in the range 12–13 at. % Ge for an epilayer thickness of h≊180 nm. Above 13 at. % Ge the dislocation density starts to change rapidly and this composition is identified as critical for h≊180 nm, a thickness which is almost a factor of 4 lower than the accepted ‘‘critical’’ thickness for this lattice mismatch. The result suggests that in low‐mismatched GexSi1−x alloys the dislocation density will increase continuously at the ‘‘critical’’ thickness, as opposed to exhibiting a sharp onset. The implications of these results to the various models of the critical thickness transition are discussed.

The Nucleation and Propagation of Misfit Dislocations aear the Critical Thickness in Ge-Si Strained Epilayers

The nucleation and propagation of misfit dislocations in Ge-Si strained epilayers on (100) Si have been investigated using transmission electron microscopy and X-ray diffraction topography at low lattice parameter mismatch (˜ 0.8%). Misfit dislocations nucleate as half loops which are predominantly unfaulted (> 90%) at the advancing growth interface. Under the driving force of the epilayer strain, unfaulted half loops glide and expand on inclined { 111 }planes toward the heterointerface (i.e. substrate/epilayer interface). These unfaulted half loops consist of a 60°-dislocation segment which lies along in a plane parallel to the heterointerface (i.e. (100)) and this segment is connected to the growth interface by two screw dislocation segments which both lie on the same inclined {111} glide plane. As 60° dislocations reach the heterointerface on each of the four inclined {111} variants, they form an orthogonal array of misfit dislocations which lie along [011] and [0 1 1]. At higher lattice parameter mismatch (˜ 2%), there appear to be some important changes in the dislocation behavior and these changes result in orthogonal arrays of heterointerface dislocations which are predominantly edge type (i.e. 90°dislocations).

Nucleation of misfit dislocations in strained-layer InGaAs on GaAs

Abstract Misfit dislocations at the interface of a thin epitaxial layer of In0-18Ga0-82As on GaAs have been studied by glancing incidence double-crystal synchrotron X-ray topography. The misfit dislocation density increases dramatically as the layer thickness increases from 17 to 18nm across the sample. Clear evidence for the nucleation of misfit dislocations at threading dislocations from the substrate is presented.

Effect of interface structure on the X-ray double crystal rocking curve peak position from very thin layers in the highly asymmetric Bragg geometry

Double crystal rocking curves from a thin epitaxial layer of GalnAsP on InP have been studied in the highly asymmetric, glancing incidence, Bragg geometry. While a simulation based on a single layer fits the data quite well, a slightly better fit to the data is obtained by assuming the presence of a thin, highly mismatched, interface layer. The interface layer, however, significantly affects the main layer peak position. Very similar rocking curves are found for substantially different structures. We examine the implications for the determination of main layer composition from mismatch measurement.

Dislocation images in X‐ray section topographs of curved crystals

The contrast of dislocation images in X-ray section topographs of curved crystals has been investigated both experimentally and by computer simulation. Good agreement is found between experiment and simulation in both symmetric and asymmetric Laue geometries. Very little change is observed in the symmetric Laue images for radii of curvature as small as 18m. In the asymmetric geometry, the background intensity rises and the Kato fringe visibility decreases as the curvature increases. The dynamical image becomes more localized and dominates the image as the direct image visibility falls. Our results explain the reversal of dislocation contrast in Lang projection topographs of curved crystals taken in asymmetric geometry. 1987). The differences reported in the literature can all be ascribed to artefacts of the bending process. In the asymmetric Laue geometry, however, very substantial differences occur, the most striking being a reversal of the dislocation contrast from thin crystals as the curvature is increased. This contrast reversal is also found in Lang topographs of curved crystals and was reported many years ago by Meieran & Blech (1972). Until now, this phenomenon has never been satisfactorily explained. We have undertaken an experimental and computer simulation study of the changes occurring in X-ray section topographs of a deliberately curved silicon wafer. From the results it is possible to understand the changes in the images of the (integrated wave) Lang and Hirst topographs.

Asymmetric Relaxation in Epitaxial Layers of III-V Compounds

Relaxation in a 3 μm epitaxial layer of GaAsSb on GaAs, a 1 μm layer of InGaAs on InP and an InGaAs superlattice on InP has been investigated by double crystal X-ray diffractometry and double crystal X-ray synchrotron topography and found to be asymmetric. The origins of assymetric relaxation are discussed and the sensitivity of diffractometry and topography to the detection of layer relaxation compared.

Assessment of Thin Heteroepitaxial Layers Using Skew Angle Asymmetrical X-Ray Double Crystal Diffraction

Asymmetric reflections in which the beam paths are skew with respect to the sample surface have been used to characterize thin heteroepitaxial layers by double axis X-ray diffractometry. By utilizing reflections with Braqg cones which are partially embedded in the sample surface it is possible to tune to grazing the angles of incidence or emergence. Enhanced layer to substrate peak intensity ratio and narrower layer peak widths can be obtained. The technique is demonstrated using skew angle reflections 333, 133 and 044 from a 400A GaInAsP layer on an InP substrate.

Misfit Dislocations at the Critical Thickness for Ingaas/Gaas Strained Layers

The contrast of misfit dislocations in an InGaAs layer, close to the critical thickness and capped with GaAs grown by MBE on a (001) oriented GaAs substrate has been investigated by double axis synchrotron X-radiation topography. The layer thickness variation as a function of position has been measured to a precision of 1A by matching interference fringes observed in the 004 symmetric reflection double crystal rocking curves with simulations. The misfit dislocation density is highly anisotropic, varying from zero to a high value with increasing thickness. The contrast of the dislocations in the 004, 224 and 044 reflections has been examined in detail. All of the long dislocation segments characterized were 60° in character with ½ Burgers vectors inclined to the specimen surface. No dislocations were found which did not appear to be of this type. A surprising difference in contrast of the background in the 22 4 and 2 2 4 reflections is discussed.

X-Ray Section Topography of Hydrogen Precipitates in Silicon

Spherical strain centres produced by annealing of silicon crystals grown in a hydrogen atmosphere have been studied by X-ray section topography. Excellent agreement has been found between experimental images and those simulated by numerical solution of Takagis equations. The contrast differences between images in four reflections are examined and recommendations made as to the most suitable conditions for the study of oxygen precipitates in processed silicon wafers.