D. M. Maher

Variation of Dislocation Morphology with Strain in Ge{sub x}Si{sub 1-x} Epilayers on (100)Si

A change in microstructure, including dislocation Burgers vector, length, and behavior, has been observed to occur when the epilayer mismatch is varied in Ge x Si 1− x layers grown on (100) Si. At low mismatches ( 2.3%) there is an orthogonal array of short edge dislocations. At intermediate mismatches (1.5 to 2.3%) there is a mixture of 60° and edge dislocations. The nature of the microstructure has a pronounced effect on the density of threading dislocations in the epilayer, which increase by a factor of ∼60× through a relatively small range of mismatch (1.7 to 2.1%, corresponding to x ranging from 0.4 to 0.5). These morphologies are discussed in the light of recent work on the sources of misfit dislocations. While mechanisms for the introduction and propagation of dislocations at low mismatch have recently been observed and explained, the high misfit case is clearly very different; i.e., surface nucleation seems to be likely in the latter case as opposed to operation of an internal source in the former. A mechanism for edge dislocation formation is proposed.

Convergent-beam imaging−a transmission electron microscopy technique for investigating small localized distortions in crystals

Abstract A new transmission electron microscopy technique is described, convergent-beam imaging (CBIM), in which an image is formed using a convergent beam focused above or below a thin crystalline specimen. Superimposed on the image are higher-order Laue zone (HOLZ) lines, the displacements of which map the spatial variation of strains, lattice parameters and crystallographic rotations. The image has its normal resolution, the superimposed diffraction information has an angular resolution of about 10−4 rad, and this diffraction information comes from regions of the specimen defined approximately by the cross-over size of the electron probe size, broadened by beam spreading. The visibility of HOLZ lines in the CBIM image is significantly improved by cooling the specimen. The CBIM technique is illustrated by its application to a Si/GexSi1−x strained-layer structure, where spatial changes in the lattice parameter and crystal symmetry are revealed by the relative positions of the HOLZ lines in the Si and all...

Electron energy loss spectroscopy: Detectable limits for elemental analysis

Abstract In this paper equations which govern the detectable limits in a transmission electron energy loss experiment, where inner-shell edges are used for micro-elemental analysis, are derived. The minimum detectable number of atoms and the minimum mass fraction are evaluated for several model cases, including the variation with spectrometer acceptance angle and accelerating voltage.

CBED and CBIM from semiconductors and superconductors

Abstract The applications of Convergent Beam Electron Diffraction (CBED) and Convergent Beam Imaging (CBIM) to the study of semiconductors and superconductors are reviewed. These applications include the use of well established methods such as point and space group determinations (with YBa 2 Cu 3 O 7−δ ceramic superconductors being demonstrated to be Pmmm in spite of a very high density of symmetry-breaking defects) and lattice parameter determinations in strained crystals (where CBED allows us to observe the small monoclinic distortions present in partially relaxed GeSi strained epilayers). The use of CBIM is demonstrated for studies of strains in GeSi-Si strained-layer superlattices and interface between Si and SiO 2 . Finally, CBED is also shown to have application in the study of local compositions (on a scale which is significant both in modern semiconductor devices and in the new ceramic superconductors), and two methods for microanalysis are demonstrated: zone-axis critical voltage effects are studied in the ceramic superconductor La 2−x Sr x CuO y , as a function of y and HOLZ branch structures are studied in Al x Ga 1−x as a function of x .

Detection and measurement of local distortions in a semiconductor layered structure by convergent‐beam electron diffraction

It is shown that local tetragonal distortion in a Si Ge0.05Si0.95 strained‐layered structure can be detected and quantified from analyses of the high‐order Laue zone lines which are present in the bright‐field disks of [001] convergent‐beam electron diffraction patterns. The detection of a tetragonal distortion is based on symmetry arguments, whereas the quantification requires a detailed analysis which is based on computer simulated patterns, assuming a kinematical approximation of the scattering process and including surface relaxation. The result of this quantitative analysis is in good agreement with the tetragonal distortion which is predicted by isotropic elasticity theory.

The quantitation of electron energy loss spectra

An interactive computer program which allows the quantitation of electron energy loss spectra is described. The program performs a standardless analysis using hydrogenic cross‐section models. The accuracy of the results obtained for a wide range of experimental conditions is typically within 20% of expected values for well characterized samples. The lack of agreed thin film standards is a limit to the improvement of the cross‐section models, but under most practical operating conditions the largest errors arise from instrumental factors such as the effect of finite beam convergence and chromatic errors in the coupling optics.

Topographical contrast in the transmission electron microscope

An adaptation of the Foucault method for topographical imaging in the transmission electron microscope is described in detail. The image contrast is produced by selection of electrons which have suffered differential phase retardations in the specimen inner potential. Surface or interface displacements produce bright or dark image contrast, and the ultimate resolution approaches that of the atomic scale. The imaging method is applied in studies of both amorphous and crystalline objects. The possibility of performing quantitative measurements is demonstrated by the estimation of the inner potential of crystalline MgO.

A practical electron spectrometer for chemical analysis

This paper describes the basic parameters which have been considered in the design of a practical electron spectrometer optimized for micro‐chemical analysis in a 100 kV transmission electron microscope fitted with a scanning unit. Constructional factors and the operation of this spectrometer are discussed, as well as data acquisition. Typical results are presented to illustrate the operative modes. Both the prototype spectrometer and subsequent copies of it now are being utilized sucessfully in studies related to materials and biological sciences.

The core structure of extrinsic stacking faults in silicon

The core structure of oxidation‐induced extrinsic stacking faults in silicon has been studied by high‐resolution transmission electron microscopy. The stacking faults were viewed edge‐on in thin (011) crystals so that two sets of (111) lattice planes were imaged simultaneously. The images confirm the stacking sequence A‖aCcB‖b (vertical bars denote twinning planes) that was predicted by Hornstra for the stacking‐fault core.

Tetragonal and monoclinic forms of GexSi1−x epitaxial layers

The effect of strains on the local crystallographic symmetry and structure of a Si/GexSi1−x model heterostructure grown on a (100) silicon substate was studied using convergent beam electron diffraction techniques and a cross‐sectional specimen geometry. The alloy layers are seen to distort into relaxed tetragonal and monoclinic structures which are dependent on position and/or alloy composition. These observations can be explained in terms of strain relaxation in a thin‐film specimen and deviations of the substrate from a perfect (100) orientation. The results have implications not only for the use of cross‐sectioned specimens in the characterization of strained‐layer heterostructures, but also for the band engineering of Si/GexSi1−x strained‐layer superlattices and other materials which are grown on vicinal (100) and other low‐symmetry substrate orientations. In particular relaxed tetragonal and monoclinic structures may be quite relevant to the emerging science of strain‐induced lateral confinement of ...