D. J. Eaglesham

Dislocation nucleation near the critical thickness in GeSi/Si strained layers

Abstract A theoretical and experimental study is presented of dislocation nucleation processes in the initial stages of coherency breakdown in GeSi/Si(100) strained layers. It is shown that the use of dislocation core parameters appropriate to semiconductors leads to far higher predictions than in previous studies for the activation energy for half-loop nucleation. Homogeneous nucleation is thus unlikely at low misfit in the absence of stress concentrations at large surface steps. Experimentally the actual nucleation processes in GeSi epilayers at low (less than 1%) misfit are deduced from the microstructure at the earliest stages of dislocation introduction. The first dislocations are shown to be 60[ddot] (a/2) half-loops on the inclined [111] planes. Detailed investigation shows that these can be attributed to the operation of a completely new type of (heterogeneous) regenerative nucleation source. This involves the dissociation of a pre-existing (a/6) stacking fault (the ‘diamond defect’) to ...

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...

New phases in the superconducting Y:Ba:Cu:O system

An electron diffraction and microscopy study is presented of a variety of phases in the Y:Ba:Cu:O system in which superconductivity occurs. The superconducting phase is demonstrated by convergent beam electron diffraction to be centrosymmetric with space group Pmmm, in contrast to a previous determination of Pmm2. This discrepancy arises from local symmetry‐breaking defects. In addition to this phase and a cubic BaCuO2 phase, we characterize two other phases. One is the Y‐rich orthorhombic phase: Pnma with a=13.5 A, b=6.3 A, and c=7.6 A. The second occurs by a phase transition of the superconducting Pmmm phase to P4/mmm with a=3.85 A, c=11.7 A. The superconducting phase may now be described as either an ordered array of oxygen vacancies in the perovskite structure, or an ordered array of oxygen interstitials in the new tetragonal phase, which may explain how the material can lose oxygen reversibly.

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.

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 .

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).

Nanolithography using field emission and conventional thermionic electron sources

Abstract This paper presents a number of new results on direct nanometre-scale electron beam drilling and writing of inorganic materials. It is demonstrated that drilling thresholds for a number of materials can be exceeded using a conventional thermionic tungsten filament in a standard electron microscope. Results are presented not only for materials (such as sodium beta-alumina) in which drilling is a well established phenomenon, but also for a number of materials in which drilling has not previously been reported (including K 3 CuF 6 , MoO 3 , and an amorphous Y-Ba-Cu-O phase close to the superconducting composition). Preliminary results on the temperature dependence of electron drilling are given, and it is shown that it is easier to drill amorphous alumina at liquid nitrogen temperature than at room temperature. It is demonstrated that using condensed gases upon a substrate, reactive electron beam etching of the substrate on a nanometre scale may be performed; this chemically-assisted drilling process allows us to write in a number of materials (such as C films) which are not normally beam sensitive. Although an electron beam of circular cross-section normally drills circular holes, it is shown that square holes may be drilled in MgO crystals using a circular cross-section beam.

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 ...

High temperature superconducting ceramics

Abstract We review some of the recent developments in our empirical understanding of the materials science of high T c materials. First, the distinguishing phenomenological Ginzburg-Landau features of these materials are identified, and the possible technological implications discussed. We then focus on two key issues for processing the 1-2-3 materials: oxygen stoichiometry and grain boundary phases. Finally, we briefly examine the impact of the recent discoveries of bismuth-based and thallium-based materials on the field.

Compositional modulations in GexSi1−x heteroepitaxial layers

A study is presented of compositional modulations (i.e., so‐called banding) in heteroepitaxial strained layers of nominally uniform Ge0.1Si0.9 grown by molecular‐beam epitaxy (MBE). The oscillatory nature of the modulations is studied by transmission electron microscopy (TEM) techniques and the results show that the contrast modulations have a periodicity of ∼23 nm when 1‐μm‐thick epilayers are grown on a rotated substrate. For growth on an unrotated substrate, contrast modulations are still observed but they are less regular. It is further shown that the electron microscope image features are dominated by strain contrast and not structure factor contrast. The presence of strain is consistent with there being a small change in lattice parameter between adjacent bands of material, together with elastic relaxations afforded by the proximity of the thin‐film surfaces. It is concluded that these variations in the lattice parameter are due to the compositional modulations. Limits on the magnitude (Δx) of the c...