C. J. Humphreys

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 ...

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.

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.u2009% Ge for an epilayer thickness of h≊180 nm. Above 13 at.u2009% 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.

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.

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...

Strains and Misfit Dislocations at Interfaces

In recent years there have been great advances in the heteroepitaxial growth of nonlattice- matched epilayers and strained-layer superlattices (SLSs). The role of lattice strain in these structures is extremely important. Not only does strain exert a large influence on band gaps, band offsets, effective masses and mobilities, but strain can be used deliberately to “fine-tune” these device properties. Hence it is essential to be able to measure local strains in order to understand and quantify the physical properties of the material. In this paper we will illustrate the use of Convergent Beam Electron Diffraction (CBED) for the measurement of local strain, and we will describe the use of a new technique, Convergent Beam Imaging (CBIM), for detecting, mapping and measuring small crystalline distortions.

Dislocation Behaviour in Ge x Si 1-x Epilayers on (001)Si

We have observed that the nature of misfit dislocations introduced near the critical thickness in Ge x Si 1-x alloys on (001)Si changes markedly in the region 0.4 ≤ x ≤ 0.5. At or below the lower end of this compositional range, the observed microstructure is comprised almost entirely of 60° type dislocations, while at the high end, the dislocation structure is almost entirely Lomer edge type. Concurrent with this change, the dislocation density at the top of the epilayer varies by a factor of about 60X. Similarly, several other observables (e.g. dislocation length and spacing) also change appreciably. Part of the reason for the morphological variation seems to be a change in the source for dislocation introduction, in conjunction with a change in glide behaviour of dislocations as a function of film thickness. Evidence will be presented that indicates strain, as well as thickness, has a critical value for some dislocation introduction mechanisms, and that these together determine the resulting microstructure. Furthermore, it appears unlikely that the edge-type Lomer dislocations which appear at about x = 0.5 are either introduced directly, by climb, or grown in, as in the three-dimensional island growth and coalescence which occurs when x approaches unity. Instead, a two-step mechanism involving glissile dislocations is proposed and discussed.

High Resolution Electron Microscopy and Convergent Beam Electron Diffraction of Semiconductor Quantum Well Structures

For the structural characterisation of Low-Dimensional Structures the parameters we need to know include the following,: (i) the precise thickness of each layer, (ii) the presence of interface steps, (iii) the positions of atoms in the layers and at the interfaces, (iv) whether or not crystallographic defects such as dislocations and planar faults are present, (v) the local chemical composition on a nanometre scale, (vi) the electron configuration of the atoms (particularly the d-band occupancy of transition metals in magnetic superlattices) and (vii) local strains with nanometre scale spatial resolution. In addition it would be very useful to have a ‘strain map’ superimposed on the image.

Analytical electron microscopy of [Ni38Pt6(CO)48H]5–

Approximately uniform size particles, corresponding to the dimension of the metallic skeleton of [Ni38Pt6(CO)48H]5–, have been observed in the electron microscope at low beam intensities and electron energy loss spectroscopy (e.e.l.s.) is consistent with analytical data expected for this anion; at higher electron beam intensities particle agglomeration occurs together with formation of graphitic carbon and a face-centred cubic Ni/Pt alloy.