M. S. Abrahams

Dislocation morphology in graded heterojunctions: GaAs1−xPx

The details of the formation, propagation, interaction, and densities of misfit dislocations are combined into a simple model quantitatively predicting dislocation densities for both abrupt and graded heterojunctions. Three key concepts are introduced: (1) misfit dislocations are segmented; (2) accordingly, they must give rise to a density of inclined dislocations, nI, that propagate through the growing layer; and (3) these inclined dislocations can bend in and out of any subsequently formed misfit plane to relieve the strain, and when bent in, serve as strain-relieving misfit dislocations. Thus, the value of nI is expected to remain constant with thickness. Also, nI is predicted to vary directly with the compositional gradient at the heterojunction. It is pointed out that there are two general classes of misfit dislocations, pure-edge and mixed and that their intersections, which cause the misfit dislocations to appear to bend within their plane, can be simply classified into three general types.Transmission electron microscopy was used for a comprehensive study of dislocations in a series of GaAs1−xPx heterojunctions prepared by a vapour phase growth technique. The main features of the above model were corroborated. The value of nI was found to be constant with growth distance as postulated, and in quantitative agreement with prediction, nI decreased from 4 × 107 cm−2 to 106 cm−2 as the compositional gradient decreased from 5% phosphorus/μm to 0.2% phosphorus/μm. Note that these values can far exceed the dislocation density of the substrates. Of particular significance, the inclined dislocations nI were found to propagate through a constant-composition region grown on top of a compositionally graded region, so that formation of the heterojunction must affect subsequently grown layers. Finally, it is shown that the misfit dislocations are, indeed, a combination of pure-edge and mixed, and all three postulated general interactions between these dislocations are shown to occur.

Cross‐sectional specimens for transmission electron microscopy

Cross‐sectional views of epitaxial structures yield much information when examined by transmission electron microscopy. Since the growth direction then lies in the plane of observation, rather than normal to it (as is usual), the overgrowth, original growth interface, and substrate can be imaged either simultaneously or individually. A realization of the suitable technique for preparing thin cross‐sectional samples is described. Applications to continuously graded GaAs x P1−x /GaAs and step‐graded In x Ga1−x P/GaP are shown.

Like‐sign asymmetric dislocations in zinc‐blende structure

Symmetry considerations reveal that an asymmetry exists relative to orthogonal 60° dislocations of the same sign in zinc‐blende structure. The effect of this asymmetry has been observed in compositionally graded crystals of In1−xGaxP and GaAs1−xPx grown from vapor phase. It is observed that the spatial arrangement of the two sets of misfit dislocations in an orthogonal array is different. In one 〈110〉 direction, the misfit dislocations tend to be uniformly distributed, while in the other 〈110〉 direction there is a marked tendency for periodic banding of the dislocations.

Cross‐sectional electron microscopy of silicon on sapphire

Cross sections of (100)Si/(012)sapphire were examined by transmission electron microscopy. The foil plane was (011)Si and contained the [100]Si growth direction. The number of faults (i.e., microtwins and isolated stacking faults) per cm measured in the [022]Si direction, FD, decreases with increasing distance d from the silicon/substrate growth interface according to the equations FD= (3.1×107)/d0.63 (440⩽d⩽2400 A), and FD= (1.3×1011)/d1.7 (2400?d?4.3×104 A). Misfit dislocations were not observed, and no evidence for the presence of an Al‐bearing phase was seen in the proximity of the interface.

Reduction of dislocation densities in heteroepitaxial III−V VPE semiconductors

Electron microscope evidence from {011} cross−sectional samples of InxGa1−xP/GaP is presented to demonstrate the reduction of dislocation densities in constant composition regions of {100} III−V samples prepared by vapor−phase epitaxy with compositional grading having over−all misfit strains as high as 3.7%. Dislocations are shown to be confined to the graded region by an abrupt compositional step in both step− and continuous−graded samples. The crystal growth technique described works well for epitaxial layers which are in compression but not for those which are in tension.

Observations Concerning Radiative Efficiency and Deep‐Level Luminescence in n‐Type GaAs Prepared by Liquid‐Phase Epitaxy

A study was made of n‐type GaAs prepared by liquid‐phase epitaxy doped with Si, Ge, Sn, Te, and Se by photoluminescence and Te‐doped material by transmission‐electron microscopy. A broad emission band centered at 1.2 eV (band B) is observed in LPE materials doped with group VI elements. Band B increases in intensity relative to the bandgap radiation with increasing dopant concentration in the 1018 cm−3 range. It is suggested that the recombination centers responsible for band B are the neutral (VGa+3 Te) complexes postulated by Vieland and Kudman, and that these represent the solid solution of Ga2Te3 in GaAs. With increasing dopant content, the solubility limit is eventually exceeded, and precipitates of this compound are then formed. These have been observed and identified by transmission‐electron microscopy. The radiative efficiency falls off sharply with increasing dopant content beyond 2–3×1018 cm−3 in materials doped with Se and Te. It is suggested that this fall off is partly due to nonradiative rec...

Detection of Selenium Clustering in GaAs by Transmission Electron Microscopy

Direct evidence has been found for the existence of coherent particles and small precipitates in bulk Se‐doped GaAs (n=2×1018 cm−3) and in Se‐doped, vapor‐grown epitaxial GaAs (n=4×1018; 1×1019 cm−3). Data obtained from moire fringe contrast indicate a probable particle composition of Ga2Se3. In samples with more than 2×1018 carriers cm−3, the major part of the total Se content is in the form of particles, and this accounts for the difference between the total Se concentration and the electrically active Se concentration. Diffusion in the solid can account for particle growth in the Bridgman‐grown crystal, but not in the vapor‐grown epitaxial material.

The Si/SiO2 interface examined by cross‐sectional transmission electron microscopy

Thin (200–300 A) cross sections of Si/SiO2 have been examined by transmission electron microscopy at a resolution of better than 10 A to search for crystalline Si protrusions into, or islands in, the thermally grown SiO2. Within the resolution obtained, no evidence was found for any phase separation within the amorphous oxide layer.

Early growth of silicon on sapphire. I. Transmission electron microscopy

The early growth of Si on (0112) ‐oriented sapphire has been examined by transmission electron microscopy. The Si was deposited at 1000 °C by pyrolyzing silane in H2. The nominal growth rate was 0.4 μm/min. The morphology of the initial Si growth islands was determined after 0.5, 1.0, 2.5, and 3.5 sec of growth. The basic Si orientation is (100) with [011] Si∥[2110] Al2O3. Extensive faulting and twinning is observed leading to {221} orientations. These defects apparently form at coalescence sites of adjacent islands. Also present are four {110} orientations occuring as twin‐related pairs. The observation of isolated {110} domains indicates that they nucleate on the sapphire independently of the (100) domains. The {110} and (100) domains grow at about the same rate. Eventually, the {110} domains become trapped by the surrounding (100) domains. The volume percentage of the {110} domains is constant with growth time and equals 7% up to coverages of 90%.

Dislocations and Precipitates in GaAs Injection Lasers

The new A‐B etchant has been employed to reveal interfacial dislocations near the junctions of GaAs injection lasers. Injection lasers were examined parallel and perpendicular to the junction region. Dislocations are seen predominantly in the junction region, lying in the (001) plane, and in only two directions, the [110] and [110]. Their source is ascribed to the lattice mismatch between the substrate and grown layer. Decoration of the dislocations parallel to [110] is observed, but very little decoration of the [110] dislocations is seen. The decorating impurity is believed to be Zn. The difference in decoration of the [110] and [110] dislocations is accounted for by a difference in the structure of these two dislocations. A direct correlation between lasing filaments and decorated interfacial dislocations was found in the [110] direction. This is believed to be due to the local distortion of the current lines near the defects.