C. J. Buiocchi

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.

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.

Dislocations in vapor‐grown compositionally graded (In,Ga)P

Dislocation morphologies in compositionally graded vapor‐phase epitaxial (VPE) layers of (In,Ga)P deposited on (100) GaP substrates have been determined via transmission electron microscopy using (011) cross‐sectional samples. Evidence for the introduction of misfit dislocations by slip and from substrate dislocations has been found. These processes lead to the familiar crossed array of dislocations at an interfacial misfit plane. Evidence for Frank‐Read sources, which are pinned dislocation segments in the crossed array, and dislocation annihilation is also presented. Asymmetric arrays of dislocations at compositional steps have been observed.

Interdependence of strain, precipitation, and dislocation formation in epitaxial Se‐doped GaAs

The defect morphology of Se‐doped GaAs grown by CVD on undoped 100‐oriented GaAs substrates has been studied by transmission electron microscopy. Two types of samples are discussed: The first is nearly saturated, n ≅ NSe ≅ 4 × 1018 cm−3; the second is supersaturated, n ≅ 1.5 × 1019 cm−3, NSe ≅ 4 × 1019 cm−3. The nearly saturated samples exhibit an array of small features (probably Frank loops) near the original interface and have a density of misfit relieving dislocations ≲103 cm−1. The supersaturated samples show a large number of Frank loops, whose density decreases monotonically with distance away from the original interface while their diameter increases in this direction. The loops seem to be invariably connected with one or more very small precipitate particles of Ga2Se3. Most of the Frank loops are of the intrinsic type; these are sometimes accompanied by closely neighboring loops of extrinsic nature. Pure edge misfit relieving dislocations with a density ≅ 104 cm−1 are observed at the interface, r...

Effect of Substrate Imperfections on GaAs Injection Lasers Prepared by Liquid‐Phase Epitaxy

A study was made of the correlation between substrate flaws and the performance of GaAs injection lasers fabricated by liquid‐phase epitaxy. These devices are p+/n structures where the p+ region is deposited epitaxially, followed by heat treatment to displace the p‐n junction about 1–2 μ into the n‐type substrate. The melt‐grown substrates were studied with the aid of transmission electron microscopy, infrared transmission, x‐ray topography, etching studies, and photoluminescence. It is concluded that the dislocation density of the substrate is not the controlling factor in laser performance unless it exceeds 105 cm−2. A dislocation density of 103–104 cm−2 is not excessive. Precipitates and impurity striations in the substrate are probably the most damaging imperfections. In Te‐doped GaAs the presence of Ga2Te3 precipitates is to be avoided. In the case of Si‐doped materials, the precipitation problem appears to be less severe. Nevertheless, small clusters have been detected by electron transmission micro...