E. Bauser

Microscopic growth mechanisms of semiconductors: Experiments and models

Abstract This paper describes the microscopic growth mechanisms of facet growth, dislocation-controlled facet growth, near facet growth, terrace growth and terrace-free growth. It investigates the growth mechanisms by studying (i) the micromorphology of the solid/fluid interfaces and (ii) impurity striations in corresponding regions in the bulk of the crystals. It deals with experiments and models, which define the distinguishing features of each of the mechanisms. The studies are based on epitaxial layers of GaAs and Si, grown from solution and on silicon crystals, grown from the melt by the Czochralski technique: They demonstrate the influence of the growth mechanisms on the distribution of impurities and dopants for epitaxial layers and bulk crystals. In addition, experiments are reported which illustrate the influence of the growth mechanisms on the morphological stability of the growth interface under otherwise identical crystal growth conditions.

Analysis of dislocations creating monomolecular growth steps

Abstract During liquid-phase epitaxial growth of GaAs monomolecular growth steps, created at the growth interface, arrange themselves into regular patterns around well-defined centres. The nature of these centres is investigated by high-voltage electron microscopy. Each centre has been found to correspond to a dislocation. In addition to examples in which dislocations possess non-vanishing Burgers vector components perpendicular to the growth interface (in accordance with Franks model), others have been found whose Burgers vectors do not have a component normal to the growth interface. The latter examples, denoted “transverse step sources” in this paper, show that even dislocations without a Burgers vector component perpendicular to the growth interface create growth steps. The efficiency of these transverse step sources is similar to that of Franks screw-dislocation step sources, correspondingly denoted “longitudinal step sources”.

Substrate orientation and surface morphology of GaAs liquid phase epitaxial layers

The surface morphology of GaAs LPE layers grown in a modified tipping system on (100)-oriented GaAs substrates has been studied. The experiments demonstrate straightforwardly that the frequently observed terraced surface structures are determined by the substrate orientation: layers grown on slightly misoriented substrates (misorientation about 30 min of arc) exhibit a terrace structure, whereas layers grown on carefully oriented substrates (misorientation less than 5′) show flat and smooth surfaces. Our experiments are in agreement with observations on GaP by Peters, and by Saul and Roccasecca.

Extremely flat layer surfaces in liquid phase epitaxy of GaAs and AlxGa1−xAs

Abstract Optical micrographs were taken of the surfaces of LPE grown layers of GaAs and A1GaAs. They show patterns of mono-molecular growth steps on facets in square-shaped mesa areas up to 1 mm 2 in size. Step pattern details are discussed as well as the influence of growth temperature on the shape of step patterns, step edges, and on step interdistances.

Steps on facets of solution grown GaAs epitaxial layers

Abstract The micromorphology of facets on solution grown GaAs epitaxial layers consists of monomolecular, equidistant steps. The steps are arranged in spirals or closed loops around the sites where dislocations emerge on the surface. Optical micrographs are presented which show that the step patterns exhibit a strict spatial periodicity. The steps have uniform height of atomic dimensions. A decoration technique is used to make the steps visible and to facilitate microscopic studies on large facets. The influence of the periodic arrangement of steps on impurity segregation and interface stability during growth is discussed.

X-ray topography of growth step sources in LPE gallium arsenide

Abstract X-ray topographs, taken of (001) oriented growth facets of GaAs layers grown by liquid phase epitaxy (LPE), were compared with interference-contrast micrographs of the same areas. The emergence points of dislocations coincide exactly with the apexes of very low-profile growth-step pyramids on the facet surface. The coincidence gives direct experimental evidence for dislocations acting as step sources on growth facets (Frank mechanism).

Microstep structure on vapour grown NbSe4I0.33 crystals

Abstract Quasi-one-dimensional NbSe 4 I 0.33 single crystals with tetragonal symmetry were grown by chemical vapour transport utilizing the temperature difference of 730–670°C. On the c -planes of the crystals microstep lines were abundantly present and were observed directly in an interference contrast microscope. Steps of two different heights, either 32 ± 2 A or 16 ± 1 A, were found to contribute to the growth of the crystals. The regular arrangement of the microstep lines generated at spiral dislocations or at crystal edges reveals that the crystal grows by a lateral growth process on vicinal faces. At the centre of the spiral patterns an anomalous structure with a rise or a pit was often observed. A tentative explanation for these particular spiral step centres is given.

Liquid phase epitaxy of GaAs quantum well structures

Abstract Gallium arsenide quantum wells are grown by liquid phase epitaxy. The growth parameters are carefully adjusted in order to prevent any dissolution of the GaAlAs layer by the GaAs growth solution as well as to eliminate solution carryover. The interfaces between the thin GaAs layer and the GaAlAs cladding layers are therefore chemically abrupt. Due to a specific difference in the growth morphologies of GaAlAs and GaAs, the GaAlAs/GaAs and the GaAs/GaAlAs heterointerfaces differ considerably. The different interface morphologies cause quantum well thickness fluctuations which influence the line-widths of the quantum well related optical transitions. Photoluminescence measurements presently reveal minimum line-widths of 2.5 meV, and discrete n = 1 heavy-hole, n = 1 light-hole, and n = 2 heavy-hole exciton transitions are clearly detected by photoluminescence-excitation spectroscopy.

Crystal growth by microsteps on vapour grown NbSe4I0.33

Abstract Single crystals of NbSe 4 I 0.33 with tetragonal symmetry often show elementary microsteps either 32 A or 16 A high on their c -plane. An anomalous pit at the spiral centre previously reported was confirmed to be a growth depression formed during dendritic growth by surface diffusion. Under a slowly growing condition, a screw dislocation introduced the usual simple Archimedean spiral step structure. The elementary microsteps generated spontaneously from the edge of a crystal also adopted the same step heights as those from the screw dislocations. The discrepancy of the microstep height to the unit cell lattice parameter of the base crystal is discussed tentatively.

Origin of multiple steps in vapour growth of NbSe4I0.33

Abstract Giant steps or groups of 16 A high steps in spiral line structures occur frequently on the {001} faces of vapour grown tetragonal NbSe 4 I 0.33 crystals. Step structures of this kind can be observed by differential interference contrast microscopy. We apply this method to observe steps occurring in a series of growing crystals in order to discuss the genesis of the step structures. Initially, dendritic crystals develop. When the skeleton grows, the crystal walls draw together. Before these parts bond, a considerable misalignment may, however, exist between different, separate crystals. The misalignment brings about many dislocations when, during growth, the space between the crystals fills up. To illustrate the process how crystals are completed, typical examples are given here.