Growth Striae Research Articles

Garnet substrate preparation by homoepitaxy

The surface preparation of rare earth gallium garnet substrates has been improved by liquid phase homoepitaxy. An examination of the resulting surface by means of X-ray diffraction topography has revealed that the influence of growth striae on the resulting substrate can be reduced by this method.

Effects of natural and forced convection in vapor phase growth systems

Convective temperature oscillations (CTO) introduce growth striae and other inhomogeneities in melt grown crystals, whereas a literature review suggests that the cause of such defects in materials grown from the vapor phase by heterogeneous chemical reaction, either in closed tubes or in flow-through systems, is not satisfactorily understood. Although CTO occur in room temperature gases at values of Rayleigh number ( R) greater than about 6 × 10 3, it was not clear whether CTO occur in practical open tube chemical vapor deposition systems at elevated temperature, because numerical calculations indicated that at constant pressure the R of gases depends on temperature as T -n, with n ≈ 4.5. Measurements carried out in the present work in flowing systems similar to those used in open tube chemical vapor deposition have shown that, in general agreement with hydrodynamic predictions, CTO with amplitude up to 20 °C and frequency up to 0.25 cycles/sec occur both in vertical and horizontal apparatus with cold walls, but not in apparatus with hot walls. Hydrogen and helium are found experimentally to be very much more stable than argon or nitrogen under equivalent conditions, also in agreement with Rayleigh number calculations. The observed superiority from a hydrodynamic stability viewpoint, of hydrogen compared to helium, is an important anomaly, since Rayleigh number calculations indicate helium should be slightly more stable than hydrogen over the range from room temperature to 1200 °K.

Lang Topographic Studies of III-V Heteroepitaxial Films Grown on Sapphire and Spinel

Single-crystal layers of GaAs and GaP grown by chemical vapor deposition on sapphire and magnesium aluminate spinel substrates have been characterized by Lang x-ray topography and x-ray diffraction methods. Separate topographs and diffraction peak profiles of the substrate and layer were obtained without removing the layer from the substrate. Epitaxial layers grown on substrates prepared by different growth techniques were examined and compared. Layers shown to be monocrystalline by back-reflection Laue methods were revealed by Lang topography to be composed of small grains which were misoriented ±0.1° from the nominal orientation of the layer. Polishing scratches and growth striae in the substrates were found to influence the perfection of the layer. The Lang camera was used as a diffractometer to measure the misorientation of the layers from exact parallel epitaxy and to determine the radius of curvature of warped substrates.

Barium sodium niobate: Constitutional studies and crystal growth

Phase equilibria of barium sodium niobate, stoichiometric formula Ba2 NaNb5O15 (10 Na2O, 40 BaO, 50 Nb2O5), in the ternary Na2O-BaO-Nb2O5 system have been studied and related to observations made on Czochralski grown single crystals. Extensive solubility occurs in Nb2O5-rich specimens, the liquidus and solidus surfaces being extremely flat and close together over the complete homogeneity range. Crystals grown from the non-stoichiometric congruent melting composition (7.2 Na2O, 42.2 BaO, 50·6 Nb2O5) are completely free from growth striae.

Ferroelectric domains and growth striae in barium sodium niobate single crystals

The relationship between the ferroelectric domain structure and growth striae is examined in barium sodium niobate (Ba2NaNb5O15) single crystals. Growth striae stabilise a particular form of domain structure and are further shown to be caused by temperature fluctuations occurring in the melts from which such crystals are grown. These fluctuations and the associated striae can be eliminated by reducing the melt level in the crucible used to contain the molten material. Evidence is presented to show that this effect cannot simply be regarded as a reduction in the Rayleigh Number of the melt system, as convective effects occurring in the gas ambient above the melt also exert an influence.

Elastic properties of Ba 2NaNb 5O 12

Crystals of Ba 2NaNb 5O 12 are found to be elastooptic, strongly piezoelectric and have acoustic Q's of the order of 10 5 at 500 Mcs. The attenuation of elastic wave propagation is anisotropic below ∼ 100°K and is explained on the basis of uniform growth striae.

Avoidance of Growth-striae in Semiconductor and Metal Crystals grown by Zone-melting Techniques

IT has recently been shown1 that the growth striae found in semiconductor crystals grown by zone-melting techniques are caused by fluctuations in temperature in the melt from which the crystal is being grown. (The term growth-striae is used to describe a banded distribution of impurity revealed on etched crystal sections.) One of us has shown2 that the temperature fluctuations were a characteristic property of a liquid metal contained in a horizontal boat on which was impressed an axial temperature gradient. At lower temperature gradients, but still above some critical gradient, the temperature varied sinusoidally with time.

Temperature oscillations in molten metals and their relationship to growth striae in melt-grown crystals

Experiments are reported which show that the temperature distribution in molten gallium contained in a horizontal boat on which is impressed an axial temperature gradient becomes oscillatory when t...

Constitutional supercooling during crystal growth from stirred metls: III. The morphology of the germanium cellular structure

The morphology of the germanium cellular structure obtained by growing crystals under conditions of constitutional supercooling has been examined using metallographic techniques. A study of the growth striae delineated by etching shows that the cellular interface is composed of an array of small {111} planes. The particular {111} planes which predominate are those which lie nearest the plane of the interface. If the plane of the interface is itself a {111} plane, the interface becomes faceted with macro-steps on it. The size and distribution of the cells is shown to depend on the fluid flow and the gradient of constitutional supercooling adjacent to the interface. Evidence is presented which indicates that the liquid is trapped at intervals in the re-entrant corners formed by the {111} planes and that this liquid subsequently climbs down the crystal by temperature gradient zone melting. It is demonstrated that, in order to avoid transient cell formation during growth from heavily doped melts, a high stability against growth rate fluctuations is required.

Spiral growths on large crystal surfaces

Means have been developed for maintaining observation of the individual faces of a large crystal while it is in process of growing or dissolving under controlled conditions. This has made possible an investigation into the cause of twinning occurring in lithium sulphate monohydrate crystals. Growth spirals have been detected on the two pairs of [110] wedge faces at opposite ends of the polar axis. The spirals have been shown to have characteristic shapes which are similar to the etch figures which distinguish the respective ends of the polar axis. The spirals and etch figures on the adjacent faces of a wedge pair are oppositely oriented and their approximate angular relations to the crystallographic axes have been ascertained. An estimate of the step height of the spirals indicates them to be about 3000Å. The width of the twinned growth striae which are liable to occur during crystal growth is of almost the same magnitude. It has been established in this work that the incidence of twinning is associated with a condition of high supersaturation in the nutrient solution and is frequently accompanied by spontaneous microcrystallization. The present observations therefore are considered to provide evidence that the growth mechanism is by the addition to an active edge of large unit blocks which have been preformed in solution as ordered clusters.