Interferometric studies on some aspects of crystal growth

Abstract

Surface structure of the spiral is studied employing multiple beam interferometric techniques. The spiral has a slope perpendicular to a radius vector equal to h/2piA per radian, where h is the step height of the spiral. The surface of the spiral is highly curved; the curvature decreasing with distances away from the dislocation. The angle of inclination of the growth surface with the underlying (0001) face of SiC crystal is determined at various sections and is found to be greater than the theoretical radial slope Tan-1 h/a where d is the distance between tv/o consecutive loops of the spiral. The hollow dislocations predicted by Prank are associated with surface distortions of the sinking in type. Sometimes the large stress accompanying a giant dislocation produces a percussion figure on the crystal surface. Employing interferometric, X-ray diffraction and electron-microscopic techniques, it is shown that multi-molecular steps do exist in SiC and that they are sharp, single and vertical. Analysing the structure of multi-molecular steps, it is shown that the average distance between two kinks along the step edge should be about 1000A in SiC. It is established that at least in SiC, the step height is determined by the Burgers vector of the dislocation. In addition to the movement of unit dislocation causing unit slip, evidence for the movement of giant dislocations has been obtained for the first time. The experimental evidence for the operation of a modified cone source as an alternative to the Frank-Read source which is also common in SiC is presented. In this type of source, the slip plane of the sweeping dislocation rotates as the dislocation spirals. This surface study has also revealed the nature of dislocations present in SiC. In addition to the pure screw dislocations with Burgers vector perpendicular to the crystal surface, inclined dislocations which can he resolved into edge and screw components have been observed. In the light of these inclined dislocations, a modification to Frank's theory of polytypism is suggested. This explains the production of almost all the rhombohedral polytypes in SiC without invoking periodic break in the stacking sequence. Dislocations having screw component usually equal to 15 A have been observed on the step lines of the growth spiral. Deep channels terminating on these dislocations have been observed along the step lines. These dislocations cause among other things, fault lines, dissociation and grouping of steps and disorder in rhombohedral class of crystals. This latter has been confirmed by X-rays. Optical and interferometric study of the dendrites of germanium and tin grown from undercooled melt and by the action of Zn on SnCl2 solution respectively has been carried out. In the case of tin, tv/o types of crystals, viz: (1) flat dendrites with perpendicular branching and (2) needles without branching have been found to occur. It has been possible both in the case of Ge and Sn to correlate the surface features with their space lattice. Since most of the observations have been made employing the multiple beam interferometric techniques, the contouring efficiency of thin films used in these techniques is studied. It is shown for the first time that silver films accurately contour the underlying topography. Dielectric multilayers can be used for qualitative work but for quantitative measurements they do not seem to satisfy contouring requirements. The thesis commences with a brief account of the theories of crystal growth. The previous observations are reviewed in brief in the light of the present investigation.