GROWTH FORMS OF BETA-RHOMBOHEDRAL BORON WHISKERS AND PLATELETS PREPARED IN A LOW-PRESSURE B2H6+HE PLASMA IN TERMS OF PERIODIC BOND CHAIN METHOD

As a result of SEM observations of beta-rhombohedral boron crystallites grown in a low pressure B-2H-6 + He plasma, the following was found: (1) some whisker samples are bounded by {11BAR0} and they grow towards [111]; (2) platelet-like or planar crystallites show extensive growth of {111}; (3) in some platelet-like crystallites, twinning was found to occur on (111). (In this paper, we use crystallographic notations according to the rhombohedral unit cell). Application of the periodic bond chain (PBC) method indicates that the PBCs of beta-rhombohedral boron are [2BAR11], [12BAR1] and [112BR]; then, the F (flat) faces are {111}, the S (stepped) faces are {11BAR0}, and K (kinked) faces are {111BAR}, {110}, {100}, and so forth. This can partly explain the results, the extension of {111} in the platelet-like crystallites; the whisker growth bounded by {11BAR0} seems to suggest a surface-stabilizing mechanism of the S faces by chemisorption of species in PECVD atmosphere. The following growth mechanisms of the whiskers and platelets are considered: when only one kind of crystallographically equivalent faces is inactive to the growth reaction under certain CVD conditions and these faces cannot form a geometrically closed shape by themselves, the whiskers or platelets should grow for those of {11BAR0} or {111}, respectively; this may be applied to the other hexagonal crystals.