No abstract

We have detected a Si hexagonal phase in wafers implanted at high dose rate, P-type, 2-12 Ωcm, <001> oriented Si wafers were implanted by 80 keV As+ ions to a dose of 1016/cm2 at a current of 100 μA. These samples, without further heat treatment, were analyzed by TEM observations. The hex Si appears as a few hundred Å long rods elongated in the matrix <110> direction, see Fig. 1. They produce weak but observable diffraction effects which led to their identification, Fig. 2. The orientation relationship of the hex and dc Si is that [000l]hex | | [110]dc and [1010]hex | | [001]dc. Because of their small sizes, we could not identify their habit planes. Mechanical deformation experiments, however, showed that large size hex Si platelets with {115}dc habit planes may be produced.3 We have performed such experiments to confirm this point. We now propose a dc to hex phase transformation scheme for Si. Assume that the transformation driving force is a large compressive stress in the dc [110] direction. Then the bond between an atom and one of its two nearest neighbors on the (110) plane may be broken and a new bond between this atom and one of its two second nearest neighbors along the [110] direction may be formed. In this way, six membered atomic rings appropriate for the hex type structure are formed, Fig. 3(a). Relaxation of these new rings proceeds simultaneously to give the structure in Fig. 3(b), which is the hex Si structure with proper orientation relationship w.r.t. dc Si. After the transformation, the material expanded by ~18% in the [110]dc direction, contracted by ~18% in the [110]dc direction. Thus, the transformation facilitates a stress relieving mechanism which provided a relief of the equivalent of ~18% of a pure elastic compression in the dc [110]dc direction. We estimated that the hex Si is ~1011 ergs/cm3 higher in energy than the dc Si, and an activation energy of ~1011 ergs/cm3 is needed to induce this transformation. We have also constructed an atomic model for the dc-hex Si interface, the (115) habit plane. This interface is composed of 5-7 membered atomic rings with no dangling bonds or long range strains and it is therefore a low energy configuration, Fig. 4. Hexagonal Si were generated in ion implanted samples, we believe, because of direct transfer of momentum from the the impinging ions provided enough energy to overcome the nucleation barrier. Once they are generated, stress provided by dislocations and amorphous zones stabilized the transformed regions.