An energetic electron beam has been used to stimulate crystallization of spatially isolated amorphous regions in Si, Ge, GaP, and GaAs at 30 and 300 K. In the four materials it was found that crystallization was induced even when the energy of the electron beam was less than that required to create point defects in the crystalline structure. The rate of crystallization depended on the material and on the electron energy. In all materials, the rate decreases as the electron energy increases from 50 keV (the lowest electron energy used), reaching a minimum value at an electron energy slightly below the displacement threshold voltage. Above the displacement threshold, the regrowth rate again increases with increasing electron energy. The possible role of electron-beam heating was studied both theoretically and experimentally. Calculations suggested heating effects were negligible and this was confirmed by in situ ion implantations and electron irradiations performed at 30 K, where subthreshold electrons stimulated crystallization. The subthreshold and low-temperature results are consistent with the model that the crystallization process is dependent on the creation of defects (dangling bonds and kinks) at the crystalline-amorphous (c-a) interface. The crystallization stimulated by the subthreshold electron beams suggests that electronic excitation of the bonds along the c-a interface can induce the amorphous to crystalline transition.
The damage produced in GaAs by implantation with low energy heavy ions has been studied as a function of ion mass and implantation temperature (30 and 300 K). The experiments were performed in situ in the microscope-accelerator facility at Argonne National Laboratory. In samples implanted and examined at 30 K, spatially isolated amorphous regions were produced by the direct impact of 50 keV Ar, Kr, and Xe ions. The probability that the impact of an individual ion formed an amorphous zone increased as the ion mass increased from Ar to Kr but not from Kr to Xe. The average dimension of the amorphous zones also increased with ion mass, being greater for the Xe than for the Kr ion implantation. On warming to room temperature, the amorphous zones decreased in size and density as the sample temperature was increased above 200 K. In samples implanted and examined at 300 K, the probability of forming an amorphous zone by direct impact increased as the ion mass increased from Kr to Xe, although the probability was always less than at 30 K. The density of amorphous zones produced at 300 K was similar to that remaining in a sample implanted at 30 K and then warmed to room temperature. With time at 300 K the amorphous zones decreased in size and eventually crystallized completely, leaving no trace of their prior existence.
Neutron incoherent quasielastic scattering experiments on the liquid crystal MBBA in the nematic phase are reported. Taking advantage of published NMR results it is shown that the motion of an oriented MBBA molecule is composed of two parts. The 12 protons of the butyl end and alkoxy chains perform uniaxial rotational diffusion with a diffusion constant of 1010s−1 while the 9 protons of the rigid benzylideneaniline core are restricted to a uniaxial simple rotational diffusion on a segment of a circle of an apex angle of 57° with an associated rate constant of 2.3 × 1010s−1.
Spatially isolated amorphous regions in Si and Ge have been regrown at room temperature by using an electron beam with an energy less than that required to cause displacement damage in crystalline material. The rate at which the zones regrow is a function of the energy of the electron beam. As the electron energy is increased from 25 keV (lowest energy employed), the regrowth rate decreases and reaches a minimum below the threshold displacement voltage. With further increases in the electron energy, the rate again increases. It is suggested that at the lower electron energies this room temperature regrowth process is stimulated by electronic excitation rather than by displacive-type processes.
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