Layer by Layer Amorphization in Si: Temperature, Ion Mass and Flux Effects
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411 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
phization in a temperature range for which the pure crystallization kinetics is negligible. The effects of ion flux, temperature, and energy deposited into elastic collisions are investigated and the experimental results are explained on the basis of a phenomenological model. EXPERIMENTAL Si samples (100) in orientation, were amorphized by multiple Ge or Xe implantations in order to obtain samples with pre-existing a-layers both at the surface (-, 17 nm thick) and buried (,- 170 nm below the surface). A crystalline layer, - 100 nm thick, was thus embedded between the two amorphous layers. Ion-assisted layer-by-layer amorphization was induced by irradiation with 600 keV Kr++ ions at ion fluxes ranging from 3 x 10 11/cm 2 sec to 5 X 1012 /cm 2 sec. During irradiations the samples were mounted onto a resistively heated copper block whose temperature, controlled by a thermocouple, was varied in the range 50-140 *C. The kinetics of amorphization was determined by in situ reflectance measurements described elsewhere [13]. Some selected samples were analyzed by 2.0 MeV He+ channeling in the backscattering geometry in order to further check the accuracy of the method. The amorphization velocities, determined by the two methods, differ by less than 10 % and thus all the measured values will be quoted without referring to a particular measurement method. In another set of experiments, layer-by-layer amorphization was induced by implanting different ions at a fixed ion flux (1 X 1012ions/cm 2 ) through a c-a interface lying at , 600nm from the surface. The process of amorphization was characterized by 2.0 MeV He+ Rutherford Backscattering Spectroscopy (RBS) in combination with the channeling technique. ION FLUX AND TEMPERATURE EFFECTS At a given the substrate temperature we have investigated the flux dependence of the layer by layer amorphization process. In Fig.1 values for the interface velocity as a function of the ion flux (€) are reported for two different irradiation temperatures. Data can be fitted by assuming a 2/13 law; the rate of amorphization therefore depends less than linearly upon ion flux at temperatures at which the process of crystallization is fully inhibited. The temperature dependence has been investigated in the range 80-140 'C and the data are summarized in Fig.2. The absolute values of the interface velocity are reported as a function of 1/T for different fluxes. Data taken by 600 keV Kr++ irradiation, refer to both crystallization [13] and amorphization. At each ion flux an exponential increase in the amorphization velocity by decreasing temperature is observed. In the investigated temperature range an activation energy of ,-- 0.48 eV is determined.
The most widely accepted model [12] for ion beam induced crystal-amorphous transition in silicon assumes the rate R as given by the balance between crystallization (Re) and amorphization (Ra) rates:
R=R -R1
(1)
The model assumes that the rate of crystallization is governed by r
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