Ion-Implanted Amorphous Silicon Studied by Variable Coherence TEM
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Fig.1. RBS spectrum for a-Si samples annealed at 580'C foi 2.5 mins and 600'C for 5 mins. RESULTS We obtained several preliminary results for ion-implanted a-Si with variable coherence microscopy, as shown in Fig. 2. The positions of the two peaks in the variance curve are the same ring positions in the diffraction pattern of a-Si. The atomic correlation length can be extracted from the peak height". In paracrystalline models 9, the variance was calculated for different paracrystaliine structures of a-Si, and the simulated variance curve is similar to that of sputtered and as-implanted a-Si in Fig. 2. It follows that the ion-implanted a-Si and sputtered a-Si:H are paracrystalline structures, but the degree of paracrystallity is slightly different, though we examined the near surface area of the ion-implanted sample. We noticed the spatial resolution on sputtered a-Si:H data is 0.5nm (controlled by larger objective aperture), while the spatial resolution on as-implanted a-Si data is 2.2nm. Meanwhile, we have not corrected the variance offset on as-implanted data points. We need further experiments and more reproducible results to prove that the ion-implanted a-Si has smaller correlation length than sputtered a-Si:H. We have succeeded in observing the structural relaxation in ion-implanted a-Si samples upon annealing. Gibson and Treacy have also observed the same relaxation phenomena on the evaporated a-Ge5 . This is the direct evidence of relaxation to support the assumption of the paracrystalline model for the oc-Si system. The paracrystal with high free energy, stored in terms of strain energy or heat, will transit towards the more disordered state, like a continuous random network, which is energetically favorable upon thermal annealing. The thermal relaxation behavior of oc-Si has been confirmed by calorimetric experiments ' . It is thought the unrelaxed and relaxed a-Si are made up of continuous random networks in short range order, and the degree of randomness, due to point defect annihilation, changed upon relaxation 3. However, the failure of standard techniques to detect the subtle structural change, especially in medium range order, is obvious. As with X-ray diffraction experiments3 , the difference of the diffraction pattern between as-implanted and annealed a-Si is hardly noticed. The average distribution of the diffracted intensity is the Fourier transformation of the pair correlation function, which is only sensitive to the short range ordering. Intrinsically, diffraction techniques are not able to probe the atomic ordering in 2nm, which corresponds to the grain size of the paracrystalline structure, whereas variable coherence microscopy has demonstrated its capability at this point.
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