Boron Induced Changes in the Structural Properties of Amorphous Silicon Boron Alloys
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RESULTS Figures 1 (a) and (b) show the x-ray spectra for the films with boron contents of 1.5% and 3%. In both the figures, x-ray spectra of the as deposited films show them to be completely amorphous. The spectra of the oxidized films show sharp diffraction peaks due to silicon dioxide and boron oxide. This indicates that crystallization of the films has taken place. Figures 1 (c) and (d) show the x-ray spectra for the films with boron content of 15 and 20% respectively. For the oxidized films, the broad peak at 20=13.50 indicates that significant microcrystallization has taken place [9]. Figures 1 (e) and (f) show the x-ray spectra for the films with boron contents of 35% and 40% respectively. For the x-ray spectra of the oxidized film with 35% B, only a diffraction peak due to boron oxide can be seen. No diffraction peak corresponding to silicon dioxide is visible. With 40% B, the x-ray signature of the oxidized film is seen to be completely amorphous, indicating that no crystallization has occurred. Figure 2 shows the Raman spectra of the a-Si:B alloy with different B contents, annealed at 7000C for 30 minutes in N2 . The unannealed film shows a spectrum containing a broad peak at 480cm-', very similar to that of amorphous silicon. No structure at 520cm , which corresponds to crystalline silicon, is visible [10]. (The structure below 100cm" is due to the air ambient of the Raman scattering experiment.) Upon annealing, the films with 0.5 and 7% B crystallize, indicated by a sharp peak at about 520cm-. The film with 45% B, however, shows a very small peak at 520cm', showing crystallization has not taken place. DISCUSSION For the LPCVD growth of a-Si:B, one is alloying two network forming elements, of preferred 3 and 4 coordination which results in an average coordination of between 3 and 4. The concept of a continuous random network (CRN) is therefore applicable. At low concentration, B is present tetrahedrally and induces a misfit strain of substantial magnitude. Dangling and floating bonds may also be introduced. As the B content is raised past the limit of four coordination (-3%) [11], significant three-coordinated boron networking is believed to occur. This can result in local geometrical constraints on the CRN, which in turn results in additional local strain due to the inability of the structure to relax. When B content is increased further, additional B atoms are able to integrate into the matrix in such a way that the total disorder does not change much. A possible explanation is that although the original order present in the a-Si is disrupted because of the replacement of tetrahedral Si with three-coordinated B centers, there is a net gain in order due to those replacements that occur in the neighborhood of other B atoms. This gain in order offsets the disorder caused by those replacements that occur away from B centers. This B networking could account for the stability of the alloy against crystallization. Above -40%, so much B is involved in the matrix that practically all the Si atoms are bonded to
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