Role of dangling bonds for the photoluminescence of Er doped in a-Si : H

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In order to see a role of Si dangling bonds in the photoluminescence(PL) mechanism of Er which is doped in a-Si : H, it is thought to be useful that the Er site is separated from the Si dangling bond site and the influence of changing the separation on the Er PL is observed. To realize the idea, Er ions are added in two kinds of wide gap materials to inhibit the optical absorption of the host materials. Crystalline LiNbO_3 or amorphous Al_2O_3 are tried as wide gap materials. On top of the Er-doped thin film of the wide gar material, a-Si : H was deposited. The Er PL was not enhanced, however, when there was only one interface between a-Si : H and the wide gap material. In order to have a better sensitivity, we prepared a multilayer structure of the formAl_2O_3 : Er/a-Si : H/Al_2O_3 : Er/a-Si : H/Al_2O_3 : Er with around 20 nm thick a-Si : H layers sandwiched between Er-doped Al_2O_3.After annealing at 300 C, the Er PL was observed, while the threefold-stacked layer without a-Si : H la … More yer did not exhibit the Er PL after the same procedure. The observed results indicate that the thin a-Si : H layer is useful to excite Er ions which locate apart from a-Si : H. We cannot conclude at present whether Si dangling bonds in a-Si : H really play a role in enhancing the Er PL or not. Further investigations by changing the thicknesses of a-Si : H and Er-doped Al_2O_3 are needed to clarify the mechanism.We also studied on the environment of an Er ion by calculating the Stark levels due to the ligand field and by comparing the results with the PL spectra observed at 19 K. A model fo the Er environment is one that an Er ion is surrounded six oxygens which are incorporated unintentionally into a-Si : H during preparation. It should be emphasized that we did not rely on fitting parameters whose physical meaning were not clear but started with the wavefunction of the Er ion. Thus the effect of changing the distance between the Er and 0 atoms on the Stark splitting can be deduced. We conlude that the environmental model is consistent with the observed Er PL spectra. The fluctuation of the structure due to the randomness of the amorphous network is most probably the origin of the linewidth of the Er PL spectra. Less