Proper surface passivation is critical for achieving stable, efficient PL from light-emitting porous silicon (LEPSi). As-anodized LEPSi is passivated by hydrogen which desorbs at a temperature as low as 400 °C. For device purposes, it is necessary that porous Si can tolerate at least 450 °C for post anodization annealing/metallization steps. We have established that, if the hydrogen at the surface is substituted by oxygen, the resulting Si-Ox passivation is significantly more stable. One way of achieving this is to implant low energy/low dose oxygen to form a thin coating of SiO2 on the surface. Post implantation FTIR data report the absence of Si-H peaks. XPS data indicate the formation of nearly stoichiometric SiO2 at the surface. Similar results were achieved by implanting with nitrogen to form Si3N4. As an alternative to implantation, we have deposited thin capping layers of SiO2, Si3N4 and SiC by plasma-enhanced chemical vapor deposition (PECVD) which resulted in a similar degree of passivation. Wafers were pre-treated at 400 °C to remove hydrogen from the surface. Finally, we carried out a low-pressure CVD (LPCVD) oxide deposition on LEPSi. Post implantation/CVD annealing was done at temperatures up to 600 °C. In most cases, little or no change was observed in the resultant PL intensities.
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Duttagupta, S.P., Tsybeskov, L., Fauchet, P.M. et al. Post-Anodization Implantation and CVD Techniques for Passivation of Porous Silicon. MRS Online Proceedings Library 358, 381 (1994). https://doi.org/10.1557/PROC-358-381