Mechanism of a remarkable enhancement in the light emission from nanocrystalline porous silicon annealed in high-pressure water vapor

To clarify the effect of surface passivation on the optical properties of nanocrystalline porous silicon (PS), the photoluminescence (PL) characteristics of PS have been investigated by employing a high-pressure water vapor annealing (HWA). PS samples with various porosities were prepared on (100)-oriented p-type (4 Ω cm) single-crystalline silicon wafers by electrochemical anodization. Some samples were then electrochemically oxidized. The HWA treatment was then applied to the prepared PS samples at 0.5–3 MPa and 200–300 °C for 2–3 h. The PL intensities, spectra, and dynamics after HWA were measured in relation to surface analyses by Fourier-transform-infrared (FTIR) spectroscopy. It is shown that the HWA treatment leads to a drastic enhancement in both the PL efficiency and stability. Under the optimum condition, the PS sample exhibits an extremely high external quantum efficiency of 23% at room temperature. According to the FTIR spectra analyses, silicon nanocrystallites in HWA-treated PS are covered with a high-quality SiO₂ tissue. The PL decays are found to be longer than those of as-prepared PS, and become closer to a single-exponential behavior near the PL peak wavelength. The observed high efficiency and stability of PL emission from HWA-treated PS is attributed to (i) suppression of nonradiative surface defect density, (ii) uniform passivation by unstrained thin oxides, and (iii) strong localization of excitons in silicon nanocrystals. This low-temperature treatment is very useful for obtaining highly efficient and stable luminescent PS and devices.