LUMINESCENCE AND STRUCTURAL STUDY OF POROUS SILICON FILMS

The luminescence properties of 3-mu-m thick, strongly emitting, and highly porous silicon films were studied using a combination of photoluminescence, transmission electron microscopy, and Fourier transform infrared spectroscopy. Transmission electron micrographs indicate that these samples have structures of predominantly 6-7 nm size clusters (instead of the postulated columns). In the as-prepared films, there is a significant concentration of Si-H bonds which is gradually replaced by Si-O bonds during prolonged aging in air. Upon optical excitation these films exhibit strong visible emission peaking at almost-equal-to 690 nm. The excitation edge is shown to be emission wavelength dependent, revealing the inhomogeneous nature of both the initially photoexcited and luminescing species. The photoluminescence decay profiles observed are highly nonexponential and decrease with increasing emission energy. The 1/e times observed typically range from 1 to 50-mu-s. The correlation of the spectral and structural information suggests that the source of the large blue shift of the visible emission compared to the bulk Si band gap energy is likely to be due to quantum confinement in the nanometer size Si clusters. The electron-hole recombination process, on the other hand, remains unclear.