In situ photoluminescence studies of photochemically grown porous silicon

We have used a variety of laser wavelengths (365, 473, 633, 685, 730 nm) to produced porous silicon thin films by photochemically etching n-type Si in 48% HF in aqueous solution. Photoluminescence (PL) has been excited either with 365 or 473 nm light. We have observed the PL spectrum as a function of fabrication wavelength (lambda(fab)) and of exposure to the PL excitation laser. We demonstrate that the in situ peak PL emission wavelength (lambda(peak)) depends on the fabrication laser wavelength for lambda(fab) greater than or equal to 473 nm and that the redder the lambda(fab), the redder lambda(peak). For lambda(fab) greater than or equal to 473 nm we find lambda(peak) approximate to 550 nm. We have also found that post-growth exposure of a film to 365 or 473 nm light rapidly (on the order of 10 s) and continuously shifts lambda(peak) to a saturation value of 530-550 nm where a steady state is reached. In addition, we have made simultaneous observations of the PL spectrum while a film is grown with lambda(fab) = 365 or 473 nm. Detectable PL is first observed after similar to 100 s. The peak then grows in intensity until a steady value is achieved, lambda(peak) is found to vary little as the peak intensity grows. The observation that redder lambda(fab) leads to redder lambda(peak) is consistent with the predictions of quantum confinement for fully hydrogen-terminated porous Si. When exposed to air, lambda(peak) = 670-700 nm irrespective of lambda(fab). The latter result suggests that chemisorbed oxygen is important in determining lambda(peak) of air-exposed films. (C) 2000 Elsevier Science S.A. All rights reserved.