Abstract

We have manufactured thin films of light-emitting porous silicon (LEPSi) by electrochemical etching in HF and subsequently lifted them off the silicon wafer by an electropolishing step [1]. The film thickness was varied from 100 μm to 0.1 μm and the porosity ranged from < 40% to ≥ 90%. To insure the structural integrity of the thinner, higher porosity films, they were deposited on sapphire windows where they remain attached by van der Waals or electrostatic forces, in a manner reminiscent of earlier work with ultrathin film of III-V semiconductors [2]. The microstructural, chemical and optical properties of these films have been investigated using a variety of tools, including AFM, XRD, FTIR, cw and time-resolved absorption and luminescence measurements. These films are optically flat (see Fig. 1), do not scatter light, show pronounced Fabry-Perot interference fringes in the transparency region which can extend to photon energies well above the bandgap of crystalline silicon (see Fig. 2) and can have a refractive index as low as 1.2 to 1.3. The peak of the luminescence spectrum, which is determined by quantum size effects and surface chemistry, has no clear relation to the onset of absorption, which is dominated by the largest nanocrystallites. The broad luminescence spectrum splits into a few narrow peaks which are produced by multiple interference (see Fig. 3).

© 1995 Optical Society of America