CORRELATION OF SURFACE-MORPHOLOGY WITH LUMINESCENCE OF POROUS SI FILMS BY SCANNING-TUNNELING-MICROSCOPY

The evolution of visible photoluminescence is demonstrated by measuring the surface morphology of thin (approximately 20 nm) electrochemically etched porous silicon (PS) films with scanning tunneling microscopy (STM). Using low current densities, three sorts of samples were prepared under different conditions: In the dark (A), under illumination with ultraviolet (UV) light (B), and in the dark followed by a postphotochemical treatment (C). Upon UV light excitation, type A samples do not emit visible light, while samples of type B and C show weak and efficient photoluminescence in the visible range, respectively. STM imaging of these PS layers reveals a considerable decrease in the lateral dimensions of the surface features from approximately 10 nm (type A) to roughly 2 nm (type C), in accordance with the quantum confinement approach in describing the luminescence properties.