EVOLUTION OF VISIBLE PHOTOLUMINESCENCE AND SURFACE-MORPHOLOGY OF ULTRATHIN POROUS SI FILMS IMAGED BY SCANNING-TUNNELING-MICROSCOPY

The surface morphology of various ultrathin porous Si (PS) films (approximately 20 nm in thickness) is systematically investigated by means of scanning tunneling microscopy (STM) and correlated with the respective photoluminescent properties. Using moderately doped Si(100) wafers, three classes of PS samples are fabricated by electrochemical etching at low current densities under different preparation conditions: in the dark (class A), under illumination with ultraviolet (UV) light (class B), and in the dark followed by a postphotochemical treatment (class C). Upon UV-light excitation, layers of class A do not emit light in the visible range, while weak and efficient photoluminescence signals are obtained from samples belonging to classes B and C, respectively. STM imaging reveals a remarkable decrease in the lateral size of characteristic surface features from approximately 10 nm for class A to roughly 2 nm for class C films, fitting excellently to the quantum confinement approach in describing the origin of luminescence. For comparison, mesoporous Si films are also employed, fabricated by electrochemical etching of highly doped Si wafers. Preparation in the dark leads to nonvisible light-emitting material and the surface morphology is comparable to that of class A films.