RELAXATION SPECTRA OF PHOTOLUMINESCENCE FROM POROUS SILICON OBTAINED BY CHEMICAL ETCHING OF LASER-MODIFIED SILICON

The distinctive features of time-resolved photoluminescence spectra of porous silicon created by chemical etching of single-crystal silicon subjected to modification by laser light has been investigated. Two ba nds were seen upon examination of the amplitude spectra: a band with a maximum at a wavelength (lambda(max)similar or equal to 530 nm) and a band with lambda(max)similar or equal to 420 nm and twice the intensi ty of the first. The relaxation curves are characterized by two sets o f times: tau(1)<2x10(-8) s and 2x10(-8) s<tau(2)x2x10(-4) s in the spe ctral ranges 420-700 nm and 500-850 nm, respectively. Upon increasing the energy density of the laser irradiation from similar to 10 to simi lar to 40 J . cm(-2), the intensity of the photoluminescence and the c ontribution of the fast component increased as well. The difference in the photoluminescence spectra compared to the spectra of layers of po rous silicon obtained by electrochemical etching is attributable to a shift in the distribution of nanocrystallite sizes toward smaller size s, which leads to an increase in the area of the Si/SiOx surface that separates the phases. Although our results can be explained within the framework of a quantum-well model, we do not rule out the contributio n of local centers at the Si/SiOx phase boundary. The slow component o f the relaxation is approximated by a ''stretched'' exponential. It is assumed that the slow component is controlled by the rate at which ph otocarriers leave the quantum-well nanocrystallites by tunneling throu gh the Si/SiOx barrier. (C) 1997 American Institute of Physics.