DEFECTS IN POROUS P-TYPE SI - AN ELECTRON-PARAMAGNETIC-RESONANCE STUDY

The defects in p+ porous silicon of low and high porosity have been studied by using electron-paramagnetic-resonance (EPR) spectroscopy and compared with an impurity analysis obtained from nuclear reaction analysis (NRA). The EPR measurements show, in both high- and low-porosity samples, the same dominant paramagnetic defect which we have identified from its g tensor and hyperfine tensor as the trigonal (111) P(b) center; the neutral dangling bond at the Si/SiO2 interface. The symmetry of the central Si hyperfine-interaction tensor has been determined. From the defect concentration it is estimated that about 50% of the surface of p-type porous Si exposed to air is oxidized. The symmetry of the EPR spectrum of the P(b) center relative to the substrate proves the monocrystalline character of the oxidized surface with a preferential (111) orientation. In as-grown and aged samples a high fraction ( > 0. 95) of the P(b) centers are passivated by H. They are depassivated by a thermal anneal at 400-degrees-C under ultrahigh vacuum, revealing total P(b)-center concentrations of some 10(11) cm-2. Independent of porosity both types of sample were found, by using NRA, to be heavily contaminated by H, C, and 0 with impurity-atom to Si-surface-atom ratios of 0.3 to 0.6, 0.1, and 0.01, respectively. The 80%-porosity samples, the only ones showing room-temperature visible photoluminescence, contain, in addition, amorphous Si inclusions; their presence is deduced from the observation of the g = 2.0055 dangling-bond centers in thermally annealed samples. As P(b) centers are recombination centers, efficient photoluminescence in 80%-porosity samples, in which surface properties are predominant, requires a minimization and stabilization of these defects.