DEFECT AND STRUCTURE-ANALYSIS OF N(-, P(+)- AND P-TYPE POROUS SILICONBY THE ELECTRON-PARAMAGNETIC-RESONANCE TECHNIQUE())

We have studied the defects and the structure of porous silicon layers prepared by electrochemical dissolution from n(+)-, p(+)- and p-type conductive substrates by electron paramagnetic resonance (EPR). As-pre pared, vacuum-annealed, thermally oxidized and anodically oxidized lay ers of various porosities (45-85%) have been analyzed. Under photoexci tation at temperatures below 50 K, a high-intensity EPR spectrum is ob served in both n(+) and p(+) layers, which we attribute to the resonan ce of photoexcited free electrons with lifetimes in the 100s range. Th e long lifetime favours the model of a spatial separation of photoexci ted electrons and holes at T < 50 K. For temperatures above 50 K their lifetimes become too short to allow EPR detection. No EPR spectrum as sociated with the dopants (phosphorus and boron) could be detected. Th e dominant paramagnetic defect observed under thermal equilibrium cond itions is the P-b centre at the (111) Si/SiO2 interfaces; its concentr ation varies strongly with annealing and oxidation in the 10(10)-10(12 )cm(-2) concentration range. Anodically oxidized layers show a differe nt EPR spectrum, which is tentatively attributed to the simultaneous p resence of Pb centres from (Ill)and other Si/SiO, interfaces. In addit ion, EX centre defects are detected in high-temperature oxidized mater ial; in anodically oxidized porous silicon E' centres, a defect studie d previously in bulk SiO2, and attributed to positively charged oxygen vacancies, are observed. The vacuum annealing, which increases the P- b centre concentration, can equally lead to the formation of amorphous /disordered inclusions, as evidenced by the detection of the g = 2.005 5 dangling bond defect. The symmetry of the P-b centre is used to veri fy the monocrystalline character of the porous layers and to determine the internal surfaces of porous layers. The electrochemical dissoluti on and the thermal oxidation lead to the preferential formation of (11 1) surfaces.