Defect and structure analysis of n +-, p +- and p-type porous silicon by the electron paramagnetic resonance technique

Abstract

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-prepared, vacuum-annealed, thermally oxidized and anodically oxidized layers of various porosities (45–85%) have been analyzed. Under photoexcitation at temperatures below 50K, a high-intensity EPR spectrum is observed in both n + and p + layers, which we attribute to the resonance of photoexcited free electrons with lifetimes in the 100 s range. The long lifetime favours the model of a spatial separation of photoexcited electrons and holes at T < 50K. For temperatures above 50K their lifetimes become too short to allow EPR detection. No EPR spectrum associated with the dopants (phosphorus and boron) could be detected. The dominant paramagnetic defect observed under thermal equilibrium conditions is the P b centre at the (111) Si/SiO 2 interfaces; its concentration varies strongly with annealing and oxidation in the 10 10-10 12 cm -2 concentration range. Anodically oxidized layers show a different EPR spectrum, which is tentatively attributed to the simultaneous presence of P b centres from (111) and other Si/SiO 2 interfaces. In addition, EX centre defects are detected in high-temperature oxidized material; in anodically oxidized porous silicon E′ centres, a defect studied previously in bulk SiO 2 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 = 2.0055 dangling bond defect. The symmetry of the P b centre is used to verify the monocrystalline character of the porous layers and to determine the internal surfaces of porous layers. The electrochemical dissolution and the thermal oxidation lead to the preferential formation of (111) surfaces.