Carriers reactivation in p+-type porous silicon accompanies hydrogen desorption

FTIR spectra of p+-type mesoporous silicon (m-PS) outgassed in the 300–600 K range show a loss of transparency with increasing temperature, more pronounced at low frequencies. This is evidence of free-carrier formation. Previous work (F. Geobaldo et al., Sensors and Actuators B, in press [1]) concerning the reversible interaction of NO2 and NH3 has shown the presence at the surface of adsorption sites involving Si/B pairs. Thermal treatment of the sample causes desorption of molecular hydrogen, released through the homolytic splitting of Si-H bonds. Besides meeting each other forming a H2 molecule, H atoms may interact with an adsorption site, by creating a new H-Si-B bond. This new bond needs one additional electron to be formed and injection of a hole takes place into the solid. At higher temperatures, surface hydrogen is almost totally removed and the sample transparency recovered.