Observations of the EX center in porous material, by means of electron spin resonance, were reported. This center had previously been observed in low-temperature thin oxides on crystalline Si, and was believed to comprise a delocalized hole on three or four O dangling bonds at a Si vacancy. In porous material, the defect was seen only in samples which had been oxidized for about 60s. The electron spin resonance intensity of the EX center could be correlated with the red photoluminescence at room temperature.

W.E.Carlos, S.M.Prokes: Journal of Applied Physics, 1995, 78[3], 2129-31