Trapping and detrapping of electrons photoinjected from silicon to ultrathin SiO2 overlayers. I. In vacuum and in the presence of ambient oxygen

Transient trapping/detrapping of electrons at the Si(100)/SiO2 outer surfac e is studied studied in vacuum or with an O-2 ambient (between 10(-3) and 3 0 Torr) following internal electron photoemission from Si. Photoemission-cu rrent (produced by a 150 fs, 800 nm laser source) and contact-potential-dif ference techniques were used to investigate a wide variety of n- and p-dope d samples at 300 K with thermally grown, steam grown, and dry oxides with t hickness less than or equal to 5 nm as well as samples with the oxide layer s removed. Characteristics of the steam grown oxide were also studied at 40 0 and 200 K. For samples in vacuum charging is attributed to direct filling of at least two families of traps, one related to the oxide and the other the Si/SiO2 interface. For samples in O-2, details of oxygen-assisted surfa ce charging as reported previously [Phys. Rev. Lett. 77, 920 (1996)] are gi ven. A fast, Coulomb-repulsion driven spillover of surface charge from the irradiated spot to the rest of the surface was detected. Oxygen aids trap f illing of the in-vacuum filled and gas-sensitive traps and also detrapping (the efficacy of which increases strongly from 400 to 200 K) when the optic al excitation source is removed. Surface transient charging and charge trap ping efficacy for the oxidized samples are not very sensitive to sample pre paration. A mobility of the trapped charges, probably hopping between traps and also Coulomb-repulsion driven, was measured. (C) 2000 American Institu te of Physics. [S0021-8979(00)05114-8].