Charge transport and trapping phenomena in off-stoichiometric silicon dioxide films

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Abstract
The electrical characteristics of off-stoichiometric silicon dioxide films have been investigated. The off-stoichiometric oxide films studied had an excess atomic silicon (Si) content in the range of 1%–6%. Raman spectroscopy and photoconductivity measurements indicate that the excess Si is present as amorphous Si islands or small crystallites embedded in silicon dioxide (SiO2) forming a two-phase material. These films differ in structure from previously reported films where dual dielectric layers of stoichiometric SiO2 and Si-rich SiO2 with ≥13% excess atomic Si were used. These dual dielectric films were observed to produce electron injection from contacting electrodes via the Si-rich SiO2 layer into the SiO2 at lower average electric fields. This injection mechanism was believed to be due to localized electric field enhancement near the SiO2–Si-rich SiO2 interface caused by the curvature of the tiny Si islands in the SiO2 matrix. The current versus voltage characteristics of the off-stoichiometric oxide films which will be discussed here were found to be highly nonohmic, showing an increase in conductivity with increasing excess silcion content in the material. At low fields (≲1 MV/cm), these films have a very small conductance with leakage current densities smaller than 10−11 A/cm2 at room temperature. Furthermore, the effect of permanent charge trapping was observed to decrease in these films with increasing Si content. It is proposed that the electron transport across the film is controlled primarily by tunneling between the silicon islands with a reversible space charge region ≲150 Å in extent near the injecting contact, limiting the current measured in the external circuit. It is also proposed that the decrease in permanent electron trapping is associated with this conduction mechanism and/or the possibility of trapped electrons in the SiO2 phase tunneling to the Si islands. With the incorporation of these off-stoichiometric oxides into electrically-alterable read-only-memory devices, extended write/erase cycling (by at least four orders of magnitude) beyond that normally observed for equivalent devices using stoichiometric SiO2 layers is demonstrated due to the reduction in permanent electron trapping in the oxide layer and its effect on the electric fields.