BULK ELECTRICAL-CONDUCTION IN THE BURIED OXIDE OF SIMOX STRUCTURES

Buried oxide (BOX) layers in separated by implantation of oxygen (SIMOX) structures exhibit well-defined and reproducible bulk (''background'') conduction which, in contrast to localized (''defect'') conduction, is area-dependent. For annealed samples the bulk conduction is quasi-ohmic and time dependent up to approximately 80 V (for approximately 400 nm thick BOX); the temperature dependence indicates 0.3 eV as the activation energy associated with trapping effects. At higher voltage the current-voltage characteristics are superlinear and resemble those observed for deposited Si-rich SiO2 and Si(x)N(y) films, except that they depend on polarity Fowler-Nordheim plots indicate an effective barrier height of 1.3 to 1.5 eV for annealed samples and 0.4 eV for unannealed ones; this range is intermediate between 3.1 eV (thermal oxide) and 0.4 to 0.6 eV (Si-rich SiO2). Poole-Frenkel or Schottky plots (log I vs. E1/2) are similar to those reported for Si(x)N(y) films with excess silicon (x/y > 0.75). However, the temperature dependence of the high-field conduction is much less than expected for these mechanisms, indicating that the conduction is controlled by tunneling between Si clusters. From the effective barrier heights of annealed samples the size of the clusters is estimated as approximately 0.5 nm (three Si atoms) and their density as 2 x 10(19) cm-3 so that the concentration of excess silicon is at least 6 x 10(19) cm-3. For unannealed BOX the order of magnitude of excess silicon concentration is estimated as 10(21) cm-3. The polarity dependence of conduction is probably associated with the asymmetry in the distribution of excess silicon at the two interfaces. For unannealed samples the current at a given voltage is several orders of magnitude higher than for the annealed ones and the onset of the superlinear regime is at lower voltage; this behavior is similar to the localized ('' defect'') conduction exhibited by both annealed and unannealed SIMOX samples. All of the observations indicate that the BOX layers contain excess silicon; this may be in the form Of O3Si-SiO3 bonds in the oxide and/or very small silicon (probably amorphous) clusters/filaments.