Interface states and electron spin resonance centers have been observed and compared in thermally oxidized (111) and (100) silicon wafers subjected to various processing treatments. The ESR Pb signal, previously assigned to interface ⋅Si≡Si3 defects on (111) wafers, was found to have two components on (100): an ⋅Si≡Si3 center oriented in accord with (100) face structure, and an unidentified center consistent with ⋅Si≡Si2O. The quantitative proportionality of Pb spin concentration to midgap interface trap density Dit is maintained on (100), and both are lower by a factor of about 3 compared to (111). This correlation persists over the range of oxidation temperatures 800–1200°C, for both n- and p-doped silicon, cooled by fast pull in oxygen, and cooled or annealed in nitrogen or argon. The correlation is independent of doping level. In samples with different oxide thickness, neither Pb nor Dit varied significantly over the range 100–2000 A, but Pb was smaller at 50 A. In general, ESR is judged to offer promise for further studies of specific interface features.

Energy distribution of Pb centers (⋅Si≡Si3) and electronic traps (Dit) at the Si/SiO2 interface in metal-oxide-silicon (MOS) structures was examined by electric-field-controlled electron paramagnetic resonance (EPR) and capacitance-voltage (C-V) analysis on the same samples. Chips of (111)-oriented silicon were dry-oxidized for maximum Pb and trap density, and metallized with a large MOS capacitor for EPR and adjacent small dots for C-V measurements. Analysis of C-V data shows two Dit peaks of amplitude 2×1013 eV−1 cm−2 at Ev+0.26 eV and Ev+0.84 eV. The EPR spin density reflects addition or subtraction of an electron from the singly occupied paramagnetic state and shows transitions of amplitude 1.5×1013 eV−1 cm−2 at Ev+0.31 eV and Ev+0.80 eV. This correlation of electrical and EPR responses and their identical chemical and physical behavior are strong evidence that ⋅Si≡Si3 is a major source of interface electronic traps in the 0.15–0.95 eV region of the Si band gap in unpassivated material.

The band-gap energy distribution of Pb centers on oxidized (100) Si wafers has been determined and compared with interface electrical trap density Dit. Two different Pb centers are observed on (100) Si: Pb0, which has the structure ⋅Si≡Si3, and is essentially identical to the sole Pb center observed on (111) Si; and Pb1, of presently uncertain identity, but clearly different in nature from Pb0. By electric field-controlled electron paramagnetic resonance (EPR) and capacitance-voltage (C-V) measurements, it is found that Pb0 has its (0↔1) electron transition at Ev+0.3 eV and its (1↔2) transition at Ev+0.85 eV. Similarly, Pb1 has its (0↔1) transition at Ev+0.45 eV and its (1↔2) transition at Ev+0.8 eV. The Pb band-gap density correlates qualitatively and quantitatively with the electrical trap density Dit from C-V analysis; nonbonded Pb orbitals are found to be the source of about 50% of the characteristic traps in dry-oxidized, unannealed (100) Si wafers.

The ESR Pb center has been observed in thermally oxidized single-crystal silicon wafers, and compared with oxide fixed charge Qss and oxidation-induced interface states Nst. The Pb center is found to be located near the interface on (111) wafers. Its g anisotropy is very similar to that of known bulk silicon defects having SiIII bonded to three other Si atoms; the Pb unpaired electron orbital, however, is exclusively oriented normal to the (111) surface. The Pb center cannot be identified with any other known defect in Si or SiO2; in particular, it is totally unlike the common E′ center of SiO2. In contrast to Qss, both Pb and Nst were found to be greatly reduced by steam oxidation and hydrogen annealing. Both Pb and Nst may be regenerated by subsequent N2 anneals at 500 °C. In a graded series of samples, Pb and Nst are found to be proportional and nearly equal in concentration. This possible confirmation of SiIII at the interface, and correlation with Nst, support the theoretical indication of an SiIII band-gap energy level. The E′ center is unobservable, and if present, exists only in a concentration well below that of Qss. Thus, in addition to a lack of strong correlation with Pb, Qss is evidently not due to E′ centers in their normal charge state. Overall, ESR is judged to be a useful technique for research on silicon wafer defects.