High Sensitivity Electron Spin Resonance Studies of the Silicon-Silicon Dioxide Interface.

In order to study the atomic structure of intrinsic defects in all crystallographic orientations and the susceptibility and spin dynamics of inversion layer charge at the silicon -silicon dioxide interface, the sensitivity of Electron Spin Resonance (ESR) is extended to allow measurement of interfaces and thin films with sensitivity from 10 to 100 times higher than previous instruments. The spectrometer is a phase sensitive X-band superheterodyne detector in which amplication of the resonator response is provided by a cryogenically cooled heterostructure transistor device with a noise temperature of 15 kelvin. Resonator response to spin resonance in thin films is increased over that of conventional cavity resonators by the use of microstrip resonators. Microstrip resonators can be fabricated on silicon wafers using conventional device techniques, and can be made from superconducting metal for unloaded Q values of 50,000. The size of the resonator is.1cm x.5cm x.04cm. The resonator metallization provides a natural way to "gate" the interface. Detection of ESR of localized defects (such as the P_{b} dangling bond center) at low temperature is only available in the dispersion phase of the spectrometer where source frequency noise is large. A microstrip bimodal resonator is described which discriminates against frequency noise and thereby maintains high spectrometer sensitivity to the dispersion ESR signal. The ultimate sensitivity of the spectrometer to Curie law spins is 1 times 10 ^8^ins per gauss over an area of 1 cm^2 at 4.2 kelvin for a 1 gauss line at the onset of saturation. Preliminary studies of conduction electron spin resonance of electrons trapped in silicon-silicon dioxide inversion layers show that over an inversion layer concentration range of 10^{11} cm ^{-2} to 10^{12 } cm^{-2} the Lande splitting factor is isotropic in magnetic field direction. Susceptibility versus temperature measurements suggest that the electrons occupy a single band of approximately six-fold degeneracy. Neither result is expected since the six degenerate valleys in bulk silicon are split by the inversion layer field, giving an anisotropic g factor and orbital degeneracy equal to two. The anomalous results are probably connected to the presence of strong disorder in these samples, but no definitive model is presented.