Electron-Spin Resonance of Type II Si-Clathrate Thin Film for New Solar Cell Material

Silicon clathrate thin films, NaxSi136 (x > 5), were prepared on Si (111) substrates. The X-ray diffraction (XRD) pattern shows a mixture of the type II Si-clathrate crystalline and amorphous phases. The electron-spin resonance (ESR) spectrum for the Si-clathrate thin film consists of two lines with a set of g value and width, (g, gamma(mT)) = (2.005, 1) and (2.002, 0.1). In comparison, the ESR spectra for NaxSi136 (x = 5.5, 11) polycrystalline powders have a fairly broad line with (g, gamma(mT)) = (similar to 2.05, 15) other than the above two lines. Such broad line may be assigned to a Na-Na pair and/or a Na-cluster in the Si-clathrate polycrystalline powders. These ESR results suggest that the dominant line with (g, gamma(mT)) = (2.005, 1) observed for the Si-clathrate thin film is assigned to electron trapped at a Si-deficit with a dangling bond created in the amorphous phase.