Real-time study of α-Si:H/c-Si heterointerface formation and epitaxial Si growth by spectroscopic ellipsometry, infrared spectroscopy, and second-harmonic generation

The performance of many devices based on Si thin films deposited on crystalline Si (c-Si) is highly governed by interface quality. For many of these applications, only fully epitaxial films or fully amorphous films having an abrupt interface with the substrate are desired. However, the realization of these perfectly sharp interfaces and the mechanisms governing their formation are not fully understood yet. In this study, the interface formation between Si thin films and c-Si has been investigated by simultaneously applying three complementary optical techniques in real time during low temperature Si film growth. The films were deposited in a hot-wire chemical vapor deposition process by using both native oxide covered and H terminated Si(100) substrates. The formation of hydrogenated amorphous Si (a-Si:H), epitaxial Si, and mixed phase Si has been detected with spectroscopic ellipsometry by measuring the optical properties of the growing films. The evolution of the hydrogen content and hydrogen bonding configurations in the films has been monitored by attenuated total reflection infrared spectroscopy. A clear dependence of the hydrogen content on film morphology is observed with the amorphous films containing significantly more hydrogen. The surface and interface sensitive technique of second-harmonic generation (SHG) has been applied both spectroscopically and in real time. The SHG spectra of a-Si:H films on Si(100) obtained in the SHG photon energy range of 2.7–3.5 eV revealed a dominant contribution originating from the film/substrate interface related to E/E1 critical point (CP) transitions of c-Si. The real-time behavior of the SHG response is shown to strongly depend on differences in initial film morphology, which allows for identification of direct a-Si:H/c-Si heterointerface formation, nanometer-level epitaxial growth, and fully epitaxial growth at a very early stage of film growth. On the basis of the results obtained by the three optical techniques, the c-Si surface passivation mechanism by a-Si:H thin films is addressed and it is demonstrated that the combination of the techniques provides a profound method to control processes occurring during Si thin film growth