Microscopic bonding and macroscopic strain relaxations at Si-SiO 2 interfaces

Abstract

Optical second harmonic generation studies of the temperature dependence of the phase angle differences from contributions of terrace and edge bonding at silicon–silicon dioxide interfaces prepared on vicinal Si(111) wafers have revealed an interface relaxation process at an annealing temperature between 850 and 900 °C. Complementary studies by synchrotron soft X-ray photoelectron spectroscopy have established that this relaxation is associated with changes in the concentration and composition of local suboxide bonding environments in an ultrathin, interfacial transition region that is approximately one molecular layer thick. This relaxation occurs at a significantly lower temperature than an approximately 990±10 °C relaxation of macroscopic compressive strain in the bulk of the SiO2 film. This paper establishes an analogy between i) the Si–SiO2 interface in which there is a transition from a ‘rigid’ substrate, Si, to an ‘ideal continuous random covalent network’, SiO2, in which the average number of bonding constraints/atom matches the network dimensionality, and ii) a concentration dependent transition between ‘under-’ and ‘over-constrained’ local bonding in non-crystalline glass alloys such as GexSe1-x. The interfacial suboxide transition region is demonstrated to have properties in common with a regime of alloy compositions in which ‘self-organization’ reduces bond constraint induced strain, thereby stabilizing these compositions against ‘aging’ as for example in time-dependent changes in the glass transition temperature. These comparisons provide important new insights into defect formation at Si–SiO2 interfaces, as well as interfaces between Si and alternative high-k dielectrics being considered for advanced Si devices including Al2O3 and transition metal silicate alloys, e.g., (ZrO2)x(SiO2)1-x. This new perspective is also extended to interfaces between GaN and SiO2, where the interfacial transition region is a suboxide of Ga, GaOx, with x<1.5.