The nature of the silicon oxide transition region in the vicinity of the Si/SiO2 interface is probed by infrared and x-ray photoelectron spectroscopies. The layer-by-layer composition of the interface is evaluated by uniformly thinning thermal oxide films from 31 Å down to 6 Å. We find that the thickness dependence of the frequencies of the transverse optical and longitudinal optical phonons of the oxide film cannot be reconciled by consideration of simple homogeneous processes such as image charge effects or stress near the interface. Rather, by applying the Bruggeman effective medium approximation, we show that film inhomogeneity in the form of substoichiometric silicon oxide species accounts for the observed spectral changes as the interface is approached. The presence of such substoichiometric oxide species is supported by the thickness dependence of the integrated Si suboxide signal in companion x-ray photoelectron spectra.

We have investigated the fundamental mechanism underlying the hydrogen-induced exfoliation of silicon, using a combination of spectroscopic and microscopic techniques. We have studied the evolution of the internal defect structure as a function of implanted hydrogen concentration and annealing temperature and found that the mechanism consists of a number of essential components in which hydrogen plays a key role. Specifically, we show that the chemical action of hydrogen leads to the formation of (100) and (111) internal surfaces above 400 °C via agglomeration of the initial defect structure. In addition, molecular hydrogen is evolved between 200 and 400 °C and subsequently traps in the microvoids bounded by the internal surfaces, resulting in the build-up of internal pressure. This, in turn, leads to the observed “blistering” of unconstrained silicon samples, or complete layer transfer for silicon wafers joined to a supporting (handle) wafer which acts as a mechanical “stiffener.”

Oriented thin films of organic semiconducting small molecules were prepared by crystallization on rubbed alignment layers. Polarized absorption spectra showed that the long axis of the conjugated backbones was highly oriented along the rubbing direction and parallel to the substrates. Transmission electron microscopy and diffraction confirmed that the molecules and in many cases the resulting crystals are aligned. Using the above aligned films as semiconducting layers, we fabricated field-effect transistors having anisotropic mobilities with ratios greater than 15. Several common organic semiconductors have been investigated, and the results indicate that this growth method is generally successful for achieving macroscopic alignment of these semiconducting molecules (and frequently their crystals, as well).

Articles you may be interested inAnalysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy Angle-resolved x-ray photoelectron spectroscopy ͑AR-XPS͒ is utilized in this work to accurately and nondestructively determine the nitrogen concentration and profile in ultrathin SiO x N y films. With furnace growth at 800-850°C using nitric oxide ͑NO͒ and oxygen, 10 13 -10 15 cm Ϫ2 of nitrogen is incorporated in the ultrathin (р4 nm͒ oxide films. Additional nitrogen can be incorporated by low energy ion ( 15 N 2 ) implantation. The nitrogen profile and nitrogen chemical bonding states are analyzed as a function of the depth to understand the distribution of nitrogen incorporation during the SiO x N y thermal growth process. AR-XPS is shown to yield accurate nitrogen profiles that agree well with both medium energy ion scattering and secondary ion mass spectrometry analysis. Preferential nitrogen accumulation near the SiO x N y /Si interface is observed with a NO annealing, and nitrogen is shown to bond to both silicon and oxygen in multiple distinct chemical states, whose thermal stability bears implications on the reliability of nitrogen containing SiO 2 .

Since the first report of the Unibond process,l there has been much interest in reproducing Si exfoliation by H implantation and in understanding the mechanism leading to such a remarkably uniform shearing. We have previously demonstrated that, contrary to the initial speculation? there are in fact three distinct aspects to the process3 i) The generation of damage to the crystalline material by the implantation; ii) The unique surface chemistry of hydrogen and silicon that drives the thermal evolution of this damage region and; iii) The creation of internal pressure that ultimately causes exfoliation of the overlying Si layer. Therefore, a detailed understanding of the exfoliation mechanism involves the study of initial damage, of H-passivation of various internal structures and of the mechanical forces exerted by trapped gases as a function of hydrogen implantation doseldepth and annealing temperature. In this work, we have used different hydrogen implantation conditions (ion energies ranging from 1 V to 1 MeV and substrate crystallographic orientations) as well as co-implantation of a variety of other elemental species, in combination with novel spectroscopic configurations, to further explore these different mechanistic aspects.Infrared spectroscopy has played a key role in elucidating the microscopic details of the process, due to its high sensitivity and selectivity and inherent non-destructiveness. However, the frequency range accessible was limited to above 1500 cm-1 so that only the Si-H stretching vibrations could be observed. Recently, we have developed novel optical configurations that allow probing of the Si-H bending modes (at -600-650 cm-1) and scissor modes (850-910 cm-I), allowing definitive identification of the different defect modes. Using this approach, in combination with a variety of other techniques, we have been able to definitively show that exfoliation consists of the following distinct mechanistic steps: Above the critical dose of 6 x 1016/cm2, the IR spectrum shows evidence for monohydride-terminated, multi-vacancy defects that are typically found in hydrogenated amorphous silicon. The formation of such a "multivancy" defect region is critical to exfoliation, because it allows both formation of agglomerated defects and the evolution of molecular H2. These defects, in turn, develop into (100) and (1 11) internal cracks which act as traps for the H2 leading to the build-up of internal pressure and subsequent shearing. It is the synergetic combination of H-passivation of internal surfaces and H2 pressure within these intemal cracks that leads to the shearing in the presence of the joined wafer, that acts as a mechanical stiffener. Importantly, in the absence of the stiffener, the surface 'blisters'; in the absence of sufficient damage (below the critical dose), the hydrogen diffuses away from the implanted region, preventing exfoliation.Recent experiments4 have isolated the physical and chemical contributions to exfoliation by co-implanting He, Li and Si along with H and demonstrated that...