Fude Liu

Transition layers at the SiO2/SiC interface

The electrical performance of SiC-based microelectronic devices is strongly affected by the densities of interfacial traps introduced by the chemical and structural changes at the SiO2∕SiC interface during processing. We analyzed the structure and chemistry of this interface for the thermally grown SiO2∕4H-SiC heterostructure using high-resolution transmission electron microscopy (TEM), Z-contrast scanning TEM, and spatially resolved electron energy-loss spectroscopy. The analyses revealed the presence of distinct layers, several nanometers thick, on each side of the interface; additionally, partial amorphization of the top SiC surface was observed. These interfacial layers were attributed to the formation of a ternary Si–C–O phase during thermal oxidation.

The mechanism for polarity inversion of GaN via a thin AlN layer: Direct experimental evidence

Lateral-polarity heterostructures of GaN on c sapphire were prepared by deposition and patterning of a thin low-temperature AlN nucleation layer. Adjacent macroscopic domains were found to have opposite polarity; domains grown on the AlN nucleation layer were Ga polar while those grown on the nitrided sapphire were N polar, as confirmed by convergent-beam electron diffraction and Z-contrast images. We directly determined the atomic interface structure between the AlN and c sapphire with an aberration-corrected scanning transmission electron microscope at ∼1.0A resolution. This is the direct experimental evidence for the origin of the polarity control in III nitrides. This understanding is an important step toward manipulating polarity in these semiconductors.

Thermal annealing effect on the interface structure of high-κ LaScO3 on silicon

The thermal stability of LaScO3 on Si was examined by various transmission electron microscopy techniques. The film remained amorphous up to 700°C and became polycrystalline at 800°C. All samples showed an interfacial layer about 3.5nm thick, except for the 1000°C-annealed sample, which had a thicker interfacial layer containing a thin silicate layer close to the interface with the substrate. Although the chemical composition of the bulk film was stoichiometric, the interfacial layer was oxygen-rich after postannealing. The interfacial layer remained amorphous up to 1000°C, indicating that this interfacial layer itself may be used as a gate dielectric.

Chemical composition changes across the interface of amorphous LaScO3 on Si (001)

An amorphous, high-dielectric-constant LaScO3 film was deposited directly on Si (001) by molecular-beam deposition at ∼100°C. Various transmission electron microscopy techniques were applied to study the interface at atomic resolution. We observed an ∼3.5-nm-thick interfacial layer that was not previously detected with other techniques. The interfacial layer contained defects and its density changes gradually. The interface was not only structurally sharp but also chemically sharp within the detection limit of the experimental methods. The chemical composition of the bulk oxide film was stoichiometric, but the interfacial layer was oxygen poor.