No abstract

The characteristics of interfacial reactions and the valence band offset of HfO2 films grown on GaAs by atomic layer deposition were investigated by combining high-resolution x-ray photoelectron spectroscopy and high-resolution electron transmission microscopy. The interfacial characteristics are significantly dependent on the surface state of the GaAs substrate. Polycrystalline HfO2 film on a clean GaAs surface was changed to a well-ordered crystalline film as the annealing temperature increased, and a clean interface with no interfacial layer formed at temperatures above 600°C. The valence band offset of the film grown on the oxidized GaAs surface gradually increased with the stoichiometric change in the interfacial layer.

Pentacene-based thin film transistors with ultrathin (6nm) (HfO2)x(SiO2)1−x gate dielectric layers (x=0.25 and 0.75) were fabricated for low-voltage operation. The devices with ultrathin (HfO2)x(SiO2)1−x as the gate dielectric layer were operated at a gate voltage lower than −4.0eV. However, the threshold voltage and drain current have different values depending on the composition of the (HfO2)x(SiO2)1−x gate dielectric layer. The device with (HfO2)0.75(SiO2)0.25 gate dielectrics, having larger capacitance, shows a higher drain current than that with (HfO2)0.25(SiO2)0.75 gate dielectrics. On the other hand, the device with (HfO2)0.25(SiO2)0.75 gate dielectrics, which has a larger work function, shows a lower threshold voltage. The in situ ultraviolet photoelectron spectroscopy shows that this is caused by the difference in electronic structures and by changes in band alignment of the interface between the pentacene and dielectric layers.

The electronic structures of tris-(8-hydroquinoline) aluminum (Alq3)∕Li2O∕Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. Inserting Li2O between Al and Alq3 led to the highest occupied molecular orbital (HOMO) of Alq3 shifting to a higher binding energy compared to that without Li2O, which resulted in an improved electron injection. We also observed that the magnitude of the secondary cutoff shift was almost identical to that of the HOMO shift with the insertion of Li2O. This implies that the energy level alignment depends on the interface dipole and ionization energy of the adsorbate. Additionally, a gap state was observed in the gap of Alq3, which is related to the interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by inserting Li2O between them.

Additional In adsorption onto the Si(111))ϫ)-In surface at room temperature has been known to induce spontaneous structural transformations into a 2ϫ2 and a ͱ7ϫ) phase, which accompany a drastic change of the surface electric property. These structural transformations have been studied by low-energy-electron diffraction and core-level photoemission spectroscopy using synchrotron radiation. The transformation from ) ϫ) to 2ϫ2 is characterized by the appearance of an extra In 4d component shifted by Ϫ0.41 eV in binding energy. The 2ϫ2 phase fully develops at the In coverage of ϳ0.8 monolayer ͑ML͒, which has two different In sites as indicated by the In 4d spectra. This and the Si 2p core-level data deny the present structural models of the 2ϫ2 phase. The In 4d line shape of the ͱ7ϫ) phase formed above ϳ1.2 ML exhibits a strong asymmetry, indicating a metallic character of this surface in clear contrast to )ϫ) and 2 2 phases. A unique Si 2p surface component, which represents the topmost Si layer, is identified for the ͱ7ϫ) phase with a surface core-level shift of Ϫ0.20 eV. These results are generally consistent with the ͱ7ϫ) structure model consisting of one planar In overlayer on top of a bulk-terminated Si͑111͒. Accompanying the structural transformations, a drastic lowering of the surface Fermi-level position is observed until the In coverage increases up to ϳ1.0 ML.