T. Komeda

Interface states at ultrathin oxide/Si(111) interfaces obtained from x‐ray photoelectron spectroscopy measurements under biases

The energy distribution of interface states for Si(111)‐based metal–oxide–semiconductor (MOS) devices with an ultrathin chemical oxide layer is obtained from measurements of x‐ray photoelectron spectra under biases. All the observed interface states have discrete energy levels. The energy deviation of the interface state levels from the midgap is smaller than that for the Si(100)‐based MOS devices; this is attributed to weaker interaction of the Si dangling bond with Si and oxygen atoms in the oxide layer. The densities of the interface states at the Si(111)/ultrathin oxide interface are comparable to those at the Si(100)/ultrathin oxide interface.

Formation of periodic step and terrace structure on Si(100) surface during annealing in hydrogen diluted with inert gas

Annealing of a Czochralski Si(100) substrate in a gas flow in which H2 is diluted to a concentration of 3% in He was investigated. The surfaces annealed at 900 °C or above showed well-developed terraces with mono-atomic steps that alternate between the straight A-step and the zigzag B-step configurations, whereas no morphological improvement occurred in the surfaces annealed at temperatures below 900 °C, as confirmed by atomic force microscopy. Reflection high-energy electron diffraction observations and Fourier-transform infrared attenuated total reflection spectra of the flat surfaces clarified that the surface Si atoms make dimers with their dangling bonds terminated by H atoms, i.e., the surface reconstruction is Si(100)2×1/1×2-H. We show that the depth of the etching was not sufficient to account for the observed smoothening of the surface during the annealing at high temperatures, and conclude that the main mechanism of surface flattening is surface migration of the Si atoms.

Interface States for Si-Based MOS Devices with an Ultrathin Oxide Layer : X-Ray Photoelectron Spectroscopic Measurements under Biases

Energy distributions of interface states for metal-oxide-semiconductor (MOS) devices have been obtained from measurements of X-ray photoelectron (XPS) spectra under biases. Upon applying bias voltages, the substrate Si(2p) peak is shifted because of a change in the occupancy of interface states by electrons, inducing a change in the potential drop across the silicon oxide layer. Devices with a native oxide layer have high interface state density near the midgap, which is attributed to isolated dangling-bond states. For MOS devices with a thermal oxide layer grown at 550° C in a wet-oxygen atmosphere, the interface states have two density maxima, one below and the other above the midgap. For devices with a thermal oxide layer formed at 700° C in wet oxygen, two density maxima of the interface states are also observed with reduced density.

Octadecyltrichlorosilane self-assembled-monolayer islands as a self-patterned-mask for HF etching of SiO2 on Si

Octadecyltrichlorosilane (OTS), self-assembled-monolayer (SAM) grown on SiO2 in the submonolayer region is investigated by atomic force microscope (AFM), which is further applied to SiO2 characterizations technique. OTS-SAM forms characteristic dendrite-shaped islands in its submonolayer region, whose shape and size significantly depend on the surface roughness of SiO2 formed at different temperatures in the range of 700–1100 °C. Moreover, OTS-SAM islands have practical usefulness as a self-patterned-mask for HF etching. When an oxidized Si wafer covered by OTS-SAM islands is dipped into HF, SiO2 in the area uncovered by the islands is selectively removed. This technique is successfully applied for the precise SiO2 thickness measurement in ultrathin (<50 A) regions by AFM. In addition, this technique enables a simultaneous observation of the morphologies of SiO2 surface and SiO2/Si interface. The result shows, for the first time, the continuity of the steps on SiO2 surface and SiO2/Si interface, indicatin...

Self-assembled-monolayer film islands as a self-patterned-mask for SiO2 thickness measurement with atomic force microscopy

A novel method for measuring ultrathin (2–12 nm) SiO2 film thickness is discussed. The process consists of: (1) formation of octadecyltrichlorosilane (OTS) self-assembled-monolayer (SAM) islands on SiO2 of which thickness to be measured, (2) removal of the SiO2 layers not covered by the OTS-SAM islands, and (3) measurement of the height difference between the etched and nonetched areas by atomic-force-microscopy. The OTS film is good resist against HF and its islands can be regarded as self-patterned-mask. Practical usefulness is demonstrated not only by the compatibility of the measured values but also by the short measurement period resulting from the directness of the method.

INTERFACE STATE-INDUCED SHIFT OF THE OXIDE AND SEMICONDUCTOR CORE LEVELS FOR METAL-OXIDE-SEMICONDUCTOR DEVICES

Measurements of x‐ray photoelectron spectra are performed for ∼3‐nm‐thick Pt/∼3.6‐nm‐thick silicon oxide/n‐Si(100) devices under biases between the Pt layer and the Si substrate. It is observed that the oxide Si 2p peak as well as the substrate peaks is shifted upon applying biases. These shifts are caused by a bias‐induced change of the potential drop across the oxide layer due to the change in the amount of the interface state charge. The amount of the shift of the oxide Si 2p peak is well correlated to that of the substrate Si 2p3/2 peak. The energy distribution of the interface states is obtained by analyzing the amount of the shift of the substrate Si 2p3/2 peak measured as a function of the bias voltage. The interface state spectrum has one peak near the midgap, and the peak is attributed to isolated Si dangling bond states.

Comb-shape step formation on Si(001)-2 × 1 surface

Abstract We report about the formation of grating-like 2 × 1 and 1 × 2 domain structures, where stripe-shape mounds and troughs with monoatomic step height difference appear alternately and cover the entire terrace part with quite uniform periodicity, on Si(001) wafer which has very small miscut angle and high P dopant concentration. The mechanism of the formation of such stripe-shape domains was considered as the combination of the very wide terrace width, and the segregation of the dopant P in the surface cleaning process. The former is due to the expansion of 2 × 1 single domain which contains surface tensile stress perpendicular to the dimer rows formed by dimers. The latter increased the surface stresses by replacing the SiSi dimers with PP or PSi dimers which have a shorter dimer bond length.

Electronic Structure of Stoichiometric and Reduced SrTiO3(100) Surfaces: A Photoemission Study

We have performed angle-resolved photoemission and X-ray photoemission studies for Stoichiometric and reduced SrTiO3(100) surfaces. In the empirical band structure of the O2p valence bands along the whole ? line in the bulk Brillouin zone, there is no essential difference between the Stoichiometric and reduced surfaces. Our results are in excellent agreement with the calculation. In addition to the O2p valence band structure, two new states at ~1 eV and ~11 eV below Fermi level appear for the reduced surface (but not for the stoichiometric surface). We show that both of these states are closely connected with oxygen vacancies near the surface.

Effect of surface oxygen vacancies on electronic states of reduced SrTiO3(110) surface

Abstract The electronic structure of reduced SrTiO3(110) surface prepared by annealing in ultrahigh vacuum (UHV), has been studied using photoemission spectroscopy (PES) and scanning tunneling microscopy/spectroscopy (STM/STS). In the PES spectra of reduced SrTiO3(110) surface after annealing in UHV at 800°C, a clear metallic state with a sharp Fermi cut-off and a broad state centered at ∼1.1eV below the Fermi level are observed in the band gap region. A Ti3p->3d resonance enhancement is occurred for the metallic state, but not for the ∼1.1eV state. On the other hand, after the sample was annealed in UHV at 1000°C, it has been shown that the metallic state becomes drastically weak, and that only the ∼1.1eV state is seen in the band gap region. Thereafter, the ∼1.1eV state exhibits a clear Ti3p->3d resonance enhancement. The difference in the spectral feature of these band gap states is directly related to the surface geometric structural change.

Atomic configuration of segregated B on Si(001) surface

The atomic configurations of segregated B atoms on a highly B-doped Si(001) surface is investigated with scanning tunneling microscopy (STM) observation and first-principle cluster calculation, on which characteristic comb-shape step structures are observed. The characteristic features in the STM image are (1) dark regions in the occupied state, and (2) paired protrusions in the unoccupied state. The calculation shows that models of a B dimer on the top surface and B atoms bonded to two neighboring Si atoms in a dimer row can reproduce STM images well for (1) and (2), respectively.