Ken Fujita

Thermal decomposition of ultrathin oxide layers on Si(111) surfaces mediated by surface Si transport

Thermal decomposition of ultrathin oxide layers (less than 1 nm thick) on Si(111) surfaces was studied by using scanning reflection electron microscopy and scanning tunneling microscopy. Void formation, where the diameter and density of the voids depend on the oxide film thickness, occurred on terraces randomly and independently of the buried steps at SiO2/Si(111) interfaces. Decomposition of the oxide layers caused by the void growth produced atomic-height holes on exposed Si surfaces. The surface roughness produced by the holes after thermal decomposition increased with the thickness of the oxide layers. The surface mass transport of Si adatoms to form volatile SiO products explains these experimental results.

Observation and creation of current leakage sites in ultrathin silicon dioxide films using scanning tunneling microscopy

We used scanning tunneling microscopy (STM) to investigate the local leakage current through ultrathin silicon dioxide (SiO2) films grown on Si substrates. Individual leakage sites, which were created by hot-electron injection from the STM tip under a high sample bias of +10 V, were identified from the local change in surface conductivity due to defect creation in the oxide films. When we reversed the stressing polarity (using a negative sample bias) no leakage sites were created in the oxide film.

NANOMETER-SCALE SI SELECTIVE EPITAXIAL GROWTH ON SI(001) SURFACES USING THE THERMAL DECOMPOSITION OF ULTRATHIN OXIDE FILMS

Nanometer-scale Si crystals were produced by selective epitaxial growth on Si(001) surfaces passivated with 0.3-nm-thick oxide films. Window areas for the growth were provided by void formation during the thermal decomposition of the oxide films. Dynamical processes of the void formation and epitaxial growth were observed at 630–730 °C by scanning tunneling microscopy. The crystal shape was a quadrangular pyramid and the typical size was 20 nm in length and 0.8 nm in height. The thin oxide films were found to act as masks for the selective epitaxial growth of nanoscale structures.

NANOMETER-SCALE SI SELECTIVE EPITAXIAL GROWTH ON SI SURFACE WINDOWS IN ULTRATHIN OXIDE FILMS FABRICATED USING SCANNING TUNNELING MICROSCOPY

Using scanning tunneling microscopy (STM), nanometer-scale Si(111) and Si(001) windows in ultrathin SiO2 films are fabricated by electron-beam-induced thermal decomposition. At 450–630 °C, the oxidized Si surfaces are irradiated with a field emission electron beam from a STM tip with an energy of 70–150 eV and a current of 10–50 nA. The smallest window size is about 40 nm. The shape of the Si crystals selectively grown on the Si(001) windows is that of a frustum of a quadrangular pyramid, while that on the Si(111) windows is an (111) oriented two-dimensional island. We discuss the influence of the field emission electrons on the fabrication and the selective growth.

Magnetodielectric effect in EuZrO3

Following recent report on antiferromagnetic ordering in EuZrO3 we performed dielectric measurements of this material as a function of temperature and magnetic field. Dielectric constant of dense EuZrO3 ceramics is 30.1 at 300 K. It gradually decreases upon cooling without any quantum paraelectric behavior; however, below TN=4.1 K it shows a pronounced drop that qualitatively resembles that observed in EuTiO3. We report that dielectric constant of EuZrO3 is magnetic field dependent. The magnitude of the magnetodielectric effect in both EuTiO3 and EuZrO3 is discussed in the light of the recently proposed coupling of the Eu–O–Eu superexchange interactions with electrons involved in partially covalent Ti(Zr)–O bond.

Nucleation along step edges during Si epitaxial growth on the Si(111) surface observed by STM

Abstract Nucleation on the Si(111) surface during Si epitaxial growth was investigated by scanning tunneling microscopy combined with chemical beam epitaxy. It was found that two-dimensional nuclei align along step edges on the Si(111) surface in the temperature range 450–550°C. In this temperature range, the 7 × 7 reconstruction is not formed in regions where epitaxial layers have grown from step edges, whereas the 7 × 7 reconstruction is preserved on original terraces. Nucleation takes place on the boundaries between the 7 × 7 reconstructed and unreconstructed regions. Nucleation is more conspicuous at [1¯1¯2]-type step edges than at [112¯]-type step edges.

Scanning tunneling microscopy study on void formation by thermal decomposition of thin oxide layers on stepped Si surfaces

We investigate void formation by thermal decomposition of thin oxide layers on stepped Si(001) and Si(111) surfaces by using high-temperature scanning tunneling microscopy. We have found that the surface roughening during void formation on stepped Si surfaces is less than that on on-axis Si surfaces. The Si atoms necessary for oxide decomposition are supplied from step edges on the stepped surface rather than by hole nucleation.

Selective thermal decomposition of ultrathin silicon oxide layers induced by electron-stimulated oxygen desorption

The mechanism of electron-beam-induced selective thermal decomposition of ultrathin oxide layers on Si surfaces was studied by scanning reflection electron microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. We found that the change in the oxide layer composition caused by electron-stimulated oxygen desorption accounted for the selective thermal decomposition, where nanometer-scale voids were densely generated at a low heating temperature (720 °C). This implies that oxygen desorption from the oxide layers promotes the formation of a volatile oxide (SiO), and generates void nucleation sites.

Nanometer-scale Ge selective growth on Si(001) using ultrathin SiO2 film

We performed nanometer-scale Ge selective growth using ultrathin silicon dioxide film on Si(001) surfaces. Growth was observed in real time by scanning tunneling microscopy (STM). Window areas with a size of 10–50 nm were fabricated using two different methods: void formation during thermal decomposition of the oxide and field-emission electron-beam irradiation from an STM tip. Selective epitaxial growth was achieved by introducing germane gas (GeH4). With the first method, 3D nucleations occurred near the periphery of the voids and several Ge clusters of irregular shape grew. With the second method, 3D nucleations occurred at the center of the window, and several clusters coalesced forming one hut cluster. The second method was used to form a nanometer-scale Ge dot array.

Electron-Beam-Induced Selective Thermal Decomposition of Ultrathin SiO2 Layers Used in Nanofabrication.

We used ultrathin SiO 2 layers less than 1 nm thick for nanofabrication. In this method, nanometer-scale patterning onto the oxide layers was achieved by electron-beam (EB)-irradiation and subsequent thermal heating (EB-induced selective thermal decomposition). We examined the delineation mechanism by using scanning reflection electron microscopy (SREM), and Auger electron and X-ray photoelectron spectroscopy (AES and XPS). We found that the change in the oxide layer composition caused by electron-stimulated oxygen desorption (ESD) from the oxide layers accounted for the selective thermal decomposition, by which nanometer-scale voids were densely generated.