Tatsuro Maeda

Atomic Control of the SrTiO3 Crystal Surface

The atomically smooth SrTiO3 (100) with steps one unit cell in height was obtained by treating the crystal surface with a pH-controlled NH4F-HF solution. The homoepitaxy of SrTiO3 film on the crystal surface proceeds in a perfect layer-by-layer mode as verified by reflection high-energy electron diffraction and atomic force microscopy. Ion scattering spectroscopy revealed that the TiO2 atomic plane terminated the as-treated clean surface and that the terminating atomic layer could be tuned to the SrO atomic plane by homooepitaxial growth. This technology provides a well-defined substrate surface for atomically regulated epitaxial growth of such perovskite oxide films as YBa2Cu3O7-δ.

Atomic‐scale formation of ultrasmooth surfaces on sapphire substrates for high‐quality thin‐film fabrication

The atomically ultrasmooth surfaces with atomic steps of sapphire substrates were obtained by annealing in air at temperatures between 1000 and 1400 °C. The terrace width and atomic step height of the ultrasmooth surfaces were controlled on an atomic scale by changing the annealing conditions and the crystallographic surface of substrates. The obtained ultrasmooth surface was stable in air. The topmost atomic structure of the terrace was examined quantitatively by atomic force microscopy and ion scattering spectroscopy as well as a theoretical approach using molecular dynamics simulations.

High mobility Ge-on-insulator p-channel MOSFETs using Pt germanide Schottky source/drain

We demonstrate, for the first time, successful operation of Schottky-barrier source/drain (S/D) germanium-on-insulator (GOI) MOSFETs, where a buried oxide and a silicon substrate are used as a gate dielectric and a bottom gate electrode, respectively. Excellent performance of p-type MOSFETs using Pt germanide S/D is presented in the accumulation mode. The hole mobility enhancement of 50%/spl sim/40% against the universal hole mobility of Si MOSFETs is obtained for the accumulated GOI channel with the SiO/sub 2/-Ge interface.

1-nm-capacitance-equivalent-thickness HfO2/Al2O3/InGaAs metal-oxide-semiconductor structure with low interface trap density and low gate leakage current density

We have studied the impact of the Al2O3 inter-layer on interface properties of HfO2/InGaAs metal-oxide-semiconductor (MOS) interfaces. We have found that the insertion of the ultrathin Al2O3 inter-layer (2 cycle: 0.2 nm) can effectively improve the HfO2/InGaAs interface properties. The frequency dispersion and the stretch-out of C-V characteristics are improved, and the interface trap density (Dit) value is significantly decreased by the 2 cycle Al2O3 inter-layer. Finally, we have demonstrated the 1-nm-thick capacitance equivalent thickness in the HfO2/Al2O3/InGaAs MOS capacitors with good interface properties and low gate leakage of 2.4 × 10−2 A/cm2.

Highly suppressed short-channel effects in ultrathin SOI n-MOSFETs

We have investigated short-channel effects of ultrathin (4-18-nm thick) silicon-on-insulator (SOI) n-channel MOSFETs in the 40-135 nm gate length regime. It is experimentally and systematically found that the threshold voltage (V/sub th/) roll-off and subthreshold slope (S-slope) are highly suppressed as the channel SOI thickness is reduced. The experimental 40-nm gate length, 4-nm thick ultrathin SOI n-MOSFET shows the S-slope of only 75 mV and the /spl Delta/V/sub th/ of only 0.07 V as compared to the value in the case of the long gate-length (135 nm) device. Based on these experimental results, the remarkable advantage of an ultrathin SOI channel in suppressing the short-channel effects is confirmed for future MOS devices.

Topmost surface analysis of SrTiO3 (001) by coaxial impact‐collision ion scattering spectroscopy

The terminating atomic plane of SrTiO3 (001) surface was investigated by means of coaxial impact‐collision ion scattering spectroscopy (CAICISS). CAICISS spectra proved that SrTiO3 (001) surfaces of as‐supplied substrates as well as of O2‐annealed substrates were predominantly terminated with TiO2 atomic plane, while the SrO atomic plane came at the topmost surface of SrTiO3 (001) homoepitaxial film. This indicates the structural conversion of the topmost atomic layer from TiO2 to SrO occurred during the SrTiO3 homoepitaxial growth. The azimuth rotational CAICISS spectra exhibited a fourfold symmetry in the surface atom alignments, showing the square lattice structure of a terminating plane.

Ge metal-insulator-semiconductor structures with Ge3N4 dielectrics by direct nitridation of Ge substrates

We have fabricated Ge metal-insulator-semiconductor structures with ultrathin pure germanium nitride (Ge3N4) films by the direct nitridation of germanium (Ge) substrates. The plasma-enhanced nitridation technique was used with dc plasma source at low temperatures. Capacitance–voltage characteristics with no hysteresis and capacitance equivalent thickness of 1.23 nm have been achieved.

Room-Temperature Epitaxial Growth of CeO2 Thin Films on Si(111) Substrates for Fabrication of Sharp Oxide/Silicon Interface

Room-temperature (20° C) epitaxy of high-melting point CeO2 thin films was achieved for the first time on Si(111) substrates. Cross-sectional high-resolution transmission electron microscopy, Rutherford backscattering spectrometry and ion scattering spectroscopy confirmed the formation of a sharp oxide/silicon heterointerface with no boundary amorphous layer and single crystallinity of the present CeO2 films. This was achieved by pulsed laser deposition in an ultrahigh vacuum under optimized oxygen partial pressures and by hydrogen termination of the Si surface.

Role of germanium nitride interfacial layers in HfO2/germanium nitride/germanium metal-insulator-semiconductor structures

The authors investigate the electrical properties of germanium nitride interfacial layers for germanium metal-insulator-semiconductor (Ge MIS) structures with HfO2 high-k dielectrics. A pure Ge nitride interfacial layer is fabricated by direct nitridation of a Ge substrate with the plasma processing before high-k deposition. The interface trap density of Au∕HfO2∕Ge niride/Ge MIS structures measured by the ac conductance method including the effect of the surface potential fluctuation is found to be as low as 1.8×1011cm−2eV−1 at the minimum. It is also found that Ge nitride interfacial layers mitigate the degradation of the accumulation capacitance during the high-temperature annealing.

Pure germanium nitride formation by atomic nitrogen radicals for application to Ge metal-insulator-semiconductor structures

We have investigated the nitridation of germanium using atomic nitrogen radicals generated by a remote rf plasma source. Pure amorphous Ge3N4 films without oxygen are obtained by the direct nitridation of clean Ge substrates. The conformal growth with smooth surface and sharp interface can be achieved in the Ge3N4 films grown at 100°C, where the maximum thickness of the Ge3N4 films is approximately 3nm. While the surfaces of the Ge3N4 films are partially oxidized by the exposure to air, the Ge3N4 films exhibit the high resistance against oxygen diffusion. The Ge3N4 films are water insoluble and soluble in HF. These results demonstrate that pure direct nitridation of Ge substrates has a possibility to be used not only as a passivation layer but also as a diffusion barrier layer against oxygen for Ge metal-insulator-semiconductor field effect transistor applications.