Mitsuo Sakashita

Praseodymium silicate formed by postdeposition high-temperature annealing

Praseodymium silicate (Pr silicate) has been synthesized by molecular-beam deposition of Pr2O3 layers on Si(111) substrates and subsequent high-temperature postdeposition annealing at 1000 °C. This thermal treatment drastically changes the film texture from the crystalline Pr2O3 epitaxially grown on Si, into Pr silicate with completely amorphized structures, resulting from intermixing of Si from the substrate. It was found that the electrical characteristics of Pr silicate were critically dependent on the quality of the as-deposited Pr2O3 films. A typical dielectric constant and a leakage current density of Pr silicate grown under an optimal condition were, respectively, 19.7 and 3×10−9Acm−2 at +1 V relative to the flatband voltage for an equivalent oxide thickness of 1.9 nm. Using x-ray photoelectron spectroscopy, a valence-band offset at the Pr-silicate/Si(111) interface of 2.75 eV and a band gap of 6.50 eV were obtained. The large band gap and the highly symmetric band alignment account for the observe...

Growth and applications of GeSn-related group-IV semiconductor materials

Abstract We review the technology of Ge1−xSnx-related group-IV semiconductor materials for developing Si-based nanoelectronics. Ge1−xSnx-related materials provide novel engineering of the crystal growth, strain structure, and energy band alignment for realising various applications not only in electronics, but also in optoelectronics. We introduce our recent achievements in the crystal growth of Ge1−xSnx-related material thin films and the studies of the electronic properties of thin films, metals/Ge1−xSnx, and insulators/Ge1−xSnx interfaces. We also review recent studies related to the crystal growth, energy band engineering, and device applications of Ge1−xSnx-related materials, as well as the reported performances of electronic devices using Ge1−xSnx related materials.

Pulsed Laser Deposition and Analysis for Structural and Electrical Properties of HfO2–TiO2 Composite Films

We have developed growth processes for HfO2–TiO2 composite films using a pulsed-laser-deposition (PLD) method and investigated the film structure which is characteristic to the growth sequences. Structural and electrical properties of the films sensitively depend on the growth conditions. It is observed that crystallization of the film is suppressed by increasing the TiO2 content. In the case that HfO2/TiO2 composition ratio is set to be 50/50, the film is completely in the amorphous phase and, after annealing, the film texture consisting of large HfTiO4 crystalline grains is formed. The dielectric constant of HfO2–TiO2 composite films increases with the increase in the TiO2 content in the film when the content is less than 50 at%. A marked change in the leakage property of the film is observed when comparing two films with 10 and 20 at% TiO2. We examine an optimum condition for the growth of HfO2–TiO2 composite film with both high dielectric constant and low leakage current.

Structural and Electrical Characteristics of HfO2 Films Fabricated by Pulsed Laser Deposition.

We investigated the structural and electrical characteristics of HfO2 films deposited by a pulsed laser deposition (PLD) method. It was found that the formation of interlayers at the HfO2/Si interface depends strongly on the oxygen pressure rather than the deposition temperature during PLD. Furthermore, we determined the electrical properties which are closely related to the structure and orientation of grains in the polycrystalline HfO2 films. It was noted that randomly oriented HfO2 films have capacitance-voltage curves with no hysteresis and low leakage current.

High hole mobility tin-doped polycrystalline germanium layers formed on insulating substrates by low-temperature solid-phase crystallization

We investigated the effects of incorporation of 0%–2% tin (Sn) into amorphous germanium (Ge) on its crystallization behavior and electrical properties. Incorporation of only 0.2% Sn caused the polycrystallization temperature of Ge to lower from 450 to 430 °C, while a polycrystalline Ge1−xSnx layer with high crystallinity compared to that of polycrystalline Ge was formed by incorporation of 2% Sn. A polycrystalline Ge1−xSnx layer with a low Sn content of 2% annealed at 450 °C exhibited a Hall hole mobility as high as 130 cm2/V s at room temperature even though it possessed a small grain size of 20–30 nm. The Hall hole mobility of a poly-Ge1−xSnx layer with an Sn content of 2% was four times higher than that of a polycrystalline Ge layer and comparable to that of single-crystalline silicon.

Control of Crystal Structure and Ferroelectric Properties of Pb(ZrXTi1-X)O3 Films Formed by Pulsed Laser Deposition

Pb(ZrXTi1-X)O3 (PZT) thin films on Pt/SiO2/Si substrates formed by pulsed laser deposition (PLD) and subsequent rapid thermal annealing (RTA) have been investigated. X-ray diffraction (XRD) and atomic force microscopy showed that the crystalline structure and the crystallographic orientation of PZT films after RTA, sensitively depended on the microstructure of as-deposited films. The preferentially (100)-oriented perovskite PZT films forming a trigonal structure were obtained from the as-deposited films that had contained small grains having a pyrochlore structure. This type of perovskite structure was found to depend also on the oxygen pressure during PLD. Chemical composition analysis by X-ray photoelectron spectroscopy clearly revealed the characteristics of a Zr-rich composition which induced the trigonal phases in the films, which were consistent with the XRD results. The capacitors made from these films showed large remanent polarization.

Formation of high-quality oxide/Ge1−xSnx interface with high surface Sn content by controlling Sn migration

In this paper, we investigated how Sn migrated during annealing for Ge1−xSnx at its surface and in its interior, as well as the Ge oxide formation on Ge1−xSnx with controlling surface oxidation. After oxidation at 400 °C, X-ray photoelectron spectroscopy and X-ray diffraction measurements revealed Sn migration from inside the epitaxial Ge1−xSnx layer to its surface. Annealing was not the primary cause of significant Sn migration; rather, it was caused mostly by oxidation near the Ge1−xSnx surface. This process formed a Ge1−xSnx oxide with a very high Sn content of 30%, inducing a wide hysteresis loop in the capacitance–voltage characteristics of its corresponding MOS device. We also found that forming a thin GeO2 layer by using a deposition method that controls Ge surface oxidation produced low densities of interface states and slow states. From these results, we conclude that controlling Sn migration is critical to forming a high-quality Ge1−xSnx gate stack.

Liquid-Sn-driven lateral growth of poly-GeSn on insulator assisted by surface oxide layer

Effects of surface oxide layers on liquid-Sn-driven GeSn crystallization on insulators at various temperatures (<475 °C) are investigated. An amorphous Ge in neighborhood of patterned-Sn (<3.5 μm) becomes to polycrystalline Ge1−xSnx (x ≈ 0.025 − 0.14) after annealing at 150–475 °C, which is independent of the surface oxide thickness. Interestingly, a 50-μm-length lateral growth of polycrystalline Ge0.99Sn0.01 layers achieved by combination of thickening of the surface-oxide treated by NH4OH and annealing above melting temperature of Sn (231.9 °C). The growth length is 15 times longer than without the treatment. The advanced process promises to achieve group-IV-based optic and electronic devices on flexible substrates and Si platforms.

Thermal Stability and Electrical Properties of (La2O3)1-x(Al2O3)x Composite Films

(La2O3)1-x(Al2O3)x composite films (La2O3:Al2O3 = 100:0, 80:20, 67:33, 50:50) were synthesized using pulsed laser deposition. We investigated their thermal stability and electrical properties as high dielectric constant gate dielectric films. La2O3 (an Al2O3 content of 0%) films were uniformly crystallized as La-silicate after rapid thermal annealing (RTA) at 1000°C for 15 s in an N2 ambient. We found that the addition of Al2O3 suppresses crystallization, and amorphous structures are retained in the samples with an Al2O3 content of 33% or greater. Furthermore, Al2O3 addition suppresses diffusion of Si into upper composite films during RTA. The smallest capacitance equivalent oxide thickness of 1.2 nm was achieved for a sample with an Al2O3 content of 20% and with leakage current density of 3.9×10-3 A/cm2 at + 1 V relative to the flat band voltage.

Charge trapping and interface state generation in metal‐oxide‐semiconductor capacitors due to Fowler–Nordheim tunneling injection at low temperatures

Electrical phenomena in metal‐oxide‐semiconductor capacitors caused by Fowler–Nordheim tunneling injection of electrons have been studied at temperatures ranging from 77 to 300 K. It has been found that the shift of flatband voltages (ΔVFB) depends on the oxidation temperature and that low‐temperature oxidation is desirable for the reduction of ΔVFB . The dependence of ΔVFB on the injection temperature shows a characteristic feature: The ΔVFB reduces with decreasing injection temperature in the range above 180 K and, on the other hand, is almost independent of the injection temperature in the range below 180 K. These results are mainly attributable to the injection temperature dependence of interface state generation. The injection temperature dependence of interface state density clearly indicates that two generation mechanisms of interface states are present. In addition, we have found a power‐law dependence of the generated interface states on ΔVFB, independent of oxidation and injection conditions.