Akira Toriumi

Kinetic pathway of the ferroelectric phase formation in doped HfO2 films

The dopant-induced ferroelectric HfO2 formation has been systematically investigated by using cation (Sc, Y, Nb, Al, Si, Ge, and Zr) and anion (N) dopants. Both differences and similarities are discussed among various dopants by focusing on two major factors, the oxygen vacancy (Vo) and the dopant ionic size. First, the doping concentration dependence of the remanent polarization in 27 (±2) nm HfO2 films is quantitatively estimated. Then, by comparing the polarization result with the structural transformation in doped HfO2, the pathway of the dopant-induced HfO2 phase transition is discussed among monoclinic, ferroelectric orthorhombic, tetragonal, and cubic phases. Finally, it is addressed that a dopant species independent phase transition route may exist in HfO2 owing to the same kinetic transition process, in which the ferroelectric phase seems to be at an intermediate state between tetragonal and monoclinic phases.

Thermal oxidation kinetics of germanium

Thermal oxidation kinetics of Ge was investigated by the 18O tracing study and re-oxidation experiments of the SiO2/GeO2 stacked oxide-layer. The results suggest that Ge oxidation kinetics is completely different from that expected from the Deal-Grove model and that Ge is oxidized by GeO2 on Ge instead of O2 at the interface. This oxidation process forms large amounts of oxygen vacancies in GeO2, which facilitate the diffusion of oxygen atoms in GeO2. This means that oxygen atoms diffuse through GeO2 with an exchange type of process. Based on experimental results, a possible kinetics for Ge oxidation is discussed.

Experimental detection of active defects in few layers MoS2 through random telegraphic signals analysis observed in its FET characteristics

Transition-metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), are expected to be promising for next generation device applications. The existence of sulfur vacancies formed in MoS2, however, will potentially make devices unstable and problematic. Random telegraphic signals (RTSs) have often been studied in small area Si metal-oxide-semiconductor field-effect transistors (MOSFETs) to identify the carrier capture and emission processes at defects. In this paper, we have systemically analyzed RTSs observed in atomically thin layer MoS2 FETs. Several types of RTSs have been analyzed. One is the simple on/off type of telegraphic signals, the second is multilevel telegraphic signals with a superposition of the simple signals, and the third is multilevel telegraphic signals that are correlated with each other. The last one is discussed from the viewpoint of the defect–defect interaction in MoS2 FETs with a weak screening in atomically confined two-dimensional electron-gas systems. Furthermore, the position of defects causing RTSs has also been investigated by preparing MoS2 FETs with multi-probes. The electron beam was locally irradiated to intentionally generate defects in the MoS2 channel. It is clearly demonstrated that the MoS2 channel is one of the RTS origins. RTS analysis enables us to analyze the defect dynamics of TMD devices.

Identifying the Collective Length in VO2 Metal–Insulator Transitions

The collective length in VO2 metal-insulator transitions is identified by controlling nanoscale dopant distribution in thin films. The crossover from the local transition to the collective transition is observed, which originates from the increased instability of the metal-insulator domain boundary. This instability renders the transition collective within the collective length, which will enable the design of collective electronic devices.

Local, isotropic, and damageless doping to oxide semiconductors by using electrochemistry

Electrochemical doping, which introduces dopant impurities via isotropic electrochemical reaction, was proposed and demonstrated in an archetypical oxide semiconductor TiO2. By controlling electrochemical potential of TiO2 in electrolyte, the technique successfully introduced dopants to increase the conductivity of TiO2 by ∼5 orders. The technique was also applied through micro-patterned photo resist, verifying nano-scale compatibility. The dopants, which are hydrogen or oxygen vacancy in this case, were stabilized with an activation energy of 1.2 eV, making this technique attractive for oxide devices in displays, sensors, solar cells, and neuromorphic circuits.

Study of SiGe oxidation kinetics for preferential SiO2 formation under a low O2 pressure condition

We have studied the oxidation kinetics of SiGe as parameters of O2 pressure and temperature. This paper first discusses the SiGe oxidation experimentally and thermodynamically. It was found that Si was predominantly oxidized in the Si0.5Ge0.5 oxidation under lower O2 pressures. This fact is thermodynamically reasonable, but the Ge remaining after Si oxidation may be a big concern in terms of SiGe gate stacks, because it should form defects at the interface or inside the SiO2 film. Therefore, it is critically important to understand how the Ge atoms behave after the SiO2 formation. Second, the GeO2/Si reaction, which might be a key part to well controlled SiGe gate stacks in the preferential SiO2 formation, is discussed. Two kinds of metallic Ge formation kinetics at the SiGe interface in the annealing of GeO2/Si are conjectured: One is the metallic Ge diffusion into the Si substrate and the other is the Ge precipitation at the interface, which should be avoided for improving the SiGe interface properties. The experimental results indicate that the former case is made possible by annealing under the low O2 pressure condition in a very thin SiO2 formation region.

n + Si/pGe Heterojunctions Fabricated by Low Temperature Ribbon Bonding With Passivating Interlayer

A bonding technique via passivating interlayer formation is proposed for bulk material heterojunction fabrication. n+Si/pGe heterojunctions were fabricated by a ribbon bonding with interfaces passivated by an amorphous interlayer. With a highest process temperature as low as 150 °C, the bonded junctions exhibited rectifying characteristics with a turn-on voltage of 0.3 V as an ideal Si/Ge heterojunction and an ideality factor of 2.15. This technique shows a great potential for bulk material heterojunction formation, especially when ultimately abrupt junctions and low temperature processes are needed.

Thermodynamic understanding and analytical modeling of interfacial SiO2 scavenging in HfO2 gate stacks on Si, SiGe, and SiC

This work thermodynamically and experimentally generalizes the interfacial SiO2 scavenging in HfO2 gate stacks from on Si to on other channel materials including SiGe and SiC and proposes a generalized formulation for this process. By paying attention to the Si chemical potential in the SiO2 interfacial layer (SiO2-IL) significantly affected by the substrate, it clarifies that Si in the substrate is indispensable to trigger the scavenging process. Thanks to this understanding, we demonstrate that the scavenging is extendable to next generation of channel materials containing Si such as SiGe and SiC with well-controlled high-k gate stacks. In addition, via formulating the diffusion-reaction-diffusion kinetics, an analytical relation like the Deal-Grove model is obtained for SiO2-IL scavenging in high-k gate stacks.