Yukio Yasuda

Reduction of threading dislocation density in SiGe layers on Si (001) using a two-step strain–relaxation procedure

A method to obtain high-quality strain–relaxed SiGe buffer layers on Si(001) substrates is presented. In this method, the strain relaxation of the SiGe layer is performed using a two-step procedure. Firstly, a low-temperature-grown SiGe layer, whose surface is covered by a thin Si cap layer, is thermally annealed. At this stage, the strain is incompletely relaxed and an atomically flat surface can be realized. Then, a second SiGe layer is grown on the first layer to achieve further strain relaxation. Transmission electron microscopy has clearly revealed that dislocations are dispersively introduced into the first SiGe/Si substrate interface and thus no pile up of dislocations occurs. The formation of a periodic undulation on the growth surface of the second SiGe layer is the key to inducing a drastic reduction in the threading dislocation density.

Electrical Properties and Solid-Phase Reactions in Ni/Si(100) Contacts.

The solid-phase reaction and electrical characteristic in Ni/Si contact systems have been investigated. We show that nickel monosilicide (NiSi) is an attractive candidate for contact materials in terms of its flat interface morphology, low sheet resistance, and low formation temperature. The contact resistivities on the order of 10-8 Ωcm2 are obtained in both n- and p-types contacts formed at 350°C.

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.

Atomistic evolution of Si1–x–yGexCy thin films on Si(001) surfaces

The initial growth process of Si1−x−yGexCy thin films on Si(001) surfaces is examined by scanning tunneling microscopy. The surface morphology of the film critically depends on the C fraction in the film. Evidence is presented on an atomic scale that the epitaxial growth of Si1−x−yGexCy films with large C fractions is dominated by phase separation between Si–C and Si–Ge, concomitant with C condensation on the surface of the growing films. We find that the addition of a thin (1–2 ML) SiGe interlayer between the Si1−x−yGexCy film and the Si substrate drastically improves the film structure, leading to a planar morphology even with large C fractions present in the film.

Contact resistivity between tungsten and impurity (P and B)-doped Si1−x−yGexCy epitaxial layer

Abstract Contact resistivity between tungsten and in situ impurity (P and B)-doped Si 1− x − y Ge x C y films with 0≤ x ≤0.7, 0≤ y ≤0.02 has been investigated. In the case of the P-doped Si 1− x − y Ge x C y films, the contact resistivity decreases with increasing the carrier concentration, independently of the Ge and C fraction. Because P atoms become electrically inactive with increasing the Ge and C fractions, lower Ge and C fractions are necessary to reduce the contact resistivity. By growing the multi-layer P-doped epitaxial Si with a high carrier concentration of 4×10 20 xa0cm −3 at a very low temperature of 450xa0°C on the P-doped Si 1− x − y Ge x C y film, very low contact resistivity of 6.5×10 −8 xa0Ωxa0cm 2 is achieved. On the other hand, in the case of the B-doped Si 1− x − y Ge x C y films, the contact resistivity decreases with increasing the Ge fraction and scarcely depends on C fraction at a specified carrier concentration, and is typically 25 and 57% lower for x =0.44 and 0.7 than that for Si, respectively. For the B-doped Si 0.56 Ge 0.44 films with a high carrier concentration of 6×10 20 xa0cm −3 , very low contact resistivity is obtained to be 3.8×10 −8 xa0Ωxa0cm 2 . These results demonstrate that low contact resistivity is caused by the lowering of schottky barrier height between metal and the B-doped film, due to the valence band shift.

Application of a Two-Step Growth to the Formation of Epitaxial CoSi2 Films on Si(001) Surfaces: Comparative Study using Reactive Deposition Epitaxy.

Epitaxial growth of 20-nm-thick CoSi2 films on Si(001) surfaces has been investigated for a two-step growth and reactive deposition epitaxy (RDE) at growth temperatures of 320–680°C using in-situ reflection high-energy electron diffraction and scanning tunneling microscopy, and ex-situ X-ray diffraction and atomic force microscopy. For the RDE, three-dimensional CoSi2 islands with a {115}-faceted structure grow along the directions and pinholes or channels with depths over ~20 nm are formed. However, for the two-step growth, no {115}-faceted islands exist and the depth of pinholes or channels decreases greatly. In the two-step growth, cobalt is solely deposited at an elevated substrate temperature on an epitaxial CoSi2(001) film formed by solid-phase epitaxy of 0.23-nm-thick Co as the first step. The first-step CoSi2 film has effects on restraining the Si diffusion from the substrate at the pinhole sites and promoting the layer-by-layer growth at the second step. The original two-step growth technique will be highly suitable for the realization of high-quality epitaxial CoSi2 contacts in the future ULSI technology.

Characterization of defect traps in SiO2 thin films influence of temperature on defects

Abstract In order to improve the electrical operation of very thin gate oxide metal-oxide-semiconductor (MOS) devices, it is necessary to understand generated defects in the non-stoichiometric SiO x area and at the Si–SiO 2 interface. For this purpose, we extended our new measurement technique, which is a temperature dependent method, to MOSFETs and Al-gate MOS devices to investigate slow-state traps and their relationship with fast-state traps (influence of technology, temperature and field dependence). With this method, activation energies for slow-state traps can be determined. Hydrogen species have been characterized. These defects are created when the MOS capacity is under a strong electric field that induces injection of electrons by tunnel effect through the silicon dioxide, SiO 2 .

Local Leakage Current of HfO2 Thin Films Characterized by Conducting Atomic Force Microscopy

l.Introduction Recenfly, many high-k materials replacing SiO, have been investigated to achieve low leakage current for next generation gate dielechic films in metal-oxidesemiconductor field-effect tansistors (MOSFETs) t1]. HfO, films are very athactive as a promising candidate for the gate dielectric materials because of its high dielectric constant (-25), good thermal stability in direct contact with Si, relatively large band gap, and reasonable barrier height for carrier iqiection into the oxide film [2]. We have studied the relationship between the electical and crystallographic properties of HfO, thin films and found that the leakage,current and the hysteresis in capacitance-voltage characteristics are sfiongly dependent upon local bonding structures such as grain boundaries and orientation of the grain [3]. In this work, we investigated the local leakage characteristics of Hm, thin films, using conducting atomic force microscopy (C-AFM) and discussed the relationship between the morphological sfucture and the local leakage current microscopically.

Growth Processes and Electrical Characteristics of Silicon Nitride Films Formed on Si(100) by Radical Nitrogen

We have investigated nitridation processes on clean Si(100)-2×1 surfaces and the electrical characteristics of metal-insulator-semiconductor (MIS) capacitors with nitride films formed by radical nitrogen. The radical nitrogen exposure to Si surfaces at 500°C initially causes both nitridation of Si and local detachment reactions on the surface. It is found from scanning tunneling microscopy and scanning tunneling spectroscopy that the nitridation proceeds in a layer-by-layer manner in which a continuous film is formed by the coalescence of two-dimensional islands. At nitride thicknesses above 0.6 nm, the nitride film is homogeneously formed over the entire surface. The MIS capacitor with a 5.4-nm-thick nitride film formed by radical nitrogen shows low hysteresis of less than 0.1 V and low leakage current density. These electrical properties indicate that the nitride films formed by radical nitrogen are applicable to gate insulators.

Characterization of Defect Traps in SiO2 Thin Films

In order to understand the degradation of the electrical operations of metal-oxide-semiconductor n(MOS) devices, this work is concerned by the defects generation processes in the non-stoichiometric SiOx, area and at the SiO2 interface. For this purpose, a new measurement technique to study slow-state traps and their relationship with fast-state traps is developed. This method considers capacitance-voltage nmeasurements and temperature effects during the hysteresis cycle.