Osamu Nakatsuka

High resolution-high energy x-ray photoelectron spectroscopy using third-generation synchrotron radiation source, and its application to Si-high k insulator systems

High-resolution x-ray photoelectron spectroscopy (XPS) at 6 keV photon energy has been realized utilizing high-flux-density x rays from the third generation high-energy synchrotron radiation facility, SPring-8. The method has been applied to analysis of high-k HfO2/interlayer/Si complementary metal–oxide–semiconductor gate-dielectric structures. With the high energy resolution and high throughput of our system, chemical-state differences were observed in the Si 1s, Hf 3d, and O 1s peaks for as-deposited and annealed samples. The results revealed that a SiOxNy interlayer is more effective in controlling the interface structure than SiO2. Our results show the wide applicability of high resolution XPS with hard x rays from a synchrotron source.

Growth of highly strain-relaxed Ge1−xSnx/virtual Ge by a Sn precipitation controlled compositionally step-graded method

We have investigated Sn precipitation and strain relaxation behaviors in the growth of Ge1−xSnx layers on virtual Ge substrates (v-Ge) for strain engineering of Ge. By varying misfit strain at Ge1−xSnx∕v-Ge and Ge1−ySny∕Ge1−xSnx interfaces, we found that a critical misfit strain controls the onset of Sn precipitation at a given thickness of the Ge1−xSnx layer. A compositionally step-graded method, in which the critical misfit strain is taken into account, was applied to the growth of strain-relaxed Ge1−xSnx layers on v-Ge. Postdeposition annealing at each growth step led to lateral propagation of threading dislocations preexisting in the layer and originating from v-Ge, which resulted in high degree of strain relaxation. An epitaxial Ge layer was grown on the strain-relaxed Ge1−xSnx layer and an in-plane tensile strain of 0.68% was achieved.

Ferromagnetism and electronic structures of nonstoichiometric Heusler-alloy Fe3-xMnxSi Epilayers grown on Ge(111).

For the study of ferromagnetic materials which are compatible with group-IV semiconductor spintronics, we demonstrate control of the ferromagnetic properties of Heusler-alloy Fe3-xMnxSi epitaxially grown on Ge(111) by tuning the Mn composition x. Interestingly, we obtain L2(1)-ordered structures even for nonstoichiometric atomic compositions. The Curie temperature of the epilayers with x approximately 0.6 exceeds 300 K. Theoretical calculations indicate that the electronic structures of the nonstoichiometric Fe3-xMnxSi alloys become half-metallic for 0.75 < or = x < or = 1.5. We discuss the possibility of room-temperature ferromagnetic Fe(3-x)Mn(x)Si/Ge epilayers with high spin polarization.

Growth and structure evaluation of strain-relaxed Ge1−xSnx buffer layers grown on various types of substrates

We have performed growth and structure evaluation of strain-relaxed Ge1−xSnx buffer layers grown on Si(0 0 1), virtual Ge(0 0 1) and bulk Ge(0 0 1) substrates. In the case of Si(0 0 1), amorphous Ge1−xSnx phases are partially formed as well as many threading dislocations in Ge0.98Sn0.02 layers. Employing virtual Ge substrates to reduce the lattice mismatch at the interface leads to epitaxial Ge0.978Sn0.022 layers with a flat surface. Most of threading dislocations in the Ge0.978Sn0.022 layer comes from pre-existing ones in the virtual Ge substrate and propagates laterally, leaving misfit segments at the Ge0.978Sn0.022/virtual Ge interface, after post-deposition annealing (PDA). This simultaneously results in the reduction of threading dislocation density and the promotion of strain relaxation. In the case of bulk Ge(0 0 1), although low threading dislocation density can be achieved, less than 106 cm−2, the film exhibits surface undulation and a lesser degree of strain relaxation even after PDA.

Mobility Behavior of Ge1-xSnx Layers Grown on Silicon-on-Insulator Substrates

We have investigated the behaviors of the carrier mobility and concentration of the undoped Ge1-xSnx layers epitaxially grown on silicon-on-insulator (SOI) substrates. Hall measurement revealed the conduction of holes excited from acceptor levels related to vacancy defects whose concentration was as high as 1018 cm-3 in Ge1-xSnx layers. The temperature dependences of the carrier mobility and concentration in the valence band was estimated by reducing the parallel conduction component in the impurity band. The incorporation of Sn at a content lower than 4.0% hardly degraded the hole mobility of heteroepitaxial Ge1-xSnx layers. In contrast, the mobility of the Ge1-xSnx layers was improved by reducing the carrier concentration of the Ge1-xSnx layers by Sn incorporation compared with that of the Ge layer formed under the same growth and annealing conditions. This result suggests that the incorporation of Sn into Ge leads to reducing the hole concentration of the electrically active vacancy defects due to the formation of Sn-vacancy pairs.

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.

Pure-edge dislocation network for strain-relaxed SiGe/Si(001) systems

We have grown strain-relaxed SiGe layers on Si(001) substrates with a pure-edge dislocation network buried at the heterointerface and analyzed dislocation morphology depending on growth conditions. The process employed here consists of pure-Ge film growth on Si(001) and subsequent high temperature annealing for solid-phase intermixing of the Ge film and Si deposited on the top to form a SiGe alloy layer. Transmission electron microscopy revealed morphological changes of shorter pure-edge dislocation segments initially formed at the Ge∕Si interface into a network structure consisting of longer and regularly spaced dislocations during post-deposition annealing. The dislocation network was explicitly preserved even after the intermixing of Si and Ge and predominantly contributed to in-plane strain relaxation of the SiGe layer. Applicability of the pure-edge dislocation network to strain-relaxed SiGe buffer layers on Si(001) substrates is discussed.

Molecular beam deposition of Al2O3 on p-Ge(001)/Ge0.95Sn0.05 heterostructure and impact of a Ge-cap interfacial layer

We investigated the molecular beam deposition of Al2O3 on Ge0.95Sn0.05 surface with and without an ultra thin Ge cap layer in between. We first studied the atomic configuration of both Ge1−xSnx and Ge/Ge1−xSnx surfaces after deoxidation by reflection high-energy electron diffraction and resulted, respectively, in a c(4×2) and (2×1) surface reconstructions. After in situ deposition of an Al2O3 high-κ gate dielectric we evidenced using time-of-flight secondary ion mass spectroscopy analyses that Sn diffusion was at the origin of high leakage current densities in the Ge1−xSnx/Al2O3 gate stack. This damage could be avoided by inserting a thin 5-nm-thick Ge cap between the oxide and the Ge1−xSnx layer. Finally, metal-oxide-semiconductor capacitors on the Ge capped sample showed well-behaved capacitance-voltage (C-V) characteristics with interface trap density (Dit) in the range of 1012 eV−1 cm−2 in mid gap and higher close to the valence band edge.

Low-Temperature Formation of Epitaxial NiSi2 Layers with Solid-Phase Reaction in Ni/Ti/Si(001) Systems

The solid-phase epitaxial growth of NiSi2 in Ni/Ti/Si systems has been investigated. Continuous epitaxial NiSi2 layers consisting of pyramidal domains with {111} facets at the Ni-silicide/Si interface can be formed by annealing at a temperature (350°C) lower than that for conventional Ni/Si systems. This NiSi2 layer is transformed to a uniform epitaxial NiSi2 layer with an atomically flat silicide/Si interface by additional annealing at 850°C, while the {111} facets at the NiSi2/Si interface remains in the Ni/Si system under the same annealing conditions. Moreover, the epitaxial NiSi2 layer formed at 350°C exhibits a high thermal robustness even after annealing at higher than 750°C, in contrast to the polycrystalline NiSi layer.

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.