Keisuke Arimoto

Fabrication of high-quality strain-relaxed thin SiGe layers on ion-implanted Si substrates

We fabricated high-quality strain-relaxed thin SiGe layers by Ar ion implantation into Si substrates before epitaxial growth. The surface of 100-nm-thick Si0.8Ge0.2 layers, the relaxation ratio of which was more than 80%, was found to be very smooth, with a rms roughness of 0.34 nm. Cross-sectional transmission electron microscopy analysis confirmed that strain-relieving dislocations were effectively generated due to the ion-implantation-induced defects and confined in the vicinity of the heterointerface, resulting in a dislocation-free SiGe surface. Moreover, in-plane strain-field fluctuation was found to be largely reduced by this ion implantation method.

Strain dependence of hole effective mass and scattering mechanism in strained Ge channel structures

Strain dependence of hole effective mass (m∗) in the strained Ge channel was systematically studied, and monotonic m∗ reduction by more than 20% was clearly observed when the strain increased from 0.8% up to 2.8%. The scattering mechanism, which strongly depended on the modulation-doping structure as well as strains, was also investigated based on the Dingle ratio evaluation, and the interface roughness scattering was found to be effectively suppressed by adopting the inverted structure even for the largely strained channels.

Low nickel germanide contact resistances by carrier activation enhancement techniques for germanium CMOS application

We fabricated and studied nickel germanide (NiGe) contacts on both n- and p-type germanium (Ge) substrates by applying the carrier activation enhancement (CAE) technique. We achieved a high electron concentration of 8.6 × 1019 cm−3 using a P/Sb co-implant and a record-high hole concentration of 8.4 × 1020 cm−3 using a Ge preamorphization implant and a boron implant. We used the circular transfer length method and two-dimensional DC simulation to determine the specific contact resistivity (ρc). Using the CAE technique, we obtained low ρc values of 6.4 × 10−7 Ω cm2 for the NiGe/n+-Ge contact and 4.0 × 10−8 Ω cm2 for the NiGe/p+-Ge contact. Theoretical calculation of ρc shows that, to achieve a ρc of 1 × 10−8 Ω cm2 as required by the International Technology Roadmap for Semiconductors for the year 2015, contacts on p+-Ge need contact process optimization, while contacts on n+-Ge need further CAE improvement and/or Schottky barrier height reduction.

Ion dose, energy, and species dependencies of strain relaxation of SiGe buffer layers fabricated by ion implantation technique

We systematically studied on ion dose, energy, and species dependencies of strain relaxation ratios for SiGe buffer layers fabricated by ion implantation technique where the epitaxial growth of SiGe layers was carried out on Si or Ar ion preimplanted Si substrates. For Si+ implantation, we found that there was an optimal ion-implantation condition to effectively enhance strain relaxation of the SiGe layers, that is, relaxation ratios increased with the ion dose but reduced remarkably when it exceeded a certain critical dose (∼1×1015 cm−2). The drop of relaxation also occurred as the implantation energy increased. Based on simulations and transmission electron microscopy (TEM) observations, it was concluded that end-of-range (EOR) defects generated by Si+ implantation crucially caused formation of high-density misfit dislocations at the heterointerface, and the observed complicated results were well understood in terms of the position of EOR defects from the heterointerface. We confirmed this conclusion by...

Elastic theory for strained heterostructures with in-plane anisotropy

An analytical formulation of elastic strain in epitaxially grown crystalline film was derived. The concept of anisotropic in-plane strain was included in the method, which is applicable to the investigation of strained films with an anisotropic defect configuration or fabrication-induced anisotropic stress. The developed method applies to arbitrary crystal classes with arbitrary surface orientations.

Acceptorlike Behavior of Defects in SiGe Alloys Grown by Molecular Beam Epitaxy

The Ge composition dependence of the densities and energy levels of the acceptorlike defect states that are generated during the solid-source molecular beam epitaxy (SSMBE) of SiGe films was investigated. Hall measurements in a wide temperature range were carried out, and a previously unreported very shallow acceptorlike state was found. We provide evidence that the observed acceptorlike states are relevant to intrinsic point defects.

Introduction of Uniaxial Strain into Si/Ge Heterostructures by Selective Ion Implantation

A selective-ion-implantation technique was developed for introducing uniaxial strain into Si/Ge heterostructures. Laterally selective ion implantation with a stripe pattern was carried out into a Si substrate, followed by SiGe overgrowth in the whole region. Large strain relaxation of SiGe occurred selectively only in the ion-implanted area. This largely relaxed SiGe was found to considerably affect the strain state of the neighboring strained SiGe in the unimplanted area, resulting in the realization of a highly asymmetric strain state, that is, uniaxial strain. This result indicates that this technique has a high potential to realize high-mobility Si/Ge channels with uniaxial strain.

Thickness Dependence of Strain Field Distribution in SiGe Relaxed Buffer Layers

The buffer thickness dependence of strain field distribution was investigated in SiGe heterostructures by micro-Raman spectroscopy. Crosshatch-like strain fluctuations were clearly observed in strained-Si and SiGe buffer layers, and the fluctuation wavelength was found to increase almost linearly with increasing buffer thickness. It was also found that SiGe homoepitaxial growth on planarized SiGe buffer layers gave rise to crosshatch roughness on the surface with almost the same morphology as the strain distribution, indicating that the strain fluctuation caused the roughness formation associated with growth kinetics. The strain fluctuation still remained on the buffer layer thicker than 7 µm, which should be taken into consideration for device applications.

Influence of Ge atoms on mobility and junction properties of thin-film transistors fabricated on solid-phase crystallized poly-SiGe

The transport properties of thin-film transistors (TFTs) fabricated on polycrystalline-SiGe films, which were formed by solid-phase crystallization of amorphous-Si1−xGex (x=0, 0.3, 0.5, and 0.7) films, were studied. The mobility of TFTs increases as the Ge concentration increases from 0% to 30% and the mobility decreases as the Ge concentration exceeds 30%, which agrees with the Ge concentration dependence of the grain size. The generation rates and the effective lifetimes of electron-hole pairs at the p-n junctions of TFTs increase and decrease monotonically with increasing Ge concentration, respectively, which means that the grain boundary defects in p-n junctions increase with increasing Ge concentration.

Effects of deposition rate on the structure and electron density of evaporated BaSi2 films

In order to control the electrical properties of an evaporated BaSi2 film, which is an emerging candidate for the absorber-layer material of earth-abundant thin-film solar cells, we have investigated the effects of deposition rate on the produced phases, microstructure, and carrier density of the thin films grown by thermal evaporation of BaSi2. X-ray diffraction results show that a high substrate temperature is necessary for BaSi2 formation at a high deposition rate, which is discussed from viewpoints of vapor composition and diffusion time. Microstructural characteristics such as grain size of 30–120 nm, oxide particle arrays present around the interface, and partial oxidation at a low substrate temperature are revealed by cross-sectional transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy combined with an energy-dispersive X-ray spectroscopy. With increasing deposition rate, the crystalline quality of BaSi2 is found to improve, as evidenced by a decrease...