Hiroya Ikeda

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.

Oxidation of H‐terminated Si(100) surfaces studied by high‐resolution electron energy loss spectroscopy

The adsorption of oxygen on H‐terminated Si(100) surfaces has been investigated by high‐resolution electron energy loss spectroscopy (HREELS). Adsorptions of atomic oxygen occur even at room temperature. Si‐OH stretching and Si‐O‐Si (B1) vibrational modes are observed in HREELS spectra, which indicates that atomic oxygen is adsorbed on sites of Si—H bonds and Si—Si back bonds. On the other hand, H‐terminated surfaces are very stable for molecular oxygen, which cannot adsorb until 380u2009°C on the surface. A dissociative adsorption of molecular oxygen is observed above 380u2009°C and the activation energy of the adsorption is 2.0 eV at 380–450u2009°C. This value coincides with the desorption energy of hydrogen atoms from a Si(100) surface with the monohydride phase. These results indicate that the dangling bonds are essential to the adsorption of molecular oxygen on Si(100) surfaces.

Surfactant effect of H atoms on the suppression of Ge segregation in Si overgrowth on Ge(n ML)/Si(100) substrates by gas source molecular beam epitaxy

Hydrogen surfactant effect on the surface segregation of Ge atoms has been studied from layer-by-layer changes in the growth rate of Si overlayers on Ge/Si(100) substrates with Ge thicknesses of 0-6 ML by gas source molecular beam epitaxy using Si 2 H 6 . The layer-by-layer growth rates were in-situ measured by reflection high-energy electron diffraction (RHEED) intensity oscillation periods. The growth rate of Si overlayers decreases exponentially with increasing Si thickness and eventually approaches that of Si/Si(100) homoepitaxial growth at substrate temperatures of 440-550°C, which corresponds to the change in surface coverages of Ge atoms. The decay length of the surface concentration of Ge atoms is suppressed by an increase in Si 2 H 6 gas pressure. The temperature dependence of the decay length has an activation energy of 1.3 eV for Si 2 H 6 gas pressures of 5×10 -6 to 5×10 -5 Torr. These facts can be explained as an effect of H atoms existing on the surface

Initial oxidation of H-terminated Si(111) surfaces studied by HREELS

Abstract We have investigated the initial oxidation process and the local bonding structure of Siue5f8Oue5f8Si bonds of H-terminated Si(111)-1 × 1 surfaces using high-resolution electron energy loss spectroscopy (HREELS) below an oxygen coverage of 2.5 ML. Oxygen atoms randomly adsorb on the sites between surface and subsurface Si atoms at room temperature in this oxidation coverage. The vibrational energy of the Siue5f8Oue5f8Si asymmetric stretching mode increases monotonously with increasing the number of adsorbed O atoms in contrast with the case of Si(100)-(1 × 1)H. The relaxation of Siue5f8Oue5f8Si structures is promoted by the existence of Siue5f8H bonds.

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.

Solid-phase epitaxial growth of CoSi2 on clean and oxygen-adsorbed Si(001) surfaces

Abstract Growth behavior of epitaxial CoSi 2 (001) films on clean Si(001) surfaces and oxidized Si(001) surfaces with oxygen coverages below 2xa0ML has been investigated by scanning tunneling microscopy. In the solid-phase epitaxy of 3xa0ML-thick Co on clean Si surfaces at 530°C, rectangular-shaped CoSi 2 (001) three-dimensional (3D) islands elongated along the 〈110〉 direction were grown on thermally stable two-dimensional layers of CoSi 2 (001). This fact suggests that the CoSi 2 /Si(001) epitaxial growth obeys a Stranski–Krastanov-like mode. The shape of 3D islands is considered to be explained by energetics to reduce the surface energy of CoSi 2 . Moreover, it is found that oxygen atoms act as a kind of surfactant to lower the surface energy of CoSi 2 . Almost pinhole-free and atomically-flat epitaxial CoSi 2 (001) films with a thickness of 6xa0ML can be realized by solid-phase epitaxy under the condition of an annealing temperature of 470°C and oxygen coverages of 1–2xa0ML.

Influences of hydrogen on initial oxidation processes of H-terminated Si(100) surfaces

Abstract The initial oxidation of H-terminated Si(100) surfaces by atomic oxygen at room temperature has been investigated by using high-resolution electron energy loss spectroscopy (HREELS). It has been found on H-terminated Si(100) surfaces with a 1 × 1 (dihydride) structure that there are two oxidation steps in the range of oxygen coverage, θ, less than unity. The adsorption of oxygen atoms on the Siue5f8Si back bond with a constant vibrational energy of about 128 meV for the Siue5f8Oue5f8Si asymmetric stretching mode occurs in the first step of θ θ > 0.4. Moreover, in the case of oxidation of clean surfaces, the Siue5f8Oue5f8Si vibrational energy is monotonously increased with increasing oxygen coverage and is identical with that on H-terminated surfaces at θ > 0.4. It is considered that the structural relaxation of Siue5f8Oue5f8Si bonds is promoted by a charge transfer from back bonds to surface Siue5f8H bonds.

Electrical properties of metal/Si1−xGex/Si(100) heterojunctions

Abstract The electrical properties of Zr/ and Ti/Si 1− x Ge x /(100)Si heterostructures have been examined. The Schottky barrier heights (SBHs) of as-deposited diodes determined fromI-V characteristics are 0.53 and 0.55 eV for Zn/ and Ti/n-Si 0.8 Ge 0.2 , respectively. It can be seen that p-Si 0.8 Ge 0.2 diodes tend to have smaller SBHs than n-Si 0.8 Ge 0.2 ones for both metals. This fact is consistent with the contact resistivity measured for Zr/n + - and Zr/p + -Si 0.8 Ge 0.2 /(100)Si.

Nucleation and growth of Ge on Si(111) in solid phase epitaxy

The nucleation and growth of Ge on Si(111) surfaces using solid phase epitaxy (SPE) have been investigated by scanning tunneling microscopy (STM). In order to clarify the growth mechanism of two-dimensional (2D) and three-dimensional (3D) Ge islands in SPE, 0.25-bilayer (BL)-thick and 3-BL-thick Ge films on Si surfaces were grown. In the case of the 2D-islanding process, initial amorphous Ge clusters formed into 2D-islands on Si surface. We observed that disordered structures on the island surface and the Si surface transformed into well-ordered surface reconstructed structures on both surfaces as annealing time increased. From these results and the dependence of the 2D-island density on the annealing temperature, we found that growth process of 2D-islands in SPE is similar to that in conventional molecular beam epitaxy (MBE). On the other hand, in case of the 3D-islanding process, crystalline 3D-islands and islands with irregular shapes were initially formed. The 3D-islands preferentially grow with the expense of the irregular island, which corresponds to the ripening process. Considering the results of STM observation, we discuss the origin of the activation energy which was obtained by annealing temperature dependence of the 3D-island density.

Interfacial reactions and electrical characteristics in Ti/SiGe/Si(100) contact systems

Abstract Schottky barrier heights (SBHs), contact resistivities and solid-phase reactions at the interface of Ti/Si0.8Ge0.2/Si(100) systems have been investigated. At annealing temperatures below 300°C, Ti atoms react with Si atoms preferentially. Germanium atoms start to react above 400°C and the formation of C54-Ti(Si0.88Ge0.12)2 is confirmed by X-ray diffraction after annealing at 650°C. The annealing behavior of SBHs suggests that Ge atoms are segregated in SiGe layers at the interface, which is consistent with the results on interfacial reactions. For both n- and p-SiGe, the lower SBHs than Ti/Si(100) are obtained at 650°C, which is considered to be due to the work function of reaction products such as C54-Ti(Si1−yGey)2. The contact resistivities smaller than those expected from the SBHs are obtained for n+- and p+-SiGe at 580°C.