Yosuke Shimura

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.

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 1012u2002eV−1u2009cm−2 in mid gap and higher close to the valence band edge.

Crystallinity Improvement of Epitaxial Ge Grown on a Ge(110) Substrate by Incorporation of Sn

We have investigated the growth behavior of Ge and Ge1-xSnx epitaxial layers on Ge(110) substrates. Many stacking faults are induced in the homoepitaxial Ge layer because of the anisotropic surface reconstruction structure of Ge(110). In contrast, we found that the crystallinity of epitaxial Ge1-xSnx layers can be improved by the incorporation of Sn atoms. This is thought to be due to the change in the reconstruction structure of the Ge(110) surface with Sn. As a result, the growth of a pseudomorphic Ge0.952Sn0.048 layer without any stacking faults has been achieved.

Sn diffusion during Ni germanide growth on Ge1-xSnx

We report on the redistribution of Sn during Ni germanide formation on Ge1–xSnx/〈Ge(100)〉 and its influence on the thin film growth and properties. These results show that the reaction involves the formation of Ni5Ge3 and NiGe. Sn redistributes homogenously in both phases, in which the Sn/Ge ratio retains the ratio of the as-deposited Ge1–xSnx film. Sn continues to diffuse after full NiGe formation and segregates in two regions: (1) at the interface between the germanide and Ge1–xSnx and (2) at the surface, which has major implications for the thin film and contact properties.

Effect of in situ Sb doping on crystalline and electrical characteristics of n-type Ge1− x Sn x epitaxial layer

We examined the molecular beam epitaxy of Ge1− x Sn x with in situ Sb doping on Ge substrates. The effects of Sb doping on the crystalline and electrical characteristics of Ge1− x Sn x epitaxial layer were investigated in detail. We found that Sb doping with a concentration of 1020 cm−3 remarkably improves the crystallinity, and surface uniformity of the Ge1− x Sn x epitaxial layer by changing the growth mode by the surfactant effect of Sb atoms. Low-temperature Ge1− x Sn x growth with in situ Sb doping realizes a very high electron concentration of 1020 cm−3, which is above the thermal equilibrium solid solubility, as a result of suppressing Sb segregation and precipitation.

Solid-phase crystallization of Si1− x − y Sn x C y ternary alloy layers and characterization of their crystalline and optical properties

The solid phase crystallization of Si1− x − y Sn x C y ternary alloy layers on an insulator has been examined. Amorphous Si1− x − y Sn x C y layers with a Sn content of 0–20% and a C content of 0–10% were deposited on quartz substrates using a radio-frequency magnetron sputtering method and annealed at temperatures from 400 to 800 °C to induce the solid-phase crystallization. The crystalline properties of the Si1− x − y Sn x C y layers and the influences of Sn and C introduction on their crystalline structures were investigated. It was found that Sn introduction effectively reduces the crystallization temperature of a Si1− x − y Sn x C y layer to 400 °C, while a Si1− y C y binary alloy layer is hardly crystallized even at 800 °C. In addition, X-ray photoelectron spectroscopy measurement revealed that the Sn introduction effectively enhances the introduction of C atoms into substitutional sites in the ternary alloys. The substitutional C content in a polycrystalline Si1− x − y Sn x C y layer was estimated to be as high as 7.2%, which exceeds the thermal equilibrium solid solubility of C in a Si matrix. The absorption spectra of Si1− x − y Sn x C y ternary alloys were also investigated.

Strained Ge and Ge1-xSnx Technology for Future CMOS Devices

We have investigated the growth and crystalline structures of Ge1-xSnx buffer and tensile-strained Ge layers for future use in CMOS technology. We have demonstrated that strain relaxed Ge1-xSnx layers with an Sn content of 12.3% and 9.2% can be grown on Ge and Si substrates, respectively. We achieved a tensile-strain value of 0.71 % in Ge layers on a Ge0.932Sn0.068 buffer layer. We have also investigated the effects of Sn incorporation into Ge on the electrical properties of Ge1-xSnx heteroepitaxial layers.

Growth and applications of GeSn-related group-IV semiconductor materials

We have developed the epitaxial growth technology of Ge<inf>1−x</inf>Sn<inf>x</inf> and related group-IV materials. The crystalline properties and energy band structure have been investigated for integrating group-IV semiconductors into Si ULSI platform.

Epitaxial Growth and Anisotropic Strain Relaxation of Ge1-xSnx Layers on Ge(110) Substrates

We have achieved the heteroepitaxial growth of a pseudomorphic Ge0.954Sn0.046 layer without Sn precipitation on a Ge(110) substrate. The strain in the Ge_1-xSn_x layer preferentially relaxed along [001] direction with annealing over 500°C due to the anisotropic layout of glide planes on Ge(110) surface. These results suggest that an uniaxially compressive strained Ge_1-xSn_x structure can be realized with the anisotropic strain relaxation.

Optical Properties of Ge1-xSnx Epitaxial Layers with Very High Sn Contents

This paper measured the direct band gap of Ge_1-xSn_x epitaxial layers with a very high Sn content over 15%, and showed the Sn content dependence of the bowing parameter for the direct gap. The bowing parameter for Ge_1-xSn_x epitaxial layers were determined to be b = -4.77 x + 2.47 (eV).