Takashi Yamaha

Near-infrared light absorption by polycrystalline SiSn alloys grown on insulating layers

High-Sn-content SiSn alloys are strongly desired for the next-generation near-infrared optoelectronics. A polycrystalline growth study has been conducted on amorphous SiSn layers with a Sn-content of 2%–30% deposited on either a substrate of SiO2 or SiN. Incorporating 30% Sn into Si permits the crystallization of the amorphous layers at annealing temperatures below the melting point of Sn (231.9 °C). Composition analyses indicate that approximately 20% of the Sn atoms are substituted into the Si lattice after solid-phase crystallization at 150–220 °C for 5 h. Correspondingly, the optical absorption edge is red-shifted from 1.12 eV (Si) to 0.83 eV (Si1−xSnx (x ≈ 0.18 ± 0.04)), and the difference between the indirect and direct band gap is significantly reduced from 3.1 eV (Si) to 0.22 eV (Si1−xSnx (x ≈ 0.18 ± 0.04)). These results suggest that with higher substitutional Sn content the SiSn alloys could become a direct band-gap material, which would provide benefits for Si photonics.

Non-uniform depth distributions of Sn concentration induced by Sn migration and desorption during GeSnSi layer formation

The distributions of Sn concentration in GeSnSi layers formed on Ge substrate at various temperatures were investigated. High deposition temperature (Td) induces significant Sn migration and desorption, which have activation energies of 0.75 eV and 0.27 eV, respectively. A model quantitatively clarified the Sn migration fluxes during the deposition, which increase not only with increasing Td but also with the layer thickness. A non-negligible Sn flux compared with the supplied flux was found at 350 °C at the surface of the 200-nm-thick layer. Consequently, designs of layer thickness and Td taking into account the appropriate Sn flux are important to form a GeSnSi layer with uniform Sn content for future optoelectronics.

Formation, crystalline structure, and optical properties of Ge1−x−ySnxCy ternary alloy layers

We have investigated the crystalline and optical properties of epitaxial layers of the ternary alloy Ge1−x−ySnxCy grown on a Si substrate. We achieved the formation of epitaxial Ge1−x−ySnxCy layers with a C content as high as 2% even with a high C incorporation efficiency. X-ray photoemission spectra and Raman scattering spectroscopy measurements revealed that C atoms preferentially bond with Sn atoms in the Ge matrix, which is considered to enhance C introduction into substitutional sites in Ge with local strain compensation. We also demonstrated the control of the energy bandgaps of epitaxial Ge1−x−ySnxCy layers by controlling Sn and C contents.

Experimental observation of type-I energy band alignment in lattice-matched Ge1−x−ySixSny/Ge heterostructures

We experimentally demonstrated the formation of type-I energy band alignment in lattice-matched Ge1−x−ySixSny/Ge(001) heterostructures and clarified the dependence of Si and Sn contents on the energy band structure. By controlling the Si and Sn contents, keeping the Si:Sn ratio of 3.7:1.0, we formed high-quality Ge1−x−ySixSny pseudomorphic epitaxial layers on a Ge substrate with the lattice misfit as small as 0.05%. The energy bandgaps of the Ge1−x−ySixSny layers, measured by spectroscopic ellipsometry, increased to 1.15 eV at Si and Sn contents of 41% and 15%, respectively. X-ray photoelectron spectroscopy indicated that the top of the valence band of Ge1−x−ySixSny was lower than that of Ge. Additionally, the energy band offsets between Ge1−x−ySixSny and Ge at both the conduction and valence band edges were estimated to be larger than 0.1 eV with an Sn content of more than 8%. These results promise that heterostructures of group-IV semiconductors using Si, Ge, and Sn can have type-I energy band alignment...

Growth and crystalline properties of Ge 1−x−y Sn x C y ternary alloy thin films on Ge(001) substrate

We achieved the worlds first epitaxial growth of a Ge_1-x-ySn_xC_y layer, and investigated the effect of Sn incorporation on the growth of Ge_1-xC_x. Sn incorporation can decrease the epitaxial temperature of Ge_1-xC_x layer. Also, Sn incorporation can make C atoms stable at the substitutional site. This Ge_1-x-ySn_xC_y layer is expected to realize the energy band engineering independently on the lattice parameter and promises to extend the potential of group-IV semiconductor materials for nanoelectronics and optoelectronic applications.

Low temperature growth of SiSn polycrystals with high Sn contents on insulating layers

Comparative study between the poly-Si and the poly-SiSn layers reveals that the Sn incorporation is effective to decrease the crystallization temperature. Also, we found that the formation of SiSn polycrystal with a substitutional Sn content of around 30%, which is expected as a direct transition semiconductor, becomes possible on SiO2 at a low temperature growth of 150°C.

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.

Growth of Si1−x−ySnxCy ternary alloy layer on Si(001) substrate and characterization of its crystalline properties

We have demonstrated the crystal growth of Si1−x−ySnxCy ternary alloy layers on Si(001) substrates by radio-frequency magnetron sputtering. We have investigated the crystalline properties of the ternary alloy layers and clarified the influence of Sn and C on the crystalline structure of Si1−x−ySnxCy layers. We found that the epitaxial growth temperature of Si1−x−ySnxCy decreased and the island growth was suppressed by the introduction of Sn. The Sn precipitation was also effectively suppressed by C introduction. The substitutional C content of Si1−x−ySnxCy was estimated to be 2.7% and exceeded the thermal equilibrium solid solubility of C into the Si matrix.

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 crystalline properties of Ge 1−x−y Si x Sn y layers on Ge(001) substrates

We investigated the crystalline structure of Ge_1-x-ySi_xSn_y layers epitaxially grown on Ge(001) substrates. The unstrained and compressive strained Ge_1-x-ySi_xSn_y layers with very flat surface and high crystallinity can be grown. We found that the control of the strain direction is important to form a high quality Ge_1-x-ySi_xSn_y layer even with small misfit strain.