Takanori Asano

Formation of high-quality oxide/Ge1−xSnx interface with high surface Sn content by controlling Sn migration

In this paper, we investigated how Sn migrated during annealing for Ge1−xSnx at its surface and in its interior, as well as the Ge oxide formation on Ge1−xSnx with controlling surface oxidation. After oxidation at 400 °C, X-ray photoelectron spectroscopy and X-ray diffraction measurements revealed Sn migration from inside the epitaxial Ge1−xSnx layer to its surface. Annealing was not the primary cause of significant Sn migration; rather, it was caused mostly by oxidation near the Ge1−xSnx surface. This process formed a Ge1−xSnx oxide with a very high Sn content of 30%, inducing a wide hysteresis loop in the capacitance–voltage characteristics of its corresponding MOS device. We also found that forming a thin GeO2 layer by using a deposition method that controls Ge surface oxidation produced low densities of interface states and slow states. From these results, we conclude that controlling Sn migration is critical to forming a high-quality Ge1−xSnx gate stack.

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.

Effect of Sn atoms on incorporation of vacancies in epitaxial Ge1−xSnx film grown at low temperature

The anomalous increase and decrease in the S-parameters of Doppler broadening spectroscopy in positron annihilation spectroscopy in a narrow range of Sn atom content were detected in a Ge1−xSnx thin film grown by MBE at low temperatures. The increase can be explained in terms of vacancies when the target content of 1.7% Sn atoms is incorporated in a Ge matrix, owing to the binding nature between them. However, the S-parameters were markedly decreased when the target content of Sn atoms in the film grown at the same temperature was 0.1%. These changes in the S-parameters correspond to the carrier concentrations obtained by Hall measurements.

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.

Robustness of Sn precipitation during thermal oxidation of Ge1−xSnx on Ge(001)

The thermal robustness of Sn segregation and precipitation in epitaxial Ge1−xSnx layers on Ge(001) substrates with a Sn content greater than the equilibrium solubility limit has been investigated for applications of Ge1−xSnx in high-performance metal–oxide–semiconductor field-effect transistors (MOSFETs). Sn segregation and precipitation occur on the Ge1−xSnx surface after epitaxial growth of the Ge1−xSnx layer at 150 °C. After the thermal oxidation of the Ge1−xSnx layer below 500 °C, there are no significant decreases in the average Sn content in the Ge1−xSnx layer and no additional Sn segregation on the Ge1−xSnx surface. However, Sn precipitation occurs at the Ge1−xSnx surface during the thermal oxidation of the Ge1−xSnx layer with an average Sn content as high as 8.7% at 600 °C, causing a decrease in the Sn content in the Ge1−xSnx layer. The Sn content in the Ge1−xSnx oxide is 1.5 times greater than that observed near the Ge1−xSnx surface for the sample with a Sn content of 8.7% after the thermal oxidation at 400 to 500 °C. The capacitance–voltage characteristics of the Al/Al2O3/Ge1−xSnx/Ge MOS capacitors treated with thermal oxidation at 400 °C indicate that the slow state density increases with the Sn content. Meanwhile, the small interface state density could be achieved via thermal oxidation of the Ge1−xSnx layer, even with a high Sn content.

Impact of hydrogen surfactant on crystallinity of Ge1−xSnx epitaxial layers

The effect of a hydrogen surfactant on the crystallinity of a Ge1−xSnx epitaxial layer was investigated. The improvement of crystallinity on the in-plane uniformity of Ge1−xSnx epitaxial layer was observed by X-ray diffuse scattering and transmission electron microscopy. We also observed the decrease in the surface roughness of the Ge1−xSnx epitaxial layer. This indicates the suppression of the three-dimensional growth mode of Ge1−xSnx epitaxial layer due to a compressive strain. In addition, we observed the reduction in acceptor-like defect density in an undoped-Ge1−xSnx epitaxial layer from the capacitance–voltage characteristics of a metal–oxide–semiconductor capacitor. Consequently, introducing hydrogen during the growth leads to the improvement of the crystalline quality of the Ge1−xSnx epitaxial layer.

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...

Formation of high-quality Ge1?xSnx layer on Ge(110) substrate with strain-induced confinement of stacking faults at Ge1?xSnx/Ge interfaces

The formation of a high-quality Ge1−xSnx layer has been examined on the basis of the understanding of the relationship between the stacking fault and the misfit strain between the Ge1−xSnx layer and the Ge substrate. We found that the crystallinity of the Ge1−xSnx layer is improved with increasing Sn content despite the increase in the lattice mismatch. This is caused by the shortening of the distance between the two dissociated partial dislocations, which indicates that the confinement of the stacking fault occurs at the Ge1−xSnx/Ge interface with increasing misfit strain.

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.

Hydrogen-surfactant-mediated epitaxy of Ge1− x Sn x layer and its effects on crystalline quality and photoluminescence property

The effect of hydrogen-surfactant-mediated molecular beam epitaxy (MBE) growth of Ge1− x Sn x layer on Ge(001) substrate on crystalline quality and photoluminescence (PL) property has been investigated. The effect of irradiation of atomic hydrogen (H) generated by dissociating molecular hydrogen (H2) were examined during the MBE growth. H irradiation significantly improves the surface morphology with the enhancement of the two-dimensional growth of the Ge1− x Sn x epitaxial layer. Enhanced diffuse scattering is observed in the X-ray diffraction profile, indicating a high density of point defects. In the PL spectrum of the H2-irradiated Ge1− x Sn x layer, two components are observed, suggesting the radiative recombination with both indirect and direct transitions, while one component related to the direct transition is observable in the H-irradiated sample. The postdeposition annealing in nitrogen ambient at as low as 220 °C decreases the PL intensity of the H-irradiated Ge1− x Sn x layer, although the intensity is recovered after annealing at 300 °C, suggesting the annihilation of point defects in the Ge1− x Sn x layer.