No abstract

In this paper, we proposed an approach to obtain a globally biaxially strained silicon-on-insulator (SOI) wafer, and the strain mechanism was discussed. By this process, both biaxially tensile and compressive strained SOI (sSOI) can be obtained. The strain introduced into the SOI layer is mainly contributed by the plastic deformation of the buried SiO2 film caused by annealing with the deposition of a high-stress SiN film. Furthermore, He+ implantation at the interface between SiO2 and the substrate Si layer is confirmed to effectively enhance the strain by the sliding of the buried SiO2 at the SiO2-substrate Si interface. Raman spectroscopy shows that the strain of the He+ implanted sSOI has a significant enhancement of more than 300% compared with the unimplanted sSOI.

Optoelectronic integration technology which utilizes CMOS process to achieve the integration of photonic devices has the advantages of high integration, high speed and low power consumption. The Ge1-xSnx alloys have been widely used in photodetectors, light-emitting diodes, lasers and other optoelectronic integration areas because they can be converted into direct bandgap semiconductors as the Sn component increases. However, the solid solubility of Sn in Ge as well as the large lattice mismatch between Ge and Sn resulting from the Sn composition cannot be increased arbitrarily:it is limited, thereby bringing a lot of challenges to the preparation and application of direct bandgap Ge1-xSnx. Strain engineering can also modulate the band structure to convert Ge from an indirect bandgap into a direct bandgap, where the required stress is minimal under biaxial tensile strain on the (001) plane. Moreover, the carrier mobility, especially the hole mobility, is significantly enhanced. Therefore, considering the combined effect of alloying and biaxial strain on Ge, it is possible not only to reduce the required Sn composition or stress for direct bandgap transition, but also to further enhance the optical and electrical properties of Ge1-xSnx alloys. The energy band structure is the theoretical basis for studying the optical and electrical properties of strained Ge1-xSnx alloys. In this paper, according to the theory of deformation potential, the relationship between Sn component and stress at the critical point of bandgap transition is given by analyzing the bandgap transition condition of biaxial tensile strained Ge1-xSnx on the (001) plane. The energy band structure of strained Ge1-xSnx with direct bandgap at the critical state is obtained through diagonalizing an 8-level kp Hamiltonian matrix which includes the spin-orbit coupling interaction and strain effect. According to the energy band structure and scattering model, the effective mass and mobility of carriers are quantitatively calculated. The calculation results indicate that the combination of lower Sn component and stress can also obtain the direct bandgap Ge1-xSnx, and its bandgap width decreases with the increase of stress. The strained Ge1-xSnx with direct bandgap has a very high electron mobility due to the small electron effective mass, and the hole mobility is significantly improved under the effect of stress. Considering both the process realization and the material properties, a combination of 4% Sn component and 1.2 GPa stress or 3% Sn component and 1.5 GPa stress can be selected for designing the high speed devices and optoelectronic devices.

Alkali metal has predicted to be a promising candidate for decorating silicene surface to obtain the high hydrogen storage capacity, owing to their physical properties of lightweight, lower cohesive energy, and appropriate strength of the interaction with H2 molecules. However, though the high potential in hydrogen storage of alkali metal adatoms-decorated silicene under the fixed adatom adsorption component is well known, the evidence for the hydrogen storage capacity of alkali metal adatoms-decorated silicene under different adatom adsorption components remains largely unexplored, which may be of great significance to make the most advantages of alkali metal adatoms-decorated silicene in hydrogen storage aspects. Herein, according to the first-principles calculation corrected by the van der Waals effect, we take Li-decorated silicene for example and perform the detailed study of the geometry structure, the stability and the hydrogen storage capacity of silicene under different Li adsorption components (LixSi1-x), aiming to maximize the hydrogen storage performance of Li-decorated silicene. The results show that the preferred site of Li changes from the hollow site to the valley site as the Li component increases from 0.11 to 0.50, and binding energy of Li is always greater than the corresponding cohesive energy, showing the high stability of Li-decorated silicene and the feasibility of the method to obtain a higher hydrogen storage capacity by increasing the Li component. The hydrogen storage of silicene under different Li adsorption components is investigated by the sequential addition of H2 molecules nearby Li atoms in a stepwise manner. It can be observed that the hydrogen storage capacity of Li-decorated silicene increases and the average adsorption energy decreases with the increase of the Li component. The corresponding hydrogen storage capacities of Li0.11Si0.89, Li0.20Si0.80, Li0.33Si0.67, Li0.43Si0.57 can reach up to 2.54 wt%, 4.82 wt%, 6.00 wt% and 9.58 wt% with 0.58 eV/H2, 0.47 eV/H2, 0.54 eV/H2 and 0.41 eV/H2 average adsorption energy, respectively. When the Li component increases up to 0.50, Li atoms are saturated with a maximum hydrogen storage capacity of 11.46 wt% and an average adsorption energy of 0.34 eV/H2, which well meet the hydrogen storage standard set by the U.S. Department of Energy and mean that the hydrogen storage can be theoretically improved by increasing the Li adsorption component to a saturated level. Furthermore, we analyze the Mulliken charge population, the charge density difference and the density of states, showing that the charge-induced electrostatic interaction and the orbital hybridization are the key factors for the hydrogen adsorption of Li-decorated silicene. Our results may enhance our fundamental understanding of the hydrogen storage mechanism and explore the applications in areas of hydrogen storage for Li-decorated silicene, which are of great importance for the usage of hydrogen in the future.

The successful introduction and micron-scale characterization of uniaxial strain with wafer level play pivotal roles in designing and optimizing of the silicon-on-insulator (SOI) microstructures for next-generation strained-Si transistors. In this paper, the successful fabrication of uniaxially strained SOI with wafer level by simply mechanical bending and annealing was realized. Employing polarized Raman measurements, the Raman intensity as a function of the angle between the crystal-axis and the polarization-direction of the scattered light for the strained top Si layer demonstrated the uniaxial character of the induced strain. Micro-Raman measurements revealed that the strain was uniaxially compressive with 0.114% strain value. The transmission electron microscopy characterization was used to reveal the little effect of process on crystallinity, which was confirmed by XRD measurements. The crystal symmetry change from cubic to tetragonal structure of the strained Si was further discussed.