MXene, 2D transition metal carbides, nitrides, and carbonitrides with a unique 2D structure, inspired a series of function applications related to energy storage and conversion, biometrics and sensing, lighting, purification, and separation. Its surface terminations are confined by the adjacent MXene layers, and form the 2D planar space with symmetrical surfaces, which is similar to a 2D nanoreactor that can be utilized and determined MXene’s function. Based on the working principle, surface and interface play critical roles in the ion intercalation, physical/chemical adsorption, and chemical reaction process, and show significant effects on MXene’s properties and functions. Although there have been some reviews on MXene, less attention has been paid to the underlying principle of the involved surface chemistry, controllable design, and resultant properties. Herein, the regulation methods, characterization techniques, and the effects on properties of MXene surface terminations were summarized to understand the surface effects, and the relationship between the terminations and properties. We expected this review can offer the route for a series of ongoing studies to address the MXene surface environment and the guidelines for MXene’s application.
The thermal stability of a nickel germanide film formed on a tensile-strained Ge epilayer on a silicon substrate with a low-temperature Si 0.77 Ge 0.23 (50-nm)/Ge (50-nm) buffer is investigated. A record temperature of 700 • C for the stability of sheet resistance of nickel germanide is reported, which is increased by about 150 • C compared to that on bulk Ge and comparable to the temperature for nickel silicide on the Si substrate. The improvement of the thermal stability is demonstrated due to the delay of the agglomeration of the nickel germanide film on Ge-on-Si, which is proposed to be attributed to the increase of the tensile strain in the Ge epilayer during thermal annealing due to the thermal mismatch between Si and Ge.
To understand the agglomeration mechanism of NiGe films grown on Ge(001), texture structures of NiGe films are revealed by X-ray pole figure measurement. Two preferred epitaxial orientations of the NiGe grains are identified to be NiGe(45̄4) ∥ Ge(001) NiGe[1̄01] ∥ Ge[110] and NiGe(130) ∥ Ge(001) NiGe[002] ∥ Ge[110]. The component of the first epitaxial alignment becomes dominating and the latter diminishing with increasing annealing temperature. The NiGe grains of the second epitaxial alignment are unstable and diminishing at high temperature due to the relatively higher interface/surface energy. The competition of grains with various epitaxial orientations has made a significant contribution to film agglomeration.
In this Letter, NiGe/SiO2/n-Ge ohmic contacts were demonstrated with Ge, rather than Ni, diffusion through the ion-implanted SiO2 films to form NiGe. The equivalent Schottky barrier height reduced from 0.58 eV for NiGe/n-Ge to ohmic contact. The anomalous diffusion behavior and accumulation of Ge in the SiO2 near the NiGe/SiO2 interface can be explained by vacancy-enhanced Ge diffusion. It is proposed that the presence of vacancies and Ge atoms embedded in the SiO2 layer play a significant role in the current enhancement by generation of multiple levels in the SiO2 band gap.
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