Interdependence of elastic strain and segregation in metallic multilayers: An x-ray diffraction study of (111) Au/Ni multilayers J.The effect of intermixing on the apparent interface stress is studied in ͗111͘-textured dc-magnetron sputtered Au/Ni multilayers by use of two methods commonly used for determining interface stress. The method using profilometry and in-plane x-ray diffraction does not take intermixing into account and yields an apparent interface stress of Ϫ8.46Ϯ0.99 J m Ϫ2 . However, observed discrepancies between model calculations and measured high-angle x-ray diffractograms indicate intermixing, and by use of the profilometry and sin 2 method the real interface stress value of Ϫ2.69Ϯ0.43 J m Ϫ2 is found. This method also reveals a significant and systematic change of the stress-free lattice parameter of both constituents as a function of modulation period which is shown to account for the difference between the two findings. The method using in-plane diffraction is thus shown to be inapplicable to interface stress determinations in systems exhibiting a modulation period-dependent stress-free lattice parameter. Finally, a deviation of the interface stress in the Au/Ni sample with the smallest modulation period as compared to specimens with larger bilayer lengths is observed to be concurrent with a significant decrease in the interface roughness measured by x-ray reflectivity, which suggests that the deviation is of geometrical origin.
Transition metal dichalcogenides (TMDs) have received widespread attention because of their excellent performances in the field of optoelectronic, nanoelectronic device and photocatalytic exploration. The structures of TMDs can be expressed by the MX2, M=Mo, W; X=S, Se, Te, etc. As a typical TMD, MoSe2 has a graphene-like two-dimensional periodic structure with perfect physical, photoelcrtonic and catalytic properties. Currently, there are various methods to prepare the nanolevel MoSe2, such as the mechanical exfoliation, physical vapor deposition (PVD), hydrothermal method, chemical vapor deposition (CVD), etc, and most studies focused on regular triangular morphologies of the surfaces of different substrates. The new morphology, such as the hexangular star bilayer, has not been systematically investigated. In this study, the hexangular star MoSe2 nanosheets are successfully synthesized by using a simple CVD method in an atmosphere of mixed H2/Ar with a flow rate ratio of 1:4. Molybdenum trioxide(MoO3) and selenium (Se) powders are chosen to be the Mo and Se source, respectively. Moreover, the structure of the obtained MoSe2 nanosheet is characterized by Raman, SEM, EDS, XRD and TEM. The results of Raman spectrum and SEM indicate that the hexangular star MoSe2 possesses a bilayer structure. The TEM characterization reveals that the MoSe2 is a single crystal with a hexagonal lattice structure and good quality. The heating time at high temperature has a remarkable influence on the MoSe2 bilayer growth process. The growth process of the hexangular star MoSe2 bilayer is inferred to experience a three-step process. First, Mo and Se sources are gasified into gaseous molecules and then the Mo molecules are selenized into the MoSe2 crystal nucleus under high temperature. Next, these crystal nucleus are in a triangular epitaxial growth under the action of carrier gas. As heating time increases, the space steric effect leads to different interlayer separations between the two MoSe2 layers in various stacking configurations, eventually forming a hexangular star bilayer. The PL result shows that the spectra split into two main emission peaks, i.e., the direct and indirect bandgaps of the hexangular star structure appearing at 1.53 eV (810.2 nm) and 1.78 eV (696.9 nm), respectively. It might be due to the spin-orbit coupling interaction between the double MoSe2 molecules. The wide spectral range of the MoSe2 bilayer indicates that it has a potencial application in the photoelectric detectors.
The first-principles method has been used to explore how to minimize the over-erase phenomenon in charge trapping memory. Over-erase phenomenon originates from the nitrogen vacancy due to its weak localization of charge on Si atoms. Therefore, we develop a defect model for studying Si3N4 supercells. The defect model consists of an N vacancy and a substitutional atom on the Si site. The substitutional atoms can be C, N, and O atoms, respectively. The Si site belongs to the N vacancy. Then, the Bader charge distribution after program/erase operation, the interaction energy and density of states are calculated for the model so as to analyze the effects of the substitutional atoms on the over-erase phenomenon. The obtained results of the Bader charge distribution show that the substitution of O for the 128th Si can minimize the over-erase phenomenon in Si3N4, and the replacement of the 128th Si by C can also reduce the over-erase phenomenon. However, the model represents a weak localization of charge due to the replacement by C, which is not preferable for charge storage. And the results also reveal that the substitution of N for the 128th Si completely fails to reduce the over-erase phenomenon. With regard to the 162th and 196th Si sites, the substitutions of the three atoms for the two sites cannot minimize the over-erase phenomenon. Furthermore, the analysis of the interaction energies indicates that the combination of each of the three atoms with the N vacancy can form stable clusters on the 128th site in the model. In particular, the attractive interaction between O and N vacancy is the weakest of the three so that the injected charge can temporarily break the stability of the O cluster to rearrange the charge distribution, realizing the localization of charge around the O cluster. And then, the results of the density of states designate that subtitutional O atom at the 128th Si atom site produces a deep-level trap in the band gap, which has a powerful ability to localize the charge. The above results suggest that substitution of O for Si is an excellent solution for the minimization of over-erase phenomenon in Si3N4. This work can provide a method for the minimization of over-erase phenomenon in charge trapping memory and also can be helpful to the improvement of charge retention and optimization of memory window in the charge trapping memory.
The initial dynamics of photogenerated carriers in a-Si:H/a-SiNx:H multilayers has been investigated with time-resolved laser spectroscopy. The mechanisms of thermalization and recombination of these carriers are analysed. The results also reveal that the decay time cutoff, the mobility edge and the band-tail width vary non-monotonously with respect to nitrogen content with a turning point near x = 0.85, which may result from the changes of the internal electric field and the structure of multilayer with nitrogen content.
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