Metastable Superlattice Research Articles

Conversion between Metavalent and Covalent Bond in Metastable Superlattices Composed of 2D and 3D Sublayers.

Reversible conversion over multimillion times in bond types between metavalent and covalent bonds becomes one of the most promising bases for universal memory. As the conversions have been found in metastable states, an extended category of crystal structures from stable states via redistribution of vacancies, research on kinetic behavior of the vacancies is highly in demand. However, it remains lacking due to difficulties with experimental analysis. Herein, the direct observation of the evolution of chemical states of vacancies clarifies the behavior by combining analysis on charge density distribution, electrical conductivity, and crystal structures. Site-switching of vacancies of Sb2Te3 gradually occurs with diverged energy barriers owing to their own activation code: the accumulation of vacancies triggers spontaneous gliding along atomic planes to relieve electrostatic repulsion. Studies on the behavior can be further applied to multiphase superlattices composed of Sb2Te3 (2D) and GeTe (3D) sublayers, which represent superior memory performances, but their operating mechanisms were still under debate due to their complexity. The site-switching is favorable (suppressed) when Te-Te bonds are formed as physisorption (chemisorption) over the interface between Sb2Te3 (2D) and GeTe (3D) sublayers driven by configurational entropic gain (electrostatic enthalpic loss). Depending on the type of interfaces between sublayers, phases of the superlattices are classified into metastable and stable states, where the conversion could only be achieved in the metastable state. From this comprehensive understanding on the operating mechanism via kinetic behaviors of vacancies and the metastability, further studies toward vacancy engineering are expected in versatile materials.

Enhanced crystallization resistance and thermal stability via suppressing the metastable superlattice phase in Ni-(Pd)-P metallic glasses

Virtually, glass formation is to avoid crystallization during solidification, i.e., a consequence of the competition between the undercooled liquid and primary crystalline phases. It is found that the crystallization resistance of the binary Ni-P system was drastically enhanced with alloying of Pd and correspondingly, the critical size for glass formation increased significantly from the micrometer to millimeter scale. Thermodynamically, the introduction of Pd could effectively increase the atomic size mismatch and heat of mixing, which are beneficial to stabilize the supercooled liquid. Kinetically, the introduction of Pd not only successfully suppresses the formation of metastable superlattice phase, which is prone to nucleation and growth in the supercooled liquid state, but also changes the crystallization mechanism from the primary to eutectic mode. The current finding sheds light on understanding glass formation of the most studied Pd-Ni-P system and the glass-forming ability in general.

Oxide nano-engineering using MBE

Molecular beam epitaxy (MBE) has achieved unparalleled control in the integration of semiconductors at the nanometer level; its use for the integration of oxides with similar nanoscale customization appears promising. This paper describes the use of reactive MBE to synthesize layered oxide heterostructures, including new compounds and metastable superlattices, involving monolayer-level integration of the dielectric and ferroelectric oxides SrO, SrTiO 3, BaTiO 3, PbTiO 3, and Bi 4Ti 3O 12. The controlled synthesis of such layered oxide heterostructures offers great potential for tailoring the dielectric and ferroelectric properties of materials. Oxide nano-engineering is accomplished by supplying the incident species in the desired layering sequence with submonolayer composition control. Comparisons between the growth of compound semiconductors and oxides by MBE are made.

The Importance of In Situ Monitors in the Preparation of Layered Oxide Heterostructures by Reactive MBE

ABSTRACTUsing a variety of in situ monitors and when possible adsorption-controlled growth conditions, layered oxide heterostructures including new compounds and metastable superlattices have been grown by reactive molecular beam epitaxy (MBE). The heteroepitaxial layers grown include Bi4Ti3 O12—SrTiO3 and Bi4Ti3O12—PbTiO3 Aurivillius phases, Srn+1TinO3n+1 Ruddlesden-Popper phases, and metastable PbTiO3 / SrTiO3 and BaTiO3 / SrTiO3 superlattices. Accurate composition control is key to the controlled growth of such structures, and to this end combinations of reflection high-energy electron diffraction (RHEED), atomic absorption spectroscopy (AA), a quartz crystal microbalance (QCM), and adsorption-controlled growth conditions were employed during growth. The structural perfection of the films has been investigated using in situ RHEED, four-circle x-ray diffraction, atomic force microscopy (AFM), and high-resolution transmission electron microscopy (TEM).

Formation of a metastable superlattice by x-ray interaction with standing surface acoustic waves

X-ray diffraction on the surface of the YZ cut of a LiNbO3 ferroelectric crystal modulated by standing surface acoustic waves (SAW) was investigated under the total external reflection conditions. It is shown that a metastable superlattice is formed in the near-surface region in the process of the interaction between the incident x-ray radiation and the ferroelectric crystal surface modulated by standing SAW. The metastable superlattice may be stored for a long time period and may be read out by x-ray and laser radiation.

An annealing study of relaxation and interface quality in Si-Si1−xGex strained-layer superlattices

The relaxation and interface quality of two representative molecular-beam-epitaxy-grown Si-Si1−xGex strained-layer superlattices have been investigated by Raman scattering spectroscopy, double-crystal x-ray diffraction, transmission electron microscopy, and charge collection microscopy in a scanning electron microscope. One of the superlattices appeared stable, and the other metastable, under thermal annealing at temperatures between 600 and 900 °C. The strain values perpendicular to the growth direction as a function of annealing temperature are obtained from a kinematical simulation of x-ray rocking curves. These results are compared with the frequency shifts of the longitudinal optical phonons in the Raman spectra. Deterioration of the crystal quality and progressive relaxation was observed in the metastable superlattice. The thermally induced relaxation occurred through formation of misfit dislocations at the first Si-Si1−xGex interface and these caused threading dislocations to form within the epilayer. The degradation of the interfaces for both superlattices on annealing is correlated with a sharp decrease in the acoustic mode Raman intensities.

Raman scattering study of folded acoustic phonons in GaAs/InxGa1−xAs strained-layer superlattices

The excitation of zone-folded longitudinal acoustic phonons in GaAs/InxGa1−xAs strained-layer superlattices is observed by Raman scattering. The Raman data agree well with theory and are consistent with high indium concentrations achieved in the fabrication of these metastable superlattices of good crystalline quality.