Lattice Mismatch-induced Strain Research Articles

Unveiling the Role of Donor Impurity Position on the Electronic Properties in Strained Type I and Type II Core/Shell Quantum Dots under Magnetic Field.

In this theoretical investigation, we delve into the significant effects of donor impurity position within core/shell quantum dot structures: type I (CdTe/ZnS) and type II (CdTe/CdS). The donor impurity's precise location within both the core and the shell regions is explored to unveil its profound influence on the electronic properties of these nanostructures. Our study investigates the diamagnetic susceptibility and binding energy of the donor impurity while considering the presence of an external magnetic field. Moreover, the lattice mismatch-induced strain between the core and shell materials is carefully examined as it profoundly influences the electronic structure of the quantum dot system. Through detailed calculations, we analyze the strain effects on the conduction and valence bands, as well as the electron and hole energy spectrum within the core/shell quantum dots. The results highlight the significance of donor impurity position as a key factor in shaping the behaviors of impurity binding energy and diamagnetic susceptibility. Furthermore, our findings shed light on the potential for tuning the electronic properties of core/shell quantum dots through precise impurity positioning and strain engineering.

Emergence of Quasi Two-Dimensional Electronic States at the Interface of LSMO/STO via Lattice Mismatch-Induced Strains

A two-dimensional interfacial electronic phase often emerges at the interface between different oxide layers, which may display remarkably distinct properties from its bulk phases. The mechanism of evolution of such an additional phase at the interface and its properties, however, are not fully understood yet. Herein, we detect an additional structural and magnetic phase and so-called quasi two-dimensional electronic states of La0.7Sr0.3MnO3 (LSMO) that interface with SrTiO3 (STO), while LSMO interfaced with a [Pb(Mg1/3Nb2/3)O3]0.7-[PbTiO3]0.3 (PMN-PT) substrate shows a single phase. We find that the additional phase exhibits a lower Curie temperature, a larger effective saturation magnetization, and a larger Gilbert damping coefficient than its bulk counterpart. We discuss that the lattice mismatch-induced strain plays an important role in the formation of the additional state. These results will give an insight into designing full-oxide electronics, including 2D spintronics.

Pseudomorphic growth of thick Al0.6Ga0.4N epilayers on AlN substrates

We report on the absence of strain relaxation mechanism in Al0.6Ga0.4N epilayers grown on (0001) AlN substrates for thickness as large as 3.5 μm, three-orders of magnitude beyond the Matthews–Blakeslee critical thickness for the formation of misfit dislocations (MDs). A steady-state compressive stress of 3–4 GPa was observed throughout the AlGaN growth leading to a large lattice bow (a radius of curvature of 0.5 m−1) for the thickest sample. Despite the large lattice mismatch-induced strain energy, the epilayers exhibited a smooth and crack-free surface morphology. These results point to the presence of a large barrier for nucleation of MDs in Al-rich AlGaN epilayers. Compositionally graded AlGaN layers were investigated as potential strain relief layers by the intentional introduction of MDs. While the graded layers abetted MD formation, the inadequate length of these MDs correlated with insignificant strain relaxation. This study emphasizes the importance of developing strain management strategies for the implementation of the single-crystal AlN substrate platform for III-nitride deep-UV optoelectronics and power electronics.

Relaxation of asymmetric crystallographic tilt: In situ x-ray diffraction studies of epitaxial electrodeposition of bismuth on GaAs (110)

High quality, epitaxial thin-films are key components of almost all modern electronic devices. During deposition, lattice mismatch between the substrate and the film generates elastic strain. The strain energy grows with film thickness until a defect is generated that relieves the strain. The strain relaxation mechanism is critical in epitaxial electrodeposition. Here, we study how a metal (bismuth) film growing via electrodeposition on a semiconductor substrate [GaAs(110)] relaxes the lattice mismatch-induced strain. Using in situ synchrotron-based X-ray techniques, we monitor the crystallographic orientation and grain size of the growing film during electrochemical deposition. We (1) confirm that a single crystallographic orientation of the film, with (011¯8) as the oriented plane, can be selected by controlling the overpotential, η, and (2) find that, after a threshold thickness is reached, the tilt angle varies monotonically with film thickness. Our data are consistent with the film relaxing the strain by forming low-energy, asymmetric tilt boundaries.

Strain-enhanced tunneling magnetoresistance in MgO magnetic tunnel junctions.

While the effects of lattice mismatch-induced strain, mechanical strain, as well as the intrinsic strain of thin films are sometimes detrimental, resulting in mechanical deformation and failure, strain can also be usefully harnessed for applications such as data storage, transistors, solar cells, and strain gauges, among other things. Here, we demonstrate that quantum transport across magnetic tunnel junctions (MTJs) can be significantly affected by the introduction of controllable mechanical strain, achieving an enhancement factor of ~2 in the experimental tunneling magnetoresistance (TMR) ratio. We further correlate this strain-enhanced TMR with coherent spin tunneling through the MgO barrier. Moreover, the strain-enhanced TMR is analyzed using non-equilibrium Green's function (NEGF) quantum transport calculations. Our results help elucidate the TMR mechanism at the atomic level and can provide a new way to enhance, as well as tune, the quantum properties in nanoscale materials and devices.

In-situ high-resolution low energy electron diffraction study of strain relaxation in heteroepitaxy of Bi(111) on Si(001): Interplay of strain state, misfit dislocation array and lattice parameter

The relief of lattice mismatch-induced strain in Bi(111) on Si(001) heteroepitaxial system was investigated in real time as the Bi film relaxes, by means of high resolution low-energy electron diffraction (LEED). The inherent lattice mismatch of 2.5% at room temperature is accommodated through the formation of an ordered misfit dislocation array confined to the interface. The strain fields of the dislocations cause a periodic height undulation of the surface in the sub-Ångström regime, which is observed through spot splitting in LEED. From a simultaneous measurement of the position of the first-order LEED spots, which corresponds to the lattice parameter of the film, and of the separation of satellite spots, which corresponds to the ordering of the dislocation array, the evolution of the strain state during annealing of a 6nm Bi film was determined. The strain is solely relaxed by full edge-type dislocations arranged in the ordered array at the interface. From the remaining strain of ε=0.6% the critical thickness for generation of misfit dislocations under equilibrium conditions can be derived.

γ-Al 2O 3 thin film formation via oxidation of β-NiAl(1 1 0)

β-NiAl(1 1 0) was oxidized in air for 1–2 h in the temperature range of 650–950 °C. The structure and morphology of the oxide films were characterized using a cross-sectional transmission electron microscopy (TEM) method. Only a thin film of aluminum oxide, γ-Al 2O 3, was formed. The epitaxial relationship between NiAl and γ-Al 2O 3 as well as the surface roughness depends on the oxidation temperature. The Nishiyama–Wassermann (NW) orientation relation (OR) between β-NiAl and γ-Al 2O 3 was noted at an oxidation temperature of 850 °C while the Kurdjumov–Sachs (KS) OR was observed at 650 °C. The changes in the epitaxial relationship between the γ-Al 2O 3 film and the NiAl substrate were caused by lattice mismatch-induced strain energy during oxide growth. It was also noted that short time oxidation at T = 750 °C created γ′ phase precipitates between the NiAl substrate and the γ-Al 2O 3 film, while oxidation at the higher temperature of 950 °C resulted in textured polycrystalline γ-Al 2O 3 films. The smoothest single crystal epitaxial γ-Al 2O 3 film formed at an oxidation temperature of 850 °C in air.

Chemisorption of pyrrole and polypyrrole on Si(001)

The functionalization of the Si(001) surface by pyrrole and polypyrrole is investigated by means of first-principles pseudopotential calculations. We find dissociative reactions, leading to the partial fragmentation of the molecule, to be energetically most favored for pyrrole adsorption. The lowest energy configuration for monolayer coverage is characterized by pyrrole rings bonded to the surface via Si-N linkage. In co-existence with alternative adsorption geometries where both N and C are bonded to the surface, this structure accounts very well for the available experimental data. Chemisorption of pyrrole is found to effectively passivate the Si(001) surface, irrespective of the details of the adsorption geometry. The formation of well-ordered polypyrrole structures on Si(001) may require chemical modifications of the polypyrrole chains in order to accommodate the lattice mismatch-induced strain.

Dielectric loss of strontium titanate thin films on different substrates

Lattice mismatch-induced strain in incipient ferroelectric strontium titanate (SrTiO3) thin films has been found to greatly influence the dielectric properties of the thin films. In this paper we report results of SrTiO3 (STO) thin films grown on different substrate which result in different strain, in particular their effects on the dielectric loss.

Strain effects on the energy bands of ZnSe films grown on GaAs substrates by spectroscopic ellipsometry

Room-temperature spectroscopic ellipsometry measurements were carried out on ZnSe thin films grown on (001)GaAs substrates by molecular-beam epitaxy for the study of the lattice mismatch-induced strain at the interface. The magnitude of the absorption coefficient at the E0+Δ0 critical point is very sensitive to the strain in the film. The variation in the magnitude of the absorption coefficient is used to estimate the critical thickness for the onset of dislocation generation. Almost complete relaxation of the films was obtained for thicknesses higher than 500 nm. Also, the strain-induced coupling between the valence subbands was found to cause additional shifting of the light-hole subband.