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.

Nanointerface Chemistry: Lattice-Mismatch-Directed Synthesis and Application of Hybrid Nanocrystals.

In the past decades, great strides have been made in the synthesis of hybrid nanocrystals (HNCs) consisting of two or more disparate subunits (such as metals and/or semiconductors) joined through nanointerfaces, which are intriguing due to their exceptional functionalities that cannot be achieved by single-component nanosystems. The promising and versatile applications of these HNCs are closely dependent on the structural and electronic properties of the nanointerface between subunits. This is because the compatibility of the lattice structures between subunits not only determines the synthetic accessibility and growth mechanisms of the HNCs on the thermodynamic basis but also influences their interfacial characteristics (atomic arrangement, lattice mismatch-induced strain or defects), configurations, crystallinity, and the synergistic interplay of different subunits at the nanoscale. As a result, nanointerface chemistry has attracted intense scientific endeavors worldwide and spurred the rapid development of the lattice-mismatch-directed precise synthesis. This review gives an overview of the main strategies developed for delicate design and fabrication of core-shell HNCs under different degrees of lattice mismatch (from 0.2% to larger than 50%), including epitaxial seeded growth, nanoscale cation exchange, cation exchange-facilitated nonepitaxial growth, etc. Moreover, as for the core-shell HNCs with small (<5%) or moderate lattice mismatch (∼5-20%), the significance of the lattice-strain control at the nanointerface in maneuvering their functions toward desired applications are discussed in detail. Regarding the core-shell HNCs with large lattice mismatch (>20%), the challenges in precise synthesis, the promising solutions enabled by cation exchange-facilitated nonepitaxial growth, and the enhanced applications of the resulting HNCs with strain-free nanointerface are elaborated. We conclude with a personal perspective on the significance and urgency of fully harnessing the effects of lattice mismatch to further advance the science of synthesis and application of HNCs.

Short range biaxial strain relief mechanism within epitaxially grown BiFeO3

Lattice mismatch-induced biaxial strain effect on the crystal structure and growth mechanism is investigated for the BiFeO3 films grown on La0.6Sr0.4MnO3/SrTiO3 and YAlO3 substrates. Nano-beam electron diffraction, structure factor calculation and x-ray reciprocal space mapping unambiguously confirm that the crystal structure within both of the BiFeO3 thin films is rhombohedral by showing the rhombohedral signature Bragg’s reflections. Further investigation with atomic resolution scanning transmission electron microscopy reveals that while the ~1.0% of the lattice mismatch found in the BiFeO3 grown on La0.6Sr0.4MnO3/SrTiO3 is exerted as biaxial in-plane compressive strain with atomistically coherent interface, the ~6.8% of the lattice mismatch found in the BiFeO3 grown on YAlO3 is relaxed at the interface by introducing dislocations. The present result demonstrates the importance of: (1) identification of the epitaxial relationship between BFO and its substrate material to quantitatively evaluate the amount of the lattice strain within BFO film and (2) the atomistically coherent BFO/substrate interface for the lattice mismatch to exert the lattice strain.

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.

Phase coexistence and pinning of charge density waves by interfaces in chromium

We study the temperature dependence of the charge density wave (CDW) in a chromium thin film using x-ray diffraction. We exploit the interference between the CDW satellite peaks and Laue oscillations to determine the amplitude, the phase, and the period of the CDW. We find discrete half-integer periods of CDW in the film and switching of the number of periods by one upon cooling/heating with a thermal hysteresis of 20 K. The transition between different CDW periods occurs over a temperature range of 30 K, slightly larger than the width of the thermal hysteresis. A comparison with simulations shows that the phase transition occurs as a variation of the volume fraction of two distinct phases with well-defined periodicities. The phase of the CDW is constant for all temperatures, and we attribute it to strong pinning of the CDW by the mismatch-induced strain at the film-substrate interface.

Lattice Mismatch Dominant Yet Mechanically Tunable Thermal Conductivity in Bilayer Heterostructures.

Heterostructures that are assembled by interfacing two-dimensional (2D) materials offer a unique platform for the emerging devices with unprecedented functions. The attractive functions in heterostructures that are usually absent and beyond the single layer 2D materials are largely affected by the inherent lattice mismatch between layers. Using nonequilibrium molecular dynamics simulations, we show that the phonon thermal transport in the graphene-MoS2 bilayer heterostructure is reduced by the lattice mismatch, and the reduction can be mitigated well by an external tension, weakening the effect of inherent mismatch-induced strain on thermal conductivity. Mechanical analysis in each layered component indicates that the external tension will alleviate the lattice mismatch-induced deformation. The phonon spectra are also softened by the applied tension with a significant shift of frequency from high to low modes. A universal theory is proposed to quantitatively predict the role of the lattice mismatch in thermal conductivity of various bilayer heterostructures and shows good agreement with simulations.

Coherently Strained Si-SixGe1-x Core-Shell Nanowire Heterostructures.

Coherently strained Si-SixGe1-x core-shell nanowire heterostructures are expected to possess a positive shell-to-core conduction band offset, allowing for quantum confinement of electrons in the Si core. We report the growth of epitaxial, coherently strained Si-SixGe1-x core-shell heterostructures through the vapor-liquid-solid mechanism for the Si core, followed in situ by the epitaxial SixGe1-x shell growth using ultrahigh vacuum chemical vapor deposition. The Raman spectra of individual nanowires reveal peaks associated with the Si-Si optical phonon mode in the Si core and the Si-Si, Si-Ge, and Ge-Ge vibrational modes of the SixGe1-x shell. The core Si-Si mode displays a clear red-shift compared to unstrained, bare Si nanowires thanks to the lattice mismatch-induced tensile strain, in agreement with calculated values using a finite-element continuum elasticity model combined with lattice dynamic theory. N-type field-effect transistors using Si-SixGe1-x core-shell nanowires as channel are demonstrated.

Physical justification for ionic conductivity enhancement at strained coherent interfaces

Introducing lattice mismatch-induced tensile strain at hetero-interfaces may be an efficient approach to enhance ionic conductivity at moderate temperatures. Extensive theoretical and experimental research has focused on strain-versus-ionic conductivity correlations. However, the correlation between strain and ionic conductivity still lacks a systematic and quantitative investigation. In this report, we derive an analytical expression to evaluate quantitatively the strain-induced enhancement in ionic conductivity. We demonstrate that simulation and experimental data in the literature, are well explained in the framework of our derived expressions. Our studies provide a powerful platform for quantitatively designing and engineering heterostructure interfaces for a number of applications, including fuel cells/batteries and optical/magnetic devices.

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.

Growth kinetics and size-dependent wetting of Ag/α-Al2O3(0001) nanoparticles studied via the plasmonic response

The growth of vapour-deposited silver nanoparticles on α-Al2O3 was studied in situ from 190 to 675 K by surface differential reflectivity spectroscopy in the UV–visible range. Changes in size, shape and density were derived from the plasmonic response modelled in the framework of interface susceptibilities by assuming that supported clusters were in the form of truncated spheres. The sticking coefficient of silver on alumina is close to one up to T ≃ 575 K before entering a regime of incomplete condensation. The Arrhenius dependence of the saturation density indicates a nucleation on defects at low temperature (T ≤ 300 K) and detrapping above. The particle size D evolution follows temporal power laws, independent of temperature and flux, which characterize the growth (D ∼ t0.31) and coalescence (D ∼ t0.55) of the film. These are indicative of the growth of isolated particles at constant density and dynamic coalescence, respectively. The wetting angle of the silver clusters is shown to increase during the growth regime, which is assigned to a combination of surface stress and mismatch-induced strain, and to decrease upon coalescence, which is attributed to plastic relaxation. For particles larger than 10 nm in size, the values of contact angle and adhesion energy level off with asymptotic limits (θc = 127.5° ± 1° and 0.48 ± 0.02 J m−2) that nicely agree with tabulated data. This work highlights the ability of nanoplasmics to monitor in situ the growth kinetics of thin supported films.

Structural characterization of La 0.9Ba 0.1MnO 3/Y-ZrO 2 film by X-ray diffraction

Perovskite manganite La 0.9Ba 0.1MnO 3(LBMO) films were deposited on (0 0 1)-oriented single crystal yttria-stabilized zirconia (YSZ) substrate by 90° off-axis radio frequency magnetron sputtering. The film thickness ranged from 10 nm to 100 nm. Grazing incidence X-ray diffraction technique and high resolution X-ray diffraction were applied to characterize the structure of LBMO films. The LBMO film mainly consisted of (0 0 1)-orientated grain as well as weakly textured (1 1 0)-orientated grain. The results indicated that an amorphous layer with thickness of about 4 nm was formed at the LBMO/YSZ interface. The strain in LBMO film was small and averaged to be about –0.14%. The strain in the film was not lattice mismatch-induced strain but residual strain due to the difference in thermal expansion coefficient between film and substrate.

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.

Evaluation of Pd-based brazes to join silicon nitride to copper-clad-molybdenum

Si 3N 4 (SN-281)/Cu-clad-Mo joints, brazed using a soft (YS: 341 MPa) and ductile (43% elongation) Pd–Co braze were sound and exhibited an interaction zone comprised of Pd 66Mo 14Cu 10Co 6Si 3 and Pd 74Mo 11Co 8Cu 6Si. Similar joints made using a less ductile and stronger Pd–Ni braze led to cracking from large CTE mismatch-induced strain energy (∼64 mJ to 348 mJ).

Growth and characterization of La2/3Ca1/3MnO3 thin films on ‘silicon on insulator’ substrates

La2/3Ca1/3MnO3 thin films have been grown on SrTiO3, LaAlO3, and yttria-stabilized zirconia buffered silicon-on-insulator (SOI) substrates by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved on the SrTiO3 and LaAlO3 substrates, a coexistence of the cube-on-cube and cube-on-diagonal epitaxy was observed in the the manganite films on SOI substrates. Besides the intrinsic four-fold magnetocrystalline anisotropy, a uniaxial anisotropy also exists in the films, which is determined by the demagnetization field and the mismatch-induced strain. A tensile strain leads to an easy plane, while a compressive strain favors an easy axis. The different magnetization configurations in the films on different substrates are the reason for their varied transport and magnetic properties. Due to a combined effect of these magnetic anisotropy, the magnetization in the two crystallography domains in the film on SOI tends to lie in the film plane but align in their respective easy axes. There are always large spin angles across the domain boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunnelling was observed. It shows a resistance change of ∼20% at 50 K in a magnetic field ∼700 Oe, which is promising for real applications.

Crack-free InAlGaN quaternary alloy films grown on Si(1 1 1) substrate by metalorganic chemical vapor deposition

Crack-free In 0.08Al 0.25Ga 0.67N quaternary films, with and without thick (>1.5 μm) high-temperature-GaN (HT-GaN) interlayer, have been grown on Si(1 1 1) substrates by a low-pressure metalorganic chemical vapor deposition (MOCVD) system. Mole fractions of In and Al in quaternary alloy layers are determined by Energy dispersive spectroscopy (EDS) and Rutherford backscattering spectrometry (RBS), which are recorded as ∼8% and ∼25–27%, respectively. High-resolution X-ray diffraction (HRXRD) and room temperature photoluminescence (RT-PL) results evidence the film's single crystal structure and the existence of local In- and/or Al-rich regions. Compared with GaN film grwon on Si(1 1 1) substrate, no crack is observed in the quaternary ones. Two explanations are proposed. First, mismatch-induced strain is relaxed significantly due to gradual changes of In concentration. Second, the weak In–N bond is likely to break when the sample is cooled down to the room temperature, which is expected to favor the releasing of thermal stress.

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.

The influence of strain on the diffusion of Si dimers on Si(001)

The influence of lattice mismatch-induced tensile strain on the diffusion of Si dimers on Si(001) has been studied. The rate of surface diffusion of a Si dimer along the substrate dimer rows is relatively insensitive to tensile strain, whereas the rate of diffusion for a Si dimer across the substrate dimer rows is significantly enhanced. The insensitivity of the along row diffusion rate for tensile strain is attributed to the presence of a dissociative intermediate state of the ad-dimer during diffusion rather than diffusion as a solid unit.