Interfacial Lattice Mismatch Research Articles

Interfacial electric field optimization and co-catalyst free LaFeO3-based p-p-type homojunction for efficient PEC water splitting

LaFeO3 (LFO), a typical p-type semiconductor, is considered as a promising material for photoelectric conversion because of its flexible composition and structure. However, its severe photogenerated carrier recombination and surface reaction kinetic hysteresis greatly limit its application in photoelectrochemistry (PEC) water splitting. Here, for the first time, a LFO p-p-type homojunction (Cu-LFO/Ni-LFO) formed by Cu-doped LFO and Ni-doped LFO is reported. Experimental investigation and density functional theory (DFT) calculations reveal that Cu and Ni doping forms a built-in electric field directed from Ni-LFO to Cu-LFO, which effectively improves the carrier separation and transport efficiency. The comparable crystal structure at the interface minimizes the photogenerated carrier recombination due to the interface lattice mismatch. In addition, Ni-LFO in contact with the electrolyte served as a co-catalyst like role in this PEC system, improving the kinetic properties of the surface reaction significantly by optimizing the adsorption and desorption of H2O molecules and HER reaction intermediates on the Fe in the active center whilst significantly reducing the energy barrier. As a result, the photocurrent density of the Cu-LFO/Ni-LFO photocathode reaches 5.94 mA cm2, which is 11 times higher than that of Cu-LFO and surpasses all LFO-based photocathodes reported so far. The LFO-based p-p homo-junction synthesized in this work is instructive for improving the carrier separation efficiency and surface reaction kinetics of LFO.

Epitaxial growth of highly symmetrical branched noble metal-semiconductor heterostructures with efficient plasmon-induced hot-electron transfer

Epitaxial growth is one of the most commonly used strategies to precisely tailor heterostructures with well-defined compositions, morphologies, crystal phases, and interfaces for various applications. However, as epitaxial growth requires a small interfacial lattice mismatch between the components, it remains a challenge for the epitaxial synthesis of heterostructures constructed by materials with large lattice mismatch and/or different chemical bonding, especially the noble metal-semiconductor heterostructures. Here, we develop a noble metal-seeded epitaxial growth strategy to prepare highly symmetrical noble metal-semiconductor branched heterostructures with desired spatial configurations, i.e., twenty CdS (or CdSe) nanorods epitaxially grown on twenty exposed (111) facets of Ag icosahedral nanocrystal, albeit a large lattice mismatch (more than 40%). Importantly, a high quantum yield (QY) of plasmon-induced hot-electron transferred from Ag to CdS was observed in epitaxial Ag-CdS icosapods (18.1%). This work demonstrates that epitaxial growth can be achieved in heterostructures composed of materials with large lattice mismatches. The constructed epitaxial noble metal-semiconductor interfaces could be an ideal platform for investigating the role of interfaces in various physicochemical processes.

Interfacial strain evolution and abnormal Poisson's ratios in Al nuclei for the Al/TiB2 heterostructure

The interfacial lattice mismatch and local elastic strains of Al/TiB2 heterostructures are quantitatively investigated by aberration-corrected transmission electron microscopy. Both in-plane atomic expansion and out-of-plane compression of Al layers stacked on (0001)TiB2 are uncovered and favor the further extension of deformed Al layers for realizing a successfully heterogeneous nucleation. These lattice strains can persist up to several Al atomic layers from the topmost (0001) surface of TiB2. Based on the compressive and expansive elastic strain relation, we estimate the Poisson's ratio in Al nuclei and find that large mismatch strains lead to abnormal changes in the Poisson's ratio. This study reveals the effect of lattice mismatch on the heterogeneous nucleation of Al on TiB2 and may have important implications in understanding the heterogeneous nucleation of Al.

Atomic insights into heterogeneous nucleation and growth kinetics of Al on TiB2 particles in undercooled Al-5Ti-1B melt

Titanium diboride (TiB2) is an effective grain refiner of Al alloys in the industry that facilitates casting processes by forming uniformly refined microstructures. Although our understanding of the underlying refinement mechanisms has advanced, the atomic kinetics of heterogeneous nucleation of Al on TiB2 remains unknown. Here, we report atomic-scale observations of the heterogeneous nucleation and growth kinetics of Al on self-formed TiB2 particles by in situ heating of undercooled Al-5Ti-1B films. We demonstrate that an ordered Al monolayer forms on the Ti-terminated {0001}TiB2 surface; then, the surrounding Al atoms are initiated to form an island-shaped Al nucleus with face-centered cubic {111} stacking without the assistance of a Ti-rich buffer layer. The interfacial lattice mismatch between {111}Al and {0001}TiB2 causes remarkable out-of-plane strain that decreases gradually with Al nucleus layers increasing to 6 atomic layers. The elastic strain energy originating from this interfacial strain increases the free energy of the Al/TiB2 heterostructure, hence impeding the rapid growth of the Al nucleus. We found that TiB2 particles stabilize the Al nuclei rather than activating their free growth into grains when the experimental undercooling ΔT is lower than the onset undercooling ΔTfgin Greer's free growth model. Our findings provide an atomic-scale physical image of the heterogeneous nucleation and growth mechanisms of Al with inoculator participation and elucidate the strain-dependent growth kinetics of Al nuclei.

AlGaN UV Detector with Largely Enhanced Heat Dissipation on Mo Substrate Enabled by van der Waals Epitaxy

The epitaxy of AlGaN on metallic substrates exhibits numerous advantages including flexibility, vertical carrier injection, and enhanced heat dissipation for optoelectronic devices; however, there are still many challenges for the growth of AlGaN according to the serious interfacial reaction and lattice mismatch by conventional epitaxial techniques. In this work, the c-oriented AlGaN is grown on polycrystalline Mo substrate by van der Waals (vdWs) epitaxy with graphene as the insertion layer. A high-temperature annealed AlN is deposited as the nucleation layer, which optimizes the crystalline quality of following AlGaN. The isolation effect of the graphene insertion layer results in a good epitaxial interface, which suppresses the atoms diffusion and chemical reaction, demonstrating by the theoretical calculation of high energy barriers for Al/Mo atoms penetrating through graphene insertion layer. The ultraviolet (UV) detector is fabricated by further growing a n-AlGaN layer as the photosensitive absorber, whose responsivity (1.7 × 10–3 A W–1) is comparable with that fabricated on conventional sapphire. Even better, the UV detector on the Mo substrate possesses enhanced heat dissipation ability due to its higher thermal conductivity, and the temperature elevation after consistently applying a drive voltage largely decreases by 50%. This work enlightens the growth of AlGaN materials and fabrication of high-power/high-voltage optoelectronic devices on polycrystalline metallic substrates by the mean of vdWs epitaxy.

Achieving a Combination of Higher Strength and Higher Ductility for Enhanced Wear Resistance of AlCrFeNiTi0.5 High-Entropy Alloy by Mo Addition

AlCrFeNiTi0.5Mox (x = 0, 0.1, 0.2, 0.3 and 0.4) high-entropy alloys (HEAs) were prepared by arc melting and investigated in terms of microstructure, mechanical properties, and wear resistance. All the as-cast HEAs are composed of one disordered BCC phase (BCC) and one ordered BCC (B2) phase. The added Mo acted as a solid solute in the BCC phase. When Mo molar ratio was more than 0.3, a new type or modified BCC phase formed at the grain boundary, which was enriched with both Mo and Ti. Strength, hardness, and ductility of AlCrFeNiTi0.5 were markedly increased with the Mo addition. The increase in hardness was caused by Mo-solute strengthened disordered BCC phase and precipitation-strengthening by precipitation of hard (Mo, Ti)-rich BCC phase at grain boundaries. The improved ductility was largely attributed to reduced interfacial lattice mismatch between the BCC and B2 phase. The Mo-free AlCrFeNiTi0.5 showed the highest wear loss, about 2.5 times as large as that of AlCrFeNiTi0.5Mo0.4 alloy, which possessed the highest hardness, yield strength, maximum strength, and ductility.

(Invited, Digital Presentation) Ultra-Low Thermal Conductance across Hetero-Structured Four-Layered Van Der Waals Materials

Tuning the thermal properties of materials and exploring their heat transport mechanisms are of critical significance for optimizing the performance of current electronic and thermoelectric devices. Regarding flexible thermoelectric, recently, gigantic power factor is achieved in carbon nanotube fibers with tuned Fermi energy level [1]. Thus, control of heat transport across van der Waals (vdW) interfaces, which are the origins of flexibility, is very important for improvement of thermoelectric conversion efficiency. Considering that thermal transport is mainly affected by phonon-boundary scattering, development a route to tune such scattering at vdW interfaces is important. Ultrathin two-dimensional (2D) materials have been widely studied in electronic and photonic fields due to the distinctly unique geometrical and electronic structures of such materials. Regarding thermal transport properties, they have a lot of degrees of freedom in their integration, and the layered vdW integrated structure of these materials, in which building blocks are physically assembled together through weak vdW interactions, provides a strategy to reduce thermal conductance. The rich interfacial lattice mismatch in layered vdW structures is promising for enhancing phonon-boundary scattering at interfaces. For instance, Vaziri et al. studied the thermal boundary resistance of various heterointerfaces via the stacking of exfoliated several-micron-sized graphene, MoS2, and WSe2 monolayers using Raman thermometry, suggesting great potential for controlling heat flow by using the interfaces of vdW heterostructures [2]. However, there has been no systematic study on how homo- and hetero-interfaces with or without lattice matches influence heat flow. Therefore, a systematic study on the thermal conductance (G) of vdW structures with various comparable interfaces is urgently required to understand how homo- and heterointerfaces and the coupling effect influence thermal transport. Here we demonstrate the thermal conductance across four layered (4L) homo- and hetero-structured MoS2 and MoSe2 using time-domain thermo-reflectance method. Compared with the thermal conductance of homo-structured sample, the thermal conductance across hetero-structured 4L sample was significantly reduced, indicating that hetero-interface is an efficient way to reduce thermal conductance [3]. In addition, we have combined electrochemical gating method and TDTR measurements, and employed our technique on the 4L sample and investigated how the intercalation can influence the thermal conductance on the 4L sample. In this presentation, I would like to discuss our recent results.

Molecular dynamics simulations on shock induced plasticity and stacking fault of coherent {001} Ni/Ni3Al laminate composite

Using molecular dynamics (MD) simulations, the shock induced plasticity and stacking fault of coherent {001} Ni/Ni3Al laminate composite based on the interfaces were studied in this work. Firstly, the influences of interfaces on shock wave propagation are studied. The results reveal “reflection-unloading” at the Ni-Ni3Al, but “reflection-loading” at the Ni3Al-Ni. Strong discontinuities of shock wave stress at the interfaces also are revealed, the discontinuities are correlated to the impedance mismatch and elastic-plastic properties of the two materials adjacent to the interfaces. Secondly, the dislocation activities and stacking faults in the Ni3Al layer were obtained through shock simulation of multilayer target, and the interfacial lattice mismatch was studied at the atomic level, the abnormal arrangement of the atomic layers on both sides of the interface was found. Thirdly, since atoms in different directions are not activated at the same time during shock, resulting in dislocation decomposition in two steps. The dislocation sheet composed of Lomer-Cottrell locks and Hirth locks can act as a barriers to dislocation transmission. The phase transition process induced by the shock was also studied. The result showed that the region where the dislocation lines are located first changed from fcc to bcc. With the slip extension of Shockley dislocations, bcc to hcp continued to occur in the stacking fault region.

Formation and Effect of Deposited Thin TiO2 Layer With Compressive Strain and Oxygen Vacancies on GaAs (001) Substrate

The integration of metal oxides and GaAs semiconductors is quite attractive for its potential applications, but interfacial diffusion and lattice mismatch usually cause huge challenges toward achieving high-performance electronic devices. In this article, we reported a thin layer of epitaxial TiO2 (110) on a GaAs (001) substrate with significant compressive strain, lattice distortion, and oxygen vacancies, where the oxygen vacancies proved to be the critical factor to induce the compressive strain and lattice distortion. In this case, the lattice mismatches between this compressed TiO2 (110) and GaAs (001) surface were calculated to be as small as 1.3 and 0.24% along the [110] and [001] orientations of TiO2, respectively. Further, no Ga-oxides or As-oxides were found at the interface, indicating that the TiO2 layer inhibited the diffusion of Ga and As atoms effectively. In summary, TiO2 film can be grown epitaxially on GaAs (001) substrates with non-negligible compressive strain, lattice distortion, oxygen vacancies, and a high-quality interface. This study also provides an approach to integrate different functional oxides on TiO2-buffered GaAs for various GaAs-based electronic devices with higher reliability and performance.

Pre-Buried Additive for Cross-Layer Modification in Flexible Perovskite Solar Cells with Efficiency Exceeding 22.

Halide perovskites have shown superior potentials in flexible photovoltaics due to their soft and high power-to-weight nature. However, interfacial residual stress and lattice mismatch due to the large deformation of flexible substrates have greatly limited the performance of flexible perovskite solar cells (F-PSCs). Here, ammonium formate (HCOONH4 ) is used as a pre-buried additive in electron transport layer (ETL) to realize a bottom-up infiltration process for an in situ, integral modification of ETL, perovskite layer, and their interface. The HCOONH4 treatment leads to an enhanced electron extraction in ETL, relaxed residual strain and micro-strain in perovskite film, along with reduced defect densities within these layers. As a result, a top power conversion efficiency of 22.37% and a certified 21.9% on F-PSCs are achieved, representing the highest performance reported so far. This work links the critical connection between multilayer mechanics/defect profiles of ETL-perovskite structure and device performance, thus providing meaningful scientific direction to further narrowing the efficiency gap between F-PSCs and rigid-substrate counterparts.

Elimination of Interfacial Lattice Mismatch and Detrimental Reaction by Self‐Assembled Layer Dual‐Passivation for Efficient and Stable Inverted Perovskite Solar Cells

AbstractInterfacial lattice mismatch and adverse reaction are the key issues hindering the development of nickel oxide (NiOx)‐based inverted perovskite solar cells (PVSCs). Herein, a p‐chlorobenzenesulfonic acid (CBSA) self‐assembled small‐molecule (SASM) is adopted to anchor NiOx and perovskite crystals to endow dual‐passivation. The chlorine terminal of SASMs can provide growth sites for perovskite, leading to interfacial strain release. Meanwhile, the sulfonic acid group from SASMs can passivate surface defects of NiOx, conducive to charge carrier extraction. In addition, the self‐assembled layer inhibits the adverse interfacial reaction by preventing NiOx contact with perovskite. Therefore, the NiOx/CBSA‐based PVSCs obtain a champion power conversion efficiency (PCE) of 21.8%. Of particular note, the unencapsulated devices can retain above 80% of their initial PCE values after storage in a nitrogen atmosphere for 3000 h, in air with a relative humidity of 50–70% for 1000 h, and heating at 85 °C for 800 h, respectively.

Control of Thermal Conductance across Vertically Stacked Two-Dimensional van der Waals Materials via Interfacial Engineering.

A comprehensive understanding of the roles of various nanointerfaces in thermal transport is of critical significance but remains challenging. A two-dimensional van der Waals (vdW) heterostructure with tunable interface lattice mismatch provides an ideal platform to explore the correlation between thermal properties and nanointerfaces and achieve controllable tuning of heat flow. Here, we demonstrate that interfacial engineering is an efficient strategy to tune thermal transport via systematic investigation of the thermal conductance (G) across a series of large-area four-layer stacked vdW materials using an improved polyethylene glycol-assisted time-domain thermoreflectance method. Owing to its rich interfacial mismatch and weak interfacial coupling, the vertically stacked MoSe2-MoS2-MoSe2-MoS2 heterostructure demonstrates the lowest G of 1.5 MW m-2 K-1 among all vdW structures. A roadmap to tune G via homointerfacial mismatch, interfacial coupling, and heterointerfacial mismatch is further demonstrated for thermal tuning. Our work reveals the roles of various interfacial effects on heat flow and highlights the importance of the interfacial mismatch and coupling effects in thermal transport. The design principle is also promising for application in other areas, such as the electrical tuning of energy storage and conversion and the thermoelectricity tuning of thermoelectronics.

Controlled Growth of Hierarchical Bi2Se3/CdSe‐Au Nanorods with Optimized Photothermal Conversion and Demonstrations in Photothermal Therapy

AbstractIntegrating multiple mechanisms to maximize photothermal conversion efficiency is a significant strategy but remains challenging to construct therapeutic agents toward photothermal tumor treatment. Here, an approach to synthesize asymmetric Bi2Se3/CdSe‐Au hierarchical nanorods with excellent photothermal conversion is reported. Ag wetting‐layer is firstly grown to help overcome the interfacial lattice mismatch and promote the site‐selective growth of AgCdSe onto one end or side surface of Au nanorods. Subsequently, extraction of Ag+ ions out of lattice is observed during cation exchange reaction and epitaxial growth of Bi2Se3 shell. Bi2Se3/CdSe heterojunction with type‐II band alignment is formed and located at the plasmonic hotspots of Au nanorods, which experiences enhanced light absorption and accelerates the charge separation of photo‐excited carriers. Under excitation of near‐infrared 808 nm laser, the matchstick‐like Bi2Se3/CdSe‐Au nanorods show an excellent photothermal conversion, with 4.3 times temperature increment (ΔT) than that of bare Au nanorods. Moreover, in vitro and in vivo experiments verify them as excellent photothermal therapeutic agents.

Nanodomain patterns in ultra-tetragonal lead titanate (PbTiO3)

Very recently, the discovery of ultra-tetragonal PbTiO3 thin films was reported [Zhang et al., Science, 361, 494 (2018)], in which the switchable out-of-plane polarization was seen to be almost twice that of any previously known ferroelectric. To understand more about this system and to explore features that might contribute to these remarkable functional properties, we have mapped the polarization microstructure on the nanoscale using piezoresponse force microscopy (PFM) (scan sizes less than 700 nm). Our study reveals an extremely fine-scale pattern, with individual domains typically 20–50 nm in size, all of which exhibit both out-of-plane and in-plane polarization components. In-plane vector PFM polarization maps show strongly charged quadrant-lattice domain arrays with fourfold domain wall junctions (vertices). The existence of fourfold vertices in ferroelectrics is rare but can be explained via the “Ashkin-Teller” model, in which very large interfacial lattice mismatch strains and local electric fields play a role. This ultra-tetragonal ferroelectric system was expected to contain straightforward c-axis out-of-plane 180° domains, but instead shows an extremely rich nanoscale domain tiling pattern; these unusual nanodomains may be important in facilitating the extremely high switchable polarization values previously reported.

Effect of Al-V-B grain refiner on refining aluminium alloys: estimation from ab initio calculations

Ab initio calculations based on density functional theory were conducted to estimate the effect of Al-V-B grain refiner on refining aluminium alloys. Al-Ti-B, an industrial grain refiner, was investigated simultaneously to act as a reference. Six interfaces formed by (111) Al with (112) Al3V(Ti), V(Ti)-terminated (0001) V(Ti)B2 and B-terminated (0001) V(Ti)B2 were studied in detail. Interfacial lattice mismatch, work of adhesion and stain-free interfacial energy were calculated for each of these interfaces, respectively. Analysis of interfacial energy demonstrated that Al3V overlayers are less likely to form on the surface of (0001) VB2 surface compared with the Ti-related system. By combination of interfacial lattice mismatch and work of adhesion, it was concluded that VB2 is much more potent in refining aluminium alloys than TiB2, while Al3V is much weaker in nucleant potency compared with Al3Ti.

The role of oxygen vacancies on Pt/NaInO2 catalyst in improving formaldehyde oxidation at ambient condition

In order to gain an understanding of the role of oxygen vacancy in enhancing gaseous formaldehyde oxidation, mesoporous NaInO2 compounds were synthesized with different amount of oxygen vacancies while maintaining similar BET surface area. The NaInO2 compounds were designed as the support for platinum (Pt) nanoparticles loaded catalytic oxidation, exhibiting the best performance with a formaldehyde degradation ratio of 96.11%. Interfacial lattice mismatch was considered as the cause of oxygen vacancies and simulated by density functional theory (DFT). Special bridged bidentate species were observed in Pt/NaInO2 with oxygen vacancies during HCHO oxidation using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Based on our observations we propose, for the first time, that superoxide (O2−) is the effective species that oxidizes formaldehyde, while oxygen vacancies provide pathways to transport O2− from Pt nanoparticles to support surface thus offering more active sites for the reaction to take place.

Seeded Heteroepitaxial Growth of Crystallizable Collagen Triple Helices: Engineering Multifunctional Two-Dimensional Core–Shell Nanostructures

Engineering free-standing 2D nanomaterials with compositional, spatial, and functional control across size regimes from the nano- to mesoscale represents a significant challenge. Herein, we demonstrate a straightforward strategy for the thermodynamically controlled fabrication of multicomponent sectored nanosheets in which each sector can be chemically and spatially addressed independently and orthogonally. Collagen triple helices, comprising collagen-mimetic peptides (CMPs), are employed as molecularly programmable crystallizable units. Modulating their thermodynamic stability affords the controlled synthesis of 2D core-shell nanostructures via thermally driven heteroepitaxial growth. Structural information, gathered from SAXS and cryo-TEM, reveals that the distinct peptide domains maintain their intrinsic lattice structure and illuminates various mechanisms employed by CMP triple helices to alleviate the elastic strain associated with the interfacial lattice mismatch. Finally, we demonstrate that different sectors of the sheet surface can be selectively functionalized using bioorthogonal conjugation chemistry. Altogether, we establish a robust platform for constructing multifunctional 2D nanoarchitectures in which one can systematically program their compositional, spatial, and functional properties, which is a significant step toward their deployment into functional nanoscale devices.

Atomic layer deposition of SnOx onto mesoporous, nanocrsytalline TiO2 and SnO2 thin films

The atomic layer deposition of tetrakisdimethylamidotin(IV) and water on mesoporous thin films of nanocrystalline anatase TiO2, rutile SnO2, and rutile TiO2 was investigated. For all deposition conditions tested, highly oxygen deficient coatings were created; purge time and temperature were found to be the factors most affecting the stoichiometry. Long purge times and a 180 °C deposition temperature produced the least oxygen-deficient coatings. Under these conditions, crystalline rutile SnO2 deposited epitaxially on rutile SnO2 substrates. As-deposited SnOx coatings on anatase TiO2 and rutile TiO2 substrates were mostly smooth and amorphous, but contained various SnOx nanocrystals along the surface of the substrate nanocrystals. Post-deposition annealing at 450–500 °C results in the delamination of the SnOx shells from the nanocrystalline cores concomitant with the formation of SnO2 nanocrystals that are speckled along the periphery of the substrate nanocrystals. On anatase substrates the SnOx coatings form the rare orthorhombic phase of SnO2 after annealing, while rutile SnO2 nanocrystals are formed on rutile TiO2 substrates. The emergence of orthorhombic SnO2 on anatase TiO2 substrates and rutile SnO2 on rutile TiO2 substrates is rationalized by interfacial lattice mismatch and strain between the core and shell materials.

Factors influencing thermal transport across graphene/metal interfaces with van der Waals interactions.

We implement non-equilibrium Green's function (NEGF) calculations to investigate thermal transport across graphene/metal interfaces with interlayer van der Waals interactions to understand the factors influencing thermal conductance across the interface. It is found that interfaces with a smaller interfacial lattice mismatch, lighter metal substrate and stronger interfacial bonding strength will show better interfacial thermal transport abilities. Strain induced by the interfacial lattice mismatch in graphene is the key factor for the decrease of interfacial phonon transmission in the main frequency range of metals, which finally results in a decrease of interfacial thermal conductance. A comprehensive interfacial influencing factor is proposed combining the factors of graphene density, metal density and interfacial binding energy to realize the prediction of interfacial thermal conductance across the graphene/metal interface. The results are hoped to promote the understanding of the thermal transport mechanism and design of graphene based 2D/3D materials interfaces.

Effect of buffer layer structure on the structural properties of GaAs epitaxial layers grown on GaP substrates

Three-μm-thick GaAs layers were grown on 2°-off (1 0 0) GaP substrates by employing various buffer layer structures, which consist of GaAsP- and InGaAs-based ternary compound semiconductors. To confirm the effects, we altered the layer thickness, the interface lattice mismatch, and number of the layers in the buffer layer structure, and also a superlattice structure was employed in some of the buffer layers. The lattice constants of the layers were controlled by changing the As/P and In/Ga compound ratios. The crystal properties of the grown GaAs layers were characterized with X-ray diffraction, photoluminescence, and etch pit density observations. The effect of the buffer layer structure on the crystallographic character of the GaAs layers was analyzed by introducing a parameter that is a function of the thickness and interface lattice mismatch of each layer in the buffer layer structure. The results suggest that the GaAs layer is relatively relaxed but contains a greater number of dislocations for smaller layer thicknesses and greater lattice mismatches in the buffer layer structure, while the GaAs layer has a smaller number of dislocations but a rather deformed lattice structure for larger layer thicknesses and smaller lattice mismatches. Our parameter is useful for developing design principles of buffer layer structures.