Small Lattice Mismatch Research Articles

A novel design of SiH/CeO2(111) van der Waals type-II heterojunction for water splitting.

Searching for economical low-dimensional materials to construct the highly efficient type-II heterojunction photocatalysts for splitting water into hydrogen is very strategic. In this study, using the first-principles calculations, we construct a novel SiH/CeO2(111) type-II heterojunction with a very small lattice mismatch of less than 1%. Based on AIMD simulation and phonon dispersion calculations, the SiH/CeO2(111) heterojunction reveals sufficient stability, and is easy to synthesize. Due to the vdW interaction between SiH and CeO2(111) components, electron and hole accumulation regions form at the heterojunction interface, which is very conducive to the separation of photoexcited electron-hole pairs. Besides, the SiH/CeO2(111) heterojunction has good visible light response, and even a strong absorption peak of up to 8.7 × 105 cm-1 in the high-energy visible region. More importantly, the SiH/CeO2(111) heterojunction exhibits good OER and HER performance because its oxidation and reduction potentials well meet the requirements of water splitting. Consequently, SiH/CeO2(111) is a potential photocatalyst for splitting water to hydrogen.

Intermetallic compounds: Formation mechanism and effects on the mechanical properties of friction stir lap welded dissimilar joints of magnesium and aluminum alloys

The formation mechanism of intermetallic compounds (IMCs) during dissimilar friction stir lap welding (FSLW) of Mg and Al alloys, as well as the effects of IMCs on the shearing properties of FSLW joints, are still ambiguous and require clarification. In this study, FSLW dissimilar joints of AZ31 Mg alloys and Al–Zn–Cu–Mg alloys are investigated to clarify the formation mechanism of IMCs and their effect on the joint strength. Microstructures and shearing properties of the joints are characterized. The results showed that new phases of Al12Mg17, Al3Mg2, MgZn2, and Al2Cu are generated at the welding interface, and the original Cu2Mg phase is dissolved into the Al matrix during FSLW. Furthermore, the formation sequence of IMCs in the same binary system is affected by the effective enthalpy of formation, while it is related to the element concentrations in different binary systems. The order of IMC formation is as follows: Al12Mg17, Al3Mg2, MgZn2, and Al2Cu. During shearing deformation of the FSLW joint, the crack is initiated at the hook zone in the retreading side and propagated along the welding interface, where IMCs aggregate and the strain concentrates. The small lattice mismatch between the IMCs and the matrix improves their interfacial bonding strength, which increases in the following order: Al12Mg17 and Al, Al12Mg17 and Mg, Al3Mg2 and Al, Al3Mg2 and Mg, MgZn2 and Al, MgZn2 and Mg, Al2Cu and Al, Al2Cu and Mg, Cu2Mg and Mg, and Cu2Mg and Al.

MoS2-on-AlN Enables High-Performance MoS2 Field-Effect Transistors through Strain Engineering.

Molybdenum disulfide (MoS2) has substantial application prospects in the field of electronic devices. The fabrication of devices of excellent quality based on MoS2 films is an important research direction. In this study, based on the atomic layer deposition technique, large-area MoS2 films were grown, and top-gate MoS2-based field-effect transistor arrays were fabricated on four substrates (AlN, GaN, sapphire, and SiO2). It was found that the interface defects that were introduced by lattice mismatch and roughness of the growth substrate could cause an exponential (102) drop in mobility. Because of the small lattice mismatch and excellent surface quality, transistors on the AlN substrate have shown an enhanced mobility (10.45 cm2 V-1 s-1) compared to transistors on the other substrates. This study proves that the AlN substrate is a superior substrate for large-area and high-performance MoS2 film synthesis. This result can also be applied in higher-level microelectronic systems, such as in digital logic circuit design.

Impact of germanium doping on the mechanical strength of low oxygen concentration Czochralski silicon wafers

ABSTRACT The lack of interstitial oxygen for locking and pinning of dislocations has made the ultra-high resistivity silicon wafers for radio frequency applications prone to slip and warpage during electronic device manufacturing. In this work, we investigate the role of germanium doping on the dislocation mobility in very low oxygen concentration, Czochralski-grown silicon wafers. Mechanical bending tests are used to study the average dislocation velocity and unlocking stress for germanium-doped, nitrogen-doped, and undoped silicon wafers with similar oxygen levels. In contrast to the previously reported effect of germanium on retarding dislocation motion in high oxygen concentration silicon wafers, we find that germanium by itself at a high concentration (7–9 × 1019 atoms/cm3) does not lock dislocations by solid solution strengthening, nor does it appreciably change the oxygen dislocation locking behaviour in silicon. The lack of dislocation pinning and locking effects by germanium in silicon is explained from the small lattice mismatch (4%) and low chemical interactions of substitutional Ge atoms with silicon comparing to interstitial impurities, such as oxygen and nitrogen.

Interfacial modulation on single-crystalline aluminum films grown on GaAs by ErAs insertion

Single-crystalline aluminum (Al) films are grown on GaAs (100) substrates by molecular beam epitaxy. The Al/GaAs interface is modified by ErAs insertion, and the Al quality is further improved. The full-width at half-maximum for Al (111) diffraction peak is 0.06°, and the root-mean-square surface roughness is 0.69 nm. In addition, the Al growth orientation can be tuned by ErAs insertion, which is attributed to the smaller lattice mismatch and higher surface symmetry provided by ErAs. The interfacial interdiffusion between Al and GaAs can be suppressed by ErAs as well. The sharp interfaces as well as the high quality of Al are confirmed by cross-sectional transmission electron microscopy studies. The lattice arrangements of the two stable Al/GaAs and Al/ErAs structures are proposed to show that the lattice matching and the interfacial bonding are two main driving forces.

Atomic-scale silicidation of low resistivity Ni-Si system through in-situ TEM investigation

Nickel silicide has many advantages, such as low resistivity and low formation temperature; therefore, it has been widely used in the fields of solar cells, transistors and complementary metal-oxidesemiconductor (CMOS) devices. To obtain high-quality nickel-silicide thin film, solid-state reaction is a convenient and efficient fabrication method. For better understanding of the dynamic formation mechanism, we used in-situ transmission electron microscopy (TEM) to record the diffusion behavior during the heating process. In this work, three-steps annealing process to synthesize different nickel silicides corresponding to the various formation temperatures were investigated systematically. At 250 °C, the product of the first-step annealing was inverted-triangle Ni2Si, embedded in the Si substrate. Then, well-distributed NiSi thin film was synthesized, having the lowest resistivity among Ni-Si system at 400 °C. Finally, NiSi2, a Si-rich product, would form during the third-step annealing at 600 °C. NiSi2 product and Si substrate have small lattice mismatch; thus, the epitaxial relationship would be observed. We provide the evidence of diffusion behaviors and structural identification of Ni-Si system. Furthermore, these results are beneficial for the formation of specific nickel silicides, which is expected to optimize the fabrication of microelectronics.

Templated epitaxy of TiO2(B) on a perovskite

The bronze-phase TiO2(B) has an open atomic framework that makes it a good candidate for applications in photochemical and electrochemical conversion of energy. However, the synthesis of bronze-phase TiO2(B) thin films on perovskite substrates, such as SrTiO3 (STO), which is one of the most conventional versatile substrates for oxide epitaxy, has been extremely challenging owing to the preferential formation of the anatase TiO2 over TiO2(B). The main reason is that the anatase TiO2 has not only a smaller lattice mismatch than TiO2(B), but also a better structural symmetry match when grown on STO. Here, we demonstrate a way to circumventing this problem by using a VO2(B) buffer layer, yielding the growth of a high-quality single crystalline TiO2(B) film on a (001)-oriented STO substrate. From the resulting TiO2(B) film, we found that this film has a large optical bandgap of ∼3.6 eV, which is close to the known theoretical value, the largest among TiO2 polymorphs, and useful for developing high-power energy devices.

Influence of Au doping concentration on structural and optical properties of ZnO:Au films fabricated through magnetron sputtering

Gold-doped ZnO (ZnO:Au) films were prepared by radio frequency (RF) magnetron sputtering and post-annealing methods. The doping concentration of Au was adjusted by controlling the area of Au wires in the sputtering area of ZnO target. It was found that the as-grown ZnO:Au films had better crystal quality than as-grown ZnO films due to small lattice mismatch between Au and ZnO. The solid solubility of Au in ZnO films was found to be 1.00 at.% for the 350oC-annealed ZnO:Au, and it decreased with the rising annealing temperature. XPS results revealed that Au exited in the ZnO:Au with chemical states of metallic Au and positive univalance Au substituted for Zn (AuZn1+). Au concentration-dependent photoluminescence (PL) quenching was observed for ZnO:Au films. Low-temperature PL measurements and schottky model analysis for ZnO:Au films indicated that AuZn1+ and its associated non-radiative recombination center could cause the PL quenching, moreover, the degree of PL quenching was related to the amount of metallic Au in the films.

Cubic-cubic perovskite quantum dots/PbS mixed dimensional materials for highly efficient CO2 reduction

The mixed dimensional materials, especially 0D/2D mixed materials have been regarded as outstanding candidates for photodetector and photovoltaic devices, while have rarely been utilized for photocatalytic reactions including photosynthesis, water splitting and CO2 reduction. In this work, a mixed dimensional material composited by cubic 0D CsPbBr3 perovskite quantum dots (PQDs) and cubic 2D-PbS is prepared for reduction of CO2 to fuels. The PQDs, as light-harvesting components, are homogeneously grown on the 2D-PbS nano-substrate which is employed for electron transport. The same cubic crystal structures and rather small lattice mismatch (Δd/dCsPbBr3=0.84%) between CsPbBr3 and PbS guarantee easy in-situ growth of PQDs on PbS nano-substrate and rapid electron migration. As a result, an improved photocurrent is obtained based on the combination of the advantages of 0D and 2D materials. In addition, the size of PQDs synthesized on the 2D-PbS is dramatically reduced in comparison to that synthesized in solvent, thus endowing them wider bandgap and greater potential enough for the photocatalytic reactions. This PQDs/2D-PbS mixed dimensional materials exhibit outstanding selectivity (over 99%) and activity (yield 81.6 and 52.7 μmol g−1) for reducing CO2 to CO and CH4, from which the CH4 yielded is ca. 9-fold than that of pure PQDs.

Integration of Single Oriented Oxide Superlattices on Silicon Using Various Template Techniques.

To benefit from the diverse functionalities of perovskite oxides in silicon-based complementary metal oxide semiconductor (CMOS) technology, integrating oxides into a silicon platform has become one of the major tasks for oxide research. Using the deposition of LaMnO3/SrTiO3 (STO) superlattices (SLs) as a case study, we demonstrate that (001) single oriented oxide SLs can be integrated on Si using various template techniques, including a single-layer buffer of STO prepared by molecular beam epitaxy (MBE) and pulsed laser deposition, a multilayer buffer of Y-stabilized zirconia/CeO2/LaNiO3/STO, and STO-coated two-dimensional nanosheets of Ca2Nb3O10 (CNO) and reduced graphene oxide. The textured SL grown on STO-coated CNO nanosheets shows the highest crystallinity, owing to the small lattice mismatch between CNO and STO as well as less clamping from a Si substrate. The epitaxial SL grown on STO buffer prepared by MBE suffers the largest thermal strain, giving rise to a strongly suppressed saturation magnetization but an enhanced coercive field, as compared to the reference SL grown on an STO single crystal. These optional template techniques used for integrating oxides on Si are of significance to fulfill practical applications of oxide films in different fields.

Tetragonal and uniaxial strains in pristine and doped half-Heusler AuMnSn alloy

Using the first-principles density functional theory calculations, we examined the effect of tetragonal strain (t-strain) and uniaxial strain (u-strain) on the electronic and magnetic properties of pristine and Sb-doped half-Heusler AuMnSn alloy. Our result shows that ferromagnetic half-Heusler AuMnSn (AMS) exhibits half metallic ferromagnetism (HMF) only from −8% to −10% u-strain. On the other hand, the Sb-doped half-Heusler AuMnSn (AMSS) exhibits HMF from −2% to 4% t-strain and from −4% to 2% u-strain while the width of the minority spin band gap remains unaltered. We also observed changes in the magnetic moments as the applied strain changes from compressive to tensile strain. Our result suggests the possibility of epitaxial growth of thin-film of half-metallic AMSS on a semiconductor substrate, even with a small lattice mismatch.

Ultraviolet optoelectronic devices based on AlGaN-SiC platform: Towards monolithic photonics integration system

Rapid advancement of wide-bandgap AlGaN semiconductor materials offers tremendous opportunities in the field of ultraviolet (UV) optoelectronics for a wide range of advanced applications. Additionally, SiC has large bandgap and excellent material properties, also making itself a suitable material for UV photodetection. More importantly, high-quality AlGaN alloys can be epitaxially grown on SiC substrates because of the very small lattice mismatch between them (less than 1%), which enables a possible monolithic integration of those two materials and allows us to take advantage of their material and physical properties to realize high-performance UV optoelectronics and eventually the integrated UV photonics systems. Herein, we review the recent progress in the development of UV optoelectronics based on AlGaN-SiC platform, mainly focusing on: (1) the growth strategies and material characterizations of AlGaN epilayers on SiC; (2) the fabrication and performance evaluation of UV optoelectronic devices built on the platform, including UV LEDs/lasers and UV photodetectors. Thereafter, we briefly discuss the initial efforts in the pursuit of monolithic integration of those UV optoelectronic devices. Finally, the challenges and potential advances associated with individual UV optoelectronic components as well as UV integrated photonics system on the prosperous AlGaN-SiC platform are outlined, providing insights and perspectives for possible device- and system-level innovation in future. The recent advances in the development of ultraviolet light emitters (LED, lasers) and photodetectors on the AlGaN-SiC platform are reviewed. By taking advantage of the extreme small lattice mismatch between the two wide bandgap semiconductors - AlGaN and SiC, together with their compatible physical and material properties, a monolithic ultraviolet photonics integration system can be built on the AlGaN-SiC platform. • AlGaN and SiC are recognized as two most promising wide bandgap semiconductors for UV optoelectronics applications. • AlGaN and SiC can be integrated to form AlGaN-SiC platform for the monolithic UV photonics integration system. • This review provides a comprehensive summary of the recent advancement of UV optoelectronics based on the AlGaN-SiC platform. • The outlooks and challenges in device- and system-level innovation of UV optoelectronics on AlGaN-SiC platform are discussed.

Effects of Al on the precipitation of B2 Cu-rich particles in Fe–Cu ferritic alloy: Experimental and theoretical study

The fundamental mechanism of the precipitation kinetics and stability of nano-sized Cu-precipitates are crucial to the development of ultra-high strength low carbon ferritic steels. The effects of Al on the formation and stabilization of B2 Cu-rich precipitates in Fe–Cu based steels were studied using experimental method combining the first-principles calculations and molecular dynamics simulations. With increasing aging time, the Cu-rich particles with B2 structure precipitated in the ferritic matrix with cube-on-cube and coherent relationship, and Al addition leads to a relatively higher precipitation hardening effect. The first-principle calculations indicate that the metastable B2 FeCu phase has relatively higher mixing energy and small lattice mismatch with the ferritic matrix, and the addition of Al leads to the lower mixing energy of B2 precipitates and the B2 structure Fe(CuAl) compound possesses more smaller lattice mismatch with ferritic matrix. To better understanding the effect of Al on the evolution process of Cu-rich particles, it is essential to investigate the migration and clustering process of Cu atoms during the isothermal tempering at the atomic scale. To this end, we first developed a new interatomic potential of Fe–Cu–Al using the force-matching method based on ab initio calculations, then the formation and evolution of Cu clusters in the Fe–Cu–Al ternary were studied using molecular dynamics method with the new EAM potential. The simulation results indicate that Al addition promotes the clustering of Cu atoms due to the attractive interaction between Cu and Al atoms which enhances nucleation rate of the Cu-rich precipitates.

Nanoscale Engineering of Polymorphism in Cu2Se-Based Composites.

Crystal polymorphism selection during synthesis is extremely challenging. However, promoting the formation of a specific metastable polymorph enables modulation of the functional properties of phase-change materials through alteration of the relative abundance of various polymorphs. Here, we demonstrate the stabilization of the superionic β-Cu2Se phase under ambient conditions and the direct control over the relative ratio between the α-Cu2Se and β-Cu2Se polymorphs in (x)CuGaSe2/(1-x)Cu2Se composites using CuGaSe2 nanoseeds. We found that the small lattice mismatch between β-Cu2Se (cubic) and the ab plane of tetragonal CuGaSe2 nanoseeds promotes the formation of low-energy coherent CuGaSe2/β-Cu2Se interfaces, leading to preferential stabilization of β-Cu2Se. Astonishingly, the hierarchical microstructure of the resulting composites enables a remarkable decoupling of charge and heat transport, which is manifested by a breakdown of the Wiedemann-Franz law.

First-principles investigation on the bonding mechanism between graphyne and the (111) surface of Cu, Ag and Au

A series of adsorption configurations with less than 2% lattice mismatch of graphyne adsorbed at different sites on the (111) surface of Cu, Ag and Au are obtained by a systematic rotation of the graphyne sheet. Besides, all the non-equivalent adsorption configurations of graphyne on metal surfaces are obtained by a structure screening method based on the translation of graphyne sheet and the calculation of the atomic overlapping area at the interface. The most stable adsorption configuration of graphyne on each metal surface is obtained by optimizing these non-equivalent adsorption configurations with a small computational cost. Then the adsorption energy, differential charge density, and partial density of states are calculated using a first-principles method in order to investigate the bonding mechanism between graphyne and the (111) surface of these three metals. The results show that the adsorption types of graphyne on the (111) surface of Cu, Ag and Au are chemisorption, chemical and physical mixed adsorption and pure physisorption, respectively. In detail, when a metal (Cu or Ag) atom is below a C atom, the C-pz orbital will interact with the s and d orbitals of the metal atom and then form a covalent bond with the metal atom. In particular, when a Cu atom lies below two carbon atoms joined by an acetylene bond, the s and d orbitals of the Cu atom hybridize with each other to form two identical sd orbitals and then form two covalent bonds with two C-pz orbitals. Our results not only provide a structure screening method for obtaining the most stable configuration with a very small lattice mismatch but also reveal the interface bonding mechanism between graphyne and metals, which will provide help for the preparation of graphyne and its application in composite materials.

Understanding the photocatalytic mechanisms of the BiOI/Bi2MoO6 and BiOCl/Bi2MoO6 heterostructures: First-principles study

First-principles calculations are used to explore the interfacial structures and photocatalytic mechanisms of the BiOI/Bi2MoO6 and BiOCl/Bi2MoO6 heterostructures. We find that both the BiOI/Bi2MoO6 and BiOCl/Bi2MoO6 composites have small lattice mismatch and good stability. The DOS analysis show that the BiOI/Bi2MoO6 belongs to type-II heterostructure, while the BiOCl/Bi2MoO6 composite is a type-I heterostructure. The analyses of the electron difference density and Mulliken charge suggest that charge transferring and redistribution occurs at the interfaces. Moreover, a built-in electric field directed from Bi2MoO6 to BiOI or BiOCl can be formed at the interfaces of the heterostructures. The band alignment and built-in electric field in the BiOI/Bi2MoO6 and BiOCl/Bi2MoO6 heterostructures can promote the separation of the photo-generated carriers, resulting in their improved photocatalytic activities compared with the single component. Our work may provide useful information for understanding the photocatalytic mechanisms of the BiOI/Bi2MoO6 and BiOCl/Bi2MoO6 heterostructures and developing more efficient heterostructure photocatalysts.

Low temperature deposition of high quality single crystalline AlN thin films on sapphire using highly oriented monolayer MoS2 as a buffer layer

AlN films are commonly deposited on sapphire substrates. However, the growth process usually requires high temperature and multi-steps to improve the film quality, due to the large lattice mismatch between the film and substrate. Here we demonstrate that using monolayer MoS2 grown on sapphire as a template, single crystalline AlN films with high crystallinity can be obtained at a much lower growth temperature using helicon sputtering system. The x-ray rocking curve of AlN films prepared on MoS2/sapphire template at 400 °C shows a full width at half maximum of 0.050°, showing a dramatic reduction compared to that of 0.803° for those grown directly on sapphire substrate. The estimated dislocation density of AlN on MoS2/sapphire is decreased to 7.7x107 cm−2, which is comparable to that prepared by metal-organic chemical vapor deposition at high temperatures. The small lattice mismatch between MoS2 and AlN, as well as the polar crystal of monolayer MoS2 apparently improve the quality of AlN films and facilitate AlN formed at lower temperature.

Nature of SrTiO3/TiO2 (anatase) heterostructure from hybrid density functional theory calculations.

In this work, we investigate the structural and electronic properties of the SrTiO3/TiO2 (anatase) heterostructure by means of hybrid density functional theory calculations. The work is motivated by several experiments that pointed to SrTiO3/TiO2 as a good system for photocatalytic applications, due to the small lattice mismatch between these two oxides and their favorable band alignment, leading to a type-II heterojunction, favoring the charge-carrier separation. The present results provide insights into the nature of the contact region and an estimation of the band offsets in the composite system. Our results are also compared with the available experimental values and with previous theoretical reports. The calculated offsets quantitatively agree with experimental measurements. In addition, we found significant interfacial effects that make the band offsets slightly increase with respect to those of the separated components. Last, we also discuss the role of point defects such as oxygen vacancies, finding that they do not remarkably affect the band alignment.

Deep insights into interface engineering by buffer layer for efficient perovskite solar cells: a first-principles study

Recent years have seen swift increase in the power conversion efficiency of perovskite solar cells (PSCs). Interface engineering is a promising route for further improving the performance of PSCs. Here we perform first-principles calculations to explore the effect of four candidate buffer materials (MACl, MAI, PbCl2 and PbI2) on the electronic structures of the interface between MAPbI3 absorber and TiO2. We find that MAX (X = Cl, I) as buffer layers will introduce a high electron barrier and enhance the electron-hole recombination. Additionally, MAX does not passivate the surface states well. The conduction band minimum of PbI2 is much lower than that of MAPbI3 absorber, which significantly limits the band bending of the absorber and open-circuit voltage of solar cells. On the other side, suitable bandedge energy level positions, small lattice mismatch with TiO2 surfaces, and excellent surface passivation make PbCl2 a promising buffer material for absorber/electron-transport-layer interface engineering in PSCs. Our results in this work thus provide deep understanding on the effects of interface engineering with a buffer layer, which shall be useful for improving the performance of PSCs and related optoelectronics.

First-principles investigation on the structural, electronic, vibrational and magnetic properties of the Co-substituted orthorhombic SrSnO3

Half-metallic (HM) ferromagnets provide an excellent possibility to design spintronic applications, yet a limited number of ferromagnetic materials are available which provide half-metallicity only in one spin channel. Moreover, often theoretically explored HM gap disappears when the material is synthesized experimentally due to small lattice mismatch happens which further hinders the advancement of spin-based applications. In this study, we have systematically explored the structural, electronic, formation, vibrational and magnetic properties of the Co-substituted SrSn(1−x)CoxO3wherex=0,0.25,0.5,0.75 (SSCO) alloys by employing first-principles methods with hybrid functionals. The Co-doping at the Sn-site induced the spontaneous magnetism into the non-magnetic orthorhombic SrSnO3 (SSO) perovskite material. Our calculations show that there are two ferromagnetic semiconductor materials found at the Co doping concentration of x = 0.5 & 0.75 with the large values of energy bandgaps. Furthermore, HM ferromagnetic ground state appears for the Co-doping of SSO at x = 0.25 which is vibrationally stable at the Г-point and has an integral magnetic moment of 1μB. The orthorhombic SrSn0.75Co0.25O3 alloy can be the prospective contender for the spintronic applications due to its thermodynamic and dynamic stability and large value of the HM gap (0.38 eV).