Role Of Lattice Mismatch Research Articles

On the van der Waals Epitaxy of Homo-/Heterostructures of Transition Metal Dichalcogenides.

Layered materials held together by weak van der Waals (vdW) interactions are a promising class of materials in the field of nanotechnology. Besides the potential for single layers, stacking of various vdW layers becomes even more promising since unique properties can hence be precisely engineered. The synthesis of stacked vdW layers, however, remains to date, hardly understood. Therefore, in this work, the vdW epitaxy of transition metal dichalcogenides (TMDs) on single-crystalline TMD templates is investigated in depth. It is demonstrated that the role of lattice mismatch is insignificant. More importantly is the role of surface energy, calculated using density functional theory, which plays an essential role in the activation energy for adatom diffusion, hence nucleation density. This in turn correlates with defect density since the stacking sequence in vdW epitaxy is generally poorly controlled. Moreover, the vapor pressure of the transition metal is also found to correlate with adatom diffusion. Consequently, the proposed study enables important and new insight in the vdW epitaxy of multilayer 2D homo-/heterostructures.

Enhanced thermal stability of electron transport layer-free perovskite solar cells via interface strain releasing

The thermal decomposition of perovskite films on ZnO surfaces is generally believed to originate from specific surface states of ZnO and the impact from the lattice mismatch between ZnO and perovskite films on this process has long been ignored. In this research, the role of lattice mismatch in the thermal degradation process of cesium-containing perovskite films on Al doped ZnO (AZO) is studied. A Ba(OH)2 buffer layer on the surface of AZO is employed to release the lattice mismatch and suppress the thermal degradation of perovskite films resulted from ZnO. Consequently, perovskite films with enhanced thermal stability and crystalline properties are obtained. Meanwhile, the Ba(OH)2 films efficiently passivate the surface trap states and reduce the vacuum level of the AZO surfaces. On this basis, electron transport layer-free perovskite solar cells yield the best efficiency of 18.18% and the thermal stability is obviously improved.

The role of lattice mismatch in heterogeneous nucleation of pure Al on Al2O3 single-crystal substrates with different termination planes

Al solidification on Al2O3 single-crystal substrates with (0001) and (11 $$ \overline{2} $$ 0) termination surfaces was performed to investigate whether the lattice mismatch can influence the heterogeneous nucleation when the same kind of substrate was employed. Nucleation undercooling was measured via differential scanning calorimeter to present the nucleation potency of Al on selected substrates. The measured results show that the lower undercooling was achieved in Al/Al2O3(11 $$ \overline{2} $$ 0), which means that the Al has a higher nucleation potency on Al2O3(11 $$ \overline{2} $$ 0) substrates. This is consistent with the calculated lattice misfit of Al/Al2O3(0001) and Al/Al2O3(11 $$ \overline{2} $$ 0) based on Bramfitt two-dimensional model because the lowest lattice misfit of Al/Al2O3(11 $$ \overline{2} $$ 0) is less than that of Al/Al2O3(0001). Moreover, the corresponding orientation relationships at the interface achieved by high-resolution transmission electron microscopy are almost the same with the predicted one from lattice misfit calculation. It is proved that the lattice misfit does affect the heterogeneous nucleation even the melted metals nucleated on the substrates made of the same kind of material.

Effect of Substrate on Memristive Switching of Pr<sub>0.7</sub>Ca<sub>0.3</sub>MnO<sub>3</sub>

. Perovskite manganite Pr0.7Ca0.3MnO3 (PCMO) thin film nanostructures were grown on different substrates by chemical solution deposition to investigate its electrical switching properties. Planar structures consisting of Ag/Pr0.7Ca0.3MnO3/Ag grown on SiO2, Si (100), LaAlO3 (100) and MgO (100) were characterized by grazing incidence X-ray diffraction, atomic force microscopy and electrical measurements. In each case, single PCMO phase formation and smooth surface morphology was confirmed by XRD and AFM, respectively. Four terminal current voltage characteristics of Ag/PCMO/Ag planar geometry exhibited a sharp transition from a low resistance state (LRS) to a high resistance state (HRS) with a high resistance switching ratios of the order of 950 for PCMO films grown on quartz was estimated at room temperature. High resistance switching ratios were found to depend on the substrate, suggesting a role of lattice mismatch for resistance switching. We have observed that higher mismatch lead to better resistance switching in this compound. The observed conduction characteristics provide direct evidence of substrate strain induce resistance switching in the Pr0.7Ca0.3MnO3.

Non-hexagonal ice at hexagonal surfaces: the role of lattice mismatch

It has long been known that ice nucleation usually proceeds heterogeneously on the surface of a foreign body. However, little is known at the microscopic level about which properties of a material determine its effectiveness at nucleating ice. This work focuses on the long standing, conceptually simple, view on the role of a good crystallographic match between bulk ice and the underlying substrate. We use grand canonical Monte Carlo to generate the first overlayer of water at the surface and find that the traditional view of heterogeneous nucleation does not adequately account for the array of structures that water may form at the surface. We find that, in order to describe the structures formed, a good match between the substrate and the nearest neighbour oxygen-oxygen distance is a better descriptor than a good match to the bulk ice lattice constant.

The role of lattice mismatch and kinetics in texture development: Co1−xNixSi2 thin films on Si(100)

Mixed Co1−xNixSi2 films (0≤x≤1) were grown by solid phase reaction of homogeneous Co1−xNix metal films, codeposited on Si(100). The texture of these films was contemplated using complementary experimental techniques: Rutherford backscattering and channeling spectrometry, x-ray pole figure measurements, and orientation imaging with electron backscattering diffraction. Based on the increasing Co1−xNixSi2 lattice parameter with increasing Ni concentration, a gradual, continuous improvement of the epitaxial quality of the film would be expected. The observed trend is significantly different. The epitaxial quality of the disilicide film indeed improves with increasing Ni concentration, but only up to 15% Ni. Moreover, the increasing epitaxial quality is due to a large volume fraction of (110)-oriented grains, instead of the anticipated (100) orientation. The most abundant texture component is not necessarily the one with the best in-plane match with the substrate, i.e., epitaxy, nor the one which assures the continuity of crystallographic planes across the plane of the interface, i.e., axiotaxy. Clearly, geometrical arguments alone cannot account for the observed large size and high volume fraction of (110)-oriented grains. On the other hand, we demonstrate that growth kinetics plays an important role in texture development and epitaxial growth during the solid phase reaction. Above 15% Ni, the epitaxial quality rapidly decreases and a polycrystalline film is formed for 40% Ni. This decrease is explained by a gradual shift of the disilicide nucleation site from the interface with the substrate to the surface of the thin film. For high Ni concentrations, i.e., ≥50% Ni, the (100) orientation dominates the thin-film texture, due to the growth of a NiSi2-rich film at the substrate interface. The changing nucleation site, due to this phase separation, and the differing growth kinetics can significantly alter the texture of ternary films. These two factors should be taken into consideration when implementing ternary alloys in devices, since their physical properties, stability, roughness, resistance, etc., depend critically on the texture of the films.

Growth of self-assembled (Zn)CdSe nanostructures on ZnSe by migration enhanced epitaxy

The growth of CdSe on ZnSe by migration enhanced epitaxy has been studied in situ by means of reflection high-energy electron diffraction (RHEED) and reflection anisotropy spectroscopy (RAS). The change from a streaky to a spotty RHEED pattern during CdSe deposition indicates the transition to a three-dimensional growth mode. This change is accompanied by a damping of RHEED intensity oscillations and a decrease in the overall intensity. RAS spectra show a pronounced change at a photon energy of about 2.2 eV with increasing CdSe thickness. To investigate the role of lattice mismatch, ternary ZnCdSe layers of different compositions have been grown. With decreasing Cd concentration the critical thickness for the growth mode transition increases and for Cd concentrations below 30% no growth mode transition could be observed.

A study of microstructures in ferroelectric Pb(Zr,Ti)O3 and KNbO3 thin films

Oxide ferroelectric thin films are receiving considerable attention due to their potential application in a wide range of devices including ferroelectric nonvolatile memory, dynamic random access memory (DRAMs) electrooptic phase modulators, and laser light source frequency doublers. However, the complexity of the oxide materials has resulted in a relatively slow development of structureprocessing property relationships. In this paper, we review in turn how the microstructures of epitaxial KNbO3 and Pb(Zr Ti)O3 have been significantly improved over a period of time.In the case of epitaxial KNbO3 on oxide single crystal substrates, defect types which need to be controlled include 221-type misorientations, low angle boundaries, multiposition domains, and inversion domains. Several of these defect types are controlled via the substrate surface preparation. The role of lattice mismatch between film and substrate on the formation of the low angle boundaries remains an unresolved issue. We are currently comparing via TEM the KNbO3(100) films on (100)MgO, (100)MGAl2O4, (100)NdGaO3 and (100)KTaO3 substrates; these substrates covering a range of values of lattice mismatch between 0.1% and 5.0%.

Growth of Epitaxial KNbO3 Thin Films

AbstractKNbO3 possesses high nonlinear optical coefficients making it a promising material for frequency conversion of infrared light into the visible wavelength range using integrated optical devices. While epitaxial thin films of KNbO3 have previously been grown using ion beam sputtering, defects (i.e. grain boundaries, domains, surface roughness) in these films resulted in high optical losses and no measurable in-plane birefringence. Previous films were grown on MgO substrates, which have a >4% lattice mismatch with KNbO3. In the work reported here, we have grown films on MgO, MgA12O4, NdGaO3, and KTaO3 to investigate the role of lattice mismatch on the resulting film quality. Films have also been grown with and without oxygen ion assistance. The orientations, morphologies, and defects in the films were examined using x-ray diffraction and AFM to determine their relationships to the growth conditions and substrate lattice mismatch.

Role of lattice mismatch and surface chemistry in the formation of epitaxial semiconductor-insulator interfaces.

The formation of ${\mathrm{SrF}}_{2}$/Si(111) and Ge/${\mathrm{CaF}}_{2}$/Si(111) interfaces is studied with photoemission and compared to previous results for the ${\mathrm{CaF}}_{2}$/Si(111) interface. The interface between ${\mathrm{SrF}}_{2}$ and Si(111) is found to be nonstoichiometric, similar to the interface between ${\mathrm{CaF}}_{2}$ and Si(111): the bonding is between Si and the cation, with a layer of fluorine missing at the interface. In the case of Ge growth on ${\mathrm{CaF}}_{2}$/Si(111), a variety of effects are noted: The ${\mathrm{CaF}}_{2}$/Si(111) valence-band offset is reduced by about 1 eV upon deposition of Ge at room temperature. The sticking coefficient of the Ge is significantly increased by preparing the ${\mathrm{CaF}}_{2}$ surface with electron bombardment to remove the top layer of fluorine. For both the irradiated and nonirradiated cases, annealing of thin room-temperature-deposited films resulted in Ge island formation.

The Role of Lattice Mismatch in Growth of Epitaxial Cubic Silicides on Silicon

AbstractThe lattice mismatch of single-crystal epitaxial nickel disilicide of orientation A and B on Si(111) has been measured in and out of the <111> growth direction using a four-circle X-ray diffractometer. In both directions the mismatch of the as-grown B-type film was found to be larger than for the A-type; after annealing at 800°C the mismatch of the A-type approached the value of the B-type, while the latter remained the same. This result suggests that the B-type NiSi2 film has relaxed more during growth than the A-type which can only be relieved by subsequent annealing. This difference in A- and B-type mismatch is correlated with TEM studies. The combined results provide evidence that the growth of either A or B orientation is strongly affected by the nature and density of the misfit dislocations.