In-plane Crystallographic Direction Research Articles

Determination of in-plane surface directions in scanning probe microscopy images.

We describe an approach to determine the in-plane crystallographic surface directions in scanning probe microscopy (SPM) images. This method is based on a one-time characterization of the SPM instrument with an appropriate test sample and is exemplified by the analysis of non-contact atomic force microscopy (NC-AFM) images on surfaces whose natural cleavage occurs along {111} planes. We introduce a two-dimensional rotation matrix relating the crystallographic surface directions known from an analysis of the macroscopic crystal to the directions in the NC-AFM images. The procedure takes into account rotations and mirror axes resulting from sample mounting, the SPM scanner rotation, the choice of scan direction, as well as data processing, storage, and display. We demonstrate the practicability of the approach by determining the [112̄] direction in topographic images of a CeO2(111) film grown on a Si(111) wafer and atomic resolution images of CaF2(111) with an instrument based on the beetle-type scanner.

Culling a Self-Assembled Quantum Dot as a Single-Photon Source Using X-ray Microscopy.

Epitaxially grown self-assembled semiconductor quantum dots (QDs) with atom-like optical properties have emerged as the best choice for single-photon sources required for the development of quantum technology and quantum networks. Nondestructive selection of a single QD having desired structural, compositional, and optical characteristics is essential to obtain noise-free, fully indistinguishable single or entangled photons from single-photon emitters. Here, we show that the structural orientations and local compositional inhomogeneities within a single QD and the surrounding wet layer can be probed in a screening fashion by scanning X-ray diffraction microscopy and X-ray fluorescence with a few tens of nanometers-sized synchrotron radiation beam. The presented measurement protocol can be used to cull the best single QD from the enormous number of self-assembled dots grown simultaneously. The obtained results show that the elemental composition and resultant strain profiles of a QD are sensitive to in-plane crystallographic directions. We also observe that lattice expansion after a certain composition-limit introduces shear strain within a QD, enabling the possibility of controlled chiral-QD formation. Nanoscale chirality and compositional anisotropy, contradictory to common assumptions, need to be incorporated into existing theoretical models to predict the optical properties of single-photon sources and to further tune the epitaxial growth process of self-assembled quantum structures.

Structural Properties of TaAs Weyl Semimetal Thin Films Grown by Molecular Beam Epitaxy on GaAs(001) Substrates

Thin crystalline layers of TaAs Weyl semimetal are grown by molecular beam epitaxy on GaAs(001) substrates. The (001) planes of the tetragonal TaAs lattice are parallel to the GaAs(001) substrate, but the corresponding in-plane crystallographic directions of the substrate and the layer are rotated by 45°. In spite of a substantial lattice mismatch (about 19%) between the GaAs(001) substrate and TaAs epilayer, no misfit dislocations are observed at the GaAs(001)/TaAs(001) interface. Only stacking fault defects in TaAs are detected by transmission electron microscopy. Thorough X-ray diffraction measurements and analysis of the in situ reflection high-energy electron diffraction images indicate that TaAs layers are fully relaxed already at the initial deposition stage. Atomic force microscopy imaging reveals the columnar structure of the layers, with lateral (parallel to the layer’s surface) columns about 20 nm wide and 200 nm long. Both X-ray diffraction and transmission electron microscopy measurements indicate that the columns share the same orientation and crystalline structure.

Giant anisotropic in-plane thermal conduction induced by Anomalous phonons in pentagonal PdSe2

In two-dimensional materials, different atomic stacking induces anisotropic atomic interactions and phonon dispersions, leading to the anisotropy of in-plane thermal transport. Here, we report an exceptional case in layered pentagonal PdSe2, where the bonds, force constants, and lattice constants are nearly-equal along the in-plane crystallographic axis directions, while the thermal conductivity is surprisingly much greater along b-axis than along a-axis with a ratio up to 1.8. Such strong anisotropy is not only unexpected in in-plane uniform structured materials, but also comparable to the record high in-plane anisotropic thermal conductivity in the nonuniform structured material reported to date (the ratio is ∼2.0 in TiS3). By combining the inelastic X-ray scattering measurement and the first-principles calculations, we attribute such high anisotropy to the low-energy phonons along a-axis, particularly their lower group velocities and “avoided-crossing” behavior. The different buckling structures between a- (zigzag-type) and b-axis (flat-type) are mainly responsible for such unique phonon dynamics properties of PdSe2. This finding helps to discover materials with high anisotropic in-plane thermal conductivity in uniform structures and reveals new physics of anisotropy of in-plane thermal conduction. Due to the unique features in structure and thermal transport properties, PdSe2 may serve as a new platform for designing novel devices to route heat flow precisely at the nanoscale.

Surface boundary-dendrite interactions in thin metallic Al-alloy samples

Crystallographic orientations and growth directions of dendrites in thin Al-15 wt.%Cu (200 µm) and Al-29 wt.%Ge (500 µm) alloys were investigated employing in situ X-radiography and post-mortem X-ray tomography and electron backscatter diffraction. The Al-Cu alloy was solidified under microgravity conditions, while the Al-Ge sample was processed horizontally on ground. It was found that (i) dendrites nucleate on both sides of the surface in microgravity, but accumulate under normal gravity near the top surface due to buoyancy, (ii) dendrites grow along preferred in-plane crystallographic directions, which are 〈100〉 for Al-15 wt.%Cu and 〈100〉, 〈110〉 and 〈210〉 for Al-29 wt.%Ge, indicating that the solid-liquid interfacial anisotropy of this Al-Ge composition is similar for these 〈xy0〉 directions, (iii) slightly inclined dendrite arms in Al-Cu grow along the sample boundary after contact, whereas tips of inclined dendrite arms in Al-Ge never touch the boundary, but develop new primary tips originating from secondary branches.

Controlling antiferromagnetic domains in patterned La0.7Sr0.3FeO3 thin films

Transition metal oxide thin films and heterostructures are promising platforms to achieve full control of the antiferromagnetic (AFM) domain structure in patterned features as needed for AFM spintronic devices. In this work, soft x-ray photoemission electron microscopy was utilized to image AFM domains in micromagnets patterned into La0.7Sr0.3FeO3 (LSFO) thin films and La0.7Sr0.3MnO3 (LSMO)/LSFO superlattices. A delicate balance exists between magnetocrystalline anisotropy, shape anisotropy, and exchange interactions such that the AFM domain structure can be controlled using parameters such as LSFO and LSMO layer thickness, micromagnet shape, and temperature. In LSFO thin films, shape anisotropy gains importance only in micromagnets where at least one extended edge is aligned parallel to an AFM easy axis. In contrast, in the limit of ultrathin LSFO layers in the LSMO/LSFO superlattice, shape anisotropy effects dominate such that the AFM spin axes at micromagnet edges can be aligned along any in-plane crystallographic direction.

Epitaxial growth, structural characterization, and exchange bias of noncollinear antiferromagnetic Mn3Ir thin films

Antiferromagnetic materials are of great interest for spintronics. Here we present a comprehensive study of the growth, structural characterization, and resulting magnetic properties of thin films of the noncollinear antiferromagnet $\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ir}$. Using epitaxial engineering on MgO (001) and $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ (0001) single-crystal substrates, we control the growth of cubic \ensuremath{\gamma}-$\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ir}$ in both (001) and (111) crystal orientations, and discuss the optimization of growth conditions to achieve high-quality crystal structures with low surface roughness. Exchange bias is studied in bilayers, with exchange bias fields as large as \ensuremath{-}29 mT (equivalent to a unidirectional anisotropy constant of $0.115\phantom{\rule{0.16em}{0ex}}\mathrm{erg}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ or $11.5\phantom{\rule{0.16em}{0ex}}\mathrm{nJ}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$) measured in $\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ir}$ (111)/Permalloy heterostructures at room temperature. In addition, a distinct dependence of blocking temperature on in-plane crystallographic direction in $\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ir}$ (001)/Permalloy bilayers is observed. These findings are discussed in the context of antiferromagnetic domain structures, and will inform progress towards chiral antiferromagnetic spintronic devices.

Electronic transport anisotropy of 2D carriers in biaxial compressive strained germanium

The anisotropic nature of carrier mobility in simple cubic crystalline semiconductors, such as technologically important silicon and germanium, is well understood as a consequence of effective mass anisotropy arising from a change in band structure along non-identical surface crystal directions. In contrast to this, we show experimentally that this type of anisotropy is not the dominant contribution. Recent advances in epitaxial growth of high quality germanium enabled the appearance of high mobility 2D carriers suitable for such an experiment. A strong anisotropy of 2D carrier mobility, effective mass, quantum, and transport lifetime has been observed, through measurements of quantum phenomena at low temperatures, between the ⟨110⟩ and ⟨100⟩ in-plane crystallographic directions. These results have important consequences for electronic devices and sensor designs and suggest similar effects could be observed in technologically relevant and emerging materials such as SiGe, SiC, GeSn, GeSnSi, and C (Diamond).

(Invited) Probing the Dielectric Response of Exfoliated Black Phosphorous in Free Standing Conditions

Ultra-thin Black Phosphorus (BP) is a 2D semiconductor characterized by a direct and tunable band gap associated to high carriers mobility (1-3). Studying suspended thin layers of pristine BP is however challenging due to its strong degradation through a thickness dependent and photo-assisted oxidation reaction by adsorbed moisture oxygen (4). Using a protective transfer procedure from glovebox to our TEM-EELS operating at 40kV, we probe the dielectric response of suspended BP down to the monolayer in the range 0.5-40 eV, that include band gaps thresholds and surface/volume plasmons energies for BP. The dispersion of the plasmons as a function of the momentum is also measured and simulated for both in-plane crystallographic directions. (1)Liu et al, ACS Nano (2014) (2)Qiao et al, Nature Comm. (2014) (3) Long et al, Nano Letter (2016) (4)Favron et al, Nature Mater. (2015)

Magnetoelectric properties of epitaxialFe3O4thin films on (011) PMN-PT piezosubstrates

We determine the magnetic and magnetotransport properties of 33 nm thick ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ films epitaxially deposited by rf-magnetron sputtering on unpoled (011) ${[{\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}]}_{0.68}\ensuremath{-}{[{\mathrm{PbTiO}}_{3}]}_{0.32}$ (PMN-PT) substrates. The magnetoresistance (MR), as well as the magnetization reversal, strongly depend on the in-plane crystallographic direction of the epitaxial (011) ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ film and strain. When the magnetic field is applied along [100], the magnetization loops are slanted and the sign of the longitudinal MR changes from positive to negative around the Verwey transition at 125 K on cooling. Along the $[01\overline{1}]$ direction, the loops are square shaped and the MR is negative above the switching field across the whole temperature range, just increasing in absolute value when cooling from 300 K to 150 K. The value of the MR is found to be strongly affected by poling the PMN-PT substrate, decreasing in the [100] direction and slightly increasing in the $[01\overline{1}]$ direction upon poling, which results in a strained film.

Molecular beam epitaxy growth and properties of Co2TiSi thin films on GaAs(0 0 1): the effect of growth temperature

Molecular beam epitaxy growth of ferromagnetic Co2TiSi films on GasAs(0 0 1) substrates is presented and it is found that the optimum growth temperature is between 300–360 °C where the film is single phase and exhibits highly (0 0 1)-ordered crystal structure. The Co2TiSi films are ferromagnetic up to 300 K and the highest value of saturation magnetization obtained is of 0.8 μB per formula unit. The Co2TiSi films also exhibit a very low degree of magnetic anisotropy along in-plane crystallographic directions. Expected values of Seebeck coefficient and resistivity measured at room temperature confirm good stoichiometry of the Co2TiSi films and indicate that this material is a promising candidate for both spincaloric and spintronic applications.

Interplay of Magnetic Anisotropies in Epitaxial Ferromagnetic Hybrids of Fe and (Ga,Mn)As

The combined magnetometric and microwave spectroscopy study of Fe/(Ga,Mn)As hybrids is reported. Temperature-dependent remnant magnetization data revealed unexpected non-monotonic behavior when measured along one of the in-plane crystallographic directions of (Ga,Mn)As, with a slope-sign change at the Curie point of the ferromagnetic semiconductor. Subsequent ferromagnetic resonance measurements presented uniaxial and cubic+uniaxial symmetries of in-plane magnetic anisotropy in Fe and (Ga,Mn)As, respectively. The apparent non-monotonicity of magnetization was explained as a manifestation of competition between the applied magnetic field and temperature-altered magnetic anisotropy in ferromagnetic metal layer.

The influence of strain on dewetting of silicon films

The influence of strain on the thermally induced dewetting mechanism of silicon films is reported. This study shows that the initial strain level in the silicon film significantly affects the final size and shape of the silicon agglomerates resulting from the film dewetting. With the increase of the biaxial strain up to 1.6%, the size of the silicon agglomerates is significantly reduced while their density increases. Moreover, the shape of the agglomerates becomes elongated when the strain favors one of the in-plane crystallographic directions to minimize the total energy of the system. A quantitative analysis of the dewetting mechanism is presented in terms of the agglomerates size and density versus the strain level. Finally, phenomenological laws are extracted, which predict the size and shape of the agglomerates.

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e., (001), (110), and (11−8). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term, with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-oriented structures, whereas LSMO films grown on nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic anisotropy, with the easy axis direction aligned parallel to residual substrate step edges. Elongated structures are also found for LSMO films grown on vicinal STO(001) substrates. These films present a well-defined uniaxial magnetic anisotropy, with the easy axis lying along the step edges, and its strength increases with the LSMO thickness. It is remarkable that this step-induced uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our results are promising for engineering novel half-metallic magnetic devices that exploit tailored magnetic anisotropy.

Anisotropic edge retraction and hole growth during solid-state dewetting of single crystal nickel thin films

At elevated temperatures, single crystal thin films sometimes undergo solid-state dewetting to form holes and islands with anisotropic morphologies and regular features. We have measured the rates of edge retraction for single crystal Ni(1 0 0) and Ni(1 1 0) films with edges patterned along different in-plane crystallographic directions. The polar plots of the edge retraction velocities have anisotropic shapes with prominent cusps in particular directions. The characteristic anisotropy in the edge retraction velocity can be understood in terms of in-plane orientation-dependent edge faceting, with single faceting of specific edges and multiple micro-faceting in other edges. Faceting occurs due to surface energy minimization, and is modified by changes in the annealing ambient. Changes in facet morphologies affect the relative edge retraction rates in different crystallographic orientations and also the initial shapes of holes growing in the film. The mechanistic understanding of edge retraction provided through these studies will provide a basis for understanding the evolution of complex morphologies that result from solid-state dewetting of unpatterned single crystal films.

Microwave dielectric properties of BaTiO3 and Ba0.5Sr0.5TiO3 thin films on (001) MgO

The microwave properties of BaTiO3 and Ba0.5Sr0.5TiO3 films were characterized as a function of in-plane film strain, crystallographic direction, film distortion, and dc bias. The strain dependence of BaTiO3 and Ba0.5Sr0.5TiO3 films showed an opposite pattern at room temperature, going from compression to tension, or vice versa. At zero bias, the dielectric constant and dielectric loss showed little dependence on direction ([100] and [110]). However, the tunability was consistently smaller along the [110] direction than along [100]. These observations agreed well with our previous work on how polarizations (both ionic and spontaneous) form and contribute to the nonlinear dielectric behavior.

Electrical Conductivity in Anisotropic Porous Silicon Films

The electrical transport in free standing porous silicon films formed from heavily boron-doped (110) Si wafers are investigated. The conductivity and photoconductivity of the films are shown to be significantly larger along the [110] in-plane crystallographic direction than that along the [001] one. The conductivity anisotropy decreases with increasing temperature and frequency of the applied voltage. The experimental results are explained by a model, which takes into account potential barriers between anisotropic Si nanocrystals assembling PS films.

Paraelectric thin films by nanoscale engineering of epitaxy and planar anisotropy for microwave phase shifter applications

Epitaxial and (110) oriented paraelectric thin films of Ba0.60Sro.40TiO3 were grown on (100) oriented NdGaO3 orthorhombic substrates, and the nonlinear dielectric properties were studied at 10 GHz along selected in-plane crystallographic directions in the film thickness range of 25-1200 nm. The measured dielectric properties show strong residual strain and in-plane directional dependence. For instance, the in-plane relative permittivity is found to vary from as much as 500 to 150 along [110] and [001], respectively, in the 600 nm film. Tunability was found to vary from as much as 54% to 20% in all films and directions. In a given film, the best tunability is observed along the compressed axis in a mixed strain state, 54% along [110] in the 600 nm film. It is shown that, by nanoscale manipulation of epitaxy and planar anisotropy, the return loss and phase shift in a paraelectric can be tuned over a rather wide range. The approach presented herein opens avenues for obtaining various degrees of phase shift on the same film, enabling one with an additional degree of freedom in device design and fabrication as well as multifunctionality.

In-plane anisotropy in the magnetic and transport properties of manganite ultrathin films

To analyze the conducting and magnetic properties near the film and substrate interface in manganites, ultrathin films $(\mathrm{thickness}\ensuremath{\le}100\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ were epitaxially grown by molecular-beam epitaxy on single-crystal (001) ${\mathrm{LaAlO}}_{3}$, (110) ${\mathrm{NdGaO}}_{3}$, and (001) ${\mathrm{SrTiO}}_{3}$ substrates. Structural, magnetic, and magnetoresistive properties were investigated. All samples exhibit a substrate-independent decrease of the $c$-lattice parameter for thinnest films. Highly anisotropic behavior in both transport and magnetic properties were measured along the in-plane directions parallel to the substrate crystallographic axes. In particular, for the thinnest films $(60\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$, the negative magnetoresistance at about $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the average current along one of the crystallographic directions, is larger than the room-temperature colossal value. In the same low-temperature range, with the current along the other in-plane crystallographic direction, the magnetoresistance changes sign (resulting to be magnetic field independent for $T\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$). Such an in-plane anisotropy of transport and magnetic properties is investigated with respect to possible intrinsic and extrinsic physical mechanisms.

Strong anisotropy of lateral electrical transport in (110) porous silicon films

We study the lateral electrical transport in free-standing porous silicon films prepared by anodic etching of heavily boron-doped (110) Si wafers. It is shown that the lateral dark conductivity and photoconductivity are significantly higher along the [10] in-plane crystallographic direction than that along the [001] one. The electrical transport anisotropy decreases under light illumination and with increasing temperature. The experimental results are explained by using an effective-medium approximation and taking into account potential barriers between anisotropic Si nanocrystals assembling the films. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)