Reciprocal Space Mapping Research Articles

Increasing the scan speed in high resolution, low energy electron diffraction measurements by presetting the gate time.

We report on a speed-up data acquisition routine for recording intensities in reciprocal space (k-space) with increased scan speed by a single point detector. It is designed for recording low energy electron diffraction (LEED) data with high resolution by a spot profile analysis LEED instrument. It counteracts the problem of long acquisition times that are encountered when larger areas in the reciprocal space are scanned. It exploits the fact that in typical LEED images of ordered surfaces, more than 90% of the data points in k-space belong to the low-intense background, which is often not of interest. Only about 10% of the data points are related to the relevant diffraction features, namely, the LEED spots. Often it is not necessary to measure the background with the same statistical significance as measuring those points that contain information. The data points belonging to the LEED spots can be discriminated from those of the background by their higher intensities. An acquisition routine that sets an increased gate time for the counting in response to higher intensities thus safeguards good statistics for data points of the LEED spots and saves measurement time when recording data points of the background with small gate times. For typical LEED images, a reduction of the total acquisition time by a factor of about 10 is obtained. We give examples of one- and two-dimensional scans from current experiments, recorded with and without the speed-up routine. We further discuss how the routine supports the measurement of energy dependent reciprocal space maps.

Dislocation arrangements in 4H-SiC and their influence on the local crystal lattice properties.

Two wafers of one 4H-silicon carbide (4H-SiC) bulk crystal, one cut from a longitudinal position close to the crystal's seed and the other close to the cap, were characterized with synchrotron white-beam X-ray topography (SWXRT) in back-reflection and transmission geometry to investigate the dislocation formation and propagation during growth. For the first time, full wafer mappings were recorded in 00012 back-reflection geometry with a CCD camera system, providing an overview of the dislocation arrangement in terms of dislocation type, density and homogeneous distribution. Furthermore, by having similar resolution to conventional SWXRT photographic film, the method enables identification of individual dislocations, even single threading screw dislocations, which appear as white spots with a diameter in the range of 10 to 30 µm. Both investigated wafers showed a similar dislocation arrangement, suggesting a constant propagation of dislocations during crystal growth. A systematic investigation of crystal lattice strain and tilt at selected wafer areas with different dislocation arrangements was achieved with high-resolution X-ray diffractometry reciprocal-space map (RSM) measurements in the symmetric 0004 reflection. It was shown that the diffracted intensity distribution of the RSM for different dislocation arrangements depends on the locally predominant dislocation type and density. Moreover, the orientation of specific dislocation types along the RSM scanning direction has a strong influence on the local crystal lattice properties.

Evaluation of Strain-Relaxation of Carbon-Doped Silicon Nanowires and Its Crystal Orientation Dependence Using X-Ray Diffraction Reciprocal Space Mapping

Evaluation of Strain-Relaxation of Carbon-Doped Silicon Nanowires and Its Crystal Orientation Dependence Using X-Ray Diffraction Reciprocal Space Mapping

Epitaxial growth of (Ba0.7Sr0.3)TiO3 thin films on GdScO3 substrates by magnetron sputtering

Epitaxial growth of Ba0.7Sr0.3TiO3 (BST70) thin films on GdScO3 (GSO) substrates had been realized using the radio frequency magnetron sputtering system with the epitaxial alignment [001]BST70||[110]GSO and [010]BST70||[001]GSO. Reciprocal space mapping and transmission electron microscope results confirm the epitaxial growth without an impurity phase at the interface. The Fourier-filtered image shows that the BST70 thin film grew well with few dislocations. The out-of-plane parameter of the as-deposited film was elongated due to strain, which was induced by the differences in the thermal expansion coefficients between the film and the substrate, and oxygen vacancies. The highly strained as-deposited BST70 films could be relaxed by the post-annealing procedure at 800 °C in an O2-rich atmosphere for better epitaxial quality.

Combining x-ray real and reciprocal space mapping techniques to explore the epitaxial growth of semiconductors

In the present work, the importance of determining the strain states of semiconductor compounds with high accuracy is demonstrated. For the matter in question, new software titled LAPAs, the acronym for LAttice PArameters is presented. The lattice parameters as well as the chemical composition of Al1−x In x N and Ge1−x Sn x compounds grown on top of GaN- and Ge- buffered c-Al2O3 and (001) oriented Si substrates, respectively, are calculated via the real space Bond’s method. The uncertainties in the lattice parameters and composition are derived, compared and discussed with the ones found via x-ray diffraction reciprocal space mapping. Broad peaks lead to increased centroid uncertainty and are found to constitute up to 99% of the total uncertainty in the lattice parameters. Refraction correction is included in the calculations and found to have an impact of 0.001 Å in the lattice parameters of both hexagonal and cubic crystallographic systems and below 0.01% in the quantification of the InN and Sn contents. Although the relaxation degrees of the nitride and tin compounds agree perfectly between the real and reciprocal-spaces methods, the uncertainty in the latter is found to be ten times higher. The impact of the findings may be substantial for the development of applications and devices as the intervals found for the lattice match the condition of Al1−x In x N grown on GaN templates vary between ∼1.8% (0.1675–0.1859) and 0.04% (0.1708–0.1712) if derived via the real- and reciprocal spaces methods.

High-Performance P- and N-Type SiGe/Si Strained Super-Lattice FinFET and CMOS Inverter: Comparison of Si and SiGe FinFET.

This research presents the optimization and proposal of P- and N-type 3-stacked Si0.8Ge0.2/Si strained super-lattice FinFETs (SL FinFET) using Low-Pressure Chemical Vapor Deposition (LPCVD) epitaxy. Three device structures, Si FinFET, Si0.8Ge0.2 FinFET, and Si0.8Ge0.2/Si SL FinFET, were comprehensively compared with HfO2 = 4 nm/TiN = 80 nm. The strained effect was analyzed using Raman spectrum and X-ray diffraction reciprocal space mapping (RSM). The results show that Si0.8Ge0.2/Si SL FinFET exhibited the lowest average subthreshold slope (SSavg) of 88 mV/dec, the highest maximum transconductance (Gm, max) of 375.2 μS/μm, and the highest ON-OFF current ratio (ION/IOFF), approximately 106 at VOV = 0.5 V due to the strained effect. Furthermore, with the super-lattice FinFETs as complementary metal-oxide-semiconductor (CMOS) inverters, a maximum gain of 91 v/v was achieved by varying the supply voltage from 0.6 V to 1.2 V. The simulation of a Si0.8Ge0.2/Si super-lattice FinFET with the state of the art was also investigated. The proposed Si0.8Ge0.2/Si strained SL FinFET is fully compatible with the CMOS technology platform, showing promising flexibility for extending CMOS scaling.

Room-temperature ferroelectric-like behavior in anatase TiO2 epitaxial films prepared by chemical solution deposition

Ferroelectrics of simple oxides have been of great interest due to they are easy to integrate with silicon semiconductors. The FE hysteresis loop was measured for the first time in pure anatase-phase titanium dioxide is reported, demonstrating that the ferroelectric-like behavior in a-TiO2 epitaxial thin films (7.5 nm to 228.8 nm) prepared by chemical solution deposition method on (001) SrTiO3 based substrates. Synchrotron based X-ray diffraction and reciprocal space mapping analysis prove that we have prepared high-quality epitaxial films. According to the results of the FE hysteresis loop, Pr can reach approximately 0.42 μC/cm2 in a-TiO2 film with a thickness of 14.5 nm. Furthermore, the PFM shows that the ferroelectric-like behavior is exhibited in a-TiO2 film of 7.5 nm and that the PFM polarization switch can be maintained for more than 100 min. All of these seem to indicate the presence of ferroelectric behavior in a-TiO2 films. We recognize that the origin of ferroelectric-like behavior is due to the distortion induced by strain and vacancy. However, since the polarization value is not very large and the PFM decays, the electrochemical effect has a non-negligible influence on the intrinsic ferroelectric performance.

Intervalley energy separation in the conduction band of InAs1−x Bi x determined by terahertz emission spectroscopy

InAsBi layers with different bismuth content were grown on InAs substrates by solid source MBE. The amount of bismuth incorporated in the layers was estimated using X-ray diffraction measurements. The relaxation degree of the grown crystalline layers was evaluated using reciprocal space map analysis. The intervalley energy separation in the conduction band of InAsBi was studied by Terahertz Excitation Spectroscopy. It has been found that this separation slightly decreases with increasing Bi content. In the studied samples with Bi content varying from 2.7% to 4.5% the Γ-L separation shifts down to about 0.9 eV.

Toward ultraclean correlated metal CaVO3

We report the synthesis and electronic properties of the correlated metal CaVO3, grown by hybrid molecular beam epitaxy. Films were grown on (100) LaAlO3 substrates at a temperature of 900 °C by supplying a flux of elemental Ca through a thermal effusion cell and metalorganic precursor, vanadium oxitriisopropoxide, as a source of vanadium. The presence of a self-regulated growth regime was revealed by the appearance of a specific surface reconstruction detected by reflection high-energy electron diffraction. Films grown within the growth window were characterized by atomically flat surfaces. X-ray reciprocal space maps revealed that the films were coherently strained to the substrate and inherited its twinned microstructure. Despite the presence of twin walls, CaVO3 thin films, grown within the stoichiometric growth window, revealed very low electrical resistivities at low temperatures, with residual resistivity ratios exceeding 90, while films grown at either Ca- or V-excess show deteriorated transport properties, attributed to the presence of extrinsic defects arising from the non-stoichiometry present in these films.

Metalorganic chemical vapor deposition of β-(AlxGa1−x)2O3 thin films on (001) β-Ga2O3 substrates

Phase pure β-(AlxGa1−x)2O3 thin films are grown on (001) oriented β-Ga2O3 substrates via metalorganic chemical vapor deposition. By systematically tuning the precursor molar flow rates, the epitaxial growth of coherently strained β-(AlxGa1−x)2O3 films is demonstrated with up to 25% Al compositions as evaluated by high resolution x-ray diffraction. The asymmetrical reciprocal space mapping confirms the growth of coherent β-(AlxGa1−x)2O3 films (x < 25%) on (001) β-Ga2O3 substrates. However, the alloy inhomogeneity with local segregation of Al along the (2̄01) plane is observed from atomic resolution STEM imaging, resulting in wavy and inhomogeneous interfaces in the β-(AlxGa1−x)2O3/β-Ga2O3 superlattice structure. Room temperature Raman spectra of β-(AlxGa1−x)2O3 films show similar characteristics peaks as the (001) β-Ga2O3 substrate without obvious Raman shifts for films with different Al compositions. Atom probe tomography was used to investigate the atomic level structural chemistry with increasing Al content in the β-(AlxGa1−x)2O3 films. A monotonous increase in chemical heterogeneity is observed from the in-plane Al/Ga distributions, which was further confirmed via statistical frequency distribution analysis. Although the films exhibit alloy fluctuations, n-type doping demonstrates good electrical properties for films with various Al compositions. The determined valence and conduction band offsets at β-(AlxGa1−x)2O3/β-Ga2O3 heterojunctions using x-ray photoelectron spectroscopy reveal the formation of type-II (staggered) band alignment.

Incommensurability and negative thermal expansion of single layer hexagonal boron nitride

The emerging field of straintronics, i.e., the control and utilization of the strain state of 2D-materials, is of great importance for their technological development, specifically in view of their future incorporation into van der Waals heterostructures. To gain fundamental insight into structural peculiarities of two-dimensional systems, single layer hexagonal boron nitride (hBN) grown on Ir(111) by chemical vapor deposition was used as a prototypical model system: High-resolution reciprocal space mapping reveals the incommensurate nature of the material system by measuring the hBN in plane lattice parameter with high precision, facilitated by the moiré magnification effect in electron diffraction. In a growth temperature (Tg) regime of 700–1150°C an average lattice parameter of 2.496 ± 0.006 Å was found. Eventually, careful disentanglement of the hBN’s and substrate’s behavior for rising Tg allowed the determination of a negative thermal expansion coefficient of αhBN = −2.4 ± 1.2 × 10-6 K−1 for free-standing hBN.

2D reciprocal space map of etched metalorganic chemical vapor deposited CdTe(001) film surface on miscut GaAs(001)

CdTe thin films have drawn continued interest in research and applications in various fields such as optoelectronics. We report on the study of a two-dimensional (2D) reciprocal space map of Br-methanol etched CdTe(001) epitaxial film grown by metalorganic chemical vapor deposition on miscut GaAs(001) using azimuthal reflection high-energy electron diffraction (ARHEED), which reveals a two-fold symmetric surface with a rectangular unit mesh. This is in contrast to the observed four-fold symmetry with a square unit mesh on an etched miscut single crystal CdTe(001). Two sets of two-fold symmetric diffraction spots with unequal intensities indicate unequal populations of two 90° orientation domains in the etched film. Based on the 2D maps and tilted azimuthal dependent RHEED patterns of the etched film, it unveils that the short length of rectangular unit mesh in the dominant domain aligns with the step edge direction. A simple method to characterize the miscut angle of a heteroepitaxial film grown on a miscut substrate is demonstrated using ARHEED. Both etched single crystal and film surfaces are Te rich with a higher Te to Cd ratio on the elongated hillocks in the etched film disclosed by scanning Auger electron spectroscopy mapping. This methodology can be applied to examine etched epitaxial film's surfaces that might deviate from bulk structure or bulk stoichiometry before interfacing with other materials to form heterojunctions or making ohmic contacts with metals.

Shear strain stabilized high-temperature metallic monoclinic VO2 variants with symmetry permission

In this work, temperature-dependent in situ x-ray diffraction reciprocal space mappings are employed to reveal three-dimensional lattice shear deformation in epitaxial VO2 variants, and linear Lagrangian strain tensors of epitaxial lattices are deduced by metric tensors of the reciprocal space at various temperatures. An equilibrium modulated-monoclinic phase is identified above the critical temperature, which is permitted in a translationengleiche subgroup from high symmetry with an index of 4. Being different from the conventional low-temperature monoclinic phase, the high-temperature monoclinic phase presents a specific metallic feature, which is ascribed to the Mott–Hubbard and charge density wave mechanisms. Under the proposed general procedure, which precisely identifies the normal and shear strain status of deformed lattices, it is demonstrated that the structural symmetry reduction under shear deformation unambiguously acts as the origin of unexpected metallic modulated-monoclinic VO2 at high temperatures. Our results highlight the significance of precise detection and contribution of shear deformation in various fields of physics.

Emergent chirality in a polar meron to skyrmion phase transition

Polar skyrmions are predicted to emerge from the interplay of elastic, electrostatic and gradient energies, in contrast to the key role of the anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. Here, we explore the reversible transition from a skyrmion state (topological charge of −1) to a two-dimensional, tetratic lattice of merons (with topological charge of −1/2) upon varying the temperature and elastic boundary conditions in [(PbTiO3)16/(SrTiO3)16]8 membranes. This topological phase transition is accompanied by a change in chirality, from zero-net chirality (in meronic phase) to net-handedness (in skyrmionic phase). We show how scanning electron diffraction provides a robust measure of the local polarization simultaneously with the strain state at sub-nm resolution, while also directly mapping the chirality of each skyrmion. Using this, we demonstrate strain as a crucial order parameter to drive isotropic-to-anisotropic structural transitions of chiral polar skyrmions to non-chiral merons, validated with X-ray reciprocal space mapping and phase-field simulations.

Demonstration of Bixbyite-Structured δ-Ga2O3 Thin Films Using β-Fe2O3 Buffer Layers by Mist Chemical Vapor Deposition

Gallium oxide (Ga2O3) possesses five polymorphs: α, β, γ, κ (ε), and δ. Although the first four polymorphs have been well-studied, there are few reports on δ-Ga2O3. Here, we demonstrate the epitaxial growth of metastable δ-Ga2O3 thin films by mist chemical vapor deposition using β-Fe2O3 buffer layers. X-ray diffraction (XRD) 2θ–ω scan pattern revealed that (004) κ-Ga2O3 grew on (111) yttria-stabilized zirconia (YSZ) without a buffer layer or with a bcc-In2O3 buffer layer, whereas (222) δ-Ga2O3 grew on (222) β-Fe2O3. The β-Fe2O3 buffer layer led to the epitaxial growth of the δ-Ga2O3 thin film. The lattice mismatch between the equivalent crystal structures of β-Fe2O3 and δ-Ga2O3 triggered this growth. XRD analysis shows that δ-Ga2O3 grew epitaxially on the β-Fe2O3 buffer layer/YSZ substrate in both the out-of-plane and in-plane orientations, and the lattice constant inferred from the diffraction peaks was estimated to be 9.255 Å. Reciprocal space mapping results indicated that the δ-Ga2O3 grown on β-Fe2O3 was fully relaxed. Selected area electron diffraction images confirmed that the δ-Ga2O3 exhibited a cubic bixbyite structure. The optical band gap of δ-Ga2O3 was 4.3 or 4.9 eV, as calculated from reflection electron energy loss spectroscopy. We successfully grew a δ-Ga2O3 epitaxial thin film for the first time.

Large negative electrocaloric response induced by nanoscale phase transition in (Bi, Na)TiO3-based thin films

Electrocaloric effect (ECE) offers an efficient and environmentally friendly route for solid-state cooling. Either positive or negative ECE could exhibit a large adiabatic temperature change (ΔT). Compared to the positive electrocaloric response, the investigation of negative ECE is lagging behind, largely due to the fact that its origin is still elusive. In this work, the negative ECE behavior of conventional ferroelectric thin films, namely 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 (BNBT), was studied. A remarkable ΔT of −26.1 K was acquired near 160 °C under a moderate electric field of 875 kV/cm, attributing to the ferroelectric phase transition in the polar nanoregions from rhombohedral (R3c) to tetragonal (P4bm), as confirmed by temperature-dependent dielectric permittivity, Raman spectra, and x-ray reciprocal space mapping. The BNBT thin film presents a high electrocaloric coefficient (ΔT/ΔE) of 0.0298 K cm kV−1, transcending that of the most reported negative electrocaloric response of thin films.

The effect of ammonia partial pressure on the growth of semipolar (11–22) InGaN/GaN MQWs and LED structures

This paper presents the effect of ammonia partial pressure at a relatively low fixed V/III (∼765) growth atmosphere on the growth of multi-quantum well (MQW) structures and the light emitting diode (LED) emission wavelength. At fixed V/III ratio, ammonia partial pressure together with the consequent group III partial pressures are shown to influence the growth of InGaN/GaN MQWs in term of growth rate and the indium incorporation for the quantum well. It is also observed that the growth of InGaN quantum well (QW) behaves differently as compared to GaN quantum barrier (QB) at low V/III ambient. For the growth of InGaN QW, the data collected shows the existence of threshold point for ammonia partial pressure of ∼300 Pa. In the regime below the threshold value, the decomposition of InGaN due to severe dissociation of In-N bonds caused the growth rate to be extremely low. As the ammonia partial pressure increases above the threshold value, the growth rate of InGaN QW increases sharply together with an improvement in indium incorporation, as proven by the XRD and (11–24) asymmetric reciprocal space mapping (RSM) results. The XRD result is consistent with the atomic force microscopy (AFM) results. As the partial pressures increases, the RMS and peak-to-valley roughness increases which is credited to the increment in indium incorporation. Furthermore, “needle-like” structures can be observed over all samples when the MQWs were grown with limited ammonia partial pressure. During the growth for the LED structure with thin layers of InGaN/GaN, the partial pressure of ammonia became a dominating factor in the formation of well-abrupt layer interfaces. Through the XRD and room-temperature photoluminescence (RTPL) results, it is shown that abrupt layers can be formed at higher ammonia partial pressures, while indium clusters tend to form at a lower partial pressure, which are most probably due to the insufficient N concentration, while the electroluminescence (EL) results were consistent as well. Therefore, it is once again confirmed that the performance of the InGaN/GaN LED is highly governed by the partial pressures of the ammonia.

Molecular beam epitaxy of KTaO3

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high-quality KTaO3 thin films by molecular-beam epitaxy. Tantalum was provided by either a suboxide source emanating a TaO2 flux from Ta2O5 contained in a conventional effusion cell or an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10% O3 + 90% O2) were simultaneously supplied with the TaO2 (or tantalum) molecular beams to grow the KTaO3 films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO3 (001) and GdScO3 (110) substrates.

HAT: a high-energy surface X-ray diffraction analysis toolkit.

This work introduces the high-energy surface X-ray diffraction analysis toolkit (HAT), an open-source cross-platform software package written in Python to allow the extraction and processing of high-energy surface X-ray diffraction (HESXRD) data sets. Thousands of large-area detector images are collected in a single HESXRD scan, corresponding to billions of pixels and hence reciprocal space positions. HAT is an optimized reciprocal space binner that implements a graphical user interface to allow the easy and interactive exploration of HESXRD data sets. Regions of reciprocal space can be selected with movable and resizable masks in multiple views and are projected onto different axes to allow the creation of reciprocal space maps and the extraction of crystal truncation rods. Current and future versions of HAT can be downloaded and used free of charge.

Coherent growth of β-(Al x Ga1−x )2O3 alloy thin films on (010) β-Ga2O3 substrates using mist CVD

Herein, we demonstrate β-(Al x Ga1−x )2O3 thin films that were coherently grown on a (010) β-Ga2O3 substrate using mist chemical vapor deposition (CVD). X-ray diffraction and reciprocal space mapping results revealed that the β-(Al x Ga1−x )2O3 thin films were of high-crystalline quality and were grown coherently to attain an Al content of 18.3% as measured by Rutherford backscattering spectroscopy. Importantly, based on their surface morphologies, the coherently grown β-(Al x Ga1−x )2O3 thin films have atomically flat surfaces. These results indicate that mist CVD is a promising technique for β-(Al x Ga1−x )2O3/β-Ga2O3 heterojunction devices.