High-resolution X-ray Diffraction Reciprocal Space Research Articles

Investigation of OP-GaP Grown on OP-GaAs Templates Using Nondestructive Reciprocal Space Mapping

Orientation-patterned gallium phosphide (OP-GaP) has been grown heteroepitaxially on OP gallium arsenide (GaAs) templates using hydride vapor phase epitaxy (HVPE). The effect of OP-GaAs template fabrication methods of epitaxial-inversion and wafer bonding on the heteroepitaxial OP-GaP growth has been investigated. OP-GaP layers with a growth rate of up to 35 µm/h and excellent domain fidelity were obtained. The growth rate and the domain fidelity have been revealed/studied by scanning electron microscope (SEM). In addition, we demonstrate that the crystalline quality of the individual domains, namely, the substrate-oriented domains (ODs) and the inverted domains (IDs), can be investigated by high-resolution x-ray diffraction reciprocal space mapping (HRXRDRSM), which can also indicate the domain fidelity. Attempts to increase the growth rate and improve the domain fidelity by increasing the III and V group precursors resulted in either an increase in the growth rate in the OP-GaP layers grown on epitaxial inversion OP-GaAs template at the expense of the domain crystalline quality and fidelity or an improvement in the crystalline quality of the domains at the expense of the growth rate in the OP-GaP layers grown on wafer-bonded OP-GaAs templates. In the case of OP-GaP grown on OP-GaAs templates prepared by epitaxial inversion, the crystalline quality of the ODs is better than that of the IDs, but it shows that the quality of the inverted layer in the template influences the quality and fidelity of the grown domains. To the authors’ knowledge, exploitation of HRXRDRSM studies on OP-GaP to establish the crystalline quality of its individual domains (ODs and IDs) is the first of its kind. OP-ZnSe grown on OP-GaAs templates has also been included in this study to further emphasize the potential of this method. We propose from this study that once the growth rate is optimized from SEM studies, HRXRDRSM analysis alone can be used to assess the structural quality and to infer the domain fidelity of the OP structures.

Single-crystal epitaxial europium iron garnet films with strain-induced perpendicular magnetic anisotropy: Structural, strain, magnetic, and spin transport properties

Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with roughness as low as 0.1 nm as observed using atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed in-plane epitaxial film growth, very smooth film/substrate interface, excellent film crystallinity with a small full width at half maximum of 0.012$^{\circ}$ in the rocking curve scans, and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The measured squarish out-of-plane magnetization-field hysteresis loops by vibrating sample magnetometry in conjunction with the measurements from angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. We have tailored the magnetic properties of the EuIG thin films, including saturation magnetization ranging from 71.91 to 124.51 emu/c.c. (increase with the (Eu/Fe) ratios), coercive field from 27 to 157.64 Oe, and the strength of PMA field ($H_\bot$) increasing from 4.21 to 18.87 kOe with the in-plane compressive strain from -0.774 to -1.044%. We have also investigated spin transport in Pt/EuIG bi-layer structure and evaluated the real part of spin mixing conductance to be $3.48\times10^{14} {\Omega}^{-1}m^{-2}$. We demonstrated the current-induced magnetization switching with a low critical switching current density of $3.5\times10^6 A/cm^2$, showing excellent potential for low-dissipation spintronic devices.

Strain-induced formation of self-assembled InGaN/GaN superlattices in nominal InGaN films grown by plasma-assisted molecular beam epitaxy

In this work, we report the spontaneous formation of superlattice structures in nominal InGaN films grown by plasma-assisted molecular beam epitaxy. A 700-nm-thick self-assembled ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}/\mathrm{GaN}$ superlattice with excellent structural quality was achieved. Strain was studied as a possible driving force for the formation of self-assembled superlattice (SASL) structure by growth of InGaN on ZnO substrate using similar growth conditions. The SASL structures were optically characterized using photoluminescence spectroscopy. Structural characterization was conducted via transmission electron microscopy and atom probe tomography. High-resolution x-ray diffraction (XRD) and XRD reciprocal space map were utilized to determine the average composition and the degree of relaxation of InGaN films. We propose that the vertical phase separation observed in the SASL structure is caused by high-temperature growth and intensified by strain. This work provides a method for engineering strain and growth of thick InGaN films for a variety of applications including solar cells and photodetectors.

Growth of GaP1 − x − yAsyNx on Si substrates by chemical beam epitaxy

Chemical beam epitaxy has been used to grow layers of GaP1 − xNx, GaP1 − yAsy, and nearly lattice-matched layers GaP1 − x − yAsyNx on Si substrates. To address the issue of antiphase domain generation associated with the growth of polar semiconductors on Si, misoriented Si(001) substrates have been used combined with a carefully designed GaP buffer layer growth. The reflection high-energy electron diffraction pattern exhibits a (2 × 4) surface reconstruction after GaP buffer layer and GaP(As,N) graded layer growth, indicating the good surface quality and planarity of the grown layers. Sample composition was obtained by simultaneous acquisition of Rutherford backscattering spectrometry and nuclear reaction analysis, indicating a linear dependence of N and As mole fractions on the flux of their respective precursor. GaP1 − x − yAsyNx layers grown on Si substrates have a lattice mismatch not larger than ±0.005 for N contents in the range 0.02 < x < 0.05. High-resolution X-ray diffraction reciprocal space maps demonstrate a good crystalline quality. Intense photoluminescence spectra have been measured in all GaP1 − xNx and GaP1 − x − yAsyNx layers, as it is expected for direct bandgap materials. Two wide overlapped emission peaks are observed in all the spectra, most likely related to near bandgap recombination. The position of the higher energy peak for GaP1 − xNx and GaP1 − x − yAsyNx layers has been compared to bandgap energy calculations using the band anticrossing model, showing good agreement.

Structural analysis and resistivity measurements of InAs and GaSb fins on 300 mm Si for vertical (T)FET

We report on the uniform selective area growth of InAs and GaSb by metal-organic vapor phase epitaxy on a patterned shallow trench isolation SiO2/Si 300 mm template. High-quality InAs fins are realized via direct InAs nucleation on Si in the trenches. 60° interfacial misfit dislocation arrays formed along the {111} oriented InAs/Si interfaces accommodate most of the lattice mismatch such that threading dislocation generation from residual strain is minimized. GaSb grown on a thick GaAs buffer (partial filling of the trench) suffers from twin formation. The twin density is found to decrease with increasing growth temperature. To maximize the volume of low-defect density material inside the trench, the GaSb growth is initiated as close as possible to the trench bottom. Therefore, uniform high-quality GaSb fins are grown on a very thin low-temperature GaAs and the newly developed InAs seed in the V-groove. High-resolution x-ray diffraction reciprocal space maps show that the InAs and GaSb fins are nearly completely relaxed. The resistivity of undoped and n-doped InAs and undoped and p-doped GaSb fins is measured on as-grown material by micro-4-point-probe without the need for additional complicated device processing. The values are compared to those required for devices as presented in modeling papers. The result is encouraging for the ultra-large-scale integration of vertical nanowire III-V (tunnel) field-effect transistor on 300 mm Si.

Control of misfit strain in ferroelectric BaTiO3 thin-film capacitors with SrRuO3-based electrodes on (Ba, Sr)TiO3-buffered SrTiO3 substrates

We investigate ferroelectric BaTiO3 capacitors with SrRuO3-based electrodes employing a (BaxSr1−x)TiO3 (BST) buffer layer on (100) SrTiO3 substrates prepared by pulsed laser deposition. Structural analysis by high-resolution X-ray diffraction reciprocal space mapping shows that the BST (x = 0.5 and 0.7) layers are relaxed and have their bulk in-plane lattices in the upper part owing to strain relief. The bottom electrodes on the buffer layers grow heteroepitaxially but cannot withstand a tensile stress and then show a diminution of the in-plane lattice. On the BST (x = 0.7) layer, compared with the capacitor with an SrRuO3 electrode, that with a Ba0.1Sr0.9RuO3 one has a reduced in-plane lattice relaxation of the BaTiO3 film and then exhibits a larger remanent polarization (Pr) of 34 μC/cm2 associated with a smaller shift of the hysteresis loop. The polarization hysteresis shift is attributed to a flexoelectric effect stemming from the coupling between out-of-plane polarization and a strain gradient in the BaTiO3 film. We conclude that a reduced misfit strain relaxation in the ferroelectric film achieved with the Ba0.1Sr0.9RuO3 electrode on the thicker BST (x = 0.7) buffer layer is the origin of an enhanced Pr with a smaller hysteresis shift.

Anomalous indium incorporation and optical properties of high indium content InGaN grown by MOCVD

The optical properties of thick InGaN epilayers grown by metal-organic chemical vapor deposition with indium (In) content range from 10% to 42% were investigated. With the increase of In content, the crystal quality of InGaN epilayers significantly degraded, particularly at increasing the edge-type dislocations density for high In content InGaN. Combining high-resolution X-ray diffraction reciprocal space mapping, the splitting of the photoluminescence (PL) peak into several PL bands was found to be caused by compositional fluctuations rather than by the strain effects. Based on the PL and optical absorption spectra, the composition-dependent Stokes shift was observed and a linear relationship between Stokes shift and Urbach tailing energy was established. Additionally, a drop of In incorporation along the growth direction was observed in InGaN epilayers with high In content. Such drop behavior of In incorporation was attributed to the formation of In droplets, which offers a competing pathway for In incorporation by gathering the incoming In atoms hence causing a decrease in the surrounding In incorporation. This will present a broader view about the nature of InGaN alloys with high In content.

Effect of thermal annealing on the crystallization of low-temperature-grown In0.42Ga0.58As on InP substrate

We present the effect of postannealing on the crystallization of low-temperature-grown (LTG) InxGa1−xAs on a (001) InP substrate. LTG In0.42Ga0.58As with a thickness of 1.0 µm was grown directly on the substrate by molecular beam epitaxy at 180 °C. High-resolution X-ray diffraction reciprocal space mapping and cross-sectional transmission electron microscopy (TEM) indicate that the as-grown LTG-In0.42Ga0.58As layer comprises an amorphous layer above the substrate and a columnar crystal layer on top of the amorphous layer. The TEM images indicate that the thermally induced crystallization of the amorphous In0.42Ga0.58As crystal lattice-matched to the InP substrate occurs during annealing above 400 °C.

Strain relaxation in semipolar (202¯1) InGaN grown by plasma assisted molecular beam epitaxy

Strain relaxation in semipolar (202¯1) InGaN layers grown by plasma assisted molecular beam epitaxy (PAMBE) was investigated with high-resolution X-ray diffraction (XRD) reciprocal space mapping, cathodoluminescence (CL), fluorescent light microscopy (FLM), and atomic force microscopy. We find that XRD detects lattice relaxation much later than its actual onset occurs. Other techniques used in this study allowed to detect local footprints of plastic relaxation before it was evidenced by XRD: at the initial stages of strain relaxation, we observed changes in layer morphology, i.e., formation of short trench line segments on the surface along the ⟨112¯0⟩ direction as well as dark lines in CL and FLM. The misfit dislocations formation and glide were observed in two slip systems: initially in basal slip system ⟨112¯0⟩{0001} and for larger amount of strain in non-basal, prismatic slip system ⟨112¯0⟩{11¯00}. Experimentally determined critical thickness for InGaN layers grown by PAMBE on semipolar (202¯1) bulk GaN substrates agrees well with literature data obtained with metalorganic vapor phase epitaxy and follows the Matthews-Blakeslee model prediction. We discuss the impact of substrate structural properties on the strain relaxation onset and mechanisms. We also describe the layer morphology and surface roughness evolution related to the increasing In content and strain relaxation of the semipolar (202¯1) InGaN layers.

Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.

Systematic study of the structure of Ni2MnGa single crystals by high-resolution X-ray diffraction reciprocal space mapping

Systematic study of the structure of Ni₂MnGa single crystals by high-resolution X-ray diffraction reciprocal space mapping

Stabilization of mixed-phase structures in highly strained BiFeO3 thin films via chemical-alloying

Chemical-alloying is demonstrated to stabilize the mixed-phase structure of highly strained epitaxial BiFeO3/LaAlO3 (001) heterostructures. Such mixed-phase structures are essential for the large electromechanical responses (4%–5% strains under applied electric field); however, films with thickness exceeding 250 nm undergo an epitaxial breakdown to a non-epitaxial bulk-like rhombohedral-phase. Such an irreversible transformation of the mixed-phase structure limits the magnitude of the net surface displacement associated with these field-induced phase transformations. Using high-resolution x-ray diffraction reciprocal space mapping and scanning-probe-based studies, we show that chemical-alloying of BiFeO3 thin films can stabilize these mixed-phase structures and delay the onset of epitaxial breakdown.

Twin wall distortions through structural investigation of epitaxial BiFeO3 thin films

Abstract

Effect of indium incorporation on optical and structural properties of m-plane InGaN/GaN MQW on LiAlO 2 substrates

We report about the optical and structural characterization of m-plane InGaN/GaN multiple quantum well (MQW) structures. All samples were grown on 2-inch LiAlO 2 substrates in AIXTRON MOVPE reactors. By lowering the MQW growth temperature, an increase of an indium fraction from 5% to 30% is achieved. High-resolution X-ray diffraction (HRXRD) reciprocal space mapping reveals fully strained growth of the quantum wells. The photoluminescence (PL) peak full width half maximum (FWHM) values increase from ∼130 to ∼240 meV at room temperature when the indium fraction rises from 10% to 30% . Also, PL peaks show good wavelength stability for indium contents up to 10% when the excitation intensity is varied by two orders of magnitude. However, at 20% indium and above, a clear excitation intensity-induced peak shift of around 50 meV is detected which is likely caused by indium segregation. Cross-section transmission electron microscopy (TEM) hints at indium fluctuation near extended defects. We conclude that an improved growth process for nonpolar InGaN/GaN MQW is necessary to achieve high-performance long-wavelength light-emitting devices.

Effect of film thickness on ferroelectric domain structure and properties of Pb(Zr0.35Ti0.65)O3/SrRuO3/SrTiO3 heterostructures

Epitaxial Pb(Zr0.35Ti0.65)O3 (PZT) thin films with tetragonal symmetry and thicknesses ranging from 45 to 230 nm were grown at 540 °C on SrRuO3-coated (001)SrTiO3 substrates by pulse-injected metalorganic chemical vapor deposition. The effect of the film thickness on the ferroelectric domain structure and the dielectric and ferroelectric properties were systematically investigated. Domain structure analysis of epitaxial PZT films was accomplished with high-resolution X-ray diffraction reciprocal space mapping and high-resolution transmission electron microscopy. Fully polar-axis (c-axis)-oriented epitaxial PZT thin films with high ferroelectric polarization values [e.g., remanent polarization (Pr) ~ 90 μC/cm2] were observed for film thicknesses below 70 nm. Films thicker than 70 nm had a c/a/c/a polydomain structure and the relative volume fraction of c-domains monotonously decreased to about 72% on increasing the film thickness up to 230 nm , and finally Pr diminished to about 64 μC/cm2 for the 230-nm-thick epitaxial film. These polarization values were in good agreement with the estimated values taking into account the volume fraction of the c-axis-oriented domains while assuming a negligible contribution of 90° domain reorientation caused by an externally applied electric field.

(111)-textured Mn-substituted BiFeO3 thin films on SrRuO3∕Pt∕Ti∕SiO2∕Si structures

(111)-textured 5%-Mn-substituted BiFeO3 (BFMO) thin films were fabricated on (111)-oriented SrRuO3(SRO)∕Pt∕Ti∕SiO2∕Si structures by chemical solution deposition. X-ray diffraction analyses (θ-2θ and pole figures) confirmed that SRO and BFMO films were highly (111) textured. High-resolution x-ray diffraction reciprocal space mapping showed that the crystal structure of the BFMO films was a rhombohedrally distorted perovskite structure. Transmission electron microscopy cross section images revealed that BFMO films were composed of columnar grains epitaxially aligned on the SRO grains. The remanent polarization of approximately 70μC∕cm2 and coercive field of approximately 300kV∕cm were observed at 900kV∕cm applied electric field in the (111)-textured BFMO thin films.

Crystal structure and microstructure of epitaxial Pb(Zr,Ti)O3 films consisting of mixed phases with tetragonal and rhombohedral symmetries grown on (100)cSrRuO3//(100)SrTiO3 substrate by metalorganic chemical vapor deposition

(100)-/(001)-oriented epitaxial Pb(Zr,Ti)O3(PZT) films with various Zr/(Zr + Ti) ratios and film thicknesses were grown on (100)cSrRuO3//(100)SrTiO3substrates at 600 °C by metalorganic chemical vapor deposition. Their crystal structure and microstructure consisting of particularly mixed phases with tetragonal and rhombohedral symmetries were investigated in detail. Films with the mixed phases with tetragonal and rhombohedral symmetries were ascertained at a Zr/(Zr + Ti) ratio of around 0.50 at a thickness of more than 200 nm from x-ray diffraction (XRD) analysis, while no mixed phase region was observed for 50 nm thick film. The relative volume fraction of the tetragonal cdomain decreased with increasing film thickness for 2-μm-thick films with mixed phases, while the tetragonal adomain and rhombohedral domain increased. These mixed phases were ascertained to have originated from the same cubic PZT grown above Curie temperature from results obtained by temperature-dependent high-resolution XRD reciprocal space mapping. The transmission electron microscopy images clearly revealed that the microstructure of the film gradually changed along the film thickness, despite the fact that the film was very uniform. Tetragonal phases were dominant in the bottom region (near the interface with the substrate) of the film, while rhombohedral phase was dominant in the upper region (near the surface), which was also ascertained by cross-sectional Raman analysis. In addition, the tetragonal a/cdomain was wider in the bottom region than that in the upper region of film with mixed phases. These data clearly demonstrate that both the crystal structure and microstructure changed along the film thickness.

Self-patterned oxide nanostructures grown by post-deposition thermal annealing on steppedsurfaces

We report here on the self-patterned growth of nanostructured epitaxial oxide thin filmsusing a bottom-up approach. Different stepped (0001) sapphire surfaces have been used astemplates to grow different nanostructured thin films of yttria-stabilized zirconiaelaborated by a chemical solution deposition technique, namely sol–gel dip-coating. Forsuch films, a morphological instability occurs during post-deposition thermal annealing thatforces the film to split into isolated islands. Taking advantage of this effect, three distincttypes of nanostructures have been produced depending on the step morphology of thesubstrate surface, and depending on the relative magnitude of the mean island diameter(D) and the meanterrace length (L). Flat-top islands, dome-shaped islands, tilted flat-top islands or self-organized arrays ofnano-islands can be generated. Atomic force microscopy was used to observenanostructures and quantitative characterizations of crystallographic and nanostructuralaspects were carried out by high-resolution x-ray diffraction reciprocal space mapping.

Asymmetric relaxation of SiGe∕Si(110) investigated by high-resolution x-ray diffraction reciprocal space mapping

Strain relaxation of SiGe∕Si(110) has been studied by x-ray reciprocal space mapping. To get information about the in-plane lattice mismatch in different directions, two-dimensional maps around, e.g., (260) and (062) reciprocal lattice points have been obtained from Si0.8Ge0.2∕Si(110) samples, which were exposed to different annealing conditions. The in-plane lattice mismatch was found to be asymmetric with the major strain relaxation observed in the lateral [001] direction. This was associated with the formation and propagation of dislocations oriented along [1¯10]. The relaxation of as-grown structures during postannealing is thus different from relaxation during growth, which is mainly along [1¯10].

Structural Characterization of BiFeO3 Thin Films by Reciprocal Space Mapping

High-resolution X-ray diffraction reciprocal space mapping (HRXRD-RSM) was applied for the structural characterization of epitaxial BiFeO3 thin film grown on SrRuO3-coated (001) SrTiO3 single crystal substrate with a variety of film thicknesses ranging from 15 to 500 nm. Distinguishing monoclinic structure from rhombohedral or tetragonal structures was accomplished with a set of HRXRD-RSMs measured for several hkl diffraction conditions. The BiFeO3 thin films showed both monoclinic and tetragonal structures depending on film thickness. Tetragonal structure was observed for film thicknesses below 50 nm and was highly strained due to epitaxial strain, while films with film thickness thicker than 50 nm showed monoclinic structure. No BiFeO3 thin films showed bulk rhombohedral structure. This structural change in BiFeO3 thin film may play an important role in the enhanced ferroelectricity observed.