X-ray Diffraction Reciprocal Space Mapping Research Articles

Engineering Domain Variants in 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 Single Crystals Using High-Frequency AC Poling.

Single crystals of (001)-oriented 0.7Pb(Mg1/3Nb2/3)-0.3PbTiO3 (PMN-30PT) with a composition near the morphotropic phase boundary have attracted considerable attention due to their superior dielectric and electromechanical performance. Recently, a new alternating current (electric field) poling approach used for the enhancement of dielectric and piezoelectric properties. However, the microscopic domain variants that govern the performance, especially under high-frequency alternating current (AC) voltages, remain largely unexplored. In this work, the domain microstructure under AC poling reveals the presence of four monoclinic (MA) domain variants using a suite of scanning probe microscopy methods, and X-ray diffraction (XRD) reciprocal space mapping is tuned. It is reported on the emergence of hierarchical fine domains - needle-shaped, and 109° domain walls under applied high-frequency AC poling. Time-resolved Kelvin probe force microscopy (KPFM) reveals the charge dynamics and relaxation behavior of these needle domains and walls. The findings provide new insight and guidance to the domain engineering by high-frequency AC poling for the development of advanced transducer technology.

Structure evolution and energy band modulation in Ba-doped BiFeO3 thin films

Bi1−xBaxFeO3 (BBFO, x = 0, 0.03, 0.1) thin films were epitaxially grown on SrRuO3-buffered SrTiO3 (001) substrates by pulsed laser deposition. With increasing Ba content, the BBFO thin films show significantly reduced leakage currents but suppressed ferroelectric polarization. X-ray diffraction reciprocal space mappings and Raman spectra indicate a structural evolution from a rhombohedral-like to tetragonal-like phase in the BBFO thin films. Optical absorption and photoelectron spectroscopy measurements demonstrate a modulation of energy band structures in the BBFO thin films. With A-site Ba acceptor doping, the BBFO thin films exhibit a blue-shift of optical bandgap and an increase in work function. The energy positions of conduction and valence bands of the BBFO thin films have been modulated, and the Fermi level shifts down to the center of the forbidden band, but acceptor-doped BFO thin films still show n-type conduction. The presence of extra oxygen vacancies by acceptor doping is supposed to make contribution to conduction behavior. This study provides a method to manipulate the functional properties and gives insights into the physics of Ba doping in BFO thin films.

Stability of Interface Morphology and Thermal Boundary Conductance of Direct Wafer Bonded GaN|Si Heterojunction Interfaces Annealed at Growth and Annealing Temperatures

The properties of thin (~2 um) GaN templates on a silicon support substrate were studied to assess the stability of the direct wafer bonded GaN / Si interface. EVG® ComBond® equipment was used for bonding under high vacuum (~10-8 mtorr) at room temperature to remove unwanted native surface oxides [1,2]. For the bonded samples, the [1010] GaN edge was aligned parallel to the Si [110] edge. An X-ray diffraction reciprocal space map of the (004) Si and (0004) GaN revealed that there is a ~0.2° tilt between the GaN and Si layers and is simply due to the relative miscut between the two wafers. The treatment of the surfaces prior to bonding produces an amorphous region at the bonded interface that has been seen in many other bonded systems [3-5]. In the as-bonded sample, high resolution scanning transmission electron microscopy revealed a ~2 nm amorphous region on the Si side of the bonded interface, as confirmed by energy dispersive x-ray spectroscopy (EDX). Subsequent annealing was performed in an effort to recrystallize the amorphous interface. Previous work has shown that recrystallization between Si|Si wafer bonded samples occurred when annealed at 450 °C for 12 hours [3]. However, in the GaN|Si system, we found that the amorphous interface did not recrystallize when annealed under those conditions. Annealing at temperatures up to 450 °C and 120 hours showed only initial stages of interdiffusion and a stable interface. However, after annealing at 700 °C for 24 hours, high resolution EDX revealed the formation of amorphous SiN as well as the diffusion of gallium into silicon.Preliminary thermal results show that the thermal boundary conductance (TBC) of the as bonded sample is ~140 MW/(m2K). The TBC results of wafer bonded GaN|Si reported here is higher than previously reported TBC values of epitaxially grown interfaces such as GaN on Si [6], GaN on SiC [7], and GaN on diamond [8]. The TBC for the annealed interface is degraded by a factor of two compared to the as-bonded interface for the sample that was annealed at 700 °C for 24 hours. These results demonstrate that high TBC can be achieved through wafer bonding of GaN with materials such as silicon and that such interfaces are stable even up to device operation up to 300 °C. However, chemically rough interfaces formed due to high temperature annealing are detrimental to thermal transport across these interfaces. V. Dragoi, et al., ECS Trans., 86(5), 23 (2018)C. Flötgen, et al., ECS Trans., 64(5), 103 (2014)M.E. Liao, et al., ECS Trans., 86(5), 55 (2018)Y. Xu, et al., Ceramics International, 45, 6552 (2019)F. Mu, et al., Appl. Surf. Sci., 416, 1007 (2017)L. Yates, et al., ASME InterPACK (2015)J. Cho, et al., Phys. Rev. B, 89, 115301 (2014)H. Sun, et al., APL, 106(11), 111906 (2015) Figure 1

Varied domain structures in 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals

Single crystals of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-30PT) with a composition close to the morphotropic phase boundary have demonstrated remarkable electromechanical properties, stimulating intensive research interest in the field of piezoelectrics. Domain structures have long been associated with their overall piezoelectric properties, thereby garnering particular research interest for the enhancement of piezoelectric properties. Here, we report on three distinct domain structures in this material. Through a combination of X-ray diffraction reciprocal space mapping and piezoresponse force microscopy measurements, an M A crystal structure has been confirmed. The three-dimensional polar vectors of the three domain structures have been reconstructed with two 4M A and one 2M A domain structures. Correlations between different domain structures and their local electrical switching properties have been revealed. Our study of the PMN-30PT single crystal examines diverse domain structures at the mesoscale in detail, which provides valuable insight into the relationship between structures and properties of the material.

Vacancy-type defects in AlInN/AlN/GaN structures probed by monoenergetic positron beam

Vacancy-type defects in AlInN(10 nm)/AlN(1–2 nm)/GaN were probed by using a positron annihilation technique. The crystal quality of the AlInN layer and atomic diffusion near heterointerfaces were also studied by x-ray diffraction reciprocal space mapping, transmission electron microscopy, and energy-dispersive x-ray spectroscopy. For an as-deposited sample without an AlN spacer layer (AlInN/GaN), Ga atoms diffused into the AlInN layer, and as a result, the concentration of Ga-vacancy-type defects in the GaN layer increased. The vacancy concentration was decreased by inserting the AlN layer, which was attributed to the suppression of out-diffusion of Ga from the GaN layer. The effect of the thickness of the AlN layer on the mobility of two-dimensional electron gas is discussed in terms of the introduction of vacancies into the channel region. The annealing behaviors of vacancies in the GaN layer and atomic exchange near heterointerfaces are also discussed.

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

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.

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.

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.

Microstructural analysis of GaN films grown on (1 0 0) MgF2 substrate by 4D nanobeam diffraction and energy-dispersive X-ray spectrometry

The use of highly efficient and solarblind GaN photocathodes as part of multichannel plate UV detectors for applications in astronomy would strongly benefit from the direct growth of GaN on typical window materials with high transmission down to the deep UV range. GaN growth on MgF2 substrates by plasma-assisted molecular beam epitaxy has recently been demonstrated. Here, we report an extensive scanning transmission electron microscopy study of the thin film microstructure for growth at 525 °C and 650 °C on (100) MgF2. These results are systematically supported by X-ray diffraction reciprocal space maps. For both growth temperatures predominant cubic (111), (115) and (110) GaN is found with no preferred nucleation on the substrate and typical grain sizes of 100–200 nm. All observed orientations can be understood as the result of first and second order twins on different {111} planes related to the underlying substrate. The higher growth temperature shows a strongly increased twin density along with a higher surface roughness. Furthermore, grains with cubic (110) GaN growth show a reduced density and a reduced size of about 20 nm. In addition, in-diffusion of Mg and F into the GaN is observed, which is accompanied by the formation of cavities in the MgF2 directly at the interface.

Controlling the electronic and magnetic properties in epitaxial LaMnO3/LaScO3 superlattices

Perovskite superlattices (SLs) have attracted considerable interest owing to their rich and diverse physical properties. In this study, epitaxial LaMnO3/LaScO3 SLs were grown using pulsed laser deposition (PLD) by controlling the number of atomic monolayers. Raman spectroscopy combined with x-ray diffraction reciprocal space maps confirmed that the introduction of LaScO3 suppressed Jahn–Teller distortion in the SLs. Electrical transport measurements revealed a thermally activated single-gap behavior which mainly depended on the thickness of LaMnO3 layers. Magnetic measurements indicated that the magnetic properties of the SLs were related to the proportions of LaMnO3 and LaScO3 layers. These results are beneficial for the further understanding of the electronic and magnetic properties of LaMnO3-based SLs.

Robust perpendicular magnetic anisotropy in off-axis sputtered europium iron garnet (EuIG) thin films

The rare earth iron garnet (REIG), Eu3Fe5O12 (EuIG) has emerged as a promising material for ferrimagnetic insulator based spintronics. Earlier studies have shown that perpendicular magnetic anisotropy (PMA) in EuIG and other REIGs can be achieved by magnetostriction and misfit strain at the film-substrate interface. Typically, the strain relaxes significantly with thickness, limiting the thickness of films that can be grown using this strategy. Here we show PMA that does not reduce substantially with thickness in epitaxial EuIG films grown by off-axis sputtering; The PMA remains robust in films > 100 nm thick. Epitaxially induced strain with minimal relaxation is confirmed using high resolution symmetric and asymmetric x-ray diffraction reciprocal space maps.

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.

Non-invasive and non-destructive characterization of MBE-grown CdZnTe/CdTe superlattice-based dislocation filtering layers

We report on the structural and optical properties of heteroepitaxial II–VI CdTe (211)B buffer layers with strained CdZnTe/CdTe superlattice layers, investigated by employing non-destructive methods including high-resolution x-ray diffraction, cathodoluminescence, and photoluminescence spectroscopy. X-ray diffraction reciprocal space mapping measurements revealed that the superlattice layers are coherently strained, leading to a spread in x-ray double-crystal rocking curve full-width at half-maximum values but better in-plane lattice-matching with HgCdTe. Both cross-sectional cathodoluminescence and photoluminescence measurements confirm the coherent growth of superlattice layers and their dislocation filtering effects. Both these techniques in CdTe layers are found to be well correlated with the dislocation density as determined by etch pit density measurements. The results indicate the potential of these non-destructive methods to be further developed into general-purpose techniques capable of characterizing the defect evolution in semiconductor heteroepitaxy.

SiGe Epitaxial Growth on Si Substrate Using Al-Ge Paste

We investigated the liquid-phase growth of silicon-germanium (SiGe) epitaxial layers formed on Si substrates by a simple and rapid process of screen-printing of Al-Ge paste followed by high temperature annealing. Aluminum (Al) and germanium (Ge) (7:3) mixed paste was prepared, screen-printed on a Si substrate and annealed in an infra-red (IR) furnace at peak temperatures between 800°C to 1000°C. After reaching the Al melting point at 660°C, the melted Al dissolves the Si surface and Ge powder in the paste forming a thick liquid phase of melted Al-Ge-Si layer on the dissolved surface of the Si substrate. During the cooling process, a SiGe layer starts to grow epitaxially at the Si interface. The formed SiGe was observed by scanning electron microscope (SEM), and energy dispersive X-ray spectrometry (EDX) and characterized by X-ray diffraction reciprocal space mapping (XRD-RSM). The thickness of the SiGe layer depended on the peak temperature reaching about 25mm at 1000oC. The results suggest the epitaxially grown SiGe is strain-relaxed with a graded Ge concentration.

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.

High conductivity n-Al0.6Ga0.4N by ammonia-assisted molecular beam epitaxy for buried tunnel junctions in UV emitters

Highly doped n-Al0.6Ga0.4N can be used to form tunnel junctions (TJs) on deep ultraviolet (UVC) LEDs and markedly increase the light extraction efficiency (LEE) compared to the use of p-GaN/p-AlGaN. High quality Al0.6Ga0.4N was grown by NH3-assisted molecular beam epitaxy (NH3 MBE) on top of AlN on SiC substrate. The films were crack free under scanning electron microscope (SEM) for the thickness investigated (up to 1 µm). X-ray diffraction reciprocal space map scan was used to determine the Al composition and the result is in close agreement with atom probe tomography (APT) measurements. By varying the growth parameters including growth rate, and Si cell temperature, n-Al0.6Ga0.4N with an electron density of 4×1019 /cm3 and a resistivity of 3 mΩ·cm was achieved. SIMS measurement shows that a high Si doping level up to 2×1020 /cm3 can be realized using a Si cell temperature of 1450 °C and a growth rate of 210 nm/hr. Using a vanadium-based annealed contact, ohmic contact with a specific resistance of 10−6 Ω·cm2 was achieved as determined by circular transmission line measurement (CTLM). Finally, the n-type AlGaN regrowth was done on MOCVD grown UVC LEDs to form UVC TJ LED. The sample was processed into thin film flip chip (TFFC) configuration. The emission wavelength is around 278 nm and the excess voltage of processed UV LED is around 4.1 V.

Growth of highly relaxed InGaN pseudo-substrates over full 2-in. wafers

A highly relaxed InGaN buffer layer was demonstrated over a full two-inch c-plane sapphire substrate by metalorganic chemical vapor deposition. The InGaN buffer layer was grown on a 100 nm GaN decomposition stop layer with a 3 nm thick high indium composition InGaN underlayer. After thermal decomposition of the underlayer at 1000 °C, a 200 nm thick In0.04Ga0.96N buffer showed 85% biaxial relaxation measured by a high resolution x-ray diffraction reciprocal space map. When used as a pseudo-substrate for the regrowth of InGaN/InGaN multi-quantum wells, the sample showed a 75 nm red-shift in room temperature photoluminescence when compared to a co-loaded GaN template reference. The longer emission wavelength is associated with higher indium incorporation in the InGaN layers from the lessening of the compositional pulling effect caused by compressive strain. Using this technique, a simple red light emitting diode was demonstrated with an active layer growth temperature of 825 °C and a peak wavelength of 622 nm at a current density of 20 A cm−2. This work represents a unique method to relax a III-nitride based layer over a full substrate.

N-polar ScAlN and HEMTs grown by molecular beam epitaxy

Molecular beam epitaxy of single-phase wurtzite N-polar ScxAl1−xN (x ∼ 0.11–0.38) has been demonstrated on sapphire substrates by locking its lattice-polarity to the underlying N-polar GaN buffer. Coherent growth of lattice-matched Sc0.18Al0.82N on GaN has been confirmed by x-ray diffraction reciprocal space mapping of the asymmetric (105) plane, whereas lattice-mismatched, fully relaxed Sc0.11Al0.89N and Sc0.30Al0.70N epilayers exhibit a crack-free surface. The on-axis N-polar crystallographic orientation is unambiguously confirmed by wet chemical etching. High electron mobility transistors using N-polar Sc0.18Al0.82N as a barrier have been grown on sapphire substrates, which present the existence of well confined two-dimensional electron gas. A Hall mobility of ∼564 cm2/V s is measured for a 15-nm-thick Sc0.18Al0.82N barrier sample with a sheet electron concentration of 4.1 × 1013 cm−2, and the corresponding sheet resistance is as low as 271 Ω/sq. The polarity-controlled epitaxy of ScxAl1−xN provides promising opportunities for applications in high-frequency and high-power electronic and ferroelectric devices.