X-ray Reciprocal Space Mapping Research Articles

Influence of Growth Temperature of the Nucleation Layer on the Growth of InP on Si (001)

InP layers grown on Si (001) were achieved by the two-step growth method using gas source molecular beam epitaxy. The effects of growth temperature of nucleation layer on InP/Si epitaxial growth were investigated systematically. Cross-section morphology, surface morphology and crystal quality were characterized by scanning electron microscope images, atomic force microscopy images, high-resolution X-ray diffraction (XRD), rocking curves and reciprocal space maps. The InP/Si interface and surface became smoother and the XRD peak intensity was stronger with the nucleation layer grown at 350 °C. The Results show that the growth temperature of InP nucleation layer can significantly affect the growth process of InP film, and the optimal temperature of InP nucleation layer is required to realize a high-quality wafer-level InP layers on Si (001).

Recombination rates in green-yellow InGaN-based multiple quantum wells with AlGaN interlayers

The recombination rates in InGaN/AlGaN/GaN multiple quantum wells (MQWs) emitting in the green-yellow and grown with different Al compositions in the AlGaN interlayer (IL) are shown. By transforming measurements on radiative efficiency, absorption, and differential carrier lifetime, the radiative and nonradiative rates are determined. The IL Al composition controls lattice relaxation of the MQWs, as determined by X-ray reciprocal space mapping, and, therefore, defect formation. For the most pseudomorphic MQWs, the Shockley-Read-Hall (SRH) A coefficient is minimized and is similar to reports at shorter (blue and green) wavelengths. It is an order of magnitude smaller than a conventional InGaN/GaN MQW and is the most significant factor behind the improvement in radiative efficiency using the IL. The radiative B coefficient is also reduced and a minimum for the most pseudomorphic MQWs due to a reduction in the electron-hole wavefunction overlap. However, the decrease in A is more significant and leads to an overall improvement in the radiative efficiency. These recombination rate measurements confirm that if the SRH recombination is controlled, then the severe reduction of radiative recombination with an increased emitting wavelength is one of the main challenges in realizing high efficiency, long-wavelength InGaN-based MQW emitters operating at low to moderate current densities.

The dynamical theory of diffraction in a crystal modulated by a surface acoustic wave in the case of spatially restricted X-ray beams

The dynamical theory of X-ray diffraction in a crystal modulated by a surface acoustic wave (SAW) is developed for spatially restricted beams. It is shown that this approach is applicable to X-ray reciprocal space mapping. Rayleigh's surface-wave model is used to describe ultrasonic excitation. Based on the recurrent relations, a numerical simulation of the dynamical diffraction in a crystal modulated by a SAW is performed. Within the framework of the triple-axis diffraction scheme, the effect of the instrumental function on X-ray diffraction data is studied.

Effect of oxygen partial pressure on the electronic and magnetic properties of epitaxial SrRuO3 thin films

We have studied the electronic and magnetic properties of SrRuO3 thin films deposited by pulse laser deposition on SrTiO3 (100) substrate. X-ray diffraction and reciprocal space mappings show the epitaxial nature of the thin films and the formation of different structural phase such as tetragonal and tetragonal + monoclinic phase with oxygen partial pressure. In magnetization versus temperature behaviour we observed the large variation in Curie temperature with oxygen partial pressure. These variations are attributed to the defect induced modification in the local structure comprising RuO6 octahedra.

Strain-stress study of AlxGa1\u2212xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of AlxGa1−xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1−xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1−xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1−xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1−xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1−xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain on the optical phonon energies of the epitaxial layers is also discussed.

Thick hydride vapor phase epitaxial growth of ZnSe on GaAs (1 0 0) vicinal and orientation patterned substrates

Zinc selenide is attractive for numerous optical applications in the mid and longwave infrared from solid state lasers to non-linear optical frequency conversion. Its use has been limited by the availability of single crystal and epitaxial materials. To meet these challenges we have developed a low-pressure hydride vapor phase epitaxy (HVPE) process for growth of high quality thick ZnSe layers on GaAs (1 0 0) vicinal and orientation-patterned GaAs (OP-GaAs) substrates. We show growth of >300 µm thick layers at growth rates >50 µm/hr. With only 0.26% lattice mismatch between ZnSe and GaAs, epitaxial layers with crystal quality comparable to bulk ZnSe substrates are achieved. X-ray reciprocal space maps show the ZnSe to be fully relaxed. Cross-sectional TEM imaging analysis shows stacking faults at the interface that propagate in to the growing film consistent with a full relaxed film. Surface morphology shows the presence of pits that have their long axis perpendicular to [1–10] direction. Low temperature photoluminescence reveals band to band emission that consisted of the 2.797 eV direct band gap along with the strong exciton emission at 2.802 eV. Growth on OP templates revealed low fidelity domains of thickness >50 µm. This work presents a process to produce high quality thick layers potentially suitable for laser and frequency conversion for defense and civilian applications.

Liquid Phase Epitaxy growth of Tm3+-doped CaF2 thin-films based on LiF solvent

Single-crystalline Tm3+-doped calcium fluoride (Tm3+:CaF2) films were grown onto (100) oriented undoped bulk CaF2 substrates by Liquid Phase Epitaxy method using LiF as a solvent. The growth was performed at the temperatures of 853–865 °C. Crack-free transparent films with a high doping concentration of ∼6 at.% and a thickness up to ∼100 μm were obtained. The film structure and morphology were characterized by confocal microscopy, X-ray diffraction and scanning electron microscopy. High resolution X-ray diffraction and reciprocal space mapping confirmed the high crystallinity and the low mosaicity of the films. Energy-Dispersive X-ray spectroscopy revealed a uniform distribution of Tm3+ ions in the films. The Tm3+:CaF2 films exhibit a broadband emission at ∼2 μm and are of interest for waveguide lasers. The use of CaF2–LiF binary system is also promising for fabrication of other rare-earth-doped CaF2 layers.

Huge impact of compressive strain on phase transition temperatures in epitaxial ferroelectric KxNa1−xNbO3 thin films

We present a study in which ferroelectric phase transition temperatures in epitaxial KxNa1−xNbO3 films are altered systematically by choosing different (110)-oriented rare-earth scandate substrates and by variation of the potassium to sodium ratio. Our results prove the capability to continuously shift the ferroelectric-to-ferroelectric transition from the monoclinic MC to orthorhombic c-phase by about 400 °C via the application of anisotropic compressive strain. The phase transition was investigated in detail by monitoring the temperature dependence of ferroelectric domain patterns using piezoresponse force microscopy and upon analyzing structural changes by means of high resolution X-ray diffraction including X-ray reciprocal space mapping. Moreover, the temperature evolution of the effective piezoelectric coefficient d33,f was determined using double beam laser interferometry, which exhibits a significant dependence on the particular ferroelectric phase.

Highly polarized photoluminescence from c-plane InGaN/GaN multiple quantum wells on stripe-shaped cavity-engineered sapphire substrate

Highly polarized photoluminescence (PL) from c-plane InGaN/GaN multiple quantum wells (MQWs) grown on stripe-shaped cavity-engineered sapphire substrate (SCES) was realized. The polarization ratio was as high as 0.74 at room temperature. High-resolution X-ray reciprocal space mapping measurements revealed that the InGaN quantum wells on GaN/SCES template were under considerable anisotropic in-plane strain states of −1.178% and −1.921% along the directions perpendicular and parallel to the stripe-pattern, respectively. The anisotropic strain states were attributed to the anisotropic alignment of cavity-incorporated sapphire nano-membranes, which accommodated both anisotropic elastic relaxation in the InGaN quantum well plane as well as the graded elastic relaxation along the vertical direction in the GaN template adjacent to the InGaN/GaN MQWs. The partial strain relaxation in the InGaN wells also contributed to reduction of quantum confined Stark effect, resulting in four times higher PL intensity than InGaN/GaN MQWs on planar sapphire substrate. From theoretical calculations based on k∙p perturbation theory, it was found that fundamental origin of the polarized optical emission was strain-induced modification of valence band structures of the InGaN/GaN MQWs on the SCES. This study will allow us to realize light emitting diodes with highly polarized emission with conventional c-plane sapphire substrates by strain-induced valence band modification.

Indium incorporation in homoepitaxial β-Ga2O3 thin films grown by metal organic vapor phase epitaxy

Homoepitaxial β-(In,Ga)2O3 thin films were grown on (100) β-Ga2O3 substrates by metal organic vapor phase epitaxy using triethylgallium (TEGa) and triethylindium (TEIn). Deposition temperatures from 650 to 825 °C and pressures from 5 to 20 mbar have been explored. The growth rate decreased linearly with increasing deposition temperature and decreased exponentially with increasing pressure. The resulting films were characterized by atomic force microscopy (AFM), high resolution x-ray diffraction (HR-XRD), and transmission electron microscopy (TEM). As the flow rate of TEIn varied from 0 to 0.13 μmol/min during the growth, AFM showed the surface roughness of about 1 nm, while HR-XRD measurements revealed an increase of the vertical lattice spacing. The maximum atomic concentration of indium incorporated in monoclinic β-(In,Ga)2O3 is about 3.5% and shifts the optical absorption edge to lower energy by ∼0.18 eV. Further increase of the indium flow rate leads to an increase of the surface roughness and a decrease in the vertical lattice spacing due to the formation of a separate cubic In2O3 phase that was confirmed by HR-TEM images. X-ray reciprocal space maps showed that the β-(In,Ga)2O3 thin films were grown coherently on β-Ga2O3.

Optimization of MBE Growth Conditions of In0.52Al0.48As Waveguide Layers for InGaAs/InAlAs/InP Quantum Cascade Lasers.

We investigate molecular beam epitaxy (MBE) growth conditions of micrometers-thick In0.52Al0.48As designed for waveguide of InGaAs/InAlAs/InP quantum cascade lasers. The effects of growth temperature and V/III ratio on the surface morphology and defect structure were studied. The growth conditions which were developed for the growth of cascaded In0.53Ga0.47As/In0.52Al0.48As active region, e.g., growth temperature of Tg = 520 °C and V/III ratio of 12, turned out to be not optimum for the growth of thick In0.52Al0.48As waveguide layers. It has been observed that, after exceeding ~1 µm thickness, the quality of In0.52Al0.48As layers deteriorates. The in-situ optical reflectometry showed increasing surface roughness caused by defect forming, which was further confirmed by high resolution X-ray reciprocal space mapping, optical microscopy and atomic force microscopy. The presented optimization of growth conditions of In0.52Al0.48As waveguide layer led to the growth of defect free material, with good optical quality. This has been achieved by decreasing the growth temperature to Tg = 480 °C with appropriate increasing V/III ratio. At the same time, the growth conditions of the cascade active region of the laser were left unchanged. The lasers grown using new recipes have shown lower threshold currents and improved slope efficiency. We relate this performance improvement to reduction of the electron scattering on the interface roughness and decreased waveguide absorption losses.

Short range biaxial strain relief mechanism within epitaxially grown BiFeO3

Lattice mismatch-induced biaxial strain effect on the crystal structure and growth mechanism is investigated for the BiFeO3 films grown on La0.6Sr0.4MnO3/SrTiO3 and YAlO3 substrates. Nano-beam electron diffraction, structure factor calculation and x-ray reciprocal space mapping unambiguously confirm that the crystal structure within both of the BiFeO3 thin films is rhombohedral by showing the rhombohedral signature Bragg’s reflections. Further investigation with atomic resolution scanning transmission electron microscopy reveals that while the ~1.0% of the lattice mismatch found in the BiFeO3 grown on La0.6Sr0.4MnO3/SrTiO3 is exerted as biaxial in-plane compressive strain with atomistically coherent interface, the ~6.8% of the lattice mismatch found in the BiFeO3 grown on YAlO3 is relaxed at the interface by introducing dislocations. The present result demonstrates the importance of: (1) identification of the epitaxial relationship between BFO and its substrate material to quantitatively evaluate the amount of the lattice strain within BFO film and (2) the atomistically coherent BFO/substrate interface for the lattice mismatch to exert the lattice strain.

Epitaxial strain adaptation in chemically disordered FeRh thin films

Strain and strain adaption mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaption mechanism in a thin film model system as function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers, which allow for large variation of the preset lattice constants, e.g. epitaxial boundary condition. It is shown by means of high resolution X-ray reciprocal space maps and transmission electron microscopy that the system reacts with a tilting mechanism of the structural units in order to adapt to the lattice constants of the buffer layer. This response explained by density functional theory calculations, which evidence an energetic minimum for structures with a distortion of c/a =0.87. The experimentally observed tilting mechanism is induced by this energy gain and allows the system to remain in the most favorable structure. In general, it is shown that the use of epitaxial model heterostructures consisting of alloy buffer layers of fully miscible elements and the functional material of interest allows to study strain adaption behaviors in great detail. This approach makes even small secondary effects observable, such as the directional tilting of the structural domains identified in the present case study.

Doping-induced strain in heavily B-doped (100) diamond films prepared by hot-filament chemical vapor deposition

Diamond semiconductor has attracted attention for use in next-generation high-power, low-loss, high-radiation proof power devices. Heavily B-doping of diamond produces low-resistivity contacts that enhance the maximum output current, however, unintended lattice strain cause defect formations, degrading the device performance. In this study, we investigated the doping-induced lattice strain of heavily B-doped (100) films fabricated by hot-filament chemical vapor deposition. X-ray reciprocal space mapping indicated coherent, pseudomorphic growth between film and substrate. The perpendicular lattice strain as a function of B concentration followed Vegard's law, which accounts for substitutional incorporation. Synchrotron X-ray absorption fine structure showed no signature of lower-coordinated B-clusters. We believe that maintaining pseudomorphic growth is a key factor in suppressing electrically non-active B impurities.

Photoelectrochemical hydrogen generation using graded In-content InGaN photoelectrode structures

We have improved the InGaN/GaN heterointerface to achieve higher energy conversion efficiency by replacing a uniform InGaN layer with a graded In-content InGaN layer. Even In0.08Ga0.92N/GaN heterostructure has a large conduction band offset, which is large enough to suppress the photocurrent in the photocatalytic system. The graded In-content InGaN structures were grown by metalorganic vapor-phase epitaxy by changing the TMIn flow rate gradually. X-ray reciprocal space mapping confirmed the graded structures. The graded InGaN/GaN structure significantly increased photocurrent and H2 generation by 50% and more compared with the conventional uniform InGaN/GaN structures.

Real-time structural analysis of InGaAs/InAs/GaAs(1 1 1)A interfaces by in situ synchrotron X-ray reciprocal space mapping

Using in situ synchrotron X-ray reciprocal space mapping, this study investigates the evolution of lattice deformations in the InGaAs/GaAs(1 1 1)A structure with or without a thin InAs layer grown by molecular beam epitaxy. During the initial growth phase, InGaAs directly grown on GaAs(1 1 1)A showed an anomalous lattice shrinkage along the c-axis, with no change in the indium composition. Conversely, the InGaAs grown on InAs/GaAs(1 1 1)A showed no initial lattice distortion, but a variable indium composition. The evolution of the diffraction peak broadening was also monitored. The results confirmed that the thin InAs layer effectively improved the crystal quality during the initial growth of InGaAs.

Anomalous tilting in InGaAs graded buffers from dislocation sources at wafer edges

Uniform strain relaxation and defect densities across the entire wafer are important aspects of lattice-mismatched or metamorphic epitaxy of semiconductors. Achieving this requires an understanding of the sources of dislocations that lead to relaxation. We show that wafer edges are a major source of misfit dislocations in InGaAs graded buffers on GaAs substrates. AlInP solar cells fabricated from regions on the wafer affected by edge sources have a markedly lower performance due to a local increase in the threading dislocation density. Spatially resolved x-ray reciprocal space maps and electron backscattered diffraction maps of the InGaAs compositionally graded buffer reveal anomalous epi-layer tilting on 2-inch substrates from these dislocation sources at the wafer edge. The tilting abruptly changes direction partially through the graded buffer growth as the dislocations from the wafer edge appear in the x-ray sampling areas. We speculate that anisotropic misfit dislocation glide in III-V materials in combination with non-uniform stress states during growth biases slip system activity. These experiments demonstrate that lattice-mismatched epitaxy requires additional controls over the growth environment to achieve uniform strain relaxation for wafer scale heterogeneous integration.

Fabrication of Si1-xGex layer on Si substrate by Screen-Printing

AbstractThe impact of the Al and Ge ratio in the Al-Ge pastes are investigated for fabricating the single crystalline Si1-xGex thick layers on large area Si substrates by screen-printing metallization process. From X-ray reciprocal space maps, Ge fraction in the fabricated Si1-xGex thick layers are found to increase up to 40% with increasing the Ge ratio in the Al-Ge pastes. On the other hand, the interface of the Si and Si1-xGex layers are getting winding with increasing the Ge ratio in the Al-Ge pastes. The Al-Si-Ge phase diagram indicated that uniform SiGe layer can be fabricated by adjusting the Al-Ge ratio in the pastes within the liquid phase region.

Time-resolved X-ray reciprocal space mapping of a crystal in an external electric field

A reciprocal space mapping technique with the use of triple-crystal time-resolved X-ray diffractometry has been developed and implemented using a laboratory X-ray source for the first time. This technique allows studying fast processes that occur in a sample under external influences that cause reversible deformations of its crystal lattice. It also allows distinguishing these processes in time and distinguishing different types of crystal deformations caused by these actions. The essence of the technique is to measure time dependences of the intensity for each point of the reciprocal space in the vicinity of the diffraction maximum in three-axis diffraction geometry by subjecting the sample to repeated and structurally identical action of a strong electric field, with the subsequent construction of the time evolution of the two-dimensional reciprocal space map. The time resolution is achieved with the use of a high-speed multichannel intensity analyzer synchronized with a high-voltage source. The results of measuring the reciprocal space maps with a laboratory radiation source with a time resolution of up to 10 ms are demonstrated for a piezoelectric crystal of lanthanum gallium silicate subjected to an external electric field with the field strength 3.08 kV mm−1, which is close to the sample breakdown value.

Effect of ferro-elastic twin domains of LaAlO3 substrate on the magnetic anisotropy and the magnetization reversal process of cobalt thin film

Abstract Present work reports the effects of ferroelastic domains (FE) on the magnetic properties of top magnetic layer. Cobalt film of 15 nm is deposited using electron beam evaporation technique on single crystalline LaAlO 3 (0 0 1) (LAO) substrate which consists FE structural twin domains. LAO substrate is in-situ heated above its FE transition temperature and cooled to room temperature before depositing cobalt film of 15 nm. Textured growth of cobalt film is observed in-situ from reflection high energy electron diffraction (RHEED) patterns. The presence of FE structural twins in LAO substrate is confirmed from the X-ray reciprocal space mapping measurements. From the optical polarization microscopy, it is observed that the different regions consist different modulations viz., lamellar like and square-like structures in LAO substrate. The magnetic anisotropy and magnetization reversal is studied ex-situ using magneto-optical Kerr effect (MOKE) microscopy with different spatial resolutions. The analysis of azimuthal variation of coercivity ( H C ) shows the superposition of two-, four- and six- fold anisotropy contribution at different regions of the sample. Twofold anisotropy and weak sixfold anisotropy may due to magneto-crystalline anisotropy, whereas the fourfold anisotropy might be coming because of square-like modulations of the substrate. Moreover, MOKE microscopy reveals the domain wall pinning along the twin boundaries (FE) due to which magnetization reversal is proceed by three distinct mechanisms when field is applied away from easy axis of magnetization and lamellar pattern. As a consequence of this, magnetization reversal proceeds by ‘labyrinth switching’ and abrupt magnetic domain wall switching within individual FE domains.