High Resolution X-ray Diffraction Characterization Research Articles

Anomalous thermal conductivity of alkaline-earth-metal-substituted EuTiO3 induced by resonant scattering

We have investigated thermal conductivities (κ) of polycrystalline Eu1–xAxTiO3 (A = Ca, Sr, Ba, 0 ≤ x ≤ 0.8) bulk materials in the temperature range of ∼ 2 K <T < 1173 K. The EuTiO3 demonstrates anomalous glass-like κ(T) behavior at low temperatures. Partial substitutions with Sr2+ and Ba2+ do not cause a significant change in the κ(T) behavior, while a κ(T) peak, which looks like a manifestation of a typical crystalline solid, appears only in the Ca-substituted samples with an orthorhombic structure when x ≥ 0.4. After excluding the magnetic effects on κ and discussing the possible phonon scattering mechanisms in depth, together with heat capacity Cp measurements and high-resolution X-Ray diffraction characterization, we find that the unusual low κ at low temperatures is attributed to resonant scattering induced by the intrinsic disordered local structure and lattice instability in EuTiO3. Due to the different lattice dynamics of ATiO3, the lattice structure of Eu1–xCaxTiO3 can be regarded as formed by a part-soft (from EuTiO3) part-rigid (from CaTiO3) sublattice. The anomalous κ(T) behavior of Eu1–xCaxTiO3 results from the combined effect of phonon transport between the normal phononic heat transport in the rigid sublattice and the strong damping of heat conduction in the soft sublattice.

Removal of Ciprofloxacin from Aqueous Solution Using Prepared Biodegradable Potato Cross-linked Starch Ester

Potato starch was modified chemically to prepare an adsorbent that is efficient for removing the ciprofloxacin antibiotic from its aqueous solution. The chemical modification process of potato starch involved cross-linking and esterification steps. The high-resolution X-ray diffraction characterization shows that the modification processes do not damage the starch crystal but reduces the crystallinity and the crystal size. In addition to that, holes on the surface of the round and oval shapes of the modified starch were noticed via scanning electron microscope. The parameters that could affect ciprofloxacin removal efficiency on cross-linked potato starch ester were investigated in detail, which included initial concentration of ciprofloxacin in aqueous solution, pH and temperature. The removal efficiency of ciprofloxacin increased with the increase in the initial pH value of the aqueous solution and reached the highest value of removal at a pH of 7, after which it was noticed that the removal efficiency decreased. The removal efficiency was decreased with increasing initial concentration of ciprofloxacin (expressed as chemical oxygen demand) from 25 to 125 ppm. Finally, the results showed that the increase in the temperature treatment from 25 to 45 °C negatively affected the removal efficiency.

Epitaxial growth of InAs/GaAs quantum dots on {113}-faceted Ge/Si (001) hollow substrate

The direct epitaxial growth of GaAs on Si suffers from their nature of lattice mismatch, thermal mismatch and polarity difference induced anti-phase domains (APDs). Here, we report the high quality and thin GaAs film grown on {113}-faceted Ge/Si (001) hollow substrate by in-situ hybrid molecular beam epitaxy. By directly growth of Ge on U-shape patterned Si (001), a strain-relaxed high-quality Ge sawtooth hollow structure with {113} facets was obtained. With an additional 400 nm GaAs deposition, an APD-free surface with a root-mean-square roughness of merely 0.67 nm is obtained on such Ge {113} /Si (001) substrate. The lattice mismatch dislocation between Ge and Si is found to terminate mostly at the sidewalls of the hollow structures. The {113}-faceted Ge surface is acting as an equivalent to the miscut substrate, which annihilates the APDs at the GaAs/Ge interface. High-resolution X-ray diffraction characterization reveals that the hollow structures can effectively reduce the thermal strain, leading to a crack-free GaAs film up to 7 µm. Five-layer InAs/GaAs quantum dots (QDs) on such virtual GaAs/Ge {113} /Si (001) substrate without any dislocation filter layers exhibits almost the same photoluminescence (PL) intensity as that on the GaAs substrate, providing a promising method for integrating III-V QD lasers with silicon photonic platform.

Strain engineering on the metal-insulator transition of VO2/TiO2 epitaxial films dependent on the strain state of vanadium dimers

In this letter, a series of vanadium dioxide (VO2) epitaxial films were deliberately deposited on TiO2 substrates with different orientations [(001), (110), and (101)], in an attempt to gain insights into the strained VO2 epitaxial film. We found in-plane [100] and [1-10] directions, obviously anisotropic metal-insulator transition (MIT) in (110)-oriented VO2 films. In combination with synchrotron radiation high-resolution x-ray diffraction characterizations, electronic transport data reveal that the critical temperature of MIT depends on the strain state of the dimeric vanadium atomic chain along the c axis of the rutile phase. The anisotropy of MIT is closely related to the orientation of the VO2 films, which is caused by the varied orientation configuration of V-V atomic chain dimerization in the films. Soft x-ray absorption spectroscopy results further indicate that this anisotropy may be driven by the directional hybridization of O 2p and V 3d orbitals with respect to the orientation of VO2 thin films. The polarization-dependent V L-edge and O K-edge XAS data suggest that the elongation of the apical V-O bond length increases the p-d orbital overlap; thus, the energy level of the d// orbital is raised relative to that of the π* orbital. These anisotropic MIT behaviors will help us to understand how the strain engineering depends on the strain state of vanadium dimers in VO2 films.

Structure and bandgap nonlinearity in BaSn1−xTixO3 epitaxial films

BaSn1−xTixO3 (BSTO, x = 0, 0.25, 0.5, 0.75, 1.0) epitaxial films were deposited on LaAlO3(001) substrates by employing a pulsed laser deposition system. Their composition dependent structure and optical properties were systematically investigated using density functional theory (DFT) calculations. High resolution x-ray diffraction characterization showed that the film lattice parameters decrease linearly with the increase in Ti concentrations, following Vegard's law. Optical property measurements revealed that all films have the optical transmittance of more than 70% in the visible and infrared wavelength range. Interestingly, the bandgap nonlinearity of BSTO films was observed, and the film at Ti doped x = 0.25 has the largest bandgap value of 4.04 eV. The calculated electronic structure and the density of states of BSTO films determined by DFT suggested that the strong hybridization between Sn 5s and Ti 3d orbitals caused the initial expansion of the bandgap, whereas upon further increasing the doping content, Ti 3d orbitals dominated the conduction band, leading to the bandgap reduction.

Structural characteristics of m-plane AlN substrates and homoepitaxial films

Homoepitaxial non-polar AlN films were realized on m-plane (101-0)-oriented AlN single crystals by metalorganic chemical vapor deposition (MOCVD). The microstructural properties of m-plane AlN substrates and homoepitaxial films were assessed by means of atomic force microscopy and high resolution x-ray diffraction characterization. Results indicated that both m-plane AlN substrates and films possessed exceptional structural quality, with some anisotropic mosaic distributions due to the quasi-bulk nature of the non-polar single crystals. An increase in the MOCVD growth temperature was noted to minimize the degree of inherited mosaic anisotropy without altering the m-plane AlN film growth rate, indicating that high temperature growth is critical to produce optimal film crystallinity. A dramatic change in the film surface morphology from heavily faceted “slate-like” features to monolayer steps was observed as the growth temperature was increased. The “slate-like” surface morphology produced low intensity cross-streaks in symmetric (101-0) reciprocal space maps, tilted about 18° away from the (101-0) crystal truncation rod. The orientation of these diffuse streaks corresponds to the physical alignment of the slates with respect to the substrate surface normal. X-ray line scans and defect-selective reciprocal space mapping confirmed that these low intensity streaks are solely dependent on this peculiar surface structure produced at low MOCVD growth temperatures and unrelated to basal plane stacking faults or other extended defects. All observations confirm that high quality III-nitride epitaxial structures on m-plane AlN substrates are attainable with controllable MOCVD growth processes, as demanded for future high performing AlN-based non-polar devices.

(Invited) High-Resolution X-Ray Diffraction Characterization and Metrology for Advanced Logic

High-Resolution X-ray Diffraction (HRXRD) is a well-established technique for the characterization and metrology of epitaxial materials encountered in a variety of important technologies such as nano, opto and power electronics. Traditionally the technique was mainly used for blanket films, and this is still a significant application. However, complex epitaxial nanostructures continue to grow in importance to enable advanced CMOS logic transistors that must combine high-performance with low power consumption and cost with scaled dimensions. Hence, the need to measure and analyze such structures becomes necessary. Several new transistor architectures being considered for logic nodes beyond 10 nm, including more sophisticated FinFETs, gate-all-around (GAA) FETs and the use of fully-depleted silicon-on-insulator (FDSOI) substrates. Equally varied is the range of epitaxial materials proposed, which include SiGe, SiP, Ge as well as III-V materials. Each of these innovations has specific challenges for both process development and control. In this presentation we will discuss the recent advances in HRXRD tools, measurements and analysis methods for the characterization and metrology of arrays of nanostructures on patterned wafers with a spot-size < 50 µm and production worthy throughputs and reliability. We will discuss how HRXRD measurements such as high-speed reciprocal space maps (RSMs) are used to determine the composition, strain, and quality of epitaxial nanostructures, and illustrate the challenges and results using representative examples drawn from our recent collaborations.

1550-nm InGaAsP multi-quantum-well structures selectively grown on v-groove-patterned SOI substrates

We report direct growth of 1550-nm InGaAsP multi-quantum-well (MQW) structures in densely packed, smooth, highly crystalline, and millimeter-long InP nanoridges grown by metalorganic chemical vapor deposition on silicon-on-insulator (SOI) substrates. Aspect-ratio-trapping and selective area growth techniques were combined with a two-step growth process to obtain good material quality as revealed by photoluminescence, scanning electronic microscopy, and high-resolution X-ray diffraction characterization. Transmission electron microscopy images revealed sharp MQW/InP interfaces as well as thickness variation of the MQW layer. This was confirmed by atom probe tomography analysis, which also suggests homogenous incorporation of the various III-V elements of the MQW structure. This approach is suitable for the integration of InP-based nanoridges in the SOI platform for new classes of ultra-compact, low-power, nano-electronic, and photonic devices for future tele- and data-communications applications.

Polymorph separation induced by angle distortion and electron delocalization effect via orbital modification inVO2epitaxial thin films

Since Morin discovered that vanadium dioxide $(\mathrm{V}{\mathrm{O}}_{2})$ undergoes a reversible and dramatic structural phase transition coupled with an abrupt metal-insulator transition, extensive attention has been paid to $\mathrm{V}{\mathrm{O}}_{2}$ due to its importance in fundamental condensed state physics and its potential technological applications. Here, we observed that the precipitated phases of $\mathrm{V}{\mathrm{O}}_{2}$ (insulating and metallic polymorphs) could be controlled by relaxing the dimerization of the vanadium-vanadium (V-V) atomic chain. In particular, the monoclinic metallic phase can be stabilized even at room temperature with the assistance of the angle-distortion-induced $(\ensuremath{\beta}={120}^{\ensuremath{\circ}})$ metallization through symmetry matching between the $\mathrm{V}{\mathrm{O}}_{2}$ epitaxial thin films and the (0001)-oriented sapphire substrates. Concomitantly, the insulating phase $(\text{M}1,\ensuremath{\beta}=122.{6}^{\ensuremath{\circ}})$ that separates from the metallic matrix may supply another driving force for stabilizing the metallic phase, as indicated by scattering-type scanning near-field optical infrared microscopy and further confirmed by synchrotron radiation high-resolution x-ray diffraction characterizations. Soft x-ray absorption spectroscopy results showed that the orbital features of the monoclinic metallic phase are analogous to those of the high-temperature metallic rutile $\mathrm{V}{\mathrm{O}}_{2}$ (R) phase. First-principles calculations further demonstrate the angle-distortion-induced reduction of the V-V atomic dimerization, which enhances the electron delocalization and thus the conductivity. Therefore, the angle distortion results in the metallic monoclinic phase and stabilizes it with the assistance of the nanoscale insulating $\mathrm{V}{\mathrm{O}}_{2}$ (M1) domains at room temperature. These results are of great importance for understanding the contributions of various polymorphs to the metal-insulator transition and for the design of novel artificially heterointerfacial devices based on $\mathrm{V}{\mathrm{O}}_{2}$ nanoscale polymorphs.

Quantum size effect on cubic PbTe nanocrystals embedded in amorphous InSe thin film matrix

PbTe nanocrystals (NCs) embedded in a multilayered structure with amorphous InSe has been prepared and investigated. PbTe/InSe multilayered structures composed of alternating InSe and PbTe layers were prepared with different numbers of PbTe NCs layers by thermal evaporation technique. The presence of cubic PbTe NCs was confirmed from transmission electron microscopy images of the film cross section. High resolution X-ray diffraction characterization demonstrated face centered cubic rock salt structure of PbTe and the average size of PbTe NCs are varied by the number of PbTe NCs layers. Quantum size effect was revealed by the blue shift in the absorption edges of the multiple PbTe NCs layers structure with respect to the reference PbTe films. The experimentally observed absorption edge shift is compared with the theoretical values predicted based on the effective mass approximation model.

Epitaxial growth of (111)-oriented ZrxTi1−xN thin films on c-plane Al2O3 substrates

A systematic study is presented on the effects of process parameters of S-gun configured DC magnetron sputtered ZrN thin films on c-plane Al2O3 substrates. Using a quartz crystal microbalance the deposition rate of ZrN is investigated as a function of Ar and N2 flow rates, target power, chamber pressure and gas injection position in the chamber. Selected growth conditions for ZrN show the interrelation of growth parameters on film orientation and crystallinity. (111) oriented ZrN thin films exhibit X-ray diffraction rocking curve FWHM as low as 0.36°. Additionally, (111) oriented ternary ZrxTi1−xN thin films (0≤x≤1) are also deposited on c-plane Al2O3 substrates. High resolution X-ray diffraction characterization shows that ZrxTi1−xN (x=0, 0.64, 0.80, 0.93, 1) layers exhibit rocking curve FWHM values of 0.0045–0.006° for the (111) reflection, indicating highly crystalline thin films. Atomic force microscopy characterizations show ZrxTi1−xN thin films with a surface roughness between 1.2nm and 2.9nm.

THE EFFECT OF In0.1Ga0.9As UNDERLYING LAYER ON THE STRUCTURAL PROPERTIES OF SELF-ASSEMBLED In0.5Ga0.5As QUANTUM DOTS

The effect of a thin In0.1Ga0.9As underlying layer on the structural properties of single layer In0.5Ga0.5As quantum dots (QDs) was investigated using atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD) characterization. The size of dots formed on the surface is uniform but the density increases with the addition of In0.1Ga0.9As underlying between In0.5Ga0.5As QDs and GaAs buffer layer. This is consistent with the TEM characterization. The existence of thin underlying layer has caused the dots to have different crystal orientation as shown in TEM characterization. From the HR-XRD characterization, broad peak of In0.1Ga0.9As underlying layer and QDs has been observed. The wider width of the layer peak than the expected one has been attributed to the strain-relaxation-induced defects. The growth of a thin In0.1Ga0.9As underlying layer in the In0.5Ga0.5As/GaAs structures strongly affects the structural properties, which was also believed to influence the optical properties of QDs.

Study of GaAsBi MOVPE growth on (1 0 0) GaAs substrate under high Bi flow rate by high resolution X-ray diffraction

GaAsBi alloy was grown on (1 0 0) GaAs substrate by metalorganic vapour phase epitaxy. GaAsBi film was elaborated with V/III ratio of 9.5, trimethyl bismuth molar flow rate of about 3 μmol/min and a growth temperature of 420 °C. The surface morphology of GaAsBi alloy was investigated by means of scanning electron microscopy and atomic force microscopy. Results show surface Bi droplets formation. High-resolution X-ray diffraction (HRXRD) curves present more diffraction peaks other than that of GaAs substrate. Detailed HRXRD characterization shows that diffraction peaks splitting do not represent a crystallographic tilting with respect to GaAs substrate. Diffraction patterns also show a remarkable stability of the alloy against thermal annealing.

Epitaxial growth of InN on c-plane sapphire by pulsed laser deposition with r.f. nitrogen radical source

We have grown InN films on c-plane sapphire substrates by pulsed laser deposition (PLD) with a radio frequency nitrogen radical source for the first time and investigated the effect of the substrate surface nitridation on the structural and electrical properties of InN films with reflection high energy electron diffraction (RHEED), atomic force microscope, the Hall effect measurements and high-resolution X-ray diffraction (HRXRD). RHEED and HRXRD characterizations revealed that high-quality InN grows epitaxially on sapphire by PLD and its epitaxial relationship is InN (0 0 0 1)∣∣sapphire (0 0 0 1) and InN [2 -1 -1 0]∣∣sapphire [1 0 -1 0]. The InN crystalline quality and the electron mobility are improved by the substrate nitridation process. The area of the pits at the InN surface is reduced by the substrate nitridation process probably due to the reduction in the interface energy between InN and the substrate. The full width at half maximum of the -1 -1 2 4 X-ray rocking curve for InN grown by the present technique without using any buffer layers was as small as 34.8 arcmin. These results indicate that the present technique is promising for the growth of the high-quality InN films.

Indication of hysteresis in AlMnN

AlN films grown by gas-source molecular beam epitaxy were doped with different levels of Mn during growth. High resolution x-ray diffraction characterization revealed good crystallinity in single phase material, with lattice constant decreasing with increasing Mn concentration. Single phase AlMnN was found to be p type while AlMnN/AlMn mixed phase material was found to be highly conductive n type. Magnetization measurements performed with a superconducting quantum interference device magnetometer indicated ferromagnetism in single phase material persisting to 300 K and showed no evidence of room temperature magnetization in multiphase material. In particular, it was shown that Mn4N second phases are not contributing to the magnetization in the AlMnN under optimized growth conditions.

Raman Scattering Study of PbSe Grown on (111) BaF2 Substrate

PbSe films were grown on (111)-oriented BaF2 substrates by using molecular beam epitaxy. High resolution x-ray diffraction characterization showed good crystalline quality of PbSe films. Both longitudinal optical phonon at 135 cm-1 and transverse optical phonon at 47.6 cm-1 were observed by Raman scattering measurements. The Raman tensor calculation demonstrates that both transverse-optical and longitudinal-optical (LO) phonons in PbSe crystal are Raman active on (111)-oriented surface. Furthermore, 2LO phonon at about 270 cm-1 and polaron at about 800 cm-1 in PbSe, were also observed. The observed Raman frequencies are in good agreement with theoretical calculations using point ion model.

High Resolution X-ray Diffraction Characterization of [111]B Oriented InGaAs/GaAs Mqw Structures

In this work we apply high-resolution X-ray diffractometry to the study of InGaAs/GaAs multiple quantum well structures on (001) and(lll)B GaAs substrates. The samples consisted of p-i-n diodes with a multiple quantum well embedded in the i-region and were simultaneously grown on (001) and (111)B substrates by molecular beam epitaxy. For the characterization we have used symmetric and asymmetric reflections at different azimuthal positions. The interpretation of the diffraction profiles has been possible thanks to our recently developed simulation model, which allows the calculation of any reflection regardless of the substrate orientation. X-ray results about composition and thickness are very similar in the samples simultaneously grown on both orientations as expected from our specific growth conditions. The information obtained from X-ray characterization is consistent with the results of photoluminescence and photocurrent measurements within the experimental uncertainty of the techniques. In (lll)B samples, X-ray diffractometry provides structural information which cannot be easily obtained from optical characterization techniques.

High resolution x-ray diffraction characterization of semiconductor structures

Abstract The high-resolution X-ray diffraction techniques, such as double-crystal rocking and triple-crystal diffraction, have become essential tools in the semiconductor materials and devices laboratory. Quick and non-destructive characterization is possible for basic layer parameters including layer composition, strain, mismatch and thickness. More sophisticated characterization of interface and quantum-well structures with submonolayer resolution is possible with the help of kinematical diffraction principles and diffraction profile simulation using the dynamical diffraction theory. These X-ray diffraction techniques are making it possible to correlate the structural properties to the epitaxial growth process, to the electrical,/optical properties and to the device performance. In this review article, I discuss some basic aspects of the instrument, simple determination of layer parameters, the dynamical and kinematical diffraction theories, and some recent results in the characterization of the heterostructure interfaces and strained quatum-well devices. I also discuss the diffuse scattering analysis of bulk semiconductors in the Bragg diffraction geometry. Interface analysis of the lattice-matched structures such as GaInAs/InP and AlGaAs/GaAs is discussed in terms of kinematical diffraction principles, namely the X-ray phase shift and the crystal truncation rod. Analysis of strained quantum wells is discussed using Bragg peak profile and Pendellosung fringe profile. Diffuse scattering in bulk semiconductors is discussed in the context of the analysis theory, namely the rocking-curve profile and the radial intensity profile around a reciprocal lattice point of the diffused scattered X-rays due to such defects as dislocation loops and spherical defect clusters.