Triple-axis X-ray Diffraction Research Articles

Thermal transport and structural improvements due to annealing of wafer bonded β-Ga2O3|4H-SiC

The impact of postbond annealing on the structural and thermal characteristics of 130 nm thick exfoliated (201) β-Ga2O3 (via H+ ion implantation) wafer bonded to (0001) 4H-SiC was studied. Thirty nanometer amorphous-Al2O3 was grown on the β-Ga2O3 substrates prior to bonding as an interlayer between β-Ga2O3 and 4H-SiC. The surface activated bonding technique was utilized for bonding, which induces a thin nanometer amorphous interfacial region at the bonded interface (Al2O3|4H-SiC). We demonstrate annealing the bonded structure at 800 °C up to 1 h is beneficial: (1) the removal of residual strain in the exfoliated β-Ga2O3 layer that was due to the exfoliation implant, (2) reduction of lattice mosaicity in the β-Ga2O3 layer, (3) nearly complete recrystallization of the amorphous bonded interfacial region, and (4) partial recrystallization of the initially amorphous-Al2O3 interlayer. The thermal characteristics correspondingly improve with the improvement in structural characteristics. The thermal conductivity of the as-bonded β-Ga2O3 layer was 2.9 W/m K, and the thermal boundary conductance of the bonded interface was 66 MW/m2 K. After annealing at 800 °C for 1 h, triple-axis x-ray diffraction ω:2θ measurements showed a reduction in strain for the β-Ga2O3 layer and the symmetric (201) rocking curve widths. We simultaneously observe a doubling of the β-Ga2O3 thermal conductivity to 6.0 W/m K and a 20% increase in the thermal boundary conductance. However, upon further annealing up to 10 h and fully recrystallizing both the Al2O3 interlayer and bonded interface, the thermal boundary conductance dropped by ∼30%. This preliminary result suggests that crystalline heterointerfaces may not necessarily be the most optimal interfacial structure for thermal transport.

Experimental evidence of auxeticity in ion implanted single crystal calcite

We report initial experimental evidence of auxeticity in calcite by ion implanting (1010) oriented single crystalline calcite with Ar+ at room temperature using an ion energy of 400 keV and a dose of 1 × 1014 cm−2. Lattice compression normal to the substrate surface was observed, which is an atypical result for ion implanted materials. The auxetic behavior is consistent with predictions that indicate auxeticity had been predicted along two crystallographic directions including [1010]. Materials with a positive Poisson’s ratio experience lattice expansion normal to the substrate surface when ion implanted, whereas lattice contraction normal to the surface is evidence of auxetic behavior. Triple-axis X-ray diffraction measurements confirmed the auxetic strain state of the implanted calcite substrates. Reciprocal space maps for the symmetric 3030 and asymmetric 1450 reflections revealed that the implanted region was fully strained (pseudomorphic) to the bulk of the substrate, as is typical with implanted single crystals. A symmetric (3030) ω:2θ line scan was used with X-ray dynamical diffraction simulations to model the strain profile and extract the variation of compressive strain as a function of depth normal to the substrate surface. SRIM calculations were performed to obtain a displacement-per-atom profile and implanted Ar+ concentration profile. It was found that the strain profile matches the displacement-per-atom profile. This study demonstrated the use of ion implantation and X-ray diffraction methods to probe mechanical properties of materials and to test predictions such as the auxeticity.

A-axis oriented Zn0.72Mg0.28O epitaxial thin films with large second-order nonlinear susceptibility

Large nonlinear susceptibility that originates from strong electronic polarization enables ultrafast nonlinear optical devices. This work discovers that Mg incorporation has important contribution for enhancing second-order nonlinear susceptibility (χ (2)) and laser-induced surface-damage threshold of wurtzite ZnO epitaxial thin films deposited by radio-frequency magnetron sputtering method. Second-harmonic generation measurements derive that as-deposited Zn0.72Mg0.28O shows a gain of 48%, 77% and 33% in χ 33, χ 31 and χ 15 with respect to as-deposited ZnO. Specially, the annealed Zn0.72Mg0.28O has a χ 33 value of −57.0 ± 1.8 pm V−1, which gets comparable to that of LiNbO3 crystals. Triple-axis x-ray diffraction measurements conclude that the Mg incorporation should increase the χ (2) under optical frequency electric field by strengthening electronic polarization rather than increasing the residual strain in the film. Furthermore, the annealed Zn0.72Mg0.28O exhibits an increase of 48% in laser-induced surface-damage threshold relative to [11–20] ZnO crystals. These findings open the way of the Zn0.72Mg0.28O thin films to ultrafast nonlinear optical devices.

Exfoliation of β-Ga2O3 Along a Non-Cleavage Plane Using Helium Ion Implantation

An important step in the successful transfer of controlled thickness, wafer-scale β-Ga2O3 layers is demonstrated through exfoliation via helium ion implantation. He+ ions were implanted into epi-ready (010) β-Ga2O3 substrates at room temperature with an ion energy of 160 keV and a dose of 5 × 1016 cm−2. The implanted substrate was then annealed at 200°C followed by 500°C. The lower temperature step initiated He bubble nucleation while the higher temperature step promoted He bubble growth at the projected range beneath the substrate surface. No surface blistering was observed after the 200°C anneal. After the 500°C anneal step, micron-sized surface blistering covering the entire implanted area was observed, confirming exfoliation of β-Ga2O3 along a non-cleavage plane. Triple-axis X-ray diffraction ω:2θ measurements showed that after annealing at 500°C, the implantation-induced strain was removed. The ω scans showed some peak broadening after the low temperature step and more extensive broadening after the high temperature step; this observation corresponds to an increase in lattice plane tilt due to the formation of He bubbles at the projected range. Prior to annealing, scanning transmission electron microscopy images showed a dark contrast at the projected range due to the implantation of a high concentration of He ions. After the 500°C anneal, large cracks at the projected range were observed. These observations match early reports of silicon blistering which, when combined with established direct wafer bonding practices, is expected to lead to large-scale transfer of controlled thickness β-Ga2O3 layers along non-cleavage-plane orientations.

Defect Characterization of Multicycle Rapid Thermal Annealing Processed p-GaN for Vertical Power Devices

Post-implantation damage and defect characterization of multicycle rapid thermal annealing (MRTA) activated Mg+ implanted (0001) GaN on (110) sapphire substrate was investigated using high resolution X-ray scattering and transmission electron microscope techniques. The initial implant-induced isolated defects produced a pseudomorphic strain that matches the calculated recoil damage profile in the GaN - along the [0001] direction. Triple axis X-ray diffraction showed removal of the implant-induced elastic strain during two different MRTA annealing processes. An improvement in crystalline quality after the different MRTA processes was also noted, indicating that the MRTA processes can reduce crystalline strain and defects in as-grown material. For the different MRTA processes, an improvement in electrical conductivity was correlated with a lower (0004) triple axis rocking curve width and with a reduced presence of extended defects observed from electron microscopy measurements. The extended defects were identified to be circular defects with an observable screw component. The defect density was on the order of 1010 cm−2. While both MRTA processes led to a high activation efficiency, a higher residue defect density was correlated with a greater X-ray scattering rocking curve width of the symmetric (0004) reflection, which was further correlated with a lower hole mobility (20 cm2/V⋅s vs 40 cm2/V⋅s).

Microstructure and strength of AlN–SiC interface studied by synchrotron X-rays

Bulk AlN crystals grown by sublimation on SiC substrates exhibit relatively high dislocation densities. The kind of defect formation at early growth stages influences the structural quality of the grown crystals. In this work, the dislocation distribution near to the interface is understood through investigation of thin (≤1.5 mm) continuous (non-cracked) freestanding crystals obtained in one process with the evaporation of the substrates. The AlN specimens were characterized using synchrotron radiation imaging techniques. We revealed by triple-axis X-ray diffraction study that, near to the former interface, randomly distributed dislocations configured to form boundaries between \(\sim \)0.02\(^{\circ }\) misoriented sub-grains (from [0001] direction). Threading dislocation structure similar to that in epitaxial GaN films was not detected. To explain these observations, a theoretical model of misfit stress relaxation near the interface is suggested.

Analyses of determination conditions of n-GaN dislocation density by triple-axis X-ray diffraction

Dislocation densities of two hydride vapor phase epitaxy-grown hexagonal GaN samples, which are Si doped and unintentionally doped respectively, are determined by triple-axis X-ray diffractometry and van der Pauw variable temperature Hall-effect measurement. The dislocation densities of these two samples should be at the same level from the X-ray testing, the -FWHM (full width at half maximum) values of all corresponding reflections for these two samples are almost the same. But from the Hall-effect measurements, the dislocation density values should be different from each other remarkably, because the unintentionally doped sample belongs to Mott transition material, while the Si-doped one does not. This fact indicates that the X-ray testing is perhaps inaccurate under some conditions, although the triple-axis X-ray diffractometry is a highly suitable technique for discriminating different kinds of structural defects such as edge and screw dislocations that lead to characteristic broadening of symmetric and asymmetric Bragg reflection. The experimental result obtained so far (say, for hot-electron bolometer) shows that the dislocation density value from mobility fitting model is in good accordance with that from -FWHM fitting using Srikant method. The anomaly that the dislocation density from -FWHM fitting is much lower than that from mobility fitting for the same sample (sample 59#), indicates that dislocations located in grain boundary may not be tested by triple-axis X-ray diffractometry. According to mosaic model, the layer is assumed to consist of single crystallites, called mosaic blocks, which are assumed to be slightly misoriented with respect to each other. The out-of-plane rotation of the block perpendicular to the surface normal is of the mosaic tilt, and the in-plane rotation around the surface normal is of the mosaic twist. The average absolute values of tilt and twist angles are directly related to the FWHM values of the corresponding distributions of crystallographic orientations. So, the X-ray testing can determine the average orientation of the grains with the same interplanar distance, excluding the information about the grain boundary at which X-ray cannot interfere because of disdortion of lattice. The experimental results and calculation analyses indicate that the dislocation density value from Srikant model is accurate when the ratio of twist angle to tilt angle exceeds 2.0, or the magnitude of the lateral coherence length is larger than 1.5 m.

Piezoelectric Ca3NbGa3Si2O14 crystal: crystal growth, piezoelectric and acoustic properties

Ca3NbGa3Si2O14 (CNGS), a five-component crystal of lanthanum-gallium silicate group, was grown by the Czochralski method. The parameters of the elementary unit cell of the crystal were measured by powder diffraction. The independent piezoelectric strain coefficients \(d{}_{11}\) and \(d_{14}\) were determined by the triple-axis X-ray diffraction in the Bragg and Laue geometries. Excitation and propagation of surface acoustic waves (SAW) were studied by high-resolution X-ray diffraction at BESSY II synchrotron radiation source. The velocity of SAW propagation and power flow angles in the \(Y\)-, \(X\)- and \(yxl/{+}36^\circ\)-cuts of the CNGS crystal were determined from the analysis of the diffraction spectra. The CNGS crystal was found practically isotropic by its acoustic properties.

Control of symmetric properties of metamorphic In0.27Ga0.73As layers by substrate misorientation**Project supported by the National Natural Science Foundation of China (Grant No. 61376065) and the Suzhou Science and Technology Project, China (Grant No. ZXG2013044).

Asymmetry in dislocation density and strain relaxation has a significant impact on device performance since it leads to anisotropic electron transport in metamorphic materials. So it is preferred to obtain metamorphic materials with symmetric properties. In this paper, we grew metamorphic In0.27Ga0.73As epilayers with symmetric low threading dislocation density and symmetric strain relaxation in two 〈110〉 directions using InAlGaAs buffer layers on 7° misoriented GaAs (001) substrates. To understand the control mechanism of symmetric properties of In0.27Ga0.73As layers by the substrate miscut angles, In0.27Ga0.73As grown on 2° and 15° misoriented substrates were also characterized as reference by atomic force microscopy, transmission electron microscopy, and high resolution triple axis x-ray diffraction. The phase separation and interaction of 60° misfit dislocations were found to be the reasons for asymmetry properties of In0.27Ga0.73As grown on 2° and 15° substrates, respectively. Photoluminescence results proved that the In0.27Ga0.73As with symmetric properties has better optical properties than the In0.27Ga0.73As with asymmetric properties at room temperature. These results imply that high quality metamorphic In0.27Ga0.73As can be achieved with controllable isotropic electron transport property.

X-ray diffraction by phase diffraction gratings

The diffraction properties of phase gratings with the period D = 1.6, 1.0 and 0.5 µm fabricated on an Si(111) crystal by e-beam lithography were studied by triple-axis X-ray diffraction. A 100 nm-thick tungsten layer was used as a phase-shift layer. It is shown that the presence of a grating as a phase-shift W layer on the surface of the Si(111) crystal causes the formation of a complicated two-dimensional diffraction pattern related to the diffraction of X-rays on the phase grating at the X-ray entrance and exit from the crystal. A model of X-ray diffraction on the W phase diffraction grating is proposed.

Chemical Mechanical Polishing of III-V Materials for Wafer Bonding Applications

ABSTRACT Wafer bonding of III-V cells offers opportunities to integrate materials with different bandgaps and different lattice parameters without forming high defect densities. However, growth conditions that produce the best cells do not necessarily produce epitaxial layer surfaces that are suitable for wafer bonding. The chemical mechanical polishing of III-V materials including GaAs, InP, InAs, and alloy layers used in high efficiency III-V solar cell structures is investigated with sodium hypochlorite and citric acid solutions. It is found that the surfaces can be polished to below 0.5 nm RMS surface roughness without the induction of crystalline damage using similar abrasive-free polishing solutions for all the materials with controlled polishing rates of 10 nm / min. The low removal rate is key for applications that include epitaxial layers that must be subsequently polished without l=excessive loss of material. The actual composition of the slurry is adjusted for the particular alloy composition. A balance between reduced sub-surface mechanical damage and smooth surface morphology is obtained by adjusting the amount of citric acid similar to Br-methanol polishing. However, the damage-free planarization in these cases may be aided by an oxide formation as predicted by Pourbaix diagrams. Subsequent bonding suing sulfur passivation on the CMP surface shows excellent electrical characteristics with low resistivity as well as high strength bonding after annealing at 400 °C. Additionally, triple axis x-ray diffraction is found to be an extremely sensitive, non-destructive method for evaluating damage induced by the CMP process.

New optical transition, structural, and ferromagnetic properties of InCrP:Zn implanted with Cr

InCrP:Zn was prepared using the implantation with Cr concentrations of 0.3% and 0.7%, respectively. It was confirmed that the photoluminescence peaks near 0.85(D, Cr)eV and 0.96(e,Cr)eV were Cr-correlated PL bands by the implantation of Cr. Especially, each 0.85(D, Cr)eV and 0.96(e,Cr)eV peaks were separately observed based on InP. In triple-axis x-ray diffraction patterns, the samples revealed a shoulder peak indicative of intrinsic InCrP. Ferromagnetic hysteresis loops measured at 10K and 300K were observed and the temperature-dependent magnetization showed ferromagnetic behavior ≥300K, which reveals obvious and enhanced ferromagnetic spin coupling mediated by hole.

X-ray diffraction analyses of RF sputtered Ba0.6Sr0.4TiO3 thin films grown on Pt/Ti/LaAlO3 (100) substrates

Ba0.6Sr0.4TiO3 ferroelectric thin films were deposited on Pt/Ti/LaAlO3 (100) substrates by RF magnetron sputtering system. This paper focused on X-ray diffraction (XRD) analyses of Ba0.6Sr0.4TiO3 thin films on single-crystal LaAlO3 (LAO) substrates by three aspects research. Triple-axis XRD was utilized to investigate the mosaic defects of single-crystal LAO substrate. Both the out-of-plane grazing incidence mapping and ω-scan techniques were applied to research the crystallographic oriented growth of polycrystalline Ba0.6Sr0.4TiO3 thin films. X-ray Φ-scans and pole figures were used to assess the orientation relationship between the film and substrate.

InGaN-based multi-quantum well light-emitting diode structure with the insertion of superlattices under-layer

This paper reports on the improved crystalline quality of In0.11Ga0.89N/In0.02Ga0.98N multi-quantum well (MQW) light-emitting diode (LED) structure with the insertion of n-AlN/n-GaN multilayer (ML) and n-Al0.06Ga0.94N/n-GaN strained-layer superlattices (SLSs) cladding under-layer. The LED sample structure was epitaxially grown on Si (111) substrate by metal organic chemical vapor deposition (MOCVD) system with laminar-flow horizontal growth reactor. From X-ray rocking curves (XRCs) omega-scan (ω-scan), the n-Al0.06Ga0.94N/n-GaN SLS under-layer facilitate in improving the crystalline quality of the LED sample structures by reducing the threading dislocation density (TDD). Reciprocal space mapping (RSM) confirms the strain state of the GaN epitaxial layers. Triple axis (TA) X-ray diffraction (XRD) omega-2Theta (ω-2θ) scan reveals a higher-order fringes indicate the ML and MQW layers with sharp layer interfaces. Photoluminescence (PL) measurement of the sample with n-Al0.06Ga0.94N/n-GaN SLS under-layer shows band-to-band photon energy of 2.97eV with narrower full-width at half-maximum (FWHM) as compared to the sample with n-Al0.03Ga0.97N under-layer. Surface morphology examination by atomic force microscopy (AFM) demonstrate smoother sample surface for the sample with n-Al0.06Ga0.94N/n-GaN SLS as compared to conventional sample without the SLS layer.

Effect of high-temperature buffer thickness on quality of AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition

The effect of an initially grown high-temperature AlN buffer (HT-AlN) layer's thickness on the quality of an AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) in a two-step growth process is investigated. The characteristics of AlN epilayers are analyzed by using triple-axis crystal X-ray diffraction (XRD) and atomic force microscopy (AFM). It is shown that the crystal quality of the AlN epilayer is closely related to its correlation length. The correlation length is determined by the thickness of the initially grown HT-AlN buffer layer. We find that the optimal HT-AlN buffer thickness for obtaining a high-quality AlN epilayer grown on sapphire substrate is about 20 nm.

Clarification of enhanced ferromagnetism in Be-codoped InMnP fabricated using Mn/InP:Be bilayers grown by molecular beam epitaxy

The p-type InMnP:Be epilayers were prepared by the sequential growth of Mn/InP:Be bilayers using molecular-beam-epitaxy and the subsequent in-situ annealing at 200–300 °C. In triple-axis x-ray diffraction patterns, the samples revealed a shoulder peak indicative of intrinsic InMnP. The ferromagnetic transition in InMnP:Be was observed to occur at the elevated temperature of ∼140 K, and the ferromagnetic spin-domains clearly appeared in magnetic force microscopy images. The improved ferromagnetic properties are attributed to the increased p–d hybridation due to high p-type conductivity of InMnP:Be (p ∼ 1020 cm−3). The results suggest that enhanced ferromagnetism can be effectively obtained from Be-codoped InMnP.

Improvement of crystal quality of AlN grown on sapphire substrate by MOCVD

AbstractThe crystalline quality of aluminum nitride (AlN) epilayers grown on sapphire substrates by MOCVD was improved by increasing hydrogen flow rate during the high temperature growth process. The AlN epilayer exhibited a root mean square (rms) of roughness was 1.944 nm from the 2×2 µm2 size atomic force microscopy (AFM) images. Full widths at half maximum (FWHMs) of (002) and (102) rocking curves of triple‐axis high resolution X‐ray diffraction (HRXRD) measurements were as narrow as 28.8 arc sec and 868 arc sec, respectively. The optical transmittance spectra showed a sharp absorption edge at a wavelength of 200 nm and strong Fabry‐Perot (FP) oscillations. It is proposed that the improvement in crystalline quality is due to the surface in the low‐temperature aluminum nitride (LT‐AlN) buffer layer is promoted to be stable Al‐polarity by the conditions of increasing hydrogen flow rate and ramping up the growth temperature. Addtionally, the parasitic reactions are effectively suppressed by increasing the hydrogen flow rate during the growth process of high temperature. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fully relaxed low-mismatched InAlAs layer on an InP substrate by using a two step buffer

The strain relaxation in low mismatched InxAl1−xAs layers has been studied by triple axis x-ray diffraction, transmission electron microscopy, and photoluminescence. Using a two step buffer, a fully relaxed top layer has been grown by adapting the composition and thickness of a first “strained layer.” The threading dislocation density in the top layer is below 106/cm2 and strain is relaxed at the substrate/first layer interface by misfit dislocations. This scheme is a promising method to limit the thickness of buffer layers and obtain fully relaxed pseudosubstrates.

Mg composition dependent strain analysis in nonpolar a-plane MgxZn1−xO films

Nonpolar a-plane (112¯0) MgxZn1−xO (a-MgxZn1−xO) films are deposited on (011¯2) r-sapphire substrates using metalorganic chemical vapor deposition with varying Mg composition (x from 0 to 0.25). Unit cell parameters with Mg composition are determined by high-resolution triple-axis x-ray diffraction. In-plane strain along the c-axis [0001] and m-axis [11¯00] in the films is anisotropic and increases with increasing Mg composition. The in-plane strain anisotropy changes with Mg composition in a-MgxZn1−xO. Calculations are carried out to determine the influence of Mg content on the residual interfacial strain.

Chemical Mechanical Polishing of Exfoliated III-V Layers

The chemical mechanical polishing of III-V materials including GaAs, InP, InAs, and GaSb is investigated with sodium hypochlorite and citric acid solutions. It is found that the surfaces can be polished to below 0.5 nm RMS surface roughness without the induction of crystalline damage using similar abrasive-free polishing solutions for all the materials with controlled polishing rates of 10 nm / min which is important for touch polishing of exfoliated III-V layers. The optimal composition of the slurry is adjusted for the particular material. A balance between reduced sub-surface mechanical damage and smooth surface morphology is obtained by adjusting the amount of citric acid. However, the damage-free planarization in these cases may be aided by an oxide formation as predicted by Pourbaix diagrams. Additionally, triple axis x-ray diffraction is found to be an extremely sensitive, non-destructive method for evaluating damage induced by the CMP process.