Synchrotron Radiation X-ray Topography Research Articles

Synchrotron radiation x-ray topography applied to nitride semiconductor crystals

Gallium nitride (GaN) and aluminum nitride (AlN), as examples of third-generation semiconductors, have attracted significant interest due to their remarkable physical attributes, including a wide bandgap, high breakdown voltage, exceptional chemical stability, and high thermal conductivity. These characteristics render GaN and AlN highly promising for use in power and (opto)-electronic devices. Consequently, there is a growing demand for high-quality GaN and AlN crystals on the centimeter scale. As the dislocation density in these materials decreases, the need for a reliable method of dislocation characterization becomes more pressing. Synchrotron radiation x-ray topography (SR-XRT) has emerged as a superior, nondestructive technique for the precise characterization of crystal defects. This review briefly introduced the principle of XRT, and its application in the analysis of dislocations in GaN and AlN crystals is summarized. By examining the relationship between the SR-XRT image contrast and the Burgers vectors of dislocations, it is possible to categorize wafer dislocations and determine the magnitude and direction of Burgers vectors. Additionally, SR-XRT facilitates the analysis of interactions between dislocations in GaN and AlN crystals. These analyses are instrumental in advancing the development of superior crystals. This review concludes with a discussion of the current challenges faced by SR-XRT and a projection of its future applications in characterizing third-generation semiconductor crystal materials. This review offers significant guidance for the characterization of nitride crystal defects using SR-XRT.

Three-dimensional distribution and propagation of dislocations in β-Ga2O3 revealed by Borrmann effect x-ray topography

Synchrotron radiation x-ray topography (XRT) in a transmission configuration based on the Borrmann effect (BE) was carried out to observe characteristic dislocation structures and three-dimensional distribution and propagation of dislocations in β-Ga2O3 grown via the edge-defined film-fed growth (EFG) method. Substrates with a range of surface orientations of (001), (010), and (2¯01), cut perpendicular or parallel to the ⟨010⟩ growth direction of the EFG, were observed to understand the whole picture of dislocations distributed in the bulk crystals. Using the (001)-oriented substrate, we found characteristic dislocation structures such as dislocation helices, damage-related (001)-plane dislocation networks, and tangled dislocation complexes, which exist universally in EFG crystals but have rarely been reported before. A careful measurement of the dislocation length in BE-XRT images taken with different g-vectors allows us to determine the crystal plane on which a dislocation lies. The BE-XRTs taken from the (010)-oriented and (2¯01)-oriented substrates suggested that the dislocations propagating along the [010] growth direction were dominant. Most of these b-axis threading dislocations had a Burgers vector of [010] or [001], and they tended to align in the (100) plane. The BE-XRT observations in this study provide valuable knowledge for understanding the structure and character of dislocations in β-Ga2O3.

Domain boundaries in ScAlMgO4 single crystal observed by synchrotron radiation x-ray topography and reticulography

We performed synchrotron radiation x-ray topography (XRT) and reticulography to observe dislocations and crystal misorientation at domain boundaries (DBs) in a scandium magnesium aluminate single-crystal substrate grown via the Czochralski method. By inserting a fine-scale x-ray-absorbing mesh between the sample and the topograph-recording medium, reticulography was clearly observed allowing for direct measurement of twist and tile domain misorientation >5 × 10−5 rad. The results show that the angular resolution can be further improved to better than 2 × 10−5 rad by applying a different mesh-to-film distance M and then comparing the relative displacement in their reticulographs. The character of dislocations aligning at the DBs revealed by XRT analysis is consistent with the reticulography results. Finally, we demonstrate a method that combines reticulography and XRT into a single reticulographic XRT image, from which dislocation observation and misorientation measurement can be performed simultaneously.

Mechanical behavior prediction of additively manufactured components based on defect evolution observation by synchrotron radiation X-ray tomography

Defects including inclusions and voids significantly affect the mechanical properties of the additive manufacturing materials. It is necessary to precisely capture the defects and determine their hazardous effects on material mechanical properties. In this paper, a damage model is developed to describe the nucleation, growth, and coalescence of voids in additive manufacturing materials, revealing the nature of true stress drop. In order to characterize the defect morphology and depict the defect evolution, an in-situ tensile test with synchrotron radiation X-ray topography (SRXT) is carried out. Statistical reconstruction of the initial voids morphology are used as input for the established damage model. Furthermore, in light of the epistemic uncertainty in the process of defect reconstruction in SRXT, Bayesian framework is adopted for parameter estimation. Finally, the above model is verified by the data form 3D defect reconstruction and the uniaxial tensile test, where the constitutive behavior as well as its scatter are well captured. This work contributes to the depiction on damage evolution and the correspondingly affected deformation performance, which can be useful in material design and defect control for additive manufactured load-bearing structures.

Bulk crystal growth and characterization of 3-HBST: A promising organic nonlinear optical crystal with high quality

High yield 3-hydroxy benzaldehyde-N-methyl 4-stilbazolium tosylate (3-HBST) was synthesized by optimizing the reaction temperature. High-quality bulk crystals of 3-HBST were grown by slow evaporation technique and spontaneous nucleation method, respectively. The crystal structure of 3-HBST was determined by single crystal X-ray diffraction (XRD) technique and belongs to monoclinic system. The defects of the 3-HBST crystals were observed by the synchrotron radiation X-ray topography. 3-HBST crystal remains stable until 305 °C, with a melting point of 214.1 °C from the TG-DTG and DSC techniques. The dielectric constant of 3-HBST crystal was also studied. The grown crystal had good optical transparency and the optical transmittance was investigated by UV-vis-NIR spectral analysis. Photoluminescence (PL) indicated green emission occurred at 478 nm. Based on the growth technical analysis and performance parameters measurements, 3-HBST crystal was verified to be a promising organic nonlinear optical crystal.

Synthesis, growth, structure analysis and performance characterization of DOST crystal: a nonlinear functional material with a wide range of applications

A new non-centrosymmetric organic ion nonlinear optical crystal, DOST.

Synchrotron radiation X-ray topography and defect selective etching analysis of threading dislocations in halide vapor phase epitaxy GaN crystal grown on ammonothermal seed

Halide vapor phase epitaxy GaN crystal is examined, in terms of threading dislocations, by two experimental techniques: synchrotron radiation X-ray topography and defect selective etching. The obtained results are analyzed and compared. Three kinds of threading dislocations are found. Other defects in the crystal are also shown. A correlation between defects determined by the two experimental methods is presented and discussed.

Effect of disordered structure and crystal defects on heat transfer behavior in Er:Yb: YCa4O(BO3)3 crystal

The anomalous glasslike behavior of heat transfer in Er:Yb: YCa4O(BO3)3 crystal was explored by means of temperature dependence Raman scattering spectra and synchrotron radiation X-ray topography. Crystal samples were prepared by the Czochralski technique, and the corresponding thermal properties, including thermal expansion, specific heat and thermal conductivity were investigated. The temperature-dependent Raman scattering results indicate the frequencies of Raman peaks as a function of crystal temperature followed a linear dependency on crystal temperature with different slopes. Nevertheless, the temperature-dependent linewidths of different Raman peaks were inconsistent and complicated. The X-ray topography firstly revealed a core situated in the center of crystal. Surrounding the core, a number of other defects like striations and dislocations were observed. The existence of such defects can lead to phonon scattering, which will introduce complicated influence into heat transfer process. As a result, the anomalous behavior of heat transfer can be influenced by disordered structure and crystal defects.

Bulk Growth of Low Resistivity n-Type 4H-SiC Using Co-Doping

The growth of n-type 4H-SiC crystal was performed by physical vapor transport (PVT) growth method by using nitrogen and aluminum (N-Al) co-doping. Resistivity of N-Al co-doped 4H-SiC was as low as 5.8 mΩcm. The dislocation densities of N-Al co-doped substrates were evaluated by synchrotron radiation X-ray topography (SXRT). In addition, epitaxial growth was performed on the N-Al co-doped substrates by chemical vapor deposition (CVD). No double Shockley type stacking fault was observed in the epitaxial layer.

MOVPE growth of GaN on 6-inch SOI-substrates: effect of substrate parameters on layer quality and strain

We demonstrate that higher crystalline quality, lower strain and improved electrical characteristics can be achieved in gallium nitride (GaN) epitaxy by using a silicon-on-insulator (SOI) substrate compared to a bulk silicon (Si) substrate. GaN layers were grown by metal–organic vapor phase epitaxy on 6-inch bulk Si and SOI wafers using the standard step graded AlGaN and AlN approach. The GaN layers grown on SOI exhibited lower strain according to x-ray diffraction analysis. Defect selective etching measurements suggested that the use of SOI substrate for GaN epitaxy reduces the dislocation density approximately by a factor of two. Furthermore, growth on SOI substrate allows one to use a significantly thinner AlGaN buffer compared to bulk Si. Synchrotron radiation x-ray topography analysis confirmed that the stress relief mechanism in GaN on SOI epitaxy is the formation of a dislocation network to the SOI device Si layer. In addition, the buried oxide layer significantly improves the vertical leakage characteristics as the onset of the breakdown is delayed by approximately 400 V. These results show that the GaN on the SOI platform is promising for power electronics applications.

Evolution of impurity incorporation during ammonothermal growth of GaN

Ammonothermally grown GaN is a promising substrate for high-power optoelectronics and electronics thanks to its scalability and high structural perfection. Despite extensive research, ammonothermal GaN still suffers from significant concentrations of impurities. This article discusses the evolution of impurity incorporation during growth of basic ammonothermal GaN, in specific whether the impurity concentration changes temporally along the growth direction and how the autoclave influences the impurity concentration. The effect of the impurities on the structural, electrical and optical properties of the grown crystal is also discussed. The chemical analysis is carried out by time of flight secondary ion mass spectroscopy (ToF-SIMS) and laser-ablation inductively-coupled plasma mass spectroscopy (LA-ICP-MS). Strain and dislocation generation caused by impurity concentration gradients and steps are studied by synchrotron radiation x-ray topography (SR-XRT). Fourier transform infrared (FTIR) reflectivity is used to determine the effect of the impurities on the free carrier concentration, and the luminescent properties are studied by low temperature photoluminescence (PL). The influence of the autoclave is studied by growing a single boule in multiple steps in several autoclaves. LA-ICP-MS and ToF-SIMS ion intensities indicate that the impurity concentrations of several species vary between different autoclaves by over an order of magnitude. SR-XRT measurements reveal strain at the growth interfaces due to impurity concentration gradients and steps. Oxygen is determined to be the most abundant impurity species, resulting in a high free carrier concentration, as determined by FTIR. The large variation in Mn concentration dramatically affects PL intensity.

Slip system analysis and X-ray topographic study on β-Ga2O3

Slip system and possible dislocation characters in a wide-bandgap semiconductor β-Ga2O3 have been studied. The space lattice is monoclinic in which oxygen sublattice has a distorted cubic close-packed structure consisting of {2¯01}, {101}, {3¯10} and {3¯1¯0} planes. The shortest and second shortest translation vectors on each plane are possible Burgers vectors of a dislocation. Dislocations in a (2¯01)-oriented wafer were observed by means of synchrotron radiation X-ray topography with reflection geometry. The observed dislocations were analyzed in relation to the slip system. Using the g→·b→ invisibility criterion and the obtained slip system, Burgers vectors of some typical dislocations were identified.

Formation of helical dislocations in ammonothermal GaN substrate by heat treatment

GaN substrate produced by the basic ammonothermal method and an epitaxial layer on the substrate was evaluated using synchrotron radiation x-ray topography and transmission electron microscopy. We revealed that the threading dislocations present in the GaN substrate are deformed into helical dislocations and the generation of the voids by heat treatment in the substrate for the first observation in the GaN crystal. These phenomena are formed by the interactions between the dislocations and vacancies. The helical dislocation was formed in the substrate region, and not in the epitaxial layer region. Furthermore, the evaluation of the influence of the dislocations on the leakage current of Schottky barrier diodes fabricated on the epitaxial layer is discussed. The dislocations did not affect the leakage current characteristics of the epitaxial layer. Our results suggest that the deformation of dislocations in the GaN substrate does not adversely affect the epitaxial layer.

Defect structure of a free standing GaN wafer grown by the ammonothermal method

White beam synchrotron radiation X-ray topography (SR-XRT) and X-ray diffraction (XRD) measurements were used to non-destructively study the defect structure of a bulk GaN wafer, grown by the ammonothermal method. SR-XRT topographs revealed high crystal quality with threading dislocation density 8.8×104cm−2 and granular structure consisting of large, slightly misaligned grains. The threading dislocations within grains were identified as mixed and screw type, while no pure threading edge dislocations were observed.

Large-area analysis of dislocations in ammonothermal GaN by synchrotron radiation X-ray topography

The large-area defect structure of high-quality ammonothermal GaN was studied by synchrotron radiation X-ray topography (SR-XRT) and high-resolution X-ray diffraction (XRD). The threading dislocation densities of mixed and screw dislocations were determined separately by SR-XRT and were 3.2 × 104 and 3.1 × 103 cm−2, respectively. Threading edge dislocations were not observed. SR-XRT images show that TMDs are clustered on a large scale and form short dislocation arrays with an average spacing between dislocations of 12 µm. XRD rocking curves were used to measure an average lattice tilt of 10 arcsec caused by the dislocation arrays.

Synchrotron radiation x-ray topography and defect selective etching analysis of threading dislocations in GaN

The crystal quality of bulk GaN crystals is continuously improving due to advances in GaN growth techniques. Defect characterization of the GaN substrates by conventional methods is impeded by the very low dislocation density and a large scale defect analysis method is needed. White beam synchrotron radiation x-ray topography (SR-XRT) is a rapid and non-destructive technique for dislocation analysis on a large scale. In this study, the defect structure of an ammonothermal c-plane GaN substrate was recorded using SR-XRT and the image contrast caused by the dislocation induced microstrain was simulated. The simulations and experimental observations agree excellently and the SR-XRT image contrasts of mixed and screw dislocations were determined. Apart from a few exceptions, defect selective etching measurements were shown to correspond one to one with the SR-XRT results.

Synchrotron-Radiation X-Ray Topography of the Rapid Grown KDP Crystals

KDP crystals were rapidly grown from point seeds by temperature reduction method. White-beam synchrotron radiation X-ray topography was used to study the growth imperfections in KDP crystals. Strong dislocation bunches originate on the prismatic faces of the seed were found. The producing reason of these dislocation bunches was discussed. Sector boundary between the prismatic sector and pyramid sector caused by the unbalance distribution of trivalent metal ions were also observed and analyzed.

Synchrotron radiation X‐ray topography and X‐ray diffraction of homoepitaxial GaN grown on ammonothermal GaN

AbstractIn this study, homoepitaxial GaN grown on ammonothermal GaN substrates is non‐destructively investigated by synchrotron radiation X‐ray topography (SR‐XRT) and X‐ray diffraction (XRD). The homoepitaxial GaN layer was found to be of excellent crystal quality with individual dislocations clearly visible in the SR‐XRT images. SR‐XRT images and XRD rocking curves suggest the dislocations are mainly of mixed type. To the best of our knowledge the measured dislocation density is the lowest reported in a homoepitaxial GaN film (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observation of Domain Structures Near the α-INC-β Phase Boundary in Quartz Using X-ray Topography

The domain structures near the α-INC-β phase boundary in quartz were simultaneously observed using synchrotron radiation X-ray topography and optical microscopy. In (1–10) topographic images of Z-cut samples, a fine structure corresponding to the “m-zone” in the optical microscopic image was observed. In (3–31) topographic image, at the “fog-zone” of the α phase side, a marble-like domain, which has never been seen before, with clear contrast was observed. A section topography of the marble-like domain showed that this domain formed a needle shape along the Z-axis. Therefore, the boundary of these domains is the origin of the property of opalescence.

X‐ray diffraction study of GaN grown on patterned substrates

AbstractOwing to its great potential in optoelectronic devices, a lot of effort has recently been put into improving the crystal quality of heteroepitaxial GaN. In this study, GaN layers grown on patterned GaN/sapphire substrates are non‐destructively investigated by X‐ray diffraction techniques. The effect of the patterning is characterized by determining the strain and lattice tilt and twist in the overgrown GaN layers.High resolution X‐ray diffraction (HRXRD) scans and reciprocal space maps (RSMs) were utilized for this purpose. Synchrotron radiation X‐ray topography (SR‐XRT) was used to examine the microstructure of the samples. The patterning was found to significantly improve the crystal quality of the GaN layers, especially by reducing the amount of edge dislocations. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)