High-resolution X-ray Topography Research Articles

Dislocation analysis of epitaxial InAsSb on a metamorphic graded layer using x-ray topography

Dislocations in compositionally graded virtual substrates and InAsSb epitaxial layers for long wavelength (8–12 μm) photodetectors have been investigated with high-resolution x-ray topography (XRT). By varying the imaging conditions, the properties of the virtual substrate and InAsSb could be individually characterized. We observe the formation of misfit dislocations near the interface predominantly along the (110) direction and with less relaxation along the (1–10) direction. The misfit dislocations do not form a uniform array of dislocations but rather appear as dislocation bundles. Threading dislocation clusters, which could limit device performance, are observed along the misfit arrays with a density ∼1 × 105 cm−2 and a total averaged density of less than 1 × 106 cm−2. The prospects for using XRT for further optimization of virtual substrate and development of low defect bulk InAsSb layers are discussed.

High crystal quality of vertical Bridgman and edge-defined film-fed growth β-Ga2O3 bulk crystals investigated using high-resolution X-ray diffraction and synchrotron X-ray topography

High crystal quality of vertical Bridgman (VB)-grown (001) β-Ga2O3 single-crystal was confirmed in comparison with edge-defined film-fed growth (EFG)-grown (001) β-Ga2O3 single-crystal. The FWHMs of VB were lower than FWHMs of EFG from X-ray diffraction rocking curve analysis. VB and EFG crystal curvatures were measured as 232.24 m and 45.83 m, respectively. By using transmission and reflection geometry in synchrotron X-ray topography, we unambiguously determined the Burgers vector ( b ) = [010] and [001]. For the VB crystal, screw dislocations with b = [010] and dislocation direction ( t ) = [010] were observed. For the EFG crystal, edge dislocations with b = [001] and t = [010] were observed. In addition, wandering dislocations on the (001) plane with b = [010] for EFG and b = [001] for both EFG and VB were observed. These phenomena indicate that the wandering dislocations were formed by a slip motion on the (001) slip plane.

X-ray diffraction analysis and X-ray topography of high-quality ScAlMgO4 substrates

X-ray diffraction analysis through high-resolution X-ray rocking curve measurement and X-ray topography was performed on Czockralski (CZ) -grown ScAlMgO4 (SAM) substrates. An extremely narrow rocking curve width of 0.002° (7.2 arcsec) was obtained, and the three-dimensional (3D) configuration of dislocations in the interior of the SAM crystal structure was analyzed through 3D section topography. Burgers vectors of dominating dislocations with slip planes of (0001) were determined to be parallel to the a-axis of the SAM crystal, and the dislocations were found to form clusters.

X-ray Topography Characterization of GaN Substrates Used for Power Electronic Devices

Gallium nitride (GaN) substrates grown by different methods were characterized by high-resolution x-ray diffraction and synchrotron x-ray topography. Using the monochromatic beam in the grazing incidence geometry, high-resolution x-ray topographs reveal the various dislocation types present. Dislocation contrasts were correlated with ray-tracing simulation results successfully so that the Burgers vectors of the dislocations could be determined. Ammonothermal-grown GaN substrate wafers show the best quality among all the wafers. These wafers, which are free of basal plane dislocations (BPDs) have threading mixed dislocations (TMDs) dominant among the threading dislocations (TDs). Images of patterned hydride vapor phase epitaxy (HVPE) GaN reveal a starkly heterogeneous distribution of dislocations with large areas containing low threading dislocation densities in between a grid of strain centers with higher threading dislocation densities and BPDs. The strain level of regular HVPE GaN substrates is very high, and the dislocation density is around 105–106 cm−2, which is much higher than 104 cm−2 of ammonothermal samples and dislocation-free areas in the patterned HVPE samples.

(Invited) X-Ray Metrology of AlN Single Crystal Substrates

As development of GaN and SiC material systems for applications in power switching progresses, the ultrawide-bandgap (UWBG) materials, with bandgaps greater than GaN, are gaining increased attention, in pursuit of improved power device performance. Substrate technology is critical to improve this field. High-quality native substrates, possessing low lattice and thermal expansion mismatches to epitaxial active layers, enable epitaxial growth with low densities of extended defects, leading to improved device performance and lifetime. Lateral and vertical power switching devices have been demonstrated in the emerging AlGaN alloy system [1]. Aluminum nitride (AlN) possesses a close lattice match to high Al mole fraction AlGaN alloys and a high thermal conductivity, which make it an excellent substrate for UWBG AlGaN-based devices. Physical vapor transport (PVT) growth has emerged as the most promising technique for the production of large, high-quality AlN single crystal substrates. Seeded growth technology developed by HexaTech enables iterative expansion of single crystal size, while suppressing low angle grain boundary (LAGB) formation, a primary source of grown-in dislocations. X-ray diffraction (XRD) is well suited to accurately and non-destructively probe the extended defect structure, composition, and strain of single crystals and epitaxial films. A variety of x-ray techniques were used throughout AlN substrate processing, from evaluation of as-grown crystals, through substrate polishing, to analysis of epitaxial films grown on AlN. High-resolution x-ray rocking curves (HRXRRC) and x-ray topography (XRT) were used to determine the primary dislocation types in AlN bulk crystals and their likely formation mechanisms [2, 3]. Synchrotron-based XRT images demonstrated that the expanded diameter of freestanding AlN boules contained low densities of threading dislocations (<104 cm-3), and no basal plane dislocations or LAGBs [4]. Furthermore, XRT images acquired on a commercial diffractometer were used for quality control of AlN seed crystals. The advent of a new generation of x-ray area detectors, capable of a wide dynamic range and rapid data acquisition, opened the possibility for using XRT in production. HRXRRC and surface-sensitive reciprocal space maps were used to monitor sub-surface damage removal during substrate polishing [5]. These results, together with atomic force microscopy and defect selective etching images, demonstrated that chemo-mechanically polished AlN surfaces were free of plastic damage [4]. Polished AlN surfaces were also studied by x-ray reflectivity, yielding long-range data about surface roughness and density. Finally, epitaxial AlGaN films and heterostructures deposited on AlN substrates were studied by a combination of these x-ray techniques, providing essential information on composition, thickness, relaxation, and heterostructure design. XRD has become a valuable metrology tool in the development of AlN technology. This talk will review the implementation of complementary x-ray based measurements in the processing of AlN, from boule growth to AlGaN-based devices.

High-resolution X-ray topography of threading edge dislocations in 4H-SiC using a novel nuclear emulsion film improved special resolution and sensitivity

We developed a nuclear emulsion film in order to improve the spatial resolution of X-ray topography (XRT). The film allowed detection of threading edge dislocations (TEDs) in a 4H-SiC single-crystal substrate over a large area by grazing-incidence monochromatic XRT at a practical level of sensitivity. The directions of the Burgers vectors of the observed TEDs were also determined.

Structural Characterization of the Growth Front of 4H-SiC Boules Grown Using the Physical Vapor Transport Growth Method

The defect structure at the growth front of 4H-SiC boules grown using the physical vapor transport (PVT) method has been investigated using high resolution x-ray diffraction and x-ray topography. The crystal parameters such as the c-lattice constant exhibited characteristic variations across the growth front, which appeared to be caused by variation in surface morphology of the as-grown surface of the boules rather than the defect structure underneath the surface. X-ray topography also revealed that basal plane dislocations are hardly nucleated at the growth front during PVT growth of 4H-SiC crystals.

Large-surface-area diamond (111) crystal plates for applications in high-heat-load wavefront-preserving X-ray crystal optics.

Fabrication and results of high-resolution X-ray topography characterization of diamond single-crystal plates with large surface area (10 mm × 10 mm) and (111) crystal surface orientation for applications in high-heat-load X-ray crystal optics are reported. The plates were fabricated by laser-cutting of the (111) facets of diamond crystals grown using high-pressure high-temperature methods. The intrinsic crystal quality of a selected 3 mm × 7 mm crystal region of one of the studied samples was found to be suitable for applications in wavefront-preserving high-heat-load crystal optics. Wavefront characterization was performed using sequential X-ray diffraction topography in the pseudo plane wave configuration and data analysis using rocking-curve topography. The variations of the rocking-curve width and peak position measured with a spatial resolution of 13 µm × 13 µm over the selected region were found to be less than1 µrad.

High Resolution X-ray Diffraction Studies of MBE-Grown HgCdTe Layers on Bulk-Grown CdZnTe Substrate

The structural properties of molecular beam epitaxially (MBE)-grown Hg1−x Cd x Te epilayers on CdZnTe (211) substrate have been investigated using high-resolution x-ray topography and rocking curves. High-resolution x-ray diffraction 2θ–θ scans of (422) reflections were utilized in calculating the out-of-plane lattice parameters of the HgCdTe layer and the CdZnTe substrate. The lattice strain of the HgCdTe layer was evaluated using the in-plane measurements of the (311) reflection. Etching seemed to improve the surface of the substrate by removing any damage caused by polishing or any post-processing. In spite of some localized line dislocations, a remarkable quality of the MBE-grown HgCdTe layer was observed. The full width at half maximum values of the HgCdTe layer and the CdZnTe substrate were determined as 43 arc-s and 16.2 arc-s, respectively.

Structural Inhomogeneities and Impurity Incorporation in Growth of High-Quality Ammonothermal GaN Substrates

Ammonothermal Gallium Nitride (GaN) substrates are the most promising substrates for homoepitaxial growth of GaN films having low dislocation density. Growth-induced structural inhomogeneities in these substrates were investigated by high-resolution X-ray topography (HR-XRT) and high-resolution X-ray diffraction (HR-XRD). A one-to-one correlation of defects was observed in photoluminescence imaging. From the HR-XRD intrinsic rocking curve widths were found to be 16 arcsecs indicating superior crystalline quality. The lattice constants were measured from symmetric and asymmetric reflections to be similar to bulk values indicting low sample strian. The true dislocation density, from the HR-XRT images, was observed to be of the order of 102 cm–2 in the samples, and the radius of curvature was greater than 600 m. Growth striations were observed in the a and m-pane samples and are attributed to inhomogeneity in impurity incorporation during growth. Photoluminescence imaging showed deep level luminescent centers along the growth striations.

Bulk growth of ninhydrin single crystals by solvent evaporation method and its characterization for SHG and THG applications

Ninhydrin is a well-known compound generally used in amino acid synthesis and also for detecting the latent fingerprints on porous surfaces. Single crystals can be grown by dissolving the compound in double distilled water at ambient temperature, and can be used as a potential material for second and third harmonic generation applications. The grown specimen was subjected to different characterization techniques in order to find out its suitability for device fabrication. Its lattice dimensions have been confirmed by X-ray powder diffraction and its crystalline quality has been assessed by high resolution X-ray diffraction and X-ray topography methods. The presence of functional groups was identified from HETCOR analysis and confirmed the absence of impurities during crystallization. Its optical properties have been examined by photoluminescence and birefringence analyses. Its thermal parameters such as thermal diffusivity, thermal conductivity and specific capacity have been carried out by following photopyroelectric method. Third order nonlinear optical measurements have been carried out using Z-scan technique and its nonlinear optical absorption coefficient has been determined.

Structural investigation of the seeding process for physical vapor transport growth of 4H–SiC single crystals

Structural investigation of the seeding process for the physical vapor transport (PVT) growth of 4H–SiC single crystals was conducted by high-resolution x-ray diffraction (HRXRD) and synchrotron x-ray topography. Characteristic lattice plane bending behavior was observed in the near-seed regions of the grown crystals. The bending of the (112̄0) lattice plane was localized near the seed/grown crystal interface, and the (0001) basal plane bent convexly in the growth direction near the interface, indicative of the insertion of extra-half planes pointing toward the growth direction during the seeding process for PVT growth. This study discusses a possible mechanism for the observed lattice plane bending and sheds light on defect formation processes during the PVT growth of 4H–SiC single crystals.

Phase matching, X-Ray topography, optical and thermal analysis of L-alanine cadmium chloride monohydrate: a nonlinear optical material

A potential semiorganic nonlinear optical material, L-alanine cadmium chloride monohydrate has been successfully synthesised and single crystals have been grown by slow evaporation solution growth technique at room temperature by using double distilled water as the solvent. The lattice dimensions of the grown crystal have been analysed by adopting powder X-ray diffraction technique and found that it crystallised in monoclinic system with space group C2. The crystalline perfection of the as-grown crystal has been assessed by high resolution X-ray diffraction and X-ray topography techniques and observed that the quality of the grown specimen is reasonably good. Its optical properties were examined by UV–Vis and photoluminescence techniques and found that there is no absorption in the entire visible range. Its functional groups were identified from FT-Raman and observed that there is no incorporation of other impurities during crystallisation. Its relative second harmonic generation efficiency has been tested with different particle size by Kurtz powder technique and found that within the coherence length the title compound is phase matchable. Its various thermal properties like thermal conductivity, specific heat, thermal effusivity, etc. have been evaluated by photopyroelectric technique and compared with other organic and inorganic materials. To confirm its piezoelectric response, its piezoelectric charge coefficient was measured using piezometer and found low. Its optical homogeneity as well as birefringence measurement of the grown specimen has been carried out by interferometric technique. The surface defects of the grown LACCM single crystal were analysed with etching at room temperature using water as an etchant.

Nucleation kinetics, growth, mechanical, thermal and optical characterization of sulphamic acid single crystal

Sulphamic acid (HSO3NH2) is a potential material that exhibits excellent piezoelectric and non-linear optical properties. Nucleation and growth kinetics give valuable information about the crystal growth process, which can be employed in the growth of large size crystals. Thus we have studied its nucleation parameters such as solubility, metastable zone width, induction period, interfacial tension, critical free energy, volume free energy change, critical nucleus and found that good quality sulphamic acid crystal can be grown at low temperature. Growth of good quality crystal was done by adopting slow evaporation solution growth technique at room temperature and as a result, good quality single crystal was grown. The grown bulk single crystal of the title compound was subjected to various characterization analyses. Its unit cell dimensions were confirmed using powder X-ray diffraction. Further its crystalline perfection was assessed using high resolution X-ray diffraction and X-ray topography techniques and it was found that it is reasonably good. Its various functional groups were identified from the Fourier transform-Raman method. Nanoindentation was performed to identify the load dependence and independence of hardness (H) and Young's modulus (E) using the Oliver–Pharr method and it was found that both H and E exhibit peak load dependence. Photopyroelectric study was performed to identify its thermal properties such as thermal diffusivity (α), thermal conductivity (k), thermal effusivity (e), and heat capacity (cp). These studies reveal that it has high thermal stability compared to other single crystals. The optical homogeneity of the grown crystal was assessed using a birefringence interferometer and it was found to be reasonably good.

Structural investigations of SiGe epitaxial layers grown by molecular beam epitaxy on Si(0 0 1) and Ge(0 0 1) substrates: I—High-resolution x-ray diffraction and x-ray topography

Epitaxial structures of different SiGe composition grown by molecular beam epitaxy on Si(001) and Ge(001) substrates have been studied by high-resolution x-ray diffraction and x-ray topography to establish correlations between epitaxial growth conditions and crystal perfection. It was confirmed that epitaxy under initial elastic stress inevitably led to the creation of extended crystal defects. The type of defects created and their density and spatial distribution, strongly depended on the value and sign of the initial elastic strain, the elastic constants of solid solutions, the temperature of deposition and growth rate, and the thickness of the epitaxial layers. All of the investigated structures were classified by their crystal perfection, using x-ray diffraction with the volume density of dislocation loops as a parameter. It was found that the accommodation and relaxation of initial elastic stress and creation of crystal defects were multistage “chain” processes, necessary to stabilize the crystal structure at a level corresponding to the particular growth conditions. Types, density and spatial distribution of crystal defects, related to each stage of defect creation and matched to structural features, as revealed by high resolution x-ray diffraction, were considered for explanation.

Interaction of 6H-Type Stacking Faults with Threading Screw Dislocations in PVT-Grown 4H-SiC Single Crystals

6H-type stacking faults (SFs) observed in PVT-grown 4H-SiC ingle crystals were investigated using Photoluminescence (PL) microscopy at room temperature. Structural analyses using high resolution X-ray topography have revealed that there exist no (n=4, 8) component in Burger’s vectors of the 6H-type SFs we observed, strongly suggesting that the 6H-type SFs are constructed either by insertions of very thin 6H-type foreign polytype inclusions or by successive repetitions of Shockley-type in-plane glides.

Structure and Morphology of Inclusions in 4° Offcut 4H-SiC Epitaxial Layers

The structure of inclusions and their influence on surface morphology, local strain, and basal plane dislocations were investigated in silicon carbide (SiC) epitaxial layers grown on 4° offcut substrates. On high-resolution x-ray topography images, strain fields were observed surrounding the inclusions. Ultraviolet photoluminescence images revealed the presence of strain-induced dislocations around the inclusions. Micro-Raman and microphotoluminescence spectroscopy showed that the inclusions exhibited a complex structure that consisted of 3C polytype regions and misoriented 4H polytype regions. The resulting lattice deformation typically propagates in the step-flow growth direction and causes distorted surface morphology.

Study of LiNbO3 and LiTaO3 ferroelectric domain structures using high-resolution x-ray diffraction under application of external electric field

Regular domain structures formed in ferroelectric LiNbO3 and LiTaO3 crystals by the aftergrowth thermoelectrical treatment technique were studied by high-resolution x-ray diffraction and topography under an external electric field applied to the crystals. The application of an external electric field to the crystal causes longitudinal and shift deformations due to the reverse piezoelectric effect and, as a result, inverse changes in the Bragg angle values in adjacent domains. The changes in the Bragg angle values lead to an increase in the x-ray topographic contrast of the domain structure images and allow the direction of the polar and piezoelectrically active axis in adjacent domains to be unambiguously determined. In addition, it was shown that the x-ray diffraction technique can be used to study the behavior of elastic deformations near domain boundaries under an external electric field applied to crystals.

Investigation of the physical properties of piezo-and ferroelectric crystals by high-resolution X-ray diffraction and topography

Investigation of the physical properties of piezo-and ferroelectric crystals by high-resolution X-ray diffraction and topography

X-ray diffractometry and topography of lattice plane curvature in thermally deformed Si wafer

The correlation between the microscopic lattice plane curvature and the dislocation structure in thermal warpage of 200 mm-diameter Czochralski Si (001) wafers has been investigated using high-resolution X-ray diffractometry and topography. It is found that the (004) lattice plane curvature is locally confined between two neighboring slip bands, with the rotation axis parallel to the slip bands. High-resolution topography reveals that the curvature resulted from a fragmented dislocation structure. The local confinement is attributed to the multiplication of the dislocations that are generated between the two slip bands.