White Beam X-ray Diffraction Topography Research Articles

Synchrotron Bragg diffraction imaging characterization of synthetic diamond crystals for optical and electronic power device applications.

Bragg diffraction imaging enables the quality of synthetic single-crystal diamond substrates and their overgrown, mostly doped, diamond layers to be characterized. This is very important for improving diamond-based devices produced for X-ray optics and power electronics applications. The usual first step for this characterization is white-beam X-ray diffraction topography, which is a simple and fast method to identify the extended defects (dislocations, growth sectors, boundaries, stacking faults, overall curvature etc.) within the crystal. This allows easy and quick comparison of the crystal quality of diamond plates available from various commercial suppliers. When needed, rocking curve imaging (RCI) is also employed, which is the quantitative counterpart of monochromatic Bragg diffraction imaging. RCI enables the local determination of both the effective misorientation, which results from lattice parameter variation and the local lattice tilt, and the local Bragg position. Maps derived from these parameters are used to measure the magnitude of the distortions associated with polishing damage and the depth of this damage within the volume of the crystal. For overgrown layers, these maps also reveal the distortion induced by the incorporation of impurities such as boron, or the lattice parameter variations associated with the presence of growth-incorporated nitrogen. These techniques are described, and their capabilities for studying the quality of diamond substrates and overgrown layers, and the surface damage caused by mechanical polishing, are illustrated by examples.

Structural, electrical, and optical properties of CdMnTe crystals grown by modified floating-zone technique

Cadmium manganese telluride (CdMnTe or CMT), a compound semiconductor, is considered a promising material for the fabrication of high-performance room-temperature x-ray and gamma-ray detectors. The presence of material defects, e.g., high density of Te inclusions, has been a long-standing issue in CMT crystals grown by various Bridgman methods, since these defects degrade the device performance via charge-trapping. To address this issue, we employed the modified floating-zone method (MFZ) to grow CMT crystals and obtained as-grown crystals free of Te inclusions. This represents a new and distinct feature, absence of Te inclusions, compared to CMT crystals grown by Bridgman methods. White-beam x-ray diffraction topography (WBXDT) measurements demonstrated the existence of a high stress field within the MFZ-grown CMT crystals, which originates from the steep temperature gradient near the growth interface. Furthermore, we achieved a resistivity of 109 Ωcm for the MFZ-grown CMT crystals. The low-temperature photoluminescence (PL) measurements show that the intensity of the dislocation-related Y band is much higher than that of the principal exciton peaks, (D0,X) and (A0,X), confirming that the crystalline quality is affected by the high stress field. A long-term in-situ or post-growth thermal annealing will help to release such stress to improve the crystalline quality. Open image in new window

White beam X-ray Diffraction Topography (WBXDT) Studies of Bridgman Grown CdZnTe Crystals

ABSTRACTCZT is a semiconductor material that promises to be a good candidate for uncooled gamma radiation detectors. However, to date, we are yet to overcome the technological difficulties in production of large size, defect-free CZT crystals. The most common problem is accumulation of Tellurium precipitates as microscopic inclusions. These inclusions influence the charge collection through charge trapping and electric field distortion. We employed high energy transmission X-ray diffraction techniques to study the quality of the CdZnTe crystals grown by Bridgman Technique. Crystallinity and defects within two different growth set-ups, i.e. with and without choked seeding, were compared by imaging the crystal orientation topography with white beam X-ray diffraction topography (WBXDT). The X-ray diffraction topography results show high correlation with large-area infrared transmission images of the crystals. Grain boundaries that are highly decorated with Te inclusions are observed. Characteristic Te inclusion arrangements as a result of growth conditions are discussed. We also measured the electronic properties of the detectors fabricated from ingots grown using two Bridgman processes, and observed a reduction in electrical resistivity of choked-seeding-grown CdZnTe crystals. Our results show that although choked seeding technique holds a promise in the realization of high quality mono-crystalline CdZnTe, current growth parameters must be improved to obtain defect-free crystals. These results are helpful to attain optimal seeding process for Bridgman-growth of large single crystals of CdZnTe.

Prismatic punching defects in CdTe compounds

The origin and nature of star-shaped prismatic punching defects (PPDs) were analyzed by white-beam X-ray diffraction topography (WBXDT) and photoluminescence (PL) measurements. PPDs were observed in CdTe-based compounds that had Te-rich secondary phase defects and were annealed only under Cd overpressure. The evolutional features of the PPDs during annealing were similar to those of a diffusion process, that is, the in-diffusion of Cd vapor via Cd vacancies. PL mapping around the PPDs reveals that the stoichiometry of PPDs is the same as that of a normal matrix, and that PPDs are just structural defects with non-radiative emission induced by the exothermic reaction between Cd and Te. We find that Te-rich secondary phase defects can be removed without the generation of star-shaped PPDs, if the exothermic reaction rate can be slowed down to reduce the thermal gradient between the Te-rich region and the surrounding matrix.

Evaluation of CdTexSe1−x crystals grown from a Te-rich solution

We characterized the structural quality of CdTexSe1−x crystals grown by the Traveling Heater Method (THM) from a Te-rich solution using Synchrotron White Beam X-ray Diffraction Topography in the reflection mode. Structural defects were also studied by chemical etching of the crystal surfaces. The crystals were found to be fairly free from strains, and they had very few sub-grain boundaries and dislocation/sub-grain boundary networks.

Drift Time Variations in CdZnTe Detectors Measured With Alpha Particles and Gamma Rays: Their Correlation With Detector Response

Homogeneity of properties related to material crystallinity is a critical parameter for achieving high-performance CdZnTe (CZT) radiation detectors. Unfortunately, this requirement is not always satisfied in today's commercial CZT material due to high concentrations of extended defects, in particular subgrain boundaries, which are believed to be part of the causes hampering the energy resolution and efficiency of CZT detectors. In the past, the effects of subgrain boundaries have been studied in Si, Ge and other semiconductors. It was demonstrated that subgrain boundaries tend to accumulate secondary phases and impurities causing inhomogeneous distributions of trapping centers. It was also demonstrated that subgrain boundaries result in local perturbations of the electric field, which affect the carrier transport and other properties of semiconductor devices. The subgrain boundaries in CZT material likely behave in a similar way, which makes them responsible for variations in the electron drift time and carrier trapping in CZT detectors. In this work, we employed the transient current technique to measure variations in the electron drift time and related the variations to the device performances and subgrain boundaries, whose presence in the crystals were confirmed with white beam X-ray diffraction topography and infrared transmission microscopy.

Characterization and evaluation of extended defects in CZT crystals for gamma-ray detectors

Material homogeneity is critical in achieving high-performance in all types of radiation detectors. This requirement is not inevitably satisfied in today's commercial detector-grade CdZnTe (CZT) material because it contains high concentrations of extended defects, in particular, Te inclusions, dislocation networks, and twin- and subgrain-boundaries that affect the energy resolution and the efficiency of the devices. Defects, such as grain boundaries and cracks that completely block charge-carrier transport are impermissible in CZT radiation-detectors at concentrations exceeding certain threshold values. Our group in Brookhaven National Laboratory (BNL) conducts systematic studies, detailing the roles of crystal defects in CZT detectors and the mechanisms underlying their formation and effects. We employ infrared transmission microscopy, white beam X-ray diffraction topography, and high-spatial-resolution X-ray response mapping to identify particular types of defects and reveal their relationship with the devices' performances. In this article, we summarize some of the most important results that our group obtained over the past 5 years.

Concentration of extended defects in CdZnTe single crystals: Effects of cooling rate after growth

We analyzed two CZT crystals cut from as-grown CdZnTe (CZT) ingots, the only difference between them being the rate of cooling after the crystal growth process. Using White Beam X-ray Diffraction Topography (WBXDT) and Infrared (IR) Transmission Microscopy, we identified and quantified the extended defects, e.g., Te inclusions, dislocations, and sub-grain boundaries. The effects of cooling rate on the size distribution and concentration were studied. The WBXDT and IR images of the fast-cooled crystal revealed very high density of dislocations and sub-grain boundaries, crisscrossing throughout its entire volume, extending from deep inside almost to the surface. In addition, IR analyses showed that the concentration of Te inclusions in the fast-cooled crystal (106cm−3) was higher than that in the slow-cooled one (105cm−3). For the latter, both the WBXDT and the IR images were bright and clear with low concentration of defects. We concluded that slow cooling rate can greatly reduce the number of Te inclusions and inclusion-decorated extended defects in as-grown CZT ingots.

Temperature-gradient annealing of CdZnTe under Te overpressure

The performance of CdZnTe (CZT) detectors is limited not only by conventional carrier-trapping to point defects but also by trapping at macroscopic Te secondary-phase defects, such as Te inclusions and Te precipitates. The aim of this research is to remove these secondary-phase defects via thermomigration, and to obtain high resistivity of the material by creating Te antisites through annealing in a high Te overpressure. We annealed Te-rich CZT samples in the temperature range between 500 and 700°C under Te overpressure with a temperature gradient of 50–60°C/cm. We investigated the effects of annealing under these conditions by IR transmission microscopy, current–voltage measurements, photoluminescence, and white-beam X-ray diffraction topography (WBXDT) measurements comparing the findings with those from isothermally annealed CZT samples under Cd overpressure. We proved experimentally that Te inclusions present in Te-rich CZT melts contain void fractions. We attributed the complex defect of Te antisites with Cd vacancies in our annealing experiment as the deep level defect that pins the Fermi near the middle of the bandgap, rather than the Te antisite.

Defect levels of semi-insulating CdMnTe:In crystals

Using photoluminescence (PL) and current deep-level transient spectroscopy (I-DLTS), we investigated the electronic defects of indium-doped detector-grade CdMnTe:In (CMT:In) crystals grown by the vertical Bridgman method. We similarly analyzed CdZnTe:In (CZT:In) and undoped CdMnTe (CMT) crystals grown under the amount of same level of excess Te and/or indium doping level to detail the fundamental properties of the electronic defect structure more readily. Extended defects, existing in all the samples, were revealed by synchrotron white beam x-ray diffraction topography and scanning electron microscopy. The electronic structure of CMT is very similar to that of CZT, with shallow traps, A-centers, Cd vacancies, deep levels, and Te antisites. The 1.1-eV deep level, revealed by PL in earlier studies of CZT and CdTe, were attributed to dislocation-induced defects. In our I-DLTS measurements, the 1.1-eV traps showed different activation energies with applied bias voltage and an exponential dependence on the trap-filling time, which are typical characteristics of dislocation-induced defects. We propose a new defect-trap model for indium-doped CMT crystals.

Crystal Growth and Defect Characterization of AlN Single Crystals

In this paper, we report results from seeded AlN PVT growth experiments carried out using SiC seeds. The purpose of the experiments was to understand the morphology and crystalline quality of PVT AlN crystals grown under the crystal growth environments investigated. AlN single crystal films of 120-650μm in thickness were grown and freestanding AlN single crystal pieces up to 4×5mm2 were obtained. Surface morphologies and crystal defects in these AlN single crystals were studied using optical microscopy. Selected AlN single crystals were studied using a high-resolution triple-axis X-ray diffraction technique and a Synchrotron White Beam X-ray Diffraction Topography technique. Defects identified in AlN crystals are cracks, dislocations, grain boundaries and crystallite inclusions.

Analysis of types of residual dislocations in the VGF growth of GaAs with extremely low dislocation density (EPD≪1000 cm −2)

GaAs single crystals (silicon-doped, 2 inch) with extremely low dislocation densities (etch pit density (EPD) 50–1000 cm −2) were grown by the vertical gradient freeze method. In these crystals we characterised different types of dislocations by the aid of white beam X-ray diffraction topography and infrared transmission microscopy. It was found for decreasing dislocation densities (EPD<200 cm −2), that dislocations having a line vector l , which is parallel to the [0 0 1] growth direction of the crystals, become more and more dominant. These residual dislocations are induced by the seeding process (so far we were using LEC-grown seed crystals). These residual dislocations cannot be avoided by minimising the thermal stress during the crystal growth process.

Real Time Observation of Material Deformation Processes by Synchrotron White Beam X-ray Topography

ABSTRACTAn experimental apparatus which exploits synchrotron white beam X-ray diffraction topography has been developed to perform real time in situ observation of deformation processes of single crystals. The apparatus consists of a fully automated and remotely controlled mini-tensile device and a CCD based X-ray imaging detector with associated video system. Both tensile data and X-ray topographic images are recorded simultaneously. The materials under study include single crystals of Si, and single crystals or bicrystals of the refractory metals W and Mo. The deformation processes of these crystals have been studied by monitoring a selected transmission Laue spot while the specimen is mechanically cycled by loading and unloading. The X-ray images have been captured by a frame grabber and recorded via a VCR. Several phenomena have been observed, including bending contours, strain concentrations, reversible anelastic effect and microyielding. The observed stress relaxations are correlated to microstructural variations shown by X-ray topography. The stability of low angle grain boundaries in bicrystals; and the effect of high strength and toughness coatings on the deformation of single crystal substrates was investigated.

X-Ray Topographic Contrast of Threading Dislocations in Silicon on Insulator Structures

Threading dislocations in a thin silicon layer of SIMOX (Separation by IMplanted OXygen) samples were studied by white and monochromatic beam X-ray diffraction topography in transmission and reflection geometry. The symmetrical transmission geometry appeared to be most suitable for the present study. The dislocation images were visible on topographs even though the Si layer thickness was very small compared to the pendellosung distance. The dislocation contrast could coherently be explained by an “interferometer like” and a “double crystal diffractometric like” formation mechanism. The first is controlled by local variations of the reciprocal lattice vector difference between the two parts of a bicrystal and the second by variations of the effective misorientation. The contrast analysis revealed that dislocations occurred in pairs with antiparallel Burgers vectors. The dislocation contrast dependence on the layer thickness, X-ray wavelength, working point on the rocking curve, position on the moire fringe pattern, and measurement geometry were also investigated.

Synchrotron white beam topographic investigation of crystalline defects in silicon on insulator materials

Samples prepared by separation by implantation of oxygen (SIMOX) with dislocation densities of about 105 cm−2 have been studied by synchrotron white beam X-ray diffraction topography (WBT) in transmission geometry. The threading dislocations in the top Si layer were detected and found to have Burgers vectors parallel to the [011] and to the [011̄] directions. From moiré fringes the lattice parameter difference between the top Si layer and the substrate was found to be about 6 × 10−8 in the surface plane.

Synchrotron topography of phase transitions in perovskite-like crystals

In situ observations have been made of the first-order phase transition occurring at approximately 145 degrees C in a large LaGaO3 single crystal, and the second-order phase transition occurring at approximately 435 degrees C in a large LaAlO3 single crystal, using synchrotron white beam X-ray diffraction topography. In the case of LaGaO3, the high-temperature rhombohedral phase reflections gradually replace the low-temperature orthorhombic reflections as the transition temperature is approached. In the case of LaAlO3, the magnitude of this image shift continuously decreased in magnitude as the temperature approaches the transition point, which along with the gradual distortion of the diffraction pattern, confirms the second-order nature of this transition. As the transition is surpassed, twinning completely disappears in the crystal, as expected, since the twin planes in the rhombohedral phase become mirror planes in the cubic phase.

A new diagnostic tool for the non-destructive characterization of damage accompanying the electromagnetic breakdown of epitaxial silicon p-n junctions

Synchrotron white beam X-ray diffraction topography, in both transmission and grazing Bragg-Laue geometries, has been applied to the investigation of the damage induced during the breakdown of p-n junctions on silicon. The technique, which is highly strain sensitive, can yield information on both the lateral and depth distributions of distortion in the devices in a non-destructive way. Both transmission and grazing Bragg-Laue images reveal clear differences in the extent and distribution of crystallographic damage accompanying the forward- and reverse-bias breakdown phenomena. Variable-penetration-depth grazing Bragg-Laue imaging has revealed variations in the distribution of damage as a function of depth in both the forward- and the reverse-bias case. The results indicate that these techniques constitute unique diagnostic tools for the investigation of these and other related phenomena. Comparison of results with optical microscopy studies indicates that the X-ray techniques yield subsurface information not revealed by optical techniques.

Synchrotron X-ray topographic studies of the changes in defect microstructure induced by rapid thermal processing of single-crystal silicon wafers

Synchrotron white beam X-ray diffraction topography in transmission geometry has been used to non-destructively investigate defect structures in silicon single-crystal wafers, both prior and subsequent to a 60 s rapid thermal processing (RTP) treatment at 1050°C. Among the effects observed were: dislocation glide and multiplication; and the enhancement of the strain field associated with A-type swirl defects.

Synchrotron Topographic Studies of the Influence of Rapid Thermal Processing on Defect Structures in Single Crystal Silicon.

ABSTRACTSynchrotron white beam X-ray diffraction topography in transmission geometry has been used to non-destructively investigate defect structures in silicon single crystal wafers, both prior and subsequent to a 60 second rapid thermal processing (RTP) treatment at 1050°C. Prior to RTP dislocations, precipitates and swirl defects were observed and characterized. Following RTP the following effects were observed: glide of individual dislocations and dislocation multiplication; and the enhancement of the strain field associated with the swirl defects. Precipitates appeared unaffected by RTP. This work shows that synchrotron topography is capable of non-destructively revealing significant dislocation motion induced by RTP under conditions were such motion is not thought to occur. This dislocation motion is likely to be detrimental to device performance. The technique enables determination of the conditions required to avoid such dislocation motion.