X-ray Transmission Topography Research Articles

Investigation of propagation and coalescence of threading screw and mixed dislocations in 4H-SiC crystals grown by the high-temperature gas source method

Propagation of threading screw dislocations (TSDs) and mixed dislocations (TMDs) in 4H-SiC crystals grown by the high-temperature gas source method were investigated. Cross-sectional samples cut in the growth direction were prepared and X-ray transmission topography with a diffraction vector g = 0004¯ was performed to reveal the dislocation lines. The topography image revealed that some pairs of TSDs/TMDs extending in the growth direction approached each other and merged in the middle of the grown crystal. Direct observations by synchrotron X-ray section topography confirmed that a pair of TSD and TMD with opposing c-component Burgers vectors had merged and the c-components of the dislocations were annihilated during crystal growth. Some pairs of TSDs/TMDs, however, showed no coalescence, although they could be annihilated topologically. The mechanism behind the success and failure of TSD/TMD coalescence was also discussed.

Synchrotron X-ray topographic characterization of dislocations in 6H-SiC axial samples

Wide bandgap semiconductor, 6H-SiC, is being applied in photoconductive semiconductor switches (PCSS) due to its semi-insulating properties. The behavior of the dislocations in 6H- SiC under the application of voltage and laser in such devices is of particular interest. In this study, synchrotron X-ray transmission topography, grazing incidence topography and rocking curve topography are applied to characterize the types and distribution of defects in (1 1 –2 0) oriented wafers. Threading edge dislocations (TED), threading screw dislocations (TSD), threading mixed dislocation (TMD) and basal plane dislocations (BPD) in 6H-SiC axial samples are revealed in the X-ray topographs. TEDs are observed to be more likely off c-axis compared to TSDs/TMDs. Contrast features of BPDs differ depending on their Burgers vectors and orientation with respect to wafer surface on rocking curve topographs and grazing incidence topographs. Applying ray tracing simulations, the configurations of BPDs can be determined, and the threading dislocations (TED, TSD and TMD) can be distinguished in grazing incidence topographs. The understanding of the nature and the distribution of these dislocations will help predict their propagation and movement under the application of voltage and laser, providing guidance for device fabrication.

Geometric determination of direction of dislocations using synchrotron X-ray transmission topography.

When performing transmission polychromatic beam topography, the extensions to the line segments of the diffraction images of a straight dislocation are shown to intersect at a single point on the X-ray film. The location of this point, together with the diffraction pattern recorded on the film by synchrotron radiation, gives the crystallographic direction [hkl] of the dislocation unambiguously. The results of two synchrotron topography experiments are presented. Very long dislocations found in the center of a large 450 mm-diameter Czochralski silicon crystal align with the growth direction [001]. In the other silicon sample, the dislocations are of mixed type and along the [011] direction.

Structural defects in ZnGeP2single crystals revealed by X-ray topography

The results of X-ray transmission topography and diffraction analysis of a ZnGeP2single crystal grown by the vertical Bridgman method in the [001] direction are presented and discussed. The FWHM of rocking curves over a large area of a (100) longitudinal slice is about 12′′, which is indicative of the high quality of the examined sample. Glow discharge mass spectrometry does not show significant content of foreign chemical elements. X-ray topography reveals growth striations and dislocations. The predominant defects are single dislocations and their pile-ups. Near to the growth-axis region, curved dislocation bundles passing through the entire crystal are observed, on which precipitates are formed. In the initial part of the crystal, dislocations are located chaotically, while towards the middle of the sample they are aligned along the growth striae. In the final part of the crystal, the dislocation density increases.

Crystalline damage in silicon wafers and 'rare event' failure introduced by low-energy mechanical impact

We used X-ray diffraction imaging to detect and characterize mechanical damage introduced to 300mm silicon wafers by low impact energy exerted on the wafer edge. Maps of crystalline damage show a correlation between the damage size, the magnitude of the impact energy and the location of the impact point. We demonstrate the existence of crystalline non-visual defects; crystalline defects that appear in the X-Ray diffraction images but not in optical microscopy or scanning electron microscope. We propose a mechanism of crystalline damage formation at low impact energies based on finite element analysis and high-resolution synchrotron white beam transmission X-ray topography. Finally, we propose the concept of 'rare-event' to described relatively low rate of occurrence of wafer failure by fracture within semiconductor manufacturing facilities.

Characterization of V-Shaped Defects Formed during the 4H-SiC Solution Growth by Transmission Electron Microscopy and X-ray Topography Analysis

Defects generated during 4H-SiC (0001) solution growth have been investigated by synchrotron X-ray topography and transmission electron microscopy (TEM). The defects unidentified before are recognized as V-shaped contrast features in the X-ray topographic images. The detailed analysis combining TEM results revealed that the newly generated defects are identified as a pair of dislocations on a basal plane with opposite Burgers vectors parallel to the [112̅0] direction. It is found that no defects in the substrate are directly associated with the formation of the V-shaped defects. Geometric analysis of the size and shape of the V-shaped defects indicates that they nucleate in pairs intermittently during the growth process. On the basis of the observed morphology of these defect configurations, a mechanism of two-dimensional nucleation during solution growth is postulated for the generation of the V-shaped defects.

Critical thickness investigation of MBE-grown GaInAs/GaAs and GaAsSb/GaAs heterostructures

GaInAs/GaAs and GaAsSb/GaAs heterostructures were grown by molecular beam epitaxy with different In/Sb compositions and thicknesses in order to obtain samples with different amounts of initial strain. High resolution x-ray diffraction was used to extract the alloys composition, specify the presence of dislocations, and determine the extent of relaxation while transmission electron microscopy and x-ray topography were used to observe these dislocations and characterize their type and density. The onset for the formation of misfit dislocations was found to be in agreement with the equilibrium theory. However, the films remained coherently strained for thicknesses far beyond this value. The onset for strain relaxation was found by considering the kinetics of plastic deformation using the approach proposed by Tsao and coworkers [Phys. Rev. Lett. 59, 2455 (1987)]. The mechanism of extended defect creation leading to measurable strain relief is described as a multistage process related with the structural stability and metastability of the epitaxial films.

Study of Defect Structures in 6H-SiC a/m-Plane Pseudofiber Crystals Grown by Hot-Wall CVD Epitaxy

Structural perfection of silicon carbide (SiC) single crystals is essential to achieve high-performance power devices. A new bulk growth process for SiC proposed by researchers at NASA Glenn Research Center, called large tapered crystal (LTC) growth, based on axial fiber growth followed by lateral expansion, could produce SiC boules with potentially as few as one threading screw dislocation per wafer. In this study, the lateral expansion aspect of LTC growth is addressed through analysis of lateral growth of 6H-SiC a/m-plane seed crystals by hot-wall chemical vapor deposition. Preliminary synchrotron white-beam x-ray topography (SWBXT) indicates that the as-grown boules match the polytype structure of the underlying seed and have a faceted hexagonal morphology with a strain-free surface marked by steps. SWBXT Laue diffraction patterns of transverse and axial slices of the boules reveal streaks suggesting the existence of stacking faults/polytypes, and this is confirmed by micro-Raman spectroscopy. Transmission x-ray topography of both transverse and axial slices reveals inhomogeneous strains at the seed–epilayer interface and linear features propagating from the seed along the growth direction. Micro-Raman mapping of an axial slice reveals that the seed contains high stacking disorder, while contrast extinction analysis (g·b and g·b×l) of the linear features reveals that these are mostly edge-type basal plane dislocations. Further high-resolution transmission electron microscopy investigation of the seed–homoepilayer interface also reveals nanobands of different SiC polytypes. A model for their formation mechanism is proposed. Finally, the implication of these results for improving the LTC growth process is addressed.

Bulk growth of ZnGeP2 crystals and their study by X-ray topography

Structural defects were studied in the nonlinear optical material ZnGeP2 (ZGP) grown by the vertical Bridgman technique from the melt. Progress in ZGP growth with sufficiently perfect structure allowed us observe the Borrmann effect and to apply X-ray transmission topography method based on this effect for the first time for ZGP. Types of defects were defined by comparison of the experimental and simulated topographs. In as-grown ZGP crystals the topography found growth striation, various dislocations, coherent and semi-coherent particles of second phases, inclusions, in the initial part of the crystal with compositions lying along ZGP–GeP2 pseudobinary cut. Inclusions forming solute trails in the end part of ZGP ingots with compositions lying along the ZGP–ZnP2 pseudobinary cut. The dislocation density changed from Nd ~3.5×103cm−2 to ~2.5×102cm−2 from the onset to the end of the ingot, respectively. FWHM of rocking curves for as-grown ZGP ranger from 13″ to 35″. A high optical absorption coefficient in the 0.65–2.5µm spectral region, always existing in as-grown ZGP, can be reduced by thermal annealing and electron irradiation down to ~0.02cm−1 at 2.06µm.

X-ray topography study of monocrystalline silicon wafers diffused with phosphorus by different methods

To reduce the cost of the emitter diffusion process, there has been increasing interest to substitute the standard process of batch POCl3 emitter diffusion used in the silicon solar-cell manufacturing industry with in-line diffusion processes such as the spray-on and screen-printing process. For this reason, it is essential to study and compare the processes of different diffusion methods from the point of view of the crystalline quality of the final wafers. X-ray transmission topography was employed to characterize the possible precipitates and other microdefects generated in Czochralski-grown silicon (Cz Si) during the emitter diffusion process carried out by screen-printing, spray-on and the standard process, in which the emitter was provided by a liquid (POCl3) source. The results indicate that the phosphorus diffusion process influences the crystalline quality of the wafers and the efficiency of the external gettering process that takes place during phosphorus diffusion depends on the diffusion method employed.

TSD Reduction by RAF (Repeated <i>a</i>-Face) Growth Method

A reduction in threading screw dislocation (TSD) density in 4H-SiC (silicon carbide) crystal is required for SiC power devices. In this study, TSD’s transformation by the RAF (repeated a-face) growth method [1] is observed by transmission X-ray topography (g=0004) of the cross-section of the crystal. Increasing the number of repetitions of a-face growth and offsetting c-face growth to an angle of several degrees reduce TSDs. TSD density is reduced to 1.3 TSD/cm2. The RAF growth method is very effective towards growing high quality SiC crystals.

Strain analysis in Si micro-electromechanical systems by dynamical X-ray diffraction

Long-range strain in nearly perfect crystalline materials can be detected by dynamical X-ray diffraction. Long-range strain due to uniform bending or torsion is undetectable by conventional methods when the specimen is vertical in a symmetrical Laue setting, but it can be detected by rotating the specimen along the scattering vector. This method is applied to Si devices for a resonating torsion mirror in a micro-electromechanical system. X-ray transmission topography clearly reveals local strain concentrations as enhancements or reductions in the diffraction intensity where non-uniform strain exists. In conjunction with reflection plane-wave topography, a model of local strain is proposed and the strain qualitatively analysed.

Characterization of 4H <000-1> Silicon Carbide Films Grown by Solvent-Laser Heated Floating Zone

ABSTRACTCommercially available bulk silicon carbide (SiC) has a high number (>2000/cm2) of screw dislocations (SD) that have been linked to degradation of high-field power device electrical performance properties. Researchers at the NASA Glenn Research Center have proposed a method to mass-produce significantly higher quality bulk SiC. In order for this bulk growth method to become reality, growth of long single crystal SiC fibers must first be achieved. Therefore, a new growth method, Solvent-Laser Heated Floating Zone (Solvent-LHFZ), has been implemented. While some of the initial Solvent-LHFZ results have recently been reported, this paper focuses on further characterization of grown crystals and their growth fronts. To this end, secondary ion mass spectroscopy (SIMS) depth profiles, cross section analysis by focused ion beam (FIB) milling and mechanical polishing, and orientation and structural characterization by X-ray transmission Laue diffraction patterns and X-ray topography were used. Results paint a picture of a chaotic growth front, with Fe incorporation dependant on C concentration.

Characterisation of Dislocations in ZnO by X –Ray Imaging

Transmission X-Ray topography (X-ray imaging) was used to characterize the density and type of the native dislocations in ZnO substrates. C-oriented ZnO bulk materials from different origins were compared, either high purity Chemical Vapor Transport (CVT) grown or commercial, hydrothermally grown wafers. Elongated dislocations lying within the substrate were found in hydrothermal substrates. In CVT crystals, the density of dislocations was found to be too high for X-ray determination of their Burgers vector. However, in hydrothermal crystals, the density of dislocations were found to be in the range < 104 / cm2 and extend mainly within the substrate. Whereas complete Burgers vector identification is not achieved, two kinds of dislocations have been evidenced : grown-in, swirled dislocations ("high" temperature) and linear, gliding dislocations, most probably developped during the cooling steps in (0 1–1 0) glide planes.

Micropipe-induced birefringence in 6H silicon carbide

The micropipe-induced birefringence of 6H silicon carbide (SiC) is measured and quantitatively modelled. A good agreement can be obtained between theory and experiment, provided that background residual stress is added to the local dislocation-induced stress. Observations are compatible with or predictable from the Burgers vector values, and birefringence is shown to be an interesting tool for probing the nature of the dislocations associated withe.g.micropipes; it is also faster than and complementary to the more involved techniques of transmission electron microscopy or X-ray topography.

Growth and defect structure of ZnGeP 2 crystals

A defect structure of nonlinear optical material ZnGeP 2, grown by the vertical Bridgman technique from the melt, was studied. The state-of-the-art results in ZnGeP 2 growth with sufficiently perfect structure allow one to register the presence of Borrmann effect and to apply the X-ray topography method based on this effect for the first time. Microscopy and X-ray transmission topography based on the Borrmann effect revealed growth striation, precipitates, forming lineage structures along the growth axis, dislocations and unknown linear defects, which should be more elaborately studied in future. The observed defects are formed because of deviation from ZnGeP 2 stoichiometry during synthesis and growth and unfavorable thermal conditions during growth. The precipitates are observed only with significant deviations from stoichiometry. The width of rocking curves for the as-grown crystals is 13–35 seconds of arc, which shows a good structural perfection, in spite of the revealed defects. The thermal annealing and electron beam irradiation decrease the optical absorption coefficient at 2.06 μm to 0.02 cm −1.

White-beam synchrotron-radiation and conventional X-ray topography of GdCa 4O(BO 3) 3:Y

The defect structure of gadolinium–calcium oxoborate single crystals highly doped with 22% Y, Gd 0.78Y 0.22Ca 4O(BO 3) 3, is studied using X-ray transmission topography both with the conventional source and synchrotron radiation. Straight dislocation diffraction contrasts, white or black, depending on absorption and on its changing while the diffraction vector is reversed, are observed. A model of formation of dislocation contrast is proposed in the case of intermediate absorption.

X-Ray High-Resolution Diffraction and Transmission Topography Study of InGaAs Grown by Liquid Encapsulated Czochralski Technique

Laser diodes, solar cells, photodetector structures and other optoelectronic applications of InGaAs ternary bulk crystals and layers stimulate constant interest in the improvement of growth methods for this material. Growth of bulk ternary crystals based on III–V compounds with chemical compositions given by formula A 1−xB III x C V is a difficult task due to the thermodynamical properties of their solutions. Most of the techniques to obtain ternary crystals are focused on layer growing on the relevant substrates. Techniques like metalorganic chemical vapor deposition (MOCVD) [1, 2], metalorganic vapor phase epitaxy (MOVPE) [3, 4] and molecular beam epitaxy (MBE) [5, 6] are often used. Growth of quantum wells and quantum wires is also focused on ternary crystals. Some theoretical discussions about bulk growth of ternaries can be found in [7]. Recently rotational Bridgman method [8], liquid phase electroepitaxy [9] and zone growth [10] were used. ∗corresponding author; e-mail: Jerzy.Gronkowski@fuw.edu.pl

Solution growth of high-quality 3C-SiC crystals

Abstract 3C (cubic)-silicon carbide (SiC) bulk crystals that are stable at relatively low temperatures are difficult to grow by the sublimation method, which requires relatively high growth temperatures. This study focused on growth from the solution, which has the advantage that low growth temperatures are applied and the resulting crystals are mostly of high crystalline quality. Firstly, a large number of 3C-SiC crystals were grown by solution growth via spontaneous nucleation in a large temperature gradient. A relatively high yield of 6.4% was achieved from the charged Si solvent. Transmission electron microscopy and X-ray topography observations indicated that the stacking fault density of these crystals was much lower than that of the substrates grown by chemical vapor deposition methods. A dipping solution technique was then applied for 3C-SiC growth, using 3C-SiC seed crystals grown via spontaneous nucleation. Growth was performed on the (1 1 1)Si or (1¯ 1¯ 1¯)C faces. As a result, 3C-SiC grew on the (1¯ 1¯ 1¯)C face, but 6H-SiC crystals grew on the (1 1 1)Si face. For the stable growth of 3C-SiC crystals without 6H-SiC precipitation, growth on the (1¯ 1¯ 1¯)C faces is necessary.

Glide and multiplication of basal plane dislocations during 4H-SiC homoepitaxy

Basal plane dislocations (BPDs) are an important category of extended defects in SiC epilayers. They act as nucleation sites for single layer Shockley-type stacking faults which account for the degradation of the bipolar devices operating under forward bias. It is well documented that most of the BPDs in the SiC epilayers propagate from the substrates. However, two characteristic types of BPDs were suggested to be due to either nucleation or multiplication during epitaxy, including interfacial dislocations and short BPD arrays connected to the epilayer surface by threading segments. Combining molten KOH etching, plan-view transmission x-ray topography, and photoluminescence mapping, both types are determined to be two parts of one defect produced by the sideway glide of a BPD under the influence of shear stress. During the glide, the down-step end of the BPD frequently produces a series of short BPD segments at the moving growth front. These BPD segments will grow into an array of dislocation half loops. At the same time, the sideway glide of the BPD in the epilayer leaves an edge-type BPD segment at the epilayer∕substrate interface, which is the interfacial dislocation. The defect morphology provides the evidence of significant level of shear stresses present in SiC homoepitaxy of typical power device structures. The magnitude of such stresses is estimated.