Reflection X-ray Topography Research Articles

Origin of irregular X-ray mirage fringes from a bent, thin crystal.

The dynamical theory of diffraction is used to analyse irregular X-ray mirage interference fringes observed in Si220 X-ray reflection topography from a weakly bent, thin crystal due to gravity. The origin of the irregular fringes is attributed to the interference between mirage diffracted beams and a reflected beam from the back surface, which is a new type of interference fringe. The irregular fringes are reproduced by calculating the reflected intensities numerically. The effects of absorption and thermal vibration are quite important for the reproduction. The result shows that the interference fringes depend on the strain as well as the thickness of the crystal, which indicates that the fringes should be useful for analysing weak strain in a crystal as an application.

Effect of temperature on conversion of basal plane dislocations to treading edge dislocations during 4H-SiC homoepitaxiy

Abstract 4H-SiC homoepitaxial layer grown on 4° off-axis substrates is performed at various growth temperature by using the traditional chemical vapor deposition. The effect of growth temperature on propagation of basal plane dislocations from substrate to epitaxy was studied using the deep KOH etching and Synchrotron reflection X-ray topography, and the conversion mechanism was also investigated. It is found that the majority basal plane dislocations are converted to threading edge dislocations during epitaxial growth. Meanwhile, the two types of threading edge dislocations in epitaxial layer are observed after KOH etch. By the optimization of growth temperature, high quality epitaxial wafer with extremely low basal plane dislocations density (

Evaluation of dislocations under the electrodes of GaN pn diodes by X-ray topography

We have successfully obtained the clear images of the dislocation mapping even under the electrodes of the GaN vertical pn diodes by reflection X-ray topography using monochromatic synchrotron radiation (SPring-8, BL16B2 & BL20B2). This is a powerful analysis because of the non-destructive and rapid tool unlike the etch-pit method and TEM (transmission electron microscopy). This allows us to directly study the relationship between the dislocation and the leakage current. Applying this method to vertical pn diodes on Na-flux GaN substrates, we found that leakage current per dislocation is of the order of 10 pA. This analysis has a potential to reveal the killer dislocation structure.

Dislocation structure of Ge crystals grown by low thermal gradient Czochralski technique

Dislocation structure of the Ge single crystals grown by Czochralski method with low thermal gradient has been studied. The selective etching technique and the X-Ray transmission and reflection topography were used. Clearly defined non-uniform dislocation distribution over the crystal cross - section is revealed. Helical dislocations and sets of prismatic dislocation loops are registered. Helical dislocations perpendicular to the ingot axis are situated near the boundary between the regions with low and high dislocation densities (102 and 103cm−2, respectively). Their length can be as much as several millimeters. Dislocation formations lying at a 35.3° to the crystal axis along <110> directions are also observed. These formations have the shape of prism confined by {111} planes.

Effects of Basal Plane Dislocation Density in 4H-SiC Substrate on Degradation of Body-Diode Forward Voltage

We investigated the effect of the basal plane dislocation (BPD) density in 4H-silicon carbide (SiC) substrates on the forward voltage (Vsd) degradation of body-diodes. Using reflection X-ray topography, the BPD density was automatically estimated from the substrates prior to fabrication of metal–oxide–semiconductor field-effect transistors (MOSFETs). A strong positive correlation was found between the Vsd shift, which was calculated from the difference before and after forward bias stress at 160 A/cm2 for ~500 hours, and the BPD density of the substrate. We show that it is possible to predict Vsd shifts from the BPD densities of SiC substrates prior to the fabrication of MOSFETs. In addition, we examined the origin of stacking faults (SFs) as a result of the application of forward bias stress. We presume that SFs are formed by BPDs converted to threading edge dislocations at the epi/sub interface, as well as by BPDs penetrating into the epitaxial layer.

Step-Bunching Free and 30 μm-Thick SiC Epitaxial Layer Growth on 150 mm SiC Substrate

The production of 150 mm-diameter SiC epi-wafers is the key to the spread of SiC power devices. Besides, step-bunching free surface leads to high-performance devices. We have developed the production technology of the epitaxial growth with smooth surface morphology for 4º off Si-face 4H-SiC epitaxial layers on 150 mm diameter substrates. The various area observations of the surface by optical surface analyzer, confocal microscope and atomic force microscope revealed that there was no conventional step-bunching in whole wafer surface. While creating step-bunching free surface is more difficult for thicker epilayer growth, we have achieved step-bunching free surface for 30-μm thick epilayer on a 150 mm diameter substrate. The typical values of thickness uniformity of the 30μm-thick epilayer are 0.5% (σ/mean) and 1.7% (range/mean). A few interfacial dislocations (IDs) were detected for the 150 mm-diameter epi-wafer by reflection X-ray topography. We have succeeded in removal of IDs by the optimized growth condition.

Annealing induced extended defects in as-grown and ion-implanted 4H–SiC epitaxial layers

The formation of extended defects in the 4H–SiC epilayer induced by the implantation/annealing process was investigated using synchrotron reflection x-ray topography, KOH etching analysis, and transmission electron microscopy. High temperature annealing was performed for the 4H–SiC epilayer with or without the implantation of nitrogen or aluminum ions. Other than the formation of platelet extrinsic Frank-type faults in the implanted region as reported previously, we find the formation modes of extended defects in following three categories: (i) dislocation formation near the epilayer/substrate interface, (ii) dislocation formation near the implanted region, and (iii) the formation of Shockley-type defects near the surface. The defect morphology and process dependence of each type are also discussed.

Condition Dependences of Extended Defect Formation in 4H-SiC by Ion-Implantation/Activation-Anneal Process

Condition dependences of defect formation in 4H-SiC epilayer induced by the implantation/annealing process were investigated using synchrotron reflection X-ray topography and transmission electron microscopy. Nitrogen, phosphorus or aluminum ions were implanted in the 4H-SiC epilayers and then activation annealing was performed. To compare the implantation/annealing process, a sample receiving only the annealing treatment without the implantation was also performed. Two different crucibles (conventional and improved) were used in the annealing process. The formation of single layer Shockley-type stacking faults near the surface was found to have no ion-implantation condition or crucible dependence. The formation of BPD half-loops and the glide of pre-existing BPDs showed clear dependence on the crucibles.

Formation of Extended Defects in 4H-SiC Induced by Ion Implantation/Annealing

Defect formation during the ion implantation/annealing process in 4H-SiC epilayers is investigated by synchrotron reflection X-ray topography. The 4H-SiC epilayers are subjected to an activation annealing process after Aluminum ions being implanted in the epilayers. The formation modes of extended defects induced by the implantation/annealing process are classified into the migration of preexisting dislocations and the generation of new dislocations/stacking faults. The migration of preexisting basal plane dislocations (BPDs) takes place corresponding to the ion implantation interface or the epilayer/substrate interface. The generation of new dislocations/stacking faults is confirmed as the formation of Shockley faults near the surface of the epilayer and BPD half-loops in the epilayer.

Investigation of Defect Formation in 4H-SiC(0001) and (000-1) Epitaxy

Defect formation in 4H-SiC(0001) and (000-1) epitaxy is investigated by grazing incidence synchrotron reflection X-ray topography and transmission electron microscopy. Frank-type faults, which are terminated by four Frank partials with a 1/4[0001] type Burgers vector with the same sign on four different basal planes, are confirmed to be formed by conversion of a 1c threading edge dislocation (TSD) in the substrate as well as simultaneous generation of a 1c TSD during epitaxy. The collation between the topography appearance and the microscopic structure and the variety of Frank faults are shown. Formation of carrot defects and threading dislocation clusters are also investigated.

Slip of Basal Plane Dislocations in 4H-SiC Epitaxy

Slip of basal plane dislocations in 4H-SiC epitaxy is observed by synchrotron reflection X-ray topography. It is verified that average slip distance and slip thickness increase with decreasing donor concentration of epilayer. Since N atom, which is incorporated at carbon site in 4H-SiC and acts as a donor, has a smaller radius than that of C atom, lattice contraction effect is expected due to incorporation of N. Considering difference of lattice contraction between the substrate and the epilayer owing to difference of donor concentration, strain in the epilayer is estimated. The strain is expected to increase with decreasing the donor concentration of epilayer and the results are explained from this point of view.

Influence of Growth Conditions and Substrate Properties on Formation of Interfacial Dislocations and Dislocation Half-loop Arrays in 4H-SiC(0001) and (000-1) Epitaxy

ABSTRACTFormation of interfacial dislocations (IDs) and dislocation half-loop arrays (HLAs) and their appearance in 4H-SiC epi-wafers are investigated by X-ray topography and KOH etching analysis. Synchrotron reflection X-ray topography demonstrates the ability to image IDs and HLAs simultaneously and reveal their densities as well as spatial distributions in the epi-wafers. The vertical location of IDs in the epi-wafer is also examined by this technique. The influence of wafer warp, in-situ H2 etching prior to epitaxial growth, substrate off-angle as well as the growth face (Si-face and C-face) on the densities and spatial distributions of IDs and HLAs are discussed.

Formation of basal plane Frank-type faults in 4H-SiC epitaxial growth

Formation of Frank-type faults on the basal plane in 4H-SiC{0 0 0 1} epitaxial growth has been investigated by grazing incidence synchrotron reflection X-ray topography and transmission electron microscopy. The Frank faults, which are formed by conversion of a 1 c threading screw dislocation (TSD) in the substrate as well as simultaneous generation of a 1 c TSD during epitaxial growth, are confirmed to be created by four Frank partials with a Burgers vector of 1/4[0 0 0 1] type having the same sign on four different basal planes. The fine features of the X-ray topography contrast are confirmed to reflect the microscopic structure of the Frank faults. A variety of Frank fault formation has been revealed by comparing topography images taken before and after epitaxial growth. The formation mechanism of the Frank faults is discussed based on the evaluated microscopic structures.

An X-Ray Topographic Analysis of the Crystal Quality of Globally Available SiC Wafers

We present herein a first comparative analysis of the quality of 50 mm and 75 mm diameter SiC wafers, purchased directly from vendors across the world, types including the most widely available configurations. Large Area White Beam Synchrotron Back Reflection X-Ray Topography was used to analyse selected ~1cm2 regions at various locations on up to 10 different bulk SiC wafers. The study concentrated particularly on the density and distribution of threading screw dislocations (TSDs). We also examined all wafers for basal plane dislocation (BPDs) densities and distributions. Alarmingly large variation in wafer quality was observed. TSD densities vary from a minimum of 0 cm-2 (in a-plane material) to values as large as over 2,000 cm-2 on some n-type 4H-SiC wafers. TSD densities on individual wafers can also vary by similar magnitudes, e.g. 500cm-2 to 2,500 cm-2 on two regions only 2 cm apart on a 50 mm diameter wafer. Computer-based image process analysis was used to present a statistical analysis of the distributions of defects. For example algorithms created in MATLAB®, Image Processing Toolbox, isolated possible TSD locations allowing rapid counting to be performed. These counts were confirmed by manual counting of selected unmodified images.

Migration of Dislocations in 4H-SiC Epilayers during the Ion Implantation Process

Synchrotron reflection X-ray topography and KOH etching were applied to investigate the effects of the ion implantation/annealing process on the existing dislocations in the 4H-SiC epilayers and second epitaxial growth on the ion implanted layer. No systematic generation of dislocations or stacking faults caused by the second epitaxial growth on the implanted layer was observed, while BPDs were confirmed to migrate in the epilayer during the implantation/annealing process. The BPDs bend markedly near the bottom of the implanted layer and tend to lie along the &lt;1-100&gt; (perpendicular to the off-cut direction) after the implantation/annealing process. The lattice mismatch strain created by the implantation is a possible driving force of the glide motion of the BPDs.

Investigation of defect formation in 4H-SiC epitaxial growth by X-ray topography and defect selective etching

Formation of extended defects during 4H-SiC{0 0 0 1} epitaxial growth has been investigated by grazing incidence synchrotron reflection X-ray topography and KOH-defect selective etching analysis. Details of the defect topography contrast features are collated with the KOH etched features. Conversion of threading screw dislocations (TSDs) into a defect on the basal plane as well as a carrot defect is tracked by performing topography before and after epitaxial growth. We found simultaneous nucleation of a TSD and a defect on the basal plane during epitaxial growth. The polytype inclusions are confirmed to be transparent in the topography condition. Formation of threading dislocation clusters during epitaxial growth is also investigated.

Comparison of Propagation and Nucleation of Basal Plane Dislocations in 4H-SiC(000-1) and (0001) Epitaxy

Propagation and nucleation of basal plane dislocations (BPDs) in 4H-SiC(000-1) and (0001) epitaxy were compared. Synchrotron reflection X-ray topography was performed before and after epitaxial growth to classify the BPDs into those propagated from the substrate into the epilayer and those nucleated in the epilayer. It was revealed that the propagation ratio of BPDs for the (000-1) epitaxy was significantly smaller than that for the (0001) epitaxy. Growing (000-1) epilayers at a high C/Si ratio of 1.2 achieves a further reduction in BPDs to only 3 cm-2 for those propagated from the substrate, and 16 cm-2 for those nucleated in the epilayer. A dramatic increase was also found in the nucleation of BPDs omitting the re-polishing and in-situ H2 etching procedure.

Assessment of heteroepitaxial ZnSe layers on GaAs by means of grazing incident X-ray topography

Grazing incident X-ray reflection topography was used to evaluate the crystal perfection of ZnSe epilayers grown by metalorganic vapour phase epitaxy on low-dislocation-density (1 0 0) GaAs substrates. Grazing incident topographs were recorded from 224 reflections at a wavelength and an incident angle of 0.1334 nm and 0 . 17 ∘ , respectively. We found that there is a remarkable difference between samples with ZnSe epilayer thinner than the critical thickness and thicker samples; lattice imperfections in thin ZnSe heteroepitaxial layers consist of small changes of the ZnSe lattice constant and bending of the crystal planes.

Observations of Lattice Distortions near Silicon Surfaces Implanted with Low-Energy Nitrogen Ions by Reflection X-ray Topography

Mirror-polished Czochralski-grown wafers with near-(100)-oriented surfaces were implanted with N+ at 8 keV energy at a dose of 1 ×1015 cm-2. Lattice distortions produced by the implantation process were observed by X-ray double-crystal topography using extremely asymmetric reflection. The intensity contrast caused by the lattice extensions in thin layers was clearly visualized. By annealing at 700°C for more than 90 min, the imperfect crystal in the ion-implanted region evidently recovers to a more perfect one, except for the boundary of the implanted region. The lattice distortions at the boundary, consisting of a narrow striated region, are thought to be due primarily to variations in the lattice spacing that has been deformed nonelastically. From a series of topographs taken by changing the glancing angle from the low-angle to the high-angle side of the Bragg peak, the differences in lattice spacing between the striated portions and the unimplanted regions were estimated.

Sublimation growth of 3C–SiC on 6H–SiC seeds

Sublimation growth conditions for 3C–SiC on (0 0 0 1) 6H–SiC were determined. Growth with silicon excess stream results in extended triangular shaped structures. The orientation relation of these polytypes was determined by selected area channelling pattern (SACP). The (1 1 1) 3C lattice plane is parallel to (0 0 0 1) 6H face and the direction [ 1 1 2 ¯ ] 3 C is parallel to [ 1 ¯ 1 . 0 ] 6 H in these planes. Therefore, the 6-fold axis of 6H–SiC is parallel to 3-fold axis of 3C–SiC resulting in double positioning twins and double positioning boundaries (DPBs) because of the ambiguous relation. By optimisation of seed temperature, silicon excess stream and supersaturation one orientation becomes dominating during growth. This is verified by single-crystal X-ray reflection topography (Lang technique).