Synchrotron White Beam X-ray Topography Research Articles

Dislocation arrangements in 4H-SiC and their influence on the local crystal lattice properties.

Two wafers of one 4H-silicon carbide (4H-SiC) bulk crystal, one cut from a longitudinal position close to the crystal's seed and the other close to the cap, were characterized with synchrotron white-beam X-ray topography (SWXRT) in back-reflection and transmission geometry to investigate the dislocation formation and propagation during growth. For the first time, full wafer mappings were recorded in 00012 back-reflection geometry with a CCD camera system, providing an overview of the dislocation arrangement in terms of dislocation type, density and homogeneous distribution. Furthermore, by having similar resolution to conventional SWXRT photographic film, the method enables identification of individual dislocations, even single threading screw dislocations, which appear as white spots with a diameter in the range of 10 to 30 µm. Both investigated wafers showed a similar dislocation arrangement, suggesting a constant propagation of dislocations during crystal growth. A systematic investigation of crystal lattice strain and tilt at selected wafer areas with different dislocation arrangements was achieved with high-resolution X-ray diffractometry reciprocal-space map (RSM) measurements in the symmetric 0004 reflection. It was shown that the diffracted intensity distribution of the RSM for different dislocation arrangements depends on the locally predominant dislocation type and density. Moreover, the orientation of specific dislocation types along the RSM scanning direction has a strong influence on the local crystal lattice properties.

The early development of synchrotron white-beam X-ray topography analysis for crystal investigations at Pohang light source-II

The early development of synchrotron white-beam X-ray topography analysis for crystal investigations at Pohang light source-II

The inclination of threading dislocation in chemical vapor deposition-grown single-crystal diamond analyzed by synchrotron white beam X-ray topography

The inclination of threading dislocation in chemical vapor deposition-grown single-crystal diamond analyzed by synchrotron white beam X-ray topography

Crystal Defect Investigation in PVT-Grown ZnSe Under Different Seeding Conditions and Growth Configurations Using Synchrotron X-Ray Topography

Zinc Selenide (ZnSe) as a prominent II-VI semiconductor is notable for its excellent performance on high efficiency electronic and optoelectronic devices including blue light-emitting diodes, lasers, and blue/ultraviolet-light-sensitive photodetectors, which are of considerable interest for applications in optical storage, lighting and illumination, biomedical sensing, etc. The effective utilization of ZnSe is owing to its superior properties such as wide bandgap, low optical absorption in the visible and infrared spectral region, and intense edge luminescence in the short-wavelength spectral region. The preliminary requisite to satisfy all those essential properties is the crystalline quality. Therefore, the realization of high-performance devices requires high-quality ZnSe bulk crystals with minimized structural defects. In order to overcome limitations and continuously fulfill the requirement of modern device applications, obtaining a comprehensive understanding on ZnSe crystallographic structure and defect formation mechanism is crucial. In this study, characterization techniques including synchrotron white beam X-ray topography (SWBXT), optical microscopy and high-resolution triple X-ray diffraction (HRTXD) were conducted on seeded and self-seeded ZnSe crystals grown by physical vapor transport (PVT) method in horizontal and vertical growth configurations. The effect of seeding condition and gravity vector orientation with respect to growth geometry on crystalline quality and defect structures is investigated. The presence of grains and the occurrence of twinning is revealed and analyzed. This study provides a clear understanding of factors that causes ZnSe crystal quality variation during the growth process, which contributes to achieving the routine production of high-quality ZnSe crystal and modern device enhancement. Figure 1

Crystal Defect Investigation in PVT-Grown ZnSe Under Different Seeding Conditions and Growth Configurations Using Synchrotron X-Ray Topography

Zinc selenide (ZnSe) is a widely utilized II-VI semiconducting material for fabricating high-performance optoelectronics devices owing to its unique electrical and optical properties. As those properties highly depend on the native point defects and impurity distribution, achieving routine production of high-quality ZnSe crystal is essential. This study conducted synchrotron white beam X-ray topography (SWBXT), optical microscopy and high-resolution triple X-ray diffraction (HRTXD) characterization on five as-grown ZnSe boules grown in physical vapor transport (PVT) method with different seeding conditions and growth configurations. Study indicated that nucleation of multiple grains is induced during seeded growth, while self-seeded boules tend to obtain single crystals. ZnSe boules grown in vertical configuration are found to have relatively poor crystalline quality comparing to horizontally grown boules that are characterized by low energy facets. The twinning mechanism is also discussed. This study successfully investigated factors that induce crystal quality variation during the growth process.

Analysis of Dislocations in PVT-Grown 6H-SiC through Grazing-Incidence X-Ray Topographic Images and Ray-Tracing Simulation

The superior electronic and physical properties of silicon carbide (SiC), such as wide band-gap, high saturated electron velocity, high breakdown electric field strength, and high thermal conductivity, enable successful applications in modern power electronic devices with high power, high frequency, and high temperature operation requirements. [1,2] Therefore, understanding the nature and behavior of different dislocations is of great significance towards crystal quality enhancement. Synchrotron X-ray has been widely adopted for generating topographic images of threading screw dislocations (TSDs) as well as threading edge dislocations (TEDs) and basal plane dislocations (BPDs). Besides, recent study in 4H-SiC revealed the presence of threading mixed dislocations (TMDs) with Burgers vectors nc+ma (where m and n are integers). [3] Since direct TMDs observation methods such as transmission electron microscopy (TEM) and synchrotron white-beam X-ray topography (SWBXT) are not suitable for commercially preferred c-plane-oriented wafers, ray-tracing simulated images of possible dislocation types are compared with X-ray topographs in order to identify the TMDs non-destructively. Except 4H-SiC, ray-tracing simulation has also been proved as an effective approach for dislocation characterization in various materials such as GaN [4], ZnO [5], and AlN [6]. The same principle is implemented in this study for the investigation of dislocations in 6H-SiC wafer.Synchrotron monochromatic beam X-ray topography (SMBXT) analysis was carried out on a PVT grown 6H-SiC wafer for recording its grazing-incidence images of all six different 11-212 reflections (Fig. 1). The distribution of TSDs, TEDs, TMDs, and BPDs are characterized, and the determination of their Burgers vectors is achieved by correlating the simulated results obtained through ray-tracing simulation with real X-ray topographic observations (Fig. 2). This study provides a clear understanding of dislocations in 6H-SiC, which is crucial for quality improvement of the crystal.Reference:[1]: C. Codreanu, et al, Comparison of 3C-SiC, 6H-SiC and 4H-SiC MESFETs Performances, Materials Science in Semiconductor Processing 3 137 (2000).[2]: J. W. Sun, et al, Shockley-Frank Stacking Faults in 6H-SiC, Journal of Applied Physics 111, 113527 (2012).[3]: J. Q. Guo, et al, Direct Determination of Burgers Vectors of Threading MixedDislocations in 4H-SiC Grown by PVT Method, Journal of Electronic Materials 45, 4 (2016).[4]: B. Raghothamachar, et al, X-ray Topography Characterization of Gallium Nitride Substrates for Power Device Applications, submitted for publication.[5]: T. Y. Zhou, “Threading Dislocation Characterization and Stress Mapping Depth Profiling via Ray Tracing Technique,” Ph.D. Thesis, SUNY Stony Brook, Advisor: Michael Dudley (2015).[6]: T. Y. Zhou, et, al, Characterization of Threading Dislocations in PVT-Grown AlN Substrates via x-Ray Topography and Ray Tracing Simulation, Journal of Electronic Materials volume 43, 838 (2014). Figure 1

Characterization and Reduction of Defects in 4H-SiC Substrate and Homo-Epitaxial Wafer

4H-SiC substrates and homo-epitaxial layers were obtained using the traditional methods of physical vapor transport and chemical vapor deposition. Defect morphology has been studied using both Synchrotron White Beam X-ray Topography and Monochromatic Beam X-ray Topography. Molten KOH etching method was adopted to further investigate the dislocation behavior mechanisms. Deflected dislocations were observed at the periphery regions in both substrate and epitaxial wafers. 3C polytypes and half loop arrays were observed in the 4H-SiC epitaxial wafer. It is also found that the majority of basal plane dislocations are converted to threading edge dislocations in the epitaxial wafer samples. The proportion of BPD to TED conversion depends on the surface step morphology and growth mode in epitaxial growth which in turn depends on the C/Si ratio. By the optimization of etching time prior to epitaxy and C/Si ratio, high-quality epitaxial wafers with extremely low basal plane dislocations densities (＜0.1 cm-2) was obtained.

Characterization of defects and strain in the (AlxGa(1−x))0.5In0.5P/ GaAs system by synchrotron X-ray topography

Abstract Synchrotron white beam X-ray topography and monochromatic beam topography have been applied to study GaAs substrates and (AlxGa(1−x))0.5In0.5P epitaxial layers grown on them that are being developed for LEDs. The GaAs substrates are 15° offcut toward [1 1 0] and mainly consist of dislocations in the {1 1 1} slip planes and low angle grain boundaries. The nominally lattice matched (AlxGa(1−x))0.5In0.5P layers contain defects such as threading dislocations replicated from the substrates, inclusions and misfit dislocations generated during organometallic vapor phase epitaxy (OMVPE) growth. The existence of misfit dislocations (MDs) indicates that strain relaxation has taken place to accommodate the in-plane lattice mismatch between epitaxial layer and substrate. The driving force for the glide of threading dislocations and relaxation efficiency of dislocations with different Burgers vectors and glide planes has been analyzed in the wafers with 15° offcut to predict the Burgers vectors and line directions of dislocations that dominate strain relaxation. Burgers vectors have been determined using ray tracing simulations. Reciprocal space maps (RSMs) recorded from the wafers to measure the strain and tilt of the epilayers show both in-plane and out-of-plane lattice mismatch as well as tilt between epilayer and substrate. A modified Nagai′s model has been developed to analyze and account for in-plane and out-of-plane tilt.

Analysis of the Basal Plane Dislocation Density and Thermomechanical Stress during 100 mm PVT Growth of 4H-SiC.

Basal plane dislocations (BPDs) in 4H silicon carbide (SiC) crystals grown using the physical vapor transport (PVT) method are diminishing the performance of SiC-based power electronic devices such as pn-junction diodes or MOSFETs. Therefore, understanding the generation and movement of BPDs is crucial to grow SiC suitable for device manufacturing. In this paper, the impact of the cooldown step in PVT-growth on the defect distribution is investigated utilizing two similar SiC seeds and identical growth parameters except for a cooldown duration of 40 h and 70 h, respectively. The two resulting crystals were cut into wafers, which were characterized by birefringence imaging and KOH etching. The initial defect distribution of the seed wafer was characterized by synchrotron white beam X-ray topography (SWXRT) mapping. It was found that the BPD density increases with a prolonged cooldown time. Furthermore, small angle grain boundaries based on threading edge dislocation (TED) arrays, which are normally only inherited by the seed, were also generated in the case of the crystal cooled down in 70 h. The role of temperature gradients inside the crystal during growth and post-growth concerning the generation of shear stress is discussed and supported by numerical calculations.

High Quality AlN Single Crystal Substrates for AlGaN-Based Devices

Aluminum nitride (AlN) single crystal boules were grown by physical vapor transport (PVT). Diameter expansion during boule growth, without the introduction of low angle grain boundaries (LAGB) around the boule periphery, was confirmed by crossed polarizer imaging, synchrotron white beam x-ray topography (SWBXT), and synchrotron monochromatic beam x-ray topography (SMBXT). The densities of basal plane dislocations (BPD) and threading edge dislocations (TED) averaged from high-magnification topographs of five regions of a high-quality substrate were 0 cm-2 and 992 cm-2, respectively. Substrates fabricated from AlN boules possessed excellent surface finishes suitable for epitaxy.

(Invited) SiC Growth Parameter Evolution Utilizing Infrared Thermal Imaging Towards Realization of Extremely Low Dislocation Bulk SiC

The NASA Glenn Research Center (GRC) has employed infrared thermal imaging technology in a hot wall chemical vapor deposition (CVD) system for thermal measurement of silicon carbide (SiC) crystals mounted vertically in the gas stream. The use of infrared thermal imaging technology revealed deficiencies in growth parameters that had previously not been observable utilizing traditional noncontact monitoring techniques. GRC has been investigating the lateral expansion (⊥ to (0001)) of pseudo SiC fiber seed crystals using a custom designed horizontal flow hot wall CVD system in proof of concept research, for a radically new bulk growth process. This bulk growth process would, ideally produce SiC boules with just one single centrally located screw dislocation (SD). Dislocations in general have a negative impact on SiC devices and device fabrication. Current commercially available SiC substrates have a dislocation density of ~1000 cm2. A key requirement of this process is the lateral homoepitaxial growth expansion of a SiC fiber without the introduction of additional defects. This requires maintaining growth temperatures along the length of the SiC fiber. In the absence of purpose grown fiber like seed crystals, pseudo seed crystals were created by mechanically dicing thin slivers from a-plane and m-plane oriented boule slices. The slivers (~17000 x 2000 x 400) µm’s were subsequently subjected to gaseous etching in hydrogen to produce needle like seed crystals with fiber like dimensions with a width of ~100 µm’s. The seed crystals were oriented vertically in a gas stream of the horizontal CVD system. Traditional optical pyrometry coupled with a small CCTV aligned along the horizontal axis was used to be provide real time thermal and optical data of the CVD. Due to the extremely small cross-sectional area of the seed crystals, the thermal radiation was insufficient to be measured with optical pyrometry and was ineffective in resolving the temperature of the vertical slivers. Only the temperature of the carrier holding the seed crystals could be resolved. A commercially available infrared thermal imaging system equipped with an extra-long working distance (530mm) lens replaced the pyrometer and CCTV. The thermal imager requires that the target be only 3 x 3 pixels to obtain accurate temperatures. When coupled with long working distance optics the temperature of the seed crystals could be resolved. A thermal gradient that was previously unobserved was now viewable. Growth parameters were adjusted to compensate for the gradient preventing nucleation of 3C-SiC at the tip of the seed crystal. A growth rate (⊥ to (0001)) of ~80 μm/hr was achieved. Total lateral growth was > 3.5mm. Initial analysis by synchrotron white beam x-ray topography (SWBXT) confirms, that the fiber growth was homoepitaxial, matching the polytype of the respective underlying region of the seed sliver. Although the as grown SiC contained a high density of basal plane dislocations (BPD’s) it appears that the BPD’s originated in the pseudo seed crystal without additional BPD’s introduced during the growth.

Dislocation Characterization in 4H-SiC Crystals

Definitive correlations are presented between specific types of dislocations identified via Synchrotron White Beam X-Ray Topography (SWBXRT) and identified via selective etching of 4H-SiC substrates. A variety of etch conditions and the results on different faces of SiC substrates and epiwafers are examined. Hillocks formed on the carbon face of the substrate after KOH etching correlate very well to TSDs identified via SWBXRT. Topography of substrates and of vertical crystal slices reveals a large proportion of Threading Mixed character Dislocations (TMDs) in the population of Threading Screw Dislocations.

Synchrotron X-Ray Topography Analysis of Double Shockley Stacking Faults in 4H-SiC Wafers

Synchrotron white beam X-ray topography studies carried out on 4H-SiC wafers characterized by locally varying doping concentrations reveals the presence of overlapping Shockley stacking faults generated from residual surface scratches in regions of higher doping concentrations after the wafers have been subjected to heat treatment. The fault generation process is driven by the fact that in regions of higher doping concentrations, a faulted crystal containing double Shockley faults is more stable than perfect 4H–SiC crystal at the high temperatures (>1000 °C) that the wafers are subject to during heat treatment. We have developed a model for the formation mechanism of the rhombus shaped stacking faults, and experimentally verified it by characterizing the configuration of the bounding partials of the stacking faults on both surfaces. Using high resolution transmission electron microscopy, we have verified that the enclosed stacking fault is a double Shockley type.

150 mm 4H-SiC Substrate with Low Defect Density

150 mm diameter 4H-SiC boules were grown by the physical vapor transport (PVT) method. Synchrotron white beam X-ray topography (SWBXT) was carried out to investigate the distribution of defects in axial slices cut from the boule. It was found that an increase of dislocations and micropipes was mainly induced by inclusions. After eliminating these inclusions, which were formed in the mid to late stage of the crystal growth, both the screw dislocation density and base plane dislocation density could be decreased down to a magnitude of 102 cm-2, which is comparable to that of high quality 100 mm diameter SiC substrates.

CVD growth and properties of boron phosphide on 3C-SiC

Improving the crystalline quality of boron phosphide (BP) is essential for realizing its full potential in semiconductor device applications. In this study, 3C-SiC was tested as a substrate for BP epitaxy. BP films were grown on 3C-SiC(100)/Si, 3C-SiC(111)/Si, and 3C-SiC(111)/4H-SiC(0001) substrates in a horizontal chemical vapor deposition (CVD) system. Films were produced with good crystalline orientation and morphological features in the temperature range of 1000–1200°C using a PH3+B2H6+H2 mixture. Rotational twinning was absent in the BP due to the crystal symmetry-matching with 3C-SiC. Confocal 3D Raman imaging of BP films revealed primarily uniform peak shift and peak widths across the scanned area, except at defects on the surface. Synchrotron white beam X-ray topography showed the epitaxial relationship between BP and 3C-SiC was (100)〈011〉BP||(100)〈011〉3C-SiC and (111)〈112̅〉BP||(111)〈112̅〉3C-SiC. Scanning electron microscopy, Raman spectroscopy and X-ray diffraction analysis indicated residual tensile strain in the films and improved crystalline quality at temperatures below 1200°C. These results indicated that BP properties could be further enhanced by employing high quality bulk 3C-SiC or 3C-SiC epilayers on 4H-SiC substrates.

Effect of Doping Concentration Variations in PVT-Grown 4H-SiC Wafers

Synchrotron white beam x-ray topography studies carried out on 4H-SiC wafers characterized by locally varying doping concentrations reveals the presence of overlapping Shockley stacking faults generated from residual surface scratches in regions of higher doping concentrations after the wafers have been subjected to heat treatment. The stacking faults are rhombus-shaped and bound by Shockley partial dislocations. The fault generation process is driven by the fact that in regions of higher doping concentrations, a faulted crystal containing double Shockley faults is more stable␣than a perfect 4H-SiC crystal at the high temperatures (>1000°C) that the wafers are subject to during heat treatment. We have developed a model for the formation mechanism of the rhombus-shaped stacking faults. Our studies show that during heat treatment of the wafer, such double Shockley faults can be generated in regions where dislocation sources are presents (e.g. scratches or low-angle boundaries) and when the nitrogen doping concentration exceeds a certain level.

Experimental verification of the model for formation of double Shockley stacking faults in highly doped regions of PVT-grown 4H–SiC wafers

We recently reported on the formation of overlapping rhombus-shaped stacking faults from scratches left over by the chemical mechanical polishing during high temperature annealing of PVT-grown 4H–SiC wafer. These stacking faults are restricted to regions with high N-doped areas of the wafer. The type of these stacking faults were determined to be Shockley stacking faults by analyzing the behavior of their area contrast using synchrotron white beam X-ray topography studies. A model has been proposed to explain the formation mechanism of the rhombus shaped stacking faults based on double Shockley fault nucleation and propagation. In this paper, we have experimentally verified this model by characterizing the configuration of the bounding partials of the stacking faults on both surfaces using synchrotron topography in back reflection geometry. As predicted by the model, on both the Si and C faces, the leading partials bounding the rhombus-shaped stacking faults are 30° Si-core and the trailing partials are 30° C-core. Using high resolution transmission electron microscopy, we have verified that the enclosed stacking fault is a double Shockley type.

Direct Determination of Burgers Vectors of Threading Mixed Dislocations in 4H-SiC Grown by PVT Method

In addition to pure threading screw dislocations (TSDs), the presence of threading mixed dislocations (TMDs) (with a component) has been reported both in 4H-SiC axial slices (wafers cut parallel to the growth axis) and in commercial offcut wafers (cut almost perpendicular to the growth axis). In this paper, we first demonstrate a method to quickly distinguish TMDs from TSDs in axial slices via synchrotron white-beam x-ray topography. Since such axial slices are usually not available for commercial purposes, a systematic method is then developed and demonstrated here to unambiguously determine the Burgers vectors of TMDs in 4H-SiC commercial offcut wafers. In this second study, both synchrotron monochromatic-beam x-ray topography and ray-tracing simulation are used. The x-ray topographs were recorded using grazing-incidence geometry. The principle of this method is that the contrast of dislocations on different reflections varies with the relative orientation of Burgers vectors with respect to the diffraction vectors. Measurements confirm that, in a commercial offcut wafer, the majority of the threading dislocations with screw component are mixed-type dislocations.

Synchrotron X-ray topographic study on nature of threading mixed dislocations in 4H–SiC crystals grown by PVT method

Synchrotron X-ray Topography is a powerful technique to study defects structures particularly dislocation configurations in single crystals. Complementing this technique with geometrical and contrast analysis can enhance the efficiency of quantitatively characterizing defects. In this study, the use of Synchrotron White Beam X-ray Topography (SWBXT) to determine the line directions of threading dislocations in 4H–SiC axial slices (sample cut parallel to the growth axis from the boule) is demonstrated. This technique is based on the fact that the projected line directions of dislocations on different reflections are different. Another technique also discussed is the determination of the absolute Burgers vectors of threading mixed dislocations (TMDs) using Synchrotron Monochromatic Beam X-ray Topography (SMBXT). This technique utilizes the fact that the contrast from TMDs varies on SMBXT images as their Burgers vectors change. By comparing observed contrast with the contrast from threading dislocations provided by Ray Tracing Simulations, the Burgers vectors can be determined. Thereafter the distribution of TMDs with different Burgers vectors across the wafer is mapped and investigated.

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.