PVT Growth Research Articles

Influence of the Size Distribution of the SiC Powder Source on the Shape of the Crystal Growth Interface during PVT Growth of 4H-SiC Boules

We compared the evolution of three different SiC sources during standard PVT growth runs. The evolution of the growing crystal and the morphological changes in the SiC source were visualized using in-situ X-ray visualization. Computer simulation was used to calculate the temperature field distributions. It is found that the densification and shrinkage of the SiC source material during the growth process can affect the growth conditions in such a way that the convexity of the growth interface is increased in an unfavorable manner. While unfavorable growth conditions can be related to thermal properties due to less favorable SiC powder evolution, predicting such behavior is a rather complex task that still relies on the support of experimental methods.

Analysis of Defect Structures during the Early-Stages of PVT Growth of 4H-SiC Crystals

To better understand the effects of various growth parameters during the early-stages of PVT growth of 4H-SiC on resulting defect structures, multiple short duration growths have been carried out under varying conditions of seed quality, nucleation rate, thermal gradients, and N incorporation. Besides the replication of TSDs/TMDs and TEDs as well as the deflection of TSDs/TMDs into Frank dislocations, synchrotron monochromatic beam x-ray topography (SMBXT) studies also reveal the formation of stacking faults bounded by Frank dislocations. Using ray tracing simulations to characterize the Frank dislocations, three types of stacking faults are revealed: Type 1 stacking fault resulting from 2D nucleation of 6H polytype on terraces; Type 2 stacking fault resulting from macrostep overgrowth of the surface growth spiral steps of TSDs/TMDs which separate into c/2 or c/4 increments; Type 3 stacking fault resulting from vicinal step overgrowth of surface growth spiral steps of TSDs/TMDs which separate into c/4 and 3c/4 increments. Analysis of atomic resolution scanning transmission electron microscopy (STEM) images reveals the mechanism of the Type 3 fault.

Characterization of Prismatic Slip in SiC Crystals by Chemical Etching Method

In 4H-silicon carbide crystals, basal plane slip is the predominant deformation mechanism. However, prismatic slip is often observed in single crystals grown by the physical vapor transport method as the diameter expands to 6 inches or larger. Thermal modeling has shown that occurrence of prismatic slip is attributed to increased radial thermal gradients. While X-ray topography can be used to characterize the presence and extent of prismatic slip, the feasibility of using the chemical etching method to assess the extent of prismatic slip in an industrial setting is investigated. The distribution of scallop shaped etch pits oriented along the directions that correspond to prismatic dislocations, correlate well with the results of the thermal model that predicts the occurrence of prismatic slip dislocations. This capability of the etch pit method to characterize prismatic slip can be used to manage radial thermal gradients during PVT growth.

The effect of thermally treated AlN powder on PVT-grown single crystals

Herein, aluminum nitride (AlN) powder is thermally treated via a thermocycle process for use in PVT growth.

<i>In Situ</i> Monitoring of Unintentionally Released Nitrogen Gas in the Initial PVT Silicon Carbide Growth Process Using Mass Spectrometry

Different initial process steps during PVT crystal growth of SiC were monitored with a mass spectrometer. To measure the gas phase composition in the PVT growth machine during these steps the continuously pumped exhaust gas was analyzed by a quadrupole mass spectrometer (PrismaPro QMG 250). In order to reduce unintentionally doping of the crystal by contaminations in the growth setup the focus was on the release of nitrogen during the initial steps of the growth process. During the heat up of the growth setup in vacuum a substantial release of molecular nitrogen was observed at 800 °C. Further, the influence of pump and purge-steps on the amount of nitrogen in the gas phase was examined. After performing a pump and purge step, the intensity of the measurable nitrogen-related signal (m/z = 28) was approximately 20 % of the initial value. In-situ monitoring of the gas phase during the initial steps of crystal growth proved to be a versatile tool for the development of a process minimizing unintentionally doping through released nitrogen.

Analysis of the Morphology of the Growth Interface as a Function of the Gas Phase Composition during the PVT Growth of Silicon Carbide

The growth conditions of 75 mm SiC crystals in the PVT process is varied by different methods while the temperature field is kept constant. The addition of graphite into the source material leads to the formation of an ordered step flow with step heights of 0.014 µm, while the addition of graphite into the source together with N2 doping changes the step kinetics on the main facet, leading to very large, bunched steps of 0.17 µm. When elemental Si was added into the source material large macro steps are formed on the whole crystal surface. While the doping induced step bunching is related to the incorporation kinetics, the large steps induced in Si-rich conditions are attributed the reduction of surface energy. With the variation of the inert gas pressure the morphology of the surface is altered, similarly. Under low pressure conditions (0.2 mbar) a fine step structure evolves, while at a high pressure (40mbar) large surface steps are formed on the whole growth interface. Large surface steps are strongly impeded in their lateral motion at defects permeating the growth interface. At these sites the formation of foreign polytypes is facilitated.

Investigation on the Threading Dislocations Formed by Lattice Misfits during Initial Stage of Sublimation Growth of 4H-SiC

We investigated the relation between the nucleation of dislocations and the lattice misfits by nitrogen concentration difference between seed and grown crystal during the initial stage of growth. 4H-SiC single crystals were grown with various nitrogen gas flow rates introduced into the crystal growing chamber under the same temperature and pressure to minimize the effect of thermal stress on the nucleation of dislocations. The nitrogen atomic concentrations of grown crystals depended on the introduced nitrogen gas ratios and they highly increased at the very early stage of growth. The generation of new threading dislocations at the interface also was affected by the nitrogen atomic concentrations differences between seed and grown crystals. Very few generated threading dislocations were observed in low nitrogen atomic concentration samples, however nucleation of threading dislocations at the interface were found in high nitrogen atomic concentrations samples. At initial stages of PVT growth process, the generation of threading dislocations induced by lattice misfits originated from nitrogen concentration difference between seed and grown crystals were investigated and found the appropriate nitrogen gas flow rates and profile at the heating and depressurized stage.

Progress in Bulk 4H SiC Crystal Growth for 150 mm Wafer Production

The thermoelastic stress, mechanical properties and defect content of bulk 4H n-type SiC crystals were investigated following adjustments to the PVT growth cell configuration that led to a 40% increase in growth rate. The resulting 150 mm wafers were compared with wafers produced from a control process in terms of wafer bow and warp, and dislocation density. Wafer shape was found to be comparable among the processes, indicating minimal impact on internal stress. Threading edge and threading screw dislocation densities increased and decreased, respectively, while basal plane dislocation densities were unaffected by the increase in growth rate. Loss of wafer planar stability was observed in certain cases. The elastic modulus was measured to be in the range of approximately 420-450 GPa for selected stable and unstable wafers, and was found to correspond to resistivity.

Optimization of the SiC Powder Source Material for Improved Process Conditions During PVT Growth of SiC Boules.

We have studied the influence of different SiC powder size distributions and the sublimation behavior during physical vapor transport growth of SiC in a 75 mm and 100 mm crystal processing configuration. The evolution of the source material as well as of the crystal growth interface was carried out using in situ 3D X-ray computed tomography (75 mm crystals) and in situ 2D X-ray visualization (100 mm crystals). Beside the SiC powder size distribution, the source materials differed in the maximum packaging density and thermal properties. In this latter case of the highest packaging density, the in situ X-ray studies revealed an improved growth interface stability that enabled a much longer crystal growth process. During process time, the sublimation-recrystallization behavior showed a much smoother morphology change and slower materials consumption, as well as a much more stable shape of the growth interface than in the cases of the less dense SiC source. By adapting the size distribution of the SiC source material we achieved to significantly enhance stable growth conditions.

Relationship between Basal Plane Dislocation and Local Basal Plane Bending in PVT-Grown 4H-SiC Crystals

Efforts to improve the quality and size of 4H-SiC wafers have resulted in size of 6” and dislocation densities of the order of 103 cm-2. The presence of significant quantities of crystallographic defects in bulk 4H-SiC crystals continues to be a major obstacle for achieving high-quality 4H-SiC substrates. Of particular note is the nucleation and distribution of basal plane dislocations (BPDs). BPDs are deformation-induced and are mostly generated during post-crystal growth cooling or high temperature annealing processes. BPDs have been associated with the formation of Shockley stacking fault leading to an increase in forward voltage degradation and are undesirable. Depending on the PVT growth conditions, internal stresses due to non-uniformity of temperature field cause lattice plane bending. During cooldown, this bending is accommodated by the inhomogeneous distribution of BPDs in a PVT grown 4H-SiC boule and persists in wafers sliced from the boule. Crystal bending affects subsequent wafer processing especially polishing and device fabrication. Therefore, analysis of the BPD distribution (sign and density) enables us to better understand basal plane bending, which provides insight into further improvement of the crystal growth process. Synchrotron Monochromatic Beam X-ray Topography (SMBXT) studies in grazing incidence geometry reveal the appearance of black and white contrast of BPDs which are associated with opposite sign of Burgers vectors. Based on the principle of ray tracing, black contrast BPDs are generated by focusing of diffracted X-rays when the extra half plane is extending away from the imaged surface, while defocusing of X-rays contribute to the generation of white contrast BPDs due to the extra half plane extending towards the surface [1]. The inhomogeneous distribution of the two types of BPD contrast will thus result in concave/convex shape of the basal planes. In our study, we have evaluated the ratio of the black/white contrast of BPDs along 11-20 direction across multiple 6-inch diameter 4H-SiC substrates sliced from the same and different boules and predicted how the lattice is tilted in different substrate wafers. High resolution X-ray diffraction measurements are currently underway to investigate the variation of rocking curve peak shift along 11-20 direction and thus to further verify our result.

Analysis of Basal Plane Dislocation Dynamics in PVT-Grown 4H-SiC Crystals during High Temperature Treatment

The deformation processes in 4H-SiC crystals are of particular interest because they generate mostly basal plane dislocations (BPDs) which are implicated in the degradation or premature breakdown of SiC power devices [1]. Fundamental understanding of the generation and multiplication of BPDs is critical for design of growth strategies to control the density of BPDs during PVT growth of 4H-SiC crystals. Recently, we reported on the direct observation of thermal gradient induced motion of BPDs through in-situ synchrotron X-ray topography imaging of PVT-grown 4H-SiC wafers subject to high temperature treatment [2]. The multiplication of BPDs by the operation and interactions of multiple Frank-Read sources leading to formation of complex configurations was reported. We have analyzed the series of images recorded during the in-situ experiment in detail to deduce the Burgers vectors and core structures of the moving dislocation segments and measure their individual velocities. Dislocations with Burgers vector along the off-cut 11-20 direction were found to be the only dislocations undergoing deformation during heat treatment. Sample was imaged after heat treatment and the segments of dislocations used for velocity measurements were found to be either pure screw or 60° dislocations comprised of Si cores. Using the kink-diffusion model, the activation energies for dislocation motion have been estimated from the velocity data for each of these dislocation types and found to be 2.21 eV for 60° and 3.28eV for pure screw segments respectively. These values are in good agreement with the macroscopic studies of yielding of semiconductor crystals during high temperature compression and indentation experiments [3,4]. Quantitative expression of the temperature dependent critical resolved shear stress required for dislocation motion has been derived from this analysis.

Application of Defect Conversion Layer by Solution Growth for Reduction of TSDs in 4H-SiC Bulk Crystals by PVT Growth

We have developed the bulk growth technique to reduce threading screw dislocations (TSDs) by combining solution growth and PVT growth methods. More than 80 % of TSDs in original seed crystals were successfully converted to Frank defects on basal planes by the solution growth on 4° off C-face with Si-5at.% Ti solvent. After PVT growth on the as-grown surface of the conversion layer, TSDs in the original seed were successfully reduced. The presence of micrometer-size macrosteps in the initial stage of PVT growth is important to continue to propagate the converted Frank defects on basal planes during PVT bulk growth.

Study on Dislocation Behaviors during PVT Growth of 4H-SiC

The generation and transformation of dislocations in 4H-SiC crystals grown by PVT were investigated. Experiments were carried out in two stages for more comprehensive observation on dislocation behaviors. For the first stage known as initial growth, we investigated mainly the seed and grown interface. The behavior and transition of the dislocations in grown crystal were observed along the length of the crystal at second stage. The formation of threading edge dislocations (TEDs) strongly depends on the surface morphologies related with internal temperature gradients during crystal growth. The basal plane dislocation (BPDs) and threading screw dislocation (TSDs) cause from the seed crystal and formed at the initial stage of growth were gradually decreased in number along the length of the crystal and under certain conditions such as distorted stresses, dislocations were converted into other types of dislocations.

Modeling of the PVT Growth Process of Bulk 3C-SiC - Growth Process Development and Challenge of the Right Materials Data Base

We report on the modeling of the temperature field and supersaturation in front of the SiC crystal growth interface of a physical vapor transport growth configuration. The data are compared with experimental results, like the growth of free standing 3C-SiC wafers with a diameter of 50 mm and a thickness of 870 µm. Special emphases is put on the precise handling of the materials properties which include the temperature dependency of the heat and electrical conductivity of the graphite parts at temperatures above 2000 °C.

Analysis of Basal Plane Dislocation Dynamics in PVT-Grown 4H-SiC Crystals during High Temperature Treatment

Fundamental understanding of the generation and multiplication of basal plane dislocations (BPDs) in PVT-grown 4H-SiC crystals is critical for design of growth strategies to control their densities during PVT growth of 4H-SiC crystals. Direct observation of thermal gradient induced motion of basal plane dislocations by in-situ synchrotron X-ray topography imaging of PVT-grown 4H-SiC wafers subject to high temperature treatment has provided an opportunity to analyze the movement of dislocations. Dislocations with Burgers vector along the off-cut [11-20] direction were found to be the only dislocations involved in deformation during heat treatment and the segments of dislocations used for velocity measurements were found to be either pure screw comprised of both Si- and C- core partials or 60° dislocations comprised of purely Si cores. Using the kink-diffusion model, the activation energies for dislocation motion have been estimated from the velocity data for each of these dislocation types and found to be 2.21 eV for 60° and 3.28 eV for pure screw segments, respectively. These values are in good agreement with the macroscopic studies of yielding of semiconductor crystals during high temperature compression and indentation experiments. Quantitative expression of the temperature dependent critical resolved shear stress required for dislocation motion has been derived from this analysis.

Studies on Doping Concentration Variations in 4H-SiC Substrates Using X-ray Contour Mapping

4H-SiC is one of the promising wide bandgap material that attracts considerable attention in power electronics industry. Several applications have benefited from the improved performances of devices made of 4H-SiC in terms of its higher breakdown fields, efficiency and reliability as well as excellent physical properties. However, the widespread commercialization of this material is still being hindered by the various defects introduced during the crystal growth which is found to degrade the device performance and lifetime. Stacking faults, for example, have been considered to be one of the most detrimental structural defects. It has been predicted theoretically that high nitrogen doping concentration level (above 2 x 1019cm-3) inside 4H-SiC crystal will increase propensity for formation of stacking faults. These can create quantum wells which can lower the free energy of the whole crystal once the barrier for partial dislocation motion is overcome by thermal energy (achieved by annealing above 1000o C) [1-2]. Moreover, from a perspective of high power electronic applications, other severe issues, such as inhomogeneous resistivity and a large {0001} surface roughness of substrate are characteristic of heavily nitrogen doped SiC crystal [3]. It is known that, the nitrogen incorporation kinetic is anisotropic in different crystallographic directions during PVT growth of SiC. Therefore, better understanding the nitrogen doping distribution inside the material is important. In our experiments, we conducted X-ray topographic contour mapping [4] using synchrotron monochromatic X-ray beam by rocking the heavily doped wafer surface successively with a fixed angular step size around the near-surface diffraction vectors 0008 and -0008 respectively. From these measurements, a strain map can be derived by deconvoluting the lattice parameter variations from the lattice tilt. The nitrogen doping concentration can be calculated from the isotropic lattice strain due to the incorporation of dopants according to the equation from H. Jacobson [5]. [1] Y. Yang, J. Guo, Ouloide Goue, B. Raghothamachar, M. Dudley, G. Chung, E. Sanchez, J. Quast, I. Manning, and D. Hansen, J. Crystal Growth, 452, 32 (2016). [2] T.A. Kuhr, J. Liu, H. J. Chung et al., Journal of Applied Physics,92, 5863 (2002). [3] N. Ohtani, M. Katsuno, M. Nakabayashi, T. Fujimoto et al., J. Crystal Growth, 311, 1475 (2009). [4] S. R. Stock, Haydn Chen and H. K. Birnbaum, Philosophical Magzine A, 53(1), 73(1986). [5] H. Jacobson, J. Birch, C. Hallin, A. Henry, R. Yakimova, T. Tuomi, E. Janzén, and U. Lindefelt, Applied Physics Letters 82 (21), 3689 (2003). Figure 1

PVT growth of exfoliated CdZnTe polycrystalline thick films based on stress mismatch mechanism

The exfoliated CdZnTe (CZT) films were grown on quartz substrates by closed tube physical vapor transport method based on stress mismatch mechanism. The structure and morphology of the exfoliated CZT thick films were characterized by X-ray diffraction and scanning electron microscope. The as-grown CZT films are polycrystalline with (111) preferential orientation and exhibit a dense pyramid surface structure and a thickness of more than 600 μm. The coplanar and planar X-ray detectors were fabricated based on the exfoliated CZT film. The discrepant dark resistivity is 3.17 × 109 and 5.14 × 1010 Ω cm with regard to the coplanar and planar structure detector, respectively. The planar structure detector exhibits higher quantum efficiency at low doses X-ray radiation.

Hotzone design and optimization for 2-in. AlN PVT growth process through global heat transfer modeling and simulations

A tungsten based reactor to grow 2-in. PVT AlN crystals by induction heating was designed. In order to investigate the effect of the hotzone structure layout on the temperature distribution in the growth chamber, a series of global quasi-steady numerical simulations with and without gas convection was performed using the FEMAG software. Simulation results show that the temperature gradient between the AlN powder sources and the deposition interface is influenced profoundly by the size of the induction heater and the crucible thickness. Also the tungsten heat shields have obvious effects on the global temperature distribution and heater power consumption during the growth process. However, the number of tungsten shield layers plays a trivial role on the temperature gradient between the ALN powder sources and the crucible top. Global heat transfer simulations show that the designed hotzone can provide an optimized and flexible environment for 2-in. AlN PVT growth.

Application of <i>In Situ</i> 3D Computed Tomography during PVT Growth of 4H-SiC for the Study of Source Material Consumption under Varying Growth Conditions

2D and 3D in-situ X-ray visualization was applied to study the behavior of the SiC source material during PVT growth under various growth conditions. Experiments were carried out in two growth chambers for the growth of 3 inch and 4 inch crystals. Growth parameters were varied in the gas room in terms of axial temperature and inert gas pressure. The study addresses the stability of the SiC source material surface. It is shown that a higher inert gas pressure (e.g. 25 mbar) inhibits an unintentional upward evolution of the SiC feedstock that interferes with the crystal growth interface. The latter is related to a suppression of a pronounced recrystallization inside the SiC source. For a low inert gas pressure (e.g. 10 mbar) it is concluded that the axial temperature gradient inside the source material needs to be decreased to less than ca. 10 K/cm.

Sublimation Growth of 4 and 6 Inch 4H-SiC Low Defect Bulk Crystals in Ta (TaC) Crucibles

Recently, the wide bandgap semiconductors, especially silicon carbide (SiC), have become more important due to the unique electrical and thermophysical properties that make them applicable to a variety of electronic devices (Schottky and PiN diodes, JFETs, MOSFETs, etc.). For these applications, the crystals need to be manufactured with highest possible quality, both structural and chemical, at reduced cost. This requirement places rather extreme constraints on the crystal growth as the simultaneous goals of high quality and low cost are generally incompatible.Refractory metal carbide technology, particularly, tantalum carbide (TaC), was originally developed for application in highly corrosive and reactive environments. Yu. Vodakov [1] demonstrated for the first time advantages of use of refractory metal carbides for PVT growth of SiC and later AlN bulk crystals. In the present paper we discuss the effect of refractory metal on PVT growth of large diameter 4H SiC bulk crystals.