PVT Method Research Articles

Ultrawide-bandgap semiconductor AlN crystals: growth and applications

This review systematically summarizes the latest research advances of AlN crystals grown by the PVT method and their applications.

Modelling and simulation of oxygen transport during AlN crystal growth by the PVT method

The oxygen impurity exists mainly in the form of Al2O during the AlN crystal growth by the PVT method. In this paper, 2D incompressible flow and mass transport models are developed by the FEM method and by taking into account the effects of three-phase (Al/N/Al2O) Stefan flow, buoyancy and vapor diffusion during the PVT AlN crystal growth process. The effects of the growth conditions, viz. the temperature and atmospheric pressure, on the Al2O transport in a 2-in. AlN crystal growth chamber are investigated by a series of numerical simulations using the developed models. The simulation results show that Al2O transport is strongly affected by temperature and atmospheric pressure. A high temperature is unfavourable for AlN crystal growth but is beneficial for the removal of oxygen from the powder source. An increase in the atmospheric pressure can decrease the Al2O partial pressure in the growth chamber and therefore reduce the oxygen content in the growing AlN crystal.

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

Investigation of Carbon Inclusions in SiC Crystals Grown by PVT Method

The carbon inclusions in SiC bulk crystals were studied by using optical microscope. The carbon inclusions were classified into three types, namely Type Ⅰ (100~1000 μm), Type Ⅱ (20~50 μm), Type Ⅲ (~5 μm) carbon inclusions based on their different morphologies. In addition, the evolution of these three type carbon inclusions with crystal growth process was traced. It was found that the number of Type Ⅰ and Type Ⅱ carbon inclusions showed declined tendency with crystal growth, while the number of Type Ⅲ carbon inclusions exhibited less dependence on crystal growth. Furthermore, the formation mechanism of carbon inclusions was clarified. This study would give out a better understanding of carbon inclusions, thus contribute to reducing or eliminating carbon inclusions in SiC crystals.

The growth mode and Raman scattering characterization of m-AlN crystals grown by PVT method

The growth mode of m-plane AlN crystal grown by physical vapor transport method is studied. The m-plane AlN crystal fabricated by spontaneous nucleation was used as a seed to grow bulk crystal, and 3-mm thick AlN crystals with a maximum lateral dimension of 20 mm were obtained. The Raman tensor elements of A1(TO), E2(high) and E1(TO) Raman models from the m-plane AlN crystal are investigated by angle-dependent polarized Raman scattering. The Raman tensor comparing with our previous report indicates the possible enhancement of the Raman tensor in the directions vertical to [0002] direction at higher growth temperature.

Application of Kalman Filter in Satellite Propellant Remaining Prediction Technology

The prediction of the propellant remaining is related to the use efficiency of the propellant, it is of great significance for the estimation of the remaining life of the spacecraft. The gauging uncertainty of the commonly used propellant remaining prediction method is high, and the single gauging method can not meet the higher requirements of long life satellites on-orbit management. In order to further improve the accuracy of the spacecraft propellant remaining prediction, combined with the characteristics of the existing propellant remaining estimation method, a real-time estimation method of propellant remaining is proposed. The algorithm combines two gauging methods to further improve the reliability of propellant consumption estimation. The practical application shows that the algorithm can effectively reduce the disadvantages of the BK method and the PVT method, and provide a more reasonable estimation of the real-time propellant remaining.

New Materials for Semi-Insulating SiC Single Crystal Growth by PVT Method

The present research was focused on the effect of porous graphite plate in TaC-coated crucible on crystal quality and resistivity of semi-insulating SiC crystals. Two SiC crystals grown with/without porous graphite plate in TaC-coated crucible were systematically compared. 6H-SiC polytype was obtained on both crystals regardless of porous graphite plate. The quality of SiC crystal grown using porous graphite plate placed in the TaC-coated crucible was slightly better than SiC crystal without porous graphite plate. SiC crystals having an average resistivity value of about 1×1010 Ωcm were obtained. In the result of COREMA measurement, the use of porous graphite plate tends to obtain wafers with better uniformity in resistivity value.

Research on the Key Problems in the Industrialization of SiC Substrate Materials

The development of semiconductor materials and devices will lead to a new industrial technology revolution, in which the Silicon Carbide (SiC) substrate material has very excellent performance and it is especially suitable for manufacturing wave length lasers, white light emitting tubes, high-frequency, high-temperature and high-power devices, etc. This paper focuses on solving the key problems for producing large size and low defects of SiC crystals by the PVT method, such as the preparation and purification of the high purity raw material, the simulation of the temperature field, the control of the crystal defects and the growth of the large size SiC crystals.It is critical for the development of SiC industry.

Variation of Vanadium Incorporation in Semi-Insulating SiC Single Crystals Grown by PVT Method

Two SiC crystals were grown using SiC source powder with different level of purity and then the effect of the purity of SiC source materials on the final electrical properties has been systematically observed. Furthermore, the variation of vanadium amount according to the growth direction of vanadium doped semi-insulated SiC single crystals has been investigated. The quality of SiC crystal grown using SiC source powder with higher purity was definitely better than SiC crystal with lower purity. SiC crystals having an average resistivity value of about 1×1010 Ωcm were successfully obtained. In the result of COREMA measurement, the use of high purity SiC powder was revealed to obtain wafers with better uniformity in resistivity value.

Control of 4H polytype of SiC crystals by moving up the crucible to adjust the temperature field of the growth interface

It is difficult to control the 4H polytype in the growth of SiC single crystals by the PVT method.

Development of adiabatic calorimetry system for enthalpy of gas absorption/adsorption and its application to H2/D2 absorption into palladium nanoparticles

We have developed a new adiabatic calorimetry system for enthalpy of gas absorption/adsorption process. The sample cell is designed to be extremely small as a calorimeter cell with a resistance thermometer; mass: 4.34 g, inner volume: 1.06 cc. A thin stainless steel tube is connected at the lid of the cell to introduce a gas. A gas handling system was also constructed to introduce a gas slowly and measure the amounts of the absorbed/adsorbed gas precisely via a pVT method. As the first work on this system, we measured the enthalpies of H2/D2 absorption into Pd nanoparticles with an average diameter of ca. 8 nm. The obtained values are ΔabsH(H) = –(18.15 ± 0.87) kJmol–1 and ΔabsH(D) = –(18.17 ± 0.38) kJmol–1 at 298 K. These values are considerably smaller than those of bulk Pd. The isotope effect are discussed semi-quantitatively on the basis of the potential energy surfaces at the H/D sites.

Thermal stress simulation of optimized SiC single crystal growth crucible structure

In order to reduce the dislocation density of SiC single crystal grown by PVT method, the thermal field was simulated based on finite element analysis. In this paper, VR-PVT SiC software was used to simulate the internal temperature gradient and thermal stress evolution caused by the different crucible structure. The change of crucible lid and sidewall structures has a better influence on radial temperature gradient and axial temperature gradient, respectively. Modified structure is a combination of these structures which improves both axial and radial temperature gradient. It reduces the maximum stress from 1.85 MPa to 1.58 MPa. The maximum shear stress and dislocation density distribution are shifted to the edge of the crystal. The optimization has only changed the external structure, so there is no conflict with the internal design.

Influence factors of methane hydrate formation from ice: Temperature, pressure and SDS surfactant

Abstract A series of experiments of forming hydrate from ice powders in different conditions have been carried out with constant volume method to evaluate the influence factors such as pressure, temperature, and SDS surfactant. The change of temperature and pressure were collected as a function of elapsed time, which were used to calculate the gas consumption and hydrate saturation during hydrate formation (pVT method). Based on the experimental results and the analysis, it is concluded that: (1) Both initial pressure and temperature have effect on the hydrate formation and temperature plays a more important role in the process; (2) heating and secondary pressurization will promote the gas hydrate formation and enhance the hydrate saturation as a result. Meanwhile, the promotion of heating seems to be more obvious than that of secondary pressurization; (3) different concentrations of SDS surfactant have clearly influence on the saturation of gas hydrate and there is an optimal concentration to promote the hydrate formation.

As-grown Surface Morphologies of SiC Single Crystals Grown by PVT Method

As-grown Surface Morphologies of SiC Single Crystals Grown by PVT Method

Reduction of Dislocation Density of SiC Crystals Grown on Seeds after H<sub>2</sub> Etching

Three-inch 6H-SiC bulk crystals were grown by the PVT method on the seeds processed by different treatments. The influences of seed surface morphology and subsurface damage on the dislocation density were investigated. The seed surface morphology was characterized by atomic force microscopy (AFM). The extent of the subsurface damage was estimated by electron back-scattered diffraction (EBSD) and Band Contrast (BC) value. The distribution and density of the dislocations were observed by optical microscopy (OM). The results showed that the pit density performed by H2 1400°C etching was nearly one order of magnitude lower than that by mechanical polishing (MP) process. So H2 etching processed at 1400°C for 2h could completely remove the subsurface damage, compared with the MP process with the deep surface damage.

Influence of Porous Media on Methane Hydrate Formation from Ice Powders

Abstract Methane hydrate formation from ice powders is a complicated process, and is affected easily by many factors. In this paper, a series of experiments were conducted to study the influence of porous media's particle size and volume proportion on the hydrate formation process. An experimental system based on PVT method was used to measure the gas consumption and the pressure change through the whole process of reaction. The pressure drop and hydrate saturation were analyzed to show the effects of porous media. The results indicated that porous media not only can improve the discrete degree of ice particles, but also increase the contact area of quasi liquid layer and methane gas. The specific effects of particle size and volume proportion were based on the two effects above.

(Invited) Study on the Role of Thermal Stress on Prismatic Slip of Dislocations in 4H-SiC Crystals Grown by PVT Method

Recent advances made towards the improvement of the quality of 4H silicon carbide (4H-SiC) have enabled many successful applications in the power device industry. Nevertheless, there is still a great interest in understanding the formation mechanism of crystal defects such as dislocations as well as their behavior during crystal growth. While basal plane slip is most frequently observed in 4H-SiC crystals grown by PVT method, prismatic slip of dislocations has also been reported before. [1] The glide of threading edge dislocations (TEDs) on the prismatic planes ({1-100}), for example, leaves long segments of prismatic dislocations in their wake, some of which, if adopting enough screw component, become screw type dislocations. On one hand, these screw dislocations can cross-slip onto the basal planes, generating more BPDs in the substrate crystals. On the other hand, the existence of screw BPDs segments with Burgers vector 1/3[11-20] in the substrate will be harmful for the quality of epilayers since such dislocations, if intersecting with substrate surface, will replicate into the epilayer without conversion to TEDs. In this study, we have observed three types of straight prismatic dislocation segments in a commercial wafer. These are three different types of dislocations with either screw or mixed characters and are formed by the prismatic slip of different types of TEDs with Burgers vectors and line directions along one of the three 1/3<11-20> directions. The three types of prismatic dislocations are each distributed non-uniformly in the wafer. We believe this non-uniformity is a result of the resolved shear stress distribution caused by the radial temperature gradient in the growing crystal boule, since TEDs only start gliding on the prismatic planes in regions where the critical resolved shear stress is reached. From thermal model simulation, we obtained the distributions of the resolved shear stress for all three types of prismatic dislocations by assuming an appropriate profile for the radial temperature variation. Based on these results, a simulated map can be generated to predict the spatial distributions of prismatic dislocations in the wafer. The map is then compared with X-ray topographic images and correlation is found between the two. It is shown that the prismatic dislocations are mainly distributed on the periphery of the boule and the exact distribution of each type depends highly on the difference in radial temperature profile. [1] Huanhuan Wang, Shayan Byrappa ,Fangzhen Wu, Balaji Raghothamachar, Michael Dudley, Edward Sanchez, Darren Hansen, Roman Drachev, Stephan G. Mueller, Mark J. Loboda, Materials Science Forum Vols. 717-720 (2012) pp327 Figure 1

Fluorescent P-Type 4H-SiC Grown by PVT Method

Fluorescent SiC, which contains donor and acceptor impurities with optimum concentrations, can work as a phosphor for visible light emission by donor-acceptor-pair (DAP) recombination. In this work, 3 inch N-B-Al co-doped fluorescent 4H-SiC crystals are prepared by PVT method. The p-type fluorescent 4H-SiC with low aluminum doping concentration can show intensive yellow-green fluorescence at room temperature. N-B DAP peak wavelength shifts from 578nm to 525nm and weak N-Al DAP emission occurred 403/420 nm quenches, when the temperature increases from 4K to 298K. The aluminum doping induces higher defect concentration in the fluorescent crystal and decreases optical transmissivity of the crystal in the visible light range. It triggers more non-radiative recombination and light absorption losses in the crystal.

Using Ray Tracing Simulations for Direct Determination of Burgers Vectors of Threading Mixed Dislocations in 4H-SiC c-Plane Wafers Grown by PVT Method

The presence of threading mixed dislocations (TMDs) (with both edge and screw component) in 4H-SiC crystals grown by PVT method has been reported both from axial slices (wafers cut parallel to the growth axis) and commercial offcut wafers (cut almost perpendicular to the growth axis). In this paper, a systematic method is developed and demonstrated to unambiguously determine the Burgers vectors of TMDs in 4H-SiC commercial offcut wafers using both Synchrotron Monochromatic X-ray Topography (SMBXT) and Ray Tracing Simulations. 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 commercial offcut wafers the majority of the threading dislocations with screw component are mixed type dislocations.

Influence of Impurities in SiC Powder on High Quality SiC Crystal Growth

SiC powders having different purities were prepared by carbothermal reduction under different conditions from traditional process and SiC single crystals were grown by the PVT method from the powders. After crystal growth, boule was cut to wafers and they were polished for chemical and defect analyses. Total impurities including Al, B and Ti which were derived from powders decreased remarkably during crystal growth. The formation of defects including micropipe and dislocations such as TED, TSD and BPD was strongly influenced by impurity content. The effect of impurity seemed to be negligible at below 1ppm level for MPD. On the other hand, dislocations continuously decreased even more when higher purity SiC powder below 1ppm level was used.