Physical Vapor Transport Growth Research Articles

Resistivity as a Witness of Local Crystal Growth Conditions

A detailed knowledge of the growth front geometry during physical vapor transport (PVT) growth of SiC single crystals is beneficial to achieve high quality n+ SiC substrates for power device applications. In this report we show how mapping of resistivity in SiC wafers can shed light on local growth conditions, which are very difficult-to-study in situ. We consider both thermodynamic quantities (absolute temperature T and partial pressure pN₂) and geometric characteristics of the growth surface relevant to growth kinetic parameters, namely atomic terrace width and atomic step velocity. Specifically, we show how an elevation map of the growth surface can be reconstructed from a spatially-resolved measurement of resistivity in a SiC wafer by integrating the spatial derivative of elevation with respect to the basal plane, which is assumed to be related to local resistivity through dependence of non-equilibrium nitrogen incorporation on the atomic step velocity.

Numerical Simulation of the Transport of Gas Species in the PVT Growth of Single‐Crystal SiC

AbstractSingle‐crystal silicon carbide (SiC) is an important semiconductor material for the fabrication of power and radio frequency (RF) devices. The major technique for growing single‐crystal SiC is the so‐called physical vapor transport (PVT) method, in which not only the thermal field but also the fluid‐flow field and the distribution of gas species can be hardly measured directly. In this study, a multi‐component flow model is proposed that includes the inside and outside of a growth chamber and a joint between the seed crystal holder and crucible which allows exchanges of the gas species. The joint is simulated as a thin porous graphite sheet. The Hertz‐Knudsen equation is used to describe the sublimation and deposition. The convection and diffusion are described by the Navier–Stokes equations and mixture‐averaged diffusion model, in which the Stefan flow is taken into account. The numerical simulations are conducted by the finite element method (FEM) with a multi‐physics coupled model, which is able to predict the fluid flow field, species distribution field, crystal growth rate, and evolution of the molar concentration of dopant gas. Using this model, the effects of several experimental conditions on the transport of gas species and the growth rate of single‐crystal SiC are analyzed.

Propagation behaviour of threading screw dislocations during 4H-SiC crystal growth using a hybrid method combined with solution growth and physical vapour transport growth on high-off-angle seeds

The propagation behaviour of threading screw dislocations (TSDs) during 4H-SiC crystal growth using high-off-angle seeds was investigated. By studying the conversion ratio of TSD to basal plane defects during physical vapour transport (PVT) growth and hybrid growth (combination of solution growth and PVT growth) on various off-angle seeds, we suggest an energy diagram showing the off-angle dependence of the dislocation energy which explains the activation barrier of dislocation conversion.

Oxygen reduction through specific surface area control of AlN powder for AlN single-crystal growth by physical vapor transport

In the physical vapor transport (PVT) growth of AlN, re-oxidation of aluminum nitride (AlN) source powder happening in the process of setting seed crystal into crucible seems to be unavoidable. This process introduces oxygen just before AlN growth and has a significant impact on the crystal quality. In this paper, a high and low-temperature alternative sintering method (HLAS) is proposed based on the idea of specific surface area control to reduce the re-oxidation of AlN source powder. This method introduces cyclic sintering between 1500 °C and 1900 °C to the conventional three-step treatment repeatedly, which utilizes possible phase-transition along with the processes of powder sintering back and forth to increase the particle size and decrease the specific surface area significantly. The scanning electron microscope and Brunauer, Emmett, and Teller results showed that the specific surface area of AlN powder treated with the HLAS method can be reduced to one-third of that with the conventional method. Thus, the secondary ion mass spectrometry confirmed the reduction of oxygen impurity in AlN single-crystals to a good level of 1.5 × 1017 cm−3. It is clear that this HLAS process is an effective way of controlling the specific surface area of AlN source powder, which contributes to the suppression of oxygen influence on PVT-AlN growth.

Effect of subsurface damages in seed crystals on the crystal quality of 4H-SiC single crystals grown by the PVT technology

This study focuses on the generation and transformation of defects associated with subsurface damages (SSDs) in seed crystals during the physical vapor transport (PVT) growth of 4H-SiC crystals.

(Invited) Application of 3D in-Situ X-Ray Visualization to Track the Formation of Dislocation Clusters during PVT Growth of SiC

SiC has become the key player among wide bandgap semiconductors for power electronic applications. Since the first description of the physical vapour transport (PVT) growth process of SiC by Tairov and Tsvetkov (J. Crystal Growth, 43, 209(1978)), there has been steady progress in SiC-based crystal growth, epitaxy and device processing. The success of SiC compared to Si is related to its superior material properties such as extremely high electrical breakdown field and high thermal conductivity compared to the standard silicon counterpart. In addition, SiC device processing utilises much of the standard Si processing equipment. A major reason for the success of SiC in power electronic applications compared to other wide bandgap counterparts such as GaN, Ga2O3 and diamond is related to the availability of large diameter SiC wafer materials (150mm = standard, 200mm = developped). Bulk SiC growth is now a very well developed process with comparatively high yields. The extraordinary physical properties also include obstacles related to the strong chemical bonding and complex phase diagram of the material, which pose challenges to the growth process. Therefore, there are still a number of open questions related to the nucleation, progression and termination of the bulk growth process that require fundamental research in materials science and technology.The aim of this presentation is (i) to give an overview of the state-of-the-art PVT growth process and (ii) to discuss a current research topic dealing with the early stage of the growth process and the defect formation that can occur during the initial nucleation of SiC. We have applied 3D in-situ visualisation of the growth process using X-ray computed tomography to visualise island formation on the large seeding area. These data are related to growth process instabilities such as temperature variations during the seeding process and axial doping level changes from the seed to the newly grown crystal. Both process instabilities induce mechanical stress on the SiC lattice and act as sources for dislocation generation and multiplication. We will show a series of growth processes with varying growth parameters that shed light on the initial growth stage of SiC.As the crystal diameter of SiC increases from 150 mm to 200 mm, the results of this study become increasingly important.

Experimental and simulation studies of surface segregation-limited nitrogen incorporation at the growth front of physical vapor transport-grown 4H-SiC crystals

Experimental and simulation studies were conducted for surface segregation-limited kinetics of nitrogen incorporation into a 4H-SiC crystal during physical vapor transport (PVT) crystal growth. It was revealed that the nitrogen incorporation is kinetically limited by the step-flow velocity on the growing crystal surface of a 4H-SiC crystal; in this study, the surface step-flow velocity at the growth front was deduced from the local inclination angle of the growth front measured from the (0001¯) plane, assuming a uniform growth rate along the c-axis (crystal growth direction) across the growth front, and the nitrogen concentration across the growth front was measured using Raman scattering microscopy. The step-flow velocity dependence of nitrogen incorporation was theoretically analyzed using a two-site-exchange model, and the simulated dependence using the model was in good agreement with the experimental data. On the basis of these experimental and simulation results, kinematical and energetical aspects of nitrogen incorporation at the growth front of a 4H-SiC crystal during PVT growth are discussed.

<i>In Situ</i> Monitoring of the Ambient Gas Phase during PVT Growth of Nominally Undoped High Resistivity SiC Boules

The aim of this study is to show the applicability of continuous residual gas analysis for growth monitoring of undoped SiC with physical vapor transport (PVT). For this purpose, two crystals were grown, one without doping and one with continuous nitrogen doping. During the processes continuous residual gas analysis were conducted and evaluated with emphasis on the temporal variations of the nitrogen content. The charge carrier concentration of the final crystals was determined by optical methods (spectrally resolved absorption measurement with UV-VIS and Raman spectroscopy) and the results were compared with the residual gas analysis during growth. A correlation was found between the measured nitrogen-related signal and the charge carrier concentration in the samples.

Nucleation of Threading Dislocations in 4H-SiC at Early Physical-Vapor-Transport Growth Stage

In this work, we identify the nucleation mechanism of threading dislocations (TDs) associated with stacking faults (SFs) and 15R-SiC at the early growth stage of 4H-SiC single crystals grown by the physical vapor transport (PVT) technology. By combining molten KOH etching and photochemical etching, we successfully reveal etch pits of TDs and linear etch patterns of SFs on the (112̅0) surface of 4H-SiC single crystals. Systematic investigations based on transmission electron microscopy (TEM) observations and Raman analysis indicate that the Si–C bilayer stacking sequence of SFs is (3, 2) in Zhdanov’s notation. The accumulation of SFs (3, 2) gives rise to the polymorph fluctuation and thus the formation of 15R-SiC at the early PVT growth stage of 4H-SiC single crystals. Quantitative stress analyses indicate that the strain field distributions along the SFs (3, 2) and the 15R-/4H-SiC interfaces are inhomogeneous, which give rise to the nucleation of TDs and low-angle grain boundaries (LAGBs), respectively. The nucleation of LAGBs releases the high stress at the 15R-/4H-SiC interface, which facilities the following 4H-SiC single-crystal growth. Our work indicates that the avoidance of polymorph fluctuation is important to the reduction of TDs at the early growth stage of PVT-grown 4H-SiC single crystals.

Prevention of Bunched Basal Plane Dislocation Arrays in 4H-SiC PVT-Growth

To prevent arrays of basal plane dislocations (BPD) forming during grown 4H-SiC single crystals, the growth cell in physical vapor transport (PVT) growth was modified by adapting the temperature gradients, the seed attachment method and the seeding phase. The resulting reduction in stress was modeled numerically and the crystals were investigated by X-ray topography (XRT) and molten potassium hydroxide (KOH) etching. Due to these modifications, the formation of BPD arrays was completely suppressed.

Investigation of the Nucleation Process during the Initial Stage of PVT Growth of 4H-SiC

Due to high growth temperatures during the physical vapor transport (PVT) it is still almost impossible to gain proper insight into the actual growth conditions. Therefore, computer tomography (CT) is used as an in-situ monitoring during the crystal growth process. With the help of this technique, it is possible to observe the nucleation centers during the initial stage of growth (CT after 0h) of a 4H-SiC single crystal. These growth islands are likely built before the actual growth conditions are reached. Raman investigations of the area around a growth island located directly on the interface between seed and grown crystal is used to support this assumption. In addition, optical analysis after KOH etching were made to reveal the defects around the growth island. The island exhibits a rough doping concentration in comparison to the surrounding grown crystal.

Effect of Growth Conditions on the Surface Morphology and Defect Density of CS‐PVT‐Grown 3C‐SiC

AbstractA systematic approach to determine the most crucial growth parameters and their effect on the surface morphology and defect density of sublimation grown (001) cubic silicon carbide (3C‐SiC) is conducted. Close space physical vapor transport (CS‐PVT) growth on 3C‐SiC and 4° off oriented homoepitaxial chemical vapor deposition (CVD) grown seeding layers is performed. For each growth run, one growth parameter, e.g., temperature, pressure, or N2 flux is varied, while the other parameters are kept constant. Raman spectroscopy, potassium hydroxide (KOH) etching, as well as optical microscopy and atomic force microscopy (AFM) are used for the sample characterization. Step‐flow‐controlled growth is observed on all grown samples, while the stacking fault density is generally reduced with respect to the seeding layers. Increased temperature as well as increased pressure leads to roughening of the growth surface attributed to step bunching, while their effect on the stacking fault density seems to be only minor in the analyzed parameter range. Areas of strongly enhanced step bunching are present on all samples, and this effect is more pronounced at higher temperature and pressure. Increased nitrogen doping leads to a decreased stacking fault density but also increases the length of remaining stacking faults. The surface morphology is influenced by N2 on a macroscopic level rather than on microscopic scale.

Comparative Studies of c- and m-Plane AlN Seeds Grown by Physical Vapor Transport.

The ultra-wide bandgap semiconductor AlN has attracted a great deal of attention owing to its wide application potential in the field of electronics and optoelectronic devices. In this report, based on the mechanism of the physical vapor transport (PVT) growth of AlN crystal, the c- and m-plane AlN seed crystals were prepared simultaneously through special temperature field design. It is proved that AlN crystals with different orientations can be obtained at the same temperature field. The structure parameter of AlN crystal was obtained through the characteristic evaluations. In detail, XPS was used to analyze the chemical states and bonding states of the surface of seed crystals. The content of oxygen varied along with distinct orientations. Raman spectrum documented a small level of compressive stress on these crystal seeds. Tested results confirmed that the prepared AlN crystal seeds had high quality.

Physical Vapor Transport Growth of Antiferromagnetic CrCl3 Flakes Down to Monolayer Thickness.

The van der Waals magnets CrX3 (X=I, Br, and Cl) exhibit highly tunable magnetic properties and are promising candidates for developing novel two-dimensional (2D) spintronic devices such as magnetic tunnel junctions and spin tunneling transistors. Previous studies of the antiferromagnetic CrCl3 have mainly focused on mechanically exfoliated samples. Controlled synthesis of high quality atomically thin flakes is critical for their technological implementation but has not been achieved to date. This work reports the growth of large CrCl3 flakes down to monolayer thickness via the physical vapor transport technique. Both isolated flakes with well-defined facets and long stripe samples with the trilayer portion exceeding 60µm have been obtained. High-resolution transmission electron microscopy studies show that the CrCl3 flakes are single crystalline in the monoclinic structure, consistent with the Raman results. The room temperature stability of the CrCl3 flakes decreases with decreasing thickness. The tunneling magnetoresistance of graphite/CrCl3 /graphite tunnel junctions confirms that few-layer CrCl3 possesses in-plane magnetic anisotropy and Néel temperature of 17K. This study paves the path for developing CrCl3 -based scalable 2D spintronic applications.

Physical-Vapor-Transport growth of 4H silicon carbide single crystals by a tiling method

Increasing the diameter and reducing the dislocation density of 4H silicon carbide (4H-SiC) single crystals are the development tendency of 4H-SiC single-crystal substrate to reduce the cost of 4H-SiC based devices. In this work, we use the growth of a 100 mm 4H-SiC single crystal as an example to demonstrate that the tiling approach is attractive to enlarge the diameter of physical-vapor-transport (PVT)-grown 4H-SiC single crystals by a single growth period. By titling four pieces of 4H-SiC seed crystals, a 100 mm 4H-SiC boule without crystal deterioration is obtained by the PVT growth method. The shape of the growth interface is convex and complete after the PVT growth, indicating the tiling gaps between neighboring seed-crystal pieces are healed after the PVT growth of 25-mm thick 4H-SiC. Wafers are obtained after sequential slicing, lapping, and chemical mechanical polishing (CMP). The high-resolution X-ray diffraction (HRXRD) rocking curve measurements of the wafer at the latest growth stage indicate that the 4H-SiC crystal is basically healed, with mesoscopic boundaries being created at the central area of the wafer. Raman spectra indicate that the healing polymorph across the tiling gaps is 4H-SiC. We find that threading edge dislocations (TEDs) dominate the dislocation types in the 4H-SiC substrate wafers at the initial, middle, and latest growth stages. As the PVT growth proceeds, the density of dislocations in 4H-SiC wafers significantly decreases, as a result of the crystalline healing. Our work opens a pathway to enlarge the size of 4H-SiC single crystals in a single PVT growth period, which paves the way for the high-efficiency growth of large-size 4H-SiC single-crystals.

(Invited) Development of 3-inch AlN Single Crystal Substrates

Aluminum nitride (AlN) holds great promise as a substrate for electronic devices such as high-power switches, high power density, high frequency and high operating temperature devices for RF. This promise is largely due to its superior properties, for instance its ultra-wide bandgap (UWBG) of 6.2 eV, high thermal conductivity (> 290 W/m*K), high melting point (> 2800°C), relatively good chemical resistance, similarity of its lattice structure and parameters to that of Gallium nitride (GaN), high hardness value of about 12 GPa (at 1 N using Vickers indenter), etc. Presently AlN substrates are successfully used for fabrication of ultraviolet (UV) light emitting diodes (LEDs) operating at wavelengths shorter than 280 nm, i.e., in the UVC region of the spectrum. The AlN-based UVC LEDs are employed in water disinfection, surface cleaning, air purification, and environmental sensing. Several factors including substrate availability, size, production capacity, quality, and cost influence the commercialization of the aluminum nitride both as an UWBG and as an opto-electronic substrate. In this work we present the present and future outlook for the AlN substrates made using Physical Vapor Transport (PVT) growth technique.The PVT crystal growth method of aluminum nitride is carried out in a Tungsten crucible that accommodate the AlN source material as well as the (0001) oriented AlN seed. The thermal gradient is provided by an rf-heating plus appropriate shielding and insulation. In this setup, the sublimed source material is transported to the Al-polar face of the seed where it incorporates into a single crystal. The AlN crystals are then oriented, sliced, and polished. The resulting 2-inch substrates are tested to meet certain specification needed for further epitaxial growth and LED processing. The two-inch AlN wafer specification as well as number of physical properties and purity were previously reported [1].In addition to the 2-inch AlN crystal growth there is an ongoing effort to increase the substrate diameter. Large-diameter (e.g. 100 mm) AlN substrates are a practical requirement benefiting high power electronic devices. Again, PVT growth method is used to expand the crystal size and diameter. However, several challenges have yet to be resolved en route to 100 mm diameter crystals. These challenges are associated with increased thermally induced stress, the need for better control of the axial/radial thermal gradients, vendor ability to provide materials, maintaining crystal quality during expansion, etc.Our PVT growth technique demonstrates high crystal growth yield which allows to contain relatively low production cost. Furthermore, we have established a high-capacity process capable to produce many thousands of 2-inch AlN substrates per year. Our current crystal growth process also demonstrated greater than 3-inch diameter single-crystal AlN substrates; properties of such wafers will be reported. Robert T Bondokov et al 2021 ECS Trans. 104 37

Seed surface orientation dependence of the defect formation at the initial stage of physical vapor transport growth of 4H-SiC crystals

The influence of the seed crystal surface orientation on the defect formation at the initial stage of physical vapor transport (PVT) growth of 4H-SiC crystals was investigated using X-ray topography and Raman scattering microscopy. To examine the detailed influence of the seed surface orientation, a seed crystal having a groove on the growth surface was used for PVT growth; the seed crystal with a groove provides a continuously varying seed surface orientation from (0 0 0 1¯) (on-axis c-face growth) to (1 1 2¯ 0) (a-face growth), and using the seed crystal, the effect of the seed surface orientation can be examined in a single growth experiment. X-ray topography observations revealed that a number of threading screw dislocations (TSDs) were formed at the bottom corners of the groove, while their formation was largely suppressed at the bottom of the groove, indicating that an off-oriented (0 0 0 1¯) seed crystal surface tends to generate TSDs. Characteristic dopant striations were also observed in the crystal portions grown on the off-oriented seed crystal surface, and the generated TSDs were found to propagate along the boundaries of these dopant striations. Based on these results, we discuss the TSD formation mechanism on an off-oriented (0 0 0 1¯) seed crystal surface at the initial stage of PVT growth of 4H-SiC crystals and suggest an important role of macrostep formation on the surface.

Enhanced nitrogen incorporation in the 〈112̄0〉 directions on the (0001̄) facet of 4H-SiC crystals

Enhanced nitrogen doping in the 〈110〉 directions on the (000) facet of 4H-SiC crystals grown by the physical vapor transport (PVT) growth method was investigated using Raman scattering microscopy and atomic force microscopy (AFM). The enhanced nitrogen doping showed either single or double peak structure of nitrogen incorporation in the azimuthal direction around 〈110〉. AFM observations revealed that these two characteristic doping structures stemmed from different propagation morphologies of spiral steps on the (000) facet. It was also unveiled that the change in the step-flow direction was always associated with the change of the terrace-width ratio on the facet, and that the interplay between these two parameters determined the nitrogen incorporation on the (000) facet. On the basis of these experimental results, the mechanism of the azimuthal dependence of nitrogen incorporation on the (000) facet during PVT growth of 4H-SiC crystals is discussed.

Impact of Mechanical Stress and Nitrogen Doping on the Defect Distribution in the Initial Stage of the 4H-SiC PVT Growth Process.

Nitrogen incorporation changes the lattice spacing of SiC and can therefore lead to stress during physical vapor transport (PVT). The impact of the nitrogen-doping concentration during the initial phase of PVT growth of 4H-SiC was investigated using molten potassium hydroxide (KOH) etching, and the doping concentration and stress was detected by Raman spectroscopy. The change in the coefficient of thermal expansion (CTE) caused by the variation of nitrogen doping was implemented into a numerical model to quantitatively determine the stress induced during and after the crystal growth. Furthermore, the influence of mechanical stress related to the seed-mounting method was studied. To achieve this, four 100 mm diameter 4H-SiC crystals were grown with different nitrogen-doping distributions and seed-mounting strategies. It was found that the altered CTE plays a major role in the types and density of defect present in the grown crystal. While the mounting method led to increased stress in the initial seeding phase, the overall stress induced by inhomogeneous nitrogen doping is orders of magnitude higher.

Characterization of prismatic slip in PVT-grown AlN crystals

The most frequently observed deformation mechanism in wurtzite aluminum nitride (AlN) grown by the physical vapor transport (PVT) method is basal plane slip. However, prismatic slip also takes place in such crystals. In this study, prismatic slip is observed in a 50 mm commercial AlN substrate wafer, leading to a sixfold symmetric BPD pattern in the wafer. Prismatic dislocations cross slip on to basal plane and multiply there, increasing the dislocation density of the wafer. A radial thermal model is established to explore the origin of the unevenly distributed dislocations. The unevenly distributed resolved shear stress across the whole radial area of the boule during PVT growth caused by radial thermal gradients offers driving force for the activation of the three sets of prismatic slip systems, producing nonuniform dislocation distribution in the boule and thus the wafer. Both experimental observations and theoretical calculations of the critical resolved shear stress (CRSS) correlate well with the model results, revealing that controlling radial thermal gradients is critical for lowering dislocation density and enhancing the crystal quality using PVT growth method.