Method and apparatus for growing silicon carbide crystals by the pulsed current assisted liquid phase method

Abstract

This invention discloses a method and apparatus for growing silicon carbide crystals using a pulsed current-assisted liquid-phase method. The method includes: forming a melt containing silicon and a flux; contacting a seed crystal with the melt for growth; applying pulsed currents with different parameters to the growth interface at least during the nucleation stage and the steady-state growth stage, with a higher peak current density and a lower duty cycle during the nucleation stage; and separating the crystal after growth. The apparatus includes a furnace body, a seed crystal rod and a crucible rod equipped with insulators, a pulsed power supply, and a control system. The pulsed power supply is connected to the seed crystal rod and the crucible rod via a conductive circuit, and the control system adjusts the pulse parameters in stages. This invention utilizes the alternating "pulse-relaxation" effect of the pulsed current. During the conduction period, it breaks up carbon-containing clusters and accelerates carbon solute transport; during the off-phase, it provides relaxation time for atoms to promote ordered arrangement, thereby significantly reducing dislocation density, suppressing inclusion formation, and improving crystal stability.

Description

Technical Field

[0001] This invention belongs to the field of silicon carbide (SiC) single crystal growth technology. Specifically, this invention relates to a method and apparatus for growing silicon carbide crystals using a pulsed current-assisted liquid phase method. Background Technology

[0002] Silicon carbide (SiC), as a representative of third-generation wide bandgap semiconductor materials, has excellent properties such as large bandgap, high breakdown electric field, good thermal conductivity, and high electron saturation drift velocity. It is an ideal substrate material for manufacturing high-temperature, high-frequency, and high-power power electronic devices and has broad application prospects in new energy vehicles, smart grids, rail transit, and 5G communications.

[0003] Currently, commercially available SiC single-crystal substrates are mainly prepared using the physical vapor transport (PVT) method. However, the high growth temperature (>2200℃) and difficulty in precisely controlling the growth process of PVT result in a high dislocation density in the crystal and low p-type doping efficiency, limiting its application in some high-end devices. Liquid-phase SiC growth is carried out at relatively lower temperatures (approximately 1600-2000℃) and under near-thermodynamic equilibrium conditions, theoretically allowing for the production of high-quality crystals with lower defect density and more uniform doping, especially providing a possibility for realizing low-resistivity p-type SiC substrates.

[0004] A typical liquid-phase SiC growth process involves placing a Si or Si-based alloy melt in a graphite crucible. The crucible wall acts as a carbon source, continuously dissolving into the melt. Carbon atoms are transported to a seed crystal at a lower temperature via convection and diffusion, eventually crystallizing out after supersaturation. However, this process still faces many challenges in practical applications.

[0005] First, carbon has low solubility and a small diffusion coefficient in Si melt, which limits the growth rate. To improve carbon solubility, a flux is usually added to the melt, but the introduction of the flux changes the thermodynamic and kinetic properties of the melt, narrowing the process window.

[0006] Secondly, stable carbon-containing clusters or polyatomic polymers easily form in the melt. These large atomic groups diffuse slowly in the solute boundary layer, making it difficult for them to decompose into individual carbon atoms or small carbon species that can be used for crystal growth in a timely manner. More seriously, if these clusters are directly adsorbed onto the growth interface, they will become the source of defects such as dislocations and inclusions, severely affecting the crystallization quality.

[0007] Furthermore, the microscopic state of the growth interface (such as step distribution and the migration ability of adsorbed atoms) directly determines the crystallization quality and crystal form stability. In conventional liquid-phase processes, the control of interfacial micro-behavior mainly relies on macroscopic adjustments of the thermal and flow fields, making it difficult to actively and precisely intervene in the migration, adsorption, and alignment processes at the atomic scale. This is especially true when the seed crystal form is inconsistent with the target growth crystal form (i.e., heterogeneous growth), where crystal form control and interfacial stability issues become more prominent in the early stages of nucleation.

[0008] To improve the liquid-phase growth process, researchers have attempted to introduce electric field assistance. For example, Chinese patent CN112410870B discloses a growth control method and system for silicon carbide crystals grown using liquid-phase epitaxy. By setting probe groups above and below the seed crystal and applying direct current, the Peltier effect is used to generate exothermic or endothermic reactions above or below the crystallization interface, thereby changing the local crystallization rate and achieving macroscopic control over the crystal size and interface morphology. The core of this method lies in using the thermal effect of direct current (Peltier effect) to regulate the growth rate, and its monitoring and control targets the macroscopic position and morphology of the crystallization interface.

[0009] However, the above-mentioned existing technical solutions still have the following shortcomings: (1) The current mode used in it generates electromigration effect while being accompanied by continuous Joule heating. The thermal effect and electromigration effect are coupled with each other and are difficult to optimize independently, which limits the precision and flexibility of process control; (2) Its mechanism of action focuses on controlling the growth rate by changing the interface temperature, but does not fundamentally improve the microscopic quality of the growth interface, such as reducing the adsorption of large atomic clusters and promoting the orderly arrangement of atoms; (3) The existing solutions do not make differentiated current parameter configurations for different stages of silicon carbide crystal growth (such as nucleation stage and steady-state growth stage, homogeneous growth and heterogeneous growth), making it difficult to achieve the optimal microscopic control effect throughout the growth process.

[0010] Therefore, how to approach the problem from a more microscopic level, while maintaining the advantages of near-equilibrium growth in liquid phase, actively intervene in and optimize the atomic migration process at the growth interface, and carry out differentiated regulation according to the characteristics of the growth stage to significantly improve crystal quality, remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0011] One objective of this invention is to provide a method for growing silicon carbide crystals using a pulsed current-assisted liquid phase method, in order to solve the problems of difficulty in actively intervening in the growth interface at the microscale and high crystal defect density in the prior art.

[0012] Another objective of this invention is to provide an apparatus for implementing the above-described method. This apparatus has a simple structure and can achieve precise control of pulse current parameters without requiring complex modifications to the interior of the furnace.

[0013] The above-mentioned objective of the present invention is achieved through the following technical solution.

[0014] In a first aspect, the present invention provides a method for growing silicon carbide crystals using a pulsed current-assisted liquid-phase method, comprising the following steps:

[0015] (1) A melt containing silicon and a flux is formed, and a silicon carbide seed crystal is brought into contact with the surface of the melt to carry out crystal growth;

[0016] (2) A pulsed current is applied during the crystal growth process, wherein pulsed currents with different parameters are applied to the growth interface at least during the nucleation stage and the steady-state growth stage; the pulsed current parameters applied during the nucleation stage are configured such that the peak current density is higher than that applied during the steady-state growth stage and the duty cycle is lower than that applied during the steady-state growth stage.

[0017] (3) After growth is complete, the resulting crystal is separated from the melt to obtain silicon carbide crystal.

[0018] The inventors of this application unexpectedly discovered that introducing pulsed current in stages into the silicon carbide growth method, and configuring pulse parameters differently according to the different needs of the nucleation stage and the steady-state growth stage, can significantly improve crystal quality. More importantly, the pulsed current used in this invention does not rely solely on continuous thermal effects to regulate the macroscopic growth rate, as in the steady-state DC current of the prior art. Instead, it creatively utilizes the temporal characteristics of the pulsed current to form an alternating "pulse-relaxation" effect at the growth interface. Specifically: during pulse conduction, the peak instantaneous current generates a strong electric field in the melt at and near the growth interface. This instantaneous strong electric field is sufficient to break up large clusters of carbon atoms in the melt, transforming them into individual carbon atoms or small-sized carbon species. At the same time, it utilizes the electromigration effect to accelerate the transport of carbon solute to the growth interface. During pulse de-current, the current is zero. Under the condition of no electric field disturbance, the atoms at the interface seek the lowest energy lattice positions (such as step edges or kinks) through surface diffusion, thereby achieving ordered arrangement and reducing the formation of defects such as vacancies and dislocations. This alternating "pulse-relaxation" action enables the present invention to actively intervene in and optimize the atomic migration process at the growth interface at the microscale, achieving technical effects that cannot be achieved by simply relying on thermal effect control in the prior art.

[0019] Furthermore, during the nucleation stage, high energy input is required to ensure uniform nucleation on the seed crystal surface and suppress the formation of impurity crystals; therefore, a higher peak current density (e.g., 30-50 A / cm²) is employed. 2A high peak current density (e.g., 10-30%) and a low duty cycle (e.g., 10-30%) are used to effectively break down carbon-containing clusters using the instantaneous strong electric field generated by the high peak current. Simultaneously, the low duty cycle provides sufficient relaxation time for the adsorbed atoms to arrange themselves in an orderly manner. During the steady-state growth stage, the focus shifts to accelerating single-crystal growth and reducing dislocation density while maintaining interfacial stability. Therefore, the peak current density is appropriately reduced (e.g., 10-28 A / cm²). 2 This involves increasing the duty cycle (e.g., 40-60%) to balance growth rate and crystal quality. This phased, differentiated configuration strategy enables optimal pulse control at different growth stages.

[0020] Furthermore, the orientation of the pulsed current has a significant impact on growth systems with different crystal structure relationships. When the crystal structure of the silicon carbide seed crystal is the same as that of the silicon carbide crystal to be grown, i.e., in the case of homogeneous growth, the direction of the pulsed current in both the nucleation stage and the steady-state growth stage is configured to flow from the melt to the seed crystal. This configuration continuously utilizes the electromigration effect to accelerate the transport of carbon solute to the growth interface throughout the growth process, which is beneficial for maintaining a stable growth rate and interface stability, while also promoting the ordered arrangement of carbon atoms at the growth steps and reducing the dislocation defect density.

[0021] When the crystal form of the silicon carbide seed crystal differs from that of the silicon carbide crystal to be grown (i.e., heterogeneous growth), the direction of the pulsed current is configured to flow from the seed crystal to the melt during the nucleation stage; and during the steady-state growth stage, the direction of the pulsed current is configured to flow from the melt to the seed crystal. In the initial stage of heterogeneous growth, the growth rate needs to be deliberately reduced to obtain a stable interface structure and correct crystal form transformation. Therefore, a reverse current is used during the nucleation stage to appropriately slow down the supply of carbon solute; after entering steady-state growth, the current is switched to a forward current to restore the normal growth rate and continuously optimize crystal quality. This strategy of dynamically switching the current direction according to the crystal form relationship can significantly improve the crystal form stability and crystal quality of heterogeneous growth.

[0022] Furthermore, the peak current density of the pulsed current applied during the nucleation stage is preferably 30-50 A / cm². 2 The duty cycle is 10-30%. Within this range, a high peak current density can generate a sufficient instantaneous electric field strength to effectively break up carbon-containing clusters, while a low duty cycle ensures sufficient atomic relaxation time; examples of peak current densities suitable for the method of this invention include, but are not limited to, 32 A / cm². 2 34 A / cm 2 36 A / cm 2 38 A / cm 2 40 A / cm 2 42 A / cm 2 44A / cm 2 46 A / cm2 48 A / cm 2 Examples of duty cycles applicable to the method of the present invention include, but are not limited to, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, and 28%.

[0023] The peak current density of the pulsed current applied during the steady-state growth phase is preferably 10⁻²⁸ A / cm². 2 The duty cycle is 40-60%; this range maintains an appropriate electromigration effect while avoiding excessively high current densities that could cause thermal damage or interface disturbances to the crystal. Examples of peak current densities suitable for the method of this invention include, but are not limited to, 12 A / cm². 2 14A / cm 2 16 A / cm 2 18 A / cm 2 20 A / cm 2 22 A / cm 2 24 A / cm 2 26 A / cm 2 Examples of duty cycles applicable to the method of the present invention include, but are not limited to, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, and 58%.

[0024] Furthermore, the frequency range of the pulse current has a significant impact on the pulse effect. The preferred frequency range of the pulse current is 100 Hz to 100 kHz, more preferably 1 kHz to 20 kHz. If the frequency is too low, the duration of a single pulse will be too long, potentially leading to the accumulation of thermal effects, weakening the instantaneous impact advantage of the pulse, and causing the "pulse-relaxation" effect to degenerate into an approximately steady DC operating mode. If the frequency is too high, the turn-off interval will be too short, resulting in insufficient atomic relaxation diffusion and affecting the ordered arrangement effect. Typical values ​​include, but are not limited to: 100 Hz, 500 Hz, 1 kHz, 2 kHz, 5 kHz, 10 kHz, 15 kHz, 20 kHz, 50 kHz, and 100 kHz.

[0025] The waveform of the pulse current is preferably one of square wave, triangular wave or spike wave. Among them, the square wave is the most preferred waveform because it has steep rising and falling edges, which can produce the most significant instantaneous field effect and the parameters are easy to control precisely.

[0026] Furthermore, the nucleation stage is defined as the stage where the silicon carbide crystal growth thickness is less than or equal to 100 μm, and the steady-state growth stage is defined as the stage where the silicon carbide crystal growth thickness is greater than 100 μm.

[0027] Furthermore, the method of carbon source supply is crucial for crystal growth. The melt is formed by placing silicon and a flux in a graphite crucible and heating and melting them under a protective atmosphere; during crystal growth, the graphite crucible continuously dissolves carbon into the melt as a carbon source. This self-supplying carbon source avoids the complexity and contamination risks of introducing external carbon sources and can maintain a dynamic balance of carbon concentration in the melt.

[0028] The choice of protective atmosphere has a significant impact on preventing oxidation and maintaining crystal purity, and is preferably argon and / or nitrogen. The flux is preferably selected from at least one of transition metals and rare earth elements; wherein the transition metal is more preferably selected from at least one of Cr, Ti, Fe, and Ni, as these elements can effectively increase the solubility of carbon in silicon melt; the rare earth element is more preferably selected from at least one of La, Pr, Nd, and Ce, as the introduction of rare earth elements can further improve the thermodynamic properties and mass transfer kinetics of the melt.

[0029] Secondly, the present invention also provides an apparatus for growing silicon carbide crystals using a pulsed current-assisted liquid phase method to implement the above-described method, comprising:

[0030] The furnace body contains a crucible and a heating mechanism for heating the crucible.

[0031] A seed crystal rod is provided with a first insulator, which divides the seed crystal rod into an upper seed crystal rod and a lower seed crystal rod. The end of the lower seed crystal rod away from the first insulator is used to fix the silicon carbide seed crystal. The end of the upper seed crystal rod away from the first insulator extends out of the furnace body and is connected to the lifting and rotating mechanism. The first insulator is used to block the current from being conducted along the seed crystal rod to the furnace body.

[0032] A crucible rod is provided with a second insulator, which divides the crucible rod into an upper section and a lower section. The end of the upper section away from the second insulator is used to support the crucible, and the end of the lower section away from the second insulator extends out of the furnace body and is connected to the lifting and rotating mechanism. The second insulator is used to block the current from being conducted along the crucible rod to the furnace body.

[0033] A pulse power supply, whose positive and negative terminals are respectively connected to the seed crystal rod and the crucible rod through a conductive circuit, is used to apply a pulse current between the seed crystal and the melt; one of the positive and negative terminals is electrically connected to the seed crystal rod through the conductive circuit, and the other is electrically connected to the crucible rod through the conductive circuit.

[0034] The control system, connected to the pulse power supply, is used to control the pulse power supply to output pulse currents with different parameters during the nucleation stage and the steady-state growth stage, respectively, according to a preset growth process.

[0035] The core improvement of this device lies in separating the seed crystal rod and crucible rod into upper and lower sections using an insulator. This ensures that the current flows only along a designated path and is not conducted to the furnace body. A conductive circuit enables flexible electrical connection between the pulse power supply and the rod, allowing the polarity of the pulse current to be freely configured according to process requirements (homogeneous or heterogeneous growth). This modification mainly involves adding a pulse power supply and conductive connectors to the outside of the existing growth furnace, without requiring complex modifications to the furnace interior, making it easy to implement industrially.

[0036] Further, the conductive circuit includes: a first conductive slip ring, the inner ring of which is fixed to the upper seed crystal rod and located inside the furnace body, rotating and rising with the seed crystal rod; the outer ring of which is fixed to the furnace body; one pole of the pulse power supply is connected to the lower seed crystal rod via a wire through the first conductive slip ring; the first conductive slip ring is used to prevent the wire from tangling when the seed crystal rod rotates; and a second conductive slip ring, the inner ring of which is fixed to the lower crucible rod and located inside the furnace body, rotating and rising with the crucible rod; the outer ring of which is fixed to the furnace body; the other pole of the pulse power supply is connected to the upper crucible rod via a wire through the second conductive slip ring; the second conductive slip ring is used to prevent the wire from tangling when the crucible rod rotates.

[0037] The use of conductive slip rings is a key technical solution to the problem of wire entanglement during the rotation and lifting of the seed crystal rod and crucible rod. The inner ring of the slip ring rotates and lifts synchronously with the rod, while the outer ring does so, but the direction of rotation is fixed to the furnace body. The wires are led out through the inner ring and connected to the target rod segment, effectively avoiding the risk of wire kinking or breakage due to continuous rod rotation. The first conductive slip ring and / or the second conductive slip ring are high-temperature resistant conductive slip rings.

[0038] Furthermore, the first insulator and / or the second insulator are preferably made of zirconia or alumina ceramics. Zirconia and alumina ceramics have excellent high-temperature resistance and electrical insulation properties, and can stably perform electrical isolation in high-temperature growth environments to ensure furnace safety.

[0039] Furthermore, the heating mechanism is preferably an induction coil or a resistance heater, which can provide a stable high-temperature heating environment. The control system is preferably also connected to temperature sensors, pressure sensors, and position sensors of the furnace body to achieve closed-loop control. This allows for comprehensive optimization of growth parameters such as temperature, pressure, and seed crystal position while simultaneously regulating pulse electrical parameters, achieving fully automated and precise crystal growth control.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] (1) Active optimization of micro-interface through the alternating action mechanism of "pulse-relaxation": This invention applies pulsed current to the liquid-phase growth of SiC single crystals. Its mechanism is not simply to use the thermal effect of current (such as the Peltier effect in the prior art) to regulate the macroscopic growth rate, but to use the instantaneous high-energy effect and relaxation effect of pulsed current to improve the quality of the growth interface at the microscale. During pulse conduction, the instantaneous strong electric field generated by the peak current breaks up the carbon-containing clusters in the melt at the front of the growth interface, promoting the diffusion and transport of carbon atoms in the solute boundary layer; during pulse turn-off, it provides surface relaxation time for adsorbed atoms, promoting their migration to the steps and orderly arrangement. This alternating action of "pulse-relaxation" is a fundamental breakthrough in the principle of this invention, forming a clear and essential distinction from the prior art.

[0042] (2) Achieving approximate decoupling between thermal and electromigration effects: By adjusting the duty cycle and frequency of the pulse, this invention achieves approximate independent control of thermal effects (related to average current) and electromigration effects (related to peak current). Extremely high instantaneous electromigration driving force can be obtained without generating severe Joule heating, solving the problem of "heat and force" being tied together in traditional DC-assisted growth, and greatly expanding the process window.

[0043] (3) Precise optimization of the entire growth process through staged pulse control: By applying pulse currents with different parameters during the nucleation and steady-state growth stages, the characteristics and requirements of each growth stage are fully utilized, and the entire process from nucleation to steady-state growth is precisely controlled. During the nucleation stage, a combination of pulse parameters with high peak current density and low duty cycle is used to effectively suppress polycrystalline nucleation and promote stable growth of a single crystal form; during the steady-state growth stage, a combination of parameters with medium peak current density and high duty cycle is used to accelerate crystal growth and significantly reduce dislocation density while maintaining interface stability.

[0044] (4) Flexible current direction configuration adapts to different crystal growth requirements: In view of the different characteristics of homogeneous growth and heterogeneous growth, by flexibly configuring the pulse current direction, it is possible to accelerate carbon solute transport by utilizing the electromigration effect to increase the growth rate, and to reduce the growth rate by using reverse current to obtain a stable interface structure, thus ensuring the crystal stability and crystal quality of various growth systems.

[0045] (5) Reducing defects at the source and significantly improving crystal quality: By breaking up large carbon clusters, inhibiting cluster adsorption, and promoting ordered atomic arrangement, this invention can reduce the formation of defects such as dislocations and inclusions at the source. Experiments show that SiC crystals grown using the method of this invention have a narrower half-width at half-maximum (FWHM) on X-ray rocking curves, a significantly reduced density of dislocation pits, and significantly improved crystal integrity and uniformity.

[0046] (6) The device is easy to modify and easy to implement in industrial applications: The device of this invention mainly adds a few components such as pulse power supply, insulator and conductive slip ring on the basis of existing liquid phase growth furnace. It makes very little change to the internal structure of the furnace, does not introduce additional pollution sources, and does not interfere with the original temperature field and flow field distribution. The modification cost is low and the industrial application prospects are good. Attached Figure Description

[0047] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0048] Figure 1 This is a schematic diagram of the device structure according to a specific embodiment of the present invention;

[0049] Figure 2 This is a schematic diagram of the pulse current waveform applied in Embodiment 1 of the present invention;

[0050] Figure 3 This is a schematic diagram of the pulse current waveform applied in Embodiment 3 of the present invention;

[0051] Figure 4 This is a schematic diagram of the pulse current waveform applied in Embodiment 4 of the present invention;

[0052] Figure 5 The X-ray rocking curve of the silicon carbide crystal obtained in Example 4 of this invention;

[0053] Figure 6 This is an optical microscope image of the surface of the silicon carbide crystal obtained in Example 4 of the present invention after KOH etching;

[0054] Figure 7 X-ray rocking curve of silicon carbide crystal prepared in Comparative Example 1 of the present invention;

[0055] Figure 8 This is an optical microscope image of the surface of the silicon carbide crystal prepared in Comparative Example 1 of the present invention after KOH etching;

[0056] Figure 9 X-ray rocking curve of silicon carbide crystal prepared in Comparative Example 4 of the present invention;

[0057] Figure 10 This is an optical microscope image of the surface of the silicon carbide crystal obtained in Comparative Example 4 of this invention after KOH etching;

[0058] The reference numerals in the attached figures are explained as follows:

[0059] 1-First conductive slip ring; 2-First insulator; 3-Seed crystal rod; 4-Insulation layer; 5-Crucible; 6-Melted material; 7-Induction coil; 8-Crucible rod; 9-Second insulator; 10-Second conductive slip ring; 11-Control system; 12-Pulse power supply; 13-Seed crystal rod connecting wire; 14-Seed crystal holder; 15-Seed crystal; 16-Furnace body; 17-Crucible rod connecting wire. Detailed Implementation

[0060] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0061] Figure 1 This is a schematic diagram of a pulsed current-assisted liquid-phase method device for growing silicon carbide crystals according to a specific embodiment of the present invention. Figure 1 As shown, the device includes a furnace body 16, within which are an insulation layer 4, a high-purity graphite crucible 5, and a heating mechanism (induction coil 7 is used in this embodiment) for heating the crucible. A first insulator 2 is provided on the seed crystal rod 3, dividing it into an upper and lower section. The lower end of the lower seed crystal rod (the lower end) is fixed to a silicon carbide seed crystal 15 via a seed crystal holder 14, while the upper end of the upper seed crystal rod (the upper end) extends outside the furnace body 16 and connects to a lifting and rotating mechanism (not shown in the figure). The first insulator 2 blocks current conduction along the seed crystal rod 3 to the furnace body 16. A second insulator 9 is provided on the crucible rod 8, dividing it into an upper and lower section. The upper end of the crucible rod, away from the second insulator 9 (upper end), supports the crucible 5. The lower end of the crucible rod, away from the second insulator 9 (lower end), extends outside the furnace body 16 and connects to the lifting and rotating mechanism (not shown in the figure). The second insulator 9 is used to block the conduction of current along the crucible rod 8 to the furnace body 16. The first insulator 2 and the second insulator 9 are preferably made of zirconium oxide or alumina ceramic.

[0062] The pulse power supply 12 is connected to the seed crystal rod 3 and the crucible rod 8 via a conductive circuit, and is used to apply a pulse current between the seed crystal 15 and the melt 6. Specifically, the conductive circuit includes a first conductive slip ring 1 and a second conductive slip ring 10. The inner ring of the first conductive slip ring 1 is fixed to the upper seed crystal rod and located inside the furnace body 16, rotating and rising with the seed crystal rod 3, while its outer ring is fixed to the furnace body 16. One pole (positive or negative) of the pulse power supply 12 is connected to the lower seed crystal rod via the seed crystal rod connecting wire 13 and the first conductive slip ring 1. The first conductive slip ring 1 is used to prevent the wire from getting tangled due to the rotation of the seed crystal rod 3. The inner ring of the second conductive slip ring 10 is fixed to the lower crucible rod and located inside the furnace body 16, rotating and rising with the crucible rod 8, while its outer ring is fixed to the furnace body 16. The other pole (negative or positive) of the pulse power supply 12 is connected to the upper section of the crucible rod via the crucible rod connecting wire 17 and the second conductive slip ring 10. The second conductive slip ring 10 is used to prevent the wire from getting tangled due to the rotation of the crucible rod 8.

[0063] The control system 11 (such as an industrial computer or a programmable logic controller) is connected to the pulse power supply 12 and is used to control the pulse power supply 12 to output pulse currents with different parameters during the nucleation stage and the steady-state growth stage, respectively, according to the preset growth process. Preferably, the control system 11 is also connected to the temperature sensor, pressure sensor, and position sensor of the furnace body 16 to realize closed-loop control.

[0064] Example 1

[0065] This embodiment illustrates the crystal growth effect under heterogeneous growth conditions when pulsed currents with different parameters are applied during the nucleation and steady-state growth stages, but the current direction is always positive (i.e., flowing from the melt to the seed crystal). Positive-axis 6H-SiC is used as the seed crystal, and the target is to grow 4H-SiC crystals (heterogeneous growth).

[0066] Adopting such Figure 1 The apparatus shown is used for crystal growth experiments. The specific steps are as follows:

[0067] (1) Raw material loading: High-purity silicon (purity 99.999%) and flux (Cr, purity 99.5%; Al, purity 99.9%) are loaded into a graphite crucible in an atomic ratio of Si:Cr:Al = 60:30:10.

[0068] (2) Vacuuming and melting: Close the furnace door and evacuate the furnace to a vacuum level of 5×10. -5 Below Pa, then high-purity Ar gas is introduced to 8 × 10⁻⁶ Pa. 4 Pa. Start the induction coil, heat to 1850℃, and hold for 2 hours to allow the raw materials to fully melt and form a uniform melt.

[0069] (3) Welding: Lower the seed crystal rod so that the lower surface of the seed crystal contacts the surface of the melt. Set the seed crystal rotation speed to 160 rpm and the pulling speed to 0.1 mm / h. Control the temperature field to stabilize the temperature at the seed crystal at 1850℃, with the temperature at the bottom of the crucible slightly higher, forming an axial temperature gradient of about 8℃ / cm.

[0070] (4) Applying pulsed current: After the seed crystal contacts the melt and crystal growth begins, the pulse power supply is activated through the control system to apply pulsed currents with different parameters at different growth stages, such as... Figure 2 As shown.

[0071] During the nucleation stage (silicon carbide crystal growth thickness ≤ 100 μm, approximately within 2 hours after liquid contact), the pulse current parameters are: peak current density 40 A / cm². 2 The duty cycle is 20%, the frequency is 2 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal.

[0072] During the steady-state growth stage (silicon carbide crystal thickness > 100 μm, approximately 2 hours after liquid contact), the pulse current parameters are: peak current density 20 A / cm². 2 The duty cycle is 50%, the frequency is 10 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal. The current direction remains consistent during the nucleation stage and the steady-state growth stage, both being positive (from the melt to the seed crystal).

[0073] (5) Crystal growth: Continue to grow for 60 hours under the assistance of the above pulsed current (including 2 hours of nucleation stage).

[0074] (6) End and cooling: After the growth is completed, turn off the pulse power supply first, then lift the crystal off the liquid surface, cool it to room temperature at 60℃ / h and then take out the crystal.

[0075] The crystal obtained in this embodiment was characterized. The crystal growth rate was 120.5 μm / h. X-ray rocking curve testing showed that the full width at half maximum (FWHM) of the rocking curve for the (0004) facet was 28.5 arcseconds (arcsec). After etching with molten KOH, the average dislocation density was approximately 4850 dislocations / cm². 2 Raman spectroscopy was used to determine the crystal form of the crystal surface, which was approximately 4H (91%) and 6H (9%).

[0076] Example 2

[0077] This embodiment illustrates the technical effect of using reverse current (flowing from the seed crystal to the melt) to reduce the growth rate and promote crystal transformation during the nucleation stage under heterogeneous growth conditions, and switching to forward current (flowing from the melt to the seed crystal) during the steady-state growth stage to restore high-speed, high-quality growth. Positive-axis 6H-SiC is used as the seed crystal, and the target is to grow 4H-SiC crystals (heterogeneous growth).

[0078] Adopting such Figure 1 The apparatus shown was used for crystal growth experiments, and steps (1)-(3) and (5)-(6) were the same as in Example 1. The difference was in step (4):

[0079] During the nucleation stage (silicon carbide crystal growth thickness ≤ 100 μm), the pulse current parameters are: peak current density 40 A / cm². 2 The duty cycle is 20%, the frequency is 2 kHz, the waveform is a square wave, and the current direction is from the seed crystal to the melt (reverse current).

[0080] During the steady-state growth stage (silicon carbide crystal thickness > 100 μm), the pulse current parameters are: peak current density 20 A / cm². 2 The duty cycle is 50%, the frequency is 10 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal (forward current).

[0081] The crystal obtained in this embodiment was characterized. The crystal growth rate was 117.2 μm / h. X-ray rocking curve testing showed that the full width at half maximum (FWHM) of the rocking curve for the (0004) facet was 24.1 arcseconds. After etching with molten KOH, the average dislocation density was approximately 3850 dislocations / cm². Raman spectroscopy was used to test the crystal form on the crystal surface, and the proportion of 4H crystal form reached 100%. Compared with Example 1, this shows that under heterogeneous growth conditions, using a reverse current during the nucleation stage appropriately reduced the initial growth rate, providing more sufficient interface reconstruction time for crystal form transformation, which is beneficial for obtaining high-quality crystals with a single crystal form.

[0082] Example 3

[0083] This embodiment illustrates the crystal growth effect under homogeneous growth conditions when pulsed currents with different parameters are applied during the nucleation and steady-state growth stages, with the current direction being positive (flowing from the melt to the seed crystal). A positive-axis semi-insulating 4H-SiC is used as the seed crystal, and the target is to grow a 4H-SiC crystal (homogeneous growth).

[0084] Adopting such Figure 1 The apparatus shown was used for crystal growth experiments, and steps (1)-(3) and (5)-(6) were the same as in Example 1. The difference was that the seed crystal was 4H-SiC and step (4) was performed.

[0085] The applied pulse current waveform in this embodiment is as follows: Figure 3 As shown.

[0086] During the nucleation stage (silicon carbide crystal growth thickness ≤ 100 μm), the pulse current parameters are: peak current density 45 A / cm². 2 The duty cycle is 20%, the frequency is 2 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal (forward current).

[0087] During the steady-state growth stage (silicon carbide crystal thickness > 100 μm), the pulse current parameters are: peak current density 20 A / cm². 2 The duty cycle is 50%, the frequency is 10 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal (forward current).

[0088] The crystal obtained in this embodiment was characterized. The crystal growth rate was 113.5 μm / h. X-ray rocking curve testing showed that the full width at half maximum (FWHM) of the rocking curve for the (0004) facet was 23.8 arcseconds. After etching with molten KOH, the average dislocation density was approximately 3500 dislocations / cm². Raman spectroscopy was used to test the crystal form on the crystal surface, and the 4H crystal form accounted for 100%. The crystal surface was bright, with regular edges and no visible inclusions. Compared with Examples 1 and 2, the crystal quality was further improved under homogeneous growth conditions, indicating that homogeneous growth does not require a crystal form transformation process, and high-quality single crystal growth can be directly obtained under the drive of a positive current during the nucleation stage.

[0089] Example 4

[0090] This embodiment illustrates a technical solution for achieving optimal crystal growth under homogeneous growth conditions by comprehensively optimizing the pulse current parameters at each stage. A positive-axis semi-insulating 4H-SiC crystal is used as the seed crystal, with the target being the growth of a 4H-SiC crystal (homogeneous growth).

[0091] Adopting such Figure 1 The apparatus shown was used for crystal growth experiments, and steps (1)-(3) and (5)-(6) were the same as in Example 3. The difference was that the pulse current parameters were more finely optimized in step (4), such as... Figure 4 As shown:

[0092] During the nucleation stage (silicon carbide crystal growth thickness ≤ 100 μm), the pulsed current parameters are: peak current density 45 A / cm², duty cycle 10%, frequency 5 kHz, waveform is square wave, and current direction is from melt to seed crystal (forward current). The combination of high peak current density and extremely low duty cycle provides a very strong instantaneous electric field crushing effect and sufficient atomic relaxation time during the nucleation stage.

[0093] During the steady-state growth stage (silicon carbide crystal growth thickness > 100 μm), the pulse current parameters are: peak current density 15 A / cm², duty cycle 60%, frequency 15 kHz, waveform is square wave, and current direction is from melt to seed crystal (forward current). The combination of moderate peak current density and high duty cycle fully utilizes the electromigration effect to accelerate growth while maintaining interface stability, and ensures efficient "pulse-relaxation" alternation through high frequency.

[0094] The crystal obtained in this embodiment was characterized. The crystal growth rate was 109.5 μm / h, and the X-ray rocking curve test results are as follows. Figure 5 As shown, the full width at half maximum (FWHM) of the rocking curve on the (0004) crystal surface is only 21.6 arcseconds. The morphology of dislocation corrosion pits on the crystal surface after molten KOH etching is as follows: Figure 6 As shown, the average dislocation density is approximately 2100 dislocations / cm². Raman spectroscopy was used to test the crystal form on the crystal surface, revealing that the 4H crystal form accounted for 100%. Microscopic observation of the polished longitudinal section of the crystal showed almost no visible inclusions inside. The crystal surface was bright and smooth, exhibiting extremely high crystal integrity. Compared to Example 3, further optimization of the nucleation stage duty cycle to 10% and the steady-state growth stage frequency to 15 kHz resulted in a further improvement in crystal quality.

[0095] Comparative Example 1

[0096] This comparative example illustrates the crystal growth effect when no staged parameter adjustment is performed during the entire crystal growth process, but instead the same pulse current with the same parameters is applied throughout, to highlight the importance of the technical feature of "staged configuration of different parameters" in this technical solution.

[0097] Adopting such Figure 1 The apparatus shown uses a positive-axis semi-insulating 4H-SiC seed crystal (homogeneous growth), and the basic growth steps are the same as in Example 3. The difference lies in that a pulsed current with the same parameters is applied throughout the entire crystal growth process (including the nucleation stage and the steady-state growth stage): peak current density 20 A / cm², duty cycle 50%, frequency 10 kHz, waveform is square wave, and the current direction is from the melt to the seed crystal.

[0098] The crystal obtained in this comparative example was characterized. The crystal growth rate was 108.2 μm / h, and the X-ray rocking curve test results are as follows. Figure 7 As shown, the full width at half maximum (FWHM) of the rocking curve on the (0004) crystal surface is 31.2 arcseconds. The morphology of dislocation corrosion pits on the crystal surface after etching with molten KOH is as follows. Figure 8As shown, the average dislocation density is approximately 5800 dislocations / cm². Raman spectroscopy was used to test the crystal form on the crystal surface, and the 4H crystal form accounted for 100%. Compared with Example 3 (homogeneous growth, staged control), the crystal quality decreased. Analysis suggests that using the same pulse parameters throughout the process cannot meet the specific requirements of a strong electric field to break up clusters and sufficient relaxation time during the nucleation stage, resulting in poor interface quality in the early nucleation stage, which in turn affects the overall quality of the crystal.

[0099] Comparative Example 2

[0100] This comparative example illustrates the adverse effects on crystal quality when the peak current density is too low during the nucleation stage.

[0101] Adopting such Figure 1 The apparatus shown uses positive-axis semi-insulating 4H-SiC (homogeneous growth) as the seed crystal, and the basic growth steps are the same as in Example 3. The difference lies in the peak current density of the pulsed current during the nucleation stage, which is only 25 A / cm². 2 The remaining parameters are the same as in Example 3: the nucleation stage duty cycle is 20%, the frequency is 2 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal; the peak current density in the steady-state growth stage is 20 A / cm³. 2 The duty cycle is 50%, the frequency is 10 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal.

[0102] The crystal obtained in this comparative example was characterized. The crystal growth rate was 110.8 μm / h. X-ray rocking curve testing showed that the full width at half maximum (FWHM) of the rocking curve on the (0004) surface of the crystal was 35.5 arcseconds. After etching with molten KOH, the average dislocation density was approximately 6500 dislocations / cm². Raman spectroscopy was used to test the crystal form on the crystal surface, and the crystal forms were 4H (92%) and 6H (8%). Compared with Example 3, the crystal quality was significantly reduced and 6H crystal form inclusions were present. Analysis suggests that the peak current density during the nucleation stage was too low, and the instantaneous electric field strength was insufficient to effectively break up the large carbon-containing clusters in the melt, causing the clusters to be directly adsorbed onto the growth interface, becoming the source of dislocations, inclusions, and heterogeneous nuclei.

[0103] Comparative Example 3

[0104] This comparative example illustrates the adverse effects of an excessively high duty cycle during the nucleation stage on crystal quality.

[0105] Adopting such Figure 1 The apparatus shown uses positive-axis semi-insulating 4H-SiC (homogeneous growth) as the seed crystal, and the basic growth steps are the same as in Example 3. The difference lies in the high pulse current duty cycle of 35% during the nucleation stage; the remaining parameters are the same as in Example 3: peak current density during the nucleation stage is 45 A / cm³. 2The frequency is 2 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal; the peak current density during the steady-state growth stage is 20 A / cm². 2 The duty cycle is 50%, the frequency is 10 kHz, the waveform is a square wave, and the current direction is from the melt to the seed crystal.

[0106] The crystal obtained in this comparative example was characterized. The crystal growth rate was 112.0 μm / h. X-ray rocking curve testing showed that the full width at half maximum (FWHM) of the rocking curve for the (0004) facet was 32.8 arcseconds. After etching with molten KOH, the average dislocation density was approximately 5200 dislocations / cm². 2 Raman spectroscopy was used to test the crystal form on the crystal surface, and the crystal forms were 4H (96%) and 6H (4%). Compared with Example 3, the crystal quality decreased. Analysis suggests that the excessively high duty cycle during the nucleation stage and the excessively short pulse off-time resulted in insufficient relaxation time for adsorbed atoms to diffuse and arrange themselves in an orderly manner on the surface, leading to disordered atomic stacking at the interface, increased defect density, and the formation of 6H polycrystalline inclusions.

[0107] Comparative Example 4

[0108] This comparative example illustrates the case where no pulsed current is applied throughout the heterogeneous growth process.

[0109] Adopting such Figure 1 The apparatus shown uses positive-axis 6H-SiC (heterogeneous growth) as the seed crystal, and the basic growth steps are the same as in Example 3. The difference is that no pulsed current is applied during the nucleation stage and the steady-state growth stage.

[0110] The crystal obtained in this comparative example was characterized. The crystal growth rate was 92.5 μm / h, and the X-ray rocking curve is shown below. Figure 9 As shown, the results indicate that the full width at half maximum (FWHM) of the rocking curve on the (0004) crystal surface is 90.0 arcseconds. The morphology of the dislocation corrosion pits on the crystal surface after molten KOH etching is as follows: Figure 10 As shown, statistically, the average dislocation density is approximately 7800 dislocations / cm². 2 Raman spectroscopy was used to determine the crystal form of the crystal surface, which was 4H (45%) and 6H (55%). A small number of macroscopic defects were visible on the crystal surface, and a certain number of inclusions were present inside the crystal. Compared to Example 3, the crystal quality was significantly reduced.

[0111] The characterization results of the examples and comparative examples are summarized in Table 1.

[0112] Table 1

[0113]

[0114] The data in Table 1 shows that:

[0115] First, all examples and comparative examples that applied pulsed current showed better growth rate, crystal quality, and crystal stability than Comparative Example 4, which did not apply any pulsed current. This strongly demonstrates the feasibility and significant advantages of applying pulsed current technology to the liquid phase growth of silicon carbide crystals.

[0116] Secondly, the comparison between Example 3 and Comparative Example 1 clearly demonstrates the core value of the "staged control" strategy. Comparative Example 1 uses fixed parameters throughout the homogeneous growth process, and its dislocation density and inclusion density are much higher than those of Example 3. This indicates that the differentiated configuration of "high peak current density and low duty cycle" during the nucleation period and "lower peak current density and high duty cycle" during the steady-state period is the key to stimulating the "pulse-relaxation" effect and laying a high-quality foundation in the early stages of nucleation, rather than simply applying current. Thirdly, Comparative Examples 2 and 3 further pinpoint the criticality of the nucleation period parameters. In Comparative Example 2, the peak current density during the nucleation period is too low, resulting in insufficient instantaneous field strength to break up the clusters and a high dislocation density; in Comparative Example 3, the duty cycle during the nucleation period is too high, resulting in insufficient atomic relaxation time, hindered ordered arrangement, and a significant deterioration in crystal quality. This strongly demonstrates the nucleation period process window (peak current density 30-50 A / cm²) defined in this invention. 2 The necessity and non-obviousness of low duty cycles (10-30%).

[0117] Crucially, the comparison between Examples 1 and 2 and Comparative Example 4 reveals the groundbreaking contribution of this invention in solving the problem of heterogeneous growth. In Comparative Example 4, during the heterogeneous growth from 6H to 4H, the proportion of the 4H crystal form was only 45%, and the defect density was extremely high, indicating that heterogeneous growth under natural conditions faces serious problems of crystal form control and interface instability. In Example 1, by applying a staged forward current, the proportion of the 4H crystal form was increased to 91%, and the crystal quality was also significantly improved. More remarkably, Example 2, by switching to a reverse current during the nucleation stage, achieved 100% complete conversion of the 4H crystal form, and the crystal defect density was also better than that of Example 1. This series of comparisons proves that the "pulse current direction switching" strategy for heterogeneous growth proposed in this invention can create an ideal window for heterogeneous crystal form conversion by actively intervening in the growth kinetics in the early stage of nucleation, thereby overcoming the problem of mixed crystal forms in heterogeneous growth that is difficult to solve by traditional methods.

[0118] Finally, Example 4 achieved an ultra-low sway curve half-width at half-maximum of 19.0 arcseconds and 2100 cells / cm by optimizing the process parameters for homogeneous growth. 2 The extremely low dislocation density. The above fully demonstrates that this invention, through two major strategies—"staged parameter control" and "flexible orientation configuration"—achieves precise control over the silicon carbide crystal growth process, significantly improving the crystal growth rate, crystal quality, and crystal form stability.

Claims

1. A method for growing silicon carbide crystals using pulsed current-assisted liquid phase method, characterized in that, Includes the following steps: (1) A melt containing silicon and a flux is formed, and a silicon carbide seed crystal is brought into contact with the surface of the melt to carry out crystal growth; (2) A pulsed current is applied during the crystal growth process, wherein pulsed currents with different parameters are applied to the growth interface at least during the nucleation stage and the steady-state growth stage; The pulse current parameters applied during the nucleation stage are configured such that the peak current density is higher than that applied during the steady-state growth stage, and the duty cycle is lower than that applied during the steady-state growth stage. (3) After growth is complete, the resulting crystal is separated from the melt to obtain silicon carbide crystal.

2. The method according to claim 1, wherein, When the crystal form of the silicon carbide seed crystal is the same as that of the silicon carbide crystal to be grown, the direction of the pulse current in both the nucleation stage and the steady-state growth stage is configured to flow from the melt to the seed crystal.

3. The method according to claim 1, wherein, When the crystal form of the silicon carbide seed crystal is different from the crystal form of the silicon carbide crystal to be grown: During the nucleation stage, the direction of the pulsed current is configured to flow from the seed crystal to the melt; and During the steady-state growth stage, the direction of the pulsed current is configured to flow from the melt to the seed crystal.

4. The method according to claim 1, wherein, The peak current density of the pulse current applied to the nucleation stage is 30-50 A / cm 2 , and the duty cycle is 10-30%. Preferably, the peak current density of the pulsed current applied during the steady-state growth phase is 10⁻²⁸ A / cm². 2 The duty cycle is 40-60%.

5. The method according to claim 1, wherein, The frequency range of the pulse current is 100 Hz to 100 kHz, preferably 1 kHz to 20 kHz; Preferably, the waveform of the pulse current is one of a square wave, a triangular wave, or a spike wave.

6. The method according to claim 1, wherein, The nucleation stage is the stage where the silicon carbide crystal growth thickness is less than or equal to 100 μm, and the steady-state growth stage is the stage where the silicon carbide crystal growth thickness is greater than 100 μm.

7. The method according to claim 1, wherein, The melt is formed by placing silicon and a flux in a graphite crucible and heating and melting them under a protective atmosphere; during crystal growth, the graphite crucible serves as a carbon source to continuously dissolve carbon into the melt. Preferably, the protective atmosphere is argon and / or nitrogen; Preferably, the co-solvent is selected from at least one of transition metals and rare earth elements; More preferably, the transition metal is selected from at least one of Cr, Ti, Fe and Ni; More preferably, the rare earth element is selected from at least one of La, Pr, Nd and Ce.

8. An apparatus for growing silicon carbide crystals using a pulsed current-assisted liquid phase method according to any one of claims 1-7, characterized in that, include: The furnace body contains a crucible and a heating mechanism for heating the crucible. A seed crystal rod is provided with a first insulator, which divides the seed crystal rod into an upper seed crystal rod and a lower seed crystal rod. The end of the lower seed crystal rod away from the first insulator is used to fix the silicon carbide seed crystal. The end of the upper seed crystal rod away from the first insulator extends out of the furnace body and is connected to the lifting and rotating mechanism. The first insulator is used to block the current from being conducted along the seed crystal rod to the furnace body. A crucible rod is provided with a second insulator, which divides the crucible rod into an upper section and a lower section. The end of the upper section away from the second insulator is used to support the crucible, and the end of the lower section away from the second insulator extends out of the furnace body and is connected to the lifting and rotating mechanism. The second insulator is used to block the current from being conducted along the crucible rod to the furnace body. A pulse power supply, whose positive and negative terminals are respectively connected to the seed crystal rod and the crucible rod through a conductive circuit, is used to apply a pulse current between the seed crystal and the melt; one of the positive and negative terminals is electrically connected to the seed crystal rod through the conductive circuit, and the other is electrically connected to the crucible rod through the conductive circuit. The control system, connected to the pulse power supply, is used to control the pulse power supply to output pulse currents with different parameters during the nucleation stage and the steady-state growth stage, respectively, according to a preset growth process.

9. The apparatus according to claim 8, wherein, The conductive circuit includes: The first conductive slip ring has its inner ring fixed to the upper seed crystal rod and located inside the furnace body. It rotates and rises and falls with the seed crystal rod. Its outer ring is fixed to the furnace body. One pole of the pulse power supply is connected to the lower seed crystal rod through the first conductive slip ring via a wire. The first conductive slip ring is used to prevent the wire from getting tangled when the seed crystal rod rotates. The second conductive slip ring has its inner ring fixed to the lower crucible rod and located inside the furnace body. It rotates and rises and falls with the crucible rod. Its outer ring is fixed to the furnace body. The other pole of the pulse power supply is connected to the upper crucible rod via a wire through the second conductive slip ring. The second conductive slip ring is used to prevent the wire from getting tangled when the crucible rod rotates.

10. The apparatus according to claim 8, wherein, The first insulator and / or the second insulator are made of zirconium oxide or alumina ceramic; Preferably, the heating mechanism is an induction coil or a resistance heater; Preferably, the control system is also connected to temperature sensors, pressure sensors, and position sensors of the furnace body to achieve closed-loop control.

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