Method and apparatus for producing conductive bulk β-Ga2O3 single crystals and conductive bulk β-Ga2O3 single crystals

Abstract

Conductive bulk β-Ga grown from the melt, especially by the Czochralski method 2 O 3 Method and apparatus for producing single crystals of β-Ga with high electrical conductivity and large diameters greater than 1 inch combined with lengths greater than 25 mm. 2 O 3 To grow the single crystal (7), the growth furnace is provided with an internal insulation (8) with low radiation reflectance (R) in the near infrared spectral range of 1-3 μm that reduces the reflection of heat back to the growing single crystal (7) and thus increases the heat dissipation from the growing single crystal (7). Low reflectance can be achieved either by high emissivity or high transmittance. Furthermore, β-Ga 2 O 3 The single crystal (7) grows at a rate that dynamically decreases as the growth proceeds, dynamically decreasing the latent heat of crystallization and the amount of heat dissipated from the growing single crystal (7).

Description

[Technical field]

[0001] Technical Field The present invention relates generally to a method and apparatus for growing bulk β-Ga2O3 single crystals by the Czochralski method, and in particular for large diameter conductive crystals. The present invention also relates to bulk β-Ga2O3 single crystals, particularly large diameter conductive crystals, grown by the Czochralski method. [Background technology]

[0002] 2. Background of the Invention The beta phase of Ga2O3 (β-Ga2O3) is an emerging ultra-wide bandgap n-type oxide semiconductor with high potential for UV optoelectronic and high power electronic applications. This is a result of the wide bandgap of 4.85 eV and the high theoretical breakdown field of 8 MV / cm that makes the compound transparent into the UV spectral region.

[0003] Devices require bulk single crystals, which act as substrates for the epitaxial layers, on which the device structures are then fabricated. Although foreign substrates can be used for such purposes, they usually degrade the quality of the layers, and thus the device performance, due to the lattice mismatch between the substrate and the layers. Therefore, natural substrates are in demand.

[0004] β-Ga2O3 based power devices such as Schottky diodes and field effect transistors (FETs) can be economically efficient in terms of energy conservation and energy management. They can find applications in several industrial sectors such as mobile transportation including electric vehicles, aircraft, and ships, as well as railroads, or power grids. In general, power switching devices can be designed in two configurations: horizontal (planar) and vertical. In the horizontal configuration, the substrate is electrically insulating and acts as a support for the layers with all contacts thereon, such as the source, drain, and gate contacts in a FET device. In the vertical configuration, the substrate is highly conductive and acts as an electrical contact in addition to supporting the epitaxial layers. Devices in the vertical configuration can switch higher voltages compared to the horizontal configuration.

[0005] Bulk β-Ga2O3 single crystals are known in the art and can be grown from the melt by a variety of techniques: Optical Floating Zone (OFZ), Edge-Defined Film-Fed Growth (EFG), Bridgman or Vertical Gradient Freeze (VGF), and the Czochralski method.

[0006] The OFZ method is a crucible-less method and can only produce small diameter crystals not exceeding 1 inch. An example of this method used to grow bulk β-Ga2O3 single crystals can be found in the paper by E.G. Villora et al. entitled "Large-size β-Ga2O3 single crystals and wafers", J.Cryst.Growth 270 (2004) 420-426, and in U.S. Pat. No. 8,747,553. The OFZ method makes it possible to grow both electrically insulating and conductive bulk β-Ga2O3 single crystals along different crystallographic directions, as the growth proceeds on top of a seed crystal.

[0007] The EFG method for growing bulk β-Ga2O3 single crystals uses an Ir crucible and an Ir die (shaper) on top of the crucible. Typically, the crystals have a (-201) or (001) major face (i.e., the large face of the slab) that can reach sizes of 4 or 6 inches. <010> The (010) oriented substrates are grown in the form of slabs along the crystallographic direction. 2 An example of this method related to bulk β-Ga2O3 single crystals is given in "High-quality β-Ga2O3 single crystals grown by edge-defined film-fed The present invention is disclosed in the paper by A.Kuramata et al. entitled "growth", Jpn.J.Appl.Phys.55(2016)1202A2, as well as in patent / patent application documents: WO 2013172227 (corresponding JP 6,421,357 A, EP 2851458 B, US 2015125699 B), WO 2013073497 (JP 5,491,483 A, EP 2,801,645 B, US 2014352604 B), and WO 2014073314 (JP 5,756,075 B, EP 2,933,359 B, US 9,926,646 B). Cylindrical β-Ga2O3 crystals grown by the EFG method are disclosed, for example, in Chinese Patent Application No. 112210823.

[0008] The Bridgman and VGF methods used to grow bulk β-Ga2O3 single crystals utilize a noble metal crucible, such as a Pt-Rh alloy (Bridgman) or Ir (VGF) crucible, in which the melt solidifies into a single crystal on top of a seed crystal, which may have different crystallographic orientations. This method allows the growth of both electrically insulating and conductive β-Ga2O3 single crystals. The VGF method, described in the paper by Galazka et al. entitled "Scaling-Up of Bulk β-Ga2O3 Single Crystals by the Czochralski Method", ECS J.Solid State Sci.Technol.6(2017)Q3007-Q3011, allows the growth of highly conductive β-Ga2O3 single crystals of almost 2 inches diameter, although they are short and do not exceed 20 mm. The growth of bulk β-Ga2O3 single crystals by the Bridgman method is disclosed in U.S. Patent No. 10,570,528 by K.Hoshikawa et al. The Bridgman technique reported by K. Hoshikawa et al. in the paper "2-inch diameter (100) β-Ga2O3 crystal growth by the vertical Bridgman technique in a resistance heating furnace in ambient air", J. Cryst. Growth, 545 (2020) 125724, allows the growth of electrically insulating crystals perpendicular to the (100) plane with a diameter of 2 inches, but their cylindrical length does not exceed about 25 mm. The same crystal dimensions relate to conductive crystals grown by the Bridgman technique with an orientation perpendicular to the (001) plane, as described in the paper "50 mm diameter Sn-doped (001) β-Ga2O3 crystal growth using the vertical Bridgeman technique in ambient air" by Hoshikawa et al., J. Cryst. Growth, 546 (2020) 125778.

[0009] The Czochralski method offers the possibility to grow the largest bulk β-Ga2O3 single crystals in terms of volume combined with high structural quality. Electrically insulating crystals with a diameter of 2 inches can be 60-100 mm in length as reported by Galazka et al. in their papers entitled "Scaling-Up of Bulk β-Ga2O3 Single Crystals by the Czochralski Method", ECS J. Solid State Sci. Technol. 6 (2017) Q3007-Q3011, "β-Ga2O3 for wide-bandGap electronics and optoelectronics", Semicond. Sci. Technol. 33 (2018) 113001, and "Bulk single crystals of β-Ga2O3and Ga-based spinels as ultra-wide bandGap transparent semiconducting oxides", Prog. Cryst. Growth Charact. Mater. 67 (2021) 100511. To grow such large crystals, a high oxygen partial pressure in the growth atmosphere is necessary to minimize the formation of elemental Ga in the Ga2O3 melt, which, if obtained at all, has a significant adverse effect on the growth process, crucible life, and crystal quality. This requirement is necessary for all melt growth methods used for β-Ga2O3 single crystals, including noble metal crucibles, as taught by Galazka et al. in European Patent Specification No. 3,242,965 (corresponding U.S. Patent No. 11,028,501).

[0010] Electrically insulating bulk β-Ga2O3 single crystals can be of large size in terms of diameter and cylinder length, but this does not concern conductive crystals, which suffer from the so-called free carrier absorption. Simply put, the heat generated during the liquid-solid phase transition (latent heat of crystallization) cannot be easily dissipated by the growing crystal, since most of the heat is absorbed by the free carriers in the growing crystal in the near-infrared spectral region, where it is transported through the growing crystal. It causes a temperature increase near the growth interface, which results in an interface inversion from convex to concave towards the melt. Such a concave growth interface leads to a growth instability that changes the growth morphology from a cylindrical shape to a corkscrew (spiral) shape. The spiral does not make it possible to produce wafers of the required diameter. This phenomenon related to bulk β-Ga2O3 single crystals grown by the Czochralski method has been reported in "On the bulk β-Ga2O3 single crystals grown by the Czochralski method", J.Cryst.Growth404(2014)184~191, "Czochralski-grown bulk β-Ga2O3 single crystals doped with mono-, di-, tri-, and tetravalent ions", J.Cryst.Growth529(2020)125297, "Bulk β-Ga2O3 single crystals doped with Ce, Ce+Si, Ce+Al, and Ce+Al+Si for detection of nuclear radiation", J.Alloy.Compd.818(2020)152842, and "Transparent Semiconducting Oxides-Bulk Crystal Growth and Fundamental Properties", Chapter 4, Jenny Stanford As a result, the free electron concentration is 10 18 cm -3Highly conductive crystals over 1000 nm can be grown by the Czochralski method as short cylinders, about 30 mm for crystal diameters of 1 inch or less, and less than 25 mm for two diameter crystals. For economic reasons, especially when considering the very high cost of crystal growth, the cylindrical portion of the crystal should be as long as possible to produce as many wafers as possible from one crystal while maintaining the desired electrical properties for electronic devices, especially in a vertical configuration. The Czochralski method is <010> It allows crystals to grow along crystallographic directions, thus allowing (010) oriented wafers to be produced from cross sections of cylindrical parts. For example, <010> The 2-inch diameter crystals grown along the axial direction make it possible to fabricate 2-inch diameter (010) oriented wafers with high electrical conductivity suitable for vertically configured power devices.

[0011] The electrical conductivity of undoped bulk β-Ga2O3 single crystals, independent of the growth method, arises from residual impurities, mainly silicon and / or hydrogen, which have a free electron concentration of about 10 18 cm -3 It is possible to induce electrical conductivity that would classify it as a normal semiconductor, not exceeding a value of 0.01%. For higher electrical conductivity, deliberate doping is required, which is usually performed, for example, with silicon (Si) or tin (Sn) doping, as explained in the state of the art above. Other dopants, such as Nb, Hf, or Zr, may also be used to induce electrical conductivity in β-Ga2O3. Summary of the Invention [Problem to be solved by the invention]

[0012] Summary of the Invention The aim of the present invention is to provide conditions for the production of bulk β-GaO single crystals from the melt, in particular by the Czochralski method, which allows to reliably grow crystals of high electrical conductivity, large size and high structural quality. [Means for solving the problem]

[0013] Another object of the present invention is to provide bulk β-Ga2O3 single crystals with high electrical conductivity and large size, preferably at least 2 inches in diameter and greater than 1 inch in length.

[0014] Yet another object of the present invention is to provide an apparatus for producing β-GaO single crystals from the melt, in particular by the Czochralski method, which makes it possible to carry out said method and thus grow crystals of high electrical conductivity, large size and high structural quality.

[0015] In a more general aspect, it is yet a further object of the present invention to provide a method and apparatus for producing different oxide compounds by the Czochralski process that have high melting points (above 1400° C.) and exhibit strong absorption in the near infrared spectral region.

[0016] According to a first aspect of the present invention, a method for producing conductive bulk β-Ga2O3 single crystals by the Czochralski method includes providing a growth chamber with a growth furnace comprising a noble metal crucible having a Ga2O3 starting material therein, a thermal insulation surrounding the crucible from all sides with a free space for accommodating the growing bulk β-Ga2O3 single crystal, and an inductive RF coil for heating the crucible and controlling the melting temperature during crystal growth, the RF coil being powered by an RF power source, and the growing crystal being fixed to a translation and rotation mechanism via a seed crystal, a seed crystal holder, and a pulling rod. The method further includes providing the Ga2O3 starting material with a dopant that forms shallow donors in the Ga2O3 single crystal, as well as providing the growth chamber, and thus the growth furnace, with a growth atmosphere comprising oxygen mixed with at least one non-reducing gas. The crucible with the Ga2O3 starting material is then heated by the RF coil until the Ga2O3 starting material melts. After melting the Ga2O3 starting material, an oriented seed crystal is immersed in the molten starting material, which is then pulled at a moving speed to achieve a predetermined growth rate while rotating at a predetermined rotational speed. During the first pull, the diameter of the seed is expanded to the final cylindrical diameter, which is pulled to a predetermined cylindrical length. Once the predetermined cylindrical length is achieved, the single crystal is separated from the melt and cooled to room temperature along with the growth furnace.

[0017] The essence of the invention is to additionally provide the growth furnace with an internal insulation with low radiation reflectance in the near infrared spectral range of 1-3 μm in order to reduce the reflection of heat back to the growing single crystal and thus increase the heat dissipation from the growing single crystal. This is combined with a dynamic reduction in the growth rate from the start of pulling the seed crystal to the crystal separation. The growth rate is reduced from an initial growth rate of 1-10 mm / h at the start of growth to a final growth rate of 0.2-1 mm / h at the end of growth, when the single crystal achieves a given cylindrical length. This solution dynamically reduces the latent heat of crystallization as the growth progresses and thus the amount of heat dissipated from the growing single crystal.

[0018] Preferably, the interior insulation having a low radiation reflectance has a high emissivity or a high reflectance in the near infrared spectral region greater than 0.3 at room temperature.

[0019] The growth rate may decrease linearly or non-linearly from the initial growth rate to the final growth rate.

[0020] In a preferred embodiment, the growth rate decreases from the initial growth rate to the final growth rate at different rates.

[0021] In another preferred embodiment, the growth rate may be decreased from the initial growth rate to the final growth rate continuously or in blocks combining constant and decreasing growth rates.

[0022] Advantageously, the growth atmosphere contains, in addition to oxygen, a non-reducing gas of high thermal conductivity, preferably He at a concentration of 10-95% by volume, to further enhance heat dissipation from the growing crystal via gas convection.

[0023] According to a second aspect of the present invention, a conductive bulk β-Ga2O3 single crystal grown by the Czochralski method is provided according to the method described above. The β-Ga2O3 single crystal has a cylindrical diameter of more than 1 inch, a cylindrical length of more than 25 mm, and the following electrical properties formed by shallow donors by Hall effect measurement: 1~10×10 18 cm -3 Free electron concentration, 50-120cm 2 V -1 s -1 and a resistivity of 0.01 to 0.04 Ω cm.

[0024] In a preferred form of the invention, the β-Ga2O3 single crystal has a cylindrical diameter of 2 inches or greater. Advantageously, the dopants forming the shallow donors are Si and / or Sn.

[0025] According to a third aspect of the present invention, there is provided an apparatus for producing a conductive bulk β-Ga2O3 single crystal by the Czochralski method having a predetermined cylinder diameter and cylinder length. The apparatus comprises a growth chamber and a growth furnace comprising a noble metal crucible having a Ga2O3 starting material therein, insulation surrounding the crucible on all sides with a free space for containing the bulk β-Ga2O3 single crystal being grown, and an induction RF coil for heating the crucible and controlling the melting temperature during crystal growth. The apparatus further comprises an RF power supply for supplying power to the RF coil, a translation and rotation mechanism coupled with the seed crystal via the seed crystal holder, and a pulling rod, and a scale connected with the pulling rod or the growth furnace for monitoring the growth rate of the bulk β-Ga2O3 single crystal.

[0026] According to the invention, the growth furnace further comprises an internal insulation with low radiation reflectance in the near infrared spectral region of 1-3 μm which reduces the reflection of heat back to the growing single crystal and thus increases the heat dissipation from the growing single crystal.

[0027] Advantageously, the interior insulation having a low radiation reflectance has a high emissivity in the near infrared spectral region of greater than 0.3 at room temperature, where the interior insulation is preferably selected from the group consisting of opaque alumina, zirconia, magnesia, and yttria.

[0028] In another advantageous solution of the invention, the internal insulation with low radiation reflectance has a high transmittance in the near infrared spectral region above 0.3 at room temperature, which is preferably filled by a transparent ceramic selected from the group consisting of alumina, yttria and yttrium aluminum garnet, or crystalline sapphire.

[0029] In a preferred embodiment of the apparatus, the growth atmosphere contains, in addition to oxygen, a non-reducing gas of high thermal conductivity, most preferably He at a concentration of 5-95% by volume, to further enhance heat dissipation from the growing crystal via gas convection.

[0030] Further advantages and other features of the inventive method and apparatus for growing bulk β-GaO single crystals or other oxide crystals exhibiting high absorption in the near infrared spectral region will become apparent from the detailed description of the embodiments with reference to the drawings. [Brief description of the drawings]

[0031] [Figure 1] FIG. 1 is a cross-sectional view of a schematic growth furnace for oxide single crystals grown by the Czochralski method. [Diagram 2] FIG. 14 shows different growth rate profiles of bulk β-Ga2O3 single crystals grown by the Czochralski method. [Diagram 3] FIG. 13 shows actual growth rate profiles output from a growth run of bulk β-Ga2O3 single crystals grown by the Czochralski method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Detailed Description of the Embodiments The growth furnace 1 shown in FIG. 1 is generally adapted for growing bulk oxide single crystals by the Czochralski method, and in particular for growing bulk β-Ga2O3 single crystals by the Czochralski method. The growth furnace 1 is housed in a water-cooled growth chamber (not shown) into which a predetermined growth atmosphere 2 is introduced. The furnace 1 is composed of a noble metal crucible 3 containing the Ga2O3 starting material 4, preferably having a lid 5 thereon, and a thermal insulation 6 surrounding the crucible from all sides. Above the crucible there is a free space (growth zone) containing the growing bulk β-Ga2O3 single crystal 7. The growth furnace 1 further comprises an internal thermal insulation 8 of predetermined optical properties, as will be further explained below. The noble metal crucible 5 is located outside the thermal insulation 6 and is inductively heated by an RF coil 9 powered by an RF power supply (not shown). A bulk β-Ga2O3 single crystal 7 is grown on a crystallographically oriented β-Ga2O3 seed crystal 10 fixed to a seed holder 11 connected to a pulling rod 12. The pulling rod 12 is connected to a pulling and rotation mechanism (not shown) for translating TR and rotating R the growing β-Ga2O3 single crystal 7, and to a scale (not shown) for weighing the growing β-Ga2O3 single crystal 7. Alternatively, the scale is coupled to the growth furnace 1. In either case, the scale indicates the growth rate of the growing single crystal 7 by directly weighing the crystal mass or the mass difference between the furnace and the growing crystal 7. The signal from the scale is coupled to a controller that controls the generator power and thus the melt temperature.

[0033] The insulation 6 is a refractory material that can withstand high temperatures depending on the melting point of the oxide crystal being grown. The melting point of Ga2O3 is around 1800 °C. The insulation 6 typically includes alumina or zirconia tubes, granules, felt, as well as quartz tubes in the outer part of the growth furnace 1. The purpose of the insulation 6 is to reduce heat losses and establish a predetermined temperature gradient in the crucible and in the growth zone to minimize thermal stresses and associated structural and point defects in the growing crystal 7. In detail, to grow bulk β-Ga2O3 single crystals, a low temperature gradient is necessary and therefore as good an insulation as possible is required. This is a result of the melt chemistry (decomposition) and monoclinic system of β-Ga2O3, which has anisotropic mechanical, thermal, and optical properties.

[0034] The noble metal crucible 3 for the β-Ga2O3 crystals is iridium and its alloys such as iridium-platinum alloys. Other crucible materials can also be considered, such as the platinum-rhodium alloy described by Hoshikawa et al. (mentioned above) for the Bridgman process.

[0035] In particular, the growth atmosphere 2 for growing large diameter β-Ga2O3 single crystals 7, greater than 1 inch, preferably contains at least 5% by volume oxygen and at least one other non-reducing gas, such as Ar, N2, CO2, He, Xe, and Ne, as described in U.S. Pat. No. 11,028,501 to Galazka et al., which is incorporated herein by reference.

[0036] For the Czochralski method, the β-Ga2O3 seed crystal 10 is crystallographically oriented along the

[0010] direction parallel to both the easy (100) and (001) cleavage planes present in the monoclinic system of β-Ga2O3. Other seed orientations tilted toward the

[0010] direction, such as

[0110] , can be used as well.

[0037] The crystal growth process is as follows. After assembling the growth furnace 1, the precious metal crucible 3 with the starting material 4 is heated by an RF coil 9 until the Ga2O3 starting material 4 melts. When the Ga2O3 melt is thermally stabilized, the oriented β-Ga2O3 seed crystal 10 is immersed into the Ga2O3 melt while rotating at a rotation speed R. Then, the oriented β-Ga2O3 seed crystal 10 is pulled upward at a travel speed TR while expanding its diameter to a predetermined cylindrical diameter D by gradually decreasing the melting temperature via an RF power source. When the crystal reaches a predetermined cylindrical length L, it is separated from the melt, slowly cooled to room temperature, and removed from the growth furnace 1.

[0038] The problem of growing conductive (normal or degraded semiconducting) bulk β-Ga2O3 single crystals by the Czochralski method as long right cylinders arises from an inherent property of the compound, namely, that heat transported through the growing crystal is absorbed by free carriers. The heat that must be dissipated from the crystal is generated at the growth interface during the liquid-solid phase transition. The interface is convex towards the melt (i.e., a conical part of the crystal is immersed in the melt) to ensure stable growth. The heat absorbed by the free carriers increases the temperature of the crystal near the growth interface, and the heat cannot be easily dissipated for a certain crystal length. When the temperature inside the crystal reaches the melting point of Ga2O3, the part of the crystal immersed in the melt melts back. This phenomenon is called interface inversion. After that interface inversion, the growth becomes unstable and the growth morphology changes from a right cylinder to a corkscrew (spiral). Such a crystal shape does not allow for the creation of wafers with a diameter as would be expected from a right cylinder, so the only part of the crystal available for wafering is its cylindrical part. The length of the cylindrical section of the highly conductive β-Ga2O3 crystal with a diameter larger than 1 inch is less than 25 mm. The high conductivity of the crystalline β-Ga2O3 crystal is 10 18 cm -3 is understood as the free electron concentration exceeding the value of

[0039] In order to increase the length of the cylindrical portion of highly conductive β-Ga2O3 crystals with diameters of over 1 inch grown by the Czochralski method, the following technical solutions have been found to be effective.

[0040] (i) A reduction in the amount of heat reflected from the internal insulation back to the growing β-Ga2O3 single crystal, and / or (ii) Dynamic decrease in the latent heat of crystallization as growth proceeds.

[0041] As shown in Figure 1, the crystal surface dissipates and exchanges heat with the surroundings in two ways, by gas convection C and radiation RA, the latter being much more effective. In radiation heat exchange, the heat leaving the crystal surface in all directions interacts with the metal crucible 3 and its lid 5 (if present), as well as the internal insulation 8, where it is partially reflected, partially absorbed, and partially transmitted. This can be described in a dimensionless scale as THA=1=R+E+T, where THA is the total heat, R is the reflectance, E is the emissivity, and T is the transmittance. Thus, the reflectance can be described as R=1-ET. To minimize the heat reflection from the internal insulation 8, it should have either a high emissivity E or a high transmittance T. These optical properties R, E, and T relate to the near infrared (NIR) region, where most of the heat is transported. Considering the melting point of Ga2O3 (1800 °C), the blackbody radiation at that temperature, and the lower temperature of the growing crystal (assuming a minimum of 1200 °C), the NIR region is between 1 and 3 μm. The requirement for the internal insulation 8 is therefore a low reflectance R in the 1-3 μm spectral region, which can be realized by high E or T values ​​in that spectral region. The optical properties of the material depend strongly on the spectral region and the temperature. For example, the emissivity E varies in the NIR spectral region in a wide range even at high temperatures, so that it is difficult to define the exact value, but smaller E values ​​in some subregions of the described NIR region do not have a large effect on the overall heat exchange. In any case, the emissivity E must be greater than 0.3 in the NIR spectral region of 1 to 3 μm, measured at room temperature. Suitable refractory materials for the internal insulation 8 are alumina, zirconia, yttria, and magnesia, preferably with a rough surface to enhance diffuse scattering.

[0042] Another solution is to increase the transmittance T of the internal insulation 8. This can be achieved by an insulation transparent to the NIR spectral region defined above. Since standard refractory materials are essentially opaque to the NIR, transparent ceramics or crystalline sapphire can be used. Both transparent ceramics, e.g. alumina and yttria, as well as crystalline sapphire in different shapes (tubes and lids) are available. For the transmittance T, the same considerations apply over wavelength and temperature as for the emissivity E. Also in this case, the transmittance T must be greater than 0.3 in the NIR spectral region from 1 to 3 μm, measured at room temperature.

[0043] Another above mentioned solution for more effective heat dissipation from the crystal is the dynamic reduction of the amount of heat generated during growth. This is done by dynamically decreasing the growth rate with the crystal length. Note that the growth rate V combines the movement rate TR of the crystal and the molten droplets that are converted into a solid.

[0044] Figure 2 shows a profile of the growth rate V that allows to reduce the latent heat of crystallization in a dynamic way. In general, at the beginning of growth, i.e. during seeding, seeding expansion (shoulder) and beginning of the cylindrical section, the growth rate V1 can be relatively high, between 1 and 10 mm / h, since the amount of heat generated is less (seeding, shoulder) or the crystal length is small enough to effectively dissipate the heat (initial cylindrical section). The growth rate is then reduced to a final value V2 of 0.2 to 1 mm / h to reduce the amount of heat generated. The reduction of the growth rate V can be done from the beginning of growth, especially when the initial growth rate is high, as shown by profiles P1, P2, P5, or later, as shown by profiles P3 and P4. It can be done linearly or nonlinearly, as a continuous reduction or in stepwise blocks shown by crystal length segments L1, L2, L3, and L4. Figure 3 shows an example of the growth rate from an actual growth experiment, where the initial V1 = 1.7 mm / h was constant at the beginning in the first blocks 0–L1, then decreased linearly in blocks L1–L3 to a final value V2 = 0.7 mm / h, which was kept constant in the last blocks L3–L4 of the crystal growth.

[0045] An additional tool for thermal management is the use of non-reducing gases with high thermal conductivity, especially He, in the growth atmosphere 2. This enhances heat dissipation from the growing crystal 7 by gas convection C. Preferably, He in the growth atmosphere 2 is used in combination with a reduction in the amount of reflected heat back to the crystal and a reduction in the amount of latent heat of crystallization. The He concentration in the growth atmosphere, in addition to oxygen, is between 10 and 95% by volume.

[0046] By using the technical solutions mentioned above, it is possible to increase the cylindrical length L of highly conductive bulk β-Ga2O3 single crystals 7 with diameters larger than 1 inch grown by the Czochralski method. Bulk β-Ga2O3 single crystals with diameter D = 2 inches could be grown to cylindrical lengths L of more than 25 mm and between 25 and 50 mm. The high electrical conductivity of bulk β-Ga2O3 single crystals can be realized by intentionally doping with elements that form shallow donors in this material. The most effective dopants are Si and Sn, but Nb, Zr, and Hf may also be used. To achieve high electrical conductivity, the Ga2O3 starting material 4 should be doped with 0.05-0.3 mol% SiO2, but due to the high partial pressure of Sn-containing species, it should be doped with 0.5-4 mol% SnO2. The dopants that form shallow donors can be used separately or in combination. The obtained electrical properties of bulk β-Ga2O3 single crystals doped with Si and / or Sn at the concentrations mentioned above were as follows (from Hall effect measurements): 1–10×10 18 cm -3 Free electron concentration, electron mobility = 50-120cm 2 V -1 s -1 , and a resistivity of 0.01-0.04 Ω cm. A 2-inch diameter bulk β-Ga2O3 single crystal7 with such electrical properties makes it possible to produce 2-inch diameter conductive wafers suitable for homoepitaxy and then for power electronic device fabrication in vertical configuration. In particular, such crystals make it possible to produce large wafers with a (010) surface orientation, which exhibits the highest growth rate of homoepitaxial films by molecular beam epitaxy. Also, other wafer orientations, such as (100) or (001), including breaks of several degrees from those orientations, can be produced. The bulk β-Ga2O3 single crystals obtained by the above-described method and apparatus are crystals of high structural quality, characterized by narrow rocking curves with full width at half maximum, typically less than 30 arc seconds, making the produced wafers suitable for high-quality homoepitaxial films.

Claims

1. Conductive bulk β-Ga by the Czochralski method with a given cylinder diameter (D) and cylinder length (L) ２ O ３ 1. A method for producing a single crystal, comprising the steps of: (i) Ga ２ O ３ A precious metal crucible (3) having therein the starting material (4) and a bulk β-Ga ２ O ３ providing a growth furnace (1) in a growth chamber, the growth furnace (1) comprising a thermal insulation (6) surrounding the crucible (3) from all sides with a free space for accommodating a single crystal (7), and an inductive RF coil (9) for heating the crucible (3) and controlling the melting temperature during crystal growth, the RF coil (9) being powered by an RF power source, the growing crystal (7) being fixed to a translation and rotation mechanism via a seed crystal (10), a seed crystal holder (11) and a pulling rod (12); (ii) The Ga ２ O ３ The dopant forming the shallow donor in the single crystal (7) is the Ga ２ O ３ Providing a starting material (4); (iii) providing to said growth chamber and said growth furnace (1) a growth atmosphere (2) comprising oxygen mixed with at least one non-reducing gas; (iv) The Ga ２ O ３ The crucible (3) containing the starting material (4) is heated, followed by the Ga ２ O ３ Melting the starting material (4); (v) immersing the oriented seed crystal (10) into the molten starting material (4); (vi) pulling said seed crystal (10) at a translation speed (TR) to achieve a predetermined growth rate (V) while rotating at a rotation speed (R); (vii) increasing the diameter of the seed crystal while pulling to a final cylindrical diameter (D) of the single crystal (7); (viii) pulling the single crystal (7) having the cylindrical diameter (D) to a predetermined cylindrical length (L); (ix) separating the single crystal (7) from the melt; and (x) cooling the growth furnace (1) containing the grown single crystal (7) to room temperature; - step (i) further comprises providing said growth furnace (1) with an internal insulation (8) having a low radiation reflectance (R) in the near infrared spectral range of 1-3 μm in order to reduce the reflection of heat back to said growing single crystal (7) and thereby increase heat dissipation from said growing single crystal (7), - the steps (vi), (vii) and (viii) of pulling the single crystal (7) from seeding to separation comprise a dynamic reduction of the growth rate (V) from an initial growth rate (V1) between 1 and 10 mm / h at the start of the growth when the single crystal (7) reaches the predetermined cylindrical length (L) to a final growth rate (V2) between 0.2 and 1 mm / h at the end of the growth, in order to dynamically reduce the latent heat of crystallization and the amount of heat dissipated from the growing single crystal (7) as the growth progresses.

2. 2. The method of claim 1, wherein the internal insulation (8) having a low radiation reflectance (R) has a high emissivity (E) in the near infrared spectral region greater than 0.3 at room temperature.

3. 2. The method of claim 1, wherein the internal insulation (8) having a low radiation reflectance (R) has a high transmittance (T) in the near infrared spectral range of greater than 0.3 at room temperature.

4. 4. The method according to claim 1, wherein the growth rate (V) decreases linearly (P1, P2) from the initial growth rate (V1) to the final growth rate (V2).

5. 4. The method according to claim 1, wherein the growth rate (V) decreases non-linearly (P5) from the initial growth rate (V1) to the final growth rate (V2).

6. 6. The method according to any one of claims 1 to 5, characterized in that the growth rate (V) decreases at different rates (P2, P5) from the initial growth rate (V1) to the final growth rate (V2).

7. 7. The method according to any one of claims 1 to 6, characterized in that the growth rate (V) decreases continuously (P1, P2, P5) from the initial growth rate (V1) to the final growth rate (V2).

8. 7. The method according to claim 1, wherein the growth rate (V) decreases (P3, P4) in blocks (L1-L4) combining constant and decreasing growth rates from the initial growth rate (V1) to the final growth rate (V2).

9. 9. The method according to any of the preceding claims, characterized in that the step (iii) of providing a growth atmosphere (2) comprises providing, in addition to oxygen, a non-reducing gas of high thermal conductivity, preferably He, in a concentration between 10 and 95% by volume.

10. β-Ga ２ O ３ Single crystals (7) formed with the cylinder diameter (D) greater than 1 inch, cylinder length (L) greater than 25 mm, and shallow donors with the following electrical properties by Hall effect measurements: 1-10×10 １８ cm －３ Free electron concentration, 50-120 cm ２ V －１ s －１ 10. A conductive bulk β-GaAsPb ... ２ O ３ Single crystal (7).

11. The bulk β-Ga according to claim 10, characterized in that its cylindrical diameter (D) is 2 inches or more. ２ O ３ Single crystal (7).

12. 12. The bulk β-GaAs semiconductor according to claim 10 or 11, characterized in that the dopants forming shallow donors are Si and / or Sn. ２ O ３ Single crystal (7).

13. Conductive bulk β-Ga by the Czochralski method with a given cylinder diameter (D) and cylinder length (L) ２ O ３ An apparatus for producing a single crystal, comprising: (i) a growth chamber; (ii) Ga ２ O ３ A precious metal crucible (3) having therein the starting material (4) and a bulk β-Ga ２ O ３ a growth furnace (1) comprising a thermal insulation (6) surrounding the crucible (3) on all sides with a free space for accommodating a single crystal (7), and an inductive RF coil (9) for heating the crucible (3) and controlling the melting temperature during crystal growth; (iii) an RF power source that supplies power to the RF coil (9); (iv) a translation and rotation mechanism coupled to the seed crystal (10) via a seed crystal holder (11) and a pulling rod (12); (v) The bulk β-Ga ２ O ３ a scale connected to the pulling rod (12) or the growth furnace (1) for monitoring the growth rate of the single crystal (7); the growth furnace further comprises an internal insulation (8) with low radiation reflectance (R) in the near infrared spectral range of 1-3 μm that reduces the reflection of heat back to the growing single crystal (7) and thereby increases the heat dissipation from the growing single crystal (7).

14. 14. The apparatus according to claim 13, wherein the internal insulation (8) having a low radiation reflectance (R) has a high emissivity (E) in the near infrared spectral range of greater than 0.3 at room temperature.

15. 15. Apparatus according to claim 13 or 14, characterized in that the internal insulation (8) having a low radiation reflectance (R) is selected from the group consisting of opaque alumina, zirconia, magnesia and yttria.

16. 14. The device according to claim 13, wherein the internal insulation (8) having a low radiation reflectance (R) has a high transmittance (T) in the near infrared spectral range above 0.3 at room temperature.

17. 17. The device according to claim 13 or 16, characterized in that the internal insulation (8) having a low radiation reflectance (R) is a transparent ceramic selected from the group consisting of alumina, yttria and yttrium aluminum garnet.

18. 17. Apparatus according to claim 13 or 16, characterized in that the internal insulation (8) having a low radiation reflectance (R) is crystalline sapphire.

19. Apparatus according to any of claims 13 to 18, characterized in that the growth atmosphere (2) contains, in addition to oxygen, a non-reducing gas of high thermal conductivity, preferably He, in a concentration between 5 and 95% by volume.

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