Patent Application
20190352798
The present invention relates to crystalline material of β-Ga2O3 doped with group VB elements and preparation method and application thereof.
β-Ga2O3 is a direct wide bandgap semiconductor material with a bandgap of approximately 4.8-4.9 eV. The material has many advantages including wide band gap, fast drift velocity of saturated electrons, high thermal conductivity, high breakdown stress, and stable chemical properties, and the region from the deep ultraviolet (DUV) to the infrared (IR) is transparent. Compared with traditional transparent conductive materials, the material based on β-Ga2O3 may be used to prepare a new generation of semiconductor optoelectronic devices with shorter wavelength.
Pure β-Ga2O3 crystals exhibit semi-insulating or weakly n-type conduction. Currently, known major method to improve the n-type conductivity of the β-Ga2O3 crystals is to carry out 4+ ion doping, which mainly includes Si, Hf, Ge, Sn, Zr and Ti plasma doping of the fourth main group and the fourth subgroup. Taking Si as an example, the main mechanism of improving carrier concentration is as follows:
The equation shows that the theoretical limitation of Group IV Element doping to provide free electrons is 1:1. With the increase in doping concentration, the difficulty of crystalline crystallization increases, and the improvement of conductivity is limited.
Among Group IV elements, Si and Sn are the two commonly used for doping. The use of Si doped crystals is disclosed in U.S. Patent Document US20070166967A1 and Japanese Patent Document JP2015083536A. Although the above two documents report resistivity in the 2.0×10−3 to 8.0×102Ω·cm range, the lowest resistance rate is only theoretical. Since the radius of Si4+ and Ga3+ is very different, with the increase of Si doping concentration, a second phase will precipitate out, leading to the decrease of crystalline quality. US20070166967A1 and JP2015083536A also only disclose Si doping concentration at about 0.2 mol % in the doped single β-Ga2O3 crystal. Referring to Applied Physics Letters, 2008, 92, 202120.
Sn doped single crystals β-Ga2O3 have been reported in the journal (Thin Solid Films, 2008, 516 (17), 5763-5767). However, due to the high volatility of tin oxides, the Sn content in the obtained crystals is only at the order of PPM even if 2-10 mol % Sn is added to the raw material ratio. It not only brings great difficulty to control its content and uniformity, but also causes equipment pollution by the volatilization of tin oxide.
Therefore, how to prepare high conductivity doping β-Ga2O3 crystalline in a simple way has become an important research topic in this field.
The present invention solves the problem by overcoming the limitation on the degree of improvement in the crystalline conductivity of the doped Group IV elements. In order to prepare high conductivity Group IV element doped crystals, it is difficult to crystallize and the process conditions are harsh. The present invention provides a class of Group VB element doped β-Ga2O3 crystalline material, preparation method and application thereof. The crystalline material doped with Group VB elements of the present invention exhibits n-type conductive characteristics, and high conductivity β-Ga2O3 materials may be prepared by conventional process.
Generally, doping ions with higher valence state than Ga3+ in β-Ga2O3 crystalline can improve the conductivity to some extent. However, it is generally believed that if the doping ion valence state is too high, the charge is difficult to balance, and it is easy to produce more doping defects. Defects can trap electrons, significantly reduce the number of carriers that can move freely, fail to effectively achieve the purpose of improving the conductivity by doping high-priced ions, and seriously affect the application performance of materials. Therefore, currently technologies usually use Group IV elements with one higher valence than Ga3+ to dope β-Ga2O3 crystals, and Group VB elements have not been reported to dope crystals.
However, through scientific design and experimental verification, the inventor of the present invention has found that certain amount of +5 valent Group VB (M=Nb, Ta, V, etc.) metal ion doped crystalline can provide more free electrons than the commonly used +4 valent ions, thus improving the carrier concentration and thereby facilitating the improvement of the conductivity. Moreover, the conductivity of the material can be regulated by the content control of doped element M. The main defect reaction mechanism is as follows:
As seen from the above equation, the theoretical limiting capacity of free electron supply by doping elements of Group VB can be 1:2, and the improvement in conductivity is significantly greater than that of Group IV elements. Therefore, crystals can be obtained at high doping concentration by using conventional process.
Further, the inventor of the present invention has found that after annealing the β-Ga2O3 crystalline doped with Group VB elements, the oxygen vacancy in the crystalline can be removed, and the control range of carrier concentration can be increased, providing a basis for expanding its application range.
The present invention solves the above technical problems through the following technical solutions.
The present invention provides a Group VB element-doped β-Ga2O3 crystalline material, doped elements M=Nb, Ta, V, etc. The doped crystalline belongs to monoclinic crystalline system and the space group is C2/m. Resistivity of doped materials described in 2.0×10−4 to 1×104Ω·cm range and/or Carrier concentrations range from 5×1012 to 7×1020/cm3.
In the present invention, the term crystalline material refers to a solid material whose internal structure presents a long range order, including solid crystal and liquid crystal where solid state material is the main component, etc. Among the crystal materials, crystalline material is categorized according to the macroscopic aggregate state and particle diameter of the crystalline particles during the crystallization process to, including, single crystal, thin film, polycrystalline (powder crystal), eutectic, microcrystalline, and nano crystalline, etc. In the present invention, there is no special limitation on the macroscopic existing form of the crystalline material, such as powder, particle, film, etc.
In the present invention, the molecular formula of the Group VB element doped crystalline material is Ga2(1-x)M2xO30.0.000000001≤x≤0.01, preferably, the range is 0.000001≤x≤0.01.
In the present invention, the M doped β-Ga2O3 crystalline material is preferably M doped β-Ga2O3 crystal, and more preferably, M doped β-Ga2O3 single crystal.
In the present invention, the resistivity range of M doped crystalline material is preferably from 2.0×10−3 to 3.6×102Ω·cm. If M is Ta, the preferred range is from 4×10−3 to 7.9Ω·cm. If M is Nb, the preferred range is from 5.5×10−3 to 36Ω·cm. If M is V, the preferred range is from 3×10−2 to 50Ω·cm.
In the present invention, the preferred carrier concentration range of M doped crystalline material is from 3.7×1015 to 6.3×1019/cm3. If M is Ta, the preferred range is from 3.7×1015 to 3.0×1019/cm3. If M is Nb, the preferred range is from 9.55×1016 to 1.8×1019/cm3. If M is V, the preferred range is from 5×1015 to 3.69×1018/cm3.
Doping scheme provided by the present invention may be obtained by the conventional crystal growth method. According to the conventional method in the field, M2O5 and Ga2O3 with purity above 4N may be mixed according to a molar ratio (0.000000001-0.01):(0.9999999-0.99) for crystal growth.
In the present invention, the term purity refers to the mass fraction of M2O5 or Ga2O3 in the sample. When the raw material purity of M2O5 or Ga2O3 is lower than the required purity, the conductivity of the final crystalline material is affected due to excessive impurities.
In the present invention, the purity of M2O5 and Ga2O3 is better than 5N, that is, the mass content of M2O5 or Ga2O3 in the sample is 99.999%. When the M doped crystalline material is M doped single crystal, the purity of Ga2O3 used in the preparation process is better than 6N, that is, the mass content of Ga2O3 in the sample is 99.9999%.
In the present invention, the M doped β-Ga2O3 crystalline material can be further annealed to remove defects such as oxygen vacancy in the crystal and increase the control range of the carrier concentration. Conventional annealing temperature and time may be used, such as 1000° C. to 1200° C. and annealing time of 3-10 hrs.
In the present invention, M doped β-Ga2O3 crystalline material can contain impurity elements that are unavoidable during the refining process of the raw materials and impurity elements that are inevitably mixed in during the process. Compared with all compositions, the above impurity element content is preferably below 10 ppm.
In the present invention, the β-Ga2O3 crystal growth method and condition used for preparing doped crystalline materials are not specifically limited, which may be conventional crystal growth method and condition in the field. When the doped β-Ga2O3 crystalline material is a single crystal, the melt method commonly used in the field is used to grow the single crystal. Generally, the melt growth method is to introduce the seed crystal into the melt and control the nucleation of the single crystal, and then carry out the phase transition at the phase interface between the seed crystal and the melt to promote the continuous growth of the crystal, including the Czochralski, edge-defined film fed growth, Bridgman-Stockbarger Method, the optical floating zone method, the flame melting method and so on. The optical floating zone method and edge-defined film fed growth method are both simple and efficient methods. The examples of the present invention adopt the optical floating region method.
Among them, the steps of preparing the M-doped β-Ga2O3 single crystal by the optical floating zone method generally include mixing, rod making, sintering, and crystal growth.
In the process, the mixing may be carried out by a mixing method conventionally used in the art, such as wet mixing. The kind and amount of the solvent to be used in the wet mixing are not particularly limited as long as the M2O5 and Ga2O3 can be uniformly mixed and easily removed later, and a volatile solvent such as ethanol is generally used. After dispersing and mixing M2O5 and Ga2O3 in a volatile solvent, the solvent can be completely volatilized by baking. In order to make the mixing of M2O5 and Ga2O3 more uniform, the wet mixing may also be carried out by a wet ball milling process, which may be conventional in the art, for example 12-24 hrs.
In the process, the pressure bar can be operated in a manner conventional in the art, and the pressure bar is generally performed using an isotactic press. Those skilled in the art know that the mixture of M2O5 and Ga2O3 is powdery and can be easily pressed, and can make the pressing uniform. Therefore, if the mixture has agglomeration before pressing, it can be ground into a powder by grinding, such as by ball milling.
In an embodiment of the present invention, M2O5 having a purity of 4N and Ga2O3 having a purity of 6N are mixed at a molar ratio of (0.000001-0.01):(0.999999-0.99), and wet ball milling is conducted by adding an appropriate amount of absolute ethanol; ball milling time is 12-24 hrs, so that M2O5 and Ga2O3 are thoroughly mixed. Then, the mixture obtained is baked at 80-100° C. for 3-6 hrs to completely evaporate the ethanol, and then the dried mixture is ball-milled into powdery shape for use in a pressure bar.
In the process, the sintering can be carried out according to the conventional sintering temperature and time in the art to remove moisture in the M2O5 and Ga2O3 mixture, and to solid phase reaction of M2O5 and Ga2O3 to form a polycrystalline material. The sintering temperature is preferably from 1400 to 1600° C., and the sintering time is preferably from 10 to 20 hours. The sintering is generally carried out in a muffle furnace.
In the process, the crystal growth atmosphere is preferably a vacuum, an inert atmosphere, or an oxidizing atmosphere to ensure the valence state of the Group VB metal M ions. The inert atmosphere may be an inert atmosphere conventional in the art, such as a nitrogen atmosphere or an argon atmosphere; the oxidizing atmosphere may be an oxidizing atmosphere conventional in the art, such as an oxygen atmosphere or an air atmosphere.
Among them, the preparation of the Group VB metal M-doped β-Ga2O3 single crystal is usually carried out by a melt method. Generally, the β-Ga2O3 crystal is used as a seed crystal, and the melt is formed by melting the sintered M2O5 and Ga2O3 polycrystalline materials, and the melt is gradually cooled and crystallized along the seed crystal to form a single crystal. Specific methods include a floating zone method, edge-defined film fed growth, a temperature ladder method, a Czochralski method, a Bridgman-Stockbarger Method, and the like.
In one embodiment of the present invention, the growing Group VB metal M-doped β-Ga2O3 single crystal is carried out by a floating zone method, and the following steps are performed. The sintered M2O5 and Ga2O3 polycrystalline rods are loaded into the floating zone furnace as the upper rod, and the Ga2O3 crystal in the <010> direction is used as the seed crystal. Firstly, raise the temperature to melt the seed crystal, then contacts the loading rod, adjusts the rotation speed and rotation direction of the rod and the seed crystal, inoculates, starts crystal growth. The growth rate of the crystal is 4.5-6 mm/hr, the rotation speed is 8-12 rpm, and the growth atmosphere is an air atmosphere. After the crystal growth is completed, the melting zone is pulled, slowly lowered to room temperature, and the crystal is taken out.
In addition to the <010> direction, the seed crystal used in the present invention may also be other crystal orientation β-Ga2O3 crystals (such as <001> direction, etc.).
The present invention also provides a Group VB metal M-doped β-Ga2O3 crystalline material obtained by the above preparation method.
The present invention also provides the use of the M-doped β-Ga2O3 crystalline material on power electronic devices, optoelectronic devices, photo catalysts, or conductive substrates. The present invention employs various applicable crystal faces including β-Ga2O3 crystalline materials including (100), (−201), and (010).
In the present invention, the optoelectronic device comprises a transparent electrode, a solar panel, a light emitting device, a photodetector, a sensor, etc. The conductive substrate includes a substrate material as a GaN-based and/or AlN-based material, a substrate material of β-Ga2O3 itself, and the like.
In addition, the Group VB element doped gallium oxide crystalline material of the present invention is also applicable to other crystalline gallium nitride crystalline materials (such as germanium α-Ga2O3, etc.) and other gallium nitride amorphous materials.
Based on the common knowledge in the art, the above various preferred conditions can be arbitrarily combined to obtain preferred embodiments of the present invention.
The reagents and starting materials used in the present invention are commercially available.
The positive progress made in the present invention are:
(1) The limit ability of the present invention for the free electrons of the germanium β-Ga2O3 doped with a 5-valent Group VB metal ion is 1:2, which is significantly higher than the ability of the +4 valence ion doping to provide free electrons (1:1). Therefore, more free electrons can be provided at the same doping concentration, which is more favorable for increasing the carrier concentration and improving the conductivity.
(2) The present invention adopts a 5-valent Group VB metal ion doped crystalline β-Ga2O3, and the conductivity of the β-Ga2O3 crystalline material can be controlled by controlling the content of the doping element M. The resistivity of Ta-doped β-Ga2O3 crystalline material can be controlled within a range of 2.0×10−4 to 1×104Ω·cmm and the carrier concentration can be controlled within a range of 5×1012 to 7×1020/cm3; the resistivity of Nb-doped β-Ga2O3 crystalline material can be controlled within a range of 2.5×10−4 to 1×104Ω·cm, and the carrier concentration can be controlled within a range of 5×1012 to 5.6×1020/cm3; the resistivity of V-doped β-Ga2O3 crystalline material can be controlled within a range of 2.0×10−4 to 1×104Ω·cm, and the carrier concentration can be controlled within a range of 5×1012 to 7×1020/cm3.
(3) The Group VB metal-doped β-Ga2O3 crystalline material of the present invention can be prepared by a conventional method in the art without expensive raw materials and demanding processes.
(4) After annealing the Group VB metal doped crystalline, the oxygen vacancies in the crystal lattice can be removed, and the control range of the carrier concentration increases, thereby providing a basis for its application.
The present invention is further illustrated in the following examples, without thereby limiting the present invention to the scope of the examples. The experimental methods in the following examples which do not specify the specific conditions are selected according to conventional methods and conditions, or according to the product specifications.
In the following examples, the starting materials and reagents used are commercially available.
A Ta-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Ta2xO3 (x=0.000001) and belonging to the monoclinic system and a space group of C2/m is prepared by the method and specific steps as follows:
(1) Ingredients: Ga2O3 with a purity of 6N or more and Ta2O5 with a purity of 4N or more are weighed according to a molar ratio of 0.999999:0.000001;
(2) Mixing: put the weighed raw materials into a clean PTFE ball mill jar, put into a high-purity corundum ball, pour the appropriate amount of absolute ethanol, seal, put into a ball mill, and mix for 12 hrs;
(3) Drying: the ball grinding tank is placed in an oven, baked at 80° C. for 6 hrs, the ethanol is completely volatilized, and then placed in a ball mill for 10 minutes to grind the dried bulk material into a powder;
(4) Pressure bar: the dried mixed powder is placed in an organic mold and pressed into a material bar by using an isotactic press;
(5) Sintering: the pressed rod is placed in a muffle furnace and sintered at 1500° C. for 10 hrs, the moisture in the raw material is removed, and the Ta2O5 and the Ga2O3 raw material are subjected to solid phase reaction to form a polycrystalline material;
(6) Crystal growth: the sintered polycrystalline rod is placed in a floating zone furnace as an upper rod, and the Ga2O3 crystal in the <010> direction is placed below as a seed crystal. First, temperature is raised to melt the seed crystal, and then the seed crystal is contacted with the upper rod to reach stabilization and initiate crystal growth. The crystal growth rate is 5 mm/hr, the rotation speed is 10 rpm, and the growth atmosphere is air atmosphere. After the crystal growth is completed, stop the dropping of the loading rod, and cause the gradual separation of the melting zone by naturally dropping the crystal underneath. After about 1 hour, the temperature slowly drops to room temperature, and the crystal is taken out. The primary crystal obtained therefrom is intact with even color and no cracking;
(7) Annealing: The resulting primary crystals are annealed at 1000° C. for 3 hrs.
A Ta-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Ta2xO3 (x=0.00005) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the doping concentration of Ta2O5 in step (1) is different, and the molar ratio of Ga2O3 to Ta2O5 is 0.99995:0.00005.
A Ta-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Ta2xO3 (x=0.001), belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the doping concentration of Ta2O5 in step (1) is different, and the molar ratio of Ga2O3 to Ta2O5 is 0.999:0.001. Additionally, no annealing operation is performed.
A Ta-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Ta2xO3(x=0.01) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the doping concentration of Ta2O5 in step (1) is different, and the molar ratio of Ga2O3 to Ta2O5 is 0.99:0.01. No annealing operation is performed.
A Nb-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Nb2xO3 (x=0.000001) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the dopant (Nb2O5) and doping concentration of Nb2O5 in step (1) are different.
A Nb-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Nb2xO3 (x=0.00001) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the dopant (Nb2O5) and doping concentration of Nb2O5 in step (1) are different.
A Nb-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Nb2xO3 (x=0.0001) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the dopant (Nb2O5) and doping concentration of Nb2O5 in step (1) are different. No annealing operation is performed.
A Nb-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Nb2xO3 (x=0.002) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the dopant (Nb2O5) and doping concentration of Nb2O5 in step (1) are different. No annealing operation is performed.
A Nb-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)Nb2xO3 (x=0.008) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the dopant (Nb2O5) and doping concentration of Nb2O5 in step (1) are different. No annealing operation is performed.
A V-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)V2xO3 (x=0.01) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the dopant (V2O5) and doping concentration of V2O5 in step (1) are different. No annealing operation is performed.
A V-doped β-Ga2O3 single crystal with a molecular formula of Ga2(1-x)V2xO3 (x=0.00001) and belonging to the monoclinic system and a space group of C2/m is prepared by the same steps and conditions as in Example 1, except that the dopant (V2O5) and doping concentration of V2O5 in step (1) are different. No annealing operation is performed.
A pure β-Ga2O3 single crystal without Ta2O5 doping is prepared by the same steps and conditions as in Example 1.
The M-doped maple-Ga2O3 single crystals obtained in Examples 1-11 and the pure Ga2O3 single crystal in the comparative example are cut into samples with the size of 5 mm×5 mm×0.3 mm. Four (4) Indium (In) electrodes are fabricated on four corners of each of the samples and Hall measurements are taken. All samples show N-type conductivity and detail results of carrier concentration and resistivity as shown in Table 1.
According to the data from Table 1, the pure crystal of Ga2O3 is almost insulated after annealing. Compared with the pure crystal, the carrier concentration and conductivity of Ga2O3 single crystal doped with Group VB elements increase significantly, and the carrier concentration increase at least 103 times and the resistivity decrease at least 500 times, indicating that M metal ions have been successfully doped into the lattice of Ga2O3 and achieve desired control effect.
Further, in order to study the relationship between different doping concentrations and carrier concentration or resistivity, the Ta2O5 doping concentration-carrier concentration-resistivity curve of samples without annealing in Examples 1-4 are shown in
It can be seen from
It should be noted that the carrier concentration and resistivity of the above Group VB elements doped Ga2O3 single crystal are obtained by the specific experiments of the present invention. Due to the influence of the purity of raw materials, preparation technology and test conditions, the measured carrier concentration and resistivity of the doped crystal will be different from the theoretical value, or there is a situation that can not be detected. Therefore, the above examples are merely illustrations. According to the Group VB element doping concentration disclosed by the present invention and the common knowledge in the field, it can be inferred that the carrier concentration of the Group VB element doped Ga2O3 crystalline material can be controlled in the range of 5×1012 to 7×1020/cm3, and the resistivity can be controlled in the range of 2.0×10−4 to 1×104Ω·cm. Taking Ta as an example, the specific calculation process is as follows.
According to the experiment of the present invention, the maximum value of Ta-doping concentration in the Ga2O3 single crystal is 1 at % (i.e. x=0.01). The volume of 1 mol-Ga2O3 is 184.44/5.94 cm3=31 cm3;
The number of Ta atoms in 1 mol Ta-doped Ga2O3 is: 1×2×1%×6.023×1023=1.2×1022; therefore, the theoretical value of carrier concentration of Ta-doped Ga2O3 is 2×1.2×1022/31=7.7×1020/cm3.
The Hall effect instrument with low resistance module is used to test the resistivity below the limit value of 105Ω·cm. The experiment of the present invention shows that the resistivity of 6N pure Ga2O3 crystal exceeds the test limit after annealing, indicating that the resistivity of 6N pure Ga2O3 crystal exceeds 105Ω·cm. Therefore, the resistivity of 6N pure Ga2O3 doped with Ta can be controlled to 1×104Ω·cm, which is 1/1266 of Example 1. The carrier concentration in the example is multiplied by 1/1266, 3×1012/cm3 can be obtained, so it is feasible to achieve 5×1012/cm3 carrier concentration in Ta-doped —Ga2O3 crystal material. The doping concentration of Ta corresponding to the doping concentration is 10−7 at %.
Therefore, the amount of Ta doping in Ta-doped Ga2O3 crystalline materials can be controlled in the range of x=0.000000001 to 0.01, the resistivity can be controlled in the range of 2.0×10−4 to 1×104Ω·cm, and the carrier concentration can be controlled in the range of 5×1012 to 7×1020/cm3.
Other doped elements including Nb and V may be calculated by the same method.
The examples of the present invention are described above. However, the examples mentioned are only examples for illustration and are not intended to limit the present invention. Technicians in the field may make a number of changes and modifications without departing from the spirit and scope of the present invention. The scope of protection advocated by the present invention shall prevail as described in the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201710061035.X | Jan 2017 | CN | national |
| 201710124917.6 | Mar 2017 | CN | national |
The subject application is a continuation-in-part of PCT/CN2018/074058 filed on Jan. 24, 2018, which in turn claims priority on Chinese Application Nos. CN201710061035.X filed on Jan. 25, 2017 and CN201710124917.6 filed on Mar. 3, 2017 in China. The contents and subject matter of the PCT international application and the Chinese priority application are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/CN2018/074058 | Jan 2018 | US |
| Child | 16508211 | US |
