JP2014031300A - Gallium oxide substrate and manufacturing method of the same - Google Patents
Gallium oxide substrate and manufacturing method of the same Download PDFInfo
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- 229910001195 gallium oxide Inorganic materials 0.000 title claims abstract description 130
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- JMASRVWKEDWRBT-UHFFFAOYSA-N Gallium nitride Chemical compound [Ga]#N JMASRVWKEDWRBT-UHFFFAOYSA-N 0.000 description 40
- 229910002601 GaN Inorganic materials 0.000 description 39
- 239000010409 thin film Substances 0.000 description 34
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- 230000000052 comparative effect Effects 0.000 description 15
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- MKYBYDHXWVHEJW-UHFFFAOYSA-N N-[1-oxo-1-(2,4,6,7-tetrahydrotriazolo[4,5-c]pyridin-5-yl)propan-2-yl]-2-[[3-(trifluoromethoxy)phenyl]methylamino]pyrimidine-5-carboxamide Chemical compound O=C(C(C)NC(=O)C=1C=NC(=NC=1)NCC1=CC(=CC=C1)OC(F)(F)F)N1CC2=C(CC1)NN=N2 MKYBYDHXWVHEJW-UHFFFAOYSA-N 0.000 description 9
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- GYHNNYVSQQEPJS-UHFFFAOYSA-N Gallium Chemical compound [Ga] GYHNNYVSQQEPJS-UHFFFAOYSA-N 0.000 description 5
- 238000002441 X-ray diffraction Methods 0.000 description 5
- 239000000463 material Substances 0.000 description 5
- 238000004528 spin coating Methods 0.000 description 5
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- HZAXFHJVJLSVMW-UHFFFAOYSA-N 2-Aminoethan-1-ol Chemical compound NCCO HZAXFHJVJLSVMW-UHFFFAOYSA-N 0.000 description 3
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- 239000004065 semiconductor Substances 0.000 description 3
- 239000002904 solvent Substances 0.000 description 3
- XNWFRZJHXBZDAG-UHFFFAOYSA-N 2-METHOXYETHANOL Chemical compound COCCO XNWFRZJHXBZDAG-UHFFFAOYSA-N 0.000 description 2
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- SMZOGRDCAXLAAR-UHFFFAOYSA-N aluminium isopropoxide Chemical compound [Al+3].CC(C)[O-].CC(C)[O-].CC(C)[O-] SMZOGRDCAXLAAR-UHFFFAOYSA-N 0.000 description 2
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- JBRZTFJDHDCESZ-UHFFFAOYSA-N AsGa Chemical compound [As]#[Ga] JBRZTFJDHDCESZ-UHFFFAOYSA-N 0.000 description 1
- BVKZGUZCCUSVTD-UHFFFAOYSA-L Carbonate Chemical compound [O-]C([O-])=O BVKZGUZCCUSVTD-UHFFFAOYSA-L 0.000 description 1
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- Crystals, And After-Treatments Of Crystals (AREA)
- Formation Of Insulating Films (AREA)
Abstract
Description
本発明は、酸化ガリウム基板及びその製造方法に関する。 The present invention relates to a gallium oxide substrate and a manufacturing method thereof.
窒化ガリウム（GaN）系の発光ダイオード（LED）は、紫外領域の重要な固体発光源である。他のLED材料（例えば、リン化インジウム、ガリウム砒素）とは異なり、高品質のGaNウエハが広く普及していないので、GaNは一般に高品質のサファイア（Al2O3）基板上で成長させており、サファイアは従来市販されているGaNエピタキシャル基板の中で最良の選択であった。 Gallium nitride (GaN) based light emitting diodes (LEDs) are important solid state light sources in the ultraviolet region. Unlike other LED materials (eg, indium phosphide, gallium arsenide), high quality GaN wafers are not widely used, so GaN is generally grown on high quality sapphire (Al 2 O 3 ) substrates. Therefore, sapphire was the best choice among GaN epitaxial substrates commercially available.
GaN on Sapphire LEDでは、ｎ型パッド電極を介してｎ型層にコンタクトさせるためにGaNの一部を除去する必要がある。これはサファイアに導電性が無いためである。電流はｐ型パッド電極、ｐ型層から量子井戸層を通り、ｎ型層を通ってｎ型パッド電極まで通る。電流はｎ型層を横方向に流れるので、このタイプの構造は横型構造と呼ばれている。 In the GaN on Sapphire LED, it is necessary to remove a part of GaN in order to contact the n-type layer via the n-type pad electrode. This is because sapphire has no conductivity. The current passes from the p-type pad electrode and p-type layer through the quantum well layer, through the n-type layer to the n-type pad electrode. Since current flows laterally through the n-type layer, this type of structure is called a lateral structure.
しかし、横型構造では、電流の流れが最適ではなく、特に高輝度を得るために大電流が流れると、わずか数μｍの厚さのｎ型層に電流が集中するため、また、サファイア基板の放熱性が不十分であるため、素子温度が上昇し、発光効率や素子寿命の低下を招いてしまう。しかも、大きなLEDチップは動作するためにより多くの入力電流を必要とするので、この問題はLEDのチップサイズが大きくなるにしたがって非常に深刻になる。 However, in the horizontal structure, the current flow is not optimal, and particularly when a large current flows in order to obtain high brightness, the current concentrates on an n-type layer that is only a few μm thick. Insufficient performance increases the element temperature, leading to a decrease in light emission efficiency and element lifetime. Moreover, this problem becomes more serious as the LED chip size increases because larger LED chips require more input current to operate.
上記の課題を踏まえて、特に高輝度型LEDにおいては、基板の裏面に電極を形成することで、大電流を広い断面積に流して電流の集中を抑えることが出来る、縦型構造の発光素子を作製するために、電気伝導性を持つ基板が求められてきた。 Based on the above issues, especially in high-brightness LEDs, a light emitting device with a vertical structure that can suppress the concentration of current by flowing a large current through a wide cross-sectional area by forming an electrode on the back surface of the substrate In order to fabricate, a substrate having electrical conductivity has been demanded.
そこでサファイアに代わる基板材料として、酸化ガリウムが考案されている（例えば、特許文献１を参照）。酸化ガリウム単結晶を基板材料に用い、その基板の表面を窒化処理してGaN層を形成することにより、発光素子を作製することが出来る。 Therefore, gallium oxide has been devised as a substrate material to replace sapphire (see, for example, Patent Document 1). By using gallium oxide single crystal as a substrate material and nitriding the surface of the substrate to form a GaN layer, a light-emitting element can be manufactured.
中でもβ型酸化ガリウム単結晶は、4.8eVのワイドバンドギャップを有して可視領域において透明であると共に、結晶中に酸素欠損が生ずることでn型半導体として導電性を有するため、垂直構造型の発光素子が作製可能となり、電流集中を抑えて従来よりも高輝度な発光素子を得ることが出来る等、サファイア基板とは異なる素子開発の可能性も備えている。 Above all, β-type gallium oxide single crystal has a wide band gap of 4.8 eV, is transparent in the visible region, and has conductivity as an n-type semiconductor due to oxygen deficiency in the crystal. A light-emitting element can be produced, and a light-emitting element with higher brightness than the conventional one can be obtained by suppressing current concentration.
酸化ガリウム単結晶はバルク状の大型単結晶が比較的容易に得られ、且つ前記の通り導電性を有し、発光領域で光透過性を有する。従って、酸化ガリウム単結晶基板から作製した縦型構造の発光素子は、サファイア基板から作製される横型構造の発光素子が有する従来の問題を解決することが可能である。 As the gallium oxide single crystal, a bulky single crystal can be obtained relatively easily, and has conductivity as described above, and has light transmission in the light emitting region. Accordingly, a vertical structure light-emitting element manufactured from a gallium oxide single crystal substrate can solve the conventional problems of a horizontal structure light-emitting element manufactured from a sapphire substrate.
ところで、GaN系薄膜を成長させる際の基板の表面粗さは、例えば特許文献２の記載によれば10nm以下が好ましいとされており、これはサファイア基板に限らず酸化ガリウム基板も同様である。 Incidentally, the surface roughness of the substrate when growing the GaN-based thin film is preferably 10 nm or less, for example, according to the description in Patent Document 2, and this is not limited to the sapphire substrate, but also the gallium oxide substrate.
また、一般的なサファイア基板の場合と同様に、酸化ガリウム基板面上にGaN系薄膜をエピタキシャル成長させる際に、基板の最表面にコンタミネーション（パーティクル、金属原子、有機汚染、自然酸化膜など）が存在すると、このコンタミネーションが核となり、エピタキシャル成長されたGaN系薄膜の結晶中に結晶欠陥が発生する。また、前記GaN系薄膜に発生した結晶欠陥の密度がそれほど大きくない場合でも、GaN系薄膜の結晶中に埋没される前記コンタミネーションが、GaN系薄膜の結晶を用いた半導体素子の特性に悪影響を与えてしまう。従って、GaN系薄膜の成長前に、酸化ガリウム基板の表面からコンタミネーションを取り除く清浄化を行わなければならない。 In addition, as in the case of a general sapphire substrate, when epitaxially growing a GaN-based thin film on the gallium oxide substrate surface, contamination (particles, metal atoms, organic contamination, natural oxide film, etc.) is present on the outermost surface of the substrate. If present, this contamination becomes a nucleus, and crystal defects are generated in the crystal of the epitaxially grown GaN-based thin film. Even if the density of crystal defects generated in the GaN-based thin film is not so high, the contamination buried in the crystal of the GaN-based thin film adversely affects the characteristics of the semiconductor device using the GaN-based thin film crystal. I will give it. Therefore, before the growth of the GaN-based thin film, it is necessary to perform cleaning to remove the contamination from the surface of the gallium oxide substrate.
酸化ガリウム基板の清浄化は、一般的なサファイア基板の場合と同様に、GaN系薄膜結晶の成長装置の外部で行われる洗浄工程、または前記成長装置の内部で行われるサーマルクリーニングによって行われる。前者の成長装置の外部で行われる洗浄工程は、基板を酸や過酸化水素等の薬液で処理することによって行われるもので、基板表面に付着しているコンタミネーションの大部分を除去するのが目的である。 As in the case of a general sapphire substrate, the gallium oxide substrate is cleaned by a cleaning process performed outside the GaN-based thin film crystal growth apparatus or by thermal cleaning performed inside the growth apparatus. The cleaning process performed outside the former growth apparatus is performed by treating the substrate with a chemical solution such as acid or hydrogen peroxide, and most of the contamination adhering to the substrate surface is removed. Is the purpose.
しかし半導体素子の製造工程中の他のプロセスとは異なり、GaN系薄膜のエピタキシャル成長時に要求される基板表面の清浄度は非常に高い。すなわち、化学洗浄工程で除去しきれないコンタミネーションを除去すること、また、化学洗浄工程を経た後に基板と大気中の酸素との反応によって形成される酸化膜や再付着したコンタミネーションを除去することが求められる。そこで、エピタキシャル成長の直前に成長装置のチャンバ内部で行われるのがサーマルクリーニングである。 However, unlike other processes in the manufacturing process of semiconductor elements, the cleanliness of the substrate surface required during the epitaxial growth of GaN-based thin films is very high. That is, to remove the contamination that cannot be removed by the chemical cleaning process, and to remove the oxide film formed by the reaction between the substrate and oxygen in the atmosphere after the chemical cleaning process and the reattached contamination. Is required. Therefore, thermal cleaning is performed inside the chamber of the growth apparatus immediately before the epitaxial growth.
サーマルクリーニングは、前記チャンバ内に酸化ガリウム基板を保持し、特定のガス雰囲気下で酸化ガリウム基板を加熱することにより行われ、基板表面を清浄化することができる。サーマルクリーニング後は、酸化ガリウム基板を成長装置外部に取り出すことなく、そのままGaN系薄膜成長工程へと移行することができる。従って、サーマルクリーニングは結晶欠陥が低減された、より高品質のGaN系薄膜の成長を行う上で、極めて重要な基板の清浄化作業である。 The thermal cleaning is performed by holding the gallium oxide substrate in the chamber and heating the gallium oxide substrate in a specific gas atmosphere, thereby cleaning the substrate surface. After the thermal cleaning, it is possible to proceed directly to the GaN-based thin film growth step without taking the gallium oxide substrate out of the growth apparatus. Therefore, thermal cleaning is an extremely important substrate cleaning operation for growing a higher quality GaN-based thin film with reduced crystal defects.
一般に、サーマルクリーニングの雰囲気には、水素や、アンモニア、窒素、不活性ガス、およびこれらの混合ガスが用いられる。中でも、雰囲気に水素を使用し、且つ雰囲気中の水素濃度を高くする程、不純物や酸化膜の除去効果が高くなることが知られているため（例えば、特許文献３参照）、サーマルクリーニングには水素を含む雰囲気を用いることが好ましい。また、加熱温度はより高温であるほど大きな清浄効果を期待でき、一般的なサファイア基板の場合には1000℃程度で行われることが多い。 In general, hydrogen, ammonia, nitrogen, an inert gas, and a mixed gas thereof are used for the atmosphere of thermal cleaning. Among them, it is known that the effect of removing impurities and oxide films increases as hydrogen is used in the atmosphere and the hydrogen concentration in the atmosphere is increased (see, for example, Patent Document 3). It is preferable to use an atmosphere containing hydrogen. Further, the higher the heating temperature is, the higher the cleaning effect can be expected. In the case of a general sapphire substrate, it is often performed at about 1000 ° C.
しかし、酸化ガリウム基板のサーマルクリーニングに水素雰囲気を用いる場合、水素と酸化ガリウムとが、以下のような反応を起こして酸化ガリウム基板がエッチングされてしまう。
前記反応が始まる温度は、雰囲気中の水素の分圧により若干の変動があるが600℃前後である。このためサファイア基板と同様のサーマルクリーニングを行おうとしても、水素との反応によって酸化ガリウムが還元されGa2O を生じると、Ga2Oは蒸気圧が高いために蒸発し、酸化ガリウム基板表面がエッチングされて荒れてしまい、基板表面の平坦性が失われてGaN系薄膜の結晶成長が困難になってしまう。 The temperature at which the reaction starts is about 600 ° C., although there is a slight variation depending on the partial pressure of hydrogen in the atmosphere. Therefore even attempting to thermal cleaning similar to the sapphire substrate and produce Ga 2 O gallium oxide is reduced by reaction with hydrogen, Ga 2 O evaporates due to the high vapor pressure, the gallium oxide substrate surface Etching is roughened, the flatness of the substrate surface is lost, and crystal growth of the GaN-based thin film becomes difficult.
GaN系薄膜の成長に用いられる基板の表面粗さは、前記特許文献２の記載によれば10nm以下が好ましいとされているが、酸化ガリウム基板表面の表面粗さを、GaN系薄膜の結晶成長に好ましい程度（前記のように10nm以下）に形成していたとしても、前記のような反応に基づき酸化ガリウム基板表面が水素によりエッチングされると、表面粗さが GaN系薄膜の結晶成長に好ましい程度を超えてしまい、GaN系薄膜の結晶成長が困難になる。 According to the description in Patent Document 2, the surface roughness of the substrate used for the growth of the GaN-based thin film is preferably 10 nm or less. However, the surface roughness of the gallium oxide substrate surface is the crystal growth of the GaN-based thin film. Even if it is formed to a preferable level (as described above, 10 nm or less), when the surface of the gallium oxide substrate is etched with hydrogen based on the reaction as described above, the surface roughness is preferable for crystal growth of a GaN-based thin film. The crystal growth of the GaN-based thin film becomes difficult.
サーマルクリーニング以降の工程においても、高温の水素によって酸化ガリウム基板がエッチングされる課題は同様に存在する。これはGaN系薄膜成長には一般にキャリアガスとして水素が広く用いられているためである。 In the processes after the thermal cleaning, the problem that the gallium oxide substrate is etched by high-temperature hydrogen similarly exists. This is because hydrogen is widely used as a carrier gas for GaN-based thin film growth.
しかし、エピタキシャルプロセスにおける水素ガスの影響については、従来多くの対策が講じられており、高品質なGaN系薄膜を作製する上での大きな課題とはなっていない。 However, with respect to the influence of hydrogen gas in the epitaxial process, many countermeasures have been taken in the past, and it has not become a major issue in producing a high-quality GaN-based thin film.
例えば、酸化ガリウム基板の窒化工程においては、アンモニアガス中で窒化処理を行う際の処理温度を適宜設定することで、アンモニアから分解する水素の量を制御し、酸化ガリウム基板の表面を荒らすこと無く窒化が可能である。 For example, in the nitridation process of a gallium oxide substrate, the amount of hydrogen decomposed from ammonia can be controlled by appropriately setting the treatment temperature when nitriding in ammonia gas, and the surface of the gallium oxide substrate is not roughened. Nitriding is possible.
また、バッファ層およびGaN層を形成する際のキャリアガスとして水素を使用せずに、酸化ガリウムと反応しない不活性ガスを用いることで、基板表面を荒らす化学反応を根本的に起こさない工夫がある。 In addition, there is a device that does not fundamentally cause a chemical reaction that roughens the substrate surface by using an inert gas that does not react with gallium oxide without using hydrogen as a carrier gas when forming the buffer layer and the GaN layer. .
結局のところ、水素を雰囲気に含むサーマルクリーニングを行える酸化ガリウム基板を提供することが出来さえすれば、結晶欠陥の少ない高品質なGaN系薄膜を成膜することが出来るということになる。 After all, as long as a gallium oxide substrate capable of thermal cleaning containing hydrogen in an atmosphere can be provided, a high-quality GaN-based thin film with few crystal defects can be formed.
本発明は上記事情に鑑みてなされたものであり、酸化ガリウム基板と水素との反応を抑制して、酸化ガリウム基板の表面が荒れることを防ぎつつ、高い清浄度と基板表面の平坦性を得ることのできる酸化ガリウム基板、およびその製造方法を提供することを目的とする。 The present invention has been made in view of the above circumstances, and suppresses the reaction between the gallium oxide substrate and hydrogen to prevent the surface of the gallium oxide substrate from being roughened, and obtains high cleanliness and flatness of the substrate surface. An object of the present invention is to provide a gallium oxide substrate that can be used and a method for manufacturing the same.
上記目的は、以下の本発明により達成される。即ち、
本発明の酸化ガリウム基板は、基板面上に、ゾル−ゲル法による(Ga1-xAlx)2O3膜が形成されたことを特徴とする。
The above object is achieved by the present invention described below. That is,
The gallium oxide substrate of the present invention is characterized in that a (Ga 1-x Al x ) 2 O 3 film is formed on the substrate surface by a sol-gel method.
本発明の酸化ガリウム基板の一実施形態は、酸化ガリウム基板が単結晶基板であることが好ましい。 In one embodiment of the gallium oxide substrate of the present invention, the gallium oxide substrate is preferably a single crystal substrate.
また、本発明の酸化ガリウム基板の他の実施形態は、(Ga1-xAlx)2O3膜のxが、0.05以上0.8以下であることが好ましい。 In another embodiment of the gallium oxide substrate of the present invention, x in the (Ga 1-x Al x ) 2 O 3 film is preferably 0.05 or more and 0.8 or less.
また、本発明の酸化ガリウム基板の他の実施形態は、(Ga1-xAlx)2O3膜のxが、0.05以上0.7以下であることが好ましい。 In another embodiment of the gallium oxide substrate of the present invention, x of the (Ga 1-x Al x ) 2 O 3 film is preferably 0.05 or more and 0.7 or less.
また、本発明の酸化ガリウム基板の製造方法は、酸化ガリウム基板の面上に、ゾル−ゲル法により(Ga1-xAlx)2O3膜を形成したことを特徴とする。 The gallium oxide substrate manufacturing method of the present invention is characterized in that a (Ga 1-x Al x ) 2 O 3 film is formed on the surface of the gallium oxide substrate by a sol-gel method.
また、本発明の酸化ガリウム基板の製造方法の一実施形態は、酸化ガリウム基板が単結晶基板であることが好ましい。 In one embodiment of the method for producing a gallium oxide substrate of the present invention, the gallium oxide substrate is preferably a single crystal substrate.
また、本発明の酸化ガリウム基板の製造方法の他の実施形態は、(Ga1-xAlx)2O3膜のxを、0.05以上0.8以下とすることが好ましい。 In another embodiment of the method for producing a gallium oxide substrate of the present invention, it is preferable that x of the (Ga 1-x Al x ) 2 O 3 film is 0.05 or more and 0.8 or less.
また、本発明の酸化ガリウム基板の製造方法の他の実施形態は、(Ga1-xAlx)2O3膜のxを、0.05以上0.7以下とすることが好ましい。 In another embodiment of the method for producing a gallium oxide substrate of the present invention, it is preferable that x of the (Ga 1-x Al x ) 2 O 3 film is 0.05 or more and 0.7 or less.
本発明に依れば、酸化ガリウム基板表面の面上に (Ga1-xAlx)2O3膜を形成することで、酸化ガリウム基板のサーマルクリーニング時に、雰囲気中の水素により(Ga1-xAlx)2O3膜の表面がエッチングされたとしても、(Ga1-xAlx)2O3膜の表面粗さRaを10nm以下に抑えることが可能となる。従って、清浄効果の高い水素を用いたサーマルクリーニングに対して耐性を有する酸化ガリウム基板を得ることが可能となる。 According to the present invention, the (Ga 1-x Al x ) 2 O 3 film is formed on the surface of the gallium oxide substrate surface, so that when the gallium oxide substrate is thermally cleaned, (Ga 1- Even if the surface of the x Al x ) 2 O 3 film is etched, the surface roughness Ra of the (Ga 1-x Al x ) 2 O 3 film can be suppressed to 10 nm or less. Therefore, it becomes possible to obtain a gallium oxide substrate having resistance to thermal cleaning using hydrogen having a high cleaning effect.
以上により本発明に依れば、結晶欠陥の少ない高品質なGaN系薄膜を成膜することが出来る。 As described above, according to the present invention, a high-quality GaN-based thin film with few crystal defects can be formed.
更に前記(Ga1-xAlx)2O3膜の形成方法をゾル−ゲル法とすることによりAlの偏析が抑制され、(Ga1-xAlx)2O3膜全体での格子定数のバラツキを抑えることが可能となる。 Furthermore, the segregation of Al is suppressed by making the formation method of the (Ga 1-x Al x ) 2 O 3 film a sol-gel method, and the lattice constant of the entire (Ga 1-x Al x ) 2 O 3 film is suppressed. It becomes possible to suppress the variation of.
以下、本発明に係る酸化ガリウム基板、および酸化ガリウム基板の製造方法を詳細に説明する。本発明に係る酸化ガリウム基板の基材料となる酸化ガリウム単結晶の製造方法の一例として、EFG（Edge-defined Film-fed Growth）法が挙げられる。なお、酸化ガリウム単結晶の製造方法としては、溶融された酸化ガリウム融液に種結晶を接触させることにより、酸化ガリウム融液から酸化ガリウム単結晶を結晶成長させる方法であれば、特に制限されない。具体的な方法としては、CZ（Czochralski）法やFZ（Floating Zone）法等が挙げられる。しかし、任意の主面での結晶成長や、組成均一性の高い酸化ガリウム単結晶の成長が可能であるという点で、EFG法が他の結晶成長方法に比べて好ましい。 Hereinafter, the gallium oxide substrate and the method for manufacturing the gallium oxide substrate according to the present invention will be described in detail. As an example of a method for producing a gallium oxide single crystal serving as a base material of the gallium oxide substrate according to the present invention, there is an EFG (Edge-defined Film-fed Growth) method. The method for producing the gallium oxide single crystal is not particularly limited as long as it is a method for growing a gallium oxide single crystal from the gallium oxide melt by bringing the seed crystal into contact with the molten gallium oxide melt. Specific methods include CZ (Czochralski) method and FZ (Floating Zone) method. However, the EFG method is preferable to other crystal growth methods in that crystal growth on an arbitrary main surface and growth of a gallium oxide single crystal with high composition uniformity are possible.
酸化ガリウム単結晶製造用の出発原料としては、工業用酸化ガリウムを用いても良いし、炭酸ガリウムのようなガリウムの塩類、水和物、もしくはこれらの混合物を用いても良い。ガリウムの塩類、水和物、もしくはこれらの混合物を用いる場合は、これらを焼成する等により酸化ガリウムを得る。また、前記焼成などにより酸化ガリウムを得る工程は、育成炉の内部で結晶育成の直前に行っても良いし、前記育成炉の外部で事前に行っても良い。 As a starting material for producing a gallium oxide single crystal, industrial gallium oxide may be used, or gallium salts such as gallium carbonate, hydrates, or a mixture thereof may be used. In the case of using gallium salts, hydrates, or a mixture thereof, gallium oxide is obtained by firing them. In addition, the step of obtaining gallium oxide by firing or the like may be performed immediately before crystal growth inside the growth furnace, or may be performed in advance outside the growth furnace.
前記何れかの製造方法により製造された酸化ガリウム単結晶、及びその酸化ガリウム単結晶から得られる酸化ガリウム基板は、β-Ga2O3単結晶から成ることが最も好ましい。β-Ga2O3単結晶は導電性を有するので、電極構造が垂直型の発光素子（LED）を作製することが可能となる。その結果、発光素子全体に電流を流すことが出来ることから、電流密度を低くすることが可能となり、発光素子の寿命を長くすることが出来る。 Most preferably, the gallium oxide single crystal produced by any of the production methods described above and the gallium oxide substrate obtained from the gallium oxide single crystal are made of β-Ga 2 O 3 single crystal. Since the β-Ga 2 O 3 single crystal has conductivity, a light emitting element (LED) having a vertical electrode structure can be manufactured. As a result, since current can flow through the entire light emitting element, the current density can be lowered and the life of the light emitting element can be extended.
更に、β-Ga2O3単結晶はGaNとの間に大きな格子定数のミスマッチが見られない。また、バンドギャップの観点においては、β-Ga2O3単結晶の場合、約260nmまで透過するので、GaNの発光領域の全波長範囲、特に紫外領域での利用が可能となる。 Further, the β-Ga 2 O 3 single crystal does not show a large lattice constant mismatch with GaN. Further, from the viewpoint of the band gap, the β-Ga 2 O 3 single crystal transmits up to about 260 nm, so that it can be used in the entire wavelength range of the GaN emission region, particularly in the ultraviolet region.
なお適宜、前記出発原料にドーパントをドーピングすることにより、酸化ガリウム基板の電気伝導性の改善を図っても良い。 The electrical conductivity of the gallium oxide substrate may be improved by appropriately doping the starting material with a dopant.
EFG法により、所定の大きさとして約２インチ以上の酸化ガリウム単結晶を製造し、その酸化ガリウム単結晶が充分に温度降下したことを確認後、例えば電着ダイヤモンドコアドリル等により抜き加工を施して、図１に示すような平板円形の酸化ガリウム基板１を切り出す。次に、必要に応じてスライシングマシン等によりオリエンテーションフラット（オリフラ）２を、酸化ガリウム基板１の一枚毎に形成して、酸化ガリウム基板１を作製する。 A gallium oxide single crystal having a predetermined size of about 2 inches or more is manufactured by the EFG method, and after confirming that the temperature of the gallium oxide single crystal has sufficiently dropped, for example, an electrodeposition diamond core drill or the like is used to perform a punching process. A flat circular gallium oxide substrate 1 as shown in FIG. 1 is cut out. Next, an orientation flat (orientation flat) 2 is formed for each gallium oxide substrate 1 by a slicing machine or the like as necessary, and the gallium oxide substrate 1 is manufactured.
更に、作製した酸化ガリウム基板１の片面を主面３とし、少なくともその主面３に研磨加工等を施して、主面３を平坦化する。本発明に於ける酸化ガリウム基板１の主面３は如何なる面であっても良い。主面の一例として、酸化ガリウム単結晶を、劈開性が最も強い(100)に平行な面をワイヤソー等でスライスし、(100)をRa＝１nm以下に鏡面研磨して用いる。但し主面３は(100)に限定されるものでは無く、劈開による欠けや割れの防止という点から(101)、(110)、または(111)を主面３として選択しても良い。 Further, one side of the manufactured gallium oxide substrate 1 is used as a main surface 3, and at least the main surface 3 is subjected to polishing or the like to flatten the main surface 3. The main surface 3 of the gallium oxide substrate 1 in the present invention may be any surface. As an example of the main surface, a gallium oxide single crystal is used by slicing a plane parallel to (100) having the strongest cleavage property with a wire saw or the like and mirror-polishing (100) to Ra = 1 nm or less. However, the main surface 3 is not limited to (100), and (101), (110), or (111) may be selected as the main surface 3 in terms of preventing chipping and cracking due to cleavage.
このようにして得られた酸化ガリウム基板１（図１では、前記酸化ガリウム単結晶から円抜き加工された基板を示す）の主面３である基板面上に、ゾル−ゲル法により(Ga1-xAlx)2O3膜を形成する。 The gallium oxide substrate 1 obtained in this way (FIG. 1 shows a substrate that has been circled from the gallium oxide single crystal) is formed on the substrate surface which is the main surface 3 by a sol-gel method (Ga 1 -x Al x ) 2 O 3 film is formed.
本発明のように(Ga1-xAlx)2O3膜成膜用の下地基板として、酸化ガリウム単結晶を用いると、基板に近い化学組成の材料の薄膜を形成することになる。従って、薄膜と基板との間で格子不整合が抑えられ、単結晶或いはそれに近い結晶配向をもつ薄膜の形成が可能となり、欠陥が少なく高品質の(Ga1-xAlx)2O3膜の形成が可能となる。 When a gallium oxide single crystal is used as a base substrate for forming a (Ga 1-x Al x ) 2 O 3 film as in the present invention, a thin film of a material having a chemical composition close to that of the substrate is formed. Therefore, lattice mismatch between the thin film and the substrate is suppressed, and it is possible to form a single crystal or a thin film having a crystal orientation close to it, and a high-quality (Ga 1-x Al x ) 2 O 3 film with few defects. Can be formed.
次に、ゾル−ゲル法による(Ga1-xAlx)2O3膜の成膜方法について詳述する。まず、図２のように、EFG法等により作成した酸化ガリウム単結晶から切り出した、前記酸化ガリウム基板１を用意する。 Next, a method for forming a (Ga 1-x Al x ) 2 O 3 film by a sol-gel method will be described in detail. First, as shown in FIG. 2, the gallium oxide substrate 1 cut out from a gallium oxide single crystal prepared by the EFG method or the like is prepared.
次に、酸化ガリウム基板１に対して湿式洗浄として例えば有機溶剤で超音波洗浄を行って、酸化ガリウム基板１の主面３面上の有機物及び無機物を除去する。 Next, ultrasonic cleaning is performed on the gallium oxide substrate 1 with, for example, an organic solvent as wet cleaning, and organic substances and inorganic substances on the main surface 3 of the gallium oxide substrate 1 are removed.
次に図３のように、酸化ガリウム基板１の主面３面上に(Ga1-xAlx)2O3膜４を、以下のようなゾル−ゲル法により形成する。 Next, as shown in FIG. 3, a (Ga 1 -x Al x ) 2 O 3 film 4 is formed on the main surface 3 of the gallium oxide substrate 1 by the following sol-gel method.
まず、ガリウムイソプロポキドとアルミニウムイソプロポキドとを所定のモル比で混合する。所定のモル比としては(Ga1-xAlx)2O3膜４のxが、0.05以上0.8以下の範囲となるように設定する。 First, gallium isopropoxide and aluminum isopropoxide are mixed at a predetermined molar ratio. The predetermined molar ratio is set so that x of the (Ga 1-x Al x ) 2 O 3 film 4 is in the range of 0.05 to 0.8.
次に、ガリウムイソプロポキドとアルミニウムイソプロポキドの前記混合物とモル比が１対１になるように、モノエタノールアミンを加える。更に、モノエタノールアミンを加えた前記混合物に、溶媒として２‐メトキシエタノールを加えて、スピンコートにより好適な流動性になるまで希釈する。 Next, monoethanolamine is added so that the molar ratio of the mixture of gallium isopropoxide and aluminum isopropoxide is 1: 1. Further, 2-methoxyethanol is added as a solvent to the mixture to which monoethanolamine has been added, and diluted by spin coating until suitable fluidity is obtained.
このゾル溶液を前記主面３上に滴下し、30〜40nmの厚さでスピンコートにより塗布する。なお、均一膜厚で塗布する限り塗布方法に制限は無く、前記ゾル溶液に酸化ガリウム基板１を浸漬し引き上げるディップコーティング法などを用いても良い。しかし、大面積で均一な膜厚を確保する上ではスピンコートで塗布することが好ましい。 This sol solution is dropped onto the main surface 3 and applied by spin coating at a thickness of 30 to 40 nm. The coating method is not limited as long as it is applied with a uniform film thickness, and a dip coating method in which the gallium oxide substrate 1 is dipped in the sol solution and pulled up may be used. However, in order to ensure a uniform film thickness over a large area, it is preferable to apply by spin coating.
ゾル溶液を塗布後、その塗膜中の溶媒を蒸発させるために酸化ガリウム基板１ごと大気中で90℃前後の温度で約10分程度乾燥させる。その後、塗膜中の有機物を除去するために280〜350℃で仮焼成する。仮焼成の温度が280℃未満では反応速度が遅く、逆に350℃を超えると、緻密性に欠けた(Ga1-xAlx)2O3膜４になりやすくなる。また、仮焼成時間は10〜30分程度とすることが好ましい。 After applying the sol solution, the gallium oxide substrate 1 is dried in the atmosphere at a temperature of about 90 ° C. for about 10 minutes in order to evaporate the solvent in the coating film. Thereafter, in order to remove organic substances in the coating film, calcination is performed at 280 to 350 ° C. When the pre-baking temperature is lower than 280 ° C., the reaction rate is slow. Conversely, when the temperature is higher than 350 ° C., the (Ga 1-x Al x ) 2 O 3 film 4 lacking in denseness tends to be formed. Moreover, it is preferable that temporary baking time shall be about 10 to 30 minutes.
前記ゾル溶液の塗布から仮焼成までの工程を繰り返すことにより、(Ga1-xAlx)2O3膜４の膜厚を変化させ、所望の膜厚を得ることができるが、その膜厚は30〜500nmの範囲に設定することが好ましい。 (Ga1-xAlx)2O3膜４の厚さは１μｍ以下に設定できるが、製造コストの観点も考慮すると、30〜500nm程度とすることが好ましい。 By repeating the steps from application of the sol solution to temporary baking, the film thickness of the (Ga 1-x Al x ) 2 O 3 film 4 can be changed to obtain a desired film thickness. Is preferably set in the range of 30 to 500 nm. Although the thickness of the (Ga 1-x Al x ) 2 O 3 film 4 can be set to 1 μm or less, it is preferably about 30 to 500 nm in view of manufacturing cost.
更に大気中で、600〜1200℃で30〜90分本焼成して(Ga1-xAlx)2O3膜４を形成する。本実施形態では、1000℃で60分本焼成を行うこととする。酸化ガリウム基板１の結晶に近い結晶構造の薄膜形成という点から、(Ga1-xAlx)2O3膜４もβ型の単結晶膜が好ましい。本焼成温度が600℃に満たないと、(Ga1-xAlx)2O3膜４の結晶化が進みにくくなる。600℃以上では焼成温度が高くなるにつれて結晶性が高くなる傾向があるが、1200℃を超えるような高温での処理による更なる効果はあまり見られず、製造コストも勘案すると得策ではない。また、(Ga1-xAlx)2O3膜４の結晶性の観点からみると、焼成時間が30分に満たない場合は、結晶性があまり高くなく不十分であり、90分を超えて焼成してもそれ以上の改善はあまり見られない。 Further, the main film is fired at 600 to 1200 ° C. for 30 to 90 minutes in the air to form the (Ga 1-x Al x ) 2 O 3 film 4. In this embodiment, the main baking is performed at 1000 ° C. for 60 minutes. In view of forming a thin film having a crystal structure close to that of the gallium oxide substrate 1, the (Ga 1-x Al x ) 2 O 3 film 4 is also preferably a β-type single crystal film. If the main firing temperature is less than 600 ° C., the crystallization of the (Ga 1-x Al x ) 2 O 3 film 4 is difficult to proceed. Above 600 ° C, the crystallinity tends to increase as the firing temperature increases. However, there is not much effect due to the treatment at a high temperature exceeding 1200 ° C, and it is not a good idea when considering the manufacturing cost. Also, from the viewpoint of crystallinity of the (Ga 1-x Al x ) 2 O 3 film 4, if the firing time is less than 30 minutes, the crystallinity is not so high and insufficient, exceeding 90 minutes. No further improvement is seen even after firing.
(Ga1-xAlx)2O3膜４中のGaの一部がAlで置換されると、(Ga1-xAlx)2O3膜４全体の格子定数が小さくなる。これは、AlとGaのイオン半径を比べると、Alの方がGaより小さいため、Al置換された原子サイトにおいて結晶が収縮するためと思われる。 When a part of Ga in the (Ga 1-x Al x ) 2 O 3 film 4 is replaced with Al, the lattice constant of the entire (Ga 1-x Al x ) 2 O 3 film 4 becomes small. This seems to be because, when the ionic radii of Al and Ga are compared, since Al is smaller than Ga, the crystal shrinks at the Al-substituted atomic site.
本発明では、水素を含む雰囲気中でサーマルクリーニングを実施可能な(Ga1-xAlx)2O3膜のx値を、0.05以上0.8以下の範囲に設定する。より好ましくは、β型酸化ガリウムへのAlの固溶限界値を上限とする、0.05以上0.7以下の範囲に設定する。 In the present invention, the x value of the (Ga 1-x Al x ) 2 O 3 film capable of performing thermal cleaning in an atmosphere containing hydrogen is set in the range of 0.05 to 0.8. More preferably, it is set in the range of 0.05 or more and 0.7 or less with the upper limit of the solid solution limit value of Al in β-type gallium oxide.
なお(Ga1-xAlx)2O3膜４は、ゾル−ゲル法の他にも、スパッタ法、PLD（Pulse Laser Deposition）法、CVD（Chemical Vapor Deposition）法、スプレー法、液相エピタキシ（エピタキシャル成長）、気相エピタキシ、分子線エピタキシ等で成膜することが出来る。しかしながら、スパッタ法、PLD法、CVD法、各種エピタキシでは、高真空など大掛かりな装置が必要となる。一方、スプレー法ではAlの偏析を抑えることが困難である。従って、大掛かりな装置が不要で低コストで成膜可能であり、且つAlの偏析を防止して(Ga1-xAlx)2O3膜４全体での格子定数のバラツキを抑制できるとの観点から、(Ga1-xAlx)2O3膜４の成膜法としてゾル−ゲル法が最も好ましい。 In addition to the sol-gel method, the (Ga 1-x Al x ) 2 O 3 film 4 is formed by sputtering, PLD (Pulse Laser Deposition), CVD (Chemical Vapor Deposition), spray, liquid phase epitaxy. (Epitaxial growth), vapor phase epitaxy, molecular beam epitaxy and the like can be used for film formation. However, sputtering, PLD, CVD, and various types of epitaxy require large-scale equipment such as high vacuum. On the other hand, it is difficult to suppress segregation of Al by the spray method. Accordingly, it is possible to form a film at a low cost without requiring a large-scale apparatus, and it is possible to prevent the segregation of Al and suppress the variation of the lattice constant in the entire (Ga 1-x Al x ) 2 O 3 film 4. From the viewpoint, the sol-gel method is most preferable as the method of forming the (Ga 1-x Al x ) 2 O 3 film 4.
こうして得られた(Ga1-xAlx)2O3膜４に特に研磨加工を行うことなく、(Ga1-xAlx)2O3膜４の表面粗さを原子間力顕微鏡（AFM：Atomic Force Microscope）にて評価したところ、Raにして0.5〜3nmとなっており、この膜上にGaN系薄膜をエピタキシャル成長させるのに好ましい状態、すなわち表面粗さが10nm以下となっていた。また、(Ga1-xAlx)2O3膜４の表面は、下地の酸化ガリウム基板の表面と同一の結晶面が強く配向しており、この膜上にGaN系薄膜をエピタキシャル成長させるのに好ましい状態となっていた。 The surface roughness of the (Ga 1-x Al x ) 2 O 3 film 4 was measured with an atomic force microscope (AFM) without particularly polishing the (Ga 1-x Al x ) 2 O 3 film 4 thus obtained. : Ra was 0.5 to 3 nm when evaluated with an atomic force microscope), and a preferable state for epitaxial growth of a GaN-based thin film on this film, that is, the surface roughness was 10 nm or less. Further, the surface of the (Ga 1-x Al x ) 2 O 3 film 4 is strongly oriented in the same crystal plane as the surface of the underlying gallium oxide substrate, and is used for epitaxial growth of a GaN-based thin film on this film. It was in a favorable state.
アルミニウム（Al）と酸素（O）との間の結合力は、ガリウム（Ga）と酸素との間の結合力よりも大きいが、本発明の(Ga1-xAlx)2O3膜４においては、酸素との結合力が強いAlが、膜を構成するGa-Oネットワーク中に均一に固溶することで、水素によってGa-Oのネットワークが切断されてGaが蒸発することを抑制する効果を生み出しているものと思われる。 The bonding force between aluminum (Al) and oxygen (O) is larger than the bonding force between gallium (Ga) and oxygen, but the (Ga 1-x Al x ) 2 O 3 film 4 of the present invention. In this case, Al, which has a strong binding force with oxygen, uniformly dissolves in the Ga-O network that constitutes the film, thereby suppressing the Ga-O network from being cut by hydrogen and causing Ga to evaporate. It seems to have produced an effect.
従って、酸化ガリウム基板１のサーマルクリーニング時に、雰囲気中の水素により(Ga1-xAlx)2O3膜４の表面がエッチングされたとしても、(Ga1-xAlx)2O3膜４の表面粗さRaを10nm以下に抑えることが可能となる。よって、基板の清浄効果の高い、水素を雰囲気に含むサーマルクリーニングを行うことが可能となり、表面の清浄な基板を得ることができるため、結晶欠陥の少ない高品質なGaN系薄膜を成膜することが出来る。 Therefore, even when the surface of the (Ga 1 -x Al x ) 2 O 3 film 4 is etched by hydrogen in the atmosphere during the thermal cleaning of the gallium oxide substrate 1, the (Ga 1 -x Al x ) 2 O 3 film 4 can be suppressed to a surface roughness Ra of 10 nm or less. Therefore, it is possible to perform thermal cleaning that has a high cleaning effect on the substrate and contains hydrogen in the atmosphere, and a substrate with a clean surface can be obtained, so that a high-quality GaN-based thin film with few crystal defects can be formed. I can do it.
(Ga1-xAlx)2O3膜４中のAlの、酸化ガリウム結晶中への分布度合いは、(Ga1-xAlx)2O3膜４の全面に亘って均等に分布されていることが、Alの偏析抑制と、(Ga1-xAlx)2O3膜全体での格子定数のバラツキ抑制という点から好ましい。前記の通り、本発明では(Ga1-xAlx)2O3膜４の成膜にゾル−ゲル法を用いることで、AlをGa2O3中にほぼ一定の比率で均一に存在させることが出来る。従って、(Ga1-xAlx)2O3膜４全体での格子定数のバラツキが抑制される。 The degree of distribution of Al in the (Ga 1-x Al x ) 2 O 3 film 4 in the gallium oxide crystal is evenly distributed over the entire surface of the (Ga 1-x Al x ) 2 O 3 film 4. It is preferable from the viewpoints of suppressing segregation of Al and suppressing variation of lattice constants in the entire (Ga 1-x Al x ) 2 O 3 film. As described above, in the present invention, by using the sol-gel method for forming the (Ga 1-x Al x ) 2 O 3 film 4, Al is uniformly present in the Ga 2 O 3 at a substantially constant ratio. I can do it. Therefore, the variation in lattice constant in the entire (Ga 1-x Al x ) 2 O 3 film 4 is suppressed.
以下に、各実施例を挙げて本発明を説明するが、本発明は以下の各実施例にのみ限定されるものではない。 Hereinafter, the present invention will be described by way of examples. However, the present invention is not limited to the following examples.
（実施例１−４及び比較例１）
実施例１−４及び比較例１の酸化ガリウム単結晶をEFG法により作製し、酸化ガリウム融液からβ-Ga2O3単結晶を作製した。そのβ- Ga2O3単結晶から、主面が(100)となるように２インチの酸化ガリウム基板を切り出した。更にその主面の表面粗さが、Raで１nmとなるまで鏡面研磨を施した。
(Examples 1-4 and Comparative Example 1)
The gallium oxide single crystals of Example 1-4 and Comparative Example 1 were produced by the EFG method, and β-Ga 2 O 3 single crystals were produced from the gallium oxide melt. A 2-inch gallium oxide substrate was cut out from the β-Ga 2 O 3 single crystal so that the main surface was (100). Further, mirror polishing was performed until the surface roughness of the main surface was 1 nm in Ra.
このようにして得られた酸化ガリウム基板の主面上に、実施例１−４として前記のようなゾル−ゲル法により(Ga1-xAlx)2O3膜を形成した。実施例１では、前記ゾル溶液を30〜40nmの厚さでスピンコートを行うと共に仮焼成までの工程を行った。更にこのスピンコート〜仮焼成までの工程を６回繰り返すことにより、200nmの膜厚の(Ga1-xAlx)2O3膜を形成した。なお、ゾル−ゲル法における溶媒蒸発のための加熱温度は90℃、加熱時間は10分とした。更に仮焼成は300℃で20分、本焼成は1000℃で60分、それぞれ行った。実施例１では(Ga1-xAlx)2O3膜のxを0.2、実施例２では前記xを0.4、実施例３では前記xを0.6、実施例４は前記xを0.8に、それぞれ設定した。 A (Ga 1-x Al x ) 2 O 3 film was formed on the main surface of the gallium oxide substrate thus obtained by the sol-gel method as described in Example 1-4. In Example 1, the sol solution was spin-coated at a thickness of 30 to 40 nm and the steps up to calcination were performed. Further, by repeating the steps from spin coating to pre-baking six times, a (Ga 1-x Al x ) 2 O 3 film having a thickness of 200 nm was formed. The heating temperature for solvent evaporation in the sol-gel method was 90 ° C., and the heating time was 10 minutes. Furthermore, pre-baking was performed at 300 ° C. for 20 minutes, and main baking was performed at 1000 ° C. for 60 minutes. In Example 1, x of the (Ga 1-x Al x ) 2 O 3 film is 0.2, in Example 2, x is 0.4, in Example 3, x is 0.6, and in Example 4, x is 0.8. Set.
一方、比較例１として前記酸化ガリウム基板の主面上に、ゾル−ゲル法により酸化ガリウム膜（Ga2O3膜、即ち実施例１−４においてx=0と見なされる膜）を形成したサンプルも用意した。比較例１のゾル−ゲル法は、ガリウムイソプロポキドとモノエタノールアミンをモル比で１対１となるように混合後、溶媒として２‐メトキシエタノールを加えて希釈し、スピンコートによりゾル溶液を酸化ガリウム基板主面上へ塗布した。溶媒蒸発のための加熱条件、及び仮焼成と本焼成の条件は、実施例１−４と同一条件とし、Ga2O3膜の膜厚は200nmと設定した。 On the other hand, as Comparative Example 1, a sample in which a gallium oxide film (Ga 2 O 3 film, that is, a film regarded as x = 0 in Example 1-4) was formed on the main surface of the gallium oxide substrate by a sol-gel method. Also prepared. In the sol-gel method of Comparative Example 1, gallium isopropoxide and monoethanolamine are mixed at a molar ratio of 1: 1, diluted by adding 2-methoxyethanol as a solvent, and the sol solution is formed by spin coating. It apply | coated on the gallium oxide board | substrate main surface. The heating conditions for solvent evaporation, and the conditions for pre-baking and main baking were the same as those in Example 1-4, and the film thickness of the Ga 2 O 3 film was set to 200 nm.
実施例１−４及び比較例１に係る(Ga1-xAlx)2O3膜の結晶構造を、X線回折により評価した結果を図４に示す。図４に示すようにx=0〜0.8において、下地基板である酸化ガリウム基板と同様に、β型酸化ガリウム構造の（h00）面のピークのみが観測された。これは形成された(Ga1-xAlx)2O3膜が、酸化ガリウム基板に対して同一の結晶面が強く配向して成長していることを示唆するものである。 FIG. 4 shows the results of evaluating the crystal structure of the (Ga 1-x Al x ) 2 O 3 film according to Example 1-4 and Comparative Example 1 by X-ray diffraction. As shown in FIG. 4, at x = 0 to 0.8, only the peak of the (h00) plane of the β-type gallium oxide structure was observed, similar to the gallium oxide substrate as the base substrate. This suggests that the formed (Ga 1-x Al x ) 2 O 3 film grows with the same crystal plane strongly oriented with respect to the gallium oxide substrate.
図４のX線回折パターンにおける（800）面ピークの2θ値からa軸の格子定数を求めたところ、図５に示すようにAl含有量の増加に応じてx=0〜0.7までは直線的に減少していることが確認された。従って、添加したAlがほぼ全て(Ga1-xAlx)2O3膜の結晶格子内に取り込まれていると判断できた。 When the a-axis lattice constant was determined from the 2θ value of the (800) plane peak in the X-ray diffraction pattern of FIG. 4, x = 0 to 0.7 was linear as the Al content increased, as shown in FIG. It was confirmed that the number decreased. Therefore, it was determined that almost all of the added Al was taken into the crystal lattice of the (Ga 1-x Al x ) 2 O 3 film.
一方、x=0.8の(Ga1-xAlx)2O3膜についてはx=0〜0.7の直線的な減少傾向から外れているので、Alの添加量が固溶限界を超えており、固溶し切れなかったAlが、X線回折のピークとしては検出されない極微量の不純物として存在している可能性が考えられる。従って、GaN系薄膜成長用の基板としてより好ましいのは、x=0.7以下という結論が得られた。 On the other hand, the (Ga 1-x Al x ) 2 O 3 film with x = 0.8 deviates from the linear decreasing trend of x = 0 to 0.7, so the Al addition amount exceeds the solid solution limit, There is a possibility that Al that was not completely dissolved is present as a trace amount of impurities that are not detected as X-ray diffraction peaks. Therefore, the conclusion that x = 0.7 or less is more preferable as a substrate for growing a GaN-based thin film.
（実施例５−１２及び比較例１）
次に、(Ga1-xAlx)2O3膜のサーマルクリーニングの水素に対する耐性を、次の手順で評価した。まず、(Ga1-xAlx)2O3膜付き酸化ガリウム基板を1000℃で30分、１気圧の水素雰囲気中で熱処理した。次に、熱処理後の(Ga1-xAlx)2O3膜の表面粗さをAFMにて測定し、Raを求めた。Raが10nm以下の(Ga1-xAlx)2O3膜付き酸化ガリウム基板を、水素に対する耐性有りと判定した。なお、実施例５では(Ga1-xAlx)2O3膜のxを0.05、実施例６では前記xを0.1、実施例７では前記xを0.2、実施例８では前記xを0.3、実施例９では前記xを0.4、実施例１０では前記xを0.5、実施例１１では前記xを0.6、実施例１２では前記xを0.8に、それぞれ設定し直した。また、酸化ガリウム単結晶とその作製方法、および(Ga1-xAlx)2O3膜とその作製方法は、前記実施例１−４と同一とした。
(Examples 5-12 and Comparative Example 1)
Next, the hydrogen cleaning resistance of the (Ga 1-x Al x ) 2 O 3 film was evaluated by the following procedure. First, a gallium oxide substrate with a (Ga 1-x Al x ) 2 O 3 film was heat-treated at 1000 ° C. for 30 minutes in a hydrogen atmosphere at 1 atmosphere. Next, the surface roughness of the (Ga 1-x Al x ) 2 O 3 film after the heat treatment was measured by AFM to determine Ra. A gallium oxide substrate with a (Ga 1-x Al x ) 2 O 3 film with an Ra of 10 nm or less was determined to be resistant to hydrogen. In Example 5, x of the (Ga 1-x Al x ) 2 O 3 film is 0.05, x is 0.1 in Example 6, x is 0.2 in Example 7, x is 0.3 in Example 8, In Example 9, x was reset to 0.4, in Example 10, x was set to 0.5, in Example 11, x was set to 0.6, and in Example 12, x was reset to 0.8. The gallium oxide single crystal and its manufacturing method, and the (Ga 1-x Al x ) 2 O 3 film and its manufacturing method were the same as those in Example 1-4.
評価の結果、図６に示すように、比較例１であるAlを含まないGa2O3膜は、水素中の熱処理によってエッチングされ、表面粗さRaが10nmを超えてしまう。一方、Alをx=0.05以上含んだ(Ga1-xAlx)2O3膜はいずれも表面粗さが10nm以下に抑えられ、水素によるサーマルクリーニングに耐性を有することを示した。従って、サーマルクリーニング後の(Ga1-xAlx)2O3膜は、GaN系薄膜の成長が可能な表面状態と結論付けられる。 As a result of the evaluation, as shown in FIG. 6, the Ga 2 O 3 film not containing Al as Comparative Example 1 is etched by the heat treatment in hydrogen, and the surface roughness Ra exceeds 10 nm. On the other hand, all of the (Ga 1-x Al x ) 2 O 3 films containing Al at x = 0.05 or more showed that the surface roughness was suppressed to 10 nm or less, and they were resistant to thermal cleaning with hydrogen. Therefore, it can be concluded that the (Ga 1-x Al x ) 2 O 3 film after thermal cleaning is a surface state capable of growing a GaN-based thin film.
以上により、0.05≦x≦0.8の範囲において、1気圧の水素を用いた1000℃でのサーマルクリーニングに対する耐性が付与され、表面粗さRa=10nm以下を実現することが可能となることが判明した。 From the above, it was found that in the range of 0.05 ≦ x ≦ 0.8, resistance to thermal cleaning at 1000 ° C. using 1 atm of hydrogen was imparted, and surface roughness Ra = 10 nm or less could be realized. .
更に実施例１−４より、0.05≦x≦0.7の範囲で、添加したAlがほぼ全て(Ga1-xAlx)2O3膜の結晶中に固溶することが判明した。酸化ガリウム基板表面にAlをx=0.05以上含んだ(Ga1-xAlx)2O3膜を形成することで、水素との反応により表面粗さRaが10nmを超えること無く清浄な表面が得られる。従って、(Ga1-xAlx)2O3膜（x=0.05以上含む）付き酸化ガリウム基板は、結晶欠陥の少ない高品質なGaN系薄膜を成膜することが出来ることが判明した。 Further, from Example 1-4, it was found that almost all of the added Al was dissolved in the crystal of the (Ga 1-x Al x ) 2 O 3 film in the range of 0.05 ≦ x ≦ 0.7. By forming a (Ga 1-x Al x ) 2 O 3 film containing Al x = 0.05 or more on the surface of the gallium oxide substrate, the surface roughness Ra does not exceed 10 nm due to reaction with hydrogen. can get. Therefore, it has been found that a gallium oxide substrate with a (Ga 1-x Al x ) 2 O 3 film (including x = 0.05 or more) can form a high-quality GaN-based thin film with few crystal defects.
（実施例１３−１７及び比較例２）
サーマルクリーニングの雰囲気ガスのコストを抑える場合や、サーマルクリーニング装置の運転上の都合次第では、サーマルクリーニングからGaN系薄膜を形成するまでの工程を、同一の水素流量・圧力を維持して行うことも考えられ、200Torr程度の減圧下でサーマルクリーニングを実施することもある。
(Examples 13-17 and Comparative Example 2)
Depending on the operating cost of the thermal cleaning device, the process from thermal cleaning to formation of the GaN-based thin film may be performed with the same hydrogen flow rate and pressure maintained, depending on the cost of the atmospheric gas for thermal cleaning. It is conceivable that thermal cleaning may be performed under a reduced pressure of about 200 Torr.
そこで、MOCVD（Metal Organic Chemical Vapor Deposition）法にてGaN系薄膜を形成する工程に則って、200Torrの水素をキャリアガスとして、(Ga1-xAlx)2O3膜が形成された酸化ガリウム基板上にGaN系薄膜を実施例１３−１７として形成した。図７は、Alを含まないx=0の(Ga1-xAlx)2O3膜（即ち、Ga2O3膜）が形成された酸化ガリウム基板について、サーマルクリーニング工程における基板温度とGa2O3膜表面の反射率の推移を示したグラフである。 Therefore, in accordance with the MOCVD (Metal Organic Chemical Vapor Deposition) method for forming a GaN-based thin film, gallium oxide with a (Ga 1-x Al x ) 2 O 3 film formed using 200 Torr of hydrogen as a carrier gas A GaN-based thin film was formed as Examples 13-17 on the substrate. FIG. 7 shows the substrate temperature and Ga in the thermal cleaning process for a gallium oxide substrate on which an Al = 0-free (Ga 1-x Al x ) 2 O 3 film (ie, Ga 2 O 3 film) is formed. is a graph showing changes in the reflectivity of the 2 O 3 film surface.
基板温度を室温（室温から400℃までは装置の性能により未測定）から950℃まで上昇させた後に保持しサーマルクリーニングを試みたが、温度上昇の過程で、水素との反応によりGa2O3膜がエッチングされて表面粗さが粗くなり、Raで10nmを超えると、反射率が急激に低下した。このときの温度をサーマルクリーニングの限界温度として求めたところ、比較例２であるAlを含まないGa2O3膜の場合は、限界温度は620℃であった。 The substrate temperature was raised from room temperature (from room temperature to 400 ° C, not measured depending on the performance of the apparatus) and held for 950 ° C, and then thermal cleaning was attempted, but during the temperature increase process, Ga 2 O 3 was reacted with hydrogen. The film was etched to increase the surface roughness. When Ra exceeded 10 nm, the reflectivity rapidly decreased. When the temperature at this time was determined as the limit temperature for thermal cleaning, the limit temperature was 620 ° C. in the case of the Ga 2 O 3 film containing no Al as Comparative Example 2.
同様の実験を、Alの添加量が異なる、x=0.05〜0.5の(Ga1-xAlx)2O3膜が形成された酸化ガリウム基板（実施例１３−１７）について行ったところ、限界温度のAl含有量依存性は図８のようになった。なお、実施例１３では(Ga1-xAlx)2O3膜のxを0.05、実施例１４では前記xを0.1、実施例１５では前記xを0.2、実施例１６では前記xを0.3、実施例１７では前記xを0.5、比較例２では前記xを0と、それぞれ設定した。また、酸化ガリウム単結晶とその作製方法、および(Ga1-xAlx)2O3膜とその作製方法は、前記実施例１−４又は比較例１と同一とした。 A similar experiment was performed on a gallium oxide substrate (Examples 13-17) on which (Ga 1-x Al x ) 2 O 3 films with different amounts of Al added and x = 0.05 to 0.5 were formed. The dependence of temperature on the Al content is as shown in FIG. In Example 13, x of the (Ga 1-x Al x ) 2 O 3 film was 0.05, in Example 14, x was 0.1, in Example 15, x was 0.2, in Example 16, x was 0.3, In Example 17, the x was set to 0.5, and in Comparative Example 2, the x was set to 0. The gallium oxide single crystal and its manufacturing method, and the (Ga 1-x Al x ) 2 O 3 film and its manufacturing method were the same as those in Example 1-4 or Comparative Example 1.
以上の結果をまとめると、Alを添加しないGa2O3膜（もしくは前記(Ga1-xAlx)2O3膜を形成しない酸化ガリウム基板）は、MOCVD法を用いた200Torrの水素ガスによるサーマルクリーニングは620℃以下で行わねばならず、これ以上の温度で実施すると、基板又はGa2O3膜の表面粗さRaが、結晶欠陥の少ない高品質のGaN系薄膜を得るのに不適となってしまうことが判明した。 To summarize the above results, the Ga 2 O 3 film without addition of Al (or the gallium oxide substrate not forming the (Ga 1-x Al x ) 2 O 3 film) is formed by 200 Torr hydrogen gas using the MOCVD method. Thermal cleaning must be performed at 620 ° C or less, and if it is performed at a temperature higher than this, the surface roughness Ra of the substrate or Ga 2 O 3 film is not suitable for obtaining a high-quality GaN-based thin film with few crystal defects. Turned out to be.
一方、Alを添加した(Ga1-xAlx)2O3膜を酸化ガリウム基板表面に形成することで、サーマルクリーニングの限界温度を620℃よりも高くすることが可能になり、Al含有量の増加に伴い、サーマルクリーニングの限界温度がx=0.05で650℃まで上昇し、更にx=0.5では810℃まで上昇出来ることが確認された。従って、本発明に係る(Ga1-xAlx)2O3膜が形成された酸化ガリウム基板を用いれば、x ≧0.05の条件でサーマルクリーニングの雰囲気を減圧水素とすることで、低コストで大きな清浄効果を得ることが出来る。 On the other hand, by forming an Al - added (Ga 1-x Al x ) 2 O 3 film on the gallium oxide substrate surface, it becomes possible to raise the limit temperature of thermal cleaning to more than 620 ° C, and Al content It was confirmed that the critical temperature of thermal cleaning increased to 650 ° C. at x = 0.05, and further increased to 810 ° C. at x = 0.5. Therefore, if the gallium oxide substrate on which the (Ga 1-x Al x ) 2 O 3 film according to the present invention is used, the thermal cleaning atmosphere is reduced pressure hydrogen under the condition of x ≧ 0.05. A large cleaning effect can be obtained.
１ 酸化ガリウム基板
２ オリエンテーションフラット
３ 主面
４ (Ga1-xAlx)2O3膜
1 Gallium oxide substrate 2 Orientation flat 3 Main surface 4 (Ga 1-x Al x ) 2 O 3 film
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