WO2021241281A1 - 結晶性の窒化ガリウム薄膜の製造方法 - Google Patents

結晶性の窒化ガリウム薄膜の製造方法 Download PDF

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WO2021241281A1
WO2021241281A1 PCT/JP2021/018432 JP2021018432W WO2021241281A1 WO 2021241281 A1 WO2021241281 A1 WO 2021241281A1 JP 2021018432 W JP2021018432 W JP 2021018432W WO 2021241281 A1 WO2021241281 A1 WO 2021241281A1
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thin film
gallium
gan
crystalline
substrate
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French (fr)
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文一 水谷
慎太郎 東
伸尚 高橋
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株式会社高純度化学研究所
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Priority claimed from JP2021079631A external-priority patent/JP2021188127A/ja
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Priority to KR1020227037542A priority Critical patent/KR20220156952A/ko
Priority to US17/919,883 priority patent/US20230160051A1/en
Publication of WO2021241281A1 publication Critical patent/WO2021241281A1/ja

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Definitions

  • the present invention relates to a method for forming a crystalline gallium nitride thin film by an atomic layer deposition (ALD) method.
  • ALD atomic layer deposition
  • Gallium nitride is an important semiconductor material used to emit blue to bluish purple light in light emitting diodes (LEDs) and blue lasers. It is highly desirable to grow a single crystal GaN layer on a silicon substrate, but due to the mismatch of the crystal lattice spacing between GaN and silicon, epitaxially grow crystalline GaN on a sapphire substrate with a lattice constant close to that of GaN. Is being done. However, since a single crystal sapphire substrate generally costs significantly more than a silicon substrate, a technique for growing a highly crystalline GaN film on a versatile and inexpensive substrate such as a silicon substrate is desired.
  • the chemical vapor deposition (CVD) method is generally used for GaN film formation.
  • ALD atomic layer deposition
  • raw materials are not deposited continuously, but are deposited layer by layer. Therefore, ultrathin films of several nanometers are deposited by a highly accurate and controlled method. Can be done.
  • Non-Patent Document 1 a method of forming a single crystal GaN thin film on a sapphire substrate at 450 to 900 ° C. using hydrogen as a carrier gas and using triethyl gallium (TEG) and ammonia has been reported (Non-Patent Document 1).
  • Sapphire can withstand high temperatures of 450 to 900 ° C. and has high heat resistance, which is also suitable for epitaxially growing GaN.
  • Patent Document 1 discloses, as a method for forming a GaN-ALD film, a method of forming a GaN device layer on a silicon substrate coated with an aluminum nitride (AlN) nucleation layer and crystallizing GaN by laser annealing. ..
  • TMG trimethylgallium
  • hydrogen radicals hydrogen radicals
  • ammonia ammonia
  • the GaN film obtained by this method has a large amount of carbon impurities of 10 to 35 at%.
  • Non-Patent Document 3 TMG, ammonia plasma, plasma of a mixed gas of nitrogen and hydrogen (hereinafter referred to as “nitrogen / hydrogen plasma”), or nitrogen plasma is referred to. It is used to form a crystalline thin film at 200 ° C. However, in the crystalline thin film, the N / Ga ratio is significantly N-rich, and the quality of the film is poor in nitrogen plasma without hydrogen. In Non-Patent Document 4, TEG and nitrogen / hydrogen plasma are used to form crystalline GaN thin films at 200 ° C, 285 ° C, and 350 ° C.
  • Non-Patent Document 5 TMG and a mixed plasma of nitrogen / hydrogen are used to perform ALD at 250 ° C. to form a crystalline film.
  • Non-Patent Document 5 even if plasma is used, ALD film formation cannot be performed at temperatures below 210 ° C., and the N / Ga ratio of the ALD film obtained at 250 ° C. is slightly Ga-rich, but contains about 9% of C impurities. ing. Crystallization using plasma is also exemplified in Patent Document 2.
  • Patent Document 2 uses TMG or TEG and nitrogen / hydrogen plasma to form a GaN thin film, and gallium halides such as GaCl 3 , GaCl, and GaI 3 are also exemplified.
  • TMG and TEG are widely used as Ga sources for forming a GaN film by the ALD method, but they are very unstable in the air and spontaneously ignite, so they are not easy to handle. Furthermore, in order to form a highly crystalline GaN thin film, it is necessary to perform ALD at high temperature, or to perform high temperature heat treatment at the same time as or immediately after ALD in combination with an annealing technique by laser or electron beam irradiation. rice field.
  • the GaN film formed on the substrate is a monovalent gallium compound. Therefore, if a trivalent gallium complex such as TMG or TEG is used as a raw material, there is a risk of C being mixed in because it is necessary to reduce gallium, and there is a problem that a GaN film having poor crystallinity is formed. rice field. Further, even if it is a monovalent gallium complex, in the case of an inorganic gallium complex, there is a possibility that an inorganic element may be mixed with GaN. Therefore, an object of the present invention is to provide a more efficient method for producing a GaN film by ALD, which comprises a method for producing a GaN film having extremely few impurities and high crystallinity.
  • ALD atomic layer deposition
  • a substrate having a lattice constant close to that of GaN such as GaN or GaN or AlN has been used, but in the present invention, nitrogen, which is a constituent element of GaN or sapphire, is used. It is also an object to obtain highly crystalline GaN even on a substrate containing neither gallium nitride nor aluminum as a main component.
  • the method for producing a crystalline gallium nitride thin film of the present invention includes step 1 of supplying a monovalent organic gallium complex to a reaction chamber having a substrate temperature of 350 ° C. or lower by using an atomic layer deposition (ALD) method. It is characterized by including a step 2 of supplying a nitride gas to the reaction chamber.
  • the nitrided gas is preferably nitrogen plasma gas.
  • the organic gallium complex is preferably a cyclopentadienyl complex.
  • the surface of the substrate does not contain any of nitrogen, gallium, and aluminum as main components.
  • a highly crystalline GaN film can be formed from an organic gallium complex without performing high-temperature heat treatment such as laser annealing.
  • FIG. 1 shows a cross-sectional TEM image of a GaN film formed on a silicon wafer with a natural oxide film in Example 1.
  • FIG. 2 shows a cross-sectional TEM image of the GaN film formed in Example 2.
  • the method for producing a gallium nitride (GaN) thin film of the present invention includes step 1 of supplying a monovalent organic gallium complex to a reaction chamber having a substrate temperature of 350 ° C. or lower using ALD, and a nitride gas in the reaction chamber. Has step 2 and.
  • thermal ALD can form a uniform film along a high aspect ratio surface.
  • PEALD can be performed at a low temperature, although the film forming ability on a high aspect ratio surface may be inferior to that of thermal ALD.
  • PEALD is suitable for the purpose of the present invention of efficiently forming a highly crystalline GaN film by using a monovalent gallium species.
  • GaN is formed by the deposition cycles of (i) to (ii) below.
  • One form of the deposition cycle is (i) the step of feeding the gas phase precursor into the reaction chamber to adsorb the precursor on the surface of the substrate, and (ii) the generation of radical species by plasma in the reaction chamber. It has a step of sending the nitriding gas and reacting it with a precursor adsorbed on the surface to form a GaN crystal layer. Each cycle is then repeated until the film formed reaches the desired thickness.
  • a monovalent organic gallium complex as a precursor is supplied in a gas phase into the reaction chamber in which the substrate is installed (step 1).
  • the substrate temperature is any temperature within the range of room temperature to 350 ° C., and the monovalent organic gallium complex is evaporated at a temperature lower than the substrate temperature so as not to condense on the substrate.
  • a nitride gas is supplied into the reaction chamber (step 2), and the precursor adsorbed on the substrate surface in step 1 reacts with the nitride gas to form a crystalline GaN thin film on the substrate.
  • the GaN thin film may be polycrystalline, but is preferably single crystal.
  • the N / Ga ratio of the thin film is 1, but in the case of polycrystal, a dense film is preferable, and an N / Ga ratio of 1 or less, which is Ga-rich, is preferable.
  • the GaN thin film there is almost no amorphous portion, but less than 1% by volume is preferable, less than 0.01% by volume is more preferable, and less than 0.0001% by volume is particularly preferable.
  • the GaN thin film is a high-purity thin film, and the carbon in the impurities is preferably 5 atomic% or less, more preferably 1 atomic% or less, still more preferably 0.01 atomic% or less, and particularly preferably. , 0.001 atomic% or less.
  • the oxygen content in the impurities is preferably 5 atomic% or less, more preferably 1 atomic% or less, still more preferably 0.01 atomic% or less, and particularly preferably 0.001 atomic% or less.
  • the nitriding gas supplied in step 2 is a gas containing nitrogen, and a gas in which radical species are generated by plasma is preferable.
  • the nitriding gas is not particularly limited as long as it can generate nitrogen radicals, but it is preferable that it does not contain carbon, ammonia / hydrogen plasma gas and nitrogen plasma gas are more preferable, and nitrogen plasma that can be easily used. Gas is particularly preferred.
  • the "ammonia / hydrogen plasma gas” refers to a plasma of a mixed gas of ammonia and hydrogen.
  • the precursor of the present invention is a monovalent organic gallium complex.
  • the monovalent gallium complex is an inorganic complex such as gallium chloride (I) or gallium bromide (I)
  • inorganic complex such as gallium chloride (I) or gallium bromide (I)
  • gallium chloride (I) or gallium bromide (I) it is not preferable because there is a risk of contamination or corrosion.
  • GaCl there is a concern that Cl may be mixed in or the substrate or chamber may be corroded by by-products.
  • Examples of the monovalent organic gallium complex include a cyclopentadienyl complex represented by the following general formula (1).
  • R 1 to R 5 are each independently an alkyl group having a hydrogen atom or a carbon atom number of 1 to 4.
  • the precursor represented by the general formula (1) is ⁇ 5 -pentamethylcyclopentadienyl gallium (I) represented by the following structural formula (hereinafter referred to as “Cp * Ga” or “Ga (C)”. It is also described as 5 (CH 3 ) 5) ”).
  • the substrate for growing GaN for example, a silicon substrate, a sapphire substrate, a silicon carbide substrate, a GaN substrate, or the like is used.
  • a GaN substrate of the same material is suitable for forming a GaN film having high crystallinity, but a sapphire substrate is also suitable in that the lattice constant is close to that of GaN.
  • a substrate containing neither nitrogen, gallium nor aluminum as a main component can be used to form a highly crystalline GaN film, and a silicon substrate is suitable as such a substrate. ..
  • the silicon substrate may be naturally oxidized in the atmosphere and its surface may be covered with a very thin film of silicon dioxide.
  • the film thickness of the gallium oxide thin film is preferably 5 nm or less, more preferably 2 nm or less, and particularly preferably 1.5 nm or less because it does not adversely affect the characteristics of the GaN film.
  • a method for forming the gallium oxide thin film an atomic layer deposition method is preferable, and since continuous film formation can be performed, it is preferable to use the same raw material as the present invention.
  • the oxidizing agent any oxidizing agent such as water, oxygen, ozone, oxygen plasma, or a combination thereof can be used as long as a gallium oxide thin film can be formed.
  • the gallium oxide thin film may be amorphous or crystalline, but is preferably amorphous, which is easy to form. Further, it may be a 1 to 5 atomic layer or a monatomic layer.
  • the temperature at the time of ALD is lower than the temperature at which the monovalent organic gallium complex adsorbed on the substrate is thermally decomposed and needs to be set to a temperature sufficient to sufficiently react with the nitride gas, preferably 50 to 350 ° C. More preferably, 150 to 250 ° C.
  • a temperature sufficient to sufficiently react with the nitride gas preferably 50 to 350 ° C. More preferably, 150 to 250 ° C.
  • pentamethylcyclopentadienyl gallium (Cp * Ga, GaC 5 (CH 3 ) 5 ) is used as the precursor, it can be said that 200 ° C. at which Cp * Ga does not thermally decompose is a suitable temperature.
  • the substrate temperature and the reaction temperature are the same.
  • a step 3 for supplying a reducing gas containing no oxygen may be further provided between the steps 1 and 2.
  • the reducing gas has the effect of desorbing the counterion of Ga and the cyclopentadienyl group from the monovalent organic gallium complex adsorbed on the substrate. That is, the reducing gas is usually supplied to supply a monovalent organic gallium complex as a precursor and adsorb it on the substrate, and then supply the reducing gas to desorb the ligand before supplying the nitride gas to react. do.
  • ammonia and / or hydrogen is preferable, and a gas introduced with nitrogen and / or an inert gas (for example, argon) at an appropriate ratio may be used. At this time, it is also preferable to generate radical species by plasma of these gases.
  • nitrogen and / or an inert gas for example, argon
  • an inert gas such as nitrogen or argon is usually introduced in order to purge the unreacted raw materials and by-products from the reaction space.
  • a precursor such as Cp * Ga, ammonia / hydrogen plasma gas, and nitrogen plasma gas are formed in this order.
  • the precursor is irradiated with ammonia / hydrogen plasma gas
  • the precursor adsorbed on the substrate reacts with the ammonia / hydrogen plasma gas and the ligand of the precursor is desorbed.
  • the NH groups remaining on the film formed on the substrate and H in the NH 2 groups are removed, and a crystalline GaN thin film is formed.
  • the nitriding gas is excited, dissociated, and ionized using a power of 400 W to generate plasma.
  • the electric power is not limited in size as long as it can generate plasma. Further, electric power may be applied directly to the vicinity of the substrate to generate plasma, or may be generated to a place slightly distant.
  • a highly crystalline GaN film can be formed from a monovalent organic gallium complex without performing high-temperature heat treatment such as laser annealing.
  • Example 1 (1) Cp * Ga Preparation Cp * Ga (pentamethylcyclopentadienyl gallium) is P. Jutzi et al., J. Organomet . Chem. 654, was synthesized according to the method described in 176 (2002). As a result of differential scanning calorimetry (DSC), an exothermic peak due to decomposition was observed at 250 ° C for Cp * Ga.
  • DSC differential scanning calorimetry
  • a silicon wafer with a natural oxide film is installed in an ALD device (FlexAL; manufactured by Oxford Instruments Co., Ltd.), and Cp * Ga is used as a precursor to reduce gas and nitride gas.
  • ALD film formation was performed using ammonia / hydrogen plasma gas and nitrogen plasma gas, respectively.
  • the temperature at which Cp * Ga was vaporized to the outside was set to 80 ° C, and the substrate temperature was set to 200 ° C. That is, in order to obtain a GaN film, ALD film formation was performed in the order of Cp * Ga ⁇ ammonia / hydrogen plasma ⁇ nitrogen plasma.
  • a silicon wafer with a natural oxide film is installed in an ALD device (FlexAL; manufactured by Oxford Instruments Co., Ltd.), Cp * Ga is used as a precursor, and water is used as an oxidizing agent. And oxygen plasma gas was used in this order to form a gallium oxide thin film having a thickness of 1.1 nm on a silicon wafer with a natural oxide film.
  • ALD film formation was performed using Cp * Ga and ammonia / hydrogen plasma gas and nitrogen plasma gas as the reducing gas and the nitride gas, respectively.
  • the temperature at which Cp * Ga was vaporized to the outside was set to 40 ° C., and Ar bubbling was performed.

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Abstract

原子層堆積（ＡＬＤ）による、より効率的なＧａＮ膜の製造方法であって、レーザーアニールなどの高温熱処理を行わずに、一価のガリウム化合物を用いて、不純物が極めて少なく結晶性の高いＧａＮ膜を製造する方法を提供する。 一価の有機ガリウム錯体を、基板温度を３５０℃以下とした反応室内に供給する工程１と、該反応室内に窒化ガスを供給する工程２とを含む、ＡＬＤを用いた結晶性の窒化ガリウム薄膜の製造方法。

Description

結晶性の窒化ガリウム薄膜の製造方法
　本発明は、原子層堆積（ＡＬＤ）法による結晶性の窒化ガリウム薄膜の形成方法に関する。
　窒化ガリウム（ＧａＮ）は、発光ダイオード（ＬＥＤ）および青色レーザーにおいて青～青紫光を放出するのに使用される重要な半導体材料である。シリコン基板の上に単結晶ＧａＮ層を成長させることが非常に望ましいが、ＧａＮとシリコンとの結晶格子面間隔の不整合のため、ＧａＮと格子定数の近いサファイア基板に結晶性のＧａＮをエピタキシャル成長させることが行われている。しかしながら、単結晶サファイア基板は一般にシリコン基板に比べて著しくコストがかかるため、シリコン基板のように汎用性で廉価な基板に結晶性の高いＧａＮ膜を成長させる技術が望まれている。
　ＧａＮの製膜には、一般に化学気相成長（ＣＶＤ）法が用いられる。ＣＶＤ法の一種である、原子層堆積（ＡＬＤ）法では、原料を連続的に堆積させず、一層ずつ堆積させるため、数ナノメータ単位の超薄膜を、高精度に制御された方法で堆積することができる。
　そこで、ＡＬＤを用いた結晶性ＧａＮの成膜方法が検討されている。例えば、水素をキャリアガスにして、トリエチルガリウム（ＴＥＧ）およびアンモニアを用いて、４５０～９００℃で、サファイア基板上に単結晶ＧａＮ薄膜を形成する方法が報告されている（非特許文献１）。サファイアは４５０～９００℃の高温に耐え、耐熱性が高い点でも、ＧａＮをエピタキシャル成長させるのに好適である。
　特許文献１では、ＧａＮ－ＡＬＤ膜の形成方法として、窒化アルミニウム（ＡｌＮ）核生成層を塗布したシリコン基板上にＧａＮデバイス層を形成し、レーザーアニールによってＧａＮを結晶化させる方法が開示されている。
　レーザーアニール以外の結晶化の方法として、トリメチルガリウム（ＴＭＧ）、水素ラジカル、およびアンモニアをこの順に導入して、１００℃でＡＬＤを実施する際に、水素ラジカル導入の後、及びアンモニア導入の後に電子線を照射して、水素を脱離させることによって結晶性のＧａＮ膜を形成する方法もある（非特許文献２）。ただし、この方法で得られたＧａＮ膜は炭素不純物が１０～３５ａｔ％と多い。
　また、プラズマを用いて結晶化する例として、非特許文献３では、ＴＭＧと、アンモニアプラズマ、窒素および水素の混合ガスのプラズマ（以下「窒素/水素プラズマ」と記す。）、または窒素プラズマとを使用して、２００℃で結晶性薄膜を形成している。ただし、前記結晶性薄膜において、Ｎ/Ｇａ比は有意にＮリッチであり、水素なしの窒素プラズマでは膜の品質が不良である。非特許文献４では、ＴＥＧおよび窒素/水素プラズマを使用して、２００℃、２８５℃、および３５０℃で、結晶性ＧａＮ薄膜を形成している。前記結晶性薄膜について、Ｓｉ（１００）面への成膜では、シリコン基板との界面から１８ｎｍ程のアモルファス層がある。いずれの膜も、Ｎ/Ｇａ比は、大幅にＮリッチである。非特許文献５では、ＴＭＧおよび窒素/水素の混合プラズマ使用して、２５０℃でＡＬＤを行い、結晶性膜を形成している。
　非特許文献５では、プラズマを用いても、２１０℃未満ではＡＬＤ成膜できず、２５０℃で得られたＡＬＤ膜のＮ/Ｇａ比はややＧａリッチであるがＣ不純物が約９％含まれている。プラズマを用いる結晶化については、特許文献２にも例示されている。特許文献２では、ＴＭＧあるいはＴＥＧと窒素/水素プラズマを使用してＧａＮ薄膜を形成していて、ＧａＣｌ３、ＧａＣｌ、ＧａＩ３等のハロゲン化ガリウムも例示されている。
特表２０１６－５３３６４５号公報 ＵＳ８，８４６，５０２公報
「Atomic layer epitaxy of GaN over sapphire using switched metalorganic chemical vapor deposition」、M. Asif Khan et al., Appl. Phys. Lett. 60,1366 (1992) 「Electron enhanced growth of crystalline gallium nitride thin films at room temperature and 100 °C using sequential surface reactions」、Jaclyn K. Sprenger et al., Chem. Mater. 2016, 28, 15, 5282-5294 「Hollow cathode plasma-assisted atomic layer deposition of crystalline AlN, GaN and AlxGa1-xNthin films at low temperatures」、Cagla Ozgit-Akgun et al., J. Mater. Chem. C, 2014, 2, 2123 「PEALD-deposited crystalline GaN films on Si (100) substrates with sharp interfaces」、San-JieLiu et al., Chin. Phys. B Vol. 28, No. 2 (2019) 026801 「Low temperature depositions of GaN thin films by plasma-enhanced atomic layer deposition」、Tang Wen-Hui et al., Acta Physica Sinica, 66, 098101 (2017)
　ＧａＮ膜をＡＬＤ法によって形成するためのＧａ源は、ＴＭＧやＴＥＧが広く用いられているが、これらは空気中で非常に不安定で自然発火するため、取り扱いが容易とはいえない。さらに、結晶性の高いＧａＮ薄膜を形成させるには、高温でＡＬＤを行うか、またはＡＬＤと同時もしくは直後に、高温熱処理をレーザー、または電子線の照射によるアニール技術とを組み合わせて行う必要があった。
　基板上に形成されるＧａＮ膜は一価のガリウム化合物である。そのため、原料にＴＭＧやＴＥＧなどの三価のガリウム錯体を用いれば、ガリウムを還元する必要があるためＣの混入のおそれがあり、ひいては結晶性の良くないＧａＮ膜になってしまうという問題があった。また、一価のガリウム錯体であっても、無機ガリウム錯体の場合、無機元素がＧａＮに混入するおそれがあった。そこで、本発明では、より効率の良いＡＬＤによるＧａＮ膜の製造方法であって、不純物が極めて少なく、結晶性の高いＧａＮ膜を製造する方法を提供することを目的とする。
　また、従来は、ＧａＮ薄膜の結晶性を高めるために、ＧａＮ、あるいはサファイアやＡｌＮなどのＧａＮと格子定数の近い基板を用いていたが、本発明では、ＧａＮやサファイアの構成元素である、窒素、ガリウム、およびアルミニウムのいずれも主成分として含まない基板上であっても、結晶性の高いＧａＮが得られることも目的とする。
　本発明の結晶性の窒化ガリウム薄膜の製造方法は、原子層堆積（ＡＬＤ）法を用いて、一価の有機ガリウム錯体を、基板温度を３５０℃以下とした反応室内に供給する工程１と、該反応室内に窒化ガスを供給する工程２とを含むことを特徴とする。
　前記窒化ガスは窒素プラズマガスであることが好ましい。
　前記有機ガリウム錯体はシクロペンタジエニル系錯体であることが好ましい。
　前記工程１と工程２との間に、さらに、酸素を含まない還元ガスを供給する工程３を含むことが好ましい。
　本発明の好ましい実施形態では、前記基板の表面は、窒素、ガリウム、およびアルミニウムのいずれも主成分として含まない。
　前記工程１の前に、基板の前処理として、前記原料と酸化剤を用いて、５ｎｍ以下の酸化ガリウムを堆積する工程を含むことが好ましい。
　本発明のＡＬＤ法によれば、レーザーアニールなどの高温熱処理を行わなくても、有機ガリウム錯体から結晶性の高いＧａＮ膜を形成させることができる。
図１は、実施例１で、自然酸化膜付きのシリコンウエハ上に形成したＧａＮ膜の断面ＴＥＭ像を表す。 図２は、実施例２で成膜したＧａＮ膜の断面ＴＥＭ像を表す。
　以下、本発明について詳細に説明する。
　本発明の窒化ガリウム（ＧａＮ）薄膜の製造方法は、ＡＬＤを用いて、一価の有機ガリウム錯体を、基板温度を３５０℃以下とした反応室内に供給する工程１と、該反応室内に窒化ガスを供給する工程２とを有する。
　ＡＬＤには、熱ＡＬＤおよびプラズマＡＬＤ（ＰＥＡＬＤ）の２つがある。熱ＡＬＤでは高アスペクト比表面に沿って均一な膜を形成することができる。一方、ＰＥＡＬＤは、高アスペクト比表面への膜形成能は熱ＡＬＤより劣る場合もあるが、低温で行うことができる。本発明ではどちらの方法を用いることもできるが、一価のガリウム化学種を用いて、効率良く結晶性の高いＧａＮ膜を形成するという本発明の目的からすると、ＰＥＡＬＤが好適である。
　本発明の好ましい実施形態であるＰＥＡＬＤでは、下記（ｉ）～（ii）の堆積サイクルによってＧａＮを製膜する。堆積サイクルの一形態は、（ｉ）基板の表面上に、前駆体を吸着するために、反応室内に気相の前駆体を送り込むステップと、（ii）該反応室内にプラズマによりラジカル種を生成させた窒化ガスを送り込み、表面に吸着した前駆体と反応させてＧａＮ結晶層を形成するステップとを有する。そして、各サイクルは、形成される膜が所望の厚さに達するまで、繰り返される。
　前記（ｉ）において、基板が設置された反応室内に、前駆体である一価の有機ガリウム錯体を気相で供給する（工程１）。基板温度は、室温～３５０℃の範囲内の任意の温度とし、一価の有機ガリウム錯体は、基板上で凝縮しないように、基板温度よりも低い温度で蒸発させる。
　次いで、前記（ii）では、反応室内に窒化ガスを供給して（工程２）、工程１で基板表面上に吸着した前駆体と窒化ガスを反応させて前記基板上に結晶性のＧａＮ薄膜を成長させる。このとき、ＧａＮ薄膜は多結晶であっても良いが、単結晶であることが好ましい。単結晶の場合、薄膜のＮ/Ｇａ比は１となるが、多結晶の場合、緻密な膜が好ましく、ＧａリッチとなるＮ/Ｇａ比１以下が好ましい。前記ＧａＮ薄膜において、アモルファス部分はほとんど存在しないが、１体積％未満が好ましく、０．０１体積％未満がより好ましく、０．０００１体積％未満が特に好ましい。また、前記ＧａＮ薄膜は高純度の薄膜であり、不純物中の炭素は、好ましくは、５原子％以下、より好ましくは、１原子％以下、さらに好ましくは、０．０１原子％以下、特に好ましくは、０．００１原子％以下である。同様に、不純物中の酸素は、好ましくは、５原子％以下、より好ましくは、１原子％以下、さらに好ましくは、０．０１原子％以下、特に好ましくは、０．００１原子％以下である。
　工程２で供給する窒化ガスは、窒素を含むガスであって、プラズマによってラジカル種を発生させたガスが好ましい。窒化ガスとしては、窒素ラジカルが発生できるガスであれば特に制限はないが、炭素を含まないことが好ましく、アンモニア/水素プラズマガス、および窒素プラズマガスがより好ましく、簡便に用いることのできる窒素プラズマガスであることが特に好ましい。なお、「アンモニア/水素プラズマガス」は、アンモニアおよび水素の混合ガスのプラズマをいう。
　ここで、本発明の前駆体は一価の有機ガリウム錯体である。一価のガリウム錯体が、塩化ガリウム（Ｉ）や臭化ガリウム（Ｉ）などの無機錯体であった場合、混入や腐食のおそれがあり好ましくない。例えば、ＧａＣｌの場合は、Ｃｌの混入や副生成物による基板やチャンバーの腐食の懸念がある。
　一価の有機ガリウム錯体としては、下記一般式（１）で表されるシクロペンタジエニル錯体が挙げられる。
Figure JPOXMLDOC01-appb-I000001
　一般式（１）中、Ｒ1～Ｒ5は、それぞれ独立に、水素原子又は炭素原子数１～４のアルキル基である。
　前記一般式（１）において、Ｒ1～Ｒ5のうち４つはメチル基であり、残りの１つはメチル基、エチル基、ノルマルプロピル基又はイソプロピル基であることがより好ましい。一般式（１）で表される前駆体は、具体的には、下記構造式で表されるη5－ペンタメチルシクロペンタジエニルガリウム（Ｉ）（以下「Ｃｐ*Ｇａ」又は「Ｇａ（Ｃ5（ＣＨ3）5）」とも記す。）であることが特に好ましい。
Figure JPOXMLDOC01-appb-I000002
　ＧａＮを成長させるための基板としては、例えば、シリコン基板、サファイア基板、炭化ケイ素基板およびＧａＮ基板などが用いられる。これらのうち、結晶性の高いＧａＮ膜を形成するのに、材質が同一であるＧａＮ基板が好適であることは言うまでもないが、ＧａＮと格子定数が近い点でサファイア基板も好適である。
　本発明では、結晶性の高いＧａＮ膜を形成するのに、窒素、ガリウム、およびアルミニウムのいずれも主成分として含まない基板を用いることができ、このような基板としては、シリコン基板が好適である。シリコン基板は、大気中で自然酸化して、非常に薄い二酸化ケイ素の膜でその表面が被覆されていてもよい。
　基板に対して、ＧａＮ膜を形成するための前処理として、ごく薄い酸化ガリウム薄膜を形成することも好ましい。この酸化ガリウム薄膜の膜厚は、ＧａＮ膜の特性に悪影響を与えないため５ｎｍ以下が好ましく、２ｎｍ以下がより好ましく、１．５ｎｍ以下が特に好ましい。酸化ガリウム薄膜を形成する方法としては、原子層堆積法が好ましく、連続して成膜できることから、本発明と同一の原料を用いることが好ましい。酸化剤としては酸化ガリウム薄膜を形成することができれば、水、酸素、オゾン、酸素プラズマ、あるいはそれらの組み合わせなど、任意の酸化剤が使用できる。この酸化ガリウム薄膜は、アモルファスでも結晶性でも良いが、形成の容易なアモルファスであることが好ましい。また、１～５原子層であっても、単原子層であっても良い。
　ＡＬＤを行うときの温度は、基板に吸着した一価の有機ガリウム錯体が熱分解する温度よりも低く、窒化ガスと十分に反応する程度の温度にする必要があり、５０～３５０℃が好ましく、１５０～２５０℃がより好ましい。前駆体にペンタメチルシクロペンタジエニルガリウム（Ｃｐ*Ｇａ、ＧａＣ5（ＣＨ3）5）を用いる場合は、Ｃｐ*Ｇａが熱分解しない２００℃が好適な温度といえる。なお、基板温度と反応温度は同一になる。
　工程１と工程２との間に、さらに、酸素を含まない還元ガスを供給する工程３を有してもよい。還元ガスは、基板上に吸着した一価の有機ガリウム錯体からＧａの対イオンやシクロペンタジエニル基を脱離させる効果を有する。つまり、還元ガスは、通常、前駆体である一価の有機ガリウム錯体を供給して基板に吸着させた後、窒化ガスを供給して反応させる前に、配位子を脱離させるために供給する。
　還元ガスとしては、アンモニアおよび/または水素が好ましく、さらにこれらに窒素および/または不活性ガス（例えば、アルゴン）を適当な比率で導入したガスを用いてもよい。このとき、これらのガスのプラズマによってラジカル種を発生させることも好ましい。
　なお、前記工程１、工程２および工程３の後には、未反応原料および副生成物を反応空間からパージするため、通常、窒素やアルゴンのような不活性ガスを導入する。
　本発明の結晶性の窒化ガリウム薄膜の製造方法の好ましい形態では、Ｃｐ*Ｇａ等の前駆体、アンモニア/水素プラズマガス、および窒素プラズマガスの順のサイクルで成膜する。前駆体にアンモニア/水素プラズマガスを照射すると、基板上に吸着した前駆体とアンモニア/水素プラズマガスとが反応して前駆体の配位子が脱離する。次いで、窒素プラズマを照射することで、基板上に形成された膜に残留するＮＨ基やＮＨ2基中のＨが除去されて、結晶性のＧａＮ薄膜が形成される。この窒化反応では、真空下に０．１～１０００ｍＴｏｒｒの窒素を含むガスを導入しながら、例えば、４００Ｗの電力を用いて窒化ガスを励起・解離・電離させてプラズマを発生させる。このとき、電力はプラズマが発生できる電力であれば大きさに制限はない。また、電力を基板付近に直接かけてプラズマを発生させても、少し離れたところにかけて発生させても良い。
　以上のとおり、本発明のＡＬＤ法によれば、レーザーアニールなどの高温熱処理を行わなくても、一価の有機ガリウム錯体から結晶性の高いＧａＮ膜を形成することができる。
　以下、本発明を実施例に基づいてさらに具体的に説明するが、本発明は下記実施例により制限されるものではない。
　［実施例１］
（１）Ｃｐ*Ｇａの調製
　Ｃｐ*Ｇａ（ペンタメチルシクロペンタジエニルガリウム）はP. Jutzi et al., J. Organomet. Chem. 654, 176 (2002)に記載された方法に従って合成した。
　示差走査熱量測定（ＤＳＣ）の結果、Ｃｐ*Ｇａは２５０℃に分解による発熱ピークが観測された。
（２）ＡＬＤによるＧａＮ膜の形成
　ＡＬＤ装置（ＦｌｅｘＡＬ；オックスフォード・インストゥルメンツ（株）製）内に自然酸化膜付きシリコンウエハを設置し、前駆体としてＣｐ*Ｇａを用い、還元ガスおよび窒化ガスとして、それぞれ、アンモニア/水素プラズマガスおよび窒素プラズマガスを用いてＡＬＤ成膜を行った。このとき、Ｃｐ*Ｇａを外部で気化する温度は８０℃とし、基板温度は２００℃とした。
　すなわち、ＧａＮ膜を得るのに、Ｃｐ*Ｇａ→アンモニア/水素プラズマ→窒素プラズマの順のサイクルでＡＬＤ成膜を行った。
　この方法で３００サイクル成膜したＧａＮ膜を透過電子顕微鏡（ＴＥＭ）によって断面を観察したところ、結晶化していた。この結果（ＸＴＥＭ像）を図１に示す。
　また、このサンプルのＧａＮ膜部分を高分解能ＲＢＳ分析装置（ＨＲＢＳ５００；（株）神戸製鋼所製）で組成分析を行った結果、Ｃ、Ｏ不純物は検出限界（Ｃ；４原子％程度、Ｏ；３原子％程度）以下であり、Ｎ/Ｇａ比は０．９であった。
　このように本発明の方法によれば、不純物が極めて少なく、結晶性の高いＧａＮ膜が製造できる。
　［実施例２］
（１）Ｃｐ*Ｇａの調製
　実施例１と同様にして調製した。
（２）ＡＬＤによるＧａＮ膜の形成
　ＡＬＤ装置（ＦｌｅｘＡＬ；オックスフォード・インストゥルメンツ（株）製）内に自然酸化膜付きシリコンウエハを設置し、前駆体としてＣｐ*Ｇａを用い、酸化剤として、水および酸素プラズマガスをこの順に用いて、自然酸化膜付きシリコンウエハ上に厚さ１．１ｎｍの酸化ガリウム薄膜を形成した。次いで、Ｃｐ*Ｇａならびに、還元ガスおよび窒化ガスとして、それぞれ、アンモニア/水素プラズマガスおよび窒素プラズマガスを用いてＡＬＤ成膜を行った。このとき、Ｃｐ*Ｇａを外部で気化する温度は４０℃とし、Ａｒバブリングを行った。このときの基板温度は２００℃とした。
　すなわち、ＧａＮ膜を得るのに、酸化ガリウム→Ｃｐ*Ｇａ→アンモニア/水素プラズマ→窒素プラズマの順のサイクルでＡＬＤ成膜を行った。この方法で５００サイクル成膜したＧａＮ膜を透過電子顕微鏡（ＴＥＭ）によって断面を観察したところ、結晶化していた。この結果（ＸＴＥＭ像）を図２に示す。

Claims (6)

  1. 　一価の有機ガリウム錯体を、基板温度を３５０℃以下とした反応室内に供給する工程１と、該反応室内に窒化ガスを供給する工程２とを含む、原子層堆積（ＡＬＤ）法を用いた結晶性の窒化ガリウム薄膜の製造方法。
  2. 　前記窒化ガスが窒素プラズマガスである、請求項１に記載の結晶性の窒化ガリウム薄膜の製造方法。
  3. 　前記有機ガリウム錯体がシクロペンタジエニル系錯体である、請求項１または２に記載の結晶性の窒化ガリウム薄膜の製造方法。
  4. 　前記工程１と工程２との間に、さらに、酸素を含まない還元ガスを供給する工程３を含む、請求項１～３のいずれか一項に記載の結晶性の窒化ガリウム薄膜の製造方法。
  5. 　前記基板の表面が、窒素、ガリウム、およびアルミニウムのいずれも主成分として含まない、請求項１～４のいずれか一項に記載の結晶性の窒化ガリウム薄膜の製造方法。
  6. 　前記工程１の前に、基板の前処理として、前記原料と酸化剤を用いて、５ｎｍ以下の酸化ガリウムを堆積する工程を含む請求項１～５のいずれか一項に記載の結晶性の窒化ガリウム薄膜の製造方法。
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