JP4480298B2 - Boride crystal, substrate for forming semiconductor layer using the same, and method for producing the same - Google Patents

Description

【００１８】
本発明のホウ化物単結晶は、少量のＣｒＢ_２を含むＸＢ_２系のＺｒＢ_２若しくはＴｉＢ_２の化合物またはこれら化合物の固溶体の単結晶であり、半導体層形成用の基板に用いられる。基板上に、半導体層として、ガリウム、アルミニウム、インジウム、硼素のいずれか少なくとも一つの窒化物半導体層を成長形成させるのに使用される。窒化物半導体層は、特に、窒化ガリウム系半導体層に利用される。これら窒化物半導体の成長層は、上述のように、この基板と良好な格子整合関係を持ち、特に、基板が単結晶であり、且つ、この単結晶基板が大きな寸法であるで、その上に形成された半導体成長層は、広い面域に亘って、格子欠陥が少なく、且つ結晶性が極めて良好になる。
【００１９】
また、表１に示すように、本発明の二硼化物単結晶から形成した基板は、サファイア基板と対比して、上記の窒化物半導体との熱膨張係数の差が小さいのも特徴であり、これにより、本発明の基板上での例えばＧａＮ膜の成長過程で、温度降下する場合にＧａＮ膜に加わる（０００１）面内方向への応力発生はサファイア基板を用いたときに比べ小さくなる。それに伴って、サファイア基板を使ったときに見られようなＧａＮ膜成長させた基板の反り変形が小さくなる。
【００２０】
上記の二硼化物単結晶は、表１に示すように、サファイアよりも良好な熱伝導性を示し、半導体成長用の基板として利用すれば、半導体装置部分からの放熱が良好となる。また、上記の二硼化物単結晶は、良好な導電性を示す。そこで、半導体装置に、二硼化物単結晶を基板に用いると、基板は導電性を有し、半導体層の上表面と共に、基板の表面、又は特に、裏面側に、接続用の電極を形成することが可能となる。特に、基板裏面に電極が形成できることは、基板上に形成される半導体装置の高効率化や小型化に有効である。さらに基板自体を電極として使用することも可能であり、リード線を直接接続することもできる。
【００２１】
本発明のＸＢ_２単結晶の製造方法は、ＸＢ_２にＣｒＢ_２を混合した原料棒からフローティング・ゾーン法により単結晶に成長させる方法が利用される。原料棒は、フローティング・ゾーン法により、混合物に溶融帯を形成しながら溶融帯を垂直方向に移動させて、溶融帯の通過した後の一方凝固により、特定方位のＸＢ_２結晶を優先的に成長させることにより、単結晶が得られる。
【００２２】
この方法では、詳しくは、ＸＢ_２粉末とＣｒＢ_２粉末の混合物は、圧粉体又は焼結体として棒状原料に成形される。帯溶融装置のホルダーで原料棒を垂直に配置して、原料棒の下方側に局部的に加熱して溶融帯を作りながら、原料棒に対して加熱位置を相対的に上方に移動させ、溶融体を移動させる。このように移動する溶融帯の下部は冷却されながら凝固し、凝固過程ては、熱勾配が一様であるで、上方に伸びるように結晶を成長させることができる。
【００２３】
単結晶化を促進するには、ＸＢ_２の単結晶片を種結晶として使用するのが好ましい。上記の例では、始めに、種結晶と原料棒とを先端同士接触又は近接させて配置し、その近接部位の原料棒から帯溶融を開始して、原料棒長手方向に沿って順次、溶融帯を移動させるようにする。
【００２４】
本発明においては、ＸＢ_２粉末にＣｒＢ_２粉末が配合され、ＣｒＢ_２が溶融帯直下の凝固過程で、単一の結晶だけを優先成長させ、大きな直径の単結晶を得ることができる。
一般には、純度の高いＺｒＢ_２など単独の化合物の融点はきわめて高く（ＺｒＢ_２の融点は３０５０±５℃であり、ＴｉＢ_２の融点は２７９０±５℃）、従って、溶融帯からの試料の放冷凝固過程では固液界面での温度勾配が大きくて、成長過程では、固液界面で幾つかの結晶が同時成長し、図３に模式的に示すように、得られた試料の断面には、結晶粒界１２により区分された多数の結晶６から成る多結晶構造が得られる。
【００２５】
これに対して、本発明においては、ＣｒＢ_２の役割は明らかでないけれども、ＸＢ_２原料中にＣｒＢ２を少量添加することによりＸＢ_２混合物の融点が低下し（１５ｍｏｌ％のＣｒＢ_２につき融点は約２００℃の低下）それにより、温度勾配が緩和されて（温度勾配はそのときの凝固温度の４乗に比例すると考えられる）、図２に模式的に示すように、単一の結晶のみが優先的に成長し、結晶体全体が単結晶６０により構成されるものと考えられる。また、種子結晶を用いない場合には、成長初期に多結晶体から単結晶化するまでの長さがＣｒＢ_２の添加がない場合には３〜５ｃｍであったものが、ＣｒＢ_２を添加することにより、２ｃｍ以下になり、単結晶化が促進される。
【００２６】
本発明の製造方法においては、ＣｒＢ_２の配合量は、ＸＢ_２とＣｒＢ_２との粉末混合物中に、１〜３０ｍｏｌ％のＣｒＢ_２の範囲とする。１ｍｏｌ％以下のＣｒＢ_２の配合では、単結晶化促進に効果が少なく、３０ｍｏｌ％を越えると、溶融帯にＣｒＢ_２が濃化し、単結晶の成長速度が低下するので好ましくない。特に、ＣｒＢ_２の配合量は、混合物中５〜１５ｍｏｌ％の範囲が好ましい。
【００２７】
フローティング・ゾーン法においては、加熱は、高輝度ランブからの収束光や、レーザ光が使用できるが、Ｔｉ又はＺｒの二硼化物は、電気伝導性があるので、好ましくは、高周波誘導加熱が利用される。図１は、高周波誘導加熱式のフローティング・ゾーン炉を示す。この装置は、密閉耐圧容器１２中に加熱部を配置しており、容器内空間１３に数気圧の不活性ガス雰囲気において結晶育成が可能なように設計されている。
【００２８】
この図において、誘導加熱には、誘導コイル４が使用されるが、この例のコイル４は、水冷銅管が水平面に渦巻き状に捲回されて構成され、そのコイル４の中心部が、試料が挿通する貫通孔を形成し、耐圧容器１２内に配置されている。誘導コイル４の上下位置には、耐圧容器１２外に昇降可能な駆動部１、９と、それぞれの駆動部１、９から伸びて耐圧容器を気密的に且つ摺動可能に貫通する上軸２及び下軸１０と、これら上下の軸２、１０にそれぞれ固定された試料握持部、即ちホルダー３、９とが配置され、上ホルダー３と下ホルダー１１との間で、誘導コイルの中心部を貫通する原料棒の端部が握持される。そして、上軸駆動部１と下軸駆動部９とは、個別に上下移動可能に制御される。
【００２９】
この例では、誘導コイル４と原料棒が耐圧容器内に収容され、数気圧〜数十気圧の圧力で不活性ガス雰囲気に制御して、溶融帯７からの揮発を抑制し、溶融帯が耐火物などと接触することなく、結晶育成がてきるようにされている。
【００３０】
帯溶融の操作においては、上ホルダー３に原料棒８の上端を握持して懸下し、原料棒８下端側を、下ホルダー１１に握持された種結晶の上端に接触させて、且つ、誘導コイルの中心部に位置付けし、誘導コイル４に高周波電力を供給して、上記導電性二硼化物原料棒８に誘導電流を生じさせ、そのジュール熱により原料棒８の下端部を局部的に加熱溶融させる。このようにして、形成された溶融帯７に上方より原料棒を送り込み、下方より単結晶６０を成長させる。
【００３１】
本発明による単結晶育成の原料棒の例としては、原料の二ホウ化物粉末ＸＢ_２（ＸはＴｉ及びＺｒの少なくとも一種を含む）と二ホウ化クロム(ＣｒＢ_２)粉末を所要の配合量にしてよく混合し、結合剤として例えば少量の樟脳を加え、ラバープレスにより静水圧（例えば、２０００ｋｇ／ｃｍ^２）で加圧して、棒状の圧粉体を作る。好ましくは、この圧粉棒を真空中又は不活性ガス中で千数百℃に加熱して焼結して、原料棒を作る。
【００３２】
作成した原料棒８を上軸２にホルダー３を介して固定し、下軸１０には種結晶（または初期融帯形成用の焼結棒）５をホルダー１１を介してセットする。つぎに、原料棒８の下端をコイル４の誘導加熱により溶融させ、溶融帯７を形成させ、上軸２と下軸１０をゆっくりと下方に移動させて、原料棒に対して相対的に溶融帯を上昇させ、溶融帯の冷却凝固に対応して単結晶６０を育成する。このとき、原料棒８の溶融帯７への供給速度は、供給原料棒の密度（理論密度の通常約５５％）と育成する結晶の直径を考慮して、設定される。このようにして、実質的に結晶粒界を含まない大径を有する単結晶棒を得ることができる。
【００３３】
雰囲気は、数気圧のアルゴンまたはヘリウムなどの不活性ガスを用いるのが好ましい。これは、誘導コイル４の部分で発生する放電を防止するためであり、さらに、溶融帯７からの蒸発を抑制するためである。
【００３４】
このようにして得られたＸＢ_２単結晶棒は、切断加工し、主面が所要の面方位に成るように切り出し、片面を研磨加工して主面とした基板に形成される。一例として、ＧａＮ系半導体層の形成用の基板では、基板主面は、通常は、結晶の（０００１）面に平行になるように調製される。
【００３５】
作成された基板は、アルミナ、シリカ砥粒を用いて化学的機械的研磨される。もし粒界が存在すれば、研摩表面を目視することにより確認できるが、本発明の方法による単結晶は、ほとんど粒界は含まれない。粒界の存在は、Ｘ線トポグラフや酸エッチングによっても確認できるが、主面を化学研磨した時点で目視にて容易に判別することができる。
【００３６】
【実施例】
［実施例１］
【００３７】
この実施例では、ＸＢ_２として、ＺｒＢ_２を選び、ＺｒＢ_２とＣｒＢ_２との混合粉中のＣｒＢ_２添加量を、０．１ｍｏｌ％から３０ｍｏｌ％との間に５水準を選び、外径１２〜１４ｍｍ、長さ２０００ｍｍの粉末棒に圧縮し、１４００℃で焼結して、原料棒とした。原料棒を、高周波加熱型のフローディングゾーン炉で、入力２８ｋＷ／ｈの高周波電力を投入し２ｃｍ／ｈの速度で直径１２〜１４ｍｍの結晶棒を作った。
【００３８】
結晶棒の縦断面を腐食して目視観察及び顕微鏡観察により、ＣｒＢ_２添加による粒界の抑制効果を調べた。ＣｒＢ_２添加量の１ｍｏｌ％から粒界抑制効果が見られ、５ｍｏｌ％以上になると、抑制効果が顕著になり、断面には実質的に結晶粒界、亜粒界のない単結晶棒が得られた。また、ＣｒＢ_２添加量２０ｍｏｌ％以上では、育成時に溶融帯直上にＣｒＢ_２−ＺｒＢ_２共融物がたまり、数ｃｍ以上の溶融帯移動は不可能であったが、粒界・亜粒界のない良質な単結晶が得られた。それ故、長尺の単結晶棒を得るには、ＣｒＢ_２添加量５〜１５ｍｏｌ％の範囲が好ましい添加量であることが判った。
【００３９】
また、ＩＣＰ発光分光分析装置（セイコー電子工業（株）製造：型式「ＳＰＳ１２００ＶＲ」）を使用して、ＣｒＢ_２とＺｒＢ_２とを分析したが、得られた結晶棒中のＣｒＢ_２含量は、原料棒中の添加量の半分以下に減少しており、ＣｒＢ_２は、ＺｒＢ_２より蒸発が激しく、添加量の大半が蒸発により失われることが判る。原料棒中３０ｍｏｌ％ＣｒＢ_２の添加量が、ＺｒＢ_２結晶育成中のＣｒＢ_２の蒸発により、結晶中のＣｒＢ_２含有量は９ｍｏｌ％以下となり、原料棒中５ｍｏｌ％ＣｒＢ_２を含有させたものは、結晶中のＣｒＢ_２の含有量は０．００１０〜０．５ｍｏｌ％である。原料棒中１ｍｏｌ％ＣｒＢ_２の添加で結晶中ＣｒＢ_２は少なくとも０．０００４ｍｏｌ％が含まれるが、原料棒中ＣｒＢ_２の無添加は、結晶中は０．０００２ｍｏｌ％ＣｒＢ_２以下となる。
【００４０】
このように、原料棒中のＣｒＢ_２の添加は、溶融帯下の結晶育成初期に導入された粒界１２や亜粒界をその後の成長した結晶中には除去する効果があることが判った。即ち、直径の大きなＺｒＢ_２結晶を成長させるに際して、ＺｒＢ_２ 原料棒中にＣｒＢ_２が添加されていないで純度が高いと、結晶育成初期に導入された粒界や亜粒界は、その後の成長した結晶中からも容易に除去されない。この場合は、溶融帯直下の凝固面では順次、幾つかの結晶が同時発生して、多結晶化するのである。しかし、ＺｒＢ_２ 粉末にＣｒＢ_２を添加すると、成長成開始時に導入された粒界が容易に除去される。即ち、ＣｒＢ_２の存在は、初期の多結晶中の特定結晶粒が成長して、帯溶融域全域を単結晶化するのを促進する作用がある。ＣｒＢ_２を５ｍｏｌ％以上添加すると、溶融開始後１〜２ｃｍの溶融帯移動により、育成初期に導入されて粒界が除かれ、特定の結晶粒のみが優先的に成長し、それ以後の溶融帯移動により良質な単結晶が得られることが判った。
【００４１】
以上のＣｒＢ_２添加育成方法は、ＦＺ法に高周波加熱以外の加熱法、例えば、レーザー加熱を使用して、単結晶の育成に適用することができる。また、このようなＣｒＢ_２による結晶性の改善方法は、フローティング・ゾーン法による結晶育成だけではなく、スカルメルト法などの他の結晶成長方法にも適用することができる。
【００４２】
比較例として、上記のＦＺ法を利用して、但し、ＣｒＢ_２を添加しないでＸＢ_２ホウ化物の単独の焼結棒を用いて単結晶を作った。１ｃｍ以上の直径をもつ結晶棒が製造されたが、図３に示すように結晶中に粒界１２や亜粒界が発生し良質な単結晶が得られなかった。
【００４３】
［実施例２］
実施例１の方法において得られた直径１５ｍｍの結晶について、Ｘ線により結晶方位を同定し、（０００１）面を主面とする厚さ０．６ｍｍの板をバンドソーを用いて切り出した。得られた板は外形が不定形をしているので、１２．７ｍｍ角に外片を研削加工し、厚さ方向にはダイヤモンド砥石を用いた研削加工を行い０．４ｍｍ厚みにした。この板の片面をコロイダルシリカを用いて化学研磨加工し、洗浄の上基板とした。
【００４４】
上記方法によりＣｒＢ_２を５〜１５ｍｏｌ％添加した原料棒から調製した結晶においては研磨後に粒界は観察されず、Ｘ線の回折試験を実施したが回折ピークも単一である。さらに、、研磨後の基板表面に塩酸・硝酸による腐食を行っても粒界は確認されず均質な単結晶となっていることが確認された。
比較例のＣｒＢ_２無添加の原料棒からＦＺ法により調製した直径１０ｍｍを越える結晶棒については、化学研磨した結晶表面に粒界が目視にて観察され、また、Ｘ線回折法にて（０００１）面を測定すると回折ピークが複数観察され、基板は多結晶体であることが判った。
【００４５】
［実施例３］
実施例２により得られた研磨後の基板を用いて、有機金属化合物成長法（以下、ＭＯＣＶＤ法という）による窒化ガリウム半導体層を気相成長により膜形成した。水素ガスを炉内に流しながらサセプタを加熱して基板を１０５０℃に加熱し、１０分保持後、９５０℃にしてトリメチルアルミニウム（ＴＭＡ）とＮＨ_３を流してアンドープＡｌＮ層を１μｍ成長した。このＡｌＮ層はいわゆる低温バッファ層とされるアモルファス層又は多結晶層ではなく、高温の環境で結晶成長した単結晶層である。さらに、ＮＨ_３を流したまま１０００℃に基板を加熱し、その後トリメチルガリウム（ＴＭＧ）を流してアンドープのＧａＮを２μｍ成長した。このアンドープＧａＮエピタキシャル結晶の電気特性をホール効果法にて測定したところ、ｎ型であって、キャリア濃度は１×１０^１６ｃｍ^−３ 程度で良好な特性の結晶が成長した。
【００４６】
［実施例４］
市販のＺｒＢ_２粉末にＣｒＢ_２粉末を１０ｍｏｌ％添加混合した後、結合剤として樟脳を少量加え、直径１５ｍｍのゴム袋に詰め円柱形とした。これを２０００ｋｇ／ｃｍ^２のラバープレスを行い圧粉体を得た。この圧粉体を真空中、１８００℃で加熱し、直径１．４ｃｍ、長さ２０ｃｍ程度の焼結棒を得た。密度は約５５％であった。
【００４７】
この焼結棒を図１に示すフローティング・ゾーン炉の上軸にホルダーを介し固定し、下軸にはＺｒＢ_２焼結棒を固定した。炉内に５気圧のアルゴンを充填した後、高周波誘導加熱により原料棒下端部を溶かし初期溶融帯を形成し、２ｃｍ／ｈの速度で３時間に下方に移動させ、全長６ｃｍ、直径１、５ｃｍの単結晶を育成した。結晶中のＣｒＢ_２量は、４ｍｏｌ％であった。得られた結晶から（０００１）面を切り出し鏡面研磨の後、エッチングを行い表面観察を行った。その結果、粒界や亜粒界のない良質な単結晶であることを確認した。
【００４８】
［実施例５］
実施例４に記載の研磨後の基板を用いて、ＭＯＣＶＤ法による窒化ガリウム系半導体の気相成長を行った。水素ガスを炉内に流しながらサセプタを加熱して基板を１０５０℃に加熱後、９５０℃にしてトリメチルガリウム（ＴＭＧ）とＮＨ_３を流してアンドープＧａＮ層を０．５μｍ成長した。このＧａＮ層は、低温バッファ層とされるアモルファス層又は多結晶層ではなく、高温の環境で結晶成長した単結晶層である。さらに、ＮＨ_３を流したまま１０００℃に基板を加熱し、その後、トリメチルガリウム（ＴＭＧ）を流してアンドープのＧａＮ層を２μｍ成長させた。
【００４９】
このようにして形成したアンドープＧａＮエピタキシャル結晶層の電気特性をホール効果法にて測定したところ、ｎ型であって、キャリア濃度は１×１０^１６ｃｍ^−３程度であって、良好な特性の結晶を成長させることができた。
【図面の簡単な説明】
【図１】 本発明の実施形態における単結晶育成に使用されるフローティング・ゾーン装置の模式的断面図。
【図２】 本発明の育成方法により育成した結晶棒の腐食断面図。
【図３】 比較例の方法により育成した結晶棒の腐食断面図。
【符号の説明】
３：ホルダー
４：誘導コイル
５：種結晶
６：結晶
６０：単結晶
７：溶融帯
８：原料棒
９：ホルダー
１２：結晶粒界[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a boride single crystal composed of ZrB ₂ or TiB ₂ or a solid solution composition thereof, and a method for producing the same, and a semiconductor layer growth substrate using the same and a method for producing the same.
[0002]
[Prior art]
In recent years, gallium nitride semiconductors have been put into practical use for light-emitting diodes and the like. The gallium nitride semiconductor is gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and mixed crystals of In _x Ga _y Al _z N (0 ≦ x, 0 ≦ y, x + y ≦ 1). , Z = 1−xy)). Conventionally, such a single crystal layer of a gallium nitride semiconductor has been formed by epitaxial growth on a sapphire substrate.
[0003]
However, sapphire has a large lattice mismatch with the gallium nitride semiconductor, and crystal defects due to the lattice mismatch are introduced into the epitaxial growth layer, so that a gallium nitride semiconductor layer with excellent crystallinity cannot be obtained. there were. Also, since the sapphire substrate is an insulator, in the structure of a light emitting diode or the like, electrodes cannot be taken out from the sapphire substrate surface side, and both the positive electrode and the negative electrode are formed only on the surface on which the gallium nitride based semiconductor is formed. There was a need. This complicates the manufacturing process of the light emitting diode and the like, and has a problem that the light emitting area is smaller than the element area.
[0004]
[Problems to be solved by the invention]
In order to solve the above-described problem of crystal mismatch between the substrate and the crystal layer to be grown, the present inventors have added a chemical formula XB ₂ (where X is Ti or Zr) to the gallium nitride based crystal layer growth substrate. Is proposed in another application (Japanese Patent Application No. 2000-228903).
[0005]
As shown in Table 1 below, the crystals of ZrB ₂ and TiB ₂ have a lattice constant and a thermal expansion coefficient almost equal to those of nitride-based semiconductors such as GaN and AlN, have a relatively high thermal conductivity, and are electrically Since it is a good conductor, it can be expected as a substrate crystal for forming these gallium nitride based semiconductor layers.
[0006]
Since crystals of ZrB ₂ and TiB ₂ (hereinafter simply referred to as XB ₂ ) have a very high melting point of around 3000 ° C., they can be grown only by the floating zone method (FZ method) and the flux method. However, it is known that the FZ method is advantageous for crystal growth (for example, S. Otani, Y. and Ishizawa: Preparation of TiB ₂ Single Crystals by the Floating Zone Method; J. Crystal Growth, 140, (1994 ) pp 451-453. and S. Otani, Y. and Ishizawa: Preparation of ZrB ₂ Single Crystals by the Floating Zone Method; J. Crystal Growth, 165, (1996) pp 319-322.)
[0007]
In order to use the XB ₂ crystal as a substrate crystal of a gallium nitride based semiconductor, a single crystal that is as large as possible and substantially free of grain boundaries is required. However, with the conventional FZ growth technique, when a crystal having a large diameter, for example, a diameter of 1 cm or more is grown, a large single crystal of good quality cannot be obtained with many grain boundaries and sub-grain boundaries in the crystal. .
[0008]
In view of the above-mentioned problem of single crystallization, the present invention provides a method for producing an XB ₂ single crystal having no grain boundary using the FZ method. An object is to provide a growth substrate for forming an excellent semiconductor layer by using a single crystal.
[0009]
[Means for Solving the Problems]
The present invention is a crystal containing CrB ₂ in XB ₂ , which prevents the generation of grain boundaries by adding CrB ₂ and creates a large single crystal. That is, the boride crystal of the present invention is a crystal mainly containing XB ₂ (X is at least one of Ti and Zr), and contains 0.0004 mol% to 9 mol % of CrB ₂ in the crystal. It is characterized by. The boride crystal is a single crystal that substantially does not include a grain boundary in the crystal cross section.
[0010]
In addition, the present invention includes a substrate made of a crystal containing CrB ₂ in XB ₂ for growing a semiconductor layer on the main surface. This diboride-based substrate is particularly preferably used for forming a gallium nitride-based semiconductor layer as a semiconductor layer, and a semiconductor layer with few lattice defects can be formed on a large single crystal substrate.
[0011]
In the present invention, the XB ₂ single crystal is grown by a levitation zone melting method (hereinafter, floating zone method, FZ method). In such a method for producing an XB ₂ single crystal, XB ₂ (X is Ti And at least one kind of Zr) and a mixed raw material containing CrB ₂ is used to grow a single crystal by a floating zone method.
[0012]
In this boride single crystal manufacturing method, the mixed raw material preferably contains an XB ₂ compound and 1 to 30 mol% of CrB _{2. A} raw material rod made of XB ₂ and CrB ₂ is vertically melted. , Grow a large single crystal in one direction. The content of CrB _{2 in} the raw material bar preferably includes a range of 5 to 20 mol%, thereby preventing the generation of crystal grain boundaries and being a large crystal having a diameter of more than 1 cm, which is substantially a crystal grain. Single crystals without boundaries can be created. Such XB ₂ single crystal is accompanied by volatilization of CrB ₂ at the time of band melting, and therefore contains 0.0004 mol% to 9 mol% of residual CrB ₂ in the crystal.
[0013]
Furthermore, the single crystal is formed on a substrate for forming a semiconductor layer, and is used as a growth substrate by performing chemical etching on the main surface, and particularly preferably used as a substrate for growing a nitride semiconductor layer such as GaN. Can do.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The boride crystal of the present invention is represented by the formula of XB _{2 and} is composed of a diboride single crystal containing at least one of Ti and Zr in X. XB ₂ of the present invention includes TiB ₂ or ZrB _2. Or these solid solutions are included. These crystal structures of XB ₂ have a hexagonal crystal structure called an AlB ₂ structure, and the warp crystal structure is very similar to the wurtzite structure of a GaN crystal.
[0015]
In the present invention, the crystal of the chemical formula XB ₂ contains at least CrB ₂ in addition to TiB ₂ or ZrB ₂ , and the crystal contains CrB ₂ in a range of 0.00004 mol% to 9 mol%. It is a single crystal substantially free of boundaries. The crystals are preferably provided as large single crystals having a diameter of more than 1 cm.
[0016]
The Ti or Zr XB ₂ crystal has a hexagonal index and is used as a growth substrate with the (0001) plane as the main surface. The main surface of the substrate is a mirror surface suitable for crystal growth, and GaN, AlN , InN or BN, or a mixed layer thereof. However, in addition to the (0001) plane, the (01-10) plane, the (11-20) plane (01-12) plane, etc. can be used as the main plane for growth of GaN and other nitride semiconductor layers. In particular, the reason why the (0001) plane of Ti or Zr XB ₂ crystal is used as the principal plane is that the lattice matching with GaN or AlN is good.
Table 1 shows lattice constants including lattice constants of ZrB ₂ and TiB ₂ measured in the following examples. In particular, TiB ₂ and ZrB ₂ exhibit higher matching with GaN and AlN than alumina. In particular, ZrB ₂ has a lattice constant difference of 2% or less for both GaN and AlN. It can be seen that this is a highly consistent combination.
[0017]
[Table 1]

[0018]
The boride single crystal of the present invention is a single crystal of an XB ₂ -based ZrB ₂ or TiB ₂ compound containing a small amount of CrB ₂ or a solid solution of these compounds, and is used for a substrate for forming a semiconductor layer. It is used to grow and form at least one nitride semiconductor layer of gallium, aluminum, indium, or boron as a semiconductor layer on the substrate. The nitride semiconductor layer is particularly used for a gallium nitride based semiconductor layer. As described above, these nitride semiconductor growth layers have a good lattice matching relationship with this substrate, and in particular, the substrate is a single crystal, and the single crystal substrate has a large size. The formed semiconductor growth layer has few lattice defects over a wide surface area and has extremely good crystallinity.
[0019]
Further, as shown in Table 1, the substrate formed from the diboride single crystal of the present invention is also characterized in that the difference in thermal expansion coefficient from the above nitride semiconductor is small compared to the sapphire substrate, As a result, during the growth process of, for example, a GaN film on the substrate of the present invention, the occurrence of stress in the (0001) in-plane direction applied to the GaN film when the temperature drops is smaller than when a sapphire substrate is used. Along with this, warpage deformation of the substrate on which the GaN film is grown as seen when using a sapphire substrate is reduced.
[0020]
As shown in Table 1, the above-described diboride single crystal exhibits better thermal conductivity than sapphire, and when used as a substrate for semiconductor growth, heat dissipation from the semiconductor device portion is improved. The diboride single crystal described above exhibits good conductivity. Therefore, when a diboride single crystal is used as a substrate for a semiconductor device, the substrate has conductivity, and a connection electrode is formed on the surface of the substrate or particularly on the back surface side together with the upper surface of the semiconductor layer. It becomes possible. In particular, the ability to form electrodes on the back surface of the substrate is effective for improving the efficiency and miniaturization of a semiconductor device formed on the substrate. Further, the substrate itself can be used as an electrode, and lead wires can be directly connected.
[0021]
As a method for producing an XB ₂ single crystal of the present invention, a method of growing a single crystal by a floating zone method from a raw material rod in which CrB ₂ is mixed with XB ₂ is used. The raw material rod is preferentially grown on a specific orientation XB ₂ crystal by moving the melt zone in the vertical direction while forming a melt zone in the mixture by the floating zone method, and by solidification after passing through the melt zone By doing so, a single crystal is obtained.
[0022]
Specifically, in this method, a mixture of XB ₂ powder and CrB ₂ powder is formed into a rod-shaped raw material as a green compact or a sintered body. Place the raw material rod vertically in the holder of the belt melting device, and locally heat the lower side of the raw material rod to create a melting zone, move the heating position relative to the raw material rod, and melt Move your body. The lower part of the moving melting zone is solidified while being cooled, and the crystal can be grown so as to extend upward because the thermal gradient is uniform during the solidification process.
[0023]
In order to promote single crystallization, it is preferable to use a single crystal piece of XB ₂ as a seed crystal. In the above example, first, the seed crystal and the raw material rod are arranged in contact with each other or in close proximity to each other, and band melting is started from the raw material rod at the adjacent portion, and the melting zone is sequentially formed along the longitudinal direction of the raw material rod. To move.
[0024]
In the present invention, the compounding CrB ₂ powder XB ₂ powder, in the solidification process right under CrB ₂ is melting zone, only a single crystal is preferential growth, it is possible to obtain a single crystal of a large diameter.
In general, a single compound such as high purity ZrB ₂ has a very high melting point (ZrB _{2 has} a melting point of 3050 ± 5 ° C. and TiB _{2 has} a melting point of 2790 ± 5 ° C.), and therefore the sample is released from the melting zone. In the cold solidification process, the temperature gradient at the solid-liquid interface is large, and in the growth process, several crystals grow at the solid-liquid interface at the same time, and as shown schematically in FIG. A polycrystalline structure composed of a large number of crystals 6 divided by the crystal grain boundaries 12 is obtained.
[0025]
In contrast, in the present invention, although the role of CrB ₂ is not clear, the melting point of the XB ₂ mixture is lowered by adding a small amount of CrB ₂ to the XB ₂ raw material (the melting point is about 200 per 15 mol% of CrB _2). As a result, the temperature gradient is relaxed (the temperature gradient is considered to be proportional to the fourth power of the solidification temperature at that time), and as shown schematically in FIG. 2, only a single crystal is preferential. It is considered that the entire crystal body is composed of the single crystal 60. Further, when seed crystals are not used, the length from the polycrystal to the single crystallization in the initial stage of growth is 3 to 5 cm when CrB ₂ is not added, but CrB ₂ is added. By this, it becomes 2 cm or less, and single crystallization is promoted.
[0026]
In the production method of the present invention, the amount of CrB ₂ is a powder mixture of XB ₂ and CrB _2, the range of CrB ₂ of 1 to 30 mol%. When the content of CrB _{2 is} 1 mol% or less, the effect of promoting single crystallization is small, and if it exceeds 30 mol%, CrB ₂ is concentrated in the melting zone, and the growth rate of the single crystal is decreased, which is not preferable. In particular, the blending amount of CrB ₂ is preferably in the range of 5 to 15 mol% in the mixture.
[0027]
In the floating zone method, the convergent light from a high-luminance lamp or laser light can be used for heating. However, since Ti or Zr diboride has electrical conductivity, preferably high frequency induction heating is used. Is done. FIG. 1 shows a high-frequency induction heating type floating zone furnace. This apparatus has a heating unit arranged in a sealed pressure vessel 12 and is designed so that crystals can be grown in an inert gas atmosphere of several atmospheres in the inner space 13 of the vessel.
[0028]
In this figure, an induction coil 4 is used for induction heating. The coil 4 in this example is configured by winding a water-cooled copper tube in a spiral shape on a horizontal plane, and the central portion of the coil 4 is a sample. Is formed in the pressure vessel 12. At the upper and lower positions of the induction coil 4, the drive units 1 and 9 that can be moved up and down outside the pressure vessel 12, and the upper shaft 2 that extends from the drive units 1 and 9 and penetrates the pressure vessel in an airtight and slidable manner. And the lower shaft 10 and the sample gripping portions fixed to the upper and lower shafts 2 and 10, that is, the holders 3 and 9, are arranged between the upper holder 3 and the lower holder 11. The end of the raw material bar penetrating through is gripped. The upper shaft driving unit 1 and the lower shaft driving unit 9 are controlled to be individually movable up and down.
[0029]
In this example, the induction coil 4 and the raw material rod are accommodated in a pressure vessel, and controlled to an inert gas atmosphere at a pressure of several to several tens of atmospheres to suppress volatilization from the melting zone 7 and the melting zone is refractory. Crystals can be grown without contact with objects.
[0030]
In the band melting operation, the upper end of the raw material rod 8 is held and suspended on the upper holder 3, the lower end side of the raw material rod 8 is brought into contact with the upper end of the seed crystal held by the lower holder 11, and The induction coil 4 is positioned at the center, high frequency power is supplied to the induction coil 4 to generate an induction current in the conductive diboride raw material rod 8, and the lower end of the raw material rod 8 is locally localized by the Joule heat. Heat to melt. In this way, the raw material rod is fed into the melt zone 7 formed from above, and the single crystal 60 is grown from below.
[0031]
As an example of a raw material rod for single crystal growth according to the present invention, raw diboride powder XB ₂ (X contains at least one of Ti and Zr) and chromium diboride (CrB ₂ ) powder are mixed in a required blending amount. Mix well and add, for example, a small amount of camphor as a binder, and press with a hydrostatic pressure (for example, 2000 kg / cm ² ) with a rubber press to make a rod-shaped green compact. Preferably, the powder bar is heated and sintered in a vacuum or in an inert gas to a few hundreds of degrees Celsius to produce a raw material bar.
[0032]
The prepared raw material rod 8 is fixed to the upper shaft 2 via a holder 3, and a seed crystal (or a sintered rod for forming an initial fusion zone) 5 is set to a lower shaft 10 via a holder 11. Next, the lower end of the raw material rod 8 is melted by induction heating of the coil 4 to form a melting zone 7, and the upper shaft 2 and the lower shaft 10 are slowly moved downward to melt relative to the raw material rod. The band is raised and the single crystal 60 is grown in response to the cooling and solidification of the molten zone. At this time, the supply speed of the raw material rod 8 to the melting zone 7 is set in consideration of the density of the raw material rod (usually about 55% of the theoretical density) and the diameter of the crystal to be grown. In this way, a single crystal rod having a large diameter substantially free of crystal grain boundaries can be obtained.
[0033]
The atmosphere is preferably an inert gas such as argon or helium at several atmospheres. This is to prevent discharge generated in the induction coil 4 and to suppress evaporation from the melting zone 7.
[0034]
The XB ₂ single crystal rod thus obtained is cut and cut so that the main surface has a required plane orientation, and one surface is polished to form a main surface. As an example, in a substrate for forming a GaN-based semiconductor layer, the substrate main surface is usually prepared so as to be parallel to the (0001) plane of the crystal.
[0035]
The produced substrate is chemically and mechanically polished using alumina and silica abrasive grains. If a grain boundary exists, it can be confirmed by visual observation of the polished surface, but the single crystal according to the method of the present invention contains almost no grain boundary. The presence of grain boundaries can be confirmed by X-ray topography or acid etching, but can be easily discriminated visually when the main surface is chemically polished.
[0036]
【Example】
[Example 1]
[0037]
In this embodiment, as XB _2, select the ZrB _2, a CrB ₂ addition amount of the powder mixture of ZrB ₂ and CrB _2, select five levels between 30 mol% from 0.1 mol%, the outer diameter 12 It was compressed into a powder bar of ˜14 mm and a length of 2000 mm and sintered at 1400 ° C. to obtain a raw material bar. The raw material rod was made into a crystal rod having a diameter of 12 to 14 mm at a speed of 2 cm / h by applying a high frequency power of 28 kW / h in a high frequency heating type floating zone furnace.
[0038]
The longitudinal effect of the crystal rod was corroded, and the effect of suppressing grain boundaries by adding CrB ₂ was examined by visual observation and microscopic observation. Grain boundary suppressing effect is seen from 1 mol% of CrB ₂ addition amount, and when it is 5 mol% or more, the suppressing effect becomes remarkable, and a single crystal rod substantially free of crystal grain boundaries and sub-grain boundaries is obtained in the cross section. It was. Further, when the CrB ₂ addition amount is 20 mol% or more, CrB ₂ —ZrB ₂ eutectic is accumulated immediately above the melting zone during growth, and movement of the melting zone of several centimeters or more is impossible. A good quality single crystal was obtained. Therefore, in order to obtain a long single crystal rod, it was found that the range of 5-15 mol% of CrB ₂ addition is a preferable addition amount.
[0039]
Moreover, ICP emission spectrometer: Use (Seiko Instruments Inc. production model "SPS1200VR"), has been analyzed and CrB ₂ and ZrB _2, CrB ₂ content in the obtained crystal rod material It can be seen that CrB ₂ is more volatile than ZrB ₂ and that most of the addition is lost due to evaporation. The addition amount of 30 mol% CrB _{2 in} the raw material rod is such that the CrB ₂ content in the crystal becomes 9 mol% or less due to the evaporation of CrB ₂ during ZrB ₂ crystal growth, and the raw material rod contains 5 mol% CrB ₂ The content of CrB _{2 in} the crystal is 0.0010 to 0.5 mol%. Although crystal CrB ₂ with the addition of feed rod in 1 mol% CrB ₂ is included at least 0.0004 mol% is additive-free feed rod in CrB _2, the crystal becomes 0.0002 mol% CrB ₂ or less.
[0040]
Thus, it has been found that the addition of CrB ₂ in the raw material rod has the effect of removing the grain boundaries 12 and sub-grain boundaries introduced in the initial stage of crystal growth under the melting zone in the grown crystal. . That is, when growing a large-diameter ZrB ₂ crystal, if the purity is high without adding CrB ₂ to the ZrB ₂ raw material rod, the grain boundaries and sub-grain boundaries introduced in the initial stage of crystal growth are It is not easily removed from the crystal. In this case, several crystals are generated simultaneously on the solidified surface immediately below the melting zone to be polycrystallized. However, when CrB ₂ is added to the ZrB ₂ powder, the grain boundaries introduced at the start of growth are easily removed. That is, the presence of CrB ₂ has the effect of promoting the growth of specific crystal grains in the initial polycrystal and the single-crystalization of the entire zone melting zone. When 5 mol% or more of CrB ₂ is added, by introducing a melting zone of 1 to 2 cm after the start of melting, the grain boundary is removed at the initial stage of growth, and only specific crystal grains grow preferentially. It was found that a good quality single crystal can be obtained by the movement.
[0041]
The above CrB ₂ addition growth method can be applied to the growth of a single crystal by using a heating method other than high-frequency heating, for example, laser heating, for the FZ method. Further, such a crystallinity improvement method using CrB ₂ can be applied not only to crystal growth by the floating zone method but also to other crystal growth methods such as a skull melt method.
[0042]
As a comparative example, a single crystal was made by using the above-described FZ method, but using a single sintered rod of XB ₂ boride without adding CrB ₂ . A crystal rod having a diameter of 1 cm or more was produced, but as shown in FIG. 3, grain boundaries 12 and sub-grain boundaries were generated in the crystal, and a high-quality single crystal could not be obtained.
[0043]
[Example 2]
With respect to the crystal having a diameter of 15 mm obtained by the method of Example 1, the crystal orientation was identified by X-ray, and a 0.6 mm-thick plate having a (0001) plane as a principal surface was cut out using a band saw. Since the outer shape of the obtained plate was indefinite, the outer piece was ground to a 12.7 mm square, and the thickness was ground to 0.4 mm by using a diamond grindstone. One side of this plate was chemically polished using colloidal silica to obtain a substrate after cleaning.
[0044]
In the crystal prepared from the raw material rod added with ₅ to 15 mol% of CrB ₂ by the above method, no grain boundary was observed after polishing, and an X-ray diffraction test was conducted, but the diffraction peak was also single. Further, even when the polished substrate surface was corroded with hydrochloric acid or nitric acid, no grain boundary was confirmed, and it was confirmed that the substrate was a homogeneous single crystal.
In the case of a crystal rod having a diameter of more than 10 mm prepared by the FZ method from the CrB _2- free raw material rod of the comparative example, the grain boundary was visually observed on the chemically polished crystal surface, and (0001 ) Surface was measured, a plurality of diffraction peaks were observed, and the substrate was found to be polycrystalline.
[0045]
[Example 3]
Using the polished substrate obtained in Example 2, a gallium nitride semiconductor layer was formed by vapor phase growth using an organometallic compound growth method (hereinafter referred to as MOCVD method). The substrate was heated to 1050 ° C. by flowing hydrogen gas into the furnace to maintain the substrate at 1050 ° C., held for 10 minutes, and then 950 ° C. to flow trimethylaluminum (TMA) and NH ₃ to grow an undoped AlN layer having a thickness of 1 μm. This AlN layer is not an amorphous layer or a polycrystalline layer that is a so-called low-temperature buffer layer, but a single-crystal layer that is crystal-grown in a high-temperature environment. Further, the substrate was heated to 1000 ° C. with NH ₃ flowing, and then trimethylgallium (TMG) was flown to grow 2 μm of undoped GaN. When the electrical characteristics of this undoped GaN epitaxial crystal were measured by the Hall effect method, crystals of good characteristics were grown with an n-type and a carrier concentration of about 1 × 10 ¹⁶ cm ⁻³ .
[0046]
[Example 4]
After adding 10 mol% of CrB ₂ powder to commercially available ZrB ₂ powder and mixing, a small amount of camphor was added as a binder, and it was packed into a rubber bag with a diameter of 15 mm to form a cylindrical shape. This was subjected to a rubber press at 2000 kg / cm ² to obtain a green compact. The green compact was heated in vacuum at 1800 ° C. to obtain a sintered rod having a diameter of about 1.4 cm and a length of about 20 cm. The density was about 55%.
[0047]
This sintered rod was fixed to the upper shaft of the floating zone furnace shown in FIG. 1 via a holder, and the ZrB ₂ sintered rod was fixed to the lower shaft. After filling the furnace with argon at 5 atm, the lower end of the raw material rod is melted by high frequency induction heating to form an initial melting zone and moved downward at a speed of 2 cm / h for 3 hours. Single crystals were grown. The amount of CrB ₂ in the crystal was 4 mol%. The (0001) plane was cut out from the obtained crystal, and after mirror polishing, etching was performed to observe the surface. As a result, it was confirmed that it was a high-quality single crystal without grain boundaries and sub-grain boundaries.
[0048]
[Example 5]
Using the polished substrate described in Example 4, vapor phase growth of a gallium nitride based semiconductor by MOCVD was performed. The susceptor was heated while flowing hydrogen gas into the furnace to heat the substrate to 1050 ° C., then 950 ° C., and trimethyl gallium (TMG) and NH ₃ were flowed to grow an undoped GaN layer by 0.5 μm. This GaN layer is not an amorphous layer or a polycrystalline layer that serves as a low-temperature buffer layer, but a single-crystal layer that is crystal-grown in a high-temperature environment. Further, the substrate was heated to 1000 ° C. with NH ₃ flowing, and then trimethylgallium (TMG) was flowed to grow an undoped GaN layer by 2 μm.
[0049]
The electrical properties of the undoped GaN epitaxial crystal layer thus formed were measured by the Hall effect method. As a result, the n-type GaN epitaxial crystal layer was n-type and had a carrier concentration of about 1 × 10 ¹⁶ cm ^−3. Was able to grow.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a floating zone device used for single crystal growth in an embodiment of the present invention.
FIG. 2 is a cross-sectional view of corrosion of a crystal rod grown by the growing method of the present invention.
FIG. 3 is a cross-sectional view of corrosion of a crystal rod grown by the method of a comparative example.
[Explanation of symbols]
3: Holder 4: Induction coil 5: Seed crystal 6: Crystal 60: Single crystal 7: Melt zone 8: Raw material rod 9: Holder 12: Grain boundary

Claims (4)

_XB 2 (X is at is least one of Ti and Zr) viewed including the a and CrB ₂ as main components, growing a single crystal by the floating zone method from a raw material mixture amount of CrB ₂ is in the range of 1 to 30 mol% A process for producing a boride single crystal characterized by comprising: The process according to claim 1, wherein the boride single crystal containing single crystal 0.0004mol% ~9mol% of CrB _2. A single crystal is grown by a floating zone method from a mixed raw material containing XB ₂ (X is at least one of Ti and Zr) and CrB ₂ as main components, and the blending amount of CrB _{2 being} in the range of 1 to 30 mol%. ,
A method for manufacturing a substrate for forming a semiconductor layer, wherein the substrate is cut out from the single crystal and used as a substrate for growing a semiconductor layer on a main surface. The method for producing a semiconductor layer forming substrate according to claim 3 , wherein the semiconductor layer is a gallium nitride based semiconductor layer.

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