JP4117402B2 - Single crystal aluminum nitride film and method for forming the same, base substrate for group III nitride film, light emitting element, and surface acoustic wave device - Google Patents

Description

【００２５】
ここで、ａｌｏｎは、（Ａｌ_(64+x)/3□_(8-x)/3Ｏ_32-xＮ_x、但し、□は陽イオン空孔、２＜ｘ＜６）と表わされる。
【００２６】
本発明に使用するサファイア基板としては、市販のものが制限なく使用できる。特に内包する気泡の直径が５０マイクロメートル以下、好ましくは１０マイクロメートル以下で、かつ純度が９９．９９％以上のものが好適に使用される。この基板表面に、ａｌｏｎ層およびＡｌＮ膜を形成させる場合、基板の結晶面としては任意の面が使用できるが、結晶の対称性の点でＣ面以外の面が好ましい。
【００２７】
カーボンとしては種々の市販品が使用できる。反応雰囲気をカーボンで飽和させる目的から、粉末状のグラファイトおよび／またはカーボンブラックが好適に利用される。カーボンの純度は９９．９％以上であることが好ましく、９９．９９９％以上であることがより好ましい。
【００２８】
窒素および一酸化炭素は、通常ガス状のものが使用されるが、９９．９９９９％以上の窒素および９９．９％以上の一酸化炭素が好ましい。
【００２９】
次に、上記反応原料を用いての、サファイア基板上へのａｌｏｎ層およびＡｌＮ膜の形成方法について説明する。
【００３０】
図１は、Ａｌ₂Ｏ₃−ＡｌＮ擬２元系状態図を示す。この状態図に示されているように、ａｌｏｎは、１９０３Ｋ（１６３０℃）以上の高温でのみ安定に存在し、広い固溶領域を有する逆スピネル構造のγ相酸窒化アルミニウムである。このａｌｏｎは、高温における化学的安定性が高く、金属精錬における耐食性が高い耐火物材料として注目されているとともに、多結晶体であっても紫外域から赤外域にわたって高い透光性を有し、かつ化学的に安定であるという特徴をもつ。
【００３１】
本発明においては、このａｌｏｎを単結晶α−アルミナ基板例えばサファイア基板の表面を直接窒化することにより形成する。即ち、例えば、図２に示す装置のＡｌ₂Ｏ₃製反応管の底部に、表面の結晶性がＡ面（１１−２０）のサファイア基板とグラファイトを装入し、Ｎ₂−ＣＯ混合ガスの組成を調節することにより、酸素ポテンシャルと窒素ポテンシャルを制御した雰囲気下で、基板を窒化させる。窒化温度はａｌｏｎが生成する１９０３Ｋ以上とする。これによりａｌｏｎ層と、ａｌｏｎ層の上のＡｌＮ膜が生成する。図３は、横軸に温度（Ｋ）をとり、縦軸にＣＯ分圧（Ｐ_CO）とＮ₂分圧（Ｐ_N2）との比をとって、図１に示す状態図を、ＣＯとＮ₂との分圧比Ｐ_CO／Ｐ_N2で書き直したものである。この図は、全圧Ｐ_CO＋Ｐ_N2が１気圧、炭素の活量ａ_cが１の条件下でのＡｌ₂Ｏ₃、ａｌｏｎ、ＡｌＮの相安定図である。図中、Ｐ_CO／Ｐ_N2が大きい領域はＡｌ₂Ｏ₃の安定領域であり、領域Ａはａｌｏｎの安定領域（１９０３Ｋ以上）であり、その下側の領域ＢがＡｌＮの安定領域である。
【００３２】
本発明においては、基板を図３の領域Ｂにて示す生成条件に保持することにより、基板上の最表面にＡｌＮ膜が生成し、基板とＡｌＮ膜との間に、ａｌｏｎ層が生成する。１９０３Ｋ以下の温度では、Ａｌ_２Ｏ_３とＡｌＮとが直接平衡するため、ａｌｏｎは生成しない。しかし、１９０３Ｋ以上の温度では、図１に示す状態図および図３に示すアルミニウム−酸素−窒素−炭素系化学ポテンシャルダイアグラムから明らかなように、必ず、平衡相として、ａｌｏｎがサファイアとＡｌＮとの間に存在することになる。なお、後述する図４のＸＲＤ（Ｘ線回折）プロファイルは、図３の●にて示す温度およびＣＯとＮ_２との分圧比の条件でａｌｏｎとＡｌＮとを成長させた場合のものである。
【００３３】
図３に示すように、サファイア基板をａｌｏｎの生成温度（１９０３Ｋ以上）以上の温度に保持することにより、サファイア基板の上にａｌｏｎ層が生成し、さらにその上に結晶性が優れたＡｌＮ膜が生成する。このａｌｏｎが生成する温度よりも低い温度では、ａｌｏｎが生成しないとともに、サファイア基板上に生成するＡｌＮ膜は多結晶化してしまう。例えば、全圧Ｐ_CO＋Ｐ_N2が１気圧、炭素の活量ａ_cが１の条件下で１９０３Ｋ以上の温度に保持してサファイア基板上にａｌｏｎおよびＡｌＮを生成する方法においては、ガスでサファイア基板の表面を窒化するため、サファイア中の酸素とガスから供給される窒素が互いにサファイア中で置換しながら、順にａｌｏｎ層および単結晶ＡｌＮ膜が形成されていく。従来のＭＯＶＰＥ法等により成膜したＡｌＮ膜は、一軸方向には成長しているものの霜柱のような柱状の集合組織となっており転位密度も高い。本発明により得られる単結晶ＡｌＮ膜は従来法で得られたＡｌＮ膜とは異なり、霜柱のような柱状の集合組織となっておらず、また転位密度も低く、電子顕微鏡観察によっても欠陥を発見できないような結晶性が優れたものである。
【００３４】
図３の化学ポテンシャルダイアグラムから理解できるように、直接窒化反応に使用されるカーボン、窒素および一酸化炭素の量やその配合比あるいは加熱温度は、この図を利用して適宜決定すればよい。
【００３５】
カーボンの使用量は、単結晶α−アルミナに対して、好ましくは重量比で０．１〜１程度使用する。
【００３６】
一酸化炭素と窒素の混合比（分圧比）は、上記化学ポテンシャルダイアグラムから決定することができるが、０．１〜０．５が好適に採用される。反応系の全圧は１気圧とするのが好ましい。そうすることが、反応装置の製作や運転の容易さから好ましい。反応に先立って、反応装置内は一度真空に排気した後、所定の分圧になるようにした混合ガスが導入される。反応中は、この混合ガスを所定の流量で流す。
【００３７】
加熱温度は、ａｌｏｎが生成する温度（１９０３Ｋ）以上からａｌｏｎとＡｌＮが直接平衡する上限の温度（２１４９Ｋ）以下の間から選ばれる。
【００３８】
加熱時間は、所望する膜厚により適宜決定される。例えば、１９７３ＫでＰ_CO／Ｐ_N2＝０．１の条件下における単結晶窒化アルミニウム膜の成長速度は、毎時０．２〜０．８マイクロメートルである。
【００３９】
反応終了後は、例えば反応管を炉から引き抜くなど、急速冷却して取り出す。冷却速度が遅いと、基板と単結晶窒化アルミニウム膜の間に生成したａｌｏｎ層が分解するので好ましくない。
【００４０】
なお、本発明においては、基板を図３の領域Ｂ内の条件に保持してａｌｏｎとＡｌＮとを同時に生成する場合に限らず、先ず、基板上にａｌｏｎ層を形成した後、生成条件を変更してＡｌＮ膜を形成することとしてもよい。この場合のａｌｏｎ層およびＡｌＮ膜の形成条件は、先ず、基板を図３の領域Ａ内の生成条件に保持することによりａｌｏｎ層のみを先に成長させ、その後、温度および／またはＰ_CO／Ｐ_N2比を変更することにより、生成条件を領域Ｂ内のものに変更し、この条件に保持すれば、先に生成しているａｌｏｎ層の上に、ＡｌＮ膜が生成する。
【００４１】
このようにして、基板上にａｌｏｎ層およびＡｌＮ膜を同一の工程で連続して形成することができる。図４は、上述のごとく、サファイア基板上にａｌｏｎ層およびＡｌＮ膜の積層膜を成長させたものについて、ＸＲＤ（Ｘ線回折）プロファイルを示すグラフ図の一例である。図４において、横軸は回折角度２θ、縦軸はＸ線強度である。なお、Ｎ₂−ＣＯ混合ガス中の分圧比は、Ｐ_CO／Ｐ_N2＝０．１であり、この雰囲気下で、サファイア基板を１９７３Ｋに２４時間加熱した。この図４に示すように、面方位がＡ面のα−Ａｌ₂Ｏ₃（サファイア基板）と（１１１）面のａｌｏｎと、Ｃ面のＡｌＮとの積層体が形成されている。
【００４２】
この図４に示すように、サファイア／ａｌｏｎ／ＡｌＮの積層体が成長し、生成したａｌｏｎとＡｌＮは単結晶であり、その結晶方位関係は、
Ａ面サファイア／／（１１１）ａｌｏｎ／／Ｃ面ＡｌＮ
であることがわかる。
【００４３】
本発明の単結晶窒化アルミニウム膜とサファイア基板の結晶方位関係は以下に示すように基板のサファイア面によってａｌｏｎおよびＡｌＮの結晶性が異なる。これはＣ面サファイアが３つの等方的な対称軸を有するため、３種類の結晶方位を有するａｌｏｎが成長して多結晶になるのに対し、Ａ面サファイアは等方的な対称軸を持たないため、１方向にしか結晶成長を許されないからである。
【００４４】
以下に、サファイア基板とａｌｏｎおよびＡｌＮ膜の結晶方位の関係を例示する。
【００４５】
Ｃ面サファイア／（１１１）ａｌｏｎ（多結晶）：オフセット角２０°
（１１１）ａｌｏｎ（多結晶）／／Ｃ面ＡｌＮ（多結晶）
【００４６】
なお、オフセット角とは結晶面間の角度を表わす。
【００４７】
Ａ面サファイア／／（１１１）ａｌｏｎ／／Ｃ面ＡｌＮ
得られるＡｌＮ膜の厚みは、通常０．１〜２０μｍであるが、必要に応じてさらに厚くすることもできる。
【００４８】
本発明の単結晶窒化アルミニウム膜中の転位密度は１０⁸個／ｃｍ²以下である。本発明で得られた単結晶窒化アルミニウム膜の透過型電子顕微鏡（ＴＥＭ）の断面写真から、貫通転位は見当たらないので、仮に写真の視野である１平方マイクロメートルの範囲に１つの転位が存在すると仮定した場合の転位密度（最も大きく見積もった値）から、１０⁸個／ｃｍ²と算出した。
【００４９】
本発明において、格子不整合を最近接陰イオン間距離ｄを用いて、
【００５０】
ｍ．ｆ．＝１００ｘ（ｄｓｕｂ−ｄｅｐｉ）／ｄｓｕｂ
【００５１】
ここでｄｓｕｂは基板の最近接陰イオン間距離そしてｄｅｐｉはエピタキシャル層の最近接陰イオン間距離を表わす、
と定義すると、それぞれの界面の格子不整合は以下のように表わされる。
・Ａ面サファイア基板／Ｃ面窒化アルミニウム：１７．０
・Ａ面サファイア基板／（１１１）酸窒化アルミニウム：４．９０
・（１１１）酸窒化アルミニウム／Ｃ面窒化アルミニウム：４．８２
すなわち、酸窒化アルミニウムを介することによって、格子不整合を約５％に減少させることができる。
【００５２】
本発明において得られる立方晶スピネル型構造のγ相酸窒化アルミニウム（ａｌｏｎ）層は単結晶で構成され、通常厚みは０．１マイクロメートル程度である。
【００５３】
なお、ＡｌＮ膜やａｌｏｎ層の結晶形態や結晶性は、Ｘ線回折、極点図形解析および透過型電子顕微鏡（ＴＥＭ）による断面視察により確認できる。またそれぞれの厚みについては、ＴＥＭによる断面観察で確認、計測できる。
【００５４】
本発明の単結晶窒化アルミニウム積層基板は、それ自体でＡｌＮ膜を発光膜とする発光素子となりうるし、さらにこのＡｌＮ膜上に単結晶窒化ガリウム（ＧａＮ）あるいはＡｌＧａＮやＩｎＧａＮなどの任意のIII族窒化物混晶膜を積層することにより、該単結晶III族窒化物膜を発光膜とする発光素子となりうる。具体的には、単結晶ＡｌＮ膜自体は紫外光の発光層として使用することができる。このＡｌＮ紫外光発光素子は、高密度光メモリへの応用が可能である。またＡｌＮ層は紫外光用の受光素子としても幅広く利用できる。
【００５５】
図５に、本発明の発光素子を示す。サファイア基板上に、この基板を直接窒化処理することにより、ａｌｏｎ層が形成されており、このａｌｏｎ層上に単結晶ＡｌＮ膜が形成されている。さらに、この単結晶ＡｌＮ膜を緩衝層として、ＡｌＮ膜上に、単結晶ｎ型III族窒化物膜が形成されている。この単結晶ｎ型III族窒化物膜は、有機金属気相成長法、ハライド化学蒸着法または分子線エピタキシャル法により形成することができる。得られたｎ型III族窒化物は、下地のＡｌＮ膜の結晶性が優れているため、極めて優れた結晶性を有している。
【００５６】
本発明の発光素子は、前述のごとく、サファイア基板上にａｌｏｎ層を形成し、その上にＡｌＮ膜を形成しているので、ＡｌＮ膜の結晶性を著しく向上させることができる。従来のように、サファイア基板上にＭＢＥまたはＭＯＶＰＥ法等によりＡｌＮ膜を形成すると、サファイア基板とＡｌＮ膜との間の格子不整合が１７％にも達するが、本発明のように、サファイア基板を直接窒化することによりａｌｏｎ層を形成した上で、このａｌｏｎ層を下地膜としてＡｌＮ膜を形成すると、サファイア基板／ａｌｏｎ膜界面およびａｌｏｎ膜／ＡｌＮ膜界面の格子不整合をそれぞれ約５％程度に抑制することができる。このため、高品質の単結晶ＡｌＮ膜を形成することができる。よって、このＡｌＮ膜上にIII族窒化物膜を形成すれば、貫通転位が極端に少ない良質なIII族窒化物薄膜を形成することができる。従って、このIII族窒化物発光層を有する発光デバイスの発光効率を飛躍的に向上させることができる。
【００５７】
本発明の単結晶窒化アルミニウム積層基板は、従来の多結晶体よりも表面弾性率の異方性が大きくなるため、高性能の表面弾性波デバイスとしても利用できる。この表面弾性波デバイスは、具体的には携帯電話、移動体通信、テレビ中間帯フィルター、衛星電話等に使用できる。
【００５８】
【実施例】
実施例１
図２に示す反応装置を用いて、１９７３ＫでＮ₂−ＣＯ混合ガスとグラファイトによってサファイア基板を窒化することにより，ＡｌＮおよびａｌｏｎ相を作製した。Ａｌ₂Ｏ₃製反応管の底部に、表面の結晶面がＡ面（１１−２０）のサファイア基板（１０ｍｍ×１０ｍｍ×１ｍｍ）およびグラファイト粉末（純度９９．９９９％）を設置した。あらかじめ反応管内を一旦ロータリーポンプで真空排気して管内の水分を完全に除去し、一酸化炭素（ＣＯ）分圧と窒素（Ｎ₂）分圧の比が０．１である混合ガスで完全に置換を行った。その後、この混合ガスを一定の流量（５５ｍｌ／ｍｉｎ）で流した。装置系内の全圧は１気圧である。反応管の底部を炉の灼熱部に挿入することにより試料を急速昇温し、１９７３Ｋに保って反応を開始した。２４時間保持した後、反応管を炉から引き抜くことによって試料を急速冷却し反応を終了した。
【００５９】
得られた反応済みのサファイア基板は、Ｘ線回折分析によりサファイア基板の（１１−２０）面以外にａｌｏｎの（１１１）面とＡｌＮの（０００２）面の回折ピークが観察された（図４）。このことから、試料全面において生成したＡｌＮおよびａｌｏｎ層は単一の結晶方位を有していることがわかった。
【００６０】
図６および図７に生成したそれぞれＡｌＮ層の｛１−１００｝面の極点図形およびａｌｏｎ層の｛１１１｝面の極点図形を示す。ここで、極点図形のφ＝０^oは、サファイアの[１−１００]方向とした。図６より、ＡｌＮ層の｛１−１００｝面は、ψ＝９０^oにφが６０^o間隔で６つの回折ピークを示していることが分かる。これは、ＡｌＮの(０００１)面がＡ面サファイア基板と水平に存在していることを表している。さらに、ＡｌＮの(０００１)面内の[１−１００]方向はサファイアの[１−１００]方向と一致した。また、図７より、ａｌｏｎ層の｛１１１｝面は、原点およびψ＝７０^oにφが６０^o間隔で6つの回折ピークを示していることが分かる。これは、サファイア基板（Ａ面）に対してａｌｏｎの(１１１)面が平行で、実線および破線で示されるａｌｏｎ(１１１)面内で互いに１８０^o回転した２つの結晶方位を有することを示している。また、ａｌｏｎ(１１１)面内の[１１−２]方向は、サファイアの[１−１００]方向に対して３０^o回転した方向に存在した。これは、ａｌｏｎ(１１１)面内の[１１０]方向がサファイアの[１−１００]方向に平行なことに対応している。このことから、基板と作製した窒化物相は、次式の結晶方位関係，
【００６１】
Ａ面サファイア／／(１１１) ａｌｏｎ／／C面 ＡｌＮ
【００６２】
に付随して次の結晶方位関係，
【００６３】
[１−１００]ＡｌＮ／／[１１０] ａｌｏｎ／／[１−１００]サファイア
【００６４】
も有することが分かった。
【００６５】
上記の結晶方位関係より，各界面における格子の回転を考慮して，サファイア／ａｌｏｎ界面およびａｌｏｎ／ＡｌＮ界面の原子のならびを図示すると図８のようになり，この図から格子不整合を算出すると、A面サファイアを窒化した場合、それぞれの界面の格子不整合は以下のように表わされる。
【００６６】
・Ａ面サファイア基板／（１１１）酸窒化アルミニウム：４．９０
・（１１１）酸窒化アルミニウム／Ｃ面窒化アルミニウム：４．８２
【００６７】
本実施例で得られたＡ面サファイア基板の断面ＴＥＭ像を図９に示す。図に示すように、結晶方位および結晶構造の違う３相が層状に形成していることがわかった。また、これら各相についてＥＤＸによる組成分析を行ったところ、サファイア基板にａｌｏｎ層、ＡｌＮ層が連続的に生成していることがわかった。さらに、生成したａｌｏｎ層およびＡｌＮ層は電子線回折図形から単結晶として生成し、少なくとも１平方マイクロメートルの範囲では転位が存在していないことがわかった。この結果から生成したＡｌＮ中の転位密度は、最も大きく見積もって１０⁸個／ｃｍ²程度であり、従来の転位密度より一桁以上小さいことがわかった。
【００６８】
本実施例で得られたＡ面サファイア基板の原子間力顕微鏡（ＡＦＭ）像を図１０に示す。図に示すとおり窒化処理を行った表面は平滑ではなく、最大で４００ｎｍ程度の起伏が存在した。
【００６９】
参考例２
図２に示す実験装置を用いて、ＡｌＮおよびａｌｏｎ相は１９７３ＫでＮ_２−ＣＯ混合ガスとグラファイトによってサファイア基板を窒化することにより作製した。Ａｌ_２Ｏ_３製反応管の底部に、表面に結晶面がＣ面のサファイア基板およびグラファイト粉末（純度９９．９９９％）を設置した。あらかじめ反応管内を一旦ロータリーポンプで真空排気して管内の水分を完全に除去し、Ｐ_ＣＯ／Ｐ_Ｎ２＝０．１の組成のＣＯ−Ｎ_２混合ガスの雰囲気下で完全に置換を行った。その後、この混合ガスを一定の流量（５５ｍｌ／ｍｉｎ）で流した。装置系内の全圧は１気圧である。反応管の底部を炉の灼熱部に挿入することにより試料を急速昇温し、１９７３Ｋに保ち反応を開始した。２４時間保持した後、反応管を炉から引き抜くことによって試料を急速冷却し反応を終了した。
【００７０】
反応済みのＣ面サファイア基板上に生成したＡｌＮ膜の極点図形を図１１に示す。その結果、Ｃ面サファイアと（１１１）ａｌｏｎ間には２０°のオフセット角が存在することがわかった。
【００７１】
Ｃ面サファイア／（１１１）ａｌｏｎ（多結晶）：オフセット角２０°
（１１１）ａｌｏｎ（多結晶）／／Ｃ面ＡｌＮ（多結晶）
【００７２】
このため、Ｃ面サファイアが３つの等方的な対称軸を有することから、３種類の結晶方位を有するａｌｏｎが成長する、つまり多結晶となり、最外面のＡｌＮもＣ面配向しているものの同様に多結晶になっているものと考えられる。
【００７３】
これに対し、実施例１で説明したように、Ａ面サファイアは等方的な対称軸を持たないため、１方向にしか結晶成長が許されず、結晶性の良好な単結晶ＡｌＮが得られたものと考えられる。
【００７４】
以上のとおり、本発明によれば、ハライド化学蒸着法におけるハライド系の有害ガスおよびＭＯＶＰＥ法における有機金属系の有害ガスを使用することなく、欠陥が少なく結晶性が優れた均一な単結晶ＡｌＮ膜を、しかも安価にサファイア基板の如き単結晶α−Ａｌ₂Ｏ₃基板上に形成することができる。
【００７５】
この単結晶ＡｌＮ膜を単結晶III族窒化物膜形成の緩衝層として使用すれば、青色発光ダイオードおよび青色レーザー等の発光デバイスの発光効率を著しく向上させることができ、また単結晶ＡｌＮ膜を紫外光の発光ダイオードとして使用すれば、紫外光の発光素子および受光素子の発光効率および受光効率を著しく向上させることができる。さらに、この単結晶ＡｌＮ膜を高密度光メモリおよび表面弾性波デバイスとして実用化することができるようになる。
【図面の簡単な説明】
【図１】Ａｌ₂Ｏ₃−ＡｌＮ擬２元系状態図を示す。
【図２】本発明の実施例で用いた反応装置の概略図である。
【図３】アルミニウム−酸素−窒素−炭素系化学ポテンシャルダイアグラムである。横軸に温度（Ｋ）をとり、縦軸にＣＯ分圧（Ｐ_CO）とＮ₂分圧（Ｐ_N2）との比をとって、図１に示す状態図を、ＣＯとＮ₂との分圧比Ｐ_CO／Ｐ_N2で書き直したものである。
【図４】サファイア基板上にａｌｏｎ層およびＡｌＮ膜の積層膜を成長させたものについて、ＸＲＤ（Ｘ線回折）プロファイルを示すグラフ図である。
【図５】本発明に係る発光素子の層構成を示す図である。
【図６】ＡｌＮ膜の極点図形（Ａ面サファイア）を示す図である。
【図７】ａｌｏｎ膜の極点図形（Ａ面サファイア）を示す図である。
【図８】サファイア／ａｌｏｎ及びａｌｏｎ／ＡｌＮ界面の格子不整合を示す図である。
【図９】基板断面のＴＥＭ（Ａ面サファイア）を示す図である。
【図１０】基板表面のＡＦＭ（Ａ面サファイア）を示す図である。
【図１１】ＡｌＮ膜の極点図形（Ｃ面サファイア）を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a single crystal aluminum nitride film capable of forming a single crystal aluminum nitride (AlN) film with high efficiency and a method for forming the same, a base substrate for a group III nitride film, a light emitting element, and a surface acoustic wave device.
[0002]
[Prior art]
Group III nitride semiconductors typified by gallium nitride (GaN) are materials that have attracted particular attention in recent years as light emitting diodes (LEDs) that emit blue to ultraviolet light and light emitting devices such as lasers. As seen in the laminated structure of blue LEDs, Group III nitrides with a high melting point must be epitaxially grown on a substrate such as sapphire, but because of the large lattice mismatch with the substrate material, the Group III nitrides still have few defects. There is a problem that it is extremely difficult to obtain a thin film. The light emission efficiency of group III nitride semiconductor devices is largely determined by the initial crystal growth on the substrate, so the development of substrate materials with good consistency is the most important issue that will bring a major breakthrough in this field. ing.
[0003]
Therefore, it is proposed to insert a so-called buffer layer (buffer layer) such as AlN between the sapphire substrate and the group III nitride film in order to alleviate the lattice mismatch between the sapphire and the group III nitride of the substrate. Has been.
[0004]
However, since the buffer layer itself such as AlN also has a large lattice mismatch with the sapphire substrate, it is difficult to obtain a uniform thin film having no defects. Regarding the formation of the buffer layer on the sapphire substrate, conventionally, a large-scale molecular beam epitaxial method (MBE), a halide chemical vapor deposition method using aluminum chloride and ammonia, or a metal organic vapor phase growth method using trimethylaluminum and ammonia ( MOVPE) is used. In either method, a large lattice mismatch between the sapphire of the substrate and the AlN thin film causes a large strain to remain at the bonding interface. For this reason, the AlN layer formed on sapphire also has a high dislocation density and has a columnar texture like a frost column. Therefore, the conventional AlN film does not sufficiently play a role as a buffer layer for growing the GaN film, and the threading dislocation in the GaN film penetrates from the substrate to the film surface. ⁸ -10 ⁷ Piece / cm ² Also exist. This is a factor that deteriorates the light emission efficiency of the light emitting device. For this reason, the light emission efficiency of the conventional blue LED is 22%, and the light emission efficiency of the ultraviolet light emitting LED is only 7.5%.
[0005]
Patent Document 1 discloses MgAl. ₂ O _Four A technique using aluminum oxynitride as a buffer layer has been proposed to alleviate lattice mismatch when GaN is stacked on a spinel substrate. However, in this prior art, the method for forming the aluminum oxynitride layer uses a metal organic chemical vapor deposition method or a molecular beam epitaxial method, and a GaN film is formed on the aluminum oxynitride layer. Therefore, it cannot be said that the lattice mismatch is sufficiently improved.
[0006]
Further, Patent Documents 2 and 3 propose a technique related to a stacked single crystal substrate in which an aluminum oxynitride film is formed on a sapphire substrate and an aluminum nitride single crystal thin film is stacked on the film. In this method, an aluminum oxynitride film or an aluminum nitride single crystal thin film is deposited and grown on a sapphire substrate by a CVD method. The formed aluminum oxynitride film was formed at a substrate temperature of 1,150 ° C. It consists of a non-equilibrium phase called a strained superlattice with an oxygen concentration of 25 mol% on the sapphire substrate side and 0 mol% on the aluminum nitride side. This is a substance different from the cubic γ spinel type aluminum oxynitride obtained at the present invention and existing at 1,630 ° C. or higher. Moreover, the evaluation of the aluminum nitride film produced through this aluminum oxynitride layer is also evaluated only by the half-value width of the X-ray rocking curve, and it has not been confirmed whether the entire sample is a single crystal.
[0007]
[Patent Document 1]
US Pat. No. 5,741,724
[Patent Document 2]
JP-A-2-141495
[Patent Document 3]
JP-A-2-1539797
[0008]
[Problems to be solved by the invention]
The present invention has been made in view of such problems, and can form a group III nitride film having a low dislocation density, a small lattice mismatch, an excellent crystallinity, and an excellent luminous efficiency. An object is to provide a single crystal aluminum nitride film and a method for forming the same, a base substrate for a group III nitride film, a light emitting element, and a surface acoustic wave device.
[0009]
In the process of conducting a thermodynamic study of aluminum nitride formation reaction using alumina, carbon, and nitrogen as reaction raw materials, the present inventors do not form a target thin film on a conventional sapphire substrate, It is found that a single crystal aluminum nitride film having good crystallinity can be formed by converting the alumina component into aluminum oxynitride and aluminum nitride from the surface of the sapphire substrate to the inside using an equilibrium reaction, and the present invention is completed. It reached.
[0010]
[Means for Solving the Problems]
According to the present invention, the object of the present invention is firstly to provide a single crystal α-Al. ₂ O _Three On the substrate, a single crystal aluminum nitride film is laminated as an outermost layer through an aluminum oxynitride layer, and the dislocation density in the single crystal aluminum nitride is 10 ⁸ Piece / cm ² This is achieved by a single crystal aluminum nitride laminated substrate characterized by:
[0011]
According to the present invention, the above object of the present invention is, secondly, single crystal α-Al ₂ O _Three A single crystal characterized in that the substrate is nitrided to form an aluminum oxynitride layer and a single crystal aluminum nitride film on the aluminum oxynitride layer, thereby producing the single crystal aluminum nitride multilayer substrate of the present invention. This is achieved by a method for manufacturing an aluminum nitride laminated substrate.
[0012]
According to the present invention, thirdly, the object of the present invention is to provide a single crystal α-Al ₂ O _Three This is achieved by a method for forming a single crystal aluminum nitride film, characterized in that the substrate is nitrided to form an aluminum oxynitride layer and an aluminum nitride film on the aluminum oxynitride layer.
[0013]
According to the present invention, the above object of the present invention is fourthly achieved by a single crystal aluminum nitride film obtainable by the above method of the present invention.
[0014]
According to the present invention, fifthly, the object of the present invention is achieved by a base substrate for a group III nitride film characterized by comprising the single crystal aluminum nitride laminated substrate of the present invention.
[0015]
Further, according to the present invention, sixthly, the object of the present invention is to provide a light emitting device comprising the single crystal aluminum nitride laminated substrate of the present invention, wherein the single crystal aluminum nitride film is a light emitting film. Achieved.
[0016]
Further, according to the present invention, seventhly, the object of the present invention is to have a semiconductor film made of a group III nitride single crystal on the single crystal aluminum nitride laminated substrate of the present invention, and the semiconductor film This is achieved by a light emitting element characterized by being a light emitting film.
[0017]
Finally, according to the present invention, the object of the present invention is eighthly achieved by a surface acoustic wave device characterized by comprising the single crystal aluminum nitride laminated substrate of the present invention.
[0018]
In the present invention, as described above, the single crystal α-Al ₂ O _Three An aluminum oxynitride (alon) layer is formed on the surface of a substrate such as a sapphire substrate by nitriding treatment, and a single crystal aluminum nitride (AlN) film is formed using the alon layer as a base film. For this reason, the obtained single crystal AlN film has a very low dislocation density and excellent crystallinity. Therefore, when this single crystal AlN film is used as a light emitting device, an ultraviolet light emitting element with extremely high luminous efficiency can be obtained, and it can be applied to a high density optical memory, and it can be widely used as a light receiving element for ultraviolet light. Applicable. Furthermore, when a group III nitride semiconductor typified by a GaN system is epitaxially grown on this single crystal AlN film having excellent crystallinity, a group III nitride semiconductor single crystal film having extremely high crystallinity and few defects is obtained. Can be formed. For this reason, the luminous efficiency of the blue light emitting element and ultraviolet light emitting element using this group III nitride semiconductor single crystal film can be remarkably improved. Furthermore, single crystal AlN can also be applied as a surface acoustic wave device, and according to the present invention, a single crystal AlN film having excellent crystallinity can be obtained, so that the driving efficiency of a surface acoustic wave device using this can be improved. Can do.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail. First, a preferred embodiment of the present invention is as follows.
[0020]
The single crystal aluminum nitride substrate of the present invention is preferably a single crystal α-Al ₂ O _Three The crystal plane of the substrate is the A plane (11-20), and the crystal plane of the single crystal aluminum nitride film is the C plane (0001).
[0021]
Here, the plane orientation of (11-20) indicates that the coordinate values are “1”, “1”, “−2”, “0” in order for convenience.
[0022]
In the present invention, the single crystal α-Al ₂ O _Three The nitriding treatment of the substrate preferably comprises heat-treating the substrate in the presence of carbon, nitrogen and carbon monoxide.
[0023]
In the present invention, the alon layer and the AlN film are made of single crystal α-Al. ₂ O _Three A significant feature is that the substrate is formed by direct nitridation of a sapphire substrate, for example. The direct nitridation is a single crystal α-Al using carbon, nitrogen and carbon monoxide as reaction raw materials. ₂ O _Three This is a method of nitriding (for example, sapphire) by the following reaction formula.
[0024]
[Expression 1]

[0025]
Where alon is (Al _{(64 + x) / 3} □ _{(8-x) / 3} O _32-x N _x Where □ is represented as cation vacancies, 2 <x <6).
[0026]
As the sapphire substrate used in the present invention, a commercially available substrate can be used without limitation. In particular, a bubble having a diameter of 50 micrometers or less, preferably 10 micrometers or less and a purity of 99.99% or more is preferably used. When an alon layer and an AlN film are formed on the surface of the substrate, any surface can be used as the crystal plane of the substrate, but a plane other than the C plane is preferable in terms of crystal symmetry.
[0027]
As the carbon, various commercial products can be used. For the purpose of saturating the reaction atmosphere with carbon, powdery graphite and / or carbon black is preferably used. The purity of carbon is preferably 99.9% or more, and more preferably 99.999% or more.
[0028]
Nitrogen and carbon monoxide are usually in a gaseous state, but 99.9999% or more of nitrogen and 99.9% or more of carbon monoxide are preferable.
[0029]
Next, a method for forming an alon layer and an AlN film on the sapphire substrate using the reaction raw material will be described.
[0030]
Figure 1 shows Al ₂ O _Three -An AlN pseudo binary system phase diagram is shown. As shown in this phase diagram, alon is a γ-phase aluminum oxynitride having an inverse spinel structure that exists stably only at a high temperature of 1903 K (1630 ° C.) or higher and has a wide solid solution region. This alon is attracting attention as a refractory material with high chemical stability at high temperatures and high corrosion resistance in metal refining, and has high translucency from the ultraviolet region to the infrared region even in the case of a polycrystal. It is also characterized by being chemically stable.
[0031]
In the present invention, this alon is formed by directly nitriding the surface of a single crystal α-alumina substrate such as a sapphire substrate. That is, for example, the Al of the apparatus shown in FIG. ₂ O _Three A sapphire substrate having a surface crystallinity of A-plane (11-20) and graphite are charged at the bottom of the reaction tube, and N ₂ By adjusting the composition of the —CO mixed gas, the substrate is nitrided in an atmosphere in which the oxygen potential and the nitrogen potential are controlled. The nitriding temperature is 1903 K or more generated by alon. As a result, an alon layer and an AlN film on the alon layer are generated. FIG. 3 shows temperature (K) on the horizontal axis and CO partial pressure (P _CO ) And N ₂ Partial pressure (P _N2 ) And the state diagram shown in FIG. ₂ Pressure ratio P _CO / P _N2 Rewritten with This figure shows the total pressure P _CO + P _N2 Is 1 atm, carbon activity a _c Al under the condition of 1 ₂ O _Three FIG. 3 is a phase stability diagram of Alon, alon, and AlN. In the figure, P _CO / P _N2 The region with large is Al ₂ O _Three The region A is an alon stable region (1903K or more), and the region B below it is an AlN stable region.
[0032]
In the present invention, an AlN film is generated on the outermost surface of the substrate by maintaining the substrate under the generation conditions indicated by region B in FIG. When An alon layer is formed between the AlN film. At temperatures below 1903K, Al ₂ O ₃ Since AlN directly equilibrates, alon is not generated. However, at temperatures above 1903 K, as is apparent from the phase diagram shown in FIG. 1 and the aluminum-oxygen-nitrogen-carbon chemical potential diagram shown in FIG. 3, alon is always between sapphire and AlN as an equilibrium phase. Will exist. Note that the XRD (X-ray diffraction) profile of FIG. 4 to be described later has the temperature, CO, and N indicated by ● in FIG. ₂ This is the case where alon and AlN are grown under the condition of the partial pressure ratio.
[0033]
As shown in FIG. 3, by maintaining the sapphire substrate at a temperature equal to or higher than the generation temperature of alon (1903K or higher), an alon layer is generated on the sapphire substrate, and an AlN film with excellent crystallinity is further formed thereon. Generate. At a temperature lower than the temperature at which alon is generated, alon is not generated and the AlN film generated on the sapphire substrate is polycrystallized. For example, the total pressure P _CO + P _N2 Is 1 atm, carbon activity a _c In the method for generating alon and AlN on a sapphire substrate by maintaining the temperature at 1903K or higher under the condition of 1 in order to nitride the surface of the sapphire substrate with gas, nitrogen supplied from the gas and oxygen in sapphire As a result, the alon layer and the single crystal AlN film are sequentially formed. An AlN film formed by a conventional MOVPE method or the like grows in a uniaxial direction but has a columnar texture like a frost column and has a high dislocation density. Unlike the AlN film obtained by the conventional method, the single crystal AlN film obtained by the present invention does not have a columnar texture like a frost column, and has a low dislocation density. It has excellent crystallinity that cannot be achieved.
[0034]
As can be understood from the chemical potential diagram of FIG. 3, the amounts of carbon, nitrogen and carbon monoxide used in the direct nitridation reaction, the mixing ratio thereof, or the heating temperature may be appropriately determined using this figure.
[0035]
The amount of carbon used is preferably about 0.1 to 1 by weight with respect to the single crystal α-alumina.
[0036]
Although the mixing ratio (partial pressure ratio) of carbon monoxide and nitrogen can be determined from the above chemical potential diagram, 0.1 to 0.5 is preferably employed. The total pressure in the reaction system is preferably 1 atm. It is preferable to do so from the viewpoint of ease of production and operation of the reaction apparatus. Prior to the reaction, the inside of the reaction apparatus is once evacuated to a vacuum, and then a mixed gas having a predetermined partial pressure is introduced. During the reaction, this mixed gas is allowed to flow at a predetermined flow rate.
[0037]
The heating temperature is selected between the temperature at which alon is generated (1903K) and the upper limit temperature (2149K) at which alon and AlN directly equilibrate.
[0038]
The heating time is appropriately determined depending on the desired film thickness. For example, P at 1973K _CO / P _N2 The growth rate of the single crystal aluminum nitride film under the condition of = 0.1 is 0.2 to 0.8 micrometers per hour.
[0039]
After completion of the reaction, for example, the reaction tube is pulled out of the furnace and rapidly cooled and taken out. A slow cooling rate is not preferable because the alon layer formed between the substrate and the single crystal aluminum nitride film is decomposed.
[0040]
In the present invention, the generation condition is changed after the alon layer is first formed on the substrate, without being limited to the case where alon and AlN are generated simultaneously while maintaining the substrate in the condition in the region B of FIG. Then, an AlN film may be formed. The formation conditions of the alon layer and the AlN film in this case are as follows. First, only the alon layer is grown first by holding the substrate at the generation conditions in the region A of FIG. 3, and then the temperature and / or P _CO / P _N2 By changing the ratio, the generation condition is changed to that in the region B, and if this condition is maintained, an AlN film is generated on the previously generated alon layer.
[0041]
In this way, the alon layer and the AlN film can be continuously formed on the substrate in the same process. FIG. 4 is an example of a graph showing an XRD (X-ray diffraction) profile for a layered film of an alon layer and an AlN film grown on a sapphire substrate as described above. In FIG. 4, the horizontal axis represents the diffraction angle 2θ, and the vertical axis represents the X-ray intensity. N ₂ The partial pressure ratio in the -CO mixed gas is P _CO / P _N2 = 0.1, and in this atmosphere, the sapphire substrate was heated to 1973 K for 24 hours. As shown in FIG. 4, α-Al whose plane orientation is the A plane ₂ O _Three A laminate of (sapphire substrate), (111) plane alon and C plane AlN is formed.
[0042]
As shown in FIG. 4, a sapphire / alon / AlN laminate is grown, and the generated alon and AlN are single crystals.
A-plane sapphire // (111) alon // C-plane AlN
It can be seen that it is.
[0043]
As for the crystal orientation relationship between the single crystal aluminum nitride film of the present invention and the sapphire substrate, the crystallinity of alon and AlN differs depending on the sapphire surface of the substrate as shown below. This is because C-plane sapphire has three isotropic symmetry axes, and alon having three kinds of crystal orientations grows to become polycrystalline, whereas A-plane sapphire has isotropic symmetry axes. This is because crystal growth is allowed only in one direction.
[0044]
Below, the relationship of the crystal orientation of a sapphire substrate and an alon and AlN film is illustrated.
[0045]
C-plane sapphire / (111) alon (polycrystal): offset angle 20 °
(111) alon (polycrystalline) // C-plane AlN (polycrystalline)
[0046]
The offset angle represents an angle between crystal faces.
[0047]
A-plane sapphire // (111) alon // C-plane AlN
The thickness of the obtained AlN film is usually 0.1 to 20 μm, but can be further increased as necessary.
[0048]
The dislocation density in the single crystal aluminum nitride film of the present invention is 10 ⁸ Piece / cm ² It is as follows. From the cross-sectional photograph of the transmission electron microscope (TEM) of the single crystal aluminum nitride film obtained in the present invention, threading dislocations are not found, so if there is one dislocation in the range of 1 square micrometer which is the field of view of the photograph. From the assumed dislocation density (the largest estimated value), 10 ⁸ Piece / cm ² And calculated.
[0049]
In the present invention, the lattice mismatch is determined using the closest anion distance d,
[0050]
m. f. = 100x (dsub-depi) / dsub
[0051]
Where dsub is the distance between the nearest anions of the substrate and depi is the distance between the nearest anions of the epitaxial layer.
, The lattice mismatch at each interface is expressed as follows.
A surface sapphire substrate / C surface aluminum nitride: 17.0
A surface sapphire substrate / (111) aluminum oxynitride: 4.90
(111) Aluminum oxynitride / C plane aluminum nitride: 4.82
That is, the lattice mismatch can be reduced to about 5% by using aluminum oxynitride.
[0052]
The cubic spinel-type γ-phase aluminum oxynitride (alon) layer obtained in the present invention is composed of a single crystal and usually has a thickness of about 0.1 micrometers.
[0053]
The crystal form and crystallinity of the AlN film and the alon layer can be confirmed by X-ray diffraction, pole figure analysis, and cross-sectional inspection using a transmission electron microscope (TEM). Moreover, about each thickness, it can confirm and measure by the cross-sectional observation by TEM.
[0054]
The single-crystal aluminum nitride multilayer substrate of the present invention can itself be a light-emitting element having an AlN film as a light-emitting film, and further, on this AlN film, single-crystal gallium nitride (GaN) or any group III nitride such as AlGaN or InGaN can be used. By stacking compound mixed crystal films, a light emitting element using the single crystal group III nitride film as a light emitting film can be obtained. Specifically, the single crystal AlN film itself can be used as an ultraviolet light emitting layer. This AlN ultraviolet light emitting device can be applied to a high-density optical memory. The AlN layer can be widely used as a light receiving element for ultraviolet light.
[0055]
FIG. 5 shows a light-emitting element of the present invention. An alon layer is formed on the sapphire substrate by directly nitriding the substrate, and a single crystal AlN film is formed on the alon layer. Further, using this single crystal AlN film as a buffer layer, a single crystal n-type group III nitride film is formed on the AlN film. This single crystal n-type group III nitride film can be formed by metal organic chemical vapor deposition, halide chemical vapor deposition, or molecular beam epitaxy. The obtained n-type group III nitride has extremely excellent crystallinity because the crystallinity of the underlying AlN film is excellent.
[0056]
As described above, since the alon layer is formed on the sapphire substrate and the AlN film is formed thereon, the light emitting element of the present invention can remarkably improve the crystallinity of the AlN film. When an AlN film is formed on a sapphire substrate by MBE or MOVPE as in the prior art, the lattice mismatch between the sapphire substrate and the AlN film reaches 17%. After forming an alon layer by direct nitridation and forming an AlN film using this alon layer as a base film, the lattice mismatch at the sapphire substrate / alon film interface and the alon film / AlN film interface is about 5%, respectively. Can be suppressed. Therefore, a high quality single crystal AlN film can be formed. Therefore, if a group III nitride film is formed on this AlN film, a high-quality group III nitride thin film with extremely few threading dislocations can be formed. Accordingly, the light emission efficiency of the light emitting device having the group III nitride light emitting layer can be dramatically improved.
[0057]
Since the single crystal aluminum nitride laminated substrate of the present invention has a larger anisotropy of surface elastic modulus than a conventional polycrystal, it can also be used as a high performance surface acoustic wave device. Specifically, the surface acoustic wave device can be used for a mobile phone, mobile communication, a TV intermediate band filter, a satellite phone, and the like.
[0058]
【Example】
Example 1
Using the reactor shown in FIG. ₂ AlN and alon phases were fabricated by nitriding a sapphire substrate with —CO mixed gas and graphite. Al ₂ O _Three A sapphire substrate (10 mm × 10 mm × 1 mm) and a graphite powder (purity 99.999%) having a surface crystallized surface A (11-20) were placed at the bottom of the reaction tube. The reaction tube is evacuated with a rotary pump in advance to completely remove the water in the tube, and the carbon monoxide (CO) partial pressure and nitrogen (N ₂ ) Complete replacement was performed with a mixed gas having a partial pressure ratio of 0.1. Thereafter, this mixed gas was allowed to flow at a constant flow rate (55 ml / min). The total pressure in the system is 1 atmosphere. The sample was rapidly heated by inserting the bottom of the reaction tube into the heating section of the furnace, and the reaction was started at 1973K. After holding for 24 hours, the sample was rapidly cooled by withdrawing the reaction tube from the furnace to complete the reaction.
[0059]
In the obtained reacted sapphire substrate, diffraction peaks of the (111) plane of alon and the (0002) plane of AlN were observed in addition to the (11-20) plane of the sapphire substrate (FIG. 4). . From this, it was found that the AlN and alon layers generated on the entire surface of the sample had a single crystal orientation.
[0060]
FIG. 6 and FIG. 7 show the pole figure of the {1-100} plane of the AlN layer and the pole figure of the {111} plane of the alon layer, respectively. Here, φ = 0 of the pole figure ^o Is the [1-100] direction of sapphire. From FIG. 6, the {1-100} plane of the AlN layer is ψ = 90. ^o Φ is 60 ^o It can be seen that six diffraction peaks are shown at intervals. This indicates that the (0001) plane of AlN exists horizontally with the A-plane sapphire substrate. Furthermore, the [1-100] direction in the (0001) plane of AlN coincided with the [1-100] direction of sapphire. Further, from FIG. 7, the {111} plane of the alon layer is the origin and ψ = 70. ^o Φ is 60 ^o It can be seen that six diffraction peaks are shown at intervals. This is because the (111) plane of alon is parallel to the sapphire substrate (plane A), and is 180 degrees within the alon (111) plane indicated by the solid line and the broken line. ^o It shows that it has two rotated crystal orientations. The [11-2] direction in the alon (111) plane is 30 with respect to the [1-100] direction of sapphire. ^o Existed in the direction of rotation. This corresponds to the fact that the [110] direction in the alon (111) plane is parallel to the [1-100] direction of sapphire. From this, the crystal phase relationship of the following equation is
[0061]
A-plane sapphire /// (111) alon // C-plane AlN
[0062]
Accompanying the following crystal orientation relationship,
[0063]
[1-100] AlN // [110] alon // [1-100] sapphire
[0064]
Also found to have.
[0065]
From the above crystal orientation relationship, taking into account the rotation of the lattice at each interface, the arrangement of atoms at the sapphire / alon interface and the alon / AlN interface is shown in FIG. 8, and the lattice mismatch is calculated from this figure. When A-plane sapphire is nitrided, the lattice mismatch at each interface is expressed as follows.
[0066]
A surface sapphire substrate / (111) aluminum oxynitride: 4.90
(111) Aluminum oxynitride / C plane aluminum nitride: 4.82
[0067]
A cross-sectional TEM image of the A-plane sapphire substrate obtained in this example is shown in FIG. As shown in the figure, it was found that three phases having different crystal orientations and crystal structures were formed in layers. Moreover, when the composition analysis by EDX was performed about each of these phases, it turned out that the alon layer and the AlN layer were continuously producing | generating on the sapphire substrate. Further, it was found that the generated alon layer and AlN layer were generated as single crystals from the electron diffraction pattern, and no dislocation was present in the range of at least 1 square micrometer. From this result, the dislocation density in the generated AlN is estimated to be 10 ⁸ Piece / cm ² It was found to be about an order of magnitude less than the conventional dislocation density.
[0068]
An atomic force microscope (AFM) image of the A-plane sapphire substrate obtained in this example is shown in FIG. As shown in the figure, the surface subjected to the nitriding treatment was not smooth, and undulations of up to about 400 nm existed.
[0069]
reference Example 2
Using the experimental apparatus shown in FIG. 2, the AlN and alon phases are 1973 K and N ₂ It was produced by nitriding a sapphire substrate with —CO mixed gas and graphite. Al ₂ O ₃ A sapphire substrate having a C-plane crystal face and graphite powder (purity 99.999%) were placed on the bottom of the reaction tube. The reaction tube is evacuated with a rotary pump in advance to completely remove the water in the tube. _CO / P _N2 = CO-N with a composition of 0.1 ₂ Complete replacement was performed under a mixed gas atmosphere. Thereafter, this mixed gas was allowed to flow at a constant flow rate (55 ml / min). The total pressure in the system is 1 atmosphere. The sample was rapidly heated by inserting the bottom of the reaction tube into the heating section of the furnace, and the reaction was started at 1973K. After holding for 24 hours, the sample was rapidly cooled by withdrawing the reaction tube from the furnace to complete the reaction.
[0070]
FIG. 11 shows a pole figure of the AlN film generated on the reacted C-plane sapphire substrate. As a result, it was found that an offset angle of 20 ° exists between C-plane sapphire and (111) alon.
[0071]
C-plane sapphire / (111) alon (polycrystal): offset angle 20 °
(111) alon (polycrystalline) // C-plane AlN (polycrystalline)
[0072]
For this reason, since C-plane sapphire has three isotropic symmetry axes, alon having three kinds of crystal orientations grows, that is, becomes polycrystalline, and the outermost AlN is also C-plane oriented. It is thought that it is polycrystalline.
[0073]
On the other hand, as described in Example 1, since the A-plane sapphire has no isotropic symmetry axis, crystal growth is allowed only in one direction, and single crystal AlN having good crystallinity was obtained. It is considered a thing.
[0074]
As described above, according to the present invention, a uniform single crystal AlN film having few defects and excellent crystallinity without using a halide-based harmful gas in a halide chemical vapor deposition method and an organometallic-based harmful gas in a MOVPE method. Single crystal α-Al such as a sapphire substrate at low cost ₂ O _Three It can be formed on a substrate.
[0075]
If this single crystal AlN film is used as a buffer layer for forming a single crystal group III nitride film, the light emission efficiency of light emitting devices such as blue light emitting diodes and blue lasers can be remarkably improved. If it is used as a light emitting diode, the light emitting efficiency and light receiving efficiency of the ultraviolet light emitting element and the light receiving element can be remarkably improved. Furthermore, this single crystal AlN film can be put into practical use as a high-density optical memory and a surface acoustic wave device.
[Brief description of the drawings]
FIG. 1 Al ₂ O _Three -An AlN pseudo binary system phase diagram is shown.
FIG. 2 is a schematic view of a reaction apparatus used in Examples of the present invention.
FIG. 3 is an aluminum-oxygen-nitrogen-carbon chemical potential diagram. The horizontal axis represents temperature (K), and the vertical axis represents CO partial pressure (P _CO ) And N ₂ Partial pressure (P _N2 ) And the state diagram shown in FIG. ₂ Pressure ratio P _CO / P _N2 Rewritten with
FIG. 4 is a graph showing an XRD (X-ray diffraction) profile of a layered film of an alon layer and an AlN film grown on a sapphire substrate.
FIG. 5 is a diagram showing a layer structure of a light emitting device according to the present invention.
FIG. 6 is a diagram showing a pole figure (A-plane sapphire) of an AlN film.
FIG. 7 is a diagram showing a pole figure (A-plane sapphire) of an alon film.
FIG. 8 shows lattice mismatch at sapphire / alon and alon / AlN interfaces.
FIG. 9 is a diagram showing a TEM (A-plane sapphire) of a substrate cross section.
FIG. 10 is a diagram showing AFM (A-plane sapphire) on the substrate surface.
FIG. 11 is a diagram showing a pole figure (C-plane sapphire) of an AlN film.

Claims (10)

On the single crystal α-Al ₂ O ₃ substrate, a single crystal aluminum nitride film is laminated as an outermost layer through an aluminum oxynitride layer, and the dislocation density in the single crystal aluminum nitride is 10 ⁸ / cm ² or less A single crystal aluminum nitride laminated substrate characterized by the above. _2. The multilayer substrate according to claim 1, wherein the crystal plane of the single crystal α-Al ₂ O ₃ substrate is an A plane and the crystal plane of the single crystal aluminum nitride film is a C plane. The single crystal aluminum nitride laminate according to claim 1 or 2, wherein the single crystal α-Al ₂ O ₃ substrate is nitrided to form an aluminum oxynitride layer and a single crystal aluminum nitride film on the aluminum oxynitride layer, thereby A method for producing a single crystal aluminum nitride laminated substrate, comprising producing a substrate. Nitriding the single crystal α-Al ₂ O ₃ substrate, carbon, in the presence of nitrogen and carbon monoxide, The method of claim 3 which comprises heating a single crystal α-Al ₂ O ₃ substrate. A method for forming a single crystal aluminum nitride film, comprising nitriding a single crystal α-Al ₂ O ₃ substrate to form an aluminum oxynitride layer and an aluminum nitride film on the aluminum oxynitride layer. Nitriding the single crystal α-Al ₂ O ₃ substrate, carbon, in the presence of nitrogen and carbon monoxide, the method according to claim 5 consisting of heat treating the single crystal α-Al ₂ O ₃ substrate.   A base substrate for a group III nitride film comprising the single crystal aluminum nitride laminated substrate according to claim 1.   A light emitting device comprising the single crystal aluminum nitride laminated substrate according to claim 1, wherein the single crystal aluminum nitride film is a light emitting film.   3. A light emitting device comprising: a semiconductor film made of a group III nitride single crystal on the single crystal aluminum nitride laminated substrate according to claim 1; and the semiconductor film being a light emitting film.   A surface acoustic wave device comprising the single crystal aluminum nitride laminated substrate according to claim 1.

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