JP3803606B2 - Method for manufacturing group III nitride semiconductor substrate - Google Patents

Description

【００６１】
のことを指し、<1-100>方向とは、
【００６２】
【数２】

【００６３】
のことを指す。
【００６４】
次に、図２（ｃ）に示す工程で、上記図２（ｂ）に示す工程で得られた基板をＨＶＰＥ炉内に導入する。次いで、８００℃、窒素およびアンモニア雰囲気中で１０分間熱処理を行なって、基板表面上の不純物を除去した後、基板上にＧａＮ層１４を形成する。ＧａＮ層１４の形成方法は、上記実施形態１の図１（ｂ）に示す工程で行なう方法と同じである。
【００６５】
このとき、ＧａＮ結晶は、Ｎｉ膜２３上には成長せず、開口部２３ａ内に露出したＧａＮ層１２上から成長を始める。ＧａＮ結晶の成長をさらに続けていくと、開口部２３ａ内に露出したＧａＮ層１２から成長したＧａＮ結晶は、Ｎｉ膜２３上をＮｉ膜２３の上面に沿って成長し、隣接する開口部２３ａから成長したＧａＮ結晶と共にＮｉ膜２３を覆う。続いて、さらにＮｉ膜２３を覆うＧａＮ結晶の上にＧａＮ結晶が成長し、ＧａＮ層１４は形成される。このようにしてＧａＮ層１４の厚さを３００μｍまで成長させた後、室温まで冷却し、ＨＶＰＥ炉内から取り出す。
【００６６】
次に、図２（ｄ）に示す工程で、上記実施形態１と同様に、レーザビームをサファイア基板１１の下面側から、サファイア基板１１を介してＧａＮ層１２の下面全体に亘って走査しながら照射する。使用するレーザビームは、Ｎｄ：ＹＡＧレーザの３次高調波（３５５ｎｍ）で、照射エネルギー０．３Ｊ／ｃｍ²、パルス幅５ｎｓ、照射時のレーザビーム径は１ｍｍである。ＧａＮの吸収端波長は３６０〜３７０ｎｍであるので、波長が３５５ｎｍであるレーザビームはＧａＮ層１２で吸収され、発熱する。このことによって、ＧａＮ層１２が分解される。
【００６７】
次に、図２（ｅ）に示す工程で、サファイア基板１１と、Ｎｉ膜２３およびＧａＮ層１４とを分離する。このとき、上記図２（ｄ）に示す工程においてＧａＮ層１２が分解されているので、サファイア基板１１と、Ｎｉ膜２３およびＧａＮ層１４とを分離することは容易である。
【００６８】
次に、図２（ｆ）に示す工程で、ＧａＮ層１４の下に形成されているＮｉ膜２３を、硝酸を用いたウエットエッチングにより除去する。このことにより、自立したＧａＮ基板１４ａが得られる。Ｎｉ膜２３が除去されていないＧａＮ基板にデバイスを作製した場合、Ｎｉ膜２３は熱伝導率が小さいので、デバイスの放熱特性が悪くなり、デバイスの性能に影響を及ぼす可能性がある。従って、Ｎｉ膜２３を除去することが好ましい。なお、Ｎｉ膜２３の除去は、研磨によって行なってもよい。
【００６９】
本実施形態の方法では、ＧａＮ層１２とＧａＮ層１４との間に、ＧａＮ層１２よりも熱伝導率の低いＮｉ膜２３形成する。ＡｇやＣｕなどの金属は熱伝導率が大きいが、Ｎｉは熱伝導率が０．８４Ｗ／ｃｍ・Ｋであり、ＧａＮの熱伝導率よりも小さい。このため、ＧａＮ層１２におけるレーザビームの吸収により発生した熱は、Ｎｉ膜２３に伝導しにくい。つまり、Ｎｉ膜２３は熱拡散を抑制する。従って、発生した熱の大半はＧａＮ層１２の分解に寄与するので、ＧａＮ層１２を効率良く分解できる。
【００７０】
さらに本実施形態では、ＧａＮ層１２よりも熱伝導率が小さいサファイア基板１１（熱伝導率０．４６Ｗ／ｃｍ・Ｋ）を用いている。このため、サファイア基板１１への熱の伝導も抑えられ、より多くの熱をＧａＮ層１２の分解に寄与させることができる。
【００７１】
ＧａＮよりも熱伝導率が小さく、且つＧａＮの成長温度（１０５０℃）以上の融点を有する金属材料として、Ｎｉの他にＰｔ（０．７１Ｗ／ｃｍ・Ｋ）、Ｔｉ（０．２２Ｗ／ｃｍ・Ｋ）が挙げられる。従って、これらの金属からなる膜を、Ｎｉ膜２３の代わりに用いてもよい。また、金属材料において、熱は格子振動と自由電子により伝えられるが、一般に自由電子による熱輸送が支配的である。しかし、合金では自由電子の数が減少し、格子振動による熱輸送が支配的になるため、熱伝導率は合金の成分金属よりも小さくなる傾向がある。従って、Ｎｉ膜２３の代わりに合金膜を用いてもよい。
【００７２】
なお、図２（ｂ）に示す工程において、Ｎｉ膜２３の表面をシリコン酸化膜などで被覆してもよい。このことによって、アンモニアあるいは水素等とＮｉ膜２３が化学反応を起こすのを防ぐことができる。勿論、Ｎｉ膜２３の代わりに上述のＰｔ膜、Ｔｉ膜、合金膜を用いた場合も、これらの金属膜の表面をシリコン酸化膜などで被覆してもよい。
【００７３】
本実施形態では、ＧａＮ層１４の形成の際に、ＧａＮ結晶がＮｉ膜２３上をＮｉ膜２３の上面に沿って成長するので、ＧａＮ層１２に発生した転位がＧａＮ層１４にはほとんど伝播しない。つまり、ＧａＮ層１４内には転位が非常に少なくなる。このため、本実施形態では良好な結晶性を有するＧａＮ基板を作製することが可能である。
【００７４】
また、本実施形態では円形の開口部２３ａを有するＮｉ膜２３を形成したが、開口部２３ａの形状はこれに限定されない。開口部２３ａの形状として、例えば、ストライプ状、矩形状であっても良い。いずれの形状の開口部を設けた場合においても、開口部内に露出するＧａＮ層１２の上面の面積が小さいほど、熱拡散を防止する効果が大きい。
【００７５】
さらに、本実施形態では開口部２３ａが、ＧａＮ層１２の<11-20>方向に一定間隔ａ（本実施形態では５μｍ）で１列に並び、ＧａＮ層１２の<1-100>方向に隣接する各列の開口部は、一定間隔ａ／２（本実施形態では２．５μｍ）だけずらして配置されている。開口部２３ａをこのように配置にすると、ＧａＮ結晶は六方最密構造であるため、結晶の成長面がＮｉ膜２３の上では開口部２３ａを中心とする六角形の各辺となる。従って、互いに隣接する開口部２３ａから成長したＧａＮ結晶が接触する際には、格子欠陥が生じにくい。つまり、より格子欠陥の少ないＧａＮ層１４を形成することができ、良質なＧａＮ基板が得られる。
【００７６】
なお、本実施形態では、開口部２３ａを有するＮｉ膜２３ｂを、ＧａＮ層１４の横方向成長のためのマスクとしても使用する。このため、新たにＧａＮ層１４の横方向成長のためのマスクを製造する必要が無く、製造工程を簡略化することができる。
【００７７】
（実施形態３）
本発明の実施形態３におけるＧａＮ基板の製造方法を、図４を参照しながら説明する。図４は、本実施形態のＧａＮ基板の製造方法を表す工程断面図である。
【００７８】
まず、図４（ａ）に示す工程で、Ｃ面を主面とするサファイア基板１１（直径２インチ、厚さ３００μｍ）をＭＯＶＰＥ炉内に導入する。次に、１０５０℃の水素雰囲気中でサファイア基板１１を熱処理した後、５００℃まで冷却する。続いて、III族原料にトリメチルガリウム（ＴＭＧ）、Ｖ族原料にアンモニアを用いて、サファイア基板１１上に厚さ２０ｎｍのＧａＮ緩衝層（不図示）を形成する。再び温度を１０５０℃まで上げ、ＧａＮ緩衝層の上に厚さ２００ｎｍのＧａＮ層１２を形成する。その後、室温まで冷却し、得られた基板をＭＯＶＰＥ炉内から取り出す。
【００７９】
次に、図４（ｂ）に示す工程で、スパッタ法により、ＧａＮ層１２上に厚さ２００ｎｍのＳｉＯ₂層を形成する。続いて、このＳｉＯ₂層をパターニングすることによって開口部４３ａを有するシリコン酸化膜４３を形成する。このとき、ＳｉＯ₂層のパターニングは、フォトリソグラフィーおよびエッチングを用いて行ない、図３に示す上記実施形態２との開口部２３ａと同様の、ＧａＮ層１２に到達する直径２．５μｍの円形の開口部４３ａを形成する。開口部２３ａは、ＧａＮ層１２の<11-20>方向に一定間隔ａ（本実施形態では５μｍ）で１列に並び、ＧａＮ層１２の<1-100>方向に隣接する各列の開口部は、一定間隔ａ／２（本実施形態では２．５μｍ）だけずらして配置されている。
【００８０】
次に、図４（ｃ）に示す工程で、上記図４（ｂ）に示す工程で得られた基板をＨＶＰＥ炉内に導入する。次いで、８００℃、窒素およびアンモニア雰囲気中で１０分間熱処理を行なって、基板表面上の不純物を除去した後、基板上にＧａＮ層１４を形成する。ＧａＮ層１４の形成方法は、上記実施形態１の図１（ｂ）に示す工程で行なう方法と同じである。
【００８１】
このとき、ＧａＮ結晶は、シリコン酸化膜４３上には成長せず、開口部４３ａ内に露出したＧａＮ層１２上から成長を始める。ＧａＮ結晶の成長をさらに続けていくと、開口部４３ａ内に露出したＧａＮ層１２から成長したＧａＮ結晶は、シリコン酸化膜４３上をシリコン酸化膜４３の上面に沿って成長し、最終的に隣接する開口部４３ａから成長したＧａＮ結晶と共にシリコン酸化膜４３を覆う。続いて、さらにシリコン酸化膜４３を覆うＧａＮ結晶の上にＧａＮ結晶が成長し、ＧａＮ層１４は形成される。このようにしてＧａＮ層１４の厚さを３００μｍまで成長させた後、室温まで冷却し、ＨＶＰＥ炉内から取り出す。
【００８２】
次に、図４（ｄ）に示す工程で、上記実施形態１と同様に、レーザビームをサファイア基板１１の下面側から、サファイア基板１１を介してＧａＮ層１２の下面全体に亘って走査しながら照射する。使用するレーザビームは、Ｎｄ：ＹＡＧレーザの３次高調波（３５５ｎｍ）で、照射エネルギー０．３Ｊ／ｃｍ²、パルス幅５ｎｓ、照射時のレーザビーム径は１ｍｍである。ＧａＮの吸収端波長は３６０〜３７０ｎｍであるので、波長が３５５ｎｍであるレーザビームはＧａＮ層１２で吸収され、発熱する。このことによって、ＧａＮ層１２が分解される。
【００８３】
次に、図４（ｅ）に示す工程で、サファイア基板１１と、シリコン酸化膜４３およびＧａＮ層１４とを分離する。このとき、上記図４（ｄ）に示す工程においてＧａＮ層１２が分解されているので、サファイア基板１１と、シリコン酸化膜４３およびＧａＮ層１４とを分離することは容易である。
【００８４】
次に、図４（ｆ）に示す工程で、ＧａＮ層１４の下に形成されているシリコン酸化膜４３を、フッ酸とフッ化アンモニウムからなる溶液を用いたウエットエッチングにより除去する。このことにより、自立したＧａＮ基板１４ａが得られる。シリコン酸化膜４３が除去されていないＧａＮ基板にデバイスを作製した場合、シリコン酸化膜４３は熱伝導率が小さいので、デバイスの放熱特性が悪くなり、デバイスの性能に影響を及ぼす可能性がある。従って、シリコン酸化膜４３を除去することが好ましい。なお、シリコン酸化膜４３の除去は、研磨によって行なってもよい。
【００８５】
本実施形態では、ＧａＮ層１２とＧａＮ層１４との間に、ＧａＮ層１２よりも熱伝導率の低いシリコン酸化膜４３を形成する。シリコン酸化膜４３を形成しているＳｉＯ₂の熱伝導率は０．０１４Ｗ／ｃｍ・Ｋであり、ＧａＮよりも熱伝導率が非常に小さく、また、上記実施形態２で用いた金属膜に比べても小さい。このため、ＧａＮ層１２におけるレーザビームの吸収により発生した熱は、シリコン酸化膜４３を伝導しにくい。つまり、シリコン酸化膜４３は熱拡散を抑制する効果が高い。従って、発生した熱の大半はＧａＮ層１２の分解に寄与するので、ＧａＮ層１２を効率良く分解できる。
【００８６】
本実施形態のシリコン酸化膜４３のような非金属材料では、自由電子による熱伝導がない。さらに、誘電体のような非結晶性材料では原子配列が不規則であるので、熱伝導の効率が悪い。このため、非金属材料であり且つ非結晶性材料の熱伝導率は一般に、金属および結晶性材料にくらべて熱伝導率は小さい。従って、本実施形態のシリコン酸化膜４３の代わりに、非金属材料であり且つ非結晶性材料からなる膜として、例えば窒化シリコン（熱伝導率：０．１８Ｗ／ｃｍ・Ｋ）からなる膜を用いてもよい。
【００８７】
さらに本実施形態では、ＧａＮ層１２よりも熱伝導率が小さいサファイア基板１１（熱伝導率０．４６Ｗ／ｃｍ・Ｋ）を用いている。このため、サファイア基板１１への熱の伝導も抑えられ、より多くの熱をＧａＮ層１２の分解に寄与させることができる。
【００８８】
また、本実施形態においても上記実施形態２と同様に、ＧａＮ結晶はシリコン酸化膜４３上をシリコン酸化膜４３の上面に沿って成長するので、ＧａＮ層１４には転位が伝播しにくい。このため、本実施形態によれば、良好な結晶性を有するＧａＮ基板を作製することができる。
【００８９】
また、本実施形態では円形の開口部４３ａを有するシリコン酸化膜４３を形成したが、開口部４３ａの形状はこれに限定されない。開口部４３ａの形状として、例えば、ストライプ状、矩形状であっても良い。いずれの形状の開口部を設けた場合においても、開口部内に露出するＧａＮ層１２の上面の面積が小さいほど、熱拡散を防止する効果が大きい。
【００９０】
さらに本実施形態では、開口部４３ａが上記実施形態２と同様に設けられているので、より格子欠陥の少ないＧａＮ層１４を形成することができ、良質なＧａＮ基板が得られる。
【００９１】
なお、本実施形態では、シリコン酸化膜４３を、ＧａＮ層１４の横方向成長のためのマスクとしても使用する。このため、新たにＧａＮ層１４の横方向成長のためのマスクを製造する必要が無く、製造工程を簡略化することができる。
【００９２】
上記実施形態１〜３において、ＧａＮおよびＧａＮよりも熱伝導率の低い材料の例を以下の表１に示す。上記実施形態１〜３における、ＧａＮよりも熱伝導率の低いＩｎＮ層１３、Ｎｉ膜２３およびシリコン酸化膜４３を、それぞれ表１に示す材料から形成される膜にしてもよい。
【００９３】
【表１】

【００９４】
（実施形態４）
本発明の実施形態４におけるＧａＮ基板の製造方法について、図５を参照しながら説明する。図５は、本実施形態のＧａＮ基板の製造方法を表す工程断面図である。
【００９５】
まず、図５（ａ）に示す工程で、Ｃ面を主面とするサファイア基板１１（直径２インチ、厚さ３００μｍ）をＭＯＶＰＥ炉内に導入する。次に、１０５０℃でサファイア基板１１を水素雰囲気中で熱処理した後、５００℃まで冷却する。続いて、III族原料にトリメチルガリウム（ＴＭＧ）、Ｖ族原料にアンモニア、キャリアガスとしてＨ₂を用いて、サファイア基板１１上に厚さ２０ｎｍのＧａＮ緩衝層（不図示）を形成する。再び温度を１０５０℃まで上げ、ＧａＮ緩衝層の上に厚さ３００ｎｍのＧａＮ層１２を形成する。その後、室温まで冷却し、得られた基板をＭＯＶＰＥ炉内から取り出す。
【００９６】
なお、サファイア基板１１上に、ＧａＮ緩衝層を形成せずにＧａＮ層２１を形成してもよい。また、キャリアガスとしてＮ₂や、Ｎ₂とＨ₂の混合ガスを用いてもよい。
【００９７】
次に、図５（ｂ）に示す工程で、スパッタ法により、ＧａＮ層１２上に厚さ６１ｎｍのＳｉＯ₂層と、厚さ３５ｎｍのＴｉＯ₂層とを交互に形成し、高反射率膜５３を形成する。本実施形態では、高反射率膜５３は、５層のＳｉＯ₂層と４層のＴｉＯ₂層の合計９層からなる膜となっている。なお、本実施形態で形成される高反射率膜５３のＮｄ：ＹＡＧレーザビームの３次高調波に対する反射率は約９７％である。
【００９８】
次に、図５（ｃ）に示す工程で、上記実施形態１と同様に、レーザビームをサファイア基板１１の下面側から、サファイア基板１１を介してＧａＮ層１２の下面全体に亘って走査しながら照射する。使用するレーザビームは、Ｎｄ：ＹＡＧレーザの３次高調波（３５５ｎｍ）であり、照射エネルギー０．２６Ｊ／ｃｍ²、パルス幅５ｎｓ、照射時のビーム径は７ｍｍである。
【００９９】
ＧａＮの吸収端波長は３６０〜３７０ｎｍであるので、レーザビームはサファイア基板１１を透過し、ＧａＮ層１２に吸収される。その結果、ＧａＮ層１２の下部は加熱され、熱分解される。このことによって、ＧａＮ層１２の下部にＧａ金属部５４が形成され、窒素ガスが発散する。このとき、サファイア基板１１とＧａＮ層１２とは、レーザビーム照射後も完全に分離せず、Ｇａ金属部５４により弱く接着している。ＧａＮ層１２は薄く、ハンドリングが困難であるため、サファイア基板１１上にＧａＮ層１２を載せたまま、以降の工程を行なう。
【０１００】
次に、図５（ｄ）に示す工程で、高反射率膜５３をパターニングすることによって開口部５３ａを有する高反射率膜５３ｂを形成する。このとき、高反射率膜５３のパターニングは、フォトリソグラフィーおよびエッチングを用いて行ない、ＧａＮ層１２に到達するストライプ状の開口部５３ａを形成する。本実施形態では、各開口部５３ａの幅は３μｍであり、各開口部５３ａの間は３μｍである。なお、エッチングの際には、高反射率膜５３を構成するＳｉＯ₂層はフッ酸を用い、ＴｉＯ₂層は熱濃硫酸を用いる。なお、開口部５３ａはストライプ状の他に、ドット状、矩形状などの形状にしてもよい。また、本工程は、レーザビーム照射工程の前に行っても良いが、その場合、開口部５３ａ内に位置するＧａＮ層１２が割れて飛んでしまうことがある。そのため、パターニングはレーザビーム照射工程の後に行なうのが好ましい。
【０１０１】
次に、図５（ｅ）に示す工程で、８００℃、窒素およびアンモニア雰囲気中で１０分間熱処理を行なって、基板表面上の不純物を除去した後、基板上にＧａＮ層１４を形成する。ＧａＮ層１４の形成方法は、上記実施形態１の図１（ｂ）に示す工程で行なう方法と同じである。
【０１０２】
このとき、ＧａＮ結晶は、高反射率膜５３ｂ上には成長せず、開口部５３ａ内に露出したＧａＮ層１２上から成長を始める。ＧａＮ結晶の成長をさらに続けていくと、開口部５３ａ内に露出したＧａＮ層１２から成長したＧａＮ結晶は、高反射率膜５３ｂの上面に沿って成長し、最終的に隣接する開口部５３ａから成長したＧａＮ結晶と共に高反射率膜５３を覆う。続いて、高反射率膜５３ｂを覆うＧａＮ結晶の上に、さらに新たなＧａＮ結晶が成長し、ＧａＮ層１４が形成される。このようにしてＧａＮ層１４の厚さを３００μｍまで成長させる。
【０１０３】
次に、図５（ｆ）に示す工程で、Ｇａ金属部５４を塩酸で除去することによって、自立したＧａＮ基板１４ａを得る。
【０１０４】
本実施形態では、図５（ｄ）に示すレーザビーム照射工程において、ＧａＮ層１２の下部で吸収されなかったレーザビームが高反射率膜５３に反射されて戻り、ＧａＮ層１２の熱分解に寄与する。ＧａＮ層１２を透過せずにＧａＮ層１２の下面に照射されるレーザビーム強度を１とし、レーザビームの３次高調波のＧａＮ層１２に対する進入長約０．３μｍと、高反射率膜５３の反射率９７％を考慮すると、反射されてＧａＮ層１２の下部に戻ってくるレーザビームの強度は、約０．１３となる。つまり、本実施形態ではＧａＮ層１２の熱分解に寄与するレーザビームの照射エネルギーの利用効率は、従来の方法の１．１３倍となる。従って、ＧａＮ層１２を効率良く熱分解することができる。
【０１０５】
また、従来の方法では、ＧａＮ層１０２を熱分解させるためのレーザビームの照射エネルギーのしきい値は約０．３０Ｊ／ｃｍ²である。しかし、本実施形態では、レーザビームの照射エネルギーが０．２６Ｊ／ｃｍ²であるが、ＧａＮ層１２の熱分解が生じている。つまり、本実施形態によれば、ＧａＮ層１２を熱分解させるために必要なレーザビームの照射エネルギーのしきい値を、従来の約８８％に低下させることができる。
【０１０６】
ここで、本実施形態におけるＧａＮ層１２の厚さとＧａＮ層１２の熱分解に寄与するレーザビーム強度との関係を図６に示す。なお、図６の縦軸は、従来方法においてＧａＮ層１０２の熱分解に寄与するレーザビーム強度に対する比で表されている。
【０１０７】
図６に示すように、本実施形態では、ＧａＮ層１２の膜厚が薄いほど、熱分解に寄与するレーザビーム強度は大きくなり、レーザビームの照射エネルギーの閾値も低下させることができる。しかし、ＧａＮ層１２を過剰に薄くすると、ＧａＮ層１２全てが熱分解してしまう。従って、ＧａＮ層１２全てが熱分解しない程度にＧａＮ層１２の膜厚を調節する必要がある。
【０１０８】
また、レーザビームの照射エネルギーのしきい値を低下させる手段として、レーザビームのビーム径を大きくすることができる。ビーム径の大きなレーザビームを用いれば、レーザビームを照射する際の走査回数を少なくすることができ、ＧａＮ基板の生産性および歩留の向上が可能となる。
【０１０９】
また、本実施形態では、高反射率膜５３を形成しているＳｉＯ₂およびＴｉＯ₂はいずれも誘電体であり、上記実施形態３で述べたようにＧａＮよりも熱伝導率が非常に小さい。このため、ＧａＮ層１２におけるレーザビームの吸収により発生した熱は、高反射率膜５３を伝導しにくい。従って、高反射率膜５３への熱拡散は小さく、発生した熱の大半はＧａＮ層１２の分解に寄与するので、ＧａＮ層１２を効率良く分解できる。
【０１１０】
さらに、本実施形態では、開口部５３ａを有する高反射率膜５３ｂを、ＧａＮ層１４の横方向成長のためのマスクとしても使用する。このため、新たにＧａＮ層１４の横方向成長のためのマスクを製造する必要が無く、製造工程を簡略化することができる。
【０１１１】
（実施形態５）
本発明の実施形態５におけるIII族窒化物半導体基板の製造方法について、図７を参照しながら説明する。図７は、本実施形態のIII族窒化物半導体基板の製造方法を表す工程断面図である。
【０１１２】
まず、図７（ａ）に示す工程で、Ｃ面を主面とするサファイア基板１１（直径２インチ、厚さ３００μｍ）をＭＯＶＰＥ炉内に導入する。次に、１０５０℃でサファイア基板１１を水素雰囲気中で熱処理した後、５００℃まで冷却する。続いて、III族原料にトリメチルガリウム（ＴＭＧ）、Ｖ族原料にアンモニア、キャリアガスとしてＨ₂を用いて、サファイア基板１１上に厚さ２０ｎｍのＧａＮ緩衝層（不図示）を形成する。再び温度を１０５０℃まで上げ、ＧａＮ緩衝層の上に厚さ５０ｎｍのＧａＮ層１２を形成する。
【０１１３】
次に、図７（ｂ）に示す工程で、III族原料にＡｌの原料ガスとしてトリメチルアルミニウムとトリメチルガリウム（ＴＭＧ）を、Ｖ族原料にアンモニアを用いて、ＧａＮ層１２上に厚さ４μｍのＡｌＧａＮ層５５を形成する。
【０１１４】
本実施形態では、ＡｌＧａＮ層５５は、Ａｌ_0.1Ｇａ_0.9Ｎから形成されているが、ＡｌとＧａとの組成は、レーザビーム照射工程で用いるレーザビームの照射エネルギーよりもバンドギャップが大きくなるように決定すればよい。その後、室温まで冷却し、得られた基板をＭＯＶＰＥ炉内から取り出す。
【０１１５】
次に、図７（ｃ）に示す工程で、スパッタ法により、ＡｌＧａＮ層５５上に厚さ６１ｎｍのＳｉＯ₂層と、厚さ３５ｎｍのＴｉＯ₂層とを交互に形成し、高反射率膜５３を形成する。本実施形態では、高反射率膜５３は、５層のＳｉＯ₂層と４層のＴｉＯ₂層の合計９層からなる膜となっている。なお、本実施形態で形成される高反射率膜５３のＮｄ：ＹＡＧレーザビームの３次高調波に対する反射率は約９６％である。
【０１１６】
次に、図７（ｄ）に示す工程で、上記実施形態１と同様に、レーザビームをサファイア基板１１の下面側から、サファイア基板１１を介してＧａＮ層１２の下面全体に亘って走査しながら照射する。このことによって、ＧａＮ層１２が分解される。使用するレーザビームは、Ｎｄ：ＹＡＧレーザの３次高調波（３５５ｎｍ）であり、照射エネルギー０．２０Ｊ／ｃｍ²、パルス幅５ｎｓ、照射時のビーム径は７ｍｍである。
【０１１７】
ＧａＮの吸収端波長は３６０〜３７０ｎｍであるので、レーザビームはサファイア基板１１を透過し、ＧａＮ層１２に吸収される。その結果、ＧａＮ層１２の下部は加熱され、熱分解される。本実施形態では、ＧａＮ層１２が非常に薄いため、ＧａＮ層１２のほぼ全体がＧａ金属部５４となり、窒素ガスが発散する。このとき、サファイア基板１１とＡｌＧａＮ層５５とは、レーザビーム照射後も完全に分離せず、Ｇａ金属部５４により弱く接着している。ＡｌＧａＮ層５５は薄く、ハンドリングが困難であるため、サファイア基板１１上にＡｌＧａＮ層５５を載せたまま、以降の工程を行なう。
【０１１８】
次に、図７（ｅ）に示す工程で、高反射率膜５３をパターニングすることによって開口部５３ａを有する高反射率膜５３ｂを形成する。このとき、高反射率膜５３のパターニングは、フォトリソグラフィーおよびエッチングを用いて行ない、ＡｌＧａＮ層５５に到達するストライプ状の開口部５３ａを形成する。本実施形態では、各開口部５３ａの幅は３μｍであり、各開口部５３ａの間は３μｍである。なお、エッチングの際には、高反射率膜５３を構成するＳｉＯ₂層はフッ酸を用い、ＴｉＯ₂層は熱濃硫酸を用いる。なお、開口部はストライプ状の他に、ドット状、矩形状などの形状にしてもよい。また、本工程は、レーザビーム照射工程の前に行っても良いが、その場合、開口部５３ａ内に位置するＡｌＧａＮ層５５が割れて飛んでいってしまうことがある。そのため、パターニングはレーザビーム照射工程の後に行なうのが好ましい。
【０１１９】
次に、図７（ｆ）に示す工程で、８００℃、窒素およびアンモニア雰囲気中で１０分間熱処理を行なって、基板表面上の不純物を除去した後、基板上にＧａＮ層１４を形成する。ＧａＮ層１４の形成方法は、上記実施形態１の図１（ｂ）に示す工程で行なう方法と同じである。
【０１２０】
このとき、ＧａＮ結晶は、高反射率膜５３ｂ上には成長せず、開口部５３ａ内に露出したＡｌＧａＮ層５５上から成長を始める。ＧａＮ結晶の成長をさらに続けていくと、開口部５３ａ内に露出したＡｌＧａＮ層５５から成長したＧａＮ結晶は、高反射率膜５３ｂの上面に沿って成長し、最終的に隣接する開口部５３ａから成長したＧａＮ結晶と共に高反射率膜５３ｂを覆う。続いて、高反射率膜５３ｂを覆うＧａＮ結晶の上に、さらに新たなＧａＮ結晶が成長し、ＧａＮ層１４が形成される。このようにしてＧａＮ層１４の厚さを３００μｍまで成長させる。
【０１２１】
次に、図７（ｇ）に示す工程で、ＡｌＧａＮ層５５およびＧａＮ層１４とが搭載されたサファイア基板１１を塩酸につけることにより、サファイア基板１１からＡｌＧａＮ層５５およびＧａＮ層１４を分離する。このことによって、ＡｌＧａＮ層５５とＧａＮ層１４とからなるIII族窒化物半導体基板が得られる。
【０１２２】
本実施形態では、図７（ｄ）に示すレーザビーム照射工程において、ＧａＮ層１２の下部で吸収されなかったレーザビームが高反射率膜５３に反射されて戻り、ＧａＮ層１２の熱分解に寄与する。このため、本実施形態よれば、ＧａＮ層１４の結晶性が非常に良好なIII族窒化物半導体基板が得られる。このことについて、以下に詳細に説明する。
【０１２３】
ＡｌＧａＮ層５５（Ａｌ_0.1Ｇａ_0.9Ｎ）のバンドギャップは約４．１ｅＶであり、Ｎｄ：ＹＡＧレーザビームの３次高調波のエネルギーより大きい。このため、ＡｌＧａＮ層５５において、レーザビームは吸収されない。また、レーザビームのＧａＮ層１２に対する進入長が約０．３μｍであり、高反射率膜の反射率が９６％である。上記の条件を考慮すると、ＧａＮ層１２の下面に照射されるレーザビーム強度を１とした場合、高反射率膜５３に反射されてＧａＮ層１２の下面に戻ってくるレーザビーム強度は約０．７となる。つまり、本実施形態ではＧａＮ層１２の熱分解に寄与するレーザビームの照射エネルギーの利用効率は、従来の方法の１．７倍となる。従って、ＧａＮ層１２を効率良く熱分解することができる。
【０１２４】
また、従来の方法では、ＧａＮ層１０２を熱分解させるためのレーザビームの照射エネルギーのしきい値は約０．３０Ｊ／ｃｍ²である。しかし、本実施形態では、レーザビームの照射エネルギーが０．２０Ｊ／ｃｍ²であるが、ＧａＮ層１２の熱分解が生じている。つまり、本実施形態によれば、ＧａＮ層１２を熱分解させるために必要なレーザビームの照射エネルギーのしきい値を、従来の約６７％に低下させることができる。
【０１２５】
また、レーザビームの照射エネルギーのしきい値を低下させる手段として、レーザビームのビーム径を大きくすることができる。ビーム径の大きなレーザビームを用いれば、レーザビームを照射する際の走査回数を少なくすることができる。このため、結晶性が良好なIII族窒化物半導体基板が得られ、更にIII族窒化物半導体基板の生産性および歩留の向上も図ることができる。
【０１２６】
なお、上記実施形態４と同様に、本実施形態においてもＧａＮ層１２の膜厚が薄いほど、熱分解に寄与するレーザビーム強度は大きくなり、レーザビームの照射エネルギーの閾値も低下させることができる。しかし、ＧａＮ層１２を過剰に薄くすると、ＧａＮ層１２全てが熱分解してしまう。従って、ＧａＮ層１２全てが熱分解しない程度にＧａＮ層１２の膜厚を調節する必要がある。
【０１２７】
さらに、本実施形態では、開口部５３ａを有する高反射率膜５３ｂを、ＧａＮ層２２の横方向成長のためのマスクとしても使用する。このため、新たにＧａＮ層２２の横方向成長のためのマスクを製造する必要が無く、製造工程を簡略化することができる。
【０１２８】
（実施形態６）
本発明の実施形態６におけるＧａＮ基板の製造方法について、図８を参照しながら説明する。図８は、本実施形態のＧａＮ基板の製造方法を表す工程断面図である。
【０１２９】
まず、図８（ａ）に示す工程で、両面研磨した直径２インチのＣ面を主面とする厚さ３００μｍのサファイア基板１１を用意する。次いで、図中の矢印に示すように、サファイア基板１１の下面からプロトン（Ｈ⁺）を注入する。プロトンの注入条件は、イオンエネルギー２００ｋｅＶ、平均飛程１．１２μｍ、注入量５．０×１０¹⁶atoms・cm^-2である。
【０１３０】
次に、図８（ｂ）に示す工程で、上述のイオン注入によって、サファイア基板１１の下面から深さ１．１２μｍを中心として約２μｍの厚さを有するイオン注入領域６２が形成される。イオン注入領域６２は、イオン注入時に損傷を受けるため、非常に欠陥の多い領域となっている。
【０１３１】
次に、図８（ｃ）に示す工程で、イオン注入領域６２が形成されたサファイア基板１１をＨＶＰＥ炉内に導入する。次いで、III族ラインからＧａＣｌ、Ｖ族ラインから窒素キャリアガスとともにアンモニアガスを供給することによって、サファイア基板１１の上面上にＧａＮ層６４を形成する。III族ラインから供給されるＧａＣｌは、具体的には、９００℃に加熱したＧａメタルが充填されたボートに、窒素をキャリアガスとしてＨＣｌガスを導入することによって生成される。このことによって、５００℃に加熱されたサファイア基板１１の上面上にＧａＮからなる厚さ３０ｎｍのバッファ層（不図示）を形成する。続いて、サファイア基板１１を１０５０℃まで加熱し、ＧａＮ層６４を５０μｍ／時の形成速度で６時間成長させ、最終的にＧａＮ層６４の厚さを３００μｍにする。この後、基板を室温まで冷却し、ＨＶＰＥ炉内から取り出す。
【０１３２】
次に、図８（ｄ）に示す工程で、レーザビームをイオン注入領域６２の下面側から、イオン注入領域６２およびサファイア基板１１を介してＧａＮ層６４の下面全体に亘って走査しながら照射する。このことによって、ＧａＮ層１２が分解される。使用するレーザビームは、Ｎｄ：ＹＡＧレーザの３次高調波（３５５ｎｍ）であり、照射エネルギー０．３Ｊ／ｃｍ²、パルス幅５ｎｓ、照射時のビーム径は１ｍｍである。
【０１３３】
このとき、レーザビームは、イオン注入領域６２を通過する。イオン注入領域６２は、非常に欠陥の多い領域となっており、結晶の均一性は失われている。このため、レーザビームはイオン注入領域６２を通過後、図８（ｄ）に示すように散乱し、ビーム径が広がる。従って、レーザビームの空間強度分布が均一化される。ＧａＮの吸収端波長は３６０〜３７０ｎｍであるので、波長が３５５ｎｍであるレーザビームはＧａＮ層６４で吸収され、発熱する。この発熱により、ＧａＮ層６４の下部が熱分解される。
【０１３４】
次に、図８（ｅ）に示す工程で、サファイア基板１１とＧａＮ層６４とを分離する。このとき、上記図８（ｄ）に示す工程においてＧａＮ層６４の下部が分解されているので、サファイア基板１１とＧａＮ層６４とを完全に分離することができ、自立したＧａＮ層６４すなわちＧａＮ基板６４ａを得ることができる。
【０１３５】
本実施形態の図８（ｃ）に示す基板と、イオン注入領域６２を形成しなかったこと以外、図８（ｃ）に示す基板と全く同様に形成された基板とを用意し、それぞれの基板に、レーザビームをサファイア基板の下面側から１パルスだけ照射し、照射痕の大きさを比較した。なお、このときのレーザビーム径は１ｍｍである。
【０１３６】
イオン注入領域６２を形成しなかった場合、ＧａＮ層における照射痕は直径約１ｍｍの円状であり、レーザビームはほとんど散乱することなく、ＧａＮ層へと到達していた。一方、イオン注入領域６２を形成した本実施形態の図８（ｃ）に示す基板では、ＧａＮ層６４における照射痕は直径約１．５ｍｍの円形状となっっていた。つまり、本実施形態の図８（ｃ）に示す基板では、レーザビームがＧａＮ層６４に到達したときに、イオン注入領域６２を形成しなかった場合に比べて約１．５倍のビーム径になるまで散乱される。
【０１３７】
上述のように、本実施形態のＧａＮ基板の製造方法によれば、サファイア基板１１に形成されたイオン注入領域６２において、レーザビームを効果的に散乱させ、ビーム径を拡大することができる。このため、レーザビームの走査回数を低減することができ、短時間でＧａＮ基板を作製することができる。
【０１３８】
なお、本実施形態ではイオン注入をサファイア基板の下面全体に対して行なったが、サファイア基板の下面の一部にイオン注入を行なってもよい。
【０１３９】
また、サファイア基板１１に注入される元素は、特に限定されるものではなく、Ｂ、Ａｌ、Ｇａ、Ｉｎ、Ｓｉ、Ｇｅ、Ｍｇ、Ｚｎ、Ｎ、Ｏ、Ｃ等でも同様の効果が得られる。勿論、これらの複数種類の元素を注入してもよい。
【０１４０】
また、イオン注入はＧａＮ層の成長前に行なったが、レーザビーム照射工程以前であれば、いつ行なってもよい。
【０１４１】
なお、ＧａＮ層６４の形成の際の加熱によって、イオン注入領域６２の結晶性が回復してしまうと散乱効果が弱くなるので、イオン注入は照射損傷を実質的に回復できない程度まで行なうことが好ましい。具体的なイオン注入条件は、注入する元素によって変わるので限定することはできないが、イオンを注入して散乱の効果を確実に得るためには、注入量が１×１０¹⁴〜１×１０²⁰atoms・cm^-2の範囲内であることが好ましい。
【０１４２】
また、得られたＧａＮ基板６４ａの下部は、レーザビーム照射時の熱分解により凹凸が生じているので、必要に応じて研磨によって平坦化を行なってもよい。
【０１４３】
（実施形態７）
本発明の実施形態７におけるＧａＮ基板の製造方法について、図９を参照しながら説明する。
【０１４４】
まず、図９（ａ）に示す工程で、両面研磨した直径２インチのＣ面を主面とする厚さ３００μｍのサファイア基板１１を用意し、ＨＶＰＥ炉内に導入する。次いで、III族ラインからＧａＣｌ、Ｖ族ラインから窒素キャリアガスとともにアンモニアガスを供給することによって、サファイア基板１１の上面上にＧａＮ層６５を形成する。III族ラインから供給されるＧａＣｌは、具体的には、９００℃に加熱したＧａメタルが充填されたボートに、窒素をキャリアガスとしてＨＣｌガスを導入することによって生成される。このことによって、５００℃に加熱されたサファイア基板１１の上面上にＧａＮからなる厚さ３０ｎｍのバッファ層（不図示）を形成する。続いて、サファイア基板１１を１０５０℃まで加熱し、ＧａＮ層６５を５０μｍ／時の形成速度で約２分間成長させ、最終的にＧａＮ層６５の厚さを１μｍにする。この後、基板を室温まで冷却し、ＨＶＰＥ炉内から取り出す。
【０１４５】
次に、図９（ｂ）に示す工程で、図中に示す矢印のようにＧａＮ層６５にプロトン（Ｈ⁺）を注入する。イオン（プロトン）の注入条件は、イオンエネルギー１８０ｋｅＶ、平均飛程１．１２μｍ、注入量５．０×１０¹⁶atoms・cm^-2である。
【０１４６】
次に、図９（ｃ）に示す工程で、上述のイオン注入によって、サファイア基板１１の上部に厚さ約０．５μｍのイオン注入領域６６が形成される。イオン注入領域６６は、イオン注入時に損傷を受けるため、非常に欠陥の多い領域となっている。
【０１４７】
次に、図９（ｄ）に示す工程で、基板を再度ＨＶＰＥ炉内に導入し、７００℃、アンモニアガス雰囲気中で熱処理を行い、ＧａＮ層６５の表面の不純物を除去する。次いで、サファイア基板１１を１０５０℃まで加熱し、５０μｍ／時で６時間成長を行い、厚さ３００μｍのＧａＮ層６７を成長させる。この後、基板を室温まで冷却し、ＨＶＰＥ炉内から取り出す。
【０１４８】
次に、図９（ｅ）に示す工程で、レーザビームをサファイア基板１１の下面側から、サファイア基板１１およびイオン注入領域６６を介してＧａＮ層６５の下面全体に亘って走査しながら照射する。このことによって、ＧａＮ層６５の下部が分解される。使用するレーザビームは、Ｎｄ：ＹＡＧレーザの３次高調波（３５５ｎｍ）であり、照射エネルギー０．３Ｊ／ｃｍ²、パルス幅５ｎｓ、照射時のビーム径は１ｍｍである。
【０１４９】
このとき、レーザビームは、イオン注入領域６６を通過する。イオン注入領域６６は、非常に欠陥の多い領域となっており、結晶の均一性は失われている。このため、レーザビームはイオン注入領域６６を通過後、図９（ｅ）に示すように散乱し、ビーム径が広がる。従って、レーザビームの空間強度分布が均一化される。ＧａＮの吸収端波長は３６０〜３７０ｎｍであるので、波長が３５５ｎｍであるレーザビームはＧａＮ層６５で吸収され、発熱する。この発熱により、ＧａＮ層６５の下部が熱分解される。
【０１５０】
次に、図９（ｆ）に示す工程で、サファイア基板１１およびイオン注入領域６６と、ＧａＮ層６５および６７とを分離する。このとき、上記図９（ｅ）に示す工程においてＧａＮ層６５の下部が分解されているので、サファイア基板１１とＧａＮ層６５とを完全に分離することができ、自立したＧａＮ層６５および６７、すなわちＧａＮ基板６７ａを得ることができる。
【０１５１】
なお、図９（ｆ）に示す工程の後、得られたＧａＮ基板６７ａの下部は、レーザビーム照射時の熱分解により凹凸が生じているので、必要に応じて研磨によって平坦化を行なってもよい。
【０１５２】
本実施形態の図９（ｄ）に示す基板と、イオン注入領域６６を形成しなかったこと以外、図９（ｄ）に示す基板と全く同様に形成された基板とを用意し、それぞれの基板に、レーザビームをサファイア基板の下面側から１パルスだけ照射し、照射痕の大きさを比較した。なお、このときのレーザビーム径は１ｍｍである。
【０１５３】
イオン注入領域６６を形成しなかった場合、ＧａＮ層における照射痕は直径約１ｍｍの円状であり、レーザビームはほとんど散乱することなく、ＧａＮ層へと到達していた。一方、イオン注入領域６６を形成した本実施形態の図９（ｄ）に示す基板では、ＧａＮ層６５における照射痕は直径約１．３ｍｍの円形状となっっていた。つまり、本実施形態の図９（ｄ）に示す基板では、レーザビームがＧａＮ層６５に到達したときに、イオン注入領域６６を形成しなかった場合に比べて約１．３倍のビーム径になるまで散乱される。
【０１５４】
上述のように、本実施形態のＧａＮ基板の製造方法によれば、サファイア基板１１に形成されたイオン注入領域６６において、レーザビームを効果的に散乱させ、ビーム径を拡大することができる。このため、レーザビームの走査回数を低減することができ、短時間でＧａＮ基板を作製することができる。
【０１５５】
なお、本実施形態ではイオン注入をサファイア基板の上面全体に対して行なったが、サファイア基板の上面の一部にイオン注入を行なってもよい。
【０１５６】
また、サファイア基板１１に注入される元素は、特に限定されるものではなく、Ｂ、Ａｌ、Ｇａ、Ｉｎ、Ｓｉ、Ｇｅ、Ｍｇ、Ｚｎ、Ｎ、Ｏ、Ｃ等でも同様の効果が得られる。勿論、これらの複数種類の元素を注入してもよい。
【０１５７】
なお、本実施形態では、ＧａＮ層６５を介してイオン注入するので、ＧａＮ層６５の損傷を低減するためには、イオン注入量を１×１０¹⁴〜１×１０¹⁸atoms・cm^-2とし、基板に直接注入する上記実施形態６よりも、イオン注入量を減らすことが好ましい。
【０１５８】
レーザビームの照射によって、ＧａＮ層６５内に熱分解した部分と熱分解していない部分とが存在する場合、ＧａＮ層６５内に局所的な応力が発生する。しかしながら、本実施形態では、ＧａＮ層６５を介してイオン注入を行なうので、ＧａＮ層６５にもイオン注入による損傷が生じ、原子間の結合が弱くなっている部分が存在する。このため、ＧａＮ層６５内に発生した局所的な応力は、原子間の結合が弱い部分で緩和される。従って、ＧａＮ層６５にクラックや割れが発生しにくい。つまり、クラックや割れの少ない良質なＧａＮ基板を得ることができる。
【０１５９】
（実施形態８）
本発明の実施形態８におけるＧａＮ基板の製造方法について、図１０を参照しながら説明する。
【０１６０】
まず、図１０（ａ）に示す工程で、両面研磨した直径２インチのＣ面を主面とする厚さ３００μｍのサファイア基板１１を用意し、ＨＶＰＥ炉内に導入する。次いで、蒸着法によりサファイア基板１１の下面上に、厚さ１００ｎｍのＮｉ層６８を形成する。
【０１６１】
次に、図１０（ｂ）に示す工程で、Ｎｉ層６８をパターニングすることによって、開口部６８ａを有するＮｉ膜６８ｂを形成する。このとき、Ｎｉ層６８のパターニングは、フォトリソグラフィーおよびエッチングによって行ない、サファイア基板１１に到達するストライプ状の開口部６８ａを形成する。なお本実施形態では、各開口部６８ａは、開口幅が０．２５μｍ、各開口部６８ａの間隔が０．２５μｍであり、サファイア基板１１の<11-20>方向に延びるように設けられている。
【０１６２】
次に、図１０（ｃ）に示す工程で、Ｎｉ膜６８ｂをマスクとするＡｒガスを用いたドライエッチングによって、サファイア基板１１をエッチングを行なう。
【０１６３】
次に、図１０（ｄ）に示す工程で、Ｎｉ膜６８ｂをウエットエッチングによって除去する。このことによって、サファイア基板１１の下部に溝６９を形成する。本実施形態では、溝６９の深さは０．２５μｍである。
【０１６４】
次に、図１０（ｅ）に示す工程で、上記実施形態６と同様の方法で、厚さ３００μｍのＧａＮ層７０をサファイア基板１１の上面上に形成する。続いて、得られた基板を室温まで冷却する。
【０１６５】
次に、図１０（ｆ）に示す工程で、レーザビームをサファイア基板１１の下面側から、サファイア基板１１を介してＧａＮ層７０の下面全体に亘って走査しながら照射する。このことによって、ＧａＮ層７０の下部が熱分解される。使用するレーザビームは、レーザの３次高調波（３５５ｎｍ）であり、照射エネルギー０．３Ｊ／ｃｍ²、パルス幅５ｎｓ、照射時のビーム径は１ｍｍである。
【０１６６】
このときレーザビームは、図９（ｅ）に示すように溝６９によって散乱し、ビーム径が広がる。従って、レーザビームの空間強度分布が均一化される。ＧａＮの吸収端波長は３６０〜３７０ｎｍであるので、波長が３５５ｎｍであるレーザビームはＧａＮ層７０で吸収され、発熱する。この発熱により、ＧａＮ層６５の下部が熱分解される。
【０１６７】
次に、図１０（ｇ）に示す工程で、サファイア基板１１と、ＧａＮ層７０とを分離する。このとき、上記図１０（ｆ）に示す工程においてＧａＮ層７０の下部が分解されているので、サファイア基板１１とＧａＮ層７０とを完全に分離することができ、自立したＧａＮ層７０、すなわちＧａＮ基板７０ａを得ることができる。
【０１６８】
本実施形態の図１０（ｅ）に示す基板と、溝６９を形成しなかったこと以外、図１０（ｅ）に示す基板と全く同様に形成された基板とを用意し、それぞれの基板に、レーザビームをサファイア基板の下面側から１パルスだけ照射し、照射痕の大きさを比較した。なお、このときのレーザビーム径は１ｍｍである。
【０１６９】
溝６９を形成しなかった場合、ＧａＮ層における照射痕は直径約１ｍｍの円状であり、レーザビームはほとんど散乱することなく、ＧａＮ層へと到達していた。一方、溝６９を形成した本実施形態の図１０（ｅ）に示す基板では、ＧａＮ層７０における照射痕は直径約１．５ｍｍの円形状となっていた。つまり、本実施形態の図９（ｃ）に示す基板では、レーザビームがＧａＮ層７０に到達したときに、溝６９を形成しなかった場合に比べて約１．５倍のビーム径になるまで散乱される。
【０１７０】
上述のように、本実施形態のＧａＮ基板の製造方法によれば、サファイア基板１１に形成された溝６９によって、レーザビームを効果的に散乱させ、ビーム径を拡大することができる。このため、レーザビームの走査回数を低減することができ、短時間でＧａＮ基板を作製することができる。
【０１７１】
なお、本実施形態では溝６９をストライプ状に形成したが、これに特に限定されるものではない。例えば、溝６９の代わりに、ドット状、格子状に凹部を設けてもよい。また、サンドブラストや切削などで生じる不規則な凹部でもよい。
【０１７２】
なお、溝６９あるいは上述の凹部が密に形成されているほど散乱の効果が大きくなるので好ましい。特に、本実施形態において、散乱の効果を得るためには、各溝６９の開口幅および間隔は、入射する光の波長以下の大きさであることが好ましい。また、溝６９の側面が斜面となっていてもよい。側面が斜面となっている溝６９の形成法としては、サファイア基板１１の下面に対して斜め方向からプラズマ照射を行なう、あるいは図１０（ｃ）に示すサファイア基板１１のエッチング工程で用いるマスクとして、エッチングによって削れ易いマスク材料（例えば、レジスト）を用いる方法等が挙げられる。
【０１７３】
（実施形態９）
本実施形態では、上記実施形態１〜８で得られるＧａＮ基板１４ａ、６４ａ、６７ａおよび７０ａのいずれかを用いて、多数のＧａＮ基板を作製する方法を説明する。
【０１７４】
例えば、上記実施形態１で得られる、２インチ径のＧａＮ基板１４ａをＨＶＰＥ炉内に導入する。７５０℃でＧａＮ基板１４ａをアンモニア雰囲気中で熱処理した後、 実施形態１と同様に、１０５０℃にまで加熱し、厚さが１０ｍｍとなるまで成長する。その後、室温まで冷却し、ＨＶＰＥ炉内から取り出す。次いで、スライサーを用いてｃ軸に直角方向に厚さ３００μｍでスライスすることによって、３０枚のＧａＮ基板を得ることができる。
【０１７５】
勿論、上記実施形態２〜８で得られたＧａＮ基板１４ａ、６４ａ、６７ａ、７０ａを用いて、上述の方法で全く同様に複数枚のＧａＮ基板を作製することができる。
【０１７６】
本実施形態の方法では、上記実施形態１〜８のいずれかで作製したＧａＮ基板を種結晶として、ＧａＮ単結晶を厚く成長させ、厚いＧａＮ単結晶をスライサにより切断するので、一挙に複数のＧａＮ基板を作製することが可能である。さらに、本実施形態の方法では、各実施形態１〜８において最初に用いられるサファイア基板１１が、１枚だけでよいので低コスト化が可能である。
【０１７７】
（その他の実施形態）
本実施形態では、上記実施形態１〜８における改変例について説明する。
【０１７８】
上記実施形態１〜８では、III族窒化物半導体基板の製造方法としてＧａＮ基板の製造方法を例に述べたが、これに限定されるものではない。III族ラインから供給される原料ガスを適宜変更することによって、Ａｌ_xＧａ_yＩｎ_1-x-yＮ（０≦ｘ≦１、０≦ｙ≦１、０≦ｘ＋ｙ≦１）からなるIII族窒化物半導体基板を製造することができる。
【０１７９】
上記実施形態１〜８では、Ｎｄ：ＹＡＧレーザビームを用いたが、III族窒化物半導体基板の吸収端波長よりも大きく、且つ基板の吸収端波長よりも小さい波長を有するものであれば良い。例えば、ＫｒＦ：エキシマレーザビーム（２４８ｎｍ）、ＸｅＣｌ：エキシマレーザビーム（３０８ｎｍ）などを用いることも可能である。
【０１８０】
上記実施形態１〜８では、サファイア基板１１の大きさは２インチとしたが、さらに面積の大きい基板であっても、割れや欠けなく、III族窒化物半導体基板を作製することが可能である。
【０１８１】
また、サファイア基板１１の代わりに、Ｎｄ：ＹＡＧレーザビームやＫｒＦエキシマレーザビームが透過可能な材料（例えば、スピネル等）を用いてもよい。
【０１８２】
また、III族窒化物半導体基板を形成する際に、II族、IV族またはVI族元素を含む原料を用いて、II族、IV族またはVI族元素をドーピングしてもよい。例えば、ＧａＮ基板にＳｉ、Ｇｅ、Ｓｅ等を不純物としてドーピングすれば、ｎ型の導電性を有するIII族窒化物半導体膜が得られる。また、ＧａＮ基板にＢｅ、Ｍｇ、Ｚｎ等を不純物としてドーピングすれば、ｐ型の導電性を有するIII族窒化物半導体膜が得られる。
【０１８３】
以上説明したように、本発明のIII族窒化物半導体基板の製造方法によれば、割れや欠けのない広い面積を有するIII族窒化物半導体基板を短時間で作製でき、それによりIII族窒化物半導体基板の量産が可能となる。
【０１８４】
本発明の窒化物半導体基板の製造方法により、窒化物半導体層の熱分解に必要なレーザビームの照射エネルギーのしきい値を低下させることが出来る。また、閾値の低下に伴い、レーザビームのビーム径を大きくすることが出来るので、母材基板と窒化物半導体層の分離を行う際に必要なレーザビームの走査回数を減らすことが出来る。そのため、レーザビーム照射工程に要する時間を短縮し、生産性を向上させることができる。また、レーザビームの照射回数が減少するので、歩留の向上にも繋がる。
【０１８５】
本実施形態で得られたIII族窒化物半導体基板は、半導体レーザや電界効果トランジスタ等の半導体装置を作製するために用いられる。
【０１８６】
【発明の効果】
本発明によれば、良質な結晶性を有するIII族窒化物半導体基板が得られる。
【図面の簡単な説明】
【図１】図１（ａ）〜（ｅ）は、本発明の実施形態１のＧａＮ基板の製造方法を表す工程断面図である。
【図２】図２（ａ）〜（ｆ）は、本発明の実施形態２のＧａＮ基板の製造方法を表す工程断面図である。
【図３】図３は、図２（ｂ）に示す基板の上面図である。
【図４】図４（ａ）〜（ｆ）は、本発明の実施形態３のＧａＮ基板の製造方法を表す工程断面図である。
【図５】図５（ａ）〜（ｅ）は、本発明の実施形態４のＧａＮ基板の製造方法を表す工程断面図である。
【図６】図６は、ＧａＮ層の厚さと、ＧａＮ層の熱分解に寄与するレーザビーム強度との関係を示す図である。
【図７】図７（ａ）〜（ｆ）は、本発明の実施形態５のIII族窒化物半導体基板の製造方法を表す工程断面図である。
【図８】図８（ａ）〜（ｅ）は、本発明の実施形態６のＧａＮ基板の製造方法を表す工程断面図である。
【図９】図９（ａ）〜（ｆ）は、本発明の実施形態７のＧａＮ基板の製造方法を表す工程断面図である。
【図１０】図１０（ａ）〜（ｇ）は、本発明の実施形態８のＧａＮ基板の製造方法を表す工程断面図である。
【図１１】図１１（ａ）〜（ｃ）は、従来のＧａＮ基板の製造方法を表す工程断面図である。
【符号の説明】
１１ サファイア基板
１２、６５ ＧａＮ層
１３ ＩｎＮ層
１４、６４、６７ ＧａＮ層
１４ａ、６４ａ、６７ａ、７０ａ ＧａＮ基板
２３ Ｎｉ膜
２３ａ、４３ａ、５３ａ、６８ａ 開口部
４３ シリコン酸化膜
５３、５３ｂ 高反射率膜
５４ Ｇａ金属部
５５ ＡｌＧａＮ層
６２、６６ イオン注入領域
６８ Ｎｉ層
６８ｂ Ｎｉ膜
６９ 溝
７０ ＧａＮ層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a group III nitride semiconductor substrate used for a semiconductor laser or a high temperature operation transistor that emits light of a short wavelength such as blue or purple.
[0002]
[Prior art]
Al _x Ga _y In _1-xy A group III nitride semiconductor represented by N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) is an optical device having a wavelength from red to ultraviolet. This material is expected to be applied to light-emitting devices and light-receiving devices. Until now, relatively high-quality group III nitride semiconductor films have been formed mainly by crystal growth on a sapphire substrate.
[0003]
However, since the sapphire substrate and the group III nitride semiconductor film do not lattice match, the group III nitride semiconductor film includes many crystal defects. As a result, there has been a problem that the characteristics of the device using the group III nitride semiconductor are poor.
[0004]
In addition, since the sapphire substrate is a substrate that does not conduct electricity, a so-called insulating substrate, when the group III nitride semiconductor film formed on the sapphire substrate is used for a semiconductor laser or transistor, all the electrodes are group III nitride. It is necessary to form it on the semiconductor film. This complicates the manufacturing process and reduces the manufacturing yield of devices made of group III nitride semiconductors.
[0005]
Therefore, in order to improve the manufacturing yield and performance of devices using Group III nitride semiconductors, it is strongly desired to obtain a Group III nitride semiconductor substrate (especially a GaN substrate) of high quality and large area. It is rare. Against this background, various methods have been proposed for removing a heterogeneous substrate after a group III nitride semiconductor film is grown on a heterogeneous substrate (such as a sapphire substrate).
[0006]
For example, a method of separating a sapphire substrate and a GaN film by irradiating with a strong laser beam (Michael K. Kelly et al., Japanese Journal of Applied Physics Vol. 38 p.L217-L219, 1999) is known. Hereinafter, this conventional method will be described with reference to FIG. FIG. 11 is a process sectional view for explaining a conventional method.
[0007]
In the step shown in FIG. 11 (a), a hydride vapor phase epitaxy (hereinafter referred to as HVPE) is used to form a 200-inch thick sapphire substrate 101 having a C-plane as the main surface. A 300 μm GaN layer 102 is formed.
[0008]
Next, in the step shown in FIG. 11B, after the sapphire substrate 101 on which the GaN layer 102 is formed is taken out from the HVPE reactor, a laser beam having a wavelength of 355 nm is passed through the sapphire substrate 101 from the sapphire substrate 101 side. Irradiation is performed while scanning over the entire lower surface of the GaN layer 102. In addition, the arrow in a figure represents a laser beam. At this time, heat is generated in the portion of the GaN layer 102 irradiated with the laser beam, and the lower portion of the GaN layer 102 is decomposed by this heat.
[0009]
Next, in the step shown in FIG. 11C, the sapphire substrate 101 and the GaN layer 102 are separated to obtain a self-standing GaN substrate 102a.
[0010]
[Problems to be solved by the invention]
However, the conventional method has the following problems.
[0011]
The thermal conductivities of GaAs and InP, which are typical III-V group compound semiconductors, are 0.54 W / cm · K and 0.68 W / cm · K, respectively. Moreover, the thermal conductivity of Si used for the submount for heat dissipation is 1.5 W / cm · K.
[0012]
On the other hand, the thermal conductivity of GaN is 1.3 W / cm · K. That is, GaN is a material that easily conducts heat, compared to the thermal conductivity of the above-described materials. Therefore, in the conventional method of irradiating GaN with a laser beam, heat generated under the GaN layer 102 due to absorption of the laser beam is likely to diffuse. For this reason, in the step shown in FIG. 11B, there is a problem in that the amount of heat for completely decomposing the portion of the GaN layer 102 irradiated with the laser beam is insufficient and the decomposition efficiency of GaN deteriorates. When the decomposition efficiency of GaN deteriorates, the number of scans of the laser beam is increased, and the sapphire substrate 101 and the GaN layer 102 are supplied by supplying heat for completely decomposing the portion of the GaN layer 102 irradiated with the laser beam. Must be separated. Therefore, the time required for the process shown in FIG. 11B is lengthened and productivity is lowered.
[0013]
In addition, since sapphire and GaN are not lattice matched, the GaN layer 102 includes many crystal defects and strains. For this reason, a crack may occur in the obtained GaN substrate 102a due to an impact that releases the stress during GaN decomposition. In particular, as the number of times the laser beam is scanned increases, the probability that the GaN substrate 102a is cracked increases.
[0014]
Furthermore, even when the GaN substrate 102a is not cracked, the crack may remain inside the GaN substrate 102a. When a device such as a light-emitting diode or a laser diode is manufactured using the GaN substrate 102a having a residual crack, there is a problem that the crack causes current leakage and the reliability of the device is lowered.
[0015]
The present invention has been made to solve the above problems, and an object of the present invention is to provide a high-quality group III nitride semiconductor substrate.
[0016]
[Means for Solving the Problems]
The method for producing a group III nitride semiconductor substrate of the present invention includes a step (a) of preparing a substrate, a step (b) of forming a first semiconductor layer made of a group III nitride semiconductor on the substrate, and the first step. A step (c) of forming a thermal diffusion suppression layer having a thermal conductivity lower than that of the first semiconductor layer on the first semiconductor layer; and a second semiconductor layer made of a group III nitride semiconductor on the thermal diffusion suppression layer. Irradiating the first semiconductor layer through the substrate with a light beam that passes through the substrate and is absorbed by the first semiconductor layer. (E).
[0017]
According to the present invention, a thermal diffusion suppression layer having a lower thermal conductivity than the first semiconductor layer is formed between the first semiconductor layer and the second semiconductor layer. For this reason, the heat generated by the absorption of the light beam in the first semiconductor layer is suppressed by the thermal diffusion suppression layer. Therefore, most of the generated heat contributes to the decomposition of the first semiconductor layer, so that the first semiconductor layer can be decomposed efficiently. Therefore, even if the number of scanning times of the light beam is smaller than before, the amount of heat for completely decomposing the first semiconductor layer can be supplied, and the productivity is improved. In addition, since the number of times of scanning with the light beam is smaller than before, the risk of cracking in the group III nitride semiconductor substrate obtained by separating the second semiconductor layer can be reduced.
[0018]
The thermal diffusion suppressing layer may be formed of a group III nitride semiconductor having a lower thermal conductivity than the group III nitride semiconductor forming the first semiconductor layer.
[0019]
The thermal diffusion suppression layer is made of In. _x Ga _1-x You may form from the semiconductor which consists of N (0 <x <= 1).
[0020]
In the step (c), it is preferable that after the thermal diffusion suppression layer is formed, an opening that penetrates the thermal diffusion suppression layer and reaches the first semiconductor layer is formed.
[0021]
Thereby, when forming the second semiconductor layer, the group III nitride semiconductor crystal forming the second semiconductor layer grows along the upper surface of the thermal diffusion suppressing layer. Therefore, dislocations generated in the first semiconductor layer hardly propagate to the second semiconductor layer. That is, there are very few dislocations in the second semiconductor layer. Therefore, a group III nitride semiconductor substrate having good crystallinity can be obtained.
[0022]
The thermal diffusion suppressing layer may be formed from a metal.
[0023]
The thermal diffusion suppression layer may be formed of at least one metal selected from Ni, Pt, and Ti.
[0024]
The thermal diffusion suppression layer may be formed from a dielectric.
[0025]
The thermal diffusion suppression layer may be formed of at least one dielectric selected from a silicon oxide film and a silicon nitride film.
[0026]
It is preferable that the process (f) which removes the said thermal-diffusion suppression layer is included after the said process (e).
[0027]
In the step (f), the thermal diffusion suppressing layer may be removed by etching.
[0028]
In the step (f), the thermal diffusion suppressing layer may be removed by polishing.
[0029]
The substrate preferably has a lower thermal conductivity than the first group III nitride semiconductor.
[0030]
As a result, heat conduction to the substrate is also suppressed, and more heat can be contributed to the decomposition of the first semiconductor layer.
[0031]
Another method for producing a group III nitride semiconductor substrate of the present invention includes a step (a) of preparing a substrate, a step (b) of forming a first semiconductor layer made of a group III nitride semiconductor on the substrate, A step (c) of forming a light reflecting layer on the first semiconductor layer, a step (d) of forming a second semiconductor layer made of a group III nitride semiconductor on the light reflecting layer, and the substrate. A step (e) of decomposing the first semiconductor layer by irradiating the first semiconductor layer with a light beam that is transmitted and absorbed by the first semiconductor layer through the substrate; The light reflecting layer reflects the light beam irradiated in the step (e).
[0032]
According to the present invention, during the light beam irradiation, the laser beam that has not been absorbed in the lower portion of the first semiconductor layer is reflected back to the light reflecting layer, contributing to thermal decomposition of the first semiconductor layer. For this reason, the threshold value of the irradiation energy of the light beam necessary for thermally decomposing the first semiconductor layer can be lowered as compared with the conventional case. Further, the beam diameter of the light beam can be increased as means for lowering the threshold value of the irradiation energy of the light beam. Therefore, even if the number of scanning times of the light beam is smaller than before, the amount of heat for completely decomposing the first semiconductor layer can be supplied, and the productivity is improved. In addition, since the number of times of scanning with the light beam is smaller than before, the risk of cracking in the group III nitride semiconductor substrate obtained by separating the second semiconductor layer can be reduced.
[0033]
The first semiconductor layer has a first layer made of a group III nitride semiconductor having a smaller band gap than the energy of the light beam, and a band gap larger than the energy of the light beam formed on the first layer. It is preferable to have a second layer made of a group III nitride semiconductor.
[0034]
As a result, the light beam is not absorbed in the second layer. Therefore, attenuation is suppressed when the light beam that has not been absorbed by the lower portion of the first semiconductor layer is reflected by the light reflecting layer. For this reason, the utilization efficiency of the irradiation energy of the light beam which contributes to the thermal decomposition of the first semiconductor layer becomes larger than before.
[0035]
In the step (c), after forming the light reflecting layer, it is preferable to form an opening that penetrates the light reflecting layer and reaches the first semiconductor layer.
[0036]
Thereby, when forming the second semiconductor layer, the group III nitride semiconductor crystal forming the second semiconductor layer grows along the upper surface of the thermal diffusion suppressing layer. Therefore, dislocations generated in the first semiconductor layer hardly propagate to the second semiconductor layer. That is, there are very few dislocations in the second semiconductor layer. Therefore, a group III nitride semiconductor substrate having good crystallinity can be obtained.
[0037]
The light reflecting layer may be formed of a dielectric.
[0038]
The light reflecting layer may be a laminated film in which silicon oxide films and titanium oxide films are alternately laminated.
[0039]
Still another method for producing a group III nitride semiconductor substrate of the present invention includes a step (a) of preparing a substrate, a step (b) of forming a light scattering portion inside the substrate, and a group III on the substrate. (C) forming a semiconductor layer made of a nitride semiconductor, and irradiating the semiconductor layer through the substrate with a light beam that passes through the substrate and is absorbed by the semiconductor layer. Decomposing the lower part of the layer (d).
[0040]
According to the present invention, the beam diameter is increased when the light beam is scattered by the light scattering portion and reaches the semiconductor layer. Therefore, even if the number of scanning times of the light beam is smaller than before, the amount of heat for completely decomposing the first semiconductor layer can be supplied, and the productivity is improved. In addition, since the number of times of scanning with the light beam is smaller than before, the risk of cracking in the group III nitride semiconductor substrate obtained by separating the second semiconductor layer can be reduced.
[0041]
In the step (b), a light scattering portion may be formed inside the substrate by implanting ions into the substrate.
[0042]
The step (c) is performed after the step (a), and in the step (b), ions are implanted into the substrate through the semiconductor layer, thereby forming a light scattering portion in the substrate. A step of forming another semiconductor layer made of a group III nitride semiconductor on the semiconductor layer may be further included between the steps (b) and (d).
[0043]
In the step (b), a plurality of concave portions may be formed in the lower portion of the substrate as the light scattering portion.
[0044]
The plurality of recesses may be formed by plasma irradiation.
[0045]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. For simplicity, components common to the respective embodiments are denoted by the same reference numerals.
[0046]
(Embodiment 1)
A method for manufacturing a GaN substrate in Embodiment 1 of the present invention will be described with reference to FIG. FIG. 1 is a process cross-sectional view illustrating a method of manufacturing a GaN substrate according to this embodiment.
[0047]
First, in the step shown in FIG. 1A, a sapphire substrate 11 (diameter 2 inches, thickness 300 μm) having a C surface as a main surface is introduced into a MOVPE furnace. Next, after heat-treating the sapphire substrate 11 in a hydrogen atmosphere at 1050 ° C., it is cooled to 500 ° C. Subsequently, trimethylgallium (TMG) is used as a group III material, ammonia is used as a group V material, and H is used as a carrier gas. ₂ Is used to form a GaN buffer layer (not shown) having a thickness of 20 nm on the sapphire substrate 11. The temperature is again raised to 1050 ° C., and a GaN layer 12 having a thickness of 200 nm is formed on the GaN buffer layer. Next, the temperature is lowered to 700 ° C., trimethylindium (TMI) is used as a group III material, and ammonia is used as a group V material, thereby forming an InN layer 13 having a thickness of 50 nm on the GaN layer 12. Then, it cools to room temperature and takes out the obtained board | substrate from the inside of a MOVPE furnace.
[0048]
Next, in the step shown in FIG. 1B, the sapphire substrate 11 on which the GaN layer 12 and the InN layer 13 are formed is introduced into the HVPE furnace. Next, the GaN layer 14 is formed on the InN layer 13 by supplying ammonia gas together with GaCl from the group III line and nitrogen carrier gas from the group V line. Specifically, GaCl supplied from the group III line is generated by introducing HCl gas into a boat filled with Ga metal heated to 900 ° C. using nitrogen as a carrier gas. At this time, the sapphire substrate 11 is heated to 1050 ° C., the GaN layer 14 is grown for 6 hours at a formation rate of 50 μm / hour, and the thickness of the GaN layer 14 is finally set to 300 μm. Thereafter, the substrate is cooled to room temperature and taken out from the HVPE furnace.
[0049]
Next, in the step shown in FIG. 1C, the laser beam is irradiated from the lower surface side of the sapphire substrate 11 while scanning over the entire lower surface of the GaN layer 12 through the sapphire substrate 11. The laser beam used is the third harmonic (355 nm) of an Nd: YAG laser, and the irradiation energy is 0.3 J / cm. ² The pulse width is 5 ns, and the laser beam diameter during irradiation is 1 mm. Since the sapphire substrate 11 is transparent to the wavelength of the laser beam, the laser beam passes through the sapphire substrate 11. Since the absorption edge wavelength of GaN is 360 to 370 nm, the transmitted laser beam is absorbed by the GaN layer 12 formed on the sapphire substrate 11, and the GaN layer 12 is decomposed.
[0050]
Next, in the step shown in FIG. 1D, the sapphire substrate 11 is separated from the InN layer 13 and the GaN layer 14. At this time, since the GaN layer 12 is decomposed in the step shown in FIG. 1C, it is easy to separate the sapphire substrate 11 from the InN layer 13 and the GaN layer 14.
[0051]
Next, in the step shown in FIG. 1E, the InN layer 13 formed under the GaN layer 14 is removed by polishing. As a result, a self-standing GaN substrate 14a is obtained. When a device is fabricated on a GaN substrate from which the InN layer 13 has not been removed, heat dissipation is worsened and the device characteristics may be affected. Therefore, it is preferable to remove the InN layer 13.
[0052]
In the method of the present embodiment described above, the InN layer 13 having a lower thermal conductivity than the GaN layer 12 is formed between the GaN layer 12 and the GaN layer 14. The thermal conductivity of InN is 0.8 W / cm · K, which is smaller than the thermal conductivity of GaN, 1.3 W / cm · K. For this reason, the heat generated by the absorption of the laser beam in the GaN layer 12 is difficult to conduct through the InN layer 13. That is, the InN layer 13 suppresses thermal diffusion. Therefore, most of the generated heat contributes to the decomposition of the GaN layer 12, so that the GaN layer 12 can be decomposed efficiently.
[0053]
Furthermore, in this embodiment, the sapphire substrate 11 (thermal conductivity 0.46 W / cm · K) having a thermal conductivity smaller than that of the GaN layer 12 is used. For this reason, conduction of heat to the sapphire substrate 11 is also suppressed, and more heat can be contributed to the decomposition of the GaN layer 12.
[0054]
Therefore, even if the number of scans of the laser beam is smaller than before, the amount of heat for completely decomposing the GaN layer 12 can be supplied, and the productivity is improved. In addition, since the number of times of scanning with the laser beam is smaller than before, the risk of cracking in the GaN substrate 14a can be reduced.
[0055]
In this embodiment, the InN layer 13 is formed as a layer having a lower thermal conductivity than the GaN layer 12. _x Ga _1-x An N layer (0 <x <1) may be formed. In general, the thermal conductivity of mixed crystal semiconductors tends to be smaller than that of binary compounds. In other words, In _x Ga _1-x Since the thermal conductivity of N is smaller than the thermal conductivity of GaN regardless of the In composition ratio, the heat generated by the absorption of the laser beam in the GaN layer 12 is less likely to be conducted than the InN layer. For this reason, the GaN layer 12 can be decomposed more efficiently.
[0056]
If the thickness of the InN layer 13 exceeds the critical film thickness, crystal defects generated inside the InN layer 13 increase and cracks may occur. When crystal defects increase or cracks occur in the InN layer 13, dislocations are likely to occur in the GaN layer 14 formed on the InN layer 13. For this reason, it is preferable that the thickness of the InN layer 13 does not exceed the critical film thickness.
[0057]
(Embodiment 2)
A method for manufacturing a GaN substrate in Embodiment 2 of the present invention will be described with reference to FIG. FIG. 2 is a process cross-sectional view illustrating the method for manufacturing the GaN substrate of the present embodiment.
[0058]
First, in the step shown in FIG. 2A, a sapphire substrate 11 (diameter 2 inches, thickness 300 μm) having a C surface as a main surface is introduced into a MOVPE furnace. Next, after heat-treating the sapphire substrate 11 in a hydrogen atmosphere at 1050 ° C., it is cooled to 500 ° C. Subsequently, trimethylgallium (TMG) is used as a group III material, ammonia is used as a group V material, and H is used as a carrier gas. ₂ Is used to form a GaN buffer layer (not shown) having a thickness of 20 nm on the sapphire substrate 11. The temperature is again raised to 1050 ° C., and a GaN layer 12 having a thickness of 200 nm is formed on the GaN buffer layer. Then, it cools to room temperature and takes out the obtained board | substrate from the inside of a MOVPE furnace.
[0059]
Next, in the step shown in FIG. 2B, a Ni layer having a thickness of 200 nm is formed on the GaN layer 12 by vapor deposition. Subsequently, this Ni layer is patterned to form a Ni film 23 having an opening 23a. At this time, the Ni layer is patterned by photolithography and etching to form a circular opening 23a having a diameter of 2.5 μm reaching the GaN layer 12 as shown in FIG. The opening 23 a is formed on the GaN layer 12. The GaN layers 12 are arranged in a row at a constant interval a (5 μm in this embodiment) in the <11-20> direction. The openings of each row adjacent in the <1-100> direction are arranged so as to be shifted by a fixed distance a / 2 (2.5 μm in this embodiment). In this specification, <11-20> direction is
[0060]
[Expression 1]

[0061]
Refers to <1-100> direction is
[0062]
[Expression 2]

[0063]
Refers to that.
[0064]
Next, in the step shown in FIG. 2C, the substrate obtained in the step shown in FIG. 2B is introduced into the HVPE furnace. Next, heat treatment is performed at 800 ° C. in a nitrogen and ammonia atmosphere for 10 minutes to remove impurities on the substrate surface, and then a GaN layer 14 is formed on the substrate. The method for forming the GaN layer 14 is the same as the method performed in the step shown in FIG.
[0065]
At this time, the GaN crystal does not grow on the Ni film 23 but starts growing on the GaN layer 12 exposed in the opening 23a. As the growth of the GaN crystal continues further, the GaN crystal grown from the GaN layer 12 exposed in the opening 23a grows on the Ni film 23 along the upper surface of the Ni film 23, and from the adjacent opening 23a. The Ni film 23 is covered together with the grown GaN crystal. Subsequently, a GaN crystal grows on the GaN crystal further covering the Ni film 23, and the GaN layer 14 is formed. Thus, after growing the thickness of the GaN layer 14 to 300 μm, it is cooled to room temperature and taken out from the HVPE furnace.
[0066]
Next, in the step shown in FIG. 2D, as in the first embodiment, the laser beam is scanned from the lower surface side of the sapphire substrate 11 over the entire lower surface of the GaN layer 12 through the sapphire substrate 11. Irradiate. The laser beam used is the third harmonic (355 nm) of an Nd: YAG laser, and the irradiation energy is 0.3 J / cm. ² The pulse width is 5 ns, and the laser beam diameter during irradiation is 1 mm. Since the absorption edge wavelength of GaN is 360 to 370 nm, the laser beam having a wavelength of 355 nm is absorbed by the GaN layer 12 and generates heat. As a result, the GaN layer 12 is decomposed.
[0067]
Next, in the step shown in FIG. 2E, the sapphire substrate 11, the Ni film 23, and the GaN layer 14 are separated. At this time, since the GaN layer 12 is decomposed in the step shown in FIG. 2D, it is easy to separate the sapphire substrate 11 from the Ni film 23 and the GaN layer 14.
[0068]
Next, in the step shown in FIG. 2F, the Ni film 23 formed under the GaN layer 14 is removed by wet etching using nitric acid. As a result, a self-standing GaN substrate 14a is obtained. When a device is fabricated on a GaN substrate from which the Ni film 23 has not been removed, the Ni film 23 has a low thermal conductivity, so that the heat dissipation characteristics of the device are deteriorated, which may affect the performance of the device. Therefore, it is preferable to remove the Ni film 23. The Ni film 23 may be removed by polishing.
[0069]
In the method of this embodiment, the Ni film 23 having a lower thermal conductivity than the GaN layer 12 is formed between the GaN layer 12 and the GaN layer 14. Metals such as Ag and Cu have a high thermal conductivity, but Ni has a thermal conductivity of 0.84 W / cm · K, which is smaller than the thermal conductivity of GaN. For this reason, the heat generated by the absorption of the laser beam in the GaN layer 12 is difficult to conduct to the Ni film 23. That is, the Ni film 23 suppresses thermal diffusion. Therefore, most of the generated heat contributes to the decomposition of the GaN layer 12, so that the GaN layer 12 can be decomposed efficiently.
[0070]
Furthermore, in this embodiment, the sapphire substrate 11 (thermal conductivity 0.46 W / cm · K) having a thermal conductivity smaller than that of the GaN layer 12 is used. For this reason, conduction of heat to the sapphire substrate 11 is also suppressed, and more heat can be contributed to the decomposition of the GaN layer 12.
[0071]
In addition to Ni, Pt (0.71 W / cm · K), Ti (0.22 W / cm ·) are used as metal materials having a lower thermal conductivity than GaN and a melting point equal to or higher than the growth temperature (1050 ° C.) of GaN. K). Therefore, a film made of these metals may be used instead of the Ni film 23. In metal materials, heat is transferred by lattice vibration and free electrons, but heat transport by free electrons is generally dominant. However, in an alloy, the number of free electrons decreases, and heat transport by lattice vibration becomes dominant, so that the thermal conductivity tends to be smaller than that of the constituent metals of the alloy. Therefore, an alloy film may be used instead of the Ni film 23.
[0072]
In the step shown in FIG. 2B, the surface of the Ni film 23 may be covered with a silicon oxide film or the like. This prevents chemical reaction between the Ni film 23 and ammonia or hydrogen. Of course, when the above-described Pt film, Ti film, or alloy film is used instead of the Ni film 23, the surface of these metal films may be covered with a silicon oxide film or the like.
[0073]
In this embodiment, since the GaN crystal grows on the Ni film 23 along the upper surface of the Ni film 23 when the GaN layer 14 is formed, dislocations generated in the GaN layer 12 hardly propagate to the GaN layer 14. . That is, there are very few dislocations in the GaN layer 14. For this reason, in this embodiment, it is possible to produce a GaN substrate having good crystallinity.
[0074]
In this embodiment, the Ni film 23 having the circular opening 23a is formed. However, the shape of the opening 23a is not limited to this. The shape of the opening 23a may be, for example, a stripe shape or a rectangular shape. In any case of providing the opening of any shape, the smaller the area of the upper surface of the GaN layer 12 exposed in the opening, the greater the effect of preventing thermal diffusion.
[0075]
Furthermore, in this embodiment, the opening 23a is formed on the GaN layer 12. The GaN layers 12 are arranged in a row at a constant interval a (5 μm in this embodiment) in the <11-20> direction. The openings of each row adjacent in the <1-100> direction are arranged so as to be shifted by a fixed distance a / 2 (2.5 μm in this embodiment). When the openings 23a are arranged in this way, the GaN crystal has a hexagonal close-packed structure, and thus the crystal growth surface is on each side of the hexagon centering on the openings 23a on the Ni film 23. Therefore, lattice defects are less likely to occur when GaN crystals grown from the openings 23a adjacent to each other come into contact with each other. That is, the GaN layer 14 with fewer lattice defects can be formed, and a high-quality GaN substrate can be obtained.
[0076]
In this embodiment, the Ni film 23b having the opening 23a is also used as a mask for lateral growth of the GaN layer 14. For this reason, it is not necessary to newly manufacture a mask for lateral growth of the GaN layer 14, and the manufacturing process can be simplified.
[0077]
(Embodiment 3)
A method for manufacturing a GaN substrate in Embodiment 3 of the present invention will be described with reference to FIG. FIG. 4 is a process cross-sectional view illustrating the GaN substrate manufacturing method of the present embodiment.
[0078]
First, in the step shown in FIG. 4A, a sapphire substrate 11 (diameter 2 inches, thickness 300 μm) having a C surface as a main surface is introduced into a MOVPE furnace. Next, after heat-treating the sapphire substrate 11 in a hydrogen atmosphere at 1050 ° C., it is cooled to 500 ° C. Subsequently, a GaN buffer layer (not shown) having a thickness of 20 nm is formed on the sapphire substrate 11 using trimethyl gallium (TMG) as a group III material and ammonia as a group V material. The temperature is again raised to 1050 ° C., and a GaN layer 12 having a thickness of 200 nm is formed on the GaN buffer layer. Then, it cools to room temperature and takes out the obtained board | substrate from the inside of a MOVPE furnace.
[0079]
Next, in the step shown in FIG. 4B, a 200 nm thick SiO 2 layer is formed on the GaN layer 12 by sputtering. ₂ Form a layer. Subsequently, this SiO ₂ A silicon oxide film 43 having an opening 43a is formed by patterning the layer. At this time, SiO ₂ The patterning of the layer is performed using photolithography and etching to form a circular opening 43a having a diameter of 2.5 μm that reaches the GaN layer 12, similar to the opening 23a in the second embodiment shown in FIG. . The opening 23 a is formed on the GaN layer 12. The GaN layers 12 are arranged in a row at a constant interval a (5 μm in this embodiment) in the <11-20> direction. The openings of each row adjacent in the <1-100> direction are arranged so as to be shifted by a fixed distance a / 2 (2.5 μm in this embodiment).
[0080]
Next, in the step shown in FIG. 4C, the substrate obtained in the step shown in FIG. 4B is introduced into the HVPE furnace. Next, heat treatment is performed at 800 ° C. in a nitrogen and ammonia atmosphere for 10 minutes to remove impurities on the substrate surface, and then a GaN layer 14 is formed on the substrate. The method for forming the GaN layer 14 is the same as the method performed in the step shown in FIG.
[0081]
At this time, the GaN crystal does not grow on the silicon oxide film 43 but starts growing on the GaN layer 12 exposed in the opening 43a. As the growth of the GaN crystal continues further, the GaN crystal grown from the GaN layer 12 exposed in the opening 43a grows on the silicon oxide film 43 along the upper surface of the silicon oxide film 43, and finally adjoins. The silicon oxide film 43 is covered together with the GaN crystal grown from the opening 43a. Subsequently, a GaN crystal grows on the GaN crystal covering the silicon oxide film 43, and the GaN layer 14 is formed. Thus, after growing the thickness of the GaN layer 14 to 300 μm, it is cooled to room temperature and taken out from the HVPE furnace.
[0082]
Next, in the step shown in FIG. 4D, the laser beam is scanned from the lower surface side of the sapphire substrate 11 over the entire lower surface of the GaN layer 12 through the sapphire substrate 11 as in the first embodiment. Irradiate. The laser beam used is the third harmonic (355 nm) of an Nd: YAG laser, and the irradiation energy is 0.3 J / cm. ² The pulse width is 5 ns, and the laser beam diameter during irradiation is 1 mm. Since the absorption edge wavelength of GaN is 360 to 370 nm, the laser beam having a wavelength of 355 nm is absorbed by the GaN layer 12 and generates heat. As a result, the GaN layer 12 is decomposed.
[0083]
Next, in the step shown in FIG. 4E, the sapphire substrate 11, the silicon oxide film 43, and the GaN layer 14 are separated. At this time, since the GaN layer 12 is decomposed in the step shown in FIG. 4D, it is easy to separate the sapphire substrate 11 from the silicon oxide film 43 and the GaN layer 14.
[0084]
Next, in the step shown in FIG. 4F, the silicon oxide film 43 formed under the GaN layer 14 is removed by wet etching using a solution made of hydrofluoric acid and ammonium fluoride. As a result, a self-standing GaN substrate 14a is obtained. When a device is fabricated on a GaN substrate from which the silicon oxide film 43 has not been removed, the silicon oxide film 43 has a low thermal conductivity, so that the heat dissipation characteristics of the device are deteriorated, which may affect the performance of the device. Therefore, it is preferable to remove the silicon oxide film 43. The removal of the silicon oxide film 43 may be performed by polishing.
[0085]
In the present embodiment, a silicon oxide film 43 having a lower thermal conductivity than the GaN layer 12 is formed between the GaN layer 12 and the GaN layer 14. SiO forming silicon oxide film 43 ₂ Has a thermal conductivity of 0.014 W / cm · K, which is much smaller than that of GaN, and smaller than that of the metal film used in the second embodiment. For this reason, the heat generated by the absorption of the laser beam in the GaN layer 12 is difficult to conduct through the silicon oxide film 43. That is, the silicon oxide film 43 is highly effective in suppressing thermal diffusion. Therefore, most of the generated heat contributes to the decomposition of the GaN layer 12, so that the GaN layer 12 can be decomposed efficiently.
[0086]
A non-metallic material such as the silicon oxide film 43 of this embodiment does not conduct heat due to free electrons. Furthermore, since the atomic arrangement is irregular in an amorphous material such as a dielectric, the efficiency of heat conduction is poor. For this reason, the thermal conductivity of non-metallic materials and non-crystalline materials is generally lower than that of metals and crystalline materials. Therefore, instead of the silicon oxide film 43 of the present embodiment, a film made of, for example, silicon nitride (thermal conductivity: 0.18 W / cm · K) is used as a film made of a non-metallic material and made of an amorphous material. May be.
[0087]
Furthermore, in this embodiment, the sapphire substrate 11 (thermal conductivity 0.46 W / cm · K) having a thermal conductivity smaller than that of the GaN layer 12 is used. For this reason, conduction of heat to the sapphire substrate 11 is also suppressed, and more heat can be contributed to the decomposition of the GaN layer 12.
[0088]
Also in the present embodiment, as in the second embodiment, the GaN crystal grows on the silicon oxide film 43 along the upper surface of the silicon oxide film 43, so that dislocations hardly propagate to the GaN layer 14. For this reason, according to the present embodiment, a GaN substrate having good crystallinity can be produced.
[0089]
In this embodiment, the silicon oxide film 43 having the circular opening 43a is formed. However, the shape of the opening 43a is not limited to this. The shape of the opening 43a may be, for example, a stripe shape or a rectangular shape. In the case of providing any shape of opening, the smaller the area of the upper surface of the GaN layer 12 exposed in the opening, the greater the effect of preventing thermal diffusion.
[0090]
Furthermore, in this embodiment, since the opening 43a is provided in the same manner as in the second embodiment, the GaN layer 14 with fewer lattice defects can be formed, and a high-quality GaN substrate can be obtained.
[0091]
In the present embodiment, the silicon oxide film 43 is also used as a mask for lateral growth of the GaN layer 14. For this reason, it is not necessary to newly manufacture a mask for lateral growth of the GaN layer 14, and the manufacturing process can be simplified.
[0092]
In Embodiments 1 to 3, examples of materials having lower thermal conductivity than GaN and GaN are shown in Table 1 below. In the first to third embodiments, the InN layer 13, the Ni film 23, and the silicon oxide film 43, which have lower thermal conductivity than GaN, may be formed from the materials shown in Table 1, respectively.
[0093]
[Table 1]

[0094]
(Embodiment 4)
A method for manufacturing a GaN substrate in Embodiment 4 of the present invention will be described with reference to FIG. FIG. 5 is a process cross-sectional view illustrating the GaN substrate manufacturing method of the present embodiment.
[0095]
First, in the step shown in FIG. 5A, a sapphire substrate 11 (diameter 2 inches, thickness 300 μm) having a C surface as a main surface is introduced into a MOVPE furnace. Next, after heat-treating the sapphire substrate 11 in a hydrogen atmosphere at 1050 ° C., it is cooled to 500 ° C. Subsequently, trimethylgallium (TMG) is used as a group III material, ammonia is used as a group V material, and H is used as a carrier gas. ₂ Is used to form a GaN buffer layer (not shown) having a thickness of 20 nm on the sapphire substrate 11. The temperature is again raised to 1050 ° C., and a GaN layer 12 having a thickness of 300 nm is formed on the GaN buffer layer. Then, it cools to room temperature and takes out the obtained board | substrate from the inside of a MOVPE furnace.
[0096]
Note that the GaN layer 21 may be formed on the sapphire substrate 11 without forming the GaN buffer layer. N as carrier gas ₂ N ₂ And H ₂ You may use the mixed gas of.
[0097]
Next, in the step shown in FIG. 5B, a 61 nm thick SiO 2 layer is formed on the GaN layer 12 by sputtering. ₂ Layer and TiO with a thickness of 35 nm ₂ The high reflectance film 53 is formed by alternately forming layers. In the present embodiment, the high reflectivity film 53 is formed of five layers of SiO. ₂ Layers and 4 layers of TiO ₂ The film consists of a total of nine layers. The reflectivity of the high reflectivity film 53 formed in this embodiment with respect to the third harmonic of the Nd: YAG laser beam is about 97%.
[0098]
Next, in the step shown in FIG. 5C, as in the first embodiment, the laser beam is scanned from the lower surface side of the sapphire substrate 11 over the entire lower surface of the GaN layer 12 via the sapphire substrate 11. Irradiate. The laser beam used is the third harmonic (355 nm) of an Nd: YAG laser, and the irradiation energy is 0.26 J / cm. ² The pulse width is 5 ns and the beam diameter during irradiation is 7 mm.
[0099]
Since the absorption edge wavelength of GaN is 360 to 370 nm, the laser beam passes through the sapphire substrate 11 and is absorbed by the GaN layer 12. As a result, the lower part of the GaN layer 12 is heated and thermally decomposed. As a result, a Ga metal portion 54 is formed below the GaN layer 12, and nitrogen gas is diffused. At this time, the sapphire substrate 11 and the GaN layer 12 are not completely separated even after the laser beam irradiation, and are weakly bonded by the Ga metal portion 54. Since the GaN layer 12 is thin and difficult to handle, the subsequent steps are performed with the GaN layer 12 placed on the sapphire substrate 11.
[0100]
Next, in the step shown in FIG. 5D, the high reflectance film 53 is patterned to form a high reflectance film 53b having an opening 53a. At this time, patterning of the high reflectivity film 53 is performed using photolithography and etching to form a stripe-shaped opening 53 a that reaches the GaN layer 12. In the present embodiment, the width of each opening 53a is 3 μm, and the distance between the openings 53a is 3 μm. In the etching, SiO constituting the high reflectivity film 53 is formed. ₂ The layer uses hydrofluoric acid and TiO ₂ The layer uses hot concentrated sulfuric acid. The opening 53a may have a dot shape, a rectangular shape or the like in addition to the stripe shape. In addition, this step may be performed before the laser beam irradiation step, but in that case, the GaN layer 12 located in the opening 53a may break and fly. Therefore, patterning is preferably performed after the laser beam irradiation process.
[0101]
Next, in the step shown in FIG. 5E, heat treatment is performed at 800 ° C. in a nitrogen and ammonia atmosphere for 10 minutes to remove impurities on the substrate surface, and then a GaN layer 14 is formed on the substrate. The method for forming the GaN layer 14 is the same as the method performed in the step shown in FIG.
[0102]
At this time, the GaN crystal does not grow on the high reflectance film 53b, but starts growing on the GaN layer 12 exposed in the opening 53a. When the growth of the GaN crystal is further continued, the GaN crystal grown from the GaN layer 12 exposed in the opening 53a grows along the upper surface of the high reflectivity film 53b, and finally from the adjacent opening 53a. The high reflectivity film 53 is covered together with the grown GaN crystal. Subsequently, a new GaN crystal is further grown on the GaN crystal covering the high reflectivity film 53b, and the GaN layer 14 is formed. In this way, the thickness of the GaN layer 14 is grown to 300 μm.
[0103]
Next, in the step shown in FIG. 5F, the Ga metal portion 54 is removed with hydrochloric acid to obtain a self-supporting GaN substrate 14a.
[0104]
In the present embodiment, in the laser beam irradiation step shown in FIG. 5D, the laser beam that has not been absorbed at the lower portion of the GaN layer 12 is reflected by the high reflectivity film 53 and contributes to thermal decomposition of the GaN layer 12. To do. The intensity of the laser beam irradiated to the lower surface of the GaN layer 12 without passing through the GaN layer 12 is 1, and the penetration length of the third harmonic of the laser beam into the GaN layer 12 is about 0.3 μm. Considering the reflectivity of 97%, the intensity of the laser beam reflected back to the lower part of the GaN layer 12 is about 0.13. That is, in this embodiment, the utilization efficiency of the irradiation energy of the laser beam contributing to the thermal decomposition of the GaN layer 12 is 1.13 times that of the conventional method. Therefore, the GaN layer 12 can be thermally decomposed efficiently.
[0105]
In the conventional method, the threshold value of the irradiation energy of the laser beam for thermally decomposing the GaN layer 102 is about 0.30 J / cm. ² It is. However, in this embodiment, the irradiation energy of the laser beam is 0.26 J / cm. ² However, thermal decomposition of the GaN layer 12 occurs. That is, according to the present embodiment, the threshold value of the laser beam irradiation energy necessary for thermally decomposing the GaN layer 12 can be reduced to about 88% of the conventional value.
[0106]
Here, FIG. 6 shows the relationship between the thickness of the GaN layer 12 and the laser beam intensity contributing to the thermal decomposition of the GaN layer 12 in the present embodiment. Note that the vertical axis in FIG. 6 is expressed as a ratio to the laser beam intensity contributing to the thermal decomposition of the GaN layer 102 in the conventional method.
[0107]
As shown in FIG. 6, in this embodiment, the thinner the GaN layer 12 is, the greater the intensity of the laser beam that contributes to thermal decomposition, and the lower the threshold of laser beam irradiation energy. However, if the GaN layer 12 is excessively thinned, the entire GaN layer 12 is thermally decomposed. Therefore, it is necessary to adjust the film thickness of the GaN layer 12 so that the entire GaN layer 12 is not thermally decomposed.
[0108]
Further, as a means for lowering the threshold value of the laser beam irradiation energy, the beam diameter of the laser beam can be increased. If a laser beam having a large beam diameter is used, the number of scans when irradiating the laser beam can be reduced, and the productivity and yield of the GaN substrate can be improved.
[0109]
In the present embodiment, the SiO film on which the high reflectivity film 53 is formed. ₂ And TiO ₂ Are dielectrics and have a much lower thermal conductivity than GaN as described in the third embodiment. For this reason, the heat generated by the absorption of the laser beam in the GaN layer 12 is difficult to conduct through the high reflectivity film 53. Accordingly, the thermal diffusion to the high reflectivity film 53 is small, and most of the generated heat contributes to the decomposition of the GaN layer 12, so that the GaN layer 12 can be decomposed efficiently.
[0110]
Furthermore, in this embodiment, the high reflectance film 53b having the opening 53a is also used as a mask for lateral growth of the GaN layer 14. For this reason, it is not necessary to newly manufacture a mask for lateral growth of the GaN layer 14, and the manufacturing process can be simplified.
[0111]
(Embodiment 5)
A method for manufacturing a group III nitride semiconductor substrate according to Embodiment 5 of the present invention will be described with reference to FIG. FIG. 7 is a process cross-sectional view illustrating the method for manufacturing the group III nitride semiconductor substrate of this embodiment.
[0112]
First, in the step shown in FIG. 7A, a sapphire substrate 11 (diameter 2 inches, thickness 300 μm) having a C surface as a main surface is introduced into a MOVPE furnace. Next, after heat-treating the sapphire substrate 11 in a hydrogen atmosphere at 1050 ° C., it is cooled to 500 ° C. Subsequently, trimethylgallium (TMG) is used as a group III material, ammonia is used as a group V material, and H is used as a carrier gas. ₂ Is used to form a GaN buffer layer (not shown) having a thickness of 20 nm on the sapphire substrate 11. The temperature is again raised to 1050 ° C., and a GaN layer 12 having a thickness of 50 nm is formed on the GaN buffer layer.
[0113]
Next, in the step shown in FIG. 7B, trimethylaluminum and trimethylgallium (TMG) are used as the group III source gas as the Al source gas, and ammonia is used as the group V source. An AlGaN layer 55 is formed.
[0114]
In the present embodiment, the AlGaN layer 55 is made of Al. _0.1 Ga _0.9 Although it is formed of N, the composition of Al and Ga may be determined so that the band gap is larger than the irradiation energy of the laser beam used in the laser beam irradiation step. Then, it cools to room temperature and takes out the obtained board | substrate from the inside of a MOVPE furnace.
[0115]
Next, in the step shown in FIG. 7C, a 61 nm thick SiO 2 layer is formed on the AlGaN layer 55 by sputtering. ₂ Layer and TiO with a thickness of 35 nm ₂ The high reflectance film 53 is formed by alternately forming layers. In the present embodiment, the high reflectivity film 53 is formed of five layers of SiO. ₂ Layers and 4 layers of TiO ₂ The film consists of a total of nine layers. The reflectivity of the high reflectivity film 53 formed in this embodiment with respect to the third harmonic of the Nd: YAG laser beam is about 96%.
[0116]
Next, in the step shown in FIG. 7D, as in the first embodiment, the laser beam is scanned from the lower surface side of the sapphire substrate 11 over the entire lower surface of the GaN layer 12 via the sapphire substrate 11. Irradiate. As a result, the GaN layer 12 is decomposed. The laser beam used is the third harmonic (355 nm) of an Nd: YAG laser, and the irradiation energy is 0.20 J / cm. ² The pulse width is 5 ns and the beam diameter during irradiation is 7 mm.
[0117]
Since the absorption edge wavelength of GaN is 360 to 370 nm, the laser beam passes through the sapphire substrate 11 and is absorbed by the GaN layer 12. As a result, the lower part of the GaN layer 12 is heated and thermally decomposed. In this embodiment, since the GaN layer 12 is very thin, almost the entire GaN layer 12 becomes the Ga metal portion 54, and nitrogen gas is emitted. At this time, the sapphire substrate 11 and the AlGaN layer 55 are not completely separated even after the laser beam irradiation, and are weakly bonded by the Ga metal portion 54. Since the AlGaN layer 55 is thin and difficult to handle, the subsequent steps are performed with the AlGaN layer 55 placed on the sapphire substrate 11.
[0118]
Next, in the step shown in FIG. 7E, the high reflectance film 53 is patterned to form a high reflectance film 53b having an opening 53a. At this time, patterning of the high reflectivity film 53 is performed using photolithography and etching to form a stripe-shaped opening 53 a that reaches the AlGaN layer 55. In the present embodiment, the width of each opening 53a is 3 μm, and the distance between the openings 53a is 3 μm. In the etching, SiO constituting the high reflectivity film 53 is formed. ₂ The layer uses hydrofluoric acid and TiO ₂ The layer uses hot concentrated sulfuric acid. In addition to the stripe shape, the opening may have a dot shape, a rectangular shape, or the like. In addition, this step may be performed before the laser beam irradiation step, but in that case, the AlGaN layer 55 located in the opening 53a may break and fly. Therefore, patterning is preferably performed after the laser beam irradiation process.
[0119]
Next, in the step shown in FIG. 7F, heat treatment is performed in an atmosphere of nitrogen and ammonia at 800 ° C. for 10 minutes to remove impurities on the substrate surface, and then a GaN layer 14 is formed on the substrate. The method for forming the GaN layer 14 is the same as the method performed in the step shown in FIG.
[0120]
At this time, the GaN crystal does not grow on the high reflectivity film 53b but starts growing on the AlGaN layer 55 exposed in the opening 53a. As the growth of the GaN crystal continues further, the GaN crystal grown from the AlGaN layer 55 exposed in the opening 53a grows along the upper surface of the high reflectivity film 53b, and finally from the adjacent opening 53a. The high reflectance film 53b is covered together with the grown GaN crystal. Subsequently, a new GaN crystal is further grown on the GaN crystal covering the high reflectivity film 53b, and the GaN layer 14 is formed. In this way, the thickness of the GaN layer 14 is grown to 300 μm.
[0121]
Next, in the step shown in FIG. 7G, the AlGaN layer 55 and the GaN layer 14 are separated from the sapphire substrate 11 by attaching the sapphire substrate 11 on which the AlGaN layer 55 and the GaN layer 14 are mounted to hydrochloric acid. As a result, a group III nitride semiconductor substrate composed of the AlGaN layer 55 and the GaN layer 14 is obtained.
[0122]
In this embodiment, in the laser beam irradiation step shown in FIG. 7D, the laser beam that has not been absorbed at the lower part of the GaN layer 12 is reflected back to the high reflectivity film 53 and contributes to thermal decomposition of the GaN layer 12. To do. For this reason, according to the present embodiment, a group III nitride semiconductor substrate having a very good crystallinity of the GaN layer 14 can be obtained. This will be described in detail below.
[0123]
AlGaN layer 55 (Al _0.1 Ga _0.9 The band gap of N) is about 4.1 eV, which is larger than the energy of the third harmonic of the Nd: YAG laser beam. For this reason, the laser beam is not absorbed in the AlGaN layer 55. The penetration length of the laser beam with respect to the GaN layer 12 is about 0.3 μm, and the reflectance of the high reflectance film is 96%. Considering the above conditions, when the intensity of the laser beam applied to the lower surface of the GaN layer 12 is 1, the intensity of the laser beam reflected by the high reflectivity film 53 and returning to the lower surface of the GaN layer 12 is about 0. 7 That is, in this embodiment, the utilization efficiency of the irradiation energy of the laser beam contributing to the thermal decomposition of the GaN layer 12 is 1.7 times that of the conventional method. Therefore, the GaN layer 12 can be thermally decomposed efficiently.
[0124]
In the conventional method, the threshold value of the irradiation energy of the laser beam for thermally decomposing the GaN layer 102 is about 0.30 J / cm. ² It is. However, in this embodiment, the irradiation energy of the laser beam is 0.20 J / cm. ² However, thermal decomposition of the GaN layer 12 occurs. That is, according to the present embodiment, the threshold value of the laser beam irradiation energy required for thermally decomposing the GaN layer 12 can be reduced to about 67% of the conventional value.
[0125]
Further, as a means for lowering the threshold value of the laser beam irradiation energy, the beam diameter of the laser beam can be increased. If a laser beam with a large beam diameter is used, the number of scans when irradiating the laser beam can be reduced. Therefore, a group III nitride semiconductor substrate having good crystallinity can be obtained, and further, the productivity and yield of the group III nitride semiconductor substrate can be improved.
[0126]
As in the fourth embodiment, also in this embodiment, the thinner the GaN layer 12, the greater the laser beam intensity contributing to thermal decomposition, and the lower the laser beam irradiation energy threshold. . However, if the GaN layer 12 is excessively thinned, the entire GaN layer 12 is thermally decomposed. Therefore, it is necessary to adjust the film thickness of the GaN layer 12 so that the entire GaN layer 12 is not thermally decomposed.
[0127]
Further, in the present embodiment, the high reflectivity film 53b having the opening 53a is also used as a mask for lateral growth of the GaN layer 22. For this reason, it is not necessary to newly manufacture a mask for lateral growth of the GaN layer 22, and the manufacturing process can be simplified.
[0128]
(Embodiment 6)
A method for manufacturing a GaN substrate in Embodiment 6 of the present invention will be described with reference to FIG. FIG. 8 is a process cross-sectional view illustrating the GaN substrate manufacturing method of the present embodiment.
[0129]
First, in the step shown in FIG. 8A, a sapphire substrate 11 having a thickness of 300 μm is prepared with a C-face having a diameter of 2 inches polished on both sides. Next, as indicated by the arrows in the figure, protons (H ⁺ ). The proton implantation conditions are ion energy 200 keV, average range 1.12 μm, implantation amount 5.0 × 10. ¹⁶ atoms · cm ^-2 It is.
[0130]
Next, in the step shown in FIG. 8B, an ion implantation region 62 having a thickness of about 2 μm with a depth of 1.12 μm as the center is formed from the lower surface of the sapphire substrate 11 by the above-described ion implantation. Since the ion implantation region 62 is damaged at the time of ion implantation, it is a region having a large number of defects.
[0131]
Next, in the step shown in FIG. 8C, the sapphire substrate 11 in which the ion implantation region 62 is formed is introduced into the HVPE furnace. Next, a GaN layer 64 is formed on the upper surface of the sapphire substrate 11 by supplying ammonia gas together with GaCl from the group III line and nitrogen carrier gas from the group V line. Specifically, GaCl supplied from the group III line is generated by introducing HCl gas into a boat filled with Ga metal heated to 900 ° C. using nitrogen as a carrier gas. As a result, a buffer layer (not shown) made of GaN and having a thickness of 30 nm is formed on the upper surface of the sapphire substrate 11 heated to 500 ° C. Subsequently, the sapphire substrate 11 is heated to 1050 ° C., and the GaN layer 64 is grown at a formation rate of 50 μm / hour for 6 hours, so that the thickness of the GaN layer 64 is finally set to 300 μm. Thereafter, the substrate is cooled to room temperature and taken out from the HVPE furnace.
[0132]
Next, in the step shown in FIG. 8D, the laser beam is irradiated while scanning over the entire lower surface of the GaN layer 64 from the lower surface side of the ion implantation region 62 through the ion implantation region 62 and the sapphire substrate 11. . As a result, the GaN layer 12 is decomposed. The laser beam used is the third harmonic (355 nm) of an Nd: YAG laser, and the irradiation energy is 0.3 J / cm. ² The pulse width is 5 ns, and the beam diameter during irradiation is 1 mm.
[0133]
At this time, the laser beam passes through the ion implantation region 62. The ion-implanted region 62 is a region with many defects, and the crystal uniformity is lost. Therefore, the laser beam is scattered after passing through the ion implantation region 62 as shown in FIG. Therefore, the spatial intensity distribution of the laser beam is made uniform. Since the absorption edge wavelength of GaN is 360 to 370 nm, the laser beam having a wavelength of 355 nm is absorbed by the GaN layer 64 and generates heat. Due to this heat generation, the lower portion of the GaN layer 64 is thermally decomposed.
[0134]
Next, in the step shown in FIG. 8E, the sapphire substrate 11 and the GaN layer 64 are separated. At this time, since the lower portion of the GaN layer 64 is decomposed in the step shown in FIG. 8D, the sapphire substrate 11 and the GaN layer 64 can be completely separated, and the self-standing GaN layer 64, that is, the GaN substrate. 64a can be obtained.
[0135]
A substrate shown in FIG. 8C of this embodiment and a substrate formed in the same manner as the substrate shown in FIG. 8C except that the ion implantation region 62 is not formed are prepared. In addition, the laser beam was irradiated by one pulse from the lower surface side of the sapphire substrate, and the sizes of the irradiation marks were compared. The laser beam diameter at this time is 1 mm.
[0136]
When the ion implantation region 62 was not formed, the irradiation trace in the GaN layer was a circle having a diameter of about 1 mm, and the laser beam reached the GaN layer with almost no scattering. On the other hand, in the substrate shown in FIG. 8C of the present embodiment in which the ion implantation region 62 is formed, the irradiation mark on the GaN layer 64 has a circular shape with a diameter of about 1.5 mm. That is, in the substrate shown in FIG. 8C of this embodiment, when the laser beam reaches the GaN layer 64, the beam diameter is about 1.5 times that in the case where the ion implantation region 62 is not formed. Scattered until
[0137]
As described above, according to the GaN substrate manufacturing method of the present embodiment, the laser beam can be effectively scattered in the ion implantation region 62 formed in the sapphire substrate 11 and the beam diameter can be enlarged. For this reason, the number of scans of the laser beam can be reduced, and a GaN substrate can be manufactured in a short time.
[0138]
In this embodiment, the ion implantation is performed on the entire lower surface of the sapphire substrate, but the ion implantation may be performed on a part of the lower surface of the sapphire substrate.
[0139]
The elements implanted into the sapphire substrate 11 are not particularly limited, and the same effect can be obtained with B, Al, Ga, In, Si, Ge, Mg, Zn, N, O, C, or the like. Of course, these plural kinds of elements may be implanted.
[0140]
Ion implantation is performed before the growth of the GaN layer, but may be performed at any time before the laser beam irradiation step.
[0141]
Since the scattering effect is weakened when the crystallinity of the ion implantation region 62 is recovered by heating at the time of forming the GaN layer 64, the ion implantation is preferably performed to such an extent that irradiation damage cannot be substantially recovered. . Specific ion implantation conditions vary depending on the element to be implanted, and thus cannot be limited. However, in order to reliably obtain the effect of scattering by implanting ions, the implantation amount is 1 × 10. ¹⁴ ~ 1x10 ²⁰ atoms · cm ^-2 It is preferable to be within the range.
[0142]
Moreover, since the unevenness | corrugation has arisen in the lower part of the obtained GaN board | substrate 64a by the thermal decomposition at the time of laser beam irradiation, you may planarize by grinding | polishing as needed.
[0143]
(Embodiment 7)
A method for manufacturing a GaN substrate in Embodiment 7 of the present invention will be described with reference to FIG.
[0144]
First, in the step shown in FIG. 9A, a 300 μm-thick sapphire substrate 11 having a C-face with a diameter of 2 inches polished on both sides is prepared and introduced into an HVPE furnace. Next, the GaN layer 65 is formed on the upper surface of the sapphire substrate 11 by supplying ammonia gas together with nitrogen carrier gas from the group III line and GaCl from the group V line. Specifically, GaCl supplied from the group III line is generated by introducing HCl gas into a boat filled with Ga metal heated to 900 ° C. using nitrogen as a carrier gas. As a result, a buffer layer (not shown) made of GaN and having a thickness of 30 nm is formed on the upper surface of the sapphire substrate 11 heated to 500 ° C. Subsequently, the sapphire substrate 11 is heated to 1050 ° C., and the GaN layer 65 is grown at a formation rate of 50 μm / hour for about 2 minutes, and finally the thickness of the GaN layer 65 is set to 1 μm. Thereafter, the substrate is cooled to room temperature and taken out from the HVPE furnace.
[0145]
Next, in the step shown in FIG. 9B, protons (H ⁺ ). The ion (proton) implantation conditions are as follows: ion energy 180 keV, average range 1.12 μm, implantation amount 5.0 × 10. ¹⁶ atoms · cm ^-2 It is.
[0146]
Next, in the step shown in FIG. 9C, an ion implantation region 66 having a thickness of about 0.5 μm is formed on the sapphire substrate 11 by the above-described ion implantation. Since the ion implantation region 66 is damaged during ion implantation, the ion implantation region 66 is a region with many defects.
[0147]
Next, in the step shown in FIG. 9D, the substrate is again introduced into the HVPE furnace, and heat treatment is performed in an ammonia gas atmosphere at 700 ° C. to remove impurities on the surface of the GaN layer 65. Next, the sapphire substrate 11 is heated to 1050 ° C. and grown at 50 μm / hour for 6 hours to grow a GaN layer 67 having a thickness of 300 μm. Thereafter, the substrate is cooled to room temperature and taken out from the HVPE furnace.
[0148]
Next, in the step shown in FIG. 9E, the laser beam is irradiated from the lower surface side of the sapphire substrate 11 while scanning over the entire lower surface of the GaN layer 65 through the sapphire substrate 11 and the ion implantation region 66. As a result, the lower portion of the GaN layer 65 is decomposed. The laser beam used is the third harmonic (355 nm) of an Nd: YAG laser, and the irradiation energy is 0.3 J / cm. ² The pulse width is 5 ns, and the beam diameter during irradiation is 1 mm.
[0149]
At this time, the laser beam passes through the ion implantation region 66. The ion implantation region 66 is a region having a large number of defects, and the uniformity of the crystal is lost. For this reason, after passing through the ion implantation region 66, the laser beam is scattered as shown in FIG. Therefore, the spatial intensity distribution of the laser beam is made uniform. Since the absorption edge wavelength of GaN is 360 to 370 nm, the laser beam having a wavelength of 355 nm is absorbed by the GaN layer 65 and generates heat. Due to this heat generation, the lower part of the GaN layer 65 is thermally decomposed.
[0150]
Next, in the step shown in FIG. 9 (f), the sapphire substrate 11 and the ion implantation region 66 are separated from the GaN layers 65 and 67. At this time, since the lower part of the GaN layer 65 is decomposed in the step shown in FIG. 9 (e), the sapphire substrate 11 and the GaN layer 65 can be completely separated, and the GaN layers 65 and 67, which are self-supporting, That is, the GaN substrate 67a can be obtained.
[0151]
After the step shown in FIG. 9 (f), the lower portion of the obtained GaN substrate 67a has irregularities due to thermal decomposition during laser beam irradiation, so that it may be planarized by polishing as necessary. Good.
[0152]
A substrate shown in FIG. 9D of this embodiment and a substrate formed in the same manner as the substrate shown in FIG. 9D except that the ion implantation region 66 is not formed are prepared. In addition, the laser beam was irradiated by one pulse from the lower surface side of the sapphire substrate, and the sizes of the irradiation marks were compared. The laser beam diameter at this time is 1 mm.
[0153]
When the ion implantation region 66 was not formed, the irradiation mark in the GaN layer was a circle having a diameter of about 1 mm, and the laser beam reached the GaN layer with almost no scattering. On the other hand, in the substrate shown in FIG. 9D of the present embodiment in which the ion implantation region 66 is formed, the irradiation mark on the GaN layer 65 has a circular shape with a diameter of about 1.3 mm. That is, in the substrate shown in FIG. 9D of the present embodiment, when the laser beam reaches the GaN layer 65, the beam diameter is about 1.3 times that of the case where the ion implantation region 66 is not formed. Scattered until
[0154]
As described above, according to the GaN substrate manufacturing method of the present embodiment, the laser beam can be effectively scattered in the ion implantation region 66 formed in the sapphire substrate 11 and the beam diameter can be enlarged. For this reason, the number of scans of the laser beam can be reduced, and a GaN substrate can be manufactured in a short time.
[0155]
In this embodiment, ion implantation is performed on the entire top surface of the sapphire substrate, but ion implantation may be performed on a portion of the top surface of the sapphire substrate.
[0156]
The elements implanted into the sapphire substrate 11 are not particularly limited, and the same effect can be obtained with B, Al, Ga, In, Si, Ge, Mg, Zn, N, O, C, or the like. Of course, these plural kinds of elements may be implanted.
[0157]
In this embodiment, since ions are implanted through the GaN layer 65, in order to reduce damage to the GaN layer 65, the ion implantation amount is set to 1 × 10. ¹⁴ ~ 1x10 ¹⁸ atoms · cm ^-2 In addition, it is preferable to reduce the ion implantation amount as compared with the sixth embodiment in which implantation is directly performed on the substrate.
[0158]
When there are a thermally decomposed portion and a non-thermally decomposed portion in the GaN layer 65 due to laser beam irradiation, local stress is generated in the GaN layer 65. However, in this embodiment, since the ion implantation is performed through the GaN layer 65, the GaN layer 65 is also damaged by the ion implantation, and there is a portion where the bond between atoms is weak. For this reason, the local stress generated in the GaN layer 65 is relaxed at a portion where the bond between atoms is weak. Therefore, cracks and cracks are unlikely to occur in the GaN layer 65. That is, a good quality GaN substrate with few cracks and cracks can be obtained.
[0159]
(Embodiment 8)
A method for manufacturing a GaN substrate according to Embodiment 8 of the present invention will be described with reference to FIG.
[0160]
First, in the step shown in FIG. 10A, a 300 μm-thick sapphire substrate 11 having a C-face with a diameter of 2 inches polished on both sides is prepared and introduced into an HVPE furnace. Next, a Ni layer 68 having a thickness of 100 nm is formed on the lower surface of the sapphire substrate 11 by vapor deposition.
[0161]
Next, in a step shown in FIG. 10B, the Ni layer 68 is patterned to form a Ni film 68b having an opening 68a. At this time, the Ni layer 68 is patterned by photolithography and etching to form a stripe-shaped opening 68a that reaches the sapphire substrate 11. In the present embodiment, each opening 68a has an opening width of 0.25 μm and an interval between the openings 68a of 0.25 μm. It is provided to extend in the <11-20> direction.
[0162]
Next, in the step shown in FIG. 10C, the sapphire substrate 11 is etched by dry etching using Ar gas using the Ni film 68b as a mask.
[0163]
Next, in the step shown in FIG. 10D, the Ni film 68b is removed by wet etching. As a result, a groove 69 is formed in the lower portion of the sapphire substrate 11. In the present embodiment, the depth of the groove 69 is 0.25 μm.
[0164]
Next, in the step shown in FIG. 10E, a GaN layer 70 having a thickness of 300 μm is formed on the upper surface of the sapphire substrate 11 by the same method as in the sixth embodiment. Subsequently, the obtained substrate is cooled to room temperature.
[0165]
Next, in the step shown in FIG. 10F, the laser beam is irradiated from the lower surface side of the sapphire substrate 11 while scanning over the entire lower surface of the GaN layer 70 via the sapphire substrate 11. As a result, the lower portion of the GaN layer 70 is thermally decomposed. The laser beam used is the third harmonic (355 nm) of the laser, and the irradiation energy is 0.3 J / cm. ² The pulse width is 5 ns, and the beam diameter during irradiation is 1 mm.
[0166]
At this time, the laser beam is scattered by the groove 69 as shown in FIG. Therefore, the spatial intensity distribution of the laser beam is made uniform. Since the absorption edge wavelength of GaN is 360 to 370 nm, the laser beam having a wavelength of 355 nm is absorbed by the GaN layer 70 and generates heat. Due to this heat generation, the lower part of the GaN layer 65 is thermally decomposed.
[0167]
Next, in the step shown in FIG. 10G, the sapphire substrate 11 and the GaN layer 70 are separated. At this time, since the lower portion of the GaN layer 70 is decomposed in the step shown in FIG. 10F, the sapphire substrate 11 and the GaN layer 70 can be completely separated, and the self-standing GaN layer 70, that is, GaN. The substrate 70a can be obtained.
[0168]
A substrate shown in FIG. 10 (e) of the present embodiment and a substrate formed in the same manner as the substrate shown in FIG. 10 (e) except that the groove 69 is not formed are prepared. The laser beam was irradiated by one pulse from the lower surface side of the sapphire substrate, and the sizes of irradiation marks were compared. The laser beam diameter at this time is 1 mm.
[0169]
When the groove 69 was not formed, the irradiation trace in the GaN layer was circular with a diameter of about 1 mm, and the laser beam reached the GaN layer with almost no scattering. On the other hand, in the substrate shown in FIG. 10E of the present embodiment in which the groove 69 is formed, the irradiation mark on the GaN layer 70 has a circular shape with a diameter of about 1.5 mm. That is, in the substrate shown in FIG. 9C of this embodiment, when the laser beam reaches the GaN layer 70, the beam diameter is about 1.5 times that in the case where the groove 69 is not formed. Scattered.
[0170]
As described above, according to the GaN substrate manufacturing method of the present embodiment, the groove 69 formed in the sapphire substrate 11 can effectively scatter the laser beam and expand the beam diameter. For this reason, the number of scans of the laser beam can be reduced, and a GaN substrate can be manufactured in a short time.
[0171]
In the present embodiment, the grooves 69 are formed in a stripe shape, but the present invention is not particularly limited thereto. For example, in place of the grooves 69, the concave portions may be provided in a dot shape or a lattice shape. Moreover, the irregular recessed part produced by sandblasting, cutting, etc. may be sufficient.
[0172]
In addition, since the effect of a scattering becomes large, it is preferable that the groove | channel 69 or the above-mentioned recessed part is formed densely. In particular, in the present embodiment, in order to obtain the scattering effect, it is preferable that the opening width and interval of each groove 69 have a size equal to or smaller than the wavelength of incident light. Moreover, the side surface of the groove 69 may be a slope. As a method of forming the groove 69 having a sloped side surface, the lower surface of the sapphire substrate 11 is irradiated with plasma from an oblique direction, or as a mask used in the etching process of the sapphire substrate 11 shown in FIG. Examples thereof include a method using a mask material (for example, a resist) that can be easily removed by etching.
[0173]
(Embodiment 9)
In the present embodiment, a method of manufacturing a large number of GaN substrates using any one of the GaN substrates 14a, 64a, 67a, and 70a obtained in the first to eighth embodiments will be described.
[0174]
For example, the 2-inch diameter GaN substrate 14a obtained in the first embodiment is introduced into the HVPE furnace. After the GaN substrate 14a is heat-treated at 750 ° C. in an ammonia atmosphere, it is heated to 1050 ° C. and grown to a thickness of 10 mm as in the first embodiment. Then, it cools to room temperature and takes out from the HVPE furnace. Next, 30 slices of GaN substrates can be obtained by slicing with a thickness of 300 μm in a direction perpendicular to the c-axis using a slicer.
[0175]
Of course, using the GaN substrates 14a, 64a, 67a, and 70a obtained in Embodiments 2 to 8 above, a plurality of GaN substrates can be fabricated in exactly the same manner as described above.
[0176]
In the method of this embodiment, the GaN single crystal produced in any of Embodiments 1 to 8 above is used as a seed crystal, the GaN single crystal is grown thickly, and the thick GaN single crystal is cut by a slicer. It is possible to produce a substrate. Furthermore, in the method of this embodiment, since only one sapphire substrate 11 is used first in each of the first to eighth embodiments, the cost can be reduced.
[0177]
(Other embodiments)
In the present embodiment, modified examples in the first to eighth embodiments will be described.
[0178]
In the first to eighth embodiments described above, the method for manufacturing a GaN substrate is described as an example of the method for manufacturing a group III nitride semiconductor substrate, but the present invention is not limited to this. By appropriately changing the source gas supplied from the group III line, Al _x Ga _y In _1-xy A group III nitride semiconductor substrate made of N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) can be manufactured.
[0179]
In the first to eighth embodiments, the Nd: YAG laser beam is used. However, any laser beam having a wavelength larger than the absorption edge wavelength of the group III nitride semiconductor substrate and smaller than the absorption edge wavelength of the substrate may be used. For example, a KrF: excimer laser beam (248 nm), a XeCl: excimer laser beam (308 nm), or the like can be used.
[0180]
In the first to eighth embodiments, the size of the sapphire substrate 11 is 2 inches. However, even if the substrate has a larger area, it is possible to produce a group III nitride semiconductor substrate without cracking or chipping. .
[0181]
Instead of the sapphire substrate 11, a material (for example, spinel) that can transmit an Nd: YAG laser beam or a KrF excimer laser beam may be used.
[0182]
Further, when forming a group III nitride semiconductor substrate, a group II, group IV or group VI element may be doped using a raw material containing a group II, group IV or group VI element. For example, if a GaN substrate is doped with Si, Ge, Se or the like as an impurity, a group III nitride semiconductor film having n-type conductivity can be obtained. Further, if a GaN substrate is doped with Be, Mg, Zn or the like as an impurity, a group III nitride semiconductor film having p-type conductivity can be obtained.
[0183]
As described above, according to the method for producing a group III nitride semiconductor substrate of the present invention, a group III nitride semiconductor substrate having a large area free from cracks and chips can be produced in a short time, whereby the group III nitride is produced. Mass production of semiconductor substrates becomes possible.
[0184]
According to the method for manufacturing a nitride semiconductor substrate of the present invention, the threshold value of the laser beam irradiation energy required for thermal decomposition of the nitride semiconductor layer can be lowered. In addition, since the beam diameter of the laser beam can be increased as the threshold value decreases, the number of scans of the laser beam necessary for separating the base material substrate and the nitride semiconductor layer can be reduced. Therefore, the time required for the laser beam irradiation process can be shortened and productivity can be improved. In addition, since the number of times of laser beam irradiation is reduced, the yield is improved.
[0185]
The group III nitride semiconductor substrate obtained in the present embodiment is used for manufacturing a semiconductor device such as a semiconductor laser or a field effect transistor.
[0186]
【The invention's effect】
According to the present invention, a group III nitride semiconductor substrate having good crystallinity can be obtained.
[Brief description of the drawings]
FIGS. 1A to 1E are process cross-sectional views illustrating a method for manufacturing a GaN substrate according to Embodiment 1 of the present invention.
FIGS. 2A to 2F are process cross-sectional views illustrating a method for manufacturing a GaN substrate according to Embodiment 2 of the present invention.
FIG. 3 is a top view of the substrate shown in FIG. 2 (b).
4A to 4F are process cross-sectional views illustrating a method for manufacturing a GaN substrate according to Embodiment 3 of the present invention.
FIGS. 5A to 5E are process cross-sectional views illustrating a method for manufacturing a GaN substrate according to Embodiment 4 of the present invention.
FIG. 6 is a diagram showing the relationship between the thickness of a GaN layer and the intensity of a laser beam that contributes to thermal decomposition of the GaN layer.
FIGS. 7A to 7F are process cross-sectional views illustrating a method for manufacturing a group III nitride semiconductor substrate according to Embodiment 5 of the present invention.
FIGS. 8A to 8E are process cross-sectional views illustrating a method for manufacturing a GaN substrate according to Embodiment 6 of the present invention.
FIGS. 9A to 9F are process cross-sectional views illustrating a method for manufacturing a GaN substrate according to Embodiment 7 of the present invention.
FIGS. 10A to 10G are process cross-sectional views illustrating a method for manufacturing a GaN substrate according to Embodiment 8 of the present invention.
FIGS. 11A to 11C are process sectional views showing a conventional method for manufacturing a GaN substrate.
[Explanation of symbols]
11 Sapphire substrate
12, 65 GaN layer
13 InN layer
14, 64, 67 GaN layer
14a, 64a, 67a, 70a GaN substrate
23 Ni film
23a, 43a, 53a, 68a Opening
43 Silicon oxide film
53, 53b High reflectivity film
54 Ga metal part
55 AlGaN layer
62, 66 ion implantation region
68 Ni layer
68b Ni film
69 groove
70 GaN layer

Claims (16)

Preparing a substrate (a);
Forming a first semiconductor layer made of a group III nitride semiconductor on the substrate (b);
Forming a thermal diffusion suppression layer made of a group III nitride semiconductor having a lower thermal conductivity than the group III nitride semiconductor forming the first semiconductor layer on the first semiconductor layer;
A step (d) of forming a second semiconductor layer made of a group III nitride semiconductor on the thermal diffusion suppressing layer;
(E) decomposing the first semiconductor layer by irradiating the first semiconductor layer through the substrate with a light beam that is transmitted through the substrate and absorbed by the first semiconductor layer;
A method for producing a group III nitride semiconductor substrate comprising: In the manufacturing method of the group III nitride semiconductor substrate according to claim 1 ,
The method of manufacturing a group III nitride semiconductor substrate, wherein the thermal diffusion suppression layer is formed of a semiconductor made of InxGa1-xN (0 <x ≦ 1). In the manufacturing method of the group III nitride semiconductor substrate according to claim 1 ,
In the step (c), after forming the thermal diffusion suppression layer, an opening that penetrates the thermal diffusion suppression layer and reaches the first semiconductor layer is formed. Production method. In the manufacturing method of the group III nitride semiconductor substrate according to claim 1,
A method for producing a group III nitride semiconductor substrate comprising a step (f) of removing the thermal diffusion suppressing layer after the step (e). In the manufacturing method of the group III nitride semiconductor substrate according to claim 4 ,
In the step (f), the method for producing a group III nitride semiconductor substrate, wherein the thermal diffusion suppressing layer is removed by etching. In the manufacturing method of the group III nitride semiconductor substrate according to claim 4 ,
In the step (f), the thermal diffusion suppressing layer is removed by polishing. In the manufacturing method of the group III nitride semiconductor substrate according to claim 1,
The method for manufacturing a group III nitride semiconductor substrate, wherein the substrate has a lower thermal conductivity than the first group III nitride semiconductor. Preparing a substrate (a);
Forming a first semiconductor layer made of a group III nitride semiconductor on the substrate (b);
Forming a light reflecting layer on the first semiconductor layer (c);
Forming a second semiconductor layer made of a group III nitride semiconductor on the light reflecting layer (d);
(E) decomposing the first semiconductor layer by irradiating the first semiconductor layer through the substrate with a light beam that is transmitted through the substrate and absorbed by the first semiconductor layer. Including
In the method of manufacturing a group III nitride semiconductor substrate, the light reflecting layer reflects the light beam irradiated in the step (e).
The first semiconductor layer has a first layer made of a group III nitride semiconductor having a smaller band gap than the energy of the light beam, and a band gap larger than the energy of the light beam formed on the first layer. A method for producing a group III nitride semiconductor substrate comprising a second layer made of a group III nitride semiconductor. In the manufacturing method of the group III nitride semiconductor substrate according to claim 8 ,
In the step (c), after forming the light reflecting layer, an opening that penetrates the light reflecting layer and reaches the first semiconductor layer is formed. . In the manufacturing method of the group III nitride semiconductor substrate according to claim 9 ,
The method for producing a group III nitride semiconductor substrate, wherein the light reflecting layer is formed of a dielectric. Preparing a substrate (a);
Forming a first semiconductor layer made of a group III nitride semiconductor on the substrate (b);
Forming a light reflecting layer on the first semiconductor layer (c);
Forming a second semiconductor layer made of a group III nitride semiconductor on the light reflecting layer (d);
(E) decomposing the first semiconductor layer by irradiating the first semiconductor layer through the substrate with a light beam that is transmitted through the substrate and absorbed by the first semiconductor layer. Including
In the method of manufacturing a group III nitride semiconductor substrate, the light reflecting layer reflects the light beam irradiated in the step (e) .
In the step (c), after forming the light reflecting layer, an opening that penetrates the light reflecting layer and reaches the first semiconductor layer is formed .
The method of manufacturing a group III nitride semiconductor substrate, wherein the light reflecting layer is a laminated film in which a silicon oxide film and a titanium oxide film which are dielectrics are alternately laminated. Preparing a substrate (a);
Forming a light scattering portion inside the substrate (b);
Forming a semiconductor layer made of a group III nitride semiconductor on the substrate (c);
(D) disassembling the lower portion of the semiconductor layer by irradiating the semiconductor layer with the light beam transmitted through the substrate and absorbed by the semiconductor layer;
A method for producing a group III nitride semiconductor substrate comprising: In the manufacturing method of the group III nitride semiconductor substrate according to claim 12 ,
In the step (b), a light scattering portion is formed inside the substrate by implanting ions into the substrate, a method for manufacturing a group III nitride semiconductor substrate.
. In the manufacturing method of the group III nitride semiconductor substrate according to claim 13 ,
The step (c) is performed after the step (a),
In the step (b), by injecting ions into the substrate through the semiconductor layer, a light scattering portion is formed inside the substrate,
A group III nitride semiconductor substrate further comprising a step of forming another semiconductor layer made of a group III nitride semiconductor on the semiconductor layer between the steps (b) and (d). Production method. In the manufacturing method of the group III nitride semiconductor substrate according to claim 12 ,
In the step (b), a plurality of recesses are formed in the lower part of the substrate as the light scattering part. In the manufacturing method of the group III nitride semiconductor substrate according to claim 15 ,
A method of manufacturing a group III nitride semiconductor substrate, wherein the plurality of recesses are formed by plasma irradiation.

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