JP2004146525A

JP2004146525A - METHOD OF MANUFACTURING P-TYPE GaN COMPOUND SEMICONDUCTOR

Info

Publication number: JP2004146525A
Application number: JP2002308673A
Authority: JP
Inventors: Hiroaki Okagawa; 岡川　広明; Takashi Tsunekawa; 常川　高志; Kazuyuki Tadatomo; 只友　一行; Hiromitsu Kudo; 工藤　広光; Yoichiro Ouchi; 大内　洋一郎
Original assignee: Mitsubishi Cable Industries Ltd
Current assignee: Mitsubishi Cable Industries Ltd
Priority date: 2002-10-23
Filing date: 2002-10-23
Publication date: 2004-05-20

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing p-type GaN semiconductor which can stably conduct the activation process of p-type impurity in view of reducing a contact resistance while hydrogen passivation is prevented. <P>SOLUTION: In this method of manufacturing p-type GaN semiconductor, a GaN-system semiconductor including p-type impurity is first grown with the chemical vapor deposition method, and the heat treatment is thereafter conducted under the atmosphere of the inert gas including ammonium in the rate of 0.1 to 5 vol%. Here, the preferable heat treatment temperature is set to 700°C or higher. Moreover, it is also preferable to set the heat treatment temperature at the range of 900 to 1100°C for the period of 5 to 180 seconds. Hydrogen passivation can be prevented and dissociation of nitrogen atom can also be prevented. Accordingly, the p-type GaN semiconductor having the low total resistance (namely, hole concentration is high) and the low contact resistance can be obtained. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、ｐ型ＧａＮ系半導体の製造方法に関する。
【０００２】
【従来の技術】
青色または紫色発光素子等に用いられる窒化ガリウム系化合物半導体（以下、「ＧａＮ系半導体」ともいう）の製造方法としては有機金属気相成長法（ＭＯＶＰＥ法）や分子線エピタキシー法（ＭＢＥ法）等が一般的に用いられている。
【０００３】
例えば、ＭＯＶＰＥ法を用いた成長方法について説明すると、サファイア等からなる基板を設置した反応炉に有機金属であるトリメチルガリウム（ＴＭＧ）、トリメチルアルミニウム（ＴＭＡ）、アンモニア等を水素ガス等を含む結晶成長用雰囲気中で供給し、６００℃程度の低温でＧａＮやＡｌＮ等のバッファ層を積層した後、１０００℃程度の高温でＧａＮやＡｌＧａＮ等のＧａＮ系半導体結晶を成長させる。このとき、必要に応じ、不純物をドープしてｐ型、ｉ型、ｎ型層を製造してダブルヘテロ構造等のデバイス構造を製造する。ｐ型不純物としてはＭｇ、Ｚｎ等が知られている。
【０００４】
ｐ型ＧａＮ系半導体を製造する場合、該ＧａＮ系半導体結晶の成長中および室温に冷却するまでの雰囲気から水素（主にアンモニアの分解により発生する）が当該結晶へ混入し、水素パッシベーションが生じてｐ型ＧａＮ系半導体が高抵抗なものになってしまう。水素パッシベーションとは、ｐ型不純物であるＭｇやＺｎの周辺で窒素と原子状水素との結合、すなわちＮ−Ｈ結合が安定化される結果、上記ｐ型不純物がアクセプターとして働かなくなる現象である。
【０００５】
水素パッシベーションによるｐ型ＧａＮ系半導体の高抵抗化を低減するために、従来の製造方法では、例えば、ｐ型ＧａＮ系半導体の成長後、「実質的に水素を含まない雰囲気下で高温での熱アニールを施し、水素をＧａＮ膜から追い出すことで低抵抗なｐ型を得る」方法がある。（特許文献１参照）。
【０００６】
【特許文献１】
特許第２５４０７９１号公報
【０００７】
【発明が解決しようとする課題】
特許文献１記載の方法により製造されたＧａＮ系半導体結晶のホール濃度は高く、該ＧａＮ系半導体自体はねらいどおり低抵抗化されていた。しかし、本発明者らが更に検討を行った結果、特許文献１記載の方法には以下の問題があることが分かった。
【０００８】
すなわち、上記製造方法により得られたＧａＮ系半導体を用いてＬＥＤを製造したところ、動作電圧が非常に高くなったのである。これは、接触抵抗が高くなったためであると考え、ＴＬＭ法による接触抵抗の測定を試みた。その結果、接触抵抗が高くなっていることを確認し、前記考えが正しいことが判明した。さらに、上記ＬＥＤでは、漏れ電流が増加して、逆耐圧が低下するといった問題も生じることが分かった。
【０００９】
本発明は、上記問題を解消する、すなわち、ｐ型不純物の活性化処理を安定的に行うことができるｐ型ＧａＮ系半導体の製造方法を提供することを目的とする。
【００１０】
【課題を解決するための手段】
本発明者らは、熱処理を行う際のガス雰囲気を不活性ガスのみとしたことに起因して、ＧａＮ系半導体から窒素原子が解離した結果、該半導体表面にｎ型の欠陥が導入され、ｐ型電極とのオーミック特性が得られにくくなったことが、動作電圧が上昇した原因であると考えた。また、上記窒素原子の解離は、ｐ型ＧａＮ系半導体の表面近傍のみならず、ｐｎ接合界面付近にまで及んでいるために漏れ電流が増加したのだと考えた。このような着想に基いて、本発明者らは、通常は水素パッシベーションの原因と考えられるアンモニアを熱処理の際のガス雰囲気に微量加えることで、水素追い出しによる低抵抗化と、窒素脱離の抑制による低い接触抵抗が得られることを両立させる本発明を完成した。
【００１１】
すなわち、本発明は下記の特徴を有する。
（１）気相成長法によってｐ型不純物を含むＧａＮ系半導体を成長させた後、アンモニアを０．１〜５ｖｏｌ％の割合で含む不活性ガス雰囲気下で熱処理を行うことを特徴とするｐ型ＧａＮ系半導体の製造方法。
（２）上記熱処理を７００℃以上で行うことを特徴とする、上記（１）に記載の製造方法。
（３）上記熱処理を９００〜１１００℃で、５〜１８０秒間行うことを特徴とする、上記（１）または（２）に記載の製造方法。
【００１２】
【発明の実施の形態】
本発明の製造方法は、気相成長法によってｐ型不純物を含むＧａＮ系半導体を成長させた後、アンモニアを０．１〜５ｖｏｌ％の割合で含む不活性ガス雰囲気下で熱処理を行うことを特徴とする。
【００１３】
上述した割合でアンモニアを含む不活性ガス雰囲気下で熱処理を行うことで、ＧａＮ系半導体からの窒素の解離を防ぐことができ、オーミック特性が低下し難くなる。しかし、熱処理時における不活性ガス雰囲気に含まれるアンモニアの割合が多すぎると、ＧａＮ系半導体から水素が抜け難くなり、低抵抗化が困難になる。本発明者らは上記不活性ガス雰囲気に含まれるアンモニアの割合について鋭意検討を行った結果、当該割合を０．１〜５ｖｏｌ％の範囲にすると、オーミック特性が低下し難く、かつ、低抵抗なｐ型半導体も得られることを見出した。さらに、アンモニアの分解で発生する水素が多すぎるとＧａＮ系半導体からの水素の追い出しが不所望に阻害される傾向があることと、アンモニアの分解で発生する窒素が少なすぎるとＧａＮ系半導体からの窒素の脱離を効果的に抑制し難くなる傾向があることから、熱処理時における不活性ガス雰囲気に含まれるアンモニアの割合は０．１〜３ｖｏｌ％が好ましく、０．１〜２ｖｏｌ％がより好ましい。前記不活性ガス雰囲気におけるアンモニア以外の成分は、特に限定はないが、水素を含まないことが好ましく、取り扱いの容易さから不活性ガスであることがより好ましい。ここで、不活性ガスとは、Ｎ_２、Ａｒ、Ｈｅまたはそれらの混合物をさす。
【００１４】
さらに、本発明者らは熱処理時間・温度についても最適値が存在するのではないかと仮定して鋭意研究した結果、以下のことを見出した。
低抵抗なｐ型ＧａＮ系半導体を得るには、熱処理温度をｐ型不純物に結合した水素を脱離させ得る温度（例えば、３５０℃）以上とすることが必要であり、好ましくは７００℃以上、より好ましくは８００〜１０００℃である。ここで、熱処理温度とは、熱処理における処理系内の最高温度をさす。熱処理時間は水素を脱離させるのに必要な時間であり、低い温度の場合は長時間が必要で、高い温度の場合は短時間でよい。このように、熱処理時間は熱処理温度により変化し、例えば、６００〜８００℃といった比較的熱処理温度が低い場合は、熱処理時間が短すぎると低抵抗化を達成し難く、熱処理時間が長すぎると接触抵抗が高くなる傾向にあるため、好ましい熱処理時間は５〜３０分であり、より好ましい熱処理時間は１０〜２０分である。一方、例えば、９００〜１１００℃といった比較的熱処理温度が高い場合は、接触抵抗を低くするためには、さらに熱処理時間を短くすることが望まれるため、好ましい熱処理時間は５〜１８０秒であり、より好ましい熱処理時間は５秒〜１分である。ここで、熱処理時間とは、ｐ型不純物を含むＧａＮ系半導体を成長させた後（後述）、再び３５０℃以上にて処理する時間のことである。
【００１５】
本発明でいうＧａＮ系とは、Ｉｎ_ｘＡｌ_ｙＧａ_{１−ｘ−ｙ}Ｎ（０≦ｘ≦１、０≦ｙ≦１）で示される化合物半導体であって、例えば、ＡｌＮ、ＧａＮ、ＡｌＧａＮ、ＩｎＧａＮなどが重要な化合物として挙げられる。これらの化合物半導体であれば、任意のものを用いることができる。
【００１６】
本発明の半導体の形成に用いられる結晶基板は、ＧａＮ系半導体結晶が成長可能なものであればよい。好ましい結晶基板としては、例えば、サファイア（Ｃ面、Ａ面、Ｒ面）、ＳｉＣ（６Ｈ、４Ｈ、３Ｃ）、ＧａＮ、ＡｌＮ、Ｓｉ、スピネル、ＺｎＯ、ＧａＡｓ、ＮＧＯ（ＮｄＧａＯ_３）などが挙げられる。また、これらの結晶を表層として有する基材であってもよい。なお、基板の面方位は特に限定されなく、更にジャスト基板でもよいしオフ角を付与した基板であってもよい。
【００１７】
本発明の半導体の製造に際し、ドープするｐ型不純物は、ＧａＮ系結晶にドープすることで正孔を生ぜしめるものであればよく、当業界で公知のものを任意に用いることができる。そのようなｐ型不純物としては、Ｍｇ、Ｚｎ、Ｂｅ、Ｃ等が例示され、制御が容易であり、毒性の問題がないなどの理由によりＭｇ、Ｚｎ等が好ましい。
【００１８】
本発明でいう気相成長法としては、ＧａＮ系半導体結晶の成長方法として公知の、ＨＶＰＥ法、ＭＯＶＰＥ法、ＭＢＥ法などが例示される。厚膜を作製する場合はＨＶＰＥ法が好ましく、薄膜を作製する場合はＭＯＶＰＥ法やＭＢＥ法が好ましい。
【００１９】
本発明において、「気相成長法によってｐ型不純物を含むＧａＮ系半導体を成長させる」とは、上述した気相成長法によってｐ型不純物の原料となる化合物を供給しつつＧａＮ系半導体結晶を成長させて、一旦室温にまで冷却することをさす。
【００２０】
ｐ型不純物を含むＧａＮ系半導体の成長の諸条件（温度プロファイル、雰囲気等）は特に限定なく、公知技術を適宜参照してよい。
【００２１】
例えば、ｐ型不純物を含むＧａＮ系半導体結晶の成長が行われる結晶成長用雰囲気は、上述した不活性ガスを５０ｖｏｌ％以上（より好ましくは７０〜９０ｖｏｌ％）の割合で有することが好ましい。このように、不活性ガスの割合の大きい結晶成長用雰囲気中、すなわち、水素供給源となる水素ガスやアンモニアの少ない結晶成長用雰囲気中でｐ型不純物を含むＧａＮ系半導体を成長させることで、当該ｐ型ＧａＮ系半導体に水素が入り難くなる。ｐ型ＧａＮ系半導体に含まれる水素が少なければ、低抵抗なｐ型ＧａＮ系半導体を得易くなる。
【００２２】
本発明の製造方法は、ｐ型ＧａＮ系半導体の製造方法としてだけでなく、ｐ型ＧａＮ系半導体を有する全てのＧａＮ系半導体素子（ＧａＮ系発光素子、ＧａＮ系受光素子、その他、ＧａＮ系半導体を用いた素子・電子デバイスなど）の製造方法として有用である。その場合、ＧａＮ系半導体レーザー、ＧａＮ系受光素子、ＧａＮ系半導体からなる電子デバイスなどの構造については、従来公知のものを参照してよい。また、ＧａＮ系半導体素子の製造のために適宜必要となる、ＧａＮ系低温成長バッファ層を用いる技術、ＧａＮ系結晶の転位密度低下のための技術（選択成長法、結晶基板面に凹凸加工して行うラテラル成長やファセット成長の技術など）、パターニング技術、素子電極材料と構造、分断技術などについては、公知の技術を参照してよい。
【００２３】
【実施例】
以下、各実施例に基づいて、本発明についてさらに詳細に説明するが、本発明は実施例のみに限定されるものではない。
【００２４】
［実施例１−１１、比較例１、２］
直径２インチのサファイア基板（ウエハ）をＭＯＶＰＥ装置に設置し、１１００℃に昇温して水素雰囲気下でサーマルエッチングを行った。次に、成長温度を３７５℃に下げ、キャリアガスとして水素および窒素を用い、トリメチルアルミニウム（以下ＴＭＡ）およびアンモニア（以下ＮＨ_３）を原料として、ＡｌＮ低温バッファー層を形成した。その後、１０００℃に昇温し、キャリアガスとして水素および窒素を用いトリメチルガリウム（以下ＴＭＧ）、ＮＨ_３を原料として、アンドープのＧａＮ層を３μｍ成長させた。さらに、キャリアガスとして水素および窒素を用いて、ｐ型不純物原料としてビスシクロペンタジエニルマグネシウム（以下ＣＰ２Ｍｇ）を加えたＧａＮ層を０．５μｍ成長させた。ｐ型不純物原料を加えた成長における雰囲気のガスの比率は窒素７０ｖｏｌ％、水素５ｖｏｌ％、ＮＨ_３２５ｖｏｌ％であった。
【００２５】
成長終了後、ＴＭＧ、ＣＰ２Ｍｇ、水素の供給を止め加熱を停止して、室温まで冷却を行った。以上の工程により、ｐ型不純物を含むＧａＮ系半導体を成長させることができた。
【００２６】
その後、サンプルをＭＯＶＰＥ装置から取り出して、熱処理装置に移し替え、下記表１記載の割合でＮＨ_３を含む不活性ガス雰囲気中、９００℃での熱処理を行ってｐ型ＧａＮ系化合物半導体を得た。熱処理時間も下記表１に記載のとおりとした。前記不活性ガス雰囲気におけるＮＨ_３以外の成分は全て窒素とした。
【００２７】
［評価］
ＭＯＶＰＥ装置から取り出した試料のＧａＮ層表面に、５ｍｍ角サイズに電極を形成しホール測定によりホール濃度を測定した。また、ＴＬＭ法により接触抵抗を測定した。結果を表１にまとめる。
【００２８】
【表１】

【００２９】
表中、「＊１」は、抵抗が高すぎるためＴＬＭ法による接触抵抗の測定が困難であったことを示し、「＊２」は、ホール測定において抵抗が高すぎて正孔濃度の測定ができなかったことを示す。
【００３０】
比較例１のようにＮＨ_３が全くないと、熱処理中に半導体中に水素が混入し難くなり、Ｍｇの水素パッシベーションは抑制できるが、表面から窒素が解離するため、測定できないほどに接触抵抗が高くなる。また比較例２のようにＮＨ_３が多すぎると、表面からの窒素解離は抑制できるが、半導体中への水素の混入が多すぎ、Ｍｇの水素パッシベーションの影響で抵抗が高くなることがわかる。また、実施例６〜１１からは以下のことが推察される。すなわち、熱処理時間が短いと半導体から水素を充分に追い出せずにホール濃度が低くなる傾向にあり、逆に熱処理時間が長いと、半導体から水素を追い出すことはできるが、表面から窒素が解離するため接触抵抗が高くなる傾向にある。
【００３１】
［実施例１２−１７］
サンプルをＭＯＶＰＥ装置から取り出した後の熱処理温度を７００℃としたことの他は実施例１と同様にして、ｐ型ＧａＮ系化合物半導体を得て、評価を行った。熱処理条件および結果を表２にまとめる。
【００３２】
【表２】

【００３３】
表２において熱処理時間が短い場合にホール濃度が低くなるのは、半導体からの水素の追い出しの程度が小さいためと考えられる。逆に熱処理時間が長い場合に、ホール濃度は高くなるものの接触抵抗も高くなってしまうのは、半導体から水素は充分に追い出されるが、表面からの窒素の脱離も多くなってしまったためであると考えられる。
【００３４】
［実施例１８］
サファイア基板（ウエハ）をＭＯＶＰＥ装置に設置し、１１００℃に昇温して水素雰囲気下でサーマルエッチングを行った。次に、成長温度を３７５℃に下げ、キャリアガスとして水素および窒素を用い、ＴＭＡおよびＮＨ_３を原料として、ＡｌＮ低温バッファー層を形成した。次に、１０００℃に昇温し、キャリアガスとして水素および窒素を用いＴＭＧ、ＮＨ_３を原料として、アンドープのＧａＮ層を１μｍ成長させた。続いてＳｉＨ_４を流し、Ｓｉドープのｎ型ＧａＮ結晶層（コンタクト層兼クラッド層）を３μｍ成長させた。
【００３５】
次いで、温度を８００℃に低下させた後、Ｓｉを５×１０^１７ｃｍ^−３添加したＧａＮ障壁層（厚さ１０ｎｍ）と、ＩｎＧａＮ井戸層（発光波長３８０ｎｍ、Ｉｎ組成は０．０３、厚さ３ｎｍ）を４周期成長した後、ｐ層に接する最後のＧａＮ障壁層（Ｓｉを５×１０^１７ｃｍ^−３添加、厚さ２０ｎｍ）を成長し、多重量子井戸構造を形成した。その後、成長温度を１０００℃に昇温して、厚さ５０ｎｍのｐ型ＡｌＧａＮクラッド層、厚さ１００ｎｍのｐ型ＧａＮコンタクト層を順に形成した後、ＴＭＧ、ＣＰ２Ｍｇ、水素の供給を止め、加熱を停止し、ＮＨ_３、窒素雰囲気で室温まで冷却を行った。
【００３６】
その後、サンプルをＭＯＶＰＥ装置から取り出して、熱処理装置に移し替え、０．５ｖｏｌ％のＮＨ_３ガスを含む不活性ガス雰囲気下（ＮＨ_３以外は全て窒素）で９００℃で６０秒間の熱処理を行った。その後、電極形成、素子分離を行い、紫外線ＬＥＤチップを得た。
【００３７】
［比較例３］
ＮＨ_３を全く含まない不活性ガス雰囲気下で熱処理を行ったことの他は実施例１８と同様にして紫外線ＬＥＤチップを得た。
【００３８】
［評価］
実施例１８および比較例３で得られた紫外線ＬＥＤチップのサンプルの２０ｍＡでの動作電流を測定したところ、実施例１８では３．６Ｖであり、比較例３では３．７６Ｖであった。また、両サンプルについて、逆バイアスでの漏れ電流が１０μＡになる電圧（逆耐圧）を測定したところ、実施例１８では２５Ｖであり、比較例３では１０Ｖであった。比較例３において逆耐圧が低下したのは、窒素の解離によるリーク電流の増大が原因と考えられる。
【００３９】
【発明の効果】
本発明の製造方法により、水素パッシベーションを防ぐとともに窒素原子の解離も防ぐことができ、半導体全体として低抵抗であり（すなわち、ホール濃度が高い）、かつ、接触抵抗も低いｐ型ＧａＮ系半導体を得ることができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a p-type GaN-based semiconductor.
[0002]
[Prior art]
Examples of a method for producing a gallium nitride-based compound semiconductor (hereinafter, also referred to as a “GaN-based semiconductor”) used for a blue or purple light-emitting device include a metal organic chemical vapor deposition (MOVPE) method and a molecular beam epitaxy method (MBE method). Is commonly used.
[0003]
For example, a description will be given of a growth method using the MOVPE method. A crystal growth including a hydrogen gas or the like containing trimethylgallium (TMG), trimethylaluminum (TMA) or ammonia as an organic metal in a reactor in which a substrate made of sapphire or the like is installed. After supply in an atmosphere for use and laminating a buffer layer such as GaN or AlN at a low temperature of about 600 ° C., a GaN-based semiconductor crystal such as GaN or AlGaN is grown at a high temperature of about 1000 ° C. At this time, if necessary, a device structure such as a double hetero structure is manufactured by doping impurities to manufacture p-type, i-type, and n-type layers. Mg and Zn are known as p-type impurities.
[0004]
When manufacturing a p-type GaN-based semiconductor, hydrogen (generally generated by decomposition of ammonia) is mixed into the crystal during the growth of the GaN-based semiconductor crystal and before cooling to room temperature, causing hydrogen passivation. The p-type GaN-based semiconductor has a high resistance. Hydrogen passivation is a phenomenon in which a bond between nitrogen and atomic hydrogen, that is, an NH bond is stabilized around Mg or Zn, which is a p-type impurity, so that the p-type impurity does not work as an acceptor.
[0005]
In order to reduce the increase in the resistance of the p-type GaN-based semiconductor due to hydrogen passivation, in a conventional manufacturing method, for example, after growth of the p-type GaN-based semiconductor, the “heat at a high temperature in an atmosphere containing substantially no hydrogen” is used. Annealing to remove hydrogen from the GaN film to obtain a low-resistance p-type. (See Patent Document 1).
[0006]
[Patent Document 1]
Japanese Patent No. 2540791
[Problems to be solved by the invention]
The hole concentration of the GaN-based semiconductor crystal manufactured by the method described in Patent Document 1 is high, and the GaN-based semiconductor itself has been reduced in resistance as intended. However, as a result of further studies by the present inventors, it has been found that the method described in Patent Document 1 has the following problems.
[0008]
That is, when an LED was manufactured using the GaN-based semiconductor obtained by the above manufacturing method, the operating voltage became extremely high. This was thought to be due to the increased contact resistance, and an attempt was made to measure the contact resistance by the TLM method. As a result, it was confirmed that the contact resistance was high, and it was found that the above idea was correct. Further, it has been found that in the above-mentioned LED, a problem that a leakage current increases and a reverse withstand voltage lowers also occurs.
[0009]
SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a p-type GaN-based semiconductor that can solve the above problem, that is, can stably activate a p-type impurity.
[0010]
[Means for Solving the Problems]
The present inventors dissociated nitrogen atoms from the GaN-based semiconductor due to the fact that only the inert gas was used as the gas atmosphere when performing the heat treatment, resulting in the introduction of n-type defects into the semiconductor surface, It was considered that the difficulty in obtaining the ohmic characteristics with the mold electrode was the cause of the increase in the operating voltage. In addition, it was considered that the dissociation of the nitrogen atoms extended not only near the surface of the p-type GaN-based semiconductor but also near the pn junction interface, thereby increasing the leakage current. Based on such an idea, the present inventors added a small amount of ammonia, which is usually considered to be a cause of hydrogen passivation, to a gas atmosphere at the time of heat treatment, thereby lowering resistance by purging hydrogen and suppressing nitrogen desorption. The present invention has been completed, in which low contact resistance can be obtained.
[0011]
That is, the present invention has the following features.
(1) After growing a GaN-based semiconductor containing a p-type impurity by a vapor phase growth method, a heat treatment is performed in an inert gas atmosphere containing 0.1 to 5 vol% of ammonia. A method for manufacturing a GaN-based semiconductor.
(2) The method according to (1), wherein the heat treatment is performed at 700 ° C. or higher.
(3) The method according to (1) or (2), wherein the heat treatment is performed at 900 to 1100 ° C. for 5 to 180 seconds.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
The manufacturing method of the present invention is characterized in that a GaN-based semiconductor containing a p-type impurity is grown by a vapor phase growth method, and then a heat treatment is performed in an inert gas atmosphere containing 0.1 to 5 vol% of ammonia. And
[0013]
By performing the heat treatment in an inert gas atmosphere containing the above-described ratio of ammonia, dissociation of nitrogen from the GaN-based semiconductor can be prevented, and the ohmic characteristics are not easily reduced. However, if the proportion of ammonia contained in the inert gas atmosphere during the heat treatment is too large, it becomes difficult for hydrogen to escape from the GaN-based semiconductor, and it becomes difficult to reduce the resistance. The present inventors have conducted intensive studies on the proportion of ammonia contained in the inert gas atmosphere. As a result, when the proportion is in the range of 0.1 to 5 vol%, the ohmic characteristics are hardly reduced and the resistance is low. It has been found that a p-type semiconductor can also be obtained. In addition, if the amount of hydrogen generated by decomposition of ammonia is too large, the eviction of hydrogen from the GaN-based semiconductor tends to be undesirably inhibited. If the amount of nitrogen generated by decomposition of ammonia is too small, GaN-based semiconductor The ratio of ammonia contained in the inert gas atmosphere during the heat treatment is preferably 0.1 to 3% by volume, and more preferably 0.1 to 2% by volume, since it is difficult to effectively suppress the desorption of nitrogen. . The components other than ammonia in the inert gas atmosphere are not particularly limited, but preferably do not contain hydrogen, and more preferably an inert gas from the viewpoint of easy handling. Here, the inert gas refers to N ₂ , Ar, He, or a mixture thereof.
[0014]
Furthermore, the present inventors have conducted intensive studies on the assumption that there are optimal values for the heat treatment time and temperature, and have found the following.
In order to obtain a low-resistance p-type GaN-based semiconductor, the heat treatment temperature needs to be higher than a temperature at which hydrogen bonded to the p-type impurity can be desorbed (for example, 350 ° C.), and preferably 700 ° C. or higher. More preferably, it is 800 to 1000 ° C. Here, the heat treatment temperature refers to the highest temperature in the treatment system in the heat treatment. The heat treatment time is a time required for desorbing hydrogen. A long time is required at a low temperature, and a short time is required at a high temperature. As described above, the heat treatment time varies depending on the heat treatment temperature. For example, when the heat treatment temperature is relatively low, for example, 600 to 800 ° C., it is difficult to achieve a low resistance if the heat treatment time is too short, and the contact becomes difficult when the heat treatment time is too long. Since the resistance tends to increase, a preferable heat treatment time is 5 to 30 minutes, and a more preferable heat treatment time is 10 to 20 minutes. On the other hand, for example, when the heat treatment temperature is relatively high such as 900 to 1100 ° C., it is desired to further shorten the heat treatment time in order to lower the contact resistance. Therefore, the preferable heat treatment time is 5 to 180 seconds. A more preferred heat treatment time is 5 seconds to 1 minute. Here, the heat treatment time refers to a time during which a GaN-based semiconductor containing a p-type impurity is grown (described later) and then treated again at 350 ° C. or higher.
[0015]
The GaN-based in the present _invention, a compound semiconductor represented by _{_{In x Al y Ga 1-x}} -y N (0 ≦ x ≦ 1,0 ≦ y ≦ 1), for example, AlN, GaN, AlGaN, InGaN is an important compound. Any of these compound semiconductors can be used.
[0016]
The crystal substrate used to form the semiconductor of the present invention may be any substrate on which a GaN-based semiconductor crystal can be grown. Preferred crystalline substrate, for example, sapphire (C plane, A-plane, R-plane), and the like SiC (6H, 4H, 3C) , GaN, AlN, Si, spinel, ZnO, GaAs, NGO (NdGaO 3) . Further, a substrate having these crystals as a surface layer may be used. The plane orientation of the substrate is not particularly limited, and may be a just substrate or a substrate having an off angle.
[0017]
In producing the semiconductor of the present invention, the p-type impurity to be doped may be any one that can generate holes by doping a GaN-based crystal, and any known p-type impurity in the art can be used. Examples of such p-type impurities include Mg, Zn, Be, C, and the like. Mg, Zn, and the like are preferable because they are easily controlled and have no toxicity problem.
[0018]
Examples of the vapor phase growth method referred to in the present invention include HVPE, MOVPE, MBE, and the like, which are known as a method for growing a GaN-based semiconductor crystal. When forming a thick film, the HVPE method is preferable, and when forming a thin film, the MOVPE method or the MBE method is preferable.
[0019]
In the present invention, “growing a GaN-based semiconductor containing a p-type impurity by a vapor phase growth method” means that a GaN-based semiconductor crystal is grown while supplying a compound serving as a source of the p-type impurity by the above-described vapor phase growth method. Let it cool to room temperature once.
[0020]
Various conditions (temperature profile, atmosphere, etc.) for growing a GaN-based semiconductor containing a p-type impurity are not particularly limited, and a known technique may be appropriately referred to.
[0021]
For example, the crystal growth atmosphere in which a GaN-based semiconductor crystal containing a p-type impurity is grown preferably contains the above-mentioned inert gas at a rate of 50 vol% or more (more preferably 70 to 90 vol%). As described above, by growing a GaN-based semiconductor containing a p-type impurity in a crystal growth atmosphere in which the proportion of an inert gas is high, that is, in a crystal growth atmosphere in which hydrogen gas or ammonia serving as a hydrogen supply source is small, Hydrogen hardly enters the p-type GaN-based semiconductor. If the amount of hydrogen contained in the p-type GaN-based semiconductor is small, it is easy to obtain a low-resistance p-type GaN-based semiconductor.
[0022]
The manufacturing method of the present invention is applicable not only to a method for manufacturing a p-type GaN-based semiconductor, but also to all GaN-based semiconductor elements having a p-type GaN-based semiconductor (GaN-based light-emitting elements, GaN-based light-receiving elements, and other GaN-based semiconductors). This is useful as a method for manufacturing the used elements and electronic devices. In this case, the structures of a GaN-based semiconductor laser, a GaN-based light receiving element, and an electronic device made of a GaN-based semiconductor may be referred to conventionally known structures. In addition, a technique using a GaN-based low-temperature growth buffer layer and a technique for lowering the dislocation density of a GaN-based crystal (selective growth method, forming a concave-convex pattern on a crystal substrate surface, which are required as appropriate for the manufacture of a GaN-based semiconductor device) Known techniques may be referred to for the lateral growth and facet growth techniques to be performed), the patterning technique, the element electrode material and structure, and the dividing technique.
[0023]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to Examples.
[0024]
[Example 1-11, Comparative Examples 1 and 2]
A sapphire substrate (wafer) having a diameter of 2 inches was set in a MOVPE apparatus, heated to 1100 ° C., and subjected to thermal etching in a hydrogen atmosphere. Next, the growth temperature was lowered to 375 ° C., hydrogen and nitrogen were used as carrier gases, and an AlN low-temperature buffer layer was formed using trimethylaluminum (hereinafter, TMA) and ammonia (hereinafter, NH ₃ ) as raw materials. Thereafter, the temperature was raised to 1000 ° C., and an undoped GaN layer was grown to a thickness of 3 μm using trimethylgallium (hereinafter, TMG) and NH ₃ as raw materials using hydrogen and nitrogen as carrier gases. Further, a GaN layer to which biscyclopentadienyl magnesium (hereinafter referred to as CP2Mg) was added as a p-type impurity material was grown to 0.5 μm using hydrogen and nitrogen as carrier gases. The ratio of the gas in the atmosphere in the growth to which the p-type impurity material was added was 70 vol% of nitrogen, 5 vol% of hydrogen, and 25 vol% of NH ₃ .
[0025]
After the growth was completed, the supply of TMG, CP2Mg, and hydrogen was stopped to stop heating, and the system was cooled to room temperature. Through the above steps, a GaN-based semiconductor containing a p-type impurity could be grown.
[0026]
Thereafter, the sample was taken out of the MOVPE apparatus, transferred to a heat treatment apparatus, and heat-treated at 900 ° C. in an inert gas atmosphere containing NH ₃ at a ratio shown in Table 1 below to obtain a p-type GaN-based compound semiconductor. . The heat treatment time was also as shown in Table 1 below. All components other than NH ₃ in the inert gas atmosphere were nitrogen.
[0027]
[Evaluation]
An electrode having a size of 5 mm square was formed on the GaN layer surface of the sample taken out from the MOVPE apparatus, and the hole concentration was measured by hole measurement. Further, the contact resistance was measured by the TLM method. The results are summarized in Table 1.
[0028]
[Table 1]

[0029]
In the table, "* 1" indicates that the contact resistance was difficult to measure by the TLM method because the resistance was too high, and "* 2" indicates that the hole concentration was too high to measure the hole concentration in the hole measurement. Indicates that it was not possible.
[0030]
If there is no NH ₃ as in Comparative Example 1, it is difficult for hydrogen to be mixed into the semiconductor during the heat treatment, and the hydrogen passivation of Mg can be suppressed. However, since nitrogen dissociates from the surface, the contact resistance becomes so large that it cannot be measured. Get higher. Also, as in Comparative Example 2, when the amount of NH ₃ is too large, the dissociation of nitrogen from the surface can be suppressed, but the amount of hydrogen mixed into the semiconductor is too large, and the resistance increases due to the influence of hydrogen passivation of Mg. In addition, the following is presumed from Examples 6 to 11. That is, if the heat treatment time is short, the hole concentration tends to be lowered without sufficiently displacing hydrogen from the semiconductor. Conversely, if the heat treatment time is long, hydrogen can be expelled from the semiconductor, but nitrogen is dissociated from the surface. Contact resistance tends to increase.
[0031]
[Example 12-17]
A p-type GaN-based compound semiconductor was obtained and evaluated in the same manner as in Example 1 except that the heat treatment temperature after the sample was taken out of the MOVPE apparatus was set to 700 ° C. Table 2 summarizes the heat treatment conditions and results.
[0032]
[Table 2]

[0033]
In Table 2, the reason why the hole concentration is reduced when the heat treatment time is short is considered to be that the degree of elimination of hydrogen from the semiconductor is small. Conversely, when the heat treatment time is long, the hole concentration is increased but the contact resistance is increased because hydrogen is sufficiently expelled from the semiconductor, but the desorption of nitrogen from the surface is also increased. it is conceivable that.
[0034]
[Example 18]
The sapphire substrate (wafer) was set in a MOVPE apparatus, and heated to 1100 ° C. to perform thermal etching in a hydrogen atmosphere. Next, the growth temperature was lowered to 375 ° C., hydrogen and nitrogen were used as carrier gases, and an AlN low-temperature buffer layer was formed using TMA and NH ₃ as raw materials. Next, the temperature was raised to 1000 ° C., and an undoped GaN layer was grown to 1 μm using TMG and NH ₃ as raw materials using hydrogen and nitrogen as carrier gases. Subsequently, SiH ₄ was flowed to grow a Si-doped n-type GaN crystal layer (contact layer / cladding layer) to 3 μm.
[0035]
Next, after the temperature was lowered to 800 ° C., a GaN barrier layer (10 nm thick) to which Si was added at 5 × 10 ¹⁷ cm ⁻³ and an InGaN well layer (emission wavelength: 380 nm, In composition: 0.03, thickness: 0.03) After growing 4 cycles of 3 nm), the last GaN barrier layer (adding 5 × 10 ¹⁷ cm ^{−3 of} Si and having a thickness of 20 nm) in contact with the p layer was grown to form a multiple quantum well structure. Thereafter, the growth temperature was raised to 1000 ° C., and a p-type AlGaN cladding layer having a thickness of 50 nm and a p-type GaN contact layer having a thickness of 100 nm were sequentially formed. Then, the supply of TMG, CP2Mg, and hydrogen was stopped, and the heating was stopped. It was stopped and cooled to room temperature in an NH ₃ , nitrogen atmosphere.
[0036]
Thereafter, the sample was taken out of the MOVPE apparatus, transferred to a heat treatment apparatus, and subjected to a heat treatment at 900 ° C. for 60 seconds in an inert gas atmosphere containing 0.5 vol% of NH ₃ gas (all nitrogen except for NH ₃ ). . Thereafter, electrode formation and element separation were performed to obtain an ultraviolet LED chip.
[0037]
[Comparative Example 3]
An ultraviolet LED chip was obtained in the same manner as in Example 18, except that the heat treatment was performed in an inert gas atmosphere containing no NH ₃ .
[0038]
[Evaluation]
When the operating current at 20 mA of the ultraviolet LED chip samples obtained in Example 18 and Comparative Example 3 was measured, it was 3.6 V in Example 18 and 3.76 V in Comparative Example 3. In addition, the voltage (reverse breakdown voltage) at which the leakage current at the reverse bias was 10 μA was measured for both samples. As a result, it was 25 V in Example 18 and 10 V in Comparative Example 3. The decrease in reverse breakdown voltage in Comparative Example 3 is considered to be due to an increase in leakage current due to dissociation of nitrogen.
[0039]
【The invention's effect】
According to the manufacturing method of the present invention, it is possible to prevent the hydrogen passivation and the dissociation of nitrogen atoms, and to provide a p-type GaN-based semiconductor having low resistance (that is, high hole concentration) and low contact resistance as a whole semiconductor. Obtainable.

Claims

A p-type GaN-based semiconductor characterized in that a GaN-based semiconductor containing a p-type impurity is grown by a vapor phase growth method, and then heat-treated in an inert gas atmosphere containing ammonia at a rate of 0.1 to 5 vol%. Manufacturing method.

The method according to claim 1, wherein the heat treatment is performed at 700 ° C. or higher.

The method according to claim 1, wherein the heat treatment is performed at 900 to 1100 ° C. for 5 to 180 seconds.