JP2004087930A - Nitride semiconductor light-emitting device - Google Patents
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【0001】本発明は、窒化物半導体発光素子に関し、特に光の取り出し効率を向上させるものである。
【0002】
【従来技術】GaN系化合物半導体を用いた青色発光ダイオード(LED)や紫外LED、青〜紫色半導体レーザ(LD)が開発され、これら発光素子と蛍光体を組み合わせた白色固体発光素子は、電球や蛍光灯等の真空管式照明光源を代替する新光源として期待されている。しかし、現在でもこれらの発光素子を照明用途に使うには更に素子の高出力化を達成する必要があり、そのための研究が種々なされている。
【0003】ところで、上記したGaN系化合物半導体は厚膜成長が基本的に難しいという特質がある。従って、一般的なGaN系化合物半導体発光素子においては、ワイヤーボンディング用の主電極から発光層までの距離が極めて短いものとならざるを得ず、他の材料系の半導体発光素子で行なわれているような電流拡散層を使っての発光の均一化(発光層全面で均一に発光が起こっているという意味での均一化)手段は通常採用することが出来ない。このため、オーミック電極を光が透過する程度の薄膜とする所謂透明電極(本件明細書内において第2のp補助電極とも呼ぶ)とし、該透明電極を素子の(p型層の)ほぼ全表面に形成し発光層全面に電流が行き渡るようにすることで均一な発光を得る等の工夫がなされている。
【0004】上記の透明電極の採用により、発光層全面が有効に活用され素子内部における発光量はいきおい増加することになる。また、併せて転位欠陥等を抑制することで、注入されるキャリアを高い割合でフォトンに変換させることが可能となり、その結果として内部量子効率を大幅に向上させることはできる。
【0005】
【発明が解決しようとする課題】しかしながら、光の取り出し効率の観点から当該素子構造から見た場合、様々な不都合が存在する。先ず、用途により異なってくるのであるが、ウエハーから切り出されるLEDダイのサイズは現在の汎用品として200〜400μm角である。このサイズであるとpnの両主電極の位置はエピ面に対し対角に位置する1対の隅部に形成される。ここで当然n電極はエピ面をエッチングし、n型半導体層上に形成することになる。ここで電流密度の最も高いところが当然に最もよく光るのであるが、電流密度が最も高いのは、p主電極からn電極に向かう部分である。全面電極(透明電極のことであり、本件明細書の第2のp補助電極に相当する。)を採用したとしても完全には全面に均等に電流が行き渡るわけではない。というのも電流は抵抗値が同じであれば最も短い距離を流れようとする性質があり、結果としてp主電極からn電極に向かう部分に電流が集中してしまう。よって全面電極の存在にもかかわらず、エピ周辺部は中央部に比べてあまり光らない。
【0006】また、p主電極による光の吸収の問題が挙げられる。実装方法等にもよるが、フェイスアップ実装ではp主電極直下で発光した光は取り出すことが難しい。また、p主電極近傍で発光し、p主電極の直下に伝播してしまいそのまま吸収される問題もある。さらに、第2のp補助電極としての透明電極による光の吸収の問題も挙げられる。即ち、透明電極の光の透過率は50%程度しか無く、また透明電極はほぼ全表面に形成されることから、素子の鉛直(表面)方向から放出されるべき光の取り出し効率を悪化させる要因となっている。ところがこの問題は、透明電極によるキャリアの注入性の向上と表裏の関係にあるため、例えば透明電極の不使用等は抜本的な問題解決とはならない。
【0007】従って本発明は、光取り出し効率を高め、外部量子効率を格段に向上させた窒化物半導体発光素子を提供することを目的とする。
【0008】
【課題を解決するための手段】本発明の窒化物半導体発光素子は、基板上に、少なくともn型窒化ガリウム系化合物半導体層と、活性層と、p型窒化ガリウム系化合物半導体層が順に積層され、同一面内に少なくともpn1対の電極が形成され、n電極がn型窒化ガリウム系化合物半導体層を一部露出させたn電極形成面上に形成され、p電極が、p主電極と、p主電極からp型窒化ガリウム系化合物半導体層全面に電流を拡散させるためのp補助電極からなる窒化物半導体発光素子において、p主電極に近接して電流阻止溝が形成され、該溝が少なくともn電極形成面よりも深いことを特徴とする。この構成にすることによって、電流としての最短距離のp主電極中心からn電極に向かう部分を電気的に切断することになり、電流の集中を抑制し、周辺部への拡散を促進することができる。尚且つp主電極直下の発光を抑制できる。これにより周辺部分の発光が増加し、光の取り出し効率がアップする。また、素子破壊は最も電圧がかかるところで起き、結果として電流が最も流れ、最も発光する部位、即ちp主電極からn電極に向かう部分の静電破壊が起こっていたがこの構成にすることによって、電流を拡散させることが出来るため、ダイ全体として静電耐圧が向上し、素子寿命が延びる。また、この構成をとることによる更なる効果としてp補助電極側に電気の拡散を促せるためp主電極直下の発光を抑制することができる。これにより発光しても効率的に取り出すことが出来なかった発光を抑制し、効率的に取り出せる部分を発光させることが出来る部分を発光させ素子全体としての発光能力を向上させることが出来る。また、このような構成を取ることにより、今まではp主電極の直下で発光した光が、n電極の方に進行した場合、エピ端面から出射される前にほとんど透明電極等により吸収され熱に変化し、取り出すことができなかったが、このようにp主電極をダイの側面と電流阻止溝で囲い込むことによって例えp主電極直下で発光したとしても光を確実に取り出すことができる。また逆に、窒化物半導体層内をp主電極方向に伝播している光は、p主電極に吸収されること無く取り出すことが出来る。
【0009】また、本発明の窒化物半導体発光素子は、前記電流阻止溝に加え、p主電極とn電極を結ぶ線上に光取り出し溝が形成されている。このような構成を取ることにより、例えばpn両主電極が形成されていない対角のエピ端面付近で発光した光がもう一方のエピ端面側に進行した場合、エピ端面から出射される前にほとんど透明電極等により吸収され熱に変化し、取り出すことができなかったが、その前に溝部よりエピ外部に取り出すことが可能になった。
【0010】さらにまた、本発明の窒化物半導体発光素子は、さらに発光面を分割する光取り出し溝を素子端面に対してほぼ垂直に付加する。溝を発光面を均等に分割するように増加させると、活性層で発光した光がどの方向に進行しても短い距離で溝部よりエピ外部に取り出すことが出来る。また溝を第1のp補助電極に対して垂直に形成することにより電流の流れを阻害することなく、全面発光させることが出来る。
【0011】さらにまた、本発明の窒化物半導体発光素子は、前記両溝部の総面積が発光面に対し5〜50%である。これより少なければ効果が少なく、これよりも多くなれば逆に発光領域が減少することになり電流密度が上がり発光効率の低下と駆動電圧の上昇を招く結果になってしまう。
【0012】さらにまた、本発明の窒化物半導体発光素子は、前記両溝の深さが基板まで達している。本発明において基板材料は特に限定される訳ではなく、サファイアやSiC、GaN、AlN等を基板とすることができ、基板の材質によって屈折率も異なるのでどこまで溝を形成すると有効かが変わってくる。ここで各材質の屈折率はサファイア:約1.8、SiC:約2.8、GaN:約2.5、AlN:約2.2である。サファイアを基板とした場合、積層されるGaNよりも屈折率が低いため、光は優先的に屈折率の高いエピ側を導波する。よってその導波する光を溝部よりエピ外部に取り出すためには基板まで溝を形成するのが効果が大きい。
【0013】さらにまた、本発明の窒化物半導体発光素子は、前記両溝部が基板を貫通している。さらにまた、本発明の窒化物半導体発光素子は、前記溝部が基板を貫通している。基板にSiC等の屈折率が高いものを使用すると、優先的に基板の方を光が導波するため、基板を貫通するように溝を形成すると光の取り出し効率の向上により効果がある。また、サファイア等の屈折率の低い基板を用いた場合であっても全ての光がエピ内を導波するのではなく、当然基板内も導波し、また、エピ厚に比べて基板の厚みはかなり厚いので基板内を導波する光のトータル量は無視できない。よって基板の屈折率によらず光の取り出し効率は向上する。
【0014】さらにまた、本発明の窒化物半導体発光素子は、前記両溝の幅(w)が基板上の積層膜厚以上である。エピ内を導波している光は基板界面とエピ表面の間、即ちエピ厚内で導波している。よって溝幅をエピ厚以上にすることによって溝から出射された光が再度エピ内に入る確率が低減する。
【0015】さらにまた、本発明の窒化物半導体発光素子は、前記両溝の幅(w)と溝の深さ(d)の関係がw/d≧1である。このような構成にすることにより、溝から出射された光が再度エピ内に入る確率が低減する。
【0016】さらにまた、本発明の窒化物半導体発光素子は、前記両溝がテーパ角を有する。テーパ角を有することによりさらに光の取り出し効率を向上することができる。この場合の溝の幅(w)は溝の底部ではなく、最表面で定義する。
【0017】さらにまた、本発明の窒化物半導体発光素子は、前記両溝側面に保護膜が製膜されている。保護膜によりショート、活性層が表面にさらされないためライフの向上等が期待できる。
【0018】さらにまた、本発明の窒化物半導体発光素子は、基板が成長面側に凹凸を有する。図6に示すような特殊加工した基板を使用するとさらに光の取り出し効率が上がる。図6においてハッチング部が凹部である。正三角形、菱形、又は正六角形の凹凸を有し、その凹凸面で光を散乱・回折・屈折させる効果がある。凹凸の段差は50nm以上で基板上に成長させる半導体層の厚さ以下の寸法であるのが重要である。この理由は少なくとも発光波長(例えば、AlGaInN系の発光層の場合、206nm〜632nm)をλとした時、λ/4以上の深さ又は段差がないと、十分に光を散乱又は回折することができない一方、凹部の深さ又は凸部の段差が基板上に成長させる半導体層の厚さを越える寸法の場合には、電流が積層構造内の横方向に流れにくくなり、発光効率が低下するからである。従って、半導体層の表面が凹状及び/又は凸状をなしてもよい。尚、十分に光を散乱又は回折させるためにはλ/4以上の深さ又は段差であることが好ましいが、λ/4n(nは半導体層の屈折率)以上の深さ又は段差であれば散乱又は回折の効果を得ることができる。
また、凹部及び/又は凸部の大きさ(即ち、凹部及び/又は凸部の構成辺となる一辺の長さ)、及び相互の間隔は、半導体中における発光波長をλとしたとき、少なくともλ/4以上の大きさであることが重要である。逆にあまりに凹凸の大きさや相互間隔が大きすぎると散乱面が減るため妥当ではない。よって0.2〜20μmが好ましい。
さらにまた、凹凸をエピ成長面に対し垂直に形成するよりも40〜50°のテーパ角を形成すればさらに取り出し効率がアップする。
【0019】さらにまた、本発明の窒化物半導体発光素子は、前記両溝がエッチングにより形成される。エッチングが最も形成しやすく、ばらつきも少ない。
【0020】
【作用】本発明者らは、窒化物半導体発光素子においてp主電極の直下で発光した光が有効に取り出すことが出来ていないこと、また逆に、窒化物半導体層内をp主電極方向に伝播している光はp主電極に吸収されること、pn両主電極間で最短距離部で発光出力が集中し全面に渡って均一に発光していないこと、端面から発せられる発光成分の割合が比較的大きいことを見出し、本発明を完成するに至った。即ち、請求項1記載の発明にあっては、通常の発光素子における端面発光部であるところの素子周辺部における第一の端面発光部に加え、p主電極の内側において発光領域の端面がエッチング加工により露出された第二端面発光部(光取り出し溝と電流狭窄溝)を形成することにより、当該第二端面発光部がp主電極直下で発光した光の光取り出し窓として作用し、補助電極下で発光した光がp主電極直下に伝播しp主電極により吸収されないように作用し、かつ、pn電極間での電気的短絡を防止し、よって、p主電極下部への電子の流れ込みを抑制しp主電極直下での発光を少なくし、結果としてエピ全面に渡って電流を拡散させ、エピ全面を光らせることができる。よって光の取り出し効率も向上するし、実際の発光量のトータル量そのものも向上するため、結果として外部量子効率を著しく向上することが出来る。
【0021】
【発明の実施の態様】以下図面に基づいて、本発明の実施の形態につき説明する。図1は本発明の窒化物半導体発光素子の一実施例を示しており、図1(a)は当該窒化物半導体発光素子を積層面側から見た平面図、図1(b)はそのA−A’線断面図を示している。図において、1は基板、21はn型窒化ガリウム系化合物半導体層、22はp型窒化ガリウム系化合物半導体層、20は活性層である。以下、本発明の製造方法について詳述する。
【0022】
半導体ウエハーとして、LED(light emitting diode)となる構成の窒化物半導体層をスピネル基板上に形成させた。具体的には、スピネル基板上に、GaNのバッファー層、n型GaNのコンタクト層、n型AlGaNのクラッド層、多重量子井戸構造となるInGaNの活性層、p型GaNのキャップ層、p型AlGaNのクラッド層及びp型GaNのコンタクト層が積層されている。この半導体ウエハーのエピ面側から電流阻止溝や光取り出し溝を形成する。溝の形成方法としてはエッチング(ドライエッチングとウエットエッチングの両方を含む)が好適で、レーザ照射による光学的方法やダイサーやスクライバーなど機械的方法によっても可能である。
【0023】
(窒化物半導体ウエハー100,200,300,400,500,600)
窒化物半導体ウエハー100,200,300,400,500,600としては、基板1上に窒化物半導体2が形成されたものである。窒化物半導体2の基板1としては、サファイア、スピネル、炭化珪素、酸化亜鉛や窒化ガリウム単結晶など種々のものが挙げられるが量産性よく結晶性の良い窒化物半導体層を形成させるためにはサファイア基板、スピネル基板などが好適に用いられる。
【0024】
窒化物半導体(InXGaYAl1−X−YN、0≦X、0≦Y、X+Y≦1)はMOCVD法やHVPE法などにより種々形成することができる。窒化物半導体にPN接合、PIN接合、MIS接合を形成させることにより半導体素子として利用することができる。半導体の構造もホモ接合、ヘテロ接合やダブルへテロ接合など種々選択することができる。また、半導体層を量子効果が生じる程度の薄膜とした単一量子井戸構造や多重量子井戸構造とすることもできる。
【0025】
【0026】
溝を形成する順序は電極を形成する直前でも直後でも良い。電極を形成する直前に溝を形成すると第2のp補助電極形成時に溝内に電極材料が入り込まないようにマスキングする必要が生じる。また、電極を形成した直後に溝を形成すると第2のp補助電極ごと溝を形成する必要が発生し溝の形成方法方法が限定される。
【0027】
電流狭窄溝と光取り出し溝は別々に形成しても良いが別々に形成する必要も特に無く、同時に形成することで工程が簡略化できる。先行技術としてはn電極を形成するためのnコンタクト層露出時に光取り出し溝を形成している例があるが(特開2002−164574、特開2002−26386等)、本発明の効果を十分に発揮するためにはnコンタクト層露出面よりも電流狭窄溝はより深い必要があるため、同じ工程で溝を形成するのは困難である。よって溝の形成方法にもよるがnコンタクト層露出時とは別工程で行なうことが好ましい。
【0028】
【実施例】
(実施例1)
厚さ425μmであり洗浄されたサファイアを基板としてMOCVD法を利用して窒化物半導体を積層させ窒化物半導体ウエハーを形成させた。窒化物半導体は発光素子とすることが可能なよう多層膜として成膜させた。まず、510℃において原料ガスとしてNH3(アンモニア)ガス、TMG(トリメチルガリウム)ガス及びキャリアガスである水素ガスを流すことにより厚さ約200オングストロームのバッファー層を形成させた。
【0029】
次に、TMGガスの流入を止めた後、反応装置の温度を1050℃に挙げ再びTMGガス、ドーパントガスとしてSiH4(シラン)ガスを流すことによりn型コンタクト層として働く厚さ約6μmのGaN層を形成させた。
【0030】
活性層は、一旦、キャリアガスとNH3のみとさせ反応装置の温度を800℃に保持し後、原料ガスとしてNH3(アンモニア)ガス、TEGガス、TMI(トリメチルインジウム)及びキャリアガスとして窒素ガスを流すことによりアンドープGaNよりなる障壁層を200オングストロームの膜厚で成長させ、続いて温度を800℃にして、TMG、TMI、アンモニアを用いアンドープIn0.4Ga0.6Nよりなる井戸層を30オングストロームの膜厚で成長させる。そして障壁+井戸+障壁+井戸・・・・+障壁の順で障壁層を5層、井戸層を4層、交互に積層して、総膜厚1120オングストロームの多重量子井戸構造よりなる活性層20を成長させる。
【0031】
活性層上にクラッド層を形成させるためTMG、TMIの流入を停止し反応装置の温度を1050℃に保持した後、原料ガスとしてNH3(アンモニア)ガス、TMA(トリメチルアルミニウム)ガス、TEGガス、ドーパントガスとしてCp2Mg(シクロペンタジエルマグシウム)ガス及びキャリアガスとして、窒素ガスを流しp型クラッド層として厚さ約0.1μmのGaAlN層を形成させた。
【0032】
最後に、反応装置の温度を1050℃に維持し原料ガスとしてNH3(アンモニア)ガス、TMGガス、ドーパントガスとしてCp2Mgガス及びキャリアガスとして水素ガスを流しp型コンタクト層として厚さ約0.5μmのGaN層を形成させた。(なお、p型窒化物半導体層は400℃以上でアニール処理してある。)
このように形成された窒化物半導体2にnコンタクト層露出を行い、n型コンタクト層を露出させ、次に第2のp補助電極33を形成し、その上に第1のp補助電極32とp主電極を同時に形成する。その後n電極を形成し、保護膜を形成する。ここまでは従来と同様であり、これを350μm角のチップとして切り出したものをリファレンスAとする。
【0033】
リファレンスAをベースとして電流狭窄溝を角度を付けず垂直に基板まで形成した(図1(a))。結果、リファレンスAと比較して5%光の取り出し効率が上昇した。
【0034】
(実施例2)
実施例1に加え、図2のようにp主電極とn電極を結ぶ線上に光取り出し溝を角度を付けず垂直に基板まで形成した。結果、リファレンスAと比較して12%光の取り出し効率が上昇した。
【0035】
(実施例3)
実施例2に加え、さらに図3のように素子端面に対してほぼ垂直に光取り出し溝を角度を付けず垂直に基板まで形成した。結果、リファレンスAと比較して16%光の取り出し効率が上昇した。
【0036】
(実施例4)
実施例3は電流狭窄溝と光取り出し溝が基板のエピ成長面側に対し垂直に形成したのに対し、テーパ加工を施した。垂直面に対して50°のテーパ角を形成した。結果、リファレンスAと比較して24%光の取り出し効率が上昇した。
【0037】
(実施例5)
実施例4と同様の加工を施し、かつ、特殊加工した基板上に形成した。図6(a)に示すパターンを使用し、正三角形の1辺の長さは10μm、隣り合う間隔は4μm、凹凸の深さは0.8μm、凹凸のテーパ角は垂直方向から40°であった。結果、リファレンスAと比較して41%光の取り出し効率が上昇した。
【0038】
(実施例6)
実施例1〜5までは350μm角のチップとして切り出したものであったのに対し、以後1mm角チップを使用した実施例を挙げる。半導体層は実施例1と同様に積層し、図7に示すような電極配置にpn両電極を形成する。この状態をリファレンスBとする。このリファレンスBに対して図4に示すように網目状に電流狭窄溝や光取り出し溝を形成する。この時の溝幅は29μm、溝深さは基板まで、溝はテーパを形成せず基板に対して垂直に形成した。結果、リファレンスBと比較して40%光の取り出し効率が上昇した。
【0039】
(実施例7)
実施例6と比較して溝形成において基板に対し垂直方向にテーパ角50°を形成する以外は同様に作製した。結果、リファレンスBと比較して62%光の取り出し効率が上昇した。
【0040】
(実施例8)
実施例7と同様の加工を施し、かつ、特殊加工した基板上に形成した。図6(a)に示すパターンを使用し、正三角形の1辺の長さは25μm、隣り合う間隔は10μm、凹凸の深さは0.8μm、凹凸のテーパ角は垂直方向から40°であった。結果、リファレンスAと比較して66%光の取り出し効率が上昇した。
【0041】
【発明の効果】
本発明の窒化物発光素子は、請求項1記載の発明にあっては、通常の発光素子における端面発光部であるところの素子周辺部における第一の端面発光部に加え、p主電極の内側において発光領域の端面がエッチング加工により露出された第二端面発光部(光取り出し溝と電流狭窄溝)を形成することにより、当該第二端面発光部がp主電極直下への電子の流入を抑制し、p主電極直下での発光を抑え、第2のp補助電極下部で発光した光がp主電極直下に入り込むのを防ぐことが出来、例えp主電極直下で発光したとしても光の光取り出し窓として作用し、かつ、pn電極間での電気的短絡を防止し、結果としてエピ全面に渡って電流を拡散させ、エピ全面を光らせることができる。よって光の取り出し効率も向上するし、実際の発光量のトータル量そのものも向上するため、結果として外部量子効率を著しく向上することが出来る。
【図面の簡単な説明】
【図1】本発明の窒化物半導体発光素子の一例を示す図であって、(a)図はその模式的平面図、(b)図は(a)図のA−A’線における模式的断面図である。
【図2】本発明の窒化物半導体発光素子の他の例を示す模式的平面図である。
【図3】本発明の窒化物半導体発光素子の他の例を示す模式的平面図である。
【図4】本発明の窒化物半導体発光素子の他の例を示す模式的平面図である。
【図5】従来の窒化物半導体発光素子の例を示す模式的平面図である。
【図6】本発明の窒化物半導体発光素子に使用可能な特殊加工した基板の模式的パターンである。
【図7】従来の窒化物半導体発光素子の例を示す模式的平面図である。
【符号の説明】
1・・・基板
2・・・窒化物半導体
20・・・活性層
21・・・n型窒化ガリウム系化合物半導体
22・・・p型窒化ガリウム系化合物半導体
31・・・p主電極
32・・・第1のp補助電極
33・・・第2のp補助電極
4・・・n電極
41・・・n主電極
42・・・n補助電極
51・・・電流狭窄溝
52・・・光取り出し溝
100,200,300,400,500,600・・・窒化物半導体ウエハー[0001] The present invention relates to a nitride semiconductor light emitting device, and more particularly to improving light extraction efficiency.
[0002]
2. Description of the Related Art Blue light-emitting diodes (LEDs), ultraviolet LEDs, and blue-violet semiconductor lasers (LDs) using GaN-based compound semiconductors have been developed. It is expected as a new light source to replace a vacuum tube illumination light source such as a fluorescent lamp. However, even now, in order to use these light-emitting elements for lighting applications, it is necessary to further increase the output of the elements, and various studies have been made for that purpose.
[0003] Incidentally, the GaN-based compound semiconductor described above has a characteristic that it is basically difficult to grow a thick film. Therefore, in a general GaN-based compound semiconductor light-emitting device, the distance from the main electrode for wire bonding to the light-emitting layer is inevitably extremely short, and is performed with another material-based semiconductor light-emitting device. Such means for uniformizing light emission using the current diffusion layer (uniformity in the sense that light emission occurs uniformly over the entire surface of the light emitting layer) cannot be usually employed. For this reason, the ohmic electrode is a so-called transparent electrode (also referred to as a second p auxiliary electrode in the present specification) having a thin film that transmits light, and the transparent electrode is formed on almost the entire surface (of the p-type layer) of the device. In order to obtain uniform light emission by making the current spread over the entire surface of the light-emitting layer, various measures have been taken.
[0004] By employing the above-mentioned transparent electrode, the entire surface of the light emitting layer is effectively utilized, and the amount of light emitted inside the device is greatly increased. In addition, by suppressing dislocation defects and the like, injected carriers can be converted into photons at a high rate, and as a result, the internal quantum efficiency can be greatly improved.
[0005]
However, from the viewpoint of the light extraction efficiency, there are various disadvantages from the viewpoint of the element structure. First, the size of an LED die cut out from a wafer is 200 to 400 μm square as a general-purpose product, though it differs depending on the application. With this size, the positions of the pn main electrodes are formed at a pair of corners located diagonally to the epi plane. Here, the n-electrode is naturally formed on the n-type semiconductor layer by etching the epi-surface. Here, the portion having the highest current density naturally emits the best light, but the portion having the highest current density is the portion from the p main electrode to the n electrode. Even if a full-surface electrode (which is a transparent electrode and corresponds to the second p-auxiliary electrode in the present specification) is employed, current is not completely and uniformly distributed over the entire surface. This is because the current tends to flow over the shortest distance if the resistance value is the same, and as a result, the current concentrates on the portion from the p main electrode to the n electrode. Therefore, despite the presence of the entire surface electrode, the periphery of the epi is less luminous than the center.
Another problem is the absorption of light by the p main electrode. Although it depends on the mounting method and the like, it is difficult to extract light emitted immediately below the p main electrode in face-up mounting. There is also a problem that light is emitted in the vicinity of the p main electrode, propagates directly below the p main electrode, and is absorbed as it is. Further, there is a problem of light absorption by the transparent electrode as the second p auxiliary electrode. That is, the light transmittance of the transparent electrode is only about 50%, and since the transparent electrode is formed on almost the entire surface, a factor that deteriorates the efficiency of extracting light to be emitted from the vertical (surface) direction of the device. It has become. However, since this problem has a front-to-back relationship with the improvement of the carrier injectability by the transparent electrode, for example, non-use of the transparent electrode does not solve the fundamental problem.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a nitride semiconductor light emitting device in which light extraction efficiency is enhanced and external quantum efficiency is significantly improved.
[0008]
According to a nitride semiconductor light emitting device of the present invention, at least an n-type gallium nitride compound semiconductor layer, an active layer, and a p-type gallium nitride compound semiconductor layer are sequentially laminated on a substrate. At least a pair of pn1 electrodes are formed in the same plane, the n-electrode is formed on the n-electrode formation surface partially exposing the n-type gallium nitride-based compound semiconductor layer, and the p-electrode is formed of a p-main electrode and a p-electrode. In a nitride semiconductor light emitting device comprising a p auxiliary electrode for diffusing current from the main electrode to the entire surface of the p-type gallium nitride based compound semiconductor layer, a current blocking groove is formed close to the p main electrode, and the groove is at least n It is characterized by being deeper than the electrode formation surface. With this configuration, a portion of the shortest distance as a current from the center of the p main electrode toward the n electrode is electrically disconnected, so that concentration of the current is suppressed and diffusion to the peripheral portion is promoted. it can. In addition, light emission immediately below the p main electrode can be suppressed. As a result, the light emission in the peripheral portion increases, and the light extraction efficiency increases. In addition, the element breakdown occurs where the highest voltage is applied, and as a result, the current flows the most, and the part that emits the most light, that is, the part from the p main electrode to the n electrode is electrostatically damaged. Since the current can be diffused, the electrostatic withstand voltage of the entire die is improved, and the life of the element is extended. Further, as a further effect of adopting this configuration, since the diffusion of electricity is promoted to the p auxiliary electrode side, light emission immediately below the p main electrode can be suppressed. As a result, it is possible to suppress the light emission that could not be efficiently extracted even when the light was emitted, and to improve the light emission capability of the entire device by emitting light from the portion that can efficiently emit light. In addition, by adopting such a configuration, when light emitted immediately below the p main electrode travels toward the n electrode, it is almost absorbed by the transparent electrode or the like before being emitted from the end face of the epi. However, by surrounding the p main electrode with the side surface of the die and the current blocking groove, light can be reliably extracted even if light is emitted immediately below the p main electrode. Conversely, light propagating in the nitride semiconductor layer in the direction of the p main electrode can be extracted without being absorbed by the p main electrode.
Further, in the nitride semiconductor light emitting device of the present invention, in addition to the current blocking groove, a light extraction groove is formed on a line connecting the p main electrode and the n electrode. By adopting such a configuration, for example, when light emitted near the diagonal epi end face where the pn both main electrodes are not formed proceeds to the other epi end face, almost no light is emitted before being emitted from the epi end face. Although it was absorbed by the transparent electrode and changed into heat and could not be taken out, it could be taken out of the epi from the groove before that.
Further, in the nitride semiconductor light emitting device of the present invention, a light extraction groove for further dividing the light emitting surface is added substantially perpendicular to the device end face. If the groove is increased so as to divide the light emitting surface evenly, the light emitted from the active layer can be extracted from the groove portion to the outside of the epi at a short distance no matter which direction the light travels. Further, by forming the groove perpendicular to the first p auxiliary electrode, it is possible to emit light over the entire surface without obstructing the flow of current.
Further, in the nitride semiconductor light emitting device of the present invention, the total area of the two groove portions is 5 to 50% with respect to the light emitting surface. If it is less than this, the effect is small, and if it is more than this, the light emitting region is conversely reduced, resulting in an increase in current density and a decrease in luminous efficiency and an increase in drive voltage.
Further, in the nitride semiconductor light emitting device of the present invention, the depth of the two grooves reaches the substrate. In the present invention, the substrate material is not particularly limited, and sapphire, SiC, GaN, AlN, or the like can be used as the substrate, and the refractive index differs depending on the material of the substrate. . Here, the refractive index of each material is about 1.8 for sapphire, about 2.8 for SiC, about 2.5 for GaN, and about 2.2 for AlN. When sapphire is used as a substrate, light has a lower refractive index than GaN to be laminated, so that light is guided preferentially on the epi side having a higher refractive index. Therefore, in order to extract the guided light to the outside of the epi from the groove, it is effective to form a groove up to the substrate.
Further, in the nitride semiconductor light emitting device according to the present invention, the two grooves penetrate the substrate. Furthermore, in the nitride semiconductor light emitting device of the present invention, the groove penetrates the substrate. If a substrate having a high refractive index, such as SiC, is used as the substrate, light is guided preferentially through the substrate. Therefore, if a groove is formed so as to penetrate the substrate, it is more effective to improve the light extraction efficiency. Even when a substrate having a low refractive index such as sapphire is used, not all light is guided in the epi, but also guided in the substrate, and the thickness of the substrate is smaller than the epi thickness. Is so thick that the total amount of light guided in the substrate cannot be ignored. Therefore, the light extraction efficiency is improved regardless of the refractive index of the substrate.
Further, in the nitride semiconductor light emitting device of the present invention, the width (w) of each of the two grooves is equal to or larger than the thickness of the laminated film on the substrate. Light guided in the epi is guided between the substrate interface and the epi surface, that is, in the epi thickness. Therefore, by setting the groove width to be equal to or larger than the epi thickness, the probability that light emitted from the groove enters the epi again is reduced.
Further, in the nitride semiconductor light emitting device of the present invention, the relationship between the width (w) of the two grooves and the depth (d) of the grooves is w / d ≧ 1. With such a configuration, the probability that the light emitted from the groove enters the epi again is reduced.
Further, in the nitride semiconductor light emitting device according to the present invention, both the grooves have a taper angle. By having a taper angle, the light extraction efficiency can be further improved. In this case, the width (w) of the groove is defined by the outermost surface, not the bottom of the groove.
Further, in the nitride semiconductor light emitting device of the present invention, a protective film is formed on both side surfaces of the groove. Since the protective film prevents the active layer from being exposed to the short circuit, the life can be improved.
Furthermore, in the nitride semiconductor light emitting device of the present invention, the substrate has irregularities on the growth surface side. The use of a specially processed substrate as shown in FIG. 6 further increases the light extraction efficiency. In FIG. 6, a hatched portion is a concave portion. It has irregularities of regular triangle, rhombus, or regular hexagon, and has the effect of scattering, diffracting, and refracting light on the irregular surface. It is important that the step of the unevenness has a size of not less than 50 nm and not more than the thickness of the semiconductor layer grown on the substrate. The reason for this is that when at least the emission wavelength (for example, 206 nm to 632 nm in the case of an AlGaInN-based emission layer) is λ, if there is no depth or a step of λ / 4 or more, light can be sufficiently scattered or diffracted. On the other hand, when the depth of the concave portion or the step of the convex portion exceeds the thickness of the semiconductor layer grown on the substrate, the current does not easily flow in the lateral direction in the laminated structure, and the luminous efficiency decreases. It is. Therefore, the surface of the semiconductor layer may be concave and / or convex. In order to sufficiently scatter or diffract light, it is preferable that the depth or the step is λ / 4 or more, but if the depth or the step is λ / 4n (n is the refractive index of the semiconductor layer) or more. Scattering or diffraction effects can be obtained.
In addition, the size of the concave portion and / or the convex portion (that is, the length of one side which is a constituent side of the concave portion and / or the convex portion) and the interval between each other are at least λ when the emission wavelength in the semiconductor is λ. It is important that the size is / 4 or more. Conversely, if the size of the unevenness or the mutual interval is too large, the scattering surface is reduced, which is not appropriate. Therefore, 0.2 to 20 μm is preferable.
Further, if a taper angle of 40 to 50 ° is formed as compared with the case where the unevenness is formed perpendicular to the epi-growth surface, the extraction efficiency is further improved.
Further, in the nitride semiconductor light emitting device of the present invention, the two grooves are formed by etching. Etching is the easiest to form, with little variation.
[0020]
The present inventors have found that in a nitride semiconductor light emitting device, light emitted immediately below the p main electrode cannot be effectively extracted, and conversely, the inside of the nitride semiconductor layer extends in the direction of the p main electrode. The propagating light is absorbed by the p main electrode, the light output is concentrated at the shortest distance between the pn main electrodes, and the light is not uniformly emitted over the entire surface, and the ratio of the light emitting component emitted from the end face Was relatively large, and the present invention was completed. That is, according to the first aspect of the present invention, the end face of the light emitting region is etched inside the p main electrode, in addition to the first end face light emitting section in the peripheral area of the element, which is the end face light emitting section in the ordinary light emitting element. By forming the second end face light emitting portion (light extraction groove and current constriction groove) exposed by the processing, the second end face light emission portion functions as a light extraction window for light emitted immediately below the p main electrode, and the auxiliary electrode The light emitted below propagates directly below the p main electrode and acts so as not to be absorbed by the p main electrode, and also prevents an electrical short circuit between the pn electrodes, thereby preventing electrons from flowing into the lower part of the p main electrode. Suppression suppresses light emission immediately below the p main electrode, and as a result, current can be diffused over the entire surface of the epi and the entire surface of the epi can be illuminated. Therefore, the light extraction efficiency is improved, and the total amount of the actual light emission is also improved. As a result, the external quantum efficiency can be significantly improved.
[0021]
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the nitride semiconductor light emitting device of the present invention. FIG. 1 (a) is a plan view of the nitride semiconductor light emitting device viewed from the lamination surface side, and FIG. FIG. 4 shows a cross-sectional view taken along the line A ′. In the figure, 1 is a substrate, 21 is an n-type gallium nitride compound semiconductor layer, 22 is a p-type gallium nitride compound semiconductor layer, and 20 is an active layer. Hereinafter, the production method of the present invention will be described in detail.
[0022]
As a semiconductor wafer, a nitride semiconductor layer having a structure to be an LED (light emitting diode) was formed on a spinel substrate. Specifically, a GaN buffer layer, an n-type GaN contact layer, an n-type AlGaN cladding layer, an InGaN active layer having a multiple quantum well structure, a p-type GaN cap layer, a p-type AlGaN And a p-type GaN contact layer. A current blocking groove and a light extraction groove are formed from the epi-face side of the semiconductor wafer. Etching (including both dry etching and wet etching) is suitable as a method for forming the groove, and an optical method using laser irradiation or a mechanical method such as a dicer or a scriber is also possible.
[0023]
(
The
[0024]
Nitride semiconductor (In X Ga Y Al 1- X-Y N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1) can be variously formed by a MOCVD method or HVPE method. A nitride semiconductor can be used as a semiconductor element by forming a PN junction, a PIN junction, and a MIS junction. The structure of the semiconductor can be variously selected such as a homo junction, a hetero junction, and a double hetero junction. In addition, a single quantum well structure or a multiple quantum well structure in which the semiconductor layer is thin enough to produce a quantum effect can be used.
[0025]
[0026]
The order of forming the grooves may be immediately before or after forming the electrodes. If the groove is formed immediately before forming the electrode, it is necessary to mask the electrode material during the formation of the second p auxiliary electrode so that the electrode material does not enter the groove. In addition, if the groove is formed immediately after the formation of the electrode, it is necessary to form the groove together with the second p auxiliary electrode, and the method of forming the groove is limited.
[0027]
The current confinement groove and the light extraction groove may be formed separately, but it is not particularly necessary to form them separately, and the steps can be simplified by forming them simultaneously. As a prior art, there is an example in which a light extraction groove is formed at the time of exposing an n-contact layer for forming an n-electrode (JP-A-2002-164574, JP-A-2002-26386, etc.). Since the current confinement groove needs to be deeper than the exposed surface of the n-contact layer in order to exhibit the effect, it is difficult to form the groove in the same process. Therefore, it is preferable to perform the step in a step different from the step of exposing the n-contact layer, although it depends on the method of forming the groove.
[0028]
【Example】
(Example 1)
Using a 425 μm-thick and washed sapphire substrate as a substrate, a nitride semiconductor was laminated by MOCVD to form a nitride semiconductor wafer. The nitride semiconductor was formed as a multilayer film so as to be a light emitting element. First, a buffer layer having a thickness of about 200 Å was formed at 510 ° C. by flowing NH 3 (ammonia) gas, TMG (trimethylgallium) gas, and hydrogen gas as a carrier gas as source gases.
[0029]
Next, after stopping the inflow of the TMG gas, the temperature of the reactor was raised to 1050 ° C., and again a TMG gas and a SiH 4 (silane) gas as a dopant gas were allowed to flow, so that GaN having a thickness of about 6 μm serving as an n-type contact layer was formed. A layer was formed.
[0030]
The active layer is once made only of carrier gas and NH 3, and the temperature of the reactor is kept at 800 ° C., then NH 3 (ammonia) gas, TEG gas, TMI (trimethylindium) as a source gas and nitrogen gas as a carrier gas. , A barrier layer made of undoped GaN is grown to a thickness of 200 angstroms, then the temperature is set to 800 ° C., and a well layer made of undoped In 0.4 Ga 0.6 N using TMG, TMI, and ammonia. Is grown to a thickness of 30 angstroms. An
[0031]
After the flow of TMG and TMI was stopped to form a cladding layer on the active layer and the temperature of the reactor was maintained at 1050 ° C., NH 3 (ammonia) gas, TMA (trimethylaluminum) gas, TEG gas, A Cp 2 Mg (cyclopentadiermagnesium) gas as a dopant gas and a nitrogen gas as a carrier gas were passed to form a GaAlN layer having a thickness of about 0.1 μm as a p-type cladding layer.
[0032]
Finally, the temperature of the reactor is maintained at 1050 ° C., and NH 3 (ammonia) gas, TMG gas, Cp 2 Mg gas as a dopant gas, and hydrogen gas as a carrier gas are flowed as a source gas, and a thickness of about 0 is formed as a p-type contact layer. A 0.5 μm GaN layer was formed. (Note that the p-type nitride semiconductor layer has been annealed at 400 ° C. or higher.)
The
[0033]
Based on the reference A, a current confinement groove was formed vertically to the substrate without making an angle (FIG. 1A). As a result, the extraction efficiency of 5% light was increased as compared with Reference A.
[0034]
(Example 2)
In addition to Example 1, a light extraction groove was formed vertically on the line connecting the p main electrode and the n electrode to the substrate as shown in FIG. As a result, the light extraction efficiency increased by 12% as compared with Reference A.
[0035]
(Example 3)
In addition to Example 2, a light extraction groove was formed almost perpendicularly to the element end face to the substrate without making an angle as shown in FIG. As a result, 16% light extraction efficiency was increased as compared with Reference A.
[0036]
(Example 4)
In Example 3, the current constriction groove and the light extraction groove were formed perpendicular to the epitaxial growth surface side of the substrate, but were tapered. A taper angle of 50 ° was formed with respect to the vertical plane. As a result, 24% light extraction efficiency was increased as compared with Reference A.
[0037]
(Example 5)
The same processing as in Example 4 was performed and formed on a specially processed substrate. Using the pattern shown in FIG. 6A, the length of one side of an equilateral triangle is 10 μm, the interval between adjacent triangles is 4 μm, the depth of the unevenness is 0.8 μm, and the taper angle of the unevenness is 40 ° from the vertical direction. Was. As a result, the light extraction efficiency increased by 41% compared to Reference A.
[0038]
(Example 6)
While Examples 1 to 5 were cut out as chips of 350 μm square, hereafter, examples using 1 mm square chips will be described. The semiconductor layers are stacked in the same manner as in the first embodiment, and both pn electrodes are formed in an electrode arrangement as shown in FIG. This state is referred to as reference B. As shown in FIG. 4, a current constriction groove and a light extraction groove are formed in the reference B in a mesh shape. At this time, the groove width was 29 μm, the groove depth was up to the substrate, and the groove was formed perpendicular to the substrate without forming a taper. As a result, 40% light extraction efficiency was increased as compared with Reference B.
[0039]
(Example 7)
In the same manner as in Example 6, a groove was formed except that a taper angle of 50 ° was formed in a direction perpendicular to the substrate. As a result, the light extraction efficiency was increased by 62% as compared with Reference B.
[0040]
(Example 8)
The same processing as in Example 7 was performed and formed on a specially processed substrate. Using the pattern shown in FIG. 6A, the length of one side of an equilateral triangle is 25 μm, the interval between adjacent triangles is 10 μm, the depth of the unevenness is 0.8 μm, and the taper angle of the unevenness is 40 ° from the vertical direction. Was. As a result, the light extraction efficiency increased by 66% as compared with Reference A.
[0041]
【The invention's effect】
The nitride light-emitting device of the present invention, according to the first aspect of the present invention, further includes an inner surface of a p-side main electrode in addition to a first end-surface light-emitting portion in a peripheral portion of the device, which is an end-surface light-emitting portion in a normal light-emitting device. Forming the second end face light emitting portion (light extraction groove and current constriction groove) in which the end face of the light emitting region is exposed by etching, the second end face light emitting portion suppresses the inflow of electrons directly below the p main electrode. Further, light emission immediately below the p main electrode can be suppressed, and light emitted below the second p auxiliary electrode can be prevented from entering directly below the p main electrode. Even if light is emitted immediately below the p main electrode, light is emitted. It acts as an extraction window and prevents an electrical short circuit between the pn electrodes. As a result, current can be diffused over the entire surface of the epi and the entire surface of the epi can be illuminated. Therefore, the light extraction efficiency is improved, and the total amount of the actual light emission is also improved. As a result, the external quantum efficiency can be significantly improved.
[Brief description of the drawings]
FIG. 1 is a view showing an example of a nitride semiconductor light emitting device of the present invention, wherein FIG. 1 (a) is a schematic plan view thereof, and FIG. 1 (b) is a schematic view taken along line AA ′ of FIG. 1 (a). It is sectional drawing.
FIG. 2 is a schematic plan view showing another example of the nitride semiconductor light emitting device of the present invention.
FIG. 3 is a schematic plan view showing another example of the nitride semiconductor light emitting device of the present invention.
FIG. 4 is a schematic plan view showing another example of the nitride semiconductor light emitting device of the present invention.
FIG. 5 is a schematic plan view showing an example of a conventional nitride semiconductor light emitting device.
FIG. 6 is a schematic pattern of a specially processed substrate that can be used for the nitride semiconductor light emitting device of the present invention.
FIG. 7 is a schematic plan view showing an example of a conventional nitride semiconductor light emitting device.
[Explanation of symbols]
DESCRIPTION OF
Claims (12)
p主電極に近接して電流阻止溝が形成され、該溝が少なくともn電極形成面よりも深いことを特徴とする窒化物半導体発光素子。At least an n-type gallium nitride-based compound semiconductor layer, an active layer, and a p-type gallium nitride-based compound semiconductor layer are sequentially stacked on a substrate, and at least one pair of pn electrodes are formed in the same plane, and the n-electrode is an n-type. A p-electrode is formed on the n-electrode formation surface partially exposing the gallium nitride-based compound semiconductor layer, and a p-electrode for diffusing current from the p-main electrode to the entire surface of the p-type gallium nitride-based compound semiconductor layer. In a nitride semiconductor light emitting device including a p auxiliary electrode,
A nitride semiconductor light emitting device, wherein a current blocking groove is formed close to a p main electrode, and the groove is deeper than at least an n electrode forming surface.
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