JP3707446B2 - White light emitting device - Google Patents

Description

【００３０】
というボーズ統計に従う。単位体積当たりの光子のエネルギーの平均値Ｅｍ＝＜ｈν＞を求めたい。波数空間の微小体積ｄｋ_ｘｄｋ_ｙｄｋ_ｚ＝１について一つの状態がある。振動数νはν＝ｃ／λ＝ｃｋ／２πである。ｋ空間において球対称なのでｄｋ_ｘｄｋ_ｙｄｋ_ｚ＝４πｋ^２ｄｋ＝３２π^４ν^３ｄν／ｃ^３であるから、確率分布は
【００３１】
【数２】

【００３２】
というように書くことができる。それが黒体輻射の光子のエネルギー分布である。それにｈνを掛けて積分すると、黒体輻射（白熱電球など）の分布となる。分母の絶対温度Ｔが色温度である。
【００３３】
分布最大を示す振動数を、νｍａｘや平均の振動数νｍとすると、それらはＴに比例する。νｍａｘ／Ｔ＝ｃｏｎｓｔ１、νｍ／Ｔ＝ｃｏｎｓｔ２となる。平均値で言えばνｍ＝ｃｏｎｓｔ２×Ｔとなる。
【００３４】
黒体輻射でない場合、分布が式（１）ではない。スペクトルをｇ（ν）として式（１）のｆ（ν）に置き換えて、νの平均値を計算する。それをνｍとし、それをｃｏｎｓｔ２で割ると色温度Ｔを計算できる。色温度がＴの白色というのは黒体輻射で分布関数の温度がＴである場合のエネルギー分布と同じ平均値を与えるという意味である。だから黒体輻射から離れたスペクトルの白色でも色温度を定義でき計算もできる。
【００３５】
白色を評価するもう一つのパラメータは演色性である。それは黒体輻射（３）からのズレを表したものである。黒体輻射は当然演色性が１００％で、白熱電球がこれにあたる。本発明では演色性をあまり問題にしないから演色性については述べない。
【００３６】
先述の（Ａ）ＧａＮ系白色発光素子のＩｎＧａＮ‐ＬＥＤは図３の色度図において、４６０ｎｍの青色（ｍ点）を発生し、ＣｅドープＹＡＧ蛍光剤は青色光を吸収して５６８ｎｍにピークのある黄色（ｄ点）を発生する。だから（Ａ）ＧａＮ系白色発光素子（ＹＡＧ＋ＩｎＧａＮ‐ＬＥＤ）は直線ｍｄ上の点に対応する複合的な色を生成できる。直線ｍｄは白色領域Ｗの左端を横切る。だから白色を作り出すことができる。先述の７０００Ｋの白色というのはＷの内部のＸ＝０．３１、Ｙ＝０．３２の点である。色温度がかなり高い白色になるのはＩｎＧａＮ‐ＬＥＤの発光する光が青色光といっても波長が短かいからである。
【００３７】
もう一つの（Ｂ）ＺｎＳｅ系白色発光素子（ＺｎＣｄＳｅ／ＺｎＳｅ基板）は活性層のＺｎＣｄＳｅが４８５ｎｍの青色光を発生し図３の色度図のｊ点に対応する。不純物（Ａｌ、Ｉｎ、Ｂｒ、Ｃｌ、Ｇａ、Ｉ）ドープＺｎＳｅの蛍光は５８５ｎｍ程度の黄色光の蛍光を発生する。図３においてそれはｃ点にあたる。活性層からの４８５ｎｍの青色光（ｊ点）と、ＺｎＳｅ基板からの５８５ｎｍの黄色光（ｃ点）が合成されると直線ｊｃ上の任意の色を作り出すことができる。好都合なことに、この直線ｊｃは白色領域Ｗを左から右まで横切る。ということはＺｎＳｅ厚み、不純物濃度を変化させて様々の色温度の白色を作り出す事ができるということである。
【００３８】
ここで１００００Ｋ、８０００Ｋ、７０００Ｋ、６０００Ｋ、５０００Ｋ、４０００Ｋ、３０００Ｋ、２５００Ｋの色温度の白色の座標を点によって示した。そのようにＺｎＳｅ系白色発光素子は直線ｊｃの傾きがゆるくて白色領域を長く横切る。そのおかげで多様な色調（色温度）の白色を作ることができる。その点で（Ａ）のＧａＮ系白色発光素子より便利である。
【００３９】
【発明が解決しようとする課題】
［１．ＺｎＳｅ系白色発光素子の利点と欠点］
ＺｎＳｅ系白色発光素子の色の合成を色度図（図３）で見ると、ＺｎＣｄＳｅ‐ＬＥＤの青色光Ｂ（４８５ｎｍ、ｊ点）とＺｎＳｅ基板の黄色光Ｙ（５８５ｎｍ、ｃ点）を結んだ直線ｊｃが、白色光の軌跡（１００００Ｋ〜２５００Ｋ）と、ほぼ一致している。ＺｎＳｅ基板厚みを変える、不純物濃度を変えるなどして青色光Ｂと黄色光Ｙの割合を変えるだけで任意の色温度（１００００Ｋ〜２５００Ｋ）の白色を得ることができる。素子構造が小型、簡単であり電極も単純であるなどの利点はある。
【００４０】
しかしながらＺｎＳｅ系ＬＥＤは劣化しやすく寿命が短いことが欠点である。発光のため大電流を流すので欠陥が増加して劣化が進行する。劣化すると発光効率が低下する。やがて発光しなくなる。短命であること、それはＺｎＳｅ基板上の発光素子に共通の難点である。
【００４１】
また青色光Ｂと黄色光Ｙを混ぜて白色Ｗを合成する場合、必要な青色光Ｂと黄色光Ｙの比率が問題である。エネルギーの高い４４５ｎｍ近辺の波長の青色光を使用したとき必要な青色光の比率が最も小さくなる。エネルギーのより低い４８５ｎｍ近辺（ｊ点）の青色光を使用したとき、４４５ｎｍの青色光と比べて２倍程度の青色光が必要となる（Ｂ：Ｙ＝２：１）。
【００４２】
ここで青色光は黄色光と比べて視感度が低いので青色光の比率が小さい方が発光効率が高くなる。したがって、青色光をより多く必要とするＺｎＳｅ系の白色発光素子は効率の点で不利である。
【００４３】
［２．ＧａＮ系白色発光素子の利点と欠点］
それに対して（Ａ）ＧａＮ系（ＩｎＧａＮ‐ＬＥＤ＋ＹＡＧ）の白色発光素子では青色が４６０ｎｍ〜４４５ｎｍでエネルギーが高く、必要なパワーの比はＢ：Ｙ＝１：１であり、青色がＺｎＳｅ系に比べ半分で済む。だから効率の点で有利である。ＧａＮ系の発光素子が長寿命であるから、それを利用した白色発光素子も長寿命である。そのような長所がある。
【００４４】
しかし反面、欠点もある。ＹＡＧ＋ＩｎＧａＮ‐ＬＥＤ白色発光素子は、図３から分かるように、青色光ｍと黄色光ｄを結んだ直線ｍｄが白色の軌跡（１００００Ｋ〜２５００Ｋ）に対して７０００Ｋで接するような勾配を持っているので、任意の色温度の白色を合成することができない。７０００Ｋ付近より低い色温度（６０００Ｋ、５０００Ｋ、２５００Ｋ等）の白色を合成できない。
【００４５】
一般に白色電球の色温度は３５００Ｋ近辺の低い色温度である。ＩｎＧａＮ系白色発光素子は、白色電球とは異なった色温度の白色しか実現できない。つまり照明用の白色発光素子としては使えない。液晶バックライトに利用できる可能性はあるが色温度が高すぎるという欠点がある。
【００４６】
そのため優れた特性（寿命、効率）を有しているにもかかわらず、白色電球の代替が充分に進んでいない。
【００４７】
色温度のより低い、具体的には６０００Ｋよりも低い色温度の白色を作り出せる小型の白色発光素子を提供することが本発明の第１の目的である。長寿命の白色発光素子を提供することが本発明の第２の目的である。長寿命でなければ白熱球や蛍光灯を代替することができない。
【００４８】
【課題を解決するための手段】
１． 本発明は、ＩｎＧａＮ‐ＬＥＤの上にＺｎＳＳｅ塊状蛍光板を積み重ねた白色発光素子を提案する。波長の短い青色光を発光するＩｎＧａＮ‐ＬＥＤ発光の一部をＺｎＳＳｅ結晶からなる塊状の蛍光板によって黄色光に変換し、青色光Ｂと黄色光Ｙを混ぜ合わせる事によって白色Ｗ（Ｗ＝Ｂ＋Ｙ）を合成する。
【００４９】
２． ＩｎＧａＮ‐ＬＥＤによって発生する青色光の波長を４１０ｎｍ〜４６０ｎｍにする。これは青色光でも短波長の方であり図３の色度図において左下のｍｎの部分の発光に対応する。そのような短波長の青色光はＺｎＳｅ系（ＺｎＣｄＳｅ活性層：４８５ｎｍ；ｊ点）では作れない。それでＧａＮ系（ＩｎＧａＮ活性層）のＬＥＤを用いる。ＩｎＧａＮ／サファイヤ‐ＬＥＤは実績、寿命、コスト、信頼性の点でも使いやすいものである。だから本発明は、ＬＥＤの点では先述の従来技術（Ａ）ＹＡＧ＋ＩｎＧａＮ‐ＬＥＤのものと共通する。しかし蛍光材がＹＡＧでない。新規な物質を用いる。
【００５０】
３． 黄色光の波長を５７０ｎｍ〜５８０ｎｍにする。これが本発明の新規な着想を波長によって表現したものである。先述の従来技術（Ａ）ＹＡＧ＋ＩｎＧａＮ‐ＬＥＤは５６８ｎｍ（ｄ点）の蛍光を発生するＣｅドープＹＡＧを使っているから７０００Ｋの白色しか合成できない。ｄ点は黄色光というよりは黄緑に近い。本発明はもっと波長の長い赤に近い黄色光を発生させる。色度図で５７０ｎｍ（ｖ点）と５８０ｎｍ（ｕ点）の間の範囲のより赤に近い黄色を発生させる。そのようにすると青色光の方では線分ｎｍの範囲で発光し、黄色光の方では線分ｕｖの範囲で蛍光を発する。線分ｎｍと線分ｕｖを結んだ直線が本発明の白色発光素子の発光に対応する。線分ｎｍと線分ｕｖを結んだ直線（一点鎖線）は、ＹＡＧに対応する線分ｍｄより深く白色領域に入る。そのように直線の黄色側を少し下げたところに本発明の工夫の一つがある。
【００５１】
４． 黄色光側を少し下げる（赤色側へずらす）ためにはＹＡＧ以外の新規な蛍光材が必要である。本発明はＺｎＳｅとＺｎＳの混晶であるＺｎＳＳｅ結晶を用いる。高純度のＺｎＳＳｅは蛍光を発しない。発光中心となるドーパントが必要である。ドーパントはＡｌ、Ｉｎ、Ｇａ、Ｃｌ、Ｂｒ、Ｉのいずれかである。本発明で用いる蛍光材は、ＺｎＳＳｅ結晶中にＡｌ、Ｉｎ、Ｇａ、Ｃｌ、Ｂｒ、Ｉの少なくとも１元素以上の不純物（ドーパント）が１×１０^１７ｃｍ^−３以上の濃度含まれているものである。これ以下だと蛍光を充分に発生しない。ドーパント濃度を変えたり、蛍光材の厚みを変えることによって黄色光（ｕｖ）の比重を変更させることができる。つまり色度図上の青色光・黄色光の線分（ｍｎ〜ｕｖ）において色を示す点を線分に沿って上下させることができる。ＹＡＧを使う従来例（Ａ；ｍｄ）より線分が白色領域に深く進入しているから、いろいろな色温度の白色を合成できる。蛍光材厚みを増やす、ドーパント濃度を上げることによって黄色光（ｕｖ）の比重を高め低い色温度の白色を作ることができる。
【００５２】
ドーパントとしてＡｌ、Ｉｎ、Ｇａ、Ｃｌ、Ｂｒ、Ｉの何れかを使うのはＺｎＳｅ基板を蛍光板として使う従来例（Ｂ）と同じである。しかし本発明はＺｎＳｅではなく、ＺｎＳＳｅを蛍光板とする。さらにＬＥＤがＺｎＣｄＳｅでなくＩｎＧａＮである。
【００５３】
５． ＺｎＳｅはバンドギャップが狭く、ＺｎＳはバンドギャップが広いので、混晶比ｘによってその中間のバンドギャップのものを作ることができる。本発明の蛍光材の材料は、ＺｎＳＳｅ結晶中のＺｎＳの組成比をｘ、ＺｎＳｅの組成比を（１−ｘ）とし、ｘを０．１〜０．４にする。正しくは混晶比を入れてＺｎＳ_ｘＳｅ_１−ｘ（０．１≦ｘ≦０．４）と書けばよいのであるが簡単のため混晶比ｘを略している。ｘ＝０ならＺｎＳｅが蛍光板となり従来例（Ｂ）の蛍光板と同一になる。しかし、それを蛍光板とすると、青色ｋｎと黄色ｃを結ぶ線分（ｋｎ〜ｃ）の色しかできないことになり白色領域Ｗの下側へずれてしまう。広い範囲の任意の色温度の白色を作る事ができない。だからＺｎＳｅ（ｘ＝０）は否定されるのである。
【００５４】
かといって全部をＺｎＳ（ｘ＝１）にすると蛍光波長が短すぎて従来例（Ａ）ＹＡＧ＋ＩｎＧａＮ‐ＬＥＤと同様に高い色温度の白色しかできないようになる。それで本発明は混晶比をｘ＝０．１〜０．４とする。蛍光の波長はバンドギャップそのものではない。その関係は後で述べる。
【００５５】
６． 先述のようにＺｎＳＳｅは塊状のものを用いる。これが本発明の重要な工夫である。粉末状ではいけない。さらに言えば蛍光板を構成するＺｎＳＳｅ結晶の平均粒径を蛍光板の厚みより大きくするのが望ましい。従来例（Ａ）のＹＡＧはＹＡＧ（yttrium aluminum garnet）の粉末を透明な樹脂に分散したものを蛍光剤として利用している。そもそも蛍光板というのは蛍光を発する材料の微粉末を透明のガラス、樹脂に分散させたものが多い。粉末ではなく塊にすると光は内部へ入らないのだから内部の蛍光剤は無駄である。できるだけ光が当たり易く変換効率が良くなるように蛍光剤は微粉末とし透明樹脂、ガラスに分散する。透明樹脂に光が通り蛍光粉末に光が当たり易く、蛍光物質濃度の調整が容易であるし、任意の形状に賦形（造形）できるからである。
【００５６】
しかし前記の不純物をドープしたＺｎＳＳｅ多結晶の微粉末を樹脂に分散させるとＹＡＧなどにない不都合があるということが分かった。ＺｎＳＳｅは吸水性がある。粉末にして樹脂に分散すると樹脂に水が進入し粉末ＺｎＳＳｅが水を吸って劣化しやすい。そのような難点を克服しなければ蛍光剤として利用できない。そこで従来の蛍光剤と違って粉末ではなく、塊状の多結晶、単結晶のＺｎＳＳｅを使うことにする。ここが本発明において重要である。塊状にすると表面積が狭いので水が入りにくく入っても表面の近くに留まる。吸水による劣化も表面近傍に限定される。そのような訳で塊のＺｎＳＳｅを用いる。
【００５７】
多結晶の場合でも粒径が大きい方が良い。それは粒界に水が進入しやすいということもあるが、それだけではない。粒界で光が乱反射され吸収されることがあり光学的な損失の原因になる。それで粒界が大きい方が良いのである。多結晶の平均粒径が蛍光板の厚み以上のものがより適している。この場合どの粒塊（grain）も厚み方向には単結晶を保ち、平均粒径は蛍光板の面方向において定義される。
【００５８】
７． より好ましくはＺｎＳＳｅ蛍光板を単結晶ＺｎＳＳｅによって構成する。多結晶の粒界（boundary）は光学的な損失の原因となるから粒界はない方が良い。粒界がない理想的なものと言えば単結晶である。だから不純物ドープＺｎＳＳｅ単結晶が本発明の蛍光板として最適である。そうはいうもののＺｎＳＳｅ単結晶は簡単に作れない。化学輸送法で作ることができるが時間がかかり高コストである。コストを下げるという点では塊状の多結晶のＺｎＳＳｅを用いることになろう。
【００５９】
８． 青色光発光ＬＥＤとして発光波長４１０ｎｍ〜４６０ｎｍのＩｎＧａＮ系ＬＥＤを用いる。これは既に述べているが色度図において青色光の発光領域をｍｎにし黄色光と青色光を結ぶ線分が白色領域Ｗを深く横切るようにするためである。それによって７０００Ｋより低い色温度の白色を得る。
【００６０】
９． ＺｎＳＳｅ結晶中のＺｎＳの組成比をｘ、ＺｎＳｅの組成比を（１−ｘ）とし、青色光発光ＬＥＤの発光波長をλ_ＬＥＤとしたとき、λ_ＬＥＤ≧１２３９／（２．６５＋１．６３ｘ−０．６３ｘ^２）ｎｍとするのが望ましい。ＺｎＳｅのバンドギャップは２．７ｅＶで吸収端波長が４６０ｎｍである。ＺｎＳのバンドギャップは．３．７ｅＶで吸収端波長は３３５ｎｍである。発光波長λＬＥＤの式の中は２．６５となっており、バンドギャップは２．７となっている。混晶ＺｎＳ_ｘＳｅ_１−ｘのバンドギャップは近似的にＥｇ＝２．７＋１．６３ｘ−０．６３ｘ^２によって与えられる。バンドギャップで１２３９（＝ｈｃ）を割ると吸収端波長をｎｍ単位で表現したものになる。つまり上の式は本発明で使う蛍光材の混晶ＺｎＳＳｅのバンドギャップより低いエネルギーを持つ（長波長の）青色光で蛍光材を励起するのが良いと言っているのである。それは色度図上でｍｎ〜ｕｖが白色領域Ｗを縦断するというのとは全く別の話しである。少し複雑であるが、この条件はＩｎＧａＮ‐ＬＥＤの青色光がＺｎＳＳｅ蛍光材の表面で吸収されず内部にまで到達して内部で吸収されて蛍光を発生するという条件である。半導体はバンドギャップより高いエネルギー（短波長）の光をすぐに吸収してしまう。塊状としたといっても蛍光材の表面は吸水の為劣化している可能性がある。だから表面を使いたくない。内部まで青色光が到達して内部でドーパントを励起して発光するようにしたいものである。そのため吸収されにくいＺｎＳＳｅのバンドギャップよりも低いエネルギーの青色光を用いるということである。
【００６１】
１０． Ｚｎ雰囲気で熱処理を施したＺｎＳＳｅ結晶を蛍光板として使用するのが良い。熱処理によって欠陥が減少し散乱や非蛍光吸収が減少するからである。
【００６２】
【発明の実施の形態】
７０００Ｋより低い色温度の白色を発生させたいという上記の課題を解決するためには、発光波長４４５ｎｍ（ｍｎ間点）近辺のＬＥＤから放出された青色光の一部を、中心波長５７５ｎｍ（ｕｖの中点）近辺の光に変換する蛍光材が存在すればよいことになる。
【００６３】
４４５ｎｍを出すＩｎＧａＮ‐ＬＥＤは存在し市販されており入手可能である。蛍光材が問題である。５７５ｎｍの波長を出す蛍光材が必要である。従来例（Ａ）のＣｅ‐ＹＡＧの蛍光は中心波長が５６８ｎｍ（ｄ点）である。それは短すぎる。従来例（Ｂ）で述べたように、ＺｎＳｅ結晶に３族元素や７族元素（Ａｌ、Ｉｎ、Ｇａ、Ｂｒ、Ｃｌ、Ｉ）を混入させた場合の発光波長は５８５ｎｍ（ｃ点）近辺である。これは長すぎる。ｄ点とｃ点の中間のｕ〜ｖの波長の蛍光が欲しい。
【００６４】
Ｃｅ‐ＹＡＧのような通常の蛍光剤はＣｅなどのドーパント濃度しかパラメータがないので蛍光の中心波長を変えることができない。Ｃｅ濃度を増やすと線分ｍｄ上で色座標がｄ点に近付くだけでありｄ点自体を動かすことはできない。それはＹＡＧのマトリックスの中で、Ｃｅ自体が孤立した色中心として働いているからである。
【００６５】
しかしＺｎＳｅとＺｎＳの混晶であるＺｎＳ_ｘＳｅ_１−ｘ結晶ではドーパント（Ａｌ、Ｉｎ、Ｇａ、Ｂｒ、Ｃｌ、Ｉ）の他に混晶比ｘが自由に選べるパラメータとして存在する。ｘ＝０のＺｎＳｅはバンドギャップＥｇ_ＺｎＳｅ＝２．７ｅＶ、ｘ＝１のＺｎＳはバンドギャップがＥｇ_ＺｎＳ＝３．７ｅＶである。つまり１ｅＶ程度大きい。ｘによってバンドギャップは変化する。ＺｎＳＳｅの蛍光はドナー・アクセプタ遷移によるらしいということが分かってきた。３族、７族ドーパントを入れることによって比較的深いドナー、アクセプタの両方が生成される。そのドナー・アクセプタ遷移によって蛍光が出る。だから蛍光中心波長Λｑはバンドギャップ波長λｇ（＝ｈｃ／Ｅｇ）よりかなり長いものとなる。
【００６６】
すると、バンドギャップＥｇを変えるとドナー・アクセプタ遷移による蛍光の波長Λｑも変化させることができるということである。蛍光中心波長ΛｑがバンドギャップＥｇによるということがＺｎＳＳｅの便利な点である。これは受動的な蛍光の話しでＺｎＳｅを能動的なＬＥＤとする場合の話ではない。複雑であるが両者を混同してはいけない。
【００６７】
不純物ドープＺｎＳｅの蛍光中心波長Λｑ_ＺｎＳｅ＝５８５ｎｍであり、所望の蛍光中心波長が５８０ｎｍ〜５７０ｎｍ（ｕ〜ｖ間）であるから１０ｎｍほど短くすれば良いだけである。ＺｎＳを蛍光材とした場合の蛍光波長は４７０ｎｍ近辺である。ＺｎＳＳｅバンドギャップと蛍光波長はｘによって連続的に変化するであろう。だとすれば、所望の蛍光波長５７０ｎｍ〜５８０ｎｍはｘを適当に選んだ混晶ＺｎＳ_ｘＳｅ_１−ｘによって得られる筈である。
【００６８】
そこでＺｎＳの組成比ｘを適当に選び、そのＺｎＳ_ｘＳｅ_１−ｘ結晶に３族元素や７族元素を混入させると５７５ｎｍ近辺での蛍光を得ることは可能である筈である。好適なｘの値については後に述べる。
【００６９】
ここで混入密度が小さいと充分な発光を得ることができない。１×１０^１７ｃｍ^−３程度以上の不純物（ドーパント）の混入（ドーピング）が必要である。これ以上の濃度であって濃度を増やすと黄色光の比重を高め、濃度を減らすと青色光の比重を高めることができる。蛍光材の厚みを変化させることによっても、そのようなことは可能である。
【００７０】
ただしＺｎＳｅ系やＺｎＳＳｅ系やＺｎＳ系の蛍光材料は耐水性に欠けるという問題がある。経年変化によって水を吸収し劣化する。そのためもあってＺｎＳＳｅ系の材料を蛍光剤としてＩｎＧａＮ系ＬＥＤと組み合わせた白色発光素子は実用化されていなかった。ＹＡＧなど通常の蛍光剤は粉末を透明樹脂、透明ガラスに分散して使う。微粉末ＺｎＳＳｅは直ちに吸水し、たちまち劣化し使用できなくなる。吸水劣化の問題を克服しなければならない。
【００７１】
ここで耐水性の問題であるが、蛍光剤の表面積が相対的に大きい場合に問題となる。耐水性を高めるためには表面積を極力狭くするのが効果的である。半径ｒの球の表面積は４πｒ^２であり、体積は４πｒ^３／３である。表面積／体積の比は３／ｒである。その比を下げようと思えば半径ｒを大きくすれば良いのである。
【００７２】
そのためには粉末状のＺｎＳＳｅを使用するより、大きい塊状の単結晶もしくは多結晶ＺｎＳＳｅの蛍光板を使用すれば良い。塊状の蛍光材というのは自己矛盾のようで聞き慣れないが、ＺｎＳＳｅは塊状にすれば蛍光材として利用できよう。そうすれば、蛍光材体積に対する表面積の割合が非常に小さくなるので、耐水性が格段に向上する筈である。
【００７３】
通常、粉末だから「蛍光剤」と書くのである。本発明では微粉末でない塊状のＺｎＳＳｅを使うから「蛍光材」または「蛍光板」と書くことにしよう。
【００７４】
ただし、仮に上記のような蛍光板を使用しても蛍光板表面近傍で青色光が全て吸収され、蛍光板内部に青色光が進入することなく、蛍光板の表面のみが蛍光を発生するような場合は、表面の影響が強く出るので、やはり耐水性の問題が顕在化する。それにＬＥＤ光が蛍光板表面で殆ど吸収されるならば内部の蛍光材は無駄になり非効率である。そもそも通常の蛍光体で蛍光剤を粉末とし樹脂に分散するのは全ての蛍光剤に光が当たるようにするための工夫であった。本発明では塊状の蛍光板としているのだから、光が表面に留まらず内部まで入るようにすることが必要である。それが通常の蛍光剤と多いに事情の異なるところである。内部までＬＥＤ光を入れるにはどうすれば良いのか？
【００７５】
ＬＥＤの青色光に対し、ＺｎＳＳｅが殆ど透明であれば良いのである。通常の蛍光剤は不透明でそんなことはないが本発明では塊状の蛍光板を使うからＬＥＤ光に対し透明のものを用いる。では透明にするにはどうすれば良いか？本発明者は、蛍光板を構成するＺｎＳＳｅ結晶の禁制帯幅（バンドギャップ；Ｅｇ）より小さなエネルギーを持った青色光を使用すれば良い、ということに気付いた。
【００７６】
幸運なことにＺｎＳはバンドギャップが広くて、ＩｎＧａＮ‐ＬＥＤの出すＬＥＤ光のエネルギーよりも高い。ＺｎＳｅのバンドギャップはＩｎＧａＮ‐ＬＥＤの光のエネルギーより低い。適当な混晶比ｘで、ＩｎＧａＮ青色発光素子の発光波長λ_ＬＥＤに対応するエネルギーに等しいバンドギャップＥｇをもつＺｎＳＳｅが存在する。その臨界混晶比より大きい混晶比ｘをもつＺｎＳＳｅを蛍光材とすれば、バンドギャップが広くなりＬＥＤ光に対し透明になる。ＬＥＤ光は蛍光板の内部まで浸透できるはずである。塊状の蛍光材をＬＥＤ光に対し透明にするという課題はそれによって鮮やかに解決される。
【００７７】
そうすれば、青色光に対する蛍光板の吸収係数が小さくなり、蛍光板内部にまで青色光が進入し、蛍光板全体で青色光が発光することになる。劣化した表面の影響が小さくなるし、内部の蛍光材も有効に利用できる。
【００７８】
反対に青色光ＬＥＤの発光波長λ_ＬＥＤがＺｎＳＳｅ混晶蛍光材のバンドギャップよりも長い（エネルギーが低い）としてもよい。
【００７９】
ＺｎＳＳｅ結晶中のＺｎＳ組成をｘとする（ＺｎＳ_ｘＳｅ_１−ｘ）と、その禁制帯幅は
【００８０】
Ｅｇ_{ＺｎＳＳｅ}＝２．７＋１．６３ｘ−０．６３ｘ^２（ｅＶ） （４）
【００８１】
で与えられる。エネルギーをｅＶで表現し、波長をｎｍで表現するとそれらは反比例し、その比例定数は１２３９であるから、ＬＥＤ青色光波長λ_ＬＥＤとＺｎＳ組成ｘの関係として、
【００８２】
【数３】

【００８３】
を満たせばよい。これは蛍光材の組成ｘが決まったとしてＩｎＧａＮ‐ＬＥＤの波長範囲を決める不等式である。混晶比ｘを変化させてＺｎＳＳｅバンドギャップＥｇ、バンドギャップ波長λｇを（５）によって計算すると次のようになる。
【００８４】
【表１】

【００８５】
Ｉｎ_ｙＧａ_１−ｙＮ‐ＬＥＤの発光波長はＩｎの混晶比ｙを変化させることによって変動させることができる。先述にようにＩｎＧａＮの好ましい発光波長は４１０ｎｍ〜４６０ｎｍとしているが、ＩｎＧａＮはそれ以上の４８０ｎｍの光まで出すことができる。Ｉｎの比率ｙが高いと長波長側に発光波長が移動し、Ｇａの比率１−ｙが高いと短波長側へ発光波長が変化する。４１０ｎｍに対応するＺｎＳ混晶比はｘ＝０．２５２である。それより大きいｘに対してバンドギャップ波長λｇは４１０ｎｍよりも短くなる。だから１＞ｘ＞０．２５２の範囲では最早式（５）はＩｎＧａＮ‐ＬＥＤの発光波長λ_ＬＥＤを限定する条件にはならない。０＜ｘ≦０．２５２の範囲ではバンドギャップ波長が４１０ｎｍ以上であるから、（５）式がＩｎＧａＮ‐ＬＥＤの発光波長λ_ＬＥＤを限定する条件となる。
【００８６】
λ_ＬＥＤは４１０ｎｍ〜４６０ｎｍとするので、例えばｘ＝０．１の場合は４６０ｎｍ＞λ_ＬＥＤ＞４４１ｎｍ、ｘ＝０．２の場合は４６０ｎｍ＞λ_ＬＥＤ＞４２０ｎｍとなる。ｘ＝０の場合は式（５）からλ_ＬＥＤ＞４６７ｎｍとなるが、それは４６０ｎｍ以下という条件を満足しない。だからｘ＝０は不適である。色度図において蛍光波長が５７０ｎｍ〜５８０ｎｍ（ｕ〜ｖ）でなければならないという点でもｘ＝０が不適であるが、それとは別に独立の（ＬＥＤ光が蛍光板内部へ入る）条件によっても不適なのである。
【００８７】
逆にＩｎＧａＮ‐ＬＥＤの波長λ_ＬＥＤが先に決まっており、それに対する蛍光板のＺｎＳ混晶比ｘを限定するものだというようにも式（５）を解釈することもできる。
【００８８】
蛍光板のＺｎＳ組成ｘはこれだけではなくて先述のように蛍光波長Λｑが５７０ｎｍ〜５８０ｎｍの範囲でなければならないのでそのような条件が全て満足されるように決めるべきである。ＺｎＳＳｅの蛍光波長ΛｑはバンドギャップＥｇによって決まるのであるが、その関係は未だ解析的にハッキリしたものではない。後に実験の結果によって、それを説明する。
【００８９】
さてＺｎＳＳｅ蛍光板であるが単結晶であるのが最適である。単結晶では粒界が存在しないという利点がある。それに加えて微細なＺｎＳＳｅ蛍光板を作成する上で加工し易いという利点がある。つまり適当な厚みを持った面方位（１００）ＺｎＳＳｅ基板を劈開することによって、任意の大きさの立方体状のＺｎＳＳｅ蛍光板を容易に作成することができる。しかし必ずしも単結晶でなくても良い。多結晶でも構わない。多結晶では劈開によって分割できないが機械的に切断すればよい。多結晶粒界での光吸収や光散乱が変換効率を低下させてしまうので、多結晶の粒界が大きい方が好ましい。できれば平均粒界がＺｎＳＳｅ蛍光板の板厚より大きいことが望ましい。
【００９０】
ＺｎＳＳｅ蛍光板の青色光が入射する側の面は、入射効率を高めるためにミラー研磨する事が望ましい。粗面であると乱反射するからである。ＺｎＳＳｅ蛍光板のそれ以外の面に関しては必ずしもミラー研磨する必要はないが、加工上の必要に応じてミラー研磨してもよい。加えて入射面に反射防止膜を形成すればよりいっそう好ましいと考えられる。反射防止膜は透明な誘電体膜で形成する。一層でも可能であるが多層膜にすると反射防止の性能が向上する。
【００９１】
またＺｎＳＳｅ蛍光板内で発生した黄色光の出射効率を高めるための表面加工を施すのも有用である。
【００９２】
青色光の波長であるが、４４５ｎｍ近辺が有利だと説明した。しかしながら必ずしも４４５ｎｍでなければならないというものではない。ＬＥＤの青色光の発光波長の違いによる発光効率の変化や、蛍光板の変換効率の変化もあるので、本当に最適の波長は青色光発光ＬＥＤの技術動向によって変化する。
【００９３】
しかしながら、ＺｎＳＳｅ蛍光板を使用して青色光の一部を黄色光に変換するのであるから変換ということから考えると青色光発光の波長が４１０ｎｍ〜４８０ｎｍの範囲になければならない。それをはずれると明らかに効率が低下してしまう。だから４１０ｎｍ〜４８０ｎｍの範囲の波長の青色光を使用すべきである。しかし色度図を見て白色を作るということから考えるとＩｎＧａＮ‐ＬＥＤ青色光の波長は４１０ｎｍ〜４６０ｎｍとするのがよい。
【００９４】
この範囲の青色光を使用して白色を実現するためには、ＺｎＳＳｅ蛍光板の発光の中心波長は５７０ｎｍ〜５８０ｎｍにすべきである。これは色度図を見て分かることである。
【００９５】
このような中心波長をもつ蛍光を示すにはＺｎＳＳｅ結晶中のＺｎＳの組成比ｘを０．１〜０．４にすればよい。その根拠は後に述べる。
【００９６】
青色ＬＥＤとしてＩｎＧａＮ系のＬＥＤを使用する場合、現在の技術では４００ｎｍ近辺の波長で最も発光効率が高く、それよりも長波長になると発光効率が低下する。発光効率から言って青色光の波長は４６０ｎｍより短い方が好ましい。 だから青色光ＬＥＤとしてＩｎＧａＮ‐ＬＥＤを用いる場合は、その発光波長は４１０ｎｍ〜４６０ｎｍということになる。だから先述の青色光の範囲の内４６０ｎｍ〜４８０ｎｍは、白色を作る５７０ｎｍ〜５８０ｎｍ蛍光を出すことはできるがＬＥＤの効率が低くなるので省かれる部分である。
【００９７】
ＺｎＳＳｅ結晶の青色光に対する吸収係数であるが、Ｚｎ雰囲気中での熱処理の温度によって調整することができる。熱処理によって青色光の吸収が増えるようになる。したがって、白色を合成するために適当な量の青色光を黄色光に変換させるためには、この吸収係数の調整が有用である。
【００９８】
【実施例】
［実施例１（ＺｎＳ混晶比ｘによる蛍光波長の変化）］
ＺｎＳＳｅ蛍光板発光のＺｎＳ組成比（ｘ）依存性を調べるために、ｘ＝０、０．１、０．２、０．３、０．４、０．５、０．６のＺｎＳＳｅ結晶をヨウ素を輸送媒体とする化学輸送法で作製した。この結晶から切り出したＺｎＳＳｅ基板に１０００℃の温度でＺｎ雰囲気で５０時間熱処理し、ＺｎＳＳｅ蛍光板を作製した。
【００９９】
このＺｎＳＳｅ基板に波長４４０ｎｍの光を照射したときに発せられる蛍光の波長分布から、中心波長（色度図上の点）を見積もった。結果を表２に示す。
【０１００】
【表２】

【０１０１】
色度図の分析から、蛍光は５７０ｎｍ〜５８０ｎｍである事が条件となる。ＺｎＳ混晶比が０．４を越えると５７０ｎｍより短くなる。０．１以下であると５８０ｎｍを越えてしまう。この結果からＺｎＳ組成比ｘは０．１〜０．４が最適であることが判明した。蛍光の中心波長というのはＬＥＤの発光波長のように鋭いピークを持つものではない。蛍光だから、なだらかな山になり、その中心波長である。
【０１０２】
［実施例２（ｘ＝０．４、λ_ＬＥＤ＝４２０ｎｍ、Λｑ＝５７０ｎｍ）］
ＺｎＳ組成ｘ＝０．４のＺｎＳ_０．４Ｓｅ_０．６単結晶から切り出した厚み２００ミクロン、面方位（１００）のＺｎＳＳｅ基板をＺｎ雰囲気中１０００℃で熱処理した。熱処理は青色光の吸収係数を調整するために行った。このＺｎＳＳｅ基板を両面ミラー研磨して厚み１００ミクロンにした。このＺｎＳＳｅ基板をスクライブブレークして、３００ミクロン角・厚み１００ミクロンのＺｎＳＳｅ蛍光板を作製した。
【０１０３】
またサファイヤ基板を使用したＩｎＧａＮ活性層を持つ発光波長４２０ｎｍの青色ＬＥＤチップを準備した。このＬＥＤチップを図４にあるように、フリップチップ型に実装し、ＬＥＤの上側（サファイヤ基板の上側）にＺｎＳＳｅ蛍光板を透明樹脂を介して張り付けた。図４において、大きいΓ型リード２４、小さいΓ型リード２５を組み合わせている。リードは複雑な組み合わせになっており、大Γリード２４の孔に小Γリード２５が挿入されるようになっている。Γ型のリード２４には窪み２６があり、その中にＩｎＧａＮ‐ＬＥＤ２７が実装される。サファイヤ基板のＩｎＧａＮなので電極３０、３２はエピタキシャル成長面の方に設けられる。
【０１０４】
通常は図１のように２本のワイヤによって電極とリードを接続するのであるが、ここではワイヤボンディングではなくて、電極３０を大Γ型リード２４に、電極３２を小Γ型リード２５に裏返して付けている。リード２４、２５は相互に浸透し合っているが接触していない。ＩｎＧａＮ‐ＬＥＤ２７は裏返しなので青色光はサファイヤ基板の方から上に向かって発射される。サファイヤ基板の上にＺｎＳＳｅ蛍光板２８が載っている。窪み２６には拡散剤（ＳｉＣ粉末）を分散した透明樹脂が充填してある。それらをモールド樹脂３６でモールドし砲弾型の発光素子を製作した。それに通電すると、ＩｎＧａＮ‐ＬＥＤ２７から青色光が出て、それが蛍光板で黄色となる。それが透明樹脂で拡散されて広がってゆく。それによって色温度が５０００Ｋの白色を得る事ができた。図５は青色光Ｂによって黄色光Ｙが励起され青色光Ｂと黄色光Ｙが混合して外部へ出てゆき白色となる様子を示す。
【０１０５】
ＩｎＧａＮ‐ＬＥＤを使いつつ、そのような低温の色温度の白色ができるのは一つには蛍光の波長が５６８ｎｍ（Ｃｅ‐ＹＡＧ）ではなくて５６９ｎｍ（ＺｎＳ_０．４Ｓｅ_０．６）だからである。もう一つはＩｎＧａＮ‐ＬＥＤの波長を４２０ｎｍと短くしているからである。
【０１０６】
［実施例３（ｘ＝０．２５、λ_ＬＥＤ＝４４０ｎｍ、Λｑ＝５７５ｎｍ）］
実施例２のＺｎＳの組成比ｘを０．２５に変え、また青色ＬＥＤの発光波長を４４０ｎｍにし、他は同様にして、図４に示すような白色発光素子を作製した。この白色発光素子に通電して発光させたところ、色温度３５００Ｋの白色を得ることができた。
【０１０７】
【発明の効果】
本発明は、４１０ｎｍ〜４６０ｎｍで発光するＩｎＧａＮ‐ＬＥＤを青色光源とし、５７０ｎｍ〜５８０ｎｍに中心波長をもつ蛍光を発するＺｎＳＳｅバルク結晶蛍光板を用い、青色ＬＥＤの青色光の一部を蛍光板によって黄色光に変換し青色と合成することによって白色を発生する白色発光素子を与える。ＹＡＧ／ＩｎＧａＮ‐ＬＥＤよりも低い色温度の白色を作り出すことができる。５０００Ｋより低い色温度の白色を合成することもできる。ＩｎＧａＮ‐ＬＥＤを使うので小型、軽量で、劣化が少なく長寿命である。二つの色の組み合わせなので演色性は良くない。しかし高輝度で低色温度の白色を発生するので液晶バックライトなどに用いることができる。
【図面の簡単な説明】
【図１】 ＣｅドープＹＡＧ蛍光剤を分散させた透明樹脂によって青色発光ＩｎＧａＮ‐ＬＥＤを囲んでＬＥＤの青とＹＡＧの黄色の組み合わせによって高い色温度の白色を生成できる従来例▲１▼にかかる砲弾型にした白色ＬＥＤ（ＹＡＧ／ＩｎＧａＮ‐ＬＥＤ）の構造を示す断面図。
【図２】 Ａｌ、Ｇａ、Ｉｎ、Ｂｒ、Ｃｌ、Ｉの何れかをドーパントとして含むＺｎＳｅ基板上にＺｎＣｄＳｅエピタキシャル層を形成し、ＺｎＣｄＳｅ発光部からの青色によってＺｎＳｅ基板の不純物を励起して黄色を発生させＺｎＣｄＳｅの青色とＳＡ発光の黄色を組み合わせることによって白色を生成する従来例▲２▼にかかる白色ＬＥＤ（ＺｎＳｅ／ＺｎＣｄＳｅ）の構造を示す断面図。
【図３】 白色をＬＥＤの青色と蛍光の黄色との組み合わせによって生成する白色発光素子の白色の原理を説明するための色度図。
【図４】 波長の短い青色を発生するＩｎＧａＮ‐ＬＥＤとＡｌ、Ｇａ、Ｉｎ、Ｂｒ、Ｃｌ、Ｉの何れかをドーパントとして含むＺｎＳＳｅ蛍光板とを組み合わせ、ＩｎＧａＮ‐ＬＥＤの青色によってＺｎＳＳｅ蛍光板を励起して黄色を発生させ５０００Ｋ以下の色温度の白色を発生させることのできる本発明の白色ＬＥＤの構造を示す断面図。
【図５】 ＩｎＧａＮ‐ＬＥＤの青色光によって、ＺｎＳＳｅ蛍光板を励起し黄色の蛍光を発生し、青色光と黄色光を混合することによって白色を得る本発明の原理を説明する図４のＬＥＤ、蛍光材の部分の拡大断面図。
【符号の説明】
２ Γ型リード
３ 凹部
４ ＩｎＧａＮ‐ＬＥＤ
５ ＹＡＧ蛍光剤を分散させた樹脂
６ 電極
７ 電極
８ ワイヤ
９ ワイヤ
１０ 直線リード
２０ 透明樹脂
２２ 不純物ドープＺｎＳｅ基板
２４ Γ型リード
２５ Γ型リード
２６ 凹部
２７ ＩｎＧａＮ‐ＬＥＤ
２８ ＺｎＳＳｅ蛍光板
２９ 拡散剤を分散した透明樹脂
３０ 電極
３２ 電極
３６ モールド樹脂[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a white light-emitting element that can generate white light that can be used for illumination, display, liquid crystal backlight, and the like with a single element structure.
[0002]
Many light emitting diodes (LEDs) and semiconductor lasers (LDs) are manufactured and sold as small light emitting elements. As LEDs having high luminance, red LEDs, yellow LEDs, green LEDs, blue LEDs, and the like are commercially available. The red LED is an LED having AlGaAs, GaAsP or the like as an active layer. Yellow and green are LEDs having GaP as a light emitting layer. Orange and yellow can be produced by an LED having a light emitting layer of AlGaInP.
[0003]
The blue color requiring wide band gap interband transition was the most difficult and difficult. SiC, ZnSe, and GaN-based materials have been tried and competed. However, it has been found that GaN-based materials with high brightness and long life are overwhelmingly superior, and wins and losses are already attached. Since the GaN-based LED actually has an active layer of InGaN, it will be referred to as an InGaN-based LED, InGaN-LED, etc., but the substrate is sapphire and the main layer structure is GaN. Since these semiconductor light emitting elements such as LEDs and LD use band gap transition, naturally they emit light of a single color with a narrow spectrum width. If it is as it is, it is not possible to make a complex color with semiconductor elements.
[0004]
[Prior art]
Illumination light sources are useless with monochromatic light sources. A liquid crystal backlight cannot be a monochromatic light source. A white light source is required for illumination. In particular, white with high color rendering properties is desirable. A liquid crystal backlight also requires a white light source. Incandescent bulbs and fluorescent lamps are still used exclusively as illumination light sources. Incandescent light bulbs are suitable for lighting because of their high color rendering properties, but they have the disadvantages of poor efficiency and short life. Fluorescent lamps have a short life span, require heavy objects, and are large and heavy equipment.
[0005]
The appearance of a white light source that is smaller, has a longer life, is more efficient, and is less expensive is awaited. If it is light weight, small size, long life, and high efficiency, it seems that it is already only a semiconductor element.
[0006]
In fact, there are blue LEDs, green LEDs, and red LEDs, so if these three light colors are combined, white should be synthesized. If three types of LEDs of blue, green, and red are uniformly attached to the panel and light is emitted at the same time, it should be white. It seems that such a three-color mixed LED has already been proposed and implemented in part. Three colors can produce a white color, but it must not appear as a separate single color, so the three types of LEDs must be distributed at a high density.
[0007]
In addition, since the three types of LEDs have different current, voltage, and light emission characteristics, they must be separated from each other, resulting in three power sources. Difficult to align due to variations in brightness. Although there is such a problem, above all, a large number of three kinds of LEDs are densely arranged in parallel, so that it becomes an expensive light source.
[0008]
Expensive light sources are not widespread and useless. We would like to make a low-cost small white light emitting element as a semiconductor device. There are two known technologies for semiconductor light-emitting devices using a single light-emitting device. One is a composite LED in which an InGaN-LED (light emitting element on a GaN substrate) is surrounded by a YAG phosphor. The other is that the ZnSe-LED ZnSe substrate is doped with impurities to form a phosphor, and the ZnSe fluorescent part is excited by the blue color of the ZnSe-LED light-emitting part (ZnCdSe) (referred to as SA emission) to generate yellow and orange colors. White is obtained by combining yellow and orange. For simplicity, the former will be referred to as a GaN-based white light-emitting element (A), and the latter will be referred to as a ZnSe-based white light-emitting element (B). Each will be described.
[0009]
(A) GaN-based white light emitting device (YAG + InGaN-LED; FIG. 1)
This uses InGaN-LED, for example
(1) “Optical Functional Materials Manual”, Optical Functional Materials Manual Editorial Board, Optoelectronics, p457, June 1997
Explained. FIG. 1 shows the structure.
[0010]
A recess 3 is provided in the horizontal portion of the Γ-type lead 2, and an InGaN-LED 4 is attached to the bottom of the recess 3. The recess 3 accommodates a resin 5 in which a Ce-added YAG fluorescent agent is dispersed. YAG fluorescent agents have the property of absorbing blue light and generating a lower energy yellow. Such a material that absorbs light with high energy and is excited to be excited to return to the original level, and light with low energy emitted is called fluorescence. The material that gives it out is called a fluorescent agent. Since it returns to the original level via various levels, the energy spreads and the fluorescence spectrum is wide. The energy loss becomes heat.
[0011]
The electrodes 6 and 7 of the InGaN-LED 4 are connected to the lead 2 and the lead 10 by wires 8 and 9. The upper portions of the leads 2 and 10 and the fluorescent agent resin 5 are covered with a transparent resin 20. Thereby, a bullet-type white light emitting element is manufactured. Since an InGaN-LED uses an insulating sapphire substrate, an n-electrode cannot be provided on the bottom surface, and an n-electrode and a p-electrode are formed at two locations on the top surface, and two wires are required.
[0012]
This is because the InGaN blue LED 4 is surrounded by a resin layer 5 in which a YAG fluorescent agent is dispersed, and a part of the blue light B is converted into yellow light Y by the fluorescent agent, and the original blue light B and yellow light are converted. By combining Y, white W (= B + Y) is realized. White can be produced with a single light-emitting element. Here, Ce-activated one is used as the YAG fluorescent agent. 460 nm light is used as the blue light B of the InGaN-LED. The center wavelength of yellow light Y converted by YAG is about 570 nm. That is, YAG absorbs blue light of 460 nm and converts it into yellow light having a broad peak at about 570 nm.
[0013]
Since the light-emitting element InGaN-LED has high brightness and long life, this white light-emitting element also has an advantage of long life. However, since YAG is an opaque material, blue light is strongly absorbed, and the conversion efficiency is not good. This realizes a white light emitting element having a color temperature of about 7000K.
[0014]
(B) ZnSe-based white light emitting device (ZnCdSe emission, ZnSe substrate fluorescent agent; FIG. 2)
This uses ZnCdSe-LED instead of InGaN-LED as a blue light source. Uses fluorescence but does not use a separate fluorescent agent. Excellent and clever element. Become the applicant,
[0015]
(2) Japanese Patent Application No. 10-316169 "White LED"
[0016]
It was first proposed by The structure of LED shown in FIG. 2 is shown. A ZnSe substrate 22 is used instead of the GaN substrate. A light emitting layer made of a ZnCdSe epitaxial layer is provided on the impurity-doped ZnSe substrate 22. ZnCdSe emits a blue color of 485 nm. The ZnSe substrate is doped with any of I, Al, In, Ga, Cl, and Br as the emission center. The impurity-doped ZnSe substrate 22 absorbs a part of blue and generates broad yellow light having a center at 585 nm. Blue light B and yellow light Y are combined to produce white W (W = B + Y).
[0017]
Actually, the ZnCdSe-LED of FIG. 2 is also attached to the lead and surrounded by a transparent resin to form a bullet-type light emitting device like the device of FIG. 1, but this is not shown. In this method, an n-type ZnSe substrate is doped with impurities and the n-type substrate itself is used as a fluorescent plate. The epilayer ZnCdSe emits blue, and the ZnSe substrate emits yellow fluorescence. Together, they become white W.
[0018]
Since it is LED, a board | substrate is essential. The substrate functions as a fluorescent plate in addition to the physical holding function of the light emitting layer. That is, it has an elaborate structure that effectively uses the substrate twice. An independent fluorescent agent such as YAG is unnecessary. That is a great advantage.
[0019]
The emission of impurity-doped ZnSe is called SA emission (self activated). This uses blue light with a wavelength of 485 nm and yellow light with a center wavelength of 585 nm, and realizes white having an arbitrary color temperature between 10,000 K and 2500 K. When the ZnSe substrate is thinned or the impurity concentration is lowered, the fluorescence becomes inferior and the blue light of the ZnCdSe light emitting layer becomes dominant. A white color having a high color temperature is obtained. When the ZnSe substrate is thickened or the impurity concentration is increased, the fluorescence becomes superior, so that white having a low color temperature can be obtained. With such a little ingenuity, whites with various color temperatures can be obtained.
[0020]
As described above, there are three semiconductors with wide band gaps: ZnSe, SiC, and GaN. SiC is indirect transition and is not efficient and does not compete from the beginning. ZnSe, which can produce a single crystal substrate, was primary. Currently, InGaN blue light from GaN and InGaN thin films on sapphire substrates is winning as a long-life, high-brightness, low-cost blue LED. InGaN / Sapphire-LEDs can generate blue light with shorter wavelengths (higher energy), have a longer lifetime, and have higher brightness.
[0021]
Since ZnSe has a short life and low energy (long wavelength), it has been delayed as a blue light LED. However, in this white light emitting element B, the substrate itself is a fluorescent plate, a special fluorescent agent is not required, and it is excellent in economic efficiency and low cost. There is a possibility of growing into a light emitting device.
[0022]
(C) Chromaticity diagram (Fig. 3)
The law of synthesis of white W = blue B + yellow Y alone is difficult to understand strictly. Let's explain with a chromaticity diagram. FIG. 3 is a chromaticity diagram. The chromaticity diagram is a two-dimensional graph representing stimulus values for the human eye for the three primary colors red, green, and blue for a visible light source or object color. Speaking of the light source, let x be the stimulus value that the human eye feels red, y the stimulus value that feels green, and z the stimulus value that feels blue. Although it is difficult to understand, human vision is the standard and it is not the energy of light (Nhν; N is the number of photons and ν is the frequency). Even with the same energy, the green visibility is high and the blue visibility is low. Two-dimensional display is not possible if there are three variables. Moreover, since the color tone is considered, the total amount of stimulation is irrelevant.
[0023]
Therefore, a two-dimensional coordinate space is assumed in which red and green are adopted and blue is discarded. Normalized red X is given by X = x / (x + y + z), and normalized green is given by Y = y / (x + y + z). A chromaticity diagram is displayed with normalized red on the horizontal axis and normalized green on the vertical axis. Of course, X + Y <1 must be satisfied. Any color is represented by a point on its two-dimensional coordinates. A single color (the spectrum becomes a δ function) is placed on an oblique horseshoe-shaped curve (shown by a solid line; abcdefghijkmmnpq). The limit a at the lower right of the horseshoe curve is the red limit. 620, 610... Represents the wavelength in nm. Although 620 to 530 nm (abcdef) is close to a straight line, since the blue component z is small, the portion becomes such a straight line with X + Y being close to 1.
[0024]
The curve falls from 520 nm (g) of green and falls to 510, 505, 500... (Hi), but this has a shape along the Y axis because there is almost no red component and strong blue and green components. It can be said that the decrease from 520 nm is the blue component. The x component also increases slightly from 495 nm (i) because the red component is included. Red and green are used for the x and y coordinates, and blue is discarded. So where did the blue go? The distance between the diagonal line of X + Y = 1 and the coordinate point is 2 ^1/2 As a result of doubling, it actually appears on the chromaticity diagram.
[0025]
The end n of the horseshoe curve is purple. A straight line npqa connecting purple and red is called a pure purple curve. The single color is on this curve abcdefghijkmmnpq. However, the pure purple curve npqa is not always monochromatic and difficult to explain. This part is not yet mature in color theory and is not a problem here.
[0026]
All the combined colors are inside this curve. When colors are mixed in a complicated manner, they are expressed by internal points. A portion surrounded by a broken line in the center is a white region W. The chromaticity diagram itself is originally only a device for displaying two-dimensional color components. However, since the linearity is maintained for blue, red, and green from the above definition, a mixed tone of two or three colors can be obtained. There is a very convenient property that corresponds to the mixing operation on the chromaticity diagram.
[0027]
Here, the color temperature will be briefly described. Even though it is white, it is variously reddish, yellowish, and bluish. As a white expression method, there is also a method of expressing with coordinate points of a chromaticity diagram. It lacks intuition, however. As mentioned above, the expression based on the color tone that is reddish or bluish is easy to understand, but lacks quantitativeness. There are various expressions depending on the color temperature. In the present invention, white is expressed by color temperature, so the definition will be described.
[0028]
Light having a frequency ν has an energy of hν. Because photons are bosons
[0029]
[Expression 1]

[0030]
Follow Bose statistics. We want to find the average value Em = <hν> of photon energy per unit volume. Micro volume dk of wave number space _x dk _y dk _z There is one state for = 1. The frequency ν is ν = c / λ = ck / 2π. dk because it is spherically symmetric in k-space _x dk _y dk _z = 4πk ² dk = 32π ⁴ ν ³ dν / c ³ So the probability distribution is
[0031]
[Expression 2]

[0032]
Can be written as follows. That is the energy distribution of photons of blackbody radiation. If it is multiplied by hν and integrated, black body radiation (incandescent light bulb etc.) distribution is obtained. The absolute temperature T of the denominator is the color temperature.
[0033]
If the frequency showing the maximum distribution is νmax or the average frequency νm, they are proportional to T. νmax / T = const1, νm / T = const2. In terms of average value, νm = const2 × T.
[0034]
If it is not blackbody radiation, the distribution is not equation (1). The average value of ν is calculated by replacing the spectrum with g (ν) and f (ν) in equation (1). By setting it as νm and dividing it by const2, the color temperature T can be calculated. White having a color temperature of T means that the same average value as that of the energy distribution when the temperature of the distribution function is T with black body radiation is given. Therefore, it is possible to define and calculate the color temperature even in the white of the spectrum away from blackbody radiation.
[0035]
Another parameter for evaluating white color is color rendering. It represents the deviation from blackbody radiation (3). Naturally, black body radiation has a color rendering of 100%, and an incandescent light bulb corresponds to this. In the present invention, color rendering is not a problem, so the color rendering is not described.
[0036]
The InGaN-LED of the GaN-based white light emitting element (A) described above generates blue (m point) of 460 nm in the chromaticity diagram of FIG. 3, and the Ce-doped YAG fluorescent agent absorbs blue light and has a peak at 568 nm. Some yellow (d point) is generated. Therefore, (A) a GaN-based white light emitting device (YAG + InGaN-LED) can generate a composite color corresponding to a point on the straight line md. The straight line md crosses the left end of the white area W. So white can be created. The white color of 7000K described above is a point of X = 0.31 and Y = 0.32 inside W. The reason why the color temperature becomes white is that the light emitted from the InGaN-LED is blue light, but the wavelength is short.
[0037]
Another (B) ZnSe-based white light-emitting element (ZnCdSe / ZnSe substrate) generates blue light having an active layer ZnCdSe of 485 nm, which corresponds to point j in the chromaticity diagram of FIG. The fluorescence of impurity (Al, In, Br, Cl, Ga, I) doped ZnSe generates yellow light fluorescence of about 585 nm. In FIG. 3, it corresponds to point c. When the 485 nm blue light (j point) from the active layer and the 585 nm yellow light (c point) from the ZnSe substrate are combined, an arbitrary color on the straight line jc can be created. Conveniently, this straight line jc crosses the white area W from left to right. This means that white of various color temperatures can be produced by changing the ZnSe thickness and impurity concentration.
[0038]
Here, white coordinates of color temperatures of 10,000K, 8000K, 7000K, 6000K, 5000K, 4000K, 3000K, and 2500K are indicated by dots. As such, the ZnSe white light emitting element has a gentle slope of the straight line jc and traverses the white region for a long time. Thanks to this, it is possible to create whites with various colors (color temperatures). In this respect, it is more convenient than the GaN-based white light-emitting element (A).
[0039]
[Problems to be solved by the invention]
[1. Advantages and disadvantages of ZnSe-based white light-emitting elements]
When the composition of the color of the ZnSe-based white light emitting element is viewed in the chromaticity diagram (FIG. 3), the blue light B (485 nm, j point) of the ZnCdSe-LED and the yellow light Y (585 nm, c point) of the ZnSe substrate are connected. The straight line jc substantially coincides with the locus of white light (10000K to 2500K). A white color having an arbitrary color temperature (10000 K to 2500 K) can be obtained simply by changing the ratio of the blue light B and the yellow light Y by changing the thickness of the ZnSe substrate or the impurity concentration. There are advantages such as a small and simple element structure and simple electrodes.
[0040]
However, ZnSe-based LEDs are liable to deteriorate and have a short lifetime. Since a large current flows for light emission, defects increase and deterioration progresses. When it deteriorates, the luminous efficiency decreases. Eventually it will stop emitting light. Being short-lived is a common problem for light-emitting elements on a ZnSe substrate.
[0041]
When the white light W is synthesized by mixing the blue light B and the yellow light Y, the necessary ratio of the blue light B and the yellow light Y is a problem. When blue light having a wavelength of around 445 nm with high energy is used, the required blue light ratio is the smallest. When blue light having a lower energy around 485 nm (j point) is used, about twice as much blue light is required as compared with 445 nm blue light (B: Y = 2: 1).
[0042]
Here, since blue light has lower visibility than yellow light, the smaller the ratio of blue light, the higher the light emission efficiency. Therefore, a ZnSe-based white light emitting element that requires more blue light is disadvantageous in terms of efficiency.
[0043]
[2. Advantages and disadvantages of GaN-based white light-emitting elements]
On the other hand, in (A) GaN-based (InGaN-LED + YAG) white light emitting element, blue has a high energy at 460 nm to 445 nm, and the required power ratio is B: Y = 1: 1. Half is enough. Therefore, it is advantageous in terms of efficiency. Since a GaN-based light emitting device has a long life, a white light emitting device using the light emitting device also has a long life. There are such advantages.
[0044]
However, there are disadvantages. As can be seen from FIG. 3, the YAG + InGaN-LED white light emitting element has a gradient such that the straight line md connecting the blue light m and the yellow light d is in contact with the white locus (10000K to 2500K) at 7000K. It is not possible to synthesize white with any color temperature. A white color with a color temperature lower than around 7000K (6000K, 5000K, 2500K, etc.) cannot be synthesized.
[0045]
In general, the color temperature of a white light bulb is a low color temperature around 3500K. The InGaN-based white light emitting element can only realize white having a color temperature different from that of the white light bulb. That is, it cannot be used as a white light emitting element for illumination. Although there is a possibility that it can be used for a liquid crystal backlight, there is a drawback that the color temperature is too high.
[0046]
Therefore, despite having excellent characteristics (lifetime and efficiency), the replacement of the white light bulb is not sufficiently advanced.
[0047]
It is a first object of the present invention to provide a small white light emitting device capable of producing white having a lower color temperature, specifically, a color temperature lower than 6000K. It is a second object of the present invention to provide a long-life white light emitting device. Unless it has a long life, incandescent bulbs and fluorescent lamps cannot be substituted.
[0048]
[Means for Solving the Problems]
1. The present invention proposes a white light emitting device in which ZnSSe bulk fluorescent plates are stacked on an InGaN-LED. Part of InGaN-LED light emission that emits blue light with a short wavelength is converted into yellow light by a bulk fluorescent plate made of ZnSSe crystal, and by mixing blue light B and yellow light Y, white W (W = B + Y) is obtained. Synthesize.
[0049]
2. The wavelength of blue light generated by the InGaN-LED is set to 410 nm to 460 nm. This is the shorter wavelength of blue light and corresponds to the light emission of the lower left part mn in the chromaticity diagram of FIG. Such a short wavelength blue light cannot be produced by a ZnSe system (ZnCdSe active layer: 485 nm; j point). Therefore, a GaN-based (InGaN active layer) LED is used. InGaN / Sapphire-LEDs are easy to use in terms of performance, lifetime, cost, and reliability. Therefore, the present invention is in common with the prior art (A) YAG + InGaN-LED described above in terms of LEDs. However, the fluorescent material is not YAG. Use new substances.
[0050]
3. The wavelength of yellow light is set to 570 nm to 580 nm. This is a novel idea of the present invention expressed by wavelength. Since the prior art (A) YAG + InGaN-LED described above uses Ce-doped YAG that generates fluorescence at 568 nm (d point), it can only synthesize white at 7000K. The point d is closer to yellowish green than yellow light. The present invention produces yellow light close to red having a longer wavelength. In the chromaticity diagram, a yellow color closer to red is generated in a range between 570 nm (point v) and 580 nm (point u). As a result, the blue light emits light in the range of the line segment nm, and the yellow light emits fluorescence in the range of the line segment uv. A straight line connecting the line segment nm and the line segment uv corresponds to light emission of the white light emitting element of the present invention. A straight line (one-dot chain line) connecting the line segment nm and the line segment uv enters the white region deeper than the line segment md corresponding to YAG. One of the ideas of the present invention is such that the yellow side of the straight line is slightly lowered.
[0051]
4). In order to lower the yellow light side slightly (shift to the red side), a new fluorescent material other than YAG is required. The present invention uses a ZnSSe crystal that is a mixed crystal of ZnSe and ZnS. High purity ZnSSe does not emit fluorescence. A dopant that becomes the emission center is required. The dopant is any one of Al, In, Ga, Cl, Br, and I. The fluorescent material used in the present invention contains 1 × 10 impurities (dopants) of at least one element of Al, In, Ga, Cl, Br, and I in the ZnSSe crystal. ¹⁷ cm ^-3 The above concentrations are included. If it is less than this, sufficient fluorescence will not be generated. The specific gravity of yellow light (uv) can be changed by changing the dopant concentration or changing the thickness of the fluorescent material. That is, the point indicating the color in the line segment (mn to uv) of blue light / yellow light on the chromaticity diagram can be moved up and down along the line segment. Since the line segment has entered deeper into the white region than in the conventional example (A; md) using YAG, white of various color temperatures can be synthesized. By increasing the thickness of the fluorescent material and increasing the dopant concentration, it is possible to increase the specific gravity of yellow light (uv) and produce a white color with a low color temperature.
[0052]
The use of Al, In, Ga, Cl, Br, or I as a dopant is the same as in the conventional example (B) in which a ZnSe substrate is used as a fluorescent plate. However, the present invention uses not ZnSe but ZnSSe as a fluorescent plate. Furthermore, the LED is InGaN instead of ZnCdSe.
[0053]
5. Since ZnSe has a narrow bandgap and ZnS has a wide bandgap, an intermediate bandgap can be produced according to the mixed crystal ratio x. In the fluorescent material of the present invention, the composition ratio of ZnS in the ZnSSe crystal is x, the composition ratio of ZnSe is (1-x), and x is 0.1 to 0.4. Correctly add ZnS and add ZnS _x Se _1-x (0.1 ≦ x ≦ 0.4) may be written, but the mixed crystal ratio x is omitted for simplicity. If x = 0, ZnSe becomes a fluorescent plate, which is the same as the fluorescent plate of the conventional example (B). However, if it is a fluorescent plate, only the color of the line segment (kn to c) connecting blue kn and yellow c can be produced, and the white region W is shifted to the lower side. It is not possible to make white with a wide range of arbitrary color temperatures. Therefore, ZnSe (x = 0) is denied.
[0054]
However, if all of them are made of ZnS (x = 1), the fluorescence wavelength is too short, and only white having a high color temperature can be produced as in the conventional example (A) YAG + InGaN-LED. Therefore, the present invention sets the mixed crystal ratio to x = 0.1 to 0.4. The wavelength of fluorescence is not the band gap itself. The relationship will be described later.
[0055]
6). As described above, ZnSSe is used in a lump. This is an important device of the present invention. It must not be in powder form. Furthermore, it is desirable to make the average grain size of the ZnSSe crystal constituting the fluorescent plate larger than the thickness of the fluorescent plate. YAG of the conventional example (A) uses YAG (yttrium aluminum garnet) powder dispersed in a transparent resin as a fluorescent agent. In the first place, many fluorescent plates are obtained by dispersing fine powders of fluorescent materials in transparent glass or resin. If it is made into a lump instead of a powder, light does not enter the interior, so the internal fluorescent agent is useless. The fluorescent agent is made into a fine powder and dispersed in a transparent resin and glass so that light can be easily hit as much as possible and conversion efficiency is improved. This is because light passes through the transparent resin and light easily strikes the fluorescent powder, the concentration of the fluorescent substance can be easily adjusted, and it can be shaped (molded) into an arbitrary shape.
[0056]
However, it has been found that if the fine powder of ZnSSe polycrystal doped with the above impurities is dispersed in the resin, there is a disadvantage that YAG does not have. ZnSSe has water absorption. When powdered and dispersed in the resin, water enters the resin and the powder ZnSSe easily absorbs water and deteriorates. It cannot be used as a fluorescent agent unless such difficulties are overcome. Therefore, unlike conventional fluorescent agents, bulk polycrystalline or single crystal ZnSSe is used instead of powder. This is important in the present invention. If it is made agglomerated, the surface area is narrow, so it is difficult for water to enter and stays close to the surface. Deterioration due to water absorption is also limited to the vicinity of the surface. For that reason, lump ZnSSe is used.
[0057]
Even in the case of polycrystal, it is better that the particle size is large. It is not only that water can easily enter the grain boundaries. Light may be diffusely reflected and absorbed at the grain boundary, causing optical loss. Therefore, the larger grain boundary is better. It is more suitable that the average grain size of the polycrystal is larger than the thickness of the fluorescent plate. In this case, every grain retains a single crystal in the thickness direction, and the average particle diameter is defined in the plane direction of the fluorescent screen.
[0058]
7. More preferably, the ZnSSe phosphor plate is made of single crystal ZnSSe. It is better that there are no grain boundaries because polycrystalline grain boundaries cause optical loss. Speaking of an ideal one without grain boundaries, it is a single crystal. Therefore, an impurity-doped ZnSSe single crystal is optimal as the fluorescent plate of the present invention. Nevertheless, a ZnSSe single crystal cannot be made easily. It can be made by chemical transport, but it is time consuming and expensive. In terms of reducing costs, bulk polycrystalline ZnSSe would be used.
[0059]
8). An InGaN-based LED having an emission wavelength of 410 nm to 460 nm is used as the blue light emitting LED. This is because, as already described, in the chromaticity diagram, the blue light emission region is set to mn so that the line segment connecting the yellow light and the blue light crosses the white region W deeply. Thereby a white color temperature lower than 7000K is obtained.
[0060]
9. The composition ratio of ZnS in the ZnSSe crystal is x, the composition ratio of ZnSe is (1-x), and the emission wavelength of the blue light emitting LED is λ. _LED Where λ _LED ≧ 1239 / (2.65 + 1.63x−0.63x ² ) Nm is desirable. ZnSe has a band gap of 2.7 eV and an absorption edge wavelength of 460 nm. The band gap of ZnS is. The absorption edge wavelength is 335 nm at 3.7 eV. The formula of the emission wavelength λLED is 2.65, and the band gap is 2.7. Mixed crystal ZnS _x Se _1-x The band gap of Eg is approximately Eg = 2.7 + 1.63x−0.63x ² Given by. When 1239 (= hc) is divided by the band gap, the absorption edge wavelength is expressed in nm. In other words, the above formula says that it is better to excite the fluorescent material with blue light (long wavelength) having energy lower than the band gap of the mixed crystal ZnSSe of the fluorescent material used in the present invention. It is a completely different story from mn to uv traversing the white region W on the chromaticity diagram. Although slightly complicated, this condition is a condition that the blue light of the InGaN-LED reaches the inside without being absorbed by the surface of the ZnSSe fluorescent material and is absorbed inside to generate fluorescence. A semiconductor immediately absorbs light having a higher energy (short wavelength) than the band gap. Even if it is said that it is a lump, the surface of the fluorescent material may be deteriorated due to water absorption. So I don't want to use the surface. The blue light reaches the inside and the dopant is excited inside to emit light. For this reason, blue light having energy lower than the band gap of ZnSSe which is difficult to be absorbed is used.
[0061]
10. A ZnSSe crystal that has been heat-treated in a Zn atmosphere is preferably used as the fluorescent plate. This is because heat treatment reduces defects and reduces scattering and non-fluorescence absorption.
[0062]
DETAILED DESCRIPTION OF THE INVENTION
In order to solve the above-described problem of generating white color having a color temperature lower than 7000 K, a part of blue light emitted from an LED near an emission wavelength of 445 nm (point between mn) is changed to a center wavelength of 575 nm (uv (Midpoint) It is sufficient that there is a fluorescent material that converts light in the vicinity.
[0063]
InGaN-LEDs that emit 445 nm exist and are commercially available. Fluorescent materials are a problem. A fluorescent material that emits a wavelength of 575 nm is required. The fluorescence of Ce-YAG in the conventional example (A) has a center wavelength of 568 nm (d point). It's too short. As described in the conventional example (B), the emission wavelength when a group 3 element or a group 7 element (Al, In, Ga, Br, Cl, I) is mixed in the ZnSe crystal is around 585 nm (point c). is there. This is too long. I want fluorescence with a wavelength between u and v between point d and point c.
[0064]
Conventional fluorescent agents such as Ce-YAG cannot change the central wavelength of fluorescence because only the concentration of dopant such as Ce has parameters. When the Ce density is increased, the color coordinate only approaches the point d on the line segment md, and the point d itself cannot be moved. This is because Ce itself acts as an isolated color center in the YAG matrix.
[0065]
However, ZnS is a mixed crystal of ZnSe and ZnS. _x Se _1-x In the crystal, in addition to the dopant (Al, In, Ga, Br, Cl, I), the mixed crystal ratio x exists as a parameter that can be freely selected. ZnSe with x = 0 is the band gap Eg _ZnSe = 2.7 eV, x = 1 ZnS has a band gap of Eg _ZnS = 3.7 eV. That is, it is about 1 eV larger. The band gap changes with x. It has been found that the fluorescence of ZnSSe appears to be due to donor-acceptor transition. By adding a Group 3 or Group 7 dopant, both a relatively deep donor and acceptor are generated. The donor-acceptor transition causes fluorescence. Therefore, the fluorescence center wavelength Λq is considerably longer than the band gap wavelength λg (= hc / Eg).
[0066]
Then, when the band gap Eg is changed, the fluorescence wavelength Λq due to the donor-acceptor transition can also be changed. The convenient point of ZnSSe is that the fluorescence center wavelength Λq depends on the band gap Eg. This is a passive fluorescence story, not a case where ZnSe is an active LED. Complex but not to be confused.
[0067]
Fluorescence center wavelength Λq of impurity-doped ZnSe _ZnSe = 585 nm, and the desired fluorescence center wavelength is 580 nm to 570 nm (between u and v). The fluorescence wavelength when ZnS is used as the fluorescent material is around 470 nm. The ZnSSe band gap and fluorescence wavelength will vary continuously with x. If so, the desired fluorescence wavelength of 570 nm to 580 nm is a mixed crystal ZnS in which x is appropriately selected. _x Se _1-x Is obtained from
[0068]
Therefore, the composition ratio x of ZnS is appropriately selected, and the ZnS _x Se _1-x If a group 3 element or a group 7 element is mixed in the crystal, it is possible to obtain fluorescence at around 575 nm. A suitable value of x will be described later.
[0069]
Here, if the mixing density is low, sufficient light emission cannot be obtained. 1 × 10 ¹⁷ cm ^-3 It is necessary to mix (doping) impurities (dopants) of a degree or more. If the concentration is higher than this and the concentration is increased, the specific gravity of yellow light can be increased, and if the concentration is decreased, the specific gravity of blue light can be increased. Such a thing is also possible by changing the thickness of the fluorescent material.
[0070]
However, there is a problem that ZnSe-based, ZnSSe-based, and ZnS-based fluorescent materials lack water resistance. It absorbs water and deteriorates over time. For this reason, a white light emitting element in which a ZnSSe-based material is combined with an InGaN-based LED as a fluorescent agent has not been put into practical use. Ordinary fluorescent agents such as YAG are used by dispersing powder in transparent resin and transparent glass. The fine powder ZnSSe immediately absorbs water and soon deteriorates and cannot be used. The problem of water absorption deterioration must be overcome.
[0071]
Although it is a water resistance problem here, it becomes a problem when the surface area of a fluorescent agent is relatively large. In order to increase the water resistance, it is effective to make the surface area as narrow as possible. The surface area of a sphere of radius r is 4πr ² And the volume is 4πr. ³ / 3. The surface area / volume ratio is 3 / r. If the ratio is to be lowered, the radius r can be increased.
[0072]
For that purpose, a large lump single crystal or polycrystalline ZnSSe fluorescent plate may be used rather than powdered ZnSSe. A bulky fluorescent material is self-consistent and unfamiliar, but ZnSSe can be used as a fluorescent material if it is made bulky. By doing so, the ratio of the surface area to the fluorescent material volume becomes very small, so that the water resistance should be remarkably improved.
[0073]
Because it is usually a powder, it is written as “fluorescent agent”. In the present invention, lump-shaped ZnSSe that is not fine powder is used, so that it will be written as “fluorescent material” or “fluorescent plate”.
[0074]
However, if all of the blue light is absorbed in the vicinity of the fluorescent plate surface even if the above fluorescent plate is used, and only the surface of the fluorescent plate generates fluorescence without entering the fluorescent plate, As a result, the problem of water resistance becomes apparent. In addition, if the LED light is almost absorbed by the fluorescent plate surface, the internal fluorescent material is wasted and inefficient. In the first place, it was an idea to make all the fluorescent agents irradiate with light by dispersing the fluorescent agent in the form of powder with a normal phosphor. In the present invention, since it is a massive fluorescent plate, it is necessary to allow light to enter the interior without staying on the surface. That is where the situation differs from ordinary fluorescent agents. What should I do to put LED light into the interior?
[0075]
It is sufficient that ZnSSe is almost transparent to the blue light of the LED. A normal fluorescent agent is opaque and is not such a thing. However, in the present invention, a bulky fluorescent plate is used, so that a transparent one for LED light is used. How can we make it transparent? The inventor has realized that it is only necessary to use blue light having an energy smaller than the forbidden band width (band gap; Eg) of the ZnSSe crystal constituting the fluorescent plate.
[0076]
Fortunately, ZnS has a wide band gap and is higher than the energy of LED light emitted by InGaN-LEDs. The band gap of ZnSe is lower than the light energy of InGaN-LED. Light emission wavelength λ of InGaN blue light emitting device with appropriate mixed crystal ratio x _LED ZnSSe having a band gap Eg equal to the energy corresponding to. If ZnSSe having a mixed crystal ratio x larger than the critical mixed crystal ratio is used as a fluorescent material, the band gap becomes wide and the LED light becomes transparent. The LED light should be able to penetrate into the fluorescent screen. The problem of making the massive fluorescent material transparent to LED light is thereby solved vividly.
[0077]
If it does so, the absorption coefficient of the fluorescent plate with respect to blue light will become small, blue light will approach into the inside of a fluorescent plate, and blue light will light-emit on the whole fluorescent plate. The influence of the deteriorated surface is reduced, and the internal fluorescent material can be used effectively.
[0078]
Conversely, the emission wavelength λ of the blue light LED _LED May be longer (low energy) than the band gap of the ZnSSe mixed crystal phosphor.
[0079]
The ZnS composition in the ZnSSe crystal is x (ZnS _x Se _1-x ) And its forbidden bandwidth is
[0080]
Eg _ZnSSe = 2.7 + 1.63x-0.63x ² (EV) (4)
[0081]
Given in. If the energy is expressed in eV and the wavelength in nm, they are inversely proportional and the proportionality constant is 1239, so the LED blue light wavelength λ _LED And ZnS composition x as
[0082]
[Equation 3]

[0083]
Should be satisfied. This is an inequality that determines the wavelength range of the InGaN-LED assuming that the composition x of the fluorescent material is determined. When the mixed crystal ratio x is changed and the ZnSSe band gap Eg and the band gap wavelength λg are calculated according to (5), the result is as follows.
[0084]
[Table 1]

[0085]
In _y Ga _1-y The emission wavelength of the N-LED can be changed by changing the mixed crystal ratio y of In. As described above, the preferable emission wavelength of InGaN is 410 nm to 460 nm, but InGaN can emit light of 480 nm beyond that. When the In ratio y is high, the emission wavelength shifts to the long wavelength side, and when the Ga ratio 1-y is high, the emission wavelength changes to the short wavelength side. The ZnS mixed crystal ratio corresponding to 410 nm is x = 0.252. For larger x, the bandgap wavelength λg is shorter than 410 nm. Therefore, in the range of 1>x> 0.252, the earliest equation (5) is the emission wavelength λ of the InGaN-LED. _LED It is not a condition to limit Since the band gap wavelength is 410 nm or more in the range of 0 <x ≦ 0.252, the equation (5) is the emission wavelength λ of the InGaN-LED. _LED It becomes the condition which limits.
[0086]
λ _LED Is 410 nm to 460 nm, for example, when x = 0.1, 460 nm> λ _LED > 441 nm, x = 0.2, 460 nm> λ _LED > 420 nm. If x = 0, λ from equation (5) _LED Although> 467 nm, it does not satisfy the condition of 460 nm or less. Therefore x = 0 is unsuitable. In the chromaticity diagram, x = 0 is unsuitable in that the fluorescence wavelength must be 570 nm to 580 nm (uv), but it is also unsuitable for independent conditions (LED light enters the inside of the phosphor plate). is there.
[0087]
Conversely, the wavelength λ of InGaN-LED _LED The equation (5) can also be interpreted so that the ZnS mixed crystal ratio x of the phosphor plate is limited to that.
[0088]
The ZnS composition x of the fluorescent plate is not limited to this, and the fluorescence wavelength Λq must be in the range of 570 nm to 580 nm as described above, so that all such conditions should be satisfied. Although the fluorescence wavelength Λq of ZnSSe is determined by the band gap Eg, the relationship is not yet analytically clear. This will be explained later by the results of the experiment.
[0089]
The ZnSSe fluorescent plate is optimally single crystal. Single crystals have the advantage that there are no grain boundaries. In addition, there is an advantage that it is easy to process in producing a fine ZnSSe fluorescent plate. That is, by cleaving a plane-oriented (100) ZnSSe substrate having an appropriate thickness, a cubic ZnSSe phosphor plate having an arbitrary size can be easily formed. However, it is not necessarily a single crystal. Polycrystal may be used. Polycrystals cannot be divided by cleavage, but may be cut mechanically. Since the light absorption and light scattering at the polycrystalline grain boundary decrease the conversion efficiency, it is preferable that the polycrystalline grain boundary is large. If possible, it is desirable that the average grain boundary is larger than the thickness of the ZnSSe fluorescent plate.
[0090]
The surface on the blue light incident side of the ZnSSe fluorescent plate is preferably mirror-polished in order to increase the incident efficiency. This is because the rough surface causes irregular reflection. Other surfaces of the ZnSSe phosphor plate need not be mirror-polished, but may be mirror-polished according to processing needs. In addition, it is more preferable to form an antireflection film on the incident surface. The antireflection film is formed of a transparent dielectric film. Even a single layer is possible, but antireflection performance is improved by using a multilayer film.
[0091]
It is also useful to perform surface processing to increase the emission efficiency of yellow light generated in the ZnSSe fluorescent plate.
[0092]
Although it is the wavelength of blue light, it was explained that the vicinity of 445 nm is advantageous. However, it does not necessarily have to be 445 nm. Since there is a change in emission efficiency due to a difference in the emission wavelength of blue light of the LED and a change in conversion efficiency of the fluorescent screen, the really optimum wavelength changes depending on the technical trend of the blue light emitting LED.
[0093]
However, since a part of blue light is converted into yellow light using a ZnSSe fluorescent plate, the wavelength of blue light emission must be in the range of 410 nm to 480 nm in consideration of conversion. Obviously, the efficiency drops. Therefore, blue light having a wavelength in the range of 410 nm to 480 nm should be used. However, considering that the white color is produced by looking at the chromaticity diagram, the wavelength of the InGaN-LED blue light is preferably 410 nm to 460 nm.
[0094]
In order to achieve white using blue light in this range, the central wavelength of light emission of the ZnSSe phosphor plate should be 570 nm to 580 nm. This can be seen by looking at the chromaticity diagram.
[0095]
In order to show fluorescence having such a central wavelength, the composition ratio x of ZnS in the ZnSSe crystal may be set to 0.1 to 0.4. The reason will be described later.
[0096]
When an InGaN-based LED is used as a blue LED, the current technology has the highest light emission efficiency at a wavelength around 400 nm, and the light emission efficiency decreases at longer wavelengths. In terms of luminous efficiency, the wavelength of blue light is preferably shorter than 460 nm. Therefore, when an InGaN-LED is used as the blue light LED, the emission wavelength is 410 nm to 460 nm. Therefore, 460 nm to 480 nm in the blue light range described above is a portion that can emit fluorescence of 570 nm to 580 nm that produces white, but is omitted because the efficiency of the LED is lowered.
[0097]
The absorption coefficient for the blue light of the ZnSSe crystal can be adjusted by the temperature of the heat treatment in the Zn atmosphere. Blue light absorption is increased by the heat treatment. Therefore, the adjustment of the absorption coefficient is useful for converting an appropriate amount of blue light into yellow light for synthesizing white.
[0098]
【Example】
[Example 1 (change in fluorescence wavelength according to ZnS mixed crystal ratio x)]
In order to examine the ZnS composition ratio (x) dependence of ZnSSe phosphor plate emission, iodine was added to ZnSSe crystals of x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6. It was produced by a chemical transport method using a transport medium. The ZnSSe substrate cut out from this crystal was heat-treated at a temperature of 1000 ° C. in a Zn atmosphere for 50 hours to produce a ZnSSe phosphor plate.
[0099]
The central wavelength (point on the chromaticity diagram) was estimated from the wavelength distribution of fluorescence emitted when the ZnSSe substrate was irradiated with light having a wavelength of 440 nm. The results are shown in Table 2.
[0100]
[Table 2]

[0101]
From the analysis of the chromaticity diagram, the condition is that the fluorescence is 570 nm to 580 nm. When the ZnS mixed crystal ratio exceeds 0.4, it becomes shorter than 570 nm. If it is 0.1 or less, it exceeds 580 nm. From this result, it was found that the optimum ZnS composition ratio x is 0.1 to 0.4. The central wavelength of fluorescence does not have a sharp peak like the emission wavelength of an LED. Because it is fluorescent, it becomes a gentle mountain and its central wavelength.
[0102]
[Example 2 (x = 0.4, λ _LED = 420 nm, Λq = 570 nm)]
ZnS with ZnS composition x = 0.4 _0.4 Se _0.6 A ZnSSe substrate having a thickness of 200 microns and a plane orientation (100) cut out from a single crystal was heat-treated at 1000 ° C. in a Zn atmosphere. The heat treatment was performed to adjust the absorption coefficient of blue light. This ZnSSe substrate was mirror-polished to a thickness of 100 microns. This ZnSSe substrate was scribed and broken to produce a ZnSSe phosphor plate having a 300-micron square and a thickness of 100 microns.
[0103]
A blue LED chip with an emission wavelength of 420 nm having an InGaN active layer using a sapphire substrate was prepared. As shown in FIG. 4, this LED chip was mounted in a flip chip type, and a ZnSSe fluorescent plate was attached to the upper side of the LED (upper side of the sapphire substrate) via a transparent resin. In FIG. 4, a large Γ-type lead 24 and a small Γ-type lead 25 are combined. The lead is a complicated combination, and the small Γ lead 25 is inserted into the hole of the large Γ lead 24. The Γ-type lead 24 has a recess 26 in which an InGaN-LED 27 is mounted. Since the sapphire substrate is InGaN, the electrodes 30 and 32 are provided toward the epitaxial growth surface.
[0104]
Normally, the electrode and the lead are connected by two wires as shown in FIG. 1, but here the electrode 30 is turned over to the large Γ-type lead 24 and the electrode 32 is turned over to the small Γ-type lead 25 instead of wire bonding. It is attached. The leads 24 and 25 penetrate each other but are not in contact with each other. Since the InGaN-LED 27 is turned upside down, blue light is emitted upward from the sapphire substrate. A ZnSSe fluorescent plate 28 is placed on the sapphire substrate. The recess 26 is filled with a transparent resin in which a diffusing agent (SiC powder) is dispersed. These were molded with a mold resin 36 to produce a shell-type light emitting element. When it is energized, blue light is emitted from the InGaN-LED 27 and turns yellow on the fluorescent screen. It spreads with a transparent resin. As a result, a white color having a color temperature of 5000K could be obtained. FIG. 5 shows a state in which the yellow light Y is excited by the blue light B, and the blue light B and the yellow light Y are mixed and go out to become white.
[0105]
One of the reasons for the white color at such a low color temperature while using an InGaN-LED is that the wavelength of the fluorescence is not 568 nm (Ce-YAG) but 569 nm (ZnS). _0.4 Se _0.6 That's why. Another reason is that the wavelength of the InGaN-LED is shortened to 420 nm.
[0106]
[Example 3 (x = 0.25, λ _LED = 440 nm, Λq = 575 nm)]
A white light emitting device as shown in FIG. 4 was produced in the same manner except that the composition ratio x of ZnS in Example 2 was changed to 0.25, the emission wavelength of the blue LED was changed to 440 nm, and the others were the same. When the white light emitting element was energized to emit light, white having a color temperature of 3500 K could be obtained.
[0107]
【The invention's effect】
The present invention uses a ZnSSe bulk crystal phosphor that emits fluorescence having a central wavelength of 570 nm to 580 nm using an InGaN-LED emitting at 410 nm to 460 nm as a blue light source, and converts a part of blue light of the blue LED to yellow light by the phosphor plate. A white light-emitting element that generates white by conversion and synthesis with blue is provided. A white color having a lower color temperature than that of YAG / InGaN-LED can be produced. It is also possible to synthesize white having a color temperature lower than 5000K. Because it uses InGaN-LED, it is small and light, has little deterioration and has a long life. Color rendering is not good because of the combination of two colors. However, it generates white color with high brightness and low color temperature, so that it can be used for a liquid crystal backlight.
[Brief description of the drawings]
FIG. 1 shows a conventional shell (1) capable of generating a white color with a high color temperature by surrounding a blue light-emitting InGaN-LED with a transparent resin in which a Ce-doped YAG fluorescent agent is dispersed and combining the blue and YAG yellow of the LED. Sectional drawing which shows the structure of white LED (YAG / InGaN-LED) made into a type | mold.
FIG. 2 shows that a ZnCdSe epitaxial layer is formed on a ZnSe substrate containing any one of Al, Ga, In, Br, Cl, and I as a dopant, and the blue color from the ZnCdSe light emitting part excites impurities in the ZnSe substrate to cause yellowing. Sectional drawing which shows the structure of the white LED (ZnSe / ZnCdSe) concerning the prior art example (2) which produces | generates white by combining the blue of ZnCdSe and yellow of SA light emission.
FIG. 3 is a chromaticity diagram for explaining the principle of white of a white light emitting element that generates white by a combination of blue of LED and yellow of fluorescence.
FIG. 4 is a combination of an InGaN-LED that generates blue with a short wavelength and a ZnSSe phosphor plate containing any one of Al, Ga, In, Br, Cl, and I as a dopant, and excites the ZnSSe phosphor plate with the blue color of InGaN-LED. Sectional drawing which shows the structure of the white LED of this invention which can generate yellow and can generate white of color temperature below 5000K.
5 illustrates the principle of the present invention in which the blue light of an InGaN-LED excites a ZnSSe phosphor plate to generate yellow fluorescence and obtains white color by mixing blue light and yellow light. FIG. The expanded sectional view of the part of material.
[Explanation of symbols]
2 Γ type lead
3 recess
4 InGaN-LED
5 Resin with YAG fluorescent agent dispersed
6 electrodes
7 electrodes
8 wires
9 wire
10 Straight lead
20 Transparent resin
22 Impurity doped ZnSe substrate
24 Γ type lead
25 Γ type lead
26 recess
27 InGaN-LED
28 ZnSSe fluorescent plate
29 Transparent resin with diffusing agent dispersed
30 electrodes
32 electrodes
36 Mold resin

Claims (5)

An InGaN-based LED that emits blue light having a wavelength of 410 nm to 460 nm and an impurity of at least one element of Al, In, Ga, Cl, Br, and I at a concentration of 1 × 10 ¹⁷ cm ⁻³ or more , It includes a bulky fluorescent plate made of ZnS _x Se _1-x crystal in which the composition ratio is x, the composition ratio of ZnSe is (1-x), and x is 0.1 to 0.4 . A part is converted into yellow light with a wavelength of 570 nm to 580 nm by a bulk ZnS _x Se _1-x crystal fluorescent plate, and the white light is mixed by mixing the blue light of 410 nm to 460 nm of the InGaN-based LED and the yellow light of 570 nm to 580 nm of the fluorescent plate. A white light emitting element which is synthesized. 2. The white light emitting device according to claim 1, wherein an average particle diameter of ZnS _x Se _1-x crystals constituting the fluorescent plate is larger than a thickness of the fluorescent plate. The white light emitting device according to claim 2, wherein the ZnS _x Se _1-x crystal phosphor plate is composed of single crystal ZnS _x Se _1-x . The white light emission according to any one of claims 1 to 3, wherein λ _LED ≧ 1239 / (2.65 + 1.63x−0.63x ² ) nm when the emission wavelength of the blue light emitting LED is λ _LED. element. The white light-emitting element according to claim 1, wherein a ZnS _x Se _1-x crystal that has been heat-treated in a Zn atmosphere is used as a fluorescent plate.

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