JP3555727B2 - Semiconductor laser device - Google Patents

Description

【００４２】
また、第１ガイド層７０４及び第２ガイド層７０６の層厚を５０ｎｍまで厚く設定した場合（従来形態２）には、閾値電流を本実施形態と同じ１５ｍＡまで低減することができる。しかし、その場合にはガイド層７０４、７０６の層厚の増大による素子抵抗の増大のために、動作電圧は２．１Ｖに増大するという問題が発生する。その結果を表２に示す。
【００４３】
【表２】

【００４４】
本実施形態１では、発振波長の設定値０．７８μｍに対して、作製した半導体レーザ素子の発振波長も０．７８μｍとなり、発振波長の制御ができる。これに対して、上記従来形態１の場合には、発振波長の設定値０．７８μｍに対して、発振波長が０．７７５μｍまで波長ずれが起こり、発振波長の制御が困難となる。
【００４５】
本実施形態１の半導体レーザ素子では、図２に示すように、第１ガイド層１０４及び第２ガイド層１０６を量子井戸層１２０に隣接させると共に第１ガイド層１０４及び第２ガイド層１０６の禁制帯幅を量子井戸層１２０の禁制帯幅より大きくし、且つ、第１ガイド層１０４及び第２ガイド層１０６の禁制帯幅を量子障壁層１２１の禁制帯幅よりも小さくしている。
【００４６】
ここで、第１ガイド層１０４及び第２ガイド層１０６の禁制帯幅を量子障壁層１２１の禁制帯幅よりも小さくすることは、ガイド層１０４、１０６のＡｌ組成比を量子障壁層１２１のＡｌ組成比より小さく設定することに相当する。このため、クラッド層１０３、１０７からガイド層１０４、１０６へのドーパント拡散が抑制される。従って、ＭＱＷ活性層１０５へのドーパント拡散が抑制されるので、ＭＱＷ活性層１０５の量子井戸層１２０のＡｌ組成比の変化が抑制され発振波長ずれが防止される。
【００４７】
また、第１ガイド層１０４又は第２ガイド層１０６の禁制帯幅を制御することは屈折率を制御することに相当する。従って、２つのガイド層１０４、１０６の屈折率をそれぞれ適当な値に設定して、垂直放射角を調整することが可能となる。
【００４８】
さらに、ガイド層１０４、１０６のＡｌ組成比を量子障壁層１２１のＡｌ組成比より小さく設定することにより、ガイド層１０４、１０６の非発光再結合準位の数を低減できる。このため、量子井戸層１２０からガイド層１０４、１０６に漏れだしたキャリヤに対して、非発光再結合を抑制できる。従って、キャリヤを発光に有効に利用でき、閾値電流の低減が可能となる。
【００４９】
本実施形態１の半導体レーザ素子において、第１ガイド層１０４及び第２ガイド層１０６のＡｌ組成比を変化させた場合、即ち、禁制帯幅を変化させた場合の閾値電流の変化を調べた。図３に、その結果をガイド層１０４、１０６の禁制帯幅と閾値電流との関係で示す。ここで、Ｅｇは第１ガイド層１０４及び第２ガイド層１０６の禁制帯幅を、Ｅｂは量子障壁層１２１の禁制帯幅を、ＥλはＭＱＷ活性層１０５のレーザ発振光のエネルギーに相当する禁制帯幅をそれぞれ表す。この結果から、ＥｇがＥｂより小さくなると閾値電流は減少し、ＥｇがＥλ以下ではＥｇ＝Ｅｂのときよりも逆に閾値電流は増大することがわかる。特に、Ｅｇが下記（２）式の条件を満たすときに閾値電流は最小になる。
【００５０】
Ｅλ＋１００ｍｅＶ≦Ｅｇ≦Ｅｂ−５０ｍｅＶ・・・・（２）
このように、閾値電流はガイド層１０４、１０６の禁制帯幅に大きく依存する。ここで、ＥｇがＥλ＋１００ｍｅＶより小さくなると閾値電流が増大するのは、ＭＱＷ活性層１０５の量子井戸層１２０に注入されたキャリヤが、第１ガイド層１０４及び第２ガイド層１０６に隣接する量子井戸層１２０から第１ガイド層１０４及び第２ガイド層１０６にキャリヤが漏れだして、量子井戸層１２０内のキャリヤの閉じ込め率が低下するために、発振に必要なキャリヤが増大して、閾値電流が増加するためである。
【００５１】
また、ＥｇがＥｂ−５０ｍｅＶより大きいときに閾値電流が増大するのは、ＥｇがＥｂ−５０ｍｅＶより大きいと第１ガイド層１０４及び第２ガイド層１０６の屈折率が低下するので、ＭＱＷ活性層１０５への光の閉じ込め率が減少し、発振に多くの電流を必要とするためである。
【００５２】
（実施形態２）
図４及び図５は、本発明の半導体レーザ素子の実施形態２を示す。
【００５３】
図４に示すように、この半導体レーザ素子は、ｎ−ＧａＡｓ基板２０１上に、層厚０．５μｍのｎ−ＧａＡｓバッファ層２０２、層厚１．５μｍのｎ−Ａｌ_０．５Ｇａ_０．５Ａｓ第１クラッド層２０３、層厚１０ｎｍのＡｌ_０．２７Ｇａ_０．７３Ａｓ第１ガイド層２０４、ノンドープＭＱＷ活性層２０５、層厚１０ｎｍのＡｌ_０．２７Ｇａ_０．７３Ａｓ第２ガイド層２０６、層厚０．２μｍのｐ−Ａｌ_０．５Ｇａ_０．５Ａｓ第２クラッド層２０７、層厚０．２μｍのｐ−Ａｌ_０．２Ｇａ_０．８Ａｓエッチングブロック層２０８、層厚０．００３μｍのｐ−ＧａＡｓエッチングストッパ層２０９、層厚１．２μｍのｐ−Ａｌ_０．５Ｇａ_０．５Ａｓ第３クラッド層２１０及び層厚０．８μｍのｐ−ＧａＡｓキャップ層２１１をＭＯＣＶＤ法により順次積層した構造を有する。
【００５４】
ここで、ＭＱＷ活性層２０５は、図５に示すように層厚１０ｎｍのＡｌ_０．１Ｇａ_０．９Ａｓ量子井戸層２２０と、層厚５ｎｍのＡｌ_０．３５Ｇａ_０．６５Ａｓ量子障壁層２２１とからなり、この量子井戸層３層と量子障壁層２層による交互の繰り返しにより構成されている。
【００５５】
次に、上記の積層体の表面にフォトレジストからなるストライプマスクを形成し、選択エッチングによりｐ−ＧａＡｓエッチングストッパ層２０９表面でエッチングを停止させて、底部のストライプ幅２．２μｍのリッジストライプ２１２を形成する。
【００５６】
リッジストライプ２１２の両側を埋め込むように、層厚０．６μｍのｎ−Ａｌ_０．７Ｇａ_０．３Ａｓ第１電流光閉じ込め層２１３、層厚０．３μｍのｎ−ＧａＡｓ第２電流閉じ込め層２１４及び層厚０．３μｍのｐ−ＧａＡｓ平坦化層２１５を順次ＭＯＣＶＤ法により成長させる。
【００５７】
次に、ｐ−ＧａＡｓキャップ層２１１及びｐ−ＧａＡｓ平坦化層２１５の表面を覆うように、層厚３μｍのｐ−ＧａＡｓコンタクト層２１６をＭＯＣＶＤ法により成長させる。ｎ−ＧａＡｓ基板２０１表面及びｐ−ＧａＡｓコンタクト層２１６表面にそれぞれｎ型電極２１７及びｐ型電極２１８を形成する。劈開法により共振器長を３７５μｍに調整し、共振器端面の光出射側の端面反射率が１２％、後側の端面反射率が９５％となるように、各端面にＡｌ_２Ｏ_３膜とＳｉ膜を形成する。
【００５８】
本実施形態２の半導体レーザ素子において、ｎ型電極２１７とｐ型電極２１８の間に順方向電圧を印加した場合、発振波長０．７８μｍ、閾値電流３０ｍＡ、電流−光出力特性のスロープ効率０．７５Ｗ／Ａ、光出力３０ｍＷにおける動作電流７０ｍＡ、動作電圧１．８Ｖが得られた。
【００５９】
本実施形態２の半導体レーザ素子では、図５に示すように、第１ガイド層２０４及び第２ガイド層２０６を量子井戸層２２０に隣接させると共に第１ガイド層２０４及び第２ガイド層２０６の禁制帯幅を量子井戸層２２０の禁制帯幅より大きくし、且つ、第１ガイド層２０４及び第２ガイド層２０６の禁制帯幅を量子障壁層２２１の禁制帯幅よりも小さくしている。また、第１クラッド層２０３と第３クラッド層２１０との間にＭＱＷ活性層２０５のレーザ発振光のエネルギーと略等しい発光エネルギーを有するｐ−ＧａＡｓエッチングストッパ層２０９を設けている。
【００６０】
ここで、層厚０．２μｍのｐ−Ａｌ_０．２Ｇａ_０．８Ａｓエッチングブロック層２０８に隣接する層厚０．００３μｍのｐ−ＧａＡｓエッチングストッパ層２０９の発光エネルギーを、ＭＱＷ活性層２０５のレーザ発振光のエネルギーに略等しくしているため、このエッチングストッパ層２０９が可飽和吸収層として機能する。このエッチングストッパ層２０９の可飽和吸収効果により、レーザ発振光がパルス状に発振する自励発振が起こる。自励発振状態ではレーザの縦モードがマルチモードになり、各縦モードのスペクトル線幅が広がる。このため、レーザのコヒーレンシーが低下して、戻り光による影響を受け難くなり、雑音を低減させることができる。
【００６１】
本実施形態２では、発振波長の設定値０．７８μｍに対して、作製した半導体レーザ素子の発振波長も０．７８μｍとなり、発振波長の制御ができる。
【００６２】
これに対して、従来例の第１ガイド層７０４と第２ガイド層７０６の禁制帯幅を量子障壁層７１１の禁制帯幅に等しくした場合（従来形態１）、即ち、第１ガイド層７０４と第２ガイド層７０６のＡｌ組成比を０．３５とした場合には、発振波長の設定値０．７８μｍに対して、発振波長が０．７７μｍまで波長ずれが起こり、発振波長の制御が困難となる。
【００６３】
これは、本実施形態２では、可飽和吸収用に低Ａｌ組成比からなるｐ−Ａｌ_０．２Ｇａ_０．８Ａｓエッチングブロック層２０８及びｐ−ＧａＡｓエッチングストッパ層２０９をｐ型第２クラッド層２０７に隣接して配置したために、ｐ型第２クラッド層２０７のドーパントがこれらの低Ａｌ組成比の層で拡散が抑制され、逆に反対側のＭＱＷ活性層２０５側への拡散が増大したことに起因する。
【００６４】
これに対し、上記従来形態１では、ガイド層７０４、７０６のＡｌ組成比が量子障壁層７１１のＡｌ組成比と同じになるために、ｐ型第２クラッド層７０７のドーパントがＭＱＷ活性層７０５の量子井戸層７１０まで拡散し易くなり、拡散により量子井戸層７１０のＡｌ組成比の変化が生じて、発振波長が短波長側にシフトしてしまう。
【００６５】
このように、本実施形態２は波長ずれを防止するのに有効である。また、ドーパント拡散によるＭＱＷ活性層２０５の量子井戸層２２０のＡｌ組成比の変化は、量子井戸層２２０の層厚、量子障壁層２２１の層厚にも影響を及ぼす。このため、波長ずれ以外にも、電気的特性及び光学的特性においても設計値とのずれが生じる。本実施形態２によれば、このような問題を解決することも可能となる。
【００６６】
尚、上記の実施形態２では、可飽和吸収層として、ｐ−Ａｌ_０．２Ｇａ_０．８Ａｓエッチングブロック層２０８に隣接してｐ−ＧａＡｓエッチングストッパ層２０９を設ける例を示したが、それ以外に、ｐ型クラッド層中に、ＭＱＷ活性層の量子準位と略等しい量子準位の単一量子井戸層を設ける形態、ＭＱＷ活性層の量子準位と略等しい量子準位の多重量子井戸層を設ける形態及びＭＱＷ活性層の量子準位と略等しい禁制帯幅の２０ｎｍよりも厚いバルク型半導体層を設ける形態とすることも可能である。上記いずれの形態においても同様の効果が得られる。
【００６７】
さらに、上記の実施形態２では、ｐ型クラッド層中に可飽和吸収層を設ける形態を示しているが、それ以外にｎ型クラッド層中に可飽和吸収層を設ける形態とすることも可能である。即ち、可飽和吸収層は、ＭＱＷ活性層を挟むｐ型クラッド層とｎ型クラッド層との間に適宜設ければ良い。
【００６８】
（実施形態３）
図６は、本発明の半導体レーザ素子の実施形態３を示す。
【００６９】
図６に示すように、この半導体レーザ素子は、ｎ−ＧａＡｓ基板３０１上に、ｎ−Ｇａ_０．５Ｉｎ_０．５Ｐバッファ層３０２、層厚１．５μｍのｎ−（Ａｌ_０．７Ｇａ_０．３）_０．５Ｉｎ_０．５ｐ第１クラッド層３０３、層厚３５ｎｍの（Ａｌ_０．４Ｇａ_０．６）_０．５Ｉｎ_０．５Ｐ第１ガイド層３０４、ノンドープＭＱＷ活性層３０５、層厚３５ｎｍの（Ａｌ_０．４Ｇａ_０．６）_０．５Ｉｎ_０．５Ｐ第２ガイド層３０６、層厚１．５μｍのｐ−（Ａｌ_０．７Ｇａ_０．３）_０．５Ｉｎ_０．５ｐ第２クラッド層３０７及び層厚０．３μｍのｐ−Ｇａ_０．５Ｉｎ_０．５ｐキャップ層３０８を分子線エピタキシャル成長法（ＭＢＥ法）により順次積層した構造を有する。
【００７０】
ここで、ＭＱＷ活性層３０５は、層厚８ｎｍのＧａ_０．５Ｉｎ_０．５Ｐ量子井戸層と、層厚５ｎｍの（Ａｌ_０．５Ｇａ_０．５）_０．５Ｉｎ_０．５Ｐ量子障壁層とからなり、この量子井戸層４層と量子障壁層３層による交互の繰り返しにより構成されている。
【００７１】
次に、上記の積層体の表面にフォトレジストからなるストライプマスクを形成し、選択エッチングを行い、ｐ−（Ａｌ_０．７Ｇａ_０．３）_０．５Ｉｎ_０．５Ｐ第２クラッド層３０７の平坦部の残し厚さが０．３μｍとなるようにエッチングを停止させて、幅５μｍのリッジストライプ３０９を形成する。
【００７２】
次に、リッジストライプ３０９の外側を埋め込むように、層厚１．２μｍのｎ−ＧａＡｓ電流光閉じ込め層３１０をＭＢＥ法により成長させる。
【００７３】
次に、ｎ−ＧａＡｓ基板３０１表面及びｐ−ＧａＡキャップ層３０８表面にそれぞれｎ型電極３１１及びｐ型電極３１２を形成する。劈開法により共振器長を５００μｍに調整し、共振器端面の光出射側端面の反射率が５０％、後側の反射率が８５％となるように、各端面にＡｌ_２Ｏ_３膜とＳｉ膜を形成する。
【００７４】
本実施形態３の半導体レーザ素子で、ｎ型電極３１１とｐ型電極３１２の間に順方向電圧を印加した場合、発振波長０．６５μｍ、閾値電流３０ｍＡ、電流−光出力特性のスロープ効率０．６Ｗ／Ａ、光出力３ｍＷにおける動作電流３５ｍＡ、動作電圧２Ｖが得られた。このように、本実施形態３では、動作電圧の増大を抑制して閾値電流を低減できる。
【００７５】
また、本実施形態３では、発振波長の設定値０．６５μｍに対して、作製した半導体レーザ素子の発振波長も０．６５μｍとなり、発振波長の制御ができる。
【００７６】
これに対して、第１ガイド層７０４及び第２ガイド層７０６の禁制帯幅を量子障壁層７１１の禁制帯幅に等しくした場合（従来形態１）、即ち、第１ガイド層７０４及び第２ガイド層７０６のＡｌ組成比を０．５とした場合には、発振波長の設定値０．６５μｍに対して、発振波長が０．６４μｍまで波長ずれが起こり、発振波長の制御が困難となる。これは、ｐ型クラッド層７０７のドーパントが拡散しやすい傾向にあるため、ｐ型クラッド層７０７のドーパントがＭＱＷ活性層７０５へ拡散し、量子井戸層７１０のＡｌ組成比が変化するために波長ずれを起こすためである。
【００７７】
このように、クラッド層の種類によっては、例えば、ＡｌＧａＩｎＰ系クラッド層では、特にＡｌＧａＡｓ系クラッド層に比べてｐ型クラッド層のドーパントがより拡散しやすい傾向にある。従って、このようなクラッド層にあっては、ｐ型クラッド層側の第２ガイド層の禁制帯幅を量子障壁層の禁制帯幅より小さくすることにより、ドーパントの活性層への拡散を抑制して、波長ずれを一層効果的に防止できる。
【００７８】
（実施形態４）
図７は、本発明の半導体レーザ素子の実施形態４を示す。
【００７９】
図７に示すように、この半導体レーザ素子は、ｎ−ＧａＡｓ基板４０１上に、層厚０．５μｍのｎ−ＧａＡｓバッファ層４０２、層厚１．２μｍのｎ−Ａｌ_０．５Ｇａ_０．５Ａｓ第１クラッド層４０３、層厚０．２μｍのｎ−Ａｌ_０．４８Ｇａ_０．５２Ａｓ第２クラッド層４０４、層厚５ｎｍのＡｌ_０．２７Ｇａ_０．７３Ａｓ第１ガイド層４０５、ノンドープＭＱＷ活性層４０６、層厚５ｎｍのＡｌ_０．２７Ｇａ_０．７３Ａｓ第２ガイド層４０７、層厚０．１５μｍのｐ−Ａｌ_０．５Ｇａ_０．５Ａｓ第２クラッド層４０８、層厚０．００２μｍのｐ−ＧａＡｓエッチングストッパ層４０９、層厚１．０μｍのｐ−Ａｌ_０．５Ｇａ_０．５Ａｓ第３クラッド層４１０及び層厚０．８μｍのｐ−ＧａＡｓキャップ層４１１をＭＯＣＶＤ法により順次積層した構造を有する。
【００８０】
ここで、ＭＱＷ活性層４０６は、層厚１０ｎｍのＡｌ_０．１３Ｇａ_０．８７Ａｓ量子井戸層と、層厚５ｎｍのＡｌ_０．３５Ｇａ_０．６５Ａｓ量子障壁層とからなり、この量子井戸層８層と量子障壁層７層による交互の繰り返しにより構成されている。
【００８１】
尚、ｎ型第２クラッド層４０４は、垂直方向の放射角の制御用に用いられるものであり、本発明の効果には影響を及ぼさない。
【００８２】
次に、上記の積層体の表面にフォトレジストからなるストライプマスクを形成し、選択エッチングによりｐ−ＧａＡｓエッチングストッパ層４０９表面でエッチングを停止させて、底部のストライプ幅２．２μｍのリッジストライプ４１２を形成する。
【００８３】
リッジストライプ４１２の両側を埋め込むように、層厚０．６μｍのｎ−Ａｌ_０．７Ｇａ_０．３Ａｓ第１電流光閉じ込め層４１３、層厚０．３μｍのｎ−ＧａＡｓ第２電流閉じ込め層４１４及び層厚０．３μｍのｐ−ＧａＡｓ平坦化層４１５をＭＯＣＶＤ法により順次成長させる。
【００８４】
次に、ｐ−ＧａＡｓキャップ層４１１及びｐ−ＧａＡｓ平坦化層４１５の表面を覆うように、層厚３μｍのｐ−ＧａＡｓコンタクト層４１６をＭＯＣＶＤ法により成長させる。ｎ−ＧａＡｓ基板４０１表面及びｐ−ＧａＡｓコンタクト層４１６表面にそれぞれｎ型電極４１７及びｐ型電極４１８を形成する。劈開法により共振器長を２００μｍに調整し、共振器端面の光出射側の端面反射率が３０％、後側の端面反射率が７５％となるように、各端面にＡｌ_２Ｏ_３膜とＳｉ膜を形成する。
【００８５】
本実施形態４の半導体レーザ素子において、ｎ型電極４１７とｐ型電極４１８の間に順方向電圧を印加した場合、発振波長０．７８μｍ、閾値電流１５ｍＡ、電流−光出力スロープ効率０．７５Ｗ／Ａ、光出力３ｍＷの動作電流１９ｍＡ、動作電圧１．８Ｖが得られた。
【００８６】
本実施形態４の半導体レーザ素子では、ストライプ外部のＭＱＷ活性層での可飽和吸収効果を利用して、レーザ発振光がパルス状に発振する自励発振が起こる。自励発振状態ではレーザの縦モードがマルチモードになり、各縦モードのスペクトル線幅が広がる。このため、レーザのコヒーレンシーが低下して、戻り光による影響を受け難くなり、雑音を低減させることができる。
【００８７】
本実施形態４の半導体レーザ素子においては、活性層に水平方向と垂直方向の放射光の最小スポット位置の差、即ち、非点隔差は５μｍとなる。
【００８８】
これに対して、従来例の第１ガイド層７０４及び第２ガイド層７０６の禁制帯幅を量子障壁層７１１の禁制帯幅に等しくした場合（従来形態１）、即ち、第１ガイド層７０４及び第２ガイド層７０６のＡｌ組成比を０．３５とした場合には、放射角の非点隔差は１５μｍに増大する。非点隔差が増大すると、放射光をレンズで集光するとき、集光スポットサイズの増大を生じ、光デイスク等のシステムで使用することが困難となる。
【００８９】
即ち、本実施形態４の半導体レーザ素子によれば、ガイド層４０５、４０７に隣接する量子井戸層のストライプ外部における可飽和吸収量を増大することができ、ストライプ内部に光を閉じ込めたまま自励発振を起こすことができる。その結果、ストライプ外部の可飽和吸収効果による波面の影響を受け難くなって、活性層に水平方向の放射光のスポット位置のずれが小さくなって、非点隔差が改善される。従って、放射光の非点隔差を低減して、光学特性を改善することが可能となる。
【００９０】
図８は、本実施形態４と従来形態１についての半導体レーザ素子の屈折率差Δｎと非点隔差ΔＺとの関係を示す。ここで、屈折率差Δｎは、ストライプ内部の活性層に閉じこめられた光の屈折率ｎ_ａと、ストライプ外部の活性層に閉じこめられた光の屈折率ｎ_ｂとの差を表す。この結果から屈折率差が非点隔差に依存することが認められ、しかも本実施形態４は従来形態１に比べ非点隔差ΔＺが低く抑えられていることがわかる。
【００９１】
自励発振を起こすには、ストライプ外部に光を滲みださせるために、屈折率差Δｎを７×１０^−３以下にする必要がある。非点隔差ΔＺは屈折率差Δｎを減少するにしたがい増大する傾向にある。本実施形態４の半導体レーザ素子で、非点隔差ΔＺを１０μｍ以下にするには屈折率差Δｎを２×１０^−３以上にする必要がある。そこで、本実施形態４では、屈折率差Δｎを下記（１）式の条件を満たす範囲に設定することにより、非点隔差１０μｍ以下の良好な光学特性を有する自励発振型の低雑音半導体レーザ素子を構成している。
【００９２】
２×１０^−３≦Δｎ≦７×１０^−３・・・・（１）
これに対して、従来形態１では自励発振を起こすのに必要な屈折率差７×１０^−３以下において、非点隔差ΔＺは１０μｍより大きくなる。従って、従来形態１では非点隔差を低減して、自励発振を起こすことはできない。
【００９３】
（実施形態５）
図９及び図１０は、本発明の半導体レーザ素子の実施形態５を示す。
【００９４】
図９に示すように、この半導体レーザ素子は、ｎ−ＧａＡｓ基板５０１上に、層厚０．５μｍのｎ−ＧａＡｓバッファ層５０２、層厚１．５μｍのｎ−Ａｌ_０．５Ｇａ_０．５Ａｓ第１クラッド層５０３、層厚１５ｎｍのＡｌ_０．３Ｇａ_０．７Ａｓ第１ガイド層５０４、ノンドープＭＱＷ活性層５０５、層厚１５ｎｍのＡｌ_０．２５Ｇａ_０．７５Ａｓ第２ガイド層５０６、層厚０．２μｍのｐ−Ａｌ_０．５Ｇａ_０．５Ａｓ第２クラッド層５０７、層厚０．００３μｍのｐ−ＧａＡｓ第１エッチングストッパ層５０８、層厚０．０１μｍのｐ−Ａｌ_０．６Ｇａ_０．４Ａｓ第２エッチングストッパ層５０９、層厚１．０μｍのｎ−Ａｌ_０．５Ｇａ_０．５Ａｓ電流光閉じ込め層５１０及び層厚０．８μｍのｎ−ＧａＡｓキャップ層５１１をＭＯＣＶＤ法により順次積層した構造を有する。
【００９５】
ここで、ＭＱＷ活性層５０５は、図１０に示すように層厚８ｎｍのＡｌ_０．１Ｇａ_０．９Ａｓ量子井戸層５２０と、層厚５ｎｍのＡｌ_０．３５Ｇａ_０．６５Ａｓ量子障壁層５２１からなり、この量子井戸層２層と量子障壁層１層による交互の繰り返しにより構成されている。
【００９６】
尚、ｐ−ＧａＡｓ第１エッチングストッパ層５０８及びｐ−Ａｌ_０．６Ｇａ_０．４Ａｓ第２エッチングストッパ層５０９の各層厚は非常に薄いために、エッチングストッパ層は光の閉じ込め作用又は内部の光吸収に影響を及ぼさない。
【００９７】
次に、上記積層体の表面にフォトレジストからなるストライプ窓を形成し、選択エッチングによりｐ−ＧａＡｓ第１エッチングストッパ層表面に到達する幅２．５μｍのストライプ溝５１２を形成する。
【００９８】
次に、ストライプ溝５１２を埋め込むように、層厚１．５μｍのｐ−Ａｌ_０．５Ｇａ_０．５Ａｓ第３クラッド層５１３及び層厚２μｍのｐ−ＧａＡｓコンタクト層５１４をＭＯＣＶＤ法により順次成長させる。
【００９９】
ｎ−ＧａＡｓ基板５０１表面及びｐ−ＧａＡｓコンタクト層５１４表面にそれぞれｎ型電極５１５及びｐ型電極５１６を形成する。劈開法により共振器長を３７５μｍに調整し、共振器端面の光出射側の端面反射率が３０％、後側の端面反射率が９５％となるように、各端面にＡｌ_２Ｏ_３膜を形成する。
【０１００】
本実施形態５の半導体レーザ素子において、ｎ型電極５１５とｐ型電極５１６の間に順方向電圧を印加した場合、発振波長０．７８μｍ、閾値電流１０ｍＡ、電流−光出力特性のスロープ効率０．７５Ｗ／Ａ、光出力３ｍＷの動作電流１４ｍＡ、動作電圧１．７Ｖが得られた。
【０１０１】
本実施形態５の半導体レーザ素子では、図１０に示すように、第１ガイド層５０４及び第２ガイド層５０６を量子井戸層５２０に隣接させると共に第１ガイド層５０４及び第２ガイド層５０６の禁制帯幅を量子井戸層５２０の禁制帯幅より大きくし、且つ、第１ガイド層５０４及び第２ガイド層５０６の禁制帯幅を量子障壁層５２１の禁制帯幅よりも小さくしている。さらに、第１ガイド層５０４と第２ガイド層５０６の禁制帯幅を異ならせている。
【０１０２】
本実施形態５では、第１ガイド層５０４と第２ガイド層５０６の禁制帯幅が異なる、即ち、Ａｌ組成比が異なる。このとき、垂直方向の放射角度は２５度である。これに対して、第１ガイド層５０４と第２ガイド層５０６の禁制帯幅を等しくした場合、即ち、Ａｌ組成比を共に０．２５とした場合には、垂直方向の放射角度は３０度になる。このように、両ガイド層５０４、５０６のＡｌ組成比を適当な値に設定することによって、レーザの放射光特性を制御することが可能となる。
【０１０３】
（実施形態６）
図１１及び図１２は、本発明の半導体レーザ素子の実施形態６を示す。
【０１０４】
図１１に示すように、この半導体レーザ素子は、サファイア基板６０１上に、層厚０．０５μｍのＧａＮバッファ層６０２、層厚３μｍのｎ−ＧａＮ第１クラッド層６０３、層厚０．１μｍのｎ−Ｉｎ_０．０５Ｇａ_０．９５Ｎ第２クラッド層６０４、層厚０．５μｍのｎ−Ａｌ_０．０５Ｇａ_０．９５Ｎ第３クラッド層６０５、層厚２０ｎｍのノンドープＩｎ_０．１Ｇａ_０．９Ｎ第１ガイド層６０６、ノンドープＭＱＷ活性層６０７、層厚２０ｎｍのノンドープＩｎ_０．１Ｇａ_０．９Ｎ第２ガイド層６０８、層厚２０ｎｍのｐ−Ａｌ_０．２Ｇａ_０．８Ｎ第４クラッド層６０９、層厚０．１μｍのｐ−ＧａＮ第５クラッド層６１０、層厚０．５μｍのｐ−Ａｌ_０．０５Ｇａ_０．９５Ｎ第６クラッド層６１１及び層厚０．２μｍのｐ−ＧａＮコンタクト層６１２をＭＯＣＶＤ法により順次積層した構造を有する。
【０１０５】
ここで、ＭＱＷ活性層６０７は、図１２に示すように層厚４ｎｍのＩｎ_０．２Ｇａ_０．８Ｎ量子井戸層６２０と、層厚８ｎｍのＩｎ_０．０５Ｇａ_０．９５Ｎ量子障壁６２１とからなり、この量子井戸層３層と量子障壁層２層による交互の繰り返しにより構成されている。
【０１０６】
次に、上記積層体の表面にストライプ状のレジストマスクを形成し、ドライエッチングにより幅２μｍのリッジストライプ６３０を形成し、ｎ−ＧａＮ第１クラッド層６０３及びｐ−ＧａＮコンタクト層６１２にそれぞれｎ型電極６４０及びｐ型電極６４１を形成する。劈開法により共振器長は７００μｍ、共振器端面の光出射側の端面反射率と後側の端面反射率３０％となるように誘電体膜のコーティングにより調整する。
【０１０７】
本実施形態６の半導体レーザ素子において、ｎ型電極６４０とｐ型電極６４１の間に順方向電圧を印加した場合、発振波長０．４１μｍ、閾値電流１００ｍＡ、電流−光出力特性のスロープ効率０．２Ｗ／Ａ、閾値電流時の動作電圧６Ｖが得られた。このように、本実施形態６では、動作電圧の増大を抑制して閾値電流を低減できる。
【０１０８】
本実施形態６の半導体レーザ素子では、図１２に示すように、第１ガイド層６０６及び第２ガイド層６０８を量子井戸層６２０に隣接させると共に第１ガイド層６０６及び第２ガイド層６０８の禁制帯幅を量子井戸層６２０の禁制帯幅より大きくし、且つ、第１ガイド層６０６及び第２ガイド層６０８の禁制帯幅を量子障壁層６２１の禁制帯幅よりも小さくしている。さらに、ガイド層６０６、６０８の禁制帯幅を量子井戸層６２０と量子障壁層６２１の中間に設定している。
【０１０９】
ここで、各半導体層の組成比と禁制帯幅との関係については、各半導体層のＩｎ組成比が増大すると、禁制帯幅は減少し屈折率は増大する関係にある。また、各半導体層のＡｌ組成比が増大すると、禁制帯幅は増大し屈折率は減少する関係にある。
【０１１０】
本実施形態６では、ガイド層６０６、６０８にノンドープのＩｎＧａＮ層を用いているので、ｎ−第３クラッド層６０５及びｐ−第４クラッド層６０９からのドーパントがＭＱＷ活性層６０７に拡散するのが、ガイド層６０６、６０８によって抑制されるので、組成比の変動にともなう発振波長ずれを防止することができる。
【０１１１】
さらに、ガイド層６０６、６０８にＩｎＧａＮを適用し、その禁制帯幅を量子井戸層６２０と量子障壁層６２１の中間に設定しているので、ガイド層６０６、６０８の屈折率を増大させることができる。このため、動作電圧の増大を抑制しつつＭＱＷ活性層６０７への光の閉じ込め率を増大させ、閾値電流を低減することが可能となる。
【０１１２】
これに対して、従来例では、ガイド層７０４、７０６にｎ型又はｐ型のＧａＮ層を用いるために、ドーパントがＭＱＷ活性層７０５に拡散して発振波長ずれを引き起こすという問題が生じる。それを防止するために、ガイド層７０４、７０６をノンドープにすると、動作電圧が増大するという別の問題が生じる。
【０１１３】
尚、上記の各実施形態では、第１ガイド層及び第２ガイド層の禁制帯幅を量子障壁層の禁制帯幅より小さくした場合について述べたが、第１ガイド層又は第２ガイド層のいずれか一方の禁制帯幅を量子障壁層の禁制帯幅より小さくすることも可能である。
【０１１４】
また、本発明は、上述の各実施形態で示した層厚、Ａｌ、Ｉｎ等の組成比、キャリヤ濃度に限定されるものではなく、それ以外の条件とすることも可能である。
【０１１５】
また、成長法については、ＭＯＣＶＤ法及びＭＢＥ法以外に、ＬＰＥ法、ガスソースＭＢＥ法、ＡＬＥ（原子線エピタキシー）法を適用することも可能である。
【０１１６】
【発明の効果】
上記の本発明の半導体レーザ素子によれば、ガイド層の禁制帯幅を量子障壁層の禁制帯幅より小さくし、即ち、ガイド層の屈折率を量子障壁層の屈折率より大きく設定しているので、ＭＱＷ活性層の量子井戸層への光の閉じ込め率を増大することができ、閾値電流を低減できる。加えて、従来例のようにガイド層の層厚を厚くする必要もないので、素子抵抗の増大に伴う動作電圧の増大も抑制することができ、閾値電流を低減できる。
【０１１７】
また、ガイド層の禁制帯幅を量子障壁層の禁制帯幅より小さくし、例えば、Ａｌ系半導体層の場合は、ガイド層のＡｌ組成比を量子障壁層のＡｌ組成比より小さく設定しているので、クラッド層からガイド層へのドーパント拡散を抑制することができる。その結果、ＭＱＷ活性層へのドーパント拡散を抑制することができるので、ＭＱＷ活性層の量子井戸層のＡｌ組成比の変化を抑制することができ発振波長ずれを防止できる。
【０１１８】
また、ガイド層の禁制帯幅を制御することは屈折率を制御することに相当し、２つのガイド層の屈折率をそれぞれ適当な値に設定して、垂直放射角を調整することができる。
【０１１９】
さらに、ガイド層のＡｌ組成比を量子障壁層のＡｌ組成比より小さく設定することにより、ガイド層の非発光再結合準位の数を低減することができるので、量子井戸層からガイド層に漏れだしたキャリヤに対して、非発光再結合を抑制することができる。その結果、キャリヤを発光に有効に利用でき、閾値電流を低減できる。
【０１２０】
また、特に請求項２及び請求項３記載の半導体レーザ素子によれば、ＭＱＷ活性層のレーザ発振光の禁制帯幅と略等しい発光エネルギーの禁制帯幅を有する可飽和吸収層を、２つのクラッド層の間に設ける構成をとるので、可飽和吸収効果によりレーザ発振光がパルス状に発振する自励発振が起こる。自励発振状態ではレーザの縦モードがマルチモードになり、各縦モードのスペクトル線幅が広がる。このため、レーザのコヒーレンシーが低下して、戻り光による影響を受け難くなり、雑音を低減することができる。
【０１２１】
尚、この可飽和吸収層を設ける構成による場合には、ＭＱＷ活性層と可飽和吸収層の間のクラッド層のドーパントが可飽和吸収層で拡散が抑制され、ＭＱＷ活性層側への拡散が増大することが考えられるが、ガイド層のＡｌ組成比を量子障壁層のＡｌ組成比より小さく設定しているため、クラッド層からガイド層へのドーパント拡散を抑制することができる。従って、ＭＱＷ活性層へのドーパント拡散を抑制することができるので、ＭＱＷ活性層の量子井戸層のＡｌ組成比の変化を抑制して発振波長ずれを防止することができる。
【０１２２】
また、特に請求項４記載の半導体レーザ素子によれば、ストライプ状の第３クラッド層を設け、上記屈折率差Δｎを上記（１）式の範囲内で設定するので、ストライプ外部の活性層における可飽和吸収量を増大させて、ストライプ内部に光を閉じ込めたまま自励発振を起こすことができる。その結果、ストライプ外部の可飽和吸収効果による波面の影響を受け難くなって、活性層に水平方向の放射光のスポット位置のずれを小さくでき、即ち、非点隔差を低減し、光学特性を改善できる。
【０１２３】
また、特に請求項５記載の半導体レーザ素子によれば、ｐ型クラッド層側の第２ガイド層の禁制帯幅を量子障壁層の禁制帯幅より小さくするので、例えば、ＡｌＧａＩｎＰ系クラッド層のようにｐ型クラッド層のドーパントが拡散しやすい傾向にあるクラッド層に対し、ドーパントの活性層への拡散を抑制して波長ずれを一層効果的に防止できる。
【０１２４】
また、特に請求項６記載の半導体レーザ素子によれば、２つのガイド層の禁制帯幅の内小さい方のガイド層の禁制帯幅を、活性層のレーザ発振光のエネルギーに相当する禁制帯幅より大きく設定するので、図３に示すように閾値電流を一層効果的に低減できる。
【０１２５】
また、特に請求項７記載の半導体レーザ素子によれば、２つのガイド層の禁制帯幅の内小さい方のガイド層の禁制帯幅Ｅｇを上記（２）式の関係を満たすように設定するので、図３に示すように閾値電流を一層効果的に低減できる。
【図面の簡単な説明】
【図１】本発明の実施形態１の半導体レーザ素子の断面構造を示す図である。
【図２】本発明の実施形態１の活性層付近のエネルギーバンドダイヤグラムである。
【図３】本発明のガイド層の禁制帯幅と閾値電流との関係を示す図である。
【図４】本発明の実施形態２の半導体レーザ素子の断面構造を示す図である。
【図５】本発明の実施形態２の活性層付近のエネルギーバンドダイヤグラムである。
【図６】本発明の実施形態３の半導体レーザ素子の断面構造を示す図である。
【図７】本発明の実施形態４の半導体レーザ素子の断面構造を示す図である。
【図８】半導体レーザ素子の屈折率差と非点隔差との関係を示す図である。
【図９】本発明の実施形態５の半導体レーザ素子の断面構造を示す図である。
【図１０】本発明の実施形態５の活性層付近のエネルギーバンドダイヤグラムである。
【図１１】本発明の実施形態６の半導体レーザ素子の断面構造を示す図である。
【図１２】本発明の実施形態６の活性層付近のエネルギーバンドダイヤグラムである。
【図１３】従来の半導体レーザ素子の断面構造を示す図である。
【図１４】従来の活性層付近のエネルギーバンドダイヤグラムである。
【符号の説明】
１０１，２０１，３０１，４０１，５０１，６０１，７０１ ｎ−ＧａＡｓ基板１０２，２０２，３０２，４０２，５０２，６０２，７０２ ｎ−ＧａＡｓバッファ層
１０３，２０３，３０３，４０３，５０３，６０３，７０３ ｎ−第１クラッド層
４０４，６０４ ｎ−第２クラッド層
６０５ ｎ−第３クラッド層
１０４，２０４，３０４，４０５，５０４，６０６，７０４ 第１ガイド層
１０５，２０５，３０５，４０６，５０５，６０７，７０５ ＭＱＷ活性層
１２０，２２０，５２０，６２０，７１０ 量子井戸層
１２１，２２１，５２１，６２１，７１１ 量子障壁層
１０６，２０６，３０６，４０７，５０６，６０８，７０６ 第２ガイド層
１０７，２０７，３０７，５０７，７０７ ｐ−第２クラッド層
４０８ ｐ−第３クラッド層
６０９ ｐ−第４クラッド層
６１０ ｐ−第５クラッド層
６１１ ｐ−第６クラッド層
２０８ ｐ−エッチングブロック層
１０８，２０９，４０９，５０８，５０９ ｐ−エッチングストッパ層
１０９，２１０，４１０，５１３ ｐ−第３クラッド層
１１０，２１１，３０８，４１１ ｐ−キャップ層
１１１，２１２，３０９，４１２，５１２ ストライプ
１１２，２１３，３１０，４１３，５１０ ｎ−第１電流光閉じ込め層
１１３，２１４，４１４ ｎ−第２電流閉じ込め層
１１４，２１５，４１５ ｐ−平坦化層
１１５，２１６，４１６，５１４，６１２，７０８ ｐ−コンタクト層
１１６，１１７，２１７，２１８，３１１，３１２，４１７，４１８，５１５，５１６，６４０，６４１ 電極[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a semiconductor laser device used as a light source in fields such as an optical disk, a laser beam printer, and optical transmission, and more particularly to a structure of a semiconductor laser device having an active layer having a multiple quantum well structure.
[0002]
[Prior art]
In order to reduce the current of a semiconductor laser device, for example, Japanese Patent Publication No. 4-67354 discloses a multiplexing method in which a quantum well layer having a thickness less than the de Broglie wavelength of electrons, that is, a quantum well layer having a thickness of about 20 nm or less is provided in an active layer. Quantum well (MQW: Multiple Quantum)
(Well) The use of an active layer is described.
[0003]
As shown in FIG. 13, this semiconductor laser device has an n-GaAs substrate 701, an n-GaAs buffer layer 702, an n-AlGaAs cladding layer 703, an n-AlGaAs guide layer 704, an MQW active layer 705, and p-AlGaAs. It has a structure in which a guide layer 706, a p-AlGaAs cladding layer 707, and a p-GaAs cap layer 708 are sequentially stacked.
[0004]
Here, the MQW active layer 705 is composed of a plurality of GaAs quantum well layers 710 and an AlGaAs quantum barrier layer 711 sandwiched between the quantum well layers 710, and the three quantum well layers and the two quantum barrier layers are alternately formed. It consists of repetitions.
[0005]
FIG. 14 is an energy band diagram of each layer near the MQW active layer 705. The forbidden band widths of the n-AlGaAs guide layer 704 and the p-AlGaAs guide layer 706 are both set to be equal to the forbidden band width of the quantum barrier layer 711. By making the forbidden bandwidths of the guide layers 704 and 706 equal to the forbidden bandwidth of the quantum barrier layer 711, the quantum well layers 710 are all sandwiched between semiconductor layers having the same forbidden bandwidth. The spread of the level becomes small and the emission spectrum becomes narrow, so that the threshold current can be reduced.
[0006]
[Problems to be solved by the invention]
By the way, in recent semiconductor laser devices, further reduction of the threshold current is required. If the threshold current is to be further reduced in the above-described conventional example, the light confinement rate of the MQW active layer to the quantum well layer can be reduced. Need to increase. Therefore, in the conventional example, the thickness of the guide layer is increased.
[0007]
However, when the thickness of the guide layer is increased, the following problem occurs.
[0008]
Generally, the dopant concentration of the guide layer is usually set to be much lower than the dopant concentration of the cladding layer or non-doped in order to suppress the diffusion of the dopant from the guide layer to the MQW active layer. Therefore, when the guide layer is thickened, the operating voltage increases due to the effect of the increase in the element resistance in the guide layer. Therefore, in the conventional example, although the threshold current can be reduced, a new problem of deterioration in characteristics due to an increase in operating voltage has occurred.
[0009]
In this conventional example, the forbidden bandwidth of the guide layer is set equal to the forbidden bandwidth of the quantum barrier layer. This corresponds to setting the Al composition ratio of the guide layer as high as the Al composition ratio of the quantum barrier layer. Therefore, in this case, the dopant from the cladding layer diffuses to the active layer, the Al composition ratio of the quantum well layer tends to change, the oscillation wavelength deviates from the set value, and wavelength control becomes difficult. There was a problem.
[0010]
An object of the present invention is to solve such a problem of the related art, and an object of the present invention is to provide a semiconductor laser device capable of reducing a threshold current without increasing an operating voltage.
[0011]
Another object of the present invention is to provide a semiconductor laser device that can also prevent oscillation wavelength shift.
[0013]
[Means for Solving the Problems]
The present invention The semiconductor laser element of Contains at least Al Each of the multiple quantum well layers has a bandgap Equally contains at least Al With an active layer of a multiple quantum well structure in which a plurality of quantum barrier layers are alternately stacked, Contains at least Al A first guide layer and Contains at least Al A semiconductor laser device provided with a second guide layer, wherein the first guide layer and the second guide layer are adjacent to the quantum well layer, and a forbidden band width of the first guide layer and the second guide layer is provided. Is larger than the forbidden band width of the quantum well layer, and Making the Al composition ratio of the first guide layer and the second guide layer smaller than the Al composition ratio of the quantum barrier layer; The forbidden band width of the first guide layer and the second guide layer is made smaller than the forbidden band width of the quantum barrier layer, and the first conductive type first band is sandwiched between the first guide layer and the second guide layer. A first cladding layer and a second cladding layer of the second conductivity type are provided, and a third cladding layer of the second conductivity type is provided outside the second cladding layer and at a position opposite to the first cladding layer. A saturable absorbing layer having an emission energy substantially equal to the energy of the laser oscillation light of the active layer is provided between the first cladding layer and the third cladding layer. the above Objective is achieved.
[0015]
Also, preferably, a first cladding layer of a first conductivity type and a second cladding layer of a second conductivity type are provided so as to sandwich the first guide layer and the second guide layer, and the outside of the second cladding layer is provided. And a third cladding layer of the second conductivity type is formed in a stripe shape at a position opposite to the first cladding layer, and a refractive index n of light confined in the active layer inside the stripe. _a And the refractive index n of light confined in the active layer outside the stripe _b Is the condition of the following equation (1).
2 × 10 ^-3 ≦ Δn ≦ 7 × 10 ^-3 ... (1)
And a configuration that satisfies
[0017]
Also, preferably, the forbidden band width of the first guide layer or the second guide layer. Are different The smaller bandgap is made larger than the bandgap corresponding to the energy of the laser oscillation light of the active layer.
[0018]
Preferably, of the forbidden band widths of the first guide layer or the second guide layer, a smaller forbidden band width is Eg, a forbidden band width of the quantum barrier layer is Eb, and laser oscillation light of the active layer is provided. When the forbidden band width corresponding to the energy of E is Eλ, Eg, Eb and Eλ satisfy the condition of the following equation (2).
Eλ + 100 meV ≦ Eg ≦ Eb−50 meV (2)
And a configuration that satisfies
[0019]
Hereinafter, the operation of the present invention will be described.
[0020]
In the above configuration, making the forbidden band width of the guide layer smaller than the forbidden band width of the quantum barrier layer is equivalent to setting the refractive index of the guide layer to be larger than the refractive index of the quantum barrier layer. Therefore, the confinement rate of light in the quantum well layer of the MQW active layer increases, and the threshold current decreases. In addition, since it is not necessary to increase the thickness of the guide layer in order to reduce the threshold current as in the above-described conventional example, an increase in operating voltage due to an increase in element resistance is suppressed.
[0021]
Making the forbidden band width of the guide layer smaller than the forbidden band width of the quantum barrier layer means that, for example, in the case of an Al-based semiconductor layer, the Al composition ratio of the guide layer is set smaller than the Al composition ratio of the quantum barrier layer. It corresponds to that. Therefore, diffusion of the dopant from the cladding layer to the guide layer is suppressed. Therefore, the diffusion of the dopant into the MQW active layer is suppressed, so that the change in the Al composition ratio of the quantum well layer of the MQW active layer is suppressed, and the oscillation wavelength shift is prevented.
[0022]
Further, controlling the forbidden band width of the guide layer corresponds to controlling the refractive index. Therefore, it is possible to adjust the vertical radiation angle by setting the refractive indexes of the two guide layers to appropriate values.
[0023]
Further, by setting the Al composition ratio of the guide layer to be smaller than the Al composition ratio of the quantum barrier layer, the number of non-radiative recombination levels of the guide layer is reduced. For this reason, non-radiative recombination of carriers leaked from the quantum well layer to the guide layer is suppressed. As a result, the carrier is effectively used for light emission, and the threshold current can be reduced.
[0024]
In addition, a saturable absorbing layer having a bandgap of emission energy substantially equal to the bandgap of laser oscillation light of the MQW active layer is provided between two clad layers, for example, the first clad layer and the second clad layer. In the case of a configuration provided between the layers or between the first clad layer and the third clad layer, self-pulsation occurs in which laser oscillation light oscillates in a pulse shape due to the saturable absorption effect. In the self-excited oscillation state, the longitudinal mode of the laser becomes a multimode, and the spectral line width of each longitudinal mode is widened. For this reason, the coherency of the laser is reduced, and the laser is hardly affected by the return light, and the noise is reduced.
[0025]
In the case where the saturable absorption layer is provided, the diffusion of the dopant in the cladding layer between the MQW active layer and the saturable absorption layer is suppressed by the saturable absorption layer, and the diffusion toward the MQW active layer increases. It is possible to do. However, since the Al composition ratio of the guide layer is set smaller than the Al composition ratio of the quantum barrier layer, dopant diffusion from the cladding layer to the guide layer is suppressed. Therefore, the diffusion of the dopant into the MQW active layer is suppressed, so that the change in the Al composition ratio of the quantum well layer of the MQW active layer is suppressed, and the oscillation wavelength shift is prevented.
[0026]
Further, when a stripe-shaped third cladding layer is provided and the refractive index difference Δn is set within the range of the above formula (1), the saturable absorption amount in the active layer outside the stripe increases, and light is confined inside the stripe. Self-excited oscillation occurs. As a result, the influence of the wavefront due to the saturable absorption effect outside the stripe is reduced, and the shift of the spot position of the radiated light in the horizontal direction on the active layer is reduced. That is, the astigmatic difference is reduced, and the optical characteristics are improved.
[0027]
Further, depending on the type of the cladding layer, for example, the dopant of the p-type cladding layer tends to diffuse more easily in the AlGaInP cladding layer than in the case of the AlGaAs cladding layer in particular. Therefore, in such a cladding layer, by making the bandgap of the second guide layer on the p-type cladding layer side smaller than the bandgap of the quantum barrier layer, diffusion of the dopant into the active layer is suppressed. The wavelength shift described above is more effectively prevented.
[0028]
When the forbidden band width of the smaller one of the two guide layers is set to be larger than the forbidden band width corresponding to the energy of the laser oscillation light of the active layer, the threshold current is increased as shown in FIG. Can be more effectively reduced.
[0029]
In particular, when the forbidden band width Eg of the guide layer having the smaller forbidden band width of the two guide layers is set so as to satisfy the relationship of the above equation (2), the threshold current can be further reduced as shown in FIG. It is possible to reduce the total.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings.
[0031]
(Embodiment 1)
1 and 2 show a first embodiment of a semiconductor laser device of the present invention.
[0032]
As shown in FIG. 1, this semiconductor laser device has an n-GaAs buffer layer 102 having a layer thickness of 0.5 μm and an n-Al layer having a layer thickness of 1.5 μm on an n-GaAs substrate 101. _0.5 Ga _0.5 As first cladding layer 103, Al having a thickness of 15 nm _0.25 Ga _0.75 As first guide layer 104, non-doped MQW active layer 105, Al having a thickness of 15 nm _0.25 Ga _0.75 As second guide layer 106, p-Al having a thickness of 0.2 μm _0.5 Ga _0.5 As second cladding layer 107, p-GaAs etching stopper layer 108 having a thickness of 0.003 μm, p-Al having a thickness of 1.2 μm _0.5 Ga _0.5 It has a structure in which an As third cladding layer 109 and a p-GaAs cap layer 110 having a layer thickness of 0.8 μm are sequentially stacked by metal organic chemical vapor deposition (MOCVD).
[0033]
Here, the MQW active layer 105 has an Al thickness of 8 nm as shown in FIG. _0.1 Ga _0.9 As quantum well layer 120 and 5 nm thick Al _0.35 Ga _0.65 An As quantum barrier layer 121 is formed by alternately repeating three quantum well layers and two quantum barrier layers. Note that the forbidden band width of each semiconductor layer is determined by the Al composition ratio.
[0034]
Since the P-GaAs etching stopper layer 108 has a very small thickness of 0.003 μm, the etching stopper layer does not affect light confinement or internal light absorption. The etching stopper layer is useful for forming a ridge stripe described below with good control by etching. However, even when there is no etching stopper layer, it is possible to control the etching with time for forming the ridge stripe.
[0035]
Next, a stripe mask made of a photoresist is formed on the surface of the laminate, and the etching is stopped at the surface of the p-GaAs etching stopper layer 108 by selective etching, and the ridge stripe 111 having a bottom stripe width of 2.2 μm is formed. To form
[0036]
An n-Al layer having a thickness of 0.6 μm is buried on both sides of the ridge stripe 111. _0.7 Ga _0.3 An As first current confinement layer 112, an n-GaAs second current confinement layer 113 having a thickness of 0.3 μm, and a p-GaAs flattening layer 114 having a thickness of 0.3 μm are sequentially grown by MOCVD.
[0037]
Next, a 3 μm-thick p-GaAs contact layer 115 is grown by MOCVD so as to cover the surfaces of the p-GaAs cap layer 110 and the p-GaAs planarization layer 114. An n-type electrode 116 and a p-type electrode 117 are formed on the surface of the n-GaAs substrate 101 and the surface of the p-GaAs contact layer 115, respectively. The cavity length was adjusted to 375 μm by the cleavage method, and Al was applied to each end face so that the end face reflectivity on the light emitting side of the resonator end face became 10% and the end face reflectivity on the rear side became 75%. ₂ 0 ₃ A film and a Si film are formed.
[0038]
In the semiconductor laser device according to the first embodiment, when a forward voltage is applied between the n-type electrode 116 and the p-type electrode 117, the oscillation wavelength is 0.78 μm, the threshold current is 15 mA, and the slope efficiency of the current-light output characteristic is 1. 0 W / A, an operating current of 50 mA at an optical output of 35 mW, and an operating voltage of 1.8 V were obtained.
[0039]
On the other hand, when the forbidden band width of the first guide layer 704 and the second guide layer 706 of the conventional example is equal to the forbidden band width of the quantum barrier layer 711 (conventional mode 1), that is, the first guide layer 704 When the Al composition ratio of the second guide layer 706 is 0.35, the threshold current is 25 mA, the operation current is 60 mA, and the operation voltage is 1.8 V.
[0040]
Thus, in the semiconductor laser device of the first embodiment, the threshold current was able to be reduced from 25 mA to 15 mA without increasing the operating voltage. Table 1 shows the results.
[0041]
[Table 1]

[0042]
When the thicknesses of the first guide layer 704 and the second guide layer 706 are set to be as large as 50 nm (conventional mode 2), the threshold current can be reduced to 15 mA, which is the same as in the present embodiment. However, in this case, there is a problem that the operating voltage increases to 2.1 V due to an increase in element resistance due to an increase in the thickness of the guide layers 704 and 706. Table 2 shows the results.
[0043]
[Table 2]

[0044]
In the first embodiment, the oscillation wavelength of the manufactured semiconductor laser device is 0.78 μm with respect to the set value of the oscillation wavelength of 0.78 μm, and the oscillation wavelength can be controlled. On the other hand, in the case of the first conventional example, a wavelength shift occurs up to 0.775 μm with respect to the set value of the oscillation wavelength of 0.78 μm, which makes it difficult to control the oscillation wavelength.
[0045]
In the semiconductor laser device of the first embodiment, as shown in FIG. 2, the first guide layer 104 and the second guide layer 106 are adjacent to the quantum well layer 120 and the first guide layer 104 and the second guide layer 106 are forbidden. The band width is larger than the forbidden band width of the quantum well layer 120, and the forbidden band width of the first guide layer 104 and the second guide layer 106 is smaller than the forbidden band width of the quantum barrier layer 121.
[0046]
Here, setting the forbidden band width of the first guide layer 104 and the second guide layer 106 to be smaller than the forbidden band width of the quantum barrier layer 121 is performed by setting the Al composition ratio of the guide layers 104 and 106 to that of the quantum barrier layer 121. This is equivalent to setting it smaller than the composition ratio. For this reason, dopant diffusion from the cladding layers 103 and 107 to the guide layers 104 and 106 is suppressed. Therefore, the diffusion of the dopant into the MQW active layer 105 is suppressed, so that the change in the Al composition ratio of the quantum well layer 120 of the MQW active layer 105 is suppressed, and the oscillation wavelength shift is prevented.
[0047]
Further, controlling the forbidden band width of the first guide layer 104 or the second guide layer 106 corresponds to controlling the refractive index. Accordingly, it is possible to adjust the vertical radiation angle by setting the refractive indexes of the two guide layers 104 and 106 to appropriate values.
[0048]
Furthermore, by setting the Al composition ratio of the guide layers 104 and 106 to be smaller than the Al composition ratio of the quantum barrier layer 121, the number of non-radiative recombination levels of the guide layers 104 and 106 can be reduced. For this reason, non-radiative recombination of carriers leaking from the quantum well layer 120 to the guide layers 104 and 106 can be suppressed. Therefore, the carrier can be effectively used for light emission, and the threshold current can be reduced.
[0049]
In the semiconductor laser device of the first embodiment, the change in the threshold current when the Al composition ratio of the first guide layer 104 and the second guide layer 106 was changed, that is, when the forbidden band width was changed was examined. FIG. 3 shows the result in the relationship between the forbidden bandwidth of the guide layers 104 and 106 and the threshold current. Here, Eg is the forbidden band width of the first guide layer 104 and the second guide layer 106, Eb is the forbidden band width of the quantum barrier layer 121, and Eλ is the forbidden band corresponding to the energy of the laser oscillation light of the MQW active layer 105. Each represents a band width. From this result, it can be seen that the threshold current decreases when Eg is smaller than Eb, and that the threshold current increases when Eg is equal to or smaller than Eλ as compared to when Eg = Eb. In particular, when Eg satisfies the condition of the following equation (2), the threshold current becomes minimum.
[0050]
Eλ + 100 meV ≦ Eg ≦ Eb−50 meV (2)
Thus, the threshold current largely depends on the forbidden bandwidth of the guide layers 104 and 106. Here, the reason why the threshold current increases when Eg becomes smaller than Eλ + 100 meV is that the carriers injected into the quantum well layer 120 of the MQW active layer 105 are caused by the quantum well layer adjacent to the first guide layer 104 and the second guide layer 106. Since carriers leak from the first guide layer 104 to the first guide layer 104 and the second guide layer 106 and the confinement ratio of carriers in the quantum well layer 120 decreases, the number of carriers required for oscillation increases and the threshold current increases. To do that.
[0051]
Also, the reason why the threshold current increases when Eg is larger than Eb-50 meV is that the refractive index of the first guide layer 104 and the second guide layer 106 decreases when Eg is larger than Eb-50 meV. This is because the confinement ratio of light to the light decreases and a large amount of current is required for oscillation.
[0052]
(Embodiment 2)
4 and 5 show a second embodiment of the semiconductor laser device of the present invention.
[0053]
As shown in FIG. 4, this semiconductor laser device has an n-GaAs buffer layer 202 having a thickness of 0.5 μm and an n-Al layer having a thickness of 1.5 μm on an n-GaAs substrate 201. _0.5 Ga _0.5 As first cladding layer 203, Al having a thickness of 10 nm _0.27 Ga _0.73 As first guide layer 204, non-doped MQW active layer 205, Al having a thickness of 10 nm _0.27 Ga _0.73 As second guide layer 206, p-Al having a layer thickness of 0.2 μm _0.5 Ga _0.5 As second cladding layer 207, p-Al having a thickness of 0.2 μm _0.2 Ga _0.8 As etching block layer 208, p-GaAs etching stopper layer 209 having a thickness of 0.003 μm, p-Al having a thickness of 1.2 μm _0.5 Ga _0.5 It has a structure in which an As third cladding layer 210 and a p-GaAs cap layer 211 having a thickness of 0.8 μm are sequentially laminated by MOCVD.
[0054]
Here, the MQW active layer 205 has an Al thickness of 10 nm as shown in FIG. _0.1 Ga _0.9 As quantum well layer 220 and 5 nm thick Al _0.35 Ga _0.65 An As quantum barrier layer 221 is formed by alternately repeating three quantum well layers and two quantum barrier layers.
[0055]
Next, a stripe mask made of a photoresist is formed on the surface of the laminate, and the etching is stopped on the surface of the p-GaAs etching stopper layer 209 by selective etching, thereby forming a ridge stripe 212 having a bottom stripe width of 2.2 μm. Form.
[0056]
An n-Al layer having a thickness of 0.6 μm is buried on both sides of the ridge stripe 212. _0.7 Ga _0.3 An As first current confinement layer 213, an n-GaAs second current confinement layer 214 having a thickness of 0.3 μm, and a p-GaAs planarization layer 215 having a thickness of 0.3 μm are sequentially grown by MOCVD.
[0057]
Next, a 3 μm-thick p-GaAs contact layer 216 is grown by MOCVD so as to cover the surfaces of the p-GaAs cap layer 211 and the p-GaAs planarization layer 215. An n-type electrode 217 and a p-type electrode 218 are formed on the surface of the n-GaAs substrate 201 and the surface of the p-GaAs contact layer 216, respectively. The cavity length was adjusted to 375 μm by the cleavage method, and Al was applied to each end face so that the end face reflectivity on the light emitting side of the resonator end face was 12% and the end face reflectivity on the rear side was 95%. ₂ O ₃ A film and a Si film are formed.
[0058]
In the semiconductor laser device of the second embodiment, when a forward voltage is applied between the n-type electrode 217 and the p-type electrode 218, the oscillation wavelength is 0.78 μm, the threshold current is 30 mA, and the slope efficiency of the current-light output characteristic is 0. 75 W / A, an operating current of 70 mA at an optical output of 30 mW, and an operating voltage of 1.8 V were obtained.
[0059]
In the semiconductor laser device of the second embodiment, as shown in FIG. 5, the first guide layer 204 and the second guide layer 206 are adjacent to the quantum well layer 220 and the first guide layer 204 and the second guide layer 206 are forbidden. The band width is larger than the forbidden band width of the quantum well layer 220, and the forbidden band width of the first guide layer 204 and the second guide layer 206 is smaller than the forbidden band width of the quantum barrier layer 221. Further, a p-GaAs etching stopper layer 209 having emission energy substantially equal to the energy of laser oscillation light of the MQW active layer 205 is provided between the first cladding layer 203 and the third cladding layer 210.
[0060]
Here, p-Al having a layer thickness of 0.2 μm was used. _0.2 Ga _0.8 Since the emission energy of the 0.003-μm-thick p-GaAs etching stopper layer 209 adjacent to the As etching block layer 208 is substantially equal to the energy of the laser oscillation light of the MQW active layer 205, this etching stopper layer 209 It functions as a saturable absorption layer. Due to the saturable absorption effect of the etching stopper layer 209, self-pulsation where laser oscillation light oscillates in a pulse shape occurs. In the self-excited oscillation state, the longitudinal mode of the laser becomes a multimode, and the spectral line width of each longitudinal mode is widened. For this reason, the coherency of the laser is reduced, the influence of the return light is reduced, and noise can be reduced.
[0061]
In the second embodiment, the oscillation wavelength of the manufactured semiconductor laser device is 0.78 μm with respect to the set value of the oscillation wavelength of 0.78 μm, and the oscillation wavelength can be controlled.
[0062]
On the other hand, when the forbidden band width of the first guide layer 704 and the second guide layer 706 in the conventional example is equal to the forbidden band width of the quantum barrier layer 711 (conventional mode 1), that is, the first guide layer 704 When the Al composition ratio of the second guide layer 706 is set to 0.35, the oscillation wavelength shifts to 0.77 μm with respect to the set value of the oscillation wavelength of 0.78 μm, which makes it difficult to control the oscillation wavelength. Become.
[0063]
This is because, in the second embodiment, p-Al having a low Al composition ratio for saturable absorption is used. _0.2 Ga _0.8 Since the As etching block layer 208 and the p-GaAs etching stopper layer 209 are arranged adjacent to the p-type second cladding layer 207, the dopant of the p-type second cladding layer 207 diffuses in these low Al composition ratio layers. This is because the diffusion to the opposite side of the MQW active layer 205 is increased.
[0064]
On the other hand, in the first conventional example, since the Al composition ratio of the guide layers 704 and 706 is equal to the Al composition ratio of the quantum barrier layer 711, the dopant of the p-type second cladding layer 707 is different from that of the MQW active layer 705. It becomes easy to diffuse to the quantum well layer 710, and the diffusion causes a change in the Al composition ratio of the quantum well layer 710, so that the oscillation wavelength shifts to the shorter wavelength side.
[0065]
As described above, the second embodiment is effective for preventing a wavelength shift. Further, the change in the Al composition ratio of the quantum well layer 220 of the MQW active layer 205 due to the dopant diffusion affects the layer thickness of the quantum well layer 220 and the layer thickness of the quantum barrier layer 221. For this reason, in addition to the wavelength shift, a shift from the design value occurs in the electrical characteristics and the optical characteristics. According to the second embodiment, such a problem can be solved.
[0066]
In the second embodiment, p-Al _0.2 Ga _0.8 Although the example in which the p-GaAs etching stopper layer 209 is provided adjacent to the As etching block layer 208 has been described, in addition to the above, a single quantum level substantially equal to the quantum level of the MQW active layer is provided in the p-type cladding layer. A mode in which one quantum well layer is provided, a mode in which a multiple quantum well layer having a quantum level substantially equal to the quantum level of the MQW active layer is provided, and a bulk type having a forbidden band width substantially equal to the quantum level of the MQW active layer is thicker than 20 nm A mode in which a semiconductor layer is provided is also possible. A similar effect can be obtained in any of the above embodiments.
[0067]
Further, in the above-described Embodiment 2, the form in which the saturable absorbing layer is provided in the p-type cladding layer is shown. is there. That is, the saturable absorption layer may be appropriately provided between the p-type clad layer and the n-type clad layer sandwiching the MQW active layer.
[0068]
(Embodiment 3)
FIG. 6 shows a third embodiment of the semiconductor laser device of the present invention.
[0069]
As shown in FIG. 6, this semiconductor laser device has an n-GaAs substrate _0.5 In _0.5 P buffer layer 302, 1.5 μm thick n- (Al _0.7 Ga _0.3 ) _0.5 In _0.5 p First cladding layer 303, 35 nm thick (Al _0.4 Ga _0.6 ) _0.5 In _0.5 P first guide layer 304, non-doped MQW active layer 305, 35 nm thick (Al _0.4 Ga _0.6 ) _0.5 In _0.5 P second guide layer 306, p- (Al _0.7 Ga _0.3 ) _0.5 In _0.5 p-second cladding layer 307 and p-Ga having a thickness of 0.3 μm _0.5 In _0.5 It has a structure in which p-cap layers 308 are sequentially stacked by molecular beam epitaxy (MBE).
[0070]
Here, the MQW active layer 305 has a Ga thickness of 8 nm. _0.5 In _0.5 P quantum well layer and 5 nm thick (Al _0.5 Ga _0.5 ) _0.5 In _0.5 A P quantum barrier layer is formed by alternately repeating four quantum well layers and three quantum barrier layers.
[0071]
Next, a stripe mask made of a photoresist is formed on the surface of the above-mentioned laminated body, and selective etching is performed, and p- (Al _0.7 Ga _0.3 ) _0.5 In _0.5 The etching is stopped so that the remaining thickness of the flat portion of the P second cladding layer 307 becomes 0.3 μm, and a ridge stripe 309 having a width of 5 μm is formed.
[0072]
Next, an n-GaAs current / light confinement layer 310 having a layer thickness of 1.2 μm is grown by MBE so as to bury the outside of the ridge stripe 309.
[0073]
Next, an n-type electrode 311 and a p-type electrode 312 are formed on the surface of the n-GaAs substrate 301 and the surface of the p-GaAs cap layer 308, respectively. The length of the resonator was adjusted to 500 μm by a cleavage method, and Al was applied to each end face such that the reflectivity of the end face of the resonator on the light emission side was 50% and the reflectivity of the rear side was 85%. ₂ O ₃ A film and a Si film are formed.
[0074]
In the semiconductor laser device of the third embodiment, when a forward voltage is applied between the n-type electrode 311 and the p-type electrode 312, the oscillation wavelength is 0.65 μm, the threshold current is 30 mA, and the slope efficiency of the current-light output characteristic is 0. 6 W / A, an operating current of 35 mA at an optical output of 3 mW, and an operating voltage of 2 V were obtained. Thus, in the third embodiment, the threshold current can be reduced by suppressing an increase in the operating voltage.
[0075]
In the third embodiment, the oscillation wavelength of the manufactured semiconductor laser device is 0.65 μm with respect to the set value of the oscillation wavelength of 0.65 μm, and the oscillation wavelength can be controlled.
[0076]
On the other hand, when the forbidden band width of the first guide layer 704 and the second guide layer 706 is made equal to the forbidden band width of the quantum barrier layer 711 (conventional mode 1), that is, the first guide layer 704 and the second guide layer When the Al composition ratio of the layer 706 is 0.5, a wavelength shift occurs up to 0.64 μm from the set value of the oscillation wavelength of 0.65 μm, and it becomes difficult to control the oscillation wavelength. This is because the dopant of the p-type cladding layer 707 tends to diffuse easily, so that the dopant of the p-type cladding layer 707 diffuses into the MQW active layer 705 and the Al composition ratio of the quantum well layer 710 changes, so that the wavelength shift occurs. In order to cause
[0077]
As described above, depending on the type of the cladding layer, for example, the dopant of the p-type cladding layer tends to diffuse more easily in the AlGaInP cladding layer than in the AlGaAs cladding layer in particular. Therefore, in such a cladding layer, the diffusion of the dopant into the active layer is suppressed by making the bandgap of the second guide layer on the p-type cladding layer side smaller than the bandgap of the quantum barrier layer. Thus, wavelength shift can be more effectively prevented.
[0078]
(Embodiment 4)
FIG. 7 shows a fourth embodiment of the semiconductor laser device of the present invention.
[0079]
As shown in FIG. 7, this semiconductor laser device has an n-GaAs buffer layer 402 having a layer thickness of 0.5 μm and an n-Al layer having a layer thickness of 1.2 μm on an n-GaAs substrate 401. _0.5 Ga _0.5 As first cladding layer 403, n-Al having a thickness of 0.2 μm _0.48 Ga _0.52 As second cladding layer 404, 5 nm thick Al _0.27 Ga _0.73 As first guide layer 405, non-doped MQW active layer 406, 5 nm thick Al _0.27 Ga _0.73 As second guide layer 407, p-Al having a layer thickness of 0.15 μm _0.5 Ga _0.5 As second cladding layer 408, p-GaAs etching stopper layer 409 having a thickness of 0.002 μm, p-Al having a thickness of 1.0 μm _0.5 Ga _0.5 It has a structure in which an As third cladding layer 410 and a 0.8 μm-thick p-GaAs cap layer 411 are sequentially stacked by MOCVD.
[0080]
Here, the MQW active layer 406 is made of Al having a thickness of 10 nm. _0.13 Ga _0.87 As quantum well layer and 5 nm thick Al _0.35 Ga _0.65 An As quantum barrier layer is formed by alternately repeating eight quantum well layers and seven quantum barrier layers.
[0081]
Note that the n-type second cladding layer 404 is used for controlling the radiation angle in the vertical direction, and does not affect the effects of the present invention.
[0082]
Next, a stripe mask made of a photoresist is formed on the surface of the laminate, and the etching is stopped on the surface of the p-GaAs etching stopper layer 409 by selective etching. Form.
[0083]
An n-Al layer having a thickness of 0.6 μm is buried on both sides of the ridge stripe 412. _0.7 Ga _0.3 An As first current confinement layer 413, an n-GaAs second current confinement layer 414 having a thickness of 0.3 μm, and a p-GaAs planarization layer 415 having a thickness of 0.3 μm are sequentially grown by MOCVD.
[0084]
Next, a 3 μm-thick p-GaAs contact layer 416 is grown by MOCVD so as to cover the surfaces of the p-GaAs cap layer 411 and the p-GaAs planarization layer 415. An n-type electrode 417 and a p-type electrode 418 are formed on the surface of the n-GaAs substrate 401 and the surface of the p-GaAs contact layer 416, respectively. The length of the resonator was adjusted to 200 μm by a cleavage method, and Al was applied to each end face so that the end face reflectivity on the light emitting side of the end face of the resonator was 30% and the reflectivity on the rear side was 75%. ₂ O ₃ A film and a Si film are formed.
[0085]
In the semiconductor laser device of the fourth embodiment, when a forward voltage is applied between the n-type electrode 417 and the p-type electrode 418, the oscillation wavelength is 0.78 μm, the threshold current is 15 mA, and the current-optical output slope efficiency is 0.75 W / A, an operating current of 19 mA with an optical output of 3 mW, and an operating voltage of 1.8 V were obtained.
[0086]
In the semiconductor laser device of the fourth embodiment, self-sustained pulsation in which laser oscillation light oscillates occurs utilizing the saturable absorption effect in the MQW active layer outside the stripe. In the self-excited oscillation state, the longitudinal mode of the laser becomes a multimode, and the spectral line width of each longitudinal mode is widened. For this reason, the coherency of the laser is reduced, the influence of the return light is reduced, and noise can be reduced.
[0087]
In the semiconductor laser device of the fourth embodiment, the difference between the minimum spot positions of the emitted light in the horizontal and vertical directions on the active layer, that is, the astigmatic difference is 5 μm.
[0088]
On the other hand, when the forbidden band width of the first guide layer 704 and the second guide layer 706 of the conventional example is equal to the forbidden band width of the quantum barrier layer 711 (conventional mode 1), that is, the first guide layer 704 and the When the Al composition ratio of the second guide layer 706 is 0.35, the astigmatic difference of the radiation angle increases to 15 μm. When the astigmatic difference is increased, when the emitted light is focused by the lens, the size of the focused spot increases, and it becomes difficult to use the light in a system such as an optical disk.
[0089]
That is, according to the semiconductor laser device of the fourth embodiment, the amount of saturable absorption outside the stripe of the quantum well layer adjacent to the guide layers 405 and 407 can be increased, and self-excitation can be performed while light is confined inside the stripe. Oscillation can occur. As a result, the influence of the wavefront due to the saturable absorption effect outside the stripe is less likely to be exerted, the shift of the spot position of the emitted light in the horizontal direction on the active layer is reduced, and the astigmatic difference is improved. Therefore, it is possible to reduce the astigmatic difference of the emitted light and to improve the optical characteristics.
[0090]
FIG. 8 shows the relationship between the refractive index difference Δn and the astigmatic difference ΔZ of the semiconductor laser device according to the fourth embodiment and the conventional example 1. Here, the refractive index difference Δn is the refractive index n of the light confined in the active layer inside the stripe. _a And the refractive index n of light confined in the active layer outside the stripe _b And the difference. From this result, it is recognized that the refractive index difference depends on the astigmatic difference, and it is understood that the astigmatic difference ΔZ of the fourth embodiment is suppressed to be lower than that of the first embodiment.
[0091]
In order to cause self-sustained pulsation, the refractive index difference Δn is set to 7 × 10 ^-3 It must be: The astigmatic difference ΔZ tends to increase as the refractive index difference Δn decreases. In order to reduce the astigmatic difference ΔZ to 10 μm or less in the semiconductor laser device of the fourth embodiment, the refractive index difference Δn is set to 2 × 10 ^-3 It is necessary to do above. Therefore, in the fourth embodiment, by setting the refractive index difference Δn in a range that satisfies the condition of the following expression (1), a self-pulsation type low-noise semiconductor laser having good optical characteristics with an astigmatic difference of 10 μm or less. The element constitutes.
[0092]
2 × 10 ^-3 ≦ Δn ≦ 7 × 10 ^-3 ... (1)
On the other hand, in the first embodiment, the refractive index difference required for causing self-sustained oscillation is 7 × 10 ^-3 In the following, the astigmatic difference ΔZ will be greater than 10 μm. Therefore, in the first embodiment, it is impossible to reduce the astigmatic difference and cause self-pulsation.
[0093]
(Embodiment 5)
9 and 10 show a fifth embodiment of the semiconductor laser device of the present invention.
[0094]
As shown in FIG. 9, this semiconductor laser device has an n-GaAs buffer layer 502 having a thickness of 0.5 μm and an n-Al layer having a thickness of 1.5 μm on an n-GaAs substrate 501. _0.5 Ga _0.5 As first cladding layer 503, Al having a thickness of 15 nm _0.3 Ga _0.7 As first guide layer 504, non-doped MQW active layer 505, Al having a thickness of 15 nm _0.25 Ga _0.75 As second guide layer 506, p-Al having a thickness of 0.2 μm _0.5 Ga _0.5 As second cladding layer 507, p-GaAs first etching stopper layer 508 having a thickness of 0.003 μm, p-Al having a thickness of 0.01 μm _0.6 Ga _0.4 As second etching stopper layer 509, n-Al having a thickness of 1.0 μm _0.5 Ga _0.5 It has a structure in which an As current confinement layer 510 and an n-GaAs cap layer 511 having a thickness of 0.8 μm are sequentially stacked by MOCVD.
[0095]
Here, the MQW active layer 505 has an Al thickness of 8 nm as shown in FIG. _0.1 Ga _0.9 As quantum well layer 520 and 5 nm thick Al _0.35 Ga _0.65 It is composed of an As quantum barrier layer 521 and is formed by alternately repeating two quantum well layers and one quantum barrier layer.
[0096]
The p-GaAs first etching stopper layer 508 and the p-Al _0.6 Ga _0.4 Since each layer thickness of the As second etching stopper layer 509 is very thin, the etching stopper layer does not affect the light confinement function or the internal light absorption.
[0097]
Next, a stripe window made of a photoresist is formed on the surface of the laminate, and a stripe groove 512 having a width of 2.5 μm reaching the surface of the p-GaAs first etching stopper layer is formed by selective etching.
[0098]
Next, a 1.5 μm-thick p-Al layer is filled so as to fill the stripe groove 512. _0.5 Ga _0.5 An As third cladding layer 513 and a p-GaAs contact layer 514 having a thickness of 2 μm are sequentially grown by MOCVD.
[0099]
An n-type electrode 515 and a p-type electrode 516 are formed on the surface of the n-GaAs substrate 501 and the surface of the p-GaAs contact layer 514, respectively. The cavity length was adjusted to 375 μm by the cleavage method, and each end face was made of Al so that the end face reflectivity on the light emitting side of the resonator end face was 30% and the end face reflectivity on the rear side was 95%. ₂ O ₃ Form a film.
[0100]
In the semiconductor laser device of the fifth embodiment, when a forward voltage is applied between the n-type electrode 515 and the p-type electrode 516, the oscillation wavelength is 0.78 μm, the threshold current is 10 mA, and the slope efficiency of the current-light output characteristic is 0. As a result, an operating current of 14 mA and an operating voltage of 1.7 V with an optical output of 3 mW were obtained at 75 W / A.
[0101]
In the semiconductor laser device of the fifth embodiment, as shown in FIG. 10, the first guide layer 504 and the second guide layer 506 are adjacent to the quantum well layer 520, and the first guide layer 504 and the second guide layer 506 are forbidden. The band width is larger than the band gap of the quantum well layer 520, and the band gap of the first guide layer 504 and the second guide layer 506 is smaller than the band gap of the quantum barrier layer 521. Further, the forbidden band widths of the first guide layer 504 and the second guide layer 506 are different.
[0102]
In the fifth embodiment, the first guide layer 504 and the second guide layer 506 have different forbidden band widths, that is, different Al composition ratios. At this time, the vertical radiation angle is 25 degrees. On the other hand, when the forbidden band widths of the first guide layer 504 and the second guide layer 506 are made equal, that is, when both the Al composition ratios are 0.25, the vertical radiation angle becomes 30 degrees. Become. Thus, by setting the Al composition ratio of the guide layers 504 and 506 to an appropriate value, it becomes possible to control the radiation characteristics of the laser.
[0103]
(Embodiment 6)
11 and 12 show a sixth embodiment of the semiconductor laser device of the present invention.
[0104]
As shown in FIG. 11, this semiconductor laser device has a GaN buffer layer 602 having a thickness of 0.05 μm, an n-GaN first cladding layer 603 having a thickness of 3 μm, and an n-GaN layer having a thickness of 0.1 μm on a sapphire substrate 601. -In _0.05 Ga _0.95 N second cladding layer 604, n-Al having a thickness of 0.5 μm _0.05 Ga _0.95 N third cladding layer 605, non-doped In with a layer thickness of 20 nm _0.1 Ga _0.9 N first guide layer 606, non-doped MQW active layer 607, non-doped In with a layer thickness of 20 nm _0.1 Ga _0.9 N second guide layer 608, p-Al having a thickness of 20 nm _0.2 Ga _0.8 N fourth cladding layer 609, p-GaN fifth cladding layer 610 having a thickness of 0.1 μm, p-Al having a thickness of 0.5 μm _0.05 Ga _0.95 It has a structure in which an N-th sixth cladding layer 611 and a p-GaN contact layer 612 having a thickness of 0.2 μm are sequentially laminated by MOCVD.
[0105]
Here, the MQW active layer 607 has a 4 nm-thick In layer as shown in FIG. _0.2 Ga _0.8 N quantum well layer 620 and 8 nm thick In _0.05 Ga _0.95 An N quantum barrier 621 is formed by alternately repeating three quantum well layers and two quantum barrier layers.
[0106]
Next, a stripe-shaped resist mask is formed on the surface of the stacked body, a ridge stripe 630 having a width of 2 μm is formed by dry etching, and the n-GaN first cladding layer 603 and the p-GaN contact layer 612 are each n-type. An electrode 640 and a p-type electrode 641 are formed. The cavity length is adjusted to 700 μm by the cleavage method, and the dielectric film coating is adjusted so that the end face reflectivity on the light emission side of the end face of the resonator and the end face reflectivity on the rear side are 30%.
[0107]
In the semiconductor laser device of the sixth embodiment, when a forward voltage is applied between the n-type electrode 640 and the p-type electrode 641, the oscillation wavelength is 0.41 μm, the threshold current is 100 mA, and the slope efficiency of the current-light output characteristic is 0. An operating voltage of 2 W / A and a threshold voltage of 6 V were obtained. As described above, in the sixth embodiment, the threshold current can be reduced by suppressing an increase in the operating voltage.
[0108]
In the semiconductor laser device of the sixth embodiment, as shown in FIG. 12, the first guide layer 606 and the second guide layer 608 are adjacent to the quantum well layer 620, and the first guide layer 606 and the second guide layer 608 are forbidden. The band width is larger than the band gap of the quantum well layer 620, and the band gap of the first guide layer 606 and the second guide layer 608 is smaller than the band gap of the quantum barrier layer 621. Further, the forbidden band width of the guide layers 606 and 608 is set between the quantum well layer 620 and the quantum barrier layer 621.
[0109]
Here, the relationship between the composition ratio of each semiconductor layer and the forbidden band width is such that as the In composition ratio of each semiconductor layer increases, the forbidden band width decreases and the refractive index increases. When the Al composition ratio of each semiconductor layer increases, the forbidden band width increases and the refractive index decreases.
[0110]
In the sixth embodiment, since non-doped InGaN layers are used for the guide layers 606 and 608, the dopant from the n-third cladding layer 605 and the p-fourth cladding layer 609 diffuses into the MQW active layer 607. Since it is suppressed by the guide layers 606 and 608, it is possible to prevent the oscillation wavelength shift due to the fluctuation of the composition ratio.
[0111]
Furthermore, since InGaN is applied to the guide layers 606 and 608 and the forbidden band width is set between the quantum well layer 620 and the quantum barrier layer 621, the refractive index of the guide layers 606 and 608 can be increased. . Therefore, it is possible to increase the light confinement rate to the MQW active layer 607 while suppressing an increase in the operating voltage, and to reduce the threshold current.
[0112]
On the other hand, in the conventional example, since an n-type or p-type GaN layer is used for the guide layers 704 and 706, there is a problem that a dopant is diffused into the MQW active layer 705 to cause an oscillation wavelength shift. If the guide layers 704 and 706 are made non-doped in order to prevent this, another problem arises in that the operating voltage increases.
[0113]
In each of the above embodiments, the case where the forbidden band width of the first guide layer and the second guide layer is smaller than the forbidden band width of the quantum barrier layer has been described. It is also possible to make one of the band gaps smaller than the band gap of the quantum barrier layer.
[0114]
Further, the present invention is not limited to the layer thickness, the composition ratio of Al, In and the like, and the carrier concentration shown in each of the above-described embodiments, and other conditions can be used.
[0115]
As the growth method, an LPE method, a gas source MBE method, and an ALE (atomic beam epitaxy) method can be applied in addition to the MOCVD method and the MBE method.
[0116]
【The invention's effect】
According to the above-described semiconductor laser device of the present invention, the forbidden band width of the guide layer is made smaller than the forbidden band width of the quantum barrier layer, that is, the refractive index of the guide layer is set to be larger than the refractive index of the quantum barrier layer. Therefore, the confinement rate of light in the quantum well layer of the MQW active layer can be increased, and the threshold current can be reduced. In addition, since it is not necessary to increase the thickness of the guide layer as in the conventional example, it is possible to suppress an increase in operating voltage due to an increase in element resistance, and to reduce a threshold current.
[0117]
Further, the forbidden band width of the guide layer is made smaller than the forbidden band width of the quantum barrier layer. For example, in the case of an Al-based semiconductor layer, the Al composition ratio of the guide layer is set smaller than the Al composition ratio of the quantum barrier layer. Therefore, dopant diffusion from the cladding layer to the guide layer can be suppressed. As a result, the diffusion of the dopant into the MQW active layer can be suppressed, so that the change in the Al composition ratio of the quantum well layer of the MQW active layer can be suppressed, and the oscillation wavelength shift can be prevented.
[0118]
Also, controlling the forbidden band width of the guide layer corresponds to controlling the refractive index, and the vertical radiation angle can be adjusted by setting the refractive indexes of the two guide layers to appropriate values.
[0119]
Further, by setting the Al composition ratio of the guide layer to be smaller than the Al composition ratio of the quantum barrier layer, the number of non-radiative recombination levels of the guide layer can be reduced, and the leakage from the quantum well layer to the guide layer can be reduced. Non-radiative recombination of carriers can be suppressed. As a result, carriers can be effectively used for light emission, and the threshold current can be reduced.
[0120]
According to the semiconductor laser device of the present invention, the saturable absorbing layer having the emission energy band gap substantially equal to the laser energy band gap of the MQW active layer is formed by the two cladding layers. Since the structure is provided between the layers, self-sustained pulsation in which the laser oscillation light oscillates in a pulse shape occurs due to the saturable absorption effect. In the self-excited oscillation state, the longitudinal mode of the laser becomes a multimode, and the spectral line width of each longitudinal mode is widened. For this reason, the coherency of the laser is reduced, the effect of the return light is reduced, and noise can be reduced.
[0121]
In the case where the saturable absorption layer is provided, the diffusion of the dopant in the cladding layer between the MQW active layer and the saturable absorption layer is suppressed by the saturable absorption layer, and the diffusion toward the MQW active layer increases. However, since the Al composition ratio of the guide layer is set smaller than the Al composition ratio of the quantum barrier layer, it is possible to suppress dopant diffusion from the cladding layer to the guide layer. Therefore, the diffusion of the dopant into the MQW active layer can be suppressed, so that the change in the Al composition ratio of the quantum well layer of the MQW active layer can be suppressed and the oscillation wavelength shift can be prevented.
[0122]
According to the semiconductor laser device of the fourth aspect, the third cladding layer in the form of a stripe is provided, and the refractive index difference Δn is set within the range of the expression (1). By increasing the saturable absorption amount, self-sustained pulsation can be generated while light is confined inside the stripe. As a result, the influence of the wavefront due to the saturable absorption effect outside the stripe is reduced, and the displacement of the spot position of the emitted light in the horizontal direction on the active layer can be reduced, that is, the astigmatic difference is reduced and the optical characteristics are improved. it can.
[0123]
According to the semiconductor laser device of the present invention, the forbidden band width of the second guide layer on the p-type cladding layer side is made smaller than the forbidden band width of the quantum barrier layer. In contrast to the clad layer in which the dopant of the p-type clad layer tends to diffuse easily, the diffusion of the dopant into the active layer can be suppressed, and the wavelength shift can be more effectively prevented.
[0124]
According to the semiconductor laser device of the present invention, the forbidden band width of the smaller one of the forbidden band widths of the two guide layers is set to the forbidden band width corresponding to the energy of the laser oscillation light of the active layer. Since the threshold current is set to be larger, the threshold current can be reduced more effectively as shown in FIG.
[0125]
According to the semiconductor laser device of the present invention, the forbidden band width Eg of the smaller one of the forbidden band widths of the two guide layers is set so as to satisfy the relationship of the above expression (2). As shown in FIG. 3, the threshold current can be reduced more effectively.
[Brief description of the drawings]
FIG. 1 is a diagram showing a cross-sectional structure of a semiconductor laser device according to a first embodiment of the present invention.
FIG. 2 is an energy band diagram near an active layer according to the first embodiment of the present invention.
FIG. 3 is a diagram showing a relationship between a forbidden band width of a guide layer and a threshold current according to the present invention.
FIG. 4 is a diagram illustrating a cross-sectional structure of a semiconductor laser device according to a second embodiment of the present invention.
FIG. 5 is an energy band diagram near an active layer according to a second embodiment of the present invention.
FIG. 6 is a diagram showing a cross-sectional structure of a semiconductor laser device according to a third embodiment of the present invention.
FIG. 7 is a diagram showing a cross-sectional structure of a semiconductor laser device according to a fourth embodiment of the present invention.
FIG. 8 is a diagram illustrating a relationship between a refractive index difference and an astigmatic difference of a semiconductor laser device.
FIG. 9 is a diagram showing a cross-sectional structure of a semiconductor laser device according to a fifth embodiment of the present invention.
FIG. 10 is an energy band diagram near an active layer according to a fifth embodiment of the present invention.
FIG. 11 is a diagram showing a cross-sectional structure of a semiconductor laser device according to a sixth embodiment of the present invention.
FIG. 12 is an energy band diagram near an active layer according to a sixth embodiment of the present invention.
FIG. 13 is a diagram showing a cross-sectional structure of a conventional semiconductor laser device.
FIG. 14 is an energy band diagram near a conventional active layer.
[Explanation of symbols]
101, 201, 301, 401, 501, 601, 701 n-GaAs substrate 102, 202, 302, 402, 502, 602, 702 n-GaAs buffer layer
103, 203, 303, 403, 503, 603, 703 n-first cladding layer
404,604 n-second cladding layer
605 n-third cladding layer
104, 204, 304, 405, 504, 606, 704 First guide layer
105, 205, 305, 406, 505, 607, 705 MQW active layer
120, 220, 520, 620, 710 Quantum well layer
121,221,521,621,711 Quantum barrier layer
106, 206, 306, 407, 506, 608, 706 Second guide layer
107, 207, 307, 507, 707 p-second cladding layer
408 p-third cladding layer
609 p-fourth cladding layer
610 p-fifth cladding layer
611 p-sixth cladding layer
208 p-etch block layer
108, 209, 409, 508, 509 p-etching stopper layer
109, 210, 410, 513 p-third cladding layer
110, 211, 308, 411 p-cap layer
111, 212, 309, 412, 512 stripe
112, 213, 310, 413, 510 n-first current light confinement layer
113, 214, 414 n-second current confinement layer
114,215,415 p-planarization layer
115,216,416,514,612,708 p-contact layer
116, 117, 217, 218, 311, 312, 417, 418, 515, 516, 640, 641

Claims (4)

Across respectively the plurality of quantum well layer containing at least Al, an active layer of multiple quantum well structure in which a plurality of quantum barrier layers are alternately stacked, including the band gap at least equal Al, the at least Al A semiconductor laser device provided with one guide layer and a second guide layer containing at least Al ,
The first guide layer and the second guide layer are adjacent to the quantum well layer, and the forbidden band width of the first guide layer and the second guide layer is larger than the forbidden band width of the quantum well layer; The Al composition ratio of the first guide layer and the second guide layer is smaller than the Al composition ratio of the quantum barrier layer, and the forbidden band width of the first guide layer and the second guide layer is reduced . Smaller than the forbidden band width of
A first cladding layer of a first conductivity type and a second cladding layer of a second conductivity type are provided so as to sandwich the first guide layer and the second guide layer, and the second cladding layer is provided outside the second cladding layer. A third cladding layer of a second conductivity type is provided at a position opposite to the one cladding layer, and light emission substantially equal to the energy of the laser oscillation light of the active layer is provided between the first cladding layer and the third cladding layer. A semiconductor laser device provided with a saturable absorption layer having energy. A first cladding layer of a first conductivity type and a second cladding layer of a second conductivity type are provided so as to sandwich the first guide layer and the second guide layer, and the second cladding layer is provided outside the second cladding layer. third cladding layer of the second conductivity type is formed in a stripe shape on the opposite side of the position and first cladding layer, confined in the active layer with a refractive index n _a and the stripe external light confined in the active layer inside the stripe The difference Δn from the refractive index n _{b of the} light satisfies the condition of the following formula (1): 2 × 10 ⁻³ ≦ Δn ≦ 7 × 10 ⁻³ (1)
The semiconductor laser device according to claim 1 . When the forbidden band widths of said first guide layer or the second guiding layer are different, the forbidden band width smaller and larger than the band gap corresponding to the energy of the laser oscillation light of the active layer according to claim 1, wherein Item 3. A semiconductor laser device according to item 2 . Of the forbidden bandwidths of the first guide layer or the second guide layer, the smaller forbidden bandwidth is Eg, the forbidden bandwidth of the quantum barrier layer is Eb, and the energy of the laser oscillation light of the active layer is equivalent. When the forbidden band width is Eλ, Eg, Eb, and Eλ satisfy the condition of the following equation (2): Eλ + 100 meV ≦ Eg ≦ Eb−50 meV (2)
The semiconductor laser device according to claim 3 .

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