JP2565909C

JP2565909C -

Info

Publication number: JP2565909C
Authority: JP
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Publication date: 1999-03-10

Description

【発明の詳細な説明】〈産業上の利用分野〉本発明は、高出力特性にすぐれた高性能半導体レーザ素子に関するものである
。〈従来の技術〉近年、ＹＡＧレーザの励起用光源等として小型軽量の半導体レーザによる１Ｗ
以上の高出力動作が求められている。一般に、非常に小さな体積で動作する半導
体レーザの高出力限界は主に大きな駆動電流時の発熱による光出力の飽和や高い
光密度による破壊限界によってきまっている。したがって高出力動作においては
数十ミクロン（μｍ）のストライプ幅を持った広ストライプレーザを用いること
により単位体積あたりの光密度を低下している。このような広ストライプレーザ
においては成長層の均一性が問題となるが近年分子線エピタキシイ法（ＭＢＥ法
）や有機金属気相成長法（ＭＯ−ＣＶＤ法）等によって大面積でかつ均一な結晶
成長が可能となっており、上述のようなストライプ幅の広いレーザによって高い
出力の発振が得られるようになっている。従来、活性層厚が５００Å以上のいわゆるダブルヘテロ（ＤＨ）型レーザを高
出力用レーザとして用いるとき、共振器両端面の反射率を後面７５％以上、前面
約５％の非対称になるようにコーティングを施し、後面への光出力の損失を少な
くしている。一般に半導体レーザにおいて発振閾値電流密度は共振器長を長くす
ると低下するが、ＤＨレーザの場合共振器長を長くすると量子効率の低下が大き
いため高出力動作時の駆動電流を上昇し発熱の効果による光出力の飽和がおこる
ため共振器長は２５０μｍ程度が最適と考えられる。〈発明の目的〉本発明は高出力動作に有利な量子井戸レーザを用いて発熱による光出力飽和の影響を少なくし、高い出力での動作を可能にするものである。活性層に量子井戸
構造を用いた場合、注入されたキャリァは、量子井戸方向に量子化され、キャリ
ァのエネルギ状態密度が階段状となるためレーザの利得係数が注入キャリァ密度
に対して急激に立ち上がり、低い閾値電流密度で発振する。〈発明の構成及び効果の説明〉本発明は、駆動電流が低く高出力動作に有利な量子井戸活性層を有し前後端面
の反射率をそれぞれ低反射率、高反射率とした高出力半導体レーザにおいて、共
振器長を３００μｍ以上１０００μｍ以下とすることにより連続発振時に発熱に
よる光出力の飽和の影響のない高出力動作を得るものである。即ち、本発明に係る半導体レーザ素子は、ＧａＡｓ基板上に、層厚が電子のド
ブロイ波長より薄い活性層を単層または多層に形成した半導体レーザ素子におい
て、前記活性層の共振端面をそれぞれ７５％以上及び１０％以下の反射率に設定
しかつ共振器長を３００μｍ以上１０００μｍ以下としたことによって上記効果
を得る。〈実施例〉第１図に本発明の１実施例の断面図を示す。ｎ−ＧａＡｓ基板１上にｎ−Ａｌ
_0.7Ｇａ_0.3Ａｓクラッド層２、Ａｌ混晶比を０．７から０．３まで放物線状に徐
々に変化させたＡｌ_xＧａ_1-xＡｓグレーディドインデックスガイド層３、層厚６
０ÅのＡｌ_0.1Ｇａ_0.9Ａｓ量子井戸活性層４、Ａｌ混晶比を０．３から０．７ま
で放物線状に徐々に変化させたＡｌ_xＧａ_1-xＡｓグレーディドインデックスガイ
ド層５、ｐ−Ａｌ_0.7Ｇａ_0.3Ａｓクラッド層６、ｐ−ＧａＡｓコンタクト層７、
ｎ−Ａｌ_0.5Ｇａ_0.5Ａｓ電流狭窄層８、ｎ−ＧａＡｓ保護層を、順次ＭＢＥ法に
よって成長した。これに一般に用いられるフォトレジストを用いた化学エッチン
グにより１００μｍ幅のストライプ状にｎ−ＧａＡｓ保護層９及びｎ−Ａｌ_0.5
Ｇａ_0.5Ａｓ電流狭窄層８を除去し、その後ｎ側，ｐ側にそれぞれオーミック電
極を形成しレーザ端面前面に反射率５％の低反射コーティング、後面に反射率９
５％の高反射コーティングを施し高出力用のグレーディドインデックスガイド層
付量子井戸レーザの１００μｍ電極ストライプ型素子を作製した。第３図にこの素子の共振器長を２５０μｍから１３００μｍの間で変えた多数
の素子の室温連続発振における最大光出力の実測値を示す。第３図に代表的な素子として共振器長２５０μｍと３７５μｍの場合の光出力
−電流特性を示す。第３図から判るように共振器長が３００μｍ末満の場合、素子の最大出力は急
激に低下する。この場合光出力−電流特性は、第４図のＬ＝２５０μｍの素子に
ついて示したように発熱によって飽和している。一方、共振器長Ｌが３００μｍ≦Ｌ≦１０００μｍの範囲では第３図のＬ＝３
７５μｍの素子に対して示したように、光出力は破壊限界までのびており発熱に
よる出力飽和の影響はほとんどない。しかしＬ＞１０００μｍ以上では、再び、最大出力は熱による飽和によって決
っている。このような熱による光出力飽和の共振器存性を第５、６、７図を用い
て説明する。第５図に共振器長２５０μｍ及び３７５μｍの素子についての熱抵抗の実測値
を示す。これよりＬ＝３００μｍ以上の素子と以下の素子では、熱抵抗には有為
差はないと考えられる。第６図に量子井戸レーザにおける光出力１Ｗにおける駆動電流密度の共振器長
依存性の理論値を示す。これは発熱による温度の上昇の効果は含まれていない。これよりＬ＝３００μｍ以下の領域で電流密度の急上昇が見られる。これより
第３図における３００μｍ以下での出力飽和は、この電流密度の上昇によるもの
と考えられる。次に第７図に光出力１Ｗを得るのに必要な駆動電流の共振器長依
存性の理論値を一般のＤＨレーザと量子井戸レーザに対して示す。発熱による駆
動電流上昇はないものと仮定している。高出力動作時の駆動電流は、素子の微分
量子効率に大きく依存する。一般に微分量子効率はに比例する。ここで、Ｒ，Ｒはレーザ両端面の反射率、Ｌは共振器長、αｉは素
子の内部損失をあらわす。したがって共振器長を長くした場合、微分量子効率は
低下し、高出力動作時の駆動電流は増大する。しかし量子井戸レーザの場合内部
損失αｉが約５ｃｍ^-1程度まで低くできＤＨレーザが約５ｃｍ^-1に比べて極めて
小さい値となり、第１式のＬ依存性が小さく共振器を長くしても駆動電流の上昇は小さい。ただしこのような効果は式（１）からも判るように内部損失αｉとミ
ラー損失１／２Ｌｌｎ（１／Ｒ₁Ｒ₂）の相対関係できまっているため、ミラー損
失が小さい場合には、この効果は現れない。実験によると後面反射率が９０％以
上の場合、前面反射率は１０％以下でないと高出力時の熱による光出力飽和を抑
制する効果はあらわれない。これは以下のような理論解析より説明できる。第８図に端面反射率を変えた場合の式（１）の値のＬ依存性の理論値を示す。
ここでは、前面より効率よく光をとり出すため後面の反射率を９５％と一定にし
てある。前面反射率を５％とした場合、量子井戸レーザの内部損失５ｃｍ^-1に対
しミラー損失が大きいため、量子効率の共振器長依存性が小さい。しかし前面の
反射率を１５％と大きくした場合、ミラー損失が内部損失に対して小さくなり、
共振器長を大きくした場合の量子効率の低下は大きくなり高出力時の駆動電流が
上昇する。比較のために内部損失１５ｃｍ^-1のＤＨレーザに関する式（１）の値
を図中に示す。これより前面反射率を大きくした場合内部損失が小さな量子井戸
レーザにおいてもＤＨレーザに近い量子効率の低下が起る。第３図において３００μｍから１０００μｍまでの広い範囲で熱による光出力
の飽和の影響が光出力１Ｗ以上でもほとんどあらわれないことは第７図の駆動電
流の上昇が非常に小さいことに起因していると考えられる。しかし共振器長が１０００μｍ以上となった場合駆動電流の上昇と共に徐々に
光出力の飽和があらわれる。以上の議論よりレーザ端面の前後面にそれぞれ５％、９５％のコーティングを
施した量子井戸レーザにおいて、共振器長を３００μｍ以上１０００μｍ以下と
した場合、発熱による光出力飽和の影響のない高出力レーザが得られる。次に本発明の第２の実施例を第２図によって説明する。ＭＢＥ法によって第１の実施例と全く同じ層構造をＭＢＥ法により成長した後
、フォトレジストを用いた化学エッチングにより第８，９層を約１００μｍ幅の
ストライプ状にエッチングする。次に８，９層を取除いたストライプ内に同じく
フォトレジストを用いた化学エッチングにより間隔１μｍ幅２μｍのメサ部が残
るように第７層及び第６層をエッチングする。この場合、エッチング部分の第６
層は約０．２μｍ残した。このようにして１００μｍ幅リッヂ導波路アレイを製
作した。なおリッヂ部側面には絶縁膜を蒸着により形成しその後ｐ・ｎ両電極を形成し
た。このウェハを用いて、共振器長の異なる多くの素子を作製したところ最大光
出力の共振器長依存性はほぼ第３図と同様の傾向をあらわし、共振器長３００μ
ｍ以上１０００μｍ以下の領域で、光出力１Ｗまで光出力の飽和はみられなかっ
た。ただしこれらの素子の端面反射率は後面９５％、前面５％とした。上記実施例においては活性領域としてＡｌの混晶比をｘ＝０．１としたが、０
≦ｘ≦０．２の範囲では、量子井戸レーザによる内部損失はＤＨレーザに比べて
非常に低い値であり、かつ混晶比依存性はないため、本実施例と同様に３００μ
ｍ≦Ｌ≦１０００μｍとした場合熱による出力飽和のない高出力レーザが得られ
る。また実施例では活性領域をＧＲＩＮ−ＳＣＨ構造としたが、量子井戸活性層を
多層に積層した多重量子井戸構造としても全く同様の効果が得られる。上記実施例においては、端面反射率を後面９５％、前面３％又は５％とする例
を示したが、本発明はこれに限定されるものではなく、後面が７５％以上、前面
が１０％以下であれば、本願の目的を達成することができる。 DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a high performance semiconductor laser device having excellent high output characteristics. <Prior Art> In recent years, a 1 W light source using a small and light semiconductor laser as a light source for exciting a YAG laser has been developed.
The above high output operation is required. In general, the high output limit of a semiconductor laser that operates with a very small volume is mainly determined by the saturation of the optical output due to heat generated by a large driving current and the destruction limit due to a high optical density. Therefore, in a high output operation, the light density per unit volume is reduced by using a wide stripe laser having a stripe width of several tens of microns (μm). In such a wide stripe laser, uniformity of a growth layer is a problem, but in recent years, a large area and uniform crystal growth has been performed by a molecular beam epitaxy method (MBE method), a metal organic chemical vapor deposition method (MO-CVD method), or the like. It is possible to obtain high-output oscillation by using a laser having a wide stripe width as described above. Conventionally, when a so-called double hetero (DH) type laser having an active layer thickness of 500 ° or more is used as a high-output laser, coating is performed so that the reflectivity at both end faces of the resonator is 75% or more at the rear face and about 5% at the front face. To reduce the loss of light output to the rear surface. In general, the oscillation threshold current density of a semiconductor laser decreases as the resonator length is increased. However, in the case of a DH laser, increasing the resonator length causes a large decrease in quantum efficiency. Since the optical output is saturated, it is considered that the resonator length is optimally about 250 μm. <Object of the Invention> The present invention reduces the influence of light output saturation due to heat generation by using a quantum well laser advantageous for high output operation, and enables operation at high output. When a quantum well structure is used for the active layer, the injected carrier is quantized in the direction of the quantum well, and the energy state density of the carrier becomes stair-like, so that the gain coefficient of the laser rises sharply with respect to the injected carrier density. Oscillate at a low threshold current density. <Description of Configuration and Effect of the Invention> The present invention is directed to a high-power semiconductor laser having a quantum well active layer having a low drive current and advantageous for high-output operation and having low and high reflectances at front and rear end faces, respectively. In the above, by setting the resonator length to be 300 μm or more and 1000 μm or less, a high output operation without influence of light output saturation due to heat generation during continuous oscillation can be obtained. That is, the semiconductor laser device according to the present invention has a layer thickness of electrons on a GaAs substrate.
In semiconductor laser devices with single or multilayer active layers thinner than the Broy wavelength
And setting the resonance end faces of the active layer to reflectivity of 75% or more and 10% or less, respectively.
And the resonator length is set to 300 μm or more and 1000 μm or less, the above-mentioned effect is obtained.
Get. <Embodiment> FIG. 1 shows a sectional view of one embodiment of the present invention. n-Al substrate on n-GaAs substrate 1
_0.7 Ga _0.3 As clad layer 2, Al _x Ga _{1 -x} As graded index guide layer 3 with an Al mixed crystal ratio gradually changed from 0.7 to 0.3 in a parabolic manner, layer thickness 6
0 ° Al _0.1 Ga _0.9 As quantum well active layer 4, Al _x Ga _{1 -x} As graded index guide layer 5 with an Al mixed crystal ratio gradually changed from 0.3 to 0.7 in a parabolic manner, p- Al _0.7 Ga _0.3 As clad layer 6, p-GaAs contact layer 7,
The n-Al _0.5 Ga _0.5 As current confinement layer 8 and the n-GaAs protection layer were sequentially grown by MBE. The n-GaAs protective layer 9 and n-Al _{0.5 are} formed into a stripe having a width of 100 μm by chemical etching using a generally used photoresist.
Ga _0.5 As current confinement layer 8 is removed, then the n-side, low-reflective coating of the p-side in the reflection ratio of 5%, respectively to form an ohmic electrode on the front laser facet reflectivity on the rear surface 9
A 100 μm electrode stripe type element of a quantum well laser with a graded index guide layer for high output with a high reflection coating of 5% was produced. FIG. 3 shows actual measured values of the maximum light output in continuous oscillation at room temperature of many devices in which the resonator length of this device was changed between 250 μm and 1300 μm. FIG. 3 shows the optical output-current characteristics when the resonator length is 250 μm and 375 μm as typical elements. As can be seen from FIG. 3, when the resonator length is less than 300 μm, the maximum output of the element rapidly decreases. In this case, the light output-current characteristic is saturated due to heat generation as shown in the element of L = 250 μm in FIG. On the other hand, when the resonator length L is in the range of 300 μm ≦ L ≦ 1000 μm, L = 3 in FIG.
As shown for a 75 μm device, the light output extends to the destruction limit and is hardly affected by output saturation due to heat generation. However, above L> 1000 μm, the maximum output is again determined by heat saturation. The existence of such a resonator of light output saturation due to heat will be described with reference to FIGS. FIG. 5 shows the measured values of the thermal resistance of the devices having the resonator lengths of 250 μm and 375 μm. From this, it is considered that there is no significant difference in the thermal resistance between the element having L = 300 μm or more and the following element. FIG. 6 shows theoretical values of the dependence of the drive current density on the cavity length at an optical output of 1 W in the quantum well laser. This does not include the effect of temperature rise due to heat generation. From this, a sharp increase in current density is observed in the region of L = 300 μm or less. From this, it is considered that the output saturation at 300 μm or less in FIG. 3 is due to this increase in current density. Next, FIG. 7 shows theoretical values of the resonator length dependence of the drive current required to obtain an optical output of 1 W for general DH lasers and quantum well lasers. It is assumed that there is no drive current increase due to heat generation. The driving current at the time of high output operation largely depends on the differential quantum efficiency of the device. Generally, the differential quantum efficiency is Is proportional to Here, R and R are the reflectivities of both end faces of the laser, L is the cavity length, and αi is the internal loss of the element. Therefore, when the resonator length is increased, the differential quantum efficiency decreases and the driving current during high-power operation increases. However DH laser can lower Ku case of a quantum well laser internal loss αi is up to about 5 cm ^-1 becomes extremely small value as compared to about 5 cm ^-1, even longer first equation L dependence is small cavity The rise in drive current is small. However, as can be seen from equation (1), such an effect can be obtained by a relative relationship between the internal loss αi and the mirror loss １／ Lln (1 / R ₁ R ₂ ). Does not appear. According to the experiment, when the rear surface reflectance is 90% or more, the effect of suppressing the light output saturation due to heat at the time of high output is not exhibited unless the front surface reflectance is 10% or less. This can be explained by the following theoretical analysis. FIG. 8 shows the theoretical value of the L dependence of the value of equation (1) when the end face reflectance is changed.
Here, in order to extract light more efficiently than the front surface, the rear surface has a constant reflectance of 95%. When the front surface reflectivity is 5%, the mirror loss is large with respect to the internal loss of the quantum well laser of 5 cm ^−1, so that the dependence of quantum efficiency on the cavity length is small. However, if the reflectivity of the front surface is increased to 15%, the mirror loss becomes smaller than the internal loss,
When the resonator length is increased, the decrease in quantum efficiency is increased, and the driving current at the time of high output is increased. For comparison, the value of equation (1) for a DH laser with an internal loss of 15 cm ^-1 is shown in the figure. When the front facet reflectance is made higher than this, even in a quantum well laser having a small internal loss, a quantum efficiency lower than that of a DH laser occurs. In FIG. 3, the effect of the saturation of the light output due to heat in a wide range from 300 μm to 1000 μm is hardly apparent even when the light output is 1 W or more, because the rise of the drive current in FIG. 7 is very small. Conceivable. However, when the cavity length is 1000 μm or more, the saturation of the optical output gradually occurs as the drive current increases. 5%, respectively front and rear surfaces of the laser facets than the above discussion, in a quantum well laser which has been subjected to 95% of co pos- sesses, when the resonator length was set to 300μm or more 1000μm or less, high no influence of light output saturation due to heat generation An output laser is obtained. Next, a second embodiment of the present invention will be described with reference to FIG. After the same layer structure as that of the first embodiment is grown by MBE by MBE, the eighth and ninth layers are etched into stripes having a width of about 100 μm by chemical etching using a photoresist. Next, the seventh layer and the sixth layer are etched by chemical etching using the same photoresist in the stripe from which the eighth and ninth layers have been removed so that a mesa portion having an interval of 1 μm and a width of 2 μm remains. In this case, the sixth portion of the etched portion
The layer remained about 0.2 μm. Thus, a 100 μm wide ridge waveguide array was manufactured. An insulating film was formed on the side surface of the lip by vapor deposition, and then both p and n electrodes were formed. When many devices having different resonator lengths were manufactured using this wafer, the dependency of the maximum optical output on the resonator length showed almost the same tendency as in FIG. 3, and the resonator length was 300 μm.
In the region of not less than m and not more than 1000 μm, no saturation of the light output was observed up to the light output of 1 W. However, the end face reflectivities of these elements were 95% on the rear face and 5% on the front face. In the above embodiment, the mixed crystal ratio of Al was set to x = 0.1 as the active region.
In the range of ≦ x ≦ 0.2, the internal loss due to the quantum well laser is much lower than that of the DH laser, and there is no dependency on the mixed crystal ratio.
When m ≦ L ≦ 1000 μm, a high-output laser without output saturation due to heat can be obtained. Although the active region has a GRIN-SCH structure in the embodiment, the same effect can be obtained by using a multiple quantum well structure in which quantum well active layers are stacked in multiple layers. In the above embodiment, the end face reflectance is set to 95% for the rear face and 3% or 5% for the front face.
However, the present invention is not limited to this.
Is 10% or less, the object of the present application can be achieved.

【図面の簡単な説明】第１図及び第２図はそれぞれ本発明の１実施例である１００μｍ電極ストライ
プ型グレーディドインデックスガイド層付量子井戸レーザの断面図である。第３
図はＧＲＩＮ−ＳＣＨレーザの最大光出力の共振器長依存性を示す特性図である
。第４図はＧＲＩＮ−ＳＣＨレーザの光出力電流特性を示す特性図である。第５
図はＧＲＩＮ−ＳＣＨレーザの熱抵抗の共振器依存性の実測値を示す特性図であ
る。第６図はＧＲＩＮ−ＳＣＨレーザの光出力１Ｗにおける駆動電流密度の共振
器長依存性の理論値を示す説明図である。第７図及び第８図はＧＲＩＮ−ＳＣＨ
レーザ及びＤＨレーザの光出力１Ｗにおける、駆動電流の共振器長依存性の理論
値を示す。１……ｎ−ＧａＡｓ基板、２……ｎ−Ａｌ_0.7Ｇａ_0.3Ａｓクラッド層、３……Ａ
ｌ_xＧａ_1-xＡｓ（ｘ＝０．３〜０．７）グレーディドインデックスガイド層、４
……Ａｌ_0.1Ｇａ_0.9Ａｓ（６０Å）活性層、５……ｐ−Ａｌ_xＧａ_1-xＡｓクラッ
ド層、７……ｐ−ＧａＡｓコンタクト層、８……ｎ−Ａｌ_0.5Ｇａ_0.5Ａｓ電流狭
窄層、９……ｎ−ＧａＡｓ保護層、１０……ｐ型オーミック電極、１１……ｎ型オーミック電極、１２……絶縁層。BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are cross-sectional views of a quantum well laser with a 100 μm electrode stripe type graded index guide layer according to an embodiment of the present invention. Third
The figure is a characteristic diagram showing the dependence of the maximum optical output of the GRIN-SCH laser on the cavity length. FIG. 4 is a characteristic diagram showing an optical output current characteristic of the GRIN-SCH laser. Fifth
The figure is a characteristic diagram showing measured values of the dependence of the thermal resistance of the GRIN-SCH laser on the resonator. FIG. 6 is an explanatory diagram showing a theoretical value of a resonator length dependence of a driving current density at an optical output of 1 W of a GRIN-SCH laser. 7 and 8 show GRIN-SCH
The theoretical value of the resonator length dependence of the driving current at the optical output of 1 W of the laser and the DH laser is shown. 1 ...... n-GaAs _{substrate, 2 ...... n-Al 0.7 Ga} 0.3 As cladding layer, 3 ...... A
l _x Ga _1-x As (x = 0.3 to 0.7) graded index guide layer, 4
... Al _0.1 Ga _0.9 As (60 °) active layer, 5 p-Al _x Ga _{1 -x} As clad layer, 7 p-GaAs contact layer, 8 n-Al _0.5 Ga _0.5 As current confinement layer , 9 ... n-GaAs protective layer, 10 ... p-type ohmic electrode, 11 ... n-type ohmic electrode, 12 ... insulating layer.

Claims

1. A semiconductor laser device having a single-layer or multi-layer active layer having a layer thickness smaller than the de Broglie wavelength of electrons formed on a GaAs substrate. A semiconductor laser device having a reflectance of not less than 10% and a resonator length of not less than 300 μm and not more than 1000 μm.