JP4991025B2 - Nitride semiconductor laser device - Google Patents

Description

【００８５】
得られるレーザ素子は、光ガイド層とｐ側キャップ層を有しているレーザ素子に比べて、駆動電流が大幅に上昇する傾向にあり、１００ｍＡ近傍のものもあった。
【００８６】
［参考例２］
参考例として、基板の上に表２に示すｎ側コンタクト層〜ｐ側コンタクト層まで順に積層し、エッチングにより、ストライプ状の導波路を形成し、更にｎ側コンタクト層を露出させ、これらのコンタクト層にｐ，ｎ電極を形成して、図６に示すレーザ素子を得る。この時、ストライプ状の導波路を形成する際のエッチング深さとしては、ｐ側クラッド層の膜厚が０．１μｍとなる位置より下（活性層に近づく方向）で、活性層よりも上（活性層に達しない深さ）となる深さである。
【００８７】
【表２】

【００８８】
得られるレーザ素子は、参考例１に比べて、駆動電流が１０〜２０ｍＡ程度低くなる傾向にある。
【００８９】
［変形例１］
変形例として、基板の上に表３に示すｎ側コンタクト層〜ｐ側コンタクト層までの各層順に積層し、エッチングにより、ストライプ状の導波路を形成し、更にｎ側コンタクト層を露出させ、これらのコンタクト層にｐ，ｎ電極を形成して、図７に示すレーザ素子を得る。この時、ストライプ状の導波路を形成する際のエッチング深さとしては、具体的にはｐ側クラッド層の膜厚が０．１μｍとなる位置より下（活性層に近づく方向）で、活性層よりも上（活性層に達しない深さ）となる深さである。好ましくは、上述したように、ストライプ状のリッジ導波路がｐ側光ガイド層２１０に達する深さで形成し、さらに具体的には実施例１と同様に、膜厚１０００Åとなる深さで形成する。
【００９０】
【表３】

【００９１】
得られるレーザ素子は、ｐ側キャップ層を有するレーザ素子に比べて、駆動電圧Ｖｆが、下がる傾向にあるものの、閾値電流が５〜６倍に上昇する傾向にあり、得られるレーザ素子の多くがレーザ発振を示さない傾向にある。また、ｐ側光ガイド層の膜厚を２．５μｍ、ｎ側光ガイド層の膜厚を１．５μｍとして、ｐ側光ガイド層をｎ側よりも厚くして得られるレーザ素子は、上記に加えて、ストライプ状の導波路による横モードの制御が良好なものとなり、アスペクト比などの光学特性に優れるレーザ素子が得られる。このように、両光ガイド層の膜厚差が、１μｍであると、上述した光分布がｐ側光ガイド層側に偏らせることによるモード制御性向上の効果が、十分に得られる傾向にある。この両光ガイド層の膜厚差としては、５００Å以上であれば前記傾向がみられる。
【００９２】
（長波長域のレーザ素子）
本発明のレーザ素子において、４５０ｎｍ以上、具体的には４５０以上５２０ｎｍ以下の、青色〜緑色の長波長領域では、以下の層構成とすることが好ましい。ただし、本発明は、この波長域に限定されるものではない。
長波長域において、活性層として、井戸層、障壁層に加えて、その間に中間層を設けることが発振特性の向上につながり好ましい。
【００９３】
短波長域、具体的には４５０ｎｍ以下の波長域、に用いる活性層では、ＩｎＧａＮからなる井戸層、その井戸層よりバンドギャップエネルギーの大きい障壁層で挟んだ量子井戸構造で、具体的にはＩｎＧａＮからなる井戸層とその井戸層とは混晶比若しくは組成の異なるＡｌＧａＩｎＮからなる障壁層を用いる。このような構造として、障壁層／井戸層／障壁層の単一量子井戸構造（ＳＱＷ）、井戸層と障壁層とを繰り返し積層した多重量子井戸構造（ＭＱＷ）が用いられている。しかし、この井戸層と障壁層とは、混晶比もしくは組成が異なるため、それぞれの層成長時に適した温度が異なることとなり、その成長が困難な傾向になる。この場合、井戸層の上に、それよりも成長温度を高くして障壁層を成長することとなる。これは、Ｉｎを有する井戸層において、障壁層成長時の昇温過程で、Ｉｎの分解が発生し、発光ピークの鋭いものが得られなくなる。また、障壁層を井戸層とほぼ同じ温度で形成したとしても、活性層の形成後に続く、他の層（クラッド層、ガイド層）を形成する際にも、良好な結晶成長のためには昇温過程が必要となる。このような成長困難性は、発振波長が長くなるにつれて、顕著なものとなる傾向にあり、上記長波長域では中間層を設けることが好ましい。
【００９４】
このため、上記中間層を介することで、上記昇温による問題を解決できる。この中間層を設けることで、上記Ｉｎの分解を部分的なものとして観察される傾向にあり、また中間層そのもののが凹凸を呈する表面形態として観察される傾向にあり、これらのことが駆動電圧や閾値電圧の大幅な低下に寄与しているものと考えられる。この中間層は、井戸層と障壁層との間に設けるものであり、そのバンドギャップエネルギーが、障壁層よりも大きいものである。この中間層は、活性層がＭＱＷである場合には、少なくとも１層の井戸層上に設ける必要があり、全ての井戸層の上に設けることで、井戸層上の障壁層の全てについて上記問題が解決でき好ましい。
【００９５】
また、中間層の膜厚としては、障壁層の膜厚より薄くして、１原子層以上１００Å以下の範囲とすることが好ましい。これは膜厚が１００Å以上となることで、中間層と障壁層との間にミニバンドが形成され、発振特性が悪化する傾向にあるためである。この時の障壁層としては、１０Å以上４００Å以下の範囲とする。更に、中間層の組成として、好ましくはＡｌ_uＧａ_1-uＮ（０≦ｕ≦１）とすることで、上記Ｉｎの部分的な分解、中間層の表面形態による駆動電圧や閾値電圧の低下傾向を示し、更に好ましくは、Ａｌ_vＧａ_1-vＮ（０．３≦ｖ≦１）とすることで上記各電圧の低下を大きくすることができる。
【００９６】
［変形例２］
基板上に、以下の表４に示すｎ側コンタクト層〜ｐ側コンタクト層を順に積層して、レーザ素子構造を形成した。次に、ストライプ幅１．８μｍ、ｐ側コンタクト層側からｐ側光ガイド層の膜厚が５００Åとなる深さまで、エッチングすることで、ストライプ状のリッジ導波路を形成し、その他は実施例と同様に、更にエッチングによりｎ側コンタクト層を露出させ、各コンタクト層の上に、ｐ，ｎ電極を形成して、チップを取り出して図８に示すようなレーザ素子を得た。なお、図中２０８ａは中間層、２０８ｂは井戸層、２０８ｃは障壁層を示すものであり、図８は、活性層２０８の構造を拡大して模式的に示している。
【００９７】
【表４】

【００９８】
得られたレーザ素子は、波長４５０nmであり、室温において閾値電流密度２．０ｋＡ／cm²、で１０００時間以上の連続発振が確認された。これは、ストライプ状の導波路形成時のエッチング深さが、ｐ側光ガイド層に達しない深さのレーザ素子に比べても、横モードの制御性、Ｆ．Ｆ．Ｐ．におけるアスペクト比に優れたものが得られる。また、ｐ側光ガイド層を１３００Åとして、ｎ側光ガイド層との膜厚差を十分なものとすることで、更に好ましいレーザ素子を得ることができる。
【００９９】
［変形例３］
基板上に積層する素子構造が、以下の表５の通りであることを除いて、変形例２と同様にして、レーザ素子を得る。
【０１００】
【表５】

【０１０１】
得られるレーザ素子は、発振波長が510nmであり、良好なレーザ素子が得られる。変形例２に比べて、活性層をＭＱＷからＳＱＷとしたことによる素子特性の低下は僅かなものとなる傾向にあるが、活性層中の中間層がＧａＮであることにより、中間層を設けることによる効果が低くなる傾向がみられる。しかしながら、変形例２と同様に、本発明のストライプ状の導波路を有することで、横モードの安定性、素子寿命に優れたレーザ素子が得られ、長波長域にも本発明は適用できる。また、ｐ側光ガイド層を１３００Åとして、ｎ側光ガイド層との膜厚差を十分なものとすることで、更に好ましいレーザ素子を得ることができる。
【０１０２】
［変形例４］
実施例１と同様に、異種基板上にバッファ層、下地層を形成した後、Ｓｉを１×１０¹⁸／cm²ドープしたＧａＮを１００μｍの膜厚で成長させる。続いて、ウエハの裏面、すなわち、窒化物半導体を成長させた異種基板の主面に対向する面側から、研磨して、基板を除去し、窒化物半導体のみとする。
次に、基板除去した面とは反対側の面の窒化物半導体２０４を主面として、図９に示すように、実施例１と同様の、ｎ側クラッド層２０６、ｎ側光ガイド層２０７、活性層２０８、ｐ側キャップ層２０９、ｐ側光ガイド層２１０、ｐ側クラッド層２１１、ｐ側コンタクト層２１２を順に積層する。続いても、実施例１と同様に、７００℃でアニールを行いｐ型導電層を更に低抵抗化し、反応容器からウェーハを取り出し、ＲＩＥ装置に移して、エッチングにより幅約３μｍのストライプ状の導波路を形成する。この時、エッチング深さは、ｐ側光ガイド層の膜厚に達する深さで、その膜厚が５００Åとなる位置となる深さで形成する。つづいて、ｐ側コンタクト層２１２の最上面にＮｉ／Ａｕからなるｐ電極２２０を形成し、このｐ電極２２０を除くエッチング露出面に、ＳｉＯ₂よりなる絶縁膜２６４を形成し、ｐ電極２２０に電気的に接続する取り出し電極２２２を絶縁膜２６４にまたがって形成し、ウェーハの裏面（ｎ側コンタクト層表面）にＴｉ／Ａｌよりなるｎ電極２２１、その上にヒートシンクとのメタライゼーション用にＡｕ／Ｓｎよりなる薄膜を形成する。最後に、ｎ電極２２１が設けられたウェーハ面側からスクライブし、ＧａＮのＭ面［（１１−００）面］でウェーハを劈開してバー状とした後、共振面を作製する。互いに対向する一対の共振面の内、少なくとも一方にＳｉＯ₂／ＴｉＯ₂よりなる誘電体多層膜のミラーを設け、最後に共振器方向にほぼ垂直に切断して、レーザ素子チップ得る。得られるレーザ素子は、実施例１に比べて、ストライプ幅が広いために、横モードの安定性に少し劣るものの、電流―光出力曲線において、キンクの発生のない良好な特性を有している。このことは、本発明がこのような設計変更に影響されず、良好な素子特性の向上を奏しうることを示唆するものである。
【０１０３】
【発明の効果】
本発明の窒化物半導体レーザ素子は、従来の出力特性を確保しながら、レーザ素子の光学特性、特にレーザ光のＦ．Ｆ．Ｐ．を良好なものが得られ、アスペクト比も大幅に改善された。また、光の閉じ込め効果が増大させることができたため、良好な導波路の形成が可能となり、素子寿命の向上も確認された。
【図面の簡単な説明】
【図１】本発明のレーザ素子を説明する断面模式図。
【図２】本発明のレーザ素子を説明する一部拡大した模式図。
【図３】本発明の一実施形態の製造方法を説明する模式図。
【図４】本発明のレーザ素子におけるエッチング深さと素子特性の関係を説明する図。
【図５】本発明の変形例１に係るレーザ素子の模式断面図。
【図６】本発明の変形例２に係るレーザ素子の模式断面図。
【図７】本発明の変形例３に係るレーザ素子の模式断面図。
【図８】本発明の変形例４，５に係るレーザ素子の模式断面図。
【図９】本発明の変形例６に係るレーザ素子の模式断面図。
【符号の説明】
１，２０１・・・異種基板
２，２０２・・・バッファ層
３，２０３・・・下地層
４，２０４・・・ｎ側コンタクト層
５，２０５・・・クラック防止層
６，２０６・・・ｎ側クラッド層
７，２０７・・・ｎ側光ガイド層
８，２０８・・・活性層
９，２０９・・・ｐ側キャップ層
１０，２１０・・・ｐ側光ガイド層
１１，２１１・・・ｐ側クラッド層
１２，２１２・・・ｐ側コンタクト層
６１，２６１・・・第１の保護膜
６２，２６２・・・第２の保護膜
６３，２６３・・・第３の保護膜
２０，２２０・・・ｐ電極
２１，２２１・・・ｎ電極
２２，２２２・・・ｐパッド電極
２３，２２３・・・ｎパッド電極[0001]
[Industrial application fields]
The present invention relates to a semiconductor laser having a short wavelength, and more particularly to a nitride semiconductor laser device capable of continuous oscillation at a high output without kinks in the field of optical information processing.
[0002]
[Prior art]
In recent years, with the development of the information society, a phi device for storing a large amount of information is required, and a short-wavelength laser light source is eagerly desired as a light source for communication and the like as a light source for large-capacity media such as DVDs. The present applicant has also announced that a nitride semiconductor laser device has achieved continuous oscillation of 10,000 hours or more at room temperature in a single mode with a wavelength of 403.7 nm.
[0003]
[Problems to be solved by the invention]
However, the application as a laser light source as described above requires further improvement of the characteristics of the laser element, particularly improvement of optical characteristics. This requires improvement of the optical waveguide of the semiconductor laser, such as the aspect ratio of the beam shape of the laser light, the improvement of the far-field image, and the prevention of light leakage.
[0004]
Specifically, the long-lived laser element is a refractive index waveguide type having a ridge waveguide structure, and the transverse mode must be controlled with high accuracy. This is because the effective refractive index of the ridge waveguide structure changes depending on the depth of etching, the height of the stripe, etc., and such a change in the structure greatly affects the device characteristics. As described above, the conventional laser element described above does not have sufficient optical characteristics in its application, and it is necessary to further improve the characteristics.
[0005]
That is, the beam shape of laser light, that is, F.R. F. P. (Far field pattern) aspect ratio improvement. Because, for application to optical disc systems and laser printers, laser light is corrected and adjusted by each optical system, but if the aspect ratio is not more than a certain value, the correction optical system becomes large-scale, its design, Loss due to manufacturing and the optical system becomes a big problem.
[0006]
Further, in the nitride semiconductor light emitting device, it is necessary to take measures against light leakage, which has been a problem in the past, and this appears as a ripple in the laser device, which causes a noise problem in the application of the laser device.
[0007]
In addition, it is necessary to further improve the yield from the viewpoint of the productivity of the laser element. Specifically, this is a problem caused by the controllability of the etching depth when forming the stripe.
[0008]
The present invention provides a nitride semiconductor laser element that has achieved improvement in element characteristics which is a problem in the application of the laser element as described above.
[0009]
[Means for Solving the Problems]
In view of the above circumstances, the present inventor pays attention to the light guide layer constituting the waveguide in order to obtain a laser element having a good beam shape, and the p-side light guide layer and the n-side light guide layer have different film thicknesses. As a result, the present invention has been completed.
[0010]
  In other words, the nitride semiconductor laser device of the present invention is such that the active layer is sandwiched in a nitride semiconductor laser device having an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer on a substrate. An n-side light guide layer and a p-side light guide layer are provided to form a waveguide, and an n-side cladding layer and a p-side cladding layer are provided so as to sandwich the waveguide,The p-side light guide layer has a stripe-shaped waveguide region by providing a striped protrusion by etching, and the p-side light guide layer is formed thicker than the film thickness of the n-side light guide layer. The p-side light guide layer has a thickness of 1 μm or less at the protruding portion, and the p-side light guide layer has a remaining etched thickness of 500 mm or more, other than the upper surface of the protruding portion. A protective film on the surface of the p-side light guide layer that is a side surface of the projecting portion, and the protective film is selected from the group consisting of Ti, V, Zr, Nb, Hf, Ta An oxide containing at least one element, or SiO ₂ , BN, SiC, AlNIt is characterized by. This realizes a good light confinement effect without increasing the threshold current and reduces ripple. In manufacturing, when a striped waveguide or the like is formed by etching, the etching depth up to the position of the p-side light guide layer is preferable because the etching accuracy is increased as compared with the conventional case.
[0011]
The p-side light guide layer has a stripe-shaped protrusion, and has a p-type nitride semiconductor layer on the protrusion, and the film thickness of the protrusion of the p-side light guide layer is 1 μm or less. It is characterized by. As a result, a stripe-shaped waveguide region is formed, and good lateral mode control is possible. Specifically, an effective refractive index difference is effectively formed. In particular, the light spread of the far field pattern in the direction parallel to the bonding surface is improved as compared with the conventional case, and the aspect ratio is also good in the application of the device. Furthermore, such an improvement in optical characteristics can be realized by keeping the threshold current equal to or higher than the conventional level low, enabling oscillation with a long lifetime, and maintaining various characteristics related to the conventional oscillation.
[0012]
The protruding portion of the p-side light guide layer and the p-type nitride semiconductor layer on the protruding portion are striped ridge waveguides formed by etching from the p-type nitride semiconductor layer side. And Since it is a ridge waveguide formed in the p-side light guide layer, it achieves good threshold current reduction, long life, stable oscillation in single mode, good lateral mode confinement, and aspect ratio This is a laser device having a ridge waveguide structure from which good laser light can be obtained.
[0013]
When the film thickness of the p-side light guide layer is 2500 mm (angstrom) or more, a more effective effective refractive index acts in laser light guiding, and the horizontal transverse mode is more effectively confined and stable. Thus, a laser beam having a good aspect ratio can be obtained.
[0014]
The p-side light guide layer has a film thickness in a region other than the protruding portion of 500 mm or more and 1000 mm or less, a laser element is stably manufactured, and there is little element variation, and an improvement in manufacturing yield can be realized.
[0015]
The stripe width of the protrusion is 1 μm or more and 3 μm or less, so that good transverse mode control is possible, especially oscillation in a single mode is possible, and generation of kinks etc. is suppressed even at high output. It is.
[0016]
Since the p-side light guide layer is InxGa1-xN (0 ≦ x <1), a good optical waveguide is formed, and the laser device has more excellent device characteristics.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to a specific example shown in FIG. Specifically, the nitride semiconductor laser device of the present invention is formed by laminating an n-side light guide layer made of an n-type nitride semiconductor, an active layer, and a p-side light guide layer made of a p-type nitride semiconductor on a substrate. In this case, the thickness of the p-side light guide layer is larger than the thickness of the n-side light guide layer.
[0018]
The nitride semiconductor laser device of the present invention has a structure in which an active layer is sandwiched between a p-side light guide layer and an n-side light guide layer, and the p-side light guide layer is formed from the thickness of the n-side light guide layer. By being thick, it has good laser light. This is because the light distribution in the film thickness direction deviates from the gain distribution due to the difference in film thickness between the p-side light guide layer and the n-side light guide layer sandwiching the active layer, and a change is made in the lateral mode control compared to the conventional case. Thus, a good laser beam is extracted. In addition, the effect of confining light is increased, and the generation of ripples is suppressed.
[0019]
(Striped waveguide region)
In the nitride semiconductor laser device of the present invention, in addition to the above-described optical guide film thickness being asymmetric across the active layer, the lateral mode is controlled by providing a striped waveguide region. It has a refractive index guided structure. In other words, an n-side light guide layer made of an n-type nitride semiconductor, an active layer, and a p-side light guide layer made of a p-type nitride semiconductor are stacked on a substrate. It has a striped protrusion and has a striped waveguide region. Furthermore, the laser element of the present invention is a laser element in which a p-type nitride semiconductor layer is formed on the protruding portion. Specifically, it has a striped waveguide region as described above, and is a refractive index guided laser element.
[0020]
(Etching depth)
Specifically, the laser device of the present invention includes an n-side light guide layer made of an n-type nitride semiconductor, an active layer, a p-side light guide layer made of a p-type nitride semiconductor, and a p-type nitride thereon. After the semiconductor layers are stacked, a part of the p-type nitride semiconductor layer and the p-side light guide layer is removed by etching from the p-type nitride semiconductor layer side to form a stripe structure. At this time, since the height of the protruding portion of the side light guide layer is determined by the etching depth, the controllability of the etching depth is improved as compared with the conventional case, as will be described later. In addition, it is important that the etching depth not reach the active layer, and in the present invention, etching is performed up to the position of the p-side light guide layer.
[0021]
In the present invention, the shape of the protrusion of the p-side light guide layer or the shape of the striped ridge waveguide is not limited to the forward mesa shape or the reverse mesa shape. This is preferable because mode control tends to be realized.
[0022]
(Etching means)
In order to etch a nitride semiconductor such as the formation of the p-side light guide layer or the ridge waveguide described above, there are methods such as wet etching and dry etching. For example, reactive ion etching (RIE) or reactive etching is used as dry etching. There are apparatuses such as ion beam etching (RIBE), electron cyclotron etching (ECR), and ion beam etching, all of which can etch a nitride semiconductor by appropriately selecting an etching gas.
[0023]
(Light guide layer)
A waveguide is formed by a structure in which an active layer is sandwiched between an n-side light guide layer and a p-side light guide layer. The laser element of the present invention has a striped waveguide region by providing a striped protrusion on the p-side light guide layer.
[0024]
(P-side light guide layer)
In the present invention, the n-side light guide layer and the p-side light guide layer sandwiching the active layer have different film thicknesses and are intended to increase the thickness of the p-side light guide layer. More preferably, the p-side light guide layer has a stripe-shaped protruding portion. Specifically, a p-type nitride semiconductor layer is formed on the protruding portion, and a stripe-shaped waveguide region is formed. The laser element which has is formed. Specifically, it is a laser element in which a ridge waveguide is formed by a p-side light guide layer. Further, as described above, this protrusion is specifically formed by etching from the p-type nitride semiconductor layer side, and is formed by stopping etching in the film of the p-side light guide layer. Here, the film thickness of the p-side light guide layer corresponds to the film thickness when the p-side light guide layer is grown. When the protrusion is formed by the etching after the p-side light guide layer is formed, the film thickness is a predetermined film thickness. Since the p-side light guide layer is formed by removing a part of the p-side light guide layer, the film thickness of the protruding portion becomes the film thickness of the p-side light guide layer. At this time, if the thickness of the p-side light guide layer is more than 1 μm, the threshold value is greatly improved, and laser oscillation becomes extremely difficult. The film thickness of the p-side light guide layer is 1 μm or less. More preferably, the film thickness of the p-side light guide layer, that is, the film thickness of the protruding portion is in the range of 1500 mm to 5000 mm. Because, if it is thinner than 1500 mm, the laser beam F.V. F. P. If the film thickness exceeds 5000 mm, the oscillation threshold current tends to increase. Specifically, when the thickness is less than 1500 mm, the horizontal and transverse modes are not sufficiently controlled. F. P. Becomes a beam shape of 10 ° or more in the x direction, and as a result, the aspect ratio exceeds 2.0. At this time, in particular, in the present invention, when the thickness of the p-side light guide layer is 2500 mm or more, the increase in the threshold current tends to be suppressed as compared with the case where the thickness of the guide layer sandwiching the active layer is the same. It has been confirmed that, as in the present invention, the p-side light guide layer has a thick asymmetric waveguide and tends to work advantageously.
[0025]
Further, when forming the ridge waveguide and the protruding portion by etching, productivity must be taken into consideration. This is because as the depth of etching increases, the accuracy, for example, variation among elements in the wafer increases, and this needs to be avoided. Specifically, when the above-described stripe-shaped protrusion (ridge structure) is formed by etching at a depth exceeding 0.7 μm, the above problem tends to occur rapidly, and etching may be performed shallower than this. preferable. That is, in the present invention, the height of the ridge is adjusted, and the laser element is preferably formed within the above range. Here, the height of the protrusion is specifically the ridge from the plane of the p-side light guide layer in the region other than the protrusion, that is, the plane exposed by etching and continuing to the side surface of the protrusion to the protrusion. Is the height in the film thickness direction up to the p-type nitride semiconductor layer formed on the protruding portion, and the uppermost surface of the p-type nitride semiconductor layer is the etching start position.
[0026]
(Height of protrusion)
Furthermore, in the p-side light guide layer of the present invention, it is preferable to increase the height of the protruding portion because the oscillation threshold current tends to decrease. That is, this increases the stability of the output as the etching becomes deeper, and greatly contributes to the application of the laser element. That is, even if the output increases, stable oscillation in a single mode is realized, and since the oscillation threshold current is good, the element deterioration is greatly suppressed, and continuous oscillation with a long life is realized. In the p-side light guide layer, when the height of the protrusion is 100 mm or more, a laser element having a good beam shape can be obtained, preferably 500 mm or more. Oscillation is possible. Therefore, the reliability of the element required in the application of the laser element is sufficiently ensured.
[0027]
In addition to the above, in the case of forming by etching, it is more preferable in consideration of the flatness of the surface exposed by etching. This is because the position of the surface of the p-side light guide layer exposed by the etching is in the film thickness direction when the stripe-shaped protrusion is formed by etching, and causes variations between elements. Since there is variation in the region, it is necessary to consider it. Specifically, the p-side light guide layer is etched to a depth that leaves a thickness in the range of 500 mm or more, preferably in the range of 500 to 1000 mm, to form the protruding portion of the p-side light guide layer. If the depth is more than 500 mm, the etching is deeper than the p-side light guide layer, and the protrusion is formed with good accuracy. On the other hand, if it is 1000 mm or more, the above-described increase in the oscillation threshold current is observed, and the controllability of the transverse mode tends to be inferior.
[0028]
In the present invention, the composition of the p-side light guide layer is not particularly limited, and may be a single film that is made of a nitride semiconductor and has an energy band gap sufficient for waveguide formation. Either a multilayer film may be used. For example, undoped GaN is used at wavelengths of 370 to 480 nm, and an InGaN / GaN multilayer structure is used at longer wavelengths.
[0029]
In the present invention, the waveguide constituted by the structure in which the active layer is sandwiched between the n-side light guide layer and the p-side light guide layer is the sum of the film thicknesses, that is, the thickness of the region sandwiched between both guide layers. However, it is preferably 5000 mm or less, more preferably 4500 mm or less. This is because if the total thickness of the waveguide exceeds 5000 mm, the threshold value increases, and if it exceeds 7000 mm, the threshold current increases rapidly, and continuous oscillation in the fundamental mode becomes extremely difficult. When it is 4500 mm or less, such an increase in the oscillation threshold current is suppressed, and continuous oscillation with a basic mode and a long lifetime is possible.
[0030]
In the present invention, the n-side light guide layer is not particularly limited except that it is thinner than the thickness of the p-side light guide layer, and a waveguide is used as a structure in which the active layer is sandwiched between both light guide layers. . Specifically, it is desirable to grow GaN and InGaN as the n-side light guide layer, specifically, undoped GaN, InGaN with an In mixed crystal ratio decreasing as it approaches the active layer, and GaN alternately. There are multi-layer films laminated on each other. Here, InGaN is a ternary mixed crystal in which GaN contains In.
[0031]
In the nitride semiconductor laser device of the present invention, the p-type nitride semiconductor layer formed on the p-side light guide layer specifically includes a p-side cladding layer and a p-side contact as shown in the examples. Layers are laminated. Therefore, in the present invention, the p-type nitride semiconductor layer formed on the protruding portion of the p-side light guide layer is formed in a stripe shape and forms a ridge waveguide.
[0032]
In the present invention, a cap layer may be formed between the light guide layer and the active layer. For example, a p-side cap layer made of AlxGa1-xN (0 ≦ x ≦ 1) doped with a p-type impurity is formed between the active layer and the p-side light guide layer. At this time, if the stripe-shaped ridge waveguide is formed at a depth reaching the p-side cap layer, the device life tends to be reduced. In such a case as well, as described above, the p-side light It is preferable to form the ridge waveguide so that the guide layer is provided with a striped protrusion.
[0033]
As for a waveguide in which a waveguide is formed by the active layer and the light guide layer as described above, or a waveguide having a cap layer thereon, as specific embodiments, examples described later, modified examples 1 to 3, and FIGS. There is something shown in 7. The light guide layer has a structure in which the active layer is sandwiched, and is provided on each of the p-type conductive layer side and the n-type conductive layer side, and a region sandwiched between both the light guide layers forms an optical waveguide region.
[0034]
Further, the p-side cap layer provided between the active layer and the p-side cladding layer, preferably between the active layer and the p-side light guide layer, reduces the threshold current, thereby facilitating easy oscillation. It is a layer that contributes and also functions as confinement of carriers in the active layer. When AlGaN is used for this p-side cap layer, it may have the above-mentioned function by being preferably doped with a p-type impurity. However, even when non-doped, it tends to function as the confinement of the carrier. . The film thickness is 500 mm or less, and Al_xGa_1-xAs the composition of N, the above effect can be sufficiently expected when x is larger than 0, preferably 0.2 or more.
[0035]
In the present invention, when the stripe width of the ridge waveguide or the stripe width of the protruding portion in the p-side light guide layer is in the range of 1 μm or more and 3 μm or less, favorable lateral mode control is possible. In this range, oscillation in a single transverse mode is realized relatively well, and a ridge waveguide is formed by the p-side light guide layer, thereby providing a stable and accurate beam shape. Control (good FP) can be realized. At this time, if the thickness is less than 1 μm, the formation of the striped ridge structure or the protruding portion becomes difficult in manufacturing, the yield is lowered, and if it exceeds 3 μm, the control of the horizontal transverse mode tends to be difficult.
[0036]
The nitride semiconductor constituting the nitride semiconductor laser device according to the present invention is represented by InxAlyGa1-xyN (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1), in addition to gallium nitride. There are ternary and quaternary mixed crystals. In the present invention, the laser element structure laminated on the substrate is made of the nitride semiconductor represented by the above composition formula, as shown in the examples. Shape control tends to be most preferred.
[0037]
Here, F.R. F. P. The horizontal direction (x direction) in FIG. 2 indicates a direction parallel to the junction surface (or pn junction surface), and is the x direction 102 of the far-field image (FFP) 101 in FIG. The transverse mode in that direction is referred to as a horizontal transverse mode.
[0038]
(Action / Effect)
The present invention has an asymmetric structure in which the p-side light guide layer is thicker than the n-side light guide layer among the light guide layers sandwiching the active layer, and in the waveguide region sandwiched between both light guide layers. By shifting the light distribution in the laser compared to the conventional laser element, that is, by shifting the light distribution and the gain distribution, the transverse mode is controlled by a method different from the conventional one to obtain a laser beam with a desired beam shape. is there.
[0039]
Preferably, the p-side light guide layer has a stripe-shaped protrusion, which forms a stripe-shaped waveguide region as described above and forms an effective refractive index distribution. . Specifically, as seen in FIG. 2, the laser spot 103 as the emitted light is biased toward the p-side light guide layer 9 side having the stripe-shaped protruding portion, and good horizontal and transverse mode control is realized. As a result, in the conventional stripe-structure laser element, F.I. F. P. The horizontal direction (x direction) is narrow and the aspect ratio is 3 or more. However, in the present invention, the horizontal mode in the horizontal direction is well controlled, so the aspect ratio is much better than before. It will be something.
[0040]
In the present invention, when the active layer is a nitride semiconductor containing In, for example, a ternary mixed crystal of InGaN, light emitted from the active layer is scattered by In in the active layer. Must be considered. That is, the light from the active layer is lost by In, which is a light scattering material in the active layer, and this is the original idea that hinders the improvement in output. In the laser element of the present invention, as described above, the laser light can be obtained by waveguide different from the conventional one because the p-side light guide layer is thick among the light guide layers sandwiched between the active layers. Loss due to the light scattering material is reduced, and the laser element compensates for the increase in threshold current due to the increase in the thickness of the light guide layer. This is mainly because the light distribution in the waveguide in the film thickness direction deviates from the gain distribution, so that the light is distributed and guided in a position deviated from the region (active layer) of the light scattering material. by. For this reason, the loss due to the light scattering is reduced, and as a result, it is considered that oscillation can be performed with a threshold current equal to or lower than that of the conventional one. This suppresses the tendency of the threshold current to increase due to the increase in the thickness of the light guide layer, particularly the thickness of the p-side light guide layer, and the loss of light scattering is reduced in the present invention. A laser element having a good laser beam shape can be obtained with the threshold value being similar to the conventional one.
[0041]
In FIG. 2, conventionally, the spot shape 103 on the exit surface has spread in a direction parallel to the joint surface. F. P. The x-direction 102 was narrow at 10 ° or less and the aspect ratio was poor. However, in the present invention, as shown in the figure, the longitudinal direction of the spot shape is in the horizontal direction as in the prior art, but the spread in the longitudinal direction is narrowed. F. P. The x direction of 101 is wider than before, specifically 12 ° to 20 °, and the aspect ratio is as good as around 2.0. As described above, the optical characteristics are improved without deteriorating the output characteristics and the element reliability. As described above, the p-side light guide layer provided with the stripe-shaped protrusions provides a good effective refractive index. This is due to the fact that the stripe-shaped waveguide region having the above is formed in the laser element. Furthermore, as described above, the thickness of the p-side light guide layer or the total thickness of both guide layers, which is obtained by adding the thickness of the n-side light guide layer, is increased as compared with the conventional case. Beam divergence in the direction perpendicular to the surface (y direction) is suppressed by the reduction of the diffraction effect, which also contributes to the improvement of the optical characteristics, particularly the aspect ratio, of the laser device of the present invention. That is, not only the horizontal / horizontal mode control as described above, but also the F.S. F. P. By suppressing the spread of the light in the y direction, laser light closer to a perfect circle can be obtained from the far field pattern 101 flattened in the vertical direction as in the prior art. As described above, the p-side light guide layer is thicker than the n-side light guide layer. F. P. In addition to the x direction, the contribution to improving the beam shape in the y direction is also important for achieving the effects of the present invention.
[0042]
【Example】
The element characteristic change (FIG. 4) with respect to the etching depth or the height of the stripe-shaped protrusion of the p-side light guide layer shown below was measured based on the laser element of Example 1, This is the case where the thickness of the p-side light guide layer is large among both the light guide layers sandwiching the active layer.
[0043]
FIG. 4 shows changes in threshold current and changes in aspect ratio with respect to a structure in which an etching depth, a p-side cap layer, a p-side guide layer, and a p-side light guide layer are stacked. As is clear from FIG. 4, by etching to a depth reaching the p-side light guide layer, a ridge structure is formed, that is, the p-side light guide layer has a stripe-shaped protrusion, so that a good threshold current can be obtained. Thus, it is possible to oscillate, and the aspect ratio of the obtained laser light tends to approach 1. As shown in FIG. 2, this is a state in which the emitted light moves to the stripe-shaped protruding portion side of the p-side light guide layer, and a good effective refractive index is formed. As a result, the horizontal and transverse modes are effectively confined. F. P. This shows that a laser element having a beam shape in the x direction of 10 ° or more, preferably 12 ° to 20 °, and having a good aspect ratio can be obtained. Further, since the threshold current and the aspect ratio tend to decrease as the film thickness in the region other than the projecting portion of the p-side light guide layer becomes thinner, that is, as the height of the projecting portion increases, The height of the protruding portion is within the above-described range.
[0044]
[Example 1]
FIG. 1 is a schematic cross-sectional view showing a structure of a laser device according to an embodiment of the present invention, and shows a laminated structure when cut along a plane perpendicular to a stripe-shaped protrusion. Hereinafter, Example 1 is demonstrated based on this figure.
[0045]
Here, in this embodiment, a different substrate different from the nitride semiconductor is used as the substrate, but a substrate made of a nitride semiconductor such as a GaN substrate may be used. Here, as the heterogeneous substrate, for example, sapphire or spinel (MgA1) whose main surface is any one of the C-plane, R-plane, and A-plane.₂O_FourIt is possible to grow a nitride semiconductor such as an insulating substrate such as SiC (including 6H, 4H, 3C), ZnS, ZnO, GaAs, Si, and an oxide substrate lattice-matched with a nitride semiconductor. A substrate material different from that of a nitride semiconductor can be used. Preferable heterogeneous substrates include sapphire and spinel. Further, the heterogeneous substrate may be off-angle, and in this case, it is preferable to use a step-off-angle substrate because the growth of the underlying layer made of gallium nitride grows with good crystallinity. Further, when a heterogeneous substrate is used, a nitride semiconductor as a base layer before forming the element structure is grown on the heterogeneous substrate, and then the heterogeneous substrate is removed by a method such as polishing to obtain a single substrate of the nitride semiconductor An element structure may be formed, or a method of removing the heterogeneous substrate after the element structure is formed may be used.
[0046]
As the substrate, a sapphire substrate having a (0001) C plane as a main surface was used. At this time, the orientation flat surface was the A surface. As a substrate for growing a nitride semiconductor, it is known for growing nitride semiconductors such as SiC, ZnO, spinel (MgAl 2 O 4), GaAs, etc. in addition to sapphire (the main surface is C-plane, R-plane, A-plane). A model substrate made of a material different from the nitride semiconductor can be used. Further, it may be laminated directly on a substrate made of a nitride semiconductor.
[0047]
(Buffer layer 2)
A 1-inch φ, heterogeneous substrate 1 made of sapphire with a C-plane as the main surface is set in a MOVPE reaction vessel, the temperature is set to 500 ° C., a buffer made of GaN using trimethylgallium (TMG) and ammonia (NH 3). The layer is grown to a thickness of 200 mm.
[0048]
(Underlayer 3)
After growing the buffer layer, the temperature is set to 1050 ° C., and the base layer 3 made of undoped GaN is grown to a thickness of 4 μm using TMG and ammonia. This layer acts as a substrate in the growth of each layer forming the device structure. Thus, when forming a nitride semiconductor device structure on a heterogeneous substrate, it is preferable to form a low-temperature growth buffer layer and a base layer to be a nitride semiconductor substrate.
[0049]
(N-side contact layer 4)
Next, an n-side contact layer 5 made of GaN doped with Si 3 × 10 18 / cm 3 is grown on the nitride semiconductor substrate 1 at 1050 ° C. with a film thickness of 4 μm using ammonia and TMG and silane gas as impurity gas. Let
[0050]
(Crack prevention layer 5)
Next, the crack prevention layer 6 made of In0.06Ga0.94N is grown to a thickness of 0.15 μm using TMG, TMI (trimethylindium), and ammonia at a temperature of 800 ° C. This crack prevention layer can be omitted.
[0051]
(N-side cladding layer 6)
Subsequently, using TMA (trimethylaluminum), TMG, and ammonia at 1050 ° C., a layer made of undoped Al0.16Ga0.84N is grown to a thickness of 25 mm, then TMA is stopped, silane gas is flowed, and Si is added to 1 A layer made of n-type GaN doped with × 1019 / cm3 is grown to a thickness of 25 mm. These layers are alternately stacked to form a superlattice layer, and an n-side cladding layer 7 made of a superlattice having a total film thickness of 1.2 μm is grown.
[0052]
(N-side light guide layer 7)
Subsequently, the silane gas is stopped and the n-side light guide layer 8 made of undoped GaN is grown at a thickness of 1000 mm at 1050 ° C. The n-side light guide layer 8 may be doped with n-type impurities.
[0053]
(Active layer 8)
Next, the temperature is set to 800 ° C., a barrier layer made of Si-doped In0.05Ga0.95N is grown to a thickness of 100 mm, and then a well layer made of undoped In0.2Ga0.8N is grown to a thickness of 40 mm at the same temperature. Grow with thickness. A barrier layer and a well layer are alternately stacked twice, and finally, an active layer of a multiple quantum well structure (MQW) having a total film thickness of 380 mm is grown by ending with the barrier layer. The active layer may be undoped as in this embodiment, or may be doped with n-type impurities and / or p-type impurities. Impurities may be doped into both the well layer and the barrier layer, or one of them may be doped. Note that if the n-type impurity is doped only in the barrier layer, the threshold value tends to decrease.
[0054]
(P-side cap layer 9)
Next, the temperature was raised to 1050 ° C., TMG, TMA, ammonia, Cp 2 Mg (cyclopentadienyl magnesium) was used, and the band gap energy was larger than that of the p-side light guide layer 11, and Mg was doped 1 × 10 20 / cm 3. A p-side cap layer 7 made of p-type Al0.3Ga0.7N is grown to a thickness of 300 mm.
[0055]
(P-side light guide layer 10)
Subsequently, Cp2Mg and TMA are stopped, and a p-side light guide layer 11 made of undoped GaN having a band gap energy smaller than that of the p-side cap layer 10 is grown at 1050 ° C. to a thickness of 2500 mm.
The p-side light guide layer 10 is grown undoped, that is, intentionally undoped, but Mg diffusion occurs from adjacent layers of the p-side cap layer and the p-side cladding layer. In practice, the Mg concentration is 5 × 10¹⁶/cm^ThreeThus, a layer doped with Mg is formed.
[0056]
(P-side cladding layer 11)
Subsequently, a layer made of undoped Al0.16Ga0.84N is grown at a thickness of 25 mm at 1050 ° C., then Cp2Mg and TMA are stopped, and a layer made of undoped GaN is grown at a thickness of 25 mm, resulting in a total thickness of 0 A p-side cladding layer 12 made of a .6 μm superlattice layer is grown. When the p-side cladding layer is made of a superlattice in which at least one nitride semiconductor layer containing Al is included and nitride semiconductor layers having different bandgap energies are stacked, impurities are heavily doped into one of the layers. Although so-called modulation doping tends to improve the crystallinity, both may be doped in the same manner. The cladding layer 12 has a nitride semiconductor layer containing Al, preferably a superlattice structure containing AlXGa1-XN (0 <X <1), more preferably a superlattice structure in which GaN and AlGaN are stacked. To do. By making the p-side cladding layer 12 a superlattice structure, the Al mixed crystal ratio of the entire cladding layer can be increased, so that the refractive index of the cladding layer itself is reduced and the band gap energy is increased. It is very effective in lowering. Furthermore, since the superlattice is used, the number of pits generated in the clad layer itself is less than that not formed in the superlattice, so that the occurrence of a short circuit can be suppressed.
[0057]
(P-side contact layer 12)
Finally, the p-side contact layer 12 made of p-type GaN doped with 1 × 10 20 / cm 3 of Mg is grown on the p-side cladding layer 11 at 1050 ° C. to a thickness of 150 μm. The p-side contact layer can be composed of p-type InXAlYGa1-X-YN (0 ≦ X, 0 ≦ Y, X + Y ≦ 1), preferably Mg doped GaN, and most preferable with the p electrode 20. Ohmic contact is obtained. Since the contact layer 12 is a layer for forming an electrode, a high carrier concentration of 1 × 10 17 / cm 3 or more is desirable. If it is lower than 1 × 10 17 / cm 3, it tends to be difficult to obtain a preferable ohmic with the electrode. Furthermore, when the composition of the contact layer is GaN, a preferable ohmic with the electrode material is easily obtained.
[0058]
The wafer on which the nitride semiconductor is grown as described above is taken out from the reaction vessel, a protective film made of SiO2 is formed on the surface of the uppermost p-side contact layer, and SiCl4 gas is used using RIE (reactive ion etching). As shown in FIG. 1, the surface of the n-side contact layer 4 where the n-electrode is to be formed is exposed. Thus, SiO2 is optimal as a protective film for deep etching of the nitride semiconductor.
[0059]
Next, a method for forming a striped ridge waveguide will be described. First, as shown in FIG. 3A, a first protective film 61 made of Si oxide (mainly SiO 2) is formed on the substantially entire surface of the uppermost p-side contact layer 12 by a PVD device to a thickness of 0.5 μm. After forming with a film thickness, a mask having a predetermined shape is put on the first protective film 61 to form a third protective film 63 made of a photoresist with a stripe width of 2 μm and a thickness of 1 μm. Here, the first protective film 61 may be any material as long as it has a difference from the etching rate of the nitride semiconductor regardless of the insulating property. For example, Si oxide (including SiO.sub.2), photoresist, or the like is used. Preferably, it is dissolved in acid rather than the second protective film in order to provide a difference in solubility from the second protective film to be formed later. Select a material that has easy properties. As the acid, hydrofluoric acid is preferably used, and therefore Si oxide is preferably used as a material that is easily dissolved in hydrofluoric acid.
[0060]
Next, as shown in FIG. 3B, after the formation of the third protective film 63, the first protective film 63 is used as a mask by using a CF4 gas and a third protective film 63 as a mask by an RIE (reactive ion etching) apparatus. The protective film is etched into a stripe shape. Thereafter, the first protective film 61 having a stripe width of 2 μm can be formed on the p-side contact layer 12 as shown in FIG.
[0061]
Further, as shown in FIG. 3D, after the first protective film 61 having a stripe shape is formed, the p-side contact layer 12, the p-side cladding layer 11, and the p-side light guide are again used by RIE using SiCl4 gas. The layer 10 is etched to form a ridge stripe as a striped waveguide region having a depth of 1000 mm in the etched region (region other than the protruding portion) of the p-side light guide layer.
[0062]
After forming the ridge stripe, the wafer is transferred to the PVD apparatus, and as shown in FIG. 3E, a second protective film 62 made of Zr oxide (mainly ZrO 2) is formed on the first protective film 61, It is continuously formed with a film thickness of 0.5 μm on the p-side light guide layer 11 exposed by etching (a region other than the protruding portion).
[0063]
Here, the material of the second protective film is a material other than SiO2, preferably an oxide containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf, Ta, SiN, BN, It is desirable to use at least one of SiC and AlN, and it is particularly preferable to use an oxide of Zr or Hf, BN, or SiC among them. Some of these materials have a property of being slightly dissolved in hydrofluoric acid. However, if an insulating layer of a laser element is used, the reliability as a buried layer tends to be considerably higher than that of SiO2. In addition, oxide thin films formed in the gas phase such as PVD and CVD are less likely to be oxides in which the element and oxygen are equivalently reacted, and thus the reliability of the insulating properties of the oxide thin films is unlikely to be insufficient. However, PVD, CVD oxides, BN, SiC, and AlN of the elements selected in the present invention tend to be more reliable in terms of insulation than Si oxides. In addition, it is very convenient as a buried layer of a laser element when an oxide whose refractive index is smaller than that of a nitride semiconductor (for example, other than SiC) is selected. Furthermore, when the first protective film 61 is made of Si oxide, since it has selectivity for hydrofluoric acid with respect to Si oxide, the side surface of the stripe waveguide as shown in FIG. When the stripe is formed continuously on the plane (etch stop layer) and the surface of the first protective film 61, when only the first protective film 61 is removed by a lift-off method, FIG. The second protective film 62 having a uniform film thickness with respect to the plane can be formed as shown in FIG.
[0064]
After forming the second protective film 62, the wafer is heat-treated at 600 ° C. When a material other than SiO 2 is formed as the second protective film in this way, after the second protective film is formed, heat treatment is performed at 300 ° C. or higher, preferably 400 ° C. or higher and below the decomposition temperature of the nitride semiconductor (1200 ° C.). By doing so, it becomes difficult for the second protective film to dissolve in the dissolved material (hydrofluoric acid) of the first protective film, and it is more desirable to add this step.
[0065]
Next, the wafer is immersed in hydrofluoric acid, and as shown in FIG. 3F, the first protective film 61 is removed by a lift-off method.
[0066]
Next, as shown in FIG. 3G, the p-electrode 20 made of Ni / Au is formed on the surface of the p-side contact layer exposed by removing the first protective film 61 on the p-side contact layer 12. To do. However, the p electrode 20 has a stripe width of 100 μm and is formed over the second protective film 62 as shown in FIG. After the formation of the second protective film, an n electrode 21 made of Ti / Al is formed in a direction parallel to the stripes on the surface of the n-side contact layer 5 that has already been exposed.
[0067]
Next, after forming a dielectric multilayer film 64 made of SiO 2 and TiO 2 by masking a desired region for providing a take-out electrode on the p and n electrodes on the surface exposed by etching to form an n-electrode. Extraction (pad) electrodes 22 and 23 made of Ni-Ti-Au (1000? -1000? -8000?) Were provided on the p and n electrodes, respectively.
[0068]
As described above, after polishing the sapphire substrate of the wafer on which the n-electrode and the p-electrode are formed to 70 μm, the substrate is cleaved in a bar shape from the substrate side in a direction perpendicular to the stripe-shaped electrode. A resonator is fabricated on the (11-00) plane, a plane corresponding to a hexagonal side surface = M plane). A dielectric multilayer film made of SiO2 and TiO2 is formed on the resonator surface, and finally a bar is cut in a direction parallel to the p-electrode to obtain a laser element as shown in FIG. At this time, the resonator length was 800 μm.
[0069]
When this laser element was placed on a heat sink and each pad electrode was wire-bonded and laser oscillation was attempted at room temperature, it was in a single transverse mode at an oscillation wavelength of 400 to 420 nm and an oscillation threshold current density of 2.9 kA / cm 2. Of room temperature continuous oscillation. Next, F. of laser light. F. P. As a result, a good horizontal transverse mode of 16 ° to 20 ° in the horizontal direction was obtained. Further, the horizontal and transverse modes were almost as good as those in Comparative Example 1, and the aspect ratio was 2. In addition, the thick light guide layer provides good light confinement, and the generation of ripples can be significantly suppressed as compared with Comparative Example 1.
[0070]
[Example 2]
A laser element is obtained in the same manner as in Example 1 except that the thickness of the n-side light guide layer is 2000 mm. The obtained laser element is slightly inferior to the transverse mode control as compared with the first embodiment. F. P. The x direction is 14 ° and the aspect ratio is 2, which is significantly improved as compared with Comparative Example 1. Since the aspect ratio is 2.5 or less, the application to optical information equipment becomes simpler. Further, the light confinement was good as in Example 1, and the occurrence of ripples was greatly reduced. As for the output characteristics, the film thickness of the waveguide region sandwiched between the p-side light guide layer and the n-side light guide layer is 5000 mm or more, so the threshold current is increased compared to Example 1, and the device is also implemented. It was inferior to Example 1.
[0071]
[Example 3]
The thickness of the p-side light guide layer is 3000 mm, the film thickness in the region other than the protrusions is 1000 mm, that is, the etching is performed at a depth where the film thickness of the p-side light guide layer is 1000 mm. Thus, a laser element is obtained. The obtained laser element can obtain laser light that is as good as that of the first embodiment. F. P. The beam spread in the x direction was 18 ° and the aspect ratio was 1.4, which was sufficient for practical use. Furthermore, when the thickness of the p-side light guide layer is 3500 mm and the height of the protrusion of the p-side light guide layer is 2500 mm (the film thickness of the region other than the protrusion is 1000 mm), the threshold current increases. Although the lifetime of the element tended to decrease, the beam shape of the emitted light was almost the same. This seems to be due to the fact that the total thickness of both light guide layers sandwiching the active layer exceeded 5000 mm.
[0072]
[Example 4]
A laser element is obtained in the same manner as in Example 1 except that the stripe width of the protruding portion of the p-side light guide layer, that is, the width of the striped ridge waveguide is 3 μm. The obtained laser element is inferior in control of the horizontal and transverse modes as compared with the first embodiment, compared with the first embodiment. F. P. The aspect ratio was 2, which was inferior to that of Example 1. In addition, the stability of oscillation in the single transverse mode was inferior to that of Example 1, and the proportion of devices that became defective products with kinks tended to increase. Therefore, more preferably, the stripe width is in the range of 2 μm ± 0.5 μm (1.5 μm or more and 2.5 μm or less), so that there is little element variation in lateral mode controllability, and the laser light aspect ratio is also good. A single-mode oscillation laser element can be obtained.
[0073]
[Example 6]
As an embodiment of the present invention, a laser element having a longer wavelength than that of Example 1, specifically, a longer wavelength of 480 nm or more will be described below. On the heterogeneous substrate 1 made of sapphire having the C-plane as the main surface, a buffer layer 2 made of GaN is grown to 200 μm, and an underlayer 3 made of undoped GaN is grown to 4 μm as in Example 1, and Si is deposited thereon. An n-side contact layer 4 made of GaN doped with 1 × 10 18 / cm 3 is grown to 4.5 μm, and an intermediate layer 5 made of Si-doped In 0.3 Ga 0.7 N is grown. At this time, the intermediate layer can be omitted.
[0074]
(N-side cladding layer 6)
Next, TMG, ammonia, and TMA (trimethylaluminum) are flowed, and a layer made of undoped Al0.15Ga0.85N is grown at 1050 ° C. to a thickness of 25 mm. Subsequently, TMA is stopped, silane gas is flowed, and Si is flown. A layer made of n-type GaN doped with 1 × 10 19 / cm 3 is grown to a thickness of 25 mm. These layers are alternately stacked to form a superlattice layer, and an n-side cladding layer 6 made of a superlattice having a total film thickness of 0.2 μm to 1.5 μm, preferably 0.7 μm, is grown. The n-side cladding layer preferably has a nitride semiconductor layer containing Al, preferably a superlattice structure containing AlXGa1-XN (0 <X <1), more preferably a superlattice structure in which GaN and AlGaN are stacked. And In the case of a superlattice, when one of the layers is doped with a large amount of impurities and so-called modulation doping is performed, the crystallinity tends to be improved, but both may be similarly doped.
[0075]
(N-side light guide layer 7)
Subsequently, the silane gas is turned off, TMI is flown, and a layer made of undoped In0.1Ga0.9N is grown at a thickness of 10 mm at 850 ° C. to 950 ° C., preferably 880 ° C., and then the TMI is turned off. The resulting layer is grown to a thickness of 10 mm. These layers are alternately stacked to form a superlattice layer, and an n-side light guide layer 7 made of a superlattice having a total film thickness of 50 to 2500 mm, preferably 500 to 800 mm, and more preferably 750 mm is grown.
[0076]
(Active layer 8)
Subsequently, TMI is flown, and a well layer composed of undoped In0.4Ga0.6N is formed at 30 ° C. and a cap layer composed of undoped In0.3Ga0.7N is formed at 10 ° C. at 750 ° C. to 850 ° C., preferably 820 ° C., followed by 850 ° C. to 950 ° C. A barrier layer made of undoped In0.1Ga0.9N is grown at 60 ° C. at 60 ° C., preferably 880 ° C., and this is used as one pair to grow an active layer 8 in which a total of 6 pairs are stacked.
[0077]
(P-side cap layer 9)
Next, TMI is stopped, TMA is flown, and a cap layer 9 made of p-type Al0.3Ga0.7N doped with 1 × 1020 / cm3 of Mg at 850 ° C. to 950 ° C., preferably 880 ° C., is 10 μm or more and 0.1 μm. Hereinafter, the film is grown preferably with a thickness of 100 mm.
[0078]
(P-side light guide layer 10)
Subsequently, TMA is stopped, TMI is flown, a layer made of undoped In0.1Ga0.9N is grown at a thickness of 10 mm at 850 ° C. to 950 ° C., preferably 880 ° C., and then TMI is stopped, A layer of GaN doped with 1 × 10 18 to 3 × 10 18 / cm 3 is grown to a thickness of 10 mm. A superlattice layer is formed by alternately laminating these layers, and a p-side light guide layer 10 made of a superlattice having a total film thickness of 50 to 2500 mm, preferably 500 to 800 mm, and more preferably 750 mm is grown.
[0079]
(P-side cladding layer 11)
Subsequently, TMA was flown to grow a layer made of undoped Al0.15Ga0.85N at a thickness of 25 mm at 850 ° C. to 1050 ° C., and then TMA was stopped, and Mg was 3 × 10 18 to 5 × 10 18 / cm 3. A layer made of doped GaN is grown to a thickness of 25 mm. These layers are alternately stacked to form a superlattice structure, and a p-side cladding layer 11 made of a superlattice having a total film thickness of 0.2 μm to 1.5 μm, preferably 0.7 μm, is grown.
[0080]
(P-side contact layer 12)
Finally, a p-side contact layer 12 made of p-type GaN doped with 1 × 10 20 / cm 3 of Mg is grown on the p-side cladding layer 10 at a temperature of 850 ° C. to 1050 ° C. to a thickness of 150 Å. The p-side contact layer can be made of p-type InXGaYAl1-X-YN (0 ≦ X, 0 ≦ Y, X + Y ≦ 1), preferably Mg-doped GaN, InGaN, and the p electrode. A preferred ohmic contact is obtained. Since the contact layer 12 is a layer constituting an electrode, it is desirable that the contact layer 12 has a high carrier concentration of 1 × 10 18 / cm 3 or more. If it is lower than 1 × 10 18 / cm 3, it tends to be difficult to obtain a preferable ohmic with the electrode. Furthermore, when the composition of the contact layer is GaN, InGaN, or a superlattice containing GaN, InGaN, a preferable ohmic with the electrode material is easily obtained.
[0081]
After laminating the above layers, etching is performed in the same manner as in Example 1 to expose the surface of the n-side contact layer 4 and further to form a striped ridge waveguide. The dielectric multilayer film 64 and the extraction electrodes 22 and 23 are formed to obtain a laser element. The obtained laser element had a threshold current density of 2.0 kA / cm 2 and a threshold voltage of 4.0 V, was confirmed to be continuously oscillated at an oscillation wavelength of 480 nm, and had a lifetime of 1000 hours or more. The laser beam is F.I. F. P. The horizontal direction (x direction) of the beam shape was as wide as about 17 °, and the aspect ratio was as good as about 1.5. Even a long-wavelength laser element can be obtained with good laser light, good oscillation characteristics, low oscillation threshold current, and good lifetime characteristics.
[0082]
[Comparative Example 1]
A laser element was formed in the same manner as in Example 1 except that the thickness of the p-side light guide layer and the n-side light guide layer was 1000 mm. The obtained laser element had the same oscillation threshold current. F. P. , The spread in the x direction was narrow and about 8 °, and the aspect ratio was about 3.2.
[0083]
[Reference Example 1]
As a reference example, each layer from the n-side contact layer to the p-side contact layer shown in Table 1 is sequentially laminated on the substrate, and a striped waveguide is formed by etching, and the n-side contact layer is further exposed. The p and n electrodes are formed on these contact layers to obtain the laser element shown in FIG. At this time, the etching depth when forming the striped waveguide is lower than the position where the thickness of the p-side cladding layer is 0.1 μm (in the direction approaching the active layer) and higher than the active layer ( This is a depth that does not reach the active layer.
[0084]
[Table 1]

[0085]
The obtained laser element has a tendency that the drive current is significantly increased as compared with the laser element having the light guide layer and the p-side cap layer, and there is a laser element in the vicinity of 100 mA.
[0086]
[Reference Example 2]
As a reference example, an n-side contact layer to a p-side contact layer shown in Table 2 are sequentially stacked on a substrate, a stripe-shaped waveguide is formed by etching, and the n-side contact layer is exposed, and these contacts A p and n electrode is formed in the layer to obtain the laser element shown in FIG. At this time, the etching depth when forming the striped waveguide is lower than the position where the thickness of the p-side cladding layer is 0.1 μm (in the direction approaching the active layer) and higher than the active layer ( This is a depth that does not reach the active layer.
[0087]
[Table 2]

[0088]
The obtained laser element tends to have a drive current lower by about 10 to 20 mA than that of Reference Example 1.
[0089]
[Modification 1]
As a modification, a layered waveguide from the n-side contact layer to the p-side contact layer shown in Table 3 is laminated on the substrate in order, and a stripe-shaped waveguide is formed by etching, and the n-side contact layer is exposed. The p and n electrodes are formed on the contact layer, and the laser element shown in FIG. 7 is obtained. At this time, the etching depth when forming the stripe-shaped waveguide is specifically below the position where the thickness of the p-side cladding layer is 0.1 μm (in the direction approaching the active layer), and the active layer It is a depth that is higher (depth that does not reach the active layer). Preferably, as described above, the stripe-shaped ridge waveguide is formed to a depth reaching the p-side light guide layer 210, and more specifically, the depth is set to a thickness of 1000 mm as in the first embodiment. To do.
[0090]
[Table 3]

[0091]
In the obtained laser element, the drive voltage Vf tends to decrease as compared with the laser element having the p-side cap layer, but the threshold current tends to increase 5 to 6 times. There is a tendency not to show laser oscillation. The laser element obtained by setting the thickness of the p-side light guide layer to 2.5 μm, the thickness of the n-side light guide layer to 1.5 μm, and making the p-side light guide layer thicker than the n-side is as described above. In addition, the transverse mode is controlled well by the striped waveguide, and a laser element having excellent optical characteristics such as an aspect ratio can be obtained. Thus, when the film thickness difference between the two light guide layers is 1 μm, the effect of improving the mode controllability due to the deviation of the above-described light distribution toward the p-side light guide layer tends to be sufficiently obtained. . As the film thickness difference between the two light guide layers, the above tendency is observed when the thickness is 500 mm or more.
[0092]
(Laser element in long wavelength range)
In the laser element of the present invention, it is preferable to have the following layer structure in a long wavelength region of 450 nm or more, specifically 450 to 520 nm, blue to green. However, the present invention is not limited to this wavelength range.
In the long wavelength region, it is preferable to provide an intermediate layer between the well layer and the barrier layer as the active layer, leading to improvement of oscillation characteristics.
[0093]
The active layer used in a short wavelength region, specifically a wavelength region of 450 nm or less, has a quantum well structure sandwiched between a well layer made of InGaN and a barrier layer having a larger band gap energy than the well layer, specifically InGaN. A barrier layer made of AlGaInN having a mixed crystal ratio or a different composition is used for the well layer made of and the well layer. As such a structure, a single quantum well structure (SQW) of barrier layer / well layer / barrier layer, or a multiple quantum well structure (MQW) in which a well layer and a barrier layer are repeatedly stacked are used. However, since the well layer and the barrier layer have different mixed crystal ratios or compositions, the temperatures suitable for the growth of the layers are different, and the growth tends to be difficult. In this case, the barrier layer is grown on the well layer at a higher growth temperature. This is because, in the well layer having In, decomposition of In occurs in the temperature rising process during the growth of the barrier layer, and a layer having a sharp emission peak cannot be obtained. Even if the barrier layer is formed at substantially the same temperature as that of the well layer, when the other layers (cladding layer, guide layer) subsequent to the formation of the active layer are formed, the barrier layer is also increased for good crystal growth. A temperature process is required. Such growth difficulty tends to become more prominent as the oscillation wavelength becomes longer, and it is preferable to provide an intermediate layer in the long wavelength region.
[0094]
For this reason, the problem by the said temperature rise can be solved by passing through the said intermediate | middle layer. By providing this intermediate layer, the above decomposition of In tends to be observed as a partial one, and the intermediate layer itself tends to be observed as a surface form exhibiting irregularities. It is thought that this contributes to a significant decrease in threshold voltage. This intermediate layer is provided between the well layer and the barrier layer, and its band gap energy is larger than that of the barrier layer. When the active layer is MQW, this intermediate layer needs to be provided on at least one well layer. By providing the intermediate layer on all well layers, the above-mentioned problem can be solved for all the barrier layers on the well layer. Can be solved.
[0095]
The thickness of the intermediate layer is preferably less than the thickness of the barrier layer and is in the range of 1 atomic layer or more and 100 mm or less. This is because when the film thickness is 100 mm or more, a miniband is formed between the intermediate layer and the barrier layer, and the oscillation characteristics tend to deteriorate. The barrier layer at this time is in the range of 10 to 400 mm. Further, the composition of the intermediate layer is preferably Al._uGa_1-uBy setting N (0 ≦ u ≦ 1), the partial decomposition of In and the tendency of the drive voltage and threshold voltage to decrease due to the surface form of the intermediate layer are shown. More preferably, Al_vGa_1-vBy setting N (0.3 ≦ v ≦ 1), the decrease in each voltage can be increased.
[0096]
[Modification 2]
On the substrate, an n-side contact layer to a p-side contact layer shown in Table 4 below were sequentially laminated to form a laser element structure. Next, a stripe-shaped ridge waveguide is formed by etching from a p-side contact layer side to a depth at which the p-side light guide layer has a thickness of 500 mm, with a stripe width of 1.8 μm. Similarly, the n-side contact layer was exposed by etching, p and n electrodes were formed on each contact layer, the chip was taken out, and a laser device as shown in FIG. 8 was obtained. In the figure, 208a indicates an intermediate layer, 208b indicates a well layer, and 208c indicates a barrier layer. FIG. 8 schematically shows an enlarged structure of the active layer 208.
[0097]
[Table 4]

[0098]
The obtained laser device has a wavelength of 450 nm and a threshold current density of 2.0 kA / cm at room temperature.²Thus, continuous oscillation for 1000 hours or more was confirmed. This is because the etching depth at the time of forming the stripe-shaped waveguide is controllability in the transverse mode even when compared with a laser element whose depth does not reach the p-side light guide layer. F. P. A product having an excellent aspect ratio can be obtained. Further, by setting the p-side light guide layer to 1300 mm and making the film thickness difference from the n-side light guide layer sufficient, a more preferable laser element can be obtained.
[0099]
[Modification 3]
A laser element is obtained in the same manner as in Modification 2 except that the element structure laminated on the substrate is as shown in Table 5 below.
[0100]
[Table 5]

[0101]
The obtained laser element has an oscillation wavelength of 510 nm, and a good laser element can be obtained. Compared to the modified example 2, the device characteristics tend to be slightly reduced by changing the active layer from MQW to SQW. However, the intermediate layer in the active layer is GaN, so that the intermediate layer is provided. There is a tendency for the effect of to decrease. However, as in the second modification, by having the striped waveguide according to the present invention, a laser element having excellent transverse mode stability and element lifetime can be obtained, and the present invention can be applied to a long wavelength region. Further, by setting the p-side light guide layer to 1300 mm and making the film thickness difference from the n-side light guide layer sufficient, a more preferable laser element can be obtained.
[0102]
[Modification 4]
As in Example 1, after forming a buffer layer and an underlayer on a different substrate, Si was added at 1 × 10¹⁸/cm²Doped GaN is grown to a thickness of 100 μm. Subsequently, the substrate is removed from the rear surface of the wafer, that is, the surface facing the main surface of the heterogeneous substrate on which the nitride semiconductor is grown, so that only the nitride semiconductor is obtained.
Next, with the nitride semiconductor 204 on the surface opposite to the surface removed from the substrate as the main surface, as shown in FIG. 9, the n-side cladding layer 206, the n-side light guide layer 207, the same as in Example 1, The active layer 208, the p-side cap layer 209, the p-side light guide layer 210, the p-side cladding layer 211, and the p-side contact layer 212 are sequentially stacked. In the same manner as in Example 1, annealing was performed at 700 ° C. to further reduce the resistance of the p-type conductive layer, the wafer was taken out from the reaction vessel, transferred to the RIE apparatus, and etched into a stripe-shaped lead having a width of about 3 μm. A waveguide is formed. At this time, the etching depth is a depth that reaches the film thickness of the p-side light guide layer, and is formed at such a depth that the film thickness is 500 mm. Subsequently, a p-electrode 220 made of Ni / Au is formed on the uppermost surface of the p-side contact layer 212, and the etching exposed surface excluding the p-electrode 220 is formed with SiO 2.₂An insulating film 264 is formed, a take-out electrode 222 electrically connected to the p-electrode 220 is formed across the insulating film 264, and an n-electrode 221 made of Ti / Al is formed on the back surface (n-side contact layer surface) of the wafer. On top of that, a thin film made of Au / Sn is formed for metallization with a heat sink. Finally, scribing is performed from the wafer surface side on which the n-electrode 221 is provided, and the wafer is cleaved with a GaN M-plane [(11-00) plane] to form a bar shape, and then a resonant surface is produced. At least one of the pair of opposing resonant surfaces is SiO.₂/ TiO₂A dielectric multilayer film mirror is provided and finally cut substantially perpendicularly to the cavity direction to obtain a laser element chip. The obtained laser device has good characteristics with no occurrence of kinks in the current-light output curve although the stability of the transverse mode is slightly inferior because of the wide stripe width as compared with Example 1. . This suggests that the present invention is not affected by such a design change and can improve the device characteristics.
[0103]
【The invention's effect】
The nitride semiconductor laser device of the present invention has the optical characteristics of the laser device, particularly the F.F. F. P. Good aspect ratio was obtained, and the aspect ratio was greatly improved. Further, since the light confinement effect could be increased, it was possible to form a good waveguide, and it was confirmed that the device life was improved.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view illustrating a laser element of the present invention.
FIG. 2 is a partially enlarged schematic diagram illustrating a laser element of the present invention.
FIG. 3 is a schematic diagram illustrating a manufacturing method according to an embodiment of the present invention.
FIG. 4 is a view for explaining the relationship between etching depth and device characteristics in a laser device of the present invention.
FIG. 5 is a schematic cross-sectional view of a laser device according to Modification 1 of the present invention.
FIG. 6 is a schematic cross-sectional view of a laser device according to a second modification of the present invention.
FIG. 7 is a schematic cross-sectional view of a laser device according to a third modification of the present invention.
FIG. 8 is a schematic cross-sectional view of a laser device according to Modifications 4 and 5 of the present invention.
FIG. 9 is a schematic cross-sectional view of a laser device according to Modification 6 of the present invention.
[Explanation of symbols]
1,201 ... Different substrates
2,202 ... Buffer layer
3,203 ... Underlayer
4,204 ... n-side contact layer
5,205 ... Crack prevention layer
6,206 ... n-side cladding layer
7,207 ... n-side light guide layer
8, 208 ... active layer
9,209 ... p-side cap layer
10, 210 ... p-side light guide layer
11, 211 ... p-side cladding layer
12, 212 ... p-side contact layer
61,261 ... first protective film
62, 262 ... second protective film
63, 263 ... Third protective film
20,220 ... p electrode
21 221 ... n electrode
22, 222 ... p-pad electrode
23, 223... N pad electrode

Claims (8)

In a nitride semiconductor laser element having an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer on a substrate,
An n-side light guide layer and a p-side light guide layer are provided so as to sandwich the active layer to form a waveguide, and an n-side cladding layer and a p-side cladding layer are provided so as to sandwich the waveguide,
The p-side light guide layer has a stripe-shaped waveguide region by providing a stripe-shaped protrusion by etching,
The p-side light guide layer is formed with a film thickness at the protruding portion larger than the film thickness of the n-side light guide layer,
The p-side light guide layer has a thickness of 1 μm or less at the protruding portion, and the p-side light guide layer has a remaining etched thickness of 500 mm or more,
In a region other than the upper surface of the protruding portion, the protective film is provided on the surface of the p-side light guide layer that is the side surface of the protruding portion,
The protective film is formed of an oxide containing at least one element selected from the group consisting of Ti, V, Zr, Nb, Hf, and Ta, or at least one of SiO ₂ , BN, SiC, and AlN. A nitride semiconductor laser device characterized by the above.   2. The nitride semiconductor laser device according to claim 1, wherein the protrusion has a stripe width of 1 μm to 3 μm. The nitride semiconductor laser device according to claim 1, wherein the p-side light guide layer is In _x Ga _1-x N (0 ≦ x <1). 2. A p-side cap layer made of Al _x Ga _1-x N (0 ≦ x ≦ 1) doped with a p-type impurity is provided between the active layer and the p-side light guide layer. 4. The nitride semiconductor laser device according to any one of 3 above. 5. The nitride semiconductor laser according to claim 1, wherein the p-side cladding layer has a superlattice structure including Al _x Ga _1-x N (0 <x <1). element. The nitride semiconductor laser element according to claim 1, wherein the active layer has a single quantum well structure.   The nitride semiconductor laser device according to claim 1, wherein the n-side cladding layer is formed thicker than a p-side cladding layer.   The nitride semiconductor laser element according to claim 1, wherein the protective film is an oxide having a refractive index smaller than that of the nitride semiconductor.

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