JP2007005720A - Semiconductor laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser device in which operation currents are reduced and a high-order lateral mode is suppressed. <P>SOLUTION: The semiconductor laser device is provided with a substrate, a first conductive first clad layer formed on the substrate, an active layer formed on the first clad layer, a second conductive overflow prevention layer formed on the active layer, and a second conductive second clad layer formed on the overflow prevention layer. The second clad layer has a ridge and a non-ridge, the thickness of the non-ridge adjacent to the ridge includes an area ≤0.1 μm, and an insulating film and a light absorption film for radiation light radiated from the active layer are formed on the upper surface of the non-ridge and the side face of the ridge. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device having a ridge waveguide.

  The next-generation DVD (Digital Versatile Disc) is being developed for long-time recording of high-definition video and large-capacity recording for computers. In order to obtain a recording capacity that is four times or more that of a conventional DVD, the wavelength of the semiconductor laser device needs to be shortened in the 400 nm band, not 650 nm. For this purpose, a gallium nitride material is used.

  In order to realize such a high-density optical disk, a so-called ridge waveguide type semiconductor laser device is used. This structure forms a ridge portion such as a stripe or a taper in a clad layer provided on a double heterojunction and controls a horizontal transverse mode, and is called a refractive index waveguide structure.

  A gallium nitride film grown on a sapphire substrate or the like is more prone to lattice mismatch and crystal defects than an InAlGaP film, and may cause a curved portion in the laminated film, and the ridge shape control is insufficient. there were. As a result, a high-order lateral mode is likely to occur, and the generation of kinks in the output characteristics has been an obstacle to the high output of the semiconductor laser device.

There is a technology disclosure example in which a light absorption film is provided in the vicinity of the ridge side surface to suppress a high-order lateral mode (Patent Document 1). Was insufficient.
JP 2002-314197 A

  The present invention provides a semiconductor laser device in which an operating current is reduced and a high-order transverse mode is suppressed.

According to one aspect of the invention,
A substrate,
A first cladding layer having a first conductivity type provided on the substrate;
An active layer provided on the first cladding layer;
An overflow prevention layer having a second conductivity type provided on the active layer;
A second cladding layer having a second conductivity type provided on the overflow prevention layer;
With
The second cladding layer has a ridge portion and a non-ridge portion, and the non-ridge portion adjacent to the ridge portion includes a region having a thickness of 0.1 micrometers or less.
There is provided a semiconductor laser device characterized in that an insulating film and a light absorption film for radiation emitted from the active layer are provided on an upper surface of the non-ridge portion and a side surface of the ridge portion.

  The present invention provides a ridge waveguide type semiconductor laser device in which the operating current is reduced and the higher-order transverse mode is suppressed.

Hereinafter, embodiments of the invention will be described with reference to the drawings.
FIG. 1 is a schematic cross-sectional view of a semiconductor laser device according to a first specific example of the present invention. On the n-type GaN substrate 20, an n-type Al _0.08 Ga _0.92 N clad layer 22 (thickness 0.5 to 2.0 micrometers), an n-type GaN light guide layer 24 (thickness 0.01 to 0.00). 1 micrometer) and the active layer 26 is laminated.

Further, on the active layer 26, a p ⁺ type Al _0.2 Ga _0.8 N overflow prevention layer 28 (thickness 5 to 20 nanometers), a p-type GaN light guide layer (thickness 0.01 to 0.1 nanometers). ), P-type Al _0.08 Ga _0.92 N clad layer 32 (thickness 0.5 to 2.0 micrometers), and p ⁺ -type GaN contact layer 34 (thickness 0.02 to 0.2 micrometers). Has been.

  These semiconductor laminated films can be sequentially grown on the n-type GaN substrate 20 using, for example, MOCVD (Metal Organic Chemical Vapor Deoisition). In general, silicon is used as the n-type impurity, and magnesium is used as the p-type impurity.

In this specification, “gallium nitride-based semiconductor” refers to a composition ratio x in a chemical formula of In _x Al _y Ga _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1). And semiconductors of all compositions in which y is changed within the respective ranges. In addition, the “gallium nitride based semiconductor” also includes those further including any of various impurities added to control the conductivity type.

  The structure illustrated in FIG. 1 belongs to a refractive index waveguide structure also called a ridge waveguide type. That is, the p-type AlGaN cladding layer 32 is formed with a ridge portion 42 represented by a broken line having a height H and a non-ridge portion 40 represented by a broken line having a thickness J. As will be described in detail later, in a high-power semiconductor laser device required for an optical disk for rewriting, it is necessary to suppress the occurrence of kinks in the optical output-operating current characteristics. Since the kink is generated mainly because a higher-order mode is generated in the horizontal and transverse mode, it is important to accurately form the width W of the ridge portion 42 within a range of, for example, 1 to 3 micrometers.

  Further, when, for example, wet etching is performed on an off-angle substrate or the like often used for threshold current reduction or the like, the ridge side surface 44 becomes asymmetrical and the inclination becomes loose. Therefore, it is preferable to use the RIE (Reactive Ion Etching) method for forming the ridge portion.

An insulating film / light absorption film is deposited on the side surface 44 of the ridge portion 42 and the upper surface of the non-ridge portion 40. As the insulating film 36 that is transparent to the radiation from the active layer 26, SiO ₂ , Al ₂ O ₃ , AlN, SiN _x , Ta ₂ O ₅ , ZrO _2, or the like can be used. For the light absorption film 38, a material containing silicon can be used. In this specific example, SiO ₂ is used as the insulating film 36 and amorphous silicon is used as the light absorption film 38. The thickness of each layer can be selected in the range of 0.001 to 0.3 micrometers. In the first specific example illustrated in FIG. 1, the SiO ₂ film 36 and the amorphous silicon film 38 are each one layer.
Here, the “light absorption film” in the present specification means a film having an absorption coefficient of 1 × 10 ⁵ m ⁻¹ or more in a wavelength band of 400 nanometers or more and 800 nanometers or less.

Furthermore, a p ⁺ -type GaN contact layer 34 is provided on the ridge portion 42, and ohmic contact with the p-side electrode 50 is formed. As the p-side electrode, a single layer, a laminate, or an alloy such as Pt, Pd, Ni, or Au is used. An n-type electrode is formed on the back surface of the n-type GaN substrate 20. As the n-type electrode, a single layer such as Ti, Pt, Au, or Al, a laminate, or an alloy is used.

Next, the operation of the laminated structure will be described in more detail. FIG. 2 is a band diagram of the semiconductor laminated structure of this example. The non-doped GaN diffusion prevention layer 27 suppresses diffusion of p-type impurities such as magnesium (Mg) from the high concentration p ⁺ -type AlGaN overflow prevention layer 28 to the active layer 26.
Further, the p ⁺ type overflow prevention layer 28 does not require an operating current due to the electron Q represented by the arrow injected from the n-type GaN substrate 20 side leaking to the p-type Al _x Ga _1-x N cladding layer 32. Suppress the increase.

That is, when the aluminum composition ratio x of the p ⁺ -type Al _x Ga _1-x N overflow prevention layer 28 is increased, the band gap difference from the active layer 26 is increased, and the electrons Q injected from the n side are emitted from the active layer 26. Leakage to the p-type Al _x Ga _1-x N cladding layer 32 can be reduced. Further, by increasing the p-type concentration of the p ⁺ -type AlGaN overflow prevention layer 28 (for example, 1 × 10 ²⁰ cm ⁻³ ), the hetero barrier on the conduction band side with the active layer 26 can be increased. Leakage can be further reduced.

On the other hand, when the aluminum composition ratio is increased, the lattice constant is generally decreased, so that the crystallinity is impaired, for example, lattice mismatch occurs. However, since the thickness of the p ⁺ type AlGaN overflow prevention layer 28 is as thin as 5 to 20 nanometers, for example, the influence of the deterioration of crystallinity can be minimized.

Next, beam characteristics when the aluminum composition ratio is increased will be described. When the aluminum composition ratio is increased, the refractive index is decreased, so that light confinement in the vertical (Y-axis) direction is enhanced. Therefore, if the aluminum concentration of the p-type AlGaN cladding layer 32 is excessively increased, the beam divergence angle (Θ _⊥ ) in the vertical direction becomes large, and the aspect ratio which is the ratio to the beam divergence angle (Θ _‖ ) in the horizontal direction. ( _Θ⊥ / _Θ‖ ) becomes large. In order to effectively use the light output, it is desirable that the aspect ratio is close to 1. Therefore, when the aspect ratio is too large, a beam shaping lens, a prism, or the like is required, and the optical system becomes complicated. In this specific example, the aspect ratio can be set to an appropriate value by setting the aluminum composition ratio of the p-type AlGaN cladding layer 32 to 0.3 or less.

Next, supplementary explanation regarding the components of the laminated structure will be given. The p-type cladding layer 32 is not limited to a p-type Al _x Ga _1-x N layer (x ≦ 0.3), and for example, a superlattice layer in which Al _y Ga _1-y N / GaN pairs are stacked. It may be. The aluminum composition ratio y is desirably larger than 0 and not larger than 0.6. When the superlattice layer is used, stress due to lattice mismatch or the like is relieved (that is, effective in preventing cracks) and the operating voltage can be reduced. For example, a clad layer having a thickness of 1 micrometer can be realized by alternately stacking 200 pairs of GaN having a width of 2.5 nanometers and Al _0.1 Ga _0.9 N having a width of 2.5 nanometers.

Similarly, the n-type cladding layer 22 is not limited to the n-type Al _0.08 Ga _0.92 N layer, but may be a superlattice layer in which Al _z Ga _1-z N / GaN pairs are stacked. good. The aluminum composition ratio z is preferably greater than 0 and 0.3 or less. The effect of the superlattice is the same as that of the p-type cladding layer.

Further, the active layer 26 made of In _x Ga _1-x N / In _y Ga _1-y N may be a single or multiple quantum well active layer. In this case, the indium composition ratio x in the well layer can be selected within the range of 0.05 to 1.0, the indium composition ratio y in the barrier layer is 0 to 1.0 and x> y. . For example, an In _0.15 Ga _0.85 N / In _0.02 Ga _0.98 N structure, a well layer thickness of 2 to 5 nanometers, a well number of 2 to 4, and a barrier layer thickness of 3 to 10 nanometers can be used. . By changing the composition and profile of the active layer, the threshold current, FFP, temperature characteristics, and the like can be adjusted. Furthermore, it is desirable to increase the overflow prevention effect by making the aluminum composition ratio in the p ⁺ -type AlGaN overflow prevention layer 28 higher than the aluminum composition ratio in the p-type AlGaN cladding layer 32.

  In the ridge waveguide structure, if the width W of the bottom surface of the ridge portion 42 is, for example, 1 to 3 micrometers, the basic horizontal transverse mode 60 has an optical confinement effect. On the other hand, the light confinement effect is insufficient for the high-order horizontal transverse mode. For example, the primary horizontal / horizontal mode 62 may occur on both sides of the basic horizontal / horizontal mode 60. These are particularly likely to occur at high output, and appear as kinks in the optical output-operating current characteristics, ie “bends”. In the rewriting of the optical disk, the irradiation power must be changed by erasing, recording, and reproduction. Therefore, it is difficult to control the irradiation power with high accuracy in a semiconductor laser device with many “bends”.

In this specific example, the primary horizontal transverse mode 62 can be attenuated in the light absorption film 38 by setting the thickness J of the non-ridge portion 40 to 0.1 micrometers or less. That is, since the p-type AlGaN overflow prevention layer 28 has a high aluminum composition ratio of 0.2, the refractive index is low (about 2.439). For this reason, since the horizontal transverse mode is shifted to the n-type GaN light guide layer 24 side, the thickness J of the non-ridge portion 40 is set to 0.1 μm or less. As a result, since the primary horizontal transverse mode 62 is in contact with the light absorption layer 38, the beam along the optical axis is attenuated, and kinks in the optical output-operating current characteristics can be suppressed. The light absorption film 38 is effective even in a horizontal or higher horizontal mode. The absorption coefficient of amorphous silicon is about 1.00 × 10 ⁸ m ⁻¹ at a wavelength of 400 nanometers.

  Further, it will be described below that when the thickness J of the non-ridge portion 40 is reduced, the operating current can be reduced in addition to the reduction of the high-order horizontal transverse mode. That is, in this specific example, the simulation was performed with the thickness J of the non-ridge portion 40 being 0.02, 0.1, and 0.14 μm.

The main specifications of other components in the simulation are shown below. The thickness of the n-type Al _0.05 Ga _0.95 N cladding layer 22 is 1.5 micrometers, the thickness of the In _0.01 Ga _0.99 N lower barrier layer is 0.04 micrometers, and In _0.13 Ga. The thickness of the _0.87 N well layer is 3 nanometers (3 layers), the thickness of the In _0.01 Ga _0.99 N barrier layer between the well layers is 10 nanometers (2 layers), and the thickness of the In _0.01 Ga _0.99 N upper barrier layer is 0.04 micrometers. Further, the thickness of the GaN diffusion prevention layer is 0.05 nanometer, the thickness of the p ⁺ type Al _0.2 Ga _0.8 N overflow prevention layer 28 is 0.01 micrometer, and the p type Al _0.05 Ga _0.95 N clad. The thickness of the layer 32 was 0.6 micrometers, and the thickness of the p ⁺ -type GaN contact layer 34 was 0.1 micrometers. The ridge width was 2.1 micrometers. The upper surface of the non-ridge portion 40 and the side surface 44 of the ridge portion 42 are coated with 0.02 micrometers of SiO ₂ and 0.4 micrometers of amorphous silicon, respectively.

  In addition, a reflective film having a reflectance of 10% is provided on the front side of the front side of the optical resonator of the gallium nitride semiconductor laser device, and a reflective film having a reflectance of 95% is provided on the rear side of the surface. The light output from the surface is increased.

  FIG. 3 is a graph showing the simulation result of the optical output-operating current characteristic when the ambient temperature (Ta) is 80 ° C. It can be seen that the operating current can be further reduced when the thickness J of the non-ridge portion 40 is smaller. From this simulation result, the dependence of the operating current at the time of 100 mW output on the thickness J of the non-ridge portion 40 is obtained and illustrated in FIG. A low operating current is possible when the thickness J of the non-ridge portion 40 is 0.10 micrometers or less, but the operating current rapidly increases when the thickness of the non-ridge portion 40 is 0.10 micrometers or more. Therefore, the thickness J of the non-ridge portion 40 is preferably 0.1 micrometers or less, not only for suppressing higher-order transverse modes but also for reducing operating current.

  FIG. 5 is a graph showing measured values of CW light output-operating current characteristics at Ta = 80.degree. In this specific example, the thickness J of the non-ridge portion 40 is 0.02 micrometers, and in the comparative example, the thickness J of the non-ridge portion 40 is 0.14 micrometers. In this specific example, it is possible to suppress the higher-order horizontal transverse mode by the light absorption film 38 and reduce the current by reducing the thickness of the non-ridge portion 40. As a result, a CW optical output of 100 mW can be realized at Ta = 80 ° C. and an operating current of about 145 mA.

  On the other hand, in the comparative example, since the thickness J of the non-ridge portion 40 is as thick as 0.14 micrometers, the distance from the active layer 26 is large. As a result, the primary horizontal / horizontal mode 62 cannot be sufficiently attenuated, and the kink W that is “bent” in the output characteristic occurs. Further, the operating current at the time of 80 mW CW light output is about 190 mA, which is about 70 mA larger than this specific example. As a result, a high output and low operating current characteristic satisfying the next-generation DVD rewriting application specification could not be obtained.

  Gallium nitride semiconductor laser devices used for next-generation DVD applications have more spontaneous emission components than semiconductor laser devices having a wavelength band of 650 nanometers. Therefore, the irradiation power to the disc is increased. For example, CW is 10 to 20 mW during reproduction, CW is 70 to 80 mW during erasure, and CW is 100 mW or more during writing. However, these examples can realize these.

  FIG. 6 shows a first modification of this example. That is, the insulating film 36 and the light absorption film 38 are provided in two layers alternately. As a result, it is easier to suppress the primary horizontal / horizontal mode 62. It is possible to provide the light absorption film 38 in contact with the p-type AlGaN cladding layer 32, but the insulating film 36 is more excellent in electrical insulation.

  FIG. 7 shows a second modification of this example. A micro trench portion 54 is provided below the side surface 44 of the ridge portion 42. The microtrenches 54 can be formed by selecting RIE process conditions when forming the ridges 42. A distance K between the lower end of the microtrench 54 and the upper surface of the p-type light guide layer 30 corresponds to the thickness J of the non-ridge portion 42 in FIGS. By setting the distance K to 0.1 micrometers or less, it is possible to reduce the operating current and suppress the higher-order horizontal transverse mode.

Next, the crystallinity of the gallium nitride based laminated film will be described. _Al x _{Ga 1-x} N layer and _In y _{Ga 1-y} N layer that constitutes the gallium nitride-based multilayer film, the lattice constant difference is large, respectively. For this reason, a crack may enter in a crystal | crystallization or a laminated film may curve. In particular, when a laminated film is formed on the sapphire substrate using ELOG (Epitaxial Lateral Over Growth) or the like, the crystallinity is improved by providing a buffer layer or the like. In this specific example as well, the above ELOG can be used. However, the provision of a laminated film on the GaN substrate 20 can achieve more lattice matching and reduce the curvature and cracks of the laminated film. As a result, it becomes easier to accurately form the non-cladding portion 40 with a thickness of, for example, 0.1 μm or less.

  Further, since the n-side electrode 52 can be formed on the back surface of the n-type GaN substrate 20, the electrodes can be arranged vertically as in the semiconductor laser device made of InGaAlP, GaAs, InP or the like. As a result, the assembly process can be simplified and the reliability can be improved.

  Further, unlike the ELOG structure, since the current path is above and below the chip, the current in the horizontal direction of the semiconductor laminated film is small, and the resistance component on the substrate side is small. As a result, the operating voltage can be reduced.

  In the above specific examples, the gallium nitride semiconductor laser device has been described. However, the present invention is not limited to this, and can be applied to compound semiconductor laser devices such as InGaAlP-based, GaAlAs-based, and InP-based.

  The embodiments of the present invention have been described above with reference to specific examples. However, the present invention is not limited to these. For example, even if those skilled in the art have made various design changes regarding the size, material, arrangement relationship, etc. of each element constituting the semiconductor laser device having the ridge portion as a waveguide, the present invention has the gist. Insofar as included in the scope of the present invention.

It is a schematic cross section of the semiconductor laser device concerning the example of this invention. It is a band figure of the semiconductor laser apparatus concerning the example of this invention. It is a graph showing the simulation result of the optical output-operating current of the semiconductor laser apparatus concerning the example of this invention. It is a graph showing the result of having simulated the non-ridge part thickness dependence of the operating current in a semiconductor laser apparatus. It is the graph which represented the CW light output-operating current characteristic measured value of this example compared with the comparative example. It is a schematic cross section of the semiconductor laser device which is the 1st modification of this example. It is a schematic cross section of the semiconductor laser device which is the 2nd modification of this example.

Explanation of symbols

20 n-type GaN substrate 22 n-type AlGaN cladding layer 24 n-type GaN light guide layer 26 active layer 28 p ⁺ -type AlGaN overflow prevention layer 30 p-type light guide layer 32 p-type AlGaN cladding layer 34 p ⁺ -type GaN contact layer 36 insulation Film 38 Light absorption layer 40 Non-ridge portion 42 Ridge portion 44 Ridge side surface 50 P-side electrode 52 n-side electrode 54 Micro-trench portion 60 Basic horizontal transverse mode 62 Primary horizontal transverse mode

Claims (5)

A substrate,
A first cladding layer having a first conductivity type provided on the substrate;
An active layer provided on the first cladding layer;
An overflow prevention layer having a second conductivity type provided on the active layer;
A second cladding layer having a second conductivity type provided on the overflow prevention layer;
With
The second cladding layer has a ridge portion and a non-ridge portion, and the non-ridge portion adjacent to the ridge portion includes a region having a thickness of 0.1 micrometers or less.
2. A semiconductor laser device according to claim 1, further comprising: an insulating film and a light absorption film for light emitted from the active layer on the top surface of the non-ridge portion and the side surface of the ridge portion. The first cladding layer and the second cladding layer may be a superlattice in which an Al _y Ga _1-y N (0 <y ≦ 0.6) layer and a GaN layer are stacked, or Al _x Ga _1-x N ( 0 <x ≦ 0.3) layer,
The active layer _has a _{In Z Ga 1-Z N /} In W Ga 1-W N multiple quantum well structure (0.05 ≦ z ≦ 1.0,0 ≦ w ≦ 1, z> w),
The overflow preventing _{layer, Al t Ga 1-t N} (t> x, and t> y) semiconductor laser device according to claim 1, wherein characterized in that it consists of layers.   The semiconductor laser device according to claim 1, wherein the light absorption film includes silicon.   The semiconductor laser device according to claim 1, wherein the insulating film includes any one of silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, and zirconium oxide.   The semiconductor laser device according to claim 1, wherein the substrate is made of GaN.

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