WO2004105202A1

WO2004105202A1 - Laser diode device

Info

Publication number: WO2004105202A1
Application number: PCT/JP2004/005821
Authority: WO
Inventors: Hiroshi Watanabe
Original assignee: Toyoda Gosei Co. Ltd.
Priority date: 2003-05-22
Filing date: 2004-04-22
Publication date: 2004-12-02
Also published as: TW200427165A; TWI237431B; JP2004349485A

Abstract

A group III nitride compound semiconductor laser diode device of ridge waveguide type is disclosed which has a ridge structure comprising a first region which includes the central portion of the structure and continuously extends in the longitudinal direction and a second region which extends on both sides of the first region and has an average layer thickness thinner than that of the first region.

Description

Description Laser diode device The present application is based on Japanese Patent Application No. 2003-145137, and the entire contents of this Japanese application are referred to and incorporated in the present application. Technical field

The present invention relates to a group III nitride compound semiconductor laser diode device. More specifically, the present invention relates to improvement of a ridge waveguide type group III nitride compound semiconductor laser diode device. Background art

Laser diode devices (GaN laser diode devices) made of group III nitride compound semiconductors are considered promising as capable of emitting short-wavelength laser light, and research and development for practical use are underway. It has been done energetically. The basic structure of a GaN-based laser diode element is a structure in which an n-type semiconductor layer and a p-type semiconductor layer are laminated on an sapphire substrate with an active layer sandwiched between them, and various types of optical waveguides have been proposed. I have.

A typical optical waveguide structure is a ridge waveguide structure having a ridge for horizontal current confinement and light confinement (hereinafter, also referred to as a “ridge structure”). Many improvements have been made on this ridge structure.For example, a GaN-based buried layer that absorbs oscillation light is formed on both sides in the width direction of the ridge to increase the ridge width without reducing the ridge width. A structure that avoids the generation of the next mode and obtains oscillation in the fundamental mode (Japanese Patent Laid-Open No. 2000-31599), a waveguide of perfect refractive index and a waveguide of effective refractive index as the waveguide region on the stripe There has been proposed a structure (JP-A-2002-374035) in which the device reliability and beam characteristics are improved by having the structure described above. The ridge waveguide structure has a relatively simple structure and is easy to fabricate. On the other hand, in the horizontal and transverse modes, the light confinement ratios of the fundamental mode (0th mode) and the higher mode (1st mode) are close ( That is, since the gain difference between the modes is small), the mode is easily shifted, and the kink is easily generated. Although it is possible to stabilize the horizontal and transverse modes by reducing the ridge width, such a structure involves an increase in the resistance of the P electrode and an increase in operating voltage. Also, the horizontal light confinement factor decreases, causing an increase in the threshold current density. Disclosure of the invention

An object of the present invention is to provide a ridge waveguide type III-nitride-based compound semiconductor laser diode device capable of stabilizing the horizontal and lateral modes without increasing the threshold current density and operating voltage. And

According to the present invention,

A ridge waveguide type group III nitride compound semiconductor laser diode device, comprising: a first region including a central portion and continuing in a longitudinal direction; and a first region sandwiching the first region from both sides and having an average layer thickness of A laser diode element having a ridge structure including a second region smaller than the average layer thickness of the first region is provided.

In this configuration, a ridge structure is provided in which a first region having a relatively large average layer thickness is sandwiched by a second region. As a result, in the ridge structure, the effective refractive index in the central portion directly involved in the fundamental mode oscillation becomes higher than that in the peripheral portion, and light is efficiently confined in the central portion. That is, the light confinement rate of the fundamental mode increases, and the light confinement rate of the higher-order mode decreases. Therefore, the stability of the horizontal and horizontal modes is increased, and the occurrence of kinks is suppressed.

On the other hand, since the total ridge width can be sufficiently secured, the threshold current density and the operating voltage do not increase. Also, the light confinement rate of the fundamental mode Increase leads to a decrease in fight current density and operating voltage.

The average layer thickness of the first region and the second region means the average thickness of each region in the stacking direction of the P-type contact layers constituting the first region and the second region. The second region is defined as a region (consisting of a P-type contact layer) having an average thickness smaller than that of the p-type contact layer constituting the first region. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a sectional view showing an example of a ridge structure employed in a laser diode device of the present invention.

FIG. 2 is a sectional view showing another example of the ridge structure employed in the laser diode device of the present invention.

FIG. 3 is a sectional view showing another example of the ridge structure employed in the laser diode device of the present invention.

FIG. 4 is a sectional view showing a configuration of a laser diode element according to an embodiment of the present invention.

FIG. 5 is a diagram showing a semiconductor laser device using the laser diode element 1 of the embodiment.

FIG. 6A is a schematic view showing a ridge structure of an element used for a simulation.

FIG. 6B is a table showing the thickness and refractive index of each layer in the element used for the simulation.

FIG. 7A is a graph showing light output characteristics in the basic mode.

FIG. 7B is a graph showing light output characteristics in a higher-order mode. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, each element of the present invention will be described. The laser diode element of the present invention has a basic structure in which a plurality of semiconductor layers made of a group III nitride compound semiconductor are stacked on a substrate, and has a ridge structure. In a laser diode element (ridge waveguide type laser diode element) having such a ridge structure, current confinement and light confinement in the horizontal direction are realized by the ridge structure. The semiconductor layers stacked on the substrate include an n-type contact layer, an n-type clad layer, an n-type guide layer, an active layer (single quantum well structure, multiple quantum well structure, etc.), a p-type guide layer, Includes a mold cladding layer and a P-type contact layer.

The substrate is not particularly limited as long as a Group III nitride compound semiconductor layer can be grown thereon, and is preferably GaN, sapphire, spinel, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, oxide magnetic Shiumu, manganese oxide, YS Z (stabilized zirconate Nia yttria), can be used a substrate made of Z r B ₂ (zirconium di Po fluoride) or the like. When a semiconductor substrate is not used, a sapphire substrate is preferable, and in that case, its c-plane is particularly preferably used. This is for growing a group III nitride compound semiconductor layer having good crystallinity.

A buffer layer can be provided between the substrate and the crystal layer made of a group III nitride compound semiconductor. The puffer layer is provided for the purpose of improving the crystallinity of the group III nitride compound semiconductor grown thereon. The buffer layer can be formed of a group III nitride compound semiconductor such as AIN, InN, GaN, AlGaN, InGaN, and A1InGaN.

Here, the group III nitride-based compound semiconductor has the general formula A 1 _X G a _Y I n! -Χ.γ 0 (0≤X≤ 1, 0≤Y≤ 1, 0≤X + Y≤ 1) So-called binary system of A 1 N, G a Ν and I η 、, A l _x G a い_X N, A 1 _x I n i- _x N, G a _x I n _X _ _X N (or more , Including the so-called ternary system of 0 <χ <1). At least a part of the group III element may be replaced by boron (B), lithium (T 1), or the like, and at least a part of nitrogen (N) may be replaced by phosphorus (P), arsenic (A s), It can be replaced by antimony (Sb), bismuth (Bi), etc. The group III nitride-based compound semiconductor may contain any dopant. Silicon (S i), germanium (G e), selenium (S e), tellurium (T e), carbon (C), or the like can be used as the n-type impurity. As the P-type impurities, magnesium (Mg), zinc (Zn), beryllium (Be), calcium (Ca), strontium (Sr), and balium (Ba) can be used. After doping with the p-type impurity, the Group III nitride-based compound semiconductor can be exposed to electron beam irradiation, plasma irradiation, or heating by a furnace or the like, but is not essential.

These semiconductor layers can be formed by a known film formation method. For example, in addition to metal organic chemical vapor deposition (MOCVD), molecular beam crystal growth (MBE), halide vapor deposition (HVPE), sputtering, and ion plating can be used. it can.

The ridge structure is a stripe-shaped convex portion and is typically constituted by a part of a p-type contact layer and a part of a p-type cladding layer. Such a ridge structure can be formed by growing each semiconductor layer on a substrate and then removing a part of the P-type semiconductor layer by etching or the like. The width of the entire ridge structure can be, for example, 1 im to 10 m.

In the laser diode device of the present invention, the first region including the center portion and continuing in the longitudinal direction, and the first region is located so as to sandwich the first region, and the average layer thickness is larger than the average layer thickness of the first region. A ridge structure including a small second region is provided. In such a configuration, the effective refractive index of the central portion (first region) is larger than that of the regions located on both sides (second region). This increases the light confinement rate in the fundamental mode, and consequently improves the stability in the horizontal and transverse modes.

FIG. 1 shows an example of a ridge structure employed in the laser diode device of the present invention. FIG. 1 is a cross-sectional view of the ridge structure 40 perpendicular to the longitudinal direction. As shown in FIG. 1, a specific example satisfying the above conditions may be a ridge structure 40 in which both sides (second regions) 42 of the central portion (first region) 41 are lower by one step toward the substrate than the central portion (first region) 41. . In this ridge structure 40, the first region 4 The ratio of the width 41 of 1: the second region (one side) 42 and the width 42 of 42 can be set to 1:10 to 10: 1. On the other hand, the ratio of the height 41b of the first region 41 to the height 42b of the second region 42 can be set to 2: 1 to 11: 1.

In the example of FIG. 1, the second region 42 is formed symmetrically on both sides of the first region 41. Such symmetry contributes to the improvement of the horizontal and transverse mode characteristics. In the example of FIG. 1, the first region 41 and the second region 42 each have a uniform layer thickness (height) over the entire area, but the layer thickness (height) in each region is continuous or stepwise. May be changed.

FIG. 2 (a) shows an example in which the layer thickness (height) in the first region changes continuously. In this example, the first region 41 is tapered on both sides with the center as a boundary. That is, the layer thickness of the first region 41 decreases continuously at a constant rate of change from the center to the periphery. The rate of change of the layer thickness need not be constant.

FIG. 2 (b) shows an example in which the layer thickness (height) in the first region changes stepwise. In this example, the layer thickness of the first region 41 changes stepwise. By forming the upper surface in a stepped shape as shown in the figure, the first region 4 1 in which the layer thickness changes stepwise (that is, the first region 41 including two or more regions having different layer thicknesses) can be configured. .

Further, the layer thickness of the first region 41 may be formed by using both a continuous change and a stepwise change.

FIG. 3 (a) shows an example in which the layer thickness (height) changes stepwise in the second region. In this example, the second region 42 has a tapered shape so that its layer thickness is continuously reduced as the distance from the first region 41 is increased. That is, the layer thickness of the second region 42 continuously decreases at a constant rate of change in the direction away from the first region 41. The rate of change of the layer thickness may not be constant.

FIG. 3 (b) shows an example in which the layer thickness (height) in the second region changes stepwise. In this example, the layer thickness of the second region 42 changes stepwise. As shown in the figure, the upper surface is stepped, so that the second layer thickness changes gradually. The region 42 (ie, the second region 42 including two or more regions having different layer thicknesses) can be formed.

Further, the layer thickness of the second region 42 may be formed by using both continuous change and stepwise change.

The ridge structure is generally constituted by a part of a P-type cladding layer and a P-type contact layer. In the present invention, it is preferable that both the first region and the second region constituting the ridge structure include a p-type contact layer. In such a structure, a p-electrode can be formed on the first region and the second region. That is, it is possible to secure a sufficiently large electrode surface. As a result, a laser diode element having a low threshold current density and a low operating voltage is formed. The method of manufacturing the ridge structure including the first region 41 and the second region 42 is, for example, as follows.

First, after growing each semiconductor layer (n-type contact layer to p-type contact layer) on the substrate, a striped protective film is formed on the surface of the p-type contact layer so as to cover the region where the ridge structure is to be formed. It is formed by lithography. Protective film is, for example, made of silicon oxide such as S i 0 _2. Next, the region where the protective film is not formed is etched partway through the P-type cladding layer. As an etching method, a reactive ion etching method (RIE) can be suitably used. Next, the protective film covering the other portion (second region) is removed so that the protective film remains only in the portion (center portion) where the first region 41 is formed. Thereafter, etching is performed in the same manner as described above to etch a part of the P-type contact layer that is not covered with the protective film. Through the above steps, a ridge structure in which the peripheral portion (second region) is one step lower than the central portion (first region) toward the substrate is formed.

[Example]

FIG. 4 shows a semiconductor laser diode device (hereinafter, also referred to as “LD device”) 1 according to an embodiment of the present invention. Details of each layer of the LD element 1 are as follows.

Layer: Composition Third p-type layer 1 G a N: Mg

Second p-type layer 1 A 1 G a N: Mg

First _p- type layer 1 G a N: Mg

MQW layer 1 6 I n G a N / G a N

Third n-type layer 1 G a N: S

2nd n-type layer 1 A I G a N: S i

1st n-type layer 1 G a N: S i

Buffer layer 1 2 A 1 N

Substrate 1 1 Sapphire

The first n-type layer 13 is an n-type contact layer, the second n-type layer 14 is an n-type cladding layer, the third n-type layer 15 is an n-type guide layer, and the MQW layer 16 is light emitting Layer, the first! The type layer 17 functions as a type guide layer, the second p-type layer 18 functions as a p-type cladding layer, and the third p-type layer 19 functions as a p-type contact layer.

The buffer layer 12 may be composed of GaN, InN, AlGaN, InGaN and A1InGaN.

Here, the n-type layers 13, 14, 15 may be composed of G aN, A1 G aN, In G aN or A 1 In G aN.

The n-type layers 13, 14, and 15 are doped with Si as an n-type impurity. In addition, Ge, Se, Te, and C are also doped as n-type impurities. You may want to

The MQW layer 16 may have a multiple quantum well structure such as A1GaN / A1GaInN, in addition to the multiple quantum well structure of InGaNZNZaN. The number of quantum well layers is preferably 1 to 30.

The P-type layers 17, 18, 19 can be made of GaN, AlGaN, InGaN or InA1GaN. Also, the p-type impurity may be Zn, .Be, Ca, Sr, Ba instead of Mg. After the introduction of the p-type impurity, the resistance of the p-type layer can be reduced by a known method such as electron beam irradiation, plasma irradiation, heating by a furnace or the like. In the LD device 1 having the above configuration, the group III nitride compound semiconductor layer above the first n-type layer 13 is formed by MOCVD, molecular beam crystal growth (MBE), or halide vapor deposition. (HVPE method), a sputtering method, an ion plating method, or the like.

After stacking the semiconductor layers, a ridge structure 20 is formed. The ridge structure 20 is formed by photolithography and etching. First, a protective film (S i 0 ₂₎ on the entire surface of the third p-type layer 1 9. Next, a part of the protective film is removed by photolithography to form a stripe-shaped protective film having a desired width (width of the ridge structure 20). Subsequently, using the remaining protective film as a mask, portions exposed without being covered with the protective film are sequentially removed by reactive ion etching from the third p-type layer 19. This etching process is continued until part of the second p-type layer 18 is removed.

Next, the convex portion (ridge structure 20) formed by the above process is processed to form a first region 20a and a second region 20b. First, using photolithography, the protective film covering the other portions is removed so that the protective film remains only in the portion where the first region 20a is to be formed (that is, in the center of the ridge structure 20). I do. As a result, a stripe-shaped protective film covering the central portion of the ridge structure 20 is formed. Subsequently, reactive ion etching is performed again to etch a part of the third p-type layer 19 that is not covered with the protective film. Finally, the protective film is removed. By the above steps, as shown in FIG. 1, the ridge structure 20 in which the peripheral portion (the second region 20 b) is lower by one step toward the substrate 11 than the central portion (the first region 20 a). Is formed. In this embodiment, both the first region 20a and the second region 20b include the third p-type layer 19.

After forming the ridge structure 20 by the above procedure, the n-electrode 22 and the p-electrode 23 are formed. The n-electrode 22 is made of a material including A 1, Ti and the like, and after forming the ridge structure 20, the third p-type layer to the second n-type layer, and the first n-type layer 1 Exposed by removing part of 3 by etching It is formed on the first n-type layer 13 by vapor deposition. The p-electrode 23 is made of a material including Ni, Pi; and Au, and is formed by vapor deposition.

Subsequently, after chipping is performed by a conventional method such as etching, a light reflecting film (not shown) is formed on the end surface (rear end surface) on the light reflecting side of the obtained laminate by sputtering.

FIG. 5 shows an example of a semiconductor laser device using the LD element 1 manufactured through the above steps. For convenience of explanation, some elements such as a p-electrode and an n-electrode are omitted in FIG.

The LD element 1 is installed on a stem 71 erected on a support 70 via a heat sink (conductive substrate) 72. The LD element 1 is mounted on the heat sink 72 with the electrode side down. An insulating material layer is formed on a part of the surface of the heat sink 72, and the insulating material layer prevents a short circuit between the n-electrode and the p-electrode.

The cap 75 has a condenser lens 77, and the laser light generated by the LD element 1 is emitted to the outside via the condenser lens 77.

6A and 6B show conditions for simulating the optical output characteristics of the laser diode device manufactured by the above-described method. FIG. 6A schematically shows a ridge structure used for the simulation. In the conventional ridge structure, the ridge width is 1.8 m in the area shown in Fig. 6A (a). In the ridge structure of the present embodiment, the ridge width is 0.6 m in part (first region) and 0.6 m in part (second region) shown in FIG. In total, 0.6 X 3 = 1.8 ΠΙ. Fig. 6 (4) shows the equivalent refractive index nefi calculated in each part (1) to (3) of the ridge structure. The equivalent refractive index neff is calculated by a calculation formula: nefi r ^ r + r ^ Γ ₂ + n 3 Γ 3 +-· + η _η Γ _η .伹, η 丄 ~]! Is the refractive index of each layer, and Γ is the proportion of light present in each layer, which can be determined by simulation.

FIGS. 7A and 7B show the simulation results. Fig. 7A is a graph showing the light output characteristics of the fundamental mode, and Fig. 7B is the higher order mode. 5 is a graph showing the light output characteristics of the optical device. As is clear from these graphs, the peak of the fundamental mode is higher in the structure of the present example, and the peak in the higher-order mode is lower than that of the conventional structure. That is, the structure of the present embodiment has a higher light confinement ratio in the fundamental mode and a lower light confinement ratio in the higher-order mode. Therefore, in the structure of this embodiment, the horizontal

-It is said that the stabilization is further stabilized and the kink level (kink level) is improved. In the structure of this embodiment, the light confinement ratio increases in the fundamental mode, and the threshold current density decreases. Although the invention has been described with respect to particular embodiments for a complete and clear disclosure, the appended claims should not be limited to the description but include the basic teachings described herein. And should be construed as including all modifications and alternatives that are within the reach of those skilled in the art. Industrial potential

According to the present invention, there is provided a ridge waveguide type III-nitride-based compound semiconductor laser diode device capable of stabilizing a horizontal / lateral mode without increasing a threshold current density and an operating voltage. .

Claims

The scope of the claims

1. A ridge waveguide type II I-nitride-based compound semiconductor laser diode,

A ridge structure including a first region including a central portion and continuing in the longitudinal direction, and a second region sandwiching the first region from both sides and having an average layer thickness smaller than the average layer thickness of the first region. , Laser diode element.

2. The laser diode element according to claim 1, wherein the second region has a symmetrical shape in a cross section perpendicular to the longitudinal direction.

3. The laser diode device according to claim 1, wherein a layer thickness of the first region is uniform throughout.

4. The laser diode according to claim 1, wherein the upper surface of the first region is formed in a stepped or tapered shape so that the layer thickness decreases continuously or stepwise from the center to the periphery. Element.

5. The laser diode device according to any one of claims 1 to 4, wherein a layer thickness of the second region is uniform throughout.

6. The laser diode device according to claim 1, wherein the second region includes two or more regions having different average layer thicknesses.

7. The upper surface of the second region is formed in a stepped or tapered shape so that the layer thickness decreases continuously or stepwise as the distance from the first region increases. 14. A laser diode element according to item 1.

8. The laser diode device according to claim 1, wherein the first region and the second region each include a p-type contact layer, and p electrodes are formed on both regions.