JP3618989B2 - Semiconductor laser device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device. More particularly, the present invention relates to a semiconductor laser device including a gallium nitride-based compound semiconductor laser element, which can remarkably improve the heat dissipation characteristics from the active layer of the element.
[0002]
[Prior art]
In recent years, materials using gallium nitride-based compound semiconductors such as GaN, InGaN, and GaAlN have been realized as light emitting diodes (LEDs) in the blue to ultraviolet range and semiconductor lasers. Gallium nitride-based compound semiconductors have attracted attention because they have a direct transition type band structure, so that high luminous efficiency can be obtained.
[0003]
A semiconductor laser using this material system is expected to be applied as a light source for high-density information processing because of its short oscillation wavelength. For example, when used as a light source for an optical disk recording apparatus, the recording density can be doubled.
[0004]
In this specification, “gallium nitride-based compound semiconductor” refers to a chemical formula of Ga _x Al _y In _z B _1-xyz N (x ≦ 1, y ≦ 1, z ≦ 1, x + y + z ≦ 1). It is assumed that semiconductors of all compositions in which the composition ratios x, y, and z are changed within the respective ranges are included. For example, GaInN (x = 0.6, y = 0, z = 0.4) is also included in the “gallium nitride compound semiconductor”.
[0005]
[Problems to be solved by the invention]
However, semiconductor lasers using these gallium nitride compound semiconductors have a problem that it is difficult to efficiently release the heat generated in the active layer to the outside. Hereinafter, this problem will be described in detail.
[0006]
First, taking a typical gallium nitride (GaN) as an example of a gallium nitride-based compound semiconductor, the band gap energy is 3.4 eV. For this reason, the operating voltage of the semiconductor laser must be a value higher than that. On the other hand, in a semiconductor laser, current concentration on the active layer is required. Therefore, even if the drive current value is the same, the light emitting diode (LED) and the semiconductor laser are greatly different in terms of current density.
[0007]
Here, the heat generation amount is determined by the product of the drive voltage and the drive current density. That is, when a semiconductor laser is formed of a gallium nitride-based compound semiconductor, the amount of heat generation is larger than that of other material systems, and the heat generation is concentrated in the oscillation region of the active layer.
[0008]
On the other hand, when forming a laminated structure of gallium nitride compound semiconductor materials, sapphire is generally used as a growth substrate. However, sapphire has a very low thermal conductivity and is not suitable for heat dissipation. In addition, the region where the amount of heat generation is the largest is the oscillation region of the active layer, which is usually relatively far from the substrate but rather close to the surface. Therefore, such a normally used device configuration is not desirable in terms of releasing heat from the substrate side.
[0009]
As described above, the conventional gallium nitride-based compound semiconductor laser device has a problem that heat generated at high density in the oscillation region of the active layer cannot be efficiently released to the outside.
[0010]
The present invention has been made in view of this point. That is, an object of the present invention is to provide a semiconductor laser device having a long life by improving a light emission characteristic, a temperature characteristic, and reliability by realizing a good heat dissipation structure.
[0011]
[Means for Solving the Problems]
That is, a semiconductor laser device according to the present invention includes a semiconductor laser element including a gallium nitride compound semiconductor and having at least one electrode pad, an electrode terminal for supplying a current to the semiconductor laser element, the electrode pad, and the electrode terminal And a plurality of wires that connect to each other, and heat can be efficiently radiated through the plurality of wires.
[0012]
More specifically, the semiconductor laser device of the present invention includes an n-type gallium nitride compound semiconductor layer, an active layer, a p-type gallium nitride compound semiconductor layer, a p-side electrode, an n-side electrode, And a plurality of wires that connect the p-side electrode and the electrode terminal, and are generated in the active layer. Heat can be efficiently released through the p-side electrode.
[0013]
The semiconductor laser device according to the present invention includes a substrate, an n-type gallium nitride compound semiconductor layer formed on the substrate, and an active layer formed on the n-type gallium nitride compound semiconductor layer. A semiconductor laser element having a p-type gallium nitride compound semiconductor layer formed on the active layer, an n-side electrode, and a p-side electrode, and an electrode terminal for supplying current to the semiconductor laser element; And a plurality of wires connecting the p-side electrode and the electrode terminal, wherein the heat generated in the active layer cannot be efficiently released through the substrate. However, it is possible to efficiently dissipate heat through a plurality of wires.
[0014]
Here, the substrate is sapphire, and the plurality of wires are connected to a portion of the p-side electrode of the semiconductor laser element above the oscillation region, thereby providing heat conduction. In a laser device having a low-rate substrate, heat can be radiated efficiently.
[0015]
Alternatively, the plurality of wires are connected along the oscillation region existing in a stripe shape of the semiconductor laser element, thereby efficiently releasing heat generated in the oscillation region of the active layer. Will be able to.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a plurality of wires are connected to the p-side electrode of a gallium nitride-based compound semiconductor laser device, thereby efficiently releasing heat generated at a high density in the oscillation region of the active layer of the semiconductor laser device to the outside. The present invention proposes a semiconductor laser device capable of performing
[0017]
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic explanatory view showing the main part of the semiconductor laser device according to the first embodiment of the present invention. FIG. 2 is a schematic view illustrating the cross-sectional structure of the semiconductor laser element.
[0018]
The semiconductor laser device 10A of the present invention includes a semiconductor laser element 20A, electrode terminals 50 and 60A, and a plurality of wires 70, 70,... And 80, 80,. These elements can be appropriately disposed on a member (not shown) such as a lead frame, a stem, a chip carrier, or an insulating substrate or a conductive substrate.
[0019]
The semiconductor laser element 20 </ b> A is a laser element including a gallium nitride-based compound semiconductor, and includes a p-side electrode 30 and an n-side electrode 40.
[0020]
The electrode terminals 50 and 60A are electrode terminals for supplying a drive current to the semiconductor laser element 20A. Specific examples thereof include an inner lead portion of a lead frame (not shown), an electrode terminal of a stem, or a part of a wiring pattern formed on an insulating substrate. Further, it may be an input / output terminal of an electric element such as an amplification element, a resistance element, a capacitance element, or an inductor (not shown).
[0021]
The p-side electrode 30 and the electrode terminal 50 of the laser element 20A are connected by a plurality of wires 70, 70,. Here, the number of wires 70 is not limited to the three illustrated, and may be two or more.
Further, the n-side electrode 40 and the electrode terminal 60A of the laser element 20A are connected by a plurality of wires 80, 80,. Here, the n-side wire 80 is not limited to the illustrated example, and may be one wire.
[0022]
According to the present invention, the heat dissipation efficiency can be greatly improved by connecting the p-side electrode 30 and the electrode terminal 50 of the laser element 20A with the plurality of wires 70, 70,.
[0023]
This effect will be described with reference to a cross-sectional view of the laser element 20A shown in FIG. That is, the laser element 20A is formed on a sapphire substrate having a thickness of about 60 μm with the c-plane as the main surface. A gallium nitride (GaN) buffer layer 102 is first formed on the sapphire substrate 101A, and a GaN quality improving layer 103 and an n-type GaN contact layer 104 are stacked thereon. Furthermore, an n-type aluminum gallium nitride (AlGaN) clad layer 105, an undoped GaN guide layer 106, an active layer 107 having a multiple quantum well (MQW) structure, a p-type GaN guide layer 108, and a p-type AlGaN clad are partially formed thereon. A layer 109 and a p-type GaN first contact layer 110 are stacked. Further, an n-type current blocking layer 111 for constricting the injected current is provided in the oscillation region of the active layer, that is, a region where current is injected and laser oscillation occurs. A p-type GaN second contact layer 112 is formed so as to cover it, and a high carrier concentration p-type GaN third contact layer 113 is formed as the uppermost layer. An SiO ₂ film 120 is formed so as to protect the side surfaces from the n-type contact layer 104 to the p-type third contact layer.
[0024]
A p-side electrode 30 is formed on the p-type contact layer 113, and an n-side electrode 40 is formed on the n-type contact layer 104. Here, the p-side electrode 30 can have, for example, a laminated structure including four layers of Pt, Ti, Pt, and Au from the semiconductor side. Further, the n-side electrode 40 can have a laminated structure of Ti and Au from the semiconductor layer side.
[0025]
According to the present invention, a plurality of wires 70, 70,... Are connected to the p-side electrode 30 of such a gallium nitride-based compound semiconductor laser device 20A as shown. More specifically, the plurality of wires 70, 70,... Are desirably bonded along the stripes immediately above the stripe-shaped oscillation region L of the active layer 107.
[0026]
In the gallium nitride compound semiconductor laser element, as illustrated in FIG. 2, heat is generated at a high density in the oscillation region L of the active layer 107 where the injection current is confined and laser oscillation occurs. Here, for example, the distance from the oscillation region L to the p-side electrode 30 in FIG. 2 is about 1 to 3 μm, whereas from L to the lower surface of the substrate 101A, the thickness of the semiconductor layer is added to the substrate thickness to 65 μm. The distance from L to the n-side electrode is about 30 μm from the lateral dimension. As can be seen from such a distance relationship, the p-side electrode 30 immediately above the oscillation region L functions extremely effectively as a discharge path for the generated heat.
[0027]
On the other hand, by connecting a plurality of wires 70, 70,..., The electrical resistance can be lowered, and the capacity and inductance can be adjusted. That is, by reducing the resistance, electric capacity, and inductance of the wire portion, the laser element can be modulated at a higher speed than before, and the transmission speed of information can be increased.
[0028]
Here, in the case of a laser element using a material system such as gallium arsenide or indium phosphide, if wire bonding is performed immediately above the active layer, a defect is induced in the active layer, causing deterioration of the element. On the other hand, a gallium nitride-based compound semiconductor has a characteristic that element deterioration hardly occurs even when the defect density is relatively high. That is, the gallium nitride compound semiconductor based semiconductor laser has an advantage that a plurality of wire bondings can be performed immediately above the active layer without causing deterioration of the element.
[0029]
Here, the diameter of the bonding wire used in the present invention may be the usual 100 μmΦ, but when considering the heat dissipation efficiency, it is desirable that the diameter be larger than that, for example, 150 μmΦ. In this way, by bonding a plurality of wires having a predetermined thickness directly above the active layer, it is possible to greatly improve the heat dissipation and improve the initial characteristics and reliability of the semiconductor laser device. it can.
[0030]
As a result of the inventor's experiment, in the laser device in which four wires of 120 μmΦ are bonded immediately above the oscillation region, the slope efficiency in the current-luminescence intensity characteristic is improved by about 30% compared to the conventional case. At the same time, the maximum emission intensity of the laser increased by about 15%. Furthermore, the upper limit temperature at which practical laser oscillation light can be obtained has also been improved from the conventional 40 ° C. to 65 ° C. For example, when used as an optical pickup of a DVD device, a laser oscillation operation at around 65 ° C. is often required as the maximum rated temperature. According to the present invention, by improving the heat dissipation efficiency of the element, the operating condition temperature can be increased as compared with the conventional case, and the application range of the semiconductor laser device can be remarkably expanded.
[0031]
Furthermore, according to the present invention, the lifetime of the laser element can be extended. As a result of experiments by the present inventors, it was possible to obtain a life that is about three times that of a conventional single bonding.
[0032]
In addition, according to the present invention, by bonding a plurality of wires, the effect of reducing the contact resistance of the electrode and improving the adhesion strength can be obtained. That is, in the wire bonding process, usually, a wire is pressed against the electrode with a predetermined load in a heated state, and an ultrasonic wave is applied. By performing such wire bonding at a plurality of locations, the adhesion between the electrode and the contact layer thereunder can be improved, and at the same time, the contact resistance can be reduced. In the case of a gallium nitride compound semiconductor, the contact resistance and adhesion strength of the electrode are often not sufficient particularly on the p side. Therefore, this effect of the present invention is remarkable.
[0033]
According to the inventor's experiment, the contact resistance of the p-side electrode 30 was reduced to about 1/5 and the adhesion strength of the electrode could be improved by 50% compared to the prior art.
[0034]
Here, the main point of the present invention is to make a plurality of wire bondings to electrodes in a laser apparatus using a gallium nitride compound semiconductor laser element, and does not depend on the laminated structure inside the laser element. However, in a structure in which the injection current is further reduced, such as a so-called BH (Buried Hetero) structure, the heat generation density in the oscillation region is further increased, so that a more remarkable effect can be obtained by applying the present invention.
[0035]
Next, a second embodiment of the present invention will be described.
[0036]
FIG. 3 is a schematic explanatory diagram showing the main part of the second semiconductor laser device 10B of the present invention.
[0037]
Here, the semiconductor laser device 10B of the present invention includes a semiconductor laser element 20B, electrode terminals 50 and 60B, and a plurality of wires 70, 70,. As in the first embodiment described above, the electrode 30 and the electrode terminal 50 of the laser element 20B are connected by a plurality of wires 70, 70,.
[0038]
Here, the difference between this embodiment and the first embodiment described above is that the laser element 20B is directly mounted on the electrode terminal 60B and connected without a wire. That is, the n-side electrode 40 is formed on the back side of the laser element 20B. This can be realized by using a conductive material as the substrate 101B of the laser element 20B. Specific examples of such materials include silicon carbide (SiC). That is, by using a SiC substrate as the substrate 101B, a gallium nitride compound semiconductor layer having a predetermined quality of crystallinity can be epitaxially grown on the substrate 101B, and a current can be passed through the substrate 101B.
[0039]
Here, it is desirable that the crystal type of SiC used as the substrate is “4H type” or “2H type” rather than the so-called “6H type”. This is because the heterobarrier with the gallium nitride-based compound semiconductor layer can be further reduced to reduce the resistance to current.
[0040]
In addition, as the n-side electrode formed on the back surface of the substrate 101B, a layer in which nickel (Ni) and gold (Au) are stacked in this order from the substrate side can be used. Alternatively, platinum (Pt) and gold (Au) may be laminated in this order. Further, instead of these gold (Au), an alloy of gold (Au) and germanium (Ge) or silicon (Si) can be used.
[0041]
Note that the stacked structure of the gallium nitride-based compound semiconductor formed over the substrate 101B can be substantially the same as that illustrated in FIG. Alternatively, various semiconductor laser element structures using gallium nitride-based compound semiconductors can be similarly employed.
[0042]
Also in this embodiment, the various effects described above with reference to FIG. 1 can be obtained similarly. Furthermore, according to the present embodiment, n-side wire bonding is not required, and the configuration can be simplified, and effects such as downsizing of the apparatus and simplification of the manufacturing process can be obtained.
[0043]
Next, a third embodiment of the present invention will be described.
[0044]
FIG. 4 is a schematic explanatory diagram showing the main part of the third semiconductor laser device 10C of the present invention.
[0045]
Here, the semiconductor laser device 10C includes a semiconductor laser element 20C, electrode terminals 50, 60C, and a plurality of wires 70, 70,. As in the first embodiment described above, the electrode 30 and the electrode terminal 50 of the laser element 20C are connected by a plurality of wires 70, 70,.
[0046]
Here, also in the present embodiment, the n-side electrode 40 is formed on the back surface side of the laser element 20C as in the second embodiment described above, and it is connected without interposing a wire by mounting directly on the electrode terminal 60C. Has been. However, in this embodiment, the laser element 20C does not have a substrate. That is, it is composed only of a laminated structure of gallium nitride compound semiconductors.
[0047]
For example, the laser element 20C can be a stacked structure above the contact layer 104 shown in FIG. Such a laser element 20C can be obtained by forming a laminated structure on a substrate (not shown) and then removing the substrate. Specifically, a laminated structure of a gallium nitride compound semiconductor is epitaxially grown on a substrate such as sapphire or SiC via a crystal layer that is easily etched under specific etching conditions. After that, the substrate and the stacked structure can be separated by etching away the crystal layer that is easily etched. Examples of the crystal layer that can be easily etched under specific etching conditions include ZnO, SiO ₂ , MgO, and AlN.
[0048]
After removing the substrate, the n-side electrode 40 can be formed on the back surface of the device by etching away the buffer layer and quality improving layer to expose the n-type contact layer 104 on the back surface of the device. Alternatively, an appropriate amount of doping can be applied to the buffer layer and the quality improvement layer to provide a function as an n-type contact layer.
[0049]
Also in this embodiment, the various effects described above with reference to FIG. 1 can be obtained similarly. Furthermore, according to the present embodiment, n-side wire bonding is not required, and the configuration can be simplified, and effects such as downsizing of the apparatus and simplification of the manufacturing process can be obtained.
[0050]
In addition to the example shown in FIG. 4, in the laser element having no substrate, a part of the n-type contact layer 104 is exposed as in the case of FIG. 1, and the n-side electrode 40 is connected to the n-type contact layer 104. You may form in the upper side of. In this case, similarly to the case of FIG. 1, the electrode terminal 60C may be provided at a position different from the laser element 20C and connected to the n-side electrode 40 through a wire.
[0051]
Next, a fourth embodiment of the present invention will be described.
[0052]
FIG. 5 is a schematic explanatory diagram showing the main part of the fourth semiconductor laser device 10D of the present invention. Here, the semiconductor laser device 10D includes a semiconductor laser element 20D, electrode terminals 50, 60D, a plurality of wires 70, 70,..., And a plurality of wires 80, 80,. As in the first embodiment described above, the electrode 30 of the laser element 20D and the electrode terminal 50 are connected by a plurality of wires 70, 70,. Further, the electrode 40 of the laser element 20D and the electrode terminal 60D are connected by a plurality of wires 80, 80,.
[0053]
In the present embodiment, the wire bonding of the electrode 30 is not formed immediately above the oscillation region L. The reason for this will be described below. That is, when a sapphire substrate is used as the substrate 101A of the laser element, since sapphire is insulative, an n-side contact is ensured by etching away a part of the structure in which the gallium nitride compound semiconductor is stacked. Is common. At this time, a current component flowing in a direction parallel to the laminated surface is inevitably generated. The distance depends on the thickness, resistivity, and structure of the layer structure. However, if the distance is too long, generally the resistance in the lateral direction increases accordingly, which is not desirable from the viewpoint of reducing the resistance. Further, it is not desirable that the length is too short from the viewpoint of the influence of damage to the oscillation region due to the controllability of etching, or the generation of leakage current due to the rise of the electrode metal due to the controllability of the electrode formation process.
[0054]
For the reasons described above, it may be desirable for the oscillation region L to have a distance from the side surface 90 of the mesa region set to about 10 to 20 μm. In such a case, if wire bonding is performed immediately above the oscillation region L, bonding to the side surface 90 may occur and adhesion may be reduced. Therefore, it is necessary to perform bonding so as to be away from the side surface 90 so as not to cause the wraparound of bonding and as close as possible to the position directly above the oscillation region L. FIG. 5 shows such a structure. That is, with the structure illustrated in the figure, the heat dissipation is reduced by about 20% compared to the case where bonding is performed immediately above the oscillation region, but certain effects can be obtained without departing from the gist of the present application. Can be obtained.
[0055]
The embodiments of the present invention have been described above with specific examples. However, the present invention is not limited to these specific examples.
[0056]
For example, the number of wires connected to the electrode 30 of the laser element is not limited to the three shown in the figure, and is appropriately determined in consideration of the laser and wire dimensions, the amount of heat generated in the active layer, and other various parameters. be able to.
[0057]
Further, the substrate 101 of the laser element 20A is not limited to sapphire, and other various materials such as SiC, Si, and GaAs can be used. However, according to the present invention, a more remarkable effect can be obtained when a substrate having low thermal conductivity is used. Examples of such a substrate include spinel, ScAlMgO ₄ , (LaSr) (AlTa) O _{3, and the} like in addition to sapphire described above. Here, it is desirable to use the (0001) plane in the case of the ScAlMgO ₄ substrate and the (111) plane in the case of the (LaSr) (AlTa) O ₃ substrate.
[0058]
Further, the structure of the laser element and its conductivity type are not limited to the specific examples described above, and for example, a structure in which p-type and n-type are inverted may be used.
[0059]
【The invention's effect】
The present invention is implemented in the form as described above, and has the effects described below.
[0060]
First, according to the present invention, the heat dissipation of the laser element can be improved by performing a plurality of wire bondings directly on the oscillation region of the active layer. As a result, various characteristics of the semiconductor laser device can be improved, the reliability and life can be extended, and the application range can be greatly expanded.
[0061]
Further, according to the present invention, the contact resistance and adhesion strength of the electrode of the laser element can be improved by bonding a plurality of wires. As a result, the driving current can be reduced, the light emission characteristics can be stabilized, and the mechanical strength against vibration and impact can be improved.
[0062]
By connecting a plurality of wires, the electrical resistance can be lowered and the capacity and inductance can be adjusted. That is, while suppressing the heat generation of the wire, the laser of the gallium nitride compound semiconductor can be operated at a high output, and the laser element can be modulated at a higher speed than the conventional one, and the transmission speed of information can be increased. become able to.
[0063]
As described above, according to the present invention, it is possible to provide a semiconductor laser device having high performance and high reliability, which has a great industrial advantage.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory diagram of a semiconductor laser device according to a first embodiment of the present invention.
FIG. 2 is a schematic view showing a cross section of a laser element 20A.
FIG. 3 is a schematic explanatory diagram of a semiconductor laser device according to a second embodiment of the present invention.
FIG. 4 is a schematic explanatory diagram of a semiconductor laser device according to a third embodiment of the present invention.
FIG. 5 is a schematic explanatory diagram of a semiconductor laser device according to a fourth embodiment of the present invention.
[Explanation of symbols]
10A, 10B, 10C, 10D Semiconductor laser device 20A, 20B, 20C, 20D Semiconductor laser element 30, 40 Electrode 50, 60 Electrode terminal 70, 80 Wire 90 Mesa side surface 101A Substrate 102 Buffer layer 103 Quality improvement layer 104 Contact layer 105, 109 Cladding layer 106, 108 Guide layer 107 Active layer 110 First contact layer 111 Block layer 112 Second contact layer 113 Third contact layer 120 Protective film L Oscillation region

Claims (2)

A substrate, an n-type gallium nitride compound semiconductor layer formed on the substrate, an active layer formed on the n-type gallium nitride compound semiconductor layer, and formed on the active layer a semiconductor laser device having a p-type gallium nitride compound semiconductor layer, an n-side electrode, and a p-side electrode;
An electrode terminal for supplying a current to the semiconductor laser element;
A plurality of wires connecting the p-side electrode and the electrode terminal;
And the plurality of wires are connected along a portion of the p-side electrode of the semiconductor laser element above the oscillation region existing in a stripe shape of the semiconductor laser element. Laser device. The semiconductor laser device according to claim 1, wherein the substrate is made of sapphire or SiC.

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