JP2002299769A - Semiconductor laser and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a GaN-based semiconductor laser and a method for manufacturing the device, which has high reliability, even if the device is operated in high-humidity and high-output state. SOLUTION: The rear surface of a substrate made of nitride semiconductor is etched to form a ruggedness surface, thus increasing its surface area by about 3 fold or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gallium nitride (GaN) -based short wavelength semiconductor laser expected to be applied to the field of optical information processing and the like, and a method of manufacturing the same.

[0002]

2. Description of the Related Art A nitride semiconductor having nitrogen (N) as a group V element is considered to be promising as a material for a short-wavelength light emitting device because of its large band gap. Among them, DVD
Aiming to increase the capacity of optical disc devices such as
A semiconductor laser having a band as an oscillation wavelength is eagerly desired. Particularly GaN-based nitride semiconductor _{(Al x Ga y In z N} (0 ≦ x,
Lasers composed of y, z ≦ 1, x + y + z = 1)) have been actively studied, and are now reaching a practical level.

FIG. 1 is a structural view of a cross section orthogonal to a resonator of a GaN-based semiconductor laser device. On a GaN substrate 101, an n-AlGaN contact layer 102, an n-AlGaN cladding layer 103, an n-Ga
N optical guide layer _{104, Ga 1- x In x N} / Ga 1-y In y N
(0 <y <x <1) multiple quantum well (MQW) active layer 105, p-GaN optical guide layer 106, p-AlG
aN cladding layer 107, p-GaN contact layer 108
Is grown. Then, a ridge stripe having a width of about 2 μm is formed on the p-GaN contact layer 108, and both sides thereof are buried with an insulating film 110. Thereafter, a p-electrode 109 made of, for example, Ni / Au is formed on the ridge stripe and the insulating film 110, and an n-electrode 111 made of, for example, Ti / Al is formed on a part of which is etched until the n-AlGaN contact layer 102 is exposed. You.

When the n-electrode 111 is grounded and a voltage is applied to the p-electrode 109 to inject a current, electrons and holes are recombined in the MQW active layer 105 to emit light, thereby producing an optical gain.
Laser oscillation occurs around a wavelength of 400 nm. The oscillation wavelength is controlled by the composition and thickness of the Ga _1-x In _x N / Ga _1-y In _y N thin film constituting the MQW active layer 105.

For example, “Jpn. J. Appl. Phys. Vol. 39, L
Page 647 "discloses that the estimated lifetime of the GaN-based semiconductor laser as described above is 15000 hours at 60 ° C. and 30 mW output CW operation. In this example,
Prior to the crystal growth of the laser structure, selective lateral growth is performed on the GaN substrate 101 to reduce the dislocation density existing in the stripe-shaped active layer region into which current is injected to 7 × 10 ⁵ cm ⁻² , The reliability of the device has been improved.

Generally, the size of a semiconductor laser device is several hundred μm square, and the laser resonator surface is formed by cleavage of a semiconductor crystal. In the crystal growth process and the device fabrication process, the substrate thickness is selected to be about several hundred μm because of easy handling of the substrate.However, in order to cleave a resonator length of several hundred μm with high yield, Prior to the cleaving step, it is necessary to polish or etch the back surface of the substrate until the thickness of the substrate becomes about 50 to 150 μm to make it thinner. The surface obtained in this way is usually flat. The electrode on the back side of the substrate is formed on this flat surface.

[0007] When power is supplied to the semiconductor laser element, the temperature of the pn junction increases. The temperature rise not only changes the oscillation characteristics such as the optical output and the oscillation wavelength, but also has a great influence on the life of the device. Therefore, when mounting a laser element requiring a high operating current, such as a high-power laser, in a package, the pn junction side of the element is bonded to a heat sink in order to improve heat radiation characteristics, that is, the back side of the element substrate. In general, mounting is performed in a so-called junction-down arrangement. An example is shown in FIG. This is achieved through the heat sink 114 via the submount 113.
This is an example implemented in.

[0008]

SUMMARY OF THE INVENTION Compared with a conventional semiconductor laser device using GaAs, InP or the like for a substrate, GaN
The thermal conductivity of the substrate is about 1.5 W / cmK, which is about twice as high and has good heat dissipation characteristics. Therefore, the thermal management of the semiconductor laser device is compared. It is easy.

However, it is expected that higher temperature operation and higher output operation will be required for nitride semiconductor lasers in the future. Therefore, as a result of examination by the present inventors,
In the GaN-based semiconductor laser device configured according to the above-described conventional technology, for example, compared with the case of operating at 60 ° C. and 30 mW output,
It was found that the device life was extremely shortened at the time of output operation at 60 ° C. and 60 mW. In the current-light output characteristics in the high temperature region,
Although the increase in the oscillation threshold current value is not remarkable (that is, the decrease in the characteristic temperature is not remarkable), it has been observed that the slope efficiency is reduced at a high output of about 50 to 100 mW, so that thermal saturation has begun to occur. It was considered. This phenomenon is considered to be due to insufficient heat radiation characteristics.

The present invention has been made in view of the above circumstances, and provides a semiconductor laser device using a nitride semiconductor laser element having high reliability even at high temperature and high output operation, and a method of manufacturing the same. . It is particularly effective in application to a laser device for optical disks.

[0011]

In order to solve the above-mentioned problems, a semiconductor laser device according to the present invention is a semiconductor laser device in which a nitride semiconductor laser device is mounted in a junction-down arrangement. The back surface of the substrate is an uneven surface. In the above configuration, the uneven surface is formed of a hexagonal pyramidal facet, a hexagonal truncated pyramid facet, or a columnar hole.

Preferably, the substrate of the nitride semiconductor laser device is made of a nitride semiconductor. In the above configuration, a p-type or n-type
Preferably, a mold electrode is formed. Further, the back surface of the substrate made of a nitride semiconductor is preferably a -c polarity plane.

Another semiconductor laser device according to the present invention is a semiconductor laser device in which a nitride semiconductor laser element is mounted in a junction-down arrangement or a junction-up arrangement. It is stuck to.

Further, a method of manufacturing a semiconductor laser device according to the present invention is a method of manufacturing a semiconductor laser device on a substrate made of a nitride semiconductor, the method comprising obtaining an uneven surface by anisotropically etching the back surface of the substrate. including.

[0015] The method further includes a step of treating the back surface of the substrate with a chemical solution containing at least phosphoric acid. In the above configuration, the phosphoric acid is preferably at least one selected from triphosphoric acid, metaphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, pyrophosphoric acid, and orthophosphoric acid.

The method also includes a step of etching the back surface of the substrate using a chemical solution containing orthophosphoric acid, nitric acid and water to obtain an uneven surface. In the above-described configuration, it is preferable that the temperature of the chemical solution is 100 ° C. or more and 250 ° C. or less.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings.

Example 1 FIG. 3 is a diagram schematically showing a semiconductor laser device according to one embodiment of the present invention. Ga
A state in which an N-based semiconductor laser device is mounted on a heat sink 114 in a junction-down arrangement via a submount 113 is schematically shown in a cross section orthogonal to the resonator. The semiconductor laser device is an n-AlGaN substrate 1
15, n-AlGaN cladding layer 103, n-GaN
Light guide layer 104, MQW made of GaInN / GaN
An active layer 105, a p-GaN optical guide layer 106, a ridge-shaped p-AlGaN cladding layer 107, and a p-GaN contact layer 108 are sequentially laminated. Both sides of the ridge are buried with an insulating film 110, and a p-electrode 109 is formed. p
-On the GaN contact layer 108, the n-electrode 111 is n-
It is formed on the back surface of the AlGaN substrate 115 (that is, a surface different from the surface on which the active layer is formed).

The point of this embodiment lies in that the back surface of the n-AlGaN substrate 115 is an uneven surface. The uneven surface is composed of a hexagonal pyramid-shaped facet 116, a hexagonal truncated pyramid-shaped facet 117 or a columnar hole 118. The manufacturing process of this laser device includes, in order, crystal growth, dry etching, p-electrode deposition,
It consists of each process such as substrate back surface polishing, substrate back surface treatment, n-electrode deposition, cleavage, end surface coating, element separation, mounting, etc.
The point of the present invention lies in the substrate back surface processing step.

The n-AlGaN substrate 115 used for manufacturing a laser device has a diameter of, for example, 2 inches and a thickness of about 300 μm.
The Al mixed crystal ratio is 3%, the carrier concentration is 1 × 10 ¹⁸ cm ⁻³ (dopant is Si), and the plane orientation is (0001). A laser element structure is formed on the front surface side of the n-AlGaN substrate 115 that has been completed up to the substrate back surface polishing step, and the substrate thickness is reduced to about 100 μm by back surface polishing. The substrate back surface treatment is performed, for example, by immersing the substrate in orthophosphoric acid at 160 ° C. for 2 minutes. The p-electrode 109 and the (0001) plane of the GaN-based semiconductor, which cover the entire surface, are hardly affected by orthophosphoric acid under this condition. However, when an element structure different from that of this embodiment is employed, or when a more severe treatment is performed. When the conditions are adopted, the entire surface can be protected by a SiO ₂ film or the like so that it is not attacked by orthophosphoric acid, if necessary.

FIGS. 4 and 5 show optical microscope images of the rear surface of the n-AlGaN substrate 115 after the processing. FIG. 4 shows FIG.
Shows a hexagonal pyramid-shaped facet 116, a hexagonal truncated pyramid-shaped facet 117 or a columnar hole 118 whose cross section is schematically shown. The entire back surface of the substrate has a diameter of about several tens to several hundred μm,
It can be seen that it is covered with a hexagonal pyramid and a hexagonal pyramid with a depth of about several to 10 μm. FIG. 5 shows a columnar hole 118. The shape of the columnar hole 118 is hexagonal or circular with a diameter of about 5 μm, and the depth is about 10 to 20 μm. The density is about 2 × 10 ⁶ cm ⁻² . In the dense part, independent columnar holes overlap to form a large hole.

The plane orientation of the back surface of the substrate is the (000-1) plane, that is, the -c polarity plane. The -c polar plane refers to three out of four Ga-N bonds in the hexagonal GaN crystal structure shown in FIG.
The book is facing down. Although the surface atomic rearrangement structure of the GaN-based semiconductor is unknown, the fact that the -c polar plane is more susceptible to orthophosphoric acid than the + c polar plane is attributed to the difference in the configuration of the dangling bond. Is more likely to react with orthophosphoric acid and to easily produce gallium phosphate such as Ga ₃ PO ₄ . The reason why the columnar holes 118 are formed is unknown, but considering the density,
It can be considered that the dislocation was promptly etched.

The surface area of the back surface of the n-AlGaN substrate 115 was measured. As a result, it was found that the surface area was 20.1 cm ² before the treatment and 75.6 cm ² after the treatment, which is more than three times as large. . It is presumed that the formation of the columnar holes 118 greatly contributes to the increase in the surface area.

After the substrate back surface treatment step, an n-electrode 111 was deposited on the back surface of the n-AlGaN substrate 115, and cleaved into bars at 750 μm intervals. Irregularities on the back surface of the substrate did not affect the cleavage yield. Further, the rear end face is subjected to high reflectance coating and element separation, and the submount 11
3 and mounted on the heat sink 114 in a junction-down arrangement.

The thermal resistance was measured to evaluate the heat radiation characteristics of the semiconductor laser device. For comparison, a semiconductor laser device manufactured without performing the substrate back surface treatment was about 25 to 3 for comparison.
In contrast to 5 ° C./W, when the substrate back surface treatment according to the present invention was performed, it was improved to about 15 to 25 ° C./W. Therefore, for example, the current-light output characteristics at the time of CW operation at 100 ° C. have been improved, and the conventional problems of 50 to 1
No decrease in slope efficiency was observed at the time of the 00 mW output operation. Further, the device life at the time of output operation at 80 ° C. and 60 mW was extended.

As described above, according to the present embodiment, the heat dissipation characteristics are improved by increasing the surface area of the back surface of the substrate, and the reliability during high-temperature high-output operation can be improved.

In this embodiment, n-Al is used as the substrate.
Although the case where GaN is used has been described, the same effect can be obtained by using GaN or another nitride semiconductor such as GaInN. Further, a substrate other than a nitride semiconductor, for example, SiC, Si, Al ₂ O ₃ or the like may be used. In this case, an appropriate back surface treatment method for forming an uneven surface on each substrate is used.

The conductivity type of the substrate is not limited to n-type, but may be p-type or semi-insulating. In addition, the polarity of the substrate may be not only the case where the back surface of the nitride semiconductor substrate is the −c polarity surface but also the case where the back surface of the nitride semiconductor substrate is the + c polarity surface. However, in this case, more severe backside processing conditions are required as described above.

Also, in this embodiment, an element having an electrode formed on the back surface of the substrate has been described.
The same effect can be obtained in the case of an element in which both p-type and n-type electrodes are formed on the substrate surface side as shown in FIG.

In this embodiment, orthophosphoric acid is used for the back surface treatment of the nitride semiconductor substrate, but orthophosphoric acid (H ₃ P
O ₄ ) is dehydrated and condensed by heating to pyrophosphoric acid (H ₄ P ₂
O ₇ ), triphosphoric acid (H ₅ P ₃ O ₁₀ ), metaphosphoric acid (HP
O ₃ ), and the metaphosphoric acid is multimerized and usually trimetaphosphoric acid (H ₃ P ₃ O ₉ ) and tetrametaphosphoric acid (H ₄ P
Since it exists as ₄ O ₁₂ ), the same effect can be obtained even when a chemical solution containing at least these phosphoric acids is used. This is because the nitride semiconductor easily reacts with these phosphoric acids to generate a chelate compound of gallium phosphate.

In order to prevent orthophosphoric acid from being dehydrated and condensed, it is preferable to add nitric acid and water thereto. Accordingly, the back surface of the substrate made of the nitride semiconductor can be etched stably and with good reproducibility to obtain an uneven surface. If the temperature of this chemical is less than 100 ° C., the reactivity is poor, and if it exceeds 250 ° C., dehydration condensation proceeds.
It is preferably used at a temperature of 0 ° C or higher and 250 ° C or lower.

Further, as a method of providing the uneven surface on the back surface of the substrate, a wet etching method that easily reacts with the nitride semiconductor, such as a molten alkali treatment, can be selected in addition to the phosphoric acid treatment. In addition, dry etching having reactivity to be anisotropic etching may be used.

Example 2 FIG. 7 is a diagram schematically showing a semiconductor laser device according to another embodiment of the present invention. G
FIG. 4 schematically shows a state in which an aN-based semiconductor laser device is mounted on a heat sink 114 in a junction-down arrangement via a submount 113 in a cross section orthogonal to the resonator. The semiconductor laser device has an n-AlGaN contact layer 102 and an n-AlGaN
Clad layer 103, n-GaN optical guide layer 104, Ga
MQW active layer 105 made of InN / GaN, p-Ga
An N light guide layer 106, a ridge-shaped p-AlGaN cladding layer 107, and a p-GaN contact layer 108 are sequentially stacked.
08, an n-electrode 111 is formed on the etched surface up to the n-AlGaN contact layer 102, respectively.

The point of this embodiment is that the GaN substrate 101
Is that a heat sink 114 is also provided on the back surface. Thereby, the heat radiation characteristic from the back surface is improved. The submount 113 and the heat sink 114 are selected from a group of substances having a high thermal conductivity in consideration of a difference in thermal expansion coefficient between the submount 113 and the semiconductor laser element so as to suppress the occurrence of thermal distortion due to mounting. Usually, diamond, S
iC, Si, AlN or the like is used. Heat sink 11
Reference numeral 4 denotes a portion corresponding to a post in the package of the semiconductor laser device, and a metal such as Cu or Kovar is used. Further, the heat sink 114 provided on the back surface of the substrate desirably has a high emissivity, and may be usually a metal.

The shape of the heat sink provided on the back surface of the substrate is as follows.
The heat sink may have a flat plate shape as shown in FIG. 7 or a comb shape as shown in FIG. 8 in order to increase the surface area of the heat sink. Further, as shown in FIG. 9, in order to enhance the thermal conductivity, it may be connected to a post heat sink 114 via a metal pad 119 covering the back surface of the substrate.

When the thermal resistance was measured in order to evaluate the heat radiation characteristics of the semiconductor laser device, the semiconductor laser device manufactured without providing a heat sink on the back surface of the substrate was compared for comparison.
In contrast to the case where the heat sink was fixed at about 25 to 35 ° C./W, about 15 to 25 ° C.
/ W. Therefore, for example, the current-light output characteristics at the time of the CW operation at 100 ° C. have been improved, and the decrease in the slope efficiency at the time of the 50 to 100 mW output operation, which has been a conventional problem, has not been observed. In addition, 80 ° C 60
The device life at the time of mW output operation was extended.

As described above, according to the present invention, the heat radiation characteristic is improved by fixing the heat sink to the back surface of the substrate, and the reliability at the time of high temperature and high output operation can be improved.

In this embodiment, the case where GaN is used as the substrate has been described. However, other nitride semiconductors, for example, AlGaN, GaInN, etc., or substrates other than nitride semiconductors, for example, SiC, Si, Al ₂ O ₃ or the like may be used.

In this embodiment, a p-type substrate is provided on the substrate surface side.
Although an element having both n-type electrodes has been described,
The same effect can be obtained in the case of an element in which an electrode is formed on the back surface of the substrate.

In this embodiment, the case where the semiconductor laser device is mounted in a junction-down configuration has been described. However, even in the case where the semiconductor laser device is mounted in a junction-up configuration, a large heat sink is fixed to the substrate surface. The effect can be obtained.

Although the present invention relates to a laser device, it can be applied to other semiconductor devices such as an electronic device requiring high-temperature high-power operation and a method of manufacturing the same, and provides high reliability.

[0042]

As described above, according to the semiconductor laser device and the method of manufacturing the same of the present invention, it is possible to provide a highly reliable GaN-based semiconductor laser device having good heat radiation characteristics even at high temperature and high output operation. .

[Brief description of the drawings]

FIG. 1 is a structural diagram of a cross section orthogonal to a resonator of a GaN-based semiconductor laser device.

FIG. 2 is a schematic diagram of a cross section orthogonal to a resonator showing a state in which a conventional GaN-based semiconductor laser device is mounted via a submount.

FIG. 3 is a schematic diagram of a cross section orthogonal to a resonator showing a state in which a GaN-based semiconductor laser device according to an embodiment of the present invention is mounted via a submount.

FIG. 4 is a diagram showing an optical microscope image of the back surface of the substrate of the GaN-based semiconductor laser device according to one embodiment of the present invention;

FIG. 5 is a view showing an optical microscope image of the back surface of the substrate of the GaN-based semiconductor laser device according to one embodiment of the present invention;

FIG. 6 shows hexagonal GaN crystal (0001) plane and (00)
Schematic diagram showing the polarity of the 0-1) plane

FIG. 7 is a schematic cross-sectional view orthogonal to the resonator showing a state in which the GaN-based semiconductor laser device according to one embodiment of the present invention is mounted via a submount.

FIG. 8 is a schematic cross-sectional view orthogonal to the resonator showing a state in which the GaN-based semiconductor laser device according to one embodiment of the present invention is mounted via a submount.

FIG. 9 is a schematic cross-sectional view orthogonal to the resonator showing a state in which the GaN-based semiconductor laser device according to one embodiment of the present invention is mounted via a submount.

[Explanation of symbols]

 Reference Signs List 101 GaN substrate 102 n-AlGaN contact layer 103 n-AlGaN cladding layer 104 n-GaN optical guiding layer 105 active layer 106 p-GaN optical guiding layer 107 p-AlGaN cladding layer 108 p-GaN contact layer 109 p electrode 110 insulating film 111 n electrode 112 solder 113 submount 114 heat sink 115 n-AlGaN substrate 116 hexagonal pyramidal facet 117 hexagonal truncated pyramid facet 118 columnar hole 119 metal pad

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5F043 AA05 BB06 DD07 FF10 GG04 5F073 AA11 AA45 AA74 BA06 CA03 CB02 DA22 DA23 DA24 DA32 EA24 EA27 EA28 FA14

Claims (11)

[Claims] 1. A semiconductor laser device in which a nitride semiconductor laser element is mounted in a junction-down arrangement.
A semiconductor laser device, wherein the back surface of the substrate of the nitride semiconductor laser element is an uneven surface. 2. The semiconductor laser device according to claim 1, wherein the uneven surface on the back surface of the substrate of the nitride semiconductor laser element comprises a hexagonal pyramid-shaped facet, a hexagonal truncated pyramid-shaped facet, or a columnar hole. 3. The semiconductor laser device according to claim 1, wherein the substrate of the nitride semiconductor laser element is made of a nitride semiconductor. 4. The semiconductor laser device according to claim 3, wherein a p-type or n-type electrode is formed on a back surface of the nitride semiconductor substrate. 5. The back surface of a substrate made of a nitride semiconductor has -c
The semiconductor laser device according to claim 3, wherein the semiconductor laser device is a polar surface. 6. A semiconductor laser device in which a nitride semiconductor laser element is mounted in a junction-down arrangement or a junction-up arrangement, wherein both the front and back surfaces of the element are fixed to a submount or a heat sink. Semiconductor laser device. 7. A method of manufacturing a semiconductor laser device on a substrate made of a nitride semiconductor, comprising a step of obtaining an uneven surface by anisotropically etching the back surface of the substrate. Production method. 8. A method for manufacturing a semiconductor laser device on a substrate made of a nitride semiconductor, comprising a step of treating the back surface of the substrate with a chemical solution containing at least phosphoric acid. Manufacturing method. 9. The semiconductor laser device according to claim 8, wherein the phosphoric acid is at least one selected from the group consisting of triphosphoric acid, metaphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, pyrophosphoric acid, and orthophosphoric acid. Manufacturing method. 10. A method of manufacturing a semiconductor laser device on a substrate made of a nitride semiconductor, comprising the steps of: etching a back surface of a substrate using a chemical solution containing orthophosphoric acid, nitric acid, and water to obtain an uneven surface. A method for manufacturing a semiconductor laser device, comprising: 11. The method for manufacturing a semiconductor laser device according to claim 10, wherein the temperature of the chemical is 100 ° C. or higher and 250 ° C. or lower.