CA2088754A1

CA2088754A1 - Semiconductor laser device and method of making same

Info

Publication number: CA2088754A1
Application number: CA002088754A
Authority: CA
Inventors: Yoichiro Ota
Original assignee: Individual
Current assignee: Mitsubishi Electric Corp
Priority date: 1992-02-04
Filing date: 1993-02-03
Publication date: 1993-08-05
Also published as: FR2687857A1; JPH05218586A

Abstract

ABSTRACT OF THE DISCLOSURE

A semiconductor laser device includes a laser chip and a submount. The laser chip includes a mesa-shaped semiconductor substrate having a surface on which a plurality of layers required for generating laser oscillations are epitaxially grown to be conformal to the surface contour of the substrate. The laser chip is soldered upside down to the submount. If solder rises up along the side surfaces of the laser chip, it does not short-circuit a PN junction contained in the epitaxially grown layers.

Description

SEMICON~UCTOR LASER DEVICE
AND MET~IOD OF MAKING SAME

The present invention relates generally to a semiconductor laser device and a method of making it, and more particularly, to a semiconductor laser device including a laser chip soldered upside down on a submount, of which a PN junction is not short-circuited by solder even when solder rises up along side surfaces of the laser chip, and also to a method of making such a semiconductor laser device.

BACKGROUND OF THE INvEnTIon In Figure 1, a cross-section of an example of a conventional semiconductor laser device is shown. A laser chip 1 includes a semiconductor substrate 2, epitaxially grown layers 3 disposed on the semiconductor substrate 2, a p-side electrode 4, and an n-side electrode 5. The laser chip 1 is bonded to a submount 7 with the epitaxially grown layers 3 facing to the submount 7, by means of solder 8. (In the present application, the manner to solder the laser chip portion on the epitaxially grown layer side to a submount is called "upside down" mounting.) The laser chip 1 shown in Figure 1 is described in greater detail. The epitaxially grown layers 3 are disposed on (a bottom surface, in Figure 1, of) the N-type semiconductor substrate 2, for example. The layers 3 include an N-type cladding layer 11, a P-type active layer 12, a P-type cladding layer 13, a P-type current blocking layer 14, and a contact layer 15 disposed in the named order, with the cladding layer 11 disposed on the substrate 2. The p-side electrode 4 is disposed on the epitaxially grown layers 3, and the n-side electrode 5 is disposed on the opposite side of the substrate 2.
When an operating voltage of a predetermined magnitude is applied between the electrodes 4 and 5 of the semiconductor laser device of Figure 1, laser oscillations take place in the 208875~

epitaxially grown layers 3 which include double-heteroiunctions therein. The epitaxially grown layers 3 may generate heat, but, since the laser chip 1 is soldered upside down on the submount 7 as shown, -the heat generated in the layers 3 is conducted ~o and dissipated rapidly through the submount 7 which acts also as a heat sink, whereby degradation of the laser chip due to heat is avoided. It should be noted that, although not shown, a laser resonator surface is coated with a film of a dielectric material, such as Al203 and SiO2, which acts as an anti-reflection film.
When the laser chip, such as the chip 1, of a conventional semiconductor laser device, such as the one shown in Figure 1, is soldered to the submount 7, the solder 8 may rise up along the side surfaces of the chip 1 beyond the active layer 12 to electrically short-circuit the PN junction formed by the active layer 12 and the N-type cladding layer 11, which undesirably makes laser oscillations impossible to occur.
For example, Japanese Unexamined Patent Publications No.
HEI 1-115187, No. HEI 2-181488 and No. SH0 60-55687 disclose semiconductor devices which can avoid, to some extent, a PN
junction from being short-circuited by solder extending up along side surfaces of a laser chip when the laser chip is soldered upside down onto a submount. All of the semiconductor laser devices disclosed in the above-cited Japanese publications are arranged such that depressions or grooves are formed in the surface of the laser chip which is to be soldered to the submount, namely, in the surface of the contact layer or the capping layer of the laser chip, and excessive solder enters into the depressions or grooves. In these semiconductor laser devices, rising up of solder to short-circuit a PN
junction may be eliminated to some extent, but it cannot completely prevent PN junction short-circuiting. In addition, it is essentially necessary to provide a thick contact layer or capping layer, which results in poor heat dissipation through the epitaxially grown layers.
Japanese Unexamined Patent Publication No. HEI 1-215088 discloses a semiconductor laser device which includes a laser chip mounted upside down to a submount by means of solder.
Insulating regions are formed in the laser chip along its side surfaces by implanting ions of, for example, boron or hydrogen from the exposed surface of the epitaxial layer of the laser chip, namely, from the surface of the second cladding layer on the side to be soldered to the submount, through the active layer and the first cladding layer to the semiconductor substrate. If solder rises up along the side surfaces of the laser chip, it does not short-circuit the PN junction, and a satisfactory result may be obtained. However, implantation of ions, such as boron ions and hydrogen ions, from the surface of the epitaxially grown layers to the semiconductor substrate of the laser chip during manufacturing it, requires large energy.
The present invention is to avoid the above-stated problems encountered in conventional semiconductor laser devices such as those described above. According to the present invention, a semiconductor laser device is provided, which includes a laser chip soldered upside down to a submount, and which has such a structure that the PN junction therein is not short-circuited by solder, even if the solder rises up along the side surfaces of the laser chip. The present invention also provides a method of manufacturing such a semiconductor laser device.

SUMMARY OF THE INVENTION
A semiconductor laser device according to one embodiment of the present invention includes a laser chip including a semiconductor substrate which has a generally mesa-shaped crosssection (hereinafter referred to as mesa-shaped substrate), and a plurality of layers required for producing laser oscillations which are epitaxially grown on and conformal to the upper surface of the mesa-shaped substrate. The laser chip is soldered upside down to a submount on the epitaxially grown layer side of the laser chip.

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A semiconductor laser device according to another embodiment of the present lnvention includes a generally mesa-shaped laser chip including a semiconductor substrate which has a flat surface, and a plurality of layers required ior producing laser oscillations which are epitaxially grown on the surface of the semiconductor substrate. Semi-insulating regions are formed along opposite sides of at least the epitaxially grown layers. This laser chip is soldered upside down to a submount on the epitaxially grown layer side of the laser chip.
A method of manufacturing the semiconductor laser device of the above-described one embodiment includes a step of forming in a surface of a flat semiconductor substrate, grooves which extend in a predetermined direction with a predetermined spacing between adjacent grooves and have a predetermined depth, a step of epitaxially growing on the surface of the semiconductor substrate and in the respective grooves a plurality of layers necessary for producing laser oscillations, a step of cleaving the semiconductor substrate with the plurality of epitaxially grown layers along the respective grooves into separate laser chips, and a step of soldering the portions of the respective laser chips on the epitaxial layer sides of the chips to the submount.
A method of manufacturing the semiconductor laser device of the above-described another embodiment includes a step of epitaxially growing on a flat surface of a semiconductor substrate a plurality of layers necessary for producing laser oscillations, a step of forming grooves which extend, in the depth direction from the surface of the uppermost one of the epitaxially grown layers to at least the layer closest to the substrate, and extend along the surface in a predetermined direction with a predetermined spacing between adjacent grooves, a step of implanting ions of, for example, boron or hydrogen into the structure through the grooved portions to thereby form semi-insulating regions along the inner side surfaces and bottom surfaces of the respective grooves, . . . , -~ . .
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cleaving the semiconductor substrate with the plurality of epi-taxially grown layers thereon along the grooves into separate laser chips, and soldering the epitaxially grown layer side of the laser chip to a submount.
According to the present invention, a semiconductor laser chip and a method of manufacturing it are provided, in which even if solder rises up along the sloping side surfaces of the laser chip when it is solder upside down to the submount, the solder does not short-circuit the PN junction in the epitaxially grown layers.

BRIEF DESCRIPTION OF THE DRAWINGS
Now, the present invention is described in detail with reference to the embodiments shown in the accompanying drawings. In the drawings:
Figure 1 is a cross-sectional view of a conventional semiconductor laser device:
Figure 2 is a cross-sectional view of a semiconductor laser device according to a first embodiment of the present invention;
Figures 3(a) and 3(b) show cross-sections of a laser chip of the semiconductor laser device of Figure 2 in different manufacturing steps;
Figure 4 is a cross-sectional view of a semiconductor laser device according to a second embodiment of the present invention;
Figure 5 is a cross-sectional view of an undesirable semiconductor laser device which could result during manufacturing semiconductor devices of the present invention shown in Figure 4:
Figure 6 is a cross-sectional view of a semiconductor laser device according to a third embodiment of the present invention: and Figures 7(a) and 7(b) are cross-sectional views of a laser chip of the semiconductor laser device of Figure 6 in different manufacturing steps.

` 20887~4 Figure 2 is a cross-sectional view of a semiconductor laser device according to a first embodiment of the present invention. A laser chip 21 includes a generally mesa-shaped semiconductor substrate 22, a set of layers 23 necessary for producing laser oscillations epitaxially grown on the upper surface of the substrate 22, a p-side electrode 24, and an n-side electrode 25.
For example, the semiconductor substrate 22 may be an N-type GaAs substrate having a thickness of about 100 ~m and having a carrier concentration of 1 x 10'fl cm 3 or higher.
The epitaxially grown layers 23 are disposed on and conformal to the contour of the upper surface (shown facing downward in Figure 2) of the N-type GaAs substrate 22, and include, for example, an ~-type Al0.44GaO.s~As cladding layer 31 having a thickness of 1-3 ~m and a carrier concentration of from 5 x 10'~ cm 3 to 5 x 10' 7 cm 3, a P-type, N-type, or intrinsic AlO~,~GaO~OAs active layer 32 having a thickness of 0.02-0.1 ~m, a cladding layer 33 having a thickness of 0.5-3 ~m at a mesa-shaped portion 331 thereof through which current flows and having a carrier concentration of from 1 x 10'3 cm 3 to 5 x 10l9 cm 3, an N-type GaAs current blocking layer 34 having such a thickness to embed therein the mesa-shaped portion 331 of the cladding layer 33 and having a carrier concentration of 5 x 10'3 cm 3 or higher, and a P-type contact layer 35 having a thickness of 2-3 ~m and having a carrier concentration of from 2 x 10l3 cm 3 to 3 x 10'9 cm 3 .
Generally, a resonator end surface of the laser chip 21 is coated with a dielectric film (not shown), such as an Al203 film and an SiOz film, which acts as an anti-reflection film.
Alternatively, an InP substrate may be used as the semiconductor substrate 22. In such a case. InGaAsP is used for the cladding layers 31 and 33 and for the active layer 32.
Between the cladding layer 33 and the current bloc~ing layer 34, a GaAs or InP buffer layer having a thickness of 0.02-0.1 ~m and a carrier concentration of 1 x 10'3 cm 3 to .
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20887~4 1 x 101~ cm 3 may be disposed.
The semiconductor laser chip 21 is placed upside down, i.e. with the bottom surface of the semiconductor substrate 22 facing upward, and the epitaxially grown layer side of the chip is soldered to a submount 27 by means of solder 28. In this soldering step, even if the solder 28 rises up along the side surfaces of the laser chip 21, it only contacts the sloping side surfaces of the contact layer 35, but does not short-circuit the PN junction. Accordingly, when an operating voltage of predetermined magnitude is applied between the electrodes 24 and 25, laser oscillations occur in the epitaxial layers 23 including a double-heterojunction structure therein.
A method of manufacturing the laser chip 21 of the semiconductor laser device of Figure 2 is described with reference to Figure 3. As shown in Figure 3(a), grooves 30 having, for example, a U-shaped cross-section and having a depth of several ~m are formed with a predetermined spacing between each other by etching one surface of the N-type GaAs substrate 22 having the electrode 25 formed on the opposite surface. As an etchant, a tartaric acid-hydrogen peroxide-water type etchant, for example, may be used Next, as shown in Figure 3(b), the epitaxially grown layers 23 including the cladding layer 31, the active layer 32, the cladding layer 33, the current blocking layer 34, and the contact layer 35, having their predetermined shapes are disposed on the substrate 22 of Figure 3(a). In this step, the layer 23 are formed conformal to the surface contour of the substrate 22, and, therefore, groove-like depressions 36 are formed in the wafer of Figure 3(b) at locations corresponding to the grooves 30.
Next, the wafer shown in Figure 3(b) is cleaved along lines I-I into separate laser chips. After that, the p-side electrode 24 is disposed on the contact layer 35 of each laser chip, and also an anti-reflection film is disposed on a resonator end surface of the chip, which results in a laser chip 21 shown in Figure 2. The laser chip 21 is then soldered .
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upside down onto the submount 27. Thus, a semieonduetor laser device shown in Figure 2 has been manufactured.
Figure 4 shows a cross-section of a semiconduetor laser device according to a second embodiment of the present invention. The structure of the semiconductor laser device of Figure 4 is substantially the same as the one shown in Figure 2, except that a semiconductor substrate 42 is mesa-shaped with its sloping side surfaces being straight.
Specifically, a laser chip 41 includes a semiconductor substrate 42, and a set of layers 43 epitaxially grown on the upper surfaee (shown facing downward in Figure 4) of the substrate 42. The epitaxial layers 43 includes a cladding layer 51 on the substrate 41, an active layer 52 on the cladding layer 51, another cladding layer 53 on the active layer 52, a current blocking layer 54 on the cladding layer 53, and a contact layer 55 on the layer 54. Electrodes 24 and 25 are disposed on the contact layer 55 and on the bottom surface of the substrate 42, respeetively. Generally, a dielectric film, such as an Al203 film and an Sio2 film, is disposed to provide an anti-refleetion film for a resonator end surface. The laser chip 41 of the semiconductor laser device shown in Figure 4 is also soldered upside down onto a submount 27. In this case, too, even if solder 28 rises up along the side surfaees of the laser chip 41, it only contacts the sloping surfaces of the contact layer 55 and does not cause electrical short-cireuiting of the Pn junetion.
The laser chip of the semieonductor laser device shown in Figure 4 is produeed by first forming V-shaped grooves in the substrate 42 by etching. However, depending on the relation of the direction of extension of the grooves to the crystallographic orientation of the substrate, the cross-sectional shape of the grooves may not become V-shaped but dovetail-shaped. If layers necessary for generating laser oscillations are epitaxially grown on such a substrate with dovetail-shaped grooves etehed therein, the resulting laser ehip will have a structure shown in Figure 5. It is likely , , '' ' ~:

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208~754 that epitaxial layers 63 may not uniformly grown on such a substrate 62 having dovetail-shaped grooves. If there is a possibility that dovetail-shaped grooves could be formed in a substrate due to the direction in which the grooves to extend, an etchant of different composition should be used.
For example, if dovetail-shaped grooves are formed when a tartaric acid-hydrogen peroxide~water type etchant of a certain composition is used, a similar type etchant which has a different composition may be used to form desired V-shaped grooves in a substrate.
Figure 6 is a cross-sectional view of a semiconductor laser device according to a third embodiment of the present invention. A laser chip 61 shown in Figure 6 includes a semiconductor substrate 72 having a flat surface (shown facing downward), as in the device shown in Figure 4. A cladding layer 81, an active layer 82, a cladding layer 83, a current blocking layer 84, and a contact layer 85 are epitaxially grown in the named order on the flat surface of substrate 72 by, for example, MOCVD. Then, the opposite sides of the epitaxially grown layers 73 are etched to form sloping surfaces so that a generally mesa-shaped chip is formed, as shown. Ions of, for example, boron or hydrogen are implanted through the thus exposed, sloping portions of the epitaxially grown layers 73 to thereby form semi-insulating regions 86. As in the case of the previously described embodiments, an anti-reflection film of a dielectric material, such as Al 203 and Sio~, is disposed on a resonator end surface.
The materials of the semiconductor substrate 72 and the epitaxial layers 73 of the semiconductor laser device of Figure 6 may be, for example, the same materials as used for the device shown in Figure 2. As in the case of the previously described embodiments, the laser chip 61 is placed upside down and soldered to a submount 27. Even if some solder 28 rises up along the side surfaces of the chip 61 during the soldering step, there is no fear that the PN junction is short-circuited so that no laser oscillations occur or no leakage current 20887~

flows, because the exposed surface portions of the epitaxial layers 73 which would be in contact with solder 28 are semi-insulating.
The laser chip 61 of the semiconductor laser device shown in Figure 6 may be manufactured in the following manner. As shown in Figure 7ta), on one surface of the flat substrate 72 having the electrode 25 pre-fabricated on the other surface, the epitaxial layers 73 including the cladding layer 81, the active layer 82, the cladding layer 83, the current blocking layer 84, and the contact layer 85 of a predetermined configuration are disposed by means of, for example, MOCVD.
Next, as shown in Figure 7(b), grooves 80 of, for example, U-shape, having such a depth that the bottom reaches at least the the cladding layer 81, are formed at predetermined locations in the surface of the epitaxial layers 73. The grooves may be V-shaped. Then, ions 88 of, for example, boron or hydrogen are implanted selectively through *he grooves 80 to thereby make semi-insulating the portions of the epitaxial layers 73 along the inner surfaces of the grooves. After that, the wafer is cleaved along the center lines II-II of the respective grooves 80 into separate laser chips. Thereafter, the p-side electrode 25 is disposed on the contact layer 85 of each laser chip, and an anti-reflection film is disposed on the resonator end surface. Thus, the laser chip 61 as shown in Figure 6 results. The laser chip 61 is soldered upside down to the submount 27, which completes the semiconductor laser device shown in Figure 6.
Because ions 88 of, for example, boron or hydrogen, are implanted into the grooves 80 in the epitaxially grown layers 73, the energy required for forming the semi-insulating regions 86 can be significantly smaller than the energy required for forming the semiconductor laser device shown in the previously discussed Japanese Unexamined Patent Publication No. HEI
1-215088.
In the device of Figure 6, a buffer layer may be additionally disposed between the cladding layer 83 and the current blocking layer 84. Further, other than GaAs type materials, InP type materials can be used as materials for the layers 73 as well as the substrate 72.
As described in detail, in all of the embodiments of the present invention, a PN junction in the epitaxially grown layers of a laser chip is never short-circuited by solder which could rise up along side surfaces of the laser chip when the laser chip is soldered upside down to a submount. Accordingly, the yield is greatly improved. Further, leakage current is minimized. In addition, heat generated by the laser chip can be efficiently dissipated into the submount 27, and, therefore, the device can operate with stability for long time.

Claims

1. A semiconductor laser device including a laser chip soldered upside down onto a submount, said laser chip comprising:
a mesa-shaped semiconductor substrate; and a plurality of epitaxially grown layers necessary for producing laser oscillations disposed on the upper surface of said substrate, said epitaxially grown layers being conformal to the contour of the upper surface of said substrate;
the laser chip portion on the epitaxially grown layer side thereof being soldered to said submount.

2. A semiconductor laser device including a laser chip soldered upside down onto a submount, said laser chip comprising:
a semiconductor substrate having a flat surface; and a plurality of epitaxially grown layers necessary for producing laser oscillations disposed on said surface of said substrate, said epitaxially grown layers being substantially mesa-shaped and having semi-insulating films on their opposite side portions;
the laser chip portion on the epitaxially grown layer side thereof being soldered to said submount.

3. A method of making a semiconductor laser device of Claim 1, comprising the steps of:
forming grooves in a flat surface of a semiconductor substrate, said grooves extending in a predetermined direction with a predetermined spacing between adjacent grooves, and having a predetermined depth;
epitaxially growing a plurality of layers necessary for producing laser oscillations on said substrate surface and on the inner surfaces of said respective grooves;
cleaving said semiconductor substrate having said plurality of epitaxially grown layers formed thereon along said grooves into separate laser chips: and soldering the portion of said laser chip on the epitaxially grown layer side thereof to a submount.

4. A method according to Claim 3 wherein the cross-section of said grooves formed in the surface of said semiconductor substrate is U-shaped.

5. A method according to Claim 3 wherein the cross-section of said grooves formed in the surface of said semiconductor substrate is V-shaped.

6. A method of making a semiconductor laser device of Claim 2, comprising the steps of:
epitaxially growing a plurality of layers necessary for producing laser oscillations on a flat surface of a substrate:
forming grooves in a flat surface of a semiconductor substrate, said grooves extending in a predetermined direction with a predetermined spacing between adjacent grooves, said grooves having such a depth as to extend to at least that one of said epitaxially grown layers closest to said substrate;
implanting ions, such as boron ions and hydrogen ions, through said grooves to form semi-insulating regions in the portions of said layers along the inner and bottom surfaces of said grooves:
cleaving said semiconductor substrate with said plurality of epitaxially grown layers along said grooves into separate laser chips; and soldering the portion of said laser chip on the epitaxially grown layer side thereof to a submount.

7. A method according to Claim 6 wherein the cross-section of said grooves formed in the surface of said semiconductor substrate is U-shaped.

8. A method according to Claim 6 wherein the cross-section of said grooves formed in the surface of said semiconductor substrate is V-shaped.