JPS6244439B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to increasing the optical output of a semiconductor laser.

In injection type double heterojunction semiconductor lasers used for high-speed optical fiber communications, video/audio disks, etc., a fundamental mode is required as a transverse mode in the direction horizontal to the active layer (hereinafter referred to as a horizontal transverse mode).

Various striped structures such as a buried hetero structure and a transverse junction structure (TJS) are known as examples of semiconductor lasers for obtaining this fundamental transverse mode. However, in order to stably obtain the fundamental mode of these semiconductor lasers, the mode size has to be small, around 2 to 3 μm, so the continuous wave optical output that can operate stably is limited to around 1 mW, and even momentarily to around 10 mW. Put it away. This is because the reflective surface is altered and destroyed by the high-density light output. Therefore, in the basic mode operation with a small spot size, the optical output is small, which has been a major obstacle in various applications including high-speed and long-distance optical fiber communications.

An object of the present invention is to provide a semiconductor laser device that can stably output a large optical output even in basic horizontal transverse mode operation with a small spot size.

The semiconductor laser of the present invention is an improved double-hetero structure TJS semiconductor laser in which the active layer around the selective diffusion film becomes the light emitting region by performing selective diffusion of impurities deeper than the active layer. The structure is such that the excitation region does not come into direct contact with the reflection surface, and the effective bandgap of the excitation region is smaller than the effective bandgap of the non-excitation region in the active layer that exists between the excitation region and the reflection surface. ing.

The principle of the present invention is conveniently applied to the horizontal transverse mode control of a structure already filed by the present inventors (Japanese Patent Application No. 72441/1983).

Optical damage that destroys the reflective surface when the optical output of a semiconductor laser is increased is said to be caused by temperature rise on the reflective surface or stimulated Brillouin scattering, and approximately
It occurs at an optical power density on the order of 10 ⁶ W/cm ² . However, the inventors have found that if the absorption coefficient for laser light is reduced in the active layer near the reflective surface, optical damage becomes less likely to occur, and it can withstand an optical output density of 10 ⁷ W/cm ² , that is, approximately an order of magnitude higher optical output density. This has recently come to light. Therefore, if this principle is applied to a structure that causes fundamental horizontal transverse mode oscillation, which reduces the spot size, the above-mentioned problem of small optical output can be solved.

It is known that the effective band gap of binary, ternary, and quaternary - group semiconductors differs depending on the type and concentration of impurities. Taking GaAs as an example, the higher the n-type concentration, the larger the effective bandgap, and the higher the p-type concentration, the smaller the effective bandgap. Also p-
The reduction in the effective bandgap is remarkable for the n-compensated p-type, and is especially remarkable for the p ⁺ -n ⁺ compensated p ⁺ -type. Therefore, depending on the combination of impurity type and concentration, the active layer in the oscillation region (excitation region) can be brought into a state with a small effective band gap, and the active layer in the region adjacent to the reflective surface (non-excitation region) can be brought into a state with a large effective band gap. Can be done. Since the photon energy of the laser beam is approximately equal to the excitation region band gap, by doing this, the absorption coefficient for the laser beam becomes small in the active layer near the reflecting surface. As a result, optical damage is less likely to occur, and an optical output that is about one order of magnitude higher than that of the conventional method can be stably obtained. Furthermore, since this structure is a TJS structure, the fundamental horizontal transverse mode can be obtained most effectively.

Next, the present invention will be explained with reference to the drawings.

Figure 1 shows an example of applying Zn diffusion to an (Al/Ga)As double heterojunction laser. In the top view of FIG. 1a, Zn diffusion is applied to region 6,
The boundary 10 between this Zn diffusion region 6 and the non-diffusion region,
11 is bent near the reflective surface 9. length 250
The boundary area 11 provided near the reflective surface 9 perpendicular to the straight boundary area 10 at the center of μm is
10μm apart. The resonator length between the reflective surfaces 9 is 300
It is μm.

FIG. 1b shows a cross-sectional structure taken along the center dotted line A-A' of FIG. 1a. The first layer of n-
Al ₀ . ₃ Ga ₀ . ₇ As layer (thickness 5 μm, carrier concentration 2
×10 ¹⁷ cm ^-3 )2, second n-GaAs active layer (thickness 0.15 μm, carrier concentration 3 ×10 ¹⁸ cm ^-3 )3,
A third n-Al _{0.3 Ga 0.7} As layer (3 μm thick, carrier _{concentration} 2×10 ¹⁷ cm ⁻³ ) 4 is provided.
A Si ₃ N ₄ film 5 is formed on _{the third n-Al 0.3 Ga 0.7} As layer ₄ in the shape shown in FIG. Zn is selectively _diffused at _, for example, ₆₅₀ °C using _a diffusion mask (formed in Area 6). Since the hole concentration near the tip of the Zn diffusion region is as high as approximately 1×10 ¹⁹ cm ^-3 , there are many non-emissive components, making it difficult for laser oscillation. For this reason, further heat treatment is performed at a higher temperature, for example, 900° C., to move the tip of the Zn diffusion region by about 2 to 3 μm. As a result, the average hole concentration in the region 7 where Zn has moved in the active layer 3 becomes approximately 5×10 ¹⁸ cm ⁻³ , and the luminous efficiency is improved, making laser oscillation easier. A p-type ohmic electrode 8 of Au/Cr and an n-type ohmic electrode 8' of Au/Au--Ge--Ni are provided in a conventional manner. A current is injected from the n-type region in the active layer 3 to the Zn pushed region 7, which becomes a light-emitting and oscillating region.

In FIG. 1a, the central linear region (excitation region) 10 that becomes the laser oscillation region has a total of 50 μm of active layer 12 (hereinafter referred to as the non-excitation region) in which Zn is not diffused up to the reflection surface 9. ). The non-excited region 12 is 3×10 ¹⁸ cm ⁻³ n ⁺ while the excited region 10 is 5×10 ¹⁸ cm ⁻³ p ⁺ −n ⁺ impurity-compensated p ⁺ . The effective bandgap of the excited region 10 is approximately larger than that of the non-excited region 12.
40 meV small. Therefore, for the laser beam generated in the excitation region 10, the absorption coefficient of the non-excitation region 12 is 20.
It is extremely small at ~30 cm ^-1 . For this reason, the excitation region 1
Laser oscillation easily occurs on a straight line along 0.

When continuous oscillation is performed at room temperature with this structure, no optical damage will appear on the reflective surface even if an optical output of about 100 mW in the fundamental horizontal transverse mode is produced. On the other hand, a structure without the non-excitation region 12, that is, the reflective surface 9 is in the excitation region 1
For structures in direct contact with zero, optical damage occurs at approximately 15-20 mW.

Note that oscillation does not occur along the excitation region 10 and region 11. This is because the radiation loss at the curved portion is large. Therefore, the region 11 is not parallel to the excitation region 10, as shown in FIG.
It may be in any shape, such as intersecting with. Further, oscillation does not occur on a straight line in the region 11. In this case, the central part has a p ⁺ of 1 × 10 ¹⁹ cm ^-3 or more
This is because they are old and difficult to emit light.

In this structure, current also flows in region 11, and the active layer there is exposed on reflective surface 9. Further, the reflective surface 9 from which the laser beam 13 is emitted is also likely to be oxidized by the spontaneously emitted light and the laser beam. Therefore, for long-term stable operation, SiO ₂ , Al ₂ O ₃ , Si ₃ N ₄ , C
It is desirable to provide a surface protective film such as the above on the reflective surface 9.

Although the distance between the excitation region 10 and the region 11 is set to 10 μm in FIG. 1, it is sufficient that the distance is at least such that the laser beam does not couple to the region 11.

FIG. 2 shows another embodiment in which the area through which current flows is restricted by proton irradiation to reduce the operating current. FIG. 2b is a cross-sectional view similar to FIG. 1b. Unlike the first embodiment, the Zn diffusion region 20 has a rectangular shape as shown in FIG. 2a. If this continues, the current will flow all over the periphery of the rectangle, so both the threshold current and the operating current will become large. Excitation region 21 This region was created in the same manner as in the first embodiment, so if proton irradiation is applied to the Zn diffusion region 20 side at a distance of about 10 μm deeper than the Zn diffusion depth, this proton irradiation region 22 is completely electrically isolated. be converted into Therefore, the current effectively flows almost only in the vicinity of the excitation region 21. In addition, in high temperature operation, in the structure shown in Fig. 1, _{the first layer n-Al 0.3 Ga 0.7 As}
Although more current flows through the wide-area pn junction present in layer 2, increasing the threshold current, the structure of FIG. 2 is advantageous because its area is small. As a result, the threshold current is reduced to about 70% compared to the structure shown in Figure 1, allowing continuous oscillation at higher temperatures. In order to avoid this in FIG. 1, it is necessary to increase the Al composition ratio of the first layer 2.

Note that the distance between the proton irradiation region 22 and the excitation region 21 is approximately 5 μm as the proximity limit, and as the distance approaches closer than this, a portion of the protons enters the excitation region 21, reducing luminous efficiency. The proton irradiation depth may be shallower than the front surface of the Zn _{diffusion in the n-Al 0.3 Ga 0.7} As layer ₂ of the first layer. In this case, the greater the electrical resistance seen from the p-type ohmic electrode 8, the more effective the proton irradiation, so the deeper the proton irradiation depth, the better. Further, in FIG. 2a, the proton irradiation region 22 may be formed to have approximately the same length as the Zn diffusion region 21 in the longitudinal direction of the resonator.

FIG. 3 shows a third embodiment, which has a structure in which the same purpose as in FIG. 2 is achieved using a semi-insulating GaAs substrate 30. Since the Zn diffusion region 31 reaches the semi-insulating GaAs substrate 30, the current flowing through the p-type ohmic electrode 33 and the n-type ohmic electrode 34 separated by the Si ₃ N ₄ film 32 is shown in FIG. It flows laterally through the epitaxial layer. Therefore, as in FIG. 2, it is possible to lower the threshold current and operate at a higher temperature. Laser oscillation naturally occurs along the excitation region 35. In addition, the board 30 is attached in order to adhere to the case.
A metal 36 such as Au/Au-Ge-Ni is deposited on the surface.

FIG. 4 shows a fourth embodiment, which has a structure in which the same purpose as in FIG. 2 is achieved by mesa etching or cleavage instead of proton irradiation. In the case of mesa etching or cleavage (side surface 41) in which the crystal is mesa etched or cleaved approximately 20 .mu.m away from the excitation region 40 and approximately parallel to it using the layer structure of FIG. 1, it is sufficient that the depth is deeper than the Zn diffusion depth.

In addition, in FIGS. 2, 3, and 4, it is desirable to provide a surface protective film on the reflective surface 9 in order to ensure long-term stable operation.

In the above embodiments, the lengths of the excited region and non-excited region, the Al composition of each layer, the layer thickness, and the carrier concentration are not limited to the above. The effects of the present invention can be sufficiently exhibited if the effective band gap difference between the excited region and the non-excited region is several meV or more. It is sufficient that the length of the non-excited region is longer than the diffusion length of the injection carrier, but if it is too long, the laser beam will be diffracted between the excitation region and the reflective surface, and some of the light reflected by the reflective surface will be lost in the excitation region. The rate of return to Therefore, the threshold current increases. Regarding the layer structure, although the most basic three-layer structure has been described, various structures are possible. 1st
In Figure b, there is a _{structure in which there are three layers of n, p, and n-Al 0.3 Ga 0.7 As instead of the} third n- _{Al 0.3 Ga 0.7} As layer 4; The GaAs substrate 1 is a p-GaAs substrate, and the first n-Al _0.3 Ga _0.7 As layer ₂ is replaced by n,p or n,p,n- _{Al 0.3 Ga 0.7 As} . Similar effects can be obtained with any structure, such as a structure in which the Zn diffusion depth reaches the p-GaAs substrate, and a structure in which the n-type electrode is taken out from the top surface of FIG. 1b.

In the example, forced diffusion of Zn was used to form the excitation region. At this time, the temperature and time can be selected arbitrarily. In addition to the forced diffusion method, the excitation region may be formed by first performing medium concentration diffusion and then performing slightly shallow high concentration diffusion. It is also possible to achieve this by only medium concentration diffusion. The diffusion impurity may be Cd instead of Zn, and the p-type region may be formed using an ion implantation method without relying on thermal diffusion. Although Zn diffusion has been taken as an example above, it is clear that the same type of structure can be realized using n-type impurity diffusion such as Si or Se, ion implantation, etc.

Also, GaAs was used as an example for the active layer.
Al _y Ga _1-y As may also be used. Furthermore (InGa) (As P)/
It goes without saying that it can be easily applied to quaternary double heterojunction crystals such as InP, (AlGa) (AsSb)/GaSb, etc.

[Brief explanation of the drawing]

FIG. 1 shows the most typical embodiment of the present invention.
A is a top view, and b is a sectional view taken along the line A-A' of a. 1: n-GaAs substrate, 2: first layer n-
Al _0.3 Ga _0.7 As layer _, 3: second layer n- _GaAs active layer,
4: Third layer n-Al _{0.3 Ga 0.7} As layer, ₅ : Si ₃ N ₄ film,
6: Zn diffusion region, 7: Zn indentation region (light emitting region), 8: Au/Cr p-type ohmic electrode, 8':
Au/Au-Ge-Ni n-type ohmic electrode, 9: reflective surface, 10: excitation region, 11: bending region, 12:
Non-excited region, 13: laser light. FIG. 2 shows a second embodiment of the present invention using proton irradiation, in which a is a top view and b is a sectional view taken along line A-A' of a. 20: Zn diffusion region, 21: excitation region, 22:
Proton irradiation insulating region. FIG. 3 shows a third embodiment of the present invention, in which a is a top view and b is a sectional view taken along line A-A'. 30: Semi-insulating GaAs substrate, 31: Zn diffusion region, 32: Si ₃ N ₄ film, 33: Au/Cr p-type ohmic electrode, 34: Au/Au-Ge-Ni n-type ohmic electrode, 35: excitation region , 36:Au/Au−Ge
−Ni metal layer. FIG. 4 shows a fourth embodiment of the present invention, in which a is a top view and b is a sectional view taken along line A-A'. 40: Excitation region, 41: Mesa etching side surface.

Claims (1)

[Claims] 1 In a semiconductor laser with a transverse junction structure, an impurity doped region is selectively provided deeper than the active layer in a semiconductor material having a double heterostructure, and the region around the impurity doped region becomes an excitation region. The effective bandgap of the linear excitation region is greater than the effective bandgap of the non-excitation region in the active layer that exists between the excitation region and the reflection surface, which is an extension of the excitation region without directly contacting the reflection surface. Characterized by small size. Large optical output fundamental transverse mode semiconductor laser device.