JPS6358390B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser whose oscillation transverse mode is stabilized and a method for manufacturing the same.

As a semiconductor laser structure that stabilizes the oscillation transverse mode, the thickness and shape of the active layer are changed during the epitaxial growth process, an optical waveguide region with a high equivalent refractive index is formed within the film plane, and this portion is used. Several structures for forming resonators of semiconductor lasers have been reported. In this type of structure, the optical waveguide can be formed with a width of about 2 μm or less, which is necessary for single transverse mode oscillation, but it is necessary to provide an injection current region with a limited width corresponding to this optical waveguide region. is difficult. Generally, after crystal growth, an electrode structure is used that uses an oxide film to limit the width of the injection current region or a structure that forms a selective diffusion region directly above the optical waveguide region. The width of the current region and the misalignment between the injected current region and the optical waveguide region cause variations in device characteristics such as oscillation threshold current.

The purpose of the present invention is to form an injection current limiting region in a positional relationship corresponding to the optical waveguide region during the epitaxial growth process, thereby reducing variations in device characteristics, and forming the active layer and the injection current limiting region in one step. An object of the present invention is to provide a semiconductor laser having a structure that is easy to manufacture by forming it through a crystal growth process.

According to the present invention, on a semiconductor substrate of a first conductivity type in which a mesa stripe narrowed by two adjacent parallel grooves is formed, a semiconductor substrate of a second conductivity type is laminated except for only the upper part of the mesa stripe. current blocking layer,
A cladding layer of a first conductivity type that is laminated with a substantially uniform thickness over the entire surface, and an active layer of the first conductivity type or a second conductivity type are sequentially formed and further laminated thereon. A semiconductor laser or the like having a structure characterized in that at least four cladding layers are formed is obtained.

Next, the present invention will be explained in detail using the drawings. FIG. 1 is a sectional view showing an embodiment of the semiconductor laser of the present invention. Width in <110> direction parallel to the resonator direction
3 μm deep on both sides of 2 μm mesa stripe 20,
Mesa substrate 1 (n
A p-type InP current blocking layer 2 (thickness at the flat part of 0.7 μm) is grown on the mesa stripe (<001> orientation), and an n-type InP cladding layer 3 is laminated over the entire surface. (Thickness at flat part 0.5μm)
A non-doped InGaAsP active layer 4 (thickness 0.2 μm, emission wavelength 1.3 μm) is laminated over the entire surface, and a p-type layer grows with decreasing film thickness just above the mesa stripe.
InP cladding layer 5 (thickness 2.5μm at flat part) and p-type layer stacked so that the surface is almost flat.
InGaAsP cap layer 6 (thickness at flat part 1μm)
are formed sequentially. Also, as an electrode, it is on the p side.
A p-side metal electrode 11 made of Au-Zn, and an n-side metal electrode 12 made of Au-Ge-Ni are formed on the n-side. When a bias voltage is applied to this device, with the p-side metal electrode 11 being positive and the n-side metal electrode 12 being negative, the p-n junction is connected to the InGaAsP active layer in the upper part of the mesa stripe 20 where the p-type InP current blocking layer 2 is interrupted. 4, current is injected into the InGaAsP active layer 4 directly above the mesa stripe 20, causing radiative recombination. However, since the region where the p-type InP current blocking layer 2 is formed forms a pnpn junction, almost no current flows below the turn-on voltage (approximately 5 V) of this pnpn junction.
Therefore, most of the injected current is concentrated in the InGaAsP active layer 4 directly above the mesa stripe 20. In addition, the InGaAsP active layer 4 has a mesa stripe 2
Since it is curved just above zero, the equivalent refractive index is high, and the optical waveguide 7 is formed at this portion. Therefore, since the region where the injected current is concentrated coincides with the optical waveguide 7, the injected current efficiently contributes to laser oscillation. A low oscillation threshold current of 20 mA at room temperature was obtained for a device with a cavity length of 250 μm. Since the optical waveguide 7 is formed in the upper part of the mesa, the stability of the oscillation transverse mode is good, and single fundamental transverse mode oscillation was exhibited up to about three times the oscillation threshold. Furthermore, in this device, the p-type InGaAsP cap layer 6 doped with a high concentration of p-type impurity is curved at the top of the mesa, so that the optical waveguide 7 is the shortest distance between the InGaAsP active layer 4 and the p-type InGaAsP cap layer 6. in the distance. Therefore, from the equipotential surface formed on the lower surface of the highly doped p-type InGaAsP cap layer 6,
The voltage drop in the p-type InP cladding layer 5 up to the active layer 4 is the smallest at the optical waveguide 7, and when the applied voltage is increased for high-output operation or high-temperature operation, the injected current flows through the path with the smallest voltage drop. The tendency of the current to flow becomes stronger, and the degree of concentration of the current from the p-side electrode side to the optical waveguide region 7 becomes higher. Due to the above factors, the temperature characteristics of this device are Ith∝
The characteristic temperature _T0 , expressed as exp(T/ _T0 ), was 70 to 80k, which was good, and the maximum cw temperature was 100°C or higher. These characteristics are similar among each element, and variations are small.

To summarize the device characteristics of the semiconductor laser according to the present invention, the oscillation threshold current is low because the optical waveguide 7 in the InGaAsP active layer 4 and the injection current limiting region formed on the upper part of the mesa stripe 20 overlap. In addition, the voltage drop in the pInP cladding layer 5 is the smallest at the top of the optical waveguide 7, so the temperature characteristics of the device are good, the maximum cw temperature is 100°C or more, and the variation between devices is small. .

Next, a method of manufacturing this device will be explained using FIG. 2. FIG. 2a shows an n-type InP substrate 100 (plane orientation <
001> shows a diagram in which a photoresist film 40 is formed. A photoresist stripe 41 having a width of 2 μm at the center and stripes 41 and 43 having a width of 5 μm on both sides and exposing the substrate surface are parallel to the <110> direction. Next, Br-methanol (concentration 0.4%)
41 and 43 stripes to a depth of 3μ.
Etching is performed to a depth of m to form a mesa stripe 20 and two grooves 31 and 32. This state is shown in FIG. 2b. FIG. 3c shows the mesa substrate 1 formed by peeling off the photoresist mask. Next, each layer is successively grown by liquid phase epitaxial growth. The crystal growth starting temperature is 630°C. 1st
The p-type InP current blocking layer 2 is grown from a two-phase solution with InP polycrystals floating on the In solution. Growth from a two-phase solution has a low degree of supersaturation, and growth on the mesa sides is fast, so p-type InP is grown on the top surface of the mesa stripe 20 in the center.
Current blocking layer 2 is not laminated. 2nd layer n-type
The InP block layer 3 is grown from a solution with a supersaturation degree of 10°C. In this case, the n-type InP block layer 3 is laminated with a substantially uniform thickness over the entire surface. next
The InGaAsP active layer 4 and the p-type InP gradient layer 5 are grown from a solution with a supersaturation degree of 5°C. When growing the p-type InP cladding layer 5, it is initially laminated in a shape that traces the InGaAsP active layer, but by the time the final film thickness is reached (2.5 μm at the flat part), the degree of supersaturation decreases.
In order to quickly fill the recesses above the trenches 31 and 32, the thickness of the p-type InP cladding layer 5 ultimately becomes the thinnest at the top of the mesa 20. Next, a p-type InGaAsP cap layer 6 is grown from a two-phase solution. In the growth of InGaAsP layers, the growth rate in the (001) plane is fast, so if the thickness of the flat part is about 1 μm,
The recesses are completely filled and the surface is flat. The state in which each layer is laminated is shown in FIG. 2d. This completes the growth process. Next, grow the crystal using the usual method.
It is polished to about 80 μm, and a metal electrode 11 of Au-Zn is formed on the p-side and a metal electrode 12 of Au-Ge-Ni is formed on the n-side. Finally, the wall is cut to a cavity length of approximately 250 μm to form a semiconductor laser chip.

The semiconductor laser and its manufacturing method according to the present invention have been described above. In the above embodiments, the InGaAsP active layer 4 was continuous throughout, but even if it was interrupted on both sides of the waveguide section 7 above the mesa. good. In addition, although InGaAsP was used as the semiconductor material, AlGaAs
It may be a system.

Finally, to enumerate the features of the present invention, the oscillation threshold current is low because the optical waveguide section and the injection current limiting region overlap, and the temperature characteristics are good because the voltage drop in the path flowing to the optical waveguide section is lower than that in the surrounding area. , small variations among each element, etc.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a semiconductor laser showing an embodiment of the present invention, and FIGS. 2a, b, c, d, and e are diagrams showing a manufacturing method of the embodiment shown in FIG. In the figure 1... (001) orientation n-type InP mesa substrate, 2
...p-type InP current blocking layer, 3...n-type InP cladding layer, 4...non-doped InGaAsP active layer,
5...p-type InP cladding layer, 6...p-type InGaAsP
Cap layer, 7... Optical waveguide in InGaAsP active layer, 11... P-side metal electrode, 12... N-side metal electrode, 20... Mesa stripe, 31, 32...
Groove portion, 40... Photoresist film, 41... Stripe of photoresist film, 42, 43... Stripe with exposed semiconductor surface, 100... (001)
It is an oriented n-type InP substrate.

Claims (1)

[Claims] 1. On a semiconductor substrate of the first conductivity type in which a mesa stripe narrowed by two adjacent parallel grooves is formed, a two-phase solution with a supersaturation degree small enough to prevent crystal growth above the mesa stripe is used. a first current blocking layer of a second conductivity type, which is laminated on the semiconductor substrate except for the top surface of the mesa stripe, and is located inside the two parallel grooves and is lower than the height of the mesa stripe. a second step of forming a first cladding layer of a first conductivity type that is laminated to a substantially uniform thickness over the current blocking layer and the upper surface of the mesa stripe; a third step of forming an active layer of a first conductivity type or a second conductivity type in a curved manner directly above the mesa stripe; and a second cladding layer of a second conductivity type laminated on the entire surface of the active layer. A method for manufacturing a semiconductor laser, characterized in that the first to fourth steps are successively performed in one growth furnace.