JPS6347356B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a novel laser device that uses a compound semiconductor and has a stable transverse mode and a low threshold.

Conventional configurations and their problems Converting semiconductor lasers into fundamental modes is essential to enable broadband, long-distance transmission, low-distortion analog modulation, etc. in optical fiber communications.

Control of the oscillation mode for this purpose was first achieved with so-called electrode stripe lasers, which confine optical energy and injection current to specific regions to minimize optical loss and wasteful recombination. Since then, various striped lasers have been developed and are still available today, but all of them have their own drawbacks and are unsatisfactory in terms of characteristics. For example, when only the current distribution is confined, such as in an electrode stripe laser, the laser light is mainly guided in the stripe direction by the gain distribution, but the waveguide effect due to this gain is unstable.
When the current is increased, high-order transverse mode oscillation easily occurs, and furthermore, the current-optical output characteristics are often distorted.
This is because the electrode stripe laser does not have a structure that confines carriers and light in the lateral direction of the active layer.

Therefore, in an attempt to overcome the above-mentioned drawbacks, a buried heterostructure as shown in FIG. 1 was created.
InP/InGaAsP semiconductors will be explained below as an example. In Figure 1, 1 is an n-InP substrate, 2 is an n-InP substrate, and 2 is an n-InP substrate.
-InP first cladding layer, 3 an InGaAsP active layer (for example, corresponding to a 1.3 μm emission wavelength), 4 a P-InP second cladding layer, and 5 an InGaAsP cap layer (for example, corresponding to a 1.05 μm emission wavelength). A mask is attached to such a four-layer structure epitaxial wafer by means of photolithography, unnecessary portions are removed in the shape of an inverted mesa by chemical etching, and P- An InP buried layer 6 and an n-InP buried layer 7 are formed.

In such a structure, the active layer has a large band gap on both the top and bottom and left and right sides, and has a small refractive index.
It is covered with an InP layer and has a narrow active layer width W ₁ of around 2 μm, which has the effect of confining light and carriers in the lateral direction, making it possible to obtain a semiconductor laser with a stable transverse mode. However, in the buried hetero laser, the active layer is already grown in the first epitaxial growth and is selectively etched chemically or exposed to air. Furthermore, this exposed active layer is exposed to high temperatures in the furnace during the second buried epitaxial growth, with the risk of thermal degradation. In addition, in order to reduce the electrical resistance at the electrode as much as possible, as shown in Figure 1, W ₁ < W ₂ , and if this shape is overetched, the area above the active layer will disappear. Furthermore, if etching is insufficient, W ₁ will not become narrow enough and higher-order modes will occur. As described above, buried hetero lasers have many problems such as instability in the manufacturing process, a reduction in essential crystallinity during the crystal growth process, and a reduction in manufacturing yield.

OBJECTS OF THE INVENTION The present invention provides a semiconductor laser with a novel DH laser structure that eliminates the drawbacks of the conventional buried heterostructure as described above, has a high yield, and enables control of the transverse mode.

Structure of the Invention In order to achieve the above object, three epitaxial layers of n-type, p-type, and n-type are provided on an n-type crystal substrate, and an epitaxial layer with a thickness at least as thick as the p-type layer is formed on the epitaxial substrate. The wafer is processed by means such as chemical etching into an inverted mesa stripe to a depth of Grow the cap layer.

Description of the embodiment As shown in FIG. 2, an n-type (001) InP substrate 11
On top, an n-type InP layer 12 with a thickness of 4 μm, a P-type InP layer 9 with a thickness of 2 μm, and an n-type InP layer 10 with a thickness of 2 μm are sequentially formed.
It was grown by liquid phase epitaxial growth. Next, 100 μm in the (110) direction of such an epitaxial wafer.
Attach SiO ₂ in stripes at intervals of 1HBr:
Using an etching solution consisting of 1HNO ₃ :5H ₂ O, unnecessary portions were selectively removed in an inverted mesa shape. This situation is shown in FIG. By using the above-mentioned etching solution, the degree θ in FIG. 3 becomes approximately 55°, that is, the etched plane becomes (111). Note that 8 is a SiO ₂ oxide film mask. What is important here is that after the active layer grows,
Part of the active layer is removed by exposure to a slightly unsaturated solution. That is,
When an active layer is liquid-phase epitaxially grown on an inverted mesa-shaped wafer, the growth rate is extremely faster at the periphery of the mesa than in other flat areas, and it is necessary to expose such a growing layer to an unsaturated solution. This makes it extremely easy to leave only the thick part around the mesa and remove the other parts. The active layer formed in this way changes from the conventional thin plate shape to a triangular prism shape, which reduces the beam spread angle due to diffraction, makes the light emitting spot almost circular, and creates an extremely stable transverse mode. This structure can be used to obtain oscillation.

Such an active layer is further formed with a P-type confinement layer and a P-type cap layer to form an epitaxial wafer for a laser device. First, an n-type active layer 15 is attached to the thus processed wafer by second liquid phase epitaxial growth. At this time, the active layer 15 has a flat portion A as shown in FIG.
The thickness of the corner portion B and the corner portion B are significantly different. In this example, 4.64 atm% of As and Ga were used for In2g.
Using a solution containing 0.83 atm% P and 0.14 atm%, the temperature was lowered from the growth starting temperature of 640°C at a cooling rate of 0.5°C/min.
When the second growth was performed, the flat area A shown in Fig. 4
The thickness was approximately 0.2 μm, but the thickest portion at corner B was 0.65 μm. The thickness ratio B/A is determined by the etching angle θ in Figure 3.
It was largest at 55°. Note that the composition of this grown layer is In _0.74 Ga _0.26 As _0.57 P _0.43 .

After forming such a growth layer, it was brought into contact for 2 seconds with a solution containing about 2 atm % of As based on In2g. By contacting with this solution and melting back, the flat part A of the active layer is removed, leaving only the active layer 15' at the corner part B, as shown in FIG. As can be seen from the figure, the shape of this active layer 15' is approximately triangular. An example of an enlarged view of this part is shown in FIG. a is approximately 0.8 μm, and b is approximately 0.5 μm. P-InP layer 13, P-In _0.89 Ga _0.11 As _0.23 P _0.77
Layer 14 was grown to obtain a wafer as shown in FIG.

The SiO ₂ oxide film 8 was removed from this wafer, and as shown in FIG. 8, an Au-Zn 5% ohmic electrode 16 was attached to the growth layer side, and an Au-Sn 10% ohmic electrode 17 was attached to the substrate side. From this weight, a chip with a resonator length of approximately 300 μm and width of 200 μm was cut out by cleavage and mounted on a Cu header plated with Au using In. of such an element
When a current was passed with the Au-Zn side set as + and the substrate side set as -, oscillation at a wavelength of 1.29 μm was exhibited at room temperature with a threshold value of 20 mA. At 25 mA or more, both the longitudinal and transverse modes became single. Furthermore, the beam divergence angle was approximately 20° both perpendicular to and perpendicular to the current direction, which was very close to a circle. In the above example, InP and InGaAsP were described, but GaAs and
It is also possible to use GaAlAs.

Effects of the Invention As described above, the semiconductor laser of the present invention has
It has the advantage of having a low threshold value, a stable mode, and a small beam divergence angle.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional buried laser according to an embodiment, and FIGS. 2 to 8 are diagrams showing the structure and manufacturing process of a semiconductor laser according to an embodiment of the present invention. 8...SiO ₂ layer, 9...P-type InP layer, 10...n
type InP layer, 11... n-type InP substrate, 12... n-type
InP layer, 13...P-InP layer, 14...P-In _0.89
Ga _0.11 As _0.23 P _0.77 layer, 15...n-type active layer.

Claims (1)

[Claims] 1. A first epitaxial growth layer is deposited on a (001) substrate surface, a (111) plane is formed on a part of the substrate, and the (001) substrate surface and (111) ) surface,
A semiconductor laser characterized by an embedded active layer. 2. The semiconductor laser according to claim 1, wherein the epitaxial layer includes at least one p-type semiconductor layer sandwiched between n-type semiconductors.