JPS6238875B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for manufacturing a semiconductor laser device.

BACKGROUND TECHNOLOGY In recent years, the major problem of semiconductor laser device deterioration has been largely solved, and devices with a lifespan of several thousand to tens of thousands of hours can now be easily obtained.
It has come to be used in optical technology application systems such as optical communication and optical information processing.

There are various types of such semiconductor laser devices.

Problems to be Solved by the Invention In order to put a semiconductor laser device into widespread practical use, it is necessary not only to extend its life span, but also to significantly improve its performance. To achieve this, the requirements include spot-like oscillation in the same mode both vertically and horizontally, the spread of the light beam to be small, the second-order distortion in the current-light output characteristics to be extremely small, and the operating current to be small. The challenge is how to satisfy this.

In conventional striped semiconductor laser devices, multi-mode oscillation occurs as the current increases, and the linearity of the current-optical output characteristics is very poor due to the occurrence of kinks.

The present invention aims to provide a method for easily manufacturing a semiconductor laser device that solves these problems.

Means for Solving the Problems The method of the present invention involves forming two ridges parallel to each other on the surface of a semiconductor substrate, forming a striped groove between them, and then forming the two ridges. A cladding layer is formed on the semiconductor substrate by making the thickness on the two ridges thinner than the thickness on other regions and curving the surface above the striped grooves downward, and further forming the active layer on the cladding layer. above,
The thickness on the ridge is thinner than the thickness on other areas.

Effect When a ridge is formed as described above and a cladding layer is formed on the ridge forming surface, the growth rate of the cladding layer is significantly higher on the side surfaces of the ridge than on the top surface of the ridge or the main surface of the substrate.
Therefore, the cladding layer is formed to be thicker in the vicinity of the outer surface of the ridge pair than on the further outer side. Therefore, when an active layer is formed on the cladding layer, the growth rate of the active layer on the sloped part of the cladding layer is higher than that on the other substantially flat part, so that the active layer becomes active. The layer is formed to be thicker near the outside of the pair of ridges than on the top surface of the ridge or on the substrate farther from the ridge.

When an electrode is placed above the striped groove between the ridges and electricity is applied to emit light, the thickness of the cladding layer on the top surface of the ridge is thin, allowing the light to be absorbed by the substrate in this area. . Furthermore, since the cladding layer and the active layer are formed thick near the outer surfaces of the bridge pair, the spread of current is effectively suppressed. Therefore, the light emitting portion is limited to a very narrow area.

Example Hereinafter, an example of the method of the present invention will be described.
This will be explained using figures.

First, as shown in FIG. 1A, an n-type GaAs substrate 1 is prepared, and a pattern of 15 μm in width with a spacing of 5 μm is formed on one main surface of the substrate using a normal photoetching technique.
A pair of striped pattern masks of 250~
Formed with a pitch of 300μm. Then, using this mask, the substrate 1 is exposed to H ₂ O:H ₂ O ₂ :H ₂ SO ₄ =
1.5-1.7μm with 8:8:1 sulfuric acid etching solution
Have sex to the depth of By this selective etching, the ridges 1a and 1b are formed in two parallel protruding shapes on a substantial plane.

In the above-mentioned low-viscosity sulfuric acid-based etching solution, the reaction rate-determining process is dominant, and not only the etching depth is proportional to the etching time, but also the etching speed is relatively slow, so the height of the ridges 1a and 1b is Easy to control.

Next, a double heterostructure is formed on the n-type GaAs substrate 1 having the ridges 1a and 1b by liquid phase epitaxial growth. The mechanism of crystal growth on a substrate with ridges 1a, 1b is different from the mechanism of crystal growth on a flat substrate. That is, in a substrate with ridges 1a and 1b, the growth rates on the top surface of the ridge portion and other flat surfaces are different even if they are the same (100) plane. For example, 250μm
In a substrate having a single ridge with a width of 15 μm on a pitch, the growth rate on the top surface of the ridge is about 1/6 of that on a flat surface. This is because a portion of the ridge melts back, and the growth in the ridge portion is more dominant on the (111) side surface than on the (100) top surface. Therefore, the ridges 1a and 1b of the substrate 1
The crystal growth rate is also higher on the intermediate portions than on the other main surfaces of the substrate 1 or the top surfaces of the ridges 1a and 1b. Taking advantage of such crystal growth properties, n
type Ga _1-x Al _x As cladding layer 2, Ga _1-y Al _y As active layer 3, p-type Ga _1-x Al _x As cladding layer 4 and p-type
GaAs layers 5 are sequentially grown. For an infrared laser, the values of x and y are 0.3 and 0.05, respectively.

The growth process of n-type Ga _1-x Al _x As layer 2 is as follows:
The difference between the sides of ridges 1a and 1b and the other surfaces is that when growth is performed for 4 minutes at a growth start temperature of 800°C and a cooling rate of 0.5°C/min, the thickness is 0.3 μm on the top surface of ridges 1a and 1b, and 0.3 μm on the top surface of ridge 1a. , 1.8μ in the part between 1b
A film thickness of m is obtained.

The rate of crystal growth on the portion of the substrate 1 between the ridges 1a and 1b varies depending on the height of the ridges 1a and 1b. That is, when the ridges 1a and 1b are low (about 1 μm or less), they grow to the same height as the ridges 1a and 1b, but when the ridges 1a and 1b are about 1 μm or more, they grow in a concave shape as shown in FIG. 1B. becomes.

A very thin Ga _1-y Al _y As active layer 3 is grown on this n-type Ga _1-x Al _x As layer 2 . In this case as well, the growth rate on the ridges 1a and 1b is slow, which is advantageous for thin film growth, and a film thickness of 0.1 μm can be obtained with good reproducibility. On the outer parts of the ridges 1a and 1b of the cladding layer 2, that is, on the sloped parts, the growth rate of the active layer 3 is faster than that on the flat surface, so the thickness of the active layer 3 is smaller than that on the top of the cladding layer 3. It is larger on the slope than on the surface. Ritsuji 1a, 1
The thickness of the active layer 3 between ridges 1a and 1b is slightly thicker than that on the top surfaces of ridges 1a and 1b, and the active layer 3 has a substantially flat surface. Further on top of the active layer 3 is a p-type layer with a thickness of 2 μm.
A Ga _1-x Al _x As layer 4 and a 1 μm thick p-type GaAs layer 5 are sequentially formed. When using hetero isolation as a striped structure, the thickness is further increased by 1
μm n-type Ga _0.5 Al _0.5 As layer 6 is required, but in the case of oxide film stripes, after growth, CVD
A film is formed by a sputtering method or a sputtering method (FIG. 1C). Here, a hetero isolation stripe structure will be explained. An oxide film is formed as a mask on the wafer shown in FIG. 1C, and a window with a width of 5 μm is formed directly above the ridges 1a and 1b by photoetching. Furthermore, a window is opened in the n-type Ga _0.5 Al _0.5 As layer 6 by selective etching using hot phosphoric acid, iodine etchant, etc. until the p-type GaAs layer 5 is exposed in a striped pattern. After additional diffusion with Zn and forming a p ⁺ layer in the window to improve contact with the electrode, Ti−Pt−
A positive electrode 8 is formed by depositing Au,
Furthermore, the back side is etched so that the total thickness is approximately 100 μm. H ₂ O is the solution for back side etching:
A solution of H ₂ O ₂ :H ₂ SO ₄ =1:1:8 is used. After etching, Au-Ge-Ni is deposited on the back surface to form a negative electrode 7 (FIG. 1D).

The characteristics of the laser device manufactured as described above will be described below. As shown in Figure 2, the ridges 1a, 1
The height d ₂ outside b and the depth d ₁ of the groove do not necessarily have to match, but as an example, d ₁ = d ₂ =
We created a laser using a 1.5 μm substrate.

When a voltage is applied between the positive electrode 8 and the negative electrode 7 and the semiconductor laser device is energized, the active layer 3
Single spot laser oscillation occurred on the groove between the ridges 1a and 1b. The relationship between the optical output and the current had good linearity. and,
The operating current required for the oscillation may also be small. This corresponds to the cladding layer 2 on the top surface of the ridges 1a and 1b.
Since the thickness of the cladding layer 2 and the active layer 3 are extremely thin, the light is absorbed by the substrate 1 in this part, and the oscillation part is specified. This is thought to be because the current spread is effectively suppressed because it is large in the vicinity.

The threshold current density for laser oscillation is 1
It depends on the dimensions of a and 1b. This will be specifically explained below.

Figure 3 shows the oscillation threshold current density J _th and the width of ridges 1a and 1b when the groove width W ₁ is 5 μm.
It shows the relationship with W ₂ and W ₃ . In the figure, the solid line is W ₂
is the same width as W ₃ (W ₂ = W ₃ ), and the dashed line is
It shows the change in J _th when W ₂ =100 μm constant. The change in J _th when W ₂ was set to 100 μm or more was almost the same as the broken line. From this result, it can be seen that J _th is almost constant when W ₃ is 100 μm or more, but J _th monotonically decreases with W ₃ when W ₃ <100 μm. However, W ₃ is 1μ
If it is formed narrower than m, the laser oscillation will not be in the form of a single spot. Therefore, practically, it is preferable that the width is 1 to 100 μm.

Regarding the distance between the ridges 1a and 1b, it is 1
When it becomes narrower than μm, the generated light becomes narrower than 1 μm.
Since the ratio of absorption into the substrate 1 through a and 1b increases, the threshold current density for laser oscillation increases. Furthermore, when the interval is wider than 10 μm, the oscillation threshold current density becomes high. In practical use, it is desirable that the width of the ridges 1a, 1b be within the range of 1 to 10 μm.

In Figure 4, d ₂ = 1.5 μm, d ₃ = 1.8 μm, W ₃ =
This figure shows the change in J _th due to d ₁ when W ₂ =1 μm. As shown in the figure, d ₁ is from 0.7μm to 1.7
Although J _th hardly changes in the range of about μm,
When d ₁ is 0.7 μm or less, J _th increases sharply. This is because when d ₁ becomes 0.7 μm or less, the ratio of light generated directly above the groove between the ridges 1a and 1b being absorbed by the n-GaAs substrate 1 increases, and the degree of absorption increases between the active region and the n-GaAs substrate 1. The shorter the distance to the substrate 1, the larger the distance. On the other hand, if d ₁ is 1.7 μm or more, the series resistance in the groove portion becomes high, making it difficult for current to flow through the region of the active layer 2 located in the groove portion.

Effects of the Invention In the method of the present invention, one main surface side of a semiconductor substrate is formed into a shape in which a pair of ridges protrudes parallel to a substantially flat surface, and then the ridges are formed on the substrate surface having these ridges. A cladding layer and an active layer are sequentially formed. Therefore, the cladding layer is thin on the top surface of the ridge, and is easily formed thicker in the vicinity of the outer surfaces of the paired ridges than in the portions farther from the ridges. Also, the active layer easily grows thicker in the vicinity of the outer surfaces of the paired ridges than in the parts further away. Therefore, according to this method, not only is it very easy to implement, but also the laser oscillation part can be limited to an extremely narrow area, so that not only can single mode oscillation be achieved, but also the operating current can be reduced. It is possible to manufacture a small number of semiconductor laser devices.

[Brief explanation of the drawing]

1A to 1D are cross-sectional views for explaining the manufacturing process of an embodiment of the method of the present invention, and FIG. 2 is a cross-sectional view for explaining the characteristics of a semiconductor laser device obtained by the method of the present invention. The enlarged cross-sectional views of FIGS. 3 and 4 are diagrams showing the relationship between the ridge dimensions and the threshold current density of the device. 1... n-GaAs substrate, 2... n-Ga _1-x Al _x As
Layer, 3...n-Ga _1-y Al _y As layer, 4...p-
Ga _1-x Al _x As layer, 5... p-GaAs layer, 6... n-
Ga _0.5 Al _0.5 As layer, 7... n-side electrode, 8... p
side electrode.

Claims (1)

[Claims] 1. Forming two parallel ridges on the surface of a semiconductor substrate and forming a striped groove between the ridges, forming a cladding layer on the semiconductor substrate having the two ridges, and forming a cladding layer on the semiconductor substrate having the two ridges. a step of forming an active layer on the cladding layer by making the thickness thinner than the thickness on other regions and curving the surface above the striped grooves downward;
and forming a current inflow portion above the active layer above the striped groove. A method for manufacturing a featured semiconductor laser device.