JPS6033320B2 - Semiconductor laser and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser, and particularly to a semiconductor laser having a reflective surface protection film for preventing deterioration of the reflective surface, and a method for manufacturing the same.

At room temperature, a GaAs-AIGaAs double heterostructure semiconductor laser can operate continuously for tens of thousands of hours or more at an output of a few mW to 1 mW, but in order to withstand practical use as a light source for optical communications, the output must decrease while the power is on. It is necessary to suppress the so-called deterioration phenomenon and realize continuous operation for 100,000 hours or more.

The main causes of deterioration of semiconductor lasers are the generation of dislocations inside the active layer due to energization and deterioration of the external reflective surface. Although the generation of dislocations causes rapid initial deterioration of semiconductor lasers, it can be suppressed by using a low dislocation density substrate and by crystal growth techniques by adding a small amount of N or P to the GaAs active layer. On the other hand, it has been confirmed that the phenomenon in which the optical output of a laser gradually decreases over a long period of time, such as one hour or more, is caused by deterioration of the reflecting surface. When a semiconductor laser is operated in an atmosphere containing oxygen, GaAs and oxygen react, and GaAs is
It has been found that the reflective surface of a ship is oxidized and an oxide film of GaAs is formed on the reflective surface. Such an oxide film causes deterioration of the reflective surface and, ultimately, deterioration of the laser. This surface oxidation reaction causes excited carriers in the active layer,
and facilitated by light. Therefore, in order to prevent such deterioration of the reflective surface, it is necessary to eliminate oxygen in the outside air, and it is necessary to cover the reflective surface with a dielectric protective film such as Sj○, Si02, AI203 film, etc. It is being done. However, since Sio has poor adhesion to the Ga carrier and is hygroscopic, it is insufficient as a protective film. Si0
2 has good adhesion with GaAs and is mechanically robust, so it has a protective function, but it also has the property of allowing Na ions to pass through easily, so Na ions in the atmosphere will adhere to the reflective surface.
Oxygen ions attracted to these Na ions cause oxidation of the reflective surface. Further, since Ga dissolves in Si02, during long-term operation, Ga on the reflective surface diffuses into Si02, the interface changes in quality, and the laser deteriorates. Si02
Since the expansion coefficient of is one order of magnitude smaller than that of the Ga carrier, stress is generated between the Si02 film and the reflective surface. The application of such stress to the reflective surface during operation of the semiconductor laser causes deterioration of the semiconductor laser. AI203 membrane is stable,
It is strong and has almost the same expansion coefficient as a semiconductor laser crystal, and does not react with Ga, so it has an excellent function as a protective film for the reflective surface of a semiconductor laser. When applying , the reflective surface and A
There is a drawback that a GaAs oxide film is formed at the interface with the I203 film. Currently known methods include CVD, sputtering, and electron beam evaporation, but all methods require a sample temperature of 300 oo or higher, so residual oxygen or oxygen separated from N203 and Ga carrier reacts, and a GaAs oxide film is formed on the surface of the GaAs reflective surface before the AI203 film is formed. Therefore, in these methods, the interface contains a CaAs oxide film that causes deterioration of the laser. Furthermore, since the normally used atomic electrode deteriorates in quality at a temperature of 300 qo or more, the electrode must be attached after the protective film is formed, which complicates the semiconductor laser manufacturing process. Protective films that do not have the above-mentioned drawbacks include high-resistance Si, high-resistance Si with additional Si02 added, and N directly applied to the reflective surface.
It has been proposed to oxidize the AI to form peaks 203, but if only high-resistance Si is attached, the Si film as a reflective surface protective film absorbs the light of the semiconductor laser.
The membrane needs to be thin enough to accommodate no more than 200 people.

Such a thin Si film contains many pinholes and has the disadvantage that it has a low ability to protect against external mechanical impact. A high-resistance Si film with an Sj02 film added is superior to a single Si film as a protective film for the reflective surface of a semiconductor laser, but as mentioned above, Sj○ and G
The stress applied to the reflective surface due to the difference in thermal expansion coefficient from aAs causes deterioration of the semiconductor laser. In addition, in the method of adding N to the reflective surface and oxidizing the AI, if the oxidation time is long, the Ga
As is also oxidized. AI without oxidizing Ga vessels
It is extremely difficult to oxidize all of them. The present invention provides a semiconductor laser that does not deteriorate even when operated for a long time by applying a protective film free from the above-mentioned defects to a reflective surface, and a method for manufacturing the same.

In other words, by taking advantage of the fact that Si has good adhesion to GaAs, Si is first attached to the GaAs reflective surface, and then a mountain 203 film is further deposited on the yielding i film to prevent oxidation of the reflective surface. To obtain a highly reliable semiconductor laser that does not deteriorate and has a sufficient protective film formed thereon.

At this time, if there is conduction in the Si film, the current-light output characteristics of the laser will be impaired, so the Si needs to have a high resistance. CVD is a method for forming the AI203 film on the Si film.
, sputtering, and electron beam evaporation require a sample temperature of about 300° C., which complicates the semiconductor laser manufacturing process. As a specific method for solving this drawback, we propose a method in which AI is deposited on the high-resistance Si film and the peaks are oxidized. In this method, the sample temperature can be lowered to 10,000 or less. Moreover, even if AI is oxidized excessively, only Si02 is formed, so G
a) The false reflective surface will not be oxidized. The present invention will be described in detail below based on examples. Figure 1 shows the striped Ga pine-AIGa obtained by the usual manufacturing method.
A plate crystal 1 separated from an As semiconductor laser crystal is shown.

Electrodes are formed by depositing Cr-Au on the p-type side and Au-Ni on the n-type side. First, a Si film 3 is deposited on the cleavage surface 2 of this plate-shaped crystal 1, that is, the reflective surface of the semiconductor laser, by electron beam evaporation, ion plating, or vapor phase growth (FIG. 2). Si film 3
The specific resistance of the semiconductor laser is set to be at least 1 mm in order to prevent deterioration of the current-light output characteristics of the semiconductor laser due to current leakage to the Si film, and the thickness of the Si film is set so that the absorption of laser light by the Si film is To the extent that the characteristics of the laser are not significantly impaired.
At this time, cover the electrode with a thin metal plate to prevent Si from adhering to the electrode. Next, cover the electrode in the same way as when forming the Si film on the Si film 3, and apply evaporation or sputtering to form the peak 4. (Figure 3). Reflective surface 2 is Si film 3
, and the plate-shaped crystal 1 covered with the N film 4 is inserted into oxygen plasma generated by high-frequency discharge in an oxygen atmosphere of about 10-3 Torr, and the AI is plasma oxidized.

At this time, the peaks are completely oxidized to the extent that some Si is oxidized. The thickness of AI may be arbitrary, but in particular, the thickness of the formed mountain 20
If the thickness of the AI film is set so that the film thickness in step 3 is 1/2 of the oscillation wavelength of the semiconductor laser, the current-light output characteristics of the laser with the Si-AI203 protective film will be the same as those with only the Si film. It can be the same as when. As above, Ga
A single semiconductor laser device 5 (FIG. 4) is manufactured by cutting a plate-shaped semiconductor laser crystal in which a Si--N203 protective film is formed on a false reflective surface.

In the semiconductor laser device 5 thus manufactured, G
GaA at the interface between the aAs reflective surface and the Si-mountain 203 protective film.
Since there is no s oxide film and the AI203 film can prevent oxygen from entering from the external atmosphere,
The reflective surface does not deteriorate during operation. Therefore, the semiconductor laser device 5 operates stably for a long time without deteriorating. In fact, after a semiconductor laser with a stripe width of 15 m is energized for 10,000 hours at an initial optical output of 5 hW, the output is N
In the case of only the 203 film, an optical output of 80% of the initial optical output was obtained, but with the Si-AI 203 protective film, this was further improved by about 10%. Furthermore, the peaks 203 have high mechanical strength and are not easily damaged by chemicals, so that damage caused by mechanical contact with the bonding system can be prevented. Furthermore, according to the method of the present invention, the sample temperature can be maintained during the formation of the AI203 film, so that the electrode does not change in quality.

Furthermore, since there is a high-resistance Si film between the GaAs reflective surface and the N film, excessive oxidation of the N film only oxidizes the Si film, and the GaAs reflective surface is not oxidized, causing deterioration of the semiconductor laser. Formation of a GaAs oxide film, which is the cause, can be prevented. In the above example, a method using oxygen plasma was described as a method for oxidizing AI, but in addition to this,
A method by anodic oxidation is also possible.

The reflective surface can also be formed by chemical etching or ion etching instead of cleavage.

[Brief explanation of the drawing]

Figures 1 to 4 are schematic diagrams showing the manufacturing process of a semiconductor laser in which a Si, AI203 film is attached to the cleavage plane of a Ga pseudo-NGaAs semiconductor laser. Here, 1 is a plate-shaped crystal of the semiconductor laser, and 2 is a Cleavage plane, that is, the reflective surface of the semiconductor laser, 3
is Sj film, 4 is AI film, 4' is mountain 203 film, 5 is Si-
A semiconductor laser has a reflective surface coated with an AI203 film, and 6 is a stripe region. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Scope of Claims] 1. A GaAs-AlGaAs semiconductor laser, characterized in that a reflective surface of the semiconductor laser is coated with a high-low anti-Si film, and the Si film is further coated with an Al_2O_3 film. 2. It is characterized by comprising the steps of forming a high-resistance Si film on the reflective surface of the semiconductor laser, further forming an Al film on this Si film, and oxidizing this Al film to form an Al_2O_3 film. A method of manufacturing a semiconductor laser.