JP3399049B2 - Semiconductor laser device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low noise semiconductor laser device suitable as a light source for an optical disk and a method for manufacturing the same.

[0002]

2. Description of the Related Art A conventional semiconductor laser device will be described below.

As shown in FIG. 4, a dielectric film 1 is formed over the entire surface of a light emitting side end surface of a conventional semiconductor laser. The dielectric film 1 is important from the following two points. The first is to prevent the crystal from being exposed at the light emitting side end face of the epitaxial growth layer 2 and thus the oxidation of the crystal to progress, thereby preventing the semiconductor laser from deteriorating. Secondly, the reflectance of the end face is adjusted by the thickness of the dielectric film 1 to control the characteristics such as the threshold and efficiency of the semiconductor laser. In a high-power laser used for an optical disc or the like, the reflectance at the light emitting portion 3 of the end face on the light emitting side is required to be about 10% or more in order to reduce noise due to returned light. Further, when an optical pickup having a structure for generating three beams by a diffraction grating is used, the reflected light of the laser light emitted from the semiconductor laser is located at a position several tens of μm above and below the light emitting portion on the light emitting side end face. Come back.
If the light reflectance of the end face of the laser chip on the light emission side is high, the reflected light is further reflected by the end face and causes tracking noise.

Therefore, as shown in FIG. 5, the thickness of the dielectric film 1 is made different from that on the light emitting portion so that the semiconductor substrate 4 side below the light emitting portion 3 has a low reflectance. Thus, the noise due to the reflected light on the end face on the emission side is reduced.

[0005]

The conventional semiconductor laser has not been constructed so as to sufficiently reduce the noise due to the reflected light at the light emitting side end face of the laser chip. This is because, in the structure shown in FIG. 5, the end surface on the light emitting side is covered with the single-layer dielectric film 1, and the light emitting portion 3 and other regions except for the vicinity are etched until the reflectance becomes about 3%. Although it is realized by making the thickness thinner, it is necessary to control the film thickness with an accuracy of ± 10 nm or less in order to make it substantially non-reflective, and it is very important to industrially manufacture with high yield. It was difficult.

Further, even if the film thickness can be controlled, the reflection itself cannot be completely eliminated, and it has not been an essential solution to the application field requiring a lower reflectance.

An object of the present invention is to solve the above problems and to provide a low noise semiconductor laser device which can be easily manufactured and a manufacturing method thereof.

[0008]

In order to achieve this object, a semiconductor laser device of the present invention comprises a semiconductor laser chip and a dielectric multilayer film formed on the light emitting side end face of the semiconductor laser chip. , The dielectric multilayer film emits light
Formed on the area and other areas except the light emitting area.
The first dielectric having a smaller refractive index than the semiconductor crystal
A body film, and on the first dielectric film and on the light emitting region.
And when formed on other areas except the light emitting area
A second dielectric having a refractive index larger than that of the first dielectric film
A body film and on the second dielectric film and on the light emitting region.
When formed, the refractive index is smaller than that of the second dielectric film.
A third dielectric film, and the reflectance of the light emitting area is higher than the reflectance of the other area.

[0009]

[0010]

With this structure, the reflection of the return light from the external optical components can be reduced at least in the area excluding the light emitting portion. In addition, since the first, second, and third thin films are formed by being sequentially laminated, the film thickness can be accurately controlled, and by selectively removing a predetermined portion of the third thin film, It has become possible to manufacture a high-performance semiconductor laser device with high yield.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

1A and 1B show the structure of a semiconductor laser chip according to an embodiment of the present invention. FIG. 1A is a perspective view and FIG.
Is a partially enlarged side view.

An alumina film 1a and a silicon film 1b are laminated as a dielectric film on the light emitting side end face of a semiconductor laser chip having an epitaxial layer 2 formed on a semiconductor substrate 4. Here, each film thickness of the alumina film 1a and the silicon film 1b is set so that the reflectance with respect to the laser light is 2% or less in two layers. In the case of this embodiment, the alumina film 1a and the silicon film 1b are controlled within a range of about ± 1 nm with respect to a predetermined film thickness by the sputtering method or the electron beam evaporation method (EB evaporation method).
Unlike the case of controlling the film thickness of the single-layer dielectric film 1 by etching as in the conventional example (see FIG. 5), a high yield is obtained, and the reflected light of the return light on the crystal surface re-enters the optical pickup. It is possible to suppress the occurrence of noise.

The design value of the film thickness will be described using the numerical calculation results. The calculation formula is shown below. The wavelength of the laser beam in a vacuum is λ, the refractive index of the crystal of the coated portion is n ₀ , the refractive index of the first layer film is n ₁ , the refractive index of the second layer film is n ₂ , and the first layer film is n ₂ . When the film thickness is d ₁ and the film thickness of the second layer is d ₂ , the end face reflectance R is given by the following equation.

[0015]

[Equation 1]

Here,

[0017]

[Equation 2]

From the calculation of the above equation, it is found that, in order to reduce the reflectance, first of all, a dielectric film having a lower refractive index than the semiconductor crystal is required for the first dielectric film. At this time, the reflectance becomes minimum when the film thickness of the dielectric film is λ / 4. Ideally, if a dielectric film having a refractive index that is the square root of the refractive index of a semiconductor crystal is formed, the reflectance can be set to 0, but it is easy to coat such a material with good control. Not really. In the case of a single layer,
In the conventional technique, the structure shown in FIG. 5 has a problem in that the yield is reduced because it is necessary to perform etching from a film thickness that provides the reflectance of the light emitting portion to a film thickness that does not cause reflection.

Therefore, in this embodiment, a method of forming the dielectric film in multiple layers was used. That is, according to the above formula, the first layer is a dielectric film having a refractive index lower than that of the semiconductor crystal, and the second layer is a dielectric film having a refractive index higher than that of the first layer. This is because the coat can be easily realized. At this time, the required refractive index of the dielectric does not have to be as strict as in the case of a single-layer film, and is usually used for coating semiconductor lasers and has a proven track record in reliability, such as alumina, silicon, titanium oxide, and silicon oxide. Materials such as are sufficient.

A specific design example will be shown below. Figure 2 shows G
For aAs (refractive index 3.6), the refractive index is 1.
This is the result when an alumina film of 66 was formed and a silicon film having a refractive index of 3.3 was formed as the second layer. The film thickness in the figure is expressed as an optical film thickness for generalization with respect to the wavelength of laser light. From FIG. 2, it is possible to obtain the reflectance of 3 by properly combining the film thicknesses of the alumina film and the silicon film.
It turns out that it can be made less than or equal to%. Moreover, the minimum value
It can be much smaller than the minimum in the alumina monolayer. That is, the minimum value in the case of the alumina single layer is about 2% when the film thickness is λ / 4, but it can be set to 0.1% or less in the case of the two-layer structure of the alumina film and the silicon film. In this example, the reflectance was set to 0.1% or less by setting the set film thickness value to 0.15λ for the alumina film 1a and 0.044λ for the silicon film 1b.

At this time, even if the respective thicknesses are made thicker than the setting shown in FIG. 2, the reflectance of R <3% can be realized by the above-mentioned calculation formula, so that the thickness may be set. For example,
The film thickness of silicon having the minimum reflectance is calculated every λ / 2 cycle when calculated by increasing the film thickness of silicon in FIG. 2. Therefore, in the above-described embodiment, 0.044λ + mλ /
A silicon film thickness of 2 (m is a natural number) may be used. Further, the film thickness of alumina is 0.15λ + mλ in the above embodiment.
The thickness may be as thick as / 2. Furthermore, the thicknesses of the alumina film and the silicon film are 0.40λ + mλ / 2 and 0.4, respectively.
It may be 55λ + mλ / 2.

Further, a dielectric film is formed on the two layers in the light emitting portion so that the reflectance in the light emitting portion has a value designed to determine the characteristics of the semiconductor laser. In this embodiment, the alumina film 1c having a thickness of 0.11λ is formed so that the reflectance of the light emitting portion is 11%.

The material of the first dielectric film is not limited to alumina and may be silicon oxide. This is because the refractive index of silicon oxide is 1.45, which is almost the same value as that of alumina, so that similar dependency of reflectance can be obtained. As design values at this time, a thickness of 0.11λ for the silicon oxide film and 0.06λ for the silicon film was obtained from the above calculation formula. Of course, it is needless to say that the element can be realized in any combination of film thicknesses where R <3% in the above calculation formula.

The material of the second dielectric film is not limited to silicon and may be titanium oxide. This is because titanium oxide has a refractive index of 2.3, which is higher than the refractive index of the first dielectric film, and a non-reflection coating can be easily realized. Further, unlike silicon, titanium oxide has an advantage that it does not absorb laser light. At this time, as design values, alumina 0.10λ, titanium oxide 0.11λ, or silicon oxide 0.084
[lambda] and titanium oxide 0.116 [lambda] were obtained from the above formula. Of course, it is needless to say that the element can be realized in any combination of film thicknesses where R <3% in the above calculation formula.

Similarly, the third dielectric film which determines the reflectance of the light emitting portion may be silicon oxide. When the first dielectric film is an alumina film having a thickness of 0.15λ and the second dielectric film is a silicon film having a thickness of 0.044λ,
The thickness of the silicon oxide film is 0.148λ when the reflectance is 11%, for example.

Also, when titanium oxide is used as the material of the second dielectric film and silicon oxide is used as the material of the third dielectric film, titanium oxide has a sufficient etching rate for the HF system. Due to its small size, titanium oxide can likewise be used as an etching stop layer.

Also, when titanium oxide is used as the second dielectric film, the third dielectric film may be formed to have a film thickness of 11% silicon oxide in addition to alumina. .

FIG. 3 is a process chart for explaining one embodiment of the manufacturing method of the present invention. First, as shown in FIG. 3A, a semiconductor laser chip having an epitaxial growth layer 2 formed on one main surface of a semiconductor substrate 4 is prepared, and an alumina film 1a and a silicon film 1b are formed on the light emitting side end surface thereof. The layers are sequentially formed by a sputtering method, an electron beam evaporation method (EB evaporation), or the like. At this time, the film thickness is set to a value at which the reflectance is 3% or less. Then, on the silicon film 1b,
The alumina film 1c is formed to a thickness such that the reflectance is 11%.

The alumina film 6 and the silicon film 7 are formed on the end surface of the semiconductor substrate 4 opposite to the end surface on the light emitting side.
Are sequentially laminated and formed.

Next, as shown in FIG. 3B, the area of the alumina film 1c on the epitaxial growth layer 2 including the light emitting portion 3 on the light emitting side end face is selectively resist film 5 by a known method. Then, the part of the alumina film 1c which is not covered with the resist film 5 is removed with an HF-based etchant. At this time, since the second silicon film 1b acts as an etching stop layer, it is not necessary to control the etching amount by the etching time. Finally, the resist film 5 is organically cleaned and removed. In this embodiment, when the alumina film 1b and the silicon film 1a are laminated and the upper alumina film 1b is etched by HF, the lower silicon film 1a is not etched, and the lower layer is not removed. I have to.

As a material for the dielectric film having a laminated structure, a combination of a first dielectric film having a refractive index lower than that of a semiconductor crystal and a dielectric film having a refractive index higher than that of the first dielectric film is used. I wish I had it. That is, in addition to silicon oxide, titanium oxide, alumina, and silicon, for example, bismuth oxide (refractive index 1.9), cerium oxide (refractive index 2.
3), europium oxide (refractive index 1.9), hafnium oxide (refractive index 2.0), indium oxide (refractive index 2.
0), lanthanum oxide (refractive index 1.9), neodymium oxide (refractive index 2.15), molybdenum oxide (refractive index 1.9),
Magnesium oxide (refractive index 1.72), praseodymium oxide (refractive index 1.93), samarium oxide (refractive index 1.9).
9), antimony oxide (refractive index 2.1), silicon monoxide (refractive index 1.9), scandium oxide (refractive index 1.
9), tin oxide (refractive index 2.0), tungsten oxide (refractive index 1.68), yttrium oxide (refractive index 1.87),
It goes without saying that zirconium oxide (refractive index 2.05), zinc oxide (refractive index 2.1) and the like may be used in a film thickness satisfying the above conditions.

[0032]

The present invention can easily realize a low-noise semiconductor laser device in which reflection of return light is suppressed.
That is, the semiconductor laser device of the present invention has a dielectric film having a refractive index lower than that of the semiconductor crystal and a refractive index higher than that of the dielectric film in at least a region of the end surface of the semiconductor laser chip excluding the light emitting portion. By having a two-layer coating of the dielectric film that is provided, the reflectance of the region with respect to the laser light is set to 3% or less, thereby reducing the reflectance on the semiconductor substrate side, Tracking noise due to reflection of return light is completely suppressed. Also in the manufacturing process, highly reliable materials such as alumina, silicon, titanium oxide, and silicon oxide can be used as the coating film of the semiconductor laser, and the mass productivity is excellent. Further, the reflectance value can be easily realized to be 1% or less with the above material. Further, when forming the difference in reflectance with the light emitting portion, since the second layer of silicon or the like can be used for the etching stop layer, strict control of the etching amount is not required, and a very high yield can be obtained. . It goes without saying that if such a semiconductor laser device is used as a light source for an optical disk, it is useful as a low-noise, low-cost light source.

[Brief description of drawings]

FIG. 1A is a perspective view of a semiconductor laser device according to an embodiment of the present invention, and FIG. 1B is a cross-sectional view of a coated portion of an end face of a light emitting side thereof.

FIG. 2 is a diagram showing a relationship between a film thickness and an end face reflectance in an alumina / silicon laminated film.

3A to 3C are manufacturing process diagrams of a semiconductor laser device according to an embodiment of the present invention.

FIG. 4 is a perspective view showing an example of a conventional semiconductor laser device.

FIG. 5 is a perspective view showing another example of a conventional semiconductor laser device.

[Explanation of symbols]

1a Alumina film 1b Silicon film 1c Alumina film 2 Epitaxial growth layer 3 Light emitting part of the end face on the light emitting side 4 Semiconductor substrate 5 Resist film

Front page continuation (72) Inventor Kunio Ito 1-1, Sachimachi, Takatsuki-shi, Osaka, Matsushita Electronic Industrial Co., Ltd. (56) Reference JP-A-62-230076 (JP, A) JP-A-56-100488 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (3)

(57) [Claims] 1. A semiconductor laser chip and a dielectric multilayer film formed on a light emitting side end surface of the semiconductor laser chip, wherein the dielectric multilayer film is on the light emitting region and the light emitting region.
It is formed on other areas except the exit area and has a refractive index
Is smaller than a semiconductor crystal, and the first dielectric film,
On the dielectric film and on the light emitting area and the light emitting area
Formed on other regions except the
A second dielectric film larger than the first dielectric film;
Formed on the dielectric film and on the light emitting region,
Third dielectric film having a refractive index smaller than that of the second dielectric film
The semiconductor laser device according to claim 1, wherein the reflectance of the light emitting region is higher than the reflectance of the other region. 2. The second dielectric film is made of silicon or titanium oxide.
It consists Tan, also the semiconductor laser device according to claim 1 in which the third dielectric film is made of alumina. 3. The second dielectric film is made of titanium oxide ,
The semiconductor laser device according to claim 1, wherein the third dielectric film is made of silicon oxide .