JP3281666B2 - Semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-pulsation type semiconductor laser which realizes low noise.

[0002]

2. Description of the Related Art Since semiconductor lasers have excellent characteristics such as small size, light weight, high efficiency, high response speed, and wide wavelength selectivity, they have been introduced into fields such as optical disks, optical communication, and laser printers. It is being actively promoted. Above all, a semiconductor laser used for an optical disk has a large problem of noise induced by a change in optical output due to return light at the time of reading information, and there is a high demand for low noise characteristics. As a semiconductor laser, there are a single longitudinal mode laser and a multi longitudinal mode laser. The single longitudinal mode laser has low noise during steady operation, but its output greatly fluctuates due to return light. On the other hand, a multi-longitudinal mode laser is unlikely to generate noise due to fluctuations in external factors such as return light, but has an astigmatic difference in light emitting points, making it difficult to reduce the convergent spot diameter, and is not suitable for pickup. Is appropriate. There is also a problem that power consumption is large.

[0003] As a method for improving the noise characteristics due to the return light, a method of reducing the coherence of the semiconductor laser element so that the return light does not react to the phase of the return light even if the return light is incident and is hardly affected by the return light. There have been various reports of self-pulsation type semiconductor lasers utilizing the self-pulsation phenomenon.
Conventionally, a self-pulsation type semiconductor laser has been obtained by changing structural parameters such as the thickness of the active layer or the thickness of the cladding layer. However, this method has a limit in reducing noise. Therefore, a method of inserting a saturable absorbing layer having large light absorption has been proposed.

FIG. 8 is disclosed in Japanese Patent Application Laid-Open No. 63-202083.
Schematic diagram showing a conventional self-sustained pulsation type semiconductor laser
It is sectional drawing. This semiconductor laser is an n-GaAs substrate
N-Ga on 21_0.5Al_0.5As clad layer 22, and
Ga_0.86Al_0.14As active layer 23, p-Ga_0.5Al
_0.5As cladding layer 24, p-Ga_0.8Al_0.2Asmo
Isolation layer 25, p-Ga_0.5Al_0.5As selective etching
The layer 26 and the p-GaAs cap layer 27 are laminated in this order.
You. Where p-Ga_0.5Al_0.5On the As cladding layer 24
Part, p-Ga_0.8Al_0.2As layer mode separation layer 25 and p
-Ga_0.5Al _0.5Both sides of the As selective etching layer 26 are n
Embedded in the GaAs layers 28; And above
The electrodes 29 and 30 are formed on the surface and the lower surface, respectively.
Here, the n-GaAs layer 28 and the undoped Ga_0.86Al_0.14
The distance from the As active layer 23 is 0.1 to 0.5 μm.
A good horizontal basic mode has been obtained.

In this semiconductor laser, a layer having a larger refractive index or a larger light absorption than the cladding layer (p-Ga in FIG. 8) is provided on the surface (or inside) of the cladding layer.
By providing the _0.8 Al _0.2 As layer mode separation layer 25), the oscillation state of the laser can take a plurality of modes, and self-sustained pulsation easily occurs due to the oscillation of the mode between these two laser spectra. , Which is resistant to return light noise.

[0006] In the active layer MQW (M ulti Q uantum
W ell; multiple quantum well) semiconductor laser of the self-excited with a structure has been proposed (IEICE technical report, IEICE Technical Report, vol.88, No.4, OQE88-5, pp33~38 ). In this semiconductor laser, the active layer, which is an optical waveguide, has an MQW structure to control the self-excited oscillation frequency. In this semiconductor laser, since the change in the refractive index with respect to the injected carrier is small, the maximum light output can be made twice or more that of a normal laser. In addition, the quantum effect enables low threshold current density operation, which leads to low power consumption. Further, the temperature dependency can be improved, and high reliability can be obtained. As described above, a semiconductor laser having low noise and high output characteristics can be realized.

[0007]

However, in the self-pulsation type semiconductor laser having the saturable absorption layer, according to the experimental results of the present inventors, the band gap of the saturable absorption layer is considerably larger than the band gap of the active layer. If the band gap of the saturable absorbing layer is much larger than the band gap of the active layer, the saturable absorbing layer becomes transparent to the oscillating light, resulting in insufficient light absorption. And self-excited oscillation may not occur. In order to solve this, it has been found that it is essential to make the bandgap energy of the saturable absorption layer substantially equal to the bandgap energy of the active layer, that is, the oscillation wavelength energy.

However, when the saturable absorbing layer has a bulk structure (the thickness of the layer is several hundred mm or more where no quantum effect occurs), the band gap is adjusted by changing the composition ratio of the saturable absorbing layer. By doing so, the bandgap energy of the saturable absorption layer is made substantially equal to the oscillation wavelength energy. However, when the composition ratio of the saturable absorbing layer is changed as described above, in particular, an AlGaInP-based or GaI
In an nAsP-based semiconductor laser device or the like, there is a problem that a crystal defect is generated in the crystal of the saturable absorption layer, and characteristics of the semiconductor laser device are deteriorated such as a high threshold current. In particular, when the oscillation wavelength is short (for example, when the active layer has an MQW structure), it is difficult to make the energy of the saturable absorption layer of the bulk structure substantially equal to the oscillation wavelength energy. That is, it was difficult to control the band gap energy of the saturable absorbing layer only by the composition ratio.

In addition, a self-excited oscillation type semiconductor laser having an MQW structure as an active layer has a problem that astigmatism is increased or kink (non-linearity of light output-current characteristics) occurs at low light output. Was. The present invention has been made in view of such circumstances, and the band gap energy of the saturable absorption layer and the band gap energy of the active layer can be easily adjusted by forming the saturable absorption layer into a quantum well structure or a strained quantum well structure. It is an object of the present invention to provide a self-excited type semiconductor laser which has a low optical output, does not easily cause a kink, has a low astigmatic difference, and can obtain a threshold current equivalent to that of a conventional semiconductor laser.

[0010]

[0011]

According to a first aspect of the present invention, there is provided a semiconductor laser including a one-conductivity-type cladding layer, an active layer,
A self-excited oscillation type semiconductor laser comprising a cladding layer of another conductivity type and a saturable absorbing layer on, under, or inside the cladding layer of one conductivity type and / or the cladding layer of the other conductivity type. Forming a strained quantum well saturable absorption layer as the saturable absorption layer to constitute the saturable absorption layer.
Valence band and amount of conductor in well layers of quantum well structure
It is noted that the energy between the
Sign.

A semiconductor laser according to a second aspect of the present invention is the semiconductor laser according to the first aspect , wherein the active layer has a quantum well structure or a strained quantum well structure.

[0013]

[0014]

According to the first aspect of the present invention, a method of introducing lattice strain by changing the composition ratio of the compound semiconductor used in the well layer by forming the saturable absorption layer into a strained quantum well structure, and the well width described above. , The band gap energy between the saturable absorbing layer and the active layer can be easily adjusted with higher accuracy. Also, as in the first invention, the number of well layers is determined by the material used and the well width, and can be singular or plural.

FIG. 9 shows Ga _x formed on a GaAs substrate.
Is a graph showing the relationship between the Ga content x and the strain amount in the In _1-x P. When the strain amount is a positive value, it indicates a compressive strain, and when the strain amount is a negative value, it indicates a tensile strain.
In this case, when the amount of compressive strain is increased, that is, when the content of Ga is reduced, the oscillation wavelength becomes longer, and conversely, when the amount of tensile strain is increased, that is, when the content of Ga is increased,
The oscillation wavelength becomes shorter. As shown in FIG. 9, since the Ga content and the strain amount have a linear relationship, the required strain amount can be easily obtained by changing the Ga content, and the band gap energy of the saturable absorption layer can be reduced. Can be controlled. If the well width is set to be equal to or less than the critical thickness (thickness at which crystal defects begin to be generated) of a desired strain amount, the problem of crystal defects does not occur.

According to the second invention, in addition to the effect of the first invention, the active layer has a quantum well structure or a strained quantum well structure, so that the self-excited oscillation frequency is controlled and a low-noise semiconductor laser is provided. Can be obtained. If the number of well layers is reduced, the operating current can be reduced, but the gain is reduced. Therefore, the number of well layers is determined in combination with the well width.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below with reference to the drawings showing the embodiments. FIG. 1 is a schematic sectional view showing a semiconductor laser according to the present invention, showing a case of a red semiconductor laser. In the figure, 1 is a substrate made of n-GaAs,
On this substrate 1, a buffer layer 2 made of n-GaInP, a clad layer 3 made of n- (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P, and a strained MQW active layer 4 are sequentially formed. Here, a strained quantum well saturable absorption layer 6 is formed in the cladding layer 3. On the strained MQW active layer 4, p- (Al
_A cladding layer 5 made of _0.7 Ga _0.3 ) _0.5 In _0.5 P and a strained quantum well saturable absorption layer 6 are formed, and a ridge-shaped cladding layer 5 and a p-GaI
A contact layer 7 made of nP is formed. Both sides of the cladding layer 5 and the contact layer 7 are n-GaAs
It is embedded in the block layers 8 and 8 made of s. Further, p-Ga is formed on the contact layer 7 and the block layer 8.
A cap layer 9 made of As is formed. A p-electrode 10 is formed on the upper surface of the cap layer 9, and an n-electrode 11 is formed on the lower surface of the substrate 1.
Are formed respectively.

FIGS. 2 and 3 are explanatory views showing a method of manufacturing the semiconductor laser shown in FIG. A buffer layer 2 (film thickness) is formed on a substrate 1 by MOCVD (metal organic chemical vapor deposition).
0.3 μm) and a clad layer 3 (0.8 μm thick) are formed, and a strained quantum well saturable absorption layer 6 is formed during the formation of the clad layer 3. This strained quantum well saturable absorption layer 6
FIG. 4 shows the energy band diagram of FIG.
_0.7 Ga _{0.3) 0.5} In a _0.5 consisting of P barrier layer 6a (thickness _{50Å), Ga x In 1-} x P consists well layer 6b (thickness
100Å, strain + 0.5 ~ 1.0%)
In this embodiment, three well layers 6b are formed.

On the cladding layer 3, a strained MQW active layer 4 is provided.
It is formed by the MOCVD method soon. Strained MQW active layer 4
FIG. 5 shows the energy band diagram of FIG.
_0.5Ga_0.5)_0.5In_0.5Guide layer 4a made of P (film
(500mm thick)_0.5Ga_0.5)_0.5In_0.5P
Layer 4b (thickness: 50 °) made of GaInP
Well layers 4c (thickness 100 mm, strain + 0.5%) are alternately stacked
And (Al) _0.5Ga_0.5)_0.5In_0.5
It is formed by forming a guide layer 4a (thickness 500 °) made of P.
In this embodiment, it is preferable that the number of the well layers 4c is 5 to 8.
In FIG. 5, five well layers 4c are provided.
You. Here, the guide layer 4a traps and guides light and emits light.
It is formed for the purpose of increasing the rate. On the strained MQW active layer 4
Includes the same strained quantum well saturable absorber layer 6 as described above,
− (Al_0.7Ga_0.3)_0.5In_0.5Crack consisting of P
Layer 5 (1.1 μm) and contact layer 7 are MOCVD
(FIG. 2A).

Next, an SiO ₂ film is formed on the contact layer 7 by an electron beam evaporation method or a CVD method, and is patterned into a stripe having a width of about 5 μm by a photolithography method to form a mask 12 (FIG. 2 ( b)). And mask
The upper part 0.8 μm of the contact layer 7 and the cladding layer 5 not covered with 12 is removed by etching (30 ° C., 30 seconds) using hydrobromic acid to form a ridge (FIG. 2 (c)). )).

Thereafter, n-GaAs is grown by selective growth to form block layers 8, 8 (1 μm) (FIG. 3).
(d)). Then, after removing the mask 12 with a buffered hydrofluoric acid solution, a cap layer 9 made of p-GaAs is formed by MOCVD (FIG. 3E), and a p-electrode is formed on the upper surface of the cap layer 9.
10 and an n-electrode 11 is formed on the lower surface of the substrate 1 (FIG. 3).
(f)).

FIG. 6 is a graph showing the light output-current characteristics of a conventional self-pulsation type semiconductor laser in which the thickness of the active layer or the cladding layer is appropriately selected and the semiconductor laser according to the present invention. FIG. 6A shows the case of a conventional semiconductor laser, and FIG. 6B shows the case of the semiconductor laser according to the present invention. The semiconductor laser according to the present invention includes a strained MQW active layer 4 having five 0.5% strained well layers 4c and a strained quantum well saturable absorption layer 6 having one 0.6% strained well layer 6b. The thickness of the cladding layer 5 below the layer 8 is 0.25 μm
Was used. As a conventional semiconductor laser,
Seven well layers (100Å) with 0.5% strain and barrier layer (50Å)
A clad layer under the block layer having a thickness of 0.35 μm was used.
FIG. 6 shows that in the present invention, the nonlinearity of the light output-current characteristic is greatly improved. Table 1 shows the threshold current, astigmatic difference, and the presence or absence of kink in these semiconductor lasers. As is clear from Table 1, the present invention also has the effect of reducing the astigmatic difference.

[0023]

[Table 1]

FIG. 7 is a graph showing a threshold current and a γ value indicating optical coherence with respect to the number of well layers 6b of the strained quantum well saturable absorber layer 6 according to the present invention. When the γ value is 1.0, the device operates as a single mode. As the γ value decreases, the self-oscillation property increases. When the γ value is 0.7 or less, the device operates well as a self-oscillation type. In the semiconductor laser used here, the thickness of the well layer 6b is 100 mm, the strain is 0.6%, the thickness of the well layer 4c is 100 mm, the thickness of the barrier layer 4b is 50 mm, the strain is 0.5%, and The side cladding layer 5 is 0.25 μm.
When the number of the well layers 6b is increased to reduce the optical coherence, the threshold current increases. Therefore, in this embodiment, it is desirable that the number of the well layers 6b formed in the strained quantum well saturable absorption layer 6 is one to three.

In this embodiment, the strained quantum well saturable absorbing layer 6
Is provided in both the cladding layer 3 and the cladding layer 5, but a good effect can be obtained in either one of them.
The formation position is determined by the required amount of light absorption,
It is not limited to the inside of the cladding layer (3 or 5). Further, in the present embodiment, it is desirable that the oscillation wavelength energy is substantially equal to the distance between the valence band and the quantized level of the conductor in the well layer of the quantum well structure constituting the saturable absorption layer . Also, not only the strained MQW structure but also the MQW structure, SQW
The present invention is also applicable to a semiconductor laser having an active layer having a (single quantum well) structure or a bulk structure. In this embodiment, GaInP and AlGaIn
Although the case of the red semiconductor laser using P has been described, it is needless to say that the present invention can be applied to any self-pulsation type semiconductor laser using another compound semiconductor.

[0026]

The semiconductor laser according to the present invention as described above, according to the present invention, the saturable absorbing layer and the strained quantum well saturable absorbing layer, wherein
In the well layer of the quantum well structure constituting the saturable absorption layer
Oscillation wavelength energy between the valence band and the quantized level of the conductor
By equalizing the saturable absorbing layer
Band gap energy of the active layer
The present invention has excellent effects, such as realizing a good semiconductor laser having low noise and high output characteristics, which is equal to energy .

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a semiconductor laser according to the present invention.

FIG. 2 is an explanatory view showing a method of manufacturing the semiconductor laser shown in FIG.

FIG. 3 is an explanatory view showing a method for manufacturing the semiconductor laser shown in FIG.

FIG. 4 is an energy band diagram of a strained quantum well saturable absorption layer in the semiconductor laser shown in FIG.

5 is an energy band diagram of a strained MQW active layer in the semiconductor laser shown in FIG.

FIG. 6 is a graph showing light output-current characteristics.

FIG. 7 is a graph showing a threshold current and a γ value with respect to the number of well layers in a strained quantum well saturable absorption layer.

FIG. 8 is a schematic sectional view showing a conventional semiconductor laser.

FIG. 9 is a graph showing the relationship between the amount of strain and Ga composition in GaInP.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 buffer layer 3 cladding layer 4 strained MQW active layer 5 cladding layer 6 strained quantum well saturable absorption layer 7 contact layer 8 block layer 9 cap layer 10 p electrode 11 n electrode 12 mask

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shoji Honda 2--18 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (56) References JP-A-63-202083 (JP, A) JP-A JP-A-3-257887 (JP, A) JP-A-5-3367 (JP, A) JP-A-2-268417 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 5 / 00-5/50

Claims (2)

(57) [Claims] 1. A cladding layer of one conductivity type, an active layer, and a cladding layer of another conductivity type are provided on a substrate in this order, and upper and lower surfaces of the cladding layer of one conductivity type and / or the cladding layer of the other conductivity type are provided. Alternatively, in a self-pulsation type semiconductor laser having a saturable absorption layer formed therein , a strained quantum well saturable absorption layer is formed as the saturable absorption layer, and the saturable absorption layer is formed.
Valence band and conduction in the well layer of the constitutive quantum well structure
The distance between the quantized levels of the conductor is equal to the oscillation wavelength energy
A semiconductor laser, comprising: 2. The method according to claim 1, wherein the active layer has a quantum well structure or a strained quantum well structure.
2. The semiconductor according to claim 1, wherein the semiconductor has a door structure.
laser.