JPS6251281A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser, which generates stable self-excited vibration, keeps a basic lateral mode oscillation and is characterized by a low-noise characteristic, good controllability and excellent reproducibility, by providing a stripe shaped carrier injecting region at the central region of a resonator other than the vicinities of both reflecting surfaces, in a current block layer that is formed at a part neighboring one of clad layers. CONSTITUTION:On an n-type GaAs substrate 10, an n-type Al0.45Ga0.55As first clad layer 11, an n-type Al0.15Ga0.85As active layer 12, a p-type Al0.45Ga0.55As second clad layer 13, an n-type Al0.3Ga0.7As first block layer 14 and an n-type GaAs second block layer 15 are continuously grown. Then entire body is covered with SiO2 film. Thereafter, a stripe shaped window is provided along the entire length of a resonator in the longitudinal direction. The second block layer 15 is etched and the upper surface of the first clad layer 14 is exposed. A stripe shaped window is formed in the resist film, with the vicinities of both reflecting surface being made to remain, so that the central lines of the widths agree. With the resist film as a mask, the first block layer 14 is etched, and the surface of the second clad layer 13 is exposed. Then the resit film 16 and the SiO2 film are removed. Thereafter a p-type Al0.45Ga0.55As third layer and p<+> type GaAs cap layer 18 are continuously grown.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a semiconductor laser for optical information processing.

(Prior art and other problems) ・Among optical information processing semiconductor lasers, when used as a light source for reading video disks and optical disks, noise characteristics, especially noise characteristics induced by return light, are a problem. becomes. In order to reduce light-induced noise and noise in semiconductor lasers,
Although various methods have been tried, reducing output coherence is particularly effective.

As one of these methods, Oishi, Kayane, and Nakamura Kazuojima proposed low-noise semiconductor lasers by high-frequency superposition in the Proceedings of the 1983 Autumn Conference on Applied Physics, p. 102, 26a-
P-6'Reduction of noise and longitudinal mode characteristics of semiconductor laser by high frequency superposition' has been proposed and shown to be effective. However, this method not only requires the addition of a high frequency drive circuit, but also has disadvantages such as high frequency leakage to external mechanisms.

On the other hand, a method to generate self-excited vibration and multiply longitudinal modes to reduce noise is proposed by Meki, Matsumoto, Tari, Watanabe, and Kurihara in IEICE Technical Report, Photon Quantum Electronics oqE84-57, p. 39 "l58S It has been proposed and attempted in the area of laser noise characteristics and self-pulse modulation mechanism.However, in this method, the characteristics of self-excited vibration depend extremely sensitively on the laser structure (layer thickness, groove width, etc.), and therefore The yield of devices exhibiting stable self-oscillation was low.

In addition to the method described above, there is a method of introducing a saturable absorber into a resonator as a method of causing self-excited vibration in a semiconductor laser. However, at present, the relationship between the magnitude of various parameters of a saturable absorber and the phenomenon that appears (self-excited vibration or bistable operation) has not been sufficiently clarified. Therefore, the purpose of the present invention is to solve the above-mentioned drawbacks. The purpose of this invention is to propose a semiconductor laser that eliminates the oscillation, generates stable self-excited vibration, has low noise characteristics, maintains fundamental transverse mode oscillation, and has excellent controllability and reproducibility.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention provides a method of forming an active layer into a double layer sandwiched between cladding layers made of a material with a wider band gap than the active layer. A semiconductor laser with a multilayer structure using a telojunction semiconductor material, in which a current blocking layer is formed adjacent to one side of the cladding layer, and a striped carrier injection region is formed in the current blocking layer near the internal reflection surface. An effective refractive index distribution region that includes the carrier injection region in the vertical direction and has a width wider than the carrier injection region is formed over the entire length of the resonator in the longitudinal direction. and the carrier diffusion length in the active layer is shorter than half the difference between the width of the effective refractive index distribution region and the width of the carrier injection region, and the height of the effective refractive index is shorter than the injection region. The active layer near the internal reflection surface is an absorption region that is larger than the amount of decrease in refractive index caused by carriers.

(Example) Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a perspective view of an embodiment of the present invention, FIG. 2 is a cross-sectional view taken along line AA' in FIG. 1, and FIG. 3 is a cross-sectional view taken along line B-B' in FIG. It is a diagram.

In the manufacturing of this example, first, as shown in FIG.
Kn-type Al, ), B Ga1) on n-type GaAs substrate 10
) HAs first cladding layer 11 is 2 μm% n-type A11),
1 @ ()a 6. BAS active layer (n-type concentration n =
1.5 x 10"d") 12 for 0.1μms
p-type A 1o, s Gao, 6 B' As second cladding layer 13 of 0.3 μm, n-type A 16.1
Ca 11. ? The thickness of the As first block layer 14 is 0.3 μm.
, the n-type GaAs second block layer 15 is 0.1 μm S M
Continuous growth using OCVD method.

Since the MOCVD method allows thin film growth and has precise film thickness controllability, it is possible to grow the above-mentioned layered structure with good controllability. In addition, as described above, the active layer 12
By setting the n-type concentration to 15×10 cML, the carrier diffusion length can be reduced to 1 μm or less. It was also revealed that at this concentration, the luminous efficiency was also the highest.

Next 810. After coating the entire surface with a film, a stripe-shaped window with a width of 4 μm is opened along the entire length of the resonator using a photoresist method, and the GaAs second block layer 15 is etched.
1°, the top surface of the B Ga04 As first block layer 14 is exposed. Furthermore, a photoresist method is performed to create striped windows with a width of 2 μm in the resist film 16, leaving 40 μm each in the vicinity of the internal reflection surface so as to match the center line of the above-mentioned width, and using this resist film as a mask, A16,
B Ga6. ? The A6 first block layer 14 is etched to expose the surface of the AIo,41()ao,5sAs second cladding layer 13 (FIG. 5).

Next, resist film 16 and 810! After removing the p-type A16.4@Ga(1,6@As) third cladding layer 17, a high concentration p-type GaAs cap layer 18 of 2.0+ μm1 is added.
．． Continuous growth of 0 μm. In the conventional liquid phase growth method for this growth, AlzGat xA8
layer A16. B Ga00sl As second cladding layer 13 and A164 Ga64 As first block layer 14
No liquid phase layer is grown on top of this, but it can be easily grown using MOCVD. Especially this MOCVD
Immediately before growing the third cladding layer 17 in the
The reproducibility and reliability of the grown device can be further improved by etching the surface of the growing surface with a small amount of gas such as .

After this, p-type ohmic contacts 19.
By attaching an n-type ohmic contact 20 to the substrate side, the semiconductor laser of this example is obtained (Figures 1, 2, and 3).
.

The structure of one embodiment of the present invention has been explained above with reference to the perspective view in Fig. 1 and the sectional views in Figs. 2 and 3. explain in detail.

In the structure of this embodiment, the current flowing from the entire surface electrode is applied to the nap layer 18. An n-type GaAs second block layer 15, which spreads over the entire surface of the third cladding layer 17 and has a different electrical polarity, is adjacent to the third cladding layer 17, and an n-type AIO, which is adjacent to this.

()a6. Since there is a first block layer 14,
The current is blocked in the first and second blocking layers and finally in the n-type Al0J()a@,yAs first blocking layer 1
A p-type Al (1,41()ao, ss As second cladding layer 1
3, n-type Al 6,1 @ Ga 6.1 @
As is implanted into the active layer 12.

The carriers injected into the active layer diffuse in the horizontal and lateral directions of the active layer, form a gain distribution, and start laser oscillation. At this time, as mentioned earlier, since the carrier diffusion length in the active layer 12 is short, the gain distribution is mainly formed in the part of the active layer under the striped window formed in the first block layer 14. The shape of the gain distribution becomes steep, and as a result, the gain is high only in the lower part of the striped window, and the outside becomes a loss region.

On the other hand, light seeps out from the active layer 12 and spreads in the vertical direction. At this time, the light that seeps into the second cladding layer 13 is transmitted to the n-type A1°, 1Ga, which has a high refractive index and is adjacent to the second cladding layer 13.
1) 4 As Since there is a first block layer 14, it is drawn into this layer 14. Furthermore, there is an n-type GaAs second block layer 15 adjacent to the first block layer 14, but this layer not only has a higher refractive index than the first block layer and draws in light, but also has a band gap for laser oscillation light. narrow ~1
It is a layer that absorbs light at a wavelength of 0,000 cm or more. Therefore, the light is drawn into the second block layer and suffers a large absorption loss there. As a result, a positive refractive index difference ΔηB occurs across the window formed in the second block layer 15. The inventor's calculation results revealed that the value is ΔηB=5×10′ in this example.

From the above, in the structure of this embodiment, the second
The light spreads over a wider window width than that in the block layer 15, where a positive refractive index guiding mechanism is built. By the way, when carriers are injected into the active layer and a gain distribution is formed, the refractive index decreases due to the negative dependence of the refractive index on the carrier density. However, since its value is about 3 to 4 x 10, in this embodiment, a refractive index of 1 to 2 x 10 is activated during laser oscillation, and this positive refractive index guiding and the above-mentioned second
Due to the synergistic effect with the rapid absorption of light by the blocking layer 15, fundamental transverse mode oscillation can be maintained.

In the structure of this example, the width of the spread of light is wider than the width of the gain distribution, so the light spreads from the gain region to the loss region outside of it, which means that there is a saturable absorber under the equality constraint. Self-excited vibration is likely to occur.

As described above, the structure of this embodiment has absorption regions not only in the horizontal direction of the active layer but also in the vicinity of the internal reflection surface. That is, in the region near this internal reflection surface, p-type Al is formed adjacent to the active layer 12. , 45 Gao, ss Since there is the Aa second cladding layer 13 and the first blocking layer 14 adjacent thereto, the injection current is partially blocked by the first blocking layer 14. In addition, if the resistance of the second cladding layer 13 is made relatively high, the current flowing through the second cladding layer 13 along the horizontal direction of the active layer can be suppressed by the synergistic effect of the thin layer thickness of the second cladding layer 13. This amount is negligibly small, since the effect is to make the entire lateral resistance of the second cladding layer extremely high.

Therefore, carriers are not injected into the active layer region near this internal reflection surface, so the bandgap in this region is narrow with respect to laser oscillation light by ~150cI! L
is in the area of absorption loss.

By the way, in this region, when carriers are excited by absorbing laser oscillation light, a phenomenon occurs in which the band gap widens and loss decreases, resulting in a so-called saturable absorber.

As described above, the structure of this embodiment has a saturable absorber not only in the horizontal and lateral directions of the active region but also in the direction along the length of the resonator, making self-oscillation possible.

Furthermore, in the structure of this example, the carrier diffusion length is less than half the width of the carrier injection region that determines the width of the refractive index distribution and the gain distribution width, and the refractive index during laser oscillation is relatively small. It has the effect of promoting self-excited vibration.

That is, first of all, since the carrier diffusion length is short, fluctuations in the injected carrier density distribution become drastic, and along with this, the width of the fundamental transverse mode fluctuates greatly, causing its contraction and expansion, and as a result, self-excited vibration is promoted.

According to the analysis results of the present inventor, calculations using carrier diffusion lengths of 1 μm and 2 μm in the structure of this example show that the self-excited vibration of carrier diffusion length of 1 μm is 5.5 to 6 times that of 2 μm. Became.

Furthermore, the fact that the refractive index during laser oscillation is relatively small also promotes fluctuations in the width of the fundamental transverse mode.

According to the analysis results of the present inventor, the ratio of the first peak intensity of self-excited vibration to the intensity at the first valley is ηg=1.
0X10-" then ηB=5X10-" for 160
I found out that it would be 195.

As a result of all the synergistic effects described above, self-excited vibration easily occurs in the structure of this example, resulting in multi-mode axial modes and reduced coherence of the axial modes, resulting in extremely low noise to reflected light and low-noise characteristics. can get. Therefore, the laser element of this embodiment becomes a low-noise laser necessary for optical reading.

In the structure of this embodiment, the first waveform is flat and uniform near the internal reflection surface.
A second layer of n-type GaAs with a striped window along one direction of the length 11 of the resonator adjacent to the block layer 14
There is a block layer 15. Of the light that has traveled in the active layer in the longitudinal direction of the resonator, the laser light that has spread in the vertical direction of the active layer undergoes rapid absorption loss in the n-type GaAs layer outside the active layer. In this way, a positive refractive index guiding mechanism works in the stripe portion, and the laser beam is controlled by this guiding mechanism in the vicinity of the internal reflection surface. As a result, there is no astigmatism, and stable laser oscillation can be maintained by the light guiding mechanism between the central region of the resonator and the vicinity of this internal reflection surface.

Note that although an n-type GaAs substrate was used in the above embodiment, a p-type GaAs substrate was used.
The present invention can be realized even if n is inverted. Also, the example is A
Although the lGaAs/GaAs double heterojunction crystal material has been described, other crystal materials such as lm
GaP/AI ImP ImGaAsP/lm
GaP eImGaAs P/ImP, AI
The present invention can be applied to many crystalline materials such as GaAs Sb/GaAs 3b.

(Effects of the Invention) As explained in detail above, the semiconductor laser of the present invention has no astigmatism, can maintain fundamental transverse mode oscillation, and also produces stable self-excited vibration. Since the permissible range of the conditions for generating excitation vibration is wide and the structure is relatively simple, it has excellent controllability and reproducibility, and can be manufactured at a high yield. Since this semiconductor laser stably generates self-excited vibration, when used as a light source for reading video discs and optical discs, it is possible to reduce return light-induced noise.

[Brief explanation of drawings]

FIG. 1 is a perspective view of an embodiment of the present invention, FIG. 2 is a cross-sectional view taken along line AA' in FIG. 1, and FIG. 3 is a cross-sectional view taken along line B-B' in FIG. 4 are cross-sectional views showing the structure of a double heterojunction crystal grown in the process of manufacturing this example. FIG. 5 shows the long axis of a resonator with a structure in which a striped current injection port was provided in the central region of the resonator except for the vicinity of the internal reflection surface in the double heterojunction crystal described above during the fabrication process of this example. FIG. 10 - n-type GaAs substrate, 11-n-type Al (1,45
()a6. . As first cladding layer, 12-n type A1o, ti()a
O,81As active layer, 13 ・p-type Alo, 45 ()
ao, 5 @ As second cladding layer, 14-...-n type A
:Lo, 3 Gao, 7 As 1st block layer, 15.
...n-type GaAs second block layer = 16...resist film, 17-p-type Ale, 4 N Gao, sa
As third cladding layer, 18... - type GaAs cap layer, 19... p-type ohmic contact, 20...
N-type ohmic contact. Agent Patent Attorney Shinsuke Honjo

Claims (1)

[Claims] In a multilayer semiconductor laser having a double heterojunction semiconductor material in which an active layer is sandwiched between cladding layers made of a material with a wider band gap than the active layer, a current blocking layer is formed adjacent to one of the cladding layers. A striped carrier injection region is provided in the current blocking layer in the central region of the resonator excluding the vicinity of both reflective surfaces, and the stripe-shaped carrier injection region includes the carrier injection region in the vertical direction and has a width wider than the carrier injection region. An effective refractive index distribution region with The active layer near both reflective surfaces is an absorption region, and the height of the effective refractive index is greater than the amount of decrease in the refractive index due to injected carriers. semiconductor laser.