JPS6239088A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To provide the low noise characteristic by producing a stable self- oscillation by determining the width of the grooves so that a carrier diffusion length in an active region becomes shorter than half a difference between a width of the second groove and that of the first groove formed in a block layer. CONSTITUTION:On an N-type GaAs substrate 10, an N-type Al0.5Ga0.5As first cladding layer 11, an N-type Al0.15Ga0.85As active layer 12, a P-type Al0.4Ga0.6As second cladding layer 13, an N-type Al0.5Ga0.5As first block layer 14, and an N-type GaAs second block layer 15 are grown. The N-type concentration of the active layer 12 is determined to be 1.5X10<18>cm<-3>, and the diffusion length of carriers 1mum or under. The substrate is coated with an SiO2 film 16 and a stripe-form window of 4mum width is opened in longitudinal direction of a resonator. The second block layer 15 is etched to form a groove and a stripe- form block layer 15 is etched to form a groove and a stripe-form window of 2mum width is opened in a resist film 17, and the first block layer 14 is etched to form a groove. The films 17 and 16 are removed and a P-type Al0.4Ga0.6As third cladding layer 18 and a P<+> type GaAs cap layer 19 are grown.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor laser for optical information processing.

(Prior art and its problems) Among optical information processing semiconductor lasers, when used as a light source for reading video disks and optical disks,
Noise characteristics, especially characteristics of noise induced by returned light, are a problem. Various methods have been tried to reduce the feedback-induced noise of semiconductor lasers, and among them, reducing output coherence is particularly effective.

As one of these methods, Oishi, Shimone, Nakamura, and Ojima proposed low-noise semiconductor lasers by high-frequency superposition in the Proceedings of the 1983 Autumn Conference on Applied Physics, p. 26a-102.
It was proposed in P-6 "Reduction of noise and longitudinal mode characteristics of semiconductor laser by high frequency superposition" and has been 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 of generating self-excited oscillation, multiplicating the longitudinal modes, and reducing the noise is proposed by Meiki, Matsumoto, Kyuzai, Watanabe, and Kurihara in the Technical Report of the Institute of Electronics and Communication Engineers, Photon Quantum Electronics 0QE84-57.39, page 15SS. Proposals and attempts have been made in the area of ``laser noise characteristics and self-pulse modulation mechanism''. However, this method has the disadvantage that the yield of devices exhibiting stable self-oscillation is low because the characteristics of self-oscillation depend extremely sensitively on the laser structure (layer thickness, groove width, etc.). .

In other words, the laser structure proposed by Megi et al., as shown in FIG. It has a structure in which a flat active layer 52 and a cladding layer 53 are formed on the buried layer. As a result, in the structure shown in Figure 5, the current injection port is farther from the active layer than the refractive index distribution region formed by the light absorption effect, so the current is injected into the active layer with a spread equal to or more than the width of the refractive index distribution. Therefore, the carrier distribution formed in the active layer has a width comparable to that of the refractive index distribution.
The light absorption effect occurring in the horizontal and lateral directions of the active layer is significantly reduced, and self-excited vibrations are less likely to occur. As will be explained later, it is essentially important to shorten the carrier diffusion length to less than half the difference between the effective refractive index distribution width and the carrier injection region width, and this causes self-excited vibration. and has a conducive effect, resulting in low noise laser characteristics. However, the structure proposed by Meiki et al. does not take such effects into account, and as a result, the allowable range in which self-excited vibration occurs becomes extremely narrow.

(Objective of the Invention) The object of the present invention is to eliminate the above-mentioned drawbacks, to propose a semiconductor laser that generates stable self-oscillation, has low noise characteristics, maintains fundamental structural mode oscillation, and has excellent controllability and reproducibility. It is in.

(Structure of the Invention) The present invention has a double heterojunction structure in which an active layer is sandwiched between cladding layers made of a material with a wider band gap than the active layer, and further includes a first groove having a first groove on the cladding layer. a block layer, a second block layer having a second groove wider than the first groove on the first block layer, and embedded in the first and second grooves; a multilayer structure including a semiconductor layer on the second block layer, and the carrier diffusion length in the active layer is less than half the difference between the width of the second groove and the width of the first groove. The width of each groove is determined so that the width of each groove is shortened, and the effective refractive index in the region of the second groove is determined to be larger than the amount of decrease in the refractive index due to carrier injection. .

(Embodiment of the invention) An embodiment of the invention will be described below with reference to the drawings. FIG. 1 is a perspective view of this embodiment, and FIG. 2 is a sectional view of FIG. 1. As shown in FIG. 3, an n-type Al layer is formed on an n-type GaAs substrate 10.
o, 5 Ga□, 5As first cladding layer 1 layer 2.5p
m.

n-type Alo, 15Gao, 55As active layer (n-type concentration n
=1.5X 1018cm-3) 12 to 0.08pm,
The p-type Alo, 4Ga0.6As second cladding layer 13 has a thickness of 0.3 pm, the n-type Al□, 5Ga□, 5As first block layer 14 has a thickness of 0.6 pm, and the n-type GaAs second block layer 15 has a thickness of 1.0 pm.

Continuous growth using MOCVD method. Since the MOCVD method allows thin film growth and has precise film thickness controllability, the layered structure described above can be grown with good control.

Further, as mentioned above, the n-type concentration of the active layer 12 is set to 1.5X 10
By setting it to 110l8, the carrier diffusion length can be made 111m or less. It was also revealed that the luminous efficiency was highest at this concentration.

Next, after coating the entire surface with a 5i02 film 16, a stripe-shaped window with a width Qm is opened in the length direction of the resonator using a 7-oto-regi method, and the GaAs second block layer 15 is etched to form a groove. The upper surface of the 5As first block layer 14 is exposed. Furthermore, a photoresist method is performed to create a striped window with a width of 2 μm in the resist film 17 so as to match the center line of the groove, and using this resist film as a mask, the AI□, 5Ga□, 5As first block layer 1
4 to form grooves by etching 1-, AI□, 4Ga□,
The surface of the 6As second cladding layer 1-3 is exposed (Figure 4).

Next, after removing the resist film 17 and the 5i02 film 16,
The p-type AN□, 4Ga□, 6As third cladding layer 18 is
．． 013. m, high concentration p-decade GaAs cap layer 19
Continuously grow 1.0 pm. In the conventional liquid phase growth method, Al, zGal-
Alo, 4Ga□, 6As second cladding layer 13 which is a zAs layer and AIo, 5Ga□, 5As 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 18 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. After this, a p-type ohmic contact 20 is attached to the entire surface of the growth, and an n-type ohmic contact 21 is attached to the substrate side to obtain the semiconductor laser of the present invention (FIGS. 1 and 2).
.

(Operations and Effects of the Invention) In the structure of the present invention, the current injected from the entire surface electrode spreads over the entire surface of the cap layer 19 and the third cladding layer 18 and flows, but adjacent to the third cladding layer 18 there is a difference in electrical polarity. An n-type GaAs second block layer 15 and an n-type AN□, 5Ga□, 5As first block layer 14 adjacent thereto.
Therefore, the current is blocked by the first and second blocking layers, and finally the p-type Al□, 4
Through the Ga□, 6As second cladding layer 1-3, the n-type A
lo, 15Gao, B5As are 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 above, since the carrier diffusion length in the active layer 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, and its shape is steep. 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 of the active layer and spreads in the vertical direction. At this time, the light seeping into the second cladding layer 13 spreads to the n-type Al□, 5GaO, 5As first blocking layer 14 adjacent to the second cladding layer 13. 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 with respect to the laser oscillation light. It is a narrow absorption layer for light of 100,100 O0" or more. Therefore, the light is drawn into the second block layer and suffers a large absorption loss there. As a result, a positive A refractive index difference Δ113 occurs.The value is Δ113 in this example.
! The inventor's calculation results revealed that IB=5×10−3.

As a result of the above, in the structure of the present invention, the first. block layer 1
4, light spreads over a wider window width than the narrow window width opened in the second block layer 15, and a positive refractive index guiding mechanism is built there. 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, the value is 3~
4X10-3, so in this example, a refractive index of 1 to 2X10-3 is built in during laser oscillation, and this positive refractive index guiding and the sudden change in light due to the second blocking layer described above Due to the synergistic effect with absorption, fundamental transverse mode oscillation can be maintained.

In the structure of the present invention, 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 it, which is isometrically equivalent to having a saturable absorber. Self-excited vibration is likely to occur. Furthermore, in the structure of the present invention, 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, which promotes self-oscillation. It has the effect of

In other words, first, because the carrier diffusion length is short, fluctuations in the injected carrier density distribution become severe, and along with this, the width of the fundamental transverse mode fluctuates greatly, causing its contraction and expansion, and as a result, the magnitude of self-excited vibration is promoted. be done. According to the analysis results of the present inventor, the carrier diffusion length is 11 in the structure of this embodiment.
As a result of calculation using 1 m and 2 pm, carrier diffusion length 1
It has been revealed that the self-excited vibration of pnn is 5.5 to 6 times that of 2pm.

Furthermore, the relatively small magnitude of the refractive index during laser oscillation 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 r18=1 as a result of calculation using a carrier diffusion length of 1 pm in the structure of this example.
．． It was found that in 0-cut 0-2, it was 160, whereas in rlB=5-cut Q-3, it was 195. As a result of all of the above synergistic effects, self-excited vibration easily occurs in the structure of the present invention, 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. . Therefore, the laser device of the present invention becomes a low-noise laser necessary for optical reading.

(Effect of the invention) The semiconductor laser of the present invention is (1) Fundamental transverse mode oscillation can be maintained.

(2) Since self-excited vibration occurs and the permissible range of its conditions is wide, it can be obtained with good reproducibility.

(3) Since the structure is relatively simple, it has the advantage of being able to be manufactured with good reproducibility and high yield.

Note that although an n-type GaAs substrate was used in the above embodiment, pn
may be reversed. Furthermore, although the embodiments have been described with reference to the AlGaAs/GaAs double heterojunction crystal material, other crystal materials such as InGaP/AIInP, InGaAsP/InG
aP, InGaAsP/InP.

It can be applied to many crystal materials such as AlGaAsSb/GaAsSb.

[Brief explanation of the drawing]

Figure 1 is a perspective view of an embodiment of the present invention, Figure 2 is a sectional view of Figure 1, Figure 3 is a side view of a double heterojunction crystal grown in the process of manufacturing this embodiment, and Figure 4. is a cross-sectional view when a stripe-shaped current injection port is provided in the double heterojunction crystal in the process of manufacturing this embodiment. FIG. 5 is a diagram showing a conventional semiconductor laser. In the figure, 10 - n-type GaAs substrate, 11 - n-type Al□, 5 Ga□, 5 As first cladding layer, 12 - n-type AlO, 15Ga0035 As active layer, 13 - p-type Al□, 4 GaO, 6As first cladding layer. 2 cladding layer, 14-n-type Alo, 5 Gao, 5 As first block layer, 15-n-type GaAs second block layer, 16...5i02 film, 17-resist film, 18-p-type Al□ , 4 Ga□, 6 As third cladding layer, 19... p 10-type GaAs cap layer, 20...
. . . p-type ohmic contact, 21 . . . n-type ohmic contact, respectively. O 2 Fig. O 4 Fig.

Claims (1)

[Claims] It has a double heterojunction structure in which an active layer is sandwiched between cladding layers made of a material with a wider band gap than the active layer, and further includes a first block layer having a first groove on the cladding layer, and the first blocking layer has a first groove. a second block layer having a second groove wider than the first groove on the block layer; a multilayer structure provided on a block layer, and each groove width is set such that the carrier diffusion length in the active layer is shorter than half the difference between the width of the second groove and the width of the first groove. and further, the magnitude of the effective refractive index in the region of the second groove is determined to be larger than the amount of decrease in the refractive index due to injection of carriers.