JP2723924B2 - Semiconductor laser device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device, and more particularly to a high-output low-noise semiconductor laser device suitable for an optical disk light source.

[Conventional technology]

2. Description of the Related Art Conventionally, a self-pulsation type semiconductor laser is suitable as a low-noise semiconductor laser required for a light source such as an optical video disk. The characteristics of the self-pulsation type semiconductor laser are discussed in, for example, the IEICE technical report OEQ84-30 pp65-72, but the kink generation light output and the self-pulsation of the conventional self-pulsation type semiconductor laser can be obtained. Light output was low, at most 7-9 mW. In addition, the control of the self-excited oscillation frequency was not studied, and the frequency was as high as about 1 GHz.

[Problems to be solved by the invention]

In the above prior art, no consideration is given to suppressing return light noise when return light is generated in the self-pulsation type semiconductor laser. There was a problem that it was not possible to obtain low noise characteristics at a level of ^-13 Hz _-1 or less.

An object of the present invention is to provide a semiconductor laser device having high output and low noise characteristics suitable as an optical disc light source while maintaining low noise characteristics up to higher output even when the amount of return light is large.

[Means for solving the problem]

The above object can be achieved by doping the multiple quantum well active layer with an n-type impurity in a self-pulsation type semiconductor laser device. More specifically, it is as follows.

(1). In a heterojunction semiconductor layer composed of a semiconductor active layer and semiconductor optical waveguide layers provided above and below this layer, a ridge step is formed by processing the upper optical waveguide layer of the semiconductor active layer, and current is applied to both sides of the ridge. In a self-pulsation type semiconductor laser fabricated by providing a confinement and light absorption layer and controlling the effective refractive index difference in the lateral direction of the active layer, an n-type impurity is contained only in the quantum barrier layer of the set multiple quantum well active layer. A semiconductor laser device obtained by modulation doping.

(2). In the semiconductor laser device according to the above mode (1), n
A semiconductor laser device wherein the width of a layer to be doped with a type impurity is smaller than the width of a quantum barrier layer, and the outside of the doping region of the quantum barrier layer is not doped with an impurity.

(3). In the semiconductor laser device according to the above mode (1), n
A semiconductor laser device in which the type impurity species is Se or Si.

(4). The semiconductor laser device according to (1), wherein the width of the quantum barrier layer of the multiple quantum well active layer is 3 to 7 nm, and the width of the quantum well layer is 5 to 20 nm.

(5). (4) The semiconductor laser device according to item (4), wherein the thickness of the multiple quantum well active layer is 0.05 to 0.09 μm.

(6). In the semiconductor laser device described in (1), the multiple quantum well active layer is a GRIN-SCH (Granded Index Sep.).
A semiconductor laser device with a rate confinement heterostructure.

(7). (1) The semiconductor laser device according to (1), wherein the plane of the semiconductor substrate on which the crystal grows is (111).

[Action]

If the amount of return light is large even with a self-pulsation type semiconductor laser with excellent low noise characteristics, return light noise is generated as shown in 3p192 The noise intensity is higher than 10 ^-13 Hz _-1 . The return light noise in this case is related to the timing of the return light of the element and is determined by the self-excited oscillation frequency of the element and the optical path length of the optical pickup system. In the case of an optical video disk having an optical path length of 60 mm, the self-excited oscillation frequency of the element needs to be 1.3 GHz or less in order not to induce return optical noise. That is, control of the self-excited oscillation frequency is important to obtain low noise characteristics.

According to the present invention, the self-excited oscillation frequency of the device can be reduced by doping the bulk active layer or the multiple quantum well active layer with an n-type impurity in the self-excited oscillation type semiconductor laser. The reason will be described below.

The self-sustained pulsation frequency can be described in the same way as the expression representing the relaxation oscillating frequency, considering that the self-sustained pulsation of the semiconductor laser tends to resonate at the same frequency as the relaxation oscillation. That is, the self-excited oscillation frequency
_P is Here, K is a constant, dg / dn is a differential gain, xi] _P is the photon lifetime, I is the drive current, I _th is the threshold current, C is the speed of light, n _r is the effective refractive index, alpha _i is the internal loss, L is Resonator lengths, R ₁ and R _2, are end face reflectivities.

When the active layer has a multiple quantum well structure, for example, as shown in IEICE Technical Report OQE86-63 pp17-24, the self-excited oscillation frequency is increased so that the relaxation oscillation frequency becomes higher because the differential gain increases. Is higher than that of a normal bulk active layer. In order to reduce the self-excited oscillation frequency, in addition to extending the photon lifetime by increasing the resonator length and increasing the end face reflectivity in the above equations (1) and (2), the differential gain Is considered to be effective. Reducing the differential gain can be achieved by doping the active layer with an n-type impurity. In other words, by doping the multiple quantum well active layer with an n-type impurity, electrons that contribute to recombination light emission can be supplied even in a state where no carrier is injected, and a gain can be obtained in a wider energy region than usual. .
For this reason, the gain spectrum is widened, and the rise of the gain with respect to the carrier injection, that is, the differential gain is smaller than that in the case of undoping the active layer.

Doping the multiple quantum well layer uniformly,
There is a possibility that the quantum level formed in the quantum well layer is deformed due to the space charge of the impurity ions, or the distribution of the gain spectrum is disturbed, thereby causing an adverse effect such as an unstable oscillation wavelength. Therefore, it is effective to modulate doping only the quantum barrier layer without doping the quantum well layer or to do δ doping only at the center of the quantum barrier layer.

Also, when the thickness of the entire multiple quantum well layer is reduced, longitudinal multi-mode self-oscillation cannot be obtained, and there is a tendency for longitudinal single mode oscillation. Therefore, the entire film thickness is suitably 0.05 to 0.09 μm. . If the quantum well layer is thinned, longitudinal single mode oscillation tends to occur, and even if self-excited oscillation is obtained, the frequency will increase. Therefore, the quantum well width is appropriately 5 to 20 nm, and especially 8 to 12 nm. Is desirable.

According to the present invention, since the self-excited oscillation frequency can be reduced,
Relative noise intensity 10 ^-14 to 10 ^-13 Hz even if 3-6% of return light occurs
A low noise level of ^-1 level was realized even at higher light output than before.

〔Example〕

Example of Study First, an example of studying the effect of doping an n-type impurity into the multiple quantum well active layer will be described with reference to the MQW self-excited oscillation semiconductor laser shown in FIG. An n-GaAs buffer layer 2 (0.5 μm thick) and an n-Al _x Ga _{1 -x} As clad layer 3 on an n-GaAs substrate 1
(Thickness 1.5 μm, x = 0.45), AlGaAs multiple quantum well structure active layer 4 ′ (Al _y Ga _1-y As quantum barrier layer y = 0.2 to 0.27, thickness 3 to
5 nm 5 layers repeated, Al _z Ga _1-z As quantum well layer z = 0.04 to 0.07
The quantum barrier layer and the quantum well layer are uniformly doped with an n-type impurity Se at 9 × 10 ⁻¹⁷ to 2 × 10 ¹⁸ cm ⁻³ by repeating four layers), and the p-Al _x Ga _{1 -x} As clad layer 5 ( 1.3-1.6μm thickness,
x = 0.45), p-GaAs layer (thickness 0.2 μm) is first formed by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (M
The crystal grows sequentially according to the BE) method. Thereafter, an insulating film is formed on the crystal, and a striped insulating film mask is formed by photolithography and etching. Using this insulating film mask, the striped ridge waveguide 13 in FIG. 1 is etched with a phosphoric acid-based solution. The thickness d of the layer 5 on both sides of the ridge formed by etching is 0.3 to 0.3.
0.6 μm. Next, while leaving the insulating film mask, n
-Selectively grow a GaAs current confinement layer 7 (0.7 to 1.1 μm in thickness). Then, after the insulating film mask is removed by etching using a hydrofluoric acid solution, the p-GaAs buried layer 8 (having a thickness of 1.0 to 1.0 μm) is formed.
(1.5 μm) is grown again. Thereafter, the p-side electrode 9,
An n-side electrode 10 was vapor-deposited and cut into a device by a cleavage scribe process.

In order to obtain a laser which oscillates stably in the fundamental transverse mode, it is appropriate that the stripe width S at the bottom of the ridge is 4 to 7 μm. For comparison, the active layer was undoped AlGa
An element having the same other parameters as the As multiple quantum well layer 4 was manufactured. In both the device in which the active layer was doped with the n-type impurity and the device in which the active layer was undoped, self-sustained pulsation was maintained up to the kink level. FIG. 3 shows the optical output-current characteristics of the active layer uniform n-doped device of this study example in comparison with that of the active layer undoped device. In a device in which the MQW active layer is uniformly n-type doped, higher gain can be obtained at a lower injection carrier density than in an undoped device.
The threshold carrier density and the threshold current density are reduced, and a low threshold current is obtained. As shown in FIG. 3, the threshold current of the uniformly n-type doped device is 25 to 30 mA, and the threshold current of the undoped device is 35 to 30 mA.
A low threshold current of 20 to 30% compared to 40 mA could be achieved. Furthermore, the kink level could be improved as compared with the undoped element, and could be increased to 25 to 30 mW. Up to this kink level, the relative noise intensity is
A level below 10 ^-13 Hz ^-1 could be obtained. Further, since the differential gain of the MQW active layer can be reduced by n-type doping, the self-excited oscillation frequency can be reduced. As shown in Fig. 4, the self-excited oscillation frequency must be reduced to 1/2 to 1/3 in the uniformly n-type doped device (△ in the figure) compared to the undoped device (□ in the diagram). Was possible. In this study example, the self-excited oscillation frequency could be further reduced by coating a high reflection film on the back surface opposite to the emission surface of the device. As a result, the self-excited oscillation frequency is increased to 1.0 GHz up to the kink level of 25 to 30 mW.
The relative noise intensity can be suppressed to a level of 10 ^-13 Hz ^-1 or less even when the return light is generated at 3 to 6%. In the case of an asymmetrically coated device, the kink level could be improved to 60 to 70 mW.

The element of this study example has an end face breakdown level of 80 to 90 mW in the element with asymmetric coating, and in the life test of the element, it does not deteriorate even if it is operated for 2000 hours or more under constant light output operation at an ambient temperature of 50 ° C and 20 mW. Was.

Embodiment 1 Embodiment 1 of the present invention will be described with reference to FIG.
The fabrication process of the device was performed in the same manner as in the study example, except that in the crystal growth, only the quantum barrier layer in the MQW active layer was doped with Se as an impurity to produce a device having a modulation n-type doping. The doping concentration of the impurity Se is 9 × 10 ¹⁷ to 2 × 10 ¹⁸
cm ^-3 . In this device, self-sustained pulsation was obtained stably and continued to the kink level. In the device of this example, the threshold current was 20 to 25 mA, which could be reduced to about the same as or less than the device of the study example. The self-excited oscillation frequency could be suppressed to the same level as or less than that of the study example. As a result, the self-excited oscillation frequency can be maintained at 1.0 GHz or less up to the kink level, and the relative noise intensity can be reduced by 10% even when the return light is generated at 3 to 6%.
It was possible to keep it below ^-13 Hz ^-1 level.

Second Embodiment An MQW self-excited oscillation laser according to a second embodiment of the present invention will be described with reference to FIG. Preparation process of the device performed in the same manner as the considered examples, respectively lower and upper MQW active layer in the crystal growth is provided an n-type and p-type AlGaAs Guredetsudo layers 11 and 12, so-called GRIN-SCH (Gr aded- In dex Waveguide S ep
The arate C onfinement H eterostructare) structure and the device was fabricated.

As for the MQW active layer, a threshold current of 10 to 20 mA was obtained by performing modulation n doping as in Example 1. In addition, self-sustained pulsation continued until the kink level was 30 mW, and the self-sustained pulsation frequency was 0.2 to 1.0 GHz in the range of light output of 2 to 30 mW. This makes it possible to reduce the relative noise intensity by 10% even if the return light is generated by 3 to 6%.
It was possible to keep it below ^-13 Hz ^-1 level.

〔The invention's effect〕

According to the present invention, the effect that the frequency of the self-pulsation type semiconductor laser can be reduced to 1/2 to 1/3 of the conventional one is obtained. When the active layer is a multiple quantum well layer, the differential gain increases and the self-sustained pulsation frequency becomes higher than in the case of a normal bulk active layer. However, according to the present invention, the self-excited oscillation frequency is equal to or lower than that of the bulk active layer. Could be lowered. The self-excited oscillation frequency of the device can be suppressed as low as about 0.2 to 1.0 GHz up to an optical output of 25 to 30 mW, and the relative noise intensity is 10 even if 3 to 6% of return light is generated.
Low noise characteristics of ^-14 to 10 ^-13 Hz ^-1 level were obtained. The asymmetrically coated device generates self-excited oscillation up to 50 to 60 mW, and low noise characteristics with relative noise intensity of 10 ^-14 to 10 ^-13 Hz ^-1 level even if return light occurs in the light output range of 5 to 50 mW. Was. According to the present invention, there is an effect of improving the high-output low-noise characteristics of the semiconductor laser as compared with the related art.

For an asymmetrically coated device, the ambient temperature is 50 ° C and 40mW.
No degradation was observed even after lapse of 2000 hours in the constant light output operation.

In the present invention, the crystal material is made of AlGaAs, but other materials are used.
It goes without saying that the present invention can be applied to AlGaInP / GaInP and InGaAsP / InP and the like.

[Brief description of the drawings]

FIG. 1 shows a study example according to the present invention and the first embodiment of the present invention.
FIG. 2 is a cross-sectional view of an MQW self-oscillation laser, FIG. 2 is a cross-sectional view of an MQW self-oscillation laser according to a second embodiment of the present invention, FIG. FIG. 4 is a diagram showing a relationship between a self-excited oscillation frequency of an element and a drive current. 1 ... n-GaAs substrate, 2 ... n-GaAs buffer layer, 3 ... n-
Al _x Ga _1-x As clad layer, 4 ... undoped AlGaAs multiple quantum well structure active layer, 4'-uniform n-type doped AlGaAs multiple quantum well structure active layer, 4 "... modulated n-type doped AlGaAs multiple quantum well structure active Layer, 5 ... p-Al _x Ga _1-x As clad layer, 6 ...
p-GaAs layer, 7 ... n-GaAs current confinement layer, 8 ... p-GaAs buried layer, 9 ... p-side electrode, 10 ... n-side electrode, 11 ... n-AlGaAs graded layer, 12 ... p-AlGaAs graded layer , 13 ... Striped ridge waveguide.

Continuation of front page (72) Inventor Yuichi Ono 1-280 Higashi Koikebo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (56) References JP-A-62-123790 (JP, A) 1988 (Showa 63) Proceedings of the 35th Annual Meeting of the Japan Society of Natural Sciences, 3rd volume, 31p-ZP-5 1988 (Showa 63) P. 708 -713

Claims (3)

(57) [Claims] 1. An active region formed by stacking a quantum well layer so as to be sandwiched between quantum barrier layers having a band gap larger than that of the quantum well layer, and formed on the active region and on the opposite side of the active region from above the bottom. An optical waveguide layer in which a stripe-shaped ridge having a small width is formed and having a band gap larger than that of the quantum barrier layer; and an optical waveguide layer in contact with both side surfaces of the ridge and separated from the optical waveguide layer on a region where the ridge is not formed. And a light absorbing layer having a band gap smaller than that of the quantum well layer, wherein the thickness of the optical waveguide layer is 1.3 μm or more and 1.6
0.3 μm or less on both sides of the ridge,
The active region is set to 0.6 μm or less, and the active region is 0.05 μm or more including a plurality of quantum well layers having a thickness of 5 nm or more and 20 nm or less.
It is formed to a thickness of 0.09 μm or less, and an n-type impurity is added to only the above-mentioned quantum barrier layer in an amount of 9 × 10 ¹⁷ cm ⁻³ or more and 2 × 10 ¹⁷
A semiconductor laser device characterized by being doped at a concentration of ¹⁸ cm ^-3 or less. 2. The semiconductor laser device according to claim 1, wherein said n-type impurity is doped only in a central portion of said quantum barrier layer. 3. The semiconductor laser device according to claim 1, wherein said n-type impurity is Se or Si.