JPH02174178A - Semiconductor laser element - Google Patents

Abstract

PURPOSE:To enable a broad output range of self-oscillator with low oscillator threshold valve by specifying the width of current injection region, the thickness of the second clad layer outside the current injection region, and the refractive index differences between the parts of the first and second light guide layers touching an active layer and the first and second clad layers, respectively. CONSTITUTION:The width of a current injection region 31, the thickness of the second clad layer 7 outside the current injection region 31, and the refractive index differencies between the parts of the first and second light guide layers 4, 6 touching an active layer 5 and the first and second clad layers 3, 7 are specified to be 2-5mum, 0.2-0.6mum, and 0.065-0.20 respectively. This lessens the variation of the refractive index of the active layer 5 by injected carriers. and the setting of the refractive index difference between the inside of a wave guide path and the outside does not vary by output greatly. Accordingly, self- oscillation becomes to occur through a broad output range and oscillation threshold current becomes smaller.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device suitable for a light source such as an optical disk system.

(Prior Art) In recent years, semiconductor laser elements have been widely used as light sources for optical discs such as compact discs. In semiconductor laser devices used for such applications, the optical output fluctuates depending on the light reflected from the disk, inducing so-called return optical noise, but it is desirable that the intensity of such noise be as small as possible. In order to reduce the intensity of return optical noise, semiconductor laser devices that utilize a phenomenon called automatic oscillation are used. An example of this is shown in FIG.

The device shown in FIG.
led 5ubstrate Inner 5trip
e) It is called a laser, and the p-GaAs substrate 1
1, a 1l-GaAS current confinement layer 16, P Ga
@, 5A l a, asAs cladding layer 12, p
G ai8sAl @, 1. AS active layer 13, nG
a 11. ssA l s, 4sA s cladding layer 14
, n-GaAs cap layer 15 are laminated. Further, 21 is a p-side electrode, and 22 is an n-side electrode. A striped current path is formed by the V-shaped groove 23 extending from the surface of the current confinement layer 16 to the substrate 11, and the region above 71A23 of the active layer 13 becomes an oscillation region.

In a semiconductor laser device having such a structure, the thickness d of the p-cladding layer 12 outside the groove 23 is set to, for example, 0.3 am, and the thickness d2 of the active layer 13 is set to 0.12 μm. The refractive index difference Δn between the inside and outside of 23 is 3X10
It is set to about -3.

Therefore, the carriers injected into the active N13 tend to fluctuate spatially and temporally, causing automatic oscillation.

(Problems to be Solved by the Invention) As described above, in the conventional semiconductor laser device, the thickness dl of the cladding layer 12 outside the layer 23 and the thickness d2 of the active layer 13 are
By controlling this, automatic oscillation is caused and the return optical noise is reduced. However, such a configuration
Outside the groove 23, the light from the active layer 13 is transferred to the layer 1 on the substrate side.
Since it has a so-called loss guide structure in which a waveguide is formed by absorption by 6 and 11, the oscillation threshold current becomes large (for example, when the resonator length is 250 μm,
In addition, in such a double heterostructure in which the thickness d2 of the active layer 13 is large, the rate of refractive index change due to injected carriers is relatively large, and the output range in which automatic oscillation occurs is narrow.

Furthermore, the LPE method is mainly used as a conventional method for growing semiconductor laser devices as described above. Therefore, the thickness of the cladding layer 12 and the active layer 13 within the wafer are
There is a problem that the non-uniformity of the thickness is large, and the probability (yield) of producing or appearing an element that produces automatic oscillation is small.

On the other hand, in order to reduce the oscillation threshold current 3, a semiconductor laser device using a quantum well structure has been proposed. Figure 13<
Figure a) shows a cross-sectional refinement of a typical example, and figure (b) shows a distribution diagram of the Al mixed crystal ratio in the vicinity of the active layer.

This exemplary semiconductor laser device includes an n-GaAs buffer layer 2, an n-Ga@, 3A
l9. TAS cladding layer 3, graded index type Ga+-, Al/As optical guide layer 4 (thickness 0.2
μm), GaAs single quantum well active layer 5 (thickness 60 μm)
, graded index type Ga, □AI, As optical guide layer 6 (thickness 0.2 μm) with A1 mixed crystal ratio X gradually changed from 0.18 to 0.7, p -G a@, 3A
I l17AS clad N7, p-GaAs cap layer 8 formed, ridge-like region 24 with d w = 3 μm
The insulating film 9 and electrodes 21 and 22 are formed except for.

Thickness del of the p-cladding layer 7 outside the ridge-like region 24,
to, 15 μm.

In the case of such a structure, a small oscillation threshold of 5 mA was obtained with a resonator length of 250 μm, but if the thickness dc was set to 0.
Since it is set as small as 15 μm, the refractive index difference Δn between the inside and outside of the ridge becomes a large value of 1×10 −2 or more. Also,
Light guide layer 4 functioning as a barrier for the quantum well structure
．． Since there is a large difference between the mixed crystal ratio of 0.18 in the part adjacent to the active layer 5 of 6 and the mixed crystal ratio of 0.7 of the cladding layer 3.7, the light is transmitted inside the active layer 5 and the light guide layer 4.6. be strongly confined. Therefore, the oscillation spectrum tends to be in a single mode, and noise is likely to occur in the returned light, so a semiconductor laser device having such a structure is not suitable as a light source for an optical disk system.

The present invention was made in view of the current situation, and
It is an object of the present invention to provide a semiconductor laser element having a structure that oscillates at a low threshold value, generates self-sustained oscillation over a wide output range, and can be manufactured at a high yield, and has low return optical noise.

(Means for Solving the Problems) A semiconductor laser device of the present invention includes a first cladding layer, a first optical guide layer, an active layer having a quantum well structure, and a second cladding layer formed on a semiconductor substrate. A layered structure having a light guide layer and a second cladding layer, and a striped current injection region formed above the layered structure, and a refractive index guide based on a refractive index difference inside and outside the current injection region. A wave path is formed, the width of the current injection region is 2 to 5 μm, and the second
The cladding layer has a thickness of 0.2 to 0.6 μm outside the injection region, and a portion of the first and second light guide layers in contact with the active layer and a portion of the first and second light guide layers that are in contact with the active layer. The refractive index difference between the second cladding layer and the second cladding layer is set to 0.065 to 0.20, thereby achieving the above object.

(Function) In the semiconductor laser device of the present invention, since Δn of the refractive index waveguide can be set to an appropriate value, the carriers injected into the active layer oscillate temporally and spatially, and Excited oscillation occurs. In addition, since the active layer has a quantum well structure, changes in the refractive index of the active layer due to injected carriers are small, and the setting of the refractive index difference Δn inside and outside the waveguide does not change greatly depending on the output. In the semiconductor laser device of the invention, self-oscillation occurs over a wide output range.

The difference in refractive index between the first and second cladding layers and the first and second light guide layers, which function as a barrier for the quantum well structure of the active layer, is between 0.065 and 0. ．．
16, so that light is appropriately confined within the active layer and the light guide layer. Therefore, self-focusing that interferes with self-oscillation does not occur.

As the light guide layer, either one having a single Al mixed crystal ratio or a so-called graded index type may be used. In addition to having such a light guide layer and having a quantum well structure in the active layer, a light absorption region (in the conventional example shown in FIG. 12 described above, the current confinement layer 16 and the substrate 11) is present. Therefore, the oscillation threshold current becomes small.

Furthermore, since the semiconductor laser device of the present invention can be manufactured by the MBE method or MOCVD method that allows precise control of layer thickness, the incidence of devices that generate self-sustained pulsation is much higher than that of conventional devices.

(Example) The invention will now be described with reference to examples.

The cross-sectional structure of the first embodiment of the present invention is shown in FIG. 1(a). The manufacturing process of the semiconductor laser device of this embodiment will be described with reference to FIGS. 2 and 3. First, a superlattice buffer layer 2 is formed using the MBE method on an n-GaAs substrate 1 having a (100) plane orientation, and is St-doped (1°Qx 10"c).
m-3) n-Ga@, 5A I 85A s cladding layer 3 was laminated to a thickness of 1 μm, and the Al mixed crystal ratio
．． Graded index type non-doped Ga,-,AlxAs optical guide layer 4 (thickness 0.15 μm) gradually changed from 5 to 0.35, three Ga@,teA
! Non-doped activity of multiple quantum well structure with e, +2A S well layer (70 people thick) and two Ga@, 66A l,..., A s buffer layers (40 people thick)
15. Non-doped Ga+-, AI, As optical guide layer 6 (with At mixed crystal ratio X gradually changed from 0.35 to 0.5)
thickness, 0.15 μm), Be doped (1,0XIO”c
m-”)p-G a,, 5A l @, 6A s cladding layer 7 (thickness 1 μm), and Be-doped (3, OX
10"am-') p-GaAS cap layer 8 (thickness 1μ
m) were sequentially laminated to form the laminated structure shown in FIG. At of the active layer 5, the optical guide layer 4.6 and the cladding layer 3.7
In such a structure, the distribution diagram of the mixed crystal ratio is shown in FIG. 1(b), the portion (Ga@, asAl
1.3sA S ) with respect to a laser beam with a wavelength of 780nm, and both cladding layers (G a , 5A l , ,
The difference from the refractive index of 6As) 3.7 is as small as 0.10.

Thereafter, by wet etching or dry etching using RIBE, the p-cap layer 8 and the p-cladding layer 7 are etched up to the middle of the p-cladding layer 7 (outside the ridge-like region 31) so that the ridge-like region 31 remains. (so that the thickness dc of the p-cladding layer 7 becomes 0.3 μm).

The width dW of the ridge-shaped region 31 serving as a current path was set to 3 μm (FIG. 3).

Other than the part that becomes the current path on the upper surface of the ridge-shaped region 31,
S i Na insulating film 9 was formed by a CVD method, a shuttering method, an electron beam evaporation method, or the like. Thereafter, the substrate 1 is lapped to a thickness of about 100 μm. AuGe/N as n1llt pole 22
i, p side electric f! 21 as A u Z n / A
A laminated structure shown in FIG. 1(a) was formed by vapor-depositing n.

After alloying treatment (450℃), the resonator length is 250℃.
The semiconductor laser device of this example was obtained by slightly opening the semiconductor laser device to a diameter of μm. The obtained device was mounted on a heat sink with the substrate 1 side facing down, and device characteristics were examined.The device of this example oscillated at a small threshold current of 20 mA, and the oscillation wavelength was 785 nm. As is clear from the oscillation spectrum at an output of 3 mW shown in Figure 4, it oscillates in multiple modes, and the width of one spectrum is 1.30.
Wider width is occurring due to self-sustained oscillation. The automatic oscillation frequency at this time is 1.2G) (Z, and the automatic oscillation is at output 2.
It was observed in a wide range from mW to 17mW.

Figure 5 shows the relative noise intensity (R
IN), which was measured under the conditions of a measurement frequency of 720 kHz, a bandwidth of 10 kHz, and an optical path length of 30 mm. As is clear from FIG. 5, according to this embodiment, when the amount of returned light is 3% or less, the relative noise intensity is -135 d.
The noise was suppressed to below B/Hz, and good noise characteristics were obtained.

The appearance rate of devices having such characteristics was approximately 70% per wafer. In the above embodiment, the thickness dc of the p-cladding layer 7 was set to 0.3 am, but the thickness dc was set to 0.3 am. Automatic oscillation was observed for any value in the range of .2 to 0.6 μm. for example,
When dc=0.2μm, the oscillation threshold current is 17
mA, and automatically oscillated with an output of 3 to 19 mW. dc=
When the diameter was 0.4 μm, the oscillation threshold current was 22 mA, and automatic oscillation occurred with an output of 1.5 mW or more. Also, d
When c=0.6μm, the oscillation threshold current is 26m
A, and self-oscillated with an output of 1 mW or more.

Automatic oscillation was also observed for the width dW of the ridge-like region 31 in the range of 2 to 5 μm. The width dW of the ridge-shaped region 31 (2 μm≦dW≦5 μm) and the thickness d of the cladding layer 7
The oscillation threshold current and the optical output at which automatic oscillation occurs vary depending on the combination with c (0.2 μm≦dc≦0.6 μm).

The cross-sectional structure of the second embodiment of the present invention is shown in FIG. 6(a). In this embodiment, n-GaA
Si-doped (1,0X1OII1cm-
”) n-GaAS buffer layer 2 was formed with a thickness of 1 μm. On top of that, Si-doped (1, 0×10” cm−3)
n-GagSA I@5AS cladding layer 3 (H 1μm
), A1 mixed crystal ratio X is 0. Non-doped Gap-, A1, As light guide layer 4 (
Thickness 0.15 μm), thickness LZ = 70 people Ga @, B
BA IeI2A An active layer 5 having a single quantum well structure having a 5-well layer, a non-doped Ga+-, AI, As optical guide layer 6 (thickness 0.15μm), Be-doped (1,0XI
O”am-3) p -G a@, 5A l , , A
S cladding layer 7 (thickness 1 μm), and Be doped (3,
0XIO”crrr3) p-GaAs cap layer 8(f
f 1 μm) were sequentially laminated.

Similar to the first embodiment described above, the p-cap layer 8 and the p-
In this embodiment, the cladding layer 7 is removed by etching to the middle of the p-cladding layer 7 to form the ridge-like region 31. FIG. 6(b) shows a distribution diagram of the At mixed crystal ratio of the active layer 5, the optical guide layer 4.6, and the cladding layers 3 and 7 formed in this example.

The width dW of the ridge-like region 31 is 3 μm, and the ridge-like region 31
When the outside thickness da of the p-cladding layer 7 was 0.3 μm, the oscillation threshold current was 12 mA, and self-sustained oscillation was observed at an output of 2 mW or more. Further, the return light noise level was 135 dB/Hz or less at an output of 3 mW, and an element with good return light noise characteristics was obtained.

A third embodiment of the present invention is shown in FIG. 7(a). This embodiment has substantially the same laminated structure as the first embodiment (FIG. 1) described above, but the cladding layer 3.7 is G a e, zs
The active layer 5 with a multi-quantum well structure has four Ga, , HA l @, 1
2As well NJ (60 people thick) and 3 Ga55Al
a. It is composed of an As barrier layer (40 layers thick). In addition, Ga1-xAl tAs optical guide layer 4.6
The A1 mixed crystal ratio X of 0.55 to 0.4 and 04
The refractive index difference for laser light with a wavelength of 780 nm corresponding to this change is 0.55.
0 and small. The ridge-like region 31 is formed by two grooves 32, 32, and the SiN insulating film 9 is formed on a surface other than the upper surface of the ridge-like region 31. A distribution diagram of the Al mixed crystal ratio of the cladding layer 3.7 is shown in FIG. 7(b).

In such a configuration, when @dW of the ridge-like region 31 is 4 μm and the thickness dc of the ρ-cladding layer 7 outside the ridge-like region 31 is 0.25 μm, the oscillation threshold current is 20 mA. Self-oscillation was observed at an output of 2 mW or more (oscillation wavelength 780 nm), and the return optical noise level was -135 dB/Hz or less at an output of 2 mW or more.

Note that even when the number of well layers in the active layer 5 is 2 to 7 and the number of barrier layers is 1 to 6, the oscillation threshold current is 15 to 2.
5 mA, and self-oscillation was observed at an output of 2 mW to 3 mW. Also, the return optical noise level is -135dB
/Hz or less.

A cross-sectional configuration of the fourth embodiment is shown in FIG. 8(a).

MB on an n-GaAs substrate 1 having a (100) plane orientation.
A superlattice buffer layer 2 is formed using the E method, and n-Ga@
, tsAl@, 5sAs cladding layer 3 is laminated to a thickness of 1 μm, and the Al mixed crystal ratio X is set from 0.55 to 0.3.
A graded index type non-doped Ga+-, AI, As optical guide layer 4 (thickness 0
, 20 μm, the refractive index difference corresponding to the change in Al mixed crystal ratio is 0.13), 3 Gas eeAl 112A
S well layer (60 people thick) and 2 Ga@, 6sA
l ++, 35A Non-doped active layer 5 with multi-quantum well structure having 3 barrier layers (thickness 50 layers), Al mixed crystal ratio X
A non-doped Ga1-xAl and As optical guide layer 6 (thickness 0.20 μm
), p G a @45A I @, 65AS cladding layer 7 (thickness: 0.3 μm) was formed. p GEL@, aAlB, 2AS buffer layer 41 on top of the cladding 17
(thickness 50-100 people), p - Ga (,,,6A
A 6.55AS cladding layer 10 and a p-GaAs cap layer 8 were laminated to form a ridge-like region 31. In forming this ridge-like region 31, the GaAs cap layer 8 was selectively etched with an ammonia-based etchant, and the cladding layer 10 was selectively etched with an HF-based etchant.

During this latter etching, p-Ga,...,A
Since the IB, 2As buffer layer 41 functions as an etching stop layer, the ridge-like region 31 is symmetrical and formed with good controllability. Furthermore, p-G a, , A I ,
, 2As If the thickness of the s buffer layer 41 is 50 to 100, there is almost no variation in the refractive index difference Δn between the inside and outside of the ridge-shaped region 31 depending on the presence or absence of the nucleus layer.

When the width dW of the ridge-like region 31 is 3.5 μm and the thickness dc of the p-cladding layer 7 outside the ridge-like region 31 is 0.3 μm, the oscillation threshold current is 18 mA, which is 2 mW.
Self-sustained oscillation was observed at outputs above. The return light noise level was -135 dB/) (2 or less). Also, the appearance rate of such elements per wafer was 80%.

In this example, the Al mixed crystal ratio y of the cladding layer 7.10
The difference Δx (=y
-x) was set to 0.20, but ΔX was set to 0.10-0.30.
range (the corresponding refractive index difference is 0.065 to 0.2
0), self-sustained oscillation was observed. The structure near the active layer 5 is the same as that shown in FIG. 8(b), and examples of combinations of Al mixed crystal ratios X and y in which self-sustained oscillation was observed are shown in Table 1 below.

Ol 35 0, 35 0, 35 0, 35 0, 35 0, 40 0, 40 0, 40 0, 40 0, 45 0, 45 0.5,0 Table 1 O250 0, 55 0, 45 0, 60 0, 65 0, 50 0, 55 0, 60 0, 65 0, 55 o, 65 0, 70 ΔX O915 0, 20 0, 10 0, 25 0, 3°O310 0, 15 0, 20 0, 25 0 , 10 0, 20 0, 20 The shape of the ridge-like region 31 may be any of the various shapes illustrated in the first to fourth embodiments.
By setting the width dW of 2 to 5 μm and the thickness da of the ρ-cladding layer 7 outside the ridge-like region 31 to a range of 0.2 to 0.6 μm, each combination of the above A1 mix ratios can be obtained. Self-oscillation and low noise characteristics were observed.

In addition, in each combination example of the above-mentioned A1 mixed crystal ratio, the refractive index difference Δn between the inside and outside of the ridge-shaped region 31 is 3×10 −4 to 1×1
It is within the range of 0-2. Although we fabricated elements with a difference ΔX in Al mixed crystal ratio of 0.3 or more (correspondingly, a refractive index difference of 0.20 or more), no automatic oscillation was observed in them, and most of the elements It oscillated in single mode. this is,
This is considered to be because the refractive index difference Δn became too large due to self-focusing due to too much light being confined in the active layer and the light guide layer.

The A1 mixed crystal ratio distribution in the vicinity of the active layer 5 of the fifth embodiment of the present invention is shown in FIG. .6 is G a a
8sA l 11. tsA S (thickness 0.15μm)
The cladding layer 3.7 is composed of layers with a single mixed crystal ratio of Ga
e, sA l 6. It was composed of an A s layer.

In this embodiment as well, the width dW of the ridge-like region 31 is set to 2 to 2.
By setting the thickness dc of the p-cladding N7 outside the ridge-like region 31 within the range of 0.2 to 0.6 μm within the range of 5 μm, the difference ΔX in the A1 mixed crystal ratio can be reduced to 0.5 μm. 10
By setting it in the range of ~0.30, self-sustained oscillation is ij!
It was measured. However, the optical output value for automatic oscillation is
It differed depending on the combination of set values of W, da, and ΔX.

Furthermore, similar results were obtained even when the structure above the ground layer 7 was made similar to that of each of the second to fourth embodiments described above.

Cross-sectional components of the sixth embodiment of the present invention This embodiment, shown in FIG. 10, has a ridge-like region 3
1 was formed f& (Fig. 3), and p was formed by MOCVD method.
-Ga@, 4AIs, sAS buried layer 42 is formed on the sA 1l15As cladding layer 7 as a current layer 9ff1 to bury the sides of the ridge-shaped region 31, and the entire surface of the buried layer 42 is filled with ρ. -〇aAs contact layer 10
a is stratified 0a. A larger than cladding layer 7
The buried layer 42 having a mixed crystal ratio of 1 also has the function of giving 0 to the difference in refractive index between the inside and outside of the ridge-shaped region 31.

In such a configuration, p- outside the ridge-like region 31
The thickness dc of the cladding layer 7 is set to 0. When the thickness was 3 μm, the refractive index difference Δn was I×10−′, and automatic oscillation was observed at an output of 2 mW or more. In addition, when the thickness dc and the refractive index difference Δn are set to values within the above-mentioned ranges, and the structure near the active layer 5 and the light guide layer 4.6 is the same as that of each of the above-mentioned 2nd to 5th embodiments. Similar results were obtained in both cases.

The cross-sectional structure of the seventh embodiment of the present invention is shown in FIG. 11. In this embodiment, after forming the laminated structure shown in FIG. 2 in the same manner as in the first embodiment, Area 3
High resistance region 43 by implanting 10 tons other than 1
was formed. Width d of striped area 31
When W and the thickness da of the p-cladding layer 7 portion outside the region 31, which is not made highly resistive, were set within the above-mentioned range, self-sustained oscillation was observed. Further, the same results were obtained when the active layer and the light guide layer were made similar to those of the second to fifth embodiments described above.

(Effects of the Invention) As described above, the semiconductor laser device of the present invention has a low oscillation threshold, generates self-sustained oscillation over a wide output range, and has a low return optical noise level, so it can be used as a light source for optical disk systems.
suitable. Further, since the semiconductor laser device of the present invention can be manufactured by the MBE method or MOCVD method, which has excellent layer thickness controllability, it is possible to manufacture a device that generates automatic oscillation with a high yield.

Figure 1 (a) is a cross-sectional view of the first example, Figure (b) is a distribution diagram of the A1 mixed crystal ratio near the active layer of that example, and Figures 2 and 3 are A cross-sectional view for explaining the manufacturing process of the first embodiment ρ1, FIG. 4 is a graph showing the oscillation spectrum of the first embodiment, and FIG. 5 is a relative noise intensity (R4N
), FIG. 6(a) is a cross-sectional view of the second example, FIG. 6(b) is a distribution diagram of the A1 mixed crystal ratio near the active layer of that example, and FIG. 7(a) is a graph showing Cross-sectional view of the third embodiment, the same figure (
b) is a distribution diagram of the A1 mixed crystal ratio near the active layer of the example,
FIG. 8(a) is a cross-sectional view of the fourth embodiment, FIG. 8(b) is a distribution diagram of the Al mixed crystal ratio near the active layer of that embodiment, and FIG. 9 is a diagram of the active layer of the fifth embodiment. Distribution diagram of nearby A1 mixed crystal ratio, 1st
0 and 11 are sectional views of the sixth and seventh embodiments, respectively, FIG. 12 is a sectional view of a conventional example, and FIG. 13 is a sectional view of another conventional example.

1... Substrate, 2... Superlattice buffer layer, 3.7...
- Cladding layer, 4.6... Optical guide layer, 5... Active layer, 8... Cap layer, 9... SiN4 insulation film, 3
1... Ridge-shaped region, 42... Buried layer.

that's all

Claims (1)

[Claims] 1. A first cladding layer formed on a semiconductor substrate;
A laminated structure having a first optical guide layer, an active layer having a quantum well structure, a second optical guide layer, and a second cladding layer;
and a striped current injection region formed above the laminated structure, a refractive index waveguide based on a refractive index difference inside and outside the current injection region is formed, and the width of the current injection region is 2 to 5 μm. The thickness of the second cladding layer outside the current injection region is 0.2 to 0.6 μm, and the thickness of the second cladding layer is 0.2 to 0.6 μm, and the thickness of the second cladding layer is 0.2 to 0.6 μm, and the thickness of the second cladding layer is 0.2 to 0.6 μm. The refractive index difference between the first and second cladding layers is 0.065 to 0.
．． 20. A semiconductor laser device.