JP2763781B2 - Semiconductor laser device and method of manufacturing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor laser device, and more particularly, to a semiconductor laser device having a ridge waveguide structure.

(Prior art) A semiconductor laser device having a ridge waveguide structure has a refractive index using a laser operation layer provided on a flat substrate by a molecular beam epitaxy (MBE) method, a metal organic chemical vapor deposition (MOVPE) method, or the like. A waveguide mechanism is formed, and has a very simple structure. Since this semiconductor laser device can employ a quantum well structure in the active layer region, a low threshold current characteristic of about 10 mA can be easily obtained.

FIG. 3 shows a typical example of a semiconductor laser device having a ridge waveguide structure. This device has an n-GaAs substrate 31 on which n-
GaAs buffer layer 32, n-AIGaAs cladding layer 33, AIGaAs graded-index optical guide layer 34, GaAs quantum well active layer 35, AlGa
As refractive index gradient type optical guide layer 36, p-AlGaAs cladding layer 3
7 and the p-GaAs cap layer 38 grown by the MBE method. After growth by the MBE method, a striped resist mask is formed on the surface of the p-GaAs cap layer 38,
A ridge portion 39 is formed by removing both sides of this mask by etching to a part of the p-clad layer 37. afterwards,
An SiN insulating film 40 is formed by a plasma CVD method, and a ridge portion 39 is formed.
Only the portion of the SiN insulating film 40 in the upper flat region is selectively removed. Next, the substrate 31 is polished to a thickness of about 100 μm to form a p-side electrode 41 and an n-side electrode 42, and then each element is obtained by a cleavage method.

In the semiconductor laser device of FIG. 3 manufactured as described above, since the active region has a quantum well structure having a gradient-index optical guide layer, its threshold current is a low value of about 10 mA. In addition, such an element is mounted in a junction-up manner, so that CW oscillation can be easily obtained. But,
When mounted in a junction-up manner, the heat dissipation characteristics are poor, so that there are disadvantages such as poor high-output characteristics or reduced reliability.

The semiconductor laser device shown in FIG. 4 is for solving those problems. In the device shown in FIG. 4, two grooves 43 defining a ridge portion 39 are formed by selectively etching the p-clad layer 37 and the p-cap layer 38. The laser characteristics of such an element depend on the shape of the ridge portion 39 (mainly the width of the ridge portion 39 and the p-cladding layer 37 on both sides thereof).
Remaining thickness). Therefore, in order to obtain desired characteristics, the shape of the ridge portion 39 must be precisely controlled. In other words, the etching for forming the groove 43 must be performed precisely.

On the other hand, in such a laser device, the p-cladding layer 37
Has a thickness of 1 μm or more, so as to reduce the loss caused by the absorption of the light leaking from the active region into the p-cladding layer 37 by the cap layer 38. Also, the thickness of the cap layer 38 is usually set to 0.5 μm or more.
Therefore, in order to form the ridge portion 39, etching must be performed to remove the semiconductor layer having a thickness of 1 μm or more. It is difficult to precisely control such etching (for example, O. Wada et al., Electron Lett., Vol. 21, p. 1025).
(1985)) By the way, semiconductor laser devices used in the field of handling analog signals such as video discs are required to have extremely good low noise characteristics. In such a field, a laser element is used in which the laser structure is appropriately controlled so that a self-excited oscillation phenomenon occurs, the oscillation spectrum is multimode, and each spectrum is widened.

FIG. 5 shows an example of such a self-pulsation laser element. The laser device in FIG. 5 is a VSIS (V-channeled Subs
trate Inner Stripe) A laser called
-N-GaAs current confinement layer 52, p-AlGaAs on GaAs substrate 51
Layer 53, p-AlGaAs active layer 54, n-AlGaAs cladding layer 55,
The n-GaAs cap layer 56 is formed by a liquid phase growth method. 57 is an n-side electrode, and 58 is a p-side electrode. A V-shaped groove 59 reaching the substrate 51 from the surface of the current confinement layer 52 forms a stripe-shaped current path.
The region above 59 is set to be the oscillation region.

In the laser device of FIG. 5, the thickness of the p-cladding layer 53 on both sides of the V-shaped groove 59 is appropriately increased (for example, 0.3 μm).
m), self-excited oscillation can be obtained by weakening the refractive index waveguide mechanism (Hayashi et al., IEICE Technical Report MW84-24, p.65 (1984)).
For that purpose, it is necessary to precisely control the crystal growth in the region above the V-shaped groove 59. However, such VSIS
In the liquid phase growth method used for laser growth, since such precise control is difficult, there is a problem that reproducibility is inferior and manufacturing yield is poor. Further, it is difficult to reduce the threshold current of the VSIS laser, and the threshold current is about 50 mA.

In the semiconductor laser device having the ridge waveguide structure shown in FIGS. 3 and 4, the self-excited oscillation described above is refracted by appropriately increasing the remaining thickness of the p-cladding layer 37 on both sides of the ridge portion 39. It can be obtained by weakening the waveguide mechanism. However, for that purpose, the width of the ridge portion 39 and the p-cladding layer 37
Must be precisely controlled.
It is difficult to accurately etch a semiconductor layer as thick as about μm.

(Problems to be Solved by the Invention) As described above, in order to obtain desired characteristics (especially self-pulsation phenomenon), in a conventional semiconductor laser device, etching must be performed precisely, which is difficult to manufacture. It is. There is also a problem that the reproducibility is poor and the yield is poor. Furthermore, there is a structure which can be mounted only by junction up, and has a drawback that high output characteristics are poor and reliability is reduced.

The present invention has been made in view of such a situation, is easy to manufacture, excellent in reproducibility, high yield,
It is an object of the present invention to provide a semiconductor laser device which has low threshold current characteristics and can be mounted with a junction down.

(Means for Solving the Problems) In a semiconductor laser device of the present invention, a laminated structure composed of at least a first clad layer, an active layer, and a second clad layer is formed on a semiconductor substrate in this order. In a semiconductor laser device in which the two cladding layers have at least an upper ridge portion and a lower flat portion, an interface between the upper ridge portion and the lower flat portion of the second cladding layer or a lower flat portion in the upper ridge portion With the vicinity of the interface with the boundary as a boundary, the impurity concentration of the upper ridge portion in the region above the boundary is made higher than the impurity concentration in the region below the boundary, thereby achieving the above object.

At this time, a flat portion may be further provided on the upper ridge portion of the second cladding layer.

In the method of manufacturing a semiconductor laser device according to the present invention, at least a first cladding layer, an active layer, a low-doping second cladding layer, and a high-doping second cladding layer are formed in this order on a semiconductor substrate. And a second step of etching the second cladding layer to form a ridge portion. The lowermost surface of the ridge portion is formed of a low-doping second cladding layer and a high-doping second cladding layer. Formed at the interface or in the low-doping second cladding layer near the interface,
Thereby, the above object is achieved.

The light guide layer preferably has a gradient refractive index configuration. The thickness of the second cladding layer is 4000Å10000Å
It is preferable to set it in the range. When the thickness of the second cladding layer is 4000-6000 °, the doping concentration of the second cladding layer is 3 × 10 ¹⁸ at a portion where the thickness in contact with the second optical guide layer is at least 1000 °. cm ^-3 or less,
It is preferable that the area on the side opposite to the second light guide layer be larger than 3 × 10 ¹⁸ cm ⁻³ .

The buried layer may be made of a high-low resistance material,
It may be made of an insulating material.

(Examples) The present invention will be described below with reference to examples.

 FIG. 1 (a) shows a cross-sectional view of an example of a real finger of the present invention.

The manufacturing process of this embodiment will be described. First, the n-GaAs substrate 1
On top are an n-GaAs buffer layer 2, an n-AlGaAs cladding layer 3, an AlGaAs graded-index optical guide layer 4, a GaAs single quantum well active layer 5, an AlGaAs graded-index optical guide layer 6, and a Be-doped p-type. -AlGaAs cladding layer 7 and Be-doped p-GaAs
The cap layer 8 was grown by MBE. p-AlGaAs
The thickness of the cladding layer 7 is about 6000Å, and the p-GaAs cap layer 8
Was about 2000 mm thick. The doping concentration of the p-cladding layer 7 is, as shown in FIG. The doping concentration is about 1 × 10 ¹⁸ cm ⁻³ and about 8 × 10 ¹⁸ cm ^{−3 in} the area on the cap layer 8 side (areas b to c). The doping concentration in the area b to c is about 5 × 10 ¹⁸ cm ^−3. 1
It is preferably set to × 10 ¹⁹ cm ^-3 . A transition region of the doping concentration was provided between the region (ab) on the light guide layer side and the region (bc) on the cap layer side. This change in doping concentration was performed by adjusting the temperature of the Be cell. The doping concentration of the cap layer 8 was the same as that of the cap layer side regions (b to c).

Next, after forming a stripe-shaped resist mask on the cap layer 8 by using the photolithography method, etching is performed halfway through the p-clad layer 7 to form the ridge 12.
(Width about 2.5 μm). The etching is performed so as to remove the semiconductor layer having a total thickness of about 4000 ° including the cap layer 8 (2000 °) and the portion having a thickness of about 2000 ° of the p-cladding layer 7. The remaining thickness of layer 7
It was about 4000Å. That is, the interface of the etching and the portion where the doping concentration of the p-cladding layer 7 changes (portion b) substantially match, or the interface of the etching is slightly closer to the substrate 1 than this portion.

The SiN insulating layer 9 is formed to a thickness of about 4000 mm by the plasma CVT method, and is formed on the ridge portion 12 by the photolithography method.
The SiN insulating layer 9 was selectively removed. As a result, the upper surface of the ridge portion 12 (that is, the surface of the cap layer 8) and the surface of the SiN insulating layer 9 substantially match. Polishing of the substrate 1, formation of the electrodes 10 and 11, and cleavage were carried out in the same manner as in the above-mentioned conventional example to obtain a semiconductor laser device.

In the present embodiment, since the thickness of the p-cladding layer 7 on both sides of the ridge portion 12 is about 4000 °, the refractive index homogenous groove formed by the ridge portion 12 is considerably weak. Further, since the interface of the etching and the portion where the doping concentration of the p-cladding layer 7 changes substantially coincide, or the interface of the etching is slightly closer to the substrate 1 than the portion, the p-cladding layer The spread of the current in the inside 7 is suppressed and small. For these reasons, the device of this embodiment can easily obtain self-sustained pulsation and output 0.5 to 8 mW.
Self-excited oscillation was observed in the range. When the return light amount is 3% or less, the relative noise intensity is suppressed to -135 dB / Hz or less.
Good noise characteristics were obtained. The oscillation threshold current is 15 ~
20 mA.

Since the upper surface of the ridge portion 12 and the surface of the SiN insulating layer 9 substantially coincide with each other, the device of this embodiment can be mounted by a junction-down method in which the active layer region side is mounted. Therefore, heat radiation characteristics are improved, and high output characteristics and reliability are improved. The maximum light output of the device of this example was 70 mW. In addition, when a reliability test was performed in a high-temperature atmosphere (50 ° C.) with an optical output of 5 mW, oscillation was stabilized for about 4000 hours or more.

Note that a configuration is adopted in which the sides of the ridge portion 12 are buried so that the upper surface of the ridge portion 12 and the surface of the SiN insulating layer 9 substantially match each other.
It is not impossible to apply to the conventional ridge waveguide structure shown in the figure. However, for this purpose, the thickness of the SiN insulating film 40 must be 1 μm or more, and the heat radiation effect is reduced, and the reliability due to the stress caused by the thick insulating layer may be reduced.

FIG. 2 (a) shows another embodiment of the present invention. The laminated structure of this embodiment is substantially the same as that of the embodiment of FIG. 1 (a), except that the cap layer 8 is not provided, and the Zn diffusion region 15 is formed at a depth of 3000 ° above the p-cladding layer 7. did. The height of the ridge portion 12 (that is, the depth at which the p-cladding layer 7 is etched) was 4000 °. The doping concentration of the p-cladding layer 7 was 1 × 10 ¹⁸ cm ⁻³ . For the Zn diffusion region 15,
Since the impurity concentration increases to about 3 × 10 ¹⁹ cm ⁻³ by Zn diffusion, the p-type impurity concentration in this region changes as shown in FIG. 2 (b). The characteristics of the present embodiment were similar to those of the above-described embodiments.

In the present embodiment, since the Zn diffusion region 15 is formed,
A good contact with low resistance can be formed. Further, since the cap layer is not formed, the etching depth can be reduced by the thickness of the cap layer, and the etching control becomes easier. Alternatively, the thickness of the p-cladding layer 7 can be increased by the thickness of the cap layer, so that the characteristics can be further improved.

Although the active layer 5 has a single quantum well structure in each of the above embodiments, it may have a multiple quantum well structure. The light guide layer does not need to be of the gradient index type, and may have a normal SCH type configuration.

(Effects of the Invention) According to the present invention, by making the ridge portion highly doped, it is possible to reduce the resistance in the ridge, which is the narrowest area of the current path, and prevent the operating voltage from increasing.
In addition, by making the flat portion low-doped, it is possible to reduce optical loss due to free electron absorption due to impurity reduction near the active layer and to reduce reactive current due to lateral current spreading in the flat portion. Thus, it is possible to reduce the laser operating current and, consequently, the operating voltage. Thus, power consumption can be reduced by reducing the operating voltage and the operating current.

Further, since the impurity distribution in the lateral direction near the active layer is made uniform, it is possible to eliminate the influence of impurities on the optical mode and realize a stable oscillation mode.

Further, since the amount of impurities near the active layer is reduced, reliability can be improved.

[Brief description of the drawings]

FIG. 1A is a sectional view of an embodiment of the present invention, and FIG.
FIG. 2A is a graph schematically showing the distribution of the doping concentration above the second cladding layer in the embodiment, FIG. 2A is a cross-sectional view of another embodiment, and FIG. FIGS. 3 to 5 are cross-sectional views of a conventional example, schematically showing the distribution of the doping concentration above the second cladding layer in the embodiment of FIG. 1... N-GaAs substrate, 2... N-GaAs buffer layer, 3.
... n-AlGaAs cladding layer (first cladding layer), 4 ...
AlGaAs refractive index gradient type light guide layer (first light guide layer),
5 GaAs single quantum well active layer, 6 AlGaAs refractive index gradient type optical guide layer (second optical guide layer), 7 p-AlGa
As cladding layer (second cladding layer), 8 ... p-GaAs cap layer, 9 ... SiN insulating layer, 12 ... ridge part, 15 ... Z
n diffusion area.

Continued on the front page (72) Inventor Mukosei Takahashi 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Masahiro Hosoda 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation ( 72) Inventor Toshiro Hayakawa 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (56) References JP-A-54-107281 (JP, A) JP-A-2-1099387 (JP, A) (58) ) Surveyed field (Int.Cl. ⁶ , DB name) H01S 3/18

Claims (2)

(57) [Claims] 1. A laminated structure comprising at least a first clad layer, an active layer, and a second clad layer is formed on a semiconductor substrate in this order, and the second clad layer comprises at least an upper ridge portion and a lower ridge portion. In a semiconductor laser device having a flat portion, an interface between an upper ridge portion and a lower flat portion of the second cladding layer or a vicinity of an interface with a lower flat portion in the upper ridge portion is set as a boundary, and is higher than the boundary. Wherein the impurity concentration of the upper ridge portion in the region is higher than the impurity concentration in the region below the boundary. 2. A first step of forming at least a first cladding layer, an active layer, a low-doping second cladding layer, and a high-doping second cladding layer on a semiconductor substrate in this order; A second step of etching the cladding layer to form a ridge portion, wherein the lowermost surface of the ridge portion is formed at an interface between the low-doping second cladding layer and the high-doping second cladding layer or at a low level near the interface. A method for manufacturing a semiconductor laser device, wherein the method is formed in a doped second cladding layer.