JPH02152292A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To provide a semiconductor laser device for an optical disk system with reduced astigmatism and with satisfactory return light noise characteristics by reducing the thickness of a second cladding layer at opposite sides of a ridge region move than that in a resonator, in the vicinity of the exit end surface of the laser light. CONSTITUTION:A ridge wavelength type semiconductor laser device includes, on an n-GaAs substrate 1, an n-GaAs buffer layer 2, an n-Al0.5Ga0.5As first cladding layer 3, an undoped AlxGa1-xAs GRIN layer 4, an undoped multiple quantum well active layer 5, an undoped AlyGa1-yAs GRIN layer 6, a p-Al0.5Ga0.5 As second cladding layer 7 (1mum thick), and a p-GaAs contact layer 8, all being grown by a MBE process. The thickness of the second cladding layer 7 on opposite sides of the ridge region is selected to be 0.4mum for example. Further, the thickness of the second cladding layer 7 on opposite sides of the ridge region located within 100mum from the laser light exit end surface is selected to be 0.2mum for example. The resulting semiconductor laser device is excellent in optical output, return light amount, and relative noise intensity, and is reduced in astigmatism.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor laser element for an optical disk system that has good return optical noise characteristics and small astigmatism.

(Prior Art) Recently, optical disc systems such as digital audio systems using compact discs (CDs) and video systems using video systems (VDs) have become widespread. The optical disks used in these optical disk systems have audio and video signals recorded thereon by providing concave pit rows on a recording film formed on, for example, a disk-shaped thin plastic substrate. The recorded signal is reproduced by irradiating the signal surface of the recording film through the substrate with a laser beam spot narrowed down by an objective lens. A semiconductor laser element is used as a light source for signal reproduction.

In the optical disk system as mentioned above.

Since it is necessary to focus the laser beam spot down to the wavelength, the optical path from the light source to the disk.

The optical path returning from the disk cannot be spatially separated. Therefore, 1. Usually, a 174 wavelength plate is used to rotate the polarization plane of the incident light and reflected light.

The above two optical paths are optically separated by a polarizing beam splitter.

However, since the plastic substrate of the optical disk has birefringence, the optical paths are not separated sufficiently. When reflected light from the disk returns to the semiconductor laser element, the semiconductor laser element and the disk form a type of resonator. Therefore, if the disk changes even slightly, the oscillation conditions of the semiconductor laser element change, making the output unstable and causing noise. Such noise is called return optical noise.

Further, generally, a semiconductor laser element has astigmatism, and the cross-sectional shape of the laser beam in a far field region sufficiently distant from the laser beam emitting end face is elliptical. The elliptical shape of the far-field image is flat.

Reliability in signal reproduction decreases.

Therefore, in order to efficiently image this elliptical laser beam as a circular minute spot on the optical disk,
An elliptical laser beam is shaped into a circle.

Compact discs are among the optical discs.

Because the audio signal has a narrow band, it is recorded digitally. Therefore, it is possible to provide redundancy to the recorded signal, and the tolerance level for the above-mentioned return optical noise is relatively high. Against this.

Video discs have a wide video signal band.

recorded in analog form. Therefore, it is not possible to provide redundancy to the recorded signals as in a compact disc, and the return optical noise directly affects the quality of the reproduced image on the video disc. Therefore, a particularly high S/N ratio is required in video disc systems.

Therefore, in optical disk systems that require a high S/N ratio, there is a method to reduce the noise level by adding a high frequency current to the drive current of a longitudinal single mode semiconductor laser element to cause longitudinal multimode oscillation. is used.

However, such a method requires an additional circuit for superimposing the high frequency current on the drive current, increasing the manufacturing cost of the optical disk system. therefore,
The semiconductor laser element itself is required to have good return optical noise characteristics.

(Problem to be solved by the invention) Generally, a refractive index waveguide type semiconductor laser device.

The oscillation spectrum is a single longitudinal mode, and the astigmatism is very small, but the return light noise characteristics are poor.

On the other hand, a gain waveguide type semiconductor laser device has a single oscillation spectrum in longitudinal multimode, but has low coherence and therefore has good return optical noise characteristics. This vertical multi-mode semiconductor laser device originally has a higher noise level than a vertical single-mode semiconductor laser device, but it also has large astigmatism, so it needs to be corrected.

The present invention is intended to solve the above-mentioned conventional problems, and its purpose is to provide a semiconductor laser element for an optical disk system that has good return light noise characteristics and small astigmatism. There is a particular thing.

(Means for Solving the Problems) A semiconductor laser device of the present invention has a first cladding layer, an active layer, a second cladding layer, and a contact layer formed above a semiconductor substrate, and includes a first cladding layer, an active layer, a second cladding layer, and a contact layer. A striped ridge region is formed in the second cladding layer.

The semiconductor laser device is of a ridge waveguide type, and the second cladding layer on both sides of the ridge region has a thickness smaller in the vicinity of the laser beam emitting end face than in the inside of the resonator.

As a result, the above objective is achieved.

Another semiconductor laser element of the present invention has a first cladding layer, an active layer, a second cladding layer, and a contact layer formed above a semiconductor substrate, and the contact layer and the second cladding layer are provided with stripes. A shaped ridge region is formed. A ridge waveguide type semiconductor laser device, wherein the second cladding layer on both sides of the ridge region has a thickness smaller in the vicinity of the laser beam emitting end surface than in the inside of the resonator, and the second cladding layer has a thickness smaller than that in the cavity. at least two semiconductor layers having a larger band gap and lower refractive index than the active layer; and at least one etching layer having a band gap larger than the photon energy of the laser beam and inserted between each of the semiconductor layers. and a stop layer, thereby achieving the above object.

Each of the above layers in the semiconductor laser device of the present invention is formed on the semiconductor substrate by, for example, molecular beam epitaxy (MBB) or metal organic chemical vapor deposition (OCVD).

As described above, in the Fuji waveguide semiconductor laser device of the present invention, the thickness of the second cladding layer on both sides of the ring region is smaller in the vicinity of the laser beam emitting end face than in the inside of the resonator. Its typical layer thickness is 3000 Å or less near the laser beam emitting end face, and 200 Å or less inside the cavity.
00 to 6000 people.

The above-mentioned etching stop layer can be etched selectively with respect to the second cladding layer, or is composed of semiconductor layers having different etching rates. Typically, the layer thickness of the etch stop layer is set to be less than 500 nm.

If such an etching stop layer is used in combination with the crystal growth method described above, the accuracy of one layer thickness will be improved compared to the method of controlling the layer thickness by setting the etching time to section #A, and the layer thickness can be controlled at the atomic level. becomes possible.

Furthermore, the quality of the processed semiconductor layer is improved without damage to the crystal, which occurs when ion beam etching is used. Therefore, the etching process is simplified and a semiconductor laser device with high reliability can be obtained with good reproducibility.

(effect) Generally, a ridge waveguide type semiconductor laser device.

It has a structure as shown in FIG. Above the semiconductor substrate a, a first cladding layer C2, an active layer C2, a second cladding layer d
, and a contact layer e are sequentially laminated, and a striped ridge region f is formed in the center of the contact layer e and the second cladding layer d.

When the layer thickness W of the second cladding layer d on both sides of the ridge region f is large, the ridge waveguide type semiconductor laser device becomes a gain waveguide type semiconductor laser device, and has a vertical multilayer structure as shown in FIG. 8a. Gives the oscillation spectrum of the mode. vice versa,
When the layer thickness W of the second cladding layer d is small, the semiconductor laser device becomes a refractive index waveguide type semiconductor laser device and provides a single longitudinal mode oscillation spectrum as shown in FIG.

As described above, the gain waveguide type semiconductor laser device has good return optical noise characteristics, but has large astigmatism. Therefore, in such a gain waveguide type semiconductor laser device,
If the thickness of the second cladding layer on both sides of the ridge region is made smaller in the vicinity of the laser beam emitting end face than in the inside of the resonator.

A portion near the laser beam emitting end face has a refractive index waveguide structure, making it possible to improve astigmatism.

(Example) Examples of the present invention will be described below.

FIG. 1 is a schematic cross-sectional view of the Norifuji waveguide type semiconductor laser device of this embodiment. The semiconductor laser device was manufactured as follows. First, an n-GaAs substrate 1 (Si
= lXIO"cm), on which is an n-GaAs buffer layer 2 (thickness 0.5μnl+si = IX
IO"cm-'), n-Alo, 5Gao, SA
S first cladding layer 3 (thickness 1 μm, Si=l
0X10l11'), undoped A1. Ga, -
, As GRTN layer 4 (thickness 0.15 μn, x =
0.5 → 0.32), undoped multiple quantum well active layer 5 (as shown in FIG. 3(b), undoped Al o, + zGao, *eAs well layer with a thickness of 70 people, and three layers, as shown in FIG. 3(b)). 40 undoped Ale, zzGao
, 611As barrier layers are stacked alternately).

To undoped, Gat-yAsGRIN layer 6 (thickness 0
．． 15μm.

)' =0.32→0.5 ), p-AIo, 5G
a6. sAS second cladding layer 7 (thickness 1 pm, B
e= 1 xlO'8am-'), and p-GaAs
Contact layer 8 (thickness 0.3 am, Be-I
x10"cm-3) using the MBE method at a growth temperature of 700
It was grown continuously at 'C' (Figure 3(a)).

Next, a striped photoresist pattern 9 (width: 3 μm, thickness: 1 μm or more) is formed using photolithography, as shown in FIG.

By irradiating and etching with an Ar ion beam, the thickness of the second cladding layer 7 on both sides of the ridge region is reduced to zero.
．． It was set to 4 μm. Furthermore, 100 mm from the laser beam emitting end face
On the second cladding layer 7 in the resonator section exceeding μm,
By forming a photoresist pattern (thickness of 1 μm or more) using photolithography technology and etching it by irradiating it with an Ar ion beam, the layer thickness of the second cladding layer 7 on both sides of the ridge region within 100 μm from the laser beam emitting end face is determined. was set to 0.271 m (Fig. 3(d)).

After removing the photoresist pattern, a SiNx insulating film 1o was first formed over the entire surface using plasma CVD, and then etching was performed using photolithography to remove the SiNx insulating film at the top of the ridge.

A two-selective etchant (
A mixed solution of HF and NH4F) was used. n-GaAs
The back surface of the substrate 1 was polished to a thickness of approximately 100 μm. Next, AuZn/Au and AuGe/Ni are deposited on the p-side and n-side, respectively, to form the p-type electrode 11 and the n-side.
A mold electrode 12 was formed. And the cavity length is 250μm
It was divided into two chips and mounted on a heat sink in a junction-up format.

The semiconductor laser device thus obtained is as follows.

Automatic oscillation occurs at a dark value current of 20 mA, and the measurement frequency is 720
Under the conditions of kHz, bandwidth 10 kHz, and optical path length 30 mm, it exhibited excellent characteristics such as an optical output of 3 mw, a return light amount of 3% or less, and a relative noise intensity ratio (1?IN) of -140 dB or less.

Furthermore, the astigmatism ΔZ was a small value of 5 μm or less. Furthermore, the characteristic temperature of the semiconductor laser element is CW
It was 150 K during operation.

In this embodiment, the ridge region was formed by irradiation with an Ar ion beam, but it can also be formed in the same way by using a reactive ion etching (RIE) method.

On the other hand, for comparison, a semiconductor was fabricated in the same manner as in Example 1 except that the second cladding layer on both sides of the ridge region near the laser beam emitting end face was not thinned and the layer thickness was made uniform at 0.4 μm. A laser device was manufactured. The semiconductor laser device thus obtained has an astigmatism ΔZ of 20 to 30a.
It was large, m, and was not suitable for a laser element for an optical disk system.

Figure 2 is a schematic cross-sectional view of the Fuji waveguide type semiconductor laser device of this example. The semiconductor laser device was manufactured as follows. First, in the same manner as in Example 1, FIG.
A structure as shown in d) was formed. Next, using the LPE method, both sides of the ridge region were coated with high resistance undoped Alo,
bsGao, 5sAs layer 13. Mg doped p- to 16.5sGao, 4sAs layer 14. Then, a Te-doped n-Alo, 5sGao, and bsΔS layer 15 was buried, and an Mg-doped p-GaAs cap layer 16 was grown over the entire surface. The back surface of the n-GaAs substrate 1 was polished to a thickness of approximately 100 μm.

Next, uZn/Au and AuGe/Ni were vapor-deposited on the p-side and n-side, respectively, to form a p-type electrode 17 and an n-type electrode 18. Then, it was divided into chips with a resonator length of 250 μm, and mounted on a heat sink in a junction-down manner.

The semiconductor laser device of this example obtained in this way exhibited the same oscillation threshold current and noise characteristics as the semiconductor laser of Example 1. Furthermore, the characteristic temperature of the semiconductor laser device of this example is (180℃ during J operation,
It was found that the temperature characteristics were 30 times higher than those of the semiconductor laser device of Example 1, and the temperature characteristics were improved.

FIG. 4 is a schematic cross-sectional view of the Norifuji waveguide type semiconductor laser device of this embodiment. The semiconductor laser device was manufactured as follows. First, an n-GaAs substrate 21 (Si
= I
XIOIllcm-”), n-AIo, 5Ga6.
sAs first cladding layer 23 (thickness 1 μm, 5i=IX
IQ"am-'), undoped Al, Gap-, A
s GRIN layer 24 (thickness 0.15 μm, x=o,
5→0.32), undoped multiple quantum well active layer 25
(As shown in Figure 6(b).

Thickness 70 people undoped AIo, + zGao, 5
3 aAs wool layers and 40mm thick undoped Al
o, zzGao, bll^S barrier layers are stacked alternately).

Undoped AlyGa+-yAs GRIN layer 26 layer thickness 60.15 pm.

y = 0.32→0.5), p-Alo, 5Ga
o, 5As first semiconductor layer 27 (thickness 0.15 μm, B
e = IX1018cm-'), p-Alo,
+5Gao, a5As first etching stop layer 28
(100 people thick.

Be=IXl0I8cl”), p-AIo, 5
Gao, sASAs 2nd Semiconductor. , r 5 Gao. 55As second cutting stop layer 30 (thickness 100, Be=IXIO”cm
-'), p-AI,, 5Gao. s to S third semiconductor layer 31 (thickness 0.6 pre, Be=I
cm-').

and p-GaAs contact l-layer 32 (thickness 0.3
pm, Be=IX1019cm-3) as MBE
It was grown continuously at a growth temperature of 700″C by the method (
Figure 6(a)). In the semiconductor laser device of this example. p-Alo. 5Gao, sAs first semiconductor layer 27, p-Alo, Is ao. 55AS 1st processing stop layer 2EL p-Alo. 5Gaa. 5A
SAs 2nd Semiconductor 29.

p-Al615Ga6, IIs/Is second etch stop layer 30;

and p-Alo. 5Gao. sAs third semiconductor layer 3
1, a second cladding layer 41 is constructed.

Next, a striped photoresist pattern 33 (width: 3 μm, thickness: 1 μm or more) was formed using photolithography. Then, as shown in Figure 6(C),
NH40t! p- using −HzO□-based etching solution
The GaAs contact layer 32 is etched and further etched with HF.
p-Ale. using a p-Ale. 5Gao. s
The As third semiconductor layer 31 was etched. p-Al6,
+5 Gao. esAs second etching stop layer 3
Because 0 is set.

p-Ale. 5Gao. The etching rate of the sAs third semiconductor layer 31 suddenly decreased when the second etching stop layer 30 was reached. Therefore, there was no need to strictly control the etching time, and the etching work was easy. Moreover, the layer thickness accuracy in the ridge region has been improved.

Next, p-Alo. +5Gaa. A photoresist pattern was formed on the a5As second etching stop layer 30 using photolithography. Then, using NH40H-HzOz etching solution, p-
A.I. ,,.

Ga. ), B5As second etching punch [30
Work/Nching. Furthermore, using HF-based etching solution,
″′Cp-AI., 5Gao.s to S second semiconductor layer 2
I engineered 9. p-Alo. +5cao.
Since the fi5^S fifth^S transistor pump layer 28 is provided, p-Ale, 5Ga,..., 5As are used.
The speed of etching the second semiconductor layer 29 sharply decreased at the point U, when the etching layer G was reached. Therefore, there was no need to strictly control the machining/chinking time, and the machining/chinking work was easy. Moreover,
The thickness accuracy of the second cladding layer 41 on both sides of the cladding region has been improved.

By the etching process as described above, the second layer on both sides of the glue/nozzle region G within 100 μm from the laser beam emitting end face is removed.
Cladding layer 41 (first stage, construction, construction, etc.)

The layer thickness of the second cladding layer 41 (excluding the second layer 28) is 0.15 μm, and the thickness of the second cladding layer 41 (excluding the second layer 28) is 0.15 μm, and the layer thickness is 0.15 μm from the emission end face. The layer thickness was set to 0.4 μm (Fig. 6(d)
)).

After removing the photoresist pattern 33, the Blazuf CV
Using method D. First, a SiNx insulating film 34 was formed over the entire surface and then etched using photolithography to remove the SiNx insulating film at the top of the ridge. For etching of SiNx insulation film.

A selective etchant (a mixed solution of HF and NHaF) was used. The back surface of the n-GaAs substrate 1 was polished to a thickness of approximately 100 μm. Next, AuZn/^U and AuGe/Ni were deposited on the p-side and n-side, respectively, to form a p-type electrode 35 and an n-type electrode 36. Then, it was divided into chips with a resonator length of 250 μm and mounted on a heat sink in a junction-up manner.

The semiconductor laser device thus obtained is as follows.

Automatic oscillation occurs at a threshold current of 20 mA. Measurement frequency 720
kHz.

Under the conditions of a bandwidth of 10 kHz and an optical path length of 30 mm, it exhibited excellent characteristics such as an optical output of 3 mW, a return light amount of 3% or less, and a relative noise intensity ratio (RIN) of -140 dB or less.

Furthermore, the astigmatism ΔZ was a small value of 5 μm or less. Furthermore, the characteristic temperature of the semiconductor laser element is CW
150 during operation.

On the other hand, for comparison, a semiconductor was fabricated in the same manner as in Example 3, except that the second cladding layer on both sides of the ridge region near the laser beam emitting end face was not thinned and the layer thickness was made uniform at 0.4 μm. A laser device was manufactured. The semiconductor laser device thus obtained has an astigmatism ΔZ of 20 to 30μ.
It was large, m, and was not suitable for a laser element for an optical disk system.

Back side flame treatment↓ FIG. 5 is a schematic cross-sectional view of the ridge waveguide type semiconductor laser device of this example. The semiconductor laser device was manufactured as follows. First, in the same manner as in Example 3, FIG.
A structure as shown in d) was formed. Then, use the LPE method to remove both sides of the ridge area.

High resistance undoped A16. -, Ga6.5As layer 36 and n-Alo, 5Ga6. It was buried with a SASAs layer, and further a p-GaAs cap layer 38 was grown over the entire surface. The back surface of the n-GaAs substrate 1 was polished to a thickness of approximately 100 μm. Next.

AuZn/8U and AuG on n side and n side respectively
A p-type electrode 39 and an n-type electrode 40 were formed by vapor depositing e/Ni. Then, it was divided into chips with a resonator length of 250 μm, and mounted on a heat sink in a junction-down manner.

The semiconductor laser device thus obtained is as follows.

It exhibited the same oscillation dark value current and noise characteristics as the semiconductor laser device of Example 3. Furthermore, the characteristic temperature of the semiconductor laser device of this example is 180℃ during CW operation, which is 30℃ higher than that of the semiconductor laser device of Example 3.
It was found that the temperature characteristics were improved.

(Effects of the Invention) According to the present invention, a ridge waveguide semiconductor laser device in which the thickness of the second ground layer on both sides of the ridge region is smaller in the vicinity of the laser beam emitting end face than in the inside of the resonator can be provided with high reproducibility. well obtained.

Such a semiconductor laser device has good return light noise characteristics, small astigmatism, and high reliability.

Therefore, the semiconductor laser device of the present invention is extremely useful as a semiconductor laser device for optical disk systems. Further, if the second cladding layer is composed of a plurality of semiconductor layers and an etching stop layer is inserted between each of the semiconductor layers, the etching process is simplified and the thickness of the second cladding layer can be easily controlled. Therefore, such a semiconductor laser device according to the present invention can be manufactured with a high yield, and greatly contributes to the development of optical disk systems.

4. Figure 1 is a schematic cross-sectional view of a ridge waveguide type semiconductor laser device which is one embodiment of the present invention, and Figure 2 is a ridge waveguide type semiconductor laser device which is another embodiment of the present invention. Schematic cross-sectional diagram of
3(a) to 3(d) are schematic diagrams for explaining the manufacturing process of the semiconductor laser device shown in FIGS. 1 and 2, and FIG. 4 is an example of an embodiment of the present invention using an etching stop layer. A schematic cross-sectional diagram of an example ridge waveguide semiconductor laser device,
FIG. 5 is a schematic cross-sectional view of a ridge waveguide semiconductor laser device according to another embodiment of the present invention using an etching stop layer, and FIGS. 6(a) to 6(d) are similar to FIGS. 7 is a schematic cross-sectional view showing the structure of a general ridge waveguide type semiconductor laser device, and FIGS. 8(a) and (b) are respectively FIG. 2 is a characteristic diagram showing the oscillation spectra of a longitudinal multimode gain waveguide semiconductor laser device and a longitudinal single mode refractive index waveguide semiconductor laser device.

1, 21・=n-GaAs substrate, 2.22-n-
GaAs banfer layer, 3,23-n-Alo,
5Gao, 5As first cladding layer.

4.24-...Undoped"AlXGa,-,AsG
RIN Jii (x = 0.5→0.32),
5.25...Undoped multiple quantum well layer, 6.2
6...Undoped AlXGa+-g As GRIN
layer (x = 0.32→0.5), 7'・'p
-AIo, 5Gao, sAS second cladding layer, 8
, 32-p-GaAs contact layer, 10.34...
・SiNx insulating film, If, 17.35.39・p pear-shaped electrode, 12.18° 36, 40...n-type electrode, 13
...High resistance undoped A1o, bsGao, zs^
S layer, 14.Mg doped I)-AIo, 55caO
, a, As layer.

15=4e doped n-Ale, 1sGao, bs8S layer, 16.Mg dolph.

p-GaAs Kip layer, 27-p-Alo, 5G
ao, sAs first semiconductor layer, 28...p-AIo,
+5 Gao, esAs first etching stop layer +2
9...p-Ale, 5Gao, =, As second semiconductor layer, 30...p-Alo, I 5Gao, 5
sAs second processing stop layer, 31...p-Al
o, 5Gao, sAs third semiconductor layer, 36-...
High resistance amplifier A1°, 5 Ga6. sAs layer.

37...n Alo,s Ga,,sAs layer.

1st figure 38...p GaAs cap layer.

41...Second Kuratu Do layer.

Below Up

Claims (1)

[Claims] 1. A first cladding layer, an active layer, a second cladding layer, and a contact layer formed above a semiconductor substrate, and a striped ridge region in the contact layer and the second cladding layer. is a ridge waveguide type semiconductor laser device in which a layer thickness of the second cladding layer on both sides of the ridge region is such that in the vicinity of the laser beam emitting end face,
A semiconductor laser element that is smaller than the inside of its resonator. 2. It has a first cladding layer, an active layer, a second cladding layer, and a contact layer formed above a semiconductor substrate, and a striped ridge region is formed in the contact layer and the second cladding layer. A ridge waveguide type semiconductor laser device, wherein the thickness of the second cladding layer on both sides of the ridge region is such that in the vicinity of the laser beam emitting end face,
smaller than the inside of the resonator, the second cladding layer has at least two semiconductor layers with a larger band gap and smaller refractive index than the active layer, and a band gap larger than the photon energy of the laser light; and at least one etching stop layer inserted between each of the semiconductor layers.