JPH0582886A - Semiconductor laser element - Google Patents

Abstract

PURPOSE:To provide a distributed feedback semiconductor laser element having the current of a small lasing threshold value which lases in a single vertical mode. CONSTITUTION:A semiconductor laser element has a double hetero structure wherein an active layer which generates guided emission rays is placed between an n-type clad layer 3 and first and second p-type clad layers 5 and 8. A high resistive layer 6 of a rattan blind type having a cycle corresponding to the diffraction grating is formed between the first and second p-type clad layers 5 and 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device capable of oscillating in a single longitudinal mode.

[0002]

2. Description of the Related Art Currently, an optical distributed feedback type semiconductor laser device is being actively researched as a light source device suitable for long-distance and large-capacity optical communication, optical information processing, optical recording, optical applied measurement and the like. Such a semiconductor laser device generally has an optical waveguide structure for efficiently generating stimulated emission light by a heterojunction in which an active layer is sandwiched between transparent clad layers, and an interface of the transparent optical waveguide layer close to the active layer. Light distribution feedback is performed by so-called refractive index coupling, in which a saw-toothed diffraction grating is formed on the active layer to periodically change the refractive index of the optical waveguide layer.

However, the above semiconductor laser device is
It is uncertain which of the two longitudinal modes near the Bragg reflection condition will oscillate, and there is also the problem that the two modes may oscillate at the same time.

As a semiconductor laser device for solving this problem, as shown in FIG. 7, optical distribution feedback is performed by gain coupling based on a periodic perturbation of a loss coefficient or gain coefficient accompanied by optical loss, and a single longitudinal mode is obtained. Appl. Phys. Lett. 55 (1989) is an optical distributed feedback semiconductor laser device that can oscillate at
It is disclosed in P1606 to P1608.

In the figure, 51 is an n ^{+ type} GaAs substrate.
On the n ^{+ type} GaAs substrate 51, n type GaAlAs is formed.
Cladding layer 52, GaAlAs active layer 53, p-type GaA
The 1As barrier layer 54 is formed in this order.

On the p-type GaAlAs barrier layer 54, a p-type GaAlAs light absorption layer 55 having a second-order diffraction grating (period 240 nm) shape is formed.

A p-type GaAlAs clad layer 56 and ap ⁺ GaAs contact layer 57 are formed in this order on the p-type GaAlAs light absorption layer 55, and SiO _{2 is} inserted on the upper surfaces of both ends of the p ⁺ GaAs contact layer 57. Layer 5
8 is formed.

On the upper surface of the p ⁺ GaAs contact layer 57 and the insertion layer 58 and on the lower surface of the n ^{+ type} GaAs substrate 51,
The p-type electrode 59 and Au-Ge made of Au-Zn, respectively.
A semiconductor laser device is formed by forming an n-type electrode 60 of.

Such a semiconductor laser device is manufactured as follows.

First, the n-type GaAlAs clad layer 52 and the GaAlAs active layer 5 are formed on the n ^{+ -type} GaAs substrate 51 by the first liquid phase epitaxial (LPE) method.
3, p-type GaAlAs barrier layer 54, and p-type GaAl
The As light absorption layer 55 is formed in this order.

Next, the p-type GaAlAs light absorption layer 55 is formed.
After applying a resist film thereon, it is exposed and developed by a holographic interference exposure method to form a pattern. Then, using the patterned resist film as a mask, the p-type GaAlA is formed by reactive ion etching (RIE).
s The light absorption layer 55 is formed in the shape of a secondary diffraction grating (period 240 nm).

Then, by the second LPE method, the p-type GaAlAs light absorption layer 55 formed in the above-mentioned diffraction grating shape.
On top, a p-type GaAlAs clad layer 56, ap ⁺ -type GaA
The s contact layer 57 is formed in this order.

Finally, after the insertion layers 58 made of SiO ₂ are formed on the upper surfaces of both ends of the p ^{+ -type} GaAs contact layer 57, the upper surfaces of the p ⁺ contact layer 57 and the insertion layer 58,
On the lower surface of the n ^{+ type} GaAs substrate 51, Au-Z
The p-type electrode 59 made of n and the n-type electrode 60 made of Au—Ge are formed to form the semiconductor laser device shown in FIG.

Such a semiconductor laser device has a characteristic that since the distributed light feedback is performed by the periodical change of the gain, the semiconductor laser device oscillates in a single longitudinal mode, and the single longitudinal mode property is not influenced by the reflection at the end facet of the field. ..

[0015]

However, in the structure as described above, p is located close to the GaAlAs active layer 53.
Since the GaAlAs light absorption layer 55 is formed,
Since the light of a specific wavelength in the As active layer 53 is also absorbed, the threshold current required for laser oscillation is usually 5 to 6
A double value, that is, a high value of 300 mA was required.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a semiconductor laser device of single longitudinal mode oscillation having a low laser oscillation threshold current.

[0017]

In a distributed feedback semiconductor laser device, a high resistance layer which does not absorb light like a blind having a period corresponding to a diffraction grating is provided in the vicinity of an active layer for generating stimulated emission light. It is characterized by

[0018]

According to the semiconductor laser device of the present invention, since the blind-shaped high resistance layer has a period corresponding to that of the diffraction grating, a current flows through the active layer in accordance with the period of the diffraction grating to cause a current to flow in the active layer. Since a current distribution having a periodicity is generated, the net gain coefficient of the light wave propagating in the axial direction of the resonator with respect to the electromagnetic field changes in the period, and distributed feedback by gain coupling is performed. Since only the longitudinal mode oscillates and light absorption does not occur in the high resistance layer, the oscillation threshold can be reduced.

[0019]

Embodiments of the present invention will be described with reference to the drawings.

1 and 2 are a sectional view showing a lateral section and a front view showing a light emitting side end face of a semiconductor laser device of a first embodiment according to the present invention, respectively.

In the figure, 1 is an n-type GaAs substrate. On this substrate 1, an n-type GaAs buffer layer 2 and an n-type Al are formed.
_{A 0.4} Ga _0.6 As clad layer 3, an undoped GaAs active layer 4, and a first p-type Al _0.4 Ga _0.6 As clad layer 5 are successively formed.

A portion of the first p-type Al _0.4 Ga _0.6 As cladding layer 5 has, for example, a width of 1500Å and a period of 3
A blind-shaped p-type GaAs antioxidant layer 6 having 770Å is formed, and a width 22 corresponding to the third-order diffraction grating is formed in the remaining portion.
70Å, Al such as Al ₂ O _{3 in the} shape of a blind with a period of 3770Å
A high resistance layer 7 made of a system oxide that does not absorb light is formed. The p-type GaAs anti-oxidation layer 6 is set to have a very thin thickness, for example, about 10 to 100 Å, which corresponds to the wavelength of the stimulated emission light generated in the active layer. Does not absorb.

A second p-type Al _0.4 Ga _0.6 As cladding layer 8 having a ridge portion 9 is formed on the p-type GaAs anti-oxidation layer 6 and the high resistance layer 7 which does not absorb light, and the ridge portion 9 is formed. A p-type GaAs cap layer 10 is formed on the upper surface of the substrate 9.

The second p-type Al_0.4Ga_0.6As Crush
The p-type G is formed on the p-type GaAs cap layer 10 and the p-type GaAs cap layer 10.
Except for the upper surface of the aAs cap layer 10, SiO 2₂, S
i₃N _FourAnd the like insulating layer 11 is formed.

A p-type G in which the insulating layer 11 is not formed
A p-type electrode 15 of Au—Cr or the like is formed on the upper surface of the aAs cap layer 10, and an n-type electrode 16 of Au—Sn or the like is formed on the lower surface of the n-type GaAs substrate 1.

A method of manufacturing such a semiconductor laser device will be described.

First, an n-type GaAs substrate 1 is prepared, and molecular beam epitaxy (MB) is formed on its (100) crystal orientation plane.
According to the method E), the substrate temperature is 650 ° C. and the growth rate is 1.2 μm.
Under the condition of m / h, the n-type carrier concentration is 1 × 10 ¹⁸ cm ⁻³ ,
Si-doped n-type GaAs buffer layer 2 having a layer thickness of 0.5 μm, n-type carrier concentration 3 × 10 ^{17 to} 5 × 10 ¹⁷ c
m ⁻³ , layer thickness 1 μm, Si-doped n-type Al _0.4 G
a _0.6 As clad layer 3, undoped undoped GaAs active layer 4 having a layer thickness of 0.05 μm, p-type carrier concentration of 2 × 10 ^{17 to} 3 × 10 ¹⁷ cm ⁻³ , first layer of Be doped with a layer thickness of 800 Å P-type Al _0.4 Ga _0.6 As clad layer 5, p-type carrier concentration 5 × 10 ^{17 to} 7 × 10 ¹⁷ cm ⁻³ ,
A Be-doped p-type GaAs antioxidant layer 6 having a layer thickness of 50 Å is formed in this order (first step).

Then, the p-type G is formed by spin coating.
A resist film having a thickness of 0.4 μm is formed on the entire upper surface of the aAs oxidation preventing layer 6. Then, this resist film is exposed and developed by a two-beam interference exposure method to form a blind pattern with a period of 3770Å (second step).

Then, using the patterned resist film as a mask, the p-type GaAs anti-oxidation layer 6 is treated with a phosphoric acid-based etchant (for example, a molar ratio of H ₃ PO ₄ : H ₂ O ₂ : H _2).
O = 4: 6: 20) for about 2 seconds to completely remove the p-type GaAs antioxidant layer 6 and further remove the p-type Al _0.4 Ga _0.6 As clad layer 5 to a depth of 100 Å. (Third step).

Then, the resist film is removed by oxygen plasma to expose the p-type GaAs oxidation preventing layer 6, and the surface of the first p-type Al _0.4 Ga _0.6 As clad layer 5 is oxidized to form Al. forming an Al-based oxides such as ₂ O ₃ 3
A blind-shaped high resistance layer 7 having a period of 3770Å corresponding to the next diffraction grating is formed. By the oxygen plasma, G
Although a and As-based oxides are also generated, these oxides can be sublimated before the next crystal growth, so that p-type GaAs is used.
The oxide layer on the antioxidant layer 6 does not substantially pose a problem (fourth step).

Subsequently, the p-type carrier concentration is 8 × 10 ¹⁷ to 1 × 10 ¹⁸ cm ⁻³ by the MBE method under the same conditions as above.
Second p-type Al _0.4 G doped with Be having a layer thickness of 1 μm
a _0.6 As clad layer 8, p-type carrier concentration 1 × 10 ¹⁹
cm ⁻³ , Be-doped p-type Ga having a layer thickness of 0.5 μm
The As cap layer 10 is formed in this order (fifth step).

Subsequently, the p-type GaAs cap layer 10 is formed.
After forming a resist film thereon by spin coating, the resist film is patterned into stripes having a width of about 4 μm. Using the patterned resist film as a mask, a phosphoric acid-based etchant (for example, a molar ratio of H ₃ PO ₄ : H ₂
O ₂ : H ₂ O = 3: 1: 1) to give the second p-type A
After forming the _0.4 Ga _0.6 As cladding layer 8 and the p-type GaAs cap layer 10 in a ridge shape with a width of about 4 μm, the resist film is removed. Accordingly, the second p-type Al _{0. 4} Ga _0.6
The ridge portion 9 is formed on the As clad layer 8 (sixth step).

Next, on the second p-type Al _0.4 Ga _0.6 As cladding layer 8 and the p-type GaAs cap layer 10, an insulating layer of SiO ₂ , Si ₃ N ₄ or the like having a layer thickness of 0.2 to 0.5 μm is formed. 1
1 is formed by a thermal CVD method, a sputtering method, or the like.
Then, after removing the insulating layer 11 formed on the upper surface of the p-type GaAs cap layer 10, a p-type electrode 15 made of Au—Cr is formed on the upper surface of the p-type GaAs cap layer 10 and the n-type GaAs layer 1 is formed. N made of Au-Sn on the lower surface
The shaped electrode 16 is formed to form a device formation wafer.
After that, the element-formed wafer is divided to complete a semiconductor laser element having a cavity length of 300 μm as shown in FIGS. 1 and 2 (seventh step).

FIG. 3 shows an applied current 8 of such a semiconductor laser device.
It is a spectrum characteristic when 0 mA is ± 10 mA modulated at a frequency of 500 MHz. From this figure, it can be seen that single longitudinal mode oscillation is well maintained.

Further, the current of the semiconductor laser device shown in FIG.
The optical output characteristics are shown. However, no coating film or the like is applied to the end surface that emits light.

It can be seen from FIG. 4 that the threshold current required for laser oscillation is as small as 60 mA, which is a significant improvement over the conventional distributed feedback semiconductor laser device utilizing gain coupling.

With the above-mentioned structure, the current flows through the active layer at a period substantially corresponding to the period of the diffraction grating due to the high resistance layer in the shape of a blind, which has a period corresponding to the period of the diffraction grating. Since a current distribution having a characteristic is generated, the net gain coefficient for the electromagnetic field of the light wave propagating in the axial direction of the resonator changes in the above period, and distributed feedback by gain coupling is performed. Only the mode oscillates, and unlike the conventional gain-coupled distributed feedback semiconductor laser device, the light absorption layer is not used and a high resistance layer that does not absorb light is used. Therefore, the oscillation threshold can be reduced.

Next, FIG. 5 shows a semiconductor laser device according to a second embodiment of the present invention. The same parts as those in the first embodiment are designated by the same reference numerals and the description thereof will be omitted.

In the figure, 16 is a p-type GaAs antioxidant layer, and a high resistance layer 17 having a period of 3770Å that does not absorb light in the form of a blind penetrates the p-type GaAs antioxidant layer 16 to form a first p-type. The Al _0.4 Ga _0.6 As clad layer 5 is also formed so as to partially penetrate. A second p-type Al _0.4 Ga layer is formed on the p-type GaAs oxidation prevention layer 16 and the high resistance layer 17.
_{A 0.6} As clad layer 8 is formed.

Next, a method of manufacturing such a semiconductor laser device will be described.

First, the same process as the second step of the first embodiment is performed.
After that, using the patterned resist film as a mask,
For example, if the acceleration voltage is 10 KeV, N₂ ⁺Ion to ion
P-type carrier concentration 5 × 10 by injection method¹⁷~ 7 × 10
¹⁷cm^-3, Be-doped p-type GaA with a layer thickness of 50Å
The p-type GaAs anti-oxidation layer is injected into the anti-oxidation layer 16.
The p-type carrier concentration under the stop layer 16 is 2 × 10.¹⁷~ 3 x 10¹⁷
cm^-3, Be-doped first p-type with a layer thickness of 800Å
Al_0.4Ga_0.6Fill the As clad layer 5 to a depth of 250Å
Enter, N₂ ⁺P-type GaAs antioxidation layer containing ions 1
6 and p-type Al _0.4Ga_0.6Connection of As clad layer 5
Absorbs light by deteriorating crystallinity and increasing resistance
The high resistance layer 17 is formed.

After that, the same steps as the third to seventh steps of the first embodiment are performed to complete the semiconductor laser device shown in FIG.

In such a semiconductor laser device, only a single longitudinal mode oscillates, and the measurement result of the current-optical output characteristics shows that the threshold current is as small as 55 mA, which is substantially the same as that of the first embodiment.

Further, FIG. 6 shows that Si is formed on both end faces of the light emitting side.
A coating layer having a reflectance of 10%, such as O _2, and a reflectance of 70
FIG. 5 is a diagram showing measurement results of current-light output characteristics of the semiconductor laser device of the present example in which a coating layer of 10% was formed.

From FIG. 6, even in the semiconductor laser device of this embodiment in which the light emitting end face is asymmetrically coated, the conventional G
It can be seen that a light output of 20 mW, which is almost double that of the aAs system distributed feedback laser device, is obtained.

In the semiconductor laser device of the present embodiment as well, as in the first embodiment, a high resistance layer which does not absorb light in the form of a blind is formed with a period corresponding to that of the diffraction grating. Can oscillate with a low oscillation threshold.

In the above embodiment, the high resistance layer 7 is p-type A.
Although the structure is such that a part of the l _0.4 Ga _0.6 As clad layer 5 is penetrated, the structure may penetrate the clad layer 5 or the clad layer 5 may not be penetrated at all.

In each of the above embodiments, the high resistance layer is made of p-type Al.
_Although it is provided in the _0.4 Ga _0.6 As clad layer, it may be provided in the vicinity of the undoped GaAs active layer, for example, it may be provided in the n-type Al _0.4 Ga _0.6 As clad layer, and further in the undoped GaAs active layer. You may form so that it may contact.

When the high resistance layer is provided in the vicinity of the active layer by ion implantation, the ions to be implanted may be other than N ₂ ⁺ ions, but it is more desirable that the mass is larger because it is hard to be implanted inside. ..

In the above embodiment, the crystal growth was carried out by using the MBE method. However, the metal organic chemical vapor deposition method (MO
CVD method), liquid phase epitaxy method or the like may be used.

Further, in the above-mentioned embodiment, the high resistance layer is formed in the cycle corresponding to the third-order diffraction grating, but it may be formed in the cycle corresponding to the diffraction gratings of other orders. However, since the current may diffuse in the cavity length direction in the active layer, it is better to form the high resistance layer so as to correspond to, for example, a third-order diffraction grating having a wide period so that the current distribution in the active layer is good. It has desirable periodicity and is desirable.

Furthermore, the present invention is not limited to a semiconductor laser device made of an AlGaAs semiconductor, and in other semiconductor laser devices, when a high resistance layer having a period corresponding to the diffraction grating is formed near the active layer, the Since the current flows in the active layer at a period that substantially matches the period of the lattice, the net gain coefficient for the electromagnetic field of the light wave propagating in the axial direction of the resonator will change at the period, and distributed feedback by gain coupling will occur. Therefore, only the single longitudinal mode oscillates and the oscillation threshold can be reduced.

The fact that the high resistance layer and the antioxidant layer do not absorb light means that they do not absorb at least stimulated emission light, and includes the case where they hardly absorb light.

[0054]

In the distributed feedback semiconductor laser device of the present invention, since the blind high resistance layer which does not absorb light has a period corresponding to that of the diffraction grating, the active layer is formed in accordance with the period of the diffraction grating. Since a current flows and a current distribution having a periodicity is generated in the active layer, the net gain coefficient with respect to the electromagnetic field of the light wave propagating in the axial direction of the resonator is changed in the above period, which results from the gain coupling. Since distributed feedback is performed, only a single longitudinal mode oscillates and the oscillation threshold can be reduced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a lateral cross section obtained by dividing a center of a semiconductor laser device according to a first embodiment of the present invention.

FIG. 2 is a front view showing a light emitting side end face of the semiconductor laser device.

FIG. 3 is a spectrum diagram showing an emission spectrum of the semiconductor laser device.

FIG. 4 is a characteristic diagram showing current-light output characteristics of the semiconductor laser device.

FIG. 5 is a cross-sectional view showing a lateral cross section obtained by cutting the center of a semiconductor laser device according to a second embodiment of the present invention.

FIG. 6 is a characteristic diagram showing current-light output characteristics of the semiconductor laser device.

FIG. 7 is a perspective view showing a conventional semiconductor laser device.

[Explanation of symbols]

 3 n-type clad layer 4 active layer 5 first p-type clad layer 8 second p-type clad layer 7 high resistance layer 17 high resistance layer

Claims (1)

[Claims] 1. A distributed feedback semiconductor laser device, characterized in that a high resistance layer which does not absorb light in a blind shape having a period corresponding to a diffraction grating is provided in the vicinity of an active layer for generating stimulated emission light.