JPH0770784B2 - Lateral injection laser and manufacturing method thereof - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser having a structure in which a current is injected into an active layer in parallel with an epitaxial growth layer.

[0002]

2. Description of the Related Art In a lateral injection laser, a pn junction is formed in the horizontal direction in an epitaxial growth layer by epitaxially growing all layers without adding impurities and diffusing the impurities in necessary portions. With this structure, the total capacitance between the pn regions can be significantly reduced as compared with the conventional type semiconductor laser in which a current is injected perpendicularly to the epitaxial growth layer, and the electrodes of p and n can be arranged on the same plane. Therefore, it is considered suitable for integration into an optical integrated circuit.

As an example of a specific structure of the lateral injection laser, Fluta et al., Electronics Letters, Vol. 24, No. 20, pp. 1282 to 1283 (A.FURUTA et a.
L., Electron. Lett. Vol. 24, No. 20, pp. 1282-1283) are known. In this reference, an undoped AlGaAs layer and multiple quantum wells are formed on a semi-insulating GaAs substrate, and then undoped Al is further formed thereon.
A GaAs layer is grown, two rows of grooves are formed in the AlGaAs layer on the upper side along the light emitting region of the active layer, and n-type and p-type impurities are diffused in the grooves to sandwich the active layer in the lateral direction. The structure is shown with a pn junction formed. Electrodes are connected to the n-type region and the p-type region, respectively, through GaAs contact layers formed in the portions of the upper AlGaAs layer other than the groove. Therefore, the current from the p-side electrode is laterally injected into the active layer via the contact layer, the p-type region formed in the upper AlGaAs layer, and the p-type region at the bottom of the groove. This current is further transferred to the n-type region at the bottom of the groove on the opposite side, AlGaAs on the upper side.
It reaches the n-side electrode through the n-type region formed in the layer and the contact layer.

[0004]

However, in the conventional structure, the width of the groove cannot be reduced so much due to processing problems.
The current has to flow for several μm along the bottom of the groove. Moreover, the composition of the bottom portion of this groove is AlG.
It is aAs, and its resistivity is considerably high even if impurities are implanted or diffused. Therefore, the element resistance is increased, and it is difficult to obtain good characteristics.

It is an object of the present invention to solve the above problems and provide a lateral injection laser having a small device resistance.

[0006]

A first aspect of the present invention is a device structure of a lateral injection laser, which comprises a plurality of semiconductor layers epitaxially formed on a semiconductor substrate without adding impurities. At least one of the plurality of semiconductor layers is formed as an active layer that generates stimulated emission light by current injection, and a layer adjacent to the active layer is formed as a light confining layer having a refractive index different from that of the active layer. In a lateral injection laser having p-type regions and n-type regions on both sides substantially in the same plane as the stimulated emission light generation direction, the optical confinement layer includes p-type regions and n-type regions. A part of the region is thinly formed, and at least one of the thinly formed parts, preferably both, is provided with a layer having a lower resistance than the light confinement layer,
An electrode is connected to this low resistance layer.

When a GaAs crystal or a quantum well is used for the active layer and an AlGaAs mixed crystal is used for the optical confinement layer, a p-type region is a p-type region and an n-type region is an n-type GaAs region as a low resistance layer. Is preferred. In the case of the InP system, InGaAs or InGaAsP mixed crystal is used for the active layer, InGaAsP mixed crystal or InP is used for the optical confinement layer, and In doped with high concentration as a low resistance layer is used.
A GaAs mixed crystal can be used.

In the present specification, for ternary or quaternary mixed crystals, AlGaAs, In are used unless otherwise required.
The mixed crystal ratio is omitted as in GaAs and InGaAsP.

It is desirable that the light confinement layer is provided with a guide layer having a refractive index different from that of the surrounding layers on a surface substantially parallel to the active layer along the direction of generation of stimulated emission light.

A second aspect of the present invention is a method of manufacturing the above lateral injection laser, in which a plurality of semiconductor layers including an active layer and a light confinement layer are epitaxially grown on a semiconductor substrate without adding impurities. A first step, a second step of thinly forming portions on both sides of the active layer along the stimulated emission light generation direction, and a second step of forming a p-type region on one of the thinned portions and an n-type region on the other A method of manufacturing a lateral injection laser including three steps, characterized by including a step of growing a low resistance layer on the thinly formed portion, following the second step.

In the second step, the ridge stripe may be left in a portion of the active layer along the stimulated emission light generation direction, and the other portion may be etched. You may grow it.

In the third step of forming the p-type region and the n-type region, impurities may be added by ion implantation or diffusion after the low resistance layer is formed, but in the step of growing the low resistance layer. Impurities may be added to at least one of them.

In the step of growing the low resistance layer, the light confinement layer is grown so as to cover the entire surface, and the p-type region and the n-type region are formed in the third step, and then the upper portion of the ridge stripe is etched. Alternatively, a low resistance layer may be selectively grown only on the side of the ridge stripe. Instead of etching, O ₂ or the like may be ion-implanted on the ridge stripe for insulation.

[0014]

By using a low resistance layer as a layer for passing a current in the lateral direction, the series resistance of the element is reduced, the carrier injection efficiency is increased, the heat generation during driving is reduced, and the oscillation characteristics of the laser element are improved. Be improved. Further, since the DC resistance is reduced, the high frequency characteristics of the laser device are improved.

[0015]

1 is a cross-sectional view showing the structure of a lateral injection laser according to an embodiment of the present invention, showing the structure of a cross section that crosses the stripe direction.

This lateral injection laser has a buffer layer 2, a superlattice buffer layer 3, a cladding layer 4, a GRIN (gradient refractive index) as a plurality of semiconductor layers epitaxially formed on the semiconductor substrate 1 without adding impurities. ) Layer 5, S
A QW (single quantum well) layer 6, a GRIN layer 7, a block layer 8, a guide layer 9 and a cladding layer 10 are provided. SQW
The layer 6 is used as an active layer for generating stimulated emission light by current injection, and the GRIN layer 5 adjacent to the SQW layer 6,
7, clad layer 4, block layer 8 and clad layer 10
Is used as an optical confinement layer having a refractive index different from that of the SQW layer 6. In addition, along the generation direction of the stimulated emission of the SQW layer 6,
A p-type region and an n-type region are provided on both sides in the same plane as the SQW layer 6.

The feature of this embodiment is that the light confinement layer, especially the cladding layer 10 is thinly formed in the p-type region and the n-type region, and the GRIN layer is formed in each of the thinly formed regions. 5, 7, clad layer 4, block layer 8
And the cap layer 11 having a resistance lower than that of the cladding layer 10, and the electrodes 12 and 13 are connected to the cap layer 11, respectively.

Since a part of the clad layer 10 is thinly formed, a ridge stripe is formed on the clad layer 10 along the direction in which the SQW layer 6 generates stimulated emission light. Between the ridge stripe and the SQW layer 6, a guide layer 9 having a refractive index different from that of the surrounding layers is provided on a surface substantially parallel to the SQW layer 6 in order to confine light in the lateral direction.

An example of the composition of each layer is shown below, and FIG. 2 shows a cross section equivalent to that of FIG. 1 of an actually manufactured device. In FIG. 2, (a) is an SEM image photograph, and (b) is a diagram illustrating each part of the photograph of (a).

Substrate 1 Semi-insulating GaAs buffer layer 2 GaAs superlattice buffer layer 3 GaAs / Al _0.6 Ga _0.4 As clad layer 4 Al _0.6 Ga _0.4 As GRIN layer 5 Al _x Ga _1-x As, x = 0.6 → 0.3 SQW layer 6 GaAs GRIN layer 7 Al _x Ga _1-x As, x = 0.3 → 0.6 Block layer 8 Al _0.6 Ga _0.4 As Guide layer 9 Al _0.3 Ga _0.7 As Cladding layer 10 Al _0.6 Ga _0.4 As cap layer 11 GaAs (impurity Si, Zn)

3 to 10 show a method of manufacturing the device shown in FIG. 2, in which a plurality of semiconductor layers including an active layer and an optical confinement layer are epitaxially grown on a semiconductor substrate without adding impurities. 3 to FIG. 5, a second step of thinly forming the portions on both sides of the active layer along the stimulated emission light generation direction is shown in FIG. 6, and subsequent to this second step, the thinly formed portion has a low resistance. The step of growing the layer is shown in FIG. 7, the third step of forming the p-type region in one of the thinly formed portions and the n-type region in the other is shown in FIG. 8, and the subsequent steps are shown in FIGS. 9 and 10.

The process shown in FIG. 3 is the first epitaxial process, and includes the buffer layer 2, the superlattice buffer layer 3, the cladding layer 4, the GRIN layer 5, the SQW layer 6 and the GRI on the substrate 1.
The N layer 7, the block layer 8 and the guide layer 9 were grown.
No impurities were added to any of the layers. MOVPE (metalorganic vapor phase epitaxy) is used for crystal growth, and the growth temperature is 78
It was performed under the conditions of 0 ° C. and a pressure of 100 Torr.

Subsequently, in the rib etching step shown in FIG. 4, the liquid temperature is 5 ° C., H ₂ SO ₄ : H ₂ O ₂ : H ₂ O = 4:
Etching was performed to a depth of about 0.04 μm using 1:90 sulfuric acid / hydrogen peroxide mixture to form a structure for confining light laterally. The etching depth was set based on the idea that "the etching should be stopped in the block layer 8". This is because if the GRIN layer 7 is also etched, the characteristics may be deteriorated, and conversely, if the guide layer 9 remains, the refractive index difference becomes small and the light confinement effect deteriorates.

After the rib etching step, the second epitaxial step shown in FIG. 5 was performed to grow the cladding layer 10 without adding impurities. Also in this step, MOVPE is used for the crystal growth, the growth temperature is 750 ° C., and the pressure is 1
It was performed under the conditions of 00 Torr. Since the composition of the clad layer 10 is the same as that of the block layer 8, these boundaries are shown by broken lines in FIG.

A ridge etching step shown in FIG. 6 is performed subsequent to the second epitaxial step, and the liquid temperature is 30 ° C. and H ₃ P is used.
The ridge stripe was formed by etching the cladding layer 10 using phosphoric acid / hydrogen peroxide mixture of O ₄ : H ₂ O ₂ : H ₂ O = 3: 1: 50. This ridge stripe was of a forward mesa type so that ions could be implanted into the entire surface in a later step.

After that, the third epitaxial step shown in FIG. 7 was performed to grow the undoped cap layer 11. Also in this step, MOVPE was used for crystal growth, and the growth temperature was 750 ° C. and the pressure was 100 Torr.

Subsequently, the ion implantation step shown in FIG. 8 was performed to implant Si on one side and Zn on the other side centering on the ridge stripe. Specifically, one side of the ridge stripe was covered with a resist and ions were implanted into the opposite side, the resist at that time was removed, and then the opposite side was covered with a resist and another type of ions was implanted again. Ion implantation energy and dose amount were set as follows: Si: implantation energy = 100 keV, dose amount = 1 × 10 ¹⁵ cm ⁻² Zn: implantation energy = 150 keV, dose amount = 1 × 10 ¹⁵ cm ⁻² . In terms of characteristics, it is considered that there is no problem if the dose is 3 × 10 ¹⁴ cm ^-2 or more.

In the ion implantation step, both Si and Zn ions are implanted into the cap layer 11 on the ridge stripe, and a pn junction is formed there. Therefore, the cap layer 11 on the ridge stripe was removed by the etching process shown in FIG. In this step, NH ₃ : H ₂ O ₂ = 1: 20 ammonia-hydrogen peroxide mixture having a liquid temperature of 20 ° C. was used to perform etching to a depth of about 0.3 μm. In FIG. 9, it is shown that only the upper portion of the ridge stripe is removed, and the inclined surface portion of the ridge stripe was intentionally covered with a mask even in the trial manufacture.
Actually, as shown in FIG. 2, the cap layer 1 on the slope portion
1 was also etched.

After this etching, as shown in FIG. 10, annealing is performed at 800 ° C. for 120 minutes to disorder part of the SQW layer 6 by thermal diffusion of impurities, and a pn junction is formed with a ridge stripe in between. Formed.

After that, the AuGe / Au n electrodes 12 and Ag are formed on the cap layers 11 on both sides of the ridge stripe, respectively.
The p-electrode 13 of the Zn / Au is provided, the total thickness of about 100μm by polishing the back surface of the substrate 1, for alloying N ₂
The device shown in FIG. 2 was obtained by heating in an atmosphere at 350 ° C. for 2 minutes.

FIG. 11 shows an example of the characteristics of a prototype device having a resonator length of 1000 μm. This characteristic is for continuous oscillation, the horizontal axis represents the injection current, and the vertical axis represents the output power and the bias voltage. In this example, laser oscillation occurred with an injection current of 38.8 mA as a threshold, and the bias voltage at that time was 2.11V. In the case of pulse oscillation, the characteristics of lower threshold current were obtained, and it was found that this element showed sufficient characteristics. Also, when the near-field image was measured, it was found that light was emitted only near the ridge.

12 to 14 show a modification of the steps shown in FIGS. 5 to 6. In this method, instead of growing a low resistance layer on the entire surface and then performing ridge etching, a ridge stripe is selectively grown along the stimulated emission light generation direction of the active layer. That is, the clad layer 10 is grown using the mask 120 having an opening in the portion where the ridge stripe is to be formed, the mask 120 is removed, and the cap layer 11 is further grown.

FIGS. 15 and 16 show a modification of the steps shown in FIGS. 7 and 9. In this method, the cap layer 1
1 is not grown on the entire surface and then the portion above the ridge stripe is not etched, but a mask 150 is provided on the ridge stripe and the cap layer 11 is selectively grown only on the necessary portion.

Although the impurity is added by ion implantation in the above description, the present invention can be similarly implemented even if the impurity is added by a diffusion method to at least one side of the ridge stripe, and the number of times of ion implantation can be reduced. . Further, the present invention can be similarly implemented even if the annealing is performed by lamp annealing. Further, when the cap layer 11 is grown also on the ridge stripe, the present invention can be similarly implemented by ion-implanting O ₂ or the like on the ridge stripe for insulation instead of etching. For the active layer, the present invention can be similarly implemented by using MQW or bulk instead of SQW.

The present invention can be similarly implemented not only when the composition is lattice-matched with GaAs but also when another material is used. For example, a composition example of each layer when lattice matching with InP is shown below.

Substrate 1 InP clad layer (corresponding to buffer layer 2 to cladding layer 4 described above) InP confinement layer (corresponding to GRIN layers 5 and 7 described above) InGaAsP (eg λ _g = 1.1, 1.2, 1.3 μm three-stage structure) SQW layer 6 InGaAs block layer 8 InP guide layer 9 InGaAsP (λ _g = 1.3 μm) Cladding layer 10 InP cap layer 11 InGaAs InGaAsP is selected to have a lattice matching with InP. λ _g is the wavelength of light corresponding to the band gap. The impurity added by ion implantation or thermal diffusion is n
Si is generally used as a mold and Be is commonly used as a p-type.

[0037]

As described above, in the lateral injection laser of the present invention, the series resistance of the device can be reduced by providing the low resistance layer in the layer in which the current flows in the lateral direction. Therefore, carriers can be easily and efficiently injected, and heat generated when the device is driven is reduced, so that the oscillation characteristics of the device are improved. Also, since the efficiency of carrier injection is high,
This has the effect of improving the high frequency characteristics of the device.

Although the lateral injection laser is structurally suitable for integration, the lateral injection laser of the present invention is further excellent in oscillation characteristics, high frequency characteristics, and generates little heat. Therefore, it is particularly effective when used for a light source integrated in an optical integrated circuit.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the structure of a lateral injection laser according to an embodiment of the present invention, showing the structure of a cross section that crosses the stripe direction.

FIG. 2 is a diagram showing a cross section of an actually manufactured device,
(A) is an SEM image photograph, (b) is a figure explaining each part of a photograph.

FIG. 3 is a diagram showing the manufacturing method of the example, showing a first epitaxial step.

FIG. 4 is a diagram showing the manufacturing method of the embodiment, and a diagram showing a rib etching step.

FIG. 5 is a diagram showing the manufacturing method of the example, and a diagram showing a second epitaxial step.

FIG. 6 is a diagram showing the manufacturing method of the example, and a diagram showing a ridge etching step.

FIG. 7 is a diagram showing the manufacturing method of the example, and a diagram showing a third epitaxial step.

FIG. 8 is a diagram showing the manufacturing method of the example, and a diagram showing an ion implantation step.

FIG. 9 is a diagram showing the manufacturing method of the example, and a diagram showing an etching step for the cap layer on the ridge stripe.

FIG. 10 is a diagram showing the manufacturing method of the example, and a diagram showing an annealing step.

FIG. 11 is a diagram showing an example of characteristics of a prototype device.

FIG. 12 is a diagram showing a modified example of the manufacturing method, which is a diagram showing mask formation for selective growth.

FIG. 13 is a diagram showing a modified example of the manufacturing method, and a diagram showing selective growth of the cladding layer.

FIG. 14 is a diagram showing a modified example of the manufacturing method, showing the growth of the cap layer.

FIG. 15 is a diagram showing a modified example of the manufacturing method, showing a mask formation for selective growth.

FIG. 16 is a diagram showing a modified example of the manufacturing method, showing a selective growth of the cap layer.

[Explanation of symbols]

 1 substrate 2 buffer layer 3 superlattice buffer layer 4, 10 clad layer 5, 7 GRIN layer 6 SQW layer 8 block layer 9 guide layer 11 cap layer 12, 13 electrode

Claims (6)

[Claims] 1. A semiconductor substrate is provided with a plurality of semiconductor layers epitaxially formed without adding impurities, and at least one of the plurality of semiconductor layers is an active layer (6) that generates stimulated emission light by current injection. ) Is formed as a light confinement layer (4, 5, 7, 8, 10) having a refractive index different from that of the active layer, and is disposed in the direction of generation of stimulated emission light of the active layer. A lateral injection laser having a p-type region and an n-type region on both sides substantially in the same plane as the active layer, wherein the optical confinement layer includes the p-type region and the n-type region. A lateral injection laser, which is characterized in that it is thinly formed, and a layer (11) having a resistance lower than that of the light confinement layer is provided on at least one of the thinly formed portions, and an electrode is connected to the layer of low resistance. . 2. The active layer is formed of a material containing GaAs as a main component, the optical confinement layer is formed of a material containing AlGaAs mixed crystal as a main component, and the low resistance layer is formed of a material containing GaAs as a main component. A lateral injection laser as defined in claim 1 formed. 3. The light confinement layer is provided with a guide layer (9) having a refractive index different from that of the surrounding layers on a surface substantially parallel to the active layer in the direction of generation of stimulated emission light. The lateral injection laser according to claim 1. 4. A first step of epitaxially growing a plurality of semiconductor layers including an active layer and a light confinement layer on a semiconductor substrate without adding an impurity, and a step of forming both sides of the active layer along a generation direction of stimulated emission light. A method of manufacturing a lateral injection laser, comprising: a second step of forming a thin portion; and a third step of forming a p-type region in one of the thinly formed portions and an n-type region in the other, the second step And a step of growing a low resistance layer on the thinly formed portion, followed by. 5. The method of manufacturing a lateral injection laser according to claim 4, wherein the second step includes an etching step of etching the other portion of the active layer while leaving the ridge stripe in the portion along the stimulated emission light generation direction. . 6. The method of manufacturing a lateral injection laser according to claim 4, wherein the second step includes a selective epitaxial step of growing a ridge stripe along a direction of generation of stimulated emission light of the active layer.