JPH0750815B2 - Method for manufacturing semiconductor optical integrated device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a semiconductor optical integrated device, which is used for optical communication, optical information processing, and the like, in which a semiconductor laser, an optical waveguide, and the like are integrated.

[Prior Art] Optical semiconductor devices used for optical communication and optical information processing are required to have higher performance and higher functionality. For that purpose, it is important not only to improve the performance and functionality of individual devices such as semiconductor lasers and photodiodes, but also to integrate them by combining them. When manufacturing a semiconductor optical integrated device (PIC) by integrating various elements, how to arrange each element on the semiconductor substrate, and how to make the structure as designed Is important.

As a conventional PIC example, a structure in which two semiconductor laser (LD) elements, a multiplexer, and an optical waveguide are integrated is shown in FIG. FIG. 6 (a) is a plan view showing the outline of the PIC, and FIG. 6 (b) is a perspective view showing the structure of the PIC. The active layer 3 exists only in the laser region, and the guide layer 10 exists in the entire laser region and the waveguide region. As an example, when InGaAsP with a wavelength of 1.55 μm is used for the active layer 3, InGaAsP with a wavelength of 1.3 μm is used for the guide layer 10. In order to flow an electric current only in the laser region and electrically insulate between the two laser elements, a high resistance embedded structure filled with high resistance InP13 is used and mesa etching is used.

The manufacturing process of this PIC will be described. The metal organic vapor phase epitaxy (MOVPE) is generally used for crystal growth. First,
On the n-InP substrate 1, n-InGaAsP guide layer 10, InGaAs
After growing the P active layer 3 and the p-InP clad layer 4, the p-InP clad layer 4 and the InGaAsP active layer 3 in the waveguide region are removed by using the SiO ₂ film as a selective mask, and the InGaAsP guide layer and the p-InP clad layer are removed. A layer (not shown in the figure) is grown by burying. Next, the SiO ₂ film is used as a mask for mesa etching to bury and grow the Fe-doped high-resistance InP layer 13. S
After removing the iO ₂ film, a p-InP clad layer 5 and a P ⁺ -InGaAs cap layer 7 are further grown on the front surface. After forming an insulating groove by etching between the laser region and the waveguide region, and between the two laser elements, SiO _{2 is} formed on the entire surface.
A film 21 is deposited, a window is opened in the upper part of the laser portion, a p-side pad electrode 32 is formed, and an n-side electrode 33 is formed on the substrate side to complete the process. In this example, the oscillation wavelengths of the two laser elements cannot be controlled, but if a distributed feedback (DFB) groove is used, the pitch of the grating can be changed, or a multi-electrode structure can be used to form a multi-wavelength light source. it can.

[Problems to be Solved by the Invention] In manufacturing such a PIC, it is important to precisely control the layer structure forming the waveguide. The layer thickness can be sufficiently controlled by using a vapor deposition method such as MOVPE, but the waveguide width is conventionally controlled by mesa etching using SiO ₂ as a mask, and is sufficiently controlled by side etching. There were problems such as lack of controllability.

In order to form the active layer in the laser region and the guide layer in the waveguide region, the active layer is grown over the entire surface, the active layer in the waveguide region is selectively etched and removed, and the guide layer is embedded and grown. It was The active layer and the guide layer must be optically coupled and have a structure that causes less scattering at the joint,
It was difficult to fabricate such a structure by buried growth.

[Means for Solving the Problems] A method for manufacturing an optical semiconductor integrated device for solving the above-mentioned problems is a method of manufacturing a semiconductor optical integrated device, which comprises two parallel stripes whose widths are changed in the stripe direction. A step of forming a dielectric thin film on the semiconductor surface so that the crystal growth region becomes a stripe-shaped opening having a constant width; and using the dielectric thin film as a mask, a quantum well structure is included in the crystal growth region by MOVPE growth. And a step of selectively crystallizing the semiconductor layer, by changing the width of the dielectric thin film,
The crystal growth rate of the semiconductor layer in the crystal growth region is controlled.

[Operation] First, the state of selective growth on a flat substrate, which is the basis of the present invention, will be described.
Shown in the figure. As shown in FIG. 1 (a), a selective growth thin film 21 is formed on a (100) oriented semiconductor substrate 1, and the thin film 21 is selectively removed in stripes in the <011> and <01> directions. When the DH structure is grown by MOVPE, as shown in FIG. 1B, the side surface of the growth layer is the (111) A plane along the <01> stripe and the (111) A plane along the <011> direction. ) Side B is formed. The surface of each growth layer forms a (100) plane, and the interface is very flat. The composition of the mixed crystal is uniform in-plane unless the stripe width of the thin film is extremely wide, and can be sufficiently applied to the active layer and the waveguide layer of the optical semiconductor element and PIC. Further, since the side surface is the (111) plane, if the patterning of the thin film 21 is precise, the controllability of the growth layer width is also very good.

Further, in this selective growth, the growth layer thickness can be changed by changing the stripe width of the thin film. .
FIG. 4 shows an example of the result of measuring the relationship between the stripe width and the growth rate. The wider the stripe width, the higher the growth rate. This is because the amount of growth species that migrate from the thin film and reach the semiconductor surface increases. From this, it becomes possible to control the layer thickness of the selective growth layer. By selectively growing the quantum well structure and changing the well layer thickness, it becomes possible to locally change the equivalent refractive index and the emission energy of the quantum well structure, which increases the degree of freedom in PIC fabrication.

[Embodiment] As a first embodiment, a result of application to a PIC in which a distributed feedback (DFB) semiconductor laser having an active layer having a multiple quantum well (MQW) structure and an electroabsorption semiconductor optical modulator are monolithically integrated will be described. Describe. This PIC oscillates and modulates the light generated from a semiconductor laser with a modulator and emits it from the side of the modulator. Compared to the case where a conventional semiconductor laser is directly modulated, the spectrum spread (chirping) during high-speed modulation It is characterized by being narrow, and is being researched and developed as a device for next-generation optical communication.

Conventionally, an active layer in the laser region (wavelength 1.55 μm composition) was grown over the entire surface, then the active layer in the modulator region was selectively etched and removed, and an absorption layer (wavelength about 1.4 μm composition) was selectively grown. . The active layer and the absorption layer have to be optically coupled to each other and have a structure that causes less scattering at the junction, and it has been difficult to produce such a structure by selective growth.

On the other hand, according to the present invention, by changing the width of the SiO ₂ film, it is possible to change the layer thickness of the selectively grown MQW structure, which makes it possible to grow the active layer and the absorption layer at the same time. Not only can the number be reduced, but a bond with high coupling efficiency can be obtained. FIG. 5 shows the calculation results of the relationship between the well layer thickness and the MQW emission wavelength when the MQW well layer is InGaAs and the barrier layer is InGaAsP. The figure shows the case where InGaAsP of the barrier has a wavelength of 1.3 μm composition and a wavelength of 1.15 μm composition. Based on this figure, when the barrier composition is 1.15 μm, the wavelength is 1.55 μm when the well thickness is about 80 Å and 1.4 μm when the well thickness is about 30 Å.
It turns out that From this calculation result and the relationship between the SiO ₂ stripe width and the growth rate in FIG. 4, the laser region has a stripe width of 20 μm, and the modulator region has a stripe width of 2 μm.

FIG. 2 (a) shows a state in which the MQW structure is selectively grown. The grating is formed only in the laser region of the n-InP substrate 1, and the n-InGaAsP guide layer 10 (wavelength 1.3 μm is formed thereon).
m composition, carrier concentration 1 × 10 ¹⁸ cm ^-3 , layer thickness 1000Å), MQ
The W active / absorbing layer 11 and the p-InP clad layer 4 (carrier concentration 5 × 10 ¹⁷ cm ⁻³ , layer thickness 500 Å) were selectively grown. In MQW, the number of wells is 4 and the layer thickness is InGaAs well thickness 78 Å, 1.
The 15 μm composition InGaAsP barrier thickness was 150Å, the modulator region had a well thickness of 34Å, and a barrier thickness of 66Å. The active layer width is
It was 2.0 μm. In order to suppress the reflectance of the modulator-side end face, a window structure is provided with an ungrown region on the end face.

Then, p-InP layer 5 (carrier concentration 5 × 10 ¹⁷ c
m ^-3 , layer thickness 0.5 μm), and a SiO ₂ film with a width of 10 μm in the laser cavity direction and a spacing of 30 μm in a double stripe shape, and a boundary between the laser and the modulator is a single stripe shape with a width of 10 μm. To a p-InP layer (carrier concentration 1 × 10 ¹⁸ cm ⁻³ , layer thickness 1.0 μm) and P ⁺ -InGaA.
s Cap layer 7 (layer thickness 0.3 μm, carrier concentration 1 × 10 ¹⁹ cm
^-3 ) was selectively grown to confine the current and electric field to the active layer and absorption layer, and to electrically insulate the laser region and the modulator region. Finally, the SiO ₂ film 21 was formed again, a window was opened above the active layer and the absorption layer, the p-side electrode 32 was formed in a pad shape, and the n-side electrode 33 was also formed on the substrate 1 side. Completion drawing is Figure 2 (b)
Is. The cleaved laser region length was 400 μm, and the modulator region length was 200 μm. In addition, the laser side end face has a reflectance of 8
A 0% highly reflective coating was applied.

The oscillation threshold current of a typical device was 18 mA, and the maximum CW optical output was 25 mW. The oscillation wavelength was 1.545 μm, and the extinction ratio when applying −5 V to the modulation region was 25 dB. In addition, the binding efficiency estimated from the quenching property was as high as 95%. As described above, it was confirmed that a good coupled waveguide structure can be easily manufactured by the technique of simultaneously growing an active layer and an absorption layer by the selective growth of the present invention.

Next, as a second embodiment, the result of producing the PIC in which the two-wavelength semiconductor laser array and the optical waveguide shown in FIG. 6 are integrated by using the selective growth of the present invention will be described with reference to FIG. FIG. 3A shows a pattern of the SiO ₂ film 21 when the DH structure is first selectively grown. SiO _{2 in} each area
The width of the growth area surrounded by stripes is 2 μm,
The O ₂ stripe width was 15 μm in one laser region, and 5 μm in the other laser region and the waveguide region. 3 (c) and 3 (d) are sectional views of the completed device (the cutting direction is shown in FIG. 3 (a)). First, n-InP substrate 1
N-InGaAsP guide layer 10 (wavelength 1.3 μm composition, carrier concentration 1 × 10 ¹⁸ cm ^-3 , layer thickness about 1000Å), n-InP etching stop layer 12 (carrier concentration 1 × 10 ¹⁸ cm ^-3 , The MQW active layer 11 and the p-InP clad layer 4 (carrier concentration 5 × 10 ¹⁷ cm ⁻³ , layer thickness about 500Å) were grown. M
QW has 4 wells, and the layer thickness is InGaAs well layer 70Å in one laser region, InGaAsP (wavelength 1.3 μm composition) barrier thickness 150
The other laser region had a well thickness of 50 Å and a barrier thickness of 110 Å. Next, as shown in FIG. 3 (b), SiO
_{2 The} film is patterned, and the p-InP clad layer 4 other than the laser region, the MQW active layer 11 and the n-InP etching stop layer 12 are selectively etched to obtain the non-doped InGaAsP waveguide layer 1
4 (wavelength 1.3 μm composition, layer thickness 1500Å) was embedded and grown. Next, as shown in FIGS. 3C and 3D, p-In
P layer 5 (carrier concentration 1 × 10 ¹⁷ cm ^-3 , layer thickness 0.5 μm), p
-InP layer (carrier concentration 1 × 10 ¹⁸ cm ^-3 , layer thickness 10 μm),
And the P ⁺ -InGaAs cap layer 7 (layer thickness 0.3 μm, carrier concentration 1 × 10 ¹⁹ cm ⁻³ ) were selectively grown, and then the p-side pad electrode 32 was formed on the upper surface of the laser active layer in which the SiO ₂ film 21 was opened. The n-side electrode 33 was formed on the substrate side. Laser cavity length 300μ
m, the laser interval was 50 μm, the waveguide length was 250 μm, and the emission end face had a window structure.

The typical oscillation threshold current of this twin laser is 15mA
The oscillation wavelengths were 1.552 μm and 1.528 μm. The maximum optical output from the end face of the waveguide was 20 mA. Thus, the oscillation wavelength of the MQW laser can be changed by changing the stripe width, and such a technique can be applied to various integrated optical devices.

In the example, the step of etching the MQW active layer 11 in the waveguide region and then burying and growing the waveguide layer 14 was used.
By changing the growth conditions such as the width of the SiO ₂ stripe, it is possible to collectively form by selective growth as in the first embodiment.

Although the SiO ₂ film is used as the dielectric film serving as the selective growth mask in each of the above-described embodiments, other dielectric films such as Si ₃ N ₄ film may be used.

[Effects of the Invention] As described above, when the manufacturing method of the present invention is used, mesa etching is not required, and uniform active layers and waveguide widths can be manufactured with good control. Not only that, the growth layer thickness can be changed by changing the mask width, and the emission wavelength and effective refractive index of the MQW structure can be changed. By using these technologies, it has become possible to fabricate various semiconductor optical integrated devices (PICs), which conventionally required complicated processes, with relative ease and controllability.

[Brief description of drawings]

FIG. 1 is a structural diagram showing the concept of the present invention. FIG. 2 is a diagram showing a manufacturing method and a device structure of a PIC in which a DFB semiconductor laser manufactured by using the present invention and a semiconductor optical modulator are integrated. FIG. 3 is a diagram showing a manufacturing method and a device structure of an integrated device of a two-wavelength semiconductor laser and an optical waveguide manufactured by using the present invention. FIG. 4 is a diagram showing the relationship between the stripe width and the growth rate. FIG. 5 is a diagram showing the relation between the well thickness of the MQW structure and the emission wavelength. FIG. 6 is a view showing the structure of the same two-wavelength semiconductor laser and optical waveguide integrated device as in FIG. 3 according to a conventional manufacturing method. In the figure, 1 ... n-InP substrate, 3 ... InGaAsP active layer, 4 ...
... p-InP clad layer, 5 ... p-InP clad layer, 6 ...
... p-InP layer, 7 ... P ⁺ -InGaAs cap layer, 10 ... n
-InGaAs guide layer, 11 ... MQW active layer, 12 ... n-InP etching stop layer, 13 ... High resistance InP buried layer, 14
... InGaAsP waveguide layer, 21 ... SiO ₂ film, 32 ... p-side electrode, 33 ... n-side electrode.

Claims (1)

[Claims] 1. A method for manufacturing a semiconductor optical integrated device, wherein two parallel stripe-shaped dielectric thin films whose widths are changed in a stripe direction are formed so that a crystal growth region becomes a stripe-shaped opening having a constant width. A step of forming a semiconductor surface, and a step of selectively crystallizing a semiconductor layer including a quantum well structure in the crystal growth region by MOVPE growth using the dielectric thin film as a mask, the width of the dielectric thin film Is controlled to control the crystal growth rate of the semiconductor layer in the crystal growth region.