JPH09202700A - Epitaxial wafer for optical element and optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare a high-quality epitaxial wafer for an optical element by a pulling method, having >=3 inches diameter and crystal defect not more than that by a boat method and to obtain an optical element using the epitaxial wafer. SOLUTION: A strain quantum well layer 3 for reducing crystal defect occurring on an interface of a base caused by lattice mismatch in a heteroepitaxial growth is formed on a base 1 obtained by a pulling method so that the transmission of the crystal defect already existing in the base 1 into an epitaxial layer 5 is reduced even if epitaxial growth is carried out on the layer 3. Consequently, an epitaxial water for an optical element having a large aperture of >=3 inches and few crystal defect is obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an epitaxial wafer for optical devices and an optical device.

[0002]

2. Description of the Related Art The demand for optical devices is increasing with the development of optical communication technology. An optical element, especially a light emitting element, is extremely sensitive to crystal defects in terms of device characteristics including long-term reliability. Therefore, an epitaxial wafer is manufactured using a high-quality substrate with few crystal defects due to the yield, etc.
A device using this epitaxial wafer is manufactured. Such a substrate with few crystal defects is usually manufactured by a crystal growth method called a boat method, and a substrate by the pulling method which is often used for electronic devices has many defects and therefore is not used.

[0003]

Generally, in order to reduce the cost of the optical element, it is advantageous to increase the diameter of the substrate in order to reduce the process cost. In particular, waveguide-type optical integrated circuit elements that will be put to practical use in the future tend to have larger element sizes than other semiconductor elements, and it is essential to increase the substrate diameter in order to keep the element price low. is there.

However, it is difficult to obtain a large-diameter substrate by the crystal growth method called the boat method, and a substrate by the pulling method capable of producing a substrate of 3 inches or more must be used. In addition, this type of substrate has a large number of crystal defects and cannot be used for optical devices.

Therefore, an object of the present invention is to solve the above problems and to fabricate a high-quality epitaxial wafer for optical devices, which has a diameter of 3 inches or more and has a crystal defect equal to or less than the substrate by the boat method, by the pulling method. Another object is to provide an optical element using the epitaxial wafer.

[0006]

In order to achieve the above object, the epitaxial wafer for an optical device of the present invention uses a compound semiconductor substrate having a diameter of 3 inches or more, which is manufactured by a pulling method, and all the compound semiconductor substrates are formed on the compound semiconductor substrate. In the epitaxial wafer for optical devices in which the layers or at least other layers except the active layer are homoepitaxially crystal-grown so as to be lattice-matched to the compound semiconductor substrate, the crystal defects existing in the compound semiconductor substrate cause At least one additional thin film is formed to reduce the propagated crystal defects.

In addition to the above structure, in the epitaxial wafer for optical devices of the present invention, a thin film for reducing crystal defects propagating from the compound semiconductor substrate is a buffer layer provided directly on the compound semiconductor substrate or on the compound semiconductor substrate. At least one layer may be additionally formed on the above.

In addition to the above structure, in the epitaxial wafer for optical devices of the present invention, the thin film for reducing the crystal defects propagating from the compound semiconductor substrate has a critical film thickness less than the critical film thickness when the compound semiconductor substrate is gallium arsenide. It may be a thin germanium thin film.

In addition to the above structure, in the epitaxial wafer for optical device of the present invention, the thin film for reducing the crystal defects propagating from the compound semiconductor substrate includes a strained quantum well layer having a thickness smaller than at least one critical thickness. It may be a thin film.

In the optical device of the present invention, homoepitaxial crystal growth is performed on a compound semiconductor substrate manufactured by the pulling method so that all layers or at least other layers except the active layer are lattice-matched to the compound semiconductor substrate. In the optical device, at least one thin film is additionally formed to reduce the crystal defects which are propagated into the epitaxial layer due to the crystal defects already existing on the compound semiconductor substrate.

With the above structure, a layer that reduces crystal defects generated at the substrate interface due to lattice mismatch in heteroepitaxial growth is formed on the substrate obtained by the pulling method, and epitaxial growth is performed on the layer. Even if the above step is performed, the propagation of crystal defects already existing in the substrate into the epitaxial layer is reduced. For this reason,
An epitaxial wafer for optical devices having a large diameter of 3 inches or more and few crystal defects can be obtained.

[0012]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a view showing an embodiment of the epitaxial wafer for optical devices of the present invention. In the present embodiment, a case of a super luminescent diode as an optical element will be described.

In FIG. 1, n-type GaAs having a thickness of about 0.1 μm formed by molecular beam epitaxy on a 4-inch size semi-insulating GaAs substrate 1 produced by the pulling method.
The buffer layer 2 is formed. On the n-type GaAs buffer layer 2, the strain quantum well structure 3 which is the defect reducing layer of the present invention is formed. This strained quantum well structure 3 includes three undoped GaAs layers and these three undoped Ga layers.
Two undoped In _0.21 Ga _0.79 A sandwiched between As layers
It is a double quantum well structure including an s-strained quantum well layer. On the strained quantum well structure 3, n-type GaA having a thickness of about 0.5 μm is formed.
The s-buffer layer / contact layer 4 is formed. An n-type Al _0.4 Ga _0.6 As lower cladding layer 5 having a thickness of about 1.5 μm and an n-type Al _0.15 Ga _0.85 As optical guide layer 6 having a thickness of about 0.1 μm are formed on the n-type GaAs buffer layer / contact layer 4. Are formed. n-type Al _0.15 Ga _0.85 As
Three undoped GaAs barrier layers 7a, 7b and 7c and two undoped In _0.21 Ga _0.79 are formed on the light guide layer 6.
An active layer 9 having a double quantum well structure composed of As strained quantum well layers 8a and 8b is formed. A p-type Al _0.15 Ga _0.85 As optical guide layer 10 having a thickness of about 0.1 μm is formed on the active layer 9. The strain quantum compressibility of the strain quantum well layers 8a and 8b was set to about 2.0%, and the film thickness of the undoped GaAs barrier layers 7a to 7c was set to about 6 nm. p-type Al _0.15 G
a _0.85 As p-type A of about 1.5 μm on the optical guide layer 10
An l _0.4 Ga _0.6 As upper clad layer 11 was formed, and a p-type GaAs electrode contact layer 12 having a thickness of about 0.2 μm was formed on the p-type Al _0.4 Ga _0.6 As upper clad layer 11.

Next, the operation will be described.

FIG. 2 is a schematic diagram showing the propagation state of crystal defects from the substrate in homoepitaxial growth when the defect reduction layer of the present invention is added.

As shown in FIG. 1, after the defect reduction layer 22 is added to the epitaxial layer 21 formed on the substrate 20, the epitaxial layer 23 is formed on the defect reduction layer 22 so that the substrate 20 is removed. Propagation of the crystal defect in the epitaxial layer 23 is drastically reduced by stopping the propagation of the defect 24 or changing the propagation direction in the direction parallel to the substrate surface. As a result, it is possible to obtain an epitaxial wafer in which optical devices can be manufactured with a high yield even when using a large-sized substrate manufactured by the pulling method.

The dislocation defect density of the epitaxial wafer manufactured as described above is about 5,000 even though a 4-inch pulling substrate (usually, dislocation defect density ≦ 100,000 cm ⁻² ) is used. ~ 8,000cm ^-2
And was lower than the usual dislocation defect density of a 3-inch substrate by the boat method ≦ 10,000 cm ⁻² .

(Comparative Example) FIG. 3 is a schematic diagram showing the state of propagation of crystal defects from the substrate in conventional homoepitaxial growth. Compound semiconductor substrate 3 by pulling method
0 already contains much more crystal defects 31 than the substrate manufactured by the boat method, and the defect density thereof generally increases as the size of the substrate increases. Many of these crystal defects, especially defects called "dislocations", propagate to the inside of the epitaxial layer 32 during normal epitaxial crystal growth, and as a result, the defect density of the epitaxial layer 32 becomes similar to that of the substrate 30. . In this case, there are too many defects and they cannot be used for optical devices.

Next, an element as shown in FIG. 4 was produced using this epitaxial wafer. As a manufacturing method, a super luminescent diode 40 having a ridge-type waveguide structure and a monitor photodiode 41 are formed by a photolithography method, and a groove 42 is formed at an interface between them to form a groove 42 between both elements. Electrical isolation. Si on the super luminescent diode 40 and the monitor photodiode 41.
After depositing the O ₂ film 43, a contact hole is formed,
Furthermore, a p-type electrode 44 and an n-type electrode 45 were formed, and an integrated device was produced by cleavage. 4 is an external perspective view of an optical element using the optical element epitaxial wafer of the present invention.

[0021]

In summary, according to the present invention, the following excellent effects are exhibited.

Since at least one thin film is additionally formed to reduce the crystal defects propagating into the epitaxial layer due to the crystal defects already existing on the compound semiconductor substrate, the diameter is 3 inches or more, and It is possible to manufacture a high-quality epitaxial wafer for an optical device having a crystal defect equal to or lower than that of the substrate by the boat method by the pulling method and realize an optical device using the epitaxial wafer.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of an epitaxial wafer for optical devices of the present invention.

FIG. 2 is a schematic diagram showing the propagation state of crystal defects from the substrate in homoepitaxial growth when the defect reduction layer of the present invention is added.

FIG. 3 is a schematic diagram showing a propagation state of crystal defects from a substrate in conventional homoepitaxial growth.

FIG. 4 is an external perspective view of an optical device using the optical device epitaxial wafer of the present invention.

[Explanation of symbols]

1 compound semiconductor substrate (semi-insulating GaAs substrate, substrate) 3 strained quantum well layer (strained quantum well structure) 5 epitaxial layer (n-type Al _0.4 Ga _0.6 As lower cladding layer)

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location // H01L 21/208 H01L 21/208 P

Claims (5)

[Claims] 1. A compound semiconductor substrate having a diameter of 3 inches or more manufactured by a pulling method is used, and all layers on the compound semiconductor substrate or at least other layers except an active layer are lattice-matched to the compound semiconductor substrate. In the epitaxial wafer for optical devices grown by homoepitaxial crystal growth, at least one additional thin film is formed to reduce crystal defects propagated into the epitaxial layer due to crystal defects already existing on the compound semiconductor substrate. An epitaxial wafer for optical devices, which is characterized by being 2. A thin film for reducing crystal defects propagating from the compound semiconductor substrate is formed at least one layer directly on the compound semiconductor substrate or on a buffer layer provided on the compound semiconductor substrate. The epitaxial wafer for an optical element according to claim 1. 3. The germanium thin film, which is thinner than the critical film thickness, only when the compound semiconductor substrate is gallium arsenide, as the thin film for reducing crystal defects propagating from the compound semiconductor substrate. Epitaxial wafer for optical devices. 4. The optical device according to claim 1, wherein the thin film for reducing crystal defects propagating from the compound semiconductor substrate is a thin film including a strain quantum well layer thinner than at least one critical film thickness. Epitaxial wafer. 5. An optical device in which homoepitaxial crystal growth is performed on a compound semiconductor substrate produced by a pulling method so that all layers or at least other layers except an active layer are lattice-matched to the compound semiconductor substrate. An optical device characterized in that at least one thin film is additionally formed to reduce crystal defects propagated into an epitaxial layer due to crystal defects already existing on the compound semiconductor substrate.