JPH0430114A - Semiconductor optical element - Google Patents

Abstract

PURPOSE:To obtain the semiconductor optical element which can operate even if light intensity is increased by disposing the end of the prescribed electrode of the semiconductor optical element, such as optical modulator or photodetector, on the side inner than the light incident end face of an optical waveguide. CONSTITUTION:This semiconductor optical element has the 1st electrode 7 disposed on one main surface of an (n) type substrate 6, a 1st semiconductor layer 5 of an (n) type disposed on the other main surface, an electric field impressing layer 4 of a semiconductor layer of a low impurity concn. disposed thereon, a 2nd semiconductor layer 3 of a (p) type disposed on the layer 4, and the 2nd electrode 10 disposed on or in proximity to the layer 3. The optical waveguide is constituted of the layers 5, 4, 3. The current density near the light incident end is lowered and the deterioration in characteristics by the generation of heat in the element is prevented if the position of this electrode end is formed on the side inner than the light incident end part. The element is thus operated even with the incident light of a large electric power.

Description

Detailed Description of the Invention [Industrial Application Field 1] The present invention relates to an optical modulator and a photodetector that can be used in the fields of optical communication and optical information processing and can operate even with high-power incident light. The present invention relates to semiconductor optical devices such as the following.

[Prior Art] The basic structure of conventionally used electroabsorption semiconductor optical modulators and photodetectors is that a non-doped core layer is placed between p-type and n-type cladding layers with high impurity concentrations and an electric field application layer. There are some that are placed as. By applying an external electric field to the electric field application layer of these elements, optical functions such as light modulation or detection are enabled. In this type of device, the operational allowable incident optical power is an important factor that determines system performance, and it is necessary to make it as large as possible.

In the structure of a conventional semiconductor optical device, a light absorption current flows intensively near the light incident end face due to its structural limitations. For this reason, local heat generation occurs due to electrical resistance, particularly in the vicinity of the incident end face. Furthermore, element deterioration such as damage to the element material is likely to occur due to leakage current caused by crystal defects in the vicinity or electric field concentration in the structure, and the upper limit of the incident optical power is restricted.

FIG. 6 is a perspective view showing an example of a conventional optical modulator.

Here, l is an electrode made of Au or the like, and 2 is a cap layer for obtaining ohmic contact between the semiconductor material and the electrode l. 3 is a p-type semiconductor layer, 4 is a non-doped semiconductor layer, 5 is an n-type semiconductor layer, and 6 is an n-type semiconductor substrate, and these layers 3, 4, and 5.6 form an optical waveguide. The p-type semiconductor layer 313 and the n-type semiconductor layers 5 and 6 become a cladding part, and the non-doped semiconductor layer 4 becomes a core part. 7
is an electrode.

In this case, if a negative external voltage (reverse bias) ■ is applied to the electrode 1 with respect to the electrode 7, depending on the magnitude of the voltage V,
An electric field is applied to the non-doped layer 4.

The absorption coefficient α of this fake changes. In normal operation, the larger ■ becomes, the larger α becomes, and the propagation loss for light becomes larger. Therefore, when light 8 of a constant intensity is incident from one end surface of the core section 4, light 9 whose intensity is modulated according to the voltage (2) is emitted from the optical modulator.

FIG. 7 is a diagram showing the distribution of the light absorption current density Ja of the guiding waveguide core section 4 and the leakage current (dark current) density Jz caused by the externally applied voltage V in the optical modulator shown in FIG.

In FIG. 6, the light intensity is highest near the light incident end face, so the light absorption current density Ja is largest near this area.
The current Ja becomes proportional to exp(-αX) with respect to the position X of the optical waveguide, and the larger V becomes, the larger the current Ja becomes. On the other hand, the leakage current Jz depends on the material of the semiconductor material,
It varies depending on crystal quality, electric field strength, etc.

[Problem to be Solved by the Invention 1] Since the optical waveguide is usually formed by cleavage of the input/output end face, etc., the semiconductor crystal is likely to be damaged, and Jz becomes large. In addition, due to the edge effect of the electrode l, electric field concentration occurs, and compared to the center of the optical waveguide, J
l becomes larger. For this reason, in conventional optical modulators, a particularly large current flows near the light input end, and a large amount of heat is generated in this vicinity due to internal resistance of the semiconductor, electrodes, etc.

Therefore, if the intensity of the incident light 8 is increased while the external voltage V is applied, the temperature of the incident end face will particularly rise, the propagation loss will increase, and furthermore, thermal damage will be added to the semiconductor crystal, causing the modulator to deteriorate. This may cause the problem of deterioration of characteristics.
There was a drawback that the light intensity could not be increased.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks and provide a semiconductor optical device that can operate even when the light intensity is increased.

[Means for Solving the Problems 1] In order to achieve such an object, the present invention provides a semiconductor substrate, a first electrode disposed on one main surface of the substrate,
an electric field application layer including a first semiconductor layer of a first conductivity type disposed on the other main surface of the substrate; and a low impurity concentration semiconductor layer disposed on the first semiconductor layer; comprising at least a second semiconductor layer of a second conductivity type disposed on the electric field application layer, and a second electrode disposed on or adjacent to the second semiconductor layer, and the first and second semiconductor layers In the semiconductor optical device in which an optical waveguide is constituted by the voltage application layer, at least an end portion of the second electrode is arranged inside a light incident end surface of the optical waveguide.

Here, a portion of the second electrode on the light incident end side can be formed into a circular shape.

A portion of the second semiconductor layer may be arranged inside the light incident end surface of the optical waveguide, corresponding to the second electrode.

Furthermore, at least one of the width and thickness of the optical waveguide in the vicinity of the light input end of the optical waveguide can be made relatively smaller than the remaining portion of the optical waveguide.

In another form of the invention, the present invention includes a semiconductor substrate, a first electrode disposed on one main surface of the substrate, and a first semiconductor of a first conductivity type disposed on the other main surface of the substrate. a layer, and the first
an electric field application layer composed of a low impurity concentration semiconductor layer disposed on the semiconductor layer; and a second electric field application layer disposed on the electric field application layer.
an optical waveguide comprising at least a second semiconductor layer of a conductivity type, and a second electrode disposed on or adjacent to the second semiconductor layer, and the first and second semiconductor layers and the voltage application layer constitute an optical waveguide. In the semiconductor optical device according to the present invention, at least one of the width and thickness of the optical waveguide near the light input end of the optical waveguide is made smaller relative to the remaining part of the optical waveguide. shall be.

[Function 1] In the present invention, on the light incident end side of the semiconductor element, the position of the electrode end is formed inside the light incident end, or the radius of curvature thereof is made large, or the optical waveguide width in the vicinity is Alternatively, by making the thickness relatively small, the current density near the light input end can be reduced compared to conventional elements.
Therefore, characteristic deterioration due to element heat generation can be prevented, and therefore operation is possible even with high-power incident light.

[Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Embodiments of the present invention will be described below, taking as an example an electro-absorption semiconductor optical modulator having a structure in which a p-1-n type semiconductor layer is formed on an n-type semiconductor substrate, and one layer serves as the core part of an optical waveguide. Explain in detail.

FIG. 1 is a perspective view showing an embodiment of the present invention, in which the electrode end position is formed on the light incidence end side inside the light incidence end. Note that the same reference numerals are given to the same components as in the conventional example.

In FIG. 1, lO is an electrode, and 11 is a cap layer for making ohmic contact. In this embodiment, the cap layer 11 and the electrode 10 are arranged in this order on the p-type semiconductor layer 3. In contrast to FIG. 6), the end portions of the electrode lO and the cap layer 11 are configured to be located inside the light incident end l.

As can be seen from FIG. 1, the electric field is not applied to the part of the input end 12 where there are many defects.
It is possible to make the leakage current Jz relatively small. That is, since the light absorption current density Ja can be increased by the amount by which the leakage current is reduced, operation is possible even when the light intensity is increased.

FIG. 2 is a perspective view χ showing another embodiment of the present invention, in which a cap layer 14 and an electrode stand are arranged in this order on a p-type semiconductor layer 3, and the electrode 13 and the cap layer By making the end of electrode 14 on the light incident end side circular, electric field concentration at this electrode end can be alleviated.

In addition, a portion of the p-type semiconductor layer 3 near the light incidence end 12 is removed by etching or the like in line with the circular ends of the layers 13 and 14 to reduce the layer thickness. Since the electric field strength at the end face can be reduced, the light absorption current density Ja and the leakage current J1 near the light incident end can be reduced. Therefore, it is possible to increase the power of the incident light using the same principle as in the embodiment shown in FIG. In the embodiment shown in FIG. 1 as well, as shown by the broken line, a portion of the p-type semiconductor layer 3 near the light incident end 12 is removed along with the layers 13 and 14 to reduce the layer thickness. You can also.

FIG. 3 is a perspective view showing still another embodiment of the present invention, in which the width W1 of the optical waveguide on the light incidence end 12 side is narrower than the width W8 of the other portion.

FIG. 4 is a diagram illustrating the principle of the waveguide construction method in the embodiment of FIG. 3.

In FIG. 3, 15 is an electrode, 16 is a cap layer, and 17
, 19 are p-type and n-type constituting the cladding part of the optical waveguide.
The shaped semiconductor layer 18 is a non-doped semiconductor layer constituting a core portion, and is laminated on the substrate 6 in the order of layers 19.18, 17, 16 and 15. Here, the layers 15 and 16 are arranged so as to be retreated inward from the light incident end 12. The optical waveguide width W is determined to be W3 in the area 1 near the light input end 12 and W in the area ▪ near the light input end, where: ! +
There is a relationship between <L.

Figure 4 shows the width W and
FIG. 6 is a diagram showing the relationship between the spot size wj5 of the optical waveguide and the light confinement coefficient r of the core portion. As can be seen from FIG. 4, in general, as W becomes smaller, `` becomes smaller monotonically, but with respect to W, there is a relationship in which the minimum value w0 is reached at a certain specific size Wo of W.

The magnitude of 1°9wo at this time depends on the material, dimensions, and shape of the optical waveguide, and the relationship therebetween is determined by ordinary optical waveguide analysis methods such as the equipolarized refractive index method and the finite element method. Is possible. The width of the optical waveguide is W l
<! When configured according to the relationship l *, its confinement coefficient r
The relationship is r l < r *.

When an external voltage is applied to the electrode 15, the absorption coefficient of the non-doped semiconductor layer 18 changes, but the absorption coefficient as an optical waveguide has a magnitude proportional to . is smaller than the absorption coefficient of However, since the change in the absorption coefficient with respect to the applied voltage generally shows a nonlinear relationship, the extinction ratio can be restored by only a slight increase in the voltage, and therefore, the extinction ratio can be restored without increasing the driving voltage. Modulators can be configured.

The optical absorption current density distribution in the optical waveguide at this time is
As shown in the figure.

In the above configuration, if the sizes of Wl and 6 are set so that the respective spot sizes are equal (w,') as shown in FIG. 4, the loss due to spot size mismatch can be reduced. Therefore, a low-loss optical modulator can be realized.

In the embodiment shown in FIG. 3, a case has been described in which the waveguide width W is changed, but the effects of the present invention can also be obtained by changing the thickness d of the waveguide core portion 18. That is, keeping W constant, the core thickness d1 of region I and the core thickness d2 of region H are d
When configured to have the relationship of + < d s, the confinement coefficient F has the relationship of + < r x as described above, so that the light absorption current near the light incidence end 12 can be relatively reduced. At this time, If d+ and dx are set so that the optical spot sizes W in regions I and II are equal, low loss characteristics can also be ensured.

Furthermore, it is obvious that the effects of the present invention can be obtained using the same principle even with a waveguide configuration in which L<L and d+<dx.

Additionally, in Embodiment 1 or 2 shown in FIG. 1 or 2, the width and/or thickness of the optical waveguide near the light input end 12 can be made relatively small.

In the embodiment shown in FIG. 3, for example, InAlAs is used as the cladding portion of the layers 17 and 19, and a multiple quantum well structure is used as the non-doped layer 18 (well layer: InGaAs, barrier layer:
In the case of a 1.5 μ-band absorption optical modulator using a semiconductor layer of InAlAs), the waveguide core thickness d2 is usually 0.3.
~0.4μm, so set dl to 0.1~O,=2μm
□If it's about □, it's a spot size. are 0.3 each
It becomes about the size of ~0.4μ mold. On the other hand, regarding the light confinement coefficient F, "1" is 20-40%, "2" is 60-7%.
Since it becomes 0%, the absorption coefficient near the light incidence end 12 can be reduced. Therefore, high power light input is possible.

In the above, the case where a p-1-n type semiconductor layer is formed on the n-type semiconductor substrate 6 has been described.
The present invention can also be applied to a device in which a 1-p type semiconductor layer is formed, or a device in which a p-1-n or n-1-p semiconductor layer is formed on a semiconductor insulating substrate.

In addition, in 1 and above, the case where the core part is composed of the non-doped semiconductor layer 4 or 18 has been explained, but the core part is made of p-
As mentioned above, the light confinement coefficient r of the i-semiconductor layer, which is composed of semiconductor layers with a 1-n structure and becomes a light absorption layer, is defined as the area ■
However, it may be made smaller by the area engineering value.

Effects of the Invention J As explained above, in the present invention, on the light incident end side of a semiconductor element, the position of the electrode end is formed inside the light incident end, or the radius of curvature is thickened. By making the width or thickness of the optical waveguide relatively small (by making the width or thickness of the optical waveguide relatively small), it is possible to improve the characteristic deterioration due to element heat generation by creating a structure that reduces the current density near the light input end. It becomes possible to operate even with incident power light, making it possible to realize a high-power semiconductor optical device.

The present invention is applicable to semiconductor modulators with a multi-quantum well structure that utilizes the quantum confinement Stark effect, semiconductor bulk crystal semiconductor modulators that utilize the Franz Keldysche effect, etc.
Can be used for electroabsorption modulators. Furthermore, it is obvious that the present invention can also be applied to semiconductor optical devices that utilize light absorption, such as waveguide type optical devices. Furthermore, the present invention can be applied not only to the individual type optical elements described above, but also to monolithic type optical elements integrally configured with a semiconductor laser element.

[Brief explanation of drawings]

1, 2 and 3 are perspective views showing the configurations of embodiments 1, 2 and 3 of the present invention, respectively, and FIG. 4 and ÷ figure are characteristic diagrams for detailed explanation of the present invention. , FIG. 6 and FIG. 6 are respectively a perspective view showing the configuration of a conventional example and a diagram showing the current distribution within the optical element. 1... Electrode, 2... Cap layer, 3... P-type semiconductor layer (cladding layer), 4... Non-doped semiconductor layer (core layer) 5... N-type semiconductor layer (cladding layer), 6... N-type semiconductor substrate, 7... Electrode, 8... Incident light, 9... Outgoing light, 10... Electrode, 11... Cap layer, 12... Light incident end surface, 13... Electrode, l4... Cap layer, 15... Electrode, 16... Cap layer, 17... P-type semiconductor layer, 18... Non-doped semiconductor layer, 19... N-type semiconductor layer. Agent for patent applicant Nippon Telegraph and Telephone Corporation

Claims (1)

[Scope of Claims] 1) A semiconductor substrate, a first electrode disposed on one main surface of the substrate, and a first electrode disposed on the other main surface of the substrate.
a first semiconductor layer of a conductivity type; an electric field application layer composed of a semiconductor layer with a low impurity concentration disposed on the first semiconductor layer; and a second conductivity type semiconductor layer disposed on the electric field application layer. A semiconductor optical device comprising at least a second semiconductor layer and a second electrode disposed on or near the second semiconductor layer, and an optical waveguide configured by the first and second semiconductor layers and the voltage application layer. 1. A semiconductor optical device, wherein at least an end portion of the second electrode is disposed inside a light incident end face of the optical waveguide. 2) The semiconductor optical device according to claim 1, wherein a portion of the second electrode on the light incident end side is formed in a circular shape. 3) The semiconductor optical device according to claim 1 or 2, wherein a part of the second semiconductor layer is arranged inside the light incident end face of the optical waveguide, corresponding to the second electrode. 4) a semiconductor substrate, a first electrode disposed on one main surface of the substrate, and a first electrode disposed on the other main surface of the substrate;
a first semiconductor layer of a conductivity type; an electric field application layer composed of a semiconductor layer with a low impurity concentration disposed on the first semiconductor layer; and a second conductivity type semiconductor layer disposed on the electric field application layer. A semiconductor optical device comprising at least a second semiconductor layer and a second electrode disposed on or near the second semiconductor layer, and an optical waveguide configured by the first and second semiconductor layers and the voltage application layer. In the semiconductor optical device, at least one of the width and thickness of the optical waveguide near the light input end of the optical waveguide is made smaller relative to the remaining part of the optical waveguide. element. 5) At least one of the width and thickness of the optical waveguide near the light input end of the optical waveguide is made smaller relative to the remaining part of the optical waveguide. 4. The semiconductor optical device according to any one of items 3 to 3.