JPH0367203A - photodetector - Google Patents

Abstract

PURPOSE:To obtain the photodetector suitable for the integration to allow the common provision of a wavelength selecting function in the photodetector by forming the photodetecting layer in the position where the 3rd waveguide mode centering at itself does not overlap much with a 1st waveguide mode and overlaps much on a 2nd waveguide mode. CONSTITUTION:The photodetecting layer 108 is formed in the position where the 3rd waveguide mode centering at itself does not overlap much with the 1st waveguide mode and where the mode overlaps much on the 2nd waveguide mode. The photodetecting layer 108 may be provided so as to face the outside part of the layer having a photoconductivity and the quantity of incident light may be detected from a change in resistance value between electrodes 204 and 205 by providing two pieces of the above-mentioned electrodes atop this layer. The formation to an FET structure by providing 3 pieces of the electrodes is equally satisfactory. The photodetector which is suitable for integration and has the high efficiency and sharp wavelength selectivity is obtd. in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a photodetector that selectively detects signal light of a specific wavelength from human-emitted light in a wavelength-multiple optical transmission system.

[Prior Art] Conventionally, this type of photodetector demultiplexes incident light to extract signal light, which is received using a photoelectric converter such as a photodiode.

FIG. 7 is a diagram showing the configuration of a conventional photodetector.

The ridge-shaped waveguide 73 configured on the substrate 71J includes a diffraction grating 741 that reflects signal lights of different wavelengths.
~743 are formed. The multiplexed signal light incident from the optical fiber 77 is reflected by a diffraction grating whose wavelength satisfies the Bragg condition. At each reflection position, a photodetector 75, such as a photodiode, is provided with similarly formed diffraction gratings 76, 763, respectively. ~753
is formed, and each signal light is detected.

Also, instead of a diffraction grating (
Some use interference filters to transmit or reflect only light of a specific wavelength.

The photodetector described above requires a long optical path for demultiplexing and requires a large area, so it has the disadvantage that the photodetector becomes large.

In addition, it has the disadvantage that the light absorption loss caused by the long optical path C and the reflection and scattering loss caused by inserting various parts are large, the wavelength resolution is low, and high-density integration has not been achieved. .

Photodetectors proposed to solve these drawbacks include Japanese Patent Application No. 1-56142 and Japanese Patent Application No. 1-5982.
There is one in size 5. What is described in Komura is that two first and second waveguide layers having different propagation constants in the direction perpendicular to the substrate (first waveguide layer, 804 and second waveguide counterfeit 806, first waveguide layer, layer 901 and light absorption layer 902), and a grating (diffraction grating 807, 907) so that optical coupling occurs between each waveguide layer for light of a desired wavelength. They are a wavelength filter that moves light from a first waveguide layer to a second waveguide layer and demultiplexes the light, and a photodetector.

In the device described in Japanese Patent Application No. 1-59825, a part of the second waveguide layer 806 is removed and replaced with a photodetection layer, which is the second waveguide layer, as shown in FIG. , patent application Hei 1-
The device described in No. 56142 uses the second waveguide layer itself as a light absorption layer 902 as a light detection layer, as shown in FIG.

In any of these, the position where the grating is formed is the even mode 1001° odd mode]002 which guides the I and second waveguide layers, respectively, as shown in FIG.
It is said that these areas overlap.

According to each of the above proposals, it is possible to obtain a photodetector that is suitable for integration, has high efficiency, and has sharp wavelength selectivity.

[Problems to be Solved by the Invention] Among the conventional examples described above, the one shown in FIG. 7 has the disadvantage that the manufactured one is large and the reflection and scattering loss is also large. Although these drawbacks are solved in the case shown in FIGS. 8 and 9, the second waveguide layer 806, 902, which is a photodetection layer, has the first layer as shown in FIG. The odd mode 1002 guided through the waveguide layers 804 and 901 extends, and some light of non-selected wavelengths is also detected by the photodetection layer. For this reason, when an extremely high wavelength selection ratio or crosstalk between wavelengths is required, there is a drawback that advanced technology is required for design and manufacturing.

Furthermore, in the case shown in FIG. 8, since the absorption layer 809, which is a photodetection section, is formed in the same layer as the second waveguide layer, it is necessary to perform regrowth after etching.
This method has disadvantages in that the number of manufacturing steps increases and the manufacturing time becomes longer.

An object of the present invention is to realize a photodetector that can eliminate the various drawbacks of the above-mentioned conventional techniques.

[Means for Solving the Problems] The photodetector of the present invention includes a photodetector in which semiconductor layers are laminated and used to separate signal light of a specific wavelength from wavelength-multiplexed optical signals. The detector includes a first waveguide layer into which an optical signal is incident, a second waveguide layer for guiding signal light of a specific wavelength, and a first waveguide layer centered on the first waveguide layer. A diffraction grating is provided in a region where the first waveguide mode and the second waveguide mode centered on the second waveguide layer overlap, and the diffraction grating absorbs the incident light to absorb the incident light. a photodetection layer for detecting the amount of light, the photodetection layer is configured such that a third waveguide mode centered on itself does not overlap much with the first waveguide mode, and It is formed in a position where it overlaps a lot with the wave mode.

In this case, the photodetection layer may be provided with a photoconductive layer facing the outside, and two electrodes may be provided on the top surface of the layer to detect the amount of incident light from the change in resistance between them. It is also possible to provide an FET structure.

Furthermore, a Pin photodiode structure may be used in which the photodetection layer is an i-layer.

[Operation] Similar to the conventional one shown in FIGS. 8 and 9, the first
Since the second waveguide layer has a laminated structure, the propagation constant can be made different depending on the materials thereof, and the difference in the propagation constant that occurs between them can be made large. Therefore, the optical power transfer occurring between the two waveguide layers is dominated by the diffraction grating, and sharp wavelength selectivity is obtained. Furthermore, in the present invention, the photodetection layer has a self-centered third waveguide mode that does not overlap much with the first waveguide mode and overlaps much with the second waveguide mode. It is formed in the position.

Therefore, most of the light detected by the photodetection layer is in the second waveguide mode, that is, the light that propagates through the second waveguide layer, and the non-selected light that propagates through the first waveguide layer. Since the absorption of wavelength light is reduced, extremely high wavelength selection ratio or crosstalk between wavelengths can be obtained. Furthermore, since the semiconductor layer has a structure in which semiconductor layers are stacked, the manufacturing process is almost the same as that of a general semiconductor element, and the reproducibility is also good.

Embodiment FIG. 1 is a diagram showing the configuration of a first embodiment of the present invention, and FIG. 2(a) is a top view showing the configuration of electrodes formed on the top surface of the first embodiment.

First, the structure of this embodiment will be explained.

In this example, a substrate 1 having a thickness of 0.5 μm is formed on a substrate 1 made of GaAs.
buffer layer 104 of GaAs, 1.5 μm thick;
Al. , ,,Ga0. , , cladding layer 105 of As, GaAs and AI. , 4Ga, , , , As are alternately stacked to form a multiple quantum well (MQW). , aGao5^S cladding layer 106,G
aAs and Alo, 2Ga6. A second waveguide layer 10 with a thickness of 0.2 μm formed by alternately stacking aAs and MQW.
2. Alo4Ga0.25μm thick. , , a diffraction grating layer 107 of As, 0.25 μm thick n-Ga
A photodetection layer 108 made of As (n = 1 x 10 "cm-') is sequentially grown by molecular beam epitaxial (MBE) method. Next, after a resist mask is prepared by photolithography using a photoresist,
Selective etching was performed using a mixed etchant of aqueous ammonia and hydrogen peroxide, and the photodetection layer 108 was removed except for the photodetection region 109 on the right side of the drawing. At this time, under the light absorption layer 109 is AI. Since the diffraction grating layer 107 is made of 4Gao, sAs, selective etching depending on the characteristics of AI can be easily performed.

Next, a resist mask is made again, and a ridge guide for waveguide light confinement is formed at a position where the photodetection region 109 and the introduction region 111 are connected, as shown in FIG. 2(a), using the reactive ion beam etching (RIBE) method. A wave path 201 was formed. Furthermore, by the same process, the ridge waveguide 201
Perpendicular to the longitudinal direction, pitch: 30 μm, area length: 550 μm
A diffraction grating 202 was formed. This diffraction grating 202 is
It is obtained by etching the diffraction grating layer 107 made of A104Gao, 6As. Therefore the depth is 0
．． It is 25 μm.

Furthermore, a second waveguide layer 1 in front of the ridge waveguide 201 is configured such that only the first waveguide layer 101 couples the light when light is input.
02 was removed by etching (see Figure 1). At this time, the ridge end face 203 is etched obliquely with respect to the longitudinal direction of the ridge waveguide 201 as shown in FIGS. I took it as a thing.

Next, a Si3N4 film with a thickness of 0.3 μm was formed on the upper surface by plasma CVD (chemical vapor deposition). Then, as shown in FIG. 2(a), a resist mask was prepared for forming an electrode window in a part of the photodetection region 109, and then a part of the Si3N4 film was removed by dry etching using the resist mask. Next, Au/Au-T
An electrode film made of he was formed, and two electrodes 204 and 205 in contact with the photodetection layer 108 shown in FIG. 1 were formed by photolithography as shown in FIG. 2(a). These two electrodes 204 and 205 are arranged so that they face each other across the longitudinal direction of the ridge waveguide 201 so that the wavelength-selected guided light is coupled at the photodetection layer 108 below between each electrode 204 and 205. It is set up to do so. The length of the electrode in the direction of light propagation is 100 μm in this example.

The length of the wavelength selection region 110 corresponds to the length of the diffraction grating 202, which is 550 μm in this embodiment, and the length of the photodetection region 109 corresponds to the length of the electrode, which is 100 μm in this embodiment.

After cutting the element produced by the above procedure, it was fixed on a sample holder, lead wires were bonded to each electrode 204 and 205, and the element was taken out to the outside.

In this way, the photodetector of this example is shown in FIGS. 1 and 2.
As shown in Figure (a), three waveguide layers (first waveguide layer 101, second waveguide layer 102, photodetection layer 108) are laminated in the layer direction, and a directional coupler is constructed by these layers. Since each waveguide layer 101, 102 and photodetection layer 108 are formed to have different thicknesses and compositions, the propagation constants of light guided through each will be different.Second waveguide layer 102
The diffraction grating 202 formed in is for selecting the wavelength to be optically coupled, and the wavelength range to be selected is determined by the pitch thereof.

Furthermore, in the photodetection region 109, since the photodetection layer 108 is formed on the second waveguide layer 102, light of the selected wavelength is absorbed and carriers are generated. For this reason, each electrode 204
．． The resistance value between 205 decreases in accordance with the amount of carriers generated due to photoconductivity, and by detecting this, the selected wavelength light can be detected.

TEO to TE in FIG. 1 indicate the optical electric field distribution of the guided mode in each region, and indicate the light intensity in the traveling direction of the guided light.

In each of the regions 109 to 111 in this embodiment, different semiconductor layers are stacked, so the waveguide modes established are also different.

As for the waveguide mode in the light introduction region 111, only the substrate mode, that is, the TEo mode is established. In the wavelength selection region 110, an even mode (T
Eo mode) and an odd mode (TE mode) having a central intensity in the first waveguide layer 101 are established.

In the photodetection region 109, two or more modes are established depending on its laminated structure, and as shown in FIG.
Three modes are established: + and TE2. The guided light of each mode is absorbed by the photodetection layer as described above, but the absorption coefficient at this time depends on how much the optical electric field distribution of each mode extends to the photodetection layer 108. They are very different.

Next, regarding the operation of this embodiment, FIGS. 1 and 2 (
Referring to a) and (b), the wavelength/l of the optical signal 112 incident on the first waveguide layer 101 is selected and received by the diffraction grating 202.

The description will be made on the assumption that it includes the wavelength λ2 and the wavelength λ2 that is desired to be cut off.

The guided light 101 that has entered the first waveguide layer 101 is guided in the TEo mode in the light introduction region 111.

Next, in the wavelength selection region 110, the wavelength λ1 is selected by the diffraction grating 202 and transferred to the second waveguide layer 102.
TE with central strength at . A TEI that strongly couples with the mode and has a central intensity in the first waveguide layer 101 for wavelength input 2.
It is strongly coupled with the mode and guided. Continuing photodetection area 10
9, the wavelength λ1 is strongly coupled with the TEo and TE+ modes, and the wavelength R2 is strongly coupled with the TE2 mode. At this time, the photodetection layer 108 absorbs light and carriers are generated, but the photodetection layer 108 is the center of the photoelectric field distribution, and the light absorbed at this time is a TE with a high absorption coefficient.
Most of them are in the o, TE, mode, that is, the wavelength R1. Regarding the wavelength input 2, the optical electric field distribution is centered on the first waveguide layer 101 and does not extend to the photodetection layer 108C, so the wave is guided as a TE2 mode with a low absorption coefficient, so it is mostly absorbed. Not done.

FIG. 2(b) shows an example of a circuit that detects the amount of light absorbed by the photodetection layer 108.

A DC power supply vl is connected between the electrodes 204 and 205 via a resistor R1 and an ammeter 206, and a change in the supplied current value is measured by the ammeter 206, thereby detecting a change in the resistance value between the electrodes 204 and 205. , and the amount of light absorbed by the photodetection layer 108 can be detected.

In FIG. 3, light having a wavelength of 0.80 μm to 0.86 μm is introduced into the first waveguide layer 101 as incident light 112, and the detection current ratio for each wavelength is measured. As shown in the figure, the full width at half maximum at the central detection peak is 77 points. Further, the crosstalk between the wavelengths of 0.83 μm and 0.85 μm was approximately 20 dB.

FIG. 4 is a perspective view showing the appearance of a second embodiment of the present invention.

In contrast to the first embodiment, which uses photoconductivity to perform photodetection, this example uses FET structure electrodes to perform photodetection. The manufacturing process and structure are the same as those of the first embodiment except for the electrode structure formed on the top surface, so the same numbers are given and explanations are omitted.

The electrodes in this example are the same as those provided in the first example.
A third electrode is further provided between the two electrodes.

That is, as shown in FIG. 4, a total of three electrodes are provided, which are a drain electrode 401, a gate electrode 402, and a source electrode 403, respectively. Drain electric F
i401, the source electrode 403 is made of Au/8u to Ge film. A gate electrode 402 provided between them is made of an At film.

The gate electrode 402 and the photodetection layer 108 are in Schottky contact, and the photodetection layer directly under the gate type rod 402 is a depletion layer. With a positive voltage applied to the drain electrode 401 and a negative voltage applied to the source electrode 403, the gate electrode 4
When a negative voltage of about 2V to -SV is applied, the depletion layer expands, and in the pinch-off state, the current flowing between the drain and the source becomes 0.

In this state, from the wavelength selection region 110, the second waveguide layer 10
When the guided light propagating through the photodetection layer 108 reaches the photodetection region 109, the guided light is absorbed by the photodetection layer 108 and carriers are generated. Holes among the generated carriers are sucked into the gate electrode 402 and the depletion layer is pushed up, so
A channel current flows between the drain and source. At this time, electrons among the carriers described above become a photocurrent flowing into the drain electrode 401.

In other words, when the guided light does not enter the photodetection layer, the drain-source electrode is in a pinch-off state,
No drain current is generated. When guided light is incident, a channel current due to depletion layer change and a photocurrent due to carrier generation are generated, and as a result, a drain current is generated and light is detected.

In this case, by controlling the gate voltage to an appropriate value, it is possible to improve crosstalk between wavelengths and amplify the detection current. Furthermore, unlike the first embodiment, the detection operation is performed using an FET structure, thereby making it possible to obtain a photodetector capable of high-speed response of GHz or higher.

FIG. 5 is a diagram showing the structure of a third embodiment of the present invention.

In this embodiment, FIG. 5 shows an example in which a Pin photodiode is formed as a photodetection mechanism using an i-layer as a photodetection layer.

First, the structure of this embodiment will be explained.

On the substrate 501, which is n''-GaAs, a 0.5μ thick
buffer layer 502 of n-GaAs with a thickness of 1.5 m;
7
The first waveguide layer 504, which is AS, has a thickness of 1.3 μm.
Cladding layer 505, n which is Alo, 5Gao, 5AS
-GaAs, n-Alo, 5Gao, and 7AS are alternately stacked to form a multiple quantum well with a thickness of 0.2μ
m second waveguide layer 506, 0.15 μm thick n Al
Diffraction grating layer 507 of e4Gao6AS, thickness 0.2
A photodetection layer 508 made of 1-GaAs with a thickness of 75 μm, a cladding layer 509 made of P Alo, 5Gao, or sAS with a thickness of 1.0 μm, and a cap layer 510 made of p”-GaAs with a thickness of 0.5 μm are formed by molecular beam epitaxy. (MBE)
They were grown sequentially using the method. After that, a resist mask that protects the four photodetection regions 109 is fabricated using a photolithography method, and then p''-GaAs, cap layer 510,
p-A1. A GaAs crat 509 layer was etched. Next, the i-GaAs photodetection layer 508 is changed to a mixed etchant of ammonia water and hydrogen peroxide water.
Selectively etched. At this time, the base of the 1-GaAs photodetection layer 508 is n-Al with an Ai content of 40%.
Since the diffraction grating layer 507 is made of o, aGaa6As, etching can be stopped at the i-GaAs layer.

Subsequently, the wavelength selection region 110 was formed by the same process as in the first example. The pitch of the diffraction grating was 22 μm, the length was 690 μm, and the depth was 0.15 μm. A protective film 51 made of Si3N except for the photodetection area 109
1 was formed into a film.

Thereafter, an electrode 512 made of Au/Cr is formed on the cap layer 510, and an electrode 512 made of Au/Cr is formed on the back surface of the substrate 501.
An electric gi513 consisting of the following was formed.

In this embodiment, of the optical signals 514 input to the first waveguide layer 504, only those of the selected wavelength are coupled to the second waveguide layer 506 in the wavelength selection region 110.

After this light is introduced into the light detection region 109, it is absorbed by the light detection layer i-GaAs 508 above the second waveguide layer. The photodetection region 109 has a p-1-n structure, and has electrodes 512 .
A reverse bias is applied between 513 and 513. Therefore,
Carriers generated by absorption are detected as a current signal.

FIG. 6 is a diagram showing the wavelength characteristics of the signal current extracted in this embodiment.

In this example, the depth of the corrugation is shallower and the spacing between the waveguide layers is wider than in the first example, so the coupling length is longer, but the full width at half maximum is approximately 50 inputs. It became narrow.

All of the above embodiments are formed by laminating the photodetection layer on the upper layer of the second waveguide layer, but in thermal theory, the photodetection layer is placed next to the second waveguide layer, that is, in parallel on the plane. The object of the present invention can also be achieved even if it is formed as follows. In other words, if the signal light is coupled from the second waveguide layer to the photodetection layer, but is not coupled from the first waveguide layer to the photodetection layer, the spatial relationship between the photodetection layer and the waveguide layer is Positional relationship is not a particular problem.

In addition, the constituent material used was GaAs-based material, but depending on the detection wavelength, other m-v group compound semiconductors such as InGaAs, semiconductors such as Si and Ge, and II-VT such as GdTe may be used. It is also theoretically possible to construct it from a group compound semiconductor or the like.

Furthermore, the layer on which the diffraction grating is formed may be any region as long as the waveguide modes (even mode and odd mode) formed around the light absorption layer and the waveguide layer overlap.

[Effects of the Invention] Since the present invention is constructed as described above, it produces the following effects.

According to the first aspect, it is possible to realize a photodetector suitable for integration in which the photodetector can also have a wavelength selection function, and has a higher wavelength selection ratio and a higher wavelength selection ratio than conventional ones. This has the effect of realizing a detector with even better inter-wavelength crosstalk.

According to the second aspect of the present invention, there is an advantage that the above-mentioned effects can be easily realized.

In the device according to claim 3, since it has a FET structure, it is possible to improve crosstalk between wavelengths and amplify detection current, and to have high-speed response of GHz or higher. It also has the effect of

In the fourth aspect of the present invention, since it has a pin structure and a current flows in accordance with the amount of human light, there is an effect that detection is facilitated in addition to the above-mentioned effect.

In addition, all of the above are suitable as photodetectors applied to wavelength division multiplexing communication systems, and are also suitable for future optical ICs and OEICs (optoelectronic ICs) because they can be easily integrated with other functional elements. be. Furthermore, it can be widely applied to current and future optical information transmission systems, optical communication systems, optical LANs, optical computing, and the like.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of the first embodiment of the present invention, FIG. 2(a) is a top view showing the configuration of electrodes formed on the top surface of the first embodiment, and FIG. 2(b) 3 is a diagram showing an example of the detection circuit using the first embodiment, FIG. 3 is a diagram showing the wavelength characteristics of the detection current in the first embodiment, and FIG. 4 is a diagram showing the wavelength characteristics of the detection current in the second embodiment of the present invention. A perspective view showing the external appearance, FIG. 5 is a diagram showing the structure of the third embodiment of the present invention, FIG. 6 is a diagram showing the wavelength characteristics of the detection current in the third embodiment, and FIGS. 7 to 9 Each figure shows the configuration of a conventional example, and FIG. 10 is a diagram for explaining the operation of the conventional example shown in FIGS. 8 and 9. 1.1 board, 101,504-...1st
Waveguide layer, 102.502--Second waveguide layer, 104,50
2--Buffer layer, 105--First cladding layer, 1
06--Second cladding layer, 107.507--Diffraction grating layer, 108,508--Photodetection layer, 109...
Photodetection region, 110...Wavelength selection region, 111-
...Introduction area, 112,514-...Optical signal, 2
01--Ridge waveguide, 202--Diffraction grating, 2
03...Ridge end surface, 204.205,512,513...Electrode, 206.
...Ammeter, 401...Drain electrode, 4
02-...Gate electrode, 403-...Source electrode,
503.505,509--cladding layer, 510...
・Cap layer, 511...protective film, old...resistance, v2...DC power supply.

Claims (1)

[Claims] 1. A photodetector in which semiconductor layers are laminated and used to separate signal light of a specific wavelength from a wavelength-multiplexed optical signal, the optical detector comprising: a first waveguide layer into which the signal light is incident, a second waveguide layer for guiding the signal light of the specific wavelength, and a first waveguide mode centered on the first waveguide layer. , a diffraction grating provided in a region where the second waveguide mode centered on the second waveguide layer overlaps, and a photodetector that absorbs incident light and detects the amount of the incident light. The photodetection layer has a third waveguide mode centered on itself that does not overlap much with the first waveguide mode, and
A photodetector characterized in that it is formed at a position that largely overlaps with the second waveguide mode. 2. The photodetector according to claim 1, wherein a photoconductive layer is formed to face the outside as the photodetection layer, and a change in resistance value therebetween is detected on the upper surface of the photodetection layer. A photodetector characterized in that it is provided with two electrodes for detecting the amount of incident light. 3. The photodetector according to claim 1, wherein a photoconductive layer is formed to face the outside as the photodetection layer, and an FET using the photodetection layer as a substrate is placed on the upper surface of the photodetection layer.
A photodetector characterized in that three electrodes are provided to form a structure. 4. The photodetector according to claim 1, wherein the photodetector has a Pin photodiode structure in which the photodetection layer is an i-layer.