JPH0396917A - Photodetection type optical function element and production thereof - Google Patents

Abstract

PURPOSE:To obtain the optical function element having a good matching property with optical fibers by laminating an optical waveguide structure having a multiple quantum well structure as a core, a reverse conductivity type layer and electrode on a semiconductor substrate of one conductive type and irradiating the core part perpendicularly cleaving the substrate with a laser beam. CONSTITUTION:An n-InAlAs clad layer 2, a light absorption layer (InGaAs clad) 3 of the non-added multiple quantum well structure MQW and a p-InGaAs gap 5 are superposed on the InP substrate 1 and are etched to cover the mesa with polyimide. The electrode 7 is provided thereon and the entire part is cloven in the prescribed manner to form a chip. The MQW 3 consists of the cycle of the multiple quantum well layers of InGaAs and barrier layers of InAlAs. The photodetecting sensitivity is improved, the absorption length is shortened and the element capacity is decreased by adopting the MQW structure of a large absorption coefft., by which the photodetector having the high sensitivity and high speed is obtd. The refractive index near the end face is lowered by the semiconductor structure of an end face incident type and the spot diameter of a waveguide mode is increased. The quantum efficiency as the photodetector is thus enhanced and the optical function element having the good matching property with the optical fibers is obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a light-receiving optical functional element that is small in size, has high coupling efficiency with an optical fiber, and can realize light reception, light modulation, and light amplification, and a method for manufacturing the same. It is something.

[Conventional technology]

Conventionally, light-receiving elements have generally been of the two-sided type, which detects light from the top surface of the element, but this has poor compatibility with so-called optical integrated circuits, in which various optical elements are arranged and connected on a flat surface. Ta. In addition, with the development of long-distance, high-capacity optical fiber transmission systems, greater high-speed response is required. Since the photodetector is used with a reverse voltage,
The resistance R is large, and 50Ω is used for matching with the circuit. On the other hand, since the capacitance is determined by the element area, it is necessary to reduce the light-receiving diameter, but as shown in Figure 3, as the light-receiving diameter decreases, the ratio of the surrounding electrode portion to the element capacitance increases. However, there is a problem with high speed. In order to compensate for these shortcomings, a so-called edge-receiving type light-receiving element, in which light enters from the side of the element, has been proposed (IEICE Photon Quantum Electronics Study Group: IEICE Technical Report 20QE).
86-181 (1986)). The edge-light receiving type photodiode can be made small because the coupling length with the incident light can be made long, and because the element capacitance is determined by the thickness of the absorption layer, it can be expected to have high-speed response. However, from the perspective of the optical waveguide structure, there is a noticeable asymmetry in the spot size of the guided light between the directions parallel to and perpendicular to the layers, which is a general drawback of semiconductor waveguides. ,
The disadvantage is that there is a large mismatch in the size of the light beam from a commonly used optical fiber, and the coupling efficiency is low.

On the other hand, as the long distance and capacity of optical fiber transmission increases, the development of high-speed, low-noise photodetectors has become important, and surface-illuminated PI
Research on PIN/FETs that integrate N photodiodes and FETs on the same substrate is becoming more active (for example, IEE3, Journal of Quantum Electronics (I
EEE J. Quantum Electronic
s) Q E 2 2, p. 805 (1986))
. Although data has been released for each of these, the manufacturing process is complex and yields have not been taken into account, so they are still at a stage where they cannot be considered practical.

In addition, recently, an optical waveguide type APD (avalanche diode) has been proposed in a multiple quantum well structure (hereinafter referred to as MQW structure) that combines Si and Si-Gea products (Temkjn et al., Applied Applied Physics Letters
ters) 4, 9, 809 (1986)). Since the above element is originally an indirect transition type, the aim was to increase the absorption length by adopting an optical waveguide structure using the above material with a small absorption coefficient, and to improve the quantum efficiency, and to some extent, the characteristics have been obtained. .

[Problem to be solved by the invention]

However, the prior art described in Section 2 has the drawbacks of large residual stress existing inside the device and large dark current.

4 Also, semiconductor optical modulators usually employ an optical waveguide structure for the purpose of increasing interaction with light.
Same as above, for example, IEICE Technical Report OCS88-11 or OQE
As shown in 8 9-3 2, it has the disadvantage of large coupling loss.

Furthermore, semiconductor optical amplification elements also have a problem in that coupling loss with optical fibers is large, and nearly half of the gain is lost (for example, IEICE Technical Report OQE89-17).

The present invention employs an MQW structure with a large absorption coefficient to increase the light receiving sensitivity, shortens the absorption length to reduce the element capacitance, and obtains a highly sensitive and high-speed light receiver. Lower the refractive index in the vicinity, increase the spot diameter of the waveguide mode, increase the coupling efficiency with the incident light, improve the quantum efficiency as a light receiver, and increase the diameter of the waveguide spot 1 to match with the optical fiber. The object of the present invention is to obtain an optical functional element with improved properties and a method for manufacturing the same.

[Means to solve the problem]

The above object is to form an optical waveguide structure having a core of a multi-quantum well structure consisting of a quantum well layer and a barrier layer that are lattice-matched to the substrate on a semiconductor substrate of one conductivity type, and to form an optical waveguide structure on the optical waveguide structure. A layer having a conductivity type opposite to that of the semiconductor substrate and an electrode for directly applying a predetermined electric field to the multi-quantum well layer are provided, and the substrate is cleaved perpendicularly to the plane consisting of the semiconductor substrate and the optical waveguide structure. This is achieved by irradiating the core of the light-receiving surface formed by the process with a predetermined laser beam to create a mixed crystal, thereby obtaining an optical waveguide mode spot diameter that is smaller than the spot diameter inside the optical waveguide. .

[For production]

The present invention uses an optical waveguide structure having an MQW structure as its core.
The main feature of an edge-receiving optical functional element in which light enters from the side surface of the element is that the refractive index near the edge is controlled to improve coupling efficiency with incident light. As a means to take advantage of the above characteristics, a laser beam of a predetermined wavelength and output is irradiated near the end face of the optical waveguide for a predetermined period of time near the core of the MQW.
The second feature is that a desired refractive index distribution can be formed in the optical waveguide by making the structure a mixed crystal.

In the MQW structure, as shown in FIG. 4, an exciton absorption peak is clearly observed even at room temperature, and the absorption coefficient at this peak is about three times larger than that of the bulk. Therefore, approximately the same amount of light absorption can be obtained with a length of about 1/3 compared to the bulk, so the element length can be shortened.
The element capacitance is also reduced, making it suitable for high-speed response. The above MQW
A heat treatment method in which a dielectric film is attached on top of the layer and rapid heating and cooling are repeated in a short period of time (Japanese Journal of
Applied Physics (J.J.A.P) Volume 28 73
0 (1989)), mixed crystal formation occurs in the MQW layer, resulting in the spectral characteristics of the absorption coefficient as shown in FIG.

That is, the peak position of the absorption coefficient shifts to the high energy, short wavelength side, and the refractive index of that portion also becomes smaller than the value before mixed crystal formation (FIG. 6). Therefore, the optical 1 waveguide conditions in the part where mixed crystallization has occurred change, and the difference in refractive index with the cladding layer becomes smaller than in the part where mixed crystallization does not occur.
The light confinement in the direction perpendicular to the layer becomes weaker, the mode spot size increases, and the difference between the horizontal and vertical spot shapes in the plane perpendicular to the light propagation direction of the optical waveguide decreases. , the shape changes from a flat ellipse to a shape close to a circle, improving the coupling efficiency with the optical fiber. However, the above method requires a dielectric film and... This poses a practical problem because it is necessary to form the film directly on the MQW layer (the MQW layer is usually used as the core of optical waveguide). In other words, the above spot size conversion section is always required at the light incident section, but since this section is formed at the wafer stage in device fabrication, after the above process, the cladding layer is re-attached to the section etched to the core section. It has to be re-lengthened to form an optical waveguide structure, which makes the process extremely complicated.

In the present invention, when the core part is irradiated with a high-power laser beam whose emission wavelength is the wavelength absorbed by the core part, the same effect as described above can be produced, and mixed crystal formation can be prevented in a relatively short time. Since it occurs only in desired areas, there is no need to apply a dielectric film or perform repeated heat treatments. By the way, GaA
In the MQW structure of s/AAGa-8 As, irradiation with Ar laser causes mixing of products (Applied Physics Letters).
ppl. Phys. Lett. ) 4 Volume 3 21
No. 24 (1986)).

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a first embodiment of a light-receiving optical functional element according to the present invention, and FIG. 2 is a diagram showing a second embodiment of the present invention.
) is a perspective view, and (b) is a side view.

In the first embodiment of the first drawing, 1 is an InP substrate, 2 is an n
InAIIAs cladding layer, 3 is non-doped MQW
Light absorption layer (InGaAs or InGanAs cladding layer, 5 is p-InGaAs cap layer,
6 is an n electrode, 7 is a p electrode, and 8 is polyimide. The manufacturing method is to place an n-cladding layer 2 on an InP substrate 1 with a thickness of 0.2
~0.4/ffi, MQWJI3 0.4~1.0μ,
P-cladding layer 4 is 1.0μ, cap layer 5 is 0.2p
m, sequentially elongated using the molecular beam epitaxial elongation method. Thereafter, using an ordinary photomask, etching is performed by reactive dry etching or chemical etching up to the InP substrate 1, leaving a width of 3 to 5 IIm, and the entire formed mesa portion is spin-coated with polyimide. After that, electrodes were provided by vapor deposition, patterned again by photolithography, and dry etched to the butt part (~50 μm).
mX5Qμ)) and @3 0 0 tn
gI is opened to a length of about 100 IM to form a chip, and a bonding wire is attached. The MQW layer 3 is I n
Quantum well layer of GaAs or InAuGaAs (thickness 60-90A). I nAuAs barrier layer (thickness 50
~70 layers), consisting of 20 to 40 periods, and each layer is not doped with impurities. The light is incident parallel to the layer and the voltage is n. It extends over the MQW layer 3 sandwiched between the p-cladding layers 2 and 4. In this first embodiment, the present invention is applied to an external modulator, and the extinction ratio is 20 dB and the frequency band degraded by 3 dB @ 20 GH at an applied voltage of 4 to 7 cm for the incident light of 1.55 mm. In this example, in which a value of z or more is obtained, the refractive index distribution gradually increases over a length of 3 to 5 capes from the light incident end face toward the inside of the optical waveguide layer. This is an Ar laser (λ=488nm)
This is because mixed crystal formation occurred in the MQW layer 3 in the core portion by irradiating the core portion with a focused lens for several seconds at an output of 500 mW. By the above irradiation, In Ga As
Ga and An are mixed between InAIIAs and InAIIAs, and InAIIAs
A mixture of GaA11As is formed, and no mixed crystal formation occurs in areas that are not irradiated with light. The refractive index of the part where mixed crystallization has occurred is In Ga As / I
In the case of n An As -based MQW, it decreases by about IXIO-'. This large change is due to the presence of exciton absorption. The refractive index of the mixed crystal region is at most close to the average refractive index of the quantum well layer and barrier layer that make up the MQW, but in this case it is a case where the mixed crystal has completely progressed. There is a change of about 0 to 3% depending on the degree of crystallization. Therefore, the refractive index difference with the cladding layer InuAs decreases,
The mode spot diameter in the direction perpendicular to the layer also increases. For example, the core is made of 1nGaAs/InAllAs cladding layer.
In the case of InAQAsMQW, the thickness of MQWM is 0.
．． 4.x, the mode-11 Subono 1 diameter increases from 0.36μ to 0.40p, so the coupling efficiency improves by about 1 dB. This becomes even more noticeable when the thickness of the MQW/W is thin, for example, when the thickness is 0
．． The mode spot diameter is 0.33ρ to 0.40um for 25-, and 0.3'71 for thickness 0.15μ.
rm to 0.45 μm, and the coupling efficiency is both about 7
Increases by dB. This tendency can be improved by increasing the mode spot diameter by increasing the refractive index of the cladding layer, lowering the refractive index of the core, or reducing the thickness of the waveguide itself to lower the equivalent refractive index. become noticeable.

FIG. 2 is an explanatory diagram showing a second embodiment of the present invention that achieves the above effects, in which the thickness of the core portion is formed in a tapered shape with respect to the traveling direction of the light at the light incident portion. . This can be formed by selective etching of semiconductor crystals, and MQ
Waveguide core section consisting of W layer]. Since the thicknesses 3 and 13' are formed smoothly, radiation loss due to unevenness etc. can be reduced. In the figure, 11 is the substrate, 12 is n
One cladding layer, King 3 is MQW#, 13' is mixed crystal M1
2-QWN, 14 is p cladding layer, 15 is cap layer, process 6
, 17 are n. p electrode, 18 polyimide, 19
is a mode converter, 20 is a semi-insulating InP or InAu
This is an As buried layer. Note that a portion of the P electrode 17 is removed so that light can be irradiated from above the optical waveguide. Therefore, compared to the method of irradiating light from the cleavage end face, it is possible to form a mixed crystal longer in the longitudinal direction of the optical waveguide. As the thickness of the trumpet-shaped core changes, the refractive index changes gradually, and there is no sudden refractive index change within the optical waveguide, and the mode spot shape of the optical waveguide is gradually transformed. , scattering loss associated with mode conversion, etc. can be reduced.

If the position of the exciton absorption peak is adjusted in advance to the above wavelength for light with a wavelength of 1.55, it is possible to change the refractive index significantly from the laser irradiation process described above, and the mode spot diameter can be matched. , a quantum efficiency of 0.3 A/W was obtained, and the frequency characteristics were 0.1 because polyimide was used under the bonding butt to reduce stray capacitance.
It is around p・F, which is 50GH as a 3dB deterioration frequency band.
z was obtained.

Furthermore, in the optical modulation element, the coupling loss was improved from around 5 dB to 10 dB or less. Furthermore, a similar reduction in coupling loss could be achieved in the optical amplification element as well.

InGaAs as quantum well layer, InAl as barrier layer
Although IAs has been explained, InGa as a quantum well layer is also used.
AsP. GaAs. AflGaAs. GaSb. Affl
Similar effects can be obtained using materials such as GaSb.

〔Effect of the invention〕

As described above, the light-receiving optical functional device according to the present invention has an optical waveguide structure having a core of a multi-quantum well structure consisting of a quantum well layer and a barrier layer that are lattice-matched to the substrate on a semiconductor substrate of one conductivity type. a layer having a conductivity type opposite to that of the semiconductor substrate and an electrode for applying a predetermined electric field perpendicularly to the multi-quantum well layer on the optical waveguide structure; By forming the light-receiving surface by cleaving the substrate perpendicular to the plane consisting of the light-receiving surface, the spot diameter of the light incident area can be adjusted to an appropriate size using the spot diameter conversion waveguide, improving coupling efficiency. This increases the external quantum efficiency. In addition, since it is an end-illuminated structure, for example, the light-receiving element can be operated with low capacitance, high speed, and low voltage, and an optical functional element suitable for optical integration can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a first embodiment of the light-receiving optical functional element according to the present invention, and FIG. 2 is a diagram showing a second embodiment of the present invention.
) is a perspective view, (b) is a side view, and FIG. 3 is a diagram showing the relationship between element junction capacitance and light receiving diameter of a conventional top-light receiving type photodetector.
Figure 4 is a diagram showing the difference in absorption coefficient between the bulk and MQW structure, Figure 5 is a diagram showing that the light absorption spectrum of the MQW structure shifts to the short wavelength side due to rapid heat treatment, and Figure 6 is the diagram shown above. 6 is a diagram showing the wavelength dependence of the refractive index of the sample showing the absorption coefficient change in FIG. 5. FIG. 1, Step 1...Semiconductor substrate 3, Step 3...Multiple quantum well structure (MQW)-15-3'. 13'...Mixed crystal MQW layer 6, l6...n electrode 7, 17...p electrode-16-

Claims (1)

[Claims] 1. An optical waveguide structure having a core of a multiple quantum well structure consisting of a quantum well layer and a barrier layer lattice-matched to the substrate is formed on a semiconductor substrate of one conductivity type, and the optical waveguide A layer having a conductivity type opposite to that of the semiconductor substrate and an electrode for applying a predetermined electric field perpendicularly to the multiple quantum well layer are provided on the structure,
A light-receiving type optical functional element in which a light-receiving surface is formed by cleaving a substrate perpendicular to a plane made of the semiconductor substrate and optical waveguide structure. 2. The optical waveguide structure is characterized in that the refractive index distribution in the portion where the equivalent refractive index decreases gradually increases from the end face toward the inside, and becomes the same as the internal refractive index at a predetermined length. A light-receiving optical functional element according to claim 1. 3. On a semiconductor substrate of one conductivity type, an optical waveguide structure having a core of a multi-quantum well structure consisting of a quantum well layer and a barrier layer lattice-matched to the substrate is formed, and on the optical waveguide structure, the above-mentioned a layer having a conductivity type opposite to that of the semiconductor substrate, and an electrode for applying a predetermined electric field perpendicular to the multiple quantum well layer;
The substrate is cleaved perpendicularly to the multi-quantum well layer to form a light-receiving surface, and a laser beam having an emission wavelength that is absorbed by the core portion is externally applied to the core portion of the light-receiving surface at a predetermined input power. A light-receiving type optical functional element in which the quantum well layer in the core portion is mixed crystallized by irradiation for a predetermined period of time, and the mode spot diameter of the optical waveguide portion is made smaller than the spot diameter within a predetermined length from the light-receiving surface. Production method. 4. On the light-receiving surface, an electrode is formed short by a predetermined length inside the cleavage plane, and in the part where the electrode is not present,
The light-receiving type optical functional device according to claim 3, wherein the substrate and the waveguide portion are irradiated with a predetermined laser beam from a vertical direction, and then the cleavage plane is formed such that the irradiated portion becomes the irradiated portion. manufacturing method.