JPH03198025A - Optical modulator and integratation type optical modulator and photodetector as well as production thereof - Google Patents

Abstract

PURPOSE:To attain ultra-high-speed modulation and the widening of a frequency range and to improve a yield and uniformity by using a high-resistance semiconductor substrate and embedding a PIN structure optical waveguide with the high-resistance layer. CONSTITUTION:Striped means 69 having the pin structure in which an I layer of a low carrier concn. is formed as the semiconductor optical waveguide layer 66, the high-resistance layers 70 on both side faces of the striped means, and means for impressing electric fields to the semiconductor optical waveguide layer 66 are provided on the high-resistance semiconductor substrate 61. The inter-electrode distance is made longer than heretofore by using the high- resistance semiconductor substrate and the embedding structure of the semiconductor high- resistance layers. The wiring capacity and pad capacity are, therefore, decreased and the capacity over the entire part of the element is determined nearly by a junction capacity and the frequency range of the optical modulator is widened. The ultra-high-speed modulation as the integration type optical modulator is executed by having the PIN structure formed on the high-resistance substrate and embedded by the high-resistance layers. Thus, the ultra- high-speed modulation and the widening of the frequency range are possible in this way and the yield is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical modulator, an integrated optical modulator, a photodetector used in a high-speed optical communication system, and a method for manufacturing the same.

(Prior Art) With the recent development of optical communication systems, there is an increasing demand for optical modulators that can operate at ultra-high speeds and low voltages, and that can be easily miniaturized and integrated. Effect of increasing optical absorption loss in the optical waveguide by applying an electric field to the optical waveguide in a semiconductor optical modulator (Franz Keldysch effect or quantum Stark effect)
The device using this has the advantage of having a modulation band of several tens of GHz if the element capacitance is reduced, and that it is easy to integrate distributed feedback semiconductor lasers and the like. As an example of a modulator, InGaAlAs/InAlAs prototyped by Wakita et al.
A 20 GHz optical modulator using an MQW structure is described in the Proceedings of the Spring National Conference of the Institute of Electronics, Information and Communication Engineers C-474. This is an absorption type modulator that utilizes the shift of the exciton peak caused by the electric field due to reverse bias to the semiconductor PIN structure.
s cladding layer, i-MQW guide layer, p-InAlAs
The cladding layer was created by the MBE method. The modulation frequency band Δf of such a modulator is approximately determined by the capacitance C of the element, and is expressed as Δf=1/(,CR).

Further, the element capacitance is expressed as the sum of the junction capacitance Cj at the pn junction, the wiring capacitance Ci, and the pad capacitance C1 at the bonding pad.

In the case of the above-mentioned modulator, since ultra-high-speed modulation is aimed at, polyimide is buried under the pad portion to reduce the capacitance, and as a result, an extremely low element capacitance of 0.2 pF is obtained. However, even in this case, the junction capacitance C inherent in the modulator
j is less than half of the total, and the remainder is essentially unnecessary wiring capacitance and pad capacitance generated between the n-InP substrate and the wiring electrode. Furthermore, the element length of this modulator is approximately 10011 m, and considering the characteristics of the switch, it is difficult to reduce the junction capacitance any further, and furthermore, a conductive substrate such as an n-InP substrate is used. Therefore, it is also difficult to further reduce wiring capacitance and pad capacitance. Therefore, in an optical modulator with a conventional structure, the modulation band is at most 20 to 25 GH.
z, future ultra-high-speed optical modulator (bandwidth ≧50GHz)
It is difficult to apply to Furthermore, as an example of integrating a modulator and a semiconductor laser, IoOshi (10
0C'89) Technical Digest 20PDB-5 (
(1989). This is a device that integrates a DFB-LD and a modulator that utilizes light absorption due to the Franz Keldeitssch effect on an n-InP substrate, and both sides of the optical waveguide of the LD and modulator are embedded with high-resistance InP. . However, since a conductive substrate is used, there is a large parasitic capacitance, and to reduce this, a polyimide layer is placed under the electrode pad. As a result, the element capacitance was 0.55pF,
A modulation band of about 10 GHz has been obtained.

(Problem to be solved by the invention) In the conventional example, since a conductive substrate is used, there is a limit to the reduction of element capacitance, and the modulation band is about 25 GHz for a modulator, and about 10 GHz for an integrated modulator with a laser. It was limited. This is because the thickness of the high resistance layer between the electrode formed on the conductive substrate and the other electrode formed on the mesa stripe and high resistance layer is only 2 to 3 pmL on both sides of the striped mesa. This is because the element capacitance cannot be reduced to 0.5 pF or less. Although element capacitance can be reduced by increasing this thickness, there is a limit to the height of the mesa due to manufacturing considerations.

An object of the present invention is to provide a broadband optical modulator, an integrated optical modulator, and a photodetector that can perform ultrahigh-speed modulation by reducing the element capacitance. Furthermore, it is an object of the present invention to provide a manufacturing method in which they can be obtained with good yield and uniformity.

(Means for Solving the Problems) The optical modulator of the present invention includes a striped mesa having a PIN structure in which one layer with a low carrier concentration is a semiconductor optical waveguide layer on a high-resistance semiconductor substrate; It is characterized by comprising a high resistance layer on both sides of the mesa and means for applying an electric field to the semiconductor optical waveguide layer. Further, the integrated optical modulator of the present invention monolithically integrates a semiconductor laser with the above-mentioned optical modulator, and arranges it so that the oscillation light of the semiconductor laser and the waveguide of the optical modulator are optically coupled. It is characterized in that the intensity of light from the semiconductor laser can be modulated by the intensity of the applied electric field.

The photodetector of the present invention has the same structure as the optical modulator, but one layer is made of a semiconductor having a narrow bandgap than the light to be detected so as to serve as a light absorption layer, and an electrode for detecting photocurrent is used. It is characterized by having the following. Further, the manufacturing method of the present invention includes at least a first conductivity type semiconductor layer, an i-type semiconductor layer, and a second conductivity type semiconductor layer on a high-resistance semiconductor substrate having a partially striped first conductivity type buffer layer and a flat surface. a step of forming a semiconductor multilayer including a semiconductor layer; a step of forming a stripe-shaped mask on the semiconductor multilayer along one boundary between the buffer layer and the high-resistance substrate; and a step of forming one of the mesa stripes using this mask. forming a mesa stripe by etching to expose the high resistance substrate on one side and the buffer layer on the other side; forming a high resistance layer on both sides of the mesa stripe to form a substantially flat surface; A first layer is formed on the buffer layer exposed by removing a portion of the high resistance layer.
forming an electrode, and forming a second electrode on a region including the mesa stripe.

One method for forming a high-resistance semiconductor substrate with a partially striped buffer layer of the first conductivity type and a flat surface is to form stripe-like grooves on the high-resistance semiconductor substrate and selectively fill the grooves. According to a method of forming a first conductivity type buffer layer in a. The second method is to form striped grooves on a high-resistance substrate.
A first conductivity type buffer layer is formed on the entire surface, a photoresist is applied so as to flatten the grooves, and then vapor phase etching is performed until the high-resistance substrate is exposed except for the grooves. . The third method is to introduce impurities of the first conductivity type into a high-resistance substrate in a striped manner by ion implantation or diffusion.

The fourth method is to form a semiconductor layer of the first conductivity type over the entire surface on a high-resistance substrate, and implant ions to increase the resistance of the buffer layer of the first conductivity type except for striped portions.

(Function) The present invention uses a high-resistance semiconductor substrate and further embeds a PIN structure optical waveguide with a high-resistance layer, thereby eliminating parts unrelated to the actual operation of an optical modulator, integrated optical modulator, or photodetector. By reducing the capacitance as much as possible, the capacitance of the entire device can be reduced, making it possible to widen the bandwidth of the optical modulator and photodetector.

Generally, the capacitance C can be expressed as C=ε8εQS/d.

Here, ε5 is the relative dielectric constant, ε0 is the vacuum permittivity, S is the electrode area (or pn junction area), and d is the interelectrode distance (or depletion layer thickness). As described in the conventional example section, the capacitance Ct of the entire element is expressed by the junction capacitance Cj, the wiring capacitance Ci, and the pad capacitance Cp as Ct=Cj+C1+C1. Since the junction capacitance Cj affects the static characteristics of the optical modulator, it is designed to the extent that it does not deteriorate.If the waveguide width is 2 μm, the waveguide length is 1100p, and the depletion layer thickness is 0.3pm, the junction capacitance Cj is approximately 74fF. Become. It is desirable to reduce the remaining wiring capacitance Ci and pad capacitance Cp in order to widen the band of the optical modulator. According to the present invention, by using a high-resistance semiconductor substrate and a high-resistance layer embedded structure of semiconductor or dielectric material, the distance d between electrodes can be increased to about 100 μm, which is longer than before. , a structure in which only the bottom of the pad is buried with a dielectric material such as polyimide (
d = 2 to 3 pm, εS-3), the structure (d
=2 to 3 pm, ε, ~12), the wiring capacitance Ci and the pad capacitance CI layer can be reduced to about 1/30. As a result, the capacitance Ct of the entire element is approximately determined by the junction capacitance Cj, making it possible to widen the band of the optical modulator.

In addition, P
An optical waveguide with an IN structure has many structural similarities with a semiconductor laser, and with a similar configuration, it can be easily applied to an integrated device of an optical modulator and a semiconductor laser, and it can be used as an integrated optical modulator with a laser. Ultra-high speed is also possible.

Furthermore, in the structure of the optical modulator according to the present invention, the composition of the optical waveguide layer is made to have a bandgap wavelength longer than the wavelength of the light source, and the photocurrent due to the light absorbed by the optical waveguide layer is transferred to the p-side electrode and the n-side electrode. It can be used as a waveguide type photodetector by detecting from . In this case as well, as shown earlier, the capacitance of the element can be greatly reduced, so
An ultra-wideband photodetector can be obtained.

In addition, in order to fully bring out the effects of using the high-resistance substrate of the optical modulator and photodetector of the present invention, a high-resistance layer is placed under the p-type electrode, and an n-type There should be no electrodes. In order to reduce the capacitance, it is necessary to form a p-type electrode on one side of the mesa stripe and an n-type electrode on the other side. This structure can be realized by performing mesa etching twice and etching one side at a time, but the manufacturing process becomes complicated. According to the manufacturing method of the present invention, it can be formed automatically. That is, on a high-resistance substrate having a striped first conductivity type semiconductor buffer layer on a part of the surface, P is applied on the edge of the stripe of the first conductivity type buffer layer.
A mesa stripe with an IN structure is formed by etching. By appropriately setting the etching depth and mesa width, it is possible to automatically expose the high resistance substrate on one side of the mesa stripe and the first conductivity type buffer layer on the other side. In this way, a conductive layer can be formed only on one side of the stripe by etching at a certain degree. A high resistance layer is formed on both sides of the mesa to make the surface substantially flat, a part of the high resistance layer is removed to expose the buffer layer, a first electrode is formed there, and a mesa stripe and a high resistance layer are formed. The optical modulator or photodetector of the present invention can be obtained by forming a second electrode on a part of the substrate.

This manufacturing method requires only one mesa etching, so the process is easy, the yield is high, and it is possible to manufacture with good uniformity. Further, a striped buffer layer of the first conductivity type can be formed on a part of a high-resistance substrate by using selective growth or vapor phase etching of a semiconductor layer and a resist at a uniform rate. Alternatively, a striped buffer layer may be formed by introducing impurities of the first conductivity type into the portion of the high resistance substrate that will become the buffer layer portion by diffusion or ion implantation. A conductive buffer layer may be formed, and then striped portions may be removed by proton implantation to increase the resistance of the buffer layer. In either method, a high-resistance substrate having a striped conductive buffer layer can be easily obtained with good yield.

(Embodiment) FIG. 1 is a perspective view showing a first embodiment of claim 1 according to the present invention, and FIG. 2 is a perspective view perpendicular to the stripe direction showing the manufacturing process of the optical modulator of the first embodiment. FIG. As for the material system, a DH structure waveguide will be explained using InGaAsP/InP system, but the material and structure are not limited to this, and may include InGaAs/InAlAs system, GaAs/AlGaA
A method for manufacturing the optical modulator shown in the first embodiment of the present invention will be described using s-based materials and FIGS. 1 and 2. FIG. On a high-resistance InP substrate 1, an n''-InP buffer layer 2 with a thickness of 0.5 pm, a 1-InGaAsP (band gap wavelength 1.475 pm) optical waveguide layer 3 with a thickness of 0.3 pm,
A p''-InP cladding layer 4 is sequentially grown using a 1.2 pm MOVPE method to form a striped 5
An i02 mask 10 is formed by a normal photolithography method. The width of the stripe at this time was 2 pm.

Furthermore, in order to take out the n-side electrode, a resist mask 1I layer is applied to only one side of the stripe as a boundary. This state is shown in FIG. 2(a). Using these masks, the side without the resist mask 11 is removed by etching, and a step with a thickness of 0.5 pm, approximately the same as that of the n''-InP layer 2, is created in advance, as shown in Figure 2(b). .

After peeling off the resist mask 1I layer, the previously formed layer 5
Using the i02 stripe mask 10, etching is performed to form a three-dimensional waveguide. If the etching depth at this time is about 1.6 pm, due to the step created earlier,
A high-resistance InP substrate 1 is exposed on one side of the stripe, and an n''-InP buffer layer 2 is exposed on the other side.Second
It is figure (c).

Next, using the 5i02 stripe mask 10 as it is as a mask for selective growth, the Fe on both sides of the mesa stripe is
When selectively buried with the doped high-resistance InP buried layer 5, the result is as shown in FIG. 2(d). After peeling off the 5i02 stripe mask 10, the top of the mesa stripe and the n+InP
A p-side electrode 6 processed as shown in FIG. A part of the high-resistance InP buried layer 5 on the side is removed by etching until the n+-InP buffer layer 2 appears on the surface, and an n-side electrode 7 is formed in that part to form the structure shown in FIG. 1 or 2(e). The structure shown is completed. The distance between the n-side electrode 7 and the p-side electrode 6 was approximately 1100p. The substrate was polished to approximately 1100p, and the element length was cleaved to 1100XX.
shall be. At this time, the area of the p-side electrode 6 is 100
The size is μm×2pm, the wiring part is 10pm×20pm, and the pad part is 1100p×100μm. In this way, the capacitance can be reduced by dividing the electrode metal into the stripe portion, the pad portion, and the wiring portion connecting them, and reducing the area.

Next, the operation of the optical modulator of this first embodiment will be explained. First, we will discuss static characteristics. The wavelength of the incident light 8 is assumed to be 1.55 pm for optical communication. p-side electrode 6 and n-side electrode 7
When no reverse bias voltage is applied between them, the incident light 8 is directly output as the output light 9. The propagation loss at this time is due to the element length of 100 μm and the wavelength difference between the incident light and the band gap of the optical waveguide layer being 75 nm.
This is a small value of about 1.5 dB. A reverse bias voltage is applied between the p-side electrode 6 and the n-side electrode 7, and i-InGaA
When an electric field is applied to the sP guide layer 3, the incident light 8 is absorbed by the entire 1-InGaAsP optical waveguide layer 3 due to the Lanz-Kjelditssch effect, but the outgoing light 9 is not output. At this time, the extinction ratio was 10 dB or more at a voltage of 3 V, and good characteristics were obtained. Next, we will discuss the modulation characteristics. As mentioned in the operation section, the band of a modulator using electric field effect is almost determined by the capacitance C of the element, and Δf=1/(,CR
). In the case of the example, the dielectric constant of the semiconductor is 12
．． 5, the junction capacitance Cj is 74 fF, the wiring capacitance Ci and the pad capacitance C1 are 12 Sho, and the capacitance of the entire element is 86 fF.

Therefore, by using the high-resistance substrate according to the present invention, the value of the element capacitance that determines the modulation speed can be reduced to about one-tenth to one-tenth of that of the conventional one, and the modulation band is 74G.
Hz, and a modulator capable of ultra-high-speed modulation can be obtained.

FIG. 3 is a perspective view showing a second embodiment of the optical modulator according to the present invention, and FIG. 4 is a diagram showing the manufacturing process of the optical modulator of the second embodiment.

First, the manufacturing method of the second embodiment of the present invention will be explained using FIGS. 3 and 4. An n-InP buffer layer 22 with a thickness of 1.0 pm is formed on a high-resistance InP substrate 21, and an i-InG
An aAsP (bandgap wavelength 1.475 pm) optical waveguide layer 23 with a thickness of 0.3 pm, a p''-InP layer 24 with a thickness of 1.2 pm were sequentially grown by the MOVPE method, and a striped 5i02 mask 3I layer was formed to form a waveguide. is formed by a normal photonography method.The width of the stripe at this time is 2 pm.5i02
Perform etching using the stripe mask 3I layer,
A three-dimensional waveguide is formed as shown in FIG. 4(a). By setting the etching depth at this time to about 1.8 μm, the layer surface 2 of the n+InP buffer layer 22 is exposed.

Next, using the 5i02 stripe mask 3I layer as it is as a mask for selective growth, both sides of the mesa stripe are selectively buried with high-resistance InP buried layers 25. 5
After peeling off the i02 stripe mask 3I layer, the radius is 20B1m.
The 5i02 stripe mask 32 is first formed so as to cover which mesa stripe, and the 5i02 stripe mask 3 is formed to cover which mesa stripe.
2 to form a high-resistance InP buried layer 25 and n+-I
The nP buffer layer 22 is etched by about 2 pm until it is slightly etched, forming a wide mesa stripe. This is shown in FIG. 4(b). Only one side including this mesa stripe is covered with a 5i02 mask 33, and the other side is etched until the high resistance InP substrate 21 is exposed.

This is shown in FIG. 4(e). After peeling off the 5i02 mask 33, a p-side electrode 2 is formed via the 5i02 passivation film 28 on the mesa stripe where the p+-InP cladding layer 24 is exposed and on the side where the n+-InP buffer layer 22 is not present.
6, and finally n+-In on the opposite side to the p-side electrode 26.
An n-side electrode 27 is placed on the part where the P buffer layer 22 is exposed on the surface.
form.

In this way, the structure shown in FIG. 3 or FIG. 4(d) is completed. As in the first embodiment, the substrate is polished to approximately 1100p, and the element length is cleaved to 1100p. In addition, the p-side electrode is 100 pm 100 pm in the stripe part.
p, 10pm x 20pm in the wiring part, 1010 in the pad part
0p 1100p.

Next, the operation of the optical modulator of this second embodiment will be explained. Since the composition and layer thickness of the i-InGaAsP optical waveguide 23 are the same as those of the first embodiment, similar results can be obtained regarding the static characteristics, and the output light 30 is The propagation loss is as small as about 1.5 dB, and the extinction ratio when a reverse bias voltage is applied between the p-side electrode 26 and the n-side electrode 27 is 10 dB or more at a voltage of 3 V, which is a good characteristic. Regarding the modulation characteristics, since the manufacturing process is different from that of the first embodiment, the element capacitance is also slightly different from 92 fF due to the difference in the structure of the wiring portion and the pad portion, and therefore the modulation band is 69 GHz. Although the band is somewhat narrower than that of the first embodiment, this degree can be improved sufficiently by changing the thickness of the substrate polishing and the area of the pad portion, and similarly in the second embodiment of the present invention, An ultrahigh-speed modulator with a band of 50 GHz or more can be obtained.

The element length, waveguide width, pad area, etc. of the optical modulator in the first and second embodiments shown here are merely examples, and are not limited to these.

Also, regarding the structure of each layer, the n-side buffer layer is made of In
Applications such as using one of the P layer and the InGaAsP layer as an etching stop layer, and providing an InGaAsP cap layer on the p-side InP cladding layer are also fully applicable.

In addition, in the first and second embodiments, both sides of the stripe of the waveguide are buried with a high-resistance InP layer, but even if a dielectric material such as polyimide is used in this part, it will not work as an optical modulator. The obtained effects are almost the same, and an optical modulator capable of ultra-high-speed modulation can be obtained by reducing the element capacitance.

Figure 5 shows a semiconductor laser (
FIG. 3 is a diagram showing an embodiment of the integrated optical modulator according to claim 2, in which a LD) and a modulator are integrated, in which (a) is a cross-sectional view in the light propagation direction, and (b) is a cross-sectional view between A and A'. Cross-sectional view, (C) shows B-B
A cross-sectional view between ' is shown. First, a method for manufacturing this integrated device will be briefly described using FIG. n+InGaAsP (
(bandgap wavelength 1.2 pm) buffer layer 42 (band gap wavelength 1.2 pm)
511 m, 1-InGaAsP (band gap wavelength 1.55 pm) active layer 43 by MOVPE of 0.311 m
i-In on the grating after being grown by the method
Only the GaAsP active layer 43 is left and the other parts are
-5i0 until InGaAsP buffer layer 42 is exposed
A 1-InGaAsP (bandgap wavelength: 1.475 pm) optical waveguide layer 44 is selectively grown on the etched portion to a thickness of 0.3 pm. As a result, the 1-InGaAsP active layer 43 and i
-InGaAsP optical waveguides 44 are optically connected in series. After removing the 5i02 mask, a p+-InP cladding layer 45 is grown to a thickness of 1.2 pm. For the subsequent manufacturing process, the same as shown in FIGS. 2(a) to 2(d) of the first embodiment of the optical modulator can be applied as is. A high resistance InP substrate 41 and an n+-InGaAsP buffer layer 42 on the other side.
A shape is made in which these are exposed, and they are buried with a high-resistance InP buried layer 46. Next, in order to electrically isolate the LD section and the modulator section, a groove 52 having a depth of 1 pm and a length of 10 pm is formed. Finally, the p-side electrode 47 and n-side electrode 48 of the LD section
, the p-side electrode 49 and the n-side electrode 50 of the modulator section are formed independently. The substrate was polished to a thickness of approximately 100 yen 1 m, and the element length was cleaved to 400 pm, of which the LD portion was 300 pm and the modulator portion was 1100 pm. Furthermore, the area of the p-side electrode 49 of the modulator is 100 pmX in the stripe portion.
100p, the wiring part is 1011m x 101l, and the pad part is 10100p x 1100p.

Next, the operation of the integrated optical modulator of this embodiment will be explained. When a current is passed in the forward direction between the electrodes 47 and 48, the LD oscillates, and an optical output 51 is obtained through the optical waveguide 44 which is optically cascaded with the active layer 43. The oscillation threshold of the LD is 50 mA, the wavelength is 1.5511 m, and the electrode 1
The optical output 51 at 00 mA is 5 mW. When a reverse bias is applied between the electrodes 49 and 50 of the modulator section, the light propagating through the optical waveguide layer 44 is absorbed and modulated by the Franz Keldysch effect. The operation of the modulator is the same as that of the embodiment shown in Fig. 1, and will not be repeated here since it has already been explained. Integrated optical modulators can be used as light sources capable of ultra-high-speed modulation.

In addition, in Fig. 5, InP-based material is used, and DFB
Although an example in which an LD and the first embodiment of the modulator according to the present invention shown in FIG. 1 are integrated is shown, the material, structure, and manufacturing method of the integrated optical modulator are not limited to this embodiment. Needless to say. For example, in order to increase the output, the output end face may be coated with a non-reflective film and the reflective surface may be a highly reflective surface.

Further, as an example of the invention of claim 3, in the structure of the optical modulator shown in FIGS. 1 and 3, the composition of the i-InGaAsP guide layer is a composition having a band gap wavelength longer than the wavelength of the light source. For example, by setting the thickness to 1.7 μm and detecting photocurrent due to light absorbed by the guide layer from the p-side electrode and the n-side electrode, it can be used as a waveguide type photodetector. The structure and manufacturing method are the same as those described in detail as an example of the optical modulator using FIGS. 1 to 4. In this case, since the element capacitance can be reduced to about 90 fF, an ultra-wideband photodetector can be obtained.

Next, the manufacturing method according to claims 4 to 8 will be described.

In the above example, mesa etching was performed twice to form a mesa stripe including a PIN structure. Claim 4
In methods 8 to 8, desired mesa stripes can be obtained by one mesa etching.

A method of manufacturing a third embodiment of the optical modulator of the present invention according to claims 4 and 5 will be described using FIGS. 6 and 7.

FIG. 6 is a perspective view of the structure of an optical modulator obtained by this manufacturing method, and FIG. 7 (aXbXcXd) is a high-resistance optical modulator with a partially striped buffer layer of the first conductivity type and a flat surface. FIG. 3 is a cross-sectional view showing a process of manufacturing a semiconductor substrate.

The optical modulator shown in FIG.
On a high-resistance InP substrate 61 including an aAsP buffer layer 64, an n-type InP lower cladding layer 65, an undoped InGaAsP optical waveguide 66 with a wavelength composition of 1.4 μm, and a p-type InP layer are formed at the end of the striped buffer layer. A mesa stripe 69 is formed from the multilayer structure of a cladding layer 67 and a p''-InGaAs cap layer 68.The thickness of each layer is 3 pm for the buffer layer 64 and 0.5 pm for the lower cladding layer 65.
5μm, optical waveguide 66 is 0.3pm, upper cladding layer 67
is 1.5 pm, and the cap layer 68 is 0.5 pm. The width of the mesa stripe is 1.5 pm. Both sides of the mesa stripe 69 are buried with a high-resistance InP layer 70 doped with Fe, except for a part above the buffer layer 64, and a p-side electrode 7I layer is placed on the high-resistance layer 70, and the exposed buffer An n-side electrode 72 is provided on the layer 64. A non-reflective coating is formed on the front and rear end surfaces, which correspond to the light exit surfaces, to suppress reflection of light. The element length is 300 μm.

Next, a method for manufacturing this device will be explained using FIG. 7. 7th
In Figure (a), a striped groove 62 with a depth of 3 pm is formed in a high-resistance InP substrate 61 by chemical etching using the 5i02 film 63 as an etching mask, and then
Using the i02 film 63 as a mask, the n-InGaAsP buffer layer 64 is selectively formed into the groove 62 using hydride VPE.
Form it until it becomes flat by the method. In Figure 7(b), 5
After removing the i02 film 63, an n-InP lower cladding layer 65, an undoped (i-layer) InGaAsP optical waveguide layer 66, a p-InP upper cladding layer 67, and a p-InP upper cladding layer 67 are formed on the entire surface.
An InGaAs cap layer 68 is successively crystal-grown by MO-VPE. In FIG. 7(C), after forming a mesa stripe 69 at the end position of the stripe of the buffer layer 64 by photolithography and etching, the MO-V
Both sides thereof are filled with Fe-doped InP high-resistance block layers 70 using the PE method. When forming the mesa stripe 69, the width of the mesa 69 is 1.511 m, and the mesa 69
The etching depth and width are assumed to be such that the high-resistance substrate 61 is exposed at one hem and the buffer layer 64 is exposed at the other hem. In FIG. 7(d), the buffer layer 64
Only a portion of the high-resistance layer 70 on the cap layer 68 and the high-resistance block layer 70 is removed using, for example, HCI, which is an etchant that selectively etches only InP. The electrode 7I layer consisting of
Furthermore, an electrode 72 made of AuGeNi is formed on the exposed buffer layer 64 by sputtering and thermal evaporation, respectively. Then, a non-reflection coating film made of SiNx is formed by sputtering on both end faces of each element separated by cleavage.

The semiconductor optical modulator shown in FIG. 7 thus obtained has an element capacitance of 0.25 pF and a wavelength of 1.5511 pF.
It was possible to obtain a modulation band of 26 GHz when inputting light of 26 GHz. These performances are about twice as improved as those of conventional devices.

In this embodiment, the composition of the buffer layer 64 is InGaAs.
Although P is used, the buffer layer 64 may be made of n-InP. In this manufacturing method, one mesa etching as shown in FIG. 7(C) can expose the high resistance substrate on one side of the mesa stripe and the buffer layer 64 on the other side. Mesa etching only needs to be done once, and photolithography during mesa etching can be easily performed, resulting in yields and uniformity that are twice as high as conventional methods.

Claims 6, 7, and 8 each relate to a method of manufacturing a high-resistance substrate having a partially striped buffer layer of the first conductivity type and having a flat surface, similar to claim 5. FIG. 8 is a diagram illustrating the method of claim 6. In FIG. 8(a), an n-InGaAsP buffer layer 6 is formed by MO-VPE on the entire surface of a high-resistance substrate 61 in which a groove 73 with a depth of 3 pm is formed.
4 to a thickness of about 3 pm. When a photoresist 74 (eg, Hoechst's AZ series) is spin-coated thereon, the photoresist 74 covers the semiconductor substrate substantially evenly. In FIG. 8(b), the photoresist 74 and the semiconductor layer are etched at a constant speed by active ion beam etching (RIBE). Such isokinetic etching uses Ar as a reactive gas, 02.
This can be achieved by using a mixed gas of MCI and adjusting the mixing ratio appropriately. The etching process ends when the high resistance substrate 61 is exposed in areas other than the grooves 73, and the remaining resist 74, if any, is removed. The substrate thus obtained has a good substrate surface flatness, so the controllability and yield of subsequent steps are good, and the in-plane uniformity is even better.

The manufacturing method according to the seventh aspect of the invention is a method of ion-implanting or diffusing impurities that convert a portion of a high-resistance substrate into n-type. In the case of ion implantation, an n-type semiconductor can be obtained by using Si+ as an impurity ion. In the case of diffusion, Si is also used as an impurity. FIG. 9 is a diagram illustrating a method of Si diffusion. In FIG. 9(a), amorphous 5i75 is partially formed on a high resistance substrate 61,
The entire surface is covered with a SiN film 76. Approximately 800 in Figure 9(b)
After heat treatment at a high temperature close to °C to diffuse Si, S
The iN film 76 and the amorphous 5i 75 are removed. Since the semiconductor substrate surface obtained by this method is completely flat, mesa stripes can be formed more easily than in the methods of claims 5 and 6. It also has the advantage that it is easy to manufacture the diffraction grating required when integrating with a DFB laser or the like. Although an example of converting a high-resistance semiconductor to an n-type has been described here, the buffer layer 4 may be of a p-type, and may be made of Si.
Instead, a p-type impurity such as Zn or Cd may be diffused. However, in this case, it is necessary to reverse the conductivity of all other semiconductor layers.

FIG. 10 shows a diagram illustrating the manufacturing method according to claim 8. In FIG. 10(a), the entire upper surface of the high resistance substrate 61 is covered with n-In.
A P buffer layer 64 is formed to a thickness of about 3 pm. 10th
In FIG. 6B, ions that increase the resistance of the semiconductor are implanted deeper than the thickness of the buffer layer 64 in a portion thereof. B+ or B+ may be used as the ion. As a result, an n-type buffer layer 64 can be partially formed on the surface of the high-resistance substrate 61. This method has the advantage that the thickness of the buffer layer 4 can be made thicker than the method of claim 7. This is B+ or B+
This is because such ions can be implanted more deeply than Si+. This increases the degree of freedom in designing the buffer layer.

The optical modulator integrated type modulator photodetector produced by the manufacturing method according to claims 4 to 8 of the present invention can obtain characteristics equivalent to those shown in the embodiments according to claims 1 to 3. Furthermore, the yield uniformity is improved by twice that of the conventional method.

In the embodiments described above, the same effect can be obtained even if the n-type and p-type are replaced. Further, the optical waveguide layer may have a multilayer quantum well structure. In addition, the material is not limited to InGaAsP active layer P, but also InGaAs/InAlAs and AlGaAs/GaA.
Materials used in normal semiconductor lasers and semiconductor heterojunction detectors, such as s-based and AIGaInP/GaInP/GaAs-based materials, can be used. Furthermore, although Fe-doped InP is used as the high-resistance layer, a high-resistance semiconductor layer doped with a dopant such as Co or Ti may be used, or a high-resistance dielectric such as polyimide may be used. Polyimide has a different coefficient of thermal expansion than semiconductors, so it is possible that stress will be applied to the semiconductor layer, but if the stress is reduced by reducing the width of the groove and the volume of polyimide, etc., reliability will not be affected. . High-resistance semiconductors are more preferable for devices that generate heat, such as semiconductor lasers, but are effective for optical modulators and photodetectors because they can be easily formed.

(Effects of the Invention) As described above in detail, according to the present invention, an optical modulator, an integrated optical modulator, and a photodetector capable of ultra-high-speed modulation can be obtained. Further, according to the manufacturing method of the present invention, the yield is twice as high as that of the conventional method.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a first embodiment of the optical modulator of the present invention, FIGS. 2(a) to (e) are diagrams showing the manufacturing process thereof, and FIG. 4(a) to 4(d) are diagrams showing the manufacturing process thereof, and FIG. 5 is a diagram showing a second embodiment of the modulator, and FIG. 5 is a diagram showing the manufacturing process of the second embodiment of the modulator. FIG. 2 is a diagram illustrating an example of an integrated optical modulator. FIG. 6 is a diagram showing a third embodiment of the optical modulator of the present invention. FIGS. 7(a) to 7(d) are diagrams showing the manufacturing process. FIG. 8, FIG. 9, and FIG. 10 are diagrams each showing a process of manufacturing a high-resistance substrate having a buffer layer of the first conductivity type partially. In each figure, 1, 21.41.61 is a high resistance substrate, 2.22.64 is a buffer layer 3, 23.44.66 is an optical waveguide layer, 4.2
4.45 is the cladding layer, 5, 25.46.70 is the high resistance layer, 6.265.47.49.71 is the p-side electrode, 7.2
7, 48, 50, 72 are n-side electrodes, 8, 29 are incident lights,
9°30 is the output light, 10.31.32.33.63 is 5
i02 film, 11.74 is photoresist, 28 is 5i0
2 passivation film, 42 is n-InGaAsP
cladding layer, 43 i-InGaAsP active layer, 51
is optical output, 52, 62, 73 is groove, 65 is lower cladding layer, 67 is upper cladding layer, 68 is cap layer, 69 is mesa stripe, 75 is amorphous silicon, 76 is Si
N film.

Claims (8)

[Claims] (1) Applying an electric field to a striped mesa having a PIN structure with an I layer as a semiconductor optical waveguide layer on a high resistance semiconductor substrate, high resistance layers on both sides of the striped mesa, and the semiconductor optical waveguide layer. An optical modulator comprising means for: (2) In the optical modulator according to claim 1, a semiconductor laser is formed on the same high-resistance semiconductor substrate, and the oscillation light of the semiconductor laser and the optical waveguide layer of the optical modulator are in a positional relationship such that they are optically coupled. An integrated optical modulator characterized by: (3) PI with I layer as light absorption layer on high resistance semiconductor substrate
A photodetector comprising a striped mesa having an N structure, high resistance layers on both sides of the striped mesa, and means for detecting photocurrent from the light absorption layer. (4) At least a first conductivity type semiconductor layer, an i-type semiconductor layer, and a second conductivity type semiconductor layer on a high-resistance semiconductor substrate having a partially striped first conductivity type buffer layer and having a flat surface. forming a striped mask on the semiconductor multilayer along one boundary between the buffer layer and the high-resistance substrate; A step of etching to expose the high resistance substrate on one side and a buffer layer on the other side to form a mesa stripe, and a step of forming a high resistance layer on both sides of the mesa stripe to form a substantially flat surface. , comprising the steps of forming a first electrode on the buffer layer exposed by removing a part of the high resistance layer, and forming a second electrode on the mesa stripe. A method for manufacturing an optical modulator, or an integrated optical modulator or photodetector. (5) The method of manufacturing a high-resistance semiconductor substrate having a partially striped buffer layer of the first conductivity type and having a flat surface according to claim 4 forms a striped dielectric film on a high-resistance semiconductor substrate. A step of forming a groove by etching using the dielectric film as a mask, and forming a first conductivity type buffer layer by crystal growth in the groove using the dielectric film as a mask until the surface becomes flat. 5. The manufacturing method according to claim 4, comprising the steps of: and removing the dielectric film by etching. (6) The method of manufacturing a high-resistance semiconductor substrate having a partially striped buffer layer of the first conductivity type and having a flat surface according to claim 4 forms a striped dielectric film on a high-resistance semiconductor substrate. a step of forming a groove by etching using the dielectric film as a mask; a step of removing the dielectric film; a step of forming a first conductivity type buffer layer with a substantially uniform thickness over the entire surface; A process of applying a photoresist on the buffer layer so as to fill the grooves on the substrate surface almost flatly, and removing the groove portions using a vapor phase etching method that exhibits a uniform etching rate for the photoresist and the semiconductor. 5. The manufacturing method according to claim 4, further comprising the steps of: removing a portion of the photoresist and buffer layer until the high-resistance substrate is exposed; and removing the photoresist remaining in the groove. (7) The step of producing a high-resistance substrate having a partially striped buffer layer of the first conductivity type and having a flat surface according to claim 4 includes forming the buffer layer of the first conductivity type only in the striped portion of the high-resistance substrate. 5. The manufacturing method according to claim 4, further comprising the step of introducing an impurity by ion implantation or diffusion. (8) The step of producing a high-resistance substrate having a partially striped buffer layer of the first conductivity type and having a flat surface according to claim 4 includes forming the buffer layer of the first conductivity type on the entire surface of the high-resistance substrate. 5. The manufacturing method according to claim 4, further comprising the step of implanting ions to partially increase the resistance of the first conductivity type buffer layer.