WO2014156480A1

WO2014156480A1 - Optical modulator

Info

Publication number: WO2014156480A1
Application number: PCT/JP2014/055206
Authority: WO
Inventors: 藤方　潤一; 重樹高橋
Original assignee: 日本電気株式会社
Priority date: 2013-03-29
Filing date: 2014-03-03
Publication date: 2014-10-02
Also published as: JPWO2014156480A1

Abstract

This optical modulator (1) is provided with: a first semiconductor layer (4) having a first conductivity type; a dielectric layer (5) which is formed upon the first semiconductor layer (4), and which extends along a top of the first semiconductor layer (4) in a first direction; and a second semiconductor layer (6) which is formed upon the dielectric layer (5), and which has a second conductivity type different to the first conductivity type. The dielectric layer (5) is provided with: a first region (5a) which, when viewed from the first direction, is positioned at a centre of the dielectric layer (5) in a width direction, and which has a relatively high electric capacitance in a thickness direction of the dielectric layer (5); and second regions (5b) which are positioned at ends of the dielectric layer (5) in the width direction, and which have a relatively low electric capacitance in a thickness direction.

Description

Light modulator

The present invention relates to an optical modulator using a carrier plasma effect.

2. Description of the Related Art Silicon-based optical communication devices that operate in optical fiber communication wavelength bands of 1310 nm and 1550 nm are known as optical communication devices used in an optical communication system such as LAN (Local Area Network). Such an optical communication device is a very promising device in that an optical functional element and an electronic circuit can be integrated on a silicon substrate by using CMOS (Complementary Metal Oxide Semiconductor) technology. .

Up to now, passive devices such as waveguides, optical couplers, and wavelength filters have been very widely studied as silicon-based optical communication devices. On the other hand, in recent years, active devices such as silicon-based optical modulators and optical switches have received much attention as important elements of means for manipulating optical signals for optical communication systems.

Some silicon-based optical modulators change the refractive index of silicon using the thermo-optic effect. However, the optical modulator using the thermo-optic effect cannot be used for an apparatus that has a low operation speed and requires a modulation frequency exceeding 1 Mb / sec. Therefore, in order to realize a high modulation frequency required in more optical communication systems, an optical modulator using the electro-optic effect is required.

Pure silicon shows no change in refractive index due to the linear electro-optic effect (Pockels effect), and the change in refractive index due to the Franz-Keldysh effect or the Kerr effect is very small. For this reason, many of the currently proposed optical modulators utilizing the electro-optic effect utilize the carrier plasma effect (see, for example, Non-Patent Document 1 and Patent Document 1). That is, by changing the free carrier density in the silicon layer by free carrier injection, accumulation, removal, or inversion, the real part and imaginary part of the refractive index are changed, and the phase and intensity of light are changed. is there.

FIG. 1 and FIG. 2 are cross-sectional views schematically showing an optical modulator using a carrier plasma effect described in Non-Patent Document 1 and Patent Document 1, respectively.

The optical modulator 101 shown in FIG. 1 is made of intrinsic semiconductor silicon, and has a silicon layer 104 that constitutes an SOI (Silicon on Insulator) substrate together with a support substrate 102 and a buried oxide layer 103. The silicon layer 104 has a rib-shaped portion. A high concentration p-type region 105 and a high concentration n-type region 106 are formed on both sides of the rib-shaped portion. The high-concentration p-type region 105 and the high-concentration n-type region 106 are regions in which the silicon layer (intrinsic semiconductor silicon) 104 is doped with p-type impurities and n-type impurities at high concentrations, respectively. The upper surface of the silicon layer 104 is covered with an oxide cladding 107.

When a forward and reverse bias is applied to such a PIN (p-intrinsic-n) diode structure, the free carrier density in the silicon layer 104 made of intrinsic semiconductor silicon changes. As a result, the refractive index of the silicon layer 104 changes, and light transmitted through the silicon layer 104 as a waveguide is phase-modulated.

On the other hand, the optical modulator 201 shown in FIG. 2 is partially formed on the p-type silicon layer 204 via the p-type silicon layer 204 that constitutes the SOI substrate together with the support substrate 202 and the buried oxide layer 203 and the dielectric layer 205. And an n-type silicon layer 206 stacked so as to overlap each other. In the p-type silicon layer 204, a high-concentration p-type region 204a doped with a p-type impurity at a high concentration is formed. In the n-type silicon layer 206, a high-concentration n-type region 206a doped with an n-type impurity at a high concentration is formed. In the p-type silicon layer 204 and the n-type silicon layer 206, the impurity concentration is defined so that the change in carrier density is controlled by the external signal voltage. The p-type silicon layer 204, the dielectric layer 205, and the n-type silicon layer 206 are covered with an oxide cladding 207.

In such a SIS (Semiconductor-Insulator-Semiconductor) capacitor structure, free carriers are accumulated, removed, or inverted in the p-type silicon layer 204 and the n-type silicon layer 206 on both sides of the dielectric layer 205, so that the light Phase modulation is performed.

Special table 2006-515082 gazette

However, the above-described optical modulator using the carrier plasma effect has a problem that it is impossible to achieve both high-speed operation, downsizing, and low power consumption as described below.

The light modulation operation speed of the light modulator shown in FIG. 1 is limited by the free carrier lifetime in the rib-shaped portion of the silicon layer 104 and carrier diffusion when the forward bias is removed. Such an optical modulator having a PIN diode structure usually has an operation speed in the range of 10 to 50 Mb / sec during forward bias operation. In contrast, the switching speed can be increased by introducing impurities into the silicon layer 104 in order to shorten the carrier lifetime. However, the introduced impurities reduce the light modulation efficiency, which hinders downsizing and low power consumption.

Also, the biggest factor affecting the operation speed is due to the RC time constant. In the optical modulator shown in FIG. 1, the electric capacity (C) when a forward bias is applied becomes very large due to a decrease in the carrier depletion layer at the PN junction. Theoretically, high speed operation of the PN junction can be achieved by applying a reverse bias. However, for that purpose, it is necessary to make the driving voltage relatively large or to make the device size relatively large.

On the other hand, in the optical modulator shown in FIG. 2, ideally, it is desirable that the optical signal electric field and the region in which the carrier density is dynamically controlled are matched, but in practice, the carrier density changes dynamically. The thickness of the area to be reduced is as thin as about several tens of nm. Therefore, in order to obtain a predetermined phase shift, the length along the light propagation direction of the optical modulator is required to be 1 mm or more, which increases the device size.

In the optical modulator shown in FIG. 2, when the thickness of the dielectric layer 206 is reduced, the optical modulation efficiency can be improved, and the device can be downsized and the power consumption can be reduced. However, reducing the thickness of the dielectric layer 206 simultaneously increases the electric capacity and increases the RC time constant, leading to a decrease in operating speed. That is, there is a trade-off relationship between the operation speed and the light modulation efficiency.

Therefore, an object of the present invention is to provide an optical modulator that achieves both high-speed operation, miniaturization, and low power consumption.

In order to achieve the above-described object, an optical modulator according to the present invention is formed on a first semiconductor layer having a first conductivity type, the first semiconductor layer, and the first semiconductor layer is formed on the first semiconductor layer. And a second semiconductor layer formed on the dielectric layer and having a second conductivity type different from the first conductivity type, the dielectric layer comprising: When viewed from the direction of the dielectric layer, located in the center of the dielectric layer in the width direction, the first region having a relatively large capacitance in the thickness direction of the dielectric layer, and the end in the width direction, And a second region having a relatively small electric capacity in the thickness direction.

As described above, according to the present invention, it is possible to provide an optical modulator that achieves both high-speed operation, miniaturization, and low power consumption.

It is sectional drawing which shows the related optical modulator schematically. It is sectional drawing which shows the related optical modulator schematically. It is sectional drawing which shows schematically the optical modulator in one Embodiment of this invention. It is a schematic sectional drawing which expands and shows a part of optical modulator shown to FIG. 3A. It is a graph which shows the relationship between the electric capacity of the dielectric material layer in this embodiment, and an applied voltage. It is a schematic sectional drawing which shows 1 process of the manufacturing method of the optical modulator of this embodiment. It is a schematic sectional drawing which shows the process of the said manufacturing method following the process of FIG. It is a schematic sectional drawing which shows the process of the said manufacturing method following the process of FIG. It is a schematic sectional drawing which shows the process following the process of FIG. 7 of the said manufacturing method. It is a schematic sectional drawing which shows the process following the process of FIG. 8 of the said manufacturing method. It is a schematic sectional drawing which shows the process of the said manufacturing method following the process of FIG. It is a schematic sectional drawing which shows the process of the said manufacturing method following the process of FIG. It is a schematic sectional drawing which shows the process following the process of FIG. 11 of the said manufacturing method. It is a schematic sectional drawing which shows the process of the said manufacturing method following the process of FIG. It is a schematic sectional drawing which shows the process of the said manufacturing method following the process of FIG. It is a schematic sectional drawing which shows the process following the process of FIG. 14 of the said manufacturing method. It is a schematic sectional drawing which shows the process following the process of FIG. 15 of the said manufacturing method. It is a schematic sectional drawing which shows the process following the process of FIG. 16 of the said manufacturing method. It is a schematic sectional drawing which shows the modification of the optical modulator of this embodiment. It is a schematic sectional drawing which shows the modification of the optical modulator of this embodiment. It is a schematic sectional drawing which shows the modification of the optical modulator of this embodiment. It is a schematic sectional drawing which shows the modification of the optical modulator of this embodiment. It is a schematic sectional drawing which shows the modification of the optical modulator of this embodiment. It is a schematic sectional drawing which shows the modification of the optical modulator of this embodiment. It is a schematic sectional drawing which shows the modification of the optical modulator of this embodiment. It is a top view which shows roughly the Mach-Zehnder type | mold optical modulator using the optical modulator of this embodiment. 1 is a schematic plan view showing a configuration in which Mach-Zehnder optical modulators of this embodiment are arranged in parallel. 1 is a schematic plan view showing a configuration in which Mach-Zehnder optical modulators of this embodiment are arranged in series.

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

First, before describing the configuration of an optical modulator according to an embodiment of the present invention, a mechanism for modulating the carrier density in the silicon layer, which is the basis of the operation of the optical modulator of the present embodiment, will be described. The optical modulator of the present embodiment, including the modifications described later, uses the carrier plasma effect described below.

As described above, pure silicon shows no change in refractive index due to the Pockels effect, and the change in refractive index due to the Franz-Keldysh effect or the Kerr effect is very small. Therefore, in pure silicon, only the carrier plasma effect and the thermo-optic effect can be used for the light modulation operation. However, an optical modulator that changes the refractive index using the thermo-optic effect has a low operating speed. Therefore, for desired high-speed operation (1 Gb / second or more), only carrier diffusion by the carrier plasma effect is effective. Carrier diffusion due to the carrier plasma effect is described by the following relational expression in the first-order approximation.

In the above formulas (1) and (2), Δn and Δk represent the real part and the imaginary part of the refractive index change of the silicon layer, respectively. Moreover, e is elementary charge, lambda is optical wavelength, epsilon ₀ is the dielectric constant in vacuum, n is the refractive index of the intrinsic semiconductor silicon, m _e is the effective mass of the electron carrier, m _h is the effective mass of the hole carriers, mu _e Is the electron carrier mobility, μ _h is the hole carrier mobility, ΔN _e is the electron carrier concentration change, and ΔN _h is the hole carrier concentration change.

An experimental evaluation of the carrier plasma effect in the silicon layer has been conducted. According to this, it is understood that the refractive index change with respect to the carrier density in the 1310 nm and 1550 nm optical fiber communication wavelength bands used in the optical communication system agrees well with the results obtained from the above formulas (1) and (2). ing. In the optical modulator using the carrier plasma effect, the amount of phase change is defined by the following equation.

In the above formula (3), L is the length along the light propagation direction of the optical modulator.

The amount of phase change due to the carrier plasma effect is larger than the amount of phase change due to the electroabsorption effect, and the optical modulator described below can basically exhibit characteristics as a phase modulator.

Next, the configuration of the optical modulator of the present embodiment will be described with reference to FIGS. 3A and 3B.

FIG. 3A is a cross-sectional view schematically showing the optical modulator of the present embodiment, showing a cross section perpendicular to the light propagation direction. FIG. 3B is a schematic cross-sectional view showing an enlarged part of the optical modulator shown in FIG. 3A.

The optical modulator 1 includes a support substrate 2, a buried oxide layer 3 formed on the support substrate 2, and a first semiconductor layer 4 formed on the buried oxide layer 3 and having the first conductivity type. Have. In the present embodiment, the first semiconductor layer 4 is formed of silicon doped with p-type impurities, that is, p-type silicon. Hereinafter, the first semiconductor layer 4 is referred to as a “p-type silicon layer”. The support substrate 2, the buried oxide layer 3, and the p-type silicon layer 4 constitute an SOI substrate.

The optical modulator 1 is formed on the p-type silicon layer 4 and extends on the light propagation direction (perpendicular to the plane of the drawing). The optical modulator 1 is formed on the dielectric layer 5 and has the first conductivity type. And a second semiconductor layer 6 having a different second conductivity type. In the present embodiment, the second semiconductor layer 6 is formed of silicon doped with n-type impurities, that is, n-type silicon. Hereinafter, the second semiconductor layer 6 is referred to as an “n-type silicon layer”. The p-type silicon layer 4, the dielectric layer 5, and the n-type silicon layer 6 form a SIS (Semiconductor-Insulator-Semiconductor) capacitor structure. Therefore, the optical modulator 1 of the present embodiment is an optical modulator having a SIS capacitor structure formed on an SOI substrate. Over the n-type silicon layer 6, an electrode lead portion 7 having the same conductivity type as that of the n-type silicon layer 6 is formed. That is, the electrode lead portion 7 is made of n-type silicon.

Each of the p-type silicon layer 4 and the n-type silicon layer 6 is composed of at least one layer of polycrystalline silicon, amorphous silicon, strained silicon, single crystal silicon, and Si _1-x Ge _x . The dielectric layer 5 is one of silicon oxide, silicon nitride, hafnium oxide, zirconium oxide, and rare earth oxide, or an alloy made of at least two of these.

On both sides of the p-type silicon layer 4 with the dielectric layer 5 interposed, high-concentration p-type regions 4a doped with p-type impurities at a high concentration are formed. An electrode contact layer 4b is formed on the high concentration p-type region 4a. An electrode wiring 4c is connected to the electrode contact layer 4b. High-concentration n-type regions 7a doped with n-type impurities at high concentrations are formed on both sides of the electrode lead-out portion 7 with the dielectric layer 5 interposed therebetween. An electrode contact layer 7b is formed on the high concentration n-type region 7a. An electrode wiring 7c is connected to the electrode contact layer 7b. The entire waveguide is covered with an oxide cladding 8.

As described above, when the thickness of the dielectric layer 5 is reduced, the light modulation efficiency is improved, and the device is reduced in size and power consumption is reduced. However, it also leads to an increase in electrical capacity and hinders high speed operation.

Therefore, in the optical modulator 1 of the present embodiment, as shown in FIG. 3B, the dielectric layer 5 is a central region located at the center in the width direction when viewed from the light propagation direction (first direction). (First region) 5a and an end region (second region) 5b located at the end. The end region 5b is thicker than the central region 5a. That is, the electric capacity in the thickness direction of the dielectric layer 5 is relatively large in the central region 5a and relatively small in the end region 5b.

In the optical field propagating through the junction interface of the SIS capacitor structure described above, the optical electric field strength is large in the central region 5a and small in the end region 5b. Therefore, by making the dielectric constant of the central region 5a of the dielectric layer 5 larger than that of the end region 5b, it is possible to reduce the electric capacity while maintaining the light modulation efficiency. That is, high-speed operation can be realized while maintaining light modulation efficiency. Hereinafter, this effect will be described with reference to FIG. 4 and Table 1.

FIG. 4 is a graph showing the relationship between the electric capacity of the dielectric layer 5 of the present embodiment and the applied voltage. The graph of FIG. 4 shows the applied voltage dependency of the capacitance in the dielectric layer 5 when the thickness of the end region 5b is 5 nm, 10 nm, and 15 nm, respectively. Here, the thickness of the central region 5a of the dielectric layer 5 is 5 nm.

As can be seen from FIG. 4, when the thickness of the end region 5b is made larger than the thickness of the central region 5a, the region where carrier accumulation occurs, that is, the applied voltage is smaller than the case where the thickness of the dielectric layer 5 is uniform. In the negative region, the electric capacity can be reduced by about 20 to 40%. Further, Table 1 shows the modulation efficiency V _π L at this time.

A small modulation efficiency V _π L means that the drive voltage and the waveguide length necessary for a predetermined phase shift are small, and therefore means that the light modulation efficiency is good.

The modulation efficiency V _π L when the thickness of the end region 5b is 10 nm is approximately the same as that when the thickness of the end region 5b is 5 nm (the thickness of the dielectric layer 5 is uniform). That is, even if the thickness of the end region 5b is twice that of the central region 5a, the light modulation efficiency can be maintained at the same level.

On the other hand, when the thickness of the end region 5b is 15 nm, the modulation efficiency V _π L is increased by about 27% compared to the case where the thickness of the end region 5b is 5 nm, and the light modulation efficiency decreases. Is seen. However, when the thickness of the end region 5b is 15 nm, an effect of reducing the electric capacity that exceeds the decrease in the light modulation efficiency is obtained (see FIG. 4), and the thickness of the dielectric layer 5 is uniform. Even if compared, it turns out that an advantageous effect is acquired.

In the present embodiment, as described above, the p-type silicon layer 4, the dielectric layer 5, and the n-type silicon layer 6 form a rib-shaped or ridge-shaped waveguide. Further, regions doped with a high concentration (a high concentration p-type region 4a and a high concentration n-type region 7a) are formed at a distance from the rib waveguide. Thereby, the light absorption loss due to the overlap between the light field and the heavily doped region is reduced, and as a result, the light modulation efficiency can be improved.

Note that if the doping density of the heavily doped region is further increased, the series resistance component in the SIS capacitor structure can be further reduced, and the RC time constant can be further reduced. As a result, a further high speed operation can be realized. From this point of view, the

electrode contact layers

4b and 7b are preferably formed of silicide.

In the region near the junction interface of the SIS capacitor structure described above, that is, on both sides of the dielectric layer 5, regions where the carrier density is modulated are generated. The thickness (maximum depletion layer thickness) W of this region is defined by the following equation in the thermal equilibrium state.

In the above equation (4), ε _s is the dielectric constant of the semiconductor layer, k is the Boltzmann constant, N _c is the carrier density, _ni is the intrinsic carrier concentration, and e is the elementary charge. For example, when the carrier density N _c is 10 ¹⁷ / cm ³ , the maximum depletion layer thickness W is about 0.1 μm, and as the carrier density N _c increases, the depletion layer thickness, that is, modulation of the carrier density occurs. The thickness of the region is reduced. In other words, the region where the carrier density is modulated becomes very small with respect to the spread of the optical signal electric field, resulting in a problem that the light modulation efficiency is deteriorated.

Therefore, the optical modulator 1 of the present embodiment is designed so that the overlap between the region where the carrier density is modulated and the optical field is maximized.

When the effective refractive index felt by the optical signal electric field is n _eff and the optical signal wavelength is λ, the optical field size is λ / n _eff . Therefore, by setting the height of the region where the carrier density is modulated to about λ / n _eff , the overlap between the optical field and the region where the carrier density is modulated is maximized, and efficient phase modulation of light is realized. The

Therefore, when the region where the optical signal electric field has the peak intensity is arranged in the region where the free carriers are accumulated, removed, or inverted on both sides of the dielectric layer 5, the highest light modulation efficiency is obtained.

Next, with reference to FIGS. 5 to 18, a method for manufacturing the optical modulator of the present embodiment will be described. 5 to 18 are cross-sectional views schematically showing the optical modulator in each step of the manufacturing method of the present embodiment, and each show a cross section perpendicular to the light propagation direction. In the following, the manufacturing method of the present embodiment will be described by taking the case where the dielectric layer 5 is made of an oxide as an example, but the present invention is not limited to this.

First, as shown in FIG. 5, an SOI substrate composed of a support substrate 2, a buried oxide layer 3, and a p-type silicon layer 4 is prepared. The buried oxide layer 3 has a thickness of 1000 nm or more in order to reduce optical loss. After laminating a silicon layer of about 100 nm to 1000 nm on the buried oxide layer 3, the surface layer is doped with phosphorus (P) or boron (B) by ion implantation. And the p-type silicon layer 4 is formed by heat-processing it. Note that an SOI substrate provided with a silicon layer doped p-type in advance may be used.

Next, as shown in FIG. 6, an oxide film mask 9 is formed on the p-type silicon layer 4.

Further, a silicon nitride (SiN _x ) layer is formed on the oxide film mask 9. Then, the SiN _x layer is patterned so as to have a width equal to the width of the rib waveguide (that is, the dielectric layer 5 and the n-type silicon layer 6), and the hard mask 10 is formed as shown in FIG.

Next, thermal oxidation is performed to increase the thickness of the oxide film mask 9 as shown in FIG. At this time, only the end portion of the oxide film mask 9 covered with the hard mask 10 is thermally oxidized. Therefore, the oxide film mask 9 is formed with a relatively thick region and a thin region.

Next, after removing the hard mask 10 using a solution such as hot phosphoric acid, the surface of the oxide film mask 9 is removed by dilute hydrofluoric acid treatment or the like. Thereafter, a thermal oxidation process is performed again to improve the film quality of the oxide film mask 9, thereby forming the dielectric layer 5 as shown in FIG.

Thereafter, as shown in FIG. 10, an n-type silicon layer 6 is formed on the dielectric layer 5. Specifically, first, a polycrystalline silicon layer is formed on the dielectric layer 5 by a CVD (Chemical Vapor Deposition) method or a sputtering method. Thereafter, the polycrystalline silicon layer is doped with P by ion implantation, whereby the n-type silicon layer 6 is formed.

Next, a SiN _x layer is formed on the n-type silicon layer 6 by a low pressure CVD method or the like. Then, by dry etching or the like, the SiN _x layer is patterned so as to have a width equal to the width of the rib waveguide, and a hard mask 11 is formed as shown in FIG.

Then, using this hard mask 11, as shown in FIG. 12, the n-type silicon layer 6 is patterned so that a portion to be a rib waveguide remains. Although the dielectric layer 5 is partially removed by the etching at this time, this etching stops at the dielectric layer 5 itself. Therefore, in practice, the dielectric layer 5 is present on the p-type silicon layer 4 in addition to the lower portion of the n-type silicon layer 6.

Next, an oxide cladding 8 is formed by plasma CVD or the like as shown in FIG. Then, the surface of the oxide cladding 8 is planarized by CMP (Chemical-Mechanical-Polishing).

Next, after removing the hard mask 11, as shown in FIG. 14, an electrode lead portion 7 of the n-type silicon layer 6 is formed on the oxide cladding 8. Specifically, first, a polycrystalline silicon layer is formed on the oxide cladding 8 by a CVD method or a sputtering method. Thereafter, the polycrystalline silicon layer is doped with P by ion implantation, whereby the n-type electrode lead-out portion 7 is formed. The electrode lead portion 7 can also be formed by performing a doping process during the formation of the polycrystalline silicon layer.

Next, as shown in FIG. 15, high-concentration n-type regions 7a in which n-type impurities are highly doped are formed at both ends of the electrode lead-out portion 7 of the n-type silicon layer 6 by ion implantation.

Next, an oxide clad 8 is further laminated so as to bury the electrode lead portion 7 of the n-type silicon layer 6 by plasma CVD or the like. Then, as shown in FIG. 16, contact holes 12 are formed in the oxide cladding 8 by reactive etching. Thereafter, a high-concentration p-type region 4a doped with a high concentration of p-type impurities is formed in the p-type silicon layer 4 by ion implantation, and the surface of the high-concentration p-type region 4a and the high-concentration n-type region 7a The

electrode contact layers

4b and 7b are formed respectively. The

electrode contact layers

4b and 7b may be silicide layers formed by stacking and annealing a metal such as Ni on the high concentration p-type region 4a and the high concentration n-type region 7a.

Next, a metal layer such as Ti / TiN / Al (Cu) or Ti / TiN / W is formed by CVD or sputtering, and patterned by reactive etching to form electrode wirings 4c and 7c. Then, by connecting the

electrode wirings

4c and 7c to the drive circuit, the optical modulator 1 is completed as shown in FIG.

18 to 24 are cross-sectional views schematically showing modifications of the optical modulator of the present embodiment, showing a cross section perpendicular to the light propagation direction. Each figure is an enlarged view of the vicinity of the dielectric layer, and corresponds to FIG. 3B.

18 and FIG. 19, the thickness of the dielectric layer 5 continuously increases from the central region 5a to the end region 5b. On the other hand, as shown in FIGS. 20 and 21, the thickness of the dielectric layer 5 may increase stepwise from the central region 5a to the end region 5b.

In the above-described modification, by making the thickness of the end region 5b thicker than the thickness of the central region 5a, the electric capacity in the thickness direction of the dielectric layer 5 is relatively large in the central region 5a. The partial area 5b is relatively small. However, the configuration for changing the electric capacity is not limited to this. According to another configuration, the electric capacity in the thickness direction of the dielectric layer 5 can be relatively large in the central region 5a and relatively small in the end region 5b.

In the modification shown in FIG. 22, the central region 5a and the end region 5b have different laminated structures, and in the modification shown in FIG. 23, the central region 5a and the end region 5b are formed with different compositions. ing. In the modification shown in FIG. 24, regions having relatively small electric capacity compared to the surroundings are discretely provided along the light propagation direction in the center in the width direction of the dielectric layer 5. With such a configuration, the electric capacity in the thickness direction of the dielectric layer 5 is relatively large in the central region 5a and relatively small in the end region 5b.

Finally, the configuration of a Mach-Zehnder optical modulator using the optical modulator of this embodiment will be described.

FIG. 25 is a plan view schematically showing a Mach-Zehnder type optical modulator using the optical modulator of the present embodiment.

The Mach-Zehnder type optical modulator 20 has first and

second arms

21 and 22 arranged in parallel to each other. The above-described optical modulator of the present embodiment is incorporated in the Mach-Zehnder optical modulator 20 as the first and

second arms

21 and 22. Connected to the input sides of the first and

second arms

21 and 22 is an optical branching structure 23 that branches an optical signal input from the outside into a first arm 21 and a second arm 22. An optical coupling structure 24 that couples optical signals output from the first arm 21 and the second arm 22 is connected to the output sides of the first and

second arms

21 and 22. The Mach-Zehnder optical modulator 20 has an electrode pad 25 for applying a voltage to each of the first and

second arms

21 and 22.

The optical signal input from the outside to the Mach-Zehnder optical modulator 20 is branched by the optical branching structure 23 and input to the first arm 21 and the second arm 22. The input optical signal is phase-modulated by the first arm 21 and the second arm 22. The phase-modulated optical signal is converted into a light intensity modulated signal by phase interference by the optical coupling structure 24.

In the Mach-Zehnder optical modulator 20 of the present embodiment, an optical signal input from the outside is branched by the optical branching structure 23 into the first arm 21 and the second arm 22 with equal power. Here, by applying a positive voltage to the first arm 21, carrier accumulation occurs on both sides of the dielectric layer of the first arm 21 (that is, the optical modulator of the present embodiment). Further, by applying a negative voltage to the second arm 22, carriers on both sides of the dielectric layer of the second arm 22 (that is, the optical modulator of the present embodiment) are removed. As a result, the refractive index felt by the optical signal electric field is small in the first arm 21 that is in the carrier accumulation mode, and the refractive index felt by the optical signal electric field is in the second arm 22 that is in the carrier removal (depletion) mode. growing. For this reason, the optical signal phase difference between the first arm 21 and the second arm 22 is maximized. In this way, the optical signals transmitted through both arms are multiplexed by the optical coupling structure 24 on the output side, whereby optical intensity modulation occurs. According to this embodiment, an optical signal of 40 Gbps or more can be transmitted.

As shown in FIGS. 26 and 27, the Mach-Zehnder type optical modulator 20 of the present embodiment is arranged in parallel or in series, so that the optical modulator or matrix optical switch having a higher transfer rate can be obtained. Application is also possible.

The present invention has been described above with reference to the embodiments, but the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention. For example, in the above-described embodiment, the case where the first conductivity type is p-type and the second conductivity type is n-type is illustrated, but the opposite may be possible. That is, the first semiconductor layer having the first conductivity type may be an n-type silicon layer, and the second semiconductor layer having the second conductivity type may be a p-type silicon layer.

This application claims priority based on Japanese Patent Application No. 2013-074179 filed on March 29, 2013, the entire disclosure of which is incorporated herein.

1 optical modulator 2 support substrate 3 buried oxide layer 4 p-type silicon layer 4a high-concentration p-type region 4b electrode contact layer 4c electrode wiring 5 dielectric layer 6 n-type silicon layer 7 electrode lead-out portion 7a high-concentration n-type region 7b electrode Contact layer 7c Electrode wiring 8 Oxide cladding 9

Oxide mask

10, 11 Hard mask 12 Contact hole 20 Mach-Zehnder optical modulator 21 First arm 22 Second arm 23 Optical branching structure 24 Optical multiplexing structure 25 Electrode pad

Claims

A first semiconductor layer having a first conductivity type;
A dielectric layer formed on the first semiconductor layer and extending in a first direction on the first semiconductor layer;
A second semiconductor layer formed on the dielectric layer and having a second conductivity type different from the first conductivity type;
When viewed from the first direction, the dielectric layer is located at the center in the width direction of the dielectric layer and has a relatively large electric capacity in the thickness direction of the dielectric layer. And a second region located at an end in the width direction and having a relatively small capacitance in the thickness direction.
The optical modulator according to claim 1, wherein the dielectric layer has a thickness of the second region larger than a thickness of the first region.
3. The optical modulator according to claim 2, wherein a thickness of the dielectric layer continuously increases from the first region to the second region.
The optical modulator according to claim 2, wherein the thickness of the dielectric layer increases stepwise from the first region to the second region.
The optical modulator according to claim 1, wherein the dielectric layer has a stacked structure in which the first region and the second region are different.
2. The optical modulator according to claim 1, wherein the first dielectric layer and the second region are formed with different compositions in the dielectric layer.
2. The light modulation according to claim 1, wherein regions having a relatively large capacitance compared to the surroundings are discretely provided along the first direction in the center in the width direction of the dielectric layer. vessel.
The dielectric layer according to any one of claims 1 to 7, wherein the dielectric layer is any one of silicon oxide, silicon nitride, hafnium oxide, zirconium oxide, and rare earth oxide, or an alloy composed of at least two. The optical modulator according to item 1.