JP2017016020A - Optical modulator, and method for manufacturing optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator that has excellent frequency characteristic and is easily formed.SOLUTION: An optical modulator has: a lower clad layer 12 having a first conductivity-type semiconductor; a non-doped layer including a core layer or a semi-insulating layer 14 including a core layer which is arranged on the lower clad layer; a first upper clad layer 16a which is arranged on the non-doped layer or the semi-insulating layer and has a second conductivity type opposite to the first conductivity type or the first conductivity type; a first electrode 24a connected to the lower clad layer; a second electrode 24ba arranged on the first upper clad layer; and a block layer 18a which is arranged between the non-doped layer or the semi-insulating layer and the first upper clad layer in at least a part ranging from an area near the end of the second electrode in a longitudinal direction to the outside, in which a first side face closer to the second electrode is covered by the first upper clad layer and etching can be performed to a lower layer contacting a lower face, and which has resistibility higher than the first upper clad layer or a conductivity type opposite to the conductivity type of the first upper clad layer.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an optical modulator and a method for manufacturing the optical modulator.

  An optical modulator that has a lower cladding layer, a core layer, and an upper cladding layer and modulates the phase of propagating light by changing the electric field strength of the core layer is known. Such an optical modulator is mounted on a semiconductor optical device such as a Mach-Zehnder optical modulator (for example, Patent Documents 1 and 2 and Non-Patent Document 1).

  A clad layer made of a semi-insulating semiconductor layer may be provided on both longitudinal sides of the upper clad layer of the optical modulator. Such a semi-insulating semiconductor clad layer is used to electrically isolate optical modulators mounted on the same substrate (for example, Non-Patent Document 1). The semi-insulating semiconductor clad layer not only electrically isolates the optical modulators but also improves the high frequency characteristics of the optical modulators (for example, Patent Document 1 and Non-Patent Document 1).

JP 2004-151590 A JP 2011-203382 A JP 2013-236067 A JP 2011-203384 A International Publication No. 2004/081638

S. Akiyam et. Al., “InP-Based Mach-Zehnder Modulator With Capacitively Loaded Traveling-Wave Electrodes”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 5, MARCH 1, 2008, pp. 608-615.

  As described above, when a semi-insulating semiconductor clad layer is provided on both sides in the longitudinal direction of the upper clad layer, the high frequency characteristics of the optical modulator are improved. However, when a semiconductor clad layer with a different material and doping concentration from the upper clad layer is grown on both sides in the longitudinal direction of the upper clad layer, there are various problems such as formation of electrodes on the surface of the grown layer, which makes it difficult to form electrodes. Occurs. Therefore, it is not easy to form an optical modulator with excellent high frequency characteristics.

  Therefore, an object of the present invention is to solve such a problem.

  In order to solve the above problem, according to one aspect of the present apparatus, a non-doped layer or a core layer including a lower clad layer having a first conductivity type semiconductor and a core layer disposed on the lower clad layer. A semi-insulating layer including: a first upper cladding layer disposed on the non-doped layer or the semi-insulating layer and having a second conductivity type opposite to the first conductivity type or the first conductivity type; and the lower cladding A first electrode connected to a layer; a second electrode disposed on the first upper cladding layer; and at least part of the second electrode from the vicinity in the longitudinal direction to the outside thereof, the non-doped layer or the The first side surface on the second electrode side, which is disposed between the semi-insulating layer and the first upper clad layer, is covered with the first upper clad layer and can be selectively etched with respect to the lower layer in contact with the lower surface. A first semiconductor layer, a second semiconductor layer disposed between the first semiconductor layer and the first upper cladding layer, wherein the second side surface on the second electrode side is covered with the first upper cladding layer, or the first semiconductor layer There is provided an optical modulator having a semiconductor layer and a block layer having a higher resistivity than the first upper cladding layer or a conductivity type opposite to the conductivity type of the first upper cladding layer.

  According to the disclosed optical modulator, an optical modulator having excellent frequency characteristics and easy to form is provided.

FIG. 1 is a plan view for explaining the optical modulator according to the first embodiment. FIG. 2 is a cross-sectional view taken along the line II-II in FIG. FIG. 3 is a cross-sectional view taken along line III-III in FIG. FIG. 4 is a diagram for explaining an example of the operation of the semiconductor optical device according to the first embodiment. FIG. 5 is a diagram for explaining the electric field distribution in the low frequency region of the optical modulator. FIG. 6 is a diagram for explaining the electric field distribution in the high frequency region of the optical modulator. FIG. 7 is a diagram illustrating the calculation result of the electric field distribution in the low frequency region of the optical modulator. FIG. 8 is a process cross-sectional view illustrating the manufacturing method of the optical modulator of the first embodiment. FIG. 9 is a process cross-sectional view illustrating the method for manufacturing the optical modulator of the first embodiment. FIG. 10 is a cross-sectional view of an optical modulator having a semi-insulating cladding layer on both sides in the longitudinal direction of the upper cladding layer. FIG. 11 is a diagram for explaining the problems of the semi-insulating clad layer. FIG. 12 is a cross-sectional view in the longitudinal direction of the optical modulator of the third modification. FIG. 13 is a cross-sectional view along the longitudinal direction of the optical modulator according to the second embodiment. FIG. 14 is a process cross-sectional view illustrating the manufacturing method of the optical modulator of the second embodiment. FIG. 15 is a longitudinal sectional view of an optical modulator that does not have a second cladding layer. FIG. 16 is a plan view of a semiconductor optical device on which the optical modulator of the third embodiment is mounted. 17 is a cross-sectional view taken along the line XVII-XVII in FIG. FIG. 18 is a plan view of a semiconductor optical device in which the optical modulator of the fourth embodiment is integrated. FIG. 19 is a cross-sectional view taken along line XIX-XIX in FIG. FIG. 20 is a process cross-sectional view illustrating an example of a method of manufacturing a semiconductor optical device according to the fourth embodiment. FIG. 21 is a process cross-sectional view illustrating an example of a method of manufacturing a semiconductor optical device according to the fourth embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and equivalents thereof.

  Even if the drawings are different, parts having the same structure are denoted by the same reference numerals, and description thereof is omitted.

(Embodiment 1)
(1) Structure FIG. 1 is a plan view for explaining an optical modulator according to the first embodiment. FIG. 1 shows a semiconductor optical device 4 including optical phase modulators 2a and 2b according to the first embodiment. The semiconductor optical device 4 of the first embodiment is a Mach-Zehnder type optical modulator.

  The semiconductor optical device 4 includes an input waveguide 6 and a 1 × 2 MMI coupler 8a (multi-mode-interference coupler) in which the input waveguide 6 is connected to an input end. The semiconductor optical device 4 further includes an optical phase modulator 2a connected to one of the output ends of the 1 × 2 MMI coupler 8a and an optical phase modulator 2b connected to the other of the output ends of the 1 × 2 MMI coupler 8a. The semiconductor optical device 4 further includes an output of a 2 × 1 MMI coupler 8b in which an optical phase modulator 2a is connected to one of the input ends and an optical phase modulator 2b is connected to the other of the input ends, and an output of the 2 × 1 MMI coupler 8b. And an output waveguide 10 connected to the end.

  The structures of the optical modulator 2a and the optical modulator 2b are substantially the same. FIG. 2 is a cross-sectional view taken along the line II-II in FIG. FIG. 3 is a cross-sectional view taken along line III-III in FIG.

  The optical modulators 2a and 2b (see FIG. 2) include a lower cladding layer 12 having a first conductivity type (for example, n-type) semiconductor (for example, n-type InP), and a non-doped layer disposed on the lower cladding layer 12. Layer 14. The lower cladding layer 12 includes, for example, a semiconductor substrate (for example, an n-type InP substrate) and a buffer layer (for example, an n-type InP layer) formed on the semiconductor substrate. The non-doped layer 14 is, for example, a semiconductor layer including a core layer made of non-doped (ie, i-type) AlGaInAs / AlGaInAs multiple quantum wells (hereinafter referred to as AlGaInAsMQW). In addition to the core layer, the non-doped layer 14 may include an i-type AlGaInAs layer and an i-type InP layer. The core layer may be an i-type InGaAsP / InGaAsP multiple quantum well (the same applies to the second to fourth embodiments).

  The optical phase modulators 2a and 2b (see FIG. 2) are further disposed on the non-doped layer 14 and have a first upper cladding layer 16a (for example, p-type) having a second conductivity type (for example, p-type) opposite to the first conductivity type. , P-type InP layer). The optical phase modulators 2a and 2b are further provided on the non-doped layer 14 and have the same conductivity type (for example, p-type) as the first upper cladding layer 16a, and the first upper cladding layer 16a and a first semiconductor layer to be described later A second upper cladding layer 16b whose upper surface is covered with 18a (see FIG. 3) is provided. The second upper cladding layer 16b is, for example, a p-type InP layer. The core layer is a semiconductor layer having a higher refractive index than the first and second upper cladding layers 16 a and 16 b and the lower cladding layer 12.

  The optical phase modulators 2a and 2b further include a non-doped layer 14 (see FIG. 3) and a first upper cladding layer in a portion (hereinafter referred to as “end portion”) from the vicinity of the longitudinal end of the upper electrode, which will be described later, to the outside. A first semiconductor layer 18a (for example, an i-type InGaAsP layer) disposed between 16a. The first semiconductor layer 18a has a first side surface 20a on the center side (that is, a second electrode side described later) of the non-doped layer 14 covered with the first upper cladding layer 16a and is selective to a lower layer in contact with the lower surface. It is a semiconductor layer that can be etched. The lower layer of the first embodiment is the second upper cladding layer 16b. “Selective etching with respect to the lower layer” means that etching can be performed at an etching rate 5 times or more (preferably 10 times or more) faster than the etching rate with respect to the lower layer. An etchant that enables such etching is called a selective etchant.

  The first semiconductor layer 18a of the first embodiment is a semiconductor layer having a higher resistivity than the first and second upper cladding layers 16a and 16b. As will be described later, the first semiconductor layer 18 a is a semiconductor layer (hereinafter referred to as a block layer) that suppresses an electric field applied to an end portion in the longitudinal direction of the non-doped layer 14.

  The optical phase modulators 2a and 2b (see FIGS. 2 and 3) further include contact layers 22a and 22b (for example, a p-type InGaAs layer) disposed on the central portion of the first upper cladding layer 16a and a second electrode 24ba. , 24bb (hereinafter referred to as upper electrode). The upper electrodes 24ba and 24bb are disposed on the central portion of the upper cladding layer 16a via the contact layers 22a and 22b. The optical phase modulators 2 a and 2 b further have a common first electrode 24 a (hereinafter referred to as a lower electrode) connected to the lower cladding layer 12.

  The upper electrodes 24ba and 24bb (see FIGS. 2 and 3) are preferably electrodes in contact with the block layer (in the first embodiment, the first semiconductor layer 18a) in plan view. The upper electrodes 24ba and 24bb may cover the end of the block layer in plan view. Alternatively, the upper electrodes 24ba and 24bb may be separated from the block layer in plan view. The overlap between the upper electrodes 24ba and 24bb and the block layer is preferably within 5 μm. Similarly, the distance between the upper electrodes 24ba and 24bb and the block layer is preferably within 5 μm.

The first upper clad layer 16a (see FIG. 3), the second upper clad layer 16b, and the lower clad layer 12 are preferably 1 × 10 ¹⁷ cm ^{−3 to} 1 × 10 ²⁰ cm ⁻³ in impurity concentration (or carrier concentration; The same applies hereinafter). More preferably, the first upper clad layer 16a, the second upper clad layer 16b, and the lower clad layer 12 are conductive semiconductor layers having an impurity concentration of 5 × 10 ¹⁷ cm ⁻³ or more and 5 × 10 ¹⁹ cm ⁻³ or less. It is. The contact layers 22a and 22b are conductive semiconductor layers having an impurity concentration higher than that of the first upper cladding layer 16a.

The non-doped layer 14 is preferably a semiconductor layer (that is, i-type semiconductor layer) having an impurity concentration of 5 × 10 ¹⁶ cm ⁻³ or less. More preferably, the non-doped layer 14 is a semiconductor layer having an impurity concentration of 2 × 10 ¹⁶ cm ⁻³ or less. The non-doped layer 14 is replaced with a semiconductor layer having a resistivity of 1 × 10 ⁵ Ωcm or more (or 5 × 10 ⁵ Ωcm or more) (that is, a semi-insulating semiconductor layer (hereinafter referred to as a semi-insulating layer)). Also good.

The optical phase modulators 2a and 2b (see FIGS. 2 and 3) have, for example, a waveguide structure having a non-doped layer 14 and a protruding portion (that is, a high mesa) extending in one direction. A resin layer 26 (for example, a benzocyclobutene layer) is preferably provided on the side of the optical phase modulators 2a and 2b. A passivation film 28a (for example, a SiO ₂ film) is preferably provided between the non-doped layer 14 and the resin layer 26. Another surface of the resin layer 26 is preferably provided with another passivation film 28b (for example, SiO ₂ film).

  In the example described with reference to FIGS. 1 to 3, the first semiconductor layer 18 a is disposed on both the region on one end side and the region on the other end side of the non-doped layer 14. However, the first semiconductor layer 18 a may be disposed on a region on one end side of the non-doped layer 14. In this case, the contact layers 22a and 22b and the upper electrodes 24ba and 24bb are arranged above the region on the other end side of the non-doped layer 14 via the first upper cladding layer 16a (the same applies to the second embodiment described later). ).

(2) Operation (2-1) Operation of Semiconductor Optical Device FIG. 4 is a diagram for explaining an example of the operation of the semiconductor optical device 4 according to the first embodiment.

  The light 30 incident on the input waveguide 6 is split into a first branched light 29a and a second branched light 29b by the 1 × 2 MMI coupler 8a. The first branched light 29a is supplied to one optical phase modulator 2a, and the phase is modulated. The second branched light 29b is supplied to the other optical phase modulator 2b. In the example shown in FIG. 4, the phase of the second branched light 29b is not modulated. Thereafter, the first branched light 29a and the second branched light 29b are combined by the 2 × 1 MMI coupler 8b to become output light. The output light 32 propagates through the output waveguide 10 and is output from the semiconductor optical device 4.

  The intensity of the output light changes according to the phase difference between the first branched light 29a and the second branched light 29b. For example, when the phase difference between the first branched light 29a and the second branched light 29b is 0 rad, the intensity of the output light is maximized. On the other hand, when the phase difference between the first branched light 29a and the second branched light 29b is π rad, the intensity of the output light is minimized. The phase of the first branched light 29a changes according to the voltage of the electrical signal supplied to one optical phase modulator 2a. The phase of the second branched light 29b is kept constant by the constant voltage supplied to the other optical modulator 2b.

  A reverse bias voltage of a PN junction including the first and second upper cladding layers 16a and 16b and the lower cladding layer 12 is applied between the upper electrodes 24ba and 24bb and the lower electrode 24a of the optical phase modulators 2a and 2b. The The reverse bias voltage is supplied by constant voltage sources 34 and 35, for example.

  By applying the reverse bias voltage, an electric field (an electric field stronger than the electric field corresponding to the build-in potential) is generated in the core layer 14. Then, for example, the refractive index of the core layer 14 changes due to the QCSE effect (Quantum Confined Stark Effect). That is, the refractive index of the core layer 14 changes according to the electric field strength in the core layer.

  The optical phase modulator 2a is further supplied with an electrical signal exceeding 10 Gb / s superimposed on the reverse bias voltage. The electrical signal is supplied by an external circuit 36, for example.

  In response to an electrical signal applied between the upper electrodes 24ba, 24bb and the lower electrode 24a, the refractive index of the core layer 14 included in one optical modulator 2a (hereinafter referred to as the core refractive index) changes. Due to the change in the core refractive index, the phase of the first branched light 29a propagating through one optical phase modulator 2a is modulated. Then, the phase difference between the first branched light 29a and the second branched light 29b changes, and the intensity of the output light of the semiconductor optical device 4 changes.

(2-2) Frequency Characteristics of Optical Modulator FIGS. 5 to 7 are diagrams for explaining the frequency characteristics of the optical modulators 2a and 2b. FIG. 5A is a longitudinal sectional view of an optical phase modulator 37 (hereinafter referred to as an optical modulator without a block layer) that does not have the first semiconductor layer 18a (block layer). The structure of the optical phase modulator 37 without block layer is substantially the same as the structure of the optical phase modulators 2a and 2b of the first embodiment except that the first semiconductor layer 18a (block layer) is not provided.

  FIG. 5A shows an electric field 38 a generated in the optical phase modulator 37 without a block layer. FIG. 5B shows an electric field 38b generated in the optical phase modulators 2a and 2b of the first embodiment.

  In the electric field 38a in FIG. 5A, a low-frequency signal (for example, a signal of 50 MHz or less) with respect to the bit rate of the modulation signal superimposed on the reverse bias voltage is supplied to the optical phase modulator 37 without a block layer. The electric field generated when Similarly, the electric field 38b in FIG. 5B is generated when a low-frequency signal (for example, an electric signal of 50 MHz or less) superimposed on the reverse bias voltage is supplied to the optical modulators 2a and 2b of the first embodiment. This is the electric field that is generated.

  A solid line indicates an electric field generated in the non-doped layer 14 (the same applies to FIG. 6 described later).

  A broken line indicates a weak electric field generated in the lower cladding layer 12 and the upper cladding layer 16 (or the first and second upper cladding layers 16a and 16b).

  As shown in FIG. 5A, the electric field 38a of the optical modulator 37 without a block layer oozes outside the upper electrode 24b in plan view. The electric field strength in the non-doped layer 14 is kept substantially constant below the upper electrode 24b, and gradually decreases outside the upper electrode 24b as the distance from the upper electrode 24b increases.

  The electric field strength in the non-doped layer 14 of the optical phase modulators 2a and 2b of the first embodiment is also kept substantially constant below the upper electrodes 24ba and 24bb. Outside the upper electrodes 24ba and 24bb, the electric field strength in the non-doped layer 14 of the first embodiment is weaker than that of the optical phase modulator 37 without a block layer described above.

  In the optical phase modulators 2a and 2b of the first embodiment, a first semiconductor layer 18a (block layer) is provided outside the upper electrodes 24ba and 24bb. The first semiconductor layer 18a is a semiconductor layer having a higher resistivity than the first and second upper clad layers 16a and 16b (hereinafter referred to as upper clad layers 16a and 16b). Accordingly, the electric field strength in the first semiconductor layer 18a (block layer) is approximately the same as the electric field strength in the non-doped layer 14. Therefore, when a reverse bias voltage is applied to the upper electrodes 24ba and 24bb, a part of the reverse bias voltage is applied to the first semiconductor layer 18a (block layer) outside the upper electrodes 24ba and 24bb. As a result, in the optical modulators 2a and 2b of the first embodiment, the electric field strength in the non-doped layer 14 is reduced outside the upper electrodes 24ba and 24bb. That is, the strength of the electric field generated at the end portion in the longitudinal direction of the non-doped layer 14 is suppressed by the first semiconductor layer 18a (block layer).

  FIG. 6A shows an electric field 40a generated in a block layerless optical phase modulator 37 to which a high-frequency signal (for example, an electrical signal of 1 GHz or more) superimposed on a reverse bias voltage is supplied. FIG. 6B shows an electric field generated in the optical modulators 2a and 2b (the optical modulator of the first embodiment) supplied with a high-frequency signal (for example, an electrical signal of 1 GHz or more) superimposed on the reverse bias voltage. 40b is shown.

  As shown in FIGS. 6 (a) and 6 (b), the electric fields 40a, 40b for the high-frequency signal are generated outside the upper electrodes 24b, 24ba, 24bb regardless of the presence or absence of the first semiconductor layer 18a (block layer). Hardly oozes.

  The dielectric loss of a semiconductor such as InP increases as the frequency of the electrical signal increases. The path that passes outside the upper electrodes 24b, 24ba, and 24bb and reaches the non-doped layer 14 is longer than the path that passes below the upper electrodes 24b, 24ba, and 24bb and reaches the non-doped layer 14. Therefore, the path outside the upper electrodes 24b, 24ba, and 24bb is easily affected by dielectric loss. As a result, in the high band (for example, a frequency region of 1 GHz or more), the electric fields 40a and 40b hardly ooze out of the upper electrodes 24b, 24ba, and 24bb.

  In the optical phase modulator 37 without a block layer, the distribution of the electric field strength of the core layer 14 with respect to a high-frequency signal (for example, an electric signal of 1 GHz or higher) and the electric field of the core layer 14 with respect to a low-frequency signal (for example, an electric signal of 50 MHz or lower). It is very different from intensity distribution. Specifically, the seepage of the electric field generated by the low frequency signal hardly occurs for the high frequency signal (see FIGS. 5A and 6A).

  Therefore, in the optical phase modulator 37 without a block layer, the region where the core refractive index substantially changes due to the high frequency signal is limited to the region below the upper electrode 24b. On the other hand, the region where the core refractive index substantially changes due to the low frequency signal is a region where the outside and the lower side of the upper electrode 24b are combined. Therefore, in the optical phase modulator 37 without a block layer, the modulation factor for the high frequency signal is smaller than the modulation factor for the low frequency signal. Specifically, for example, the modulation degree of the optical phase modulator 37 without a block layer rapidly decreases in a frequency region (hereinafter referred to as a low band) of 50 MHz to 1 GHz.

  Note that the frequency of the low frequency signal corresponding to the electric fields 38a and 38b in FIG. 5 is a frequency equal to or lower than the lower limit frequency (50 MHz) of the low band. That is, the band between the frequency of the low frequency signal corresponding to the electric fields 38a and 38b in FIG. 5 and the frequency of the high frequency signal corresponding to the electric fields 40a and 40b in FIG.

  On the other hand, as shown in FIG. 5B, in the optical modulators 2a and 2b of the first embodiment, the electric field applied to the non-doped layer is small due to the leaching of the electric field by the low frequency signal. Therefore, the amount by which the core refractive index changes due to the application of the low frequency signal is very small. Therefore, in the optical modulators 2a and 2b of the first embodiment, the amount of change in the core refractive index does not greatly depend on the frequency of the electric signal. Therefore, according to the first embodiment, the low bandwidth of the optical phase modulators 2a and 2b. Deterioration of response characteristics (modulation degree) in is suppressed.

  FIG. 7 is a diagram illustrating an example of an electric field intensity distribution in the non-doped layer 14. The vertical axis represents the electric field strength in the non-doped layer 14. The horizontal axis is the coordinate in the longitudinal direction of the non-doped layer 14. The origin of the horizontal axis is determined so that the coordinates of one end of the upper electrodes 24b, 24ba, 24bb are 0.5 mm. The region whose longitudinal coordinate is 0 to 0.5 mm is the region below the upper electrodes 24b, 24ba, and 24bb. The region where the longitudinal coordinate is larger than 0.5 mm is a region outside the upper electrodes 24b, 24ba, and 24bb.

  The data in FIG. 7 is data calculated by electromagnetic field simulation. The frequency of the electric signal used for the electromagnetic field simulation is 50 MHz. The non-doped layer 14 of the model (light modulator) used for the simulation is a layer having a thickness of 900 nm and including an i-type AlGaInAs MQW (Multiple Quantum Well) core layer and an i-type InP layer.

  The first electric field distribution 42 a is an electric field distribution in the non-doped layer 14 of the optical phase modulator 37 without a block layer. The second electric field distribution 42b is an electric field distribution in the non-doped layer 14 of the optical phase modulators 2a and 2b having the first semiconductor layer 18a (block layer) having a thickness of 200 nm. The third electric field distribution 42c is an electric field distribution in the non-doped layer 14 of the optical phase modulators 2a and 2b having the first semiconductor layer 18a (block layer) having a thickness of 500 nm.

  As shown in FIG. 7, in the optical phase modulator 37 without a block layer (see FIG. 5A), the electric field intensity outside the upper electrode 24b gradually decreases as the distance from the upper electrode 24b increases (first electric field distribution). 42a). On the other hand, in the optical phase modulators 2 a and 2 b (see FIG. 5B) having the first semiconductor layer 18 a (block layer), the electric field strength outside the upper electrodes 24 ba and 24 bb is that of the optical modulator 37 without the block layer. It is smaller than the case (see the second and third electric field distributions 42b and 42c).

  That is, according to the optical modulators 2a and 2b of the first embodiment, the amount of change in the core refractive index that occurs outside the upper electrodes 24ba and 24bb due to the application of a low-frequency signal (an electric signal of 50 MHz or less) is small. Therefore, according to the optical modulators 2a and 2b of the first embodiment, it is possible to suppress deterioration of response characteristics (modulation degree) in a low band (for example, 50 MHz to 1 GHz).

  As shown in FIG. 7, the electric field strength outside the upper electrodes 24ba and 24bb is smaller as the first semiconductor layer 18a (block layer) is thicker. Therefore, the first semiconductor layer 18a (block layer) is preferably as thick as possible.

(3) Manufacturing Method FIGS. 8 and 9 are process cross-sectional views illustrating a method of manufacturing the optical modulator of the first embodiment.

(3-1) Step of forming a non-doped layer including a core layer (see FIG. 8A)
A non-doped layer 14 including a core layer is grown on the lower cladding layer 12 having the first conductivity type semiconductor. The non-doped layer 14 is grown by, for example, a metal organic growth method or a molecular beam epitaxy growth method. The same applies to each semiconductor layer described later.

  Specifically, for example, an n-type InP buffer layer (not shown) and an i-type semiconductor layer containing AlGaInAs MQW (non-doped layer 14) are grown on the n-type InP substrate 44. The n-type InP substrate 44 and the n-type InP buffer layer form the lower cladding layer 12.

(3-2) Step of forming second upper clad (see FIG. 8B)
A second upper cladding layer 16b of a second conductivity type different from the first conductivity type is grown on the non-doped layer.

  Specifically, for example, a second p-type InP layer (second upper cladding layer 16 b) is grown on the non-doped layer 14.

(3-3) First semiconductor film formation step (see FIG. 8C)
A first semiconductor film 46a that can be selectively etched with respect to the first lower layer (second upper clad layer 16b in the first embodiment) in contact with the lower surface is grown on the second upper clad layer 16b. The first semiconductor film 46a is a block layer having a higher resistivity than the second upper cladding layer 16b and a first upper cladding layer 16a described later.

  Specifically, for example, an i-type InGaAsP film (first semiconductor film 46a) is grown on the second p-type InP layer (second upper cladding layer 16b). The i-type InGaAsP film (first semiconductor film 46a) is a semiconductor that can be selectively etched with respect to the second p-type InP clad layer (second upper clad layer 16b).

(3-4) Selective etching process (see FIG. 9A)
The central portion (hereinafter referred to as the first central portion) of the modulator formation region 48a of the first semiconductor film 46a (see FIG. 8C) is a first lower layer (specifically, the second upper cladding layer 16b). Is selectively etched. At this time, the first lower layer is hardly etched. The modulator formation region 48a is a region corresponding to the optical phase modulators 2a and 2b (see FIG. 1).

  The etching rate of the first semiconductor film 46a is preferably 5 times or more (preferably 10 times or more) that of the first lower layer. By this etching, the first semiconductor film 46a becomes the first semiconductor layer 18a (block layer) as shown in FIG.

Specifically, for example, the region corresponding to the upper electrodes 24ba and 24bb (see FIG. 3) in the i-type InGaAsP film (first semiconductor film 46a) is made of, for example, a mixed solution of HCl, H ₂ O ₂ and H ₂ O. Selectively wet etch. The second upper clad layer 16b is hardly etched, and the surface remains flat even when exposed to the mixed solution.

(3-5) First upper cladding layer and contact layer forming step (see FIG. 9B)
A second conductivity type opposite to the first conductivity type (for example, on the first semiconductor film 46a (that is, the first semiconductor layer 18a) and the first lower layer (second upper clad layer 16b) whose first central portion is etched (for example, , P-type) first upper cladding layer 16a is grown (see FIG. 9B). Thereafter, contact layers 22a and 22b (see FIG. 2) of the second conductivity type are formed on the first upper clad layer 16a.

  For example, the first p-type InP layer (first upper clad layer 16a) is further formed on the selectively etched i-type InGaAsP film (first semiconductor film 46a) and second p-type InP layer (second upper clad layer 16b). To grow. Thereafter, a p-type InGaAs layer (contact layer 122) is grown on the first p-type InP layer (first upper cladding layer 16a).

  The first p-type InP cladding layer (first upper cladding layer 16a) and the p-type InGaAs layer (contact layer 122) are grown so as to cover the entire n-type InP substrate 44.

  Thereafter, the p-type InGaAs layer is etched so that regions corresponding to the upper electrodes 24ba and 24bb remain to form contact layers 22a and 22b.

(3-6) Formation of optical waveguide (see FIG. 2)
After the contact layers 22a and 22b are formed, the growth layer is etched to the upper surface of the lower cladding layer 12 by dry etching, for example, to form an optical waveguide having a high mesa structure.

Thereafter, the etched surface is covered with a passivation film 28a (see FIG. 2). Further, a resin layer 26 is formed on the side of the formed optical waveguide. The resin layer 26 is, for example, a benzocyclobutene layer. Further, a passivation film 28b that covers the resin layer 26 is formed. The passivation films 28a and 28b are, for example, SiO ₂ films.

(3-7) Formation of electrode (see FIG. 9C)
After the formation of the waveguide, the lower electrode 24a connected to the lower cladding layer 12 and the upper electrodes 24ba and 24bb on the center of the first upper cladding layer 16a are formed.

  Specifically, for example, the lower electrode 24 a is formed on the back surface of the n-type InP substrate 44. Further, upper electrodes 24ba and 24bb are formed on the contact layers 22a and 22b. Thereafter, the semiconductor substrate 44 is cleaved and divided into a plurality of semiconductor devices each including the optical modulators 2a and 2b.

  The optical modulators 2a and 2b are formed by the above process. The optical phase modulators 2a and 2b are formed together with, for example, each component (for example, the input waveguide 6) of the semiconductor optical device 4.

  According to the manufacturing method of the first embodiment, the layer structure (for example, the first semiconductor layer 18 + the first upper cladding layer 16a) that suppresses the seepage of the electric field can be formed without using selective growth. . Therefore, the optical modulators 2a and 2b can be easily formed.

  In the manufacturing method described with reference to FIGS. 8 to 9, the first semiconductor film 46a is selectively etched at the center of the modulator formation region 48a. However, the first semiconductor film 46a may be selectively etched on one end side of the modulator formation region 48a. In this case, the contact layers 22a and 22b and the upper electrodes 24ba and 24bb are formed above the region where the first semiconductor layer 18a is selectively etched.

  In the manufacturing method described with reference to FIGS. 8 to 9, the non-doped layer 14 including the core layer is grown on the lower cladding layer 12. A semi-insulating layer including a core layer may be grown instead of the non-doped layer 14 (the same applies to the second embodiment).

(4) Optical Modulator Having Semi-Insulating Cladding Layer FIG. 10 is a cross-sectional view of an optical modulator 54 having semi-insulating cladding layers 52 on both sides in the longitudinal direction of the upper cladding layer 16. FIG. 10 shows an electric field 38c generated when a low-frequency signal (for example, an electric signal of 50 MHz or less) superimposed on the reverse bias voltage is supplied. The low frequency signal is supplied to the upper electrode 24 b disposed on the contact layer 22.

  As shown in FIG. 10, the semi-insulating cladding layer 52 suppresses an electric field (an electric field in the core layer 14) generated outside the upper electrode 24b. Therefore, according to the optical modulator 54 shown in FIG. 10, the deterioration of the response characteristic (modulation degree) in the low band of the optical modulator is suppressed.

  FIG. 11 is a diagram for explaining the problem of the semi-insulating clad layer 52. In order to form the semi-insulating clad layer 52, first, a region corresponding to the outside of the upper electrode 24b is removed from the upper clad layer 16 (see FIG. 5A). Thereafter, a semi-insulating clad layer 52 (for example, a semi-insulating InP layer) is selectively grown in a region where the upper clad layer 16 is removed by a metal organic growth method. However, when the semi-insulating cladding layer 52 is selectively grown in the region where the upper cladding layer 16 is removed, a step 56 (see FIG. 11) is formed at the boundary between the upper cladding layer 16 and the semi-insulating cladding layer 52. Such a step 56 makes it difficult to form the upper electrode 24b.

  As described above, the optical phase modulators 2a and 2b according to the first embodiment have a layer structure (first semiconductor layer 18a + first upper cladding layer 16a) that can be formed without selective growth. Suppress out. Therefore, according to the first embodiment, the frequency characteristic of the optical phase modulator can be improved without generating the protrusion 56 that makes it difficult to form the upper electrode 24b. That is, according to the first embodiment, it becomes easy to form an optical modulator having excellent frequency characteristics.

  In the above example, an example of an optical phase modulator used for a Mach-Zehnder type optical modulator has been shown. However, the structure of the present invention can be applied to all optical modulators to which a reverse bias is applied. A similar effect can be obtained with a type modulator.

(5) Modification (5-1) Modification 1
In the above example, the first semiconductor layer 18a is a block layer having a higher resistivity than the first upper cladding layer 16a and the second upper cladding layer 16b. However, the first semiconductor layer 18a may be a block layer having a conductivity type (for example, n-type) opposite to that of the first upper cladding layer 16a (for example, p-type). Hereinafter, an optical modulator in which the conductivity type of the first semiconductor layer 18a is opposite to that of the first upper cladding layer 16a is referred to as an optical modulator of the first modification.

  When a reverse bias voltage is applied to the upper electrodes 24ba and 24bb of Modification 1, a part of the reverse bias voltage includes the first upper cladding layer 16a and the first semiconductor layer 18a outside the upper electrodes 24ba and 24bb. Used to widen the depletion layer of the pn junction. As a result, the voltage applied to the core layer 14 outside the upper electrodes 24ba and 24bb is suppressed. Therefore, the modification 1 also suppresses deterioration of response characteristics (modulation degree) in the low band.

  If the impurity concentration in the first semiconductor layer 18a is sufficiently high, even if the first semiconductor layer 18a is thin, a large voltage is applied to the pn junction including the first upper cladding layer 16a and the first semiconductor layer 18a. obtain. Therefore, even if the first semiconductor layer 18a is thinned, the deterioration of the frequency characteristics can be suppressed. That is, according to the first modification, the first semiconductor layer 18a can be thinned. When the first semiconductor layer 18a is thinned, the reflection of guided light generated at the boundary between the central portion of the first upper cladding layer 16a and the first semiconductor layer 18a is suppressed.

  The optical modulator of Modification 1 can be manufactured by substantially the same procedure as the manufacturing method described with reference to FIGS. Specifically, refer to FIGS. 8 and 9 except that the conductivity type of the first semiconductor film 46a (see FIG. 8C) is opposite to the conductivity type of the first upper cladding layer 16a. The optical modulator of Modification 1 can be manufactured by the same procedure as the manufacturing method described above. Therefore, the optical modulator of the modification 1 can be easily formed as in the optical modulator described with reference to FIG.

(5-2) Modification 2
In the above example, the first upper cladding layer 16a and the second upper cladding layer 16b are semiconductors having a second conductivity type (for example, p-type) opposite to the conductivity type (for example, n-type) of the lower cladding layer 12. Is a layer. However, the first upper cladding layer 16a and the second upper cladding layer 16b may be semiconductor layers having the same conductivity type as that of the lower cladding layer 12 (for example, n-type) (Embodiments 2 to 4). The same applies to Hereinafter, an optical modulator having the same conductivity type as that of the upper clad layer and the lower clad layer is referred to as an optical modulator of Modification 2.

  Even if the upper cladding layer and the lower cladding layer have the same conductivity type, a large electric field is applied to the core layer because the layer 14 around the core layer is i-type or semi-insulating. Therefore, it is possible to modulate the phase of incident light. Furthermore, since the modulator of Modification 2 includes the first semiconductor layer 18a (block layer), it is possible to suppress deterioration of response characteristics in a low band.

  By the way, the light absorption coefficient of a semiconductor may differ with conductivity types. Therefore, the optical loss of the optical modulator can be reduced by changing the conductivity type of the upper clad layer and the lower clad layer to a conductivity type having a small light absorption coefficient. For example, the light absorption coefficient of n-type InP is smaller than the light absorption coefficient of p-type InP. Therefore, the optical loss of the optical modulator can be reduced by forming the upper cladding layer and the lower cladding layer with n-type InP.

  Similarly, semiconductor losses for high frequencies may vary by conductivity type. The high frequency characteristics of the optical modulator can be improved by changing the conductivity type of the upper cladding layer and the lower cladding layer to a conductivity type having a small loss with respect to the high frequency. For example, the loss of n-type InP with respect to high frequency is smaller than the loss of p-type InP with respect to high frequency. Therefore, the high frequency characteristics of the optical modulator can be improved by forming the upper cladding layer and the lower cladding layer with n-type InP.

  The optical modulator of Modification 2 can be manufactured by substantially the same procedure as the manufacturing method described with reference to FIGS. Specifically, the first and second upper cladding layers 16a and 16b will be described with reference to FIGS. 8 and 9 except that a semiconductor layer having the same conductivity type as that of the lower cladding layer 12 is grown. The optical modulator of Modification 2 can be manufactured by the same procedure as the manufacturing method described above. Therefore, the optical modulator of Modification 2 can be easily formed in the same manner as the optical modulator described with reference to FIG.

(5-3) Modification 3
In the above example, the optical modulators 2 a and 2 b include the second upper cladding layer 16 b (see FIG. 3) between the first semiconductor layer 18 a and the non-doped layer 14. However, the optical phase modulators 2a and 2b may not have the second upper cladding layer 16b. Hereinafter, an optical modulator that does not have the second upper cladding layer 16b is referred to as an optical modulator of Modification 3.

  FIG. 12 is a cross-sectional view in the longitudinal direction of the optical modulator 2c of the third modification. Regardless of the presence or absence of the second upper cladding layer 16b, when a reverse bias voltage is applied to the upper electrodes 24ba and 24bb, the first semiconductor layer 18a (block layer) has the same electric field as that generated in the non-doped layer 14. A strong electric field is generated. Therefore, according to the modification 3, the deterioration of the response characteristic (modulation degree) in the low band is suppressed.

  The optical modulator 2c of Modification 3 can be formed by substantially the same procedure as the manufacturing method described with reference to FIGS. Specifically, the optical modulator of Modification 3 is performed by the same procedure as the manufacturing method described with reference to FIGS. 8 and 9 except that the second upper cladding layer 16b (see FIG. 8B) is not grown. 3c can be manufactured. Therefore, the optical modulator 2c according to the modified example 3 can be easily formed in the same manner as the optical modulators 2a and 2b shown in FIG. Further, since the second upper cladding layer 16b is not grown, the manufacturing process is simplified (the same applies to the optical modulator 202c of the second embodiment).

In the case where the core layer 14 is an i-type AlGaInAs MQW and the first semiconductor layer 18a is an i-type InGaAs layer, the selective etching solution is HCl: CH ₃ COOH with the composition ratio adjusted so that the etching rate of the AlGaAsAs layer is increased, for example. H ₂ O _2: a _{H 2} O.

(5-4) Modification 4
In the above example, the first semiconductor layer 18a is an InGaAsP layer. However, the first semiconductor layer 18a may be a semiconductor layer other than the InGaAsP layer. For example, the first semiconductor layer 18a may be an AlGaInAs layer. The AlGaInAs layer is a semiconductor layer that can be selectively etched with respect to the InP layer.

  As described above, the optical modulator of the first embodiment has the first semiconductor layer 18a (block layer) that has a high resistivity (or a conductivity type opposite to that of the first upper cladding layer) and can be selectively etched. On the longitudinal end of the core layer 14. Therefore, a layer structure (first semiconductor layer 18a + first upper clad layer 16a) that suppresses the seepage of the electric field can be formed without using selective growth. Therefore, according to the first embodiment, an optical modulator having excellent frequency characteristics and easy to form is provided.

  In the above example, the optical phase modulators 2a and 2b included in the semiconductor optical device 4 shown in FIG. 1 have been described as an example of the optical modulator of the first embodiment. However, since the semiconductor optical device 4 shown in FIG. 1 is a Mach-Zehnder optical modulator including the structure shown in FIG. 3, it can be considered as the optical modulator of the first embodiment.

(Embodiment 2)
The optical modulator of the second embodiment further includes a second optical phase modulator 2a, 2b (see FIG. 3) of the first embodiment that is disposed between the first semiconductor layer 18a and the first upper cladding layer 16a. An optical phase modulator having a semiconductor layer. The optical modulator of the second embodiment has substantially the same structure as the optical modulators 2a and 2b of the first embodiment except that the optical modulator has the second semiconductor layer. Therefore, the description of the same parts as those in Embodiment 1 is omitted or simplified.

(1) Structure FIG. 13 is a cross-sectional view along the longitudinal direction of the optical modulator 202a of the second embodiment.

  The optical modulator 202a according to the second embodiment includes a second semiconductor layer 218b disposed between the first semiconductor layer 218a and the first upper cladding layer 16a.

  As shown in FIG. 13, the second side surface 20b on the upper electrode side (second electrode side) of the second semiconductor layer 218b is covered with the first upper cladding layer 16a. The second semiconductor layer 218b is a block layer having a resistivity higher than that of the first upper cladding layer 16a or a conductivity type opposite to that of the first upper cladding layer 16a.

  The second semiconductor layer 218b is preferably a semiconductor layer having a refractive index greater than or equal to the refractive index of the first upper cladding layer 16a and smaller than the refractive index of the first semiconductor layer 218a. More preferably, the second semiconductor layer 218b is a semiconductor layer having a refractive index of the first upper cladding layer 16a.

  Specifically, for example, the first upper cladding layer 16a and the second upper cladding layer 16b are p-type InP layers. The first semiconductor layer 218a is, for example, an i-type (or n-type) InGaAsP layer. The second semiconductor layer 218b is, for example, an i-type (or n-type) InP layer.

  Similar to the optical modulators 2a and 2b of the first embodiment, the optical modulator 202a includes a first semiconductor layer 218a that can be selectively etched with respect to the lower layer. Therefore, the optical modulator 202a according to the second embodiment can be easily formed in the same manner as the optical modulators 2a and 2b according to the first embodiment.

  The first semiconductor layer 218a and the second semiconductor layer 218b are block layers having high resistivity (or a conductivity type opposite to that of the upper cladding layer). Therefore, according to the second embodiment, as in the first embodiment, the oozing of the electric field to the outside of the upper electrode 24ba can be suppressed. Therefore, according to the second embodiment, as in the first embodiment, it is possible to provide an optical modulator that has excellent frequency characteristics and can be easily formed.

  Further, since the second semiconductor layer 218b is a block layer that suppresses the seepage of the electric field, the seepage of the electric field to the outside of the upper electrode 24ba can be suppressed even if the first semiconductor layer 218a is thinned.

  As described above, the second semiconductor layer 218b has a refractive index that is greater than or equal to the refractive index of the first upper cladding layer 16a (eg, p-type InP layer) and smaller than the refractive index of the first semiconductor layer 218a (eg, i-type InGaAsP layer). It is a semiconductor layer (for example, i-type InP layer) having a refractive index. Therefore, by reducing the thickness of the first semiconductor layer 218a, the difference between the waveguide mode in the region where the first semiconductor layer 218a is disposed and the waveguide mode in the electrode portion where the first semiconductor layer 218a is not present can be reduced. Therefore, according to the second embodiment, it is possible to suppress light reflection that occurs at the boundary between the end portion of the core layer and the central portion of the core layer.

(2) Manufacturing Method FIG. 14 is a process cross-sectional view illustrating a method for manufacturing the optical modulator 202a of the second embodiment.

(2-1) Core layer and second upper clad forming step (see FIGS. 8A to 8B)
By substantially the same procedure as in the first embodiment, a non-doped layer 14 including a core layer (for example, a semiconductor layer including i-type AlGaInAs MQW) is formed on the lower cladding layer 12 including a first conductivity type semiconductor (for example, n-type InP). ) And a second upper cladding layer 16b (for example, an n-type InP layer).

(2-2) Second and third semiconductor film formation steps (see FIG. 14A)
Above the non-doped layer 14, a second semiconductor film 46b that can be selectively etched with respect to a second lower layer (for example, the second upper cladding layer 16b) in contact with the lower surface, and a third semiconductor film 46c on the second semiconductor film 46b. Is grown. The second semiconductor film 46b is, for example, an i-type InGaAsP film. The third semiconductor film 46c is, for example, an i-type InP layer.

  The second semiconductor film 46b and the third semiconductor film 46c are higher in resistivity than the second upper clad layer 16b and a first upper clad layer 16a described later (or opposite to the first and second upper clad layers 16a and 16b). A semiconductor layer having a conductivity type).

(2-3) Selective etching process (see FIGS. 14B and 14C)
The third semiconductor film 46c is selectively etched with respect to the second semiconductor film 46b in the central portion (hereinafter referred to as the second central portion) of the modulator formation region 48b of the stacked film 58 (see FIG. 14B). ). Thereafter, the second semiconductor film 46b is selectively etched with respect to the second lower layer (specifically, the second upper clad layer 16b) (see FIG. 14C). The modulator formation region 48b is a region corresponding to the optical phase modulator. By the etching, the third semiconductor film 46c becomes the second semiconductor layer 218b. The second semiconductor film 46b becomes the first semiconductor layer 218a.

Specifically, for example, the second central portion corresponding to the upper electrode 24ba (see FIG. 13) in the i-type InP layer (third semiconductor film 46c) is selectively etched with concentrated HCl (FIG. 14B). reference). Thereafter, the second central portion of the i-type InGaAsP layer (second semiconductor film 46b) is selectively etched with a mixed solution of HCl, H ₂ O ₂ and H ₂ O (see FIG. 14C).

(2-4) First upper cladding and contact layer formation step (see FIG. 13)
Thereafter, the first upper cladding layer 16a and the contact layer 22a are grown by substantially the same procedure as in the first embodiment, an optical waveguide is formed, and the lower electrode 24a and the upper electrode 24ba are formed.

  According to the second embodiment, as in the first embodiment, the layer structure (first semiconductor layer 18a + second semiconductor layer 18b + first upper clad layer 16a) that suppresses leaching of the electric field is selectively grown. It can be formed without using. Therefore, according to the second embodiment, the optical modulator 202a having excellent frequency characteristics can be easily formed.

  In the manufacturing method described with reference to FIGS. 14 to 15, the third semiconductor film 46 c is selectively etched at the center of the modulator formation region 48 b. However, the third semiconductor film 46c may be selectively etched on one end side of the modulator formation region 48b. In this case, the contact layer 22a and the upper electrode 24ba are formed above the region where the third semiconductor film 46c is selectively etched.

(3) Modification In the above example, the second semiconductor layer 218b is an InP layer. However, the second semiconductor layer 218b may be a semiconductor layer other than the InP layer. For example, the second semiconductor layer 218b may be an InGaAsP layer or an AlGaInAs layer.

  In the above example, the first semiconductor layer 218a is an InGaAsP layer. However, the first semiconductor layer 218a may be a semiconductor layer other than the InGaAsP layer. For example, the first semiconductor layer 218a may be an AlGaInAs layer or an InP layer. When the first semiconductor layer 218a is an InP layer, the uppermost layer of the second upper cladding layer 16b is preferably a p-type InGaAsP layer (or a p-type AlGaInAs layer) that can be selectively etched with respect to the InP layer.

  In the above example, the first semiconductor layer 218a has a higher resistivity than the first and second upper cladding layers 16a and 16b (or a conductivity type opposite to that of the first and second upper cladding layers 16a and 16b). It is a semiconductor layer (namely, block layer) which has. However, the first semiconductor layer 218a may be a semiconductor layer (for example, a p-type InGaAsP layer) having the same conductivity type as the first and second upper cladding layers 16a and 16b (for example, a p-type InP layer). Even in such a structure, the second semiconductor layer 218b can suppress the seepage of the electric field to the outside of the upper electrode 24ba.

  In this case, the second semiconductor film 46b (see FIG. 14B) corresponding to the first semiconductor layer 218a has the same conductivity type as the first and second upper cladding layers 16a and 16b (for example, p-type InP layers). A semiconductor layer (for example, a p-type InGaAsP film).

  In the above example, the optical phase modulator 202a has the second upper cladding layer 16b (see FIG. 13). However, the optical phase modulator 202a may not have the second upper cladding layer 16b.

  FIG. 15 is a longitudinal sectional view of an optical phase modulator 202c that does not have the second upper clad layer 16b. Regardless of the presence or absence of the second upper cladding layer 16b, when a voltage is applied to the upper electrode 24ba, at least the second semiconductor layer 218b (block layer) has a strong electric field comparable to the electric field generated in the non-doped layer 14. Occur. Therefore, even if the second upper cladding layer 16b does not exist, it is possible to suppress the deterioration of the response characteristic (modulation degree) in the low band.

  The optical modulator 202c not having the second upper cladding layer 16b can be formed by substantially the same procedure as the manufacturing method of FIG. Specifically, except that the second upper cladding layer 16b (see FIG. 13) is not grown, the optical modulator 202c of FIG. 15 is manufactured by the same procedure as the method of manufacturing the optical modulator 202c described with reference to FIG. Can be manufactured.

  As described above, according to the second embodiment, as in the first embodiment, an optical modulator that is excellent in frequency characteristics and easy to form is provided. Furthermore, according to the second embodiment, the first semiconductor layer 218a that can be selectively etched with respect to the lower layer can be formed of a semiconductor layer having the same or substantially the same refractive index as that of the first upper cladding layer 16a. Therefore, light reflection at the boundary between the end portion and the portion where the upper electrode is formed can be suppressed.

(Embodiment 3)
The optical modulator of Embodiment 3 is an optical modulator having a coplanar electrode. The optical modulator of the third embodiment has substantially the same structure as the optical modulators 2a and 2b of the first embodiment except that it has a coplanar electrode. Therefore, the description of the same parts as those in Embodiment 1 is omitted or simplified.

  FIG. 16 is a plan view of the semiconductor optical device 304 on which the optical modulators 302a and 302b of the third embodiment are mounted. 17 is a cross-sectional view taken along the line XVII-XVII in FIG. The semiconductor optical device 304 is a Mach-Zehnder optical modulator having coplanar electrodes 324a, 324ba, and 324bb.

  The lower cladding layer 312 (first cladding layer) of the semiconductor optical device 304 includes a semi-insulating semiconductor substrate 344 (for example, an SI InP substrate) and a first conductivity type semiconductor layer 362 disposed on the semiconductor substrate 344. (For example, an n-type InP layer). Further, the semiconductor optical device 304 has a ground electrode 324a (first electrode) disposed on the surface of the first conductivity type semiconductor layer 362 instead of the lower electrode 24a. The first upper electrode 324ba, the second upper electrode 324bb, and the ground electrode 324a form a coplanar electrode. Except for the above points, the optical modulators 302a and 302b have substantially the same structure as the optical modulators 2a and 2b of the first embodiment.

  Similar to the optical phase modulators 2a and 2b of the first embodiment, the optical phase modulators 302a and 302b of the third embodiment have the first semiconductor layer 18a (block layer) that can be selectively etched with respect to the lower surface. Therefore, according to the third embodiment, as in the first embodiment, an optical modulator having excellent frequency characteristics and easy to form is provided.

  By the way, the semi-insulating semiconductor substrate improves the frequency characteristics of the semiconductor optical device. Therefore, according to the third embodiment, it is possible to provide an optical modulator having a frequency characteristic even better than the optical phase modulators 2a and 2b of the first embodiment.

(Embodiment 4)
(1) Structure The optical modulator of the fourth embodiment is integrated in a semiconductor optical device together with a semiconductor laser. However, the optical modulator of the fourth embodiment has substantially the same structure as the optical phase modulators 2a and 2b of the first embodiment. Therefore, the description of the same parts as those in Embodiment 1 is omitted or simplified.

  FIG. 18 is a plan view of a semiconductor optical device 404 in which the optical phase modulators 2a and 2b of the fourth embodiment are integrated. FIG. 19 is a cross-sectional view taken along line XIX-XIX in FIG. The semiconductor optical device 404 is a semiconductor device having the semiconductor laser 470 further connected to the input waveguide 6 in the semiconductor optical device 4 of the first embodiment. Except for the above points, the semiconductor optical device 404 has substantially the same structure as the semiconductor optical device 4 of the first embodiment.

  The semiconductor laser 470 is, for example, a distributed feedback semiconductor laser. The semiconductor laser 470 includes, for example, a first conductivity type lower cladding layer 12 (for example, an n-type InP cladding layer) and a diffraction grating 472 (for example, an InGaAsP diffraction grating) disposed in the lower cladding layer 12 (FIG. 19). The semiconductor laser 470 further includes an active layer 474 (for example, InGaAsP-based MQW) and a second conductivity type upper cladding layer 416 (for example, a p-type InP cladding layer). The active layer 474 has a gain in the 1.55 μm band, for example. The semiconductor laser 470 further includes a contact layer 422 (for example, a p-type InGaAs layer), an upper electrode 424, and a lower electrode 24a.

  When a forward bias voltage is applied between the upper electrode 424 and the lower electrode 24a, a current is injected into the active layer 474. As shown in FIG. 19, the semiconductor laser 470 is not provided with the first semiconductor layer 18a (block layer). Therefore, current injection into the active layer 474 is not hindered by the first semiconductor layer 18a (block layer).

  As in the first embodiment, the first semiconductor layer 18a (block layer) suppresses oozing of the electric field in the optical phase modulators 2a and 2b. The first semiconductor layer 18a (block layer) further suppresses current leakage from the semiconductor laser 470 to the optical modulators 2a and 2b. Therefore, according to the fourth embodiment, current leakage of the optical semiconductor device having the semiconductor laser and the optical modulators 2a and 2b can be suppressed.

(2) Manufacturing Method FIGS. 20 and 21 are process cross-sectional views illustrating an example of a method of manufacturing the semiconductor optical device 404 according to the fourth embodiment.

(2-1) Diffraction grating forming step (see FIGS. 20A to 20B)
A semiconductor layer 480 (for example, an InGaAsP layer) having a refractive index different from that of the semiconductor substrate 444 is grown on the first conductivity type semiconductor substrate 444 (for example, an n-type InP substrate) (see FIG. 20A). .

  Next, the semiconductor layer 480 is processed to form a diffraction grating 472 in a region corresponding to the semiconductor laser 470 (see FIG. 20B). The semiconductor layer 480 is processed using, for example, an electron beam drawing method.

(2-2) Step of forming p-type cladding and active layer (see FIG. 20C)
A first conductivity type semiconductor layer 484 (for example, an n-type InP layer) is grown on the semiconductor substrate 444 on which the diffraction grating 472 is formed. Thereafter, an MQW 486 (for example, InGaAsP-based MQW) and a second conductivity type semiconductor layer 488 (for example, a p-type InP layer) are grown on the semiconductor layer 484.

  The semiconductor substrate 444 and the semiconductor layer 484 become the lower cladding layer 12. The MQW 486 is a semiconductor layer corresponding to the active layer 474. The semiconductor layer 488 is a semiconductor layer corresponding to the upper cladding layer 416.

(2-3) Step of forming butt joint (see FIGS. 20D to 20E)
Except for the region corresponding to the semiconductor laser 470, the semiconductor layer 488 and the MQW 486 are removed (see FIG. 20D).

  The core layer 14, the second upper cladding layer 16b, and the first semiconductor film 46a are selectively grown in the region from which the semiconductor layer 488 and the MQW 486 have been removed. By this selective growth, a butt joint (butt joint) is formed (see FIG. 20E).

  For example, the core layer 14 is a semiconductor layer containing i-type AlGaInAs MQW. The second upper cladding layer 16b is, for example, a p-type InP layer. The first semiconductor film 46a is, for example, an i-type InGaAsP film.

(2-4) Block layer and first upper cladding layer forming step (see FIGS. 21 (f) to 21 (g))
A region corresponding to the upper electrodes 24ba and 24bb (see FIGS. 19 and 2) in the first semiconductor film 46a is selectively etched to form the first semiconductor layer 18a (block layer) (FIG. 21F). reference).

  Thereafter, a first upper cladding layer 16a and a semiconductor layer 446 (for example, a p-type InGaAs layer) having an impurity concentration higher than that of the first upper cladding layer 16a are grown on the entire surface of the semiconductor substrate 444.

  The semiconductor layer 446 other than the region corresponding to the upper electrodes 424, 24ba, and 24bb (see FIGS. 19 and 2) in the semiconductor layer 446 is etched to form contact layers 422, 22a, and 22b of the semiconductor laser and the optical modulator. (See FIG. 21 (g)).

(2-5) Waveguide and electrode formation process (see FIG. 21 (h))
After the growth of the contact layer, the growth layer is etched to the upper surface of the lower cladding layer 12, for example, by dry etching to form a high mesa optical waveguide.

  Thereafter, for example, a semi-insulating InP layer is formed on the side of the semiconductor laser 470. A passivation film (not shown) and a resin layer (not shown) are formed on the sides of the optical modulators 2a and 2b. Further, a passivation film 28b is formed to cover the resin layer 26 (see FIG. 2) and the first semiconductor layer 18a.

  Thereafter, upper electrodes 424, 24ba, and 24bb (see FIG. 21 (h) and FIG. 2) are formed on the contact layers 422, 22a, and 22b (see FIG. 21 (h) and FIG. 2).

  According to the fourth embodiment, as in the first embodiment, the layer structure (first semiconductor layer 18a + first upper cladding layer 16a) that suppresses the seepage of the electric field can be formed without using selective growth. it can. Therefore, the optical modulators 2a and 2b can be easily formed.

  By the way, after the formation of the butt joint (butt joint), the growth of the semiconductor layer is temporarily interrupted. This interruption is for removing the insulating film used for the selective growth. During this interruption, the first semiconductor film 46a can be selectively etched. Accordingly, the first semiconductor layer 18a (block layer) can be formed without increasing the number of growth interruptions.

  Instead of the semiconductor laser 470, a semiconductor optical amplifier or a duplexer may be integrated in the semiconductor optical device 404.

  As mentioned above, although embodiment of this invention was described, Embodiment 1-4 is illustration and is not restrictive.

  For example, the optical modulator of the embodiment is a phase modulator. However, the optical modulator of the embodiment may be an electroabsorption optical modulator that modulates the intensity of input light.

  The optical modulator of the embodiment is a part of a semiconductor optical device. However, the optical modulators of Embodiments 1 to 4 may be single optical elements.

  In the embodiment, the semiconductor substrate is InP. However, the semiconductor substrate may be a semiconductor other than the InP substrate. For example, the semiconductor substrate may be GaAs. In this case, the core layer can be formed of GaAs / AlGaAs MQW. The clad layer can be formed of AlGaAs.

  The following additional notes are further disclosed with respect to the first to fourth embodiments.

(Appendix 1)
A lower clad layer having a first conductivity type semiconductor;
A non-doped layer including a core layer disposed on the lower cladding layer, or a semi-insulating layer including the core layer;
A first upper cladding layer disposed on the non-doped layer or the semi-insulating layer and having a second conductivity type opposite to the first conductivity type or the first conductivity type;
A first electrode connected to the lower cladding layer;
A second electrode disposed on the first upper cladding layer;
A first side surface on the second electrode side disposed between the non-doped layer or the semi-insulating layer and the first upper cladding layer in at least a part from the vicinity of the end in the longitudinal direction of the second electrode to the outside. A first semiconductor layer covered with the first upper cladding layer and selectively etchable with respect to a lower layer contacting the lower surface;
A second semiconductor layer or the first semiconductor layer disposed between the first semiconductor layer and the first upper clad layer and having a second side surface on the second electrode side covered with the first upper clad layer, An optical modulator comprising: a block layer having a higher resistivity than the first upper cladding layer or a conductivity type opposite to the conductivity type of the first upper cladding layer.

(Appendix 2)
Further, a second layer is provided on the non-doped layer or the semi-insulating layer, has a conductivity type of the first upper cladding layer, and has an upper surface covered with the first upper cladding layer and the first semiconductor layer. Having an upper cladding layer,
The optical modulator according to appendix 1, wherein the block layer has a higher resistivity than the second upper cladding layer or a conductivity type opposite to the conductivity type of the second upper cladding layer.

(Appendix 3)
The optical modulator according to appendix 1 or 2, wherein the second semiconductor layer has a refractive index that is greater than or equal to the refractive index of the first upper cladding layer and less than the refractive index of the first semiconductor layer.

(Appendix 4)
The first upper cladding layer, the second upper cladding layer, and the lower cladding layer have an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ or more,
The non-doped layer has an impurity concentration (or carrier concentration) of 5 × 10 ¹⁶ cm ⁻³ or less,
The semi-insulating layer has a resistivity of 1 × 10 ⁵ Ωcm or more,
The supplementary note 2, wherein the first semiconductor layer or the second semiconductor layer has an impurity concentration (or carrier concentration) of 5 × 10 ¹⁶ cm ⁻³ or less or a resistivity of 1 × 10 ⁵ Ωcm or more. Light modulator.

(Appendix 5)
The conductivity type of the first upper cladding layer is the second conductivity type opposite to the first conductivity type of the lower cladding layer,
Between the first electrode and the second electrode, there is a reverse bias voltage of a PN junction including the first upper clad layer and the lower clad layer, and a high frequency signal exceeding 1 GHz superimposed on the reverse bias voltage. The optical modulator according to any one of appendices 1 to 4, wherein the optical modulator is supplied.

(Appendix 6)
A lower cladding layer of a first conductivity type;
A non-doped layer disposed on the lower cladding layer, or a semi-insulating layer including the core layer;
A first upper cladding layer disposed on the non-doped layer or the semi-insulating layer and having a second conductivity type opposite to the first conductivity type or the first conductivity type;
A first electrode connected to the lower cladding layer;
A second electrode disposed on the first upper cladding layer;
A first side surface on the second electrode side disposed between the non-doped layer or at least part of the semi-insulating layer from the vicinity of the end in the longitudinal direction of the second electrode to the outside and the first upper cladding layer A first semiconductor layer covered with the first upper cladding layer and selectively etchable with respect to a lower layer contacting the lower surface;
A second semiconductor layer or the first semiconductor layer disposed between the first semiconductor layer and the first upper clad layer and having a second side surface on the second electrode side covered with the first upper clad layer, An optical modulator comprising: the non-doped layer, or a block layer that suppresses an electric field applied to at least part of the semi-insulating layer from the vicinity of the end in the longitudinal direction of the second electrode to the outside.

(Appendix 7)
Growing a non-doped layer including a core layer or a semi-insulating layer including the core layer on a lower clad layer having a first conductivity type semiconductor;
Growing a first semiconductor film that can be selectively etched on the first lower layer in contact with the lower surface on the non-doped layer or the semi-insulating layer;
Selectively etching a part of the first semiconductor film with respect to the first lower layer;
After the etching step, the first upper portion having the second conductivity type opposite to the first conductivity type or the first conductivity type on the first semiconductor film partially etched and the first lower layer. A step of growing a cladding layer;
Forming a first electrode connected to the lower cladding layer and a second electrode disposed above the part via the first upper cladding layer;
The method of manufacturing an optical modulator, wherein the first semiconductor film has a higher resistivity than the first upper cladding layer or a conductivity type opposite to a conductivity type of the first upper cladding layer.

(Appendix 8)
Furthermore, before the step of growing the first semiconductor film, there is a step of growing a second upper cladding layer having the conductivity type of the first upper cladding layer on the non-doped layer or the semi-insulating layer. And
8. The optical modulator according to claim 7, wherein the first semiconductor film has a higher resistivity than the second upper cladding layer or a conductivity type opposite to a conductivity type of the second upper cladding layer. Method.

(Appendix 9)
Growing a non-doped layer including a core layer or a semi-insulating layer including the core layer on a lower clad layer having a first conductivity type semiconductor;
A laminated film including a second semiconductor film that can be selectively etched with respect to the second lower layer in contact with the lower surface and a third semiconductor film on the second semiconductor film is formed on the non-doped layer or the semi-insulating layer. A growing process;
Etching the third semiconductor film in a part of the stacked film and further selectively etching the second semiconductor film with respect to the second lower layer;
Growing a first upper cladding layer having the second conductivity type opposite to the first conductivity type or the first conductivity type on the laminated film and the second lower layer partially etched;
Forming a first electrode connected to the lower cladding layer and a second electrode disposed above the part via the first upper cladding layer;
The method of manufacturing an optical modulator, wherein the third semiconductor film has a higher resistivity than the first upper cladding layer or a conductivity type opposite to a conductivity type of the first upper cladding layer.

(Appendix 10)
Furthermore, before the step of growing the laminated film, the step of growing a second upper cladding layer having a conductivity type of the first upper cladding layer on the non-doped layer or the semi-insulating layer,
The optical modulator according to claim 9, wherein the third semiconductor film has a higher resistivity than the second upper cladding layer or a conductivity type opposite to that of the second upper cladding layer. Method.

2a, 2a, 2c ... optical phase modulator 12 ... lower cladding layer 14 ... non-doped layer or semi-insulating layer 16a ... first upper cladding layer 16b ... second upper cladding layer 18a , 218a, first semiconductor layer 18b, 218b, second semiconductor layer 20a, first side 20b, second side 24a, first electrode (lower electrode).
24ba, 24bb ... second electrode (upper electrode)
46a: first semiconductor film 46b: second semiconductor film
46c: third semiconductor films 48a, 48b: modulator formation region
58 ... Laminated film

Claims (7)

A lower clad layer having a first conductivity type semiconductor;
A non-doped layer including a core layer disposed on the lower cladding layer, or a semi-insulating layer including the core layer;
A first upper cladding layer disposed on the non-doped layer or the semi-insulating layer and having a second conductivity type opposite to the first conductivity type or the first conductivity type;
A first electrode connected to the lower cladding layer;
A second electrode disposed on the first upper cladding layer;
A first side surface on the second electrode side disposed between the non-doped layer or the semi-insulating layer and the first upper cladding layer in at least a part from the vicinity of the end in the longitudinal direction of the second electrode to the outside. A first semiconductor layer covered with the first upper cladding layer and selectively etchable with respect to a lower layer contacting the lower surface;
A second semiconductor layer or the first semiconductor layer disposed between the first semiconductor layer and the first upper clad layer and having a second side surface on the second electrode side covered with the first upper clad layer, An optical modulator comprising: a block layer having a higher resistivity than the first upper cladding layer or a conductivity type opposite to the conductivity type of the first upper cladding layer. Further, a second layer is provided on the non-doped layer or the semi-insulating layer, has a conductivity type of the first upper cladding layer, and has an upper surface covered with the first upper cladding layer and the first semiconductor layer. Having an upper cladding layer,
2. The optical modulator according to claim 1, wherein the block layer has a higher resistivity than the second upper cladding layer or a conductivity type opposite to a conductivity type of the second upper cladding layer. 3. The optical modulator according to claim 1, wherein the second semiconductor layer has a refractive index that is greater than or equal to a refractive index of the first upper cladding layer and smaller than a refractive index of the first semiconductor layer. Growing a non-doped layer including a core layer or a semi-insulating layer including the core layer on a lower clad layer having a first conductivity type semiconductor;
Growing a first semiconductor film that can be selectively etched on the first lower layer in contact with the lower surface on the non-doped layer or the semi-insulating layer;
Selectively etching a part of the first semiconductor film with respect to the first lower layer;
After the etching step, the first upper portion having the second conductivity type opposite to the first conductivity type or the first conductivity type on the first semiconductor film partially etched and the first lower layer. A step of growing a cladding layer;
Forming a first electrode connected to the lower cladding layer and a second electrode disposed above the part via the first upper cladding layer;
The method of manufacturing an optical modulator, wherein the first semiconductor film has a higher resistivity than the first upper cladding layer or a conductivity type opposite to a conductivity type of the first upper cladding layer. Furthermore, before the step of growing the first semiconductor film, there is a step of growing a second upper cladding layer having the conductivity type of the first upper cladding layer on the non-doped layer or the semi-insulating layer. And
5. The optical modulator according to claim 4, wherein the first semiconductor film has a higher resistivity than the second upper cladding layer or a conductivity type opposite to a conductivity type of the second upper cladding layer. 6. Production method. Growing a non-doped layer including a core layer or a semi-insulating layer including the core layer on a lower clad layer having a semiconductor of the first conductivity type;
A laminated film including a second semiconductor film that can be selectively etched with respect to the second lower layer in contact with the lower surface and a third semiconductor film on the second semiconductor film is formed on the non-doped layer or the semi-insulating layer. A growing process;
Etching the third semiconductor film in a part of the stacked film and further selectively etching the second semiconductor film with respect to the second lower layer;
Growing a first upper cladding layer having the second conductivity type opposite to the first conductivity type or the first conductivity type on the laminated film and the second lower layer partially etched;
Forming a first electrode connected to the lower cladding layer and a second electrode disposed above the part via the first upper cladding layer;
The method of manufacturing an optical modulator, wherein the third semiconductor film has a higher resistivity than the first upper cladding layer or a conductivity type opposite to a conductivity type of the first upper cladding layer. Furthermore, before the step of growing the laminated film, the step of growing a second upper cladding layer having a conductivity type of the first upper cladding layer on the non-doped layer or the semi-insulating layer,
The optical modulator according to claim 6, wherein the third semiconductor film has a resistivity higher than that of the second upper cladding layer or a conductivity type opposite to a conductivity type of the second upper cladding layer. Production method.

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