Semiconductor optical modulator

Abstract

The present disclosure relates to a semiconductor optical modulator (1) configured by laminating a plurality of semiconductor layers including an optical modulation layer (for example, a multi-quantum well optical modulation layer (5)) on a semiconductor substrate (InP substrate 2), the semiconductor optical modulator (1) modulating the intensity or phase of light incident on the optical modulation layer and emitting the light. The light modulation layer is configured using a digital alloy in which semiconductor layers having a layer thickness of 2 atomic layers or more and having different constituent elements or composition ratios are stacked alternately and repeatedly.

Description

Technical Field

[0001] The present application relates to semiconductor light modulators. Background Art

[0002] The progress of digital transformation using digital information has brought about a significant development of communication networks that exchange digital information and data centers that store and process data. Communication networks and communications within data centers use optical communications, and the speed and capacity have made significant progress in recent years. Among them, semiconductor optical modulators such as electric field absorption (EA: Electro-Absorption) modulators and Mach-Zehnder (MZ: Mach-Zehnder) modulators with excellent high-speed performance are used on the transmission side of optical communications (for example, refer to Japanese Patent Document 1.).

[0003] The EA modulator modulates the intensity of light by extinction (absorption) or transmission of the laser light emitted from a semiconductor laser (LD) corresponding to the digital signals 0 and 1. The laser modulated by the EA modulator can be modulated at high speed compared to the direct current modulation of the LD, and the wavelength spectrum expansion during light modulation is small, so it can be transmitted over long distances. In recent years, it has become the most important optical device in high-speed communications above 25Gbit / sec. In addition, the MZ modulator is used for multi-value modulation such as digital coherent communication, and the transmitting light source for long-distance communication because it can perform phase modulation.

[0004] Here, for intensity modulators represented by EA modulators or phase modulators represented by MZ modulators, multi-quantum well layers (MQW: multi quantum well) are mainly used in the layer (light modulation layer) that modulates the intensity or phase of light by changing the absorption coefficient or refractive index of light. Quantum well refers to a structure in which a semiconductor layer (quantum well layer) with a small band gap is sandwiched by a barrier layer with a larger band gap than the quantum well layer. A multi-quantum well layer is formed by stacking multiple quantum wells. In the quantum well layer, the energy levels of holes and electrons are discretely formed, and holes and electrons attract each other through Coulomb force to form excitons (excitons). The energy difference ΔE between the energy levels of holes and electrons is smaller than before the excitons are formed.

[0005] The energy level of heavy holes and the energy level of light holes are formed in the holes. When voltage is applied, electrons and holes move to the low energy side and the high energy side in the quantum well layer, respectively. Therefore, when voltage is applied, the light absorption energy based on excitons becomes smaller than when no voltage is applied, and the light absorption edge wavelength shifts to the long wavelength side. This is called the quantum confined Stark effect, which modulates light by using the light absorption coefficient or the change in refractive index caused by the shift of the absorption edge wavelength (for example, refer to non-patent document 1.). The EA modulator is used to make MQW function as a light modulation layer to change the light absorption coefficient, and the MZ modulator is used to make MQW function as a light modulation layer to change the refractive index.

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 3-290614

[0007] Non-patent document 1: JOURNALOF LIGHTWAVE TECHNOLOGY, (USA), 1988, VOL 6, NO 6, pp. 743

[0008] However, a single semiconductor layer, i.e., bulk (random alloy), is usually used in the quantum well layer of the above-mentioned light modulation layer. Therefore, near the absorption edge wavelength, in addition to the exciton absorption of heavy holes, the exciton absorption of light holes and the electrical interaction between heavy holes and light holes occur, so that the wavelength half-value width of the exciton absorption becomes wider. As a result, there are problems such as the loss near the absorption edge wavelength becomes larger, the change of the absorption coefficient or refractive index becomes smaller, and the propagation loss of light increases. Summary of the invention

[0009] The present application discloses a technology for solving the above-mentioned problems, and its object is to obtain a semiconductor light modulator that increases the change in absorption coefficient or refractive index and reduces the propagation loss of light.

[0010] The semiconductor optical modulator disclosed in the present application is constructed by stacking a plurality of semiconductor layers including an optical modulation layer on a semiconductor substrate, modulating the intensity or phase of light incident on the optical modulation layer and emitting the light, and is characterized in that the optical modulation layer is constructed using a digital alloy formed by alternately and repeatedly stacking semiconductor layers having a layer thickness of more than 2 atomic layers and different constituent elements or composition ratios.

[0011] According to the semiconductor light modulator disclosed in the present application, by using digital alloy in the light modulation layer, the half-value width of the wavelength of exciton absorption can be narrowed, thereby increasing the change in absorption coefficient or refractive index and reducing light propagation loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1Aand Figure 1B 1 and 2 are respectively a schematic cross-sectional view showing the structure of the semiconductor optical modulator according to Embodiment 1 and a cross-sectional view enlarging a portion of a multi-quantum well optical modulation layer.

[0013] Figure 2A and Figure 2B These are respectively an energy band diagram of a well layer constituting the semiconductor optical modulator according to the first embodiment and a line graph showing the wavelength dependence of the light absorption coefficient and the refractive index change amount.

[0014] Figure 3A and Figure 3B These are respectively an energy band diagram of a well layer constituting a semiconductor light modulator according to a comparative example, and a line graph showing wavelength dependence of a light absorption coefficient and a refractive index change amount.

[0015] Figure 4 This is an enlarged cross-sectional view of a portion of a multi-quantum well optical modulation layer of a semiconductor optical modulator according to the second embodiment.

[0016] Figure 5A and Figure 5B These are respectively an energy band diagram of a well layer constituting the semiconductor optical modulator according to the second embodiment and a line graph showing the wavelength dependence of the light absorption coefficient.

[0017] Figure 6 This is an enlarged cross-sectional view of a portion of a multi-quantum well optical modulation layer of a semiconductor optical modulator according to the third embodiment.

[0018] Fig. 7A and Figure 7B These are respectively an energy band diagram of a well layer constituting the semiconductor optical modulator according to the third embodiment and a line graph showing the wavelength dependency of the light absorption coefficient.

[0019] Figure 8 This is an enlarged cross-sectional view of a portion of a multi-quantum well optical modulation layer of a semiconductor optical modulator according to the fourth embodiment. DETAILED DESCRIPTION

[0020] Implementation Method 1

[0021] Figure 1A and Figure 1B as well as Figure 2A and Figure 2B is a diagram for explaining the structure of a semiconductor optical modulator according to Embodiment 1. Figure 1A is a schematic cross-sectional view showing the structure of an EA modulator as a semiconductor optical modulator, Figure 1B This is a cross-sectional view of a portion of a multi-quantum well optical modulation layer in the layers constituting the semiconductor optical modulator. Figure 2AThe energy band diagram of the quantum well layer constituting the multi-quantum well optical modulation layer of the semiconductor optical modulator when no voltage is applied and when voltage is applied. Figure 2B It is a line graph showing the wavelength dependence of the light absorption coefficient and the refractive index change.

[0022] in addition, Figure 3A and Figure 3B is a diagram for explaining the properties of a semiconductor optical modulator having a multi-quantum well optical modulation layer of a general structure as a comparative example, Figure 3A The energy band diagram of the quantum well layer constituting the multi-quantum well optical modulation layer of the semiconductor optical modulator when no voltage is applied and when voltage is applied. Figure 3B It is a line graph showing the wavelength dependence of the light absorption coefficient and the refractive index change.

[0023] The semiconductor optical modulator of the present application is an EA modulator or an MZ modulator formed by stacking a plurality of semiconductor layers on a substrate in a manner of having a multi-quantum well optical modulation layer, and is characterized in that a digital alloy is applied to the multi-quantum well optical modulation layer. The definition of digital alloy will be described later, and the basic structure and operation characteristics of the semiconductor optical modulator will be described.

[0024] The semiconductor optical modulator according to the first embodiment can be formed by applying a general method for forming a semiconductor layer, such as metal organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE).

[0025] use Figure 1A The case where an EA modulator is formed as the semiconductor optical modulator 1 is described. Using the MOVPE method or the MBE method, a 0.1 to 1 μm thick semiconductor optical modulator having a carrier concentration of about 1 to 5×10 18 cm ―3 An n-type cladding layer 3 (or buffer layer) such as n-type InP or AlInAs.

[0026] After growing an n-type light confinement layer 4 composed of a single layer or stacked layers of InGaAsP, InAlAs, InGaAlAs, etc., a multi-quantum well light modulation layer 5 is formed. The multi-quantum well light modulation layer 5 is a multi-quantum well layer that functions as a light modulation layer. Growing a p-type light confinement layer 6 composed of a single layer or stacked layers of InGaAsP, InAlAs, InGaAlAs, etc., with a carrier concentration of about 1 to 5×10 18 cm ―3Then, an n-electrode 8N is formed on the n-type InP substrate 2 side, and a p-type InGaAs contact layer and a p-electrode 8P (not shown) are formed on the p-type cladding layer 7 side.

[0027] Here, in view of the increase in the half-value width of the wavelength of exciton absorption described in the background art, the inventors of the present application focused on preventing the expansion of the energy level of holes in the quantum well layer or the generation of multiple energy levels in order to narrow the half-value width of the wavelength. Therefore, for example, considering that it is sufficient to limit the energy level of the valence band or conduction band of the material constituting the quantum well layer itself, materials that can limit the energy level through a micro gap were studied, and as a result, a material called digital alloy was discovered.

[0028] Therefore, in the semiconductor optical modulator 1 according to the first embodiment, Figure 1B As shown, in the multi-quantum well optical modulation layer 5, a digital alloy is used for the plurality of quantum well layers 51 sandwiching the barrier layer 52. Specifically, an i-type InAl2O3 having a layer thickness of 2 atomic layers and a composition ratio of z is used. z Ga (1－z) The first composition layer 511 composed of As and the i-type InAl with a layer thickness of 2 atomic layers and a composition ratio of x x Ga (1－x) The second composition layer 512 composed of As is alternately grown and stacked to a total thickness of several nm. The composition ratio z is different from the composition ratio x, and the average composition ratio of the two (=(z+x) / 2) is set to be approximately the same as that of InAlGaAs of a general random alloy.

[0029] The barrier layers 52 each have a thickness of several nanometers and are formed of i-type InAlAs or InAlGaAs having a larger band gap than the average composition ratio of InAlGaAs in the quantum well layer 51. The overall thickness of the multi-quantum well optical modulation layer 5 is approximately 0.1 μm. The width and composition of the quantum well layer 51 are set so that a wavelength several to several tens of nanometers shorter than the wavelength of the modulated light becomes a light absorption edge.

[0030] Here, the multi-quantum well optical modulation layer 5 is recorded as i-type, but even if it is p-type or n-type, as long as the carrier concentration is low and an electric field is applied to a portion of the multi-quantum well optical modulation layer 5. In addition, in order to be able to exert the effect without the microgap becoming too large, a thickness of about 2 to 6 atomic layers is preferred. Furthermore, in order to have a margin relative to the critical film thickness, a thickness of 2 to 4 atomic layers is further preferred, at which crystal dislocations caused by the lattice mismatch of each layer are difficult to produce and the microgap does not affect the energy level of heavy holes. In addition, in order to have the most margin relative to the critical film thickness, 2 atomic layers are most preferred, which is the minimum thickness that does not produce dislocation proliferation and produces microgaps during long-term operation.

[0031] Based on the above structure, the characteristics of the semiconductor optical modulator 1 (Example 1) of the present application and the characteristics of the semiconductor optical modulator involved in the comparison example in which the quantum well layer is composed of a random alloy are described using the energy band diagram of the quantum well layer, the light absorption coefficient α, and the wavelength dependence of the refractive index change Δn.

[0032] The quantum energy level ΔEn measured from the energy band bottom can be roughly expressed by equation (1).

[0033] ΔEn≒((h·h)) / (2m ＊ ))·((π·n) / Lz)(1)

[0034] Here, h is Planck's constant, m ＊ is the effective mass of a hole or electron, n is a positive integer corresponding to an energy level, and Lz is the effective quantum well width.

[0035] like Figure 3A As shown, in the valence band of the quantum well layer 51R formed of the random alloy of the semiconductor light modulator according to the comparative example, a valence band based on the effective mass m is formed. ＊ The quantum energy levels of the heavy hole (solid line) and the effective mass m ＊ Quantum energy level of light holes (dashed line). Since each hole is attracted to an electron by Coulomb force to form an exciton, the actual quantum energy level ΔEn is slightly smaller than the value calculated by equation (1).

[0036] On the other hand, in the semiconductor optical modulator 1 of the first embodiment using the digital alloy layer for the quantum well layer 51, as shown in FIG. Figure 2A As shown in FIG. 1 , since a micro gap is generated in the quantum well layer 51, the quantum energy level based on the light hole cannot be formed or becomes weak. As a result, in the comparative example, Figure 3B As shown in FIG. 1 , two absorption peaks due to excitons appear in the wavelength dependence of the light absorption coefficient α. In contrast, in Example 1, as shown in FIG. Figure 2B As shown in the figure, it becomes one absorption peak, or even if it is two peaks, the peak based on light holes becomes smaller. nv represents the light absorption coefficient in the absence of voltage, α av Indicates the light absorption coefficient when voltage is applied. As a result, the half-value width of the wavelength of exciton absorption becomes narrower.

[0037] In the EA modulator, light with incident wavelength longer than the absorption peak wavelength of the exciton is modulated. When voltage is applied to the quantum well layer, the absorption peak of the exciton shifts to the long wavelength side and absorbs the incident light due to the quantum confinement Stark effect described in the background technology. Figure 2BAs shown in the change amount of light absorption coefficient α (absorption coefficient change amount Δα), if the wavelength half-value width of exciton absorption is narrow, the absorption coefficient change amount Δα with or without applied voltage is compared with the comparative example ( Figure 3B ) becomes larger. As a result, even if the voltage applied to the EA modulator is low, a sufficient change in the light absorption coefficient α can be obtained. In addition, since the wavelength half-value width of exciton absorption is narrow, Figure 2B The loss L shown EA Thus, light absorption in a state where no voltage is applied becomes smaller.

[0038] In the MZ modulator, the incident wavelength is longer than that of the EA modulator, so that the light phase is modulated without much absorption, forming a light modulation layer. The phase of light changes because the refractive index changes (refractive index change Δn) according to the Kramers-Kronig relationship as the absorption spectrum of light changes. Therefore, the greater the absorption coefficient change Δα, the greater the refractive index change Δn. Figure 2B , Figure 3B The dotted line represents the curve of the refractive index change Δn. Figure 3B ) compared with Example 1 ( Figure 2B ) has a large absorption coefficient change Δα, so the refractive index change Δn also becomes large, and even if the voltage applied to the MZ modulator is low, a sufficient refractive index change Δn can be obtained. In addition, since the wavelength half-value width of exciton absorption is narrow, Figure 2B The loss L shown MZ In this way, light absorption in a state where no voltage is applied becomes smaller. This can also be applied to phase modulators other than MZ type modulators.

[0039] That is, in the digital alloy, a microgap is formed in the valence band or the conduction band, and the quantum energy level is limited. Since the digital alloy is used for the quantum well layer 51 instead of the usual random alloy, the energy level width of the exciton formed in the quantum well layer 51 is limited, and the half-value width of the wavelength of the exciton absorption is narrowed. In addition, in the quantum well layer 51R (comparative example) formed by the random alloy, in addition to the exciton absorption of the heavy holes, the exciton absorption of the light holes is also generated, and the wavelength spectrum of the exciton absorption becomes two peaks, and the half-value width of the wavelength of the exciton absorption is widened, but in the digital alloy (embodiment), by setting it to one peak, the half-value width of the wavelength can be narrowed.

[0040] As described above, the definition of the digital alloy used in the semiconductor light modulator 1 of the present application, which produces microgaps and exhibits characteristics different from those of random alloys, is described using the easily explained ternary InAlAs as a specific example. Conventional InAlAs is a material in which AlIn is randomly arranged while maintaining a certain composition ratio of aluminum (Al), indium (In), and arsenic (As), and is also called a bulk or random alloy. On the other hand, a material in which binary AlAs and InAs are alternately stacked with a thickness of several atomic layers (2 to 6 atomic layers) to form InAlAs is called a digital alloy (for example, refer to Digital Alloy: JOURNAL OF LIGHTWAVE TECHNOLOGY, (USA), 2018, VOL.36, NO.17, pp.3580-3585.).

[0041] In other words, digital alloy refers to a substance formed by alternately stacking multiple semiconductors with different constituent elements or composition ratios in a layer thickness of several atomic layers, which is the same as substances called "pseudoalloy (for example, refer to the specification of U.S. Patent No. 6326650)" and "ultra-short period superlattice". Among them, although there are general multi-quantum well layers or superlattices in similar structures, they are essentially different from digital alloys because they maintain a certain composition ratio in each layer while the elements are randomly arranged.

[0042] Since digital alloys are stacked in layers of several atomic layers, unlike general multi-quantum well layers or superlattices, they do not exhibit the properties of each layer of AlAs and InAs, and have energy bands close to those of bulk InAlAs with an averaged composition ratio. However, in digital alloys, since the elements are stacked regularly and have atomic periodicity that random alloys do not have, they have the characteristic of forming microgaps in the valence band or conduction band. In other words, digital alloys are epitaxial layer structures formed by alternately stacking materials by selecting materials in a way that produces microgaps and designing and adjusting the stacking period in units of several atomic layers. In the semiconductor optical modulator 1 of the present application, it is characterized by utilizing the microgaps unique to digital alloys.

[0043] The digital alloy is not limited to the stacking of binary AlAs and InAs as described above, but may also be a material in which InAs and GaAs are stacked in binary form. InAlGaAs, which is represented by stacking ternary InAlAs and InGaAs alternately in several atomic layers, may be a material in which the quaternary InAlAs shown in Embodiment 1 is stacked. z Ga (1－z) As and InAl x Ga (1－x)As and other materials are stacked in several atomic layers alternately. In addition, it is also possible to stack binary (AlAs) and ternary (InAl z Ga (1－z) As) stacked InAlGaAs, the ternary (In z Ga (1－z) As) and quaternary (InAl x Ga (1－x) As) stacked material.

[0044] Furthermore, it is also possible to combine binary (GaAs, InAs, etc.) and ternary (In z Ga (1－z) As) stacked InGaAs, binary (InAs, GaAs, or AlAs) and quaternary (InAl x Ga (1－x) Here, the Al of the above materials may be replaced by phosphorus (P). In addition, it may be a material system (InAlAsSb) to which antimony (Sb) is added. Furthermore, it may be a combination of materials such as quinary InAlGaAsP formed by stacking quaternary InAlGaAs and InGaAsP, which is selected in a way to produce the micro gap described later. Here, although there is a concern about the problem of crystal strain, since each layer is several atomic layers thick, even if the lattice mismatch of each layer is plus or minus several percent, as long as the cumulative lattice mismatch is small, crystal growth can be performed.

[0045] Thus, in the semiconductor optical modulator 1 involved in Embodiment 1, a digital alloy structure is applied to the quantum well layer 51, that is, a structure in which the stacking period is set in units of several atomic layers is applied, thereby generating a micro gap to suppress the absorption based on light holes. As a result, compared with a general EA modulator or MZ modulator using a random alloy, the operating voltage can be reduced, and the propagation loss of light can also be reduced.

[0046] In addition, the differences between the materials similar to the digital alloy defined in the present application and the differences in characteristics are additionally described. For example, in Patent Document 1, a scheme is shown in which InAs and GaAs are alternately stacked in 1 atomic layer to form a quantum well layer to reduce the energy fluctuation of excitons. However, in the quantum well layer formed by alternately stacking 1 atomic layer, the energy band structure is substantially the same as that of a random alloy, and the micro gaps like those in the digital alloy of the present application do not appear.

[0047] When the thickness (number of atoms) of each layer is set to the range of 2 atoms to 8 atoms, the width of the microgap becomes larger. If the microgap does not appear, the exciton absorption of light holes cannot be suppressed, so the wavelength spectrum becomes two peaks, and the half-value width of the wavelength of the exciton absorption cannot be narrowed. In other words, the case where the thickness of each layer is 1 atom is different from the digital alloy defined in this application.

[0048] In addition, a structure example of a single quantum well composed of a short-period superlattice array whose quantum confinement potential is an equivalent quadratic curve is disclosed (for example, refer to Japanese Patent Publication No. 04-250428). However, in such a structure with gradually changing potential, microgaps with a certain energy level, such as the digital alloy of the present application, do not appear.

[0049] The total thickness of the layers repeated at the same period in order to make microgaps appear needs to be several nanometers, and the period needs to change in stages rather than gradually. If microgaps do not appear, the exciton absorption of light holes cannot be suppressed, so the wavelength spectrum becomes two peaks, and the wavelength half-value width of exciton absorption cannot be narrowed.

[0050] In addition, a method is disclosed in which a 3-atomic-layer thick Al layer is provided on both sides of an individual quantum well of a rectangular potential formed by a GaAs layer with a thickness of 38 atomic layers in a basic unit quantum well. 0.3 Ga 0.7 The structure of a pair of quantum barrier layers formed by As layers (for example, refer to Japanese Patent Publication No. 07-261133). However, the layer with a thickness of 3 atomic layers only exists on both sides, and it is not a repeated structure. Therefore, unlike the digital alloy of the present application, there is no micro gap, and the exciton absorption of light holes cannot be suppressed.

[0051] Furthermore, a structure in which the amount of strain changes gradually within the well layer can also be considered, but in this case, it is the same as the structure in which the potential changes gradually, and is not a periodically repeated structure, so no microgaps appear, which is a different structure from the digital alloy of the present application.

[0052] Implementation Method 2

[0053] In Embodiment 1, an example is described in which each quantum well layer is composed entirely of digital alloys. In Embodiment 2, an example is described in which each quantum well layer is composed of two layers composed of a digital alloy layer and a random alloy layer.

[0054] Figure 4 ,as well as Figure 5A and Figure 5B is a diagram for explaining the structure of a semiconductor optical modulator according to Embodiment 2. Figure 4 This is a cross-sectional view of a portion of a multi-quantum well optical modulation layer in the layers constituting the semiconductor optical modulator. Figure 5A The energy band diagram of the quantum well layer constituting the multi-quantum well optical modulation layer of the semiconductor optical modulator when no voltage is applied and when voltage is applied. Figure 5B It is a line graph showing the wavelength dependence of the light absorption coefficient. However, the same reference numerals are given to the same parts as those in Embodiment 1, and the description of the same parts is omitted.

[0055] The semiconductor optical modulator 1 as the EA modulator according to the second embodiment has a structure similar to that of the semiconductor optical modulator 1 described in the first embodiment ( Figure 1A ) are substantially the same, but the structure of the multi-quantum well optical modulation layer 5 that absorbs light is different. In the semiconductor optical modulator 1 involved in the second embodiment, Figure 4 As shown, the quantum well layer 51 is not entirely a digital alloy, but a double-layer structure of a digital alloy layer 51a and a random alloy layer 51b.

[0056] The digital alloy layer 51a is composed of i-type InAl with a layer thickness of 2 atomic layers and a composition ratio of z. z Ga (1－z) The first composition layer 511 composed of As and the i-type InAl with a layer thickness of 2 atomic layers and a composition ratio of x x Ga (1－x) The second component layer 512 composed of As is grown alternately to have a thickness of about half the thickness of the well layer (about 2 to 7 nm). y Ga (1－y) The composition ratio z is different from the composition ratio x, and the average composition ratio (=(z+x) / 2) of the digital alloy layer 51a is set to be the same as that of the random alloy layer 51b (InAl y Ga (1－y) ) part is roughly the same. That is, (z+x) / 2≒y.

[0057] As in the first embodiment, the barrier layers 52 each have a thickness of several nanometers and are formed of i-type InAlAs or InAlGaAs having a larger band gap than the average composition ratio of InAlGaAs of the quantum well layer 51. The thickness of the multi-quantum well optical modulation layer 5 is about 0.1 μm. The width and composition of the quantum well layer 51 are set so that a wavelength shorter than the wavelength of the modulated light by several nanometers to several tens of nanometers becomes a light absorption edge.

[0058] In the second embodiment, the multi-quantum well optical modulation layer 5 is also described as i-type, but even if it is p-type or n-type, as long as the carrier concentration is low, an electric field can be applied to a part of the multi-quantum well optical modulation layer 5. In addition, each layer in the digital alloy layer 51a (the first component layer 511 and the second component layer 512) is set to 2 atomic layers, but the thickness can be about 2 to 8 atomic layers.

[0059] In addition, in order to achieve the effect without excessively increasing the microgap, a thickness of about 2 to 6 atomic layers is preferred. Furthermore, in order to have a margin relative to the critical film thickness, a thickness of 2 to 4 atomic layers is further preferred, at which crystal dislocations caused by the lattice mismatch of each layer are not easily generated and the microgap does not affect the energy level of heavy holes. Furthermore, in order to have the most margin relative to the critical film thickness, 2 atomic layers are most preferred, which is the minimum thickness for generating microgaps without generating dislocation proliferation during long-term operation.

[0060] Even if the semiconductor light modulator 1 (Example 2) has a double-layer structure consisting of the digital alloy layer 51a and the random alloy layer 51b as the quantum well layer 51, Figure 5A As shown, micro gaps are also generated in the quantum well layer 51. Therefore, quantum levels based on light holes cannot be formed or become weak.

[0061] Furthermore, the energy level of heavy holes is biased toward the random alloy layer 51b in the quantum well layer 51 as in the case of no voltage in the energy band diagram. When a voltage is applied so that the digital alloy layer 51a side becomes negative, the energy level of heavy holes is biased toward the digital alloy layer 51a as in the case of voltage in the energy band diagram. Since there is a micro gap in the digital alloy layer 51a part, the energy level of heavy holes is biased toward the digital alloy layer 51a side as in the case of voltage in the energy band diagram. Figure 3B ), the energy level energy is shifted upward (to the conduction band side). That is, the energy shift amount ΔEh of heavy holes caused by the applied voltage is larger than that of the comparative example.

[0062] On the other hand, the amount of electron energy shift ΔEe caused by the applied voltage does not change. As a result, Figure 5B As shown in FIG. 1 , in the wavelength dependency of the light absorption coefficient α, the shift amount ΔE (=ΔEh+ΔEe) of the peak wavelength of exciton absorption caused by heavy holes is larger than that of the comparative example.

[0063] In the EA modulator, light with an incident wavelength longer than the peak absorption wavelength of the exciton is modulated. When a voltage is applied to the quantum well layer, the absorption peak of the exciton shifts to the long wavelength side to absorb the incident light due to the quantum confined Stark effect described in the background technology. If the shift ΔE of the peak wavelength absorbed by the exciton is large, the change Δα in the absorption coefficient becomes larger. As a result, even if the voltage applied to the EA modulator is low, a sufficient change in the absorption coefficient can be obtained. In addition, if the shift ΔE of the peak wavelength absorbed by the exciton is large, the wavelength of the light incident on the EA modulator can be set longer than before, so Figure 5B As shown in the figure, the light absorption becomes smaller when no voltage is applied.

[0064] In the MZ modulator, the incident wavelength is longer than the wavelength of the EA modulator, and the phase of the light is modulated without much absorption. The reason why the phase of the light changes is that as the absorption spectrum of the light changes, the refractive index changes by Δn according to the Kramers-Kronig relationship. Therefore, the greater the absorption coefficient change Δα, the greater the refractive index change Δn. Figure 5B ) compared to the comparative example ( Figure 3B ) The absorption coefficient change Δα is large, so the refractive index change Δn also becomes large, and even if the voltage applied to the MZ modulator is low, a sufficient refractive index change Δn can be obtained. In addition, similar to the EA modulator, if the shift ΔE of the peak wavelength of exciton absorption is large, the wavelength of light incident on the MZ modulator can be set longer than that of the comparative example, so that the light absorption in the state where no voltage is applied becomes smaller.

[0065] Thus, compared with the EA modulator or MZ modulator involved in the comparative example in which the quantum well layer 51R is composed only of random alloys, or compared with the embodiment 1 in which the quantum well layer 51 is composed entirely of digital alloys, in the embodiment 2, the operating voltage can be reduced by the above-mentioned action, and the propagation loss of light can also be reduced. In addition, in the epitaxial growth of the digital alloy, it is necessary to open and close the shutter of the epitaxial device and switch the gas every several atomic layers, which leads to a longer epitaxial growth time. However, as in the present embodiment 2, since only a part of the quantum well layer 51 is set as the digital alloy layer 51a, the epitaxial growth time can be greatly reduced compared with the embodiment 1 in which the quantum well layer 51 is composed entirely of digital alloys. In addition, the consumption of the epitaxial growth device can also be suppressed.

[0066] Implementation 3

[0067] In Embodiment 1 or 2, an example in which the first component layer and the second component layer in the digital alloy layer are stacked with the same repeating thickness is described. In this Embodiment 3, an example in which a quantum well layer is composed of two types of digital alloy layers with different repeating thicknesses is described.

[0068] Figure 6 ,as well as Fig. 7A and Figure 7B is a diagram for explaining the structure of a semiconductor optical modulator according to Embodiment 3. Figure 6 This is a cross-sectional view of a portion of a multi-quantum well optical modulation layer in the layers constituting the semiconductor optical modulator. Fig. 7A The energy band diagram of the quantum well layer constituting the multi-quantum well optical modulation layer of the semiconductor optical modulator when no voltage is applied and when voltage is applied, Figure 7B It is a line graph showing the wavelength dependence of the light absorption coefficient. However, the same reference numerals are given to the same parts as those in Embodiment 1, and the description of the same parts is omitted.

[0069] The semiconductor optical modulator 1 as the EA modulator according to the third embodiment is also similar to the semiconductor optical modulator 1 described in the first embodiment ( Figure 1A ) are roughly the same, but the structure of the multi-quantum well optical modulation layer 5 that absorbs light is different. Figure 6 As shown, in the semiconductor optical modulator 1 according to the third embodiment, the quantum well layer 51 is entirely composed of digital alloys, but has a two-layer structure of a long period layer 51 c having a long thickness, ie, a period, and a short period layer 51 d having a short period.

[0070] The long period layer 51c is composed of i-type InAl with a layer thickness of 4 atomic layers and a composition ratio of z. z Ga (1－z) The first composition layer 511 composed of As and the i-type InAl with a layer thickness of 4 atomic layers and a composition ratio of x x Ga (1－x) The second composition layer 512 composed of As is alternately grown to have an 8-atom period (repeating thickness). Then, the short-period layer 51d is composed of i-type InAl with a layer thickness of 2 atomic layers and a composition ratio of z. z Ga (1－z) The first composition layer 511 is composed of As and the i-type InAl with a layer thickness of 2 atomic layers and a composition ratio of x. x Ga (1－x) The second composition layer 512 composed of As is grown alternately to form a 4-atomic period (repeating thickness). The thickness of the long period layer 51c of 8 atomic periods is about half the thickness of the well layer (about 2 to 7 nm), and the remaining thickness is formed by the short period layer 51d of 4 atomic periods.

[0071] The composition ratio z of the first composition layer 511 is different from the composition ratio x of the second composition layer 512 , and the average composition ratio (=(z+x) / 2) of the digital alloy is set to be substantially the same as when the quantum well layer is composed of a random alloy.

[0072] As in the first embodiment, the barrier layers 52 each have a thickness of several nanometers and are formed of i-type InAlAs or InAlGaAs having a larger band gap than the average composition ratio of InAlGaAs of the quantum well layer 51. The thickness of the multi-quantum well optical modulation layer 5 is about 0.1 μm. The width and composition of the quantum well layer 51 are set so that a wavelength shorter than the wavelength of the modulated light by several nanometers to several tens of nanometers becomes a light absorption edge.

[0073] In the third embodiment, the multi-quantum well optical modulation layer 5 is also described as i-type, but even if it is p-type or n-type, as long as the carrier concentration is low and an electric field is applied to a portion of the multi-quantum well optical modulation layer 5, it will suffice. In addition, each layer (first component layer 511, second component layer 512) in the long period layer 51c and the short period layer 51d is set to 2 atomic layers and 4 atomic layers, respectively, but it is sufficient to select a combination within the thickness range of about 2 to 8 atomic layers. In addition, in order to further exert the effect, it is preferably distributed within the thickness range of about 2 to 6 atomic layers, and it is more preferably distributed within the thickness range of 2 to 4 atomic layers.

[0074] The repetition thickness of each layer in the long period layer 51c and the short period layer 51d can also be gradually changed in the quantum well layer 51, such as a 2-atomic layer period, a 4-atomic layer period, a 6-atomic layer period, etc., but since a micro gap needs to be formed, each repetition thickness needs to be at least 1 to 2 nm in total. In addition, in order to reliably form a micro gap, the repetition structure of the same period needs a thickness of about several nm. In addition, the period needs to be changed in stages.

[0075] Even if the semiconductor optical modulator 1 (Example 3) uses a double-layer structure consisting of a long-period layer 51c and a short-period layer 51d of a digital alloy as the quantum well layer 51, Fig. 7A As shown, micro gaps are also generated in the quantum well layer 51. Therefore, quantum levels based on light holes cannot be formed or become weak.

[0076] In the quantum well layer 51, the microgap width of the short period portion (short period layer 51d) is narrower than the microgap width of the long period portion (long period layer 51c).

[0077] Furthermore, the energy level of heavy holes is biased toward the short period layer 51d with a short period, as in the case of no voltage in the energy band diagram. If a voltage is applied so that the long period layer 51c with a long period becomes negative, the energy level of heavy holes is biased toward the long period layer 51c, as in the case of voltage in the energy band diagram. Since the micro gap on the side with a long period is wide, the micro gap is larger than that in the comparative example ( Figure 3B ), the energy level energy is shifted upward (to the conduction band side). That is, the energy shift amount ΔEh of heavy holes caused by the applied voltage is larger than that of the comparative example.

[0078] The results, such as Figure 7B As shown in FIG. 1 , in the wavelength dependency of the light absorption coefficient α, the shift amount ΔE of the peak wavelength due to the exciton absorption by heavy holes becomes larger.

[0079] In the EA modulator, light with an incident wavelength longer than the peak absorption wavelength of the exciton is modulated. When a voltage is applied to the quantum well layer, the absorption peak of the exciton shifts to the long wavelength side to absorb the incident light due to the quantum confined Stark effect described in the background technology. If the shift ΔE of the peak wavelength absorbed by the exciton is large, the change Δα in the absorption coefficient becomes larger. As a result, even if the voltage applied to the EA modulator is low, a sufficient change in the absorption coefficient can be obtained. In addition, if the shift ΔE of the peak wavelength absorbed by the exciton is large, the wavelength of the light incident on the EA modulator can be set longer than before. Therefore, if Figure 7B As shown in the figure, the light absorption becomes smaller when no voltage is applied.

[0080] In the MZ modulator, the incident wavelength is longer than the wavelength of the EA modulator, and the phase of the light is modulated without much absorption. The phase of the light changes because the refractive index changes Δn according to the Kramers-Kronig relationship as the absorption spectrum of the light changes. Therefore, the greater the absorption coefficient change Δα, the greater the refractive index change Δn. Figure 3B ) compared with Example 3 ( Figure 7B ) has a large absorption coefficient change Δα, so the refractive index change Δn also becomes large, and even if the voltage applied to the MZ modulator is low, a sufficient refractive index change Δn can be obtained. In addition, similar to the EA modulator, if the shift ΔE of the peak wavelength of exciton absorption is large, the wavelength of light incident on the MZ modulator can be set longer than that of the comparative example, so that the light absorption in the state where no voltage is applied becomes smaller.

[0081] Thus, compared with the EA modulator or MZ modulator involved in the comparative example in which the quantum well layer 51R is composed of only random alloys, in Example 3, the operating voltage can be reduced by the above-mentioned action, and the propagation loss of light can also be reduced. Moreover, compared with the EA modulator or MZ modulator involved in Example 1 in which the quantum well layer 51 is composed of a digital alloy uniformly formed in the quantum well layer at the repetition period of each atomic layer, the EA modulator or MZ modulator of Example 3 can reduce the operating voltage by the above-mentioned action, and can also reduce the propagation loss of light.

[0082] In addition, in digital alloys, the energy level of microgaps changes depending on the repetition period of each atomic layer. In the epitaxial growth of digital alloys, it is necessary to correctly switch the constituent elements and composition ratios every few atomic layers to alternately stack them, but in practice, unevenness or deviation occurs in the switching of constituent elements or composition ratios. When unevenness or deviation occurs, the energy level of microgaps will not appear as expected, resulting in a situation that is inconsistent with the energy level of light holes.

[0083] However, in the third embodiment, since the repetition period is changed in the quantum well layer 51, the energy level of the micro gap in any repetition period is consistent with the energy level of the light holes, which can suppress the absorption of the light holes and thus reduce the deviation of the absorption spectrum. Therefore, the characteristics are stabilized, the yield is improved, and the productivity is improved.

[0084] Implementation 4

[0085] In Embodiments 1 to 3, examples were shown in which a multi-quantum well optical modulation layer is formed by a barrier layer made of random alloy and a quantum well layer made of digital alloy. In this Embodiment 4, an example in which a digital alloy is applied to a barrier layer will be described.

[0086] Figure 8 This is a diagram for explaining the structure of a semiconductor optical modulator according to Embodiment 4, and is a cross-sectional view of an enlarged portion of a multi-quantum well optical modulation layer in the layers constituting the semiconductor optical modulator. The same reference numerals are given to the same parts as those in Embodiment 1, and descriptions of the same parts are omitted.

[0087] The semiconductor optical modulator 1 as the EA modulator according to the fourth embodiment is also similar to the semiconductor optical modulator 1 described in the first embodiment ( Figure 1A ) are roughly the same, but the structure of the multi-quantum well optical modulation layer 5 that absorbs light is different. Figure 8 As shown, in the semiconductor optical modulator 1 according to the fourth embodiment, the quantum well layer 51 may be InAlGaAs of a digital alloy or InAlGaAs of a random alloy, but the barrier layer 52 is made of a digital alloy.

[0088] The barrier layer 52 is i-type InAlAs or InAlGaAs having a larger band gap than the InAlGaAs of the quantum well layer 51. Figure 8 As shown, in the case of the digital alloy of InAlAs, a first component layer 521 composed of i-type AlAs with a layer thickness of 2 atomic layers and a second component layer 522 composed of i-type InAs with a layer thickness of 2 atomic layers are alternately stacked.

[0089] In the case of the digital alloy of InAlGaAs, the i-type InAlAs with a layer thickness of 2 atomic layers and a composition ratio of z is z Ga (1－z) The first composition layer 521 composed of As and the i-type InAl with a layer thickness of 2 atomic layers and a composition ratio of x x Ga (1－x) The second composition layer 522 composed of As is formed by alternate growth. The composition ratio z is different from the composition ratio x, and the band gap when the random alloy is formed with the average composition ratio (=(z+x) / 2) as a digital alloy is set larger than the band gap of the quantum well layer 51 .

[0090] The overall thickness of the multi-quantum well optical modulation layer 5 is approximately 0.1 μm. The width and composition of the quantum well layer 51 are set so that a wavelength that is several nm to several tens of nm shorter than the wavelength of the modulated light becomes the light absorption edge. Among them, in the present embodiment 4, the multi-quantum well optical modulation layer 5 is also recorded as i-type, but as long as an electric field is also applied to a part of the multi-quantum well optical modulation layer 5, it can be p-type or n-type. In addition, each layer of the digital alloy in the barrier layer 52 (the first component layer 521, the second component layer 522) is set to 2 atomic layers respectively, but the thickness can be about 2 to 8 atomic layers.

[0091] In addition, in order to achieve the effect without the microgap becoming too large, a thickness of about 2 to 6 atomic layers is preferred. Furthermore, in order to have a margin relative to the critical film thickness, a thickness of 2 to 4 atomic layers is further preferred, at which crystal dislocations caused by the lattice mismatch of each layer are not easily generated and the microgap does not affect the energy level of heavy holes. Furthermore, in order to have the most margin relative to the critical film thickness, 2 atomic layers are most preferred, which is the minimum thickness that does not generate dislocation proliferation and generates microgaps during long-term operation.

[0092] In the EA modulator, if dopants such as zinc (Zn) and sulfur (S) that exist in high concentration in the InGaAs contact layer or the InP substrate 2 diffuse into the multi-quantum well layer, the multi-quantum well layer is p-type or n-type. In this case, even if a voltage is applied, the electric field is not applied to each quantum well layer or it becomes weak. If no electric field is applied, the operating voltage rises, so it is important to prevent the diffusion of dopants into the multi-quantum well layer.

[0093] because Figure 8 The InAs and AlAs shown are not lattice matched with the InP substrate, so compressive strain and tensile strain are repeated in several atomic layer periods, but the total strain amount cancels out and crystal growth is possible. However, since the lattice constants of InAs and AlAs differ by more than 6%, if each layer is observed locally, a large crystal strain is applied. Since dopants such as Zn have the property of not being easy to diffuse into layers with large strain, if digital alloys are applied to the barrier layer 52, diffusion can be prevented. In addition, digital alloys can also be applied to light confinement layers (n-type light confinement layers 4, p-type light confinement layers 6), cladding layers (n-type cladding layers 3, p-type cladding layers 7) and other layers in the semiconductor layer that are closer to the multi-quantum well optical modulation layer 5 than the InGaAs contact layer or the InP substrate 2 where dopants such as zinc and sulfur exist at a high concentration, and the same effect can be obtained.

[0094] In addition, although an example of applying a superlattice layer to a barrier layer is disclosed (for example, refer to Japanese Patent Publication No. 04-088322), in this case, each layer of the superlattice layer has a different band gap. On the other hand, in the semiconductor optical modulator 1 involved in the fourth embodiment, the barrier layer 52 is made of a digital alloy formed by stacking very thin layers such as several atomic layers. Therefore, since a band gap does not appear in each layer, the band structure is different from the example of applying a superlattice layer to the barrier layer 52. In addition, since layers with a large strain amount whose lattice constant difference is several percent can be alternately stacked, the diffusion prevention effect of the dopant is large.

[0095] In addition, if microgaps exist as in the digital alloy used in the semiconductor light modulator 1 of the present application, the electron or hole confinement effect is expanded and the absorption half-value width of the exciton is narrowed. In addition, similar to the quantum well layer of the third embodiment, if the repetition period of each atomic layer of the digital alloy in the barrier layer 52 is changed, a plurality of microgaps can be formed according to the repetition period, thereby expanding the electron or hole confinement effect and increasing the effect of narrowing the absorption half-value width of the exciton.

[0096] Thus, compared with the general EA modulator or MZ modulator shown in the comparative example, in the semiconductor optical modulator 1 involved in the fourth embodiment, the diffusion of dopants can be suppressed and the operating voltage can be reduced. Since the barrier layer 52 is designed as a digital alloy structure, that is, the absorption based on light holes is suppressed by adjusting the stacking period according to several atomic layer units to generate a micro gap, the effective barrier layer height (= energy barrier height) is increased by the action of the micro gap. As a result, the confinement effect of the exciton is increased, so even if the operating temperature increases, the half-value width of the exciton does not widen, suppressing the increase of light absorption loss at high temperature and the increase of the operating voltage, and expanding the temperature range in which the EA modulator and the MZ modulator can operate.

[0097] In addition, in the embodiments of the present application, a case where a multi-quantum well layer formed by stacking a plurality of quantum well layers is used as a light modulation layer is described. In addition, when a single quantum well layer having one quantum well layer is used as a light modulation layer, it is obvious that the same effect can be obtained as in embodiments 1 to 4, and is included in the technical scope of the present application.

[0098] In addition, in each embodiment of the present application, a quantum well layer composed of a digital alloy with a thickness of about several nm to 20 nm is sandwiched by a barrier layer as a light modulation layer, but a semiconductor layer composed of a digital alloy with a thickness of about 20 nm to 500 nm that does not constitute a quantum well can also be used as a light modulation layer.

[0099] In the case of a semiconductor layer of a general random alloy, when measuring the wavelength dependence of the light absorption coefficient, the absorption edge wavelength of light equivalent to the band gap energy is used as the boundary. If the wavelength is shorter than it, the light absorption coefficient increases. On the other hand, in the case of a digital alloy, as described in Example 1, a micro gap is generated in the valence band, and the absorption coefficient at a wavelength equivalent to the energy level of the micro gap decreases. On the contrary, the light absorption coefficient near the absorption edge of the light increases. This is obvious from the sum law that the sum of the absorption coefficients is constant.

[0100] As a result, when the wavelength becomes shorter than the absorption edge wavelength, the light absorption coefficient in the digital alloy increases sharply compared to the random alloy. Therefore, even when the light modulation layer is set as a semiconductor layer with a thickness of about 20nm to 500nm instead of a quantum well layer, the effects of reducing the operating voltage and reducing the propagation loss can be obtained in the same manner as in embodiments 1 to 4, and since it is not set as a quantum well layer, it also has the effect of easy manufacturing, and is included in the technical scope of the present application.

[0101] In addition, the light modulation layer may be a single quantum well composed of a well layer using a digital alloy with a thickness of about 20nm to 500nm and a barrier layer in contact with the well layer. In this case, the above-mentioned effects of reducing the operating voltage and propagation loss, as well as the effect of a small number of quantum well layers and easy manufacturing are also achieved, and are included in the technical scope of the present application.

[0102] In addition, various illustrative embodiments and examples are described in this application, but the various features, methods, and functions described in one or more embodiments are not limited to the application of specific embodiments, and can be applied to the embodiments alone or in various combinations. Therefore, countless unillustrated variations can be imagined within the technical scope disclosed in this application specification. For example, it includes the case where at least one constituent element is deformed, added, or omitted, and also includes the case where at least one constituent element is extracted and combined with constituent elements of other embodiments. .

[0103] As described above, according to the semiconductor optical modulator 1 of the present application, a plurality of semiconductor layers including an optical modulation layer (e.g., a multi-quantum well optical modulation layer 5) are stacked on a semiconductor substrate (InP substrate 2), and the intensity or phase of light incident on the optical modulation layer (e.g., a multi-quantum well optical modulation layer 5) is modulated and emitted. The optical modulation layer (e.g., a multi-quantum well optical modulation layer 5) is composed of a digital alloy in which semiconductor layers having a layer thickness of more than 2 atomic layers and different constituent elements or composition ratios are alternately and repeatedly stacked. As a result, the half-value width of the wavelength absorbed by the exciton can be narrowed, and therefore, a semiconductor optical modulator 1 can be obtained that increases the change in the absorption coefficient or refractive index and reduces the propagation loss of light.

[0104] Here, if the semiconductor optical modulator 1 is of an electric field absorption type or a Mach-Zehnder type, a stable and highly reliable semiconductor optical modulator 1 can be surely obtained.

[0105] The optical modulation layer is a structure (multi-quantum well optical modulation layer 5) formed by alternatingly stacking quantum well layers 51 and barrier layers 52 with a band gap larger than that of the quantum well layers 51. At least one of the quantum well layers 51 and the barrier layers 52 is formed using the above-mentioned digital alloy, so that the half-width of the wavelength absorbed by the exciton can be reliably narrowed, thereby obtaining a semiconductor optical modulator 1 that increases the change in the absorption coefficient or refractive index and reduces the propagation loss of light.

[0106] At this time, if the quantum well layer 51 is formed by stacking digital alloys (digital alloy layer 51a) and random alloys (random alloy layer 51b), the epitaxial growth time can be greatly reduced compared to the case where the quantum well layer 51 is entirely composed of digital alloys, and the consumption of the epitaxial growth device can also be suppressed.

[0107] The light modulation layer may also be composed of a single quantum well, which is composed of a quantum well layer and a barrier layer, wherein the quantum well layer is composed of a digital alloy having a thickness of 20 nm or more and 500 nm or less, and the barrier layer is in contact with the quantum well layer and has a larger band gap than the quantum well layer. In this case, the half-value width of the wavelength of exciton absorption can also be reliably narrowed, thereby obtaining a semiconductor light modulator 1 that increases the change in the absorption coefficient or refractive index and reduces the propagation loss of light.

[0108] When the barrier layer 52 is formed of a digital alloy, the half-value width of the wavelength of exciton absorption can be narrowed, so that the semiconductor light modulator 1 can be obtained in which the change in the absorption coefficient or the refractive index is increased and the propagation loss of light is reduced.

[0109] Alternatively, even if the light modulation layer is composed of a digital alloy having a thickness of more than 20nm and less than 500nm, the half-value width of the wavelength of exciton absorption can be narrowed, thereby obtaining a semiconductor light modulator 1 that increases the change in absorption coefficient or refractive index and reduces light propagation loss.

[0110] If the digital alloy is composed of multiple layers (long period layer 51c, short period layer 51d) of different repeated thicknesses of semiconductor layers with different constituent elements or composition ratios, the energy level of the microgap of any period will be consistent with the energy level of the light holes, which can suppress the absorption of light holes, thereby reducing the deviation of the absorption spectrum, improving the yield and stabilizing the production volume.

[0111] If the semiconductor layers (for example, the light confinement layers (n-type light confinement layer 4, p-type light confinement layer 6), n-type cladding 3) among the multiple semiconductor layers that are closer to the light modulation layer (for example, the multi-quantum well light modulation layer 5) than the ends in the stacking direction (for example, the n-electrode 8N, the p-electrode 8P) are composed of digital alloys, the diffusion of dopants such as zinc and sulfur can be prevented.

[0112] When the layer thickness of the digital alloy is 8 atomic layers or less, the width of the micro gap that brings about the above-mentioned effect becomes larger.

[0113] Description of Reference Numerals

[0114] 1...semiconductor optical modulator; 2...InP substrate (semiconductor substrate); 3...n-type cladding layer; 4...n-type optical confinement layer; 5...multi-quantum well optical modulation layer (optical modulation layer); 51...quantum well layer; 51a...digital alloy layer; 51b...random alloy layer; 51c...long period layer (digital alloy layer); 51d...short period layer (digital alloy layer); 52...barrier layer; 6...p-type optical confinement layer; 7...p-type cladding layer; 8...electrode.

Claims

1. A semiconductor light modulator, comprising a plurality of semiconductor layers including a light modulation layer stacked on a semiconductor substrate, which modulates the intensity or phase of light incident on the light modulation layer and emits the light, characterized in that: The light modulation layer is formed using a digital alloy in which semiconductor layers having a layer thickness of two atomic layers or more and having different constituent elements or composition ratios are alternately and repeatedly stacked.

2. The semiconductor optical modulator according to claim 1, characterized in that The semiconductor optical modulator is of electric field absorption type or Mach-Zehnder type.

3. The semiconductor optical modulator according to claim 1 or 2, characterized in that: The light modulation layer is a structure in which quantum well layers and barrier layers with a band gap larger than the band gap of the quantum well layers are alternately stacked. At least one of the quantum well layer and the barrier layer is formed using the digital alloy.

4. The semiconductor optical modulator according to claim 3, characterized in that The quantum well layer is formed by stacking the digital alloy and the random alloy.

5. The semiconductor optical modulator according to any one of claims 1 to 4, characterized in that: The optical modulation layer is a single quantum well composed of a quantum well layer and a barrier layer, the quantum well layer is composed of the digital alloy having a thickness of greater than 20 nm and less than 500 nm, the barrier layer is in contact with the quantum well layer and has a band gap larger than that of the quantum well layer.

6. The semiconductor optical modulator according to any one of claims 3 to 5, characterized in that: The barrier layer is made of digital alloy.

7. The semiconductor optical modulator according to claim 1 or 2, characterized in that: The light modulation layer is the digital alloy having a thickness of 20 nm or more and 500 nm or less.

8. The semiconductor optical modulator according to any one of claims 1 to 7, characterized in that: The digital alloy is formed by stacking a plurality of layers having different repetition periods.

9. The semiconductor optical modulator according to any one of claims 1 to 8, characterized in that: Among the plurality of semiconductor layers, a semiconductor layer closer to the light modulation layer than an end portion in a stacking direction is formed of the digital alloy.

10. The semiconductor optical modulator according to any one of claims 1 to 9, characterized in that: The layer thickness of the digital alloy is less than 8 atomic layers.

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