JP3527078B2 - Light control element - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small-sized, high-speed, high-performance optical control element that performs optical modulation, optical switching, and the like.

[0002]

2. Description of the Related Art In recent years, a semiconductor multiple quantum well structure and a superlattice structure have been introduced due to the progress of compound semiconductor ultrathin film fabrication technology, and it has become possible to significantly improve the characteristics of optical devices as compared with conventional bulk semiconductors. . Among these, in optical control elements such as optical modulation and optical switches, methods that mainly use changes in absorption coefficient and refractive index change due to the electric field absorption effect are used, and optical control elements driven at high speed and low voltage are realized. ing.
In optical control, two types of physical quantities, an absorption coefficient and a refractive index, are used. Of these, the operation wavelength is set in a region where the change in the absorption coefficient is small and the change in the refractive index there is used as phase modulation. As an example of the refractive index control type element using the phase modulation, a Mach-Zehnder type modulator having a chirp-free characteristic, which uses the interference effect due to the phase of light, is manufactured. In order to realize a small size and low voltage drive in this element, it is necessary to greatly change the change in the refractive index in the modulation portion with the shortest propagation distance.

[0003]

By the way, the change in the refractive index of a semiconductor with respect to an electric field is as small as about 0.05%. Therefore, in the waveguide type refractive index control element, the element length must be increased in order to obtain a desired change in the refractive index by propagation. On the other hand, since the semiconductor absorbs a large amount of light, if the element length is increased, the light is absorbed by the propagation loss before the refractive index is changed. Further, since the element length must be increased, there is a problem that the applied voltage becomes high.

Therefore, the present invention has been made in order to solve the above-mentioned problems, and an object thereof is to make the light control element smaller in size and capable of being driven at a low voltage.

[0005]

In the light control element of the present invention, the light absorption layer is constructed as follows. That is, the first barrier layer and the first barrier layer having a tensile lattice strain and having an n-type impurity introduced therein are formed in contact with the barrier layer.
Well layer, a second barrier layer formed in contact with the first well layer, and a second well layer formed in contact with the second barrier layer having a compressive lattice strain, The third barrier layer is formed in contact with the second well layer. When the electric field is not applied to the light absorption layer, the ground level of the conduction band of the first well layer is lower than the Fermi level, and the ground level of the conduction band of the second well layer is Fermi level. Further, when a predetermined electric field is applied to the light absorption layer, the ground level of the conduction band of the first well layer is higher than the Fermi level, and the ground level of the conduction band of the second well layer is Fermi level. The state is lower than the level. As a result, when an electric field is applied to the first well layer in this light absorption layer, TM
A peak of exciton absorption occurs when polarized light is transmitted. Further, in the second well layer, an absorption peak of excitons is generated when TE polarized light is transmitted, and disappears when an electric field is applied. In the light control element of the present invention, the light absorption layer has the following structure. That is, the first barrier layer, the first well layer that has a compressive lattice strain, and is formed in contact with the barrier layer by introducing an impurity imparting n-type, and the first well layer in contact with the first well layer. A formed second barrier layer,
The second well layer has a tensile lattice strain and is formed in contact with the second barrier layer, and the third barrier layer is formed in contact with the second well layer. . And
When no electric field is applied to the light absorption layer, the ground level of the conduction band of the first well layer is lower than the Fermi level and the ground level of the conduction band of the second well layer is higher than the Fermi level. When a predetermined electric field is applied to the light absorption layer, the ground level of the conduction band of the first well layer is higher than the Fermi level, and the ground level of the conduction band of the second well layer is Fermi level. I made it lower. As a result, in this light absorption layer,
In the first well layer, when an electric field is applied, a peak of exciton absorption occurs when TE polarized light is transmitted. Also,
In the second well layer, a peak of exciton absorption occurs when TM polarized light is transmitted, and disappears when an electric field is applied.

[0006]

Embodiment 1 (Claim 3) A first embodiment of the present invention will be described below with reference to the drawings. 1 is a perspective view and a sectional view showing a configuration of a light control element according to a first embodiment of the present invention. here,
An example in which the light control element is applied to a Mach-Zehnder type modulator is shown. Explaining the structure of this light control element, an optical waveguide 110 having a ridge structure is formed on a substrate 100 made of n-InP. In addition, the optical waveguide 110 includes an incident portion 111, a demultiplexing portion 112, a modulating portion 11
3, a multiplexing section 114 and an emission section 115.
Then, the signal light (laser light) incident from the incident unit 111
Are branched by the branching unit 112, an electric field corresponding to the signal is applied in the modulating unit 113 to generate a phase difference, and
The intensity changes due to the phase difference at the time of merging at
It is emitted from 5.

The modulator 113 of the optical waveguide 110 is constructed as shown in the sectional view of FIG. First,
A cladding layer 120 made of n-InP is formed on the substrate 100.
Is arranged on the light absorption layer 1 of the multiple quantum well structure.
30 is formed. Then, on this, p-type In
The clad layer 140 made of P is formed, and the substrate 1
An n-side electrode 160 is formed on the back surface of the metal layer 00, and a p-side electrode 170 is formed on the cladding layer 140.

Next, the structure of the light absorption layer 130 will be described in more detail. The light absorption layer 130 has the following layer structure as a minimum unit. That is, as shown in FIG. 1C, first, n-InG having tensile strain is formed on the barrier layer (first barrier layer) 131 made of InGaAsP.
A well layer (first well layer) 132 made of aAsP is formed. InGaA is formed on the well layer 132.
A barrier layer (third barrier layer) 133 made of sP is formed. Then, a well layer (second well layer) 1 made of non-doped InGaAsP having compressive strain is formed thereon.
34, and a barrier layer (second barrier layer) 135 made of InGaAs is formed to form a multiple quantum well structure.

Then, the band structure of this multiple quantum well structure is set to the state shown in FIG. In FIG. 2, (a) is a state where no voltage is applied, and (b) is
Is a state in which a voltage is applied. First, as shown in FIG. 2A, in the well layer 132, the ground level of the conductor is lower than the Fermi level, and in the well layer 134, the ground level of the conductor is slightly higher than the Fermi level. And

In addition, when a voltage is applied, as shown in FIG.
As shown in, the ground level of the conduction band of the well layer 132 is higher than the Fermi level, and the ground level of the conduction band of the well layer 134 is lower than the Fermi level. With the configuration described above, first, the exciton absorption by electrons in the quantum well can be made transparent, and the change in the absorption coefficient of this light control element can be sharply changed with a large change in the vicinity of the operating wavelength (operating wavelength). It is possible to realize a state in which there is a rise and there is almost no change in the absorption coefficient at the wavelength used. As a result, the change in the refractive index can be increased at the used wavelength.

First, the mechanism of the change in the refractive index of the semiconductor will be described. For example, in the case of an optical modulator, the incident signal light is optically modulated in the light absorption layer sandwiched between the upper and lower clad layers. This change in the refractive index in the light absorption layer can be calculated from the change in absorption coefficient by Kramers =
It is derived using the Kronig relationship.

[0012]

[Equation 1]

In Equation 1, Δα represents the amount of change in absorption coefficient, Δn represents the amount of change in refractive index, and λ ₀ represents the operating wavelength. By the way, FIG. 3A shows an example of an absorption coefficient change due to an electric field in a semiconductor quantum well structure. From this, a change in the refractive index of a light absorption layer adopting the semiconductor quantum well structure can be derived by Equation 1 It becomes like 4 (a). At this time, the change in the refractive index becomes maximum near the band gap of the quantum well structure, but both the absorption coefficient and the change in absorption coefficient are large near the wavelength corresponding to this band gap.
On the contrary, in the region where the change in the absorption coefficient is small, the change in the refractive index is slight.

However, in the refractive index control type optical element, as described above, the absorption coefficient / absorption coefficient change is suppressed to be small,
In addition, a property contradictory to this case, that is, a large change in the refractive index is obtained, is required. Here, the reason why a large change in the refractive index cannot be obtained is, as can be understood from Equation 1,
The integration is most effective in the vicinity of the operating wavelength, but the integrand of Equation 1 has positive and negative regions that are similar to each other, and therefore it is canceled out during the integration and the refractive index change cannot be increased.

Therefore, in order to realize the absorption coefficient spectrum in which the change in the refractive index remains at the used wavelength, from the property of the integral of equation 1, as shown in FIG. It is necessary to obtain an absorption coefficient change spectrum in which the balance between the positive region and the negative region where the absorption coefficient change is as large as possible and the absorption coefficient change peak is as close as possible to the used wavelength. Then, if such an absorption coefficient change spectrum is obtained, as shown in FIG. 4B, the change in the refractive index can be increased near the wavelength used.

Then, as described above, the absorption coefficient change spectrum can be realized by using the multiple quantum well structure in which the exciton absorption by the electrons in the quantum well can be made transparent. Here, the effect of making the exciton absorption transparent by the electrons in the well and the effect of the difference in the absorption characteristics with respect to the polarization distortion lattice will be described. When the light absorption layer has a quantum well structure, when the transition from the valence band to the conduction band occurs when signal light enters it, the generated holes and electrons are confined in the well and Coulomb interaction occurs. Excitonic absorption occurs. Then, the absorption spectrum of the quantum well structure shows a sharp rise with respect to the wavelength axis due to the exciton absorption.

However, when the quantum well contains electrons due to impurities, this time the electron shielding effect occurs and exciton absorption does not occur. Therefore, as described above, exciton absorption can be eliminated by injecting electrons into the quantum well by introducing n-type impurities. This is called transparent exciton absorption. That is, in the first embodiment, as shown in FIG. 1, the well layer 132 is made of n-InGaAsP,
Since the exciton absorption is extinguished, it is possible to realize a large change in the absorption coefficient (absorption coefficient change spectrum) having a sharp rise in the vicinity of the wavelength used as the signal light. As a result, in the light control element according to the first embodiment, the change in the refractive index can be increased near the wavelength used.

By the way, the signal light has components of which polarization directions are perpendicular to each other (TE polarized light and TM polarized light). When these signal lights are used after being modulated, the original signals having different polarization directions must be equally modulated. Therefore, in addition to the above, in the first embodiment, as described above, first, the well layer 132 is made of n-InGaAsP having tensile strain,
The well layer 134 is made of non-doped InG having compressive strain.
It was composed of aAsP. Since the strained lattice is used for the quantum well in this way, it becomes possible to control the absorption change for light of different polarization directions.

The strained lattice and the change in absorption will be described below. The strained grating has a polarization dependence of the absorption coefficient spectrum. That is, TE-polarized light has a larger absorption for a compressive strain crystal than a lattice-matched crystal, but a small absorption for a tensile strain crystal. On the contrary, the TM polarized light has a larger absorption for the tensile strained crystal as compared with the lattice-matched crystal, but has a smaller absorption for the compression strained crystal.

Therefore, in the case of a quantum well structure using a compressive strained crystal in the well layer, a larger exciton absorption spectrum is generated for TE polarized light than for a quantum well structure using a lattice matching crystal, but for TM polarized light. Does not cause large exciton absorption. On the other hand, in the case of the quantum well structure using the tensile strained crystal in the well layer, an exciton absorption spectrum larger than that of the quantum well structure using the lattice matching crystal is generated for TM polarized light, but large for TE polarized light. Does not cause exciton absorption.

Therefore, in the case of the band structure shown in FIG. 2A, since TE polarized light has a large absorption with respect to compressive strain as described above, a peak of exciton absorption for the well layer 134 of compressive strain occurs, while Absorption of the tensile strain in the well layer 132 is small, and an exciton absorption spectrum does not occur. On the other hand, as shown in FIG. 2B, the absorption spectrum distribution when an electric field is applied will be described. When an electric field is applied, the Fermi level becomes tensile strained well layer 1.
It is located below the ground level of 32 and above the ground level of the compressively strained well layer 134. Therefore, the n-type carriers (electrons) on the well layer 132 side having tensile strain
201 moves to the compressive strained well layer 134 side at high speed.

At this time, the TE polarized light has a large absorption in the well layer 134 with compressive strain, but since the carriers are present in the well layer 134, the exciton becomes transparent due to the shielding effect, and the exciton absorption is absorbed. No peak occurs. In addition, the TE-polarized light absorption in the well layer 132 having a tensile strain is small, and an exciton absorption spectrum in the well layer 132 does not occur. Therefore, the change in the absorption coefficient spectrum when a voltage is applied from a voltage of 0 reflects the transparency of excitons with respect to the TE polarized light, and FIG. 3B (at this time, the change in absorption coefficient Δα is in the negative direction). Represents the change of),
It is possible to take a large portion where the absorption coefficient change is negative.

Therefore, when the refractive index change is derived for the TE polarized light by the Kramers-Kronig transformation, it becomes as shown in FIG. 4B, which suppresses the change of the absorption coefficient at the used wavelength λ ₀ , as compared with the conventional quantum well structure. Therefore, the change in refractive index can be increased. At this time, the refractive index change Δn represents a change in the negative direction (FIG. 4). Also,
The speed of modulation is mainly determined by the movement of electrons between the two quantum wells, but the movement occurs due to the tunnel effect, so the movement of electrons is high. On the other hand, the effect of distortion of absorption on TM polarized light is opposite to that described above, and brings about an effect opposite to that of TE polarized light on change in absorption coefficient and change in refractive index. That is, when an electric field is applied, the refractive index greatly changes in the positive direction. The action is also reversed with respect to the TM polarized light, and like the case with the TE polarized light, the change in the absorption coefficient at the used wavelength λ ₀ can be suppressed and the change in the refractive index can be made larger than in the conventional case.

As described above, in the first embodiment, by making the exciton absorption by the carriers in the quantum well transparent, a large absorption spectrum change having a sharp rise near the operating wavelength is realized, The effect of the strained grating is combined and the effect is made to correspond to different polarized lights. Hereinafter, the characteristics of the modulator 113 of the light control element (Mach = Zehnder modulator) according to the first embodiment will be compared with a conventional light modulator. FIG. 5 shows the relationship between the absorption coefficient and the change in refractive index. As a factor for determining the characteristics for phase modulation, the ratio of the magnitude of the change in refractive index to absorption loss can be considered. The larger this value is, the shorter the element length can be modulated.

As shown in FIG. 5, in the configuration (b) according to the first embodiment, the amount of change in the refractive index with respect to the absorption coefficient is larger than that in the conventional configuration (a). As a result, according to the first embodiment, the length of the waveguide for changing the phase by π can be made shorter than the conventional one. Further, FIG. 6 shows the relationship between the voltage and the phase change. However, since the amount of change in the refractive index is large, the structure (b) of the first embodiment is thus different from the structure (b) of the related art as described above. ) Can operate at a lower voltage.

Second Embodiment (Claims 1 and 2) A second embodiment of the present invention will be described below with reference to the drawings. In the first embodiment, the tensile strain well layer 13
2 and the compressively strained well layer 134 are formed via the barrier layer 133, but the same effect can be obtained without this barrier layer. That is, as shown in FIG. 7, first, a well layer 132 made of n-InGaAsP having tensile strain is formed on a barrier layer 131 made of InGaAsP, and a well layer made of non-doped InGaAsP having compressive strain is formed thereon. Alternatively, a barrier layer 135 made of InGaAs may be formed on top of this to form a multiple quantum well structure.

In this configuration as well (FIG. 7 (a)), TE
Since polarized light has a large absorption with respect to compressive strain, the exciton absorption peak for the compressive strained well layer 134 occurs, while the absorption for the tensile strained well layer 132 is small and an exciton absorption spectrum does not occur. On the contrary,
As shown in FIG. 7B, when an electric field is applied, the Fermi level is located below the ground level of the tensile strained well layer 132 and above the ground level of the compressive strained well layer 134. Like Therefore, the tensile strain well layer 132
The n-type carrier (electron) 201 located on the side moves to the well layer 134 side of compressive strain at high speed.

At this time, the TE polarized light has a large absorption in the well layer 134 with compressive strain, but since the carriers are present in the well layer 134, the exciton becomes transparent due to the shielding effect, and the exciton absorption is absorbed. No peak occurs. In addition, the TE-polarized light absorption in the well layer 132 having a tensile strain is small, and an exciton absorption spectrum in the well layer 132 does not occur. Therefore, the change in the absorption coefficient spectrum when a voltage is applied from 0 becomes a reflection of exciton transparency with respect to the TE polarized light, as shown in FIG. Can be big.

Therefore, also in this embodiment,
Deriving the refractive index change for TE polarized light, FIG.
As a result, compared to the conventional quantum well structure, it is possible to suppress the change in absorption coefficient and increase the change in refractive index at the used wavelength λ ₀ . At this time, the refractive index change Δn
Represents the change in the negative direction (FIG. 4). Further, the modulation speed is determined mainly by the movement of electrons between the two quantum wells, the movement of electrons is high, and a high modulation speed can be obtained. On the other hand, again, the effect on the distortion of absorption is opposite to the TM polarized light, and the effect opposite to that of the TE polarized light is brought about to the change of the absorption coefficient and the change of the refractive index. That is, when an electric field is applied, the refractive index greatly changes in the positive direction. The action is also reversed with respect to the TM polarized light, and like the case with the TE polarized light, the change in the absorption coefficient at the used wavelength λ ₀ can be suppressed and the change in the refractive index can be made larger than in the conventional case.

By the way, in the first and second embodiments, the n-type impurity is introduced into the well layer having the tensile type lattice strain. However, the present invention is not limited to this, and the compressive type lattice strain is provided. An n-type impurity may be introduced into the well layer, and the well layer having tensile type lattice strain may be undoped. For example, if the structure of the quantum well structure is I
nGaAsP barrier layer / compressive strain n-InGaAsP well layer / InGaAsP barrier layer / tensile strain InGaAs
A structure in which P well layers / InGaAsP barrier layers are multiply stacked may be used. In addition, the quantum well structure is InGaAs
A structure in which P barrier layer / compressive strained n-InGaAsP well layer / tensile strained InGaAsP well layer / InGaAsP barrier layer are stacked in multiple layers may be used. Even with those configurations, the same effects as those of the above-described first and second embodiments can be obtained.

[0031]

As described above, in the present invention, the light absorption layer has the following structure. That is, the first barrier layer has a tensile or compressive lattice strain, and n
A first well layer formed in contact with the barrier layer by introducing a shape-imparting impurity, a second barrier layer formed in contact with the first well layer, and compressive or tensile lattice strain And a second well layer formed in contact with the second barrier layer,
The third barrier layer is formed in contact with the second well layer. When the electric field is not applied to the light absorption layer, the ground level of the conduction band of the first well layer is lower than the Fermi level, and the ground level of the conduction band of the second well layer is Fermi level. Further, when a predetermined electric field is applied to the light absorption layer, the ground level of the conduction band of the first well layer is higher than the Fermi level, and the ground level of the conduction band of the second well layer is Fermi level. The state is lower than the level.

With this structure, in the first strain layer with tensile strain, when an electric field is applied, a peak of exciton absorption occurs when TM polarized light is transmitted. In the second well layer having compressive strain, an absorption peak of excitons is generated when TE polarized light is transmitted, and disappears when an electric field is applied. On the other hand, in the compressive strained first well layer, when an electric field is applied, a peak of exciton absorption occurs when TE polarized light is transmitted. In the second strain layer having tensile strain, a peak of exciton absorption occurs when TM polarized light is transmitted, and disappears when an electric field is applied. Therefore, a larger change in the refractive index can be obtained for each of TE polarized light and TM polarized light than that of the conventional semiconductor optical device, the light control device can be made smaller, and low voltage driving can be performed.

[Brief description of drawings]

FIG. 1 is a perspective view and a cross-sectional view showing a configuration of a light control element according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a band structure of the light control element according to the first embodiment.

FIG. 3 is an explanatory diagram showing an absorption coefficient change spectrum with respect to wavelength in an absorption layer of a light control element.

FIG. 4 is an explanatory diagram showing a refractive index change spectrum with respect to wavelength in the absorption layer of the light control element.

FIG. 5 is an explanatory diagram comparing the ratio of the change in the refractive index with respect to the wavelength and the absorption coefficient between the structure according to the first embodiment of the present invention and the conventional structure.

FIG. 6 is an explanatory diagram comparing a phase change with respect to an applied voltage between the structure according to the first embodiment of the present invention and the conventional structure.

FIG. 7 is an explanatory diagram showing a band structure of a light control element in the second embodiment of the present invention.

[Explanation of symbols]

100 ... Substrate, 111 ... Incident part, 112 ... Demultiplexing part, 11
3 ... Modulator, 114 ... Multiplexer, 115 ... Ejector, 120
... clad layer, 130 ... light absorption layer, 131 ... barrier layer (first barrier layer), 132 ... well layer (first well layer), 13
3 ... Barrier layer (third barrier layer), 134 ... Well layer (second well layer), 135 ... Barrier layer (second barrier layer), 140 ...
Clad layer, 160 ... N-side electrode, 170 ... P-side electrode.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-8-242039 (JP, A) JP-A-7-104222 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/00-1/125 H01S 5/00-5/50

Claims (3)

(57) [Claims] 1. A light control device comprising a light absorption layer having a multiple quantum well structure sandwiched between cladding layers, wherein the light absorption layer has a first barrier layer and a tensile lattice strain. a first well layer formed in contact with the barrier layer by introducing an impurity imparting n-type conductivity, and a second well formed in contact with the first well layer having a compressive lattice strain Layer and a second barrier layer formed in contact with the second well layer, the base of the conduction band of the first well layer when no electric field is applied to the light absorption layer. When the level is lower than the Fermi level, the ground level of the conduction band of the second well layer is higher than the Fermi level, and when a predetermined electric field is applied to the light absorption layer, the first well layer Of the conduction band of the second well layer is higher than the Fermi level of the conduction band of Light control element, characterized in that less than Mi level. 2. A light control element comprising a light absorption layer having a multiple quantum well structure sandwiched between cladding layers, wherein the light absorption layer has a first barrier layer and a compressive lattice strain, a first well layer formed in contact with the barrier layer by introducing an impurity imparting n-type conductivity, and a second well formed in contact with the first well layer and having a tensile lattice strain Layer and a second barrier layer formed in contact with the second well layer, the base of the conduction band of the first well layer when no electric field is applied to the light absorption layer. When the level is lower than the Fermi level, the ground level of the conduction band of the second well layer is higher than the Fermi level, and when a predetermined electric field is applied to the light absorption layer, the first well layer The conduction band of the second well layer is higher than the Fermi level, and the conduction band of the second well layer is lower than the Fermi level. Light control element, characterized in that less than Mi level. 3. The light control element according to claim 1, wherein a third barrier layer is formed in contact with each of the first well layer and the second well layer. An optical control element characterized by.