JPH09179081A - Optical modulator and optical semiconductor switch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the driving with a low voltage by using a semiconductor possible and to make the integration with a semiconductor optical element possible by presenting the modulator or optical switch element by longitudinal electric field type exciton erasure. SOLUTION: This optical modulator has a lower clad layer 2, multiple quantum well structures 3 and an upper clad layer 4 which are successively formed on a substrate 1. Electrodes are formed on at least either of the rear surface of the substrate 1 or the upper clad layer 4 via an insulating layer 6 by which pa p-i-n junction is constituted in the direction perpendicular to the multiple quantum well structure 3. An exciton absorption peak disappears when the voltage in a forward direction is impressed on the p-i-n junction. The light of the wavelength corresponding to the excition absorption within the well layers of the multiple quantum well structure 3 is then subjected to light intensity modulation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator which can be integrated with a semiconductor laser for optical communication capable of high speed modulation, and an optical switch element necessary for constructing an optical circuit.

Recently, in the field of communication, it has been required to increase the communication capacity in order to build an advanced information society, and for this reason, an element capable of high-speed modulation is required.

On the other hand, in the field of information processing, there is an interest in information processing using light such as optical computers and optical interconnections. In response to this application, it is required to develop an optical arithmetic circuit, which requires a compact optical arithmetic element that can be integrated with a semiconductor light emitting element and a light receiving element.

[0004]

2. Description of the Related Art As a method of performing optical modulation using a semiconductor laser, a direct modulation method in which the light emitted from the laser is changed by changing the current injected into the laser and an optical modulator provided outside the laser are used. There is an external modulation method that changes the light intensity and the refractive index.

Of these, in the direct modulation method, wavelength chirping (fluctuation of oscillation wavelength) occurs due to relaxation oscillation of the laser when high-speed modulation is performed. In order to avoid this, it is desirable to use an external modulation method as the high speed modulation element, and therefore an external modulator capable of high speed modulation is required.

Among the modulators, as an electroabsorption type optical modulator,
Conventionally, those using the Franz-Keldysh effect have been developed. However, for a system with a transmission capacity of 10 Gb / s, which is expected to be realized in the future, this modulator is not sufficient, and quantum confinement Stark with multiple quantum wells, which is considered to be capable of higher speed modulation recently, is expected. Research on modulators using the Quantum Confinement Stark Effect (QCSE) has begun. In order to enable high-speed modulation, two points are required: (1) reduction of modulator capacitance and (2) reduction of drive voltage.

On the other hand, the most basic optical operation element is an optical switching element, but LiNb is currently the most widely used optical switching element.
Some use O ₃ crystals.

The optical switch element using this crystal has a larger size than that using a semiconductor, and cannot be integrated with a semiconductor laser, making it difficult to compact the optical switch. Moreover, since monolithic integration is not possible, alignment of the optical axis with the semiconductor laser is troublesome, an optoelectronic integrated circuit (OEIC) cannot be configured, and an optical computer cannot be manufactured.

Further, in order to realize an optical computer that operates at high speed, it is necessary to have an element capable of high-speed switch operation. To this end, (1) reduction of electrostatic capacitance of optical switch / arithmetic element, (2) Two points are required to reduce the driving voltage.

[0010]

At present, sufficient characteristics have not yet been obtained even in the above-mentioned multiple quantum well optical modulator utilizing the quantum confined Stark effect (QCSE).

As a modulator that may have sufficient characteristics, the change in the refractive index with respect to the voltage is large.
There are some that use LiNbO ₃ crystals, but this is disadvantageous because it is larger than the modulator using semiconductors and cannot be assembled with a semiconductor laser.

There is also a current injection type optical switch element, but this is because in the case of performing high speed modulation, the injected carriers cannot follow the modulation speed, so that in terms of high speed response compared to the one using voltage. Inferior.

In order to solve these problems, it is conceivable to use an electro-optical effect that can give a change above the quantum confined Stark effect (QCSE) using a semiconductor. As an example thereof, it is conceivable to use the transverse electric field exciton erasing type modulator shown in FIG. 11 (see Japanese Patent Application Laid-Open No. 06-148699 of the present invention). This modulator performs refractive index modulation by an electric field in the lateral direction. However, with this modulator, the optical waveguide width must be made quite short (about 2 μm for 2 V operation) in order to reduce the operating voltage, making it difficult to fabricate the device. Receive.

On the other hand, in order to solve the problems that the size of the optical switching element is large and that it is difficult to integrate the optical switching element with the light emitting element or the light receiving element, it is conceivable to fabricate an optical switching element using a semiconductor. .

However, an optical switch element using a bulk semiconductor has a change in refractive index with respect to a voltage which is inferior to that using a LiNbO ₃ crystal. The length of the optical operation element (optical waveguide length) must be increased.

However, this makes it difficult to perform a high-speed optical switch (optical exchange) / optical calculation operation, and to increase the loss of light or to make the optical calculation elements compact. Problems arise.

There is also a current injection type optical switching element, but this is because in the case of applying high speed modulation, the injected carriers cannot follow the modulation speed, and therefore in terms of high speed response compared to the one using voltage. Inferior.

In order to solve the above problems, the present invention presents a modulator or an optical switch element by longitudinal electric field type exciton erasing, which can be driven at a low voltage using a semiconductor and is integrated with a semiconductor optical element. The purpose is to enable.

[0019]

Means for Solving the Problems To solve the above problems, 1) a lower clad layer, a multiple quantum well structure, and an upper clad layer, which are sequentially formed on a substrate, are provided on the back surface of the substrate or the upper clad layer. Electrodes are formed on the back surface of the substrate and on the upper clad layer via an insulating layer on at least one of the upper side, and p is formed in a direction perpendicular to the multiple quantum well structure.
When an in-junction is formed and a forward voltage is applied to the pin-junction, the exciton absorption peak disappears, and light with a wavelength corresponding to exciton absorption in the well layer of the multiple quantum well structure is emitted. An optical modulator with intensity modulation, or 2) The multiple quantum well structure uses a type II quantum well that spatially separates an electron confinement layer and a hole confinement layer,
2. The optical modulator according to 1 above, wherein the electron confinement layer is adjacent to an adjacent electron confinement layer through a barrier layer having a thickness smaller than that of the electron confinement layer, or 3) a lower clad layer sequentially formed on the substrate, Multiple quantum well structure, having upper clad layer, and forming electrodes on the back surface of the substrate and on the upper clad layer via an insulating layer on the back surface of the substrate or on at least one of the upper clad layer And the p direction is perpendicular to the multiple quantum well structure.
When an in-junction is formed and a forward voltage is applied to the pin-junction, excitons in the well layer of the multiple quantum well structure are extinguished, and light of a wavelength corresponding to the exciton refractive index peak is emitted. 4) A semiconductor optical switch in which refractive index modulation is performed, or A pin junction is formed in a direction perpendicular to the multi-quantum well structure, and the multi-quantum well structure in the groove-shaped region is used as an optical waveguide. One optical waveguide is branched into a plurality of optical waveguides. , An electrode is formed on the back surface of the substrate and on the upper cladding layer via an insulating layer on the back surface of the substrate or on at least one of the upper cladding layers on the branched optical waveguide Semiconductor optical switch Or 5) a semiconductor optical switch having a plurality of waveguide terminals by combining a plurality of the semiconductor optical switches according to 3 or 4, or 6) the optical modulator according to 1 above, wherein the substrate is a semi-insulating semiconductor substrate. Or 7) the above 3 or 4 wherein the substrate is a semi-insulating semiconductor substrate
This is achieved by the semiconductor optical switch described.

FIG. 1 is a structural diagram of the modulator of the present invention. This structure is almost the same as an ordinary MQW optical modulator using QCSE, but a semi-insulating substrate 1 is used as the substrate to reduce the capacitance of the modulator. The guide layer 1A is stacked on the substrate, and the lower clad layer 2, the quantum well layer with the potential profile shown in Fig. 2 are stacked on the substrate in multiple cycles. Form. Electrodes 10 on the n-type region and p-type region,
Form 7. In the figure, 11 is a buried layer.

Here, the cladding layer and the MQW layer are undoped intrinsic semiconductors. Between the electrode and the contact layer
This is different from the usual MQW optical modulator using QCSE in that it is used by applying a forward bias voltage through the SiO ₂ insulating films 6 and 9.

The present invention presents an optical switch element having a structure shown in FIG. 5 based on a principle completely different from the conventional example. Figure 5 (a) is a plan view showing the waveguide structure of the optical switch.
FIG. 5 (b) is a sectional view taken along the lines (A) and (D) of FIG. 5 (a).

In this optical switching element, a lower clad layer 2 is formed on a semi-insulating substrate 1, and a quantum well layer and a barrier layer having an energy gap larger than that are laminated on the lower clad layer 2 in multiple cycles to form a multiple quantum layer. The well (MQW) structure 3 and the upper clad layer 4 are sequentially stacked.

This structure is almost the same as an ordinary MQW optical modulator using QCSE, but the cross section of α and β parts in FIG. 5 (a) is on the upper cladding layer 4 as shown in FIG. 6 (a). On contact layer
5, a silicon dioxide (SiO ₂ ) insulating film 6 and a p-side electrode 7 are formed, and a contact layer 8 and an n-side electrode 10 are formed on the back surface of the substrate. Here, the cladding layer and MQW layer are undoped intrinsic semiconductors. Si between the electrode and the contact layer
It differs from the normal MQW optical modulator using QCSE in that it is used by applying a forward bias voltage with an O ₂ insulating film 6 interposed.

Next, the operation of the modulator of the present invention will be described. In the present invention, a quantum well structure having the potential profile shown in FIG. 2 is used as the MQW layer. Layers (1) and (3) in Fig. 2 are composed of materials with the same composition and thickness. This figure is based on the usual notation. The upper line shows the energy at the conduction band edge and the lower line shows the energy at the valence band edge.

In this structure, the built-in voltage with respect to the pin structure remains unchanged even when no current is applied to the optical modulator.
Since it joins the MQW layer, the potential profile of the MQW layer is as shown in Fig. 3 (a). In this case, (3) layer electrons and (4)
The holes in the layer have a relatively large transition probability while being localized in each layer, and the absorption spectrum has a large exciton peak as shown by the dotted line in Fig. 4.

However, when a forward bias V ₁ that cancels the built-in voltage is applied to this structure, the potential profile becomes as shown in FIG. 3 (b). In this state, even in a normal type II well using the (2), (3), (4), and (5) layers as the quantum well structure,
Since the holes in the (4) layer are spatially separated, the transition probability is reduced, but in the present invention, the thin barrier layer (2) is used to (3)
Because the electronic level of the layer and the electronic level of the (1) layer are strongly coupled,
The wavefunctions of these electrons spread from the (1) layer to the (3) layer. Due to this effect, the transition probabilities of electrons in layer (3) and holes in layer (4) are reduced more than in the case of a normal type II well. As a result, the absorption spectrum has no peak as shown by the solid line in Fig. 4.

Therefore, when λ _OP in FIG. 4 is set as the operating wavelength, the difference in absorption amount between the non-biased state and the V ₁ bias becomes large at this wavelength, and this can be utilized to perform the modulation operation. This operation is possible even if a normal type II well is used, but since the difference in absorption amount at the time of V ₁ bias is not as large as that of the present invention, the modulation operation is inferior to the present invention.

In the present invention, no current flows even if a forward bias is applied because the insulating layer is provided between the electrode and the element. Next, the operation of the optical switching element of the present invention will be described.

In this optical switch, the same type I quantum well as that of an ordinary optical modulator is used for the layer indicated by reference numeral 3 in FIGS. 5 and 6 through a thin barrier layer. In the structure shown in Fig. 6 (a), the built-in voltage of the pin structure is added to the MQW structure even when no voltage is applied to the optical modulator.
The potential profile of W is as shown in Fig. 7 (a).

In this case, the electron level in each well is
Since they are energetically separated from the electron levels of the wells on both sides, mutual coupling is weak and each electron level is localized in each well. Therefore, an exciton level is formed, and the relationship between the wavelength of light incident on the MQW structure and the refractive index is shown by the solid line in Fig. 8 when no voltage is applied.

However, when a forward bias V ₁ that cancels the built-in voltage is applied to this structure _{, the} potential profile becomes as shown in Fig. 7 (b), and the electron level of each well is strongly coupled to the adjacent well, This wavefunction becomes spread over the entire MQW structure. Therefore, in this case, the exciton level disappears, the refractive index changes as shown by the dotted line in Fig. 8, and when the incident light wavelength is set to λ _OP in Fig. 8, the refractive index decreases when a voltage is applied.

Since the voltage is applied only under the electrode in the α (or β) part and is applied only to the concave part in FIG. 6 (a), the refractive index in this direction becomes as shown in FIG. 6 (b) before the voltage is applied. However, when a voltage is applied to the electrodes, a difference in refractive index occurs as shown in Fig. 6 (c).

Therefore, if a voltage is applied only to the α part and no voltage is applied to the β part in FIG. 5A, a difference in refractive index occurs only between the waveguide A and the branched waveguide B. Therefore, the waveguide A
The light incident from is unable to propagate in the (B) direction due to the difference in refractive index, and propagates only in the (C) direction. On the other hand, when a voltage is applied to the β part, light from the waveguide A conversely propagates only in the (B) direction and does not propagate in the (C) direction.

Therefore, by switching the applied voltage between the electrode α and the electrode β, the propagation direction of light can be switched, and the device can be operated as an optical switch. In addition, by combining a plurality of waveguide structures shown in Fig. 5 (a) to form a structure as shown in Fig. 10, by applying a voltage to each electrode of this structure, the propagation direction of light at each branch point It is possible to obtain a multi-terminal optical switch that can be freely switched.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION First, an embodiment of a modulator will be described.
I do. InAs layers (1) and (3) in Fig. 2_0.43P_0.53, (2),
(5) In as a layer_0.73Ga_0.27As _0.49P_0.51Using the (4) layer
As In_0.53Ga_0.47As_0.71P_0.29, The film thickness is (1),
Layer (3) is 9 nm, layer (2) is 2 nm, layer (4) is 12 nm, and layer (5) is
 According to the present invention when 5 nm is repeated for 5 cycles
The operation of the optical modulator will be described.

Here, the thickness of the clad layer is 50 nm on the p-side,
The n side is 30 nm. An InGaAs layer is used as the contact layer.
Assuming that the doping concentration is 3.0 × 10 ¹⁷ cm ⁻³ for the p layer and the background concentration of the i layer is 1.0 × 10 ¹⁵ cm ⁻³ , the built-in current in this case is −0.8 V. Therefore, in this embodiment, a modulator that can operate at an extremely low voltage of 0.8 V can be obtained.

In this case, a large exciton peak in the case of no voltage occurs near the wavelength of 1.34 μm, so λ _OP = 1.34 μm
Then, a large absorption change can be obtained. Since this modulator can be manufactured by the same process as a normal MQW modulator, no new process development is required. Also,
This modulator can be operated in various wavelength regions by changing parameters such as material system, composition, and layer thickness.

Next, an embodiment of the optical switch element will be described. First, a lower clad layer 2 made of InP is grown on a semi-insulating substrate 1 made of InP, covered with a resist film 91 as shown by the shaded portion in FIG. 9 (1), and then the substrate surface is covered. Etching.

As a result, the sectional structure of FIG. 9 (2) is shown in FIG. 5 (a).
It is formed on the (A) and (D) lines of. Next, metalorganic vapor phase growth
(MOCVD) method, metalorganic vapor phase epitaxial growth (MOVPE)
The MQW layer 3 in Fig. 6 (a) is grown by the method.

The MQW layer 3 is made of In _1-x Ga _x As _y P with a thickness of 90 Å.
_1-y (x = 0.430, y = 0.920) well layer, 50 Å thick InGaAsP
(x = 0.150, y = 0.330) The barrier layers are laminated alternately and the period is set to 10.

An upper cladding layer 4 made of InP is grown on the MQW layer 3. Next, a contact layer 5 made of InGaAs is grown on the entire surface of the device, and a resist mask 92 having an opening at a region 93 in FIG. 9C is formed. Then, a focused ion beam device is used to implant ions such as Zn in the region 93 to form a p-type region.

Next, the resist film 52 is removed, and the insulating film 6 made of SiO ₂ is formed on the entire surface of the element. Finally, Figure 9
Deposition mask with the same pattern as resist mask 92 in (3)
(Not shown) is formed, and a p-side electrode (Ti / Pt from the lower layer) is formed in the region 93 by vapor deposition. After that, the mask is removed.

A contact layer 8 is also formed on the back side of the substrate, an n-type region is formed by ion implantation or diffusion using Sn or Si as a dopant, and an n-side electrode (AuGe / Au from the lower layer) is formed.
Forming a nine.

Since the waveguide described in the embodiment has the same structure as that of various semiconductor lasers, this optical switch can be easily integrated with the semiconductor laser and the light receiving element. Therefore, it is not necessary to align the optical axis with the light emitting element, which is necessary in the conventional optical switch element.

Next, the lower clad layer in the embodiment
If the film thickness of 2 is 30 nm, the film thickness of the upper cladding layer 4 is 100 nm, and the doping concentration is 5.0 × 10 ¹⁷ cm ^{-3 for} the p layer and the background concentration of the i layer is 1.0 × 10 ¹⁵ cm ^-3. ， In this case, the built-in voltage is −0.89V. Therefore, in the example of this embodiment, an optical switch element that can operate at a voltage as low as 0.89 V is obtained.

When the composition, thickness, and period of the MQW layer are as described above, when excitons are eliminated, the refractive index becomes maximum near the wavelength of 1.51 μm (corresponding to λ _OP in FIG. 8), and light of this wavelength You can make an optical switch that works with.

It is needless to say that the composition, film thickness and period of the MQW layer may be changed depending on the wavelength band used, and therefore the present invention is not limited to this embodiment. Further, in the description of the embodiments, since the semi-insulating substrate is used, the inter-electrode capacitance can be reduced, and high speed operation is possible.

FIG. 10 is an explanatory view of the multi-terminal optical switch of the present invention. By combining the four waveguide structures shown in Fig. 5 (a) and applying a voltage to each electrode (hatched area) of this structure, it is possible to freely switch the propagation direction of light at each branch point. You get a switch.

[0050]

EFFECTS OF THE INVENTION By using the longitudinal electric field type exciton erasing optical modulator according to the present invention which uses the type II type quantum well structure, it becomes possible to perform a modulation operation which can be operated at a lower voltage than before, and the present invention According to this, by adopting a new structure as shown in FIG. 6, a compact optical switch that can be driven at a low voltage and uses a semiconductor can be obtained, and it can be integrated with a semiconductor laser or the like, and optical axis alignment is unnecessary. can do.

Further, a multi-terminal optical switch can be constructed by combining a plurality of optical switches.

[Brief description of the drawings]

FIG. 1 is a structural explanatory view of a modulator of the present invention (1)

FIG. 2 is a structural explanatory view of a modulator of the present invention (2)

FIG. 3 is an explanatory diagram of the principle of the modulator of the present invention.

FIG. 4 is an explanatory diagram of an absorption spectrum of a quantum well structure.

FIG. 5 is a structural explanatory view of the optical switch of the present invention.

FIG. 6 is an explanatory view of the principle of the optical switch of the present invention.

FIG. 7 is a potential shape diagram of the multiple quantum well of the present invention.

FIG. 8 is an explanatory diagram of refractive index modulation.

FIG. 9 is a diagram illustrating an embodiment of a manufacturing process of the present invention.

FIG. 10 is an explanatory diagram of a multi-terminal optical switch of the present invention.

FIG. 11 is an explanatory view of a conventional example.

[Explanation of symbols]

1 Semi-insulating semiconductor substrate 2 Lower clad layer 3 MQW layer 4 Upper clad layer 5 Contact layer 6 Insulating layer is SiO ₂ film 7 p-side electrode 8 Contact layer 9 Insulating layer is SiO ₂ film 10 n-side electrode 11 Embedded layer

Claims (7)

[Claims] 1. A lower clad layer, a multiple quantum well structure, and an upper clad layer which are sequentially formed on a substrate, and an insulating layer is provided on the back surface of the substrate or on at least one of the upper clad layer. , An electrode is formed on the back surface of the substrate and on the upper clad layer to form a pin junction in a direction perpendicular to the multiple quantum well structure, and is excited when a forward voltage is applied to the pin junction. An optical modulator, characterized in that light intensity modulation is performed on light having a wavelength corresponding to exciton absorption in a well layer of the multi-quantum well structure when the child absorption peak disappears. 2. The multi-quantum well structure uses a type II quantum well that spatially separates an electron confinement layer and a hole confinement layer, and the electron confinement layer includes a barrier layer thinner than the electron confinement layer. 2. The optical modulator according to claim 1, wherein the optical modulator is adjacent to the adjacent electron confinement layer. 3. A lower clad layer, a multiple quantum well structure, and an upper clad layer, which are sequentially formed on a substrate, and an insulating layer is provided on the back surface of the substrate or on at least one of the upper clad layer. , An electrode is formed on the back surface of the substrate and on the upper cladding layer to form a pin junction in a direction perpendicular to the multiple quantum well structure, and a forward voltage is applied to the pin junction, A semiconductor optical switch, wherein excitons in a well layer of the multiple quantum well structure are extinguished, and light having a wavelength corresponding to an exciton refractive index peak is subjected to refractive index modulation. 4. A multi-quantum well structure in which a groove-shaped lower clad layer is formed on a substrate, and an upper clad layer is sequentially formed on the lower clad layer and is perpendicular to the multi-quantum well structure. Has a structure in which a pin junction is formed in the direction, and the multi-quantum well structure of the groove-shaped recessed region is used as an optical waveguide, and one optical waveguide is branched into a plurality of optical waveguides, and on the branched optical waveguide In the semiconductor optical device described above, an electrode is formed on the back surface of the substrate and on the upper cladding layer via an insulating layer on at least one of the back surface of the substrate and the upper cladding layer. switch. 5. A semiconductor optical switch comprising a plurality of semiconductor optical switches according to claim 3 or 4 coupled to each other and having a plurality of waveguide terminals. 6. The optical modulator according to claim 1, wherein the substrate is a semi-insulating semiconductor substrate. 7. The semiconductor optical switch according to claim 3, wherein the substrate is a semi-insulating semiconductor substrate.