JPH0961763A - Semiconductor optical modulation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor optical modulation device which lessen the effect of absorption saturation, has a high extinction ratio and has a wide modulation frequency band by specifying the angle between the direction of the electric field impressed on a light absorption layer and a lamination direction of the light absorption layer. SOLUTION: This semiconductor optical modulation device is provided with the light absorption layer 3 and an n-type clad layer 1 and p-type clad layer 5 across the light absorption layer 3. The angle θ formed by the direction of the electric field 10 impressed on the light absorption layer 3 generated by voltage impression and the lamination direction of the light absorption layer 3 is specified to 0 deg.<θ<90 deg.. The thickness of the i-type clad layers 2, 4 holding the light absorption layer 3 along the Y direction orthogonal with the lamination direction of the light absorption layer 3 varies and, therefore, the value of the effective electric field impressed on the light absorption layer 3 varies and the band diagram of the light absorption layer 3 inclines in accordance with the difference in the electric fields as well. The electrons and holes generated by light absorption move in directions reverse from each other according to this inclination and move out of the light absorption layer 3 beyond the energy barrier with a semi-insulating embedment layer 7 and, therefore, the formed internal electric fields are diminished and the deterioration of the extinction rate is substantially eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical modulator, and more particularly to a semiconductor optical modulator used as an external modulator in long-distance optical fiber communication of several 100 km or more.

[0002]

2. Description of the Related Art In recent years, optical fiber communication has become widespread in the trunk line system of telephone lines, etc., but since the current system uses a direct modulation system of a distributed feedback semiconductor laser, the temperature rise due to carrier injection. Alternatively, if the wavelength fluctuates significantly due to the fluctuation of the refractive index and the wavelength fluctuates because the transmission speed in the optical fiber varies depending on the wavelength, the pulse waveform becomes dull during transmission, so long-distance light without a regenerator is used. Fiber transmission was difficult.

In order to solve such a problem, an external modulation method has been studied in which the direct current optical output from the semiconductor laser is modulated by an external optical modulator to minimize the wavelength fluctuation, and the length is several hundred km. Achieves non-relay transmission over distance.

Conventionally, an electroabsorption type optical modulator has been used as such an external optical modulator, and therefore the structure of a conventional semiconductor optical modulator will be described with reference to FIG. See FIG. 6. This semiconductor optical modulator is an n-type In that also serves as a cladding layer.
The i-type InGaAsP light absorption layer 30, the p-type InP clad layer 25, and the p ⁺ -type InGaAsP contact layer 26 are sequentially deposited on the P substrate 21, and then the n-side electrode 28 and the p-side electrode 29 are provided. . In this case, a thick InGaAsP layer is used as the light absorption layer, but there is also one using a quantum well structure light absorption layer.

By applying a reverse bias to such a semiconductor light modulation device, the light absorption characteristics of the i-type InGaAsP light absorption layer 30 are controlled. In this case, the light incident direction is n-type. There are two directions, that is, a direction perpendicular to the InP substrate 21 and a light guiding direction in the i-type InGaAsP light absorption layer 30. In any case, i-type In
The electric field 31 applied to the GaAsP light absorption layer 30 is in a direction perpendicular to the stacking direction of the i-type InGaAsP light absorption layer 30 as indicated by an arrow in the figure.

Next, the modulation principle and modulation characteristics of such a conventional semiconductor optical modulator will be described with reference to FIG. See FIG. 7A. If the solid line in FIG. 7A is the absorption spectrum when no electric field is applied to the optical modulator, the equivalent band gap is narrowed by applying an electric field to the optical modulator. The spectrum shifts to the long wavelength side as shown by the alternate long and short dash line.

See FIG. 7B. If the wavelength of the light modulated in this optical modulator is λ _in , then the light is not absorbed and is transmitted as it is when no electric field is applied, and the wavelength of λ _in is applied when an electric field is applied. The input light is absorbed and does not pass through, and the extinction ratio characteristic as shown in FIG. 7B is seen.

[0008]

Problems to be Solved by the Invention Here, FIG. 8 and FIG.
The problem to be solved by the invention in the present invention will be described with reference to FIG. 8 is an explanatory diagram of the deterioration of the modulation characteristic in the conventional semiconductor optical modulator, and FIG. 9 is an explanatory diagram of the electric field screening in the conventional semiconductor optical modulator.

Referring to FIGS. 8A and 8B, in the conventional semiconductor optical modulator, the extinction ratio deteriorates as the light input increases, as compared with the extinction ratio at the low light input indicated by the alternate long and short dash line. As shown in FIG. 8B, the modulation frequency band at the time of high light input was narrowed, and high-speed modulation of 10 Gb / sec could not be performed. The extinction ratio at the time of low light input indicated by the alternate long and short dash line corresponds to the extinction ratio indicated by the solid line in FIG.

Referring to FIG. 9A, the external electric field 31 applied to the light absorption layer 30 of the conventional semiconductor optical modulator is only a component in the direction perpendicular to the light absorption layer 30, and electrons generated by the absorption of light. 11 and 12
Was moving toward the electrodes by the external electric field 31.

In this case, the electrons 11 and the holes 12 are i-type I.
nGaAsP light absorption layer 30, n-type InP substrate 21 and p
It is necessary to pass through the energy barrier due to the heterojunction formed between the n-type electrode and the p-type InP clad layer 25 and escape to the n-side electrode and the p-side electrode. i-type InGaA
It will be accumulated inside the sP light absorption layer 30.

Electrons 11 and holes 12 thus accumulated
Due to the influence of the external electric field 31, the electrons 11 become n-type InP substrate 2
On the 1st side, the holes 12 are unevenly distributed on the p-type InP clad layer 25 side, respectively. Therefore, due to the uneven distribution of the electrons 11 and the holes 12, an internal electric field 32 that cancels the external electric field 31 is generated and an energy band structure. Will be deformed, and such a phenomenon is called electric field screening. Then, the shift amount due to voltage application of the absorption spectrum by the deformation of the energy band structure becomes small, lambda _in
The absorption coefficient decreases at the wavelength of.

9 (b) and 9 (c) In this case, since the amount of electrons 11 and holes 12 accumulated increases as the light input increases, the internal electric field 32 increases as the light input increases. , With it,
The absorption spectrum when the same voltage is applied changes from the absorption spectrum shown by the one-dot chain line to the absorption spectrum shown by the broken line, the shift amount of the absorption edge wavelength decreases, and the absorption coefficient further decreases to the wavelength of λ _in. It will be. The solid line in FIG. 9 (c) is the absorption spectrum when there is no bias.

Such a phenomenon is called an absorption saturation phenomenon, and as the light input increases, the shift amount of the absorption edge wavelength decreases, so the extinction ratio deteriorates, and the light absorption layer accumulates. If the amount of remaining electrons and holes is large, the absorption response to the next optical signal also deteriorates, and the modulation band also decreases.

Therefore, an object of the present invention is to provide a semiconductor optical modulator having a high extinction ratio and a wide modulation frequency band by reducing the influence of the absorption saturation phenomenon.

[0016]

FIG. 1 shows a principle structure of a semiconductor optical modulator for explaining the principle structure of the present invention. The problem in the present invention will be solved with reference to FIG. The means for doing so will be described.

Referring to FIG. 1, (1) In the semiconductor optical modulator of the present invention, the light absorption layer 3 and semiconductor layers having opposite conductivity types sandwiching the light absorption layer 3, that is, the n-type cladding layer 1 and the p-type semiconductor layer are provided. In addition to having the cladding layer 5, the angle θ formed by the direction of the electric field 10 applied to the light absorption layer 3 generated by voltage application and the stacking direction of the light absorption layer 3
Is 0 ° <θ <90 °.

(2) Further, the present invention is characterized in that in the above (1), the angle θ is 40 ° <θ <50 °.

(3) Further, in the present invention according to the above (1) or (2), a semi-insulating semiconductor layer, that is, a semiconductor layer between the light absorption layer 3 and the semiconductor layers 1 and 5 having opposite conductivity types, that is, The i-type clad layers 2 and 4 are provided.

(4) Further, the present invention is characterized in that, in any one of the above (1) to (3), the light absorption layer 3 has a single quantum well structure or a plurality of quantum well structures.

(5) Further, the present invention is characterized in that in any one of the above (1) to (3), the light absorption layer 3 is made of a semiconductor layer having a uniform composition as a whole.

(6) Further, according to the present invention, in any one of the above (1) to (5), the semiconductor laser is formed monolithically and integrally, and the active layer of the semiconductor laser and the light absorption layer 3 are separately exposed. The feature is that the axes are substantially aligned.

[0023]

Next, referring to FIG. 2, which shows an energy band diagram along the X direction, which is the stacking direction of the light absorption layers 3 in the semiconductor optical modulator shown in FIG. 1, and the Y direction orthogonal to the X direction, FIG. The operation of the invention will be described.

Referring to FIG. 2A, the angle θ formed by the direction of the electric field applied to the light absorption layer 3 and the stacking direction of the light absorption layer 3 caused by the voltage application is 0 ° <θ <90 °.
As a result, the electrons 11 and holes 12 generated by light absorption are generated in the n-type cladding layer 1 and p-type, respectively, as in the conventional case.
It moves to the mold cladding layer 5 side.

See FIG. 2B. However, the direction orthogonal to the stacking direction of the light absorption layer 3, that is,
I-type cladding layers 2 and 4 sandwiching the light absorption layer 3 along the Y direction
, The effective electric field value applied to the light absorption layer 3 is different, and the band diagram of the light absorption layer 3 is also inclined based on the difference in the electric field.

With the inclination of this band diagram,
The electrons 11 and the holes 12 move in directions opposite to each other, pass through the energy barrier with the semi-insulating buried layer 7, and escape from the light absorption layer 3.

Also in this case, some of the electrons 11 and holes 12 are accumulated in the vicinity of the energy barrier with the semi-insulating burying layer 7, but the distance in the X direction, that is, the thickness of the light absorbing layer 3. Since the distance in the Y direction, that is, the stripe width, is larger than that in, the internal electric field formed by the accumulated electrons 11 and holes 12 becomes extremely small, the shift amount of the absorption edge wavelength almost disappears, and the extinction Almost no deterioration of the ratio.

The angle θ formed by the direction of the electric field applied to the light absorption layer 3 and the stacking direction of the light absorption layers 3 is 40 ° <θ <50 °.
By setting the above, the generation of the internal electric field can be suppressed more effectively. That is, when θ is small, the inclination of the electric field shown in FIG. 2B becomes small and the amount of accumulated electrons 11 and holes 12 does not decrease. Therefore, the formation of the internal electric field is not suppressed, and θ is large. Then, the effective electric field applied to the light absorption layer 3 becomes small and the extinction characteristic deteriorates.

By providing the semi-insulating semiconductor layers 2 and 4 between the light absorption layer 3 and the semiconductor layers 1 and 5 of opposite conductivity types, the direction of the electric field applied to the light absorption layer 3 and the light absorption. Layer 3
The angle θ with the stacking direction of can be arbitrarily set.

Further, when the light absorption layer 3 is formed of a single quantum well structure or a plurality of quantum well structures, light absorption occurs due to transition between quantum levels, so that the light absorption characteristic becomes sharp and small. Since the extinction ratio can be made large by the voltage and the extinction characteristic can be improved when the light absorption layer 3 is thick, the extinction characteristic can be controlled by the number of quantum well structures.

Further, since the light absorption layer 3 is composed of a semiconductor layer having a uniform composition as a whole, the manufacture becomes easy,
In addition, the extinction characteristic can be controlled by the thickness of the semiconductor layer.

Further, by integrally forming the semiconductor laser monolithically, the light source and the modulation means can be integrated, and the optical axis of the active layer of the semiconductor laser and the light absorption layer 3 in the conventional assembly can be integrated. The alignment with the optical axis of is unnecessary. Further, in such a direct coupling type device, the optical coupling rate is very large, and the absorption saturation phenomenon remarkably occurs, so that the application of the present invention becomes more effective.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to FIGS.
The manufacturing process of the example semiconductor light modulation device will be described. See FIG. 3A. First, the impurity concentration is 1 × 10.¹⁸~ 5 × 10¹⁸cm^-3, Good
Suitably 2 × 10¹⁸cm ^-3On the n-type InP substrate 21 of
Pure substance concentration is 3 × 10¹⁷~ 7 × 10¹⁷cm^-3, Preferably 5
× 10¹⁷cm^-3N-type InP buffer layer (not shown)
After growing, undoped i-type InP clad layer 2
Grow two.

Next, referring to FIG. 3B, an inclined portion is formed in the i-type InP clad layer 22 by using a dry etching method or a wet etching method. For example, when the wet etching method is used, the inclined portion is formed by etching using the photoresist as a mask.

When the dry etching method is used, dry etching is performed using a tapered SiO ₂ layer formed by wet etching with a photoresist as a mask. In this case, the SiO ₂ layer is also etched at the same time when the InP layer is etched, and the mask edge recedes.

As a result, an inclined portion is formed, and this inclination angle can be adjusted by the taper angle of the SiO ₂ mask layer and the etching rate of SiO ₂ and InP. The structure and method of forming the inclined portion are similar to those of the conventional TS type semiconductor laser, and the inclination angle θ is 0 ° <θ <90 °, preferably 40 ° <θ <50.
The angle is, for example, 45 °.

Next, referring to FIG. 4C, a 10 nm-thick InGaAsP barrier layer having a composition of 1.1 μm and a 6 nm-thickness of 6 nm are formed on the i-type InP clad layer 22 having the inclined portion. InGaA
An i-type MQW light absorption layer 23 and an i-type InP clad layer 24 having a multiple quantum well structure in which a quantum well structure in which ten sP well layers are alternately repeated is sandwiched between InGaAsP optical guide layers having a composition of 1.1 μm and a thickness of 100 nm. To grow sequentially.

When the i-type InP clad layer 24 is grown, the i-type InP clad layer 24 is grown by growing under the growth condition that the growth rate at the sloped portion is high.
Surface is flattened and the thickness of the flat part above the step is 0.4
˜0.5 μm, preferably 0.4 μm.

Next, the impurity concentration is 1 × 10 ¹⁷ to 1 × 1.
0 ¹⁸ cm ^-3 , preferably 5 × 10 ¹⁷ cm ^-3 with a thickness of 0.8-
1.2 μm, preferably 1.0 μm p-type InP clad layer 25 and an impurity concentration of 1 × 10 ^{19 to} 5 × 10 ¹⁹ c
m ⁻³ , preferably 2 × 10 ¹⁹ cm ⁻³ and a thickness of 0.3 to 0.5
A p ⁺ -type InGaAsP contact layer 26 having a thickness of μm, preferably 0.4 μm is grown.

Next, as shown in FIG. 4D, the p ⁺ -type InGaAsP contact layer 26 to the n-type InP buffer layer are formed by a dry etching method or a wet etching method using a photoresist (not shown) as a mask. Part (in the figure, n-type In
The surface of the P substrate 21 is mesa-etched in a stripe shape to form a stripe-shaped mesa having a width d of 1.0 to 1.8 μm, preferably 1.5 μm.

5E and 5F, the stripe-shaped mesas are filled with the Fe-doped InP burying layer 27, and ohmic electrodes to be the n-side electrode 28 and the p-side electrode 29 are formed, respectively. The semiconductor optical modulator is completed.

Although the Fe-doped InP burying layer 27 is used as the burying layer, other InP containing impurities such as Cr and Ti forming a deep level may be used. Also,
As the crystal growth method, MOVPE method (metal organic chemical vapor deposition method) is used, but other crystal growth method such as MBE method may be used.

In this semiconductor light modulation device, as described with reference to FIG. 2, the angle formed by the direction of the electric field applied to the i-type MQW light absorption layer 23 and the stacking direction of the i-type MQW light absorption layer 23, that is, , The inclination angle θ of the inclined portion is 0 ° <θ <90
Since it is set to 40 °, preferably 40 ° <θ <50 °, for example, 45 °, the electrons and holes generated by light absorption as shown in FIG. 2B are absorbed in the i-type MQW light absorption layer 23. Due to the inclination of the band diagram formed along the lines, they move in the opposite directions and pass through to the Fe-doped InP buried layer 27.

In this case, some of the electrons and holes are i-type MQW.
Although it remains and accumulates in the light absorption layer 23, the i-type MQ
Thickness of W light absorption layer 23 (10 nm × 9 + 6 nm × 10 +
Since the stripe width d (1.5 μm) is 4 times or more as compared with 100 nm × 2 = 350 nm, that is, 0.35 μm, the internal electric field formed by electrons and holes becomes extremely small, and the electric field screening is performed. Will not occur.

Therefore, even if the optical input becomes large, the extinction ratio does not deteriorate and the modulation frequency band does not narrow, so that high-speed modulation of 10 Gb / sec becomes possible.

Next, a semiconductor laser integrated semiconductor light modulator, which is a modification of the semiconductor light modulator, will be described. First, by the same manufacturing process as that of a conventional distributed feedback semiconductor laser having a BH (buried heterojunction) structure, I
A distributed feedback semiconductor laser having an nGaAsP-based MQW layer as an active layer is formed.

Then, the growth layer in the portion forming the semiconductor optical modulator is selectively removed by etching to expose the n-type InP substrate or the n-type InP buffer layer, and then the i-type InP clad layer is exposed at this exposed portion. Grow.

Then, as described with reference to FIG. 3B, i
The type InP clad layer is etched to form an inclined portion having a predetermined inclination angle. In this case, the position and height of the inclined portion need to be etched so that the optical axis of the light absorption layer formed on the inclined portion substantially coincides with the optical axis of the stripe-shaped active layer of the semiconductor laser.

Next, as shown in FIGS. 4C to 5F, the light absorption layer, the i-type InP clad layer, and the p-type In.
After the P clad layer and the p ⁺ -type InGaAsP contact layer are sequentially deposited and mesa-etched, the stripe-shaped mesas are filled with the Fe-doped InP burying layer, and finally, the semiconductor optical modulation device section and the semiconductor laser section, respectively. By forming the n-side electrode and the p-side electrode, the semiconductor laser integrated semiconductor optical modulator is completed.

In such a direct coupling type semiconductor light modulation device integrated with a semiconductor laser, the optical axes of the active layer of the semiconductor laser and the light absorption layer of the semiconductor light modulation device are aligned at the time of element formation. It is not necessary to align the optical axes at the time of assembly as in the conventional case, and the size of the entire apparatus can be reduced.

Further, since the optical coupling efficiency is large in the direct coupling type semiconductor optical modulation device, the absorption saturation phenomenon remarkably occurs. However, by adopting the configuration of the present invention, the absorption saturation phenomenon is effectively suppressed. be able to.

Although the semiconductor optical modulator using the n-type substrate has been described in the above embodiment, the semiconductor optical modulator using the p-type substrate may be used, and the material system is AlGaAs system. Other material systems such as the above may also be used.

Although the i-type MQW light absorption layer 23 is used as the light absorption layer in the above embodiment, an SQW light absorption layer having a single quantum well structure may be used. In this case, the MQW light absorption layer is used. The extinction characteristic deteriorates as compared with the case where the light absorption layer is used.

As the light absorption layer, a thick film InGaAsP light absorption layer having a uniform composition may be used. In this case, the extinction characteristic can be improved by increasing the thickness of the InGaAsP light absorption layer. In some cases, InG
The aAsP light absorption layer may be divided into a plurality of layers in the stacking direction by an i-type InP barrier layer to form a multilayer structure.

[0055]

According to the present invention, the angle θ formed by the direction of the electric field applied to the light absorption layer of the semiconductor light modulation device and the stacking direction of the light absorption layers is set to 0 ° <θ <90 °, whereby absorption saturation is achieved. Since the phenomenon can be suppressed and the deterioration of the extinction ratio and the decrease of the modulation frequency band can be suppressed, the ultra-high speed modulation of 10 Gb / sec or more becomes possible, which greatly contributes to the development of the long distance non-relay communication technology.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a principle configuration of the present invention.

FIG. 2 is an explanatory diagram of the operation of the present invention.

FIG. 3 is an explanatory view of a manufacturing process up to the middle of the semiconductor optical modulation device of the present invention.

FIG. 4 is an explanatory view of a manufacturing process of the semiconductor optical modulation device of the present invention up to the middle of FIG.

FIG. 5 is an explanatory diagram of the manufacturing process of the semiconductor optical modulation device of the present invention after FIG. 4;

FIG. 6 is an explanatory diagram of a conventional semiconductor light modulation device.

FIG. 7 is an explanatory diagram of a modulation principle and a modulation characteristic of a conventional semiconductor optical modulation device.

FIG. 8 is an explanatory diagram of deterioration of modulation characteristics of a conventional semiconductor optical modulation device.

FIG. 9 is an explanatory diagram of electric field screening in a conventional semiconductor optical modulation device.

[Explanation of symbols]

1 n-type clad layer 2 i-type clad layer 3 light absorption layer 4 i-type clad layer 5 p-type clad layer 6 p-type contact layer 7 semi-insulating buried layer 8 n-side electrode 9 p-side electrode 10 electric field 11 electron 12 positive Hole 21 n-type InP substrate 22 i-type InP clad layer 23 i-type MQW light absorption layer 24 i-type InP clad layer 25 p-type InP clad layer 26 p ⁺ -type InGaAsP contact layer 27 Fe-doped InP buried layer 28 n-side electrode 29 p-side electrode 30 i-type InGaAsP light absorption layer 31 external electric field 32 internal electric field

Claims (6)

[Claims] 1. A light absorption layer and semiconductor layers of opposite conductivity type sandwiching the light absorption layer are provided, and a direction of an electric field applied to the light absorption layer caused by voltage application and a stacking direction of the light absorption layers are provided. The semiconductor optical modulation device, wherein the angle θ formed is 0 ° <θ <90 °. 2. The semiconductor optical modulation device according to claim 1, wherein the angle θ is 40 ° <θ <50 °. 3. The semiconductor light modulation device according to claim 1, wherein a semi-insulating semiconductor layer is provided between the light absorption layer and the semiconductor layers of opposite conductivity types. 4. The semiconductor light modulation device according to claim 1, wherein the light absorption layer has a single quantum well structure or a plurality of quantum well structures. 5. The semiconductor light modulation device according to claim 1, wherein the light absorption layer is made of a semiconductor layer having a uniform composition as a whole. 6. The semiconductor optical modulator and the semiconductor laser are monolithically formed integrally with each other, and the optical axes of the active layer of the semiconductor laser and the optical absorption layer of the semiconductor optical modulator are substantially aligned with each other. The semiconductor light modulation device according to claim 1, wherein the semiconductor light modulation device is a device.