JP2010217373A - Absorption type semiconductor optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact and high speed absorption type semiconductor optical modulator having improved reliability. <P>SOLUTION: In the absorption type semiconductor optical modulator, an optical waveguide including a core layer having at least a function absorbing light is formed on a semiconductor substrate, the optical waveguide has a light incident end surface, and light made incident from the light incident end surface is absorbed in the core layer in a light transmitted process by the optical waveguide to generate a photocurrent. The core layer is so formed that the width on the light incident end surface side thereof is made wide and is made narrow toward a light transmission direction so as to discharge the Joule heat generated by the generated photocurrent to the semiconductor substrate, to avoid thermal destruction due to the Joule heat, and the optical waveguide on the light incident end surface side constitutes a multi mode optical waveguide. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention belongs to the field of small-sized, high-speed, and highly reliable absorption type semiconductor optical modulators.

(First prior art)
FIG. 9 shows a schematic perspective view of a high-mesa optical modulator shown in Non-Patent Document 1 as a first conventional technique of a so-called absorption semiconductor optical modulator having a function of absorbing light.

1 is an n ⁺ -InP substrate, 2 is an n-InP lower cladding layer (or simply, an n-InP cladding layer), 3 is an i-InGaAsP well, and an i-InGaAsP barrier having a different band gap energy from the well. Multiple quantum wells (InGaAsP-InGaAsP Multiple Quantum Well: InGaAsP-InGaAsP MQW) core layer (this multi-quantum well layer 4 is a non-dope layer, hereinafter abbreviated as i-MQW core layer), 4 is p -InP upper clad layer (or simply p-InP clad layer), 5 is a p ⁺ -InGaAs contact layer, 6 is a p-electrode for applying an electric signal, 7 is an n-electrode, 8 is a polyimide, 9 is a bonding It is a wire.

  Of the p-electrode 6, a portion 10 to which a bonding wire is connected is called a bonding pad portion. In this high mesa type semiconductor optical modulator, in order to reduce the electric capacitance of the bonding pad portion, polyimide 8 having a small relative dielectric constant is used underneath.

  The p-InP cladding layer 4, the i-MQW core layer 3, and the n-InP cladding layer 2 constitute an optical waveguide that guides light. Here, the i-MQW core layer 3 has a higher refractive index than the p-InP cladding layer 4 and the n-InP cladding layer 2 and has a central role of guiding light. The width and length of the ridge portion constituting the high mesa optical waveguide are about 2 μm and 200 μm, respectively.

  The operation of the first prior art will be described. FIG. 10 shows the absorption coefficient of light when no electric field is applied and when a reverse bias is applied, with the wavelength as a variable. In the figure, the wavelength for operating the optical modulator is shown as the operating wavelength.

  As can be seen from FIG. 10, when no electric field is applied, the absorption coefficient of light is sufficiently small at the wavelength at which the semiconductor optical modulator operates, and the end face (hereinafter referred to as light incident) on the semiconductor optical modulator where light enters. The light incident on the end face is emitted from the end face from which the opposite light exits (hereinafter abbreviated as the light exit end face). On the other hand, when a reverse bias is applied between the p-electrode 6 and the n-electrode 7, the light absorption spectrum shifts to the longer wavelength side, so that the light absorption coefficient increases at the operating wavelength of the optical modulator, and the light is absorbed. The Thus, by turning the reverse bias voltage between the p electrode 6 and the n electrode 7 ON or OFF, the light is also turned OFF or ON, and an electric signal can be converted into an optical signal.

In this high-mesa optical modulator, the i-MQW core layer 3 having a function of absorbing light is surrounded by air having a small relative dielectric constant ε _r (ε _r = 1), and below the bonding pad portion 10. Since polyimide 8 (ε _r = 3 to 4) having a small relative dielectric constant ε _r is used, its electric capacitance C is small. Accordingly, the 3 dB optical modulation band (= 1 / πRC, or simply referred to as the optical modulation band) by the so-called CR constant limit, which is limited by the electric capacitance C and the load resistance R, is the right and left of the i-MQW core layer 3. The high mesa type optical modulator is suitable for high-speed optical modulation because it is higher than that of an embedded structure in which all are embedded with a semiconductor.

  FIGS. 11A and 11B show distributions in the longitudinal direction of the optical modulator and in the width direction from the mesa center, respectively, with respect to the heat generated due to light absorption during reverse bias application.

First, consider the longitudinal direction. In the longitudinal direction, that is, in the light propagation direction, the intensity P of the light propagates through the optical waveguide composed of the i-MQW core layer 3, the p-InP clad layer 4, and the n-InP clad layer 2, and P = P ₀ exp (−αz) (1)
In this way, it is absorbed by the i-MQW core layer 3 in accordance with the exponential function equation. Here, P ₀ is the power of incident light at the light incident end face, α is an absorption coefficient, and z is a distance from the light incident end face. Since the exponential function is a function that rapidly attenuates with respect to a variable, light is rapidly absorbed within a short distance from the light incident end face, and converted into a current I _p (referred to as a photocurrent).

  On the other hand, in the lateral direction (horizontal direction on the substrate surface), the cross-sectional structure is a high mesa structure, that is, the left and right sides of the ridge portion are air. Since the thermal conductivity of air is several orders of magnitude smaller than that of semiconductors, the generated heat is hardly transmitted to the outside of the ridge.

Accordingly, heat is accumulated in the short direction from the light incident end face in the longitudinal direction and in the ridge portion of the high mesa in the lateral direction. In particular, when high-power light is incident, Joule heat (P _J = V · I _P ) represented by the product of the applied voltage V and the photocurrent I _P is released to the substrate 1 through the n-InP cladding layer 2. It cannot be fully communicated to 1 and accumulates in the ridge of High Mesa. Therefore, the ridge portion is broken due to the high Joule heat. Experimentally, the maximum optical input power to the high-mesa optical modulator is limited to about 10 mW.

(Second prior art)
This thermal breakdown due to photocurrent can be avoided by adopting the second prior art structure whose schematic perspective view is shown in FIG. Here, 1 is an n ⁺ -InP substrate, 11 is an n-InP lower cladding layer (or simply, an n-InP cladding layer), 12 is an i-MQW core layer, and 13 is a p-InP upper cladding layer (or simply) P-InP cladding layer), 14 is a p ⁺ -InGaAs contact layer, 15 is an i-InP buried layer, 16 is a p-electrode for applying an electric signal, and 7 is an n-electrode.

In the first prior art shown in FIG. 9, the heat generated as a result of the light being absorbed by the i-MQW core layer 3 and the generation of the photocurrent _Ip is air with extremely low thermal conductivity on the left and right sides of the ridge. Therefore, they could not escape and accumulated in the ridge, resulting in element destruction. On the other hand, in the second prior art, Joule heat generated as a result of light absorption by the i-MQW core layer 12 is released to the substrate 1 via the n-InP cladding layer 11 and left and right i-InP It escapes to the n ⁺ -InP substrate 1 through the buried layer 15.

  FIG. 13A shows the heat generated along the light propagation axis along with the light propagation in the second prior art shown in FIG. 12, and FIG. 13B shows the light incidence of the semiconductor light modulator. The heat distribution in the horizontal direction in the vicinity of the end face is shown.

In general, since a semiconductor has high thermal conductivity, heat generated through a semiconductor such as the n-InP cladding layer 11 and the i-InP buried layer 15 surrounding the top, bottom, left and right of the i-MQW core layer 12 is rapidly n ⁺ −. As a result, it can be seen that the peak value of the heat distribution centered on the i-MQW core layer 12 is lower than that of the first prior art. For this reason, it can be said that the second prior art has a structure with excellent light-proof input characteristics.

However, this second prior art is so-called limited by the electric capacitance C and the load resistance R formed by the p-InP cladding layer 13, the i-InP buried layer 15, and the n ⁺ -InP substrate 1 having a large area. The 3 dB optical modulation band (= 1 / πRC or modulation band) due to the CR constant limit is significantly lower than that of the high-mesa optical modulator, and there is a problem that it cannot be applied to high-speed optical modulation.

  Further, although the second conventional technique is relatively simple crystal regrowth, there is still a problem that crystal regrowth involves a long manufacturing process. Next, this will be described.

  FIG. 14 shows an excerpt of the manufacturing process of the second prior art.

In the step (a), an n-InP clad layer 17 and an i-MQW core layer 18 are crystal-grown on the n ⁺ -InP substrate 1. Next, after a SiO ₂ mask 19 is deposited, a photoresist 20 is spin-coated.

In the step (b), the photoresist 20 in the step (a) is patterned as 22, and the SiO ₂ mask 19 in the step (a) is etched as 21 using this photoresist 22.

In the step (c), the i-MQW core layer 18 and the n-InP clad layer 17 in the step (b) are respectively formed as 12, 11 using the photoresist 22 and the SiO ₂ mask 21 defined in the step (b). Etch into.

In step (d), after removing the photoresist 22 in step (c), the i-InP buried layer 15 is regrown using the SiO ₂ mask 21 as a mask for crystal regrowth.

In step (e), after removing the SiO ₂ mask 21 used in step (d), the p-InP cladding layer 13 and the p ⁺ -InGaAs contact layer 14 are grown.

In the actual semiconductor optical modulator shown in FIG. 12, after the step (e), the n ⁺ -InP substrate 1 is thinly polished to about 100 μm. Next, the p electrode 16 is formed on the p ⁺ -InGaAs contact layer 14, and the n electrode 7 is formed on the back of the n ⁺ -InP substrate 1.

  As described above, the second conventional technique is a light-modulated CR constant limit, and although crystal re-growth is simpler than the third conventional technique described below, crystal recrystallization growth is still possible. Since it is necessary, there is a problem that the manufacturing process is long.

(Third prior art)
In order to reduce the electrical capacitance C of the pin junction formed by the wide area p-InP cladding layer 13, the i-InP buried layer 15, and the n ⁺ -InP substrate 1 shown in FIG. The third prior art as shown in FIG. 15 reported in Patent Document 2 is widely used.

  In the third prior art, after the i-MQW core layer 12 is grown on the n-InP clad layer 11 in substantially the same procedure as the process step shown in FIG. 12, a p-InP clad layer 24 is further grown. To do. After these are etched into a high mesa structure, the sides thereof are buried with an Fe-doped InP buried layer 24.

  Reference numeral 24 denotes an Fe-doped InP buried layer (referred to as a semi-insulating InP buried layer or Fe—InP buried layer) in which insulation resistance is increased by doping Fe atoms with InP.

  Since the Fe—InP buried layer 24 also has a high thermal conductivity, it can be said that the third prior art also has a structure with excellent light-proof input characteristics. And since the Fe—InP buried layer 24 is used in the third prior art, the electrical capacitance is small compared to the second prior art of FIG. 12 where the electrical capacitance is large due to the pin junction. There is a big advantage.

  However, in the third conventional technique, when the Fe—InP buried layer 24 is regrown, the high mesa formed by the n-InP cladding layer 11, the i-MQW core layer 12, and the p-InP cladding layer 23 is used. Therefore, not only is crystal regrowth difficult, but there is a big problem. That is, in order to form the Fe—InP buried layer 24, it is necessary to have a dedicated crystal growth furnace in which the dopant is Fe. Generally, in order to have a crystal growth device, a large capital investment of several hundred million yen is required including safety devices.

  Further, the Fe atoms and Zn as the dopant of the p-InP clad layer 12 are easily diffused to each other. As a result, the electrical capacitance is remarkably increased, and the electrical high-speed operation becomes difficult. Therefore, there is a problem that it takes a long time to determine the conditions for the crystal regrowth. In general, finding the crystal regrowth condition for the Fe-InP buried layer 24 is much more difficult than finding the crystal regrowth condition for the i-InP buried layer 15 shown in FIG. That is, in order to realize the third prior art shown in FIG. 15, there is a problem that not only a large capital investment but also a long development period is required.

(Fourth prior art)
Without using an Fe—InP buried layer that requires expensive capital investment as in the third prior art and it is difficult to find out the conditions for crystal re-growth, the crystal regrowth is relatively difficult as in the second prior art. FIG. 4 is a schematic perspective view of the structure disclosed in Patent Document 3 in order to reduce the electrical capacitance due to the pin junction, which is a problem in the second prior art, using a simple i-InP buried layer. 16 shows. A top view thereof is shown in FIG.

Here, 11 is an n-InP clad layer, 12 is an i-MQW core layer, 25 is a p-InP clad layer, 26 is a p ⁺ -InGaAs contact layer, 27 is an i-InP buried layer, and 28 is an electric signal applied. The p electrode 7 is an n electrode.

As can be seen from FIGS. 16 and 17, the fourth prior art has the same structure as the second prior art shown in FIG. 12 in the vicinity of the light incident end face where the amount of heat generated by light absorption is large. ing. As a result, the generated heat can be effectively released to the n ⁺ -InP substrate 1, so that the element is not destroyed by the generated heat. On the other hand, in the region where the photocurrent is small, the mesa width is narrowed to reduce the electrical capacitance due to the pin junction. As a result, by adopting the fourth conventional technique, it was possible to avoid element destruction and to operate at high frequency.

  However, as described in the explanation of FIG. 15, although it is easier than the Fe—InP buried layer 24, crystal regrowth is still necessary. A modulator needs to be developed.

IEEE, Journal of Quantum Electronics, vol. 28, pp. 224-230, 1992 1993 IEICE Spring Conference C-153

Japanese Patent No. 3913161

As described above, in the high mesa type semiconductor optical modulator as in the first prior art, when high power light is incident, a large Joule heat caused by the applied voltage and the current generated in the light absorbing portion is generated in the high mesa ridge. The ridge is thermally destroyed. However, if it is embedded with i-InP in order to avoid this, the electric capacitance becomes large and high-speed operation becomes difficult. In order to reduce this electric capacitance, it is effective to embed it with Fe-InP, but a long development period is required to determine a large amount of capital investment and crystal regrowth conditions. Furthermore, in the fourth prior art in which the width of the buried layer is increased in the vicinity of the end surface where light is incident and the width of the mesa is reduced along the direction of light propagation, thermal breakdown is avoided in the fourth conventional technique. Although high-speed operation was possible, crystal regrowth using SiO ₂ or SiN _x as a mask is still necessary. In the crystal regrowth, the yield does not reach 100%, and the manufacturing process is long, so that the cost as an absorption semiconductor optical modulator increases. Therefore, in order to reduce the cost, it is necessary to develop an absorption type semiconductor optical modulator that can be manufactured by a simpler manufacturing process.

  In order to solve the above problems, in the absorption semiconductor optical modulator according to claim 1 of the present invention, an optical waveguide including a core layer having a function of absorbing at least light is formed on a semiconductor substrate, The optical waveguide has a light incident end face, and an absorption-type semiconductor optical modulator in which a photocurrent is generated by absorption of light incident from the light incident end face into the core layer in the process of propagating through the optical waveguide The core layer is formed with a wide width on the light incident end face side so as to release the Joule heat to the semiconductor substrate in order to avoid thermal destruction due to Joule heat caused by the generated photocurrent. In addition, the optical waveguide is formed narrower in the light propagation direction, and the optical waveguide on the light incident end face side constitutes a multimode optical waveguide.

  In order to solve the above problems, in the absorption type semiconductor optical modulator according to claim 2 of the present invention, in the absorption type semiconductor optical modulator according to claim 1, the width of the core layer is set to a curved shape and a straight line. It is characterized by narrowing according to at least one of a shape and a staircase shape.

  In order to solve the above-described problems, in the absorption semiconductor optical modulator according to claim 3 of the present invention, in the absorption semiconductor optical modulator according to claim 1 or 2, the optical waveguide is a high mesa optical waveguide. It is characterized by being.

  In order to solve the above problems, in the absorption type semiconductor optical modulator according to claim 4 of the present invention, in the absorption type semiconductor optical modulator according to claim 1 or 2, the optical waveguide is a buried type optical waveguide. It is characterized by being.

  In the present invention, by providing a wide core in a region where a large amount of heat is generated by absorbing light, the generated Joule heat is released to the substrate. Since the Joule heat generated in the region where the photocurrent is small is small, a narrow core is provided to escape to the substrate. Therefore, by using the present invention, it is possible to minimize the core area viewed from the upper surface along the light propagation direction, so that the electrical capacitance is not increased unnecessarily, and due to Joule heat. It is possible to avoid element destruction and to perform high-speed optical modulation.

  Also, since the core width is wide so that the optical waveguide on which light is incident propagates in multimode, the coupling loss of light is small, resulting in a large insertion loss as an absorption semiconductor optical modulator. There are advantages.

1 is a perspective view showing a schematic configuration of an absorption semiconductor optical modulator according to a first embodiment of the present invention. 1 is a top view of an absorption semiconductor optical modulator according to a first embodiment of the present invention. Wafer cross-sectional view of crystal growth to produce the first embodiment of the present invention 2A shows the mode of light propagating near the end surface on the BB ′ side in FIG. 2, and FIG. 2B shows the mode of light propagating near the end surface on the AA ′ side in FIG. The perspective view which shows schematic structure of the absorption type semiconductor optical modulator concerning the 2nd Embodiment of this invention. The top view of the absorption type semiconductor optical modulator concerning the 2nd Embodiment of this invention The top view which shows schematic structure of the absorption type semiconductor optical modulator concerning the 3rd Embodiment of this invention The perspective view which shows schematic structure of the absorption type semiconductor optical modulator concerning the 4th Embodiment of this invention. The perspective view which shows schematic structure of the absorption type semiconductor optical modulator concerning 1st prior art The figure explaining the principle of operation of the 1st prior art The figure explaining the problem of 1st prior art The perspective view which shows schematic structure of the absorption type semiconductor optical modulator concerning a 2nd prior art The figure explaining the advantage of the 2nd prior art The figure explaining the manufacturing process of the 2nd prior art The perspective view which shows schematic structure of the absorption type semiconductor optical modulator concerning a 3rd prior art The perspective view which shows schematic structure of the absorption type semiconductor optical modulator concerning a 4th prior art Top view of the absorption type semiconductor optical modulator according to the fourth prior art

  Hereinafter, embodiments of the present invention will be described. Since the same numbers as those in the prior art shown in FIGS. 9 to 17 correspond to the same function units, the description of the function units having the same numbers is omitted here.

(First embodiment)
FIG. 1 shows a schematic perspective view of the absorption type semiconductor optical modulator according to the first embodiment of the present invention. Here, 30 is an n-InP lower clad layer (or simply, an n-InP clad layer), 31 is an i-MQW core layer, and 32 is a p-InP upper clad layer (or simply, a p-InP clad layer). , 33 are p ⁺ -InGaAs contact layers, and 34 is a p-electrode for applying an electric signal. Reference numeral 35 denotes a bonding pad portion.

A top view of the first embodiment is shown in FIG. In order to maximize the effects of the present invention, the optical waveguide has a high mesa structure in the first embodiment. The width W _{in the} vicinity of the end face where the light enters is about 22 μm, the width W _out near the end face where the light is emitted is about 2 μm, and the length L _in of the tapered region is about 30 μm. Thus, heat broad areas widths to convey widely the width of the semiconductor material near the end surface on which light is incident (generated significant heat to n ⁺ -InP substrate 1 in the first embodiment This is called the conduction semiconductor part). Then, the optical waveguide on the light incident end face side with the wide width is a multimode optical waveguide.

Therefore, as in the fourth prior art, this embodiment can avoid the destruction of the element even if a large amount of heat is generated when converting light into photocurrent. Note that these dimensions are merely an example, absorbed by the width W _in the i-MQW core 31 on the light incident side to generate large heat, semiconductors width is large light includes a core other than the region where the incident It goes without saying that values other than these may be used as long as they are wider than (the width W _out of the i-MQW core 31 on the light emitting side).

The first In the fabrication of the embodiment, first, as shown in FIG. ^3, n + -InP on the substrate 1, n-InP lower cladding layer (or simply, n-InP cladding layer) 36, i- After the MQW core layer 37, the p-InP upper clad layer (or simply the p-InP clad layer) 38, and the p ⁺ -InGaAs contact layer 39 are crystal-grown, as shown in FIG. Etching is performed in a curved taper shape, and then a p-electrode 34 and an n-electrode 7 are formed. As described above, in the manufacture of the first embodiment of the present invention, the crystal regrowth is not required, so that the manufacturing process is very simple.

  Further, as can be seen from FIG. 2, in the present invention, as in the fourth prior art, the width of the mesa is reduced as light propagates. Therefore, as in the fourth prior art, the electrical capacitance C can be greatly reduced as compared with the second prior art. That is, as in the fourth prior art, the optical modulation band defined by the CR constant by the electric capacitance C and the load resistance R can be greatly improved as compared with the second prior art.

  As described above, in the present invention, since it can be formed by one crystal growth, the manufacturing process becomes extremely simple, and the cost as an absorption semiconductor optical modulator can be remarkably reduced as compared with the fourth prior art. Further, since the i-MQW core 31 is wide, there is a great advantage that even if light of high power is absorbed, it is not broken by heat generation, and high-frequency light modulation is possible.

  Next, discussion will be made from the viewpoint of mode propagation and the resulting coupling loss (or insertion loss) of light, which is a major feature of the present invention. Actually, as shown in FIG. 2, in the present invention, light is incident from the AA 'side on the incident side and is emitted from the BB' side on the outgoing side. It is assumed that light is incident from the BB ′ side that is the emission side, and that light is originally emitted from the AA ′ side of the incident side.

  Reference numeral 40 in FIG. 4A denotes a mode (or light field distribution) of light propagating through a high-mesa optical waveguide in the vicinity of BB ′ where light is incident. This shows that a single mode is propagating. Reference numeral 41 in FIG. 4B denotes a mode (or light field distribution) when light incident from the BB ′ side is emitted from the AA ′ side, and the light mode on the BB ′ side. The shape is very similar to 40. The reason will be described below.

In FIG. 2, the width of the mesa on the AA ′ side is wide and a multimode optical waveguide is formed in the horizontal direction. Therefore, if the eigenmodes on the A-A ′ side are φ ₀ , φ _1, ..., Φ _n−1 , φ _n , the light ψ propagating on the BB ′ side is the eigenmode φ on the A-A ′ side. _{Using 0} , φ ₁ ,..., Φ _n−1 , φ _n ψ = c ₀ φ ₀ + c ₁ φ ₁ +... + C _n−1 φ _n−1 + c _n φ _n (2)
Can be deployed. Here, c ₀ , c ₁ ,... C _n−1 , c _n are expansion coefficients.

That is, since the width of the mesa on the AA ′ side is wider than the width of the mesa on the BB ′ side and more modes can be propagated in the expression (2), the light ψ propagating on the BB ′ side can be propagated. This means that the expansion coefficients c ₀ , c ₁ ,... C _n−1 , c _n are automatically adjusted so that the shape can be reproduced. Accordingly, the region indicated by Lin _in FIG. 2 can be regarded as if it were a free space.

The above is the same when light is incident from the AA ′ side, which is the original method of use of the present embodiment. In other words, even if light is incident from the A-A ′ side, the incident light is developed as shown in Equation (2) using the multi-mode on the A-A ′ side, and the shape of the incident light mode is substantially maintained. The region indicated by Lin _in FIG. 2 propagates like a free space.

  When these are expressed strictly in terms of electromagnetic field analysis, incident light does not propagate in free space, but the modes that make up the multimode share the energy of the incident light and synthesize those multimodes. As a result, it can be said that it seems to propagate in free space.

Further, when the width of the p-InP clad 32 is increased in order to increase the efficiency of Joule heat conduction to the n ⁺ -InP substrate 1, the first-order higher-order mode whose field distribution is an odd function at that time is excited. If the width of the p-InP clad 32 is increased so as to be excited to a higher order mode, it is disadvantageous from the viewpoint of optical coupling loss, but against thermal breakdown. The effect of the present invention to become strong can be exhibited.

  The single-mode property and multi-mode property of the optical waveguide are not simply determined by the width of the mesa of the high mesa optical waveguide or the width of the core in which the embedded optical waveguide is embedded. That is, in the case of high mesa, the thickness of the core and the difference in refractive index between the core and the upper and lower claddings, and in the case of an embedded optical waveguide, the thickness of the embedded core, and the embedded core with respect to the upper and lower or left and right The difference in refractive index from the cladding has an important effect. Therefore, even if the width of the mesa or the embedded core is widened, a single mode can be obtained. However, in the present invention, by combining the optical waveguide with a multimode, the coupling loss of incident light, and hence the absorption semiconductor light modulation, can be achieved. The insertion loss as a device is reduced.

In addition, since the length of Lin _in FIG. 2 is as short as 10 μm to 30 μm, or at most about 50 μm, the light incident on AA ′ propagates while maintaining almost the mode shape (strictly spreading a little). The insertion loss of light as an absorption semiconductor optical modulator does not increase significantly.

On the other hand, unlike the present invention, when the mesa width is widened while maintaining the single mode condition on the AA ′ side (for example, by reducing the thickness of the i-MQW core layer 31), the single mode becomes extremely flat. shape and become, a-A'in the optical coupling efficiency, and significantly deteriorated coupling efficiency of light from the end face of the light incident side in FIG. 2 to the L _in the region towards the B-B'-side in terms of the distance As a result, the insertion loss increases. That is, in the present invention, such a problem is solved by setting the AA ′ side to the multimode. Further, since the area of Lin _in FIG. 2 is tapered, an increase in insertion loss is effectively suppressed.

(Second Embodiment)
The width of the semiconductor portion for heat conduction in the present invention is not as effective as in the case of an exponential function or a power function, but the width of the semiconductor portion for heat conduction is set to a curve or a straight line other than the exponential function or the power function. The width may be narrowed along the propagation direction. As an example, the absorption semiconductor optical modulator of the second embodiment of the present invention is a case where the curved shape of the taper portion of the semiconductor portion for heat conduction is a straight shape in the first embodiment of the present invention shown in FIG. The schematic perspective view is shown in FIG. 5, and the top view is shown in FIG. In the figure, the same parts as those in the absorption semiconductor optical modulator of the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted.

  In the above description, the exponential function and the power curve and the straight line have been described as the shape for narrowing the mesa. However, any other curve such as a trigonometric function or a combination of a straight line and a curved line may be used. Needless to say. Furthermore, the width of the mesa may be narrowed discontinuously in a step shape instead of a straight line.

(Third embodiment)
FIG. 7 is a top view of an absorption semiconductor optical modulator according to the third embodiment of the present invention. As can be seen from FIG. 7, in this embodiment, the shape of the multi-mode region on which light is incident is rectangular. When the rectangular shape is used compared to the tapered shape, the insertion loss is slightly increased, but there is no problem in the effect of the present invention.

(Fourth embodiment)
FIG. 8 is a schematic perspective view of an absorption semiconductor optical modulator according to the fourth embodiment of the present invention. Here, 42 is an n-InP lower cladding layer (or simply, an n-InP cladding layer), 43 is an i-MQW core layer, and 44 is a p-InP upper cladding layer (or simply, a p-InP cladding layer). , 45 is a p ⁺ -InGaAs contact layer, 46 is an i-InP buried layer, and 47 is a p-electrode for applying an electric signal. Reference numeral 48 denotes a bonding pad portion.

  Unlike the first embodiment of the present invention which is the structure of the high mesa optical waveguide shown in the fourth embodiment and FIG. 1, in the fourth embodiment, the i-MQW core layer 43 as shown in FIG. There are i-InP buried layers 46 on the left and right sides, so to speak, this embodiment has a structure of a buried optical waveguide. Also in this embodiment, the width of the i-MQW core layer 43 on the light incident end face side is configured to be wider than the width of the i-MQW core layer 43 on the light exit end face side, and the optical waveguide on the light incident end face side is formed as a multi-wavelength. It is a mode optical waveguide.

  In this embodiment, as can be seen from FIG. 8, it is necessary to perform crystal regrowth in this embodiment. Therefore, the semiconductor optical modulator manufactured according to this embodiment is disadvantageous from the viewpoint of cost as compared with the other embodiments of the present invention, but the optical insertion loss because the optical waveguide on the light incident side is multimode. Has the advantage of being small.

(Various embodiments)
The substrate is assumed to be an n ⁺ -InP material, but it is not dependent on the type of substrate such as a p ⁺ -InP substrate or a semi-insulating InP substrate, and i-InGaAsP-InGaAsP is used as a material for the light absorbing portion. Although MQW is assumed, other multiple quantum wells such as i-InGaAs / InGaAsP MQW, i-InGaAs / InP MQW layer, i-InGaAlAs / InAlAs MQW, and bulk materials such as i-InGaAsP and i-InGaAlAs may be used. .

  In addition, although the embedding apparatus using the semi-insulating InP such as Fe-doped InP and the embedding technique are not necessary, of course, it may be used, and other materials can be used as long as the generated Joule heat can be released to the substrate. It goes without saying that may be used.

  Furthermore, since the present invention is for dispersing the heat generated, it can be applied to other semiconductor devices such as a semiconductor light receiver that generates heat as a result of absorbing light and converting it into a current.

1: n ⁺ -InP substrate 2, 11, 17, 30, 36, 42: n-InP lower clad layer 3, 12, 18, 31, 37, 43: i-MQW core layer 4, 13, 23, 25, 32, 38, 44: p-InP upper clad layer 5, 14, 26, 33, 39, 45: p ⁺ -InGaAs contact layer 6, 16, 28, 34, 47: p electrode 7: n electrode 8: polyimide 9 : Bonding wire 10, 29, 35, 48: Bonding pad 15, 27, 46: i-InP buried layer 19, 21: Mask 20, 22: Photoresist 24: Fe-InP buried layer 40, 41: Light mode ( Or distribution of light field)

Claims (4)

An optical waveguide including a core layer having at least a function of absorbing light is formed on a semiconductor substrate, and the optical waveguide has a light incident end surface, and light incident from the light incident end surface propagates through the optical waveguide. In an absorption semiconductor optical modulator in which a photocurrent is generated by being absorbed by the core layer in the process of
The core layer is formed to have a wide width on the light incident end face side so as to release the Joule heat to the semiconductor substrate in order to avoid thermal destruction due to Joule heat caused by the generated photocurrent. Formed to narrow toward the light propagation direction,
The absorption type semiconductor optical modulator characterized in that the optical waveguide on the light incident end face side constitutes a multimode optical waveguide.   2. The absorption semiconductor optical modulator according to claim 1, wherein the width of the core layer is narrowed according to at least one of a curved shape, a linear shape, and a stepped shape.   3. The absorption semiconductor optical modulator according to claim 1, wherein the optical waveguide is a high mesa optical waveguide.   3. The absorption semiconductor optical modulator according to claim 1, wherein the optical waveguide is a buried optical waveguide.

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