JP3816924B2 - Semiconductor waveguide type light control element - Google Patents

Description

  The present invention relates to various light control elements such as an ultrafast optical switch and a wavelength conversion element, and more particularly to a semiconductor waveguide type light control element using intersubband transition in a nitride semiconductor quantum well.

  Due to the rapid increase in the amount of information accompanying the spread of the Internet, a large amount of information of several hundreds of gigabits to several hundreds of terabits per second is expected to fly over the optical fiber in the near future in the backbone transmission system. In order to realize such an ultra-high capacity optical network, an optical control element such as an optical switching element or a broadband variable wavelength conversion element capable of an ultra-high speed operation on the order of 1 Tb / s is required. By increasing the processing speed and bandwidth, it is possible to reduce the number of parts, downsize, simplify control, simplify wavelength management, and so on. In particular, semiconductor optical control elements are smaller, lighter, and more stable than optical control elements that use optical fibers, etc., from the viewpoint of mass production, cost reduction, integration, low delay time, high-speed repetitive operation, etc. It is advantageous.

  As an operating principle of a 1 Tb / s class ultrahigh-speed semiconductor optical control device, the use of saturable absorption based on intersubband transition in a nitride semiconductor quantum well has been proposed (see, for example, Patent Document 1). The relaxation time between subbands is about three orders of magnitude faster than the relaxation time between carriers (carrier life). So far, inter-subband transitions in the near-infrared wavelength region used for optical fiber communication have been realized in InGaAs / AlAsSb, CdS / ZnSe / BeTe, and GaN / Al (Ga) N quantum well structures. Yes. Among them, the GaN system has a large interaction between the LO phonon and the electron, so that the intersubband relaxation time can be increased by an order of magnitude or more compared to the InGaAs system.

  For example, the intersubband relaxation time in the GaN quantum well is 140 to 400 fs at room temperature and in the optical communication wavelength band. Using saturable absorption of the transition between subbands (a phenomenon in which the absorption coefficient decreases when strong light is incident), pattern effects (element response time compared to signal time slots) even at ultra-high-speed repetitions of about 1 Tb / s Therefore, it is possible to perform an optical switching operation without causing a response waveform or the like to change or become distorted depending on the pattern.

However, there is generally a trade-off between the high-speed response of an optical switch and the energy consumption, and an optical pulse with a higher energy density is required for a high-speed optical switch. The saturation light energy intensity (input optical pulse energy density at which the absorption coefficient of intersubband absorption is reduced by half) near the absorption peak wavelength of GaN / AlN intersubband absorption in the optical communication wavelength band is the pulse width (here, 100). ˜500 fs) and absorption spectrum width (depending on the uniformity of the quantum well, flatness of the interface, ionized impurity density, electron density of the well layer, etc.), but on the order of several hundreds fJ / μm ² .

  In order to perform switching with a small energy pulse, it is necessary to reduce the mode cross-sectional area of the optical waveguide and increase the energy density. However, an optical waveguide with a small cross-sectional area has low optical coupling efficiency with the outside, and energy required for switching cannot be coupled into the waveguide unless strong pulse energy is input. Such an ultrashort optical pulse with a large energy makes it difficult to stably transmit an optical fiber without waveform deterioration, and a usable light source is limited to an impractical one.

  Therefore, in order to realize a practical optical switch, a structure with a so-called spot conversion function in which the cross-sectional area of the optical input / output unit is large but the cross-sectional area of the main part of the optical switch is small is essential. A mere taper of width cannot achieve both sufficient optical coupling efficiency and strong optical confinement in the multiple quantum well layer. An ideal spot conversion structure is a structure in which a tapered GaN core layer is embedded with an Al (Ga) N cladding layer (see, for example, Patent Document 2).

  However, it is difficult to grow a high-quality crystal with a thick AlN layer or a stepped AlN layer. One reason for this is that there is a 2.4% lattice mismatch with the GaN layer. When an AlN thick film is grown or an AlN layer is grown on a step, a large number of defects are generated. As a result, the crystal quality deteriorates and the substrate warps. In addition, the AlN layer is easily affected by residual oxygen in the growth chamber, and it is difficult to grow a high-quality thick film from this point.

  In addition, when an AlN thick film is grown under slightly III-rich conditions for growing a flat film by MBE, many Al droplets are often formed on the surface. Since the droplets are not pure Al but contain Al compounds, it is difficult to remove them by hydrochloric acid treatment or the like later. In the state where the droplets remain, it is extremely difficult to form a fine pattern. Even if the droplets can be removed, crystal defects such as dislocations are usually formed under the droplets, and large pits are formed during etching or electrons are collected at the dislocations, resulting in serious light propagation loss. Cause.

Therefore, it has been difficult to produce a spot diameter conversion structure such as a structure in which a tapered GaN core layer is embedded with an AlN cladding layer without deteriorating crystal quality so much and with a high yield.
JP-A-8-179387 JP 2002-107776 A

  As described above, the semiconductor waveguide optical control device using the GaN-based intersubband transition has high speed and wide bandwidth, but has problems of low efficiency and large switching energy, which is a practical obstacle. It was. Further, since the lattice mismatch is large and it is difficult to grow a high-quality thick AlN layer or a stepped AlN layer, it is difficult to produce a spot diameter conversion structure for solving the above-described problems.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a semiconductor waveguide light control element that is ultrafast, has low switching energy, and is easy to manufacture. is there.

  In order to solve the above problems, the present invention adopts the following configuration.

  That is, the present invention relates to the optical control waveguide via the tapered waveguide section on at least one of the input side or the output side of the optical control waveguide section that generates a nonlinear optical response due to intersubband transition in the nitride semiconductor multiple quantum well. A semiconductor waveguide type optical control device having an input / output waveguide portion having a larger mode cross-sectional area than the waveguide portion, a nitride semiconductor multiple quantum well layer that causes intersubband transition on the substrate, A GaN core layer sandwiching the quantum well layer from above and below, a nitride semiconductor lower cladding layer having an average refractive index lower than that of the GaN core layer, and an average provided on the GaN core layer. And a nitride semiconductor upper cladding layer having a refractive index lower than that of the GaN core layer, and the light control waveguide section includes at least the upper cladding layer of the multilayer structure. The GaN core layer and the multi-quantum well layer are formed of a mesa-shaped single mode optical waveguide formed by removing the multi quantum well layer outside the mesa, and the input / output waveguide portion and the tapered waveguide portion are formed on the multilayer structure. A tapered ridge type optical waveguide formed with a loading layer having a refractive index higher than that of the substrate, wherein the tapered waveguide portion is a width of the mesa having the multilayer structure from the input / output waveguide portion to the light control waveguide portion. And the thickness of the loading layer continuously or intermittently decreases, and extends from the portion in contact with the input / output waveguide portion toward both sides of the tapered waveguide portion toward the light control waveguide portion. The thickness of the slab portion is reduced corresponding to a change in the thickness of the loaded layer in the tapered waveguide portion.

  The present invention is also directed to at least one of the input side and the output side of the optical control waveguide unit that generates a nonlinear optical response due to intersubband transition in the nitride semiconductor multiple quantum well, via the tapered waveguide unit. A semiconductor waveguide light control element provided with an input / output waveguide portion having a larger mode cross-sectional area than the waveguide portion, wherein the optical waveguide extending from the light control waveguide portion to the input / output waveguide portion is the substrate The main components are at least a lower clad layer made of a nitride semiconductor, a GaN lower core layer having an average refractive index higher than that of the lower clad layer, a nitride semiconductor multiple quantum well layer that causes intersubband transition, and a GaN layer. The light control waveguide section is composed of a multilayer structure formed by laminating an upper core layer as a component and an upper clad layer having an average refractive index higher than that of the substrate and lower than that of the upper core layer. A mesa-shaped single mode optical waveguide formed by removing at least the upper cladding layer, the upper core layer, the multiple quantum well layer, and the lower core layer of the layer structure outside the mesa, and the input / output waveguide The waveguide section is composed of a ridge-type optical waveguide in which the upper core layer is formed thicker than the light control waveguide section, and the taper waveguide section extends from the input / output waveguide section to the light control waveguide section. The width of the mesa of the multilayer structure and the thickness of the upper core layer are continuously or intermittently reduced toward the light control waveguide portion from the portion in contact with the input / output waveguide portion. The thickness of the slab portion extending on both side surfaces of the tapered waveguide portion is reduced corresponding to the change in the thickness of the upper core layer in the tapered waveguide portion.

  According to the present invention, light can be efficiently coupled from the outside of the device to the light control waveguide portion having a small mode cross-sectional area by the input / output waveguide portion and the tapered waveguide portion. In addition, the optical control waveguide is a single-mode high-mesa optical waveguide with a small mode cross-sectional area and a strong optical confinement effect. Therefore, the optical electric field strength in the nitride semiconductor multiple quantum well layer can be increased, and nonlinear optics such as absorption saturation The effect can be produced efficiently with a relatively small power input light. The input / output waveguide portion is a ridge type optical waveguide that does not increase the number of propagation modes even if the mode cross-sectional area is large, and can be used as a pseudo single mode by appropriate alignment. As a result, instability such as light output fluctuation due to interference between modes can be suppressed. In the present invention, in the tapered waveguide portion, the width of the ridge type optical waveguide is reduced from the input / output waveguide portion to the light control waveguide portion, and the thickness of the slab portion is reduced. Can be suppressed to a low level.

  Therefore, since the epitaxial growth of a thick AlN film that has deteriorated the process yield and optical waveguide characteristics is not required, the manufacturing is facilitated, the yield is improved, and the cost and environmental load can be reduced. As a result, it is possible to realize a semiconductor waveguide light control element that is ultrafast, has low switching energy, and is easy to manufacture.

  The details of the present invention will be described below with reference to the illustrated embodiments. In the following description, unless otherwise specified, it is assumed that the optical wavelength is a near-infrared wavelength in the vicinity of 1.55 μm, and the polarization is TM mode (the electric field is perpendicular to the well layer).

(First embodiment)
FIGS. 1-7 is a figure for demonstrating the semiconductor waveguide type optical control element (optical switch) concerning the 1st Embodiment of this invention.

  The optical switch according to this embodiment includes a nitride semiconductor waveguide layer 2 stacked on the sapphire substrate 1 (0001) surface as a main component. The nitride semiconductor waveguide 2 includes, from the input side, an input waveguide section 3 having a length of about 30 μm, an input taper waveguide section 4 having a length of 40 μm, an optical switch main section (light control waveguide section) 5 having a length of 360 μm, The output taper waveguide section 6 has a length of 40 μm and the output waveguide section 7 has a length of about 30 μm. Here, to simplify the explanation, it is assumed that the input waveguide portion 3 and the output waveguide portion 7 and the input tapered waveguide portion 4 and the output tapered waveguide portion 6 have the same structure symmetrical with respect to the propagation direction. Detailed description on the output side will be omitted. In an actual optical switch, it is not necessary that the input side and the output side have a symmetrical structure.

  The feature of the optical switch described in this embodiment is that an insulator film having a small refractive index difference from that of the nitride semiconductor layer is used above the layer structure constituting the optical waveguide. Specifically, an example in which an AlN film formed by sputtering is used will be described.

  FIG. 1 is a diagram schematically showing the structure of the vicinity of the optical input portion of this semiconductor waveguide optical switch, that is, the portion from the input waveguide portion 3 to the entrance of the main portion 5 of the optical switch (the scale is not accurate). ). Since the input side and the output side are symmetric, the input waveguide section 3 and the input tapered waveguide section 4 in FIG. 1 correspond to the output waveguide section 7 and the output tapered waveguide section 6, respectively, when viewed from the output side. Although not shown in the drawing, an antireflection coating is applied to the end face of the optical waveguide.

  The input waveguide section 3 is a ridge type optical waveguide having two propagation modes, and the optical switch main section 5 is a single mode high mesa type optical waveguide. In the tapered waveguide section 4, the optical waveguide width decreases from 3 μm to 0.8 μm in the range of 40 μm from the input waveguide section 3 to the optical switch main section 5. At three points in between, a step 8 having a thickness of 400 nm per one point is provided. Depending on the resist baking conditions, etching conditions, and the like at the time of forming the step, the step is devised so as to be as gentle as possible. Although the optical waveguide portions 3 and 5 having different structures are connected by the tapered waveguide portion 4, the tapered waveguide portion 4 may be a multimode optical waveguide. The slab portions 9 of the nitride semiconductor layer exist on both side surfaces of the ridge type optical waveguide.

  FIG. 2 is a diagram schematically showing a cross-sectional structure perpendicular to the light propagation direction of the main part 5 of the optical switch. The optical switch main part 5 includes, from the sapphire substrate 1 side, an undoped AlN lower cladding layer 11 having a thickness of 200 nm, an undoped GaN lower core layer 12 having a thickness of 200 nm, an undoped AlN barrier layer having a thickness of 4 nm, and a Si having a thickness of about 2 nm. It consists of a multiple quantum well layer 13 in which 10.5 pairs of doped GaN well layers are alternately stacked, an undoped GaN layer upper core layer 14 having a thickness of 200 nm, and an undoped AlN upper cladding layer 15 having a thickness of 200 nm. This nitride semiconductor multilayer structure is a single crystal epitaxial growth layer formed by molecular beam epitaxial growth (MBE). This layer structure is processed into a high-mesa optical waveguide having a width of 0.8 μm by dry etching.

  In order to make this high mesa type optical waveguide into a single mode, it is necessary to make the mesa width 0.9 μm or less and the mesa layer thickness 0.9 μm or less. Since this optical waveguide has a small cross-sectional area and a peak of a mode hits the multiple quantum well layer 13, the light intensity in the multiple quantum well layer 13 can be increased, and absorption saturation occurs efficiently with relatively small power input light. be able to. Even when the slab part remains on the side surface of the optical waveguide, this condition is satisfied if the thickness of the slab part is 0.2 μm or less.

  FIG. 3 is a diagram schematically showing a cross-sectional structure perpendicular to the light propagation direction of the input waveguide section 3. The layer structure below the input waveguide section 3 is the same as the layer structures 11 to 15 of the optical switch main section 5, and an AlN film 16 having a thickness of 1.2 μm is formed thereon as a loading layer. The AlN film 16 is an insulating film formed not by MBE growth but by sputtering. Although there is a tendency to be easily oriented due to the influence of the underlayer, since it is not a single crystal film, the generation of defects and the increase in propagation loss caused by it can be suppressed as in the case of a thick AlN epitaxial growth layer. The refractive index is 2.0 to 2.1, and functions in the same manner as an AlN epitaxial growth layer as a cladding layer. This layer structure is processed by dry etching into a ridge type optical waveguide having a width of 3 μm and a height of 1 μm.

  In this optical waveguide, there are no propagation modes having three or more antinodes in the lateral direction in the thickness direction. In addition to the basic mode, there are also two propagation modes (odd mode) in the horizontal and height directions, but the cross-sectional size is relatively large, so the alignment of incident light is adjusted appropriately so as not to excite the odd mode. Then, it can be used as a pseudo single mode optical waveguide.

  Despite its large size, there can be no higher-order modes with three or more antinodes in the thickness direction or in the lateral direction because radiation occurs due to coupling with the slab mode and does not become a propagation mode. is there. If the mesa is made too deep with respect to the overall layer thickness, the coupling with the slab mode is weakened, resulting in multiple modes. On the other hand, if the mesa is too low with respect to the layer thickness, light confinement becomes weak and radiation loss increases. If the mesa width is too wide, it becomes multimode, and if it is too narrow, the coupling efficiency decreases and the radiation loss increases. If the layer thickness and mesa width are not more than twice that of the main part 5 of the optical switch, the effect of the taper structure is insufficient. Although there are some differences depending on the structure, the standard of mesa height is about 80% to 130% of the layer thickness. Furthermore, the standard of mesa width is about 80% to 200% of the layer thickness.

  Since this optical waveguide has a large mode cross-sectional area, it can be highly efficiently coupled to the outside via a tip spherical taper fiber or a lens. Therefore, even if there is a slight error in the open position of the input waveguide section 3, variations in the optical switch characteristics and the optical coupling characteristics can be suppressed to be small.

  4 is a diagram showing a cross-sectional layer structure in which the input tapered waveguide portion 4 is vertically cut along the light propagation direction. FIG. 5 is a sectional view of the slab portion 9 outside the tapered waveguide portion 4 taken along a plane parallel to FIG. Cross-sectional layer structure. 4 and 5, the scale is greatly different between the propagation direction and the vertical direction. The sputter-formed AlN film 16 of the tapered waveguide section 4 is gradually thinned by steps 8a, 8b, and 8c having a height of 400 nm provided every 10 μm in length, and the upper AlN cladding layer 15 is formed beyond the last step 8c. Exposed. Corresponding to the step of the tapered waveguide portion 4, steps 8d, 8e, 8f are also formed in the slab portion 9, and the sapphire substrate 1 is exposed at the final step 8f.

  In the above-described structure, the AlN lower cladding layer 11 to the sputtered AlN film 16 are laminated on the entire surface of the substrate, and then three steps 8 are sequentially formed perpendicular to the light propagation direction, and then the optical waveguide portions 3 and 4 are formed. , 5, 6 and 7 can be produced. Since there is no growth of a thick AlN film having a thickness of 250 nm or more and no growth on the steps, the crystal quality is not lowered and droplets are not generated, and the production is easy. Since the inclination angle of the step 8 is small, a process with good reproducibility is possible even though the optical waveguide pattern is thin.

Although there is a difference between polycrystal and single crystal, the sputtered AlN film 16 and the other epitaxial growth layers 11 to 15 are simultaneously etched by Cl ₂ / BCl ₃ / Ar dry etching. However, since the etch rate differs depending on the material depending on conditions, the heights of the steps 8d, 8e, and 8f of the slab portion 9 corresponding to the steps 8a, 8b, and 8c of the waveguide portion are different. In order to simplify the description, the case where the mesa side surface of the optical waveguide is vertical is illustrated, but in an actual device, the side surface may be slightly inclined.

FIG. 6 is a diagram schematically showing the conduction band structure of the multiple quantum well layer 13. In Group III polarity, the left side is the front side and the right side is the substrate side. Although the number of wells is 10, FIG. 6 shows only two periods near the center. As described above, the multiple quantum well layer 13 includes the undoped AlN barrier layer 17 having a thickness of 4 nm and the GaN well layer 18 having a thickness of about 2 nm, and the GaN well layer 18 has Si of about 4 × 10 ¹⁹ cm ^−3. Doped.

  A large electric field (several MV / cm) is generated in the wurtzite nitride semiconductor heterostructure whose principal surface is the (0001) plane due to the piezoelectric effect and spontaneous polarization. The direction of this electric field is reversed between AlN and GaN. Due to the redistribution of surface charge and the spatial movement of carriers across the well layer, the voltage drop per cycle is almost zero in the well near the center. Although five subbands E1, E2, E3, E4, and E5 are formed in the well layer 18, almost all electrons exist only in E1 in a thermal equilibrium state. The intersubband absorption wavelength between E1 and E2 is in the vicinity of 1.6 μm. The levels of E1 and E2 do not change so much even if the internal electric field fluctuates, but the level of E3 or higher is sensitive to fluctuations in the internal electric field (stress, thickness, etc.) and varies greatly depending on the location. Absorption due to transitions other than between E1 and E2 is very weak.

  The state of light propagation in the vicinity of the light input portion of this optical switch was simulated. However, in order to shorten the calculation time, the length of the input waveguide portion 3 was shortened to 10 μm, and the beam diameter of incident light was assumed to be 3 μm. As a result, interference between modes (including non-propagating modes) is observed inside the input waveguide section 3 and the input tapered waveguide section 4, and radiation loss to the substrate 1 and the slab section 9 is also observed. It was found that about 60% of the light coupled to the input waveguide section 3 was coupled to the fundamental mode of the optical switch main section 5. Although the input beam shape was not optimized, the optical coupling loss from the outside to the main part of the optical switch was 8 dB.

  FIG. 7 shows the light transmission characteristics of the optical switch of this embodiment in comparison with a conventional optical switch. The wavelength of the input switch light is 1.55 μm, the wavelength and pulse energy of the switched light are 1.7 μm and 10 fJ, respectively, and the pulse width is 200 fs.

  As a comparative example, consider the ridge type optical waveguide (RWG) shown in FIG. This optical waveguide includes a thin AlN buffer layer 41, a lower GaN layer 42 having a thickness of 1 μm, and a GaN / AlN multiple quantum layer having intersubband absorption in the vicinity of a wavelength of 1.57 to 1.58 μm. The well layer 43 is composed of an upper GaN layer 44 having a thickness of 1 μm. The upper GaN layer 44 is processed into a mesa shape having a width of 3 μm, and a waveguide type optical device having a length of 500 μm is formed by opening the top.

  In the waveguide type optical control element having the structure shown in FIG. 13, transmission / absorption with respect to a signal light pulse with a wavelength of 1.65 μm is switched by saturable absorption with a strong control light pulse with a wavelength of 1.55 μm. Although the optical coupling loss to this optical waveguide is about 5 dB, the incident light spreads and propagates in a region of about 2 μm in thickness and about 3 μm in width, so that the light intensity (energy density) in the multiple quantum well layer 43 is increased. I can't take it. Therefore, as indicated by a dotted line (RWG) in FIG. 7, the optical pulse energy required for switching with an extinction ratio of 10 dB is as large as about 23 pJ, which is not practical.

  Therefore, a small high mesa (HM) optical waveguide having a cross section of 0.8 μm square is considered. In the case of this optical waveguide, the optical confinement in the multiple quantum well layer 43 is strong, but the optical coupling loss from the outside is about 15 dB. Therefore, as indicated by a one-dot chain line (HM) in FIG. 7, the optical pulse energy to be supplied from the outside in order to obtain an extinction ratio of 10 dB or more is 23.4 pJ, and the insertion loss is the ridge type optical waveguide of FIG. It was much bigger.

  On the other hand, in the optical switch (solid line, TAPER) of this embodiment, the switching light pulse energy required to obtain an extinction ratio of 10 dB was improved to 4.8 pJ. Thus, according to the present embodiment, the pulse energy required for ultrafast optical switching can be greatly reduced.

  In order to realize stable single mode propagation in the main part 5 of the optical switch, the core layers 12 and 14 including the multiple quantum well layers 13 are formed into the cladding layers 11 and 15 having a refractive index lower than that of the core layer. It is essential to be sandwiched. When this condition is not satisfied (including the case where the clad layer is only on one side), the light component propagating in a zigzag manner becomes significant, and the long optical switch main part 5 cannot be stably propagated. An increase in the radiation component to the substrate 1 and a decrease in the optical confinement coefficient in the multiple quantum well layer 13 occur, resulting in a decrease in optical switching characteristics.

  On the output sides 6 and 7, the light beam spreads, the emission angle becomes small, and the optical coupling loss with the outside is reduced. If the coupling method is the same as that on the input side, almost the same coupling efficiency (8 dB) is realized due to the reciprocity of light propagation. Compared to the case where there is no output taper waveguide portion 6 and no output waveguide portion 7, the improvement is about 7 dB.

The semiconductor waveguide type optical waveguide element of this embodiment can be variously modified. For example, if the AlN film 16 is deposited as much as 1.2 μm even by sputtering, depending on the deposition method, the stress applied to the epitaxial growth layer becomes large, causing problems such as generation of cracks. As a countermeasure, the upper half (about 600 nm) of the AlN film can be replaced with a SiN _x film (n = 1.9 to 2.1) having a refractive index close to that of the AlN layer. By using a multilayer structure, the influence of stress can be reduced. If the deposition method of the SiN _x film is controlled so that the refractive index is substantially equal or not larger than that of the AlN film, the light propagation characteristics can be made substantially the same as in the above-described embodiment.

Various modifications, combinations, and applications are possible, such as an extra epitaxially grown AlN layer of about 200 nm and an SiN _x film of 1 μm laminated thereon. The use of a SiN _x film having poor heat conduction is slightly disadvantageous from the viewpoint of heat dissipation, but in the input waveguide section 3 and the input tapered waveguide section 4, heat generation due to intersubband absorption is small. As long as it is limited to the use of, there is no substantial problem.

  If the uppermost layer of the MBE growth layer is AlN, the surface is easily oxidized, and defects that adversely affect the optical waveguide characteristics may occur near the interface between the AlN upper cladding layer 15 and the AlN film 16 as the loading layer. In order to avoid the influence, a thin GaN layer may be sandwiched between the upper cladding layer 15 and the loading layer 16.

Further, the surface of the optical waveguide or the substrate may be covered with a dielectric film or an insulating film having a refractive index smaller than that of the nitride semiconductor (or SiN _x film). For example, even when the periphery of the optical waveguide is covered with a SiO ₂ film having a thickness of about 600 nm or when the optical waveguide is flattened with a polyimide film, the above-mentioned propagation characteristics do not show so much difference.

  A part or all of the AlN barrier layers 17 of the AlN cladding layers 11 and 15 and the multiple quantum well layer 13 may be an AlGaN layer or a GaN / AlN multilayer structure. For example, if a GaN / AlN multilayer structure is provided between the AlN cladding layer and the GaN core layer, graded index light confinement can be realized. The GaN / AlN multilayer structure is also effective in reducing dislocations. Further, for the purpose of increasing the difference in refractive index or deepening the well, at least a part of the core layer or the well layer may be an InGaN layer.

  In some cases, such as when integrating in series with other elements, the tapered waveguide section may be required only on either the input side (4) or the output side (6). The input waveguide section 3 and the output waveguide section 7 may be used as a long passive waveguide. Of course, such a passive waveguide may have a curved shape. Various other modifications can be considered without departing from the scope of the present invention.

  As described above, according to the present embodiment, light can be efficiently coupled from the outside of the device to the optical switch main portion 5 having a small mode cross-sectional area by the input / output waveguide portion 3 and the tapered waveguide portion 4. Since the optical switch main part 5 is a single-mode high-mesa optical waveguide having a small mode cross-sectional area and a strong optical confinement effect, the optical electric field strength in the nitride semiconductor multiple quantum well layer can be increased, and nonlinear optical effects such as absorption saturation can be achieved. Can be efficiently generated with input light of relatively small power. Further, the input / output waveguide section 3 does not have a large number of propagation modes even if the mode cross-sectional area is large, the mesa height is 80% to 130% of the layer thickness, and the mesa width is 80% to 200% of the layer thickness. By using a ridge type optical waveguide, it can be used as a pseudo single mode by appropriate alignment. As a result, instability such as light output fluctuation due to interference between modes can be suppressed. Since the cross-sectional size is large, alignment is relatively easy.

  Although the waveguide structure is different between the input / output waveguide portion 3 of the ridge waveguide and the optical switch main portion 5 of the high mesa waveguide, the fluctuation of the mesa height in the tapered waveguide portion 4 is suppressed to be relatively small. A low loss connection is realized. Generally, when the width of the ridge-type optical waveguide is narrowed, the radiation loss to the slab portion 9 is increased. However, in this embodiment, the thickness of the slab portion 9 is also thinned, so that the radiation loss can be suppressed to a low level. .

  That is, according to the present embodiment, the epitaxial growth of the thick AlN film, which has deteriorated the process yield and the optical waveguide characteristics through the generation and increase of droplets, dislocations, warpage, etc., is no longer necessary, which facilitates the production. In addition, the yield can be improved, and the cost and environmental load can be reduced.

(Second Embodiment)
FIG. 8 is a diagram schematically showing a cross-sectional structure of the input waveguide portion 3 of the semiconductor waveguide light control element (optical switch) according to the second embodiment of the present invention.

  This embodiment is different from the first embodiment described above only in that a GaN epitaxial growth layer 21 is used instead of the AlN insulating film 16. Since the configuration of the other parts is exactly the same, the description thereof is omitted. Even if GaN is somewhat thick, a high-quality single crystal layer can be grown by MBE.

  The state of light propagation in the vicinity of the light input portion of the optical switch according to the present embodiment was simulated. As in the case of the first embodiment, the beam diameter of the incident light is assumed to be 3 μm, and the length of the input waveguide section 3 is 10 μm. In the input waveguide section 3, the GaN layer 21 having a large refractive index is also provided on the upper cladding layer 15, so that it is a kind of coupled optical waveguide. Compared to the case of the first embodiment, the light distribution in the input waveguide section 3 and the input tapered waveguide section 4 tends to be closer to the thick GaN layer 21 having a high refractive index. For this reason, although the loss component reflected by the level | step difference 8 and radiated | emitted to the board | substrate 1 increases, the radiation loss to the slab part 9 reduces. As a result, the proportion of light that can be coupled to the fundamental mode of the optical switch main part 5 is almost the same as in the first embodiment. Accordingly, optical switch characteristics substantially equivalent to those of the optical switch of the first embodiment can be obtained.

  As described in the section of the first embodiment, the upper cladding layer 15 in the optical switch main part 5 is essential. When a simulation of light propagation was performed for a structure in which the upper cladding layer 15 was also replaced with a GaN layer, stable propagation in the fundamental mode was not obtained, and light confinement in the multiple quantum well layer 13 was also deteriorated. I understood.

  The optical switch of the present embodiment can be variously modified and applied without departing from the spirit of the invention. For example, as shown in FIG. 9, a thin AlN layer 22 for controlling step etching may be sandwiched between GaN layers 21. For example, the step etching depth can be controlled by properly using an etching condition in which the etching rate of AlN is smaller than that of GaN and an etching condition in which the selectivity of AlN and GaN is small. Alternatively, the etching depth can be controlled by using the AlN layer 22 as a marker layer and monitoring the etching product (Al compound). Since the AlN layer 22 is thin, the influence on the crystal quality is small.

  Although not shown in FIG. 9, the AlN layer 22 may also be present on the top GaN layer 21. Of course, the same effect can be obtained by using an AlN / GaN multilayer structure or an AlGaN layer instead of these AlN layers 22. Further, the loading layer is not limited to AlN or GaN, and a layer having a refractive index higher than that of the sapphire substrate can be used.

(Third embodiment)
FIG. 10 schematically shows a cross-sectional layer structure in which the input tapered waveguide section 4 of the semiconductor waveguide light control element (optical switch) according to the third embodiment of the present invention is cut vertically along the light propagation direction. FIG. FIGS. 11 and 12 are diagrams schematically showing the cross-sectional structures of the input waveguide section 3 and the optical switch main section 5 of the optical switch. Since the basic configuration is similar to that of the first embodiment, the description here will focus on the differences from the first embodiment.

  The optical switch of this embodiment is manufactured as follows. First, nitride semiconductor layers 11, 31, 12, 13, and 34 are grown on the flat (0001) sapphire substrate 1 by MBE. More specifically, a 50 nm thick AlN cladding layer 11 serving as a buffer layer for dislocation reduction, surface planarization, and polarity control is laminated on the substrate 1, and a GaN layer and an AlN layer are formed thereon. An AlN / GaN multilayer structure 31 having a total thickness of 150 nm grown by 15 pairs is stacked. The AlN / GaN multilayer structure 31 has a relative relationship between the GaN layer and the AlN layer so that the AlN layer is thicker on the lower AlN cladding layer 11 side and the GaN layer is thicker on the upper GaN core layer 12 side. This is a gradient refractive index layer in which a proper ratio is modulated. This multilayer structure 31 is also effective in reducing dislocations. The GaN lower core layer 12 and the multiple quantum well layer 13 are the same as those in the first embodiment, but an upper GaN layer 34 having a thickness of 1.4 μm is stacked thereon.

Thereafter, three steps 8a, 8b and 8c having a length of about 400 nm are formed in the input / output tapered waveguide portions 4 and 6 at intervals of 10 μm by patterning and Cl ₂ / BCl ₃ / Ar dry etching three times. To do. Etching at this time is performed under the condition that the inclination of the step is 20 degrees or less. As a result, the thickness of the upper GaN layer 34 of the optical switch main part 5 becomes 200 nm. After that, by patterning once and Cl ₂ / BCl ₃ / Ar dry etching, the width of the input / output waveguide portions 3 and 7 is about 2.5 μm, and the width of the optical switch main portion 5 is about 0.5 μm. Then, an optical waveguide mesa having tapered waveguide portions 4 and 6 is formed. Since the etch rates are slightly different between GaN and AlN, the mesa height is about 800 nm in the input / output waveguide portions 3 and 7 and about 500 nm in the optical switch main portion 5. In the figure, the side surface is drawn vertically, but in reality, the side surface is trapezoidal with an inclination of about 80 degrees.

On this, an AlN film 35 is deposited to a thickness of about 300 nm by sputtering. Actually, since the mesa side wall is slightly inclined, an AlN film 35 having a thickness of 150 to 200 nm is also deposited on the side wall. The AlN film 35 functions as an upper and side cladding layer in the optical switch main part 5. Further, the whole may be covered with a SiO ₂ film or the like. Thereafter, the chip that has passed the inspection is mounted on the module with the polarization maintaining fiber through processes such as back surface polishing, widening / cutting, end surface non-reflective coating formation, inspection / selection, and the like.

  According to this manufacturing method, since there is no growth process of a thick AlN layer exceeding 50 nm, a crystal with higher quality and less optical loss than the first and second embodiments can be obtained. In the tapered waveguide portion 4, the cross-sectional dimension of the GaN layer having a large refractive index is gradually reduced, so that the loss can be kept low. In addition, since the optical switch main part 5 has the AlN film 35 serving as a cladding on the side surface, the confinement effect in the width direction becomes larger than that of the optical switch of the first or second embodiment, and the multiple quantum well layer 13 The light intensity of the light can be increased, and more stable fundamental mode light propagation is guaranteed. The other points are almost the same as those in the first and second embodiments.

  The optical switch of the present embodiment can be variously modified and applied without departing from the spirit of the invention. For example, as in the modification of the second embodiment, a thin AlN layer for step etching control (or for monitoring), a GaN / AlN multilayer film, or the like may be inserted into the GaN layer 44.

(Modification)
The semiconductor waveguide type optical control element of the present invention is not only an ultrafast optical switch (repetition to 1 Tb / s), but also an ultrafast optical modulator that modulates light with light (modulation band to about 2 THz), and a variable conversion wavelength. The present invention can also be applied to a broadband wavelength batch conversion element (maximum wavelength shift amount to about 100 nm). The multiple quantum well may be replaced with a coupled multiple quantum well and used as an optically pumped intersubband optical amplifier. It is effective in reducing necessary optical pulse switching energy and pumping light power.

  The substrate is not limited to sapphire. For example, if a high-quality AlN substrate can be used in the future, the lower cladding layer can be replaced with an AlN substrate. However, when a substrate having a higher refractive index than that of a nitride semiconductor such as Si is used, it is necessary to sandwich a layer having a refractive index sufficiently lower than that of the nitride semiconductor between the substrate and the lower cladding layer by about 1 μm.

  The present invention can be variously modified and applied without departing from the spirit of the invention, in addition to the above-described embodiments and modifications thereof.

(Summary)
As described above, the present invention is a semiconductor waveguide light control element using a nonlinear optical response due to intersubband transition in a nitride semiconductor multiple quantum well, and includes a light control waveguide portion that generates a nonlinear optical response. An input / output waveguide portion having a mode cross-sectional area larger than that of the optical waveguide constituting the light control waveguide portion is provided on at least one of the input side or the output side of the optical waveguide constituting the optical waveguide portion via the tapered waveguide portion. And in particular,
(1a) On a substrate, a nitride semiconductor multiple quantum well layer causing intersubband transition, a GaN core layer having this multiple quantum well layer inside, and an average refractive index provided below the GaN core layer are A multilayer structure comprising a nitride semiconductor lower cladding layer lower than the GaN core layer and a nitride semiconductor upper cladding layer having an average refractive index lower than the GaN core layer provided on the GaN core layer is formed. ,
(1b) The light control waveguide section is a mesa-shaped single mode optical waveguide formed by removing at least the upper cladding layer, the GaN core layer, and the multiple quantum well layer of the multilayer structure outside the mesa. Consists of
(1c) The input / output waveguide section and the tapered waveguide section have a loading layer having a thickness higher than the total layer thickness of the multilayer structure and a higher refractive index than the substrate, on the multilayer structure. Consisting of a ridge-type optical waveguide,
(1d) In the tapered waveguide portion, the width of the mesa of the multilayer structure and the thickness of the loading layer decrease continuously or intermittently from the input / output waveguide portion to the light control waveguide portion. And
(1e) Further, the thickness of the slab portion extending from the portion in contact with the input / output waveguide portion toward both sides of the tapered waveguide portion toward the light control waveguide portion is equal to the thickness of the loading layer in the tapered waveguide portion. Decreasing in response to changes in thickness,
It is characterized by this.

  Here, preferred embodiments of the present invention include the following.

  (1-1) The loading layer is an insulating film.

(1-2) The insulating film is an AlN film, a SiN _x film, or a laminated structure of an AlN film and a SiN _x film.

  (1-3) The loading layer is composed of a GaN layer or a laminated structure of a GaN layer and an AlN layer having a thickness of 50 nm or less.

  (1-4) The total layer thickness of the multilayer structure is 0.9 μm or less, and the mesa width is 0.9 μm or less.

  (1-5) The refractive index of the loading layer in the input / output waveguide section is 1.9 or more.

  (1-6) The height of the mesa in the ridge type optical waveguide is 80% to 130% of the total layer thickness of the multilayer structure and the loading layer, and the width of the mesa is 80% to the total layer thickness of the multilayer structure and the loading layer. 200%.

  (1-7) The substrate is any one of sapphire, SiC, Si, GaN, and AlN.

  (1-8) In the ridge type optical waveguide, the loading layer is formed thicker than the total layer thickness of the multilayer structure.

The present invention also relates to a semiconductor waveguide type optical control element using a nonlinear optical response due to transition between subbands in a nitride semiconductor multiple quantum well, and an optical waveguide that constitutes an optical control waveguide section that generates a nonlinear optical response At least one of the input side or the output side of the is provided with an input / output waveguide portion having a larger mode cross-sectional area than the optical waveguide constituting the light control waveguide portion via the tapered waveguide portion, In particular,
(2a) An optical waveguide extending from the light control waveguide section to the input / output waveguide section includes, from the substrate side, a lower cladding layer made of a nitride semiconductor, and a GaN lower core having an average refractive index higher than that of the lower cladding layer A layer, a nitride semiconductor multiple quantum well layer that causes intersubband transition, an upper core layer having a GaN layer as a main component, and an average refractive index provided above the GaN upper core layer is higher than that of the substrate. A multilayer structure comprising at least an upper cladding layer that is higher than the upper core layer,
(2b) The light control waveguide section is a mesa-shaped structure formed by removing at least the upper cladding layer, the GaN upper core layer, the multiple quantum well layer, and the GaN lower core layer of the multilayer structure outside the mesa. Consisting of a single-mode optical waveguide,
(2c) In the input / output waveguide section and the tapered waveguide section, the upper core layer is thicker than the light control waveguide section, and the total layer thickness of the multilayer structure in the input / output waveguide section is A ridge-type optical waveguide that is at least twice the total layer thickness of the multilayer structure in the light control waveguide portion;
(2d) In the tapered waveguide portion, the width of the mesa of the multilayer structure and the thickness of the upper core layer decrease continuously or intermittently from the input / output waveguide portion toward the light control waveguide portion. And
(2e) and the thickness of the slab portion extending from the portion in contact with the input / output waveguide portion toward both sides of the tapered waveguide portion toward the light control waveguide portion is an upper core layer in the tapered waveguide portion. In response to changes in the thickness of the
It is characterized by that.

  Here, preferred embodiments of the present invention include the following.

(2-1) The upper cladding layer has an AlN film, a SiN _x film, or a laminated structure of an AlN film and a SiN _x film.

  (2-2) The mesa side surface of the optical control waveguide section is also covered with the same material as the upper cladding layer.

  (2-3) The refractive index of the upper cladding film is 1.9 or more.

  (2-4) The height of the mesa in the ridge type optical waveguide is 80% to 130% of the total layer thickness of the multilayer structure and the loading layer, and the width of the mesa is 80% to the total layer thickness of the multilayer structure and the loading layer. 200%.

  (2-5) The substrate is any one of sapphire, SiC, Si, GaN, and AlN.

  (2-6) The total layer thickness of the multilayer structure is 0.9 μm or less, and the mesa width is 0.9 μm or less.

  (2-6) The total layer thickness of the multilayer structure in the ridge-type optical waveguide is at least twice the total layer thickness of the multilayer structure in the light control waveguide section.

The perspective view which shows typically the structure of the optical input part vicinity of the semiconductor optical switch concerning 1st Embodiment. The figure which shows typically the cross-sectional structure perpendicular | vertical to the optical propagation direction of the optical switch main part of the semiconductor optical switch concerning 1st and 2nd embodiment. The figure which shows typically the cross-sectional structure perpendicular | vertical to the light propagation direction of the input waveguide part of the semiconductor optical switch concerning 1st Embodiment. The figure which shows typically the cross-sectional layer structure which cut | disconnected the input taper waveguide part up and down along the optical propagation direction in the semiconductor optical switch concerning 1st Embodiment. In the semiconductor optical switch concerning 1st Embodiment, the figure which shows typically the cross-sectional layer structure which cut | disconnected the slab part extended on both sides of a taper waveguide part up and down in the surface parallel to the light propagation direction of a taper waveguide. The figure which shows typically the conduction-band structure of a part (for 2 periods of quantum wells) of the multiple quantum well layer of the semiconductor optical switch concerning the 1st to 3rd embodiment. The figure which compares the optical pulse input / output characteristic (TAPER) of the semiconductor light guide switch concerning 1st Embodiment with the characteristic (RWG, HM) of the semiconductor waveguide type optical control element of a prior art. The figure which shows typically the cross-sectional structure perpendicular | vertical to the light propagation direction of the input waveguide part 3 of the semiconductor optical switch concerning 2nd Embodiment. The figure which shows typically the cross-sectional layer structure which cut | disconnected the input taper waveguide part up and down along the optical propagation direction in the semiconductor light guide switch concerning the modification of 2nd Embodiment. The figure which shows typically the cross-sectional layer structure which cut | disconnected the input taper waveguide part up and down along the optical propagation direction in the semiconductor optical switch concerning 3rd Embodiment. The figure which shows typically the cross-sectional structure perpendicular | vertical to the light propagation direction of the input waveguide part of the semiconductor optical switch concerning 3rd Embodiment. The figure which shows typically the cross-sectional structure perpendicular | vertical to the optical propagation direction of the optical switch main part of the semiconductor optical switch concerning 3rd Embodiment. The figure which shows typically the cross-sectional structure perpendicular | vertical to the optical propagation direction of the conventional semiconductor optical switch as a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Sapphire substrate 2 ... Nitride semiconductor layer 3 ... Input waveguide part 4 ... Input taper waveguide part 5 ... Optical switch main part 6 ... Output taper waveguide part 7 ... Output waveguide part 8 ... Level difference 9 ... Slab part 11 ... AlN lower cladding layer 12 ... GaN lower core layer 13,43 ... Multiple quantum well layer 14 ... GaN upper core layer 15 ... AlN upper cladding layer 16,35 ... Sputtered AlN film 17 ... AlN barrier layer 18 ... Si doped GaN well Layers 21, 34 ... GaN layer 22 ... AlN layer for etching control 31 ... GaN / AlN multilayer structure

Claims (8)

A mode more than the light control waveguide section through the tapered waveguide section on at least one of the input side or the output side of the light control waveguide section that generates a nonlinear optical response due to intersubband transition in the nitride semiconductor multiple quantum well. A semiconductor waveguide light control element provided with an input / output waveguide portion having a large cross-sectional area,
A nitride semiconductor multiple quantum well layer that causes intersubband transition on a substrate, a GaN core layer sandwiching the multiple quantum well layer from above and below, and an average refractive index provided below the GaN core layer is a GaN core layer A multilayer structure comprising a lower nitride semiconductor lower cladding layer, and a nitride semiconductor upper cladding layer having an average refractive index lower than the GaN core layer provided on the GaN core layer;
The light control waveguide portion is a mesa-shaped single mode optical waveguide formed by removing at least the upper cladding layer, the GaN core layer, and the multiple quantum well layer of the multilayer structure outside the mesa,
The input / output waveguide portion and the tapered waveguide portion are formed of a ridge-type optical waveguide in which a loading layer having a refractive index higher than that of the substrate is formed on the multilayer structure.
In the taper waveguide portion, the width of the mesa of the multilayer structure and the thickness of the loading layer are continuously or intermittently reduced from the input / output waveguide portion toward the light control waveguide portion,
Further, the thickness of the slab portion extending from the portion in contact with the input / output waveguide portion toward both sides of the tapered waveguide portion toward the light control waveguide portion is equal to the thickness of the loading layer in the tapered waveguide portion. A semiconductor waveguide type light control element characterized by decreasing in response to a change.   2. The semiconductor waveguide light control element according to claim 1, wherein the loading layer is an insulating film. 3. The semiconductor waveguide light control element according to claim 2, wherein the insulating film has an AlN film, a SiN _x film, or a laminated structure of an AlN film and a SiN _x film.   2. The semiconductor waveguide light control element according to claim 1, wherein the loading layer is composed of a GaN layer or a laminated structure of a GaN layer and an AlN layer having a thickness of 50 nm or less. Light control via a tapered waveguide section on at least one of the input side or the output side of a light control waveguide section that generates a nonlinear optical response due to intersubband transition in a nitride semiconductor multiple quantum well stacked on a substrate A semiconductor waveguide light control element provided with an input / output waveguide portion having a larger mode cross-sectional area than the waveguide portion,
An optical waveguide from the light control waveguide section to the input / output waveguide section includes at least a lower cladding layer made of a nitride semiconductor from the substrate side, and a GaN lower core layer having an average refractive index higher than that of the lower cladding layer A nitride semiconductor multiple quantum well layer that causes intersubband transition, an upper core layer mainly composed of a GaN layer, an upper cladding layer having an average refractive index higher than that of the substrate and lower than that of the upper core layer, , And has a multilayer structure
The optical control waveguide section is a mesa-shaped single mode formed by removing at least the upper cladding layer, the upper core layer, the multiple quantum well layer, and the lower core layer of the multilayer structure outside the mesa. An optical waveguide,
The input / output waveguide section is composed of a ridge type optical waveguide in which the thickness of the upper core layer is formed thicker than the light control waveguide section,
In the taper waveguide portion, the mesa width of the multilayer structure and the thickness of the upper core layer are continuously or intermittently reduced from the input / output waveguide portion toward the light control waveguide portion,
The thickness of the slab portion extending from the portion in contact with the input / output waveguide portion toward both sides of the tapered waveguide portion toward the light control waveguide portion is the thickness of the upper core layer in the tapered waveguide portion. A semiconductor waveguide type light control element characterized by decreasing in response to a change in.   6. The semiconductor waveguide light control element according to claim 1, wherein a total layer thickness of the multilayer structure in the light control waveguide portion is 0.9 μm or less. 6. The semiconductor waveguide light control device according to claim 5, wherein the upper cladding layer has an AlN film, a SiN _x film, or a laminated structure of an AlN film and a SiN _x film.   6. The semiconductor waveguide light control element according to claim 5, wherein the mesa side surface of the light control waveguide portion is covered with the same material as that of the upper clad layer.

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