Method for manufacturing optical device

Abstract

Ions (105) are implanted into a multiple quantum well structure (102), and the multiple quantum well structure (102) in a second region (152) is then disordered through heating to form a disordered layer (106) (second step). Since a mask layer (104) is formed, the ions (105) are not implanted into the multiple quantum well structure (102) in a first region (151). By diffusing impurities introduced by ion implantation through heating, semiconductor materials constituting a well layer and barrier layer can be mutually diffused, the multiple quantum well is disordered, the band gap of the multiple quantum well structure (102) in the second region (152) is increased, and only an absorption edge wavelength is shortened.

Description

Method for manufacturing an optical device

[0001] The present invention relates to a method for manufacturing an optical device.

[0002] Compound semiconductors typified by InP and GaAs are often direct transition semiconductors, unlike Si which is an indirect transition semiconductor, and have strong interaction with light, so they have excellent light emission and absorption characteristics. Taking advantage of this property, they are used in active devices such as lasers, optical amplifiers, and optical modulators.

[0003] On the other hand, for optical control, in addition to the light amplification and absorption functions, passive elements such as wavelength filters, polarization filters, polarizers, and mirrors are also important for functional expansion. Since it is generally desirable that the excess loss of these passive elements is minimized, the material constituting the passive element is desirably free of light absorption, that is, transparent. Therefore, a technology for integrating both an active functional part made of a material having light absorption and amplification and a passive part made of a material that is infinitely transparent on the same substrate becomes important.

[0004] Y. Ueda et al., "Partial Regrowth of Optical-Gain Section for Improved Wafer Process Flexibility of InP Photonic Integrated Circuits", Journal of Lightwave Technology, vol. 40, no. 8, pp. 2465-247, 2021.

[0005] When integrating an optically passive region and an active region, semiconductor regrowth technology is often used. For example, there is a method of partially forming semiconductors with different bandgaps (Non-Patent Document 1). However, this technology has problems in that the number of process steps increases and the yield decreases due to an increase in the number of crystal growths. In addition, this technology also has problems such as structural discontinuities at the interfaces of different semiconductor layers. Structural discontinuities not only cause scattering losses of the guided light, but often lead to higher-order mode excitation and reflection that cause instability in the operation of optical circuits. This reflection causes laser noise in semiconductor lasers, which are typical of optical waveguide devices.

[0006] A typical example of a passive function integration into a semiconductor laser, a representative active element, where both higher-order mode excitation and reflection are problematic, is the integration of a spot size converter that changes the spot size of the light at the laser output end. In semiconductor lasers, increasing the spot size at the chip output end leads to suppression of the beam spread of the emitted light, which in turn leads to a relaxation of optical implementation tolerance in optical fibers or an improvement in fiber output.

[0007] The purpose of a spot size converter is to expand the waveguide modes, so naturally, the waveguide material needs to be transparent. However, when a spot size converter is formed from semiconductor material produced by semiconductor regrowth, the output beam shape becomes distorted as a result of the higher-order mode excitation described above, which hinders the achievement of the spot size converter's original purpose.

[0008] Therefore, especially in the integration of a semiconductor laser with a spot size converter, minimizing structural discontinuities in the semiconductor material is crucial in two ways: stabilizing the oscillation characteristics of the semiconductor laser and shaping the emitted beam.

[0009] As mentioned above, from the standpoint of ensuring specific characteristics, there is a problem in that it is not easy to fabricate spot size converters that connect to semiconductor optical elements such as semiconductor lasers using conventional semiconductor regrowth.

[0010] This invention was made to solve the above-mentioned problems, and aims to make it easier to manufacture spot size converters connected to semiconductor optical elements while ensuring their characteristics.

[0011] The present invention relates to a method for manufacturing an optical device comprising a waveguide-type semiconductor optical element formed in a first region of a substrate and a spot size converter formed in a second region of the substrate continuous with the first region. The method comprises: a first step of forming a multiple quantum well structure made of a compound semiconductor on the substrate from the first region to the second region; a second step of disordering the multiple quantum well structure in the second region by ion implantation and heating to form a disordered layer; and a third step of etching the multiple quantum well structure and the disordered layer through the thickness direction to form a core, thereby forming a core shape in which the multiple quantum well structure and the disordered layer extend from the first region to the second region. The semiconductor optical element is formed in the first region which has been formed into a core shape, and the spot size converter is formed in the second region which has been formed into a core shape.

[0012] As explained above, according to the present invention, since the second region of the multiple quantum well structure formed from the first region to the second region is disordered by ion implantation, a spot size converter connected to a semiconductor optical element can be manufactured more easily while ensuring its characteristics.

[0013] Figure 1A is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 1 of the present invention. Figure 1B is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 1 of the present invention. Figure 1C is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 1 of the present invention. Figure 1D is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 1 of the present invention. Figure 1E is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 1 of the present invention. Figure 1F is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 1 of the present invention. Figure 2A is a characteristic diagram showing the simulation results of a far-field image of an optical waveguide formed from a core formed from a disordered multiple quantum well structure. Figure 2B is a characteristic diagram showing the simulation results of a far-field image of an optical waveguide formed from a core formed from a disordered multiple quantum well structure. Figure 3A is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 2 of the present invention. Figure 3B is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 2 of the present invention. Figure 3C is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 2 of the present invention. Figure 3D is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 2 of the present invention. Figure 3E is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 2 of the present invention. Figure 3F is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 2 of the present invention. Figure 3G is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 2 of the present invention. Figure 3H is a cross-sectional view showing the state of an optical device in an intermediate step for explaining the manufacturing method of an optical device according to Embodiment 2 of the present invention.Figure 4A is a characteristic diagram showing the simulation results of the far-field image of an optical waveguide when the thickness of the core formed from a disordered multiple quantum well structure is varied. Figure 4B is a characteristic diagram showing the simulation results of the far-field image of an optical waveguide when the thickness of the core formed from a disordered multiple quantum well structure is varied.

[0014] The following describes a method for manufacturing an optical device according to an embodiment of the present invention.

[0015] [Embodiment 1] First, a method for manufacturing an optical device according to Embodiment 1 of the present invention will be described with reference to Figures 1A to 1F. Figures 1A to 1D show cross-sections parallel to the waveguide direction, while Figures 1E and 1F show cross-sections perpendicular to the waveguide direction. This manufacturing method is a method for manufacturing an optical device comprising a waveguide-type semiconductor optical element formed in a first region 151 of a substrate 101 and a spot size converter formed in a second region 152 of the substrate 101 continuous with the first region 151.

[0016] First, as shown in Figure 1, a multi-quantum well structure 102 made of a compound semiconductor is formed on the substrate 101 from the first region 151 to the second region 152 (first step). In this example, an upper semiconductor layer 103 made of a compound semiconductor such as InP is formed on the multi-quantum well structure 102. For example, the substrate 101 can be made of InP. The multi-quantum well structure 102 can be made by alternately stacking well layers and barrier layers made of InGaAsP, with a thickness of 300 nm. For example, a multi-quantum well structure 102 made of InGaAsP can have a photoluminescence peak wavelength of 1520 nm. Alternatively, a lower semiconductor layer (not shown) made of InP can be formed on the substrate 101, and the multi-quantum well structure 102 can be formed on this lower semiconductor layer.

[0017] Next, as shown in Figure 1B, a mask layer 104 is formed on the multiple quantum well structure 102 (upper semiconductor layer 103) of the first region 151. The mask layer 104 can be made of, for example, silicon oxide.

[0018] Next, as shown in Figure 1C, ions 105 are implanted into the multiple quantum well structure 102, and then heating is performed to disorder the multiple quantum well structure 102 in the second region 152, creating a disordered layer 106 (second step). Since the mask layer 104 is formed, ions 105 are not implanted into the multiple quantum well structure 102 in the first region 151. As is well known, by diffusing impurities (e.g., Zn, Si, P, B, etc.) introduced by ion implantation through heating, the semiconductor materials constituting the well layer and the barrier layer can be mutually diffused (intermixed). Due to this mutual diffusion, the multiple quantum wells become disordered, the band gap of the multiple quantum well structure 102 in the second region 152 increases, and only the absorption edge wavelength becomes shorter.

[0019] Next, the multiple quantum well structure 102 and the disordered layer 106 are etched through in the thickness direction to form a core, so that the multiple quantum well structure 102 and the disordered layer 106 form a core shape extending from the first region 151 to the second region 152 (third step). In this example, first, as described above, the multiple quantum well structure 102 in the second region 152 is disordered, and then the mask layer 104 is removed. Next, the upper semiconductor layer 103 is thickened by regrowth or the like. After this, the multiple quantum well structure 102 and the disordered layer 106 are etched through in the thickness direction to form a core, so that the multiple quantum well structure 102 forms the core shape in the first region 151 and the disordered layer 106 forms the core shape in the second region 152.

[0020] In the etching process, the thickened upper semiconductor layer 103, the multiple quantum well structure 102, and the disordered layer 106 are etched through in the thickness direction, while the substrate 101 is etched partway to form a core shape (ridge shape). This process forms the active layer 102a of the multiple quantum well structure in the first region 151. This process also forms the core 106a in the second region 152.

[0021] As shown in Figures 1D, 1E, and 1F, in the first region 152, an upper semiconductor layer 103a is formed on the active layer 102a by processing a thickened upper semiconductor layer 103 into a ridge shape. As mentioned above, a lower semiconductor layer can be formed, and an embedded heterostructure can be created in which the active layer 102a is sandwiched between the lower semiconductor layer and the upper semiconductor layer 103a. In the second region 152, there is a core 106a in which the multiple quantum well structure is disordered. An upper semiconductor layer 103a is also formed on the core 106a.

[0022] As described above, after forming the core-shaped active layer 102a and core 106a, a compound semiconductor layer is formed to sandwich them and embed the core-shaped portion.

[0023] Next, a semiconductor optical element is formed in the first region 151 which has the active layer 102a. For example, a current injection structure can be formed by forming a compound semiconductor layer sandwiching the active layer 102a, thereby forming a semiconductor optical element that includes a semiconductor laser or an optical modulator. In the case of a semiconductor laser, a resonator such as a diffraction grating can be formed in the first region 151.

[0024] Furthermore, a spot size converter is formed in the second region 152 which contains the core 106a. For example, a spot size converter can be formed by creating an optical waveguide structure using layers of compound semiconductors formed so as to sandwich the core 106a.

[0025] Here, for example, light emitted by a semiconductor laser using an active layer 102a made of undisordered multiple quantum wells is not absorbed in the disordered core 106a. Furthermore, in core 106a, where the band gap is increased by disordering and the effective absorption edge wavelength is shortened, the refractive index of the light decreases as described above. In this way, the refractive index of core 106a decreases, so that light confinement is weakened, and the modes broaden in the second region 152, making it possible to use it as a spot size converter.

[0026] Here, the simulation results of the far-field pattern (FFP) of an optical waveguide formed from the disordered multiple quantum well structure described above are shown in Figures 2A and 2B. Figure 2A shows the FFP in the waveguide direction (x-axis direction), and Figure 2B shows the FFP in the direction perpendicular to the waveguide direction (y-direction).

[0027] The wavelength of light was set to 1520 nm, and the polarization state was set to TE (transversal electric field) polarization. The refractive index of the multiple quantum well structure before disordering was set to 3.49, based on experimental values ​​from a semiconductor laser using this structure as the active layer. The refractive index of the disordered core of the multiple quantum well structure was set to 3.43 and 3.39, respectively, based on theoretical values, assuming that it can be approximated as a lattice-matched InGaAsP system with PL peak wavelengths of 1420 nm and 1320 nm. The FFP at a refractive index of 3.49 before disordering is shown by a dashed line, the FFP at a refractive index of 3.43 after disordering is shown by a dotted line, and the FFP at a refractive index of 3.39 is shown by a solid line.

[0028] Under the condition of a refractive index of 3.49 before disordering, the full-width half maximum (FWHM) was 47.4° and 51.5° in the x and y directions, respectively. In contrast, when the refractive index was reduced to 3.43 by disordering, the FWHM became 47.3° and 45.8° in the x and y directions, respectively. Furthermore, when the refractive index was reduced to 3.39 by disordering, the FWHM became 47.2° and 40.5° in the x and y directions, respectively. Thus, it can be seen that the full-width at half maximum of the FFP decreases as the refractive index decreases.

[0029] Therefore, the optical waveguide in the second region 152, formed by the core 106a consisting of a disordered multiple quantum well structure, is considered to be passive and also function as a spot size converter.

[0030] [Embodiment 2] Next, a method for manufacturing an optical device according to Embodiment 2 of the present invention will be described with reference to Figures 3A to 3H. Figures 3A to 3F show cross-sections parallel to the waveguide direction, while Figures 3G and 3H show cross-sections perpendicular to the waveguide direction. This manufacturing method is a method for manufacturing an optical device comprising a waveguide-type semiconductor optical element formed in a first region 151 of a substrate 101 and a spot size converter formed in a second region 152 of the substrate 101 continuous with the first region 151.

[0031] First, in the same manner as in Embodiment 1 described above, ions 105 are injected into the multiple quantum well structure 102, and then the multiple quantum well structure 102 in the second region 152 is disordered by heating to form a disordered layer 106 (second step; Figure 1C).

[0032] Next, as shown in Figure 3A, a second upper semiconductor layer 107, 150 nm thick, made of InGaAsP, is formed on an upper semiconductor layer 103, 150 nm thick, made of InP. Then, the second upper semiconductor layer 107 is selectively etched using a first mask layer 108 that covers the entire first region 151 and extends into a portion of the second region 152 in the waveguide direction, to form an upper semiconductor mask layer 107', as shown in Figure 3B.

[0033] For example, the second upper semiconductor layer 107 can be selectively etched by wet etching using a mixed solution of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) (piranha solution) as the etching solution. InGaAsP dissolves in the piranha solution, but InP hardly dissolves in the piranha solution, so the second upper semiconductor layer 107 can be selectively etched using the upper semiconductor layer 103 as an etching stop layer.

[0034] Next, after removing the first mask layer 108, the upper semiconductor layer 103 is selectively etched using the second mask layer 109, which covers the entire area of ​​the first region 151 and extends beyond the upper semiconductor mask layer 107' into a portion of the second region 152 in the waveguide direction, thereby forming a semiconductor mask layer 103' as shown in Figure 3C. The upper semiconductor mask layer 107' extends into a portion of the second region 152 in the waveguide direction, and the semiconductor mask layer 103' extends beyond the upper semiconductor mask layer 107' into a portion of the second region 152 in the waveguide direction.

[0035] For example, the upper semiconductor layer 103 can be selectively etched by wet etching with a mixed solution of hydrochloric acid and phosphoric acid. InP dissolves in the mixed solution of hydrochloric acid and phosphoric acid, but InGaAsP hardly dissolves in the mixed solution of hydrochloric acid and phosphoric acid. Therefore, the disordered layer 106 can be used as an etching stop layer to selectively etch the upper semiconductor layer 103.

[0036] Next, after removing the second mask layer 109, a third mask layer 110 is formed on top of the upper semiconductor mask layer 107', as shown in Figure 3D. By forming the third mask layer 110, a stepped structure is formed by the third mask layer 110, the semiconductor mask layer 103', and the upper semiconductor mask layer 107'.

[0037] Next, the upper semiconductor mask layer 107', the semiconductor mask layer 103', and the disordered layer 106 are etched by a dry etching process using the third mask layer 110 as a mask. For example, the upper semiconductor mask layer 107', the semiconductor mask layer 103', and the disordered layer 106 are etched by dry etching using a chlorine-based plasma.

[0038] As a result of the dry etching described above, the disordered layer 106 is processed in a stepped manner, reflecting the stepped structure formed by the third mask layer 110, the semiconductor mask layer 103', and the upper semiconductor mask layer 107'. Consequently, as shown in Figure 3E, the disordered layer 106' in the second region 152 becomes thinner as it moves away from the first region 151 (fourth step).

[0039] Next, after removing the third mask layer 110, as shown in FIG. 3F, an upper cladding layer 111 made of InP is formed. After that, the disordered layer 106' and the multiple quantum well structure 102 are etched so as to penetrate in the thickness direction and processed into a core shape. As shown in FIGS. 3G and 3H, the multiple quantum well structure 102 and the disordered layer 106' are formed into a core shape extending from the first region 151 to the second region 152 (third step).

[0040] In the etching process, the semiconductor mask layer 103', the multiple quantum well structures 102, and the disordered layer 106' are etched so as to penetrate in the thickness direction, and the substrate 101 is etched so as to reach halfway and processed into a core shape (ridge shape).

[0041] By this processing, in the first region 151, an active layer 102a of the multiple quantum well structure is formed. On the active layer 102a, an upper semiconductor layer 103'a obtained by processing the semiconductor mask layer 103' into a ridge shape is formed. As described above, a lower semiconductor layer can be formed, and an embedded hetero structure in which the active layer 102a is sandwiched between the lower semiconductor layer and the upper semiconductor layer 103'a can be formed.

[0042] Also, in the second region 152, a core 106'a in which the multiple quantum well structure is disordered is formed. An upper semiconductor layer 103'a is also formed on the core 106'a. The core 106'a reflects the cross-sectional shape of the disordered layer 106' and has a thinner shape as it is farther from the first region 151.

[0043] As described above, after forming the core-shaped active layer 102a and the core 106'a, a compound semiconductor layer is formed so as to sandwich them, and the core-shaped portion is filled.

[0044] After that, a semiconductor optical element is formed in the first region 151 including the active layer 102a. For example, by forming a current injection structure with the compound semiconductor layer formed so as to sandwich the active layer 102a, a semiconductor optical element including a semiconductor laser or an optical modulator can be formed. When it is a semiconductor laser, a resonator using a diffraction grating or the like can be formed in the first region 151.

[0045] Further, a spot size converter is formed in the second region 152 including the core 106'a. For example, by forming an optical waveguide structure using a compound semiconductor layer formed so as to sandwich the core 106'a, it can be used as a spot size converter.

[0046] For example, light oscillated by a semiconductor laser with an active layer 102a composed of an un-disordered multiple quantum well does not undergo absorption or the like in the disordered core 106'a. Also, in the core 106'a where the bandgap is increased and the effective absorption edge wavelength is shortened by disordering, the refractive index of the above-described light decreases. Thus, since the refractive index of the core 106'a decreases, the light confinement becomes weaker, and in the second region 152, the mode spreads, and it can be used as a spot size converter.

[0047] Further, since the core 106'a is made thinner as it is farther from the first region 151, it is considered that in the second region 152, the mode spreads more as it is farther from the first region 151.

[0048] Here, simulation results of the far-field pattern (FFP) of an optical waveguide with a core formed from the above-described disordered multiple quantum well structure are shown in FIGS. 4A and 4B. In this example, simulation results when the thickness of the core is changed are shown. Note that FIG. 4A shows the FFP in the waveguide direction (x-axis direction), and FIG. 4B shows the FFP in the direction perpendicular to the waveguide direction (y direction).

[0049] The FWHM of the condition (solid line) where the thickness of the core formed from the disordered multiple quantum well structure is 0.3 mm was 47.3° and 45.8° in the x direction and the y direction, respectively. On the other hand, the FWHM of the condition (dotted line) where the core thickness was 0.2 mm was 47.2° and 40.9° in the x direction and the y direction, respectively. Also, the FWHM of the condition (broken line) where the layer thickness was reduced to 0.1 mm was 44.8° and 26.1° in the x direction and the y direction, respectively. From these results, it can be seen that by thinning the core, the FFP becomes sufficiently small.

[0050] Therefore, the optical waveguide in the second region 152, which is composed of a disordered multiple quantum well structure and whose core 106'a is thinner as it moves away from the first region 151, is considered to be passive and also functions as a spot size converter.

[0051] Incidentally, the semiconductor optical element formed in the first region 151 may include a semiconductor amplifier positioned between the semiconductor laser and the spot size converter. A resonator is formed in the region designated as the semiconductor laser, while a resonator is not formed in the portion designated as the semiconductor amplifier. In this way, a semiconductor laser and a semiconductor amplifier can be formed in the first region 151, and a spot size converter can be formed in the second region 152 adjacent thereto, without using semiconductor regrowth.

[0052] Because semiconductor amplifiers have an optical amplification effect, reflections at the light output end face are also amplified. As mentioned above, reflections are a source of laser noise in semiconductor lasers. Introducing a spot size converter at the light output end of a semiconductor amplifier by semiconductor regrowth creates a structural discontinuity, which causes reflections. In contrast, according to this embodiment, a spot size converter can be formed without using semiconductor regrowth, leading to the suppression of reflections at the output end face.

[0053] As described above, according to the embodiment of the present invention, the second region of the multiple quantum well structure formed from the first region to the second region is disordered by ion implantation, so that a spot size converter connected to a semiconductor optical element can be manufactured more easily while ensuring its characteristics.

[0054] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be implemented within the technical concept of the present invention by those with ordinary skill in the art.

[0055] 101...Substrate, 102...Multiple quantum well structure, 102a...Active layer, 103...Upper semiconductor layer, 103a...Upper semiconductor layer, 104...Mask layer, 105...Ion, 106...Disordered layer, 106a...Core, 151...First region, 152...Second region.

Claims

1. A method for manufacturing an optical device comprising a waveguide-type semiconductor optical element formed in a first region of a substrate and a spot size converter formed in a second region of the substrate continuous with the first region, comprising: a first step of forming a multiple quantum well structure made of a compound semiconductor on the substrate from the first region to the second region; a second step of disordering the multiple quantum well structure in the second region by ion implantation and heating to form a disordered layer; and a third step of etching the multiple quantum well structure and the disordered layer so as to penetrate in the thickness direction to process them into a core shape, thereby forming the multiple quantum well structure and the disordered layer into a core shape extending from the first region to the second region, wherein the semiconductor optical element is formed in the core-shaped first region and the spot size converter is formed in the core-shaped second region.

2. A method for manufacturing an optical device according to claim 1, further comprising a fourth step of making the disordered layer thinner as it moves away from the first region.

3. A method for manufacturing an optical device according to claim 1 or 2, wherein the semiconductor optical element comprises a semiconductor laser.

4. A method for manufacturing an optical device according to claim 3, wherein the semiconductor optical element comprises a semiconductor amplifier disposed between the semiconductor laser and the spot size converter.

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