Semiconductor optical device and manufacturing method thereof

Abstract

To provide a semiconductor optical device capable of realizing a higher modulation rate and a manufacturing method of the same.SOLUTION: A higher modulation rate is achieved if a ridge semiconductor structure 16 forming a diode of a semiconductor optical device, together with a semiconductor substrate 14 and an active layer 18 formed between the ridge semiconductor structure and the semiconductor substrate, is designed to extend from the active layer towards a surface of the ridge semiconductor structure facing opposite the active layer, and the active layer is restricted laterally so as to lie in a projection of the surface onto the active layer in a direction perpendicular to the semiconductor substrate. In particular, the above means makes it possible to structure the active layer in a self-aligned manner by using the surface of the ridge semiconductor structure opposite to the active layer as a mask and thereby structuring the active layer during dry etching.SELECTED DRAWING: Figure 2e

Description

[Technical Field]

[0001] This application relates to semiconductor optical devices, such as light emitting devices, such as, but not limited to, DFB (distributed feedback) lasers, and their fabrication. [Background technology]

[0002] Optical data transmission in communication systems, such as in data centers or high-performance computing systems, requires high-speed transmitter components. High-speed DFB lasers, especially in the o-band wavelength region, are key components for these applications.

[0003] Within the past decade, significant research and development efforts have been made to increase the modulation speed of DFB lasers by optimizing the InGaAlAs MQW (multiple quantum well) active layer and DFB lattice for high relaxation frequencies, as well as by reducing the laser's series resistance and parasitic capacitance. Thus, DFB lasers with frequency bandwidths up to 30 GHz have been achieved at room temperature.

[0004] However, more recently there have been some new developments in DFB lasers with frequency bandwidths of 40 GHz and above.

[0005] One approach to achieve this is the use of so-called membrane lasers with transverse current injection. Such devices have achieved modulation bandwidths exceeding 40 GHz, and even 100 GHz using photon-photon resonance effects [1.1]. However, such membrane lasers have the disadvantage of very low optical output power (less than 1 nW), which precludes their use in many applications. A second approach is to extend the modulation bandwidth by increasing the ratio between the optical confinement factor and the mode-field diameter, a method already proposed in 1993 [1.2]. At the time, this was extremely difficult to achieve. However, with further improvements in InP technology, this proposed method has proven very promising.

[0006] A third approach is the use of a grooved trench waveguide structure [1.3]. After ridge formation, a groove is etched into the active layer, which consists of an InGaAlAs MQW, and passivated with SiO2. However, technological limitations mean that Al-containing materials can be oxidized, which can result in increased threshold current and reduced reliability in narrow grooved trench devices. To date, no significant increase in modulation bandwidth has been demonstrated using this approach.

[0007] Since 2020, LUMENTUM has published several papers on DFB lasers with modulation bandwidths exceeding 40 GHz [1, 4 and 1-5]. These lasers have an active stripe width of less than 1 μm, and by modifying the BH laser design, they enabled the optimization of the confinement / mode diameter ratio mentioned above. On the other hand, the narrow ridge results in a high series resistance, thus limiting the modulation bandwidth. However, this effect is mitigated because the ridge used here is made of n-type InP, which has low resistivity.

[0008] For information on the fabrication of these devices, see references 1.6 and 1.7.

[0009] The use of a V-shaped InP ridge was proposed as early as 1997 to reduce the series resistance of narrow ridge waveguide lasers [1.8].

[0010] However, there is a need to provide semiconductor optical devices that allow for even greater increases in modulation speed. Summary of the Invention [Problem to be solved by the invention]

[0011] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a semiconductor optical device that can achieve a higher modulation speed and a method for manufacturing the same. [Means for solving the problem]

[0012] This object is achieved by the subject matter of the independent claims.

[0013] The fundamental finding underlying the present invention is that higher modulation speeds can be achieved if the semiconductor substrate and the ridge semiconductor structure, which together with an active layer formed between the ridge semiconductor structure and the semiconductor substrate form the diode of a semiconductor optical device, are designed to extend from the active layer towards the face of the ridge semiconductor structure facing away from the active layer, and the active layer is laterally confined to lie in the projection of a plane onto the active layer in a direction perpendicular to the semiconductor substrate. In particular, this measure makes it possible to structure the active layer in a self-aligned manner by using the face of the ridge semiconductor structure facing away from the active layer as a mask during dry etching and thereby structuring the active layer.

[0014] According to one embodiment of the present application, the active layer is formed such that its laterally opposing side surfaces are laterally disposed between opposing side surfaces of the surface of the ridge semiconductor structure, on the one hand, and a side surface of the interface between the active layer and the ridge semiconductor structure, on the other hand. In other words, the active layer can have a width between the width of the surface, on the one hand, and the width of the interface, on the other hand. By this means, the ridge semiconductor structure is in full contact with the active layer, but the active layer is still narrower than the surface of the ridge semiconductor structure that serves as a mask for defining the active layer. Additionally or alternatively, the width of the active layer across the longitudinal axis of the ridge semiconductor structure may be less than 1 μm.

[0015] In one embodiment, the ridge semiconductor structure has a trapezoidal cross section. The ratio of the width of the active layer to the width of the facets of the ridge semiconductor structure may be, for example, greater than 7 / 5. This means that the active layer is highly confined, but the width of the ridge semiconductor structure is extended, allowing efficient light propagation within the ridge.

[0016] According to one embodiment, the semiconductor substrate comprises a protrusion on which the active layer is disposed, the protrusion having a lateral extent coinciding with the active layer. In other words, the active layer may be formed on the protrusion of the semiconductor substrate such that its opposite lateral sides coincide with the opposite lateral sides of the protrusion. By this measure, light is effectively confined laterally in the active layer. A protective layer may cover the opposite lateral sides of the active layer. This protective layer may further cover a portion of the surface of the semiconductor substrate that laterally surrounds the active layer. According to one embodiment, this protective layer has a thin thickness, such as less than 100 nm or less than 50 nm, or the protective layer is thinner than the height of the protrusion or thinner than the active layer. By this measure, the optically effective width of the active layer can be kept small, and the resulting light confinement can improve the modulation speed.

[0017] Additionally or alternatively, in one embodiment, the material of the ridge semiconductor structure and the semiconductor substrate is indium phosphide (InP). Additionally or alternatively, in one embodiment, the material of the ridge semiconductor structure is P-type (e.g., p-InP). It is also possible for the material of the ridge semiconductor structure to be N-type (e.g., n-InP). Furthermore, the material of the ridge semiconductor structure and the semiconductor substrate may be GaAs, AlGaAs, InGaAsP, InGaAsSb, or InAlGaAs.

[0018] In the manufacture of semiconductor optical devices, the surface of the ridge semiconductor structure may be used as a mask when dry etching the active layer such that the active layer is laterally disposed within the projection of the surface of the ridge semiconductor structure onto the active layer in a direction perpendicular to the semiconductor substrate.

[0019] In the following description, the embodiments are described in detail, but it should be understood that the embodiments provide many applicable concepts that can be implemented in a wide variety of semiconductor optical device fields. The specific embodiments described are merely illustrative of particular ways to implement and use the concepts and do not limit the scope of the embodiments. In the following description of the embodiments, the same or similar element or elements having the same function are given the same reference numeral or identified by the same name, and repeated descriptions of elements given the same reference numeral or identified by the same name are typically omitted. In the following description, numerous details are set forth to provide a more complete description of the embodiments of the present disclosure.

[0020] However, it will be apparent to those skilled in the art that other embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in diagrammatic form, rather than in detail, in order to avoid obscuring the examples described herein. Furthermore, features of different embodiments described herein may be combined with each other unless specifically stated otherwise.

[0021] Embodiments of the present invention are described in detail below with reference to the accompanying drawings. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a schematic 3D cross-sectional view of a semiconductor optical device according to an embodiment. [Figure 2a] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2b] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2c] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2d]1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2e] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2f] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2g] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2h] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2i] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 2j] 1A to 1C are cross-sectional views of a semiconductor optical device, here embodied as a DFB laser, in various states reached during the manufacture of the device; [Figure 3] 2 is a schematic longitudinal cross-sectional view of a variation of the semiconductor optical device of FIG. 1, shown here as a DFB laser with a diffraction grating. [Figure 4] 1 is a graph showing the dependency of the relaxation frequency increase rate of a DFB laser on the active layer width. DETAILED DESCRIPTION OF THE INVENTION

[0023] A semiconductor optical device according to one embodiment will now be described with reference to Figure 1. The semiconductor optical device 10 of Figure 1 comprises a diode 12 formed by a semiconductor substrate 14 of the semiconductor optical device 10, a ridge semiconductor structure 16, and an active layer 18 formed between the semiconductor substrate 14 and the ridge semiconductor structure 16. The semiconductor substrate 14 and the ridge semiconductor structure 16 are of opposite conductivity type and will be described in more detail below. With reference to Figures 2a-2j, the active layer 18 may itself be a layer stack structured in a particular MQW structure and / or sandwiched between particular waveguide layers.

[0024] As shown in FIG. 1 , the ridge semiconductor structure 16 has a cross section perpendicular to the active layer, i.e., perpendicular to the lateral extent of the active layer and the longitudinal extent of both the ridge semiconductor structure 16 and the active layer, which cross section extends from the active layer 18 towards a face 20 of the ridge semiconductor structure 16 facing away from the active layer 18.

[0025] As shown in Figure 1, the ridge semiconductor structure 16 and the active layer 18 both extend longitudinally along a longitudinal axis 22. This results in the active layer 18 being formed in a stripe shape on a top surface 24 of the semiconductor substrate 14, with the ridge semiconductor structure 16 being formed above the active layer 18 and spaced apart from the semiconductor substrate 14 via the active layer 18. Figure 1 shows a portion or longitudinal section of the semiconductor optical device 10 along the longitudinal axis 22, with the front surface shown in cross section perpendicular to the longitudinal axis 22. As shown in Figure 1, the ridge semiconductor structure 16 can have a constant cross section 16a perpendicular to the longitudinal axis 22, although variations are of course possible.

[0026] In diode 12, semiconductor substrate 14 is of a first conductivity type and ridge semiconductor structure 16 is of a second conductivity type. In the embodiments described in more detail below, for example, ridge semiconductor structure 16 is p-type and semiconductor substrate 14 is n-type, although this configuration can be switched, and even the embodiments described below can be switched in this regard.

[0027] As shown in FIG. 1 , the cross section 16 a of the ridge semiconductor structure 16 widens from the active layer 18, i.e., from the interface 26 where the ridge semiconductor structure 16 abuts the active layer 18, toward the face 20. This widening, as already mentioned above, reduces the series resistance and therefore improves the modulation speed of the device 10. In the example of FIG. 1 and in the embodiments described further below, the cross section of the ridge semiconductor structure 16 has a trapezoidal shape with overhanging sidewalls 16 b along the lateral sides of the longitudinal axis 22 connecting the top face 20 with the interface 26. However, this V-shape may, of course, be modified. As will become clear from the further description, the increased width of the face 20 compared to the interface 26, i.e., the width measured laterally and perpendicularly to the longitudinal axis 22, allows for self-aligned lateral structuring of the active layer 18 by using the ridge semiconductor structure 16, or more precisely, the face 20, as a mask during dry etching of the active layer 18.

[0028] 1 , the active layer 18 has a narrow shape, i.e., is formed as a stripe, and is laterally constrained to be within the projection 28 of the surface 20 onto the active layer 18 in a direction perpendicular to the semiconductor substrate 14; i.e., constrained to be within the area cast by the surface 20 when projected toward the semiconductor substrate 14 along a vertical direction 30 orthogonal to the lateral plane of the substrate 14. Further stated, the lateral extent of the active layer 18 in a direction perpendicular to the longitudinal axis 22 is constrained to be within or as the lateral extent 32 of the area resulting from the projection of the surface 20 onto the surface 24 of the substrate 14. The narrow width of the active layer 18 further enhances the modulation bandwidth of the device 10, and the width of the active layer 18 can be designed to be small because the surface 20 can be used for self-aligned lateral structuring of the active layer 18.

[0029] This allows the semiconductor optical device 10 of FIG. 1 to function as a light-emitting device. In particular, light is emitted in response to application of a voltage signal between the ridge semiconductor structure 16, on the one hand, and the semiconductor substrate 14, on the other hand, and the structure described and shown in FIG. 1 allows for improved modulation speeds in modulating the light thus emitted. As will be explained in more detail with respect to embodiments further described below, a contact may be located on face 20 and another contact may be located on the semiconductor substrate 14 opposite face 24, such as face 24a. As will be explained in more detail below, the light-emitting device thus formed may be a diode laser, such as a DFB laser, or the device 10 as shown in FIG. 1 may be part of a DFB laser or some other type of laser, as will be explained in more detail below. It should be noted, however, that the semiconductor optical device 10 of FIG. 1 may alternatively be implemented to function as a light-receiving device, i.e., a photosensor, that converts modulated light into a modulated electrical signal that is output, for example, across the diode 12.

[0030] Although examples of possible production / manufacturing of the semiconductor optical device 10 of FIG. 1 or more detailed embodiments thereof are presented below, such methods will now be briefly described with reference to FIG. 1. In particular, a possible manufacturing method can involve depositing an active layer 18 in an unstructured manner on the top surface 24 of a semiconductor substrate 14, and then forming a structure 16 on the active layer 18 in the form shown in FIG. 1. In this situation, the active layer 18 does not yet have its stripe shape and needs to be laterally structured. The structuring is then carried out using dry etching, in particular by using the face 20 of the ridge semiconductor structure 16 as a mask when dry etching the active layer, so that the active layer 18 is laterally positioned in the projection as already described, as shown in FIG. 1. As described in embodiments further below, the dry etch can be performed such that it stops just short of the semiconductor substrate 14 and its top surface 24, respectively, but the dry etch can also extend into or past the active layer 18, thereby removing semiconductor material of the semiconductor substrate 14 on either side of the projection area 28, resulting in the semiconductor substrate 14 forming a protrusion that projects from the top surface 24 toward the semiconductor structure 16, with the active layer 18 on top. Additionally or alternatively, as described in embodiments further below, a wet etch performed after the dry etch can be used to further reduce the lateral width 34 of the active layer 18, thereby bringing laterally opposing side surfaces 18 a of the active layer 18 closer to each other and to the lateral side surfaces 26 a of the interface 26. This wet etching may further contribute to the removal of semiconductor material at the upper surface 24 of the semiconductor substrate 14 to increase the height of or define the above-mentioned protrusion of the semiconductor substrate 14 having the active layer 18 thereon, or may even form the protrusion.

[0031] The protrusion is indicated in Figure 1 by a dashed line and reference number 36. This protrusion can have a lateral width or extent perpendicular to the longitudinal axis 22 that coincides with the active layer 18, as shown in Figure 1. In other words, the active layer 18 can completely cover the side of the protrusion 36 facing the ridge semiconductor structure 16, since the protrusion is formed at the same time as the active layer 18 is laterally structured in dry etching and / or wet etching, respectively.

[0032] As can be seen in FIG. 1 and described below, the side surface 18 a of the active layer 18 and, if present, the protrusion 36 of the semiconductor substrate 14 may remain unburied, i.e., semiconductor material may not surround the side surface 18 a of the active layer 18. According to embodiments, for example, a protective layer 62 may be formed to protect the surface of the active layer 18 outside the interface 26 that is not covered by either the ridge semiconductor structure 16 or the semiconductor substrate 14, e.g., the side surface 18 a, and optionally, the portion 18 b of the surface of the active layer 18 facing the ridge semiconductor structure 16. This protective layer 62 may act as a passivation layer laterally abutting the active layer 18, and may be formed thin, as described in more detail below, thereby keeping the effective width of the active layer 18 small and thereby contributing to improving the modulation bandwidth of the device 10. However, the protective layer is optional. For example, the semiconductor optical device 10 may be dedicated to or housed for operation in a non-reactive or inert environment, such as a vacuum. For example, semiconductor optical device 10 may be operated within an enclosure, such as a vacuum-sealed cavity or a chamber filled with an inert gas.

[0033] After describing one embodiment of the present application with respect to Figure 1, the present application proceeds to a description of an embodiment of a semiconductor optical device 10, here a DFB laser. In particular, with respect to Figures 2a-2j below, the following description describes an embodiment of a DFB laser and its fabrication.

[0034] In particular, Figures 2a-2j show a semiconductor optical device 10 according to this embodiment as a series of cross sections perpendicular to the longitudinal axis 22 of the semiconductor device at successive stages in the manufacturing process.

[0035] The description begins with the situation shown in Figure 2a, in which the following elements have been formed one on top of the other in an unstructured manner, from bottom to top: a semiconductor substrate 14, an active layer 18, a layer 40 that has not yet been structured to form a ridge semiconductor structure 16, followed by a contact layer 44. As can be seen in Figure 2a, the active layer 18 may be sandwiched between waveguide layers 42a and 42b, which separate the active layer 18 from the semiconductor substrate 14 on the one hand and from the semiconductor layer 40 on the other hand. Above the contact layer 44, a dielectric layer 46 has been formed, which is arranged in the ridge semiconductor structure 16 and is laterally structured to define the position of the ridge semiconductor structure. The contact layer 44 may be formed of, for example, a metal.

[0036] 2b shows the state of the production of the semiconductor optical device 10 reached from the state of FIG. 2a by structuring the layer 40, e.g. by dry etching, using the structured dielectric layer 46 as a mask, and subsequently structuring the semiconductor material of the layer 40 by wet etching, e.g. by chemically selective wet etching, so that the semiconductor ridge structure 16 emerges from the layer 40 and the active layer 18 is arranged laterally in the projection of the face 20 of the ridge semiconductor structure 16 onto the active layer 18 in a direction perpendicular to the semiconductor substrate 14, i.e. in a vertical direction 30 orthogonal to the longitudinal axis 22.

[0037] As a result of the wet etching performed in this manner, overhanging sidewalls 16b are obtained. As shown, a trapezoidal or V-shaped ridge semiconductor structure 16 may be obtained as a result, with the top surface 20 of the ridge semiconductor structure 16 being wider than the bottom surface 50 of the ridge semiconductor structure 16. In other words, the ridge semiconductor structure 16 has a cross section 16a perpendicular to the active layer 18, which widens from the active layer 18 toward the top surface 20 of the ridge semiconductor structure 16 facing away from the active layer. In yet other words, the ridge semiconductor structure 16 narrows from the top surface 20 facing away from the active layer 18 toward the bottom surface 50 facing toward the active layer. The wet etching removes the semiconductor material, excluding the ridge semiconductor structure 16, down to the waveguide layer 42a, exposing a portion 18b of the top surface of the waveguide layer.

[0038] 2b, as a result of the dry etching and subsequent wet etching, the face 20 of the semiconductor material of the ridge semiconductor structure 16 facing away from the active layer, the contact layer 44, and the dielectric layer 46 formed on top of the semiconductor material of the ridge semiconductor structure 16 have the same lateral extent. In other words, in the state of FIG. 2b, the contact layer 44 and the dielectric layer 46 form a top surface 20 that is wider than the interface 26.

[0039] Next, as shown in FIG. 2c, a masking layer 52 is conformally deposited on the surface of the semiconductor optical device 10, thereby achieving the state shown in FIG. 2c from the state shown in FIG. 2b. This deposition is performed so that the masking layer 52 covers the portion 18b of the waveguide layer 42a, the side surface 16b of the ridge semiconductor structure 16, i.e., the overhanging sidewall 16b of the ridge semiconductor structure 16, and the face 20 of the ridge semiconductor structure, i.e., the upper surface of the contact layer 44. That is, the deposition may cover the entire upper surface of the semiconductor optical device 10 in the state shown in FIG. 2b. According to one embodiment, the masking layer 52 may be made of a dielectric material. In particular, according to one embodiment, the deposition of the masking layer may be performed using a chemical vapor deposition process or a sputtering deposition process.

[0040] FIG. 2d shows the state achieved from FIG. 2c by structuring the masking layer 52 of the semiconductor optical device 10 using etching and the upper surface 20 of the ridge as a mask. In particular, this etching can be performed using dry etching. The dielectric layer 46 and the waveguide layer 42a can serve as etch stops for the dry etching. Due to anisotropy, the masking layer 52 therefore remains on the overhanging sidewalls 16b of the ridge semiconductor structure 16 and on the projection area 28 within the portion 18b of the upper surface of the waveguide layer 42a. In other words, the masking layer 52 is removed from the portion located at the top of the ridge semiconductor structure 16 and outside the projection area 28, i.e., outside the projection cast on the waveguide layer 42a by the lateral extent 56 of the surface 20.

[0041] That is, etching of the masking layer 52 can be performed by a maskless dry etch in that it takes advantage of the overhanging properties of the ridge structure and uses an intrinsic mask for its upper surface 20. When a dielectric material is used for the masking layer, the etching step is sometimes described as a maskless dielectric intrinsic dry etch.

[0042] As a result of the steps performed to reach the state of FIG. 2d, the dielectric layer 46 and the one or more portions 52a, 52d of the masking layer 52 on the overhanging sidewalls and the region 28 of portion 18b, respectively, completely cover the ridge structure 16, thereby protecting the ridge structure from the wet etchant and allowing the structuring of the active layer 18 and the ridge semiconductor structure 16 in a self-aligned manner without alignment problems.

[0043] In the embodiment described with respect to Figures 2a to 2d, the structuring of the active layer 18 is carried out by using dry etching followed by wet etching, although alternatives using one of these etching steps are also possible.

[0044] 2e shows the state of device 10 during fabrication, achieved by applying dry etching to the state shown in FIG. 2d. Dry etching is performed using face 20 of ridge semiconductor structure 16 as a mask, whereby dry etching etches the material of waveguide layers 42a and 42b and active layer 18 beyond these layers and into a portion of semiconductor substrate 14 outside projection area 28, resulting in a protrusion 36 of semiconductor substrate 14 in which active layer 18 is located and whose lateral extent 36a coincides with the lateral extent 34 of active layer 18. In other words, dielectric layer 46, together with masking layer 52, acts as a mask for dry etching, and if dry etching is performed for a time longer than is necessary to remove layers 42a, 42b, and 18, the protrusion 36 of semiconductor substrate 14 is formed as a result of dry etching into semiconductor substrate 14. In particular, as a result of the dry etching, the areas bounded by the active layer 18, the protrusion 36 of the semiconductor substrate 14, the top surface 20 of the ridge semiconductor structure 16, and the outer side of the portion 52a of the masking layer 52 a abutting the waveguide layer 42a in the projection area 28 each have substantially identical lateral extents or widths. In particular, the side surface 18a of the active layer 18 and the side surface 36b of the protrusion 36 of the semiconductor substrate 14 are aligned with one another in a vertically spaced apart configuration parallel to the vertical axis 30. In other words, they are substantially coplanar.

[0045] It will also be appreciated from FIG. 2e that the lateral width 50a of the bottom surface 50 of the ridge semiconductor structure, which here interfaces with the active layer 18 via the waveguide layer 42a, may be less than or equal to the lateral width 34 of the active layer 18.

[0046] In principle, the protruding portion of the substrate 14 can be interpreted as forming a bottom ridge on which the ridge semiconductor structure 16 is formed, separated by the active layer 18 and the stack of waveguide layers 42a,b.

[0047] 2f shows the state of the semiconductor optical device 10 during fabrication, achieved by applying wet etching to the state of FIG. 2e. The wet etching is performed using the face 20 of the ridge semiconductor structure 16 as a mask to reduce the lateral extent 34 of the active layer 18 and the lateral extent 36a of the protrusion 36 of the semiconductor substrate 14. The masking layer 52 covering the ridge semiconductor structure 16 and the dielectric layer is resistant to the wet etching performed, thereby protecting the ridge semiconductor structure 16.

[0048] The wet etching is performed so that, relative to the projection of the face 20 of the ridge semiconductor structure 16 onto the active layer 18, the side surfaces 18 a of the active layer 18 and the side surfaces 36 b of the protrusion 36 of the semiconductor substrate 14 approach each other, i.e., so that their lateral widths perpendicular to the longitudinal axis 22 decrease and both of their lateral widths are located between the opposing side surfaces 20 a of the top surface 20 of the ridge semiconductor structure 16 and the side surface 26 a of the interface 26 between the active layer 18 and the ridge semiconductor structure 16. In other words, as a result of the wet etching, the lateral extent 34 of the active layer 18 and the lateral extent 36 a of the protrusion of the semiconductor substrate 14 are limited within the range from the lateral width 50 a of the bottom surface 50 of the ridge semiconductor structure 16 to the lateral width 20 b of the top surface 20 of the ridge semiconductor structure 16.

[0049] As can be seen in FIG. 2f, after the wet etching, the width of the lateral extent 34 of the active layer 18 and the waveguide layers 42a, 42b may be equal to the width of the lateral extent 36a of the protrusion 36 of the semiconductor substrate 14. However, if the etching rates between the waveguide layers 42a, 42b and the active layer 18 are slightly different, their widths may also be slightly different. As can be seen in FIG. 2f, the laterally extending portion 52a of the masking layer 52 forms an overhang 60 on the active layer 18 as a result of the wet etching. That is, the portion 60 of the masking layer 52 extends laterally outward from the active layer 18 such that the portion 60 overhangs the active layer 18. In other words, the wet etching results in under-etching, where a portion 60 of the laterally extending portion 52a of the masking layer 52 is not supported by the active layer 18.

[0050] In particular, the wet etching may be performed using a non-selective wet chemical etching solution. As explained, the wet etching can selectively etch everything except the dielectric layer 46 and the masking layer 52, but in principle the wet etching can be performed to have an effective etching rate, i.e., to effectively etch only the waveguide layers 42 a, b and the active layer 18. For example, other alternatives may exist in which only the semiconductor material of the substrate 14 is etched, thereby causing the protrusion 36 to be narrower than the active layer 18 and the sandwiching layers 42 a, b. By this means, the lateral extent 34 of the active layer 18 (and of the waveguide layers 42 a, b) and the lateral extent 36 a of the protrusion 36 of the semiconductor substrate 14 may no longer coincide but may differ.

[0051] Because the wet etching may be isotropic, the wet etching not only reduces the width of the protrusion and its top end formed by the active layer 18 and layers 42a, b, but also increases the height 36c of the protrusion, or more precisely, etches down to the substrate 14 outside the protrusion 36, as shown in FIG. 2f.

[0052] FIG. 2g illustrates the state during fabrication of device 10 achieved by applying selective growth of protective layer 62 to the state illustrated in FIG. 2f. The selectivity is selected so that growth of protective layer 62 is not performed on dielectric layer 46 and masking layer 52, or, stated another way, only on the laterally opposed side surfaces 18a of active layer 18, the portion of surface 24 of substrate 14 surrounding protrusion 36, and the sidewalls of protrusion 36 of semiconductor substrate 14. Thus, protective layer 62 is selectively grown to cover surface 24 of semiconductor substrate 14 other than protrusion 36, the side surfaces 36b of protrusion 36 of semiconductor substrate 14, and the side surfaces 18a of the stack formed by active layer 18 and its sandwiching layers 42a,b. If it is not already clear from the preceding discussion, note that protective layer 62 abuts active layer 18 along both the left and right side surfaces 18a. In other words, protective layer 62 is in direct contact with active layer 18.

[0053] For example, the protective layer 62 can be thinned such that it is thin compared to the height 36c of the protrusion 36 of the semiconductor substrate 14, and / or the protective layer 62 is thin compared to the thickness 18c of the active layer 18, and / or the protective layer 62 is thin compared to 100 nm or even 50 nm. Due to the thin thickness covering the side surface 18a of the active layer 18, high optical confinement can be achieved, resulting in an increased modulation bandwidth.

[0054] In an exemplary alternative, the lateral extent 36a of the protrusion 36 of the semiconductor substrate 14 may be narrow compared to the lateral extent 34 of the active layer 18. In this case, the protective layer 62 may cover a portion of the face 18d of the active layer 18 facing towards the semiconductor substrate 14. According to the described embodiment and those further described below, the protective layer may be made of any one of InGaAsP, InP or a dielectric material.

[0055] 2h illustrates the state of device 10 during fabrication reached by removal of dielectric layer 46 and masking layer 52, followed by deposition of contact metallization element 64, as illustrated in FIG. 2g. That is, dielectric layer 46 over top surface 20 of ridge semiconductor structure 16 and masking layer 52 covering overhanging side surfaces 16b of ridge semiconductor structure 16 and upper surface 18b of waveguide layer 42a are removed. This is followed by deposition of contact metallization element 64, which may have a lateral extent 64a equal to or narrower than the lateral extent 20b of top surface 20 of ridge semiconductor structure 16. Removal of masking layer 52 and dielectric layer 46 can be performed using a wet etch suitable for removing the particular dielectric or masking material.

[0056] 2i illustrates the state achieved from FIG. 2h by depositing a passivation layer 66 on the semiconductor optical device 10 such that the passivation layer 66 covers laterally opposed side surfaces 64b of the contact metallization elements 64, the overhanging sidewalls 16b of the ridge semiconductor structure 16, a portion 18b of the upper surface of the waveguide layer 42a adjacent to the ridge semiconductor structure 16, a top surface 62a of a portion 62b of the protective layer 62 laterally adjacent to the active layer 18, and a portion 62c of the protective layer 62 covering the face 24 of the semiconductor substrate 14, the side surfaces 36b of the protrusions 36 of the semiconductor substrate 14, and the side surfaces 18a of the active layer 18. In other words, the deposition may be selective to result in deposition only in areas outside the contact metallization elements 64, such that the deposited passivation layer 66 is open over the laterally extending faces 64c of the contact metallization elements. For example, the passivation layer 66 may be formed of a dielectric material. Additionally, the passivation layer 66 may be composed of the same material as the masking layer 52. Additionally, the passivation layer 66 may partially cover the laterally extending surfaces 64c of the contact metallization elements 64.

[0057] 2j shows the state of the semiconductor optical device 10 achieved from the state of FIG. 2i by disposing the semiconductor optical device 10 with a contact structure 68. As can be seen in FIG. 2j, the contact metallization element 64, whose lateral side surfaces 64b are surrounded by a passivation layer 66, is further structured with the contact structure 68, such that the contact structure 68 is disposed on the contact metallization element 64 and extends laterally from one side 66a of the passivation layer 66 surrounding the ridge semiconductor structure 16 to the other side 66a of the passivation layer 66 surrounding the ridge semiconductor structure 16. It can be seen from FIG. 2j that the contact structure 68 extends laterally beyond the ridge semiconductor structure 16 forming an air bridge contact 68c, and that the contact structure is supported by a stabilization structure 70 sandwiched between the semiconductor substrate 14 and the portion 68b of the contact structure 68.

[0058] As further shown in FIG. 2j, a portion 68a of the contact structure 68 extending laterally from one side 66a of the passivation layer 66 to the other side 66a of the passivation layer 66 may extend longitudinally along the ridge semiconductor structure 16 in a direction parallel to the longitudinal axis 22. A portion 68b of the contact structure 68 extending laterally in the longitudinal plane beyond the ridge semiconductor structure 16 may be shaped to provide an electrical connection, e.g., a contact pad 68d, as in FIG. 2j. The structuring of the contact structure 68 can be performed using a deposition process. Furthermore, the contact structure 68 may be made of a metallic material, e.g., Ti, Pt, or Au, among other metals.

[0059] 2j is a stabilization structure 70, which is disposed on the top surface 24 of the semiconductor substrate 14 such that the portion 68b of the contact structure 68 that extends away from the ridge semiconductor structure 16 is supported by the stabilization structure 70. In other words, the stabilization structure 70 is sandwiched between the semiconductor substrate 14 and the portion of the contact structure 68 that forms the contact pad 68d. In particular, the stabilization structure 70 may be made of a polymer material such as benzocyclobutene (BCB), or a polymer such as SiN x Alternatively, it can be made from other dielectric materials such as SiO2.

[0060] As shown in FIG. 2j, after device 10 is in the fabricated state, thinning of semiconductor substrate 14 from the back surface facing away from ridge structure 16 can be followed by deposition of back-contact metallization element 72. Semiconductor substrate 14 of semiconductor optical device 10 as shown in FIG. 2j can be thinned by etching, after which back-contact metallization element 72 can be deposited on back surface 24a of semiconductor substrate 14, perpendicularly opposite face 24. Contacts 68 and 72 located on top of ridge semiconductor structure 16 and below semiconductor substrate 14 provide electrical connection to semiconductor optical device 10. This electrical connection can then be used to electrically activate semiconductor optical device 10, thus enabling field injection into diode 12 of semiconductor optical device 10.

[0061] 2a-2j thus illustrate a possible method for fabricating a semiconductor optical device 10, here a DFB laser. While some possible modifications have already been mentioned in describing these figures, further variations are possible. For example, it should be noted that the top surface of the semiconductor optical device 10 may be completely covered with a polymer or the like and then planarized, such that the top surface of the semiconductor optical device 10 thus fabricated is flat and the contact elements 64 are exposed. The contact structures 68 may be omitted or formed differently. Alternatively, the polymer may be planarized, including the contact structures 68, such that the contact structures 68 are exposed. Because the polymer material BCB can be used, an additional support structure 70 is not required.

[0062] FIG. 3 illustrates a variation of the semiconductor optical device 10 of FIG. 1 in which a grating 74 is etched into the waveguide layer 42a separating the active layer 18 from the ridge semiconductor structure 16. Thus, FIG. 3 may represent an embodiment in which the semiconductor optical device 10 is embodied as a DFB laser. FIG. 3 illustrates a cross-sectional view along the longitudinal axis 22 so that the grating 74 is visible. In other words, one of the waveguide layers 42a, 42b—here, the upper layer 42a—forms a grating layer 75, with the grating 74 formed on the face of the layer 42a facing away from the active layer 18. Additionally or alternatively, the grating layer may be etched into the waveguide layer 42b separating the active layer 18 and the semiconductor substrate 14. Alternatively, it is feasible to etch the grating layer through the ridge semiconductor structure 16 or through the semiconductor substrate 14 into the active layer 18. According to one embodiment, the grating may be a phase-shifted index-coupled grating or a compound coupled grating. The grating may be composed of different sections with different optical properties, such as, for example, coupling coefficients. In particular, the longitudinal schematic of a DFB laser in Figure 3 can form a single-section DFB laser. Furthermore, variations of single-section DFB lasers or alternative embodiments of semiconductor optical device 10 can be used as building blocks to form multi-section devices along longitudinal axis 22.

[0063] The following describes, by way of example, various modifications or alternatives to the semiconductor optical devices described herein.

[0064] Considering the single-section DFB laser seen in FIG. 3 above, multi-section distributed Bragg reflector (DBR) lasers may be possible. In such cases, the multi-section DBR laser may include one or more single DBR sections with a DBR grating or one or more single DBR sections without a DBR grating. For example, a three-section DBR laser may include an active section, i.e., an active layer sandwiched between two semiconductor structures of opposite conductivity type as described in many embodiments of the present invention, and a passive section, i.e., an active section in which the active layer is replaced by a passive waveguide layer, and the passive section may include an implemented DBR grating.

[0065] Other examples of semiconductor optical devices integrated with other devices or laser sections include, but are not limited to, an electrically modulated laser including a DFB laser section and an electroabsorption modulator (EAM) section; an electrically modulated laser with a semiconductor optical amplifier (SOA) including a DFB laser section, an EAM section, and an SOA section; a passive feedback laser including a DFB laser and a passive section with a highly reflective facet, or a multi-section device such as a DFB laser, a facet section, and a DBR section; an active feedback laser including a multi-section device such as a first DFB section with a first diffraction grating, an active section, and a second DFB section with a second diffraction grating.

[0066] FIG. 4 shows a graph illustrating the percent change in relaxation frequency increase as a function of the width 34 of the active layer 18 in μm for two different values ​​of the width 50a of the bottom surface 50 of the ridge semiconductor structure 16, denoted by ws, i.e., ws = 1.0 μm and ws = 0.5 μm, in a simulation of an embodiment in which the semiconductor optical device 10 is embodied as a DFB laser. FIG. 4 illustrates that a significant increase in relaxation frequency is achieved when the width 34 of the active layer 18 is reduced to less than 1 μm. As can be seen from FIG. 4, when the width 34 of the active layer 18 is reduced to less than 1 μm and the bottom ridge width 50a is 0.5 μm, the relaxation frequency can be increased by as much as 35%. Therefore, reducing the width 34 of the active layer 18 and the thin protective layer covering the side surfaces 18a of the active layer 18 in embodiments of the present invention provides high optical confinement, resulting in a significant increase in relaxation frequency. 4, an n-substrate 14 is used, in which case most of the series resistance comes from the p-InP ridge 16, in which case the V-shaped or otherwise flared-top ridge 16 helps to reduce the series resistance. However, a p-InP substrate and an n-InP ridge may alternatively be used.

[0067] While the present invention has been described with respect to certain embodiments, there are alterations, permutations, and equivalents that fall within the scope of the present invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted to include all such alterations, permutations, and equivalents that fall within the true spirit and scope of the present invention.

[0068] References [1.1] S. Yamaoka et. al. “Uncooled 100-GBaud Operation of Directly Modulated Membrane Lasers on High-Thermal-Conductivity SiC Substrate”, ECOC 2020, paper We1E.3 [1.2] K. Uomi et. al.; Dependence of High-Speed Properties on the Number of Quantum Wells in 1.55μm InGaAs-InGaAsP MQW l / 4 shifted DFB-laser, IEEE Journal of QE, vol. 29, 1993, pp355…360 [1.3] Yu Han, Qi Tian, Sikang Yang, Jing Luan, Ruigang Zhang, Pengxiang He, Deming Liu, Minming Zhang, "Direct modulation bandwidth enhancement of DFB laser with groove-in-trench waveguide structure," Proc. SPIE 12154, 13th International Photonics and OptoElectronics Meetings (POEM 2021), 121540I (20 January 2022); https: / / doi.org / 10.1117 / 12.2625456 [1.4] K. Suga et. al., “Analysis of TDECQ Dependence on Skew and Extinction Ratio with 106-Gb / s PAM-4 modulation of Directly Modulated Submicron Ridge Localized Buried Heterostructure Lasers”, OFC 2020, paper T3C.2 [1.5] K. Nahakara et.al., “112-Gb / s PAM-4 Uncooled (25°C to 85°C) Directly Modulation of 1.3-μm InGaAlAs-MQW DFB BH Lasers with Record High Bandwidth”, ECOC 2020, paper PD2.4 [1.6] US20180090910A1 [1.7] US20230106955A1 [1.8] M. Aoki et.al. “InP-Based Reversed-Mesa Ridge-Waveguide Structure for High-Performance Long-Wavelength Laser Diodes” IEEE J. of. Quantum electron., vol. 3., 1997, pp672…683 [Explanation of symbols]

[0069] 10 Semiconductor optical devices 12 Diodes 14 Semiconductor substrate, n-substrate 16 Ridge semiconductor structure, p-Inp ridge, semiconductor ridge structure 16a cross section 16b Overhanging side wall, overhanging side 18 Active layer 18a side 18b Face, face part 18c thickness 18d side 20 Top surface 20a Side 20b Horizontal width, horizontal spread 22 Longitudinal axis 24 Top surface, top surface 24a back side 26 Interface 26a side 28 Projection area, projection 30 vertical axis, vertical direction 34 Horizontal width, horizontal spread 36 Protrusion 36a Lateral spread 36b Side 36c height 40 Semiconductor layer 42a Waveguide layer, upper layer, sandwich layer 42b Waveguide layer 44 Contact layer 46 Dielectric Layer 50 bottom 50a Horizontal width, bottom ridge width 52 Masking Layer Part 52a, horizontal spreading part 52d part 56 Lateral spread 60 part, part, overhang 62 Protective layer 62a top surface 62b part 62c part 64 Contact Meterization Elements 64a Horizontal spread 64b side 64c side 66 Passivation Layer 66a side 68 Contact Structure 68a part 68b part 68c Air Bridge Contact 68d Contact Pad 70 Stabilizing and supporting structures 72 Back Contact Meterization Elements 74 Diffraction Grating 75 Diffraction Grating Layer

Claims

1. a semiconductor substrate of a first conductivity type; a ridge semiconductor structure of a second conductivity type opposite to the first conductivity type; an active layer formed between the semiconductor substrate and the ridge semiconductor structure; a diode formed by the ridge semiconductor structure has a cross section perpendicular to the active layer, the cross section extending from the active layer towards a face of the ridge semiconductor structure facing away from the active layer, the active layer being laterally constrained to lie in a projection of the face onto the active layer in a direction perpendicular to the semiconductor substrate. Semiconductor optical devices.

2. 2. The semiconductor optical device of claim 1, wherein laterally opposing side surfaces of the active layer are laterally disposed between opposing side surfaces of the face and sides of an interface between the active layer and the ridge semiconductor structure.

3. 3. The semiconductor optical device of claim 1, wherein the active layer has a width across the longitudinal axis of the ridge semiconductor structure of less than 1 [mu]m.

4. The semiconductor optical device according to claim 1 , wherein the ridge semiconductor structure has a trapezoidal cross section.

5. 5. The semiconductor optical device according to claim 1, wherein a ratio of the width of the active layer to the width of the facet of the ridge semiconductor structure is greater than 7 / 5.

6. 6. The semiconductor optical device of claim 1, wherein the active layer comprises a multiple quantum well structure made of any one of InGaAsP, InGaAlAs or InGaAsP / InGaAlAs, InGaAsP bulk material, InGaAlAs bulk material, InAs quantum dot layers, or InAs quantum dash layers.

7. 7. The semiconductor optical device according to claim 1, wherein the semiconductor substrate comprises a protrusion on which the active layer is disposed and whose lateral extent coincides with that of the active layer.

8. 8. The semiconductor optical device according to claim 1, further comprising protective layers covering laterally opposing side surfaces of the active layer.

9. The semiconductor optical device according to claim 8 , wherein the protective layer further covers a surface of the semiconductor substrate in a portion that laterally surrounds the active layer.

10. 10. The semiconductor optical device according to claim 7, wherein the protective layer is thin compared to one or more of the height of the protrusion of the semiconductor substrate, the thickness of the active layer, and 100 nm.

11. The semiconductor optical device of claim 1 , wherein the ridge semiconductor structure further comprises a contact structure on the facet of the ridge semiconductor structure.

12. 12. The semiconductor optical device of claim 1, further comprising a masking layer covering the semiconductor substrate, the active layer, laterally opposing side surfaces of the contact structure, and laterally opposing side surfaces of the ridge semiconductor structure, the active layer being laterally confined to lie within the projections of opposing side surfaces of the face of the ridge semiconductor structure covered by the masking layer.

13. 13. The semiconductor optical device according to claim 1, wherein the material of the ridge semiconductor structure and the semiconductor substrate is indium phosphide.

14. 14. The semiconductor optical device of claim 13, wherein said material of said ridge semiconductor structure is P-type indium phosphide.

15. A DFB laser, a semiconductor substrate of a first conductivity type; a ridge semiconductor structure of a second conductivity type opposite to the first conductivity type; an active layer formed between the semiconductor substrate and the ridge semiconductor structure; a diode formed by the ridge semiconductor structure has a cross section perpendicular to the active layer, the cross section extending from the active layer towards a face of the ridge semiconductor structure facing away from the active layer, the active layer being laterally constrained to lie in a projection of the face onto the active layer in a direction perpendicular to the semiconductor substrate such that a side of the active layer is located laterally between an opposing side of the face and a side of an interface between the active layer and the ridge semiconductor structure; the active layer includes a multiple quantum well structure; the semiconductor substrate includes a protrusion on which the active layer is disposed and whose lateral extent coincides with that of the active layer; The DFB laser further includes a protective layer covering laterally opposing side surfaces of the active layer, the protective layer comprising: the height of the protrusion on the semiconductor substrate; the thickness of the active layer, and 100 nm thin compared to one or more of DFB laser.

16. depositing an active layer on a semiconductor substrate; forming a ridge semiconductor structure on the active layer such that the ridge semiconductor structure extends from the active layer toward a surface of the ridge semiconductor structure facing away from the active layer; performing a dry etching process using the surface of the ridge semiconductor structure as a mask such that the active layer is laterally disposed within a projection of the surface of the ridge semiconductor structure onto the active layer in a direction perpendicular to the semiconductor substrate; A manufacturing method comprising:

17. performing a wet etch to reduce a lateral width of the active layer relative to the projection of the facet of the ridge semiconductor structure onto the active layer; The method of claim 16 further comprising:

18. performing a wet etch to reduce a lateral width of the active layer relative to the projection of the face of the ridge semiconductor structure onto the active layer, such that a side of the active layer is located between an opposing side of the face and a side of an interface between the active layer and the ridge semiconductor structure; The method of claim 16 or 17, further comprising:

19. 19. The method of claim 16, wherein the wet etching isotropically etches the semiconductor substrate in addition to the active layer, resulting in a protrusion of the semiconductor substrate on which the active layer is disposed.

20. providing a masking layer on overhanging side surfaces of the ridge semiconductor structure and on a portion of a surface of the active layer facing the overhanging side surfaces and disposed laterally within a projection of the surface of the ridge semiconductor structure onto the active layer, the masking layer being resistant to the wet etching so as to prevent the ridge semiconductor structure from narrowing laterally as a result of the wet etching; providing the masking layer by depositing the masking layer using vapor deposition followed by a maskless, dielectric-specific dry etch; performing the dry etching using the surface of the ridge semiconductor structure as a mask, with the masking layer provided on the overhanging side surface of the ridge semiconductor structure and on the portion of the surface of the active layer facing the overhanging side surface and disposed laterally within a projection of the surface of the ridge semiconductor structure onto the active layer; performing the dry etching using the face of the ridge semiconductor structure as a mask, whereby the dry etching etches beyond the active layer down to a portion of the semiconductor substrate, thereby creating a protrusion of the semiconductor substrate in which the active layer is located and whose lateral extent coincides with that of the active layer; The method of any one of claims 16 to 19, further comprising:

21. performing the wet etching to reduce the lateral extent of the protrusion of the semiconductor substrate so that a side surface of the protrusion of the semiconductor substrate is located between an opposing side surface of the face of the ridge semiconductor structure and a side surface of the interface between the active layer and the ridge semiconductor structure; The method of any one of claims 16 to 20, further comprising:

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