Surface-emitting laser and method for manufacturing the same

Abstract

To provide a surface emitting laser that is compact, fast, low power consumption, and operates stably even during temperature changes and high power operation, and a method for manufacturing the same.SOLUTION: A surface emitting laser of the present embodiment has, on a substrate, a lower semiconductor DBR layer, an active layer, an oxidation narrowing layer that forms an oxidation aperture for vertical light emission, an upper semiconductor DBR layer, a lateral propagation reflection layer that includes the oxidation aperture and forms a coupling resonance aperture in a wider area than the oxidation aperture, a dielectric DBR layer whose first layer thickness counted from the upper semiconductor DBR layer side is approximately half of the optical wavelength.SELECTED DRAWING: Figure 1

Description

Technical Field

[0005] ,

[0001] Embodiments of the present invention relate to a surface emitting laser and a method for manufacturing the same.

Background Art

[0002] In modern society where communication devices such as smartphones and IoT (Internet of Things) equipped with various measurement sensors are abundant, increasing communication capacity is an essential issue. For electronic devices, further high speed, miniaturization, low power consumption and high output are inevitable issues. Also, in the era of the 5th and 6th generation (5G, 6G) wireless communication standards, innovations in AI (Artificial Intelligence) technologies such as autonomous driving, robots, and drones are progressing, and high-speed communication at various frequencies is being utilized. Therefore, further high functionality, high-speed operation, and high reliability of electronic devices are required.

[0003] Originally, a surface emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser) has characteristics of being small and having low power consumption. In recent years, it has been used in high-function devices such as face authentication for smartphones and light sources for LIDAR (Light Detection And Ranging). In order to achieve high functionality, high speed, and high output, various attempts have been made to solve problems. For example, in Patent Document 1, by integrating a micro resonator horizontally in a surface emitting laser, the modulation band is expanded by optical feedback.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] This disclosure is made in such circumstances, and one of its exemplary objectives is to provide a surface-emitting laser that is small, fast, and low-power, and that operates stably even under temperature changes and high-power operation, as well as a method for manufacturing the same. [Means for solving the problem]

[0006] A surface-emitting laser according to one aspect of the present disclosure includes, on a substrate, a lower semiconductor DBR layer, an active layer, an oxide constriction layer forming an oxide aperture for vertical light emission, an upper semiconductor DBR layer, a transverse propagation reflection layer including the oxide aperture and forming a coupled resonance aperture in a region wider than the oxide aperture, and a dielectric DBR layer having a thickness of approximately half the optical wavelength for the first layer counted from the upper semiconductor DBR layer side.

[0007] Furthermore, the method for manufacturing a surface-emitting laser according to this embodiment includes the steps of forming a lower semiconductor DBR layer, an active layer, an oxide constriction layer that forms an oxide aperture for vertical light emission, and an upper semiconductor DBR layer on a substrate; forming a transverse propagation reflection layer that includes the oxide aperture and forms a coupled resonance aperture in a wider area than the oxide aperture; and forming a dielectric DBR layer in which the thickness of the first layer counted from the upper semiconductor DBR layer side is approximately half the optical wavelength. [Effects of the Invention]

[0008] According to one aspect of the present invention, stable single-mode oscillation operation is possible even for large optical apertures, and the modulation bandwidth can be significantly expanded. [Brief explanation of the drawing]

[0009] [Figure 1] This is a cross-sectional structural diagram of a surface-emitting laser in the first embodiment. [Figure 2] This is a schematic diagram illustrating the coupled resonance of a surface-emitting laser in an embodiment. [Figure 3A] This is an example of calculating the mode profile of a surface-emitting laser in an embodiment. [Figure 3B]This is an example of calculating the mode profile of a surface-emitting laser in an embodiment. [Figure 4] This figure shows the relationship between the thickness of the first dielectric DBR layer and the resonant wavelength in the first embodiment. [Figure 5] This is a cross-sectional structural diagram of a surface-emitting laser in the second embodiment. [Figure 6] This figure shows the relationship between the thickness of the first dielectric DBR layer and the resonant wavelength in the second embodiment. [Figure 7A] This figure shows the oscillation spectrum of a surface-emitting laser in an embodiment. [Figure 7B] This figure shows the frequency characteristics of a surface-emitting laser in an embodiment. [Modes for carrying out the invention]

[0010] The embodiments will be described in detail below with reference to the drawings. In the following description, components having substantially the same function and configuration will be denoted by the same reference numerals, and redundant descriptions will be given only when necessary. Also, the scale of the drawings is not the same as the scale of the actual device when it is used to emphasize the features of the embodiments for clarity.

[0011] (First embodiment) Figure 1 is a cross-sectional view of a surface-emitting laser in the first embodiment. The surface-emitting laser 100 of this embodiment has a lower semiconductor DBR (Distributed Bragg Reflector) layer 11, an active layer 12, an oxide constriction layer 13, an upper semiconductor DBR layer 14, a transverse propagation reflection layer 15, and a dielectric DBR layer 16 on a substrate (not shown).

[0012] The substrate is not shown because it is not an essential component in this embodiment, and there is an embodiment in which the substrate is finally removed. Terms such as upper, lower, vertical, and horizontal are used with reference to the substrate, but do not mean absolute directions. Further, as an embodiment of the present invention, a surface-emitting laser in the 1060 nm band of the InGaAs / GaAs system will be exemplified, but it is not limited thereto. A surface-emitting laser using other compound semiconductors has the same effect.

[0013] The lower semiconductor DBR layer 11 is formed by stacking a predetermined number of pairs of AlGaAs layers with different high and low refractive indexes having an Al composition of (0 to 1), for example. The optical thickness of each layer is a quarter wavelength, and a high-reflectivity mirror is formed.

[0014] The active layer 12 is composed of, for example, an InGaAs / GaAs quantum well active layer.

[0015] The oxidation confinement layer 13 is composed of, for example, AlGaAs or AlAs having a higher Al composition than the upper semiconductor DBR layer 14 and the lower semiconductor DBR layer 11 so that oxidation proceeds selectively faster in the selective oxidation process. The inside of the oxidation confinement layer 13 is divided into a non-oxidized region 13a and an oxidized region 13b, and an oxidation aperture 17 for emitting light in the vertical direction is formed. The refractive index of the non-oxidized region of the oxidation confinement layer 13a is about 3.0, but when a material such as AlGaAs or AlAs with a high Al composition is oxidized, the refractive index decreases to about 1.6. The oxidation confinement layer 13 confines the current and at the same time confines light to the non-oxidized region 13a due to this refractive index difference. Here, the oxidation aperture १७९ is the size of the non-oxidized region 13a, and this size (diameter) is denoted as Doa. In this embodiment, the non-oxidized region 13a and the oxidation aperture 17 are described as circular, but are not limited to that shape.

[0016] The upper semiconductor DBR layer 14 is formed by stacking a predetermined number of pairs of AlGaAs layers with different high and low refractive indexes having an Al composition of (0 to 1), for example. The optical thickness of each layer is a quarter wavelength.

[0017] The lateral propagation reflection layer 15 can be made of various materials such as metals, semiconductors, and dielectrics. That is, it may be an optically discontinuous member that causes reflection when light propagates in the lateral direction (horizontal direction). When the lateral propagation reflection layer 15 is formed of a metal, it can be used as an electrode for the drive current. The lateral propagation reflection layer 15 forms a coupling resonance aperture 18 that includes the region of the oxidation aperture 17 and is wider than the oxidation aperture 17. The portion where the optically discontinuous members are not laminated becomes the coupling resonance aperture 18, and the size (diameter) thereof is denoted as Dra. In the present embodiment, the coupling resonance aperture 18 is described as being circular, but it is not limited to this shape.

[0018] Let the widths of the ring-shaped regions around the oxidation aperture 17 within the coupling resonance aperture 18 be R1 and R2. When the oxidation apertures 17 within the coupling resonance aperture 18 are concentric, R1 and R2 are equal. Here, the ring-shaped region from around the oxidation aperture 17 to the coupling resonance aperture 18 is defined as the periphery of the oxidation aperture 19.

[0019] Normally, the dielectric DBR layer 16 is configured such that the optical wavelength thickness of all layers is 1 / 4 wavelength. For example, a high refractive index layer (Ta2O5) and a low refractive index layer (SiO2) are used as a pair layer, and a predetermined number of these are laminated to achieve a desired reflectance. In the present embodiment, the layer thickness of the first layer 16a counted from the upper semiconductor DBR layer 14 side is configured to be approximately 1 / 2 of the optical wavelength.

[0020] FIG. 2 is a schematic diagram for explaining the coupling resonance of the surface-emitting laser in the embodiment. The upper figure is a plan view seen from above and represents the resonance state of light in the lateral direction (horizontal direction). The lower figure is a cross-sectional view and represents the resonance state of light in the longitudinal direction (vertical direction). The explanation will be made in conjunction with FIG. 1.

[0021] Light is reflected at the end of the oxidation region 13b of the oxidation narrowing layer 13 in the lateral direction. Therefore, optical resonance occurs within the oxidation aperture 17. Also, reflection occurs at the end of the lateral propagation reflection layer 15, and optical resonance also occurs at the coupling resonance aperture 18. These two optical resonances form a coupling resonator.

[0022] For light to propagate laterally from the oxidation aperture 17 to the periphery 19 of the oxidation aperture, the following conditions are necessary. In the cross-sectional view below, if the longitudinal resonance wavelength of the oxidation aperture 17 is λoa ​​and the longitudinal resonance wavelength of the periphery 19 of the oxidation aperture is λra, then it is necessary that λra is greater than λoa (λra > λoa).

[0023] When this condition is met, some of the light confined in the oxidation aperture 17 propagates laterally as slow light from the oxidation aperture 17 to the periphery of the oxidation aperture 19, and is reflected by the lateral propagation reflection layer 15, causing optical resonance within the coupled resonance aperture 18.

[0024] Light is almost completely confined to the oxidation aperture 17 due to the current constriction of the oxidation constriction layer 13 and the refractive index difference between the non-oxidized region 13a and the oxidized region 13b. Therefore, considering the mode profile of the light, it is best to set the values ​​of R1 and R2 to values ​​between approximately 1 and 2 μm. If they are 3 μm or less, the feedback of light from the transverse propagation reflection layer 15 (periphery of the oxidation aperture 19) is performed efficiently. The values ​​of R1 and R2 are determined so that the phase of the light fed back to the oxidation aperture 17 is optimal.

[0025] In this way, the resonant light within the coupled resonant aperture 18 is fed back to the oxidation aperture 17, improving the frequency characteristics and expanding the modulation bandwidth. Furthermore, a coupled resonator is formed by the optical resonance within the coupled resonant aperture 18 and the optical resonance of the oxidation aperture 17, resulting in a Vernier effect. This Vernier effect allows for stabilization of transverse modes even with a large oxidation aperture 17.

[0026] Figure 3 shows an example of a calculated mode profile for a surface-emitting laser in an embodiment. Figure 3A shows the case where λra < λoa, and the electric field distribution is confined to the region of the oxide aperture 17. Figure 3B shows the case where λra > λoa, and it can be seen that the light propagates laterally around the periphery 19 of the oxide aperture.

[0027] Figure 4 shows the relationship between the thickness of the first layer of the dielectric DBR layer 16 and the resonance wavelength. The horizontal axis represents the optical wavelength ratio of the thickness of the first layer 16a of the dielectric DBR layer 16, and the vertical axis represents the resonance wavelength in the vertical direction. The solid line shows the resonance wavelength when the thickness of the oxidation region 13b is 30 nm, and the dotted line shows the resonance wavelength of the oxidation opening 17 (non-oxidation region 13a).

[0028] If the optical wavelength thickness of all layers of the dielectric DBR layer 16 is set to 1 / 4 wavelength (optical wavelength ratio 0.25), then, as shown in dotted region A, the longitudinal resonance wavelength λra of the periphery of the oxide aperture 19 becomes smaller than the longitudinal resonance wavelength λoa of the oxide aperture 17. As mentioned earlier, under these conditions, light does not propagate transversely, resulting in the mode profile shown in Figure 3A. When the region of the oxide aperture 17 is wide, the transverse modes are unstable, and since no optical feedback occurs, the modulation bandwidth is not wide.

[0029] When the optical wavelength thickness of the first layer of the dielectric DBR layer 16 is close to half a wavelength (optical wavelength ratio 0.5), curves for higher-order modes appear on the shorter wavelength side. As shown in the dotted line region B, the longitudinal resonance wavelength λra of the periphery of the oxide aperture 19 is greater than the longitudinal resonance wavelength λoa of the oxide aperture 17. This resonant wavelength inversion occurs in the region of optical wavelength ratio approximately 0.47 to 0.53. At this time, as shown in the mode profile in Figure 3B, light propagates transversely, and optical feedback occurs at the oxide aperture 17, improving the frequency characteristics and expanding the modulation bandwidth. In addition, a Vernier effect occurs due to the coupled resonator, and the transverse modes remain stable even when the region Doa of the oxide aperture 17 is widened.

[0030] As shown in the dotted region C, near an optical wavelength ratio of 0.5, the thickness of the first layer of the dielectric DBR layer 16 is such that a resonance wavelength exists on the longer wavelength side. It is necessary to prevent laser oscillation on the longer wavelength side. Therefore, the composition of the active layer 12 is adjusted so that the gain is higher at the shorter wavelength resonance wavelength compared to the longer wavelength resonance wavelength, thereby setting it to oscillate stably on the shorter wavelength side.

[0031] As described above, according to the first embodiment, by making the thickness of the first layer of the dielectric DBR layer 16 approximately half the optical wavelength, lateral propagation occurs, and optical feedback occurs to the oxide aperture 17. This improves the frequency characteristics and expands the modulation bandwidth. In addition, because the vernier effect occurs due to the coupled resonator formed by the oxide aperture 17 and the coupled resonant aperture 18, the lateral modes can be stabilized even if the region of the oxide aperture 17 is widened. High power output can also be achieved because the region of the oxide aperture 17 can be widened. Furthermore, the manufacturing process is simplified because there is no need to perform surface relief as described later.

[0032] (Second embodiment) Figure 5 is a cross-sectional view of the surface-emitting laser in the second embodiment. The surface-emitting laser 200 in this embodiment, like the first embodiment, has a lower semiconductor DBR layer 11, an active layer 12, an oxide constriction layer 13, an upper semiconductor DBR layer 14, a transverse propagation reflection layer 15, and a dielectric DBR layer 16 on a substrate (not shown).

[0033] The oxidation constriction layer 13 is composed of AlGaAs or AlAs, which has a higher Al composition than the upper semiconductor DBR layer 14 and the lower semiconductor DBR layer 11, so that oxidation proceeds more rapidly in the selective oxidation process. The oxidation constriction layer 13 is divided into a non-oxidized region 13a and an oxidized region 13b, and an oxidation opening 17 is formed for light emission in the vertical direction. The refractive index of the non-oxidized region of the oxidation constriction layer 13a is approximately 3.0, but when a material such as AlGaAs or AlAs with a high Al composition is oxidized, the refractive index drops to about 1.6. The oxidation constriction layer 13 constricts the current, and at the same time, this refractive index difference confines light to the non-oxidized region 13a. Here, the oxidation opening 17 is the size of the non-oxidized region 13a, and this size (diameter) is denoted as Doa. In this embodiment, the non-oxidized region 13a and the oxidation opening 17 are described as circular, but their shape is not limited to this.

[0034] In the upper semiconductor DBR layer 14, a portion of the semiconductor surface in the region corresponding to the non-oxidized region 13a is removed (hereinafter referred to as surface relief). This surface relief portion is shown by a dotted line region, and an enlarged view thereof is shown in the upper right of Figure 5. Here, the depth of this surface relief is denoted as Sr.

[0035] The transverse propagation reflection layer 15 can be made of various materials such as metals, semiconductors, and dielectrics. In other words, it can be an optical discontinuous member that causes reflection when light propagates transversely. If the transverse propagation reflection layer 15 is made of metal, a drive current can be injected from the transverse propagation reflection layer 15. The transverse propagation reflection layer 15 includes an oxide opening 17, forming a coupled resonance opening 18 in a region wider than the oxide opening 17. The portion where the optical discontinuous member is not laminated becomes the coupled resonance opening 18, and its size (diameter) is denoted as Dra. In this embodiment, the coupled resonance opening 18 is described as circular, but its shape is not limited to that.

[0036] Let R1 and R2 be the widths of the ring-shaped regions around the oxidation opening 17 within the coupled resonance opening 18. If the oxidation opening 17 within the coupled resonance opening 18 lies on a concentric circle, then R1 and R2 are equal. Here, the ring-shaped region from around the oxidation opening 17 to the coupled resonance opening 18 is defined as the oxidation opening peripheral region 19.

[0037] Normally, the dielectric DBR layer 16 is constructed with an optical wavelength thickness of 1 / 4 wavelength for all layers. However, in this embodiment, the first layer 16a, counting from the upper semiconductor DBR layer 14 side, is configured to have a thickness of approximately 1 / 2 wavelength of the optical wavelength.

[0038] In other words, the second embodiment has a structure in which, in addition to the first embodiment, a surface relief is applied to a part of the upper semiconductor DBR layer 14. The surface relief is applied in order to shorten the resonance wavelength of the region corresponding to the non-oxidation region 13a.

[0039] Figure 6 shows the relationship between the thickness of the first dielectric DBR layer and the resonance wavelength. The horizontal axis represents the optical wavelength ratio of the thickness of the first layer 16a of the dielectric DBR layer 16, and the vertical axis represents the resonance wavelength in the vertical direction. The solid lines show the resonance wavelengths when the oxide layer thickness of the oxide region 13b is 70 nm and 100 nm, and the resonance wavelength of the oxide opening 17 (non-oxidized region 13a) is shown when the surface relief amount Sr is varied to 15 nm and 30 nm.

[0040] As shown in the dotted line region D of Figure 6, when the optical wavelength ratio of the thickness of the first layer of the dielectric DBR layer 16 is 0.25, even with a surface relief of 15-30 nm, λra > λoa does not occur in thick oxide layers of 70 nm or more. That is, light does not propagate laterally, resulting in the mode profile shown in Figure 3A. When the region Doa of the oxide aperture 17 is wide, the transverse modes are unstable, and since no optical feedback occurs, the modulation bandwidth is not wide.

[0041] As shown in the dotted line region E, when the optical wavelength ratio of the thickness of the first layer of the dielectric DBR layer 16 is 0.5, λra > λoa can be achieved by applying a surface relief of about 15 to 30 nm. This resonant wavelength inversion state occurs in the region of optical wavelength ratio of about 0.47 to 0.53. At this time, as shown in the mode profile in Figure 3B, light propagates laterally, and optical feedback occurs at the oxide aperture 17, improving the frequency characteristics and expanding the modulation bandwidth. In addition, a Vernier effect occurs due to the coupled resonator, and the transverse mode remains stable even when the region Doa of the oxide aperture 17 is widened.

[0042] When the thickness of the first layer of the dielectric DBR layer 16 is near an optical wavelength ratio of 0.5, a resonant wavelength also exists on the long-wavelength side. It is necessary to prevent laser oscillation on the long-wavelength side. Therefore, the composition of the active layer 12 is adjusted so that the gain is higher at the short-wavelength resonant wavelength compared to the long-wavelength resonant wavelength, thereby setting it to stably oscillate on the short-wavelength side.

[0043] As described above, when using conventional dielectric DBR, it was difficult to obtain lateral coupling resonance when the oxide layer thickness of the oxide region 13b was thick, such as 70 nm or more. According to the second embodiment, by making the thickness of the first layer of the dielectric DBR layer 16 approximately half the optical wavelength and further applying a shallow surface relief, lateral light propagation can be generated even when the oxide layer thickness is thick, such as 70 nm or more. As a result, lateral light feedback occurs to the oxide aperture 17, improving the frequency characteristics and expanding the modulation bandwidth. In addition, since the vernier effect is generated by the coupled resonator formed by the oxide aperture 17 and the coupling resonance aperture 18, the lateral mode can be stabilized even if the region of the oxide aperture 17 is widened. High power output can also be achieved because the region of the oxide aperture 17 can be widened. Furthermore, since there is no need for deep surface relief, several challenges in the manufacturing process are solved.

[0044] Figure 7A shows the oscillation spectrum of a surface-emitting laser with an oxide aperture diameter Doa of 10 μm in this embodiment, and Figure 7B shows the frequency characteristics of the same device. In both cases, the drive current is 8 mA. As shown in Figure 7A, stable single operation is obtained even with a relatively wide oxide aperture of 10 μm. Furthermore, as shown in Figure 7B, a broadband frequency response is observed, and a modulation bandwidth (-3 dB point) of 22 GHz is obtained.

[0045] As described above, according to the embodiment, stable single-mode oscillation operation is possible even with large oxidation apertures, and the modulation bandwidth can be significantly expanded.

[0046] Although several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. The novel embodiments shown in the present invention can be carried out in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. [Explanation of symbols]

[0047] 11…Lower semiconductor DBR layer 12…Active layer 13…Oxidized constriction layer 14…Upper semiconductor DBR layer 15…Lateral propagation reflection layer 16…Dielectric DBR layer 17…Oxidation opening 18...Coupling resonance aperture

Claims

1. On the substrate, a lower semiconductor DBR layer and The active layer, An oxidized constriction layer that forms an oxidized opening for light emission in the vertical direction, The upper semiconductor DBR layer, A transverse propagation reflection layer including the oxidation opening and forming a coupled resonance opening in a region wider than the oxidation opening, The dielectric DBR layer, counting from the upper semiconductor DBR layer side, has a thickness of 0.47 to 0.53 times the optical wavelength, A laser that emits light from a single point of contact.

2. The surface-emitting laser according to claim 1, wherein the thickness of the first layer of the dielectric DBR layer is set such that the resonance wavelength at the coupled resonance aperture around the oxidation aperture is longer than the resonance wavelength in the vertical direction at the oxidation aperture.

3. The surface-emitting laser according to claim 2, wherein there are resonant wavelengths on the short-wavelength side and resonant wavelengths on the long-wavelength side, and the laser oscillates at the resonant wavelength on the short-wavelength side.

4. The surface-emitting laser according to claim 3, wherein the active layer has a higher gain at the short-wavelength resonance wavelength than at the long-wavelength resonance wavelength.

5. The surface-emitting laser according to any one of claims 1 to 4, wherein a portion of the surface of the upper semiconductor DBR of the oxidation opening is removed.

6. The surface-emitting laser according to claim 1, wherein the transverse propagation reflection layer is made of metal.

7. The surface-emitting laser according to claim 1, wherein the transverse propagation reflection layer is a semiconductor.

8. The surface-emitting laser according to claim 1, wherein the transverse propagation reflection layer is a dielectric.

9. The process involves forming a lower semiconductor DBR layer, an active layer, an oxidation constriction layer that forms an oxidation opening for vertical light emission, and an upper semiconductor DBR layer on a substrate. A step of forming a transverse propagation reflection layer that includes the oxidation opening and forms a coupled resonance opening in a region wider than the oxidation opening, A step of forming a dielectric DBR layer in which the thickness of the first layer, counted from the upper semiconductor DBR layer side, is 0.47 to 0.53 times the optical wavelength, A method for manufacturing a surface-emitting laser having

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