阅读说明：本技术 垂直腔面发射激光器制备方法及垂直腔面发射激光器 (Vertical cavity surface emitting laser and preparation method thereof ) 是由 向宇 赖铭智 高逸群 岳光礼 许聪基 于 2021-09-14 设计创作，主要内容包括：本发明公开了一种垂直腔面发射激光器制备方法。本发明方法包括：在衬底上的层结构中蚀刻沟槽以形成被所述沟槽分隔出的柱状主动区平台的步骤,以及对所述柱状主动区平台进行侧壁氧化以在所述柱状主动区平台内形成氧化孔的步骤；所述沟槽为非闭合沟槽,所述柱状主动区平台与沟槽外的层结构之间具有至少一处连接部；所形成氧化孔的横截面形状呈边缘为平滑曲线的非圆形。本发明还公开一种垂直腔面发射激光器。相比现有技术,本发明可以极低的成本和代价同时实现垂直腔面发射激光器高速传输性能和可靠性的大幅提高。(The invention discloses a preparation method of a vertical cavity surface emitting laser. The method comprises the following steps: etching a trench in a layer structure on a substrate to form a pillar-shaped active region mesa partitioned by the trench, and performing sidewall oxidation on the pillar-shaped active region mesa to form an oxidation hole in the pillar-shaped active region mesa; the groove is a non-closed groove, and at least one connecting part is arranged between the columnar active region platform and the layer structure outside the groove; the cross-sectional shape of the oxidized pores formed is non-circular with smooth curves at the edges. The invention also discloses a vertical cavity surface emitting laser. Compared with the prior art, the invention can greatly improve the high-speed transmission performance and the reliability of the vertical cavity surface emitting laser at very low cost and price.)

1. A method for preparing a vertical cavity surface emitting laser comprises the following steps: etching a trench in a layer structure on a substrate to form a pillar-shaped active region mesa partitioned by the trench, and performing sidewall oxidation on the pillar-shaped active region mesa to form an oxidation hole in the pillar-shaped active region mesa; the structure is characterized in that the groove is a non-closed groove, and at least one connecting part is arranged between the columnar active region platform and the layer structure outside the groove; the cross-sectional shape of the oxidized pores formed is non-circular with smooth curves at the edges.

2. A method according to claim 1, wherein said pillar shaped active region mesa is cylindrical.

3. A method according to claim 1, wherein two connection portions of different widths are symmetrically disposed between the pillar-shaped mesa and the layer structure outside the trench.

4. A method of fabricating a vertical cavity surface emitting laser according to claim 1, wherein said oxide hole is formed to have a cross-sectional shape that approximates an ellipse, a drop, or a pear.

5. A method of fabricating a vertical cavity surface emitting laser according to claim 1, further comprising: and forming an electrode on the columnar active region platform and leading out the electrode through one of the connecting parts.

6. A method of fabricating a vertical cavity surface emitting laser according to claim 5, wherein said trench is not filled.

7. A vertical cavity surface emitting laser using the method according to any one of claims 1 to 6.

Technical Field

The invention relates to a method for preparing a Vertical-Cavity Surface-Emitting Laser (VCSEL for short), belonging to the technical field of semiconductor lasers.

Background

The VCSEL is a semiconductor device, and unlike a general edge emitting type laser in which laser light is emitted from an edge, the laser light is emitted perpendicularly to a top surface. Compared with the traditional edge-emitting laser, the VCSEL has a smaller far-field divergence angle, emits a narrow and round light beam, and is easy to couple with an optical fiber; the threshold current is low; the modulation frequency is high; the single longitudinal and transverse mode works in a wide temperature and current range; the process manufacturing and detection can be completed without cleavage, and the cost is low; the method has the advantages of easy realization of large-scale array, photoelectric integration and the like, so that the method is more and more applied, and the preparation process is rapidly developed.

In the manufacturing process of the existing VCSEL, an optical hole is mostly defined by adopting an oxidized aperture method. The main process steps comprise: the epitaxial growth of the wafer, in the epitaxial growth process of the wafer, AlGaAs layers with high Al components are arranged on the lower Bragg reflector layer and/or the upper Bragg reflector layer close to the resonant cavity as oxidation limiting layers, the layer structure after the epitaxial growth is shown in figure 1, and the VCSEL chip structure mainly comprises an N-type doped DBR reflector, a resonant cavity containing a quantum well/quantum dot active region and a P-type doped DBR reflector from bottom to top; as shown in fig. 2a and 2b, a closed trench 2 is etched in the epitaxially grown layer structure to form a pillar-shaped (usually cylindrical) active region mesa 1, which is required to ensure that the oxide confinement layer 3 is exposed on the sidewall of the active region mesa 1; oxidizing the side wall of the active region platform 1, wherein during the oxidation, the oxidation is carried out along the lateral direction of the oxidation limiting layer, and the oxidized oxidation limiting layer forms Al_xO_yThe layer, and the areas which are not oxidized in the middle form an oxidation hole, namely a light outlet hole and a current injection area of the VCSEL; and then, carrying out surface passivation, a planarization process of the groove 2 (namely filling the groove 2 with polymers such as polyimide, benzocyclobutene and the like), manufacturing electrodes, leading out and the like.

In order to achieve the purpose that the shape of the oxidation hole is close to the circular shape as much as possible to obtain better light-emitting effect, the prior art generally considers that some improvement processes are provided; for example, researchers have proposed actively compensating for the difference in the oxidation rate of the sidewall in all directions by adjusting the cross-sectional shape of the mesa to be elliptical or providing an oxidation diffusion barrier layer in the sidewall portion of the mesa to achieve the final oxide hole as close to a circle as possible. Fig. 3 is an SEM electron micrograph of a typical slice of an active region mesa obtained by combining a conventional cylindrical active region mesa with a lateral wet oxidation process, and it can be seen from fig. 3 that the shape of the oxidized hole (the portion in the dashed line frame in the figure) therein is nearly circular, and theoretically should conform to the ideal shape of the oxidized hole sought in the prior art. However, the inventors found through long-term studies that: the diameter of the oxidized hole of the high-speed VCSEL laser chip is generally 6 microns or more, the high-speed VCSEL laser chip also presents a multi-transverse mode lasing state, similar to a guided wave mode in optical fiber transmission, and along with the shape of the circular oxidized hole, the phenomena similar to polarization degeneracy and mode degeneracy in multimode optical fiber exist, which can aggravate gain competition and polarization competition, thereby causing noise deterioration and influencing high-speed transmission performance. Therefore, the inventors considered that the circular oxidized pores are not the optimal shape, and further optimization of the shape of the oxidized pores is necessary.

On the other hand, as shown in fig. 2a and 2b, after the active region mesa is formed by etching, the active region mesa is completely separated from the periphery, and in the subsequent process of forming a passivation layer by plating, compressive stress or/and tensile stress caused by a dielectric film is introduced, so that the active region mesa is easily broken or even collapsed due to the influence of the stress, thereby causing the chip to completely fail.

Disclosure of Invention

The invention aims to overcome the technical defects and provide a preparation method of a vertical cavity surface emitting laser, which can greatly improve the high-speed transmission performance and the reliability of the vertical cavity surface emitting laser at extremely low cost and price.

The invention specifically adopts the following technical scheme to solve the technical problems:

a method for preparing a vertical cavity surface emitting laser comprises the following steps: etching a trench in a layer structure on a substrate to form a pillar-shaped active region mesa partitioned by the trench, and performing sidewall oxidation on the pillar-shaped active region mesa to form an oxidation hole in the pillar-shaped active region mesa; the groove is a non-closed groove, and at least one connecting part is arranged between the columnar active region platform and the layer structure outside the groove; the cross-sectional shape of the oxidized pores formed is non-circular with smooth curves at the edges.

Preferably, the pillar-shaped active region mesa is cylindrical.

Preferably, two connecting portions with different widths are symmetrically arranged between the columnar active region platform and the layer structure outside the groove.

Preferably, the cross-sectional shape of the formed oxidation pores is approximately elliptical, drop-shaped, or pear-shaped.

Further, the method further comprises: and forming an electrode on the columnar active region platform and leading out the electrode through one of the connecting parts.

Preferably, the trench is not filled.

Based on the same inventive concept, the following technical scheme can be obtained:

a vertical cavity surface emitting laser using the method of any of the above technical solutions.

Compared with the prior art, the technical scheme of the invention and the further improvement or preferred technical scheme thereof have the following beneficial effects:

the structure of the active area platform is improved, the traditional island structure is abandoned, and at least one connecting part is arranged between the active area platform and the layer structure outside the groove, so that the structure strengthening effect can be achieved, the stress influence caused by the subsequent coating process can be prevented, and the reliability is effectively improved; on the other hand, the improved active region platform structure can influence the subsequent oxidation hole generation process, so that the cross section of the oxidation hole obtained by oxidizing the side wall is in a non-circular shape with a smooth curve at the edge, and the improvement of the noise coefficient and the great improvement of the high-speed transmission performance are realized;

the method has strong adaptability to the existing process conditions, only needs to slightly adjust the original photoetching pattern, does not need to change the rest process conditions such as etching, oxidation and the like, hardly generates additional cost, and is convenient for large-scale popularization and application;

because at least one connecting part is arranged between the active region platform and the layer structure outside the groove, the subsequent electrode on the top of the active region platform can be directly led out through the connecting part, and the step of filling the groove outside the active region platform with polymer in the traditional process can be completely abandoned, thereby further improving the production efficiency and reducing the production cost.

Drawings

FIG. 1 is a schematic layer structure diagram of a VCSEL;

FIGS. 2a and 2b are a schematic vertical cross-sectional view and a schematic top-view of an active region mesa, respectively;

FIG. 3 is an SEM electron micrograph of a typical slice of an active area platform from a prior art process;

FIG. 4 is a schematic top view of an active region platform according to a preferred embodiment of the present invention;

FIG. 5 is an SEM electron micrograph of a typical section of an active area platform obtained in a preferred embodiment of the present invention;

FIG. 6 is a graph showing the Relative Intensity Noise (RIN) of the preferred embodiment of the present invention in comparison to the prior art;

fig. 7a and 7b are graphs showing the results of laser output optical power pf in accelerated aging tests of VCSEL chips in the prior art and the preferred embodiment of the present invention, respectively.

The designations in the figures have the following meanings: 1. active region mesa, 2, trench, 3, oxide confinement layer.

Detailed Description

Aiming at the defects in the prior art, the invention aims to improve the platform structure of the active area, abandons the traditional island structure, and arranges at least one connecting part between the platform of the active area and the layer structure outside the groove, thereby playing a role in strengthening the structure, preventing the stress influence caused by the subsequent film coating process and effectively improving the reliability; on the other hand, the improved active region platform structure can influence the subsequent oxidation hole generation process, so that the cross section of the oxidation hole obtained by oxidizing the side wall is in a non-circular shape with a smooth curve at the edge, and the improvement of the noise coefficient and the great improvement of the high-speed transmission performance are realized.

The preparation method of the vertical cavity surface emitting laser provided by the invention comprises the following steps: etching a trench in a layer structure on a substrate to form a pillar-shaped active region mesa partitioned by the trench, and performing sidewall oxidation on the pillar-shaped active region mesa to form an oxidation hole in the pillar-shaped active region mesa; the groove is a non-closed groove, and at least one connecting part is arranged between the columnar active region platform and the layer structure outside the groove; the cross-sectional shape of the oxidized pores formed is non-circular with smooth curves at the edges.

In the above technical solution, the cross-sectional shape of the columnar active region platform may be a conventional circle, or may be an ellipse, a semicircle, a rectangle, a polygon or other suitable shapes according to actual needs; to achieve minimal modification to existing process conditions, the columnar active region mesas are preferably most conventional cylindrical.

The specific number, position, shape, width and other parameters of the connecting parts can be optimized according to the shape of the oxidation hole to be realized, the used oxidation process and the like; for example, there may be only 1 connecting part, or there may be a plurality of connecting parts distributed uniformly or non-uniformly; the shape and width of each connecting part can be the same or different; the connecting part can be rectangular, or trapezoidal, or concave or convex water bottle shape.

The invention utilizes the structure reinforcement of the platform of the active area to simultaneously realize the optimization of the shape of the oxidation hole. Some schemes for actively correcting the oxidation holes exist in the prior art, for example, according to the difference of oxidation rates of the side wall of the active region platform in different directions, the difference of the oxidation rates is compensated by correcting the shape of the active region platform or arranging a diffusion barrier layer in some areas of the side wall of the active region platform; however, the inventor finds in practice that such a modified oxide hole scheme is liable to cause discontinuous abrupt changes at the edges of the generated oxide hole, wherein the discontinuous abrupt changes at the edges are represented by obtuse angles or acute angles in the cross-sectional shape of the oxide hole, and at the sharp points of the obtuse angles or the acute angles, not only the material stress action between the oxide layer and the adjacent semiconductor layers above and below is higher than that in other areas, but also the current crowding effect is more severe under the action of the injected current, and the combined effect is represented by that the device is more liable to fail due to the cracking or defect propagation of the oxide layer. Therefore, the cross-sectional shape of the optimized oxidation hole should be non-circular with smooth curve at the edge, and preferably approximate to ellipse, drop or pear shape, etc.

Because at least one connecting part is arranged between the active region platform and the layer structure outside the groove, the electrode prepared at the top of the active region platform can be directly led out through the connecting part, and the invention can further cancel the step of filling the groove outside the active region platform with polymer in the traditional process, thereby further improving the production efficiency and reducing the production cost.

For the public to understand, the technical scheme of the invention is explained in detail by a preferred embodiment and the accompanying drawings:

the preparation process of the VCSEL in this embodiment is specifically as follows:

step 1, forming an active region platform described by the invention through a dry etching process or a wet etching process, wherein the platform is not completely separated from other layer structures at the periphery of an etched groove;

step 2, through a wet oxidation process, reacting water vapor at high temperature with a high Al layer in the active area platform through an etching groove to form an oxidation hole;

step 3, manufacturing a ring-shaped positive electrode on the top of the active area platform through a metallization process, and simultaneously leading the ring-shaped electrode to a peripheral area through a connecting part of the active area platform;

and 4, completely covering one or more layers of passivation protective films by a film coating process and protecting the active region platform.

The active region mesa formed by etching in this embodiment is different from the conventional island mesa, and as is clear from the top view shown in fig. 4, the trench 2 in this embodiment is not closed, the main body of the active region mesa 1 is circular, but there is a connection portion connected to the layer structure outside the trench at the position symmetrical up and down, the two connection portions have different widths d1 and d2, and d2> d 1. Through the two connecting parts, the structural strength of the platform 1 in the active area is greatly enhanced, and the influence of stress caused by a subsequent coating process is favorably prevented, so that the reliability of a product is effectively improved.

The two connecting parts can directly influence the formation of subsequent oxidation holes, so that the optimization of the oxidation holes is realized. The oxidized pores formed in the active region mesa 1 after sidewall oxidation are shown by the dotted frame portion in fig. 5, and the entire shape is approximately elliptical, and due to the difference between d1 and d2, the symmetry of the approximately elliptical oxidized pores is further broken, so that the existence of mode union and polarization union can be better suppressed, and the noise figure is improved.

In order to verify the technical effect of the technical scheme of the invention, the VCSEL obtained by using the embodiment is compared with the VCSEL in the prior art in terms of relative intensity noise and effective service life. As can be seen from fig. 6, compared with the conventional VCSEL with a circular oxide aperture, the VCSEL with an approximately elliptical oxide aperture of the present invention suppresses the RIN value of the relative intensity noise by more than 3dB, which plays a significant role in improving the error rate caused by the noise of the multimode VCSEL in high-speed transmission; meanwhile, the effective service life of the chip is further prolonged compared with that of the traditional round oxide hole VCSEL, as shown in FIGS. 7a and 7b, FIG. 7a is a VCSEL verification result that the active region platform is not connected with the outer layer structure of the etched groove completely, and it can be seen that the optical power of the to-be-tested device is gradually attenuated to below 90% after 1000 hours of aging; and as can be seen from the comparison of fig. 7b, the optical power of the to-be-measured part is not attenuated to below 90% after the to-be-measured part is aged for 2500 hours because the active platform is connected with the outer layer structure of the etching groove.