A vertical cavity surface emitting laser

Abstract

The application discloses a vertical cavity surface emitting laser, which comprises a circular light emitting window, the light emitting window is divided into a circular central area which is concentric with the light emitting window but has a slightly smaller radius and a peripheral annular area which surrounds the circular central area, a light extraction layer is arranged on the upper surface of the peripheral annular area, the light extraction layer is composed of a circular ring part with a thickness of an odd multiple of λ / 4 at an outer ring and a bevel part at an inner ring, λ is the emitting laser wavelength of the vertical cavity surface emitting laser, the thickness of the bevel part linearly changes from 0 to the thickness of the circular ring part with the horizontal distance from the central axis of the light emitting window, the included angle between the bevel surface of the bevel part and the central axis of the light emitting window is 35°-65°, the material refractive index of the light extraction layer is smaller than the material refractive index of the light emitting window, and the bevel surface of the bevel part is provided with a light reflection structure. Compared with the prior art, the application can effectively inhibit the generation of transverse high-order mode, thereby improving the performance and reliability of the laser.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, and more particularly to a vertical-cavity surface-emitting laser. ( Vertical-Cavity Surface-Emitting Laser, referred to as VCSEL) . Background Technology

[0002] Vertical-cavity surface-emitting lasers (VCSELs) possess high optical power and excellent transverse mode control, making them highly promising for applications in optical communication, attitude sensing sensors, printing, and magnetic storage. However, their structure suffers from drawbacks such as a thin active region, short cavity length, and low single-layer gain. To improve their effective photon confinement capability, oxide-confined DBR (Distributed Bragg Reflector) structures are commonly used. The oxide aperture formed by the oxide-confined structure provides excellent transverse control over the current injected into the active region, resulting in virtually no transverse current. Simultaneously, this oxide aperture structure can also transversely confine the light emitted from the laser's active region, reducing the number of laser modes. This mode reduction effectively stabilizes the laser.

[0003] In vertical-cavity surface-emitting lasers (VCSELs), the current injected into the active region through the electrodes is non-uniform during operation, with the current being highest at the electrode edges. This non-uniformity worsens with increasing injected current, resulting in uneven light intensity distribution within the laser's output aperture. The light intensity in the outer region is higher than that in the inner region. Excessively strong transverse light on the outer side, when reflected back to the DBR layer, can potentially cause oscillations and generate laser light, especially under high-current conditions. This strong transverse light intensity on the outer side, reflected back to the DBR, and under the influence of oscillations, forms multiple higher-order transverse modes, causing the laser to switch between multiple modes during operation, thus deteriorating the laser's performance and reliability. Therefore, it is necessary to perform mode modulation on VCSELs to minimize undesirable modes. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a vertical cavity surface-emitting laser that can effectively suppress the generation of transverse high-order modes, thereby improving the performance and reliability of the laser.

[0005] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems:

[0006] A vertical-cavity surface-emitting laser (VCSEL) includes a circular light-emitting window. The light-emitting window is divided into a central circular region concentric with the light-emitting window but with a slightly smaller radius, and an outer annular region surrounding the central circular region. A light extraction layer is disposed on the upper surface of the outer annular region. The light extraction layer consists of an outer annular portion with a thickness that is an odd multiple of λ / 4 and an inner beveled portion, where λ is the emitted laser wavelength of the VCSEL. The thickness of the beveled portion changes linearly from 0 with the horizontal distance from the central axis of the light-emitting window to the thickness of the annular portion. The angle between the beveled surface of the beveled portion and the central axis of the light-emitting window is 35° to 65°. The refractive index of the material of the light extraction layer is less than that of the material of the light-emitting window. A light-reflecting structure is disposed on the beveled surface of the beveled portion.

[0007] Preferably, the light reflection structure includes a set of dielectric refractive layers disposed on the inclined surface of the inclined portion, wherein the refractive index of the material is greater than that of the light extraction layer, the thickness of each dielectric refractive layer is λ / 2, and the refractive index of each dielectric refractive layer increases progressively in the direction away from the light extraction layer.

[0008] More preferably, the light reflection structure further includes a metal reflective layer disposed on the outer surface of the outermost dielectric refractive layer.

[0009] Preferably, the light-reflecting structure is a metal reflective layer disposed on the inclined surface of the inclined portion.

[0010] More preferably, the thickness of the metal reflective layer is 10–200 nm.

[0011] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0012] This invention, through VCSEL A light extraction layer with an inner bevel structure is provided on the outer region of the light output window surface, and a light reflection structure is provided on the bevel surface. This extracts the transverse light outside the laser's light output window, greatly reducing the probability that the transverse light outside the light output window will be reflected back to the DBR, thus reducing the possibility of the laser generating transverse higher-order modes. At the same time, the light reflection structure provided on the surface of the bevel area can reflect the extracted light out of the laser's light output window, so that the transverse light outside the light output window will not be reflected back to the DBR, avoiding the possibility of the laser generating transverse higher-order modes, and improving the laser's working stability and reliability. Attached Figure Description

[0013] Figure 1 This is a top view of the VCSEL of the present invention;

[0014] Figure 2 This is a schematic cross-sectional view of the VCSEL of the present invention with two dielectric refractive layers along the AA direction.

[0015] Figure 3 This is a schematic cross-sectional view of the VCSEL of the present invention with three dielectric refractive layers along the AA direction.

[0016] Figure 4 A schematic cross-sectional view of the VCSEL of the present invention, which has three dielectric refractive layers and one metallic reflective layer, along the AA direction.

[0017] Figure 5 This is a schematic cross-sectional view of the VCSEL of the present invention with two dielectric refractive layers along the BB direction.

[0018] Figures 6 to 27 This is a schematic diagram of the preparation process of the VCSEL of the present invention.

[0019] The meanings of the reference numerals in the figure are as follows:

[0020] 1. GaAs substrate; 2. Buffer layer; 3. N-type DBR layer; 4. Quantum well active layer; 5. Oxide confinement layer; 6. P-type DBR layer; 7. Passivation layer; 8. P-type metal; 9. N-type metal; 10. Light extraction layer; 11. Light reflection structure; 11-1 to 11-3. Dielectric refractive layer; 11-4. Metal reflection layer. Detailed Implementation

[0021] To suppress higher-order transverse modes of the laser, the solution proposed in this invention is... VCSEL A light extraction layer with an inner inclined structure is set on the outer area of ​​the light output window surface to extract the transverse light outside the laser light output window. The extracted light is then reflected out of the laser light output window by a light reflection structure set on the inclined surface, so that the transverse light outside the light output window will not be reflected back to the DBR, thus avoiding the possibility of the laser generating transverse higher-order modes.

[0022] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems:

[0023] A vertical-cavity surface-emitting laser (VCSEL) includes a circular light-emitting window. The light-emitting window is divided into a central circular region concentric with the light-emitting window but with a slightly smaller radius, and an outer annular region surrounding the central circular region. A light extraction layer is disposed on the upper surface of the outer annular region. The light extraction layer consists of an outer annular portion with a thickness that is an odd multiple of λ / 4 and an inner beveled portion, where λ is the emitted laser wavelength of the VCSEL. The thickness of the beveled portion changes linearly from 0 with the horizontal distance from the central axis of the light-emitting window to the thickness of the annular portion. The angle between the beveled surface of the beveled portion and the central axis of the light-emitting window is 35° to 65°. The refractive index of the material of the light extraction layer is less than that of the material of the light-emitting window. A light-reflecting structure is disposed on the beveled surface of the beveled portion.

[0024] Preferably, the light reflection structure includes a set of dielectric refractive layers disposed on the inclined surface of the inclined portion, wherein the refractive index of the material is greater than that of the light extraction layer, the thickness of each dielectric refractive layer is λ / 2, and the refractive index of each dielectric refractive layer increases progressively in the direction away from the light extraction layer.

[0025] More preferably, the light reflection structure further includes a metal reflective layer disposed on the outer surface of the outermost dielectric refractive layer.

[0026] Preferably, the light-reflecting structure is a metal reflective layer disposed on the inclined surface of the inclined portion.

[0027] More preferably, the thickness of the metal reflective layer is 10–200 nm.

[0028] To facilitate public understanding, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings:

[0029] The structure of the VCSEL of this invention is as follows: Figures 1-4 As shown, it includes: GaAs substrate 1, buffer layer 2, N-type DBR layer 3, quantum well active layer 4, oxide confinement layer 5, P-type DBR layer 6, passivation layer 7, P-type metal 8, and N-type metal 9. The oxide confinement layer has an oxide hole in the middle, and the circular light-emitting window of the laser is surrounded by the P-type metal 8. This part of the structure is the same as that of the existing oxide-confined VCSEL. The present invention differs from existing VCSELs in that the light emission window is divided into a circular central region concentric with the light emission window but with a slightly smaller radius, and an outer annular region surrounding the circular central region. A light extraction layer 10 is disposed on the upper surface of the outer annular region. The light extraction layer 10 consists of an outer annular portion with a thickness of (2n+1)λ / 4 and an inner beveled portion, where n is a natural number and λ is the emitted laser wavelength of the VCSEL. The thickness of the beveled portion changes linearly from 0 with the horizontal distance from the central axis of the light emission window to the thickness of the annular portion. The angle between the beveled surface of the beveled portion and the central axis of the light emission window is 35° to 65°. The refractive index of the material of the light extraction layer is less than that of the material of the light emission window. A light reflection structure 11 is disposed on the beveled surface of the beveled portion. The light extraction layer 10 can extract the transverse light outside the laser's light emission window and reflect the extracted light out of the laser's light emission window through the light reflection structure 11, so that the transverse light outside the light emission window is not reflected back to the DBR, avoiding the possibility of the laser generating higher-order transverse modes.

[0030] The light reflection structure 11 can be composed of a single metal reflective layer disposed on the inclined surface of the inclined portion; or it can be composed of a group of dielectric refractive layers disposed on the inclined surface of the inclined portion, wherein the refractive index of the material is greater than that of the light extraction layer, and the thickness of each dielectric refractive layer is λ / 2, and the refractive index of each dielectric refractive layer increases layer by layer in the direction away from the light extraction layer; or it can be composed of a combination of the above-mentioned group of dielectric refractive layers and the outermost metal reflective layer. Figures 2-4 Three different configurations of the light-reflecting structure 11 are shown. Figure 2 The light-reflecting structure 11 consists of two dielectric refractive layers 11-1 and 11-2. Figure 3 The light-reflecting structure 11 consists of three dielectric refractive layers 11-1, 11-2, and 11-3. Figure 4 The light-reflecting structure 11 consists of three dielectric refractive layers 11-1, 11-2, and 11-3 and the outermost metal reflective layer 11-4.

[0031] The light extraction layer 10 can be made of dielectric materials such as Al2O3, SiON, SiO2, SiNx, TiO2, TaO2, and SiO2; the dielectric refractive layers 11-1, 11-2, and 11-3 can be made of dielectric materials such as SiNx, TiO2, TaO2, and SiON, and the refractive index of each layer can meet the above requirements; the metal reflective layer can be made of metal materials such as Ag and Al, and the thickness is preferably 10 to 200 nm.

[0032] In this invention, the light extraction layer only needs to cover the outer annular region of the light emission window. However, considering the difficulty of manufacturing, the outer edge of the light extraction layer can extend beyond the outer edge of the light emission window, preferably covering the entire surface of the active region platform except for the circular central region of the light emission window. The annular electrode covered by the light extraction layer can be led out through conductive channels, such as... Figure 5 As shown.

[0033] The VCSEL of this invention can be fabricated using a few additional steps on the basis of existing VCSEL fabrication processes. The light extraction layer 10 and the light reflection structure 11 can be fabricated using existing mature semiconductor processes, resulting in lower costs. The fabrication process is further described in detail below with a specific embodiment:

[0034] The VCSEL preparation in this embodiment includes the following steps:

[0035] Step 1: Coat the epitaxial wafer (which includes, from bottom to top, a GaAs substrate, a buffer layer, an N-type DBR layer, a quantum well active layer, and a P-type DBR layer) with photoresist, the photoresist film thickness being 5-15µm; expose and develop the photoresist to obtain a circular active platform photoresist, with no photoresist remaining in the remaining areas. See [link to relevant documentation]. Figure 6 ;

[0036] Step 2: Etch the epitaxial wafer obtained in Step 1 using ICP dry etching process, etching down to the 1-10 pairs of P-DBRs below the quantum well layer. The etching gas is Cl2 / BCl3 or Cl2 / SiCl4, resulting in a stepped active region platform that exposes the high-alumina layer to be oxidized. See [link to relevant documentation]. Figure 7 Remove the photoresist to obtain the active region platform of the stepped structure, see [link / reference]. Figure 8 ;

[0037] Step 3: The active region platform is sidewall oxidized using a wet oxidation process. This oxidizes the outer Al layer of the exposed high-alumina layer (AlxGa1-xAs), while the unoxidized portions in the center form oxidation pores, resulting in an active region platform with an oxidation-confined structure. (See [link to relevant documentation]). Figure 9 ;

[0038] Step 4: Deposit a passivation layer on the surface of the epitaxial wafer obtained in Step 3. The deposition process is PECVD or ALD. The film layer is made of SiNx, SiO2, SiON, Al2O3, TiO2, etc. The film layer can be a single layer or a stack of the above materials. The film thickness is 20-1000 nm. See [link to relevant documentation]. Figure 10 ;

[0039] Step 5: Etch the passivation layer metal holes on the epitaxial wafer completed in Step 4 using CF4+Ar or BOE as the etching gas, to obtain an epitaxial wafer with filled metal holes. See [link to relevant documentation]. Figure 11 ;

[0040] Step 6: Deposit metal onto the epitaxial wafer obtained in Step 5 to fill the metal holes. The metal may be Au, Pt, Ag, Al, etc. (See [link]). Figure 12 The circular passivation layer surrounded by the ring-shaped P-type metal is the light-emitting window of the laser; the existing oxide-confined VCSEL fabrication process can be completed at this point, and it is only necessary to divide the wafer into independent lasers or laser arrays as needed;

[0041] Step 7: Deposit a light extraction layer material on the surface of the epitaxial wafer obtained in Step 6. The thickness of the light extraction layer material is (2n+1)λ / 4. This layer material is a dielectric material such as Al2O3, SiON, SiO2, SiNx, TiO2, TaO2, or SiO2. Its refractive index should be lower than the refractive index of the material of the light extraction window (i.e., the refractive index of the passivation layer). See [link to relevant documentation]. Figure 13 ;

[0042] Step 8: Perform photolithographic patterning on the light extraction layer material deposited in Step 7, such as... Figure 14 As shown, this design ensures that only the central area of ​​the laser emission window is free of photoresist, and the inner ring of the annular photoresist has a sloping structure with an angle θ between the sloping surface and the surface of the emission window ranging from 35 degrees to 65 degrees.

[0043] Step 9: Etch the light extraction layer material on the surface of the epitaxial wafer patterned in Step 8. See [link to step 8]. Figure 15 After etching, the photoresist is removed. The central area of ​​the laser emission window lacks a light extraction layer, and the inner ring of the light extraction layer has a beveled structure. The angle θ between the bevel and the surface of the emission window is 35 to 65 degrees. See [link to documentation]. Figure 16 ;

[0044] Step 10: Deposit a first dielectric refractive layer on the epitaxial wafer surface completed in Step 9. The dielectric refractive layer has a thickness of λ / 2 and is made of materials such as SiNx, TiO2, TaO2, or SiON. The refractive index of the first dielectric refractive layer should be greater than that of the light extraction layer. (See also...) Figure 17 ;

[0045] Step 11: Coat the epitaxial wafer surface as completed in Step 10 with photoresist, such as... Figure 18 As shown, this ensures that no photoresist remains in the central area of ​​the light-emitting window;

[0046] Step 12: Etch the epitaxial wafer obtained in Step 11 using ICP dry etching. The etching gas is Cl2 / BCl3, Cl2 / SiCl4, or CF4+Ar, etc., so that only the first dielectric refractive layer material in the central region of the light-emitting window is etched away. See [link to relevant documentation]. Figure 19 After etching, the photoresist is removed, resulting in an epitaxial wafer without the first dielectric refractive layer material in only the central region of the light-emitting window. See [link to documentation]. Figure 20 ;

[0047] Step 13: Deposit a second dielectric refractive layer on the surface of the epitaxial wafer obtained in Step 12. The thickness of the second dielectric refractive layer is λ / 2, and the material is SiNx, TiO2, TaO2, SiON, etc. The refractive index of the material of the second dielectric refractive layer is greater than that of the first dielectric refractive layer. See [link to relevant documentation]. Figure 21 ;

[0048] Step 14: Coat the epitaxial wafer surface as completed in Step 13 with photoresist, such as... Figure 22 As shown, this ensures that no photoresist remains in the central area of ​​the light-emitting window;

[0049] Step 15: Etch the epitaxial wafer obtained in Step 14 using ICP dry etching. The etching gas is Cl2 / BCl3, Cl2 / SiCl4, or CF4+Ar, etc., so that only the second dielectric refractive layer material in the central region of the light-emitting window is etched away. See [link to relevant documentation]. Figure 23 After etching, the photoresist is removed, resulting in an epitaxial wafer with no second dielectric refractive layer material in the central region of the light-emitting window. See [link to documentation]. Figure 24If more layers of dielectric refractive layer are needed or a metal reflective layer needs to be set on the outermost layer of the inclined plane, repeat steps 13 to 15 above to obtain an epitaxial film with no dielectric refractive layer material and no metal reflective layer material in the central area of ​​the light-emitting window.

[0050] Step 16: Coat the surface of the epitaxial wafer obtained in Step 15 with photoresist, such as... Figure 25 As shown, this arrangement ensures that only the funnel-shaped space above the center area of ​​the light-emitting window retains photoresist, while the rest of the space retains no photoresist.

[0051] Step 17, as follows Figure 26 As shown, the epitaxial wafer obtained in step 16 is etched using an ICP dry etching process. The etching gas is Cl2 / BCl3, Cl2 / SiCl4, or CF4+Ar, etc. All dielectric refractive layer material and / or metal reflective layer material in other areas are etched away, leaving only the dielectric refractive layer material and / or metal reflective layer material on the light extraction layer slope. After etching, the photoresist is removed, resulting in a laser with a light-reflecting structure consisting of two dielectric refractive layers only on the light extraction layer slope. Figure 27 As shown.

Claims

1. A vertical-cavity surface-emitting laser, comprising a circular light-emitting window, characterized in that, The light-emitting window is divided into a circular central region concentric with the light-emitting window and an outer annular region surrounding the circular central region. A light extraction layer is provided on the upper surface of the outer annular region. The light extraction layer consists of an outer annular portion with a thickness that is an odd multiple of λ / 4 and an inner beveled portion, where λ is the emitted laser wavelength of the vertical cavity surface emitter (VCSEL). The thickness of the beveled portion changes linearly from 0 to the thickness of the annular portion along the horizontal distance from the central axis of the light-emitting window. The angle between the beveled surface of the beveled portion and the central axis of the light-emitting window is 35° to 65°. The refractive index of the material of the light extraction layer is less than that of the material of the light-emitting window. A light-reflecting structure is provided on the beveled surface of the beveled portion.

2. The vertical-cavity surface-emitting laser as described in claim 1, characterized in that, The light reflection structure includes a set of dielectric refractive layers disposed on the inclined surface of the inclined portion, wherein the refractive index of the material is greater than that of the light extraction layer. The thickness of each dielectric refractive layer is λ / 2, and the refractive index of each dielectric refractive layer increases progressively in the direction away from the light extraction layer.

3. The vertical-cavity surface-emitting laser as described in claim 2, characterized in that, The light reflection structure also includes a metal reflective layer disposed on the outer surface of the outermost dielectric refractive layer.

4. The vertical-cavity surface-emitting laser as described in claim 1, characterized in that, The light-reflecting structure is a metal reflective layer disposed on the inclined surface of the inclined section.

5. The vertical-cavity surface-emitting laser as described in claim 3 or 4, characterized in that, The thickness of the metal reflective layer is 10–200 nm.

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