Mode-controllable vertical cavity surface emitting laser and method for manufacturing the same

Abstract

The application discloses a mode-controllable vertical cavity surface emitting laser. The light window surface of the laser is divided into a first area located in the middle and a second area surrounding the first area, at least one mode-adjusting composite layer is arranged on the second area, the mode-adjusting composite layer is composed of a lower high work function oxide layer and an upper low work function metal material layer, the thickness of the high work function oxide layer is (2n+1)λ / 4, the upper surface of the high work function oxide layer is roughened, λ is the wavelength of the vertical cavity surface emitting laser, n is a natural number, part or all of the high work function oxide is in an under-oxidized state, and the low work function metal material layer is composed of a low work function metal and part of under-oxidized low work function metal oxide or is entirely composed of under-oxidized low work function metal oxide. The application further discloses a preparation method. The application can effectively adjust and control the mode of the laser, and does not need to change the basic structure of the existing VCSEL laser, and can be realized on the basis of the existing process.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, and in particular to a mode-controllable vertical-cavity surface-emitting laser (VCSEL) and its fabrication method. 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 structures are currently widely 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 optical communication applications, the differentiated rise and fall times between different modes cause deterministic jitter in the eye diagram. When a vertical-cavity surface-emitting laser (VCSEL) is coupled to a multimode fiber, differential mode delay occurs, exacerbating this deterministic jitter. Theoretically, reducing the oxide aperture can reduce the number of laser modes; the smaller the oxide aperture, the fewer the laser modes, and the closer the emitted laser is to single-mode. However, it is difficult to control the size and uniformity of the oxide aperture during the oxidation process. Furthermore, the strong scattering of light by a small oxide aperture leads to strong light absorption, significantly reducing the laser's optical power and affecting its reliability.

[0004] To address this issue, some manufacturers have modified the active region or DBR structure to suppress unwanted higher-order modes as much as possible. However, such technical solutions require significant changes to the basic structure of VCSEL lasers and existing manufacturing processes, and may alter fundamental parameters such as laser wavelength. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a mode-controllable vertical cavity surface-emitting laser, which can effectively control the mode of the laser without changing the basic structure of the existing VCSEL laser, and can be realized on the basis of existing processes.

[0006] The specific technical solution proposed in this invention is as follows:

[0007] A mode-controllable vertical-cavity surface-emitting laser (VCSEL) comprises, from top to bottom, an upper DBR layer, an active layer, a lower DBR layer, and an oxide layer sandwiched within the upper DBR layer with an oxide hole formed in the middle. An optical window for laser emission is formed on the top surface of the upper DBR layer. The surface of the optical window is divided into a first region located in the middle and a second region surrounding the first region. At least one mode-tuning composite layer is disposed above the second region. The mode-tuning composite layer is composed of a lower high-work-function oxide layer and an upper low-work-function metal material layer. The thickness of the high-work-function oxide layer is (2n+1)λ / 4 and its upper surface is roughened. λ is the wavelength of the VCSEL, and n is a natural number. Some or all of the high-work-function oxides in the high-work-function oxide layer are in a sub-oxidized state. The low-work-function metal material layer is composed of a low-work-function metal and some low-work-function metal oxides in the sub-oxidized state, or is composed entirely of low-work-function metal oxides in the sub-oxidized state.

[0008] Preferably, the high work function oxide is a stack of any one or at least two of the following materials: SiO2, TiO2, and TaO2.

[0009] Preferably, the low work function metal is a stack of any one or at least two of the following metal materials: Mg, Na, Ca, K, and Li.

[0010] Preferably, the thickness of the low work function metal material layer is 10–20 nm.

[0011] Preferably, the roughness of the upper surface of the high work function oxide layer after roughening treatment is 10–50 nm.

[0012] A method for fabricating a mode-controllable vertical-cavity surface-emitting laser as described in any of the above technical solutions includes the following steps:

[0013] A high work function oxide layer with a thickness of (2n+1)λ / 4 is formed on the second region of the light window surface;

[0014] The upper surface of the high work function oxide layer is roughened by a plasma surface treatment method using Ar as the process gas.

[0015] A film composed of a low work function metal is formed on a roughened high work function oxide layer to obtain the first mode-tuning composite layer.

[0016] Preferably, a film composed of a low work function metal is formed on a roughened high work function oxide layer using a vacuum evaporation method.

[0017] Preferably, the process parameters of the plasma surface treatment method are as follows: power 50-500 W, Ar flow rate 10-50 sccm, pressure 0.5-0.1 Pa, and treatment time 10-60 s.

[0018] Furthermore, the preparation method further includes: under vacuum, on the first modality-modulating composite layer, firstly depositing a high work function oxide layer with a thickness of (2n+1)λ / 4, then performing roughening treatment, and then depositing a film layer composed of low work function metal on the roughened high work function oxide layer to obtain a second modality-modulating composite layer; and so on, to obtain a structure with at least two modality-modulating composite layers stacked.

[0019] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0020] This invention, without altering the internal structure of existing VCSEL lasers, incorporates a mode-tuning composite layer at the edge of the optical window. This layer consists of a high-work-function oxide layer and a low-work-function metal material layer. When the low-work-function metal material contacts the high-work-function oxide material, it extracts oxygen from the high-work-function oxide, simultaneously generating both oxygen-deficient low-work-function metal oxides and oxygen-deficient high-work-function oxides near the interface. This composite structure of two oxygen-deficient oxides exhibits strong light absorption characteristics. Combined with roughening at the interface, this significantly increases the absorption of transverse mode light emitted from the edge of the laser optical window while reducing the reflection of the fundamental mode light. This avoids the possibility of reflected light causing laser oscillations, reduces the likelihood of forming multiple transverse modes, and enhances the laser's mode control capability. This invention requires only a few simple additional steps after the existing VCSEL laser fabrication process, resulting in low cost. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a specific embodiment of the mode-controllable vertical-cavity surface-mount laser of the present invention; it includes the following reference numerals: 1. GaAs substrate; 2. Buffer layer; 3. N-type DBR; 4. Quantum well active layer; 5. Oxide layer; 6. P-type DBR layer; 7. N-type metal; 8. P-type metal; 9. Passivation layer; 10. Dielectric protection layer; 11. Bilayer mode-tuning composite layer; 11-1. High work function oxide layer; 11-2. Low work function metal material layer; 11-3. High work function oxide layer; 11-4. Low work function metal material layer;

[0022] Figures 2 to 17 for Figure 1 The diagram shows the fabrication process of a vertical cavity surface laser. Implementation

[0023] To address the shortcomings of existing technologies, the present invention addresses this issue by adding a mode-tuning composite layer composed of a high work function oxide layer and a low work function metal material layer at the edge of the optical window, while maintaining the existing internal structure of the VCSEL laser. When the low work function metal material contacts the high work function oxide material, it extracts oxygen from the high work function oxide, thereby simultaneously generating both oxygen-deficient low work function metal oxide and oxygen-deficient high work function oxide near the contact interface. This composite structure of two oxygen-deficient oxides exhibits strong light absorption characteristics. Combined with roughening treatment at the interface, this significantly increases the absorption of transverse mode light emitted from the edge of the laser optical window while reducing light reflection. This avoids the possibility of reflected light causing laser oscillation, reduces the likelihood of forming multiple transverse modes, and enhances the laser's mode control capability.

[0024] The specific technical solution proposed in this invention is as follows:

[0025] A mode-controllable vertical-cavity surface-emitting laser (VCSEL) comprises, from top to bottom, an upper DBR layer, an active layer, a lower DBR layer, and an oxide layer sandwiched within the upper DBR layer with an oxide hole formed in the middle. An optical window for laser emission is formed on the top surface of the upper DBR layer. The surface of the optical window is divided into a first region located in the middle and a second region surrounding the first region. At least one mode-tuning composite layer is disposed above the second region. The mode-tuning composite layer is composed of a lower high-work-function oxide layer and an upper low-work-function metal material layer. The thickness of the high-work-function oxide layer is (2n+1)λ / 4 and its upper surface is roughened. λ is the wavelength of the VCSEL, and n is a natural number. Some or all of the high-work-function oxides in the high-work-function oxide layer are in a sub-oxidized state. The low-work-function metal material layer is composed of a low-work-function metal and some low-work-function metal oxides in the sub-oxidized state, or is composed entirely of low-work-function metal oxides in the sub-oxidized state.

[0026] Preferably, the high work function oxide is a stack of any one or at least two of the following materials: SiO2, TiO2, and TaO2.

[0027] Preferably, the low work function metal is a stack of any one or at least two of the following metal materials: Mg, Na, Ca, K, and Li.

[0028] Preferably, the thickness of the low work function metal material layer is 10–20 nm.

[0029] Preferably, the roughness of the upper surface of the high work function oxide layer after roughening treatment is 10–50 nm.

[0030] A method for fabricating a mode-controllable vertical-cavity surface-emitting laser as described in any of the above technical solutions includes the following steps:

[0031] A high work function oxide layer with a thickness of (2n+1)λ / 4 is formed on the second region of the light window surface;

[0032] The upper surface of the high work function oxide layer is roughened by a plasma surface treatment method using Ar as the process gas.

[0033] A film composed of a low work function metal is formed on a roughened high work function oxide layer to obtain the first mode-tuning composite layer.

[0034] Preferably, a film composed of a low work function metal is formed on a roughened high work function oxide layer using a vacuum evaporation method.

[0035] Preferably, the process parameters of the plasma surface treatment method are as follows: power 50-500 W, Ar flow rate 10-50 sccm, pressure 0.5-0.1 Pa, and treatment time 10-60 s.

[0036] Furthermore, the preparation method further includes: under vacuum, on the first modality-modulating composite layer, firstly depositing a high work function oxide layer with a thickness of (2n+1)λ / 4, then performing roughening treatment, and then depositing a film layer composed of low work function metal on the roughened high work function oxide layer to obtain a second modality-modulating composite layer; and so on, to obtain a structure with at least two modality-modulating composite layers stacked.

[0037] It should be noted that the terms "high work function" and "low work function" used in this article are relative concepts.

[0038] By employing the technical solution of this invention, the specific modal control characteristics of the laser can be adjusted by adjusting parameters such as the shape and size of the first and second regions on the surface of the optical window.

[0039] To facilitate public understanding, the technical solution of the present invention will be described in detail below through a specific embodiment and in conjunction with the accompanying drawings:

[0040] The VCSEL laser structure in this embodiment is as follows: Figure 1 As shown, the structure includes, from bottom to top: GaAs substrate 1, buffer layer 2, N-type DBR 3, quantum well active layer 4, oxide layer 5, P-type DBR layer 6, N-type metal 7, P-type metal 8, passivation layer 9, and dielectric protection layer 10. Unlike existing VCSEL lasers, a dual-layer mode-tuning composite layer 11 is formed on the laser emission window on the top surface of the P-type DBR layer 6 to modulate the emitted laser mode. Figure 1As shown, the dual-layer mode modulation composite layer 11 is disposed above the outer ring of the optical window, forming a circular ring concentric with the circular optical window. The lower mode modulation composite layer consists of a lower high work function oxide layer 11-1 and an upper low work function metal material layer 11-2, and the upper mode modulation composite layer consists of a lower high work function oxide layer 11-3 and an upper low work function metal material layer 11-4; the high work function oxide layers 11-1 and 11-3... It is composed of high work function oxides such as SiO2, TiO2, and TaO2, with a thickness of (2n+1)λ / 4 and its upper surface is roughened, where λ is the wavelength of the vertical cavity surface-emitting laser and n is a natural number; some or all of the high work function oxides in the high work function oxide layers 11-1 and 11-3 are in a sub-oxidized state; the low work function metal material layers 11-2 and 11-4 are composed of low work function metals such as Mg, Na, Ca, K, and Li and some low work function metal oxides in a sub-oxidized state, or are composed entirely of low work function metal oxides in a sub-oxidized state.

[0041] The main mechanism of the VCSEL laser mode controllability of this invention is to selectively increase the Bragg mirror loss of higher-order transverse modes, thereby reducing reflected light and lowering or eliminating the possibility of laser oscillation caused by reflected higher-order transverse modes, thus achieving the goal of laser mode controllability. Specifically, a high-work-function oxide layer of a specific thickness is applied to the second region of the laser window. This layer increases the Bragg transmittance of higher-order transverse modes, increasing the Bragg mirror loss. After surface treatment of this portion of the film, the surface roughness of the oxide layer increases, and the surface becomes oxygen-deficient. The large surface roughness and oxygen deficiency further increase the Bragg mirror loss of higher-order transverse modes. Then, a low-work-function metal is deposited on the surface of the surface-treated portion. The low-work-function metal is reactive and will capture some oxygen from the oxygen-deficient oxide, further increasing the oxygen deficiency of the high-work-function oxide, and simultaneously forming a low-work-function metal-deficient oxide layer. These layers can further increase the Bragg mirror loss of higher-order transverse modes, ultimately achieving mode controllability of the laser.

[0042] The specific fabrication process of this VCSEL laser is as follows:

[0043] Step 1, in Figure 2 The epitaxial wafer shown is coated with photoresist, with a photoresist film thickness of 5-15 μm; the photoresist is exposed and developed to obtain a P-Mesa ring-shaped photoresist, see [reference]. Figure 3 ;

[0044] Step 2: Use ICP dry etching process to etch the epitaxial wafer obtained in Step 1. The etching gas is Cl2 / BCl3 or Cl2 / SiCl4. Etch the P Mesa step structure to expose the high-alumina layer to be oxidized. See [link to relevant documentation]. Figure 4Etch down to the 1-10 pairs of P-DBRs below the quantum well layer; remove the photoresist to obtain the P Mesa stage, see [reference]. Figure 5 ;

[0045] Step 3: Oxidize the high-alumina layer Al from Step 4 using a wet oxidation process. x Ga 1-x Al oxidation in the As layer yields a P-Mesa stepped structure with oxidation-confined structure, see... Figure 6 ;

[0046] Step 4: Deposit a passivation layer on the surface of the epitaxial wafer obtained in Step 3. The deposition process is PECVD or ALD, and the film layer is SiN. x Materials include SiO2, SiON, Al2O3, and TiO2. The film layer can be a single layer or a stack of the above materials, with a film thickness of 20-1000 nm and a water vapor barrier capacity (WVTR) of 5E. -2 ~1E -4 Oxygen barrier function (OTR) is 5E. 0 ~1E -1 See Figure 7 ;

[0047] Step 5: Perform metal via etching on the epitaxial wafer completed in Step 4. The etching gas is CF4+Ar or BOE to obtain an epitaxial wafer with vias. See [link to relevant documentation]. Figure 8 ;

[0048] Step 6: Deposit metal to fill the Via holes of the epitaxial wafer obtained in Step 5. The metal can be Au, Pt, Ag, Al, etc. See [link to relevant documentation]. Figure 10 ;

[0049] Step 7: Deposit pad metal on the epitaxial wafer obtained in Step 6. The metal may be Au, Pt, Ag, Al, etc. See [link to relevant documentation]. Figure 10 ;

[0050] Step 8: Deposit a high work function oxide layer on the epitaxial wafer obtained in Step 7. The physical thickness of the high work function oxide layer is (2n+1)λ / 4, and the material is a high work function oxide dielectric material such as SiO2, Al2O3, or TiO2. The deposition process is PECVD or ALD. See [link to relevant documentation]. Figure 11 ;

[0051] Step 9: Coat the surface of the epitaxial wafer obtained in Step 8 with photoresist, the photoresist film thickness being 1-15 μm; expose and develop the photoresist until only the outer ring of the optical window is free of photoresist, and the photoresist-free area is annular, concentric with the circular optical window, as shown. Figure 12 As shown;

[0052] Step 10: Perform Ar Plasma treatment on the surface of the epitaxial wafer obtained in Step 9. The Ar Plasma power is 50-500W, the Ar flow rate is 10-50 sccm, the surface treatment pressure is 0.5-0.1 Pa, and the surface treatment time is 10-60 s. The Ar Plasma method is used for roughening, which on the one hand roughens the surface of the high work function oxide material (in this embodiment, the roughness after treatment is 10-50 nm), and on the other hand causes some oxygen to be lost from the surface of the high work function oxide material, thus forming a low-oxygen layer with a certain degree of roughness on the surface of the high work function oxide layer. See [link to relevant documentation]. Figure 13 ;

[0053] Step 11: Remove the photoresist to obtain a high work function oxide layer with a roughened surface. The roughened high work function oxide layer has a ring shape and is concentric with the optical window. (See [link]). Figure 14 ;

[0054] Step 12: Coat the epitaxial wafer surface completed in Step 11 with photoresist, the photoresist film thickness being 1-15 μm; expose and develop the photoresist, ensuring that only the upper side of the optical window is covered with photoresist, while the rest of the laser is free of photoresist; perform patterning etching of the high work function oxide layer on the epitaxial wafer using CF4+Ar, CHF3+Ar, CF4+CHF3+Ar, BCl3+Cl2+Ar, or SiCl4+Cl2+Ar as the etching gas; after etching, remove the photoresist to obtain an epitaxial wafer with a high work function oxide layer on the optical window, wherein the upper part of the high work function oxide layer in the outer ring region of the optical window is rough and in a deoxygenated state, see [link to relevant documentation]. Figure 15 ;

[0055] Step 13: In the first vacuum chamber, low work function metals such as Mg, Na, Ca, K, and Li are vapor-deposited onto the laser surface obtained in Step 11. The metal film thickness is 10-20 nm. During the vapor deposition process, a physical mask is used to block other parts of the laser, so that only the parts of the surface treated with Ar Plasma are vapor-deposited with low work function metal materials. The rest of the laser is blocked by the physical mask and will not be vapor-deposited with low work function metal materials. Since low work function metal materials such as Mg, Na, Ca, K, and Li are chemically active and easily oxidized to form metal oxides, when they come into contact with the high work function oxide layer, they will capture oxygen from the high work function oxide layer. The high work function oxide near the contact interface will be further in a low oxygen state, while the vapor-deposited low work function metal material will be partially or completely oxidized into a low oxide. This forms the first mode-tuning composite layer.

[0056] Step 14: Depending on actual needs, multiple vacuum chamber devices can be used to prepare a double-layer or more modally modulated composite layer. Evaporation between different film layers does not require leaving the vacuum environment; it can be achieved simply by transferring the layers between different coating chambers. Specifically, the laser obtained in Step 13 is transferred to the second coating chamber for the evaporation of the second high work function oxide layer. The evaporation material is a high work function oxide material such as SiO2, Al2O3, or TiO2. Then, steps 9 to 12 are repeated to perform surface treatment on the second high work function oxide layer. Next, step 13 is repeated to perform the evaporation of the second low work function metal material layer.

[0057] Step 15: Finally, the laser with the completed single-layer or multi-layer mode-tuning composite layer is subjected to dielectric protective layer deposition. The dielectric protective layer material is SiO2, Al2O3, TiO2, or SiN. x Oxide dielectric materials such as SiON.

Claims

1. A mode-controllable vertical cavity surface emitting laser comprising, from top to bottom, an upper DBR layer, an active layer, a lower DBR layer, and an oxidation layer interposed in the upper DBR layer and having an oxidation hole formed therein, and a light window for laser emission formed on a top surface of the upper DBR layer; characterized in that, The light window surface is divided into a first region in the middle and a second region surrounding the first region, at least one modal adjustment composite layer is arranged on the second region, the modal adjustment composite layer is composed of a lower high work function oxide layer and an upper low work function metal material layer, the thickness of the high work function oxide layer is (2n+1)λ / 4, the upper surface of the high work function oxide layer is roughened, λ is the wavelength of the vertical cavity surface emitting laser, n is a natural number, part or all of the high work function oxides in the high work function oxide layer are in an under-oxidized state, the low work function metal material layer is composed of a low work function metal and a low work function metal oxide in an under-oxidized state or is entirely composed of a low work function metal oxide in an under-oxidized state.

2. The mode-controllable vertical cavity surface emitting laser of claim 1, wherein, The high work function oxide is any one of the following materials or a stack of at least two of the following materials: SiO2, TiO2, TaO2.

3. The mode-controllable vertical cavity surface emitting laser of claim 1, wherein, The low work function metal is any one of the following metal materials or a stack of at least two of the following metal materials: Mg, Na, Ca, K, Li.

4. The mode-controllable vertical cavity surface emitting laser of claim 1, wherein, The thickness of the low work function metal material layer is 10-20 nm.

5. The mode-controllable vertical cavity surface emitting laser of claim 1, wherein, The roughness of the upper surface of the high work function oxide layer after roughening is 10-50 nm.

6. A method of manufacturing a mode-controllable vertical cavity surface emitting laser according to any one of claims 1 to 5, characterized in that The method comprises the following steps: forming a high work function oxide layer with a thickness of (2n+1)λ / 4 on the second region of the light window surface; roughening the upper surface of the high work function oxide layer by a plasma surface treatment method using Ar as a process gas; forming a film layer composed of a low work function metal on the roughened high work function oxide layer to obtain a first layer of modal adjustment composite layer.

7. The preparation method according to claim 6, characterized in that, forming a film layer composed of a low work function metal on the roughened high work function oxide layer by a vacuum evaporation method.

8. The preparation method according to claim 6, characterized in that, The process parameters of the plasma surface treatment method are as follows: power 50-500 W, Ar flow rate 10-50 sccm, pressure 0.5-0.1 Pa, and treatment time 10-60 s.

9. The preparation method according to claim 6, characterized in that, The method further comprises: under vacuum, first evaporating a high work function oxide layer with a thickness of (2n+1)λ / 4 on the first layer of modal adjustment composite layer, then roughening, and then evaporating a film layer composed of a low work function metal on the roughened high work function oxide layer to obtain a second layer of modal adjustment composite layer; by analogy, obtaining a structure in which at least two layers of modal adjustment composite layer are stacked.

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