A controllable modal vertical cavity surface-emitting laser

Abstract

This invention discloses a controllable modal vertical-cavity surface-emitting laser (VCSEL). It 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 consists of a lower light extraction layer and an upper polysilicon light-absorbing layer. The thickness of both the light extraction layer and the polysilicon light-absorbing layer is an odd multiple of one-quarter of the emission wavelength of the VCSEL. The refractive index of the material of the light extraction layer is less than that of the material of the upper DBR layer. Compared with the prior art, this invention has better modal control capability and stability.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, and more particularly to a controllable modal vertical-cavity surface-emitting laser (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 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 solutions require significant changes to the basic structure of VCSEL lasers and existing manufacturing processes, and may alter fundamental parameters such as laser wavelength. Other researchers have proposed fabricating an amorphous silicon light-absorbing layer on the surface of the laser's optical window to absorb the transverse light emitted from the window edge, reducing its intensity and thus effectively decreasing the likelihood of transverse light forming laser beams, thereby achieving mode control over the laser. However, the inventors discovered that amorphous silicon materials have numerous defects, such as vacancies, microvoids, and silicon dangling bonds. Furthermore, amorphous silicon contains a large amount of hydrogen, which exists in a free or gaseous state. These defects and the chemical instability of hydrogen in amorphous silicon lead to interactions between light and the amorphous silicon material during light absorption, releasing hydrogen gas and causing changes in material properties, resulting in instability, affecting light absorption, and ultimately impacting the stable operation of the laser. In addition, the amorphous silicon light absorption layer has a smooth interface, which reflects a certain amount of light. This reflection reduces the light absorption capacity of the amorphous silicon light absorption layer. The reflected light interacts to form laser oscillations, resulting in multiple transverse modes and weakening the control capability of the laser modes. 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 controllable modal vertical cavity surface-emitting laser, which has better modal control capability and stability compared with the existing scheme using an amorphous silicon light-absorbing layer.

[0006] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems:

[0007] A controllable modal vertical cavity surface-emitting laser (VCSEL) includes, 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 light extraction layer and an upper polycrystalline silicon light-absorbing layer. The thickness of both the light extraction layer and the polycrystalline silicon light-absorbing layer is an odd multiple of one-quarter of the emission wavelength of the VCSEL. The refractive index of the material of the light extraction layer is less than that of the material of the upper DBR layer.

[0008] Preferably, the polycrystalline silicon light-absorbing layer is obtained by laser crystallization of amorphous silicon material.

[0009] More preferably, the laser crystallization method is as follows: First, the amorphous silicon material is heat-treated to reduce the hydrogen content in the amorphous silicon material. The heat treatment temperature is 350-450 ℃ and the time is 30-60 min. Then, the heat-treated amorphous silicon material is irradiated with blue laser or excimer laser to melt the amorphous silicon material and transform it into polycrystalline silicon.

[0010] Preferably, the material of the light extraction layer is any one or a combination of at least two of the following dielectric materials: SiN x , SiO2, SiON, Al2O3, TiO2.

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

[0012] This invention replaces the existing amorphous silicon light-absorbing layer with a composite light-emitting layer and a polycrystalline silicon light-absorbing layer. Because polycrystalline silicon has a very low hydrogen content and fewer defects such as vacancies, microvoids, and silicon dangling bonds, its chemical properties are very stable. During light absorption, polycrystalline silicon does not release hydrogen gas, and its material properties do not change, enabling continuous and stable absorption of light emitted by the laser and stabilizing the laser's mode control. Furthermore, polycrystalline silicon has numerous grain boundaries and a certain degree of roughness. These grain boundaries and rough structures form non-smooth interfaces at the interfaces, scattering incident light, reducing reflection, and increasing absorption. Reduced reflection further weakens the interaction of reflected light, avoiding the possibility of laser oscillation caused by reflected light, reducing the formation of multiple transverse modes, and enhancing the laser's mode control capability. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a specific embodiment of the controllable mode vertical cavity surface-emitting laser of the present invention; the figure 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. Light extraction layer; 11. Polycrystalline silicon light absorption layer;

[0014] Figures 2 to 17 for Figure 1 The diagram shows the fabrication process of a controllable modal vertical cavity surface-emitting laser. Implementation

[0015] To address the shortcomings of existing technologies that use amorphous silicon light-absorbing layers for mode modulation, this invention proposes to replace the amorphous silicon light-absorbing layer with a composite light-emitting layer and a polycrystalline silicon light-absorbing layer, thereby improving the mode control capability and performance stability of VCSEL lasers.

[0016] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems:

[0017] A controllable modal vertical cavity surface-emitting laser (VCSEL) includes, 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 light extraction layer and an upper polycrystalline silicon light-absorbing layer. The thickness of both the light extraction layer and the polycrystalline silicon light-absorbing layer is an odd multiple of one-quarter of the emission wavelength of the VCSEL. The refractive index of the material of the light extraction layer is less than that of the material of the upper DBR layer.

[0018] Because polycrystalline silicon has a very low hydrogen content and even lower levels of defects such as vacancies, microvoids, and silicon dangling bonds, its chemical properties are extremely stable. During light absorption, polycrystalline silicon does not release hydrogen gas, and its material properties remain unchanged, enabling it to continuously and stably absorb light emitted by the laser and stabilize the laser's mode control. Furthermore, polycrystalline silicon has numerous grain boundaries and a certain degree of roughness. These grain boundaries and rough structures form non-smooth interfaces at the points of contact, scattering incident light and reducing reflection while simultaneously increasing absorption. Reduced reflection further weakens the interaction between reflected light and the laser, preventing the formation of laser oscillations and minimizing the formation of multiple transverse modes, thus enhancing the laser's mode control capability.

[0019] The polycrystalline silicon light-absorbing layer can be prepared using existing polycrystalline silicon preparation methods, such as solid-state crystallization, metal-induced crystallization, and laser crystallization. Considering the existing mature amorphous silicon thin film preparation process and the process of preparing polycrystalline silicon from amorphous silicon, preferably, the polycrystalline silicon light-absorbing layer is obtained by laser crystallization of amorphous silicon material.

[0020] The specific process flow and process parameters of the laser crystallization can be reasonably set according to the actual film thickness. More preferably, the laser crystallization method is as follows: First, the amorphous silicon material is heat-treated to reduce the hydrogen content in the amorphous silicon material. The heat treatment temperature is 350-450 ℃ and the time is 30-60 min. Then, the heat-treated amorphous silicon material is irradiated with blue laser or excimer laser to melt the amorphous silicon material and transform it into polycrystalline silicon.

[0021] Preferably, the material of the light extraction layer is any one or a combination of at least two of the following dielectric materials: SiN xSiO2, SiON, Al2O3, and TiO2 are all commonly used materials in existing VCSEL fabrication processes. They are relatively easy to obtain and the fabrication processes and equipment are mature.

[0022] 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:

[0023] The specific structure of the controllable modal vertical cavity surface-emitting laser in this embodiment is as follows: Figure 1 As shown, it includes: 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 an annular double-layer mode-tuning composite layer located above the edge region of the laser window. Each mode-tuning composite layer is composed of a lower light extraction layer 10 and an upper polysilicon light absorption layer 11. The thickness of both the light extraction layer 10 and the polysilicon light absorption layer 11 is (2n+1)λ / 4, which is an odd multiple of one-quarter of the emission wavelength λ of the vertical cavity surface-emitting laser. The refractive index of the material of the light extraction layer is less than that of the material of the upper DBR layer.

[0024] In this embodiment, the light extraction layer material is SiN. x In the preparation of the annular bilayer mode-tuning composite layer, dielectric materials such as SiO2, SiON, Al2O3, and TiO2 are used. First, a dielectric material with a physical film thickness of (2n+1)λ / 4 is deposited on the surface of the laser window. The dielectric material is then dry-etched to form a pattern, retaining only the dielectric material above the laser window. Next, an amorphous silicon material with a physical film thickness of (2n+1)λ / 4 is deposited on the surface of the patterned dielectric material. The amorphous silicon material is then dry-etched so that it is retained only in the outer region of the laser window (i.e., the second region on the surface of the laser window), and the retained amorphous silicon is concentric with the circular window, forming a ring shape. Finally, the patterned amorphous silicon material is heat-treated at a temperature of 350-450 ℃ for 30-60 seconds. The process involves a process called min, which reduces the hydrogen content in the amorphous silicon material. The dehydrogenated amorphous silicon material is then irradiated with a blue laser or excimer laser. The laser energy is absorbed by the amorphous silicon, causing it to melt. After the laser irradiation stops, the amorphous silicon becomes a polycrystalline silicon light-absorbing layer. By repeating the above process, the modal modulation composite layer or multiple modal modulation composite layers can be obtained.

[0025] The complete process flow of this controllable mode vertical cavity surface-emitting laser is as follows:

[0026] 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 ;

[0027] Step 2: Employ ICP dry etching 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 4 Etch 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 ;

[0028] Step 3: Oxidize the high-alumina layer Al to be oxidized 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 [link to documentation]) Figure 6 ;

[0029] 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 thickness of 20-1000 nm. The water vapor barrier capacity (WVTR) of the passivation layer is 5E. -2 ~1E -4 Oxygen barrier function (OTR) is 5E. 0 ~1E -1 See Figure 7 ;

[0030] 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 ;

[0031] Step 6: Deposit metal to fill the Via holes on the epitaxial wafer obtained in Step 5. The metal can be Au, Pt, Ag, Al, etc. See [link to epitaxial wafer description]. Figure 9 ;

[0032] 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 epitaxial wafer description] for the resulting epitaxial wafer. Figure 10 ;

[0033] Step 8: Deposit a light extraction layer on the epitaxial wafer obtained in Step 7. The physical thickness of the light extraction layer is λ(2n+1) / 4, and the material is SiN. x Materials include SiO2, SiON, Al2O3, and TiO2. The deposition process is PECVD or ALD. The resulting epitaxial wafers are shown in [reference needed]. Figure 11 ;

[0034] Step 9: Deposit an amorphous silicon film on the epitaxial wafer obtained in Step 8. The physical thickness of the amorphous silicon film is λ / 4. See [link / reference]. Figure 12 ;

[0035] Step 10: Coat the surface of the epitaxial wafer obtained in Step 9 with photoresist, the photoresist film thickness being 5-15 μm; expose and develop the photoresist to obtain a ring-shaped photoresist covering the outer area of ​​the optical window, see [link to relevant documentation]. Figure 13 ;

[0036] Step 11: Perform patterning etching of the light absorption layer on the epitaxial wafer completed in Step 10. The etching gas is CF4+Ar, CHF3+Ar, or CF4+CHF3+Ar to obtain an epitaxial wafer with a ring-shaped light absorption layer pattern. See [link to relevant documentation]. Figure 14 The patterned annular light-absorbing layer is subjected to heat treatment at a temperature of 350-450 ℃ for 30-60 min to reduce the hydrogen content in the amorphous silicon material of the annular light-absorbing layer.

[0037] Step 12: Irradiate the epitaxial wafer completed in Step 11 with blue laser or excimer laser. The excimer laser wavelength is 308 nm, and the blue laser wavelength is 445 nm. The laser energy is 1-5 W, and the irradiation time is 10-60 s. See [link / reference]. Figure 15 The laser transforms the light-absorbing layer material from amorphous silicon to polycrystalline silicon. The resulting polycrystalline silicon material has a refractive index of 3.9-4.3, a roughness of 20-50 nm, and a grain size of 0.3-1 μm. See [link to documentation]. Figure 16 ;

[0038] Step 13: Repeat steps 8 through 12 to obtain the following result. Figure 17 The image shows a controllable modal vertical cavity surface-emitting laser with a dual-layer modal modulation composite layer.

Claims

1. A controllable modal vertical cavity surface-emitting laser, comprising, 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, wherein an optical window for laser emission is formed on the top surface of the upper DBR layer; characterized in that, The surface of the optical window is divided into a first region in the middle and a second region surrounding the first region. The second region is completely covered by at least one mode modulation composite layer disposed thereon. The mode modulation composite layer consists of a lower light extraction layer and an upper polycrystalline silicon light absorption layer. The thickness of both the light extraction layer and the polycrystalline silicon light absorption layer is an odd multiple of one-quarter of the emission wavelength of the vertical cavity surface emission laser. The refractive index of the material of the light extraction layer is less than that of the material of the upper DBR layer.

2. The controllable modal vertical cavity surface-emitting laser as described in claim 1, characterized in that, The polycrystalline silicon light-absorbing layer is obtained by laser crystallization of amorphous silicon material.

3. The controllable modal vertical-cavity surface-emitting laser as described in claim 2, characterized in that, The laser crystallization method is as follows: First, the amorphous silicon material is heat-treated to reduce the hydrogen content in the amorphous silicon material. The heat treatment temperature is 350-450 ℃ and the time is 30-60 min. Then, the heat-treated amorphous silicon material is irradiated with blue laser or excimer laser to melt the amorphous silicon material and transform it into polycrystalline silicon.

4. The controllable modal vertical cavity surface-emitting laser as described in claim 1, characterized in that, The material of the light extraction layer is any one or a combination of at least two of the following dielectric materials: SiN x , SiO2, SiON, Al2O3, TiO2.

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