GaN-based integrated laser and preparation method thereof

Abstract

The invention discloses a GaN-based integrated laser and a preparation method thereof, and belongs to the field of semiconductor laser chip manufacturing processes. According to the invention, a strip-shaped waveguide optical path with rectangular or Gaussian selected area refractive index distribution is arranged in a laterally epitaxial mask layer by using impurity engineering, and then a GaN-based laser chip is formed on the surface of a substrate by using a lateral epitaxial technology to realize monolithic integration. The problems that in the prior art, alignment precision of a GaN laser and a passive light path is low, and bonding medium loss is large can be solved, the GaN laser is compatible with an existing CMOS technology, the structure is compact, stability is high, and the integration density of a chip can be effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor laser chip fabrication technology, specifically relating to a design and manufacturing method for integrating GaN-based lasers using lateral epitaxy and mask processing techniques. Background Technology

[0002] Since the 1990s, third-generation semiconductors, represented by GaN-based materials, have gradually entered the laser application arena. Compared to materials such as GaAs and InP, GaN materials have a wider bandgap and higher temperature stability, which can extend the lasing wavelength limit of lasers to the visible light and even the ultraviolet band. In integrated optical systems, shorter wavelength lasers can not only increase the compactness of chips, but also play a unique application value in biosensing and quantum technology. First, shorter wavelengths mean higher theoretical resolution limits and higher optical field confinement capabilities. GaN-based lasers can not only help to further reduce the waveguide size in the optical path and achieve higher density and larger capacity photonic integration, but also effectively reduce the propagation loss of light in the waveguide and enhance the interaction strength between light and matter. Second, since the fluorescence excitation bands of biomolecules such as proteins and DNA are mostly located in the blue-green or ultraviolet regions, and the manipulation of qubits such as diamond nitrogen vacancy centers and Yb and Ca ion traps usually requires the participation of ultraviolet or blue-green lasers, GaN-based short-wavelength integrated lasers are the best light source choice for the above-mentioned on-chip laboratory systems.

[0003] Although GaN materials are relatively mature in light-emitting devices such as LEDs and lasers, the monolithic integration of GaN-based laser sources with passive devices such as optical waveguides / routers has long faced numerous technical challenges. This is mainly because, within the same epitaxial structure, the absorption energy of GaN passive waveguides is often lower than the lasing wavelength of the corresponding structure, leading to significant transmission losses through direct coupling. Furthermore, GaN materials also suffer from weak electro-optic and thermo-optic effects and limited modulation performance. To address these challenges, a hybrid integration approach is often adopted, utilizing coupling devices to introduce GaN-based laser sources into photonic platforms made of other materials. Existing hybrid integration approaches include: 1. Chip flip-chip integration, where GaN lasers are directly diced from a wafer and flipped to align the light-emitting point with the integrated optical path coupler. 2. Micro-transfer technology, where the GaN laser chip is first "pre-released" from the substrate using a sacrificial layer structure, then a micro-stamp is used to pick up the GaN thin-film laser and move it to the designated position within the integrated optical path. 3. Bonding and lift-off technology, which involves bonding the entire GaN wafer to the silicon photonics wafer directly or indirectly, then removing the substrate using chemical etching or laser lift-off methods, and finally using common photolithography, etching, and coating processes to fabricate structures such as lasers and modulators.

[0004] The above-mentioned solutions have achieved certain improvements in practical applications, but they still have limitations and shortcomings in terms of cost and yield. For example, although the flip-chip integration method is relatively simple, it involves solder bonding, which leads to lower integration efficiency and excessive chip volume in the integrated optical path. Most importantly, the flip-chip solution has lower precision, which easily leads to large coupling losses. In addition, while micro-transfer technology or bond stripping can improve integration precision and efficiency, they all involve the stripping of GaN wafers, requiring additional sacrificial layers, which has a certain impact on laser performance. More importantly, the bonding between GaN and the passive optical path requires direct optical coupling, with the coupling distance reaching the wavelength level, and the coupling medium needing to have an extremely low absorption coefficient; otherwise, it will lead to significant coupling losses. Summary of the Invention

[0005] To overcome the above difficulties, this invention proposes a GaN-based integrated laser and its fabrication method. This invention can fabricate a GaN laser and a passive optical path in one go and on a monolithic basis, and effectively realize the alignment and coupling of the GaN-based laser with the bottom buried optical waveguide.

[0006] The technical solution provided by this invention is as follows: An integrated GaN laser comprises a mask layer on a substrate, the mask layer having periodic windows. Epitaxial GaN crystals are grown longitudinally to drill out the window regions and laterally to cover the mask layer. A laser functional layer is grown on the epitaxial GaN layer and fabricated into a laser chip structure. The mask layer contains a striped optical routing pattern with a rectangular or Gaussian refractive index cross-section. The laser chip structure, from bottom to top, includes an n-type GaN lower waveguide layer, an InGaN multiple quantum well, an unintentionally doped GaN upper waveguide layer, a p-type AlGaN electron blocking layer, a p-type AlGaN upper confinement layer, a p-type GaN contact layer, and n-type and p-type electrodes. The mask layer is in close contact with the bottom of the n-type GaN lower waveguide layer, and optical guidance and mode modulation are achieved by evanescent wave coupling.

[0007] Furthermore, the substrate is a GaN, SiC, Si, or sapphire single crystal substrate.

[0008] Furthermore, the thickness of the mask layer is between 100 nm and 3 μm, and the material of the mask layer is an insulating material such as SiO2, TiO2, or Al2O3.

[0009] Furthermore, the optical routing pattern includes an optical waveguide, a coupler, a resonator, and a grating mode converter.

[0010] Furthermore, the GaN laser functional layer has an island-like structure with a thickness ranging from 1 to 2 μm and a width ranging from 6 to 50 μm.

[0011] Furthermore, the periodic window is H-shaped, and the central strip growth window is further defined by the direction of the central strip being the <10-10> direction of the GaN material, with a width ranging from 1 to 5 μm, a period ranging from 5 to 50 μm, and a length ranging from 600 to 1500 μm.

[0012] Furthermore, the n-type and p-type electrodes are Ti / Pt / Au, Ti / Al / Ni / Au, or Ni / Au.

[0013] Furthermore, a method for fabricating a GaN laser integrated laser is provided, the steps of which are as follows: 1) Deposit a mask layer of amorphous dielectric film material on the substrate surface; 2) The mask layer is etched using photolithography and dry etching processes to form periodic windows; 3) Photolithography and doping processes are used to selectively adjust the refractive index distribution inside the mask layer to form a striped optical path pattern with a rectangular or Gaussian refractive index cross-section. 4) The substrate with the optical path mask layer is introduced into the MOCVD chamber and annealed in an NH3 atmosphere to modify the morphology of the window area; 5) Grow a low-temperature GaN buffer layer in the window region to allow the GaN crystal to grow vertically and drill out of the window region; 6) Increase the temperature of the lateral epitaxial n-type GaN waveguide layer to increase the lateral growth ratio to cover the bottom mask and form the first wing region and the second wing region; 7) An InGaN multiple quantum well, an unintentionally doped GaN waveguide layer, a p-type AlGaN electron blocking layer, a p-type AlGaN confinement layer, and a p-type GaN contact layer are sequentially grown on the first wing region and the second wing region. 8) An insulating layer is grown on the surface of the first and second wing regions using PECVD, and a portion of the n-type GaN lower waveguide layer and the injection window of the p-type current are exposed in the first and second wing regions using photolithography and etching processes, respectively. 9) Deposit n and p electrode structures in the injection window region respectively to complete the fabrication of the laser chip structure on the epitaxial GaN layer. Furthermore, the doping process in step 3) is thermal diffusion, ion implantation, or plasma treatment. The doping elements are H, Ar, N, O, C, B, Al, and Ti. Specifically, the temperature range for thermal diffusion is 300–600 °C, the energy range for ion implantation is 100 keV–1 MeV, and the RF power range for plasma treatment is 100–600 W. The width of the surface doping ranges from 2 to 5 μm, and the depth ranges from 50 to 500 nm.

[0014] Furthermore, the annealing temperature range in step 4) is 1000~1100℃, and the NH3 flow rate range is 1000~2000 sccm.

[0015] Furthermore, in step 5), the GaN buffer layer has a thickness of 1~3 μm, a growth temperature range of 500~700℃, and a V / III ratio range of 2000~5000.

[0016] Furthermore, in step 6), the growth temperature range of the GaN lower waveguide layer is 1000~1100℃, and the V / III ratio range is 1000~3000.

[0017] This invention utilizes impurity engineering to create a rectangular or Gaussian-shaped strip waveguide optical path with a selected refractive index distribution within the mask layer during lateral epitaxy, while maintaining a flat mask surface. Subsequently, a GaN-based laser chip is formed on the substrate surface using lateral epitaxy technology, achieving monolithic integration. This invention not only solves the problems of low alignment accuracy between the GaN laser and the passive optical path and high bonding medium loss in existing technologies, but also maintains compatibility with existing CMOS processes, features a compact structure and high stability, and can effectively improve chip integration density.

[0018] The beneficial effects of this invention are as follows: (1) The GaN laser structure completely covers the optical path mask layer through lateral growth. The laser and the bottom waveguide are firmly and tightly bonded together, and there is no bonding medium between the interface. This can greatly reduce the coupling loss caused by the absorption of the bonding medium or the thickness.

[0019] (2) The spatial position of the laser structure and waveguide prepared by the present invention is highly bound by photolithography, the planar alignment accuracy between the light-emitting area and the optical path structure is higher, the preparation efficiency is higher, and it is suitable for mass production.

[0020] (3) The present invention uses lateral epitaxial growth technology to grow GaN material, which effectively filters out the dislocation density above the mask, resulting in higher crystal quality and applicability to a variety of different substrates.

[0021] (4) The present invention uses thermal diffusion, ion implantation, ion replacement and other methods to prepare buried waveguide structures, avoiding sidewall roughness and scattering loss caused by direct etching, and theoretically can achieve extremely low loss optical transmission.

[0022] (5) The present invention replaces the high-resistivity n-type AlGaN confinement layer in the traditional laser with an oxide mask material, which can effectively reduce the operating voltage and provide stronger optical confinement and coupling effects. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the GaN-based integrated laser structure of the present invention; Figure 2 This is a flowchart illustrating the fabrication process of the GaN-based integrated laser of this invention. Figure 3 This is a curve showing the variation of the coupling coefficient between the GaN-based laser and the bottom waveguide in Embodiment 1 of the present invention; Figure 4 This is a diagram showing the optical field distribution of the GaN-based laser coupled to the bottom waveguide in Embodiment 1 of the present invention. In the figure: 10, sapphire substrate; 11, SiO2 mask; 12, strip buried waveguide; 13, GaN lower waveguide layer; 14, InGaN multiple quantum wells; 15, GaN upper waveguide layer; 16, AlGaN electron blocking layer; 17, AlGaN upper confinement layer; 18, p-type contact layer; 19, SiO2 insulating window; 20, I-shaped window; 21, first wing region; 22, second wing region; 23, p-type current confinement window; 24, n-type current confinement window; 25, p-type contact electrode; 26, n-type contact electrode. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0025] Example 1 This embodiment uses a 450 nm GaN-based edge-emitting integrated laser as an example. Figure 1As shown, the GaN-based integrated laser includes a sapphire substrate, a SiO2 optical path mask layer, a low-temperature GaN buffer layer, a lower waveguide layer, multiple quantum wells, an upper waveguide layer, an electron blocking layer, an upper confinement layer, a p-type contact layer, an insulating window, and n-type and p-type electrodes. The SiO2 mask contains periodic windows, from which the GaN-based laser structure emerges and laterally covers the SiO2 mask to form a first and second wing region. A strip-shaped waveguide optical path is located inside the SiO2 mask, situated between the sapphire substrate and the GaN laser chip. The upper surface of the SiO2 optical path substrate is in close contact with the lower waveguide of the GaN-based laser. The GaN-based laser cavity has a length of 1200 μm and a width of 40 μm. The top of the p-type layer of the GaN laser is flat, and a 200 nm SiO2 insulating layer covers the first wing region, with a 3 μm current injection window in the center. A p-type electrode covers the window. The second wing region is formed by ICP etching into an n-type GaN waveguide layer to create a mesa, which is then covered with an n-type electrode.

[0026] The fabrication steps of the 450 nm GaN-based edge-emitting integrated laser provided by this invention are as follows: Figure 2 As shown: 1) First, a 200 nm thick SiO2 mask is deposited on a sapphire substrate using PECVD, such as... Figure 2 As shown in (a). 2) I-shaped windows with a period of 50 μm are formed on the SiO2 mask by photolithography and RIE etching to expose the bottom sapphire substrate, such as... Figure 2 As shown in (b), the window width of the I-shaped structure is 3 μm, the length of the central window area is 1200 μm, and the width of the two ends extending to the left and right is 5 μm.

[0027] 3) Subsequently, a strip-shaped buried waveguide optical path is fabricated in the unetched SiO2 mask area. The buried waveguide depth is 100 nm, the waveguide width is 4 μm, the waveguide length is 1500 μm, and the waveguide refractive index cross-section is Gaussian, such as... Figure 2 As shown in (c), the buried waveguide fabrication method is as follows: 1. A window with a width of 4 μm and a length of 1500 μm is formed in the mask area of ​​SiO2 by photolithography. 2. Using H... + Ions were implanted into the waveguide window at an energy of 100 keV and a dose of 5 × 10⁻⁶. 16 ions / cm². 3. Place the substrate into a tube furnace and anneal it in an N2 atmosphere at 1000°C for 30 minutes to restore the lattice.

[0028] 4) The optical path substrate is introduced into the MOCVD chamber and annealed in an NH3 atmosphere at 1100°C to modify the morphology of the window area. The NH3 flow rate is 1500 sccm.

[0029] 5) A 200 nm thick low-temperature GaN buffer layer was grown at 550 °C by longitudinal drilling from the mask window, with a V / III ratio of 4000 and H2 as the growth carrier gas.

[0030] 6) A 200 nm thick n-type GaN lower waveguide layer was laterally grown at 1100℃, increasing the proportion of lateral growth to both sides of the window, covering the bottom optical path mask and forming the first and second wing regions. The total lateral expansion width of the first and second wing regions was 40 μm, the growth V / III ratio was 1000, and the growth carrier gas was H2.

[0031] 7) A 13 nm thick In0.20Ga0.80N / GaN multiple quantum well, a 200 nm thick GaN waveguide layer, a 10 nm thick p-type AlGaN electron blocking layer, a 300 nm thick p-type AlGaN confinement layer, and a 10 nm thick p-type GaN contact layer are sequentially grown on the first and second wing regions. The growth temperature range is 700~1100℃. Figure 2 As shown in (d).

[0032] 8) A 200 nm SiO2 insulating layer was grown on the surface of the island structure using PECVD. Photolithography and etching processes were then used to expose the p-type portion of the n-type GaN lower waveguide layer and the p-type current injection window in the first and second wing regions, respectively. The p-type current injection window has a width of 3 μm and a length of 1200 μm, and its horizontal position is aligned with the bottom waveguide structure. The n-type current injection window has a width of 20 μm and a length of 1200 μm, as shown below. Figure 2 As shown in (e).

[0033] 9) Subsequently, Ti / Pt / Au layers with thicknesses of 10 / 100 / 200 nm were deposited as contact electrodes in the n and p window regions, respectively. Figure 2 As shown in (f).

[0034] In a specific embodiment of the present invention, the quantum well active region of the GaN laser chip consists of 2 pairs of In... 0.20 Ga 0.80 The structure is composed of N / GaN, with a quantum well thickness of 2.5 nm and a quantum barrier thickness of 4 nm. The lower waveguide is a 200 nm thick n-type GaN with a doping concentration of 1 × 10⁻⁶. 18 cm -3The upper waveguide layer is an unintentionally doped GaN layer with a thickness of 200 nm. The p-type electron blocking layer is an Al layer with a thickness of 10 nm. 0.3 Ga 0.7 N, with a doping concentration of 1×10 19 cm -3 The upper confinement layer is an Al layer with a thickness of 300 nm. 0.1 Ga 0.9 N, with a doping concentration of 1×10 19 cm -3 .

[0035] This embodiment utilizes the evanescent wave coupling principle to transmit the stimulated emission mode in the active region of a GaN laser to the bottom buried waveguide. For example... Figure 3 As shown, when the propagation constant β1 of the laser chip matches the propagation constant β2 of the support mode in the bottom buried waveguide, the coupling coefficient has a maximum value, and most of the energy will be distributed in the bottom waveguide. Figure 4 This is a simulation of the optical field distribution of a 450 nm GaN-based laser coupled to a bottom SiO2 buried waveguide under this condition, using finite element analysis. It can be clearly seen that most of the optical field in the active region is guided into the bottom Gaussian waveguide structure, with a near-elliptical near-field spot and a waveguide coupling coefficient reaching 48%. Compared with existing technologies, the laser structure and the bottom waveguide structure of this invention are directly bonded, requiring no bonding medium. The nanoscale coupling distance significantly reduces transmission loss and medium absorption. Furthermore, in the horizontal direction, this invention uses photolithography to bind the bottom waveguide position to the laser's active region, achieving sub-micron level alignment accuracy.

[0036] Example 2 The difference between this embodiment and Embodiment 1 is that the buried strip waveguide has a 1×3 beam combining structure, which can combine the stimulated emission of GaN lasers of different wavelengths into one beam. The beam combining structure of this embodiment can combine multiple low-power lasers into a high-power beam. In addition, it can combine multiple lasers of different wavelengths, such as red, green, and blue, and achieve different color displays in a micro-projection by modulating the intensity of each laser beam.

[0037] Example 3 The difference between this embodiment and Embodiment 1 is that the buried strip waveguide contains a third-order Bragg grating structure pattern. The period of the Bragg grating satisfies Λ=λ / 2m×n. eff In this embodiment, the Bragg grating can interact with the active region of the laser to form a DFB laser, effectively filtering out the phase-matched longitudinal modes in the resonant cavity, thus realizing a single-mode, narrow-linewidth laser.

[0038] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. A GaN laser integrated laser, characterized in that, A mask layer with periodic windows is provided on the substrate. Epitaxial GaN crystals are grown longitudinally to drill out the window regions and laterally to cover the mask layer. A laser functional layer is grown on the epitaxial GaN layer and fabricated into a laser chip structure. The mask layer has a striped optical routing pattern with a rectangular or Gaussian refractive index cross-section. The laser chip structure, from bottom to top, includes an n-type GaN lower waveguide layer, an InGaN multiple quantum well, an unintentionally doped GaN upper waveguide layer, a p-type AlGaN electron blocking layer, a p-type AlGaN upper confinement layer, a p-type GaN contact layer, and n-type and p-type electrodes. The mask layer is in close contact with the bottom of the n-type GaN lower waveguide layer, and optical guidance and mode modulation are formed by evanescent wave coupling.

2. The GaN laser integrated laser as described in claim 1, characterized in that, The substrate is a GaN, SiC, Si, or sapphire single crystal substrate.

3. The GaN laser integrated laser as described in claim 1, characterized in that, The thickness of the mask layer is 100nm~3μm, and the material of the mask layer is SiO2, TiO2, or Al2O3 insulating material. The optical routing pattern includes an optical waveguide, a coupler, a resonator, and a grating mode converter.

4. The GaN laser integrated laser as described in claim 1, characterized in that, The periodic window is I-shaped, and further, the central strip growth window is oriented in the <10-10> direction of the GaN material, with a width ranging from 1 to 5 μm, a period ranging from 5 to 50 μm, and a length ranging from 600 to 1500 μm.

5. The GaN laser integrated laser as described in claim 1, characterized in that, The n-type and p-type electrodes are Ti / Pt / Au, Ti / Al / Ni / Au, or Ni / Au.

6. A method for fabricating a GaN laser integrated laser, comprising the following steps: 1) Deposit a mask layer of amorphous dielectric film material on the substrate surface; 2) The mask layer is etched using photolithography and dry etching processes to form periodic windows; 3) Photolithography and doping processes are used to selectively adjust the refractive index distribution inside the mask layer to form a striped optical path pattern with a rectangular or Gaussian refractive index cross-section. 4) The substrate with the mask layer is introduced into the MOCVD chamber and annealed in an NH3 atmosphere to modify the morphology of the window area; 5) Grow a low-temperature GaN buffer layer in the window region to allow the GaN crystal to grow vertically and drill out of the window region; 6) Increase the temperature of the lateral epitaxial n-type GaN waveguide layer to increase the lateral growth ratio to cover the bottom mask and form the first wing region and the second wing region; 7) An InGaN multiple quantum well, an unintentionally doped GaN waveguide layer, a p-type AlGaN electron blocking layer, a p-type AlGaN confinement layer, and a p-type GaN contact layer are sequentially grown on the first wing region and the second wing region. 8) An insulating layer is grown on the surface of the first and second wing regions using PECVD, and a portion of the n-type GaN lower waveguide layer and the injection window of the p-type current are exposed in the first and second wing regions using photolithography and etching processes, respectively. 9) Deposit n and p electrode structures in the injection window region respectively to complete the fabrication of the laser chip structure on the epitaxial GaN layer.

7. The method for fabricating a GaN laser integrated laser as described in claim 6, characterized in that, The doping process in step 3) is thermal diffusion, ion implantation, or plasma treatment. The doping elements are H, Ar, N, O, C, B, Al, and Ti. The temperature range for thermal diffusion is 300~600℃, the energy range for ion implantation is 100 keV~1 MeV, the RF power range for plasma treatment is 100~600W, the width range for surface doping is 2~5μm, and the depth range is 50~500 nm.

8. The method for fabricating a GaN laser integrated laser as described in claim 6, characterized in that, The annealing temperature range in step 4) is 1000~1100℃, and the NH3 flow rate range is 1000~2000 sccm.

9. The method for fabricating a GaN laser integrated laser as described in claim 6, characterized in that, The GaN buffer layer in step 5) has a thickness of 1~3μm, a growth temperature range of 500~700℃, and a V / III ratio range of 2000~5000.

10. The method for fabricating a GaN laser integrated laser as described in claim 6, characterized in that, In step 6), the thickness of the GaN lower waveguide layer ranges from 1 to 2 μm, the width ranges from 6 to 50 μm, the growth temperature range of the GaN lower waveguide layer ranges from 1000 to 1100℃, and the V / III ratio ranges from 1000 to 3000.

Images