Cladding-free GAN-based thin film edge emitting laser

Abstract

A method for manufacturing a cladding-free GaN-based thin film edge emitting laser includes the steps of: attaching a typical LED wafer to a Si substrate; removing the original sapphire substrate of the LED wafer to expose the n-GaN buffer layer and the u-GaN buffer layer; reducing the thickness of the film to maximize the overlap factor between the resonance mode distribution and the active region; another reflective metal layer is deposited on the surface of the n-GaN and used for light confinement and electric contact; and defining the pattern of the edge emission cavity through a nanometer photoetching technology; the etching is performed by an ICP etch to transfer the pattern to the thin film.

Description

[0001] Cross - reference to related patent applications

[0002] This application claims the benefit of priority of U.S. Application No. 63 / 428,993, filed on November 30, 2022, under 35 U.S.C. § 119(e), which is hereby incorporated by reference in its entirety. Technical Field

[0003] The present invention relates to thin - film edge - emitting lasers, and more particularly, to cladding - free GaN - based thin - film edge - emitting lasers. Background Art

[0004] There is a growing interest in photonic integrated circuits (PICs) that monolithically or heterogeneously integrate multiple photonic devices at the system level. The reasons for this interest relate to the comparison of the speed of light with the speed of electrons in electronic integrated circuits (ICs), especially when the scaling of electronic ICs predicted by Moore's law approaches its physical limits. These photonic integrated systems offer many advantages, including increased functionality, scalability, speed, efficiency, size reduction, improved stability, robustness, and reduced power consumption. All of these advantages are crucial for the development of new optical integrated systems such as optical communication, optical sensing, and optical computing. PICs have been demonstrated using various material systems including Si[1,2], GaAs[3,4], InP[5,6], and GaN[7 - 9]. While each material has its own advantages, GaN with In and Al alloys is a comprehensive material system that has a wide and direct bandgap, giving a wide wavelength coverage from the deep ultraviolet to the visible light band.

[0005] GaN - based visible - light - emitting laser diodes have been used as the basis for optical storage (blue light), laser displays, and visible - light communication. Typical forms of such laser diodes include vertical - cavity surface - emitting lasers (VCSELs) and edge - emitting lasers [10,11]. Utilizing horizontal stimulated emission, edge - emitting lasers are ideal light sources for on - chip photonic communication or integrated circuits, which have attracted increasing interest due to their advantages over their electronic counterparts, including functionality, scalability, speed, efficiency, size reduction, improved stability, robustness, and reduced power consumption.

[0006] An edge - emitting laser is a type of laser that has mirrors (or equivalents) on both sides of a waveguide that contains a gain medium (the active region of a semiconductor laser). The quality of the laser beam emission from the laser depends on (a) optical confinement, i.e., the degree to which the light beam is confined within the waveguide, and (b) the effective gain of the gain medium, which typically depends on the internal quantum efficiency of the active region (MQW) and how much of the guided beam overlaps within the active region.

[0007] Typically, for a semiconductor edge-emitting laser, a pair of cladding layers must be grown above and below the active region (e.g., multiple quantum wells (MQWs)) to confine light waves near the active region. Unfortunately, in terms of growth quality and MQW design, inserting this pair of cladding layers into the epitaxial structure is detrimental to the internal quantum efficiency (IQE). Therefore, the growth of high-quality laser epitaxial structures with cladding layers is generally complex and expensive, and has low yields.

[0008] Traditionally, edge-emitting lasers had to be fabricated on expensive custom-grown wafers, where epitaxial cladding layers were inserted to improve the optical confinement and overlap between the active region and the modal distribution. Without the inserted cladding layers, laser emission would be impossible within the normal operating range, thus limiting the possibility of integrating multiple photonic components on a single platform for applications such as photonic integrated circuits. Low-refractive-index layers grown epitaxially are typically inserted above and below the InGaN / GaN MQWs and the GaN waveguide layer to facilitate optical confinement. Due to the lattice matching with GaN epitaxy, AlGaN materials are usually chosen as the cladding material to maintain the high material quality of the GaN-based epitaxial structure, as shown in Figure 1. However, although AlGaN materials do have a lower refractive index than GaN materials, the small difference in refractive index (usually about 0.1 - 0.2, depending on the composition

[13] ) limits the effectiveness of the waveguide effect. In addition, the growth of the layers can cause various problems. The high growth temperature of the AlGaN layer adversely affects the fragile InGaN / GaN MQWs. The tensile strain caused by the layer may potentially lead to cracking. Therefore, the performance of the device will be affected by the lower conductivity of the thick AlGaN layer [14, 15].

[0009] Figure 2 shows a conventional thin-film semiconductor edge-emitting laser, which has cladding layers grown on the left and right sides and a native (as-grown) substrate before the thin-film process. The illustration on the right is shown upside down for side-by-side comparison with the structure on the left. Note that the dashed line shows the guided light.

[0010] U.S. Application Publication No. 2022 / 0231477 discloses a flip-chip optoelectronic device in which a semiconductor die is attached to a substrate or carrier with the bond pad side down. In the process used for this design, the original substrate is not removed after flip-chip. Therefore, the film thickness cannot be optimized. U.S. Patent No. 10,554,017 discloses a method and device related to a III-nitride edge-emitting laser diode with a high confinement factor having lattice-matched cladding layers. The laser structure is fabricated on a wafer with an epitaxial AlGaN cladding layer grown. The lift-off process is not involved. Summary of the Invention

[0011] One embodiment of a solution to the problem of constructing an edge-emitting laser diode is to build an edge-emitting laser structure through an outer cladding by means of a thin-film strategy. A typical light-emitting epitaxial structure can be bonded to a silicon substrate by a reflective metal bond, and then its original substrate is removed. Thus, the thickness of the customized epitaxy can be adjusted by etching before depositing the outer metal cladding for optical confinement.

[0012] This embodiment of the present invention relates to a method of manufacturing a cladding-free edge-emitting laser cavity structure. By fabricating a conventional edge-emitting laser cavity structure on a thin-film platform, the film thickness can be adjusted to maximize laser performance. The thin-film platform is formed by bonding a typical GaN-based LED wafer to a Si substrate and removing the original substrate. After substrate removal, the exposed bottom buffer layer (grown to minimize material defects) can be removed by etching while keeping the LED active region intact. The waveguide of the laser is formed without adding any cladding layers. Instead, the structure of the LED is trimmed so that it becomes a thin cavity. The top GaN-air interface is naturally a waveguide structure, while the bottom GaN / metal or GaN / DBR (distributed Bragg reflector) is another waveguide. The thinness of the cavity results in a very high overlap of the cavity mode with the gain region (MQW). Therefore, by thinning the film, the laser performance is greatly improved, and the laser threshold is reduced to a feasible value for normal operation.

[0013] Therefore, this embodiment utilizes a thin-film platform to fabricate an edge-emitting laser structure. The thin-film platform is formed by attaching the LED film of a conventional commercially available LED wafer to a Si substrate. Since the film thickness can be finely adjusted by etching after removing the original substrate, the laser performance can be improved and the laser threshold can be reduced, making normal operation feasible.

[0014] Although both thin-film platforms and edge-emitting lasers already exist, using a thin-film platform to fabricate an edge-emitting laser is novel. By adopting a thin-film platform that can be assembled from commercially available LED wafers for laser fabrication, the performance of the laser can be improved without using expensive custom-grown wafers or epitaxial claddings.

[0015] As a second way to solve the problem of the cladding in the prior art, the second embodiment of the present invention relates to a cladding-free process method. The manufacturing process starts with a conventional LED epitaxial structure that does not have any cladding to support a limited waveguide. The use of the LED epitaxial structure provides the following advantages: (a) high IQE due to a mature LED growth process and (b) lower cost due to the large-scale production and huge market of LED epitaxy.

[0016] The thin film process of this second embodiment transforms an LED epitaxial structure into a laser diode with high optical confinement and gain overlap. The LED epitaxial structure is bonded to a substrate, typically Si, with a reflective metal or a conductive distributed Bragg reflector. This layer acts as a reflective layer that replaces the p-cladding layer for bottom optical confinement and simultaneously provides a contact for electrical injection. After bonding and substrate removal, the bottom of the LED epitaxy is exposed for etching. The thin film process of this embodiment of the present invention involves removing the thick semiconductor layer below the active region of the LED epitaxy to enhance optical confinement and gain overlap. This is achieved by etching the layer to an optimal thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] This patent or application document contains at least one color drawing. Copies of this patent or patent application publication with (multiple) color drawings will be provided by the patent office upon request and payment of the necessary fees.

[0018] The foregoing and other objects and advantages of the present invention will become more apparent when considered in conjunction with the following detailed description and the drawings, in which like reference numerals represent like elements in each view, and in which:

[0019] Figure 1A is a cross-sectional TEM image showing an InGaN / GaN MQW sandwiched between a pair of AlGaN cladding layers in a laser diode structure, and Figure 1B is Figure 1A an enlarged view of the active region shown in the box in; Figure 2A is a 3D schematic diagram, Figure 2B is the corresponding layer structure of the thin film edge-emitting laser proposed by the present invention, with a reflective metal layer at the bottom, and Figure 2C shows the simulated electric field distribution of the fundamental resonance mode of the thin film edge-emitting laser; and

[0020] Figure 3A is a 3D schematic diagram, and Figure 3B is the corresponding layer structure of the thin film edge-emitting laser proposed by the present invention, with a distributed Bragg reflector (DBR) at the bottom;

[0021] Figure 4A is a photoluminescence (PL) spectrum, and Figure 4B is the corresponding linewidth and integrated PL intensity as a function of excitation energy density of the fabricated thin film edge-emitting laser according to an embodiment of the present invention;

[0022] Figure 5A shows a diagram of a conventional thin film semiconductor edge-emitting laser with a grown cladding layer, and Figure 5B shows an inverted diagram of the native substrate before the thin film process;

[0023] Figure 6AShows a second embodiment, in which an LED epitaxial structure is bonded to a Si substrate using a reflective metal or a conductive DBR, Figure 6B Shows an inverted view of a native LED epitaxy before the thin film process of the second embodiment of the present invention, and Figure 6C is a perspective view of the second embodiment of the present invention;

[0024] Figure 7A Shows the electroluminescence (EL) spectrum of a cladding-free edge-emitting laser of the second embodiment of the present invention, and Figure 7B shows the full width at half maximum (FWHM) and intensity of the laser peak plotted against the current density.

[0025] Figure 8 is a cross-sectional SEM image of the DBR structure of the present invention;

[0026] Figure 9 is a graph of the reflectivity spectrum simulated and measured from a DBR sample of the present invention. Detailed Description

[0027] The design of a thin film edge-emitting laser according to an embodiment of the present invention eliminates the need to grow a cladding within the epitaxial structure and instead relies on an external reflector for optical confinement. Such a reflector (including a metal mirror) also serves as an electrical contact or a dielectric distributed Bragg reflector (DBR) and provides excellent optical confinement to further enhance the mode gain.

[0028] Figure 2A and 2B Show a 3D perspective view and the corresponding layer structure of a platform for creating an edge-emitting laser of this embodiment of the present invention, respectively. The first step in obtaining the laser structure of this embodiment of the present invention is to attach a typical LED wafer to a Si substrate by eutectic bonding. Gold (Ag) is preferred as the metal bonding material due to its high reflectivity. The original sapphire substrate of the LED wafer is then removed by a laser lift-off process, thereby exposing the n-GaN and u-GaN buffer layers. Inductively coupled plasma (ICP) etching can then be employed to thin the film thickness to maximize the overlap factor between the resonant mode distribution and the active region (i.e., the multiple quantum well (MQW)). In fact, the flexibility to fine-tune the cavity thickness for overlap factor optimization is a major advantage of this thin film approach. A reflective metal layer is then deposited on the n-GaN surface for optical confinement and electrical contact. Finally, the pattern of the edge-emitting cavity can then be defined by nano-lithography techniques, such as laser direct write lithography, electron beam lithography, or nanoimprint lithography, followed by ICP etching to transfer the pattern to the thin film.

[0029] Two-dimensional finite difference time domain (FDTD) simulations are performed to analyze the resonant modes of the edge-emitting laser cavity. Figure 2CThe simulated electric field distribution of the fundamental resonance mode is shown, demonstrating a high Q factor of 26000, with a confinement factor of 0.71. To verify the feasibility of the design, an edge-emitting laser with a Fabry–Perot cavity was fabricated on a thin-film MQW sample with an emission wavelength of 444 nm. First, the LED wafer on the sapphire sample was bonded to the Si substrate, and subsequently, the sapphire substrate was removed by laser lift-off (LLO) etching. Inductively coupled plasma (ICP) etching was performed to thin the sample to a thickness of approximately 500 nm. Then, a laser direct-write lithography machine was used to pattern rectangular blocks, and then ICP etching was performed to form the edge-emitting laser. Preliminary experimental data were obtained by optically pumping the fabricated edge-emitting laser with a diode-pumped solid (DPSS) laser that emits at 349 nm, has a pulse width of 4 ns, and a repetition rate of 1 kHz. As Figure 4A and 4B shown, at a wavelength of ~444 nm, lasing was observed at a threshold of 1.1 mJ / cm 2 , with a Q factor of ~1100.

[0030] The simulation and experimental studies verified the feasibility of the thin-film edge-emitting laser design. The fabrication of such a device is relatively simple and can be easily adapted for different applications for potential commercialization. As Figure 3A and 3B shown, the performance of the design can be improved by replacing the reflective metal layer with a pair of distributed Bragg reflectors (DBRs).

[0031] As a second method to address the problem of the cladding in the epitaxial structure, a second embodiment of the present invention relates to a cladding-free process method. The manufacturing process begins with a conventional LED epitaxy without any cladding layer that supports a limited waveguide. See Figure 6A . Any guided wave will diffuse across the entire thick LED epitaxial structure, thus having little overlap with the active region and not causing population inversion for laser beam generation. In contrast, a conventional laser epitaxial structure with a cladding supports laser beam generation even without a thin-film process, as Figure 5A shown. The use of the LED epitaxial structure provides the following advantages: (a) high IQE due to the mature LED growth process and (b) lower cost due to the large-scale production and huge market of the LED epitaxy.

[0032] The thin-film process of this second embodiment transforms the LED epitaxial structure into a laser diode with high optical confinement and gain overlap. As Figure 6A shown, the LED epitaxial structure is bonded to a Si substrate with a reflective metal or conductive DBR. This layer serves as the reflective layer, which replaces Figure 5AThe p-cladding layer shown in [Fig. 0] is used for bottom optical confinement and at the same time provides contacts for electrical injection. After bonding, the Si substrate is removed. Then the bottom of the LED epitaxy is exposed for etching. Note that even in conventional laser epitaxy, the thick semiconductor layers below the cladding are useless for optical confinement; however, they are crucial for reducing defects such as dislocations and stacking faults (i.e., improving film quality) during growth. Therefore, the thin-film process of the proposed second embodiment allows these layers to be removed from the LED epitaxy to enhance optical confinement and gain overlap. This is achieved by etching to an optimal thickness.

[0033] For a typical edge-emitting laser or waveguide, the cavity thickness is usually N times the emission wavelength divided by the refractive index of the semiconductor material used. Taking into account the limitation that the thickness of the p-doped semiconductor layer and MQW cannot be removed, N is usually chosen as the smallest number (to maximize the overlap between the guided beam and the active region) such that the MQW will be close to the center of the epitaxy after etching. The actual value will depend on the thickness of the p-doped semiconductor layer and MQW in the initial LED epitaxy.

[0034] In Figure 6A the proposed thin-film semiconductor, cladding-free edge-emitting laser is shown. Figure 6B The native LED epitaxy before the thin-film process is shown, but it is shown upside down for side-by-side comparison. Note that the dashed line shows the guided light. Figure 6C is a perspective view of the device.

[0035] Comparing the proposed thin-film semiconductor, cladding-free edge-emitting laser ( Figure 6A and 6B ), with a conventional thin-film semiconductor edge-emitting laser ( Figure 5A and 5B ), it is obvious that the design of the present invention is much better in terms of optical confinement compared to conventional laser epitaxy.

[0036] Due to the limitations of the growth of conventional laser epitaxy, the choice of cladding material is very limited because it must have lattice parameters similar to those of the semiconductor material of the epitaxy to avoid deterioration of film quality. Therefore, the difference between the refractive indices of the cladding material and the semiconductor material is very low, resulting in low reflectivity and thus low optical confinement.

[0037] On the other hand, the bottom reflector layer of the proposed design uses a reflective metal (>90% reflectivity) or a conductive DBR (>95% reflectivity), which has a much higher reflectivity than that of a conventional cladding. For the top surface, the light is guided by total internal reflection caused by the high refractive index contrast between air and the semiconductor material (instead of between the semiconductor material and the cladding material).

[0038] Figure 7A and7B Shows the results of an experimental demonstration of the proposed thin-film semiconductor unclad edge-emitting laser based on InGaN / GaN LED epitaxy according to the present invention. Figure 7A Shows the EL spectrum, and Figure 7B shows the full width at half maximum (FWHM) and intensity of the laser peak plotted against the current density.

[0039] An updated prototype of the proposed design has been fabricated using a blue-emitting InGaN / GaN LED wafer. The experimental results are shown in the Figure 7B inset, demonstrating the ability of the designed laser generation under electrical injection.

[0040] To verify the effect of implementing DBR in the thin-film approach, 8 pairs of SiO2 / TiO2 dielectric DBR were deposited on the p-GaN surface of a GaN LED wafer located on a Si substrate. Then the structure was annealed. The SiO2 / TiO2 DBR structure exhibited a reflectivity of >99% at the emission wavelength of the LED epitaxy. Figure 8 is the SEM image of the TiO2 / SiO2 dielectric DBR structure, and Figure 9 is the graph of the reflection spectra simulated and measured from the DBR sample.

[0041] Therefore, the combination of the thin-film structure and the high-reflection DBR contributes to a high overlap factor and strong light confinement, significantly enhancing the Q factor of the GaN-based edge-emitting laser and reducing the laser threshold. Thus, the design of the present invention has great potential for realizing high-performance unclad edge-emitting lasers using conductive DBR.

[0042] References

[0043] The references cited in this application are incorporated herein by reference in their entirety and are as follows:

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[0059] Although the invention has been explained with reference to certain embodiments, it should be understood that various modifications will become apparent to those skilled in the art after reading the specification. Accordingly, it is to be understood that the invention disclosed herein is intended to cover such modifications.

Claims

1. A method for manufacturing a cladding-free GaN-based thin-film edge-emitting laser, comprising the following steps: Attach a typical LED wafer to a Si substrate; Remove the original sapphire substrate of the LED wafer to expose its n-GaN and u-GaN buffer layers; Reduce the film thickness to maximize the overlap factor between the resonant mode distribution and the active region; Define the pattern of the edge-emitting cavity by nano-lithography technology; Perform etching by ICP etching to transfer the pattern to the thin film; And Deposit another reflective metal layer on the n-GaN surface for optical confinement and electrical contact.

2. The method for manufacturing a cladding-free GaN-based thin-film edge-emitting laser according to claim 1, wherein the attaching step is achieved by eutectic bonding.

3. The method for manufacturing a cladding-free GaN-based thin-film edge-emitting laser according to claim 2, wherein the metal bonding material is Ag.

4. The method for manufacturing a cladding-free GaN-based thin-film edge-emitting laser according to claim 1, wherein the step of removing the original sapphire substrate is achieved by laser lift-off process.

5. The method for manufacturing a cladding-free GaN-based thin-film edge-emitting laser according to claim 1, wherein the step of thinning the film thickness is achieved by inductively coupled plasma (ICP) etching.

6. The method for manufacturing a cladding-free GaN-based thin-film edge-emitting laser according to claim 1, wherein the nano-lithography technology includes one of laser direct writing lithography, electron beam lithography or nanoimprint lithography.

7. The method for manufacturing a cladding-free GaN-based thin-film edge-emitting laser according to claim 1, wherein the film thickness is thinned to fine-tune the cavity thickness for overlap factor optimization.

8. A method for manufacturing a cladding-free GaN-based thin-film edge-emitting laser, comprising the following steps: Bond an LED epitaxial structure to a Si substrate using one of a reflective metal and a conductive distributed Bragg reflector (DBR) for bottom optical confinement and for providing electrical injection contact; Remove the Si substrate after the bonding step to expose the bottom of the LED epitaxy; And Etch the LED epitaxy to an optimal thickness for laser emission.

9. A cladding-free GaN-based thin-film edge-emitting laser, comprising: A conductive distributed Bragg reflector (DBR) for bottom optical confinement and for providing electrical injection contact; A p-doped semiconductor layer and an n-doped semiconductor layer, the p-doped semiconductor layer and the n-doped semiconductor layer forming an active region having multiple quantum wells (MQW); And A contact layer on the top.

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