A composite structure for enhanced second harmonic generation and a method of manufacturing and using the same

Abstract

This invention provides a composite structure for enhanced second harmonic generation, its fabrication method, and its application. The composite structure includes a substrate and a GaN-based structure and an AlScN layer sequentially disposed along a direction away from the substrate; the GaN-based structure includes an n-type GaN layer, and the AlScN layer is heterojunction-coupled to the n-type GaN layer; furthermore, the AlScN layer contains a material with the chemical formula Al. 1‑ x Sc x The N and x values ​​are 0.1~0.43, and the crystal structure is hexagonal wurtzite with a preferred c-axis orientation. The thickness of the AlScN layer is greater than 60 nm. The composite structure provided by this invention can achieve second harmonic enhancement in the 1200nm~1300nm band, solving the problem of insufficient second harmonic intensity in pure GaN bulk materials or single epitaxial GaN-based structures in the prior art.

Description

Technical Field

[0001] This invention belongs to the field of second harmonic generation materials technology, specifically relating to a composite structure for enhanced second harmonic generation, its preparation method, and its application. Background Technology

[0002] Second harmonic generation (SHG) is an important nonlinear optical effect. It refers to the generation of 2ω-frequency harmonic light when a fundamental frequency light of frequency ω is incident on a nonlinear optical material. Its core characteristics are a non-centrosymmetric crystal structure and a high second-order nonlinear polarizability (χ²). （2） Second harmonic generation technology is widely used in laser frequency conversion, nonlinear optical sensing, optical communication, and biological imaging. In particular, second harmonic devices in the 1200-1300nm near-infrared band play an irreplaceable role in scenarios such as infrared laser frequency doubling and optical network monitoring.

[0003] GaN, as a third-generation semiconductor material, possesses a stable hexagonal wurtzite structure (non-centrosymmetric), providing a structural basis for second-harmonic generation. It is also compatible with mainstream semiconductor manufacturing processes, making it an ideal substrate material for fabricating integrated nonlinear optical devices. Currently, research on second-harmonic generation based on GaN mainly focuses on sapphire-based epitaxial GaN films or GaN-based quantum well structures. The intensity of the second harmonic is enhanced by optimizing the epitaxial growth process of GaN (such as temperature and doping concentration), but the enhancement effect is limited by the upper limit of GaN's own second-order nonlinear polarizability. Furthermore, in practical applications, GaN often exists in the form of "sapphire-based epitaxial GaN substrates" rather than pure GaN bulk materials. However, under femtosecond laser excitation in the 1200-1300nm near-infrared band, the second-harmonic peak intensity of such sapphire-based epitaxial GaN films is relatively weak, making it difficult to meet the application requirements of high sensitivity and high power output. Summary of the Invention

[0004] To solve all or part of the above-mentioned technical problems, the present invention provides the following technical solutions:

[0005] A first aspect of the present invention provides a composite structure for enhanced second harmonic generation, the composite structure comprising a substrate and a GaN-based structure and an AlScN layer disposed sequentially along a direction away from the substrate;

[0006] The GaN-based structure includes an n-type GaN layer, and the AlScN layer is heterojunction-coupled to the n-type GaN layer; furthermore, the material contained in the AlScN layer has the chemical formula Al. 1-x Sc x The N and x values ​​are 0.1 to 0.43, and the crystal structure is hexagonal wurtzite with a preferred c-axis orientation. The thickness of the AlScN layer is more than 60 nm.

[0007] The composite structure provided by this invention achieves second harmonic enhancement in the 1200nm~1300nm wavelength band through heterojunction coupling of an epitaxial GaN-based structure and AlScN, solving the problem of insufficient second harmonic intensity in existing pure GaN materials or single epitaxial GaN-based structures. The concentration, crystal orientation, and crystal structure of the Sc component in the AlScN layer affect the second harmonic enhancement effect. If the Sc component concentration in the AlScN layer is low, such as below 10% molar concentration, the second-order nonlinearity is weak, resulting in weak second harmonic intensity; if the Sc component concentration is too high, such as above 43% molar concentration, its crystal structure changes from hexagonal wurtzite structure to cubic rock salt phase, increasing symmetry and drastically decreasing nonlinear response. Furthermore, the AlScN layer needs to be c-axis oriented (002) to ensure noncentrosymmetry and strong spontaneous polarization, while effectively reducing the lattice mismatch with the GaN-based structure. On the one hand, AlScN layers with the above-mentioned Sc concentration range, crystal structure, and crystal orientation have high χ². （2） On the one hand, its properties enhance the second harmonic distortion (HHD) through interfacial coupling with the epitaxial GaN-based structure. On the other hand, the synergistic effect of these two aspects leads to HHD enhancement. In some embodiments, under the same testing conditions, the HHD peak intensity of the composite structure of the present invention is more than twice that of the sapphire-based epitaxial GaN-based structure, and in some preferred embodiments, the enhancement factor can reach 2-4 times.

[0008] In some embodiments, the thickness of the AlScN layer is 60 nm to 200 nm. Second harmonic generation, as a second-order nonlinear optical effect, depends on the accumulation of second-order polarization intensity in the nonlinear medium, and thickness is a key factor in this accumulation. If the AlScN layer thickness is too low, effective second harmonic intensity cannot be generated. A second harmonic enhancement effect within the 60 nm to 200 nm range is superior and therefore a preferred solution.

[0009] In some embodiments, the GaN-based structure includes a buffer layer, an intermediate layer, and the n-type GaN layer sequentially disposed along a direction away from the substrate.

[0010] In some embodiments, the material of the buffer layer and the intermediate layer can be undoped gallium nitride (uGaN).

[0011] In some embodiments, the thickness of the buffer layer can be 10~50nm.

[0012] In some embodiments, the thickness of the intermediate layer can be 1~5 μm.

[0013] In some embodiments, the thickness of the n-type GaN layer is 1~10 μm.

[0014] In some embodiments, the doping element contained in the n-type GaN layer can be any one or more of Si, Ge, and O, and the doping concentration can be 10. 17 cm -3 ~10 20 cm -3 Within the range.

[0015] In some embodiments, the substrate is a sapphire substrate, a Si substrate, or a SiC substrate. That is, the composite structure provided by this invention can solve the problem of insufficient second harmonic intensity in the 1200-1300 nm wavelength band of existing substrate-epitaxy GaN thin films.

[0016] A second aspect of the present invention provides a method for preparing a composite structure for enhanced second harmonic generation, the method comprising:

[0017] Provide substrate;

[0018] A GaN-based structure is epitaxially grown on the substrate, and the GaN-based structure includes an n-type GaN layer with a preferred (002) crystal orientation;

[0019] An AlScN layer is formed on the n-type GaN layer, wherein the chemical formula of the material contained in the AlScN layer is Al. 1-x Sc x The N and x values ​​are 0.1 to 0.43, and the crystal structure is hexagonal wurtzite with a preferred c-axis orientation. The thickness of the AlScN layer is more than 60 nm.

[0020] In some embodiments, the method for forming the AlScN layer on the n-type GaN layer includes at least one of magnetron sputtering (MS method) and pulsed laser deposition (PLD method). Some second-harmonic enhancement materials in the prior art (such as LiNbO3) are incompatible with GaN-based semiconductor processes, making it difficult to fabricate integrated devices. However, the AlScN layer of this invention can be fabricated using MS and PLD processes, which are fully compatible with mainstream fabrication processes for GaN-based semiconductor devices. It requires no modification to existing equipment and can be directly integrated into integrated optical device production lines, reducing industrialization costs.

[0021] In some embodiments, the process conditions of the magnetron sputtering method include: using an AlSc target as the target material, a substrate temperature of 200℃~400℃, a sputtering power of 1kW~10kW, and an Ar to N2 gas flow ratio of 20:20~20:130.

[0022] In some embodiments, the process conditions of the pulsed laser deposition method include: using an AlSc target as the target material, a substrate temperature of 200℃~400℃, a pulse energy of 200mJ~400mJ, a laser frequency of 1Hz~3Hz, and a nitrogen pressure of 0.5Pa~3Pa.

[0023] Under the aforementioned magnetron sputtering and pulsed laser deposition process conditions, the grown AlScN thin film exhibits superior quality and c-axis orientation, making these preferred process conditions. More preferably, the AlScN layer is prepared using the aforementioned magnetron sputtering method, resulting in an AlScN layer of even higher quality and superior second harmonic enhancement effect.

[0024] The deposition time of the magnetron sputtering and pulsed laser deposition methods can be adjusted according to the desired AlScN layer thickness. In some embodiments, the deposition time of the magnetron sputtering method can be 90s to 350s. The deposition time of the pulsed laser deposition method can be 60min to 300min. Within the above deposition time ranges, AlScN layers with a thickness of 60nm to 200nm can be obtained.

[0025] In some embodiments, the preparation method includes: making the surface roughness Ra of the substrate below 0.5 nm, and then growing a GaN-based structure on the substrate to ensure the flatness of subsequent epitaxial growth.

[0026] In some embodiments, a buffer layer, an intermediate layer, and an n-type GaN layer are epitaxially grown sequentially on the substrate to obtain the GaN-based structure.

[0027] In some embodiments, the surface roughness Ra of the n-type GaN layer is below 0.7 nm, and using it as the growth substrate for the AlScN layer can improve the lattice matching and interface bonding quality.

[0028] In some embodiments, the substrate may be made of sapphire, Si, or SiC.

[0029] The further details of the AlScN layer, buffer layer, intermediate layer, and n-type GaN layer involved in the "Preparation Method of an Enhanced Second Harmonic Generation Composite Structure" provided in the second aspect of this invention, such as thickness range, material, doping elements, and doping concentration, have been specifically described in the first aspect of this invention, "An Enhanced Second Harmonic Generation Composite Structure," and therefore will not be repeated here. The preparation method provided in the second aspect of this invention can produce the enhanced second harmonic generation composite structure described in the first aspect of this invention. The preparation methods for the buffer layer, intermediate layer, and n-type GaN layer can employ any known preparation method in the art, such as metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), magnetron sputtering, etc., and this invention does not impose any particular limitation on these methods.

[0030] A third aspect of the present invention provides a composite structure for enhanced second harmonic generation, which is prepared by the method for preparing a composite structure for enhanced second harmonic generation as described in any of the technical solutions.

[0031] A fourth aspect of the present invention provides a method for enhancing the second harmonic of a GaN-based structure, comprising:

[0032] A GaN-based structure is provided, comprising an n-type GaN layer with a preferred (002) crystal orientation;

[0033] An AlScN layer is fabricated on the GaN-based structure, and the AlScN layer is heterojunctionally coupled to the n-type GaN layer.

[0034] The chemical formula of the material contained in the AlScN layer is Al. 1-x Sc x The N and x values ​​are 0.1 to 0.43, and the crystal structure is hexagonal wurtzite with a preferred c-axis orientation. The thickness of the AlScN layer is more than 60 nm.

[0035] In some embodiments, an AlScN layer is fabricated on the GaN-based structure using at least one of magnetron sputtering and pulsed laser deposition, with magnetron sputtering being preferred.

[0036] In some embodiments, the thickness of the AlScN layer is 60nm~200nm.

[0037] A further embodiment of the "method for enhancing second harmonics in a GaN-based structure" described in the fourth aspect of this invention has been specifically described in the second aspect of this invention, "method for preparing a composite structure for enhancing second harmonic generation," and will not be repeated here.

[0038] The fifth aspect of this invention provides the application of the composite structure for enhanced second harmonic generation described in any of the technical solutions, or the method for enhancing second harmonics of GaN-based structures described in any of the technical solutions, in the fabrication of laser frequency conversion devices, nonlinear sensors, or optical communication devices. The composite structure provided by this invention provides a second harmonic enhancement effect covering the 1200nm~1300nm near-infrared key band, and can be directly applied to laser frequency conversion, nonlinear sensing, optical communication, and other fields, solving the problem of poor performance of pure GaN-based second harmonic materials in this band.

[0039] A sixth aspect of the present invention provides a second harmonic device comprising a composite structure for enhanced second harmonic generation as described in any of the technical solutions.

[0040] Compared with the prior art, the present invention has at least some or all of the following beneficial effects: The enhanced second harmonic generation composite structure provided by the present invention is based on a high χ² value of an AlScN layer with a specific Sc ​​concentration, crystal orientation, and specific thickness. （2） The characteristics and the interface coupling enhancement effect with the epitaxial GaN-based structure show that the second harmonic enhancement effect is significant in the 1200nm~1300nm band, which solves the problem of insufficient second harmonic intensity of pure GaN materials or single epitaxial GaN-based structures in the prior art. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 This is a schematic diagram of the composite structure for enhanced second harmonic generation obtained in Embodiment 1 of the present invention;

[0043] Figure 2 This is a schematic diagram of the optical path during the second harmonic generation test;

[0044] Figure 3 These are second harmonic signal graphs of the composite structures of Examples 1, 6, and Comparative Example 1, measured under a fundamental frequency wavelength of 1200nm; wherein, the "AlScN (MS)" curve corresponds to Example 1, the "AlScN (PLD)" curve corresponds to Example 6, and the "GaN" curve corresponds to Comparative Example 1;

[0045] Figure 4 Examples 1, 6, and Comparative Example 1 (corresponding to) were measured under a fundamental wavelength of 1300nm. Figure 4The second harmonic signal diagram of the composite structure of "GaN" (with the "GaN" curve); wherein, the "AlScN (MS)" curve corresponds to Example 1, the "AlScN (PLD)" curve corresponds to Example 6, and the "GaN" curve corresponds to Example 1;

[0046] Figure 5 This is the XRD pattern of the AlScN layer prepared in Example 1;

[0047] Figure 6 This is the XRD pattern of the AlScN layer prepared in Example 6. Detailed Implementation

[0048] The invention will be more fully understood through the following detailed description, which should be read in conjunction with the accompanying drawings. Detailed embodiments of the invention are disclosed herein; however, it should be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, the specific functional details disclosed herein should not be construed as limiting, but rather as the basis for the claims and as intended to teach those skilled in the art to employ the representative basis of the invention in different ways in any suitable detailed embodiment.

[0049] In addition, unless otherwise specified, all raw materials used in the following embodiments can be purchased from the market or other sources, and all production and testing equipment used are known in the art, as are the testing methods used.

[0050] Example 1

[0051] This embodiment provides a composite structure for enhanced second harmonic generation. Figure 1 This is a schematic diagram of the composite structure for enhanced second harmonic generation obtained in this embodiment, as shown below. Figure 1 As shown, it includes a sapphire substrate and, in a direction away from the sapphire substrate, a buffer layer (i.e., a uGaN buffer layer, with a thickness of 25 nm), an intermediate layer (i.e., a uGaN intermediate layer, with a thickness of 2.5 μm), an n-type GaN layer (with a thickness of 2 μm), and an AlScN layer (with a thickness of 100 nm) stacked sequentially. The AlScN layer contains a material with the chemical formula Al. 0.8 Sc 0.2 The molar concentration of N, i.e., Sc component, is 20%, the crystal structure is hexagonal wurtzite structure, the crystal orientation is (002) preferred orientation, and the AlScN layer thickness is 100nm.

[0052] The method for preparing the above-mentioned composite structure for enhanced second harmonic generation includes the following steps:

[0053] (1) Prepare a sapphire substrate with a surface roughness Ra of less than 0.5 nm to ensure the smoothness of subsequent epitaxial layer growth.

[0054] (2) A 25 nm thick uGaN layer (i.e., undoped gallium nitride) is grown on the sapphire substrate as a buffer layer; then, a 2.5 μm thick uGaN layer and a 2 μm thick n-type GaN layer are epitaxially grown on the buffer layer to obtain a sapphire substrate GaN substrate. The crystal orientation of the n-type GaN layer is (002) preferred orientation, and the surface roughness Ra is less than 0.7 nm, serving as the growth substrate for the AlScN layer to improve lattice matching and interface bonding quality.

[0055] (3) The above-mentioned sapphire substrate GaN substrate is cleaned in preparation for the subsequent growth of AlScN layer.

[0056] The specific cleaning steps are as follows: First, take a glass beaker and a polytetrafluoroethylene basket of appropriate capacity, rinse them repeatedly with deionized water, and blow dry the excess water on the inner wall of the beaker and the basket with a nitrogen gun; then take out the sample and put it into the basket, add acetone solution to the beaker, immerse the basket and the sample together in the solution, and then put it into an ultrasonic cleaner for 5 minutes. Repeat this operation twice; next, change to isopropanol solution and clean the substrate for 5 minutes, repeat twice, and other operations are the same as the previous step; finally, rinse the substrate repeatedly with deionized water, blow dry with nitrogen, and put it into the sample box.

[0057] (4) An AlScN layer is grown on the n-type GaN layer of the sapphire substrate GaN substrate described above using radio frequency magnetron sputtering (MS method). The process parameters of the MS method include: Al 0.8 Sc 0.2 Using an alloy target as the target material, a substrate temperature of 200℃, a sputtering power of 2.5kW, an Ar / N2 flow ratio of 20 sccm / 100sccm, and a deposition time of 150 seconds, an AlScN layer with a thickness of 100nm was obtained, thereby fabricating the aforementioned composite structure for enhanced second harmonic generation.

[0058] Example 2

[0059] Example 2 is basically the same as Example 1, except that the MS process conditions for growing the AlScN layer in step (4) of Example 2 include: using Al 0.8 Sc 0.2 An alloy target was used as the target material. The substrate temperature was 300℃, the sputtering power was 2.5kW, the Ar / N2 flow ratio was 20 sccm / 20sccm, and the deposition time was 90 seconds to obtain an AlScN layer with a thickness of about 60nm.

[0060] The remaining embodiments are the same as those in Embodiment 1, and will not be repeated here.

[0061] Example 3

[0062] Example 3 is basically the same as Example 1, except that the MS process conditions for growing the AlScN layer in step (4) of Example 3 include: using Al 0.8 Sc 0.2 An alloy target was used as the target material, the substrate temperature was 400℃, the sputtering power was 2.5kW, the gas flow ratio of Ar to N2 was 20sccm / 130sccm, the deposition time was 350 seconds, and an AlScN layer with a thickness of about 200nm was obtained.

[0063] The remaining embodiments are the same as those in Embodiment 1, and will not be repeated here.

[0064] Example 4

[0065] Example 4 is basically the same as Example 1, except that in step (4) of Example 4, during the growth of the AlScN layer, the sputtering power is controlled to be 1 kW. The rest is the same as Example 1, and will not be repeated here.

[0066] Example 5

[0067] Example 5 is basically the same as Example 1, except that in step (4) of Example 5, during the growth of the AlScN layer, the sputtering power is controlled to be 10 kW. The rest is the same as Example 1, and will not be repeated here.

[0068] Example 6

[0069] Example 6 is basically the same as Example 1, except that:

[0070] The chemical formula of the material contained in the AlScN layer of Example 6 is Al 0.7 Sc 0.3 N, that is, the molar concentration of component Sc, is 30%;

[0071] Step (4) of Example 6 involves growing an AlScN layer using pulsed laser deposition (PLD), and the process parameters include: using Al 0.7 Sc 0.3 The alloy target was used as the target material, the substrate temperature was 400℃, the pulse energy was 300mJ, the frequency was 2Hz, the target-substrate distance was 6.5cm, the nitrogen pressure was 1Pa, the deposition time was 146 minutes, and the thickness of the AlScN layer was 100nm.

[0072] Example 7

[0073] Example 7 is basically the same as Example 6, except that the process parameters for growing the AlScN layer in step (4) of Example 6 include: using Al 0.7 Sc 0.3The alloy target was used as the target material, the substrate temperature was 200℃, the pulse energy was 200mJ, the frequency was 1Hz, the target-substrate distance was 6.5cm, the nitrogen pressure was 1Pa, the deposition time was 60 minutes, and the thickness of the AlScN layer was 60nm.

[0074] The rest is the same as in Example 6, and will not be described again here.

[0075] Example 8

[0076] Example 8 is basically the same as Example 6, except that the process parameters for growing the AlScN layer in step (4) of Example 8 include: using Al 0.7 Sc 0.3 The alloy target was used as the target material, the substrate temperature was 300℃, the pulse energy was 400mJ, the frequency was 3Hz, the target-substrate distance was 6.5cm, the nitrogen pressure was 1Pa, the deposition time was 300 minutes, and the thickness of the AlScN layer was 200nm.

[0077] The rest is the same as in Example 6, and will not be described again here.

[0078] Example 9

[0079] Example 9 is basically the same as Example 1, except that in the enhanced second harmonic generation composite structure provided in Example 9, the chemical formula of the material contained in the AlScN layer is Al. 0.9 Sc 0.1 N, i.e., the molar concentration of component Sc, is 10%. The rest is the same as in Example 1, and will not be repeated here.

[0080] Example 10

[0081] Example 10 is basically the same as Example 1, except that in the enhanced second harmonic generation composite structure provided in Example 10, the chemical formula of the material contained in the AlScN layer is Al. 0.57 Sc 0.43 N, i.e., the molar concentration of component Sc, is 43%. The rest of the procedure is the same as in Example 1, and will not be repeated here.

[0082] Comparative Example 1

[0083] The only difference between Comparative Example 1 and Example 1 is that step (4) of growing the AlScN layer is not performed, that is, the final structure of Comparative Example 1 is only a sapphire substrate GaN substrate.

[0084] Comparative Example 2

[0085] The only difference between Comparative Example 2 and Example 1 is that the AlScN layer in Comparative Example 2 contains a material with the chemical formula Al. 0.92 Sc 0.08N, i.e., the molar concentration of component Sc, is 8%. The rest is the same as in Example 1, and will not be repeated here.

[0086] Comparative Example 3

[0087] The only difference between Comparative Example 3 and Example 1 is that the AlScN layer in Comparative Example 3 contains a material with the chemical formula Al. 0.5 Sc 0.5 N, i.e., the molar concentration of component Sc, is 50%. The rest is the same as in Example 1, and will not be repeated here.

[0088] Comparative Example 4

[0089] The only difference between Comparative Example 4 and Example 1 is that the AlScN layer grown in Comparative Example 4 has a thickness of 20 nm. The rest of the process is the same as in Example 1 and will not be repeated here.

[0090] The composite structures prepared in the above embodiments and comparative examples were tested for second harmonic generation. The test method was as follows: a femtosecond laser (pulse width 50 fs, repetition frequency 25 kHz, incident power 2 mW) was used, with fundamental wavelengths of 1200 nm and 1300 nm, respectively. The fundamental wavelength light was perpendicularly irradiated onto the surface of the AlScN layer of the composite structure, with either p-polarized or s-polarized polarization. Detection conditions: a spectrometer (detection range 400-750 nm, resolution ≤0.1 nm) was used to collect the second harmonic signal in the reflection direction. A simplified optical path diagram during the test is shown below. Figure 2 As shown.

[0091] Figure 3 This is Example 1 (corresponding to) measured under the condition of a fundamental wavelength of 1200nm. Figure 3 The “AlScN (MS)” curve in the middle, Example 6 (corresponding to) Figure 3 The “AlScN(PLD)” curve in the middle, and Comparative Example 1 (corresponding to) Figure 3 The second harmonic signal diagram of the composite structure of GaN (in the "GaN" curve). Figure 4 This is Example 1 (corresponding to) measured under the condition of a fundamental wavelength of 1300nm. Figure 4 The “AlScN (MS)” curve in the middle, Example 6 (corresponding to) Figure 4 The “AlScN(PLD)” curve in the middle, and Comparative Example 1 (corresponding to) Figure 4 The second harmonic signal diagram of the composite structure of GaN (using the "GaN" curve). Combined with... Figure 3 , Figure 4It can be seen that the second harmonic intensity of the composite structures obtained in Examples 1 and 6 is significantly higher than that of the structure in Comparative Example 1. Among them, compared to the composite structure obtained by the PLD method in Example 6, the AlScN layer grown by the MS method in Example 1 has a higher film quality and therefore a better second harmonic enhancement effect. However, compared to Comparative Example 1, the second harmonic enhancement factor of the composite structure in Example 6 also reaches 2 times. The second harmonic intensity of the composite structures obtained in Examples 2 and 3 is basically equivalent to that of Example 1. Comparing Examples 1, 4, and 5, it was found that the AlScN crystal orientation obtained by controlling the sputtering power to 2.5 kW in the MS method preparation process is better, thus resulting in a better second harmonic enhancement effect. The second harmonic intensity of the composite structures obtained in Examples 7 and 8 is basically equivalent to that of Example 6.

[0092] Comparing Examples 1, 9-10, and 2-3, it was found that the second harmonic enhancement effect of Examples 1 and 9-10 was better than that of Comparative Examples 2-3. In the AlScN layer, the core role of Sc doping is to break the charge distribution symmetry through lattice distortion, enhance the bonding ionicity, and thus improve the x-ray diffusivity. （2） In Comparative Example 2, when the Sc molar concentration was 8%, the lattice distortion was slight (the AlN lattice was only partially expanded by Sc atoms), the electron cloud distribution remained close to the symmetry of AlN, and the nonlinear polarization capability was insufficient, resulting in a low second-order nonlinear polarization rate (χ²). （2） The χ² value is too low. In Comparative Example 3, when the Sc molar concentration is 50%, the large number of Sc atoms embedded in the AlN lattice will cause severe lattice distortion and stress accumulation, exceeding the lattice's capacity limit. This will lead to defects such as dislocations, twins, and elemental segregation in the film, resulting in a decrease in crystal quality; the crystal symmetry will deviate from the optimal range. At a high Sc concentration of 50%, excessive lattice distortion may cause a slight shift in the crystal point group, or the increase in defects may cause local regions to exhibit "quasi-centrosymmetry" characteristics, leading to an increase in χ² value. （2） Tensor components are suppressed (some components tend to zero), resulting in a significant reduction in nonlinear response efficiency.

[0093] In Comparative Example 4, the AlScN layer was too thin, resulting in almost no second harmonic enhancement effect. This indicates that to obtain a good enhancement effect, the thickness of the AlScN layer needs to be controlled within a suitable range. Tests conducted by this invention revealed that when the thickness of the AlScN layer reaches 60 nm or more, the second harmonic enhancement effect is more significant, with a preferred thickness of 60 nm to 200 nm.

[0094] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0095] In summary, the second-order nonlinear polarizability (χ²) of the AlScN thin film with specific Sc ​​concentration range, crystal structure, and crystal orientation described in this invention... （2） AlScN has a higher potential value than GaN and a low lattice mismatch with epitaxial GaN (which can be controlled to within 5% through composition optimization), making it easy to form high-quality heterojunctions. Combining AlScN thin films with epitaxial GaN-based structures on substrates achieves second-harmonic enhancement. Under femtosecond laser excitation at a fundamental wavelength of 1200–1300 nm, the second-harmonic peak intensity of the composite structure provided by this invention is higher than that of epitaxial GaN thin films on substrates under the same testing conditions, with an enhancement factor of 2–4 times. The enhancement effect covers the key near-infrared band of 1200–1300 nm, and can be directly applied to laser frequency conversion, nonlinear sensing, optical communication, and other fields, filling the technological gap in GaN-based enhanced second-harmonic materials in this band.

[0096] All aspects, embodiments, features, and examples of this invention should be considered illustrative and used to explain and illustrate the invention, but not to limit the invention. The scope of the invention is defined only by the claims.

[0097] Although the invention has been described with reference to illustrative embodiments, those skilled in the art will understand that various other changes, omissions, and / or additions can be made without departing from the spirit and scope of the invention, and that elements of the described embodiments can be substituted with substantially equivalents. Furthermore, many modifications can be made without departing from the scope of the invention to adapt particular situations or materials to the teachings of the invention. Therefore, this invention is not intended to be limited to the specific embodiments disclosed for carrying out the invention, but rather is intended to encompass all embodiments falling within the scope of the appended claims.

Claims

1. A composite structure for enhanced second harmonic generation, characterized in that, Includes a substrate and a GaN-based structure and an AlScN layer sequentially disposed in a direction away from the substrate; The GaN-based structure includes an n-type GaN layer, and the AlScN layer is heterojunction-coupled to the n-type GaN layer; furthermore, the material contained in the AlScN layer has the chemical formula Al. 1-x Sc x The N and x values ​​are 0.1 to 0.43, and the crystal structure is hexagonal wurtzite with a preferred c-axis orientation. The thickness of the AlScN layer is more than 60 nm.

2. The composite structure for enhanced second harmonic generation according to claim 1, characterized in that: The thickness of the AlScN layer is 60nm~200nm; And / or, the GaN-based structure includes a buffer layer, an intermediate layer, and the n-type GaN layer sequentially disposed along a direction away from the substrate; And / or, the substrate is a sapphire substrate, a Si substrate, or a SiC substrate.

3. A method for preparing a composite structure for enhanced second harmonic generation, characterized in that, include: Provide substrate; A GaN-based structure is epitaxially grown on the substrate, and the GaN-based structure includes an n-type GaN layer with a preferred (002) crystal orientation; An AlScN layer is formed on the n-type GaN layer, wherein the chemical formula of the material contained in the AlScN layer is Al. 1-x Sc x The N and x values ​​are 0.1 to 0.43, and the crystal structure is hexagonal wurtzite with a preferred c-axis orientation. The thickness of the AlScN layer is more than 60 nm.

4. The preparation method according to claim 3, characterized in that, The method for forming the AlScN layer on the n-type GaN layer includes at least one of magnetron sputtering and pulsed laser deposition.

5. The preparation method according to claim 4, characterized in that, The process conditions for the magnetron sputtering method include: using an AlSc target as the target material, a substrate temperature of 200℃~400℃, a sputtering power of 1kW~10kW, and an Ar to N2 gas flow ratio of 20:20~20:

130. And / or, the process conditions of the pulsed laser deposition method include: using an AlSc target as the target material, a substrate temperature of 200℃~400℃, a pulse energy of 200mJ~400mJ, a laser frequency of 1Hz~3Hz, and a nitrogen pressure of 0.5Pa~3Pa.

6. The preparation method according to any one of claims 3-5, characterized in that: The thickness of the AlScN layer is 60nm~200nm; And / or, the GaN-based structure includes a buffer layer, an intermediate layer, and the n-type GaN layer sequentially disposed along a direction away from the substrate; And / or, the substrate is a sapphire substrate, a Si substrate, or a SiC substrate.

7. A composite structure for enhanced second harmonic generation, characterized in that, It is prepared by any one of claims 3-6.

8. A method for enhancing the second harmonic of a GaN-based structure, characterized in that, include: Provides a GaN-based structure comprising an n-type GaN layer with a preferred (002) crystal orientation; An AlScN layer is fabricated on the GaN-based structure, and the AlScN layer is heterojunctionally coupled to the n-type GaN layer. The chemical formula of the material contained in the AlScN layer is Al. 1-x Sc x The N and x values ​​are 0.1 to 0.43, and the crystal structure is hexagonal wurtzite with a preferred c-axis orientation. The thickness of the AlScN layer is more than 60 nm.

9. The application of the composite structure for enhanced second harmonic generation as described in any one of claims 1-2 and 7, or the method described in claim 8, in the fabrication of laser frequency conversion devices, nonlinear sensors, or optical communication devices.

10. A second harmonic device, characterized in that, The composite structure for enhanced second harmonic generation as described in any one of claims 1-2 and 7.

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