A mid-infrared photoelectric detector based on ultrathin supersurface and its preparation method

Abstract

The present invention belongs to the technical field of mid-infrared photodetectors, and particularly relates to a mid-infrared photodetector based on an ultrathin metasurface and a preparation method thereof. First, a substrate and a meta-atom material are provided, and ellipsometry is performed to obtain the dielectric functions of the substrate and the meta-atom material. The dielectric functions are used as inputs and brought into finite element software to perform theoretical modeling and design of the metasurface. After obtaining appropriate metasurface dimensions, the metasurface is fabricated. Subsequently, ellipsometry is used to measure the reflectivity and transmittance of the metasurface under different polarizations, and a standard electron beam evaporation process is used to deposit a metal layer on the metasurface as an electrode. Finally, a CO2 laser is selected to irradiate the metasurface in a photodetection experiment. The present invention realizes mid- and far-infrared broadband absorption, efficient photothermal conversion, and photoelectric response by optimizing the structural parameters and material properties of the metasurface, thereby solving the problems of short corresponding wavelengths, heavy devices, and insufficient detection efficiency in the prior art.

Description

Technical Field

[0001] The present invention belongs to the technical field of mid-infrared photoelectric detectors, and in particular relates to a mid-infrared photoelectric detector based on an ultra-thin supersurface and a preparation method thereof. Background Art

[0002] Mid-infrared photodetectors have important applications in military, medical and deep space exploration. However, traditional two-dimensional infrared photodetectors are limited by the narrow band gap of materials and complex designs, with limited wavelength coverage and high manufacturing costs. In addition, typical mid-infrared detectors require cooling devices to reduce the device temperature, resulting in a bulky structure and affecting miniaturization. In recent years, metasurfaces have become a strong candidate for the next generation of photodetection technology due to their strong light-matter interaction, especially in combination with surface phonon polaritons (SPhPs) technology, which can enhance light absorption in the long-wave range and achieve efficient photothermal conversion. However, the current SPhPs-based metasurfaces usually have a narrow Reststrahlen band and only support light-phonon response in a wavelength range of less than 10µm, limiting the range of action. In addition, in order to effectively excite SPhPs, such metasurfaces require a large meta-atom height (about 1µm), which not only increases the difficulty of manufacturing, but also significantly increases the optical loss. Therefore, the development of a metasurface-type photodetector that can achieve strong light absorption in the long-wave range and has low lateral and longitudinal dimensions is of great significance for achieving room temperature, miniaturized, and high light absorption efficiency mid-infrared photodetection. Summary of the invention

[0003] In view of the various shortcomings of the prior art, the inventors have researched and designed a mid-infrared photoelectric detector based on an ultra-thin metasurface and a preparation method thereof in long-term practice. By optimizing the structural parameters and material properties of the metasurface, mid-infrared and far-infrared broadband absorption, efficient photothermal conversion and photoelectric response are achieved, thereby solving the problems of short corresponding wavelengths, heavy devices and insufficient detection efficiency in the prior art.

[0004] The method for preparing the mid-infrared photoelectric detector of the present invention comprises the following steps:

[0005] Step 1, providing a polar material substrate.

[0006] Step 2, providing a high refractive index material to form the meta-atoms above the substrate.

[0007] Step 3, perform ellipsometry testing to obtain the real and imaginary parts of the dielectric function of the polar substrate and top meta-atom materials.

[0008] Step 4: The dielectric function obtained from the ellipsometry test is used as input into the finite element software to perform theoretical modeling and design of the metasurface, and the geometric parameters of the metasurface are selected to achieve the excitation of the SPhPs resonance mode in the long-wave infrared range.

[0009] Step 5, after obtaining the appropriate metasurface size through theoretical design, the metasurface is manufactured, including the following sub-steps.

[0010] Sub-step 5.1: Prepare the sample on a single-side polished polar substrate: first, deposit a meta-atom film of different target thicknesses onto the substrate by electron beam evaporation, and then spin-coat PMMA photoresist on the sample to obtain a PMMA photoresist layer.

[0011] Sub-step 5.2, using an electron beam lithography system to expose the designed disk pattern onto the PMMA photoresist layer, developing the sample after exposure, using the developed PMMA photoresist pattern as a mask, and etching through a subsequent inductively coupled plasma enhanced reactive ion etching process to obtain a meta-atom disk array.

[0012] Sub-step 5.3, immersing the obtained metasurface in an acetone solution to wash away the residual PMMA photoresist to obtain a mixed medium metasurface.

[0013] Step 6: Use the ellipsometric test method to test the reflectivity and transmittance of the obtained metasurface under different polarizations, and obtain the absorptivity based on 1-reflectivity-transmittance, and compare and verify the ellipsometric experimental results and finite element calculation results.

[0014] Step 7: Use standard electron beam evaporation technology to deposit a metal layer on the metasurface as an electrode, and then construct a metal wire loop to make the equipment a testable device.

[0015] Step 8, a CO2 laser is used to illuminate the metasurface in the photoelectric detection experiment.

[0016] Furthermore, in step 1, the polar material substrate is one of SrTiO3, BaTiO3, and TiO2; the polar material substrate has two Reststrahlen bands in the infrared range of >10μm, one of which spans a wavelength of more than 20μm, and in the wavelength range covered by the Reststrahlen band, the real part of the dielectric function of the material is less than 0.

[0017] Furthermore, in step 2, the high refractive index material is Ge or Si.

[0018] Furthermore, in step 4, the geometric parameters of the metasurface include the period of the metasurface, the height and diameter of the meta-atom.

[0019] Furthermore, in step 5.1, the meta-atom films of different target thicknesses are deposited onto the substrate by electron beam evaporation, with a deposition rate of 1 Å / s and a vacuum chamber pressure of 9×10 -7 Torr.

[0020] Furthermore, in step 5.1, PMMA photoresist is spin-coated on the sample at a rotation speed of 3500 rpm for 60 seconds, and baked at 180° C. for 90 seconds to obtain a photoresist layer with a thickness of 70 nm, 100 nm or 250 nm.

[0021] Furthermore, in step 5.2, a 50KV electron beam lithography system is used to expose the designed disk pattern onto the PMMA photoresist layer.

[0022] Furthermore, in step 5.2, the sample is developed using a 1:3 mixed solution of methyl isobutyl ketone and isopropanol for 35 seconds, followed by washing with an isopropanol solution for 30 seconds.

[0023] Furthermore, in step 5.2, the period of the meta-atom disk array is 5 μm, the diameter is 4 μm, and the height of the meta-atom is no more than 300 nm.

[0024] Furthermore, in step 5.3, the metasurface was immersed in an acetone solution for 5 minutes to wash away the residual PMMA photoresist, and then cleaned with alcohol.

[0025] Furthermore, in step 6, the ellipsometric test results and the finite element calculation results are compared and verified, and the difference between the ellipsometric test value and the finite element simulation value is controlled to be lower than 10% of the finite element simulation value itself.

[0026] Furthermore, in step 7, the specific steps of depositing the metal layer on the super surface are:

[0027] Sub-step 7.1: Cover a portion of the metasurface with a thin and long aluminum foil whose width is smaller than the lateral dimension of the metasurface, and then perform electron beam evaporation on the entire surface to coat the metal layer.

[0028] Sub-step 7.2: Remove the aluminum foil, and a part of the metasurface is exposed without the metal layer, which is used as the light absorption area. The remaining areas of the metasurface containing the metal layer are used as electrodes. Spot welding is performed on the metal-covered areas to build conductive nanowires, thereby forming a pathway.

[0029] Furthermore, in step 8, a dual-channel digital source meter is used to measure the photoelectric response of the device, using a homemade 10.6 μm mid-infrared CO2 laser as the light source. All measurements are performed in ambient air, with a test power range of 200 mW to 700 mW and a bias voltage range of -5 V to 5 V.

[0030] The present invention also provides a mid-infrared photoelectric detector based on an ultra-thin supersurface, which is made by the above-mentioned preparation method. The mid-infrared photoelectric detector includes a supersurface, and the supersurface includes a polar material substrate and a meta-atom array etched on the surface of the polar material substrate.

[0031] The beneficial effects of the present invention are:

[0032] 1) It can effectively solve the defect that the conventional two-dimensional materials have narrow band gaps and can only detect wavelengths less than 15um. By stimulating the resonant absorption of SPhPs in the range of 15μm, 25μm and more wavelengths, it broadens the detection range of infrared photoelectric detection to longer wavelengths. Moreover, the method of the present invention does not require a cooler, and the device is simple and easy to carry.

[0033] 2) The prepared metasurface can achieve an absorption of 0.89 under the conditions of a height less than 0.36% of the wavelength and a period less than 25% of the wavelength. Therefore, efficient light absorption can be achieved in an extremely small area, which contributes to the development of device miniaturization and improves the efficiency of photoelectric detection.

[0034] 3) By optimizing the structure and material properties, the responsiveness and photocurrent of the photodetector are significantly improved, which is particularly suitable for efficient detection in the MIR band. Therefore, the solution provided by the invention can provide basic support for the fields of microelectronic information integration, medical treatment, deep space exploration, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is the dielectric function of Ge film and SrTiO3 obtained by ellipsometry test of the present invention.

[0036] FIG. 2( a ) is a schematic diagram of the structure of a supersurface prepared according to an embodiment of the present invention.

[0037] Figure 2(b) is a local enlarged view of the metasurface structure.

[0038] Figure 3 is a SEM image of a supersurface prepared according to an embodiment of the present invention.

[0039] Figure 4 It is a comparison between the absorptivity of the metasurface measured by the ellipsometry experiment of the present invention and the absorptivity obtained by simulation calculation.

[0040] Figure 5 It is the photoelectric detection performance of different microstructures obtained from the photoelectric detection experiment of the present invention. DETAILED DESCRIPTION

[0041] The present invention will be further described in detail below in conjunction with the accompanying drawings and implementation examples. These implementation examples are described in sufficient detail so that those skilled in the art can understand and practice the present invention. Without departing from the spirit and scope of the present invention, logical, achievable and other changes may be made to the implementation. Therefore, the following detailed description should not be construed as limiting, and the scope of the present invention is limited only by the claims.

[0042] In view of the problems of narrow wavelength range and low absorption efficiency of current mid-infrared photoelectric detectors, the present invention proposes a mid-infrared photoelectric detector based on an ultra-thin metasurface and a preparation method thereof, and the preparation method specifically comprises the following steps:

[0043] Step 1, providing a polar material substrate. The polar material substrate has two Reststrahlen bands in the infrared range of >10 μm, one of which spans a wavelength of more than 20 μm, and in the wavelength range covered by the Reststrahlen band, the real part of the dielectric function of the material is less than 0. The polar material substrate of the present invention is one of SrTiO3, BaTiO3, and TiO2.

[0044] Step 2, providing a high refractive index material to form the meta-atoms above the substrate. The high refractive index material of the present invention is Ge or Si, both of which are CMOS compatible materials with low manufacturing cost and suitable for large-scale integration.

[0045] Step 3, perform ellipsometry testing to obtain the real and imaginary parts of the dielectric function of the polar substrate and top meta-atom materials.

[0046] Step 4: Take the dielectric function obtained from the ellipsometry test as input and bring it into the finite element software (Comsol Multiphysics) to theoretically model and design the metasurface, and design a metasurface that meets expectations from a theoretical level. Through structural optimization design, the geometric parameters of the metasurface are selected to achieve the excitation of the SPhPs resonance mode in the long-wave infrared range, ensuring that the metasurface has strong absorption enhancement in the target band. The geometric parameters of the metasurface include the period of the metasurface, the height of the meta-atom, and the diameter.

[0047] By performing optical simulations on the constructed hybrid dielectric metasurface, the structural parameters of the metasurface are explored, so that there is at least one SPhPs resonance in each of the spectral ranges contained in the two Reststrahlen bands, thus exhibiting strong absorption characteristics.

[0048] Step 5, after obtaining the appropriate metasurface size through theoretical design, the metasurface is manufactured. Specifically, it includes the following sub-steps:

[0049] Substep 5.1, prepare the sample on a single-side polished polar substrate. First, the meta-atom films of different target thicknesses were deposited onto the substrate by electron beam evaporation (DETECH, DE500C) with a deposition rate of 1 Å / s and a vacuum chamber pressure of 9 × 10 -7 Then, PMMA photoresist (950 A4 model of KayakuAM) was spin-coated on the sample at a rotation speed of 3500 rpm for 60 seconds, and then baked at 180°C for 90 seconds to obtain a PMMA photoresist layer of 70 nm, 100 nm, or 250 nm thickness.

[0050] Sub-step 5.2, use a 50KV electron beam lithography (EBL) system (Raith Voyager) to expose the designed disk pattern onto the PMMA photoresist layer. After exposure, the sample is developed with a 1:3 mixed solution of MIBK (methyl isobutyl ketone) and IPA (isopropyl alcohol) for 35 seconds, followed by IPA cleaning for 30 seconds. The developed PMMA photoresist pattern is used as a mask, and the meta-atom disk array is etched by a subsequent inductively coupled plasma enhanced reactive ion etching (ICP-RIE) process (PlasmaPro 80 Cobra). The period of the meta-atom disk array is 5μm and the diameter is 4μm. In the metasurface of the present invention, the height of the meta-atom is not more than 300nm.

[0051] In sub-step 5.3, the metasurface is immersed in an acetone solution for 5 minutes to wash away the residual PMMA photoresist, and then cleaned with alcohol to obtain a mixed medium metasurface.

[0052] Step 6: Use the ellipsometry method to test the reflectivity and transmittance of the obtained metasurface under different polarizations, and obtain the absorptivity according to 1-reflectivity-transmittance. Compare and verify the ellipsometry test results and the finite element calculation results, and control the difference between the ellipsometry test value and the finite element simulation value to be less than 10% of the finite element simulation value itself to ensure the reliability of sample preparation.

[0053] Step 7: Use standard electron beam evaporation technology to deposit a metal layer on the metasurface as an electrode, and then build a metal wire structure loop at a later stage to make the equipment a testable device.

[0054] The specific steps for depositing a metal layer on a metasurface are:

[0055] Sub-step 7.1: Cover a portion of the metasurface with a thin and long aluminum foil whose width is smaller than the lateral dimension of the metasurface, and then perform electron beam evaporation on the entire surface to coat the metal layer, wherein the metal layer is 30nm Cr and 100nm Au.

[0056] Sub-step 7.2: Remove the aluminum foil, and a portion of the metasurface is exposed without a metal layer, which is used as a light-absorbing area, and the remaining areas of the metasurface containing a metal layer are used as electrodes. Spot welding is performed on the areas covered with metal to build conductive nanowires, thereby forming a path. Further assembly is performed to obtain a device that can perform photoelectric detection.

[0057] Step 8: A 10.6μm CO2 laser was used to illuminate the metasurface in the photoelectric detection experiment. The photoelectric response of the device was measured using a Keithley 2636B dual-channel digital source meter, and a homemade 10.6μm mid-infrared CO2 laser was used as the light source. All measurements were performed in ambient air, with a test power range of 200mW to 700mW and a bias voltage range of -5V to 5V.

[0058] A mid-infrared photodetector based on an ultrathin metasurface is prepared by the above-mentioned preparation method. The mid-infrared photodetector includes a metasurface, which includes a polar material substrate and a meta-atom array etched on the surface of the polar material substrate. The photocurrent of the metasurface composed of the polar material substrate and the meta-atom is greater than the photocurrent of the unetched meta-atom thin film system of the same thickness. Combining the meta-atom array with the polar material substrate to construct a metasurface can not only achieve broadband light absorption, but also improve the response speed, sensitivity and miniaturization capability of the detector.

[0059] The method of the present invention can effectively solve the defect that the conventional two-dimensional material has a narrow band gap and is less than 15um wavelength detection. By stimulating the resonance absorption of SPhPs in the range of 15μm, 25μm and more wavelengths, the detection range of infrared photoelectric detection is broadened to longer wavelengths. Moreover, the method of the present invention does not require a cooler, and the device is simple and easy to carry.

[0060] The metasurface prepared by the method of the present invention can achieve an absorption of 0.89 under the conditions of a height less than 0.36% of the wavelength and a period less than 25% of the wavelength. Therefore, efficient light absorption can be achieved in an extremely small area, which is conducive to the development of device miniaturization and improves the efficiency of photoelectric detection.

[0061] The present invention significantly improves the responsiveness and photocurrent of the photodetector by optimizing the structure and material properties, and is particularly suitable for efficient detection in the MIR band. Therefore, the solution provided by the invention can provide basic support for the fields of microelectronic information integration, medical treatment, deep space exploration, etc.

[0062] The following is a specific experimental case:

[0063] 1) Purpose of the experiment

[0064] A broadband mid-infrared photodetector based on an ultrathin mixed dielectric metasurface is designed with reference to the technical solution of the present invention to verify the effectiveness and feasibility of the technical solution of the present invention. In this case, SrTiO3 polar material is used as the substrate and Ge material is used as the meta-atom of the metasurface.

[0065] 2) Implementation steps

[0066] Firstly, the dielectric function of SrTiO3 bulk single crystal and the dielectric function of Ge films with different thicknesses (70nm, 100nm and 250nm) were tested by ellipsometry.

[0067] See also Figure 1 , the dielectric functions of Ge film and SrTiO3 obtained by ellipsometry test. Figure (a) shows the real part of the dielectric function of Ge film at 100nm and 200nm thickness; Figure (b) shows the imaginary part of the dielectric function of Ge film at 100nm and 200nm thickness. Figure (c) shows that the dielectric function of SrTiO3 has two regions where the real part of the dielectric function is less than 0. The real part of the dielectric function of the Ge film with a thickness of 100nm is greater than that of the film with a thickness of 200nm; the imaginary parts of the two are basically the same and close to 0.

[0068] The geometric structure is modeled in finite element software to build a metasurface physical model. The dielectric function of the obtained material is used as input to assign corresponding material properties to different structural positions. Then, the conditions for exciting SPhPs are simulated and explored.

[0069] See Figure 2(a) and Figure 2(b), which show the composition of the designed metasurface: a periodic array of meta-atoms is etched on top of the substrate.

[0070] See also Figure 3 According to the simulation conditions, a metasurface was prepared. Ge films of different thicknesses (70nm, 100nm, 250nm) were deposited on a single crystal SrTiO3 substrate by electron beam evaporation. Then, electron beam lithography and reactive ion etching techniques were used to form a meta-atom array of corresponding height on the Ge film.

[0071] See also Figure 4, the absorptivity of the metasurface measured by ellipsometry experiment is compared with the absorptivity obtained by simulation calculation. In the figure, (a) shows the experimental and simulated absorptivity of the metasurface with a height of 250nm in the transverse electric (TE) polarization direction; (b) shows the experimental and simulated absorptivity of the metasurface with a height of 250nm in the transverse magnetic (TM) polarization direction; (c) shows the absorptivity and simulated absorptivity of the metasurface with a height of 70nm obtained by Fourier infrared spectrometer test. When the meta-atom height is 70nm, the resonant absorption of SPhPs at 14μm can be stimulated, and when the meta-atom height is 250nm, the resonant absorption of SPhPs at 14μm and 25μm are both stimulated. The ellipsometry experimental spectrum is consistent with the calculated spectrum, verifying the correctness of the design.

[0072] See also Figure 5 , which are the photoelectric detection performances of different microstructures obtained from the photoelectric detection experiment. In the figure, (a) is the responsivity and (b) is the detection rate. The electrode electrical and photoresponse measurements were performed using a Keithley 2636B dual-channel digital source meter, and a 10.6μm CO2 laser was used for the excitation wavelength. The experimental results show that the ultra-thin hybrid medium metasurface not only has significant light absorption capacity in a wide wavelength range, but also significantly improves the photocurrent and response.

[0073] The present invention has been described in detail above. The above description is only a preferred embodiment of the present invention and should not limit the scope of implementation of the present invention. That is, all equivalent changes and modifications made within the scope of this application should still fall within the scope of the present invention.

Claims

1. A method for preparing a mid-infrared photodetector based on an ultrathin supersurface, characterized in that: The following steps are involved: Step 1, providing a polar material substrate; Step 2, providing a high refractive index material to form the meta-atoms above the substrate; Step 3, performing ellipsometry testing to obtain the real and imaginary parts of the dielectric function of the polar substrate and the top meta-atom material; Step 4: Take the dielectric function obtained by the ellipsometry test as input and bring it into the finite element software to perform theoretical modeling and design of the metasurface, select the geometric parameters of the metasurface, and realize the excitation of the SPhPs resonance mode in the long-wave infrared range; Step 5, after the metasurface size is obtained by theoretical design, the metasurface is manufactured, including the following sub-steps: Sub-step 5.1: Prepare a sample on a single-side polished polar substrate: first, deposit a meta-atom film of different target thicknesses onto the substrate by electron beam evaporation, and then spin-coat PMMA photoresist on the sample to obtain a PMMA photoresist layer; Sub-step 5.2, using an electron beam lithography system to expose the designed disk pattern onto the PMMA photoresist layer, developing the sample after exposure, using the developed PMMA photoresist pattern as a mask, and etching to obtain a meta-atom disk array through a subsequent inductively coupled plasma enhanced reactive ion etching process; Sub-step 5.3, immersing the obtained metasurface in an acetone solution to wash away the residual PMMA photoresist to obtain a mixed medium metasurface; Step 6, using the ellipsometry method to test the reflectivity and transmittance of the obtained metasurface under different polarizations, and obtaining the absorptivity according to 1-reflectivity-transmittance, and comparing and verifying the ellipsometry experimental results and the finite element calculation results; Step 7: Use standard electron beam evaporation process to deposit a metal layer on the metasurface as an electrode, and then build a metal wire loop to make the equipment a testable device; Step 8: A CO2 laser is used to illuminate the metasurface in the photoelectric detection experiment.

2. The preparation method according to claim 1, characterized in that: In step 1, the polar material substrate is one of SrTiO3, BaTiO3, and TiO2; the polar material substrate has two Reststrahlen bands in the infrared range of >10μm, one of which spans a wavelength of more than 20μm, and in the wavelength range covered by the Reststrahlen band, the real part of the dielectric function of the material is less than 0.

3. The preparation method according to claim 1, characterized in that: In step 2, the high refractive index material is Ge or Si.

4. The preparation method according to claim 1, characterized in that: In step 4, the geometric parameters of the metasurface include the period of the metasurface, the height and diameter of the meta-atom.

5. The preparation method according to claim 1, characterized in that: In step 5.1, the meta-atom films of different target thicknesses were deposited onto the substrate by electron beam evaporation at a deposition rate of 1 Å / s and a vacuum chamber pressure of 9×10 -7 Torr; In step 5.1, PMMA photoresist is spin-coated on the sample at a rotation speed of 3500 rpm for 60 seconds, and baked at 180° C. for 90 seconds to obtain a photoresist layer with a thickness of 70 nm, 100 nm or 250 nm.

6. The preparation method according to claim 1, characterized in that: In step 5.2, a 50KV electron beam lithography system is used to expose the designed disk pattern onto the PMMA photoresist layer; In step 5.2, the sample is developed using a 1:3 mixed solution of methyl isobutyl ketone and isopropanol for 35 seconds, followed by washing with an isopropanol solution for 30 seconds; In step 5.2, the period of the meta-atom disk array is 5 μm, the diameter is 4 μm, and the height of the meta-atom is no more than 300 nm; In step 5.3, the metasurface was immersed in acetone solution for 5 min to wash away the residual PMMA photoresist and then cleaned with alcohol.

7. The preparation method according to claim 1, characterized in that: In step 6, the ellipsometric test results and the finite element calculation results are compared and verified, and the difference between the ellipsometric test value and the finite element simulation value is controlled to be lower than 10% of the finite element simulation value itself.

8. The preparation method according to claim 1, characterized in that: In step 7, the specific steps of depositing a metal layer on the super surface are: Sub-step 7.1: Covering a portion of the metasurface with a thin and long aluminum foil having a width smaller than the lateral dimension of the metasurface, and then performing electron beam evaporation on the entire surface to coat the metal layer; Sub-step 7.2: Remove the aluminum foil, and a part of the metasurface is exposed without the metal layer, which is used as the light absorption area. The remaining areas of the metasurface containing the metal layer are used as electrodes. Spot welding is performed on the metal-covered areas to build conductive nanowires, thereby forming a pathway.

9. The preparation method according to claim 1, characterized in that: In step 8, a dual-channel digital source meter is used to measure the photoelectric response of the device, using a homemade 10.6μm mid-infrared CO2 laser as the light source. All measurements are performed in ambient air, with a test power range of 200mW to 700mW and a bias voltage range of -5V to 5V.

10. A mid-infrared photodetector based on an ultrathin supersurface, made by the preparation method according to any one of claims 1 to 9, characterized in that: It comprises a super surface, which comprises a polar material substrate and a meta-atom array etched on the surface of the polar material substrate.

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