A method for designing a photodetector with reduced two-photon excitation threshold

Abstract

The application discloses a kind of photodetector design methods for reducing two-photon excitation threshold, it is related to photoelectric detection technical field.The method comprises: providing nanometer thickness, microscale size semiconductor strip;Preparation ohmic contact electrode at both ends of strip;Form non-uniform light on strip to generate temperature difference.Nanometer size restriction effect is utilized, two-photon absorption and photothermal effect are simultaneously excited, and positive feedback cascade mechanism is formed by two, directional migration of carrier is driven, and excitation threshold is greatly reduced.The application does not need PN junction, and is suitable for flexible non-uniform light illumination mode such as converging light spot or shielding, and can work under low-power continuous laser, which is beneficial to low-cost array preparation.

Description

Technical Field

[0001] This invention relates to the field of photoelectric detection technology, and in particular to a design method for a photoelectric detector that reduces the two-photon excitation threshold. Background Technology

[0002] Nonlinear absorption is an important quantum effect of intense light-matter interaction. Two-photon absorption (TPA), as a representative nonlinear absorption process, is theoretically not limited by the inherent band gap of semiconductors. However, exciting TPA usually requires simultaneously satisfying extremely high photon energy and high light intensity conditions. Currently, this phenomenon mainly relies on high-energy femtosecond pulsed lasers, resulting in a persistently high excitation threshold, which severely restricts its practical application in photodetectors.

[0003] On the other hand, the photothermoelectric effect (PTE), as a type of intrinsic photoelectric conversion mechanism, is based on the formation of a photovoltage caused by localized temperature differences in materials under non-uniform illumination. Traditional approaches often involve constructing physical barriers in the material structure to achieve asymmetric illumination distribution; for example, some existing ultrathin silicon imaging chip technologies often employ semi-shielded optical structures to achieve temperature differences. However, such designs lack utilization of the deep-seated synergistic mechanisms of physical effects, and their responsivity and excitation threshold remain limited by the physical limits of their individual effects.

[0004] How to achieve a synergistic superposition of two-photon absorption effect and photothermoelectric effect through ingenious structural design and mechanism innovation, thereby significantly reducing the excitation threshold of two-photon absorption, is a key technical problem that urgently needs to be solved in the field of photoelectric detection. Summary of the Invention

[0005] To address this, embodiments of the present invention provide a photodetector design method for reducing the two-photon excitation threshold, thereby solving the problems of high two-photon absorption excitation threshold and dependence on high-intensity pulsed lasers in the prior art.

[0006] To address the aforementioned technical problems, embodiments of the present invention provide a method for designing a photodetector to reduce the two-photon excitation threshold, the method comprising: A semiconductor strip is provided, the semiconductor strip having a nanometer-scale thickness, a micrometer-scale width, and a micrometer-scale length; Ohmic contact electrodes are fabricated at both ends of the semiconductor strip; A non-uniform illumination distribution is formed on the semiconductor strip to generate a temperature difference along the length of the semiconductor strip; By utilizing the nanoscale thickness confinement effect of the semiconductor strip, under incident light irradiation, the semiconductor strip is simultaneously excited by a two-photon absorption effect and a photothermal-electric effect. The two-photon absorption effect generates photogenerated carriers in the semiconductor strip, and the photothermal-electric effect generates a thermoelectric potential in the semiconductor strip. The thermoelectric potential drives the photogenerated carriers to migrate directionally along the length of the semiconductor strip, and the generated Seebeck electric field further modulates and enhances the two-photon absorption, forming a positive feedback cascade mechanism of the two-photon absorption effect and the photothermal-electric effect, thereby reducing the excitation threshold of the two-photon absorption effect.

[0007] Preferably, the thickness of the semiconductor strip is Width is , length is .

[0008] Preferably, the semiconductor strip is made of a material with a Seebeck coefficient greater than 1. Semiconductor materials.

[0009] Preferably, the two-photon absorption coefficient of the semiconductor material is at least greater than 1E-11m / W.

[0010] Preferably, the method for forming a non-uniform illumination distribution on the semiconductor strip includes: irradiating a local area of ​​the semiconductor strip with incident light through a converging spot; or, providing a semi-shading structure on a local area of ​​the semiconductor strip, the semi-shading structure being composed of a polymer material or a stacked structure of metal and dielectric.

[0011] This invention also provides a photodetector fabricated based on the design method described above, comprising: Semiconductor strips with nanometer-scale thickness, micrometer-scale width, and micrometer-scale length; Ohmic contact electrodes are respectively disposed at both ends of the semiconductor strip; An asymmetric illumination unit is used to form a non-uniform illumination distribution on the semiconductor strip to generate a temperature difference; Under incident light, the semiconductor strip generates both two-photon absorption and photothermal-electric effects based on its nanoscale thickness limitation effect, forming a positive feedback cascade mechanism of two-photon absorption and photothermal-electric effects.

[0012] Preferably, the thickness of the semiconductor strip is 10 nm. 70nm, width 1μm 5μm, length 10μm 20μm.

[0013] Preferably, the semiconductor strip is made of a material with a Seebeck coefficient greater than 1. The semiconductor material having a two-photon absorption coefficient of at least 1E-11m / W.

[0014] Preferably, the asymmetric illumination unit is a semi-occluded structure, and the reflectivity of the semi-occluded structure is greater than that of the semi-occluded structure. It is composed of a layered structure of metal and dielectric or a polymer material.

[0015] Preferably, the fabrication steps of the photodetector include: thinning a semiconductor material to form a thickness of... The semiconductor thin film is photolithographically and etched to form the semiconductor strip; the ohmic contact electrode is fabricated by electron beam evaporation or magnetron sputtering; and the asymmetric illumination unit is formed by depositing a metal-dielectric stack or coating with a polymer material. As can be seen from the above technical solutions, this invention application has the following beneficial effects: First, this invention utilizes the nanoscale thickness size confinement effect and the positive feedback cascade amplification mechanism between two-photon absorption and photothermal-electric effects to change the traditional mode that relies on high-energy femtosecond pulsed lasers, enabling the photodetector to stably generate a two-photon absorption response under continuous low-power laser excitation.

[0016] Second, the present invention adopts a non-junctional working mode, which does not require a PN junction or heterojunction; at the same time, the temperature difference construction method is flexible and not limited to a specific rigid physical shield. Non-uniform illumination can be achieved through various forms such as focusing light spots or coating with high polymers. The range of materials to be selected is wide and the working spectrum is not limited by the semiconductor band gap.

[0017] Third, by adopting an ultra-thin semiconductor strip configuration with nanometer-level thickness, combined with its non-junctional structural characteristics, it not only avoids the performance bottlenecks caused by doping concentration and interface defects in traditional junction devices, but also has strong process compatibility and is easy to realize large-scale array fabrication, which is very conducive to realizing low-cost, high-performance large-scale photoelectric detection applications.

[0018] Fourth, based on the aforementioned combined action mechanism of intrinsic processes, this invention can extend the response wavelength of the photodetector to... (nm), where The bandgap width (eV) is the material's width. Furthermore, a key feature of the detector is that it contains no junction structure, i.e., it does not contain any PN junctions or heterojunctions. Its signal generation and carrier separation rely entirely on the intrinsic physical processes of the semiconductor strip itself, resulting in extremely high structural simplicity. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly described below. Referring to the drawings will make the features and advantages of the present invention clearer. The drawings are illustrative and should not be construed as limiting the present invention in any way. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 This is a schematic diagram illustrating the principle of the positive feedback cascade mechanism of two-photon absorption effect and photothermoelectric effect in an embodiment of the present invention; Figure 2 This is a flowchart of a photodetector design method for reducing the two-photon excitation threshold provided by an embodiment of the present invention; Figure 3 These are the Z scan test results of silicon thin film samples in the embodiments of the present invention, where a, b, c, and d are the Z scan test results of silicon thin film samples with different thicknesses d, the equilateral triangle and the inverted triangle represent the data of the forward and reverse tests, respectively, and the horizontal coordinate Z represents the position along the z-axis. Figure 4 These are the Z-scan test results of gallium nitride samples in the embodiments of the present invention, where a, b, c, d, and e are the Z-scan test results of gallium nitride materials with different thicknesses d, the equilateral triangle and the inverted triangle represent the data of the forward and reverse tests, respectively, and f is the effect of the thickness d of semiconductor gallium nitride on the two-photon absorption (TPA) coefficient β; Figure 5 In this embodiment of the invention, the semiconductor silicon thickness d affects the two-photon absorption (TPA) coefficient β and the temperature difference. A diagram illustrating the effect of T; Figure 6 This is a schematic diagram of the simulation results of the responsivity of silicon strip devices of different thicknesses at a wavelength of 1310nm in the embodiments of the present invention; Figure 7 This is a schematic diagram showing the response rate test results of silicon strip devices of different thicknesses at wavelengths of 1100nm-1800nm ​​in embodiments of the present invention; Figure 8 This is a schematic diagram of the simulation results of the responsivity of silicon strip devices of different widths at a wavelength of 1310nm in the embodiments of the present invention; Figure 9 This is a schematic diagram of an experiment used in this embodiment of the invention to demonstrate two-photon absorption and photothermoelectric effects. In this diagram, 'a' represents the photocurrent response intensity of the ultrathin silicon strip device under a 1V bias voltage at the zero point of the photothermoelectric effect thermoelectric potential of the ultrathin silicon strip. incThe plot shows the changing double logarithmic curves. The inset is a schematic diagram of the zero point of the PTE effect potential when incident light shines on an ultrathin silicon strip. b is the surface photocurrent distribution of the strip region under bias voltages of +1V, 0V, and -1V. c is the photoelectric response intensity distribution along the center line of the strip. The incident light wavelength is 1310nm. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1: Physical Mechanism and Structure of a Detector Based on Size Confinement Effect This embodiment provides a method for designing a photodetector to reduce the two-photon excitation threshold, and a photodetector fabricated based on this design method. Figure 1 As shown, the core of this detector is to construct a semiconductor strip with specific size constraints to excite the mutual modulation and enhancement between the two-photon absorption effect and the photothermoelectric effect.

[0022] like Figure 2 As shown, this embodiment provides a photodetector design method for reducing the two-photon excitation threshold, including: A semiconductor strip is provided, the semiconductor strip having a nanometer-scale thickness, a micrometer-scale width, and a micrometer-scale length; Ohmic contact electrodes are fabricated at both ends of the semiconductor strip; A non-uniform illumination distribution is formed on the semiconductor strip to generate a temperature difference along the length of the semiconductor strip; By utilizing the nanoscale thickness confinement effect of the semiconductor strip, under incident light irradiation, the semiconductor strip is simultaneously excited by a two-photon absorption effect and a photothermal-electric effect. The two-photon absorption effect generates photogenerated carriers in the semiconductor strip, and the photothermal-electric effect generates a thermoelectric potential in the semiconductor strip. The thermoelectric potential drives the photogenerated carriers to migrate directionally along the length of the semiconductor strip, and the generated Seebeck electric field further modulates and enhances the two-photon absorption, forming a positive feedback cascade mechanism of the two-photon absorption effect and the photothermal-electric effect, thereby reducing the excitation threshold of the two-photon absorption effect.

[0023] This invention also provides a photodetector fabricated based on the design method described above, comprising: Semiconductor strips with nanometer-scale thickness, micrometer-scale width, and micrometer-scale length; Ohmic contact electrodes are respectively disposed at both ends of the semiconductor strip; An asymmetric illumination unit is used to form a non-uniform illumination distribution on the semiconductor strip to generate a temperature difference; Under incident light, the semiconductor strip generates both two-photon absorption and photothermal-electric effects based on its nanoscale thickness limitation effect, forming a positive feedback cascade mechanism of two-photon absorption and photothermal-electric effects.

[0024] To further clarify the preconditions for the effective establishment of the aforementioned positive feedback cascade mechanism, this invention has verified through extensive experiments and simulations that specific size parameters of the semiconductor strip are necessary conditions for activating this physical mechanism. Based on this, this embodiment has specifically optimized the size and material of the semiconductor strip, as follows: (1) Core dimensions and material selection In this embodiment, the thickness of the semiconductor strip is preferably... The width is preferably , length is Reducing the thickness to the tens of nanometers scale allows the structural feature size to approach or even be smaller than the mean free path of the charge carriers. Under this size constraint, the ballistic transport mode of the charge carriers is activated, enabling them to travel long distances without scattering. This avoids the adverse effects of longitudinal thermoelectric potential on charge carrier diffusion, thus providing an ideal physical space for the subsequent cascade enhancement of the two-photon absorption effect and the photothermoelectric effect.

[0025] In terms of material selection, the semiconductor strip is made of a material with a Seebeck coefficient greater than [value missing]. The semiconductor material is a semiconductor material having a two-photon absorption coefficient at least greater than 1E-11m / W, such as silicon or gallium nitride. In a preferred embodiment, the semiconductor material is P-type single-crystal silicon with a doping concentration of [missing information]. This choice of light doping with low doping concentration can effectively reduce internal defects in the material. Combined with the nanoscale thickness limitation, it maximizes the synergistic effect of carrier-dominated thermoelectric properties and nonlinear optical properties.

[0026] (2) Implementation of asymmetric lighting (temperature difference construction) To generate a stable temperature difference along the length of the semiconductor strip, this embodiment forms a non-uniform illumination distribution on the semiconductor strip. It should be noted that the method for forming the non-uniform illumination distribution in this embodiment is not limited to a single physical semi-shading structure. As a feasible implementation, the incident light can be directly converted into a converging spot using an optical system, so that the spot only illuminates a localized area of ​​the photosensitive region of the semiconductor strip; alternatively, a semi-shading structure can be provided on a portion of the semiconductor strip. The semi-shading structure is made of a polymer material (such as polyimide or polymethyl methacrylate) or a stacked structure of metal and dielectric, and its reflectivity is preferably greater than [missing value]. Regardless of the specific physical method used, as long as a high energy density gradient and temperature difference can be formed locally in the semiconductor strip, it is acceptable.

[0027] (3) Physical mechanism: Positive feedback cascade of two-photon absorption and photothermal-electric effect Under the ultra-thin size constraints of this embodiment, when incident light irradiates a non-uniformly illuminated area, the following physical processes occur inside the device due to the nanometer-scale thickness limitation: First, due to the extremely thin strip, the localized high light intensity easily excites the two-photon absorption effect (TPA), generating a large number of photogenerated carriers. Simultaneously, the non-uniform illumination creates localized temperature differences at both ends of the strip. Thermoelectric potential (photothermoelectric effect, PTE) is generated based on the Seebeck effect.

[0028] Most importantly, these two effects are not independent but form a positive feedback cascade mechanism. Specifically, the thermoelectric electromotive force generated by the photothermoelectric effect drives the newly generated photogenerated carriers to migrate directionally along the strip length; and the directional migration of carriers is not merely a passive response, but can further disrupt the thermal equilibrium within the strip, enhancing the local temperature difference along the strip length. This cascaded amplification loop of "two-photon absorption, photogenerated carrier generation, increased temperature difference, enhanced photothermoelectric effect, accelerated carrier migration, and optimized two-photon absorption conditions" enables the two-photon absorption effect, which originally required extremely high peak power femtosecond lasers to trigger, to be effectively excited under continuous low-power laser excitation, thereby significantly reducing the excitation threshold of two-photon absorption. This non-junctional operating mode of the present invention eliminates the need to construct traditional PN junctions or heterojunctions.

[0029] (4) Experimental verification and parameter optimization To verify the above mechanism, this embodiment uses Z-scan testing to measure the nonlinear absorption coefficients of silicon and gallium nitride thin films of different thicknesses (e.g., Figure 3 , Figure 4As shown in the figure). Test results show that when the material thickness is reduced to tens of nanometers, its two-photon absorption coefficient increases by four orders of magnitude compared to the bulk material. Combined with finite element simulation (such as... Figure 5 As shown), at an optical power of of Under continuous laser irradiation, as the thickness decreases, the temperature difference increases. While significantly increasing, the two-photon absorption coefficient It also shows a synchronous increasing trend, which confirms the shift in the carrier-dominated heating mode and the activation of the positive feedback mechanism.

[0030] To further determine the optimal dimensional parameters, this embodiment uses a fixed width of... , length is The device underwent response rate simulation and testing (e.g. Figure 6 , Figure 7 (As shown). The results indicate that the thickness is Ultrathin silicon strips exhibit the best detection responsivity. Furthermore, width is also a key parameter affecting performance (e.g., Figure 8 As shown), narrower strips (e.g.) It can more effectively guide the directional migration of charge carriers, thereby obtaining a higher response rate.

[0031] In the two-photon absorption dominance verification experiment, this embodiment focuses on a thickness of... Width is , length is Ultrathin silicon devices were tested (e.g.) Figure 9 (As shown). The fitting result of the slope of the double logarithmic power response curve is as follows: The value is close to the eigenvalue 2, proving that photogenerated carriers are mainly generated by two-photon absorption. Simultaneously, photocurrent mapping tests show that the current at the center of the strip is 0 under zero bias, while the response is strongest near the electrode at 2 μm, which is a typical characteristic of the photothermoelectric effect. The above experiments comprehensively verify the combined working mode of the two-photon absorption effect and the photothermoelectric effect.

[0032] Example 2: Fabrication method of photodetector This embodiment provides a method for fabricating a photodetector with a reduced two-photon excitation threshold as described in Embodiment 1. Based on the device structure of Embodiment 1, the method specifically includes the following steps: Step 1: Providing semiconductor materials and fabricating semiconductor strips Choose a Seebeck coefficient greater than The semiconductor material, wherein the two-photon absorption coefficient of the semiconductor material is at least greater than 1E-11m / W (e.g., P-type single-crystal silicon, with a doping concentration of...). First, the semiconductor material is thinned using a deep silicon etching process to precisely reduce its thickness to a specific value. A semiconductor thin film is then formed. Subsequently, the semiconductor thin film undergoes photolithography (electron beam lithography or ultraviolet lithography) and etching (reactive ion etching or inductively coupled plasma etching) to form a width of... , length is Ultra-thin semiconductor strips.

[0033] Step 2: Fabricate ohmic contact electrodes at both ends of the semiconductor strip At both ends of the semiconductor strip prepared in step one, a first electrode and a second electrode are fabricated using electron beam evaporation or magnetron sputtering, respectively. When the semiconductor material is P-type single-crystal silicon, a chromium-gold composite layer is preferably used as the electrode material. The initial deposition thickness is... The chromium layer serves as an adhesion layer and forms a good ohmic contact interface, with a redeposition thickness of [missing information]. A gold layer is used as the conductive layer. After fabrication, excess metal is removed by a lift-off process to ensure an ohmic contact between the electrode and the semiconductor strip. The first and second electrodes overlap the two ends of the semiconductor strip, with each overlap area having a length of [length missing]. .

[0034] Step 3: Construct an asymmetric light distribution to generate a temperature difference After electrode fabrication, an asymmetric illumination distribution is constructed on a portion of the semiconductor strip. This step can be implemented flexibly: a focusing lens can be directly used in the optical path system to converge the incident light into a spot that illuminates a localized area of ​​the strip, relying on the gradient of the spot intensity distribution to create a temperature difference; alternatively, a coating process can be used to fabricate a semi-shading structure on a portion of the semiconductor strip, such as spin-coating and curing a polymer material (e.g., polyimide or polymethyl methacrylate), or depositing a metal-dielectric laminate structure (e.g., silicon dioxide / aluminum, silicon dioxide / gold) via electron beam evaporation / thermal evaporation to form a reflectivity greater than... A light-shielding and heat-insulating layer is provided. Through any of the above methods, an asymmetric illumination distribution is formed in the semiconductor strip, thereby creating a temperature difference along its length. This temperature difference will excite the synergistic cascade mechanism of the two-photon absorption effect and the photothermal-electric effect under subsequent illumination, achieving a detection effect that reduces the two-photon absorption excitation threshold under continuous low-power laser light.

[0035] The order of the steps in the above preparation method is not strictly limited. Those skilled in the art can adjust the steps according to actual process needs (e.g., prepare the shielding structure first and then prepare the electrode). The specific implementation of this embodiment is only an illustrative description of the technical solution of the present invention and is not intended to limit the scope of protection of the present invention. Other equivalent modifications made based on the concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for designing a photodetector to reduce the two-photon excitation threshold, characterized in that, include: A semiconductor strip is provided, the semiconductor strip having a nanometer-scale thickness, a micrometer-scale width, and a micrometer-scale length; Ohmic contact electrodes are fabricated at both ends of the semiconductor strip; A non-uniform illumination distribution is formed on the semiconductor strip to generate a temperature difference along the length of the semiconductor strip; By utilizing the nanoscale thickness confinement effect of the semiconductor strip, under incident light irradiation, the semiconductor strip is simultaneously excited by a two-photon absorption effect and a photothermal-electric effect. The two-photon absorption effect generates photogenerated carriers in the semiconductor strip, and the photothermal-electric effect generates a thermoelectric potential in the semiconductor strip. The thermoelectric potential drives the photogenerated carriers to migrate directionally along the length of the semiconductor strip, and the generated Seebeck electric field further modulates and enhances the two-photon absorption, forming a positive feedback cascade mechanism of the two-photon absorption effect and the photothermal-electric effect, thereby reducing the excitation threshold of the two-photon absorption effect.

2. The photodetector design method for reducing the two-photon excitation threshold according to claim 1, characterized in that, The thickness of the semiconductor strip is Width is , length is .

3. The photodetector design method for reducing the two-photon excitation threshold according to claim 1, characterized in that, The semiconductor strip is made of a material with a Seebeck coefficient greater than 1. Semiconductor materials.

4. The photodetector design method for reducing the two-photon excitation threshold according to claim 3, characterized in that, The two-photon absorption coefficient of the semiconductor material is at least greater than 1E-11m / W.

5. The photodetector design method for reducing the two-photon excitation threshold according to claim 1, characterized in that, A method for forming a non-uniform illumination distribution on the semiconductor strip includes: irradiating a local area of ​​the semiconductor strip with incident light through a converging spot; or, providing a semi-shading structure on a local area of ​​the semiconductor strip, the semi-shading structure being composed of a polymer material or a stacked structure of metal and dielectric.

6. A photodetector fabricated based on the design method according to any one of claims 1 to 5, characterized in that, include: Semiconductor strips with nanometer-scale thickness, micrometer-scale width, and micrometer-scale length; Ohmic contact electrodes are respectively disposed at both ends of the semiconductor strip; An asymmetric illumination unit is used to form a non-uniform illumination distribution on the semiconductor strip to generate a temperature difference; Under incident light, the semiconductor strip generates both two-photon absorption and photothermal-electric effects based on its nanoscale thickness limitation effect, forming a positive feedback cascade mechanism of two-photon absorption and photothermal-electric effects.

7. The photodetector according to claim 6, characterized in that, The thickness of the semiconductor strip is Width is , length is .

8. The photodetector according to claim 6, characterized in that, The semiconductor strip is made of a material with a Seebeck coefficient greater than 1. The semiconductor material having a two-photon absorption coefficient of at least 1E-11m / W.

9. The photodetector according to claim 6, characterized in that, The asymmetric illumination unit is a semi-occluded structure, and the reflectivity of the semi-occluded structure is greater than that of the semi-occluded structure. It is composed of a layered structure of metal and dielectric or a polymer material.

10. The photodetector according to claim 6, characterized in that, The fabrication steps of the photodetector include: thinning a semiconductor material to form a thickness of... The semiconductor thin film is photolithographically and etched to form the semiconductor strip; the ohmic contact electrode is prepared by electron beam evaporation or magnetron sputtering; and the asymmetric illumination unit is formed by depositing a metal-dielectric stack or coating a polymer material.

Images