A photoconductive terahertz optical mixer chip, its fabrication method and application

Abstract

The present invention relates to a photoconductive terahertz light mixing chip and a preparation method and application thereof. The photoconductive terahertz light mixing chip comprises a photoconductive active region, wherein the photoconductive active region comprises a substrate, a semiconductor layer, an electrode and a transmission enhancement layer from bottom to top; the electrode is a forked metal grating electrode. The photoconductive terahertz light mixing chip of the present invention maximizes the incident light absorption efficiency and the local field enhancement effect of the semiconductor layer by exciting a high-order coupled plasmon effect. At the same time, the field enhancement effect acts on the forked metal grating electrode, and the electric field under the forked metal grating electrode realizes the effect of light field enhancement, which will generate a higher density of photogenerated carriers under the forked metal grating electrode, and the carriers generated near the forked metal grating electrode are directly transported to the forked metal grating electrode, further enhancing the transport of carriers.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor optoelectronic devices, and more specifically, to a photoconductive terahertz light mixing chip and a preparation method and application thereof. Background Art

[0002] High-speed photodetectors (PDs) are key components of high-speed optoelectronic systems. For example, in wireless communication systems, optical signals are transmitted and amplified over long distances via optical fibers, and then converted into electrical signals of millimeter-wave or terahertz carrier waves by high-speed photodetectors. In radar systems, photodetectors are used for heterodyne frequency conversion, and combined with delay design, radar signals can be flexibly beam-controlled. Optical signals are mixed by high-speed photodetectors, and terahertz signals can be generated based on spectrum-spectrum mapping. At this time, the high-speed photodetector used for optical mixing is also called an optical mixer.

[0003] The high-speed photodetectors reported so far mainly include PIN photodetectors (PIN-PD), single carrier photodetectors (UTC-PD) and avalanche photodetectors (APD). Among them, the high-speed performance of APD is poor, which is due to the long time it takes to establish an avalanche; because the fast response of PIN-PD depends not only on the fast transport of photogenerated electrons, but also on the slower transport of photogenerated holes, the bandwidth is limited by the hole transport and it is difficult to achieve a major breakthrough. For example, in 2020, HHI in Germany prepared a PIN-PD with a bandwidth of 145GHz, which is the best bandwidth value that PIN-PD can achieve at present; and the photodetector based on the single carrier structure (UTC-PD) mainly relies on the fast electron transport process and has the advantage of broadband. For example, in 2000, NTT of Japan obtained a UTC-PD with a bandwidth of 310GHz, which has a greatly improved bandwidth value compared with PIN-PD, but due to the limitation of junction capacitance, the response speed of the photodetector is difficult to improve.

[0004] The photoconductive terahertz optical mixer can respond to and generate terahertz frequency signals due to the picosecond lifetime of the semiconductor material, and has the advantage of ultra-high-speed response. However, the low light absorption efficiency of the semiconductor material and the low photoelectric conversion efficiency are key issues that restrict its application. In addition, the output of the traditional photoconductive mixer mainly utilizes the antenna radiation structure, and the device is usually designed to facilitate the radiation (or collection) of terahertz signals. One is to use a large-area antenna, and the signal is radiated in the direction of the substrate. When the frequency is high, it is easy to produce a surface effect on the surface of the medium, thereby reducing the radiation efficiency. Alternatively, the electrode and load antenna fusion design is adopted, which is conducive to the radiation of high-frequency electromagnetic fields but not conducive to the output of current. However, most of the existing high-frequency devices are waveguide interface outputs. Therefore, the existing photoconductive mixer design is not conducive to the waveguide output of high-frequency current, is not conducive to the transmission of high-frequency current, and is difficult to be compatible with the interface of existing high-frequency equipment, resulting in increased losses.

[0005] In order to improve the light absorption of the photoconductive mixer, there are existing schemes that use nanoscale metal grating electrodes to excite surface plasmons to enhance light localization and absorption. However, the preparation of such photoconductive mixers is difficult: first, the preparation of nanoscale electrodes is difficult, and second, due to the low tolerance of the structural size, a higher preparation accuracy is required, which reduces the application of the surface plasmon effect in photoconductive mixers to a certain extent. In addition, a Chinese patent entitled "A Plasmonic Electrode Photodetector Suitable for Non-Polarized Light" enhances the absorption efficiency of incident light by setting the electrode into a specific circular array metal grating electrode, but it still cannot overcome the problem of low tolerance of the structural size.

[0006] Therefore, it is necessary to solve the problems of low light absorption efficiency of the current photoconductive terahertz optical mixer or the difficulty in preparing components and low structural size tolerance for improving the light absorption of the photoconductive terahertz optical mixer by using nanoscale metal grating electrodes. Summary of the invention

[0007] The primary purpose of the present invention is to overcome the problems of low light absorption efficiency of the above-mentioned existing photoconductive terahertz light mixers or the difficulty in preparing components and low structural size tolerance of improving the light absorption of photoconductive terahertz light mixers by nanoscale metal grating electrodes, and to provide a photoconductive terahertz light mixing chip.

[0008] A further object of the present invention is to provide a method for preparing the above-mentioned photoconductive terahertz light mixing chip.

[0009] A further object of the present invention is to provide an application of the above-mentioned photoconductive terahertz light mixing chip in a wireless communication system or a large bandwidth radar.

[0010] The above-mentioned object of the present invention is achieved by the following technical solutions:

[0011] A photoconductive terahertz light mixing chip comprises a photoconductive active region, wherein the photoconductive active region comprises a substrate, a semiconductor layer, an electrode and a transmission-enhancing layer from bottom to top; the electrode is an interdigitated metal grating electrode;

[0012] A localized surface plasmon resonance mode is formed at the contact interface between the electrode (2) and the semiconductor layer (3);

[0013] Multiple sub-FP-like modes are excited below the electrode (2).

[0014] Preferably, the period of the interdigitated metal grating electrode is 1-6 μm, and the width of a single interdigitated finger is 1000-2000 nm.

[0015] By regulating the width of a single finger of the interdigitated metal grating electrode, it is possible to form a localized surface plasmon resonance mode and excite multi-level FP-like modes.

[0016] It should be understood that the period is equal to the sum of the minimum distance between two adjacent fingers and the width of a single finger.

[0017] Preferably, the height of the interdigitated metal grating electrode is 100-400 nm.

[0018] Preferably, the thickness of the substrate is ≥50 μm.

[0019] Preferably, the material of the substrate is Si, Cu, Ni, GaAs or InP.

[0020] Preferably, the semiconductor layer has a thickness of 1000-2000 nm.

[0021] Preferably, the material of the semiconductor layer is at least one of GaAs, AlAs, InAs, InGaAs, AlGaAs or InGaAsP.

[0022] Preferably, the metal material of the interdigitated metal grating electrode is at least one of Au, Ag, Pt, Ni, Cr, Ti, Ge or Cu.

[0023] Preferably, the thickness of the transmission enhancing layer is 250-650 nm.

[0024] Preferably, the material of the transmission layer is TiO 2 、SiN x 、SiO 2 , ITO or IZO.

[0025] Preferably, the electrode forms an ohmic contact with the semiconductor layer.

[0026] Preferably, the interdigitated metal grating electrodes have a structure in which adjacent metal grating electrodes have opposite polarities and the electrodes are staggered in positive and negative directions, and metal grating electrodes of the same polarity are connected to serve as positive electrodes or negative electrodes.

[0027] Preferably, the interdigitated metal grating electrode is embedded in the semiconductor layer and located on the top of the semiconductor layer.

[0028] Preferably, the photoconductive terahertz light mixing chip further comprises a coplanar waveguide transmission line for transmitting high-frequency current. The transmission line is designed to transmit high-frequency current, which is conducive to application in more high-frequency devices.

[0029] Preferably, the wavelength of the incident light is 1200-1800 nanometers.

[0030] The interlaced positive and negative interdigitated metal grating electrodes can shorten the transport distance of photogenerated carriers from tens of microns to several microns or even hundreds of nanometers, thereby improving the carrier transport efficiency and further improving the responsiveness of the chip.

[0031] The method for preparing the above-mentioned photoconductive terahertz light mixing chip is characterized by comprising the following steps:

[0032] S1. growing a semiconductor epitaxial layer on a substrate;

[0033] S2. Spin-coating a photoresist on the semiconductor epitaxial layer in step S1 and performing photolithography on the photoresist to form a pattern of interdigitated metal grating electrodes on the semiconductor epitaxial layer;

[0034] S3. forming an interdigitated metal grating electrode on the semiconductor epitaxial layer according to the pattern described in step S2, and stripping off the remaining photoresist;

[0035] S4. Etching the semiconductor epitaxial layer to form a semiconductor layer;

[0036] S5. A transmission-enhancing layer is formed on the interdigitated metal grating electrode to obtain the photoconductive terahertz light mixing chip.

[0037] The application of the above-mentioned photoconductive terahertz light mixing chip in the preparation of wireless communication systems or large-bandwidth radars is also within the protection scope of the present invention.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) The photoconductive terahertz light mixing chip of the present invention realizes the excitation of high-order coupled plasmon modes, which are the coupling of the multi-level sub-FP modes (high-order sub-FP modes) below the interdigitated metal grating electrodes and the local surface plasmon resonance formed on the contact interface between the interdigitated metal grating electrodes and the semiconductor layer, thereby maximizing the incident light absorption efficiency and local field enhancement effect of the semiconductor layer. At the same time, the field enhancement effect acts on the interdigitated metal grating electrodes, and the electric field below the interdigitated metal grating electrodes realizes the effect of light field enhancement, which will generate a higher density of photogenerated carriers below the interdigitated metal grating electrodes, and the carriers generated near the interdigitated metal grating electrodes are directly transported to the interdigitated metal grating electrodes, further enhancing the transport of carriers.

[0040] (2) The photoconductive terahertz light mixing chip of the present invention has excellent tolerance in size. When preparing the device, a lithography machine with ordinary lithography resolution can be selected for preparation, thereby improving the success rate and stability of the preparation.

[0041] (3) Compared with traditional PIN and UTC-PD, the photoconductive terahertz light mixing chip of the present invention can achieve a wider bandwidth, usually at the terahertz level. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a schematic diagram of the overall structure of the proposed photoconductive terahertz light mixing chip.

[0043] Figure 2 It is a schematic cross-sectional view of the photoconductive active region of the photoconductive terahertz light mixing chip proposed in Example 1.

[0044] Figure 3 It is a schematic cross-sectional view of a single unit structure in the photoconductive terahertz light mixing chip proposed in Example 1.

[0045] Figure 4 3. It is a top view of the photoconductive terahertz light mixing chip proposed in Example 1.

[0046] Figure 5 It is a unit structure in the photoconductive terahertz optical mixing chip structure proposed in Example 2 and a light power absorption curve of the semiconductor material layer without high-order coupling plasmon effect.

[0047] Figure 6 It is a schematic diagram of light absorption of semiconductor layer materials within a range of 300 nanometers below different metal grating electrode widths at a wavelength of 1550 nanometers in the unit structure of the photoconductive terahertz light mixing chip proposed in Example 2.

[0048] Figure 7 It is a schematic diagram of the electric field enhancement of the unit structure in the photoconductive terahertz light mixing chip proposed in Example 2.

[0049] Figure 8 It is a schematic diagram of absorption of the unit structure in the photoconductive terahertz light mixing chip proposed in Example 2 at different grating heights (a) and different grating widths (b).

[0050] Fig. 9 It is a curve diagram showing the responsivity of the photoconductive terahertz light mixing chip proposed in Example 2 at a voltage of 0 to 0.5V and the responsivity of the fingerless metal grating electrode at a voltage of 0 to 0.5V amplified by 10 times.

[0051] Fig.10 It is a schematic cross-sectional view of the optical conductivity active region of the photoconductive terahertz light mixing chip proposed in Example 3.

[0052] Fig.11 It is a cross-sectional schematic diagram of a single unit structure of the photoconductive terahertz light mixing chip light proposed in Example 3.

[0053] Fig.12 It is a light power absorption curve diagram of the semiconductor material layer of the unit structure of the photoconductive terahertz light mixing chip proposed in Example 4.

[0054] Fig.13 It is a schematic diagram of the electric field enhancement of a unit structure in the photoconductive terahertz light mixing chip structure proposed in Example 4.

[0055] Figure 1 and Fig.10 Middle: 1 is the anti-reflection layer; 2 is the interdigitated metal grating electrode; 3 is the semiconductor layer; 4 is the substrate. DETAILED DESCRIPTION

[0056] In order to more clearly and completely describe the technical solution of the present invention, the present invention is further described in detail through specific embodiments below. It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention. Various changes can be made within the scope of the rights of the present invention.

[0057] Example 1

[0058] This embodiment provides a photoconductive terahertz light mixing chip, such as Figure 1 As shown, the photoconductive terahertz optical mixing chip includes a photoconductive active region and a coplanar waveguide transmission line for transmitting high-frequency current. Figure 2 As shown, the photoconductive active region of the photoconductive terahertz light mixing chip includes, from bottom to top, a substrate 4, a semiconductor layer 3, a forked metal grating electrode 2 and a transmittance-enhancing layer 1. The transmittance-enhancing layer 1 is used to increase the transmission of incident light and reduce reflection. After the incident light is absorbed, photogenerated carriers are generated in the semiconductor layer 3. Under the action of an external bias voltage, the carriers drift to generate transient photocurrents, which are finally collected by the forked metal grating electrode 2.

[0059] like Figure 3 As shown, in a single unit structure of the photoconductive terahertz light mixing chip, the thickness of the substrate 2 is a, the thickness of the semiconductor layer 3 is b, the thickness of the transmission layer 1 is c, the period of the interdigitated metal grating electrode 2 is P, the width of a single electrode (grating width) is W, and the height (grating height) is H. Optionally, the thickness a of the substrate 2 is usually greater than 50 microns, the thickness b of the semiconductor layer 3 is 1000-2000 nanometers, the thickness c of the transmission layer 1 is 250-650nm, the period P of the interdigitated metal grating electrode 2 is 1-6μm, the width W of a single electrode is 1000-2000nm, and the height H is 100-400nm.

[0060] Figure 4 As shown, the electrodes of the photoconductive terahertz light mixing chip are interdigitated metal grating electrodes.

[0061] Example 2

[0062] This embodiment provides a photoconductive terahertz light mixing chip. Based on the embodiment 1, the semiconductor material used for the semiconductor layer 3 is InGaAs, the material used for the interdigitated metal grating electrode 2 is Au, the material used for the substrate 4 is InP, and the material used for the anti-reflection layer 1 is Si. 3 N 4 Among the specification parameters, P = 3000 nm, b = 1000 nm, c = 490 nm, H = 230 nm, W = 1000 nm. When simulating parameter P, the test wavelength selects lambda = 1200 ~ 2000 nm band.

[0063] Figure 5 1 is a light power absorption curve of the semiconductor material layer of the unit structure of the photoconductive terahertz light mixing chip proposed in this embodiment. After determining the size of the unit structure and the incident light wavelength of 1400-1800nm, the light absorption of a unit structure of the photoconductive terahertz light mixer structure is simulated. The light absorption of the semiconductor layer is as follows: Figure 5 As shown in the figure, it can be seen that near the 1550 nanometer band, the light absorption of the semiconductor material layer reaches about 41%. Compared with the absorption of the semiconductor material without the high-order coupled plasmon effect (removing the interdigitated metal grating electrode), its absorption efficiency is increased by about 78%, which can greatly improve the device conversion efficiency.

[0064] Figure 6 Schematic diagram of light absorption of semiconductor layer materials within 300 nanometers below different metal grating electrode widths at a wavelength of 1550 nanometers in the unit structure of the photoconductive terahertz light mixing chip proposed in Example 2. Figure 6 It can be seen that the light absorption intensity varies with the metal width, showing a FP-like cavity effect, which proves the existence of FP-like modes.

[0065] Figure 7It is a schematic diagram of the section electric field enhancement of the unit structure of the photoconductive terahertz light mixing chip proposed in this embodiment. In the structure, the excitation of the high-order coupled plasmon mode is realized, and the mode is the coupling of the local surface plasma resonance and the high-order FP-like mode on the contact interface of the metal grating and the semiconductor layer. According to the plasmon theory, the plasmon mode is a near-field electromagnetic wave propagating along the metal surface when the light wave is incident on the interface between the metal and the dielectric, and the free electrons on the metal surface oscillate collectively, and the electromagnetic wave is coupled with the free electrons on the metal surface to form a near-field electromagnetic wave propagating along the metal surface. If the periodic metal grating makes the oscillation frequency of the electrons match the frequency of the incident light wave at this time, resonance will be generated. In the resonant state, the energy of the electromagnetic field is effectively converted into the collective vibration energy of the free electrons on the metal surface. At this time, the electromagnetic field is confined to a very small range on the metal surface and is enhanced. It can be seen from the figure that there is an obvious local electric field enhancement under the interdigitated metal grating electrode, and 4 FP-like modes can be observed under the electrode, realizing the excitation of high-order coupled plasmons, and then realizing the enhancement of the local electric field under the electrode.

[0066] Figure 8 This is a schematic diagram of the absorption of the unit structure in the photoconductive terahertz light mixing chip proposed in this embodiment at different grating heights and different widths. It can be seen from the figure that even within a larger dimensional error range, the light absorption efficiency can still be maintained at more than 80% of the peak value, which further proves the robustness and reliability of the structure.

[0067] Fig. 9 The graph is a graph of the experimentally measured responsivity of the photoconductive terahertz light mixing chip proposed in this embodiment at a voltage of 0 to 0.5V and a 10-fold magnification of the responsivity of the non-interdigitated metal grating electrode at a voltage of 0 to 0.5V. In the entire voltage test range, the responsivity of the interdigitated metal grating electrode structure is 37 to 58 times higher than that of the non-interdigitated metal grating electrode structure. This further proves that the interdigitated metal grating electrode of this embodiment effectively shortens the distance between the carriers and improves the transport efficiency of the carriers, thereby improving the responsivity of the chip.

[0068] Example 3

[0069] This embodiment provides a photoconductive terahertz light mixing chip, which is different from the embodiment 1 in that: Fig.10 As shown, the interdigitated metal grating electrode 2 is embedded in the semiconductor layer 3 and located on the top of the semiconductor layer 3 . Fig.11 It is a schematic cross-sectional view of a single unit structure of the photoconductive terahertz light mixing chip of this embodiment.

[0070] Example 4

[0071] This embodiment provides a photoconductive terahertz light mixing chip. Based on the embodiment 3, the semiconductor material used in the semiconductor layer 3 is InGaAs, the material used in the interdigitated metal grating electrode 2 is Au, the material used in the substrate 4 is InP, and the material used in the anti-reflection layer 1 is Si. 3 N 4 Among the specification parameters, P = 1520 nm, b = 1000 nm, c = 180 nm, H = 200 nm, W = 1000 nm. When simulating parameter P, the test wavelength selects lambda = 1200 ~ 1800 nm band.

[0072] The contact surface between the embedded interdigitated metal grating electrode and the semiconductor layer of the photoconductive terahertz light mixing chip of this embodiment has a stronger localized plasma resonance mode, and has a more obvious local field enhancement region inside the semiconductor compared to Example 2, and also realizes a high-order coupled plasmon mode similar to Example 2, which is the coupling of the localized surface plasma resonance on the interface between the metal grating and the semiconductor layer and the high-order FP mode.

[0073] Fig.12 1 is a light power absorption curve of the semiconductor material layer of the unit structure of the photoconductive terahertz light mixing chip of this embodiment. After determining the size of the unit structure and the incident light wavelength of 1400-1800nm, the light absorption of a unit structure of the photoconductive terahertz light mixing chip is simulated. Fig.12 As shown, it can be seen from the figure that near the 1550 nanometer band, the light absorption of the semiconductor material layer reaches about 45%, which can greatly improve the device conversion efficiency.

[0074] Fig.13 Schematic diagram of electric field enhancement of a cross section of a unit structure of a photoconductive terahertz light mixing chip of this embodiment. It can be seen from the figure that there is an obvious electric field enhancement under the interdigitated metal grating electrode.

[0075] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A photoconductive terahertz light mixing chip, characterized in that: The photoconductive active region comprises a substrate (4), a semiconductor layer (3), an electrode (2) and a transmission-enhancing layer (1) from bottom to top; the electrode (2) is an interdigitated metal grating electrode; A localized surface plasmon resonance mode is formed at the contact interface between the electrode (2) and the semiconductor layer (3); The bottom of the electrode (2) excites multiple sub-FP-like modes; The localized surface plasmon resonance mode and the multi-level FP-like mode are coupled to achieve excitation of a high-order coupled plasmon mode; The thickness of the substrate (4) is greater than 50 μm, and the thickness of the semiconductor layer (3) is 1000-2000 nm; The width of a single interdigital finger of the interdigital metal grating electrode is 1000-2000 nm, the height is 100-400 nm, and the width of one period of the interdigital metal grating electrode is 1-6 μm; The semiconductor layer (3) is suitable for a photoconductive terahertz light mixing chip.

2. The photoconductive terahertz light mixing chip according to claim 1, characterized in that: The material of the semiconductor layer (3) is at least one of GaAs, AlAs, InAs, InGaAs, AlGaAs or InGaAsP.

3. The photoconductive terahertz light mixing chip according to claim 1, characterized in that: The metal material of the interdigitated metal grating electrode is at least one of Au, Ag, Pt, Ni, Cr, Ti, Ge or Cu.

4. The photoconductive terahertz light mixing chip according to claim 1, characterized in that: The material of the transmission layer (1) is at least one of TiO2, Si3N4, SiO2, ITO or IZO.

5. The photoconductive terahertz light mixing chip according to claim 1, characterized in that: The electrode (2) forms an ohmic contact with the semiconductor layer (3).

6. The photoconductive terahertz light mixing chip according to claim 1, characterized in that: The interdigitated metal grating electrode is embedded in the semiconductor layer (3) and is located on the top of the semiconductor layer (3).

7. The photoconductive terahertz light mixing chip according to claim 1, characterized in that: The photoconductive terahertz optical mixing chip also includes a coplanar waveguide transmission line for transmitting high-frequency current.

8. The method for preparing the photoconductive terahertz light mixing chip according to any one of claims 1 to 7, characterized in that: The steps include: S1. growing a semiconductor epitaxial layer on a substrate; S2. Spin-coating a photoresist on the semiconductor epitaxial layer in step S1 and performing photolithography on the photoresist to form a pattern of interdigitated metal grating electrodes on the semiconductor epitaxial layer; S3. Forming an interdigitated metal grating electrode on the semiconductor epitaxial layer according to the pattern in step S2, and stripping the remaining photoresist; S4. Etching the semiconductor epitaxial layer to form a semiconductor layer; S5. A transmission-enhancing layer is formed on the interdigitated metal grating electrode to obtain the photoconductive terahertz light mixing chip.

9. Application of the photoconductive terahertz light mixing chip according to any one of claims 1 to 7 in a wireless communication system.

10. The use according to claim 9, characterized in that: The wireless communication system is a large bandwidth radar.

Images