WO2015188736A1 - 一种基于低维电子等离子体波的太赫兹调制器及其制造方法 - Google Patents

一种基于低维电子等离子体波的太赫兹调制器及其制造方法 Download PDF

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WO2015188736A1
WO2015188736A1 PCT/CN2015/081026 CN2015081026W WO2015188736A1 WO 2015188736 A1 WO2015188736 A1 WO 2015188736A1 CN 2015081026 W CN2015081026 W CN 2015081026W WO 2015188736 A1 WO2015188736 A1 WO 2015188736A1
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terahertz
grating
cavity
plasmon
dimensional electron
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PCT/CN2015/081026
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English (en)
French (fr)
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黄永丹
秦华
张志鹏
余耀
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中国科学院苏州纳米技术与纳米仿生研究所
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Priority to JP2017516023A priority Critical patent/JP6431978B2/ja
Priority to US15/317,669 priority patent/US9927675B2/en
Publication of WO2015188736A1 publication Critical patent/WO2015188736A1/zh

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Definitions

  • the present invention relates to a terahertz modulator and a method of fabricating the same, and more particularly to a terahertz modulator capable of realizing a high-speed large modulation depth based on a low-dimensional electron plasma wave and a method of fabricating the same.
  • the invention also relates to a modulation method based on the terahertz modulator.
  • Terahertz waves have very important applications in electronics, communications, life sciences, defense, aerospace and medical.
  • Terahertz functional devices occupy a central position in terahertz systems.
  • High-performance modulators play an important role in terahertz high-speed imaging and communication systems.
  • the existing technical scheme of the solid-state terahertz modulator is mainly based on the non-resonant absorption mechanism of the terahertz wave caused by the change of the Drude conductivity of the single electron behavior.
  • modulators based on semiconductor two-dimensional electron gas, semiconductor composite metamaterials and graphene.
  • Two-dimensional electron gas modulator The conductivity of the two-dimensional electron gas is controlled by the gate to change its conductivity, thereby changing the transmission intensity of the incident terahertz radiation.
  • the method can work at room temperature, but its maximum modulation depth is only 3%, which is far from the actual application.
  • This type of modulator does not utilize plasma wave characteristics.
  • the principle is that the transmission intensity of a terahertz wave is related to the electrical conductivity of a two-dimensional electron gas.
  • a metamaterial is an artificial medium having an electromagnetic resonance response composed of structural units ("atoms") that are smaller than the wavelength of the excited electromagnetic wave.
  • the fabrication of semiconductor composite metamaterials can be adjusted in geometric design and metamaterial electromagnetic structure parameters to change its resonance characteristics. This principle can be used to achieve effective regulation of terahertz radiation.
  • a metamaterial is formed on the doped semiconductor epitaxial layer to form a Schottky diode structure, and the resonance intensity is changed by a carrier concentration of a semiconductor substrate layer near a slit of a voltage-modulating Split-Ring Resonator (SRR) structural unit. Therefore, the terahertz wave transmission intensity at the resonance frequency can be achieved by electrical modulation.
  • SRR voltage-modulating Split-Ring Resonator
  • the method can achieve terahertz modulation with a modulation depth of 50% and a modulation speed of 2 MHz at room temperature.
  • Another composite metamaterial structure integrates a high electron mobility transistor (HEMT) at the slit of the SRR, and changes the electron density of the channel by the gate to change the capacitance of the SRR to adjust the SRR resonance intensity.
  • the device has a modulation depth of up to 33% and a maximum modulation speed of 10MHz.
  • the patented technology of the existing modulator is mainly realized by using metamaterials. For example, in 2009, Houtong Chen et al. applied for a US invention patent called “active terahertz metamaterial device”.
  • Graphene Modulator In the terahertz band, the in-band transition of electrons in graphene plays a major role.
  • the large-area single-layer graphene can achieve 15% intensity modulation depth at room temperature and a modulation frequency of 20 kHz.
  • the modulation depth of the prior art is generally not high, and the highest can only reach 50%, indicating that the energy loss mode of these mechanisms is not very effective; in addition, high-speed modulation is one of the most important performance indicators of the modulator, and the prior art
  • the modulation speed is not high (the maximum modulation speed is 10MHz).
  • the carrier layer of two-dimensional electron gas and graphene is very thin, the interaction time between electromagnetic wave and carrier is short, and the modulation efficiency is not high if strong coupling is not achieved;
  • Drude The dissipation of the conductivity model is caused by external free carriers (phonons, impurities, defects, etc.).
  • the electromagnetic wave is coupled with a single particle, so the loss mechanism of the terahertz wave is not very efficient;
  • the modulation speed of the large-area device is limited by the parasitic capacitance and resistance of the device, such as the consumption of the semiconductor composite metamaterial plus the gate voltage. Make up the capacitance and resistance.
  • the technical problem to be solved by the present invention is to provide a terahertz modulator capable of realizing high speed and large modulation depth, which utilizes the plasmon polariton formed by strong coupling of the terahertz wave cavity mode and the plasma wave mode to achieve Efficient modulation of Hertz waves.
  • a possible solution is provided for the modulators necessary for terahertz high-speed imaging and high-speed communication.
  • the terahertz modulator of the present invention includes a plasmon and a resonant cavity; wherein
  • the plasmons include:
  • a high electron mobility transistor of GaN/AlGaN comprising: a source, a drain, and a grating gate;
  • the resonant cavity includes a surface of the grating in contact with the sample to a dielectric resonator formed by the lower surface of the thinned sapphire substrate.
  • a terahertz modulator as described above, wherein the resonant cavity may be a Fabry-Pérot (abbreviated as F-P) resonator formed by thinning the interface of the sample itself or a symmetric cavity structure composed of the same dielectric layer.
  • F-P Fabry-Pérot
  • the invention also proposes a high-speed modulation method based on a high electron mobility transistor, comprising:
  • the transistor has a grating gate, and the grating gate strongly couples the two-dimensional plasma wave and the terahertz resonant cavity mode in the two-dimensional electron gas to form a plasmon polariton;
  • the invention also proposes a manufacturing method of a terahertz modulator, comprising the following steps:
  • the back side of the sapphire substrate is thinned and polished to form a resonant cavity structure.
  • the invention also discloses a manufacturing method of a terahertz modulator with a symmetric dielectric cavity, comprising the steps of manufacturing a dielectric resonator and bonding the dielectric resonator to a two-dimensional electronic gas chip, wherein the manufacturing medium
  • the step of the resonant cavity includes: thinning and polishing the cavity plate material; performing pattern transfer of the wafer bonding region on the cavity plate material; and transferring the flip chip pattern in the bonded metal region.
  • the invention has remarkable characteristics. Since the plasma wave in the two-dimensional electron gas is a collective oscillation of electrons, the inherently strong bound electric field can achieve strong coupling with the terahertz wave, so the modulation efficiency of the terahertz wave is high. .
  • the loss of the collective oscillation mode of the present invention is more effective than the loss mechanism of single electron behavior, which is advantageous for generating a large modulation depth.
  • the plasma wave of two-dimensional electron gas is integrated into the terahertz resonator to achieve strong coupling, and the regulation of the plasmon resonance mode is adopted to achieve efficient control of the resonance mode of the cavity.
  • the invention utilizes the plasmon polariton formed by the strong coupling of the terahertz wave cavity mode and the plasma wave mode to realize efficient modulation of the terahertz wave. Modulation at high speed (about 1 GHz) and large modulation depth (70% or more) allows a possible solution for terahertz high-speed imaging and high-speed communication.
  • FIG. 1 is a schematic view showing the structure of a terahertz modulator according to the present invention
  • FIG. 2 is a schematic diagram of a grating gate in a terahertz modulator of the present invention, wherein FIG. 2a is a one-dimensional grating gate, and FIG. 2b is a interdigital grating gate;
  • FIG. 3 is a schematic structural view of a symmetric resonant cavity in a terahertz modulator according to the present invention.
  • FIG. 4 is a schematic diagram of biasing of electrodes of the terahertz modulator in an operating state according to the present invention
  • Figure 5 is a dispersion curve of plasmon and free space terahertz light
  • Figure 6 is a graph showing the two branches ⁇ + and ⁇ - curves of the plasmon excited with the cavity mode to form a plasmon polariton;
  • Figure 7 is a calculated eigenmode of the plasmon polaritons
  • Figure 9 is a flow chart showing the fabrication of the terahertz modulator of the present invention.
  • Figure 10 is a detailed flow chart of a fabrication process of a basic terahertz modulator structure.
  • Terahertz modulator A device or device capable of effectively regulating the electromagnetic characteristics (electric field strength or phase) of a terahertz wave.
  • Terahertz cavity refers to an electromagnetic medium structure capable of supporting a specific terahertz wave standing wave mode, including a metal waveguide cavity and a dielectric waveguide cavity.
  • the Fabry resonator is a metal waveguide resonator or a dielectric waveguide resonator filled with a low-loss dielectric material or vacuum (air).
  • the cavity surface can be a metal coating or a medium-air interface with high reflectivity.
  • Two-dimensional electron gas (2DEG) The movement of a certain spatial dimension is limited, while in the other two dimensions, it is free to move with a certain concentration of electronic system.
  • the solid is generally a quasi-two-dimensional electron layer formed on the side of the narrow band gap semiconductor at the semiconductor heterojunction, such as the GaAs on the GaAs side of the GaAs/AlGaAs heterojunction interface and the GaN at the GaN/AlGaN heterojunction interface.
  • Two-dimensional electron gas on one side. Since the electrons in the two-dimensional electron gas in the heterojunction can be spatially separated from the doping impurities, the two-dimensional electron gas has a higher mobility than the carriers in the corresponding semiconductor body material.
  • Plasma wave refers to the density fluctuation of the same kind of polar charge in the opposite polarity charge background, has the characteristics of wave, is a collective excitation of charge.
  • the quasiparticle of the plasma wave oscillation is a plasmon, which is actually a charge density vibration.
  • the swaying element is excited.
  • it refers to a charge density oscillating wave in a two-dimensional electron gas of a semiconductor, which is called a two-dimensional plasma wave or a two-dimensional plasmon.
  • Ionic Polarization A coherent mixed state formed by strong coupling of cavity photons and matter elements.
  • it refers to a mixed state formed by the strong coupling of a two-dimensional electron gas medium and a terahertz resonant cavity mode, which is the eigenstate of the strongly coupled system. It is characterized in that an anti-crossing feature occurs when the plasmon resonates with the cavity mode.
  • High Electron Mobility Transistor A field effect transistor fabricated using a semiconductor heterojunction material. Since electrons are separated from the doped regions, electrons in the channel of the transistor have high mobility, so Transistors are called high electron mobility transistors.
  • the invention provides a terahertz modulator whose basic principle is based on a resonance absorption mechanism caused by electron collective oscillation (Plasmon, ie, plasma wave), in order to enhance the coupling of terahertz waves and plasmons.
  • a HEMT structure with a grating-coupled two-dimensional electron gas is integrated in a terahertz resonator, thereby proposing a terahertz modulator.
  • the terahertz modulator of the invention utilizes a strong coupling effect to minimize the transmission coefficient of the cavity mode at the resonance point.
  • the oscillation frequency of the plasmon is adjusted so that the system switches between the resonance point and the non-resonance point, the resonance mode of the cavity can be realized. Effective modulation.
  • Figure 1 shows the structure of the most basic terahertz modulator device, which mainly includes two units of plasmon and resonator.
  • the plasmons include:
  • GaN/AlGaN HEMT structure with grating gate including source 1, drain 2, grating gate 3;
  • a quasi two-dimensional electron layer formed on one side of the narrow band gap semiconductor at the semiconductor heterojunction that is, a two-dimensional electron gas 4, which is a carrier of the plasma wave 6, or it is used to excite the plasma wave;
  • the resonant cavity is a dielectric resonator 5 formed of a thinned sapphire dielectric material.
  • the present invention uses a sapphire material as a cavity dielectric material after being thinned.
  • the resonant cavity should include a dielectric resonator formed by the surface of the grating in contact with the sample to the lower surface of the substrate.
  • a dielectric resonator formed by the surface of the grating in contact with the sample to the lower surface of the substrate.
  • the two-dimensional electron gas 4 there is a very thin layer of gate barrier layer, which is also included in the resonance.
  • the layer is very thin and the refractive index is very close to the substrate, so it can be collectively referred to as a dielectric resonator.
  • the source 1 and the drain 2 can be realized by a conventional ohmic contact process, so that the contact resistance is as small as possible.
  • the two-dimensional electron gas formed on the GaN side of the GaAs side at the GaAs/AlGaAs heterojunction interface and the GaN side of the GaN/AlGaN heterojunction interface there are two main parameters for the two-dimensional electron gas formed on the GaN side of the GaAs side at the GaAs/AlGaAs heterojunction interface and the GaN side of the GaN/AlGaN heterojunction interface.
  • high electron mobility the higher the mobility, the higher the quality factor of the plasma wave, and the stronger coupling can be achieved.
  • the room temperature mobility is on the order of 2000 cm 2 /Vs.
  • the highest temperature of the plasmon resonance absorption observed in the experiment is close to 200K.
  • the formation of effective plasmon oscillations requires electron mobility of the order of 10000 cm 2 /Vs and above.
  • the second parameter is the concentration of high two-dimensional electron gas. The higher the concentration, the better the quality factor is obtained, and the higher the oscillation frequency under other conditions,
  • Two-dimensional electron gas materials include: GaAs/AlGaAs, InGaAs/AlGaAs, graphene, MoS 2 and the like.
  • the cavity mode electric field 7 is coupled to the bound electric field of the plasma wave 6 via the near field enhancement of the grating to achieve strong coupling.
  • the two-dimensional electron gas of the non-gate control region in the structure shown in FIG. 1 may be etched away, for example, by Inductive Coupled Plasma (ICP) etching.
  • ICP Inductive Coupled Plasma
  • Reference numeral 8 in Fig. 1 denotes an incident terahertz wave, and reference numeral 9 passes through a transmission terahertz wave after the terahertz modulator.
  • Figure 2 shows two embodiments of a grating gate in a terahertz modulator of the present invention.
  • Figure 2a shows a one-dimensional grating in which the source of the device is S, the drain is D, and the gate is represented by G.
  • the gate length is W
  • L is the grating period, which is equal to the sum of the gate length and the gate pitch.
  • Fig. 2b is an interpolated grating in which the grating G1 has a gate length of W1, the grating G2 has a gate length of W2, and the grating pitch is S.
  • the grating acts as a coupling.
  • the period must be much smaller than the terahertz wavelength (grating period L ⁇ ⁇ THz / 100).
  • the loss of the grating in the terahertz band is small, and the metal of the grating is required.
  • the thickness must be greater than the skin depth ( ⁇ 120 nm).
  • the role of the grating gate is to regulate the plasma wave by adjusting the grating gate voltage.
  • the grating grid also includes a planar two-dimensional grid grating, a concentric circular grating, a terahertz metamaterial, and a periodic structure or pattern of a two-dimensional electronic gas table.
  • the resonator structure shown in Figure 1 uses the simplest substrate and belongs to the basic planar resonator. Therefore, the present invention also proposes a symmetric cavity structure composed of a substrate and another piece of the same medium of the same thickness as shown in FIG.
  • the dielectric material is selected based on the absorption of terahertz waves as small as possible, and also the requirement for high electron mobility two-dimensional electron gas material heterojunction growth.
  • the dielectric material is selected based on the absorption of terahertz waves as small as possible, and also the requirement for high electron mobility two-dimensional electron gas material heterojunction growth.
  • sapphire substrates have low absorption of terahertz waves and high mechanical strength, which is more advantageous for processing into smaller-sized resonators.
  • the size of the resonant cavity is typically required to be on the order of the wavelength of the band of interest, primarily to facilitate the formation of discrete cavity modes.
  • Fabry-Pérot plate cavity may also be a cavity that is coplanar with the two-dimensional electron gas, such as a Co-planar Waveguide (CPW), a slot antenna (Slot Antenna). ) and Terahertz Metamaterial Resonator.
  • CPW Co-planar Waveguide
  • Slot Antenna slot antenna
  • Terahertz Metamaterial Resonator Terahertz Metamaterial Resonator
  • the quasi two-dimensional electron layer is an electron in a two-dimensional electron channel of the semiconductor, and the resonance absorption of the terahertz wave is realized by the collective oscillation of the electron, and the absorption is further enhanced by strong coupling with the cavity mode.
  • the resonance absorption of terahertz waves is realized by the collective oscillation of electrons in the two-dimensional electron channel, and the absorption is further enhanced by strong coupling with the cavity mode. This physical mechanism is the essence of the present invention.
  • 4 to 8 are for explaining in detail the high-speed modulation method based on the high electron mobility transistor HEMT structure proposed by the present invention.
  • FIG. 4 is a schematic diagram of voltage biasing during operation of the terahertz modulator of the present invention. As shown in FIG. 4, it is a GaN/AlGaN High Electron Mobility Transistor (HEMT) structure having a grating gate. .
  • HEMT High Electron Mobility Transistor
  • the modulation principle of the terahertz modulator of the present invention is that the grating strongly couples the plasma wave in the two-dimensional electron gas with the terahertz resonant cavity mode, thereby forming a plasmon polariton.
  • the grating gate voltage regulates the resonance conditions of the plasmon and the cavity mode, so that the system switches between the resonance point and the non-resonance point to achieve the purpose of regulating the transmission of the terahertz cavity mode.
  • the source and drain are grounded, providing a negative DC gate voltage V G and an AC modulated signal to the gate (amplitude ),As shown in Figure 4.
  • the electron concentration is controlled by the gate voltage, and the relationship between the electron concentration and the gate voltage obtained by the gradient channel approximation of the physical model of the device is (where Vth is the threshold voltage at which the electron concentration is depleted, C is the unit capacitance of the gate, ⁇ 0 is the vacuum dielectric constant, ⁇ AlGaN is the dielectric constant of the gate barrier layer, and d is the thickness of the barrier layer , that is, the distance of the two-dimensional electron from the surface of the sample).
  • the alternating modulation signal causes the resonant frequency of the plasmon to constantly change.
  • the dispersion relation curve of the terahertz wave of the plasmon and the free space is shown in Fig. 5.
  • the broken line indicates resonance of both, that is, resonance absorption of plasmons occurs.
  • Fabry-Pérot (FP) resonant cavity mode and refractive index of cavity medium Related to the thickness D, the resonance frequency is
  • the gate voltage When the gate voltage is regulated, the electron concentration of the two-dimensional electron gas is substantially regulated, so that the gate-controlled plasmon resonates with the cavity mode, and the cavity mode which is the transmission peak is plasmon resonance. Absorbed. Therefore, when the gate voltage is switched between the resonant and non-resonant points, effective modulation of the cavity mode is achieved. This is the basic working principle of the terahertz modulator of the present invention.
  • the low temperature time domain transmission spectra of the device demonstrate its great potential as a terahertz modulator.
  • the bias diagram of the device during operation At the resonance point, the transmission of the cavity mode is minimal, and when the applied modulation signal adjusts the gate voltage to the non-resonant point, the transmission of the cavity mode is maximized.
  • the figure shows the DC operating voltage of the device and the best applied modulated high frequency signal.
  • Fig. 8 is a graph showing the transmission coefficient of the cavity mode as a function of the gate voltage when the frequency is 1.07 THz in the experimental results.
  • Figure 8 shows the device DC operating gate voltage and the applied high-frequency modulation signal.
  • the transmittance has a good linearity with the gate voltage change, that is, the transmittance varies with the gate voltage.
  • the change is linear.
  • the present invention also provides a method of fabricating a terahertz modulator, including the following steps:
  • the back side of the sapphire substrate is thinned and polished to form a resonant cavity structure.
  • a two-dimensional electronic gas table adopting ultraviolet exposure technology (referred to as photolithography) to realize pattern transfer of two-dimensional electron gas table surface, and then etching two-dimensional electron gas material by inductively coupled plasma (ICP) to form a device Source area countertop.
  • photolithography ultraviolet exposure technology
  • ICP inductively coupled plasma
  • ohmic contact photolithography, using an electron beam evaporation process, vapor deposition of a ohmic contact multilayer metal structure on a two-dimensional electron gas table surface, after stripping, forming an ohmic contact metal pattern.
  • Ti/Al/Ni/Au (20/120/70/100 nm) is generally used.
  • annealing was performed at 900 ° C for 30 seconds in a nitrogen atmosphere in a rapid annealing furnace to form a source-drain ohmic contact.
  • grating gate photolithography, realize the pattern transfer of the grating structure, evaporate Ti/Au or Ni/Au (20/100nm) by electron beam evaporation, and form a metal grating gate structure after stripping.
  • lead electrode photolithography, lead electrode of grating gate, ohmic contact lead electrode and pattern transfer for wafer bonding.
  • Ti/Au or Ni/Au (20/300 nm) was evaporated by electron beam evaporation, and after peeling, a corresponding electrode structure was formed.
  • the substrate cavity is polished by a thinning machine and a chemical machine, and thinned and polished from the back side of the sample to form a cavity structure, and the thickness after thinning is generally between 100 and 200 microns;
  • the lobes form a single grating-coupled two-dimensional electron gas chip: using laser dicing and manual dissociation, the 1.5 cm small square piece is divided into independent 2D electron gas prototype chips of about 6 ⁇ 6 mm 2 size.
  • FIG. 3 Another specific structure besides the most basic substrate resonator structure is shown in Figure 3. It is a terahertz modulator with a symmetric dielectric cavity, two symmetric cavities. Sharing a plasmon, that is, a two-dimensional electronic gas chip finally produced in the flow of FIG. 10, symmetrically disposed with another resonant cavity 10, which is "shared" with the resonant cavity of the two-dimensional electronic gas chip. A plasmon. Accordingly, the present invention also provides an embodiment of a method of fabricating a terahertz modulator of such a different resonant cavity.
  • the manufacturing method for a terahertz modulator with a symmetric dielectric cavity is as follows:
  • a dielectric resonator is added, thereby forming a symmetric dielectric cavity.
  • the manufacturing method includes manufacturing the dielectric cavity 10 and the two-dimensional cavity
  • the step of bonding the electronic gas chips together, the dielectric resonator described herein refers to the dielectric resonator 10 symmetrically disposed with the two-dimensional electronic gas chip, and the steps of manufacturing the dielectric resonator are briefly described as follows:
  • the dielectric resonator prepared through the above steps and the two-dimensional electronic gas chip finally obtained through the steps of FIG. 10 are bonded together to form a terahertz with a symmetric dielectric cavity. Modulator.
  • the step of pattern transfer of the wafer bonding region on the cavity plate material is carried out by evaporating Ti/Au or Ni/Au by electron beam evaporation and peeling off the metal regions forming the wafer bonding.
  • the step of transferring the flip chip pattern in the bonded metal region is performed by vapor deposition of indium by thermal evaporation and stripping to obtain an indium column required for flip chip bonding.
  • the step of bonding the two-dimensional electronic gas chip and the dielectric resonator together can be realized by flip chip bonding technology, that is, the positional relationship between the two is aligned as shown in FIG. 3, and then pressure is applied after heating to pass the two.
  • the gold-indium solid solution is combined and finally reflowed in the reflow oven to make the bond stronger.
  • the two-dimensional electron gas in the GaN/AlGaN heterojunction has a high electron concentration, and can cover a wider terahertz band through gate regulation;
  • the device works for narrowband, it can work at multiple frequency points, and the working frequency and working voltage can be designed according to theory and requirements, so the flexibility of the device is high;
  • the sapphire substrate itself has little absorption of terahertz waves, so it is less lossy than doped semiconductors;
  • the sapphire substrate is very strong, so the device can be thinned to between 100 and 200 microns without damage and sufficient mechanical strength.
  • the present invention is based on a resonance absorption mechanism caused by electron collective oscillations (plasma waves, ie, plasmons, Plasmon).
  • a HEMT High Electron Mobility Transistor
  • the strong coupling effect makes the transmission coefficient of the cavity mode minimum at the resonance point, and the resonance condition of the plasmon and the cavity mode is controlled by the change of the grating gate voltage to achieve the purpose of regulating the transmission of the terahertz wave.
  • the effective modulation of the resonant mode of the resonant cavity is achieved when the oscillating frequency of the plasmon is adjusted such that the system switches between the non-resonant point and the resonant point.
  • the invention introduces the physical mechanism and implementation technology of the terahertz modulator in detail, and provides a possible solution for related applications.
  • the present invention has outstanding substantial features and significant advances due to the strong coupling of "light” (terahertz waves) and "substance” (plasmon).

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Abstract

一种基于低维电子等离子体波的太赫兹调制器和制造方法以及一种高速调制方法。太赫兹调制器包括等离激元和谐振腔（5）。原理基于电子集体振荡引起的共振吸收机制。为了增强太赫兹波和等离激元的耦合强度，将具有光栅栅极（3）的GaN/AlGaN高电子迁移率晶体管结构集成在一个太赫兹法布里-玻罗谐振腔里。等离激元与谐振腔模式的强耦合形成了等离极化激元。强耦合作用使得谐振腔模式的透射系数在共振点最小。通过光栅栅压的改变，调控等离激元和腔模的共振条件，实现对太赫兹波的高效、高速调制。

Description

一种基于低维电子等离子体波的太赫兹调制器及其制造方法 技术领域
本发明涉及一种太赫兹调制器及其制造方法，特别是涉及一种基于低维电子等离子体波的能实现高速的大调制深度的太赫兹调制器及其制造方法。本发明还涉及基于所述太赫兹调制器的调制方法。
背景技术
太赫兹波在电子、通信、生命科学、国防、航天和医疗等方面具有非常重要的应用。太赫兹功能器件在太赫兹系统中占据核心地位。在太赫兹高速成像和通信系统中，高性能调制器占据重要地位。目前国际上还没有研制成电驱动的高调制速度和大调制深度的太赫兹调制器，故阻碍了其发展和应用。
现有的固态太赫兹调制器的技术方案主要是基于单电子行为的Drude电导率变化引起的太赫兹波非共振吸收机制。按实现的材料分，主要有基于半导体二维电子气、半导体复合超材料和石墨烯等材料的调制器。
二维电子气调制器：通过栅极电控制二维电子气的浓度来改变其电导率，从而改变入射太赫兹辐射的透射强度。该方法可以在室温下工作，但是其最大调制深度只有3％，距离实际推广应用还有很大差距。该类调制器没有利用等离子体波特性。原理是太赫兹波的透射强度与二维电子气的电导率有关。
半导体复合超材料调制器：超材料是一种由小于激励电磁波波长的结构单元(“atoms”)构成的、具有电磁共振响应的人工介质。制作半导体复合超材料，可以在几何设计和超材料电磁结构参数上进行调节从而改变其共振特性，利用该原理可以实现对太赫兹辐射的有效调控。在掺杂半导体外延层上制作超材料形成肖特基二极管结构，通过电压调节劈裂环共振器(Split-Ring Resonator，SRR)结构单元缝隙处附近的半导体衬底层载流子浓度来改变共振强度，故在共振频点的太赫兹波透射强度可以通过电调制实现。该方法可以实现室温下调制深度达50％和调制速度达2MHz的太赫兹调制。另外一种复合超材料结构是将高电子迁移率晶体管(HEMT)集成在SRR的缝隙处，通过栅极改变沟道的电子浓度来改变SRR的电容进而调节SRR共振强度。该器件的调制深度可达33％，最高调制速度达10MHz。在太赫兹波段，现有调制器的专利技术主要利用超材料实现，例 如，2009年Houtong Chen等人申请的名为“主动型太赫兹超材料器件”的美国发明专利。
石墨烯调制器：在太赫兹波段，石墨烯中电子的带内跃迁起主要作用，利用大面积单层石墨烯可以实现室温下15％的强度调制深度和20kHz的调制频率。
现有技术的缺点主要有：
现有技术的调制深度普遍不高，最高的也只能达50％，说明这些机制的能量损耗方式不是很有效；此外，高速调制是调制器最重要的性能指标之一，而现有技术的调制速度不高(最高调制速度为10MHz)。
导致这些缺点的根本原因主要有：a)二维电子气和石墨烯的载流子层很薄，电磁波与载流子相互作用时间短，如果没有实现强耦合则调制效率不高；b)Drude电导率模型的耗散是单个自由载流子受到外界散射(声子、杂质和缺陷等)引起的。电磁波是与单粒子耦合的，故太赫兹波的这种损耗机制不是很高效；c)大面积器件的调制速度受限于器件的寄生电容和电阻，如半导体复合超材料加栅压形成的耗尽层电容和电阻。
目前尚未见有采用低维电子等离子体波实现太赫兹调制器的相关文献报道。
发明内容
本发明要解决的技术问题是提供一种能实现高速和大调制深度的太赫兹调制器，利用了太赫兹波腔模与等离子体波模式强耦合后形成的等离极化激元实现对太赫兹波的高效调制。为实现太赫兹高速成像和高速通信所必须的调制器提供一种可能的解决方案。
本发明所述的太赫兹调制器，包括等离激元和谐振腔；其中，
所述等离激元包括：
GaN/AlGaN之高电子迁移率晶体管，其包括：源极、漏极和光栅栅极；
位于所述晶体管的半导体异质界面处窄带隙半导体一侧形成的准二维电子层，用于激发等离子体波；
所述谐振腔，包括光栅与样品接触的表面到经减薄蓝宝石衬底的下表面形成的介质谐振腔。
如上所述的太赫兹调制器，其中所述光栅栅极为一维光栅或者插指型光栅。
如上所述的太赫兹调制器，其中所述谐振腔可以为减薄样品本身界面形成的Fabry-Pérot(简称F-P)谐振腔或者由相同介质层组成的对称谐振腔结构。
本发明还提出一种基于高电子迁移率晶体管的高速调制方法，包括：
将所述晶体管的源极和漏极接地，向所述晶体管的栅极提供一负直流栅压VG和交流调制信号，该信号幅值为
Figure PCTCN2015081026-appb-000001
所述晶体管具有光栅栅极，光栅栅极将二维电子气中二维等离子体波和太赫兹谐振腔模式进行强耦合形成等离极化激元；
调节光栅栅压，使其达到等离激元和谐振腔模的共振条件；
将栅压在共振和非共振点之间进行切换，实现对谐振腔模式的有效调制。
如上所述的高速调制方法，其中，等离激元的共振频率依赖于电子浓度ns、波矢q以及有效介电常数
Figure PCTCN2015081026-appb-000002
即
Figure PCTCN2015081026-appb-000003
(W为栅长，m＝1,2,3...)。
如上所述的高速调制方法，其中，Fabry-Pérot(F-P)谐振腔模式与谐振腔介质的折射率
Figure PCTCN2015081026-appb-000004
和厚度D有关，其共振频率为
Figure PCTCN2015081026-appb-000005
其中，当等离激元与谐振腔模式共振强耦合时，出现等离极化激元的两分支，这是强耦合的重要特征。
本发明还提出了一种太赫兹调制器的制造方法，包括步骤如下：
一、在GaN/AlGaN二维电子气材料小片上形成器件有源区的二维电子气台面；
二、形成源、漏欧姆接触；
三、形成光栅栅极；
四、形成光栅栅极和欧接触姆的引线电极以及用于晶片键合的图形转移；
五、对蓝宝石衬底的背面进行减薄并抛光，以形成谐振腔结构。
本发明还公开一种带有对称介质谐振腔的太赫兹调制器的制造方法，包括制造介质谐振腔和将介质谐振腔与二维电子气芯片键合在一起的步骤，其中，所述制造介质谐振腔的步骤包括：对谐振腔平板材料进行减薄并抛光；在谐振腔平板材料上进行晶片键合区的图形转移；在键合的金属区域实现倒装焊图形的转移。
本发明具有显著的特点，由于二维电子气中的等离子体波是电子的集体振荡，其固有的强束缚电场能实现与太赫兹波的强耦合，故其对太赫兹波的调制效率很高。相对于单电子行为的损耗机制，本发明的集体振荡模式的损耗则更为有效，这有利于产生大的调制深度。将二维电子气的等离子体波集成在太赫兹谐振腔里以实现强耦合，通过对等离激元的调控以实现对谐振腔共振模式的高效调控。
本发明利用了太赫兹波腔模与等离子体波模式强耦合后形成的等离极化激元实现对太赫兹波的高效调制。能实现高速(约1GHz)和大调制深度(70％即10dB以上)的调制，为实现太赫兹高速成像和高速通信所必须的调制器提供一种可能的解决方案。
附图说明
图1为本发明所述太赫兹调制器的结构原理示意图；
图2为本发明所述太赫兹调制器中的光栅栅极示意图，其中图2a为一维光栅栅极，图2b为插指光栅栅极；
图3为本发明所述太赫兹调制器中对称谐振腔的结构示意图；
图4为本发明所述太赫兹调制器在工作状态下的各电极偏置示意图；
图5为等离子激元和自由空间太赫兹光的色散关系曲线；
图6为等离子激元与谐振腔模式强耦合形成等离极化激元的两分支ω+和ω-曲线图；
图7为计算得出的等离极化激元本征模式；
图8为实验测得频率为1.07THz的腔模透射系数随栅压的变化关系图；
图9为本发明所述太赫兹调制器的制作流程图；
图10为基本太赫兹调制器结构制造工艺的具体流程图。
具体实施方式
为使本发明的目的、技术方案和优点更加清楚，下面结合附图对本发明的具
体实施方式
进行示例性的详细说明。
为了便于本领域技术人员理解，本发明提供的相关术语解释如下。
1)太赫兹波(terahertz wave，terahertz radiation)：太赫兹波是指频率为0.1-10THz(1THz＝1000GHz＝1012Hz)、波长为30微米至3毫米的电磁波，处在电磁波频谱的毫米波波段和红外光波段之间，也称为亚毫米波或远红外线。
2)太赫兹调制器(terahertz modulator)：是指能够对太赫兹波的电磁特性(电场强度或相位)进行有效调控的器件或装置。
3)太赫兹谐振腔(terahertz cavity)：指能够支持特定太赫兹波驻波模式的电磁介质结构，包括金属波导谐振腔和介质波导谐振腔。法布里谐振腔同属金属波导谐振腔或介质波导谐振腔，腔内填充低损耗介电材料或真空(空气)，腔面可以是具有高反射率的金属镀膜或介质-空气界面。
4)二维电子气(two-dimensional electron gas，2DEG)：在某一空间维度的运动受到限制，而在另外两个维度则可以自由运动的具有一定浓度的电子体系。固体中一般是在半导体异质界面处窄带隙半导体一侧形成的准二维电子层，如GaAs/AlGaAs异质结界面处GaAs一侧的二维电子气和GaN/AlGaN异质结界面处GaN一侧的二维电子气。由于异质结中二维电子气中的电子可以与掺杂杂质在空间上有效分离，因此二维电子气具有比相应半导体体材料中载流子更高的迁移率。
5)等离子体波(plasma wave，plasmon)：是指同一种极性电荷的集合体在相反极性电荷背景下产生的密度波动，具有波的特性，是一种电荷的集体激发。等离子体波振荡的准粒子是等离激元(plasmon)，实际上就是电荷密度振 荡的元激发。在此专利中指半导体二维电子气中的电荷密度振荡波，称为二维等离子体波或二维等离激元。
6)等离极化激元：腔光子与物质元激发强耦合形成的相干混合态。在本专利中指二维电子气中等离激元与太赫兹谐振腔腔模强耦合形成的混合态，它是该强耦合体系的本征态。其特征在于，当等离激元与谐振腔模式共振时会出现反交叉特征。
7)高电子迁移率晶体管(High Electron Mobility Transistor，HEMT)：用半导体异质结材料制作的场效应晶体管，由于电子与掺杂区分离，晶体管沟道中的电子具有很高的迁移率，故该晶体管称作高电子迁移率晶体管。
8)幅值调制深度：电磁波电场幅值的变化ΔA与原电场幅值A比的百分值，即η＝ΔA/A×100％。
本发明提出了一种太赫兹调制器，其基本原理是基于电子集体振荡(Plasmon，即等离子激元，或称等离子体波)引起的共振吸收机制，为了增强太赫兹波和等离子激元的耦合强度，将具有光栅耦合二维电子气的HEMT结构集成在太赫兹谐振腔里，从而提出一种太赫兹调制器。本发明太赫兹调制器利用强耦合作用使得谐振腔模式的透射系数在共振点最小，当调控等离子激元的振荡频率使得系统在共振点和非共振点切换时，能够实现对谐振腔共振模式的有效调制。
图1所示的是最基本的太赫兹调制器器件结构示意图，主要包括等离激元和谐振腔两个单元。
所述等离激元，包括：
具有光栅栅极的GaN/AlGaN之HEMT结构，包括源极1、漏极2、光栅栅极3；
半导体异质界面处窄带隙半导体一侧形成的准二维电子层，即二维电子气4，它是等离子体波6的载体，或者说，它用于激发等离子体波；
所述谐振腔，则是由经减薄的蓝宝石介质材料形成的介质谐振腔5。优选地，本发明采用蓝宝石材料经减薄处理后作为谐振腔介质材料。
具体地说，谐振腔应该包括光栅与样品接触的表面到衬底的下表面形成的介质谐振腔，二维电子气4的上面，有很薄一层栅极势垒层，其也包含在谐振腔里，主要原因是该层很薄且折射率与衬底很接近，故可以统一称之为介质谐振腔。
其中，源极1和漏极2可采用传统的欧姆接触工艺实现，使其接触电阻越小越好。
其中，对于形成在GaAs/AlGaAs异质结界面处GaAs一侧的二维电子气和GaN/AlGaN异质结界面处GaN一侧的二维电子气而言，其主要有两个关键参数。一是高电子迁移率，迁移率越高，等离子体波的品质因子就越高，就能实现更强的耦合。一般而言，对于GaN/AlGaN二维电子气，其室温迁移率在2000cm2/Vs量级。实验上能观察到的等离激元共振吸收的最高温度接近200K。形成有效的等离激元振荡要求电子迁移率在10000cm2/Vs量级及以上。第二个参数是高二维电子气的浓度，浓度越高越有利于得到较高的品质因子，并且在其它条件相同的情况下具有更高的振荡频率，故器件可以覆盖更宽的频谱范围。
其它的二维电子气材料包括：GaAs/AlGaAs、InGaAs/AlGaAs、石墨烯、MoS2等。
图1中还示意出，腔模电场7经过光栅的近场增强与等离子体波6的束缚电场耦合，可实现强耦合。
为了提高耦合强度和调制深度，可以将图1所示结构中的非栅控区的二维电子气刻蚀掉，例如，可以通过感应耦合等离子体(Inductive Coupled Plasma，简称ICP)刻蚀实现。
图1中的附图标记8表示入射的太赫兹波，附图标记9通过所述太赫兹调制器后的透射太赫兹波。
图2给出了关于本发明所述的太赫兹调制器中的光栅栅极的两个实施例。
图2a给出的是一维光栅，其中，器件的源极为S，漏极为D，栅极用G表示。栅长为W，L为光栅周期，其等于栅长与栅间距之和。
图2b则为插指型光栅，其中，光栅G1的栅长为W1，光栅G2的栅长为W2，栅间距为S。
需要说明的是，光栅起耦合作用，一方面其周期必须远小于太赫兹波长(光栅周期L≤λTHz/100)，另一方面，光栅在太赫兹波段的损耗很小，则要求光栅的金属厚度必须大于趋肤深度(≥120nm)。
光栅栅极的作用在于：通过调整光栅栅压来调控等离子体波。
除了上述的一维光栅和插指金属光栅外，光栅栅极还包括平面二维网格光栅、同心圆形光栅、太赫兹超材料和二维电子气台面的周期性结构或图形。
图1给出的谐振腔结构采用了最简单的衬底，属于基本的平板式谐振腔。因此，本发明还提出了如图3所示的利用衬底和另外一块相同厚度的相同介质组成的对称谐振腔结构。
对于上述的介质谐振腔来说，介质材料选取的依据是对太赫兹波具有尽可能小的吸收，同时也需要满足高电子迁移率二维电子气材料异质结生长的要求。对于GaN/AlGaN材料，蓝宝石衬底对太赫兹波吸收小，并且其机械强度高，更有利于加工成更小尺寸的谐振腔。一般要求谐振腔的尺寸在所考虑频段波长的量级，这主要是为了方便形成分立的腔模。
除图3所述的Fabry-Pérot平板谐振腔外，还可以是与二维电子气共面的谐振腔，如太赫兹共面波导(Co-planar Waveguide，即CPW)、狭缝天线(Slot Antenna)和太赫兹超材料共振器(Terahertz Metamaterial Resonator)等。
所述准二维电子层为半导体二维电子沟道中的电子，利用所述电子的集体振荡来实现对太赫兹波的共振吸收，通过与谐振腔模式强耦合进一步增强吸收。
基于二维等离体波共振吸收的原理：利用二维电子沟道中电子的集体振荡来实现对太赫兹波的共振吸收，通过与谐振腔模式强耦合进一步增强吸收。该物理机制是本发明的本质。
图4至图8用来详细说明本发明提出的所述的基于高电子迁移率晶体管HEMT结构的高速调制方法。
图4为本发明所述太赫兹调制器工作时电压偏置示意图，如图4所示，它是一个具有光栅栅极的GaN/AlGaN高电子迁移率晶体管(High Electron Mobility Transistor，简称HEMT)结构。
本发明所述的太赫兹调制器的调制原理是，光栅将二维电子气中等离子体波与太赫兹谐振腔模式进行强耦合，由此形成等离极化激元。通过光栅栅压调控等离激元和腔模的共振条件，使体系在共振点和非共振点切换以达到调控太赫兹腔模透射的目的。
当太赫兹调制器器件工作时，源极和漏极接地，向栅极提供一负直流栅压VG和交流调制信号(幅值为
Figure PCTCN2015081026-appb-000006
)，如图4所示。等离激元的共振频率依赖于电子浓度ns、波矢q以及有效介电常数
Figure PCTCN2015081026-appb-000007
(屏蔽效应)，即
Figure PCTCN2015081026-appb-000008
(W为栅长，m＝1,2,3...)。电子浓度受栅压调控，器件物理模型的缓变沟道近似得到的电子浓度和栅压的关系为
Figure PCTCN2015081026-appb-000009
(其中，Vth为电子浓度耗尽的阈值电压，C为栅极的单位电容，ε0为真空介电常数，εAlGaN为栅极势垒层的介电常数，d为势垒层的厚度，即二维电子气离样品表面的距离)。交流调制信号使得等离激元的共振频率不断变化。等离激元与自由空间的太赫兹波的色散关系曲线见图5所示。
图5中，虚线表示两者共振，即发生等离激元的共振吸收。Fabry-Pérot(F-P)谐振腔模式与谐振腔介质的折射率
Figure PCTCN2015081026-appb-000010
和厚度D有关，其共振频率为
Figure PCTCN2015081026-appb-000011
参考图6，当等离激元与谐振腔模式共振强耦合时，就会出现等离极化激元的两个分支，即图6中所示的ω+和ω-。如图7所示，为计算出一结构的强耦合本征模式，图7标出了某个等离极化激元(m＝1，k＝2)的上支(UPP)和下 支(LPP)，竖直的直线标出了等离激元与谐振腔模式的共振耦合点所对应的直流栅压，大约是在-2.5V右。
当通过栅压调控时，实质上调控了二维电子气的电子浓度，使得栅控等离激元与谐振腔模式发生共振，原来为透射峰的谐振腔模式就会被等离激元共振所吸收掉。故当将栅压在共振和非共振点之间进行切换时，就实现了对谐振腔模式的有效调制。这就是本发明所述太赫兹调制器的基本工作原理。
器件的低温时域透射光谱证明了其作为太赫兹调制器具有很大潜力。为更加明确地阐述调制方法，如图8所示为器件工作时的偏置示意图。在共振点时，腔模的透射最小，当外加的调制信号将栅压调到非共振点时，腔模的透射达到最大。图中标出了器件的直流工作电压和最佳外加调制高频信号。图8为实验结果中取频率为1.07THz时，腔模的透射系数随栅压变化的曲线。图8中标出了器件直流工作栅压以及所加高频调制信号，从图8中可以看出，在共振点附近，透射率随栅压变化有很好的线性度，即透射率随栅压变化呈线性关系。图8中的虚线所表示的“类似正弦波”的波形代表被调制的太赫兹波透射系数，其幅值调制深度为η1.07THz＝66％。该调制深度均高于以往的现有技术中所公开的太赫兹调制器性能。
参考图9，本发明还提出了一种太赫兹调制器的制造方法，包括步骤如下：
一、在GaN/AlGaN二维电子气材料小片上形成的器件有源区的二维电子气台面；
二、形成源、漏欧姆接触；
三、形成光栅栅极；
四、形成光栅栅极和欧姆接触的引线电极以及用于晶片键合的图形转移；
五、对蓝宝石衬底的背面进行减薄并抛光，以形成谐振腔结构。
基于图9的技术思路，图10给出的更为详尽的太赫兹调制器的制造工艺流程图，适用于批量化生产，具体说明如下：
S1001、裂片：将2寸GaN/AlGaN二维电子气原片通过激光划片机切成长宽都为1.5厘米的方型小片。
S1002、形成二维电子气台面：采用紫外曝光技术(简称光刻)实现二维电子气台面的图形转移，然后，采用感应耦合等离子体(ICP)刻蚀二维电子气材料，以形成器件有源区台面。
S1003、欧姆接触：光刻，采用电子束蒸发工艺，在二维电子气台面上蒸镀欧姆接触的多层金属结构，经过剥离后，形成欧姆接触金属图形。对于GaN/AlGaN HEMT，一般采用Ti/Al/Ni/Au(20/120/70/100nm)。最后在快速退火炉里的氮气环境下，在900℃退火30秒，即形成源漏欧姆接触。
S1004、光栅栅极：光刻，实现光栅结构的图形转移，采用电子束蒸发蒸镀Ti/Au或Ni/Au(20/100nm)，经剥离后形成金属光栅栅极结构。
S1005、引线电极：光刻，实现光栅栅极的引线电极、欧姆接触引线电极和用于晶片键合的图形转移。采用电子束蒸发蒸镀Ti/Au或Ni/Au(20/300nm)，经剥离后形成相应的电极结构。
S1006、对衬底谐振腔采用减薄机和化学机械进行抛光，从样品背面进行减薄并抛光，以形成谐振腔结构，减薄后的厚度一般在100至200微米之间；
最后，裂片形成单个的光栅耦合二维电子气芯片：采用激光划片和手工解离，将1.5厘米的小方片分割成独立的、约6×6mm2大小的二维电子气原型芯片。
如前所述，图3中给出了除了最基本的衬底谐振腔结构之外的另外一种具体结构，它是带有对称介质谐振腔的太赫兹调制器，对称的两个谐振腔“共用”一个等离激元，即在图10流程中最终制得的二维电子气芯片上，对称设置有另一谐振腔10，该谐振腔10与二维电子气芯片的谐振腔“共用”一个等离激元。因此，本发明还提供了该种不同谐振腔的太赫兹调制器的制造方法实施例。
关于带有对称介质谐振腔的太赫兹调制器的制造方法如下所示：
对于如图3所示的带有对称介质谐振腔的太赫兹调制器，相当于在图1给出的典型的太赫兹调制器基础上，增加一个介质谐振腔，由此形成具有对称介质谐振腔的太赫兹调制器。其制造方法包括制造介质谐振腔10和将该谐振腔与上述二维 电子气芯片键合在一起的步骤，在这里所述的介质谐振腔是指与二维电子气芯片对称设置的介质谐振腔10，该制造介质谐振腔的步骤简述如下：
1)采用减薄机和化学机械抛光工艺，对谐振腔平板材料(蓝宝石)进行减薄并抛光；
2)光刻，在谐振腔平板材料上实现晶片键合区的图形转移；可通过电子束蒸发蒸镀Ti/Au或Ni/Au(20/300nm)形成晶片键合的金属区域；
3)光刻，在键合的金属区域实现倒装焊图形的转移；可通过热蒸发蒸镀铟，并剥离得到倒装焊所需铟柱；
最后，通过倒装焊技术，将经上述步骤所制得的介质谐振腔和经过图10步骤最终制得的二维电子气芯片键合在一起，就形成了带有对称介质谐振腔的太赫兹调制器。
其中，在谐振腔平板材料上进行晶片键合区的图形转移的步骤，通过电子束蒸发蒸镀Ti/Au或Ni/Au并剥离形成晶片键合的金属区域实现。
在键合的金属区域进行倒装焊图形的转移的步骤，通过热蒸发蒸镀铟，并剥离得到倒装焊所需铟柱实现。
将二维电子气芯片和介质谐振腔键合在一起的步骤包括，可通过倒装焊技术实现，即将两者如图3所示的位置关系对准，加热后再施加压力，使二者通过金-铟固熔结合在一起，最后在回流炉里回流使得键合更牢固。
本发明的优点具体体现在以下几个方面：
1)由于等离子体波与太赫兹腔模处于强耦合状态，且等离子体波是电子的集体振荡(集体响应极快)，所以它具有调制深度大和速度高的优点；
2)GaN/AlGaN异质结中二维电子气的电子浓度较高，通过栅极调控可以覆盖更宽的太赫兹频带；
3)器件虽为窄带工作，但是可以工作在多个频点，并且工作频点和工作电压可以跟据理论和需求来设计，故该器件的灵活性较高；
4)当等离激元的品质因子高时，调制器工作时需要的调制信号幅值相对较小，这将有利于高速应用和降低功耗；
5)蓝宝石衬底本身对太赫兹波的吸收很小，故其相对于掺杂半导体损耗更小；
6)蓝宝石衬底很结实，故器件可减薄至100至200微米之间也不会造成损坏并且有足够的机械强度。
综上所述，与现有技术中的单电子行为的技术方案不同的是，本发明基于电子集体振荡(等离子体波，即等离激元，Plasmon)引起的共振吸收机制。为了增强太赫兹波和等离激元的耦合强度，将具有光栅耦合二维电子气的HEMT(高电子迁移率晶体管)结构集成在太赫兹谐振腔里。强耦合作用使得谐振腔模式的透射系数在共振点最小，通过光栅栅压的改变，调控等离激元和腔模的共振条件，达到调控太赫兹波透射的目的。当调控等离激元的振荡频率使得系统在非共振点和共振点切换时就实现了对谐振腔共振模式的有效调制。本发明对太赫兹调制器的物理机制和实现工艺技术都做了详细的介绍，为相关应用提供一种可能的解决方案。由于采用“光”(太赫兹波)和“物质”(等离激元)的强耦合，故本发明具有突出的实质性特点和显著地进步。
以上所述的具体实施方式是用于帮助理解本发明的目的、技术方案和有益效果，应理解的是，以上所述仅为本发明的具体实施方式而已，并不用于限定本发明的保护范围，凡在本发明的精神和原则之内，所做的任何修改、等同替换、改进等，均应包含在本发明的保护范围之内。

Claims (20)

  1. 一种基于低维电子等离子体波的太赫兹调制器，包括等离激元和谐振腔；其特征在于，
    所述等离激元包括：
    GaN/AlGaN之高电子迁移率晶体管，其包括：源极、漏极和光栅栅极；
    位于所述晶体管的半导体异质界面处窄带隙半导体一侧形成的准二维电子层，用于激发等离子体波；
    所述谐振腔，包括光栅与样品接触的表面到经减薄蓝宝石衬底的下表面形成的介质谐振腔。
  2. 根据权利要求1所述的太赫兹调制器，其中所述光栅栅极为一维光栅或者插指型光栅。
  3. 根据权利要求1所述的太赫兹调制器，其中所述谐振腔为减薄样品本身界面形成的Fabry-Pérot谐振腔或者由相同介质层组成的对称谐振腔结构。
  4. 根据权利要求1所述的太赫兹调制器，其中所述光栅栅极还包括平面二维网格光栅、同心圆形光栅、太赫兹超材料光栅。
  5. 根据权利要求1所述的太赫兹调制器，其中所述谐振腔为二维电子气共面的谐振腔。
  6. 根据权利要求1所述的太赫兹调制器，所述准二维电子层为半导体二维电子沟道中的电子，利用所述电子的集体振荡来实现对太赫兹波的共振吸收，通过与谐振腔模式强耦合进一步增强吸收。
  7. 一种基于高电子迁移率晶体管的高速太赫兹波调制方法，其特征在于，包括：
    将所述晶体管的源极和漏极接地，向所述晶体管的栅极提供一负直流栅压VG和交流调制信号，该信号幅值为
    Figure PCTCN2015081026-appb-100001
    所述晶体管具有光栅栅极，光栅栅极将二维电子气中二维等离子体波和太赫兹谐振腔模式进行强耦合形成等离极化激元；
    调节光栅栅压，使其达到等离激元和谐振腔模的共振条件；
    将栅压在共振和非共振点之间进行切换，实现对谐振腔模式的有效调制。
  8. 根据权利要求7所述的高速太赫兹波调制方法，其中，所述等离激元的共振频率依赖于电子浓度ns、波矢q以及有效介电常数
    Figure PCTCN2015081026-appb-100002
    即
    Figure PCTCN2015081026-appb-100003
    Figure PCTCN2015081026-appb-100004
    其中W为栅长，m＝1,2,3...。
  9. 根据权利要求7或8所述的高速太赫兹波调制方法，其中，所述谐振腔为Fabry-Pérot谐振腔，所述Fabry-Pérot谐振腔模式与谐振腔介质的折射率
    Figure PCTCN2015081026-appb-100005
    和厚度D有关，其共振频率为
    Figure PCTCN2015081026-appb-100006
  10. 根据权利要求7或8所述的高速调制方法，其中，当等离激元与谐振腔模式共振强耦合时，出现等离极化激元的两分支。
  11. 一种太赫兹调制器的制造方法，其特征在于，包括步骤如下：
    步骤一、在GaN/AlGaN二维电子气材料小片上形成器件有源区的二维电子气台面；
    步骤二、形成源、漏欧姆接触；
    步骤三、形成光栅栅极；
    步骤四、形成光栅栅极和欧姆接触的引线电极以及用于晶片键合的图形转移；
    步骤五、对蓝宝石衬底的背面进行减薄并抛光，以形成谐振腔结构。
  12. 根据权利要求11所述的太赫兹调制器的制造方法，其中步骤一中，所述GaN/AlGaN二维电子气材料小片，是通过激光划片机将2寸GaN/AlGaN二维电子气原片切成长宽都为1.5厘米的方型小片；所述二维电子气台面则采用紫外曝光技术实现二维电子气台面的图形转移，然后，采用感应耦合等离子体刻蚀二维电子气材料，以形成器件有源区台面。
  13. 根据权利要求11所述的太赫兹调制器的制造方法，其中步骤二中，进一步包括：光刻，采用电子束蒸发工艺，在二维电子气台面上蒸镀欧姆接触的多层金属结构，经过剥离后，形成欧姆接触金属图形，放置在快速退火炉里进行退火，形成源漏欧姆接触。
  14. 根据权利要求11所述的太赫兹调制器的制造方法，其中步骤三中所述形成光栅栅极，包括光刻，采用电子束蒸发蒸镀Ti/Au或Ni/Au，经剥离后形成的金属光栅栅极结构；其中步骤四中，引线电极的形成是先用光刻，采用电子束蒸发蒸镀Ti/Au或Ni/Au，经剥离后形成相应的电极结构。
  15. 根据权利要求11或12所述的太赫兹调制器的制造方法，其中，对样品衬底采用减薄机减薄和化学机械进行抛光，以形成谐振腔结构。
  16. 根据权利要求11或12所述的太赫兹调制器的制造方法，其中，在步骤五之后，还包括采用激光划片和手工解离，裂片形成单个的光栅耦合二维电子气芯片。
  17. 一种带有对称介质谐振腔的太赫兹调制器的制造方法，其特征在于，包括制造介质谐振腔和将所述介质谐振腔与二维电子气芯片键合在一起的步骤，其中，
    所述制造介质谐振腔的步骤包括：
    对谐振腔平板材料进行减薄并抛光；
    在谐振腔平板材料上进行晶片键合区的图形转移；
    在键合的金属区域实现倒装焊图形的转移。
  18. 根据权利要求17所述的带有对称介质谐振腔的太赫兹调制器的制造方法，其中，所述在谐振腔平板材料上进行晶片键合区的图形转移的步骤，通过电子束蒸发蒸镀Ti/Au或Ni/Au并剥离形成晶片键合的金属区域实现。
  19. 根据权利要求17所述的带有对称介质谐振腔的太赫兹调制器的制造方法，其中，所述在键合的金属区域进行倒装焊图形的转移的步骤，通过热蒸发蒸镀铟，并剥离得到倒装焊所需铟柱实现。
  20. 根据权利要求17所述的带有对称介质谐振腔的太赫兹调制器的制造方法，其中，所述将介质谐振腔和二维电子气芯片键合在一起的步骤包括，将两者对准，加热后再施加压力，使二者通过金-铟固熔结合在一起，最后在回流炉里回流。
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