WO2016136186A1 - Élément photoconducteur, son procédé de fabrication, et appareil de mesure - Google Patents

Élément photoconducteur, son procédé de fabrication, et appareil de mesure Download PDF

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Publication number
WO2016136186A1
WO2016136186A1 PCT/JP2016/000801 JP2016000801W WO2016136186A1 WO 2016136186 A1 WO2016136186 A1 WO 2016136186A1 JP 2016000801 W JP2016000801 W JP 2016000801W WO 2016136186 A1 WO2016136186 A1 WO 2016136186A1
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Prior art keywords
diffraction grating
unit
antenna unit
layer
photoconductive
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PCT/JP2016/000801
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English (en)
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Takayuki Koizumi
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Canon Kabushiki Kaisha
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Priority claimed from JP2016016447A external-priority patent/JP2016164974A/ja
Application filed by Canon Kabushiki Kaisha filed Critical Canon Kabushiki Kaisha
Publication of WO2016136186A1 publication Critical patent/WO2016136186A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/09—Devices sensitive to infrared, visible or ultraviolet radiation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0232—Optical elements or arrangements associated with the device
    • H01L31/02327—Optical elements or arrangements associated with the device the optical elements being integrated or being directly associated to the device, e.g. back reflectors

Definitions

  • the present invention relates to a photoconductive element for generating or detecting a terahertz (THz) wave, a method for manufacturing the photoconductive element, and a measuring apparatus using the photoconductive element.
  • THz terahertz
  • a THz wave is an electromagnetic wave of frequencies from 30 GHz to 30 THz.
  • An exemplary element for generating or detecting a THz wave is a photoconductive element.
  • a photoconductive element is constituted by a special semiconductor with relatively high mobility and short carrier life of 1 picosecond or less, and antenna electrodes (conductive units) which function also as electrodes provided on a semiconductor. When a gap portion between the antenna electrodes is irradiated with ultrashort pulsed light while a voltage is being applied to the antenna electrodes, a current flows momentarily between the electrodes by the excited optical carrier and a THz wave is emitted.
  • a method for increasing generation efficiency of an optical carrier by reinforcing an electric field of excitation light on a semiconductor surface of the gap portion between the antenna electrodes and increasing light absorption in the semiconductor.
  • a diffraction grating of a material with a negative dielectric constant is disposed at a period of submicron order between the antenna electrodes.
  • the diffraction grating is irradiated with ultrashort pulsed light to produce surface plasmon resonance and diffraction.
  • the generation efficiency of the optical carrier is increased by the electric field reinforcement near an interface between the diffraction grating and the semiconductor and the electric field concentration by diffraction.
  • a photoconductive element in which a diffraction grating for generating surface plasmon resonance is provided between bow tie antennas is disclosed in PTL 1 and NPL 1.
  • comparison is made on the capability of generation and detection of a THz wave in both cases with or without the diffraction grating, and it is described that provision of the diffraction grating increases generation output and detection sensitivity.
  • An exemplary measuring apparatus using a photoconductive element is a terahertz time-domain spectroscopy (THz-TDS) apparatus which measures a time waveform of a THz wave pulse by THz-TDS.
  • THz-TDS terahertz time-domain spectroscopy
  • the THz wave pulse becomes a plurality of pulse waves because the THz wave pulse passes through or reflects off a measurement target under the influence of physical properties, shape, and the like of the measurement target. Since a plurality of pulses are detected at different times depending on the optical path lengths along which each pulse has propagated, the obtained time waveform has a plurality of pulses.
  • Information about the measurement target can be acquired by splitting the plurality of pulses on the time waveform and then analyzing.
  • the time waveform of the THz wave pulse only has a single pulse. That is, it is desirable to convert a THz wave pulse into a monopulse.
  • a time waveform with a plurality of pulses may be obtained.
  • a photoconductive element including: a photoconductive layer; and a plurality of conductive units disposed on the photoconductive layer, each of the plurality of conductive units including an electrode; an antenna unit electrically connected with the electrode; and a diffraction grating unit which has a plurality of periodically arranged gratings and which is in contact with the antenna unit, wherein the plurality of conductive units are disposed to face each other with a gap left therebetween between the diffraction grating units of the plurality of conductive units, and wherein a current reflection coefficient in a contact portion of the antenna unit and the diffraction grating unit is ⁇ 5% or less.
  • a photoconductive element including: a photoconductive layer; and a plurality of conductive units disposed on the photoconductive layer, each of the plurality of conductive units including an electrode; an antenna unit electrically connected with the electrode; and a diffraction grating unit having a plurality of periodically arranged gratings; and a connecting portion disposed between the antenna unit and the diffraction grating unit and in contact with the diffraction grating unit, wherein the plurality of conductive units are disposed to face each other with a gap left therebetween between the diffraction grating units of the plurality of conductive units, and wherein the current reflection coefficient in the contact portion of the connecting portion and the diffraction grating unit is ⁇ 5% or less.
  • Fig. 1A is a top view illustrating a configuration of a photoconductive element according to a first embodiment.
  • Fig. 1B is a cross-sectional view of the photoconductive element along line IB-IB according to the first embodiment.
  • Fig. 1C is a cross-sectional view of the photoconductive element along line IC-IC according to the first embodiment.
  • Fig. 2A is a perspective view illustrating a configuration of a photoconductive element according to a second embodiment.
  • Fig. 2B is a cross-sectional view of the photoconductive element along line IIB-IIB according to the second embodiment.
  • Fig. 3A is a top view illustrating a configuration of a photoconductive element according to a third embodiment.
  • FIG. 3B is a cross-sectional view of the photoconductive element along line IIIB-IIIB according to the third embodiment.
  • FIG. 3C is a schematic diagram illustrating a configuration near a grating portion of the photoconductive element and a connecting portion according to the third embodiment.
  • Fig. 4 is a top view illustrating a configuration of a photoconductive element according to a fourth embodiment.
  • Fig. 5 is a cross-sectional view illustrating a configuration of a photoconductive element according to a fifth embodiment.
  • Fig. 6 is a flowchart illustrating a method for manufacturing a photoconductive element according to a sixth embodiment.
  • Fig. 7 is a schematic diagram illustrating a configuration of a measuring apparatus according to a seventh embodiment.
  • FIG. 8A is a top view illustrating a configuration of a photoconductive element used for calculation of a characteristic impedance.
  • FIG. 8B is a cross-sectional view of a photoconductive element along line VIIIB-VIIIB used for calculation of a characteristic impedance.
  • FIG. 1A is a top view illustrating a configuration of the element 100
  • Fig. 1B is a cross-sectional view of the element 100 of Fig. 1A along line IB-IB
  • Fig. 1C is a cross-sectional view of the element 100 of Fig. 1A along line IC-IC.
  • the element 100 has a photoconductive layer 110, a substrate 111, and a plurality of conductive units (antenna electrodes) 112.
  • the photoconductive layer 110 is a semiconductor which generates a career upon entrance of ultrashort pulsed light as excitation light.
  • the "ultrashort pulsed light” herein is light of which pulse width is 100 fs or less. Especially, ultrashort pulsed light of which pulse width is 1 to 100 fs is referred to as femtosecond pulsed light.
  • a material of which wavelength corresponding to bandgap energy of the photoconductive layer 110 is shorter than the wavelength of excitation light may be selected as a material of the photoconductive layer 110 to use career excitation by one-photon absorption.
  • a material of which wavelength corresponding to bandgap energy is longer than the wavelength of the excitation light may also use career excitation by multiphoton absorption which is one of nonlinear optical effects.
  • LT low temperature grown
  • LT-InGaAs or LT-GaAs may be used.
  • the photoconductive layer 110 is disposed on the substrate 111. If the photoconductive layer 110 is made to grown epitaxially on the substrate 111, it is only necessary to select a substrate which is lattice-matching with the photoconductive layer 110 to be used as the substrate 111. Besides the method for making the photoconductive layer 110 grow epitaxially, other techniques may be used: for example, the photoconductive layer 110 and the substrate 111 may be joined together by, for example, transferring.
  • a plurality of conductive units 112 are disposed on the photoconductive layer (the photoconductive layer 110) with a gap (a gap portion) formed between the diffraction grating units (the diffraction grating units 101) of the conductive units 112.
  • Each of a plurality of conductive units 112 has a diffraction grating unit 101, an antenna unit 102, and a plurality of electrodes 103.
  • the antenna unit 102 connects the diffraction grating unit 101 with the electrodes 103 electrically.
  • Each antenna unit 102 has a projecting portion. The projecting portions face each other at an arbitrary distance.
  • the antenna unit 102 has a function as an electrode and a voltage can be applied between the projecting portions of the antenna units 102.
  • the electrodes 103 are electrically connected with the antenna units 102, and connected with unillustrated signal lines.
  • the antenna units 102 and the electrode 103 are desirably made of a highly conductive and hardly oxidized metal material. Specifically, gold (Au) is used for example.
  • the shape of the antenna unit 102 is determined in consideration of the frequency, output, and the like of the THz wave to be generated or detected. In the present embodiment, a dipole antenna capable of generating and detecting a THz wave over a wide band is used as an example.
  • the diffraction grating unit 101 is further disposed at a tip of the projecting portion of each antenna unit 102.
  • the diffraction grating unit 101 increases optical absorption efficiency of the excitation light in the photoconductive layer 110 by a surface plasmon resonance effect or a diffraction effect.
  • the diffraction grating unit 101 has a plurality of gratings 150 made of a highly conductive material. Each of a plurality of gratings 150 is rectangular parallelepiped in shape. Adjoining gratings 150 are arranged periodically at certain intervals. That is, in the diffraction grating unit 101, a conductive member disposed on the photoconductive layer 110 has a plurality of periodically formed grooves.
  • the width of the gap portion between the diffraction grating units 101 of the conductive units 112 is desirably determined in consideration of voltage withstanding of the photoconductive layer 110.
  • the photoconductive layer 110 is made of LT-GaAs, it is only necessary to determine the width of the gap between the diffraction grating units 101 to be about 1.0 to tens of micrometers.
  • the diffraction grating unit 101 is desirably made of a highly conductive material. Specifically, Au is used for example.
  • An unillustrated antireflection film layer for controlling reflection of the excitation light may be provided in a region including the photoconductive layer 110 and the diffraction grating unit 101 of the gap portion.
  • the surface plasmon resonance is excited most efficiently when a polarization direction of the excitation light is parallel to the longitudinal direction of the antenna unit 102 (the direction of IC-IC in Fig. 1A).
  • the condition of the surface plasmon resonance is expressed by Expression (1).
  • a speed of light is denoted by c
  • a wavelength of the excitation light is denoted by ⁇
  • an angular frequency of the excitation light is denoted by ⁇
  • an incidence angle of the excitation light to the element 100 is denoted by ⁇
  • a real part of the dielectric constant of the diffraction grating unit 101 in the wavelength of the excitation light is denoted by ⁇ g
  • a period of the diffraction grating unit 101 is denoted by L
  • a real part of an effectual complex dielectric constant determined by an area and a complex dielectric constant of a substance in contact with the diffraction grating unit 101 is denoted by ⁇ p.
  • Expression (1) m is a positive integer.
  • Expression (2) is obtained.
  • the real part of the dielectric constant of the material of the diffraction grating unit 101 should take a negative value, and the absolute value thereof should be equal to or lower than the effectual complex dielectric constant determined by the area and the complex dielectric constant of the substance in contact with the diffraction grating unit 101.
  • the diffraction grating unit 101 contains a material having a negative dielectric constant.
  • the diffraction grating unit 101 contains silver (Ag), gold (Au), copper (Cu), platinum (Pt), aluminum (Al), and the like as its material.
  • the diffraction grating unit 101 may contain a material partially having a positive dielectric constant, and it is only necessary that the real part of the dielectric constant as the entire diffraction grating unit 101 is negative, and that the absolute value thereof is higher than the effectual complex dielectric constant determined by the area and the complex dielectric constant of the substance in contact with the diffraction grating unit 101.
  • the period L of the diffraction grating unit 101 that enables the surface plasmon resonance is uniquely determined by the effectual complex dielectric constant which is determined by the wavelength of the excitation light, the real part of the dielectric constant of the material of the grating 150 of the diffraction grating unit 101, and the area and the complex dielectric constant of the substance in contact with the diffraction grating unit 101.
  • the photoconductive layer 110 is made of LT-GaAs
  • the diffraction grating unit 101 is made of Au is discussed.
  • the period L of the diffraction grating units 101 at which the surface plasmon resonance is excited is about 200 nm.
  • the period L of the diffraction grating unit 101 at which the surface plasmon resonance is excited is about 700 nm.
  • the diffraction grating unit 101 is constituted by a plurality of gratings 150 of a constant period which can excite the surface plasmon resonance to the excitation light. Therefore, the period L of the diffraction grating units 101 is desirably the same as the period at which the gratings 150 are arranged. If the wavelength of the excitation light is a broadband as in the femtosecond pulsed light, the period L of the diffraction grating units 101 is desirably set in accordance with the wavelength of which output is the highest.
  • each of the gratings 150 of the diffraction grating units 101 is desirably about 1/10 of the wavelength of the excitation light, but it is only necessary that the thickness of each of the gratings 150 is at least 1/3 or less of the wavelength of the excitation light. If the thickness is greater than the value described above, reflection of the excitation light generated within the diffraction grating unit 101 becomes larger, and efficient surface plasmon resonance becomes difficult. If an antireflection film is provided, it is only necessary that the thickness of each of the gratings 150 is 1/3n or less of the wavelength of the excitation light in consideration of a refractive index n of the antireflection film in the wavelength of the excitation light.
  • the antenna units 102 may be stripline antennas, bow tie antennas, spiral antennas, and the like depending on applications.
  • the present inventor has found that a plurality of pulses exist on the time waveform in the related art, and that one of the causes of deformation is a characteristic impedance mismatch between the contact portion (a connection face) 160 of the grating 150 of the diffraction grating unit 101 and the antenna unit 102. If the characteristic impedance mismatch occurs in the contact portion 160, a photoelectric current flowing from the diffraction grating unit 101 is reflected to generate a reflected wave, which may induce resonance and loss of a THz wave of a frequency different from a desired frequency. This may cause a THz wave pulse with a plurality of peaks or deformation of pulse shape. In the present embodiment, the characteristic impedance mismatch is reduced to thereby reduce the reflected wave of the photoelectric current in the contact portion 160.
  • a relationship between a difference in the characteristic impedances and the reflected wave of the photoelectric current in a case where a surface with a characteristic impedance mismatch exists between the diffraction grating unit 101 and the antenna unit 102 is described. Since the diffraction grating unit 101 and the antenna unit 102 are connected electrically and in contact with each other in the present embodiment, a relationship between a difference in the characteristic impedances and the reflected wave of the photoelectric current in the contact portion 160 between the diffraction grating unit 101 and the antenna unit 102 is described.
  • a plurality of gratings 150 of the diffraction grating unit 101 can be considered as a plurality of transmission lines arranged in parallel and in a periodic manner.
  • the diffraction grating unit 101 having a plurality of parallelly arranged transmission lines forms the connection face 160 with the antenna unit 102 which is a single transmission line and is connected electrically with the antenna unit 102.
  • the characteristic impedance of the entire diffraction grating unit 10 is denoted by Z1 and the characteristic impedance of the antenna unit 102 is denoted by Z2
  • a current reflection coefficient R of the antenna unit 102 can be expressed by Expression (3).
  • Expression (3) the orientation from the antenna unit 102 to the diffraction grating unit 101 is positive.
  • the "current reflection coefficient" herein is defined as a ratio of the amplitude of the reflected wave to the amplitude of the incident wave in a case where the incident wave traveling from the diffraction grating unit 101 to the antenna unit 102 is reflected in the direction of the diffraction grating unit 101 in a contact portion of the two members which are in contact with each other and are electrically connected each other, and the reflected wave is generated.
  • the reflected wave is generated in the contact portion due to different characteristic impedances of the two members.
  • one of the two members located on the side of the antenna unit 102 is referred to as a first member and the other located on the side of the diffraction grating unit 101 is referred to as a second member.
  • a ratio of the amplitude of the reflected wave generated in the contact portion of the first member and the second member to the amplitude of the incident wave is referred to as a current reflection coefficient in the contact portion of the first member and the second member.
  • the current reflection coefficient may be simply referred to as the "current reflection coefficient of the first member” but the same in meaning.
  • the ratio of the amplitude of the reflected wave generated by reflecting in the direction of the diffraction grating unit 101 in the contact portion 160 to the amplitude of the incident wave traveling from the diffraction grating unit 101 to the antenna unit 102 is referred to as a "current reflection coefficient in the contact portion 160 of the diffraction grating unit 101 and the antenna unit 102.”
  • the "current reflection coefficient in the contact portion 160 of the diffraction grating unit 101 and the antenna unit 102" may be referred to as a "current reflection coefficient in the contact portion 160" or a "current reflection coefficient in the contact portion 160.”
  • the contact portion 160 herein refers to a contact portion of the entire diffraction grating unit 101 and the antenna unit 102, and the diffraction grating unit 101 includes a surface on which each of the plurality of gratings 150 and the antenna unit 102 are in contact with each other.
  • the characteristic impedance of the diffraction grating unit 101 and the characteristic impedance of the antenna unit 102 are determined depending mainly on the frequency of the incident wave, the width, the thickness, and the arrangement of the diffraction grating units 101 and the antenna unit 102, the dielectric constant of the photoconductive layer 110, and the like. Therefore, in the present embodiment, the reflected wave of the photoelectric current in the contact portion 160 is reduced by adjusting the width and the thickness of the gratings 150 in the diffraction grating unit 101, and the width and the thickness of the antenna unit 102.
  • the refractive index of the measurement target acquires, as a reference, an amplitude spectrum obtained from the time waveform of the reflected pulse from a metal mirror of which reflectance is about 100%. If the reflectance is acquired from the ratio of this amplitude spectrum and the amplitude spectrum obtained from the time waveform of the reflected pulse from the measurement target, the refractive index from that reflectance is obtainable.
  • the measurement value of the reflected pulse from the measurement target is generally significantly small compared with the measurement value of the reflected pulse from a metal mirror as a reference. Therefore, there is a case where the reflected wave of the photoelectric current is confirmed in the time waveform of the reflected pulse from the metal mirror but a signal of the reflected wave of the photoelectric current may become noise or below noise, and cannot be confirmed in the time waveform of the reflected pulse from the measurement target.
  • an error of measurement of the refractive index will be about 0.03 at the maximum if the refractive index of the measurement target is about 1.5 when the current reflection coefficient is ⁇ 5%. Similarly, if the current reflection coefficient R is ⁇ 1%, the error of measurement of the refractive index will be 0.01 or less at the maximum.
  • the current reflection coefficient R in the contact portion 160 is desirably set to be as small as possible.
  • the current reflection coefficient R in the contact portion 160 is desirably set to be ⁇ 5% or less, and more preferably set to be ⁇ 1% or less.
  • the current reflection coefficient R of the antenna unit 102 is ⁇ 5% or less if the diffraction grating unit 101 and the antenna unit 102 are considered to be transmission lines is described.
  • a case where the thickness of the diffraction grating unit 101 and the thickness of the antenna unit 102 are the same is supposed as illustrated in Fig. 1B.
  • the diffraction grating unit 101 and the antenna unit 102 of the present embodiment are disposed on a 500- ⁇ m-thick GaAs substrate.
  • the GaAs substrate is grounded on the back side.
  • the characteristic impedance of the diffraction grating unit 101 and the characteristic impedance of the antenna unit 102 at 30 GHz are calculated independently with the period L of the diffraction grating unit 101 being 700 nm as a fixed value, and the current reflection coefficient R of the antenna unit 102 is calculated from the obtained characteristic impedances.
  • a coplanar waveguide with ground calculator is applied to calculate the characteristic impedance of the diffraction grating unit 101, and a microstrip line calculator with ground is applied to the antenna unit 102.
  • the characteristic impedance of the diffraction grating unit 101 is about 60 ohm. If the width of the grating 150 is reduced to as narrow as about 100 nm, the distance between the gratings 150 becomes about 600 nm, and the characteristic impedance is increased to about 90 ohm. If the width of the antenna unit 102 is about 50 ⁇ m, the characteristic impedance of the antenna unit 102 is about 90 ohm, whereas, if the width of the antenna unit 102 is increased as large as 180 ⁇ m, the characteristic impedance is decreased to about 60 ohm.
  • the width of the antenna unit 102 is set to be equal to or greater than 170 ⁇ m to equal to or less than 290 ⁇ m if the width of the grating 150 is 350 nm to satisfy the conditions under which the current reflection coefficient R of the antenna unit 102 becomes ⁇ 5% or less. If the width of the grating 150 is reduced to as narrow as 100 nm, the current reflection coefficient R of the antenna unit 102 can be set to ⁇ 5% or less by setting the width of the antenna unit to be equal to or greater than 30 ⁇ m to equal to or less than 70 ⁇ m.
  • the current reflection coefficient R of the antenna unit 102 can be set to be ⁇ 5% or less by adjusting the current reflection coefficient R by adjusting the widths of the grating 150 of the diffraction grating unit 101 and the width of the antenna unit 102. It is considered that, with this configuration, the ratio at which a component originating from the reflected wave of the photoelectric current mixes in the amplitude spectrum obtained from the time waveform of the reflected pulse by Fourier-transformation using a measuring apparatus including the element 100 can be reduced.
  • a case where a plurality of connection faces with different characteristic impedances are provided is also included in the present invention. That is, a case where the diffraction grating unit 101 and the antenna unit 102 are not in contact with each other, and the diffraction grating unit 101 and the antenna unit 102 are electrically connected with each other via another structure of which characteristic impedance is different from that of the diffraction grating unit 101 or that of the antenna unit 102 is also included in the present invention.
  • Another structure is, for example, a connecting portion disposed between the diffraction grating unit 101 and the antenna unit 102, and electrically connecting the diffraction grating unit 101 and the antenna unit 102.
  • An exemplary connecting portion includes an extending portion for extending the width of the grating 150 toward the antenna unit 102.
  • the current reflection coefficient R in each contact portion is desirably set to be in the above-described range. That is, if the current reflection coefficient R in each contact portion is set to be in the above-described range, a configuration in which contact portions with different characteristic impedances are provided between the diffraction grating unit 101 and the antenna unit 102 is also included in the present invention. Details about this configuration are described in a second embodiment.
  • the characteristic impedance of the diffraction grating unit 101 can be estimated by calculation.
  • a calculation method supposing that the diffraction grating unit 101 is a uniform resistor body is described.
  • each of the diffraction grating unit 101 and the antenna unit 102 are considered to be a structure which is small enough relative to the wavelength of the THz wave.
  • a calculation method supposing that each of the diffraction grating unit 101 and the antenna unit 102 is a resistor body is described.
  • a photoconductive element 800 as illustrated in Fig. 8 is considered as an example.
  • the photoconductive element 800 has diffraction grating units 801 and antenna units 102.
  • the material and the like of the diffraction grating unit 801 are the same as those of the diffraction grating unit 101.
  • the diffraction grating unit 801 can be regarded as a lumped constant circuit in which each of the gratings 150 in the diffraction grating unit 101 is regarded as a resistor body and a plurality of gratings 150 are arranged in parallel.
  • the ratio of the width of the grating 150 of the diffraction grating unit 801 to the distance between the gratings 150 is 1:1, and the period L is 700 nm.
  • the length of the grating 150 is 3 ⁇ m
  • the thickness of the grating 150 is 100 nm
  • five gratings 150 are arranged in a periodic manner.
  • the material of the grating 150 is Au. If resistivity of the grating 150 is known, the gratings 150 can be regarded as a plurality of resistor bodies which are arranged in parallel, and the entire resistance, i.e., the characteristic impedance, of the diffraction grating unit 801 can be estimated from the resistance values of the resistor bodies. If Au is a thin layer of about 100 to 300 nm in thickness, the resistivity is about 0.03 ohm micrometer.
  • resistance of the grating 150 is 2.57 ohm and resistance of the entire diffraction grating unit is about 0.5 ohm. If the thickness of the antenna unit 102 is 300 nm, the width of the antenna unit 102 is 10 ⁇ m, and the length of the projecting portion of the antenna unit 102 is 20 ⁇ m, the resistance value of the antenna unit 102 is about 0.2 ohm. In accordance with these calculation results, the structure of the diffraction grating unit 801 and the structure of the antenna unit 102 are determined in a manner such that the current reflection coefficient R in the contact portion 160 becomes ⁇ 5% or less.
  • a method for determining the structure of the diffraction grating unit 801 and the structure of the antenna unit 102 in accordance with the calculation results and measurement results of the characteristic impedances of the diffraction grating unit 801 and the antenna unit 102 is also effective. That is, for the design of the element 100, it is not necessary to use the calculation method of the characteristic impedance in which the above-described transmission line is supposed, but a calculation method supposing that the diffraction grating unit 101 is a lumped constant circuit, a method using a measurement result, and the like can be used.
  • a plurality of electrodes 103 connect with an external apparatus via unillustrated signal lines for the application of a voltage between the antenna units 102 or for the measurement of a current flowing between the antenna units 102.
  • a metal wire used also as the signal line to the electrode portion 103 by wire bonding and the like is often fused and joined.
  • FIG. 1A it is only necessary that two electrodes 103 and a signal line are connected so that a voltage may be applied to the gap portion or a current in the gap portion may be detected.
  • the diffraction grating unit 101 and the antenna unit 102 are made of the same material and formed to the same thickness.
  • the metal wire material, the diffraction grating unit 101 and the antenna unit 102 may be different in melting point and thermal conductivity, may have no bonding affinity due to factors, such as surface oxidization, or may be insufficient in thickness of the electrode 103 necessary for bonding.
  • an electrode 103 of the thickness necessary for the bonding is desirably formed on the material of the antenna unit 102 using a material which is easy to join to an unillustrated signal line as illustrated in Fig. 1C.
  • a propagation distance (a transmission line length) of the photoelectric current from the center of the gap portion to the electrode 103 is desirably set to be 1.5 mm or longer. The reason is discussed below.
  • the effective specific dielectric constant ⁇ r is at least a value of about 9 or greater depending on the structure of the antenna unit 102, such as the width and the thickness, and the frequency of the THz region. Therefore, if the transmission line length d is 1.5 mm or longer, a time difference after the THz wave is generated or detected, the reflected wave of the photoelectric current generated at the electrode 103 returns to the gap portion, and until the reflected wave is generated or detected becomes 30 ps or longer.
  • the time difference of 30 ps or longer is sufficiently long delay time in the time waveform when evaluating the form and physical properties of a measurement target in the THz-TDS apparatus. For example, if the transmission line length d is 1.5 mm, a time difference after the THz wave is generated or detected, the reflected wave of the photoelectric current generated at the electrode 103 returns to the gap portion, and until the reflected wave is generated or detected becomes about 30 ps. Therefore, even if a time waveform including a reflected wave is Fourier-transformed and a frequency spectrum is obtained, the resonance frequency thereof is near 15 GHz and, it is considered that, which does not affect the measurement result and the analysis result when the frequency resolution is about 30 GHz.
  • the reflected wave of the photoelectric current can be reduced by matching the characteristic impedance of the diffraction grating unit 101 and the characteristic impedance of the antenna unit 102, and increasing the transmission line length between the electrode 103 and the diffraction grating unit 101 to an extent not to affect the measurement result.
  • the reflected wave of the photoelectric current in the contact portion of the diffraction grating unit 101 and the antenna unit 102 can be reduced.
  • influences of the reflected wave in the contact portion can be reduced, and the THz wave can be made closer to the monopulse than before. That is, in the photoconductive element using surface plasmon resonance, a THz wave pulse closer to the monopulse than before can be obtained.
  • FIG. 2A is a perspective view illustrating a configuration of the element 200.
  • Fig. 2B is a cross-sectional view of the element 200 along line IIB-IIB.
  • the diffraction grating unit 101 and the antenna unit 102 are disposed in contact with each other and are directly connected electrically with each other in each of a plurality of conductive units 112.
  • a conductive unit 212 of the element 200 of the present embodiment has a connecting portion 104 between a diffraction grating unit 101 and an antenna unit 102. That is, the diffraction grating unit 101 is not disposed in contact with the antenna unit 102.
  • the same configurations as those of the first embodiment are denoted by the same reference numerals and description thereof will be omitted.
  • the diffraction grating unit 101 and the antenna unit 102 are connected in contact with each other as in the first embodiment, it may not be easy to reduce the reflected wave of the photoelectric current by adjusting the structure of the diffraction grating unit 101 and the structure of the antenna unit 102.
  • another configuration such as a connecting portion 104 is provided between the diffraction grating unit 101 and the antenna unit 102 to increase junction points with different characteristic impedances. With this configuration, impedance mismatch points can be distributed and the reflected wave of the photoelectric current at each junction point can be reduced.
  • the connecting portion 104 is an extending portion which extends a line width toward the antenna unit 102 from the width of the grating 150.
  • the connecting portion 104 is disposed between the diffraction grating unit 101 and the antenna unit 102, and extends the line width from the gratings 150 toward the antenna unit 102 in a plurality of stages in a range smaller than the width of the antenna unit 102.
  • the connecting portion 104 has a plurality of extending portions and, as illustrated in Fig. 2A, two adjoining gratings 150 are connected in contact with a first extending portion which is wider than the grating 150. Two adjoining first extending portions are connected in contact with a second extending portion which is still wider than the first extending portion.
  • the connecting portion 104 of the present embodiment extends the line width in a plurality of stages so that the impedance of the diffraction grating unit 101 becomes closer to the impedance of the antenna unit 102.
  • the connecting portion 104 has a first contact portion 170 with the diffraction grating unit 101, a second contact portion 171 and a third contact portion 172 at which lines of different line widths are connected in the connecting portion 104, and a contact portion 173 of the connecting portion 104 and the antenna unit 103.
  • the reflected wave of the photoelectric current due to the characteristic impedance mismatch is distributed by extending the line width in a plurality of stages by the connecting portion 104 from the width of each grating 150 and finally connecting to the antenna unit 102.
  • the magnitude of the reflected wave of the photoelectric current due to the characteristic impedance mismatch in each of the contact portions 170 to 173, i.e., the current reflection coefficient in the contact portion can be reduced.
  • the width of extension for each extension event in the connecting portion 104 should be as narrow as possible depending on various restrictions, such as the structure of the diffraction grating unit 101, the structure of the antenna unit 102, and the line width of the machining limit of the connecting portion 104, and the like.
  • the connecting portion 104 is desirably configured so that the current reflection coefficient in each of the contact portions 170 to 173 is ⁇ 5% or less, and more preferably ⁇ 1% or less.
  • An exemplary configuration of the element 200 is described below. The element 200 is configured so that the current reflection coefficient in each of a plurality of contact portions 170 to 173 is ⁇ 5% or less.
  • the thickness of the diffraction grating unit 101 and the thickness of antenna unit 102 are the same as in the first embodiment.
  • the diffraction grating unit 101 and the antenna unit 102 are disposed on a 500- ⁇ m-thick GaAs substrate grounded on the back side thereof.
  • the period L of the gratings 150 of the diffraction grating unit 101 is 700 nm and the width of each of a plurality of gratings 150 is 350 nm as a fixed value.
  • the characteristic impedance of the diffraction grating unit 101 is about 56 ohm.
  • the characteristic impedance thereof is about 61 ohm. If a portion between the contact portion 171 and the contact portion 172 of the connecting portion 104 (the second extending portion) has a periodical structure with a width of about 1000 nm at a period of 2800 nm, the characteristic impedance thereof is about 67 ohm.
  • the characteristic impedance thereof is about 73 ohm. Therefore, the current reflection coefficient in each of the contact portions 170 to 172 can be made ⁇ 5% or less. Further, by setting the width of the antenna unit 103 to be in the range of 80 ⁇ m to 140 ⁇ m, the characteristic impedance becomes in the range of 65 to 80 ohm, and the current reflection coefficient in the contact portion 173 of the antenna unit 102 and the connecting portion 104 can also be set to be ⁇ 5% or less.
  • the width of the antenna unit at which the current reflection coefficient becomes ⁇ 5% or less is in the range of 170 ⁇ m to 290 ⁇ m.
  • the width of the antenna unit 102 at which the current reflection coefficient becomes ⁇ 5% or less can be reduced to the range of 80 ⁇ m to 140 ⁇ m by providing the connecting portion 104. That is, by providing the connecting portion 104, a degree of freedom of the structure, such as the width of the grating 150 of the diffraction grating unit 101, and the width and the thickness of the antenna unit 102, can be increased.
  • the reflected wave of the photoelectric current in the contact portion of the diffraction grating unit 101 and the antenna unit 102 can be reduced.
  • the time waveform of the THz wave pulse generated in the element 200 and the THz wave pulse detected in the element 200 can be made closer to the monopulse than before. That is, in the photoconductive element using surface plasmon resonance, a THz wave pulse closer to the monopulse than before can be obtained.
  • the connecting portion 104 between the diffraction grating unit 101 and the antenna unit 102 the reflected wave of the photoelectric current is distributed in the region of the tapered portion 105, whereby the magnitude of the reflected wave in each contact portion, i.e., the current reflection coefficient can further be reduced.
  • the degree of freedom of the structures such as the width of the grating 150 of the diffraction grating unit 101, and the width and the thickness of the antenna unit 102, can be increased.
  • FIG. 3A is a top view illustrating a configuration of the element 300
  • Fig. 3B is a cross-sectional view of the element 300 of Fig. 3A along line IIIB-IIIB
  • Fig. 3C is a schematic diagram illustrating a configuration near a diffraction grating unit 101 of Fig. 3A.
  • a conductive unit 312 of the element 300 further has a tapered portion 105 as a connecting portion between the diffraction grating unit 101 and the antenna unit 102.
  • the diffraction grating unit 101 and the antenna unit 102 are connected electrically via the tapered portion 105.
  • Other configurations are the same as those of the first embodiment, and description thereof is omitted.
  • the tapered portion 105 is an extending portion which extends a line width toward the antenna unit 102 from the width of the grating 150.
  • the tapered portion 105 has a tapered structure of which thickness and width becomes gradually larger from the diffraction grating unit 101 toward the antenna unit 102.
  • the tapered portion 105 is disposed between the diffraction grating unit 101 and the antenna unit 102. Therefore, the tapered portion 105 has a contact portion 180 with the diffraction grating unit 101 and a contact portion 181 with the antenna unit 102.
  • a characteristic impedance in the tapered portion 105 changes continuously.
  • the current reflection coefficient of the antenna unit 102 (the current reflection coefficient on an interface between the antenna unit 102 and the tapered portion 105) is set to be ⁇ 5% or less, and more preferably, set to be ⁇ 1% or less.
  • the current reflection coefficient of the tapered portion 105 (the current reflection coefficient on an interface between tapered portion 105 and the diffraction grating unit 101) is set to be ⁇ 5% or less, and more preferably, set to be ⁇ 1% or less.
  • the reflected wave of the photoelectric current due to an impedance mismatch in the tapered portion 105 can be distributed to the tapered portion 105 more uniformly than in the structure in which the thickness or the width are formed as steps like in the first and the second embodiment described above.
  • the tapered portion 105 it can be regarded that countless transmission lines with different impedances are connected electrically in the tapered portion 105. Therefore, by distributing the reflected wave of the photoelectric current generated in an area from the diffraction grating unit 101 to the antenna unit 102 in the region of the tapered portion 105, the magnitude, of the reflected wave in each connection face, i.e., the current reflection coefficient, can further be reduced. It is better to set the inclination of the tapered portion in the thickness direction and the width direction to be as small as possible, and more preferably to be in the range of 1 to 45 degrees, but the range is not restrictive.
  • the element 300 can reduce the reflected wave of the photoelectric current produced due to the difference in the characteristic impedance between the diffraction grating unit 101 and the antenna unit 102.
  • the time waveform of the THz wave pulse generated in the element 300 and the THz wave pulse detected in the element 300 can be made closer to the monopulse than before. That is, in the photoconductive element using surface plasmon resonance, a THz wave pulse closer to the monopulse than before can be obtained.
  • the tapered portion having the tapered structure as the connecting portion, the reflected wave of the photoelectric current is distributed in the region of the tapered portion 105, whereby the magnitude of the reflected wave in each contact portion, i.e., the current reflection coefficient can further be reduced.
  • the degree of freedom of the structures such as the width of the grating 150 of the diffraction grating unit 101, and the width and the thickness of the antenna unit 102, can be increased.
  • FIG. 4 is a top view illustrating a configuration of the element 400.
  • a conductive unit 412 of the element 400 includes an arc-shaped portion 106 which is formed as an arc shape at a part of an antenna unit 402.
  • Other configurations are the same as those of the first embodiment, and description thereof is omitted.
  • the antenna unit 102 and the electrode 103 are rectangular as illustrated in Figs. 1A and 3A in the simplest configuration.
  • the length of the long side of the rectangular shape may hinder the reduction in size of the photoconductive element.
  • reflected wave of the photoelectric current may be generated at the angled portion, which may induce resonance of a terahertz wave, loss of a detection current, and the like.
  • the antenna unit 402 with no angled portion is formed, and the antenna unit 402 reduces the size of the element 400.
  • the propagation length (the transmission line length) of the photoelectric current from the gap portion between the antennas to the electrode 103 is desirably set to 1.5 mm or longer as in the first embodiment.
  • the curvature radius of the arc-shaped portion 106 may be equal to or greater than the width of the antenna unit 402 in the length side, and may be about several micrometers to about several millimeters.
  • a plurality of arc-shaped portions 106 may be provided between one electrode 103 and the diffraction grating unit 101.
  • the element 400 can reduce the reflected wave of the photoelectric current produced due to the difference in the characteristic impedance between the diffraction grating unit 101 and the antenna unit 402.
  • the time waveform of the THz wave pulse generated in the element 400 and the THz wave pulse detected in the element 400 can be made closer to the monopulse than before. That is, in the photoconductive element using surface plasmon resonance, a THz wave pulse closer to the monopulse than before can be obtained.
  • the size of the element 400 can be reduced while keeping the transmission line length being 1.5 mm or longer.
  • FIG. 5 is a cross-sectional view illustrating a configuration of the element 500.
  • the element 500 of the present embodiment uses a Si substrate 511 as a substrate 111 in the first embodiment, and further has a lattice matching layer 501.
  • Other configurations are the same as those of the first embodiment, and description thereof is omitted.
  • the generated terahertz wave is emitted toward the substrate 111 or the terahertz wave is detected from the side of the substrate 111 in the element 100 of the first embodiment, the band is restricted by phonon absorption of the terahertz wave in the substrate 111. Therefore, in the present embodiment, the Si substrate 511 with no phonon absorption is used in the frequency range of the terahertz wave is used as the substrate 111.
  • the photoconductive layer 110 is formed on the lattice matching layer 501 after the lattice matching layer 501 which lattice-matches with the photoconductive layer 110 is formed on the Si substrate 511.
  • the lattice matching layer 501 is made of AlGaAs, or formed as a layer containing AlAs, GaAs, and Ge. With this configuration, a lattice mismatch between the Si substrate 511 and the photoconductive layer 110 can be absorbed.
  • the lattice matching layer 501 is desirably made of a material with higher resistance than that of the Si substrate 511.
  • the element 500 can reduce the reflected wave of the photoelectric current produced due to the difference in the characteristic impedance between the diffraction grating unit 101 and the antenna unit 102.
  • the time waveform of the THz wave pulse generated in the element 500 and the THz wave pulse detected in the element 500 can be made closer to the monopulse than before. That is, in the photoconductive element using surface plasmon resonance, a THz wave pulse closer to the monopulse than before can be obtained.
  • the substrate 111 with no phonon absorption in the frequency range of the THz wave is applicable by forming the lattice matching layer 501 between the substrate 111 and the photoconductive layer 110.
  • a method for manufacturing the element 100 of the first embodiment is described with reference to Fig. 6.
  • Fig. 6 is a flowchart illustrating the method for manufacturing the element 100. Description of the components common to those described above is omitted.
  • a method for collectively exposing the diffraction grating unit 101, the antenna unit 102, and the electrode 103 provided in the conductive unit 112 is described.
  • LT-GaAs formed on a Si substrate as illustrated in the fifth embodiment is selected as a photoconductive layer 110, and Au is selected as the material of the diffraction grating unit 101, the antenna unit 102, and the electrode 103.
  • SiO 2 is selected as the antireflection film, the excitation light wavelength is set to be 1.55 ⁇ m, and the career excitation method in LT-GaAs is multiphoton absorption.
  • the career generation efficiency by multiphoton absorption which is a kind of nonlinear optical effects is proportional to square of the electric field generated in the photoconductive layer 110. Therefore, the effect of improvement in the career generation efficiency by an electric field enhancement effect by surface plasmon resonance and an electric field concentration effect by diffraction are greater in the case of multiphoton absorption compared with the case of one-photon absorption.
  • the dielectric constant of Au is -135, and the dielectric constant of LT-GaAs is 11.4 by the Drude model.
  • a real part ⁇ p of an effectual complex dielectric constant determined by an area of a substance in contact with the diffraction grating unit 101, such as LT-GaAs, and a complex dielectric constant is calculated to be 6.
  • the thickness of the diffraction grating unit 101 is about 100 nm in consideration of the refractive index of the antireflection film SiO 2 .
  • a method for manufacturing the element 100 is described with reference to the flowchart of Fig. 6.
  • a 5-nm-thick titanium (Ti) film is formed on LT-GaAs to increase adhesiveness between LT-GaAs as the photoconductive layer 110 and Au as the diffraction grating unit 101, the antenna unit 102, and the electrode 103 (S601).
  • a 100-nm-thick Au layer containing Au is formed on the Ti film (S602).
  • Film formation is conducted by EB vapor deposition, sputtering, and other methods.
  • the Ti film has a positive dielectric constant at 1.55 ⁇ m, but has a low abundance ratio to the Au layer. Therefore, since the entire diffraction grating unit 101 has a negative dielectric constant and the absolute value thereof is higher than that of the real part ⁇ p of the effectual complex dielectric constant, surface plasmon resonance can be excited.
  • a SiN layer containing SiN used as a mask material when etching the Au layer is formed on the Au layer by plasma CVD (S603).
  • metal masks such as Nl, SiO 2 , or Cr, may be used.
  • photoresist is applied to the SiN layer (S604) and the SiN layer is exposed (S605). Exposure may be conducted by, besides an electron beam exposure apparatus, an exposure apparatus capable of exposing fine line widths of about 300 nm, such as a i-line stepper and a KrF stepper. After the exposure, development is conducted and a resist pattern is formed (S606). Then, the SiN layer is dry-etched (S607).
  • Etching gas for the SiN layer is desirably CF-based gas of which selection ratio in the photoresist and Au is high.
  • the pressure during the etching is desirably reduced as much as possible to increase anisotropy of the etching.
  • the photoresist is removed by ashing with O 2 (S608).
  • the Au layer and the Ti film are etched by using the SiN layer as a mask (S609).
  • Argon (Ar) gas or mixed gas of Ar and chlorine (Cl2) is used as etching gas.
  • the Ti film which is the diffraction grating unit 101 and the SiN layer used as the mask of the Au layer are dry-etched (S610).
  • SiO 2 is sputtered in the gap portion between the antennas to form the antireflection film (S611).
  • the element 100 can be manufactured by the method described above.
  • the antenna unit 102 and the diffraction grating unit 101 are desirably made of the same material and formed to the same thickness.
  • the diffraction grating unit 101 and the antenna unit 102 are desirably manufactured collectively by, for example, photolithography.
  • the element 200 and the element 300 in which the connecting portion, such as the connecting portion 104 and the tapered portion 105, is provided between the diffraction grating unit 101 and the antenna unit 102 may also be manufactured collectively.
  • the diffraction grating unit 101, the antenna unit 102, and the electrode 103 can be manufactured collectively.
  • a measuring apparatus 700 (hereafter, "apparatus 700") employing any one of the photoconductive elements of the above embodiments is described with reference to Fig. 7. Description of the components common to those described above is omitted.
  • Fig. 7 is a schematic diagram illustrating a configuration of the apparatus 700.
  • the apparatus 700 is a THz-TDS apparatus for acquiring a time waveform of a THz wave pulse by THz-TDS. Since the THz-TDS apparatus itself is basically the same apparatus as those of the related art, only outline thereof will be described.
  • the apparatus 700 has a light source 701 which outputs ultrashort pulsed light as excitation light, a beam splitter 702, a delayer 705, lenses 706 and 707, a generator 710, and a detector 711. Excitation light from the light source 701 is split into pump light 703 and probe light 704 by the beam splitter 702. The pump light 703 enters the generator 710 via the lens 706. The probe light 704 enters the detector 711 via the delayer 705 and the lens 707.
  • fiber type laser which outputs femtosecond pulsed light of a band of 1.5 ⁇ m is used as the light source 701.
  • the generator 710 generates a THz wave pulse upon entrance of pump light.
  • the detector 711 detects a THz wave pulse upon entrance of the THz wave pulse and the probe light.
  • any one of the photoconductive elements of the first to the fifth embodiments is used for at least one of the generator 710 and the detector 711.
  • the photoconductive layer 110 may be made of LT-GaAs, and can generate a career by the multiphoton absorption effect at both cases where the element 500 is used for the generator 710 and the detector 711.
  • the THz wave pulse generated from the generator 710 is guided to a measurement target 708 and is reflected off the measurement target 708.
  • the reflected THz wave pulse is detected by the detector 711.
  • the THz wave pulse reflected off the measurement target 708 includes information about, for example, absorption spectrum of the measurement target 708.
  • the delayer 705 is an optical delay system which changes an optical path length of the probe light 704, and adjusts a difference in the optical path length between the pump light and the probe light by moving the delayer 705. Therefore, timing at which the THz wave pulse is detected at the detector 711 can be controlled.
  • the time waveform of the THz wave pulse is acquirable by using the detection result of the detector 711.
  • a THz wave pulse reflected off the measurement target 708 is detected in the present embodiment, a THz wave pulse passed through the measurement target 708 may be detected by the detector 711.
  • the apparatus 700 uses any of the photoconductive elements of the first to the fifth embodiments for at least one of the generation unit 710 and the detection unit 711.
  • the photoconductive element of each embodiment can reduce the reflected wave of the photoelectric current produced due to the difference in the characteristic impedance between the diffraction grating unit and the antenna unit. Therefore, the apparatus 700 is capable of measuring using a THz wave pulse closer to a monopulse than before, whereby measurement accuracy is increased.

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Abstract

L'invention concerne un élément photoconducteur comprenant : une couche photoconductrice ; et une pluralité d'unités conductrices disposées sur la couche photoconductrice, la pluralité d'unités conductrices comprenant chacune une électrode ; une unité d'antenne électriquement connectée à l'électrode ; et une unité à réseaux de diffraction qui comporte une pluralité de réseaux agencés périodiquement et qui est en contact avec l'unité d'antenne, la pluralité d'unités conductrices étant disposées en regard l'une de l'autre avec un espace laissé entre elles entre les unités à réseaux de diffraction de la pluralité d'unités conductrices, et un coefficient de réflexion de courant dans une partie de contact de l'unité d'antenne et de l'unité à réseaux de diffraction étant inférieur ou égal à ± 5 %.
PCT/JP2016/000801 2015-02-26 2016-02-17 Élément photoconducteur, son procédé de fabrication, et appareil de mesure WO2016136186A1 (fr)

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