WO2010006440A1 - Terahertz photoconductive antennas having transparent conductor electrodes and methods of making same - Google Patents
Terahertz photoconductive antennas having transparent conductor electrodes and methods of making same Download PDFInfo
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- WO2010006440A1 WO2010006440A1 PCT/CA2009/001010 CA2009001010W WO2010006440A1 WO 2010006440 A1 WO2010006440 A1 WO 2010006440A1 CA 2009001010 W CA2009001010 W CA 2009001010W WO 2010006440 A1 WO2010006440 A1 WO 2010006440A1
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Classifications
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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
- H01L31/10—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 characterised by at least one potential-jump barrier or surface barrier, e.g. phototransistors
- H01L31/101—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
- H01L31/08—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
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q9/00—Electrically-short antennas having dimensions not more than twice the operating wavelength and consisting of conductive active radiating elements
- H01Q9/04—Resonant antennas
- H01Q9/16—Resonant antennas with feed intermediate between the extremities of the antenna, e.g. centre-fed dipole
- H01Q9/28—Conical, cylindrical, cage, strip, gauze, or like elements having an extended radiating surface; Elements comprising two conical surfaces having collinear axes and adjacent apices and fed by two-conductor transmission lines
- H01Q9/285—Planar dipole
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S1/00—Masers, i.e. devices using stimulated emission of electromagnetic radiation in the microwave range
- H01S1/02—Masers, i.e. devices using stimulated emission of electromagnetic radiation in the microwave range solid
Definitions
- TITLE TERAHERTZ PHOTOCONDUCTIVE ANTENNAS HAVING TRANSPARENT CONDUCTOR ELECTRODES AND METHODS OF
- a photoconductive antenna comprising: providing a substrate; applying a first electrode to the substrate; applying a layer of photoconductive material to the first electrode; and applying a second electrode to the photoconductive layer.
- the second electrode is spaced from the first electrode by the photoconductive layer.
- the method may further comprise attaching a first antenna contact to a first surface of the first electrode; and attaching a second antenna contact to a second surface of the second electrode.
- the transparent conductor electrodes 14 and 16 are transparent to laser beam 15 having optical or infrared wavelengths, and may either be transparent or reflective to the THz radiation 18 depending on their thicknesses. Generally, a thick transparent conductor electrode or metallic conductor electrode can reflect THz radiation, while a thin transparent conductor electrode can transmit THz radiation.
- the embodiment shown in Figure 1 comprises a thin top transparent conductor electrode 14 and a thin bottom transparent conductor electrode 16, and the generated THz wave 18 is transmitted downward through the thin bottom transparent conductor electrode 16 and upward through the top transparent conductor electrode 14, as there is no reflection.
- Step 310 includes providing a substrate.
- the substrate may be any known suitable material, for example high resistive silicon, semi-insulating GaAs, InP, Silica, or Sapphire.
Abstract
A photoconductive antenna includes a first electrode made of a first conductor material having a low resistivity, a second electrode made of a second conductor material having a low resistivity, and a photoconductor layer of photoconductor material extending between the first electrode and the second electrode. The second electrode is spaced from and generally parallel to the first electrode. The photoconductor material is capable of generating or detecting terahertz radiation when impinged upon by incident laser radiation having an optical or infrared wavelength. At least one of the first conductor material and the second conductor material comprises a transparent conductor material that is transparent to the incident laser radiation.
Description
TITLE: TERAHERTZ PHOTOCONDUCTIVE ANTENNAS HAVING TRANSPARENT CONDUCTOR ELECTRODES AND METHODS OF
MAKING SAME
FIELD [0001] The embodiments herein relate to apparatus and methods for generating and detecting terahertz radiation, and in particular, to terahertz photoconductive antennas.
BACKGROUND
[0002] Terahertz (THz) photoconductive antennas are among the most promising candidates for developing compact, portable, low power consuming, rugged, and low cost THz spectroscopy and imaging systems for variety of applications. In THz photoconductive antennas, a THz wave is generated by impinging pulsed or continuous wave laser beams onto a DC voltage biased photo-absorbing layer made of appropriate photoconductive material. The incident laser beam is absorbed by the photoconductive material and generates free carriers (electrons and holes) by exciting the electrons from a valance band into their excited states in a conduction band. Under the influence of a voltage bias, the free carriers accelerate, generating and radiating a THz wave. [0003] Metallic electrodes printed on the photoconductive layers using lithography techniques are conventionally used for applying the DC voltage bias. A conventional photoconductive antenna contains a planar metallic antenna structure and a small gap (active area) at the feed point of the antenna, where a laser beam is focused to generate free carriers. The antenna structure can be in the form of a dipole, bow-tie, spiral, log-periodic, and other known patterns. The voltage bias is connected to the planar electrodes and an electric field is induced in the gap (active area).
[0004] Increasing the applied electric field can increase the power of the generated THz radiation, and the applied electric field is generally proportional to the amplitude of the applied voltage bias. Therefore, to maximize the power of the generated THz radiation, the applied electric field (and hence the voltage bias) should be as high as possible over the entire volume of the active region of the photoconductor layer. However, the
maximum applicable electric field is limited by material properties of the photoconductor layer such as the breakdown field and thermal failure point. The latter limits the maximum generated DC photocurrent and the applicable optical excitation power. [0005] The electric field distribution between two planar electrodes is not uniform, and can be much larger at areas close to the edges of the electrodes in comparison to the regions away from the electrodes. Increasing the applied voltage bias beyond a certain limit can degrade the material properties of areas of the photoconductor layer close to the electrodes and eventually break down the material, while the electric field at other regions of the photoconductor layer such as the regions between the electrodes, is far below the breakdown limit. This puts an upper limit on the maximum applicable voltage bias to the electrodes and tends to lower the efficiency of the photoconductive antenna. [0006] For a given applied voltage bias, the greater the distance between the two planar electrodes, the lower the electric field between the two electrodes, while the electric field close to the electrodes can still be high. To increase the uniformity of the applied electric field between the two electrodes, interdigitated electrodes have been widely used in photoconductive antennas. In these structures, interdigitated electrode fingers with sub-micron widths and sub-micron gaps between adjacent electrode fingers are formed on a photoconductor layer. While interdigitated electrodes increase the efficiency of the photoconductive antennas to some extent, they have their own limitations and drawbacks. The sub-micron size for the electrode fingers makes the required lithography process for manufacturing prohibitively difficult and expensive. The electric filed is still large close to the electrode edges and thus limits the maximum applicable voltage bias, which wears the device out over the time. More importantly, the interdigitated pattern of the electrodes exhibits a parasitic characteristic capacitance, which drastically limits the maximum operation frequency of the photoconductive antennas. This electrode parasitic capacitance causes the power of the generated THz radiation to experience an extra 6 dB drop per octave (4 times drop as the frequency doubles). While photoconductive antennas with
interdigitated electrodes have been able to generate radiation with more than 10 mW of power at 100 MHz frequency, the power of the generated radiation drops to about 1 μw at 1 THz, mainly due to the electrode capacitance limitation. The interdigitated electrodes block a part of the impinging laser beams (up to 50%) and hence reduce the optical-to-THz power conversion efficiency of conventional photoconductive antennas.
[0007] Despite enormous efforts by researchers to make the photoconductive antennas more efficient in terms of their output power and maximum operation frequency, photoconductive antennas still suffer from their existing limitations. This makes the conventional photoconductive antennas unable to address the technical requirements for the THz radiation sources in spectroscopy and imaging systems for many applications. In addition to the existing limitations that include material properties, such as carrier lifetime, carrier mobility, material resistivity and such, limitations imposed by the planar metallic electrodes used in conventional photoconductive antennas play an important role in this deficiency.
[0008] There is accordingly a need for improved terahertz photoconductive antennas, which do not suffer from at least some of the limitations associated with known photoconductive antennas. SUMMARY
[0009] According to one aspect of the invention, there is provided a photoconductive antenna comprising a first electrode made of a first conductor material having a low resistivity, a second electrode made of a second conductor material having a low resistivity, the second electrode being spaced from and generally parallel to the first electrode, and a photoconductor layer of photoconductor material extending between the first electrode and the second electrode, the photoconductor material being capable of generating or detecting terahertz radiation when impinged upon by incident laser radiation having an optical or infrared wavelength, wherein at least one of the first conductor material and the second conductor material comprises a transparent conductor material that is transparent to the incident laser radiation.
[0010] The transparent conductor material may comprise an indium oxide material such as Sn-doped indium tin oxide or Zn stabilized amorphous indium oxide, or any other known transparent conducting materials and/or transparent conducting polymers. [0011] At least one of the first electrode and the second electrode may comprise a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation, or a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation. In some embodiments, the thin layer of the transparent conductor material has a thickness of about 10 nm, and the thick layer of the transparent conductor material has a thickness greater than about 200 nm.
[0012] In some embodiments, the first electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation, and the second electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation. In other embodiments, the first electrode comprises a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation, and the second electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation.
[0013] In other embodiments, the first electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation, or a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation, and the second electrode comprises a metallic electrode made of a layer of metallic material that reflects the terahertz radiation.
[0014] According to another aspect of the invention, there is provided a photoconductive antenna, comprising a substrate, a bottom electrode layer of
bottom conductor material applied to a portion of the substrate, the bottom conductor material having a low resistivity, a photoconductor layer of photoconductor material applied to the bottom electrode layer, the photoconductive material being capable of generating and detecting pulsed and continuous-wave terahertz radiation when impinged upon by incident laser radiation having an optical or infrared wavelength, and a top electrode layer of top conductor material applied to the photoconductor layer, the top conductor material having a low resistivity, the top electrode layer being spaced from the bottom electrode layer by the photoconductor layer, wherein the top conductor material comprises a transparent conductor material that is transparent to the laser radiation.
[0015] In some embodiments, the first electrode and the second electrode comprise a transparent conductor material, and the antenna further comprises at least one first antenna contact attached to a first surface of the first electrode; and at least one second antenna contact attached to a second surface of the second electrode; wherein the first and second antenna contacts cooperate with the first and second electrodes and the photoconductive layer so as to radiate or emit generated THz radiation and detect incident THz radiation during use of the antenna. [0016] In some embodiments, the first antenna contact and second antenna contact are arranged in a dipole configuration.
[0017] In some embodiments, the first antenna contact has a first arm extending inwardly to a first end, the second antenna contact has a second arm extending inwardly to a second end, and the first end and the second end are spaced apart so as to define a gap therebetween. In some embodiments, the gap is between about 10 micrometers and 20 micrometers in length.
[0018] In some embodiments, the at least one first antenna contact includes a pair of first antenna contacts, and the at least one second antenna contact includes a pair of second antenna contacts, and the pairs of first and second antenna contacts are arranged in a circular aperture configuration.
[0019] According to another aspect, there is provided method of fabricating a photoconductive antenna comprising: providing a substrate;
applying a first electrode to the substrate; applying a layer of photoconductive material to the first electrode; and applying a second electrode to the photoconductive layer. The second electrode is spaced from the first electrode by the photoconductive layer. [0020] The method may further comprise attaching a first antenna contact to a first surface of the first electrode; and attaching a second antenna contact to a second surface of the second electrode.
[0021] According to yet another embodiment, there is provided a method of fabricating a photoconductive antenna comprising providing a first substrate; providing a sacrificial layer to a surface of the first substrate; applying a photoconductive layer to the sacrificial layer; applying a first electrode to the photoconductive layer; removing the sacrificial layer and the first substrate, then applying a second electrode to the photoconductive layer. The second electrode is spaced from the first electrode by the photoconductive layer.
[0022] The method may further comprise, before removing the sacrificial layer and the first substrate: attaching a first antenna contact to a first surface the first electrode; providing an adhesive to a surface of a second substrate; and applying the second substrate to at least the first antenna contact using the adhesive.
[0023] The method may further comprise, after removing the sacrificial layer and the first substrate, attaching a second antenna contact to a second surface of the second electrode.
BRIEF DESCRIPTION OF THE DRAWINGS [0024] The invention will now be described, by way of example only, with reference to the following drawings, in which:
[0025] Figure 1 is a schematic diagram of a photoconductive antenna with transparent conductor electrodes, made in accordance with a first exemplary embodiment of the present invention;
[0026] Figure 2 is a schematic diagram of a photoconductive antenna with transparent conductor electrodes, made in accordance with a second exemplary embodiment of the present invention;
[0027] Figure 3 is a schematic diagram of a photoconductive antenna with transparent conductor electrodes, made in accordance with a third exemplary embodiment of the present invention;
[0028] Figure 4 is a schematic diagram of a photoconductive antenna with transparent conductor electrodes, made in accordance with a fourth exemplary embodiment of the present invention; [0029] Figure 5 is a schematic diagram of a photoconductive antenna with transparent conductor electrodes, made in accordance with a fifth exemplary embodiment of the present invention;
[0030] Figure 6 is a schematic diagram of a photoconductive antenna with transparent conductor electrodes, made in accordance with yet another exemplary embodiment of the present invention;
[0031] Figure 7 is a schematic diagram of a photoconductive antenna according to another embodiment;
[0032] Figure 8 is a schematic diagram of a photoconductive antenna according to another embodiment; [0033] Figure 9 is a flowchart showing a method of producing a photoconductive antenna according to another embodiment;
[0034] Figure 10 is a schematic side view showing a photoconductive antenna being made according to one embodiment;
[0035] Figure 11 is a schematic side view showing the photoconductive antenna of Figure 10 at a subsequent step in the fabrication; and
[0036] Figure 12 is a schematic side view showing the photoconductive antenna of Figure 10 completed.
DETAILED DESCRIPTION
[0037] For a high efficiency photoconductive antenna, it is beneficial to have both a strong and uniform DC electric field over as much of the volume
of the active region of the photoconductor substrate as possible, and an optical excitation light source impinging onto the active region and being absorbed by the photoconductor material for free carrier generation. Therefore, the absorption area of the active region should be large (in some cases as large as possible) in order to apply high power optical excitation without burning out or damaging the device.
[0038] It is possible to create a strong and uniform DC electric field over a large volume of a photoconductor material by sandwiching a layer of the photoconductor material between two generally parallel electrodes and applying a voltage bias to the electrodes. However, conventional electrodes would block the impinging laser beam, making it impossible, or at least very difficult, to excite the photoconductive material between the two conductive electrodes.
[0039] Referring now to the Figures, illustrating photoconductive antennas made in accordance with exemplary embodiments of the present invention, the photoconductive antennas of the present invention are directed to solving the aforementioned problem by utilizing transparent conductor electrodes that are generally highly transparent at the visible, infrared wavelengths, instead of normal conventional electrodes that block incident laser radiation. The transparent conductor electrodes of the present invention, depending on their thicknesses and other doping properties, may also be either transparent or reflective to THz radiation.
[0040] The transparent conductor electrodes of the present invention are preferably made of indium oxide materials, which are environmental stable and have relatively low resistivity and high transparency to various wavelengths. The indium oxide (ln2O3) materials may include Sn-doped indium oxide (ITO) and Zn stabilized amorphous indium oxide (IZO). Crystalline indium tin oxide (c-ITO) has a low resistivity and may be deposited onto substrates at 250-350 0C using DC magnetron sputter deposition or thermal evaporation techniques. Amorphous indium tin oxide (a-ITO) and amorphous indium zinc oxide (a-IZO) have higher resistivity, but they can be easier to work with in lithography stages and both can be deposited at room
temperature. Other known transparent conducting materials and transparent conducting polymers can also be used.
[0041] Using transparent conductor electrodes in photoconductive antennas addresses at least some of the limitations of conventional planar metallic electrodes. A thin layer of transparent conductor material (approximately 10 nm) is highly transparent at infrared and THz wavelengths, while a thicker TC layer (generally > 200 nm) is highly transparent at infrared and optical wavelengths and is reflective at the THz frequencies. In both cases, the transparent conductor material is a good conductor, and transparent conductor electrodes made from these materials can be used for applying a voltage bias to generate an electric field in the photoconductor material sandwiched between the two transparent conductor electrodes or between a transparent conductor electrode and a metallic electrode.
[0042] Referring now to Figure 1 , a photoconductive antenna 10, made in accordance with a first embodiment of the invention, comprises a first electrode 14 made of a first conductor material having a low resistivity, a second electrode 16 made of a second conductor material having a low resistivity, and a thin layer of photoconductor material 12 extending or sandwiched between the two electrodes 14 and 16. The second electrode 16b is spaced from and generally parallel to the first electrode 14. Both the first conductor material and the second conductor material comprise a transparent conductor material that is transparent to the incident laser radiation. The photoconductor material 12 may comprise a 0.1 μm to 500 μm thick layer of low-temperature-grown (LTG) GaAs, S.I. GaAs, ion implanted GaAs, ion implanted InGaAs, LTG-InGaAs, and other known suitable photoconductive materials.
[0043] As shown in Figure 1 , a pulsed or continuous wave incident laser beam 15 passes through the top transparent conductor electrode 14 and is absorbed by the layer of photoconductor material 12. The generated free carriers are accelerated under the influence of the applied electric field 17 and generate a pulsed or continuous wave THz wave 18. This structure results in a strong and uniform DC electric field, and allows the incident laser beam to be applied over the entire volume of the photoconductor material 12. The
power of the generated THz wave 18 can be orders of magnitude higher than that of conventional photoconductive antennas such as those with interdigitated electrodes. The generated THz wave 18 can be extracted and coupled into the free space or into a circuit using a variety of extraction and coupling methods, depending on the form of the designed structures and properties of the transparent conductor material used.
[0044] The transparent conductor electrodes 14 and 16 are transparent to laser beam 15 having optical or infrared wavelengths, and may either be transparent or reflective to the THz radiation 18 depending on their thicknesses. Generally, a thick transparent conductor electrode or metallic conductor electrode can reflect THz radiation, while a thin transparent conductor electrode can transmit THz radiation. The embodiment shown in Figure 1 comprises a thin top transparent conductor electrode 14 and a thin bottom transparent conductor electrode 16, and the generated THz wave 18 is transmitted downward through the thin bottom transparent conductor electrode 16 and upward through the top transparent conductor electrode 14, as there is no reflection.
[0045] Referring now to Figure 2, shown therein is a photoconductive antenna 20 made in accordance with a second embodiment of the invention, having a thick top transparent conductor electrode 24 and a thin bottom transparent conductor electrode 26. The generated terahertz radiation 18 is transmitted downward through the thin bottom transparent conductor electrode 26 as there is no reflection.
[0046] Referring now to Figure 3, shown therein is a photoconductive antenna 30 made in accordance with a third embodiment of the invention, having a thin top transparent conductor electrode 34 and a thick bottom transparent conductor electrode 36. The generated terahertz radiation 18 is transmitted upward through the thin top transparent conductor electrode 34 due to reflection caused by the thick bottom transparent conductor electrode 36.
[0047] Referring now to Figure 4, shown therein is a photoconductive antenna 40 made in accordance with a fourth embodiment of the invention,
having a thin top transparent conductor electrode 44 and a metallic bottom electrode 19. The generated terahertz radiation 18 is transmitted upward through the thin top transparent conductor electrode 44 due to reflection caused by the metallic bottom electrode 19. [0048] Referring now to Figure 5, shown there is a photoconductive antenna 50 made in accordance with a fifth embodiment of the invention, having a thick top transparent conductor electrode 54 and a metallic bottom electrode 19. The generated terahertz radiation 18 is transmitted out the sides due to repeated reflections caused by the thick top transparent conductor electrode 54 and the metallic bottom electrode 19. In some embodiments, the metallic bottom electrode 19 may be replaced by a thick bottom transparent conductor electrode.
[0049] Unlike planar configurations, for which patterning structures with sub-micron feature size can become very difficult and extremely costly, vertically grown or deposited layers associated with the photoconductive antennas 10, 20, 30, 40 and 50 can be as thin as a few nanometers, and the fabrication costs are even lower for thinner layers. So, it is easy to create very thin layers of transparent conductor electrodes and photoconductor layers stacked on top of each other, either by growing or depositing the layers on a native substrate or by transferring a grown layer onto a host substrate. The existing lithography and deposition technologies have been well developed to address the requirements for making the photoconductive antennas 10, 20, 30, 40 and 50 with very low cost. Since there are no sub-micron planar structures in photoconductive antennas 10, 20, 30, 40 and 50, the device fabrication cost tends to be much lower than the conventional photoconductive antennas such as those with interdigitated metallic electrodes.
[0050] Referring now to Figure 6, illustrated therein is a photoconductive antenna 60 made in accordance with another exemplary embodiment of the present invention. The photoconductive antenna 60 comprises a substrate 62, a bottom transparent conductor electrode 64 applied to a portion of the substrate 62, a photoconductor layer 66 applied to a portion of the bottom transparent conductor electrode 64, and a top
transparent conductor electrode 68 applied to the photoconductor layer 66. The antenna 60 may also comprise a first metal contact 70 affixed to a portion of the substrate 62 which does not have the bottom electrode 64 applied hereto, a second metal contact 72 affixed to the bottom transparent conductor electrode 64, a third metal contact 74 affixed to the top transparent conductor electrode 68, and a connector 76 for connecting first metal contact 70 to the third metal contact 74.
[0051] The substrate 62 may be made of any suitable material for example high resistive Si, GaAs and InP and may be 1 cm long, 1 cm wide and 0.5 mm tall (Is = 1 cm, ws = 1 cm, hs = 0.5 mm), although the dimensions could be different. The photoconductor layer 66 may be made from material such as LTG-GaAs or other suitable known materials and may be 2 mm long by 2mm wide (Ip = 2 mm, wp = 2 mm). The bottom transparent conductor electrode 64 may be made of a thin layer transparent conductor material layer such as ITO having a thickness of about 10 nm. The top transparent conductor electrode 68 may be made of a thicker layer of transparent conductor material such ITO having a thickness of about 400 nm. Both transparent conductor electrodes 64, 68 are transparent to the incident laser beam, and the bottom transparent conductor electrode 64 is transparent to the terahertz radiation generated by the photoconductor layer 66. The metal contacts 70, 72, 74 may be made from gold, and have a thickness of about 400 nm.
[0052] For generating terahertz radiation, a DC voltage bias is applied between metal contact 70 and metal contact 72, which induces a strong and generally uniform electric field between transparent conductor electrodes 64, 68, and a pulsed or continuous wave laser beam 77 is applied to the top surface of top transparent conductor electrode 68. The laser beam 77 passes through the top transparent conductor electrode 68 and impinges on the photoconductor layer 66, thus modulating the conductance of the photoconductor layer 66. Pulsed or continuous wave terahertz radiation 78 will be generated in the photoconductor layer 66 and transmitted through the bottom transparent conductor electrode 64 and the substrate 62.
[0053] For detecting terahertz radiation, the laser beam 77 applied to the top surface of transparent conductor electrode 68 passes through the electrode 68 and impinges on the photoconductor layer 66, which modulates the conductance of the photoconductor layer 66. Incident terahertz radiation 79 incoming from underneath photoconductive antenna 60, passes through the substrate 62 and the bottom transparent conductor electrode 64 as is received by the photoconductor layer 66, which results in a time-varying induced voltage causing a time-varying current. The time-varying current can be measured from between metal contact 70, 72 for analysis. [0054] In some embodiments, the photoconductive antenna 60 may be fabricated by following the substrate preparation process and the active layer preparation process set out below.
[0055] Substrate preparation process
1) forming the bottom transparent conductor electrode 64 by depositing a thin layer of transparent conductor on the substrate 62, wherein the substrate 62 may be any known suitable material for example high resistive silicon, semi- insulating GaAs (S.I. GaAs), InP, Silica, and Sapphire;
2) depositing the metal contact pad 72 on the bottom electrode 64 for final wire bonding to a chip carrier circuit board; 3) etching away a part of the bottom transparent conductor electrode 64 to form portion 65 of the substrate 62; and
4) depositing the metal contact pad 70 on the revealed portion 65 of the substrate 62 for final wire bonding to a chip carrier circuit board.
[0056] Active layer preparation process 1) forming the photoconductor layer 66 by growing a thin LTG-GaAs layer on an epi-ready S.I. GaAs wafer with a thin AIAs sacrificial layer grown underneath the LTG-GaAs layer;
2) forming the top transparent conductor electrode 68 by depositing a thick - transparent conductor layer on the LTG-GaAs layer; 3) depositing a metal contact pad 74 on the top transparent conductor electrode 68 for later wire bonding to the metal pad 70 on the substrate 62;
4) etching away the - transparent conductor and LTG-GaAs layers around a 2mm by 2mm area;
5) etching away the AIAs sacrificial layer underneath the LTG-GaAs layer which results in a free standing LTG-GaAs layer 66 with an - transparent conductor layer 68 and a metal contact pad 74 on it; and
6) transferring the LTG-GaAs layer onto the host substrate 62 and fixing it in place by epoxy.
[0057] Referring now to Figure 7, illustrated therein is a photoconductive antenna 160 made in accordance with another embodiment. The photoconductive antenna 160 comprises a substrate 162, a first electrode 164 (having a transparent conductor material that is transparent to the incident laser radiation), a second electrode 168 (having a transparent conductor material that is transparent to the incident laser radiation), and a photoconductive layer 166 extending between the two electrodes 164 and 168. In some embodiments, at least one of the first electrode 164 or the second electrode 168 may be replaced by an opaque conductor layer, such as a metal, where transparency to light is not required.
[0058] The photoconductive antenna 160 also includes antenna contacts provided on opposite outer sides of the first and second electrodes 164, 168. For example, as shown a first antenna contact 170 is attached to a first surface of the electrode 164 (shown in Figure 7 as being below the first electrode 164) and a second antenna contact 172 is attached to a second surface of the second electrode 168 (shown in Figure 7 on top of the second electrode 168). The first and second antenna contacts 170, 172 generally cooperate with the electrodes 164, 168 and photoconductive layer 166 so as to radiate the generated THz radiation and detect incident THz radiation during use of the antenna 160.
[0059] As illustrated, the first antenna contact 170 and second antenna contact 172 may be arranged in a dipole antenna configuration. However, other planar antenna structures such as bow-tie, spiral, log-periodic, and so on can also be realized.
[0060] For example, the first antenna contact 170 may have a first main body 170a laterally offset from the first electrode 164, and a first arm 170b extending inwardly away from the first main body 170a to a first end 170c (with the first end 170c and first electrode 164 overlapping and in contact). [0061] Similarly, the second antenna contact 172 may have a second main body 172a laterally offset from the second electrode 168, and a second arm 172b extending inwardly away from the second main body 172a to a second end 172c (with the second end 172c and second electrode 168 overlapping and in contact). [0062] The first end 170c end may be spaced apart from the second end 172c so as to define a lateral gap 180 therebetween.
[0063] In some embodiments, the first surface may be located between the body of the first electrode 164 and the substrate 162. In some embodiments, the second surface may be provided on the second electrode 168 generally opposite the substrate 162.
[0064] In some embodiments the first and second antenna contacts
170, 172 may be metallic. In other embodiments, the first and second antenna contacts may be made from any other suitable conductive material.
[0065] Referring now to Figure 8, illustrated therein is a photoconductive antenna 260 made in accordance with another exemplary embodiment of the present invention. The photoconductive antenna 260 is generally similar to the photoconductive antenna 160 in Figure 7, having antenna contacts 270, 272 on opposite sides of a first electrode 264, a second electrode 268, and a photoconductor layer 266 of photoconductor material extending between the first electrode 264 and the second electrode 268. At least one of the electrodes 264 and 268 should be made of a thin transparent conductor layer to transmit incident laser beam and the terahertz radiation.
[0066] In this embodiment, however, the first and second electrodes 264, 268 and photoconductive layer 266 are circular in shape, and the contacts 270, 272 have a circular aperture antenna configuration.
[0067] In particular, the photoconductive antenna 260 comprises a first pair of contacts 270 attached to a first surface of the first electrode 264 and a second pair of contacts 272 attached to the second surface of the second electrode 268. Each of the first antenna contacts 270 have a partially annular shape extending between two ends 270a. The first antenna contacts 270 as shown may be symmetrically aligned so as to generally surround the first transparent conductor 264, the photoconductor layer 266, and the second electrode 268.
[0068] The opposing ends 270a of the first antenna contacts 270 are also offset from each other such that the second antenna contacts 272 may be located therebetween. Furthermore, the offset of the first antenna contacts 270 and the width of the second antenna contacts 272 is selected to define four gaps 280.
[0069] In other embodiments, the photoconductive antennas as generally described herein may have contacts with other configurations, such as square, rectangular, and oval configurations, for example.
[0070] Referring now to Figure 9, in some embodiments the photoconductive antennas as described herein (e.g. photoconductive antennas 160 and 260 described above) may be fabricated using a fabrication method 300.
[0071] Step 310 includes providing a substrate. The substrate may be any known suitable material, for example high resistive silicon, semi-insulating GaAs, InP, Silica, or Sapphire.
[0072] Step 312 includes attaching a first antenna contact applied to the substrate. The first antenna contact may be any conductive material, such as metal. The first antenna contact may be applied, formed and patterned using any suitable method, such as wet or dry etching.
[0073] Step 314 includes applying a first electrode (e.g. a first transparent conductor layer) to the substrate. The first electrode may be applied to at least partially cover the first antenna contact so as to attach the first antenna contact to the first surface of the first electrode. The first surface
may be being located between the substrate and the first electrode. The first electrode generally has a low resistance.
[0074] In some embodiments, the first electrode may be a thick layer so as to reflect terahertz waves. In other embodiments, the first electrode may be a thin layer. In some embodiments, the first electrode may be replaced by an opaque conductor layer, such as a metal, where transparency to light is not required. The first electrode may be applied, formed and patterned using any suitable method, such as wet or dry etching.
[0075] Step 316 includes applying a layer of photoconductive material to the first electrode. The photoconductive layer may be any suitable material, such as amorphous GaAs, amorphous silicon, polycrystalline GaAs or polycrystalline silicon. The first electrode may be deposited and patterned using any suitable method.
[0076] Step 318 includes applying a second electrode (e.g. a second transparent conductor layer) to the photoconductive layer. The second electrode has a low resistance and is spaced from the first electrode by the photoconductive layer. The second electrode may be applied, formed and patterned using any suitable method, such as wet or dry etching. In some embodiments, the second electrode may be replaced by an opaque conductor layer, such as a metal, where transparency to light is not required.
[0077] Step 320 includes attaching a second antenna contact to the second electrode so as to attach the second antenna contact to a second surface of the second electrode. The second surface may be located on top of the second electrode. The second antenna contact may be any conductive material, such as metal.
[0078] The second antenna contact may be applied, formed and patterned using any suitable method, such as wet or dry etching. In some embodiments, the second antenna contact may be formed and patterned separately and then applied to the second electrode using a lift-off method. Using the lift-off method may avoid damage to the previously patterned layers.
[0079] In some embodiments, the first antenna contact may be applied after applying the first electrode such that the photoconductive layer will at least partially cover the first antenna contact. Similarly, the second antenna contact may be applied before applying the second electrode such that the second electrode will at least partially cover the second antenna contact.
[0080] Referring to Figures 10-12, in some embodiments the photoconductive antennas as described herein (e.g. photoconductive antennas 160 and 260 described above) may be fabricated using another fabrication method. [0081] Referring to Figure 10, a first substrate 462 may be provided having a sacrificial layer 463 applied to a surface of the first substrate 462.
[0082] Next, a photoconductive layer 466 may be applied to the sacrificial layer 463. The photoconductive layer 466 is spaced from the first substrate 462 by the sacrificial layer 463. The photoconductive layer 466 may be low-temperature grown GaAs or other materials suitable for generation and detection of terahertz waves. The sacrificial layer 463 may be AIAs when photoconductive material is GaAs.
[0083] Next, a first electrode 464 (e.g. a first transparent conductor layer) is applied to the photoconductive layer 466. The first electrode 464 is spaced from the sacrificial layer 463 by the photoconductive layer 466. In some embodiments, the first electrode 464 may be replaced by an opaque conductor layer, such as a metal, where transparency to light is not required.
[0084] Next, a first antenna contact 470 is attached to a first surface of the first electrode 464. As illustrated, the first antenna contact 470 may be applied to, and partially cover, both the photoconductive layer 466 and the first electrode 464. The resulting structure is shown generally in Figure 10.
[0085] Referring now to Figure 11 , the fabrication of the antenna may continue by providing an adhesive 490 to a surface of a second substrate 492. The second substrate 492 may be any suitable material for example high resistive silicon, semi-insulating GaAs, InP, Silica, Sapphire, or another suitable substrate. The adhesive 490 may be any suitable adhesive such as benzocyclobutene (BCB) polymer or other kinds of ultraviolet curable glues.
[0086] Next, the second substrate 492 is applied to the structure shown in Figure 10 such that the adhesive 490 attaches the second substrate 492 to at least the first transparent conductor 464. As shown, the adhesive may also attach the second substrate 492 to the first antenna contact 470 and the photoconductive layer 466.
[0087] Referring now to Figure 12, the method continues by removing the first substrate 462 and the sacrificial layer 463 to expose a surface of the photoconductive layer 466. The first substrate 462 and the sacrificial layer 463 may be removed using any suitable method, such as chemical etching, grinding, or a combination thereof.
[0088] Next, a second electrode 468 (e.g. a second transparent conductor layer) is applied to the exposed surface of the photoconductive layer 466. The second electrode 468 is spaced from the first electrode 464 by the photoconductive layer 466. In some embodiments, the second electrode 468 may be replaced by an opaque conductor layer, such as metal, where transparency to light is not required.
[0089] Next, a second antenna contact 472 is attached to a second surface of the second electrode 468. As illustrated, the second antenna contact 472 may be applied to, and partially cover, both the photoconductive layer 466 and the second electrode 468.
[0090] In some embodiments, one or more holes may be formed in the photoconductive layer 466 to partially expose the first antenna contact 470. This may allow connection of the first antenna contact 470 with outside circuitry. For example, a connector 494 may be attached to the first antenna contact 470 by soldering or another suitable method.
[0091] In some embodiments, the first antenna contact 470 and the second antenna contact 472 may be electrodes, part of the antenna structure, part of a transmission line, and/or part of a terahertz circuit.
[0092] In some embodiments, the fabrication methods described above may be used to fabricate a linear (1 x N), or two dimensional (M x N), array of antennas on a single substrate. A micro-lens array or a bundle of optical
fibers, for example, may then be used to excite the individual antennas within the array.
[0093] In some embodiments, the fabrication methods described above may be used to fabricate planar structures such as micro-strip and coplanar strip-line terahertz circuits and terahertz transmission lines, where the terahertz signal generation and detection sites incorporate transparent conductors as their electrodes. The metallization patterns for the terahertz circuits and terahertz transmission lines can be defined by modifying the application of the first and second antenna contacts. [0094] While the above description includes a number of exemplary embodiments, it should be apparent to those skilled in the art that changes and modifications can be made to these embodiments without departing from the present invention, the scope of which is defined in the appended claims.
Claims
1. A photoconductive antenna, comprising a first electrode made of a first conductor material having a low resistivity, a second electrode made of a second conductor material having a low resistivity, the second electrode being spaced from and generally parallel to the first electrode, and a photoconductor layer of photoconductor material extending between the first electrode and the second electrode, the photoconductor material being capable of generating or detecting terahertz radiation when impinged upon by incident laser radiation having an optical or infrared wavelength, wherein at least one of the first conductor material and the second conductor material comprises a transparent conductor material that is transparent to the incident laser radiation.
2. The antenna defined in claim 1 , wherein the transparent conductor material comprises an indium oxide material.
3. The antenna defined in claim 2, wherein the indium oxide material comprises Sn-doped indium tin oxide.
4. The antenna defined in claim 2, wherein the indium oxide material comprises Zn stabilized amorphous indium oxide.
5. The antenna defined in claim 1 , wherein at least one of the first electrode and the second electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation.
6. The antenna defined in claim 5, wherein the thin layer of the transparent conductor material has a thickness of about 10 nm.
7. The antenna defined in claim 1 , wherein at least one of the first electrode and the second electrode comprises a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation.
8. The antenna defined in claim 7, wherein the thick layer of the transparent conductor material has a thickness greater than about 200 nm.
9. The antenna defined in claim 1 , wherein the first electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation, and the second electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation.
10. The antenna defined in claim 1, wherein the first electrode comprises a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation, and the second electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation.
11. The antenna defined in claim 1 , wherein the first electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation, and the second electrode comprises a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation.
12. The antenna defined in claim 1 , wherein the first electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation, and the second electrode comprises a metallic electrode made of a layer of metallic material that reflects the terahertz radiation.
13. The antenna defined in claim 1 , wherein the first electrode comprises a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation, and the second electrode comprises a metallic electrode made of a layer of metallic material that reflects the terahertz radiation.
14. The antenna defined in claim 1 , wherein the first electrode comprises a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation, and the second electrode comprises a thick transparent conductor electrode made of a thick layer of the transparent conductor material that reflects the terahertz radiation
15. A photoconductive antenna, comprising
(a) a substrate;
(b) a bottom electrode layer of bottom conductor material applied to a portion of the substrate, the bottom conductor material having a low resistivity;
(c) a photoconductor layer of photoconductor material applied to the bottom electrode layer, the photoconductor material being capable of generating or detecting terahertz radiation when impinged upon by incident laser radiation having an optical or infrared wavelength; and
(d) a top electrode layer of top conductor material applied to the photoconductor layer, the top conductor material having a low resistivity, the top electrode layer being spaced from the bottom electrode layer by the photoconductor layer;
(e) wherein at least one of the top electrode and the bottom electrode comprises a transparent conductor electrode made of a transparent conductor material that is transparent to the incident laser radiation.
16. The antenna defined in claim 15, wherein the transparent conductor material comprises an indium oxide material.
17. The antenna defined in claim 15, wherein at least one of the top or the bottom electrode layer comprises a thin layer of the transparent conductor material that transmits the terahertz radiation.
18. The antenna defined in claim 17 wherein the thin layer of the transparent conductor material has a thickness of about 10 nm.
19. The antenna defined in clam 15, wherein at least one of the top or the bottom electrode layer comprises a thick layer of the transparent conductor material that reflects the terahertz radiation.
20. The antenna defined in claim 19, wherein the thick layer of the transparent conductor material has a thickness greater than about 200 nm.
21. The antenna defined in claim 15, further comprising a first metal contact affixed to a second portion of the substrate that does not have the bottom electrode layer applied thereto, a second metal contact affixed to the bottom electrode layer, a third metal contact affixed to the top electrode layer, and a connector for electrically connecting the first metal contact to the third metal contact.
22. The antenna defined in claim 1 , wherein at least one of the first electrode and the second electrode comprise a transparent conductor material, and further comprising:
(a) at least one first antenna contact attached to a first surface of the first electrode; and
(b) at least one second antenna contact attached to a second surface of the second electrode;
(c) wherein the first and second antenna contacts cooperate with the first and second electrodes and the photoconductive layer so as to radiate generated THz radiation or detect incident THz radiation during use of the antenna.
23. The antenna defined in claim 22, wherein the first antenna contact and second antenna contact are arranged in a dipole configuration.
24. The antenna defined in claim 23, wherein the first antenna contact has a first arm extending inwardly to a first end, the second antenna contact has a second arm extending inwardly to a second end.
25. The antenna defined in claim 22, wherein at least one of the first electrode and the second electrode comprises a thin transparent conductor electrode made of a thin layer of the transparent conductor material that transmits the terahertz radiation, and wherein the first and second antenna contacts are arranged in a circular aperture configuration.
26. The antenna defined in claim 22, wherein the antenna contacts are planar structures.
27. The antenna defined in claim 22, wherein the first and second antenna contacts are arranged in a configuration selected from the group consisting of.
(a) bow-tie;
(b) spiral;
(c) log-periodic;
(d) square;
(e) rectangular; and
(f) oval.
28. A method of fabricating a photoconductive antenna comprising:
providing a substrate;
applying a first electrode to the substrate;
applying a layer of photoconductive material to the first electrode; and
applying a second electrode to the photoconductive layer, the second electrode being spaced from the first electrode by the photoconductive layer.
29. The method of claim 28, further comprising: attaching a first antenna contact to a first surface of the first electrode; and attaching a second antenna contact to a second surface of the second electrode.
30. A method of fabricating a photoconductive antenna comprising: providing a first substrate; applying a sacrificial layer to a surface of the first substrate; applying a photoconductive layer to the sacrificial layer; applying a first electrode to the photoconductive layer; removing the sacrificial layer and the first substrate, then applying a second electrode to the photoconductive layer, the second electrode being spaced from the first electrode by the photoconductive layer.
31. The method of claim 30, further comprising: before removing the sacrificial layer and the first substrate: attaching a first antenna contact to a first surface the first electrode; providing an adhesive to a surface of a second substrate; and applying the second substrate to at least the first antenna contact using the adhesive.
32. The method of claim 30, further comprising, after removing the sacrificial layer and the first substrate, attaching a second antenna contact to a second surface of the second electrode.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US8183108P | 2008-07-18 | 2008-07-18 | |
| US61/081,831 | 2008-07-18 |
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| Publication Number | Publication Date |
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| WO2010006440A1 true WO2010006440A1 (en) | 2010-01-21 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/CA2009/001010 WO2010006440A1 (en) | 2008-07-18 | 2009-07-20 | Terahertz photoconductive antennas having transparent conductor electrodes and methods of making same |
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| WO (1) | WO2010006440A1 (en) |
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| EP2550680A4 (en) * | 2010-03-24 | 2017-08-02 | Mina Danesh | Integrated photovoltaic cell and radio-frequency antenna |
| WO2018046254A1 (en) * | 2016-09-09 | 2018-03-15 | Helmholtz-Zentrum Dresden - Rossendorf E. V. | Thz antenna and device for transmitting and/or receiving thz radiation |
| US10872730B2 (en) | 2018-07-20 | 2020-12-22 | Samsung Electronics Co., Ltd. | Electronic device including variable capacitor including photo-conductive material and method for controlling the same |
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