EP3335248A1 - Détecteur servant à détecter un rayonnement électromagnétique - Google Patents

Détecteur servant à détecter un rayonnement électromagnétique

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Publication number
EP3335248A1
EP3335248A1 EP15748250.6A EP15748250A EP3335248A1 EP 3335248 A1 EP3335248 A1 EP 3335248A1 EP 15748250 A EP15748250 A EP 15748250A EP 3335248 A1 EP3335248 A1 EP 3335248A1
Authority
EP
European Patent Office
Prior art keywords
top gate
gate contact
conducting layer
contact
detector according
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP15748250.6A
Other languages
German (de)
English (en)
Inventor
Sergey Mikhailov
Gennady GUSEV
Ze Don KVON
Nadezda SAVOSTIYANOVA
Alexandre LEVIN
E. E. Rodyakina
A. V. Latyshev
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Universidade de Sao Paulo USP
Universitaet Augsburg
Original Assignee
Universidade de Sao Paulo USP
Universitaet Augsburg
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Universidade de Sao Paulo USP, Universitaet Augsburg filed Critical Universidade de Sao Paulo USP
Publication of EP3335248A1 publication Critical patent/EP3335248A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • 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 potential barriers, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation

Definitions

  • the invention refers to an detector for detecting electromagnetic radiation according to the preamble of claim 1.
  • Detecting electromagnetic radiation generally is based on the registration of a dc response (photo-voltage or photo-resistance) of a solid state device, in particular a semiconductor heterostructure, to electromagnetic radiation. Such detectors are able to register changes of a small current through the device upon irradiation with electromagnetic radiation.
  • Highly sensitive detectors capable of detecting electromagnetic radiation in the microwave, terahertz and infrared frequency range can be used in many different areas, including security, medicine, chemical and biological agent detection, material characterization, THz imaging, telecommunication, astronomy, and other fields.
  • semiconductor devices which are comprising a two-dimensional (2D) charge carrier gas, in particular an electron gas, which is confined in a conducting layer arranged between dielectric layers, wherein the conducting layer is electrically connected at their longitudinal ends to a source contact and a drain contact, respectively and on top of the upper dielectric layer a gate contact in the form of a split gate is arranged.
  • 2D two-dimensional
  • the split gate contact is thus defining a quantum point contact in the conducting layer through which the charge carriers (electrons) can tunnel from one side of the depletion zone to the other side, if the gate voltage applied to the split gate contact is high enough (as disclosed for example in van Wees et.al, Phys. Rev. Lett. 60, 848 (1988) and in Wharam et.al, Solid Stade Phys. 21, L209 (1988)).
  • Such split gate structures can be used for building semiconductor detector devices for electromagnetic radiation, for example as a GaAs/AlGaAs heterostructure. Dorozhkin et.al, Appl. Phys. Lett 87, 092107 (2005) describe for example such a system for detection of microwave radiation.
  • the sensitivity of the known devices to electromagnetic radiation is often poor. It is therefore the object of the invention to provide a detector for electromagnetic radiation with an improved sensitivity. This object is achieved with a detector comprising the features of claim 1. Preferred embodiments of the detector according to the invention are shown in the dependent claims.
  • the device of the invention comprises a solid state structure which is confining a two- dimensional (2D) charge carrier gas, in particular an electron gas, and has at least a source contact, a drain contact and a gate contact, with the gate contact operating as a quantum point contact.
  • a substantially enhanced sensitivity of the device to electromagnetic radiation is achieved through the use of a special shape of the gate contact which is formed as a bridged gate.
  • the detector according to the invention is comprising a substrate, a conducting layer which is confining a two-dimensional charge carrier gas, a dielectric layer lying on the conducting layer, at least two electric contacts (namely a source contact and a drain contact) being electrically connected with the conducting layer and at least one top gate contact covering a part of the dielectric layer, wherein the top gate contact is comprising a first section and a second section which are connected with each other by a bridge section having a minimum width (d) smaller than the width (D) of the first and second sections.
  • the bridge section thereby is forming a constricted portion of the top gate contact, which is defining a quantum point contact (QPC) in the conducting layer when the top gate contact is electrically biased with respect to the conducting layer.
  • QPC quantum point contact
  • the detector according to the invention preferably can be manufactured as a solid-state device, in particular a heterostructure, with the conducting layer being a semiconductor layer which is confining a two-dimensional charge carrier gas, in particular an electron gas.
  • the conducting layer also can be a transition metal dichalcogenide monolayer, e.g. materials of the type MX 2 , with M a transition metal (Molybdenum, Tungsten) and X a chalcogen atom (Sulphur, Selenium, or Tellurium), which also can confine a two-dimensional hole gas.
  • a (positive or negative) bias is applied to the top gate contact with respect to the conducting layer (depending on whether the charge carriers are holes or electrons, respectively).
  • the electrical resistance or conductance and/or a photovoltage between the contacts developing upon irradiation of the detector with an electromagnetic radiation is measured, in order to measure the intensity of the electromagnetic radiation.
  • the first and second section of the top gate contact at least substantially have the form of a stripe having a predetermined width (D) and the bridge section is having a minimum width (d) smaller than the width (D) of the stripe.
  • the bridge section which is defining the constricted portion of the top gate contact, can be realized by known etching or lithography techniques in a semiconductor structure and may be formed by at least one substantially U- or V-shaped recess in the top gate contact.
  • the conducting layer is for example extending in a longitudinal direction (x) and is having a first longitudinal end which is electrically connected with one of the electric contacts (source contact) and a second longitudinal end which is connected with the other one of the electric contacts (drain contact).
  • the top gate contact preferably and substantially has the form of a stripe which is extending in a transversal direction (y), wherein the transversal direction (y) is (at least substantially) perpendicular to the longitudinal direction (x).
  • the first section and the second section of the top gate contact are inclining an angle of lower than 180° with each other, for example an angle between 10° and 90°.
  • the top gate contact When electrically biased with respect to the conducting layer, the top gate contact is dividing the conducting layer into two regions (sides) separated by a depletion layer and the charge carriers confined in the conducting layer are enabled to quantum mechanically tunnel from one region (side) to the other region (side) through the quantum point contact (QPC).
  • QPC quantum point contact
  • the minimum width (d) of the constricted portion of the top gate contact is preferably in the order of the electronic wavelength (de Broglie wavelength) of the charge carriers confined in the conducting layer, or it is several times the charge carrier's de Broglie wavelength. In preferred embodiments, the minimum width (d) of the constricted portion of the top gate contact is smaller than 0,5 ⁇ and preferably is smaller than 0,1 ⁇ . The minimum width (d) of the constricted portion of the top gate contact is defining the tunneling resistance for the charge carriers tunneling from one region (side) of the conducting layer to the other region (side).
  • the tunnelling resistance (or conductance) of the quantum point contact (QPC) is controllable by a gate voltage (V ga te) applied to the top gate contact which biasing the top gate contact with respect to the conducting layer. Since the tunnelling resistance or conductance of the quantum point contact (QPC) is sensitive to irradiation by electromagnetic radiation, the electrical resistance or conductance between the contacts is responsive to the intensity of the electromagnetic radiation irradiated on the detector. Therefore, by measuring the photovoltage induced between the contacts (source and drain contacts) by electromagnetic radiation irradiated on the detector, the intensity of the electromagnetic radiation can be determined.
  • irradiation of the detector by an electromagnetic radiation induces an ac local electric field near the edges of the top gate contact, wherein the (polarization) direction of the induced local electric field in the region of the constricted portion of the top gate contact is at least substantially parallel to the tunnelling direction of the charge carriers which are tunnelling through the quantum point contact (QPC).
  • QPC quantum point contact
  • the polarization direction of the locally induced ac electric fields therefore is at least mainly parallel to the tunnelling direction of the charge carriers which are tunnelling through the quantum point contact (QPC). For this reason, the tunnelling charge carriers are much more influenced by the locally induced ac electric fields in the region of the quantum point contact (QPC) and therefore the detector according to the invention is more sensitive than known devices having a split gate contact.
  • the detector is comprising a number of top gate contacts with a constricted portion arranged on top of the dielectric layer.
  • These multi-gate structures can be used to implement frequency-sensitive detectors.
  • further gate contacts are arranged on top of the dielectric layer and between a first top gate contact and a second top gate contact, wherein both top gate contacts preferably are arranged in longitudinal direction (x) one behind the other and in a distance from each other and have the form of stripes which are running parallel to each other.
  • QPC quantum point contact
  • the size of the area and the charge carrier density are preferably selected to enable the charge carriers confined in this area to oscillate with their plasma frequency upon irradiation of an electromagnetic radiation having a frequency which is equivalent to the plasma frequency (cop).
  • the predetermined plasma frequency (cop) of the charge carriers confined in said area of the conducting layer can be controlled by external magnetic fields or electric fields, as for example by biasing a back gate electrode arranged on the backside of the substrate with which the charge carrier density in the conducting layer can be controlled.
  • This embodiment of the detector therefore is sensitive to the frequency of the incident electromagnetic radiation.
  • Figure 1 Basic geometry of a detector according to the invention in a side view (figure la) and a top view (Figure lb);
  • Figure 2 preferred embodiments of a detector according to the invention implemented as a GaAs/AlGaAs heterostructure for confining a 2D electron gas;
  • Figure 3 Basic geometry of a detector according to the invention implemented in a
  • QPC developed in the conducting layer of a detector according to the invention having a bridged gate contact in a side view ( Figure 5 a) and in a top view ( Figure 5b);
  • Figure 9 implemented in a GaAs/AlGaAs heterostructure with the central part of one of the top gate contacts shown i an enhanced representation in the insert (bottom left part of the picture).
  • the detector according to the invention is comprising a substrate 3, a (semi)conducting layer 1, a dielectric layer 4 lying on the (semi)conducting layer 1, at least two electric contacts, namely a source contact 2a and a drain contact 2b, and at least one top gate contact 5 covering a part of the dielectric layer 4.
  • the (semi)conducting layer 1 is made of a conducting or a semiconducting material. When in the following a reference is made to the conducting layer 1, this means that this layer also can be a semiconductor layer.
  • the conducting layer 1 is confining a two-dimensional charge carrier gas, which can be an electron gas or a hole gas, depending on the type of carriers present in the conducting layer 1.
  • the conducting layer 1 is lying in a horizontal plane (x-y-plane) and is extending in longitudinal direction x between a first end la and a second end lb.
  • the first end la of the conducting layer 1 is electrically connected with the source contact 2a and the second end lb is electrically connected with the drain contact 2b.
  • the top gate contact 5 is formed as a "bridged-gate" and is comprising a first section 5 a and a second section 5b which are connected with each other by a bridge section 5c.
  • the top gate contact 5 in general is made of an electrically conducting material, in particular a metal or a semimetal, and the sections 5 a, 5b and the bridge section 5 c are preferably all made of the same material.
  • the first and the second section 5a, 5b both have substantially the form of a stripe with a (medium) width D and the bridge section 5 c is having a minimum width d smaller than the general width D of the sections 5a, 5b.
  • the bridge section 5c is thus forming a constricted portion of the top gate contact 5.
  • a depletion zone is developed in the conducting layer 1 in the areas under the top gate contact 5 having a lower carrier density than in the other areas of the conducting layer 1.
  • the depletion layers are thus defining a quantum point contact (QPC) in the conducting layer 1 in the region of the conducting layer 1 under the constricted portion (in the bridge section 5c) of the top gate contact 5.
  • a negative or positive bias is applied to the top gate contact 5 with respect to the 2D conducting layer 1, depending on the type of carriers in the conducting layer 1 (a negative bias is applied when a 2D electron gas is confined in the conducting layer and a positive bias is applied in the case of a 2D hole gas).
  • the bias of the top gate contact 5 leads to a reduction of the charge carrier density under the top gate contact 5 and thus a depletion layer is formed in the conducting layer in the area under the top gate contact 5.
  • the depletion layer is hence separating the conducting layer 1 in two regions (sides a and b, shown in Figure lb), so that the charge carriers have to quantum-mechanically tunnel from the left region (left side a) to the right region (right side b).
  • the resistance of the QPC can be made very high (Mega-Ohms scale).
  • the tunnelling resistance of the QPC can be controlled by the gate voltage V gat e applied to the top gate contact 5 for biasing it against the conducting layer 1.
  • V gat e applied to the top gate contact 5 for biasing it against the conducting layer 1.
  • the resistance (conductance) or the induced photovoltage between the two contacts 2 (source contact 2a, drain contact 2b) is measured. Due to the constricted portion of the top gate contact 5 having a minimum width d, as shown in Figure lb, the charge carriers preferably tunnel through this narrow region.
  • the constricted portion of the top gate contact 5 is thus forming a quantum point contact (QPC).
  • the detector according to the invention now comprises a bridged gate as the top gate contact 5 (instead of of a split gate known from the prior art). Due to physical effects which will be explained in the following, the QPC of the detector according to the invention is substantially more sensitive to electromagnetic radiation. In the tunnel regime, the QPC devices are sensitive to irradiation of an electromagnetic radiation and can be used for detecting it. Since the top gate contact 5 is conducting, the electromagnetic wave of the incident electromagnetic radiation with its electric field 6 induces near the edges of the top gate contact 5 strong local ac electric fields 7 which can be much larger than the field of the incident wave itself (known as the lightning-rod effect: the field strongly increases near sharp edges of metallic (conducting) objects).
  • the reason is that, independent of the electric field polarization of the incident electromagnetic wave, the electric field 7 locally induced in the region of the edges of the top gate contact 5 turns out to be polarized normally with respect to the edge of the top gate contact 5, as shown in Figure 5.
  • the direction of the ac electric field 7 turns out to be mainly perpendicular to the tunneling direction of the charge carriers (shown by the large horizontal arrow in Figures 5 and 6).
  • Figure 7 shows the measured and normalised photoconductance as a function of the normalised intensity (power) of an incident electromagnetic radiation. It can be seen that the detector according to the invention reveals a much better sensitivity than the prior art device. One observes a more than one-order-of-magnitude difference of the photo-conductance.
  • Figure 4a shows a top gate contact 5 in the form a stripe having two recesses approximately in the middle of the stripe length at opposite sides. Between these two recesses a bridge section 5c is arranged which is connecting (bridging) a first stripe section 5a with a second stripe section 5b of the top gate contact 5.
  • the recesses substantially can have a V- shape (as shown in Figure 4a) or a U- shape and can be realized by etching the stripe of the top gate contact 5 and/or by electron beam or photolithography.
  • the smallest width d of the bridge section is mostly limited by technological restrictions of the used etching or photolithography techniques and is preferably smaller than 0,5 ⁇ and more preferred smaller than 0,1 ⁇ , in order to be small enough to enable the charge carriers to tunnel through the QPC.
  • the width D of the stripe sections 5a, 5b is not crucial and can be for example in the range of one up to a few ⁇ .
  • Figure 4b shows an alternative embodiment for the shape of the top gate contact 5 with only one U-shaped recess in the generally stripe-formed contact, building a bridge section 5c lying at the outer edge of the contact 5 and which again is connecting the two stripe sections 5 a and 5b of the top gate contact 5.
  • FIG 4c a modification of the embodiment of Figure 4b is shown.
  • an additional element in the form of a conducting stripe 9 is placed on top of the dielectric layer 4 on one side (here: left side a) of the structure.
  • the conducting stripe 9 is directed towards the bridge section 5c of the top gate contact 5 and preferably is of metal.
  • the conducting stripe 9 is not biased (no dc voltage is applied between the stripe 9 and the underlying 2D charge carrier gas in the conducting layer 1). As described above, a dc voltage is applied between the top gate contact 5 and the underlying 2D charge carrier gas in the conducting layer 1.
  • the density of charge carriers under the top gate contact 5 is close to zero, thereby forming a depletion layer in the conducting layer 1 under the top gate contact 5 which is separating the conducting layer 1 into two sides, a and b.
  • the charge carriers have to tunnel from one side (left side a) of the structure to the other side (right side b).
  • Under the stripe 9 the density of charge carriers is the same as in the whole left side a.
  • the role of the stripe 9 is to serve as an antenna which concentrates the electric field of the incident electromagnetic radiation in the quantum-point-contact (QPC) of the device, which is arranged in the conducting layer 1 under the bridge section 5c of the top gate contact 5. This further can improve the sensitivity of the detector, since the tunnelling resistance (conductance) of the charge carriers tunnelling from one side (a) to the other side (b) is now even more influenced by the electric field of the incident electromagnetic radiation.
  • QPC quantum-point-contact
  • the width of the narrowest area in the QPC (which is defined by the minimum width of the bridge section 5c of the top gate contact 5) can be a in the range of 100 nm or smaller, and up to some ⁇ .
  • the optimal length of the stripe 9 in the horizontal direction is depending on the wavelength ⁇ of the incident radiation to be detected and is in the order of ⁇ ⁇ /2, wherein the wavelength is i.e. ⁇ ⁇ 0.15 mm for the frequency 1 THz and ⁇ 0.75 ⁇ for the telecommunication wavelength 1.55 ⁇ .
  • the width of the stripe 9 indeed is not crucial and can be for example in the range of one up to a few ⁇ .
  • FIG 4d a modification of the embodiment of Figure 4c is shown with a top gate contact 5 (which is biased against the conducting layer 1) and a stripe 9 (which again is not biased as in the embodiment of Figure 4c) serving as an antenna to direct the incident electromagnetic radiation towards the top gate contact 5.
  • the shape of the top gate contact 5 is different in the embodiment of Figure 4d.
  • the top gate 5 has now the form as shown in Figure 4d with a first section 5a and a second section 5b, which are both formed here as conical stripes which are inclining an angle of for example between 10° and 90°.
  • the conical form of the stripes is not necessary or crucial for their function, any other shape of the sections 5a, 5b would work just as well.
  • the inner ends of the first and second sections 5a, 5b which are directing towards the centre of the structure are connected (bridged) with each other by a bridge section 5c.
  • a bridge section 5c Approximately in the middle of the bridge section 5c, it has a minimum width d (which again is in the order of 0,1 ⁇ or smaller).
  • the minimum width d of the bridge section 5c is preferably smaller than the lowest width of the sections 5a and 5b of the top gate contact 5 (due to the conical form of the stripes defining the sections 5a and 5b, the lowest width of the sections 5a and 5b here is at their inner ends at which they are connected with the bridge section 5 c, as shown in Figure 4d).
  • the first longitudinal end la of the conducting layer 1 is connected with a source contact 2a and its second longitudinal end lb is connected with a drain contact 2b, which now is arranged between the outer ends of the first and second section 5a, 5b of the top gate contact 5. Between the inner end of the stripe 9 and the bridge section 5c of the top gate contact 5, a gap area 7 is formed.
  • the charge carrier gas under the gate 5 is depleted and the charge carriers have to tunnel through the QPC defined by the restricted portion of the top gate contact 5 in the bridge section 5 c, in order to move from one side (left side a) to the other side (right side b) of the 2D charge carrier gas confined in the conducting layer 1.
  • the left side a is placed outside of the first and second sections 5a, 5b of the top gate contact 5 and the right side b is placed inside of them, as shown in Figure 4d.
  • the shown shape of the top gate contact 5 now also serves as an antenna and is further directing the electric field of the incident radiation towards the region of the QPC formed in the conducting layer 1 under the bridge section 5c.
  • both the top gate contact 5 and the additional conducting stripe 9 serve as antennas focusing the electric field in the gap area 7 between the inner end of the stripe 9 and the bridge section 5 c of the top gate contact 5, which gap area 7 is quite in the vicinity of the QPC. This enhances the electric field induced in the region of the gap 7, as compared to the field of the incident wave of the electromagnetic radiation, and leads to an increase of the sensitivity of the detector.
  • Figure 2 shows two different embodiments of suitable modulation doped GaAs/AlGaAs heterostructures which can be used for implementing the invention.
  • a GaAs/AlGaAs heterostructure suitable for realising a 2D charge carrier layer in the form of an electron gas (two dimensional electron gas, 2DEG) confined in a conducting layer.
  • 2DEG two dimensional electron gas
  • the semiconductor structure of the embodiment of Figure 2(a) comprises a GaAs substrate 10, typically with a thickness of 0.3 to 0.5 mm (however, this is not critical).
  • a GaAs buffer layer 12 is grown on the GaAs substrate and is followed by a superlattice structure 14 comprising alternating layers of AlGaAs and GaAs, with the AlGaAs layers typically being 7 nm thick and GaAs layers typically being 3 nm thick.
  • the total thickness of the buffer structure 12, 14 typically amounts to 1 ⁇ , however, this is again not critical. The idea is to prevent impurities from the GaAs substrate migrating into the active part of the device.
  • the active part of the device comprises the layer 16 of GaAs (intrinsic material) which is followed by an intrinsic layer 18 of AlGaAs. After the intrinsic layer 18 has been grown it is followed by a silicon-doped AlGaAs layer 20 and a capping layer 22 of GaAs. On top of the capping layer 22 a metallic top gate contact is placed (not shown on the Figure), wherein this top gate contact is formed according to the invention and as described above.
  • the density of 2D electrons under the top gate contact can be changed and controlled by the gate voltage V ga t e .
  • an additional conducting layer separated by at least one barrier from the 2DEG, may be inserted in the layer structure of Fig. 2(a) below the 2D charge carrier layer (also not shown in the figures). It can serve as a back-gate for changing the density of charge carriers (electrons) in the 2DEG: By applying a back gate voltage V back gate between this back-gate and the 2DEG, the density of 2D electrons in the 2DEG can be changed.
  • the semiconductor heterostructure is, for example, grown by molecular beam epitaxy (MBE) or chemical vapour deposition (CVD), as are all other structures described here.
  • MBE molecular beam epitaxy
  • CVD chemical vapour deposition
  • the conduction band scheme for the structure of Fig. 2(a) is shown in the same figure in an insert right from the structure geometry. It can be seen that the interface between the undoped AlGaAs layer 18 and the undoped GaAs layer 16 forms a potential well 24.
  • the potential well 24 has a quantised energy level 26 for electrons which are induced into the potential well by the silicon donor atoms provided in the layer 20. The electrons are thus sharply localised in the potential well 24 and this results in the two-dimensional electron gas illustrated as the layer 26 in the left part of Fig. 2 (a).
  • the GaAs layer 16 therefore is equivalent to the conducting layer 1 of the basic geometry of a detector according to the invention as shown in Figure 1, since it is confining a two dimensional charge carrier
  • this two-dimensional electron gas is localised in the GaAs layer 16 having a thickness of approximately 5-10 nm. Although this is a very thin layer it is probably better described as a quasi two-dimensional electron gas rather than a strict two-dimensional electron gas which would have zero thickness.
  • the layers 18, 20 and 22 provided on top of the GaAs layer 16 have a total thickness typically in the range from 35 nm to 250 nm, with the relative proportions being as shown in the drawing. For the detector operation these dimensions should be possibly small.
  • the GaAs layer 16 typically has a thickness of approximately 1 ⁇ , but again this is not critical.
  • FIG. 2(b) there can be seen an alternative embodiment for realising a two- dimensional electron gas.
  • the structure of Fig. 2(b) is similar to that of Fig. 2(a) and the layers which are common to both structures have been given the same reference numerals.
  • the layer 16 of GaAs has been reduced in this structure to a thin layer having a thickness of ⁇ 10 nm and that it is sandwiched between two layers 18 of intrinsic AlGaAs. These two layers 18 are in turn sandwiched between two layers 20 of AlGaAs doped with silicon.
  • the conduction band diagram for this structure in the vicinity of the thin intrinsic GaAs layer 16 is shown in the insert of Fig. 2(b), right.
  • the thin layer 16 of intrinsic GaAs forms a quantum well 24 with a quantised energy level 26 in which electrons are localised.
  • the electrons are induced by the presence of the silicon donors in the two layers 20.
  • the GaAs layer 16 is equivalent to the conducting layer 1 of the basic geometry of a detector according to the invention as shown in Figure 1, since it is confining a two dimensional charge carrier (electron) gas.
  • n of the charge carriers confined in the conducting layer 1 (2D electrons) and their mobility ⁇ typically is n 3 ⁇ 4 (1 - 8) x 10 11 cm 2 and ⁇ 3 ⁇ 4 (1 - 3) x 10 6 cm 2 /Vs, corresponding to a mean free path of « 60 ⁇ which is substantially exceeding the dimensions of the QPC (defined by the minimum width (d) of the bridge section 5c).
  • the top gate contact 5 can be fabricated on the surface of the heterostructure (dielectric layer 4) using electron beam lithography.
  • the distance between the top gate contact 5 and the 2DEG typically is 90 nm.
  • the samples were irradiated by microwaves with the frequency in the range from 110 to 170 GHz and a power density 3 ⁇ 4 (1 - 10) mW/cm 2 , corresponding to the microwave electric field of ⁇ ⁇ 3 ⁇ 4 (0.6 - 2) V/cm.
  • the sample was rotated with respect to the orientation of the waveguide but no polarization dependence has been observed.
  • Nine devices were studied and similar results have been obtained for all of them.
  • detectors according to the invention could be demonstrated at microwave frequencies f 3 ⁇ 4 110 - 170 GHz and at low (4.2 K) temperatures, but the operating frequency range can be extended both to lower and higher (terahertz, IR) frequencies, and the working temperatures can be increased at least up to the liquid nitrogen temperature (experimentally verified) and probably up to room temperature.
  • the estimated internal responsivity of the detector should be about 10 6 V/W. This is several orders of magnitude larger than the responsivity of terahertz Schottky-diode detectors (which is between 100 and 1000 V/W).
  • the detectors can operate in the resonant (frequency sensitive) regime.
  • the operation principle of the detectors according to the invention is based on electronic transitions. Its response time is therefore restricted by only very short electron relaxation times. This is substantially shorter than, for example, the response time of bolometers whose operation is based on the thermal response.
  • the effective area of a single working element of the detector is about 100 x 100 nm 2 , together with lead wires - smaller than 10 x 10 ⁇ 2 .
  • the device can be realized in the form of a line or a matrix of detectors for sub-wavelength detection of microwave/terahertz radiation (sensitive microwave/terahertz/infrared camera).
  • the detectors implemented in the GaAs/AlGaAs heterostructure system comprising the above described bridged-gate structure (top gate contact 5 with a bridge section 5 c) revealed to be substantially more sensitive to the irradiation than traditional structures having a split-gate as the top gate contact.
  • This is illustrated in Figure 7, where measurement results of detectors with a split-gate (according to the prior art) and a bridged-gate (according to the invention) are shown.
  • the maximum power Po was also approximately the same both in the split- and bridged-gate devices which was independently controlled by the amplitude of the microwave induced resistance oscillations measured on the Hall bar.
  • the bridged-gate devices according to the invention demonstrate a more than one order of magnitude higher sensitivity (normalized photo-conductance ⁇ / ⁇ 0 ) as compared to the split-gate detectors according to the prior art.
  • the detectors also can be build using other materials, in particualar any other suitable semiconductor material system for confining a 2DEG in a (semi)conducting layer.
  • Another option is to use recently discovered atomically thin two-dimensional crystals like, for example, transition metal dichalcogenides (MoS2, WSe2, etc) as the (semi) conducting layer 1 (according to the basic geometry of Fig. 1), boron nitride as the dielectric layer 4 and graphene for building the top gate contact 5. Since the thickness of these layers (- 0.5 - few nm) is 10 - 100 times smaller than in GaAs/AlGaAs structures, the local electric fields 7 ( Figure 5) near atomically sharp conducting edges are expected to be orders of magnitude larger. This should lead to an orders- of-magnitude larger responsivity of the detector.
  • transition metal dichalcogenides MoS2, WSe2, etc
  • Figure 3 shows an example for the geometry of a detector based on a Graphene Related Material (GRM) in a side view.
  • a semiconducting layer 1 is arranged on top of a substrate 3 and is electrically connected with metallic contacts 2a and 2b.
  • the substrate is a dielectric material, e.g. quartz, Si0 2 and should be sufficiently thick (in the range of -0.5 mm or thicker) to be mechanically stable.
  • the structure can also lay, e.g. on top or side of a wave- guiding structure, for example on an optical wave guide in which the presence of the waves should be detected.
  • the semiconducting layer 1 is made out of, e.g., a mono-atomic layer of molybdenum disulfide, MoS 2 , with a thickness of -0.65 nm, and serves as a conducting layer in the sense of the basic structure of the detector according to the invention shown in Figure 1 (instead of a GaAs quantum well, as described above with reference to Figures 2(a) and 2(b)).
  • MoS 2 molybdenum disulfide
  • TMDC transition metal dichalcogenide
  • M a transition metal atom Mo, W, etc.
  • X a chalcogen atom S, Se, or Te
  • a dielectric layer 4 On top of the semiconducting layer 1 lies a dielectric layer 4, e.g., of hexagonal boron nitride (h-BN) which serves as a dielectric and should have a thickness of only a few atomic layers, e.g. in the range of 5-10 nm.
  • the top gate contact 5 is arranged on top of the dielectric layer 4, which is preventing the tunnelling of electrons from the top gate contact 5 to the semiconducting layer 1.
  • the top gate contact 5 can be made out of (a) graphene or (b) a metallic material.
  • the top gate contact 5 should be sufficiently thick to be a good conductor and to serve as an antenna (e.g. a metallic layer - 50 - 100 nm thick).
  • the above described detector devices are comprising a single QPC formed in the conducting layer 1 under the (bridged-gate) top gate contact 5. With such detector devices, it is possible to detect the intensity of electromagnetic radiation, but not its frequency. Using embodiments of the detector with two or more bridged-gate QPCs, it is possible to realize detectors which are sensitive on the frequency of the incident radiation. The frequency-sensitivity of the devices can be electrically and/or magnetically controlled. In the following, such an embodiment of the invention is described in detail: A preferred embodiment of a detector comprising two-bridged-gate top gate contacts 5.1 and 5.2 is shown in Figure 9.
  • the detector device shown in Figure 9 is comprising a first top gate contact 5.1 and a second top gate contact 5.2, in addition to the source and drain contacts 2a and 2b, wherein the first top gate contact 5.1 and the second top gate contact 5.2 both have a constricted portion 5c with a minimum width (d) smaller than the width (D) of the remaining portions of the respective top gate contact (as in the previous embodiments), wherein the constricted portion 5c of each top gate contact 5.1 , 5.2 is defining a quantum point contact (QPC) in the conducting layer 1, when the respective top gate contact 5.1 , 5.2 is electrically biased with respect to the conducting layer.
  • QPC quantum point contact
  • the first top gate contact 5.1 and the second top gate contact 5.1 are arranged in longitudinal direction (x) one behind the other and in a distance from each other, wherein both top gate contacts 5.1 , 5.2 have the form of stripes which are running parallel to each other.
  • Two further gate contacts 5.3, 5.4 are arranged on top of the dielectric layer and between the first top gate contact 5.1 and the second top gate contact 5.2.
  • the charge carriers in this well-defined area 9 of the conducting layer 1 are thus confined horizontally (x- y-plane) within the depletion layers surrounding the area and vertically (z-direction) within the conducting layer 1. Since the charge carriers confined in this area 9 of the conducting layer 1 have a predetermined charge carrier density (n) and a predetermined plasma frequency (cop), the size of the area 9 and the charge carrier density (n) can be selected to enable the charge carriers confined in the area to oscillate with their plasma frequency upon irradiation of an electromagnetic radiation having a frequency which is equivalent to the plasma frequency (cop).
  • the predetermined plasma frequency (cop) of the charge carriers confined in the conducting layer can be controlled by external magnetic fields or electric fields, as for example by biasing a back gate electrode arranged on the backside of the substrate with which the charge carrier density in the conducting layer can be controlled.
  • This embodiment of the detector therefore is also sensitive to the frequency of the incident electromagnetic radiation.
  • the voltage applied to the bridged-gate contacts 5.1 and 5.2 controls the current flow between the source and drain contacts 2a, 2b.
  • a voltage applied to the further gates 5.3 and 5.4 creates depletion layers under these gates and can control the effective size of the area 9 occupied by 2D electrons. Inside the area 9 electrons can oscillate with the plasma frequency (co p ), dependent on the electron density (n) and the size of the area 9.
  • FIG 10 shows a photographic representation of a realized sample of a GaAs/AlGaAs heterostructure with a double top gate contact structure as schematically shown in Figure 9 and as described above, in a top view. The central part of one of the top gate contacts is show in an enhanced representation in the insert (bottom left part of the figure).
  • the resonance frequency (plasma frequency C0p) can be controlled by external magnetic fields: in the absence of a magnetic field B the plasma frequency C0p is proportional to -v/n/R and hence depends on the electron density n and the radius R of the well-defined area 9, which can be tuned by the gate voltages V5.3 and V5.4 applied to the further gates 5.3 and 5.4, respectively. If the external magnetic field B ⁇ 0, the plasma frequency C0p depends, in addition, on the magnetic field and particularly is proportional to n/BR (so called edge magnetoplasmons).
  • n/BR edge magnetoplasmons
  • the resonance frequency (plasma frequency C0p) can also be controlled by external electric fields: Therefore a back gate electrode can be arranged on the substrate, By biasing the back gate electrode, an electric field is produced in vertical direction (z) which is controlling the charge carrier density (n) in the conducting layer 1 and hence the plasma frequency co p oc Vn/R .
  • z vertical direction
  • n charge carrier density
  • the detector devices according to the invention with a number of bridged- gate top gate contacts 5 can thus be used in a matrix to build a fast terahertz/infrared camera with a sub- wave length resolution and extremely high responsivity.

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Abstract

L'invention se rapporte à un détecteur servant à détecter un rayonnement électromagnétique avec une sensibilité élevée. Le détecteur comprend un substrat (3), une couche conductrice (1) qui confine un gaz bidimensionnel porteur de charge, une couche diélectrique (4) qui se trouve sur la couche conductrice, au moins deux contacts électriques (2a, 2b) qui sont électriquement connectés avec la couche conductrice et au moins un contact de grille supérieure (5) recouvrant une partie de la couche diélectrique, lequel contact de grille supérieure comprend une première section (5a) et une seconde section (5b) qui sont reliées l'une à l'autre par une section de pont (5c) ayant une largeur minimale (d) inférieure à la largeur (D) des première et seconde sections. La section de pont forme ainsi une partie resserrée du contact de grille supérieure, qui définit un point de contact quantique (QPC) dans la couche conductrice lorsque le contact de grille supérieure est électriquement polarisé par rapport à la couche conductrice.
EP15748250.6A 2015-08-11 2015-08-11 Détecteur servant à détecter un rayonnement électromagnétique Withdrawn EP3335248A1 (fr)

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