WO2019084609A1 - Passivation stable à l'air de semi-métaux de dirac topologiques - Google Patents

Passivation stable à l'air de semi-métaux de dirac topologiques Download PDF

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WO2019084609A1
WO2019084609A1 PCT/AU2018/051173 AU2018051173W WO2019084609A1 WO 2019084609 A1 WO2019084609 A1 WO 2019084609A1 AU 2018051173 W AU2018051173 W AU 2018051173W WO 2019084609 A1 WO2019084609 A1 WO 2019084609A1
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layer
dirac semimetal
topological
topological dirac
oxygen barrier
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PCT/AU2018/051173
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English (en)
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Michael Sears FUHRER
Mark Thomas EDMONDS
John Thery HELLERSTEDT
James Lee Richard Jessee COLLINS
Chang Liu
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Monash University
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Priority claimed from AU2017904418A external-priority patent/AU2017904418A0/en
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Publication of WO2019084609A1 publication Critical patent/WO2019084609A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66977—Quantum effect devices, e.g. using quantum reflection, diffraction or interference effects, i.e. Bragg- or Aharonov-Bohm effects
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y15/00—Nanotechnology for interacting, sensing or actuating, e.g. quantum dots as markers in protein assays or molecular motors
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02109—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates
    • H01L21/02112—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
    • H01L21/02617—Deposition types
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic Table
    • H01L29/1606—Graphene
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N50/00—Galvanomagnetic devices
    • H10N50/80—Constructional details
    • H10N50/85—Magnetic active materials
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N52/00—Hall-effect devices

Definitions

  • the invention relates to structures including a topological Dirac semimetal, methods for forming such structures, and methods for modulating electric field or band gap in such structures.
  • TDS Topological Dirac semimetals
  • Control of carrier density and electronic conduction via electric fields is an important step toward electronic devices based on TDS.
  • useful structures including Dirac semimetal layers have not been reported. This is, in part, as a result in the difficulty associated with fabricating TDS layers and/or structures that include TDS layers.
  • the present invention starts from the inventors' realisation that the charge carrier density of a topological Dirac semimetal layer can be changed by subjecting the topological Dirac semimetal layer to an electric field.
  • This effect can advantageously be used to control the current passing through the topological Dirac semimetal layer to enable the topological Dirac semimetal to act as a structure in an electronic component, such as, amongst other things, a diode or transistor.
  • a number of these materials are unstable in air, and undergo oxidation which can have a deleterious effect on the properties of the topological Dirac semimetal.
  • the inventors have now found that providing an oxygen barrier layer of the form MF 2 (wherein M is an alkaline earth metal) mitigates oxidation of the topological Dirac semimetal layer and preserves the desirable electronic properties of the topological Dirac semimetal layer.
  • a structure including: means for generating an electric field; a topological Dirac semimetal layer, wherein the topological Dirac semimetal layer is non-conductively separated from the means for generating the electric field, and the means for generating the electric field is configured to apply the electric field to at least a portion of the topological Dirac semimetal layer; and an oxygen barrier layer for preventing oxidation of the topological Dirac semimetal layer, the oxygen barrier layer formed of a material of the form MF 2 , wherein M is an alkaline earth metal.
  • the structure includes a conductor as the means for generating an electric field; and an insulating layer non-conductively separating the topological Dirac semimetal layer from the conductor.
  • a structure for altering the charge carrier density in a topological Dirac semimetal comprising: a conductor; an insulating layer; a topological Dirac semimetal layer separated from the conductor by at least the insulating layer; an oxygen barrier layer for preventing oxidation of the topological Dirac semimetal layer, the oxygen barrier layer formed from a material of the form MF 2 , wherein M is an alkaline earth metal; and at least one electrode contacting the topological Dirac semimetal layer; wherein the conductor and the at least one electrode are configured to apply an electric field to at least a portion of the topological Dirac semimetal layer to alter the charge carrier density of the topological Dirac semimetal layer.
  • a structure for altering the band gap of a topological Dirac semimetal comprising: a conductor; an insulating layer; a topological Dirac semimetal layer separated from the conductor by at least the insulating layer; an oxygen barrier layer for preventing oxidation of the topological Dirac semimetal layer, the oxygen barrier layer formed from a material of the form MF 2 , wherein M is an alkaline earth metal; and wherein the conductor is configured to apply an electric field to at least a portion of the topological Dirac semimetal layer to alter the band gap of the topological Dirac semimetal layer.
  • the oxygen barrier layer is formed from a material selected from the group consisting of: BeF 2 , MgF 2 , CaF 2 , SrF 2 , or BaF 2 .
  • the oxygen barrier layer is formed from a material selected from the group consisting of: MgF 2 , CaF 2 , SrF 2 , or BaF 2 .
  • the oxygen barrier layer is formed from MgF 2 .
  • the oxygen barrier layer comprises, consists of, or consists essentially of a material selected from the group consisting of: BeF 2 , MgF 2 , CaF 2 , SrF 2 , or BaF 2 .
  • the material is selected from the group consisting of: MgF 2 , CaF 2 , SrF 2 , or BaF 2 .
  • the material is MgF 2 .
  • the oxygen barrier layer is a multilayered structure comprising at least one layer comprising, consisting of, or consisting essentially of a material of the form MF 2.
  • the multilayered structure may, for example, be formed from two, three, four, or more layers of oxygen barrier materials.
  • Other oxygen barrier materials include metal oxides, and semimetal oxides.
  • suitable metal and semimetal oxides include MoO 3 , SiO 2 , and AI 2 O 3 .
  • the layer of the form MF 2 is in direct contact with the surface of the Dirac semimetal.
  • the other oxygen barrier layers in the multilayered structure are not in contact with the Dirac semimetal surface, and are separated from the surface of the Dirac semimetal by the MF 2 layer.
  • This multilayered arrangement may be suitable in situations where the MF 2 provides short term protection, and thus the deposition of another layer on top of the MF 2 is desirable for long term protection.
  • the oxygen barrier layer has a layer thickness of at least 5 nm.
  • the layer thickness is at least 10 nm. More preferably, the layer thickness is at least 15 nm. Most preferably, the layer thickness is at least 18 nm.
  • the oxygen barrier layer has a layer thickness of at most 1000 nm.
  • the layer thickness is at most 500 nm. More preferably, the layer thickness is at most 300 nm. Even more preferably, the layer thickness is at most 100 nm. Most preferably, the layer thickness is at most 50 nm.
  • the oxygen barrier layer has a layer thickness of from about 5 nm to about 1000 nm.
  • the layer thickness is from about 10 nm to about 300 nm. More preferably, the layer thickness is from about 15 nm to about 100 nm. Most preferably the layer thickness is from 20 nm to 50 nm.
  • the oxygen barrier layer is substantially uniform and/or substantially free from defects, such as pinholes. Defects, such as pinholes can reduce the effectiveness of the oxygen barrier layer.
  • the above mentioned oxygen barrier layer may be applied to a range of different topological Dirac semimetal layers
  • the function of the oxygen barrier layer is to prevent exposure of the topological Dirac semimetal layer to oxygen to thereby prevent oxidation.
  • the invention is particularly suited to topological Dirac semimetals that are unstable in air, and for example, undergo oxidation on exposure to air.
  • the topological Dirac semimetal layer is formed from a material of the form A 3 Bi, where A is an alkali metal.
  • A is selected from the group consisting of Na, K and Rb. Most preferably, A is Na.
  • An oxygen barrier layer of the form MF 2 is particularly well suited for topological Dirac semimetals of the form A 3 Bi.
  • M0O3 molybdenum trioxide
  • the inventors trialled the application of molybdenum trioxide (M0O3) to Na 3 Bi and found that this resulted in large n-type doping and degradation of the Na 3 Bi the film even without exposure to air.
  • M0O3 molybdenum trioxide
  • the inventors are of the view that metal oxides may adversely react with the surface of topological Dirac semimetals of the form A 3 Bi, and in particular Na 3 Bi.
  • the topological Dirac semimetal layer is an A 3 Bi layer.
  • the thickness of the A 3 Bi layer is at least 7 nm. More preferably, the thickness of the topological Dirac semimetal layer is at least 10 nm. Even more preferably, the thickness of the topological Dirac semimetal layer is at least 15 nm. Most preferably, the thickness of the topological Dirac semimetal layer is at least 18 nm. There is no particular upper limit on the thickness of the topological Dirac semimetal layer. However, it is preferred that the thickness is no more than 1000 nm, more preferably no more than 500nm, and most preferably no more than 200 nm.
  • the topological Dirac semimetal layer has a thickness of less than 20nm.
  • the topological Dirac semimetal layer is an ultrathin film and has a thickness of 3 unit cells or less. More preferably, the Dirac semimetal film layer has a thickness of 2 unit cells or less. Most preferably, the Dirac semimetal film layer has a thickness of 1 .5 unit cells or less.
  • the topological Dirac semimetal layer undergoes a transition from topological insulator to convention insulator with increasing electric field.
  • the transition occurs at an electric field strength value in the range of from about 0.5 V/nm to about 2 V/nm. More preferably, the transition occurs at an electric field strength value in the range of from about 1 .0 V/nm to about 1.5 V/nm.
  • a surface of the topological Dirac semimetal layer exhibits point defects.
  • the point defects are vacancy defects.
  • the vacancy defects are due to missing A atoms.
  • the oxygen barrier layer is disposed in direct layered physical contact with the Dirac semimetal layer.
  • the structure may include one or more intermediate layers located between the oxygen barrier layer and the Dirac semimetal layer.
  • the one or more intermediate layers may include, for example, a layer for donating or accepting charge carriers, such as an electron accepting layer or an electron donating layer; or an insulating layer.
  • the oxygen barrier layer is a capping layer, being an outermost layer of the structure.
  • one or more further layers are disposed on a surface of the oxygen barrier layer, by way of example, an insulating layer or a conductive layer.
  • an electrode is disposed either directly or indirectly on a surface of the oxygen barrier layer.
  • the structure includes a layer for donating or accepting charge carriers from the topological Dirac semimetal layer. This layer is preferably deposited on a surface of the topological Dirac semimetal layer.
  • the oxygen barrier layer may be disposed in direct layered physical contact with the layer for donating or accepting charge carriers from the topological Dirac semimetal layer; or the structure may further include one or more additional layers disposed on a surface of the layer for donating or accepting charge carriers from the topological Dirac semimetal layer (such as an insulating layer), with the oxygen barrier layer disposed in direct layered physical contact with the one or more additional layers.
  • the layer for donating or accepting charge carriers may be an organic layer, such as an organic electron accepting layer or an organic electron donating layer.
  • the layer is an electron accepting layer, such as an organic electron accepting layer.
  • the layer is a tetrafluorotetracyanoquinodimethane (F4-TCNQ) layer.
  • F4-TCNQ doping can achieve charge neutrality and even a net p-type doping. That is, under certain conditions, F4-TCNQ allows the topological Dirac semimetal layer to undergo an n- type to p- type transition.
  • the layer has a thickness of 0.25 A or greater. More preferably, the layer has a thickness of 0.85 nm or greater. Most preferably, the layer has a thickness that is about 1 nm or greater. It is also preferred that the layer has a thickness of less than 2 nm due to saturation of the charge transfer process.
  • modulation of the electrical field can affect an n- to p- type transition in the topological Dirac semimetal layer.
  • the structure is configured such that the n- to p- type transition occurs at an applied voltage between the conductor and the electrode of between -100 and 100 V. More preferably, the n- to p- type transition occurs at an applied voltage of between -75 and 75 V. Most preferably, n- to p- type transition occurs at an applied voltage of between -60 and 60 V.
  • the structure includes an insulating layer non- conductively separating the topological Dirac semimetal layer from the conductor
  • this insulating layer is formed directly on a surface of the conductor. It is further preferred that the topological Dirac semimetal is formed directly on the surface of the insulating layer.
  • the insulating layer is a passivation layer that is formed on the surface of the conductor. That is, where the conductor is a metal, the insulator may be an oxide of that metal formed, for example due to natural oxidation. However, it will be appreciated that other forms of passivation may be used.
  • the insulating layer may be aluminium oxide, such as ⁇ - ⁇ 2 0 3 .
  • the conductor may be a metalloid, such as Si in which case the insulating layer may also be an oxide of that metalloid, for example S1O2.
  • other forms of passivation may also be employed which result in non-oxide passivation layers, an example of this is silicon nitride on a silicon conductor.
  • the advantage of having a passivated insulating layer is that the insulating layer can be formed as a very thin film, which is particularly useful in certain applications.
  • the insulating layer has a thickness in the range of from about 1 nm to about 1000 nm in thickness. Minimising the thickness of the insulator is desirable as larger distances between the conductor and the topological Dirac semimetal reduces the strength of the applied electric field at a given applied voltage between conductor and electrode.
  • the structure may be a layered structure exhibiting a variety of different layered arrangements. A non-limiting disclosure of different layered arrangements is provided below.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, oxygen barrier layer.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, electron donating or electron accepting layer and oxygen barrier layer.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, electron donating or electron accepting layer, insulator and oxygen barrier layer.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, oxygen barrier layer and insulator.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, electron donating or electron accepting layer, oxygen barrier layer and insulator.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, electron donating or electron accepting layer, insulator, oxygen barrier layer and insulator.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, oxygen barrier layer and insulator, conductor.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, electron donating or electron accepting layer, oxygen barrier layer, insulator and conductor.
  • the device has the following layered arrangement: conductor, insulator, topological Dirac semimetal, electron donating or electron accepting layer, insulator, oxygen barrier layer, insulator and conductor.
  • the oxygen barrier layer may include multiple layers of different oxygen barrier materials, at least one layer of which is of the form MF 2 . In particular embodiments where the oxygen barrier layer is in direct contact with a surface of the topological Dirac semimetal, only the oxygen barrier material of the form MF 2 is in contact the surface of the topological Dirac semimetal.
  • topological Dirac semimetal is of the form A 3 Bi
  • a surface of the topological Dirac semimetal that is facing the oxygen barrier layer is terminated with A atoms.
  • the structure further comprises: at least two conductive electrodes in contact with the topological Dirac semimetal layer, the at least two electrodes spaced apart from each other and between which current may flow from a current or voltage source through the topological Dirac semimetal layer; and at least one electrode of the at least two electrodes is configured to be connected to the current or voltage source.
  • This arrangement is particularly useful where the structure is a transistor.
  • an electronic device including the structure described above.
  • a method of forming a topological Dirac semimetal layer on a substrate including: (a) providing constituent elements of a Dirac semimetal to a surface of a substrate at a first temperature to nucleate the Dirac semimetal on the surface of the substrate forming a nucleation layer of the topological Dirac semimetal layer having a first thickness; (b) increasing the first temperature to a second temperature and further providing constituent elements of the Dirac semimetal at the second temperature to grow the thickness of the Dirac semimetal layer to a final thickness; and (c) depositing a material of the form MF 2 onto the surface of the topological Dirac semimetal layer to form an MF 2 oxygen barrier layer to prevent oxidation of the topological Dirac semimetal layer, wherein M is an alkaline earth metal; wherein steps (a), (b), and (c) are conducted under ultra-high vacuum conditions.
  • UHV ultra-high vacuum
  • the oxygen barrier layer is formed from a material selected from the group consisting of: BeF 2 , MgF 2 , CaF 2 , SrF 2 , or BaF 2 .
  • the oxygen barrier layer is formed from a material selected from the group consisting of: MgF 2 , CaF 2 , SrF 2 , or BaF 2 .
  • the oxygen barrier layer is formed from MgF 2 .
  • the step of depositing the material of the form MF 2 forms an oxygen barrier layer having a layer thickness of at least 5 nm.
  • the layer thickness is at least 10 nm. More preferably, the layer thickness is at least 15 nm. Most preferably, the layer thickness is at least 18 nm.
  • the step of depositing the material of the form MF 2 forms an oxygen barrier layer having a layer thickness of at most 1000 nm.
  • the layer thickness is at most 500 nm. More preferably, the layer thickness is at most 300 nm. Even more preferably, the layer thickness is at most 100 nm. Most preferably, the layer thickness is at most 50 nm.
  • the step of depositing the material of the form MF 2 forms an oxygen barrier layer having a layer thickness of from about 5 nm to about 1000 nm.
  • the layer thickness is from about 10 nm to about 500 nm. More preferably, the layer thickness is from about 15 nm to about 300 nm. Most preferably the layer thickness is 20 nm to 50 nm.
  • step (c) further comprises depositing the material of the form MF 2 to form the oxygen barrier layer at a rate of from about 0.1 to about 10 A/sec. Preferably, the rate is from about 0.5 to about 2 A/sec.
  • the step (c) is conducted at ambient temperature, e.g. room temperature.
  • ambient temperature e.g. room temperature.
  • room temperature is well understood in the art, but typically refers to a temperature in the range of from about 15 °C to 25 °C.
  • the Dirac semimetal layer is a A 3 Bi layer, and the constituent elements are A and Bi, wherein A is an alkali metal, and the method further comprises: providing A and Bi atoms with a Bi:A flux ratio of at least 1 : 10, and with a Bi flux rate of at least about 0.0lA/sec to about 0.05 A/sec and an A flux rate of at least 0.23A/sec to about 1 .17.
  • the Bi flux rate is about 0.03 A/sec and the A flux rate is 0.7 A/sec.
  • the method further includes, after step (b) and prior to step
  • the annealing temperature is from about 240°C to about 400°C. More preferably, the annealing temperature is from about 250°C to about 390°C. Even more preferably, the annealing temperature is the second temperature. Additionally, or alternatively it is preferred that the annealing time is less than 1 hour. Preferably, less than 30 minutes. More preferably, the annealing time is less than 20 minutes. Most preferably, the annealing time is about 10 minutes or less.
  • the step of annealing the topological Dirac semimetal layer is conducted under a flux of A.
  • a flux of Bi is not provided during step of annealing.
  • the topological Dirac semimetal layer does not increase in thickness during step of annealing.
  • the substrate is an electrically insulating material, and/or the substrate includes a layer of an electrically insulating material disposed thereon, and wherein the topological Dirac semimetal layer is formed on a surface of the electrically insulating material.
  • the method includes prior to step (c), providing an electron donating or accepting layer for donating or accepting charge carriers from the topological Dirac semimetal layer. This may include applying the electron accepting layer over the topological Dirac semimetal layer as discussed previously. In some embodiments the method can include tuning charge carrier donating or accepting properties of the topological Dirac semimetal layer by altering a property of the electron donating or accepting layer. For example, in certain embodiments the thickness of the electron accepting or electron donating layer can affect the charge carrying properties of the Dirac semimetal layer.
  • the electron accepting or electron donating layer has a thickness that 0.25 A or greater. More preferably, the electron accepting or electron donating layer has a thickness greater than 0.85 nm. Most preferably, the electron accepting or electron donating layer has a thickness greater that is about 1 nm. It is also preferred that the electron accepting or electron donating layer has a thickness of less than 2 nm.
  • a structure formed according to the disclosed method in a sixth aspect of the invention, there is provided a structure formed according to the disclosed method.
  • a method of altering the charge carrier density in a topological Dirac semimetal comprising: providing a structure according to the second aspect of the invention; and applying a voltage to the conductor relative to the at least one electrode to subject at least a portion of the topological Dirac semimetal layer to an electric field to alter the charge carrier density of the topological Dirac semimetal layer.
  • the method further includes switching the voltage between a first voltage and a second voltage to increase or decrease the charge carrier density in the topological Dirac semimetal layer.
  • the method further includes switching the voltage between a first voltage and a second voltage to change a charge carrying mechanism from n- type to p- type, or from p- type to n- type.
  • a method of altering the band gap of a topological Dirac semimetal comprising: providing a structure according to the third aspect of the invention, and applying a voltage to the conductor relative to the at least one electrode to subject at least a portion of the topological Dirac semimetal layer to an electric field to alter the band gap of the topological Dirac semimetal layer.
  • the method further includes varying a strength of the electric field to vary the band gap.
  • the topological Dirac semimetal layer has a thickness of less than 20nm.
  • the topological Dirac semimetal layer is an ultrathin film and has a thickness of 3 unit cells or less. More preferably, the Dirac semimetal film layer has a thickness of 2 unit cells or less. Most preferably, the Dirac semimetal film layer has a thickness of 1 .5 unit cells or less.
  • the method further includes providing at least two electrodes in contact with the topological Dirac semimetal, the at least two electrodes spaced apart from each other and between which current may flow from a current or voltage source through the topological Dirac semimetal layer; wherein when current is passing between the at least two electrodes, the step of subjecting the topological Dirac semimetal to an electric field alters the magnitude of the voltage between the at least two electrodes.
  • the method further includes providing at least two electrodes in contact with the topological Dirac semimetal, the at least two electrodes spaced apart from each other and between which current may flow from a voltage source through the topological Dirac semimetal layer; wherein when current is passing between the at least two electrodes, the step of subjecting the topological Dirac semimetal to an electric field alters the magnitude and/or direction of the current passing between the at least two electrodes.
  • Figure 1 illustrates the structure of Na 3 Bi thin films. All data corresponds to 20 nm-thick Na 3 Bi grown at 345 °C on a-AI 2 03 [0001 ].
  • Figure 1 (a) is a schematic representation of device geometry showing location of Ti/Au pads used to make electrical contact with the Na 3 Bi film grown on sapphire. The inset shows the crystal structure of Na 3 Bi with the in-plane and c-axis lattice constants labelled, where Na and Bi atoms are coloured grey and black, respectively.
  • Figure 1 (c) is a LEED image taken at 17.5 eV showing the 1x1 symmetry of the Na 3 Bi surface.
  • Figure 2 illustrates Hall mobility and Hall carrier density of Na 3 Bi thin films measured at a temperature of 5 K for samples grown with different thermal profiles.
  • Figure 2(a) illustrates mobility and
  • Figure 2(b) illustrates Hall carrier density (n-type) plotted against the final growth temperature for the various samples.
  • the 120-345 °C growth profile achieves the highest measured mobility (6,310 cm 2 /vs) and lowest carrier density (4.6 x 10 17 cm “3 ).
  • the grey shaded region represents the range of final growth temperatures for which the sample quality significantly degrades.
  • Figure 3 illustrates the transverse magnetoresistance (about 0.5T) of Na 3 Bi thin films at a temperature of 5 K.
  • Figure 2(b) shows the quadratic coefficient of MR A from fits to Eqn. 1 in Figure 2(a) as a function of carrier density for the various films measured.
  • Figure 4a and 4b show, diagrammatically, devices used in film deposition.
  • Figure 4b is a device diagram, showing Hall bar film geometry defined by a surface stencil mask affixed on the Si:Si02 substrate that serves as a back gate.
  • Figure 5 plots Hall effect carrier density against gate voltage for various top surface coverages of molecular dopant F4-TCNQ. For coverages of +.85nm and +1 .08nm F4-TCNQ, the ambipolar, n- to p- type transition is clearly visible at +50 Vg and -35 Vg, respectively.
  • Figure 6 is an STM topographic image (400 nm x 400 nm) of few atomic layer Na3Bi(001 ) film grown on Si(1 1 1 ).
  • Figure 7(a) shows differential conductance, dl/dV spectra taken on a pristine region of Na 3 Bi at different tip-sample separations.
  • Figure 7(b) shows extracted bandgap as a function of relative height (or tip- sample separation height) from the dl/dV spectra in Figure 7(a).
  • Figure 9(a) shows zero field conductivity (S / cm) and Figure 9(b) shows low- field Hall carrier density (10 18 cm “3 ) vs. back gate voltage (Vg), for the Na 3 Bi film as- grown (circles), and after the deposition of 6.2A F4-TCNQ on the surface (squares)
  • Figure 10(a) shows the gate dependence of the zero-field resistivity ⁇ ( ⁇ -cm) after top-gating with F4-TCNQ.
  • Figure 10(b) shows transverse magnetoresistance p xy ( ⁇ /sq) plotted versus perpendicular applied magnetic field (T), for back gate voltages (Vg) indicated in the legend (V).
  • Figure 10(c) shows theoretical fits to the data in Figure 10(b) using charge puddling theory.
  • Figure 11 shows carrier density plotted as a fit parameter to the curves in Figure 10(b) against V g (line).
  • Figure 12(a) is a large area (400nm x 380 nm) topographic STM image of 20 nm
  • Figure 12(e) is an area-averaged STS spectra (vertically offset for clarity) corresponding to four different regions of the sample.
  • Figure 13(d) are upper, middle and lower panels representing histograms of the Dirac point energy maps in Figures 13(a)-(c) respectively.
  • Figure 14 is a graph showing the sheet resistance of a 20nm Na 3 Bi layer capped with 20nm MgF 2 layer.
  • Figure 15 is a graph showing the sheet resistance of: (i) a 100 nm thick Na 3 Bi layer, (ii) a 100 nm thick Na 3 Bi layer capped with a 20 nm MgF 2 layer, and (iii) a 100 nm thick Na 3 Bi layer capped with a 20 nm MgF 2 layer after exposure to air.
  • Figure 16 is a graph showing Hall carrier density and mobility of: (i) a 100 nm thick Na 3 Bi layer, (ii) a 100 nm thick Na 3 Bi layer capped with a 20 nm MgF 2 layer, and (iii) a 100 nm thick Na 3 Bi layer capped with a 20 nm MgF 2 layer after exposure to air.
  • Figure 17 is a graph showing longitudinal magnetoresistance measured by low temperature cryostat. This graph shows the resistance as a function of magnetic field at two different temperatures after exposure to air.
  • Figure 18 is a graph showing transverse magnetoresistance measured by low temperature cryostat. This graph shows the resistance as a function of magnetic field at two different temperatures after exposure to air.
  • Figure 19 is a graph showing the change in sheet resistivity of two separate
  • Figure 20 is an optical microscope image showing clear shadowing of the MgF 2 film as applied to a Na 3 Bi Dirac semimetal layer due to wire bonds.
  • Figure 21 is a graph comparing Hall carrier density and mobility for a 20 nm Na 3 Bi film before and after deposition of an MgF 2 capping layer.
  • Figure 22 is graph showing sheet resistance of 20 nm Na 3 Bi films with and without an MgF 2 capping layer versus air exposure time.
  • Figure 23 is a graph showing the temperature dependent magnetoresistance for an MgF 2 capped Na 3 Bi film after exposure to air.
  • Figure 24(c) is a 1x1 LEED image of few-layer Na 3 Bi taken at 32 eV.
  • Figure 24(e) Normalized XPS of Si 2p core level taken at 350 eV (left panel) and 850 eV (right panel). Each panel shows the Si 2p of the clean Si substrate and with few- layer Na 3 Bi grown on top. The spectra have been offset in intensity for clarity.
  • Figure 25(a) Normalized dl/dV spectra displayed on a logarithmic plot corresponding to ML and BL Na 3 Bi. The conduction and valence band edges are reflected by the sharp onset of dl/dV intensity.
  • Figure 25(c) STM topography of a region of bi-layer Na 3 Bi, monolayer Na 3 Bi ⁇ , and the underlying Si(1 1 1 ) substrate.
  • the line represents the region over which the dl/dV measurements were performed in Figure 25(e).
  • Figure 25(e) dl/dV colour map taken at and then moving away from the step edge where the dashed vertical lines reflect the spectra shown in (d) and the horizontal lines represent the averaged dl/dV signal region that is shown in Figure 25(f).
  • Figure 25(f) Shows the corresponding intensity profile of dl/dV in the bulk gap showing the exponential decay away from the step edge.
  • Figure 27(a) Two-dimensional Brillouin zone for Na 3 Bi layered structures. Here we also show the projected 1 D Brillouin zone used for studying the edge spectrum.
  • Figure 27(b-g) Results for monolayer (b-d) and bilayer (e-g) Na 3 Bi with Na(2) vacancies [with one Na(2) vacancy in a 2x2 supercell].
  • (b,e) Electronic band structures, where the energy zero is set to be at the valence band maximum.
  • Figure 28(a) Electron-band Fermi-surface of few-layer Na 3 Bi after 30 minutes of K-dosing.
  • Figure 28(b) Radially averaged momentum profile through the Fermi surface, showing the ring structure at k F .
  • the hole band is located ⁇ 140 meV below E F ;
  • K dosing equivalent to a 1 .44 Vnm "1 displacement field has n-type doped the system to an extent that an electron band has now emerged, separated from the hole band by ⁇ 100 meV;
  • K dosing equivalent to 2.18 Vnm "1 displacement field results in further n-type doping with the band separation 90 meV.
  • Figure 30(a) ARPES intensity plot along ⁇ - ⁇ - ⁇ direction after 30 minutes of K- dosing.
  • Figure 30(b) stack plots of MDCs for the valence band (left panel) and conduction band (right panel) extracted from Figure 30(a).
  • Figure 30(c) EDCs extracted from Figure 30(a).
  • Figure 31 Electric displacement field-dependence of topological insulator ML/BL
  • Figure 32(a) Schematic representation of a metallic tip (with work function ⁇ ⁇ ) at a fixed distance above the surface of Na 3 Bi (with work function ⁇ 3 ⁇ ), with the difference in work function generating a localized electric field.
  • the potential difference of ⁇ 1 .2 eV is much larger than the bias applied for d//d V measurements.
  • Figure 32(c) Bandgap extracted from d//d V spectra as a function of electric field for monolayer (squares) and bilayer (triangles). At a critical field of ⁇ 1 .5 Vnm "1 the system is no longer gapped, above this a bandgap reopens in the conventional regime.
  • the left shaded region and the right shaded region represent guides to the eye. Insets represent projected edge state bandstructures calculated by DFT below and above the critical field.
  • Figure 33(a) and (b) Tunnelling current as a function of relative tip-sample distance Z for (a) Au(1 1 1 ) (Bias +500 mV) and (b) thin film Na 3 Bi (Bias -300 mV).
  • the top axis in (b) represents the total distance, s, between tip and sample.
  • the black lines in (a) and (b) are exponential fits.
  • Figure 33(c) Energy-level schematic illustrating the effects of an image potential on the (trapezoidal between tip and sample over separation distance s) junction barrier; progression from upper curve to lower curve indicating modification of the apparent barrier height due to the imaging potential at decreasing tip-sample distances s.
  • Figure 33(d) Illustrative tunnelling current curve without (lower curve) and with (upper curve) imaging potential effects.
  • a reference tunnelling current l 0 at tip-sample separation s 0 is shown for comparison.
  • Figure 36 Individual dl/d V spectra taken on ML Na 3 Bi at different tip-sample separations (electric field). The spectra have been normalized and offset for clarity.
  • Figure 37(a) Bandgap variation as a function of electric field for monolayer Na 3 Bi with Na(2) vacancy. The gap closes and reopens at ⁇ 1 .85 V/A.
  • Figure 37(b) 2D Brillouin zone and the projected 1 D boundary Brillouin zone.
  • Figure 37(c-f) Results for the system at the electric field of (c,d) 0.0 V/A and (e,f) 2.5 V/A, which are marked by points A and B in (a),
  • the A dots represent the contribution from the Na-s and Bi-s atomic orbitals, and the B dots represent the contribution from the Bi- x/ y atomic orbitals.
  • Topological Dirac semimetals are three-dimensional analogues of graphene, with linear electronic dispersions in three dimensions.
  • the inventors have found a method for forming a topological Dirac semimetal layer on a range of substrates, including electrically insulating materials, which allows the characterisation and analysis of the properties of these Dirac semimetals, and exploitation of their unique electronic properties, such as in an electronic component.
  • the inventors have also found that providing an oxygen barrier layer of the form MF 2 , where M is an alkaline earth metal is particularly beneficial for mitigating oxidation of a TDS layer.
  • examples 1 to 5 illustrate the formation of layer of Na 3 Bi and/or devices include this Na 3 Bi layer and the subsequent characterisation of the electrical and magnetic properties of that layer. None of Examples 1 to 6 includes the oxygen barrier layer of the present invention, but rather these examples demonstrate the synthesis of the Na 3 Bi layer and subsequent characterisation of the topological properties of this Na 3 Bi layer, and show that an electric field may be applied to the Na 3 Bi layer to modulate or alter the charge carrier density, charge carrier type, and/or the band gap of the Na 3 Bi layer. Examples 7 and 8 illustrate the synthesis of a Na 3 Bi layer, which is then capped with an MgF 2 layer to prevent oxidation. Examples 7 and 8 go on to demonstrate that the Na 3 Bi layer retains its topological properties even on exposure to air. Before turning to the examples, a more detailed explanation for each of the examples is provided below.
  • Example 1 reports the synthesis of an uncapped Na 3 Bi layer of an Al 2 0 3 substrate and the characterisation of the resultant crystalline quality, as well as electrical and magnetic properties including charge carrier density, charge mobility, and magnetoresistance for different synthesis conditions.
  • the results of Example 1 demonstrate the growth of electrically isolated, highly oriented, large area thin film Na 3 Bi on a-AI 2 0 3 .
  • the high sample quality is reflected in a record high mobility and near-ideal weak anti-localization behaviour.
  • Example 2 reports the fabrication of a device including a Na 3 Bi layer formed on top of a Si:Si0 2 substrate with an organic electron accepting layer applied to a surface of the Na 3 Bi layer, but without the oxygen barrier layer of the present invention.
  • Example 3 reports results relating to controlling the bandgap of ultra-thin topological Dirac semimetal Na 3 Bi without the oxygen barrier layer of the present invention.
  • Example 4 reports results from a device including high quality thin films of Na 3 Bi grown on amorphous Si:Si0 2 substrates that are doped with molecular dopant F4- TCNQ. As with the previous examples, the device of Example 4 does not include the oxygen barrier layer of the present invention. This example again demonstrates the growth of high quality, large area thin film of Na 3 Bi directly on amorphous S1O2. This example also illustrates that the gate dependent MR shows two distinct features, a nonzero minimum of the zero-field conductivity, and a non-linear Hall response indicating the presence of electrons and holes with different mobilities.
  • Example 5 reports the use of 20 nm Na 3 Bi thin films grown via molecular beam epitaxy (MBE) on both semiconducting [S i( 1 )] and insulating [a-AI 2 O 3 (0001 )] substrates in ultra-high vacuum (UHV) to probe E F fluctuations using scanning tunneling microscopy and spectroscopy (STM/STS).
  • MBE molecular beam epitaxy
  • UHV ultra-high vacuum
  • STM/STS scanning tunneling microscopy and spectroscopy
  • This example 6 reports the formation of ultrathin mono- and bi-layers of Na 3 Bi on a semiconducting Si(1 1 1 ) substrate and the characterization of those layers, and the properties of those layers when exposed to electric field.
  • the properties of these ultrathin Na 3 Bi layers are explored using angle-resolved photoelectron spectroscopy (ARPES) and scanning tunnelling spectroscopy STS.
  • ARPES angle-resolved photoelectron spect
  • Example 1 relate to a Na 3 Bi layer.
  • Na 3 Bi is extremely air sensitive and degrades rapidly on exposure to air.
  • the inventors have found that providing an oxygen barrier layer of the form MF 2 (where M is an alkaline earth metal) is effective to prevent the oxidation of the Na 3 Bi.
  • Example 7 reports the fabrication of a device including a Na 3 Bi layer that has been capped with MgF 2 layer to mitigate oxidation of the Na 3 Bi layer. This example demonstrates that, even when capped with an MgF 2 layer, the Na 3 Bi layer continues to exhibit the charge carrier density, charge mobility, and magnetoresistance properties that allow the charge carrier density and bandgap of the Na 3 Bi layer to be modulated or altered with an electric field.
  • Example 8 reports the fabrication of a device including a Na 3 Bi layer that has been capped with MgF 2 layer to mitigate oxidation of the Na 3 Bi layer. This example demonstrates the properties and stability of the MgF2-capped Na 3 Bi layer.
  • This example demonstrates the formation of high quality, c-axis oriented thin film Na 3 Bi on insulating Al 2 0 3 [0001 ] substrates. It will be appreciated that a wide variety of different substrates can be selected for formation of the Na 3 Bi Dirac semimetal layer. However, in this instance and as discussed above, the use of insulating Al 2 0 3 is advantageous as it allows the properties of the Na 3 Bi to be analysed in a controlled manner in the absence of a conductive layer.
  • Films are grown using the two-step method. Briefly, a thin (2 nm) nucleation layer is deposited under simultaneous Bi and Na flux at low (120 °C) temperature, followed by additional growth at a higher final temperature of between 250 and 390 °C. The resultant films were subsequently characterized using low temperature magneto- transport and scanning tunnelling microscopy and spectroscopy (STM and STS) in situ in ultrahigh vacuum (UHV).
  • STM and STS low temperature magneto- transport and scanning tunnelling microscopy and spectroscopy
  • AI 2 O3[0001 ] substrates were annealed in air at 1350 °C for three hours, and subsequently annealed at 1050 °C in pure O2 atmosphere, to provide an atomically flat surface for film growth.
  • Ti/Au contacts (5/50 nm) were deposited through a stencil mask onto the corners of the substrate, and wire bonded to a contact busbar on the sample plate.
  • Substrates were introduced into ultra-high vacuum (UHV) immediately after wire bonding to minimise exposure to ambient conditions, and then annealed at 400 °C for 1 hour to remove adsorbed atmospheric species.
  • UHV ultra-high vacuum
  • Effusion cells were used to simultaneously evaporate elemental Bi (99.999%, Alfa Aesar) in an overflux of Na (99.95%, Sigma Aldrich) with a Bi:Na flux ratio not less than 1 : 10, calibrated by quartz microbalance.
  • the Bi rate used was about 0.03 A/sec, and Na was about 0.7 A/sec.
  • the pressure during growth was less than 3 x 10 "9 Torr.
  • the first 2 nm of Na 3 Bi was deposited with the substrate temperature at 120 °C. The substrate temperature was then increased to a value ranging between 250-390 °C over the next 5 nm of film growth as determined by the bismuth deposition rate.
  • a Ptlr STM tip was prepared and calibrated using an Au(1 1 1 ) single crystal and the Shockley surface state before all measurements.
  • STM differential conductance (dl/dV) was measured using a 5 mVrms AC excitation voltage (673 Hz) that was added to the tunnelling bias. Differential conductance measurements were made under open feedback conditions with the tip in a fixed position above the surface.
  • the data in Figure 1 was analysed and prepared using WSxM software. LEED measurements were done with the commercial 8" system (OCI) on the soft X-ray endstation of the Australian synchrotron. Transport measurements were carried out in the Createc LT-STM using van der Pauw geometry and standard DC electrical measurements in a magnetic field up to 0.5T at 5K.
  • Figure 1 shows the crystalline quality of Na 3 Bi thin films formed according to the present invention.
  • the structure comprises single crystal AI2O3 [0001 ] pre-patterned with electrical contacts in van der Pauw geometry, and the Na 3 Bi is deposited on top, making electrical contact.
  • Figure 1 (a) inset shows the crystal structure of Na 3 Bi.
  • Figure 1 (b) shows STM topography at a temperature of 5 K, showing a large area (about 80x80 nm) atomically flat terrace, with a step height of 4.7 A, consistent with the half-unit cell distance of 4.83 A between NaBi planes.
  • Atomic resolution of the surface (inset) shows a (1x1 ) termination with the expected in-plane lattice constant of 5.45 A.
  • Figure 1 c) shows low energy electron diffraction (LEED) image taken at 17.5 eV.
  • the low background and sharp hexagonal diffraction pattern confirm that the (1x1 ) structure is coherent across the LEED spot size of about 200 pm. Faintly visible is the same pattern rotated 30°, indicating the presence of a small fraction of the sample with that alignment. Similar quality LEED patterns were observed on samples grown across the range of final temperatures tested.
  • Figure 1(d) shows STS (differential conductance vs bias voltage) averaged over an area of 400 nm 2 . STS reflect the energy dependent local density of states of the sample.
  • the n-type minimum carrier density should not exceed 4.4x10 15 cm "3 .
  • the mean free path L can be calculated, which ranged between 75-135 nm for the prepared samples. Increasing mobility with decreasing carrier density is consistent with expectations assuming that the impurities that give rise to doping are also responsible for the disorder limiting the mobility. To better understand the relationship between the mobility and carrier density, these parameters were plotted against one another ( Figure 2(c)).
  • the hashed region is the theoretical prediction for a TDS using the random phase approximation (RPA) assuming that all the impurities are dopants of a single sign, i.e. the impurity density n imp equals the carrier density n.
  • An overall positive magnetoresistance (MR) is observed, with a cusp below about 0.1 T.
  • MR magnetoresistance
  • Figure 3(b) shows the prefactor A determined from fits to Eqn. 1 plotted as a function carrier density n.
  • the low-field MR data are well described by the strong spin-orbit coupling limit of the Hikami-Larkin-Nagaoka (HLN) formula:
  • Equation (2) where the only fit parameter is the phase coherence field B 0 .
  • Figure 3(d)) shows the phase coherence length as a function of carrier density for the various films:
  • the coherence length is substantially larger than the thickness of the samples (20 nm), consistent with the assumption of two-dimensionality.
  • Efforts to fit this data using a three-dimensional theory for weak localization yielded poor fits regardless of the limiting cases in temperature and magnetic field used, again illustrating a deviation between practice and theory.
  • 3D weak anti-localization in bulk crystals of TDS have previously been predicted. However, this was for a bulk crystal and not a thin film. The observations of 2D weak anti-localization in these films is a new and unexpected property.
  • the coherence length L 0 at densities above 6 x 10 18 cm "3 exceeds 1 pm, but is suppressed in lower carrier density samples, a phenomena that has been previously observed in other Dirac materials such as graphene and the Dirac surface state of bismuth selenide.
  • the fact that the obtained weak field MR is well described by the HLN formula indicates that Na 3 Bi films of the present invention are well described by non- interacting Dirac cones, i.e. intervalley scattering is weak.
  • the growth of electrically isolated, highly oriented, large area thin film Na 3 Bi on a-AI 2 0 3 [0001 ] substrates has been demonstrated.
  • the high sample quality is reflected in a record high mobility and near-ideal weak anti-localization behaviour.
  • High quality TDS thin films on insulators open a route toward novel topological phenomena and devices, including electric field control via gate electrodes, as illustrated in Example 2.
  • FIG 4(a) and Figure 4(b) illustrate a stencil mask 17 and prepared substrate structure 10 used to prepare an uncapped Na3Bi layer.
  • the substrate structure 10 includes a sample plate 1 1. An insulating substrate was applied over it to electrically insulate the created structure. Next a back gate electrode 14 is provided on top of which an Si:Si0 2 substrate is placed.
  • Ti/Au contacts (5/50 nm) 18 were deposited through a stencil mask 17 onto the solvent-cleaned pieces of 1 pm oxide thickness Si:Si0 2 substrate pieces 16.
  • a stencil mask 17 in the Hall bar pattern was affixed to the substrate surface, and the electrodes were subsequently wire bonded to a contact busbar on the sample plate.
  • Substrates were introduced into UHV immediately after wire bonding to minimise exposure to ambient conditions, and then annealed at 450 °C for 90 minutes to remove adsorbed atmospheric species. Effusion cells were used to simultaneously evaporate elemental Bi (99.999%,
  • Ultrathin Na 3 Bi films were grown in a ultra-high vacuum (UHV) (10 "1 ° Torr) molecular beam epitaxy (MBE) chamber and then transferred immediately after the growth to the interconnected Createc LT-STM operating in UHV (10 "11 Torr) for STM/STS measurements at 5 K.
  • UHV ultra-high vacuum
  • MBE molecular beam epitaxy
  • For Na 3 Bi film growth effusion cells were used to simultaneously evaporate elemental Bi (99.999%, Alfa Aesar) in an overflux of Na (99.95%, Sigma Aldrich) with a Bi: Na flux ratio not less than 1 : 10, calibrated by quartz microbalance.
  • the Bi rate used was ⁇ 0.03 A/s, and Na was ⁇ 0.7 A/s.
  • the pressure during growth was less than 3x10 "9 Torr.
  • Si(1 1 1 ) - a Si(1 1 1 ) wafer was flash annealed in order to achieve 7 x 7 surface reconstruction, confirmed using STM and low energy electron diffraction (LEED).
  • the substrate temperature was 345°C for successful crystallization.
  • the sample was left at 345°C for 10 min in a Na overflux to improve the film quality.
  • the sample was cooled to 315°C in Na overflux to minimise Na vacancies due to desorption at elevated temperature before cooling to room temperature.
  • a Ptlr STM tip was prepared and calibrated using an Au(1 1 1 ) single crystal and the Shockley surface state at ⁇ 0.5V and flat LDOS near the Fermi level before all measurements.
  • STM differential conductance (dl/d V) was measured using a 5 mV AC excitation voltage (673 Hz) that was added to the tunnelling bias. Differential conductance measurements were made under open feedback conditions with the tip in a fixed position above the surface.
  • Figure 6 shows the successful growth of a continuous few atomic layer thick Na 3 Bi film grown via molecular beam epitaxy on a Si(1 1 1 ) substrate.
  • the unit cell of Na 3 Bi is shown with the c-axis height of 0.96 nm (reflecting the thickness of a (001 ) film).
  • the zero of the height scale represents the Si(1 1 1 ) substrate.
  • the majority of the film has a thickness of 0.96 nm and 1.44 nm, demonstrating continuous film with atomically flat terraces larger than 40nm.
  • the spectra lines represents an increase in the tip- sample separation of 0.3 nm, 0.9 nm, 1 .5 nm and 2.7 nm respectively with respect to the initial tip-sample separation.
  • Changing the tip-sample separation changes the electric field induced from the potential difference between tip and sample.
  • the system has a dl/dV that possesses a sharp dip near the zero in energy but does not go all the way to zero (which is taken as 0.03 in the dl/dV scale and represents the noise floor of the instrument).
  • This behaviour reflects a semi-metal (with zero bandgap) expected for bulk topological Dirac semimetal Na 3 Bi.
  • the tip is moved further from the sample (i.e.
  • This example reports data from high quality thin films of Na 3 Bi grown on amorphous Si:Si0 2 substrates.
  • the films were grown using the two-step thermal process.
  • FIG. 4(b) is a sample diagram, showing the stencil mask used to define the Hall bar geometry for thin film growth on the Si:Si0 2 substrate. Substrates were introduced into UHV immediately after wire bonding to minimise exposure to ambient conditions, and then annealed at 450°C for 90 minutes to remove adsorbed atmospheric species.
  • Effusion cells were used to simultaneously evaporate elemental Bi (99.999%, Alfa Aesar) in an overflux of Na (99.95%, Sigma Aldrich) with a Bi:Na flux ratio not less than 1 :10, calibrated by quartz microbalance.
  • the Bi rate used was about 0.03_A/sec, and Na was about 0.7 A/sec.
  • the pressure during growth was less than 3 10 "9 Torr.
  • the first 2 nm of Na 3 Bi was deposited with the substrate temperature at 120°C.
  • the substrate temperature was then increased to 345°C over the next 5 nm of film growth as determined by the bismuth deposition rate.
  • the samples were annealed at the growth temperature for an additional 10 minutes in Na flux only, before cooling to room temperature for subsequent transfer to the analysis chamber.
  • Sample characterization was carried out in a Createc LT-STM operating in ultra-high vacuum (UHV) (10 "11 Torr) with base temperature 4.8K using a Hall bar geometry and standard DC electrical measurements in a magnetic field up to 1 T at 5K.
  • UHV ultra-high vacuum
  • the sample was transferred back to the growth chamber for sequential deposition of molecular dopant F4-TCNQ (Sigma- Aldrich) onto the surface, and subsequent transport characterization.
  • F4-TCNQ molecular dopant
  • the organic molecule F4-TCNQ acts as a p-type dopant due to its high electron affinity (as previously discussed in Example 2) and was deposited onto the surface of the as-grown Na3Bi to induce a depletion layer at the surface of the film, before further modulation of the carrier density using the S1O2 back gate.
  • Figure 9 demonstrates the effects of depositing F4-TCNQ on the sample surface.
  • the anomalously large coefficient of the quadratic magnetoresistance, the change in slope sign, and non-linearity of the Hall response all indicate that these samples are in the charge inhomogeneous regime near the Dirac point.
  • the MR response at each applied gate voltage is fitted using three global fit parameters: the proportionalities between electron/hole conductivity and n 4 3 , denoted A e , A h , and n rm s, the measure of the disorder induced carrier density fluctuations.
  • the carrier density n is taken as the only V g dependent fit parameter.
  • the change in carrier density can be treated as proportional to the change in gate voltage, ⁇ is proportional to AVg.
  • ⁇ is proportional to AVg.
  • a non-linear gate response on the negative side was observed, which is attributed to charge traps likely introduced as a result of the exposure to Na flux intrinsic to the sample growth process.
  • Figure 10(c) and Figure 11 show the results of using this theory to model the measured gate response of these samples.
  • the measured zero (magnetic) field conductivity shows a non-zero minimum value of 5.6 e 2 /h.
  • Figure 10(b) and Figure 10(c) when there is asymmetry between the electron and hole mobilities, and that the appropriate proportionality constant is between ⁇ and n 4 3 /n imp .
  • the hole conductivity coefficient is an order of magnitude larger than that for electrons: this interpretation is in qualitative agreement with the non-linear Hall response on the p-type side of the charge neutrality point: the high mobility, low field slope is p-type, but the (low mobility carrier) curvature at higher field is n-type. This observation could be taken as evidence that the Dirac point in this material is generated by the band inversion of the S and P states of different total angular momentum.
  • the values of the carrier density from the theory are all n-type, but are within the Gaussian profile of the inhomogeneous regime, characterized by n rm s- This is shown in Figure 11 , where the (only) V g dependent fit parameter n is plotted (see the plotted line), indicating that the sample remains on the n-type side of the Dirac point, but is in the inhomogeneous regime.
  • the (dashed) Gaussian envelope (arbitrary height) is the size of the charge inhomogeneous region determined by the global fit parameter n rm s.
  • the dots are the as-grown carrier density for comparison.
  • the overlap of the profile and the carrier density is a rough indication of how far into the inhomogeneous regime the sample approaches.
  • this example demonstrates the growth of high quality, large area thin film of Na 3 Bi directly on amorphous S1O2.
  • the gate dependent MR shows two distinct features, a non-zero minimum of the zero-field conductivity, and a non-linear Hall response indicating the presence of electrons and holes with different mobilities. These fits indicate that our samples are on the n-type side of the Dirac point, in the inhomogeneous regime, and unexpectedly, that the hole conductivity in this material is two orders of magnitude higher than the electron conductivity.
  • TDS such as Na 3 Bi and Cd 3 As 2 express the pseudorelativistic physics of two- dimensional Dirac material graphene, but extended to three dimensions. TDS can yield ultra-high mobilities as well as new physics such as the chiral anomaly.
  • the carriers in puddles in turn screen the disorder, with E F determined self-consistently by the disorder and the screening properties of the Dirac materials. Puddles have been visualized in graphene using scanning single electron transistor microscopy and scanning tunnelling spectroscopy, where the fluctuations are largely governed by the underlying substrate, and have also been measured in the Dirac surface state of a topological insulator.
  • Figure 12(a) shows a large area (400 nm x 380 nm) topographic STM image of a thin 20nm film of Na 3 Bi on Si(1 1 1 ), with several atomically flat terraces >100 nm in size.
  • Figure 12(b) (45 nm x 45 nm taken immediately after growth) and Figure 12(c) (30nm x 30nm taken seven days after growth) show the topography of two atomically flat regions away from step edges or the screw dislocations seen in Figure 12(a).
  • Figure 12(d) shows an atomically flat region (30 nm x 30 nm) of Na 3 Bi grown on sapphire (a-AI 2 O 3 (0001 ). Whilst atomically flat regions of Na 3 Bi up to 100 nm x 100 nm can be obtained on Si(1 1 1 ), sparse defect cluster sites (few per 100 nm x 100 nm) give rise to tip-induced ionization ring features that will be discussed further below. This necessitated focusing on smaller areas free of ionization rings in order to unambiguously determine the variation in Dirac point.
  • Figure 12(e) shows area-averaged scanning tunneling spectra (STS) of the
  • the Dirac point is located ⁇ 20 meV above the Fermi level indicating p-type doping. Similar doping ( ⁇ 25 meV) has been reported on similar thickness Na 3 Bi films on Si(1 1 1 ) measured with angle-resolved photoelectron spectroscopy (ARPES), validating our assumption that the minimum in the DOS reflects the Dirac point. Seven days after growth, the Dirac point has shifted to approximately 15 meV below the Fermi level, reflecting a gradual global n-type doping of the Na 3 Bi due to the adsorption of atomic species present in UHV. This adsorption results in the formation of intermittent impurity clusters on the surface, as such all topography and spectroscopic measurements were deliberately performed away from such sites.
  • the resonance feature D is unambiguously tied to E D , and not to the E F , as the relative energy shift of D with respect to E D remains unchanged within experimental accuracy during the transition from p-type to n-type doping.
  • the spatial variation of the Dirac point energy, E D can be found by tracking the position of the minimum differential conductance in STS (alternatively, the shift in the defect resonance D in each spectrum gives similar results).
  • Figure 13(a) and Figure 13(b) show the spatial variation of E D for regions 'A' and 'B' corresponding to Figure 12(b) and Figure 12(c) respectively.
  • a clear, continuously connected local potential modulation emerges, correlated on a scale much larger than the crystal lattice or point-spectroscopy grid. This modulation in E D represents the puddling of charge density at the surface.
  • FIG. 13(c) shows the local E D of 20 nm Na 3 Bi on a- AI 2 O3(0001 ) (labelled Region C) which possesses a larger n-type doping.
  • Region C shows the local E D of 20 nm Na 3 Bi on a- AI 2 O3(0001 ) (labelled Region C) which possesses a larger n-type doping.
  • the upper, middle and lower panels of Figure 13(d) show histograms of E D relative to E F for the scans in Figures 13(a)-(c) respectively.
  • This example reports the formation of ultrathin mono- and bi-layers of Na 3 Bi on a semiconducting Si(1 1 1 ) substrate and the characterization of those layers, and the properties of those layers when exposed to electric field.
  • the properties of these ultrathin Na 3 Bi layers are explored using angle-resolved photoelectron spectroscopy (ARPES) and scanning tunnelling spectroscopy STS.
  • ARPES angle-resolved photoelectron spectroscopy
  • STS scanning tunnelling spectroscopy
  • TDS are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases.
  • This example reports the fabrication and characterization of an ultrathin layer of the TDS material Na 3 Bi.
  • Ultra-thin Na 3 Bi thin films were grown in ultra-high vacuum (UHV) molecular beam epitaxy (MBE) chambers and then immediately transferred after the growth under UHV to the interconnected measurement chamber (i.e. Createc LT-STM at Monash University, Scienta R-4000 analyser at Advanced Light Source and SPEC Phoibos 150 at Australian Synchrotron).
  • UHV ultra-high vacuum
  • MBE molecular beam epitaxy
  • FIG. 24 shows the characteristic RHEED pattern for Si(1 1 1 ) 7 x 7 reconstruction along T - M
  • Figure 24(b) shows the RHEED pattern for few-layer Na 3 Bi along - K, consistent with RHEED reported on films of 15 unit cell thickness, where the lattice orientation of Na 3 Bi is rotated 30° with respect to the Si(1 1 1 ) substrate.
  • Figure 24(c) shows the 1x1 LEED pattern consistent with growth of Na 3 Bi in the (001 ) direction. The sharpness of the spots and absence of rotational domains indicates high-quality single crystal few-layer Na 3 Bi over a large area.
  • the spectra have been normalized to the maximum in intensity and energy-corrected (to account for the small interfacial charge transfer that occurs) in order to overlay the core levels.
  • the spectra have been offset for clarity. It is clear there is negligible change to the Si 2p core level after Na 3 Bi growth, with no additional components or significant broadening arising, verifying that Na 3 Bi is free-standing on Si(1 1 1 ). This is consistent with the fact that the ARPES measurements on ultrathin Na 3 Bi showed no features with the Si(1 1 1 ) 7 x 7 symmetry (discussed below).
  • Figure 34(c) shows the second derivative of the spectra in order to enhance low intensity features. This has been overlaid with density functional theory (DFT) calculations for ML (broken line) and BL (full line) Na 3 Bi showing qualitatively good agreement, consistent with the STM topography which shows coexisting ML and BL regions.
  • DFT density functional theory
  • photon energy-dependent ARPES can be utilized to determine whether a material possesses a 3D band dispersion, i.e. the binding energy E B depends not only on in-plane wavevectors k x and / y , but also on out-of-plane wavevector k z .
  • To determine the momentum perpendicular to the surface requires measuring energy distribution curves as a function of the photon-energy in order to measure E B vs k z , using the nearly free-electron final state approximation:
  • Eqn. (5A) simplifies to
  • Figure 35 shows a plot of k z as a function of binding energy (and reflects energy distribution curves taken at normal emission for photon energies between 45-55 eV.
  • a flat band is observed near 0 eV (the Fermi energy) and represents the valence band maximum.
  • This band possesses no dispersion in k z (i.e. no bulk band dispersion), verifying that few-layer Na 3 Bi is indeed electronically 2D, unlike its thin-film and bulk counterparts.
  • XPS Depth dependent X-ray photoelectron spectroscopy
  • FIG. 25 illustrates the bandgap in mono- and bilayer Na 3 Bi and edge state behaviour.
  • Figure 25(a) shows typical dl/d V spectra for ML and BL with bandgaps corresponding to 0.36 ⁇ 0.025 eV and 0.30 ⁇ 0.025 eV respectively.
  • Determining the bulk electronic bandgap of mono- and bilayer Na 3 Bi was achieved by performing scanning tunnelling spectroscopy (dl/dV as a function of sample bias V) more than 5 nm away from step edges.
  • the valence and conduction band edges in the local density of states (LDOS) are defined as the onset of differential conductance intensity above the noise floor. Due to the large variation in dl/dV signal near a band edge, it was useful to plot the logarithm of the dl/dV curves for accurate band gap determination as shown in Figure 26(a) and Figure 26(b).
  • dl/dV spectra were taken over a wide range of tunnelling currents (0.01 -1 nA), resulting in large changes in signal at band edges and a change in the relative magnitude of signal to noise.
  • spectra were normalized to a relatively featureless point in the LDOS away from the band edge onset and the dl/dV signal corresponding to a bias of -400 meV was chosen for normalization. After the normalization procedure was completed for all spectra, the band edges were taken as the point at which the dl/dV has fallen to 0.01 of the normalized value.
  • Tip-induced band bending (TIBB) effects have the potential to overestimate the size of the electronic bandgap due to unscreened electric fields and can strongly influence the interpretation of STM data.
  • the absence/presence of TIBB is usually verified by performing dl/dV spectra at different initial current set points (different tip- sample separations). In the absence of TIBB there will be negligible change in the band edges of the spectra, however, if the spectra are strongly influenced by TIBB increasing the current set point (reducing tip-sample separation) will lead to increased band bending, and overestimation of the bandgap.
  • the exact opposite is observed for few-layer Na 3 Bi. In this case the bandgap becomes smaller, closes and then re-opens upon increasing the current (and consequently electric field), and is clearly not consistent with TIBB.
  • TIBB tip-induced band bending
  • Figure 25(b) plots the experimental bandgap (squares) in comparison to DFT calculated values using the generalized gradient approximation (GGA) for pristine Na 3 Bi (crosses) and Na 3 Bi layers that contain an Na(2) surface vacancy (circles).
  • the edge state spectrum is shown in Figure 27(e) for ML and Figure 27(j) for BL.
  • the projected 1 D Brillouin zone is shown in Figure 27(a) in which a Kramers pair of topological edge states can be seen.
  • Figure 25(c) shows a BL Na 3 Bi region decorated with Na surface vacancies and a ⁇ 1 .2 nm step edge to the underlying Si substrate, with a small monolayer Na 3 Bi protrusion ⁇ 0.7 nm above the substrate.
  • Figure 25(d) shows dl/d V spectra for BL Na 3 Bi far away from the edge (black curve) and at the edge (blue curve). In contrast to the gap in the bulk, the dl/d V spectrum at the edge is quite different, with states filling the bulk gap along with a characteristic dip at 0 mV bias.
  • FIG. 25(e) shows dl/d V spectra as a function of distance away from the edge, tracing the profile in Figure 25(c), demonstrating the extended nature of the edge state feature, with Figure 25(f) showing that the average dl/d V signal within the bulk bandgap moving away from the edge follows the expected exponential decay for a 1 D topological ⁇ non- trivial state.
  • Potassium deposited on the Na 3 Bi surface donates electrons leaving a positive K + ion behind, producing a uniform planar charge density. This is equivalent to a parallel plate capacitor, allowing the electric displacement field, D to be calculated across the Na 3 Bi film using Gauss' law via:
  • n(fc F ) D/hv F .
  • n(k F ) as a function of K-dosing is approximately 2 x 10 12 cm "2 between consecutive K-dosing until the 50-minute mark (where charge saturation occurs).
  • Figure 29(a)-(d) shows the bandstructure along ⁇ - ⁇ for values of the electric field of 0.0, 0.72, 1 .44 and 2.18 V/nm respectively, with dots points reflecting the extracted maxima from energy distribution curves (EDC) and momentum distribution curves.
  • EDC energy distribution curves
  • Energy dispersion curves (EDCs) and momentum dispersion curves (MDCs) are slices through constant-momentum and constant-energy of the photoemission spectra (such as Figure 30(a)) along high-symmetry directions (M - ⁇ - M) or (K - ⁇ - K) respectively.
  • Band energy and momentum coordinates are extracted by Gaussian fitting of the photoemission intensity on a flat background (as shown in Figure 30(b) and Figure 30(c) by the circles). We find that band edges are extracted more reliably from EDCs, whilst MDC peak positions are used at larger binding energies where clearly distinct peaks can be resolved (see left panel of Figure 30(b)).
  • v F i measured for the valence and conduction bands for both films are nearly independent of K dosing, with near-isotropic dispersion in k x , k y (also shown in Figure 28).
  • v F n and v F p to be a global fit parameter, with best fit values v Fxi * l x 10 6 m/s; v FiP « 3 x 10 5 m/s.
  • the photoemission intensity of the electron band is two orders of magnitude less than that of the valence band - possibly due to the different orbital characters of the two bands resulting in a lower intensity due to matrix element effects.
  • the conduction band lies well above the Fermi level in the as- grown film, meaning that significant charge transfer from K-dosing is needed to n-type dope the film in order to observe the conduction band.
  • the fitting parameter ⁇ ⁇ for the electron bands can only be determined once the conduction band is resolvable below E F , and in addition further seen to match with the valence band determined value for D.
  • the band dispersion near ⁇ displays the clear cusp of a band edge indicating a gapped system, with 140 meV separation between the valence band edge and the Fermi energy E F .
  • the effect of K dosing in Figure 29(b)-(d) is to n-type dope the sample and consequently increase the displacement field.
  • the separation from the valence band edge to E F has increased to ⁇ 257 meV.
  • the bandgap must be at least this amount, consistent with STS, though we cannot determine its exact magnitude since the conduction band lies above E F (although it can be estimated, see Figure 31 (a)).
  • the DFT results clearly demonstrate the topological phase transition as shown in Figure 37(c)-(f).
  • Figure 37(c) at 0 V/A the orbital resolved bandstructure clearly shows a band inversion of the s and p atomic orbitals at ⁇ induced by SOC. This clearly indicates a non-trivial 2D topological insulator, as shown in the corresponding projected edge spectrum in Figure 37(d) with the observation of topological edge states.
  • the orbital resolved bandstructure above the critical field value where the gap has reopened has undergone a band ordering change at ⁇ as compared with Figure 37(c). This indicates a topological to trivial insulator phase transition with electric field, and is confirmed by the disappearance of the topological edge states in Figure 37(f).
  • Figure 33 shows logarithmic plots of the tunnelling current as a function of relative distance for (a) Au(1 1 1 ) (bias 500 mV) and (b) thin film Na 3 Bi (bias -300 mV) where the absolute current is plotted.
  • Au(1 1 1 ) the characteristic exponential dependence of /(z) (straight line on the semi-log plot) is observed with current increasing from 0.01 nA to 10 nA by moving the tip 3A closer to the surface.
  • (b) At a bias of -300 mV has a tunnelling current of 26 pA. From the / vs z plot taken at -300 mV this corresponds to relative z distance of 1 .3 A. Adding 13.2 A to account for point contact and mirror potential yields a tip-sample separation of 14.5 A. This provides an estimate of the tip-sample separation, so to calculate a displacement field for each of our dl/d V measurements the potential difference needs to be calculated, i.e. the work function difference between the metallic tip and the few-layer Na 3 Bi.
  • the Kelvin probe technique was utilized to measure the work function of few- layer Na 3 Bi.
  • the work function was determined by measuring the contact potential difference of the Na 3 Bi relative to a gold reference of known work function (determined by photoelectron spectroscopy secondary electron cutoff measurements).
  • a work function for few-layer Na 3 Bi of 1 .7 ⁇ 0.05 eV was measured using this technique.
  • This value and the 2.3 eV potential barrier gives a tip work function of 2.9 ⁇ 0.05 eV, and a potential difference between tip and sample of 1 .2 ⁇ 0.1 eV.
  • Figure 32(b) shows normalized dl/d V spectra taken on BL Na 3 Bi that are offset for clarity at various tip-sample separations (electric fields). A large modulation occurs upon increasing the electric field strength, with the bandgap reducing from 300 meV to completely closed (and exhibiting the characteristic V-shape of a Dirac-semimetal) at ⁇ 1 .1 V/nm and then reopening above this to yield a bandgap of ⁇ 90 meV at ⁇ 1 .2 V/nm.
  • Inset of Figure 32(b) shows the dl/d V spectra without any offset, highlighting the clear non-zero density of states at the minimum in conductance (i.e.
  • FIG. 32(c) plots the bandgap as a function of electric field for ML and BL Na 3 Bi, with both exhibiting a similar critical field where the bandgap is closed and then reopened into the trivial/conventional regime with increasing electric field. DFT calculations also predict such a transition, arising from a Stark effect-induced rearrangement of s- and p-like bands near the gap.
  • monolayer and bilayer Na 3 Bi are quantum spin Hall insulators with bulk bandgaps above 300 meV, offering the potential to support dissipationless transport of charge at room temperature.
  • An electric-field tunes the phase from topological to conventional insulator with bandgap of ⁇ 90 meV due to a Stark effect-driven transition.
  • This bandgap modulation of more than 400 meV is larger than has been achieved in atomically thin semiconductors such as bilayer graphene and similar to phosphorene, and may be useful in optoelectronic applications in the mid-infrared.
  • Na 3 Bi is extremely air sensitive, especially for thin films which exhibit essentially instantaneous degradation upon exposure to air. For bulk crystals this may be circumvented via fabrication of the bulk crystals in a glove box under an inert atmosphere and then immersion in paraffin oil. However, this option is not available for thin film growth in ultra-high vacuum conditions.
  • the inventors have found that a protective oxygen barrier layer is useful to prevent such degradation.
  • MgF 2 is transparent with a large band gap 10.8 eV, dielectric constant of 5 and a relatively low evaporation temperature ( ⁇ 800°C) compared with other insulators such as Si0 2 . These properties make MgF 2 an excellent oxygen barrier material.
  • Na 3 Bi thin films were grown on insulating AI2O3 [0001 ] substrates in an ultra-high vacuum (UHV) molecular beam epitaxy chamber, followed by deposition of the oxygen barrier layer in the same chamber.
  • UHV ultra-high vacuum
  • effusion cells were used to simultaneously evaporate elemental Bi in an overflux of Na with a Bi:Na flux ratio not less than 1 :10, calibrated by quartz microbalance.
  • a two-step growth method was used where the first 2nm were grown at 120°C, with the remainder of the film grown at 330°C.
  • the oxygen barrier layer, magnesium difluoride MgF 2 was deposited on the Na 3 Bi film at room temperature at a rate of 0.1 A/s. A thickness of at least 20 nm was evaporated to ensure protection.
  • the film was then removed from ultra-high vacuum into atmosphere, where the resistivity was recorded as a function of time exposed to air.
  • the inventors found that using a low deposition rate, such as below 0.1 A/s, resulted in problems with the MgF 2 layer at thicknesses above 40 to 50 nm.
  • the inventors observed that at these thicknesses, the Na 3 Bi went from metallic to insulating during the deposition of MgF 2 within minutes.
  • Figure 19 shows the change in sheet resistivity of two separate Na 3 Bi films as a function of MgF 2 deposition time, with a MgF 2 deposition rate in the range of 0.05 to 0.06 A/s.
  • Figure 14 shows the sheet resistivity of a 20nm Na 3 Bi film capped with 20nm of MgF 2 as a function of air exposure. It is immediately clear that the MgF 2 oxygen barrier layer provides protection to the underlying Na 3 Bi layer, with the sheet resistivity only doubling from 3500 to 7000 over a 10 hour period in comparison with instantaneous insulating behaviour of Na 3 Bi without protection. With reference to Figure 20, examination of the device under the optical microscope revealed the shadowing by the wire bonds meant there were small lines of unprotected Na 3 Bi. This is a likely explanation for the slow increase in resistivity.
  • Exposure of a 100nm Na3Bi film capped with a 20 nm MgF 2 layer to air Figure 15, Figure 16, Figure 17, and Figure 18 provide results from a 100 nm thick Na 3 Bi film capped with a 20 nm MgF 2 layer. Transport measurements of the resistivity and Hall Effect were performed for as-grown, as-capped and after a two minutes air exposure. As shown in Figure 15 and Figure 16, the sheet resistance increases after applying the oxygen barrier layer - before and after air exposure. The MgF 2 oxygen barrier layer slightly reduces the doping however the film was significantly n-type doped after air exposure. This suggests that some degradation of the Na 3 Bi occurred due to incomplete MgF 2 coverage or migration of atmospheric species from the edges of the sample.
  • Magneto-transport measurements were attempted in a high magnetic field low temperature cryostat for the samples after exposure to air, with the results shown in Figure 17 and Figure 18.
  • the magnetoresistance behaviour is similar to low mobility bulk Na 3 Bi crystals, indicating MgF 2 is a suitable candidate for passivating the surface of Na 3 Bi thin films.
  • Figure 22 is graph showing sheet resistance of 20 nm Na 3 Bi films with and without an MgF 2 capping layer versus air exposure time. This graph shows that Na 3 Bi thin films capped with MgF 2 remain stable in air (e.g. are not oxidised) for hours. In contrast, the uncapped Na 3 Bi film oxidises within a few seconds of exposure to air to become an insulator.
  • Figure 23 is a graph of results from the analysis of an MgF 2 capped film in a low temperature cryostat to determine electronic properties at high magnetic field and over a wide temperature range. The results demonstrate the strong weak anti-localization of the air-exposed MgF 2 capped Na 3 Bi film.

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Abstract

L'invention concerne une structure comprenant : des moyens pour générer un champ électrique ; une couche semi-métallique de Dirac topologique, la couche semi-métallique de Dirac topologique étant séparée de manière non conductrice des moyens pour générer le champ électrique, et le moyen de génération du champ électrique est configuré pour appliquer le champ électrique à au moins une partie de la couche semi-métallique de Dirac topologique ; et une couche barrière à l'oxygène pour empêcher l'oxydation de la couche semi-métallique de Dirac topologique, la couche barrière à l'oxygène étant formée d'un matériau de la forme MF2, où M est un métal alcalino-terreux. L'invention concerne également une structure permettant de modifier la densité de porteurs de charge dans un semi-métal de Dirac topologique, la structure comprenant : un conducteur ; une couche isolante ; une couche semi-métallique de Dirac topologique séparée du conducteur par au moins la couche isolante ; une couche barrière à l'oxygène pour empêcher l'oxydation de la couche semi-métallique de Dirac topologique, La couche barrière à l'oxygène étant formée d'un matériau de la forme MF2, M étant un métal alcalino-terreux ; et au moins une électrode en contact avec la couche semi-métallique de Dirac topologique ; le conducteur et l'au moins une électrode étant configurés pour appliquer un champ électrique à au moins une partie de la couche semi-métallique de Dirac topologique pour modifier la densité de porteurs de charge de la couche semi-métallique de Dirac topologique. L'invention concerne en outre une structure pour modifier la bande interdite d'un semi-métal de Dirac topologique, la structure comprenant : un conducteur ; une couche isolante ; une couche semi-métallique de Dirac topologique séparée du conducteur par au moins la couche isolante ; et une couche barrière à l'oxygène pour empêcher l'oxydation de la couche semi-métallique de Dirac topologique, la couche barrière à l'oxygène étant formée d'un matériau de la forme MF2, où M est un métal alcalino-terreux ; le conducteur étant configuré pour appliquer un champ électrique à au moins une partie de la couche semi-métallique de Dirac topologique pour modifier la bande interdite de la couche semi-métallique de Dirac topologique. L'invention concerne en outre des procédés associés de formation d'une couche semi-métallique de Dirac topologique sur un substrat.
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CN112420861A (zh) * 2020-11-18 2021-02-26 长春理工大学 二维材料异质结结构及其制备方法和应用、光电器件
WO2022094666A1 (fr) * 2020-11-06 2022-05-12 Monash University Transistor à effet de champ à quantum topologique
CN115548691A (zh) * 2022-11-23 2022-12-30 云南农业大学 一种基于狄拉克半金属和二氧化钒的三频段双调谐吸波体
US11563078B2 (en) 2020-03-12 2023-01-24 The University Of Utah Research Foundation Ultra-compact inductor made of 3D Dirac semimetal

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US20120032227A1 (en) * 2010-08-09 2012-02-09 University Of Notre Dame Du Lac Low voltage tunnel field-effect transistor (tfet) and method of making same

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US20120032227A1 (en) * 2010-08-09 2012-02-09 University Of Notre Dame Du Lac Low voltage tunnel field-effect transistor (tfet) and method of making same

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US11563078B2 (en) 2020-03-12 2023-01-24 The University Of Utah Research Foundation Ultra-compact inductor made of 3D Dirac semimetal
WO2022094666A1 (fr) * 2020-11-06 2022-05-12 Monash University Transistor à effet de champ à quantum topologique
CN112420861A (zh) * 2020-11-18 2021-02-26 长春理工大学 二维材料异质结结构及其制备方法和应用、光电器件
CN115548691A (zh) * 2022-11-23 2022-12-30 云南农业大学 一种基于狄拉克半金属和二氧化钒的三频段双调谐吸波体
CN115548691B (zh) * 2022-11-23 2024-05-07 云南农业大学 一种基于狄拉克半金属和二氧化钒的三频段双调谐吸波体

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