Transparent Resonant Tunnel Diode and Methods for its Fabrication

Abstract

Resonant tunnel diode (10) comprising an electrically insulating substrate (12), a metal layer (14) and a transition metal oxide layer (16), wherein the metal layer (14) is applied to the substrate (12) and the transition metal oxide layer (16) is applied to the metal layer (14), and wherein the metal layer (14) and the transition metal oxide layer (16) have an amorphous structure.

Description

[0001] The invention relates to a resonant tunnel diode comprising an electrically insulating substrate, a metal layer and a transition metal oxide layer.

[0002] Furthermore, the invention relates to a method for manufacturing the resonant tunnel diode.

[0003] A tunnel diode is a semiconductor device that, due to the quantum mechanical effect of tunneling, exhibits a current-voltage characteristic with a negative differential resistance in a specific region. In this region, increasing the voltage does not lead to an increase in current, as is typical for ordinary diodes, but rather to a decrease in current.

[0004] A resonant tunnel diode enables this behavior because its potential distribution exhibits multiple potential barriers, which the electrodes must tunnel through to generate an electric current. The probability of tunneling depends on the level of the quantized energy state between the tunnel barriers, resulting in the negative differential resistance.

[0005] Resonant tunnel diodes typically have a complex structure with multiple layers of different materials. Some tunnel diodes are made from an n-doped germanium or gallium arsenide layer into which a smaller layer of indium is alloyed. Due to this complex layer structure, the fabrication of resonant tunnel diodes is also labor-intensive.

[0006] Patent US 10 600 961 B2 describes a vanadium dioxide (VO2) based threshold switching device having a current-controlled negative differential resistance, and a manufacturing method for the threshold switching device.

[0007] Patent US 6,534,784 B2 describes an electron tunneling device in which two different metal oxide layers are formed between the two non-insulating layers for the transport of electrons between them.

[0008] The publication US 2014 / 0302310A1 describes nanolaminates consisting of alternating layers of amorphous multi-component metal films (AMMFs) and metal oxide films. The nanolaminates are described as metamaterials whose physical properties can be engineered to tailor the resulting electrical, dielectric, and thermal characteristics. In certain configurations, the AMMFs could be used in optical or electronic devices, such as metal-insulator-metal (MIM) diodes.

[0009] Publication WO 2018 / 009 901 A1 describes the use of amorphous multi-component metal films to improve the performance of electronic devices such as resistors, diodes, and thin-film transistors. An amorphous hot-electron transistor (HET) with coplanar emitter and base electrodes offers electrical properties and performance advantages over existing vertical HET structures.

[0010] Publication GB 1 408 122 A relates to thin-film devices and in particular to vanadium oxide thin-film devices and their manufacture.

[0011] The publication ABDEL-RAHMAN, M. [et al.]: High sensitivity vanadium-vanadium pentoxide aluminum metal-inulator-metal diode, from Micro & Nano Letters, 2018, Vol. 13, Iss. 5, 680–683 (DOI: 10.1049 / mnl.2017.0728) describes the fabrication of metal-insulator-metal (MIM) diodes with improved sensitivity. It describes a diode with an asymmetric structure featuring vanadium, vanadium pentoxide, and aluminum thin films in a cascaded configuration (V-V₂O₅-Al). The MIM diode is fabricated using electron beam lithography, sputtering, and a metal lift-off process.

[0012] It is therefore an object of the invention to provide a resonant tunnel diode with a simple structure. Furthermore, it is an object of the invention to provide a method by which a resonant tunnel diode can be manufactured without significant effort.

[0013] This problem is solved by the subject matter of the independent claims. Preferred embodiments of the invention are described in the dependent claims.

[0014] According to the invention, a resonant tunnel diode is thus provided comprising an electrically insulating substrate, a metal layer and a transition metal oxide layer, wherein the metal layer is applied to the substrate and the transition metal oxide layer is applied to the metal layer, and wherein the metal layer and the transition metal oxide layer have an amorphous structure.

[0015] Furthermore, according to the invention, a method for producing a resonant tunnel diode comprising an electrically insulating substrate, a metal layer and a transition metal oxide layer is provided, comprising the steps: a) Providing the substrate, b) Coating the provided substrate with the metal layer by direct sputtering such that the metal layer has an amorphous structure, c) Coating the metal layer with the transition metal oxide layer by means of reactive sputtering such that the transition metal oxide layer has an amorphous structure, and d) Fabrication of the resonant tunnel diode described above.

[0016] A key aspect of the invention is that the metal layer and the transition metal oxide layer have an amorphous structure. Resonant tunnel diodes typically have complex structures with at least three layers deposited on the substrate. In the present invention, however, only two layers, namely the metal layer and the transition metal oxide layer, are deposited on the substrate. Because the metal layer and the transition metal oxide layer are amorphous in structure, the present invention achieves the current-voltage characteristic of a resonant tunnel diode with only these two layers deposited on the electrically insulating substrate. The structure of the resonant tunnel diode is thus greatly simplified, making its fabrication less complex.

[0017] In this context, a layer is understood to be a three-dimensional object whose extent in a two-dimensional plane is many times greater than its extent perpendicular to it. In contrast to a transition metal oxide layer, in which the transition metal atoms are oxidized, the metal atoms in a metal layer are preferably in the zero oxidation state. Thus, the metal layer preferentially exhibits metallic properties such as electrical and thermal conductivity. A transition metal oxide is understood to be a compound of a transition metal with oxygen.

[0018] In this context, a resonant tunnel diode is understood to be a semiconductor component whose current-voltage characteristic exhibits at least one region with a negative differential resistance.

[0019] The resonant tunnel diode thus comprises the electrically insulating substrate, the metal layer, and the transition metal oxide layer. The substrate preferably provides at least one two-dimensional surface onto which the metal layer and the transition metal oxide layer are applied. The two-dimensional surface can be flat or curved. The metal layer is preferably applied to the substrate such that the substrate is completely covered by the metal layer. The transition metal oxide layer is applied to the metal layer. Preferably, the transition metal oxide layer is also applied to the metal layer such that the metal layer is completely covered by the transition metal oxide layer. In other words, the resonant tunnel diode has a structure in which the metal layer is located between the electrically insulating substrate and the transition metal oxide layer.Preferably, no other layers besides the metal layer are located between the substrate and the transition metal oxide layer.

[0020] As already mentioned, the metal layer and the transition metal oxide layer exhibit an amorphous structure. This means that the atoms in the metal layer and the atoms in the transition metal oxide layer lack long-range order, and in particular, translational symmetry as found in a crystalline layer. The structural state of the metal layer and / or the transition metal oxide layer can preferably be determined by transmission electron microscopy and / or grazing-incidence X-ray diffractometry.

[0021] The resonant tunnel diode according to the invention is particularly suitable for use in self-tuning switches, high-speed oscillators, memory cells and / or logic circuits.

[0022] According to a preferred embodiment of the invention, the thickness of the metal layer is between 10 nm and 25 nm and / or the thickness of the transition metal oxide layer is between 15 nm and 50 nm. Preferably, the thickness of the metal layer is 20 nm ± 2 nm and the thickness of the transition metal oxide layer is 40 nm ± 2 nm. The layer thickness is preferably the extent of the layer parallel to the surface normal of the two-dimensional area provided by the substrate. Such thin layers have the advantage that the resonant tunnel diode can be miniaturized particularly well. Furthermore, very little material is required to produce the resonant tunnel diode.

[0023] According to a further preferred embodiment of the invention, the optical transmittance through the metal layer and the transition metal oxide layer perpendicular to the layers is greater than 50%. The optical transmittance is a measure of the permeability of the metal layer and the transition metal oxide layer to light waves and indicates the intensity of the light wave after passing through the two layers compared to the initial intensity of the light wave. Preferably, the optical transmittance for light with wavelengths between 420 nm and 750 nm is greater than 50%, and particularly preferably greater than 55%. Preferably, the optical transmittance for light in the wavelength range between 420 nm and 750 nm can be increased beyond 55% by varying the thickness of the metal layer, the transition metal oxide layer, and / or the oxidation state of the transition metal.The metal layer and the transition metal oxide layer are therefore transparent to the human eye. This gives the resonant tunnel diode the advantage of being suitable as a component in photonic systems, which use optical methods and technologies for transmitting, storing, and processing information. In particular, the resonant tunnel diode is well-suited as a component in optical neural networks, where neural networks are implemented using optical circuits. Thus, the transparent, resonant tunnel diode is especially suitable for transparent self-tuning switches, high-speed oscillators, memory cells, and / or logic circuits in displays, optical communication systems, smart windows, and / or smart glass. Optically transparent switches are essential for such applications.

[0024] Regarding the metal layer and the transition metal oxide layer, fundamentally different metals and transition metal oxides can be used. According to a preferred embodiment of the invention, the metal layer is an aluminum layer and / or the transition metal oxide layer is a vanadium(V) oxide layer. It has been found that with these materials, the described layer structure exhibits a current-voltage characteristic of a resonant tunnel diode even at room temperature. Vanadium(V) oxide, also known as vanadium pentoxide, is a stable vanadium-oxygen compound with the formula V₂O₅. Due to the stability of the material, the resonant tunnel diode is not susceptible to oxidation and is therefore very robust and durable.Furthermore, it is not necessary to protect the vanadium(V) oxide layer from environmental influences with an additional layer, making the construction of the resonant tunnel diode very simple. The transition metal oxide layer V₂O₅ can contain small amounts of its own oxide impurities. The minor oxidation states can be V₂O₅, V₂O₃, or other typical vanadium-containing oxide phases. The purity of the V₂O₅ in the vanadium(V) oxide layer can vary between 90% and greater than 99%.

[0025] Alternatively or additionally to aluminum as the metal layer, the metal layer can consist of combinations or composites of the following elements or compounds: Pt, Pd, Cu, Ni, Mo, Ta, W, and / or Si. Furthermore, TiN, TaN, and / or WN can be used as materials for the metal layer. Although the metal in these nitride compounds is not in the zero oxidation state, these materials exhibit metallic properties and are therefore suitable as materials for the metal layer.

[0026] As already mentioned, the substrate is an electrically insulating, i.e., non-conductive, substrate. This preferably means that the electrical conductivity of the substrate is less than 10⁻⁶. -8 S cm -1In this context, a further preferred embodiment of the invention provides that the substrate is optically transparent and / or made of glass and / or a polymer. By selecting an optically transparent substrate, the resonant tunnel diode is optically transparent. Preferably, the resonant tunnel diode has an optical transmission of > 50%, wherein the optical transmission is particularly preferably > 50% perpendicular to the layers, i.e., parallel to the surface normal of the two-dimensional area provided by the substrate, at a wavelength between 420 nm and 750 nm. This has the advantage that the resonant tunnel diode can be used in situations where the components are intended to absorb as little light as possible, such as in photonic systems or solar cells. Glass has the advantage of being readily available and robust. Polymers have the advantage of being inexpensive.Furthermore, polymers can be flexible, which allows the two-dimensional surface of the substrate to be curved particularly easily. The substrate is preferably made of quartz glass or the polymer polyimide.

[0027] According to a further preferred embodiment of the invention, the resonant tunnel diode comprises two electrodes, the electrodes being applied to the transition metal oxide layer at a distance from each other. This enables particularly simple contacting of the resonant tunnel diode. The electrodes can, in principle, be made of any electrically conductive material. However, it is preferred that the electrodes be made of gold, silver, platinum, copper, indium tin oxide (ITO), and / or OFHC copper, or of alloys and / or composites of these materials. OFHC copper is a copper alloy that exhibits high thermal conductivity and preferably has an oxygen content of less than 0.001%. Electrodes made of indium tin oxide are particularly preferred, as this material has the advantage of providing optically transparent electrodes.In this way, the optical transmission of the resonant tunnel diode and / or the metal layer and transition metal oxide layer can be maintained. Preferably, the electrodes are applied to the transition metal oxide layer using a lithographic process, resulting in planar physical contact between the transition metal oxide layer and the electrode. Alternatively, the electrodes can be brought into physical contact with the transition metal oxide layer as measuring tips. The phrase "the electrodes are spaced apart" preferably means that the two electrodes are not in physical contact with each other on the transition metal oxide layer. For example, the distance between the electrodes can range from a few nanometers to several centimeters.

[0028] In this context, according to a further preferred embodiment of the invention, when a voltage is applied to the electrodes, the resonant tunnel diode exhibits a current-voltage characteristic with at least one negative differential resistance at room temperature. This means that when the current is plotted against the voltage, the current-voltage characteristic shows at least one region where the current decreases despite increasing voltage. Room temperature in this context means a temperature between 0 °C and 40 °C.

[0029] Further technical effects and advantages of the resonant tunnel diode become apparent to the person skilled in the art from the description of the method for manufacturing the resonant tunnel diode, as well as from the description of the exemplary embodiments and the figures.

[0030] As mentioned previously, the process involves applying the metal layer by direct sputtering and the transition metal oxide layer by reactive sputtering. Sputtering, also known as cathode sputtering, is a coating process in which a material source is converted into the gas phase by bombardment with high-energy ions, i.e., sputtered, and subsequently deposited as a layer on the substrate. Preferably, a plasma is ignited in a vacuum chamber by applying a voltage between a cathode and an anode. The ions, accelerated primarily at the cathode, can, due to their high kinetic energy, knock atoms from the surface of the material source upon impact, thus sputtering it. Reactive sputtering occurs when one or more reactive gases, such as oxygen and / or nitrogen, are added to an inert working gas.The reactive gases react with the atomized material source and form new materials that subsequently settle on the substrate as a layer.

[0031] The inventive method provides, in a first step, the electrically insulating substrate is supplied. In a further step, the metal layer is applied to the substrate by direct sputtering. The metal layer is applied to the substrate in such a way that an amorphous structure of the metal layer is formed. In a further step, the transition metal oxide layer is applied to the metal layer by reactive sputtering. The transition metal oxide layer is also applied to the metal layer in such a way that it has an amorphous structure. The method thus enables the simple and robust production of the layer structure of the resonant tunnel diode.

[0032] According to a preferred embodiment of the invention, step b) and / or step c) is carried out at room temperature. This makes it particularly easy to create the amorphous structure of the metal layer and / or the transition metal oxide layer. It also makes the process particularly easy to perform, since neither heating nor cooling is required during the coating process. In this context, room temperature means a temperature between 0 °C and 40 °C.

[0033] In connection with the amorphous structure of the metal layer, a further preferred embodiment of the invention provides that the metal layer is an aluminum layer and is applied in step b) in an inert working gas under an applied power of 15 W to 30 W. Preferably, the inert working gas is argon with a purity of 99.998%. More preferably, the applied power is 25 W ± 3 W. It has been shown that under these conditions, an amorphous aluminum layer can be produced on the substrate particularly well and reliably. More preferably, the layer thickness can be controlled over the duration of the coating process.

[0034] In connection with the amorphous structure of the transition metal oxide layer, a further preferred embodiment of the invention provides that the transition metal oxide layer is a vanadium(V) oxide layer and, in step c), the oxygen content in an inert working gas is between 33% V / V and 67% V / V, and / or the vanadium(V) oxide layer is applied to the metal layer at an applied power of between 35 W and 45 W. As in step b), the inert working gas is preferably argon. It has been shown that an amorphous vanadium(V) oxide layer can be produced particularly well and reliably on the metal layer, and especially preferably on the aluminum layer, at an oxygen content of preferably 35% V / V and a power of 35 W to 40 W.

[0035] It is further preferred that step b), i.e., coating the substrate with the metal layer and step c), i.e., coating the metal layer with the transition metal oxide layer, take place directly one after the other, without the metal layer applied to the substrate being exposed to an oxygen-containing atmosphere in the meantime.

[0036] With regard to contacting the transition metal oxide layer with the electrodes, a preferred embodiment of the invention provides that the method additionally comprises the following step: e) Lithographic application of two spaced-apart electrodes onto the transition metal oxide layer.

[0037] In other words, the electrodes are preferably applied to the transition metal oxide layer using a lithographic process. This preferably includes creating a lithographic mask comprising open mask areas on the transition metal oxide layer. More preferably, the lithographic mask is created by applying a photosensitive photoresist to the transition metal oxide layer and transferring an image of the lithographic mask onto the photoresist using an exposure process. In a further step, the exposed areas of the photoresist are preferably dissolved, or alternatively, the unexposed areas of the photoresist are dissolved to create the lithographic mask. Subsequently, electrode material is preferably introduced into the open mask areas of the lithographic mask to deposit the electrodes onto the transition metal oxide layer.

[0038] Further technical effects and advantages of the method become apparent to the person skilled in the art from the description of the resonant tunnel diode, as well as from the description of the exemplary embodiments and the figures.

[0039] The invention will now be described in more detail with reference to the drawings and a preferred embodiment of the invention.

[0040] The drawings show Fig. 1 a schematic representation in two views of a transparent resonant tunnel diode according to a preferred embodiment of the invention and Fig. 2 a current-voltage characteristic of the transparent resonant tunnel diode Fig. 1 during a layer resistance measurement.

[0041] In Fig. 1a) is schematically an exploded view and in Fig. 1b) A schematic cross-sectional view of a transparent resonant tunnel diode 10, according to a preferred embodiment of the invention, is shown. The resonant tunnel diode 10 comprises an electrically insulating substrate 12, a metal layer 14, and a transition metal oxide layer 16. In this case, the substrate 12 is a glass substrate 12, the metal layer 14 is an aluminum layer 14, and the transition metal oxide layer 16 is a vanadium(V) oxide layer 16 with the empirical formula V₂O₅. The metal layer 14 and the transition metal oxide layer 16 are amorphous in their structure, i.e., the atoms do not exhibit long-range order. Fig. 1a) and Fig. 1b) It is evident that the metal layer 14 is applied over a flat surface 18 of the substrate 12 and that the transition metal oxide layer 16 covers the metal layer 14 over its entire surface. In the preferred embodiment presented here, the thickness 20 of the metal layer 14 is 20 nm and the thickness 22 of the transition metal oxide layer is 40 nm, wherein the values ​​for the thicknesses 20 and 22 refer, respectively, to an extent perpendicular to the flat surface 18 and parallel to the surface normal of the surface 18.

[0042] The metal layer 14 was produced by direct sputtering and the transition metal oxide layer 16 by reactive sputtering. Both direct and reactive sputtering processes were carried out consecutively at room temperature (20 °C). The coating of the substrate 12 with the metal layer 14 was performed in an inert atmosphere under argon gas with a purity of 99.998%. An electrical power of 25 W was applied between the cathode and anode. During the coating of the metal layer 14 with the transition metal oxide layer 16, the oxygen content in the argon gas was 33% by volume. In this step, an electrical power of 40 W was applied between the cathode and anode, and the metal layer was not exposed to an oxygen-containing atmosphere prior to the coating with the transition metal oxide layer 16.

[0043] Also in Fig. 1b) It is evident that the resonant tunnel diode 10 is optically transparent. In this case, the resonant tunnel diode 10 exhibits an optical transmission of 55% for light 24 with a wavelength of 532 nm. This means that an intensity I t The intensity of the light 24 after passing through the resonant tunnel diode 10 perpendicular to the surface 18 is still 55% of the initial intensity I0.

[0044] Fig. Figure 2 shows a current-voltage characteristic 26 of the transparent resonant tunnel diode 10. Fig. 1 in a film resistance measurement. In the film resistance measurement, two electrodes (not shown) were physically brought into contact with the transition metal oxide layer 16 as measuring tips. The distance between the measuring tips of the electrodes was 0.5 cm. A voltage was applied to the electrodes and the current flowing between the electrodes was recorded. In the diagram in Fig.Figure 2 shows the applied voltage in volts on the x-axis (28) and the current flow in milliamperes on the y-axis (30). The current-voltage characteristic curve (26) shows a region with negative differential resistance (32) at approximately 4.5 to 5 V. In this region, the current flow decreases despite increasing voltage. The current-voltage characteristic curve (26) was determined using a Semiconductor Characterization System (model: Keithley SCS-4200) at room temperature (20 °C). Reference symbol list 10 transparent resonant tunnel diode 12 Substrate, glass substrate 14 Metal layer, aluminum layer 16 Transition metal oxide layer, vanadium(V) oxide layer 18 area 20 Layer thickness of the metal layer 22 Layer thickness of the transition metal oxide layer 24 lights 26 Current-voltage characteristic 28 X-axis, applied voltage in volts 30 Y-axis, current flow in milliamperes 32 Area with negative differential resistance I0 Initial intensity of the light I t Light intensity according to transmission

Claims

[1] Resonant tunnel diode (10) comprising an electrically insulating substrate (12), a metal layer (14) and a transition metal oxide layer (16), wherein the metal layer (14) is applied to the substrate (12) and the transition metal oxide layer (16) is applied to the metal layer (14), and wherein the metal layer (14) and the transition metal oxide layer (16) have an amorphous structure. [2] Resonant tunnel diode (10) according to claim 1, wherein the thickness (20) of the metal layer (14) is between 10 nm and 25 nm and / or the thickness (22) of the transition metal oxide layer (16) is between 15 nm and 50 nm. [3] Resonant tunnel diode (10) according to claim 1 or 2, wherein the optical transmittance through the metal layer (14) and the transition metal oxide layer (16) perpendicular to the layers (14, 16) is > 50%. [4] Resonant tunnel diode (10) according to any of the preceding claims, wherein the metal layer (14) is a layer of a metal selected from the group comprising aluminium (14), platinum, palladium, copper, nickel, molybdenum, tantalum, tungsten, titanium silicon and / or mixtures thereof and / or the transition metal oxide layer (16) is a vanadium(V) oxide layer (16). [5] Resonant tunnel diode (10) according to any of the preceding claims, wherein the substrate (12) is optically transparent and / or the substrate (12) is made of glass and / or a polymer. [6] Resonant tunnel diode (10) according to one of the preceding claims comprising two electrodes, wherein the electrodes are applied at a distance from each other on the transition metal oxide layer (16). [7] Method for producing a resonant tunnel diode (10) comprising an electrically insulating substrate (12), a metal layer (14) and a transition metal oxide layer (16), comprising the steps: a) Providing the substrate (12), b) Coating the provided substrate (12) with the metal layer (14) by direct sputtering such that the metal layer (12) has an amorphous structure, c) Coating the metal layer (14) with the transition metal oxide layer (16) by means of reactive sputtering such that the transition metal oxide layer (16) has an amorphous structure, and d) Manufacturing a resonant tunnel diode (10) according to claim 1. [8] Method according to claim 7, wherein step b) and / or step c) is carried out at room temperature. [9] Method according to claim 7 or 8, wherein the metal layer (14) is an aluminium layer (14) and the aluminium layer is applied to the substrate in step b) in an inert working gas under an applied power of 15 W to 30 W. [10] Method according to any one of claims 7 to 9, wherein the transition metal oxide layer (16) is a vanadium(V) oxide layer (16) and in step c) an oxygen content in an inert working gas is between 33 %V / V and 65 %V / VO:Arg and / or the vanadium(V) oxide layer (16) is applied to the metal layer at an applied power between 35 W and 45 W. [11] Method according to any one of claims 7 to 10, comprising the step: e) Lithographic application of two spaced-apart electrodes onto the transition metal oxide layer (16).

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