Multifunctional thermal emitter transistor based on graphene / germanium Schottky junction and construction method

Abstract

The invention relates to the field of research, development and application of multifunctional devices, in particular to a multifunctional thermal emitter transistor based on a graphene / germanium Schottky junction and a construction method. The transistor comprises a top metal electrode, an aluminum oxide insulating layer, a graphene channel with a nanometer gap, a hafnium oxide insulating layer, a germanium substrate, a base metal electrode and an emitter metal electrode from top to bottom, wherein the base metal electrode and the emitter metal electrode are located on the two sides of the graphene channel and make contact with the graphene channel, and the metal electrodes are of a gold / titanium laminated composite structure. According to the invention, a Schottky junction of graphene and germanium is constructed, a multifunctional thermal emitter transistor is constructed, and an injection-induced hot carrier emission mechanism is utilized, so that the subthreshold swing lower than the Boltzmann limit of 60mV / decand the negative differential resistance with the peak-valley current ratio greater than 10 at room temperature and atmospheric pressure are realized; the invention relates to a THz transistor prototype device with a common emitter current gain greater than 100.

Description

Technical Field

[0001] The present invention relates to the research, development and application field of a multifunctional device, specifically to a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction and a construction method thereof. Background Art

[0002] Hot carriers are widely used in various technologies such as electronic devices, optoelectronics, thermoelectrics, and energy technology because of their higher kinetic energy than the average thermal motion energy. With the development of electronic devices, many high-performance devices have been prepared based on hot carriers, such as hot electron transistors (HETs) and real space transfer transistors (RSTTs). HETs use metals or semiconductors as the base. Since the energy of electrons in the emitter is higher than that in the base, when electrons are injected from the emitter into the base, they become hotter and faster, thereby shortening the base transit time and increasing the transfer rate. Recently, two-dimensional materials such as graphene and molybdenum disulfide have shown the potential for preparing terahertz devices because of their atomic-level thickness. When used as base materials, they further reduce the base transit time. On the other hand, RSTT uses an electric field to accelerate carriers. When the carriers become hot enough, they will transfer from one conductive layer to another, resulting in negative differential resistance and high-speed operation. Both HET and RSTT show great potential as high-performance devices. However, the general mechanism of hot carrier generation is carrier injection or acceleration, which may limit the performance and function of the device. For HET, the height and thickness of the tunneling barrier or the quantum scattering effect of the Schottky barrier limit further increases in device speed; while for RSTT, on the one hand, the thickness of the bulk material limits the device speed, and on the other hand, when the device is manufactured using Si / Ge technology compatible with mainstream semiconductor processes, the negative differential resistance is also limited. In addition, both devices cannot provide effective low-power operation.

[0003] Recently, hybrid dimensional electronic devices made by combining bulk materials and low-dimensional materials have received great attention. Hybrid dimensional heterostructures can take advantage of the geometric scale, electrical and optical properties of materials of different dimensions, and high-performance electronic and optoelectronic devices have been constructed. In particular, for low-dimensional materials, their atomic-level thickness reduces the vertical carrier transit time, and their rich band structure can be combined with bulk materials to form various barriers to control carrier transport. Therefore, hybrid dimensional heterostructures can be used to construct devices with new hot carrier mechanisms. Here, we construct a hybrid dimensional hot emitter transistor (HOET) through the generation mechanism of injection-induced hot carrier emission. The present invention is based on the preparation method of graphene-germanium Schottky junction, uses carrier injection and acceleration, and relies on the advantages of atomic-level thickness of low-dimensional materials to construct a multifunctional hot emitter transistor, achieving multifunctional characteristics such as ultra-low subthreshold swing, obvious negative differential resistance and potential terahertz frequency, which provides potential for the development of high-performance, multifunctional devices with low power consumption and high frequency characteristics in the post-Moore era.

[0004] References

[0005] [1]Sharma,S.et al.Aggregation-induced negative differentialresistance in graphene oxide quantum dots.Phys.Chem.Chem.Phys.23,16909-16914(2021).

[0006] [2] Dragoman, M., Dinescu, A. & Dragoman, D. Negative differential resistance in graphene-based ballistic field-effect transistor with oblique topgate. Nanotechnology 25, 415201 (2014).

[0007] [3]Sharma,P.,Bernard,LS,Bazigos,A.,Magrez,A.&Ionescu,AMGraphenenegative differential resistance circuit with voltage-tunable highperformance at room temperature.IEEE Electron Device Lett.36,865-867(2015).

[0008] [4]Britnell,L.et al.Resonant tunnelling and negative differentialconductance in graphene transistors.Nat.Commun.4,1794(2013).

[0009] [5]Nguyen,P.D.et al.Negative differential resistance effect in planargraphene nanoribbon break junctions.Nanoscale 7,289-293(2015).

[0010] [6]Burg,G.W.et al.Coherent interlayer tunneling and negativedifferential resistance with high current density in double bilayer graphene–WSe2 heterostructures.Nano Lett.17,3919–3925(2017).

[0011] [7]Fallahazad,B.et al.Gate-tunable resonant tunneling in doublebilayer graphene heterostructures.Nano Lett.15,428-433(2015).

[0012] [8]Bastani,P.,Mohseni,S.M.,Jamilpanah,L.,Azizi,B.&Gharehbagh,J.S.E.Interface-induced negative differential resistance and memristivebehavior in Gr / MoSe2heterostructure.J.Mater.Sci.Mater.Electron.33,6403-6410(2022).

[0013] [9] Antonova, IV, Shojaei, S., Sattari-Esfahlan, SM & Kurkina, IINegative differential resistance in partially fluorinated graphene films. J. Appl. Phys. 111, 043108 (2017). Summary of the invention

[0014] The purpose of the present invention is to provide a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction and a construction method thereof, wherein the graphene hot emitter transistor is constructed by utilizing the natural gap of graphene or by cutting the complete graphene in the channel by an atomic force microscope (AFM) and a helium ion microscope (HIM), and utilizing the injection-induced hot carrier emission mechanism so that carriers can cross the Schottky junction between graphene and germanium, thereby preparing a multifunctional transistor device having a subthreshold swing lower than the Boltzmann limit at room temperature and atmospheric pressure, a negative differential resistance with a peak-to-valley current ratio exceeding 10, and a common emitter current gain greater than 100.

[0015] The technical solution of the present invention:

[0016] A multifunctional hot emitter transistor based on a graphene / germanium Schottky junction, which includes, from top to bottom, a top metal electrode, an aluminum oxide insulating layer, a graphene channel with a nanogap, a hafnium oxide insulating layer and a germanium substrate, as well as base and emitter metal electrodes located on both sides of the graphene channel and in contact with the graphene channel, wherein the metal electrodes are a gold / titanium laminated composite structure.

[0017] The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction comprises the following steps:

[0018] (1) ultrasonically cleaning the germanium substrate with acetone and isopropanol to remove surface impurities, and then washing with diluted hydrofluoric acid and deionized water to remove surface oxides;

[0019] (2) depositing a hafnium oxide insulating layer on a germanium substrate by atomic layer deposition;

[0020] (3) using photolithography technology to prepare a photoresist mask layer, and exposing the base and emitter metal electrode patterns to obtain the metal electrode area on the hafnium oxide insulating layer, and using vacuum electron beam evaporation to evaporate gold / titanium electrodes;

[0021] (4) using photolithography technology to make a photoresist mask layer, and exposing the required window pattern, and using reactive ion etching to etch out the hafnium oxide window to expose the germanium substrate;

[0022] (5) growing a graphene film on a platinum foil by chemical vapor deposition, and transferring the graphene film to a prepared germanium substrate by bubbling transfer;

[0023] (6) using photolithography technology to prepare a photoresist mask layer, and exposing the desired graphene channel pattern to obtain a graphene channel region;

[0024] (7) Using oxygen ion etching technology, oxygen is introduced to clean away the excess graphene film exposed outside the base and emitter metal electrodes;

[0025] (8) The nanogaps in the graphene channels are formed naturally or prepared using atomic force microscopy or helium ion microscopy;

[0026] (9) depositing an aluminum oxide insulating layer on a germanium substrate by atomic layer deposition;

[0027] (10) A photoresist mask layer is prepared by photolithography technology, and a top gate metal electrode pattern is exposed to obtain a metal electrode area on the aluminum oxide insulating layer, and a gold / titanium electrode is evaporated by vacuum electron beam evaporation.

[0028] The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction comprises the following steps: in step (1), the germanium substrate is a p-type lightly doped germanium sheet having a thickness of 500 μm and a resistivity of 1 to 10 Ω·cm; the diluted hydrofluoric acid is obtained by diluting the original concentration hydrofluoric acid with deionized water in a volume ratio of 1:8, the concentration of the original hydrofluoric acid is 40 wt%, and the treatment time in the dilute hydrofluoric acid and the deionized water is 1 min.

[0029] In the method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction, in step (2), an atomic layer deposition is used to deposit a hafnium oxide insulating layer at 200° C. with a thickness of 30 nm.

[0030] The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction comprises the following steps: in step (3) and step (10), LOR3A and S-1813 double-layer photoresists are sequentially spin-coated and exposed for 4.5 seconds; an Au layer of a top metal electrode is evaporated using an electron beam evaporator to a thickness ranging from 50 to 60 nm, and then a Ti layer is evaporated to a thickness ranging from 5 to 6 nm.

[0031] The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction comprises the following steps: in step (4), LOR3A and S-1813 double-layer photoresists are sequentially spin-coated on the hafnium oxide insulating layer, and exposed for 4.5 seconds to obtain a desired window pattern; reactive ion etching is used to form the window, and the conditions are: tetrafluoromethane with a flow rate of 50 sccm is used as the etching gas, the pressure is 5.0 Pa, the power is 200 W, and the etching time is 5 minutes; then, the dilute hydrofluoric acid used in step (1) is used for cleaning for 30 seconds.

[0032] In the method for constructing a multifunctional hot emitter transistor based on graphene / germanium Schottky junction, in step (6), S-1813 photoresist is spin-coated on the graphene and exposed for 4.5 seconds.

[0033] In the method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction, in step (7), an oxygen ion cleaning machine is used for etching, and the etching conditions are: using oxygen with a flow rate of 180 sccm as the etching gas, a pressure of 1.5 to 2.5 Pa, a power of 200 W, and an etching time of 2 min.

[0034] The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction, in step (8), using an atomic force microscope to cut graphene, the cutting parameters are: in the nano-etching Nanoman mode, the speed in the XY direction is 0.03-0.05 μm / s, and the set voltage is 0.7-1.2 V; or, using a helium ion microscope to cut graphene, the cutting conditions are: on the patterned graphene, using atomic layer deposition, depositing aluminum oxide with a thickness of 30 nm at 150° C., and then using a helium ion microscope neon source for cutting, the current size is 0.31-0.98 pA, the cutting time is 15-45 s, and the graphene nanogap width is 1-45 nm.

[0035] In the method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction, when preparing a common emitter gain exceeding 100, in step (9), atomic layer deposition is used to deposit aluminum oxide at 150° C. with a thickness of 70 to 100 nm.

[0036] The design idea of ​​the present invention is:

[0037] Hybrid dimensional electronic devices fabricated by combining bulk materials and low-dimensional materials have attracted great attention because they can take advantage of the advantages of different dimensional materials in terms of geometric scale, electrical and optical properties. However, multifunctional hybrid dimensional electronic devices are rarely reported. Based on this, we propose a multifunctional hot emitter transistor based on a hybrid dimensional graphene / germanium Schottky junction.

[0038] The present invention proposes a multifunctional hot emitter transistor based on graphene / germanium Schottky junction, which utilizes the injection induced hot carrier emission mechanism, that is, utilizing the base graphene / germanium Schottky junction to inject hot carriers to further enhance the energy of hot carriers in the emitter graphene that have been accelerated by the channel electric field, thereby realizing the mechanism of hot carrier emission in the emitter. A terahertz transistor prototype device is constructed, which realizes a subthreshold swing of less than 60mV / dec, which is lower than the Boltzmann limit, a negative differential resistance with a peak-to-valley current ratio greater than 10, and a common emitter current gain greater than 100 at room temperature and atmospheric pressure, providing a new type of multifunctional mixed-dimensional prototype device for the post-Moore era.

[0039] The advantages and beneficial effects of the present invention are:

[0040] 1. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction involved in the present invention has a simple process flow, does not involve complex processes such as high-temperature annealing and ion implantation, and the required process technology is mature, and the experimental results can be well repeated.

[0041] 2. The invention relates to a method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction, in which graphene is grown on a platinum foil using a CVD method and then transferred to a target substrate. This process is the mainstream process for graphene transfer today, and the process flow is simple, effective and mature. Since the transfer method is mild, there is almost no damage to the surface of the Ge substrate, thereby ensuring device performance.

[0042] 3. The present invention constructs a multifunctional hot emitter transistor based on graphene / germanium Schottky structure. Different from the traditional tunneling transistor, the transistor uses the injection-induced hot carrier emission mechanism to enable the device to achieve a subthreshold swing of 60mV / dec below the Boltzmann limit at room temperature and atmospheric pressure, as well as a negative differential resistance with a peak-to-valley current ratio greater than 10, and a common emitter current gain greater than 100. It is a multifunctional high-speed device (THz transistor prototype device).

[0043] 4. The graphene channel gap involved in the present invention can be prepared by natural fracture of graphene, AFM probe, and HIM processing technology. The use of HIM processing technology expands the application of HIM in the field of micro-nano processing, and the use of the passivation layer protection method reduces the damage of helium / neon ions to the substrate and the influence of irradiation on the Schottky junction. The processing method is simple and can be prepared in a controllable manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 (a) Schematic diagram of the structure of a graphene channel transistor with a nanogap transferred to a Ge substrate.

[0045] Figure 1(b) Optical microscope photograph of transistors with channels of different lengths and widths (scale bar: 20 μm).

[0046] Figure 1 (c) SEM image of the nanogap (about 95 nm) after patterning in graphene (scale bar: 4 μm).

[0047] Figure 1 (d) is the IV characteristic curve of the graphene channel, where the very small emitter current I e , indicating that the emitter and base graphene have been separated. In the figure, the horizontal axis V c , V e They represent the collector and emitter voltages (V) respectively, and the vertical axis Current represents the current (A).

[0048] Figure 1 (e) IV characteristic curves of emitter graphene / p-type germanium heterojunction and base graphene / p-type germanium heterojunction. c represents the collector voltage (V), and the vertical axis Current represents the emitter voltage (I e ) and the base (I b )Current (A).

[0049] Figure 1 (f) IV characteristic curves of a typical graphene / p-type germanium heterojunction measured at different temperatures from 224.2 to 229.3 K. In the figure, the abscissa Voltage represents the voltage applied to the germanium substrate (V), and the ordinate Current represents the current at the graphene end (A).

[0050] Figure 2 (a) is a diagram of a transistor operating in common emitter form.

[0051] Figure 2 (b) is a diagram of hot holes passing through the graphene / p-type germanium Schottky barrier at the critical voltage.

[0052] Figure 2 (c) Transmission characteristics I c -V b curve, showing a sudden negative change in collector current I c In the figure, the horizontal axis V b represents the base voltage (V), the vertical axis I c Represents the collector current (μAμm -1 ).

[0053] Figure 2 (d) is the collector current characteristic curve, showing the subthreshold swing beyond the Boltzmann limit at room temperature. b represents the base voltage (V), the ordinate abs(Ic ) represents the collector current (μAμm -1 ).

[0054] Figure 2 (e) for different V c The SS statistics under voltage, the subthreshold swing is about 3.3 to 12.5mV / dec. In the figure, the horizontal axis V c represents the collector voltage (V), and the vertical axis SS represents the subthreshold swing (mV dec -1 ).

[0055] Figure 2 (f) is the temperature-dependent transfer characteristic, indicating that current transfer is a hot carrier-dominated mechanism. b represents the base voltage (V), the vertical axis I c Represents the collector current (μAμm -1 ).

[0056] Figure 3 (a) is the output characteristic curve showing the peak-to-valley ratio of the current. c represents the collector voltage (V), the ordinate I c Represents the collector current (μAμm -1 ).

[0057] Figure 3 (b) shows the peak and valley current curves as the base bias increases. b represents the base voltage (V), and the vertical axis Current represents the collector current (μAμm -1 ).

[0058] Figure 3 (c) is a statistical diagram of the peak-to-valley ratio under different base bias voltages, and the peak-to-valley current ratio is about 10. b represents the base voltage (V), and the ordinate PVR represents the peak-to-valley ratio.

[0059] Figure 3 (d) is a comparison of the peak-to-valley current ratio of other graphene devices at room temperature and pressure. In the figure, the horizontal axis Peak Current represents the peak current (A), and the vertical axis PVR represents the peak-to-valley ratio.

[0060] Figure 4 (a) is I c Slowly changing current transfer characteristic curve. In the figure, the horizontal axis V b represents the base voltage (V), the vertical axis I c Represents the collector current (μAμm -1 ).

[0061] Figure 4 (b) is I c is the output characteristic curve of saturation current. In the figure, the horizontal axis V c represents the collector voltage (V), the ordinate I c Represents the collector current (μAμm -1 ).

[0062] Figure 4 (c) with top gate V g The input and transfer characteristic curves of the device show I c With V g In the figure, the horizontal axis V b represents the base voltage (V), and the vertical axis Current represents the base (I b ) and the collector (I c )Current (μAμm -1 ).

[0063] Figure 4 (d) is the current gain statistics diagram, the common emitter current gain β is greater than 100. In the figure, the horizontal axis V b represents the base voltage (V), and the vertical axis β represents the common emitter current gain. DETAILED DESCRIPTION

[0064] In the specific implementation process, the present invention proposes a multifunctional hot emitter transistor based on graphene / germanium Schottky junction and a manufacturing method thereof, wherein the graphene channel is transferred to a germanium substrate, a graphene / germanium Schottky junction is constructed, and a graphene nanogap in the channel is prepared by natural fracture, AFM probe assistance or HIM processing. The carriers in the graphene are heated by an electric field to obtain high-speed hot carriers, thereby obtaining a multifunctional device having a subthreshold swing of less than 60mV / dec, a negative differential resistance ratio of a peak-to-valley current ratio of more than 10, and a common emitter gain greater than 100 at room temperature and normal pressure.

[0065] The preparation method of the multifunctional hot emitter transistor at the graphene / germanium Schottky junction prepared by the present invention is as follows:

[0066] It is necessary to pre-treat p-type germanium to prepare the required p-type lightly doped germanium substrate and transfer graphene to the target substrate; it is necessary to determine whether the nanogaps in the graphene channel can be stably and repeatedly prepared; it is necessary to determine the test and evaluation methods for the performance of the multifunctional hot emitter transistor in different operating modes. Among them, the p-type lightly doped germanium substrate is doped with Ga element with a doping concentration of 1.2×10 15 ~1.9×10 16 cm -3 , resistivity is 1~10Ω.

[0067] In the specific experiment, the following steps are taken respectively:

[0068] (1) Pre-treat the p-type germanium substrate. Use p-type germanium with a resistivity of 1 to 10 Ω·cm as the substrate. First, use acetone and isopropanol to ultrasonically clean it for 5 minutes to remove surface impurities, and then soak it in diluted hydrofluoric acid solution and deionized water for 1 minute each to remove the natural oxide on the germanium surface. The diluted hydrofluoric acid aqueous solution used is obtained by diluting the original concentration of hydrofluoric acid and deionized water in a volume ratio of 1:8, and the concentration of the original concentration of hydrofluoric acid is 40wt%. Then, use atomic layer deposition (ALD) to deposit a 30nm thick HfO2 insulating layer on the germanium substrate, and evaporate gold / titanium electrodes. On the germanium substrate with evaporated gold / titanium electrodes, use reactive ion etching (RIE) to etch the HfO2 on the substrate to open a window to expose the germanium substrate, and then use the above diluted hydrofluoric acid solution to clean the etched germanium substrate for 30s to remove the residual HfO2 in the window. A graphene film is grown on a platinum foil using the CVD method, and then the graphene film is transferred to a target substrate (p-type germanium substrate) using a bubbling transfer method to construct a graphene / germanium Schottky junction.

[0069] (2) Preparation of nanogaps in graphene channels: On the germanium substrate with transferred graphene, S-1813 photoresist was spin-coated, heated at 120°C for 2 minutes, and then photolithographically developed to obtain the graphene channel area. Excess graphene was etched away using an oxygen ion cleaner (O-plasma). The etching parameters were: oxygen flow rate of 180 sccm, pressure of 1.5-2.5 Pa, power of 200 W, and etching time of 2 minutes. There were some natural nanogaps in the prepared graphene channels. In addition to natural fracture, the graphene in the channels could also be cut by AFM probe-assisted cutting or HIM processing. The cutting parameters of the AFM probe were as follows: In Nanoman mode, a voltage of 0.7-1.2 V was applied to the probe, and then the graphene was slowly cut at a speed of 0.03-0.05 μm / s to construct graphene nanogaps. The cutting parameters of the HIM processing technology are as follows: A 30nm thick Al2O3 is deposited as a protective layer using ALD on the germanium substrate with the graphene channel prepared in advance, and then it is cut using HIM. The ion source used is a neon source. The cutting is performed for 15 to 45 seconds at a current of 0.31 to 0.98pA to prepare the nanogap in the graphene channel. Then, 70nm thick Al2O3 is deposited on the cut germanium substrate using ALD, and then Au / Ti electrodes are deposited on top of the Al2O3 as gates.

[0070] (3) Testing and analyzing the multifunctional hot emitter transistor in different operating modes: First, verify whether there is a Schottky junction between graphene and germanium; then test the nanogap of graphene in the channel; apply different voltages at different ends to explore the electrical performance of the multifunctional hot emitter transistor in different operating modes, and select multiple devices for multiple experimental tests and analyses to prove the repeatability of the experiment and the accuracy of the data.

[0071] The feasibility of the present invention is further demonstrated below by examples.

[0072] Example

[0073] In this embodiment, the construction method of the multifunctional hot emitter transistor based on graphene / germanium Schottky junction is as follows:

[0074] (1) A p-type germanium substrate with a resistivity of 1 to 10 Ω·cm was ultrasonically cleaned with acetone and isopropanol to remove surface impurities, and then the substrate was cleaned with a dilute hydrofluoric acid solution and deionized water to remove surface oxides. The washing time of the dilute hydrofluoric acid and deionized water was 1 min. The dilute hydrofluoric acid used was obtained by diluting the original concentration of 40wt% hydrofluoric acid with deionized water in a volume ratio of 1:8. Then, ALD was used to deposit 30nm thick HfO2 on the cleaned germanium substrate at 200°C.

[0075] (2) The back of the HfO2-deposited germanium substrate was scratched with a diamond cutter to produce defects, and then ultrasonically cleaned with acetone and isopropanol in turn, with the cleaning parameters of 60W power and 5 minutes. Subsequently, the metal collector at the bottom of the substrate was evaporated with an electron beam evaporator, and the thickness of the Ti layer was 5 to 6 nm, followed by the Au layer with a thickness of 50 to 55 nm to form a better ohmic contact at the bottom.

[0076] On the germanium substrate with HfO2, LOR3A and S-1813 double-layer photoresists are sequentially spin-coated, and the desired base and emitter electrode patterns are obtained by photolithography development, and then the base and emitter metal electrodes are evaporated by electron beam evaporation. The Au layer thickness of the top metal electrode evaporated by electron beam evaporation ranges from 50 to 60 nm, and then the Ti layer thickness ranges from 5 to 6 nm.

[0077] (3) The germanium substrate with HfO2 on the top is again spin-coated with LOR3A and S-1813 double-layer photoresists, developed by photolithography and exposed for 4.5 seconds to obtain the desired window pattern on the HfO2, and then etched by RIE to expose the desired germanium substrate portion. The etching parameters are: tetrafluoromethane with a flow rate of 50 sccm as etching gas, a pressure of 5.0 Pa, a power of 200 W, and an etching time of 5 minutes; then the etched germanium substrate is cleaned with the dilute hydrofluoric acid in step (1) for 30 seconds to remove residual HfO2.

[0078] (4) Graphene was prepared on a platinum foil substrate by chemical vapor deposition. Then, polymethyl methacrylate (PMMA) was spin-coated on the platinum foil substrate on which graphene was grown by bubbling transfer, and then the PMMA / graphene layer was separated from the platinum foil in a 1 mol / L sodium hydroxide aqueous solution, and the PMMA / graphene layer was transferred to a target germanium substrate, and then PMMA was washed off with acetone at 50°C.

[0079] (5) Use photolithography technology to make a photoresist mask layer on the germanium substrate with transferred graphene, and expose the desired channel graphene pattern to obtain the graphene channel area; spin-coat S-1813 and expose it for 4.5 seconds before developing. The development parameters are: use CD-26 developer for development, the development time is 15 seconds, and then wash in deionized water for 1 minute to remove the residual developer. Use oxygen ion etching technology to etch the excess graphene in an oxygen ion cleaning machine after photolithography. The etching parameters are using oxygen with a flow rate of 180sccm as the etching gas, a pressure of 1.5-2.5Pa, a power of 200W, and an etching time of 2 minutes.

[0080] (6) After the graphene channel is prepared, the nanogaps in the graphene channel can be generated naturally or prepared by AFM and HIM. The preparation parameters using the AFM probe-assisted method are as follows: In Nanoman mode, a voltage of 0.7 to 1.2 V is applied to the probe, and then the graphene is slowly cut at a speed of 0.03 to 0.05 μm / s to construct the graphene nanogaps. The preparation parameters using the HIM processing technology are as follows: On the germanium substrate with the graphene channel prepared in advance, a 30 nm thick Al2O3 layer is deposited at 150°C using ALD as a protective layer, and then it is cut using HIM. The ion source used is a neon source, and the current is 0.31 to 0.98 pA, and the cutting is performed for 15 to 45 seconds to prepare the nanogaps in the graphene channel.

[0081] Then, a 70 nm thick Al2O3 insulating layer was deposited on the cut germanium substrate by ALD at 150°C. After the preparation, a photoresist mask layer was made by photolithography technology, and LOR3A and S-1813 double-layer photoresists were spin-coated in sequence and exposed for 4.5 seconds to form a top gate metal electrode pattern to obtain a metal electrode area on the aluminum oxide insulating layer. Ti with a thickness range of 5 to 6 nm and Au electrodes with a thickness range of 50 to 55 nm were evaporated by vacuum electron beam evaporation (EBE) above the Al2O3 insulating layer using an electron beam evaporator as a gate.

[0082] The multifunctional hot emitter transistor includes a top metal gate electrode, a base and emitter metal electrodes, a channel graphene, a germanium substrate and a metal collector at the bottom of the substrate. Its basic structure is from top to bottom: a top metal gate electrode; an Al2O3 insulating layer; a graphene channel. A metal electrode is arranged on each side of the graphene channel, which are the emitter and the base respectively; the above metal electrodes are all Au / Ti top-down laminated composite structures; below the emitter and base are HfO2 insulating layers and p-type germanium substrates, and a metal collector is arranged at the bottom of the p-type germanium substrate, and the metal collector is a Ti / Au top-down laminated composite structure.

[0083] like Figure 1 As shown in (a), graphene is transferred to a prepared germanium substrate and patterned, and then a nanogap in the graphene channel is prepared using an AFM probe-assisted or HIM-assisted process. The graphene channel is divided into an emitter and a base by the nanogap, and Au / Ti on HfO2 are electrodes for the two parts. V e and V b The Au / Ti electrode is set as the collector at the bottom of the Ge substrate, and V is applied to it. c Bias voltage. Figure 1 As shown in (b), the graphene channel of the transistor is the contact part between the graphene in the window and the germanium substrate, and its length is 5 to 80 μm and its width is 5 to 20 μm. Figure 1 As shown in (c), the nanogaps in the graphene channels can be formed naturally or artificially by AFM probe and HIM processing assisted patterning. Figure 1 As shown in (d), at V e and V c The same voltage of 0 to -3V is applied to the terminal and V b The grounded terminal gives an I c =I b represents the reverse leakage current of the base graphene / p-type germanium Schottky junction, while the very small I e This means that the graphene between the base and emitter in the channel is separated. Figure 1 As shown in (e), for V cApply a voltage between -3 and 3V, and V b With V c Set to 0V, the IV characteristic curves of the emitter-Gr / p-Ge junction and the base-Gr / p-Ge junction show that the on-off current ratio is about 10 3 , indicating that there is a Schottky barrier between Gr and Ge. Figure 1 As shown in (f), the IV characteristics of the junction are measured at 224.2 K, 232.5 K, 241.5 K, 251.2 K, 261.7 K, 273.2 K, 285.6 K and 299.3 K, and the barrier height is calculated. The temperature-dependent IV characteristics of the junction indicate that the junction is dominated by thermionic emission.

[0084] like Figure 2 As shown in (a), the emitter V e Grounded, the transistor is in common emitter state. Figure 2 As shown in (b), since the graphene in this device is p-type, hot holes are conduction carriers. When the base bias voltage V b When the voltage increases from 0 to -10 V, the hot holes in the emitter graphene are accelerated. However, these hot holes are not hot enough to pass through the graphene / p-Ge Schottky barrier. Figure 2 (c) As shown, V b Apply a voltage between 0 and -10 V, V c Increment from -2V to -9V in -1V steps. For each V c , when V b Greater than V c When the base graphene / p-type germanium heterojunction becomes positively biased, holes are injected from the collector substrate into the base graphene, and these holes are transferred to the emitter graphene by scattering across the potential barrier caused by the gap. Since these holes also have high energy, they induce hot carriers in the emitter graphene. When the emitter and collector are reverse biased, they overcome the potential barrier between graphene / p-germanium and are emitted into the germanium substrate, resulting in a sudden hole current. From this, it can be observed that the negative collector current I generated by the hot holes collected by the collector c , and the current changes quite suddenly. Figure 2 (d) Figure 2 As shown in (e), the collector current is plotted on a logarithmic scale, and the current jump exceeds the Boltzmann limit at room temperature, where the current increases with V c With the increase of , the subthreshold swing (SS) increases from 3.3 to 12.5 mV / dec. Different from the ultra-low subthreshold swing transistors reported previously, the reason why the transistor involved in the present invention produces ultra-low subthreshold swing is injection-induced hot carrier emission, which is also consistent with its temperature-dependent current behavior. Figure 2 As shown in (f), select Vc When V is -3V, b When the range is -3 to -5V, the current mutation is observed under different temperature conditions. Different from the tunneling behavior, the current mutation of the transistor involved in the present invention only occurs above room temperature, and room temperature is also the temperature range in which most electronic systems work.

[0085] like Figure 3 As shown in (a), for each V b Bias voltage, output characteristics I c -V c Displays negative differential resistance, the peak-to-valley current ratio (PVR) of the maximum current to the minimum current. c Apply a voltage between 0 and -10 V, V b Increase from -3V to -9V in steps of -1V. c When the base graphene / p-type germanium heterojunction is forward biased, I c is positive. As V c As the bias increases, the hot holes generated at the emitter graphene are collected by the collector, generating a relatively large negative I c , and the current gradually reaches its peak value. Thereafter, when V c When it is further increased, the bias voltage between the base graphene / p-type germanium changes from positive bias to reverse bias, carrier injection stops, resulting in the appearance of valley current, and when the temperature decreases, the negative differential resistance gradually disappears, which is consistent with the hot carrier mechanism. Figure 3 (b) Figure 3 (c) shows that due to the larger V b More hot carriers are generated, so the peak and valley currents increase with V b The PVR value increases with the increase of bias voltage, and under different bias voltages, the PVR value always remains around 10, and the maximum value can reach 13.5. Figure 3 As shown in (d), compared with other works [references 1-9], the PVR of the transistor in the present invention is the maximum value of current graphene devices.

[0086] The subthreshold swing in the transfer characteristics and the negative differential resistance in the output characteristics can be adjusted by covering the graphene channel with an insulating layer and a gate. Figure 4 As shown in (a), when the device is covered with a 100nm thick insulating layer, V b Apply a voltage between 0 and -10 V, V c If the voltage increases from -3V to -9V in steps of -1V, the collector current I c changes very slowly, and when V c When it is greater than -6V, the current I c The value of V bThis is essential for generating transconductance over a wide bias voltage range, which increases from 0V to -10V. Figure 4 As shown in (b), the negative differential resistance in the output characteristic also disappears, V c Apply a voltage of -2 to -10V, V b Increasing from 0V to -9V with a -1V interval, we can see that I c With V c The increase of the electric field increases with the increase of the electric field, and then tends to saturation. This is because the electric field distribution in the graphene is changed after the insulating layer is covered. And because of its limited state density, the quantum capacitance effect of graphene is very obvious. Therefore, when the top gate is added to control the Fermi level of graphene, the collector current can be further increased to increase the transconductance. Figure 4 (c) shows that at V c When the gate bias voltage V is -8V, g As the voltage increases from 0V to -18V, the Fermi level of graphene decreases, and the barrier height between graphene and p-type germanium decreases, thus generating a larger I c Current. Figure 4 As shown in (d), due to the presence of nanogaps in the graphene channel, I b Relatively small, so current amplification is possible, the common emitter current gain β is ΔI c / ΔI b , its value can be greater than 100, indicating that the transistor cutoff frequency has the potential to be greater than THz.

[0087] Through the above method, the multifunctional hot emitter transistor based on graphene / germanium Schottky junction proposed in the present invention was successfully prepared.

[0088] The results of the examples show that the method for preparing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction proposed in the present invention constructs a graphene channel with a graphene nanogap, and generates a multifunctional transistor with an ultra-low subthreshold swing, a peak-to-valley current ratio greater than 10, and a common emitter gain greater than 100 at room temperature and atmospheric pressure by injection-induced hot carrier emission. This multifunctional transistor provides a hybrid dimensional multifunctional device for the post-Moore era, combining digital and analog technologies, and has a very broad application prospect.

Claims

1. A multifunctional hot emitter transistor based on graphene / germanium Schottky junction, characterized in that: The transistor includes, from top to bottom, a top metal electrode, an aluminum oxide insulating layer, a graphene channel with a nanogap, a hafnium oxide insulating layer and a germanium substrate, as well as base and emitter metal electrodes located on both sides of the graphene channel and in contact with the graphene channel. The metal electrodes are a gold / titanium stacked composite structure.

2. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 1, characterized in that: The steps include: (1) ultrasonically cleaning the germanium substrate with acetone and isopropanol to remove surface impurities, and then washing with diluted hydrofluoric acid and deionized water to remove surface oxides; (2) depositing a hafnium oxide insulating layer on a germanium substrate by atomic layer deposition; (3) using photolithography technology to prepare a photoresist mask layer, and exposing the base and emitter metal electrode patterns to obtain the metal electrode area on the hafnium oxide insulating layer, and using vacuum electron beam evaporation to evaporate gold / titanium electrodes; (4) using photolithography technology to make a photoresist mask layer, and exposing the required window pattern, and using reactive ion etching to etch out the hafnium oxide window to expose the germanium substrate; (5) growing a graphene film on a platinum foil by chemical vapor deposition, and transferring the graphene film to a prepared germanium substrate by bubbling transfer; (6) using photolithography technology to prepare a photoresist mask layer, and exposing the desired graphene channel pattern to obtain a graphene channel region; (7) Using oxygen ion etching technology, oxygen is introduced to clean away the excess graphene film exposed outside the base and emitter metal electrodes; (8) The nanogaps in the graphene channels are formed naturally or prepared using atomic force microscopy or helium ion microscopy; (9) depositing an aluminum oxide insulating layer on a germanium substrate by atomic layer deposition; (10) A photoresist mask layer is prepared by photolithography technology, and a top gate metal electrode pattern is exposed to obtain a metal electrode area on the aluminum oxide insulating layer, and a gold / titanium electrode is evaporated by vacuum electron beam evaporation.

3. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 2, characterized in that: In step (1), the germanium substrate is a p-type lightly doped germanium sheet with a thickness of 500 μm and a resistivity of 1 to 10 Ω·cm; the diluted hydrofluoric acid is obtained by diluting the original concentration hydrofluoric acid and deionized water in a volume ratio of 1:8, the concentration of the original hydrofluoric acid is 40 wt %, and the treatment time in the dilute hydrofluoric acid and deionized water is 1 min.

4. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 2, characterized in that: In step (2), an insulating layer of hafnium oxide is deposited at 200° C. using atomic layer deposition to a thickness of 30 nm.

5. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 2, characterized in that: In step (3) and step (10), LOR3A and S-1813 double-layer photoresists are spin-coated in sequence and exposed for 4.5 seconds; an Au layer of the top metal electrode is evaporated using an electron beam evaporator to a thickness ranging from 50 to 60 nm, and then a Ti layer is evaporated to a thickness ranging from 5 to 6 nm.

6. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 2, characterized in that: In step (4), LOR3A and S-1813 double-layer photoresists are spin-coated on the hafnium oxide insulating layer in sequence, and exposed for 4.5 seconds to obtain the desired window pattern; the window is etched out using reactive ion etching under the following conditions: tetrafluoromethane with a flow rate of 50 sccm is used as the etching gas, the pressure is 5.0 Pa, the power is 200 W, and the etching time is 5 minutes; then, the dilute hydrofluoric acid used in step (1) is used for cleaning for 30 seconds.

7. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 2, characterized in that: In step (6), S-1813 photoresist is spin-coated on the graphene and exposed for 4.5 seconds.

8. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 2, characterized in that: In step (7), an oxygen ion cleaning machine is used for etching, and the etching conditions are: oxygen with a flow rate of 180 sccm is used as the etching gas, the pressure is 1.5-2.5 Pa, the power is 200 W, and the etching time is 2 min.

9. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 2, characterized in that: In step (8), an atomic force microscope is used to cut graphene, and the cutting parameters are: in the nano-etching Nanoman mode, the speed in the XY direction is 0.03-0.05 μm / s, and the set voltage is 0.7-1.2 V; or, a helium ion microscope is used to cut graphene, and the cutting conditions are: on the patterned graphene, using atomic layer deposition, alumina with a thickness of 30 nm is deposited at 150° C., and then a helium ion microscope neon source is used for cutting, the current size is 0.31-0.98 pA, the cutting time is 15-45 s, and the graphene nanogap width is 1-45 nm.

10. The method for constructing a multifunctional hot emitter transistor based on a graphene / germanium Schottky junction according to claim 2, characterized in that: When preparing a common emitter gain exceeding 100, in step (9), atomic layer deposition is used to deposit aluminum oxide at 150° C. with a thickness of 70 to 100 nm.

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