Mxene-based ohmic electrical contacts: preparation and applications

Abstract

The invention relates to an ohmic MXene-based electrical contact, comprising a p-type III-V semiconductor on which a layer of conductive MXene material is deposited, and on which MXene layer an additional contact made of a conductive material is deposited, characterized in that the p-type III-V semiconductor (1) is a material selected from the group consisting of p-AlxGa1-xN, p-InxGa1-xN, and p-AlxIn1-xN, where x has a value from 0 to 1, the conductive MXene layer (2) is a MXene with a high work function greater than 4.5 eV, and the additional contact made of a conductive material is an ohmic contact (3), which is silver paste. The invention also relates to a method for producing the electrical contact as defined above, characterized in that it comprises a step of preparing a solution of MXene material (2) with a work function greater than 4.5 eV in a monohydric alcohol at a concentration from 250 mg / 10 ml to 500 mg / 10 ml, and, after applying the prepared solution onto the surface of a p-type doped III-V semiconductor layer (1) and allowing it to dry, annealing the resulting material under ultra-high vacuum conditions in the range from 10⁻⁶ to 10⁻¹² mbar at a temperature from 600°C to 900°C. The invention further relates to the use of the electrical contact as defined above as an electrical contact in diodes, photodetectors, and transistors.

Description

[0001] MXene-based ohmic electrical contacts : preparation and applications

[0002] The subj ect of the invention is an MXene-based ohmic electrical contact, its fabrication method, and its applications . The invention belongs to the field of electronics and semiconductor technology.

[0003] AlGaN-based LED diodes are of key importance for applications involving deep-UV radiation, such as medical sterilization, UV sensors , and water and air purification. These diodes exhibit low emission efficiency, which results from the high density of dislocations propagating in the AlGaN layer and the low hole concentration in the p-AlGaN layer .

[0004] To ensure transparency of LEDs , p-type contact layers that are transparent to UV radiation and simultaneously exhibit low resistivity are used. From the prior art, the use of p-GaN and p-AlGaN is known; however, p-GaN absorbs UV radiation, while p-AlGaN, although it improves light extraction, results in poor contact parameters and shorter device lifetime . Effective contacts to the p-AlGaN layer require p-type electrodes with a high work function, but the electrodes currently used, such as Ni / Au or Ni / Al, are characterized by high forward voltage, which generates heat and reduces the lifetime of LED diodes . Additionally, the large lattice mismatch between GaN and metallic electrodes leads to discontinuous interfaces with defects , increasing interfacial resistance and reducing carrier mobility.

[0005] In the prior art, electrical contacts based on semiconductors combined with MXene materials are known.

[0006] Chinese patent application CN110635027 discloses a semiconductor device based on an MXene electrode and a method for its fabrication. The structure of the solar cell consists of a substrate material, a thin conductive MXene layer, a semiconductor layer, and a top electrode layer . According to the invention, the conventional three-layer structure of semiconductor, dielectric layer, and electrode— commonly used in the prior art to mitigate Fermi-level pinning— is replaced with a two-layer structure of semiconductor and electrode, which simplifies the process and reduces costs . The type and content of surface groups in the MXene electrode material can be modified through simple tuning or treatment, allowing the material' s performance to be intentionally optimized and improved, while the Fermi-level pinning effect between the electrode and semiconductor is minimized .

[0007] The Chinese patent CN113097315B discloses a multi-quantum MSM photodetector utilizing an MXene-GaN Schottky junction and a method for its preparation. It belongs to the technical field of photodetectors and addresses the problem of high dark current in MSM-type photodetectors . To overcome the issue of low sensitivity, the photodetector described in the invention includes a structured sapphire substrate . On this substrate, a GaN layer is deposited, followed by n-GaN and GaN / InGaN quantum wells . Finally, an MXene layer is applied. The invention combines an MXene material with a structured sapphire substrate to reduce dark current and improve responsivity, and it can be used in fields such as underwater optical detection and underwater communication .

[0008] The application CN115692538A discloses an ultraviolet photodetector based on an MXene / AlInGaN hetero unction and a method for its preparation . The substrate material is an AlInGaN semiconductor epitaxially grown on a sapphire substrate, with a metal electrode, a thin MXene layer, and a test electrode arranged on the detector surface, where the MXene and AlInGaN form a hetero j unction .

[0009] The difference in work function between MXene and AlInGaN enables the formation of a Schottky barrier at the heterojunction interface, allowing photoinduced electron-hole pairs to be separated at zero bias, producing a photocurrent— meaning that the detector operates in a self-powered mode .

[0010] The thin MXene layer exhibits good ultraviolet transparency and can increase the device' s light detection area, while the cathode forms an ohmic contact with AlInGaN after annealing, enabling rapid electron collection and improving detector performance . The invention further provides a laser lift-off transfer method for the MXene-AlInGaN heterojunction, allowing fabrication of a flexible ultraviolet photodetector .

[0011] Luo et al . demonstrated that the use of MXene electrodes improves the sensitivity and reduces the dark current of a multi-quantum-well photodetector based on MXene -GaN-MXene compared with traditional metal-semiconductor-metal photodetectors (Cr / Au electrodes )

[0001] . Yi et al . fabricated photodetectors and stable orange LEDs based on Ti3C2Tx / (n / p) -GaN [3] . Song et al . reported a self-powered, high-performance Ti3C2TxMXene / GaN van der Waals ultraviolet photodiode based on a hetero unction

[0004] . Wang et al . used Ti3C2TxMXene as a gate contact in GaN HEMTs , demonstrating enhanced gate controllability with an exceptionally low off-state current ( IOFF) and a record-high ION / IOFF ratio of «1013( six orders of magnitude higher than conventional Ni / Au contacts )

[0005] .

[0012] State-of-the-art solutions describe the integration of semiconductors with MXene materials ; however, the parameters of these interfaces are suboptimal for many potential applications . In particular, there are currently no solutions that provide electrical contacts to p-type gallium nitride-based semiconductors exhibiting good electrical properties , especially low forward voltage .

[0013] The technical problem to be solved is therefore to provide electrical contacts to p-type gallium nitride-based semiconductors that exhibit low forward voltage, so that the resulting materials have minimal defects , distortions , and atomic disruptions typically caused during the standard metal deposition process on a semiconductor, while simultaneously ensuring transparency to UV radiation .

[0014] The present invention addresses the known problems of the prior art by integrating MXenes with a high work function (>4.5 eV) with p-type GaN-based materials, thereby improving the interface between the electrode and the p-type semiconductor . Moreover, the van der Waals (vdW) interaction between the MXenes and the GaN surface limits the formation of defects, distortions , and atomic disruptions during the typical metal deposition process .

[0015] The subject of the invention is an ohmic MXene electrical contact, comprising a p-type III-V semiconductor on which a layer of conductive MXene material is deposited, and on which MXene layer an additional contact made of a conductive material is deposited . The invention is characterized in that the p-type III-V semiconductor is a material selected from the group consisting of p-AlxGai-xN, p-InxGaixN, and p-AlxInixN, where x has a value from 0 to 1 . The conductive MXene layer is a MXene with a high work function greater than 4 . 5 eV, and the additional contact made of a conductive material is an ohmic contact, specifically silver paste .

[0016] Preferably, the high work function MXene material is selected from the group consisting of V2C, V4C3, V3C2, Cr2C, and Mo2C . Considering the wide range of work functions ( 1. 6-6.2 eV) offered by MXene materials, a natural direction for research is their effect on p-type contacts with wide-bandgap GaN-based materials . It is therefore advantageous when the p-type III-V semiconductor is p- (Al ) GaN .

[0017] The invention also relates to a method for producing the ohmic electrical contact described above . The method according to the invention is characterized in that it comprises the following steps :

[0018] a) preparing a solution of MXene material with a work function greater than 4 . 5 eV in a monohydric alcohol at a concentration from 250 mg / 10 ml to 500 mg / 10 ml;

[0019] b) applying the obtained solution onto the surface of a p-type doped III-V semiconductor layer using the "drop-casting" method and allowing it to dry at room temperature; c) annealing the resulting material under ultra-high vacuum conditions in the range from 10-6to 10-12mbar at a temperature from 600°C to 900 °C;

[0020] d) depositing an ohmic contact on the semiconductor layer, which is made of an electrically conductive material .

[0021] Preferably, in step a) , the MXene solution is prepared in a monohydric alcohol at a concentration of 300 mg / 10 ml . Furthermore, the monohydric alcohol is preferably ethanol .

[0022] Preferably, the p-type doped III-V semiconductor is any compound selected from the group consisting of p-AlxGaixN, p-InxGaixN, and p-AlxInixN, where x has a value from 0 to 1 .

[0023] Preferably, the annealing in step c) is carried out for 1 minute under a vacuum of 10-11mbar at a temperature of 800 °C .

[0024] Preferably, the MXene material with a work function greater than 4 .5 eV is any compound selected from the group consisting of V2C, V4C3, V3C2, Cr2C, or Mo2C .

[0025] The invention also relates to the use of the ohmic contact described above as an electrical contact in diodes , photodetectors, and transistors . Furthermore, it is advantageous when the diode is a UV diode .

[0026] An advantage of the present invention is that it provides improved electrical properties of the p-type contact to Al-rich AlGaN :Mg through the integration of p-AlGaN semiconductor with suitable MXene materials .

[0027] Another advantage is that the materials according to the invention, Mn+iXnTx(where M is a transition metal, X denotes C and / or N, T denotes a surface terminating group such as -O, -OH, -F, and n = 1-3 ) , exhibit metallic properties and excellent electrical conductivity, making them suitable for a wide range of applications, e . g . , as electrodes in optoelectronic and electronic devices . A further undeniable advantage of the present invention is that it can be applied in deep-UV emitters with enhanced emission efficiency, thereby increasing their application potential compared to mercury lamps , which are environmentally toxic .

[0028] Additionally, MXene materials as electrodes can be used in other electronic and optoelectronic devices such as detectors , transistors , or Schottky diodes .

[0029] The invention in the exemplary embodiment is illustrated in the figure, in which:

[0030] fig . 1 shows the electrical contact according to the invention, where a layer of conductive MXene material 2 is applied on a semiconductor 1 (p-type III-V) , on which a contact 3 made of a conductive material (e . g . , silver paste) is placed;

[0031] fig . 2 shows the results of current-voltage ( I-V) measurements for the electrical contact according to the invention;

[0032] fig . 3 shows a band alignment diagram for the investigated MXene / van Hoof GaN structures .

[0033] The implementation of the subject of the invention in its advantageous embodiments is presented in the following examples .

[0034] Example 1 - Method for obtaining a MXene electrode Commercially available MXene powders characterized by a high work function are, in the first step, mixed with a monohydric alcohol, preferably ethanol (with a purity >99% ) due to its chemical and thermal stability, its ability to efficiently disperse the powder, and the absence of impurities , to obtain a preferred concentration of 300 mg / 10 ml . In other embodiments, solutions with concentrations in the range of 250 mg / 10 ml to 500 mg / 10 ml were also prepared, and isopropanol was also used as the monohydric alcohol; however, a skilled person may easily use any other monohydric alcohol that provides effective dispersion of the MXene powder .

[0035] In this embodiment, the MXene powders used were V2C, V4C3, V3C2, Cr2C, and Mo2C .

[0036] The obtained MXene solution is then applied onto the surface of a p-type III-V material 1 and left to dry naturally (the method known as "drop casting" ) . The thickness of the applied layer is not uniform, but it should not exceed 1 micrometer .

[0037] In this embodiment, the p-type III-V materials 1 used were p-AlxGal-xN, p-InxGal-xN, and p-AlxInl-xN, where x has a value from 0 to 1 .

[0038] The resulting contact is then annealed in the next step under ultra-high vacuum conditions at a temperature ranging from 600 °C to 900°C . In this embodiment, annealing was carried out under a vacuum ranging from 10-6to 10-12mbar, most preferably at 10-11mbar . The annealing time was 1 minute .

[0039] An ohmic contact 3 made of a conductive material is then deposited on the resulting structure . In a preferred embodiment, the contact 3 was silver paste . It will be evident to a skilled person that other conductive materials commonly used in the field may also be employed. The thickness of the conductive material layer 3 is not critical and may vary to achieve the same effect .

[0040] Example 2 - Construction of the MXene electrode

[0041] The construction of the electrode obtained by the method of Example 1 is shown schematically in Fig . 1. The electrode consists of a p-type doped III-V semiconductor 1 , grown using epitaxial techniques, a layer of conductive MXene material 2 , and an ohmic contact 3 deposited on the MXene material 2 . The ohmic contact 3 is made of silver paste or another conductive material, providing proper ohmic connection . Example 3 - Results of investigations of electrodes according to the invention

[0042] Figure 2 shows the measurement results for the electrical contact according to the invention, which were carried out on three samples : one containing a p-GaN (75 nm) / A10 . 6^-OGaO . 4^-lN gradient ( 50 nm) structure with metallic Ni / Au contacts, and two other gradient structures with V2C MXene contacts, both unannealed and annealed at 800 °C .

[0043] Preliminary studies were performed for V2C MXene contacts on p-GaN ( 75 nm) / A10 . 6^-OGaO . 4^-lN gradient (50 nm) , showing improved ohmic characteristics for the annealed V2C contacts compared to the metallic Ni / Au contacts . Hall effect measurements were carried out using a Lake Shore HMS 8407 to compare the electrical parameters (hole concentration, mobility, and resistivity) of the p- (Al ) GaN layers with different MXene electrodes .

[0044] Before performing Hall effect measurements , each sample was prepared in a van der Pauw geometry with MXene contacts covered with silver paste, and their I-V characteristics (ohmic behavior) were verified. Hall effect measurements were performed comparatively on three samples : one containing a p-GaN (75 nm) / A10 . 6^-OGaO . 4^-lN gradient (50 nm) structure with metallic Ni / Au contacts, and the other two gradient structures with V2C MXene contacts, unannealed and annealed at 800 °C . The MXene contact structure underwent an annealing process . The studies showed that annealing MXene can increase their work function .

[0045] All samples exhibited p-type conductivity. For the sample with Ni / Au electrodes , the hole concentration and resistivity were 1. 9 -1018cm-3and 0. 93 Q -cm, respectively. For the sample with unannealed V2C contacts, these values were 1.4 -1017cm-3and 1. 16 Q -cm. After annealing the V2C contacts, the measured hole concentration and resistivity were 3.7 -1018cm-3and 0.52 Q -cm, respectively. The I-V measurement results for all samples are shown in Fig . 2 . The annealed V2C contacts exhibit the lowest resistance among all electrodes and show a more linear dependence compared to the unannealed contacts .

[0046] The conducted studies indicate the potential application of MXene electrodes, taking into account further optimization of material transfer onto the p- (Al) GaN surface and subsequent thermal treatment . Additionally, experiments were performed for a V2C layer on glass and for a p-GaN ( 75 nm) / AID . 6^-OGaO . 4^-lN gradient ( 50 nm) sample with only silver paste contacts . However, due to the high resistance of these samples exceeding the measurement limits, the Hall effect could not be measured . Therefore, the observed Hall effect results from vertical current flow from the semiconductor rather than lateral flow within the MXene layer .

[0047] Example 4 - Results of studies on the built-in electric field in MXene / GaN structures

[0048] In this study, the non-contact electroreflectance method was used to analyze the built-in electric field in MXene / GaN structures, aiming to understand carrier transport across the MXene / GaN interface . The physicochemical properties of the MXene / GaN structures were examined using X-ray and UV photoelectron spectroscopy. The van Hoof GaN structures were grown by metal-organic chemical vapor deposition, while MXenes dispersed in an ethanol solution were deposited on the sample surface and allowed to dry naturally in air .

[0049] Five different MXene structures were studied, including Cr2C, MO2C , V2C, V4C3, and Ti3C2. The parameters obtained from UV photoelectron spectroscopy measurements enabled the construction of band diagrams for all tested samples, as shown in Fig . 3. The ionization energies for all materials refer to the same vacuum level position. The results showed that Ti3C2appears to be a favorable material for forming a Schottky contact with p-GaN, whereas V2C and MO2C exhibited a favorable configuration for forming a Schottky contact with n-GaN . Conversely, Ti3C2is a favorable material for achieving an ohmic contact with n-GaN, while V2C and MO2C are suitable for establishing an ohmic contact with p-GaN.

[0050] The obtained Mn+IXnTx materials exhibit metallic properties and excellent electrical conductivity, making them suitable for a wide range of applications , particularly as electrodes in optoelectronic and electronic devices such as detectors , transistors , or Schottky diodes . The invention can also be applied in deep-UV emitters with enhanced emission efficiency. References

[0051]

[0001] L . Luo , Y . Huang, K . Cheng, A . Alhassan, M . Alqahtani , L . Tang, Z . Wang, J . Wu, MXene-GaN van der Waals metalsemiconductor j unctions for high performance multiple quantum well photodetectors , Light Sci Appl 10 ( 2021 ) 177 . https : / / doi . org / 10 . 1038 / s41377 -021-00619-1 .

[0052]

[0002] J . Wang , X . Zhou, M . Yang, D . Cao , X . Chen, H . Shu, Interface and polarization effects induced Schottky-barrier-free contacts in two-dimensional MXene / GaN hetero unctions , J . Mater . Chem. C 8 ( 2020 ) 7350-7357 . https : / / doi . org / 10 . 1039 / D0TC01405B .

[0053]

[0003] C . Yi , Y . Chen, Z . Kang, Y . Ma, Y . Yue , W . Liu, M . Zhu, Y . Gao , MXene-GaN van der Waals Heterostructures for High-Speed Self-Driven Photodetectors and Light-Emitting Diodes , Advanced Electronic Materials 7 ( 2021 ) 2000955 . https : / / doi . org / 10 . 1002 / aelm . 202000955 .

[0054]

[0004] W . Song , J . Chen, Z . Li , X . Fang, Self-Powered MXene / GaN van der Waals Heteroj unction Ultraviolet Photodiodes with Superhigh Efficiency and Stable Current Outputs , Advanced Materials 33 ( 2021 ) 2101059 . https : / / doi . org / 10 . 1002 / adma . 202101059 .

[0055]

[0005] C . Wang , X . Xu, S . Tyagi , P . C . Rout , U . Schwingenschlbgl , B . Sarkar , V . Khandelwal , X . Liu, L . Gao , M . N . Hedhili , H . N . Alshareef , X . Li , Ti3C2Tx MXene van der Waals Gate Contact for GaN High Electron Mobility Transistors , Advanced Materials 35 ( 2023 ) 2211738 . https : / / doi . org / 10 . 1002 / adma . 202211738 . List of reference numerals - p-type III-V semiconductor

[0056] - MXene

[0057] - ohmic contact

Claims

Claims1. An ohmic MXene electrical contact, comprising a p-type III- V semiconductor on which a layer of conductive MXene material is deposited, and on which MXene layer an additional contact made of a conductive material is deposited, characterized in that :the p-type III-V semiconductor ( 1 ) is a material selected from the group consisting of p-AlxGaixN, p-InxGaixN, and p- AlxInixN, where x has a value from 0 to 1,the conductive MXene layer (2 ) is a MXene with a high work function greater than 4 . 5 eV,and the additional contact made of a conductive material is an ohmic contact ( 3 ) , which is silver paste .2 . The ohmic contact according to claim 1 , characterized in that the MXene material (2 ) with a high work function is selected from the group consisting of V2C, V4C3, V3C2, Cr2C, and MO2C .

3. The ohmic contact according to any of claims 1 to 2 , characterized in that the p-type III-V semiconductor ( 1 ) is p- (Al ) GaN .4 . A method for producing the ohmic electrical contact according to any of claims 1 to 3, characterized in that it comprises the following steps :a) preparing a solution of MXene material ( 2 ) with a work function greater than 4 . 5 eV in a monohydric alcohol at a concentration from 250 mg / 10 ml to 500 mg / 10 ml; b) applying the obtained solution onto the surface of a p-type doped III-V semiconductor layer ( 1 ) using the "drop-casting" method and allowing it to dry at room temperature ;c) annealing the resulting material under ultra-high vacuum conditions in the range from 10-6to 10-12mbar at a temperature from 600 °C to 900 °C;d) depositing an ohmic contact (3 ) made of an electrically conductive material onto the MXene layer ( 2 ) .5 . The method according to claim 4 , characterized in that in step a) the MXene solution is prepared in a monohydric alcohol at a concentration of 300 mg / 10 ml .

6. The method according to claim 4 or 5 , characterized in that the monohydric alcohol is ethanol .7 . The method according to claim 4 , characterized in that the p-type doped III-V semiconductor ( 1 ) is any compound selected from the group consisting of p-AlxGaixN, p-InxGaiXN, and p-AlxInixN, where x has a value from 0 to 1 .8 . The method according to claim 4 , characterized in that the annealing in step d) is carried out for 1 minute under a vacuum of 10-11mbar at a temperature of 800°C .

9. The method according to any of claims 4 to 8 , characterized in that the MXene material ( 2 ) with a work function greater than 4 .5 eV is any compound selected from the group consisting of V2C, V4C3, V3C2, Cr2C, or Mo2C .

10. The use of an ohmic electrical contact as defined in any of claims 1 to 3 as an electrical contact in diodes , photodetectors, and transistors .

11. The use according to claim 10 , characterized in that the diode is a UV diode .

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