Method for improving the interface between two-dimensional mose2 and metal by plasma oxidation-reduction treatment

Abstract

The application discloses a method for improving the contact interface between two-dimensional MoS2 and metal by plasma oxidation-reduction treatment. 3‑x The method comprises the following steps: performing oxygen plasma treatment on the surface of two-dimensional MoS2 to form a MoO3 oxide layer; and then performing hydrogen plasma reduction treatment to convert the MoO3 into MoO2 or MoO 3‑x The application overcomes the thermodynamic and kinetic barriers of the difficulty in stable formation of conductive MoO2 by a two-step method of oxidation first and then reduction, and forms an atomic-level continuous low-valence metal oxide layer with high conductivity on the MoS2 surface in situ. The layer can be used as a contact buffer layer of a subsequent metal electrode, effectively reduces the contact resistance, avoids interface damage, and protects the MoS2 channel integrity. The application is controllable, compatible with existing semiconductor processes, and suitable for various two-dimensional transition metal sulfide material systems.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor devices and two-dimensional materials technology, specifically relating to a method for improving the interface between two-dimensional transition metal sulfide MoS2 and metal electrodes. Background Technology

[0002] With the widespread application of two-dimensional transition metal sulfides such as MoS2 in electronic devices, optoelectronic devices, and sensors, effectively addressing the resistance problem at the interface between two-dimensional materials and metals has become crucial for improving device performance. Particularly in MoS2-based transistors, the poor matching between the band structure of MoS2 and the work function of the metal electrode results in typically high contact resistance between the two-dimensional material and the metal, severely limiting the device's on-state current, transconductance, and operating frequency.

[0003] In existing technologies, researchers have attempted to reduce contact resistance by selecting low work function metals such as Ti and Sc, introducing buffer layers such as graphene, and doping the surface of MoS2 with elements. However, these methods still face insurmountable technical challenges in practical applications, as detailed below.

[0004] First, existing technologies have failed to effectively solve the problem of high contact resistance. The fundamental reason lies in the strong Fermi level pinning effect that occurs at the interface when metal is directly deposited on the MoS2 surface. This is due to the combined effects of sulfur vacancies on the MoS2 surface, interstitial states formed by metal atom diffusion, and a metal-induced interfacial dipole layer, which pin the metal Fermi level deep within the band gap of MoS2. This pinning effect results in the Schottky barrier height remaining almost unchanged with the metal work function, making it impossible to form an ideal ohmic contact even with Ti, which has a low work function of approximately 4.3 eV. Furthermore, in traditional electron beam evaporation or thermal deposition processes, the bombardment of two-dimensional MoS2 surfaces by high-energy metal atoms such as Au and Ni introduces numerous interface defects and structural disorder, further increasing tunneling resistance and barrier width. These physical mechanisms collectively make it difficult to reduce the contact resistance to a practical level for devices.

[0005] Furthermore, two-dimensional MoS2 is composed of three atomic layers, S-Mo-S, stacked by van der Waals forces, making its lattice integrity extremely sensitive to external energy input. There are two main sources of damage in the traditional metal electrode fabrication process: First, high-energy particle bombardment during physical vapor deposition (PVD): During sputtering or evaporation, the kinetic energy of metal atoms can reach several electron volts to tens of electron volts, far exceeding the monolayer atomic binding energy of approximately 1-2 eV / atom in MoS2. This bombardment causes sulfur atoms in the MoS2 lattice to be ejected, forming sulfur vacancies, and even creating nanoscale holes, disrupting channel integrity. Second, metal diffusion and alloying: To reduce contact resistance, high-temperature annealing (>300℃) is often required. At this temperature, metal atoms, especially Ti and Ni, diffuse into the MoS2 lattice along sulfur vacancies or grain boundaries, forming metal sulfides such as TiS2 and NiS alloy layers. Although these alloy layers have some conductivity, their formation process is uncontrollable and consumes some channel material, leading to threshold voltage drift and subthreshold swing degradation in the device. The essence of the aforementioned damage mechanism lies in the fact that existing metal deposition and annealing processes are designed for three-dimensional bulk semiconductors and are not suitable for two-dimensional materials with atomic-level thickness. Directly applying these processes inevitably causes interface damage, which is a long-standing but unresolved technical obstacle in this field.

[0006] Furthermore, theoretically, if a layer of highly conductive MoO2 or substoichiometric MoO2 can be formed in situ between MoS2 and the metal, it would be ideal. 3-x This can significantly reduce contact resistance. However, this technical approach faces the following fundamental obstacles: (1) Thermodynamic barrier: In the competitive reaction between Mo-S and Mo-O bonds, MoS2 preferentially forms the more thermodynamically stable MoO3 in an oxygen atmosphere, rather than MoO2. Conventional oxidation processes inevitably produce the insulating state of MoO3 first, with a resistivity as high as 10 Ω·cm. 6 On the order of Ω·cm, it will actually worsen the contact performance.

[0007] (2) Kinetic barriers: The formation of MoO3 is a diffusion-controlled process. Oxygen atoms first replace surface sulfur atoms to form a Mo-O layer, followed by inward diffusion of oxygen and outward diffusion of sulfur. The generation of MoO2 inside MoS2 requires precise control of the oxygen partial pressure and temperature window to keep it in the metastable region of "partial oxidation". However, this window is extremely narrow and difficult to control stably using conventional thermal oxidation or single plasma treatment.

[0008] (3) Challenges in the reduction pathway: Although theoretically MoO3 can be reduced to MoO2 with hydrogen, traditional thermal reduction requires high temperatures (>700℃) and long processing times, which severely damages the channel structure of MoS2. If the temperature is too low, the reduction reaction is incomplete, generating a large amount of intermediate valence oxides, whose conductivity is much lower than that of MoO2.

[0009] In summary, conventional oxidation methods directly generate MoO3, while MoO2 cannot be stably formed through simple processes. The challenge lies in how to controllably obtain MoO2 or MoO2 with high conductivity and low defect density while maintaining the integrity of the MoS2 channel. 3-x The interface layer has long been a persistent technical bottleneck. In existing technologies, high contact resistance, interface damage, and the difficulty in forming conductive oxides are interconnected, stemming from the fundamental incompatibility between the atomic-level thickness of two-dimensional MoS2 and traditional semiconductor processes. Finding a novel processing method that can both avoid damage and controllably form a conductive interface layer is key to improving the performance of MoS2 transistors. Summary of the Invention

[0010] This invention aims to address several problems existing in the prior art regarding the contact between two-dimensional MoS2 and metal electrodes: high contact resistance and its root cause, the Fermi level pinning effect; irreversible interface damage to two-dimensional MoS2 caused by the metal deposition process; and the conductivity of MoO2 or MoO2. 3-x Thermodynamic and kinetic barriers that are difficult to form stably.

[0011] To achieve the above objectives, the present invention provides a method for improving the interface between two-dimensional MoS2 and a metal using plasma oxidation-reduction treatment, comprising the following steps: Step 1, oxygen plasma oxidation treatment. Specifically, the two-dimensional MoS2 substrate is placed in a plasma treatment chamber and subjected to oxygen plasma treatment in an oxygen-containing atmosphere to oxidize the MoS2 surface and form a MoO3 oxide layer with a thickness of 1~20nm.

[0012] Preferably, the process conditions for the oxygen plasma treatment are as follows: treatment temperature of 300~500℃, treatment gas pressure of 0.1~0.5 Torr, and treatment time of 10~60 min; the treatment atmosphere is O2 gas or a mixture of O2 and an inert gas, and the total gas flow rate is 1~10 L / min. In the mixed gas, the inert gas includes one or more of Ar, N2, or He, and the volume fraction of oxygen is 1%~20% of the mixed gas.

[0013] The selection of the above parameter range in the oxidation step of this invention is based on the following technical considerations. First, oxygen plasma is used instead of thermal oxidation because the high-energy oxygen free radicals (O·) and oxygen ions (Oˉ, O2⁺) in the plasma have much higher oxidizing activity than molecular oxygen, enabling them to react rapidly with MoS2 at a moderate temperature of 300~500℃, avoiding damage to the two-dimensional material caused by high temperatures (>600℃). Second, the oxygen volume fraction is controlled between 1% and 20%, ensuring a sufficient oxidation rate while avoiding over-oxidation that would result in an excessively thick MoO3 layer, such as exceeding 20 nm, making complete conversion difficult in subsequent reduction steps. Third, the processing temperature is limited to 300~500℃ because below 300℃, the oxidation reaction kinetics are slow, making it difficult to form a continuous oxide layer within a reasonable time; above 500℃, thermal decomposition or sulfur volatilization may occur in the MoS2 channel region, and the generated MoO3 is prone to sublimation, disrupting the uniformity of the layered structure. The product of this step is MoO3, which is thermodynamically the most stable. Although it is an insulator, it has a fast formation rate, controllable thickness, and atomic-level continuity with the interface of the underlying MoS2, providing an ideal precursor for the subsequent conversion to the conductive state.

[0014] Step 2, hydrogen plasma reduction treatment, specifically, the product obtained in step (1) is subjected to hydrogen plasma treatment in a hydrogen atmosphere to at least partially reduce the MoO3 oxide layer to a conductive low-valence metal oxide layer, wherein the low-valence metal oxide layer comprises MoO2 and / or MoO 3-x .

[0015] Preferably, the process conditions for the hydrogen plasma treatment are as follows: treatment temperature of 400~600℃, treatment gas pressure of 0.1~0.5 Torr, and treatment time of 10~60 min; the treatment atmosphere is H2 gas or a mixture of H2 and an inert gas, and the total gas flow rate is 1~10 L / min. Wherein, the inert gas in the mixed gas is one or more of Ar, N2, or He, and the volume fraction of hydrogen in the mixed gas is 1%~20%.

[0016] This step converts insulating MoO3 into conductive MoO2 / MoO. 3-xThe key lies in the precise control of reduction thermodynamics and kinetics. Thermodynamically, the reduction of MoO3 to MoO2 by H2 is: H2 + MoO3 → MoO2 + H2O. At room temperature, ΔG > 0, requiring higher temperatures for spontaneous reduction. Traditional thermal reduction typically occurs above 700℃, which severely damages the two-dimensional MoS2 channel. This invention utilizes hydrogen plasma, dissociating H2 into high-energy reactive species such as hydrogen radicals H· and hydrogen ions H⁺ via radio frequency or microwave discharge. The activation energy of these species' reaction with MoO3 is significantly reduced, allowing the reduction reaction to proceed rapidly at moderate temperatures of 400–600℃. Below 400℃, the reduction rate is too slow, resulting in incomplete MoO3 reduction; above 600℃, although reduction is complete, sulfur vacancies or thermal decomposition may occur in the MoS2 channel. The hydrogen gas fraction should be controlled between 1% and 20%: too low a fraction results in insufficient concentration of active hydrogen species and slow reduction; too high a fraction may lead to over-reduction to form metallic Mo, which has a large lattice mismatch with MoS2, thus increasing interface defects. A processing time of 10 to 60 minutes is sufficient to ensure that a MoO3 layer with a thickness of 1 to 20 nm is completely reduced to MoO2 or MoO. 3-x Furthermore, plasma reduction exhibits anisotropic characteristics, with active hydrogen species preferentially reacting with the MoO3 surface and progressing layer by layer inward. This facilitates the formation of a uniform conductive layer and avoids the generation of intermediate valence state oxides such as Mo4O. 11 A processing pressure of 0.1~0.5 Torr can maintain stable plasma glow discharge while ensuring that the mean free path of active species is sufficient to reach the sample surface.

[0017] After the above steps, the MoO2 / MoO 3-x The layer is located on the surface of two-dimensional MoS2, with a thickness of 1–20 nm, and forms an atomically continuous interface with the underlying MoS2. The sulfur content within the interface decreases along the direction away from MoS2. This structural feature stems from the synergistic effect of sulfur being gradually replaced by oxygen during oxidation and oxygen being partially removed during reduction, ensuring efficient carrier transport between MoS2, conductive oxide, and metal after subsequent deposition of the metal electrode.

[0018] The oxidation and reduction steps of this invention can be completed within the same plasma chamber by switching gases and adjusting power, achieving in-situ or quasi-in-situ processing and avoiding sample contamination from air exposure. For MoS2 materials with different numbers of layers and masses, optimal contact optimization can be achieved by independently adjusting parameters such as oxidation and reduction temperature, time, and gas ratio. This method is also applicable to other two-dimensional transition metal sulfides, such as WS2, MoSe2, and WSe2, requiring only corresponding adjustments to the processing parameters to suit the redox characteristics of different materials.

[0019] Compared with the prior art, the present invention has the following beneficial effects. This invention achieves highly conductive MoO2 or MoO2 in situ between MoS2 and a metal. 3-x The layer effectively shields the metal-induced interface states, mitigating the Fermi level pinning effect. This conductive oxide layer provides a low-barrier tunneling path, reducing the contact resistance by approximately 30%–50% compared to untreated MoS2, and lowering the Schottky barrier height from 0.5–0.8 eV to below 0.2 eV, resulting in a simultaneous increase in device on-state current and transconductance.

[0020] In this invention, the metal electrode is deposited on a conductive oxide buffer layer, rather than in direct contact with the MoS2 channel, thus avoiding bombardment damage from high-energy metal atoms and diffusion erosion. Simultaneously, hydrogen plasma reduction is performed at a moderate temperature of 400–600°C, far lower than the 700°C or higher required for traditional thermal reduction, effectively preventing thermal decomposition of the MoS2 channel and sulfur loss. After treatment, the MoS2 channel maintains its intact lattice structure, and the subthreshold swing is close to the theoretical value.

[0021] This invention circumvents the stringent thermodynamic window for the direct generation of MoO2 from MoS2 by employing a two-step "oxidation followed by reduction" strategy. The oxygen plasma step rapidly forms a uniform MoO3 precursor within a relatively wide parameter window, while the hydrogen plasma step utilizes active hydrogen species to lower the reduction temperature from above 700℃ to 400-600℃, completing the reduction within 10-60 minutes and avoiding the formation of intermediate valence oxides. The final result is MoO2 / MoO3 with excellent metallic conductivity. 3-x The layer has a resistivity of approximately 10. -3 It has an Ω·cm content and forms an atomically continuous gradient interface with MoS2.

[0022] The method of this invention is simple in procedure, compatible with existing semiconductor processes, and can selectively process source and drain regions without adding photomasks. The processing parameters can be adjusted independently to adapt to MoS2 materials of different thicknesses and qualities, and can be extended to other two-dimensional transition metal sulfide systems such as WS2, MoSe2, and WSe2. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the process of forming a MoO3 layer and a MoO2 layer on the surface of MoS2 by oxidation and reduction in this invention.

[0024] Figure 2 This is a graph showing the change in contact resistance under different oxidation / reduction treatment temperatures according to the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and do not constitute a limitation on the scope of protection of this invention. Any equivalent substitutions or improvements made within the spirit and principles of this invention, based on the concept of this invention, should be included within the scope of protection of this invention.

[0026] like Figure 1 As shown, the core of this invention lies in achieving controllable oxidation-reduction conversion on the MoS2 surface through a two-step plasma treatment. Figure 1 The left side shows the original two-dimensional MoS2 crystal structure (S-Mo-S three-atom layers). After oxygen plasma treatment ( Figure 1 (Central part), oxygen free radicals (O·) and oxygen ions (Oˉ, O2) ⁺ The sulfur atoms react with the MoS2 surface, gradually being replaced by oxygen atoms to form a MoO3 oxide layer. This process is accompanied by the escape of sulfur as SO2 or S2 gas. The thickness of the MoO3 layer can be precisely controlled within 1–20 nm by adjusting the processing time. This is followed by hydrogen plasma reduction treatment. Figure 1 On the right side, highly reactive hydrogen radicals (H·) remove some oxygen atoms from MoO3, generating water vapor (H2O) which then evaporates, transforming MoO3 into MoO2 with metallic conductivity or substoichiometric MoO3. 3-x The final structure is as follows: the bottom consists of the original MoS2 channels, above which is an atomically continuous MoO2 / MoO2 layer. 3-x In the conductive layer, the sulfur content decreases in a gradient away from MoS2, ensuring efficient carrier transport. Example 1

[0027] In this embodiment, oxygen plasma and hydrogen plasma were used to treat the monolayer MoS2 to improve its interface with the metal. The specific steps are as follows.

[0028] Step 1, Material Preparation: A high-quality monolayer MoS2 thin film was grown on a SiO2 / Si substrate using chemical vapor deposition (CVD). The active region of the transistor was defined by photolithography and oxygen plasma etching, and the source and drain regions were defined by a second photolithography. The channel length was approximately 2 μm and the width was approximately 10 μm.

[0029] Step 2, Oxygen plasma oxidation treatment: The above sample is placed in a plasma reaction chamber, with a base vacuum of less than 1×10⁻⁶. -2Torr was introduced, with an O2 / Ar mixed gas concentration of 3% by volume and a total flow rate of 5 SLM. The chamber pressure was set to 0.2 Torr, the RF power to 100 W, and the substrate temperature to 300 °C. The processing time was 30 minutes. This step formed a MoO3 layer with a thickness of approximately 5 nm on the MoS2 surface in the source / drain region.

[0030] Step 3, Hydrogen Plasma Reduction Treatment: After oxidation, switch the sample to the hydrogen plasma reaction chamber while maintaining a vacuum environment, or switch gas paths within the same chamber. Introduce an H2 / Ar mixed gas, with an H2 volume fraction of 5% and a total flow rate of 5 SLM. Set the chamber pressure to 0.2 Torr, RF power to 150 W, and substrate temperature to 500°C. The treatment time is approximately 30 minutes. This step reduces MoO2 to a MoO2 layer.

[0031] Step 4: After reduction treatment, the sample is removed, and the source and drain patterns are defined by a third photolithography step. Metal electrode deposition is then performed using conventional processes, such as electron beam evaporation to deposit Ti (10nm) / Au (50nm). Finally, the sample is annealed at 200°C for 10 minutes in a N2 atmosphere to stabilize the contacts.

[0032] Contact resistance was measured using the four-probe method. The results showed that the contact resistance of the control sample, on which electrode metal was deposited directly on a monolayer of MoS2, was approximately 3.2 kΩ·μm. The contact resistance of the sample treated in this embodiment was reduced to approximately 1.9 kΩ·μm, representing a reduction of approximately 41% compared to the control sample. Example 2

[0033] Materials preparation: Same as in Example 1.

[0034] Oxidation treatment conditions were varied: oxidation temperatures were set to 200℃, 250℃, 350℃, 450℃, 500℃, and 550℃, respectively, with an oxidation time of 30 minutes for each temperature and an O2 concentration of 5%. Other conditions were the same as in Example 1.

[0035] Reduction treatment: The reduction temperature was uniformly set to 450℃, the H2 concentration was 10%, and the treatment time was 30 minutes.

[0036] The contact resistance test results are as follows Figure 2 As shown, the contact resistance is lowest when the oxidation temperature is 400℃. If the oxidation temperature is too low, such as 200℃, the oxidation is incomplete, and the residual MoS2 leads to discontinuity of the conductive layer after reduction. If the oxidation temperature is too high, such as 550℃, the MoO3 layer is too thick and dense, the reduction is insufficient, and there is still MoO3 residue on the surface, which increases the contact resistance. Therefore, the preferred oxidation temperature is 300~500℃, and the most preferred is 350-450℃. Example 3

[0037] Material preparation and oxidation treatment: Same as in Example 1, except that the oxidation temperature is 350°C and the oxidation time is 30 minutes.

[0038] Reduction treatment temperature variation: The reduction temperature was set to 400℃, 500℃, 600℃ and 700℃ respectively, and other conditions were the same as in Example 1.

[0039] Test results show that at a reduction temperature of 400℃, some MoO3 remains unreduced, resulting in a relatively high contact resistance of approximately 2.8 kΩ·μm; at 500℃, reduction is complete, with the lowest contact resistance of 1.9 kΩ·μm; at 600℃, although reduction is complete, slight sulfur loss occurs in the MoS2 channel, and the contact resistance increases slightly by 2.1 kΩ·μm; at 700℃, significant pitting and blistering appear in the MoS2 channel, leading to device failure. Therefore, the preferred reduction temperature is 400~600℃, with 450-550℃ being the most preferred.

[0040] The method of this invention can be implemented in existing plasma processing equipment, such as inductively coupled plasma (ICP) systems, without special modifications. The processed sample can be directly fed into a metal evaporation chamber, making it compatible with standard CMOS processes. This method is not only applicable to MoS2 but can also be extended to other two-dimensional transition metal sulfides such as WS2, MoSe2, and WSe2, requiring only corresponding adjustments to the plasma processing parameters.

[0041] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various improvements and modifications without departing from the spirit and principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment, characterized in that, At least the following steps are included: Oxygen plasma oxidation treatment: The two-dimensional MoS2 surface is treated with oxygen plasma in an oxygen-containing atmosphere to form a MoO3 oxide layer on the MoS2 surface. Hydrogen plasma reduction treatment: The MoO3 oxide layer is subjected to hydrogen plasma treatment in a hydrogen-containing atmosphere to at least partially reduce the MoO3 oxide layer to a conductive low-valence metal oxide layer, wherein the low-valence metal oxide layer comprises MoO2 and / or MoO2. 3-x .

2. The method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment according to claim 1, characterized in that, The process conditions for the oxygen plasma treatment are: treatment temperature 300~500℃, treatment pressure 0.1~0.5 Torr, and treatment time 10~60 minutes.

3. The method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment according to claim 1, characterized in that, The oxygen plasma treatment atmosphere is O2 gas or a mixture of O2 and an inert gas, with a total gas flow rate of 1~10 L / min; wherein the inert gas in the mixture includes one or more of Ar, N2 or He, and the volume fraction of O2 in the mixture is 1%~20%.

4. The method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment according to claim 1, characterized in that, The process conditions for hydrogen plasma treatment are: treatment temperature 400~600℃, treatment gas pressure 0.1~0.5 Torr, and treatment time 10~60 minutes.

5. The method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment according to claim 1, characterized in that, The hydrogen plasma treatment atmosphere is H2 gas or a mixture of H2 and an inert gas, with a total gas flow rate of 1~10 L / min; wherein, in the mixture, the inert gas is one or more of Ar, N2 or He, and the volume fraction of hydrogen in the mixture is 1%~20%.

6. The method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment according to claim 1, characterized in that, The thickness of the MoO3 oxide layer formed by the oxygen plasma treatment is 1~20 nm.

7. The method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment according to claim 1, characterized in that, After the hydrogen plasma reduction treatment, the thickness of the low-valence metal oxide layer is 1~20nm, and an atomically continuous interface is formed between it and the two-dimensional MoS2 below. The sulfur content in the interface decreases along the direction away from the two-dimensional MoS2.

8. The method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment according to claim 1, characterized in that, The contact resistance between the treated 2D MoS2 and the subsequently deposited metal electrode is reduced by 30% to 50% compared to the contact resistance of untreated MoS2.

9. The method for improving the interface between two-dimensional MoS2 and metals through plasma oxidation-reduction treatment according to claim 1, characterized in that, In the method, two-dimensional MoS2 is replaced with any one of two-dimensional transition metal sulfides among WS2, MoSe2, and WSe2.

10. A two-dimensional MoS2 transistor, characterized in that, The contact interface between its source and drain electrodes and MoS2 is formed by the method of improving the two-dimensional MoS2-metal contact interface by plasma oxidation-reduction treatment as described in any one of claims 1-9.

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