Let’s Explore the Two-Dimensional World---Introduction to 2D Materials

Let’s Explore the Two-Dimensional World---Introduction to 2D Materials

Professor Wen-Shi Lee

Ph.D. Student : ShiXun Chen

Department of Electrical Engineering, National Cheng Kung University

-

The 2010 Nobel Prize in Physics was awarded to the discoverers of graphene, Andre Geim and Konstantin Novoselov of the University of Manchester in the UK. In 2004, they successfully used 3M tape to peel graphene from graphite blocks, kicking off a research boom in the field of two-dimensional materials. The application of graphene’s many interesting properties in two-dimensional material systems has brought revolutionary products and opportunities into our lives in fields ranging from seawater desalination technology [1] and electrical vehicle super batteries [2] to textiles [3].

The surface of graphene repels water. This feature means that water can pass quickly through graphene channels. Furthermore, an epoxy encapsulant can be used to limit how much the grapheme (GO) channels expand after being soaked in water. In this way, it is possible to create a screen capable of filtering out 97% of the salt in water. (Figure 1)

When used in battery electrodes, graphene can effectively improve the battery’s energy efficiency and density while charging and discharging. The additives, such as glue, required to fix traditional electrode materials in place reduce the conductivity, making the charge and discharge rates less ideal. Furthermore, the functional groups on the electrode materials react with the electrolyte at higher voltages, which also limits the energy density of the input. Graphene can help with both of these issues. It has no functional groups on the surface and can be grown directly on the substrate, eliminating the need for adhesives. It can ameliorate the two fatal flaws of supercapacitors and greatly shorten the the time needed to charge electric vehicle batteries. (Figure 2)

Activated carbon has been widely used in the textile industry for antibacterial and deodorizing purposes. Carbon at the two-dimensional scale, graphene, can add new features to textiles with its strength, ductility and high-speed electrical and thermal conductivity. By mixing it into fabric fibers, its high thermal conductivity can be used to spread absorbed heat throughout an entire garment to achieve a uniform temperature. It can also be used as a coating to create tiny, heat-generating circuits to ward off the cold. (Figure 3)

In addition to grapheme, which is comprised of carbon, there are many 2D system materials composed of other elements that need to be explored. They have the potential to endow many of the products we use in our daily lives with truly magical new features.

Figures 1 to 3. Current Applications of 2D Materials

The lattice arrangement of materials are divided into zero to three dimensions according to their points, lines, surfaces and volumes. The three-dimensional blocks that we see in our everyday lives are comprised of the three dimensions of length, width and height. 2D materials are planer, layered materials comprised of two dimensions: length and width. (Figure 4) [4] When we use process technology to arrange a material in these two-dimensional sheets instead of the common three-dimensional stacks, the material’s characteristics become obviously different.

Figure 4. Material Lattice Dimensions

First, most of the atoms in the material will be exposed to the outside world, resulting in a large effective surface area for chemical reactions, which is advantageous in the field of catalysis. Second is the bonding position. 2D layered materials do not have vertical floating bonds. Having one less interface with other materials means one less source of interference, which, in turn, means there is less need for passivation processes. Then there is the influence of thickness. The material has only one layer of molecules in the vertical direction, which eliminates the interaction force and bonding of a large number of molecules and atoms that occur during three-dimensional stacking, allowing the weak van der Waals force to dominate in the two-dimensional material. When different 2D materials are stacked on top of each other, the layers are also connected through this force, creating what is called Van Der Waals heterostructures. (Figure 5)[5] The second effect of a low layer thickness is the change in band gap. For example, molybdenum disulfide has an indirect band gap of 1.2eV when used in multiple layers. As a single layer, this changes to a direct band gap of 1.8eV, thus resulting in different optical, electrical and semiconductor properties.

The 2D material family exhibits a wide range of properties, including those of metals, semi-metals, semiconductors with various band gaps and insulators. Just like 3D materials, 2D materials with various properties can be combined to create a variety of components. (Figure 5)[10]

Figure 5. (a) Van Der Waals Heterostructure [5]; (b) Common 2D Materials [10]

Today’s technological development requires high speed computing of vast amounts of data. Over the past four decades, lithography technology has been used to define ever smaller patterns and achieve smaller transistor sizes in order to increase operating speeds and reduce power consumption. However, when the channel length is reduced to less than 10 nanometers, problems arise that lead to a sudden drop in transistor performance, serious leakage currents, reduced threshold voltages, increased sub-threshold swing, carrier surface scattering, velocity saturation and the hot carrier effect. These changes are referred to collectively as the short channel effect.

The IRDS semiconductor development roadmap indicates that the size of computing components will continue to shrink, and FinFET transistors and gate-all-around structures have been proposed to help improve gate control and limit current leakage. Nowadays, silicon-based bulk materials have been shrunk to their physical limit. Their carrier mobility has been greatly reduced, but the leakage current is still quite high. The source-drain ohmic contact also has insufficient resistance due to complex manufacturing practices. As such, there is an urgent need to explore new systems and materials.

Among the many emerging 2D semiconductor materials, the most widely studied are the transition metal dichalcogenides (TMDs). They are comprised mainly of molybdenum and tungsten combined with sulfur, selenium and tellurium from the chalcogen group of elements. At advanced process scales below 2nm, TMDS can provide higher carrier mobility and lower leakage currents than silicon-based 3D bulk materials. This makes them suitable options for helping to achieve low-power goals.

Figure 6. Potential Applications of 2D Materials in Electronic Components [14]

Various 2D material semiconductor transistor structures are being developed and discussed (Figure 6, 7). No matter how they are altered and adjusted, however, the basic structure remains the same. Current flows through the channel into the metal source-drain wire, and the gate controls the passage of the current. The following topics need to be addressed in order: (1) channel modulation, (2) ohmic contact and (3) dielectric material integration.

The core indicator for a transistor channel is its current performance, which depends on the channel material having a suitable effective mass and band gap. After the channel material is selected, its electrical, optical and magnetic properties are modulated via doping engineering. The process techniques will be discussed in the materials’ post-processing section.

The effective mass of a material is inversely proportional to the maximum current that can pass through it. The lower the effective mass, the higher the current limit. However, if the effective mass is too low, it becomes too easy for an induced tunnel current to occur between the source and the drain. The occurrence of current in the off-state when there should be no current signal will cause the computing component to calculate incorrectly. MoS2 from the TMD 2D semiconductor material family has a large effective mass of about 0.5 m0. It can suppress the direct tunneling of current between the source and drain in ultra-short channel components, effectively reducing leakage current problems.

The band gap directly affects the ratio between the on-state current and off-state leakage current. TMDs have a direct band gap within a single layer. As the number of layers increases, the TMDs’ band gap gradually decreases and changes from a direct band gap to an indirect band gap, which varies from 1.1 to 2.1 eV. Another potential channel material that has attracted attention is black phosphorene (BP). It has a direct band gap that can be regulated between 0.3eV and 2.0eV. Another difference between traditional silicon semiconductors and two-dimensional materials is that 2D materials can naturally exhibit n-type, p-type and bipolar behavior without doping. The most widely studied MoS2, for instance, is n-type, black phosphorene and MoTe2 are p-types, and WSe2 offers bipolar operation.

Semiconductor ohmic contact engineering is used mainly to solve electronic transmission issues at the interface between adjacent but dissimilar materials. When electrons pass through the interface between a metal and a semiconductor, a high resistance will make it difficult for a current to pass. This type of material contact is called a Schottky contact. This phenomenon occurs due to the mismatch between the work functions and Fermi levels of metals and semiconductor materials. Free electrons and holes will continue to transfer locally until the Fermi level reaches equilibrium. This process blocks external input currents and is referred to as Fermi level pinning. Two strategies have been proposed for solving this problem: heavily doping the semiconductor or introducing a thin, dielectric layer at the interface to decouple the metal-semiconductor interaction. The first strategy is extremely challenging in terms of 2D material process technology. The second, which depends on architecture, relies on the use of lower-resistance tunnels, which does not meet the needs of small line-width devices.

A third strategy was proposed more recently that uses semi-metal/semiconductor contacts to suppress metal-inducted gap states (MIGS) to avoid gap state pinning. [17] (Figure 8 Left) By aligning the Fermi level of the semi-metal with the minimum conduction band of the semiconductor, the MIGS contributed by the conduction band can be greatly reduced. This means that the MIGS will be contributed entirely by the valence band and can therefore be filled and saturated, achieving gap-state saturation and realizing the ohmic contact. MoS2 components can achieve excellent performance when paired with contrasting semi-metallic Bi, with a contact resistance of 123 ohm/microns and an on-state current density of 1,135 microamps/micron.

The dielectric material placed between the gate and the channel also has a key impact on the transistor’s electrical performance. Improving the quality of that dielectric layer can reduce the threshold voltage (Vth), which is beneficial to reducing a component’s power consumption. It can reduce hysteresis and improve component stability. In silicon processes, the process technology for the high k dielectric material hafnium oxide has already matured. New challenges arise, however, when it comes to application in two-dimensional material transistors. The surface of 2D materials is clean and has no floating bonds, eliminating the need to passivate floating bonds. The new issue is that there are no nucleation points to adhere to when the dielectric material is deposited, and so the film layer formed is uneven. Charge flowing through the channel is trapped by defects in the dielectric layer, causing device hysteresis and leakage problems.

One possibility for adhering to a smooth, inert surface is to choose a 2D material insulating layer that also has a Van der Waals surface. Though hexagonal boron nitride h-BN can be used as a gate dielectric layer, it can also be used as a packaging material to isolate a 2D semiconductor from the external environment, greatly improving intrinsic properties such as mobility and stability. [19] At present, however, the growth of h-BN is still a challenge. For research, the transfer printing method is used to manufacture single components for the verification of principles, but large-scale production is not yet possible. Furthermore, h-BN has a dielectric constant of about 5, which is similar to silicon dioxide, and is not a high k dielectric. As such, current leakage occurs when the gate control capability of a smaller effective oxide thickness (EOT) is required. Another solution that has been proposed is to connect the dielectric layer by depositing a seed layer on the channel. For this, the effects of Y2O3[16] and organic PTCDA[20] have been discussed.

The material properties of 2D materials can be adapted to existing architectures and meet future needs. However, to achieve large scale integrated application, more research on process technology is needed to improve large area performance and uniformity.

Optoelectrical components can be divided into photodetectors (which absorb light to provide switching signals), solar cells and photovoltaic device (which absorb light and convert it into electrical energy), and light emitting devices or LEDs (which emit light) (Figure 10)[27]. The main indicators of good component performance are high responsivity, short response times, high sensitivity, large photo gains, and changes in linearity.

Discussions on components that absorb light can be divided into three stages: (1) light absorption, (2) carrier generation and (3) carrier transmission. The goal for a light absorbing material is to have a large bandwidth reception range, which depends on the material’s band gap. 2D semiconductor materials are able to receive a wide range of optical signals [6]. There are corresponding materials available for everything from mid-infrared light to visible light. However, the light absorption of 2D atomic layer materials is comparatively less than that of 3D bulk materials. The reception efficiency of different bandwidths can be increased by stacking heterogeneous layers of 2D materials, but component designs need to pay attention to improving the gain.

In the carrier generation stage, the material absorbs light and generates carrier electron-hole pairs. To increase gain, attention should be paid to reducing the combination of electron-hole pairs so as to increase the number of carriers taken out. One common approach is to add another material to direct the carriers to move in different directions. For example, [7] in the literature, indium atoms are absorbed and placed atop the 2D semiconductor tungsten disulfide WS2. Photo-generated electrons are directed to transfer into the tungsten disulfide channel while the holes are trapped by the indium atoms.

The main challenge in carrier transport is the interface between the semiconductor channel and the metal wire. Contact resistance causes losses and results in low responsivity. Since there is no method to repair the damage caused by the traditional doping process of two-dimensional materials, choosing a bandwidth-matching metal and using the quantum tunneling mechanism are currently the main methods employed to reduce contact resistance. Graphene, which has semi-metallic properties, is often used to connect 2D material semiconductors and metal wires. It can form low contact resistance with 2D materials. At the same time, its ultra-high carrier mobility further impedes the meeting and combining of separated electrons and holes.

Figure 9. 2D Materials Applicable at Each Frequency Band [6]   Figure 10. Common Optoelectrical Component Structures [27]

Currently, popular light emitting devices mostly make use of the principles of photoluminescence (PL) and electroluminescence (EL). Structures that use DC power include light emitting diodes (LED) and single photon emission quantum dot LEDs (QLED). 2D materials have many advantageous properties for application in light emitting devices. The QLED manufacturing process is cumbersome, and their reliance on long, hydrophobic insulating ligands hinders their stability and conductivity. 2D materials’ self-terminated surfaces, on the other hand, prevent ligands from interfering with the carriers during device operation. Organic light emitting diodes (OLED) have low carrier transport and exciton recombination capabilities, which hinders the improvement of brightness. The excellent exciton luminescence capabilities of 2D material semiconductor TMDs, however, can achieve high brightness at room temperature. [8]

The quantum confinement effect that occurs in thin layer materials reduces the state density and carrier concentration of thin layer three-dimensional materials. 2D material semiconductor TMDs can provide high carrier concentrations due to their high effective mass. Under such conditions, higher order exciton quasi-particles, such as excitons and trions, etc., can be observed. These TMDs also have a strong Coulomb force, which makes the excitons combine tightly, resulting in a high exciton binding energy that can be observed even at room temperature. The binding energy of typical III-V semiconductor GaAs is 4.76 meV, so excitons can only be observed at low temperatures. In contrast, the binding energy of molybdenum disulfide MoS2 of 2D TMDs is 240meV.

Defects in traditional semiconductors can capture carriers, preventing electrons and holes from combining to produce light and greatly reducing the Photoluminescence Quantum Yield (PLQY), which is a key indicator for determining the photoelectric performance of components. 2D material semiconductor TMDs usually have a large intrinsic defect density after processing, and repairing defects is a major process challenge. However, studies have found that neutral exciton recombination is radiative, so it can have high PLQY performance even when there is a high defect density [8]. As such, 2D TMDs have great application potential in the field of optoelectronics.

In addition to the above mentioned DC input LED structure, a structure using AC power has been proposed (Figure 11) [9]. By switching the frequency of alternating currents suitable for the material, positive and negative charges can be made to meet and combine in the material to emit light. LED structures use the PN interface of a material to emit light, but narrow material interfaces and complex structures limit large-area application. The structure in Figure 11, however, is simple and less affected by the Schottky barrier at the material interface, providing a potential solution for large-area transparent display applications.

2D materials also have advantages when it comes to the heterogeneous integration of components and control circuits made of dissimilar materials. The circuit of control components are mainly silicon-based CMOS circuits. When components comprised of HgCdTe and III-V elements are integrated with control circuits, lattice mismatches caused by the manufacturing process will result in unsuccessful bonding. 2D materials, however, can be transferred directly onto other materials and attached via Van der Waals forces, so they do not rely on lattice matching. This property can also be used to make control circuits based on 2D materials for use in III-V displays [12] and wearable displays that require transparency and flexibility [11]. Furthermore, regarding the problem of limited light absorption in 2D materials, research has fortunately found that the interaction between the 2D material layer and the optical mode field propagating along the optical waveguide can be greatly enhanced by extending the interaction length. [13] As interactions between light and matter increase, the application potential of optoelectric components that integrate silicon and 2D materials through waveguides in various photonic integrated circuits attracts ever more widespread attention. (Figure 11 Right)[26]

Figure 11. On the left is an AC LED; On the right is an optoelectrical component with waveguide integrated silicon and 2D materials

Common methods for making materials that are only a few molecules thick can be divided into the following types: exfoliation, chemical vapor deposition (CVD), and post-annealing.

Exfoliation is when an appropriate force is introduced to overcome the weak Van der Waals force between the layers of a 2D material, separating a bulky block of raw material into several thin sheets while the strong covalent, ionic or metallic bonds within keep the 2D layers intact. Ultrasonication and high-sheer mixing, for instance, are direct methods to produce 2D materials in the liquid phase by introducing sheering forces. The electrochemical exfoliation method, on the other hand, achieves the same effect by introducing an electric field to increase the distance between layers.

The principle of the chemical vapor deposition method is to use high temperatures to vaporize solid raw materials. The raw material vapors meet to cause a gas-based chemical reaction and then deposited on the target substrate. Take, for example, the 2D material semiconductor molybdenum disulfide. Solid molybdenum trioxide and sulfur powders are heated to 600~800°C. After the gas phase reaction, a thin layer of molybdenum disulfide is formed on the substrate. The challenge is to suppress vertical deposition while enhancing horizontal growth. Parameters such as temperature, pressure, holding time, substrate and precursors all have a significant impact on the reaction.

The post-annealing growth method is comprised of two steps. First, the precursor is deposited. Then, through the post-annealing reaction, it becomes the target material. At the same time, the crystallinity of the material is improved to optimize its electrical properties. Sputtering is a method suitable for large-scale manufacturing. It is considered a type of physical vapor deposition (PVD). It is a fast, cheap and scalable method to fabricate tungsten-based 2D materials that typically require higher process temperatures. However, with the low number of atomic layers required for 2D materials, it is difficult to precisely control film thickness, roughness and crystallinity. Therefore, it has to be combined with CVD for post-annealing treatment to improve crystallinity and repair defects.

It is important to choose the right post-processing method based on the properties you wish to enhance your control over. Common process techniques include annealing and doping.

Traditional annealing methods are carried out in a vacuum or inert gas environment. However, many defects will occur when 2D materials are annealed in this type of environment. For example, the temperature needed to improve the crystallinity of molybdenum telluride is over 650 degrees, but the tellurium in the film begins to detach at 250 degrees. Therefore, the annealing of this material needs to be undertaken in an atmosphere filled with tellurium. This characteristic can also be used to fill the material with the elements to be doped during the annealing process (Figure 12.1) [21]. In addition, an annealing method that would not be affected by the atmosphere has been proposed: solid phase crystallization (SPC). The sputtered MoTe2 is encapsulated by a SiO2 covering layer then raised to a high temperature. The solid phase crystallization process can easily be performed in a Te-free atmosphere (Figure 12.2) [22].

Figure 12. 2D Material Post-Processing Techniques: (1) Low-Temperature Annealing, (2) Solid Phase Crystallization Annealing and (3) Laser Treatment

Aside from the annealing method mentioned above, another additive-free method is laser treatment. Laser treatment can be targeted to specific locations. Take, for instance, the molybdenum telluride in Figure 12.3. After laser treatment, it changes from the 2H semiconductor phase to the 1T semi-metallic phase. This approach can be applied to the ohmic contact issue. (Figure 12.3)[23]

As for methods that use additives, the main strategies currently used in the doping engineering of TMDs are (1.) substitutional doping, (2.) charge transfer doping and (3.) electrostatic field effect doping. Semiconductors with conventional 3D crystal structures are typically doped with impurity atoms at substitutional or interstitial sites. In contrast, the weak Van der Waals interaction between 2D film layers leads to a greater interlayer distance, which is beneficial to the embedding of doping atoms. Also, at such ultra-thin thicknesses, they can also be easily doped via surface charge transfer and external electrostatic field effects.

Substitutional doping can be achieved by mixing in dopants during the material growth stage or by filling in dopants through the atmosphere after creating vacancies in the film layer through annealing, plasma or laser treatment. According to thermodynamics, in the presence of sulfur vacancies, doping reactions of group seven (F, Cl, Br) and group five (N, P, As) elements are more likely to occur. At metal sites, dopant formation strongly depends on the concentration of metal vacancies, such as in the case of the Re doping of MoS2. Therefore, whether the defects are formed during growth or post-processing, it is relatively easy to use in situ methods in place of doping.

Charge transfer doping methods for regulating the electronic behaviors of semiconductors have attracted widespread attention. In contrast to substitutional doping, which incorporates foreign dopant atoms into the crystal lattice, charge transfer doping makes use of charge transfer interactions between the host material and adjacent media (including surface adatoms, ions, molecules, particles and substrates). This approach can avoid lattice structure distortion and enable high mobility transport in low-dimension materials.

Due to their extreme thinness, 2D material thin films are particularly susceptible to external field effects. Electrostatic doping strategies make use of this characteristic to adjust the carrier doping concentration and polarity in TMDs. The external electric field required for electrostatic doping can be provided using an additional gate or floating gate. [24] In metal-insulator-semiconductor (MIS) structures, when the component is driven by a large bias voltage, the free charge in the channel will pass through the insulating layer to the metal floating gate to be captured by another dielectric layer. Because the floating gate is completely surrounded by a high-resistance material, the amount of charge it contains remains the same for a long time. This trapped charge will continue to provide an electric field through capacitive coupling to affect the conductivity of the semiconductor channel until that charge is discharged from the floating gate via the application of a large, opposing bias.

Many excellent properties of 2D material systems have already been reported. 2D semiconductor hardware systems that integrate sensing, storage and processing will revolutionize the architecture of electronic applications in the future. At present, a lot of research still needs to be completed in order to develop integrated circuit mass production and realize commercial application. The basic properties of 2D semiconductor materials as transistors are not yet understood, and the energy band and parasitic capacitance model needs further exploration. We look forward to more breakthroughs in process challenges, ohmic contact technology, large-area quality uniformity, and doping techniques for controlling material properties.

Reference: 

[1] Abraham, Jijo, et al. "Tunable sieving of ions using graphene oxide membranes." Nature nanotechnology 12.6 (2017): 546-550.

[2] Zhou, Guangmin. "Graphene–pure sulfur sandwich structure for ultrafast, long-life lithium-sulfur batteries." Design, Fabrication and Electrochemical Performance of Nanostructured Carbon Based Materials for High-Energy Lithium–Sulfur Batteries. Springer, Singapore, 2017. 75-94.

[3] Ba, Housseinou, et al. "Cotton fabrics coated with few-layer graphene as highly responsive surface heaters and integrated lightweight electronic-textile circuits." ACS Applied Nano Materials 3.10 (2020): 9771-9783.

[4] Liu, Ran, Jonathon Duay, and Sang Bok Lee. "Heterogeneous nanostructured electrode materials for electrochemical energy storage." Chemical Communications 47.5 (2011): 1384-1404

[5] Geim, Andre K., and Irina V. Grigorieva. "Van der Waals heterostructures." Nature 499.7459 (2013): 419-425.

[6] Chaves, A., et al. "Bandgap engineering of two-dimensional semiconductor materials." npj 2D Materials and Applications 4.1 (2020): 1-21.

[7] Yeh, Chao-Hui, et al. "Ultrafast monolayer In/Gr-WS2-Gr hybrid photodetectors with high gain." ACS nano 13.3 (2019): 3269-3279.

[8] Lien, Der-Hsien, et al. "Electrical suppression of all nonradiative recombination pathways in monolayer semiconductors." Science 364.6439 (2019): 468-471.

[9] Lien, Der-Hsien, et al. "Large-area and bright pulsed electroluminescence in monolayer semiconductors." Nature communications 9.1 (2018): 1229.

[10] Joksas, Dovydas, et al. "Memristive, Spintronic and 2D-Materials-Based Devices to Improve and Complement Computing Hardware." arXiv preprint arXiv:2203.06147 (2022).

[11] Choi, Minwoo, et al. "Flexible active-matrix organic light-emitting diode display enabled by MoS2 thin-film transistor." Science Advances 4.4 (2018): eaas8721.

[12] Hwangbo, Sumin, et al. "Wafer-scale monolithic integration of full-colour micro-LED display using MoS2 transistor." Nature Nanotechnology 17.5 (2022): 500-506.

[13] Koester, Steven J., and Mo Li. "Waveguide-coupled graphene optoelectronics." IEEE Journal of Selected Topics in Quantum Electronics 20.1 (2013): 84-94.

[14] Huang, Xiaohe, Chunsen Liu, and Peng Zhou. "2D semiconductors for specific electronic applications: from device to system." npj 2D Materials and Applications 6.1 (2022): 51.

[15] Liu, Menggan, et al. "Large‐Scale Ultrathin Channel Nanosheet‐Stacked CFET Based on CVD 1L MoS2/WSe2." Advanced Electronic Materials (2022): 2200722.

[16] Shrivastava, Mayank, and V. Ramgopal Rao. "A roadmap for disruptive applications and heterogeneous integration using two-dimensional materials: State-of-the-art and technological challenges." Nano Letters 21.15 (2021): 6359-6381.

[17] Shen, Pin-Chun, et al. "Ultralow contact resistance between semimetal and monolayer semiconductors." Nature 593.7858 (2021): 211-217.

[18] Kim, Changsik, et al. "Fermi level pinning at electrical metal contacts of monolayer molybdenum dichalcogenides." ACS nano 11.2 (2017): 1588-1596.

[19] Cui, Xu, et al. "Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform." Nature nanotechnology 10.6 (2015): 534-540.

[20] Yu, Zhihao, et al. "Reliability of ultrathin high-κ dielectrics on chemical-vapor deposited 2D semiconductors." 2020 IEEE International Electron Devices Meeting (IEDM). IEEE, 2020.

[21] Choi, Min Sup, et al. "Chemical Dopant‐Free Doping by Annealing and Electron Beam Irradiation on 2D Materials." Advanced Electronic Materials 7.10 (2021): 2100449.

[22] CHuang, Jyun-Hong, et al. "Polymorphism control of layered MoTe2 through two-dimensional solid-phase crystallization." Scientific reports 9.1 (2019): 1-8.

[23] Cho, Suyeon, et al. "Phase patterning for ohmic homojunction contact in MoTe2." Science 349.6248 (2015): 625-628.

[24] Vu, Quoc An, et al. "Two-terminal floating-gate memory with van der Waals heterostructures for ultrahigh on/off ratio." Nature communications 7.1 (2016): 12725.

[25] Luo, Peng, et al. "Doping engineering and functionalization of two-dimensional metal chalcogenides." Nanoscale Horizons 4.1 (2019): 26-51.

[26] Flöry, Nikolaus, et al. "Waveguide-integrated van der Waals heterostructure photodetector at telecom wavelengths with high speed and high responsivity." Nature Nanotechnology 15.2 (2020): 118-124.

[27] Ezhilmaran, Bhuvaneshwari, et al. "Recent developments in the photodetector applications of Schottky diodes based on 2D materials." Journal of Materials Chemistry C 9.19 (2021): 6122-6150.