Optoelectronic components can be divided into photodetectors that absorb light as switching signals, solar cells or photovoltaic devices that absorb light and convert it into electrical energy, and light emitting devices that emit light. The most common one is luminescence. Diode LED)[27].
The main indicators of component performance include high responsivity, short response time, high sensitivity, large gain (photo gain), and linearity.

Two-dimensional materials are used in optoelectronic components
The components that absorb light are discussed in three steps: (1) light absorption (2) carrier generation (3) carrier transmission. The goal of material optical absorption is to cover a large reception frequency band (bandwidth) range, depending on the energy gap of the material.The optical signals that two-dimensional material semiconductors can receive cover a large frequency range [6], and corresponding materials are available from mid-infrared light to visible light.However, the light absorption of two-dimensional atomic layer materials is relatively less than that of three-dimensional bulk materials. The reception efficiency of different frequency bands can be increased by stacking heterogeneous layers of two-dimensional materials, and the design needs to pay attention to improving the gain.

In the photogenerated carrier stage, the material absorbs light and generates carriers—electron-hole pairs. To improve the gain, attention should be paid to reducing the combination of electron-hole pairs, so that the increased number of carriers can be extracted. A common approach is to add another material to guide the carriers to move in different directions. For example, [7] uses indium atom adsorption in the literature Placed above the two-dimensional semiconductor tungsten disulfide WS2, the photogenerated electrons are guided to transfer to the tungsten disulfide channel, while the holes are trapped in the indium atoms.

The main challenge in carrier transport lies in the interface between the semiconductor channel and the metal wire, where contact resistance causes losses and results in low responsivity. Since there is no method to repair the damage caused by two-dimensional materials in traditional doping processes, choosing a band-matching metal and using quantum tunneling mechanisms are currently the main methods to reduce contact resistance. Graphene with semi-metallic properties is often used to connect two-dimensional material semiconductors and metal wires. It can form low contact resistance with two-dimensional materials. At the same time, its ultra-high carrier mobility (mobility) characteristics also further reduce The separated electrons and holes meet and combine.

Popular lighting fixtures
Today's popular light-emitting devices mostly apply the principles of photoluminescence (PL) and electroluminescence (EL). Structures that use DC power sources include light-emitting diodes (LEDs) and single photon emission (single photon emission) quantum dots (Quantum dot LEDs, QLEDs).There are many advantages to applying 2D material properties to light-emitting devices. The manufacturing process of QLED is cumbersome, and its reliance on long hydrophobic insulating ligands also hinders their stability and conductivity.The self-terminated surface characteristics of the two-dimensional material make the device carrier-free during operation. No interference from ligands. The carrier transport capacity and exciton recombination capacity of organic light-emitting diodes (OLEDs) are low, which hinders the improvement of brightness, while the excellent exciton luminescence capacity of two-dimensional material semiconductor TMDs can achieve high brightness at room temperature. [8]

The quantum confinement effect in thin-layer materials reduces the state density and carrier concentration of thin-layer three-dimensional materials. Two-dimensional material semiconductor TMDs bring high carrier concentration due to their high effective mass. Under such conditions, higher-order exciton quasiparticles, such as excitons (Exciton) and charged excitons (Trion), can be observed. etc.Two-dimensional material semiconductor TMDs have a strong Coulomb force, which tightly combines excitons and brings high exciton binding energy, and can even be observed at room temperature.The binding energy of typical III-V semiconductor GaAs is 4.76 meV, and excitons can only be observed at low temperatures, while the molybdenum disulfide MoS in two-dimensional TMDs is  240 meV.

Defects in traditional semiconductors can capture carriers, hinder the combination of electrons and holes to emit light, and greatly reduce the photoluminescence quantum yield (PLQY), which is a key indicator that determines the optoelectronic performance of components.Two-dimensional material semiconductor TMDs usually have a large native defect density after processing, and repairing defects is a major process challenge. However, studies have found that neutral exciton recombination is radiative, and high PLQY performance can still be achieved even with high defect density [ 8], giving two-dimensional TMDs great potential for optoelectronic applications.

In addition to the above-mentioned DC input LED structure, a structure using AC power has also been proposed[9]. Through the AC switching frequency suitable for the material, positive and negative charges meet and combine in the material and emit light.The LED structure uses the PN interface (PN Junction) of the material to emit light. The narrow material interface and complex structure limit large-area applications. The structure is simple and is less affected by the Schottky barrier at the material interface, providing a solution for large-area transparent displays.

Two-dimensional materials also have advantages when it comes to heterogeneous integration between components and control circuits made of dissimilar materials.The circuit of the control element is mainly a silicon-based CMOS circuit.When components composed of HgCdTe and Group III and V elements are integrated with control circuits, there will be lattice mismatch in the manufacturing process, resulting in unsmooth joining.Two-dimensional materials can be transferred to other materials through a transfer process and attached to other materials through pan-derval force without relying on lattice matching.

This feature can also be used to create controls for two-dimensional material substrates. The circuit is used in III-V displays [12] and wearable displays that need to be transparent and flexible [11].For the problem of limited light absorption in two-dimensional materials, research has found that the interaction between the two-dimensional material layer and the optical mode field propagating along the optical waveguide can be greatly enhanced by extending the interaction length. [13]With the enhanced interaction between light and matter, the application potential of optoelectronic components integrating silicon and two-dimensional materials through waveguides in various functional photonic integrated circuits has attracted widespread attention. [26]

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

1.  Exfoliation

The peeling method introduces a force of appropriate size to overcome the weak Pan-Devar force between the layers of the two-dimensional layered material, and separates the multi-layer stacked large-volume raw material blocks into several thin-layered flakes, while the Covalent, ionic, or metallic bonds are strong enough to keep the 2D layer intact.For example, ultrasonication and high-sheer mixing are direct methods to produce 2D materials in the liquid phase by introducing shear forces. The electrochemical peeling method achieves the effect by introducing an electric field to increase the distance between layers.

2. Chemical vapor deposition  ( CVD )

The principle of the chemical vapor deposition method is to use high temperature to vaporize solid raw materials. The raw material vapors meet to cause a gas-phase chemical reaction and are deposited on the target substrate.Taking the two-dimensional material semiconductor molybdenum disulfide as an example, solid powders of molybdenum trioxide and sulfur are heated to 600~800°C, and a thin layer of molybdenum disulfide is formed on the substrate after a gas phase reaction.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.

3. Post-annealing method  (post-annealing)

The post-annealing method is a two-step growth method. First, the precursor (precursor) is deposited, and then the target material is formed through the post-annealing reaction. At the same time, the crystallinity of the material is improved to optimize the electrical properties of the material. Sputtering is a method suitable for large-scale manufacturing.It belongs to physical vapor deposition (PVD). It has the advantages of fast, cheap and scalable. It can manufacture tungsten-based diodes that usually require higher process temperatures. dimensional materials. However, with the low number of atomic layers required for 2D materials, it is difficult to control precise film thickness, roughness, and crystallinity. Therefore, it is combined with CVD for post-annealing treatment to improve crystallinity and repair defects.

2D material post-processing process
It is important to choose a post-processing method appropriate to the material for the type of properties for which enhanced control is desired. Common process techniques include annealing and doping.

Traditional annealing methods are carried out in a vacuum or inert gas environment. Many defects will occur when annealing two-dimensional materials in this environment.For example, molybdenum telluride, the temperature that can improve crystallinity is above 650 degrees, and the tellurium element in the film layer It starts to detach at 250 degrees, so the annealing of this material needs to be carried out in an atmosphere filled with tellurium element. This characteristic can also be used to fill the material with the element to be doped during the annealing process[21 ].

In addition, an annealing method that is not affected by the atmosphere has been proposed, solid phase crystallization (SPC), which encapsulates sputtered MoTe2 through a SiO2 covering layer and then raises it to a high temperature. The solid-state crystallization process can be carried out in a Te-free atmosphere. Proceed easily[22].

Additive-free methods include laser processing. Laser processing can be performed on specific locations. For example, the molybdenum telluride changes from the 2H semiconductor phase to the 1T semimetal phase after laser processing, and can be applied to Ohmic contact issue. [23]

Among the methods of using additives, the main strategies currently used in TMDs doping engineering are: (1) substitution doping (2) charge transfer doping (3) electrostatic field effect doping.

Semiconductors with conventional three-dimensional crystal structures are typically doped with impurity atoms at substitutional or interstitial sites. In contrast, the weak van der Waals interaction between two-dimensional film layers results in a larger interlayer distance, which is beneficial to the embedding of dopant atoms. And at such ultrathin thicknesses, they can also be easily doped via surface charge transfer and external electrostatic field effects.

(1) Substitution doping
Substitution doping can be achieved by mixing dopants during the material growth stage, or by filling dopants through the atmosphere after creating vacancies in the film layer through annealing, plasma, or laser. In the presence of sulfur vacancies, doping reactions of Group 7 (F, Cl, Br) and Group 5 elements (N, P, As) are thermodynamically more likely to occur.At metal sites, the formation of dopants strongly depends on the concentration of metal vacancies, such as Re doping of MoS2. Therefore, whether it is the growth of defects or post-processing, it is relatively easy to implement the replacement doping process using in situ methods.

(2) Charge transfer doping
Charge transfer doping has attracted widespread attention in regulating the electronic behavior of semiconductors. Compared with alternative doping that incorporates foreign dopant atoms into the crystal lattice, charge transfer doping utilizes the interaction between the host material and the adjacent medium (including the surface). charge transfer interactions between adatoms, ions, molecules, particles, and substrates) in a manner that avoids distortion of the lattice structure and enables high-mobility transport in low-dimensional materials.

(3) Electrostatic field effect doping
Due to their ultrathin nature, thin films of 2D materials are particularly susceptible to external field effects. The electrostatic doping strategy uses this characteristic to adjust the carrier doping concentration and polarity in TMDs (Transition Metal Dichalcogenides). The external electric field required for electrostatic doping can be provided using an additional gate or a floating gate. [24]

In the metal-insulator-semiconductor (MIS) structure, when the element is driven by a large potential bias, the free charges in the channel will pass through the insulating layer to the metal floating gate and be captured by another dielectric layer. Since the floating gate is completely surrounded by high-resistance material, the amount of charge contained in it will remain unchanged for a long time.

These trapped charges will continue to provide an electric field through capacitive coupling to affect the conductivity of the semiconductor channel until these charges are applied with an opposite direction. Large potential discharges the floating gate.

Conclusion
Two-dimensional material systems have many excellent properties. 2D semiconductor hardware systems that integrate sensing, storage, and processing will subvert the architecture of electronic applications in the future. At this stage, there is still a lot of research work to be completed in order to develop integrated circuit mass production and even commercial applications.

The basic properties of two-dimensional semiconductor materials as transistors have not yet been understood, and the energy band and parasitic capacitance models still need to be further explored. Process challenges include ohmic contact, large-area quality uniformity, and doping techniques to control material properties. We look forward to more breakthroughs.

(First image source: Shutterstock)
※ This article is from Professor Li Wenxi and doctoral student Chen Shixun of the Department of Electrical Engineering at National Cheng Kung University. It was edited by Science and Technology News in "Adventure to the Flat World - An Introduction to Two-Dimensional Materials". Science and Technology News has organized it into two articles. This is the next article.