Semiconductor traps states
In semiconductor physics, traps are regions in a material where charge carriers (electrons or holes) can become trapped, affecting the transport properties of the material. Traps can come from various sources, including impurities, defects, and interfaces in the material. Traps can have a significant impact on the electrical conductivity of bulk semiconductors or 2D semiconductors. For example, If a significant number of charge carriers become trapped in the material, the overall conductivity of the material can be reduced. This can lead to a reduction in the efficiency of electronic devices based, such as transistors and solar cells. Mobile electrons can be trapped in the intrinsic traps inside the semiconductor or even in oxide traps in the dielectric materials that are normally used as gate dielectrics for transistors.
Optoelectrical properties
Traps can also affect the optoelectrical properties of a semiconductor. For example, in a semiconductor used in a solar cell, if a significant number of charge carriers become trapped, it will reduce the overall efficiency of the solar cell. Additionally, the presence of traps can affect the recombination of electron-hole pairs, which can lead to a reduction in the overall photoresponsivity of the material. Or they can increase the photoresponsivity of the material if the created traps can bring faster recombination possibilities. For example in the case of the negative photoconductivity effect.
Transport properties
Another important effect of traps in semiconductors is on the thermal stability of the material. Traps can act as a sink for charge carriers, making it more difficult for them to move through the material. This can lead to a reduction in the overall thermal stability of the material, making it more susceptible to degradation at high temperatures. The effect of traps on the transport properties of charge carriers in 2D materials can be studied using low-temperature measurements. At low temperatures, the thermal energy of the charge carriers is reduced, making it more likely that they will become trapped. This can lead to a reduction in the overall conductivity of the material. Additionally, the presence of traps can affect the efficiency of optoelectronic devices based on 2D materials, such as photodetectors and solar cells, by reducing the number of charge carriers available for transport. In fact, the carriers must overcome higher potential barriers in the crystal if the crystal is measured at low temperatures.
Detecting traps
Overall, traps play a significant role in the electrical and optoelectrical properties of semiconductors. Understanding the sources and effects of traps is important for optimizing the performance of electronic devices based on semiconductors. Techniques such as deep level transient spectroscopy (DLTS) and thermally stimulated current (TSC) measurements can be used to study the presence and properties of traps in a semiconductor. There is another way to estimate the number of traps, that is by performing conventional transistor transport measurements, the total number of traps from crystal and oxide can be estimated.
Traps in 2D materials
In 2D materials, such as MoS2 and WS2, traps can have a significant impact on the optoelectrical properties of the material. Because of the confinement in two dimensions of the crystal, each trap can scatter the carriers better than the bulk materials, due to the higher potential fluctuations around the trap relative the potential fluctuations around the traps in bulk materials.
Trap sources
One of the main sources of traps in 2D materials is defect states in the crystal lattice. These defects can occur during the growth process, or as a result of damage to the material during processing, device fabrication, or degradation. Additionally, grain boundaries in 2D materials can also act as traps for charge carriers. Grain boundaries are the regions where different grains of the material in a certain crystal direction meet each other, and they can have a significant impact on the photoresponsivity of the material. As mentioned before, by bringing more potential fluctuations in the periodicity of the crystal, the chance of scattering the carriers increases.
Summary
In summary, traps play a significant role in the transport and optoelectrical properties of 2D materials, such as MoS2 and WS2. The presence of defects and grain boundaries in these materials can act as traps for charge carriers, affecting the overall conductivity and photoresponsivity of the material. Low-temperature measurements can be used to study the effect of traps on the transport properties of charge carriers in 2D semiconductors and metals.
Author: Δ Emad Najafidehaghani Δ
