Sonia Fernandez
A condition long considered to be unfavorable to electrical conduction in semiconductor materials can actually be beneficial in 2D semiconductors, according to the new discoveries of researchers from the UC Santa Barbara published in the review Physical Review Letters.
Electron-phonon interactions-collisions between load-bearing electrons and heat-bearing vibrations in the atomic network of the material-are considered to be the main cause of the slowdown in electrons when they move in the semiconductor material. But according to Bolin Liao and Yujie Quan, mechanical engineers at the UCSB, when electrons and phonons are considered a single system, these interactions in an atomically fine material prove to be preserved in total momentum and energy, which could have important implications for the design of semiconductors in 2D.
« This contrasts strongly with three -dimensional systems where there are a lot of processes of loss of momentum “Said Mr. Liao, who specializes in thermal and energy sciences.
Hydrodynamic diffusion and flow
Two main types of energy transport underlies the concept of researchers: hydrodynamic flow, a collective kinetic process in which the individual components of a fluid move together in a general direction, such as the flow of water in a pipe, and the diffusion, in which the particles undergo random steps under the effect of a gradient of concentration or temperature, as in smoke. As a rule, the conduction of heat in solids is considered a diffusion process.
« These are two very different physical processes “Added Mr. Liao. In the case of hydrodynamic flow, which is more effective in transporting energy than diffusion, the total amount of movement of the particles is preserved during their collision processes. Although they can collide with each other, they exchange their amount of movement and continue to move together.
But when we think of the conduction of heat in the materials, it is not transported by ” real particles “, Continues Mr. Liao. Phonons, which we can consider as ” heat particles “, Are the result of the collective vibrations of the atoms of the material, and they tend to spread, with microscopic collisions which do not keep the amount of movement – a less effective process for the transport of energy. It is the interactions between the phonons and the electrons which cause the slowdown in electrons or the relaxation of their amount of movement. This is the reason why the electrical resistance of conductors decreases at low temperature: the absence of thermal energy decreases the resistance that the electrons would encounter in the material.
However, according to Liao and Quan, physics is different in two dimensions. “” They have unusual properties “Said Mr. Liao about atomically thin semiconductors. “” For example, in these materials, such as graphene, when phonons disperse between them, we know that their amount of movement is largely preserved. This is due to the different dimensionality which requires a certain constraint on the way they can interact with each other ».
In their 2D semiconductor simulations with strong electron-phonon interactions, the researchers found that when they treated load and heat carriers as part of the same system, the interactions between electrons and phonons resulted in collective hydrodynamic flow behavior.
« They start to move together like molecules in a fluid flow Said Mr. Liao. “” They derive together at the same speed, such as the flow of a fluid in a pipe. This process, called “electron-phonon coupled hydrodynamics”, reflects the way in which this combined system flows as a classic liquid. In this process, flow of fluids, heat diffusion and even electrical conduction “can become very similar ».
« We can show that if we take into account this hydrodynamic behavior, the transport of loads on a two -dimensional material can be very effective “He explained,” much more effective than we could expect by simply looking at the frequency of collisions with heat carriers ».
These results have important implications for the design of 2D semiconductors and the possibility of obtaining a very effective electrical conductivity, even at room temperature. One of the means of promoting such efficiency would be to lower temperatures to reduce collisions, explains Mr. Liao. “” Our new idea is that instead of trying to reduce the frequency of collisions, we can simply design the material so that most collision processes preserve the momentum. Thus, even if the load carriers can still lose momentum due to collisions with the heat particles, the total conservation of the momentum of the system will result in the end by low dissipation and very effective transport.
In their article, the researchers demonstrate their concept by studying the disulfide of Molybdenum (MOS2) of atomic thickness, and note that the mobility of the loads can be multiplied by almost seven when taking into account hydrodynamic behavior.
« This is a very significant improvement He concluded. In addition to offering a more practical alternative to super-temperature superconductivity, focusing on the capacity of the material to accommodate hydrodynamic electron-phonon behaviors “ can be very promising for microelectronic applications ».
Article : « Coupled Electron-Phonon Hydrodynamics in Two-Dimensional Semiconductors » – DOI : 10.1103/PhysRevLett.134.226301
Source : UCSB
