Understanding high-field electron transport properties and strain effects of monolayer transition metal dichalcogenides

Abstract

Monolayer transition metal dichalcogenides (MX2) are promising candidates for future electronics. Although the transport properties (e.g. mobility) at low electric field have been widely studied, there are limited studies on high-field properties, which are important for many applications. Particularly, there is lack of understanding of the physical origins underlying the property differences across different MX2. Here by combining first-principles calculations with Monte Carlo simulations, we study the high-field electron transport in defects-free unstrained and tensilely strained MX2 (M=Mo, W and X=S, Se). We find that WS2 has the highest peak velocity (due to its smallest effective mass) that can be reached at the lowest electric field (owing to its highest mobility). Strain can increase the peak velocity by increasing the scattering energy. After reaching the peak velocity, most MX2 demonstrates negative differential mobility (NDM). WS2 shows the largest NDM among unstrained MX2 due to the strongest effect of electron transfer from the low-energy small-mass valley to the high-energy large-mass valley. The tensile strain increases the valley separation, which on one hand suppresses the electron transfer in WS2, on the other hand allows the electrons to access the non-parabolic band region of the low-energy valley. The latter effect leads to an NDM for electrons in the low-energy valley, which can significantly increase the overall NDM at moderate strain. The valleyseparation induced NDM in the low-energy valley is found to be a general phenomenon. Our work unveils the physical factors underlying the differences in high-field transport properties of different MX2, and also identifies the most promising candidate as well as effective approach for further improvement.

Directly Visualizing Photoinduced Renormalized Momentum-Forbidden Electronic Quantum States in an Atomically Thin Semiconductor

“…Our result with a relatively moderate excitation fluence of approximately ten μJ/s cm –2 experimentally confirms that the energy renormalization effect can be effective even below the Mott threshold of 10s of meV, which is also reasonably supported by the theoretical prediction . This finding supports the potential valley-electronic applications of TMDs at the Q valley, such as the Mott transition, , Q valley TMD transistors, , and negative differential conductance. , …”

“…In 2D TMDs, photoexcitation or hot carrier injection has provided a powerful tool to control the valley degree of freedom. , The excessive carriers confined in the layered structure of TMDs lead to considerable many-particle interactions. − This many-body system, bringing the energy renormalization effect and shrinking the quasiparticle bandgap in the valley band structure at the K points, has been studied using optical technologies in the past decade. , Recently, another local minimum of the conduction band (CB) has been noted to lie midway between the K and Γ points, higher by a few meV than the K point, which is called the Q valley , and has been theoretically reported to strongly couple with the K valley on the CB edge. − Intriguingly, the Q valley was theoretically predicted to be more sensitive to the energy renormalization effect than the K valley due to its differing orbital character and the nonsignificant hole population at the Q point . Therefore, the CB K - Q valley coupling will be dramatically strengthened under the energy renormalization effect with increased carriers, altering the electronic transport and optoelectronic properties in TMD devices. − Thus, providing a complete understanding of the underlying physics and developing control of the electronic behavior at the Q valley become key to future valleytronic technologies.…”

Electronic transport properties of WS2 using ensemble Monte Carlo method

“…On the other hand, Jin et al [44] show that WS 2 has a higher saturation velocity at fields > 100 kV cm −1 and it appears that there is no NDC behavior in their results. At the same time, in a study conducted by Zhang et al, it is shown that WS 2 has the highest peak velocity and the strongest NDC among MoS 2 , MoSe 2 , and WSe 2 [54].…”

“…The electron transport properties of WS 2 at high fields are investigated by several researchers [33,54,60] where they represented the velocity-field characteristics of the material, studied the relative electron populations of two valleys (K and Q valleys) as a function of applied electric field and reported on the negative differential conductance (NDC) phenomenon observed in experiments [33]. In addition, the same authors studied the effect of strain on the energy separation between K and Q valleys which is also investigated by [45,54,[60][61][62][63][64][65]. Ferry [60] found that MoS 2 has a higher saturation velocity than WS 2 at high fields and WS 2 begins to show NDC for ∆E KQ > 100 meV where ∆E KQ is the energy separation between the minima of K and Q valleys (see figure 1).…”

Role of flexural phonons in carrier mobility of two-dimensional semiconductors: free standing vs on substrate

“…Alternatively, one can first calculate the g for a sparse grid using DFPT, and then interpolate to the fine grid. Wannier interpolation using EPW software [27][28][29] can interpolate the short-range part of the g, while it misses some long-range parts including the 2D Fröhlich [16,30,31] and quadrupole interactions [32][33][34][35] that dominate the g with long phonon wavelength (i.e. small |q|).…”