Inter-valley phonon scattering limited performance of n-channel WS₂ monolayer transistors

Abstract In addition to the K conduction band valley of monolayer WS 2 , there is a remote Q valley located 0.1 eV above the K valley. We investigate the effects of intra- and inter-valley phonon scattering, including <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>K</mml:mi> <mml:mo stretchy="false">↔</mml:mo> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> inter-valley scattering, on monolayer WS 2 device performance using a recursive Green’s function algorithm and the Büttiker probe scattering model. Our analysis reveals that ballisticity drops to 54% when only intra-valley phonon scattering is considered. When inter-valley phonon scattering (excluding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>K</mml:mi> <mml:mo stretchy="false">↔</mml:mo> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> inter-valley scattering) is added, ballisticity decreases to 42%. This further declines to 12% with the inclusion of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>K</mml:mi> <mml:mo stretchy="false">↔</mml:mo> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> inter-valley scatterings. Similarly, low-field mobility values decrease successively from 395 cm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>2</mml:mn> </mml:msup> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mtext>V</mml:mtext> <mml:mo>⋅</mml:mo> <mml:mtext>s</mml:mtext> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> to 223 cm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>2</mml:mn> </mml:msup> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mtext>V</mml:mtext> <mml:mo>⋅</mml:mo> <mml:mtext>s</mml:mtext> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> and then to 110 cm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>2</mml:mn> </mml:msup> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mtext>V</mml:mtext> <mml:mo>⋅</mml:mo> <mml:mtext>s</mml:mtext> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> at a sheet density of 10 9 cm −2 . The primary scattering mechanism responsible for this degradation is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>K</mml:mi> <mml:mo stretchy="false">→</mml:mo> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> inter-valley acoustic phonon scattering, characterized by a high deformation potential of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>7.3</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>8</mml:mn> </mml:msup> <mml:msup> <mml:mrow> <mml:mi>eV</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:mi>cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> and a low energy of 16.76 meV. Additionally, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>K</mml:mi> <mml:mo stretchy="false">→</mml:mo> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> inter-valley scattering significantly populates the Q valley, which presents a higher potential barrier in the on-state conduction band profile, resulting in a lower on-state current. Our analysis shows that approximately 40% of the on-state total power dissipated within the device is due to phonon scattering effects, with the remainder being released through thermalization in the device’s contacts.