Effects of insertion of an h-AlN monolayer spacer in Pt-WSe2-Pt field-effect transistors

The growth of two-dimensional hexagonal aluminum nitride (h-AlN) on transition metal dichalcogenide (TMD) monolayers exhibits superior uniformity and smoothness compared to HfO <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mn>2</mml:mn> <mml:mrow></mml:mrow></mml:mmultiscripts> </mml:math> on silicon substrate. This makes an h-AlN monolayer an ideal spacer between the gate oxide material and the WSe₂ monolayer in a two-dimensional field effect transistor (FET). From first principles approaches, we calculate and compare the transmission functions and current densities of Pt-WSe₂-Pt nanojunctions without and with the insertion of an h-AlN monolayer as a spacer in the gate architecture. The inclusion of h-AlN can alter the characteristics of the Pt-WSe₂-Pt FET in response to the gate voltage ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>V</mml:mi> <mml:mtext>g</mml:mtext></mml:msub> </mml:math> ). The FET without (or with) h-AlN exhibits the characteristics of a P-type (or bipolar) transistor: an on/off ratio of around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>2.5</mml:mn> <mml:mo>×</mml:mo> <mml:msup><mml:mn>10</mml:mn> <mml:mn>6</mml:mn></mml:msup> </mml:mrow> </mml:math> (or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>1.7</mml:mn> <mml:mo>×</mml:mo> <mml:msup><mml:mn>10</mml:mn> <mml:mn>6</mml:mn></mml:msup> </mml:mrow> </mml:math> ); and an average subthreshold swing (S.S.) of approximately 109 mV/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mtext>dec.</mml:mtext></mml:math> (or 112 mV/ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mtext>dec.</mml:mtext></mml:math> ), respectively. We observe that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>V</mml:mi> <mml:mtext>g</mml:mtext></mml:msub> </mml:math> shifts the profile of the transmission function by an energy of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>α</mml:mi> <mml:mo>(</mml:mo> <mml:mi>e</mml:mi> <mml:msub><mml:mi>V</mml:mi> <mml:mtext>g</mml:mtext></mml:msub> <mml:mo>)</mml:mo></mml:mrow> </mml:math> , where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math> represents the gate-controlling efficiency. We observed that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>α</mml:mi> <mml:mtext>in</mml:mtext></mml:msub> <mml:mo>=</mml:mo> <mml:mn>83</mml:mn> <mml:mo>%</mml:mo></mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>α</mml:mi> <mml:mtext>out</mml:mtext></mml:msub> <mml:mo>=</mml:mo> <mml:mn>33</mml:mn> <mml:mo>%</mml:mo></mml:mrow> </mml:math> , corresponding to whether the Fermi energy is located inside or outside the band gap. Therefore, we construct an effective gate model based on the Landauer formula, with the transmission function at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>V</mml:mi> <mml:mtext>g</mml:mtext></mml:msub> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn></mml:mrow> </mml:math> as the baseline. Our model generates results that are consistent with those obtained through first principles calculations. The relative error in current densities between model and first-principles calculations is within <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mrow><mml:mo>[</mml:mo> <mml:mfrac><mml:mrow><mml:mi>l</mml:mi> <mml:mi>n</mml:mi> <mml:mo>(</mml:mo> <mml:mn>10</mml:mn> <mml:mo>)</mml:mo></mml:mrow> <mml:mrow><mml:mi>S</mml:mi> <mml:mo>.</mml:mo> <mml:mi>S</mml:mi> <mml:mo>.</mml:mo></mml:mrow> </mml:mfrac> <mml:mo>]</mml:mo></mml:mrow> <mml:mrow><mml:mo>|</mml:mo> <mml:mi>Δ</mml:mi> <mml:msubsup><mml:mi>V</mml:mi> <mml:mrow><mml:mtext>G</mml:mtext></mml:mrow> <mml:mtext>eff</mml:mtext></mml:msubsup> <mml:mo>|</mml:mo></mml:mrow> </mml:mrow> </mml:math> . The 2D atomistic FETs show excellent device specifications and the ability to compete with existing transistors based on traditional silicon technology. Our findings could help advance the design of TMD-based FETs.