Strain-Modulated L-Valley Ballistic-Transport in (111) GaAs Ultrathin-Body nMOSFETs

Abstract

Electron transport through the quantized L valley of (111) surface orientation is a possible solution to density-of-states bottleneck of GaAs. Quantization splits the L valley into L[111] valley pair and remote L valley pairs, L[111], L[111], and L[111]. The L[111] valley pair projects to the 2-D Brillouin zone center above the Γ valley and its population enhances the drive current. However, for a typical 5-nm thin body, the Γ-L[111] energy separation is still ≈0.17 eV and the Γ valley primarily governs the electron transport. We analyze the compressive biaxial strain effects on the energy levels and effective masses of Γ, L[111], and remote L valleys of a 5-nm (111) GaAs ultrathin body using a sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s*d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> orbital basis tight binding model. We then evaluate the ballistic performance of a 10-nm double-gate device using an effective mass Schrodinger's equation with the extracted masses and band energies from sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s*d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> model. Compressive strain promotes the L[111] and remote L valleys transport by reducing the Γ-L[111] and Γ-remote L valleys energy offsets. This results in a significant improvement in charge density and drive current. A compressive stress larger than 2.5 GPa brings the remote L valleys even below the L[111] valley. Under this stress condition, the remote L[1̅11] and L[11̅1] valleys govern the ON-state transport. The different wavefunction symmetries of L[111] and remote L valleys are the physics behind the stress modulated enhanced L valley transport.