(Invited) Mechanism of Low Temperature ALD of Al2O3 on Graphene Terraces

Abstract

2-D materials are single or trilayer networks of atoms, such as carbon, hBN, and MoS2. 2D materials are attractive for future electronic devices because they provide ballistic transport of charge carriers as well as carrier confinement. These novel devices require ultrathin and defect-free dielectric layers as gate or channel insulators. However 2D semiconductors such as graphene, boron nitride, and metal dichalcogenides (MDCs) are un-reactive; thus, the dielectric layer selectively nucleates on defect sites or step edges. In the conventional atomic layer deposition (ALD) process on graphene or other 2D semiconductors, such non-uniformly grown oxides can create large leakage currents. In the present report, polar metal phthalocyanine molecules have been employed as an ALD template for high nucleation density deposition of dielectric layers. By combining in-situ scanning tunneling microscopy (STM) and fabrication graphene devices, the insulating property and growth mechanism of the dielectric layer have been elucidated. Metal Ptholocyanines have been used to deposit gate oxides on graphene, MoS2, and WSe2. To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy (MBE) as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with four-fold symmetry as measured by scanning tunneling microscopy (STM). ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistor (FETs) formed using this process have a leakage current of 0.046 pA/µm2 at 1 V gate bias with 3.0 nm EOT, which is lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade.