Mechanically transferred large-area Ga${}_2$O${}_3$ passivates graphene and suppresses interfacial phonon scattering

We demonstrate a large-area passivation layer for graphene by mechanical transfer of ultrathin amorphous Ga${}_2$O${}_3$ synthesized on liquid Ga metal. A comparison of temperature-dependent electrical measurements of millimetre-scale passivated and bare graphene on SiO${}_2$/Si indicate that the passivated graphene maintains its high field effect mobility desirable for applications. Surprisingly, the temperature-dependent resistivity is reduced in passivated graphene over a range of temperatures below 220 K, due to the interplay of screening of the surface optical phonon modes of the SiO${}_2$ by high-dielectric-constant Ga${}_2$O${}_3$, and the relatively high characteristic phonon frequencies of Ga${}_2$O${}_3$. Raman spectroscopy and electrical measurements indicate that Ga${}_2$O${}_3$ passivation also protects graphene from further processing such as plasma-enhanced atomic layer deposition of Al${}_2$O${}_3$.