First-principles nonequilibrium analysis of STM-induced molecular negative-differential resistance on Si(100)

We report on theoretical investigations of scanning tunneling microscopy (STM)- induced molecular negative-differential resistance (NDR) on heavily p-type doped Si(100). Calculations are performed using the density functional theory (DFT) local density approximation (LDA) within the Keldysh nonequilibrium Green's function (NEGF) formalism. The nonequilibrium Hamiltonian is determined self-consistently for molecules on a Si(100) substrate and below a Pt(100) STM tip. We investigate in detail the nonequilibrium conditions which are likely to produce electronic p-type resonant NDR. The discussion is divided into two parts. First, we discuss STM distance dependence and its relation to p-type resonant NDR. It is shown that under high bias conditions electron tunneling is dominated by tunneling near the top of the vacuum barrier thereby preventing resonant NDR at large STM imaging distances. Second, we discuss the self-consistent bias profile and its effect on p-type NDR. It is shown that molecular charging effects may prevent the highest occupied molecular orbital from passing the silicon electrochemical potential, though bistable effects beyond the self-consistent NEGF-LDA method cannot be ruled out.