First-principles simulations of STM images: From tunneling to the contact regime

Abstract

The operation of the STM on metallic surfaces from the tunneling to the contact regime has been explored with a combination of first-principles total energy methods and a calculation of the electronic currents based on nonequilibrium Keldish-Green's function techniques. Our calculations for the behavior of the total energy, forces, atomic relaxations, and currents for an Al tip on an Al(111) surface as a function of the tip-sample distance indicate that atomic relaxations and saturation effects become relevant in a similar distance range where the onset of a short-range chemical interaction between the tip apex and the surface atoms is taking place. These two factors, that have an opposite influence in the current, lead to corrugations of the order of 0.2 Å, similar to the ones found experimentally in other (111) metal surfaces, for the closer distances (around 4.25 Å) where stable operation can be achieved.

Origin of contrast in STM images of oxygen on Pd(111) and its dependence on tip structure and tunneling parameters

“…In our approach, 5,6 the electrical current flowing through the tip-sample system, ͑assumed to be described in an orthogonal local orbital basis͒ in the limit of small T ts that is relevant to the typical tip-sample distances in the experiments, is given by…”

“…12 As mentioned above and as discussed in detail in Ref. 6, the connection between the structural and transport calculations is done through a fast local-orbital DFT technique ͑Fireball2003͒. 7,8 In Fireball, the valence wave functions for either O or the transition metal atoms are expanded in a set of strictly localized pseudoatomic orbitals.…”

“…3 In this paper, we combine standard density functional theory ͑DFT͒ plane-wave ͑PW͒ pseudopotential methods for the accurate determination of the tip and sample structure and electronic properties, with a calculation of STM currents based on nonequilibrium Keldysh-Green function techniques. [4][5][6] The connection between the structural and transport calculations is done through a fast local-orbital DFT technique ͑Fireball2003͒, 7,8 which efficiently maps the electronic Hamiltonian determined from the PW calculation, and can be naturally linked with the transport formalism, which is expressed in terms of localized orbitals. 6 This approach incorporates the influence of the bias and current conditions as well as a realistic description of the tips chemical nature ͑either Pt or Ir͒ used in the experiments and their possible contamination.…”

Atomic scale images of acceptors in III-V semiconductors

“…[48]). Especially when the interaction between tip and sample cannot be neglected the theoretical framework often provides a qualitative description only.…”

Electron transport through electrically induced nanoconstrictions in HfSiON gate stacks

“…10͒ or, in terms of electron transport in QPCs, from tunneling to contact regime. 11,12 Under this condition, there is no electron backscattering and the conductance reaches one quantum unit. 13 In phase B, the creation of new defects leads to the progressive opening of a second bridge ͑dashed line͒ until it reaches phase C. To get an idea about the size of the defects in HfSiON, recall that the radius of conduction considered in percolation models is 0.45 nm.…”