Controlling the Thermal Stability of Thin Films by Interfacial Engineering

Thin films are the basic building blocks for solid-state electronic devices, and their simple geometry makes them ideal models for fundamental scientific studies. When the thickness of a film decreases to the atomic scale, the confinement of the film's electrons by its boundaries gives rise to quantized electronic states. These quantum well states are characterized by discrete energies that are highly sensitive both to film thickness and interface conditions. Quantum well states cause the electronic and physical film properties to deviate sharply from those of bulk systems. Controlling these characteristics at a quantum level is critical for thin film applications. Interfactants can modify the boundary condition as a means to deterministically alter such properties of thin films as the thermal stability temperature, the threshold annealing temperature at which the film structure begins to roughen.

This work is an angle-resolved photoemission study of Pb films grown on In-, Au-, and Pb-terminated Si(111) surfaces. By maintaining the same film and substrate while varying the interfactant, we have isolated the interface effect upon the thermal stability temperature. In photoemission spectra, quantum well states appear as sharp, intense peaks that are fully developed at integer monolayer thicknesses. When a smooth layer-resolved film breaks down due to increasing temperature, the intensity of the quantum well peak corresponding to that film thickness displays a sudden drop from which the thermal stability temperature is extracted. The energies of quantum well states are governed by the Bohr-Sommerfeld quantization rule such that the sum of the phase accumulated by an electron traversing the film and the phase shifts it experiences at the film-substrate and film-vacuum interfaces must be an integer multiple of 2 pi. The phase accumulated in the film is dependent upon the Pb band structure and is known from first-principles calculations, as is the phase shift at the vacuum. We fit the observed quantum well energy levels for all systems simultaneously and thus obtained the film-substrate phase shifts for each set of films.

Thermal stability temperatures were measured over a range of thicknesses for Pb films grown on the metal-terminated Si(111) surfaces. All three systems exhibit approximate bilayer thickness-dependent oscillations arising from the periodic crossing of the Fermi level by the quantum well states with increasing thickness. The relative amplitudes and phases differ significantly among the systems. When Pb is grown on the In/Si(111) surface, films composed of odd numbers of atomic layers are more stable than the even ones. This trend is reversed for Pb films grown on both the Pb/Si(111) and Au/Si(111) surfaces in that they display greater stability at even film thicknesses. In both the Pb/In/Si(111) and Pb/Pb/Si(111) systems, the most stable films break down around 250 K. In contrast, the amplitude of the bilayer oscillations is much larger for Pb on Au/Si(111) such that these films exhibit maximum stability temperatures in excess of room temperature. These stability behaviors reflect the differing electronic structures among the systems produced by the interfactants.

The Pb/In/Si(111) system has an electronic structure resembling that of Pb/Pb/Si(111) at thicknesses offset by one atomic layer. Each interfactant creates a unique electronic phase shift at the substrate boundary. The phase shift parameters derived from a fit of the thermal stability curves with a simple Friedel-like functional form compare well to the phase shift constants calculated through quantum well energy analysis. The phase shift difference between the Pb/In/Si(111) and Pb/Pb/Si(111) systems is found to be roughly pi through both methods, which corresponds to the phase reversal of the stability oscillations. These results demonstrate that the thermal stability of thin films can be controlled by means of interfactants. These findings offer important insight into and predictive power about the basic atomic-level physics of thin film electronic and physical properties that are important to the future of device applications.



D. A. Ricci, T. Miller, and T.-C. Chiang