1. Controlling the Thermal Stability of Thin Films by Interfacial Engineering
T.-C. Chiang , University of Illinois at Urbana-Champaign
The behavior of electrons in artificial solid-state structures is radically modified when the structures shrink to atomic (nanoscale) dimensions, directly affecting the electronic operation of device elements. Actually, all other material properties are impacted as well. Fig. 1 shows synchrotron-radiation photoemission data from thin films of Pb grown on an Au-terminated Si substrate, where quantized states show up as bright spots. This quantization leads to a variation in the stability of the films to breakdown at high temperatures as shown in Fig. 2b; films with even numbers of layers are seen to be more stable than those with odd numbers of layers. The quantized states can be altered by adding atoms (here In, Au, or Pb) between the substrate and the film. This changes, in turn, the thermal stability. In Fig. 2, we see that using In atoms at the interface the even/odd pattern of stability can be completely reversed, and by using Au, films stable at room temperature are possible.
In a metallic crystalline solid, the outermost (valence) electrons of the constituent atoms drift off, creating a sea of electrons surrounding a regular array of partially ionized atoms (ion cores). In the simplest picture, electrons within the solid can be regarded as independent, free particles, although they cannot escape the solid as a whole because of the positive charge of the lattice of ion cores. The freedom of movement enjoyed by these electrons is what makes metals good conductors of electricity. Metals are used to interconnect electronic elements inside integrated microcircuits. The intricate patterns of conductors needed are formed by depositing thin metallic films over a stencil pattern. Generally, smaller circuits lead to better performance, greater information storage capacities, and lower costs. Even at microscopic dimensions, this picture of metallic bonding and electrical conduction holds. But when the size approaches atomic dimensions, quantum-mechanical effects from the crystal’s “walls” begin to effect the behavior of electrons.
Thin films are studied here as prototypes for small structures in general. An advantage of a planar film is that it is small in only one dimension: we already know the behavior in the large in-plane directions, so these can be ignored. Quantum mechanics says that the electron has both particle and wave properties, and that its wavelength is inversely proportional to its momentum. Electrons traveling perpendicular to the plane of the film are reflected by the two boundaries, bouncing back and forth. Inside the film, the electron wave function interferes with itself as it is reflected repeatedly. Wave functions can exist only if there is constructive interference, loosely speaking this means that an integral number of whole wavelengths must fit into a round trip through the film. In turn only certain momenta, and energies, are allowed. The situation is analogous to waves on a string of a musical instrument: only certain frequencies occur such that the standing waves fit onto the length of the string. The discrete states in films of different thicknesses can be seen as bright spots in Fig. 1.
This “quantization” of allowed energies obviously will effect the electrical properties of the film, but in fact it impacts all physical properties. An analogy can be made to the periodic table of the elements, on which we can follow the periodic variation in various physical properties, for example, the chemical reactivity or the ionization potential, with increasing atomic number. We know this is due to the filling of discrete energy levels or shells. In the current work, the resistance of a lead (Pb) film, prepared on a silicon (Si) substrate, to breakdown at high temperatures was measured, and found to vary in a regular way with the thickness of the film, as shown in Fig. 2. Like the elements of the periodic table, this is due to the filling of the allowed energy levels as they evolve with increasing film thickness. But unlike the elements, we can modify the pattern of stability by changing the reflective properties of the film’s boundaries. By introducing “interfactant” atoms at the substrate/film interface, the phase of the reflected wave is altered, which changes the wavelengths of the standing waves, which in turn shifts the spectrum of allowed energies. The effect on the thermal stability of adding just a small amount of interfactant atoms is dramatic, as shown in Fig. 2. The even/odd pattern of relatively stable thicknesses is completely reversed when indium (In) atoms are used, whereas using gold (Au) the differences in stability appear to be amplified compared to the other cases.