Ion bombardment of gallium thin films

Abstract

There has recently been much interest in the structural changes in solids during ion bombardment, including reports of the production of amorphous metallic alloys by implementation of the alloying element to high doses [1] (~ 1017 ions cm-2). Although it has been known for some time that helium ion bombardment of certain crystalline semiconductors (e.g. silicon [2]) can produce an amorphous phase, it is only recently [3] that evidence for the amorphization of a metallic element, gallium, has been seen. In this letter we describe bombardment and annealing experiments (first reported in [4]) carried out independently on gallium. Fig. 1 shows a schematic diagram of the free energy of a general system as a function of temperature. In addition to the stable solid phase (a), the liquid (1) and vapour (v) phases, the curves describing the free energy of various metastable phases (b, c, d) are shown. We see that to form a metastable phase by the cooling of a vapour or liquid it is necessary to supercool the system below the melting-point of the metastable phase (e.g. below Tmb for the b phase). In practice, the phase which then crystallizes from the undercooled system is decided by "kinetic" factors controlling nucleation and growth of the solid [5]. A system can be undercooled in two ways: by "static" or slow undercooling, usually of micronsized droplets [6], or by "dynamic" undercooling during the rapid quenching of a fluid. This latter process occurs during thin-film deposition or splatquenching of liquids. Both processes are known to be effective in producing metastable (crystalline and amorphous) phases. Although the dynamic processes occurring during ion bombardment are still not fully understood, it is clear that a system immediately after the impact of a high-energy ion is in a highly excited and thermodynamically unstable state. While it may be necessary to describe the damage "spike" as a hydrodynamic process (involving pressure and density changes), rather than a simple local melting or vapourization of the solid, it is likely that the system can explore one or more of the metastable structural states shown in Fig. 1 during its return to thermodynamic equilibrium. Thus systems such as gallium which show metastable phase formation during static or dynamic undercooling might also be expected to form metastable phases during ion bombardment. Bombardment experiments were carried out using gallium or argon ions produced on a 100 kV heavy-ion accelerator [7]. Gallium samples were evaporated in situ from alumina boats onto sapphire (for electrical measurements) and thinfilm carbon (for electron diffraction) substrates during the same evaporation. After deposition, both samples were bombarded in turn at 13K with the same ion dose. Electron diffraction, 4-probe a.c. and 2-probe d.c. electrical measurements (using pre-evaporated chromium contacts