Formation of resonant bonding during growth of ultrathin GeTe films

A highly unconventional growth scenario is reported upon deposition of GeTe films on the hydrogen passivated Si(111) surface. Initially, an amorphous film forms for growth parameters that should yield a crystalline material. The entire amorphous film then crystallizes once a critical thickness of four GeTe bilayers is reached, subsequently following the GeTe(111) || Si(111): GeTe[-110] || Si[-110] epitaxial relationship rigorously. Hence, in striking contrast to conventional lattice-matched epitaxial systems, a drastic improvement in atomic order is observed above a critical film thickness. Raman spectra show a remarkable change of vibrational modes above the critical thickness that is attributed to a change in the nature of the bonds: While ordinary covalent bonding is found in ultrathin films, resonant bonding can prevail only once a critical thickness is reached. This scenario is further supported by density functional theory calculations showing that ultrathin films do not utilize resonant bonding in contrast to the bulk phase. These findings are important not only for ultrathin films of phase-change materials such as GeTe and GeSbTe, which are employed in phase-change memories, but also for thermoelectrics and topological insulators such as Bi2Te3 and Sb2Te3, where resonant bonding might also have a significant role.