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. A notable change in bonding mechanism tunes the crystalline order of energy-harvesting films above a critical nanoscale thickness. Turning chalcogenide compounds, such as germanium telluride, into thermoelectric materials that convert waste heat into electricity requires careful balancing of conflicting material properties. Raffaella Calarco from the Paul-Drude-Institut für Festkörperelektronik in Germany and colleagues now report that ultrathin films of germanium telluride have thickness-dependent structures that are tunable using different surface passivation techniques. Using Raman spectroscopy, the team found that at a thickness of four bilayers amorphous germanium telluride undergoes a remarkable phase transition to a crystalline state. Theoretical analysis helped deduce that this effect was due to the emergence of resonant bonding, a form of chemical bonding characterized by pronounced electron delocalization. Initially formed amorphous GeTe layer crystallizes during growth by molecular beam epitaxy, as critical film thickness is reached. A remarkable change in bonding mechanism and improvement in atomic order are observed, in striking contrast to conventional lattice-matched epitaxial systems. Supported by density function theory calculations, resonant bonding is shown to be less favorable in an ultrathin GeTe film.