Abstract
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.
Highlights
Innovations in materials synthesis often enable breakthroughs in realizing novel technologies
Epitaxial growth has been reported for a class of resonantly bonded chalcogenide compounds, including GeTe, Sb2Te3 and GeSbTe alloys (GST).[2,3,4]
Resonant bonding can be described as a superposition of electronic configurations featuring two center–two electron bonds, somewhat resembling the electronic configuration in, for example, benzene.[5]
Summary
Innovations in materials synthesis often enable breakthroughs in realizing novel technologies. Resonant bonding can be described as a superposition of electronic configurations featuring two center–two electron bonds, somewhat resembling the electronic configuration in, for example, benzene.[5] Resonant bonding in chalcogenides is accompanied by characteristic features, which include large values of the Born effective charge (Z*) and the optical dielectric constant (ε∞).[6] the transition from the amorphous to the crystalline state in materials such as GeTe or Ge2Sb2Te5 is accompanied by a significant increase of ε∞ and Z*.6–8 This observation is in line with a transition from ordinary covalent bonding in the amorphous state, where the atoms have on average three nearest neighbors,[9] that are held by saturated bonds, to resonant bonding in the crystalline state.
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