Formation of resonant bonding during growth of ultrathin GeTe films

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 Bi 2 Te 3 and Sb 2 Te 3 , where resonant bonding might also have a significant role.

Chalcogenides by Design: Functionality through Metavalent Bonding and Confinement

“…[ 72 ] In contrast, when GeTe is grown using the same conditions on the Si(111)–(√3 × √3)R30°–Sb surface, even at lower temperatures like 230 °C, it directly grows with a crystalline structure. [ 72 ] When the GeTe film is grown beyond four BLs on the Si(111)–(1 × 1)–H surface it switched to an epitaxial structure (for the entire film). The observed behavior can be well explained by (de)stabilizing effects of MVB, where, e.g., above a critical thickness a transition from localized covalent bonding in the amorphous phase to MVB in the crystalline phase is observed.…”

“…However, ultrathin films of GeTe grown by molecular beam epitaxy (MBE) on a Si(111)–(1 × 1)–H substrate surface at a temperature as high as 260 °C, well above the crystallization temperature of bulk GeTe of 200 °C, turn out to be initially (up to four bilayers (BLs)) amorphous. [ 72 ] In contrast, when GeTe is grown using the same conditions on the Si(111)–(√3 × √3)R30°–Sb surface it directly grows with a crystalline structure. [ 72 ] When the GeTe film is grown beyond four BLs on the Si(111)–(1 × 1)–H surface it switches to an epitaxial structure (for the entire film).…”

“…[ 72 ] In contrast, when GeTe is grown using the same conditions on the Si(111)–(√3 × √3)R30°–Sb surface it directly grows with a crystalline structure. [ 72 ] When the GeTe film is grown beyond four BLs on the Si(111)–(1 × 1)–H surface it switches to an epitaxial structure (for the entire film). These results show that the details of the initial surface are crucial for the type of structure that is formed in the thin film and therefore its properties (see another example in ref.…”

A Review on Disorder-Driven Metal–Insulator Transition in Crystalline Vacancy-Rich GeSbTe Phase-Change Materials

“…In addition to thermal annealing, pressure [85,107,108,109,110,111], strain [112,113], chemical composition [14,16], ion bombardment [114], focused electron beam irradiation [92], electric field [115,116], voltage pulses [117,118,119], laser and photonic excitation [120,121], and interface template [93,122,123] are shown to be effective approaches in tuning disorder in GST and related compounds. It is also worth mentioning that the remaining disorder in ultrathin hexagonal GST films ~7–14 nm would result in a weak anti-localization phenomenon.…”

Growth of crystalline phase change materials by physical deposition methods

“…Recent studies also observed this transition for the growth of GeTe on Si(1 1 1)-7 × 7, [49] but not for the growth of GeTe on Si(1 1 1)-√3 × √3-Sb [50], which is consistent with this picture. Wang et al related this transition to the different types of bonding present in conventional semiconductors, such as GaSb and Si, and PCM, such as GST [51].…”