Structural transition in sputter-deposited amorphous germanium films by aging at ambient temperature

Abstract

The structure of amorphous Ge (a-Ge) films prepared by sputter-deposition and the effects of aging at ambient temperature and pressure were studied by pair-distribution-function (PDF) analysis from electron scattering and molecular dynamics simulations. The PDFs of the as-deposited and aged samples for 3–13 months showed that the major peaks for Ge-Ge bonds decrease in intensity and broaden with aging for up to 7 months. In the PDFs of a-Ge of molecular dynamics simulation obtained by quenching liquid at different rates, the major peak intensities of a slowly cooled model are higher than those of a rapidly cooled model. Analyses on short- and medium-range configurations show that the slowly cooled model includes a certain amount of medium-range ordered (MRO) clusters, while the rapidly cooled model includes liquid-like configurations rather than MRO clusters. The similarity between experimental and computational PDFs implies that as-deposited films are similar in structure to the slowly cooled model, whereas the fully aged films are similar to the rapidly cooled model. It is assumed that as they undergo room-temperature aging, the MRO clusters disintegrate and transform into liquid-like regions in the same matrix. This transition in local configurations is discussed in terms of instability and the non-equilibrium of nanoclusters produced by a vapor-deposition process.

Thermal crystallization of sputter-deposited amorphous Ge films: Competition of diamond cubic and hexagonal phases

“…Takamori et al 24 interpreted that amorphous structure is inhomogeneous and strain-induced mechanism is operative. We observed a similar phenomenon: the coarse particles of a hexagonal structure grow rapidly and inhomogeneously in irradiation-induced crystallization (previous studies), 10,11 and thermal crystallization (this study). It can be explained in terms of strain in the films including a certain amount of MRO clusters, which assists the rapid growth of coarse particles.…”

“…It can be explained in terms of strain in the films including a certain amount of MRO clusters, which assists the rapid growth of coarse particles. 10 We suggest that the explosive crystallization is of the same nature as the rapid growth of coarse particles of a hexagonal structure, and both originate from the structural instability of the films prepared by vapor deposition.…”

“…1), which were reported in previous papers. 10,11 The crystallization temperatures for the CP and NG microstructures are significantly different: 500 • C and 600 • C, respectively.…”

“…In the previous paper, 10 we suggested that sputter-deposited a-Ge films contain medium-range ordered (MRO) clusters in a continuous random network (CRN) matrix, as described by the paracrystalline (PC) model. 9 The MRO clusters introduced by vapor deposition are probably distorted, 26 and are thus named d-MRO.…”

“…8 Recently, we have found that a structural transition occurs in sputter-deposited a-Ge films by aging at room temperature, which can be understood in terms of the instability of MRO clusters. 10,11 Analyses of pair distribution function (PDF) revealed that the structure of as-deposited samples includes some ordered regions, which becomes more random/disordered with aging at room temperature up to 7 months. In response to the change in the amorphous structure by aging, the crystallization microstructure induced by electron irradiation changes: coarse, spherical particles with a diameter of approximately 100 nm appear in the as-deposited samples and samples aged for 3 or 4 months, while homogeneous, fine nanograins appear in samples aged for 7 months or longer.…”