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

Abstract

Following our previous studies on crystallization induced by electron irradiation, we have investigated the crystallization of sputter-deposited amorphous germanium films by heat treatments. On continuous heating, samples aged for 3 days and 4 months at room temperature crystallized at 500°C to form coarse spherical particles of a hexagonal structure, of about 100 nm in diameter, whereas samples aged for 7 months turned to homogeneous nanograins of the diamond cubic structure at 600°C. When the films aged for 4 months at room temperature were annealed at 350°C for 2 h and then heated, they crystallized at 550°C to form a mixture of the two microstructures, and those annealed at 350°C and further at 500°C for 1 h crystallized at 600°C mostly to nanograins. Crystallization by electron irradiation at 350°C to 4-month-aged samples has also been studied. With increasing annealing time at 350°C, coarse particles of a hexagonal structure ceased to appear, and were replaced by fine nanograins of the diamond cubic structure. These observations can be understood in terms of structural instability of sputter-deposited amorphous films. Medium-range ordered clusters must initially be present in the films and serve as nuclei of the metastable hexagonal phase. They are unstable, however, and are eliminated by annealing, resulting in the reduction in size and number of coarse particles with a metastable structure.

Structural relaxation probed by resistance drift in amorphous germanium

“…They pointed out that transformation from MRO to CRN occurs only at higher annealing temperature (larger than 500°C for ion-implanted a-Si) [15,16]. Nakamura group found that there was a structural transition from MRO to CRN due to aging at ambient temperature [20], electron irradiation [21], and thermal annealing [22], respectively. We believe that this is the third SR, referred to as MRO-to-CRN Sr.…”

“…Subsequently, Roorda [17] questioned Cheng's opinion, and argued that structure disordering and thermal relaxation are essentially unrelated two processes. In 2016, Okugawa studied SR in a-Ge films by aging at ambient temperature [20], and crystallization of a-Ge by electron irradiation [21], and thermal annealing [22]. They argued that a small fraction of metastable MRO clusters exist in as-deposited samples, and there is a reduction of MRO due to aging at ambient temperature, electron irradiation, and thermal annealing, respectively.…”

Structure of crystallized particles in sputter-deposited amorphous germanium films

“…We have recently reported the relationship between the structure of amorphous germanium (a-Ge) films prepared by sputtering and their crystallization behaviour (Okugawa et al, 2016a,b,c). Sputter-deposited a-Ge films are initially unstable and are stabilized by ageing at room temperature (Okugawa et al, 2016a), thermal treatment (Okugawa et al, 2016c) and lowflux electron irradiation (Okugawa et al, 2016b). Unstable a-Ge crystallizes inhomogeneously to coarse crystalline particles of 100-200 nm in diameter, whereas stabilized a-Ge (Okugawa et al, 2016b) indicated that the fine nanograins were of the diamond cubic structure.…”

Molecular Dynamics Study on Structural Relaxation Processes in Amorphous Germanium

“…The theoretical electric capacities of Si and Ge are 4200 mAh/g (Li 22 Si 5 ) and 1624 mAh/g (Li 22 Ge 5 ), respectively, much larger than that of graphite (372 mAh/g for LiC 6 ), and therefore these materials are anticipated as anode materials for the next generation of lithium-ion batteries. However, large volume changes (>400%) are induced in Si and Ge during charge (lithiation) and discharge (de-lithiation), and consequently exfoliation from the collector and pulverization of these materials occur 1) .…”

“…The electron-beam-induced structures of crystallized Ge depend on the aging time: coarse crystal grains are formed in short-period aged specimens, while a uniform nanoporous structure is formed in long-period aged ones. It has also been con rmed that hexagonal Ge, which is a metastable phase, may be formed by controlling the structural relaxation of amorphous Ge 6) . Although structural relaxation plays an important role in controlling nanostructure, the details of the process on the atomic scale are not clear.…”