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.
Highlights
A fundamental understanding of the crystallization process of amorphous silicon (a-Si) and germanium (a-Ge) is relevant to the synthesis of a variety of technologically important thin-film devices used in electronic and opto-electronic industries such as solar cells, thin film transistors, etc.[1,2] In addition, the crystallization process is applicable for producing the array of nanocrystalline quantum dots with efficient photoluminescence.[3,4,5,6] It is assumed that crystallization behavior depends largely on the initial amorphous structure
The present study revealed that crystallization temperature of a-Ge films, of the fixed thickness of 40 nm, varies from 500◦C to 600◦C depending on thermal history, with corresponding variations in the amorphous structure and the crystallization behavior from coarse particles with a hexagonal structure to homogeneous nanograins of the diamond cubic structure
The present results indicate that the effect of isothermal annealing at 350◦C and 500◦C is similar to that of long-term aging and low-flux electron irradiation at room temperature: it leads to disintegration of the d-medium-range ordered (MRO) clusters
Summary
A fundamental understanding of the crystallization process of amorphous silicon (a-Si) and germanium (a-Ge) is relevant to the synthesis of a variety of technologically important thin-film devices used in electronic and opto-electronic industries such as solar cells, thin film transistors, etc.[1,2] In addition, the crystallization process is applicable for producing the array of nanocrystalline quantum dots with efficient photoluminescence.[3,4,5,6] It is assumed that crystallization behavior depends largely on the initial amorphous structure From both a scientific and technological perspectives, it is interesting and important to clarify progressive changes from prepared amorphous structures to crystalline phases. We examined changes in the irradiation-induced crystallization microstructure and in the amorphous structure through PDF during isothermal annealing at 350◦C, where the disordering of MRO clusters occurs.[11] On the basis of these results, we discuss the relationship between the instability of MRO clusters and the crystallization behavior
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