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Abstract
Single-crystalline Si1−xGex thin films on Si (100) with low threading dislocation density (TDD) are highly desired for semiconductor industrials. It is challenging to suppress the TDD since there is a large mismatch (4.2%) between Ge and Si—it typically needs 106–107/cm2 TDD for strain relaxation, which could, however, cause device leakage under high voltage. Here, we grew Si1−xGex (x = 0.5–1) films on Si (001) by low temperature molecular beam epitaxy (LT-MBE) at 200 °C, which is much lower than the typical temperature of 450–600 °C. Encouragingly, the Si1−xGex thin films grown by LT-MBE have shown a dramatically reduced TDD down to the 103–104/cm2 level. Using transmission electron microscopy (TEM) with atomic resolution, we discovered a non-typical strain relaxation mechanism for epitaxial films grown by LT-MBE. There are multiple-layered structures being introduced along out-of-plane-direction during film growth, effectively relaxing the large strain through local shearing and subsequently leading to an order of magnitude lower TDD. We presented a model for the non-typical strain relaxation mechanism for Si1−xGex films grown on Si (001) by LT-MBE.
Highlights
Single-crystalline Si1−x Gex alloy films have been an important material system due to their tunable bandgaps, strains, and lattices, and they can be tuned to match III-V semiconductors [1,2]
We presented a model for the non-typical strain relaxation mechanism for Si1−x Gex films grown on Si
The threading dislocation density (TDD) in Si1−x Gex (x = 0.5–1) films grown on Si (001) can be controlled to the level of 106 –107 /cm2 [14]
Summary
Single-crystalline Si1−x Gex alloy films have been an important material system due to their tunable bandgaps, strains, and lattices, and they can be tuned to match III-V semiconductors [1,2]. They have become a more attractive topic for several reasons, such as to integrate GeSn on Si to produce direct bandgap Si photonic devices and to produce high-frequency microwave devices on Si [3,4,5,6,7]. The LT-MBE (a few hundred degrees lower than the typical values [8]) was reported to be helpful for growing single-crystalline thin films with a high mismatch between substrate and the film [24,25]. TEM with atomic resolution has been used to characterize these films, and the film strain relaxation mechanism has been proposed
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