Abstract
We discuss the impact of free carriers on the zone-center optical phonon frequency in germanium (Ge). By taking advantage of the Ge-on-insulator structure, we measured the Raman spectroscopy by applying back-gate bias. Phonon softening by accumulating holes in Ge film was clearly observed. This fact strongly suggests that the phonon softening in heavily-doped Ge is mainly attributed to the free carrier effect rather than the dopant atom counterpart. Furthermore, we propose that the free carrier effect on phonon softening is simply understandable from the viewpoint of covalent bonding modification by free carriers.
Highlights
Germanium (Ge) is recognized as a promising candidate to replace silicon (Si) for future low-power and high-speed complementary metal-oxide-semiconductor (CMOS) devices because of its higher carrier mobility than that of Si.[1]
This fact strongly suggests that the phonon softening in heavily-doped Ge is mainly attributed to the free carrier effect rather than the dopant atom counterpart
We propose that the free carrier effect on phonon softening is understandable from the viewpoint of covalent bonding modification by free carriers
Summary
Germanium (Ge) is recognized as a promising candidate to replace silicon (Si) for future low-power and high-speed complementary metal-oxide-semiconductor (CMOS) devices because of its higher carrier mobility than that of Si.[1]. The phonon softening was considered to be due to the high free carrier density in heavily-doped semiconductors, as first predicted by Keyes.[9] From this viewpoint, we can expect that phonon properties in modern nano-scale semiconductor devices should be significantly affected by free carrier accumulation in the on-state of FETs because whole thin semiconductor channels are full of free carriers in devices that are called volume-inversion or volume-accumulation. No experiment has been conducted on focusing on free carrier effects in phonon softening This is predominantly due to the fact that distinguishing the free carrier effect from the dopant atom counterpart in heavily-doped semiconductors is difficult.
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