Characteristics of strained-germanium (Ge) p-
and n-channel field effect transistors directly on Si (1 1 1) substrates have
been investigated. A strained-Ge layer with a thickness of ~4 nm has been grown
on the relaxed Si/Si (1 1 1) substrate by ultra-high-vacuum chemical vapour
deposition. To improve the oxide/strained-Ge interface, a thin Si-cap layer
with a thickness of 3 nm has been grown on the strained-Ge layer. After the
device process, 1 nm thickness of Si-cap layer remains on the strained-Ge
layer. Thicknesses of all epitaxial layers have been measured by transmission
electron microscopy. Raman spectroscopy measurement on the Si-cap/strained-Ge
layer shows that the strained-Ge layer has a compressive strain of ~1.25%. A
hole confinement shoulder on the capacitance–voltage curve at the accumulation
region has been observed due to carrier confinement at the Si-cap/strained-Ge
hetero-interface. A metal–oxide–semiconductor (MOS) structure on the
strained-Ge layer shows a moderate interface trap charge density of ~2.8 × 1011
cm−2 eV−1. Strained-Ge p- and n-channel field effect transistors show low
off-state leakage currents of ~3.8 × 10−13 A µm−1 and ~6.5 × 10−13 A µm−1,
respectively. Drive currents of strained-Ge p- and n-channel field effect
transistors are enhanced by ~100% and ~40%, respectively, as compared with bulk
Si (1 1 1) transistors. Peak hole and electron mobility of strained-Ge (1 1 1)
field effect transistors at the low effective field are found to be ~110% and
~30% enhancement, respectively, as compared with bulk Si (1 1 1) transistors,
due to high hole and electron mobility enhancement factor as well as
strain-induced lower conduction mass in the strained-Ge channel.
Source:IOPscience
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