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 νm-1 and ∼6.5 × 10-13 A νm-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.
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