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 or Si 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.
Critical thickness and strain relaxation of c-plane GaN layers grown by molecular beam epitaxy on AlN were studied as a function of growth temperature and threading dislocation density (TDD). For this purpose we used AlN/sapphire templates and AlN single crystals with TDDs of ~109 cm−2 and ~103 cm−2, respectively. Whereas at high growth temperature (900 °C) the critical thickness for plastic relaxation is only 3 monolayers (MLs) for both substrates, this value drastically increases when decreasing the growth temperature. It reaches ~30 MLs when GaN is deposited at 750 °C on AlN single crystals. We also observed that the strain relaxation rate strongly depends on TDD. These results exemplify the lack of efficient gliding planes in III-nitrides when grown along the c-axis, which, combined with low kinetics, allows for plastic relaxation to be frozen out. Achieving pseudomorphic GaN layers on AlN is of interest for two-dimensional electron gases based on AlN/GaN/AlN heterostructures lattice-matched to AlN single crystal substrates. Source:IOPscience