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The capped Si expels completely the Ge dimers on top of the epi Ge(001)-2 × 1 and exhibits a multiphase electronic structure consisting of strained surface atoms bonded with Ge in the film. The deposition of molecular oxygen at room temperature removes a portion of the segregated Ge atoms and oxidizes some of the rest. The surface Si atoms exhibit 1+ to 4+ charge states. Heat eliminates the embedded Ge in the capped Si film and transfers the oxygen bonded with the surface Ge to Si. The oxidation reactions occur primarily on the capped Si surface region without involving the Si/Ge interface.

By using synchrotron radiation photoemission, this study investigates the room-temperature surface electronic structure of Si1−xGex(001)-2 × 1 grown by molecular-beam epitaxy on an epi Ge(001) substrate. The electronic structure of the Ge content, which ranges from 10% to 90%, shows similar surface behaviors. The Ge 3d core-level spectra of Si1−xGex(001)-2 × 1 are essentially the same as that of epi Ge(001)-2 × 1, terminating with tilted Ge–Ge dimers and Ge atoms in the subsurface layer. The strain effect is manifest only for the Si atoms, for which three types are revealed, namely two near the surface region and one in the bulk.

Si1–x Ge x with Ge contents higher than x = 0.5 is expected to boost on-current and improve reliability of p-channel metal-oxide-semiconductor (pMOS) devices. However, unavoidable GeO x formation at the high-κ/Si1‑x Ge x interface with high Ge-contents (HGC) has caused high interfacial trap density, posing a challenge for employing the HGC Si1‑x Ge x as the channel in sub-3 nm complementary MOS technology. In this work, we have deposited epitaxial Si (epi-Si) of six monolayer thickness on Si1–x Ge x at temperatures of 260–280 °C to minimize Ge diffusion and segregation. Si1–x Ge x layers with a wide range of HGC (0.5 < x ≤ 0.8) were grown to investigate the effectiveness of the epi-Si on the Si1–x Ge x . To minimize the GeO x formation caused by the oxidation of the segregated Ge on the epi-Si surface, HfO2 was subsequently deposited via e-beam evaporation on the epi-Si. The measurements using reflection high-energy electron diffraction, high-resolution synchrotron radiation X-ray diffraction, and scanning transmission electron microscopy with high-angle annular-dark-field imaging have revealed the high crystallinity of the epi-Si, the Si1–x Ge x layers, and abrupt interfaces of the high-κ/epi-Si/Si1–x Ge x layers. The well-controlled interfaces have enabled the achievement of low interfacial trap densities (Dit ) of (3–6) × 1011 eV–1 cm–2 in these high-κ/epi-Si/(HGC)­Si1–x Ge x samples. The minimum Dit values remained at 3 × 1011 eV–1 cm–2 regardless of the Ge content, confirming the effective passivation of the low-temperature deposited epi-Si. By extracting the effective charge sheet densities for the Si1–x Ge x gate stacks via examination of capacitance–voltage (C–V) hysteresis with decreasing stress voltage in the accumulation region of the MOS capacitors, we have attained very high acceleration factors of 8–12, indicating high reliability of the HfO2/epi-Si/SiGe pMOS gate stacks.

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