Reliable metal-graphene contact formation process flows in a CMOS-compatible environment.

The possibility of exploiting the enormous potential of graphene for microelectronics and photonics must go through the optimization of the graphene-metal contact. Achieving low contact resistance is essential for the consideration of graphene as a candidate material for electronic and photonic devices. This work has been carried out in an 8'' wafer pilot-line for the integration of graphene into a CMOS environment. The main focus is to study the impact of the patterning of graphene and passivation on metal-graphene contact resistance. The latter is measured by means of transmission line measurement (TLM) with several contact designs. The presented approaches enable reproducible formation of contact resistivity as low as 660 Ω µm with a sheet resistance of 1.8 kΩ/□ by proper graphene patterning, passivation of the channel and a post-processing treatment such as annealing.

Influence of plasma treatment on SiO2/Si and Si3N4/Si substrates for large-scale transfer of graphene.

One of the limiting factors of graphene integration into electronic, photonic, or sensing devices is the unavailability of large-scale graphene directly grown on the isolators. Therefore, it is necessary to transfer graphene from the donor growth wafers onto the isolating target wafers. In the present research, graphene was transferred from the chemical vapor deposited 200 mm Germanium/Silicon (Ge/Si) wafers onto isolating (SiO2/Si and Si3N4/Si) wafers by electrochemical delamination procedure, employing poly(methylmethacrylate) as an intermediate support layer. In order to influence the adhesion properties of graphene, the wettability properties of the target substrates were investigated in this study. To increase the adhesion of the graphene on the isolating surfaces, they were pre-treated with oxygen plasma prior the transfer process of graphene. The wetting contact angle measurements revealed the increase of the hydrophilicity after surface interaction with oxygen plasma, leading to improved adhesion of the graphene on 200 mm target wafers and possible proof-of-concept development of graphene-based devices in standard Si technologies.

Mid-infrared optical characterization of thin SiNx membranes.

The investigation of the optical constants (e.g., the refractive index n and the extinction coefficient κ) has been performed in the mid-infrared spectrum for various silicon nitride (SiNx) configurations. By exploiting the transfer matrix method formulation, photometric measurements of transmission and reflection have been used for iteratively calculating the optical parameters of interest. To ensure the reliability of the n and κ, the same material from which such parameters were extracted was deposited for three different thicknesses, e.g., 600, 200, and 100 nm. While the former is optically characterized, the remaining two are used for validation purposes. For each experimental/calculated comparison, the average (made over the whole considered spectrum interval) of the relative error never exceeds 1.5%, which ensures the correctness of the given n and κ. For the sake of completeness, a detailed analysis of the intrinsic limitations arising from the very nature of the method will also be conducted.

Metal-Free CVD Graphene Synthesis on 200 mm Ge/Si(001) Substrates.

Good quality, complementary-metal-oxide-semiconductor (CMOS) technology compatible, 200 mm graphene was obtained on Ge(001)/Si(001) wafers in this work. Chemical vapor depositions were carried out at the deposition temperatures of 885 °C using CH4 as carbon source on epitaxial Ge(100) layers, which were grown on Si(100), prior to the graphene synthesis. Graphene layer with the 2D/G ratio ∼3 and low D mode (i.e., low concentration of defects) was measured over the entire 200 mm wafer by Raman spectroscopy. A typical full-width-at-half-maximum value of 39 cm-1 was extracted for the 2D mode, further indicating that graphene of good structural quality was produced. The study also revealed that the lack of interfacial oxide correlates with superior properties of graphene. In order to evaluate electrical properties of graphene, its 2 × 2 cm2 pieces were transferred onto SiO2/Si substrates from Ge/Si wafers. The extracted sheet resistance and mobility values of transferred graphene layers were ∼1500 ± 100 Ω/sq and µ ≈ 400 ± 20 cm2/V s, respectively. The transferred graphene was free of metallic contaminations or mechanical damage. On the basis of results of DFT calculations, we attribute the high structural quality of graphene grown by CVD on Ge to hydrogen-induced reduction of nucleation probability, explain the appearance of graphene-induced facets on Ge(001) as a kinetic effect caused by surface step pinning at linear graphene nuclei, and clarify the orientation of graphene domains on Ge(001) as resulting from good lattice matching between Ge(001) and graphene nucleated on such nuclei.

Tensile Ge microstructures for lasing fabricated by means of a silicon complementary metal-oxide-semiconductor process.

In this work we study, using experiments and theoretical modeling, the mechanical and optical properties of tensile strained Ge microstructures directly fabricated in a state-of-the art complementary metal-oxide-semiconductor fabrication line, using fully qualified materials and methods. We show that these microstructures can be used as active lasing materials in mm-long Fabry-Perot cavities, taking advantage of strain-enhanced direct band gap recombination. The results of our study can be realistically applied to the fabrication of a prototype platform for monolithic integration of near infrared laser sources for silicon photonics.

Hepatitis C: a disease with a wide morphological spectrum?

We describe the pathological findings of 50 biopsies of Mexican (Mestizo, or mixed race, usually Indian and white heritage) patients with hepatitis C infection confirmed by a second generation test. Although half of the patients were asymptomatic, the histological examination revealed advanced stages of disease. Chronic active hepatitis was found in 26 cases, cirrhosis in 23, and acute hepatitis in one case. Common histological changes included steatosis, lymphoid aggregates, and apoptotic bodies, whereas indisputable bile duct damage was observed in only four cases. Comparison with other series in which different types of populations were analyzed revealed a wide morphological spectrum with respect to some histological changes and the type of hepatitis reported. The apparently contradictory results found in the literature indicate the need to apply universal histological criteria in biopsies of patients with hepatitis C.