Measurement of dielectric function and bandgap of germanium telluride using monochromated electron energy-loss spectroscopy.

Using a monochromator in transmission electron microscopy, a low-energy-loss spectrum can provide inter- and intra-band transition information for nanoscale devices with high energy and spatial resolutions. However, some losses, such as Cherenkov radiation, phonon scattering, and surface plasmon resonance superimposed at zero-loss peak, make it asymmetric. These pose limitations to the direct interpretation of optical properties, such as complex dielectric function and bandgap onset in the raw electron energy-loss spectra. This study demonstrates measuring the dielectric function of germanium telluride using an off-axis electron energy-loss spectroscopy method. The interband transition from the measured complex dielectric function agrees with the calculated band structure of germanium telluride. In addition, we compare the zero-loss subtraction models and propose a reliable routine for bandgap measurement from raw valence electron energy-loss spectra. Using the proposed method, the direct bandgap of germanium telluride thin film was measured from the low-energy-loss spectrum in transmission electron microscopy. The result is in good agreement with the bandgap energy measured using an optical method.

Rapid and mass-producible synthesis of high-crystallinity MoSe2 nanosheets by ampoule-loaded chemical vapor deposition.

MoSe2 is an attractive transition-metal dichalcogenide with a two-dimensional layered structure and various attractive properties. Although MoSe2 is a promising negative electrode material for electrochemical applications, further investigation of MoSe2 has been limited, mainly by the lack of MoSe2 mass-production methods. Here, we report a rapid and ultra-high-yield synthesis method of obtaining MoSe2 nanosheets with high crystallinity and large grains by ampoule-loaded chemical vapor deposition. Application of high pressure to an ampoule-type quartz tube containing MoO3 and Se powders initiated rapid reactions that produced vertically oriented MoSe2 nanosheets with grain sizes of up to ∼100 µm and yields of ∼15 mg h-1. Spectroscopy and microscopy characterizations confirmed the high crystallinity of the obtained MoSe2 nanosheets. Transistors and lithium-ion battery cells fabricated with the synthesized MoSe2 nanosheets showed good performance, thereby further indicating their high quality. The proposed simple scalable synthesis method can pave the way for diverse electrical and electrochemical applications of MoSe2.

Method of Ga removal from a specimen on a microelectromechanical system-based chip for in-situ transmission electron microscopy.

In-situ transmission electron microscopy (TEM) holders that employ a chip-type specimen stage have been widely utilized in recent years. The specimen on the microelectromechanical system (MEMS)-based chip is commonly prepared by focused ion beam (FIB) milling and ex-situ lift-out (EXLO). However, the FIB-milled thin-foil specimens are inevitably contaminated with Ga+ ions. When these specimens are heated for real time observation, the Ga+ ions influence the reaction or aggregate in the protection layer. An effective method of removing the Ga residue by Ar+ ion milling within FIB system was explored in this study. However, the Ga residue remained in the thin-foil specimen that was extracted by EXLO from the trench after the conduct of Ar+ ion milling. To address this drawback, the thin-foil specimen was attached to an FIB lift-out grid, subjected to Ar+ ion milling, and subsequently transferred to an MEMS-based chip by EXLO. The removal of the Ga residue was confirmed by energy dispersive spectroscopy.

Characteristics of an Amorphous Carbon Layer as a Diffusion Barrier for an Advanced Copper Interconnect.

The size of the advanced Cu interconnects has been significantly reduced, reaching the current 7.0 nm node technology and below. With the relentless scaling-down of microelectronic devices, the advanced Cu interconnects thus requires an ultrathin and reliable diffusion barrier layer to prevent Cu diffusion into the surrounding dielectric. In this paper, amorphous carbon (a-C) layers of 0.75-2.5 nm thickness have been studied for use as copper diffusion barriers. The barrier performance and thermal stability of the a-C layers were evaluated by annealing Cu/SiO2/Si metal-oxide-semiconductor (MOS) samples with and without an a-C diffusion barrier at 400 °C for 10 h. Microstructure and elemental analysis performed by transmission electron microscopy (TEM) and secondary ion mass spectroscopy showed that no Cu diffusion into the SiO2 layer occurred in the presence of the a-C barrier layer. However, current density-electric field and capacitance-voltage measurements showed that 0.75 and 2.5 nm thick a-C barriers behave differently because of different microstructures being formed in each thickness after annealing. The presence of the 0.75 nm thick a-C barrier layer considerably improved the reliability of the fabricated MOS samples. In contrast, the reliability of MOS samples with a 2.5 nm thick a-C barrier was degraded by sp2 clustering and microstructural change from amorphous phase to nanocrystalline state during annealing. These results were confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy and TEM analysis. This study provides evidence that an 0.75 nm thick a-C layer is a reliable diffusion barrier.

Amorphous TaxMnyOz Layer as a Diffusion Barrier for Advanced Copper Interconnects.

An amorphous TaxMnyOz layer with 1.0 nm thickness was studied as an alternative Cu diffusion barrier for advanced interconnect. The thermal and electrical stabilities of the 1.0-nm-thick TaxMnyOz barrier were evaluated by transmission electron microscopy (TEM) and current density-electric field (J-E) and capacitance-voltage (C-V) measurements after annealing at 400 °C for 10 h. X-ray photoelectron spectroscopy revealed the chemical characteristics of the TaxMnyOz layer, and a tape peeling test showed that the TaxMnyOz barrier between the Cu and SiO2 layers provided better adhesion compared to the sample without the barrier. TEM observation and line profiling measurements in energy-dispersive X-ray spectroscopy after thermal annealing revealed that Cu diffusion was prevented by the TaxMnyOz barrier. Also, the J-E and C-V measurements of the fabricated metal-oxide-semiconductor sample showed that the TaxMnyOz barrier significantly improved the electrical stability of the Cu interconnect. Our results indicate that the 1.0-nm-thick TaxMnyOz barrier efficiently prevented Cu diffusion into the SiO2 layer and enhanced the thermal and electrical stability of the Cu interconnect. The improved performance of the TaxMnyOz barrier can be attributed to the microstructural stability achieved by forming ternary Ta-Mn-O film with controlled Ta/Mn atomic ratio. The chemical composition can affect the atomic configuration and density of the Ta-Mn-O film, which are closely related to the diffusion behavior. Therefore, the 1.0-nm-thick amorphous TaxMnyOz barrier is a promising Cu diffusion barrier for advanced interconnect technology.

Evaluation of ion/electron beam induced deposition for electrical connection using a modern focused ion beam system.

Focused ion beam method, which has excellent capabilities such as local deposition and selective etching, is widely used for micro-electromechanical system (MEMS)-based in situ transmission electron microscopy (TEM) sample fabrication. Among the MEMS chips in which one can apply various external stimuli, the electrical MEMS chips require connection between the TEM sample and the electrodes in MEMS chip, and a connected deposition material with low electrical resistance is required to apply the electrical signal. Therefore, in this study, we introduce an optimized condition by comparing the electrical resistance for C-, Pt-, and W- ion beam induced deposition (IBID) at 30 kV and electron beam induced deposition (EBID) at 1 and 5 kV. The W-IBID at 30 kV with the lowest electrical resistance of about 30 Ω shows better electrical properties than C- and Pt-IBID electrodes. The W-EBID at 1 kV has lower electrical resistance than that at 5 kV; thus, confirming its potential as an electrode. Therefore, for the materials that are susceptible to ion beam damage, it is recommended to fabricate electrical connections using W-EBID at 1 kV.

Quantification of crystallinity using zero-loss filtered electron diffraction.

The quantity of the crystalline phases present in a nanomaterial is an important parameter that governs the correlation between its properties and microstructure. However, quantification of crystallinity in nanoscale-level applications by conventional methods (Raman spectroscopy and X-ray diffraction) is difficult because of the spatial limitations of sampling. Therefore, we propose a technique that involves using energy-filtered electron diffraction in transmission electron microscopy which offers improved spatial resolution. The degree of crystallinity (DOC) was calculated by separating the crystalline and amorphous intensities from the total intensity histogram acquired by the azimuthal averaging of the zero-loss filtered signals from electron diffraction. In order to validate the method, it was demonstrated that the DOC calculated by zero-loss filtered electron diffraction was consistent with the DOC measured by the area ratio using an amorphous silicon on crystalline silicon standard sample. In addition, the results obtained from zero-loss filtered and conventional electron diffractions were compared. The zero-loss filtered electron diffraction successfully provided the reliable results of the crystallinity quantification. In contrast, the DOC measured using conventional electron diffraction yielded extremely variable results. Therefore, our results provide a crystallinity quantification technique that can extract quantitative information about crystallinity of nanoscale devices by using zero-loss filtered electron diffraction with better reliability than conventional electron diffraction. RESEARCH HIGHLIGHTS: The degree of crystallinity can be measured by separating the crystalline and amorphous intensities from the total intensity histogram acquired by the azimuthal averaging of the zero-loss filtered signals from selected area electron diffraction.