A.T.S. Wee

Effect of Tip Size on Force Measurement in Atomic Force Microscopy

An atomic force microscope (AFM) has been used to study solvation forces at the solid-liquid interface between highly oriented pyrolytic graphite (HOPG) and the liquids octamethylcyclotetrasiloxane (OMCTS), n-hexadecane (n-C16H34), and n-dodecanol (n-C11H23CH2OH). Oscillatory solvation forces (F) are observed for various measured tip radii (Rtip=15-100 nm). It is found that the normalized force data, F/Rtip, differ between AFM tips with a clear trend of decreasing F/Rtip with increasing Rtip.

Roughening behavior in Si/SiGe heterostructures under O2+ bombardment

Abstract The roughening behavior of 10-layer Si/SiGe heterostructures under 1 keV O 2 + sputtering at 44° incidence angle is accounted for using a modified MRI model. The MRI curve-fitted roughness parameters obtained from the Ge depth profiles are compared with the root-mean-square roughness from AFM scans of crater bottom topography. This comparison indicates that the surface roughening behavior during ion sputtering can be reasonably described by the MRI model. The MRI curve-fitted roughness parameter is found to be systematically larger than the AFM roughness values, and this is explained by additional roughness contributions from mixing straggling and interface roughness during the surface morphology evolution under O 2 + sputtering.

Study of Ta as a Diffusion Barrier in Cu/SiO2 Structure

Tantalum (Ta) thin films of 35 nm thickness were investigated as diffusion barriers as well as adhesion-promoting layers between Cu and SiO 2 using X-ray diffractometry (XRD), Scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). After annealing at 600°C for 1h in vacuum, no evidence of interdiffusion was observed. However, XPS depth profiling indicates that elemental Si appears at the Ta/SiO 2 interface after annealing. In-situ XPS studies show that the Ta/SiO 2 interface was stable until 500°C, but about 32% of the interfacial SiO 2 was reduced to elemental Si at 600°C. Upon cooling to room temperature, some elemental Si recombined to form SiO 2 again, leaving only 6.5% elemental Si. Comparative studies on the interface chemical states of Cu/SiO 2 and Ta/SiO 2 indicate that the stability of the Cu/Ta/SiO 2 /Si system may be ascribed to the strong bonding of Ta and SiO 2 , due to the reduction of SiO 2 through Ta oxide formation.

Effect of ITO Carrier Concentration on the Performance of Organic Light-Emitting Diodes

The indium tin oxide (ITO) anodes for organic light emitting diode (OLED) were made from an oxidised target with In2O3 and SnO2 in a weight proportion of 9:1 using the RF magnetron sputtering method. The comparable ITO anodes with different carrier concentrations were prepared by varying the hydrogen partial pressure during film deposition. The current-luminance-voltage characteristics of the devices indicated that a high carrier concentration in ITO plays a role in improving OLED performance. A maximum efficiency of 3.8 cd/A was achieved when an ITO anode with a higher carrier concentration of 9×10 cm was used in a fluorene based OLED. This efficiency is about 1.5 times higher than that of an identical device made with an ITO anode having a lower carrier concentration of 5×1020 cm-3. The increase in electroluminescent efficie ncy reflects an enhanced hole-injection in the device. We consider that enhanced hole injection is due to the reduced band bending in ITO when it has a high carrier concentration.

Fe-INDUCED CHANGE OF ELECTRON AFFINITY AND SECONDARY ELECTRON YIELD ON DIAMOND

Fe films grown on clean diamond (100) have been studied using synchrotron-based X-ray photoemission spectroscopy (XPS) and ultra-violet photoemission spectroscopy (UPS). The work function and secondary electron yield (SEY) in UPS of the sample were found to change strongly as Fe film thickness increases. After the deposition of about one monolayer Fe, which was found to react with diamond by XPS, work function reaches its minimum while SEY reaches its maximum due to the formation of a dipole layer between the chemisorbed Fe and C atoms at the interface. This study could help the development of high emittance spin-polarized electron source based on magnetic thin films grown on diamond with low or negative electron affinity.

Research Frontiers at SSLS

Singapore Synchrotron Light Source operates five experimental facilities for micro/nanomanufacturing and the characterization of materials and processes. Highlights of results include THz electromagnetic metamaterials, the elucidation of C bonds in the so‐called carbon nanomesh, and the high precision determination of the geometric parameters and the density of thin layers in multilayer systems.

Shaping And Characterising Materials — Recent Results From Singapore Synchrotron Light Source

SSLS offers facilities for micro/nanofabrication and the analytical characterization of devices, materials and processes. Selected applications are presented including the manufacturing of electromagnetic metamaterials for the THz spectral range up to the near infrared, core‐hole clock spectroscopy of the charge transfer from BBB and BFF molecules to an Au electrode, and the characterization of Co and ITO thin films.