First, Al thin films were deposited onto (100) Si substrate by ra

First, Al thin films were deposited onto (100) Si substrate by radio frequency (RF) magnetron sputtering at room temperature. The base pressure of the chamber was about 10−7 Torr and the process pressure was 14 mTorr (Ar flow rate, 16 sccm). Controlling the RF power (50, 100 W) and the Q-VD-Oph sputtering time (10, 20 min), Al films with varying thicknesses (15, 40, 90 nm) were prepared. In the next step, the Al films on Si substrates were subjected to thermal annealing inside a quartz tube that was connected

to vacuum pumps. The pressure inside the tube showed a change within a range of 2 × 10−5 to 8.7 × 10−6 Torr during annealing. Annealing temperature (400°C, 550°C) and time (3, 6, 9 h) were controlled as a variable. Due to the higher thermal expansion coefficient of Al (23.1 × 10−6/K) than that of Si (3 × 10−6/K), the system is slightly bent and compressive stress is stored in Al film. To relieve the compressive stress, diffusional surface flow of Al atoms and outward diffusion of Si atoms occur at elevated temperatures, leading to the formation of Al-Si microparticles. This process is similar

to the on-film formation of nanowire growth (OFF-ON) previously reported [18], but microparticles rather than nanowires are formed as the diffusivities of Al and Si are much larger than those materials used in OFF-ON [19, 20]. Finally, Al films with Al-Si microparticles on Si substrates were naturally cooled down to room temperature. During this cooling step, vacuum pumps were not operated so that surface oxidation occurs. selleckchem Figure 1 Al-Si microparticle formation from Al thin film on Si substrate. Step 1: Al thin film deposition by RF sputtering, step 2: high-temperature annealing, and step 3: cooling down to room temperature. In step 2, compressive stress is stored in Al film due to the difference in thermal expansions of Al film and Si substrate, and interdiffusion of Al and Si atoms is accelerated to relieve this stress, leading to granulation. As a consequence of granulation, the original Al film new is almost exhausted.

The surface morphology of Al films on Si substrates was examined first at micrometer scale using a laser-scanning microscope (LSM, Olympus CLS 4000; Olympus Corporation, Tokyo, Japan). This was conducted on both as-deposited films and heat-treated films. More in-depth morphology study was performed employing a field emission scanning electron microscope (FE-SEM, click here Hitachi S4300; Hitachi High-Tech, Tokyo, Japan) equipped with energy-dispersive x-ray spectrometer (EDX). The electron acceleration voltage was set at 15 kV. Atomic force microscopy (AFM, Veeco Metrology, Santa Barbara, CA, USA) was also utilized for nanoscale analysis and step height measurement. The structure and the composition of heat-treated samples were analyzed using x-ray diffraction (XRD, Philips X’Pert PW3040; Koninklijke Philips N.V., Amsterdam, Netherlands).

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