Investigation on Laser-Induced Effects in Nanostructure Fabrication with Laser-Irradiated Scanning Tunneling Microscope
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Investigation on Laser-Induced Effects in Nanostructure Fabrication with Laser-Irradiated Scanning Tunneling Microscope Tips in Air Ambient Z. H. Mai, Y. F. Lu, W. D. Song, and W. K. Chim Laser Microprocessing Laboratory, Department of Electrical Engineering and Data Storage Institute, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ABSTRACT In this paper, we report our investigation on the kinetics of nanostructure fabrication on gold films and on H-passivated Ge surfaces. The relationship between the current and the tipsample distance of the STM junction was measured for both gold films and H-passivated Ge surfaces. The tip-sample distance for gold films under a electrochemically etched W tip is approximately 2 nm, while that for H-passivated Ge sufaces is more than 27 nm. The thermal expansion length of the tip under laser irradiation was calculated. From the comparison of the thermal expansion length and the tip-sample distance, we can reach the conclusion that for gold films, thermal mechanical indention is the primary reason of nanostructure formation, while for H-passivated Ge surfaces, optical enhancement is the only reason. 1. INTRODUCTION Various techniques have been developed for nanostructure fabrication since the invention of the STM. Applications of the laser-assisted STM arouse wide interest after Cutler, et al., proposed the use of a laser-STM combination for the measurement of a tunneling time and suggested possible laser-STM studies. One of the promising applications of the laser-assisted STM is nanostructure fabrication. Recently, several researches on the nanostructure fabrication using pulsed lasers in combination with an STM have been reported. 2-10 In this paper, we investigate mechanism of nanostructure fabrication on gold films and H-passivated Ge and Si surfaces using lasers in combination with an STM. Current-distance curves for tip-sample junctions were measured. An analytical model was established to distinguish the dominant mechanism. 2. EXPERIMENTAL Gold films with a thickness of 100 nm were deposited with physical vapor deposition. ptype Ge (100) wafers with a resistivity of 8-12 Qcm were H-passivated in a 50% HF solution. ntype Si (100) wafer with a resistivity of 10-30 Qcm were H-passivated in a 5% HF solution. Electrochemically etched STM tips were homemade from a 0.5 mm W wire Nanostructures, such as dots and lines, were created on gold films and H-passivated Ge and Si surfaces. The dependence of the height or depth of the nanostructures was measured. Current-distance curves of the STM junctions were measured and compared with calculated thermal expansion of the tip. In the air ambient, thin water layer forms on any solid surface.'' Water layers will influence the tip-sample distance for STM imaging. The curves of current versus tip-sample
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distance for an EC etched W tip on a gold film and on an H-passivated Ge surface were measured in air at a relative humidity of 88%. The tip, firstly, was automatically approached the sample surface at a pres
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