Generation of Silicon Nanocolumns by Nanosecond Pulsed-Excimer Laser Irradiation and Their Field Emission Properties
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Generation of Silicon Nanocolumns by Nanosecond Pulsed-Excimer Laser Irradiation and Their Field Emission Properties Yingfeng Guan, A. J. Pedraza, E. D. Ellis1 and L. R. Baylor2 Dept. of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996 1 Dept. of Electrical and Computer Eng., The University of Tennessee, Knoxville, TN 37996 2 Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT Laser-generated silicon microcone arrays were used as templates for the growth of nanocolumns using laser irradiation as well. The formation mechanism of the microstructure is briefly reviewed, and the origin and growth of nanocolumns are discussed. The formation mechanism of nanocolumns requires highly localized melting, which explains why they fail to form on a flat surface but can grow atop the microcones. Field emission properties from both microcolumns and nanocolumns have been measured. The high aspect ratio (height/tip radius) of nanocolumns makes them suitable for various field emission applications. INTRODUCTION Laser-induced surface modification strongly depends on fluence, as well as on the nature and pressure of the background atmosphere used in the irradiation chamber [1, 2, 3]. For instance, after a few hundred laser pulses at a fluence of 2.5 J/cm2 , shallow depressions exhibiting some crystallographic features appear on an initially flat silicon surface [4]. On additional irradiation under 500 mbar of sulfur hexafluoride (SF6 ), microholes start forming at the bottom of the undulations due to laser ablation enhanced by etching. SF6 promotes the etching process by forming volatile compound of silicon and fluorine [5]. Continuing irradiation, microcones start forming surrounding the holes. Finally, after a total of 800 pulses, a microcone-microhole structure develops on the surface. The microholes can reach a depth of over 100 µm and the microcones can protrude 30 µm relative to the initial surface [4]. It has been proposed that the growth of microcones is due to the transport of silicon-rich molecules in the vapor phase, from the bottom of the holes to the microcone tips. The melted microcone tips are preferential sites for deposition because, in the liquid state, they have a large sticking coefficient [2]. Field emission (FE) arrays made of silicon have been investigated extensively for applications in microelectronics, flat panel displays and electron multipliers devices [6, 7]. In particular, Carey et al [8] investigated FE from silicon microspike microstructures produced by femtosecond laser. In this paper, the formation of nanocolumns using a microcone microstructure as a template is investigated and the field emission properties of these nanostructures are assessed. EXPERIMENTAL DETAILS Test grade p-type, boron doped silicon wafers of (111) orientation were used in the experiments. The native oxide layer present on the silicon wafer was removed by etching in 10:1 HF solution. Silicon substrates were irradiated using a Lambda Physik LPX-305i KrF excimer laser (wavelength=2
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