Ultrafast Laser Deposition of Semiconductor Nanowires
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Ultrafast Laser Deposition of Semiconductor Nanowires Samuel S. Mao1,2 1 Lawrence Berkeley National Laboratory 2 Department of Mechanical Engineering University of California at Berkeley Berkeley, CA 94720, U.S.A. Email: [email protected] ABSTRACT Pulsed laser deposition (PLD) has been applied to fabricate thin films of a variety of materials. However, formation of micron-sized particulates during conventional nanosecond laser-based deposition process makes it unsuitable for growing high quality nanoscale materials. Owing to a nonequilibrium non-thermal ablation mechanism and characterized by their short pulse duration compared to thermal diffusion time (tens of picoseconds), ultrafast laser pulses are able to produce particulate-free precursor vapor for nanoscale material deposition. In this article, the foundation of non-thermal ultrafast laser ablation will be examined by both experimental and theoretical investigations. Ultrafast laser-induced high-density electron ejection and subsequent build-up of a strong electric field above the target material were observed. Mass spectrometry and electron microscopy measurements confirmed the particulate-free nature of ultrafast laser ablation. Using ultrafast laser-based particulate-free PLD approach, high quality ZnO nanowires were grown on sapphire and silicon substrates. The optical properties of ZnO nanowires, including the external and internal quantum efficiency of nanowire nanolasers, were experimentally determined. In addition, a nanowire UV photodiode based on p-Si/n-ZnO nanowires was successfully fabricated. This first nanowire photodiode device shows good photocurrent characteristics when operated under reverse bias.
INTRODUCTION Pulsed laser deposition (PLD) is a well-established technique for growing thin films from the vapor phase [1]. The process of PLD involves applying a pulsed laser beam to ablate a target material, followed by depositing the precursor vapor onto a substrate. PLD has been successfully employed to fabricate various types of films, for example, semiconductors [2], superconductors [3], and ferroelectrics [4], and magnetoresistant materials [5]. Conventional PLD systems apply excimer lasers for ablation, which typically emit nanosecond pulses at ultraviolet (UV) wavelength. The main technical obstacle for nanosecond PLD is the formation of micron-sized particulates (or droplets) during the ablation process. Particulate generation and subsequent landing on the deposited film surface has so far limited commercial interest of PLD because many thin film applications demand the density of micronsized particles to be less than one per cm2 . Although PLD may look simple, understanding the physics behind particulate formation has proved to be a challenging task.
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THE NEED OF ULTRAFAST LASERS Thermal Ablation Process For ablation with laser pulses longer than the characteristic time for thermal diffusion (a few tens of picoseconds), energy is transferred from the laser-excited electrons (and holes in the case of nonmetals) to the mate
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