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Spectroscopic characterization of laser-induced plasma during deposition of nanostructures in various background gases

Laser induced plasma or pulsed laser deposition techniques are recently used for deposition of very thin films and nanostructures on various substrates. In this work we have studied the influence of various background gases on nanostructures formation. The experimental apparatus consists of a vacuum chamber for deposition (see figure 1) and nanosecond lasers which are used for ablation [1]. Nd-YAG laser with the wavelength of 1064 nm and pulse width of 4 ns was used to ablate GaAs, Ge or Ti targets at repetition rate of 5 Hz. Substrate to target distance was about 6.5 cm. Laser energy at the target was about 140 mJ. Laser beam was focused with a 33 cm lens. The target was at floating potential while substrate holder was grounded. Laser ablation was performed in vacuum (<10-3 mbar) or in the presence of various gases: argon, helium, oxygen. The background gas was kept at constant flow of about 70 mL/min. The light from plasma plume was collected by lenses to optical fibre coupled to a time gated miniature optical spectrometer. Typical spectrum of GaAs plasma plume is shown in figure 1, right panel. Spectral signatures of neutrals and single and double ionized species could be readily observed. Time resolved spectroscopy allows determination of effective lifetimes for species within the plume [2]. Depo-sition was performed at gas pressures 10-2 mbar, 1 mbar, and 10 mbar for different number of pulses (100, 500, 1000). At each gas pressure images of plasma plume were taken to record the size of plasma plume as shown in figure 2. As background gas increases plasma plume is confined to a smaller volume. There is a range of pressures where plasma ball practically touches the substrate. Samples were analysed by GISAXS and AFM [3]. Nanoparticles were formed on Si substrate by pulsed laser deposition and their growth was studied depending on the deposition conditions. As an example figure 3 shows typical AFM images of GaAs deposited on Si substrate in different helium gas pressures. Comparison of optical emission spectroscopy data and optical plume imaging with various post characterization techniques data for samples allows us to find optimal condition for nano particles formation. Production of nano-scaled materials by means of pulsed laser plasma deposition offers a simple and convenient method accompanied with optical spectroscopy control of the processes.

laser induced plasma; deposition of nanostructures; GaAs

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