Surface Morphology Studies of Sub-Ps Pulsed-Laser-Deposited AlN Thin Films
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A. Klini, V. Zorba, and A. Manousaki Foundation for Research and Technology—Hellas, Institute of Electronic Structure and Laser (FORTH-IESL), Vasilika Vouton, Heraklion, Crete, 711 10, Greece
C. Fotakis Foundation for Research and Technology—Hellas, Institute of Electronic Structure and Laser (FORTH-IESL), Vasilika Vouton, Heraklion, Crete, 711 10 Greece; and University of Crete, Heraklion, Crete, Greece (Received 23 September 2003; accepted 20 November 2003)
Aluminum nitride thin films were deposited by multipulse KrF* ( ⳱ 248 nm, ∼450 fs) excimer laser ablation of AlN targets in low-pressure nitrogen. We investigated the morphology of the deposited films by scanning as well as transmission electron microscopy, as a function of laser fluence and ambient nitrogen pressure. The AlN films entirely consist of grains (clusters) with average diameters of a few tens of nanometers. In addition, particulates several hundreds of nanometers in diameter (spherical droplets) were observed on the surfaces of the deposited films. Besides these particulates, we noticed the presence of micrometer-size whiskers, or dendritic- and wave-like structures, consisting of agglomerates of nanoparticles. The particulates density decreases with the decrease of the laser fluence, or with the increase of the ambient nitrogen pressure, while their average size increases. This indicates that clustering is the dominant particulates formation mechanism, as a result of the enhanced number of collisions in the fs laser generated ablation plasma.
I. INTRODUCTION
Pulsed laser deposition (PLD) is considered to be a promising technique for thin-films synthesis due to its advantages such as high reproducibility, control of the film growth rate and stoichiometry, low impurity concentration in the composition of deposited films, and the possibility to use different substrate materials.1–4 Moreover, the short interaction time and nonequilibrium conditions allow for the formation of films with properties superior to those obtained by conventional deposition methods. Nevertheless, the main shortcoming of the thin films synthesized by PLD for high-performance electronic, optical, and opto-electronic devices is related to the presence of particulates both on the surface as well as embedded into the bulk of the films. Their origin was attributed to different processes3,5–7: (i) hydrodynamic instabilities evolving across the molten target’s surface and (ii) liquid phase expulsion, under the action of the lasergenerated plasma recoil pressure that causes the ejection from the irradiated surface of spherical droplets with diameters of up to a few micrometers; (iii) explosive dislocation of the substance caused by the subsurface overboiling of the target, and (iv) blast-wave explosion at the liquid–solid 820
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J. Mater. Res., Vol. 19, No. 3, Mar 2004 Downloaded: 18 Mar 2015
interface leading to irregular solid particulates with dimensions in the micrometer range; as well as (v) gas phase condensation (clustering) of the ablated su
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