Growth Mechanism and Film Properties in Pulsed Laser-Plasma Deposition
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GROWTH MECHANISM AND FIL•f PROPERTIES IN PULSED LASER-PLASMA DEPOSITION S.
METEV, K. METEVA BIAS, Bremen Institute of Applied Beam Technology, 33, Germany
2800 Bremen
ABSTRACT In the paper the results of a theoretical investigation of the growth process of laser-plasma deposited thin films are discussed. A kinetic approach has been used to establish direct relation between experimental conditions (laser flux density, substrate temperature) and film properties (thickness, structure). The results of some experimental investigations of the deposition process are presented confirming the general conclusions of the developed theoretical model. 1.
INTRODUCTION
The pulsed laser-plasma method is a fast developing technique for synthesis of thin-film materials [1-3]. This deposition method is based on physical processes, arrising from the impact of high-power pulsed laser radiation on solid targets and leading to removal of partially ionized material (plasma) from the impact zone [1]. In the last several years, very interesting and sometimes unique results have been obtained due to the possibility of varying over a wide range the process parameters such as plasma flux density, ionization degree, ion energy, substrate temperature and geometrical arrangement in the deposition chamber [1-3]. The properties of the deposited films depend strongly on the mechanism of film growth during the deposition process. The aim of this paper is to establish through theoretical investigation of the growth process a direct relation between experimental conditions and some film properties. 2.
DEPOSITION CHARACTERISTICS
Growth of thin films is a process which depends on many factors such as density, energy, ionization degree and type of the condensing particles, as well as temperature, and physicochemical properties of the substrate. The two main thermodynamic parameters which determine to a great extend the growth mechanism (and the film properties respectively) are the film temperature T and the supersaturation am
Am = kTln(R/Re)
(1)
where k is the Boltzmann constant, R is the actual deposition rate and Re is its equilibrium value at temperature T). It has been shown [1,2] that plasma fluxes in the laserplasma deposition process are cha racterized by a high degree of supersaturation (up to 10D J.mol- ) and ionization (up to 70%), and by a high particle energy (from several eV up to 1000 eV).
Mat. Res. Soc. Symp. Proc. Vol. 236. Q1992 Materials Research Society
418
These characteristics depend on the experimental conditions and could be changed controllably by changing the laser parameters or the geometrical arrangement in the deposition chamber driving in this way (see section 3) the growth mechanism of the thin film and its properties respectively. Another peculiarity of the laser-plasma deposition process, related to the high degree of supersaturation, is the small size of the nuclei (practically one atom) [3] resulting in two-dimentional (2D) nucleation of monotomic height. This 2D-nucleation is favorable for the socalled "la
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