R. Kelly et al., GAS-DYNAMIC EFFECTS IN THE LASER-PULSE SPUTTERING OF ALN - IS THERE EVIDENCE FOR PHASE EXPLOSION, Applied surface science, 133(4), 1998, pp. 251-269
Citations number
77
Categorie Soggetti
Physics, Applied","Physics, Condensed Matter","Chemistry Physical","Materials Science, Coatings & Films
The overall light intensity (fluorescence) of the sputtered atoms, ion
s, and molecules has been measured for polycrystalline AlN which was b
ombarded with 248 nm laser pulses in the presence of a background pres
sure of N-2. AlN is unusual in that, in spite of a similar to 6 eV ban
d gap, it is easily rendered highly absorptive of 248, 308, or 694 nm
laser pulses. In fact, since it is well established in other work that
similar to 1.5 J/cm(2) (308 nm) brings AIN to the melting temperature
, similar to 3050 K, we will assume that the fluence used here(similar
to 20 J/cm(2)) was more than enough to bring the target surface first
to a temperature sufficient for normal vaporization but finally to th
e vicinity of the thermodynamic critical temperature, T-tc. As a resul
t a significant quantity of particles can be assumed to have been expe
lled by phase explosion. The tentativeness in the argument rests in th
e problem that some part of the incident fluence beyond similar to 1.5
J/cm(2) will have been consumed in laser-plume interaction. Neverthel
ess there is evidence in work by Pedraza et al. [A.J. Pedraza, J-Y. Zh
ang, H. Esrom, Mater. Res. Sec. Symp. Proc. 285 (1993) 209] that both
AIN and Al respond linearly to the fluence up to at least 6 J/cm(2). I
t was found that the assumed phase-exploded particles decelerated rapi
dly, possibly due to their encounter with the normally vaporized parti
cles, or possibly due to an electric field arising from positive charg
ing of the target surface. The fluorescence maximum (which can be safe
ly assumed to be also a density maximum) was then nearly stationary, a
situation which characterized the lowest background pressures of N-2
(less than or equal to 3.5 Pa). At higher pressures (greater than or e
qual to 3.5 Pa) a second fluorescence maximum appeared nearer the cont
act front and was found to move. Following the suggestion of Horwitz (
1) we take this feature as being an artifact of electrons near the con
tact front diffusing (or scattering) backwards and causing fluorescenc
e which is unrelated to the particle density. From the velocity of the
contact front one obtains explicit information on the mean kinetic en
ergies (E-4) of the particles in the plume (1.5-2 eV), Another estimat
e of E-4 follows from the initial expansion observed from 0-200 ns (1.
5-3 eV). Such energies suggest, independently of the fact that the flu
ence was high (similar to 20 J/cm(2)), that a temperature near T-tc wa
s reached and that phase explosion may have occurred. We finally note
that, however tentative is the claim for phase explosion, it is certai
n that a close relative of phase explosion, due to subsurface heating,
was not involved. This is because the numerical demonstrations of sub
surface heating have been flawed. (C) 1998 Elsevier Science B.V. All r
ights reserved.