Non-Stoichiometric Transfer of Complex Oxides by Pulsed Laser Deposition at Low Fluences
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to ensure a uniform spot. The deposition rate from a stationary single phase target is measured by a quartz crystal monitor (QCM) located 3.5 cm above the target surface. By determining the weight loss of the target after ablation, we estimate that a deposition rate of 0.15 imn/shot corresponds to a target ablation depth of around 15 nm/shot. The repetition rate of the laser is kept constant at 5 Hz. The target surface composition after laser irradiation is investigated i) by X-ray diffraction in a 0-20 geometry, ii) by energy dispersive X-ray analysis (EDX) using 15keV primary electrons and averaging over a 100x100 lim2 area and iii) by Rutherford Backscattering (RBS) using 2MeV He'-ions. The composition of the resulting thin films deposited on Si at room temperature is determined by RBS using the RUMP simulation program. LASER INDUCED TARGET DECOMPOSITION AND THE PHASE DIAGRAM In order to understand the laser-target interaction, let us have a look at the ternary phase diagram of the Y-Ba-Cu-O-system [5]. Taking the tie line (fig. Ia) along YBa 2Cu 3O7 -. Y2BaCuO 5 , it appears that both YBa 2Cu 3OTma (123) and Y2BaCuO5 (211) have incongruent melting points. YBa 2 Cu 3 07-5 melts partially, forming solid Y2BaCuO 5 , which then has to transform into solid Y20 3 before the melting process is completed. Upon resolidification, Y20 3 crystallizes from the melt first. Y2BaCuO 5 is subsequently formed at the expense of Y 20 3, while at around T=-1015 TC in turn Y2BaCuO 5 has to 'dissolve' in order to form YBa 2Cu 3O7 . Ideally, this is a reversible process. Typically, however, during resolidification inclusions of the high melting temperature phase are formed within the YBa 2Cu3 -,07 matrix and low melting temperature phases fill up between the grains. The reactants to form YBa 2Cu 3O 7 are thus separated, making the high melting point inclusions very stable. T "C
T"C TY203 +LLIQUID
1900
1200 211 + L
1000
1440
211+123 13 L
Y2 0 3 + BaO
211
123
BaCuO 2 + CuO
SrO
SrTiO 3
TiO2
FIG. Ib) Schematic phase diagram of the SrO-TiO 2 system showing a congruent melting point for SrTiO3 (after [6]).
FIG. I a) Schematic phase diagram along the 123-211 tie-line in the Y-Ba-Cu-O ternary phase diagram showing the incongruent melting behaviour of 123 and 211 (after [5]).
Applying this line of reasoning to the laser ablation process, it is clear that, at low fluences, phase separation may occur due to incomplete melting of the irradiated target surface, assuming that the consecutive resolidification is too quick to allow for the reverse phase transformation. Once impurity nuclei have formed, these secondary phases will become increasingly predominant upon consecutive laser shots. We expect that this phase separation occurs only at low fluences.
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Since the induced thermal gradients increase with fluence, incomplete melting is less likely at high fluence. Although phase separation is then still possible upon resolidification, these nuclei will be destroyed by the next laser pulse, unless their thermal properties differ
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