This study focused on the deposition of superconductor thin films
by aerosol assisted MOCVD. The substrate was glued to a stainless steel
susceptor which was inductively heated in a cold wall type reactor. We
used metalorganic precursors such as triphenylbismuth, Bi(C6H5)3,
and the beta-diketone derivates of 2,2,6,6-tetramethyl-3,5-heptanodione
(tmhd), Sr(tmhd)2, Ca(tmhd)2 and Cu(tmhd)2.
Their quality particularly in the case of Sr(tmhd)2 was fundamental
for obtaining compositions likely to be reproduced. We used LaAlO3,
SrTiO3 and MgO (100) monocrystals as substrates. We investigated
the effects of deposition parameters on the relative amount of phases present
on the films, on epitaxy, nucleation and microstructure as well as the
effects of the in situ annealing on film properties, namely the transition
temperature into the superconductor state, Tc.
We deposited Bi2Sr2Ca2Cu4Oy
nominal composition films at a deposition rate of 35 nm/min. On the best
deposition conditions, T=760°C, pO2=500 Pa, we obtained
Bi-2212 phase films with intergrowths of the Bi-2223 phase. The 2212/2223
intergrowth level depended on the film’s Sr/(Sr+Ca+Cu) composition ratio.
Sr/(Sr+Ca+Cu) ratios close to 0,28 promoted a high level of Bi-2223 phase
on the 2212/2223 intergrowths. The films deposited on the MgO substrates
also revealed the presence of the CuO phase.
In all substrates the Bi-2212 phase grew with the c-axis perpendicular
to the surface. This phase’s growth was epitaxial on LaAlO3 and SrTiO3
substrates, with in plane orientations of [100]Bi-2212||[110]LaAlO3 and
[100]Bi-2212||[110]SrTiO3.
On MgO substrates the Bi-2212 phase was misoriented with the following
in plane orientations: [100]Bi-2212 || [110]MgO, [100]Bi-2212 || [100]MgO
and [700]Bi-2212 || [920]MgO.
The fact that the deposition results strongly depended on the substrate
temperature led to the development of a pulsed deposition technique called
pulsed aerosol assisted MOCVD. This alternative technique allows the reduction
of secondary phase deposition and the improvement of reproductability.
On the deposition initial stages a Frank van der Merwe (2D) type nucleation
was found which formed a film covering the whole substrate. Both the substrate
and the recently form film’s defects promote the nucleation of trimensional
particles which increase as the film grows thicker. For films thicker than
100 nm these particles make for a high level of roughness.
In situ annealing, pO2>81 kPa, T=760°C, led to the Tc onset at
115 K revealing the presence of Bi-2223 phase. With an annealing time of
60 hours we found a zero resistivity Tc of 74 K. Long annealing times at
temperatures over 760°C caused Bi-2212 phase to decompose and the Tc
to decrease. After a 2 min annealing at a temperature of 863°C, we
obtained a transition into zero resistivity at 73 K, without the Bi-2212
phase being decomposed.
The critical current density, measured at 5 K, was Jc=3,5x106
A cm-2.
We concluded that the films deposited, according to the studied conditions
interval, still show limited values in the transport properties as superconductors,
namely a large transition temperature, as a consequence of the high deposition
rate used. In order to optimize the film properties and to obtain a Bi-2223
single phase, the film composition must be carefully controlled and the
deposition rate must be lower than 3 nm/min.
In future studies, the pulsed aerosol assisted technique may be used
for multilayers deposition with recourse to independent aerosol generators,
allowing for the manufacture of heterostructures likely to be applied in
electronic devices.