In recent years, the possibility to grow High Temperature Superconducting(H
TS) or ferroelectric oxide films by MOCVD techniques has been demonstrated
by several authors. These oxide layers (essentially YBa2Cu3O7, BaTiO3, SrTi
O3, ...) can be used in the field of microelectronics (memories, microwave,
antennas, squids, bolometers, ...) but also, with an emerging interest tod
ay, in high current devices (wires, tapes, ...). For all these applications
MOCVD can be attractive, if the growth process can be sufficiently control
led in order to ensure a good homogeneity and reproducibility in the produc
ed layers; but also if high growth rates can be reached. tinder these condi
tions, the advantages of MOCVD are manifold : good growth control, depositi
on on non-planar objects, rather inexpensive set-up compatible with an indu
strial environment.
Nevertheless, during a long time,the lack of suitable precursor materials (
for Barium essentially) has been detrimental for the rapid development of M
OCVD and, despite several important developments in the chemistry of novel
precursors [2,3], only limited evaporation rates and a poor stability can b
e reached today. Most of the metalorganic precursors used belong to the -di
ketonate family, with an extensive use of Y(tmhd)3, Ba(tmhd)2 and Cu(tmhd)2
. The precursors for yttrium and copper have reasonable volatility and stab
ility at moderate temperatures (around 100 degrees C). Only Ba(tmhd)2 has t
o be heated to temperatures higher than 200 degrees C, which affects its lo
ng term vaporisation stability. Oligomerisation can occur which decreases v
olatility, leading to a compositional shift in the gas phase and in the fil
m during oxide deposition. The evaporation temperature for Barium must ther
efore be very precisely controlled and kept relatively low, thus reducing t
he maximum available Barium partial pressure into the deposition zone and l
imiting the growth rate by mass transport towards the substrate.
In order to increase the stability of Chemical Vapour reactions and to impr
ove the growth rate in the deposition process, alternative MOCVD techniques
have though been developed in the last ears.
These processes are largely described in the present paper and carefully an
alysed in terms of Chemical reaction pathways and experimental parameter de
pendence. Their fundamental principle is based on the evaporation of Mixed
Liquid Sources, where the metalorganic precursors are associated with a sui
table solvent and conditioned in small droplets with a controlled size. The
main advantage of the Mixed Liquid Source (MLS) MOCVD, against conventiona
l MOCVD, is that metal-organic precursors are exposed to elevated temperatu
res only during the short time necessary for their evaporation. The composi
tion control and the reproducibility of the process are therefore substanti
ally improved. Furthermore, the Mixed Liquid Source CVD process, due to the
possibility to transport a large amount of precursors to the preheating zo
ne, yields higher partial pressures of the reacting species in the gas phas
e and, consequently, gives rise to an improved growth rate.
The dominating technique is actually computer-controlled injection MOCVD.
This technique has been used for the synthesis of various functional oxides
and for the growth of multilayered nanostructures.
- Amorphous or heteroepitaxial oxides YBa2Cu3O7-x, PrBa2Cu3O7-x, BaTiO3, Sr
TiO3, Ba(1-x)SrxTiO3, MgO, CeO2, Ta2O5, La(1-x)MnO3, La(1-x)(ou Nd)SrxMnO3,
Y2O3, Al2O3, LaAlO3, SiO2, TiO2, ZrO2(Y), TiN, AlN, TiAlN...
- Multilayers, heterostructures and nanostructures : (YB2Cu3O7-x/PrBa2Cu3O7
-x)(n), Al2O3/CeO2/YBa2Cu3O7-x, Ba(1-x)SrxTiO3/YBa2Cu3O7-x, (Ta2O5/SiO2)(n)
, (BaTiO3/SrTiO3)(n), (La(1-x)MnO3/CeO2)(n) ...