CHEMICAL VAPOR DEPOSITION OF THIN FILM MATERIALS FOR ...

4.2 DLI-CVD Growth of NiFe2O4 Films
Nickel ferrite (NiFe2O4) is a well known insulating and high permeability magnetic
material that is commonly used in transformer cores and microwave applications.[73] It
possesses a spinel (Fd 3 m) structure (a = 0.834 nm) with the tetrahedral A sites occupied by half
of the Fe 3+ cations, whereas the remaining Fe 3+ and Ni 2+ cations are distributed over the
octahedral B sites (Fig.15). Recently, much attention has been attracted to the growth of epitaxial
spinel ferrite thin films for magnetoelectric applications and monolithic microwave integrated
circuits (MMIC).[43,77] For instance, the magnetoelectric effect in magnetic/ferroelectric
heterostructures is promising for tunable microwave devices, such as filters and phase shifters,
offering much faster tuning response and lower power consumption.[46]
As mentioned in chapter 2, a wide variety of thin film growth techniques have been
developed and investigated in the past for nickel ferrite (and with divalent cation substitutions).
However, the achievement of both high quality (structural, morphological, magnetic and
microwave properties) and thick (1-10 micron range) epitaxial films for microwave device
applications remains challenging.[73,78] Chemical Vapor Deposition (CVD) offers a number of
advantages over other deposition methods, but its use for nickel ferrite film deposition has been
limited by the low volatility and thermal stability of available chemical precursors. The direct
liquid injection (DLI) technique, which can generate vapor from a liquid solution source at
relatively low temperatures, has in recent years been successfully utilized for a number of metal-
organic chemical vapor deposition processes.[110-112] DLI-CVD is a particularly useful method
for multi-component systems as several precursors can be dissolved in the same solution and the
relative delivery rates of precursors are defined by the solution composition. Thin film growth
rate can be extremely high due to the possible high rate of precursor introduction into the thin
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