156
5 Compositionally Modulated and Multilayered Deposits
were produced with constant and varying Cu and Co–Ni–Cu layer thicknesses,
respectively. When the Co/(Co+Ni) atomic ratio is displayed as a function of the
Co
2+ /(Co
2+ +Ni
2+ ) ion ratio in the solution, the composition lines deviate the more
from the d.c. composition line, the thinner the Co–Ni–Cu layer.
The explanation is very straightforward in the light of the above discussion.
Namely, the Cu pulse can be taken as a “break” in the Co+Ni deposition, during
which the ion concentration of the less noble metal ions can relax. The shorter the
deposition pulse for the layer containing Co and Ni, the least the depletion of the
preferentially deposited Co can impact the layer composition. Therefore, the analysis of the mean composition of a multilayer series with varying layer thickness can
reveal much useful information on the deposition preference, and, indirectly, on the
composition gradient within an alloy nanolayer.
The spontaneous variation in composition along the growth direction at submicrometre scale can cause long-range composition fluctuations, too, but the origin
of the composition change in such a case is not related to an initial transient. It was
demonstrated for electrodeposited Ni–P alloys [90] that nearly random composition
fluctuations occur over several tens of micrometre. The explanation was the variation
in the hydrogen evolution rate, its complex interrelation with the depletion of the
cathode diffusion layer and the dependence of the hydrogen evolution rate on the
deposit composition.
5.4.2 Structural Features of Deposits With Modulated
Composition
It is very scarce that a conformal full monolayer is produced when the deposition of
the subsequent layer starts. This means that the growth of each new layer starts with a
nucleation process, also in the case when the growth is heteroepitaxial; i.e., when the
crystalline structure does not necessarily change with the composition modulation.
In order to provide a full coverage of the previous layer, the nuclei of the newly
formed layer (that are often called islands when their thickness is much smaller than
their lateral diameter) must coalesce. If the nominal layer thickness is around one
monolayer of even less and the components do not segregate, the resulting deposit
shows a multilayer-to-alloy transition [91].
The coalescence threshold limit is around 1 nm even for metal pairs that exhibit
a lattice misfit of less than 5% but it can be much larger if the conditions of the
heteroepitaxial growth are not fulfilled and there is a significant lattice misfit between
the neighbouring deposit layers. If the nominal layer thickness is smaller than the
threshold thickness of the formation of a full layer, the result of the deposition
can be a granular material. Such a structural transition can be observed in the row
of Co/Cu [92, 93], Co/Ag [38] and Co/Pb [94] deposits where the lattice misfit is
about 2, 14 and 39%, respectively. Co/Cu multilayers show very nice layer structures
over hundreds of nanometres along the direction parallel to the substrate with layer
5 Compositionally Modulated and Multilayered Deposits
were produced with constant and varying Cu and Co–Ni–Cu layer thicknesses,
respectively. When the Co/(Co+Ni) atomic ratio is displayed as a function of the
Co
2+ /(Co
2+ +Ni
2+ ) ion ratio in the solution, the composition lines deviate the more
from the d.c. composition line, the thinner the Co–Ni–Cu layer.
The explanation is very straightforward in the light of the above discussion.
Namely, the Cu pulse can be taken as a “break” in the Co+Ni deposition, during
which the ion concentration of the less noble metal ions can relax. The shorter the
deposition pulse for the layer containing Co and Ni, the least the depletion of the
preferentially deposited Co can impact the layer composition. Therefore, the analysis of the mean composition of a multilayer series with varying layer thickness can
reveal much useful information on the deposition preference, and, indirectly, on the
composition gradient within an alloy nanolayer.
The spontaneous variation in composition along the growth direction at submicrometre scale can cause long-range composition fluctuations, too, but the origin
of the composition change in such a case is not related to an initial transient. It was
demonstrated for electrodeposited Ni–P alloys [90] that nearly random composition
fluctuations occur over several tens of micrometre. The explanation was the variation
in the hydrogen evolution rate, its complex interrelation with the depletion of the
cathode diffusion layer and the dependence of the hydrogen evolution rate on the
deposit composition.
5.4.2 Structural Features of Deposits With Modulated
Composition
It is very scarce that a conformal full monolayer is produced when the deposition of
the subsequent layer starts. This means that the growth of each new layer starts with a
nucleation process, also in the case when the growth is heteroepitaxial; i.e., when the
crystalline structure does not necessarily change with the composition modulation.
In order to provide a full coverage of the previous layer, the nuclei of the newly
formed layer (that are often called islands when their thickness is much smaller than
their lateral diameter) must coalesce. If the nominal layer thickness is around one
monolayer of even less and the components do not segregate, the resulting deposit
shows a multilayer-to-alloy transition [91].
The coalescence threshold limit is around 1 nm even for metal pairs that exhibit
a lattice misfit of less than 5% but it can be much larger if the conditions of the
heteroepitaxial growth are not fulfilled and there is a significant lattice misfit between
the neighbouring deposit layers. If the nominal layer thickness is smaller than the
threshold thickness of the formation of a full layer, the result of the deposition
can be a granular material. Such a structural transition can be observed in the row
of Co/Cu [92, 93], Co/Ag [38] and Co/Pb [94] deposits where the lattice misfit is
about 2, 14 and 39%, respectively. Co/Cu multilayers show very nice layer structures
over hundreds of nanometres along the direction parallel to the substrate with layer