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4 Nanolayers and Multilayered Systems

4 Nanolayers and Multilayered Systems

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3 Synthesis

Fig. 3.28 Diagram of a multiple-target ion beam sputtering system used to produce multilayers

[3.64]. The target-gun plasma sputters the atoms from the target onto the substrate which may be

cleaned by the substrate gun. (Reprinted with permission from [3.64]. © 1985 American Institute

of Physics)

Sputtering involves the collision of energetic ions (e.g., of a noble gas as Ar)

with the surface of a target material, leading to the ejection of target atoms that are

collected in their film form on a substrate (Fig. 3.28). Sputtering is used for ceramic

or metal–ceramic multilayers or, because of its high quenching rate, for amorphous

metal or amorphous semiconductor layers.

In evaporation systems the films are deposited by electron beam evaporization or a resistively heated source. Molecular beam epitaxy (MBE, Fig. 3.29) is

employed to prepare single-crystal superlattices [3.65]. An MBE facility is a sophisticated and expensive evaporator where deposition occurs under ultrahigh vacuum

(UHV) conditions (Fig. 3.30) with in situ characterization as reflection high-energy

electron diffraction (RHEED) and Auger electron spectroscopy (AES). A computercontrolled shutter system is used to control the layering and an ion gun for cleaning

the substrate prior to deposition of the film.

Pulsed laser deposition also referred to as laser ablation makes use of short laser

pulses to rapidly vaporize material from the surface of a target. The material is then

collected unto a substrate [3.67] with a material thickness per pulse of about 0.1 nm.

This allows the deposition of a multicomponent film of the target composition for,

e.g., high-temperature superconductor oxides.

Chemical vapor deposition (CVD) is commonly used for the preparation of semiconductor superlattices such as GaAl − Alx Ga1−x As [3.65]. A volatile gaseous

compound is adsorbed on a substrate and reacted by pyrolysis, reduction or

oxidation to semiconductors and metals or oxides and nitrides.


Nanolayers and Multilayered Systems


Fig. 3.29 Deposition of atoms on a substrate surface during molecular beam epitaxy (MBE)

(schematic drawing). (Reprinted with permission from [3.66]. © 1998 Max-Planck-Society)

Fig. 3.30 MBE facility for the preparation of ln1−x Gax As−GaSb1−y Asy superlattices [3.68]. The

Knudsen cells are labeled by the evaporants In, Ga, As, Sb where the Sn cell is used for doping.

(Reprinted with permission from [3.68]. © 1980 Elsevier)


3 Synthesis

Metal–organic chemical vapor deposition (MOCVD) is a basic technique for

depositing compound semiconductor films and superlattices [3.65] by reactions

such as

Ga(CH3 )3 (g) + AsH3 (g) → GaAs(s) + 3CH4 (g)

xAl(CH3 )3 (g) + (1 − x)Ga(CH3 )3 (g) + AsH3 (g) → Alx Ga1−x As(s) + 3CH4 (g).

By precisely controlling temperature, pressure, and gas composition, highquality superlattices can be prepared on an atomic thickness level.

With respect to the microstructure multilayers are often referred to as compositionally modulated materials with a composition modulation wavelength of

(Fig. 3.31). Multilayers composed of single-crystal layers with perfect interfacial

atomic registry are called superlattices. An ideal superlattice without structural

variation at the interface is called coherent. A lattice parameter misfit of a

semicoherent interface may be accommodated by misfit dislocations. A pseudomorphic change of the crystal structure may occur within multilayers with, e.g., bcc

Ge formed in Mo–Ge multilayers as an example [3.69].

Multilayers are specifically characterized by x-ray diffraction. In addition to the

patterns from the crystal lattice diffraction of the single layers, satellite peaks appear

due to the superlattice. The higher the order of the detectable diffraction peaks the

Fig. 3.31 Composition

profiles for an artificially

multilayered material of

bilayer repeat length . (a)

Artificially layered material

with alternating A and B

layers; (b) ideal rectangular

composition profile; (c)

sinusoidal composition

profile; (d) composition

profile of a multilayer with a

third interfacial phase of, e.g.,

an intermetallic. (Reprinted

with permission from [3.58].

© 1996 Institute of Physics)


Nanolayers and Multilayered Systems


Fig. 3.32 X-ray diffraction of an amorphous Si–amorphous Ge multilayer with a 5.8 nm bilayer

repeat length. (Reprinted with permission from [3.70]. © 1985 American Institute of Physics)

sharper are the interfaces. In amorphous multilayers the crystalline diffraction peaks

are missing and only the satellites appear (Fig. 3.32).

From the diffraction patterns with satellite peaks, structural information on the

bilayer repeat length

and on the real part nr of the index of refraction can be

derived. The roughness of the interfaces can be derived by fitting simulations of

the x-ray interference function. Neutron and electron diffractions yield additional


From diffraction studies information only on the average layer quality can

be derived. Investigations on a more atomistic scale can be performed by

high-resolution electron microscopy HRTEM (see Fig. 3.33) or by the analysis of

the extended x-ray absorption fine structure (EXAFS) . Thin-film characterization

methods such as Rutherford back scattering (RBS) or Auger-electron spectroscopy

(AES) can be used for the chemical analysis of artificially multilayered thin films.

3.4.1 Layered Oxide Heterostructures by Molecular Beam

Epitaxy (MBE)

Molecular beam epitaxy has achieved unparalleled control in the integration of

semiconductors at the nanometer level. Metal oxides and their interfaces can now

be prepared with the same atomic precision due to advances in heteroepitaxy of

complex oxides [3.71, 3.72]. At complex oxide interfaces electrons interact and

order in unique ways, so that novel types of quantum Hall systems and unique

superconductors can be obtained (see Sect. 4.3 and [3.72]).

MBE systems dedicated to the layer growth of oxides (see [3.71]) are equipped

with molecular beams of metals (Sr, Ba, Pb, Bi, etc.,) and oxidation is achieved

by purified ozone or molecular oxygen. The composition of the atomic oxide layers on substrates can be controlled to within less than 1% of the cation ratio and


3 Synthesis

Fig. 3.33 HRTEM

cross-sectional transmission

electron micrograph of an

InAs–GaSb superlattice with

the numbers indicating the

monolayers per layer.

(Reprinted with permission

from [3.58]. © 1996 Institute

of Physics)

the absolute dose of cations. The films can be characterized by RHEED, by x-ray

diffraction, and by HRTEM (see [3.71]).

In Fig. 3.34a, b HRTEM images of PbTiO3 /SrTiO3 and BaTiO3 /SrTiO3 superlattices with a high degree of uniformity in the structural order are shown. Superlattices

digitally graded from pure SrTiO3 to pure BaTiO3 by linearly increasing the fraction

of BaTiO3 unit cell thick layers are shown in Fig. 3.34c.

3.4.2 Atomic Layer Deposition (ALD)

Atomic layer deposition can be used to process extremely thin insulating layers or Zr and Hf high-k (high dielectric constant) materials such as silicates and

aluminates [3.73] onto large-area silicon substrates and is additionally useful for

surface modification of complex nanostructures [3.74]. The technique is based on

successive, surface-controlled reactions from the gas phase to produce thin films and

overlayers in the nanometer range with high homogeneity and controllability. The

principle of ALD is schematically shown in Fig. 3.35a where the thin film growth

cycle for a binary compound (TiO2 ) from gaseous precursors (TiCl4 and H2 O) is

presented. The reactant does not need to be controlled as the process is surface


Nanolayers and Multilayered Systems


Fig. 3.34 High-resolution transmission electron micrographs (HRTEM) of (a) a PbTiO3 /SrTiO3

superlattice and (b) a BaTiO3 /SrTiO3 superlattice grown by molecular beam epitaxy (MBE).

(c) HRTEM image of a digitally graded BaTiO3 /SrTiO3 multilayer that goes from pure SrTiO3

to pure BaTiO3 in unit cell thick increments. (Reprinted with permission from [3.71]. © 2001


controlled. Currently studied ALD deposition materials are, e.g., HfO2 as a high-k

insulator (see Sect. 9.7), to serve as gate dielectrics in MOSFETS, and also metal

oxides and nitrides including ZnO, TiO2 , WNx , or Al2 O3 .

The high potential of ALD for deposition of thin layers of material into high

aspect ratio (AR) nanopores and trenches was demonstrated with the fabrication

of uniform arrays of TiO2 nanotubes, where the precursors TiCl4 and H2 O were

deposited into the nanopores of Al2 O3 on a silicon substrate (Fig. 3.35b). The Al2 O3

template was then chemically removed to reveal nanotubes with controllable tube


3 Synthesis


Nanolayers and Multilayered Systems


Fig. 3.36 Images of the alumina replicas of butterfly wing scales prepared by atomic layer

deposition (ALD). (a), (b) Scanning electron micrographs (SEM) of the alumina replicas of the

butterfly wing scales on a silicon substrate after the butterfly template was completely removed.

(c) Micrographs of the replicas with various thicknesses of deposited Al2 O3 . (Reprinted with

permission from [3.76]. © 2006 American Chemical Society)

diameter, spacing, and wall thickness (see [3.74]). By low-temperature ALD for

organic nanostructures, butterfly wings were covered with Al2 O3 , showing a change

in coloration with the thickness of the alumina layer (see Fig. 3.36) which may be

of interest for the development of optical elements.

Fig. 3.35 (a) Schematic of an atomic layer deposition (ALD) process. In step (a) the substrate is

exposed to the molecules of precursor 1 which adsorb ideally as a monolayer on the surface. In

step (b) the excess precursor 1 in the gas phase is removed by inert gas purging. In step (c) the

substrate is exposed to precursor 2 which reacts with the adsorbed precursor 1 to form a layer of

the desired material. In step (d) the excess precursors 2 and the reaction by-products are removed

by purging. The cycle is repeated (yellow-arrow) until the desired thickness of the deposit [3.74]

is obtained. (b) Schematic of the process to create TiO2 nanotube arrays on a substrate. (1)

Formation of the nanoporous Al2 O3 template by anodization of an aluminum film. (2) TiO2 deposition on the template by ALD. (3) Top layer of TiO2 on alumina removed by mechanical polishing.

(4) Alumina template chemically etched away to reveal an array of titania nanotubes on the substrate [3.75]. (Reprinted with permission from [3.74] (a) and [3.75] (b). © 2007 Wiley-VCH (a)

and © 2004 Wiley-VCH (b))


3 Synthesis

Metallic multilayers with thin individual layers date back to the medieval forging

of Damascus [3.77] or Japanese [3.78] swords composed of layers of wrought iron

and hardened steel that were repeatedly folded and rolled. Metallic multilayers of

Ni–Er were produced with 5 nm individual layer thicknesses and a total thickness

up to 200 μm [3.79].

Multilayered polymer composites such as polystyrene–polyethylene multilayers

can be composed of thousands of layers with an individual layer thickness of a few

tens of nanometers [3.80].

3.5 Shape Control of Nanoparticles

It has been shown that the shape of nanoparticles may be modified by the reaction conditions, particle irradiation, doping, etc. The various nanoparticle shapes

obtained making use of various reaction conditions are schematically shown in

Fig. 3.37.

The sensitivity of the nanoparticle shape to the reaction conditions implies

that the reaction is under kinetic rather than thermodynamic control [3.81]. Silver

nanocubes can be synthesized with edge lengths between 70 and 175 nm depending

on the reaction time (Fig. 3.38).

By shape-controlling synthesis, nanosized Au octahedra [3.83], Au decahedra

[3.84], triangular Pd plates [3.85], Pt multipods [3.86], and Ag nanowires with

pentagonal cross section (see [3.87]) can be prepared.

The shape of silica particles can be modified by irradiation with Xe ions

(Fig. 3.39). The particles shrink in beam direction and expand perpendicular to this

direction [3.88]. This is ascribed to the melting of the silica material along the ion

track giving rise to a perpendicular shear stress with local plastic expansion and

expansion of the particle normal to the ion beam. In contrast to that an initially

spherical Au particle coated by silica expands in ion beam direction (Fig. 3.40) due

to the silica shell because this effect is not observed without this shell. The surface

Fig. 3.37 A variety of

nanoparticle shapes (right)

can be synthesized from

precursors (left) depending on

the reaction conditions.

(Reprinted with permission

from [3.81]. © 2002 AAAS)


Shape Control of Nanoparticles


Fig. 3.38 Transmission

electron microscopy (TEM)

image of silver nanocubes

with {100} faces synthesized

by a reduction of AgNO3

with ethylene glycol at

160◦ C. Scale bar, 100 nm.

(Reprinted with permission

from [3.82]. © 2002 AAAS)

Fig. 3.39 SEM imaging of initially spherical silica particles on a Si substrate after irradiation with

4 MeV Xe ions to 3×1018 Xe ions/m2 at 85 K. (a) View along the Xe beam. The lateral expansion

of the spheres is indicated by a comparison with the dashed sphere before irradiation. (b) View

perpendicular to the beam with the shrinkage of the particle in beam direction. (Reprinted with

permission from [3.88]. © 2007 Elsevier)

Fig. 3.40 TEM images of a 14 nm Au particle coated by a 54 nm silica shell. (a) Before ion irradiation. (b) After irradiation with 30 MeV Se ions to a fluence of 2×1018 ions/m2 at 85 K, giving

rise to an elongation of the coated Au nanoparticle in beam direction. (Reprinted with permission

from [3.89]. © 2004 Elsevier)

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