Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications

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196

Nanostructures and Nanomaterials

employed to enhance or assist the chemical reactions or deposition, and

two

mechanisms are involved: pyrolytic and photolytic proces~es.~~*In

the pyrolytic process, the laser heats the substrate to decompose gases

above it and enhances rates of chemical reactions, whereas in the photolytic

process, laser photons are used

directly dissociate the precursor mole-

cules in the gas phase. Aerosol assisted CVD is developed for the systems

where

gaseous precursors are available and the vapor pressures of liquid

and solid precursors are too In this process, liquid precursors are

mistified to form liquid droplets that are dispersed in a carrier gas and

delivered to the deposition chamber. Inside the deposition chamber, pre-

cursor droplets decompose, react and grow films on substrate.

In addition to the growth of thin films on a planar substrate, CVD

methods have been modified and developed to deposit solid phase from

gaseous precursors on highly porous substrates or inside porous media.

Two most noticeable deposition methods are known as electrochemical

vapor deposition (EVD) and chemical vapor infiltration (CVI). EVD has

been explored for making gas-tight dense solid electrolyte films on porous

substrate^,^^^^^

and the most studied system has been the yttria-stabilized

zirconia films on porous alumina substrates for solid oxide fuel cell appli-

cations and dense

membrane^.^^-^^

In the EVD process for growing solid

oxide electrolyte films, a porous substrate separates metal precursor(s)

and oxygen source. Typically chlorides are used as metal precursors,

whereas water vapor, oxygen, or air or a mixture of them is used as the

source of oxygen. Initially, the two reactants inter-diffuse in the substrate

pores and react with each other only when they concur to deposit the cor-

responding solid oxides. When the deposition conditions are appropriately

controlled, the solid deposition can be located at the entrance of pores on

the side facing metal precursors, and plug the pores. The location of the

solid deposit is mainly dependent on the diffusion rate of the reactants

inside the pores as well

the concentrations of the reactants inside the

deposition chamber. Under typical deposition conditions, reactant mole-

cules difhsion inside pores is in the Knudsen diffusion region, in which

the diffusion rate is inversely proportional to the square root of the molec-

ular weight. Oxygen precursors diffuse much faster than metal precursors,

and consequently the deposit occurs normally near the entrance of pores

facing the metal precursor chamber. If the deposit solid is an insulator,

the deposition by CVD process stops when pores are plugged by the

deposit, since no further direct reaction between the

two

reactants occurs.

However, for solid electrolytes, particularly ionic-electronic mixed con-

ductors, the deposition would proceed further by the means of EVD, and

the film may grow on the surface exposed to the metal precursor vapor.

Two-Dimensional Nanostructures: Thin Films

197

In this process, the oxygen or water is reduced at the oxygedfilm inter-

face, and the oxygen ions transfer in the film, as the oxygen vacancies

diffuse in the opposite direction, and react with the metal precursors at the

fildmetal precursor interface to continuously form metal oxide.

CVI involves the deposition of solid products onto a porous medium,

and the primary focus of CVI is on the filling of voids in porous graphite

and fibrous mats to make carbon-carbon

composite^.^^,^^

Various CVI

techniques have been developed for infiltrating porous substrates with the

main goals to shorten the deposition time and to achieve homogeneous

deposition:

(a) Isothermal and isobaric infiltration,

(b) Thermal gradient infiltrati~n,~~

(d) Forced flow infiltration:'

(e) Pulsed infiltration,"2

(f)

Plasma enhanced infiltration?'

Various hydrocarbons have been used as precursors for CVI and typical

deposition temperatures range from

850

to 1100°C and the deposition

time ranges from

hr, and is rather long as compared to other vapor

deposition methods. The long deposition time is due to the relatively low

chemical reactivity and gas diffision into porous media. Furthermore, the

gas diffusion will progressively get smaller as more solid is deposit inside

the porous substrates. To enhance the gas diffusion, various techniques

have been introduced and include forced flow, thermal and pressure gra-

dient. Plasma has been used to enhance the reactivity; however, preferen-

tial deposition near surfaces resulted in inhomogeneous filling. The

complete filling

difficult and takes very long time, since the gas diffu-

sion becomes very slow in small pores.

5.5.5.

Diamond films

CVD

Diamond is a thermodynamically metastable phase at room tem~erature,

synthetic diamonds are made at high temperatures under high pressures

with the aid of transition metal catalysts such as Ni, Fe and CO.'"',~~ The

growth of diamond films under low pressure (equal to or less than 1 atm)

and low temperatures

800°C) is not a thermodynamically equilibrium

process, and differs from other CVD processes. The formation of diamond

from gas phase at low pressure was initially reported in late

1960~.4~3

The typical CVD process of diamond films is illustrated schematically

198

Nanostructures and Nanomaterials

Fig. 5.1448 and can be described as follows. A gaseous mixture of hydro-

carbon (typically methane) and hydrogen is fed into an activation zone of

the deposition chamber, where activation energy is introduced to the mix-

ture and causes the dissociation of both hydrocarbon and hydrogen

molecules to form hydrocarbon free radicals and atomic hydrogen. Many

different activation schemes have been found effective in depositing

diamond films and include hot-filament,

and microwave plasma and

flames. Upon arrival on the growth surface, a generic set of surface reac-

tions would occur?

(5.32)

(5.33)

(5.34)

Reaction (5.32) is to activate a surface site by removal of a surface hydrogen

atom linked to carbon atom on the diamond surface. An activated surface

site readily combines with either a hydrocarbon radical (reaction 5.33) or an

Fig.

5.14.

Schematic showing the principal elements in the complex diamond

CVD

process: flow of reactants into the reactor, activation of the reactants

the thermal and

plasma processes, reaction and transport of the species to the growing surface, and surface

chemical processes depositing diamond and other forms of carbon.

[J.E.

Butler and

D.G. Goodwin, in

Properties,

Growth

and Applications

Diamond,

eds.

M.H.

Nazare and

A.J.

Neves, INSPEC, London, p.

262,2001.1

Two-Dimensional Nanostructures:

Thin

Films

199

unsaturated hydrocarbon molecule (e.g. C2H2, reaction

5.34).

A high con-

centration of atomic hydrogen has proven a key factor in the successful

growth of diamond films, and atomic hydrogen is believed to constantly

remove the graphite deposits on the diamond growth surface,

as to ensure

continued deposition of diamond.47 Oxygen species have also proven to be

important in the deposition of diamond films by atmospheric combustion

flames using oxygen and a~etylene."~,~~ Other hydrocarbon fuels including

ethylene, propylene and methyl acetylene can all

used

precursors for

the growth of diamond

film^.^'-^^

5.6.

Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD) is a unique thin film growth method and

differs significantly from other thin film deposition methods. The most

distinctive feature of ALD has a self-limiting growth nature, each time

only one atomic or molecular layer can grow. Therefore, ALD offers the

best possibility of controlling the film thickness and surface smoothness

in truly nanometer or sub-nanometer range. Excellent reviews on ALD

have been published by Ritala and Le~kela.~~?~~ In the literature, ALD is

also called atomic layer epitaxy (ALE), atomic layer growth (ALG),

atomic layer CVD (ALCVD), and molecular layer epitaxy (MLE). In

comparison with other thin film deposition techniques, ALD is a relatively

new method and was first employed to grow ZnS film.57 More publica-

tions appeared in open literature in early

1980~.~*-~~

ALD can be consid-

ered as a special modification of the chemical vapor deposition, or a

combination of vapor-phase self-assembly and surface reaction. In a typi-

cal ALD process, the surface is first activated by chemical reaction. When

precursor molecules are introduced into the deposition chamber, they

react with the active surface species and form chemical bonds with the

substrate. Since the precursor molecules do not react with each other, no

more than one molecular layer could be deposited at this stage. Next,

the monolayer of precursor molecules that chemically bonded to the sub-

strate is activated again through surface reaction. Either the same or dif-

ferent precursor molecules are subsequently introduced to the deposition

chamber and react with the activated monolayer previously deposited.

As the steps repeat, more molecular or atomic layers are deposited one

layer at a time.

Figure

5.15

schematically illustrates the process of titania film

growth by ALD. The substrate is hydroxylated first, prior to the intro-

duction of precursor, titanium tetrachloride. Titanium tetrachloride will

200

Nanostructures and Nanomaterials

OH OH OH

HCI

CI CI

I I

CI--Ti-Cl Cl-Ti-CI CI-Ti-CI

H~O~

Purge\

HCI

cycle

I I

HO- Ti-OH HO- Ti-OH HO- Ti-OH

OH OH OH

HO-

Ti-

ipurget

Ti-

Fig.

5.15.

Schematic illustrating the principal reactions and processing steps

for

the

formation

titania film

ALD.

react with the surface hydroxyl groups through a surface condensation

reaction:

TiCI4

HOMe

C13Ti-O-Me

HCI

(5.35)

where Me represents metal or metal oxide substrates. The reaction will

stop when all the surface hydroxyl groups reacted with titanium tetra-

chloride. Then the gaseous by-product, HCl, and excess precursor

molecules are purged, and water vapor is subsequently introduced to the

system. Titanium trichloride chemically bonded onto the substrate surface

undergo hydrolysis reaction:

C1,Ti-0-Me

H20

(HO),Ti-O-Me

HC1

(5.36)

Neighboring hydrolyzed Ti precursors subsequently condensate to form

Ti-0-Ti linkage:

(HO),Ti-O-Me

(H0)3Ti-O-Me

+Me-O-Ti(OH),-O-Ti (H0)2-O-Me

H20

(5.37)

The by-product HC1 and excess

H20

will be removed from the reaction

chamber. One layer of TiOz has been grown by the completion of one

Two-Dimensional Nanostructures: Thin Films

cycle of chemical reactions. The surface hydroxyl groups are ready to

react with titanium precursor molecules again in the next cycle. By repeat-

ing the above steps, second and many more TiO2 layers can be deposited

in a very precisely controlled way.

The growth of ZnS film is another often used classical example for the

illustration of the principles of ALD process. ZnC1, and H2S are used as

precursors. First, ZnC1, is chemisorbed on the substrate, and then H2S is

introduced to react with ZnC1, to deposit a monolayer of ZnS on the sub-

strate and HCl is released as a by-product. A wide spectrum of precursor

materials and chemical reactions has been studied for the deposition of

thin films by ALD. Thin films of various materials including various

oxides, nitrides, florides, elements, 11-VI, 11-VI and 111-V compounds, in

epitaxial, polycrystalline or amorphous form deposited by ALD are sum-

marized in Table

5.2.55,56

The choice of proper precursors is the key issue in a successful design

of an ALD process. Table

5.3

summarizes the requirements for ALD

precursor^.^^^^^

A variety of precursors have been used in ALD. For example,

elemental zinc and sulfur were used in the first ALD experiments for the

growth

ZnS.57 Metal chlorides were studied soon after the first demon-

strations of ALD? Metalloragnic compounds including both organometal-

lic compounds and metal alkoxides are widely used. For non-metals, the

simple hydrides have mostly been used: H20, H202, H2S, H2Se, H2Te, NH,,

N2H4, PH3,

ASH,,

SbH3 and HF.

In comparison to other vapor phase deposition methods, ALD offer

advantages particularly in the following aspects: (i) precise control of film

Table

5.2.

Thin film materials deposited

ALD.55s56

11-VI compounds

11-VI based phosphors

111-V compounds

Nitrides

Oxides

ZnS, ZnSe, ZnTe, ZnSI_,Se,, CaS, SrS, Bas, SrSI_,Se,CdS,

CdTe, MnTe, HgTe, Hg, -,Cd,Te, Cdl_,Mn,Te

ZnS:M (M

Mn, Tb, Tm), CaS:M (M

Eu, Ce, Tb, Pb),

SrS:M (M

Ce, Tb, Pb, Mn, Cu)

GaAs, AIAs, Alp, InP, Gap, InAs, AI,Ga,-,As, Ga,Inl-,As,

Ga,Inl-,P

AIN, GaN, InN, SiN,, TiN, TaN, Ta3N5, NbN, MoN, WzN,

Ti-Si-N

A1203, TiOz, ZrOz,

HfOz,

TazO5, NbzO5, YzO3, MgO, CeOz,

SiOz, LaZO3, SrTi03, BaTi03, Bi,Ti,O,,

Inz03,

Inz03:Sn,

In2O3:F, Inz03:Zr, SnOZ, SnOz:Sb, ZnO, ZnO:Al, Gaz03,

NiO, COO,, YBa2Cu3O7.,, LaCo03, LaNi03

Si, Ge, Cu, Mo, Ta, W

Laz&, PbS, InzS3, CuGaSz, Sic

Fluorides CaFz, SrFz, ZnFz

Elements

Others

202

Nanostructures

and

Nanomaterials

Table

5.3.

Requirements for

ALD

precursor^.^^

Requirement Comments

Volatility

self-decomposition

Aggressive and complete reactions

etching of the film or

No dissolution to the film

substrate material

Un-reactive byproduct

Sufficient purity

Inexpensive

Easy to synthesize

handle

Nontoxic and environmentally

friendly

For efficient transportation, a rough limit of

0.1 torr at the applicable maximum source

temperature

Preferably liquids or gases

Would destroy the self-limiting film growth

Ensure fast completion of the surface

Lead to high film purity

No problems of gas phase reactions

competing reaction pathways

Would prevent the film growth

Would destroy the self-limiting film growth

avoid corrosion

Byproduct re-adsorption may decrease

the growth rate

meet the requirements specific to each process

mechanism

reactions and thereby short cycle times

mechanism

thickness and (ii) conformal coverage. Precise control of film thickness

is due to the nature of self-limiting process, and the thickness of a film

can be set digitally by counting the number

reaction cycles. Conformal

coverage is due to the fact that the film deposition is immune to variations

caused by nonuniform distribution of vapor or temperature in the reaction

zone. Figure 5.16 shows the X-ray diffraction spectra and the cross-

sectional

SEM

image of 160 nm Ta(Al)N(C) film on patterned silicon

wafer.62 The film is polycrystalline and shows perfect conformality.

The deposition temperature was

350°C

and precursors used were TaCl,,

trimethylaluminum (TMA) and NH3. However, it should be noted that

excellent conformal coverage can only be achieved when the precursor

doses and pulse time are sufficient for reaching the saturated state at each

step at all surfaces and no extensive precursor decomposition takes place.

ALD has demonstrated its capability of depositing multilayer structures or

nanolaminates and

Fig.

5.17, as an example, shows such a schematic rep-

resentation of nanolaminates prepared onto glass substrates by

ALD.63

ALD is an established technique for the production

large area elec-

troluminescent displays,64 and is a likely future method for the production

Two-Dimensional Nanostructures: Thin Films

203

Fig.

5.16.

(a) X-ray diffraction spectra and

(b)

cross-sectional

SEM

image

160nm

Ta(Al)N(C) film on patterned silicon wafer.

[P.

AIICn,

Juppo,

Ritala,

Sajavaara,

Keinonen, and

Leskela,

Electrochem.

SOC.

148,

G566

(2001).]

of very thin films needed in microele~tronics.~~ However, many other

potential applications of

ALD

are discouraged by its low deposition rate,

typically <0.2 nm (less than half a monolayer) per cycle. For silica depo-

sition, completing a cycle of reactions typically requires more than

min.66,67 Some recent efforts have been directed towards the develop-

ment of rapid

ALD

deposition method. For example, highly conformal

layers of amorphous silicon dioxide and aluminum oxide nanolaminates

were deposited at rates of 12nm or <32 monolayers per cycle, and the

method has been referred to as “alternating layer depo~ition”.~~ The exact

mechanism for such a multilayer deposition in each cycle is unknown, but

obviously different from the self-limiting growth discussed above. The

precursor employed in this experiment, tris(tert-butoxy)silanol, can react

with each other, and thus the growth is not self-limiting.

204

Nanostructures and Nanomaterials

bilayer

Ta205

HfO:,

Ta,O,

HfO,

Ta205

+-underlayer

A1203

~~ ~

glass

substrate

Fig.

5.17.

Schematic representation

the nanolaminates prepared onto

cm2 glass

subtrates by

ALD.

The

AI2O3

layer serves as an ion barrier against sodium diffusion from

soda lime glass substrate, [K. Kukli,

Ihanus,

Ritala, and

Leskela,

Appl.

Phys.

Left.

68,

3737

9961.1

5.7.

perlatt

ices

Superlattices in this chapter are specifically referred to as thin film

structures composed of periodically alternating single crystal film layers;

however,

should be noted that the term superlattice was originally used

to describe homogeneous ordered alloys. Composite film supperlattices

are capable of displaying a broad spectrum of conventional properties

as well as a number of interesting quantum effects. When both layers are

relatively thick, properties of bulk materials are observed due to the

frequently synergistic extensions of the laws of property mixtures that are

operative. However, when the layers are very thin, quantum effects

emerge, since the wavefunctions of charge carriers in adjacent thin layers

penetrate the barriers and couple with one another. Such structures are

mostly fabricated by

MBE;

however, CVD methods are also capable of

making superlattices. ALD is another unique technique in the fabrication

of superlattice structures. Organic superlattices can be fabricated using

technique or by self-assembly, which are to be discussed in the fol-

lowing sections. Some of semiconductor superlattice systems are listed

in Table

5.4.69

Semiconductor superlattices can be categorized into

compositional superlattices and modulation doping, i.e. selective periodic

doping, superlattice. The fabrication

semiconductor superlattices is

basically the controlled synthesis of band gap structures, which is also

known as band gap engir~eering.~@-~~ Esaki and Tsu were the pioneers in the

Two-Dimensional

Nunostructures:

Thin

Films

205

Table

5.4.

Examples

superlattice systems.69

Film materials Lattice mismatch Deposition

methods

GaAs-AsxGal _,As

Inl -,G%As-CaSbl -yA~y

GaSb-AISb

InP-Ga,Inl -,As,PI

-y

InP-Inl

_$%,As

GaP-GaP1-,Asx

GaAs-GaAsI -,PX

Ge-GaAs

Si-Si,

-,Ge,

CdTsHgTe

MnSe-ZnSe

PbTe-Pbl -,Sn,Te

0.16%

for

0.6

0.66%

O%,

0.47

1.86%

1.79%,

0.5

0.08%

0.92%,

0.22

0.74%

4.7%

0.44%,

0.2

MBE, MOCVD

MBE

MBE, MOCVD, LPE

MOCVD

MOCVD, CVD

MBE

MBE, CVD

MBE

CVD

synthesis of semiconductor thin film superlattices in

970.73

Figure

5.18

shows the TEM images of InGa03(ZnO)5 superlattice ~tructure?~

far all the methods discussed are vapor phase deposition methods.

Thin films can also be made through wet chemical processes. There are

many methods developed and examples include electrochemical deposi-

tion, sol-gel processing and self-assembly. In comparison with vacuum

deposition methods, solution based film deposition methods offer a wide

range of advantages including the mild processing conditions

that they

are applicable and widely used for the fabrication of thin films of temper-

ature sensitive materials. Mild processing conditions also lead to stress-

free films.

5.8.

Self-Assembly

Self-assembly is a generic term used to describe a process that ordered

arrangement of molecules and small components such as small particles

occurred spontaneously under the influence of certain forces such as

chemical reactions, electrostatic attraction and capillary forces. In this

section, we will focus

our

discussion on the formation

monolayer or

multiple layers of molecules through self-assembly. In general, chemical

bonds are formed between the assembled molecules and the substrate sur-

face, as well as between molecules in the adjacent layers. Therefore, the

major driving force here is the reduction of overall chemical potential.

Further discussion on self-assembly of nanoparticles and nanowires will

be presented in Chapter

variety

interactions or forces have been

Guozhong Cao. Nanostructures &amp; Nanomaterials: Synthesis, Properties &amp; Applications

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Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications