Chapter 2

Techniques of Thin Film Deposition

2.1 Thin film deposition techniques

A solid material is said to be in thin film form when it is grown as a thin layer on

a solid substrate by controlled condensation of the individual atomic, molecular or ionic

species either by physical process or chemical process. Thin film deposition techniques

have been broadly classified into two main categories:

(i) Physical process

(ii) Chemical process

Physical method covers the deposition techniques which depend on the evaporation or

ejection of the material from a source whereas chemical methods depend on certain

specific chemical reactions. The techniques of thin film deposition are briefly

summarised in Table 1.

2.1.1 Physical vapour deposition

Physical vapour deposition (PVD) is a variety of vacuum deposition technique

used for deposition of metal, alloy and many compound films [1-3]. The primary

requirement for this technique is to achieve a high vacuum of 10-5 torr or more to allow

the vapour to reach the substrates without scattering against other gas phase atoms

present in the chamber and reduce the incorporation of impurities from the residual gas

in the vacuum chamber. This technique involves four major steps:

(i) Evaporation, (ii) Transportation, (iii) Reaction and (iv) Deposition.

57
 Table 1: Classification of thin film deposition techniques

Firstly, the material to be deposited evaporates or sublimates in vacuum due to thermal

energy (resistive heating) and then the vaporised atoms or molecules is transported from

the source to the substrate. Finally, the vapour atoms or molecules are condensed on a

cooler substrate so as to form a continuous and adherent film of desired thickness [4-7].

PVD is classified into thermal evaporation, electron beam evaporation, radio frequency

induction heating, laser beam evaporation, molecular beam epitaxy (MBE), activated

reactive evaporation (ARE), electron gun heating, etc. [8-10].

Multicomponent alloys or compounds which tend to distil fractionally may be

evaporated by flash evaporation [11-13], in which fine particles or powder of the

58
material is dropped continuously onto a hot boat or surface so that numerous discrete

evaporations may occur to form the film on a substrate.

2.1.2 Sputtering deposition

Sputtering deposition is a widely used technique to deposit thin film on various

substrates. This technique is based upon ion bombardment of a source material, the

target. When a solid target material is bombarded with energetic particles, surface atoms

are ejected due to the collisions between the surface atoms and the energetic particles.

This phenomenon is known as sputter or sputtering [14]. This technique may be

described as a sequence of the given steps:

(i) Ions are generated and directed at a target material,

(ii) The ions sputter atoms from the target,

(iii) The sputtered atoms get transported to the substrate through a region of reduced

pressure and

(iv) The sputtered atoms condense on the substrate, forming a thin film.

Sputtering process produces films of higher purity and homogeneity. If the

ejection is due to bombardment by positive ion, the process is known as cathodic

sputtering. The ejection of atoms from the cathode surface by impinging of energetic

positive ions of noble gases of high purity (Ar, He etc) at a reduced pressure under a

high dc voltage gives rise to the sputtering phenomenon. If the process does not involve

any chemical reaction between the bombarding gas ions and the cathode it is known as

physical sputtering. The ions required for bombardment is usually obtained by

maintaining a glow discharge due to an applied electric field within the vacuum

59
chamber. Over the years various sputtering techniques have been developed. Sputtering

can be classified into d.c sputtering, r.f sputtering, magnetron sputtering and ion beam

sputtering etc. [15-17]. High pressure oxygen sputtering and facing target sputtering are

the two new methods introduced for deposition of thin films for applications in

superconducting and magnetic films [18].

2.1.3 Chemical vapour deposition (CVD)

When a volatile compound of the substance to be deposited is vaporized and the

vapour is thermally decomposed or reacted with other gases, vapours or liquids at the

substrate to yield non-volatile reaction products which deposit atomistically (atom by

atom) on the substrate, the process is called chemical vapour deposition (CVD) [19, 20].

CVD is a versatile and flexible technique in producing deposits of pure metals,

semiconductors and insulators.

In general, the CVD process involves the following key steps [21]:

(i) Generation of active gaseous reactant species,

(ii) Transport of the gaseous species into the reaction chamber,

(iii) Gaseous reactants undergo gas phase reactions forming intermediate species,

(iv) Absorption of gaseous reactants onto the heated substrate and the heterogeneous

reaction occurs at the gas-solid interface (i.e heated substrate) which produces the

deposit and by-product species,

(v) The deposit will diffuse along the heated substrate forming the crystallisation centre

and growth of the film,

(vi) Gaseous by product are removed from the boundary layer through diffusion or

convection and

60
(vii) The unreacted gaseous precursors and by products will be transported away from

the deposition chamber.

Various types of chemical reactions utilised in CVD for the formation of solids

are pyrolysis, reduction, oxidation, hydrolysis, synthetic chemical transport reaction etc.

[22]. There are numerous forms of CVD methods used such as Atmospheric pressure

chemical vapour deposition (APCVD), Low pressure chemical vapour deposition

(LPCVD), Metal-organic chemical vapour deposition (MOCVD), Plasma assisted

chemical vapour deposition (PACVD) or Plasma enhanced chemical vapour deposition

(PECVD), Laser chemical vapour deposition (LCVD), Photochemical vapour

deposition (PCVD), Chemical beam epitaxy (CBE) etc. [23].

2.1.4 Chemical bath deposition (CBD)

Many studies have been conducted over about three decades on chemical bath

deposition (CBD) method for the preparation of thin films. Chemical bath deposition

technique is the most important method for the growth of films owing to its versatility

for depositing a very large number of elements and compounds at relatively low

temperatures. The chemical bath deposition method is a low cost process and the films

are found to be comparable of good quality to those obtained by more sophisticated and

expensive physical deposition process. Chemical bath deposition (CBD), which is also

known as solution growth, controlled precipitation or simply chemical deposition

recently has emerged as the method for the deposition of metal chalcogenide thin films

[24]. The reaction takes place between the dissolved precursors generally in aqueous

solution at low temperature (30 to 80oC). Thiourea, thioacetamide, thiosulphate and

61
sodium sulphide are generally used as sulphide precursors. In this technique, films are

deposited on a solid substrate from a reaction occurring in a solution (mostly aqueous)

in the presence of appropriate complexing agent. Many studies have been conducted on

chemical bath deposition (CBD) method for the preparation of thin films as it has

extremely simple set up, great flexibility in substrates selection, low temperature

requirements, and ability to produce uniform, adherent, and reproducible large area

good quality thin films [25, 26]. It is widely used in the deposition of conducting and

semiconducting thin films. The films can be deposited on different substrates like glass,

ceramic, metallic surfaces, etc. It produces good deposits on suitable substrates by the

controlled precipitation of the compounds from the solution. The CBD method offers

many advantages over other well-known vapour phase synthetic routes. It may allow us

to easily control the growth factors such as film thickness, deposition rate and quality of

crystallites by varying the preparative parameters such as solution pH, temperature and

bath concentration [27]. It does not require high voltage equipment, works at room

temperature, and hence it is inexpensive. The only requirement for this deposition route

is an aqueous solution consisting of a few common chemicals and a substrate for the

film to be deposited. Due to good productivity of this techniques on a large scale and

simplicity of the apparatus, it offers most attractive way for the formation of thin films

of metal oxides, metallic, semiconducting films etc.

2.2 Selection of deposition technique for the present study

Choice and selection of deposition process plays a vital role in the formation of

good quality films, and while selecting a particular technique following aspects should

be kept in mind

62
(i) Cost effectiveness,

(ii) Ability to deposit the desired films,

(iii) Operation at reduced temperature and pressure,

(iv) Easy control over deposition parameters such as deposition temperature, deposition

rate, thickness,

(v) Reliable adhesion to different substrates,

(vi) Retention of the stoichiometry in compounds and

(vii) Ability to deposit over a wide range of deposition parameters.

Among the various techniques discussed above, chemical bath deposition method

is employed in the present investigation for the preparation of thin films and junctions

owing to its most simple experimental set up and ability for large area depositions at

relatively low temperatures. The CBD process requires only simple instruments such as

digital balance, pH meter and hot plate with magnetic stirrer, thermometer and

commonly available chemicals.

2.3 Factors influencing in the chemical bath deposition process

In chemical bath deposition technique, the deposition and nature of the films

depends upon the preparative parameters such as

(i) Concentration of reactants and complexing agent,

(ii) Deposition temperature,

(iii) pH value and duration of the reaction and

(iv) Nature of the substrates.

63
2.3.1 Concentration of reactants and complexing agent

The deposition of the films depends on the concentration of the reactants as well

as on the complexing agent. At low concentrations due to less number of ions available

for reactions, the films deposited are thin and mostly non-uniform. The growth rate and

thickness of the deposited films initially increase with an increase in the concentration

of the reactants. This condition is valid up to a certain level of concentration and then

become saturation in the growth process. At higher concentration the precipitation

occurs very fast and the ions does not get sufficient time to stick on the substrates

resulting to decrease in the films thickness [28-30].

In chemical bath deposition, a complexing agent is used to bind the metallic ions to

avoid the homogeneous precipitation of the corresponding compound. Formation of

complex ion is essential to control the rate of the reaction and to avoid the immediate

precipitation of the compound in the solution [31, 32].

2.3.2 Deposition temperature

Deposition temperature is another factor that influences the growth rate and

thickness of the deposited films. At low deposition temperature the reaction does not

occur due to insufficient reactant ions present in the solution. As the temperature

increases, dissociation of the complex metal ions increases resulting to faster reaction.

At relatively higher temperature more and more ions are released but all the ions do not

get chance to be deposited on the glass substrate surface, they settle down at the bottom

of the reaction container. This results in decrease in the film thickness [33-36].

64
2.3.3 pH value and duration of the reaction

The pH value of the solution is another important factor in the chemical bath

deposition. To obtain a desired film the pH value is optimized to avoid deleterious

rate of the reaction increases with increase in pH value [37]. Growth of the thin films by

chemical bath deposition is time dependent. The thickness of the films increases up to

certain duration and then become saturated. After prolonged duration the reactant ions

available in the solution is exhausted leading to decrease in the thickness of the film

[38].

2.3.4 Nature of the substrates

Ideally, the substrate should provide only mechanical support but not interact

with the film except for sufficient adhesion. The surface of the substrate plays a major

role in the nucleation and growth process of the film and thereby influences the thin

film properties considerably. An ideal substrate is expected to possess the following

properties

(i) Atomically smooth surface, to ensure film uniformity,

(ii) Inertness to chemicals used in processing,

(iii) High thermal conductivity to provide surface temperature control during

processing,

(iv) High mechanical strength to enable the substrate withstand strain in processing,

(v) Similar co-efficient of expansion as that of the deposited film so as to prevent film

stress that may arise due to difference in expansion co-efficient,

(vi) High resistivity and

65
(vii) Low cost.

Rough surfaces are better to obtain an adherent film, probably due to greater

actual surface area of contact per pores of the substrate [24]. Higher deposition rates and

terminal thickness are observed for those substrates whose lattice and lattice parameters

match well with those of the deposited material [39].

2.4 Solubility and Ionic Product

When a sparingly soluble salt AB is placed in water, a saturated solution

containing A and B ions in contact with the undissolved solid AB is obtained and an

equilibrium is established between the solid phase and the ions in the solution. Thus

AB(S) = A + + B- (2.01)

Applying law of mass action to this equilibrium

C +A .C -B
K= (2.02)
C AB(S)

where C +A , C B- and C AB are concentrations of A, B, and AB in the solution

respectively.

The concentration of a pure solid phase is a constant number i,e

C AB(s ) = a constant = K

C +A .C -B
K= (2.03)
K

or KK = C+A .C-B (2.04)

as K and K are constant, the product KK is also constant, say K S

66
 K S = C +A .C -B (2.05)

The constant K s is called the solubility product (SP) and the expression C +A .C -B is called

as the ionic product (IP). When the ionic product is less than the solubility product, the

solution will be unsaturated. When the solution is saturated the ionic product is equal to

the solubility product. When the ionic product exceeds the solubility product i.e. IP/SP

= S>1, the solution is supersaturated (S=degree of supersaturation), precipitation occurs

and ions combine on the substrate and in the solution to form nuclei. Once the stable

nuclei formed, adsorption of ionic species from the solution onto the nucleus occurs to

forming thin film [40].

2.5 Nucleation and Particle Growth in the Solution

The formation of solid phase from a solution involves two steps: nucleation and

particle growth.

2.5.1 Nucleation

Nucleation is defined as the series of atomic or molecular processes by which

the atoms or molecules of a reactant phase rearrange into a cluster of the product phase

large enough as to have the ability to grow irreversibly to a macroscopically larger size.

There are two types of nucleation take place in the solution: homogeneous nucleation

and heterogeneous nucleation.

67
2.5.2 Homogeneous nucleation

Homogeneous nucleation occurs due to the local fluctuations in concentration,

temperature and other variables in the solution. The collision between individual ions or

molecules results in the formation of embryos which is the initial stage of growth. These

embryos grow by collecting individual species that collide with them. The adsorption of

ions on the embryo seems to be the most probable growth mechanism. These embryos

may redissolve in the solution before they have a chance to grow into stable particles,

called nuclei. Because of high surface areas, and therefore high surface energies of such

small nuclei, they are thermodynamically unstable against redissolution. They may,

however, be kinetically stabilized by low temperatures, which increase their lifetime,

possibly enough for them to grow to a size where they are thermodynamically stable.

The critical radius, Rc, is the size where the embryo has a 50% chance of redissolving

(i.e. a 50% chance of growing into a stable nucleus). It is determined by the balance

between the surface energy required to form the embryo, Es = 4R2 (where is the

surface energy per unit area, defined as the excess energy at the surface of a material

compared to the bulk) and the energy released when a spherical particle is formed, Er =

4R3L/3 (where is the density of the solid and L [KJ/mol] is the heat of the solution).

Solvent molecules can adsorb on the embryos and change their surface energy: the

critical radius will therefore depend not only on the material of the nucleating phase but

also on the solution phase [24].

2.5.3 Heterogeneous nucleation

Heterogeneous nucleation process occurs in the presence of foreign particles in

the solution. The unstable embryos with radius smaller than Rc or even individual ions

68
can adsorb onto the substrate. The energy required forming an interface between an

embryo and the solid substrate will usually be less than the energy required for

homogeneous nucleation, where no such interface exists, due to the catalytic function of

the substrate. Therefore, heterogeneous nucleation is energetically preferred over

homogeneous nucleation. These sub-critical nuclei can grow either by surface diffusion

or by material addition from the solution [24].

2.5.4 Crystal growth

Crystal growth is the series of processes by which an atom or a molecule is

incorporated into the surface of a crystal. Once the stable nuclei are formed, adsorption

of ionic species from the solution onto the nuclei will occur and leading to increase in

their sizes. This type of crystal growth can therefore be considered a self-assembling

process. In the case of ionic solids, the process involves the combination of cation and

anion on appropriate sites [41].

(XY) n + X + + Y - (XY) n+1 (2.06)

(XY)n+1 + X + + Y - (XY)n+2 (2.07)

where n is the number of X+ and Y- required for forming stable phase (XY)n. The rate of

growth is directly proportional to the supersaturation.

Rate of growth = Ko a(Q-S) (2.08)

where a is the surface area of exposed solid and Ko is a constant which is the

characteristics of particular precipitate. Q is the excess concentration above saturation

and S is the concentration at saturation. If the supersaturation is maintained at low level

69
throughout the process, the precipitation occurs and it results in a large number of

nucleation centres, upon which growth can occur. As a consequence, none of the

particle grows very large and a colloidal suspension consisting of finely divided solid

particles is formed.

Collision between particles can also play an important role in the crystal growth.

If the concentration of particles is high, the probability of a collision between two

particles increases, this may result in either aggregation or coalescence. In the process

of aggregation, when two particles approach each other under the presence of van der

Waals force, they will tend to stick together forming a large particle called aggregate.

Grain boundaries are found between individual crystals in an aggregate. However,

under certain conditions, surface diffusion may occur where two or more particles have

aggregated, resulting in the formation of a neck. This process is called coalescence and

may continue until one large particle is formed from the original aggregate as shown in

Fig.2.01 [24].

Figure 2.01: (a) Aggregation and (b) Coalescence of individual particles. [Adapted from 24]

70
2.6 Basic Mechanisms of Chemical Bath Deposition

There are four basic mechanisms for the compound formation. They are

i. Simple ion-by-ion mechanism,

ii. Simple cluster (hydroxide) mechanism,

iii. Complex-decomposition ion-by-ion mechanism and

iv. Complex decomposition cluster mechanism.

2.6.1 Ion-by-Ion mechanism

The simplest mechanism is usually called the ion-by-ion mechanism, since it

occurs in sequential ionic reactions. For the case of lead complexed with ammonia,

firstly there is a dissociation of the complex to release Pb2+ ions

Pb( NH3 ) 24+ = Pb2+ + 4NH3 (2.09)

At the same time the sulphide ion is formed

SC( NH2 ) 2 + 2OH- S2- + CH 2 N 2 + 2H 2O (2.10)

The ionic reactions of Pb 2+ and S2- form PbS molecules.

Pb2+ + S2- = PbS (2.11)

[ ][ ]
If the ion product Pb2+ S2- exceeds the solubility product K of PbS, then, neglecting

kinetics problem of nucleation PbS will be formed as a solid phase. To control the

number of ions and rate of reaction, a complex is needed to keep the metal ion in the

solution and to prevent the hydroxide from precipitating. The complex dissociates to

give the controlled the number of Pb 2+ ions which then combine with S2- ions to form

PbS films. The deposition mechanism of PbS is illustrated in Fig.2.02

71
Figure 2.02: Schematic diagrams for film deposition involved in the ion-by-ion
mechanism for the case of PbS: (a) Diffusion of S and Pb ions onto the substrate, (b)
Formation of PbS nuclei facilitated by the substrate, (c) Growth of the PbS nuclei by
adsorption of Pb and S ions from the solution while new PbS are nucleate, (d) Growth
of PbS crystal which adhere to each other. [Adapted from 24]

2.6.2 Simple cluster (hydroxide) mechanism

In Chemical bath deposition, optimum preparative parameters are chosen to

avoid the formation of metal hydroxide. However, metal hydroxides also play an

important role as reaction intermediates in the CBD process. At the start of the CBD

process (i.e reaction between Pb2+ and S-2 ions source), a precipitate of metal hydroxide

Pb(OH)2 is formed as a colloid rather than a precipitate or as an absorbed species on the

substrate but not in the bulk of the solution [42]. Then PbS is formed by reaction of

slowly generated S2- ions with Pb(OH)2 as shown in Fig.2.03.

nPb2+ + 2nOH-1 [Pb(OH)2 ]n (2.12)

SC( NH2 ) 2 + 2OH- S2- + CH 2 N 2 + 2H 2O (2.13)

72
 [Pb(OH)2 ]n + nS2- nPbS + 2nOH- (2.14)

Figure 2.03: Schematic diagram describing the probable steps in the hydroxide mechanism for
the case of PbS: (a) Diffusion of hydroxide colloids to the substrate, (b) Adhesion to the
substrate and reaction with sulphur ions, (c) Exchange of the hydroxide by sulphide, probably
starting at the surface of the colloids, (d) Reaction continues until most of the hydroxide is
converted to sulphide and (e) PbS particles adhere to each other and form an aggregated film.
Non adsorbed particles will also aggregate and precipitate out of the solution. [Adapted from
24]

2.6.3 Complexdecomposition ion-by-ion mechanism

Thiourea complexion (here (CH3COO)2Pb.3H2O and CH4N2S as Pb+2 ans S-2 ions

source respectively)

Pb2+ + (NH2 )2 CS = [(NH2 )2 CS - Pb]

This ion could be hydrolyzed by breaking the S-C bond to form PbS.

[(NH2 )2 CS - Pb]2+ + 2OH_ PbS + CN 2 H2 + 2H2O

73
If the Pb 2+ ion is absorbed on the substrate or on previously deposited PbS, the same

reaction would occur. If the PbS so formed remains bound to the substrate, the result

would be a film growth by an ion-by-ion complex-decomposition mechanism [42].

2.6.4 Complex-decomposition cluster mechanism

The basic mechanism of complex-decomposition cluster mechanism is based on

the formation of solid phase instead of reacting directly with a free anion, it form an

intermediate complex with the anion-forming reagent.

- Pb(OH)2 + (NH2 )2 CS = Pb(OH)2 - S - C(NH2 )2 (2.17)

where Pb(OH)2 is one molecule in the solid phase Pb(OH)2 cluster. This complex then

decomposes to PbS.

Pb(OH) 2 - S - C( NH2 ) 2 PbS + CN 2 H 2 + 2H 2O (2.18)

which means that the S-C bond of the thiourea breaks, leaving the S bound to Pb. It is

suggested that Pb(OH)2 forms initially on the substrate and catalyzes the thiourea

decomposition. The catalytic effect of the solid surface could be to decompose thiourea

to sulphide ion and not necessarily to catalyse the complex-decomposition mechanism

[42]. The process has been shown schematically in Fig.2.04.

74
Figure 2.04 Schematic diagrams showing the possible steps involved in the complex
decomposition mechanism for the case of PbS. The complex PbSLig, where Lig is a ligand
(or part of the S-forming species) decomposes to PbS on the substrate and homogeneously in
the solution (a) & (b). PbS nuclei formed grow by adsorption and decomposition of more
complex species (c) until a film of aggregated crystals is formed (d). [Adapted from 24].

75
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