CHAPTER 1
CHAPTER 1
INTRODUCTION
None of the myriad scientific papers I'd read prepared me for the patience and
diligence that go into scientific research. None had prepared me for the acute
attention to minutiae that keeps science accurate, and scientific integrity intact.
"Or for the tedium."
Vaughn Edelson “Concentration Crisis”
In Brown Alumni Magazine, July/August 2007
1
2
CHAPTER 1
1.1
PREAMBLE
Transition metal chalcogenides: oxides, sulfides, selenides and tellurides, are important
technological materials. Their potential is increasingly being recognised, with current and
potential applications of transition metal chalcogenides including: solar energy conversion,
solar control coatings, microelectronic devices, catalysts, sensors, optical filters and laser
sources [1-4].
Lead and cadmium chalcogenide compounds have been the focus of a
significant number of investigations for these types of applications. Copper sulfides also are
potentially useful in a range of areas, including solar control coatings, solar energy
conversion, electronic and low-temperature gas sensor applications. More recently, ternary
copper chalcogenides, such as copper indium selenides (CIS) or copper indium gallium
selenides (CIGS) are being used in solar cell technology. These ternary copper chalcogenides
have the potential for widespread use in a range of areas, particularly solar energy conversion,
and electronic sensor applications.
A variety of methods have traditionally been used to prepare high quality transition metal
chalcogenides. Each technique has its limitations. For example, solid state reactions require
the use of high temperatures to ensure the transition of solid reactants into the molten state.
Spray pyrolysis, chemical vapour deposition and vacuum evaporation reactions also require
high temperatures to enable the successful formation of the required chalcogenide. The heat
is applied either directly by heating the reactants or indirectly through heating of the substrate.
Chemical solution methods such as hydrothermal, solvothermal, and chemical bath deposition
(CBD) generally require lower temperatures for successful generation of quality films [5].
One of the earliest reports of the use of CBD was in the preparation of lead sulfide and lead
selenide thin films for use as photoconductive detectors during World War II [5, 6]. Since
then, CBD has become an attractive method due to its suitability for large scale deposition
and the ability to deposit onto polymer substrates. It is characterised by simple formulation,
ease of set up and low temperature requirements. The process generally operates under
3
ambient conditions and has the potential to replace expensive energy and equipment intensive
techniques. In CBD, the chalcogenide film forms when the substrate is immersed into a
dilute, generally alkaline, solution containing metal ions and an appropriate source of
chalcogenide ions. Complexation of the metal ions enables the rate of the reaction to be
controlled. The chalcogen anion is usually generated by the decomposition or hydrolysis of
an organic or inorganic precursor.
Despite the advances that have been made using CBD, the full development of the technology
has been hampered by poor understanding of the relationships between process chemistry and
film structure, factors that are dependent on the properties of the bath and deposition
precursors.
The process is sensitive to precursor concentrations and to the substrate
used [7-9]. The optimal deposition parameters are generally different for each compound
deposited.
Although there have been numerous papers published reporting the preparation of
chalcogenide thin films using CBD, Kaur et al. point out that the process has remained recipe
oriented with little understanding of the kinetics of the process [10]. With a few exceptions,
this is also true regarding a mechanistic understanding of the process. There is, therefore, a
need for careful investigation of the CBD process and identification of the conditions that
favour high quality coherent deposits.
1.2
AIMS AND OBJECTIVES
It has been proposed that the mechanism by which a film grows can determine both the
quality and the properties of the resulting film. Copper sulfide thin films deposited by CBD
generally exhibit poor adhesion but it is not clear if this is an inherent function of the process
or of other factors such as the substrate choice or bath composition. The aim of the research
project described in this thesis was to investigate the deposition and modification of CBD
copper sulfide in order to elucidate the mechanisms that take place during the processes.
4
CHAPTER 1
In order to achieve the aim of this research project, the following objectives were adopted:
i.
To investigate the effect of different bath parameters on the resulting deposit.
ii.
To investigate the deposition and modification processes by in-situ techniques.
These objectives were achieved by the use of a range of experimental methods including
scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman
spectroscopy, neutron reflectometry and small angle X-ray scattering (SAXS).
Surface enhanced Raman scattering (SERS) was used to investigate the very early stages of
the deposition process and is a novel application of this experimental technique. By using a
roughened gold electrode as the substrate, the presence of a monolayer or less of the deposit
was able to be detected.
Neutron reflectometry was also used to monitor the film deposition. Using this technique the
deposition process was monitored over a period of several hours, enabling the collection of
data relating to film thickness, scattering length density (SLD) and the number of layers
present. SAXS was trialled as a method to monitor the very early stages but was too energetic
and decomposed components of the chemical bath on the substrate surface.
1.3
STRUCTURE OF THESIS
This thesis contains eight chapters.
The early chapters (1–3) provide introductory
information: CBD basics, literature review, and experimental techniques. The later chapters
provide experimental results and discussion of the results followed by conclusions in the final
chapter.
Chapter 2 consists of an introduction to CBD, which is followed by a literature review of
CBD chalcogenide materials.
The conditions used for deposition provide the lead in to chapter 3. This is followed by a
summary of the experimental techniques used in this project which includes an introduction to
5
the theory behind the main techniques, the applications of each technique reported in the
literature and the experimental conditions used in this project.
Chapter 4 focuses on Raman characterisation of reference compounds, including precursors
and possible products.
Chapter 5 considers the structure and morphology of CBD copper sulfide thin film and the
precipitate using the microscopic techniques, optical, SEM, TEM and atomic force
microscopy (AFM).
An introduction to the composition of the deposits using energy
dispersive spectroscopy (EDAX) and electron diffraction (ED) is also included.
Characterisation of thin films and the precipitate provides the basis for chapter 6. Data
obtained from analytical techniques such as X-ray diffraction (XRD), Raman, SERS and
infrared spectroscopy (IR) are presented and discussed. Electrochemical data obtained from
thin films is also included.
Neutron scattering length densities (SLD)’s for a range of copper compounds are presented in
chapter 7, followed by neutron reflectometry and SAXS data relating to the structure and
early stages of CBD copper sulfide.
The final chapter presents conclusions that can be made from the results that have been
presented in preceding chapters. Suggestions relating to possible future work complete the
thesis.
1.4
THE COPPER-SULFUR SYSTEM
The copper-sulfur (CuxS) system is very complex, with five stable phases known to exist in
nature: covellite (Cu1.00S), anilite (Cu1.75S), digenite, (Cu1.80S), djurleite (Cu1.97S), and
chalcocite (Cu2.00S). Rosebloom prepared the first phase diagram of the various phases in
1966, although Djurle had previously used X-ray techniques to define the various phases [11].
Another two phases, yarrowite (Cu1.12S) and spionkopite (Cu1.40S), also referred to as
blaubleibender (blue remaining) covellite, were identified by Goble [12]. However, the
6
CHAPTER 1
composition of the blaubleibender covellites has been shown to vary within the range
x = 1.1 - 1.4 [13, 14]. In addition to the phases already mentioned, another phase, geerite
(Cu1.60S), has been identified by Goble and Robinson [15].
In chalcocite, the value of x in the formula CuxS is known to vary between 1.997 – 2.
Samples with higher x values are referred to as high chalcocite and those with lower values
low chalcocite. Djurleite also has a range of compositions, with x = 1.94 - 1.97, and again
they are referred to as high djurleite and low djurleite.
1.4.1
CRYSTAL STRUCTURE
In nature, chalcocite and djurleite are physically difficult to distinguish from each other and
are often found intermittently intertwined [11]. Their crystal structures are similar, with both
based on a monoclinic unit cell [11]. The representation of the crystal structure of chalcocite
from Evans is presented in Figure 1.1 [16]. Inspection of the model shows that the structure
is composed of hexagonal layers, with alternating copper and sulfur ions forming the rings.
Figure 1.1 Representation of the crystal structure of chalcocite [16].
The configuration of covellite has been the topic of much discussion over a number of years.
It has an unusual and complicated structure [17]. Covellite contains 6 formula units in the
unit cell, with four copper ions having tetrahedral coordination and two with triangular
coordination (Figure 1.2). For the sulfur atoms, four form S2 groups and two single sulfide
ions [18]. According to Wyckoff, covellite has an elongated elementary cell and belongs to
7
the hexagonal crystal system with space group P63/mmc(D46h) [19]. The crystal structure is
characterised by covalent S-S bonds which join alternate layers of CuS3-Cu3S together [20].
The blaubleibender covellites have hexagonal symmetry with covalent S-S bonds, but they
are distinctly different from either covellite or digenite [12, 21]
The oxidation state of copper ions in covellite is often perceived to be copper(II) because of
the stoichiometry of the compound [22]. However, it has commonly been considered as
having both copper(I) and copper(II) ions. For example, Vaughan and Tossell proposed a
structure where copper(II) ions occupy triangular and copper(I) ions tetrahedral sites [23].
However, Goh et al. have provided X-ray photoelectron spectroscopy (XPS) evidence that all
the copper ions in covellite are present as copper(I), a view that is now generally
accepted [24].
Figure 1.2 Representation of the crystal structure of covellite (CuS) [25].
8
CHAPTER 1
1.4.2
SYNTHETIC COPPER SULFIDE
Copper sulfide particles have traditionally been prepared by a variety of methods including
solid
state
reactions,
processes [10, 26].
high
temperature
synthesis
methods,
and
hydrothermal
The formation of copper sulfide from aqueous solution is well
established, although the details of the solutions vary mostly in the choice of copper salt
and/or sulfiding agent. Hydrogen sulfide is often used as the sulfur source in aqueous copper
solutions and is either bubbled through from an external source or generated in-situ [27, 28].
Thin films of copper sulfide have been prepared by dipping copper films in a solution of
thiourea (Tu) [29]. CBD is another method of forming copper sulfide thin films from aqueous
solution and is the focus of this thesis.
The complexity of the copper sulfur system leads to mixed phase products during the
formation process of many synthetic methods, including CBD. Because many applications of
copper sulfide require one specific phase, a streamlined method and production of selected
copper sulfides of the desired phase only is required.
9
1.5
REFERENCES
1.
Yamaguchi, T., Y. Yamamoto, T. Tanaka, Y. Demizu and A. Yoshida, (Cd,Zn)S thin
films prepared by chemical bath deposition for photovoltaic devices. Thin Solid
Films, 1996. 281-282: p. 375-378.
2.
Teteris, J., Holographic recording in amorphous chalcogenide thin films. Current
Opinion in Solid State and Materials Science, 2003. 7: p. 127-134.
3.
Savadogo, O., Chemically and electrochemically deposited thin films for solar energy
materials. Solar Energy Materials and Solar Cells, 1998. 52: p. 361-388.
4.
Sang, B., W.N. Shafarman and R.W. Birkmire. Investigation of chemical-bathdeposited ZnS buffer layers for Cu(InGa)Se2 thin film solar cells. in 29th IEEE
Photovoltaic Specialists Conference. 2002.
5.
Chopra, K.L., R.C. Kainthla, D.K. Pandya and A.P. Thakoor, Chemical solution
deposition of inorganic films, in Physics of Thin Films, G. Hass, Editor. 1982,
Academic Press: New York. p. 167-235.
6.
Hodes, G. and G. Calzaferri, Chemical solution deposition of silver halide films.
Advanced Functional Materials, 2002. 12: p. 501-505.
7.
Shandalov, M. and Y. Golan, Microstructure and morphology evolution in chemical
solution deposited semiconductor films: 2. PbSe on As face of GaAs(111). The
European Physical Journal - Applied Physics, 2004. 28: p. 51-57.
8.
Shandalov, M. and Y. Golan, Microstructure and morphology evolution in chemical
solution deposited semiconductor films: 3. PbSe on GaAs vs Si substrate. The
European Physical Journal - Applied Physics, 2005. 31: p. 27-30.
9.
Shandalov, M. and Y. Golan, Microstructure and morphology evolution in chemical
deposited PbSe films on GaAs(100). The European Physical Journal - Applied
Physics, 2003. 24: p. 13-20.
10.
Kaur, I., D.K. Pandya and K.L. Chopra, Growth kinetics and polymorphism of
chemically deposited CdS films. Journal of the Electrochemical Society, 1980. 127:
p. 943-948.
11.
Evans, H.T., Copper coordination on low chalcocite and djerleite and other copperrich sulfides. American Mineralogist, 1981. 66: p. 807-818.
12.
Goble, R.J., The relationship between crystal structure, bonding and cell dimensions
in the copper sulfides. Canadian Mineralogist, 1985. 23: p. 61-76.
13.
Potter, R.W., An electrochemical investigation of the system copper-sulfur. Economic
Geology, 1997. 72: p. 1524-1542.
14.
Goble, R.J. and D.G.W. Smith, Electron microprobe investigation of copper
sulphides in the Precambrian Lewis series of S.W, Alberta, Canada. Canadian
Mineralogist, 1973. 12: p. 95-103.
15.
Goble, R.J. and G. Robinson, Geerite, Cu1.60S, a new copper sulfide from Dekalb
township, New York. Canadian Mineralogist, 1980. 18: p. 519-523.
16.
Evans, H.T., The crystal structures of low chalcocite and djurleite. Zeitschrift fuer
Kristallographie, 1979. 150: p. 229-320.
17.
Lavrentyev, A.A., B.V. Gabrelian, I.Y. Nikiforov, J.J. Rehr and A.L. Ankudinov, The
electron energy structure of some sulfides of iron and copper. Journal of Electron
Spectroscopy and related Phenomena, 2004. 137-140: p. 495-498.
10
CHAPTER 1
18.
Evans, H.T. and J.A. Konnert, Crystal structure refinement of covellite. American
Mineralogist, 1976. 61: p. 996-1000.
19.
Wyckoff, R.W.G., Crystal Structures. 1965, New York: Interscience.
20.
Pattrick, R.A.D., J.F.W. Mosselmans, J.M. Carnock, K.E.R. England, G.R. Helz,
C.D. Garner, and D.J. Vaughan, The structure of amorphous copper sulfide
precipitates: an X-ray absorption study. Geochimica et Cosmochimica Acta, 1997.
61: p. 2023-2036.
21.
Whiteside, L.S. and R.J. Goble, Structural and compositional changes in copper
sulfides during leaching and dissolution. Canadian Mineralogist, 1986. 24:
p. 247-258.
22.
Ruetschi, P. and R.F. Amile, The Electrode Potential of the Semiconductor CuS in
solutions of copper ions and sulfide ions. Journal of the Electrochemical Society,
1965. 112: p. 665-670.
23.
Vaughan, D.J. and J.A. Tossell, The chemical bond and the properties of sulfide
minerals. I. Zn, Fe and Cu in tetrahedral and triangular coordinations with sulfur.
Canadian Mineralogist, 1980. 18: p. 157-163.
24.
Goh, S.W., A.N. Buckley and R.N. Lamb, Copper(II) sulfide? Minerals Engineering,
2006. 19: p. 204-208.
25.
Takeuchi, Y., Y. Kudoh and G. Sato, The crystal structure of coellite CuS under high
pressure up to 33 kbar. Zeitschrift fur Kristallographie, 1985. 173: p. 119-128.
26.
Froment, M., H. Cachet, H. Essaaudu, G. Maurin and R. Cortes, Metal chalcogenide
semiconductors growth from aqueous solutions. Pure & Applied Chemistry, 1997. 69:
p. 77-82.
27.
Silvester, E.J., F. Grieser, B.A. Sexton and T.W. Healy, Spectroscopic studies on
copper sulfide sols. Langmuir, 1991. 7: p. 2917-2922.
28.
Drummond, D.M., F. Grieser, T.W. Healy, E.J. Silvester and M. Giersig, Steady-state
radiolysis study of the reductive dissolution of ultrasmall colloidal CuS. Langmuir,
1999. 15: p. 6637-6642.
29.
Leon, M. and F. Arjona, Electron diffraction analysis of CuxS films obtained through
a sulphurisation process. Journal of Physics D: Applied Physics, 1986. 19:
p. 1529-1534.
11
12
CHAPTER 2
CHAPTER 2
BASIC THEORY AND
LITERATURE REVIEW OF CHEMICAL
BATH DEPOSITION
I do not feel obliged to believe that the same God who has endowed us with sense,
reason, and intellect has intended us to forgo their use.
Galileo Galilei
13
14
CHAPTER 2
2.1
CBD: INTRODUCTION
A review of CBD as early as 1982 listed approximately twenty chalcogenide materials that
had been synthesised by the technique [1]. The number of known materials now exceeds 50,
most of which are semiconductors. The reasons for the increase can be related to the
advantages of the technique such as:
i.
Great flexibility in substrate selection and significantly reduced manufacturing
costs [2]
ii.
Ease of application to a wide range of chalcogenide compounds [3]
iii.
Low temperature processing temperatures and the possibility for large scale
deposition [4].
iv.
The ease with which binary chalcogenides can be doped or ternary compounds
prepared.
In the years succeeding the review in 1982, numerous reports have been published reporting
the preparation of chalcogenide thin films using CBD, including at least three further
reviews [5-7]. Despite the large volume of published literature, Kaur et al. point out that the
process has remained recipe oriented with little understanding of the kinetics of the
process [8]. With a few exceptions, this also appears to be true with respect to a mechanistic
understanding of the process.
Cadmium, lead and zinc chalcogenides are the compounds most frequently prepared by the
CBD technique [9-14].
In addition, a number of ternary chalcogenides have been
prepared [15-18].
15
2.2
CBD: BASIC PRINCIPLES
The basic principles behind the CBD process are similar to those for all precipitation
reactions, and are based on the relative solubility of the product.
The equilibrium
concentration of ions in solution is defined by the solubility product (SP) expression
(equation 2.1):
K sp = [M n+ ]a [X m- ]b
(2.1)
where Ksp is the solubility constant and [Mn+]a[Xm-]b is the ionic product (IP).
aMn+ ions and bXm- ions are formed from the solid as in equation (2.2).
M a X b ( s ) aM n + (aq ) + bX m − (aq )
(2.2)
The preparation of metal sulfides by introducing S2- ions into an aqueous solution of a metal
salt to effect chemical precipitation is well established. S2- ions can be generated in-situ by
the hydrolysis of hydrogen sulfide gas in aqueous solutions [1]. The equations for this
hydrolysis are presented in equations 2.3 and 2.4:
H 2S + OH - HS- + H 2 O
(2.3)
HS- + OH - S2- + H 2 O
(2.4)
The sulfide ions formed in equation 2.4 react with the metal ions in solution to produce the
metal sulfide. Precipitation occurs when IP > Ksp.
In CBD, this process is modified such that precipitation is controlled to eliminate or reduce
spontaneous precipitation. The first method of controlling the reaction is by complexing the
metal ions such that only a controlled number of free ions are available. As the complex
dissociates, the following equation applies (assuming a metal ion with a charge of 2+):
M(A) 2+ M 2+ + A
At any given temperature, the concentration of free metal ions is given by equation 2.6.
16
(2.5)
CHAPTER 2
Ki =
[M 2+ ][A]
[M(A) 2+ ]
(2.6)
In this equation, Ki is the stability constant of the complex ion. A complex with a small K
value is most stable, and will therefore have a lower concentration of metal ions in solution
than one with a higher constant.
By controlling the concentration and temperature of
complexing reagent, the concentration of metal ions in solution can also be controlled [1]. If
these ions can be generated at a surface, then it leads to CBD of a film.
The second method of controlling the reaction is by the slow and uniform generation of the
chalcogen ions in solution. Thiourea is frequently chosen as the sulfide ion source for metal
sulfides and is considered to produce sulfide ions according to equations 2.7 and 2.8 [1].
Equation
2.8
can
(NH 2 ) 2CS + OH - CH 2 N 2 + H 2 O + HS-
(2.7)
HS- S2- + H + , Ka = 10-17.3
(2.8)
also
be
written
in
terms
of
hydroxide
ion
concentration: HS- + OH - S2- + H 2 O , Ka = 10-3.3. In mildly alkaline solutions with
pH~11, which is the pH of many chemical baths for CBD, the sulfide ion concentration can
be written in terms of HS-, giving S2- = 10-4.3 [HS- ] . This shows that most of the sulfur ions
will be present in the form of HS- rather than S2- [19]. Pentia et al. proposed that the main
factors that control the generation rate of M2+ and S2- precursors were: precursor
concentration, pH and temperature [3].
In the preparation of metal selenides, sodium selenosulfate has often been used as a source of
selenide ions with the ions formed according to equation 2.9 [20]:
Na 2SeSO3 + 2OH - Na 2SO 4 + H 2O + Se 2-
(2.9)
Telluride compounds are more difficult to synthesise than other chalcogenides because of the
instability of telluride ions. However, it has been reported that compounds such as sodium
dithionite can dissolve telluride, thereby producing telluride ions in-situ [1].
17
As with the homogenous precipitation of metal sulfides, the formation of thin films only
occurs when the ionic product of the metal and chalcogen ions exceeds the solubility product
of the corresponding chalcogenide. Thin films form by heterogenous precipitation on the
surface of the substrate which is immersed in the solution.
2.3
CBD: LITERATURE REVIEW
2.3.1
BINARY CHALCOGENIDES
The wide range of binary chalcogenides prepared using CBD is summarised in Table 2.1. It
can be seen from the table that while there are a number of compounds with only one
reference, there are others that have several references. A significant portion of the literature
relates to cadmium sulfide, a wide band gap material, which is well suited to photovoltaic
applications. CBD has been shown to be particularly successful in preparing high quality
cadmium sulfide films, with the mechanisms and kinetics extensively investigated. Cadmium
sulfide thus provides a good reference position for understanding the general principles of
CBD. Cadmium sulfide deposition will be discussed in more detail in section 2.6.1.
18
CHAPTER 2
Table 2.1 Summary of binary chalcogenide thin films that have been prepared by CBD with key
references.
BINARY CHALCOGENIDES
As2S3 [21-23]
Ag2S [5, 24-30]
BeS [31]
Bi2S3 [21, 28, 32-44]
Bi2Se3 [33, 35, 45, 46]
CdS [2, 4, 8, 10, 21, 33, 47-74]
CdSe [20, 33, 51, 75-82]
CoS [33, 83]
CoSe [33]
CuxS [6, 15, 16, 21, 43, 84-108]
CuxSe [75, 109-112]
HgS [113]
HgSe [76]
MnS [114]
In2S3 [6, 61, 115-118]
MoS2 [119]
MoSe2 [119]
NiS [33, 120]
NiSe [33]
PbS [17, 121]
PbSe [11, 20]
PdS2 [21, 122-130]
Sb2S3 [21, 131]
Sb2Se3 [33, 131]
SnSx [21]
SnSex [132]
TlS [33, 133]
TlSe [33, 96]
ZnS [12-14, 21, 33, 56, 61, 134-142]
ZnSe [33]
19
2.3.2
TERNARY CHALCOGENIDES
Table 2.2 Summary of ternary chalcogenide thin films that have been prepared by CBD with key
references
TERNARY CHALCOGENIDES
AgBiS2 [28]
AgSbSe2 [143]
CdCr2S4 [144, 145]
CdHgSe [146-148]
CdxPb1-xS [3, 149]
Cd1-xPbxSe [150]
CdxZn1-xS [18, 61, 151]
CdxZn1-xSe [152]
CdSxSe1-x [153-156]
CdS1-xTex [157]
CuBiS2 [158]
CuInS2 [106]
CuInSe2 [159-162]
CuSnSx [100]
CuSbS2 [163]
HgCr2S4 [145, 164]
In(OH)xSy [165-168]
Pb1-xFexS [169-171]
Pb1-xHgxS [172]
PbSnS3 [173]
ZnSx(OH)1-x [174]
ZnSxSe1-x [175]
An important extension of CBD is the preparation of ternary semiconductors such as copper
indium sulfide (CuInS2) and copper indium selenide (CuInSe2), which have significant
technological application in electronic and optoelectronic devices [156]. Semiconductors of
mixed composition enable semiconductor properties to be tuned – this is particularly useful
for photoconductive detectors where a specific sensitivity range is desired [19]. Accordingly a
summary of currently known ternary chalcogenides prepared with CBD is provided in Table
2.2.
20
CHAPTER 2
The preparation of ternary compounds by CBD has been undertaken in several ways. The
simplest method is to prepare bi-layers of two different chalcogenide materials such as
Bi2S3-CuxS [16], PbS-CdS [176, 177] and PbS-CuxS [17, 178]. Formation of the ternary
chalcogenide then occurs by interaction of the two layers during the annealing process. For
example, Oladeji and Chow prepared the ternary Cd1-xZnxS by growing thin layers of zinc
sulfide and cadmium sulfide by CBD, which were subsequently annealed to form the ternary
compound [179].
CBD can also be used in conjunction with other techniques to form ternary compounds. For
example, Bindu et al. combined CBD with vacuum evaporation in order to provide a less
toxic pathway to the production of indium selenide (In2Se3) and copper indium selenide
(CuInSe2) [180-182]. Indium or indium and copper were evaporated sequentially onto CBD
selenium thin films, and the resulting films annealed at temperatures up to 723 K to form the
binary or ternary compound.
CBD ternary chalcogenides can also be prepared by the combination of all reactants in a
single bath. This method was used by Pentia et al. to synthesis the ternary chalcogenide
cadmium lead sulfide (CdxPb1-xS) on glass substrates [3]. Using a range of Cd:Pb ratios, the
metal ions were stabilised with EDTA (ethylenediaminetetraacetic acid) in an alkaline
solution to form [Cd (EDTA)]2- and [Pb(EDTA)]2- complexes. As the metal complexes
dissociated, metal ions released slowly in solution reacted with sulfide ions formed from the
decomposition of thiourea. The resulting films were annealed in air at 100 °C, and showed a
variance in crystallinity which was dependent on the value of x. It was noted that for
0.4 > x > 0.8 ternary chalcogenide films were formed, but for x ~ 0.5, the resulting films were
a mixture of cadmium sulfide and lead sulfide.
The single bath technique was also employed by Kainthla et al. to synthesise thin films of
cadmium selenide sulfide (CdSe1-xSx) [156]. Varying ratios of selenosulfate and thiourea
were added to a cadmium salt complexed with ammonia, which led to the formation of
polycrystalline thin films with a grain size of ~600 Å. It was noted that the rate of formation
21
of cadmium sulfide was greater than the rate of formation of cadmium selenide, which meant
that in the resulting films, the ratio S:Se was greater in the films than it was in solution. For
all ratios of S:Se, alloy films of cadmium selenide sulfide resulted.
The examples provided above generally relate to chalcogenides other than copper. The
preparation of ternary copper chalcogenides is discussed in more detail in section 2.5.
2.4
CBD: COPPER SULFIDE
Copper sulfide is the major focus of this dissertation and was one of the first compounds
reported to be formed using a form of CBD [19].
A wide range of chemical bath
compositions have been used in the preparation of CBD copper sulfide thin films, with quality
usable films reportedly formed from each bath solution, an indication of the versatility of the
process. Table 2.3 provides a summary of bath formulations: copper salt, complexing reagent
and sulfiding reagent as reported in the literature. The pH of the chemical bath is also
included where it was reported.
The complexation of metal ions is an important aspect of CBD. The decomposition of the
formed complex is a major factor that controls the rate of the reaction. Ammonia and
triethanolamine (TEA) are commonly used complexing reagents for CBD copper sulfide, with
sodium citrate, EDTA, ethylenediame (en) and 1,4,8,11-tetraazacyclotetradecane (cyclam)
also having been used. While the metal complex was generally formed in situ prior to the
addition of the sulfiding reagent, in the case of ethylenediame and cyclam, previously
prepared copper complexes were used. Although not specified, it was assumed that these
were dissolved in water prior to use. The use of two different complexing reagents in the
same chemical bath was also common. In eleven of the bath solutions cited in Table 2.3,
copper ions were complexed with both ammonia and triethanolamine, which led to the
coexistence of several different copper complexes in the chemical bath [89].
Possible
complexes in baths using both triethanolamine and ammonia as complexing reagents for
copper and with thiourea as the sulfide source include: [Cu(TEA)n]2+, [Cu(NH3)m]2+, and
22
CHAPTER 2
[Cu(Tu)6]2+, and equilibrium combinations of these. The relative concentrations would be
expected to change during the deposition process [89].
Table 2.3 Summary of reported bath compositions for CBD copper sulfide.
EDTA =
ethylenediaminetetraactetic acid; TEA = triethanolamine; NH3 = ammonia solution; Tu = thiourea;
DiTu = dimethylthiourea; TA = thioacetamide; SC = sodium citrate; en = ethylenediamine; cyclam =
1,4,8,11-tetraazacyclotetradecane
Copper salt
Complexing
reagents
Sulfide
sources
pH
Reference
CuCl
EDTA
Tu
8.5 -11.5
[94]
CuCl2
TEA/NH3
Tu
CuCl2
TEA/NH3
Tu
CuSO4
TEA/NH3
Tu
[84]
CuSO4,
NH3
Tu
[96]
Tu
[15, 103]
Cu(en)2(ClO4)2,
[16, 43, 86, 89, 90,
98-100, 108]
9.8
[106]
CuCl2
Na2S2O3
DiTu
[101]
CuCl2
SC
TA
[102]
CuSO4/Cu(NO3)2
Na2S2O3
5
[88, 104]
CuSO4
Na2S2O3
2.5
[105]
CuSO4
Na2S2O3
2.2
[21]
CuSO4
Na2S2O3
0.5
[87]
CuSO4
Na2S2O3
[97]
[Cu(cyclam)]2+
Na2S
[107]
23
The pH of the bath solution has been shown to be important in the preparation of many
chalcogenide materials, with alkaline baths essential for the controlled decomposition of
thiourea. In many papers, the pH was not cited for copper sulfide deposition baths, but the
use of sodium hydroxide in most solutions was sufficient to ensure that the CBD baths were
alkaline. Varkey reported pH values in the range between 8.5 – 11.5, with a pH of 10
considered to be the optimum value for the system which he was using [94]. The use of a
buffer to control the pH during the deposition process would be a logical proposition but did
not appear to be considered in the literature.
Four papers reported the CBD of copper sulfide from acidic baths composed of copper sulfate
and thiosulfate with pH values between 0.5 – 5. The pH value of 0.5, quoted for one bath, is
questionable. This bath composition contained only copper sulfate and sodium thiosulfate
with no added acid [87]. This would infer a pH value of ~5 as reported in other papers
[88, 104]. A bath of similar formulation required phosphoric acid to adjust the pH to a value
of 2.5 [105].
The sulfide source in CBD is often the factor which determines if the bath needs to be acidic
or alkaline. Of the five different sulfur sources listed in Table 2.3, thiosulfate was the only
sulfide source reported to be used in acid solutions. In alkaline solutions, the most widely
used sulfide source for the formation of copper sulfide thin films was thiourea. Other sulfur
containing sources that were used were dimethylthiourea, thioacetamide and sodium sulfide.
The use of thiosulfate often eliminated the need for a separate copper complexing reagent as
the thiosulfate served that purpose as well.
Preparation of a CBD copper sulfide differs from most other chalcogenide materials in that
the resulting product has copper present as copper(I). This means that when copper(II) salts
are used as the copper ion source, reduction of the copper(II) ion is required at some stage
during the deposition process. When thiourea is used in the CBD process, copper(II) will be
reduced to copper(I) and the ion stabilised as a thiourea complex [183].
While the
formulation of the copper salt used does not appear to be critical, they are generally simple
24
CHAPTER 2
inorganic salts such as copper(II) chloride and copper(II) sulfate. These were the most
extensively used salts. The bath composition reported by Varkey is interesting in that a
copper(I) salt rather than a copper(II) salt was used as the copper ion source [94]. The
solubility of copper(I) chloride was increased by the addition of sodium chloride.
Several compounds used as the sulfide source are also known to be reducing agents [19].
Luther et al. proposed that when copper sulfide particles are synthesised from aqueous
solutions, reduction of the copper(II) ion occurs on addition of the sulfide ion and before the
appearance of any particles [184]. Lincot et al. proposed a similar timing of the reduction of
the copper(II) ion in the CBD of copper sulfide thin films [6].
2.4.1
SUBSTRATES AND SUBSTRATE PREPARATION
One of the advantages of CBD thin films is the range of substrates that can be used on which
films deposit. This is shown by the wide range of substrates that have been used for copper
sulfide thin film deposition. The main criteria for substrate selection are that the growing film
will attach to the substrate and that the substrate will not dissolve in the chemical bath.
Glass substrates are commonly used; microscope slides for small scale projects and glass
sheets for large scale projects [87, 89, 90, 94, 101, 102, 104, 108]. Glass generally has been
immersed in a vertical position in the bath solution, although with large coating areas, two
sheets have been supported with a set distance between them and the CBD solution filled in
the gap [90, 108].
The use of other substrates have also been trialled. Various polymer substrates were also
used: 25 µm Kapton polyimide [98], modified low density polyethylene (LDPE) [105],
transparent polyester sheets (overhead transparencies) [88, 104], PMMA (polymethyl
methacrylate) sheets [99], PET (polyethylene terephthalate) films [97] and PES
(polyethersulfone) [102]. The polymer substrates were commonly floated on top of the CBD
solution during the deposition process.
Other materials include PZT (ferroelectric
piezoelectric transducer) films [88], iron, steel, aluminium, zinc and copper [94].
25
The preparation of substrates is a critical aspect that can contribute to film adherence. Several
cleaning regimes have been proposed. Glass substrates have been cleaned with detergent,
chromic acid, rinsed with water and dried [89], ultrasonically cleaned [87], or treated in a
solution of tin(II) chloride (this assisted with adhesion of the copper sulfide) [101, 105].
Overhead transparencies have been ultrasonically cleaned, soaked in tin(II) chloride, washed
with water and dried in air [88, 104].
The modification of polymer surfaces have contributed to enhanced film quality. PET films
were soaked in a methanol (1 %) solution of PEI (poly(ethyleneimine)) for 24 hours prior to
the CBD of copper sulfide to promote the formation of electrically conductive films [97]. It
was noted that without this treatment, there was a poor covering of copper sulfide on the PET
films. Thin sheets of LDPE (low density polyethylene) were functionalised by immersing in
an aqueous permanganate/hydrochloric acid solution for 8 hours prior to the chemical
deposition of copper sulfide [105]. Hu and Nair had observed that while a thin layer of zinc
sulfide on glass substrates promoted adhesion of copper sulfide to the substrate a similar
treatment undertaken on sheets of PMMA had the opposite effect, with the copper sulfide
films being less adhesive [99].
2.4.2
FILM DEPOSITION
The control of chemical bath temperatures has been proposed for the deposition of CBD
copper sulfide.
This influences the film formation rate and requires a corresponding
deposition time. Table 2.4 provides a summary of deposition times reported as a function of
bath temperature with film thickness where provided. The range of reported times and film
thicknesses reflect differences in bath composition as well as the substrates used. Film
thickness can also be affected by the use of modified substrates which provide nucleation sites
on which the growing film can form. Pre induced nucleation sites enabled any induction
period to be reduced or bypassed.
26
CHAPTER 2
Nair et al. noted that at room temperature, the deposition time could be extended from 12 – 20
hours if the thiourea concentration was reduced by half [90].
It is interesting to note that
deposition times reported for films deposited at 10 °C and 22 °C are generally less than that
given for 25 °C [89, 90, 101, 103].
It is possible that this may be a function of bath
composition or substrate preparation for a particular system.
Table 2.4 Summary of deposition times for CBD copper sulfide thin films at various temperatures.
BATH
TEMPERATURE (°C)
DEPOSITION
TIME (HOURS)
FILM
THICKNESS (nm)
REFERENCE
10
5.5
[103]
22
3 - 4.5
[89]
25
2 - 48
[101]
25
12
100 - 350
[90]
26
3; 6
100, 300
[99]
27
4; 8
75, 100
[102]
30
5.5
50
2.5
100 - 350
[90]
50
1
180
[104]
50
0.5 - 2.5
60
0.67
60
0.5; 2
[97]
70
0.5; 2
[97]
[103]
[16, 185]
370
[87]
Nair and Nair observed that for constant bath composition, an increase in bath temperature
reduced the induction period, but produced films with poor adhesion to glass substrates [89].
Films deposited onto glass or titanium substrates from acidic copper sulfate/thiosulfate baths,
appeared uneven and non adherent [21].
A growth mechanism for the formation of CBD copper sulfide thin films was proposed by
Fatas et al. [84]. This was based on the formation of copper(II) hydroxide nuclei on the
27
substrate surface as in Kaur’s model [8]. Interaction of thiourea with these surface hydroxide
nuclei stimulated the oxidation of thiourea to formamidine disulfide (FDS), with the
corresponding simultaneous reduction of copper(II) to copper(I).
Dissociation of
formamidine disulfide formed sulfide ions which then reacted with the copper(I) ions to form
chalcocite (Cu2S).
2.4.3
FILM MODIFICATION AND PHASE DETERMINATION
The most common method reported for the modification of binary copper sulfide was
annealing at various temperatures in either air or an inert atmosphere, with the atmosphere
and temperature determining the copper sulfide phases that formed. Annealing in air at
temperatures up to 200 °C was shown to form covellite (CuS) while at temperatures over
220 °C in air, the formed CuS decomposed to form sulfates, oxosulfates etc. [102]. When an
inert atmosphere, such as nitrogen was used, phases such as digenite (Cu1.8S) and
djurleite (Cu1.96S) were observed to form at 300 °C and 400 °C respectively [101].
A range of techniques have been used to identify the copper sulfide phases obtained through
CBD. SEM was often used to provide a visual representation of film topography and to give
an estimate of grain size [87, 103]. Electron microscopy was often combined with EDAX
analyses to provide an overview of the elemental composition of the films. Other techniques
that were used to assist in identification of the copper sulfide phases were optical
transmission [90,
102,
104,
108],
Raman
spectroscopy
[186],
IR
spectroscopy,
XRD [103, 104] and Rutherford back scattering [88, 104].
Nair and Nair used a combination of optical transmission and specular reflectance
measurements to calculate the resistivity of the CBD copper sulfide films [89]. From these
measurements, they estimated the extent of deviation from the stoichiometric chalcocite.
They acknowledged, though, that there is a range of literature values for the same
composition due to variations in grain size and film thickness. The authors determined, using
this method, that during the initial phase of the deposition, the films had a composition close
28
CHAPTER 2
to that of chalcocite. They also observed that when multiple depositions were undertaken on
the same substrate, each successive layer appeared to follow the same composition pattern of
the first deposition.
2.5
TERNARY AND BI-LAYER COPPER CHALCOGENIDES
A range of copper ternary chalcogenides, including CuInS2 [106], CuInSe2 [159, 161, 162,
187, 188], CuIn1-xGaxSe2 [189], CuBiS2 [158], and CuSbS2 [163], have been prepared
following the methods outlined in section 1.3.2.
The sequential deposition method followed by annealing was used by RodriguezLazcano et al. to produce thin films of CuSbS2 [163]. Nair et al. successfully produced thin
films of Cu4SnS4 by annealing together, under nitrogen, sequentially deposited films of tin
sulfide and copper sulfide (CuS) [100].
A novel method was used by Grijalva and Inoue who produced Ag2S/CuS thin films by
immersing CBD copper sulfide (CuS) films in a silver nitrate solution [15].
Ag+ ions
exchanged with Cu+ ions in the copper sulfide thin film to form mixed Ag2S/CuS films.
The single bath method was used by Suarez and Nair, who prepared good quality (PbS)1x(CuS)x
thin films by combining varying ratios of separately prepared chemical baths for lead
sulfide and copper sulfide (CuS) deposition [178]. Adjustment of the ratios Pb2+:Cu2+ was
shown to change the properties of the resulting film, with quality films resulting for
deposition times of up to 25 hours at 25 °C. A typical growth pattern was observed: initial
slow growth during which nucleation centres formed, a steady growth stage and finally a
slower growth period.
2.6
CBD: MECHANISTIC STUDIES
Investigations into the mechanisms of CBD thin film growth have focused on the
chalcogenides of cadmium, lead and zinc, with two growth mechanisms identified [79, 190]:
29
• Ion by ion growth.
• Cluster by cluster growth.
Ion by ion growth is defined as the formation of chalcogenide nuclei on the surface of the
substrate followed by subsequent growth on those nuclei. Conversely, cluster by cluster
growth results from the formation of colloids in solution, which then adsorb onto the substrate
and coagulate to form a film [79, 190]. The colloidal particles can also aggregate in solution
to form stable clusters that do not deposit onto a surface.
O’Brien and McAleese suggest that the two growth mechanisms are not well-defined, but that
there are more pathways such as mixed processes whereby the two main mechanisms operate
concurrently [56]. Table 2.5 presents four possible chemical reactions, proposed by O’Brien
and McAleese, by which films may form. The description of the third and fourth steps as
thiourea metathesis reactions is questionable, as it would be more correct to describe these
steps as thiourea decomposition reactions.
The sulfide ions, formed during the
decomposition, could then combine with the metal ions to form the metal sulfide film.
Table 2.5 Four possible reaction steps proposed by O’Brien and McAleese leading to the formation of
high quality thin films [56].
M2+ + S2- films → 'MS' nuclei → 'MS' particulate
2+
M
-
+ OH → 'M(OH)2'nuclei → 'M(OH)2'particulate
M(OH)2'nuclei + (NH2)2CS → 'MS'nuclei → MSparticulate
2+
MS'nuclei + (NH2)2CS + M
→ MSparticulate
Ion by ion growth, nucleation and growth
Ion by ion growth in solution, nucleation
and growth of hydroxide
Thiourea ‘metathesis’
hydroxide
at
surface
of
Thiourea ‘metathesis’ at surface of sulfide
Mechanistic studies undertaken on CBD metal chalcogenides provide useful background
information that may assist in the identification of growth mechanisms operating for copper
sulfide.
However, there are differences between copper sulfide and other metal
chalcogenides.
For example, for cadmium sulfide, the mechanisms merely involve the
combination of cadmium ions and sulfide ions and the formation of only one crystal structure.
30
CHAPTER 2
In the copper sulfide system, however, there are extra steps to consider, the most significant
being the required reduction of Cu2+ to Cu+ for the formation of all sulfide phases
2.6.1
CADMIUM SULFIDE
There has been strong evidence presented for the existence of two separate growth
mechanisms operating for cadmium sulfide: ion by ion growth next to the substrate, forming a
dense, compact inner layer, and cluster by cluster growth which formed a porous less adherent
outer layer [2, 4]: Other reports have confirmed the ion-by-ion growth mechanism in the
early stages of nucleation and growth [64].
The role of metal hydroxide groups has been shown to play a key role in the deposition
process [1, 9, 20, 64, 190]. High quality films have been reported when there has been visible
hydroxide observed in the chemical bath [1, 8]. It was proposed that hydroxide particles
provided a catalytic surface for the decomposition of thiourea, with thin film deposition based
on colloidal formation and adsorption of colloidal particles on the surface [1].
The role of hydroxide in solution can be represented graphically (Figure 2.1) by plotting the
results of the following two equations using pH and p[Cd2+] axes:
where:
Cd 2+ + 2OH - → Cd(OH)2
(2.10)
[Cd(NH 3 ) 4 ]2+ → Cd 2+ + 4NH3
(2.11)
2
[Cd 2+ ][NH 3 ]4
Cd 2+  OH -  = 2.2x10-14 and
= 7.56x10-8
[Cd(NH3 )4 ]2+
The point at which the hydroxide and complex lines cross divide the graph into two sections.
Chopra et al. proposed that A-quality films only form where the bath composition lies above
the hydroxide line (in regions A1 and A2), while for bath compositions which lie below the
hydroxide line (regions B1 and B2), powdery, non adherent substandard films were predicted
to form [1].
31
Figure 2.1 A complex/hydroxide plot to indicate the premium conditions for high quality CBD
cadmium sulfide thin films. Adapted from [1].
The absence or presence of hydroxide particles in solution was considered by Gorer and
Hodes to be the indicator which governs the transition between the two main growth
mechanisms for CBD cadmium sulfide [190]. They proposed a critical ratio (Rc) between the
concentrations of the complexing reagent and the cadmium ion that defines the growth
process by which film deposition occurs: a ratio below Rc leads to a colloidal route while
above it, ion by ion growth is the dominating mechanism. The actual range of Rc is small and
is temperature dependent.
Not all reports have advocated the presence of visible hydroxide particles in solution. Rieke
and Bentjen observed that hard, physically coherent and specularly reflecting films of
cadmium sulfide resulted when hydroxide groups formed on the substrate surface, but not
when there were visible hydroxide particles in the solution [9]. The authors claimed that the
formation of hydroxide groups on the substrate surface provided a catalytically active surface
toward thiourea decomposition.
32
It was maintained that these conditions controlled the
CHAPTER 2
morphology and kinetics of film formation. Ortega-Borges and Lincot also proposed a model
which involved the decomposition of thiourea molecules by the formation of a surface
intermediate complex with cadmium hydroxide on the substrate surface [59].
The kinetics of CBD cadmium sulfide was investigated by Voss et al., who observed that both
the rate of growth and the terminal thickness were a function of the stirring rate [60]. An
example of a growth curve as obtained by Voss et al. using a quartz crystal
microbalance (QCM) is presented as Figure 2.2. It is characterised by a slow induction
period (I) followed by a linear compact layer growth period (II). A kink in the growth curve
indicates the transition from ion-by-ion growth to a particle based mechanism (III). The final
region signifies depletion of the reactants and termination of the growth process (IV).
1200
I
II
IV
III
Film thickness (Å)
1000
800
600
400
kink
200
0
0
5
10
15
20
25
30
35
40
45
Time (h)
Figure 2.2 A growth rate curve similar to that obtained by Voss et al. using a QCM [60].
The effect of the substrate on film roughness was investigated by Oliva et al. [50]. They
concluded that in the early stages of growth, the substrate choice was a significant factor in
the observed roughness of the films. For longer deposition times, however, the choice of
33
substrate was a less important factor in film roughness, although it was important in
determining the adherence of the films. The authors claimed that preferential adsorption of
cadmium hydroxide (Cd(OH)2) on silicon substrates led to films of a poor quality. On
substrates such as corning glass and indium tin oxide (ITO), higher quality films were
produced.
Kostoglou et al. determined that the significant factors affecting the development, adhesion
and quality of deposited films were the solution pH and substrate characteristics [49]. A
noteworthy observation was that films deposited onto commercial glass substrates
disintegrated on rinsing with water, while those deposited onto tin oxide coated glass
substrates were tightly adherent. They further noted that surface nucleation rate depended on
both the supersaturation of the solution and the substrate material. In contrast, surface particle
growth rate was influenced by supersaturation alone. The substrate material also determined
the contact angle between the particle and substrate and hence the shape of the surface
particles.
Two vastly different nucleation processes, instantaneous and constant, were
proposed to explain the processes occurring during the early stages of film deposition.
2.6.2
CADMIUM SELENIDE
A two step deposition process for the deposition of cadmium selenide was proposed by
Froment et al. [79]. The first step involved 2D growth of a thin layer (50-85 nm) which was
initiated by the instantaneous nucleation of cylindrical nuclei. The second step involved a 3D
growth process at random sites of the first layer. The kinetics of the process and the
photovoltaic properties of the resulting film were enhanced by the addition of small quantities
of silicotungstic acid (SiW12O40H4).
The choice of substrate was shown to be an important factor in the determination of adhesion
properties and also the morphology of cadmium selenide thin films [82]. It was claimed that
the presence of reactive hydroxyl groups on the glass substrate promoted the adhesion and
34
CHAPTER 2
formation of high quality films. In contrast, when carbon coated glass substrates were used,
poor adhesion of the films was observed.
2.6.3
LEAD SELENIDE
Gorer et al. investigated the effect of the complexing reagent on CBD lead
selenide (PbSe) [11].
Their investigations revealed that the deposition mechanism was
dependent, not only on the complexing reagent used, but also on the ratio of complex to metal
ion. They demonstrated that these two factors affected the presence or absence of hydroxide
in solution and consequently the mechanism by which the resulting films formed. The
presence of a colloidal hydroxide species in solution led to formation of films by an ion by
ion growth mechanism while the absence of hydroxide led to films formed by a cluster by
cluster mechanism. In complexing reagents, such as potassium nitrotrilotriacetate (NTA),
which form strong complexes with lead, the ratio of complexing reagent to lead was required
to be low to initiate an ion-by-ion growth mechanism. In contrast, with weaker complexing
reagents, the ratio was less critical.
2.6.4
ZINC SULFIDE
While the formation of hydroxide species in solution is an important factor for the formation
of high quality films of the chalcogenides of cadmium and lead, this is not the case for zinc
films. The relatively small difference in the solubility products of zinc sulfide and zinc
hydroxide leads to possible competition between the formation of sulfide and hydroxide in
alkaline solutions, with the presence of significant quantities of oxides or hydroxides
observed in CBD zinc sulfide films [56, 174]. For this reason, hydroxide formation needs to
be minimised in order to form quality CBD zinc sulfide thin films. The addition of a second
ligand, namely an amine such as hydrazine, triethanolamine or ethanolamine was shown to
increase the growth rate. O’Brien and McAleese suggested that the purpose of using an
amine in addition to ammonia was to increase the hydrolysis of thiourea, while Mokili et al.
35
asserted that the addition of an amine increased the growth rate and participated in the
reaction mechanism [56,174].
In acidic media, the growth of zinc sulfide was shown to occur in three stages: an initial
induction period, a rapid growth period and finally a slower growth process [134].
Yamaguchi et al. determined that in solutions with pH~6.5, zinc sulfide grows via a cluster
by cluster mechanism, with film growth controlled by both the rate of homogeneous growth
in solution and the rate of aggregation of the particles [14]. The cluster by cluster mechanism
was confirmed by Sartale et al., who compared the homogeneously formed precipitate with
particles within the film matrix [191]. They also determined that the films that formed were
of the cubic (zincblende) structure and had a particle size <100 nm.
2.7
OPTIMISATION TECHNIQUES
Although CBD has a number of advantages over other more intensive methods of film
deposition, it also has limitations. It has been claimed that in the case of CBD cadmium
sulfide, only approximately 2% of the initial cadmium concentration is used in film
formation, resulting in high levels of cadmium waste [63, 66]. This, along with the volatility
of ammonia (commonly used in bath solutions) in larger scale CBD operations, lead to a
significant environmental hazard if not addressed [49, 63, 66]. Homogeneous precipitation
and deposition of the chalcogenide material on the walls of the reactor vessel are also
limitations of the method, which have the effect of depleting the chemical bath of vital
reagents which, in turn, leads to a reduction in both film thickness and film quality [63, 67].
The presence of contaminants, introduced during the deposition process can also result in a
reduction in film quality and performance [49].
In order to make the CBD process more economical, not only financially but also in terms of
maximum yield, various methods have been employed. These methods serve to minimise the
extent of particulate growth within the solutions. To achieve this aim, techniques such as the
36
CHAPTER 2
careful spacing of substrates in the bath solution [108, 192] and the minimisation of the initial
concentrations of reagents [58, 62] have been reported in the literature.
In an attempt to minimise the environmental impact of cadmium residues from CBD baths, a
method was developed whereby excess cadmium could be recovered and the reagents
recycled [63, 66]. It was also noted that by keeping the bath solution at a lower temperature
and heating only the substrate, both homogeneous precipitation and deposition onto the walls
of the reaction chamber could be minimised.
2.8
CBD: CONCLUSIONS
This chapter has outlined the basic principles of CBD and provided an overview of the use of
CBD today. CBD has been used to successfully prepare thin films of transition metal
chalcogenides, including the chalcogenides of copper. However, while most transition metal
chalcogenides form in only one or at most two different structures, for copper sulfide, there
are at least five different stable phases that may form.
Copper sulfides are generally prepared from alkaline baths although there were also reports of
acidic chemical baths. The composition of chemical baths, even where the pH was similar
varied greatly. This demonstrates the wide range of solutions from which CBD CuxS thin
films can be prepared.
The mechanistic investigations reported in section 2.6 relate to chalcogenides other than
copper, but the information gained during these studies may also provide insights into
processes occurring during the deposition of copper sulfides. While adhesion to the substrate
is a problem, it appears that this can be resolved partly by pre-treating substrates to provide
nucleation sites for the growing film.
Within chemical baths, there is a range of possible competing reactions. This can lead to
unwanted phases forming or the presence of unwanted inclusions within the resulting thin
films. These factors make it vital to determine the processes that occur during the deposition
process to enable quality films composed of a single wanted phase. The determination of
37
these processes is more difficult for copper sulfides because of the large number of possible
phases and also the extra reduction step required to produce these phases.
If CBD is to become a viable method of producing thin films of high quality, suitable for
electronic devices, there needs to be a greater understanding of the processes by which copper
sulfide thin films form. With this understanding, it may be possible to consistently produce
copper sulfides which exhibit the desired properties.
38
CHAPTER 2
2.9
REFERENCES
1.
Chopra, K.L., R.C. Kainthla, D.K. Pandya and A.P. Thakoor, Chemical solution
deposition of inorganic films, in Physics of Thin Films, G. Hass, Editor. 1982,
Academic Press: New York. p. 167-235.
2.
Breen, M.L., J.T. Woodward IV, D.K. Schartz and A.W. Apblett, Direct evidence for
an ion-by-ion deposition mechanism in solution growth of CdS thin films. Chemistry
of Materials, 1998. 10: p. 710-717.
3.
Pentia, E., V. Draghici, G. Sarau, B. Mereu, L. Pintilie, F. Sava, and M. Popescu,
Structural, electrical, and photoelectrical properties of CdxPb1-xS thin films prepared
by chemical bath deposition. Journal of the Electrochemical Society, 2004. 151:
p. G729-G733.
4.
Lincot, D. and R. Ortega Borges, Chemical bath deposition of cadmium sulfide thin
films. In situ growth and structural studies by combined quartz crystal microbalance
and electrochemical impedance techniques. Journal of the Electrochemical Society,
1992. 139: p. 1880-1889.
5.
Lokhande, C.D., Chemical deposition of metal chalcogenide thin films. Materials
Chemistry and Physics, 1991. 27: p. 1-43.
6.
Lincot, D., M. Froment and H. Cachet, Chemical deposition of chalcogenide thin
films from solution. Advances in Electrochemical Science and engineering, 1999. 6:
p. 165-235.
7.
Mane, R.S. and C.D. Lokhande, Chemical deposition method for metal chalcogenide
thin films. Materials Chemistry and Physics, 2000. 65: p. 1-31.
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