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Dense CdS Thin Films on Fluorine-doped Tin Oxide Coated Glass by Highrate Microreactor-Assisted Solution Deposition
Yu-Wei Sua' f*, Sudhir Ramprasadb' f, Seung Yeol Han' f, Wei Wang' f, Si Ok Ryuc, Daniel R.
Palos, Brian K. Paul' f and Chih-hung Chang' f
'School of Chemical, Biological & Environmental Engineering, Oregon State University,
Corvallis, OR 97330, USA
b
Energy Processes and Materials Division, Pacific Northwest National Laboratory, Corvallis,
OR 9730, USA
cSchool of Display and Chemical Engineering, Yeungnam University, 214-1 Dae-dong,
Gyeonsan, Gyeongbuk 712-749, Republic of Korea
d
Barr Engineering Co., Hibbing, MN 55747, USA
'School of Mechanical, Industrial & Manufacturing Engineering, Oregon State University,
Corvallis, OR 97330, USA
fMicroproducts Breakthrough Institute and Oregon Process Innovation Center, Corvallis, Oregon
USA 97330
Keywords: CdS thin films, microreactor, chemical bath deposition
*Corresponding author at: School of Chemical, Biological & Environmental Engineering,
Oregon State University, Corvallis, OR 97330, USA. Tel: 541-908-3271
E-mail address: suyuweiwayne@gmail.com
Abstract
Continuous microreactor-assisted solution deposition is demonstrated for the deposition
of CdS thin films on fluorine-doped tin oxide (FTO) coated glass. The continuous flow
system consists of a microscale T-junction micromixer with the co-axial water circulation
heat exchanger to control the reacting chemical flux and optimize the heterogeneous surface
reaction. Dense, high quality nanocrystallite CdS thin films were deposited at an average rate
of 25 2 nm/min, which is significantly higher than the reported growth rate from typical batch
chemical bath deposition process. Focused-ion-beam was used for transmission electron
microscopy specimen preparation to characterize the interfacial microstructure of CdS and
FTO layers. The band gap was determined at 2.44 eV by UV-Vis absorption spectroscopy. Xray photon spectroscopy show the binding energies of Cd 3d312, Cd 3d512, S 2P3/2 and S 2P12
at 411.7eV, 404.8 eV, 162.1 eV and 163.4 eV, respectively.
1. Introduction
Cadmium sulfide (CdS) is an important semiconductor material with potential
applications for photo-detectors [1], thin-film transistors [2, 3], and thin film solar cells. CdS
is currently being used in Cu(In,Ga)Se2 and CdTe thin film solar cells for the junction
formation [4]. In the early 1990's, the CdTe/CdS solar cell efficiency reached 15% [5] and
the module product was commercialized by First Solar Inc. within 10 years. Many papers [511] have reported that chemical bath deposition (CBD) is a low-cost and simple method for
the deposition of CdS thin films on transparent conducting coated glass. However, the
conventional batch CBD process suffers from large volume to surface ratio and the formation
of particle in the bath. These issues result in a non-linear growth rate and low material
utilization. Efforts have been made by various groups of researchers to reduce the bath-tosurface volume with the use of cover plates [12] or including filtration and recycling with the
batch [13]. Microreactor provides an opportunity to overcome some of the issues of batch
CBD processes [13]. Microreactor-assisted solution deposition (MASD) process makes use of
a microreactor system for efficient mixing of the reactant streams and helps to control the
homogeneous reaction before the solution impinges on a substrate [3, 14]. MASD has
demonstrated better surface coverage and uniformity of CdS films on Si02/Si substrate in
comparison with the conventional CBD process using same solution chemistry [15]. In this
paper, dense and high quality CdS thin films on fluorine-doped tin oxide (FTO) coated glass
substrate were deposited by MASD process at high rate. The films were characterized by Xray diffraction (XRD), Atomic Force Microscopy (AFM), Raman spectroscopy, X-ray
Photoelectron Spectroscopy (XPS), UV-Vis absorption spectroscopy, and High resolution
Transmission Electron Microscopy (HRTEM).
2. Experiment
The microreactor setup makes use of a micromixer for efficient mixing of two
reactant streams, and contorl the homogeneous reaction in the heat exchanger before
delivering the solution to the substrate. Fig. 1 illustrates the schematic diagram of MASD
process. The MASD setup consists of a microprocessor-controlled peristaltic pump
(Ismatec REGLO Digital) for pumping two reactant streams through a 1.22 mm ID Tygon
ST tube (Upchurch Scientific). A T-junction micromixer (Upchurch Scientific) was used
for mixing these two reactant streams. Stream A was prepared by dissolving 0.073 g
CdC12 (Mw = 183.31 g/mole), 0.214 g of NH4C1(Mw = 53.49 g/mole), and 4.16 ml of
14.82 M (28 wt%) NH4OH in de-ionized (DI) water. The total volume was added to 50
mL Stream B was prepared by dissolving 0.305 g of thiourea (Mw = 76.12 g/mole) in 50
mL DI water. With this recipe, the stream A stock solution contains 0.008 M CdC12, 0.08
M NH4C1, and 1.23 M NH4OH; the stream B stock solution contains 0.08M thiourea.
After mixing at an equal volume ratio, the concentration of each component in the
polycaryl-ether etherketone (PEEK) tube was reduced to the half of the stock solution.
The PEEK tube was enclosed co-axially in a tygon-tube, which serves as a shell and tube
heat exchanger with hot water circulation maintained at 80-85 °C by a constant
temperature bath. The flow rate of each stream was controlled at 0.434 mL/min. The
mixed solution was delivered to the FTO glass substrate (Pilkington TEC 8), which was
taped on a hotplate to maintain a surface temperature between 80 to 90
The residence
time of the solution was controlled at 35 seconds. Once the depostion process was
completed, the substrate was rinsed by DI water and dried using a nitrogen gun. There are
some overlapping XRD peaks between the CdS and Sn20. To better understand the CdS
crystal growth structure, a second CdS deposition process was performed on the asdeposited CdS/FTO/glass. The first and the second depostion were both performed at a
deposition time of 5 minutes. XRD (Bruker D8 Discover, CuKa=1.54056A) was applied to
characteize the crystal structure of the CdS films after the first and the second
depositions.
Focused ion beam (PEI Helios Dual Beam Microscope) was used to prepare cross-sectional
samples for TEM analysis. Platinum was deposited on a rectangular area of 10 p.m (length) x 2
p.m (width) with a 0.5 p.m thickness by electron beam (1.4 nA), which was cleaned by an ion-
beam with a current of 0.2 nA. Two areas with a size of 11 p.m x 6 pm, close to the platinum
deposited area, were milled down to a 3 p.m depth by ion-beam (2.8 nA). After ion milling, the
surface was cleaned by ion-beam with a current of 0.93 nA Finally, a small piece of specimen
was mounted on an omniprobe and then transferred to a supported grid. The thickness of the
specimen was again milled down to less than 0.1 pm for TEM (Philip CM12) and HRTEM (FEI
Titan) observation. The surface morphology and roughness of the CdS films were evaluated by
taping mode AFM (Bruker, Innova Scanning Probe Microscope), and chemical bonding were
examined by Raman spectroscopy (Witec, alpha 500), and XPS (ESCALAB 250Xi). The optical
property was characterized by UV-vis absorption spectroscopy using an Ocean Optics USB2000,
spectrometer with a halogen lamp as the light source.
3. Results and Discussions
3.1 XRD
Fig. 2 shows the XRD patterns of bare FTO glass substrate (a); CdS films fabricated
by single deposition (b); and CdS films fabricated by double deposition (c). Compared to the
tetraganol Sn02 (JCPDS #411445, d), the three major peaks at a 20 value of 26.37°, 37.66°,
and 51.42° of FTO substrate (a) shifted 0.2°-0.4° degree lower. The reason could be
attributed to the larger d-spacing of fluorine doped SnO2 than the pure Sn02. All the XRD
peaks from the FTO glass substrate are similar to the peaks of Sn02, and can be indexed
according to the tetraganol structure. The XRD patterns still resembled the patterns of bare
FTO substrate after both the single deposition and the double deposition except the peaks
around 26°. It can be observed from the patterns that the peak shift from 26.39° to 26.63° and
26.71°, respectively (inset figure) after the CdS deposition. The inset in the figure magnifies
the 20 range from 25.5° to 27.5° and shows a small peak at 26.41° along with a main peak at
26.63° for the CdS film after the single deposition process (b). For thicker CdS films
fabricated by the double deposition process (c), the main peak shifted to 26.71° along with an
increase of peak intensity in comparison with the thinner film fabricated by the single
deposition (b). Mazon-Montijo et al. [1] reported that the underneath substrate peaks can still
be seen when the top film is not sufficiently thick. Therefore, the small peak (26.41°) of from
the sample after the single deposition process could be assigned to the 26.39° of FTO
substrate, which corresponds to the T(110) plan. The peak at 26.63° (single deposition) and
26.71° (double deposition) from the CdS films could be assigned to either C(111) plane of
cubic CdS (JCPDS #760581, e) or H(002) plane of hexagonal CdS (JCPDS #653414, f).
Similar finding has also been reported by Chu et al [17]. Peaks at 20 values of 43.8° and 52°
could be assigned as C(220)/H(110) and C(311)/H(112). However, the peak at 43.8° was not
observed in these samples. The other peak at 52° of FTO substrate is indexed as T(211) peak
of tetragonal Sn02(JCPDS #411445, d). Other peaks at 33.60°, 37.68°, 51.38°, 61.42°, and
65.40° from the CdS films fabricated by the single-pass and double-pass deposition processes
could be assigned to the underneath FTO glass substrate as T(101), T(200), T(211), T(310),
and T(301) peaks. Ikhmayies et al. [18] reported the shifting of T(200) peak of CdS:In/FTO
sample annealed at 480°C was caused by the formation of solid solution CdSi_xSnx. We did
not observe a shift of T(200) peak collected from the as-deposited CdS films.
3.2 TEM
Fig. 3(a) and (b) show the TEM images of the interfacial region between the CdS and
FTO layers at low magnification (75,000X, operating voltage: 80 kV) and high magnification
(620,000X, operating voltage: 200 kV), respectively. The thickness of the CdS layer after the
single-pass deposition process is around 251.72 nm on top of a FTO layer with a thickness of
344.83 nm The average growth rate of CdS thin film from our MASD process could be
calculated to be around 25.2 nm/min This rate is significantly higher than typical batch CBD
process. For example, Kokotov et al. [19] reported highly-textured, columnar CdS films with
an average deposition rate of 2.5 nm/min from a CdSO4 and EDA bath and 1.67 nm/min from
CdC12 and ammonia bath, respectively. In fig. 3(b), the FTO layer exhibits large single
crystalline structure. At the interface, the deposited CdS layer consists of several nano-size
grains and shows columnar structure. A few grains are attached closely to the FTO boundary.
Kim et al. [20] reported that CdS nanocrystalline structure is cubic due to a strong correlation
with the FTO substrate. It implies that lattice mismatch is less between cubic-CdS and
tetragonal FTO. Fig. 3(c) shows the selected area diffraction pattern of CdS (region 1) with a
fractured ring pattern, which is consistent with the nanocrystalline structure of the CdS film.
The FTO substrate (fig. 3(d): region 2) presents a highly oriented dot pattern, which
represents highly crystalline material.
3.3 Surface Property
The Raman scattering measurement was performed at room temperature by using an
Ar+ laser (100mW) with an excitation wavelength at 514.5 nm and the grating is 1800
grid/mm The integration time was set at 30 seconds Fig. 4 shows the Raman spectrum of the
deposited CdS films on FTO coated glass substrate. Two phonon peaks at 298.5 cm-1, and
592.5 cm-irepresent 1LO (longitudinal optical) mode and 2L0 mode. The full with at half
maximum of the 1LO is 24.1 cm-1. The phonon peak's positions agree well with the reported
results in earlier studies [21, 22]. Compared to bulk CdS (1LO: 305 cm-1), it shows the down-
frequency shift of the 1LO Raman peak. This may come from the grain-size effect. Tong et
al. [23] attributed deformation potential caused by lattice mismatch and thermal expansion
coefficient mismatch between the CdS film and the substrate can degrade the intensity of TO
mode. This could be the reason that causes the absence of TO peak in CdS films on FTO
coated glass substrate.
Fig. 5 (a)
(c) show AFM (taping mode) scanned 2D images of the bare FTO glass
with a root-mean-square (RMS) roughness of 8.2 nm The CdS films from a single-pass and
double-pass deposition show RMS roughness of 11.3 nm and 8.1 nm, respectively. The RMS
roughness of the single-pass sample is 11.32 nm, which is higher than the 7 nm for the CdS
film deposited on indium tin oxide (ITO) glass. This difference comes from the FTO glass
substrate which has much higher roughness than the ITO glass surface. Kim et al. [24] also
reported the RMS roughness of CdS films between 7 and 15 nm depending on the thiourea
concentration in chemical solution deposition. The results show that a lower RMS value is
obtained from a higher thiourea concentration. Another interesting finding is that the RMS
value of the CdS films from the double-pass deposition is lower than the film from the singlepass deposition process.
XPS (ESCALAB 250 Xi) was performed to obtain the chemical binding information
of the CdS films. The X-ray source used was Al Ka (1486.6 eV) radiation and the spot size
was 400m. The sputtering ion beam (Art) was set at an energy of 3000 eV and current
density of 0.8 mA/cm2. In Fig. 6(c), the XPS data were corrected by the difference of the
binding energy of C is (284.9 eV) and the binding energy (282.9 eV) of the measured result,
which has the highest intensity in the as-received condition. The resulting value, 2 eV (284.9
eV-282.9 eV = 2 eV), was used to scale the binding energy of all other spectra data (as-
received), which means the whole spectra needed to be blue shifted 2 eV for correction. The
resulting value, 3 eV (284.9 eV-281.9 eV = 3 eV), of the etched C is spectra was used to
scale the binding energy of all other spectra data (etched). Fig. 6 (a) shows the oxygen is
peak at 532.2 eV due to the formation of hydroxide (Cd(OH)2) layer. For the as-received
condition, the binding energies of Cd 3d312 and Cd 3d512 shows at 411.7 eV and 404.8 eV
respectively. These spectrums of Cd and S after etching can be fitted by Gaussian function
(red dot lines). For Cd 3d energy level, the binding energy of 3d312 and 3d512 orbital are 412.7
eV and 405.9 eV respectively, with the splitting energy of 6.8eV. For S 2p energy level, the
binding energies of S 2133/2 and S 2p1 /2 orbitals are 162.2 eV and 163.4eV respectively, with
the splitting energy of 1.2eV. The obtained result shows excellent agreement with previously
published results [25, 26].
3.4 Optical property
The optical band gap (Eg) of CdS thin films was characterized by absorption
spectroscopy and determined from the formula
ahv = A(hv-Eg)n
(1)
where by is the incident photon energy, A is a constant, a(cm-1) is the absorption coefficient,
and the exponent n depends on the type of transition, n = 1/2 and 2 for direct and indirect
transition, respectively. Using equation (2), the absorption coefficient a can be obtained by
the measured wavelength-dependent transmittance (T), and the film thickness (t) was
determined by TEM observation.
T= exp(-at)
(2)
Fig. 7 (insert: photo) shows the Tauc plot of the two-pass deposited CdS films (251.72 nm in
thickness) through continuous MASD process. The optical band gap is determined by
extrapolating the curve at around 2.44eV.
4. Conclusion
The continuous MASD process was demonstrated to deposit dense CdS thin films on
FTO substrate. The achieved average growth rate of 25 2 nm/min is significantly higher than
the reported growth rate from batch CBD processes. The CdS films shows a preferred
orientation in cubic-(111), which become more significant than the tetragonal-(110) from
FTO substrate with increasing the film thickness. The nanocrystalline structure was observed
by HRTEM through FIB specimen preparation. Two-pass deposition can fill out the voids
created by the one-pass deposition. The roughness of two-pass deposited film (RMS = 8.1
nm) shows equivalent as the bare FTO substrate (RMS = 8.2 nm).
Acknowledgement
The work was funded by the US Department of Energy, Industrial Technologies Program
through award #NT08847, under contract DE-AC-05-RL01830 to PNNL and ONAMI
matching grant. Oregon Process Innovation Center is supported by Oregon BEST equipment
grant. We are thankful to Dr. Yi Liu in Oregon State University Microscope Facility for his
assistance on FIB sample preparation and TEM operation.
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Lists of Figures and Table
Fig. 1. Schematic diagram of microreactor-assisted solution deposition (MASD) process.
Fig. 2. XRD patterns of of (a) bare FTO glass substrate, (b) CdS layer on FTO after one-pass
deposition and (c) two-pass deposition, (d) SnO2 (#411445, Tetragonal), (e) CdS (#750581,
Cubic), and (f) CdS (#653414, Hexagonal).
Fig. 3. (a) Low magnification image of CdS/FTO structure (magnification: 75,000X) (b)
HRTEM images of the CdS/FTO boundary (magnification: 620,000X). (c) Fast Fourier
transformed diffraction pattern of CdS layer (region 1). (d) Fast fourier transformed
diffraction pattern of FTO layer (region 2).
Fig. 4. Raman spectra of CdS layer deposited on FTO substrate.
Fig. 5. AFM images of (a) bare FTO substrate (RMS = 8.2 nm), (b) one-pass deposited CdS
films (RMS = 11.3 nm), and (c) two-pass deposited CdS films (RMS = 8.1 nm) (insert: 3D
morphological images).
Fig. 6. XPS spectra (dashed line: as received, solid line: after etching, red dot line: Gaussian
fitting) of the CdS films deposited by continuous MASD (a) 0 ls, (b) Cd 3d, (c) C ls, (d) S
2p.
Fig. 7. Plot of (ahv)2 versus hv showing the band gap energy of 251.72 nm CdS films by
continuous MASD process.
CdC12
Cd2+ +
0H- + NH4 g NH3 + H2O
fit, T)
Cd2+ + 4NH3 ,= Cd(NH3)4+
CS(NH2)2 0HHS- + CH2N2 + H2O
HS- I- 0H- g S2- H2O
Cd2' + S2- 4 CdSto (Part cle precipitation)
Hot water
circulation
Ismatec REGLO
Digital pump
Heater
Cd(NH3),2,+ + 20H- + site 4 Cd(OH),.b +nNH3
CdC1,
Thiourea
Cd(OH)2,d, + SC(NH2)2,1CdSC(NH2)2(OH)2,.dsl*
I [CdSC(NH2)2(OH)2..,6I -CdS + CH2N2 + 2H2 + site
CdSts Particle Slicking
L.
Fig. 1. Schematic diagram of the microreactor-assisted solution deposition (MASD) process.
1000
800
C(111)/H(002)
T(110),
TA 600
TA
cL)
la'
400
25.5
26.5
26
27
27.5
(c): two-pass deposition
200
1000
800
:74 600
4,4 400
200
1000
(a): bare FTO
800
TA 600
O
1.
C
1.
j400
o
e
1.
O
1.
200
O
O
er,
0
20
25
30
35
40
50
45
55
60
65
70
(d): Sn02 (#411445, Tetragonal)
(e): CdS (#750581, Cubic)
O
O
O
(f): CdS (#653414, Hexagonal)
O
ee;-,"
O
x
I
20
25
30
35
40
I
45
50
55
60
65
70
2-theta (degree)
Fig. 2. XRD patterns of (a) bare FTO glass substrate, (b) CdS layer on FTO after one-pass
deposition and (c) two-pass deposition, (d) Sn02 (#411445, Tetragonal), (e) CdS (#750581,
Cubic), and (f) CdS (#653414, Hexagonal).
two-pass layer
(148.8 nm)
one-pass layer
(102.9 nm)
CdS
-------------------FTO
FTO (344.83 nm)
Fig. 3. (a) Low magnification image of CdS/FTO structure (magnification: 75,000X) (b)
HRTEM images of the CdS/FTO boundary (magnification: 620,000X). (c) Fast Fourier
transformed diffraction pattern of CdS layer (region 1). (d) Fast fourier transformed
diffraction pattern of FTO layer (region 2).
3500
3000
2500
k" 2000
:44
1500
1000
500
0
100 200 300 400 500 600 700 800 900 1000 1100 1200
Raman Shift (cm-1)
Fig. 4. Raman spectra of CdS layer deposited on FTO substrate.
27.5 mn
1.51 pm
mn
-31.2 nm
0.75 pm
0 pm
0 pm
0.75 pm
1.51 pm
7
1.51 pin
40.4 nm
-0.4 nm
-41.4 mn
0.75 pm
14f
4(
4.411*
ll
0
L51 pin
0.75 pm
1.51 pm
30.9 mn
5.5 mn
-20.0 mn
0.75 pm
0µm
0 pm
1.51 pm
Fig. 5. AFM images of (a) bare FTO substrate (RMS 8.2 nm), (b) one-pass deposited CdS
RMS 8.1 nm) (insert: 3D
film (RMS 11.3 nm), and (c) two-pass deposited CdS
morphological images).
7000
as- received
etched
as-received
(a)
(b)
etched
6000
Cd 3d512
=
8 5000
4-
I
t')'
Cd 3d312
0
.'Ig 4000
0
c
IN
r..1
t 3000
2-
J
1
Z')
it/6/'v1biAbi
Is
2000
to
1
0..,
- --"- - - -,:
1000
540
535
530
525
420
Binding energy (eV)
.e
1 +.7. - -- /
;-
ti
410
415
405
400
Binding energy (eV)
7000
5000
as-received
etched
4500
(d)
S 2p3/2
4000
g 3500
..
c.)
.e.)' 3000 -
i., 4000
F.)
:..'
0
3000
z 2000
t 1500
a' 2000
2
O'
g 1000
©
.0
0..,
S 2P 1 /2
0
2.t 2500
1000
O.,
500 ----..,MM -v N...4.4, si'
%
:
1.N.v...... It p t.-. . .1
0
290
285
Binding energy (eV)
280
170
165
160
155
Binding energy (eV)
Fig. 6. XPS spectra (dashed line: as received, solid line: after etching, red dot line: Gaussian
fitting) of the CdS films deposited by continuous MASD (a) 0 I s, (b) Cd 3d, (c) C I s, (d) S
2p.
6
5
1
0i
/
.
2.30
.
2.35
2.40
2.45
Eg (eV)
2.50
2.55
2.60
Fig. 7. Plot of (ahv)2 versus hv showing the band gap energy of CdS films (251.72 nm) by
two-pass continuous MASD process.