Dopping materials CdS

8/18/2019 Dopping materials CdS

1/6

CdS amorphous thin films photochemical synthesisand optical characterization

Marisol Tejos a,, Bárbara G. Rolón b, Rodrigo del Rı́o c, Gerardo Cabello d

a Facultad de Ciencias, Universidad de Valparaı́so, Av. Gran Bretaña 1111, Playa Ancha Valparaı́so, Chileb FIMEE, Universidad de Guanajuato, 36730 Salamanca, Mexicoc Departamento de Quı́mica Inorgánica, Facultad de Quı́mica, Pontificia Universidad Católica de Chile, Santiago, Chiled Departamento de Ciencias Básicas, Facultad de Ciencias, Universidad del Bı́o-Bı́o, Campus Fernando May, Chillán, Chile

a r t i c l e i n f o

Keywords:

Amorphous materials

Photochemical synthesis

Thin film

Optical materials

a b s t r a c t

Thin amorphous nanostructured CdS films were photochemically obtained via direct UV

radiation (l ¼ 254 nm) of complex Cd[(CH3)2CHCH2CH2OCS2]2 on Si(10 0) and ITO-

covered glass substrate by spin coating. Thin cadmium xanthate complex films’ UV

photolysis results in loss of all ligands from the coordination sphere. X-ray photoelec-

tron spectra for as-deposited CdS thin films show the most representative signals of Cd

3d5/2 located at 405 eV, Cd 3d3/2 located at 412 eV and a small signal S 2p located at

162eV. The surface morphology of the films was examined via atomic force microscopy.

This can be described as a fibrous-type surface without structural order, which is

characteristic of an amorphous deposit. The optical band gap value was 2.85 and

3.1570.1 eV.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Cadmium sulfide has been an attractive material due to

its optoelectronics and photoconductive properties [1–3].

The fine tuning of bands, band gaps, and surface states,

known as band gap engineering, is an essential require-

ment for constructing semiconductors based on advanced

devices. These strategies have many useful applications

including photoresistors, window layers in thin-film solar

cells, light detectors, display panels, LEDs and photocon-ductors [4].

One of the most important properties of CdS to be

considered as a photoconductive material is the energy

band gap, E g. The E g value defines the primary light

absorption edge and thus the range of the visible light

spectrum that can be converted into electricity by a solar

cell [5]. The E g value for bulk CdS is 2.5 eV; however, for

CdS thin films the E g changes depending mainly on the

deposition technique and in the particular preparation

conditions [5,6]. In this approach, the properties of the

films depend on the details of the preparation conditions

used for growth.

CdS thin films have been prepared using numerous

deposition techniques, such as chemical bath deposition

(CBD) [5,7–9], metal organic chemical vapor deposition

(MOCVD) [10], sol–gel [11], electrodeposition (ED) [12]

and photochemical deposition (PCD) from aqueous solu-

tions [13–17]. We are herein disclosing the synthesis of amorphous cadmium sulfide based on a novel photo-

chemical method to produce thin films [18–24].

2. Experimental procedures

The photochemical method used allows the deposition

of very thin films of metal oxides. The method requires

that precursor complexes produce stable amorphous thin

films by spin coating onto an inert substrate. Film

photolysis produces photoextrusion (photochemical re-

lease) of the ligands, leaving inorganic products on the

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/mssp

Materials Science in Semiconductor Processing

ARTICLE IN PRESS

1369-8001/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.mssp.2009.04.004

Corresponding author.

E-mail address: [email protected] (M. Tejos).

Materials Science in Semiconductor Processing 11 (2008) 94–99

http://www.sciencedirect.com/science/journal/matscihttp://www.elsevier.com/locate/mssphttp://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.mssp.2009.04.004mailto:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.mssp.2009.04.004http://www.elsevier.com/locate/mssphttp://www.sciencedirect.com/science/journal/matsci


2/6

surface (Eq. (1)):

(1)

Routine infrared spectra of the films of CdS wererecorded on a Perkin Elmer Model 1605 FT-IR (Fourier

transformed infrared) spectrophotometer, with 4 cm1

resolution. UV spectra were obtained with a Hewlett-

Packard 8452-A diode array spectrophotometer.

X-ray photoelectronic spectra (XPS) and Auger electro-

nic spectra (AES) were obtained in a PHI 1257 XPS-Auger

Perkin Elmer, which included an ultra-high vacuum

chamber, a hemispherical electron energy analyzer and

an X-ray source providing unfiltered Ka radiation from its

Al anode (hv ¼ 1486.6 eV). The pressure of the main

spectrometer chamber during data acquisition was kept

at ca 107 Pa. The binding energy (BE) scale was calibrated

using the peak of adventitious carbon, setting it to284.6 eV. The accuracy of the BE scale was 70.1 eV.

Measurements of atomic force microscopy (AFM)

were performed using a Nanoscope IIIa (Digital Instru-

ments, Santa Barbara, CA) in contact mode. Film thick-

ness was obtained with a Leica DMLB optical microscope

with a Michelson interference attachment. X-ray diffrac-

tion (XRD) was obtained with a Siemens D 5000, Cu

l ¼ 1.54 Å .

Solid-state photolysis was carried out at room tem-

perature under a UVS-38 at 254 nm wavelength (lamp

equipped with two 8W tubes), at room temperature.

Reaction progress was monitored via FT-IR spectra at

different time intervals, following the IR absorptiondecrease of the complexes.

ITO-covered glass substrates (22 cm) and n-type

silicon (10 0) wafers (11 cm) from Wafer Net, San Diego,

CA, were used.

2.1. Synthesis of cadmium (II) xanthates complexes

Alkali metal salts of the o-isoamyl dithiocarbonate

were prepared by reacting carbon disulfide with isoamyl

alcohol and potassium hydroxide (Chugaev reaction [25]).

The reactants purchased from Merck p.a. were used as

obtained.Potassium o-isoamyl dithiocarbonate (0.010 mol) dis-

solved in 25 mL methanol (Merck p.a.) was mixed with Cd

(NO3)24H2O (Merck p.a.) (0.030 mol) dissolved in 25 mL

water. The mixture was stirred at 25 1C until a white solid

was obtained. The product was removed in CH2Cl2 and

dried over anhydrous Na2SO4.

2.2. Preparation of amorphous thin films

Thin films of the precursor complexes were prepared

using the following procedure: the wafers were cleaned

successively with ether, methylene chloride, ethanol,aqueous HF (50:1) for 30 s, and finally with deionized

water, dried in an oven at 110 1C and stored in sealed glass

containers.

A silicon chip was placed on a spin coater at 1500 RPM

rotation. A portion of xanthate complex solution (0.1 ml)

in THF was dispensed onto the silicon chip or ITO-covered

glass substrate and allowed to spread. Upon engine stop, a

thin film of the complex remained on the chip. The film

quality was assayed under an optical microscope(1000 ).

2.3. Photolysis of metal complex films on si (1 0 0) surfaces

All photolysis experiments were done following the

same procedure. Here is the description of a typical

experiment. A film of the xanthate complex was deposited

on n-type Si(10 0) by spin-coating from a THF solution.

This resulted in the formation of a smooth, uniform

coating on the chip. The FT–IR spectrum of the starting

film was first obtained. The chip was then placed under a

UVS-38 lamp setup equipped with two 254 nm 8 W tubes,in an air atmosphere. Progress of the reactions was

monitored by determining the FT–IR spectra at different

time intervals, following the decrease in the IR absorp-

tion of the complex. After the FT–IR spectrum showed

no evidence of the starting material, the chip was

rinsed several times with dry THF to remove any

organic products remaining on the surface. During

photolysis the temperature was monitored with a data

logging multimeter with a temperature probe and kept

under 28 1C.

2.4. Annealing of films

Films were annealed under argon atmosphere at 500 1C

for 1 h in a Lindberg furnace and allowed to return slowly

to room temperature (2 1C/min).

ARTICLE IN PRESS

800

0.00

0.05

0.10

0.15

0.20

0.25

0.30t = 0

A b s o r b a n c

e

Frequency (cm-1)

900 1000 1100 1200 1300 1400 1500

Fig. 1. FT-IR spectra of precursor at various stages in its conversion to

CdS, by 24 h photolysis on Bis (o-isobutylxanthate)Cd (II) film depositedon substrate-oriented Si(10 0).

M. Tejos et al. / Materials Science in Semiconductor Processing 11 (2008) 94–99 95


3/6

3. Results and discussion

3.1. Cd (II) xanthate complexes thin-film photochemistry

Fig. 1 shows cadmium(II) o-isoamylxanthates complex,

Cd[(CH3)2CHCH2CH2OCS2]2 precursor IR spectra at various

conversion stages to CdS as reported by the (FT-IR)

absorption spectrum. This exhibits a strong band at1044 cm1, which is characteristic of the cadmium

xanthate complex CQS group stretching modes. The bands

at 1063 and 1178 cm1 correspond to the C–O–C group,

both symmetric and asymmetric stretching, respectively.

As a result of the photolysis, these bands, associated

with the complex, decrease in intensity and no new bands

become apparent in the spectrum. The decrease in intensity

of each absorption band appears to be consistent with the

original intensity, indicating the absence of a thermally

stable intermediate species. Exhaustive photolysis results in

the loss of all bands associated with the starting complex.

At the end of the photolysis (24 h irradiation) there were no

detectable absorptions in the IR spectrum.

3.2. Structural properties

Elemental analyses of the as-deposited CdS thin films

were performed via XPS, which has proven a useful tool to

investigate the chemical nature of the films [26]. Unique

information about the amount and chemical state of the

cadmium ion and sulfide ion was acquired using this

technique. Table 1 provides the XPS data obtained for

photochemically deposited CdS thin films. Photoelectron

peak assignments were made by comparison with

literature values [27].

Fig. 2 shows the spectra for the as-deposited CdS thinfilm without sputtering and after 2 min low sputtering.

Spectra show that sputtering reduces carbon and oxygen

impurities in the film. Both spectra show Cd 3d5/2 at

405eV, Cd 3d3/2 at 412eV, and a small signal S 2p at

162 eV signals. This last signal is more typical for CdS

bonding, confirming the presence of CdS [27,28]. However,

the l S 2p signal shift to 168–170 eV corresponds to SO xspecies settled on the surface, and can be associated with

oxidation products [29–31].

The presence of carbon as an impurity in the thin films

is to be noticed; the carbon detected on the surface was

probably the result of organic residues from the precursor

complexes rather than of inefficient photolysis. Never-theless, after 120s Ar+ sputtering, the carbon was

considerably reduced on the film surface.

Wartburton et al. [32] explained that depending on the

working temperature, SO x has been found as SO2, SO32, or

as SO42 using near-edge extended X-ray absorption fine

structure (NEXAFS). Ferrizz et al. [33] found that the

associated peak to 160–170 eV at room temperature is

related to molecularly adsorbed SO2, while higher tem-

perature peaks are due to decomposition of adsorbed

sulfates. XPS results show that a small fraction of the

adsorbed SO2 is oxidized at room temperature to SO42

using metal oxide oxygen. Finally, Astorino et al. [34]

demonstrated that SO2 adsorbs in the form of sulfitecomplexes, which transform into sulfate complexes, both

in the presence and in the absence of oxygen.

Solid-phase photochemistry was used in this work to

yield CdS and a cadmium xanthate thin film was used for

this purpose. Cadmium xanthate complex precursor has

oxygen close to the coordination sphere, which leads us to

believe that this reaction generates oxygenated species as

a secondary product; for example cadmium oxide, the

ARTICLE IN PRESS

Table 1

Photoelectron binding (eV) peak assignments for as-deposited and

surface with sputtering films.

Photoelectron

peaks

CdS (as-

deposited)

CdS (2min Ar+

sputtering)

Cd 3d5/2 405.8 405.5

Cd 3d3/2 412.5 412.2

S 2pa 168/170 162

O 1s 532.0 531.8

C 1s 285.0 284.8

a S 2p components are observed as a broad peak.

1000

S i 2 p

O K

L L

C d 3 s

C d 3 p 1

C d 3 p 3

O 1

s

C d 3 d 3

C d 3 d 5

C

1 s

S 2

s

S 2

p

C d

4 d

O K

L L

C d 3 s

C d 3 p 1

C d 3 p 3

C d 4 d

S 2 p

S 2 s

C

1 s

C d 3 d 5

C d 3 d 3

O 1

s

sputtering

surface

I n t e n s i t y [ a r b . u n

. ]

Binding Energy (eV)

900 800 700 600 500 400 300 200 100 0

Fig. 2. XPS spectra of as-deposited CdS amorphous film surface and CdSthin film Ar+ 2 min. sputtering.

20

20

40

60

80

100

120

140

S*S**

S

S

S

S

S

CdOCdO

CdS

CdS

CdS

A r b i t r a r y U n i t s

2θ (degrees)

22 24 26 28 30 32 34 36 38 40

Fig. 3. The X-ray diffraction patterns of photochemically deposited CdSfilm annealed at 500 1C.

M. Tejos et al. / Materials Science in Semiconductor Processing 11 (2008) 94–9996


4/6

531 eV signal, did not reduce as the oxygenated species as

expected, and was present only in a superficial nature.

Fig. 3 shows the XRD patterns of CdS, S and CdO of the

samples annealed at 500 1C. The XRD peaks observed at

2y ¼ 24.827, 26.528 and 28.206 are associated with the

(10 0), (0 0 2) and (1 0 1) planes, respectively, assigned to

the hexagonal phase of wurtzite. Other authors [14,35]

have reported similar XRD patterns for CdS samples whendeposits have been thermally treated (annealed) at 500 1C.

These patterns have a perfect match with our results.

The most intense XRD peaks for CdO were

2y ¼ 33.0291 and 38.3201 referred to (111) and (2 0 0)

planes, while the most intense peaks for S were observed

at 2y ¼ 21.9841, 27.5941, 30.8081 and 31.7041 referred to

(0 8 0), (40 1), (13 3) and (24 3) planes, respectively. Goto

et al. [13] also showed a peak 2y ¼ 22.61 (S*) referred to

the (13 1) plane allotted to the rhombic phase of S when it

is not bonded either to cadmium or to oxygen. Goto also

encountered the peak 2y ¼ 23.271 (S**) referred to the

(2 2 2) plane, allotted to an orthorhombic phase. These last

two peaks noted by Goto were found in our XRD, whichreveals that our sample contains sulfur in a polycrystalline

form.

According both to all previous data and to the

calculations made by the Multipak program and consider-

ing the sensitivity factors, it was determined that the S/Cd

ratio was 1.35 and the O/Cd ratio was 2.45. These results

clearly show the existence of excess oxygen as well as

sulfur at a superficial level, which is corroborated by the

deconvolution of the S 2p peak, which showed signals at

164.9, 166.83 and 169.02 eV corresponding to the presence

of sulfur, sulfite and sulfate, respectively.

Anyhow, it is important to state that, according to

surfaces analysis experts, these calculations are littletrustworthy because the elements in the deposits’ first

layers may have adsorbed, desorbed or simply absent

elements. This opinion is shared by Robles et al. [36],

because in the indium sulfide synthesis he also encoun-

ters the presence of oxygen, which does not allow him to

determine the exact stoichiometry of the film via XPS.

Robles mentions that upon running an XRD study at

different film thicknesses, he concluded that the crystal-

lographic structure is kept, although not the preferential

orientation. He adds that in films under 300 nm the

oxygen is present in a free form and then are introduced

during annealing. Furthermore, the diffusion of the

oxygen would make the sulfur escape from the film,thermodynamically favoring the oxygen bonds with the

other present species.

The behavior of the superficial oxygen observed by

Robles is similar to the one found in our XRD analysis

wherein while we showed the existence of sulfur and CdO

in a very little amount, it is the CdS signals that prevail in

the spectrum despite the thinness of the film.

We have found the nature of the ligand to be a very

important factor in the photochemical deposition of metal

sulfides. We used xanthate and dithiocarboxylate com-

plexes with the same metal ion.

The Auger spectral revealed that the deposits prepared

by photodeposition of the dithiocarboxylate complexesshowed the presence of oxygen, which, after sputtering

with Ar+ions, reduced almost completely, showing super-

ficial oxygen. Otherwise, when xanthates complexes were

used, the change in oxygen concentration before and after

the sputtering was not significant. This suggests that

existing oxygen in the sphere of coordination of the

complex will generate oxide species.

The surface morphology of the as-deposited and

annealed films (5001C for 4 h) was assayed by AFM.

Fig. 4(a) shows the AFM image for a 400-nm-thick as-

deposited film, whereas Fig. 4(b) corresponds to the AFM

image of a 110-nm-thick film annealed at 500 1C. The

estimated values of thickness are based on the Z -axis of 3D-AFM topography images. A scan across various regions

of as-deposited film showed uniform application with a

root-mean-square (rms) roughness of 11.4 nm and a

maximum height Rmax of 90.8 nm. This surface is fibrous

and lacks structural order, which is characteristic of an

amorphous deposit. In the annealed CdS films a non-

uniform but smoother surface was observed with an rms

roughness of 3.27 nm and an Rmax of 60.7 nm.

3.3. Optical properties

Fig. 5 shows the spectral transmittance curve fordeposited CdS films grown on ITO glass substrates over

ARTICLE IN PRESS

1

0 0 0 . 0

0 n m

2 0 0 . 0

0 n

m

2

4

6

8

µ m

2

4

6

8

µ m

Fig. 4. Three-dimensional AFM images of the CdS films deposited on Si

(10 0). Image size 1010mm, with z -scale (a) 1000.0nm for as-

deposited and (b) 200.0 nm for the CdS films annealed at 5001

C.



5/6

a wavelength range from 300 to 800 nm. The film presents

an absorption edge of approx. 300 nm. In the NIR spectralregion, the transmission shows an increasing l and a

maximum transmission (75%), which shows the high

transparency of the present film in this range. After

annealing the film at 500 1C for 4 h, transmittance rose to

more than 90%. The absorption in the close infrared is

likely to correspond to the spectra-trapped charge

carriers, increasing these results from the accumulation

of trapped conduction-band electrons and possibly im-

proving the stoichiometry and the films roughness.

The absorption coefficient a, near the band edge inmany amorphous semiconductors, shows an exponential

dependence on photon energy usually obeying Urbach’s

empirical relation (Eq. (2)) [37–39]ðahvÞ ¼ bðhv E gÞ

n (2)

where b1 is the band edge parameter, n is a number that

features the transition process, which takes values of 12, 1,

2, or 32 depending on the nature of the electronic

transitions responsible for the absorption. For the allowed

direct transitions the coefficient n is equal to 12 and for

indirect allowed transitions, n ¼ 2 [40].

The width of the optical band gap E g can be determined

from a fitted straight line in an (ahv)1/n versus (hv) plot.Fig. 5 is a plot of (ahv)1/2 vs. hv for amorphous, newlydeposited CdS, while Fig. 6 is a plot for amorphous CdS

subsequently annealed at 5001C. Since the data points fall

on a straight line, the transition is not direct [39]. The E gvalues obtained, for the as-deposited and the annealed

amorphous CdS films, were 2.8570.1 eV and 3.1570.1 eV,

respectively, as shown in Figs. 6 and 7.

Values reported for the amorphous material are

E g ¼ 2.270.1 eV [41]. This increase might be due to the

defects of the deposits because of the partial oxidation of

these films, as shown in the XPS analysis. Anyhow, it has

been reported that the energy band gap of the CdS film

decreases with annealing [9]. The annealing process for

thin films normally improves the crystallization. Never-

theless, the processing heat applied (500 1C for 1h) has

not been sufficient to crystallize the film. In amorphousmaterial at higher energies, the total absorption might be

due to contributions from all three possible transitions,

including localized states to extended states, extended

states to localized states, and extended states to extended

states. The ln a vs. energy curve would be affected by all of

these transitions, and therefore the steepest point in thecurve is not necessarily indicative of one particular

transition, but may be a result of several contributing

transitions.

Whether the possible oxidation products are impu-

rities in annealed samples remains unclear, requiring the

detection of trace CdO in films of CdS related to oxidation

of cadmium sulfide. It is known that simple CdS is covered

by CdO at temperatures over 500 1C [42,43]; e.g., the

composition of amorphous materials may vary with the

method used.

On the other hand, we have not considered the

presence of sulfur as a variable in the band gap increase.

The existence of a series of peaks in XRD (Fig. 3) shows thepresence of this element.

ARTICLE IN PRESS

300

20

30

40

50

60

70

80

90

100

(b) CdS 500°C

(a) as-deposited

(b)

(a)

T r a n s m

i t t a n c e ( % )

Wavelength (nm)

400 500 600 700 800

Fig. 5. Transmission spectra of (a) CdS films and (b) CdS films annealed

at 500 1C.

Fig. 6. Dependence of (ahv)1/2 as function of the photon energy hv, for anamorphous CdS thin film.

Fig. 7. Dependence of (ahv)1/2 as function of the photon energy hv, for aCdS amorphous and annealed at 500 1C for 4 h thin film.

M. Tejos et al. / Materials Science in Semiconductor Processing 11 (2008) 94–9998


6/6

Song et al. [44] noted a wavelength absorption

displacement at shorter values when sulfur was present,

thereby generating an energy increase of the band gap.

Something similar was noted by Walter et al. [45] who

determined a band gap interval from 1.0 to 1.4 eV; these

values were directly dependent on the films’ sulfur

concentration, i.e. the maximum value corresponded to a

film with high contents of sulfur. Accordingly, we canpresent as argument that sulfur present in our samples

could be the cause of the increased band gap.

The preceding assertion is based on the fact that the

film was treated at 500 1C for 1h, during which time a

great part of the sulfur might have evaporated. Anyhow, it

is quite possible that the thermal treatment period might

have not been enough for its complete evaporation, which

produced a film with sulfur impurities leading to the band

gap increase.

As mentioned before, the S/Cd ratio was 1.35 and the

O/Cd ratio was 2.45; it also has an effect on the band gap.

Mahanty et al. [46] states that the presence of oxides in

semiconductors increases the band gap. According toRobles et al. [36] the increase of 0–8.5% oxygen in the

stoichiometry of the indium sulfide molecule can make

the band gap vary between 2.1 and 2.9 eV; the reasons

would be summarized in the effect of the size of the grain

or the thickness of the films [47–49] and also the sulfur

excess as Song et al. had studied.

Mahanty et al. also found alterations in the band gap

when the S/Cd rate varied due to a thermal treatment.

Experimentally it was found that the CdS gap could vary

from 2.03 to 2.34 eV when the Cd/S stoichiometric rate

varied from 1.01 to 1.56.

4. Conclusions

CdS was synthesized using a direct, fast, simple, quick

and inexpensive photochemical method. These films

contain sulfur impurities and cadmium oxide as sub-

products. This method is a good alternative for deposition

of the amorphous metal sulfides by thin films. We have

found that the reduction in particles size, impurities, and

the amorphous nature of the films increase the optical

band. Further study is necessary to optimize the process,

investigating the nature of the precursor complex, the

effects of temperature, and the growing of the films in an

inert atmosphere.

Acknowledgement

We thank Universidad de Valparaı́so (Project DIPUV

08-2003) for the financial support.

References

[1] Mahdavi SM, Irajizad A, Tilaki RM. Opt Mater 20 05;27:1583.[2] Isaiah O, Chow L. Thin Solid Films 2005;474:77.[3] Amalnerkar DP. Mater Chem Phys 1999;60:1.[4] Deokjoon C, Sunmi K, Huang NK. Mater Sci Eng B 2004;10:663.

[5] Ortuño-Lopez MB, Sotelo-Lerma M, Mendoza-Galván A, Ramirez-Bon R. Vacuum 2004;76:181.

[6] Oliva AI, Castro-Rodriguez R, Solı́s-Canto O, Victor Sosa P, QuintanaP, Peña JL. Appl Surf Sci 20 03;205:56.

[7] Zhang H, Xiangyang M, Deren Y. Mater Lett 2003;58:5.[8] Wenyi L, Xun C, Qiulong C, Zhibin Z. Mater Lett 2005;59:1.[9] Metin H, Esen R. J Cristallogr Growth 2003;258:141.

[10] Uda H, Yonezawa H, Ohtsubo Y, Kosaka M, Sonomura H. Sol EnergyMater Sol C 2003;75:219.

[11] Capoen B, Gacoin T, Nedelec JM, Turrell S, Bouazaqui M. J Mater Sci2001;36:2565.[12] Pethkar S, Patil RC, Kher JA, Vijayamohanan K. Thin Solid Films

1999;349:105.[13] Goto F, Ichimura M, Arai E. J pn J Appl Phys 1997;36:L1146.[14] Wang CY, Mo X, Zhou Y, Zhu YR, Liu HT, Chen ZY. J Mater Chem

2000;10:607.[15] Soundeswaran S, Senthil Kumar O, Moorthy Babu S, Ramasamy P,

Dhanasekaran R. Mater Lett 2005;591:795.[16] Soundeswaran S, Senthil Kumar O, Ramasamy P, Kabi Raj D, Avasthi

DK, Dhanasekaran R. Physica B 2005;355:222.[17] Padmavathy R, Rajesh NP, Arulchakkaravarthi A, Gopalakrishnan R,

Santhanaraghavan, Ramasamy P. Mater Lett 2002;53:321.[18] Tejos M, Schrebler R, Dı́az FR, del Valle MA. Thin Solid Films

2002;409:172.[19] Tejos M, Schrebler R, Dı́az FR, del Valle MA. Bol Soc Chil Quı́ m

2002;47:113.[20] Buono-Core GE, Tejos M, Cabello G, Aros F, Hill RH. Bol Soc Chil

Quı́m 2002;47:495.[21] Buono-Core GE, Tejos M, Alveal G. J Mater Sci 2000;35:4873.[22] Buono-Core GE, Tejos M, Lara J, Aros F, Hill RH. Mater Res Bull

1999;34:2333.[23] Buono-Core GE, Cabello G, Torrejon B, Tejos M, Hill RH. Mater Res

Bull 2005;40:1765.[24] Buono-Core GE, Tejos M, Cabello G, Guzmán N, Hill RH. Mater Chem

Phys 2006;96:98.[25] Vogel’s textbook of practical organic chemistry, 5th ed. Singapore:

Addison Wesley Longman Singapore Pte Ltd.; 1998. p. 792.[26] Hao E, Sun H, Zhou Z, Liu J, Yang B, Shen J. Chem Mater

1999;11:3096.[27] National Institutes of Standards and Technology X-ray Photoelec-

tron Spectroscopy Database. National Institutes of Standards andTechnology Standard Reference Database 20, version 3.4, 2003./http://srdata.nist.gov/xps/S.

[28] Ristova M, Ristov M. Appl Surf Sci 2001;181:68.[29] Boyle DS, O-Brien P, Otway DJ, Robbe O. J Mater Chem 1999;9:725.[30] Boyle DS, Hearne S, Johnson DR, O-Brien P. J Mater Chem

1999;9:2879.[31] Henestreit ELD, Hebenestreit W, Diebold U. Surf Sci 2000;46:187.[32] Warburton DR, Purdie D, Muryn CA, Prrabhakaran K, Wincott PL,

Thornton G. Surf Sci 1992;269/270:305.[33] Ferrizz RM, Gorte RJ, Vohs JM. Catal Lett 2002;82:123.[34] Astorino E, Busca G, Ramis G, Willey RJ. Catal Lett 2004;23:353.[35] Razik N. J Mater Sci Lett 1987;6:1443.[36] Robles R, Barreau N, Vega A, Marsilla S, Bernède JC, Mokrani A. Opt

Mater 2005;27(4):647.[37] Urbach F. Phys Rev 1953;92:1434.[38] Gonzalez Rolon B, Ortiz A. Mater Chem Phys 2004;85:462.[39] Mott NF, Davis EA. Electronic processes in non-crystalline materials.

Oxford: Claredon Press; 1971. p. 2.[40] Madhuri KV, Naidu BS, Hussain OM. Mater Chem Phys 2002;

77:22.[41] Wohlgemuth JH, Brodie DE, Eastman PC. Can J Phys 1976;54:785.[42] Dimitrov RI, Moldovanska N, Bonev IK. Thermochim Acta

2002;385:41.[43] Sebastian PJ, Calixto ME. Thin Solid Films 2000;360:128.[44] Song J, Adurodija FO, Yoon KH, Kang KH, Lee JC. J Korean Phys Soc

1999;34:143.[45] Walter T, Ruchk M, Vethaus KO, Schok HW, Bloss WH. 11th E.C.

photovoltaic solar energy conference, Montreuax. Dordrecht:Kluwer; 1993. p. 124.

[46] Mahanty S, Basak D, Rueda F, Leon M. J Electron Mater 1999;28(5):559.

[47] Yoshida T, Yamaguchi K, Toyoda H, Asao K, Suiwra T, Minoura H,Nosaka Y, et al. Electrochem Soc Proc 1997;97:37.

[48] Brus L. J Phys Chem 1986;90.[49] Kim W-T, Kim C-D. J Appl Phys 1986;60:2631.

ARTICLE IN PRESS

http://srdata.nist.gov/xps/http://srdata.nist.gov/xps/

Documents

Dopping materials CdS