Nanocrystallin CdS films with controlled stoichiometry deposited by CSVS were investigated by meth-
od of the current-voltage characteristics in ITO/CdS /In structures. It was shown that in the case of cadmi-
um excess (S <Cd) charge flow mechanism both in the dark and under illumination is determined by bimo-
lecular recombination in the material. In the case of excess sulfur (S>Cd) charge flow mechanism is deter-
mined by monomolecular recombination. In the band gap of CdS with excess of cadmium there was detected localized states with energy Et = 0.514 ± 0.026 eV, while in the material with Excess sulfur there are two localized states with energy Et1 = 0.514 ± 0.026 eV and Et2 = 0.700 ± 0.026 eV. Full concentration of localized states is more than 2·1021 m-3 – 5·1022 m-3. Dependence of injection in parameters and nature of injection in the structures based on nanostructured CdS films on their stoichiometry was determined.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3390

Dimitriev, O.P.

Fedoryak, O.M.

Grynko, D.O.

Noskov, Yu.V.

Ogurtsov, N. A.

Opanasiuk, Anatolii Serhiiovych

Pud, A.A.

Smertenko, P.S.

Tyrkusova, Nadiia Volodymyrivna

Гринько, Д.О.

Дмитриев, О.П.

Носков, Ю.В.

Огурцов, Н.А.

Опанасюк, Анатолий Сергеевич

Опанасюк, Анатолій Сергійович

Пуд, A.A.

Смертенко, П.С.

Тиркусова, Надія Володимирівна

Тыркусова, Надежда Владимировна

Федоряк, O.M.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 1, 01NTF41(4pp) (2013) 
 
 
2304-1862/2013/2(1)01NTF41(4) 01NTF41-1  2013 Sumy State University 
Injection Spectroscopy of Deep Traps in Nanostructured Films of Cadmium Sulfide 
 
D.O. Grynko1, O.P.Dimitriev1, P.S. Smertenko1*, O.M. Fedoryak1, А.S. Оpanasyuk2†, N.V. Тirkusova2, 
Yu.V.Noskov3, N. A. Ogurtsov3, A.A. Pud3‡ 
 
1 V. Lashkaryov Institute of Semiconductor Physics, NAS Ukraine, Prospect Nauky, 41, Kyiv, Ukraine; 
2 Sumy State University, 2, Rymsky Korsakov Str., 40007 Sumy, Ukraine 
3 Institute of Bioorganic Chemistry and Petrochemistry, NAS of Ukraine, 50, Kharkivske sjose, Kyiv, Ukraine 
 
(Received 30 July 2013; published online 02 September 2013) 
 
Nanocrystallin CdS films with controlled stoichiometry deposited by CSVS were investigated by meth-
od of the current-voltage characteristics in ITO/CdS /In structures. It was shown that in the case of cadmi-
um excess (S <Cd) charge flow mechanism both in the dark and under illumination is determined by bimo-
lecular recombination in the material. In the case of excess sulfur (S>Cd) charge flow mechanism is deter-
mined by monomolecular recombination. In the band gap of CdS with excess of cadmium there was detect-
ed localized states with energy Et = 0.514 ± 0.026 eV, while in the material with Excess sulfur there are 
two localized states with energy Et1 = 0.514 ± 0.026 eV and Et2 = 0.700 ± 0.026 eV. Full concentration of lo-
calized states is more than 2·1021 m-3 – 5·1022 m-3. Dependence of injection in parameters and nature of in-
jection in the structures based on nanostructured CdS films on their stoichiometry was determined. 
 
Keywords: CdS film,Nanostructured layer, Current-voltage characteristic, Differential approach, Injection 
spectroscopy, Trap 
 
 PACS numbers: 84.60.Jt, 73.61.Ga 
 
 
                                                          
* smertenko@isp.kiev.ua  
† opanasyuk_sumdu@ukr.net  
‡ alexander.pud@gmail.com  
1. INTRODUCTION 
 
Recently CdS films have been intensively investigat-
ed due to the possibility of using it as base layers of pho-
to- and gas sensors, thin films transistors, vidicons, 
LEDs, electrochemical cells, electroluminescent devices, 
window layers of high efficiency solar cells on the base of 
absorbing layers of CdTe, CuInSe2 (CIS),  CuIn1-xGaxSe2 
(CIGS), Cu2ZnSnSe4 (CZTSe) and others [1-2]. 
For practical use in micro-and optoelectronics, solar 
energy CdS films require optimization of their optical 
and electrical characteristics which determined by the 
ensemble of intrinsic point defects and uncontrollable 
impurities [3]. If these defects are charged, they form 
traps and recombination localized states (LS) in band 
gap (BG). The need of focused control of such properties 
led to considerable interest of researchers to studying 
the defect structure of CdS and determining its impact 
on the physical characteristics of the material. At the 
same time, despite the large number of works devoted 
to this issue, the energy spectrum of LS in CdS BG 
remains insufficiently explored, first of all, it refers to 
film samples obtained by different techniques of con-
densation in vacuum. 
The method based on the analysis of steady-state 
current-voltage characteristics (CVC) in regime of 
space charge limited currents (SCLC) [4-5] is one of the 
most suitable methods of studying of LS in semiconduc-
tors such as CdS. It combines the simplicity of the ex-
perimental equipment and their high sensitivity to low 
concentrations [4-5]. 
This method in various modifications [6-8] is widely 
used for determined parameters of traps in a large 
group of wide semiconductors and dielectrics. However, 
in classic form, CVC SCLC method has several disad-
vantages that lead to a significant reduction of in-
formativeness and some incorrectness of obtained re-
sults [9]. 
That is why in some studies [10-12] there were pro-
vided approaches that can rid disadvantages of existing 
method of analysis of CVC SCLC and receive the most 
detailed information about LS in the range of wide BG 
material directly from the experimental CVCs by their 
differential processing (injection spectroscopy (IS) 
method), i.e. by solving the inverse problem of research. 
Differential approach gives also the possibility to de-
termine the charge flow mechanism in multilayer 
structures by analyzing the CVC [13-15]. 
This paper presents the results of calculation of LS 
parameters in nanocrystalline CdS films depending on 
the stoichiometry and conditions of obtaining samples 
using the low-temperature approximation of differen-
tial approach, as well as the results of the charge flow 
mechanism. 
 
2. EXPERIMENTAL DETAILS 
 
CdS films were prepared by vacuum evaporation of 
the CdS powder of semiconductor-quality purity by 
vacuum evaporation of the semiconductor-quality puri-
ty by close-spaced vacuum sublimation (CSVS) at sub-
strate temperature of Тs = 600-640 С and evaporator 
temperature of Тe = 720-750 С. To control the stoichi-
ometry of layers during the process of deposition there 
was co-evaporation of sulfur at temperatures TeS = 60-
200 C. The deposition was performed onto glass sub-
strate covered by conductive ITO underlayer. Peculiari-
ties of the films obtained have been described in detail 
 D.O. GRYNKO, O.P.DIMITRIEV, P.S. SMERTENKO ET AL. PROC. NAP 2, 01NTF41 (2013) 
 
 
01NTF41-2 
in [16]. In contact layer (area ~ 0.02 cm2) was obtained 
by vacuum evaporation. This material allows to realize 
the injection of charge carriers into CdS layer. 
Surface morphology and stoichiometry of the films 
obtained were studied using electron microscope Hita-
chi S-806 and Superprobe-733 JEOL, respectively.  
Dark and light quasi steady-state CVCs in forward 
and reverse directions were measured at room temper-
ature by the standard automated tester 14 TKS-100 
(Russia). The voltage was changed step by step with 
the period of 160 ms, and current measurement was 
carried out at 90 ms after supplying voltage. Measuring 
the light CVC was carried out using white light from a 
halogen lamp with intensity of ~30 W/m2. 
 
3. THEORETICAL PART 
 
Recognition of charge flow mechanism in the sam-
ples was carried out by the method of differential pro-
cessing of CVC, developed in the works [10-17]. This 
method allows us to determine distinction between 
concomitant and competing charge-flow mechanisms in 
the structures and allocate high field processes among 
them. It is possible by joint analysis of j – U, η – U and 
d(lnη)/d(lnU) – U dependencies, where j is the current 
density; U is the applied voltage and η  d(ln j)/d(lnU) 
 
 
2 3
2 3
(ln ) (ln ) (ln )
, ,
(ln ) (ln ) (ln )
d j d j d j
d U d U d U
 
 
The IS method was used only for the processing of 
CVC in the SCLC regime. The basic ratio of IS method 
derived in [10-12] is the following: 
 
 
2 1
fL
jL
n
e U
, (1) 
 
 0
2
2 1 1
1
2 1 1( )( )
L
sL
U
n
e eL
, (2) 
 
where e is the electron charge;  is the mobility of 
charge carriers; L is the sample thickness; 0 is dielec-
tric constant;  is the dielectric permittivity of the ma-
terial; nfL is the concentration of free charge carriers at 
the anode (later the index L in the ratio will be not 
used); nsL is the actual concentration of carriers cap-
tured by local states near anode; L is the space charge 
density at anode responsible for limiting the current 
through the sample. 
Using the well known relation between the concen-
tration of free carriers and the position of the Fermi 
level, it is easy to write an equation to determine the 
energy scale of the LS study  
 
 
2 1
( )
ln ln ln
c
F
N e j
E kT kT kT
L U
, (3) 
 
where EF is the position of the quasi Fermi level for 
injection; k is the Boltzmann constant; T is the temper-
ature; Nc( ) is free effective density of states in the con-
duction (valence) band.  
Distribution function of carriers localized at deep cen-
ters, is obtained by differentiation of (2) for the energy 
 
2
0
2 2
1 2 1 3 1 3
1
2 1 1 2
( )
( )( )
s
F
dn
U
dE kT eL
. (4) 
 
The distribution function of LS density for the ener-
gy h(E) = dNt/dE at low-temperature approximation is 
thought to coincide the function that describes the dis-
tribution of localized carriers. However, this leads to 
errors during playback the shape of distributions of 
traps and determining their depth (Et) and concentra-
tion (Nt) [9, 12, 17]. 
At a high temperature approximation function h(E) 
can be found from the convolution: 
 
 
( )
( )
( )
fs
Ef f
df E Edn
h E dE
dE d E E
, (5) 
 
where f (E – EF) is the Fermi-Dirac function. 
The joint solving of equations (3), (4) and (5) allows 
to find h(E) function in BG of material directly from the 
experimental CVC SCLC measured at arbitrary tem-
perature. For this purpose it is necessary to define 
three derivatives at each point of CVC in log-log scale. 
Mathematically, the task is reduced to the construc-
tion of cubic spline that approximates the experimental 
data. Then the cubic spline is differentiated in nodes and 
finally h(E) function is found from the convolution (5). 
Physically, the ability to scan the energy distribu-
tion of traps at the injection of charge carriers from the 
contact was caused by the following. With the increas-
ing of applied voltage the quasi Fermi level moves in 
BG of the material and it occurs the filling of LS by free 
carriers and changing of the conductivity of the struc-
ture. Since there is a correlation between the values 
that can be experimentally measured (j, U), and those 
that characterize the distribution of traps, (EF, ), thus 
each point CVC carrying an information about corre-
sponding section of the energy spectrum of LS. 
In this paper, in order to determine parameters of 
LS in CdS films the low-temperature approximation of 
IS method was used. In this case, the Fermi-Dirac sta-
tistics in (5) replaced by the Heaviside function. In or-
der to calculate the energy levels of traps in BG and its 
concentration the methods proposed in [11, 18] was 
used. Obtained results are verified by solving the direct 
problem of the experiment and building CVC SCLC by 
the known parameters of LS. Coincidence of form of 
input and reconstructed CVC was indicating about the 
accuracy of determining of the parameters of deep 
traps in the material [17]. 
 
4. RESULTS AND DISCUSSION 
 
Electron microscope images of surface of CdS films are 
presented in Fig. 1. As we expected layers were continu-
ous and had finely dispersed nanocrystalline structure 
(Fig. 1a). Further deposition led to form nanorods on the 
surface of CdS films (Fig. 1b). Data of element composi-
tion studying proves that layers obtained at ТeS <80 C 
contain the excess of cadmium (Cd > S by 3-5%), and at 
ТeS >120 C there is the excess of sulfur (Cd < S by 2-3%). 
 
 INJECTION SPECTROSCOPY OF DEEP TRAPS IN… PROC. NAP 2, 01NTF41 (2013) 
 
 
01NTF41-3 
 
 a b 
 
Fig. 1 – SEM images of CdS film surface: short- (a) and long-
term growth phase (b) 
 
Fig. 2a displays dark and light CVC (in forward 
and reverse bias) of IТО/CdS/In multilayer struc-
ture, where CdS films have excess of cadmium  
(Cd > S by 3-5%). in Fig. 2b shows the forward and 
reverse CVC curves of IТО/CdS/In multilayer struc-
ture with excess of sulfur (Cd < S by 2-3%). The 
analysis of CVCs indicates that the resistivity 
IТО/CdS/In is equal about (5 106-107) Ohm cm. Fig. 2 
shows that the high content of sulfur in CdS film 
leads to increase the photoconductivity of the struc-
ture. Conductivity of illuminated material increases 
almost about three orders of magnitude at the excess 
of sulfur (Fig. 2b), whereas at the deficiency of sulfur 
increase of photoconductivity i only about one order 
in magnitude. The shortage of cadmium was found to 
reduce the dark conductivity of the structure by two 
orders of magnitude. 
Several almost linear sections with different slope 
were observed in the dark CVC of the structure ("+" 
ITO) both with excess and deficiency of sulfur at the 
region of high applied bias. Usually the areas with de-
pendence of current on voltage with type: I ~ U, 
I ~ U 1,5-2, I ~ U 3-4 were allocated. In some cases, after 
super linear behavior there was quadratic one. 
 
 
 
Fig. 2 – CVCs of ITO / CdS / In multilayer structures: a) case 
with the excess of cadmium (Cd>S by 3-5%); b) case with the 
excess of sulfur (Cd<S by 2-3%); 1.3 - "+" ITO, 2, 4 - "+" In. 
Full symbols correspond to dark current, open ones corre-
spond to light current under illumination of white light from a 
halogen lamp with intensity of ~30 W/m2 
 
It should be noted that the features of CVC can be 
seen at η – logU dependences which allows to reveal 
the fine structure of CVC. Each point of the graph de-
termines the angle of the current-voltage curve in a 
log-log scale. η – logU dependence were obtained by 
finding the derivative at each experimental point of 
CVC SCLC. As already mentioned, mathematically 
task was to build a cubic smoothing spline that approx-
imates the experimental data and its differentiation in 
nodes. The results of the differential processing of dark 
and light CVCs are shown in Fig. 3 а–d and 3 e–h re-
spectively. 
Differential analysis of CVC slope (Fig. 3 c, d) 
[13-15] revealed two mechanisms in the nature of 
injected carriers: in the case of excess of cadmium 
(S<Cd) there is a bimolecular recombination in the 
material in the dark and under illumination, and it 
provides a differential slope of the CVC η =1.5. In 
case of sulfur excess (S > Cd) the principal mecha-
nism of charge flow is monomolecular recombination, 
namely it provides a differential slope of current-
voltage curves η = 2. 
 
 
a 
 
b 
 
c 
 
d 
 
e 
 
f 
 
g 
 
h 
 
Fig. 2 – Differential slope α of CVCs: a, c, e, g films with the 
excess of cadmium; b, d, f, h films with the excess of sulfur; 1,3 
- "+" ITO, 2, 4 - «+» In; full symbols (a-d) correspond to dark 
currents, open ones (e-h) correspond to currents under white 
light illumination 
 
In all cases, there are jumps of current that charac-
terized for SCLC. Mathematical processing of the cor-
responding curves gives the possibility to construct 
dependences ns – EF (а) and dns/dEF – EF (Fig. 4) and to 
determine the Et, Nt values of traps which are present-
ed in the wide BG material. 
 
0,01 0,1 1 10
0
1
2
3
4
=3
=1,5
=2
=1
Voltage, V
1
 
0,01 0,1 1 10
0
1
2
3
4
=3
=1,5
=2
=1
1
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
I
Voltage, V
 
0,01 0,1 1 10
0
1
2
3
4
=0,5
=1,5
=2
=1
2
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
I
Voltage, V
 
0,01 0,1 1 10
0
1
2
3
4
5
6
=0,5
=1,5
=2
=1
2
Voltage, V
 
0,01 0,1 1 10
0
1
2
3
4
=1,5
=2
=1
3
Voltage, V
 
0,01 0,1 1 10
0
1
2
3
4
=1,5
=2
=1
3
Voltage, V
 
0,01 0,1 1 10
0
1
2
3
4
=0,5
=1,5
=2
=1
4
Voltage, V
 
0,01 0,1 1 10
0
1
2
3
4
=0,5
=1,5
=2
=1
4
Voltage, V
 
 D.O. GRYNKO, O.P.DIMITRIEV, P.S. SMERTENKO ET AL. PROC. NAP 2, 01NTF41 (2013) 
 
 
01NTF41-4 
 
a 
 
b 
 
Fig. 4 – Typical differential curves ns – EF (а) and dns/dEF – EF 
(b), obtained as a result of processing the CVC SCLC, which 
allowed to determine the parameters of traps in the material  
 
It was found that CdS with excess of cadmium 
characterized by LS with energy Et = 0.514  0.026 eV. 
While in the material with excess of sulfur there were 
existed two LS in BG with energy of Et1 = 0.514  
0.026 eV and Et2 = 0.700 0.026 eV. Concentration of 
refill charge carriers on these LS was varied from 
2 1021 m-3 to 5 1022 m-3. Respectively, the full concentra-
tion Nt of these LS is more than determined values. 
Thus it was determined dependence of injection pa-
rameters and nature of injection in the structures based 
on nanostructured CdS films on their stoichiometry. 
 
 
 
5. CONCLUSIONS 
 
Nanocrystalline textured films of CdS with regulated 
stoichiometry deposited by CSVS method were investi-
gated. The CdS layers obtained at ТeS < 80 oC was found 
to contain the excess of cadmium (Cd>S by 3-5%), 
whereas the CdS layers obtained at ТeS >120 oC contain 
the excess of sulfur (Cd<S by 2-3%). It was shown that in 
the case of excess of cadmium (S < Cd) charge flow 
mechanism in the structure ITO/CdS/In was determined 
by bimolecular recombination both in the dark and un-
der illumination. In the case of sulfur excess main 
mechanism of charge flow was monomolecular recombi-
nation. There were determined LS with energy of  
Et = 0.514 0.026 eV in CdS with excess of cadmium and 
with excess of sulfur - Et1 = 0.514 0.026 eV and  
Et2  = 0.700 0.026 eV by IS method.  
Full concentration of localized states is more than 
2 1021 m-3 - 5 1022 m-3. Dependence of injection parame-
ters and nature of injection in the structures based on 
nanostructured CdS films on their stoichiometry was 
determined. 
 
ACKNOWLEDGEMENTS 
 
This work is supported by the The Ukrainian State 
Agency for Science, Innovation and Informatization 
and by the CRDF Global grant No. UKE2-7035-KV-11. 
 
REFERENCES 
 
1. S. Kasap, P. Capper, The Springer Handbook of Electronic 
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3. Handbook of Physical Properties of Semiconductors 
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4. M. Lampert, P. Mark, Current Injection in Solids (Aca-
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5. Charles K. Kao, W. Hwang, Electrical Transport in Solids 
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Solar Energy Materials & Solar Cells 76 (4), 613 (2003). 
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yavleniya v poluprovodnikah (Kiev: Naukova dumka, 
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triev, J. Phys. D: Appl. Phys. 42, 195104 (2009). 
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Dosl. 4 (2), 208 (2000) (in Russ.). 
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sol. a 36 (2), 569 (1976). 
 
-0,60 -0,55 -0,50 -0,45 -0,40 -0,35
0,0
2,0x10
22
4,0x10
22
6,0x10
22
8,0x10
22
n
s
, 
m
-
3
E
F
, eV
 
-0,60 -0,55 -0,50 -0,45 -0,40 -0,35
0,0
2,0x10
23
4,0x10
23
6,0x10
23
d
n
s
/d
E
F
 ,
 e
V
-
1
m
-
3
E
F
, eV
 


English

Injection Spectroscopy of Deep Traps in Nanostructured Films of Cadmium Sulfide

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 1, 01NTF41(4pp) (2013)   2304-1862/2013/2(1)01NTF41(4) 01NTF41-1  2013 Sumy State University Injection Spectroscopy of Deep Traps in Nanostructured Films of Cadmium Sulfide  D.O. Grynko1, O.P.Dimitriev1, P.S. Smertenko1*, O.M. Fedoryak1, А.S. Оpanasyuk2†, N.V. Тirkusova2, Yu.V.Noskov3, N. A. Ogurtsov3, A.A. Pud3‡  1 V. Lashkaryov Institute of Semiconductor Physics, NAS Ukraine, Prospect Nauky, 41, Kyiv, Ukraine; 2 Sumy State University, 2, Rymsky Korsakov Str., 40007 Sumy, Ukraine 3 Institute of Bioorganic Chemistry and Petrochemistry, NAS of Ukraine, 50, Kharkivske sjose, Kyiv, Ukraine  (Received 30 July 2013; published online 02 September 2013)  Nanocrystallin CdS films with controlled stoichiometry deposited by CSVS were investigated by meth-od of the current-voltage characteristics in ITO/CdS /In structures. It was shown that in the case of cadmi-um excess (S <Cd) charge flow mechanism both in the dark and under illumination is determined by bimo-lecular recombination in the material. In the case of excess sulfur (S>Cd) charge flow mechanism is deter-mined by monomolecular recombination. In the band gap of CdS with excess of cadmium there was detect-ed localized states with energy Et = 0.514 ± 0.026 eV, while in the material with Excess sulfur there are two localized states with energy Et1 = 0.514 ± 0.026 eV and Et2 = 0.700 ± 0.026 eV. Full concentration of lo-calized states is more than 2·1021 m-3 – 5·1022 m-3. Dependence of injection in parameters and nature of in-jection in the structures based on nanostructured CdS films on their stoichiometry was determined.  Keywords: CdS film,Nanostructured layer, Current-voltage characteristic, Differential approach, Injection spectroscopy, Trap   PACS numbers: 84.60.Jt, 73.61.Ga                                                             * smertenko@isp.kiev.ua  † opanasyuk_sumdu@ukr.net  ‡ alexander.pud@gmail.com  1. INTRODUCTION  Recently CdS films have been intensively investigat-ed due to the possibility of using it as base layers of pho-to- and gas sensors, thin films transistors, vidicons, LEDs, electrochemical cells, electroluminescent devices, window layers of high efficiency solar cells on the base of absorbing layers of CdTe, CuInSe2 (CIS),  CuIn1-xGaxSe2 (CIGS), Cu2ZnSnSe4 (CZTSe) and others [1-2]. For practical use in micro-and optoelectronics, solar energy CdS films require optimization of their optical and electrical characteristics which determined by the ensemble of intrinsic point defects and uncontrollable impurities [3]. If these defects are charged, they form traps and recombination localized states (LS) in band gap (BG). The need of focused control of such properties led to considerable interest of researchers to studying the defect structure of CdS and determining its impact on the physical characteristics of the material. At the same time, despite the large number of works devoted to this issue, the energy spectrum of LS in CdS BG remains insufficiently explored, first of all, it refers to film samples obtained by different techniques of con-densation in vacuum. The method based on the analysis of steady-state current-voltage characteristics (CVC) in regime of space charge limited currents (SCLC) [4-5] is one of the most suitable methods of studying of LS in semiconduc-tors such as CdS. It combines the simplicity of the ex-perimental equipment and their high sensitivity to low concentrations [4-5]. This method in various modifications [6-8] is widely used for determined parameters of traps in a large group of wide semiconductors and dielectrics. However, in classic form, CVC SCLC method has several disad-vantages that lead to a significant reduction of in-formativeness and some incorrectness of obtained re-sults [9]. That is why in some studies [10-12] there were pro-vided approaches that can rid disadvantages of existing method of analysis of CVC SCLC and receive the most detailed information about LS in the range of wide BG material directly from the experimental CVCs by their differential processing (injection spectroscopy (IS) method), i.e. by solving the inverse problem of research. Differential approach gives also the possibility to de-termine the charge flow mechanism in multilayer structures by analyzing the CVC [13-15]. This paper presents the results of calculation of LS parameters in nanocrystalline CdS films depending on the stoichiometry and conditions of obtaining samples using the low-temperature approximation of differen-tial approach, as well as the results of the charge flow mechanism.  2. EXPERIMENTAL DETAILS  CdS films were prepared by vacuum evaporation of the CdS powder of semiconductor-quality purity by vacuum evaporation of the semiconductor-quality puri-ty by close-spaced vacuum sublimation (CSVS) at sub-strate temperature of Тs = 600-640 С and evaporator temperature of Тe = 720-750 С. To control the stoichi-ometry of layers during the process of deposition there was co-evaporation of sulfur at temperatures TeS = 60-200 C. The deposition was performed onto glass sub-strate covered by conductive ITO underlayer. Peculiari-ties of the films obtained have been described in detail  D.O. GRYNKO, O.P.DIMITRIEV, P.S. SMERTENKO ET AL. PROC. NAP 2, 01NTF41 (2013)   01NTF41-2 in [16]. In contact layer (area ~ 0.02 cm2) was obtained by vacuum evaporation. This material allows to realize the injection of charge carriers into CdS layer. Surface morphology and stoichiometry of the films obtained were studied using electron microscope Hita-chi S-806 and Superprobe-733 JEOL, respectively.  Dark and light quasi steady-state CVCs in forward and reverse directions were measured at room temper-ature by the standard automated tester 14 TKS-100 (Russia). The voltage was changed step by step with the period of 160 ms, and current measurement was carried out at 90 ms after supplying voltage. Measuring the light CVC was carried out using white light from a halogen lamp with intensity of ~30 W/m2.  3. THEORETICAL PART  Recognition of charge flow mechanism in the sam-ples was carried out by the method of differential pro-cessing of CVC, developed in the works [10-17]. This method allows us to determine distinction between concomitant and competing charge-flow mechanisms in the structures and allocate high field processes among them. It is possible by joint analysis of j – U, η – U and d(lnη)/d(lnU) – U dependencies, where j is the current density; U is the applied voltage and η  d(ln j)/d(lnU)   2 32 3(ln ) (ln ) (ln ), ,(ln ) (ln ) (ln )d j d j d jd U d U d U  The IS method was used only for the processing of CVC in the SCLC regime. The basic ratio of IS method derived in [10-12] is the following:   2 1fLjLne U, (1)   022 1 112 1 1( )( )LsLUne eL, (2)  where e is the electron charge;  is the mobility of charge carriers; L is the sample thickness; 0 is dielec-tric constant;  is the dielectric permittivity of the ma-terial; nfL is the concentration of free charge carriers at the anode (later the index L in the ratio will be not used); nsL is the actual concentration of carriers cap-tured by local states near anode; L is the space charge density at anode responsible for limiting the current through the sample. Using the well known relation between the concen-tration of free carriers and the position of the Fermi level, it is easy to write an equation to determine the energy scale of the LS study    2 1( )ln ln lncFN e jE kT kT kTL U, (3)  where EF is the position of the quasi Fermi level for injection; k is the Boltzmann constant; T is the temper-ature; Nc( ) is free effective density of states in the con-duction (valence) band.  Distribution function of carriers localized at deep cen-ters, is obtained by differentiation of (2) for the energy  202 21 2 1 3 1 312 1 1 2( )( )( )sFdnUdE kT eL. (4)  The distribution function of LS density for the ener-gy h(E) = dNt/dE at low-temperature approximation is thought to coincide the function that describes the dis-tribution of localized carriers. However, this leads to errors during playback the shape of distributions of traps and determining their depth (Et) and concentra-tion (Nt) [9, 12, 17]. At a high temperature approximation function h(E) can be found from the convolution:   ( )( )( )fsEf fdf E Ednh E dEdE d E E, (5)  where f (E – EF) is the Fermi-Dirac function. The joint solving of equations (3), (4) and (5) allows to find h(E) function in BG of material directly from the experimental CVC SCLC measured at arbitrary tem-perature. For this purpose it is necessary to define three derivatives at each point of CVC in log-log scale. Mathematically, the task is reduced to the construc-tion of cubic spline that approximates the experimental data. Then the cubic spline is differentiated in nodes and finally h(E) function is found from the convolution (5). Physically, the ability to scan the energy distribu-tion of traps at the injection of charge carriers from the contact was caused by the following. With the increas-ing of applied voltage the quasi Fermi level moves in BG of the material and it occurs the filling of LS by free carriers and changing of the conductivity of the struc-ture. Since there is a correlation between the values that can be experimentally measured (j, U), and those that characterize the distribution of traps, (EF, ), thus each point CVC carrying an information about corre-sponding section of the energy spectrum of LS. In this paper, in order to determine parameters of LS in CdS films the low-temperature approximation of IS method was used. In this case, the Fermi-Dirac sta-tistics in (5) replaced by the Heaviside function. In or-der to calculate the energy levels of traps in BG and its concentration the methods proposed in [11, 18] was used. Obtained results are verified by solving the direct problem of the experiment and building CVC SCLC by the known parameters of LS. Coincidence of form of input and reconstructed CVC was indicating about the accuracy of determining of the parameters of deep traps in the material [17].  4. RESULTS AND DISCUSSION  Electron microscope images of surface of CdS films are presented in Fig. 1. As we expected layers were continu-ous and had finely dispersed nanocrystalline structure (Fig. 1a). Further deposition led to form nanorods on the surface of CdS films (Fig. 1b). Data of element composi-tion studying proves that layers obtained at ТeS <80 C contain the excess of cadmium (Cd > S by 3-5%), and at ТeS >120 C there is the excess of sulfur (Cd < S by 2-3%).   INJECTION SPECTROSCOPY OF DEEP TRAPS IN… PROC. NAP 2, 01NTF41 (2013)   01NTF41-3   a b  Fig. 1 – SEM images of CdS film surface: short- (a) and long-term growth phase (b)  Fig. 2a displays dark and light CVC (in forward and reverse bias) of IТО/CdS/In multilayer struc-ture, where CdS films have excess of cadmium  (Cd > S by 3-5%). in Fig. 2b shows the forward and reverse CVC curves of IТО/CdS/In multilayer struc-ture with excess of sulfur (Cd < S by 2-3%). The analysis of CVCs indicates that the resistivity IТО/CdS/In is equal about (5 106-107) Ohm cm. Fig. 2 shows that the high content of sulfur in CdS film leads to increase the photoconductivity of the struc-ture. Conductivity of illuminated material increases almost about three orders of magnitude at the excess of sulfur (Fig. 2b), whereas at the deficiency of sulfur increase of photoconductivity i only about one order in magnitude. The shortage of cadmium was found to reduce the dark conductivity of the structure by two orders of magnitude. Several almost linear sections with different slope were observed in the dark CVC of the structure ("+" ITO) both with excess and deficiency of sulfur at the region of high applied bias. Usually the areas with de-pendence of current on voltage with type: I ~ U, I ~ U 1,5-2, I ~ U 3-4 were allocated. In some cases, after super linear behavior there was quadratic one.    Fig. 2 – CVCs of ITO / CdS / In multilayer structures: a) case with the excess of cadmium (Cd>S by 3-5%); b) case with the excess of sulfur (Cd<S by 2-3%); 1.3 - "+" ITO, 2, 4 - "+" In. Full symbols correspond to dark current, open ones corre-spond to light current under illumination of white light from a halogen lamp with intensity of ~30 W/m2  It should be noted that the features of CVC can be seen at η – logU dependences which allows to reveal the fine structure of CVC. Each point of the graph de-termines the angle of the current-voltage curve in a log-log scale. η – logU dependence were obtained by finding the derivative at each experimental point of CVC SCLC. As already mentioned, mathematically task was to build a cubic smoothing spline that approx-imates the experimental data and its differentiation in nodes. The results of the differential processing of dark and light CVCs are shown in Fig. 3 а–d and 3 e–h re-spectively. Differential analysis of CVC slope (Fig. 3 c, d) [13-15] revealed two mechanisms in the nature of injected carriers: in the case of excess of cadmium (S<Cd) there is a bimolecular recombination in the material in the dark and under illumination, and it provides a differential slope of the CVC η =1.5. In case of sulfur excess (S > Cd) the principal mecha-nism of charge flow is monomolecular recombination, namely it provides a differential slope of current-voltage curves η = 2.   a  b  c  d  e  f  g  h  Fig. 2 – Differential slope α of CVCs: a, c, e, g films with the excess of cadmium; b, d, f, h films with the excess of sulfur; 1,3 - "+" ITO, 2, 4 - «+» In; full symbols (a-d) correspond to dark currents, open ones (e-h) correspond to currents under white light illumination  In all cases, there are jumps of current that charac-terized for SCLC. Mathematical processing of the cor-responding curves gives the possibility to construct dependences ns – EF (а) and dns/dEF – EF (Fig. 4) and to determine the Et, Nt values of traps which are present-ed in the wide BG material.  0,01 0,1 1 1001234=3=1,5=2=1Voltage, V1 0,01 0,1 1 1001234=3=1,5=2=11                              IVoltage, V 0,01 0,1 1 1001234=0,5=1,5=2=12                              IVoltage, V 0,01 0,1 1 100123456=0,5=1,5=2=12Voltage, V 0,01 0,1 1 1001234=1,5=2=13Voltage, V 0,01 0,1 1 1001234=1,5=2=13Voltage, V 0,01 0,1 1 1001234=0,5=1,5=2=14Voltage, V 0,01 0,1 1 1001234=0,5=1,5=2=14Voltage, V  D.O. GRYNKO, O.P.DIMITRIEV, P.S. SMERTENKO ET AL. PROC. NAP 2, 01NTF41 (2013)   01NTF41-4  a  b  Fig. 4 – Typical differential curves ns – EF (а) and dns/dEF – EF (b), obtained as a result of processing the CVC SCLC, which allowed to determine the parameters of traps in the material   It was found that CdS with excess of cadmium characterized by LS with energy Et = 0.514  0.026 eV. While in the material with excess of sulfur there were existed two LS in BG with energy of Et1 = 0.514  0.026 eV and Et2 = 0.700 0.026 eV. Concentration of refill charge carriers on these LS was varied from 2 1021 m-3 to 5 1022 m-3. Respectively, the full concentra-tion Nt of these LS is more than determined values. Thus it was determined dependence of injection pa-rameters and nature of injection in the structures based on nanostructured CdS films on their stoichiometry.    5. CONCLUSIONS  Nanocrystalline textured films of CdS with regulated stoichiometry deposited by CSVS method were investi-gated. The CdS layers obtained at ТeS < 80 oC was found to contain the excess of cadmium (Cd>S by 3-5%), whereas the CdS layers obtained at ТeS >120 oC contain the excess of sulfur (Cd<S by 2-3%). It was shown that in the case of excess of cadmium (S < Cd) charge flow mechanism in the structure ITO/CdS/In was determined by bimolecular recombination both in the dark and un-der illumination. In the case of sulfur excess main mechanism of charge flow was monomolecular recombi-nation. There were determined LS with energy of  Et = 0.514 0.026 eV in CdS with excess of cadmium and with excess of sulfur - Et1 = 0.514 0.026 eV and  Et2  = 0.700 0.026 eV by IS method.  Full concentration of localized states is more than 2 1021 m-3 - 5 1022 m-3. Dependence of injection parame-ters and nature of injection in the structures based on nanostructured CdS films on their stoichiometry was determined.  ACKNOWLEDGEMENTS  This work is supported by the The Ukrainian State Agency for Science, Innovation and Informatization and by the CRDF Global grant No. UKE2-7035-KV-11.  REFERENCES  1. S. Kasap, P. Capper, The Springer Handbook of Electronic and Photonic Materials (Berlin: Springer, 2007). 2. R. Scheer, H. Werner Schock, Chalcogenide Photovoltaics. Physics, Technologies, and Thin Film Devices (Weinheim: Wiley-VCH, 2011). 3. Handbook of Physical Properties of Semiconductors (Ed. S. Adachi) (Boston: Kluwer Academic Publishers: 2004). 4. M. Lampert, P. Mark, Current Injection in Solids (Aca-demic Press Inc., 1970). 5. Charles K. Kao, W. Hwang, Electrical Transport in Solids (United Kingdom, Pergamon Press, 1980). 6. A. Ashour, M.O. Abdel-Hamid, N.Z. El-Sayed, Chalco-genide Letters 9(9), 371 (2012).  7. C. Viswanathan, V. Senthilkumar, R. Sriranjini et al., Cryst. Res. Technol. 40 (7), 658 (2005). 8. S. Darwish, Physica B 349, 296 (2004). 9. A.S. Оpanasyuk, N.N. Оpanasyuk, N.V. 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Injection Spectroscopy of Deep Traps in Nanostructured Films of Cadmium Sulfide

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