Mechanisms of carrier transport in CdTe polycrystalline films
Cadmium telluride (CdTe) polycrystalline films grown on glass ceramic substrates by a modified close-spaced vapor transport technique have been investigated. The as-grown films have columnar structure with the average grain sizes about 10 μm. The in-plain direct current conductivity as a function...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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irk-123456789-1182362017-05-30T03:05:46Z Mechanisms of carrier transport in CdTe polycrystalline films Sukach, A.V. Tetyorkin, V.V. Krolevec, N.M. Cadmium telluride (CdTe) polycrystalline films grown on glass ceramic substrates by a modified close-spaced vapor transport technique have been investigated. The as-grown films have columnar structure with the average grain sizes about 10 μm. The in-plain direct current conductivity as a function of temperature and electric field has been investigated. The percolation conductivity is shown to be dominant at low temperatures (T < 250 K). At room temperature, the dominant transport mechanism is an activated process such as thermionic emission. The carrier transport across barriers is influenced by traps in the surface barrier regions. The non-annealed films exhibited stable electrical parameters and high photosensitivity during five-year storage under laboratory conditions. 2010 Article Mechanisms of carrier transport in CdTe polycrystalline films/ A.V. Sukach, V.V. Tetyorkin and N.M. Krolevec // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2010. — Т. 13, № 2. — С. 221-225. — Бібліогр.:19 назв. — англ. 1560-8034 PACS 78.30.L http://dspace.nbuv.gov.ua/handle/123456789/118236 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Cadmium telluride (CdTe) polycrystalline films grown on glass ceramic
substrates by a modified close-spaced vapor transport technique have been investigated.
The as-grown films have columnar structure with the average grain sizes about 10 μm.
The in-plain direct current conductivity as a function of temperature and electric field has
been investigated. The percolation conductivity is shown to be dominant at low
temperatures (T < 250 K). At room temperature, the dominant transport mechanism is an
activated process such as thermionic emission. The carrier transport across barriers is
influenced by traps in the surface barrier regions. The non-annealed films exhibited
stable electrical parameters and high photosensitivity during five-year storage under
laboratory conditions. |
| format |
Article |
| author |
Sukach, A.V. Tetyorkin, V.V. Krolevec, N.M. |
| spellingShingle |
Sukach, A.V. Tetyorkin, V.V. Krolevec, N.M. Mechanisms of carrier transport in CdTe polycrystalline films Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Sukach, A.V. Tetyorkin, V.V. Krolevec, N.M. |
| author_sort |
Sukach, A.V. |
| title |
Mechanisms of carrier transport in CdTe polycrystalline films |
| title_short |
Mechanisms of carrier transport in CdTe polycrystalline films |
| title_full |
Mechanisms of carrier transport in CdTe polycrystalline films |
| title_fullStr |
Mechanisms of carrier transport in CdTe polycrystalline films |
| title_full_unstemmed |
Mechanisms of carrier transport in CdTe polycrystalline films |
| title_sort |
mechanisms of carrier transport in cdte polycrystalline films |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2010 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118236 |
| citation_txt |
Mechanisms of carrier transport in CdTe polycrystalline films/ A.V. Sukach, V.V. Tetyorkin and N.M. Krolevec // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2010. — Т. 13, № 2. — С. 221-225. — Бібліогр.:19 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT sukachav mechanismsofcarriertransportincdtepolycrystallinefilms AT tetyorkinvv mechanismsofcarriertransportincdtepolycrystallinefilms AT krolevecnm mechanismsofcarriertransportincdtepolycrystallinefilms |
| first_indexed |
2025-07-08T13:36:16Z |
| last_indexed |
2025-07-08T13:36:16Z |
| _version_ |
1837086044655190016 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 221-225.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
221
PACS 78.30.L
Mechanisms of carrier transport in CdTe polycrystalline films
A.V. Sukach, V.V. Tetyorkin and N.M. Krolevec
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
Phone: 38 (044) 525-18-13, e-mail: teterkin@isp.kiev.ua
Abstract. Cadmium telluride (CdTe) polycrystalline films grown on glass ceramic
substrates by a modified close-spaced vapor transport technique have been investigated.
The as-grown films have columnar structure with the average grain sizes about 10 m.
The in-plain direct current conductivity as a function of temperature and electric field has
been investigated. The percolation conductivity is shown to be dominant at low
temperatures (T < 250 K). At room temperature, the dominant transport mechanism is an
activated process such as thermionic emission. The carrier transport across barriers is
influenced by traps in the surface barrier regions. The non-annealed films exhibited
stable electrical parameters and high photosensitivity during five-year storage under
laboratory conditions.
Keywords: CdTe, polycrystalline film, percolation conductivity.
Manuscript received 19.02.10; accepted for publication 25.03.10; published online 30.04.10.
1. Introduction
Cadmium telluride (CdTe) is recognized as a promising
photovoltaic material for diverse optoelectronic
applications [1]. Due to its near-optimum direct energy
gap (1.5 eV at room temperature) and high absorption
coefficient, it allows a high theoretical maximum
efficiency of 29% under AM1.5G conditions for CdTe-
based solar cells [2]. Recently CdS/CdTe thin-film solar
cells with efficiencies over 16.4% [3] and module
performance over 11% [4] were demonstrated. In
addition, CdTe has suitable physical properties like high
atomic number and high density for utilization in X-ray
and γ-ray detection [5, 6]. There is growing interest to
digital X-ray images for obtaining and processing digital
images in a real time regime. Polycrystalline CdTe as a
material with high density and high stopping power can
be successfully used for this purpose as well.
The key problems of CdS/CdTe solar cells such as
high series resistance, formation of ohmic contacts,
degradation of solar cell performance are closely
connected with conduction mechanisms in CdTe
polycrystalline films. At the same time, the conductivity
in these films has not been investigated systematically so
far. Obviously, the lack of fundamental scientific
knowledge of transport properties of CdTe
polycrystalline films impedes further progress towards
improving CdS/CdTe solar cells performance.
Usually, polycrystalline semiconductors are
regarded as having two different phases. One of them is
the crystalline material inside a grain. Its physical
properties is assumed to be similar to bulk material. The
other one is the intergrain material, that is the grain
boundaries (GBs). Accordingly, there are two
conduction channels in polycrystalline materials
connected with grains and GBs [7-9]. The conductivity
in the first channel is controlled by potential barriers at
the GBs. The grain boundary conductivity was
investigated in ZnO varistors [9]. Information on similar
investigations in other semiconductors is rather poor
because of crystalline structure and electronic properties
of GBs are not well understood so far. They were
considered to be either between ordered crystal and
disordered amorphous material [7, 8]. In CdTe
polycrystalline films of n-and p-type conductivity, GBs
were modeled by n-p-n structure [7] and dielectric
layer [10].
It is generally accepted that the direct current
conductivity in polycrystalline semiconductors is
controlled by potential barriers and space charge regions
at GBs [7, 8]. There are two distinct mechanisms
whereby mobile carriers can overcome a potential
barrier at a grain boundary: quantum-mechanical
tunneling and barrier emission. Earlier studies have
shown that if the conductivity of a polycrystalline
semiconductor exhibits Arrhenius behavior, then the
thermally excited conduction mechanism like thermionic
emission is dominant [7, 8, 11]. Frequently observed
deviations from this plot can be attributed to fluctuations
of potential barriers or to structural and electronic
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 221-225.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
222
10-1 100 101 102
10-9
10-8
10-7
10-6
10-5 - 298
- 279
- 258
- 196
I, A
U, V
Fig. 1. Current-voltage characteristics of a polycrystalline
film at different temperatures. The straight line is shown
as an eye guide.
inhomogeneity of GBs [11]. If the scatter of the potential
barrier heights is large enough, the direct current
conductivity of a polycrystalline semiconductor is
percolation in its nature [12]. An attempt to study the
percolation conductivity in a sample of bulk
polycrystalline CdTe has been performed
previously [13].
The aim of this work is to study electrical and
photoelectrical properties of thin CdTe polycrystalline
films grown on dielectric substrates in a wide range of
voltage biases and temperatures in order to clarify
mechanisms of the direct current conductivity.
2. Preparation of samples and experimental details
Preparation of CdTe polycrystalline films with stable
and predictable characteristics still to be an actual task
for device applications. Different kinds of vapor
deposition techniques [1] have been used for growing
CdTe polycrystalline films on semiconductor and
dielectric substrates. Among others, the close-spaced
vapor transport technique possesses several advantages
[14]. Since the as-grown films have poor electrical
properties, they are usually annealed in CdCl2
atmosphere.
In this study, CdTe polycrystalline films were
prepared by a close-spaced vapor transport technique
modified in our laboratory. Samples of mechanically
polished glass ceramic were used as substrates. The
glass ceramic possesses such advantages as
manufacturability, high transparency in a wide spectral
range, thermal and chemical stability. The substrates
were etched in aqueous solution of hydrofluoric acid and
washed in distilled water. For deposition, polycrystalline
CdTe of n-type conductivity doped with indium was
used as a source. The substrate holder and source
container were done from a high-density graphite block.
Both have individual heater which allows one to regulate
their temperatures with the accuracy ±3 С. The distance
h between the substrate and the surface of CdTe source
was adjusted by one to three millimeters. The developed
technological module was placed in a quartz ampoule
and mounted in a vacuum chamber (P = 10-4 Pa) using
vertical arrangement.
The source and substrate temperatures (Tso, Tsub) as
well as the distance h were varied in order to determine
optimal deposition conditions. Directly before
deposition, the substrate was heated to approximately
600 С and maintained at this temperature during a half-
hour. After that, the substrate temperature was gradually
decreased up to the required value 350-400 С.
Simultaneously, the source temperature was increased to
values 500-600 C. The films investigated in this study
had the mean grain size 9±2 μm and resistance ρ = (2-
3)×105 Ohm·cm. They were grown for approximately
15 min at Tsub = 400 С and Tso = 550 С.
The as-prepared films were of n-type conductivity.
The type of conductivity was checked by measuring the
polarity of photoresponse signal in Schottky contacts
deposited on the films. Previously, the polarity of
photoresponse signal was calibrated using a sample of
CdTe film with the known type of conductivity. The
Schottky contacts were prepared by electrolyte
deposition of Au. Ohmic contacts were deposited by
thermal vacuum evaporation of indium. The
measurements of the direct current conductivity were
made using the standard four-probe technique. The
distance between potential electrodes was within the
range 1-2 mm.
It is necessary to note that the investigated films
were not annealed. They exhibited high photosensitivity
and stable values of resistance after temperature cycling
in the range T = 77-300 K. Comparatively small changes
of electrical and photoelectrical characteristics were
observed in the investigated films after storage for five
years under ambient laboratory conditions. The
deposition method used in this investigation is charac-
terized by high transfer efficiency of the starting material
in the growth chamber (losses were less than 5%).
3. Experimental results and discussion
The current-voltage characteristics were measured in the
temperature range T = 196-298 K. Shown in Fig. 1 are
the current-electric field characteristics in the
logarithmic scale. The solid lines represent linear UI
dependences proving that the initial region of the
measured characteristics obeys the Ohm law. With
increase in electric field, the measured UI curves
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 221-225.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
223
3 4 5 6
10-10
10-9
10-8
10-7
10-6
I, A
5
4
3
2
1
103/T, K-1
Fig. 2. Direct current vs reverse temperature dependences
measured at different bias voltages, V: 1 – 2, 2 – 15, 3 – 50,
4 – 80, 5 – 120.
start to deviate from the linear dependence. It is
necessary to note that the observed non-linearity has
different character for the characteristics measured at
different temperatures. At room temperature T = 298 K,
they consist of the sub-linear region at lower values of
the electric field followed by the super-linear one at
higher values of bias voltages. To our knowledge, such
UI dependence is observed in CdTe polycrystalline
films for the first time. The sub-linear region is not
observed on characteristics measured at low
temperatures. It should be also stressed that the current-
voltage non-linearities are observed at rather low values
of the electric field F = 100-300 V/cm.
The dependence of the direct current on
temperature was measured at several fixed values of the
electric field, Fig. 2. Experimental data are shown in the
standard semilogarithmic scale, with I versus 103/T. Two
temperature regions for all the measured curves can be
distinguished: a sharp increase in current with
temperature increasing at T > 250 K and a rather slow
variation of current at lower temperatures. Obviously,
the high temperature region corresponds to a thermally
activated conduction mechanism. The activation energy
of this mechanism is represented by a straight line in
coordinates FE (Fig. 3). The value of ΔE0 =
0.63 eV at zero electric field is obtained using
extrapolation procedure. Finally, Fig. 4 shows the
current-electric field characteristics plotted in
coordinates 2/1UI . As seen, at low temperatures
experimental data are well linearized within two orders
of magnitude. The linearization is worse at high
temperatures.
Experimental results shown in Figs 1 and 2 clearly
indicate that different transport mechanisms operate in
the investigated samples. The current-voltage
characteristic measured at room temperature can be
interpreted in terms of the grain boundary trapping
theory firstly developed for silicon bicrystals [15]. The
capture of mobile carriers on partially occupied trapping
states at a grain boundary and subsequent rise of the
barrier height results in the sub-linear current-voltage
dependence. With electric field increase, these states
become fully occupied, and direct current starts to
increase exponentially. However, the field dependence
of the direct current linear dependence of the dark
current on the square root of electric field can’t be
explained within this theory [15]. In ordered
semiconductors, these dependences are usually
explained by the well-known Frenkel-Poole emission of
carriers from deep-defect states in the gap [17]. As a
rule, this emission is observed in the strong electric field
of the order of 104 V/cm. The current-voltage
characteristics with a form close to the Frenkel-Poole
law are also observed in amorphous and polycrystalline
materials [9, 18]. The theory of this phenomena in
highly doped and compensated semiconductors has been
developed by Shklovskii [18]. This theory was also used
for interpretation of the dependence of conductivity on
electric field in a polycrystalline semiconductor with the
large-scale fluctuations of the potential barrier height
[12]. However, its experimental verification in
application to polycrystalline semiconductors is
insufficient.
0 5 10 15 20 25
0
1
2
3
4
5
6
7
E(x10), eV
F1/2, V1/2 cm-1/2
Fig. 3. Field dependence of the activation energy.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 221-225.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
224
2 4 6 8 10 12 14 16
10-9
10-8
10-7
10-6
I, A
298
279
258
196
U1/2, V1/2
Fig. 4. Current-voltage characteristics in Schottky coordinates
measured at different temperatures. The straight line represents
the dependence I – U1/2.
The analysis of experimental data given below is
based on the Shklovskii theory [18]. The conductivity of
such a semiconductor at high temperatures is determined
by thermal transition of carriers on the percolation level
kT
EE
kT
E Fpp
expexp 00 , (1)
where Ep is the percolation energy and EF is the Fermi
energy. If fluctuations of potential barriers U0 are close
to Ep–EF, the non-ohmic conductivity becomes apparent
at rather low electric field F in comparison with the
ordered semiconductors:
0
9.0
0
U
kTkT
U
kT
kTeFL . (2)
Here L is the length of the large-scale fluctuations.
In this case, the field dependence of the dark current has
a form close to the Frenkel-Poole law
kT
F
II
21
0 exp , (3)
but with a different coefficient in the exponent. For the
Frenkel-Poole emission from deep traps in ordered
semiconductors this coefficient is given by the known
formula [16]
2/1
0
3 )/( eFP , (4)
Table. Experimental and calculated material parameters.
Т, K 298 279 269 258 247 217 196
βexp,
V–1/2·сm1/2
0.110 0.125 0.134 0.145 0.155 0.166 0.173
βSH,
V–1/2·сm1/2
0.089 0.094 0.097 0.101 0.106 0.120 0.133
βFP,
V–1/2·сm1/2
0.0089 0.0095 0.0099 0.0103 0.0107 0.0122 0.0135
L×106, сm 5.8 5.1 4.8 6.3 5.8 6.9 5.7
where all symbols have usual meanings. For the
Schottky emission, its value is a factor of 0.5 lower. In
the Shklovskii theory, this coefficient is expressed as
2/1
0 )(CreUSE , (5)
where the numerical coefficient C is ranged from 0.25
to 1.0 [18].
If percolation conductivity is dominant in the
investigated films at low temperatures, the coefficient β
can be extracted from the slope of 2/1UI
characteristics shown in Fig. 4. The list of
experimentally determined and calculated values of β is
shown in Table. In the calculation of βSE, two
assumptions were made: i) the magnitude of potential
barrier fluctuations U0 is equal to ΔE0 and ii) the length
of the large-scale fluctuations L is determined for the
minimal electric field values at which the non-ohmic
conductivity begins. As seen from Table, there is a
pronounced difference between experimental values of β
and those calculated for the Frenkel-Poole and Schottky
emission. Much better coincidence is observed for the
values calculated in accordance with the Shklovskii
theory.
The calculated values of the length L needs
additional explanation. In a polycrystalline
semiconductor, the distance between potential barriers
are equal to the average size of grains. The physical
meaning of L in this case is the distance between the key
barriers that determine the non-ohmic conductivity in
low electric fields [11]. As seen from Table, the
calculated values of L are far less than the average size
of grains (~10 μm) in the investigated films. In order to
consistently explain experimental results obtained in this
study, it has been assumed that the GBs in the
investigated films can be treated as a highly doped and
compensated semiconductor. This assumption is based
on the well-known fact of a very high density of defect
states (>1018 cm-3) at the GBs in CdTe polycrystalline
films [19]. These high defect state densities in the GBs
have major contribution to both the high density of point
defects and the high degree of compensation commonly
observed in cadmium telluride devices. In this case, the
length L has a meaning commonly used in the theory of
highly doped and compensated semiconductors [18].
Lowering the potential barriers in the applied electric
field shown in Fig. 3 results in emission of carriers from
the electron-hole drops onto the percolation level. This
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 221-225.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
225
result may be interpreted as analogue of Frenkel-Poole
effect in disordered semiconductors.
As seen from Fig. 4, the percolation conduction is
dominant at low temperatures. In fact, this process could
be also at room temperature, but another conduction
channel provided by the thermally excited movement of
carriers over the potential barriers can mask it.
4. Conclusions
From the study of the direct current as a function of
temperature and electric field the possible conduction
mechanisms were established in CdTe polycrystalline
films grown on glass ceramic substrates. In the low
temperature region T < 250 K, the percolation
conductivity is a dominant conduction mechanism. At
room temperature, the dominant conduction mechanism
becomes the thermally activated process like thermionic
emission or drift-diffusion transport of mobile carriers
over potential barriers at the grain boundaries.
References
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Plenum Press, New York, 1983.
2. D.P. Holliday, J.M. Eggelston, K. Duarose, A
photoluminescence study of polycrystalline thin-
film CdTe/CdS solar cells // J. Cryst. Growth 186,
p. 543-549 (1998).
3. Record 16.4% efficient CdTe PV cell // III-Vs
Review, 14(5), p. 22 (2001).
4. D.W. Cunningham, Appolo thin film process
development, in: Final Technical Report, p. 34
(2002).
5. K. Zanio, Cadmium Telluride, in: Semiconductors
and Semimetals, 13, Eds. R.K. Willardson and
A.C. Beer. Academ. Press, NY, 1978, p. 1-235.
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Ye.V. Korbutyak, M.M. Borisyuk, Cadmium
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Ivan Fedorov Publ., Kyiv, 2000.
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Thin Films and Devices, Ed. L.L. Kazmerski.
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11. J.H. Werner, Origin of curved Arrhenius plots for
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Theory of non-linear percolation
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tela 24 (5), p. 1352-1359 (1982), in Russian.
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R. Srivastava, and P.C. Mathur, Hopping
conduction ip polycrystalline semiconductors // J.
Appl. Phys. 57(6), p. 2026-2029 (1985).
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close-spaced vapor transport technique // J. Vac.
Sci. Technol. A 8(2), p. 979-983 (1990).
15. E.I. Goldman, A.G. Zhdan, Electrical conductivity
of semiconductors with intergrain barriers // Fizika
tekhnika poluprovodnikov 10(10), p. 1839 (1976),
in Russian.
16. E.I. Goldman, I.B. Gulayev, A.G. Zhdan,
V.B. Sandomirskii, Field characteristics of
electrical conductivity in semiconductor films with
intergrain barriers // Fizika tekhnika polupro-
vodnikov 10(11), p. 2089 (1976), in Russian.
17. A.G. Milnes, Deep Impurities in Semiconductors.
Wiley, New York, 1973.
18. B.I. Shklovskii, Percolation conductivity in strong
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13(1), p. 93 (1979), in Russian.
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 221-225.
PACS 78.30.L
Mechanisms of carrier transport in CdTe polycrystalline films
A.V. Sukach, V.V. Tetyorkin and N.M. Krolevec
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
Phone: 38 (044) 525-18-13, e-mail: teterkin@isp.kiev.ua
Abstract. Cadmium telluride (CdTe) polycrystalline films grown on glass ceramic substrates by a modified close-spaced vapor transport technique have been investigated. The as-grown films have columnar structure with the average grain sizes about 10 (m. The in-plain direct current conductivity as a function of temperature and electric field has been investigated. The percolation conductivity is shown to be dominant at low temperatures (T < 250 K). At room temperature, the dominant transport mechanism is an activated process such as thermionic emission. The carrier transport across barriers is influenced by traps in the surface barrier regions. The non-annealed films exhibited stable electrical parameters and high photosensitivity during five-year storage under laboratory conditions.
Keywords: CdTe, polycrystalline film, percolation conductivity.
Manuscript received 19.02.10; accepted for publication 25.03.10; published online 30.04.10.
1. Introduction
Cadmium telluride (CdTe) is recognized as a promising photovoltaic material for diverse optoelectronic applications [1]. Due to its near-optimum direct energy gap (1.5 eV at room temperature) and high absorption coefficient, it allows a high theoretical maximum efficiency of 29% under AM1.5G conditions for CdTe-based solar cells [2]. Recently CdS/CdTe thin-film solar cells with efficiencies over 16.4% [3] and module performance over 11% [4] were demonstrated. In addition, CdTe has suitable physical properties like high atomic number and high density for utilization in X-ray and γ-ray detection [5, 6]. There is growing interest to digital X-ray images for obtaining and processing digital images in a real time regime. Polycrystalline CdTe as a material with high density and high stopping power can be successfully used for this purpose as well.
The key problems of CdS/CdTe solar cells such as high series resistance, formation of ohmic contacts, degradation of solar cell performance are closely connected with conduction mechanisms in CdTe polycrystalline films. At the same time, the conductivity in these films has not been investigated systematically so far. Obviously, the lack of fundamental scientific knowledge of transport properties of CdTe polycrystalline films impedes further progress towards improving CdS/CdTe solar cells performance.
Usually, polycrystalline semiconductors are regarded as having two different phases. One of them is the crystalline material inside a grain. Its physical properties is assumed to be similar to bulk material. The other one is the intergrain material, that is the grain boundaries (GBs). Accordingly, there are two conduction channels in polycrystalline materials connected with grains and GBs [7-9]. The conductivity in the first channel is controlled by potential barriers at the GBs. The grain boundary conductivity was investigated in ZnO varistors [9]. Information on similar investigations in other semiconductors is rather poor because of crystalline structure and electronic properties of GBs are not well understood so far. They were considered to be either between ordered crystal and disordered amorphous material [7, 8]. In CdTe polycrystalline films of n-and p-type conductivity, GBs were modeled by n-p-n structure [7] and dielectric layer [10].
It is generally accepted that the direct current conductivity in polycrystalline semiconductors is controlled by potential barriers and space charge regions at GBs [7, 8]. There are two distinct mechanisms whereby mobile carriers can overcome a potential barrier at a grain boundary: quantum-mechanical tunneling and barrier emission. Earlier studies have shown that if the conductivity of a polycrystalline semiconductor exhibits Arrhenius behavior, then the thermally excited conduction mechanism like thermionic emission is dominant [7, 8, 11]. Frequently observed deviations from this plot can be attributed to fluctuations of potential barriers or to structural and electronic inhomogeneity of GBs [11]. If the scatter of the potential barrier heights is large enough, the direct current conductivity of a polycrystalline semiconductor is percolation in its nature [12]. An attempt to study the percolation conductivity in a sample of bulk polycrystalline CdTe has been performed previously [13].
The aim of this work is to study electrical and photoelectrical properties of thin CdTe polycrystalline films grown on dielectric substrates in a wide range of voltage biases and temperatures in order to clarify mechanisms of the direct current conductivity.
2. Preparation of samples and experimental details
Preparation of CdTe polycrystalline films with stable and predictable characteristics still to be an actual task for device applications. Different kinds of vapor deposition techniques [1] have been used for growing CdTe polycrystalline films on semiconductor and dielectric substrates. Among others, the close-spaced vapor transport technique possesses several advantages [14]. Since the as-grown films have poor electrical properties, they are usually annealed in CdCl2 atmosphere.
3456
10
-10
10
-9
10
-8
10
-7
10
-6
I, A
5
4
3
2
1
10
3
/T, K
-1
In this study, CdTe polycrystalline films were prepared by a close-spaced vapor transport technique modified in our laboratory. Samples of mechanically polished glass ceramic were used as substrates. The glass ceramic possesses such advantages as manufacturability, high transparency in a wide spectral range, thermal and chemical stability. The substrates were etched in aqueous solution of hydrofluoric acid and washed in distilled water. For deposition, polycrystalline CdTe of n-type conductivity doped with indium was used as a source. The substrate holder and source container were done from a high-density graphite block. Both have individual heater which allows one to regulate their temperatures with the accuracy ±3 (С. The distance h between the substrate and the surface of CdTe source was adjusted by one to three millimeters. The developed technological module was placed in a quartz ampoule and mounted in a vacuum chamber (P = 10-4 Pa) using vertical arrangement.
The source and substrate temperatures (Tso, Tsub) as well as the distance h were varied in order to determine optimal deposition conditions. Directly before deposition, the substrate was heated to approximately 600 (С and maintained at this temperature during a half-hour. After that, the substrate temperature was gradually decreased up to the required value 350-400 (С. Simultaneously, the source temperature was increased to values 500-600 (C. The films investigated in this study had the mean grain size 9±2 μm and resistance ρ = (2-3)×105 Ohm·cm. They were grown for approximately 15 min at Tsub = 400 (С and Tso = 550 (С.
The as-prepared films were of n-type conductivity. The type of conductivity was checked by measuring the polarity of photoresponse signal in Schottky contacts deposited on the films. Previously, the polarity of photoresponse signal was calibrated using a sample of CdTe film with the known type of conductivity. The Schottky contacts were prepared by electrolyte deposition of Au. Ohmic contacts were deposited by thermal vacuum evaporation of indium. The measurements of the direct current conductivity were made using the standard four-probe technique. The distance between potential electrodes was within the range 1-2 mm.
It is necessary to note that the investigated films were not annealed. They exhibited high photosensitivity and stable values of resistance after temperature cycling in the range T = 77-300 K. Comparatively small changes of electrical and photoelectrical characteristics were observed in the investigated films after storage for five years under ambient laboratory conditions. The deposition method used in this investigation is characterized by high transfer efficiency of the starting material in the growth chamber (losses were less than 5%).
3. Experimental results and discussion
The current-voltage characteristics were measured in the temperature range T = 196-298 K. Shown in Fig. 1 are the current-electric field characteristics in the logarithmic scale. The solid lines represent linear
U
I
-
dependences proving that the initial region of the measured characteristics obeys the Ohm law. With increase in electric field, the measured
U
I
-
curves start to deviate from the linear dependence. It is necessary to note that the observed non-linearity has different character for the characteristics measured at different temperatures. At room temperature T = 298 K, they consist of the sub-linear region at lower values of the electric field followed by the super-linear one at higher values of bias voltages. To our knowledge, such
U
I
-
dependence is observed in CdTe polycrystalline films for the first time. The sub-linear region is not observed on characteristics measured at low temperatures. It should be also stressed that the current-voltage non-linearities are observed at rather low values of the electric field F = 100-300 V/cm.
10
-1
10
0
10
1
10
2
10
-9
10
-8
10
-7
10
-6
10
-5
- 298
- 279
- 258
- 196
I, A
U, V
The dependence of the direct current on temperature was measured at several fixed values of the electric field, Fig. 2. Experimental data are shown in the standard semilogarithmic scale, with I versus 103/T. Two temperature regions for all the measured curves can be distinguished: a sharp increase in current with temperature increasing at T > 250 K and a rather slow variation of current at lower temperatures. Obviously, the high temperature region corresponds to a thermally activated conduction mechanism. The activation energy of this mechanism is represented by a straight line in coordinates
F
E
-
D
(Fig. 3). The value of ΔE0 = 0.63 eV at zero electric field is obtained using extrapolation procedure. Finally, Fig. 4 shows the current-electric field characteristics plotted in coordinates
2
/
1
U
I
-
. As seen, at low temperatures experimental data
are well linearized within two orders of magnitude. The linearization is worse at high temperatures.
Experimental results shown in Figs 1 and 2 clearly indicate that different transport mechanisms operate in the investigated samples. The current-voltage characteristic measured at room temperature can be interpreted in terms of the grain boundary trapping theory firstly developed for silicon bicrystals [15]. The capture of mobile carriers on partially occupied trapping states at a grain boundary and subsequent rise of the barrier height results in the sub-linear current-voltage dependence. With electric field increase, these states become fully occupied, and direct current starts to increase exponentially. However, the field dependence of the direct current linear dependence of the dark current on the square root of electric field can’t be explained within this theory [15]. In ordered semiconductors, these dependences are usually explained by the well-known Frenkel-Poole emission of carriers from deep-defect states in the gap [17]. As a rule, this emission is observed in the strong electric field of the order of 104 V/cm. The current-voltage characteristics with a form close to the Frenkel-Poole law are also observed in amorphous and polycrystalline materials [9, 18]. The theory of this phenomena in highly doped and compensated semiconductors has been developed by Shklovskii [18]. This theory was also used for interpretation of the dependence of conductivity on electric field in a polycrystalline semiconductor with the large-scale fluctuations of the potential barrier height [12]. However, its experimental verification in application to polycrystalline semiconductors is insufficient.
0510152025
0
1
2
3
4
5
6
7
E(x10), eV
F
1/2
, V
1/2
cm
-1/2
Fig. 3. Field dependence of the activation energy.
246810121416
10
-9
10
-8
10
-7
10
-6
I, A
298
279
258
196
U
1/2
, V
1/2
Fig. 4. Current-voltage characteristics in Schottky coordinates measured at different temperatures. The straight line represents the dependence I – U1/2.
The analysis of experimental data given below is based on the Shklovskii theory [18]. The conductivity of such a semiconductor at high temperatures is determined by thermal transition of carriers on the percolation level
÷
÷
ø
ö
ç
ç
è
æ
-
D
-
s
=
÷
÷
ø
ö
ç
ç
è
æ
D
-
s
=
s
kT
E
E
kT
E
F
p
p
exp
exp
0
0
,
(1)
where Ep is the percolation energy and EF is the Fermi energy. If fluctuations of potential barriers U0 are close to Ep–EF, the non-ohmic conductivity becomes apparent at rather low electric field F in comparison with the ordered semiconductors:
÷
ø
ö
ç
è
æ
@
÷
÷
ø
ö
ç
ç
è
æ
³
0
9
.
0
0
U
kT
kT
U
kT
kT
eFL
.
(2)
Here L is the length of the large-scale fluctuations. In this case, the field dependence of the dark current has a form close to the Frenkel-Poole law
÷
÷
ø
ö
ç
ç
è
æ
b
=
kT
F
I
I
2
1
0
exp
,
(3)
but with a different coefficient ( in the exponent. For the Frenkel-Poole emission from deep traps in ordered semiconductors this coefficient is given by the known formula [16]
2
/
1
0
3
)
/
(
pee
=
b
e
FP
,
(4)
Table. Experimental and calculated material parameters.
Т, K
298
279
269
258
247
217
196
βexp,
V–1/2·сm1/2
0.110
0.125
0.134
0.145
0.155
0.166
0.173
βSH,
V–1/2·сm1/2
0.089
0.094
0.097
0.101
0.106
0.120
0.133
βFP,
V–1/2·сm1/2
0.0089
0.0095
0.0099
0.0103
0.0107
0.0122
0.0135
L×106, сm
5.8
5.1
4.8
6.3
5.8
6.9
5.7
where all symbols have usual meanings. For the Schottky emission, its value is a factor of 0.5 lower. In the Shklovskii theory, this coefficient is expressed as
2
/
1
0
)
(
CreU
SE
=
b
,
(5)
where the numerical coefficient C is ranged from 0.25 to 1.0 [18].
If percolation conductivity is dominant in the investigated films at low temperatures, the coefficient
EMBED Equation.3 β can be extracted from the slope of
2
/
1
U
I
-
characteristics
EMBED Equation.3 shown in Fig. 4. The list of experimentally determined and calculated values of β is shown in Table. In the calculation of βSE, two assumptions were made: i) the magnitude of potential barrier fluctuations U0 is equal to ΔE0 and ii) the length of the large-scale fluctuations L is determined for the minimal electric field values at which the non-ohmic conductivity begins. As seen from Table, there is a pronounced difference between experimental values of β and those calculated for the Frenkel-Poole and Schottky emission. Much better coincidence is observed for the values calculated in accordance with the Shklovskii theory.
The calculated values of the length L needs additional explanation. In a polycrystalline semiconductor, the distance between potential barriers are equal to the average size of grains. The physical meaning of L in this case is the distance between the key barriers that determine the non-ohmic conductivity in low electric fields [11]. As seen from Table, the calculated values of L are far less than the average size of grains (~10 μm) in the investigated films. In order to consistently explain experimental results obtained in this study, it has been assumed that the GBs in the investigated films can be treated as a highly doped and compensated semiconductor. This assumption is based on the well-known fact of a very high density of defect states (>1018 cm-3) at the GBs in CdTe polycrystalline films [19]. These high defect state densities in the GBs have major contribution to both the high density of point defects and the high degree of compensation commonly observed in cadmium telluride devices. In this case, the length L has a meaning commonly used in the theory of highly doped and compensated semiconductors [18]. Lowering the potential barriers in the applied electric field shown in Fig. 3 results in emission of carriers from the electron-hole drops onto the percolation level. This result may be interpreted as analogue of Frenkel-Poole effect in disordered semiconductors.
As seen from Fig. 4, the percolation conduction is dominant at low temperatures. In fact, this process could be also at room temperature, but another conduction channel provided by the thermally excited movement of carriers over the potential barriers can mask it.
4. Conclusions
From the study of the direct current as a function of temperature and electric field the possible conduction mechanisms were established in CdTe polycrystalline films grown on glass ceramic substrates. In the low temperature region T < 250 K, the percolation conductivity is a dominant conduction mechanism. At room temperature, the dominant conduction mechanism becomes the thermally activated process like thermionic emission or drift-diffusion transport of mobile carriers over potential barriers at the grain boundaries.
References
1. K.L. Chopra, S.R. Das, Thin- Film Solar Cells. Plenum Press, New York, 1983.
2. D.P. Holliday, J.M. Eggelston, K. Duarose, A photoluminescence study of polycrystalline thin-film CdTe/CdS solar cells // J. Cryst. Growth 186, p. 543-549 (1998).
3. Record 16.4% efficient CdTe PV cell // III-Vs Review, 14(5), p. 22 (2001).
4. D.W. Cunningham, Appolo thin film process development, in: Final Technical Report, p. 34 (2002).
5. K. Zanio, Cadmium Telluride, in: Semiconductors and Semimetals, 13, Eds. R.K. Willardson and A.C. Beer. Academ. Press, NY, 1978, p. 1-235.
6. D.V. Korbutyak, S.V. Mel’nichuk, Ye.V. Korbutyak, M.M. Borisyuk, Cadmium Impurity-Defect States and Detector’s Properties. Ivan Fedorov Publ., Kyiv, 2000.
7. L.L. Kazmerski, in: Polycrystalline and Amorphous Thin Films and Devices, Ed. L.L. Kazmerski. Acad. Press, New York, 1980.
8.
Polycrystalline Semiconductors. Physical Properties and Applications, Ed. G. Harbeke. Springer, Heidelberg, 1985.
9. V.B. Kvaskov, Semiconductor Devices with Bipolar Conductivity. Energoatomizdat, Moscow, 1988 (in Russian).
10. K.M. Doschanov, Mecahanism of abnormally large photovoltage in polycrystalline semiconductors // Fizika tekhnika poluprovodnikov 24(7), p. 1251-1258 (1990), in Russian.
11. J.H. Werner, Origin of curved Arrhenius plots for the conductivity of polycrystalline semiconductors // Solid State Phenomena 37-38, p. 213-218 (1994).
12. A.Ya. Vinnikov, A.M. Meshkov, V.N. Savushkin, Theory of non-linear percolation electroconductivity in disordered semiconductor system with intergrain barriers // Fizika tverdogo tela 24 (5), p. 1352-1359 (1982), in Russian.
13. R.P. Sharma, A.K. Shukla, A.K. Kapoor, R. Srivastava, and P.C. Mathur, Hopping conduction ip polycrystalline semiconductors // J. Appl. Phys. 57(6), p. 2026-2029 (1985).
14. V. Sosa, R. Castro, J.L. Pena, Pressure and temperature influence on CdTe thin-film deposit by close-spaced vapor transport technique // J. Vac. Sci. Technol. A 8(2), p. 979-983 (1990).
15. E.I. Goldman, A.G. Zhdan, Electrical conductivity of semiconductors with intergrain barriers // Fizika tekhnika poluprovodnikov 10(10), p. 1839 (1976), in Russian.
16. E.I. Goldman, I.B. Gulayev, A.G. Zhdan, V.B. Sandomirskii, Field characteristics of electrical conductivity in semiconductor films with intergrain barriers // Fizika tekhnika poluprovodnikov 10(11), p. 2089 (1976), in Russian.
17. A.G. Milnes, Deep Impurities in Semiconductors. Wiley, New York, 1973.
18. B.I. Shklovskii, Percolation conductivity in strong electric fields // Fizika tekhnika poluprovodnikov 13(1), p. 93 (1979), in Russian.
19. A.S. Gilmore, V. Kaydanov, T.R. Ohno, D. Grecu, and D. Rose, Impedance spectroscopy and Hall measurements on CdTe thin polycrystalline films, in: II-VI Compound Semiconductor Photovoltaic Materials, MRS Proceedings, 668 (2001).
� EMBED Origin50.Graph ���
Fig. 2. Direct current vs reverse temperature dependences measured at different bias voltages, V: 1 – 2, 2 – 15, 3 – 50, 4 – 80, 5 – 120.
� EMBED Origin50.Graph ���
Fig. 1. Current-voltage characteristics of a polycrystalline film at different temperatures. The straight line is shown as an eye guide.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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