A novel type heterojunction photodiodes formed junctions of Au/LiZnSnO and LiZnSnO/p-Si in series

Article abstract of DOI:10.1016/j.jallcom.2014.11.035

Lithium-zinc-tin-oxide thin films were prepared by sol gel method. The structural and optical properties of the films were investigated. The optical band gaps of the LiZnSnO films were found to be 3.78 eV for 0 at.% Li, 3.77 eV for 1 at.% Li, 3.87 eV for 3 at.% Li and 3.85 eV for 5 at.% Li, respectively. Au/LiZnSnO/p-Si/Al photodiodes were fabricated using a lithium-zinc-tin-oxide (LZTO, Li-Zn-Sn-O) layer grown on p-Si semiconductor. The electrical characteristics of the photodiodes were analyzed by current-voltage, capacitance-voltage and conductance-voltage measurements. The reverse current of the diodes increases with both the increasing illumination intensity and Li content. It was found that the Li-doped ZTO photodiodes exhibited a better device performance than those with an undoped ZTO.

Full text of DOI:10.1016/j.jallcom.2014.11.035

Accepted Manuscript
A novel type heterojunction photodiodes formed junctions of Au/LiZnSnO and
LiZnSnO /p-Si in series
H. Aydin, A. Tataroğlu, Ahmed A. Al-Ghamdi, F. Yakuphanoglu, Farid El-
Tantawy, W.A. Farooq
PII:
S0925-8388(14)02678-4
DOI:
http://dx.doi.org/10.1016/j.jallcom.2014.11.035
JALCOM 32584
Reference:
Journal of Alloys and Compounds
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 September 2014
1 November 2014
4 November 2014
Please cite this article as: H. Aydin, A. Tataroğlu, A.A. Al-Ghamdi, F. Yakuphanoglu, F. El-Tantawy, W.A. Farooq,
A novel type heterojunction photodiodes formed junctions of Au/LiZnSnO and LiZnSnO /p-Si in series, Journal of
Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.11.035
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A novel type heterojunction photodiodes formed
junctions of Au/LiZnSnO and LiZnSnO /p-Si in series
H. Aydina, A. Tataroğlub, Ahmed A. Al-Ghamdic , F. Yakuphanoglua,c, Farid El-
Tantawyd, W.A. Farooqe
aDepartment of Metallurgical and Materials Science, Faculty of Engineering, Tunceli University,
Tunceli, Turkey
b Department of Physics, Faculty of Science, Gazi University, Ankara, Turkey
cDepartment of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
dDepartment of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypt
ePhysics and Astronomy Department, College of Science, King Saud University, Riyadh, Saudi
Arabia
Abstract
Lithium-zinc-tin-oxide thin films were prepared by sol gel method. The structural
and optical properties of the films were investigated. The optical band gaps of the
LiZnSnO films were found to be 3.78 eV for 0 %at. Li, 3.77 eV for 1 %at. Li, 3.87 eV for
3 %at. Li and 3.85 eV for 5 %at. Li, respectively. Au/LiZnSnO/p-Si/Al photodiodes were
fabricated using a lithium-zinc-tin-oxide (LZTO, Li-Zn-Sn-O) layer grown on p-Si
semiconductor. The electrical characteristics of the photodiodes were analyzed by current-
voltage, capacitance-voltage and conductance-voltage measurements. The reverse current
of the diodes increases with both the increasing illumination intensity and Li content. It
was found that the Li-doped ZTO photodiodes exhibited a better device performance than
those with an undoped ZTO.
Keywords: Photodiode; Lithium-zinc-tin-oxide (LZTO); Sol gel
Corresponding author: fyhanoglu@firat.edu.tr (F.Yakuphanoglu)
Tel:+90 4242370000-3496
Fax:+90 424 2330062
1

1. Introduction
Zinc-tin-oxide (ZTO, Zn-Sn-O) thin film attracts much attention due to its wide band
gap, high visible transmittance, good n-type electrical conductivity, as well as its
cheapness [1, 2]. Moreover, attractive attributes of ZTO include its tendency to possess an
exceedingly smooth surface in thin films [3]. Thus, ZTO films can become the promising
amorphous oxide semiconductors for oxide thin-film transistor applications. The ZTO
films can be doped with various elements such as Zr, Ga, Al, Mg, La, and Ti to obtain the
electrical performance devices [4-9].
A photodiode is a semiconductor device that transforms light into current and is
designed to operate in reverse bias. Also, the photodiode has a depleted semiconductor
region with a high electric field that serves to separate photo-generated electron-hole pairs.
When the photodiode exposed illumination intensity, the electron-hole pairs occur near the
junction interface and then they separated under electric field but this separated is more
effective at in the reverse bias region due to the high value of electric field. Electron-hole
pairs, which are absorbed in the quasi-neutral regions, can still contribute to the
photocurrent generated within one diffusion length of the depletion region. Furthermore,
carriers photo-generated in the depletion region are swept by the built-in field and flow as
a drift current. Therefore, the photo-response effect in the reverse bias region is
considerable higher than the forward bias region and the variation of photocurrent with
light intensity was investigated in the reverse bias [10-13].
In present paper, we have fabricated LiZnSnO/p-Si photodiodes for the first time.
The effects of Li addition on electrical characteristics of the photodiodes have been
investigated using current-voltage, capacitance-voltage and conductance-voltage
measurements.
2

2. Experimental details
The used precursors are lithium nitrate, tin chloride and zinc acetate. ZnSnO films
were doped with various molar ratios of lithium: Li:(Zn+Sn) = 1:3:5. Firstly, the lithium
nitrate, tin chloride and zinc acetate precursors were dissolved in 2-methoxyethanol under
stirring for 30 min and then, ethanolamine was added to this solution and then, was stirred
o
for 30 min at 60 C on a hot plate. The LiZnSnO gel solutions were obtained. To prepare
the diodes, firstly, p-Si substrates were used and an ohmic contact was prepared on back
side of p-Si substrates by evaporating of Al metal. The prepared Al/p-Si contact was
o
annealed at 570 C for 5 min in nitrogen atmosphere. The prepared gel solutions of
LiZnSnO samples were coated on p-type silicon wafer having ohmic Al contact using a
o
spin coater with 4000 rpm for 30 s and dried at 150 C for 10 min. The coating procedure
o
was repeated for 10 times. Then, the obtained solid films were annealed at 400 C for 1h.
The top contact was prepared by evaporating of gold metal using a sputtering system to
obtain a novel type heterojunction diode formed junctions of Au/LiZnSnO and LiZnSnO
/p-Si in series. The film thicknesses of the 0at.% LiZnSnO , 1at.% LiZnSnO, 3at.%
LiZnSnO and 5at.% LiZnSnO were determined using a PARK system XE100 atomic force
microscopy and were found to be 356 5, 221 2, 295 3 and 300 4 nm, respectively.
The I-V, C-V and G-V measurements of the prepared diodes were carried out using a
Keithley 4200 semiconductor characterization system. The photoresponse characteristics
of the diodes were measured under a solar simulator and the intensity of the light was
measured by solar power meter (Model TM-206). The reflectance spectra of the films were
performed directly using a Shimadzu 3600 UV-VIS-NIR spectrometer attached an
integrating sphere. X-ray diffraction patterns (XRD) of the LiZnSnO films were performed
by BRUKER D8 diffractometer at room temperature.
3

3. Results and Discussion
3.1. Structural and optical properties of LiZnSnO films
Fig.1 shows AFM images of the LiZnSnO films. As seen in Fig.1, the surface
morphology of the films is changed with Li dopant. The surface roughness values of 0at.%
LiZnSnO, 1at.% LiZnSnO, 3at.% LiZnSnO and 5at.% LiZnSnO films were determined to
be 7.32 nm, 15.77 nm, 16.86 nm and 8.11 nm, respectively. The grain sizes of the 0at.%
LiZnSnO, 1at. LiZnSnO, 3at.% LiZnSnO and 5at.% LiZnSnO films were found to be 50-
65 nm, 170-184 nm, 110-121 nm and 80-132 nm, respectively. The grain size is increased
with Li dopant with respect to the grain size of ZnSnO film.
-a-
-b-
4

-c-
-d-
Fig.1. AFM images of the LiZnSnO films a) 0at.% LiZnSnO b) 1at.% LiZnSnO c) 3at.%
LiZnSnO d) 5at.% LiZnSnO
X-ray diffraction patterns (XRD) of the LiZnSnO films are shown in Fig.2. As seen in
Fig.2, XRD patterns indicate some peaks. The first broad peak observed from 15 to 30o and
second peak observed from 30 to 40o are resulted from the glass substrate. Another
prominent peak was not observed in XRD patterns of LiZnSnO films. This suggests that
the LiZnSnO films have an amorphous phase.
5

0 % Li
1% Li
3% Li
5 % Li
10
20
30
40
50
60
70
80
90
2
θ (Degree)
Fig.2 X-ray diffraction patterns of the LiZnSnO films
The transmittance spectra of the LiZnSnO films are shown in Fig. 3. It is seen that the
films are transparent and their transparency is changed from 20 % to 75% between 300 and
1200 nm. The optical band gap of the films can be determined by the well-known relation
αhν = B(hν − Eg )n
(1)
where α is the optical absorption coefficient, B is a constant coefficient, hν is the photon
energy and Eg is the optical band gap. In order to determine the optical band gap of the
films, the plots of (αhv)2 versus hν are shown in Fig.4.
The optical band gaps of the LiZnSnO films were found to be 3.78 eV for 0at.% Li, 3.77
eV for 1at.% Li, 3.87 eV for 3at.% Li and 3.85 eV for 5at.% Li. The optical band gap of
the films is changed with Li content. The optical band gap of the studied zinc tin oxide was
found to be 3.78 eV. It is well known that the optical band gap of zinc tin oxide thin films
is the range of 3.3–3.9 eV with a direct optical band gap [3]. The optical band gap of zinc
tin oxide (ZTO) thin films deposited by thermal atomic layer deposition is increased from
3.4 to 4.6 eV with tin content and then, it is decreased [14]. These results indicate that the
increase in optical band gap of the films is results from the chemical composition or
structural variations of the zinc tin oxide thin films with Li content.
6

2x1015
2x1015
1x1015
0% Li
1% Li
3% Li
5% Li
5x1014
0
3.0
3.2
3.4
3.6
ν (eV)
3.8
4.0
4.2
h
Fig.4a Plots of (αhv)2 vs. hν of LiZnSnO films
5
4
3
2
1
0% LiZnSn
1% LiZnSn
3% LiZnSn
5% LiZnSn
300
400
500
600
700
800
Wavelength (nm)
Fig.4b Plots of R vs. wavelength of the LiZnSnO films
The reflectance of the films is changed with Li content. After 500 nm, the
reflectance is increased with decreasing wavelength and also with Li content. The increase
in reflectance of the films is due to decrease in absorption of the films and this change
confirms that Li atoms incorporated in Zn-Sn-O lattice. For optical applications of these
films, the optical constant such as refractive index should be determined. For this, the
complex refractive index of the films is as follows:
7

n*(λ) = n(λ) + ik(λ)
(2)
where n is the refractive index and k is the extinction coefficient. The refractive index of
the films can be determined by the following relation [15].
1+ R
1− R
4R






− k2
2
(3)
n =
+
(
1− R)
Where R is the reflectance and k is the extinction coefficient. The refractive index of the
films are determined using Eq.3 and is shown in Fig.5. As seen in Fig.5, the refractive
index is increased with Li content.
1.6
%0 Li
%1 Li
%3 Li
1.5
%5 Li
1.4
1.3
1.2
400
450
500
(nm)
550
600
λ
Fig.5 Plots of refractive index of the LiZnSnO films
3.2. Current-voltage characteristics of the diodes
Figs. 6(a), (b) and (c) show semi-logarithmic I-V characteristics of the photodiodes
under dark and various illuminations. As seen in Fig. 6(a-c), the diodes exhibited a
rectifying behavior. This behavior of the diodes can be analyzed using thermionic emission
(TE) theory. In this theory, the electrons transports over a potential barrier. Thermionic
emission current assumes that only electrons with energies greater than the energy of the
8

potential barrier add to the current flow. According to this theory, the current (I) is
described by the following equation [16-17]
qΦ B0
qV







I = A A∗T 2 exp −
exp
(4)

kT
nkT




where I0 is the saturation current and n is the ideality factor, A is the diode contact area
(3.14x10-2 cm2), A* is the Richardson constant of 32 Acm-2K-2 for p-type silicon and ΦB0 is
the zero-bias barrier height. A change in the ideality factor over the life of the diode is an
indicative of the variation in metal-semiconductor interface. This parameter is a measure of
the conformity of a diode current to be pure TE.
As seen in Fig. 6(a), (b) and (c), the reverse current at a given voltage of the
photodiode with different Li content under illumination is higher than that of the current
under dark. In other words, the reverse current of the photodiodes increases with the
increasing illumination intensity. This indicates that the prepared diodes exhibit a
photodiode behavior. When the semiconductor device is illuminated, electrons in the
valence band of the semiconductor absorb energy, and they are able to jump to the
conduction band. Also, the ln(I)-V curves of the photodiodes deviate from linearity at
higher forward-bias voltages due to the effects of some factors such as series resistance,
interface states, etc [15,18-20]. The values of barrier height (ΦB0) were calculated from the
intercept of the forward bias In(I)-V plots of the photodiodes and were found to be about
0.69 for 0at.% Li, 0.67 for 1at.% Li, and 0.65 eV for 3at.% Li, respectively. It is seen that
the barrier height is decreased with increasing Li content.
The value of ideality factor (n) were calculated using Eq. (4) from the slope of the
forward bias In(I)-V plots of the photodiodes with different Li content for all conditions
and were found to be about 4.50 for 0at.% Li, 4.55 for 1at.% Li and 4.60 for 3at.% Li,
respectively. The obtained n value of the diodes is higher than unity. The high value of n is
attributed to the presence of inhomogenities of the barrier height, the particular distribution
9

of the interface states, series resistance, the image force effect, generation-recombination
currents within the space-charge region, wide distribution of low Schottky barrier height
patches at metal/semiconductor interface, and tunneling [20-30].
Fig. 6. Semi-logarithmic I-V characteristics of the photodiodes with different the addition
of Li to the ZTO under dark and various illuminations: a) 0at.% LiZnSnO b) 1at.%
LiZnSnO c) 3at.% LiZnSnO
Fig. 7 shows semi-logarithmic I-V characteristics of the photodiodes at 100 mW/cm2
illumination intensity. As seen in Fig. 6, the value of reverse current increases with the
10

increasing Li content. This might be attributed to the increase of carrier concentration
caused by the addition of Li.
Fig. 7. Semi-logarithmic I-V characteristics of the photodiodes at 100 mW/cm2
illumination intensity.
The variation in the photocurrent with illumination intensity can be analyzed by the
following relation [31-33],
IPH = APm
(5)
where IPH is the photocurrent, A is a constant, m is an exponent and P is the illumination
intensity. The obtained m value suggests that the photoconduction mechanism of the
diodes exhibited a linear behavior. Also, the obtained m value in the range of 0.5 to 1
implies the presence of continuous distribution of trap levels. As seen in Fig. 6, the reverse
current under a certain illumination is higher than the current under dark. This confirms the
photodiode behavior. To analyze the photoconducting mechanism of the photodiodes, the
photocurrent plots are plotted as a function of Li content and are given in Fig. 8. The
11

values of m for 0, 1 and 3% Li contents were found to be about 0.39, 0.78 and 0.75,
respectively.
Fig. 8. Plots of log(IPH) vs log(P) of the photodiodes with different Li contents (at -5 V)
In addition, the transient photocurrent study is a well-known technique using to
understand the transport mechanism of any photodiode. The transient photocurrent
measurement of the photodiodes with different Li content was performed under
illumination intensity of 100 mW/cm2 and is given in Fig. 9. As seen in Fig. 9, after
turning on the illumination, the photocurrent of the diode rapidly increases to a certain
level and then gradually tends to the maximum value. When the photodiode was
illuminated, the number of photogenerated charge carries increase with the illumination
and the electrons contribute to the current. On the other hand, after turning on the
illumination, the photocurrent of the diode decreases due to the trapping of the charge
carriers in the deep levels and comes back to its initial state. As a result, the fabricated
photodiodes exhibits a good photoconducting behavior [34-38].
12

Fig. 9. Plots of photocurrent-time of the photodiodes with different Li content under 100
mW/cm2 illumination intensity.
3.3. Capacitance-voltage and conductance-voltage characteristics
The capacitance-voltage plots of the photodiodes with various Li contents are given
in Fig. 10. As seen in Fig. 10 (a), (b) and (c), the value of C remains almost constant with
the decreasing reverse bias voltage. In forward bias voltage region, the value of C is
decreased with the increasing frequency. The higher values of C at lower frequencies can
be attributed to the presence of continuous distribution of the interface states. In other
words, at lower frequencies, the interface charges can follow the ac signal and they
contribute to the capacitance [39-45].
13

Fig. 10. The capacitance-voltage (C-V) plots of the photodiodes with various Li contents
a) 0at.% LiZnSnO b) 1at.% LiZnSnO c) 3at.% LiZnSnO
In addition, the forward bias C-V curves of the photodiodes with different Li content
show an abnormal behavior. After the crossing point, the C values increase with increasing
frequency. The negative capacitance (NC) behavior of the C-V curves at each frequency
may be explained by considering the loss of interface charges at occupied states below
Fermi level. The NC has been detected in many electronic and optoelectronic devices, such
as polymer light emitting diodes, metal-semiconductor interface, metallic nanoparticles
embedded in dielectric, p-n junction, quantum well infrared photodetectors, etc. [46-51].
14

The microscopic physical mechanisms of the negative capacitance in various devices are
obviously different and have been ascribed mainly to the contact injection, interface states,
or minority-carrier injection effects.
The conductance-voltage (G-V) plots of the photodiodes with various Li contents are
shown in Fig. 11. As seen in Fig. 11(a), (b) and (c), the value of G increases with the
increasing frequency in the forward bias voltage region. This confirms the presence of the
interface states. The value of G remains almost constant in the reverse bias voltage region.
Fig. 11. The conductance-voltage (G-V) plots of the photodiodes
a) 0at.% LiZnSnO b) 1at.% LiZnSnO c) 3at.% LiZnSnO
15

The series resistance (Rs) is the most important parameter that causes the electrical
parameters of semiconductor device. In order to determine the values of Rs , we can use the
total impedance of the parallel RC circuit model. The measured impedance (Zma) at strong
accumulation region of device is equivalent to the total circuit impedance as [17,19]:
1
(6)
Zma
=
Gma + jωCma
Comparing the real and imaginary part of the impedance, the real value of Rs of the device
can be subtracted from the measured capacitance (Cma) and conductance (Gma) in the
strong accumulation region at sufficiently high frequency. The value of Rs is calculated by
the following relation,
Gma
2
Gma + ωCma
Rs =
(7)
2
)
(
The series resistance values are determined and are given in Fig.12 (a), (b) and (c). It is
seen that the series resistance decreases with the increasing frequency. This frequency
dependence of Rs is the result of frequency-dependent charges such as interface charge,
fixed oxide charge, oxide-trapped charge and mobile oxide charge [52-56]. Moreover, the
Rs-V plots give a peak at about -1 V for 0 and 1at.% Li and the peak intensity is decreased
with the increasing frequency. This indicates the presence of interface states in the
interface of the photodiodes.
16

Fig. 12. The series resistance-voltage (Rs-V) plots of the photodiodes with various Li
contents at various frequencies.
4. Conclusions
The effects of Li content on the electrical characteristics of the photodiodes with
LZTO interfacial layer. The photocurrent of the diodes is increased with the increasing Li
content. The I-V characteristics of the diodes under various illumination intensities suggest
that the photoresponse of the studied photodiodes is very sensitive to illumination
intensity. In addition, the C-V and G-V measurements indicate that the value of C and G/ω
depends on applied voltage and frequency. The obtained results suggest that the
17

photodiodes with Li-doped ZTO interfacial layer exhibited a better device performance
than those with an undoped ZTO.
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80
60
40
20
0
%0 LiZnSn
%1 LiZnSn
%3 LiZnSn
%5 LiZnSn
200
400
600
800
1000
1200
Wavelength (nm)
Fig.3. Transmittance spectra of the LiZnSnO films
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Highlights
1. Lithium-zinc-tin-oxide thin films were prepared by sol gel method.
2. The Au/LiZnSnO/p-Si/Al photodiodes were fabricated using a lithium-zinc-tin-oxide
(LZTO, Li-Zn-Sn-O) layer grown on p-Si semiconductor.
3. The photodiodes with Li-doped ZTO interfacial layer exhibited a better device
performance than those with an undoped ZTO.
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