The Au/Cu2WSe4/p-Si photodiode: Electrical and morphological characterization

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The Au/Cu

₂

WSe

₄

/p-Si photodiode: Electrical and morphological

characterization

Adem Kocyigit

a

, Murat Y

ıldırım

b,*

, Adem Sar

ılmaz

c

, Faruk Ozel

c,d,** a_{Igdir University, Engineering Faculty, Department of Electrical Electronic Engineering, 76000, Igdir, Turkey}

b_{Selcuk University, Faculty of Science, Department of Biotechnology, 42130, Konya, Turkey}

c_{Karamanoglu Mehmetbey University, Faculty of Engineering, Department of Metallurgical Science and Materials Engineering, 70200, Karaman, Turkey} d_{Karamanoglu Mehmetbey University, Scientiﬁc and Technological Research and Application Center, 70200, Karaman, Turkey}

a r t i c l e i n f o

Article history: Received 18 August 2018 Received in revised form 21 November 2018 Accepted 28 November 2018 Available online 29 November 2018

Keywords: Cu2WSe4

Copper tungsten selenide Schottky devices

Au/Cu2WSe4/p-Si photodiode Photodetector

a b s t r a c t

Cu2WSe4nanosheets were synthesized by the hot-injection method and put as interfacial layers between

Au metal and p-Si by spin coating technique to investigate their photoresponse and capacitor properties via I-V and C-V measurements, respectively. The XRD were operated to conﬁrm crystalline structure of the Cu2WSe4. The TEM image revealed that the crystalline nanosheet structures of the Cu2WSe4. The I-V

measurements were performed under dark and light illumination in the range 20 mWe100 mW light intensities with 20 mW interval. In addition, some diode parameters such as ideality factor, barrier height and series resistance were extracted via a various method and discussed in the details. The C-V mea-surements were employed for various frequency and voltages. The C-V characteristics of the device conﬁrmed the strong dependence on the frequency and voltage. The results imparted that Au/Cu2WSe4/

p-Si can be employed for photodiode, photodetector and capacitor applications.

1. Introduction

Chalcogenides materials contain one or two chalcogen atoms [1]. They are covalently bonded and can be crystal or amorphous form [2]. In addition, the chalcogenides are semiconductors mate-rials and their bandgap energies can be adjustable in the range 1e3 eV depending composition atoms [3]. The chalcogenides are performed for various applications because of their optical, elec-trical and thermal behaviors [4e7]. Among the chalcogenides, ternary transition metal ones (TTMC) have gained great interest in a couple of years [8e10]. These materials can be employed photo-catalysis, electrochemical hydrogen reactions and solar energy applications [11].

Ternary transition metal chalcogenides are formulized as AB2X4

(A, B are transition metals, X¼ S, Se, Te) [12]. One of the TTMC is Cu2WSe4 and has superior absorption of visible light and

optoelectronic properties as semiconductor materials [13]. There are some techniques to the synthesis of TTMC such as solvothermal [14], hydrothermal [15], chemical vapor deposition [16], chemical vapor transport [17] and hot-injection method [11]. The hot-injection method is cheaper and easy way to produce TTMC.

Metal-semiconductor devices via various interfacial layers have great interest for three decades [18e20]. The interfacial layer is really important to control the currentﬂow between the metal and semiconductor [21,22]. This is also provided to increase the breakdown voltage and decrease the amount of leakage currents [23]. TTMCs can be employed as an interfacial layer to control electrical properties of the metal-semiconductor contacts because of their superior properties [24e26]. When the literature surveyed, it can be seen only a few reports about Cu2WSe4interfacial layer

[11].

In this study, we synthesized the Cu2WSe4structures by

hot-injection method and used the structures as thin_{ﬁlm interfacial} layer by spin coating technique between the Au metal and p-Si to fabricate Au/Cu2WSe4/p-Si photodiodes. The obtained photodiodes

were characterized by XRD, TEM, I-V and C-V measurements by various light illumination and frequencies to see the photoresponse and capacitor properties.

* Corresponding author. ** Corresponding author.

E-mail addresses:muratyildirim@selcuk.edu.tr(M. Yıldırım),farukozell@gmail. com(F. Ozel).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : h t t p : / / w w w . e ls e v i e r . c o m / l o c a t e / j a l c o m

https://doi.org/10.1016/j.jallcom.2018.11.372

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2. Experimental

Single crystalline Cu2WSe4nanosheets have been synthesized

by hot-injection in the protective argon atmosphere. The nano-sheets were synthesized according to previously reported methods [11,27]. 170 mg CuCl2$2H2O and 163 mg WCl4 were blended in

12 ml oleylamine (OLA) in a ﬂask which had three-neck. The

mixture was heated up to 180C and we saw a color change from black to blue via increasing temperature. For adding Se element to the expected compound, 198 mg Se powder solved in OLA by magnetic stirring in a different bottle, and then the solution was heated up to 300C. The Se solution was injected toﬁrst solution as rapidly. The combined solution was stirred 30 min for the reaction and stayed to cool down. When the mixture temperature decreased to 120C, 2 ml oleic acid was added to the mixture ﬂask. The mixture was centrifuged minute in ethanol and toluene. Thus, Cu2WSe4crystal structures were obtained and dried 70C.

The Si wafer which was (100) p-type were sliced to 1.5 cm2 pieces and cleaned in acetone and propanol by an ultrasonic cleaner and then immersed in HF:H2O (1:1) solution to remove

impurities and oxide layer from the surfaces. Al layer which, had 100 nm thicknesses, was evaporated to the back surface of the wafers for ohmic contact, and they were annealed for 5 min in N2

atmosphere at 500C. The synthesized Cu2WSe4 solution were

deposited on front surfaces of the pieces as thinﬁlm form by a spin coater. To complete Au/Cu2WSe4/p-Si device, Au layer was

vapor-ized on the_{ﬁlm surfaces as rectifying contact.}

The XRD patterns of the structures were collected via a Bruker, D8 X-ray diffractometer with Cu-K_a radiation (

l

¼ 0.15418 nm). Transmission electron microscopy (TEM) images were taken with a JEOL JEM-2100 with an accelerating voltage of 200 kV. The composition and EDS pattern were determined via Zeiss-Evo model scanning electron microscope (SEM - EDX). While Fytronix FY-5000 were performed for I-V measurements under dark and illumination conditions, a Keithley 4200 were employed for C-V measurements. The performed light intensity changed from 20 mW to 100 mW by 20 mW interval.

3. Results and discussion 3.1. Structural properties

In order to identify the phase structure and purity of the Cu2WSe4nanosheets, XRD patterns were obtained.Fig. 1a depicted

the XRD pattern of Cu2WSe4nanosheets. The characteristic (001),

(110/101), (111), (210/201), (220) and (300/221) diffractions were attributed to a pure tetragonal Cu2WSe4structure with pure P42m

symmetry, which is similar to previously published structure [13]. It indicated that the elemental copper and tungsten was success-fully combined with selenium during the synthesis process. The XRD peaks were found to be in the range of standard Cu2WS4

(JCPDS no: 081-1159) although a slight shift towards lower numbers was observed due to the fact that the structure has larger selenium atoms instead of smaller sulfur atoms [28e30]. As shown inFig. 1b, the crystal structure of the Cu2WSe4formed by

inter-linked copper (Cu) and tungsten (W) atoms tetrahedrally coordi-nated to bridging Se atoms. The structures have a square planar arrangement and each Se coordinates to two Cu atoms and one W [11,31,32]. Moreover, diffraction angles did not display any other characteristic peaks from the other phase which could be attributed to the high purity of Cu2WSe4nanosheets.

The EDS analysis was applied for analyzing the chemical composition most of the CWSe particles. As shown inFig. 1c, the elemental composition of the samples was copper, tungsten, and selenium. The atomic ratio of Cu to Se and W to Se, calculated from the integrated peak intensities, are estimated to be 0.55 and 0.27 for CWSe compound, respectively. These semiquantitative values are reasonably close to 0.5 and 0.25 that is expected for the phase Cu2WSe4. Furthermore, the average elemental composition (%)

ra-tio of the nanosheets was found to be Cu2W0.9Se3.6as a result of the

quantitative analyses by EDS in combination with SEM, which re-veals that the sample's stoichiometry has been very close to being ideal (2:1:4).

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3.2. Morphological properties

The microstructure and morphology of the synthesized Cu2WSe4 were characterized by TEM. As shown in Fig. 2a,

Cu2W0.9Se3.6 displays a 2D nanosheet architecture with a few

layers. In this phase, although, most of the nanoparticles tend to agglomerate into irregular large secondary particles, the sheet length ranging from 20 to 40 nm. HRTEM images inFig. 2b reveal further the microscopic phase of the nanosheets. Lattice fringes of Cu2WSe4demonstrated their highly crystalline nature. Also,

inter-planar spacing of Cu2WSe4 nanosheets was calculated as 5.73 Å

which corresponds to (100) plane. The interplanar spacing of CWSe were shifted slightly toward higher values compared with standard CuWS, indicating the increased lattice constants of CWSe due to ionic size differences between Se (1.98A) and S (1.84A) ions in the same coordination number [29]. These results conﬁrmed the XRD results.

3.3. Electrical properties

The Cu2WSe4structures were used as interfacial thinﬁlm layer

between the Au and p-Si to fabricate the Au/Cu2WSe4/p-Si device.

The I-V measurements were performed to characterize its electrical properties under dark and illumination conditions. I-V character-istics of the Au/Cu2WSe4/p-Si device have been displayed inFig. 3

for dark and various illumination intensities. According toFig. 3, the device exhibited well rectifying property both dark and illu-mination conditions. The rectifying ratio of the Au/Cu2WSe4/p-Si

device is 1.83 105_{under dark and 4.71}₁₀3_{under 100 mW light}

intensity.

The device has good response to the light illumination at reverse biases because the current values increased via increasing light intensities from 5.29 109 A to 2.15 107 A (100 times). This property caused from generation of the electron-hole pairs in the interface of the device owing to light illumination [33]. In here, the effect of the interfacial layer Cu2WSe4is to prevent to

recombina-tion of the electron-hole pairs as well as to provide the increase electron-hole pairs numbers owing to lower energy band gap values (1.64 eV) [32]. Moreover, the device can be employed as photodiode and photodetector applications owing to increasing current values at the reverse biases [22,34].

The diode parameters provide to investigate the electrical properties of these kinds of devices. For that reason, the diode parameters such as barrier height (f_b), ideality factor (n) and series resistance (Rs) should be determined from I-V measurements. The

three techniques is employed to calculate the diode parameters: (1) thermionic emission theory, (2) Cheungs and (3) Norde methods. The current (I) is calculated from I-V measurements via following formula according to thermionic emission theory:

I¼ I0exp qV nkT 1 exp qV nkT (1)

where I0shows saturation current determined from the linear zone

of the I-V graph. I0also is calculated as next equation:

I₀¼ AA*_T2_exp qfb kT (2)

In equation(2), A, A*and T indicate diode area (in here A is equal to 7.85 103_cm2_{), Richardson constant (32 A cm}2_K2_{for p-type}

Si) and temperature, respectively. q and k represent the charge of electron and Boltzmann's constant. The determined saturation current value for Au/Cu2WSe4/p-Si device is 3.23 1010. While the

saturation current help to calculate fb, slope of dV/dlnI provide to

determine n values of the device for V_{3kT/q region via relevant} formulas shown in below:

n¼ q kT dV dlnI (3) and f_b¼kT_qln A*AT2 I0 (4)

The n and f_bvalues were determined and listed inTable 1for Au/Cu2WSe4/p-Si device. The device has 4.84 ideality factor and

0.82 eV barrier height. The higher n values than unity can be attributed to various reasons such as barrier inhomogeneity, dis-tribution of the carriers, image force effects and series resistances of the devices [35e37]. The higher ideality factors at this device can be attributed to barrier inhomogeneity and series resistance effects because of nanosheet structures of the Cu2WSe4[36,38].

The resistance of the device or junction (Rj) affects the I-V

characteristics of the metal-semiconductor devices and it should be determined. The Rjconsists of series resistance and shunt

resis-tance (Rsh). While Rsis relevant interfacial inter layer, Rshis related

to contact-semiconductor interface [39]. The determining of these

Fig. 2. TEM a) and HR-TEM b) images of the Cu2WSe4nanosheets.

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parameters is really important to understand the device charac-teristics [40]. The Rjis given as following equation:

R_j¼vV

vI (5)

The obtained Rsand Rshvalues is 30.7 k

U

and 2.00 105k

U

for

Au/T-CuSbS2/p-Si device. According to these results, device has

higher Rshvalues than Rs, and Rshvalues are 108

U

level. These

ob-tained Rsand Rshmade the device ideal for high performance

de-vices [41].

The second method to calculate the diode parameters is Cheung method, and the method also provides to determine the series resistance of the device. Normally, all the metal-semiconductor devices possess Rsevery time, but it cannot be calculated from

thermionic emission theory. While Cheung method is performed, the current is written as the next equation including voltage drop in the current because of Rs:

I¼ I0exp qðV IRsÞ nkT (6)

In this equation, IRsrepresents the voltage drop. While the Rsis

left alone at the left side of Eq.(6), Cheung's functions are trans-formed to below formula:

dV dlnI¼ IRsþ n kT q (7) HðIÞ ¼ V n kT q ln I AA*_T2 (8)

If H(I) is rearranged as:

HðIÞ ¼ IRsþ nfb (9)

Eq.(9)are obtained. Eqs.(7) and (9)are called Cheung functions and derived by Cheung and Cheung [42]. While the graphs of Chung functions are plotted versus current, the graphs exhibit straight lines. The dV/dlnI versus I graph provides to determine ideality factor from y-intercept and series resistance value from slope. The H(I)-I graph is used to determine barrier height from y-intercept and another Rsvalue from the slope. The two Rsvalues is important

for the consistency of the Cheung method [43].

Fig. 4shows the dV/dlnI vs. I and H(I) vs. I graphs of the Au/ Cu2WSe4/p-Si device. The determined n, fband Rsvalues are given

inTable 1. According toTable 1, there is some deviation both ideality factor and barrier height values of the device. These differences can be attributed to non-ideal diode structures, non-uniform distribu-tion of the interface states and approximadistribu-tion differences of the methods [43,44]. Furthermore, there are some differences also at the Rsvalues for both devices obtained from dV/dlnI vs. I and H(I) vs.

I graphs because of barrier inhomogeneity or native Cu2WSe4

layers.

The alternative method to calculate the series resistance value as well as barrier height is Norde method. According to Norde method, Norde function is expressed as below formula [45]:

FðVÞ ¼V

_g

kT q ln IðVÞ AA*T2 (10)

where

g

shows closest integer (dimensionless) value of the n. I(V) represents the currents corresponding to applied voltages. If Eq.

(10)is retyped for fband Rs, next formulas are obtained:

f_b¼ FðV0Þ þ V0

g

kTq (11) Rs¼

g

n_I kT_q (12)

where V0 is the minimum voltage value depending on Norde

function.Fig. 5displays F(V) versus V plot of the Au/Cu2WSe4/p-Si

device. The obtained fband Rsvalues are presented inTable 1.

Fig. 6shows Log I and photoresponse versus Log P of the Au/ Cu2WSe4/p-Si device. Both LogI and photoresponse of the device

changed almost linearly by increasing illumination intensity. This result highlighted that the obtained device can be employed for photodetector and photodiode applications [46].

Fig. 7 exhibits C-V graphs of the Au/Cu2WSe4/p-Si device for

various frequencies in the range of 10 kHz to 1 MHz depending voltage from5 V to þ5 V. The capacitance values did not change in inversion region via frequency and voltage but increased suddenly at the depletion region and showed the peaks at accumulation region. The peak positions changed to depletion region via increasing frequency. In addition, there are other peaks at higher accumulation region, the intensity of the peaks decreased with increasing frequency. The existence of the peaks can be attributed to interface states and series resistances or interfacial Cu2WSe4

layer [47,48]. The decrease at the peak intensities can be ascribed to that the interface states cannot follow ac signal at higher fre-quencies [49].

G-V plots of the Au/Cu2WSe4/p-Si device have been indicated in

Fig. 8depending on frequency. The conductance values did not alter

Table 1

The diode parameters of the Au/Cu2WSe4/p-Si device for various methods. Device I0(A) n (I-V)

e

n Cheung e

f_b(I-V) (eV)

f_bCheung (eV) f_bNorde (eV) RsCheung (kU(H(I))) RsCheung ((kU(dlnI) RsNorde (kU)

- 3.23 1010 _4.84 _5.32 _0.82 _0.79 _0.84 _55.7 _57.5 _38.8

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at the inversion region via changing frequency and voltage but started to increase in depletion region via increasing frequency and exhibited peaks at depletion and accumulation region. The peak positions changed towards to depletion region. In other words, while the conductance values showed peaks at higher accumula-tion region for lower frequencies (10 kHze300 kHz), the peak po-sitions shifted to depletion region for higher frequencies. The increase at the conductance values with increasing frequency can be attributed to interface states [50,51].

Fig. 9displays C2-V graphs of the Au/Cu2WSe4/p-Si device for

various frequencies. The plots showed almost good straight lines for wide range voltages from5 V to 0. The deviation from linearity can be attributed to interfacial Cu2WSe4 layer [52]. The C2-V

graphs provide to calculate various electric parameters such as doping concentration (Na), barrier heights and fermi energy level

(EF) [53]. Some electric parameters were calculated and listed in

Table 2for various frequency. Especially, the number of interface states and series resistance values is too important to understand conduction mechanism and polarization process.

According toTable 2, the doping concentration of the acceptor

Fig. 6. Log I and Photoresponse versus Log P of the Au/Cu2WSe4/p-Si device.

Fig. 7. The C-V graphs of the Au/Cu2WSe4/p-Si device.

Fig. 8. The G-V graphs of the Au/Cu2WSe4/p-Si device.

Fig. 9. The C2-V plots of the Au/Cu2WSe4/p-Si device. Fig. 5. The F(V)-V plot of the Au/Cu2WSe4/p-Si device.

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atoms (Na) decreased usually via increasing frequency. The series

resistance (Rs) and barrier height (

F

b) values also decreased by

increasing frequency, but fermi energy level (EF) increased slightly.

The decrease at the Rscan be attributed to interface states of the

device [54]. In addition, while both maximum electric (Em) and

depletion width (Wd) decreased with increasing frequency, number

of interface states (Nss) increased. The increase values of the Nsscan

be attributed to increasing conductivity of the device because the interface states can be easily followed ac signal at the preferred frequency range.

Fig. 10 indicates Ri-V plots of Au/Cu2WSe4/p-Si device for

changing frequency. The resistance values exhibited peaks close to depletion region due to the interface states effect [55]. The Rivalues

decreased with increasing frequencies since the interface states cannot reply towards to higher frequencies [49]. The insetﬁgures at

Fig. 10clearly indicated that the Rivalues is effective both inversion

and accumulation regions.

4. Conclusions

We synthesized the Cu2WSe4crystals by hot-injection method

and used it as the interfacial layer between the Au metal and p-type Si to obtain Au/Cu2WSe4/p-Si devices. The devices were

charac-terized by XRD, TEM, I-V and C-V measurements. The XRD mea-surements revealed the crystal structure of the Cu2WSe4. The TEM

images conﬁrmed Cu2WSe4 nanosheet and crystalline structures.

The I-V measurements were performed on the Au/Cu2WSe4/p-Si

devices under dark various light intensities. The diode parameters of the devices were calculated via different techniques and dis-cussed in the details. While ideality factor value is 4.84, the barrier height value is 0.82 eV according to thermionic emission theory. The higher ideality factor is due to barrier inhomogeneity and se-ries resistance effects. Furthermore, the device has good photo-response to the light illumination. According to the C-V measurements, the capacitance values were affected by frequency and voltage strongly. In addition, various electrical parameters were extracted by the help of the C-V measurements. The obtained device can be thought as photodiode, photodetector and capacitor applications.

Acknowledgements

The authors would like to thank to Selçuk University BAP ofﬁce (Project Number 17401159) and Karamanoglu Mehmetbey Univer-sity (Grand Number: 32-M-16) for Scientiﬁc Research Foundation. References

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Table 2

The electrical parameters which were calculated from C2-V plots of Au/Cu2WSe4/p-Si device. f (kHz) Na(1014_cm3₎ _R s(U) Fb(eV) EF(eV) Em(103V/cm) Wd(105cm) Nss(1012_eV1_cm2₎ 10 7.70 821.5 0.540 0.204 8.651 7.691 0.460 50 7.58 432.4 0.531 0.205 8.447 7.640 0.401 100 7.05 210.9 0.493 0.207 7.604 7.434 0.592 200 7.21 88.9 0.473 0.206 7.399 7.099 1.394 300 7.26 55.8 0.462 0.206 7.277 6.943 2.192 400 7.23 45.4 0.453 0.206 7.110 6.824 2.878 500 7.21 43.0 0.443 0.206 6.941 6.702 3.295 600 7.23 41.7 0.433 0.206 6.794 6.552 3.596 700 7.07 41.0 0.414 0.207 6.391 6.338 3.669 800 7.13 39.6 0.404 0.206 6.243 6.167 4.038 900 7.13 37.1 0.394 0.206 6.067 6.015 4.413 1000 7.15 35.1 0.384 0.206 5.897 5.849 4.774

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