The dielectric performance of Au/CuCo
5
S
8
/p-Si
heterojunction for various frequencies
A. Kocyigit1,* , D. E. Yıldız2, A. Sarılmaz3, F. Ozel3,4, and M. Yıldırım5 1
Department of Electrical and Electronics Engineering, Faculty of Engineering, Igdir University, 76000 Igdir, Turkey 2
Department of Physics, Faculty of Arts and Sciences, Hitit University, 1903 Corum, Turkey
3Department of Metallurgical and Materials Engineering, Faculty of Engineering, Karamanoglu Mehmetbey University, 70200 Karaman, Turkey
4
Scientific and Technological Research & Application Center, Karamanoglu Mehmetbey University, 70200 Karaman, Turkey 5
Department of Biotechnology, Faculty of Science, Selcuk University, 42130 Konya, Turkey
Received:6 September 2020 Accepted:22 October 2020
Ó
Springer Science+Business Media, LLC, part of Springer Nature 2020ABSTRACT
CuCo5S8 thiospinel nanocrystals were synthesized by a modified colloidal method, and then it was used as an interfacial layer in the Au/CuCo5S8/p-Si heterojunction device to characterize the dielectric performance of the CuCo5S8 thiospinel. X-ray diffractometer (XRD) was performed to investigate structural behaviors of the CuCo5S8, and the results confirmed the crystalline structure of the CuCo5S8. While the detailed structures of the CuCo5S8 thiospinel were investigated by transmission electron microscope (TEM), the surface morphol-ogy was obtained by scanning electron microscope (SEM). Furthermore, the composition of the CuCo5S8structures was studied and confirmed by the energy dispersive X-ray (EDX). The CuCo5S8thiospinel were deposited between the Au and p-Si to obtain Au/CuCo5S8/p-Si heterojunction. The impedance spec-troscopy technique was employed to determine the voltage- and frequency-dependent dielectric properties of the Au/CuCo5S8/p-Si heterojunction. While the frequency was changed from 100 kHz to 1 MHz with 100 kHz interval, the voltage was altered from - 2.5 V to ? 2.5 V. The various dielectric parameters such as complex electric permittivity (dielectric constant (e0) and dielectric loss (e00)), electric modulus (M0 and M00), and ac electrical conductivity (r) were extracted from the C–V and G–V measurements and discussed in details. The results highlighted that the Au/CuCo5S8/p-Si heterojunction device has the frequency- and voltage-dependent dielectric characteristics, and can be con-sidered as switching applications.
Address correspondence toE-mail: adem.kocyigit@igdir.edu.tr
1 Introduction
Ternary transition metal chalcogenides (TTMCs) have exceptional potentials for various applications such as photodetectors and photovoltaic devices [1–3]. TTMCs can be employed especially in the applications of device stability and energy conversion efficiency applications [4]. The thiospinel nanocrys-tals form of TTMCs exhibits superior optical, elec-trical, and thermal behaviors [5]. Due to having semiconducting behaviors, their band gaps can be changeable based on composition atoms [6]. The various methods such as hydrothermal, solvother-mal, hot injection, and modified colloidal solution methods can be performed to synthesize TTMCs [7]. Among them, the modified colloidal solution method can be used to obtain the TTMCs for metal and semiconductor device applications as an interfacial layer [8].
The native or external interfacial layers have an important role in metal–semiconductor devices which are used in many electronic devices because their characteristics can be controlled by various interfacial layers such as metal oxide, polymer, or insulator [9–12]. These interfacial layers affect the performance and reliability of these devices depending on their formations, inhomogeneous bar-rier heights together with parasitic resistances and the interface states distribution [13–15]. Thus, several interfacial layers are used and investigated to passi-vate the active dangling bonds at the semiconductor surface and improve the electrical characteristics of these devices [16]. TTMC thin films may offer new developments for passivation and controlling elec-trical characteristics of the metal semiconductor devices. To understand the conduction and polar-ization mechanisms of an interfacial layers, the dielectric characterization can be performed on the MS heterojunctions [13,17].
We synthesized the CuCo5S8thiospinel nanocrys-tals via a modified colloidal solution method and inserted them between the Au and p-Si by spin-coating method. The structural and morphological properties of the CuCo5S8 thiospinel nanocrystals were revealed by XRD, SEM, and TEM analyses. In our previous work, we reported experimental results related to the electrical properties of Au/CuCo5S8 /p-Si photodiode [18]. However, the dielectric properties of the Au/CuCo5S8/p-Si heterojunction diodes have not been examined comprehensively yet. Therefore,
we aimed to examine the effect of bias voltage and frequency on the dielectric properties of the Au/ CuCo5S8/p-Si heterojunction diodes using impe-dance spectroscopy technique over a wide-range frequency. The frequency-dependent dielectric behaviors of the Au/CuCo5S8/p-Si heterojunction diode were studied for the first time according to the web science database.
2 Experimental details
2.1 Colloidal synthesis of CuCo
5S
8thiospinel nanocrystals
The CuCo5S8 thiospinel nanocrystals were synthe-sized based on the modified colloidal method using metal acetates and octadecene precursors as starting materials and surfactants, respectively [19]. Typi-cally, 45 mg of Copper acetate, 221 mg of Cobalt acetate, and 676 mg TOPO were mixed via 10 mL of ODE in a separately two-neck flask. The mixture was heated up to 210 °C under argon flow with magnetic stirring, but freshly prepared sulfur mixture (0.875 mL (t-DDT) and 0.125 mL (DDT)) was added to the reaction medium when the temperature reached to 120 °C. Then, the reaction medium was kept at 210 °C for 30 min to complete reaction. Then, the obtained blend was left to cool down to 80 °C, and toluene-ethanol mixture (7:1) was included to the reaction flask. The black precipitate of the thiospinel nanocrystals was obtained and separated by cen-trifugation for one minute at 4000 rpm. Finally, the CuCo5S8 nanocrystals were rinsed to remove the excess amount of surfactants by three times in alco-hol, and they were allowed to dry in an oven.
2.2 Fabrication of the Au/CuCo
5S
8/p-Si
heterojunction
The (100)-oriented p-type Si wafer was cut to 2 9 2 cm slices. Then, cleaning of the slices was achieved by an ultrasonic cleaner in acetone, water, and propanol for ten minutes for every solvent. The wafer slices were submerged in the HF:H2O (1:1) solution for removing SiO2 layers and impurities from the surfaces, and rinsed in water and dried by nitrogen after cleaning. An Al layer with 100 nm thickness was vaporized to the back surface of the slices by thermal evaporation technique. The
Al-coated slices were annealed in N2 filled oven at 500 °C to obtain ohmic contact. Using a spin coater, the dispersed nanocrystals of the CuCo5S8in ethanol were deposited on the empty surface of the slices as the film layer. In order to complete the fabrication of the heterojunctions, 99.99% pure Au was thermally vaporized onto the CuCo5S8/p-Si junctions by a hole array mask. The diameters of the contact areas were adjusted as 1 mm. The obtained heterojunction was schematically illustrated in Fig.1.
2.3 Characterization of the CuCo
5S
8thiospinels and Au/CuCo
5S
8/p-Si
heterojunction
The synthesized CuCo5S8 thiospinels nanocrystals were characterized by Bruker D8 XRD instrument as powder form. The detailed structures were obtained by JEOL JEM-2100 TEM. Surface morphology and elemental composition of the coated films were investigated by Zeiss-Evo SEM with Bruker EDAX part. The impedance spectroscopy measurements were collected by Keithley 4200 semiconductor characterization system (SCS).
3 Results and discussion
The X-ray diffraction (XRD) pattern was employed to illuminate the crystal structure and phase composi-tion of the synthesized CuCo5S8 thiospinel nanocrystals. Comparative XRD patterns of the standard ICDD card for the synthesized nanoparti-cles have been given in Fig.2a. The results imparted that the XRD pattern of the nanocrystals is compati-ble with standard ICDD cards. The main diffraction peaks at around 26.90°, 31.50°, 38.18°, 46.90°, 50.01°, and 55.77° correspond to the respective (220), (311),
(400), (422), (511), and (440) planes of cubic crystal structure with a space group of Pd3m (ICDD 01-077-7463). The chemical structure of the CuCo5S8 was obtained by VESTA software and given in Fig.2b. The cubic structure of the CuCo5S8 nanocrystals is composed of CuS4 tetrahedra and CoS6 octahedra structures. At this stage, each copper and cobalt atoms bonded to 4 and 6 sulfur atoms, respectively. In the lattice of the CuCo5S8, the Cu atoms fill 1/8 tetrahedra with the S–Cu–S bond via angle of 109.47°, and the Co atoms occupy 1/2 octahedra with a covalent bonding of S [20]. Moreover, according to the Scherrer equation [21], the broadening of the peaks is in accordance with their small crystallite sizes. The crystallite sizes of CuCo5S8 thiospinel nanocrystals were calculated as 8.5 nm using the Scherrer equation for most striking peak.
The morphology and detailed structures of the CuCo5S8thiospinel nanocrystals were investigated by TEM and SEM instruments. Figure 3a shows the TEM image of the CuCo5S8 nanocrystals, and the synthesized CuCo5S8 nanocrystals have clustered shapes. The average particle sizes were measured as 8 nm, which are desirable, particularly for applica-tions in nanotechnology due to large surface areas. In addition, the calculated particle sizes are consistent with the calculated sizes from XRD patterns by the Scherrer equation. Figure 3b displays the typical FE-SEM images of the CuCo5S8nanocrystals. According to Fig.3b, all the CuCo5S8 thiospinel nanocrystals exhibited an agglomerated morphology due to elec-trostatic-steric-electrosteric forces and magnetic effects as well as and narrow size distribution [22]. Figure3c indicates the related electron diffraction patterns of the nanocrystals. The diffraction pattern are coherent with the (400), (422), (440), (642) and (733) reflections for cubic CuCo5S8 structure. From the high-resolution TEM observation in Fig.3d, the CuCo5S8thiospinel nanocrystals showed lattice frin-ges with well-resolved and pointing out a highly crystalline structure. The interplanar distance of the nanocrystals was measured as 2.70 A˚ , which corre-sponds to the interplanar distance of the (222) planes of the thiospinel copper cobalt sulfide crystal struc-ture. Figure3e–h indicates the SEM and related mapping images of different elements that are pre-sent in the sample. The Co, Cu, and S atoms are homogeneously distributed all over the sample. The result highlights that the synthesized nanocrystals
Fig. 1 The device diagram of the Au/CuCo5S8/p-Si heterojunction
have good homogeneity for various optoelectronic applications.
Figures4a and b display the C/A-V and G/Aw-V plots of the Au/CuCo5S8/p-Si heterojunction depending on the frequency between 100 kHz and 1 MHz by 100 kHz interval. The capacitance values stayed almost without change in the accumulation region, but they increased with decreasing frequency and exhibited peaks in the inversion region. The intensity of peaks decreased and shifted towards depletion region at about 0.5 V with increasing fre-quency. The showing peaks at the capacitance values and decreasing peak intensity can be ascribed to existence of the interface states in the device and decreasing response ability of them with increasing frequency, respectively [23]. Furthermore, the capac-itance behavior exhibited different tendency for the 100, 200, and 300 kHz frequencies and rest of other frequencies due to that interface states had different response to low and higher frequencies [24]. According to Fig.4b, the conductance values
increased in the inversion and depletion regions with increasing frequency but stayed constant in the accumulation region. This case can be depended on the series resistance effect of the Au/CuCo5S8/p-Si heterojunction [25].
The conduction and polarization mechanisms can be better understood by the analysis of the complex dielectric permittivity (e ¼ e0 je00) of the Au/ CuCo5S8/p-Si heterojunction, and they should be studied for a wide-range frequency. Complex dielectric permittivity consists of the real (e0) and imaginary parts (e00), whereas the real part of the dielectric permittivity shows the strength of the dipole against to the applied bias and stored energy, and the imaginary part indicates absorbed energy because of frictional dampening [14,26].
The complex permittivity and its components are given via below formula:
Fig. 2 a The XRD pattern and b schematic chemical structure of the synthesized CuCo5S8nanocrystals
e ¼ e0 je00¼ C C0
j G xC0
; ð1Þ
where the G, C, and C0are the conductance, capaci-tance, and free capacitor capacicapaci-tance, respectively. The e0 and e00 are given below formula for the free capacitor [27]: e0¼ C C0 ¼Cdi e0A; ð2Þ e00¼ G xC0 ¼ Gdi e0xA; ð3Þ
where e0 and di are vacuum permittivity and inter-layer thickness. The maximum capacitance is given by the formula of Cac¼ Ci¼ e0e0A=di depending on the interlayer capacitance in the accumulation region. Furthermore, the loss tangent (tan d) is addressed the
Fig. 3 a Low-magnification TEM image, b SEM image, c SAED pattern, d High-resolution TEM image and e–h SEM and corresponding elemental mapping images of the synthesized CuCo5S8nanocrystals
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 10 20 30 40 50 60 70 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz C/ A ( n F /cm 2) V (V) (a) -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 0 40 80 120 160 200 240 (b) 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz G/ A ( m F /c m 2) V (V)
ratio of imaginary part to real part of the dielectric constant: tan d¼e 00 e0 ¼ G xC: ð4Þ
Figure5shows the e0-V plots of the Au/CuCo5S8 /p-Si heterojunction for the various frequencies. The determined e0 values of the device reached the max-imum value at around 15.0 for 100 kHz frequency. The e0 values increased in the inversion region by the decreasing frequency and showed peak behaviors due to interface states and series resistance [28]. Furthermore, the peak intensity and peak positions were affected by the frequency and voltage changes depending on the Maxwell–Wagner polarization. The Maxwell–Wagner polarization can be referred to accumulating of the charge carriers at the boundaries [29].
Figure6shows e00-V plots of the Au/CuCo
5S8/p-Si heterojunction for the frequency range of 100 kHz and 1 MHz. The e00 values exhibited a decreasing profile by the increasing frequency and changing bias voltage from reverse to forward biases. Although the e00values were affected by the frequency and voltage changes in the inversion region, they stayed without change in the accumulation region. These changes at the e00values can be ascribed to the series resistance effect and interface states which cannot be able to follow the ac signal towards higher frequencies [15,30–32].
The tan d -V plots of Au/CuCo5S8/p-Si hetero-junction have been given in Fig.7 for changing the frequency from 100 kHz to 1 MHz. The tan d values stayed constant in the accumulation region with changing voltage and frequency, but decreased in the inversion region by an increasing frequency and changing bias from the reverse to forward. The inset of Fig.7 clearly revealed that the profile of the tan d values was influenced by the changing frequency and voltage.
The complex electric modulus (M) is obtained by the reciprocal of the complex dielectric constant
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 2 4 6 8 10 12 ε 14 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz Dielectri c Constant ( ') V (V)
Fig. 5 The e0-V plots of the Au/CuCo5S8/p-Si heterojunction
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 0 8 16 24 32 40 48 -1.5 -1.0 -0.5 0.0 0.5 0 4 8 12 16 20 24 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz Die lectr ic Loss ( '') V (V) Diel ectr ic Loss ( '' ) V (V) ε ε
Fig. 6 The e00-V plots of the Au/CuCo5S8/p-Si heterojunction
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 0 5 10 15 20 δ 25 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 0 2 4 6 8 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz Lo ss Ta ge t ( ta n V (V) Loss T aget ( tan V (V) ) δ )
Fig. 7 The tan d against voltage graphs of the Au/CuCo5S8/p-Si heterojunction
(e¼ 1=M). The M
is given by the formula of M¼ M0þ jM00depending on the real and imaginary parts, and serves to get more information about the relax-ation process of a dielectric material [15]. We can type the equation of the real and imaginary parts electric modulus by the below formula depending on the e0 and e00values [33]: M¼ 1 e¼ e0 e02þ e002þ j e00 e02þ e002¼ M 0þ jM00: ð5Þ
Figure8 displays the M0-V graphs of the Au/ CuCo5S8/p-Si heterojunction for various frequencies and voltages. The M0 values remained almost con-stant by changing bias and frequency in the inversion region. However, they increased exponentially in the depletion region with increasing voltage but not affected from the frequency changes. While the M0 values became independent of frequency changes in the inversion and depletion regions, they slightly affected in the accumulation region. The small changes at the M0values in the accumulation region can be depended on the frequency-related relaxation time of the charges [30].
Figure9 shows the M00-V plots of the Au/CuCo5 S8/p-Si heterojunction for changing frequency from 100 kHz to 1 MHz. There is increase at the M00values via increasing frequency in the inversion region for a given voltage. However, their values stayed almost constant for a given frequency. The peaks were observed in the depletion region, and they shifted towards to accumulation region for increasing fre-quency. The peak behaviors of the M00 values in the
depletion region can be based on the particular dis-tribution of the interface states such as mobile ions, stable oxide charges, or ionized traps and relaxation time of the charges in the interface [29]. There is also fluctuation at the M00 values for forward biases because the M00 values decreased with increasing frequency up to 400 kHz and then increased towards 1 MHz. This fluctuation can be attributed to that interface states have different response to low and higher frequencies.
The following formula gives the ac electric con-ductivity (r) depending on the imaginary dielectric constant:
r¼ d A
xCtand ¼ e00xe0: ð6Þ
Figure10exhibits the r-V plot of the Au/CuCo5S8 /p-Si heterojunction for the changing voltage and fre-quency. The r values decreased with increasing voltage and frequency in the inversion region, but the decreasing occurred slowly for higher frequencies. They were not affected in the depletion and accu-mulation regions by the voltage and frequency changes. The decrease at the r values by increasing frequency is depended on increasing series resistance and the decreasing eddy currents of the heterojunc-tion [31]. Nevertheless, the decrease of the r values can be referred to that the interface states at higher frequencies cannot be able to follow the ac signal [27]. Figure11a and b presents the real and imaginary impedance plots of the Au/CuCo5S8/p-Si hetero-junction against voltage for various frequencies,
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.3 0.6 0.9 1.2 1.5 1.8 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz Real Electri c M o dulus ( M' ) V (V)
Fig. 8 TheM0-V plots of the Au/CuCo
5S8/p-Si heterojunction -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 0.00 0.05 0.10 0.15 0.20 0.25 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz Imag inary El ectric Modul us ( M' ') V (V)
Fig. 9 TheM00-V plots of the Au/CuCo
respectively. The real and imaginary parts of the impedance in the inversion region stayed almost constant by the changing of frequency and voltage. However, they were affected in the depletion and accumulation regions by the voltage and frequency changes. The increasing frequency caused to decrease the impedance value due to increase conductivity of the heterojunction. The real impedance values also exhibited peaks in the depletion region. The peak behaviors can be based on the increasing series resistance effect [34]. Furthermore, the real and imaginary impedance values were obtained as about 1 MX for 10 kHz frequency (data not shown here), but their values decreased to a couple of kX
impedance values with increasing frequency up to 1MHz as shown in Fig.11.
4 Conclusion
The CuCo5S8 thiospinel nanocrystals were obtained by the modified solution method and studied by XRD, SEM, and TEM instruments. The XRD result confirmed the cubic structure of the CuCo5S8 nanocrystals. The clustered and crystalline structures of the CuCo5S8nanocrystals were confirmed by TEM images. The SEM images also indicated that the surfaces of the CuCo5S8 nanocrystals have narrow size distribution and agglomerated morphology. Furthermore, the CuCo5S8 thiospinel nanocrystals were deposited on the p-Si substrate as a thin film layer to obtain Au/CuCo5S8/p-Si heterojunction by Au metal electrode. The fabricated heterojunction was characterized by impedance spectroscopy tech-nique with changing frequency from 100 kHz to 1 MHz. According to impedance spectroscopy results, while the tan d values usually increased, the e0and e00values decreased with increasing frequency. In the inversion region, the obtained e0 values exhibited peaks, and the intensities of the peaks decreased with increasing frequency, whereas the M00 values are effective in the inversion, depletion and accumulation regions, the M0 values are effective in the depletion and accumulation regions. The r values exhibited a decreasing profile by the increasing fre-quency in the inversion region. The real and imagi-nary parts of the impedance values affected
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 0.00 0.05 0.10 0.15 0.20 σ μ 0.25 0.30 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz AC El ect ri c Con d uc ti vi ty ( ac ( S /cm )) V (V)
Fig. 10 The r versus voltage plots of the Au/CuCo5S8/p-Si heterojunction -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 0 10 20 30 Ω 40 50 100 kHz 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz Z re al (k ) V (V) (a) -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 0 50 100 150 200 250 300 100 kHz (b) 200 kHz 300 kHz 400 kHz 500 kHz 600 kHz 700 kHz 800 kHz 900 kHz 1000 kHz Z imag iner (k ) V (V) Ω
Fig. 11 Thea real and b imaginary parts of the impedance versus voltage graphs of the Au/CuCo5S8/p-Si heterojunction for changing frequency
frequency and voltage changes in the depletion and accumulation regions. The results highlighted that the frequency and voltage changes strongly influ-enced the dielectric parameters of the Au/CuCo5S8/ p-Si heterojunction.
Acknowledgements
TUBITAK (The Scientific and Technological Research Council of Turkey) supported this study with the Grand Number of 217M212. Authors would like to thank TUBITAK for supporting.
Compliance with ethical standards
Conflict of interest All authors declare that they have no conflict of interest.
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