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Radiation Effects and Defects in Solids
Incorporating Plasma Science and Plasma Technology
ISSN: 1042-0150 (Print) 1029-4953 (Online) Journal homepage: https://www.tandfonline.com/loi/grad20
Synthesis and characterization of p-GaSe thin films
and the analyses of I–V and C–V measurements
of p-GaSe/p-Si heterojunction under electron
irradiation
K. Çınar Demir, Ş. Aydoğan, Emre Gür, C. Coşkun & Z. Aygün
To cite this article: K. Çınar Demir, Ş. Aydoğan, Emre Gür, C. Coşkun & Z. Aygün (2017) Synthesis and characterization of p-GaSe thin films and the analyses of I–V and C–V
measurements of p-GaSe/p-Si heterojunction under electron irradiation, Radiation Effects and Defects in Solids, 172:7-8, 650-663, DOI: 10.1080/10420150.2017.1377713
To link to this article: https://doi.org/10.1080/10420150.2017.1377713
Published online: 12 Oct 2017.
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VOL. 172, NOS. 7–8, 650–663
https://doi.org/10.1080/10420150.2017.1377713
Synthesis and characterization of p-GaSe thin films and the
analyses of I–V and C–V measurements of p-GaSe/p-Si
heterojunction under electron irradiation
K. Çınar Demira, Ş. Aydoğanb,c, Emre Gürb, C. Coşkundand Z. Aygüne
aOltu Earth Sciences Faculty, Department of Mining Engineering, Atatürk University, Erzurum, Turkey; bFaculty of Sciences, Department of Physics, Ataturk University, Erzurum, Turkey;cFaculty of Engineering, Department of Environmental Engineering, Ardahan University, Ardahan, Turkey;dFaculty of Arts and Sciences, Department of Physics, Giresun University, Giresun, Turkey;eVocational School of Technical Science, Bitlis Eren University, Bitlis, Turkey
ABSTRACT
Gallium Selenide (GaSe) thin films were grown by the electrochem-ical deposition (ECD) technique on Indium tin oxide (ITO) and p-Si (100) substrates. The Electron paramagnetic resonance (EPR) spec-trum of GaSe thin films’ growth on ITO was recorded at room temper-ature. According to EPR results, the g value of an EPR signal obtained for GaSe deposited on ITO is 2.0012± 0.0005. In/GaSe/p-Si het-erojunction was irradiated with high-energy (6 MeV) and low-dose (1.53× 1010e−cm−2) electrons. The ideality factor of the In/GaSe/p-Si device was calculated as 1.24 and barrier height was determined as 0.82 eV from I–V measurements before irradiation. Acceptor con-centration, built-in potential and barrier height of the In/GaSe/p-Si device were also obtained as 0.72× 1014cm−3, 0.65 eV and 0.97 eV from C–V measurements, respectively. After irradiation, the ideal-ity factor n and barrier heightbvalues of the In/GaSe/p-Si device were calculated as 1.55 and 0.781 eV, respectively. Acceptor con-centration, the built-in potential and barrier height values of the In/GaSe/p-Si device have also shown a decrease after 6 MeV electron irradiation. This change in heterojunction device parameters shows that current transport does not obey thermionic emission, and thus tunneling could be active due to the defects formed by irradiation at the In–GaSe interface.
ARTICLE HISTORY Received 28 March 2017 Accepted 30 August 2017 KEYWORDS Irradiation; heterojunction; electrodeposition; GaSe; XRD; AFM 1. Introduction
Gallium Selenide (GaSe) belongs to the AIIIBIVgroup layered semiconductor having an indi-rect bandgap of 2.0 eV, which shows a p-type conductivity. The energy difference between direct and indirect bandgap is so small and can be easily overcome by the room tem-perature thermal energy (1). Its structure is characterized by the strong covalent bonds within the layer between Se-Ga-Ga-Se atoms and weak van der Waals bonds between each of the layers. Bulk GaSe has many important application areas due to its charac-teristic properties such as high birefringence, broadband transparency (0.62–1.8 μm and
CONTACT K. Çınar Demir kubra.cinar@atauni.edu.tr; Ş. Aydoğan saydogan@atauni.edu.tr
above 50 μm), large non-linear optical coefficient, high thermal conductivity and high dam-age threshold in non-linear optics, terahertz radiation and detection, particle radiation detector and optoelectronics (2–4). On the other hand, an increasing trend of the layered two-dimensional semiconductors has recently also renewed interest in the GaSe semicon-ductors (5). Photodetectors and field effect transistor applications of the two-dimensional GaSe semiconductor has already been shown successfully (1,6). This also brings up the importance of the thin film form of the GaSe semiconductor. In addition, difficulties in working with the bulk form of GaSe owing to its poor mechanical strength and thermal features, growth mechanism and quality of the grown films have become important. The high damage threshold property of the GaSe also makes it a very attractive material for the particle detectors. It is shown that GaSe shows better performance compared to that of Si-based devices in neutrino experiments (4). Therefore, it is of interest to have GaSe radiation detectors operating in the thin film forms.
This study reports the growth of the p-GaSe thin film on both Indium tin oxide (ITO) and the p-Si substrates by electrochemical deposition (ECD). Structural characterization of the ECD grown p-GaSe layers was examined with atomic force microscopy (AFM) and X-ray diffraction (XRD). Also, the electrical characteristics of p-GaSe/p-Si heterojunction are investigated through the current–voltage (I–V) and the capacitance–voltage (C–V) mea-surements. Finally, the effect of high-energy electron irradiation (6 MeV) on the electrical parameters of p-GaSe/p-Si heterojunction is presented.
2. Experimental
A GaSe thin film was grown by ECD on the p-Si and commercial ITO glass substrates. ITO glass has 15cm resistivity while p-Si has 1–10 cm resistivity corresponding to a 3.1× 1016cm−3hole concentration. Prior to growth, the ITO substrate was cleaned by dip-ping in acetone for 2 min and methanol for 2 min. Then, in order to supply homogeneity on the substrate, the ITO glass was annealed for 30 min at 300°C temperature. The p-Si sub-strate was cleaned by using the RCA procedure (retained in boiling NH3+H2O2+6H2O for
10 min and then HCl+H2O2+6H2O2at 60°C for 10 min) as reported elsewhere (7).
Prior to the growth of GaSe on the p-Si substrate, the Al element was evaporated on the unpolished side of the p-Si in a Univex-300 Pump system with a pressure of 4× 10−5Torr and p-Si/Al was annealed under N2gas flow at 580°C for 3 min to ensure its ohmic
char-acteristic. The ECD process was performed by using an electrochemical cell having three electrodes which are reference (Ag/AgCl), counter (Pt) and working electrodes (p-Si or ITO). A Gamry Reference 600 Potentiostat-Galvanostat/ZRA (Zero Resistance Ammeter) was used during the growth process. For this purpose, a solution of Ga(NO3)2xH2O with 0.05 M and
0.12 M SeO2and 0.1 M H2SO4in DI (de-ionized) water was prepared. To prevent oxidation
before and during the growth, nitrogen was introduced inside the cell. The ECD growth was performed under the cathodic potential of−0.570 V and lasted one hour at a solu-tion temperature of 85°C (pH= 2). After electrodeposition, the samples were cleaned with DI water and dried using nitrogen gas. The conductivity type of the GaSe thin films was determined by the Hall measurement (in van der Pauw configuration) and hot probe tech-niques. The Hall measurements were obtained in a home-made Hall kit using a Varian 2901 Regulated Magnet Power Supply. The GaSe thin films were also analyzed by XRD technique structurally using a Rigaku D/Max-IIIC diffractometer, with Cu Kα radiation of 1.54 Å, within
the 2θ angle ranging from 20° to 80°. AFM images of the grown thin films were taken by Nanomagnetics Instruments Ambient AFM/MFM. Only the p-GaSe sample grown on the p-Si substrate was subjected to the electron irradiation in order to investigate the irradiation effect on the interface of the heterostructure. The irradiation process was carried out by a Siemens-Primus linear electron accelerator providing us acceleration of the electrons up to 21 MeV with a 1.14× 1012e−cm−2dose. Then, In metal was evaporated on GaSe to obtain the In/GaSe/p-Si heterojunction. The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of the In/GaSe/p-Si heterojunction were obtained by using a Keithley 487 picoammeter and an HP-4192 A impedance analyzer at 500 kHz, respectively. Furthermore, these characteristics were repeated under electron irradiation too.
An X-band Electron paramagnetic resonance (EPR) spectrometer with 100 kHz modu-lation field and 9.54 GHz frequency was used for recording EPR spectra of GaSe. Diphenyl pecryl hydrazyl with a g value of 2.0036 is used for determining the central field. GaSe elec-trodeposited on ITO was stuck on the top of a diamagnetic rod with a diamagnetic glue and the EPR experiment was performed at room temperature.
3. Results and discussion
Just one crystal plane of (103) belongs to the GaSe confirmed by the JCPDS file (65-4036), and can be clearly seen in Figure1(a), which demonstrates the crystalline structure of the film for the thin film grown on p-Si. On the other hand, Figure1(b) shows the crystal planes of (107), (008), (205) and (210) of the GaSe thin film grown on the ITO substrate. Surface morphologies of the resulting films were measured by scanning electron microscopy (SEM-Nova Nanosem JSM-6400) and an SEM image of GaSe on p-Si is shown in Figure1(c). The full width at half maximum (FWHM) value of the peak corresponding to the (103) plane is 0.23 degree corresponding to the grain size of 364 Å obtained from Scherrer’s (8) formula. The FWHM values of the peaks corresponding to the (107), (008), (205) and (210) planes are 0.224, 0.350, 0.328 and 0.418 degrees, respectively. Calculated grain size values for (107), (008), (205) and (210) peaks are 375.6, 245.3, 287.3 and 244.8 Å, respectively. Apparently, the substrate has a huge effect on the crystal structure of the grown GaSe thin films. The SEM image in Figure1(c) shows the surface that is covered by the spherical grains in which the sizes change from 80 to 400 nm. Although the grain sizes calculated by Scherrer’s formula are 30–40 nm, the grain size obtained from SEM images differs from that obtained from XRD measurements. Generally, it is possible to make an error by using Scherrer’s formula for the grain sizes of a few tens of nm, since the FWHM of the XRD peaks belongs to the planes, which arises due to the contributions from strain distributions, small crystallite sizes and instrumental broadening.
Figure2(a) shows the redshift of the absorption spectra measured for GaSe thin film growth on the ITO substrate. The absorption edge of the p-GaSe thin film confirms the opti-cal quality of the films. Figure2(b) showsα2vs. energy plot. Absorption coefficient values have been determined by using the conventional equations considering the intensity losses of incident light because of the reflection from the back and front surfaces of the material. As shown in Figure2(b), the band gap energy value has been calculated as 1.85 eV for the GaSe thin film.
The EPR spectrum recorded for the GaSe electrodeposited on ITO is given in Figure3. The g-factor of the signal has been determined from the equation hυ = gβH with h being the
(a) (b)
(c)
Figure 1.(a) XRD measurement results of the p-type GaSe thin film deposited on the p-Si substrate, (b)
the crystal planes of (107), (008), (205) and (210) of GaSe thin film grown on the ITO substrate and (c) an SEM image of GaSe on p-Si.
Planck constant,β the electron bohr magneton, H the magnetic field and υ the microwave frequency. As seen in Figure6, GaSe deposited on ITO gives an EPR signal with linewidth of about 1.0 mT at a g value of 2.0012± 0.0005 which can be attributed to a trapped elec-tron similarly reported before (9–11). Except from this EPR signal, any significant magnetic property cannot be detected with the EPR method.
Figure4shows 2D and 3D 4 μm× 4 μm AFM images of the grown GaSe thin films on p-Si and ITO substrates. All measurements were carried out in tapping mode. The prop-erties of the used tips are 27 kHz, C= 1.6 N/m and the mode of the tip is DF3. Also, the height profile of the surface is pointed out through the drawn line in Figure (c,d). Figure 4(a,c,e) shows the thin film grown on the p-Si substrate while (b,d,f) shows the film grown on the ITO substrate. Figure4(a,b) shows the diverse morphology of the grown GaSe thin films on different substrates. Triangular shapes of the grown GaSe thin film on the ITO sub-strate have been observed, and round shapes on p-Si. Figure4(b) confirms 3D triangular features such as island-like grown structures. These features are very common in a few lay-ers of the grown GaSe thin films (5). The average height of the triangular shapes is about
Figure 2.(a) Absorption spectrum and (b)α2vs. energy plot of the grown GaSe on the ITO substrate.
100 nm which means that there are about hundred GaSe layers. Also some coalescence of the triangular shapes is seen in the figure. Perhaps, increasing the growth time would have resulted in the 2D growth with the coalescence of the island structures. On the other hand, round shapes of the GaSe thin films grown on p-Si substrates having 38 nm surface rough-ness, seen in Figure4(a). In this case, it might be speculated that the growth rate on the p-Si is much higher than that on the ITO substrates.
P-type conductivities of the grown GaSe thin films on p-Si and ITO substrates have been verified by using both hot probe and Hall measurements with the van der Pauw config-uration. In both cases, Hall measurements were used to carry out this analysis. Since the
Figure 3.EPR spectrum of GaSe deposited on the ITO substrate.
measured resistivity of the p-GaSe thin film grown on the p-Si is almost three orders lower than the substrate p-Si, the current will pass from the top layer of GaSe and the conductivity type characterization will be possible. In the other case of the GaSe thin film grown on the ITO substrate, a depletion layer is formed between the n-type ITO and p-GaSe. This deple-tion layer will prevent the current passing from the bottom layer and the characterizadeple-tion will be possible. The carrier concentration, resistivity and mobility of the p-type GaSe grown on p-Si were measured as 1.7× 1014cm−3, 0.01cm and 0.037 cm2V−1s−1, respectively.
For the thin film grown on ITO, carrier concentration, resistivity and mobility of p-GaSe thin films were 1.6× 1015cm−3, 0.88cm and 53 cm2V−1s−1, respectively. One order differ-ence in the hole concentration was obtained by just changing the substrate. Also, a few orders of difference in the mobility was obtained. These differences might be due to the 2D and 3D growth mechanism in ITO and p-Si substrates, since less grain boundaries formed in the 2D growth compared to the 3D growth which caused less scattering to occur in the grain boundaries. It is seen that the deposited films are the different XRD peaks even though the same thin film was grown on both ITO and p-Si. This also explains that the orientations of these substrates are different from each other.
We have analyzed the p-GaSe/p-Si heterostructure using the semi-log I–V and the C–V measurements at room temperature. Figures5and6show the semi-log I–V plot and C–V plot of In/p-GaSe/p-Si heterostructure, respectively. Four orders of rectification can be seen from the I–V plot. Deviation from the linearity in the forward bias is owing to the high value of series resistance of the p-GaSe/p-Si heterostructure. Voltage-independent leakage current of about 1× 10−9has been observed from the I–V curves. The defect density in the interface of the heterojunction increases with electron irradiation. According to many theories, the Schottky barrier height is insensitive to the interface structure and quantum mechanically, the barrier height depends on the interface structure in Schottky diodes. Series resistance can affect the I–V characteristics for the barrier height, which is an inhomo-geneous property. Another explanation for an inhomoinhomo-geneous interface is a higher ideality factor (12,13). As seen from Table2, the series resistance and ideality factor have increased after electron irradiation, which means the nature of the barrier for the In/GaSe/p-Si Schot-tky diode has shifted to have an inhomogeneous property. This results in the reduction of the barrier height and an increase of the ideality factor. Perhaps, the parameters such as image forces, tunneling, generation-recombination and interfacial oxide layer play an
Figure 4.AFM images of GaSe corresponding to the surface (a), cross-line section profiles (b) and 3D AFM images.
important role in the increase of theideality factor. The decrease in barrier height corre-sponds to the increase in leakage current. The increase in the reverse saturation current after electron irradiation may be attributed to the irradiation-induced crystal lattice defects that stem from the variation of interface states and these may act as recombination centers and/or trapping. Furthermore, in Schottky diode devices, the reverse current is proportional to the density of minority carriers near the junction, which affects the reverse bias current (14). A change in the ideality factor or in barrier height with irradiation indicates a modifica-tion in the electrical properties of the interface of metal/semiconductor structure (15) and an increase in the density of interface states (16). As a result, all of these changes switched
Figure 5.AFM surfaces (a) 2D, (b) line cross-section and (c) 3D images of the GaSe thin films growth on the ITO substrate.
the structure of the heterojunction. It may be acceptable that the current passed with the thermionic emission (TE) theory in the device.
The current flow through the In/GaSe/p-Si Schottky diode can be defined by TE theory as follows (17): I= I0exp qV nkT 1− exp −qV kT , (1)
where q is the electronic charge, k the Boltzmann constant, T the temperature, Vthe applied voltage, n the ideality factor, which is given by:
n= q kT dV d(ln I) , (2)
Figure 6.(a, b) The current–voltage characteristics of In/GaSe/p-Si/Al Schottky diode before and after 6 MeV electron irradiation.
I0is the reverse saturation current which is determined from the straight line intercept
of ln I at V= 0 in Equation (1) and is given as follows: I0= A ∗ AT2exp −qB kT , (3) B= kT q ln AA∗ T2 Is , (4)
where A∗ is the effective Richardson constant, A is the diode area and B is the barrier
height. A∗ is equal to 32 Acm−2K−2for p-Si (18,19), Acm−2K−2247 for GaSe and A diode area is around 2× 10−3cm2.
The In/GaSe/p-Si Schottky diode parameters obtained from the I–V and C–V measure-ments (see Figures5and6) before and after 6 MeV electron irradiation (AEI) are given in Table 1. The table shows that the ideality factor increases with electron irradiation whereas the barrier height decreases. Tunneling, image forces, interfacial oxide layer and generation-recombination are usually accepted as important agents for large ideality fac-tors (n> 1) (20,21). We have already explained that a high ideality factor of a Schottky diode increases with increasing irradiation energy in one of our previous works (22). On the other hand, the barrier heights determined from C–V measurements are generally greater than those from I–V measurements (23,24). This difference is generally attributed to the interface states. These types of defects affect the I–V characteristics since they can act as recombina-tion centers for trap-assisted tunnel currents. Thus, the ideality factor can increase through this mechanism and there can be a reduction in the barrier heights. The C–V technique is less prone to such kind of defects as they are related to the ac technique and so it yields the average barrier height (25). With the influence of irradiation, the atoms of the contact
Table 1.. The Schottky diode parameters for In/GaSe/p-Si, obtained from theI–V and C–V measure-ments for before (BEI) and after electron irradiations (EIs).
Barrier height (eV) Acceptor concentration (cm−3)× 1014 Diffusion potential (eV) Fermi energy
level (eV) (C–V) (I–V) Ideality factor BEI 0.72± 0.03 0.65± 0.07 0.32± 0.02 0.97± 0.06 0.82± 0.05 1.24± 0.05 EI1 (6 MeV) 0.643± 0.04 0.58± 0.04 0.32± 0.03 0.90± 0.06 0.78± 0.04 1.55± 0.06
materials can diffuse into the semiconducting material and create traps and recombination centers at the metal/semiconductor interface. These states induce the trap-assisted tunnel currents and turn the current mechanism to tunneling. These results may have practical importance in space electronics as high energetic electrons are often encountered in the low earth orbit.
Cheung and Cheung (26) suggested that considering the voltage drop across the Schot-tky diode due to the series resistance Rsand the series resistance values of Schottky diodes
can be calculated as follows:
I= AJ = AA∗ T2exp −e( B− IRs) kT × exp e(V − IRs) nkT , (5)
where the term IRsindicates the voltage drop across the device. The values of the series
resistance can be determined from the following functions using Equation (5). dV
d ln(I)= nkT
e + IRs, (6)
and H(I) is given as follows:
H(I) = V −nkT e ln I AA∗ T2 , (7) H(I) = nB+ IRs. (8)
Figure7(a,b) shows dV/dln(I) vs. I and H(I) vs. I plots of the In/GaSe/p-Si heterostruc-ture using the Cheung method. Some diode parameters calculated using Cheung, Norde (which is given below) and C–V measurements for before (BEI) and after electron irradiation (AEI) are given in Table2. It can be seen that after electron irradiation the ideality fac-tor and series resistance increased whereas the barrier height decreased. These variations have been attributed to irradiation-induced defects between In and GaSe and within the space-charge region of the device. Furthermore, increasing the ideality factor denotes that the current transport mechanism of the device no longer obeys the thermionic emission theory. Therefore, tunneling might be dominant. Our results show that the experimental series resistance RSvalues are high in accordance with the literature (27–30). Besides
-1.0 -0.5 0.0 0.5 1.0 Voltage (V) 0 3 5 8 10 13 15 18 20 Ca p ac it an ce (p F ) (a) (b)
Figure 7.(a, b) The capacitance–voltage and the 1/C2–V plot of the In/GaSe/p-Si/Al diode at 500 kHz
before and after irradiation.
Table 2.The series resistance parameters for the In/GaSe/p-Si Schottky diode calculated from different
methods with Cheung, Norde andC–V measurements for before (BEI) and after electron irradiations (EIs).
Series resistance (kohm) Barrier height (eV) Ideality factor BEI EI1 (6 MeV) BEI EI1 (6 MeV) BEI EI1 (6 MeV) dV/dln(I) 17± 0.04 203± 0.05 – – 3.1± 0.07 3.5± 0.05
H(I) 18± 0.04 215± 0.06 0.736± 0.02 0.718± 0.07 – –
F(V) 53± 0.05 279± 0.04 0.909± 0.03 0.852± 0.03 – –
C–V 302± 0.09 313± 0.07 – – – –
obtained with the help of the following equation: F(V) = V γ − kT e In I(V) AA∗ T2 , (9)
whereγ is an integer greater than ideality factor n and I(V) is the current determined from the forward bias I–V plots. Also, the barrier height and RSvalues are determined by
B= Fm + (γ − n) n Vm γ − kT e , (10) RS= (γ − n) kT eIm . (11)
Figure8shows the F(V) vs. V curve of the In/GaSe/p-Si heterostructure. The junction parameters calculated from the Norde method are given in Table2. It is observed that the RSvalues calculated from both Cheung and Norde methods increased whereas the barrier
height decreased with electron irradiation. The decrease in barrier height corresponds to a reduction in leakage current. An increase in series resistance indicates a decrease in free carrier concentration and mobility because of the irradiation-induced defects or owing to the dopant compensation of the semiconductor. The series resistance of Schottky diodes
(a)
0.0E+0 5.0E-6 1.0E-5 1.5E-5 2.0E-5 Current (A) 0.10 0.20 0.30 0.40 0.50 0.60 dV /d ln (I ) (V ) AEI BEI (b)
Figure 8.(a, b) The dV/dln(I) vs. I and H(I) vs. I plots.
Figure 9.F(V) vs. V plots for unirradiated and irradiated diodes.
can be determined using C–V characteristics too. The following equation is used for this purpose:
Rs= Gma/(G2ma+ (wCma)2), (12)
where Cmaand Gmaare the values of the capacitance and conductance in the maximum
point, respectively. These parameters were determined by using a 500 kHz measurement frequency. Figure9(a,b) shows the plots of conductance and series resistance vs. voltage for the In/GaSe/p-Si Schottky diode in processes before irradiation and after irradiation.
Figure 10.The conductance-voltage (a) and the series resistance-voltage (b) plots of the In/GaSe/p-Si/Al diode at 500 kHz is given in Figure10.
4. Conclusion
GaSe thin films were grown on both ITO and p-Si (100) substrates by the ECD technique. The structural and electrical features of the films have been analyzed. With the help of absorption measurement, the band gap energy value has been calculated as 1.85 eV for the GaSe thin film. According to EPR results, the only property obtained with the EPR tech-nique for GaSe deposited on ITO is an EPR signal assigned to a trapped electron with a g value of 2.0012± 0.0005 and about 1.0 mT linewidth. In/GaSe/p-Si Schottky diodes
were irradiated with high-energy and low-dose electrons of 6 MeV. While ideality factor and series resistance of In/GaSe/p-Si Schottky diode increase, the barrier height decreases with 6 MeV electron irradiation. These findings show that current transport does not obey thermionic emission and tunneling could be active because of the defects formed by electron irradiation at the interface.
Disclosure statement
No potential conflict of interest was reported by the authors. References
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