- Email: [email protected]
n-CuxIn1−xO heterojunction diode prepared by sol-gel spin coating
Materials Science in Semiconductor Processing 46 (2016) 46–52
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Photoresponse characteristics of p-Si/n-CuxIn1 xO heterojunction diode prepared by sol-gel spin coating K. Mageshwari, Jinsub Park n Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 10 October 2015 Received in revised form 31 December 2015 Accepted 5 February 2016
In the present work, we report on the fabrication and detailed electrical characterization of p-Si/nCuxIn1 xO heterojunction prepared via the deposition of nanocrystalline CuxIn1 xO thin films on p-type silicon substrate by sol-gel method using spin coating technique. X-ray diffraction and Raman spectroscopy results revealed the polycrystalline nature of CuxIn1 xO thin films consisting diffractions peaks and vibration modes, respectively, corresponding to CuO and In2O3. Field-emission scanning electron microscopy showed compact surface morphology while UV–vis absorption spectra exhibited sharp absorption between 300 and 425 nm along with a long tail extending in the visible region. The currentvoltage (I-V) characteristics of the fabricated heterojunction demonstrated obvious rectifying behavior in the dark and under illumination. The heterojunction exhibited low reverse leakage current ( 10 7), and upon illumination, the forward current and rectification ratio of the junction was improved while the forward threshold voltage lowered. By fitting the experimental data we have observed that the forward current conduction is dominated by the space charge limited current mechanism. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Heterojunction CuxIn1 xO thin films Sol-gel spin coating Electrical properties Ideality factor Series resistance
1. Introduction Transparent conducting oxides (TCOs) are an important group of materials used in optoelectronics due to their unique properties combining the low electrical resistance (typicallyo10 3 Ω cm) with the high optical transparency (4 85%) in the visible range. In particular, TCOs based on Cu-In-O materials system have gathered special interest for their applications in flat panel displays, solar cells, thermoelectrics and thin films transistors [1,2]. Copper oxide (CuO) is a p-type semiconductor having a narrow energy band gap of 1.2–1.5 eV, high optical absorption in the visible region and environment friendly properties like non-toxicity and low cost; while indium oxide (In2O3) is a wide band gap material ( 3.7 eV) possessing n-type conductivity, high visible transparency and free carrier mobility [3–6]. By combining both CuO and InO either p-type or n-type character could be produced in the Cu-In-O system by tuning the film composition [1]. Although Cu-In-O system has been extensively employed in various device applications satisfactory understanding of the alloy in all details has still not been achieved. Assessing the p-n junction characteristics is an efficient way to determine the semiconductor behavior of the materials. Nevertheless, less attention has been paid to the electrical characteristics of Cu-In-O system, and the reports on their n
Corresponding author. E-mail address: [email protected] (J. Park).
http://dx.doi.org/10.1016/j.mssp.2016.02.003 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
electrical properties are rather limited. Under these perspectives, herein we made a noteworthy and maiden attempt by depositing n-CuxIn1 xO thin films on p-Si substrate, and demonstrated the current-voltage (I-V) characteristics of the heterojunction in the dark and under photon illumination. Up to now, remarkable progresses have been made on the thin film deposition technologies, and a plethora of physical and chemical methods have been developed. Compared with the physical protocols, chemical protocols are recognized as the desired choice for the thin film preparation as they offer a potentially low-cost and scalable manufacturing method for the large area deposition and roll-to-roll processing even on flexible substrates [7]. Owing to its simplicity, homogeneity, low crystallization temperature, and easy control of the film thickness and composition, sol-gel spin coating method has become a competitive technique for the thin film fabrication [5,8]. Accordingly, in the present work, we employed sol-gel spin coating for the deposition of nanocrystalline CuxIn1 xO thin films on Si substrate with the final goal of understanding the electrical characteristics of the heterojunction. X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FESEM), UV–vis absorption spectroscopy and photoluminescence (PL) measurements were used to investigate the structural, morphological and optical properties of CuxIn1 xO thin films. The electrical properties of p-Si/n-CuxIn1 xO heterojunction were investigated by dark and photocurrent-voltage characteristics and the electronics parameters controlling the heterojunction, such as barrier height ( φb ), ideality factor (n) and
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
47
series resistance (Rs) was obtained by combining the modified Norde's function with conventional I-V and Cheung-Cheung methods. The fabricated p-Si/n-CuxIn1 xO heterojunction showed well-defined rectifying I-V characteristics with low reverse leakage current ( 10 7), excellent rectification ratio ( 1.1 104) and a high ideality factor (n 42).
2. Experimental details CuxIn1 xO thin films were deposited on p-Si substrate by solgel method using spin coating technique. Briefly, a solution was prepared by dissolving copper (II) acetate hydrate and indium (III) acetate with a weight ratio of 20:80 in an equal molar ratio (1.4 M) of 2-methoxyethanol and monoethanolamine. The solution was first stirred at 65 °C for 1 h and then stirred continuously for 24 h at room temperature. Finally, the solution was aged for 24 h. The precursor solution was deposited in two steps: spin coating at 600 rpm for 10 s and 2000 rpm for 20 s, followed by drying at 200 °C for 10 min on a hot plate. The deposition was repeated for 5 times and then the films were post-annealed at 500 °C for 2 h in air. The typical CuxIn1–xO thin film prepared for 5 spin coating cycles has an average thickness of around 120 nm. Under identical deposition conditions CuxIn1 xO thin films were deposited on the glass substrate for the structural, morphological and optical analysis. Structural analysis of CuxIn1 xO thin films was carried out using XRD (Rigaku D/MAX/2500/PC X-ray diffractometer) equipped with Cu Kα radiation (λ ¼ 0.154 nm), and Raman spectroscopy (750i/ELT1000 micro confocal Raman spectrophotometer, Excitation wavelength¼532 nm).The morphology of CuxIn1 xO thin films was examined by Hitachi S-4800 FESEM. The optical absorption properties of CuxIn1 xO thin films were analyzed by UV– vis absorption spectra recorded using a Cary UV/vis/NIR spectrophotometer (λ ¼300–800 nm, sampling area 2.5 2.5 cm2). Photoluminescence (PL) measurements were performed at room temperature using a He-Cd laser (Excitation wavelength ¼ 325 nm, Power ¼5 mW). The I-V measurements were carried out at roomtemperature using a probe station system equipped with Keithley series 2400 digital source meter. Platinum (Pt) metal contacts were deposited through a shadow mask onto p-Si and n-CuxIn1 xO to form bottom and top electrodes, respectively. The photoresponse behavior of the heterojunction was studied in the dark and under illuminated conditions (using a 140 W Xenon lamp).
3. Results and discussion The crystal structure of CuxIn1–xO thin films was investigated by XRD, and the corresponding diffraction pattern is shown in Fig. 1. All the diffraction peaks in the XRD pattern could be indexed to either CuO or In2O3, demonstrating the co-existence of CuO and In2O3 characteristics in Cu-In-O system. The diffraction peaks of CuO (002), (111), ( 112) and ( 113) planes matches with the monoclinic crystal structure of CuO (JCPDS card no. 45-0937). Similarly, the In2O3 characteristics peaks (221), (222), (332) and (440) planes matches with the cubic structure of In2O3 (JCPDS card no. 06-416). No other characteristic peaks of impurities such as Cu (OH)2, Cu2O and In(OH)3 were observed, which suggest that the CuxIn1 xO thin films are purely composed of CuO and In2O3. The intensity of the diffraction peaks corresponding to In2O3 is significantly higher when compared to CuO peaks, indicating the predominance of In2O3 in CuxIn1 xO thin films. The mean crystallite size of CuO and In2O3 nanoparticles were estimated for the predominant (002) and (222) planes, respectively, using the well-
Fig. 1. XRD spectrum of CuxIn1 xO thin films.
Fig. 2. SEM images of CuxIn1 xO thin films.
known Scherrer's formula [9], and found to be 12 nm and 8 nm, respectively. The surface morphology of CuxIn1 xO thin films analyzed by FESEM is shown in Fig. 2. The films exhibited a uniform coverage over the entire substrate and possessed a compact morphology without any cracks. The higher magnification image in the inset clearly shows the denser packing of the nanoparticles, which are beneficial as they can reduce the grain boundary effect thereby enhancing the charge carrier mobility, leading to high conductivity [10]. The electronic band structure and the electronic transition within the CuxIn1 xO thin films were investigated by UV–vis optical absorption and PL measurements. The UV–vis absorption spectrum of CuxIn1 xO thin films showed sharp absorption band around 300–425 nm along with a long absorption tail in the visible region. This long absorption tail entering the visible region indicates the visible light absorption capability of CuxIn1 xO thin films in addition to the UV light absorption. The optical band gap of CuxIn1 xO thin films was estimated from (αhυ)2 vs hυ plot based on the direct transition as shown in Fig. 3(b). The band gap energy was determined by extrapolating the straight-line portion to the energy axis for zero absorption co-efficient, and found to be 2.9 eV. Compared with the bulk band gap value of In2O3 (Eg 3.6 eV) the estimated lower band gap value can be attributed to the influence of CuO in the mixed CuO-In2O3 system. The PL spectrum of CuxIn1 xO thin films (Fig. 3(c)) show several emission peaks in the visible region (409, 535 and 775 nm). The emissions
48
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
Fig. 4. Raman spectra of CuxIn1 xO thin films on (a) silicon, and (b) glass substrate.
Fig. 3. (a) UV–vis absorption spectra, (b) (αhυ)2 vs hυ plot and (c) PL spectrum of CuxIn1 xO thin films.
in the visible regions have been largely considered to be associated with the intrinsic or extrinsic defects in the semiconductor oxides. The appropriate assignment of PL bands is little complex since the origin of luminescence in CuO and In2O3 are still unclear. The large variation in the electron and hole effective mass limits the precise estimation of PL emissions in CuO [11]. The PL emissions in In2O3 are generally attributed to the presence of vacancies (indium or oxygen) or defects (interstitial indium or antisite oxygen) [6]. Cubic In2O3 has an oxygen-deficient fluorite structure with the
twice the unit cell edge of the corresponding fluorite cell and with ¼ of the anions missing in an ordered way, which would induce the formation of new energy levels in the band gap region of In2O3 [12]. Hence, the emissions occur due to the radiative recombination of photogenerated hole with an electron occupying the oxygen vacancy, which are commonly referred as a deep level or trap state emissions due to the oxygen vacancies. The chemical structure of CuxIn1 xO thin films deposited on the silicon as well as on the glass substrate was analyzed by Raman spectra, and is shown in Fig. 4. The spectra show broad Raman peaks at 291, 478 and 521 cm 1 along with some minor peaks at 369 and 608 cm 1. The peak at 521 cm 1 is from the Si substrate. The peaks at 291 and 608 cm 1 corresponds to the Ag and B2g modes of monoclinic CuO, respectively, which are attributed to the vibrations of oxygen atoms in CuO [13]. The peaks observed at 369 cm 1 (stretching vibrations of In-O-In linkages) and 478 cm 1 (octahedral stretching vibrations ν(InO6)) corresponds to the body-centered cubic In2O3 [14]. The vibrational modes of CuO and In2O3 exhibited slight shift due to the disorder caused in their unit cells. The I-V characteristics were measured to investigate the heterojunction quality and to obtain the effective values of the electronics parameters that control the heterojunction. The linear I-V curve in Fig. 5(a) shows good ohmic contact characteristics between a pair of Pt electrodes deposited on CuxIn1 xO thin films, indicating no potential barrier heights. Hence, the observed rectification characteristics originate from the p-Si/n-CuxIn1 xO heterojunction and not from the semiconductor/metal contacts. Fig. 5 (b) shows the I-V characteristics of the p-Si/n-CuxIn1–xO heterojunction measured at room temperature in the dark and under illumination in the voltage range, 10 V to þ10 V. The inset in Fig. 5(b) shows the schematic diagram of the fabricated p-Si/nCuxIn1 xO heterojunction. The device showed distinct non-linear and rectifying characteristics both in the dark and under
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
49
enhanced charge transfer between the end layers due to the favorable band alignment of Si/CuO/In2O3 layer system, which is crucial for the photoresponse. It should be noted that this qualitative analysis neglects the effects of band offset at the junction, and further work is needed to understand this heterojunction interface, which is currently underway. The device characteristics are highlighted when the I-V data are plotted in semi-logarithmic scales as shown in Fig. 7(a), where the magnitude of current (|I|) is used to present I-V plot on the log scale. The I-V characteristics of the heterojunction can be analyzed by the following relation, [8,18,19].
⎡ ⎤ ⎛ q(V − IRs ) ⎞ ⎟ − 1⎥ I = Is⎢ exp⎜ ⎝ ⎠ ⎣ ⎦ nkT
(1)
where, ‘Is’ is the reverse saturation current given by:
⎛ −qφb ⎞ ⎟ IS = AA*T2exp⎜ ⎝ kT ⎠
(2)
where, q, V, Rs, n, k, T, A, A* and φb are the electronic charge, applied bias voltage, series resistance, ideality factor, Boltzmann constant, absolute temperature, rectifier contact area, effective Richardson constant and apparent barrier height, respectively. The values of ‘n’ and ‘ φb ’ can be obtained from the slope and the intercept of ln I vs V plot in the forward bias condition, respectively, using the following expressions obtained from Eqs. (1) and (2),
n=
q ⎛ dV ⎞ ⎟ ⎜ kT ⎝ d(ln I ) ⎠
φb =
Fig. 5. (a) Ohmic contact characteristics of Pt electrodes on CuxIn1 xO thin films, (b) linear I-V characteristics of p-Si/n-CuxIn1–xO heterojunction measured in the dark and under illumination. (Insets shows the corresponding device structures).
illumination and the current increasing rapidly under forward biases and effectively blocked under reverse biases. It can be seen that under reverse bias conditions, no significant change in the current takes place after illumination, whereas under forward bias the current increases drastically. A small magnitude of the leakage current and the bias independence of reverse current plateau confirm the good rectification behavior. Also, the current at a given voltage under illumination is much higher than under dark condition, indicating that the illumination increased the generation of electron-hole pairs. The forward threshold voltage obtained by extrapolating the linear fit in the high current regimes at I ¼0, was found to decrease from 3.4 V in the dark to 2.2 V under illumination. The rectification ratio (IF/IR, IF and IR are forward and reverse currents, respectively) at 75 V increases from 6.7 103 in the dark to 1.1 104 under illumination. The rectifying behavior of the p-Si/n-CuxIn1 xO heterojunction can be explained by the energy band structure. The tentative band diagram for p-Si/n-CuxIn1 xO heterojunction under the thermal equilibrium conditions illustrated in Fig. 6 was constructed based on the electron affinities (χ) and the band gap (Eg) values. The values of Eg and χ for Si are 1.12 eV and 4.05 eV, for CuO is 1.35 eV and 4.07 eV, and for In2O3 are 3.6 eV and 4.5 eV, respectively [15– 17]. Initially, when CuxIn1 xO films are deposited on the surface of p-Si the electrons will flow from CuxIn1 xO into Si at the interface due to the higher fermi energy level of In2O3. The flow of electrons stops when the fermi levels become equal, leading to the junction formation as shown in the figure. The CuO acting as an interfacial layer reduces the energy barrier between Si and In2O3, facilitating
kT ⎛ AA*T2 ⎞ ⎟ ln⎜ q ⎝ IS ⎠
(3)
(4)
The values of n and φb obtained using the above equations are given in Table 1. According to Sah-Noyce-Shockley theory, the value of n for an ideal p-n junction should be 1 at a low forward voltage and up to 2 at a higher voltage [20]. In the present case, the value of n greater than 2 clearly indicates that the junction is far from ideal. The deviation of n from unity may be attributed to various factors such as, barrier height inhomogeneity, current mechanism in the structure like tunneling and generation-recombination currents within the space-charge region, presence of surface or interface states, and series resistance [21,22]. Also, the higher n indicates that the current-transport mechanism of the heterojunction could not be explained by thermionic emission only and could be due to the presence of non-thermionic secondary transport mode at the interface. The charge transport mechanism which influences the device characteristics was investigated using the forward bias log I vs log V plot as shown in Fig. 7(b). The current was found to increase with increasing the applied voltage, and depending on the applied voltage the plot shows two distinct regions separated by a transition segment both in the dark and under illumination. The slopes of the curves both in the dark and under illumination varies from 1.82 to 3.64, indicating that the current-transport mechanism of our device is apparently dominated by trap-assisted space charge limited current (SCLC) [23]. The SCLC mechanism is normally observed in the wide bandgap semiconductors and is due to the presence of trapping centers in the semiconductors [24]. Also, the increase in the values of the slope after light illumination indicates the high carrier injection, leading to the high current gain of the heterojunction. Series resistance (Rs) is one of the parameter responsible for the non-linear behavior of the heterojunction. Generally, the forward bias I-V characteristics are linear in the semi-logarithmic scale at low voltages, but deviates considerably from linearity due
50
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
Fig. 6. Schematic illustration of the energy band diagram of p-Si/n-CuxIn1–xO heterojunction.
Table 1 Electrical parameters of p-Si/n-CuxIn1 xO heterojunction determined from I-V characteristics by different methods. Conditions
I-V method n
Dark Illumination
2.67 2.44
Φb (eV)
0.80 0.76
Cheung-Cheung
Norde's
dV/dln I
Φb (eV)
Rs (Ω)
0.74 0.77
423.0 397.4
H(I)
n
Rs (Ω)
Φb (eV)
Rs (Ω)
2.32 1.59
229.7 117.3
0.86 0.79
273.7 229.9
where, ‘IRs’ is the voltage drop across the series resistance of the heterojunction diode. The values of Rs, n and φb can be calculated using the following equations,
⎛ kT ⎞ dV = IRs + n⎜ ⎟ d(ln I ) ⎝ q⎠
(6)
and,
⎛ nKT ⎞ ⎛ I ⎞ ⎟ H (I ) = V − ⎜ ⎟ln⎜ ⎝ q ⎠ ⎝ AA*T2 ⎠
(7)
where,
H (I ) = IRs + nφb
Fig. 7. (a) Semi-logarithmic scales I-V characteristics of p-Si/n-CuxIn1 xO heterojunction measured in the dark and under illumination, and (b) log I vs log V plot of p-Si/n-CuxIn1 xO heterojunction.
to the effects of interfacial layer and Rs. The value of Rs can be calculated using the method developed by Cheung and Cheung [25], in the high voltage range where the forward bias I-V characteristics are not linear. According to Cheung-Cheung method, the forward bias I-V characteristics due to the thermionic emission having the series resistance can be expressed as, [25]
⎡ q(V − IRs ) ⎤ I = Is exp⎢ ⎥ ⎣ ⎦ nkT
(5)
(8)
The dV/d(ln I) vs I and H(I) vs I plots were given in Fig. 8. The slope and the y-intercept of the dV/d(ln I) vs I plot (Fig. 8(a)) gives the values of Rs and n, respectively. Similarly, from H(I) vs I plot (Fig. 8(b)) the values of Rs and φb are obtained from the slope and the y-intercept, respectively. The estimated values of n, Rs and φb are given in Table 1. Thus, it can be clearly seen that there is relative difference between the values of n obtained from the forward bias ln I-V plot and that obtained by Cheung-Cheung method (dV/d(ln I) vs I curve). This difference may be attributed to the existence of effects such as series resistance, interface states, and to the voltage drop across the interfacial layer [8]. Also, the values of Rs obtained from the two different plots of Cheung-Cheung method are in agreement with each other, showing the consistency of the Cheung-Cheung method. Besides Cheung-Cheung method, the Rs of the heterojunction can also be calculated by Norde's function. Norde method is an alternative method to determine the values of Rs and φb , and this method was proposed since high Rs can hinder an accurate evaluation of the barrier height from the standard ln I-V plot [26]. The modified Norde's function was expressed as, [27].
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
φb = F (VO ) +
Rs =
VO kT − γ q
kT (γ − n) qIO
51
(10)
(11)
where, F(V0), is the minimum point of F(V) and V0 is the corresponding voltage. The Rs and φb values calculated from the Norde plot are given in Table 1. The difference in the Rs values calculated from Cheung-Cheung method and Norde plot is attributed to the voltage regime used for the estimation, ie., Cheung-Cheung model is applicable in the high voltage region whereas Norde plot is applied to the full voltage range of the forward bias ln I-V characteristics. Hence, the significant photoresponse characteristics of p-Si/n-CuxIn1 xO heterojunction clearly indicate the remarkable potential for photodetector applications, and it is anticipated that our results emphasize the importance of processing novel materials for low-cost electronics and optoelectronics applications.
4. Conclusion
Fig. 8. The forward bias (a) dV/d(ln I) vs I and (b) H(I) vs I plot of p-Si/n-CuxIn1 xO heterojunction.
CuxIn1 xO nanocrystalline thin films were deposited on Si substrate by sol-gel spin coating technique and the device properties of the fabricated p-Si/n-CuxIn1 xO heterojunction diode was investigated. Structural and morphological analysis by XRD, Raman and FESEM confirmed the formation of monoclinic CuO and cubic In2O3 exhibiting compact surface morphology. Optical analysis by UV–vis absorption spectra showed a sharp absorption band around 300–425 nm along with an absorption tail in the visible region while the PL studies showed the defect-related emission peaks in the visible region. The I-V characteristics of the p-Si/n-CuxIn1 xO heterojunction showed good rectifying behavior, and under light illumination the forward current increases while the reverse current hardly changed. Conventional I-V, CheungCheung and Norde methods were used in order to determine the n, φb , and Rs values for the heterojunction. The values of n and φb obtained from the different methods are in good agreement with each other, whereas the values of Rs obtained from CheungCheung and Norde's functions differs, which is attributed to the voltage regimes used for the estimation of Rs.
Acknowledgments This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF2015R1A1A1A05027848).
References Fig. 9. Norde plot of F(V) vs V for p-Si/n-CuxIn1 xO heterojunction.
F (V ) =
VO kT ⎛ I (V ) ⎞ ⎟ − ln⎜ ⎝ A*AT2 ⎠ γ q
(9)
where, γ is the integer (dimensionless) greater than n, and q is the charge of an electron. I(V) is the current obtained from the I-V curve. The Norde plot for the p-Si/n-CuxIn1 xO heterojunction is shown in Fig. 9. Once the minimum of F(V) vs V graph is obtained, the values of Rs and φb can be obtained using the following equations,
[1] O.J. Gregory, I.M. Tougas, M. Amani, E.E. Crisman, Thermoelectric properties and microstructure of Cu-In-O thin films, ACS Comb. Sci. 15 (2013) 580–584. [2] Z.H. Hafeezullah, J. Yamani, A. Iqbal, A. Hakeem Qurashi, Rapid sonochemical synthesis of In2O3 nanoparticles their doping optical, electrical and hydrogen gas sensing properties, J. Alloy. Compd. 616 (2014) 76–80. [3] L. Yang, D. Chu, L. Wang, Porous hexapod CuO nanostructures: precursormediated fabrication, characterization, and visible-light induced photocatalytic degradation of phenol, Mater. Lett. 160 (2015) 246–249. [4] F. Li, X. Liu, Q. Zhang, T. Kong, H. Jin, Fabrication and photocatalytic property of CuO nanosheets via a facile solution route, Cryst. Res. Technol. 47 (2012) 1140–1147. [5] L.N. Lau, N.B. Ibrahim, H. Baqiah, Influence of precursor concentration on the structural, optical and electrical properties of indium oxide thin film prepared by a sol-gel method, Appl. Surf. Sci. 345 (2015) 355–359. [6] M. Seetha, P. Meena, D. Mangalaraj, Y. Masuda, K. Senthil, Synthesis of indium
52
[7]
[8]
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
oxide cubic crystals by modified hydrothermal route for application in room temperature flexible ethanol sensors, Mater. Chem. Phys. 133 (2012) 47–54. K.P. Bhandari, P. Koirala, N.R. Paudel, R.R. Khanal, A.B. Phillips, Y. Yan, R. W. Collins, M.J. Heben, R.J. Ellingson, Iron pyrite nanocrystal film serves as a copper-free back contact for polycrystalline CdTe thin film solar cells, Sol. Energy Mater. Sol. Cells 140 (2015) 108–114. S. Aksoy, Y. Caglar, Structural transformations of TiO2 films with deposition temperature and electrical properties of nanostructure n-TiO2/p-Si heterojunction diode, J. Alloy. Compd. 613 (2014) 330–337. S. Kahraman, S. Cetinkaya, H.A. Cetinkara, H.S. Guder, A comparative study of Cu2ZnSnS4 thin films growth by successive Ionic layer adsorption-reaction and sol-gel methods, Thin Solid Films 550 (2014) 36–39. B.G. Nair, G.S. Okram, J. Naduvath, T. Shripathi, A. Fatima, T. Patel, R. Jacob, K. Keerthi, S.K. Remillard, R.R. Philip, Low temperature thermopower and electrical conductivity in highly conductive CuInO2 thin films, J. Mater. Chem. C 2 (2014) 6765–6772. R. Shabu, A. Moses Ezhil Raj, C. Sanjeeviraja, C. Ravidhas, Assessment of CuO thin films for its suitability as window absorbing layer in solar cell fabrications, Mater. Res. Bull. 68 (2015) 1–8. G. Liu, Synthesis, characterization of In2O3 nanocrystals and their photoluminescence property, Int. J. Electrochem. Sci. 6 (2011) 2162–2170. M. Lamri Zeggar, L. Chabane, M.S. Aida, N. Attaf, N. Zebbar, Solution flow rate influence on properties of copper oxide thin films deposited by ultrasonic spray pyrolysis, Mater. Sci. Semicond. Process. 30 (2015) 645–650. S. Elouali, L.G. Bloor, R. Binions, I.P. Parkin, C.J. Carmalt, J.A. Darr, Gas sensing with nano-indium oxides (In2O3) prepared via continuous hydrothermal flow synthesis, Langmuir 28 (2012) 1879–1885. M.A. Qaeed, K. Ibrahim, K.M.A. Saron, M.S. Mukhlif, A. Ismail, Nezar G. Elfadill, Khaled M. Chahrour, Q.N. Abdullah, K.S.A. Aldroobi, New issue of GaN nanoparticles solar cell, Curr. Appl. Phys. 15 (2015) 499–503. A. Zainelabdin, S. Zaman, G. Amin, O. Nur, M. Willander, Optical and current
[17]
[18] [19] [20] [21]
[22]
[23]
[24]
[25] [26]
[27]
transport properties of CuO/ZnO nanocoral p-n heterostructure hydrothermally synthesized at low temperature, Appl. Phys. A 108 (2012) 921–928. B. Wang, X. Jina, Z.B. Ouyang, Synthesis, characterization and cathodoluminescence of self-assembled 1D ZnO/In2O3 nano-heterostructures, CrystEngComm 14 (2012) 6888–6903. S.M. Sze, Physics of Semiconductor Devices, Second edition, John Wiley & Sons, New York, 1981. E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, Second edition, Clarendon, Oxford, 1988. C.T. Sah, R.N. Noyce, W. Shockley, Carrier generation and recombination in P-N junctions and P-N junction characteristics, Proc. IRE 45 (1957) 1228–1243. N.N. Halder, P. Biswas, S. Kundu, P. Banerji, Au/p-Si Schottky junction solar cell: effect of barrier height modification by InP quantum dots, Sol. Energy Mater. Sol. Cells 132 (2015) 230–236. A. Jana, S. Ghosh, P. Sujatha Devi, N.R. Bandyopadhyay, M. Ray, Tunable charge transport through n-ZnO nanorods on Au coated macroporous p-Si, J. Mater. Chem. C 2 (2014) 9613–9619. M. Zheng, Q.X. Zhu, X.Y. Li, X.M. Li, R.K. Zheng, Magnetic and electrical properties of three dimensional (La,Pr,CA)MnO3 nanofilm/ZnO nanorod p-n junctions, RSC Adv. 4 (2014) 32622–32627. N. Sinha, B. Roul, S. Mukundan, G. Chandan, L. Mohan, V.M. Jali, S. B. Krupanidhi, Growth and electrical transport properties of InGaN/GaN heterostructures grown by PAMBE, Mater. Res. Bull. 61 (2015) 539–543. S.K. Cheung, N.W. Cheung, Extraction of Schottky diode parameters from forward current-voltage characteristics, Appl. Phys. Lett. 49 (1986) 85–89. S. Aksoy, Y. Caglar, Effect of ambient temperature on electrical properties of nanostructure n-ZnO/p-Si heterojunction diode, Superlattice Microstruct. 51 (2012) 613–625. H. Norde, A modified forward I–V plot for Schottky diodes with high series resistance, J. Appl. Phys. 50 (1979) 5052–5053.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Photoresponse characteristics of p-Si/n-CuxIn1 xO heterojunction diode prepared by sol-gel spin coating K. Mageshwari, Jinsub Park n Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 10 October 2015 Received in revised form 31 December 2015 Accepted 5 February 2016
In the present work, we report on the fabrication and detailed electrical characterization of p-Si/nCuxIn1 xO heterojunction prepared via the deposition of nanocrystalline CuxIn1 xO thin films on p-type silicon substrate by sol-gel method using spin coating technique. X-ray diffraction and Raman spectroscopy results revealed the polycrystalline nature of CuxIn1 xO thin films consisting diffractions peaks and vibration modes, respectively, corresponding to CuO and In2O3. Field-emission scanning electron microscopy showed compact surface morphology while UV–vis absorption spectra exhibited sharp absorption between 300 and 425 nm along with a long tail extending in the visible region. The currentvoltage (I-V) characteristics of the fabricated heterojunction demonstrated obvious rectifying behavior in the dark and under illumination. The heterojunction exhibited low reverse leakage current ( 10 7), and upon illumination, the forward current and rectification ratio of the junction was improved while the forward threshold voltage lowered. By fitting the experimental data we have observed that the forward current conduction is dominated by the space charge limited current mechanism. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Heterojunction CuxIn1 xO thin films Sol-gel spin coating Electrical properties Ideality factor Series resistance
1. Introduction Transparent conducting oxides (TCOs) are an important group of materials used in optoelectronics due to their unique properties combining the low electrical resistance (typicallyo10 3 Ω cm) with the high optical transparency (4 85%) in the visible range. In particular, TCOs based on Cu-In-O materials system have gathered special interest for their applications in flat panel displays, solar cells, thermoelectrics and thin films transistors [1,2]. Copper oxide (CuO) is a p-type semiconductor having a narrow energy band gap of 1.2–1.5 eV, high optical absorption in the visible region and environment friendly properties like non-toxicity and low cost; while indium oxide (In2O3) is a wide band gap material ( 3.7 eV) possessing n-type conductivity, high visible transparency and free carrier mobility [3–6]. By combining both CuO and InO either p-type or n-type character could be produced in the Cu-In-O system by tuning the film composition [1]. Although Cu-In-O system has been extensively employed in various device applications satisfactory understanding of the alloy in all details has still not been achieved. Assessing the p-n junction characteristics is an efficient way to determine the semiconductor behavior of the materials. Nevertheless, less attention has been paid to the electrical characteristics of Cu-In-O system, and the reports on their n
Corresponding author. E-mail address: [email protected] (J. Park).
http://dx.doi.org/10.1016/j.mssp.2016.02.003 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
electrical properties are rather limited. Under these perspectives, herein we made a noteworthy and maiden attempt by depositing n-CuxIn1 xO thin films on p-Si substrate, and demonstrated the current-voltage (I-V) characteristics of the heterojunction in the dark and under photon illumination. Up to now, remarkable progresses have been made on the thin film deposition technologies, and a plethora of physical and chemical methods have been developed. Compared with the physical protocols, chemical protocols are recognized as the desired choice for the thin film preparation as they offer a potentially low-cost and scalable manufacturing method for the large area deposition and roll-to-roll processing even on flexible substrates [7]. Owing to its simplicity, homogeneity, low crystallization temperature, and easy control of the film thickness and composition, sol-gel spin coating method has become a competitive technique for the thin film fabrication [5,8]. Accordingly, in the present work, we employed sol-gel spin coating for the deposition of nanocrystalline CuxIn1 xO thin films on Si substrate with the final goal of understanding the electrical characteristics of the heterojunction. X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FESEM), UV–vis absorption spectroscopy and photoluminescence (PL) measurements were used to investigate the structural, morphological and optical properties of CuxIn1 xO thin films. The electrical properties of p-Si/n-CuxIn1 xO heterojunction were investigated by dark and photocurrent-voltage characteristics and the electronics parameters controlling the heterojunction, such as barrier height ( φb ), ideality factor (n) and
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
47
series resistance (Rs) was obtained by combining the modified Norde's function with conventional I-V and Cheung-Cheung methods. The fabricated p-Si/n-CuxIn1 xO heterojunction showed well-defined rectifying I-V characteristics with low reverse leakage current ( 10 7), excellent rectification ratio ( 1.1 104) and a high ideality factor (n 42).
2. Experimental details CuxIn1 xO thin films were deposited on p-Si substrate by solgel method using spin coating technique. Briefly, a solution was prepared by dissolving copper (II) acetate hydrate and indium (III) acetate with a weight ratio of 20:80 in an equal molar ratio (1.4 M) of 2-methoxyethanol and monoethanolamine. The solution was first stirred at 65 °C for 1 h and then stirred continuously for 24 h at room temperature. Finally, the solution was aged for 24 h. The precursor solution was deposited in two steps: spin coating at 600 rpm for 10 s and 2000 rpm for 20 s, followed by drying at 200 °C for 10 min on a hot plate. The deposition was repeated for 5 times and then the films were post-annealed at 500 °C for 2 h in air. The typical CuxIn1–xO thin film prepared for 5 spin coating cycles has an average thickness of around 120 nm. Under identical deposition conditions CuxIn1 xO thin films were deposited on the glass substrate for the structural, morphological and optical analysis. Structural analysis of CuxIn1 xO thin films was carried out using XRD (Rigaku D/MAX/2500/PC X-ray diffractometer) equipped with Cu Kα radiation (λ ¼ 0.154 nm), and Raman spectroscopy (750i/ELT1000 micro confocal Raman spectrophotometer, Excitation wavelength¼532 nm).The morphology of CuxIn1 xO thin films was examined by Hitachi S-4800 FESEM. The optical absorption properties of CuxIn1 xO thin films were analyzed by UV– vis absorption spectra recorded using a Cary UV/vis/NIR spectrophotometer (λ ¼300–800 nm, sampling area 2.5 2.5 cm2). Photoluminescence (PL) measurements were performed at room temperature using a He-Cd laser (Excitation wavelength ¼ 325 nm, Power ¼5 mW). The I-V measurements were carried out at roomtemperature using a probe station system equipped with Keithley series 2400 digital source meter. Platinum (Pt) metal contacts were deposited through a shadow mask onto p-Si and n-CuxIn1 xO to form bottom and top electrodes, respectively. The photoresponse behavior of the heterojunction was studied in the dark and under illuminated conditions (using a 140 W Xenon lamp).
3. Results and discussion The crystal structure of CuxIn1–xO thin films was investigated by XRD, and the corresponding diffraction pattern is shown in Fig. 1. All the diffraction peaks in the XRD pattern could be indexed to either CuO or In2O3, demonstrating the co-existence of CuO and In2O3 characteristics in Cu-In-O system. The diffraction peaks of CuO (002), (111), ( 112) and ( 113) planes matches with the monoclinic crystal structure of CuO (JCPDS card no. 45-0937). Similarly, the In2O3 characteristics peaks (221), (222), (332) and (440) planes matches with the cubic structure of In2O3 (JCPDS card no. 06-416). No other characteristic peaks of impurities such as Cu (OH)2, Cu2O and In(OH)3 were observed, which suggest that the CuxIn1 xO thin films are purely composed of CuO and In2O3. The intensity of the diffraction peaks corresponding to In2O3 is significantly higher when compared to CuO peaks, indicating the predominance of In2O3 in CuxIn1 xO thin films. The mean crystallite size of CuO and In2O3 nanoparticles were estimated for the predominant (002) and (222) planes, respectively, using the well-
Fig. 1. XRD spectrum of CuxIn1 xO thin films.
Fig. 2. SEM images of CuxIn1 xO thin films.
known Scherrer's formula [9], and found to be 12 nm and 8 nm, respectively. The surface morphology of CuxIn1 xO thin films analyzed by FESEM is shown in Fig. 2. The films exhibited a uniform coverage over the entire substrate and possessed a compact morphology without any cracks. The higher magnification image in the inset clearly shows the denser packing of the nanoparticles, which are beneficial as they can reduce the grain boundary effect thereby enhancing the charge carrier mobility, leading to high conductivity [10]. The electronic band structure and the electronic transition within the CuxIn1 xO thin films were investigated by UV–vis optical absorption and PL measurements. The UV–vis absorption spectrum of CuxIn1 xO thin films showed sharp absorption band around 300–425 nm along with a long absorption tail in the visible region. This long absorption tail entering the visible region indicates the visible light absorption capability of CuxIn1 xO thin films in addition to the UV light absorption. The optical band gap of CuxIn1 xO thin films was estimated from (αhυ)2 vs hυ plot based on the direct transition as shown in Fig. 3(b). The band gap energy was determined by extrapolating the straight-line portion to the energy axis for zero absorption co-efficient, and found to be 2.9 eV. Compared with the bulk band gap value of In2O3 (Eg 3.6 eV) the estimated lower band gap value can be attributed to the influence of CuO in the mixed CuO-In2O3 system. The PL spectrum of CuxIn1 xO thin films (Fig. 3(c)) show several emission peaks in the visible region (409, 535 and 775 nm). The emissions
48
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
Fig. 4. Raman spectra of CuxIn1 xO thin films on (a) silicon, and (b) glass substrate.
Fig. 3. (a) UV–vis absorption spectra, (b) (αhυ)2 vs hυ plot and (c) PL spectrum of CuxIn1 xO thin films.
in the visible regions have been largely considered to be associated with the intrinsic or extrinsic defects in the semiconductor oxides. The appropriate assignment of PL bands is little complex since the origin of luminescence in CuO and In2O3 are still unclear. The large variation in the electron and hole effective mass limits the precise estimation of PL emissions in CuO [11]. The PL emissions in In2O3 are generally attributed to the presence of vacancies (indium or oxygen) or defects (interstitial indium or antisite oxygen) [6]. Cubic In2O3 has an oxygen-deficient fluorite structure with the
twice the unit cell edge of the corresponding fluorite cell and with ¼ of the anions missing in an ordered way, which would induce the formation of new energy levels in the band gap region of In2O3 [12]. Hence, the emissions occur due to the radiative recombination of photogenerated hole with an electron occupying the oxygen vacancy, which are commonly referred as a deep level or trap state emissions due to the oxygen vacancies. The chemical structure of CuxIn1 xO thin films deposited on the silicon as well as on the glass substrate was analyzed by Raman spectra, and is shown in Fig. 4. The spectra show broad Raman peaks at 291, 478 and 521 cm 1 along with some minor peaks at 369 and 608 cm 1. The peak at 521 cm 1 is from the Si substrate. The peaks at 291 and 608 cm 1 corresponds to the Ag and B2g modes of monoclinic CuO, respectively, which are attributed to the vibrations of oxygen atoms in CuO [13]. The peaks observed at 369 cm 1 (stretching vibrations of In-O-In linkages) and 478 cm 1 (octahedral stretching vibrations ν(InO6)) corresponds to the body-centered cubic In2O3 [14]. The vibrational modes of CuO and In2O3 exhibited slight shift due to the disorder caused in their unit cells. The I-V characteristics were measured to investigate the heterojunction quality and to obtain the effective values of the electronics parameters that control the heterojunction. The linear I-V curve in Fig. 5(a) shows good ohmic contact characteristics between a pair of Pt electrodes deposited on CuxIn1 xO thin films, indicating no potential barrier heights. Hence, the observed rectification characteristics originate from the p-Si/n-CuxIn1 xO heterojunction and not from the semiconductor/metal contacts. Fig. 5 (b) shows the I-V characteristics of the p-Si/n-CuxIn1–xO heterojunction measured at room temperature in the dark and under illumination in the voltage range, 10 V to þ10 V. The inset in Fig. 5(b) shows the schematic diagram of the fabricated p-Si/nCuxIn1 xO heterojunction. The device showed distinct non-linear and rectifying characteristics both in the dark and under
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
49
enhanced charge transfer between the end layers due to the favorable band alignment of Si/CuO/In2O3 layer system, which is crucial for the photoresponse. It should be noted that this qualitative analysis neglects the effects of band offset at the junction, and further work is needed to understand this heterojunction interface, which is currently underway. The device characteristics are highlighted when the I-V data are plotted in semi-logarithmic scales as shown in Fig. 7(a), where the magnitude of current (|I|) is used to present I-V plot on the log scale. The I-V characteristics of the heterojunction can be analyzed by the following relation, [8,18,19].
⎡ ⎤ ⎛ q(V − IRs ) ⎞ ⎟ − 1⎥ I = Is⎢ exp⎜ ⎝ ⎠ ⎣ ⎦ nkT
(1)
where, ‘Is’ is the reverse saturation current given by:
⎛ −qφb ⎞ ⎟ IS = AA*T2exp⎜ ⎝ kT ⎠
(2)
where, q, V, Rs, n, k, T, A, A* and φb are the electronic charge, applied bias voltage, series resistance, ideality factor, Boltzmann constant, absolute temperature, rectifier contact area, effective Richardson constant and apparent barrier height, respectively. The values of ‘n’ and ‘ φb ’ can be obtained from the slope and the intercept of ln I vs V plot in the forward bias condition, respectively, using the following expressions obtained from Eqs. (1) and (2),
n=
q ⎛ dV ⎞ ⎟ ⎜ kT ⎝ d(ln I ) ⎠
φb =
Fig. 5. (a) Ohmic contact characteristics of Pt electrodes on CuxIn1 xO thin films, (b) linear I-V characteristics of p-Si/n-CuxIn1–xO heterojunction measured in the dark and under illumination. (Insets shows the corresponding device structures).
illumination and the current increasing rapidly under forward biases and effectively blocked under reverse biases. It can be seen that under reverse bias conditions, no significant change in the current takes place after illumination, whereas under forward bias the current increases drastically. A small magnitude of the leakage current and the bias independence of reverse current plateau confirm the good rectification behavior. Also, the current at a given voltage under illumination is much higher than under dark condition, indicating that the illumination increased the generation of electron-hole pairs. The forward threshold voltage obtained by extrapolating the linear fit in the high current regimes at I ¼0, was found to decrease from 3.4 V in the dark to 2.2 V under illumination. The rectification ratio (IF/IR, IF and IR are forward and reverse currents, respectively) at 75 V increases from 6.7 103 in the dark to 1.1 104 under illumination. The rectifying behavior of the p-Si/n-CuxIn1 xO heterojunction can be explained by the energy band structure. The tentative band diagram for p-Si/n-CuxIn1 xO heterojunction under the thermal equilibrium conditions illustrated in Fig. 6 was constructed based on the electron affinities (χ) and the band gap (Eg) values. The values of Eg and χ for Si are 1.12 eV and 4.05 eV, for CuO is 1.35 eV and 4.07 eV, and for In2O3 are 3.6 eV and 4.5 eV, respectively [15– 17]. Initially, when CuxIn1 xO films are deposited on the surface of p-Si the electrons will flow from CuxIn1 xO into Si at the interface due to the higher fermi energy level of In2O3. The flow of electrons stops when the fermi levels become equal, leading to the junction formation as shown in the figure. The CuO acting as an interfacial layer reduces the energy barrier between Si and In2O3, facilitating
kT ⎛ AA*T2 ⎞ ⎟ ln⎜ q ⎝ IS ⎠
(3)
(4)
The values of n and φb obtained using the above equations are given in Table 1. According to Sah-Noyce-Shockley theory, the value of n for an ideal p-n junction should be 1 at a low forward voltage and up to 2 at a higher voltage [20]. In the present case, the value of n greater than 2 clearly indicates that the junction is far from ideal. The deviation of n from unity may be attributed to various factors such as, barrier height inhomogeneity, current mechanism in the structure like tunneling and generation-recombination currents within the space-charge region, presence of surface or interface states, and series resistance [21,22]. Also, the higher n indicates that the current-transport mechanism of the heterojunction could not be explained by thermionic emission only and could be due to the presence of non-thermionic secondary transport mode at the interface. The charge transport mechanism which influences the device characteristics was investigated using the forward bias log I vs log V plot as shown in Fig. 7(b). The current was found to increase with increasing the applied voltage, and depending on the applied voltage the plot shows two distinct regions separated by a transition segment both in the dark and under illumination. The slopes of the curves both in the dark and under illumination varies from 1.82 to 3.64, indicating that the current-transport mechanism of our device is apparently dominated by trap-assisted space charge limited current (SCLC) [23]. The SCLC mechanism is normally observed in the wide bandgap semiconductors and is due to the presence of trapping centers in the semiconductors [24]. Also, the increase in the values of the slope after light illumination indicates the high carrier injection, leading to the high current gain of the heterojunction. Series resistance (Rs) is one of the parameter responsible for the non-linear behavior of the heterojunction. Generally, the forward bias I-V characteristics are linear in the semi-logarithmic scale at low voltages, but deviates considerably from linearity due
50
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
Fig. 6. Schematic illustration of the energy band diagram of p-Si/n-CuxIn1–xO heterojunction.
Table 1 Electrical parameters of p-Si/n-CuxIn1 xO heterojunction determined from I-V characteristics by different methods. Conditions
I-V method n
Dark Illumination
2.67 2.44
Φb (eV)
0.80 0.76
Cheung-Cheung
Norde's
dV/dln I
Φb (eV)
Rs (Ω)
0.74 0.77
423.0 397.4
H(I)
n
Rs (Ω)
Φb (eV)
Rs (Ω)
2.32 1.59
229.7 117.3
0.86 0.79
273.7 229.9
where, ‘IRs’ is the voltage drop across the series resistance of the heterojunction diode. The values of Rs, n and φb can be calculated using the following equations,
⎛ kT ⎞ dV = IRs + n⎜ ⎟ d(ln I ) ⎝ q⎠
(6)
and,
⎛ nKT ⎞ ⎛ I ⎞ ⎟ H (I ) = V − ⎜ ⎟ln⎜ ⎝ q ⎠ ⎝ AA*T2 ⎠
(7)
where,
H (I ) = IRs + nφb
Fig. 7. (a) Semi-logarithmic scales I-V characteristics of p-Si/n-CuxIn1 xO heterojunction measured in the dark and under illumination, and (b) log I vs log V plot of p-Si/n-CuxIn1 xO heterojunction.
to the effects of interfacial layer and Rs. The value of Rs can be calculated using the method developed by Cheung and Cheung [25], in the high voltage range where the forward bias I-V characteristics are not linear. According to Cheung-Cheung method, the forward bias I-V characteristics due to the thermionic emission having the series resistance can be expressed as, [25]
⎡ q(V − IRs ) ⎤ I = Is exp⎢ ⎥ ⎣ ⎦ nkT
(5)
(8)
The dV/d(ln I) vs I and H(I) vs I plots were given in Fig. 8. The slope and the y-intercept of the dV/d(ln I) vs I plot (Fig. 8(a)) gives the values of Rs and n, respectively. Similarly, from H(I) vs I plot (Fig. 8(b)) the values of Rs and φb are obtained from the slope and the y-intercept, respectively. The estimated values of n, Rs and φb are given in Table 1. Thus, it can be clearly seen that there is relative difference between the values of n obtained from the forward bias ln I-V plot and that obtained by Cheung-Cheung method (dV/d(ln I) vs I curve). This difference may be attributed to the existence of effects such as series resistance, interface states, and to the voltage drop across the interfacial layer [8]. Also, the values of Rs obtained from the two different plots of Cheung-Cheung method are in agreement with each other, showing the consistency of the Cheung-Cheung method. Besides Cheung-Cheung method, the Rs of the heterojunction can also be calculated by Norde's function. Norde method is an alternative method to determine the values of Rs and φb , and this method was proposed since high Rs can hinder an accurate evaluation of the barrier height from the standard ln I-V plot [26]. The modified Norde's function was expressed as, [27].
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
φb = F (VO ) +
Rs =
VO kT − γ q
kT (γ − n) qIO
51
(10)
(11)
where, F(V0), is the minimum point of F(V) and V0 is the corresponding voltage. The Rs and φb values calculated from the Norde plot are given in Table 1. The difference in the Rs values calculated from Cheung-Cheung method and Norde plot is attributed to the voltage regime used for the estimation, ie., Cheung-Cheung model is applicable in the high voltage region whereas Norde plot is applied to the full voltage range of the forward bias ln I-V characteristics. Hence, the significant photoresponse characteristics of p-Si/n-CuxIn1 xO heterojunction clearly indicate the remarkable potential for photodetector applications, and it is anticipated that our results emphasize the importance of processing novel materials for low-cost electronics and optoelectronics applications.
4. Conclusion
Fig. 8. The forward bias (a) dV/d(ln I) vs I and (b) H(I) vs I plot of p-Si/n-CuxIn1 xO heterojunction.
CuxIn1 xO nanocrystalline thin films were deposited on Si substrate by sol-gel spin coating technique and the device properties of the fabricated p-Si/n-CuxIn1 xO heterojunction diode was investigated. Structural and morphological analysis by XRD, Raman and FESEM confirmed the formation of monoclinic CuO and cubic In2O3 exhibiting compact surface morphology. Optical analysis by UV–vis absorption spectra showed a sharp absorption band around 300–425 nm along with an absorption tail in the visible region while the PL studies showed the defect-related emission peaks in the visible region. The I-V characteristics of the p-Si/n-CuxIn1 xO heterojunction showed good rectifying behavior, and under light illumination the forward current increases while the reverse current hardly changed. Conventional I-V, CheungCheung and Norde methods were used in order to determine the n, φb , and Rs values for the heterojunction. The values of n and φb obtained from the different methods are in good agreement with each other, whereas the values of Rs obtained from CheungCheung and Norde's functions differs, which is attributed to the voltage regimes used for the estimation of Rs.
Acknowledgments This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF2015R1A1A1A05027848).
References Fig. 9. Norde plot of F(V) vs V for p-Si/n-CuxIn1 xO heterojunction.
F (V ) =
VO kT ⎛ I (V ) ⎞ ⎟ − ln⎜ ⎝ A*AT2 ⎠ γ q
(9)
where, γ is the integer (dimensionless) greater than n, and q is the charge of an electron. I(V) is the current obtained from the I-V curve. The Norde plot for the p-Si/n-CuxIn1 xO heterojunction is shown in Fig. 9. Once the minimum of F(V) vs V graph is obtained, the values of Rs and φb can be obtained using the following equations,
[1] O.J. Gregory, I.M. Tougas, M. Amani, E.E. Crisman, Thermoelectric properties and microstructure of Cu-In-O thin films, ACS Comb. Sci. 15 (2013) 580–584. [2] Z.H. Hafeezullah, J. Yamani, A. Iqbal, A. Hakeem Qurashi, Rapid sonochemical synthesis of In2O3 nanoparticles their doping optical, electrical and hydrogen gas sensing properties, J. Alloy. Compd. 616 (2014) 76–80. [3] L. Yang, D. Chu, L. Wang, Porous hexapod CuO nanostructures: precursormediated fabrication, characterization, and visible-light induced photocatalytic degradation of phenol, Mater. Lett. 160 (2015) 246–249. [4] F. Li, X. Liu, Q. Zhang, T. Kong, H. Jin, Fabrication and photocatalytic property of CuO nanosheets via a facile solution route, Cryst. Res. Technol. 47 (2012) 1140–1147. [5] L.N. Lau, N.B. Ibrahim, H. Baqiah, Influence of precursor concentration on the structural, optical and electrical properties of indium oxide thin film prepared by a sol-gel method, Appl. Surf. Sci. 345 (2015) 355–359. [6] M. Seetha, P. Meena, D. Mangalaraj, Y. Masuda, K. Senthil, Synthesis of indium
52
[7]
[8]
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
K. Mageshwari, J. Park / Materials Science in Semiconductor Processing 46 (2016) 46–52
oxide cubic crystals by modified hydrothermal route for application in room temperature flexible ethanol sensors, Mater. Chem. Phys. 133 (2012) 47–54. K.P. Bhandari, P. Koirala, N.R. Paudel, R.R. Khanal, A.B. Phillips, Y. Yan, R. W. Collins, M.J. Heben, R.J. Ellingson, Iron pyrite nanocrystal film serves as a copper-free back contact for polycrystalline CdTe thin film solar cells, Sol. Energy Mater. Sol. Cells 140 (2015) 108–114. S. Aksoy, Y. Caglar, Structural transformations of TiO2 films with deposition temperature and electrical properties of nanostructure n-TiO2/p-Si heterojunction diode, J. Alloy. Compd. 613 (2014) 330–337. S. Kahraman, S. Cetinkaya, H.A. Cetinkara, H.S. Guder, A comparative study of Cu2ZnSnS4 thin films growth by successive Ionic layer adsorption-reaction and sol-gel methods, Thin Solid Films 550 (2014) 36–39. B.G. Nair, G.S. Okram, J. Naduvath, T. Shripathi, A. Fatima, T. Patel, R. Jacob, K. Keerthi, S.K. Remillard, R.R. Philip, Low temperature thermopower and electrical conductivity in highly conductive CuInO2 thin films, J. Mater. Chem. C 2 (2014) 6765–6772. R. Shabu, A. Moses Ezhil Raj, C. Sanjeeviraja, C. Ravidhas, Assessment of CuO thin films for its suitability as window absorbing layer in solar cell fabrications, Mater. Res. Bull. 68 (2015) 1–8. G. Liu, Synthesis, characterization of In2O3 nanocrystals and their photoluminescence property, Int. J. Electrochem. Sci. 6 (2011) 2162–2170. M. Lamri Zeggar, L. Chabane, M.S. Aida, N. Attaf, N. Zebbar, Solution flow rate influence on properties of copper oxide thin films deposited by ultrasonic spray pyrolysis, Mater. Sci. Semicond. Process. 30 (2015) 645–650. S. Elouali, L.G. Bloor, R. Binions, I.P. Parkin, C.J. Carmalt, J.A. Darr, Gas sensing with nano-indium oxides (In2O3) prepared via continuous hydrothermal flow synthesis, Langmuir 28 (2012) 1879–1885. M.A. Qaeed, K. Ibrahim, K.M.A. Saron, M.S. Mukhlif, A. Ismail, Nezar G. Elfadill, Khaled M. Chahrour, Q.N. Abdullah, K.S.A. Aldroobi, New issue of GaN nanoparticles solar cell, Curr. Appl. Phys. 15 (2015) 499–503. A. Zainelabdin, S. Zaman, G. Amin, O. Nur, M. Willander, Optical and current
[17]
[18] [19] [20] [21]
[22]
[23]
[24]
[25] [26]
[27]
transport properties of CuO/ZnO nanocoral p-n heterostructure hydrothermally synthesized at low temperature, Appl. Phys. A 108 (2012) 921–928. B. Wang, X. Jina, Z.B. Ouyang, Synthesis, characterization and cathodoluminescence of self-assembled 1D ZnO/In2O3 nano-heterostructures, CrystEngComm 14 (2012) 6888–6903. S.M. Sze, Physics of Semiconductor Devices, Second edition, John Wiley & Sons, New York, 1981. E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, Second edition, Clarendon, Oxford, 1988. C.T. Sah, R.N. Noyce, W. Shockley, Carrier generation and recombination in P-N junctions and P-N junction characteristics, Proc. IRE 45 (1957) 1228–1243. N.N. Halder, P. Biswas, S. Kundu, P. Banerji, Au/p-Si Schottky junction solar cell: effect of barrier height modification by InP quantum dots, Sol. Energy Mater. Sol. Cells 132 (2015) 230–236. A. Jana, S. Ghosh, P. Sujatha Devi, N.R. Bandyopadhyay, M. Ray, Tunable charge transport through n-ZnO nanorods on Au coated macroporous p-Si, J. Mater. Chem. C 2 (2014) 9613–9619. M. Zheng, Q.X. Zhu, X.Y. Li, X.M. Li, R.K. Zheng, Magnetic and electrical properties of three dimensional (La,Pr,CA)MnO3 nanofilm/ZnO nanorod p-n junctions, RSC Adv. 4 (2014) 32622–32627. N. Sinha, B. Roul, S. Mukundan, G. Chandan, L. Mohan, V.M. Jali, S. B. Krupanidhi, Growth and electrical transport properties of InGaN/GaN heterostructures grown by PAMBE, Mater. Res. Bull. 61 (2015) 539–543. S.K. Cheung, N.W. Cheung, Extraction of Schottky diode parameters from forward current-voltage characteristics, Appl. Phys. Lett. 49 (1986) 85–89. S. Aksoy, Y. Caglar, Effect of ambient temperature on electrical properties of nanostructure n-ZnO/p-Si heterojunction diode, Superlattice Microstruct. 51 (2012) 613–625. H. Norde, A modified forward I–V plot for Schottky diodes with high series resistance, J. Appl. Phys. 50 (1979) 5052–5053.