- Email: [email protected]
Electrical bistable properties of nonvolatile memory device based on hybrid ZCIS NCs:PMMA film
Materials Science in Semiconductor Processing 57 (2017) 105–109
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
crossmark
Electrical bistable properties of nonvolatile memory device based on hybrid ZCIS NCs:PMMA film ⁎
Xuguang Zhanga, Jianping Xua, , Xiaosong Zhanga, Shaobo Shib, Xiangguo Zhaoa, Lan Lia,
⁎
a Institute of Material Physics, Key Laboratory of Display Materials and Photoelectric Devices, Ministry of Education, Tianjin University of Technology, Tianjin 300384, People's Republic of China b School of Science, Tianjin University of Technology and Education, Tianjin 300222, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: Quaternary ZCIS NCs Nanocomposite Electrically bistable memory device Write-once-read-many-times
A write-once–read–many-times (WORM) electrical bistable device was fabricated utilizing hybrid quaternary Zn-Cu-In-S nanocrystals (ZCIS NCs) and poly(methyl methacrylate) (PMMA) as active layer. A low turn-on voltage of 1.0 V and the maximum ON/OFF current ratio of 4×103 were found from current-voltage (I-V) characteristics. The current stability in the ON and OFF state has a fluctuation loss less than 5% in the measurement duration of 104 s. The electron transport and memory mechanism are described based on the fitted I-V curves and the energy band diagrams. The conduction models in the OFF state from 0 V to 1.1 V are described as thermionic emission (TE), space-charge-limited current (SCLC) and Fowler–Nordheim (FN) tunneling, respectively. And the Ohmic conduction dominates when the device is transited to the ON state at the voltage over 1.1 V. The high-density defects in quaternary ZCIS NCs and the high barrier between ZCIS NCs and the PMMA are considered to contribute to the electron retention behavior. For device performance optimization, an additional PMMA layer is pre-deposited on the ITO substrate surface. The 50% reduction at turn-on voltage and over 50 times enhancement in ON/OFF ratio are reasonably attributed to the better film quality and decreased series resistance from the decrease of root-mean-square average surface roughness (RMS) of the ITO surface from 0.877 to 0.216 nm. The better retention capacity is considered to relate to thickened tunnel barrier by PMMA inserted layer.
1. Introduction The resistive random access memory (RRAM) based on organic/ inorganic nanocomposites have attracted considerable attention for its fast response, high data storage density, non-destructive program/ erase operations, low power consumption and simple and feasible preparation approach [1–5]. In this consideration, semiconductor nanocrystals (NCs) with high-density defects and tunable band gap were hybridized into organic materials to act as storage medium for realization of the ideal ON/OFF current ratio and low power consumption [6–10]. As described by Shin et al., the CdSe quantum dots (QDs) was labeled as the hole-trapping sites in hybrid pentacene/CdSe QDs memory device by hysteretic capacitance-voltage response and energy band structure [11]. Onlaor et al.. valuated the device performance of the hybrid PVP:ZnO NPs in which ZnO NPs is responsible for electron capture. At optimal concentration of ZnO NPs, the formation of electron transport channel between nanoparticles is prevented and maximum ON/OFF current ratio of 3×103 is obtained [9]. Environment-friendly quaternary Zn-Cu-In-S NCs (ZCIS NCs) is considered to contain high density defects, such as Zn interstitials at Cu ⁎
sites, Cu vacancies (VCu), interstitial copper (Cui), indium substituted at a copper site (InCu), sulfur vacancy (VS), etc [12], which provides a better capability for carrier trapping than binary or ternary NPs [5]. A polymer layer such as polymethylmethacrylate (PMMA) had been usually utilized as a tunneling insulator preventing the release of trapped charge carriers from active sites [2,3,5]. In this communication, the WORM device with the configuration of Al/ZCIS NCs embedded in the PMMA layer/ITO is fabricated. The high-density electron traps existing in quaternary ZCIS NCs and high energy barrier between ZCIS NCs and PMMA are cooperatively considered to have a positive effect on the better memory characteristics [8,13–15]. The fitting of the I-V curves and energy band structure are introduced to discuss electron transport process and memory mechanism. The device performance optimization for lower turn-on voltage and the better ON/ OFF ratio are performed by the pre-treatment strategy for the ITO substrate. 2. Experimental details and characterization
Corresponding authors.
http://dx.doi.org/10.1016/j.mssp.2016.09.031 Received 26 April 2016; Received in revised form 5 August 2016; Accepted 26 September 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
The synthesis of ZCIS NCs was carried out as our previous reported
Materials Science in Semiconductor Processing 57 (2017) 105–109
X. Zhang et al.
forward current was defined by a positive bias applied to the ITO substrate. All the measurements were carried out in ambient atmosphere without any encapsulation. 3. Results and discussion The XRD pattern for as-synthesized sample in Fig. 2(a) is consistent with the results addressed by Zhang et al. [17] in which three broad diffraction peaks are located between the standard patterns of CuInS2 (JCPDS 75-0106) and ZnS (JCPDS 79-0043). The average size of the samples is about 5 nm from TEM photos in the inset. Fig. 2(b) presents the absorption spectrum of ZCIS NCs chloroform solution. The optical band gap of ZCIS NCs is calculated to be 1.95 eV using Tauc's relation [18]: where the Eg is the optical bandgap energy for direct band-gap semiconductor [12], α is the absorption coefficient and hv is the photon energy of absorption edge. The AFM surface view of as-prepared ZCIS NCs film is depicted in the inset of Fig. 2(b). The average size of the ZCIS NCs is also approximated to 5.0 nm. The I-V curves of the device A are shown in Fig. 3(a) with the voltage sweeping direction denoted by the arrows. With the applied voltage increasing from 0 V to −1 V, the device current is found to increase by about three orders of magnitude, which is described as a transition from OFF state to ON state [1,3,5]. And the maximum ON/ OFF current ratio is up to 4×103. In the bias direction from −1.0 V→ −3.0 V→0 V→3.0 V, the device maintains at ON state with the current order of 10−4 A, indicating a typical WORM memory effect [9,15]. The device B has shown a negligible hysteresis in the Fig. 3(b), revealing that the resistive switch characteristic in device A is relevant to the ZCIS NCs or the interaction between PMMA and ZCIS NCs. To investigate the electron transport in this hybrid thin-film based device, the fitted I-V curves are shown in Fig. 4. Three dominating conduction models are concluded in the OFF state for the device A at the negative scanning range from 0 V to −1.1 V. For the voltage from 0 V to −0.25 V, the small current is compatible with the linear relation of ln (I) versus V1/2 in the Fig. 4(a), indicating the thermionic emission (TE) model as a result of the electron tunneling through Schottky barrier between ITO electrode and the PMMA layer [1,19,20]. In the region between −0.25 V and −0.75 V in the Fig. 4(b), the equation of the fitted curve is I∝Va with a value of 2.0, corresponding to spacecharge-limited current (SCLC) behavior [20,21]. When the applied voltage is raised to −0.9 V, the conductive mechanism is dominated by the strong trap-charge-limited current (TCLC) with a of 8.0 (a≫2) [19,20]. According to Jung and Wu et al., the large slope value indicates that the traps are exponentially distributed in forbidden gap
Fig. 1. The structure schematic diagram of Al/ZCIS NCs embedded in the PMMA layer/ ITO.
method [16]. The 0.1675 g as-obtained ZCIS NCs was mixed with 4 ml PMMA chloroform solution at the concentration of 5 mg/ml by ultrasonication. The ITO coated glass substrates were ultrasonically cleaned in detergent, distilled water, acetone, isopropanol and methanol for 15 min in succession and then dried in vacuum for 3 h. Next, the blended solution was spin-coated on ITO substrates at 700 rpm for 9 s and 2000 rpm for 30 s, following by vacuum drying at the 80 °C for 1 h. The thickness of hybrid film was about 80 nm. Finally, a 130-nmthick Al electrode was evaporated on as-prepared film at a pressure of 3×10−4 Pa, where the measured area of 2×2 mm2 was designed with a shadow mask. The schematic diagram of fabricated device was shown in Fig. 1 and it is marked as the device A. Meanwhile, a reference device Al/PMMA-only/ITO was prepared at the same condition and labeled as device B. We also fabricated a device C with the structure of Al/ZCIS NCs embedded in the PMMA layer/PMMA/ITO in which ITO substrate was modified by a 10 nm PMMA insert layer. The spin speed was programed to be 700 rpm for 9 s and then 4000 rpm for 30 s by using the 5 mg/ml PMMA chlorobenzene solution. The film thickness was measured from the Alpha-step surface profiler (D-100, KLA Tencor). Rigaku D/MAX-2500 V/PC diffractometer with Cu Ka radiation was applied to confirm the sample structure. JEM-2100F field emission transmission electron microscope were used to characterize the morphology and the dimension for the/ ZCIS NCs. Atomic force microscopy (BRUKER INNOVA) measurements were also performed to obtain film surface roughness. To estimate the band gap of nanocrystal, the absorption spectrum was obtained by a Hitachi U-4100 spectrophotometer. Current-voltage (IV) and current-time (I-t) characteristics were measured by the Keithley 2400 digital source meter. In the voltage-sweep measurements, the
Fig. 2. (a) The XRD patterns of as-synthesized nanocrystal sample with the characteristic peaks of ZnS and CuInS2 and the TEM image in the inset. (b) The UV–vis absorption spectrum of ZCIS NCs in chloroform solution and the AFM surface view of as-prepared ZCIS NCs film (the inset).
106
Materials Science in Semiconductor Processing 57 (2017) 105–109
X. Zhang et al.
Fig. 3. (a) Current-voltage curves for the device A in multiple sweeps from −3.0 to 3.0 V (solid arrows denote the sweeping sequence). (b) Current-voltage curves for the reference device B with the structure of Al/ PMMA-only /ITO.
Fig. 4. The fitted current-voltage curves at the different scanning regions. (a) The thermionic-emission conduction mechanism with the linear relation between ln (I) and V1/2 in the lower voltage from 0 V to −0.25 V. (b) The SCLC and TCLC mechanism in the equation I∝Va with a of 2.0 and 8.0 when the voltage changes from −0.25 V to −0.90 V. (c) The Fowler– Nordheim behavior based on the linear relationship between ln (I/V2) and 1/V changes from −0.9 V to −1.1 V. (d) The Ohmic conduction due to linear relation ln (I) vs. ln (V) when the applied voltage is over −1.1 V.
bending-induced triangulated barrier caused by higher field strength can assist electron tunneling [9]. For the ON state in the Fig. 4(d), the plot gives a slope of ~1 from plotting ln (I) vs. ln (V). The Ohmic conduction behavior is dependent upon the electron moving directly to the counter electrode due to the full-filled traps. The electron transportation and device operation are described by
of the NCs or discrete confined quantum states of the conduction band [1,19]. A considerable current transition more than three orders of magnitude is observed when the supplied voltage changes from −0.9 V to −1.1 V in the Fig. 4(c). The linear relationship between ln (I/V2) and 1/V suggests that the electron conduction in this region may be related to the FN tunneling [1,9]. Onlaor et al. has proposed that a band 107
Materials Science in Semiconductor Processing 57 (2017) 105–109
X. Zhang et al.
Fig. 5. Energy band diagrams and electron transport process for the device A under (a) the negative and (b) positive voltage.
Fig. 6. Planar AFM patterns of the ITO film (a) without PMMA coating and (b) with PMMA coating at the tapping-mode.
Fig. 7. (a) Current-voltage curves for the device C in multiple sweeps from −3.0 to 3.0 V. The I-V characteristics in the selected negative voltage region for the device C and the device A (the inset). (b) The retention characteristic of the device C (read voltage of −0.1 V) and device A (read voltage of −0.2 V) in the time duration of 104 s (in the inset).
showing the weak current. With the voltage raised, a sharp current increase is attributed from gradually filled traps by the injected electrons. The triangular-deformation of PMMA band along the direction of the applied voltage [9,19] promotes the electrons directly to the Al electrode in the way of FN tunneling (Fig. 5(a)), which is regarded as
energy band diagram under the negative and positive bias in Fig. 5, respectively. The LUMO and HOMO energy levels of PMMA are chosen as −1.8 eV and −7.3 eV from the reported literature [19,20]. Under the low applied voltage, the thermally-generated electrons are injected to the active layer and successively captured by the traps in ZCIS NCs, 108
Materials Science in Semiconductor Processing 57 (2017) 105–109
X. Zhang et al.
writing operation. By applying a reverse electric field (Fig. 5(b)), the high barrier between the PMMA and ZCIS NCs layer prevents the escape of captured electrons. The procedure has been described as electrons “frozen” by Huang et al. [13].. The interface properties in a multilayer film device, which can be modulated by surface modification or functionalization, has a significant influence on device behavior. Son et al. found that the surface roughness of PEDOT:PSS layer was decreased from 1.61 to 1.16 nm after PMMA layer coating onto ITO substrate and the device ON/OFF current ratio was raised by one order of magnitude [22]. With the PMMA modification on the ZnO film in the device ITO/ZnO/PMMA/ PEDOT:PSS/Ag, Nawrocki et al. reported a decrease in turn-on voltage from 7 V to 0.8 V and an increased memory window [23]. Tripathi group deposited an additional PVP layer on the Al substrate in the device Ag/CdSe:PVP/Al, they found a field-dependent charge carrier tunneling-induced time retention improvement [10]. To protect against film ‘pinhole’, a device C with a 10 nm PMMA layer was precoated on patterned ITO surface before active layer film fabrication. The AFM patterns of the ITO surface with and without PMMA inserted layer are comparatively shown in Fig. 6. The RMS average surface roughness of ITO surface demonstrates a decrease from 0.877 to 0.216 nm by additional PMMA insert layer. The I-V curves of device C in Fig. 7(a) have shown a similar current transition behavior with device A but a reduced turn-on voltage by 50% and an increased ON/ OFF current ratio by 50 times. A lower turn-on voltage was attributed to the smooth film and lower pinhole by wetting improvement of the hybrid ZCIS NCs:PMMA layer. The selected amplified I-V curves in negative voltage region are given in the semi-logarithmic scale as an inset. The average current of device C has two orders of magnitude larger than that of the device A, indicating the lower contact resistance due to inserted PMMA layer. The inevitable negative effect on device from double PMMA layer can be compensated by the better quality film. The retention characteristic of device C is displayed in Fig. 7(b) inset for that of device A. The I-t curves of OFF state in both devices demonstrate the similar stability. It is noticed that the current fluctuations of ON state in device C is as low as 3% in the time range of 104 s at the read voltage of −0.1 V, while it is around 5% for the device A. It may be associated with the restraint of electron release by the thick barrier from PMMA insert layer.
PMMA. When a PMMA insert layer was previously coated on the ITO substrate, the device shows the lower turn-on voltage of 0.5 V and increased ON/OFF current ratio by 50 times. The device stability is likely improved with a much minor current fluctuation as the operation time. The phenomenon is considered to associate with the restraint of electron release by thickened PMMA layer. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 Program) (Grant no. 2013AA014201), the Natural Science Foundation of Tianjin (Grant nos. 14JCZDJC31200, 15JCYBJC16700 and 15JCYBJC16800), the National Key Foundation for Exploring Scientific Instrument of China (Grant no. 2014YQ120351) and International Cooperation Program from Science and Technology of Tianjin (Grant no. 14RCGHGX00872). References [1] J.H. Shim, J.H. Jung, M.H. Lee, T.W. Kim, D.I. Son, A.N. Han, S.W. Kim, Org. Electron. 12 (2011) 1566–1570. [2] K.W. Han, M.H. Lee, T.W. Kim, D.Y. Yun, S.W. Kim, Appl. Phys. Lett 99 (2011) 193302-1–193302-4. [3] D.Y. Yun, T.W. Kim, S.W. Kim, Thin Solid Films 544 (2013) 433–436. [4] S. Valanarasu, A. Kathalingam, J.K. Rhee, R. Chandramohan, T.A. Vijayan, M. Karunakaran, J. Nanosci. Nanotechnol. 15 (2015) 1416–1420. [5] D.Y. Yun, N.S. Arul, D.U. Lee, N.H. Lee, T.W. Kim, Org. Electron. 24 (2015) 320–324. [6] F. Li, D.I. Son, H.M. Cha, S.M. Seo, B.J. Kim, H.J. Kim, J.H. Jung, T.W. Kim, Appl. Phys. Lett 90 (2007) 222109-1–222109-4. [7] K.H. Park, F. Li, J.H. Jung, D.I. Son, S.W. Cho, T.W. Kim, J. Nanosci. Nanotechnol. 10 (2010) 4801–4804. [8] T.T. Dao, V.T. Tran, K. Higashimine, H. Okada, D. Mott, Appl. Phys. Lett 99 (2011) 233303-1–233303-4. [9] K. Onlaor, T. Thiwawong, B. Tunhoo, Org. Electron. 15 (2014) 1254–1262. [10] R. Kaur, S.K. Tripathi, Microelectron. Eng. 133 (2015) 59–65. [11] I.S. Shin, J.M. Kim, J.H. Jeun, S.H. Yoo, Z. Ge, J.I. Hong, J.H. Bang, Y.S. Kim, Appl. Phys. Lett 100 (2012) 183307-1–183307-5. [12] W.D. Xiang, H.L. Yang, X.J. Liang, J.S. Zhong, J. Wang, L. Luo, C.P. Xie, J. Mater. Chem. C 1 (2013) 2014–2020. [13] M. Yi, L. Zhao, Q. Fan, X. Xia, W. Ai, L. Xie, X. Liu, N. Shi, W. Wang, Y. Wang, W. Huang, J. Appl. Phys. 110 (2011) 063709-1063709-5. [14] C. Wu, F. Li, T. Guo, T.W. Kim, Org. Electron. 13 (2012) 178–183. [15] D.Y. Yun, J.K. Kwak, J.H. Jung, T.W. Kim, D.I. Son, Appl. Phys. Lett 95 (2009) 143301-1–143301-4. [16] X. Liu, X. Zhang, L. Li, X. Wang, L. Yuan, Chin. Phys. B 11 (2014) 566–571. [17] W. Zhang, X. Zhong, Inorg. Chem. 50 (2011) 4065–4072. [18] K. Sharma, A.S. Al-Kabbi, G.S.S. Saini, S.K. Tripathi, Mater. Res. Bull. 47 (2012) 1400–1406. [19] C. Wu, F. Li, T. Guo, Appl. Phys. Lett 104 (2014) 183105-1–183105-6. [20] Z. Ma, C. Wu, D.U. Lee, F. Li, T.W. Kim, Org. Electron. 28 (2016) 20–24. [21] D.Y. Yun, H.M. Park, S.W. Kim, S.W. Kim, T.W. Kim, Carbon 75 (2014) 244–248. [22] J.M. Son, W.S. Song, C.H. Yoo, D.Y. Yun, T.W. Kim, Appl. Phys. Lett. 100 (2012) 183303. [23] R.A. Nawrocki, E.M. Galiger, D.P. Ostrowski, B.A. Bailey, X. Jiang, R.M. Voyles, N. Kopidakis, D.C. Olson, S.E. Shaheen, Org. Electron. 15 (2014) 1791–1798.
4. Conclusion In summary, a WORM device with the structure configuration of Al/ZCIS NCs embedded in the PMMA layer/ITO is fabricated by using the spin-coating method. The turn-on voltage is 1.0 V and the maximum ON/OFF state current ratio is 4×103 at a read voltage of −0.2 V. The fitted I-V curves and energy band diagrams are applied to discuss electron transport process and memory mechanism. The apparent hysteresis behavior is related to the electron capture by high-density defects in ZCIS NCs and the high barrier between ZCIS NCs and the
109
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
crossmark
Electrical bistable properties of nonvolatile memory device based on hybrid ZCIS NCs:PMMA film ⁎
Xuguang Zhanga, Jianping Xua, , Xiaosong Zhanga, Shaobo Shib, Xiangguo Zhaoa, Lan Lia,
⁎
a Institute of Material Physics, Key Laboratory of Display Materials and Photoelectric Devices, Ministry of Education, Tianjin University of Technology, Tianjin 300384, People's Republic of China b School of Science, Tianjin University of Technology and Education, Tianjin 300222, People's Republic of China
A R T I C L E I N F O
A BS T RAC T
Keywords: Quaternary ZCIS NCs Nanocomposite Electrically bistable memory device Write-once-read-many-times
A write-once–read–many-times (WORM) electrical bistable device was fabricated utilizing hybrid quaternary Zn-Cu-In-S nanocrystals (ZCIS NCs) and poly(methyl methacrylate) (PMMA) as active layer. A low turn-on voltage of 1.0 V and the maximum ON/OFF current ratio of 4×103 were found from current-voltage (I-V) characteristics. The current stability in the ON and OFF state has a fluctuation loss less than 5% in the measurement duration of 104 s. The electron transport and memory mechanism are described based on the fitted I-V curves and the energy band diagrams. The conduction models in the OFF state from 0 V to 1.1 V are described as thermionic emission (TE), space-charge-limited current (SCLC) and Fowler–Nordheim (FN) tunneling, respectively. And the Ohmic conduction dominates when the device is transited to the ON state at the voltage over 1.1 V. The high-density defects in quaternary ZCIS NCs and the high barrier between ZCIS NCs and the PMMA are considered to contribute to the electron retention behavior. For device performance optimization, an additional PMMA layer is pre-deposited on the ITO substrate surface. The 50% reduction at turn-on voltage and over 50 times enhancement in ON/OFF ratio are reasonably attributed to the better film quality and decreased series resistance from the decrease of root-mean-square average surface roughness (RMS) of the ITO surface from 0.877 to 0.216 nm. The better retention capacity is considered to relate to thickened tunnel barrier by PMMA inserted layer.
1. Introduction The resistive random access memory (RRAM) based on organic/ inorganic nanocomposites have attracted considerable attention for its fast response, high data storage density, non-destructive program/ erase operations, low power consumption and simple and feasible preparation approach [1–5]. In this consideration, semiconductor nanocrystals (NCs) with high-density defects and tunable band gap were hybridized into organic materials to act as storage medium for realization of the ideal ON/OFF current ratio and low power consumption [6–10]. As described by Shin et al., the CdSe quantum dots (QDs) was labeled as the hole-trapping sites in hybrid pentacene/CdSe QDs memory device by hysteretic capacitance-voltage response and energy band structure [11]. Onlaor et al.. valuated the device performance of the hybrid PVP:ZnO NPs in which ZnO NPs is responsible for electron capture. At optimal concentration of ZnO NPs, the formation of electron transport channel between nanoparticles is prevented and maximum ON/OFF current ratio of 3×103 is obtained [9]. Environment-friendly quaternary Zn-Cu-In-S NCs (ZCIS NCs) is considered to contain high density defects, such as Zn interstitials at Cu ⁎
sites, Cu vacancies (VCu), interstitial copper (Cui), indium substituted at a copper site (InCu), sulfur vacancy (VS), etc [12], which provides a better capability for carrier trapping than binary or ternary NPs [5]. A polymer layer such as polymethylmethacrylate (PMMA) had been usually utilized as a tunneling insulator preventing the release of trapped charge carriers from active sites [2,3,5]. In this communication, the WORM device with the configuration of Al/ZCIS NCs embedded in the PMMA layer/ITO is fabricated. The high-density electron traps existing in quaternary ZCIS NCs and high energy barrier between ZCIS NCs and PMMA are cooperatively considered to have a positive effect on the better memory characteristics [8,13–15]. The fitting of the I-V curves and energy band structure are introduced to discuss electron transport process and memory mechanism. The device performance optimization for lower turn-on voltage and the better ON/ OFF ratio are performed by the pre-treatment strategy for the ITO substrate. 2. Experimental details and characterization
Corresponding authors.
http://dx.doi.org/10.1016/j.mssp.2016.09.031 Received 26 April 2016; Received in revised form 5 August 2016; Accepted 26 September 2016 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
The synthesis of ZCIS NCs was carried out as our previous reported
Materials Science in Semiconductor Processing 57 (2017) 105–109
X. Zhang et al.
forward current was defined by a positive bias applied to the ITO substrate. All the measurements were carried out in ambient atmosphere without any encapsulation. 3. Results and discussion The XRD pattern for as-synthesized sample in Fig. 2(a) is consistent with the results addressed by Zhang et al. [17] in which three broad diffraction peaks are located between the standard patterns of CuInS2 (JCPDS 75-0106) and ZnS (JCPDS 79-0043). The average size of the samples is about 5 nm from TEM photos in the inset. Fig. 2(b) presents the absorption spectrum of ZCIS NCs chloroform solution. The optical band gap of ZCIS NCs is calculated to be 1.95 eV using Tauc's relation [18]: where the Eg is the optical bandgap energy for direct band-gap semiconductor [12], α is the absorption coefficient and hv is the photon energy of absorption edge. The AFM surface view of as-prepared ZCIS NCs film is depicted in the inset of Fig. 2(b). The average size of the ZCIS NCs is also approximated to 5.0 nm. The I-V curves of the device A are shown in Fig. 3(a) with the voltage sweeping direction denoted by the arrows. With the applied voltage increasing from 0 V to −1 V, the device current is found to increase by about three orders of magnitude, which is described as a transition from OFF state to ON state [1,3,5]. And the maximum ON/ OFF current ratio is up to 4×103. In the bias direction from −1.0 V→ −3.0 V→0 V→3.0 V, the device maintains at ON state with the current order of 10−4 A, indicating a typical WORM memory effect [9,15]. The device B has shown a negligible hysteresis in the Fig. 3(b), revealing that the resistive switch characteristic in device A is relevant to the ZCIS NCs or the interaction between PMMA and ZCIS NCs. To investigate the electron transport in this hybrid thin-film based device, the fitted I-V curves are shown in Fig. 4. Three dominating conduction models are concluded in the OFF state for the device A at the negative scanning range from 0 V to −1.1 V. For the voltage from 0 V to −0.25 V, the small current is compatible with the linear relation of ln (I) versus V1/2 in the Fig. 4(a), indicating the thermionic emission (TE) model as a result of the electron tunneling through Schottky barrier between ITO electrode and the PMMA layer [1,19,20]. In the region between −0.25 V and −0.75 V in the Fig. 4(b), the equation of the fitted curve is I∝Va with a value of 2.0, corresponding to spacecharge-limited current (SCLC) behavior [20,21]. When the applied voltage is raised to −0.9 V, the conductive mechanism is dominated by the strong trap-charge-limited current (TCLC) with a of 8.0 (a≫2) [19,20]. According to Jung and Wu et al., the large slope value indicates that the traps are exponentially distributed in forbidden gap
Fig. 1. The structure schematic diagram of Al/ZCIS NCs embedded in the PMMA layer/ ITO.
method [16]. The 0.1675 g as-obtained ZCIS NCs was mixed with 4 ml PMMA chloroform solution at the concentration of 5 mg/ml by ultrasonication. The ITO coated glass substrates were ultrasonically cleaned in detergent, distilled water, acetone, isopropanol and methanol for 15 min in succession and then dried in vacuum for 3 h. Next, the blended solution was spin-coated on ITO substrates at 700 rpm for 9 s and 2000 rpm for 30 s, following by vacuum drying at the 80 °C for 1 h. The thickness of hybrid film was about 80 nm. Finally, a 130-nmthick Al electrode was evaporated on as-prepared film at a pressure of 3×10−4 Pa, where the measured area of 2×2 mm2 was designed with a shadow mask. The schematic diagram of fabricated device was shown in Fig. 1 and it is marked as the device A. Meanwhile, a reference device Al/PMMA-only/ITO was prepared at the same condition and labeled as device B. We also fabricated a device C with the structure of Al/ZCIS NCs embedded in the PMMA layer/PMMA/ITO in which ITO substrate was modified by a 10 nm PMMA insert layer. The spin speed was programed to be 700 rpm for 9 s and then 4000 rpm for 30 s by using the 5 mg/ml PMMA chlorobenzene solution. The film thickness was measured from the Alpha-step surface profiler (D-100, KLA Tencor). Rigaku D/MAX-2500 V/PC diffractometer with Cu Ka radiation was applied to confirm the sample structure. JEM-2100F field emission transmission electron microscope were used to characterize the morphology and the dimension for the/ ZCIS NCs. Atomic force microscopy (BRUKER INNOVA) measurements were also performed to obtain film surface roughness. To estimate the band gap of nanocrystal, the absorption spectrum was obtained by a Hitachi U-4100 spectrophotometer. Current-voltage (IV) and current-time (I-t) characteristics were measured by the Keithley 2400 digital source meter. In the voltage-sweep measurements, the
Fig. 2. (a) The XRD patterns of as-synthesized nanocrystal sample with the characteristic peaks of ZnS and CuInS2 and the TEM image in the inset. (b) The UV–vis absorption spectrum of ZCIS NCs in chloroform solution and the AFM surface view of as-prepared ZCIS NCs film (the inset).
106
Materials Science in Semiconductor Processing 57 (2017) 105–109
X. Zhang et al.
Fig. 3. (a) Current-voltage curves for the device A in multiple sweeps from −3.0 to 3.0 V (solid arrows denote the sweeping sequence). (b) Current-voltage curves for the reference device B with the structure of Al/ PMMA-only /ITO.
Fig. 4. The fitted current-voltage curves at the different scanning regions. (a) The thermionic-emission conduction mechanism with the linear relation between ln (I) and V1/2 in the lower voltage from 0 V to −0.25 V. (b) The SCLC and TCLC mechanism in the equation I∝Va with a of 2.0 and 8.0 when the voltage changes from −0.25 V to −0.90 V. (c) The Fowler– Nordheim behavior based on the linear relationship between ln (I/V2) and 1/V changes from −0.9 V to −1.1 V. (d) The Ohmic conduction due to linear relation ln (I) vs. ln (V) when the applied voltage is over −1.1 V.
bending-induced triangulated barrier caused by higher field strength can assist electron tunneling [9]. For the ON state in the Fig. 4(d), the plot gives a slope of ~1 from plotting ln (I) vs. ln (V). The Ohmic conduction behavior is dependent upon the electron moving directly to the counter electrode due to the full-filled traps. The electron transportation and device operation are described by
of the NCs or discrete confined quantum states of the conduction band [1,19]. A considerable current transition more than three orders of magnitude is observed when the supplied voltage changes from −0.9 V to −1.1 V in the Fig. 4(c). The linear relationship between ln (I/V2) and 1/V suggests that the electron conduction in this region may be related to the FN tunneling [1,9]. Onlaor et al. has proposed that a band 107
Materials Science in Semiconductor Processing 57 (2017) 105–109
X. Zhang et al.
Fig. 5. Energy band diagrams and electron transport process for the device A under (a) the negative and (b) positive voltage.
Fig. 6. Planar AFM patterns of the ITO film (a) without PMMA coating and (b) with PMMA coating at the tapping-mode.
Fig. 7. (a) Current-voltage curves for the device C in multiple sweeps from −3.0 to 3.0 V. The I-V characteristics in the selected negative voltage region for the device C and the device A (the inset). (b) The retention characteristic of the device C (read voltage of −0.1 V) and device A (read voltage of −0.2 V) in the time duration of 104 s (in the inset).
showing the weak current. With the voltage raised, a sharp current increase is attributed from gradually filled traps by the injected electrons. The triangular-deformation of PMMA band along the direction of the applied voltage [9,19] promotes the electrons directly to the Al electrode in the way of FN tunneling (Fig. 5(a)), which is regarded as
energy band diagram under the negative and positive bias in Fig. 5, respectively. The LUMO and HOMO energy levels of PMMA are chosen as −1.8 eV and −7.3 eV from the reported literature [19,20]. Under the low applied voltage, the thermally-generated electrons are injected to the active layer and successively captured by the traps in ZCIS NCs, 108
Materials Science in Semiconductor Processing 57 (2017) 105–109
X. Zhang et al.
writing operation. By applying a reverse electric field (Fig. 5(b)), the high barrier between the PMMA and ZCIS NCs layer prevents the escape of captured electrons. The procedure has been described as electrons “frozen” by Huang et al. [13].. The interface properties in a multilayer film device, which can be modulated by surface modification or functionalization, has a significant influence on device behavior. Son et al. found that the surface roughness of PEDOT:PSS layer was decreased from 1.61 to 1.16 nm after PMMA layer coating onto ITO substrate and the device ON/OFF current ratio was raised by one order of magnitude [22]. With the PMMA modification on the ZnO film in the device ITO/ZnO/PMMA/ PEDOT:PSS/Ag, Nawrocki et al. reported a decrease in turn-on voltage from 7 V to 0.8 V and an increased memory window [23]. Tripathi group deposited an additional PVP layer on the Al substrate in the device Ag/CdSe:PVP/Al, they found a field-dependent charge carrier tunneling-induced time retention improvement [10]. To protect against film ‘pinhole’, a device C with a 10 nm PMMA layer was precoated on patterned ITO surface before active layer film fabrication. The AFM patterns of the ITO surface with and without PMMA inserted layer are comparatively shown in Fig. 6. The RMS average surface roughness of ITO surface demonstrates a decrease from 0.877 to 0.216 nm by additional PMMA insert layer. The I-V curves of device C in Fig. 7(a) have shown a similar current transition behavior with device A but a reduced turn-on voltage by 50% and an increased ON/ OFF current ratio by 50 times. A lower turn-on voltage was attributed to the smooth film and lower pinhole by wetting improvement of the hybrid ZCIS NCs:PMMA layer. The selected amplified I-V curves in negative voltage region are given in the semi-logarithmic scale as an inset. The average current of device C has two orders of magnitude larger than that of the device A, indicating the lower contact resistance due to inserted PMMA layer. The inevitable negative effect on device from double PMMA layer can be compensated by the better quality film. The retention characteristic of device C is displayed in Fig. 7(b) inset for that of device A. The I-t curves of OFF state in both devices demonstrate the similar stability. It is noticed that the current fluctuations of ON state in device C is as low as 3% in the time range of 104 s at the read voltage of −0.1 V, while it is around 5% for the device A. It may be associated with the restraint of electron release by the thick barrier from PMMA insert layer.
PMMA. When a PMMA insert layer was previously coated on the ITO substrate, the device shows the lower turn-on voltage of 0.5 V and increased ON/OFF current ratio by 50 times. The device stability is likely improved with a much minor current fluctuation as the operation time. The phenomenon is considered to associate with the restraint of electron release by thickened PMMA layer. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (863 Program) (Grant no. 2013AA014201), the Natural Science Foundation of Tianjin (Grant nos. 14JCZDJC31200, 15JCYBJC16700 and 15JCYBJC16800), the National Key Foundation for Exploring Scientific Instrument of China (Grant no. 2014YQ120351) and International Cooperation Program from Science and Technology of Tianjin (Grant no. 14RCGHGX00872). References [1] J.H. Shim, J.H. Jung, M.H. Lee, T.W. Kim, D.I. Son, A.N. Han, S.W. Kim, Org. Electron. 12 (2011) 1566–1570. [2] K.W. Han, M.H. Lee, T.W. Kim, D.Y. Yun, S.W. Kim, Appl. Phys. Lett 99 (2011) 193302-1–193302-4. [3] D.Y. Yun, T.W. Kim, S.W. Kim, Thin Solid Films 544 (2013) 433–436. [4] S. Valanarasu, A. Kathalingam, J.K. Rhee, R. Chandramohan, T.A. Vijayan, M. Karunakaran, J. Nanosci. Nanotechnol. 15 (2015) 1416–1420. [5] D.Y. Yun, N.S. Arul, D.U. Lee, N.H. Lee, T.W. Kim, Org. Electron. 24 (2015) 320–324. [6] F. Li, D.I. Son, H.M. Cha, S.M. Seo, B.J. Kim, H.J. Kim, J.H. Jung, T.W. Kim, Appl. Phys. Lett 90 (2007) 222109-1–222109-4. [7] K.H. Park, F. Li, J.H. Jung, D.I. Son, S.W. Cho, T.W. Kim, J. Nanosci. Nanotechnol. 10 (2010) 4801–4804. [8] T.T. Dao, V.T. Tran, K. Higashimine, H. Okada, D. Mott, Appl. Phys. Lett 99 (2011) 233303-1–233303-4. [9] K. Onlaor, T. Thiwawong, B. Tunhoo, Org. Electron. 15 (2014) 1254–1262. [10] R. Kaur, S.K. Tripathi, Microelectron. Eng. 133 (2015) 59–65. [11] I.S. Shin, J.M. Kim, J.H. Jeun, S.H. Yoo, Z. Ge, J.I. Hong, J.H. Bang, Y.S. Kim, Appl. Phys. Lett 100 (2012) 183307-1–183307-5. [12] W.D. Xiang, H.L. Yang, X.J. Liang, J.S. Zhong, J. Wang, L. Luo, C.P. Xie, J. Mater. Chem. C 1 (2013) 2014–2020. [13] M. Yi, L. Zhao, Q. Fan, X. Xia, W. Ai, L. Xie, X. Liu, N. Shi, W. Wang, Y. Wang, W. Huang, J. Appl. Phys. 110 (2011) 063709-1063709-5. [14] C. Wu, F. Li, T. Guo, T.W. Kim, Org. Electron. 13 (2012) 178–183. [15] D.Y. Yun, J.K. Kwak, J.H. Jung, T.W. Kim, D.I. Son, Appl. Phys. Lett 95 (2009) 143301-1–143301-4. [16] X. Liu, X. Zhang, L. Li, X. Wang, L. Yuan, Chin. Phys. B 11 (2014) 566–571. [17] W. Zhang, X. Zhong, Inorg. Chem. 50 (2011) 4065–4072. [18] K. Sharma, A.S. Al-Kabbi, G.S.S. Saini, S.K. Tripathi, Mater. Res. Bull. 47 (2012) 1400–1406. [19] C. Wu, F. Li, T. Guo, Appl. Phys. Lett 104 (2014) 183105-1–183105-6. [20] Z. Ma, C. Wu, D.U. Lee, F. Li, T.W. Kim, Org. Electron. 28 (2016) 20–24. [21] D.Y. Yun, H.M. Park, S.W. Kim, S.W. Kim, T.W. Kim, Carbon 75 (2014) 244–248. [22] J.M. Son, W.S. Song, C.H. Yoo, D.Y. Yun, T.W. Kim, Appl. Phys. Lett. 100 (2012) 183303. [23] R.A. Nawrocki, E.M. Galiger, D.P. Ostrowski, B.A. Bailey, X. Jiang, R.M. Voyles, N. Kopidakis, D.C. Olson, S.E. Shaheen, Org. Electron. 15 (2014) 1791–1798.
4. Conclusion In summary, a WORM device with the structure configuration of Al/ZCIS NCs embedded in the PMMA layer/ITO is fabricated by using the spin-coating method. The turn-on voltage is 1.0 V and the maximum ON/OFF state current ratio is 4×103 at a read voltage of −0.2 V. The fitted I-V curves and energy band diagrams are applied to discuss electron transport process and memory mechanism. The apparent hysteresis behavior is related to the electron capture by high-density defects in ZCIS NCs and the high barrier between ZCIS NCs and the
109