ZnO-porous silicon nanocomposite for possible memristive device fabrication
© Martínez et al.; licensee Springer. 2014
Received: 20 May 2014
Accepted: 12 August 2014
Published: 27 August 2014
Preliminary results on the fabrication of a memristive device made of zinc oxide (ZnO) over a mesoporous silicon substrate have been reported. Porous silicon (PS) substrate is employed as a template to increase the formation of oxygen vacancies in the ZnO layer and promote suitable grain size conditions for memristance. Morphological and optical properties are investigated using scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. The proposed device exhibits a zero-crossing pinched hysteresis current-voltage (I-V) curve characteristic of memristive systems.
The memristor, known as the fourth fundamental circuit element, is a device whose main characteristic is the dependance of resistance according to the flux of charge passing through it and has the ability to remember its last resistance state. It was hypothesized by Chua  in 1971, but it was not until 2008 that it was first fabricated at HP Labs . Since then, the fabrication and study of memristive devices have become very popular due to their applications in information storage, non-volatile memories, neural networks, etc. [3–5] Memristive switching behavior has been observed in many metal oxides [6, 7] and attributed to the migration of oxygen vacancies within the oxide layers and grain boundaries [8, 9], but still, transport mechanisms are being studied and different models have been suggested [7–9]. Zinc oxide (ZnO) possesses several interesting properties and has been extensively studied for its technological applications, specifically in electronic and optoelectronic devices such as photodetectors [10, 11], light-emitting diodes , solar cells [13, 14], and gas sensing . On the other hand, porous silicon (PS)-ZnO composites have been used for white light emission  and to tune ZnO grain size for possible sensing applications . This leads to the possibility to fabricate a tunable memristive device made of ZnO deposited on a PS template for optimizing the conditions of grain size, oxygen vacancies, defects, etc. to achieve tunable response from the device. The memristive behavior is demonstrated and explained through scanning electron microscopy (SEM) and photoluminescence (PL) characterization. The effect of annealing on morphology and photoluminescence response is also studied.
PS samples were obtained by wet electrochemical etching using p++-type (100) Si wafers with a resistivity of 0.002 to 0.005 Ω cm. The anodization process was carried out using an electrolyte solution composed of hydrofluoric acid (48 wt% HF) and ethanol (99.9 %) in a volumetric ratio of 1:1. The bilayer porous structure was fabricated with a current density of J1 = 31.64 mA/cm2 (refractive index, n1 = 1.5) and J2 = 13.3 mA/cm2 (refractive index, n2 = 1.8). ZnO thin films were deposited on PS using sol-gel spin coating. In this process, zinc acetate dehydrate [Zn(CH3COO)2 · H2O] was first dissolved into the ethanol solution along with monoethanolamine (MEA). A homogeneous transparent solution with a concentration of 0.2 M zinc acetate and a 1:1 molar ratio of MEA/zinc acetate dehydrate was prepared. This solution was kept for hydrolysis for 48 h and spin coated onto the PS substrate seven times to get the desired film thickness. In order to study the stability and the good quality of ZnO, thin films were deposited on a Corning glass substrate (Corning Inc., Corning, NY, USA) and the transmittance measurements were taken with a PerkinElmer UV-Vis-NIR (Lambda 950) spectrophotometer (PerkinElmer, Waltham, MA, USA). To study the effect of annealing on the morphology of the ZnO film, samples were annealed in air atmosphere at 700°C for 30 min inside a tubular furnace. The orientation and crystallinity of the ZnO crystallites were measured by an X-ray diffraction (XRD) spectrometer (X'Pert PRO, PANalytical B.V., Almelo, The Netherlands) using CuKα radiation having a wavelength of 1.54 Å. The morphological effect of ZnO thin films with annealing was analyzed with a scanning electron microscope. The PL studies were carried out using a Varian fluorescence spectrometer (Cary Eclipse, Varian Inc., Palo Alto, CA, USA) under 3.8-eV excitation of a xenon lamp. The effect of the PS substrate on the electrical properties of the device (ZnO-PS) was studied by the acquisition of current-voltage curves applying DC voltage in a cyclic scan (from −10 to 10 V) at room temperature. Contacts were made of conductive carbon in two different configurations: lateral and transversal. A reference sample was fabricated and characterized by depositing ZnO on crystalline silicon.
Results and discussion
where T is the optical transmittance and d is the thickness of the ZnO thin film. For direct bandgap semiconductors, Eg can be estimated using the equation (αhv)2 = A(hv − Eg), where h is the Planck constant, v is the frequency of incident photon, A is a constant, and Eg is the optical gap. Figure 1 shows the Tauc plot: (αhv)2 vs. phonon energy (hv) for measuring the direct bandgap of ZnO (3.34 eV) .
Figure 1b shows a typical XRD pattern (corresponding to the ZnO-PS structure annealed at 700°C). The graph exhibits the prominent peaks at 2θ = 32.0°, 34.61°, and 36.58° corresponding to the (100), (002), and (101) planes of ZnO, respectively. The XRD pattern of ZnO shows a hexagonal wurtzite structure and polycrystalline nature (JCDPS card number: 36-1451). The films are oriented perpendicular to the substrate surface in the c-axis. The c-axis orientation can be understood due to the fact that the c-plane of zinc oxide crystallites corresponds to the densest packed plane.
To optically characterize the composite, the luminescent properties of ZnO/PS structures were studied before and after annealing. Generally, all the characterized ZnO thin films exhibit two bands, one centered at 380 nm and the second one around 520 nm. The spectral position of the peak at 380 nm (3.27 eV) is attributed to the near-band edge excitonic recombinations in ZnO films , whereas the blue-green emission band peaking at 520 nm (2.38 eV) has been reported as the most common band for ZnO , typically attributed to the non-stoichometric composition of ZnO (defects mainly due to oxygen vacancies) .
In the literature, there are basically two possible mechanisms acting in the system for the transport of oxygen vacancies, which are responsible for the demonstration of memristive characteristics: (a) the filamentary conducting path [7–9] and (b) the interface-type conducting path . The first one proposes that conductive and non-conductive zones in the oxide layers are created by the distribution of oxygen vacancies within the material due to its morphology and the applied bias voltage. The second one explains the resistive switching by the creation of conducting filaments made of oxygen vacancies across the dielectric material (ZnO) under an applied bias voltage. In the present study, the effect can be attributed to the fact that the use of porous silicon as a substrate increases the effective surface area (refer to Figure 2e; granular labyrinth patterns formed on the surface after annealing) and hence the oxygen vacancies in ZnO, which leads to the memristive behavior of the composite structure. Conductive channels (filamentary conducting paths) are formed within the ZnO layer and grain boundaries . In both configurations, the presence of memristive behavior suggests that a suitable grain size can promote the diffusion of oxygen vacancies in any direction of the device.
In this paper, the ZnO-mesoPS nanocomposite is demonstrated as a potential structure in the fabrication of memristive devices. Deposition of ZnO onto the mesoporous silicon substrate and post-annealing treatment resulted in the formation of regular labyrinth patterns with granular appearance. Mesoporous silicon as a substrate was found to promote the modification of ZnO grain size and consequently a significant enhancement of oxygen vacancies, which are responsible for resistive switching. Typical memristive behavior is demonstrated and analyzed. Future work is being carried out to study the tunability of the device as a function of substrate porosity/morphology.
LM and OO are PhD and M. Tech students, respectively, in a material science and technology program in a research institute (CIICAp-UAEM) in Cuernavaca. YK is a postdoctoral fellow in UNAM. VA is working as a professor-scientist in CIICAp-UAEM.
This work was financially supported by a CONACyT project (#128953). We acknowledge the technical help provided by Jose Campos in acquiring the SEM images.
- Chua L: Memristor-the missing circuit element. Circuit Theory IEEE Transact On 1971, 18(5):507–519.View ArticleGoogle Scholar
- Strukov DB, Snider GS, Stewart DR, Williams RS: The missing memristor found. Nature 2008, 453(7191):80–83. 10.1038/nature06932View ArticleGoogle Scholar
- Park J, Lee S, Lee J, Yong K: A light incident angle switchable ZnO nanorod memristor: reversible switching behavior between two non‒volatile memory devices. Adv Mater 2013, 25(44):6423–6429. 10.1002/adma.201303017View ArticleGoogle Scholar
- Yoon SM, Warren SC, Grzybowski BA: Storage of electrical information in metal–organic‒framework memristors. Angew Chem Int Ed 2014, 53(17):4437–4441. 10.1002/anie.201309642View ArticleGoogle Scholar
- Wang ZQ, Xu HY, Li XH, Yu H, Liu YC, Zhu XJ: Synaptic learning and memory functions achieved using oxygen ion migration/diffusion in an amorphous InGaZnO memristor. Adv Funct Mater 2012, 22(13):2759–2765. 10.1002/adfm.201103148View ArticleGoogle Scholar
- Yang JJ, Pickett MD, Li X, Ohlberg DA, Stewart DR, Williams RS: Memristive switching mechanism for metal/oxide/metal nanodevices. Nat Nanotechnol 2008, 3(7):429–433. 10.1038/nnano.2008.160View ArticleGoogle Scholar
- Sawa A: Resistive switching in transition metal oxides. Mater Today 2008, 11(6):28–36. 10.1016/S1369-7021(08)70119-6View ArticleGoogle Scholar
- Zoolfakar AS, Kadir RA, Rani RA, Balendhran S, Liu X, Kats E, Bhargava SK, Bhaskaran M, Sriram S, Zhuiykov S, O'Mullane AP, Zadeh KK: Engineering electrodeposited ZnO films and their memristive switching performance. Phys Chem Chem Phys 2013, 15(25):10376–10384. 10.1039/c3cp44451aView ArticleGoogle Scholar
- Liu L, Chen B, Gao B, Zhang F, Chen Y, Liu X, Kang J: Engineering oxide resistive switching materials for memristive device application. Appl Phys A 2011, 102(4):991–996. 10.1007/s00339-011-6331-2View ArticleGoogle Scholar
- Ridhuan NS, Lockman Z, Aziz AA, Khairunisak AR: Properties of ZnO nanorods arrays growth via low temperature hydrothermal reaction. Adv Mater Res 2012, 364: 422–426.View ArticleGoogle Scholar
- Yao I, Tseng TY, Lin P: ZnO nanorods grown on polymer substrates as UV photodetectors. Sensors Actuators A Phys 2012, 178: 26–31.View ArticleGoogle Scholar
- Rusli NI, Tanikawa M, Mahmood MR, Yasui K, Hashim AM: Growth of high-density zinc oxide nanorods on porous silicon by thermal evaporation. Materials 2012, 5(12):2817–2832. 10.3390/ma5122817View ArticleGoogle Scholar
- Cai F, Wang J, Yuan Z, Duan Y: Magnetic-field effect on dye-sensitized ZnO nanorods-based solar cells. J Power Sources 2012, 216: 269–272.View ArticleGoogle Scholar
- Tao R, Tomita T, Wong RA, Waki K: Electrochemical and structural analysis of Al-doped ZnO nanorod arrays in dye-sensitized solar cells. J Power Sources 2012, 214: 159–165.View ArticleGoogle Scholar
- Aroutiounian V, Arakelyan V, Galstyan V, Martirosyan K, Soukiassian P: Hydrogen sensor made of porous silicon and covered by TiO or ZnO Al thin film. Sens J IEEE 2009, 9(1):9–12.View ArticleGoogle Scholar
- Prabakaran R, Peres M, Monteiro T, Fortunato E, Martins R, Ferreira I: The effects of ZnO coating on the photoluminescence properties of porous silicon for the advanced optoelectronic devices. J Non Cryst Solids 2008, 354(19):2181–2185.View ArticleGoogle Scholar
- Kumar Y, Garcia JE, Singh F, Olive-Méndez SF, Sivakumar VV, Kanjilal D, Agarwal V: Influence of mesoporous substrate morphology on the structural, optical and electrical properties of RF sputtered ZnO layer deposited over porous silicon nanostructure. Appl Surf Sci 2012, 258(7):2283–2288. 10.1016/j.apsusc.2011.09.131View ArticleGoogle Scholar
- Harris L, Arthur Loebal L: Evaluation and analysis of optical and electrical constants of thin films as functions of reflectance and data by electronic digital computation. J Opt Soc Am 1955, 45(3):179–188. 10.1364/JOSA.45.000179View ArticleGoogle Scholar
- Monch W: On the band structure lineup of ZnO heterostructures. Appl Phys Lett 2005, 86: 162101. 10.1063/1.1897436View ArticleGoogle Scholar
- Cai H, Shen H, Yin Y, Lu L, Shen J, Tang Z: The effects of porous silicon on the crystalline properties of ZnO thin films. J Phys Chem Solid 2009, 70(6):967–971. 10.1016/j.jpcs.2009.05.004View ArticleGoogle Scholar
- Wu XL, Siu GG, Fu CL, Ong HC: Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films. Appl Phys Lett 2001, 78: 2285–2287. 10.1063/1.1361288View ArticleGoogle Scholar
- Djurišić AB, Leung YH: Optical properties of ZnO nanostructures. Small 2006, 2(8–9):944–961.Google Scholar
- Dai L, Chen XL, Wang WJ, Zhou T, Hu BQ: Growth and luminescence characterization of large-scale zinc oxide nanowires. J Phys Condens Matter 2003, 15(13):2221. 10.1088/0953-8984/15/13/308View ArticleGoogle Scholar
- Yang CL, Wang JN, Ge WK, Guo L, Yang SH, Shen DZ: Enhanced ultraviolet emission and optical properties in polyvinyl pyrrolidone surface modified ZnO quantum dots. J Appl Phys 2001, 90(9):4489–4493. 10.1063/1.1406973View ArticleGoogle Scholar
- Hassan NK, Hashim MR, Mahadi MA, Allam NK: A catalyst-free growth of ZnO nanowires on Si (100) substrates: effect of substrate position on morphological, structural and optical properties. ECS J Solid States Sci Technol 2012, 1: 86–89.View ArticleGoogle Scholar
- Umar A, Kim SH, Al-Hajry A, Hahn YB: Temperature-dependant non-catalytic growth of ultraviolet-emitting ZnO nanostructures on silicon substrate by thermal evaporation process. J Alloys Comp 2008, 463: 516–521. 10.1016/j.jallcom.2007.09.065View ArticleGoogle Scholar
- Yang JH, Zhend JH, Zahai HJ, Yang LL: Low temperature hydrothermal growth an optical properties of ZnO nanorods. Cryst Technol 2009, 44: 87–91. 10.1002/crat.200800294View ArticleGoogle Scholar
- Chew ZJ, Li L: A discrete memristor made of ZnO nanowires synthesized on printed circuit board. Mater Lett 2013, 91: 298–300.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.