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.
KeywordsMemristor Zinc oxide Porous silicon Composite Nanocrystallites
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.
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