Abstract
A cadmium selenide/zinc sulfide (CdSe/ZnS) quantum dot (QD)-based multi-level memory device with the structure [ITO/PEDOT:PSS/QDs/ZnO/Al:Al2O3/QDs/Al] was fabricated via a spin-coating method used to deposit thin films. Two layers of QD thin films present in the device act as charge storage layers to form three distinct states. Zinc oxide (ZnO) and aluminum oxide (Al2O3) were added to prevent leakage. ZnO NPs provide orthogonality between the two QD layers, and a poly(3,4-ethylenedioxythio-phene): poly(styrenesulfonate) (PEDOT:PSS) thin film was formed for effective hole injection from the electrodes. The core/shell structure of the QDs provides the quantum well, which causes the trapping of injected charges. The resistance changes according to the charging and discharging of the QDs’ trap site and, as a result, the current through the device also changes. There are two quantum wells, two current changes, and three stable states. The role of each thin film was confirmed through I–V curve analysis and the fabrication conditions of each thin film were optimized. The synthesized QDs and ZnO nanoparticles were evaluated via X-ray diffraction, transmission electron microscopy, and absorbance and photoluminescence spectroscopy. The measured write voltages of the fabricated device were at 1.8 and 2.4 V, and the erase voltages were −4.05 and −4.6 V. The on/off ratio at 0.5 V was 2.2 × 103. The proposed memory device showed retention characteristics of ≥100 h and maintained the initial write/erase voltage even after 200 iterative operations.
Highlights
A quantum dots (QDs)-based multi-level memory device was proposed in which a plurality of charge storage layers (CSL) was formed using QDs and aluminum oxide (Al2O3) film
Charges are confined to the quantum well existing in a QDs having a core/shell structure, and a current difference occurs according to the charge/discharge of these trap sites
The QDs and zinc oxide nanoparticles (ZnO NPs) used in this study were synthesized directly, and the synthesis was confirmed by measuring photo-luminescence (PL), absorbance, transmission electron microscopy (TEM), and X-ray diffraction (XRD) of the synthesized material
Summary
A charge barrier can be formed outside the CSL in order to suppress the light emission characteristics that may occur in the QDs and to suppress leakage in the shell of the partially weak QDs [31,32,33,34] Through such isolation, a plurality of CSL (quantum wells) can be formed, which enables the formation of a plurality of states and the development of a memory device capable of expressing multiple levels. The retention characteristics and the operating voltage characteristics were analyzed according to 200 repetitions
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