Modulation of the electroluminescence emission from ZnO/Si NCs/p-Si light-emitting devices via pulsed excitation

In this work, the electroluminescence (EL) emission of zinc oxide (ZnO)/Si nanocrystals (NCs)-based light-emitting devices was studied under pulsed electrical excitation. Both Si NCs and deep-level ZnO defects were found to contribute to the observed EL. Symmetric square voltage pulses (50-μs period) were found to notably enhance EL emission by about one order of magnitude. In addition, the control of the pulse parameters (accumulation and inversion times) was found to modify the emission lineshape, long inversion times (i.e., short accumulation times) suppressing ZnO defects contribution. The EL results were discussed in terms of the recombination dynamics taking place within the ZnO/Si NCs heterostructure, suggesting the excitation mechanism of the luminescent centers via a combination of electron impact, bipolar injection, and sequential carrier injection within their respective conduction regimes

Light-activated electroforming in ITO/ZnO/p-Si resistive switching devices

We report on light-activated electroforming of ZnO/p-Si heterojunction memristors with transparent indium tin oxide as the top electrode. Light-generated electron-hole pairs in the p-type substrate are separated by the external electric field and electrons are injected into the active ZnO layer. The additional application of voltage pulses allows achieving different resistance states that end up in the realization of the low resistance state (LRS). This process requires much less voltage compared to dark conditions, thus avoiding undesired current overshoots and achieving a self-compliant device. The transport mechanisms governing each resistance state are studied and discussed. An evolution from an electrode-limited to a space charge-limited transport is observed along the electroforming process before reaching the LRS, which is ascribed to the progressive formation of conductive paths that consequently induce the growth of conductive nanofilaments through the ZnO layer. This work was financially supported by the Spanish Ministry of Economy and Competitiveness (Project Nos. TEC2012-38540-C02-01 and TEC2016-76849-C2-1-R). O.B. also acknowledges the subprogram "Ayudas para Contratos Predoctorales para la Formación de-Doctores" from the Spanish Ministry of Economy and Competitiveness for economical support. J.L.F. acknowledges the subprogram "Ayudas para la Formación de Profesorado Universitario" (No. FPU16/06257) from the Spanish Ministry of Education, Culture and Sports for economical support. X.P., C.L., and C.G. are grateful to C. Frilay for his expertise in the maintenance of the sputtering setup used for the growth of the ZnO films

Toward RGB LEDs based on rare earth-doped ZnO

By using ZnO thin films doped with Ce, Tb or Eu, deposited via radiofrequency magnetron sputtering, we have developed monochromatic (blue, green and red, respectively) light emitting devices (LEDs). The rare earth ions introduced with doping rates lower than 2% exhibit narrow and intense emission peaks due to electronic transitions in relaxation processes induced after electrical excitation. This study proves zinc oxide to be a good host for these elements, its high conductivity and optical transparency in the visible range being as well exploited as top transparent electrode. After structural characterization of the different doped layers, a device structure with intense electroluminescence is presented, modeled, and electrically and optically characterized. The different emission spectra obtained are compared in a chromatic diagram, providing a reference for future works with similar devices. The results hereby presented demonstrate three operating monochromatic LEDs, as well as a combination of the three species into another one, with a simply-designed structure compatible with current Si technology and demonstrating an integrated red-green-blue emission

Photoelectrical reading in ZnO/Si NCs/p-Si resistive switching devices

The increasing need for efficient memories with integrated functionalities in a single device has led the electronics community to investigate and develop different materials for resistive switching (RS) applications. Among these materials, the well-known Si nanocrystals (NCs) have demonstrated to exhibit RS properties, which add to the wealth of phenomena that have been studied on this model material platform. In this work, we present ZnO/Si NCs/p-Si resistive switching devices whose resistance state can be electrically read at 0 V under the application of low-power monochromatic illumination. The presented effect is studied in terms of the inner structural processes and electronic physics of the device. In particular, the creation of conductive filaments through the Si NC multilayers induces a low-resistance path for photogenerated carriers to get extracted from the device, whereas in the pristine state charge extraction is strongly quenched due to the insulating nature of the NC-embedding SiO2 matrix. In addition, spectral inspection of the generated photocurrent allowed unveiling the role of Si NCs in the reported effect. Overall, the hereby shown results pave the way to obtain memories whose RS state can be read under low-power conditions

Green electroluminescence of Al/Tb/Al/SiO2 devices fabricated by electron beam evaporation

In this work, the fabrication and the structural, optical and electrical properties of Al-Tb/SiO2 nanomultilayers have been studied. The nanomultilayers were deposited by means of electron beam evaporation on top of p-type Si substrates. Optical characterization shows a narrow and strong emission in the green spectral range, indicating the optical activation of Tb3+ ions. The electrical characteriza-tion revealed conduction limited by the electrode, although trapped-assisted mechanisms can also contribute to transport. The electroluminescence analysis revealed also emission from Tb3+ ions, yielded promising results to in-clude this material in future optoelectronics applications as integrated emitting devices

Effect of Si3N4-mediated inversion layer on the electroluminescence properties of silicon nanocrystal superlattices

The achievement of an efficient all-Si electrically-pumped light emitter is a major milestone in present optoelectronics still to be fulfilled. Silicon nanocrystals (Si NCs) are an attractive material which, by means of the quantum confinement effect, allow attaining engineered bandgap visible emission from Si by controlling the NC size. In this work, SiO2-embedded Si NCs are employed as an active layer within a light-emitting device structure. It is demonstrated that the use of an additional thin Si3N4 interlayer within the metal-insulator-semiconductor device design induces an enhanced minority carrier injection from the substrate, which in turn increases the efficiency of sequential carrier injection under pulsed electrical excitation. This results in a substantial increase in the electroluminescence efficiency of the device. Here, the effect of this Si3N4 interlayer on the structural, optical, electrical, and electro-optical properties of a Si NC-based light emitter is reported, and the physics underlying these results is discussed

Structural and optical properties of Al-Tb/SiO2 multilayers fabricated by electron beam evaporation

Light emitting Al-Tb/SiO2 nanomultilayers (NMLs) for optoelectronic applications have been produced and characterized. The active layers were deposited by electron beam evaporation onto crystalline silicon substrates, by alternatively evaporating nanometric layers of Al, Tb, and SiO2. After deposition, all samples were submitted to an annealing treatment for 1 h in N2 atmosphere at different temperatures, ranging from 700 to 1100 °C. Transmission electron microscopy confirmed the NML structure quality, and by complementing the measurements with electron energy-loss spectroscopy, the chemical composition of the multilayers was determined at the nanoscopic level. The average composition was also measured by X-ray photoelectron spectroscopy (XPS), revealing that samples containing Al are highly oxidized. Photoluminescence experiments exhibit narrow emission lines ascribed to Tb3+ ions in all samples (both as-deposited and annealed ones), together with a broadband related to SiO2 defects. The Tb-related emission intensity in the sample annealed at 1100 °C is more than one order of magnitude higher than identical samples without Al. These effects have been ascribed to the higher matrix quality, less SiO2 defects emitting, and a better Tb3+ configuration in the SiO2 matrix thanks to the higher oxygen content favored by the incorporation of Al atoms, as revealed by XPS experiments

Silicon nanocrystals-based electroluminescent resistive switching device

In the last few years, the emergence of studies concerning the resistive switching (RS) phenomenon has resulted in the finding of a large amount of materials being capable of acting as an active layer in such devices, i.e., the layer where the change in resistance takes place. Whereas the normal operation consists of the electrical readout of the modified resistance state of the device after electrical writing, electro-photonic approaches seek the involvement of light in these devices, be it either for the active Set or Reset operations or the readout. We propose in this work silicon nanocrystal multilayers (Si NC MLs) as an active material for being used in RS devices, taking advantage of their outstanding optical properties. The resistance states of Si NC MLs were obtained by electrical excitation, whose readout is carried out by electrical and electro-optical means, thanks to a distinguishable electroluminescence emission under each state. To achieve this, we report on an adequate design that combines both the Si NC MLs with ZnO as a transparent conductive oxide, whose material properties ensure the device RS performance while allowing the electro-optical characterization. Overall, such an occurrence states the demonstration of a Si NCs-based electroluminescent RS device, which paves the way for their future integration into photonic integrated circuits

Resistive switching and charge transport mechanisms in ITO/ZnO/p-Si devices

The resistive switching properties of ITO/ZnO/p-Si devices have been studied, which present welldefined resistance states with more than five orders of magnitude difference in current. Both the high resistance state (HRS) and the low resistance state (LRS) were induced by either sweeping or pulsing the voltage, observing some differences in the HRS. Finally, the charge transport mechanisms dominating the pristine, HRS, and LRS states have been analyzed in depth, and the obtained structural parameters suggest a partial re-oxidation of the conductive nanofilaments and a reduction of the effective conductive area. Published by AIP Publishing. https://doi.org/10.1063/1.504691

Modulation of the electroluminescence emission from ZnO/Si NCs/p-Si light-emitting devices via pulsed excitation

In this work, the electroluminescence (EL) emission of zinc oxide (ZnO)/Si nanocrystals (NCs)-based light-emitting devices was studied under pulsed electrical excitation. Both Si NCs and deep-level ZnO defects were found to contribute to the observed EL. Symmetric square voltage pulses (50-μs period) were found to notably enhance EL emission by about one order of magnitude. In addition, the control of the pulse parameters (accumulation and inversion times) was found to modify the emission lineshape, long inversion times (i.e., short accumulation times) suppressing ZnO defects contribution. The EL results were discussed in terms of the recombination dynamics taking place within the ZnO/Si NCs heterostructure, suggesting the excitation mechanism of the luminescent centers via a combination of electron impact, bipolar injection, and sequential carrier injection within their respective conduction regimes