Silicon-rich Oxide Layers Research Articles

Nanocomposites of Silicon Oxides and Carbon: Its Study as Luminescent Nanomaterials

In this work, hybrid structures formed by nanostructured layers, which contain materials, such as porous silicon (PSi), carbon nanotubes (CNTs), graphene oxide (GO), and silicon-rich oxide (SRO), were studied. The PSi layers were obtained by electrochemical etching over which CNTs and GO were deposited by spin coating. In addition, SRO layers, in which silicon nanocrystals are embedded, were obtained by hot filament chemical vapor deposition (HFCVD) technique. Photoluminescence (PL) spectra were obtained from the hybrid structures with which a comparative analysis was completed among different PL ones. The SRO layers were used to confine the CNTs and GO. The main purpose of making these hybrid structures is to modulate their PL response and obtain different emission energy regions in the PL response. It was found that the PL spectra of the CNTs/SRO and GO/SRO structures exhibit a shift towards high energies compared to those obtained from the PSi layers; likewise, the PSi/CNTs/SRO and PSi/GO/SRO structures show a similar behavior. To identify the different emission mechanisms originated by PSi, GO, CNTs, and SRO, the PL spectra were deconvolved. It was found that the Psi/CNTs/SRO and Psi/GO/SRO structures exhibit a PL shift in respect to the PSi layers, for this reason, the modulation of the PL emission of the structures makes these hybrid structures promising candidates to be applied in the field of photonic and electroluminescent devices.

Effect of the Graded Silicon Content in SRN/SRO Multilayer Structures on the Si Nanocrystals and Si Nanopyramids Formation and Their Photoluminescence Response.

Two multilayer (ML) structures, composed of five layers of silicon-rich oxide (SRO) with different Si contents and a sixth layer of silicon-rich nitride (SRN), were deposited by low pressure chemical vapor deposition. These SRN/SRO MLs were thermally annealed at 1100 °C for 180 min in ambient N2 to induce the formation of Si nanostructures. For the first ML structure (MLA), the excess Si in each SRO layer was about 10.7 ± 0.6, 9.1 ± 0.4, 8.0 ± 0.2, 9.1 ± 0.3 and 9.7 ± 0.4 at.%, respectively. For the second ML structure (MLB), the excess Si was about 8.3 ± 0.2, 10.8 ± 0.4, 13.6 ± 1.2, 9.8 ± 0.4 and 8.7 ± 0.1 at.%, respectively. Si nanopyramids (Si-NPs) were formed in the SRO/Si substrate interface when the SRO layer with the highest excess silicon (10.7 at.%) was deposited next to the MLA substrate. The height, base and density of the Si-NPs was about 2–8 nm, 8–26 nm and ~6 × 1011 cm−2, respectively. In addition, Si nanocrystals (Si-ncs) with a mean size of between 3.95 ± 0.20 nm and 2.86 ± 0.81 nm were observed for the subsequent SRO layers. Meanwhile, Si-NPs were not observed when the excess Si in the SRO film next to the Si-substrate decreased to 8.3 ± 0.2 at.% (MLB), indicating that there existed a specific amount of excess Si for their formation. Si-ncs with mean size of 2.87 ± 0.73 nm and 3.72 ± 1.03 nm were observed for MLB, depending on the amount of excess Si in the SRO film. An enhanced photoluminescence (PL) emission (eight-fold more) was observed in MLA as compared to MLB due to the presence of the Si-NPs. Therefore, the influence of graded silicon content in SRN/SRO multilayer structures on the formation of Si-NPs and Si-ncs, and their relation to the PL emission, was analyzed.

ZnO/Silicon-Rich Oxide Superlattices with High Thermoelectric Figure of Merit: A Comprehensive Study by Experiment and Molecular Dynamic Simulation.

ZnO is a direct band gap material that has numerous optoelectronic applications. Recently, the thermoelectric behavior of ZnO has drawn much attention because it is expected to enrich the multifunctional application of ZnO. However, the high thermal conductivity nature of ZnO (∼50 W/(m·K)) is a challenge to further increase its thermoelectrtic figure of merit ( ZT). In this paper, a way to increase the ZT of ZnO thin films by insertion of silicon-rich oxide (SRO) interlayers is reported. All of the constituents are earth-abundant and environmental friendly. The effects of the number of SRO layers, thickness, grain size, heat treatment, and crystallinity of ZnO of the superlattices on the thermoelectric behaviors of ZnO were investigated. The thermoelectric ZT was determined by the transient Harman method by measuring the Seebeck voltage. The thermal conductivity of the ZnO/SRO superlattices that is crucial to elucidate the ZT behaviors is calculated using molecular dynamic simulation, in which the Zn-O and Zn-Zn interactions were described by the Born-Mayer potential and the short-range non-Coulombic O-O interaction was described by the Morse potential. For a given total ZnO/SRO thickness, the grain size of the ZnO decreases monotonically with the increasing number of SRO layers, thus leading to a decrease of the thermal conductivity and an increase of the ZT of the superlattices. As the best result, the annealed 45 nm thick ZnO thin film with three SRO interlayers presents a high ZT of ∼0.16 at room temperature. A comprehensive study on the ZnO/SRO superlattice-based thermoelectrtic devices was carried out by the experiment and theoretical simulation. The results imply potential thermoelectric application of the ZnO/SRO superlattices.

Al-Doped ZnO/Silicon-rich Oxide Superlattices with High Room-Temperature Thermoelectric Figure of Merit

This research reports the thermoelectric properties of the Al-doped ZnO (AZO)/silicon-rich oxide (SRO) superlattices. The thermoelectric figure of merit (ZT) as functions of the grain size, thickness of the superlattices, the number of SRO layers, and conductance were studied. The use of the SRO layers markedly improved the thermoelectric ZT. Moreover, the replacement of ZnO by AZO further increased the ZT. The ZT value of the AZO/SRO superlattices was as high as 0.44 when the total thickness was 45 nm and three SRO interlayers were inserted. The improvement of ZT was contributed by the reduction of the grain size, formation of the Si nanocrystals and the increase in the electrical conductance caused by using SRO interlayers and Al doping.

Size‐Controlled Si Nanocrystals Fabricated by Electron Beam Evaporation

Multilayers consisting of silicon nanocrystals (Si NCs) and SiO2 are successfully fabricated by electron beam evaporation, using pure Si and SiO2 targets in an oxygen‐rich atmosphere for alternately depositing silicon‐rich oxide (SRO) layers and SiO2 barriers, respectively. A post‐deposition annealing process is carried out at different temperatures in order to achieve the Si precipitation in the form of nanocrystals. The stoichiometry of the layers is determined by X‐ray photoelectron spectroscopy, which confirms the controlled silicon oxidation in order to attain SRO layers. Transmission electron microscopy and Raman‐scattering measurements confirm the presence of crystalline Si‐nanoprecipitates. Photoluminescence spectra from the Si NC samples can be deconvolved into two contributions, whose dynamics suggest that two different luminescent centers are responsible for the optical emission of the samples.

Enhancing emission and conduction of light emitting capacitors by multilayered structures of silicon rich oxide

This work reports the morphological, electrical and luminescent characteristics of Light Emitting Capacitors (LECs) composed by Silicon Rich Oxide (SRO) multilayers. These multilayers alternate four conductive SRO layers (silicon excess of 12 or 14at%) with three light emitting SRO layers (silicon excess of 6at%). Transmission electron microscopy reveals that multilayers present well-defined nanoscale layers and interfaces in the multilayers. Furthermore, it is found that layers with high silicon content induce the formation of bigger silicon nanocrystals in the emitting layers as compared to single layers. When LECs are biased through a current-voltage measurement, an electroforming effect, where the current increases abruptly, is observed. This electroforming effect produces conductive trajectories arrays with high silicon content in the conductive SRO layers. These conductive paths allow to the LECs achieve higher current values with lower voltages preserving the luminescent characteristics of the emitting SRO layers. Consequently, it is demonstrated that silicon nanocrystals also act as sources of electro-hole pairs increasing the radiative recombination in the SRO defects. LECs that combine conductive and light emitting SRO films in a multilayered structure allow to enhance brightness and electrical characteristics compared to single layers. Depending on the conductive SRO layer preferential electroluminescence is produced in red or blue region.

Study of narrow and intense UV electroluminescence from ITO/SRO/Si-p and ITO/SRN/SRO/Si-p based light emitting capacitors

In this work, multiple narrow and highly intense ultraviolet (UV) electroluminescent (EL) bands were observed in light emitting capacitors (LECs) using silicon rich oxide (SRO) films as active layer. Besides, the effect of a thin silicon rich nitride (SRN) film on top of the SRO (as SRN/SRO bilayer) layer was also studied. LECs were fabricated using simple metal–insulator–semiconductor (MIS) structures with indium tin oxide (ITO) and aluminum as gate and substrate electrodes, respectively. SRO and SRN films contain 41.85±1.1 and 46.96±1.1at% of silicon, respectively. Both structures exhibited a resistance switching (RS) behavior from a high conduction state (HCS) to a low conduction state (LCS), enhancing an intense UV EL. This RS behavior produces structural changes in the active layer and probably in the ITO contact. Seven narrow bands with half-peak width of 7±0.6nm at ~250, 270, 285, 305, 325, 415 and 450nm were clearly observed once the LCS was reached. These bands could be related to a combination of emissions through defects inside SRO (252, 288.2 and 415nm), and characteristic radiation of neutral tin (252.39 and 286.33nm), neutral indium (271.02, 303.93 and 325.85nm), single (444.82nm) and doubly ionized indium (403.07nm). Furthermore, red EL was observed at the HCS and it was similar to the PL spectra indicating the same radiative process involved. The charge transport is improved when the SRN/SRO bilayer is used as active layer in the LEC. An EL band at ~590nm is observed when the SRN/SRO bilayer is formed at both conduction states. This band has been observed before and attributed to transitions from the minimum conduction band to K° centers in SRN films. The conduction mechanism responsible of the EL at both conduction states was also studied.

Structural and optical properties of silicon rich oxide films in graded-stoichiometric multilayers for optoelectronic devices

Silicon nanocrystals (Si-ncs) are excellent candidates for the development of optoelectronic devices. Nevertheless, different strategies are still necessary to enhance their photo and electroluminescent properties by controlling their structural and compositional properties. In this work, the effect of the stoichiometry and structure on the optical properties of silicon rich oxide (SRO) films in a multilayered (ML) structure is studied. SRO MLs with silicon excess gradually increased towards the top and bottom and towards the center of the ML produced through the variation of the stoichiometry in each SRO layer were fabricated and confirmed by X-ray photoelectron spectroscopy. Si-ncs with three main sizes were observed by a transmission electron microscope, in agreement with the stoichiometric profile of each SRO layer. The presence of the three sized Si-ncs and some oxygen related defects enhances intense violet/blue and red photoluminescence (PL) bands. The SRO MLs were super-enriched with additional excess silicon by Si+ implantation, which enhanced the PL intensity. Oxygen-related defects and small Si-ncs (<2 nm) are mostly generated during ion implantation enhancing the violet/blue band to become comparable to the red band. The structural, compositional, and luminescent characteristics of the multilayers are the result of the contribution of the individual characteristics of each layer.

Impact of the Structural Characteristics on the Performance of Light Emitting Capacitors Using Nanometric SRO Multilayers Fabricated by LPCVD

This work relates the electrical, luminescent and morphological characteristics of two light emitting capacitors composed by multilayers of Silicon Rich Oxide (SRO). Multilayers alternate four conductive SRO layers (silicon excess of 12 or 14 at %) with three emitting SRO layers (silicon excess of 6 at %). Transmission electron microscopy reveals that multilayers present well-defined layers. Furthermore, it was found that layers with high silicon content induce growing of Si-nanocristals size on layers with lower silicon excess. After the first current versus voltage measurement, electroforming produces arrays of trajectories with high silicon content. These conductive paths allow that the LEC achieves higher currents with lower voltages, preserving the emitting characteristics of SRO layers. Consequently, the electroluminescence intensity is improved as well as the blue emission depending on the conductive SRO layers.

Enhancing the electroluminescence efficiency of Si NC/SiO2 superlattice-based light-emitting diodes through hydrogen ion beam treatment.

This paper presents a novel method for enhancing the electroluminescence (EL) efficiency of ten-period silicon-rich oxide (SRO)/SiO2 superlattice-based light-emitting diodes (LEDs). A hydrogen ion beam (HIB) was used to irradiate each SRO layer of the superlattices to increase the interfacial roughness on the nanoscale and the density of the Si nanocrystals (Si NCs). Fowler-Nordheim (F-N) tunneling was the major mechanism for injecting the carriers into the Si NCs. The barrier height of the F-N tunneling was lowered by forming a nano-roughened interface and the nonradiative Pb centers were passivated through the HIB treatment. Additionally, the reflectance of the LEDs was lowered because of the nano-roughened interface. These factors considerably increased the slope efficiency of EL and the maximum output power of the LEDs. The lighting efficiency increased by an order of magnitude, and the turn-on voltage decreased considerably. This study established an efficient approach for obtaining bright Si NC/SiO2 superlattice-based LEDs.

Synthesis and memory properties of a self-assembled Al@Al2O3 core-shell nanoparticle layer for floating gate devices

A simple method for fabricating an Al@Al2O3 core-shell nanoparticle (NP) layer with narrow spatial distributions is demonstrated, and the resultant NP layer was examined for possible application to floating gate memory devices. The Al@Al2O3 core-shell NP layer was self-assembled by annealing a sandwich structure composed of a thin Al film sandwiched between two silicon-rich oxide layers within SiO2. Capacitance-voltage (C-V) measurements on a metal-oxide-semiconductor capacitor with an Al@Al2O3 core-shell NP floating gate are indicated to have a large C-V memory window and good data retention.

Analysis of carrier injection in Si nanoparticle‐SiOx film based MOS devices

AbstractThe electrical properties of silicon rich oxide (SRO) layers integrated in metal‐oxide‐semiconductor device are analysed. The layers are deposited using the magnetron co‐sputtering of a pure SiO2 and Si targets under a pure argon plasma. Each SRO layer embedding silicon nanoparticles (Si‐np‐SiOx) was subsequently submitted to an optimized annealing treatment. Three types of Al/Si‐np‐SiOx/p‐Si devices are fabricated with different incorporated silicon excesses (9, 11, or 16 at.%) in the active layer. Analysis of static electrical properties of the devices showed a semi‐resistive behaviour for each device. Carrier injection into the SiOx matrix in terms of Poole‐Frenkel and Fowler‐Nordheim mechanisms is analysed in relation with the incorporated silicon excess. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced white photoluminescence in silicon-rich oxide/SiO_2 superlattices by low-energy ion-beam treatment

This study presents the crystalline and luminescence properties of silicon-rich oxide (SRO)/SiO2 superlattices in which the SRO layers were prepared with a low-energy (<60 eV) argon ion-beam treatment. Experimental results evidenced that density of the Si nanocrystals (NCs) in the SRO layer was increased by ion-beam treatment after annealing, increasing the surface roughness. The stoichiometry of the as-prepared SRO layer was unchanged but the phase separation of the annealed SRO layer was enhanced by the ion-beam treatment, yielding visible white photoluminescence from the E' centers and Si NCs.

Cellular Automation for the Self-Organization Processes at the Annealing of Silicon Rich Oxide Layer

Process of phase separation at the annealing (~10000C) of nonstoichiometric SiOm (m<2) layer is simulated by means of a synchronous cellular automate. The suggested model is opened for introduction of ab initio data concerning SixOy-clusters. Simulation shows migration small Si-nanoclusters inside of this layer.

Resistive switching in silicon suboxide films

We report a study of resistive switching in a silicon-based memristor/resistive RAM (RRAM) device in which the active layer is silicon-rich silica. The resistive switching phenomenon is an intrinsic property of the silicon-rich oxide layer and does not depend on the diffusion of metallic ions to form conductive paths. In contrast to other work in the literature, switching occurs in ambient conditions, and is not limited to the surface of the active material. We propose a switching mechanism driven by competing field-driven formation and current-driven destruction of filamentary conductive pathways. We demonstrate that conduction is dominated by trap assisted tunneling through noncontinuous conduction paths consisting of silicon nanoinclusions in a highly nonstoichiometric suboxide phase. We hypothesize that such nanoinclusions nucleate preferentially at internal grain boundaries in nanostructured films. Switching exhibits the pinched hysteresis I/V loop characteristic of memristive systems, and on/off resistance ratios of 104:1 or higher can be easily achieved. Scanning tunneling microscopy suggests that switchable conductive pathways are 10 nm in diameter or smaller. Programming currents can be as low as 2 μA, and transition times are on the nanosecond scale.

Si solid-state quantum dot-based materials for tandem solar cells

The concept of third-generation photovoltaics is to significantly increase device efficiencies whilst still using thin-film processes and abundant non-toxic materials. A strong potential approach is to fabricate tandem cells using thin-film deposition that can optimise collection of energy in a series of cells with decreasing band gap stacked on top of each other. Quantum dot materials, in which Si quantum dots (QDs) are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin-film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approximately 2-nm diameter QDs, these result in an effective band gap of 1.8 eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. A modified modulation doping model for the doping mechanisms in these materials is discussed which relies on doping of a sub-oxide region around the Si QDs.

Cellular Automation for the Self-Organization Processes at the Annealing of Silicon Rich Oxide Layer

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Si and Other Group IV Quantum Dot Based Materials for Tandem Solar Cells

Quantum dot materials, in which Si QDs are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approx. 2nm diameter QDs these result in an effective band gap of 1.8eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open-circuit voltage of almost 500mV. However, the doping behaviour of P and B in these QD materials is not well understood. In addition P and B have large but opposite effects on QD crystallisation, with P (B) doped material forming larger (smaller) QDs than for undoped material. Alternative materials for quantum dots are Ge and Sn. These allow lower processing temperatures to be used, more compatible with underlying layers. Alternative matrices to SiO2 such as SiNx or SiC offer higher tunnelling probability and hence lower resistance. These alternative matrix materials can also be used as hetero interlayers to improve the transport in the growth direction whilst maintaining quantum confinement. Group IV alloys can also be used to modify band gap. GeC in particular looks to have useful band gap and sputtering properties. Such alloy materials could be used in hetero-junction or homojunction devices in combination with SiQD based materials to fabricate all thin film tandem cells.

Activation of Photoluminescence of Multilayer Arrays of Silicon Rich Oxide by Oxidation at Different Temperatures

Multilayer structures composed of layers of silicon rich oxide (SRO) with high and low Si content have been prepared by low pressure chemical vapor deposition. These multilayers were oxidized at 800, 900, 1000, and 1100 °C under dry oxygen to study the possibility of photoluminescence production. Only the samples oxidized at 1000 and 1100 °C present this effect. The spectra of the two photoluminescent samples can be deconvoluted in two asymmetric peaks positioned at 1.53 and 1.73 eV. The peak at 1.73 eV has been previously found in Silicon Rich Oxide with low Si content and without Si-nanocrystals, and has been ascribed to defects in the silicon oxide matrix. The other peak has been attributed to defects in the Si-nanocrystals/SiO2 interface.

Intrinsic Resistive Switching in Bulk SiOx Films

ABSTRACTWe report a study of resistive switching in a silicon-based memristor/resistive RAM (RRAM) device in which the active layer is silicon-rich silica. The resistive switching phenomenon is an intrinsic property of the silicon-rich oxide layer and does not depend on the diffusion of metallic ions to form conductive paths. Both unipolar and bipolar programming is demonstrated.Switching exhibits the pinched hysteresis I/V loop characteristic of RRAM/memristive systems, and on/off resistance ratios of 104:1 or higher can be easily achieved. Scanning Tunnelling Microscopy suggests that switchable conductive pathways are 10nm in diameter or smaller.