J. López-Vidrier

Determining the crystalline degree of silicon nanoclusters/SiO2 multilayers by Raman scattering

We use Raman scattering to investigate the size distribution, built-in strains and the crystalline degree of Si-nanoclusters (Si-nc) in high-quality Si-rich oxynitride/SiO2 multilayered samples obtained by plasma enhanced chemical vapor deposition and subsequent annealing at 1150 °C. An initial structural characterization of the samples was performed by means of energy-filtered transmission electron microscopy (EFTEM) and X-ray diffraction (XRD) to obtain information about the cluster size and the presence of significant amounts of crystalline phase. The contributions to the Raman spectra from crystalline and amorphous Si were analyzed by using a phonon confinement model that includes the Si-nc size distribution, the influence of the matrix compressive stress on the clusters, and the presence of amorphous Si domains. Our lineshape analysis confirms the existence of silicon precipitates in crystalline state, in good agreement with XRD results, and provides also information about the presence of a large com...

Absence of quantum confinement effects in the photoluminescence of Si3N4–embedded Si nanocrystals

Superlattices of Si-rich silicon nitride and Si3N4 are prepared by plasma-enhanced chemical vapor deposition and, subsequently, annealed at 1150 °C to form size-controlled Si nanocrystals (Si NCs) embedded in amorphous Si3N4. Despite well defined structural properties, photoluminescence spectroscopy (PL) reveals inconsistencies with the typically applied model of quantum confined excitons in nitride-embedded Si NCs. Time-resolved PL measurements demonstrate 105 times faster time-constants than typical for the indirect band structure of Si NCs. Furthermore, a pure Si3N4 reference sample exhibits a similar PL peak as the Si NC samples. The origin of this luminescence is discussed in detail on the basis of radiative defects and Si3N4 band tail states in combination with optical absorption measurements. The apparent absence of PL from the Si NCs is explained conclusively using electron spin resonance data from the Si/Si3N4 interface defect literature. In addition, the role of Si3N4 valence band tail states as...

Charge transport and electroluminescence of silicon nanocrystals/SiO2 superlattices

Charge transport and electroluminescence mechanisms in Si-rich Si oxynitride/silicon oxide (SRON/SiO2) superlattices deposited on p-type Si substrate are reported. The superlattice structures were deposited by plasma-enhanced chemical-vapor deposition and subsequently annealed at 1150 °C to precipitate and crystallize the Si excess into Si nanocrystals. The dependence of the electrical conduction on the applied voltage and temperature was found to be well described by a Poole-Frenkel transport mechanism over a wide voltage range. On the other hand, the observed dependence of the electroluminescence on the SRON layer thickness is a clear proof of quantum confinement and was attributed to an excitonic radiative recombination taking place in the confined states within the Si quantum dots. A model is proposed based on thermal hopping of electrons between the quantum dots acting as trap states (Poole-Frenkel). A correlation between carrier transport and electroluminescence has been established considering impa...

Flexible inkjet printed high-k HfO2-based MIM capacitors

The soaring global demand for flexible, wearable and transparent devices has created an urgent need for new fabrication technologies that are both cost-competitive and eco-friendly. Printed electronics holds the promise of enabling low-cost, scalable solutions exploiting the ability of innovative materials to be used as processed inks onto a large area substrate. In this article, we demonstrate the direct drop-on-demand inkjet printing technology as a viable method for the fabrication of fully-printed metal–insulator–metal capacitors on a flexible substrate (Kapton®), where the high-k hafnium oxide (HfO2) was selected as the dielectric. After a low-temperature annealing process, the deposited nanoparticle (NP)-based ink of HfO2 showed high homogeneity and good integrity of the printed thin film by microscopy and spectroscopy studies. The fully-printed capacitors were characterized by field-emission scanning and transmission electron microscopies. X-ray diffraction patterns, as well as Raman scattering and Fourier-transform infrared spectra, revealed the presence of a polycrystalline solid layer, without solvent organic ink remains. The bonding structure of the HfO2 layer and the interface with the Ag electrode was studied by X-ray photoelectron spectroscopy. The good performance of the thin film was proved by its relative permittivity, k = 12.6, and dielectric loss tangent, tan δ = 0.0125 at 1 MHz. Finally, the electrical current density–voltage and capacitance–voltage measurements have been studied in the frequency range from 10 kHz to 1 MHz. The obtained results indicate that MIM capacitors based on inkjet-printed flexible HfO2 NPs work properly within the ITRS 2016 roadmap requirements.

Annealing temperature and barrier thickness effect on the structural and optical properties of silicon nanocrystals/SiO2 superlattices

The effect of the annealing temperature and the SiO2 barrier thickness of silicon nanocrystal (NC)/SiO2 superlattices (SLs) on their structural and optical properties is investigated. Energy-filtered transmission electron microscopy (TEM) revealed that the SL structure is maintained for annealing temperatures up to 1150 °C, with no variation on the nanostructure morphology for different SiO2 barrier thicknesses. Nevertheless, annealing temperatures as high as 1250 °C promote diffusion of Si atoms into the SiO2 barrier layers, which produces larger Si NCs and the loss of the NC size control expected from the SL approach. Complementary Raman scattering measurements corroborated these results for all the SiO2 and Si-rich oxynitride layer thicknesses. In addition, we observed an increasing crystalline fraction up to 1250 °C, which is related to a decreasing contribution of the suboxide transition layer between Si NCs and the SiO2 matrix due to the formation of larger NCs. Finally, photoluminescence measuremen...

Defect-Induced Luminescence Quenching vs. Charge Carrier Generation of Phosphorus Incorporated in Silicon Nanocrystals as Function of Size

Phosphorus doping of silicon nanostructures is a non-trivial task due to problems with confinement, self-purification and statistics of small numbers. Although P-atoms incorporated in Si nanostructures influence their optical and electrical properties, the existence of free majority carriers, as required to control electronic properties, is controversial. Here, we correlate structural, optical and electrical results of size-controlled, P-incorporating Si nanocrystals with simulation data to address the role of interstitial and substitutional P-atoms. Whereas atom probe tomography proves that P-incorporation scales with nanocrystal size, luminescence spectra indicate that even nanocrystals with several P-atoms still emit light. Current-voltage measurements demonstrate that majority carriers must be generated by field emission to overcome the P-ionization energies of 110–260 meV. In absence of electrical fields at room temperature, no significant free carrier densities are present, which disproves the concept of luminescence quenching via Auger recombination. Instead, we propose non-radiative recombination via interstitial-P induced states as quenching mechanism. Since only substitutional-P provides occupied states near the Si conduction band, we use the electrically measured carrier density to derive formation energies of ~400 meV for P-atoms on Si nanocrystal lattice sites. Based on these results we conclude that ultrasmall Si nanovolumes cannot be efficiently P-doped.

Boron-Incorporating Silicon Nanocrystals Embedded in SiO 2 : Absence of Free Carriers vs. B-Induced Defects

Boron (B) doping of silicon nanocrystals requires the incorporation of a B-atom on a lattice site of the quantum dot and its ionization at room temperature. In case of successful B-doping the majority carriers (holes) should quench the photoluminescence of Si nanocrystals via non-radiative Auger recombination. In addition, the holes should allow for a non-transient electrical current. However, on the bottom end of the nanoscale, both substitutional incorporation and ionization are subject to significant increase in their respective energies due to confinement and size effects. Nevertheless, successful B-doping of Si nanocrystals was reported for certain structural conditions. Here, we investigate B-doping for small, well-dispersed Si nanocrystals with low and moderate B-concentrations. While small amounts of B-atoms are incorporated into these nanocrystals, they hardly affect their optical or electrical properties. If the B-concentration exceeds ~1 at%, the luminescence quantum yield is significantly quenched, whereas electrical measurements do not reveal free carriers. This observation suggests a photoluminescence quenching mechanism based on B-induced defect states. By means of density functional theory calculations, we prove that B creates multiple states in the bandgap of Si and SiO2. We conclude that non-percolated ultra-small Si nanocrystals cannot be efficiently B-doped.

Memristive behaviour of Si-Al oxynitride thin films: the role of oxygen and nitrogen vacancies in the electroforming process

The resistive switching properties of silicon-aluminium oxynitride (SiAlON) based devices have been studied. Electrical transport mechanisms in both resistance states were determined, exhibiting an ohmic behaviour at low resistance and a defect-related Poole-Frenkel mechanism at high resistance. Nevertheless, some features of the Al top-electrode are generated during the initial electroforming, suggesting some material modifications. An in-depth microscopic study at the nanoscale has been performed after the electroforming process, by acquiring scanning electron microscopy and transmission electron microscopy images. The direct observation of the devices confirmed features on the top electrode with bubble-like appearance, as well as some precipitates within the SiAlON. Chemical analysis by electron energy loss spectroscopy has demonstrated that there is an out-diffusion of oxygen and nitrogen ions from the SiAlON layer towards the electrode, thus forming silicon-rich paths within the dielectric layer and indicating vacancy change to be the main mechanism in the resistive switching.

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.

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 emiss...