Larysa Khomenkova

Towards an optimum coupling between Er ions and Si-based sensitizers for integrated active photonics

Series of Er-doped Si-rich silicon oxide layers were studied with the aim of optimizing the coupling between Er ions and the Si-based sensitizers. The layers were grown at substrate temperature between 400 and 600 °C by the cosputtering of three confocal targets: Si, SiO2, and Er2O3. The influence of Si excess (5–15 at. %) and annealing temperature (500–1100 °C) was examined for two concentrations of Er ions (3.5x1020 and ~1021 cm−3). We report the first observation of significant Er photoluminescence (PL) from as-grown samples excited by a nonresonant 476 nm line, with a lifetime in the range of 1.3–4 ms. This suggests the occurrence of an indirect excitation of Er through Si-based entities formed during the deposition. A notable improvement was observed for both Er PL intensity and lifetime after annealing at 600 °C. This temperature is lower than that required for phase separation, suggesting the formation of “atomic scale” sensitizers (Si agglomerates, for example) considered in recent work. For high Er doping (~1021 cm−3), an optimum Er PL was obtained for the sample grown at 500 °C, annealed at 600 °C, and containing ~13 at. % of Si excess. This high PL should correspond to an optimum fraction of coupled Er for this series, which was roughly estimated to about 11% of the total Er content for an excitation photon flux of few 1019 ph cm−2 s−1. For the moderately Er-doped series (3.5x1020 cm−3) grown at 500 °C, the optimum Er PL was found for the samples containing about 9 at. % silicon and annealed in the 600–900 °C range. The time decay reached a value as high as 9 ms for low Si excess (<6 at. %) and 6–7.5 ms for high values of Si excess. The fraction of Er ions coupled to sensitizers was similarly estimated for the best sample of this series and found to be as high as 22% of the total Er content.

Hf-based high-k materials for Si nanocrystal floating gate memories

Pure and Si-rich HfO2 layers fabricated by radio frequency sputtering were utilized as alternative tunnel oxide layers for high-k/Si-nanocrystals-SiO2/SiO2 memory structures. The effect of Si incorporation on the properties of Hf-based tunnel layer was investigated. The Si-rich SiO2 active layers were used as charge storage layers, and their properties were studied versus deposition conditions and annealing treatment. The capacitance-voltage measurements were performed to study the charge trapping characteristics of these structures. It was shown that with specific deposition conditions and annealing treatment, a large memory window of about 6.8 V is achievable at a sweeping voltage of ± 6 V, indicating the utility of these stack structures for low-operating-voltage nonvolatile memory devices.

Thermal stability of high-k Si-rich HfO2 layers grown by RF magnetron sputtering

The microstructure and optical properties of HfSiO films fabricated by RF magnetron sputtering were studied by means of x-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry and attenuated total reflection infrared spectroscopy versus annealing treatment. It was shown that silicon incorporation in the HfO(2) matrix plays an important role in the structure stability of the layers. Thus, the increase of the annealing temperature up to 1000 degrees C did not lead to the crystallization of the films. The evolution of the chemical composition as well as a decrease of the density of the films was attributed to the phase separation of HfSiO on HfO(2) and SiO(2) phases in the film. An annealing at 1000-1100 degrees C results in the formation of the multilayer Si-rich/Hf-rich structure and was explained by a surface-directed spinodal decomposition. The formation of the stable tetragonal structure of HfO(2) phase was shown upon annealing treatment at 1100 degrees C.

High-k Hf-based layers grown by RF magnetron sputtering

Structural and chemical properties of Hf-based layers fabricated by RF magnetron sputtering were studied by means of x-ray diffraction, transmission electron microscopy and attenuated total reflection infrared spectroscopy versus the deposition parameters and annealing treatment. The deposition and post-deposition conditions allow us to control the temperature of the amorphous-crystalline phase transition of HfO(2)-based layers. It was found that silicon incorporation in an HfO(2) matrix plays the main role in the structural stability of the layers. It allows us not only to decrease the thickness of the film/substrate interfacial layer to 1 nm, but also to conserve the amorphous structure of the layers after an annealing treatment up to 900-1000 degrees C.

Optically active Er3+ ions in SiO2 codoped with Si nanoclusters

Optical properties of directly excited erbium (Er3+) ions have been studied in silicon rich silicon oxide materials codoped with Er3+. The spectral dependence of the direct excitation cross section (σdir) of the Er3+ atomic 4I152→4I112 transition (around 0.98 μm) has been measured by time resolved µ-photoluminescence measurements. We have determined that σdir is 9.0±1.5 x10−21 cm2 at 983 nm, at least twice larger than the value determined on a stoichiometric SiO2 matrix. This result, in combination with a measurement of the population of excited Er3+ as a function of the pumping flux, has allowed quantifying accurately the amount of optically active Er3+. This concentration is, in the best of the cases, 26% of the total Er population measured by secondary ion mass spectrometry, which means that only this percentage could provide optical gain in an eventual optical amplifier based on this material.

Nanoscale evidence of erbium clustering in Er-doped silicon-rich silica

Photoluminescence spectroscopy and atom probe tomography were used to explore the optical activity and microstructure of Er3+-doped Si-rich SiO2 thin films fabricated by radio-frequency magnetron sputtering. The effect of post-fabrication annealing treatment on the properties of the films was investigated. The evolution of the nanoscale structure upon an annealing treatment was found to control the interrelation between the radiative recombination of the carriers via Si clusters and via 4f shell transitions in Er3+ ions. The most efficient 1.53-μ m Er3+ photoluminescence was observed from the films submitted to low-temperature treatment ranging from 600°C to 900°C. An annealing treatment at 1,100°C, used often to form Si nanocrystallites, favors an intense emission in visible spectral range with the maximum peak at about 740 nm. Along with this, a drastic decrease of 1.53-μ m Er3+ photoluminescence emission was detected. The atom probe results demonstrated that the clustering of Er3+ ions upon such high-temperature annealing treatment was the main reason. The diffusion parameters of Si and Er3+ ions as well as a chemical composition of different clusters were also obtained. The films annealed at 1,100°C contain pure spherical Si nanocrystallites, ErSi3O6 clusters, and free Er3+ ions embedded in SiO2 host. The mean size and the density of Si nanocrystallites were found to be 1.3± 0.3 nm and (3.1± 0.2)×1018 Si nanocrystallites·cm−3, respectively. The density of ErSi3O6 clusters was estimated to be (2.0± 0.2)×1018 clusters·cm−3, keeping about 30% of the total Er3+ amount. These Er-rich clusters had a mean radius of about 1.5 nm and demonstrated preferable formation in the vicinity of Si nanocrystallites.

SiOx/SiNy multilayers for photovoltaic and photonic applications

Microstructural, electrical, and optical properties of undoped and Nd3+-doped SiOx/SiNy multilayers fabricated by reactive radio frequency magnetron co-sputtering have been investigated with regard to thermal treatment. This letter demonstrates the advantages of using SiNy as the alternating sublayer instead of SiO2. A high density of silicon nanoclusters of the order 1019 nc/cm3 is achieved in the SiOx sublayers. Enhanced conductivity, emission, and absorption are attained at low thermal budget, which are promising for photovoltaic applications. Furthermore, the enhancement of Nd3+ emission in these multilayers in comparison with the SiOx/SiO2 counterparts offers promising future photonic applications.PACS: 88.40.fh (Advanced materials development), 81.15.cd (Deposition by sputtering), 78.67.bf (Nanocrystals, nanoparticles, and nanoclusters).

Atomic scale observation of phase separation and formation of silicon clusters in Hf higk-κ silicates

Hafnium silicate films were fabricated by RF reactive magnetron sputtering technique. Fine microstructural analyses of the films were performed by means of high-resolution transmission electron microscopy and atom probe tomography. A thermal treatment of as-grown homogeneous films leads to a phase separation process. The formation of SiO2 and HfO2 phases as well as pure Si one was revealed. This latter was found to be amorphous Si nanoclusters, distributed uniformly in the film volume. Their mean diameter and density were estimated to be about 2.8 nm and (2.960.4) 1017 Si-ncs/cm3, respectively. The mechanism of the decomposition process was proposed. The obtained results pave the way for future microelectronic and photonic applications of Hf-based high-j dielectrics with embedded Si nanoclusters

Efficient energy transfer from Si-nanoclusters to Er ions in silica induced by substrate heating during deposition

This study investigates the influence of the deposition temperature Td on the Si-mediated excitation of Er ions within silicon-rich silicon oxide layers obtained by magnetron cosputtering. For Td exceeding 200 °C, an efficient indirect excitation of Er ions is observed for all as-deposited samples. The photoluminescence intensity improves gradually up to a maximum at Td=600 °C before decreasing for higher Td values. The effects of this “growth-induced annealing” are compared to those resulting from the same thermal budget used for the “classical” approach of postdeposition annealing performed after a room temperature deposition. It is demonstrated that the former approach is highly beneficial, not only in terms of saving time but also in the fourfold enhancement of the Er photoluminescence efficiency.

Microstructure and optical properties of Pr3+-doped hafnium silicate films

In this study, we report on the evolution of the microstructure and photoluminescence properties of Pr3+-doped hafnium silicate thin films as a function of annealing temperature (TA). The composition and microstructure of the films were characterized by means of Rutherford backscattering spectrometry, spectroscopic ellipsometry, Fourier transform infrared absorption, and X-ray diffraction, while the emission properties have been studied by means of photoluminescence (PL) and PL excitation (PLE) spectroscopies. It was observed that a post-annealing treatment favors the phase separation in hafnium silicate matrix being more evident at 950°C. The HfO2 phase demonstrates a pronounced crystallization in tetragonal phase upon 950°C annealing. Pr3+ emission appeared at TA = 950°C, and the highest efficiency of Pr3+ ion emission was detected upon a thermal treatment at 1,000°C. Analysis of the PLE spectra reveals an efficient energy transfer from matrix defects towards Pr3+ ions. It is considered that oxygen vacancies act as effective Pr3+ sensitizer. Finally, a PL study of undoped HfO2 and HfSiOx matrices is performed to evidence the energy transfer.