A.V. Emelyanov

Visible luminescence from hydrogenated amorphous silicon modified by femtosecond laser radiation

Visible luminescence is observed from the composite of SiO2 with embedded silicon nanocrystallites produced by femtosecond laser irradiation of hydrogenated amorphous silicon (a-Si:H) film in air. The photoluminescence originates from the defect states at the interface between silicon crystallites and SiO2 matrix. The method could be used for fabrication of luminescent layers to increase energy conversion of a-Si:H solar cells.

Effect of the femtosecond laser treatment of hydrogenated amorphous silicon films on their structural, optical, and photoelectric properties

The effect of the femtosecond laser treatment of hydrogenated amorphous silicon (a-Si:H) films on their structural, optical, and photoelectric properties is studied. Under the experimental conditions applied in the study, laser treatment of the film with different radiation intensities induces structural changes that are nonuniform over the film surface. An increase in the radiation intensity yields an increase in the contribution of the nanocrystalline phase to the structure, averaged over the sample surface, as well as an increase in the conductance and photoconductance of the samples. At the same time, for all of the samples, the absorption spectrum obtained by the constant-photocurrent method has a shape typical for those of amorphous silicon. Obtained results indicate the possibility of a-Si:H films photoconductance increase by femtosecond pulse laser treatment.

Features of the structure and defect states in hydrogenated polymorphous silicon films

The structural and electronic properties of thin hydrogenated polymorphous silicon films obtained by plasma-enhanced chemical vapor deposition from hydrogen (H2) and monosilane (SiH4) gas mixture have been studied by means of transmission electron microscopy, electron paramagnetic resonance (EPR) spectroscopy, and Raman spectroscopy. It has been established that the studied films consist of the amorphous phase containing silicon nanocrystalline inclusions with the average size on the order of 4–5 nm and the volume fraction of 10%. A signal was observed in the hydrogenated polymorphous silicon films during the EPR investigation that is attributed to the electrons trapped in the conduction band tail of microcrystalline silicon. It has been shown that the introduction of a small fraction of nanocrystals into the amorphous silicon films nonadditively changes the electronic properties of the material.

Photoluminescence study of the structural evolution of amorphous and crystalline silicon nanoclusters during the thermal annealing of silicon suboxide films with different stoichiometry

The effect of the stoichiometry of thin silicon suboxide films on the processes of the formation and evolution of silicon nanoclusters during thermal annealing is studied by photoluminescence measurements. The samples are produced by the thermal sputtering of a SiO powder in an oxygen atmosphere, with the subsequent deposition of a 500 nm-thick SiOx layer onto a Si substrate. The morphological properties and size of Si nanoclusters are explored by analyzing the photoluminescence spectra and kinetics. A comparative study of the luminescence properties of thin SiOx layers with different stoichiometric parameters, x = 1.10, 1.29, 1.56, and 1.68, is accomplished for samples annealed at different temperatures in the range 850 to 1200°C. The dependences of the photoluminescence decay time on the annealing temperature, the stoichiometric parameter of the initial silicon suboxide film, and the nanocluster size are studied.

Influence of the fabrication conditions of polymorphous silicon films on their structural, electrical and optical properties

The structural, optical, and photoelectric properties of polymorphous silicon films produced by plasma-enhanced chemical vapor deposition from a mixture of monosilane and hydrogen at high pressure are studied. Variations in the pressure of the gas mixture used for film production barely change the Raman spectra of the films, but induce changes in the photoconductivity and in the absorption spectrum obtained by the constant-photocurrent technique. The experimentally observed change in the optical and photoelectric parameters of the films is attributed to some structural changes induced in the films by variations in the deposition parameters.

Modification of the structure and hydrogen content of amorphous hydrogenated silicon films under conditions of femtosecond laser-induced crystallization

We have studied the Raman spectra of initially amorphous hydrogenated silicon (a-Si:H) films upon their exposure to femtosecond laser-radiation pulses with the fluence varied within 30–155 mJ/cm2. The distribution of the volume fraction of a crystalline phase over the surface of processed films is determined for the first time and a correlation is established between changes in this value and the hydrogen content in a-Si:H films upon the crystallization induced by femtosecond laser radiation.

Magnetic Metal-Nonstoichiometric Oxide Nanocomposites: Structure, Transport, and Memristive Properties

Abstract Magnetic metal-insulator nanocomposites (NCs) consist of an ensemble of ultrafine ferromagnetic nanogranules chaotically distributed in a nonmagnetic dielectric matrix. Depending on chemical compositions of constituent materials, a volume fraction of metal close or far from the percolation threshold, technological parameters during fabrication these materials exhibit a wide diversity of magnetic, transport, high frequency, optical and magneto-optical properties important for fundamental research and promising for various applications. In this chapter structural, magnetic, magneto-transport, and memristive properties of NCs thin films (Co40Fe40B20)х(Al2O3–z)100–х and (Co40Fe40B20)х(LiNbO3–z)100–х fabricated by ion-beam sputtering are discussed focusing on novel features such as presence of a large amount of dispersed metal ions in the matrix, logarithmic temperature dependence of electrical resistivity, tunneling anomalous Hall effect, resistive switching.

Structural and electrophysical properties in LiNbO3/CoFeB nanocomposite films

Memristive devices are the key contenders for the development of multilevel nonvolatile analog memories and parallel neuromorphic computing architectures [1]. In particular, metal oxide memristors have emerged as promising candidates for hardware implementation of artificial synapses in spiking networks due to their excellent scaling prospects [2]. Memristors with fast switching speed, high endurance and long retention time have been reported in various oxides. Among metal oxides the LiNbO3 based system has emerged as a promising candidate for the functional oxide layer in memristive devices [3]. In the present work we study structural and electrophysical properties of (Co41Fe39B20)x(LiNbO3)100-x nanocomposite (LNO NC) films and their possible application in spiking neuromorphic networks. The LNO NC films of thickness 3 μm were synthesized by ion-beam sputtering of a composite target, allowing in a single cycle formation of Co41Fe39B20 nanoparticles in the LiNbO3 matrix with oxygen vacancies. The factor x was varied in a range of 5 – 48 at. %. For the film with x ~ 10 at. % resistive switching (RS) effect was observed. RS weakly dependent on the contacts material (Cu, Cr) and the thickness of the LNO NC layer. The number of switching cycles (endurance) exceeds Nmax > 105, and the high-resistance to low-resistance state ratio was Roff/Ron = 65. The obtained value of Nmax is comparable with those got in HfO2-based memristors in which, nevertheless, Roff/Ron ratio is significantly smaller, ~ 6. Observed RS effect is described by the significant influence of oxygen vacancies on tunneling conductivity of chains of metal nanoclusters, determining the electrical resistance of structures below the percolation threshold. The ability of LNO NC memristive devices to support the spike-time-dependent plasticity was demonstrated. These results give every hope for stable operation of future large neuromorphic networks based on LNO NC memristive devices. The work was partially supported by the Russian Science Foundation (16-19-10233) and performed on the Resource Center facility (Kurchatov Institute).

PANI-based neuromorphic networks first results and close perspectives

Perceptron is an artificial neural network that can solve simple tasks such as invariant pattern recognition, linear approximation, prediction and others. We report on the hardware realization of the perceptron with the use of polyaniline-based memristive devices as the analog link weights. An error correction algorithm was used to get the perceptron to learn to implement the NAND and NOR logic functions as examples of linearly separable tasks. The conceptual scheme of two-layer perceptron is proposed to implement all possible logic functions including linearly inseparable ones (as XOR, for example). It is also shown how organic memristive links between two layers of neurons could be made on the base of stochastic block copolymer matrices which greatly simplifies and makes cheaper the mass-production of such networks. The physical realization of a perceptron demonstrates the ability to form the hardware-based neuromorphic networks with the use of organic memristive devices. This holds a great promise towards new approaches for very compact, low-volatile and high-performance neurochips that could be made for a huge number of intellectual products and applications.

Investigation of the dependence of the photoluminescence properties of silicon nanoclusters on their volume fraction in a silicon oxide matrix

The photoluminescence (PL) spectra and kinetics of amorphous and crystalline silicon nanoclusters are investigated. The given nanoclusters are formed by thermal annealing of thin suboxide silicon films with different volume fractions of silicon. It is demonstrated that the PL intensity and lifetime of the ensembles of silicon nanocrystals have a steplike dependence on the silicon volume fraction in the film. The influence of the percolation effect on the photoluminescence properties of the structures under study is discussed.