The features of photoluminescence from nanograins of gallium-doped cubic silicon carbide

The photoluminescence (PL) and photoexcitation spectra of carbonized porous silicon (por-Si) doped with gallium in the course of a high-temperature annealing were studied. It is shown that carbonization leads to the formation of a heterojunction between 3C-SiC nanograins and silicon quantum wires. The spectrum of PL from gallium-doped silicon carbide nanograins is shifted by 0.35 eV toward higher energies relative to the spectrum of bulk por-Si and exhibits several features related to the radiative annihilation processes involving phonons and donor-acceptor (N-Ga) pairs. The PL excitation spectra of carbonized por-Si display two resonance bands with the energies E ₁=2.8–3.1 eV and E ₂=3.2–3.7 eV.     

The mechanism of photoluminescence quenching in porous silicon by electron irradiation of various intensity

The charged particle density in an electron beam irradiating porous silicon (por-Si) affects the kinetics of desorption of the surface complexes from por-Si and, accordingly, the degree of photoluminescence quenching in this material. Electron irradiation at a beam density above 5.5×10¹³ cm⁻² s⁻¹ leads to charging of the por-Si surface and to a decrease in the adsorption capacity for the donor molecular groups.     

The effect of adsorption complexes on the electron spectrum and luminescence of porous silicon

Effects of the surface atomic structures on the electron spectrum and luminescent properties of porous silicon (por-Si) were studied by methods of photoluminescence spectroscopy, UV photoelectron spectroscopy, and Fourier-transform IR spectroscopy. An analysis of evolution of the por-Si characteristics in the course of thermal treatment in vacuum revealed a correlation between the photoluminescence spectrum and intensity, on the one hand, and the electron spectrum and surface atomic structure, on the other hand. The thermodesorption of adsorbate from por-Si leads to atomic rearrangements on the sample surface, which is accompanied by changes in the electron structure and, hence, in the luminescent properties of the material.     

Argon-oxygen ion-plasma treatment modifies the surface composition and photoluminescence spectrum of porous silicon

We have studied changes in the surface composition and photoluminescence spectrum of porous silicon (por-Si) during the ion-plasma etching of samples in an argon-oxygen gas mixture. This treatment leads to the passivation of the surface of quantum filaments by residual fluorine and the formation of silicon oxide. The source of fluorine atoms are HF molecules retained in the volume of pores upon the por-Si structure formation by chemical etching. Increase in the fluorine concentration is accompanied by the growth in intensity of the blue-green band and broadening of the red band in the photoluminescence spectrum of por-Si.     

Features of the current-voltage characteristics and temperature dependences of electric conductivity in porous silicon layers

A method of separating porous silicon (por-Si) layer without deformation from a silicon substrate, which virtually excludes almost any risk of subsequent degradation of the isolated por-Si layer is proposed. The current-voltage characteristics of por-Si have been measured in a temperature interval of 300–255 K. Direct measurements of the conductivity of por-Si have been performed for the first time and it is established that the conductivity has an activation nature for the current passage parallel to the por-Si sample surface.     

On the relationship between the optical transmission and photoluminescence characteristics of porous silicon

The characteristics of free-standing porous silicon (por-Si) films were studied using their optical transmission, photoluminescence (PL), and photoluminescence excitation spectra. The transmission spectra exhibit no features within the emission bands or near PL excitation thresholds, which is evidence that nonluminescent por-Si fragments play a dominating role in the process of light absorption. This fact indicates that the optical transmission spectra cannot be used as a source of information on the bandgap energy of charge carriers in luminescent silicon nanocrystallites. The required energy spectrum parameters can be roughly evaluated using the PL excitation spectrum.     

Energy transfer under photoexcitation of porous silicon-fullerene nanocomposite in oxygen-containing ambient

The mutual effect of two nanodimensional subsystems, porous silicon (por-Si) and fullerene C₆₀, on the course of photoinduced processes in a por-Si-C₆₀ composite material has been observed. It is demonstrated for the first time that the photoexcitation of this nanocomposite in the atmosphere of normal (triplet) molecular oxygen leads to the self-insulation of silicon nanocrystallites as a result of their oxidation by singlet oxygen species generated under the action of the excited fullerene subsystem. This repassivation of the por-Si surface leads to a significant recovery of the luminescent activity of por-Si.     

Rutherford backscattering analysis of oxide layers formed by ion implantation into single-crystal silicon

Single-crystal silicon was implanted with 40 keV and 60 keV oxygen ions. Rutherford backscattering (RBS) analysis was used to determine the oxygen profile, the ratio of oxygen to silicon in the implanted layer and the extent of radiation damage for doses up to 3 × 10¹⁸ ions cm^-2. The measured oxygen profiles appear to agree with theory at low doses. The radiation damage, as characterized by the damage peak and the yield _χmin behind the implanted layer, saturates at a dose of approximately 1 × 10¹⁶ ions cm^-2. The oxygen content of the layer is directly proportional to dose until the peak value of the oxygen:silicon ratio reaches 2.0. At higher doses the oxygen concentration only increases in the region between the peak and the surface, resulting in a uniform layer of thickness 2R_p (SiO₂). Infrared transmission measurements also indicate that stoichiometric SiO₂ is formed. Annealing at 900 °C has no effect on the RBS spectrum from the implanted layer but the area of the interfacial damage peak is reduced by 60%. Two interesting effects occur: (a) _χmin rises to a peak when the oxygen:silicon ratio is approximately unity; (b) the interfacial damage peak decreases with increasing dose once a uniform layer has been formed.     

Nanocomposite por-Si/SnOx layers formation for gas microsensors

Hetero-phase nanocomposite layers based on porous silicon and nonstoichiometric tin oxide (por-Si/SnO_x) were obtained by the chemical vapor deposition (CVD), magnetron sputtering, and molecular layer deposition methods. The structure, and the atomic and phase compositions of the nanocomposites were studied by means of transmission electron microscopy, energy-dispersive X-ray analysis (EDX), scanning electron microscopy, Raman spectroscopy, Auger spectroscopy, and X-ray photoelectron spectroscopy. The obtained data were indicative of the formation of por-Si/SnO_x nanocomposite layers up to 2 μm thick with x = 1.0-2.0. According to EDX data, in magnetron sputtering process the formation of por-Si/SnO_x nanocomposite layers proceeds on the externally exposed surface of polycrystalline por-Si skeleton elements with subsequent diffusion of tin atoms into the pores along the por-Si walls. The other two methods lead to formation of large SnO_x islands covering pores in the por-Si structure. Enhanced diffusion of tin atoms into porous matrix with D_eff ≈ 1 × 10⁻¹⁴ cm²/s was observed in samples annealed at 500 °C. Sensor heterostructures based on magnetron sputtered por-Si/SnO_x nanocomposite layers show high sensitivity to NO₂ environmental molecules and remarkable stability, thus offering promise in gas sensing applications.     

Structural and chemical characteristics and oxidation behaviour of chromium-implanted single crystal silicon carbide

High-dose chromium implantation resulted in complex changes in the structure, chemistry, and oxidation behaviour of beta-type single-crystal silicon carbide. Detailed analytical studies indicated that, in addition to the primary process of surface doping, chromium implantation of silicon carbide to 3.90×10¹⁷ ions cm⁻² at 200 keV was accompanied by many secondary processes such as surface sputtering, lattice damaging, and silicon depletion/carbon enrichment in the implanted region. These changes resulted in accelerated oxidation of the implanted samples by a factor of 1.14 as compared with the unimplanted crystals in 1 atm of flowing oxygen at 1100°C. The oxidation layer exhibited interesting structural and compositional inhomogeneity which could be explained based upon chromium mobility variation in the implanted region. The presence of densely populated chromium oxide precipitates in the outer region of the oxidation layer played a significant role in keeping the degree of oxidation acceleration low under the detrimental influence of lattice damages and silicon deletion/carbon enrichment. It was concluded that the potential of chromium implantation to improve the oxidation resistance of silicon carbide can be realized only when the implantation-induced secondary effects are suppressed.     

Photoluminescence of porous silicon layers formed in ion-implanted silicon wafers

Implantation of the B⁺ and N⁺ ions or a B⁺ + N⁺ combination into silicon substrates affects the photoluminescence properties of porous silicon (por-Si) layers prepared on the ion-modified wafers. The postimplantation anneals lead to significant changes in the por-Si emission bands. Models explaining the observed phenomena are suggested.     

Investigation of nanostructured silicon as a candidate for heat sensitive material

Nanoscale pores are fabricated on the surface of silicon by simple metal-assisted etching process. The resistance of nanostructured silicon depends obviously on temperature. The temperature coefficient of resistance is ?2.835 %/°C, which is as large as that of some heat sensitive materials, for instance vanadium oxide, amorphous silicon, used for uncooled infrared (IR) detectors. Considering with the enhanced near-IR absorption of nanostructured silicon, it is demonstrated that nanostructured silicon can be a promising heat sensitive material for uncooled IR detection. The sheet carrier concentration is slightly reduced, whereas carrier mobility is drastically decreased from 367.5 to 273.7 cm² V^?1 s^?1 after nanostructuring process.     

IR and x-ray studies of ion-beam-synthesized aluminium nitride films

IR transmission spectroscopy in conjunction with X-ray diffraction was used to characterize the phase composition of aluminium films after nitrogen ion implantation. Aluminium films deposited onto single-crystal silicon and implanted with 30 keV nitrogen ions (¹⁴N₂⁺) to a dose of 10¹⁷-10¹⁸ ions cm^-2 were subsequently characterized for aluminium nitride (AIN) formation by IR spectroscopy. The formation of a stoichiometric AIN layer was evident from the IR absorption band observed at 648 cm^-1. Furthermore, X-ray diffraction of an aluminium foil after nitrogen implantation at 110 keV to a dose of 5.0 × 10¹⁷ ions cm^-2 on each side revealed the presence of a polycrystalline AIN phase. A thermal treatment at 700°C did not yield any new crystalline phases.     

Defect formation in silicon implanted with ∼1 MeV/nucleon ions

Defect formation processes in silicon implanted with ∼1 MeV/nucleon boron, oxygen, and argon ions have been studied using microhardness and Hall effect measurements. The results indicate that ion implantation increases the surface strength of silicon single crystals owing to the formation of electrically inactive interstitials through the diffusion of self-interstitials from the implantation-damaged layer to the silicon surface. The radiation-induced surface hardening depends significantly on the nature of the ion, its energy, and the implant dose. In the case of low-Z (boron) ion implantation, the effect had a maximum at an implant dose of ∼5 × 10¹⁴ cm⁻², whereas that for O⁺ and Ar⁺ ions showed no saturation even at the highest dose reached, 1 × 10¹⁶ cm⁻². When the ion energy was increased to ∼3 MeV/nucleon (210-MeV Kr⁺ ion implantation), we observed an opposite effect, surface strength loss, due to the predominant generation of vacancy-type defects.     

Pulsed plasma chemical synthesis of carbon-containing titanium oxide-based composite

The carbon-containing titanium oxide-based composite was first obtained using a pulsed plasma chemical method. The composite was obtained from the following reagents: TiCl₄, CH₄, and O₂. The physical and chemical properties of the Ti_xC_yO_z composite powders were studied (morphology, chemical, elemental and phase composition). The presence of spherical particles and the cubic and prismatic particles were typical for the synthesised carbon-containing titanium oxide-based composites. The large particles are observed (the average size exceeds 150 nm) and smaller particles (the average size is 15–30 nm). The presence of the dense layer of amorphous carbon (10–15 nm thick) around particles is typical for the composites. The peak with a maximum of 1080 cm^?1 is registered in IR absorption spectrum of the Ti_xC_yO_z synthesised composite. The presence of IR radiation in this region of the spectrum is typical for the deformation of atomic oscillations in the Ti-O-C bond, which indicates that carbon and titanium in the composite are bound through oxygen. The content of the defined amount of titanium carbide has not been detected.     

Preparation of porous silicon nanocomposites with iron and cobalt and investigation of their electron structure by X-ray spectroscopy techniques

A method for the galvanic deposition of iron-group metals onto porous silicon (por-Si) substrates has been developed. The morphology and phase composition of por-Si nanocomposites containing galvanically deposited particles of Fe, Co, and their mixtures have been studied by scanning electron microscopy (SEM), ultrasoft X-ray emission spectroscopy (USXES), and X-ray absorption near-edge structure spectroscopy (XANES) techniques. It is established that iron uniformly covers the surface of porous silicon, whereas cobalt penetrates deep into pores in the form of nanoparticles. During the galvanic codeposition of both metals from a mixed solution of their salts, cobalt favors the penetration of iron in depth of the pores.     

Hybrid porous nanotube crystal networks for nanostructured device applications

A set of new porous materials, namely zeolite nanocage schwarzite-like crystals with the elements of both nanotubes and fullerenes in the structure is proposed as a result of ab initio and density-functional theory calculations. Twelve new Extradiamond phases of boron nitride, carbon, silicon and silicon carbide are calculated as three different hybridized crystals. The details of recently synthesized Explosion-BN (E-BN) phase are highlighted for the first time with electronic structure and vibrational frequency analysis. E-BN is supposed to be sp ²/sp ³-hybridized FAU-zeolite structure with calculated unit cell of 12.177 Å and a band gap of 3.2 eV. Calculated IR bands for E-BN₁₂₀ cluster and observed experimentally E-BN absorption spectrum are well-correlated with appropriate IR spectra of FAU-zeolite. Armchair and zig-zag nanotubes are classified as (n,n,k) and (n,0,k), respectively, where k is the number of hexagons along the nanotube axis. Novel materials are proposed as (n,m,k)-FTC, where FTC stands for framework type code. We also indicate the possibility of creation of filled hybrid networks of different segment lengths, radii and compositions for thermoelectric and novel device applications.     

Point defects as the centers of titanium dioxide sensitization in the visible spectral range

The formation of surface point defects in the initial stage of TiO₂ reduction (x ≤ 10¹² cm⁻²) has been studied by mass spectrometry and ultraviolet photoelectron spectroscopy (UPS). Heating to 720 K or UV illumination in ultrahigh vacuum creates surface color centers in TiO₂ with an energy spectrum extending from the Fermi level to the valence band top. The continuous photoelectron spectrum exhibits a peak at 2.7 eV, which varies in a manner correlated with the behavior of the optical absorption bands at 2.55 and 2.81 eV assigned to oxygen vacancies in the TiO₂ crystal structure. The interaction of the surface point defects with molecular oxygen has been studied and a special form of the photoadsorbed oxygen (with E _des = 1.37 eV) is found. It is shown that the surface color centers may serve as centers of TiO₂ sensitization in the visible spectral range.     

Effect of underlying silicon film on the evolution of microstructure and hardness of the high-temperature annealed carbon film on Si substrate

Effects of an amorphous silicon underlayer on the evolution of microstructure and hardness of an amorphous carbon film annealed at 900 °C for 0.5-1.5 h were investigated. The two-layer carbon/silicon film after annealing resulted in higher sp²/sp³ bonding ratio but lower hardness reduction compared to the single carbon film at the same total film thickness. The improved hardness reduction of the high-temperature annealed carbon film is attributed to the formation of polycrystals of the amorphous silicon together with the residual compressive stress of the two-layer C/Si films.     

Layer by layer composite film of dimyristoyl-phosphatidylcoline and bacteriorhodopsin fabricated by multilayer molecular thin film method using fatty acid and lipid

A composite film of dimyristoyl-phosphatidylcoline (DMPC) and bacteriorhodopsin (BR) was fabricated by multilayer molecular thin film method using fatty acid and lipid on a quartz substrate or a hydrogenated amorphous silicon thin film. FTIR reflection absorption spectrums and UV absorption spectrums of the films were characterized on the detail of surface structure of the films. The spectroscopic data exhibited a specific layer by layer interaction of BR and environmental molecule DMPC above fatty acid. Especially, 4 layer composite LB films DMPC and BR exhibited an entirely different feature of IR reflection absorption spectrum depend on fatty acid species.