Size dependent photoluminescence of SiC nanocrystals

The paper presents the results of porous SiC study using photoluminescence and scanning electronic microscopy. It is shown that the intensity of defect-related PL bands (2.08, 2.27, 2.44 and 2.63 eV) increases monotonically with the rise of PSiC thickness from 2.1 up to 12.0 μm. These luminescence centers are assigned to surface defects which appear at the PSiC etching process. Photoluminescence intensity stimulation for surface defects is attributed to rise of defect concentrations with increasing of porous layer thickness and to realization of the hot carrier ballistic mechanism at surface defect excitation. Intensity enhancement for exciton-related PL bands (2.79, 2.98 and 3.26 eV ) is attributed to increasing the exciton recombination rate as result of exciton weak confinement in big size SiC NCs of different polytypes (6H–PSiC with inclusions of 15R- and 4H–PSiC).     

Generating ordered Si nanocrystals via atomic force microscopy

We describe two successful routes for generating ordered arrays of Si nanocrystals by using atomic force microscopy (AFM) and amorphous silicon thin films (200–400 nm) on Ti/Ni coated glass substrates. First, we show that field-enhanced metal-induced solid phase crystallization at room temperature can be miniaturized to achieve highly spatially localized (below 100 nm) current-induced crystallization of the amorphous silicon films using a sharp tip in AFM. In the second route, resistive nano-pits are formed at controlled positions in the amorphous silicon thin films by adjusting (lowering and/or stabilizing) the exposure currents in the AFM process. Such templated substrates are further used to induce localized growth of Si nanocrystals in plasma-enhanced chemical vapor deposition process. In both cases the crystalline phase is identified in situ as features of enhanced current in current-sensing AFM maps.     

Dependence of photoluminescence properties on excitation power in ZnO and ZnCdO microrods

ZnO and ZnCdO microrods have been prepared through a chemical bath deposition method. The structure of microrods has been characterized using X‐ray diffraction (XRD) and transmission electron microscopy (TEM). Photoluminescence spectra were recorded for ZnO and ZnCdO microrods at different excitation powers. The intensity of UV emission is enhanced with increasing excitation power. The width of UV emission increases for spectra at higher excitation powers. In particular, the paper shows that the influence of excitation power on the shift of emission band for ZnCdO microrods is more remarkable than that of ZnO microrods with the increase of excitation power. The definite experimental evidence demonstrated that the temperature coefficient β of ZnCdO microrods is much larger than the temperature coefficient of ZnO microrods. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time-resolved photoluminescence of implanted SiO2:Si+ films

In this paper we present results of a low-temperature time-resolved photoluminescence (PL) investigation of thin SiO₂ films implanted with silicon ions. In addition to the luminescence of well-known ODCs, some other bands are present in the low-energy region of PL spectra that are attributed to silicon nanoclusters (quantum dots – SiQDs), excitons and hydrogen-related species (HRS). Specific features of SiQD and HRS bands are the nanosecond kinetics and unusual “stepped” PL excitation spectrum in the 3.5–7.5 eV range. The possible origin of discovered phenomena is discussed. The obtained results are interpreted taking into account the interference of exciting radiation and dimensional quantization effects.     

Correlation between the photoluminescence and different types of Si nano-clusters in amorphous silicon

This paper presents the results of PL spectrum study for Si nano-clusters in amorphous silicon matrix. The hydrogenated amorphous Si layers were prepared by the hot-wire CVD method on glass substrates. The layers were deposited at different wafer temperatures 280, 360, 420 and 460 °C and at one filament temperature of 1650 °C. The joint analysis of PL and X-ray diffraction spectra in dependence on the technological conditions and on different sizes of Si nano-clusters has been done. The mechanisms of PL are discussed as well.     

Temperature quenching of photoluminescence of ordered GaInP2 alloy under different excitation densities

The photoluminescence (PL) emission from an epi‐structure containing an atomically ordered GaInP₂ layer and a GaAs layer was studied under excitation power densities of 0.03 – 3 W/cm²at temperatures of 10 to 300 K. The quenching of the integrated PL intensity from both: the GaInP₂and the GaAs layers is stronger under low excitation, than under high excitation density. The temperature dependence, however, have different shapes being the PL decay observed for the GaInP₂ layer stronger than that for the GaAs layer. Comparing the temperature dependence of the PL intensity from the ordered GaInP₂ and the GaAs layers under different excitation densities and analyzing them together, we conclude that the inhomogeneity of the ordered layer is responsible for the different temperature behavior of the GaInP₂ alloy PL emission. To explain the experimentally observed PL intensity temperature dependence an additional nonradiative recombination mechanism due to a thermally activated escape of the carriers from its confinement within regions of lower bandgap has to be taken into account. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron-paramagnetic resonance and photoluminescence study of Si nanocrystals-photosensitizers of singlet oxygen molecules

Si nanocrystals formed by electrochemical porosifying of c-Si wafers are investigated by means of the electron-paramagnetic resonance (EPR) and photoluminescence (PL) techniques. The PL spectra and transients give evidence of the photosensitization of singlet oxygen molecules by the energy transfer from excitons confined in Si nanocrystals to oxygen molecules adsorbed on the nanocrystal surfaces. The EPR experiments show that the singlet oxygen generation is accompanied by a slowing down of the spin-spin relaxation time of Si dangling bonds on the nanocrystal surfaces. This effect is explained by the transition of a large part of the adsorbed O₂ molecules in their ground (triplet) states to the excited (singlet) ones. The EPR data allow us to estimate the concentration of the photosensitized singlet oxygen molecules to be on the order of 10¹⁸ cm⁻³.     

High-density carrier dynamics in Ge/Si quantum dots studied by time-resolved photoluminescence spectroscopy

We studied the temperature dependence of high-density carrier dynamics in as-grown and thermally annealed Ge quantum dots (QDs) in silicon crystals. In as-grown and thermally annealed samples, photoluminescence (PL) intensity exhibited a power-law dependence on the excitation intensity and its power-law index was ~ 0.7 at low temperatures. With increasing measurement temperature, PL intensity decreased and the index of the power-law function increased up to ~ 1.5, in which carrier recombination dynamics is dominated by a single carrier trapping. Moreover, in thermally annealed QDs, the index of the power law increased more rapidly than as-grown QDs, suggesting that the carrier recombination dynamics drastically changed in thermally annealed QDs. Effects of Ge/Si interface on high-density carrier recombination process are discussed.     

Observation of a new photoluminescence band at 320 nm under 270 nm excitation in Ge-doped silica glass

We investigate the influence of temperature on photoluminescence (PL) in Ge-doped silica glass. Under 270 nm excitation, we observe only one PL band at 424 nm at room temperature (RT). This band shifts to 436 nm with cooling (4 K), and a new PL band is recorded at 320 nm. We assign these PL bands to triplet-to-singlet and singlet-to-singlet transitions of a same Ge-related defect, whose structure is still unknown. The shift of the PL band (from 424 nm at RT to 436 nm at 4 K) is explained by the decrease of the overlap between PLs from different centers.     

Temperature and excitation intensity tuned photoluminescence in Tl4GaIn3S8 layered single crystals

Photoluminescence spectra of Tl₄GaIn₃S₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 500–780 nm and in the temperature range of 26–130 K with extrinsic excitation source (λ_exc = 532 nm), and at T = 26 K with intrinsic excitation source (λ_exc = 406 nm). Three emission bands A, B and C centered at 514 nm (2.41 eV), 588 nm (2.11 eV) and 686 nm (1.81 eV), respectively, were observed for extrinsic excitation process. Variations in emission spectra have been studied as a function of excitation laser intensity in the 0.9‐183.0 mW cm^–2 range for extrinsic excitation at T = 26 and 50 K. Radiative transitions from the donor levels located at 0.03 and 0.01 eV below the bottom of the conduction band to the acceptor levels located at 0.81 and 0.19 eV above the top of the valence band were proposed to be responsible for the observed A‐ and C‐bands. The anomalous temperature dependence of the B‐band peak energy was explained by configurational coordinate model. From X‐ray powder diffraction and energy dispersive spectroscopic analysis, the monoclinic unit cell parameters and compositional parameters of Tl₄GaIn₃S₈ crystals were determined, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth behavior and photoluminescence properties of ZnO nanowires on gold nano-particle coated Si surfaces

ZnO nanowires arrays were synthesized on gold coated Si by the hydrothermal technique. The density of the ZnO nanowires array is found to be inversely proportional to the particle size of gold films. The initial nucleation of ZnO nanowires is found to nucleate at the boundaries of the gold nano-particle films. The density of the ZnO nanowires can be controlled through the particle size of the gold nano-particle films. Anomalous photoluminescence properties were observed from the as-grown ZnO nanowires.     

Fabrication and photoluminescence characteristics of Er3+-doped optical fiber sensitized by Si particles

We report for the first time the development of Er³⁺/Si-doped optical fiber amplifier operating at 1550 nm upon 512 nm pumping. Er³⁺-doped silica glass optical fibers incorporated with silicon particles as the sensitizer were fabricated by the modified chemical vapor deposition and the solution doping technique. The effect of silicon incorporation into the fiber core on the optical absorption and photoluminescence properties was investigated. Upon pumping with the CW Ar-ion laser at 512 nm, a photoluminescence band appeared near 770 nm and 1560 nm. The peak intensity of the photoluminescence at 1560 nm in the Er³⁺/Si-doped fiber was found to enhance by about 70% compared to the Er³⁺-doped fiber.     

Dislocation‐related photoluminescence imaging of mc‐Si wafers at room temperature

We report on a method for fast detection of defect rich areas in multicrystalline silicon solar wafers. It is based on photoluminescence imaging of the whole wafers and detects both the band‐to‐band radiation as well as the dislocation specific radiation D1. To illustrate the capabilities of the method we examined 5.0 × 5.0 cm² wafer pieces in different stages of their processing. The achieved resolution of the D1 images was ∼120 μm, within a total recording time of 550 ms.     

Facets formation of pyramidal Si nanocrystals selectively grown on Si(0 0 1) windows in ultrathin SiO2 films

We have used in situ scanning tunneling microscopy (STM) to study the facet formation in the selective growth of pyramidal Si nanocrystals on Si(0 0 1) windows in ultrathin 0.3-nm-thick SiO₂ films. Broad (0 0 1) surfaces developed as the top of the crystals, and {1, 1, (2n+1)} (n=1–6) facets formed the sidewalls. As growth continued, the slope angle of sidewall facets increased, and {1, 1, 9} and {1, 1, (2m+1)} (0 <m < 4) facets often came to coexist on the sidewalls. On well-oriented Si(0 0 1) surfaces, layer-by-layer growth in the [0 0 1] direction was dominant. On vicinal Si(0 0 1) surfaces, lateral step growth took place in the initial stage, and the layer-by-layer growth was suppressed until after a large (0 0 1) surface had formed as the top of the crystal.     

Blue emission of ZrO2:Tm nanocrystals with different crystal structure under UV excitation

In this paper, thulium doped ZrO₂ nanocrystals with different phases were prepared by Pechini sol-gel complex method. The effects of host structure, phases and concentration on luminescence properties of Tm³⁺ ions were investigated. With the increase of the concentration of Tm³⁺ ions, the phosphors phase transformed from monoclinic to tetragonal structure and all the phosphors emitted blue light. However, because the environment symmetry around Tm³⁺ in m-ZrO₂ (C_2h) is worse than that of t-ZrO₂ (D_4h), there were still obvious differences in the emission and excitation. The reason was investigated with the help of energy level diagram. In addition, the quenching concentration of Tm³⁺ in ZrO₂ host is 1 m/m%.     

Composition of Ge+ and Si+ implanted SiO2/Si layers: Role of oxides in nanocluster formation

X-ray photoelectron spectroscopy (XPS) has been used to examine the atomic content of implanted SiO₂/Si layers. In particular, an XPS analysis permits to identify elemental Ge and Si, as well as GeO₂ precipitations in SiO₂ matrices. The XPS results reveal valuable information not only about the formation mechanism of Ge and Si nanoclusters but also on the annealing kinetics of SiO₂ whose properties are known to be significantly altered during the process of ion implantation and subsequent annealing. The composition of ion beam-modified SiO₂ layers strongly depends on the annealing temperature. With respect to germanium implanted samples a possibility of Ge nanocrystals formation appears at high (above 1000 °C) annealing temperatures. It has been shown that an intermediate step in the Ge oxide formation is necessary for the creation of Ge nanoclusters. Additionally, the presence of a subsurface zone GeO_x (about 100 nm thick) predicted in kinetic three-dimensional lattice simulations has been confirmed. In the case of Si⁺ implanted samples substoichiometric silicon oxide lines in the XPS spectra of a SiO₂ layer for all samples have been observed. No evidence of a line connected to the Si–Si bonding has been observed even at the highest annealing temperatures, at which only stoichiometric SiO₂ has been detected.     

Thickness-dependent structural transition in GaAs nanocrystals grown on Si (1 1 1) surface

Structural stabilities in GaAs nanocrystals grown on the Si (1 1 1) substrate have been studied by transmission electron microscopy in order to see the structure and growth mechanism. The GaAs nanocrystals grown epitaxially on the Si (1 1 1) surface kept at 573 K have thin shapes consisting of a flat surface which is parallel to the Si (1 1 1) surface. The crystalline structure of the initial growth layer approximately below 5 nm in thickness is the zincblend structure, but with increasing thickness the structure changes to the wurtzite structure by formation of orderly-arranged stacking faults. The small difference in the driving force between the wurtzite structure and the zincblende structure could bring about a situation, where the kinetic rate of nucleus formation is high for the wurtzite structure than for the zincblende structure. It would highly increase the probability that the wurtzite structure is formed as a non-equilibrium state.