Photoluminescence of Si nanocrystallites in different types of matrices

This paper presents the comparative investigation of photoluminescence (PL) and its temperature dependence for rf-magnetron co-sputtered Si-enriched SiO_x systems and amorphous Si films prepared by hot-wire CVD method with Si nanocrystallites of different sizes. It is shown that PL spectra of Si–SiO_x films consist of the five PL bands peaked at 1.30, 1.50, 1.76, 2.05 and 2.32 eV. Amorphous Si films with Si nanocrystallites are characterized by three PL bands only peaked at 1.35, 1.50 and 1.76 eV. The peak position of the 1.50 eV PL band shifts with the change of Si quantum dot sizes and it is attributed to exciton recombination inside of Si quantum dots. The nature of four other PL bands is discussed as well.     

Intense luminescence of amorphous Eu2O3 prepared by aqueous sol–gel method

Amorphous Eu₂O₃ was prepared by an aqueous sol–gel method. Emission due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2) transitions of Eu³⁺ ions were observed. The dominant transition was the ⁵D₀ → ⁷F₂ red emission of Eu³⁺. The properties of the as-prepared samples were different with changes in the annealing temperature. To investigate the luminescence properties of the amorphous Eu₂O₃, the temperature-dependent photoluminescence (PL) spectra of samples annealed at 600 °C were measured in the temperature range 77–300 K. PL peak positions were unchanged with the change of temperature.     

Broad blue-green-red emissions of SnO2/Pr3+ co-doped phosphate glasses

The photoluminescence properties of SnO₂/Pr^3 + co-doped strontium phosphate glasses (75P₂O₅-25SrO) are studied. An ultrabroad emission band covering blue, green and red is observed in co-doped glasses. In co-doped samples, three downward peaks appear in blue emission region, these coincide with Pr^3 + excitation peaks, indicating the energy transfer through cross-relaxation between SnO₂ and Pr^3 +. The mechanism has been detailed based on the energy level diagrams of SnO₂ and Pr^3 +. The chromaticity coordinates of the co-doped samples with varying doping ratio of SnO₂ to Pr^3 + are calculated. The result demonstrates the possibility of generating white light in the SnO₂/Pr^3 + co-doped phosphate glasses.     

Porous silicon grains in SiO2 matrix: Ultrafast photoluminescence and optical gain

We search for the presence of stimulated emission in samples of porous silicon embedded in the sol–gel derived SiO₂ matrix. By modifying the etching conditions of the porous silicon using hydrogen peroxide, we decrease substantially the nanocrystal size and produce a significant blue shift of the PL emission. Femtosecond variable-stripe length experiments combined with the shifting-excited spot technique demonstrates positive optical gain (modal gain ∼25 cm⁻¹) in the range 550–730 nm. Ultrafast photoluminescence dynamics indicates the origin of the stimulated emission as possibly due to recombination of excitonic states inside silicon nanocrystals.     

Photoluminescence in silicon rich oxide thin films under different thermal treatments

Photoluminescence (PL) was studied in silicon rich oxide (with the atomic percentage ranges of Si from 35% to 75%) thin film samples, fabricated by the plasma assisted CVD technique. A broad PL peak, blue-shifted from the bulk silicon band edge of ～1.1 eV, was observed. In one typical sample, the PL peak intensity shows a non-monotonic temperature dependence. This non-monotonic dependence was also observed in previous work by others and attributed to an energy splitting between the excitonic singlet and triplet levels in silicon nanocrystals, a consequence of quantum confinement effect. Finally, in more than 20 samples under different thermal treatments (with the annealing temperature range from 800 °C to 1100 °C), the wavelength of PL peak was observed to be pinned between ～900 and ～1000 nm, independent of thermal budget. This pinning effect, we believe, is probably due to the formation of oxygen-related interface states.     

Nanostructural and optical features of amorphous AlxSi0.45−xO0.55 (0 ⩽ x ⩽ 0.05) thin films prepared by RF-magnetron sputter techniques

The nanostructural, chemical, and optical features of Al_xSi_0.45−xO_0.55 (0 ⩽ x ⩽ 0.05) thin films were investigated in terms of Al concentration and post-deposition annealing conditions; the films were prepared by co-sputtering a Si main target and Al-chips, and the annealing was carried out at temperatures of 400–1100 °C. The a-Si_0.45O_0.55 films prepared without Al-chips and annealed at 800 °C contain ∼3.5 nm-sized Si nanocrystallites. The photoluminescence (PL) intensity as well as the volume fraction of Si nanocrystallites increased with increasing the concentration of Al to a certain level. In particular, the intensity of the PL spectra of the Al_0.025Si_0.425O_0.550 films which were annealed at 800 °C increased significantly at wavelengths of ∼580 nm. It is highly likely that the observed increase in the PL intensity is caused by the raise in the total volume of the ∼3.5 nm-sized nanocrystallites in the films. The addition of Al as well as the post-deposition annealing allow adjustment and control of the nanostructural and light-emission features of the a-SiO_x films.     

Growth and characterization of GaSb/AlGaSb multi-quantum well structures on Si (0 0 1) substrates

GaSb/AlGaSb multi-quantum well (MQW) structures with an AlSb initiation layer and a relatively thick GaSb buffer layer grown on Si (0 0 1) substrates were prepared by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated definite MQW structures. The photoluminescence (PL) emission around 1.55 μm wavelength was observed for 10.34 nm GaSb/30 nm Al_0.6Ga_0.4Sb MQW structure at room temperature. Dependence of PL emission energy on GaSb well width was well explained by finite square well potential model.     

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.     

Photoluminescence of Si or Ge nanocrystallites embedded in silicon oxide

This paper presents the results of photoluminescence, its temperature dependence and Raman scattering investigations on magnetron co-sputtered silicon oxide films with (or without) embedded Si (or Ge) nanocrystallites. It is shown the oxide related defect origin of the visible PL centers peaked at 1.7, 2.06 and 2.30 eV. The infrared PL band centered at 1.44–1.58 eV in Si–SiO_x, system has been analyzed within a quantum confinement PL model. Comparative PL investigation of Ge–SiO_x system has confirmed that high energy visible PL bands (1.60–1.70 and 2.30 eV) are connected with oxide related defects in SiO_x. The PL band in the spectral range of 0.75–0.85 eV in Ge–SiO_x system is attributed to exciton recombination inside of Ge NCs.     

Defect-related emission characteristics of nonpolar m-plane GaN revealed by selective etching

A comprehensive study of the morphology and luminescence characteristics of nonpolar m-plane GaN etched in hot acids was presented. It was found that many four-sided pyramidal pits were distributed on the etched GaN surface with the long side perpendicular to the [1 1 2? 0] direction, corresponding to the threading dislocations. When compared to the as-grown GaN, DAP emission intensity and its LO-phonon coupling phenomenon in the etched GaN were greatly attenuated, whereas the intensity of BSF-related band almost kept constant due to its immunity to chemical etching. Especially, a new PSF-related emission at 3.32 eV emerged in CL spectra of etched GaN. Simultaneously, partial relaxation of compressive stress happened for the etched GaN epilayer according to the red shift of NBE emission in photoluminescence (PL) and E₂(high) phonon peak in the Raman spectra. Contrary, the DAP peak in etched GaN was blueshifted, likely due to the reduced impurity level fluctuation by etching. In addition, the different behaviors were discussed for NBE and defect-related transitions in the etched GaN, characterized by excitation power- and temperature-dependent PL.     

Recombination luminescence of oxygen-deficient centers in silica

The luminescence of silica glass, prepared by plasma chemical vapor deposition (PCVD) and quartz glass of type IV (trade mark KS-4V) methods, were studied while irradiated with pulses of ArF laser (193 nm) light in the range of sample temperatures between 10 and 300 K. The samples contain less than 0.1 ppm metallic and hydroxyl impurities. The samples synthesized by PCVD were of two kinds. The first one (amorphous) was as-deposited from plasma at a substrate tube temperature of ～1200 °C. The second one (fused) was prepared from the first by the tube collapsing with an external burner. In this process, a section of the substrate tube with the deposited glass was installed in a lathe and processed at a temperature of ～2100 °C during ～20 min until the tube was transformed to a rod. After such processing, the rod was cooled down to room temperature in air at an average rate of about 400 °C per min. The only observed luminescence possesses two broad bands, with not well defined position, one at 2.6–2.9 eV (a blue band) and another in the range of 4.4 eV (an UV band). There is a correspondence in luminescence properties between KS-4V silica and fused PCVD silica. Those bands have been attributed to oxygen deficient centers (ODC). No luminescence is observed in amorphous PCVD silica under irradiation with 193 nm laser light. So, formation of the sample by melting at least stimulates formation of ODCs at 193 nm. The blue band decays obeys to power law ～t^?1 and is detected in the range of time 10 ns to 300 μs. The UV band possesses a fast, practically repeating excitation pulse, and a slow component (～30 μs). The obtained new kinetics data are compared with known in literature for lone twofold-coordinated silicon having exponential decay for the blue band equal to 10 ms and 4.5 ns for the UV band. That shows the blue band of new studied samples under ArF laser possesses decay component faster and the UV band slower than that of the twofold-coordinated silicon center. This corresponds to the recombination process of luminescence excitation by laser. We propose a model of the processes as charge separation under excitation with creation of a nearest self-trapped hole and electron trapped on the twofold-coordinated silicon, modified by its surrounding atoms or ions. This pair is recombining then with luminescence.     

1.5 μm Emission and infrared-to-visible frequency upconversion in Er+3/Yb+3-doped phosphoniobate glasses

Sodium phosphoniobate glasses with the composition (mol%) 75NaPO₃–25Nb₂O₅ and containing 2 mol% Yb³⁺ and x mol% Er³⁺ (0.01 ⩽ x ⩽ 2) were prepared using the conventional melting/casting process. Er³⁺ emission at 1.5 μm and infrared-to-visible upconversion emission, upon excitation at 976 nm, are evaluated as a function of the Er³⁺ concentration. For the lowest Er³⁺ content, 1.5 μm emission quantum efficiency was 90%. Increasing the Er³⁺ concentration up to 2 mol%, the emission quantum efficiency was observed to decrease to 37% due to concentration quenching. The green and red upconversion emission intensity ratio was studied as a function of Yb³⁺ co-doping and the Er³⁺–Er³⁺ energy transfer processes.     

Luminescence properties and energy transfer of Eu/Tb ions codoped aluminoborosilicate glasses

Eu/Tb codoped aluminoborosilicate glasses were fabricated by high temperature melting-quenched technique and their luminescence properties were investigated by excitation and emission spectra. Under 376 nm excitation, blue, green and red emission bands were simultaneously observed at 425 nm, 485 nm, 540 nm and 611 nm, respectively. The broad blue emission band centered at 425 nm was originated from the reduced Eu²⁺ ions, which were reduced from Eu³⁺ ions at high temperature in an ambient atmosphere and the reduction process may be related with the optical basicity of glass matrix. A complex bright white light emission was obtained for 0.5 mol% Eu₂O₃, 0.5 mol% Tb₂O₃ codoped aluminoborosilicate glass with CIE-X = 0.31 and CIE-Y = 0.33. The energy transfer among Eu³⁺, Eu²⁺ and Tb³⁺ ions was also discussed.     

Role of diffusing molecular hydrogen on relaxation processes in Ge-doped glass

Temperature dependencies of steady-state and time-resolved photoluminescence (PL) from triplet state at 3.1 eV and singlet state at 4.2 eV ascribed to the twofold-coordinated Ge have been measured in unloaded and H₂-loaded Ge-doped silica samples under 5.0 eV excitation in the 10–310 K range. Experimental evidences indicate that diffusing molecular hydrogen (H₂) depopulates by a collisional mechanism the triplet state, decreasing both its lifetime of about 14% and the associated triplet PL intensity, whereas those of the singlet are insensitive to the presence of H₂.     

On the optical absorption and photoluminescence of Er-doped Ge–S–Ga glasses

We have studied the absorption and photoluminescence (PL) of (GeS₂)₈₀(Ga₂S₃)₂₀ glasses doped with 0.17, 0.35 and 1.05 at.% Er. The sharp bands centered at around 660, 810, 980 and 1540 nm in the absorption spectra can be associated with intra 4f-shell transitions in Er³⁺ ions from ⁴I_15/2 level to ⁴F_9/2, ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 levels, respectively. It has been observed that the absorption edge shifts towards lower energies with increasing Er concentration. A decrease in the absorption coefficient in the range of weak absorption, as well as the host luminescence in more heavily doped samples has been established, which may be associated with less native defects in the glassy structure. The role of excitation wavelength (λ_ex) on the PL emission band at 1540 nm using different Er³⁺-doping level has been evaluated. It has been found that the total PL band remains almost the same under direct excitation of Er³⁺ ions (at λ_ex = 644, 770 and 982 nm), while it becomes narrower under the host excitation (at λ_ex = 532 nm).     

Temperature dependence of the photoinduced fatigue-recovery phenomena of photoluminescence under prolonged irradiation in GeS2 chalcogenide glass

We report the time-dependent intensities for the photoluminescence (PL) at various temperatures (10 K ? T ? 300 K) in amorphous GeS₂, which is known to exhibit not only PL fatigue but also PL recovery behavior. A difference in reversibility in fatigue-recovery process was found between room temperature (RT) and those at 110 K or below. Another small band of PL was also observed at 10 K or less, which also showed a time-dependence in intensity. A functional form to describe the time dependence was adopted based on a simplified model which have been obtained in a previous study and extended to describe the fatigue-recovery behaviors for all temperature range which have measured below RT.     

Enhancement of photoluminescence in SiCxNy nanoparticle films by addition of a Ni buffer layer

This work reports the effect of the presence of a Ni buffer layer on the photoluminescence (PL) of SiC_xN_y nanoparticle films prepared by RF plasma magnetron sputtering process in a reactive N₂ + Ar + H₂ gas mixture. An introduction of a Ni buffer of 80 nm or thicker remarkably improves the PL of the films. Annealing in a temperature range of 400–1100 °C is found to significantly affect the PL intensity. Optimal PL is achievable at 600 °C. X-ray photoelectron and Fourier-transform infrared spectroscopy suggest that the strong PL is directly related to the composition of the SiC_xN_y nanoparticle and the concentration of Si–O, and Si–N bonds. The results are relevant to the development of wide bandgap optoelectronic devices.     

Structure and optical properties of light-emitting Si films fabricated by neutral cluster deposition and subsequent high-temperature annealing

As a new development of our previous study on the production of light-emitting amorphous Si (a-Si) films by the neutral cluster deposition (NCD) method, we have fabricated light-emitting Si films with improved emission intensity by the combined methods of NCD and subsequent high-temperature annealing. The structure of these films is best characterized by Si nanocrystals, surrounded by an interfacial a-SiO_x (x < 2) layer, embedded in an a-SiO₂ film. These improved Si films were observed by atomic force microscopy and high-resolution transmission electron microscopy, and analyzed by means of X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence (PL) and Fourier transform infrared-attenuated total reflection measurements. The PL curves of the annealed samples exhibit peaks around 600 nm, at almost the same position as the unannealed samples. Their PL intensities, however, have increased to approximately five times those of the unannealed samples. The source of the luminescence is most likely due to electron-hole recombination in the a-SiO₂/Si interfacial a-SiO_x layer.     

Cathodoluminescence study of the excitons localization in AlGaN/GaN and InGaN/GaN quantum wells grown on sapphire

Al_0.1Ga_0.9N(5 nm)/GaN(2 nm) and In_0.2Ga_0.8N/GaN quantum wells (QWs) grown on GaN/sapphire have been studied by cathodoluminescence (CL) spectroscopy and imaged using an experimental setup especially developed for scanning near-field CL microscopy, which combines a scanning force microscope and a scanning electron microscope. The CL spectra show the characteristic band edge emission peak of GaN at λ= 364 nm and the emission peaks related to the presence of QWs, at λ= 353 and 430 nm for the AlGaN/GaN and the InGaN/GaN samples, respectively. Monochromatic CL images reveal that the emission of the AlGaN/GaN and InGaN/GaN QWs is localized at the level of the grains observed by SFM. A cross sectional analysis of the InGaN/GaN sample gives insight into its growth and an estimation of the exciton diffusion length of about L=180 nm.     

Photoluminescence and microstructure of Mn-doped ZnS phosphor powder co-fired with ZnS nanocrystallites

In this study, we correlated the photoluminescence (PL) with the microstructure of ZnS:Mn phosphor powders prepared by firing ZnS with MnO (1 mol%), NaCl (1 mol%) and ZnS nanocrystallites (NCs) in the range of 0–100 wt% at 600–1000 °C for 2 h in the atmosphere of 3%H₂/Ar. ZnS NCs of 10–30 nm in size were produced by co-precipitation of zinc nitrate and sodium sulfide solutions at room temperature. Thermal analysis (DTA/TG) and X-ray diffraction (XRD) results indicated that the cubic-hexagonal transformation temperature of ZnS NCs was lowered to approximately 600 °C, which was much lower than that of bulk ZnS (1020 °C). PL measurements revealed that ZnS:Mn fired with 1 wt% ZnS NCs showed the optimal luminescence intensity when compared to those without or with higher ZnS NCs (>1 wt%). An appropriate amount of ZnS NCs (1 wt%) acting as the flux in the firing process was inferred to avoid the inhomogeneous distribution of Mn²⁺ as well as the migration of excitation energy to quenching sites and therefore to result in the enhanced PL intensity.