High-efficiency erbium ion luminescence in silicon nanocrystal systems

The photoluminescence spectra and kinetics of both erbium-doped and undoped multilayer structures of quasi-ordered silicon nanocrystals in a silicon dioxide matrix were studied. It was shown that the optical excitation energy of silicon nanocrystals 2–3 nm in size can be practically completely transferred to Er³⁺ ions in the oxide surrounding the nanocrystals, with its subsequent radiation at 1.5 μm. Possible reasons for the high excitation efficiency of the Er³⁺ ions are discussed, and the conclusion is drawn that the F?rster mechanism is dominant in the energy transfer processes occurring in these structures. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 105–109. Original Russian Text Copyright ? 2004 by Kashkarov, Kamenev, Lisachenko, Shalygina, Timoshenko, Schmidt, Heitmann, Zacharias.     

Model of photoluminescence from ion-synthesized silicon nanocrystal arrays embedded in a silicon dioxide matrix

A four-level model of photoluminescence from Si nanocrystal arrays embedded in a SiO₂ matrix is suggested. The model allows for thermally activated transitions between singlet and triplet levels in the exchange-split energy state of an exciton in an excited silicon nanocrystal. An expression is derived for the temperature dependence of the intensity of photoluminescence monochromatic components. A correlation is found between the amount of splitting and the emitted photon energy by comparing model data with our experimental data for ion-synthesized Si nanocrystals in a SiO₂ matrix. The model explains the finiteness of the photoluminescence intensity at temperatures close to 0 K and the nonmonotonicity of the temperature run of the intensity.     

Synthesis of silicon dioxide layers by high dose ion implantation

Single crystal <100> silicon was implanted with molecular oxygen with energies ranging from 80 to 240 keV in a non-channeling direction. Rutherford backscattering (RBS) analysis was used to obtain the oxygen/silicon atomic ratio depth profiles and the thickness of the buried oxide layer, for doses ranging from 10¹⁶ to 1. 5 × 10¹⁸ O₂+/cm². This work links the early low energy work and the more recent higher energy work, and generally excellent agreement has been obtained. The minimum energy for formation of buried silicon dioxide has been identified as 160 keV per oxygen molecule and corresponding oxygen dose of 6 × 10¹⁷ O₂+/cm₂.     

Erbium electroluminescence excitation in amorphous hydrogenated silicon under thermally stimulated deep-center tunneling ionization

A study of the electroluminescence of erbium-doped, amorphous hydrogenated silicon, a-Si:H 〈Er〉, is reported. It has been found that the electroluminescence intensity at the wavelength λ=1.54 μm corresponding to the ⁴ I _13/2→⁴ I _15/2 intra-4f shell transition in Er passes through a maximum near room temperature. The unusual temperature and field dependences of the electroluminescence indicate electric-field induced multi-phonon tunneling emission of electrons from deep centers. The electroluminescence of Er³⁺ ions is due to their becoming excited as conduction-band electrons are captured by neutral dangling bonds (D ⁰ centers), which form when erbium is incorporated into the amorphous matrix. This Auger process transforms the center from its neutral state, D ⁰, to a negatively charged state, D ⁻, and the energy released in the capture is transferred by Coulomb interaction into the erbium-ion 4f shell. The steady-state current through the electroluminescent structure is supported by the reverse process of multi-phonon tunneling-electron emission from the D ⁻ center to the conduction band. The proposed theoretical model is in a good agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 210–217 (February 1999)     

Erbium in silicon carbide crystals: EPR and high-temperature luminescence

Erbium ions have been incorporated for the first time in bulk 6H-SiC crystals during growth, and they were unambiguously identified from the ¹⁶⁷Er EPR hyperfine structure. High-temperature luminescence of erbium ions at a wavelength of 1.54 μm has been detected. The observed luminescence exhibits an increase in intensity with increasing temperature. The observation of Er luminescence in 6H-SiC offers a promising potential for development of semiconductor light-emitting devices at a wavelength within the fiber-optics transparency window. Fiz. Tverd. Tela (St. Petersburg) 41, 38–40 (January 1999)     

Modified Maxwell-Bloch equations for systems under strong optical excitation

The response to a strong monochromatic electromagnetic field is investigated for a system with two energy bands. Modified Maxwell-Bloch equations are derived using a Master-equation approach in the rotating frame. Our results imply the vanishing of the stationary polarization in the rotating frame and are equivalent to the replacement of the equilibrium population in the ordinary linear response with the actual population determined by the strong excitation.     

外电场作用下二氧化硅分子的光激发特性研究

采用密度泛函B3P86和组态相互作用方法在6-311G**基组水平上计算了二氧化硅分子从基态到前5个激发态的跃迁波长、振子强度、自发辐射系数A_n0和吸收系数B_0n（n=1—5）.研究了外电场对二氧化硅分子激发态的影响规律. 结果表明,随外电场强度增大,最高占据轨道与最低空轨道能隙变小,占据轨道的电子易于激发至空轨道. 因而在外场作用下分子易于激发. 关键词： 2')" href="#">SiO₂ 激发态 外电场     

Scanning near-field optical images of ordered polystyrene particle layers in transmission and luminescence excitation modes

Scanning near-field optical images of hexagonally close-packed layers of polystyrene spherical particles with a diameter of 1.0 microm have been investigated. The layers were composed of particles that were doped either totally or partially with an organic fluorescent dye. Observations were made in the transmission and luminescence excitation modes with a scanning near-field optical microscope (SNOM) with a spatial resolution shorter than the wavelength of light. The patterns observed in the SNOM images are significantly dependent on the microstructures of layers, that is, the layers are either single or double layered, and the particles are either totally or partially doped. These results are discussed in terms of specific modes of electromagnetic waves transmitting across and along the layers after the local excitation at the tip end of the scanning microprobe.     

Kinetics of photoconductivity in a semiconductor under strong interband optical excitation (possible stochasticity)

Stationary states and the kinetics of change carriers are studied in an almost intrinsic semiconductor containing deep levels of one kind and subjected to exposure under photon energy greater than the forbidden bandwidth. The exposure intensity is assumed sufficiently large, such that the concentration of the photoexcited charge carriers would exceed equilibrium significantly. Taken into account are both the charge carrier heating by the light and the dependence of the forbidden bandwidth on the electron and hole concentration and temperature. The conditions are mentioned for which stochastic or periodic electron and hole temperature and concentration self-oscillations should occur in the specimen.Deceased.Translated from Izvestiya Vysshikh Uchebynykh Zavedenii, Fizika, No. 6, pp. 87–95, June, 1987.     

Electron-excited luminescence of SiO2 layers on silicon

An analysis of cathodoluminescence and electroluminescence spectra of Si-SiO₂ structures suggests a conclusion concerning the processes involved in excitation of the luminescence centers generated in the UV spectral region and their localization. The electroluminescence observed in this region of the spectrum is generated in excitation of luminescence centers localized in the immediate vicinity of the Si-SiO₂ phase boundary. In the case of cathodoluminescence, the observed emission bands at ??4.3 and ??2.7 eV appear in excitation of the luminescence of silylene centers at the Si-SiO₂ phase boundary.     

Production of nanocrystal layers in monocrystal silicon in the beam-type plasma of a high-voltage discharge

The results of investigations on the formation of nanostructured hydrogen–silicon complexes in the nearsurface layer of monocrystal silicon exposed to hydrogen-containing plasma are presented. By using infrared and Auger spectroscopy, it has been shown that the formation of nanolayers and, hence, an extreme change of the resistivity of the near-surface layer take place only at short treatment times.     

Photoluminescence of Er3+ ions in amorphous silicon layers under intensive laser excitation

Transients of the photoluminescence (1.54 μm) of Er³⁺ ions embedded in an amorphous silicon matrix excited with intensive laser pulses are simulated using a phenomenological model which takes into account both the defect-related excitation mechanism and stimulated optical transitions in the ions. The simulated transients are compared with the experimental ones observed in Er-doped amorphous silicon layers under pulsed laser excitation. The modeling and the experimental results demonstrate a possibility to realize a regime of superradiance in the system of Er³⁺ ions pumped via an electronic excitation of the amorphous matrix. Received: 7 August 2001 / Revised version: 1 November 2001 / Published online: 17 January 2002     

Erbium excitation in a SiO<Subscript>2</Subscript>: Si-nc matrix under pulsed pumping

The photoluminescence of Er³⁺ ions in a SiO₂ matrix containing silicon nanocrystals 3.5 nm in diameter is studied under resonant and nonresonant pulsed pumping with pulses 5 ns in duration. The effective erbium excitation cross section under pulsed pumping, σ_eff = 8.7 × 10^?17 cm₂, is close to that for nanocrystals. Comparison of the erbium photoluminescence intensity obtained for a SiO₂ matrix with and without nanocrystals made it possible to determine the absolute concentration of optically active nanocrystals capable of exciting erbium ions, the concentration of optically active erbium, and the average number of erbium ions excited by one nanocrystal. The study revealed that excitation transfer from one erbium ion to another is a relatively slow process, which accounts for the low efficiency of erbium ion excitation under pulsed pumping in a SiO₂ matrix containing silicon nanocrystals.     

Subnanosecond luminescence spectroscopy of Cd1−xMnxSe under high intensity optical excitation

Transient photoluminescence of Cd_1?xMn_xSe has been investigated for compositions x = 0, .05 and .10, in zero magnetic field, under high excitation intensity. The spectra yield information about the temporal evolution of inelastic scattering processes in a dense exciton gas, with a likely contribution by an electron-hole plasma. The short carrier lifetimes measured, less than 100 psec for x = .10, and a possible plasma expansion, might inhibit the generation of carrier densities sufficient for suggested exchange induced magnetic ordering.     

Effective excitation cross section of erbium in amorphous hydrogenated silicon under optical pumping

The effective excitation cross section of erbium embedded in an amorphous silicon matrix and the total lifetime of erbium ions in the excited state are determined by measuring the photoluminescence rise time of erbium ions under pulsed excitation of erbium-doped amorphous hydrogenated silicon. An analysis of the rate equations describing the excitation and deexcitation of erbium ions in a semiconducting matrix sheds light on the physical meaning of the effective excitation cross section. It is shown that measurement of the effective excitation cross section permits evaluation of the concentration of optically active erbium ions in the amorphous silicon matrix.     

Photoluminescence of Er<Superscript>3+</Superscript> ions in layers of quasi-ordered silicon nanocrystals in a silicon dioxide matrix

The spectra and kinetics of photoluminescence from multilayered structures of quasi-ordered silicon nanocrystals in a silica matrix were studied for undoped samples and samples doped with erbium. It was shown that the optical excitation energy of silicon nanocrystals could be effectively transferred to Er³⁺ ions, which was followed by luminescence at a wavelength of 1.5 µm. The effectiveness of energy transfer increased as the size of silicon nanocrystals decreased and the energy of exciting light quanta increased. The excitation of erbium luminescence in the structures was explained based on dipole-dipole interaction (the Förster mechanism) between excitons in silicon nanocrystals and Er³⁺ ions in silica surrounding them.     

Energy dissipation of heated electrons in silicon dioxide layers

The field and thickness dependence of the conductivity, charge state, and electroluminescence of Si-SiO₂ structures is investigated with the aim of studying the energy-dissipation mechanisms of electrons heated by an electric field in SiO₂ films. It is shown that, as well as the previously known dissipation channels (interaction with the phonon subsystem and impact ionization in the SiO₂ volume, with characterisic energy losses ₁<0.153 eV and ₂>9 eV, respectively), account must also be taken of processes of defect excitation at the Si-SiO₂ boundary, which are accompanied by radiative relaxation in the electroluminescence bands and characterized by intermediate energy-loss values (₁<<₂).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 36–40, January, 1991.     

Relative enhancement of photoluminescence intensity of passivated silicon nanocrystals in a silicon dioxide matrix

Photoluminescence (PL) intensity of passivated silicon nanocrystals (Si NCs) embedded in a SiO 2 matrix is compared with that of unpassivated Si NCs. We investigate the relative enhancement of PL intensity (I R ) as a function of annealing temperature and implanted Si ion dose. The I R increases simultaneously with the annealing temperature. This demonstrates an increase in the number of dangling bonds (DBs) with the degree of Si crystallization varying via the annealing temperature. The increase in I R with implanted Si ion dose is also observed. We believe that the near-field interaction between DBs and neighboring Si NCs is an additional factor that reduces the PL efficiency of unpassivated Si NCs.     

Photoluminescence and paramagnetic defects in silicon-implanted silicon dioxide layers

Thermally grown SiO₂ layers on Si substrates implanted with Si⁺ ions with a dose of 6×10¹⁶ cm⁻² were studied by the techniques of photoluminescence, electron paramagnetic resonance (EPR), and low-frequency Raman scattering. Distinct oxygen-vacancy associated defects in SiO₂ and non-bridging oxygen hole centers were identified by EPR. The luminescence intensity in the 620 nm range was found to correlate with the number of these defects. The low-frequency Raman scattering technique was used to estimate the average size of the Si nanocrystallites formed after the implantation and thermal annealing at T>1100°C, which are responsible for the photoluminescence band with a maximum at 740 nm. The intensity of this band can be significantly enhanced by an additional treatment of the samples in a low-temperature RF plasma.     

Q-factor of spherical optical silicon dioxide nanoresonators

The Q-factor of the bound optical modes of silicon dioxide spheres with a radius smaller than one micron has been studied in the visible spectrum, showing that local photonic states with a nonzero orbital momentum yield a thin structure in glow spectra, whose maxima positions depend on the diameters of SiO₂ nanospheres. A theoretical model to calculate the Q-factor of nanoresonators depending on the wavelengths of bound modes and the radius value of spheres has been proposed. It has been shown that the quality of an optical resonator increases as its diameter grows. The Q-factor values have been calculated for the visible light of the order of 10 for spheres smaller than 1 μm in diameter and 10⁶ for spheres of more than 7 μm in diameter. A dependence of bound modes’ radiation propagation outside of the sphere on their wavelengths and orbital momenta has been detected.