Interrelation between microstructure and optical properties of erbium-doped nanocrystalline thin films

Nanocrystalline silicon thin films codoped with erbium, oxygen and hydrogen have been deposited by co-sputtering of Er and Si. Films with different crystallinity, crystallite size and oxygen content have been obtained in order to investigate the effect of the microstructure on the photoluminescence properties. The correlation between the optical properties and microstructural parameters of the films is investigated by spectroscopic ellipsometry. PL response of the discussed structures covers both the visible wavelength range (a crystallite size-dependent photoluminescence detected for 5–6 nm sized nanocrystals embedded in a SiO matrix) and near IR range at 1.54 μm (Er-related PL dominating in the films with 1–3 nm sized Si nanocrystals embedded in a-Si:H). It is demonstrated that the different PL properties can be also discriminated on the basis of ellipsometric spectra.     

Radiative recombination of holes in delta-p-doped gallium arsenide

Photoluminescence spectra of δ-p-doped GaAs structures of different doping levels are studied experimentally. It is found that set of PL bands observed in δ-p-doped samples recently is regularly broadened with doping concentration increase due to appearance of additional low-energy bands and their subsequent red shift at higher doping levels. A red shift of the bands and a change of their relative intensities were caused also by excitation laser intensity decrease and/or temperature increase. These results confirm our previous assumption that the bands are due to radiative recombination of spatially separated photoelectrons with holes occupying size-quantization levels of δ-layer potential well.     

Amorphization and recrystallization of ion implanted Si nanocrystals probed through their luminescence properties

In this paper the amorphization of Si nanocrystals (nc) by ion beam irradiation and the subsequent recrystallization are investigated in detail. The luminescence properties of Si nanocrystals embedded within a SiO₂ matrix are used as a probe of the damaging effects generated by high-energy ion beam irradiation. Samples have been irradiated with 2 MeV Si⁺ ions at different doses, in the range between 1×10⁹ and 1×10¹⁶ cm⁻². By increasing the ion dose, the nc-related photoluminescence (PL) strongly decreases after a critical dose value. It is shown that the lifetime quenching alone cannot quantitatively explain the much stronger PL drop, but the total number of emitting centers has to diminish too. Moreover, we studied the recovery of the amorphized Si nc by performing thermal annealings. It is demonstrated that the recovery of the PL properties of completely amorphized Si nc is characterized by a single activation energy, whose value is 3.4 eV. Actually, this energy is associated to the transition between the amorphous and the crystalline phases of each Si grain. The recrystallization kinetics of Si nanostructures is demonstrated to be very different from that of a bulk system.     

Impact of interface relaxation on the nanoscale corrugation in Pb/Si(1 1 1) islands

The nanoscale hexagonal pattern observed in scanning tunneling microscopy (STM) for 3-layer and 4-layer Pb islands on Si(1 1 1) is studied theoretically. We found that besides thickness the atomic rearrangement at the Pb/Si interface plays an important role in determining the STM patterns. Electronic structures of the Pb film on Si(1 1 1) obtained from fully relaxed and unrelaxed Pb films are qualitatively different. Simulated STM images for Pb films with different stacking also show that the corrugation patterns are sensitive to the buried Pb-Si interfacial structure.     

Optical properties of InAs quantum dots in a Si matrix

We report on the optical properties of nanoscale InAs quantum dots in a Si matrix. At a growth temperature of 400°C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range with a high sheet density. Samples with such InAs quantum dots show a luminescence band in the 1.3 μm region for temperatures up to 170 K. The PL shows a pronounced blue shift with increasing excitation density and decays with a time constant of 440 ns. The optical properties suggest an indirect type II transition for the InAs/Si quantum dots. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

nc-Ge/SiN_x多层膜的光致发光特性

通过将a-Ge∶H/a-SiN_x多层膜进行氧化,制备了nc-Ge/SiN_x多层膜。观察到了室温下的强烈可见光发射,发光波长为500nm。通过分析,排除了与量子限制效应有关的光发射机制,也排除了与Si和N相关的缺陷产生的复合机制,认为该发光源于氧化后的a-SiN_x介质层中带尾态之间的辐射复合,最有效的激发能量约为介质层的带隙。     

Enhanced photoluminescence intensity of 1.3-μm multi-layer InAs/InGaAs dots-in-well structure using the high growth temperature spacer layer step

The effects of growth temperature of the GaAs spacer layers (SPLs) on the photoluminescence (PL) efficiency of multi-layer GaAs-based 1.3-μm InAs/InGaAs dots-in-well (DWELL) structures have been investigated. It is found that the PL intensity of DWELLs is enhanced by incorporating a high growth temperature step for GaAs SPLs. This improved PL efficiency could be understood in term of reducing the non-radiative recombination centers. An extremely low continuous-wave room-temperature threshold current density of 35 A/cm² is achieved for an as-cleaved 5-layer device with emission at 1.31 μm by using this growth technique.     

GaN epilayers on nanopatterned GaN/Si(1 1 1) templates: Structural and optical characterization

Template-based nanoscale epitaxy has been explored to realize high-quality GaN on Si(1 1 1) substrates. We have employed polystyrene-based nanosphere lithography to form the nano-hole array patterns on GaN/Si(1 1 1) template and then, subsequent regrowth of GaN is carried out by metalorganic chemical vapor deposition (MOCVD). During the initial growth stage of GaN on such nanopatterned substrates, we have observed formation of nanoislands with hexagonal pyramid shape due to selective area epitaxy. With further epitaxial regrowth, these nanoislands coalesce and form continuous GaN film. The overgrown GaN on patterned and non-patterned regions is characterized by high-resolution X-ray diffraction (HRXRD) and high-spatial resolution optical spectroscopic methods. Micro-photoluminescence (PL), micro-Raman scattering and scanning electron microscopy (SEM) have been used to assess the microstructural and optical properties of GaN. Combined PL and Raman data analyses show improved optical quality when compared to GaN simultaneously grown on non-patterned bulk Si(1 1 1). Such thicker GaN templates would be useful to achieve III-nitride-based opto- and electronic devices integrated on Si substrates.     

Defect-related light emission from processed He-implanted silicon

Photoluminescence (PL) from He⁺-implanted Si (Si:He, He⁺ dose—2×10¹⁶ cm⁻², at 150 keV) is related to its microstructure; it has been tuned by processing at 720-1400 K under hydrostatic Ar pressure (HP, up to 1.2 GPa). Processing of Si:He at 720 K for 10 h results in an appearance of the D2 and D3 dislocation-related PL lines, these last of the highest intensity. Only the D1 dislocation-related line of intensity increasing with HP has been detected after processing at 920-1070 K. The D1 (of the highest intensity), D2 and D3 PL lines are observed after the treatment at 1270 K. No dislocation-related PL has been detected for Si:He processed at 1400 K. The treatment of Si:He at 720-1270 K under HP makes it possible to produce Si:He of specific microstructure resulting in strong PL at 0.81, 0.87 or 0.94 eV.     

Electrical conduction properties of Si δ-doped GaAs grown by MBE

The temperature dependent Hall effect and resistivity measurements of Si δ-doped GaAs are performed in a temperature range of 25–300 K. The temperature dependence of carrier concentration shows a characteristic minimum at about 200 K, which indicates a transition from the conduction band conduction to the impurity band conduction. The temperature dependence of the conductivity results are in agreement with terms due to conduction band conduction and localized state hopping conduction in the impurity band. It is found that the transport properties of Si δ-doped GaAs are mainly governed by the dislocation scattering mechanism at high temperatures. On the other hand, the conductivity follows the Mott variable range hopping conduction (VRH) at low temperatures in the studied structures.     

The influence of substrate on the properties of Er2O3 films grown by magnetron sputtering

The structural properties and the room temperature luminescence of Er₂O₃ thin films deposited by RF magnetron sputtering have been studied. Films characterized by good morphological properties have been obtained by using a SiO₂ interlayer between the film and the Si substrate. The evolution of the properties of the Er₂O₃ films due to rapid thermal annealing processes in O₂ ambient performed at temperatures in the range 800-1200 °C has been investigated in details. The existence of well-defined annealing conditions (temperature of 1100 °C or higher) allowing to avoid the occurrence of extensive chemical reactions with the oxidized substrate has been demonstrated and an increase of the photoluminescence (PL) intensity by about a factor of 40 with respect to the as deposited material has been observed. The enhanced efficiency of the photon emission process has been correlated with the longer lifetime of the PL signal. The same annealing processes are less effective when Er₂O₃ is deposited on Si. In this latter case interfacial reactions and pit formation occur, leading to a material characterized by stronger non-radiative phenomena that limit the PL efficiency.     

Anisotropic relaxation and motion of half-integer quadrupole nuclei studied by central transition nuclear magnetic resonance spectroscopy

An efficient theoretical formalism and advanced experimental methods are presented for studying the effects of anisotropic molecular motion and relaxation on solid-state central transition NMR spectra of half-integer quadrupole nuclei. The theoretical formalism is based on density operator algebra and involves the stochastic Liouville–von Neumann equation. In this approach the nuclear spin interactions are represented by the Hamiltonian while the motion is described by a discrete stochastic operator. The nuclear spin interactions fluctuate randomly in the presence of molecular motion. These fluctuations may stimulate the relaxation of the system and are represented by a discrete relaxation operator. This is derived from second-order perturbation theory and involves the spectral densities of the system. Although the relaxation operator is valid only for small time intervals it may be used recursively to obtain the density operator at any time. The spectral densities are allowed to be explicitly time dependent making the approach valid for all motional regimes. The formalism has been applied to simulate partially relaxed central transition ¹⁷O NMR spectra of representative model systems. The results have revealed that partially relaxed central transition lineshapes are defined not only by the nuclear spin interactions but also by anisotropic motion and relaxation. This has formed the basis for the development of central transition spin-echo and inversion-recovery NMR experiments for investigating molecular motion in solids. As an example we have acquired central transition spin-echo and inversion-recovery ¹⁷O NMR spectra of polycrystalline cristobalite (SiO₂) at temperatures both below and above the α–β phase transition. It is found that the oxygen atoms exhibit slow motion in α-cristobalite. This motion has no significant effects on the fully relaxed lineshapes but may be monitored by studying the partially relaxed spectra. The α–β phase transition is characterized by structural and motional changes involving a slight increase in the Si–O–Si bond angle and a substantial increase in the mobility of the oxygen atoms. The increase in the Si–O–Si angle is supported by the results of ¹⁷O and ²⁹Si NMR spectroscopy. The oxygen motion is shown to be orders of magnitude faster in β-cristobalite resulting in much faster relaxation and characteristic lineshapes. The measured oscillation frequencies are consistent with the rigid unit mode model. This shows that solid-state NMR and lattice dynamics simulations agree and may be used in combination to provide more detailed models of solid materials.     

Photoluminescence properties of Er-doped β-FeSi2 grown by ion implantation

Photoluminescence (PL) properties of Er-doped β-FeSi₂ (β-FeSi₂:Er) and Er-doped Si (Si:Er) grown by ion implantation were investigated. In PL measurements at 4.2 K, the β-FeSi₂:Er showed the 1.54 μm PL due to the intra-4f shell transition of ⁴I_13/2→⁴I_15/2 in Er³⁺ ions without a defect-related PL observed in Si:Er. In the dependence of the PL intensity on excitation photon flux density, the obtained optical excitation cross-section σ in β-FeSi₂:Er (σ=7×10⁻¹⁷ cm²) is smaller than that in Si:Er (σ=1×10^-15 cm²). In the time-resolved PL and the temperature dependence of the PL intensity, the 1.54 μm PL in β-FeSi₂:Er showed a longer lifetime and larger activation energies for non-radiative recombination (NR) processes than Si:Er. These results revealed that NR centers induced by ion implantation damage were suppressed in β-FeSi₂:Er, but the energy back transfer from Er³⁺ to β-FeSi₂ was larger than Si:Er.     

An ab initio calculation of O and Si NMR parameters for SiO2 polymorphs

Ab initio molecular orbital calculations (Hartree–Fock, HF and density functional theories, DFTs) have been carried out for SiO₂ polymorphs coesite, low cristobalite, and α-quartz, in order to investigate the reliability of this method for predicting ²⁹Si and ¹⁷O nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the ²⁹Si and ¹⁷O chemical shifts (δ_i^Si and δ_i^O), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the δ_i^Si and δ_i^O have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated δ_i^Si values agree well (within ±1 ppm) with experimental data. The calculated ¹⁷O electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in δ_i^O for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting ²⁹Si and ¹⁷O NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method.     

Magnetocrystalline symmetry, spin statics and dynamics in high-temperature superconductors

The symmetry analysis of the magnetic structures of La₂CuO₄ and YBa₂Cu₃O_6+δ (δ 0.4) antiferromagnetic oxide has been carried out. The spin configurations and spin-reorientation transition fields have been found. The frequencies of uniform spin oscillations and the external field dependence of their polarization properties have been calculated. It has been shown that the spin excitation branches may include exchange modes among which some dipole-active modes can be present. The spectrum of the inelastic light scattering accompanied with the excitation of spin and libration (tilting) freedom degrees has been discussed.     

Photoluminescence and optical absorption properties of silicon quantum dots embedded in Si-rich silicon nitride matrices

Silicon nitride (SiN_x) films were prepared with a gas mixture of SiH₄ and NH₃ on Si wafers using the plasma-enhanced chemical vapor deposition (PECVD) method. High-resolution transmission electron microscopy and infrared absorption have been used to reveal the existence of the Si quantum dots (Si QDs) and to determine the chemical composition of the silicon nitride layers. The optical properties of these structures were studied by photoluminescence (PL) spectroscopy and indicate that emission mechanisms are dominated by confined excitons within Si QDs. The peak position of PL could be controlled in the wavelength range from 1.5 to 2.2 eV by adjusting the flow rates of ammonia and silane gases. Absorbance spectra obtained in the transmission mode reveal optical absorption from Si QDs, which is in good correlation with PL properties. These results have implications for future nanomaterial deposition controlling and device applications.     

Visible photoluminescence of co-sputtered Ge–Si duplex nanocrystals

The photoluminescence (PL) characteristics of co-sputtered Ge–Si duplex nanocrystal films were examined under excitation by a 325-nm HeCd laser, combined with Raman and Fourier-transform infrared reflection spectra analysis. A broad visible PL spectrum from the as-deposited Ge–Si nanocrystal films was observed in the wavelength range 350–700 nm. Basically, the PL spectrum can be considered to consist of two distinct parts originating from different emission mechanisms: (i) the spectrum in the range 350–520 nm, consisting of characteristic double peaks at 410 and 440 nm with PL intensities decreasing after vacuum annealing, probably due to vacancy defects in Si nanocrystals; and (ii) the spectrum in the range 520–700 nm, consisting of a characteristic peak at 550 nm with a PL intensity not affected by vacuum annealing, probably due to Ge-related interfacial defects. No size dependence of PL peak energy expected from quantum confinement effects was observed in the wavelength range investigated. However, with an increase of crystal size, the PL peak intensity in the blue zone decreased. The PL intensity is found to be strongly affected by silicon concentration. A film heated in air has a different PL mechanism from the as-deposited and vacuum-annealed films. PACS 78.67.Bf; 81.05.Cy; 81.15.Cd     

Dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies

The dielectric properties of nano Si/C/N composite powder and nano SiC powder at high frequencies have been studied. The nano Si/C/N composite powder and nano SiC powder were synthesized from hexamethyldisilazane ((Me₃Si)₂NH) (Me:CH₃) and SiH₄–C₂H₂, respectively, by a laser-induced gas-phase reaction. The complex permittivities of the nano Si/C/N composite powder and nano SiC powder were measured at a frequency range of 8.2–12.4 GHz. The real part (′) and imaginary part (″) of the complex permittivity, and dissipation factor (tg δ=″/′) of nano Si/C/N composite powder are much higher than those of nano SiC powder and bulk SiC, Si₃N₄, SiO₂, and Si, especially the tg δ. The promising features of nano Si/C/N composite powder would be due to more complicated Si, C, and N atomic chemical environment than in a mixture of pure SiC and Si₃N₄ phase. The charged defects and quasi-free electrons moved in response to the electric field, diffusion or polarization current resulted from the field propagation. Because there exists graphite in the nano Si/C/N composite powder, some charge carries are related to the sp³ dangling bonds (of silicon and carbon) and unsaturated sp² carbons. The high ″ and tg δ of nano Si/C/N composite powder were due to the dielectric relaxation. The nano Si/C/N composite powder would be a good candidate for electromagnetic interface shielding material.     

Defect-free growth of epitaxial silicon at low temperatures (500–800 °C) by atmospheric pressure plasma chemical vapor deposition

Epitaxial Si growth at low temperatures (500–800 °C) by atmospheric pressure plasma chemical vapor deposition has been investigated. Silicon films are deposited on (001) Si wafers using gas mixtures containing He, H₂, and SiH₄. The effects of deposition parameters (composition of reactive gases, very high frequency (VHF) power, and substrate temperature) on film properties are investigated by reflection high-energy electron diffraction, atomic force microscopy, cross-sectional transmission electron microscopy, and plasma emission spectroscopy. It is found that epitaxial temperature can be reduced by increasing VHF power, and that an optimum range of VHF power exists for Si epitaxy, depending on the substrate temperature and the composition of the reactive gases. The result of the H₂ concentration dependence of H_α emission intensity, shows that hydrogen atoms generated in the atmospheric pressure plasma play an important role in Si epitaxial growth. Under the optimized growth conditions, defect-free epitaxial Si films (as observed by transmission electron microscopy) with excellent surface flatness are grown at 500 °C with an average growth rate of approximately 0.25 μm/min. PACS 81.05.Cy; 81.15.Gh; 68.55.Jk