Substrate temperature dependence of the photoluminescence efficiency of co-sputtered Si/SiO2 layers

Si particles embedded in an SiO₂ matrix were obtained by co-sputtering of Si and SiO₂ at various deposition temperatures T_d (200–700°C) and annealing at different temperatures T_a (900–1100°C). The systems were characterized by X-ray photoelectron, Raman scattering, infrared absorption and photoluminescence spectroscopy techniques. The results show that the photoluminescence efficiency is strongly dependent on the degree of phase separation between the Si nanocrystals and the SiO₂ matrix. This is likely connected with the Si/SiO₂ interface characteristics, together with the features indicating the involvement of quantum confinement.     

Si–SiO2 interface formation in low-dose low-energy separation by implanted oxygen materials

The evolution of the Si–SiO₂ interface morphology of low-dose low-energy separation by implanted oxygen materials was investigated by transmission electron microscopy and atomic force microscopy. The Si–SiO₂ interface morphology and the RMS roughness are strongly affected by the implantation conditions and the annealing process. Three main types of the domains including round, square, and pyramid shapes with the step-terrace structure were observed on the buried SiO₂ surface. Round domains are observed in the early stage of the annealing process, while the square and pyramid domains are observed after the high temperature annealing. The mean RMS roughness decreases with increasing time and annealing temperature, while in the 1350 °C 4-h annealed samples, the mean RMS roughness decreases with either increasing the implantation dose or decreasing implantation energy. The scaling analysis shows that the Si–SiO₂ interfaces were found to be self-affine on the short length scales with a roughness exponent above 0.50. Qualitative mechanisms of Si–SiO₂ surface flattening are presented in terms of the variations of morphological features with the processing conditions.     

Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Amorphous Si/insulator multilayers grown by vacuum deposition and electron cyclotron resonance plasma treatment

a-Si/insulator multilayers have been deposited on (0 0 1) Si by electron gun Si evaporation and periodic electron cyclotron resonance plasma oxidation or nitridation. Exposure to an O or N plasma resulted in the formation of a thin SiO₂ and SiN_x layer whose thickness was self-limited and controlled by process parameters. For thin-layer (2 nm) Si/SiO₂ and Si/SiN_x multilayers no visible photoluminescence (PL) was observed in most samples, although all exhibited weak “blue” PL. For the nitride multilayers, annealing at 750°C or 850°C induced visible PL that varied in peak energy with Si layer thickness. Depth profiling of a-Si caps on thin insulating layers revealed no detectable contamination for the SiN_x layers, but substantial O contamination for the SiO₂ films.     

Cathodoluminescence properties of silicon nanocrystallites embedded in silicon oxide thin films

Cathodoluminescence (CL) spectra for the Si nanocrystallites embedded in a matrix of silicon oxide films are measured at room temperature. The CL spectra consist of two principal bands whose peak energies are in a near-infrared (NIR) region (<1.6 eV) and in a blue region (2.6 eV), respectively. The spectral feature of the NIR CL band is similar to the corresponding PL spectra. The strong correlation between the presence of Si nanocrystallites and the formation of the NIR CL band are found as well as the PL spectrum. The peak energy of the blue CL band is slightly lower than that of the luminescence band originating from oxygen vacancies (≡Si–Si≡) in SiO₂. Therefore, the blue CL band is considered to come from Si_n clusters with n3 in the oxide matrix. Under irradiation of electron beams, degradation of the intensity is observed for both the CL bands but the decay characteristics are different.     

Enhanced magnetic properties of Nd–Fe–B thin films crystallized by heat treatment

Nd₂Fe₁₄B Φ phase crystallites were formed in Nd_16.7Fe_65.5B_17.8 thin films prepared by RF sputtering with subsequent heat treatment. The 2 μm-thick films were deposited onto 0.1 mm Mo sheets at an average substrate temperature (T_s) of 365°C. The enhanced magnetic properties of the magnetically anisotropic thin films were investigated using different heating rates (h_r) of 10°C, 20°C, 50°C and 100°C/min in an annealing experiment. Transformation from the amorphous phase to the crystalline phase is clearly manifested by the formation of fine crystallites embedded as a columnar matrix of Nd₂Fe₁₄B phase. High-resolution scanning electron microscope data of the cross-section of the annealed films show columnar stacking of Nd₂Fe₁₄B crystallites with sizes <500 nm. Transmission electron microscope observations revealed that the microstructure of these films having out-of-plane magnetization consists of uniformly distributed Φ phase with grain size around 400 nm together with small Nd rich particles. This grain size of Φ phase is comparable to the single domain particle diameter of Nd₂Fe₁₄B. Significant change in _iH_c, 4πM_r and 4πM_s with h_r was confirmed. Annealing conditions with a heating rate of 50°C/min to an annealing temperature (T_a) of 650°C for 30 min was consequently found to give optimum properties for the NdFeB thin films. The resulting magnetic properties, considered to be the effect of varying h_r were _iH_c= 1307–1357 kA/m, 4πM_r=0.78–1.06 T and 4πM_s=0.81–1.07 T.     

The origin of photoluminescence in Ge-implanted SiO2 layers

Ge ions were implanted at 100 keV with 3×10¹⁶ cm⁻² into a 300  nm thick SiO₂ layer on Si. Visible photoluminescence (PL) around 2.1 eV from an as-implanted sample is observed, and faded out by subsequent annealing at 900°C for 2 h. However, PL shows up again after annealing above 900°C at the same peak position. Compared with the as-implanted sample, significant increase of Ge–Ge bonds is measured in X-ray photoelectron spectroscopy, and the formation of Ge nanocrystals with a diameter of 5 nm are observed in transmission electron microscopy from the sample annealed at 1100°C. We conclude that the PL peak from the sample annealed above 900°C is caused by the quantum confinement effects from Ge nanocrystals, while the luminescence from the as-implanted sample is due to some radiative defects formed by Ge implantation.     

Visible and infrared photoluminescence from Er-doped SiOx

The annealing behaviors of photoluminescence of SiO_x and Er-doped SiO_x grown by molecular beam epitaxy in the wavelength range of visible and infrared light are studied. For SiO_x, four PL bands located at 510, 600, 716 and 810 nm, respectively, are observed. For Er-doped SiO_x, the 716 nm band, which is believed to be originated from the electron–hole recombination at the interface between crystalline Si and amorphous SiO₂, disappears in the annealing temperature range of 500–900°C. It is suggested the enhancement of Er luminescence is partially due to the energy transfer from the recombination at the interface between crystalline Si and SiO₂ to Er ions.     

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.     

Reversible H2 passivation of Si ≡ Si3 interface defects in (1 1 1)Si/SiO2

K-band electron spin resonance (ESR) at 4.3 K has revealed the dipole-dipole (DD) interaction effects between [1 1 1]P_b centers (^*Si ≡ Si₃ defects with unpaired sp³ hybrid [1 1 1]) at the 2 dimensional (1 1 1)Si/SiO₂ interface. This has been enabled by the perfectly reversible H₂ passivation of P_b, which affects the defect's spin state. Sequential hydrogenation at 253–353°C and degassing treatments in high vacuum at 743–835°C allowed to vary the P_b density in the range 5 × 10¹⁰ < [P_b] (1.14 ± 0.06) × 10¹³ cm^-2. With increasing [P_b] fine structure doublets are clearly resolved. It is found that (1 1 1)Si/SiO₂ interfaces, dry thermally grown at ≈920°C, naturally comprise a ^*Si ≡ Si₃ defect density — passivated or not — of 1.14 × 10¹³ cm^-2.     

SiO2 passivation film effects on microwave characteristics of YBa2Cu3O7−x-based resonators

SiO₂ film coated as a passivation layer for YBa₂Cu₃O_7−x (YBCO)-based microwave devices is investigated by measuring the microwave characteristics of microstrip line resonators. The SiO₂ film is deposited with its 0.3 to 0.4 μm thickness by a sputtering method using Ar + 30%O₂ plasma. These deposition conditions do not degrade the microwave characteristics and the critical temperature (T_c). Next, the SiO₂ film coated resonators are compared with the uncoated ones for two kinds of degradation conditions: a 200°C annealing in air, and an exposure to air at 85°C and 85% RH (relative humidity). We find that the SiO₂ passivation film prevents the YBCO thin film from the surface degradation and reacting with water.     

Excess molar heat capacity of isobutyric acid–water binary liquid mixtures near and far away from the critical temperature

The heat capacity of the liquid–liquid mixture isobutyric acid–water has been measured for the first time near and far away from its critical point using an adiabatic calorimeter. The measurements were performed at atmospheric pressure, in the one phase region as a function of three temperatures: (1) T − T_C = 0.055 °C, (2) T − T_C = 3.055 °C, (3) T − T_C = 8.055 °C and of the composition X in acid (IA). The heat capacity C_p decreases rapidly when X increases at the used temperatures. Near the critical composition, C_p is not affected by the correlation of the concentration fluctuations.

The molar excess heat capacity of the system under investigation was analysed along the phase diagram and considered as a structural transformation effect.     

Annealing effects on structure and laser-induced damage threshold of Ta2O5/SiO2 dielectric mirrors

The effects of annealing on structure and laser-induced damage threshold (LIDT) of Ta₂O₅/SiO₂ dielectric mirrors were investigated. Ta₂O₅/SiO₂ multilayer was prepared by ion beam sputtering (IBS), then annealed in air under the temperature from 100 to 400 °C. Microstructure of the samples was characterized by X-ray diffraction (XRD). Absorption of the multilayer was measured by surface thermal lensing (STL) technique. The laser-induced damage threshold was assessed using 1064 nm free pulsed laser at a pulse length of 220 μs.

It was found that the center wavelength shifted to long wavelength gradually as the annealing temperature increased, and kept its non-crystalline structure even after annealing. The absorbance of the reflectors decreased after annealing. A remarkable increase of the laser-induced damage threshold was found when the annealing temperature was above 250 °C.     

Photoluminescence and electroluminescence investigations at Ge-rich SiO2 layers

Strong blue and violet photo (PL) and electroluminescence (EL) at room temperature was obtained from SiO₂-films grown on crystalline Si, which were either single (SI) or double implanted (DI) with Ge ions and annealed at different temperatures. The PL spectra of Ge-rich layers reach a maximum after annealing at 500–700°C for DI layers or 900–1000°C for SI layers, respectively. Both, PL and EL of 500 nm thick Ge-rich layers are easily visible by the naked eye at ambient light due to their high intensity. Based on excitation spectra we tentatively interpret the blue PL as due to the oxygen vacancy in silicon dioxide.

The EL spectrum of the Ge-implanted oxide correlates very well with the PL one and shows a linear dependence on the injected current over three orders of magnitude. For DI layers much higher injection currents than for SI layers can be achieved. An EL efficiency in the order of 10⁻⁴ for Ge⁺-implanted silicon dioxide was determined.     

The effects of substrate temperature on the superconducting properties of Tl2Ca2Ba2Cu3O10 films sputter-deposited from stoichiometric oxide targets

The annealing characteristics and the superconducting properties of Tl₂Ca₂Ba₂Cu₃O₁₀ thin films sputter-deposited onto yttrium- stabilized ZrO₂ substrate at up to 500°C from two stoichiometric oxide targets are reported. The films deposited at 400–500°C were found to require a lower post-annealing temperature than the films deposited at lower temperatures to attain the highest T_c superconducting state, due to a more pronounced Ba diffusion toward the substrate as indicated by their secondary ion mass spectrometry depth profiles. The highest T_c achieved tends to degrade with increasing substrate temperatures, a zero resistance T_c of 121 and ≈90 K, respectively, being observed for the films deposited at -ambient temperature and at 500°C. The formation of the highest T_c phase (Tl₂Ca₂Ba₂Cu₃O₁₀) generally is associated with a sheet type of crystal growth morphology with smooth and aligned surfaces which can be obtained only from the films capable of sustaining prolonged annealing at 900°C. Annealing at lower temperatures (≈860°C) results in the formation of rod or sphere type of morphologies with rough and randomly oriented crystals and the lower T_c phases such as Tl₂Ca₁Ba₂Cu₂O₈.     

Superconductivity, Ca content and oxygen deficiency of Ga(Sr,Ca)2(Yb,Ca)Cu2O7

Superconducting transition temperature (T_c), Ca content and oxygen deficiency are studied on GaSr_1.8Ca_0.2Yb_1−xCa_xCu₂O₇ (x≤0.35). Superconducting samples with T_c=52 K are prepared after the annealing at 20 MPa of oxygen. The T_c is reduced through a slight oxygen loss accompanied by annealing in air above 650°C. The oxygen loss suggests the presence of short Cu–O chains in the GaO₄ slab. The formal valence of planar Cu required for the appearance of superconductivity depends on oxygen and Ca contents. The critical formal Cu valences are 2.105 and 2.125 for the samples annealed in air at 600°C and at 835°C, respectively. The values are higher than those of usual high-T_c superconductors. This can be explained by a high concentration of localized holes in the CuO₅ slab.     

Competing thermal relaxation processes in response to intrinsic defects produced by exposing SiO2 to synchrotron radiation

Irradiation of SiO₂ with soft X-ray photons (hν>100 eV) produces a variety of defects, of which E₁′ centers and neutral Si–Si bonds are mainly responsible for the dielectric response change. The thermal processes that modify the structures around the defect sites have been investigated by in situ spectroscopic ellipsometry. Annealing the irradiated SiO₂ film diminishes the number of defects which are assigned to E₁′ centers by about half. The competing channels for annihilation of E₁′ centers are the recovery of the Si–O–Si bonding configuration and, in the opposite direction, the decomposition of the material into volatile products until the network is completely restructured. The other half of the defects are converted to Si–Si bond units and precipitates as nanocrystalline particles of Si.     

O implantation in ZnSe: lattice distortion by Raman measurement

We probe the “bulk” and near-surface regions of O-implanted heteroepitaxial ZnSe films, using the full-width at half-maximum (FWHM) of the Raman line. The results are compared to those after N implantation. A tensile stress attributed to a deformation in the lattice around O is found. Under rapid thermal annealing, the optimum temperature for the recovery of the implantation damage without suffering from thermally generated point defects is T_a = 500°C. It corresponds here to the maximum FWHM from the uncompensated tensile stress, while identical FWHM's are found for the top and the “bulk” part of the film just after implantation, and after implantation and T_a = 600°C.     

Evolution of step-terrace structure at Si–SiO2 interface in SIMOX substrate during annealing

The step-terrace structures at the interface between the Si layer and the buried SiO₂ layer of a Separation by IMplanted OXygen substrate has been observed by using atomic force microscopy (AFM) after removing the SiO₂ and Si layers. The time evolution of the Si–SiO₂ interface roughness during high-temperature annealing was analyzed by the scaling analysis of AFM data. The correlation length exhibited a nice correspondence to the size of square domain structures. Decreasing in the index of the length scale indicates that the growth mechanism changes as the annealing proceeds.     

Laser ablated bismuth cuprate thin films preparation and effect of oxygen nonstoichiometry upon superconductivity

Thin films of Bi₂Sr₂CaCu₂O₈ and (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀ have been prepared on monocrystalline (100) MgO substrates, using a laser ablation method with post annealing treatment. The influence of substrate temperature and oxygen pressure during deposition were investigated. SEM observations, EDS analysis, electric and magnetic measurements have been used to characterize the films. Superconducting “2212” films, with T_c(R = 0) at 80–83 K and J_c (50 K) up to 5 × 10⁵ A/cm², have been currently achieved, while Pb-doped “2223” films exhibit T_c as high as 110 K with J_c = 5 × 10⁴ A/cm² at 77 K. The effect of annealing at low temperature (350°C) in an argon flow has been studied for the 2212 phase, it shows the influence of the oxygen non-stoichiometry, i.e. of the hole carrier density upon T_c's which can be measured up to 89 K (zero resistance).