The effect of annealing under hydrostatic pressure on the visible photoluminescence from si-ion implanted SiO2 films

We have studied the influence of the hydrostatic pressure during annealing on the intensity of the visible photoluminescence (PL) from thermally grown SiO₂ films irradiated with Si⁺ ions. Post-implantation anneals have been carried out in an Ar ambient at temperatures T_a of 400°C and 450°C for 10 h and 1130°C for 5 h at hydrostatic pressures of 1 bar–15 kbar. It has been found that the intensity of the 360, 460 and 600 nm PL peaks increases with rising hydrostatic pressure during low-temperature annealing. The intensity of the short-wavelength PL under conditions of hydrostatic pressure continues to rise even at T_a=1130°C. Increasing T_a leads to a shift in the PL spectra towards the ultraviolet range. The results obtained have been interpreted in terms of enhanced, pressure-mediated formation of ≡Si–Si≡ centres and small Si clusters within metastable regions of the ion-implanted SiO₂.     

Depth dependence of photoluminescence and chemical bonding in porous silicon

Porous silicon (PS) is studied by stepwise peeling of the surface layer to clarify the non-uniformity in the photoluminescence (PL) and correlate it with the in-depth chemical bonding and structure of the 30 μm thick layer. The PL intensity grows by an order of magnitude after the peeling off of the first 10 μm and decreases five times in the next 5 μm while the peak maximum position shifts from 730 to 800 nm. X-ray photoelectron spectroscopy (XPS) measurements show that Si–Si and Si–O bonds are present both on the surface and below, and the preferential oxidation state of silicon changes from 3+ and 4+ on the surface to 1+ and 2+ below 10 μm. Using Raman spectroscopy silicon nanocrystals are shown to exist. Their mean size can be estimated at about 3 nm. These results show that the strongest PL comes from a region in the PS layer where silicon nanocrystallites are surrounded by oxides with a low level of oxidation and not from the strongly oxidized surface layer.     

Visible photoluminescence and its mechanisms from a-SiOx : H films with different stoichiometry

The photoluminescence (PL) properties of H-rich amorphous silicon oxide thin films prepared by dual-plasma chemical vapor deposition have been studied. The three commonly reported PL bands centered around 1.7, 2.1 and 2.9 eV have been detected from the same type of a-SiO_x : H material, only by varying the oxygen content (x≈1.35, 1.65, 2). In order to characterize the PL bands, the samples in as-prepared and annealed states up to 900°C have been analyzed by XPS, FT-IR, gas effusion, ESR and ellipsometry. Temperature quenching experiments are crucial to distinguish the 1.7 eV band, fully consistent with a bandtail-to-bandtail transition, from radiative defect luminescence mechanisms attributed either to defects related to Si–OH groups (2.9 eV) or to oxygen vacancy defects (2.1 eV).     

Characterization of high-k gate dielectric/silicon interfaces

We have shown that, for thermally evaporated Ta₂O₅ or ZrO₂ thin films on Si(1 0 0), O₂ annealing at 300–500 °C causes the formation of an interfacial silicon oxide layer as thin as 1–2 nm which can be interpreted in terms of their high permeability to oxygen. And we have demonstrated how useful the energy loss spectra of photoexcited electrons from core levels such as O 1s are to measure the energy bandgaps of very thin insulators. With the combination of measured bandgaps and valence band lineups determined for X-ray photoelectron spectroscopy valence band spectra, we have determined the energy band alignments of Ta₂O₅ and ZrO₂ with Si(1 0 0) before and after the O₂ annealing at 500 °C. In addition, we have demonstrated that total photoelectron yield spectroscopy provides us direct information to quantify the energy distributions of both the defect states in the high-k dielectrics and the dielectric/Si(1 0 0) interface states over nearly entire Si bandgap.     

Correlation between structural and optical properties of ion beam synthesized β-FeSi2 precipitates in Si

The structural and optical properties of β-FeSi₂ precipitates produced by ion beam synthesis have been investigated by transmission electron microscopy, photoluminescence (PL) analysis and near infrared transmission measurements. The PL spectrum of β-FeSi₂ precipitates in a dislocation free sample has been observed to consist of a sharp line at 1.54 μm and a weak peak at 1.46 μm. Optical transmission measurements showed a direct band gap about 0.8 eV smaller than in continuous β-FeSi₂ film. Calculation of the electronic bands of β-FeSi₂ for different values of the lattice parameters indicates that this reduction can be ascribed to band distortion provided by the lattice strain.     

Penetration of electronic states from silicon substrate into silicon oxide

The depth profiling of O 1s energy loss in silicon oxide near the SiO₂/Si interface was performed using extremely small probing depth. As a result, the energy loss of O 1s photoelectrons with threshold energy of 3.5 eV was found. This value of 3.5 eV is much smaller than the SiO₂ bandgap of 9.0 eV, but quite close to direct interband transition at Γ point in energy band structure of silicon. This can be explained by considering the penetration of electronic states from silicon substrate into silicon oxide up to 0.6 nm from the interface. In addition, the penetrating depth is larger than the thickness of the compositional transition layer.     

Visible photoluminescence from a-Si : H/SiO2 superlattices fabricated by UHV evaporation

Hydrogenated amorphous-Si/SiO₂ (a-Si:H/SiO₂) superlattices with different a-Si : H thickness in the range of a few nanometers have been fabricated by ultra high vacuum evaporator (UHV evaporator). The photoluminescence (PL) of our superlattices is observed in the visible spectral region and the peak energy shifts to higher energy as the a-Si : H layer thickness decreases. The temperature dependence of the PL spectra reveals four sub-bands by fitting. Bands at 2.2, 1.9, 1.65 and 1.45 eV are detected and are attributed to E′δ centers, nonbridging-oxygen–hole centers (NBOHC), Si/SiO₂ interface and a-Si : H layer, respectively. We explain the overall blueshift of the PL spectra by the modification of the contribution of these sub-bands.     

Structural and optical features of nanoporous silicon prepared by electrochemical anodic etching

Nanoporous silicon (NPS) samples were prepared by electrochemical anodic etching of p-type (0 0 1) silicon wafers in HF solution, and some of them were aged in air. The nanostructural, optical and chemical features of the NPS were investigated in terms of etching and aging conditions. The surface of the porous Si exhibits an etched layer with a thickness of 30–40 nm; this layer appears to consist of aggregates of 5–10 nm size nano-crystallites. The NPS exhibited broad photoluminescence (PL) spectra with its peak in the red light region (740 nm). After aging the porous samples for 4 weeks in air, we observed the PL intensity became approximately a fifth of that of the as-prepared one, along with a blue shift. It is very likely that the blue shift of the PL peak was caused by the shrinkage of the Si nano-crystallites due to the oxidation in the surface of the nano-crystallites.     

Optical characterization of InGaN/AlGaN/GaN diode grown on silicon carbide

Electroluminescence (EL) properties of In_xGa_1−xN/Al_yGa_1−yN/GaN/SiC diode were studied. The spectral range for which EL spectra were recorded is 1–3.5 eV. Room temperature EL was obtained for forward bias (3.18 V, 220 μA) at 446.067 nm (blue luminescence band), 606.98 nm (yellow luminescence band) and 893.84 nm (Infrared luminescence band). The EL temperature dependence shows that, BL band is mostly given by e–h recombination corresponding to indium composition equal to 0.17 ± 0.01 and 0.14 ± 0.02 obtained theoretically and experimentally, respectively. The yellow band is generally weak and absent at low temperature. The IRL band is more consistent with the DAP recombination and could be explained by the thermal activation of Mg states. The luminescence bands shift to lower energies is due probably to the larger potential fluctuations effect.     

Si nanocrystallites in SiO2 with intense visible photoluminescence synthesized from SiOx films deposited by laser ablation

We observed very intense and highly reproducible photoluminescence (PL) spectra for SiO_x films obtained by laser ablation of Si targets in 50-mTorr oxygen gas followed by proper annealing. It was found that the PL peak continuously changes from 1.4 eV at the center of the samples to 1.8 eV at the sample edge. The optimum values of the oxygen component in SiO_x was x=1.3-1.4 and the optimum annealing temperature was 1000 °C for intense PL. From transmission electron microscopy images of annealed films, Si nanocrystallites are found to be formed in the matrix of SiO₂ grown from the SiO_x and have diameters of 2-3 nm. These indicate that a high density of Si nanocrystallites with diameters of 2-3 nm in the SiO₂ phase are probably responsible for the PL and that the Si nanostructure is well formed from the as-deposited, metastable SiO_x (x=1.3-1.4) films by annealing at 1000 °C.     

Effect of oxygen implantation on ionoluminescence of porous silicon

We report on ionoluminescence investigations of porous Si prepared from the p⁺-type Si, which exhibited, after prolonged ambient air exposure, moderate photon emission with a maximum in the red–orange region. In an attempt to activate a shorter wavelength emission, the samples were implanted with 225 keV O⁺ ions at the dose of 1×10¹⁷ cm⁻². The strong blue band at 2.7 eV, well known in silica, has emerged in the ionoluminescence spectra following the oxygen implantation. The results of the comparative ionoluminescence experiments, performed on both porous Si and two forms of silica, show the important role of SiO₂ defect-related states in ion-induced optical emission from porous Si.     

Comparative investigation of photoluminescence of silicon wire structures and silicon oxide films

Photoluminescence spectra and their dependence on temperature as well as Raman scattering spectra and Atomic Force Microscopy investigations have been used to study the peculiarities of the red photoluminescence band in low-dimensional Si structures, such as porous silicon and silicon oxide films. It has been shown that the red photoluminescence band of porous silicon is complex and can be decomposed into two elementary bands. It was discovered that elementary band intensities depend very much on surface morphology of porous silicon. The same positions of the photoluminescence bands are also observed in silicon oxide films for different oxide composition. Comparative investigation of the PL temperature dependences in porous silicon and silicon oxide films indicates that silicon-oxide defect related mechanisms of some elementary photoluminescence bands are involved.     

Blue–violet photoluminescence from colloidal suspension of nanocrystalline silicon in silicon oxide matrix

We report room temperature visible photoluminescence (PL), detectable by the unaided eye, from colloidal suspension of silicon nanocrystals (nc-Si) prepared by mechanical milling followed by chemical oxidation. The PL bands for samples prepared from Si wafer and Si powder peak at 3.11 and 2.93 eV respectively, under UV excitation, and exhibit a very fast (～ns) PL decay. Invasive oxidation during chemical treatment reduces the size of the nc-Si domains distributed within the amorphous SiO₂ matrix. It is proposed that defects at the interface between nc-Si and amorphous SiO₂ act as the potential emission centers. The origin of blue–violet PL is discussed in relation to the oxide related surface states, non-stoichiometric suboxides, surface species and other defect related states.     

Time-resolved photoluminescence study of the red emission in nanoporous SiGe alloys

Porous Si_1−xGe_x (PSiGe) layers with efficient room temperature visible photoluminescence (PL) were elaborated by anodical etching from p-type doped epitaxial layers with Ge contents from 5 to 30%. The luminescence is characterised by a broad PL band centred at 1.8 eV. Time resolved photoluminescence decay is studied in porous silicon germanium as a function of germanium content, temperature, emission energies and surface passivation. The PL decay line shape is well described by a stretched exponential in all cases. The effective lifetime at low temperature in as prepared porous Si_1−xGe_x is 400 μs, i.e. an order of magnitude less than in porous silicon. After the formation of a 20 Å thick oxide surface layer we observe a decrease of the effective lifetime to 20 μs at T=4 K.     

Luminescent properties of GaN films grown on porous silicon substrate

GaN films have been grown on porous silicon at high temperatures (800-1050 °C) by metal organic vapor phase epitaxy. The optical properties of GaN layers were investigated by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. PL spectra recorded at 5 K exhibit excitonic emissions around 3.36-3.501 eV and a broad yellow luminescence at 2.2 eV. CL analysis at different electron excitation conditions shows spatial non-uniformity in-depth of the yellow and the band-edge emissions. These bands of luminescence are broadened and red- or blue-shifted as the electron beam penetrates in the sample. These behaviors are explained by a change of the fundamental band gap due to residual strain and the local thermal effect. It was found that the use of AlN buffer layer improves the crystalline quality and the luminescence property of GaN.     

Covalent binding of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7

The adsorption reactions and binding configurations of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7 were studied using high-resolution electron energy loss spectroscopy (HREELS), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and DFT calculation. The covalent attachments of these unsaturated hydrocarbons to Si(1 1 1)-7 × 7 through the formation of Si–C linkages are clearly demonstrated by the observation of the Si–C stretching mode at 450–500 cm⁻¹ in their HREELS spectra. For chemisorbed cyclohexene, the involvement of π_C=C in binding is further supported by the absence of C=C stretching modes and the disappearance of the π_C=C photoemission. The chemisorption of both 1,3-cyclohexadiene and 1,4-cyclohexadiene leads to the formation of cyclohexene-like intermediates through di-σ bonding. The existence of one π_C=C bond in their chemisorbed states is confirmed by the observation of the C=C and (sp²)C---H stretching modes and the UPS and XPS results. DFT calculations show that [4 + 2]-like cycloaddition is thermodynamically preferred for 1,3-cyclohexadiene on Si(1 1 1)-7 × 7, but a [2 + 2]-like reaction mechanism is proposed for the covalent attachment of cyclohexene and 1,4-cyclohexadiene.     

Visible photoluminescence in porous GaAs capped by GaAs

A typical porous structure with pores diameters ranging from 10 to 50 nm has been obtained by electrochemical etching of (1 0 0) heavily doped p-type GaAs substrate in HF solution. Room temperature photoluminescence (PL) investigations of the porous GaAs (π-GaAs) reveal the presence of two PL bands, I₁ and I₂, located at 1.403 and 1.877 eV, respectively. After GaAs capping, the I₁ and I₂ PL bands exhibit opposite shift trends. However, the emission efficiency of these two bands is not strongly modified. Low temperature PL of capped porous GaAs versus injection levels shows that the I₁ PL band exhibits a red shift while the I₂ PL band exhibits a blue shift with increasing injection levels. The I₂ PL band intensity temperature dependence shows an anomalous behaviour and its energy location shows a blue shift as temperature increases. The observed PL bands act independently and are attributed to electron – hole recombination in porous GaAs and to the well-known quantum confinement effects in GaAs nanocrystallites. The I₂ PL band excitation power and temperature dependencies were explained by the filling effect of GaAs nanocrystallites energy states.     

Cu掺杂氧化锌薄膜的发光特性研究

通过射频反应溅射法在Si（111）衬底上制备了不同Cu掺杂量的ZnO薄膜.室温下测量了样品的光致发光（PL）谱，所有样品的PL谱中均观察到435?nm左右的蓝光发光带，该发光带的强度与Cu掺杂量和溅射功率有关.当溅射功率为150?W，Cu掺杂量为2.5%时，ZnO薄膜的PL谱中出现了较强的蓝光双峰，而溅射功率为100?W，Cu掺杂量为1.5%时，出现了位于437nm（2.84eV）处较强的蓝光峰，后者的取向性较好.还研究了掺杂量和溅射功率对发光特性的影响，并对样品的蓝光发光机制进行了探讨. 关键词： ZnO薄膜 Cu掺杂 光致发光谱 射频反应共溅射     

A comparative infrared study of H2O reactivity on Si(1 0 0)-(2 × 1), (2 × 1)-H, (1 × 1)-H and (3 × 1)-H surfaces

The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H₂O → H–S–Si–OH + H₂, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm⁻¹. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H₂O → H₂Si  HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces.     

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.