Si-based solid blue emitters from 3C-SiC nanocrystals

A procedure for preparing 3C-SiC/SiO₂ composite nanocrystals embedded in Si matrix that emit blue light is reported. Through electrochemical etching of polycrystalline 3C-SiC wafers followed by ultrasonic treatment in water bath, we fabricated luminescent colloidal 3C-SiC nanocrystals. Porous Si samples that have been naturally oxidized in air for 12 h were immersed in agitated aqueous suspension of 3C-SiC nanocrystals for 10 min and then dried in air, followed by annealing in argon atmosphere to form core-shell structured 3C-SiC/SiO₂ nanocrystals embedded in Si matrix. Our result shows that the luminescence of 3C-SiC/SiO₂ composite nanocrystals is very stable over time or under high temperature. As robust and stable Si-based solid blue-emitters, they have important implications for engineering photonic components in optoelectronics and photonics. PACS 78.67.Bf; 78.55.Hx; 78.66.Sq     

Formation of epitaxial SiC nanocrystals

Epitaxial 3C-SiC grains are formed at 1190 °C in the top region of silicon, when Si wafers coated by SiO₂ are annealed in CO atmosphere. The formed SiC grains are 40-50 nm high and 100 nm wide in cross-section and contain only few defects. Main advantage of the method is that the final structure is free of voids.The above method is further developed for the generation of SiC nanocrystals, embedded in SiO₂ on Si, and aligned parallel with the interface. The nanometer-sized SiC grains were grown into SiO₂ close to the Si/SiO₂ interface by a two-step annealing of oxide covered Si: first in a CO, than in a pure O₂ atmosphere. The first (carbonization) step created epitaxial SiC crystallites grown into the Si surface, while the second (oxidation) step moved the interface beyond them. Conventional and high resolution cross-sectional electron microscopy showed pyramidal Si protrusions at the Si/SiO₂ interface under the grains. The size of the grains, as well as their distance from the Si/SiO₂ interface (peak of pyramids) can be controlled by the annealing process parameters. The process can be repeated and SiC nanocrystals (oriented in the same way) can be produced in a multilevel structure.     

Fabrication of polycrystalline silicon films by Al‐induced crystallization of silicon‐rich oxide films

Polycrystalline silicon (poly‐Si) films were fabricated by aluminum (Al)‐induced crystallization of Si‐rich oxide (SiO_x) films. The fabrication was achieved by thermal annealing of SiO_x /Al bilayers below the eutectic temperature of the Al–Si alloy. The poly‐Si film resulting from SiO_1.45 exhibited good crystallinity with highly preferential (111) orientation, as deduced from Raman scattering, X‐ray diffraction, and transmission electron microscopy measurements. The poly‐Si film is probably formed by the Al‐induced layer exchange mechanism, which is mediated by Al oxide.     

Defect reduction and surface passivation of SiO2/Si by heat treatment with high-pressure H2O vapor

Heat treatment with high-pressure H₂O vapor was applied to improve interface properties of SiO₂/Si and passivate the silicon surface. Heat treatment at 180–420 °C with high-pressure H₂O vapor changed SiO_x films, 150 nm thick formed at room temperature by thermal evaporation in vacuum, into SiO₂ films with a Si-O-Si bonding network similar to that of thermally grown SiO₂ films. Heat treatment at 130 °C with 2.8×10⁵ Pa H₂O for 3 h reduced the recombination velocity for the electron minority carriers from 405 cm/s (as-fabricated 150-nm-thick SiO_x/Si) to 5 cm/s. Field-effect passivation was demonstrated by an additional deposition of defective SiO_x films on the SiO₂ films formed by heat treatment at 340 °C with high-pressure H₂O vapor. The SiO_x deposition reduced the recombination velocity from 100 cm/s to 48 cm/s. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

XPS depth profiling studies of L-CVD SnO2 thin films

In this paper we present the results of the XPS atomic depth profile analysis, using ion beam sputtering, of L-CVD SnO₂ thin films grown on an atomically clean SiO₂ substrate after annealing at 400 °C in dry atmospheric air. From the evolution of the Sn 3d_5/2, O 1s, Si 2p and C 1s core level peaks our experiments allowed the determination of the in depth atomic concentration of the main components of the SnO₂/SiO₂ interface. Thin (few nm) nearly stoichiometric SnO₂ films are present at the topmost layer of the thin films, and progressive intermixing with SnO and silicon oxide is observed at deeper layer. The interface between the Sn and the Si oxide layers (i.e. the effective Sn oxide thickness) is measured at 13 nm.     

Nitric acid oxidation of Si (NAOS) method for low temperature fabrication of SiO2/Si and SiO2/SiC structures

We have developed low temperature formation methods of SiO₂/Si and SiO₂/SiC structures by use of nitric acid, i.e., nitric acid oxidation of Si (or SiC) (NAOS) methods. By use of the azeotropic NAOS method (i.e., immersion in 68 wt% HNO₃ aqueous solutions at 120 °C), an ultrathin (i.e., 1.3-1.4 nm) SiO₂ layer with a low leakage current density can be formed on Si. The leakage current density can be further decreased by post-metallization anneal (PMA) at 200 °C in hydrogen atmosphere, and consequently the leakage current density at the gate bias voltage of 1 V becomes 1/4-1/20 of that of an ultrathin (i.e., 1.5 nm) thermal oxide layer usually formed at temperatures between 800 and 900 °C. The low leakage current density is attributable to (i) low interface state density, (ii) low SiO₂ gap-state density, and (iii) high band discontinuity energy at the SiO₂/Si interface arising from the high atomic density of the NAOS SiO₂ layer.For the formation of a relatively thick (i.e., ≥10 nm) SiO₂ layer, we have developed the two-step NAOS method in which the initial and subsequent oxidation is performed by immersion in ∼40 wt% HNO₃ and azeotropic HNO₃ aqueous solutions, respectively. In this case, the SiO₂ formation rate does not depend on the Si surface orientation. Using the two-step NAOS method, a uniform thickness SiO₂ layer can be formed even on the rough surface of poly-crystalline Si thin films. The atomic density of the two-step NAOS SiO₂ layer is slightly higher than that for thermal oxide. When PMA at 250 °C in hydrogen is performed on the two-step NAOS SiO₂ layer, the current-voltage and capacitance-voltage characteristics become as good as those for thermal oxide formed at 900 °C.A relatively thick (i.e., ≥10 nm) SiO₂ layer can also be formed on SiC at 120 °C by use of the two-step NAOS method. With no treatment before the NAOS method, the leakage current density is very high, but by heat treatment at 400 °C in pure hydrogen, the leakage current density is decreased by approximately seven orders of magnitude. The hydrogen treatment greatly smoothens the SiC surface, and the subsequent NAOS method results in the formation of an atomically smooth SiO₂/SiC interface and a uniform thickness SiO₂.     

Introducing CdS into two- and three-dimensional photonic crystals

SiO₂/air three-dimensional (3D) periodic structures were fabricated by removing Si layers partially from Si/SiO₂ 3D photonic crystals (PhCs) formed by using autocloning. CdS/SiO₂ 3D periodic structures were formed by introducing CdS into the SiO₂/air structures by the TEA method and photoluminescence (PL) was observed from the introduced CdS. TiO₂/air/CdS two-dimensional (2D) PhCs were also fabricated by introducing CdS into the voids of TiO₂/air 2D periodic structures, in which SiO₂ layers were partially etched out from TiO₂/SiO₂ 2D PhCs fabricated by using autocloning. PL radiating normal to the surface was measured and large polarization dependence was observed.     

Fabrication and characterization of Au/SiO2 nanocomposite films grown by radio-frequency cosputtering

Au/SiO₂ nanocomposite films were prepared on Si wafers by cosputtering of SiO₂ and gold wires. Au/Si atomic ratios in Au/SiO₂ nanocomposite films were varied from 0.53 to 0.92 by controlling the length of gold wire to study the evolution of the crystallization of gold, the size of Au/SiO₂ nanocomposite particles, and the optical properties of as-deposited Au/SiO₂ nanocomposite films. An X-ray photoelectron spectroscopy reveals that Au exists as a metallic phase in the bulk of SiO₂ matrix. Dome-shaped Au/SiO₂ nanocomposite particles and both Au (1 1 1) and (2 0 0) planes were observed in a field-emission scanning electron microscopy and X-ray diffraction studies respectively. With an ultraviolet-visible, absorption peaks of Au/SiO₂ nanocomposite films were observed at 525 nm.     

Layer uniformity in glucose oxidase immobilization on SiO2 surfaces

The goal of this work was the characterization, step by step, of the enzyme glucose oxidase (GOx) immobilization on silicon oxide surfaces, mainly by means of X-Ray photoelectron spectroscopy (XPS). The immobilization protocol consists of four steps: oxide activation, silanization, linker molecule deposition and GOx immobilization. The linker molecule, glutaraldehyde (GA) in this study, must be able to form a uniform layer on the sample surface in order to maximize the sites available for enzyme bonding and achieve the best enzyme deposition. Using a thin SiO₂ layer grown on Si wafers and following the XPS Si2p signal of the Si substrate during the immobilization steps, we demonstrated both the glutaraldehyde layer uniformity and the possibility to use XPS to monitor thin layer uniformity. In fact, the XPS substrate signal, not shielded by the oxide, is suppressed only when a uniform layer is deposited. The enzyme correct immobilization was monitored using the XPS C1s and N1s signals. Atomic force microscopy (AFM) measurements carried out on the same samples confirmed the results.     

Study of the interface Si-nc/SiO2 by infrared spectroscopic ellipsometry and X-ray photoelectron spectroscopy

SiO_x films (1<x<2), 0.5 μm thick, have been elaborated by electron-gun evaporation. A thermal annealing of these films induced a phase separation leading to the formation of Si nanocrystals embedded in a SiO₂ matrix. These films have been studied by infrared spectroscopic ellipsometry and by X-ray photoelectron spectroscopy (XPS). The effective dielectric function of the thin films has been extracted in the 600–5000 cm⁻¹ range which allowed us to deduce the dielectric function of the matrix surrounding the Si-nc. A study of the Transverse Optical (TO) vibration mode has revealed the presence of SiO_x into the matrix. Before XPS measurements, the films have been etched in fluorhydric acid to remove the superficial SiO₂ layer formed during air exposure. The Si 2p core-level emission has been recorded. The decomposition of the Si 2p peak into contributions of the usual five tetrahedrons Si-(Si_4−nO_n) (n=0–4) has also revealed the presence of a SiO_x phase. Consistency between infra-red and XPS results is discussed.     

Synchrotron investigations of the electron structure of silicon nanocrystals in a SiO2 matrix

Silicon ions are implanted into silicon oxide thin films obtained by the thermal oxidation of silicon wafers in wet oxygen. The implantation dose is accumulated either once or cyclically, and the samples are annealed in dry nitrogen every time after implantation. The second series of samples is prepared in a similar way, but the technology for obtaining the oxide films includes additional annealing at 1100°C in air for three hours. X-ray absorption near-edge structure (XANES) spectra are obtained using synchrotron radiation. In all the Si L _2,3 spectra, two absorption edges are observed, the first corresponding to elemental silicon, and the second corresponding to the SiO₂ matrix. The fine structure of the first edge indicates that nanocrystalline silicon (nc-Si) can form in the SiO₂ matrix, whose atomic and electronic structure depends on the technology of its formation. In both series, the cyclic accumulation of the total dose (Φ = 10¹⁷ cm^?2) and the annealing time (2 h) gives rise to the most pronounced fine structure in the region of the absorption edge of elemental silicon. The probability of forming silicon nanocrystals decreases for the denser silicon oxide in the second series of samples.     

Corrosion of Ultrathin Iron Layers and Titanium Nitride Coated Iron

In this paper results are presented on the corrosion resistance against SO₂-polluted atmospheres of ultrathin Fe layers (<2.5 nm) deposited on SiO₂/Si wafers by two methods: vacuum evaporation and Langmuir–Blodgett (LB) technique. It is shown that the corrosion resistance of the evaporated films is similar to that shown by massive iron while the corrosion resistance of the LB films is remarkably higher. We also show that the corrosion resistance of iron evaporated films can be greatly enhanced by using TiN coatings. Results on the influence of the SO₂ concentration, the atmospheric relative humidity and the exposure time on the corrosion behaviour of thin (30 nm) TiN layers deposited onto evaporated Fe films on SiO₂/Si wafers are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nitric acid method for fabrication of gate oxides in TFT

We have developed low temperature formation methods of SiO₂ layers which are applicable to gate oxide layers in thin film transistors (TFT) by use of nitric acid (HNO₃). Thick (>10 nm) SiO₂ layers with good thickness uniformity (i.e., ±4%) can be formed on 32 cm × 40 cm substrates by the two-step nitric acid oxidation method in which initial and subsequent oxidation is performed using 40 and 68 wt% (azeotropic mixture) HNO₃ aqueous solutions, respectively. The nitric acid oxidation of polycrystalline Si (poly-Si) thin films greatly decreases the height of ridge structure present on the poly-Si surfaces. When poly-Si thin films on 32 cm × 40 cm glass substrates are oxidized at azeotropic point (i.e., 68 wt% HNO₃ aqueous solutions at 121 °C), ultrathin (i.e., 1.1 nm) SiO₂ layers with a good thickness uniformity (±0.05 nm) are formed on the poly-Si surfaces. When SiO₂/Si structure fabricated using plasma-enhanced chemical vapor deposition is immersed in 68 wt% HNO₃, oxide fixed charge density is greatly decreased, and interface states are eliminated. The fixed charge density is further decreased by heat treatments at 200 °C, and consequently, capacitance-voltage characteristics which are as good as those of thermal SiO₂/Si structure are achieved.     

SrTiO3(110) thin films grown directly on different oriented silicon substrates

We have grown (110)-oriented SrTiO₃ (STO) thin films on silicon without any buffer layer, by means of pulsed laser deposition technique. The crystal structures of the grown films were examined by X-ray diffraction analysis including θ–2θ scan and rocking curve as well as Laue diffraction methods. STO films with single (110) out-of-plane orientation were formed on all (100), (110) and (111)-oriented Si substrates. The in-plane alignments for the epitaxial STO films grown directly on Si (100) were found as STO[001]//Si[001] and STO[11̄0]//Si[010]. The results should be of interest for better understanding of the growth of perovskite oxide thin films on silicon wafers. PACS 77.55.+f; 68.55.JK; 81.15Fg     

A new nanoporous material based on amorphous silicon dioxide

Processes for making nanoporous SiO₂ layers on Si via the irradiation of thermally oxidized silicon wafers with fast ions followed by chemical treatment in a solution or vapor of hydrofluoric acid are presented. It is shown that the density, shape, diameter, and length-to-diameter ratio of channels etched in silicon dioxide can be controlled by varying the regimes of fast ion irradiation or chemical treatment of SiO₂/Si structures. Track parameters calculated using the thermal spike model are compared with the chemical etching data.     

Formation of 10-30 nm SiO2/Si structure with a uniform thickness at ∼120 °C by nitric acid oxidation method

Silicon dioxide (SiO₂) layers with a thickness more than 10 nm can be formed at ∼120 °C by direct Si oxidation with nitric acid (HNO₃). Si is initially immersed in 40 wt.% HNO₃ at the boiling temperature of 108 °C, which forms a ∼1 nm SiO₂ layer, and the immersion is continued after reaching the azeotropic point (i.e., 68 wt.% HNO₃ at 121 °C), resulting in an increase in the SiO₂ thickness. The nitric acid oxidation rates are the same for (1 1 1) and (1 0 0) orientations, and n-type and p-type Si wafers. The oxidation rate is constant at least up to 15 nm SiO₂ thickness (i.e., 1.5 nm/h for single crystalline Si and 3.4 nm/h for polycrystalline Si (poly-Si)), indicating that the interfacial reaction is the rate-determining step. SiO₂ layers with a uniform thickness are formed even on a rough surface of poly-Si thin film.     

Microstructure and dielectric properties of pulsed-laser-deposited CaCu 3 Ti 4 O 12 thin films on LaNiO 3 buffered Pt/Ti/SiO 2 /Si substrates

CaCu₃Ti₄O₁₂ (CCTO) thin films have been prepared by a pulsed-laser-deposition method on LaNiO₃ buffered Pt/Ti/SiO₂/Si substrates, and their microstructure and dielectric properties have been compared with those of the films deposited directly on Pt/Ti/SiO₂/Si substrates. The crystalline structure and the surface morphology of the CCTO thin films were markedly affected by the bottom electrodes. Both the films show temperature-independent dielectric properties in a wide temperature range, which is similar to those properties obtained in single-crystal or epitaxial thin films, while the room-temperature dielectric constant of the 350-nm-thick CCTO films on LaNiO₃/Pt/Ti/SiO₂/Si substrates at 100 kHz was found to be 2300, which was increased significantly compared with that obtained in the films on Pt/Ti/SiO₂/Si substrates. Using the impedance spectroscopy technique, it has been suggested that the high dielectric constant response of the CCTO thin films originates from the grain boundary layer mechanism as found in internal barrier layer capacitors. PACS 77.55.+f; 81.15.Fg; 68.55.-a     

Oxidation of silicon surface with atomic oxygen radical anions

The surface oxidation of silicon （Si） wafers by atomic oxygen radical anions （O- anions） and the preparation of metal-oxide-semiconductor （MOS） capacitors on the O-oxidized Si substrates have been examined for the first time. The O- anions are generated from a recently developed O- storage-emission material of [Ca24Al2sO64]^4＋·4O^- （Cl2A7-O^- for short）. After it has been irradiated by an O- anion bean： （0.5 μA/cm^2） at 300℃ for 1-10 hours, the Si wafer achieves an oxide layer with a thickness ranging from 8 to 32 nm. X-ray photoelectron spectroscopy （XPS） results reveal that the oxide layer is of a mixture of SiO2, Si2 O3, and Si2O distributed in different oxidation depths. The features of the MOS capacitor of 〈Al electrode/SiOx/Si〉 are investigated by measuring capacitance-voltage （C - V） and current-voltage （I - V） curves. The oxide charge density is about 6.0 × 10^1 cm^-2 derived from the （C - V curves. The leakage current density is in the order of 10^-6 A/cm^2 below 4 MV/cm, obtained from the I - V curves. The O- anions formed by present method would have potential applications to the oxidation and the surface-modification of materials together with the preparation of semiconductor devices.     

Soft X-ray fluorescence and photoluminescence of Si nanocrystals embedded in SiO2

We have used ion-beam mixing to form Si nano-crystals in SiO₂ and SiO₂/Si multilayers, and applied photoluminescence and soft-X-ray emission spectroscopy to study the nanoparticles. Ion-beam mixing followed by heat treatment at 1100 °C for 2 h forms the Si nanocrystals. The ion-beam-mixed sample shows higher PL intensity than that of a Si-implanted SiO₂ film. Photon and electron-excited Si L_2,3 X-ray emission measurements were carried out to confirm the formation of Si nanocrystal in SiO₂ matrix after ion-beam mixing and heat treatment. It is found that Si L_2,3 X-ray emission spectra of ion-beam-mixed Si monolayers in heat-treated SiO₂ films lead to noticeable changes in the spectroscopic fine structure. Received: 20 November 1999 / Accepted: 17 April 2000 / Published online: 5 October 2000     

Optical properties of multilayer structures

The possibility of using the ellipsometry method for investigation of the optical properties of multilayer films and structures is shown. The optical properties of structures HfO₂/SiO₂/Si, HfO₂/Si, ZrO₂/Si, Ta₂O₅/Si, and Al₂O₃/Si are studied. It is found that a layer of hafnium silicate is formed at the interface between the HfO₂ film and Si. Annealing of the structures in oxygen shows that oxides studied are oxygen-permeable and that the thickness of SiO₂ at the film-substrate interface increases. The growth rate of SiO₂ layers depends on the chemical nature of an oxide. Al₂O₃ films are impermeable for oxygen diffusion. The production of layers of alloys (Al₂O₃) _x (HfO₂)_{1 ? x} is optimized, which allows one to obtain layers with a homogeneous distribution of elements over the thickness.