Kinetics of laser-induced oxidation of silicon near room temperature

The growth of ultra-thin (<6 nm) silicon-dioxide films on Si(100):H, Si(111):H, and a-Si:H surfaces in a dry oxygen atmosphere (0.1–10 Pa) at low temperatures (35–200 °C) was investigated. Oxidation was induced by pulsed F₂-laserradiation at 157 nm. The thickness and composition of the growing films were monitored in real time by spectroscopic ellipsometry in the photon energy range of 1.15–4.75 eV. The kinetics of low-temperature oxidation was similar for the Si surfaces investigated and differs from that of high-temperature thermal oxidation (900–1200 °C) that can be described by the Deal–Grove model. To explain the faster growth at the initial stage, it is proposed that oxidation occurs by diffusion of oxygen atoms O and/or ions O^-rather than oxygen molecules. The recombination of diffusive species to oxygen molecules limits their penetration into the bulk. A diffusion model is developed for low-temperature oxidation which takes into account the recombination process of the diffusive species. Good agreement between theory and experiment is found. The activation energy of diffusion of the active species was found to be 0.15 eV, in agreement with previous results and recent calculations for O^- ions. PACS 82.65.+r; 07.60.Fs; 81.65.Mq; 82.50.Hp     

Size control of Si nanocrystals by two-step rapid thermal annealing of sputtered Si-rich oxide/SiO2 superlattice

Controllable size of silicon (Si) nanocrystals can be achieved by a two-step rapid thermal annealing technique consisting of rapid annealing at 1000°C in nitrogen ambient and rapid oxidation at 600–800°C of a radio frequency magnetron co-sputtered Si-rich oxide/SiO₂ superlattice structure. The photoluminescence (PL) spectra related to Si nanocrystals were observed in the visible range (600–900 nm). After rapid oxidation, the size of the nanocrystals was reduced and the quality of the Si nanocrystal/SiO₂ interface was improved, resulting in a blue shift and an increase of the PL peak intensity. Finally, annealing in air increases the PL intensity further.     

Photoluminescence of a-Si:H films grown by plasma-enhanced chemical vapor deposition and doped with erbium from the metallorganic compound Er(HFA)3·DME

Using standard low-temperature (<300 °C) plasma-enhanced chemical vapor deposition (PE CVD) technology, films of a Si(Er): H were obtained that emitted light in the neighborhood of 1.54 μm at room temperature. The Er source was the specially synthesized fluorine-containing metallorganic complex Er(HFA)₃·DME where HFA=CF₃C(O)CHC(O)CF₃ and DME=CH₃OCH₂CH₂OCH₃, which possesses a low transition temperature to the gas phase (of order 100 °C) at working pressures (0.1–0.5 Torr) for the PE CVD method. Distinctive features of the photoluminescence spectrum of a-Si(Er):H were investigated in the range 0.5–1.7 μm for T=77 and 300 K. The presence of photoconductivity in the synthesized films is evidence of their satisfactory electronic quality. Fiz. Tverd. Tela (St. Petersburg) 40, 1433–1436 (August 1998)     

Aluminum-assisted crystallization and <Emphasis Type="Italic">p</Emphasis>-type doping of polycrystalline Si

Aluminum-doped p-type polycrystalline silicon thin films have been synthesized on glass substrates using an aluminum target in a reactive SiH₄+Ar+H₂ gas mixture at a low substrate temperature of 300 °C through inductively coupled plasma-assisted RF magnetron sputtering. In this process, it is possible to simultaneously co-deposit Si–Al in one layer for crystallization of amorphous silicon, in contrast to the conventional techniques where alternating metal and amorphous Si layers are deposited. The effect of aluminum target power on the structural and electrical properties of polycrystalline Si films is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and Hall-effect analysis. It is shown that at an aluminum target power of 100 W, the polycrystalline Si film features a high crystalline fraction of 91%, a vertically aligned columnar structure, a sheet resistance of 20.2 kΩ/□ and a hole concentration of 6.3×10¹⁸ cm⁻³. The underlying mechanism for achieving the semiconductor-quality polycrystalline silicon thin films at a low substrate temperature of 300 °C is proposed.     

Oxidation behaviour of SiC coatings

Amorphous silicon carbide (SiC) films were deposited on silicon substrates by radio-frequency magnetron sputtering. The films were oxidized in air in the temperature range 400–900 °C and for times from 1 to 16 h. Neutron reflectivity measurements provided information on the thickness, density and roughness of the SiC and on the formed SiO₂ layers. Fourier transform infrared spectroscopy was used to determine the bond structure of the formed SiO₂ and changes in the bonding of SiC after exposure at the oxidation temperature. The surface morphology of the oxidized films was characterized by atomic force microscopy measurements. The oxidation kinetics is initially fast and as the SiO₂ layer is formed it slows down. The SiC consumption varies linearly with time at all oxidation temperatures. Exposure of the SiC at the oxidation temperature affects its density and to some degree its bond structure, while the formed SiO₂ has density and bond structure as that formed by oxidation of Si under the same conditions. PACS  66.30.Ny; 68.47.Gh; 68.55.J-     

Structure and electrical properties of HfO<Subscript>2</Subscript> high-<Emphasis Type="Italic">k</Emphasis> films prepared by pulsed laser deposition on Si (100)

High-k gate dielectric hafnium dioxide films were grown on Si (100) substrate by pulsed laser deposition at room temperature. The as-deposited films were amorphous and that were monoclinic and orthorhombic after annealed at 500°C in air and N₂ atmosphere, respectively. After annealed, the accumulation capacitance values increase rapidly and the flat-band voltage shifts from −1.34 V to 0.449 V due to the generation of negative charges via post-annealing. The dielectric constant is in the range of 8–40 depending on the microstructure. The I–V curve indicates that the films possess of a promising low leakage current density of 4.2×10⁻⁸ A/cm² at the applied voltage of −1.5 V.     

Wetting behavior of Al–Si–Mg alloys on Si3N4/Si substrates: optimization of processing parameters

The wetting behavior of Al–Si–Mg alloys on Si₃N₄/Si substrates has been investigated using the sessile drop technique. Based on a Taguchi experiment design, the effect of the following processing parameters on the contact angle (θ) and surface tension (σ_LV) was studied: processing time and temperature, atmosphere (Ar and N₂), substrate surface condition (with and without a silicon wafer), as well as the Mg and Si contents in the aluminium alloy. In nitrogen, non-wetting conditions prevail during the isothermal events while in argon a remarkable non-wetting to wetting transition leads to contact angles θ as low as 11±3° and a liquid surface tension σ_LV of 33± 10×10^-5 kJ/m². According to the multiple analysis of variance (Manova), the optimum conditions for minimizing the values of θ and σ_LV are as follows: temperature of 1100 °C, processing time of 90 min, argon atmosphere, no use of a silicon wafer, and the use of the Al-18% Mg-1% Si alloy. A verification test conducted under the optimized conditions resulted in a contact angle of θ=9±3° and a surface tension of σ_LV=29± 9×10^-5 kJ/m², both indicative of excellent wetting. PACS 68.03.Cd; 81.05.Bx; 68.08.Bc; 05.70.-a; 61.10.Nz     

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     

On new two-dimensional structures produced on the Si (111) and Si (100) surface upon molecular-beam epitaxy of cobalt and silicon

Highly perfect epitaxial heterostructure CoSi₂ films have been grown on the surface of Si (111) and Si (100) single crystals by the method of molecular-beam epitaxy. The optimal regimes of the film growth with different thicknesses have been determined. It has been shown that short-term annealing of epitaxial films at T = 900–950 K leads to the formation of new CoSi₂/Si(111)-2 × 2 and CoSi₂/Si(100)−2 × 4 superstructures.     

On a current mechanism in Ta2O5 thin films

Electrical conduction in the temperature range of 120–370 K has been studied in sandwiched structures of Al/Ta₂O₅/Si. The tantalum oxide films were prepared by evaporation of tantalum on a p-Si crystal substrate, followed by oxidation at a temperature of 600°C. The temperature-dependent current-voltage (I–V) characteristics are explained on the basis of a phonon-assisted tunnelling model. The same explanation is given for I–V data measured on Ta₂O₅ films by other investigators. From the comparison of experimental data with theory the density of states in the interface layer is derived and the electron-phonon interaction constant is assessed.      

The imperfection of electrical contact — Thin film

This paper describes the imperfection of electrical contact and its influence on the measured value of ohmic resistance in the case of very thin samples. System used for the study of this problem are two thin YSZ films. The investigated films were deposited on n-doped Si (111) substrates using e-beam evaporation of (YO_1.5)_0.18(ZrO₂)_0.82 pellets at 150 °C. The imperfection of the electrical contact, in the case of thin films of the thicknesses of ≈ 100 nm, can dramatically influence the magnitude of the measured ohmic resistance. Such influence of imperfect contact on the measured value of resistance and on the shape of impedance diagram is described. Paper presented at the 7th Euroconference of Ionics, Calcacoggio, Corsica, France, Oct. 1–7, 2000.     

Zirconia-based planar NO2 sensor using ultrathin NiO or laminated NiO–Au sensing electrode

The nanostructured thin NiO films with the thicknesses of 30–180 nm were examined as a sensing electrode (SE) for the planar mixed-potential-type yttria-stabilized zirconia (YSZ)-based NO₂ sensor. The sensing characteristics were examined in the temperature range of 600–800 °C under the wet condition (5 vol.% water vapor). Among the NiO-SEs tested, the 60 nm-thick NiO-SE sintered at 1,000 °C was found to give the highest NO₂ sensitivity in the NO₂ concentration range of 50–400 ppm accompanying with fast response/recovery at the operating temperatures of 600–700 °C. The high NO₂ sensitivity was attributed to the high catalytic activity for both electrochemical reactions of O₂ and NO₂ at the interface of NiO-SE/YSZ. The ultrathin gold layer with the thickness of about 60 nm was additionally formed on the 60 nm-thick NiO-SE to fabricate the laminated-type (60 nm NiO/60 nm Au)-SE. It was demonstrated that the use of this laminated (NiO–Au)-SE improved both the sensitivity and the selectivity to NO₂.     

Photoluminescence of terbium-organic complex in aerosol–gel-derived planar waveguides

This work deals with the low-temperature preparation of optically active silica-based materials. Tb³⁺-doped silica monolithic gels were elaborated at 40 °C. Tb³⁺–SSA-doped SiO₂–TiO₂ thin films were deposited by an original sol–gel approach, the aerosol–gel process, and heat-treated at 150 °C. Organic complexation of terbium ions was used to improve the active properties of doped silica gels and thin-film samples. Spectroscopic characterisations are reported for these samples. Photoluminescence increase by a factor two was observed for complexation by sulphosalicylic acid. Received: 16 May 2001 / Revised version: 31 August 2001 / Published online: 23 October 2001     

Development of conductive transparent indium tin oxide (ITO) thin films deposited by direct current (DC) magnetron sputtering for photon-STM applications

Highly conductive and transparent indium tin oxide (ITO) thin films, each with a thickness of 100 nm, were deposited on glass and Si(100) by direct current (DC) magnetron sputtering under an argon (Ar) atmosphere using an ITO target composed of 95% indium oxide and 5% tin oxide for photon-STM use. X-ray diffraction, STM observations, resistivity and transmission measurements were carried out to study the formation of the films at substrate temperatures between 40 and 400 °C and the effects of thermal annealing in air between 200 and 400 °C for between1 and 5 h. The film properties were highly dependent on deposition conditions and on post-deposition film treatment. The films deposited under an Ar atmosphere pressure of ∼1.7×10^-3 Torr by DC power sputtering (100 W) at substrate temperatures between 40 and 400 °C exhibited resistivities in the range 3.0–5.7×10^-5 Ω m and transmissions in the range 71–79%. After deposition and annealing in air at 300 °C for 1 h, the films showed resistivities in the range 2.9–4.0×10^-5 Ω m and transmissions in the range 78–81%. Resistivity and transmission measurements showed that in order to improve conductive and transparent properties, 2 h annealing in air at 300 °C was necessary. X-ray diffraction data supported the experimental measurements of resistivity and transmission on the studies of annealing time. The surface roughness and film uniformity improve with increasing substrate temperature. STM observations found the ITO films deposited at a substrate temperature of 325 °C, and up to 400 °C, had domains with crystalline structures. After deposition and annealing in air at 300 °C for 1 h the films still exhibited similar domains. However, after deposition at substrate temperatures from 40 °C to 300 °C, and annealing in air at 300 °C for 1 h, the films were shown to be amorphous. More importantly, the STM studies found that the ITO film surfaces were most likely to break after deposition at a substrate temperature of 325 °C and annealing in air at 300 °C for 2 or 3 h. Such findings give some inspiration to us in interpreting the effects of annealing on the improvement of conductive and transparent properties and on the transition of phases. In addition, correlations between the conductive/transparent properties and the phase transition, the annealing time and the phase transition, and the conductive/transparent properties and the annealing time have been investigated. Received: 10 July 2000 / Accepted: 27 October 2000 / Published online: 9 February 2001     

Specific features and mechanisms of photoluminescence of nanostructured silicon carbide films grown on silicon in vacuum

The light-emitting properties of cubic silicon carbide films grown by vacuum vapor phase epitaxy on Si(100) and Si(111) substrates under conditions of decreased growth temperatures (T _gr ∼ 900–700°C) have been discussed. Structural investigations have revealed a nanocrystalline structure and, simultaneously, a homogeneity of the phase composition of the grown 3C-SiC films. Photoluminescence spectra of these structures under excitation of the electronic subsystem by a helium-cadmium laser (λ_excit = 325 nm) are characterized by a rather intense luminescence band with the maximum shifted toward the ultraviolet (∼3 eV) region of the spectral range. It has been found that the integral curve of photoluminescence at low temperatures of measurements is split into a set of Lorentzian components. The correlation between these components and the specific features of the crystal structure of the grown silicon carbide layers has been analyzed.     

Enhanced photosensitivity in sol–gel derived 20GeO2:80SiO2 thin films

We investigate the photosensitivity of binary 20GeO₂:80SiO₂ (germanosilicate) inorganic films. The samples have been fabricated by the sol–gel spin-coating method and the densification has been performed by rapid thermal annealing at various temperatures ranging from 500 °C to 1000 °C. The –OH absorption bands in the Fourier-transform infrared (FTIR) spectra and the refractive-index data show that the films annealed below 900 °C are porous and the films annealed at 900 °C and above are dense. An ultraviolet (UV) KrF laser at 248 nm has been used to induce the change in the refractive index of the samples. We have achieved a large refractive-index change (Δn) of 0.0098 after UV illumination in excess of 1 min for our dense germanosilicate films. This UV-induced refractive-index change is attributed to the formation of GeE’/SiE’ centers from Ge–Ge/Si–Si (neutral oxygen monovacancy) and Ge²⁺ centers and to the creation of oxygen deficiency related defects. From our experiments, the oxygen deficiency related defects correspond to the absorption band at 620–740 cm^-1 in the FTIR spectra and these are the defects which make a large contribution to Δn. The attenuation coefficient of the as-deposited and UV-illuminated dense samples is about 0.42 dB/cm at 1550 nm. For porous samples, UV exposure has densified the samples to some extent. PACS 82.50.Hp; 71.23.Cq; 81.20.Fw     

Synchrotron investigations of electronic and atomic-structure peculiarities for silicon-oxide films’ surface layers containing silicon nanocrystals

Films obtained using molecular-beam deposition of SiO powder on c-Si (111) substrates for the purpose of SiO2 system formation with silicon nanocrystals were investigated before and after 900–1100°C annealing by photoluminescence, ultrasoft X-ray emission spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy, and X-ray diffraction. The appearance of (111)-oriented luminescent silicon nanoclusters in considerable amounts upon annealing at T = 1000–1100°C is established in the investigated films. An anomalous phenomenon of X-ray absorption quantum yield intensity reversal for the L _2,3 elementary silicon edge is detected. Models for this phenomenon are suggested.     

Preparation and investigation of composite SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> films

SiO₂-TiO₂ films [Si:Ti = 1:(0.06–2.3)] are obtained by the sol-gel method. The structural and photoluminescent properties of the films and powders heat-treated at different temperatures are studied. It is shown that after 700°C the composite consists of TiO₂ crystallites that are structurally similar to anatase and distributed in an amorphous SiO₂ matrix. The photoluminescence spectra have maxima at 450–500 nm. The photoluminescence intensity depends on the treatment temperature and TiO₂ content. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 357–361, May–June, 2007.     

Hydrogenation effect on enhancement of photoluminescence of Er and Si nanocrystallites in Er-doped SiO2 synthesized by laser ablation

Significant enhancement of photoluminescence (PL) was attained for Er ions and Si nanocrystallites (nc-Si) in SiO₂ films by two kinds of hydrogenation, using H₂ molecules or H atoms. Er-doped SiO₂ films containing Er impurities and a high density of nc-Si were fabricated by laser ablation of Er films deposited on Si substrate in an O₂ gas atmosphere, followed by annealing at high temperatures in flowing Ar gas. Hydrogenation at 300–500 °C was found to effectively increase the PL intensity of Er ions as well as that of nc-Si. In particular, the hydrogen atom treatment dramatically shortens the hydrogenation time for the enhancement of Er PL compared to the hydrogen molecule treatment. Spectra of electron spin resonance showed a decrease in residual defects, namely, P_b-type defects located at the interfaces between nc-Si and SiO₂ by hydrogenation. These results clearly show the effectiveness of hydrogen passivation for Si nanostructures; i.e., the increase in the Er PL and nc-Si PL due to hydrogen passivation of the nonradiative recombination centers located at the interfaces between nc-Si and SiO₂. PACS 78.67.Bf; 71.20.Eh; 76.30.Mi; 81.15.Fg     

Temperature-dependent characteristics of pulse-laser-deposited (Pb,Sr)TiO<Subscript>3</Subscript> films at low temperatures

Pulsed laser deposition (PLD) of (Pb,Sr)TiO₃ (PSrT) film on Pt/SiO₂/Si at low substrate temperatures (T_s), ranging from 300–450 °C, has been investigated. As T_s increases, the films reveal coarsening clusters, improved crystallization of the perovskite phase, distinct capacitance–electric field (C–E) hysteretic loops and a larger dielectric constant. The 350 °C-deposited film shows strong (100) preferred orientation and optimum dielectric properties with the dielectric constant of ∼620. The current density increases as the measurement temperature and the electric field increase. Moreover, PSrT films exhibit a strong negative temperature coefficient of resistance (NTCR) behavior at temperatures ranging from 100 to 390 °C. PACS 81.15.Fg; 77.22.Ch; 68.60.Dv