Grown SiO2 Films Research Articles

Ultralow-threshold erbium-implanted toroidal microlaser on silicon

We present an erbium-doped microlaser on silicon operating at a wavelength of 1.5 μm that operates at a launched pump threshold as low as 4.5 μW. The 40 μm diameter toroidal microresonator is made using a combination of erbium ion implantation, photolithography, wet and dry etching, and laser annealing, using a thermally grown SiO2 film on a Si substrate as a starting material. The microlaser, doped with an average Er concentration of 2×1019 cm−3, is pumped at 1480 nm using an evanescently coupled tapered optical fiber. Cavity quality factors as high as 3.9×107 are achieved, corresponding to a modal loss of 0.007 dB/cm, and single-mode lasing is observed.

Detection and characterization of silicon nanocrystals embedded in thin oxide layers

Silicon nanocrystals embedded in a thin oxide layer can be used as charge storage elements in nonvolatile memory devices. The structural characteristics of the nanocrystals and their position in the oxide determine the electrical properties of the devices. In this work, silicon nanocrystals have been formed by ultralow-energy implantation (0.65–2.0 keV) of silicon in a 10 nm thin thermally grown SiO2 film on Si (100) followed by a thermal treatment. A time of flight secondary ion mass spectrometry (TOF-SIMS) methodology has been developed to detect the presence of silicon nanocrystals and to characterize them. The methodology allows one to obtain relevant information, such as the bandwidth and tunneling distance of Si nanocrystals. Chemical information about the presence of impurities introduced into the SiO2 layer during implantation and annealing have also been obtained. The advantages and disadvantages of this technique, based on TOF-SIMS in comparison with transmission electron microscopy, are discussed as well.

Arrangement of Si and O Atoms in Thermally Grown SiO2 Films

Dispersion analysis was performed on Fourier transform infrared transmission spectra taken on thermally grown films on Si(100) substrates at 950°C in dry oxygen with thicknesses between 14 and 100 nm. It was found that within the range these spectra were best described by four Lorentzian oscillators located near 1060, 1089, 1165, and and within the range with two oscillators at 795 and contrary to vitreous silica where the former range is described by two oscillators only and the latter by one. The eventualities that some of these oscillators are due to the existence of oxygen in the Si substrate or to that of phases stable at higher temperatures in films were excluded. It was suggested that in thermal films the distribution of the angles is not a simple Gaussian but can be approximated by a superposition of two Gaussians with area ratio of 25/75, distant between them by 6.3° and with full widths at half maxima of 13 and 26.8°. © 2003 The Electrochemical Society. All rights reserved.

Time of flight secondary ion mass spectrometry study of silicon nanoclusters embedded in thin silicon oxide layers

Si nanoclusters have been formed by 5 keV Si+ implantation at a fluence of 1×1016 atoms/cm2 into a 200 Å thin thermally grown SiO2 film on Si (100), followed by thermal treatment at 1000 °C with different annealing times. All the annealed samples show a broad photoluminescence spectrum with increasing intensity as function of annealing time. The use of a dual beam time of flight secondary ion mass spectrometry in negative mode with Cs+ ions at low energy for sputtering allows us to observe variations in Si− signal due to excess of silicon atoms introduced by implantation. With the high sensitivity achieved using this instrumental configuration it is possible to follow Sin− signals which give information about the chemical enviroment of the Si atoms. The possibility of studying the time evolution of the nucleation and growth of nanoclusters has been investigated.

Characterization of silicon dioxide film by high spatial resolution cathodoluminescence spectroscopy

We have measured the cathodoluminescence (CL) spectra of silicon dioxide (SiO2) films prepared by a variety of methods and observed peaks at 280, 445, 480, 520, and 640 nm. A comparison of the CL and electron spin resonance spectra of Si-doped quartz glass showed that the 445 and 480 nm peaks originated from oxygen vacancy centers, which were different from E′ centers. From CL measurement of a cross section of thermally grown SiO2 film deposited on a Si substrate, we found that the relative intensities of the 445 and 480 nm peaks were stronger than that of the CL peak at 640 nm. This CL peak was attributed to the nonbridging oxidation hole centers or their precursors. Furthermore, CL measurement of a cross section of a 16 M dynamic random-access memory device revealed that the relative intensity in the gate oxide film was stronger than that in the interlayer oxide film. These data suggest that the content of oxygen vacancy centers increases in the vicinity of the interface between the SiO2 film and the Si substrate relative to that of the nonbridging oxidation hole centers. CL spectroscopy provides us with a large amount of data on defects in the SiO2 film of Si-based electronic devices at the 0.1 μm level.

Erbium-doped optical waveguide materials based on Si nanocrystals formed bymetal vapour vacuum arc ion implantation

Dual implantation of erbium and silicon into thermally grown SiO2 film on Siwas performed using a metal vapour vacuum arc ion source; this wasfollowed by rapid thermal annealing at 500–1100°C for 20 s. 1.54 μ mphotoluminescence (PL) was observed at 77 K. Rutherford backscatteringspectrometry shows that Er ions mainly distribute within 80 nm of thesurface. The highest Er peak concentration obtained exceeded 5 × 1021 cm−3,which is the highest Er concentration reported in Si-based materials.Transmission electron microscopy demonstrates that nanocrystallinesilicon (nc-Si) embeds in SiO2 matrices. The Er3 + excitation energy isobtained from the electron–hole pairs nonradiatively combining at defectsand impurities in SiO2 matrices or at the interface of the nc-Si/SiO2(or c-Si/SiO2), and energy transferring to Er3 + ions results in 1.54 μ mlight emission. The dependence on the Si-ion dose and annealing temperature ofthe PL intensity has also been investigated.

High-quality SiO2 film formation by highly concentrated ozone gas at below 600 °C

Highly concentrated (>93 vol %) ozone (O3) gas was used to oxidize silicon for obtaining high-quality SiO2 film at low temperature. Compared to O2 oxidation, more than 500 °C lower temperature oxidation (i.e., from 830 to 330 °C) has been enabled for achieving the same SiO2 growth rate. A 6 nm SiO2 film, for example, could be grown at 600 °C within 3 min at 900 Pa O3 atmosphere. The temperature dependence of the oxidation rate is relatively low, giving an activation energy for the parabolic rate constant of 0.32 eV. Furthermore, a 400 °C grown SiO2 film was found to have satisfactory electrical properties with a small interface trap density (5×1010 cm−2/eV) and large breakdown field (14 MV/cm).

Growth of single-crystal SiO2 film on Ni(111) surface

We grew a single-crystal SiO2 film on a Ni(111) surface. Atomic hydrogen during initial growth of an amorphous SiO2 film and an ambient oxygen atmosphere during annealing of this film were essential in forming this single-crystal SiO2 film. The film grew with a (√3×√3) R30° crystallographic orientation, and its structure was the high-temperature phase of quartz, i.e., β quartz. In addition, a polycrystalline silicide phase was formed at the SiO2/Ni(111) interface.

Local atomic environment of Si suboxides at the SiO2/Si(111) interface determined by angle-scanned photoelectron diffraction.

Local environments of Si suboxides at the interface between a thermally grown SiO2 film and Si(111) were studied by angle-scanned photoelectron diffraction. Si 2p core-level spectra containing chemically shifted components were recorded. The components were deconvoluted by least squares fitting and assigned to different Si oxidation states. The obtained diffraction patterns of the various suboxides exhibit different features. Comparison of these patterns with multiple scattering calculations including a multipole R-factor analysis shows that a simple chemical abrupt interface model describes well the environment of the suboxides and indicates ordered SiO2 close to the interface.

THE TRANSITION FROM Eu3+TO Eu2+ IN SiO2(Eu) THIN FILMS PREPARED BY ION IMPLANTATION AND CO-SPUTTERING

Eu ions doped SiO2 thin films, SiO2(Eu), were prepared by co-sputtering of SiO2 and Eu2O3 and Eu ion implantation into thermally grown SiO2 films. The Eu-L3-edge X-ray absorption near edge structure (XANES) spectra of SiO2(Eu) films show a doublet absorption peak structure with energy difference of 7 eV, which indicates the conversion of Eu3+ to Eu2+ at-high annealing temperature in N2. The strong blue luminescence of SiO2(Eu) films prepared by ions implantation after films annealed above 1100 ℃ confirms the above argument.

Ultrathin silicon dioxide films grown by photo-oxidation of silicon using 172 nm excimer lamps

We report the low temperature growth of ultrathin SiO2 films on crystalline Si by photo-oxidation with an array of Xe2∗ excimer vacuum ultraviolet (VUV) lamps operating at a wavelength of 172nm. Ultrathin layers from 1.2 to 3.3nm thickness were grown at time intervals from 5 to 40min at 100–400°C at an O2 pressure of 1000mbar. Growth rates of up to 0.2nmmin−1 have been achieved at 400°C, while the chemical bonding of the films has been analysed by Fourier transform infrared (FTIR) spectroscopy and found to be SiO2. The as-grown 3.3nm films exhibited good dielectric properties, comparible to SiO2 films of identical thickness, grown by RTP at 800°C.

Characterization of Si nanocrystals grown by annealing SiO2 films with uniform concentrations of implanted Si

We have performed physical and optical characterization of Si nanocrystals grown by ion implantation of Si+ ions at multiple energies with varying doses into thermally grown SiO2 films. The purpose of multiple implants was to achieve uniform composition of the added Si profile throughout the SiO2 film to produce Si particles with a narrow size distribution upon annealing at 1000 °C in a nitrogen atmosphere. The depth distribution of the composition and sizes of the Si particles in SiO2 films before and after the anneal were determined using Rutherford backscattering (RBS), forward recoil spectroscopy, small-angle x-ray diffraction (SXRD), and high-resolution transmission electron microscopy (HRTEM). From RBS we concluded that the amount of free silicon was reduced by annealing, presumably due to oxidation in the annealing process. The mean cluster sizes of the annealed samples were determined by SXRD. HRTEM was also employed to determine the average size of Si particles. Photoluminescence spectra (PL) from these samples were broad and the peak positions of the PL spectra were blue-shifted with decreasing cluster size. The line shapes of the PL spectra were calculated with a quantum confinement model assuming a log-normal size distribution of Si nanoparticles and (1/D)1.25 dependence of the band gap energy as a function of particle size D. The band gap energy and the average particle size obtained from the calculated line shape spectra agree well with the quantum confinement model.

Influence of hydrogen on the stability of positively charged silicon dioxide clusters

Spectra of positively charged secondary ions from thermally grown SiO2 films were recorded in a time-of-flight secondary ion mass spectrometry scheme. Ablation of cluster ions was induced by the impact of slow (4 keV/u) Au69+ projectiles. The intensities of SixOyHz+, (x=1–22, y=1–44, z=0–7) clusters are found to depend sensitively on the oxygen to silicon ratio and also on the hydrogen content. We find that oxygen rich clusters, y=2x+1, and, in one case, y=2x+2, can be stabilized by the incorporation of two additional hydrogen atoms in the cluster.

XPS analysis of ultrathin SiO2 film growth on Si by ozone

We investigated the initial oxidation of Si(100) with ozone by comparison with oxidation by oxygen molecules using a specially fabricated ozone generator that could supply concentrated (typically 30 at.%) ozone gas at 1 atm pressure. We measured the thickness of the SiO2 film on the Si by XPS, using a thermal oxide film with a known thickness as a reference. We observed the growth of an SiO2 film on Si by ozone even at a sample temperature of 200 °C, verifying the strong oxidation power of ozone. The SiO2 film was ∼2.3 nm thick for 100 min of exposure. The kinetics of oxidation with ozone showed a power law dependence relative to exposure time, with almost the same power value for the ozone gas at 1 atm as at 2000 Pa. However, the growth of the SiO2 film was saturated at the level of one monolayer when the ozone gas was supplied at 10−4 Pa, which suggests to us a rapid decrease of the sticking (or reaction) probability of ozone on monolayer SiO2. Copyright © 2000 John Wiley & Sons, Ltd.

Origin of MeV ion irradiation-induced stress changes in SiO2

The 4 MeV Xe ion irradiation of a thin thermally grown SiO2 film on a Si substrate leads to four different effects in which each manifests itself by a characteristic change in the mechanical stress state of the film: densification, ascribed to a beam-induced structural change in the silica network; stress relaxation by radiation-enhanced plastic flow; anisotropic expansion and stress generation; and transient stress relaxation ascribed to the annealing of point defects. Using sensitive wafer-curvature measurements, in situ measurements of the in-plane mechanical stress were made during and after ion irradiation at various temperatures in the range from 95 to 575 K, in order to study the magnitude of these effects, the mechanism behind them, as well as their interplay. It is found that the structural transformation leads to a state with an equilibrium density that is 1.7%–3.2% higher than the initial state, depending on the irradiation temperature. Due to the constraint imposed by the substrate, this transformation causes a tensile in-plane stress in the oxide film. This stress is relaxed by plastic flow, leading to densification of the film. The anisotropic strain-generation rate decreases linearly with temperature from (2.5±0.4)×10−17 cm2/ion at 95 K to (−0.9±0.7)×10−17 cm2/ion at 575 K. The spectrum of irradiation-induced point defects, measured from the stress change after the ion beam was switched off, peaks below 0.23 eV and extends up to 0.80 eV. All four irradiation-induced effects can be described using a thermal spike model.

Surface Sol−Gel Synthesis of Ultrathin Semiconductor Films

Ultrathin films of ZnS, Mn-doped ZnS, ZnO, and SiO2 were grown on silicon substrates using surface sol-gel reactions, and the film growth process was characterized by ellipsometry, atomic force microscopy, X-ray photoelectron spectroscopy, UV-visible absorbance, and photoluminescence (PL) spectroscopy. The Si substrates were pretreated by chemical oxidation, or by derivatization with 4-((dimethylmethoxy)silyl)butylamine. On the oxidized Si/SiOx surface, nanoparticulate films of ZnS and Mn-doped ZnS were grown by sequential immersion in aqueous metal acetate and sodium sulfide solutions. During the first four adsorption cycles, there was little film growth, but thereafter the amount of material deposited was linear with the number of adsorption cycles. This behavior is consistent with the formation of ZnS nuclei at low coverage, followed by particle growth in subsequent cycles. PL spectra are consistent with incorporation of Mn 2+ into the ZnS nanoparticles. In contrast, the growth of SiO2 films from nonaqueous SiCl4 on the same Si/SiOx substrates was regular from the first adsorption cycle, indicating a high density of nucleation sites. On aminederivatized substrates, ZnO thin films grew as relatively smooth islands, suggesting that the interaction of Zn 2+ ions or primary ZnO clusters with the amine surface priming layer was sufficiently strong to prevent the formation of isotropic nanoparticles upon exposure to aqueous base.

XPS analysis of ultrathin SiO2 film growth on Si by ozone

We investigated the initial oxidation of Si(100) with ozone by comparison with oxidation by oxygen molecules using a specially fabricated ozone generator that could supply concentrated (typically 30 at.%) ozone gas at 1 atm pressure. We measured the thickness of the SiO2 film on the Si by XPS, using a thermal oxide film with a known thickness as a reference. We observed the growth of an SiO2 film on Si by ozone even at a sample temperature of 200 °C, verifying the strong oxidation power of ozone. The SiO2 film was ∼2.3 nm thick for 100 min of exposure. The kinetics of oxidation with ozone showed a power law dependence relative to exposure time, with almost the same power value for the ozone gas at 1 atm as at 2000 Pa. However, the growth of the SiO2 film was saturated at the level of one monolayer when the ozone gas was supplied at 10−4 Pa, which suggests to us a rapid decrease of the sticking (or reaction) probability of ozone on monolayer SiO2. Copyright © 2000 John Wiley & Sons, Ltd.

Epitaxial crystallization of amorphous SiO2 films deposited on single-crystalline α-quartz

The epitaxial α-quartz thin film could be a promising material for fabricating optical devices because of its unique optical and mechanical properties and processing advantages compared to bulk materials. This letter reports on the solid-phase epitaxial growth of thin amorphous SiO2 films deposited by electron gun evaporation on single-crystalline α-quartz substrates. This was achieved by high-dose Cs+-ion implantation and subsequent thermal annealing in air. Also, a thin amorphous layer produced by Si+-ion implantation on α-quartz was epitaxially regrown, thus indicating that the epitaxy is independent of the preparation history of the amorphous layer. The results are explained on the basis of network modifications induced by alkali and oxygen in the SiO2 structure.

Defect reduction and surface passivation of SiO 2 /Si by heat treatment with high-pressure H 2 O 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 SiOx 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 SiOx/Si) to 5 cm/s. Field-effect passivation was demonstrated by an additional deposition of defective SiOx films on the SiO2 films formed by heat treatment at 340 °C with high-pressure H2O vapor. The SiOx deposition reduced the recombination velocity from 100 cm/s to 48 cm/s.

Depth distribution of luminescent Si nanocrystals in Si implanted SiO2 films on Si

Depth-resolved measurements of the photoluminescence of Si implanted and annealed SiO2 films on Si have been performed to determine the depth distribution of luminescent Si nanocrystals. Si nanocrystals with diameters ranging from ∼2 to 5 nm were formed by implantation of 35 keV Si ions into a 110-nm-thick thermally grown SiO2 film on Si(100) at a fluence of 6×1016 Si/cm2, followed by a thermal anneal at 1100 °C for 10 min. The photoluminescence spectrum is broad, peaks at λ=790 nm, and contains contributions from both recombination of quantum confined excitons in the nanocrystals and ion-implantation-induced defects. By chemical etching through the SiO2 film in steps and analyzing the changes in the photoluminescence spectrum after each etch step, the depth from which each of the two luminescence features originate is determined. The etch rate of the oxide, as derived from Rutherford backscattering spectrometry data, varies from 1.3 nm/s in the regions of small excess Si to 0.6 nm/s at the peak of the concentration profile (15 at. % excess Si). It is found that the defect luminescence originates from an ∼15-nm-thick near-surface region. Large nanocrystals luminescing at long wavelengths (λ=900 nm) are mainly located in the center of the film, where the Si concentration is highest (48 at. %). This is corroborated by transmission electron microscopy that shows a high density of Si nanocrystals in the size range of 2–5 nm in the center of the film. The largest density of small luminescent nanocrystals (λ=700 nm), not detectable by electron microscopy is found near the SiO2 surface and the SiO2/Si interface. This is attributed to either the fact that the surface and the SiO2/Si interface affect the Si nanocrystal nucleation kinetics in such a way that small nanocrystals are preferentially formed in these regions, or an optical interaction between nanocrystals of different sizes that quenches the luminescence of small nanocrystals in the center of the film.