Centers In SiO2 Research Articles

EPR study of paramagnetic centers in SiO2:C: Zn nanocomposites obtained by infiltration of fumed silica with luminescent Zn(acac)2 solution

Silica-carbon with zinc (SiO2:C:Zn) nanocomposites obtained via infiltration with aged luminescent zinc acetylacetonate (Zn(acac)2) ethanol solution of two concentrations (1 or 4%) into the fumed silica (SiO2) matrix have been studied using EPR within the temperature range 6…296 K before and after thermal annealing. The EPR spectrum of SiO2:C:Zn nanocomposites consists of three signals with the Lorentzian lineshape corresponding to paramagnetic centers with S = 1/2, which are related to carbon dangling bonds (CDB) (g = 2.0029(3)), silicon dangling bonds (g = 2.0062(3)) and oxygen-centered carbon-related radicals (CRR) (g = 2.0042(3)). A small EPR linewidth (<1 mT) observed for CDB and oxygen-centered CRR allows us to conclude that they are in the sp3-hybridized state. It was found that the temperature dependence of the EPR signal integrated intensity of the CDB and oxygen-centered CRR follows the Curie–Weiss law with a small positive value of the Curie–Weiss constant, which indicates that the weak ferromagnetic exchange interaction takes place in the spin system of CDB and oxygen-centered CRR. It was supposed that the carbon-related centers are clustered in SiO2:C:Zn nanocomposites. We assume that the oxygen-centered CRR in the sp3-hybridized state are associated with luminescent centers in previously reported aged Zn(acac)2/C2H5OH solution.

Identification of Channel Hot Carrier Stress-Induced Oxide Traps Leading to Random Telegraph Signals in pMOSFETs

We investigated the effect of channel hot carrier (CHC) stressing on the creation of gate oxide defect centers causing random telegraph signals (RTSs) in pMOSFETs. To identify these stress-induced hole traps in the oxide and fully characterize their trapping properties, variable temperature RTS measurements were performed from room temperature down to 215 K and repeated after each stressing interval, up to 1200 s. The trapping properties of the stress-induced defect sites and those present before stress are quantified, including the hole capture and emission activation energies, structural relaxation in the oxide network due to trapping/detrapping, and entropy change as well as the energy level and position of the trap in the oxide. Based on this information, disassociated III-Si and hydrogen bridge defects are identified as the hole trapping centers in SiO2. Difference was observed between the process- and stress-induced traps in the entropy change upon hole emission from the oxide trap to Si channel (electron capture from the Si valence band), implying the possibility of stress-induced structural defects being different than the native ones.

The origin of near-IR luminescence in bismuth-doped silica and germania glasses free of other dopants: First-principle study

First-principle study of possible bismuth-related centers in SiO2 and GeO2 glass model hosts is performed and the results are compared with the experimental data. The following centers are modeled: trivalent and divalent Bi substitutional centers; BiO interstitial molecule; interstitial ion, Bi+, and atom, Bi0; Bi··· ≡Si–Si≡ and Bi··· ≡Ge–Ge≡ complexes formed by interstitial Bi atoms and glass intrinsic defects, ≡Si–Si≡ or ≡Ge–Ge≡ oxygen vacancies; interstitial dimers, Bi20 and Bi2−. Experimental data available on bismuth-related IR luminescence in SiO2:Bi and GeO2:Bi glasses, visible (red) luminescence in SiO2:Bi glass and luminescence excitation are analyzed. A comparison of calculated spectral properties of bismuth-related centers with the experimental data shows that the IR luminescence in SiO2:Bi and GeO2:Bi is most likely caused by Bi··· ≡Si–Si≡ and Bi··· ≡Ge–Ge≡ complexes, and divalent Bi substitutional center is responsible for the red luminescence in SiO2:Bi.

Investigation of emitting centers in SiO2 codoped with silicon nanoclusters and Er3+ ions by cathodoluminescence technique

This study reports on the investigation and characterization of the different emitting centers within SiO2 codoped by Er3+ ions and silicon-excess. Erbium doped silicon-rich silicon oxide (SRSO:Er) thin films, fabricated by magnetron cosputtering at 500 °C, were analyzed by means of cathodoluminescence. The CL spectra of SRSO, Er-doped SiO2 and SRSO:Er were recorded and compared for various annealing temperatures. It was found that some specific optically-active point-defects called silicon-oxygen-deficient centers (SiODCs) are present in all kinds of samples. In the layers containing some excess Si, the phase separation between Si nanoclusters (Si-ncs) and SiO2 is observed when the annealing temperature reaches and exceeds 900 °C. The formation of Si-nc increases with annealing at the expense of SiODCs that was assumed to act as seeds for the growth of Si-nc. For SRSO:Er samples, the contribution of SiODCs overlaps that due to Er3+ transitions in the visible range. The emissions from SiODCs are drastically reduced when an SRSO sample is doped with Er ions, whereas the Er emissions in the visible range start to be distinctly observed. We propose a scenario of energy transfer from SiODCs toward the Er ions, especially as the emissions from the Si-based entities (SiODCs, Si-nc) and from some transitions of Er ions are located in a same visible broad range.

Statics and dynamics of excited states of oxygen-deficient centers in SiO2

The parameters of excited states of oxygen-deficient centers (ODCs) in high-energy-electron irradiated crystalline and glassy SiO2 have been studied using optical absorption, luminescence, and photoelectron emission spectroscopy. Additional evidence has been gained in support of the model of a neutral oxygen vacancy in ODCs, the diagram of electronic transitions has been refined, and their characteristics have been quantified. The possibility of ionization of the singlet and triplet defect states at a transition to the anomalously relaxed configuration has been demonstrated using the particular example of α-ODCs. Nonradiative excitation transfer from nonbridging oxygen centers to the triplet ODC state has been observed.

Optical investigations of germanium nanoclusters – Rich SiO2 layers produced by ion beam synthesis

Abstract In this work, refractive index and extinction coefficient spectra of germanium nanoclusters – rich SiO2 layers have been determined using variable angle spectroscopic ellipsometry (VASE) in the 250–1000 nm range. The samples were produced by Ge+ ion implantation into SiO2 layers on Si substrates and subsequent annealing at temperatures from 700 to 1100 °C. It is known from previous investigations of similar samples that the Ge nanoclusterization process starts already at 800 °C and spherical Ge nanocrystallites 5–8 nm in diameter are observed in the SiO2 layers after annealing for 1 h at even higher temperatures of 1000–1100 °C. Rutherford backscattering spectrometry (RBS) was employed to measure the Ge atom concentration depth profiles in the studied samples. The RBS results helped us choose realistic models for the VASE analysis which were necessary for a proper interpretation of the VASE data. It has been found that the refraction index value for the SiO2/Si layer increases after Ge implantation. This effect can be explained by a defect-dependent compaction of ion-bombarded layers. A band’s tail in the extinction coefficient spectra for all the samples is observed which originates from a strong ultraviolet absorption band at 6.8 eV due to a Germanium Oxygen-Deficiency Center (GeODC) and/or a Ge-E’center in SiO2. The annealing process results in the emergence of weaker extinction coefficient bands in the 400–600 nm region, associated with direct band-to-band transitions in Ge nanostructures. Transformation of these bands, including their blue-shift with the increasing annealing temperature could be explained via a quantum-confinement mechanism, by size and structural changes in Ge nanostructures.

Er+ implantation in SnO2:SiO2 layers: Structure changes and light emission

Structure changes and light emission behavior in Er+ implanted SnO2:SiO2 layers are studied, using transmission electron microscopy (TEM), Rutherford backscattering (RBS) and cathodoluminescence (CL). SnO2:SiO2 layers of different composition deposited with RF magnetron sputtering on Si wafers were implanted with 200keV Er+ to a fluence of 3×1015cm−2 at room temperature. The implanted structures were then annealed at 600–1000°C for 30min, resulting in the formation of crystalline SnO2 nanoclusters. Cross-section TEM revealed a strong reduction of the SnO2 crystallite size down to several nanometers in the implanted area of the SnO2:SiO2 layer as compared to the undoped layer. In addition, a very narrow layer of SnO2 nanocrystals appears at the SiO2/Si interface. Several narrow CL emission peaks and wide bands were found which could be related to the decay of SnO2 free excitons, to oxygen deficiency centers in SiO2 and to transitions between the energy levels in the Er ions, apparently located at nanoclusters. The mechanisms of nanostructuring as well as the emission process are discussed.

β-Band Photoluminescence and Sn-E' Center Generation from Twofold-Coordinated Sn Centers in SiO2 Glasses Produced via Sol–Gel Method

Violet-blue photoluminescence (β-band emission) of twofold-coordinated tin (O–Sn0–O, >Sn0) atoms in silica (SiO2) glasses, synthesized via a sol–gel route, was observed with a large band tail to longer wavelength. The presence of >Sn0 centers more easily enabled us to produce Sn-associated E'-centers with X-ray irradiation, which is expected for a photosensitivity similar to that in the case of Ge-doped SiO2 glasses fiber. In the initial state of the new fabrication process, stannic oxide (SnO2) nanocrystals were precipitated in SiO2 glasses and an adequate annealing of SnO2–SiO2 glass ceramics in H2 gas led to the decomposition of SnO2 nanocrystals and concurrently the production of >Sn0 point defects. Although the quantum efficiencies of the photoluminescence were very low to be 0.95% (external) and 1.3% (internal) under near-ultraviolet excitation owing to the triplet nature of the emissive level of >Sn0 center, we could clarify the photoluminescence properties, by means of time-resolved fluorescence spectroscopy, and the >Sn0→Sn-E' conversion.

Photoelectron spectroscopy of E′ centers in crystalline and glassy silicon dioxide

Radiation-induced E′ centers in SiO2 were studied to test the possibility of applying optically stimulated electron emission (OSEE) to the spectroscopy of excited states of point defects in dielectrics. The spectral responses of the OSEE of crystalline α quartz and silica glass irradiated by 10-MeV electrons were measured and studied. It was established that volume E′ centers in the crystalline and glassy SiO2 modifications are dominant emission-active defects. Surface E’s (1) centers were also detected in glassy SiO2. A model of the energy structure of E′ centers accounting for the absence of luminescence and taking into account the presence of two nonradiative (intracenter and ionization) relaxation channels is proposed. This model was used to explain the mechanism of photothermal decay of the E′ centers and to determine the ionization activation barriers and quantum yields of these centers. The emission, spectral, and kinetic parameters of the volume and surface E′ centers in glassy SiO2 were obtained, showing the excited states of these defects to have identical atomic configurations.

Transport and electroluminescence mechanism in Au/(Si/SiO2)/P-Si film

The samples of Au/(Si/SiO2)/p-Si structure were fabricated by using the R. F magnetron sputtering technique. Its carrier transport and electroluminescence mechanism were studied from the I–V curves and EL spectra by using the Configuration Coordinate as a theoretical model. The result indicates that there are two defect centers in SiO2 films., The electron in Au and the hole in p-Si went into SiO2 film by the Fowler-Nordheim tunneling model at a high bias voltage and recombined through these defect centers in SiO2 film.

Detection and manipulation of statistical polarization in small spin ensembles.

We report the detection of the square root of N statistical polarization in a small ensemble of electron spin centers in SiO2 by magnetic resonance force microscopy. A novel detection technique was employed that captures the statistical polarization and cycles it between states that are either locked or antilocked to the effective field in the rotating frame. Using field gradients as high as 5 G/nm, we achieved a detection sensitivity equivalent to roughly two electron spins, and observed ultralong spin-lock lifetimes, as long as 20 s. Given a sufficient signal-to-noise ratio, this scheme should be extendable to single electron spin detection.

Modeling Si nanoprecipitate formation in SiO2 layers with excess Si atoms

Computer simulations based on the Monte Carlo method are used to analyze processes leading to the formation of luminescence centers in SiO2 implanted with Si ions. The simulations, which take place in a two-dimensional space, mimic the growth of silicon nanoprecipitates in layers containing several at.% of excess silicon. It is assumed that percolation clusters made up of neighboring Si atoms form first. As the annealing temperature increases, these clusters grow and compactify into nano-sized inclusions of a well-defined phase. It is shown that a dose dependence arises from an abrupt enhancement of the probability of forming direct Si-Si bonds when the concentration of silicon exceeds ∼1 at. %. Under these conditions, percolation chains and clusters form even before annealing begins. The effect of the temperature of subsequent anneals up to 900 °C is modeled via the well-known temperature dependence of Si diffusion in SiO2. It is assumed that annealing at moderate temperatures increases the mobility of Si atoms, thereby facilitating percolation and development of clusters due to an increase in the interaction radius. Intrinsic diffusion processes that occur at high temperatures transform branching clusters into nanoprecipitates with well-defined phase boundaries. The dose and temperature intervals for the formation of precipitates obtained from these simulations are in agreement with the experimental intervals of dose and temperatures corresponding to the appearance of and changes in luminescence.

Bulk oxide traps and border traps in metal–oxide–semiconductor capacitors

Thermally stimulated current (TSC) and capacitance–voltage measurements are combined via a newly developed analysis technique to estimate positive and negative oxide-trap charge densities for metal–oxide–semiconductor (MOS) capacitors exposed to ionizing radiation or subjected to high-field stress. Significantly greater hole trapping than electron trapping is observed in 3% borosilicate glass (BSG) insulators. Two prominent TSC peaks are observed in these BSG films. A high-temperature peak near 250 °C is attributed to the Eγ′ defect, which is a trivalent Si center in SiO2 associated with an O vacancy. A lower temperature positive charge center near 100 °C in these films is likely to be impurity related. The higher temperature Eγ′ peak is also observed in 10, 17, and 98 nm thermal oxides. A much weaker secondary peak is observed near ∼60 °C in some devices, which likely is due to metastably trapped holes in the bulk of the SiO2. Negative charge densities in these thermal oxides are primarily associated with electrons in border traps, which do not contribute to TSC, as opposed to bulk electron traps, which can contribute to TSC. Ratios of electron to hole trap densities in the thermal oxides range from ∼30% for radiation exposure to greater than 80% for high-field stress. It is suggested that the large densities of border traps associated with trapped holes in these devices may be due to high space-charge induced electric fields near the Si/SiO2 interface. In some instances, border traps can reduce near-interfacial electric fields by local compensation of trapped positive charge. This may provide a natural explanation for the large densities of border traps often observed in irradiated or electrically stressed MOS capacitors.

First-principles exploration of possible trap terminators in SiO2

Oxygen vacancies (O vacancies) are considered to be charge trap centers in SiO2. This letter explores possible terminators of Si-dangling bonds associated with the O vacancies by performing first-principles total energy calculations. The present exploration is focused on the terminators which retain their chemical bonds after hole trapping. We have found that Cl atoms, NH and OH molecules firmly terminate the Si-dangling bond while H atoms are dissociated from a Si–H bond after hole trapping. The H dissociation is accompanied by the generation of gap states which can contribute to the leakage currents in the SiO2 films.

Visible Light Emission from the Silicon-Doped SiO2 Thin Film Deposited by Sputtering

Visible light emission strong enough to be seen with the naked eye was obtained at room temperature from the Si-doped SiO2 thin films deposited on Si substrates by cosputtering of Si and SiO2 when they were annealed in Ar at high temperature. The luminescence model with radiative recombination centers at the surface of a Si nanocrystal and nonradiative recombination centers in SiO2 has been developed to explain the results of the combined x-ray diffraction and photolumines cence measurements.

Relaxation of pseudo-Jahn-Teller centers in SiO2

Potential energy profiles for strongly relaxed defects in SiO2 are interpreted in terms of a pseudo-Jahn-Teller effect. Defect conversion rates involving tunelling transitions betveen energy minima are calculated using a reaction-rate method. Obtained relaxation time vs. temperature dependences call for further thermal bleaching experiments.

π-bonded model of an oxygen-vacancy center in SiO2

Tight-binding total energy calculations of the two possible states of the neutral oxygen-vacancy center in SiO2 have been performed within the bond orbital approximation. It has been shown that the lowest energy state is realized by a transfer of one electron from a nonbonding p oxygen orbital (next to the vacancy) into an opposite silicon dangling hybrid, and then by the formation of a π bond between the remaining oxygen p electron and the nearest silicon dangling hybrid. The new π-bonded model of the oxygen-vacancy center is discussed and the results obtained are compared with experimental data. The most important aspect of this work is the demonstration that for defects in SiO2, the π interactions are very important and should not be neglected in describing the physics of many problems in SiO2.

The indirect hyperfine interaction of a29Si nearest neighbour to an oxygen defect in SiO2

A recent calculation determined the indirect aluminium hyperfine interactions for a number of oxygen-aluminium trapped-hole centres (Al3+-O-) in ionic crystals. One of these crystals was SiO2 where Al3+ is incorporated as an impurity. The theory showed that the dominant mechanism of the Al3+ transferred hyperfine interactions is exchanged polarisation. Since experimental data also exist for the Si4+ magnetic nuclei that are nearest neighbours to the Al3+-O- centre in SiO2, it is of interest to see if this same theory can account for the Si4+ transferred hyperfine interactions. This Letter reports on the results of calculating the Si4+ transferred hyperfine interactions for this system. The model has much the same level of success for the Si hyperfine interactions as for the Al.

Hole traps and trivalent silicon centers in metal/oxide/silicon devices

We report electron spin resonance (ESR) measurements of E′-center (a ‘‘trivalent silicon’’ center in SiO2) density as well as capacitance versus voltage (C-V) measurements on γ-irradiated metal/oxide/silicon (MOS) structures. We also report a considerable refinement of earlier ESR measurements of the dependence of radiation-induced Pb -center (a ‘‘trivalent silicon’’ center at the Si/SiO2 interface) occupation as a function of the Fermi level at the Si/SiO2 interface. These measurements indicate that the Pb centers are neutral when the Fermi level is at mid-gap. Since the Pb centers are largely responsible for the radiation-induced interface states, one may take ΔVmg Cox/e (where ΔVmg is the ‘‘mid-gap’’ C-V shift, Cox is the oxide capacitance, and e is the electronic charge) as the density of holes trapped in the oxide. We find that radiation-induced E′ density equals ΔVmg Cox/e in oxides grown in both stream and dry oxygen. Etch-back experiments demonstrate that the E′ centers are concentrated very near the Si/SiO2 interface (as are the trapped holes). Furthermore, we have subjected irradiated oxide structures to a sequence of isochronal anneals and find that the E′ density and ΔVmg annealing characteristics are virtually identical. We conclude that the E′ centers are largely responsible for the deep hole traps in thermal SiO2 on silicon. This observation coupled with observations regarding the Pb center indicates that two intrinsic centers, both involving silicon atoms lacking one bond to an oxygen atom, are largely responsible for the two electrically significant aspects of radiation damage in MOS devices: charge buildup in the oxide and interface-state creation at the Si/SiO2 interface.

Creation and annealing kinetics of magnetic oxygen vacancy centers in SiO2

Dose and energy dependence of the E′1 defect density/cm2 created in SiO2 by implantation of Ar ions has been determined by electron paramagnetic resonance. A dose dependent region followed by a saturated, dose independent region is found for all energies studied (50–150 keV). In the low dose limit for 100-keV Ar ions we estimate E1 creation to be 35/implant ion. The defect density/cm2 is found to be a linear function of the longitudinal atomic collisional damage distribution consistent with a picture of overlapping lateral damage distributions. A simple model for defect creation and annihilation gives a good quantitative explanation of the observed energy and dose dependence of the defect density. Isothermal annealing studies have been performed and the results do not follow those expected for a simple exponentially activated process. A model assuming correlated defect/interstitial recovery gives a better description of the experimental results and suggests an activation energy for recovery of approximately 0.85 eV.