A.N. Trukhin

Cathodoluminescence of Ge+, Si+, and O+ implanted SiO2 layers and the role of mobile oxygen in defect transformations

Abstract Thermally grown SiO 2 layers of thickness d =500 nm have been implanted by Ge + , Si + , and O + ions of energy 350, 150, and 100 keV, respectively, and a uniform implantation dose of D i =5×10 16 ions/cm 2 . Thus the implantation profiles are expected with a concentration maximum of nearly 4 at.% at the half-depth d m ≅250 nm of the SiO 2 layers. After thermal annealing to 900 °C for 1 h in dry nitrogen or vacuum the typical violet luminescence band ( λ =400 nm) of the Ge + implanted centers is increased more than 200-fold and the Ge luminescent center depth profile is shifted from about 250 to 170 nm towards the surface as determined by cathodoluminescence (CL) depth profiling. Implanting oxygen increases the red band ( λ =650 nm) but does not affect the blue band ( λ =460 nm). Silicon surplus increases the amplitude of the blue (B) luminescence, but reduces the amplitude of the red (R) one. Studying the irradiation dose dependence of these blue and red bands we have established defect kinetics in SiO 2 including six main defects and precursors, including the non-bridging oxygen hole center for the red luminescence, the twofold-coordinated silicon as the oxygen deficient center ODC(2) for the blue luminescence and the mobile oxygen as the main transmitter between precursors and the radiation induced defects. The kinetics are described by a set of eight differential equations which predict the dose dependence of the CL.

Excitons in SiO2: a review

Abstract In this paper, excitonic properties of crystalline and glassy SiO 2 are reviewed. Experimental spectroscopic data (optical absorption and reflection spectra, as well as spectra of luminescence and its excitation), luminescence decay kinetics at different temperatures, and photoelectric properties — photoconductivity and photoelectron emission — were used to determine excitons in SiO 2 . Information on migration of excitons was obtained on the basis of energy transport to impurity luminescence centers, the latter being detectors of quasiparticles. Determination of excitonic properties in glassy SiO 2 was based on the comparison of the observed phenomena in crystalline and glassy materials in approximately the same conditions. Structural peculiarities are analyzed comparing optical phenomena in crystalline α-quartz and cristobalite and in fused silica of four types. Models of excitons are based on propositions of chemical bond theory and total energy adiabatic potential.

Cathodoluminescence of crystalline and amorphous SiO2 and GeO2

Abstract Cathodoluminescence (CL) and its temperature-dose behaviour are presented for different crystalline and amorphous modifications of SiO 2 and GeO 2 as well as for Ge-doped SiO 2 layers. The crystalline samples include four-fold coordinated Si and Ge in hexagonal quartz and quartz-like crystals, respectively, as well six-fold coordinated atoms in tetragonal rutile-like crystals. The detected luminescence bands, in general, are attributed to three optical active luminescence centres: the two-fold coordinated silicon (=Si:) and germanium (=Ge:) centre, respectively, the non-bridging oxygen hole centre (NBOHC) and the self trapped exciton (STE). The first ones, the oxygen deficient centres (ODC), are especially developed in both, in the tetragonal crystal rutile-like modifications as well as in glassy states. The huge violet luminescence in Ge-implanted SiO 2 -layers is attributed to the two-fold coordinated Ge in the silica matrix.

Silicon dioxide thin film luminescence in comparison with bulk silica

Abstract The luminescence of the self-trapped exciton (STE) in SiO2 films was measured at low temperatures on the background of defect luminescence under cathodoexcitation and compared with bulk silica luminescence. The defect luminescence is mainly caused by non-bridging oxygen centers (a red luminescence band at 1.8 eV) and twofold coordinated silicon centers (blue and ultraviolet luminescence with 2.7 and 4.4 eV bands, respectively). The STE luminescence with a band at 2.3 eV is uniformly distributed within SiO2 film volume. Contrary to defect luminescence, whose intensity increases with irradiation time, the STE luminescence decreases almost to zero in a few seconds of irradiation time. The defect luminescence increase is attributed to transformation of precursors whereas STE luminescence is produced in the continuous network. The decrease of STE luminescence is attributed to radiation damage in the continuous network.

Investigation of optical and radiation properties of oxygen deficient silica glasses

The deficiency of oxygen in pure silica manifests an absorption band at 5 eV as well as an absorption band of higher intensity at 7.6 eV. The band at 5 eV is associated with lone twofold-coordinated silicon centers. The nature of the main band at 7.6 eV has been studied using silica samples with different levels of oxygen deficiency. The excitation via the 7.6 eV band produces a photoelectric response as well as inner center and recombination type luminescence. Two main luminescence bands of the twofold-coordinated silicon center appear: a blue band (2.7 eV) and a UV band (4.4 eV). Induced absorption with several bands as well as thermally stimulated luminescence with complex peak structure were observed. Analyzing these data, the nature of the 7.6 eV band cannot be ascribed to a lone point defect, rather, it can be ascribed to the localized states of the disordered structure of silica modified by an oxygen deficit.

Localized states in wide-gap glasses. Comparison with relevant crystals☆☆☆

In this paper, the electronic properties of localized states in wide-gap oxide glasses such as sodium silicate, sodium germanate, lead silicate, silica and germania are reviewed. Experimental spectroscopic data (optical absorption, as well as luminescence and its excitation) and luminescence decay kinetics at different temperatures were used to determine localized states in these glasses. The corresponding data for crystals of these materials were used for comparison. The spectrum of localized states is determined by many different types of a short-range order in the glass structure. In the glasses studied the localized states create an ensemble of structurally non-equivalent anisotropic luminescence centers. Their concentration is on the level of 1%. These states disappear after glass crystallization. The structural elements of localized states are the precursors of radiation-induced defects. In the case of silica, the optical absorption of the precursor is covered by the excitonic absorption band. Reducing conditions transform the localized states into defects.

The correlation of the 7.6 eV optical absorption band in pure fused silicon dioxide with twofold-coordinated silicon

Abstract The optical absorption band at 7.6 eV, which appears in oxygen deficient pure silica, does not correlate with any ESR signal in non-irradiated samples. Longlasting illumination at 80 K in the range of its absorption leads to an increase of the absorption band at 5 eV. Subsequent heating to 290 K restores the initial absorption. These data can be explained as photodissociation and thermal recreation of a complex defect containing a twofold-coordinated silicon defect. This complex defect is responsible for the 7.6 eV absorption band.

Cathodoluminescence and cathodoelectroluminescence of amorphous SiO2 films

Abstract Cathodoluminescence of amorphous SiO 2 films thermally grown on a Silicon substrate has been observed in a scanning electron microscope using wavelength dispersed registration by a charge coupled device (CCD) camera. Spectra have three bands: at 650 nm (red), 460 nm (blue), and 285 nm (UV) whose intensities change during the initial period of electron beam excitation. Luminescence peak dose dependence has been investigated in a wide range of current density (10 −5 to 10 −3 A cm −2 ) and temperature (90 to 500 K). An interpretation of the dose-temperature dependence is made by a model of precursor transformation via a metastable interlevel. Application of an electric field during continuous electron excitation (cathodoelectroluminescence) causes an enhancement up to five times of the blue band intensity. On the other hand, the red band decreases in the electric field. Based on these phenomena, the UV and the blue luminescence band are attributed to an internal electron impact excitation within a localized center, probably twofold-coordinated silicon in the SiO 2 network, whereas the red band is ascribed to band-to-band-recombination via localized levels attributed to non-bridging oxygen.

Cathodoluminescence and IR absorption of oxygen deficient silica – influence of hydrogen treatment

Abstract The cathodoluminescence (CL) and IR absorption of silica samples with normal stoichiometry as well as with an extremely high level of oxygen deficit were studied. Additionally, some samples have been treated in hydrogen at 800°C. Crystalline quartz was used for reference measurements and the CL data have been compared with those of X-ray excited luminescence (XL). The luminescence spectra of silica have a band at 1.85 eV due to non-bridging oxygens and the two bands at 2.7 and 4.4 eV due to twofold-coordinated silicons. The energetic yield for CL is about 0.1%, for XL it approaches 0.15%. Cathodoluminescence of quartz at temperatures >130 K exhibits the self-trapped exciton luminescence. Under electron beam irradiation there is a production and destruction of luminescence centers, whereas X-ray excitation mainly leads to electron–hole recombinations on existing centers. After hydrogen treatment the IR spectra contain bands due to Si–H and Si–O–H, independent of the oxygen deficiency. The hydrogen treatment affects the cathodoluminescence properties of oxygen deficient silica by modifying the luminescence centers themselves.

Luminescence of γ-radiation-induced defects in α-quartz

Optical transitions associated with γ-radiation-induced defects in crystalline α-quartz were investigated by photoluminescence excited by both pulsed synchrotron radiation and steady-state light. After a 10 MGy γ-dose we observed two emissions at 4.9 eV (ultraviolet band) and 2.7 eV (blue band) excitable in the range of the induced absorption band at 7.6 eV. These two luminescence bands show a different temperature dependence: the ultraviolet band becomes bright below 80 K; the blue band increases below 180 K, but drops down below 80 K. Both emissions decay in a timescale of a few ns under pulsed excitation, however the blue band could also be observed in slow recombination processes and it afterglows in about 100 s at the end of steady-state excitation. The origin of the observed luminescence bands and the comparison with optical features of oxygen-deficient centres in silica glass are discussed in the framework of different models proposed in the literature.