Concentration Of Defect Centers Research Articles

Laser modification of Au–CuO–Au structures for improved electrical and electro-optical properties

CuO nanomaterials are one of the metal-oxides that received extensive investigations in recent years due to their versatility for applications in high-performance nano-devices. Tailoring the device performance through the engineering of properties in the CuO nanomaterials thus attracted lots of effort. In this paper, we show that nanosecond (ns) laser irradiation is effective in improving the electrical and optoelectrical properties in the copper oxide nanowires (CuO NWs). We find that ns laser irradiation can achieve joining between CuO NWs and interdigital gold electrodes. Meanwhile, the concentration and type of point defects in CuO can be controlled by ns laser irradiation as well. An increase in the concentration of defect centers, together with a reduction in the potential energy barrier at the Au/CuO interfaces due to laser irradiation increases electrical conductivity and enhances photo-conductivity. We demonstrate that the enhanced electrical and photo-conductivity achieved through ns laser irradiation can be beneficial for applications such as resistive switching and photo-detection.

Influence of nickel ion concentration on the free volume defects entrenched in an alkali sulphophosphate glass system by means of positron annihilation characterization technique

In this study we have evaluated the concentration of defect centers like voids, their size (up to nanoscale) and the fraction of free volume space integrated in Li2SO4−MgO−P2O5:NiO glass system as a function of NiO content. Positron annihilation lifetime (PAL) technique with 22Na positron source of radio activity 100 kBq is used for this study. The size, shape and the concentration of entrenched defects in the glass matrix are predicted to influence profoundly the physical properties of the glass. Cavity radius (R) and the entrenched free volume fraction (fv) are found to be the maximal in the sample doped with 0.8 mol% of NiO. This observation suggested that Ni2+ ions predominantly occupied octahedral positions (acted as modifiers along with SO42– ions) and induced large concentration of structural defects. The obtained information is found to be in concurrent with inferences drawn from dielectrics and spectroscopic studies of these glasses reported earlier.

Study of trap levels in β-Ga2O3 by thermoluminescence spectroscopy

Electronic defects with shallow and deep levels in β-Ga2O3 single crystals were investigated by thermoluminescence (TL) spectroscopy. Undoped, Fe-doped, Sn-doped, and Mg-doped β-Ga2O3 single crystals grown by different methods were studied, and thermal activation energies of defects were calculated using the initial rise method. Hall-effect measurements and optical absorption spectroscopy were performed to determine the electrical transport properties and optical bandgaps. It was found that the dopants do not have any effect on the bandgap energy, which is important for comparing the trap levels in the samples. Three deep trap levels were found in the undoped crystals; the activation energy, ED, and concentration of defect centers for all of them have slightly changed after doping with Fe and Mg. Fe doping induced an additional defect center with activation energy of 0.62 eV. The measurements revealed the absence of TL emission in Sn doped crystals indicating that Sn doping may quench luminescence centers or modified some original electronic defects to inactive electron traps. The second interpretation “decrease of traps” may align with the successful incorporation of Sn as a donor and the high conductivity of Sn doped crystals revealed from Hall-effect measurements. This work also illustrates that the semi-insulating characteristics of Fe and Mg doped Ga2O3 are associated with the increase of the concentration of original traps in the crystal as well as the formation of new electron traps acting as deep acceptors. Recombination centers in all crystals are assumed to be associated with iron impurities.

Microscopy and Cathodoluminescence Spectroscopy Characterization of Quartz Exhibiting Different Alkali–Silica Reaction Potential

Different quartz types from several localities in the Czech Republic and Sweden were examined by polarizing microscopy combined with cathodoluminescence (CL) microscopy, spectroscopy, and petrographic image analysis, and tested by use of an accelerated mortar bar test (following ASTM C1260). The highest alkali-silica reaction potential was indicated by very fine-grained chert, containing significant amounts of fine-grained to cryptocrystalline matrix. The chert exhibited a dark red CL emission band at ~640 nm with a low intensity. Fine-grained orthoquartzites, as well as fine-grained metamorphic vein quartz, separated from phyllite exhibited medium expansion values. The orthoquartzites showed various CL of quartz grains, from blue through violet, red, and brown. Two CL spectral bands at ~450 and ~630 nm, with various intensities, were detected. The quartz from phyllite displayed an inhomogeneous dark red CL with two CL spectral bands of low intensities at ~460 and ~640 nm. The massive coarse-grained pegmatite quartz from pegmatite was assessed to be nonreactive and displayed a typical short-lived blue CL (~480 nm). The higher reactivity of the fine-grained hydrothermal quartz may be connected with high concentrations of defect centers, and probably with amorphized micro-regions in the quartz, respectively; indicated by a yellow CL emission (~570 nm).

Effect of Nitrogen-Doping on the Properties of Radiation Defect Centers in FZ Silicon

High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to determining the properties and concentrations of radiation defect centers formed in FZ silicon single crystals subjected to doping with phosphorus in neutron transmutation processes. The studies were performed immediately after the neutron irradiation on the samples with the resistivity of the order of 105 Ωcm. The preset target resistivity was around 2000 Ωcm. The N-doping effect on the material radiation damage has been demonstrated. It is shown that in the N-enriched material, the radiation defect centers with activation energies of 81, 125, 160, 185, and 390 meV are not observed and the concentrations of the defect centers with activation energies of 46, 84, 133, 140, 265, 300, 380, 460, and 546 meV are significantly lower than those in the N-free material. We also show that the majority of defect centers produced by the irradiation of FZ silicon with fast neutrons are also observed after neutron transmutation doping.

Influence of annealing on green luminescence from Cu：ZnO thin films

ZnO thin films are prepared by sol-gel method on Si substrates. The structural and optical properties of the films annealed at different temperatures are analyzed by X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy and photoluminescence. The results of XRD show that each of all the ZnO thin films has a wurtzite phase and is preferentially oriented along the c-axis direction. The sample annealed at 900℃ exhibits a better crystalline quality. Bright and stable structured green luminescence is achieved from the Cu-doped ZnO thin film. The intensity of the green emission increases significantly after annealing at 800℃, while starts to decrease with further increasing temperature. Green luminescence is correlated with the creation of Zn vacancies. Green emission peaks are found to be dependent on the relative concentration of defect centers. The substitution of Cu2+ by Cu+ will increase concentration of defects in the Cu:ZnO thin film and result in very strong green emission.

Gamma Radiation‐Induced Changes in Trombay Nuclear Waste Glass Containing Iron

Alkali‐based barium borosilicate glasses having similar composition to the Trombay nuclear research reactor waste base glass containing iron were prepared and irradiated by gamma rays. The radiation‐induced defect centers were investigated using electron paramagnetic resonance (EPR) technique. The results showed the formation of silicon hole centers and electron trap centers apart from boron‐based oxy hole centers in the glass due to the irradiation. The EPR Hamiltonian parameters for these radicals were evaluated by simulation technique using Bruker SIMFONIA computer program. Glasses with varying iron content were irradiated with the same dose to evaluate the effect of iron content on the defect center concentration. A spin counting technique was employed to evaluate the number of defect centers produced in each glass. It was observed that the defect center concentration reduced as a function of increasing iron content. This was attributed to the charge trapping process of ferric ions in the amorphous system.

Tailoring the Electrical Properties of Undoped GaP

The charge compensation in undoped GaP single crystals is investigated by modeling the Fermi level position for various concentrations of shallow and deep donors and acceptors. The model is based on the numerical solution of the charge neutrality equation and allows for calculating the Fermi energy in the temperature range of 1 –1000 K. The experimental studies of the electronic properties and concentrations of grown-in defect centers are performed by the high-resolution photoinduced transient spectroscopy (HRPITS). We show that at the shallow acceptor concentration below 1x1015 cm-3 and the concentration of deep-level defects ~3x1015 cm-3 obtaining undoped GaP with the semi-insulating (SI) properties is possible by substantial reducing the residual concentration of shallow donor impurities.

Correlated barrier hopping in CdS nanoparticles and nanowires

The ac conduction in CdS nanoparticles and nanowires was investigated in the frequency range 102 to 106 Hz and in the temperature range 303–573K. The behavior of ac conductivity was found to agree with that reported for amorphous materials and doped semiconductors. The values of ac conductivities were found to be higher than those reported for bulk CdS. The experimental results were analyzed with the CBH model proposed by Elliott. This model provided reasonable values for the maximum barrier height and characteristic relaxation time. The electrical properties of CdS nanowires and nanoparticles were compared. It was also found that the concentration of charged defect centers was large in nanowires and nanoparticles and it tended to decrease with an increase in temperature. The density of neutral defect, formed by the conversion of charged defects was also calculated. The defects in both types of nanostructures were studied by photoluminescence spectroscopy. It was found that nanoparticles possessed higher defect density than nanowires.

Formation and conversion of defect centers in low water peak single mode optical fiber induced by gamma rays irradiation

The formation and conversion processes of defect centers in low water peak single mode optical (LWPSM) fiber irradiated with gamma rays were investigated at room temperature using electron spin resonance. Germanium electron center (GEC) and self-trapped hole center (STH) occur when the fibers are irradiated with 1 and 5 kGy cumulative doses, respectively. With the increase in irradiation doses, the GEC defect centers disappear, and new defect centers such as E′ centers (Si and Ge) and nonbridge oxygen hole centers (NBOHCs) generate. The generation of GEC and STH is attributed to the electron transfer, which is completely balanced. This is the main reason that radiation-induced attenuation (RIA) of the LWPSM fiber is only 10 dB/km at communication window. The new defect centers come from the conversion of GEC and STH to E′ centers and NBOHC, and the conversion processes cause bond cleavage, which is the root cause that the RIA of the LWPSM fiber significantly increases up to 180 dB/km at working window. Furthermore, the concentration of new defect centers is saturated easily even by increasing cumulative doses.

Far-Action Radiation Defects and Gettering Effects in 4H-SiC Implanted with Al Ions

Structural features of 4H-SiC structures with CVD epitaxial layers, subjected to high-dose Al ion implantation and short high-temperature pulse annealing, have been studied using secondary-ion mass-spectroscopy, transmission electron spectroscopy, local cathodoluminescence and cathodoluminescence imaging on cross-sectionally cleaved surfaces of the structures. An accelerated diffusion of radiation defects, a “long-range action effect”, with a diffusion coefficient of 10 -9 cm2 s-1 after high-dose Al ion implantation and the gettering effect after subsequent pulsed thermal annealing have been observed for the first time. After a short high-temperature annealing, the quality of the starting material is improved in the course of formation of implantation-doped p+-n junctions due to defect gettering. As a result of the decrease in the concentration of optical active defect centers as well of deep centers by an order of magnitude in CVD layer, an increase in the diffusion length of minority carriers (Lp) by a factor of 1.5-2 was obtained.

Identification of trapping defects in 4H-silicon carbide metal-insulator-semiconductor field-effect transistors by electrically detected magnetic resonance

In conventional Si∕SiO2-based metal oxide semiconductor devices, performance-limiting semiconductor/dielectric interface traps are localized precisely at the Si∕SiO2 boundary. The authors show that in high-quality SiC∕SiO2-based devices, this is not necessarily the case. Magnetic resonance and electrical measurements indicate that in relatively high quality 4H-SiC metal-insulator-semiconductor field-effect transistors, there exist relatively high concentrations of intrinsic deep level defect centers extending below the SiC∕SiO2 interface into the SiC bulk. The primary defect observed is almost certainly an intrinsic defect of high symmetry, most likely a silicon vacancy center.

Bright Fluorescent Nanodiamonds: No Photobleaching and Low Cytotoxicity

Diamond nanocrystals emit bright fluorescence at 600-800 nm after irradiation by a 3 MeV proton beam (5 x 1015 ions/cm2) and annealing at 800 degrees C (2 h) in vacuum. The irradiation/annealing process yields high concentrations of nitrogen-vacancy defect centers ( approximately 107 centers/mum3), making possible visualization of the individual 100 nm diamond crystallites using a fluorescence microscope. The fluorescent nanodiamonds (FND) show no sign of photobleaching and can be taken up by mammalian cells with minimal cytotoxicity. The nanomaterial can have far-reaching biological applications.

Optical defect centers and surface morphology of isotopically enriched diamond layers in designer diamond anvils

We have studied optical defect centers and surface morphology of isotopically enriched layers grown on diamond anvils by microwave plasma chemical vapor deposition for applications as designer diamond anvils in high-pressure diamond anvil cell devices. Various mixtures of methane isotopes were used to grow homoepitaxial diamond with C13 molar fractions of 0.01, 0.41, 0.83, and 0.99 as determined from Raman spectroscopy. Defect centers were studied at temperatures between 80 and 320K using micro-photoluminescence (PL) spectroscopy with an argon ion and krypton laser excitation source. The defect spectra were dominated by zero phonon lines (ZPL) from nitrogen-related defect centers at nominal energies of 1.945eV (640nm defect) and 2.156eV (575nm defect), especially for the non-(100) surfaces. Polished (100) surfaces fluoresced weakly. ZPL’s at 1.77 and 1.68eV are observed, but not for all isotopically mixed samples. The 1.77eV ZPL appears to be associated from the original diamonds, while the 1.68eV ZPL is known to originate with silicon-based defects. Atomic force microscopy of as-grown isotopically enriched layers show rough growth steps in areas with surface roughness of hundred nanometers and smooth areas with surface roughness of few nanometers. Our studies indicate that (100) polished surfaces of isotopically enriched designer diamonds with low concentration of nitrogen defect centers can be fabricated for a variety of applications in high pressure research.

Equilibrium concentration of oxygen rich and deficient defect centers in germanosilicate glasses

Equilibrium concentrations of oxygen rich and deficient defect centers are calculated as a function of temperature and glass redox condition for germanosilicate glasses. We have here used the approach of Silin and Lace [J. Non-Cryst. Solids 149 (1992) 54–61] but extended it to include the case of binary system of germanosilicate glasses. A set of 23 reactions is identified as the possible pathway for formation of different defect centers. Each of these reactions are represented by forward and backward steps with Arrhenius kinetics and their activation energies are estimated based on the relevant bond energies of the involved species. Equilibrium concentrations are determined by setting rates of each of these reactions equal to zero. Typical results are presented for glasses which are: (i) stoichiometric, (ii) glasses with excess oxygen levels of 1016 to 1020cm−3, and (iii) glasses which are oxygen deficient by 1016 to 1018cm−3.

Dependence of H 2/O 2 ratio and GeO 2 content on the enhancement of second-order non-linearity related defects in Ge-doped optical fiber preforms

In this research, we studied the influence of the process conditions through the H 2/O 2 ratio and the GeO 2 content, on the formation of germanium oxygen deficient centers and paramagnetic defects center induced by X-ray irradiation in SiO 2:GeO 2 glass preform prepared by the vapor-phase axial deposition method. The absorption band at around 5.1 eV, characteristic of germanium oxygen defect center was observed in all glass preform sample absorption spectra. The absorption coefficient increased when the GeO 2 concentration increased, however, the absorption was higher in samples prepared with high H 2/O 2 ratio. The electronic spin resonance spectrum profiles suggest that paramagnetic structures, such as, electron trapped centers and positively charged germanium oxygen deficient centers, assigned as Ge(1) and Ge(2), respectively, are induced in all samples after being X-ray irradiated for 1 h. It was found that the concentration of induced paramagnetic centers is linearly proportional to germanium oxygen defect center concentration and that the efficiency in the enhancement of paramagnetic defect centers associated with second-order optical non-linearity increased as H 2/O 2 ratio used in the fabrication process was decreasing. It was concluded that SiO 2:GeO 2 glass preform with low H 2/O 2 ratio and higher GeO 2 content must be prepared for the enhancement of the second-order non-linearity.

Dependence of H2/O2 ratio and GeO2 content on the enhancement of second-order non-linearity related defects in Ge-doped optical fiber preforms

In this research, we studied the influence of the process conditions through the H2/O2 ratio and the GeO2 content, on the formation of germanium oxygen deficient centers and paramagnetic defects center induced by X-ray irradiation in SiO2:GeO2 glass preform prepared by the vapor-phase axial deposition method. The absorption band at around 5.1 eV, characteristic of germanium oxygen defect center was observed in all glass preform sample absorption spectra. The absorption coefficient increased when the GeO2 concentration increased, however, the absorption was higher in samples prepared with high H2/O2 ratio. The electronic spin resonance spectrum profiles suggest that paramagnetic structures, such as, electron trapped centers and positively charged germanium oxygen deficient centers, assigned as Ge(1) and Ge(2), respectively, are induced in all samples after being X-ray irradiated for 1 h. It was found that the concentration of induced paramagnetic centers is linearly proportional to germanium oxygen defect center concentration and that the efficiency in the enhancement of paramagnetic defect centers associated with second-order optical non-linearity increased as H2/O2 ratio used in the fabrication process was decreasing. It was concluded that SiO2:GeO2 glass preform with low H2/O2 ratio and higher GeO2 content must be prepared for the enhancement of the second-order non-linearity.

Color centers vs electrolytes for Si-based porous anodic alumina

Anodic oxidation of Al film on silicon substrate in three different electrolytes is investigated through j– t curves and photoluminescence (PL) of the grown porous anodic alumina (PAA) under same voltage. Their growth is analyzed with three typical stages according to j– t curve. The influence of the electrolyte on color centers is shown through comparison of the PL spectra of Si-based PAA fabricated in different electrolytes. From sulfuric, oxalic to phosphoric acids, The PL shows blueshift behavior with different electrolytes due to mechanical stress and concentration of defect centers, while the change of PL intensity reduced for decreasing of the contrast across barrier layer, which is helpful to understand PL mechanism and growth mechanism of porous anodic alumina.

Role of Eu ions and defect centers in formation of high-temperature persistent spectral holes in glass

The mechanism of room temperature persistent spectral hole burning was investigated using Eu-doped sodium borate glass (35Na 2O–65B 2O 3). The concentrations dependence of defect centers on the extent of hole burning was investigated for several Eu concentrations because the mechanism proposed so far are of two types: electron transfer to Eu 3+ from Eu 2+ and from defect centers in the glass. When the defect concentration increased due to X-ray irradiation, a high-temperature hole burning was observed at a Eu concentration (0.3 mol%) ; where the phenomenon is not observed in a normal sample without X-ray irradiation. When the Eu concentration was very low (0.1 mol%) , a spectral hole was observed only at low temperatures even when the concentration of defect centers became saturated due to X-ray irradiation. Furthermore, a concentration quenching of fluorescence from Eu 3+ became obvious at Eu concentrations of more than ca. 1 mol% . Hole burning became obvious at the same concentration. These findings indicate that the mechanism is an energy transfer from selectively excited Eu 3+ to adjacent Eu 2+ and subsequent electron transfer from Eu 2+ to Eu 3+. Defect centers in the glass reduce the barrier height of this electron transfer.

Photostimulated luminescence: A tool for the determination of optical properties of defect centers

The photostimulated luminescence (PSL) can be used to determine the defect center concentration in X-ray or electron irradiated materials, which is not easily accessible by other means. This opens the unique possibility of determining oscillator strengths of radiation induced defect centers. Applications of this method are presented.