Vacancy-oxygen Complexes Research Articles

VOn Complexes in RTA Treated Czochralski Silicon Wafers Investigated by FTIR Spectroscopy

Oxygen-vacancy complexes formed after rapid thermal annealing in silicon wafers were investigated by FTIR spectroscopy at 6 K. It was found that VO4 is the only detectable complex. The concentration of VO4 complexes increases with increasing temperature of RTA treatment in the temperature range between 1250°C and 1400°C. The concentration at maximal temperature is equal to 4.5 × 1013 cm−3. The experimental results were compared with concentrations of VOn complexes in silicon wafers obtained using ab-initio calculations combined with rate equation modelling. The simulated concentration of VO4 corresponds well to the measured concentration. The bulk microdefect density increases with increasing VO4 concentration. The vacancies stored in VOn complexes after RTA are slowly released during further annealing and enhance oxide precipitation.

Structural and electronic properties of hydrogen doped Wurtzite ZnO

We have carried out comprehensive studies on the various types of hydrogen doping including, interstitial and substitutional (for oxygen) position (Hi, HO), HO + Hi, and oxygen vacancy plus interstitial hydrogen (VO + Hi) complexes using DFT + U calculations. The effect of Hi, HO, HO + Hi, and VO + Hi dopants on structural and electronic properties of Wurtzite ZnO at 0.020, 0.0625, and 0.125 nH/nZn concentrations, were studied. We found that cell parameters and volume of the supercells decrease in Hi, HO, and HO + Hi defects, whereas these values increase in the VO + Hi case. Bader analysis shows that H− ion locates at VO+2 in HO doping and it also places beside VO0 in VO + Hi. Furthermore, the HO + Hi complex is a H2 molecule that is almost neutral, electrically inactive and is trapped in VO+2. So, it seems that this complex is an appropriate model for formation of H2 molecule at high temperature. ZnO band gap is slightly influenced by Hi in all concentrations while, HO has prominent effect on increment of it which is consistent with experimental results too. In addition, it is shown that growth of H-doped sample under O-poor condition leads to improved optical properties which is applicable in transparent conductive oxide devices. It is predicted that band gap of hydrogen doped samples will decrease by annealing due to trapping of HO by Hi and formation of H2 molecule. Finally, hydrogen beside oxygen vacancy can create two deep levels, decreasing the optical band gap and shifting absorption edge to lower energy, which leads to a red shift and possible interest in photocatalytic applications.

Influence of Europium Doping on Various Electrical Properties of Low-Temperature Sintered 0.5Ba0.90Ca0.10TiO3-0.5BaTi0.88Zr0.12O3-0.1%CuO-xEu Lead-Free Ceramics

0.5Ba0.90Ca0.10TiO3-0.5BaTi0.88Zr0.12O3-0.1%CuO-xEu (BCT-BZT-Cu-xEu; x = 0–0.90%) lead-free ceramics were sintered at 1220°C with as-synthesized nanoparticles by a modified Pechini method. The structural characteristics and electrical properties of the ceramics that were influenced by varying europium-doping were investigated. All the ceramics featured high densification (relative density: ∼ 96%). X-ray powder diffraction results indicated the samples possessed pure orthorhombic phase. The maximum relative permittivity (e r, 10869) was found at x around 0.30%. Europium ions could dope on different substitution sites in the ABO3 lattice, which evidently influenced electrical properties with various volumes of oxygen vacancy. Moreover, the formation mechanisms of oxygen vacancy and defect electron complexes were stated. The piezoelectric properties were impacted by defect electron complexes, internal stress, ionic electronegativity, etc. The optimal electrical properties, i.e., d 33 = 384 pC/N, Q m = 92, and k p = 0.36, were detected at x = 0.45%.

Effect of germanium doping on the formation kinetics of vacancy-dioxygen complexes in high dose neutron irradiated crystalline silicon

The effect of germanium (Ge) doping on the formation kinetics of vacancy-dioxygen (VO2) complexes in high dose neutron irradiated crystalline silicon (c-Si) has been quantitatively investigated using infrared spectroscopy at 10 K. It is observed that Ge doping of 1019 cm−3 enhances the formation of vacancy-oxygen (VO) complexes by ∼15% during neutron irradiation and slightly suppresses the conversion of VO into VO2 complexes. By studying the generation kinetics of VO2 complexes in the temperature range of 300–345 °C, it is found that the activation energies of VO2 generation are determined to be 1.52 and 1.71 eV in the reference and Ge-doped c-Si, respectively. According to the theory for diffusion limited reactions, it is suggested that Ge doping can retard the VO diffusion in c-Si and therefore reduce the capture probability of Oi for VO complexes. This may be attributed to the temporary trapping of vacancies by Ge atoms. Hence, the formation of VO2 complexes in c-Si is slightly suppressed by Ge doping.

Sensitivity enhanced FTIR investigation of defects introduced by RTA pre-treatment in Czochralski silicon wafers

The investigation of vacancy oxygen complexes in silicon wafers by FTIR is not easy because their concentration is close to the detection limit. In order to enhance the sensitivity of the FTIR measurement we investigated stacked samples of about 1 cm thickness at temperature close to liquid helium temperature. This method was applied to study the absorption bands of defects in as-grown silicon wafers, rapid thermal annealing (RTA) pre-treated wafers, and in RTA pre-treated wafers with subsequent anneals at 800 °C for short periods. We found that the RTA pre-treatment at 1250 °C could not fully annihilate the thermal double donors which were present in the as-grown wafer. By RTA at 1100 °C annihilation was possible. In the wafer pre-treated by RTA at 1250 °C we found the absorption bands of VO4 at 985 cm−1 and 991 cm−1 in the measurements carried out at room temperature and at 6 K, respectively. In this wafer we also detected an unknown band at 1030 cm−1. The VO4 band and the unknown band at 1030 cm−1 disappeared immediately after annealing at 800 °C for 10 min. Instead, the bands at 1096 and 1099 cm−1, both assigned to VO5,6, appeared. These bands are already present in the as-grown sample but their absorption coefficient decreases during RTA at 1100 °C. In samples annealed at 800 °C for 30 min or longer a new absorption band at 1053 cm−1 appears which can be also assigned to VO5,6 complexes.

Carbon effect on the survival of vacancies in Czochralski silicon during rapid thermal anneal

Rapid thermal anneal (RTA) at high temperatures can be employed to introduce vacancies to control oxygen precipitation (OP) behavior in Czochralski (CZ) silicon. Such excessive vacancies survive from the recombination of silicon-interstitials and vacancies (V-I recombination) during the RTA. In this work, we aim to elucidate the carbon effect on the survival of vacancies in CZ silicon during the high temperature RTA by means of gold diffusion in combination with deep-level transient spectroscopy. It is revealed that the existence of ∼1017 cm−3 carbon atoms significantly increases the amount of survival vacancies in the form of vacancy-oxygen (VOm, m ≥ 2) complexes in CZ silicon when subjected to the 1250 °C/60 s RTA. Moreover, such an increase in the number of vacancies becomes more significant with the increase in the cooling rate of RTA. The density functional theory calculations suggest that the V-I recombination is to some extent unfavorable as a carbon atom is close to the silicon-interstitial. Alternatively, it is believed that the substitutional carbon (Cs) atoms tend to trap the silicon-interstitials, thus forming Ci complexes (Cs + I → Ci) in CZ silicon during the RTA. In this context, the V-I recombination is suppressed in a manner, leading to the survival of more vacancies, thus generating more VOm complexes. Furthermore, after the 1250 °C/60 s RTA, the oxide precipitate nucleation based on the VOm complexes is more significant in carbon-rich CZ (CCZ) silicon than in the conventional CZ counterpart. Hence, when subjected to the same OP anneal consisting of the nucleation anneal at 650 or 800 °C for 4 h and the subsequent growth anneal at 1000 °C for 16 h, CCZ silicon possesses a higher density of bulk microdefects and therefore stronger internal gettering capability than CZ silicon. However, the nucleation temperature for OP should be carefully selected as 650 °C for CCZ silicon in order to form an oxide precipitate-free denuded zone.

PL and ESR study for defect centers in 4H-SiC induced by oxygen ion implantation

Radiation damage in 4H-SiC samples implanted by 70 keV oxygen ion beams was studied using photoluminescence and electron spin resonance techniques. ESR peak of g = 2.0053 and two zero-phonon lines were observed with the implanted samples. Combined with theoretical calculations, we found that the main defect in the implanted 4H-SiC samples was oxygen-vacancy complex. The calculated defect formation energies showed that the oxygen-vacancy centers were stable in n-type 4H-SiC. Moreover, the \( {\text{V}}_{\text{Si}} {\text{O}}_{\text{C}}^{ 0} \) and \( {\text{V}}_{\text{Si}} {\text{O}}_{\text{C}}^{ - 1} \) centers were optically addressable. The results suggest promising spin coherence properties for quantum information science.

A negatively charged VSiON center for implementation as qubit

γ-Si3N4 is a nitrogen-based ultra-hard ceramic with Si atoms occupying both tetrahedral and octahedral sites in a spinel structure. Based on first-principles calculations, we investigate spin-polarized electronic structures and energetic stabilities of oxygenvacancy complex center (VSiON) consisting of a substituted oxygen atom and an adjacent tetrahedrally coordinated silicon vacancy in spinel silicon nitride (γ-Si3N4) with different charge states. We find that the negatively charged VSiON-1 center is stable in the p-type γ-Si3N4 and the defect center possesses an S=1 triplet ground state and a spin-conserved excited state with low excitation energy. By using a mean-field approximation, we estimate that the spin coherence time of VSiON is 0.4 s at T=0 K, which indicates that the VSiON-1 center is a promising candidate for spin coherent manipulation and qubit operation.

Thermal expansion and elastic moduli of electrolyte materials for high and intermediate temperature solid oxide fuel cell

The development of thermal stresses in solid oxide fuel cells (SOFCs), and thus their structural stability and reliability, depends directly on the thermal expansion and elastic moduli of the constituent materials. Therefore, it is important to study these properties of SOFC materials. In this study, the thermal expansion and elastic properties of common electrolyte materials, namely yttria stabilized zirconia (YSZ), scandia and ceria stabilized zirconia (SCSZ) and gadolinia doped ceria (GDC), are reported. High temperature X-ray diffraction (HT-XRD) was used to show that the cubic structure of YSZ and GDC samples is stable throughout the temperature range of 30–800°C. However, SCSZ undergoes partial cubic to rhombohedral phase transition at around 300°C but transferred completely back to cubic phase at around 500°C upon heating. The coefficient of thermal expansion (CTE) of electrolyte materials was measured using thermo-mechanical analyzer (TMA). It was found that the CTE of SCSZ is almost identical to that of YSZ, but lower than that of GDC. Elastic properties (Young's and shear moduli) were determined in the 25–900°C temperature range using resonant ultrasound spectroscopy (RUS). Young's and shear moduli of GDC decrease almost linearly with temperature, with an exception of the small anomaly between 100°C and 300°C. However, the variation of elastic moduli with temperature was found to be highly non-linear for YSZ and SCSZ with minimum values measured at around 600°C. The deviation from the linear decrease of elastic moduli with increasing temperature is related to the relaxation of oxygen vacancy complexes and phase transformations.

Evidence for room-temperature in-diffusion of nickel into silicon

Interstitial nickel in crystalline Si is shown to be a fast diffuser at room temperature. In this study, Ni is incorporated in Si by wet chemical etching in nickel-contaminated alkaline solutions. Nickel in-diffusion is observed by means of detecting the electrically active NiVO defect, which is formed due to Ni capture to the vacancy–oxygen complex in electron-irradiated Si. The depth profiles of the NiVO concentration measured by the deep-level transient spectroscopy technique extend to ∼15 μm in the samples doped with Ni at 35 °C for 30 min. This allows us to get a lower estimate for the nickel diffusivity at this temperature as 10−9 cm2/s. The activation energy for electron emission from the NiVO level and the apparent capture cross section are equal to 371 meV and 3 × 10−15 cm2, respectively. The NiVO complex dissociates at 300 °C reestablishing the initial concentration of the VO centers.

Stability of vacancy-oxygen complexes in bulk nickel: Atomistic and ab initio calculations

This work is concerned with the interactions of multiple oxygen atoms with vacancy clusters Vm of size m in bulk nickel. The calculations were carried out using molecular statics, employing a reactive potential developed recently for the NiO system containing vacancies. In particular, segregation of oxygen inside a mono- and a divacancy are considered for the first time, to complete earlier work on the subject. Overall, we find strong attractive interactions between the oxygen atoms and the vacancy clusters. We argue that this leads to increasing dramatically the solubility of oxygen in nickel. Regarding the complexes VmO1, the results indicate that atomic oxygen inside the vacancy clusters is more stable than at the conventional interstitial sites of the perfect Ni system. This stability is further enhanced as the size of the cavity increases. We have found that a monovacancy (V1) and a divacancy (V2) can trap up to 14 and 22 oxygen atoms, respectively. We have also carried out ab initio calculations, for the purposes of both comparison and validation of the potential used in the current work. We find a very good qualitative agreement between the ab initio results and the predictions obtained from the reactive potential. Both agree well with the experimentally observed facts establishing the influence of point defects on the oxidation processes observed in nickel and its alloys.

The effect of millisecond flash lamp annealing on electrical and structural properties of ZnO:Al/Si structures

The effect of millisecond flash lamp annealing (FLA) on aluminum doped ZnO (AZO) films and their interface with Si have been studied. The AZO films were deposited by magnetron sputtering on Si (100) substrates. The electrical and structural properties of the film and AZO/Si structures were characterized by current–voltage, capacitance–voltage, and deep level transient spectroscopy measurements, X-ray diffraction, and secondary ion mass spectrometry. The resistivity of the AZO film is reduced to a close to state-of-the-art value of 2 × 10−4 Ω cm after FLA for 3 ms with an average energy density of 29 J/cm2. In addition, most of the interfacial defects energy levels are simultaneously annealed out, except for one persisting shallow level, tentatively assigned to the vacancy-oxygen complex in Si, which was not affected by FLA. Subsequent to the FLA, the samples were treated in N2 or forming gas (FG) (N2/H2, 90/10%mole) ambient at 200–500 °C. The latter samples maintained the low resistivity achieved after the FLA, but not the former ones. The interfacial defect level persisting after the FLA is removed by the FG treatment, concurrently as another level emerges at ∼0.18 eV below the conduction band. The electrical data of the AZO films are discussed in term of point defects controlling the resistivity, and it is argued that the FLA promotes formation of electrically neutral clusters of Zink vacancies (VZn's) rather than passivating/compensating complexes between the Al donors and VZn's.

Investigation of structural and electrical properties of ZnMn2O4 synthesized by sol–gel method

In this present study, we have reported the synthesis of ZnMn2O4, transitional metal oxide by sol–gel method. The structural and morphological properties are confirmed using various characterization techniques namely XRD, FT-IR and SEM with EDX. Dielectric studies of ZnMn2O4 are measured at the frequency varying from 50 Hz to 5 MHz for the temperature range of 303–573 K. The temperature dependent electrical parameters like impedance and modulus exhibit a strong correlation with the grains, grain boundaries and space charge effects in the synthesized material. Diffusion of oxygen vacancies in the dipoles and defects in the material due to oxygen vacancy complexes are investigated by the activation energy obtained from Arrhenius plot. It was found that the relaxation process was dominated by the hopping mechanism between the Mn3+ and Mn4+. Nyquist plot of impedance was attributed to the existence of space charge interface, grain boundary and grain conduction mechanism of the material.

Vacancy-related defects in n-type Si implanted with a rarefied microbeam of accelerated heavy ions in the MeV range

Deep level transient spectroscopy (DLTS) has been used to study vacancy-related defects formed in bulk n-type Czochralski-grown silicon after implantation of accelerated heavy ions: 6.5MeV O, 10.5MeV Si, 10.5MeV Ge, and 11MeV Er in the single ion regime with fluences from 109cm−2 to 1010cm−2 and a direct comparison made with defects formed in the same material irradiated with 0.7MeV fast neutron fluences up to 1012cm−2. A scanning ion microprobe was used as the ion implantation tool of n-Cz:Si samples prepared as Schottky diodes, while the ion beam induced current (IBIC) technique was utilized for direct ion counting. The single acceptor state of the divacancy V2(−/0) is the most prominent defect state observed in DLTS spectra of n-CZ:Si samples implanted by selected ions and the sample irradiated by neutrons. The complete suppression of the DLTS signal related to the double acceptor state of divacancy, V2(=/−) has been observed in all samples irradiated by ions and neutrons. Moreover, the DLTS peak associated with formation of the vacancy-oxygen complex VO in the neutron irradiated sample was also completely suppressed in DLTS spectra of samples implanted with the raster scanned ion microbeam. The reason for such behaviour is twofold, (i) the local depletion of the carrier concentration in the highly disordered regions, and (ii) the effect of the microprobe-assisted single ion implantation. The activation energy for electron emission for states assigned to the V2(−/0) defect formed in samples implanted by single ions follows the Meyer–Neldel rule. An increase of the activation energy is strongly correlated with increasing ion mass.

Synchrotron VUV-UV and positron lifetime spectroscopy study of vacancy-type defects in reactor neutron-irradiated MgO��nAl2O3 (n = 2)

We investigated neutron-irradiation-induced point defects in spinel single crystals using a synchrotron VUV-UV source and positron lifetime spectroscopy. Photoexcitation (PE) spectra near 230 nm and their corresponding photoluminescence (PL) spectra at 475 nm were attributed to F-centers. With increasing irradiation temperature and fluence, PE efficiency and PL intensity decreased dramatically. Positron lifetimes (PLT) of neutron-irradiated and non-irradiated samples were measured to identify the cation vacancies. A PLT measurement of 250 ps was obtained in a neutron-irradiated (20 K) sample which is tentatively attributed to an aluminum monovacancy. Decreasing PLT with higher irradiation indicates the formation of oxygen-vacancy complex centers.

Low symmetry configurations of vacancy-oxygen complexes in irradiated silicon

Two new electron paramagnetic resonance (EPR) spectra, labeled KU2 and KU3, emerge as a result of annealing of γ-ray irradiated silicon crystals in the temperature range 360–560 °C. The EPR intensities of KU2 and KU3 increase in the upper range of the annealing temperature where the EPR peaks of the vacancy-oxygen complex (A-center) diminish. The angular dependence study shows that the EPR spectra of KU2 and KU3 resemble that of SL1, the excited triplet (spin S = 1) state of A-centers, but with slightly different g and D-tensors representing lowering in symmetries. Microscopic lattice distortions caused by capturing of extra interstitial oxygen atoms by the A-centers are proposed to be the source of lowering of the symmetries associated with KU2 and KU3.

An atomistic vision of the Mass Action Law: Prediction of carbon/oxygen defects in silicon

We introduce an atomistic description of the kinetic Mass Action Law to predict concentrations of defects and complexes. We demonstrate in this paper that this approach accurately predicts carbon/oxygen related defect concentrations in silicon upon annealing. The model requires binding and migration energies of the impurities and complexes, here obtained from density functional theory (DFT) calculations. Vacancy-oxygen complex kinetics are studied as a model system during both isochronal and isothermal annealing. Results are in good agreement with experimental data, confirming the success of the methodology. More importantly, it gives access to the sequence of chain reactions by which oxygen and carbon related complexes are created in silicon. Beside the case of silicon, the understanding of such intricate reactions is a key to develop point defect engineering strategies to control defects and thus semiconductors properties.

Improvement of Luminescence Properties and Particle Growth of AlN Phosphors By Si-Doping

Aluminum Nitride (AlN) has been regarded as a promising material for ceramic phosphors. Recently, we have developed a high efficiency blue AlN phosphor by co-doping with Si and Eu [1-2]. It has been proven that particle have grown rapidly with the highest luminescent intensity on the proper Si doping. Now, it is important to understand the role of Si on AlN particle growth and luminescence enhancement. For this purpose, AlN powders doped with Si from 0 to 4 at. % were prepared using gas-pressure sintering (GPS) furnace at 1950°C in N2 atmosphere. Firstly, particle size, phases and luminescence spectra of Si-doped AlN powder were investigated using SEM, XRD and CL. Secondly, the defect were evaluated by EPR to identify the defects transitions. Finally, the particle growth mechanism was clarified by cross-sectional CL-EDS. From 0.4 to 2.0 % Si, particle growth of AlN powders as well as ultraviolet emissions at 290 and 350 nm are enhanced. The 350 nm emission may be attributed to native AlN defects such as vacancy-oxygen complexes. The shoulder at 290 nm and 450 nm can be ascribed to the different states of Si related defects in AlN by EPR analysis. Cross sectional CL images show that the 1.6 % Si-doped AlN yields 280 nm emission at the surfaces of AlN particles (Fig. 1). Many particles are connected to each other with these bright surfaces. It suggests that the Si-rich surface may act as the binder of AlN particles and enhance the Ostwald growth.

Rapid thermal processing induced vacancy-oxygen complexes in Czochralski-grown Si1−xGex

Rapid thermal processing (RTP) induced vacancy-oxygen (VOm m ≥ 2) complexes can act as the precursors for oxide precipitate nucleation thus enhancing oxygen precipitation (OP) in Czochralski silicon (Cz-Si). In our preceding work, we have reported that 1020 cm−3 germanium (Ge)-doping in Cz-Si facilitates the formation of VOm complexes during the high temperature RTP. Then, how the even higher Ge-doping at 1021 cm−3 in Cz-Si affects the formation of VOm complexes during the RTP is an interesting issue to be addressed. Actually, such high Ge-doping in Cz-Si forms Cz-Si1−xGex alloys. In the present work, we have investigated the formation of VOm complexes in Cz-Si1−xGex (x = 0.035–0.038) wafers subjected to the high temperature RTP. The Cz-Si1−xGex wafers are found to exhibit much weaker OP than Cz-Si counterparts when subjected to the same two-step (nucleation-growth) anneal following the high temperature RTP, indicating that the formation of VOm complexes during the RTP is suppressed in Cz-Si1−xGex wafers. The ‘gold-marker method’ in combination with deep level spectroscopy further confirms the suppressed formation of VOm complexes in Cz-Si1−xGex wafers during the RTP. Besides, the characterization of the oxygen out-diffusion depth profiles in Cz-Si1−xGex and Cz-Si wafers annealed at different temperatures in the range 650–1000 °C indicates that the oxygen diffusivity in Cz-Si1−xGex is a little smaller. The calculations based on density functional theory reveal that in Cz-Si1−xGex the vacancies incline to locate at the first nearest neighboring sites of the Ge atoms and the capture of Oi atoms by the vacancies is less energetically favorable. The aforementioned adverse conditions are believed to be responsible for the suppressed formation of VOm complexes in Cz-Si1−xGex.

Oxygen precipitation in 1020 cm−3 germanium-doped Czochralski silicon

We have investigated the effects of germanium (Ge)-doping at the level of 1020 cm−3 on oxygen precipitation (OP) behaviors in Czochralski (CZ) silicon subjected to different low-high two-step anneals without or with prior high temperature rapid thermal processing (RTP). It is found that Ge-doping remarkably suppresses OP in CZ silicon without prior RTP. However, Ge-doping significantly enhances OP in CZ silicon with prior RTP. The suppressed OP in the case of the absence of prior RTP is primarily due to the fact that the 1020 cm−3 Ge-doping introduces compressive strain into silicon crystal lattice, which increases the critical size of oxygen precipitate nuclei for a given nucleation temperature. Moreover, it is revealed that the 1020 cm−3 Ge-doping facilitates the formation of vacancy-oxygen (V-O) complexes and may introduce Ge-V-O complexes in CZ silicon during high temperature RTP. More vacancy-related complexes in CZ silicon not only reduce the critical size of oxygen precipitate nuclei but also provide more precursors for oxygen precipitate nucleation. Therefore, the 1020 cm−3 Ge-doping enhances OP in CZ silicon subjected to the two-step anneals following high temperature RTP.