Verification of a Distortion in the Microstructure of GaN Detected by EXAFS Using Ab Initio Density Functional Theory Calculations

X-ray absorption fine structure (XAFS) measurements on a series of epitaxially grown GaN samples have shown a distortion in the microstructure of GaN. More specifically the central N atom is 4-fold coordinated but the four Ga atoms are not equidistant. It has been shown that 2.9 to 3.5 of them (depending on the growth conditions) are found in the expected from XRD distance of 1.94 A and the remaining are at a distance longer by approximately 15%. Second derivative calculation of the conformation energy using the Density Functional Theory (DFT) is used to investigate if the symmetric GaN cluster as given by XRD is the most energetically favorable configuration and if not which distorted structure corresponds to the most energetically favorable one. A very good agreement between DFT results and experimental XAFS spectra has been found. Generalization this technique to other dislocated clusters is also discussed

Dose - dependent bonding environment of oxygen implanted in GaN

The bonding environment of oxygen implanted in GaN is studied using Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The implantation of 70 keV O ions in GaN results in the formation of a 200 nm \u2013 thick subsurface layer that is highly defective or amorphous depending on the implantation fluence which ranges from 1 1015 to 1 1017 cm2. The NEXAFS spectra are simulated using the FEFF8 code assuming models that account for the formation of point defects (various configurations of O interstitial and O substitutional in N and Ga sites) as well as chemical effects such as the formation of various polymorphs of Ga oxides and oxynitrides. The implantation-induced lattice disorder is modeled by displacing atoms from their equilibrium positions by adding to their Cartesian coordinates random numbers that belong to normal distributions. The simulations reveal that when the fluence is 1 1015 cm2, the O implants occupy interstitial sites preferentially in the empty channels aligned parallel to the c-axis in the plane that contains the Ga atoms and/or in the columns that consist of Ga and N atoms along the c-axis. When the fluence is equal to 1 1016 cm2 the O ions substitute for N while at 1 1017 cm2 they participate in the formation of mixed GaOxNy phases

Local bonding geometry of oxygen implanted in GAN: a depth \u2013 dependent study

The bonding environment of oxygen implanted in GaN is studied using near edge X-ray absorption fine structure spectroscopy at the O-K-edge. The 70 keV oxygen ions form a 200 nm-thick subsurface layer that is highly defective or amorphous depending on the implantation fluence which ranges from 1 71015 cm−2 to 1 71017 cm−2. The information depth of the fluorescence photons varies from 50 to 63 nm, depending on the angle of incidence. The spectra are simulated using the FEFF8 code and assuming various models, e.g., O interstitial, O substitutional in N sites, Ga and N vacancies, and various polymorphs of Ga2O3. The lattice disorder is modelled by displacing atoms from their equilibrium positions by adding to their Cartesian coordinates random numbers that belong to normal distributions. The simulation results reveal that at the low fluence limit, the O atoms occupy interstitial sites preferentially in the empty channels aligned to the c-axis and in the atomic planes containing the Ga atoms. When the fluence is equal to 1 71016 cm−2 the O atoms substitute N while at 1 71017 cm−2 they form mixed GaOxNy phases with the N/O ratio decreasing with increasing depth, i.e., as we approach the peak of the implanted O profile

Local bonding geometry of oxygen implanted in GAN: a depth – dependent study

The bonding environment of oxygen implanted in GaN is studied using near edge X-ray absorption fine structure spectroscopy at the O-K-edge. The 70 keV oxygen ions form a 200 nm-thick subsurface layer that is highly defective or amorphous depending on the implantation fluence which ranges from 1×1015 cm−2 to 1×1017 cm−2. The information depth of the fluorescence photons varies from 50 to 63 nm, depending on the angle of incidence. The spectra are simulated using the FEFF8 code and assuming various models, e.g., O interstitial, O substitutional in N sites, Ga and N vacancies, and various polymorphs of Ga2O3. The lattice disorder is modelled by displacing atoms from their equilibrium positions by adding to their Cartesian coordinates random numbers that belong to normal distributions. The simulation results reveal that at the low fluence limit, the O atoms occupy interstitial sites preferentially in the empty channels aligned to the c-axis and in the atomic planes containing the Ga atoms. When the fluence is equal to 1×1016 cm−2 the O atoms substitute N while at 1×1017 cm−2 they form mixed GaOxNy phases with the N/O ratio decreasing with increasing depth, i.e., as we approach the peak of the implanted O profile

Study of the modification of the microstructure of GaN after high \u2013 dose Si implantation

Rare earth oxides are among the materials which are presently studied as possible replacements of amorphous silicon dioxide as gate insulators in nanometric Si devices; in fact, they generally exhibit high values of the dielectric constant (\u201chigh-k\u201d), a necessary requirement to obtain a high capacitance with layer thickness greater than the value below which tunnelling currents become unacceptably high. Lu2O3 is one of the rare earth oxides which may have the required properties in view of its quite high values of  and forbidden band gap. Since the envisaged dielectric layers are only a few nm thick a description and a physical understanding of the atomic and electronic structure of the interface are of great importance. In this paper, we report a study by synchrotron radiation photoemission and transmission electron microscopy of the growth of Lu2O3 on Si(001). We provide evidence of a rather complex structure in which all silicon suboxides and SiO2 are present at the same time, along with a silicate \u2013 like phase and Lu2O3 itself; moreover, both crystalline and amorphous portions are present. The valence band discontinuity is found to be 3.16 \ub1 0.16 eV. These findings are discussed in the context of available theoretical predictions of thermodynamic stability versus the formation of silicon oxide, silicates and silicides and of the band discontinuity problem

X-ray absorption fine structure study of In implanted GaN: Effect of annealing

GaN implanted with 700 keV In ions with fluence 5×1015 cm−2 is studied using X-ray absorption fine structure (XAFS) spectroscopies at the N and Ga K edges. Rutherford backscattering (RBS) reveals that implantation renders the top 200nm of the GaN film amorphous while the underlying 250nm of the film are highly defective. The increase of the static disorder due to the implantation induced lattice damage is evident both in the N K edge NEXAFS and the Ga K edge EXAFS spectra. Indium implantation also induces the formation of N2 evinced by the evolution of a sharp resonance line (RL) that corresponds to 1s→* transitions of molecular nitrogen. TheN2 molecules dissociate after annealing at temperatures higher than 800 ◦C. The bonding environment of Ga is also affected by the implantation and annealing: a marginal increase of the Ga–Ga distance is observed due to the incorporation of In atomsand/or formation of defects. The coordination number of the second nearest neighboring (NN) shell is reduced by more than 50% after the implantation but recovers after annealing at 800 ◦C. Further increase of the annealing temperature causes nitrogen loss as it is deduced by the reduction of the coordination number in the first NN shell by about 20%

Probing the structural role of Cr in stabilized tannery wastes with X ray absorption fine structure spectroscopy

The effective stabilization of tannery sludge wastes is explored using X Ray Absorption Fine Structure XAFS spectroscopies. Solidification of the Cr rich waste was realized via vitrification of the incinerated sludge with silica and flux agents. It is demonstrated that the effective reduction of Cr VI and the structural role of Cr are strongly modulated by the chemical composition of the waste. Eskolaite microcrystallites are embedded in the silica matrix of all vitrified samples and the extent of microcrystalline formation is strongly related to the glass basicity. Both Cr VI and Cr III species are identified, corresponding to Cr VI O4 glass formers and Cr III O6 network modifiers. The toxic Cr VI prevails only in the glasses with the highest basicity index and lowest waste content, nevertheless it is safely incorporated and immobilized in the silica matrix. However, the detected abundance of Cr VI increases glass basicity and as a result, glass polymerization is hindered. Thermal treatment, a process that leads to glass ceramics transforms almost all Cr VI to Cr III , while eskolaite formation is promoted concurrently. Nevertheless, microcrystalline growth proceeds mainly via depletion of Cr III from the silica matrix and not from the reduced Cr VI ; yet, Cr removal from the glass matrix does not impair the chemical stability of the devitrified product