Atomic scale characterization of ion-induced amorphization of GaAs and InAs using perturbed angular correlation spectroscopy

The perturbed angular correlation technique has been utilized to understand the production and nature of the implantation induced crystalline to amorphous transformation in GaAs and InAs. This technique, which is based upon the nuclear hyperfine interaction of the electric-quadrupole moment of the probe nucleus with the electric field gradient from extra nuclear charges, requires introduction of radioactive probe nuclei in host material. The radioactive probes ¹¹¹In/¹¹¹Cd were produced with the 14UD heavy-ion accelerator via nuclear reaction that recoil implants the ¹¹¹In nuclei deep into single crystals of GaAs (1 0 0) and InAs (1 0 0). After removal of radiation damage, caused by recoil implantation, single crystals of GaAs (1 0 0) and InAs (1 0 0) were implanted with stable ⁷⁴Ge ions (MeV) over a wide dose range at liquid nitrogen temperature. The irradiated samples were investigated with respect to the damage production. The crystalline, disordered and amorphous probe environments were identified from the measurement. The evolution of damage is described within the framework of different amorphization models. In GaAs, amorphization is obtained by direct-impact amorphization and by the growth of amorphous zones due to defect-stimulation at crystalline/amorphous interface. In InAs, the amorphization is first initiated by accumulation of simple point defects and then direct-impact/defect-stimulated mechanism contributes to further stimulate the transformation.     

Ion beam studies of InAs/GaAs quantum dots after annealing

The microstructure changes of self-assembled InAs/GaAs quantum dots during RTA treatment was investigated using ion channeling and photoluminescence (PL). A small blueshift of the PL emission is observed for annealing temperatures of 650-800 °C and an obvious blueshift at 850 °C. The yield of channeled spectra decreased as annealing temperature was increased, but the yield increased while temperature above 800 °C in RTA. These results imply the strain of QD varied during RTA treatments. In addition, the As/Ga atomic ratio near the surface was determined from the surface peaks of the channeled spectrum.     

Monte Carlo calculation of energy loss of hydrogen and helium ions transmitted under channelling conditions in silicon single crystal

In this work, we report on calculations of the electronic channelling energy loss of hydrogen and helium ions along Si〈1 0 0〉 and Si〈1 1 0〉 axial directions for the low energy range by using the Monte Carlo simulation code. Simulated and experimental data are compared for protons and He ions in the 〈1 0 0〉 and 〈1 1 0〉 axis of silicon. A reasonable agreement was found.Computer simulation was also employed to study the angular dependence of energy loss for 0.5, 0.8, 1, and 2 MeV channelled ⁴He ions transmitted through a silicon crystal of 3 μm thickness along the 〈1 0 0〉 axis.     

Simulating radiation damage in δ-plutonium

Radiation events in δ-Pu (fcc) have been simulated in an attempt to understand the fundamental mechanisms that contribute to the Pu ageing process. The Pu interactions are modelled using a potential based on the modified embedded atom method (MEAM). The energetics of point defects have been investigated using static calculations together with molecular dynamics (MD) to simulate radiation events. All MD simulations were carried out with Pu initially in the face-centred-cubic (fcc) structure, although this is not the lowest energy configuration for the pure metal.The point defect study suggests that the mono-vacancy has the lowest formation energy (0.46 eV), with interstitial defects favouring the - split orientation over occupation of the native fcc octahedral site. Displacement threshold energy calculations at room temperature give a minimum value of between 5 and 6 eV, increasing to 8-14 eV along the major crystallographic directions.Low energy collision cascades, initiated with energies in the range of 0.4-1 keV, show that the cascades form in a similar manner to other fcc metals with a vacancy rich zone at the cascade core, surrounded by isolated interstitial defects. Higher energy cascades show similar features but with occasional channelling of energetic atoms and sub-cascade branching which significantly reduces defect production. A common trait observed across all the cascades was the relatively slow annealing period, compared to cascades in other fcc metals, with simulations at energies above 5 keV requiring many 10’s of picoseconds before the ballistic phase was completed.     

Radiation damage in ZnO ion implanted at 15 K

Commercial O-face (0 0 0 1) ZnO single crystals were implanted with 200 keV Ar ions. The ion fluences applied cover a wide range from 5 × 10¹¹ to 7 × 10¹⁶ cm⁻². The implantation and the subsequent damage analysis by Rutherford backscattering spectrometry (RBS) in channelling geometry were performed in a special target chamber at 15 K without changing the target temperature of the sample. To analyse the measured channelling spectra the computer code DICADA was used to calculate the relative concentration of displaced lattice atoms.Four stages of the damage evolution can be identified. At low ion fluences up to about 2 × 10¹³ cm⁻² the defect concentration increases nearly linearly with rising fluence (stage I). There are strong indications that only point defects are produced, the absolute concentration of which is reasonably given by SRIM calculations using displacement energies of E_d(Zn) = 65 eV and E_d(O) = 50 eV. In a second stage the defect concentration remains almost constant at a value of about 0.02, which can be interpreted by a balance between production and recombination of point defects. For ion fluences around 5 × 10¹⁵ cm⁻² a second significant increase of the defect concentration is observed (stage III). Within stage IV at fluences above 10¹⁶ cm⁻² the defect concentration tends again to saturate at a level of about 0.5 which is well below amorphisation. Within stages III and IV the damage formation is strongly governed by the implanted ions and it is appropriate to conclude that the damage consists of a mixture of point defects and dislocation loops.     

Formation of InAs nanocrystals in Si by high-fluence ion implantation

We have studied the formation of InAs precipitates with dimensions of several nanometers in silicon by means of As (245 keV, 5 × 10¹⁶ cm⁻²) and In (350 keV, 4.5 × 10¹⁶ cm⁻²) implantation at 500 °C and subsequent annealing at 900 °C for 45 min. RBS, SIMS, TEM/TED, RS and PL techniques were used to characterize the implanted layers. The surface density of the precipitates has been found to be about 1.2 × 10¹¹ cm⁻². Most of the crystallites are from 3 nm to 6 nm large. A band at 1.3 μm has been registered in the low-temperature PL spectra of (As + In) implanted and annealed silicon crystals. The PL band position follows the quantum confinement model for InAs.     

Studies of InP nano dots formation after keV Ar irradiation

Temporal evolution of nano dots fabricated, in off-normal geometry but in the absence of rotation, on InP(1 1 1) surfaces by 3 keV Ar ion sputtering is reported here. After 10 min of sputtering, self-assembled nano dots with mean diameter of 24 ± 4 nm display square short range weak ordering. Fully developed square celled arrays of dots with mean diameter of 90 ± 26 nm, are seen beyond the non-linear coarsening regime at the critical time of 40 min. Inverse coarsening of dots in conjunction with surface smoothening, never seen in earlier studies of dot evolution, is observed beyond the critical time.     

Dynamic dependence of the interaction potentials for grazing scattering of fast atoms from metal and insulator surfaces

For scattering of fast atoms from metal and insulator surfaces under axial channeling conditions pronounced peaks in the angular distributions of scattered projectiles are interpreted in terms of rainbow scattering. The angular position of such “rainbow peaks” are closely related to the interaction potential and its corrugation in the topmost surface region. We have scattered N and O atoms, with energies ranging from 10 to 70 keV, from clean and flat Al(0 0 1) and LiF(0 0 1) surfaces along low index axial directions in the surface plane and studied the positions of the rainbow peaks as function of the kinetic energy of the atomic projectiles normal to the surface. For the insulator surface the rainbow angle does not depend on projectile energy for constant normal energy, whereas for the metal surface we find pronounced dynamic effects. We interpret this different behaviour as arising from a projectile energy dependent contribution to the underlying interaction potentials owing to embedding the projectiles into the free electron gas in the selvedge of the surfaces, which is present for the metals but absent for insulators.     

Positron annihilation characteristics of ODS and non-ODS EUROFER isochronally annealed

Yttrium oxide dispersion strengthened (ODS) and non-ODS EUROFER produced by mechanical alloying and hot isostatic pressing have been subjected to isochronal annealing up to 1523 K, and the evolution of the open-volume defects and their thermal stability have been investigated using positron lifetime and coincidence Doppler broadening (CDB) techniques. Transmission electron microscopy (TEM) observations have also been performed on the studied samples to verify the characteristics of the surviving defects after annealing at 1523 K. The CDB spectra of ODS EUROFER exhibit a characteristic signature that is attributed to positron annihilation in Ar-decorated cavities at the oxide particle/matrix interfaces. The variation of the positron annihilation parameters with the annealing temperature shows three stages: up to 623 K, between 823 and 1323 K, and above 1323 K. Three-dimensional vacancy clusters, or voids, are detected in either materials in as-HIPed condition and after annealing at T ? 623 K. In the temperature range 823-1323 K, these voids’ growth and nucleation and the growth of other new species of voids take place. Above 1323 K, some unstable cavities start to anneal out, and cavities associated to oxide particles and other small precipitates survive to annealing at 1523 K. The TEM observations and the positron annihilation results indicate that these cavities should be decorated with Ar atoms absorbed during the mechanical alloying process.     

Grain boundary influence on displacement cascades in UO2: A molecular dynamics study

The influence of grain boundaries on the primary damage state created by a recoil nucleus in UO₂ matrix is studied here by molecular dynamics simulations. This study is divided in two steps: (1) the study of the structural properties of several symmetrical tilt boundaries for different misorientation angles ranging from 12.7° to 61.9°; and (2) the study of displacement cascades near these grain boundaries. For all the grain boundaries studied, the structure around the interface up to about 2 nm presents a perturbed but stable fluorite lattice. The type of defect at the interface depends directly on the value of the misorientation angles. For the small angles (12.7° and 16.3°) the interface defects correspond to edge dislocations. For higher misorientation angles, a gap of about 0.3 nm exists between the two halves of the bicrystal. This gap is composed of Schottky defects involving numerous vacancies along the interface. About 10 keV displacement cascades were initiated with an uranium projectile close to the interface. In all the cases, numerous point defects are created in the grain boundary core, and the mobility of these defects increases. However, cascade morphologies depend strongly on the grain boundary structure. For grain boundaries with edge dislocations, the evolution of the displacement cascades is similar to those carried out in monocrystals. On the other hand, cascades initiated in grain boundaries with vacancy layer defects present an asymmetry on the number of displaced atoms and the number of point defects created.     

Study on C-W interactions by molecular dynamics simulations

By means of molecular dynamics simulations using bond-order potential (BOP), we have investigated the interactions between carbon (C) atoms and bcc tungsten (W). At finite temperature (T = 300 K) with incident energy of C atoms ranging from 0.5 to 100 eV at normal incidence, the projected range distribution as a function of incident energy and the average depth have been depicted. The properties of vacancy, vacancy migration, interstitial and substitutional C atoms in W have been determined. The most stable configuration for an interstitial C atom in W is in octahedral position and the lattice distortion around the C atom in octahedral interstitial configuration occurs along 〈1 0 0〉 and 〈1 1 0〉 directions. The mutual interaction between a vacancy and near interstitial C atom is also studied.     

Ion implantation in 4H-SiC

Silicon carbide offers unique applications as a wide bandgap semiconductor. This paper reviews various aspects of ion implantation in 4H-SiC studied with a view to optimise ion implantation in silicon carbide. Al, P and Si ions with keV energies were used. Channelling effects were studied in both a-axis and c-axis crystals as a function of tilts along major orthogonal planes and off the major orthogonal planes. Major axes such as [0 0 0 1] and the and minor axis like the showed long channelling tails and optimum tilts for minimising channelling are recommended. TEM analyses of the samples showed the formation of (0 0 0 1) prismatic loops and the loops as well,in both a and c-cut crystals. We also note the presence of voids only in P implanted samples implanted with amorphising doses. The competing process between damage accumulation and dynamic annealing was studied by determining the critical temperature for the transition between crystalline and amorphous SiC and an activation energy of 1.3 eV is extracted.     

Quantum mechanical calculations of uranium phases and niobium defects in γ-uranium

Depleted uranium (U) from fuel enrichment processes has a variety of applications due to its high density. With the addition of a small concentration of niobium (Nb), U becomes stainless. Nb is fully miscible with the high-temperature γ phase of U and tends to segregate upon cooling below 1050 K. The starting point of segregation is the configuration of Nb substitutional or interstitial defects. Using quantum mechanical calculations, the authors find that the formation energy of a single vacancy is 1.08 eV, that of Nb substitution 0.59 eV, that of Nb interstitial at octahedral site 1.58 eV, and that of Nb interstitial at tetrahedral site 2.35 eV in the dilute limit of isolated defects; all with reference to a reservoir of the pure γ phase U and pure Nb. The analysis of electronic structures reveals the correlation of formation energies of Nb defects with the local perturbations of electron distribution. Higher formation energy of Nb defects correlates with larger perturbation. Based on this study, Nb atoms thermodynamically prefer to occupy substitutional sites in the γ phase U.     

Effects of MeV Si ions bombardment on the thermoelectric generator from SiO2/SiO2 + Cu and SiO2/SiO2 + Au nanolayered multilayer films

The defects and disorder in the thin films caused by MeV ions bombardment and the grain boundaries of these nanoscale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We prepared the thermoelectric generator devices from 100 alternating layers of SiO₂/SiO₂ + Cu multi-nano layered superlattice films at the total thickness of 382 nm and 50 alternating layers of SiO₂/SiO₂ + Au multi-nano layered superlattice films at the total thickness of 147 nm using the physical vapor deposition (PVD). Rutherford Backscattering Spectrometry (RBS) and RUMP simulation have been used to determine the stoichiometry of the elements of SiO₂, Cu and Au in the multilayer films and the thickness of the grown multi-layer films. The 5 MeV Si ions bombardments have been performed using the AAMU-Center for Irradiation of Materials (CIM) Pelletron ion beam accelerator to make quantum (nano) dots and/or quantum (quantum) clusters in the multilayered superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after Si ion bombardments we have measured Seebeck coefficient, cross-plane electrical conductivity, and thermal conductivity in the cross-plane geometry for different fluences.     

Classical and quantum mechanical rainbow-scattering of fast He atoms from a KCl(0 0 1) surface

Fast He atoms with energies from 200 eV up to 16 keV are scattered under grazing polar angles of incidence from a flat and clean KCl(0 0 1) surface. For scattering along low-index directions (axial surface channeling) we observe pronounced peaks in the angular distributions of scattered projectiles which can be attributed to rainbow scattering. From classical and semiclassical trajectory calculations based on individual Hartree-Fock pair and density functional theory (DFT) potentials, we obtain corresponding rainbow angles for comparison with the experimental data. The calculations were performed taking into account the rumpling of K and Cl in the topmost surface layer. Fair agreement with the experimental data is found for scattering along 〈1 0 0〉 for DFT as well as individual pair potentials calculated from Hartree-Fock wave functions.     

Angular distribution of GaAs sputtered under oblique Cs bombardment

The angular distribution of Ga and As sputtered from Gallium Arsenide (1 0 0) by a Cs⁺ ion beam was experimentally measured through a collector technique allowing modifications of the energy and incidence angle of the ion beam. The impact energy was varied in the range of 2-10 keV and the angle of incidence from 30° to 60°.The angular distributions of emitted matter are determined by means of SIMS depth profiles. Our series of experiments show an evolution of the preferential direction of emission as well as the spreading around this direction in function of the characteristics of the ion beam.The second objective is the study of the evolution of the stoichiometry of the deposit in function of the emission angle. A decrease of the As/Ga ratio around the preferential direction of emission and an increase of this ratio for oblique emission are observed for different conditions of primary bombardment. Considering that the angular distribution depends on the depth of origin, our results suggest that the Cs⁺ bombardment changes the stoichiometry of the near-surface layers of the sample with an enrichment of As in the outmost layers while the sub-surface region is impoverished in As due to preferential sputtering.     

Self-diffusion near symmetrical tilt grain boundaries in UO2 matrix: A molecular dynamics simulation study

Molecular dynamics simulations have been carried out to study the influence of grain boundaries in stoichiometric UO₂ on uranium and oxygen self-diffusions over a large range of temperature varying from 300 K to 2100 K. The study was carried out on two symmetrical tilt grain boundaries, Σ5 and Σ41, which have respectively two different atomic structures. Firstly, the study of the temperature effect on the grain boundary core structure is presented. With the raise of temperature, the grain boundary core grows with an increase of disorder. Secondly, self-diffusion near both grain boundaries is studied. It has been found that grain boundaries accelerate the uranium and oxygen self-diffusion rates over several nanometres from the grain boundary interface. Uranium and oxygen self-diffusion are anisotropic, with a high acceleration along the grain boundary interface. Using the self-Van Hove correlation functions, hopping mechanisms were identified for Σ41 in all directions while for Σ5 hopping mechanism takes place along the grain boundary interface and random diffusion appears in the perpendicular direction of the grain boundary plane.     

Synthesis of CdS nanocrystals by electron beam irradiation of a Cd-mercaptopropionic acid aqueous solution

We investigated a simple route for synthesizing CdS nanocrystals (NCs) via irradiation of a CdCl₂ + mercaptopropionic acid (MPA) aqueous solution with a 10-MeV electron beam without using any additional sulfur precursors or stabilizers. When the concentration of the MPA was fixed to 2.5 mM, the morphology and optical properties of the NCs were changed by varying the CdCl₂ concentration. Based on transmission electron microscopy (TEM) imaging, a 0.4 mM concentration of CdCl₂ (the-MPA rich case) created only spherical quantum dots (QDs) with an average diameter of 6.6 nm. When the concentration of CdCl₂ was increased to 5 mM (the cadmium-rich case), nanorods with an average diameter of 5.3 nm and an average aspect ratio of 8.2 were formed along with QDs with diameter of 4.9 nm. The crystalline structures of the NCs were verified to be hexagonal.     

Energy dependence relations for positron channeling critical angle and dechanneling probability due to stacking faults

Energy dependence studies of channeling critical angle and dechanneling probabilities are essential for defect analysis. Present study reveals the fact that the well known energy dependence relations are not always applicable, particularly for the case of light relativistic particles. Here the energy dependence for positron channeling angular scans and dechanneling probability due to stacking faults is obtained using quantum mechanical models. The channeling critical angle calculated using this method (∼1.1° for 1 MeV e⁺ in Au along (1 1 1) planes) is consistent with the experimental value measured by Andresen et al. [Phys. Rev. B 3 (1971) 705]. New energy dependence relations derived in this study are found to be consistent with standard formulae as and when the system enters into classical regime.     

Interaction of 〈1 0 0〉 and ½〈1 1 1〉 dislocation loops with point defects in ferritic alloys

The interaction between dislocation loops of interstitial nature with ½〈1 1 1〉 and 〈1 0 0〉 Burgers vectors and point defects in Fe has been studied molecular dynamics. Comparative calculations have been carried out using two interatomic potentials for pure Fe ([G.J. Ackland, M.I. Mendelev, D.J. Srolovitz, S. Han, A.V. Barashev, J. Phys.: Condens. Mater. 16 (2004) 1; S. Dudarev, P. Derlet, J. Phys.: Condens. Mater. 17 (2005) 7097]). The results of this study are range and energy of the interaction as functions of size and mutual position of defects. The applied potentials predict somewhat different strain field structure for 〈1 0 0〉 loops and therefore different lengths of interaction. However, both potentials suggest that, contrary to common belief, the distance of cluster-defect interaction within the glide prism of a ½〈1 1 1〉 cluster is significantly longer than that of a 〈1 0 0〉 cluster of similar size, in spite of the longer Burgers vector in the latter case.