Porous structure formation in ion irradiated germanium Steinbach, T.; Wesch, W.
Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms,
01/2014, Letnik:
319
Journal Article
Recenzirano
The ion beam induced modification of amorphous germanium is characterised by the formation of voids close to the sample surface and the transformation into a sponge-like porous surface layer at high ...ion fluences. This extreme structural modification of the sample surface is independent of the (heavy) ion species used and accompanied by a strong volume expansion. Nevertheless, recently it was demonstrated that buried voids (and buried sponge-like layers) can be formed in the depth of the projected ion range, however, only for the irradiation with I-ions at high ion fluences. Thus, the ion species and their chemical properties seem to play an important role in the structural modification around the projected ion range. In this paper we investigate the influence of the ion species on the ion beam induced void formation in Ge for room temperature irradiation with 380keV I- and Au-ions as a function of the ion fluence. Independent of the ion species, a strong volume expansion is observed caused by void formation and the transformation into a sponge-like porous surface layer. For both ion species used, the final porous layers are structurally identical as established by cross section and plan view electron microscopy investigations. Further ion irradiation of the sponge-like porous structure, however, leads to significant differences in the ion beam induced structural evolution. For the Au-ion irradiation the porous layer remains nearly unchanged, whereas for the irradiation with I-ions a transformation from sponge-like to netlike porous layers occurs which is accompanied again by an extreme volume expansion. The underlying mechanism will be discussed based on chemical properties of the implanted ions.
The unstable N=42 nucleus 72Zn has been studied using multiple safe Coulomb excitation in inverse kinematics. The experiment was performed at the REX-ISOLDE facility at CERN making first use of the ...silicon detector array C-REX in combination with the γ-ray spectrometer Miniball. The high angular coverage of C-REX allowed to determine the reduced transition strengths for the decay of the yrast 01+, 21+ and 41+ as well as of the 02+ and 22+ states in 72Zn. The quadrupole moments of the 21+, 41+ and 22+ states were extracted. Using model independent quadrupole invariants, the ground state of 72Zn was found to have an average deformation in the γ degree of freedom close to maximum triaxiality. In comparison to experimental data in zinc isotopes with N<40, the collectivity of the 41+ state in neutron-rich 72Zn is significantly larger, indicating a collective yrast band based on the ground state of 72Zn. In contrast, a low experimental B(E2;02+→21+) strength was determined, indicating a different structure for the 02+ state. Shell-model calculations propose a 02+ state featuring a larger fraction of the (spherical) N=40 closed-shell configuration in its wave function than for the 01+ ground state.
The results were also compared with beyond mean field calculations which corroborate the large deformation in the γ degree of freedom, while pointing to a more deformed 02+ state. These experimental and theoretical findings establish the importance of the γ degree of freedom in the ground state of 72Zn, located between the 68,70Ni nuclei that have spherical ground states, and 76Ge, which has a rigid triaxial shape.
Heavy-ion irradiation of crystalline germanium (c-Ge) results in the formation of a homogeneous amorphous germanium (a-Ge) layer at the surface. This a-Ge layer undergoes structural modification such ...as a strong volume expansion accompanied by drastic surface blackening with further ion irradiation. In the present paper we investigate the mechanism of this ion-induced structural modification in a-Ge basically for the irradiation with I ions (3 and 9 MeV) at room and low temperature as a function of ion fluence for the ion incidence angles of {Theta}=7 deg. and {Theta}=45 deg. For comparison, Ag- and Au-ion irradiations were performed at room temperature as a function of the ion fluence. At fluences two orders of magnitude above the amorphization threshold, morphological changes were observed for all irradiation conditions used. Over a wide range of ion fluences we demonstrate that the volume expansion is caused by the formation of voids at the surface and in the depth of the projected ion range. At high ion fluences the amorphous layer transforms into a porous structure as established by cross section and plan view electron microscopy investigations. However, the formation depth of the surface and buried voids as well as the shape and the dimension of the final porous structure depend on the ion fluence, ion species, and irradiation temperature and will be discussed in detail. The rate of the volume expansion (i.e., porous layer formation) depends linearly on the value of {epsilon}{sub n}. This clearly demonstrates that the structural changes are determined solely by the nuclear energy deposited within the amorphous phase. In addition, at high ion fluences all perpendicular ion irradiations lead to a formation of a microstructure at the surface, whereas for nonperpendicular ion irradiations a nonsaturating irreversible plastic deformation (ion hammering) without a microstructure formation is observed. For the irradiation with ion energies of several MeV, the effect of plastic deformation shows a linear dependence on the ion fluence. Based on these results, we provide an explanation for the differences in surface morphology observed for different angles of incidence of the ion beam will be discussed in detail.
The dominating modification of crystalline solids by energetic ions is the formation of lattice defects, which accumulate with ongoing irradiation. Many materials exhibit a phase transition from ...crystalline to the amorphous state at higher ion fluence. However, this ion-beam-induced structural modification involves the formation of mechanical stress, which is generally disadvantageous for the successful application of ion irradiation in the micro-device technology. Hence, a fundamental understanding of the ion-beam-induced stress evolution is crucial for the effective use of ion beam technology. Lithium niobate (LiNbO3) is a promising candidate for the application of integrated photonic structures. However, for the fabrication of such structures in LiNbO3 ion irradiation is indispensable. In order to get a fundamental and comprehensive understanding of the ion-beam-induced defect and stress evolution in LiNbO3, irradiations with varying parameters (ion energy and irradiation temperature) over a wide range of ion fluence for different crystallographic orientations were performed. The ion-beam-induced defect and stress evolution were studied by means of in situ Rutherford backscattering spectrometry and laser reflection technique, respectively. The investigations demonstrate that ion-beam-induced defect and stress evolution in LiNbO3 is highly anisotropic. Moreover, a complex stress evolution is observed, i.e. with increasing ion fluence different stress formation and stress relaxation processes occur. It will be demonstrated that effects such as radiation-induced viscosity or anisotropic deformation that were suggested by previous models cannot explain the stress evolution in LiNbO3. This work presents a new approach that describes the anisotropic stress and defect evolution in LiNbO3 by a complex defect formation mechanism, i.e. the presence of different defect types and their transformation into each other. Each defect type strains the surrounding crystal matrix and leads to a macroscopic deformation. The total stress is the superposition of the individual stress caused by different defect types.
In this work we present the first time to our knowledge a large area Si-energy filter for ion implantation based on a 150 mm SOI Wafer Flow Process. The filter consists of a microstructured 125 mm ...diameter Si-membrane with 6.5 μm thickness supported by the remaining silicon wafer ring. These filters are of large importance in the cost effective doping of SiC for high power device fabrication. We will present in this work the Si-energy filter fabrication process, characterization results of single process steps and SIMS depth profiles after ion implantation which are in good accordance to the calculated simulation model. Additionally the impact of defects on the substrate is discussed in this work and mandatory steps are described to reduce the amount of holes in the finished membrane.
The influence of high electronic energy deposition in cadmium telluride (CdTe) was investigated using Rutherford backscattering spectrometry in channelling configuration as well as high-resolution ...transmission electron microscopy. Swift heavy ion irradiation was performed at room temperature with 185 MeV Au ions at perpendicular and non-perpendicular ion incidence. Independent of the ion incidence angle, neither ion beam induced point defects nor amorphous ion tracks were observed along the ion path. In contrast, the irradiated layer possesses a high crystalline quality even after irradiation with high ion fluences, i.e. multiple ion overlap. Nevertheless, irradiation with swift heavy ions leads to the formation of extended defects in a thin layer close to the sample surface. With increasing ion fluence the concentration of these extended defects increases continuously. This suggests that high electronic energy deposition causes the formation of defects, however, the combination of the high defect mobility within the thermal spike and the high ionicity of CdTe may benefit an effective recovery of the ionic bonds and consequently an easy recovery of the lattice, i.e. a nearly perfect recrystallization. The experimentally observed high defect resistivity enables a high doping of foreign atoms in CdTe over a wide depth range without the formation of lattice defects which is important for the use of CdTe as an effective absorber for solar cells.
Double-sided silicon strip detectors (DSSSD) are commonly used for event-by-event identification of charged particles as well as the reconstruction of particle trajectories in nuclear physics ...experiments with stable and radioactive beams. Intersecting areas of both p- and n-doped front- and back-side segments form individual virtual pixel segments allowing for a high detector granularity. DSSSDs are employed in demanding experimental environments and have to withstand high count rates of impinging nuclei. The illumination of the detector is often not homogeneous. Consequently, radiation damage of the detector is distributed non-uniformly. Position-dependent incomplete charge collection due to radiation damage limits the performance and lifetime of the detectors, the response of different channels may vary drastically. Position-resolved charge-collection losses between front- and back-side segments are investigated in an in-beam experiment and by performing radioactive source measurements. A novel position-resolved calibration method based on mutual consistency of p-side and n-side charges yields a significant enhancement of the energy resolution and the performance of radiation-damaged parts of the detector.
Brunner’s glands are submucosal glands located in the proximal duodenum. Hyperplasia of the Brunner’s gland has been reported rarely in humans and animals. We examined sections of the Brunner’s gland ...from 63 sand rats submitted for necropsy over 2 years. Of the 63 animals necropsied, 45 (71%) had evidence of hyperplasia defined as nodular expansion, dilated ducts, or intraductal papillary proliferation. The hyperplasia was graded as mild in 22 (49%) of the cases, moderate in 15 (33%), and marked in 8 (18%). We found an association with both increased age and evidence of gastric ulceration and hyperplasia of the Brunner’s gland. In sand rats with marked hyperplasia, 8 of 8 (100%) had evidence of gastric ulceration, compared to 13 of 18 (72%) in animals with no hyperplasia. Animals with marked hyperplasia were, on average, 8.4 months older than animals with no hyperplasia. There was no association with gender. The lesion in sand rats is histologically similar to that in humans.
We demonstrate an approach to measure the total fusion cross-section for beams of low-intensity light nuclei. Fusion residues resulting from the fusion of 20,16O+12C are directly measured and ...distinguished from unreacted beam particles on the basis of their energy and time-of-flight. The time-of-flight is measured between a microchannel plate (MCP) detector, which serves as a start, and a segmented silicon detector, which provides a stop. The two main difficulties associated with the initial implementation of this approach are charge trapping in the silicon detector and slit scattering in the MCP detector. Both these obstacles have been characterized and overcome. To reduce atomic slit scattering in the measurement we have eliminated wires from the beam path by developing a gridless MCP detector. The total fusion cross-section for 16O+12C in the energy range ECM=8.0–12.0MeV has been measured in agreement with established literature values (down to the 100mb level).
.
A bismuth-germanate (BGO) escape-suppression shield for the high-purity germanium triple-cluster detector of the MINIBALL
γ
-ray spectrometer was designed and built. Monte Carlo simulations with ...the simulation code GEANT4 were performed to guide the construction and to determine the detector geometry of the new BGO shield. After the first measurements concerning mechanical properties of the BGO housing and the performance of the photomultiplier tubes at the Institut de Physique Nucléaire, Orsay, the prototype BGO escape-suppression shield was combined with a MINIBALL triple-cluster detector at the Institut für Kernphysik, Cologne. A dedicated electronics and digital data-acquisition system was put into operation in order to determine timing properties of the combined coincidence measurement and to measure values for the energy resolution of the BGO detectors, for the BGO low-energy threshold, and for the crucial peak-to-total ratio (P/T). The measured P/T value for a standard
60
Co
γ
-ray source compares well with expectations and will allow to proceed with the amendment of the MINIBALL triple-cluster detectors with an escape-suppression shield for improved in-beam
γ
-ray spectroscopy especially at the new HIE-ISOLDE accelerator for radioactive ion beams at CERN.
PDF
