Claudia Schnohr

Energy dependent saturation width of swift heavy ion shaped embedded Au nanoparticles

P.K. and M.C.R. thank the Australian Research Council for support. P.K., R.G., D.J.S., and M.C.R. were supported by the Australian Synchrotron Research Program, funded by the Commonwealth of Australia via the Major National Research Facilities Program.

Structural modification of swift heavy ion irradiated amorphous Ge layers

Swift heavy ion (SHI) irradiation of amorphous Si (a-Si) at non-perpendicular incidence leads to non-saturable plastic flow. The positive direction of flow suggests that a liquid phase of similar density to that of the amorphous solid must exist and accordingly a-Si behaves like a conventional glass under SHI irradiation. For room-temperature irradiation of a-Si, plastic flow is accompanied by swelling due to the formation of voids and a porous structure. For this paper, we have investigated the influence of SHI irradiation at room temperature on amorphous Ge (a-Ge), the latter produced by ion implantation of crystalline Ge substrates. Like a-Si, positive plastic flow is apparent, demonstrating that liquid polymorphism is common to these two semiconductors. Porosity is also observed, again confined to the amorphous phase and the result of electronic energy deposition. Enhanced plastic flow coupled with a volume expansion is clearly responsible for the structural modification of both a-Si and a-Ge irradiated at room temperature with swift heavy ions.

X-ray absorption spectroscopy of semiconductors

Introduction to XAS.- Crystalline Semiconductors.- Disordered Semiconductors.- Nanostructures.- Magnetic Semiconductors.

Ion-beam-induced damage formation in CdTe

Damage formation in 〈111〉- and 〈112〉-oriented CdTe single crystals irradiated at room temperature and 15 K with 270 keV Ar or 730 keV Sb ions was investigated in situ using Rutherford backscattering spectroscopy (RBS) in channeling configuration. Defect profiles were calculated from the RBS spectra using the computer code DICADA and additional energy-dependent RBS measurements were performed to identify the type of defects. At both temperatures no formation of a buried amorphous layer was detected even after prolonged irradiation with several 1016 ions/cm2. The fact that CdTe is not rendered amorphous even at 15 K suggests that the high resistance to amorphization is caused by the high ionicity of CdTe rather than thermal effects. The calculated defect profiles show the formation of a broad defect distribution that extends much deeper into the crystal than the projected range of the implanted ions at both temperatures. The post-range defects in CdTe thus do not seem to be of thermal origin either, but are...

Nano-porosity in GaSb induced by swift heavy ion irradiation

Nano-porous structures form in GaSb after ion irradiation with 185 MeV Au ions. The porous layer formation is governed by the dominant electronic energy loss at this energy regime. The porous layer morphology differs significantly from that previously reported for low-energy, ion-irradiated GaSb. Prior to the onset of porosity, positron annihilation lifetime spectroscopy indicates the formation of small vacancy clusters in single ion impacts, while transmission electron microscopy reveals fragmentation of the GaSb into nanocrystallites embedded in an amorphous matrix. Following this fragmentation process, macroscopic porosity forms, presumably within the amorphous phase.

Ion-beam induced effects at 15K in α-Al2O3 of different orientations

Ion-beam induced effects in α-Al2O3 of c, a, and r orientations were studied by Rutherford backscattering spectrometry (RBS) in channeling configuration using 1.4MeV He ions. 150keV Ar, 150keV K, or 80keV Na ions were step by step implanted at 15K followed immediately by the RBS analysis without changing the sample environment. Defect annealing was observed during the RBS measurement, which is attributed to the electronic energy loss of the He ions. A similar effect occurs due to the electronic energy loss of the implanted ions, resulting in a reduced defect concentration between surface and profile maximum. The electronic energy loss of ions may change the charge state of defects, thus enhancing their mobility and causing defect annealing. The results suggest that within the collision cascade of individual ions in perfect sapphire only point defects are produced, the concentration of which is well reproduced by SRIM calculations taking into account suggested values of the displacement energies of EdAl=20...

Ion irradiation effects on metallic nanocrystals

We have investigated structural and morphological properties of metallic nanocrystals (NCs) exposed to ion irradiation. NCs were characterized by transmission electron microscopy in combination with advanced synchrotron-based analytical techniques, in particular X-ray absorption spectroscopy and small-angle X-ray scattering. A number of different effects were observed depending on the irradiation conditions. At energies where nuclear stopping is predominant, structural disorder/amorphization followed by inverse Ostwald ripening/dissolution due to ion beam mixing was observed for Au and Cu NCs embedded in SiO2. The ion-irradiation-induced crystalline to amorphous transition in the NCs, which cannot be achieved in the corresponding bulk metals, was attributed to their initially higher structural energy as compared to bulk material and possibly preferential nucleation of the amorphous phase at the NC/SiO2 interface. At very high irradiation energies (swift heavy ion irradiation), where the energy loss is nearly entirely due to electronic stopping, a size-dependent shape transformation of the NCs from spheres to rod like shapes was apparent in Au NCs. Our preliminary results are in good agreement with considerations on melting of the NCs in the ion track as one mechanism involved in the shape transformation.

The influence of annealing conditions on the growth and structure of embedded Pt nanocrystals

We thank the Australian Synchrotron Research Program and the Australian Research Council for financial support.

Improved Ga grading of sequentially produced Cu(In,Ga)Se2 solar cells studied by high resolution X-ray fluorescence

There is particular interest to investigate compositional inhomogeneity of Cu(In,Ga)Se2 solar cell absorbers. We introduce an approach in which focused ion beam prepared thin lamellas of complete solar cell devices are scanned with a highly focused synchrotron X-ray beam. Analyzing the resulting fluorescence radiation ensures high resolution compositional analysis combined with high spatial resolution. Thus, we are able to detect subtle variations of the Ga/(Ga + In) ratio down to 0.01 on a submicrometer scale. We observed that for sequentially processed solar cells a higher selenization temperature leads to absorbers with almost homogenous Ga/(Ga + In) ratio, which significantly improved the conversion efficiency.

Local versus global electronic properties of chalcopyrite alloys: X-ray absorption spectroscopy and ab initio calculations

Element-specific unoccupied electronic states of Cu(In, Ga)S2 were studied as a function of the In/Ga ratio by combining X-ray absorption spectroscopy with density functional theory calculations. The S absorption edge shifts with changing In/Ga ratio as expected from the variation of the band gap. In contrast, the cation edge positions are largely independent of composition despite the changing band gap. This unexpected behavior is well reproduced by our calculations and originates from the dependence of the electronic states on the local atomic environment. The changing band gap arises from a changing spatial average of these localized states with changing alloy composition.