Ion-implantation Generated Nanovoids in Si and MgO Monitored by High Resolution Positron Beam Analysis
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Ion-implantation Generated Nanovoids in Si and MgO Monitored by High Resolution Positron Beam Analysis
S.W.H. Eijt, C.V. Falub, A. van Veen, H. Schut, P.E. Mijnarends, M.A. van Huis and A.V. Fedorov Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands ABSTRACT The formation of nanovoids in Si(100) and MgO(100) by 3He ion implantation has been studied. Contrary to Si in which the voids are generally almost spherical, in MgO nearly perfectly rectangular nanosize voids are created. Recently, the 2D-ACAR setup at the Delft Positron Research Center has been coupled to the intense reactor-based variable-energy positron beam POSH. This allows a new method of monitoring thin layers containing nanovoids or defects by depth-selective high-resolution positron beam analysis. The 2D-ACAR spectra of Si with a buried layer of nanocavities reveal the presence of two additional components, the first related to para-positronium (p-Ps) formation in the nanovoids, and a second one most likely related to unsaturated Si-bonds at the internal surface of the voids. The positronium is present in excited kinetic states with an average energy of 0.3 eV. Refilling of the cavities by means of low dose 3 He implantation (1×1014 cm-2) followed by annealing reduces the formation of Ps and the width of the Ps peak in the ACAR spectrum. This width reduction is due to collisions of Ps with He atoms in the voids. In MgO, p-Ps formed with an initial energy of ~3 eV shows a final average energy of 1.6 eV at annihilation due to collisions with the cavity walls. Possibilities of this new, non-destructive method of monitoring the sizes of cavities and the evolution of nanovoid layers will be discussed.
INTRODUCTION Positrons are a very sensitive probe for detecting the presence of defects such as mono- or divacancies. Owing to their positive charge, positrons injected in matter have a high likelihood of being trapped in small open spaces where the positive nuclei are absent. Defect concentrations as low as about 10-7 can be detected. Further, in materials containing nanocavities with a typical size of ~ 8 Å or more, Ps-formation often occurs. This can be observed distinctly in, e.g., lifetime or ACAR (angular correlation of annihilation radiation) studies, and allows the study of nanocavity size distributions and their evolution. By selecting the incident energy with which the positrons are injected into a material, an average probe depth can be chosen, typically in the µm range. The sensitivity to defects and the depth selectivity have contributed to the wide application of Doppler Positron Beam Analysis (see e.g. [1]) in studies of layered (defect) structures. 2D-ACAR is a two-dimensional technique which offers an improved resolution, by at least a factor of 4, over Doppler studies. However, depth-selective 2D-ACAR studies are rare since they require a much higher positron beam intensity than commonly available, of the order of at least 107 e+/s. Previous depth-selective 2D-ACAR experiments reli
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