Material-dependent emission mechanism of secondary atomic ions from solids under MeV-energy heavy ion bombardment
Introduction
The emission process of secondary ions has been widely studied using heavy ions for various target materials. However, most of studies are concerned with secondary ion mass spectrometry (SIMS) at nuclear-collision-dominant low energies [1], [2], [3], [4], [5], [6], [7]. We have measured yields and emission energies of secondary ions emitted from Al and Si targets at electronic-collision-dominant high energies and found that the most probable and mean energies of atomic ions are proportional to their electric charge [8], [9]. The observed facts can be explained by a simple combined mechanism of the simultaneous recoiling and ionization by the projectile and the Coulomb repulsion by the short-lived ionized track region. The aim of the present work is to extend the measurements of yields and emission energies of secondary ions to semiconductive target species and to reveal the dynamic mechanism of secondary ion emission in a MeV-energy range.
Section snippets
Experimental
Measurements of the yields and emission energies of secondary ions were carried out in a similar way previously described using a conventional time-of-flight (TOF) technique at the Kyoto University 1.7 MV tandem Cockcroft–Walton accelerator facilities [8], [9]. An Si ion beam was chopped every 100 μs to a width of 50 ns and collimated to a spot of 2 mm in diameter on the GaP and GaAs targets at an incident angle of 60°. The resultant secondary ions were detected with a channel electron
Positive and negative secondary ion mass spectra
Examples of mass spectra of positive secondary-ion species are shown in Fig. 1 for the GaP and GaAs targets bombarded by Si ions. Dominant species found were 69Ga+ and 71Ga+ ions of the natural abundance for both targets. Other species found were light molecular and cluster ions such as GaO+, Ga2+, Ga2O+, Ga3+, Ga3O+ and Ga3O2+. As to the respective partners of Ga, P ions were unambiguously observed as single- to triple-charged positive ions but As ions were not observed as positive nor
Emission process of singly charged atomic ions
The important observed fact is that the emission energy distribution of the singly charged secondary atomic ions from the targets is very similar to that for the secondary atoms produced by keV-energy incident ions even in the present MeV-energy region. The distributions are represented well by the linear cascade model of Thompson–Sigmund with reasonable surface potential energies.
In the MeV-energy region, the electronic stopping power is higher than the nuclear one. That is, the fast incident
Summary
The yield and axial emission energy distribution have been measured for Ga+, Pq+ and As+ secondary ions produced from the GaP and GaAs targets in the electronic collision dominant MeV-energy region. The obtained results of the yield and emission energy distribution for singly charged ions including the previous results for the Al and Si targets are explained qualitatively by the thermalization model. However, the results for the multiply charged ions differ from those of the low energy
Acknowledgements
This work was done with the Experimental System for Ion Beam Analysis at Kyoto University. We thank A. Itoh, K. Yoshida, and K. Norizawa for their useful advice and technical supports during the experiments. It has been supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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