Near-surface muonium states in germanium

We used the low-energy mu SR technique (LE-mu SR) to extend our previous studies on the energy dependence of muonium (Mu) states in Si and insulators [T. Prokscha, E. Morenzoni, D.G. Eshchenko, N. Garifianov, H. Gluckler, R. Khasanov, H. Luetkens, A. Suter, Phys. Rev. Lett. 98 (2007) 227401] to investigations on an undoped 0.15-mm-thick Ge (100) crystal. The Mu formation in the near-surface region from about 10 to 150 nm is probed with mean implantation energies between 2.5 and 17.4 keV. In this energy range the number of track electron-hole pairs varies as a function of energy between a few hundred and several thousand [R.C. Alig, S. Bloom, Phys. Rev. Lett. 35 (1975) 1522]. Similar behavior as in Si is observed between 30 and 150 K, i.e., a doubling of the diamagnetic fraction F(D) (Mu(+) or Mu(-)) on lowering the energy E from 17.4 to 2.5 keV, corresponding to mean implantation depths of 130 and 17 nm, respectively. The fraction of Mu at the tetrahedral interstitial site (Mu(T)) does not show a pronounced energy dependence. The change of FD therefore can be attributed to a corresponding change of the bond-center Mu (Mu(BC)) formation probability. This demonstrates that also in Ge the formation of Mu(BC) clearly depends on the availability of excess charge carriers which the muon creates during the stopping process. Surprisingly, below 50K F(D) starts to increase again for E>4 kev. Additionally, bulk mu SR studies on a piece cut from the same sample shows the opposite trend in F(D) below 50 K, and distinct final charge states. More investigations are necessary to clarify this difference. (C) 2008 Elsevier B.V. All rights reserved.