From anti‐dots to sponge: Positron Annihilation Spectroscopy of Ge+ irradiated Ge


From anti‐dots to sponge: Positron Annihilation Spectroscopy of Ge+ irradiated Ge

Liedke, M. O.; Böttger, R.; Liedke, B.; Facsko, S.; Anwand, W.; Wagner, A.

Ion irradiation of Ge drives the surface morphology throughout a rich variety of nanostructure formations, i.e., from ordered nanohole and nanodot patterns to nanoporous and sponge-like structures [1]. Moreover, due to ion energy depth resolution functional modifications of Ge are not only limited to the surface but extend easily to several tens of nm depth. That is especially relevant for fuel [2], and solar cells [3], as well as for filters [4], and gas sensors [5] applications. Possible fundamental implications are under a debate as well, e.g., does the mechanism for porous Ge formation originate mainly from the vacancy clustering [1] or microexplosions [6]? In addition, surprisingly porous structures have not been found in ion irradiated Si that is in appearance virtually a very similar material to Ge.
The driving force for the irradiation induced morphology evolution is mainly related to the kinetics of ion beam induced defects. For Ge+ self-irradiation with low ion energies the sample surface remains first (i) smooth (EGe<4keV), followed by (ii) the self-organized formation of nanoholes (EGe=5-7keV), and finally (iii) at EGe>8keV porous/sponge structures develop. Increasing ion energy allows to tailor depth and porosity. For Bi+ irradiation surface morphology evolves in a similar fashion besides for EBi=7-12keV, where initial hexagonally ordered nanohole patterns reorganize into homogeneously distributed nanodots [1]. Again for larger ion energies porous and sponge-like structures evolve. The kinetics beneath ion irradiation of Ge can be drawn as follows: (i) defect distribution and number at a certain sample depth scales with ion energy, whereas (ii) increasing ion fluence forms amorphous Ge layer due to continuous creation of interstitials and vacancies. The former (iii) because of their mobility, formation volume and energy compare to interstitials can cluster and grow into pores. Once, such a small void is created and is far enough from the surface it grows by attracting additional vacancies [7]. The overall surface morphology evolution has been simulated by means of kinetic Monte Carlo (kMC) modeling [1]. Both hole and sponge structures have been visualized without, however, hexagonal ordering of patterns nor the nanodot formation found from Bi+ irradiation could be realized.
Positron annihilation lifetime spectroscopy (PALS) and Doppler Broadening (DB) measurements will allow to probe the open-volume distribution and its complexes as well as the pores size as a function of depth. Thus, it should give an insight into the evolution of surface morphology during ion irradiation of semiconductors. These results can be of importance for further kMC modeling. Preliminary investigations by DB will be performed at the SPONSOR/AIDA setup in order to estimate positron annihilation line parameters as a function of positron energy.

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