Ion microscopy (IBIC, IBIL).

The IBICC (Ion Beam Induced Charge Collection) and IBIL (Ion Beam Induced Luminescence) techniques allow the electronic properties of semiconductor and insulator materials to be mapped. Si, GaAs, CdTe and diamond devices and detectors have been characterised by means of focused (1-5 m m spot size) ion beams (1.5-7 MeV protons and alpha particles) at the National Laboratories of INFN (National Institute for Nuclear Physics) of Legnaro (I) and at the Rudjer Boskovic Institut of Zagabria (HR).  

• the "ALCHIMIA" experiment of INFN ) (see LNL ANNUAL REPORT 1999),
• the collaboration RD42 of CERN (Draft)
• the project ICI 5-CT 97-0719 (DG 12-SNRD) "Synthesis and characterisation of extra-hard materials" of the INCO-COPERNICUS founded by the European Union
• the INFM (National Institute of Physics of Matter) research line: "Ion beam Induced luminescence in diamond"

We used two geometrical configurations: the most commonly used frontal geometry (frontal IBIC) in which the ions hit the frontal electrode surface and lateral IBIC where ions are incident on the cross section of the sample.

Maps of charge collection efficiency can be obtained by recording the pulse heights as a function of the ion incident position. These maps allows both the analysis of homogeneity in polycristalline (CVD diamond) and monocrystalline (GaAs) materials, the evaluation of the electric field profile in highly resistive materials (CdTe), in junction diodes (Si) or the analysis of the "pumping state" in diamond.

IBIL , with respect to better known CL (Cathodo-Luminescence), displays the great advantage to be a real bulk diagnostic tool : as a matter of fact, protons range in diamond is 27 mm for 2 MeV energy and 160 mm for 6 MeV energy respectively [3],[4]. Protons of 2 MeV energy as produced by a standard microbeam, generate about 1.5x10⁵ hole-electron pairs along a cylindrical region about 2 mm in diameter, giving a free carrier density of about 4x10¹⁵ cm^-3 mainly concentrated at the Bragg's peak towards the end of the range. The locally produced photon flux can be followed in time ( time-resolved IBIL ), as a function of wavelength (monochromatic IBIL) or simply by counting the total number of pulses per pixel above a certain threshold (panchromatic "count" IBIL). The experimental apparatus for panchromatic IBIL mapping is shown in figure : two phototubes (Hamamatsu R647-01, with typical spectral response ranging from 350 to 600 nm) are mounted 1 cm away from the sample cross section ("detector grade" 400 mm thick CVD diamond from Norton Diamond), at angles 135 ° and 225 ° respectively with respect to the beam direction. About 1 % of the total solid angle is used for light collection. IBIL signals were obviously weighted over the spectral response of the photocathodes. Since the external radiative quantum efficiency could be below 10^-5, a coincidence set-up is used in order to improve the signal-to-noise ratio and to allow for single photon counting. The resolving time is typically below 20 ns.

By means of this apparatus, combined IBIC/IBIL maps have been obtained relevant to a "detector grade" CVD diamond sample [5].

[5] C. Manfredotti , F. Fizzotti, A. Lo Giudice, P. Polesello ,E.Vittone, M.Truccato, P. Rossi, "Ion beam induced luminescence maps in CVD diamond as obtained by coincidence measurements" , Diamond and Related Materials, 8 (1999), 1592-1596

[6] E.Vittone, A.Lo Giudice, C.Manfredotti, G.Egeni, V.Rudello, P.Rossi, G.Gennaro, G.Pratesi, M.Corazza, "Light detection with spectral analysis at the Legnaro Nuclear Microprobe:applications in material and earth sciences"Submitted at 7^th Int. Conf. on Nuclear Microprobe Technology and Applications, Bordeaux, France, September 2000, and to be published on Nuclear instruments and Methods in Physics Research B.

Poster (file .pdf): Light detection with spectral analysis at the nuclear microprobe: applications in material and earth sciences., presented at the INFMMETTING, Rome, June 2001