Albrecht, F

Dietrich, M

Pasold, G

Reislöhner, U

Witthuhn, W

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN-INTC-2003-006  INTC-P-167 20. JANUARY 2003 PROPOSAL TO THE INTC COMMITTEE PRODUCTION OF RARE EARTH ISOTOPE BEAMS  FOR RADIOTRACER- DLTS ON SIC  Jena – Konstanz – CERN Collaboration   G. Pasold1, F. Albrecht1, U. Reislöhner1, M. Dietrich2, M. Deicher3, W.Witthuhn1 and the REX-ISOLDE Collaboration4  1 Institut für Festkörperphysik, Friedrich-Schiller-Universität-Jena, 07743 Jena, Germany 2 Technische Physik, Universität des Saarlandes, 66041 Saarbrücken, Germany 3 Fachbereich Physik, Universität Konstanz, 78457 Konstanz, Germany 4 ISOLDE, CERN, CH-1211 Geneve 23, Switzerland Spokesperson: G.Pasold Contactperson: M. Dietrich Summary Electrical properties of semiconductors are extremely sensitive to minor traces of impurities and defects. This fact allows to intentionally modify material properties and is thus the very basis of semiconductor electronics and optoelectronics. In the present project, electronic properties and doping effects of rare-earth elements in the technologically important semiconductor SiC are to be investigated using optical and electrical characterization techniques like Photoluminescence, Deep Level Transient Spectroscopy and Thermal Admittance Spectroscopy. By using the elemental transmutation of radioactive isotopes as a tracer, it will be guaranteed that the impurity-related band gap states can definitively be distinguished from intrinsic or process-induced defects. For SiC up to now only detailed investigation of Er- related deep levels have been reported, preliminary data exist for Sm- and Gd- impurities. In this project we propose the implantation of Pr and Eu isotopes for detailed level studies.   1. Introduction  The electrical and optical properties of semiconductors are governed by defects and impurities. Even extremely diluted concentrations will determine the type and magnitude of electronic conductivity, luminescence properties or the mean life time of charge carriers if the impurity or defect in question give rise to a localized electron state with an energy- level within the band gap of the semiconductor. Because of this sensitivity, a detailed information on impurity related band gap states is highly desirable to understand electronic behavior of semiconductors. The standard process of doping semiconductors is ion implantation. There concentration, depth and lateral distribution of the dopants can be precisely controlled. However, ion implantation is always accompanied by structural damage to the crystal requiring thermal annealing to remove the radiation damage and achieve electrical activation of the dopants. The reconstruction of the SiC lattice requires annealing temperatures up to 1900K. Using standard spectroscopic techniques to characterize the electronic and optical properties of the impurity levels, it is essential to discriminate between states created by the dopant and states due to remaining implantation defects or defects introduced by the annealing procedure itself. Therefore the production of  pure radioactive beams of rare earth- isotopes is of large interest for the further investigation of the behaviour of these materials implanted in SiC.   Semiconductors doped with rare-earth elements are especially attractive for photonic applications in the case of indirect band gap semiconductors such as silicon or silicon carbide. Substantial research was concentrated on doping with Er3+ ions because of the emission wavelength at 1.54 µm due to the 4f-shell 4I13/2 ®  4I15/2  transition, which coincides with the attenuation minimum of silica based optical fibres. [1,2,3]  Despite the great progress in enhancing the emission intensity of Er-doped Si, the drastic loss of luminescence efficiency at room temperature as compared to that at low temperatures remains as a serious obstacle on the way to applications. However, it has been shown for GaN and, recently, for silicon carbide (SiC) too [3], that this thermal quenching decreases with increasing band gap energy of the semiconductor. Whereas in GaN and heavily doped SiC (Er concentrations of 1019 cm-3 the integrated luminescence intensity remains, as expected, nearly constant up to about 500 K (for GaN) and 350 K (for SiC), respectively, in moderately implantation doped 4H- and 6H-SiC (Er concentrations from 5´ 1015 to 6´ 1016 cm-3) the situation is quite different [3]. Here, the integrated low temperature intensity (2 K) is drastically reduced at room temperature. These results indicate the importance of the identification of the Er related band gap states, aiming at a detailed understanding of the energy-transfer process which leads to the rare-earth excitation. Therefore, we propose the investigation of rare-earth related deep levels in 4H- and 6H-SiC by electrical and optical techniques.   2. Experimental methods Most spectroscopic techniques in semiconductor physics which are able to detect and characterize band gap states do not reveal information about their microscopic origin. On the other hand, structurally or chemically sensitive methods, like magnetic resonance techniques, PAC, or Mossbauer effect, generally do not reveal the energetic position of a state within the band gap. To overcome the chemical “blindness”, radioactive isotopes are being used as “tracers”, i.e. a doping experiment is done with a radioactive isotope instead of a stable isotope of the element of interest.  Because of its characteristic concentration change according to the nuclear decay, the involvement of a radioisotope in an experimentally observed effect can be confirmed or denied definitely by several subsequent measurements during the elemental transmutation. Also band gap states related to either a parent or daughter isotope are uniquely identified by their decreasing or increasing concentration, respectively.  To detect band gap states or, more generally speaking, impurity related effects, standard techniques of semiconductor physics are used in these radiotracer studies: spectroscopic techniques like Deep Level Transient Spectroscopy (DLTS) and Thermal Admittance Spectroscopy (TAS) as well as capacitance-voltage (CV) profiling. These electrical techniques directly yield absolute concentrations. Hence, the observable signal change is expected to be proportional to the concentration, i.e. it directly reflects the nuclear decay law. [4]  Recent experiments with the isotope 160Er show one deep level in p-type SiC material only, 0,75eV above the valence band edge [5]. Also the use of 149Gd as a tracer decaying to Eu shows significant deep levels in SiC with concentrations correlated to the nuclear decay time.   3. Proposed Experiments The first experiments outlined in our letter of intent in 2002 [6] about experiments using 140Nd and 153Sm show a partial success in only one sample implanted where we could clearly reproduce the decay effect of one deep level in p-type 6H-SiC caused by the decaying  153Sm. Continuing these experiments, we propose to use  the radioactive decay of 143Pr into 143Nd with a half life of 13.57d and 156Eu into 156Gd with a half life of 15.2d, respectively. A new set up of the implantation chamber using a carbon foil together with a silicon-detector will help to control and adjust the exact beam energy on the samples.  Using the power of the classical semiconductor spectroscopy techniques in combination with radioactive dopants, we will be able to discriminate between electrical properties created by the dopants and defects related to the implantation and annealing procedure. The use of radioactive dopants also allows the accurate determination of the fraction of electrically activated dopants for different annealing procedures. Especially the investigation of the behaviour of the elements Pr and Eu in SiC are very interesting, because Pr and Eu implanted in GaN have been utilized to produce red light emmissions [7]. The ion energies have to be in the range of 2..6 MeV to reach implantation- depths of about 0.5..1.5µm, where the DLTS is sensitive.   4. Experimental requirements The collections will be performed at the 20° beam line of the REX-ISOLDE post-accelerator. The RFQ will be used to accelerate the ions to an energy of 300keV/u that drift through further down the linac. Since the corresponding total energy is too high for the implantation depth needed, the beam will be decelerated by passing through a carbon foil of the right thicknesses. In addition we are going to add the possibility to tilt the foil against the beam direction to change the effective thickness (E~cos a). Furthermore a silicon-detector behind that setup will be used to measure the resulting energy.  The sample preparation before (cutting, cleaning, etching etc.) and after the implantation (thermal annealing, electrical contacts etc.) will be done in the solid state physics laboratory in building 115 at ISOLDE or in our laboratories in Jena belonging to the half life of the implanted isotope. For electrical measurements on short half life isotopes we would need up to 10 m2 in the ISOLDE hall or in a lab nearby.  5. Beam time request Depending on the used spectroscopic technique, the total required number of implanted ions ranges from 109 up to 1011 ions per cm². i.e. assuming a fluence after the LINAC of ca. 106 s-1 implantation times between minutes and a few hours per sample are required. Most of the beam-times for a certain target-ion-source combination can and should be shared with other experiments. We request a total of 16 shifts for the next two years for the following isotopes:  Isotope  Half life Target Ion source Shifts Energy 143Pr 13,57 d UC2 W surface 8 3..6 MeV 156Eu 15,2d UC2 W surface or Laser 8 3..6 MeV  6. References [1]  Rare Earth Doped Fibre Lasers and Amplifiers, edited by M. J. F. Digonet (Marcel Dekker, New York,1993). [2] H.Kühne, G.Weiser, E.I.Terukov, A.N.Kusnetsov, V.Kh.Kudoyarova, J. Appl. Phys. 86, 896 (1999). [3]  Y.Shishkin, W.J.Choyke, R.P.Devaty, N.Achtziger, Th.Opfermann, W.Witthuhn, Mater. Sci. Forum 338-342, 639 (2000). [4] N.Achtziger, H.Gottschalk, T.Licht, J.Meier, U.Reislöhner, M.Rüb, W.Witthuhn  Appl. Phys. Lett. 66 (1995),  p. 2370.  [5] G.Pasold et.al. JAP, accepted. [6] experiment nr. CERN-INTC-2002-19  INTC-I-042 [7] A.Steckl, J.Zavada, MRS Bull. (09/1999) pp.16. 

Production of rare earth isotope beams for radiotracer- DLTS on SiC

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