DLTS measurements of radiation induced defects in epitaxial and MCz silicon detectors
Introduction
Silicon detectors will be largely employed in the tracking area of future colliding beam experiments. The proven technology and large-scale availability make them the favorite choice. However, the very large hadron fluence of up to hadrons per expected for the upgrade of the Large Hadron Collider (S-LHC) will pose an unprecedented challenge as to their radiation tolerance [1]. Small-sized pixel detectors will be used in the innermost layers of the vertex detectors to cope with the much larger multiplicity of events. Thus a reduced thickness could be tolerated for keeping the capacitance at a low level. Thin detectors would then allow a much higher doping concentration at moderate depletion voltages. A large donor reservoir in n-type silicon would in turn delay the type inversion effect by radiation generated acceptors. These considerations had been the main motivation to start systematic studies with diodes processed on thin epitaxial silicon layers [2], [3], [4], [5], [6], [7]. On top of the expected benefits it was found that shallow bistable donors (BDs) are created by radiation in epi-diodes, compensating the build up of negative space charge by deep acceptors [6], [7]. In Refs. [6], [7] it is suggested that the radiation induced formation of BDs might be connected with the presence of oxygen dimers . The oxygen dimer is not electrically active and is, therefore, not detectable by Deep Level Transient Spectroscopy (DLTS). In principal it could be seen in IR absorption spectroscopy via its local vibration modes, however, only in thick homogeneous samples with a concentration exceeding . The method is hence not applicable in thin epi-layers on Cz substrates [8]. As based on the assumption that the radiation induced defect complex is formed via the reaction the present work focuses on the relative estimation of the oxygen dimer concentration in different epitaxial and magnetic Czochralski (MCz) silicon via the detection of the defect as measured by DLTS and its possible correlation with the oxygen content revealed by SIMS profiling.
Section snippets
Experimental details
Silicon diodes manufactured on different kinds of epitaxial layers as well as on high resistivity MCz material were investigated in this work. The n-type epitaxial layers with thicknesses of 25 and were grown on highly Sb doped Cz substrates by ITME [9] in Warsaw. The resistivity of the layer was , while that of the thick layer was . The MCz wafers with a resistivity of nominal were produced by the company Okmetic [10] in Finland. The processing of the
Results and discussions
The DLTS spectra of a EPI-ST and an EPI-DO diode, as irradiated with 26 MeV protons up to an equivalent fluence of , are presented in Fig. 2. The DLTS peaks at about 200 and 120 K are typically observed in silicon after irradiation with charged hadrons or neutrons. They are attributed to both charge states of the divacancy , whereby the strong reduction of the signal compared with that of the single charge state is a well-known effect of hadron
Conclusions
Detailed DLTS studies on proton and neutron irradiated epitaxial silicon layers with and without a deliberate oxygen enrichment have been performed. For comparison also oxygen-rich MCz silicon was included in these investigations. The concentration of the radiation induced interstitial-oxygen dimer defect complex had been studied in correlation with the respective oxygen content. The results for the epi-sample without oxygen enrichment and the MCz sample suggested a quadratic dependence as
Acknowledgments
One of the authors (Ioana Pintilie) expresses many thanks to the Alexander von Humboldt foundation for obtaining a research fellowship, which enabled her participation in this project. This work was performed in the frame of the CiS-MUZ project under contract 0518/03/05 and within the CERN-RD50 collaboration. Many thanks are due to the team of the research reactor of the Jozef Stefan Institute in Ljubljana for the neutron irradiation and to A. Furgeri and the team at the cyclotron of the
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