DLTS measurements of radiation induced defects in epitaxial and MCz silicon detectors

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Abstract

n-Type epitaxial silicon layers of different thickness and resistivity, grown on highly Sb doped CZ-substrate by ITME (Warsaw), and n-type MCz silicon supplied by Okmetic (Finland) were used for the processing of planar diodes at CiS (Erfurt). For the epi-diodes a standard as well as a diffusion oxygenation process was employed. Irradiations had been performed with 26 MeV protons at the cyclotron of the Karlsruhe University and with neutrons at the TRIGA reactor of the Ljubljana University. Microscopic investigations using the DLTS method were done. The correlation of the IO2i-defect and the oxygen concentration was studied by a depth-profile measurement. The annealing behavior of the IO2i-defect at different temperatures was investigated and the activation energy extracted.

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 1016 hadrons per cm2 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 O2i. 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 1015cm-3. The method is hence not applicable in thin epi-layers on Cz substrates [8]. As based on the assumption that the radiation induced IO2i defect complex is formed via the reaction I+O2i 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 IO2i 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 72μm were grown on highly Sb doped Cz substrates by ITME [9] in Warsaw. The resistivity of the 25μm layer was 50Ωcm, while that of the 72μm thick layer was 150Ωcm. The 300μm MCz wafers with a resistivity of nominal >600Ωcm were produced by the company Okmetic [10] in Finland. The processing of the

Results and discussions

The DLTS spectra of a 72μm EPI-ST and an EPI-DO diode, as irradiated with 26 MeV protons up to an equivalent fluence of 8.1×1011cm-2, 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 (V2(-/0),V2(=/-)), whereby the strong reduction of the V2(=/-) signal compared with that of the single charge state V2(-/0) 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 IO2i 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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