Investigation of trapping levels in standard, nitrogenated and oxygenated Si p–n junctions by thermally stimulated currents

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Abstract

The trapping levels induced in p–n Si junctions by irradiation with 24 GeV proton were investigated using Thermally Stimulated Currents (TSC) methods in the 90–300 K temperature range. Several trapping levels, with activation energies between 0.27 and 0.57 eV, were put into evidence. The spatial distribution of the traps was investigated using different wavelengths for the light used to fill the traps. The results lead to the conclusion that the deepest trapping levels are not uniformly distributed in the volume of the sample. For the most important trapping level the average introduction rate was estimated to 0.9–1.2 cm−1. The activation energy and the capture cross-section of this trapping level seems to depend on the impurity element introduced in Si.

Introduction

Strong efforts are spent to develop radiation hard detectors that can operate beyond the limits of present devices [1], [2], [3]. Works are focused on finding possible solutions to limit the radiation-induced damages in silicon detectors to be used in high-level radiation environments as LHC. The key idea of the ROSE (RD48) collaboration in particular is to improve the radiation tolerance of silicon by defect engineering [4], [5]. This means deliberate addition of impurities during the growth process in order to affect the formation of electrically active defect centers and thus to control the macroscopic parameters of devices. It is believed that an impurity atom as oxygen, carbon, nitrogen, tin, acts as a vacancy sink during and following irradiation, thus limiting the detector bulk damage. The first step in understanding how microscopic defects influence the macroscopic parameters of a Si p–n junction (leakage current, depletion voltage, effective concentration of carriers) is to investigate the trapping levels associated with these defects and to find their evolution with the irradiation fluence [6], [7], [8], [9], [10]. In this paper the trapping levels induced in standard, oxygenated and nitrogenated p–n Si junctions, were investigated by the Thermally Stimulated Currents (TSC) method.

Section snippets

Experimental details

The junctions under investigation were manufactured in planar technology using different sorts of Polodovice (Czech Republic) Si wafers. The standard and oxygenated p–n junctions were processed by SINTEF (Norway) using Si wafers of 2.5  cm. The nitrogenated p–n junctions were processed by ITME-Warsaw using Si wafers of 5  cm resistivity. The oxygen concentration in oxygenated junctions is around 1016 cm−3 and the nitrogen concentration in nitrogenated junctions is of about 1015 cm−3. The area of

Results and discussion

No clear peak arises from TSC measurements performed on unirradiated samples since the current magnitude touches the detection limit of our experimental set-up. The TSC spectra for the highest irradiation fluence are shown in Fig. 1. It can be seen that the shape is preserved no matter what kind of starting Si had been used (standard, oxygenated or nitrogenated). This could be an evidence that defects induced by proton irradiation are of the same nature. There are some differences in the values

Conclusions

The TSC measurements performed on samples manufactured from standard, oxygenated and nitrogenated Si reveal the existence of three deep trapping levels induced by proton irradiation, which are located near the mid-gap, with an average introduction rate in the range of 0.9–1.2 cm−1. The enthalpy determined from Arrhenius plots is in the 0.48–0.57 eV range, in case of the dominant level 4, depending on the Si type. This level is assumed to be almost uniformly distributed in the sample. The

Acknowledgements

The authors would like to thank to Dr. F. Lemeilleur (CERN) for providing us the samples investigated in this paper.

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