Radiation hardness of silicon—a challenge for defect engineering
Introduction
In future high-energy physics experiments silicon detectors will be widely used for the necessary precise measurements of elementary particle tracks close to their origin at the interaction of colliding high-energy beams. Due to their unsurpassed properties silicon detectors have been found to be best suited for this purpose. However for the forthcoming experiments at CERN they face an unprecedented challenge by the extremely severe radiation fields in which they have to be operated. In fact detectors processed using standard high resistivity float zone (FZ) silicon would suffer a nontolerable deterioration of their performance quality. Therefore, a continuous effort had been initiated to considerably enhance the radiation tolerance above its present levels using an appropriate defect engineering of the starting material. Appreciable improvements had already been obtained for charged hadrons or γ-irradiation by enriching high resistivity FZ silicon with oxygen as demonstrated by the CERN RD48 [1], [2] collaboration. The beneficial oxygen effect is maximal in the case of 60Co-γ irradiation where the radiation damage is caused by point defects only. Present defect models attribute this effect to the formation of the V2O defect, which should be largely suppressed in oxygen rich material [3], [4].
Two defect levels have been detected by DLTS and thermally stimulated current (TSC) measurements, a deep acceptor (I) [5], [6] and a bistable donor (BD) [7], both having a strong influence on the detector performance after γ-irradiation. In this work, we also include findings after proton irradiation and focus on the material dependence of these two defects in detectors processed from standard FZ (STFZ), oxygen enriched FZ (DOFZ), high resistivity Cz and epitaxial silicon layers grown on a low resistivity Cz substrate (Epi/Cz). Recently, it has been shown that detectors processed on Epi/Cz are much radiation harder than all other investigated types [8]. It is therefore of special interest to study this unexpected result.
Section snippets
Experimental procedure
For our investigations we used diodes from 3–5 kΩcm STFZ and DOFZ with a thickness of about 290 μm, 1.2 kΩ cm Cz with a thickness of 280 μm and 50 Ω cm epi-layers with a thickness of 50 μm grown on Cz substrates (300 μm, 0.05 Ω cm). The oxygen concentration in the STFZ material is in the range of a few 1016 cm−3, in DOFZ about 1017 cm−3, in the epi-layers 6×1016 cm−3 (via in-diffusion from the Cz substrate during growth) and in the Cz-diodes 8×1017 cm−3. The carbon concentration of all diodes is below the
60Co-γ irradiation
STFZ, DOFZ and Epi/Cz were irradiated with a dose of around 500 Mrad, while the dose for the Cz material was 360 Mrad.After the irradiation the samples have been stored at room temperature for at least 2 weeks.
The TSC spectra recorded after forward injection are shown in Fig. 1. Many defects are visible in the spectra after irradiation, most of them being already very well characterized (VOi, CiCs, COi, V2) by different methods [11], [12]. As mentioned before, we will just focus on the two
Conclusions
Four different silicon materials (STFZ, DOFZ, Cz and Epi/Cz) have been investigated after γ- and proton-irradiation in order to understand the better radiation tolerance of Epi/Cz. Especially the defects having a strong influence on the device performance have been monitored. It was found that the main difference between the investigated materials consists in the generation of deep acceptors (I-defect) and shallow donors (BD). In oxygen rich material BDs are enhanced and the I-defects are
Acknowledgments
Many thanks are due to Z. Li and E. Verbitskaja for providing the 60Co facility at the BNL and help in the gamma irradiations, also to M. Moll and M. Glaser for their help with the high-energy proton irradiation at CERN. The detectors are provided by CiS and the work was financed by the DFG under the contract FR 1547/1-1. This work has been performed in the framework of the CERN-RD50 collaboration.
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