Recent advancements in the development of radiation hard semiconductor detectors for S-LHC

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Abstract

The proposed luminosity upgrade of the Large Hadron Collider (S-LHC) at CERN will demand the innermost layers of the vertex detectors to sustain fluences of about 1016 hadrons/cm2. Due to the high multiplicity of tracks, the required spatial resolution and the extremely harsh radiation field new detector concepts and semiconductor materials have to be explored for a possible solution of this challenge. The CERN RD50 collaboration “Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders” has started in 2002 an R&D program for the development of detector technologies that will fulfill the requirements of the S-LHC. Different strategies are followed by RD50 to improve the radiation tolerance. These include the development of defect engineered silicon like Czochralski, epitaxial and oxygen-enriched silicon and of other semiconductor materials like SiC and GaN as well as extensive studies of the microscopic defects responsible for the degradation of irradiated sensors. Further, with 3D, Semi-3D and thin devices new detector concepts have been evaluated. These and other recent advancements of the RD50 collaboration are presented and discussed.

Introduction

For the luminosity upgrade of the Large Hadron Collider (LHC) at CERN to 1035 cm−2 s−1 (Super-LHC or SLHC) [1] the presently available silicon detector technology cannot match the extreme requirements with respect to the necessary radiation tolerance. The innermost vertex detectors have to face fluences above 1016 cm−2 of fast hadrons after five years operation accumulating an integrated luminosity of 2500 fb−1. This is a ten times higher radiation level than expected for the vertex detectors of the LHC experiments. Due to the required very high rate capability and spatial resolution together with the demand for a significantly improved radiation hardness new detector concepts and possibly new sensor materials have to be explored and developed. The CERN RD50 research program “Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders” [2] started in 2002 with the aim to develop semiconductor sensors matching the requirements for SLHC tracking detectors. Special emphasis is put on the development of more radiation tolerant silicon like high resistivity Czochralski (Cz) or Magnetic Czochralski (MCz) silicon, epitaxial (epi) silicon layers or other defect engineered silicon. In addition, the compound semiconductor materials SiC and GaN are explored regarding their possible ability to sustain such extremely high radiation levels. On the other hand, new device concepts like 3D and semi-3D as well as thin sensors are under development and investigation. A further research field of the collaboration is the characterization and identification of radiation-induced defects and the study of the reaction kinetics in order to understand their impact on the degradation of the detector properties. Only this way a deliberate defect engineering of the material for an improved radiation hardness could be successfully achieved. Only specific topics of the overall RD50 research program and recent results are presented and discussed in this paper. More detailed information can be found in Refs. [3], [4].

Section snippets

Defect engineering in silicon

The aim of defect engineering is to influence the formation of microscopic defects in such a way that their detrimental effect on the macroscopic device properties by radiation is reduced or suppressed. This can be achieved by an incorporation of impurities or gettering sites into the silicon bulk before, during or after the processing of the detector.

Oxygen in silicon

A successful example for defect engineered silicon is the oxygen enrichment of high resistivity FZ (DOFZ) silicon by diffusion of oxygen into the

Device engineering

After an irradiation up to 1016 cm−2 fast hadrons the effective drift length of charge carriers is drastically reduced by trapping and, therefore, the produced signal does no longer depend linearly on the detector thickness or the electrode distance. In order to improve the charge collection properties different detector geometries are under development and investigation in the frame of RD50, which are described in 3.1 Thin silicon detectors, 3.2 3D and semi-3D detectors.

Conclusions

Recent developments and advancements of the RD50 collaboration related to defect engineered silicon, new semiconductor materials and new detector concepts have been presented. It is demonstrated, that especially Cz and epi-Si devices exhibit an unprecedented radiation tolerance. This is mainly due to the high concentration of oxygen as interstitial atoms (Oi) and as so-called oxygen dimers (O2). Defect studies indicate that a high Oi concentration leads to a suppression of deep acceptors, while

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