The fusion project based on tokamak requires on-line hot plasmas diagnostics using a topical spectrometric detection system operating in soft X-ray region. The used detectors must be resistant to the damage by fast neutrons and gamma rays produced by hot plasmas. The required detection performances include: (i) operation in the soft X-ray range: (0.5) 1 ÷ 5 (10) keV, (ii) energy resolution: < 1 (0.7) keV FWHM, (iii) noise threshold: < 1 (0.5) keV and resistance to overall neutrons up to a total fluency of ~ 1×1014 cm-2 and gamma rays. The best available spectrometric detectors such as Si drift chambers or Si pin diodes are out of consideration due to the large damage induced in these devices by relatively low doses (about 1 kGy) of fast neutrons. Among the possible suitable candidates for soft X-ray spectrometry, 4H-SiC detectors show excellent spectrometric performances and also higher resistance to damage by neutrons and gamma rays [1]. However, since high purity epitaxial SiC, required for detector fabrication, is very expensive, a lot of efforts have been addressed to the investigation of lower cost materials. Semi-insulating (SI) GaAs became an important candidate for fabrication of low cost X-ray detectors (and other devices) due to its good physical characteristics, high quality of base material, developed technology and relatively high hardness to radiation and neutron damage [2]. On the other hand SI GaAs exhibits short carriers lifetimes (in a range of ns), so that the drift length of carriers in the best material reaches only a few hundreds of ?m. This property must be taken into account in the design of novel spectrometric detector based on SI GaAs. The novel design is expected to bring: (i) higher detector volume, (ii) more homogenous electric field distribution in the active volume, (iii) lower bias operation (appx. 1/2 of the single-sided blocking electrode arrangement with the same base thickness), and (iv) lower detector capacitance and hence smaller contribution to the detector noise figure. In this work we present the results of a study of a novel radiation detector potentially applicable in soft X-ray spectrometry. The novel topology includes two blocking electrodes symmetrically arranged on the top and the bottom of a semiconductor wafer connected in parallel and a third (virtual) ohmic electrode(s) placed around the small area blocking contact(s). Such idea was originally proposed by Gatti and Rehak [3] and applied in the development of Si drift chamber topology. However, the idea was used in a different way: the 29 ohmic n+ contact, cathode, was formed as a small collection electrode placed in the center of the top anode barrier contacts. Such electrode arrangement is ineffective in semiconductors with short carriers lifetimes, such as SI GaAs. In addition to the novel topology, recent developments in SI GaAs contacts metallization [4, 5] could also be used to obtain further improvements of SI GaAs detector spectrometry. The presented study includes a basic numerical modeling of the novel detector which considers Si, for simplicity, as the base detector material. I-V and C-V characteristics, 2D distribution of the electric field, current and space charge density are simulated. Numerical modeling and optimization of the detector structure is supported by the advanced 2-D mixed mode electro-physical simulation. The novel design is expected to bring a higher detector volume with a more homogenous distribution of the electric field in the active volume and a lower detector capacitance, hence a lower contribution to the detector noise figure. A preliminary test detector, fabricated on the basis of the novel concept applied to SI GaAs, shows reverse current at room temperature as low as 1×10-10 A and low capacitance less than 0.4 pF at bias of -200 V. The energy resolution and the noise threshold are limited by the noise of the used preamplifier (~ 3 keV).
Novel concepts of soft X-ray detectors based on semi-insulating GaAs
Enos Gombia;
2012-01-01
Abstract
The fusion project based on tokamak requires on-line hot plasmas diagnostics using a topical spectrometric detection system operating in soft X-ray region. The used detectors must be resistant to the damage by fast neutrons and gamma rays produced by hot plasmas. The required detection performances include: (i) operation in the soft X-ray range: (0.5) 1 ÷ 5 (10) keV, (ii) energy resolution: < 1 (0.7) keV FWHM, (iii) noise threshold: < 1 (0.5) keV and resistance to overall neutrons up to a total fluency of ~ 1×1014 cm-2 and gamma rays. The best available spectrometric detectors such as Si drift chambers or Si pin diodes are out of consideration due to the large damage induced in these devices by relatively low doses (about 1 kGy) of fast neutrons. Among the possible suitable candidates for soft X-ray spectrometry, 4H-SiC detectors show excellent spectrometric performances and also higher resistance to damage by neutrons and gamma rays [1]. However, since high purity epitaxial SiC, required for detector fabrication, is very expensive, a lot of efforts have been addressed to the investigation of lower cost materials. Semi-insulating (SI) GaAs became an important candidate for fabrication of low cost X-ray detectors (and other devices) due to its good physical characteristics, high quality of base material, developed technology and relatively high hardness to radiation and neutron damage [2]. On the other hand SI GaAs exhibits short carriers lifetimes (in a range of ns), so that the drift length of carriers in the best material reaches only a few hundreds of ?m. This property must be taken into account in the design of novel spectrometric detector based on SI GaAs. The novel design is expected to bring: (i) higher detector volume, (ii) more homogenous electric field distribution in the active volume, (iii) lower bias operation (appx. 1/2 of the single-sided blocking electrode arrangement with the same base thickness), and (iv) lower detector capacitance and hence smaller contribution to the detector noise figure. In this work we present the results of a study of a novel radiation detector potentially applicable in soft X-ray spectrometry. The novel topology includes two blocking electrodes symmetrically arranged on the top and the bottom of a semiconductor wafer connected in parallel and a third (virtual) ohmic electrode(s) placed around the small area blocking contact(s). Such idea was originally proposed by Gatti and Rehak [3] and applied in the development of Si drift chamber topology. However, the idea was used in a different way: the 29 ohmic n+ contact, cathode, was formed as a small collection electrode placed in the center of the top anode barrier contacts. Such electrode arrangement is ineffective in semiconductors with short carriers lifetimes, such as SI GaAs. In addition to the novel topology, recent developments in SI GaAs contacts metallization [4, 5] could also be used to obtain further improvements of SI GaAs detector spectrometry. The presented study includes a basic numerical modeling of the novel detector which considers Si, for simplicity, as the base detector material. I-V and C-V characteristics, 2D distribution of the electric field, current and space charge density are simulated. Numerical modeling and optimization of the detector structure is supported by the advanced 2-D mixed mode electro-physical simulation. The novel design is expected to bring a higher detector volume with a more homogenous distribution of the electric field in the active volume and a lower detector capacitance, hence a lower contribution to the detector noise figure. A preliminary test detector, fabricated on the basis of the novel concept applied to SI GaAs, shows reverse current at room temperature as low as 1×10-10 A and low capacitance less than 0.4 pF at bias of -200 V. The energy resolution and the noise threshold are limited by the noise of the used preamplifier (~ 3 keV).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
