A summary is presented of the present status of several compound semiconductor radiation detectors, including detectors fabricated from GaAs, HgI2, CdTe, Cd1−xZnxTe and PbI2

Room-temperature compound semiconductor radiation detectors

We present an analysis of the electronic noise contributions which limit the resolution of X- and γ-ray spectrometers employing semiconductor detectors operating at room temperature and at signal processing times in the sub-microsecond range. The figures of merit of the front-end transistors, relevant to attain the highest resolution, are put in evidence. It is shown how the correlation between the gate and drain current noises in FETs plays a significant role in the equivalent noise charge of a charge preamplifier. Some state of the art devices, belonging to different technologies, JFET, MOSFET, MESFET and HFET, are examined.

Criteria of choice of the front-end transistor for low-noise preamplification of detector signals at sub-microsecond shaping times for X- and γ-ray spectroscopy

A radiated energy to electrical energy conversion device and technology is provided where there is a single absorber layer of semiconductor material. The thickness of the absorber layer is much less than had been appreciated as being useful heretofore in the art. Between opposing faces the layer is about ½ or less of the carrier diffusion length of the semiconductor material which is about 0.02 to 0.5 micrometers. The thickness of the absorber layer is selected for maximum electrical signal extraction efficiency and may also be selected to accommodate diffusion length damage over time by external radiation.

Fabrication of single absorber layer radiated energy conversion device

Gallium Arsenide pixel detectors with an area of 170/spl times/320 /spl mu/m/sup 2/ and thickness of 5 /spl mu/m, realized by molecular beam epitaxy, have been designed and tested with X- and /spl gamma/ rays. No significant charge trapping effects have been observed, and a charge collection efficiency of 100% has been measured. At room temperature an energy resolution of 671 eV full width at half maximum (FWHM) at 59.54 keV has been obtained, with an electronic noise of 532 eV FWHM. With the detector cooled to 243 K, the electronic noise is reduced to 373 eV FWHM, and the K/sub /spl alpha// and K/sub /spl beta// lines of the /sup 55/Fe spectrum can be resolved. The Fano factor for GaAs has been measured at room temperature with 59.5 keV photons yielding F=0.12/spl plusmn/0.01.

Pixel X-ray detectors in epitaxial gallium arsenide with high-energy resolution capabilities (Fano factor experimental determination)

In the framework of the Medipix1 collaboration the PCC, a single photon counting pixel chip, has been developed with the aim of improving the contrast resolution in medical imaging applications. The PCC consists of a matrix of 64]64 square pixels with 170 lm side length, each pixel comprising a 15-bit counter and a pulse-height discriminator. The chip has been bump bonded to equally segmented 200 lm thick SI-LEC GaAs detectors showing a very high absorption energy for X-rays used in diagnostics. An absolute calibration of the system with a radioactive source and a synchrotron beam are described resulting in the value of the test input capacitance of &24.7 fF. Using this value a full characterisation of the system from electrical measurements is presented. The entire system can reach a minimum threshold of &2100e~ with &250e~ rms noise. One of the characteristic features of the PCC is the possibility to adjust the thresholds of all pixels on a pixel-by-pixel basis with 3-bit precision. The threshold distribution after adjustment is &120e~ rms. The spatial resolution of the system has been measured to be 3.6 lp/mm. A comparison of a tooth image taken with the PCC and with a screen-CCD-system demonstrates its imaging capabilities. ( 2001 Elsevier Science B.V. All rights reserved.

/pdf/characterisation-of-a-single-photon-counting-pixel-system-3b9849ttay.pdf

Characterisation of a single photon counting pixel system for imaging of low-contrast objects

In recent papers, we have investigated, within the context of the RD-8 experiment, the behaviour as a function of bias of the active region of particle detectors made by Alenia SpA on semi-insulating liquid encapsulated Czochralski gallium arsenide: the active region width depends linearly on the bias voltage. The diodes were found to break down as soon as the field reached the back ohmic contact. This suggested that the ohmic contact was injecting holes into the diode, therefore we have decided to develop a new, non-injecting, non-alloyed ohmic contact. This new contact allows us to go far beyond, five times, the voltage bias necessary to have a fully active detector. The higher voltage reached by the detectors helps us improve the charge collection efficiency, up to more than 95% for alphas and more than 90% for beta (mips) particles and X-rays, giving a more stable operation of the detectors. For the first time we can explore the characteristics of a GaAs detector beyond the voltage needed for it to be completely active.

Performance of a new ohmic contact for GaAs particle detectors

The slow component of the output pulse of Semi-Insulating (SI) Gallium Arsenide (GaAs) particle detectors, which affects charge collection efficiency (cce), has been generally attributed to trapping/detrapping effects. However, most of the detectors analyzed in the literature can only be operated below the voltage V/sub d/ necessary to extend the electric field all the way to the ohmic contact, making difficult to distinguish between the effect of the non-active part of the detector and that of trapping/detrapping. To do that, we have carefully analyzed the output signals of SI GaAs detectors, operated below and above V/sub d/ and irradiated with /sup 241/Am /spl alpha/ particles. When the detector is biased below V/sub d/ the output signals are affected also by the non-active part of the detector itself, while, when the detector is operated above V/sub d/, the output signals are only affected by trapping/detrapping of charge carriers. We found that trapping/detrapping is only relevant for the hole contribution to the signal. Trapping/detrapping effects are in agreement with the characteristics of deep levels present in the detectors as analyzed by means of PICTS (Photo Induced Current Transient Spectroscopy) and P-DLTS (Photo Deep Level Transient Spectroscopy).

Performances of SI GaAs detectors fabricated with implanted ohmic contacts

We report on the performance and limitations of a 7 mm2, 100 μm thick SI LEC GaAs detector, operated in overdepleted conditions with applied bias of 400 V at 20°C and −30°C, in detecting X- and γ-rays. At 20°C performance is limited to a charge collection efficiency of 92% and to an energy resolution of about 16.8 keV FWHM on the 59.5 keV line of 241Am. At −30°C the charge collection efficiency is 85% and a resolution in between 1.8 and 2.2 keV FWHM in the photon energy range 8–60 keV has been achieved. The latest data represent to our knowledge the best ones yet reported for GaAs LEC detectors.

Performance of SI LEC GaAs detectors at 20°C and −30°C for X- and γ-ray spectroscopy

Semi-insulating GaAs detectors have been irradiated with 24 GeVc protons with 3.87 and 5.6 × 1013cm−2 doses. Both electron and hole lifetimes are affected by the radiation damage, the hole lifetime much more dramatically. Nevertheless, by operating at high reverse bias voltage (600 V), 70% of the alpha-particle-generated charge carriers are collected on average. A few traps for electrons (0.14 eV) and holes (0.54 eV) have been detected only in the irradiated detectors. These findings have been correlated to the performance of the irradiated GaAs detectors.

Proton radiation effects on SI LEC GaAs detector performances

Observations have been made of the behaviour of high resistivity silicon and semi-insulating gallium arsenide ionizing radiation detectors after exposure of up to 30 Mrad 60 Co photons. Results are presented on leakage current and deep level defects of the substrate material of photon damage devices. These findings have been related to the charge collection efficiency of the detectors.

Photon radiation damage in high purity silicon and lec si gallium arsenide detectors

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