C. Lanzieri

Performance of a new ohmic contact for GaAs particle detectors

Abstract 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.

Epitaxial silicon carbide charge particle detectors

Abstract The radiation detection properties of Schottky detectors made on epitaxial layers of 4H silicon carbide were evaluated. Exposure to 5.48xa0MeV alpha particles from a 241 Am source in vacuum led to robust signals from the detectors. The collection of the charge carriers was found to increase linearly with the square root of the applied reverse bias.

Deep levels in silicon carbide Schottky diodes

Abstract Native or process-induced defective states may significantly affect the transport properties of silicon carbide devices. For this reason, it is of major importance to detect them and, when possible, to identify their origin. This contribution deals with the deep levels detected by deep level transient spectroscopy analyses in silicon carbide Schottky detectors. Current–voltage and capacitance–voltage characteristics have also been studied to investigate Schottky barrier properties and diode quality. On the basis of the comparison with literature data, some of the deep levels found can be attributed to impurities introduced during growth.

Influence of electron traps on charge-collection efficiency in GaAs radiation detectors

Abstract This paper deals with the interpretation of the charge-collection efficiency for minimum ionizing particles in terms of electron traps in semi-insulating liquid encapsulated Czochralski gallium arsenide detectors fabricated with thicknesses of 80 μm, 200 μm, and 300 μm. The results, obtained within the context of the RD-8 project, show that low values of charge-collection efficiency cannot be ascribed primarily to the presence of high concentrations of EL2 defects. Possible ways of increasing the charge-collection efficiency are briefly addressed.

Performances of SI GaAs detectors fabricated with implanted ohmic contacts

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).

An optical-beam-induced-current study of active region and charge collection efficiency of GaAs particle detectors

Abstract In a recent paper, it was described how semi-insulating (SI) liquid encapsulated Czochralski (LEC) gallium arsenide particle detectors made by Alenia SpA have been tested and found to be understandable in terms of one function: the local charge collection efficiency. We have now measured, always within the context of the RD8 experiment, the local charge collection efficiency for these diodes using a technique called OBIC (optical beam induced current) and derived the active region width ( W ) as a function of bias in a range of values lower than those obtainable with particles. Using data both from OBIC and from the previous paper, W is found to behave approximately linearly as a function of bias larger than about 50 V. Below this value, W follows a square root law as a function of bias.

Gallium arsenide particle detectors: a study of the active region and charge-collection efficiency

Abstract Gallium arsenide particle detectors made by Alenia SpA have been tested within the context of the RD-8 project and found to have a high charge-collection efficiency (75%). We have analysed the data and found the overall behaviour of these detectors to be coherent and understandable in terms of one function: the local charge-collection efficiency. Two ways have been used to measure the width of the active region and the increase of such a width as a function of bias is found not to be consistent with a simple square-root law.

Influence of substrate on the performances of semi-insulating GaAs detectors

Abstract A study of the carrier transport mechanism, the charge collection efficiency and the energy resolution has been carried out on semi-insulating GaAs X-ray detectors realised on substrates with concentrations of acceptor dopants Na, varying from 1014 to 1017xa0cm−3. The electron collection efficiency (ECE) and the reverse current were found to decrease with increasing Na, while the resistivity of the material was found to increase. At room temperature, the best collection efficiency (95%) and the best energy resolution (13.7xa0keV FWHM) for 59.5xa0keV X-rays of the 241 Am source, have been achieved with the less doped detectors (Na∼1014xa0cm−3). The concentrations of ionised EL2+, determined by optical measurements in IR regions, was shown to increase with Na and to be quasi-inversely proportional to the ECE values. This behaviour strongly supports the hypothesis that the EL2 defects play a main role in the compensation of the material and in the limitation of the detection properties.

PROTON RADIATION EFFECTS ON SI LEC GAAS DETECTOR PERFORMANCES

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.

Influence of Ohmic Contacts on Semi-Insulating GaAs Detector Performances

Ohmic contact has been observed to play an important role in the performances ofLiquid Encapsulated Czochralski gallium arsenide particle detectors since it is possible to control the injection of charge carriers which determines the device performances. Alloyed and implanted contacts characteristics have been compared to relate the presence of electrically active defects induced during contact preparation to the final detector efficiency. In-situ optical probing investigations have been carried out to analyze the electric field distribution. Spectroscopic investigations show that the contact fabrication process significantly influences the trap density, whilst it does not change their type.