Charge Collection Efficiency Of Detectors Research Articles

GaAs radiation-degraded detectors: gamma spectrometry at lowered temperatures

The GaAs detectors are radiation hard against a few MGy of high-energy (MeV) electrons. The main reason for detector functionality degradation is the reduction of their charge collection efficiency and together with reverse current increase these facts cause the drop of the signal to noise ratio down to close to 1, disabling detector functionality. In this paper, the lowering of detector temperature has improved the spectra of less ionizing particles, the 59.5 keV gamma rays, measured by semi-insulating GaAs pad detectors, which were first degraded by 8 MeV electrons with a dose of 200 kGy. The signal to noise ratio improved by about 30%, from 1.09 at room temperature up to 1.42 at 276 K. The photo-peak was easily distinguishable from the noise during cooling and the calculated detector relative energy resolution improved from 46% at room temperature to 34% at 276 K. A mild decrease in detector charge collection efficiency was observed, by about 5% of its initial value, when cooling the detector to 276 K.

Design, electrical properties and fabrication method study of a novel 3D-Compound-Shell-Electrode silicon detector

In this work, a novel three-dimensional (3D) structure of silicon detector: 3D-Compound-Shell-Electrode detector (3DCSED), based on the 3D-Trench-Electrode detector and 3D-Open-Shell-Electrode detector (3DOSED), is proposed. In a 3DCSED, an open trench electrode will be etched about 10% of the detector thickness from the bottom side of the detector to meet the close trench electrode etched about 90% of the detector thickness from the top side. This not only makes the unit structure stable in the fabrication process, but also improves the detection efficiency through the optimization of the electrode structure compared to the conventional 3D-Trench-Electrode detector. In order to optimize the 3DCSED structure, it is important to study 3DCSED’s electrical properties by full 3D technology computer-aided design (TCAD) simulations. From the electric field distribution results, detector charge collection efficiency has been simulated and optimized by incorporating charge trapping caused by irradiation. Due to the highly doped trench walls, each pixel cell is electrically isolated to ensure a uniform electric field distribution. However, each pixel cell is physically connected to its neighboring cells, therefore also connected to the Si substrate, through the small portion of broken electrodes in the 3DOSED bulk (about 10% of the detector thickness from the bottom side of the detector). Furthermore, current–voltage (I–V) characteristics and full depletion voltage have been analyzed to study the detector’s properties. We also propose a method to fabricate the 3DCSED. In this method, the deep etching is processed by Deep Reacting Ion Etching (DRIE) or laser with respective processing procedures.

Radiation hardness limits in gamma spectrometry of semi-insulating GaAs detectors irradiated by 5 MeV electrons

A radiation hardness study of developed Schottky barrier semi-insulating (SI) GaAs detectors against 5 MeV electrons is described in this paper. The influence of cumulative dose (up to 1000 kGy) and the applied dose rate (20, 40 or 80 kGy/h) on spectrometric and electrical properties of detectors is presented. The spectrometric properties were evaluated from measured 241Am gamma spectra and the electrical properties were obtained measuring the current-voltage characteristics. The cumulative dose has negatively affected all spectrometric properties. The detector CCE (Charge Collection Efficiency) exponentially decreased with dose from initial 74% down to 24% at maximum applied dose of 1000 kGy. The relative energy resolution was increasing from 17% before irradiation up to almost 50% at maximum dose applied. The peak-to-valley ratio firstly increased from 10 to 17 at 24 kGy and then rapidly decreased below 2 at 200 kGy. The electrical properties were obtained from measured current-voltage characteristics of detectors. The breakdown voltage of detectors firstly decreased to 60% of its original value and at few hundred kGy it started to increase exceeding the initial value twice. The reverse current exhibited similar behaviour decreasing up to 200 kGy and then increasing. Concerning obtained results we can determine the limit of sufficient functionality of developed SI GaAs detectors against 5 MeV electrons to be 1000 kGy for 59.5 keV particles. The second investigated parameter of irradiation, the dose rate in chosen ranges, did not greatly alter the spectrometric properties of studied detectors.

Design and optimization of a novel 3D detector: The 3D-open-shell-electrode detector

A new type of three-dimensional (3D) detector, namely 3D-Open-Shell-Electrode Detector (3DOSED), is proposed in this study. In a 3DOSED, the trench electrode can be etched all the way through the detector thickness, totally eliminating the low electric field region existed in the conventional 3D-Trench-Electrode detector. Full 3D technology computer-aided design (TCAD) simulations have been done on this novel silicon detector structure. Through comparing of the simulation results of the detector, we can obtain the best design of the 3SOSED. In addition, simulation results show that, as compared to the conventional 3D detector, the proposed 3DOSED can improve not only detector charge collection efficiency but also its radiation hardness with regard to solving the trapping problem in the detector bulk. What is more, it has been shown that detector full depletion voltage is also slightly reduced, which can improve the utility aspects of the detector. When compared to the conventional 3D detector, we find that the proposed novel 3DOSED structure has better electric potential and electric field distributions, and better electrical properties such as detector full depletion voltage. In 3DOSED array, each pixel cell is isolated from each other by highly doped trenches, but also electrically and physically connected with each other through the remaining silicon bulk between broken electrodes.

Infrared LED Enhanced Spectroscopic CdZnTe Detector Working under High Fluxes of X-rays.

This paper describes an application of infrared light-induced de-polarization applied on a polarized CdZnTe detector working under high radiation fluxes. We newly demonstrate the influence of a high flux of X-rays and simultaneous 1200-nm LED illumination on the spectroscopic properties of a CdZnTe detector. CdZnTe detectors operating under high radiation fluxes usually suffer from the polarization effect, which occurs due to a screening of the internal electric field by a positive space charge caused by photogenerated holes trapped at a deep level. Polarization results in the degradation of detector charge collection efficiency. We studied the spectroscopic behavior of CdZnTe under various X-ray fluxes ranging between and photons per mm per second. It was observed that polarization occurs at an X-ray flux higher than mm·s. Using simultaneous illumination of the detector by a de-polarizing LED at 1200 nm, it was possible to recover X-ray spectra originally deformed by the polarization effect.

Comparative study of deep levels in HVPE and MOCVD GaN by combiningO-DLTS and pulsed photo-ionization spectroscopy

Operational characteristics of sensors made of GaN significantly depend on technologically introduced defects acting as rapid traps of excess carriers which reduce charge collection efficiency of detectors. In order to reveal the prevailing defects in HVPE and MOCVD grown GaN, the carrier lifetime and photo-ionization spectra have been simultaneously measured by using microwave probed photo-conductivity transient technique. Several traps ascribed to impurities as well as vacancy and anti-site type defects have been identified in HVPE GaN material samples by combining photo-ionization and electron spin resonance spectroscopy. The optical deep level transient spectroscopy technique has been applied for spectroscopy of the parameters of thermal emission from the traps ascribed to technological defects in the Schottky barrier terrace structures fabricated on MOCVD GaN.

A method to simulate the observed surface properties of proton irradiated silicon strip sensors

During the scheduled high luminosity upgrade of the LHC, the world's largest particle physics accelerator at CERN, the position sensitive silicon detectors installed in the vertex and tracking part of the CMS experiment will face a more intense radiation environment than the present system was designed for. To upgrade the tracker to the required performance level, extensive measurements and simulation studies have already been carried out. A defect model for the bulk properties of proton irradiated silicon has been created with the Synopsys Sentaurus TCAD simulation package which produces simulated results that closely match the measured properties of silicon strip detectors. However, the expected behaviour due to increased surface damage is not so well predicted by the model. The solution requires an approach that does not affect the accurate bulk properties produced by the proton model, but only adds to it the required radiation induced properties close to the surface. These include the observed position dependence of the strip detector's charge collection efficiency (CCE). In this paper a procedure to find a defect model that reproduces the correct CCE loss, along with other surface properties of a strip detector up to a fluence 1.5×1015 1 MeV neq cm−2 (Φeq), will be presented. When applied to CCE loss measurements at different fluences, this method may provide a means for the parametrization of the accumulation of oxide charge at the SiO2/Si interface as a function of dose.

GaAs detectors irradiated by low doses of electrons

Semi-insulating (SI) GaAs detectors were irradiated by 5 MeV electrons up to a dose of 69 kGy, in order to test their radiationhardness. The electric and spectrometric stability of detectors was examinedas a function of the absorbed dose. Investigated detectors showed a verygood detector radiation resistance within a dose up to 40 kGy followed bydeterioration of some spectrometric and electric properties. However, thereverse current and the detector charge collection efficiency showed minimumchanges with the overall applied doses. The obtained results will be used asa preliminary study for further radiation-hardness investigations of GaAsdetectors against high energy electrons. This will complete our previousstudies of GaAs detector radiation hardness against fast neutrons andγ-rays.

Cryogenic Si detectors for ultra radiation hardness in SLHC environment

Radiation hardness up to 10 16 n eq/cm 2 is required in the future HEP experiments for most inner detectors. However, 10 16 n eq/cm 2 fluence is well beyond the radiation tolerance of even the most advanced semiconductor detectors fabricated by commonly adopted technologies: the carrier trapping will limit the charge collection depth to an effective range of 20–30 μm regardless of depletion depth. Significant improvement of the radiation hardness of silicon sensors has been taken place within RD39. Fortunately the cryogenic tool we have been using provides us a convenient way to solve the detector charge collection efficiency (CCE) problem at SLHC radiation level (10 16 n eq/cm 2). There are two key approaches in our efforts: (1) use of the charge/current injection to manipulate the detector internal electric field in such a way that it can be depleted at a modest bias voltage at cryogenic temperature range (⩽230 K); and (2) freezing out of the trapping centers that affects the CCE at cryogenic temperatures lower than that of the LN 2 temperature. In our first approach, we have developed the advanced radiation hard detectors using charge or current injection, the current injected diodes (CID). In a CID, the electric field is controlled by injected current, which is limited by the space charge, yielding a nearly uniform electric field in the detector, independent of the radiation fluence. In our second approach, we have developed models of radiation-induced trapping levels and the physics of their freezing out at cryogenic temperatures. In this approach, we intend to study the trapping effect at temperatures below LN 2 temperature. A freeze-out of trapping can certainly help in the development of ultra-radiation hard Si detectors for SLHC. A detector CCE measurement system using ultra-fast picosecond laser with a He cryostat has been built at CERN. This system can be used to find out the practical cryogenic temperature range that can be used to freeze out the radiation-induced trapping levels, and it is ready for measurements on extremely heavily irradiated silicon detectors. Initial data from this system will be presented.

Effects of bulk and surface conductivity on the performance of CdZnTe pixel detectors

We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 A) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metal-semiconductor-metal system with two back-to-back Schottky-barrier contacts. The high surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics which differ considerably from those of the bulk material. This surface layer has a profound effect on the charge collection efficiency in detectors with multi-contact geometry; some fraction of the electric field lines originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect we place a grid of thin electrodes between the pixel contacts; when the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump bonded to a custom-built VLSI readout chip.

Strong impact of x-ray radiation associated with electron beam metallization of diamond devices

Electron-beam evaporation of metal contact is commonly used in the fabrication of semiconductor devices. We have observed that device irradiation by x-ray photons, which are generated by electrons striking the metal (titanium or gold) to be evaporated, has a strong impact on the characteristics of diamond devices used for radiation detection. It results in an improvement of the detector charge collection efficiency by a factor of 1.5 with respect to nonirradiated devices (standard thermal evaporation). Thermally stimulated current measurements showed that this effect is related to deep trap filling by free carriers generated in diamond by x-ray photons impinging the device during e-beam evaporation. Trap filling results in an increase of the free carrier drift distance before trapping. Study of contact annealing and the thermal stability of trap filling showed that charge detrapping at temperatures above 200 °C annihilates the observed detector sensitivity improvement.

Study on the performance of SI-GaAs and SI-InP surface barrier detectors for alpha and gamma detection

Undoped semi-insulating gallium arsenide and iron (Fe)-doped indium phosphide radiation detectors have been fabricated. The electrical characteristics (I–V and C–V) of the detectors have been carried out at room temperature under dark conditions. By analysing the I–V characteristics of the detectors, the barrier height, ideality factor and reverse leakage current have been evaluated. The SI-GaAs detector has been tested using 241Am alpha particle source of 5.48 MeV energy under a vacuum of 1×10−3 Torr at room temperature. An alpha energy resolution of 6% has been obtained. The detector charge collection efficiency was measured using the source and was found to be 98%. The SI-InP detector has been tested using 57Co(122keV) and 137Cs(662keV) gamma sources and the energy response of the InP detector has been analysed. In this article, details on the performance of our SI-GaAs and SI-InP surface barrier detectors for alpha- and gamma-ray detection are presented.

Low temperature annealing effects on the performance of proton irradiated GaAs detectors

Semi-insulating, undoped, Liquid Encapsulated Czochralski (SI-U LEC) GaAs detectors have been irradiated with 24 GeV/c protons at the fluence of 5.6×10 13 p/cm 2. The detector charge collection efficiency, cce, for both electrons and holes is remarkably reduced after irradiation while the reverse current increases. The effect of annealing the detectors at temperatures ranging from 220 °C to 280 °C has been seen to reduce the reverse current and to increase the electron cce, while the recovery of the hole cce is negligible in irradiated detectors. Deep electron traps have been followed in their evolution with the heat treatment temperature by P-DLTS and C-V measurements. They recover by increasing the heat treatment temperature and this can explain the restoration observed in electron cce.

Radiation detector properties of CdTe0.9Se0.1:Cl crystals grown under microgravity in a rotating magnetic field

The influence of growth conditions on radiation detector performance was studied by examining Cd(Te,Se):Cl crystals. They were grown by the travelling heater method under microgravity conditions during the PHOTON 8 mission. An additional forced convection was produced by a rotating magnetic field (400 Hz) of 2 mT. The influence on crystal quality was analyzed by several characterization methods such as time dependent charge measurement, photo-induced current transient spectroscopy and admittance spectroscopy. The charge collection efficiency of detectors was increased from 10% without magnetic field up to 66% by the influence of the rotating magnetic field. The improved detector performance is explained by the increased resistivity and a reduced number of deep levels.