Charge Collection In 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.

Simulations of electrical properties of cylindrical 3d-trench electrical si detectors under different radiation fluences and mip incident position

In this work, we investigate the dependence of charge collection on the MIP(minimum ionizing particle) incident position for cylindrical 3D-Trench electrode Si detectors being irradiated to various fluences. Simulation results are: (1) For non-irradiated detectors, detector charge collection does not depend on the MIP incident position, i.e. the charge collection is always a constant, there is no effect of trapping; (2) For irradiated detectors, detector charge collection depends on the MIP incident position; (3) As radiation fluence increases, detector charge collection decreases, regardless of MIP incident position. This is due to the fact that charge trapping by radiation induced defects also increases with radiation fluences, resulting in less charge collection.

Optical and electrical characterization of Cadmium Telluride X-ray pad detectors

Tellurium defects in CdTe and CdZnTe detectors are known to degrade the detector performance by trapping passing charges. This causes losses in charge collection and ultimately degrades the energy resolution. The amount and size of the defects has previously been studied for small areas of these materials, and small defects (<5μ m) have been identified as the main cause for signal degradation. However, previous studies concentrated on the evaluation of a view regions of interest. In this study we present a system capable of scanning larger volumes of CdTe and CdZnTe and associate the locations of the found defects with the charge collection of detectors. Further, we show that the signal degradation is not uniform over the whole detector, but linked to local densities of defects. This results in variations of charge collection even within single 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.

3D Simulation and Modeling of Ultra-fast 3D Silicon Detectors

3D detectors with very small electrode spacing can provide ultra-fast detection due to their extremely small charge collection time. Since the detector full depletion voltage and charge collection time are independent to the detector thickness, ultra-fast 3D detectors can be made relatively thick (or not too thin, ~200 μm) to ensure a large signal. The results of the 3D simulations and modeling of 3D silicon detectors with very small electrode spacing and relatively large thickness will be shown in this paper. The column spacing LP is in the range of 5 μm to 10 μm. At a bias voltage of only a few volts, the electric field in the detector can be large enough to ensure the carrier saturation drift velocity in most volume of the detector, and the detector charge collection time there can be as short as 10’s of ps. In this paper, we will analysis the simulated electrical characteristics of this detector structure through systematic 3D simulations using the Silvaco’s TCAD tool. Profiles of detector electric potential and electric field will be presented. We will investigate the region of low electric field (the “slow region”) in the detector. We will also exam whether the detector reach the breakdown condition at operation voltages suggested in this work.

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.

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.

Proximity Charge Sensing With Semiconductor Detectors

Semiconductor radiation detectors are routinely used for the detection, imaging, and spectroscopy of gamma-ray, x-ray, and charged particles. In basic form, a detector is comprised of a semiconductor crystal with two or more electrodes formed on its surfaces. Besides allowing for the application of bias voltage, one or more of the electrodes on a detector also serve as readout electrode. Charge carriers drifting across the detector induce a charge signal on the electrode, which can then be measured by a charge-sensitive amplifier connected to the electrode. Although in general the readout electrodes of a detector are formed on the detector itself, charge can be induced on any electrode, even if the electrode is not physically in contact with the semiconductor. Such proximity charge sensing effects can be utilized to achieve a variety of advantages in applications involving semiconductor detectors. In this paper, we report on the experimental verification of signal readout using proximity electrodes and demonstrate several possible applications of this technique, including the position-sensitive readout of detectors and the sensing of incomplete charge collection in detectors as a means to reduce spectral background.

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.

Electric field distribution and charge transport properties in diode-like CdTe X-ray detectors

The physical behavior of electrode contacts and their technology is an active field of interest in room-temperature semiconductor detectors, where low currents at high electric fields are extremely desirable. Recently, CdTe detectors having a diode-like In/CdTe/Pt structure have been realized which are able to sustain very high applied reverse voltages while limiting the dark current at low values; moreover, in terms of energy resolution, they show excellent characteristics respect to conventional devices based on symmetric contacts. In this work, we investigate the electric field distribution inside these diode-like detectors by using the linear electro-optic effect (Pockels effect). We also discuss their local charge collection properties, accessed by performing IR pulsed laser experiments. These techniques, in conjunction with the analysis of current–voltage curves, allow us to consistently evidence the strong hole-blocking nature of the In contact. In order to analyze the different characteristics of the In and Pt contacts and how these contacts affect the detector charge collection properties, we compare the results with those obtained from a conventional Pt/CdTe/Pt structure.

Comprehensive device Simulation modeling of heavily irradiated silicon detectors at cryogenic temperatures

Radiation hardness is a critical design concern for present and future silicon detectors in high energy physics. Tracking systems at the CERN Large Hadron Collider (LHC) are expected to operate for ten years and to receive fast hadron fluences equivalent to 10/sup 15/cm/sup -2/ 1-MeV neutrons. Recently, low temperature operating conditions have been suggested as a means of suppressing the negative effects of radiation damage on detector charge collection properties. To investigate this effect, simulations have been carried out using the ISE-TCAD DESSIS device simulator. The so-called "three-level model" has been used. A comprehensive analysis of the influence of the V/sub 2/, C/sub i/O/sub i/ and V/sub 2/O capture cross sections on the effective doping concentration (N/sub eff/) as a function of temperature and fluence has been carried out. The capture cross sections have been varied in the range 10/sup -18/-10/sup -12/ cm/sup 2/. The simulated results are compared with charge collection spectra obtained with 1064-nm laser pulses on devices irradiated with 23-GeV protons as a function of detector bias voltage. To validate the model, a wide range of temperature and fluence has been studied using a one-dimensional (1-D) simplified structure. Thousands of simulation results have been cross checked with the experimental data. The data between 190 K (the lower limit for simulations due to computational difficulties) and 290 K are well reproduced for all of the fluences considered. We conclude that the three-level model can be successfully used to predict irradiated detector behavior down to a temperature of at least 190 K.

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.

Theory of ion beam induced charge collection in detectors based on the extended Shockley–Ramo theorem

An analysis of the charge collection process induced by focused MeV ion beams in semiconductor devices is presented. It is based on the extended Shockley–Ramo theorem that provides a rigorous mathematical tool for the calculation of the induced charge and current under the assumption of a quasi-steady-state operation of the semiconductor device. A complete description of the theory and underlying assumption is given as well as a simple application of the method aimed to evaluate the main transport properties of fully depleted semiconductors from the analysis of frontal and lateral ion beam induced charge collection (IBICC) measurements.

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.

Variable temperature performance of a Si(Li) detector stack

New experimental data is presented which displays /sup 137/Cs resolution of both single Si(Li) devices and a detector stack 2 cm in height as a function of temperature (85 K/spl les/T/spl les/245 K). We also discuss variations in photopeak shape which indicate that detector charge collection may be temperature dependent over the range of interest.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Status at Harwell of Opto-Electronic and Time-Variant Signal Processing for High Performance Nuclear Spectrometry with Semiconductor Detectors

A review of the advantages and problems associated with these signal processing methods is followed by a summary of recent developments at Harwell. A new spectrometry system for use with solid state charged particle and gamma ray detectors which can handle input pulse rates in excess of 106 per second with a high throughput rate and almost complete insensitivity to detector charge collection time will be described. The paper will discuss the benefits obtained in various applications and the remaining limitations.

Time spreads of pulses generated by beta-particles in silicon drifted detectors

Results of a study of time spreads of pulses from silicon drifted detectors, irradiated by 976 keV ( 207Bi) electrons at their positive electrode side are presented. A number of 0.8–2.4 mm thick detectors were tested at room temperature and at liquid nitrogen temperature. Two components of the time spreads have been found to exist; one of them being proportional to the detector pulse rise time and inversely proportional to the reduced average range of the beta particles, and the other one independent on these quantities, however, characteristic for the individual detector. Initial risetimes of the pulses (up to 20% of the total amplitude) were determined. The results are in disagreement with the predictions of the available approximate theory of charge collection in detectors.