Silicon Radiation Research Articles

Measurements and simulations of silicon semi 3D radiation detectors

New results on the semi three-dimensional (3D) silicon radiation pixel detectors are reported. In particular, improved capacitance, leakage current measurements and 3D simulations are considered. In addition, comparisons between the experimental data and simulations are performed. The results support the anticipated beneficial features of 3D detector structures like low depletion bias, low leakage currents combined with high breakdown voltage and fast collection speed at low bias voltages.

Technology development of p-type microstrip detectors with radiation hard p-spray isolation

A technology for the fabrication of p-type microstrip silicon radiation detectors using p-spray implant isolation has been developed at CNM-IMB. The p-spray isolation has been optimized in order to withstand a gamma irradiation dose up to 50 Mrad (Si), which represents the ionization radiation dose expected in the middle region of the SCT-Atlas detector of the future Super-LHC during 10 years of operation. The best technological options for the p-spray implant were found by using a simulation software package and dedicated calibration runs. Using the optimized technology, detectors have been fabricated in the Clean Room facility of CNM-IMB, and characterized by reverse current and capacitance measurements before and after irradiation. The average full depletion voltage measured on the non-irradiated detectors was V FD=41±3 V, while the leakage current density for the microstrip devices at V FD+20 V was 400 nA/cm 2.

Annealing of ion-implanted SiC by laser-pulse-exposure-generated shock-waves

Athermal annealing in phosphorus, boron, and aluminum/boron implanted 4H-SiC is explored by launching mechanical shock waves, induced by exposure to a laser pulse. Annealing is observed in a 3.2 mm outer diameter donut-shaped area surrounding the laser exposed spot. The minimum sheet-resistance within this area in phosphorus-implanted sample is close to the thermally annealed value of 400 Ω/□. Unlike thermal annealing, shock annealing did not cause any redistribution of the implant, including boron, a known transient enhanced diffuser in SiC. Optimization of parameters like laser power and/or pulse duration or investigation of an alternate mechanical shockwave launching method is required to achieve athermal annealing far away from the center of the exposed region and similar to that of the thermal annealed material. Results of this study might help in the manufacturing of silicon carbide high-power, high-temperature, and radiation hardened devices in the future.

Active-edge planar radiation sensors

Many systems in medicine, biology, high-energy physics, and astrophysics require large area radiation sensors. In most of these applications, minimizing the amount of dead area or dead material is crucial. We have developed a new type of silicon radiation sensor in which the device is active to within a few microns of the mechanical edge. Their perimeter is made by a plasma etcher rather than a diamond saw. Their edges can be defined and also passivated by growing, in an intermediate step, a field oxide on the side surfaces. In this paper, the basic architecture and results from a synchrotron beam test are presented.

Precise dose evaluation using a commercial phototransistor as a radiation detector

An experimental arrangement and a circuitry based on an NPN phototransistor-type silicon radiation detector have been used for evaluating the X-ray beam dose in the diagnostic range. The circuitry was built to allow alteration of the electric field in the phototransistor internal structure, with some devices that have an available base connection. By changing the transistor base bias it is possible to alter its operation point to obtain a response gain from the selected photon energy range. In this way we have made an electronic energy-domain discretisation and we are investigating a model to calculate the dose contribution from each energy discretised into 10 keV steps. The method has been tested in filtered radiation beams generated from an HF-160 Pantak X-ray unit and compared with the usual dosimetry method. Our results have demonstrated that it is possible to make such a dose deconvolution from 40 to 140 keV energies by controlling the phototransistor base bias properly.

Investigation of voltages and electric fields in silicon radiation detectors using a scanning electron microscope

The paper describes qualitative and quantitative methods to measure voltages and electric fields in a biased silicon p +/n −/n + radiation detector with a scanning electron microscope using voltage-contrast phenomenon. The contrast is converted to voltage mathematically using simple equations. After splitting the detector, voltages and electric fields inside the detector can be imaged and measured. The results are compared with capacitance–voltage measurements and 2D electrical simulations.

Study of the Doppler broadening of positron annihilation radiation in silicon

We report the measurement of Doppler broadening annihilation radiation in silicon, using 22Na as a positron source, and two Ge detectors arrangement. The two-dimensional coincidence energy spectrum was fitted using a model function. The model function included at rest positron annihilation with valence band, 2p, 2s, and 1s electrons. In-flight positron annihilation was also fitted. The detectors response functions included backscattering, and a combination of Compton effects, pileup, ballistic deficit, and pulse shaping problems. The obtained results agree well with the literature.

Investigation of type inversion of n-bulk in 10 MeV proton-irradiated FZ silicon detectors using a scanning electron microscope

Based on the results of capacitance–voltage measurements and transient current technique, it was earlier deduced that the n-type bulk of float zone silicon radiation detectors changes type in heavy irradiation. This paper describes the results of measuring the voltages and electric fields with a scanning electron microscope using the voltage–contrast effect, inside radiation detectors that were irradiated with 10 MeV protons with several fluences. The results confirm the earlier observations and give more accuracy to the electric field measurements.

FTIR investigation of porous silicon formed in deutrofluoric acid based solutions

AbstractAbsorption of infrared radiation in the porous silicon (PS) films prepared by common electrochemical etching in DF‐ethanol‐D6 solutions was studied. We report our results of the effect of deuterium termination on the process of surface states changes of the PS after their exposure to ambient air for few months. In addition to these results, we also show that the replacement of hydrogen with deuterium leads to decreasing the concentration of the silicon dangling bonds at the surface of the PS. As a result, more stable passivation layer was formed. There were not any detectable quantities of the hydrogen atoms at the surface of PS during few months after its preparation. This fact is explained from the point of view of the dissociative water adsorption mechanism. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of magnetic Czochralski silicon radiation detectors

Silicon wafers grown by the Magnetic Czochralski (MCZ) method have been processed in form of pad diodes at Instituto de Microelectrònica de Barcelona (IMB-CNM) facilities. The n-type MCZ wafers were manufactured by Okmetic OYJ and they have a nominal resistivity of 1 kΩ cm. Concentrations of oxygen and carbon in MCZ wafers were measured by Secondary Ion Mass Spectroscopy (SIMS) technique to be compared to standard and oxygenated float zone (FZ) silicon material. The diodes were characterized by leakage current and capacitance measurements. The average full depletion voltage, V FD, of the pad detectors is 290±10 V, while the leakage current density at V FD+20 V is 7±3 nA/cm 2. Minority carrier lifetime of MCZ, standard FZ and oxygenated FZ substrates was measured by using a quasi steady-state photoconductance technique. The uniformity of the MCZ wafers was studied before the fabrication by mapping the resistivity of the wafer and after the fabrication process by measuring the full depletion voltage and leakage current as a function of diode position on the wafer. It was found that the fabrication process did not change the resistivity of the material although no donor-killing heat treatment was performed on the wafers.

Epitaxy — a new technology for fabrication of advanced silicon radiation detectors

Twenty five years after the introduction of the planar process to the fabrication of silicon radiation detectors a new technology, which replaces the ion implantation doping by silicon epitaxy is presented. The power of this new technique is demonstrated by fabrication of silicon drift detectors (SDDs), whereby both the n-type and p-type implants are replaced by n-type and p-type epi-layers. The very first SDDs ever produced with this technique show energy resolutions of 150 eV for 55Fe at −35 °C. The area of the detectors is 10 mm 2 and the thickness 300 μm. The high potential of epitaxy for future detectors with integrated complex electronics is described.

3D silicon detectors and radiation hardness issues

3D silicon detectors and radiation hardness issues

Silicon radiation detectors

The rapid progress of the past 20 years in silicon radiation detectors is reviewed. The availability of silicon as an almost ideal semiconductor material is one of the main reasons for this progress. The well-defined properties of the silicon-silicon dioxide interface allowed the development of detector structures beyond the structure of a classical diode detector, which was practically the only silicon detector structure 20 years ago. All new structures make use of an electric field parallel to the large surface of the detector. Improvements in the production of silicon detectors has lead to a substantial decrease of the detector leakage current. These detectors can achieve their best performance very close to or at room temperature.

Effect of Combined Oxygenation and Gettering on Minority Carrier Lifetime in High-Resistivity FZ Silicon

High oxygen concentration in silicon increases the resistance of radiation detectors to high radiation doses. Unfortunately, high-resistivity float zone (FZ) silicon, needed for radiation detectors, has too low an oxygen content. The solution to this is silicon oxygenation. There are different ways of incorporating oxygen into silicon. The most accepted one is by high-temperature diffusion from a thick layer. In this paper, we investigate the impact of this silicon oxygenation technique as well as the application of different gettering techniques to improve the minority carrier lifetime of high-resistivity FZ silicon substrates for radiation detectors. The minority carrier lifetimes before and after surface etching have been measured on samples subjected to different oxygenation and gettering treatments by using a quasi steady-state photoconductance technique. A lifetime improvement efficiency factor is defined for each treatment process. The lifetime efficiency factors behave independently, so that the lifetime efficiency factors associated with different sequential combinations of treatments can be estimated by a multiplicative combination. Different gettering techniques that improve or degrade the minority carrier lifetime are analyzed, and the best options for silicon radiation detector fabrication are determined. Oxygenated silicon with a minority carrier lifetime close to 1 ms can be obtained. © 2004 The Electrochemical Society. All rights reserved.

Response of a Si-diode-based device to fast neutrons

Semiconductor devices based on a Si-detector are frequently used for charged particle's detection; one application being in the investigation of cosmic radiation fields. From the spectra of energy deposition events in such devices, the total energy deposited by the radiation in silicon can be derived. This contribution presents the results of studies concerning the response of this type of detector to fast neutrons. First, the spectrum of energy deposition was established in fast neutron radiation fields with average energies from 0.5 to 50 MeV. It was found that these spectra vary significantly with the neutron energy. The comparison with the spectra registered in photon beams permitted an estimation of the part of energy deposited that could be attributed to neutrons. It was found that this part increases rapidly with neutron energy. The possibilities to use this type of detector for neutron detection and dosimetry for radiation protection are analysed and discussed.

Characterization and TCAD modelling of termination structures for silicon radiation detectors

We have recently proposed a novel junction termination structure for silicon radiation detectors, featuring all-p-type multiguard and scribe-line implants, with metal field-plates completely covering the gap between the implanted rings. The structure is intended for detector long-term stability enhancement even in adverse ambient conditions and for fabrication-process simplification. A thorough static characterization, including stability measurements in varying humidity conditions, has been carried out on a variety of samples fabricated at ITC-irst. Comparisons with diodes featuring an n-type implant along the border—or no edge structure at all—have been performed. The new structures show stable behaviour at relatively high bias (∼200 V) , also in the presence of wide humidity changes (1–90%). A good qualitative agreement has been obtained between experimental results and simulation predictions, allowing to gain deep insight into the physical behaviour of the device.

An improved termination structure for silicon radiation detectors with all-P-type multiguard and cut-line implants

A junction termination structure for silicon radiation detectors is investigated, featuring all-p-type multiguard and scribe-line implants, with metal field-plates providing complete coverage of the oxide upper surface above nonimplanted regions. The sensitive interface between oxide and n-type substrate is thus electrostatically screened from the external environment, resulting in improved long-term stability of the device and excellent insensitivity to ambient conditions both before and after X-ray and neutron irradiations. Careful design of the multiguard layout enables high-voltage operation to be achieved. With respect to a previously proposed structure, the adoption of alternate outward and inward field plates between adjacent rings allows a significant improvement in the voltage handling capability.

Device simulations of silicon detectors: a design perspective

Device simulation allows for accurate analysis of device behavior, accounting for several physical details that cannot easily be taken into account within compact, equivalent-circuit models. This is especially true for some issues typical of the design of silicon radiation detectors, where silicon properties are exploited in a non-conventional way and radiation damage raises severe reliability concerns. In this paper, a couple of significant applications of device simulation to the investigation and design of advanced solid-state radiation sensors are presented. More specifically, (i) radiation damage influence on detectors operating at cryogenic temperatures is successfully modeled and (ii) features of an innovative scheme for CMOS active pixel sensors are analyzed by means of mixed-mode simulation tools. From these examples, the usefulness and potentiality of advanced simulation techniques in the perspective of radiation detectors can be appreciated.

Detection of hydrogen impurity in silicon radiation detectors

The behavior of silicon particle detectors irradiated with 2.5-MeV electrons during subsequent annealing is studied. Annealing at 100–250°C was found to result in the formation of two types of traps with the levels E c −0.32 eV and Ev+0.29 eV. Increasing the annealing temperature to 300°C makes both traps disappear. On the basis of data obtained, it was concluded that these traps are related to hydrogen-containing complexes. The presence of hydrogen in a crystal results in a decrease in the annealing temperature for vacancy-oxygen (VO) complexes and complexes consisting of carbon and oxygen interstitials (CiOi). The reason for this phenomenon is the passivation of these complexes by hydrogen, which results in the formation of electrically active VOH centers {with the level E c −0.32 eV} in an intermediate stage of this process. It is assumed that hydrogen penetrates the structures under investigation in one of the stages of their fabrication.

Reduction of the dead region for edge on strip detector by a guard ring structure

p-i-n structures and high-resistivity silicon crystals are frequently used for silicon radiation detectors. The depletion layer of the reversed biased pn junction spreads vertically to the back side of the detector while it spreads laterally to a distance comparable to the thickness of the wafer during operation. As a result, a volume of nonactive silicon has to be introduced at the periphery of the active of the detector. The distance between the of the wafer and the ends of the strips, or the so-called dead region, can pose serious problems to the use of strip detectors in edge on configurations for soft X-ray applications. It may also degrade the resolution at abutting detector edges for detectors that are pieced together by multiple detectors. In this paper, a guard ring structure has been introduced in a 1 /spl times/ 8 edge on silicon strip detector. The guard ring is biased at the same potential as the strip p-n junctions. It has been demonstrated that the length of the dead region of the edge on strip detector can be dramatically reduced without significant increase in the leakage of current in the detector.