V. Eremin

Development of transient current and charge techniques for the measurement of effective net concentration of ionized charges (Neff) in the space charge region of p-n junction detectors☆

Abstract Transient current and charge techniques (TCT/TChT) have been developed as alternatives to the standard C-V measurements for measurements of the effective net concentration of ionized charges (Neff) in the space charge region (SCR) of Si p-n junction detectors, especially for heavily irradiated detectors. This paper contains the physical background of the techniques, modeling of current and charge pulse response, and applications of the methods to the characterizations of silicon planar detectors designed for high energy physics.

Improved neutron radiation hardness for Si detectors: application of low resistivity starting material and/or manipulation of N/sub eff/ by selective filling of radiation-induced traps at low temperatures

Radiation-induced electrical changes in both the space charge region (SCR) of Si detectors and bulk material (BM) have been studied for samples of diodes and resistors made on Si materials with different initial resistivities. The space charge sign inversion fluence (/spl Phi//sub inv/) has been found to increase linearly with the initial doping concentration (the reciprocal of the resistivity), which gives improved radiation hardness to Si detectors fabricated from low resistivity material. The resistivity of the BM, on the other hand, has been observed to increase with the neutron fluence and approach a saturation value in the order of hundreds k/spl Omega/cm at high fluences, independent of the initial resistivity and material type. However, the fluence (/spl Phi//sub s/), at which the resistivity saturation starts, increases with the initial doping concentrations and the value of /spl Phi//sub s/ is in the same order of that of /spl Phi//sub inv/ for all resistivity samples. Improved radiation hardness can also be achieved by the manipulation of the space charge concentration (N/sub eff/) in SCR, by selective filling and/or freezing at cryogenic temperatures the charge state of radiation-induced traps, to values that will give a much smaller full depletion voltage. Models have been proposed to explain the experimental data.

Elevated temperature annealing of the neutron induced reverse current and corresponding defect levels in low and high resistivity silicon detectors

A new aspect of degradation phenomena of neutron irradiated silicon detectors has been revealed which consists in the significant influence of carbon related defect transformation on the detector reverse current (I/sub rev/). The annealing of the reverse current at elevated temperatures and the corresponding changes of the deep level transient spectroscopy (DLTS) spectra of defects for fast neutron irradiated silicon detectors, fabricated on high (4-6) k/spl Omega/-cm, moderate (0.5-1.0 k/spl Omega/-cm) and low ( >

Semi-insulating LEC GaAs as a material for radiation detectors: materials science issues

Abstract Semi-insulating (SI) GaAs is now being reconsidered as a promising material for radiation detectors, mostly due to greatly improved quality of the material. In this paper we shall describe the properties of the state-of-the-art SI GaAs crystals grown by LEC method as relevant for such applications. Specifically, we shall concentrate on the assessment of the spectra and density of residual impurities, on the measurements of deep levels spectra and on studying the impact of these centers on material parameters. The spectra of deep centers as studied by PICTS and DLTS indicate that, in LEC crystals grown from stoichiometric melts, besides the most prominent EL2 center, other electron and hole traps are usually observed. By comparing the DLTS and PICTS results we show that PICTS often tends to overestimate the relative contributions of traps shallower than EL2. Two approaches to decreasing EL2 concentration in LEC grown crystals have been proposed in literature: to grow crystals from Ga-rich melts and to anneal the material at high temperatures. We have studied the changes in deep level spectra and transport parameters for both cases. In annealing experiments the possibility of thick samples modification by arsenic diffusion was also investigated.

A comparative study of EL2 and other deep centers in undoped SI GaAs using optical absorption spectra and photoconductivity measurements

The performance of radiation detectors fabricated from semi-insulating (SI) GaAs is highly sensitive to EL2 + concentration in the material. Near-infrared optical absorption measurements are commonly used to determine the EL2-concentration and to roughly estimate the EL2 + -concentration under the assumption that the optical absorption is mainly determined by the photoionization and the photoneutralization of EL2 0 and EL2 + , respectively. However, the presence of different native defects can contribute to optical absorption and reduce the precision of determination of EL2-concentration. In this work, we evaluate the contributions into optical absorption from EL2 and other deep center namely EL3 defect (0.55 eV) using near-infrared optical absorption and photoconductivity (PC) measurements in the photon energy interval 0.5–1.4 eV for SI GaAs crystals grown by the liquid encapsulated Czochralski method from melts with As content changing from 50% to about 46%. The photoelectrical spectra were measured on p–i–n structure detectors with heavily doped p + and n + layers grown by Liquid Phase Epitaxy and on Schottky diodes. The short circuit photocurrent spectra were registered for all detectors in the energy interval 0.65–1.4 eV. Unexpectedly, the current sensitivities in the regions of the extrinsic and intrinsic absorption were comparable. A comparative study of optical absorption, PC and short circuit photocurrent spectra resulted in determination of EL2 + -concentration. It was concluded that contribution of additional deep centers, particularly the ionized EL3 + defect could be comparable to the EL2-contribution. The EL3 centers were attributed to oxygen-related defects based on published results and on some indirect evidence in our experimental data. r 2003 Elsevier B.V. All rights reserved.

New experimental and analysis methods in I-DLTS

Abstract A simple modification to traditional experimental apparatus to perform I-DLTS measurements is presented. This setup is shown to be faster and more sensitive than traditional double boxcar I-DLTS systems. A novel analysis technique utilising multiple exponential fits to the I-DLTS signal from a highly neutron irradiated silicon sample is presented along with a discussion of the results. It is shown that the new method has better resolution and can deconvolute overlapping peaks more accurately than previous methods.

Current injected detectors (CID) - a new approach for detector operation in very high radiation environment

For the upcoming Super LHC (SLHC) experiments on the LHC upgrade, Si detectors will still be considered as the main detectors for inner tracker. However, the radiation level in SLHC will be up to 10 times more than that in LHC due to the increase of luminosity from 1034 cm-2s-1 to 1035 cm-2s-1. In this study a new approach for radiation hard detectors based on the current injection in the detector bulk is considered. It is shown that the symmetric p+-n-p+ structures as well as regular p+-n-n+ silicon detectors after irradiation by neutrons up to 5middot1014 cm-2 can operate at self-stabilized current injection (SSCI) or space charge limited current (SCLC) mode. This provides a stable electric field profile at any higher fluences with the electric field distributed in the entire detector thickness (full depletion mode). In this study operation of heavily irradiated Si detectors in the SSCI mode is confirmed by the measurements of I-V characteristics, which show the region with square I on V dependence and then the sharp current rise at a threshold voltage. It is shown that the threshold voltage rises linearly with the irradiation fluence that allows biasing the detectors by higher voltage with the fluence increase. This is obviously helpful for reduction of the collection time and consequently the related charge loss due to trapping.

New high sensitivity silicon photodetectors for medical imaging applications

We have developed a new silicon photodiode design that reduces the dark current and can improve the sensitivity of low noise silicon photodetector arrays for medical imaging applications. The reduction in dark current eliminates the need for cooling, which facilitates the mechanical design and allows for the optimum performance of the scintillators that are coupled to the photodiodes. The photodetectors are based on a planar p+ planar entrance window on n-type silicon substrates, with n+ pixels surrounded by a p+ separating grid. Using this approach, dark currents on the order of 0.2 nA/cm/sup 2/ have been achieved (for 0.3 mm thick devices, at 23/spl deg/C). The quantum efficiency of the p+ entrance window contact is 83% at 560 nm, which makes it an excellent match for CsI(Tl) scintillators. The current/voltage characteristics, response of the photodiodes to x-rays and gamma-rays when coupled to scintillators has been measured. These prototypes will be evaluated with respect to their applications in gamma cameras for medical imaging.

A comparative study of heavily irradiated silicon and non irradiated SI LEC GaAs detectors

Silicon p/sup +/n junctions irradiated with neutron and proton fluences in the range 5/spl times/10/sup 11/ 4/spl times/10/sup 15/ cm/sup -2/ and non-irradiated SI LEC GaAs Schottky barriers have been analyzed. In silicon the concentration N, of the main radiation-induced deep traps (Et/spl ap/0.4/spl divide/0.53 eV) is found to increase as N, /spl alpha/ f/sup 3/2/ achieving values up to 5/spl times/10/sup 15/ cm/sup -3/ and a mobility saturation at 100 cm/sup 2//Vs has been observed at the highest fluences. A quantitative comparison between heavily irradiated silicon and non-irradiated GaAs evidenced similar charge collection efficiencies, a quasi-intrinsic bulk and similar concentrations of deep defects. On this basis, a unique model, correlating the lattice disorder and the detector performance, is suggested.

Analytical Solutions Of Minimum Ionization Particle Induced Current Shapes Of Silicon Detectors And Simulation Of Charge Collection Properties

A new analytical, one dimensional method to obtain the induced current shapes and simulation of chasrge shapes for p{sup +} {minus}n{minus}n{sup +} silicon detectors in the case of minimum ionization particle has been developed here. jExact solutions have been found for both electron and hole current shapes. Simulations of induced charge shapes of detectors have also been given. The results of this work are consistent with the earlier work where a semi-analytical method had been used.