M. Bruzzi

Radiation damage in silicon detectors for high-energy physics experiments

Radiation effects in silicon detectors are discussed in view of their application in future high-energy physics experiments. An overview is given of the major changes in the operational parameters due to radiation damage and their origin in the radiation-induced microscopic disorder in the silicon bulk. The principal radiation hardening technologies are described that have been adopted by the high-energy physics community to face the hostile radiation environment where silicon pixel and microstrip detectors will operate in the Large Hadron Collider.

Modelling of observed double-junction effect☆

Abstract New TCT measurements reveal the existence of a strong electric field, before full depletion, near both p+ and n+ side of high- and medium-resistivity silicon detectors, irradiated over space-charge sign inversion. More, by injecting carriers near the low-field side, double-peaked TCT current pulses are observed. This fact can be justified by assuming the presence of two deep levels in the gap, an acceptor like above mid-gap, and a donor like in the lower half of the gap, which can support the existence of two depleted regions. Particularly, the theoretical analysis of the TCT current profiles has been developed, and the second peak existence has been explained as the effect of carriers re-injection from ENB inside depleted regions.

Minimum ionizing and alpha particles detectors based on epitaxial semiconductor silicon carbide

The relatively high value of the energy required to produce an electron-hole pair in silicon carbide, SiC, by a minimum ionizing particle (MIP) against the value for Si, imposes severe constrains in the crystallographic quality, the thickness and the doping concentration of the SiC epitaxial layer used as the detection medium. In this work, a 40 /spl mu/m thick 4 H-SiC epitaxial layer with a low doping concentration of /spl sim/5/spl times/10/sup 13/ cm/sup -3/ was used in order to have a relatively high number (/spl sim/2200) of e-h pairs generated by a MIP and to deplete the total active layer at relatively low reverse bias (60 V). The detectors are realized by the formation of a nickel silicide (Ni/sub 2/Si) on the silicon surface of the epitaxial layer (Schottky contact) and of the ohmic contact on the backside of a 4 H-SiC heavily doped substrate. We present experimental data on the charge collection properties with /spl alpha/-particles from /sup 241/Am and /spl beta/-particles from /sup 90/Sr. In both cases, a 100% charge collection efficiency, CCE, is demonstrated and the diffusion contribution of the minority charge carriers to CCE is pointed out. The charge spectrum for MIPs from /sup 90/Sr shows a full detection efficiency with the pedestal (noise) clearly separated by the signal (Landau distribution) at reverse bias values comparable and higher than the one needed to totally deplete the layer. Moreover, no degradation was observed at 94/spl deg/C in the CCE and in the energy resolution of the /sup 241/Am alpha-signal from the SiC detector.

Review of the development of diamond radiation sensors

Abstract Diamond radiation sensors produced by chemical vapour deposition are studied for the application as tracking detectors in high luminosity experiments. Sensors with a charge collection distance up to 250 μm have been manufactured. Their radiation hardness has been studied with pions, proton and neutrons up to fluences of 1.9×10 15 π cm −2 , 5×10 15 p cm −2 and 1.35×10 15 n cm −2 , respectively. Diamond micro-strip detectors with 50 μm pitch have been exposed in a high-energy test beam in order to investigate their charge collection properties. The measured spatial resolution using a centre-of-gravity position finding algorithm corresponds to the digital resolution for this strip pitch. First results from a strip tracker with a 2×4 cm 2 surface area are reported as well as the performance of a diamond tracker read out by radiation-hard electronics with 25 ns shaping time. Diamond pixel sensors have been prepared to match the geometries of the recently available read-out chip prototypes for ATLAS and CMS. Beam test results are shown from a diamond detector bump-bonded to an ATLAS prototype read-out. They demonstrate a 98% bump-bonding efficiency and a digital resolution in both dimensions.

Deep-level transient spectroscopy measurements of majority carrier traps in neutron irradiated n-type silicon detectors

Abstract The effect of damage produced in silicon detectors by neutron irradiation at room temperature is examined by using the experimental technique of deep-level transient spectroscopy. The production of three defects, the A centre, the E centre, and the divacancy, is reported. The divacancy is especially important in neutron damage in silicon. There is evidence of some defects generated during annealing. It has been found that the properties of the point defects outside the clusters are the main results obtained with DLTS methods.

Characterisation of silicon carbide detectors response to electron and photon irradiation

Abstract A preliminary study of the response of SiC devices to 22 MeV electrons and 6 MV photon beams from a linear accelerator is presented in view to assess the feasibility of SiC-based dosimeters. The devices used are 4H–SiC epitaxial n-type layers deposited onto a 4H–SiC n+-type substrate wafer doped with nitrogen. Schottky contacts have been formed by gold deposition on the epitaxial layer. The released charge has been observed to increase linearly with the electron dose up to 10 Gy. A linear dependence of the current response of the devices has been also observed as a function of the photon dose-rate in the 2–7 Gy/min range. A preliminary study of the photoconductive response to UV irradiation of semi-insulating 6H–SiC substrates is also reported on samples, with a bulk resistivity of ≈1011 Ω cm, produced with a modified Lely technique.

Characterisation of CVD diamond dosimeters in on-line configuration

Abstract The high sensitivity and the nearly tissue equivalence of diamond make it a material suitable as detector for on-line dosimetry. The tremendous development of the CVD diamond technology allows to employ polycrystalline diamond films with controlled dimensions and a potential low cost as on-line radiation dosimeters. In this paper a characterisation and a comparison of the response of two “detector-grade” CVD diamond films under photon and electron beams are presented. The results show that both samples can be used as on-line dosimeters for applications in radiotherapy.

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.

Radiation hardness after very high neutron irradiation of minimum ionizing particle detectors based on 4H-SiC p/sup +/n junctions

In this work we analyzed the radiation hardness of SiC p/sup +/ n diodes used as minimum ionizing particle (MIP) detectors after very high 1 MeV neutron fluences. The diode structure is based on ion implanted p/sup +/ emitter in an n-type epilayer with thickness equal to 55 /spl mu/m and donor doping N/sub D/=2/spl times/ 10/sup 14/cm/sup -3/. The diode breakdown voltages were above 1000 V. At 1000 V the leakage currents are of the order of 1 nA for all the measured diodes. The full depletion voltage is near 220-250 V. The charge collection efficiency to minimum ionizing particle has been investigated by a /sup 90/Sr /spl beta/ source. At 250 V the collected charge of the unirradiated diodes saturates near 3000 e/sup -/. At bias voltages over 100 V the energy spectrum of the collected charge was found to consist of a signal peak well separated from the noise. At around 250 V the signal saturates, in agreement with CV results. These devices have been irradiated at 6 different fluences, logarithmically distributed in the range 10/sup 14/-10/sup 16/ (1 MeV) neutrons/cm/sup 2/. The leakage current after irradiation decreases. The collected charges decrease for increasing fluences, remaining very high only until some 10/sup 14/ n/cm/sup 2/.

Deep levels and trapping mechanisms in chemical vapor deposited diamond

Detector-grade undoped chemical vapor deposited (CVD) diamond samples have been studied with thermally stimulated currents (TSC) and photoinduced current transient spectroscopy (PICTS) analyses in the temperature range 300–650 K. Two previously unknown defects have been identified, characterized by activation energies E1=1.14 eV and E2=1.23 eV, cross sections of about σ≈10−13 cm2 and concentrations of Nt≈1016 cm−3. They have been clearly observed by PICTS and isolated in TSC measurements by use of a fractional annealing cycle in the temperature range 300–400 K. Due to their trap parameters, in particular the high cross section, the levels corresponding to E1 and E2 are characterized by capture times of the order of 10–100 ps. A dominant TSC peak observed at ≈500 K has been also investigated and has been resolved into four components with activation energies of the order of 1 eV and cross sections in the range 10−19–10−17 cm2. Three of these levels exhibit a fast capture rate (0.1–10 ns) in spite of their ...