S. Sciortino

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.

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.

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 ...

Pulse height distribution and radiation tolerance of CVD diamond detectors

The paper reviews measurements of the radiation tolerance of CVD diamond for irradiation with 24 GeV/c protons, 300 MeV/c pions and 1 MeV neutrons. For proton and neutron irradiation, the measured charge signal spectrum is compared with the spectrum calculated by a model. Irradiation by particles causes radiation damage leading to a decrease of the charge signal. However, both the measurements and the outcome from the model shows that for tracker applications this drawback is at least partly counterbalanced by a narrowing of the distribution curve of the charge signal. In addition, we observed after proton irradiation at the charge signal spectrum a decrease of the number of small signals. As a result, the efficiency of a CVD diamond tracker is less affected by irradiation than the mean charge signal. (11 refs).

Three-dimensional diamond detectors: Charge collection efficiency of graphitic electrodes

Implementation of 3D-architectures in diamond detectors promises to achieve unreached performances in the radiation-harsh environment of future high-energy physics experiments. This work reports on the collection efficiency under β-irradiation of graphitic 3D-electrodes, created by laser pulses in the domains of nanoseconds (ns-made-sensors) and femtoseconds (fs-made-sensors). Full collection is achieved with the fs-made-sensors, while a loss of 25%–30% is found for the ns-made-sensors. The peculiar behaviour of ns-made sensors has been explained by the presence of a nano-structured sp3-carbon layer around the graphitic electrodes, evidenced by micro-Raman imaging, by means of a numerical model of the charge transport near the electrodes.

Characterisation of epitaxial SiC schottky barriers as particle detectors

Abstract Epitaxial SiC devices have been tested as radiation detectors for minimum ionising particles. The devices used are based on a commercial 4H–SiC epitaxial n-type layer deposited onto a 4H–SiC n + type substrate wafer doped with nitrogen. Single-pad Schottky contacts have been produced by deposition of a 1000-A gold film on the epitaxial layer using a lift-off technology and ohmic contacts have been deposited on the rear substrate side. The capacitance–voltage characteristics have been measured to determine the net effective doping in the space charge layer and the maximum active thickness of the devices. The measurements showed possible non-uniformity in the net doping of the epitaxial layer. The charge collection efficiency (CCE) has been tested by means of a 0.1 mCi 90 Sr β-source. A 100% CCE is measured at the maximum active thickness, which is achieved above approximately 400 V. The charge signal of the SiC devices is stable and reproducible, with no evidence of priming or polarisation effects, due to the high crystalline quality of the epitaxial layer.

Thermoluminescence analysis of - and -irradiated chemical vapour deposited diamond films

The thermoluminescent (TL) response of a set of synthetic diamond films obtained by chemical vapour deposition was investigated using gamma and beta radioactive sources over the dose range from 60 mGy to 3.5 Gy. A TL response linear with dose was generally observed. A numerical curve-fitting procedure was applied to deconvolute the complex-structured TL glow curves, which were found to be composed at most by five peaks in the temperature range between C to C. A short preheat treatment after irradiation allowed us to obtain a very stable smooth-shaped glow curve composed of a unique peak close to C. This isolated structure followed first-order kinetics over the whole dose range. The activation energy of the main peak, eV, was determined considering all samples and all irradiation doses using both the peak shape method and the fitting procedure. The corresponding frequency factor was estimated to be within the range -. The TL sensitivity of the diamond films calculated considering the isolated high-temperature peak compares favourably with that measured with a set of LiF TLD100 and TLD700 standard dosimeters.