F. Nava

Silicon carbide and its use as a radiation detector material

We present a comprehensive review of the properties of the epitaxial 4H silicon carbide polytype (4H–SiC). Particular emphasis is placed on those aspects of this material related to room, high-temperature and harsh environment ionizing radiation detector operation. A review of the characterization methods and electrical contacting issues and how these are related to detector performance is presented. The most recent data on charge transport parameters across the Schottky barrier and how these are related to radiation spectrometer performance are presented. Experimental results on pixel detectors having equivalent noise energies of 144 eV FWHM (7.8 electrons rms) and 196 eV FWHM at +27 °C and +100 °C, respectively, are reported. Results of studying the radiation resistance of 4H–SiC are analysed. The data on the ionization energies, capture cross section, deep-level centre concentrations and their plausible structures formed in SiC as a result of irradiation with various particles are reviewed. The emphasis is placed on the study of the 1 MeV neutron irradiation, since these thermal particles seem to play the main role in the detector degradation. An accurate electrical characterization of the induced deep-level centres by means of PICTS technique has allowed one to identify which play the main role in the detector degradation.

Temperature dependence of semiconducting and structural properties of Cr‐Si thin films

Electrical and structural properties of coevaporated Cr‐Si thin alloy films and bilayer Cr/Si films as a function of annealing temperature from 10 to 1000 °K have been studied by in situ electrical resistivity and Hall measurements, and structural analysis including MeV 4He+ ion backscattering, x‐ray diffraction, Auger electron spectroscopy combined with Ar sputtering, electron microprobe, and scanning and transmission electron microscopy. In the as‐deposited state, the coevaporated alloy film was amorphous. Upon annealing, a sharp increase in resistivity occurred near 270 °C and the increase has been determined to be amorphous to crystalline CrSi2 phase transformation. The resistivity increased further with annealing up to 550 °C then a gradual decrease took place beyond 600 °C. In cooling, the resistivity increased monotonically with decreasing temperature. For the bilayer Cr/Si films, the annealing behavior is similar except the sharp increase in resistivity occurred around 450 °C due to the formation ...

Electrical and optical properties of silicide single crystals and thin films

Abstract Electrical transport and optical properties of transition-metal silicides are reviewed. They are integrated with thermal properties of single-crystal silicides. Most of these compounds behave as metals while some of them behave as semiconductors. The former show an increasing electrical resistivity ρ with increasing temperature. Several of them show a non-classical deviation of ρ ( T ) from linearity in the high-temperature limit. This deviation, related to intrinsic properties of the compound, can be affected both in sign and in amount by the presence of foreign atoms (impurities) and structural defects. Moreover, defects dominate the electrical transport at low temperatures both in metallic and semiconducting compounds. Therefore, the interpretation of the electrical properties measured as a function of temperature may give a non-realistic description of silicide intrinsic properties. Since also other physical properties, like thermal and optical ones, can be strongly affected by impurities and defects, results about single-crystal silicides will be first illustrated. Single-crystal preparation and structural characterization are described in detail, with emphasis on crystalline quality in terms of residual resistivity ratio. The electrical quantities, resistivity and magnetoresistance, are measured as a function of temperature and along the main crystallographic directions. The effect of impurities and defects on the transport properties is then evaluated by examining the electrical transport of polycrystalline thin-film silicides. The different contributions to the total resistivity are measured by changing: (i) film stoichiometry, (ii) impurity concentration, (iii) texture growth and (iv) film thickness. Hall-coefficient measurements are briefly discussed with the main purpose to evidence that great caution is necessary when deducing mobility and charge-carrier density values from these data. The theoretical models currently used to interpret the low- and high-temperature resistivity behavior of the metallic silicides are presented and used to fit the experimental resistivity curves. The results of these studies reveal that in several cases there are well-defined temperature ranges in which a specific electron—phonon scattering mechanism dominates. This allows a more detailed study of the microscopic processes. The optical functions from the far-infrared to the vacuum ultraviolet, derived from Kramers—Kronig analysis of reflectance spectra or directly measured by spectroscopic ellipsometry, are presented and discussed for some significant metallic disilicides, both single crystals and polycrystalline films. Different physical phenomena are distinguished in the spectra: intraband transitions at the lowest photon energies, interband transitions at higher energies, and collective oscillations. In particular, the free-carrier response derived from this analysis is compared with the transport results. The interpretation of the experimental spectra is based on the calculated electronic structures or optical functions. Moreover, it is shown how the optical studies contribute to assess definitively the semiconducting character of some disilicides. Specific-heat measurements on single crystals between 0.1 and 8 K are reported. The Debye temperature and the density of electronics states at the Fermi surface are deduced from the lattice and electronic contributions, respectively. Some silicides have been found superconductors with small electron—phonon coupling constants. Emphasis is given to the comparison between the properties deduced from these studies and those obtained from the analysis of electrical transport data. The final part of this review is devoted to the calculation of some microscopic physical quantities, as for example the electron mean free path, the charge-carrier density, the Fermi velocity. The parameters of the best fit to the experimental resistivity curves, the free-carrier parameters obtained from infrared spectra and the density of electronic states at the Fermi surface determined from specific-heat measurements were used in such evaluations.

Electrical transport properties of transition‐metal disilicide films

Electrical resistivity in the temperature range of 2–1100 K and Hall‐effect measurements from 10 to 300 K of CoSi2, MoSi2, TaSi2, TiSi2, and WSi2 polycrystalline thin films were studied. Structure, composition, and impurities in these films were investigated by a combination of techniques of Rutherford backscattering spectroscopy, x‐ray diffraction, transmission electron microscopy, and Auger electron spectroscopy. These silicides are metallic, yet there is a remarkable difference in their residual resistivity values and in their temperature dependence of the intrinsic resistivities. For CoSi2, MoSi2, and TiSi2, the phonon contribution to the resistivity was found to be linear in temperature above 300 K. At high temperatures, while a negative deviation from the linearity followed by a quasisaturation was observed for TaSi2, the resistivity data of WSi2 showed a positive deviation from linearity. It is unique that the residual resistivity, ρ(2 K), of the WSi2 films is quite high, yet the temperature depend...

Deep levels by proton and electron irradiation in 4H–SiC

The effects on 4H-silicon carbide epilayers of irradiation with protons and electrons having particle energies, respectively, of 6.5 and 8.2MeV were carefully studied and critically compared. In detail, the electronic levels associated with the irradiation-induced defects were analyzed by current-voltage characteristics and deep-level transient spectroscopy (DLTS) measurements up to 550K. In the same temperature range the apparent free-carrier concentration was measured by capacitance-voltage characteristics in order to monitor compensation effects due to the deep levels associated with the induced defects. Introduction rate, enthalpy, and capture cross section of such deep levels were compared. We found that a set of deep levels (at ET=0.39eV, ET=0.65eV, and ET=0.75eV) is the same in both cases of proton and electron irradiations, whereas two other pairs of levels (S1, ET=0.20eV and S1*, ET=0.23eV; S5, ET=1.09eV and S5*, ET=0.89eV) appearing in the same temperature range within the DLTS spectra should be...

Characterization of the Transport Properties of Halogen-Doped CdTe Used for Gamma-Ray Detectors

The mobilities, trapping times, activation energies, and trap concentration have been measured for both holes and electrons in Br and Cl-doped CdTe using the time-of-flight technique. Two electron traps 25 and 50 meV below the conduction band and two hole traps 140 and 350 meV above the valence band have been found. The 10 times larger concentration of levels found in the Br-doped CdTe can be explained using a model that describes the association of cadmium vacancies and substitutional halogens. The physical interpretation of the ??+ product when two levels are present is discussed for this case where the mobility is reduced and the lifetime is increased by trapping-detrapping phenomena. The measurements demonstrate that the material has excellent potential for ?-ray detectors that do not polarize with the proper surface preparation and make good detectors (6 keV FWHM for 122 keV ?-ray).

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.

Structural and electronic transport properties of ReSi2−δ single crystals

We investigated some structural and transport properties of semiconducting ReSi2−δ . In the literature this silicides is reported to crystallize in an orthorhombic structure and to be stoichiometric ReSi2. Our investigations clearly show that the stable composition is ReSi1.75 crystallizing in the space group P1. Transport measurements show thermally activated behavior at high temperatures with one (or two) energy gap Eg=0.16 (0.30 eV). We also report Hall‐effect measurements on this material: we found that RH is positive between 30 and 660 K and at room temperature the Hall number nH=1/eRH is equal to 3.7×1018 cm−3. The Hall mobility at room temperature is relatively high (μH=370 cm2/V s) for a single crystal.

Epitaxial silicon carbide for X-ray detection

We present the first experimental results of X-ray detection and spectroscopy by means of Schottky junctions on epitaxial silicon carbide (SiC). The devices have a junction area of 3 mm/sup 2/ on an n-type 4H-SiC layer 30 /spl mu/m thick with a dopant concentration of 1.8/spl times/10 cm at 300 K, the reverse current density of the best device varies between 2 pA/cm/sup 2/ and 18 pA/cm/sup 2/ as the mean electric field is increased from 40 kV/cm up to 170 kV/cm. The devices have been tested with X and /spl gamma/ rays from /sup 241/Am; the best measured energy resolution is 2.7 keV FWHM at room temperature.

Transport Properties of Natural Diamond Used as Nuclear Particle Detector for a Wide Temperatue Range

A thorough investigation has been done about the behavior of natural diamond as a radiation detector material for a wide temperature range. Drift velocities and mean free drift time have been determined at temperatures ranging between 85 K and 700 K and with electric fields up to 60 KV/cm. Average energy required to create an electron-hole pair and energy resolution have been measured in the 100 K - 400 K interval. The spectroscopic features of diamond detectors have also been analyzed on a broad temperature interval. It was found that as T approaches 500 K polarization phenomena begin, thus establishing the upper temperature limit for the detector operation.