Results 1 - 10 of 67
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[en] Burning voltage of a self-sustained volume discharge (SSVD) is studied as a function of the specific energy deposition in SF5 with C2-H6 and H2 mixtures, which are working media of a non-chain electrochemical HF laser. It is established that the voltage rises linearly with increasing the specific energy deposition, the relative voltage rise in the SF6-C2H6 mixtures being noticeably higher than in pure SF6 and SF6-H2 mixtures. An assumption is suggested and substantiated on determining the role of molecule dissociation by the electron impact leading to the observed voltage rise. From experimental data we have found approximate energy expenditures of producing dissociation fragments including atomic fluorine in a discharge in pure SF6: E-tilde d(F)= 5±1 eV. The values of Ed well agree with literature data obtained by other experimental methods. A conclusion is drawn that the dissociation process is the main mechanism limiting the current density, which implies SSVD realisation without preliminary gas ionisation in working mixtures of a non-chain HF laser and determines a higher stability of the volume discharge in mixtures of SF6 with hydrocarbons (deuterocarbons) as compared to mixtures with hydrogen (deuterium). A method is suggested and substantiated for numerical estimation of the limitation effect of the current density and its influence on the SSVD stability. (control of radiation parameters)
[en] Effect of the cycle 'introduction of defects, annealing, and repeated introduction of defects' on the SiC properties has been studied by means of nuclear spectrometry for an example of degradation of characteristics of a p-n nuclear radiation detector. The defects were introduced by irradiation with 8-MeV protons in two equal fluences of 3 x 10 14 cm-2. The total fluence of 6 x 1014 cm-2 corresponded to an introduction of 2.4 x 10 17 cm-3 primary knocked-out atoms. The annealing was made in two stages, each 1 h long, at temperatures of 600 and 700 oC. The detectors were tested with 5.4-MeV α particles, with the charge collection efficiency and specific features of the amplitude spectrum determined. The measurements were performed in the temperature range of 20-250 deg. C. It was shown that the effect of the first irradiation and the subsequent annealing does not significantly change the radiation hardness of SiC. The effective concentration of centers introduced in the course of the second irradiation (at the same fluence) is higher by a factor of 1.2. The nonequivalence of the fluences can also be attributed to the effect of the high total proton fluence of 6 x 1014 cm-2.
[en] Dependence of the short-circuit photocurrent on the voltage Vg applied to the gate of the 6H-SiC planar field-effect transistor is studied. The negative differential photoconductivity appeared at a certain value of Vg; the parameters of this photoconductivity corresponded to those of the Wannier-Stark ladders in the natural 6H-SiC super lattice. At the same value of Vg, a fairly abrupt decrease to zero of the source-drain current Isd is observed, which is indicative of cutoff at the voltage that is much lower than the expected cutoff voltage for this structure. The effect is attributed to a decrease in mobility in the mode of the Wannier-Stark ladders, a decrease in the rate of ionization of the donor atoms, and a reduction in the screening of the field
[en] Detectors based on modern CVD-grown films were irradiated with 8 MeV protons at a fluence of 3 x 1014 cm-2. The concentration of primary radiation defects was ∼1017 cm-3, which is three orders of magnitude higher than the concentration of the initially present uncompensated donors. The resulting deep compensation of SiC enabled measurements of detector parameters in two modes: under reverse and forward bias. The basic parameters of the detectors degraded by no more than a factor of 1.7, compared with the fluence of 1 x 1014 cm-2. However, there appeared a polarization voltage, which indicates that a space charge is accumulated by radiation defects
[en] The efficiency of charge transfer in SiC detectors irradiated with 8 MeV protons at a dose of 1014 cm-2 has been studied. The number of defects originally created in this irradiation mode is equivalent to that of disruptions produced in the lattice of SiC detectors used in experiments on the modernized SLHC collider (CERN). Methods of nuclear spectrometry were employed, with the detectors tested with 5.4 MeV alpha particles. Taking into account the deep compensation of SiC in the course of irradiation, the use of structures in the unconventional forward-biased mode is suggested. In this mode, the electric field strength is distributed across the detector thickness more uniformly. An illustrative model of carrier transport is suggested for processing of experimental data
[en] Operation of detectors in strong radiation fields or an imperfect fabrication technology gives rise to a considerable amount of structural defects in materials. Using a p+-v-n+ detector structure in the unconventional forward-bias mode has been considered as a way to minimize the space-charge accumulation caused by long-term capture of carriers to deep levels. The efficiency of carrier transport and the signal amplitude spectrum were studied as functions of the applied voltage and time of signal shaping by the electronic circuit for the example of a CVD-grown 4H-SiC film containing up to 2.5 x 1017 cm-3 of primary knocked-out atoms. The detector was tested with 5.4 MeV α particles in the temperature range 20-140oC. It was demonstrated that, despite the transport of only one-fourth of the whole amount of charge, the main line of the spectrum has a high energy resolution (7%) for the given conditions.
[en] Al ion implantation of chemical-vapor deposition (CVD)-grown n-type films has been used to fabricate p+–n junctions for nuclear particle detectors. The junction formation mode is characterized by a high dose of Al ions and a short-duration (15 s) activating annealing. Specific structural features of the implanted p+–n junctions, revealed using modern analytical techniques, are accounted for by the joint action of the high-dose Al ion implantation and unconventional rapid thermal annealing mode. The detector characteristics were studied up to a temperature of 375 °C under irradiation with 3–8 MeV alpha particles in a vacuum chamber of special design. An improvement of the energy resolution and nonequilibrium charge collection efficiency was observed with increasing temperature. The results obtained are explained in terms of specific structural features of the p+–n junctions formed in the chosen implantation and thermal annealing modes. The behavior of the detector noise level was analyzed with the working temperature raised up to 375 °C
[en] p+-n-n+ detector structures based on CVD films with an uncompensated donor concentration of 2 x 1014 cm-3 have been studied. The p+-region was created by implantation of Al ions. The detectors were preliminarily irradiated with 8-MeV protons at a fluence of 3 x 1014 cm-2 and then annealed at 600 deg. C for 1 h. In measurements performed in the temperature range 20-150 deg. C, the forward-and reverse-bias modes were compared. It is shown that the annealing leads to a higher collection efficiency of carriers generated by nuclear radiation and to a decrease in the amount of charge accumulated by traps in the course of testing. Despite the positive effect of the annealing, a considerable amount of radiation defects remain, which is manifested, in particular, in the kinetics of the forward current
[en] The operation of detectors irradiated with 8-MeV protons at a fluence of 3 x 1014 cm-2 has been studied. The detectors were based on modern CVD-grown n-4H-SiC films with a concentration of uncompensated donors equal to ∼2 x 1014 cm-3 and a thickness of 55 μm. The high concentration of primary radiation defects (∼2 x 1017 cm-3) determined the deep compensation of the films. The basic characteristics of the detectors-pulse amplitude and resolution-exhibited temporal instability. This effect is due to prolonged capture of nonequilibrium carriers by radiation centers and the resulting appearance of a polarization voltage in the bulk of the detector. The kinetics of attainment of steady values by the quantities specified above was analyzed
[en] To determine the effect of a metal substrate on the electronic state of quasi-free-standing graphene, a simple model taking into account the presence of the double electric layer on the metal surface is proposed. The arising electrostatic field shifts the Dirac point of graphene, which leads to its doping. An analytical expression for the charge of graphene atoms is obtained. Numerical estimations are made for (111) faces of Cu, Ag, Au, and Pt. The agreement of the obtained estimations with the available results of numerical calculations is discussed.