Papers by Author: Alexander M. Ivanov

Abstract: Polarization effect characteristically occurs in detectors based on wide-bandgap materials at considerable concentrations of radiation defects. The appearance of an electromotive force in the bulk of a detector is due to the long-term capture of carriers at deep levels related to radiation centers. The kinetics and strength of the polarization field have been determined. The capture can be controlled by varying the detector temperature, with a compromise reached at the "optimal" temperature between the generation current and the position of the deepest of the levels whose contribution to the loss of charge via capture is negligible. It has been found that the depth of a level (related to the energy gap width) is close to 1/3, irrespective of a material. The optimal temperatures are strictly individual for materials.

Abstract: Non-uniformities of electrical properties of 4H-SiC CVD films have been revealed using physico-chemical reactions occurring upon introduction of radiation-induced structural defects. Primary knocked-out atoms and vacancies actively interact with defects of the starting material and thereby form the final system of radiation centers. The samples were irradiated with 900 keV electrons and 8 MeV protons at doses not leading to conductivity compensation ( 7.5  1012 cm–2) and a dose of 6  1014 cm–2 causing deep compensation. Capacitance methods demonstrated that characteristics of samples ~3 mm in size are not identical. The nuclear spectrometry technique, which enables microprobing of samples, demonstrated individual behavior of separate parts of a film with areas of tens of square micrometers (the dimension of α-particles track cross-section).

Abstract: Nonequilibrium-charge transport has been studied in a structure with a Schottky barrier fabricated on a CVD-grown n-4H-SiC film. The charge introduced by single α-particles was recorded by nuclear spectrometric techniques. The maximum electric field strength in the structure was 1.1 MV/cm. The recorded charge as a function of the reverse bias applied to the structure shows a superlinear rise. Simultaneously, the width of the amplitude spectrum increased superlinearly, too. The observed effect is attributed to the initial stage of impact ionization. The manifestation of the process at unusually low fields (~1.0 MV/cm) is accounted for by specific features of the charge generation process. The carriers generated by a α-particle are found to be originally "heated". The results obtained allow prognostication of the appearance of SiC detectors of the "proportional counter" type in the near future. This is enabled by the advances made in the field of high-voltage electronics in obtaining in practice the required electric field strengths.

Abstract: The effect of a cycle "introduction of defects – annealing – introduction of defects" on the SiC properties has been studied to know the degradation of characteristics of p-n- nuclear radiation detectors. The irradiation with 8 МeV protons at fluences of about 3×1014 сm-2 was used. The annealing was carried out in two stages one-hour at temperatures of 600 and 700 °С. Nuclear spectrometric techniques with 5.8 MeV -particles were employed to test the detectors. The charge collection efficiency and features of the amplitude spectrum were determined to study the capture of charge carriers by radiation-induced defects. Measurements were made in the temperature range of 20–250 °С. It is shown that at 250 °С there is a decrease in the carriers capture. The form of the amplitude spectrum essentially improves. The first irradiation and the subsequent annealing do not change significantly the radiation hardness of SiC. During the second irradiation the effective concentration of the introduced centers is 1.3 times higher. This result may be due to the high total fluence of protons, 6×1014 cm-2.

Abstract: The spectrometric characteristics of detectors based on 4H-SiC films with ion-doped p+–n junctions in a temperature range from 25 to 375 °C have been studied. The experiments with 5.8-MeV α-particles in a high-temperature chamber were performed. The interference factors of the detectors operation in a mode of spectrometry are established. The energy resolution of 1.35% is received. An increase of the efficiency of the diffusion–drift charge transport with increasing temperature has been observed. The last is explained by an increase in the diffusion length of minority carriers.

Abstract: P+–n–n+-detector structures based on CVD films with an uncompensated donor concentration of 2×1014 cm-3 have been studied. The p+-region was created by implantation of Al ions. Preliminarily, the detectors were irradiated with 8 MeV protons at a fluence of 3×1014 cm-2 and then annealed in a vacuum at 600°C for 1 h and 700°C for 1 h. Nuclear spectrometric techniques with 5.4 MeV a-particles were employed to test the detectors. In measurements performed in the temperature range 20–150°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, there remains a considerable amount of radiation defects, which is manifested, in particular, in the kinetics of the forward current.

Abstract: The charge collection efficiency (ССЕ) of SiC-detectors preliminarily irradiated with 8 MeV protons at a fluence of 1014 cm-2 has been studied. Nuclear spectrometric techniques with 5.4 MeV α-particles were employed to test the detectors. The concentration of primarily created defects was estimated to be 4×1016 cm-3. A strong compensation of SiC was observed, which allowed connection of the structure in the forward mode. The experimental data obtained were processed using a simple two-parameter model of signal formation. The model makes it possible to separate the contributions of electrons and holes to the ССЕ. An additional irradiation at a fluence of 2×1014 cm-2 reduced the ССЕ value by a factor of 2 and gave rise to polarization. The latter indicates that radiation-induced centers are not only actively involved in carrier localization (with a decrease in the lifetime), but also in transformation of the electric field within the detector.

Abstract: The detector structures based on Al ion-implanted p+-n junctions in 4H-SiC have been manufactured and tested at temperatures up to 170oC by α-particles with energies of 3.9 and 5.5 MeV. Structural peculiarities of thin Al high dose ion implanted layers before and after short high temperature activation annealing were studied by combination of Rutherford back scattering/channeling spectrometry and cross-sectional transmission electron microscopy. The detector structures fabricated on this thin ion implanted p+-n junctions operated in the temperature range of 16-170 oC with reproducible stable spectrometric characteristics. The charge collection efficiency and the energy resolution of detectors improved with rising temperature up to 170 oC, that was obtained in SiC detectors for the first time.

Abstract: A full modeling of deceleration of α-particles in SiC is carried out using a Monte-Carlo method. The distribution of energy losses in nuclear elastic collisions is calculated. The spectrum has a characteristic asymmetric form and the line width at half peak maximum is 4.62 keV. The final form of a spectral line is obtained by convolution with a Gaussian peak, and including the contribution of ionization fluctuations and noise. The resulting value of the line width was 8.75 keV (at a noise dispersion of detector and equipment of 1.7 keV). The resolution of detectors reached in practice is twice the lowest calculated value. It is shown that charge losses during transport of nonequilibrium carriers through the volume of the detector are insignificant, while a resolution divergence may result from a non-optimized “entrance window”.

Abstract: The effect of irradiation with protons, electrons, neutrons, x-ray radiation and gamma-ray photons as well as with different ions on properties of starting SiC material and devices based on it was studied. The rectifying properties of the diode structures, which degraded as a result of irradiation with high energy particles, were recovered at higher operation temperatures. The transistor structure SiC-based detectors were realized with the signal amplification by a factor of tens under irradiation. The energy resolution of 0.34 %, commensurable with Si-detectors, has been achieved for SiC detectors and is correct for all classes of short range ions. The maximum signal amplitude corresponds, in SiC, to a mean electron-hole pair creation energy of 7.7 eV.