E. V. Kalinina

Temperature dependence of the photoelectric conversion quantum efficiency of 4H–SiC Schottky UV photodetectors

Ultraviolet Schottky photodetectors based on n-4H–SiC (Nd − Na = 4 × 1015 cm−3) epitaxial layers of high purity have been fabricated. Their spectral sensitivity range is 3.2–5.3 eV peaking at 4.9 eV (quantum efficiency is about ~0.3 electron/photon), which is close to the bactericidal ultraviolet radiation spectrum. The temperature dependence of the quantum efficiency of 4H–SiC Schottky structure has been investigated to determine the temperature stability and the mechanism of the photoelectric conversion process. At low temperatures (78–175 K) the quantum efficiency increases with increasing temperature for all photon energy values and then tends to saturate. We suppose that some imperfections in the space-charge region act as traps that capture both photoelectrons and photoholes. After some time the trapped electron–hole pairs recombine due to the tunnelling effect. At high temperatures (more than 300 K), the second enhancement region of the quantum efficiency is observed in the photon energy range of 3.2–4.5 eV. It is connected with a phonon contribution to indirect optical transitions between the valence band and the M-point of the conduction band. When the photon energy is close to a direct optical transition threshold this enhancement region disappears. This threshold is estimated to be 4.9 eV. At photon energies more than 5 eV a drastic fall of the quantum efficiency has been observed throughout the temperature interval. We propose that in this case the photoelectrons and photoholes are bound to form hot excitons in the space-charge region due to the Brillouin zone singularity, and do not contribute to the following photoelectroconversion process.

The effect of irradiation on the properties of SiC and devices based on this compound

Issues related to the production of radiation defects in silicon carbide of various polytypes and with differing conductivity types and concentrations of charge carriers as a result of irradiation with high-energy particles in a wide range of their energies and masses (from electrons to heavy Bi ions) are considered. The effect of irradiation with high-energy particles on the optical and electrical characteristics of the devices based on SiC are also considered, including the devices that operate as detectors of nuclear radiation. Systematic trends (common to other semiconductors and characteristic of SiC) in the radiation-defect formation in SiC are established. The high radiation resistance of SiC is verified; it is shown that this radiation resistance can be increased at increased energies of incident particles and at higher temperatures of operation.

High Energy Resolution Detectors Based on 4H-SiC

The spectrometric characteristics of the detectors based on 4H-SiC using 4.8-7.7 MeV a-particles were determined. The Cr Schottky barriers with areas of 1×10-2 cm2 were performed^by vacuum thermal evaporation on 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) with thickness 26 and 50 µm. The concentrations of the uncompensated donors into CVD epitaxial layers were (6-10) ×1014 cm-3, that allowed to develop a detector depletion region up to 30 µm using reverse bias of 400 V. The energy resolution less than 20 keV (0.34%) for lines of 5.0- 5.5 MeV was achieved that is twice as large of the resolution of high-precision Si-based detectors prepared on specialized technology. The maximum signal amplitude of 4H-SiC - detectors corresponding to the average electron-hole pair generation energy was found to be 7.70 eV.

Optical and Electrical Properties of 4H-SiC Irradiated with Fast Neutrons and High-Energy Heavy Ions

Photoluminescence and deep-level transient spectroscopy are used to study the effect of irradiation with fast neutrons and high-energy Kr (235 MeV) and Bi (710 MeV) ions on the optical and electrical properties of high-resistivity high-purity n-type 4H-SiC epitaxial layers grown by chemical vapor deposition. Electrical characteristics were studied using the barrier structures based on these epitaxial layers: Schottky barriers with Al and Cr contacts and p+-n-n+ diodes fabricated by Al ion implantation. According to the experimental data obtained, neutrons and high-energy ions give rise to the same defect-related centers. The results show that, even for the extremely high ionization density (34 keV/nm) characteristic of Bi ions, the formation of the defect structure in SiC single crystals is governed by energy losses of particles due to elastic collisions.

Effect of irradiation with fast neutrons on electrical characteristics of devices based on CVD 4H-SiC epitaxial layers

The effect of irradiation with 1-MeV neutrons on electrical properties of Al-based Schottky barriers and p+-n-n+ diodes doped by ion-implantation with Al was studied; the devices were formed on the basis of high-resistivity, pure 4H-SiC epitaxial layers possessing n-type conductivity and grown by vapor-transport epitaxy. The use of such structures made it possible to study the radiation defects in the epitaxial layer at temperatures as high as 700 K. Rectifying properties of the diode structures were no longer observed after irradiation of the samples with neutrons with a dose of 6×1014 cm−2; this effect is caused by high (up to 50 GΩ) resistance of the layer damaged by neutron radiation. However, the diode characteristics of irradiated p+-n-n+ structures were partially recovered after an annealing at 650 K.

Mechanism of the current flow in Pd-(heavily doped p-AlxGa1−xN) ohmic contact

The physical mechanism of the current flow in Pd-(heavily doped p-AlxGa1−xN) ohmic contact is studied. Chloride-hydride epitaxy was used to grow the p-Al0.06Ga0.94N solid solution with uncompensated acceptor concentration Na–Nd ranging from 3×1018 up to 1019 cm−3. Thermal vacuum deposition and subsequent thermal treatment were used to form an ohmic Pd contact. It is shown that, after the thermal treatment, the Pd-p-Al0.06Ga0.94N barrier contact with a potential barrier height of about 2.3 V becomes ohmic and the barrier height decreases to approximately 0.05 V. For uncompensated acceptor concentration Na–Nd=3×1018 cm−3, thermionic emission is found to be the main mechanism of the current through the Pd-p-Al0.06Ga0.94N ohmic contact. An increase in Na–Nd to approximately 1019 cm−3 in the solid solution leads to a transition from thermionic emission (at high temperatures) to tunneling (at low temperatures).

Performance of p-n 4H-SiC film nuclear radiation detectors for operation at elevated temperatures (375 °C)

The spectrometric characteristics of nuclear radiation detectors based on 4H-SiC films with iondoped p+-n junctions have been studied for the first time in a temperature range from 25 to 375°C. The experiments with 5.8-MeV α particles were performed in a high-temperature chamber of special design. Factors related to the structural characteristics of both the initial silicon carbide and the ion-doped p+-n junctions are established, which limit from above the temperature interval of detector operation in a spectrometric regime. An increase in the efficiency of the diffusion-drift charge transport with increasing temperature has been observed, which is explained by an increase in the diffusion length of minority carriers.

Influence of Gamma-Ray and Neutron Irradiation on Injection Characteristics of 4H-SiC pn Structures

The effect of gamma-ray and neutron irradiation on recombination current, injection electroluminescense and the value of the lifetime of nonequilibrium carriers for 4H-SiC pn structures was investigated. The irradiation was carried out with gamma-ray (dose 5x106 rad) and 1 MeV neutrons in the doses range from 1.2x1014 cm-2 to 6.24x1014 cm-2. Neutron irradiation with a dose 1.2x1014 cm-2 increased the recombination current, decreased the lifetime for deep-level recombination in the space charge region and decreased the intensity of the edge injection electroluminescense (hnmax » 3.16 eV) by 1.5-2 orders of magnitude; the neutron irradiation with high dose (6.24x1014 cm-2) resulted in increase of the recombination current up to 2 orders of magnitude and decrease of lifetime at least up to 2 orders of magnitude. Gamma-ray irradiation and annealing at temperatures in the range 350-650 K left the recombination current and lifetime practically unchanged.

High-resolution short range ion detectors based on 4H-SiC films

The energy resolution of SiC detectors has been studied in application to the spectrometry of α particles with 5.1–5.5 MeV energies. The Schottky barrier structure of the detector was based on a CVD-grown epitaxial n-4H-SiC film with a thickness of 26 μm and an uncompensated donor concentration of (1–2)×1015 cm−3. An energy resolution of 0.5% achieved for the first time with SiC detectors allows fine structure of the α particle spectrum to be revealed. The average energy of the electron-hole pair formation in 4H-SiC is estimated at 7.71 eV.

Structure and characteristics of the high-temperature SiC detectors based on Al ion-implanted p+–n junctions

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.