The results of investigations of initial n-4H–SiC structures by various methods are presented. The structures represent a highly doped n+ substrate with epitaxial layers 5 μm thick grown by chemical vapor deposition (CVD). The concentration of uncompensated donors in the epitaxial layer is in the range of Nd – Na = (1–50) × 1014 cm–3. Using the results of X-ray diffraction analysis, it is found that, in order to obtain efficient 4H–SiC ultraviolet photodetectors, it is desirable to have structures of epitaxial layers in which the effect of point-defect gettering, leading to an increase in the lifetime of charge carriers and in the values of quantum efficiency, is observed. The photosensitivity of the investigated samples significantly depends on the degree of imperfection of the CVD epitaxial layer, which results in a change in the lifetime of charge carriers and, as a consequence, in a change in the quantum efficiency of 4H–SiC ultraviolet photodetectors.

The influence exerted by the carrier concentration in the range (1–50) × 1014 cm–3 in n-4H-SiC chemical-vapor deposited (CVD) epitaxial layers on the spectral characteristics of UV photodetectors with Cr Schottky barriers in the range of 200–400 nm is revealed and explained in terms of the photoconductivity theory. Schottky barriers with a Cr film thickness of 20 nm and diameter of 8 mm are formed by thermal evaporation in vacuum through masks. A noticeable effect of the carrier concentration in the CVD epitaxial layers on the spectral characteristics of photodetectors upon heating to 200°C is also observed and accounted for by a difference between the generation-recombination processes. The irradiation of photodetectors with 15-MeV protons at a fluence of 4 × 1012 cm–2 and a temperature of 200°C leads to an increase in the quantum efficiency as compared to samples irradiated in similar modes at 25°C. This is indicative of an increase in the radiation hardness and service life of 4H-SiC devices at elevated temperatures.

Представлены результаты исследования различными методиками исходных структур n-4H-SiC, представляющих высоколегированную n+-подложку с выращенным химическим осаждением из газовой фазы эпитаксиальным слоем толщиной 5 мкм. Концентрация носителей заряда в эпитаксиальном слое в диапазоне Nd-Na=(1-50)·1014 см-3. Привлекая результаты рентгеноструктурного анализа, было установлено, что для получения эффективных 4H-SiC ультрафиолетовых фотоприемников желательно иметь структуры эпитаксиальных слоев, в которых наблюдается эффект геттерирования точечных дефектов, что приводит к росту времени жизни носителей заряда и увеличению значений квантовой эффективности. Фоточувствительность исследованных образцов значительно зависит от степени дефектности в CVD эпитаксиальном слое, приводящей к изменению времени жизни носителей заряда и, как следствие, к изменению квантовой эффективности 4H-SiC ультрафиолетовых фотоприемников. Ключевые слова: карбид кремния, рентгеноструктурный анализ, эффект геттерирования, квантовая эффективность.

Abstract The influence exerted by the carrier concentration in the range (1–50) × 10^14 cm^–3 in n -4 H -SiC chemical-vapor deposited (CVD) epitaxial layers on the spectral characteristics of UV photodetectors with Cr Schottky barriers in the range of 200–400 nm is revealed and explained in terms of the photoconductivity theory. Schottky barriers with a Cr film thickness of 20 nm and diameter of 8 mm are formed by thermal evaporation in vacuum through masks. A noticeable effect of the carrier concentration in the CVD epitaxial layers on the spectral characteristics of photodetectors upon heating to 200°C is also observed and accounted for by a difference between the generation-recombination processes. The irradiation of photodetectors with 15-MeV protons at a fluence of 4 × 10^12 cm^–2 and a temperature of 200°C leads to an increase in the quantum efficiency as compared to samples irradiated in similar modes at 25°C. This is indicative of an increase in the radiation hardness and service life of 4 H -SiC devices at elevated temperatures.

AbstractFor the first time, comprehensive comparative investigations of ultraviolet photodetectors with Cr Schottky barriers formed on 4 H -SiC epitaxial layers are carried out by the X-ray and optical methods before and after irradiation with 15-MeV protons with fluences in the range of (1–4) × 10^12 cm^–2. When increasing the fluence of proton irradiation, the formation of localized regions with negative deformation is observed along with the unperturbed silicon-carbide matrix. Agreement between the X-ray and optical studies is obtained, which makes it possible to explain the features of the spectral changes in the photosensitivity of detectors in the range of 200–400 nm with an increase in the fluence of proton irradiation. The ultraviolet Cr/4 H -SiC photodetectors withstand irradiation by 15-MeV protons with a fluence of 4 × 10^12 cm^–2 virtually without any changes in the photosensitivity due to the gettering of simple defects by cluster and amorphous formations, which lead to partial structural improvement of the irradiated material.

For the first time, comprehensive comparative investigations of ultraviolet photodetectors with Cr Schottky barriers formed on 4 H -SiC epitaxial layers are carried out by the X-ray and optical methods before and after irradiation with 15-MeV protons with fluences in the range of (1–4) × 10 12 cm –2 . When increasing the fluence of proton irradiation, the formation of localized regions with negative deformation is observed along with the unperturbed silicon-carbide matrix. Agreement between the X-ray and optical studies is obtained, which makes it possible to explain the features of the spectral changes in the photosensitivity of detectors in the range of 200–400 nm with an increase in the fluence of proton irradiation. The ultraviolet Cr/4 H -SiC photodetectors withstand irradiation by 15-MeV protons with a fluence of 4 × 10 12 cm –2 virtually without any changes in the photosensitivity due to the gettering of simple defects by cluster and amorphous formations, which lead to partial structural improvement of the irradiated material.

Electrical and spectrometric characteristics of 4H-SiC detectors with Cr Schottky barriers in the spectral ranges of 114–175 and 210–400 nm are studied. It is demonstrated that the quality of commercially available 4H-SiC layers is sufficient to construct UV radiation detectors with their external quantum efficiency exceeding 20% in the studied spectral ranges.

Ultraviolet (UV) photodetectors based on Schottky barriers to 4 H -SiC are formed on lightly doped n -type epitaxial layers grown by the chemical vapor deposition method on commercial substrates. The diode structures are irradiated at 25°C by 167-MeV Xe ions with a mass of 131 amu at a fluence of 6 × 10 9 cm −2 . Comparative studies of the optical and electrical properties of as-grown and irradiated structures with Schottky barriers are carried out in the temperature range 23–180°C. The specific features of changes in the photosensitivity and electrical characteristics of the detector structures are accounted for by the capture of photogenerated carriers into traps formed due to fluctuations of the conduction-band bottom and valence-band top, with subsequent thermal dissociation.

Schottky barriers, 10 −2 cm 2 in area, have been prepared by thermal deposition of Cr in vacuum on 50-μm-thick 4 H -SiC epitaxial layers grown by chemical vapor deposition. The uncompensated donor concentration in these films is (4–6) × 10 14 cm −3 , which makes it possible to extend the depletion region of the detector to ≈30 μm by applying a reverse bias of 400 V. The spectrometric characteristics of the detectors are determined using α particles in the energy range 4.8–7.7 MeV. The energy resolution attained for the 5.0-to 5.5-MeV lines is higher than 20 keV (0.34%), which, by a factor of 2, is second only to precision silicon detectors fabricated by specialized technology. The maximum signal amplitude corresponds, in SiC, to a mean electron-hole pair creation energy of 7.70 eV.

The possibilities for using of Al ion implanted p(+)n junctions for different 4H-SiC devices were investigated. It has been shown that a thin low resistivity p(+)-layers can be formed in 4H-SiC pure CVD epitaxial layers with Al ion implantation followed by short high temperature annealing. These layers provide for reduction of resistance in diode structures and the increase of the hole injection in forward direction in them. Al ion implanted p(+)nn(+) diode structures with differencial resistance of 3x10(-3) Omega cm(-2) were obtained. Also the advantages of shallow Al ion implanted p(+)n junctions for the detectors of UV radiation and alpha-particles were determined.

The photoelectric properties of p+-n junctions that were based on 4H-SiC ion-implanted with aluminum and were formed in lightly doped n-type epitaxial layers grown by chemical vapor deposition were studied. It is shown that such photodetectors combine in full measure the advantages of photostructures formed on the basis of Schottky barriers and epitaxial p-n junctions. The results of the theoretical calculation of spectral characteristics of ion-implanted photodetectors are in good agreement with experimental data. The structures feature an efficiency of collection of nonequilibrium charge carriers close to 100% in the spectral range of the photon energies of 3.5–4.25 eV.

The results of studying 4 H -SiC p + - n junctions ion-implanted with aluminum as detectors of high-energy particles are reported. The junctions were formed in SiC epitaxial films grown by chemical vapor deposition. The concentration of uncompensated donors was (3–5)×10 15 cm −3 , and the charge-carrier diffusion length was L p =2.5 µm. The detectors were irradiated with 4.8–5.5-MeV alpha particles at 20°C. The efficiency of collection of the induced charge was as high as 0.35. The possibilities of operating SiC detectors at elevated temperatures (∼500°C) are analyzed.

Results of complex studies of structural, optical, and electrical characteristics of 4H–SiC n-type low-doped epitaxial layers grown by the chemical vapor deposition (CVD) method are presented. Characteristics of CVD layers grown on commercial wafers with and without a thin (<0.1 μm) buffer layer grown by liquid phase epitaxy (LPE) between the commercial wafer and CVD epitaxial layer were compared. It has been shown that the LPE filling process caused a significant improvement in the structural quality of CVD layers. Cathodoluminescence (CL) data and electrical characteristics of Schottky barriers (SB) revealed that the LPE layer also made the concentration profiles of the uncompensated donors and recombination centers for holes as well as the hole diffusion lengths more uniform over the CVD layer. According to CL and secondary ion mass-spectroscopy measurements the presence of aluminum as an impurity was detected in initial commercial wafers, as well as having LPE and CVD epitaxial layers. Forward current–voltage characteristics of SBs formed on CVD layers with and without a LPE buffer layer followed an exponential relationship with an ideality factor of 1.04–1.06 over six to seven orders of magnitude in current density. A noticeable increase of the breakdown voltage was also observed in samples with a LPE layer.