High-temperature nuclear-detector arrays based on 4 <Emphasis Type="Italic">H</Emphasis>-SiC ion-implantation-doped <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-<Emphasis Type="Italic">n</Emphasis> junctions

Results obtained in a study of spectrometric characteristics of arrays of four detectors based on 4H-SiC ion-implantation-doped p ⁺-n junctions in the temperature range 25–140 °C are reported for the first time. The junctions were fabricated by ion implantation of aluminum into epitaxial 4H-SiC layers of thickness ≤45 μm, grown by chemical vapor deposition with uncompensated donor concentration N _d ? N _a = (4–6) × 10¹⁴ cm^?3. The structural features of the ion-implantation-doped p ⁺-layers were studied by secondary-ion mass spectrometry, transmission electron microscopy, and Rutherford backscattering spectroscopy in the channeling mode. Parameters of the diode arrays were determined by testing in air with natural-decay alpha particles with an energy of 3.76 MeV. The previously obtained data for similar single detectors were experimentally confirmed: the basic characteristics of the detector arrays, the charge collection efficiency and energy resolution, are improved as the working temperature increases.     

Silicon carbide detectors of high-energy particles

The results of studying 4H-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)×10¹⁵ 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. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 6, 2002, pp. 750–753. Original Russian Text Copyright ? 2002 by Violina, Kalinina, Kholujanov, Kossov, Yafaev, Hallén, Konstantinov.     

Radiation hardness of SiC ion detectors under relativistic protons

Schottky diodes based on 6H-SiC epitaxial films exposed to 1000 MeV protons at a dose of 3×10¹⁴ cm^?2 have been studied by precision alpha spectrometry. Parameters of deep levels introduced by protons were determined by deep-level transient spectroscopy. The number of vacancies generated in proton tracks was found using TRIM software. The width of the space charge region and the hole diffusion length before and after irradiation were obtained by processing the alpha-spectrometry and capacitance measurements. Minor variations in the charge transport properties of epitaxial 6H-SiC detectors were observed.     

Photoelectric properties of p +-n junctions based on 4H-SiC ion-implanted with aluminum

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.     

Specific features of use of wide-gap semi-insulating materials for recording of nuclear radiation

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 × 10¹⁷ cm^?3 of primary knocked-out atoms. The detector was tested with 5.4 MeV α particles in the temperature range 20–140°C. 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.     

A study of n +-6H/n-3C/p +-6H-SiC heterostructures grown by sublimation epitaxy

The n ⁺-6H/n-3C/p ⁺-6H-SiC structure was fabricated for the first time by sublimation epitaxy, with mesa diodes formed on its base and their electrical characteristics were studied. It was found that a green band dominates in the spectrum of injection electroluminescence (EL) of these diodes. The band is close by its parameters to that associated with free-exciton recombination in bulk 3C-SiC, but is shifted by ～0.06 eV to shorter wavelengths. A similar effect was observed previously for triangular quantum wells in an n ⁺-6H-SiC/p-3C-SiC heterojunction. An analysis of the experimental data obtained demonstrated that the structure can be regarded as two independent heterojunctions. The EL spectrum observed may be associated with radiative recombination at the n ⁺-6H-SiC/n-3C-SiC heterointerface.     

Optical and electrical properties of 4H-SiC irradiated with Xe ions

Structures with aluminum-ion-implanted p ⁺-n junctions formed in 26-μm-thick chemicalvapor-deposited-epitaxial 4H-SiC layers with an uncompensated donor concentration N _d ?N _a = (1–3) × 10¹⁵ cm^?3 are irradiated with 167-MeV Xe ions at fluences of 4 × 10⁹ to 1 × 10¹¹ cm^?2 and temperatures of 25 and 500°C. Then as-grown and irradiated structures are thermally annealed at a temperature of 500°C for 30 min. The as-grown, irradiated, and annealed samples are analyzed by means of cathodoluminescence, including the cross-sectional local cathodoluminescence technique, and electrical methods. According to the experimental data, radiation defects penetrate to a depth in excess of several tens of times the range of Xe ions. Irradiation of the structures at 500°C is accompanied by “dynamic annealing” of some low-temperature radiation defects, which increases the radiation resource of 4H-SiC devices operating at elevated temperatures.     

Optical and electrical properties of 4<Emphasis Type="Italic">H</Emphasis>-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.     

Spectrometry of short-range ions using detectors based on 4<Emphasis Type="Italic">H</Emphasis>-SiC films grown by chemical vapor deposition

Schottky barriers, 10^?2 cm² in area, have been prepared by thermal deposition of Cr in vacuum on 50-μm-thick 4H-SiC epitaxial layers grown by chemical vapor deposition. The uncompensated donor concentration in these films is (4–6) × 10¹⁴ 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.     

High-voltage (1800 V) planar 4<Emphasis Type="Italic">H</Emphasis>-SiC <Emphasis Type="Italic">p-n</Emphasis> junctions with floating guard rings

Planar 4H-SiC p-n junctions with floating guard rings have been fabricated. The main junction and the rings were formed by room temperature boron implantation followed by high temperature annealing. The breakdown voltage of the p-n junctions is 1800 V, which twice exceeds that of similar junctions without guard rings and reaches 72% of the calculated breakdown voltage of a plane-parallel p-n junction with the same epitaxial layer parameters     

Radiation defects in n-6H-SiC irradiated with 8 MeV protons

Capacitance methods and electron spin resonance (ESR) were applied to study deep centers in n-6H-SiC irradiated with 8 MeV protons. Schottky diodes and p-n structures grown by sublimation epitaxy or commercially produced by CREE Inc. (United States) were used. The type of the irradiation-induced centers is independent of the material fabrication technology and the kind of charged particles used. Irradiation results in an increase in the total concentration of donor centers. The possible structure of the centers is suggested on the basis of data on defect annealing and ESR.     

Quantum wells on 3<Emphasis Type="Italic">C</Emphasis>-SiC/<Emphasis Type="Italic">NH</Emphasis>-SiC heterojunctions. Calculation of spontaneous polarization and electric field strength in experiments

The results of experiments with quantum wells on 3C-SiC/4H-SiC and 3C-SiC/6H-SiC heterojunctions obtained by various methods are reconsidered. Spontaneous polarizations, field strengths, and energies of local levels in quantum wells on 3C-SiC/NH-SiC heterojunctions were calculated within a unified model. The values obtained are in agreement with the results of all considered experiments. Heterojunction types are determined. Approximations for valence band offsets on heterojunctions between silicon carbide polytypes and the expression for calculating local levels in quantum wells on the 3C-SiC/NH-SiC heterojunction are presented. The spontaneous polarizations and field strengths induced by spontaneous polarization on 3C-SiC/4H-SiC and 3C-SiC/6H-SiC heterojunctions were calculated as 0.71 and 0.47 C/m² and 0.825 and 0.55 MV/cm, respectively.     

A 4<Emphasis Type="Italic">H</Emphasis>-SiC<Emphasis Type="Italic">p-i-n</Emphasis> diode fabricated by a combination of sublimation epitaxy and CVD

The possibility of fabricating heavily doped (N _a ?N _d ≥ 1 × 10¹⁹ cm_?3) p⁺-4H-SiC layers on CVD-grown lightly doped n-4H-SiC layers by sublimation epitaxy has been demonstrated. It is shown that a Au/Pd/Ti/Pd contact, which combines a low specific contact resistance (～2 × 10^?5 Ω cm²) with high thermal stability (up to 700°C), is the optimal contact to p-4H-SiC. The p-n structures obtained are used to fabricate packaged diodes with a breakdown voltage of up to 1400 V.     

Conductivity compensation in n-4H-SiC (CVD) under irradiation with 0.9-MeV electrons

The effect of electron irradiation on n-4H-SiC is studied by the methods of capacitance-voltage characteristics and photoluminescence. The carrier-removal rate is found to be V _d ≈ 0.25 cm^?1. Total conductivity compensation in samples with an initial carrier concentration of (1–2) × 10¹⁵ cm^?2 is observed at irradiation doses of ～5 × 10¹⁵ cm^?2. Simultaneously with an increase in the compensation, a rise in the intensity of defect-related luminescence characteristic of 4H-SiC is observed. The sample parameters before irradiation and after irradiation and annealing are compared. The physical mechanisms of compensation in the samples under study are analyzed.     

Effect of electron and proton irradiation on characteristics of SiC surface-barrier detectors of nuclear radiation

Structures with a Schottky barrier based on CVD-grown 4H-SiC films were irradiated with 8 MeV protons and 900 keV electrons. The maximum fluences were 10¹⁴ and 3 × 10¹⁶ cm^?2, respectively. It was found that, in the case of electrons, the primarily introduced radiation defects are closely spaced Frenkel pairs. Changes in the electrical characteristics of the structures were compared. Capacitance methods and nuclear spectrometry were employed. The latter technique was used to determine the charge collection efficiency under pulsed ionization with α-particles. It was determined that, under proton irradiation, the charge collection efficiency steadily decreases as the fluence increases. For electrons, the efficiency remains unchanged in the fluence range (1–3) × 10¹⁶ cm^?2. However, a fluence of 3 × 10¹⁶ cm^?2 leads to a pronounced increase in the non-uniformity of charge transport conditions throughout the sample volume.     

Structural peculiarities of 4H-SiC irradiated by Bi ions

X-ray diffraction, photoluminescence, micro-cathodoluminescence, and scanning and transmission electron spectroscopy were used to study the 710 MeV Bi ion irradiation effect in the fluence range of 1.4 × 10⁹−5 × 10¹⁰ cm⁻² on the structural and optical characteristics of pure high-resistivity n-type 4H-SiC epitaxial layers grown by chemical vapor deposition. It was established that the distribution of structural damage along the ion trajectory follows the computed profile of radiation defects formed in elastic collisions. The high-density ionization effect on the material characteristics has not been found under the irradiation conditions used. Optical methods revealed a wide spectrum of radiation-induced defects, with some of them contributing to the recombination process. The damaged 4H-SiC crystal lattice party recovers after annealing at 500°C. The text was submitted by the authors in English.     

Radiation resistance of transistor-and diode-type SiC detectors irradiated with 8-MeV protons

Nuclear-particle detectors based on SiC with a structure composed of an n⁺-type substrate, a p-type epitaxial layer, and a Schottky barrier are studied. Structures with a ～10-µm-thick 6H-SiC layer exhibit transistor properties, whereas those with a ～30-µm-thick 4H-SiC layer exhibit diode properties. It is established that a more than tenfold amplification of the signal is observed in the transistor-type structure. The amplification is retained after irradiation with 8-MeV protons with a dose of at least 5×10¹³ cm^?2; in this case, the resolution is ≤10%. Amplification of the signal was not observed in the structures of diode type. However, there were diode-type detectors with a resolution of ≈3%, which is acceptable for a number of applications, even after irradiation with the highest dose of 2×10¹⁴ cm^?2.     

Effect of electron irradiation on carrier removal rate in silicon and silicon carbide with 4H modification

Comparative study of the effect of successive (up to fluences of 3 × 10¹⁶ cm^?2) irradiation with 900 keV electrons of samples made of FZ-Si and 4H-SiC (CVD) has been performed for the first time. Measurements on initial and irradiated samples were made using the van der Pauw method for silicon and the capacitance-voltage technique at a frequency of 1 kHz for silicon carbide. In addition, the spectrum of the defect levels introduced was monitored by the DLTS method for SiC. The carrier removal and defect introduction rates were determined for the two materials. It was found that the rates of defect introduction into FZ-Si and 4 H-SiC (CVD) are close to eachy other (～0.1 cm^?1), which is largely due to the almost identical threshold energies of defect generation.     

Electrical properties of <Emphasis Type="Italic">n-</Emphasis>GaN/<Emphasis Type="Italic">p</Emphasis>-SiC heterojunctions

Epitaxial GaN layers were grown by hydride vapor phase epitaxy (HVPE) on commercial (CREE Inc., USA) p⁺-6H-SiC substrates (Na ? Nd ≈ 7.8 × 10¹⁷ cm^s?3) and n⁺-6H-SiC Lely substrates with a predeposited p⁺-6H-SiC layer. A study of the electrical properties of the n-GaN/p-SiC heterostructures obtained confirmed their fairly good quality and demonstrated that the given combination of growth techniques is promising for fabrication of bipolar and FET transistors based on the n-GaN/p-SiC heterojunctions.     

Similarities and distinctions of defect production by fast electron and proton irradiation: Moderately doped silicon and silicon carbide of <Emphasis Type="Italic">n</Emphasis>-type

Effects of irradiation with 0.9 MeV electrons as well as 8 and 15 MeV protons on moderately doped n-Si grown by the floating zone (FZ) technique and n-SiC (4H) grown by chemical vapor deposition are studied in a comparative way. It has been established that the dominant radiation-produced defects with involvement of V group impurities differ dramatically in electron- and proton-irradiated n-Si (FZ), in spite of the opinion on their similarity widespread in literature. This dissimilarity in defect structures is attributed to a marked difference in distributions of primary radiation defects for the both kinds of irradiation. In contrast, DLTS spectra taken on electron- and proton-irradiated n-SiC (4H) appear to be similar. However, there are very much pronounced differences in the formation rates of radiation-produced defects. Despite a larger production rate of Frenkel pairs in SiC as compared to that in Si, the removal rates of charge carriers in n-SiC (4H) were found to be considerably smaller than those in n-Si (FZ) for the both electron and proton irradiation. Comparison between defect production rates in the both materials under electron and proton irradiation is drawn.