Irradiation of 4<Emphasis Type="Italic">H</Emphasis>-SiC UV detectors with heavy ions

Ultraviolet (UV) photodetectors based on Schottky barriers to 4H-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 × 10⁹ 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.     

The effect of irradiation with high-energy protons on 4<Emphasis Type="Italic">H</Emphasis>-SiC detectors

The effect of irradiation of 4H-SiC ionizing-radiation detectors with various doses (as high as 10¹⁶ cm^?2) of 24-GeV protons is studied. Isotopes of B, Be, Li, He, and H were produced in the nuclear spallation reactions of protons with carbon. Isotopes of Al, Mg, Na, Ne, F, O, and N were produced in the reactions of protons with silicon. The total amount of the produced stable isotopes varied in proportion with the radiation dose from 1.2 × 10¹¹ to 5.9 × 10¹³ cm^?2. It is shown that, at high radiation doses, the contact characteristics of the detectors change appreciably. The potential-barrier height increased from the initial value of 0.7–0.75 eV to 0.85 eV; the rectifying characteristics of the Schottky contacts deteriorated appreciably. These effects are attributed to the formation of a disordered structure of the material as a result of irradiation.     

3<Emphasis Type="Italic">C</Emphasis>-SiC <Emphasis Type="Italic">p-n</Emphasis> structures grown by sublimation on 6<Emphasis Type="Italic">H</Emphasis>-SiC substrates

Sublimation epitaxy in a vacuum has been employed to grow n-and p-type 3C-SiC layers on 6H-SiC substrates. Diodes have been fabricated on the basis of the p-n structure obtained, and their parameters have been studied by measuring their current-voltage and capacitance-voltage characteristics and by applying the DLTS and electroluminescence methods. It is shown that the characteristics of the diodes studied are close to those of diodes based on bulk 3C-SiC. A conclusion is made that sublimation epitaxy can be used to fabricate 3C-SiC p-n structures on substrates of other silicon carbide polytypes.     

Effect of irradiation with fast neutrons on electrical characteristics of devices based on CVD 4<Emphasis Type="Italic">H</Emphasis>-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.     

Subnanosecond 4<Emphasis Type="Italic">H</Emphasis>-SiC diode current breakers

Mesa epitaxial 4H-SiC-based p ⁺-p-n ₀-n ⁺ diodes have been fabricated and their reverse recovery characteristics have been measured in modes typical of fast semiconductor current breakers, drift step recovery diodes, and SOS diodes. It has been found that, after the short (～10 ns) pulsed injection of nonequilibrium carriers by a forward current with a density of 200–400 A cm^?2 and the subsequent application of a reverse voltage pulse (with a rise time of 2 ns), diodes can break a reverse current with a density of 5–40 kA cm^?2 in a time of about (or less than) 0.3 ns. A possible mechanism for ultrafast current breaking is discussed.     

Temperature dependence of the band structure of 3<Emphasis Type="Italic">C</Emphasis>, 2<Emphasis Type="Italic">H</Emphasis>, 4<Emphasis Type="Italic">H</Emphasis>, and 6<Emphasis Type="Italic">H</Emphasis> SiC polytypes

The temperature dependences of significant energy extrema at the high-symmetry points Γ, X, L, K, M, A, and H of the Brillouin zone in the cubic and hexagonal modifications of SiC, as well as the energies of the main interband transitions at these points, were calculated for the first time by the empirical-pseudopotential method. The effect of the temperature dependence of the electron-phonon interaction on the crystal band structure was taken into account via the Debye-Waller factors, and the contribution of the linear expansion of the lattice was accounted for via the temperature dependence of the linear-expansion coefficient. The special features of the temperature dependences of the energy levels and of energies of the interband and intraband transitions are analyzed in detail. The results of the calculations are in good agreement with the known experimental data on the characteristics of SiC-based p-n structures operating in the breakdown mode. For example, the temperature coefficient of the energy of the X_1c–X_3c transition, which is responsible for the narrow violet band in the breakdown-electroluminescence spectra of reverse-biased p-n junctions, was found to be significantly smaller than the temperature coefficients for the interband transitions (from the conduction to valence band). This fact is quite consistent with the experimental curve of the temperature coefficient of the emission spectrum, which has a minimum in the same wavelength range.     

Current flow mechanism in ohmic contact to <Emphasis Type="Italic">n</Emphasis>-4<Emphasis Type="Italic">H</Emphasis>-SiC

Current flow in an In-n-4H-SiC ohmic contact (n ≈ 3 × 10¹⁷ cm⁻³) has been studied by analyzing the temperature dependence of the per-unit-area contact resistance. It was found that the thermionic emission across an ∼0.1-eV barrier is the main current flow mechanism and the effective Richardson constant is ∼2 × 10⁻² A cm⁻² K⁻¹.     

Low-temperature (77 K) impurity breakdown in <Emphasis Type="Italic">p</Emphasis>-type 4<Emphasis Type="Italic">H</Emphasis>-SiC

The impact ionization of acceptors in aluminum-doped 4H-SiC epitaxial films (Al concentration 2 × 10¹⁵ cm^?3) at a temperature of 77 K is studied. It is found that the impact-ionization coefficient exponentially depends on the reverse electric field: α _p = α*_pexp(?F*/F). The largest ionization coefficient is α* _p = 7.1 × 10⁶ cm^?3 s^?1, and the threshold field is F* = 2.9 × 10⁴ V/cm.     

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.     

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     

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.     

Excess leakage currents in high-voltage 4<Emphasis Type="Italic">H</Emphasis>-SiC Schottky diodes

The high-voltage 4H-SiC Schottky diodes are fabricated with a nickel barrier and a guard system in the form of “floating” planar p-n junctions. The analysis of I–V characteristics measured in a wide temperature range shows that the forward current is caused by thermionic emission; however, the current is “excessive” in the reverse direction. It is assumed that the reverse current flows locally through the points of the penetrating-dislocation outcrop to the Ni-SiC interface. The shape of reverse I–V characteristics makes it possible to conclude that the electron transport is governed by the monopolar-injection mechanism (the space-charge limited current) with participation of capture traps.     

On the Spatial Localization of Free Electrons in 4<Emphasis Type="Italic">H</Emphasis>-SiC MOSFETS with an <Emphasis Type="Italic">n</Emphasis> Channel

The problem of the spatial localization of free electrons in 4H-SiC metal—oxide—semiconductor field effect transistors (MOSFETS) with an accumulation- and inversion-type n channel is theoretically analyzed. The analysis demonstrates that, in optimally designed accumulation transistors (ACCUFETs), the average distance from the surface, at which free electrons are localized, may be an order of magnitude larger than that in inversion MOSFETs. This can make 4H-SiC ACCUFETs advantageous as regards the effective carrier mobility in a conducting channel.     

High-voltage (900 V) 4 <Emphasis Type="Italic">H</Emphasis>-SiC Schottky diodes with a boron-implanted guard <Emphasis Type="Italic">p-n</Emphasis> junction

High-voltage (900 V) 4H-SiC Schottky diodes terminated with a guard p-n junction were fabricated and studied. The guard p-n junction was formed by room-temperature boron implantation with subsequent high-temperature annealing. Due to transient enhanced boron diffusion during annealing, the depth of the guard p-n junction was equal to about 1.7 μm, which is larger by approximately 1 μm than the projected range of 11 B ions in 4H-SiC. The maximum reverse voltage of fabricated 4H-SiC Schottky diodes is found to be limited by avalanche breakdown of the planar p-n junction; the value of the breakdown voltage (910 V) is close to theoretical estimate in the case of the impurity concentration N = 2.5 × 10¹⁵ cm^?3 in the n-type layer, thickness of the n-type layer d = 12.5 μm, and depth of the p-n junction r _j = 1.7 μm. The on-state diode resistance (3.7 mΩ cm²) is controlled by the resistance of the epitaxial n-type layer. The recovery charge of about 1.3 nC is equal to the charge of majority charge carriers that are swept out of an epitaxial n-type layer under the effect of a reverse voltage.     

Bistable low temperature (77 K) impurity breakdown in <Emphasis Type="Italic">p</Emphasis>-type 4<Emphasis Type="Italic">H</Emphasis>-SiC

Low-temperature (77 K) forward current-voltage characteristics of 4H-SiC p ⁺-p-n ⁺-n (substrate) mesa epitaxial diode structures have been measured. The characteristics are S-shaped, which is accounted for by the bistable nature of the impact ionization of frozen-out acceptor atoms of aluminum.     

Experimental 4<Emphasis Type="Italic">H</Emphasis>-SiC junction-barrier Schottky (JBS) diodes

4H-SiC junction-barrier Schottky (JBS) diodes have been fabricated with local p–n junctions under the Schottky contact formed by nonequilibrium diffusion of boron. Static and dynamic characteristics of the JBS diodes are compared with those of similar 4H-SiC Schottky diodes. It is shown that, compared with ordinary Schottky diodes, the JBS diodes have leakage currents that are, on average, a factor of 200 lower at the same reverse bias. The reverse recovery charge is the same for both types of diodes and equal to the charge of majority carriers removed from the n-type base region in switching.     

Conductivity compensation in <Emphasis Type="Italic">p</Emphasis>-6<Emphasis Type="Italic">H</Emphasis>-SiC in irradiation with 8-MeV protons

Carrier removal rate (V _d ) in p-6H-SiC in its irradiation with 8-MeV protons has been studied. The p-6H-SiC samples were produced by sublimation in vacuum. V _d was determined by analysis of capacitance-voltage characteristics and from results of Hall effect measurements. It was found that complete compensation of samples with initial value of N _a − N _d ≈ 1.5 × 10¹⁸ cm⁻³ occurs at an irradiation dose of ∼1.1 × 10¹⁶ cm⁻². In this case, the carrier removal rate was ∼130 cm⁻¹.     

The <Emphasis Type="Italic">U</Emphasis> peak in the DLTS spectra of <Emphasis Type="Italic">n</Emphasis>-GaAs irradiated with fast neutrons and 65-MeV protons

The origin of a broad U band in spectra obtained using deep-level transient spectroscopy (DLTS) of n-GaAs irradiated with fast neutrons and 65-MeV protons was investigated. It is believed that this band is presumably a superposition of two peaks related to two defects P2 and P3 which have been well documented in GaAs and reside within defect clusters. The DLTS spectra were calculated taking into account the nonuniform distribution of these defects in a sample and the built-in electric fields induced by corresponding inhomogeneities.     

Role of spontaneous polarization in the formation of <Emphasis Type="Italic">NH</Emphasis>-SiC/3<Emphasis Type="Italic">C</Emphasis>-SiC/<Emphasis Type="Italic">NH</Emphasis>-SiC structures based on silicon carbide polytypes

The effect of the electrostatic field caused by spontaneous polarization in hexagonal plates of a heterostructure based on NH-SiC/3C-SiC/NH-SiC silicon carbide polytypes on relative positions of energy bands is considered. It was shown that the asymmetry arising in the system is associated with the superposition of polarization and contact fields.     

Giant burst of impact ionization in a <Emphasis Type="Italic">p-n</Emphasis> junction of the 6<Emphasis Type="Italic">H</Emphasis>-SiC polytype

A study of the electron component of impact ionization in the p ⁺-n ^?-n ⁺ junction in the 6HSiC polytype made it possible to detect a giant burst of impact ionization and origination of an extra early avalanche breakdown. The electric field of this breakdown is lower by ～20% than the electric field of the breakdown arising as a result of a steady development of the impact ionization. It is of interest that this phenomenon occurs abruptly, without any apparent causes, in particular, without an increase in the dark current characteristic of a prebreakdown state of the p-n junction. Conditions for origination of an unusual breakdown and its properties made it possible to assume that there are nonlinear processes that give rise to a streamer. In the p-n junction plane, the anomalous breakdown is seen as a narrow glowing track with a width of ≈10 μm. This effect takes place in the conditions of the Wannier-Stark ladder of states. The latter can stimulate a local accumulation of charge and formation of a streamer structure.