T. P. Samsonova

Excess leakage currents in high-voltage 4H-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 possibility of creating a superfast-recovery silicon carbide diode

The possibility of a superfast (<1 ns) termination of the reverse current during the recovery of a 4H-SiC diode with a p+p0n+ structure is experimentally demonstrated for the first time. It is shown that the gate recovery process is much like that taking place in inverse-recovery silicon diodes.

I-V characteristics of high-voltage 4H-SiC diodes with a 1.1-eV Schottky barrier

The I-V characteristics of high-voltage 4H-SiC diodes with a Schottky barrier ∼1.1 eV in height are measured and analyzed. The forward I-V characteristics proved to be close to “ideal” in the temperature range of 295–470 K. The reverse I-V characteristics are adequately described by the model of thermionic emission at the voltages to 2 kV in the temperature range of 361–470 K if, additionally, a barrier lowering with an increase in the band bending in the semiconductor is taken into account.

Experimental 4H-SiC junction-barrier Schottky (JBS) diodes

Abstract4H-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.

Direct bonding of silicon carbide wafers with a regular relief at the interface

The direct bonding of two oxide-free 6H-SiC(0001) silicon carbide single crystal wafers, one smooth and another bearing an artificial microscopic relief, has been studied. According to the X-ray topography data, the bonded surface fraction reaches 85% of the total area. The pattern of stress distribution at the interface is aperiodic, which is indicative of an inhomogeneous microroughness of the surface of bonded wafers.

Current-voltage characteristics of isotype SiC-SiC junctions fabricated by direct wafer bonding

Results obtained in a study of current-voltage characteristics of isotype SiC-SiC structures fabricated by direct bonding of single-crystal n-type 6H-SiC wafers with a donor concentration of ∼1016 cm−3 are presented. The initial wafer bonding was done in deionized water. To enhance the adhesion, the structure was thermally annealed at 1250°C. All the features of the current-voltage characteristics measured are consistently explained in terms of the hypothesis that the SiC-SiC interface is a variable-thickness channel filled with a native SiOx oxide of thickness 10–100 nm. The minimum experimentally measured differential resistance of the structure (6 Θ cm2) is limited by the current transport in the oxide layer, which occurs by the mechanism of space-charge-limited currents.

Leakage currents in 4H-SiC JBS diodes

Leakage currents in high-voltage 4H-SiC diodes, which have an integrated (p-n) Schottky structure (Junction Barrier Schottky, JBS), have been studied using commercial diodes and specially fabricated (based on a commercial epitaxial material) test Schottky diodes with and without the JBS structure. It is shown that (i) the main role in reverse charge transport is played by SiC crystal structure defects, most probably, by threading dislocations (density ∼104 cm−2), and (ii) the JBS structure, formed by the implantation of boron, partially suppresses the leakage currents (by up to a factor of 10 at optimal separation, 8 μm between local p-type regions).

High-voltage (1800 V) planar 4H-SiC p-n 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

High-voltage (3.3 kV) 4H-SiC JBS diodes

High-voltage 4H-SiC junction-barrier Schottky (JBS) diodes have been fabricated and studied. The working area of the diodes (anode contact area) is 1.44 mm2. At currents in the range from 10−11 to 1.5 A, the forward current-voltage characteristic of the diodes is described in terms of the thermionic emission model, with the series resistance taken into account: Schottky barrier height ΦB = 1.16 eV, ideality factor n = 1.01, and series resistance Rs = 2.2 Ω (32 mΩ cm2). The value of Rs is governed by the resistance of the blocking epitaxial n-base (impurity concentration N = 9 × 1014 cm−3, n-layer thickness d = 34 μm). The diodes can block a reverse voltage of at least 3.3 kV (with a leakage current at room temperature on the order of 1 μA). It is suggested that the leakage mechanism is associated with crystal lattice defects (dislocations) in SiC. It is shown that the reverse-recovery characteristics of the diodes are determined by the flow of a purely capacitive reverse current.

Analysis of forward current-voltage characteristics of nonideal Ti/4H-SiC Schottky barriers

Forward current-voltage characteristics of nonideal Ti/4H-SiC Schottky contacts with an ideality factor n = 1.1–1.2 in the exponential portion of the characteristics have been analyzed. The nonideality was considered to be a result of the formation of a thin dielectric layer between the deposited titanium layer and 4H-SiC. The following electrical parameters of the contacts were determined from experimental current-voltage characteristics: energy barrier height, thickness of the intermediate dielectric layer, and energy distribution of the density of states at the insulator-semiconductor interface.