Materials Science Forum Vols. 600-603

Abstract: A 10 kV 4H-SiC PiN diode with an improved junction termination structure has been fabricated. An improved bevel mesa structure, nearly vertical side-wall at the edge of pn junction and rounded corner at mesa bottom, has been formed by reactive ion etching (RIE). The junction termination extension (JTE) region has been optimized by device simulation, and simulated breakdown voltage has been compared with experimental results. The locations of electric field crowding and diode breakdown have been discussed.

Abstract: A major crystalline defect which causes a pn junction reverse leakage current has been identified. A faintish stripe defect (FSD), the main cause of the leakage current, was observed in about 90% of the current leak points of our pn diodes. Double shell pits were observed at the edge of the FSD after molten KOH etching, indicating that the FSD is elongated on a basal plane and crosses the epilayer surface. The FSDs are sorted into several groups in terms of the shapes and arrangements of the etch pits. A cross-sectional TEM image of an FSD shows an eight-hold stacked structure, demonstrating that the defect contains a stacking fault. Etch pit observation after repetitive RIE of an epilayer revealed that FSDs originate both in threading dislocations in SiC substrates and from an SiC epitaxial growth process itself.

Abstract: The impact of anode layers on the electrical characteristics of 10kV 4H-SiC PiN diodes has been evaluated in this work. Co-fabricated diodes with various epitaxial anode layer designs as well as those employing P+ implanted injecting layers are used to experimentally investigate the device conduction mechanisms. The role of the injecting layer is demonstrated via electrical characteristics and numerical simulations, showing the importance of maintaining sufficient carrier recombination lifetime in the device anode region.

Abstract: Self-heating in high-voltage 4H-SiC PiN diodes has been studied experimentally and theoretically in dc and 8-ms single pulse modes. To simulate the self-heating, an electro-thermal model was used to calculate non-isothermal current-voltage characteristics at dc and current-time dependences at pulsed measurements. The dynamic instability of N-type was observed: the current decreases in spite of increasing of bias applied to the structure. At dc, irreversible diode degradation was found to occur at a current density of about 1700 A/cm2. Under a single current surge 8-ms pulse, the loss of thermal stability has been found at a current density of approximately 9000 A/cm2. Comparison of experimental data and simulations showed that the local temperature in the diode base at the end of the 8-ms, 9000-A/cm2 pulse reaches 2000 – 2300 K.

Abstract: This paper presents a comparison of the reverse characteristics of mesa terminated PiN diodes fabricated on n- and p-type 4H-SiC substrates. For n-type the attained breakdown voltages are higher and for p-type lower than expected. This is likely to be explained by the presence of negative charges at the interface between passivation oxide and SiC. Supported by XPS data we come to the conclusion that the RIE process creates surface charges which have an impact on the breakdown voltage of the fabricated diodes.

Abstract: This paper reports on the achievement of high-power 4H-SiC Zener diodes which have a high-doped pn junction with a large active area of 4 mm x 4 mm. The temperature coefficient of the breakdown voltage is as small as 5.7x10-5 1/K (positive) in the temperature range 20-300°C. In addition, reverse power capabilities of 6.3 kW (40 kW/cm2) at 20°C and 6.0 kW (38 kW/cm2) at 300°C during rectangular pulsed power operation (tw = 1 ms) have been achieved without device failure.

Abstract: Multi-diode broadband microwave signal modulators based on 4H-SiC p-i-n diodes were fabricated and fully characterized. The three-diode modulators are characterized by a transmission loss of 1-2 dB and isolation of 27-34 dB in the 2-7 GHz frequency range. Two-diode modulators were specially designed for high-temperature operation. These modulators are characterized by a transmission loss of 1.1-2.6 dB and isolation of 33-44.5 dB, in the 2-7 GHz frequency range at temperatures up to 300°C.

Abstract: The I-V characteristics of p+n 4H-SiC diode formed by Al ion implantation have been investigated as a function of annealing temperature. Al ions are implanted at the elevated sample temperature of 500 oC in order to fabricate p-type doped layer on the n-type epitaxial layer, grown on n+ 4H-SiC substrate. The implanted sample is annealed using electron bombardment annealing system in the annealing temperature ranging from 1700 to 1900 oC. The Al implanted sample, annealed below 1800 oC shows the deteriorated I-V characteristics in which the forward current includes the resistive current components and the reverse current is in the order of 10-4 A/cm2. The p+n diode formed by annealing at 1900 oC reveals the forward current without extra-current components and the reverse current as low as 10-6 A/cm2. It is suggested that the annealing above 1900 oC is effective in reducing the implantation-induced defect at the interface between Al implanted p+ layer and the underlying n-type epitaxial layer.

Abstract: The current-voltage characteristics of Al+ implanted 4H-SiC p+n junctions show an important reduction of leakage currents with diode aging at room temperature. The case of a family of diodes that immediately after manufacture had forward current density increasing from 10-9 to 10-6 A/cm2 when biased from 0 and 2 V, and had a reverse leakage current density of @ 5×10-7 A/cm2 when biased at 100 V, is here presented and discussed. During diode manufacturing a post implantation annealing at 1600 °C for 30 min was followed by a 1000 °C 1 min treatment for metal contacts alloying. After 700 days of storage at room temperature, the diode reverse current density reached an asymptotical value of @ 4×10-11 A/cm2 that is four order of magnitude lower than the initial one. A 430 °C annealing that was made after 366 days is responsible of a decrease of one of these four orders of magnitude, but it does not interrupt the decreasing trend versus increasing time. This same annealing has been effective also for minimizing forward current for bias < 2 V, and sticking the diode turn-on voltage on 1.4 V and the current trend on an ideality factor of 2. These results show that in Al+ implanted 4H-SiC p+n junction there are defects that have an annihilation dynamic at very low temperatures, i.e. room temperature and 430 °C.

Abstract: A novel experimental set-up is developed and validated to characterize high voltage diodes in transient switching mode. Parameters extracted from DMTVCA and OCVD techniques, like ambipolar lifetime, epilayer thickness and doping level, diode area, are validated in a buck converter with resistive load. The experimental set-up allows to measure the current and voltage transient characteristics without noise and influence of high parasitic wiring. Experimental results are compared with device simulations and a good correlation is found.