Materials Science Forum Vols. 740-742

Abstract: This paper describes the physical and electrical properties of a p-n Si/on-axis SiC vertical heterojunction rectifier. A thin 400nm p-type silicon layer was wafer-bonded to a commercial on-axis SiC substrate by room temperature hydrophilic wafer bonding. Transmission electron microscopy was used to identify the crystallographic orientation as (0001)SiC//(001)Si and to reveal an amorphous interfacial layer. Electrical tests performed on the p-n heterodiodes revealed that, after an additional 1000oC anneal, the rectifier exhibit remarkably low leakage current (10nA/cm2 at an anode voltage of V=-6V), improved on-resistance due to bipolar injection and a turn-on voltage close to the p-n heterojunction theoretical value of 2.4V.

Abstract: This paper presents the results of research and development of two types diode structures based on wide bandgap 4H-SiC: drift step recovery diodes (DSRDs) and field emission diodes (FED). Diodes’ structure and manufacturing methods are reviewed. Diode’s characteristics were obtained (static current-voltage characteristics and capacitor-voltage characteristic, switching properties’ characteristics for DSRDs). Field emission 4H-SiC structures illustrated high (≥10² А/сm²) current densities at electric field intensity of approximately 10V/um. 4H-SiC DSRDs in the generator structure with a single oscillating contour allowed to form sub nanosecond impulses at a load 50 Ohm and 1,5-2kV amplitude for a single diode (current density at V=2kV J= 4•10³ А/сm²),what is significantly higher than similar DSRD’s parameters obtained for silicon.

Abstract: Carcinogenic (bactericidal) radiation (λ = 200–300 nm with a peak at 254 nm) is present in natural (Sun) and artificial (lamps) source of UV radiation. Its intensity is very low as compared to other types of radiation, but it strongly affects the health of human beings. To prevent oncological diseases, it is important to monitor the carcinogenic radiation level; i.e., selective photodetectors are required. A UV photodetectors based on n-4H-SiC Schottly barriers and p+-n junctions are proposed. The quantum efficiency spectrum of such detectors is very close to the spectrum of relative action of carcinogenic radiation on human beings due to the direct optical transition at 4.9 eV in 4H-SiC. The quantum efficiency (at the spectral peak 254 nm) amounts to about 0.3 electrons/photon for virtually zero sensitivity in other spectral regions. Quantum efficiency in the wavelength range 247–254 nm is practically independent of temperature in the range from −100 to +300°C.

Abstract: This paper deals with optical and electrical simulations of 4H-SiC UV-Photodetectors based on pn junctions. The simulations are performed under the UV light, with wavelengths varying between 200 nm and 300 nm. Under reverse bias, the simulation results point out the influence of surface patterns on the current density. The studied structures of the patterns consist in a semicircle with or without a flat surface. The patterned surfaces are parametrized according to the semicircle radius R and the flat surface length L. We show that the optical absorption strongly depends on these parameters, giving a maximum value whatever the wavelength with R = 100 nm and L = 0 nm (no flat surface). However, to optimise the carrier harvest, it is important for the space charge region to be situated in a zone where the optical generation is high. This study shows that the photodetector current density increases within three orders of magnitude (from 9x10^-14 A.cm^-2 to 3x10^-10 A.cm^-2), by using the specific surface pattern given above.

Abstract: Surface nanocones on 6H-SiC have been developed and demonstrated as an effective method of enhancing the light extraction efficiency from fluorescent SiC layers. The surface reflectance, measured from the opposite direction of light emission, over a broad bandwidth range is significantly suppressed from 20.5% to 1.0 % after introducing the sub-wavelength structures. An omnidirectional light harvesting enhancement (>91%), is also achieved which promotes fluorescent SiC as a good candidate of wavelength converter for white light-emitting diodes.

Abstract: Epitaxial graphene grown on semiinsulating silicon carbide was used to fabricate side gate graphene transistors. The transconductance of the side gate transistors is comparable to top gate designs. The transconductance decreases with increasing gate width independently on the gate to channel distance in agreement with the transconductance reduction in top gate transistor configu¬rations with increasing channel length. The transconductance of the side gate transistors decreases with increasing channel width due to a decreased specific gate capacitance.

Abstract: The mechanical properties of die attach system SiC/Au-Ge/Au-Ni-Cu-Si3N4 using the eutectic Au-Ge solder (Teut = 356°C) were investigated in a temperature range up to 300°C. The as-resulting structure of the solder is observed to be lamellar with pockets of high concentration of Au close to the interfaces. The shear strength of joint decreases with temperature but, even at 300°C, its value is well higher than the IEC standard. The creep behavior of Au-Ge solder alloy was also investigated at 300°C for different strain levels. The creep curves show a high creep resistance even for high stress level.

Abstract: Insulating properties of package for ultrahigh-voltage, high-temperature devices have been investigated. While all the packages have enough insulating strength at room temperature, deterioration of the insulating property at high temperature has been found with some packages. The authors have found that this deterioration is attributed to degrade the insulation property of AlN ceramics for DBC substrate at high temperature and that there is a various degree in the deterioration.

Abstract: A new SiC power module package structure is proposed that is capable of withstanding greater ΔTj cycle stress. Its most notable feature is the use of a SiN substrate having Cu/Invar/Cu foils (C/I/C thickness ratio of 1/8/1) brazed on both sides as conductor plates. The CIC foils show a very low coefficient of thermal expansion (CTE) of 5.1 ppm/°C and therefore can significantly reduce package degradation resulting from the larger CTE mismatch of the conductor to SiC and SiN. A thermal cycle test (TCT) was conducted between -40°C and 250°C (ΔTj = 290°C). It was found that the SiC/Au-Ge/CIC-SiN die attachment maintained joint strength of 78 MPa even after 3000 cycles.

Abstract: A comparison of radiated noise for Silicon and Silicon Carbide converters is presented. SiC JBS diodes were used in this evaluation to enable fast switching times, whilst minimizing the transistor junction temperature. Radiated electromagnetic-interference measurements showed the highest noise signature for the SiC JFET and lowest for the SiC MOSFET. The negative gate voltage requirement of the SiC MOSFET introduces up to 6 dBµV increase in radiated noise, due to the induced current in the high frequency resonant stray loop in the negative power plane of the gate drive. The SiC JFET and MOSFET have shown overall converter efficiencies of 96% and 95.5% respectively. This efficiency shows only a weak frequency dependence, in contrast to the CoolMOS/SiC JBS diode combination which demonstrated an efficiency drop from 95% to 92.5% when increasing the frequency from 100kHz to 250kHz.