Materials Science Forum Vols. 717-720

Abstract: Modern power conditioning systems require large active area devices which can support high currents. Though the breakdown and thermal properties of SiC make it an excellent choice for power switching applications, active area size is currently limited due to material and processing defects. One alternative is to parallel discrete diced die to achieve large active areas. However, this increases cost and complexity through dicing, soldering, and forming multiple wire bonds. Furthermore, paralleling discrete devices increases package volume/weight and reduces power density. To overcome these issues and achieve devices of high current switching capabilities, thyristors were designed and fabricated for the purpose of wafer-scale interconnection - which avoids the need of dicing and bonding and can achieve significant current density improvement over the paralleled diced device approach. Discrete thyristors fabricated for interconnection exhibited excellent yields and good uniformity of both blocking and on-state characteristics, showing great promise for large-scale interconnection.

Abstract: Bipolar degradation in 4H-SiC thyristors subjected to high current density stress is reported. The thyristor device structure, its fabrication process as well as testing conditions are described. The Electron Beam Induced Current (EBIC) technique was used for defect analysis in testing of both degraded and non-degraded devices. Possible nucleation sites responsible for the generation of observed defects in degraded devices are discussed

Abstract: This paper deals with the pulse capabilities of 4H-SiC optically triggered thyristors. The device structure and the fabrication process are presented. The results of pulse characterizations are shown. Two types of current pulses were used, a short (pulse width of 10 µs) and a long (pulse width of 650 µs). Peak current densities of 17 kA.cm-2 and 4 kA.cm-2 were attained with short and long pulses respectively. The failures and degradation caused by these experiments are also shown in this paper.

Abstract: The development of new semiconductor designs requires that extensive testing be completed in order to fully understand the device’s characteristics and performance capabilities. This paper describes the evaluation of experimental Silicon Carbide high power Super Gate Turn Off Thyristors (SiC SGTOs) in a unique test bed that is capable of stressing the devices with very high energy/power levels while at the same time mimicking a realistic, real world application for such devices.

Abstract: SiC based capacitive devices have the potential to operate in high temperature, chemically corrosive environments provided that the electrical integrity of the gate oxide and metallization can be maintained in these environments. We report on the performance of large area, up to 8 x 10^-3 cm², field-effect capacitive sensors fabricated on both the 4H and 6H polytypes at 600°C. Large area capacitors improve the signal/noise (S/N) ratio which is proportional to the slope of the capacitance-voltage characteristic. At 600 °C we obtain a S/N ~ 20. The device response is independent of polytype, either 4H or 6H-SiC. These results demonstrate the reliability of our field-effect structure, operating as a simple potentiometer at high temperature.

Abstract: This paper describes efforts towards the transition of existing high temperature hydrogen and hydrocarbon Schottky diode sensor elements to packaged sensor structures that can be integrated into a testing system. Sensor modifications and the technical challenges involved are discussed. Testing of the sensors at 500°C or above is also presented along with plans for future development.

Abstract: An uncooled SiC-based electro-optic device is developed for gas sensing applications. P-type dopants Ga, Sc, P and Al are incorporated into an n-type crystalline 6H-SiC substrate by a laser doping technique for sensing CO₂, CO, NO₂ and NO gases, respectively. Each dopant creates an acceptor energy level within the bandgap of the substrate so that the energy gap between this acceptor level and the valence band matches the quantum of energy emitted by the gas of interest. The photons of the gas excite electrons from the valence band to the acceptor level, which alters the electron density in these two states. Consequently, the refractive index of the substrate changes, which, in turn, modifies the reflectivity of the substrate. This change in reflectivity represents the optical signal of the sensor, which is probed remotely with a laser such as a helium-neon laser. Although the midwave infrared (3-5 mm) band is studied in this paper, the approach is applicable to other spectral bands.

Abstract: We report measurements and modeling of silicon carbide (SiC) based ultraviolet photodetectors for the detection of light in the mid-to-short ultraviolet range where SiC’s absorption coefficients are high and the corresponding penetration depths are low. These large absorption coefficients result in increased susceptibility of photo-generated electron and holes to surface recombination and therefore give rise to lower quantum efficiencies. To increase responsivity and extend the detection capability of these photodetectors to short ultraviolet wavelengths (or UVC), we measure an existing silicon carbide avalanche photodiode (APD) designed and fabricated for 280 nm operation by General Electric Global Research Center, and then develop models and techniques to increase their operation range to lower UV wavelengths. The measurements aid the development and calibration of a silicon carbide modeling and design suite that is currently being used to assist the design of a new silicon carbide APD for UVC detection. Here the design considerations require low operating voltages, low noise, low dark count rate and high responsivity. We plan to satisfy design criteria by engineering thickness and doping of stacked layers as well as by designing an APD surface that gives rise to minimal recombination of electrons and holes generated by the incident light.

Abstract: This paper presents a study of 4H-SiC UV photodetectors based on p+n thin junctions. Two kinds of p+ layers have been implemented, aiming at studying the influence of the junction elaborated by the ion implantation process (and the subsequent annealing) on the device characteristics. Aluminum and Boron dopants have been introduced by beam line and by plasma ion implantation, respectively. Dark currents are lower with Al-implanted diodes (2 pA/cm2 @ - 5 V). Accordingly to simulation results concerning the influence of the junction thickness and doping, plasma B-implanted diodes give rise to the best sensitivity values (1.5x10-1 A/W @ 330 nm).

Abstract: This paper reports on fabrication and modeling of 4H- and 6H-SiC metal-semiconductor-metal (MSM) photodetectors (PDs). MSM PDs have been fabricated on 4H-SiC and 6H-SiC epitaxial layers, and their performance analyzed by MEDICI simulation and measurements. The simulations were also used to optimize the sensitivity by varying the width and spacing of the interdigitated electrodes. The fabricated PDs with 2 µm wide metal electrodes and 3 µm spacing between the electrodes exhibited, under UV illumination, a peak current to dark current ratio of 105 and 104 in 4H-SiC and 6H-SiC, respectively. The measured spectral responsivity of 6H-SiC PDs was higher compared to that of 4H-SiC PDs, with a cutoff at 407 nm compared to 384 nm in 4H-SiC PDs. Also the peak responsivity occurred at a shorter wavelength in 6H material. A high rejection ratio between the photocurrent and dark current was found in both cases. These experimental results were in agreement with simulation.