Materials Science Forum Vols. 556-557

Abstract: The area where 4H-SiC SBDs showed high reverse currents was extracted. After KOH etching, the in-grown SF on the basal plane, composed of a straight etch line with a pair of tilted oval pits and additional etch pits forming an isosceles triangle, was observed on some devices. All of the devices containing this SF structure showed large reverse leakage currents in spite of the good forward I-V characteristics. We speculate that this in-grown SF includes another planar fault on the {1-100} plane besides the basal plane which has a great influence on reverse currents of SBDs.

Abstract: In this contribution we summarize measurements on bipolar high voltage SiC-diodes which were fabricated on 4H-SiC wafers preferentially cut 4° off the [0001] basal plane, whereas the p-emitter thickness was varied in predetermined ratios to the n-base thickness. The switching behaviour of optimized 6.5 kV-Diodes at a current level of 25 A is shown at DC link voltages up to 4 kV and at a junction temperature of 125°C. Experimental results are discussed in terms of snappiness.

Abstract: The development of 4H-SiC PiN diodes capable of blocking to greater than 10 kV while having current ratings of 20 A at 100 A/cm2 is continuing in earnest. VF instability of these diodes continues to be a roadblock, but progress is being made, and a 20 A/10 kV 4H-SiC PiN diode wafer with an overall device yield of 40% has been fabricated. The latest device characteristics are discussed, along with details of approaches in improving the reverse recovery characteristics of these diodes to satisfy the requirements needed for implementation into high voltage inverter modules capable of switching at up to 20 kHz.

Abstract: Silicon carbide devices limitations often originate from the quality of the substrate material. Therefore it is interesting to investigate devices fabricated on alternative source materials. Currently, CREE is the world market leader of SiC wafers. Nowadays, some new companies begin to propose alternative material. The European manufacturer SiCrystal furnishes now some epiwafers for the fabrication of 1,2kV devices. In this paper we present 4H-SiC 1.2 kV pin diodes with a JTE termination realized on a SiCrystal epiwafer. The devices exhibit a blocking voltage of 1.2 kV, a current density of 420 A.cm-2 and a specific differential series resistance of 4.4 m-⋅cm2. The yield of fabricated diodes with a breakdown voltage greater 600 V is superior to 75%.

Abstract: We simulated and measured the forward characteristics of 4H-SiC pin diodes in a wide temperature range from 300K to 700K. Our simulations are based on the stationary drift-diffusion model including a model for incomplete ionization of the dopants. Physically based models for Auger recombination and Shockley-Read-Hall recombination are used as well. For the mobility model the empirical relation of Caughey-Thomas is used. The model parameters to be calibrated in the simulation are the electron and hole minority lifetimes and the electron and hole bulk mobilities. Employing temperature-dependent carrier lifetimes we achieved very good agreement between simulations and measured data. For the temperature- and doping-dependent carrier mobilities we found that the best fit is obtained for a bulk mobility value much smaller than that suggested by standard parameters for 4H-SiC. With the calibrated parameters we simulated the internal carrier distributions for temperatures up to 700 K and for different carrier lifetimes.

Abstract: Raman spectroscopy and photoemission microscopy were coupled as two complementary non-destructive optical techniques in order to study biased 4H-SiC pin diodes. These two characterization tools have been largely used for the study of semiconductors but the combination of these two techniques has hardly been reported so far. Some structural defects inducing the same electrical damage could be discriminated and identified. Temperature could be measured in operating devices and the influence of the diode operating mode on the Raman signal could be evidenced.

Abstract: The charge generated in 6H-SiC n+p diodes by gold (Au) ion irradiation at an energy of 12 MeV was evaluated using the Transient Ion Beam Induced Current (TIBIC). The signal peak of the transient current increases, and the fall-time decreases with increasing applied reverse bias. The value of collected charge experimentally obtained is smaller than the ideal value. The Charge Collection Efficiency (CCE) of 6H-SiC n+p diodes irradiated with Au ions is approximately 50 % in spite that the CCE of 100 % is obtained in the case of oxygen (O) ion irradiation.

Abstract: In this work we analyzed the radiation hardness of SiC p+n diodes after very high 1 MeV neutron fluence. The diode structure is based on a p+ emitter ion implanted in n-type epilayer with thickness equal to 5 %m and donor doping ND = 3×1015 cm-3. Before irradiation, the average leakage current density at 100 V reverse bias was of the order of 3 nA/cm2. These devices were irradiated at four different fluence values, logarithmically distributed in the range 1014-1016 (1 MeV) neutrons/cm2. After irradiation the epilayer material became more resistive, as indicated by the reduction of the forward and reverse current density at a given voltage. In particular, after a neutron fluence of 1×1014 n/cm2 the epilayer active doping concentration decreased to 1.5×1015 cm-3. After irradiation at 1016 n/cm2, i.e. the highest fluence value, the average leakage current density at 100 V reverse bias decreased to values of the order of 0.1 nA/cm2. This very low noise even after very high fluence is very important to obtain a high signal to noise ratio even at room temperature.

Abstract: Forward current-voltage (I-V) characteristics and non-equilibrium carrier lifetime, τ were measured in 4H-SiC pin diodes (10-kV rated, 100 μm base width). The τ value was found to be 3.7 μs at room temperature by measurements of open circuit voltage decay. To the best of the authors' knowledge, the above lifetime value is the highest reported for 4H-SiC. The forward voltage drops were measured to be 3.44 V at current density of 100 A/cm2 and 5.45 V at 1000 A/cm2 showing a very deep modulation of the blocking base by injected carriers. Diodes operated well at elevated temperatures up to 400oC. No essential forward degradation was detected after 300- A×min current stress at 400oC.

Abstract: An investigation concerning suitable termination techniques for 4H-SiC trench JFETs is presented. Field plates, p+ floating rings and junction termination extension techniques are used to terminate 1.2kV class PiN diodes. The fabricated PiN diodes evaluated here have a similar design to trench JFETs. Therefore, the conclusions for PiN diodes can be applied to JFET structures as well. Numerical simulations are also used to illustrate the effect of the terminations on the diodes’ blocking mode behaviour.