Materials Science Forum Vols. 645-648

Abstract: This paper describes the characteristics of polycrystalline 3C-SiC micro resonators with 3 ×1017 - 1×1019 cm-3 in-situ N-doping concentrations. In this work, the 1.2 μm thick cantilevers and the 0.4 μm thick doubly-clamped beam micro resonators with various lengths were implemented using in-situ doping poly 3C-SiC thin films. The characteristics of the poly 3C-SiC micro resonators were evaluated using quartz actuator and optical read-out vibrometer under vacuum conditions at room temperature. The resonant frequencies of the SiC micro resonators decreased with doping concentrations owing to the reduction of the Young's modulus of the poly 3C-SiC thin films. It was confirmed that the resonant frequencies of the poly 3C-SiC micro- resonators are controllable by adjusting the doping concentrations.

Abstract: The main rectifier device structures for power electronics based on SiC and on GaN are compared and the main issues for each structure are evaluated in terms of performance and manufacturability. The driving volume markets for power electronics devices correspond to the systems working on 127, 240 and 400 V energy supply networks, setting the device voltage handling to 300, 600, and 1200V respectively. We have limited the scope hereafter to the 600 V typical target, for which SiC Schottky rectifiers are now commercially available from at least 3 sources. The key physical properties for any semiconductor material used as the active layer of a unipolar device for power electronics are the breakdown field and carriers mobility. The bulk values are very similar for SiC and GaN. Two main other key issues are related to quality of the ohmic and Schottky contacts. For the ohmic contacts, adequate solutions have been found for both SiC and GaN. Surprisingly, on hetero-epitaxial GaN layers on sapphire despite of the very high crystal defects density ( ≥ 109cm-2 ), the ideality factor of the best Schottky contacts seems very promising. On the other hand, improving this ideality factor and the reverse leakage current for Schottky contacts on GaN layers grown on silicon substrate remains a fierce challenge. For the SiC Schottky rectifiers, cost and availability of the SiC substrates appear as the main residual limiting factors today. For GaN based rectifiers, although engineering device prototypes have already been published [1], there are both basic issues to be validated regarding reverse leakage current and reliability, and also difficult manufacturing issues to be solved in relation with device reliability, directly resulting from the nature of the possible substrates: mainly sapphire and silicon.

Abstract: With the help of an improved die attach the Rth,jc of SiC Schottky diodes can be reduced by 40-50% at a given chip size. This enables a significant higher power density for these SiC diode chips, resulting in a chip shrink of ~ 35% for a given nominal current. This has a significant impact not only on the cost position of the device but also on the switching performance of the diodes, as their capacitive charge directly scales with the chip area. Of course these advantages are accompanied by a small penalty in static losses as the Vf of the diodes at nominal current also slightly increases by the chip shrink. However, the reduction of switching losses dominates upon the marginally increased static losses besides full load operation conditions (which are pretty exceptional in today’s SiC Schottky diode applications) combined with frequencies below 130 kHz. This allows a better competitive positioning against fast Si-based diodes and improved system efficiency at the same time.

Abstract: SiC schottky diodes take advantage of the material's superior reverse breakdown voltage when compared to Silicon (Si) [1]. However, when considered for MOSFET applications, the high concentration of interface traps at the SiC/SiO2 interface reduce the material's already low channel mobility [2]. Therefore, a Ge/SiC heterojunction solution becomes an attractive prospect, whereby the Ge forms the control region after being epitaxially grown on the SiC. With a well established Ge-High K dielectric technology [3], a carbon-free oxide would exist, leaving a channel-region with a mobility approximately four times that of SiC.

Abstract: It is known that a Schottky barrier height (b) of metal/C-face 4H-SiC Schottky barrier diode (SBD) differ from b of metal/Si-face 4H-SiC SBD. Furthermore, b of metal/4H-SiC SBD varies with annealing temperature. We fabricate 0.231mm2 SBD with Ti/SiC interface using Si-face and C-face 4H-SiC. These SBDs are annealed at several temperatures after a formation of the Ti/SiC interface. As a result, b of Ti/C-face 4H-SiC interface annealed at 400 oC is nearly equal to b of Ti/Si-face 4H-SiC interface annealed at 500 oC and the n-values of these SBDs are nearly equal to the ideal value (unity). Using that annealing condition, we fabricated 25mm2 junction barrier Schottky (JBS) diodes with Ti/SiC interface on Si-face and C-face 4H-SiC epitaxial substrate. b of Si-face and C-face JBS diodes are 1.26eV and 1.24eV, respectively. The leakage currents for both Si-face and C-face JBS diodes are less than 1mA/cm2. The current of 100A is obtained at the forward bias voltage of 1.95V and 2.16V for the Si-face JBS and the C-face JBS.

Abstract: 4H-SiC diodes with 0.60 mm2 nickel silicide Schottky contacts were fabricated on commercial epitaxial layers. At room temperature, the diodes have specific on-resistances (RON-SP) down to 10.5 mΩcm2 and blocking voltages (VBL) up to 4.6 kV, which is equal to 93 % of the calculated parallel plane breakdown voltage for used epitaxial structure. The corresponding figure-of-merit, defined as (VBL)2/RON-SP, is equal to 2015 MW/cm2 and is among the highest FOM values reported to date. The diodes demonstrated stable operation at forward current of 1 A and VBL value in excess of 3.3 kV at ambient temperatures up to 200 °C.

Abstract: The paper compares static and dynamic characteristics of 6.5 kV SiC PiN diodes fabricated with different p-emitters. The version with the thickest p-emitter (4 µm) showed the lowest forward voltage (3.4 V at 100 A/cm²) and the lowest (negative) temperature coefficient. Forward voltage DC stress tests revealed a stability within the measurement error of the test apparatus (<50 mV). The dynamic performance showed a soft recovery even at 4 kV. The reverse recovery charge Qrr is analyzed for different forward currents and junction temperatures. The dynamic losses of the SiC PiN diode are marginal with view to the application in industrial inverters.

Abstract: Correlation between carrier lifetime and forward voltage drop in 4H-SiC PiN diodes has been investigated. PiN diodes from the drift layer of 20 m shows breakdown voltage of 3.3 kV and forward voltage drop as low as 3.13 V at 100A/cm2. Variation of calculated forward voltage drop ( ) from measured carrier lifetimes is very comparable to measured of fully processed PiN diodes. Measured carrier lifetime and of PiN diodes also show good spatial correlation. Wafer level lifetime mapping can be employed to assess and predict of PiN diodes.

Abstract: In this work we discuss measurements of the breakdown voltage of diodes with non-punch-through (NPT)- and punch-through (PT)-designs. From the experimental results we deduce the temperature dependent Fulop constants of the effective ionization rate. The data of this work agree very well with ionization rates for electrons and holes determined recently.

Abstract: The authors fabricated pn diodes with Al+ implantation in p-type epitaxial layers, and investigated the influence of the implantation dose on reverse leakage currents. Only in the highest dose with the Al concentration of 2x1020cm-3, more than 90% of the devices showed high leakage currents above 10-4A at the maximum electric field of 3MV/cm. In such devices, almost all of the emissive spots corresponded to threading screw dislocations (TSDs) by the analysis of emission microscopy and X-ray topography. These TSDs were defined as killer defects with the estimated density of 500cm-2 in the case of the highest dose. The emissions were supposed to be due to microplasmas, since the spectra of the emissions were different from those of heat radiation. Condensation of Al atoms, nitrogen atoms and DI defects were excluded as the origin of the emissions by secondary ion mass spectrometry and low temperature photoluminescence analyses.