Papers by Author: Konstantinos Zekentes

Abstract: Silicon microwires (MWs) previously synthesized using the VLS method with gold catalyst are being carburized at 1100°C under methane aiming to their conversion to SiC. SEM, TEM as well as XPS and Raman spectroscopy were used for structural and morphological characterization. After carburization achievement, SiC is found to be polycrystalline with a high density of stacking faults associated to an increase of surface roughness. Directions for the carburization process optimization are given.

Abstract: The behavior of 200nm Cr Schottky contacts on n-type 4H-SiC has been investigated with photoelectron spectroscopy (XPS) and standard (I-V and C-V) electrical measurements at different measurement temperatures. A barrier height close to 1.2 eV was calculated from XPS data under no-current and no-bias conditions on ultra-thin Cr films grown in-situ under UHV conditions. The I-V measurements on as-deposited contacts resulted in a barrier height of 1.06 eV while a value of 1.2 eV has been extracted from the C-V measurements.

Abstract: 4H-SiC p-i-n diodes were designed, fabricated and characterized for use in microwave applications. The diodes exhibited a blocking voltage of 1100 V, a 100 mA differential resistance of 1-3 &, a capacitance below 0.5 pF at a punchthrough voltage of 100 V and a carrier effective lifetime between 15-27 ns. Single 4H-SiC p-i-n diode switches, operating in X-band, exhibited insertion loss 0.7 dB, isolation up to 25 dB and were able to handle microwave power up to 2.2 kW in pulsed mode of operation. The switching speed of the switches has not exceeded 20 ns.

Abstract: 4H-SiC p-i-n diodes were fabricated on epitaxial layers grown by Sublimation Epitaxy in Vacuum (SEV) and were evaluated for microwave power switching applications. Full electrical characterization (C-V, DC I-Vs, reverse recovery characteristics, low and high power microwave testing) has been performed. The results showed that SEV-grown SiC material is suitable for bipolar device fabrication. A doping higher than 1019 cm-3 for the p-type contact layer and lower than 1016 cm-3 for the n-type base layer is necessary to demonstrate microwave p-i-n diodes with similar performance as the ones fabricated on commercially available CVD-grown material.

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: A classical implementation of the field plate technique is the oxide ramp termination. This paper presents for the first time a comparison between SiC and diamond Schottky barrier diodes (SBD) using this termination. The influences of the ramp angle and oxide thickness on the diodes electrical performance are investigated for both punch-through (PT) and non punch-through (nPT) structures. The efficiency of the termination is also evaluated.

Abstract: A classical implementation of the field plate technique is the oxide ramp termination. This paper presents improvements of the breakdown voltage for both SiC JBDs and SBDs, obtained by using high-k dielectrics. A study regarding the influence of the dielectric permittivity and thickness on the off-state performances of the diodes is included. It is shown that Si3N4 is to be preferred to SiO2 for the dielectric ramp. Termination efficiencies up to 96% are reported.

Abstract: The packaged microwave 4H SiC pin diode chips (with i-region length of 6 μm, mesa diameter of 80 μm and blocking voltage of 1000 V) were investigated. We studied the parameters of diode I−V curve (in particular, the diode resistance RS at forward current) and the processes of diode switching from forward current of 50 mA to reverse voltage of 15 V, as well as C−V curves, in the 20−700 °C temperature range. At a voltage of 300 V, the diode reverse current was 10 (180) μA when temperature was 600 (700) °C. At a forward current of 40 mA, the diode resistance first decreases smoothly as temperature is increased from 20 up to 300 °C, and then grows up. As temperature is increased from 20 up to 700 °C, the effective lifetime τeff grows from 7 up to 50 ns, while the diode capacitance (in the 0−40 V reverse voltage range) grows smoothly as temperature is increased from 20 up to 400 °C.

Abstract: A 248 nm (KrF) excimer laser with a repetition rate of 10 Hz, pulse duration of 30 ns and beam energy up to 450 mJ was employed to form vias in 4H-SiC substrates and Lely platelets. SEM micrographs have been used to evaluate etched material quality as well as etch rate. The area surrounding the via-holes is covered by nanoparticles, which are debris from the laser ablation and are removed by chemical cleaning and agitation. The etch-rate exhibits a perfect linear behaviour versus the number of laser pulses showing the possibility of an all-laser via-hole formation. A slight tapering along the via-holes, useful for the subsequent metallization process is also observed. Finally, a defective,15 μm wide, zone is formed nearby the sidewalls.

Abstract: The forward current was investigated in 4H-SiC p+n structures grown by sublimation epitaxy. The doping level, Nd-Na, of the n-layer was about (3-4)x1016 cm-3 and the diode area was in the range from 1x10-5 to 2x10-4 cm2. The observed current can be considered as current due to bulk recombination in the space charge region of the pn junction via deep level center or due to surface recombination. The criterion which was performed in this study to differentiate such currents was the investigation of recombination current versus perimeter/area ratio dependence. It was found that no pronounced difference in the recombination current parameters for diodes with different perimeter/area ratio was observed, i.e. current due to surface recombination was not observed for the 4H-SiC pn structures investigated.