Normally off 4H-SiC MOSFET devices have been fabricated on a p-type semiconductor and electrically characterized at different temperatures. A gate oxide obtained by nitrogen ion implantation performed before the thermal oxidation of SiC has been implemented in n-channel MOSFET technology. Two samples with a nitrogen concentration at the SiO2/SiC interface of 5 X 1018 and 1.5 X 1019 cm-3 and one unimplanted sample have been manufactured. The sample with the highest N concentration at the interface presents the highest channel mobility and the lowest threshold voltage. For increasing temperature, in all the samples, the threshold voltage decreases, and the electron channel mobility increases. The latter case attains a maximum value of about 40 cm2/V ldr s at 200degC for the sample with the highest N concentration. These trends are explained by the reduction of interface electron traps in the upper half of the band gap toward the conduction band edge. These results demonstrate that N implantation can be effectively used to improve the electrical performances of an n-type surface channel 4H-SiC MOSFET.

Nitrogen Implantation to Improve Electron Channel Mobility in 4H-SiC MOSFET

Detailed investigations on the pre-oxidation nitrogen implantation process for the improvement of channel mobility in 4H-SiC MOSFETs are reported. Comparisons with conventional thermally nitrided gate oxides are highlighted. The results of a nitrogen dose dependence study indicate that higher N implantation doses lead to higher peak field-effect mobilities but lower threshold voltages. This apparently correlates with the interface trap density near the conduction band of 4H-SiC, as previous work suggests, but an alternate mechanism associated with counter doping of the MOSFET p-well via N implantation is proposed here. Analysis of the experimental results suggests that the counter-doping mechanism is more likely to be the dominant effect.

A Study on Pre-Oxidation Nitrogen Implantation for the Improvement of Channel Mobility in 4H-SiC MOSFETs

Electron states at the SiO2/4H–SiC interface have been investigated using capacitor structures and especially, the influence of excess nitrogen, introduced by ion implantation, at the interface is studied in detail. Implanted and nonimplanted n-type samples with an interfacial concentration of nitrogen of ∼1019 cm−3 and 1016 cm−3, respectively, were analyzed by capacitance-voltage (C-V) measurements, performed at different temperatures and probe frequencies, and thermal dielectric relaxation current (TDRC) measurements performed in the temperature range of 35–295 K. Three main categories of electron states are disclosed, true interface states (Dit), fast near interface states (NIToxfast) and slow near interface states (NIToxslow). The density versus energy distributions of Dit and NIToxfast have been deduced from the TDRC data and they are shown to give a close quantitative agreement with the shape and frequency dependence of the C-V curves. Further, the amount of NIToxslow extracted from TDRC is demonstr...

Analysis of electron traps at the 4H–SiC/SiO2 interface; influence by nitrogen implantation prior to wet oxidation

In this paper we present a novel highly efficient approach to determine material properties from measurement results. We apply our method to thermal properties of thin-film multilayers with three different materials, amorphous silicon, silicon nitride and silicon oxide. The individual material properties are identified by solving an optimization problem. For this purpose, we build a parameterized reduced-order model from a finite element (FE) model and fit it to the measurement results. The use of parameterized reduced-order models within the optimization iterations speeds up the transient solution time by several orders of magnitude, while retaining almost the same precision as the full-scale model.

http://iopscience.iop.org/article/10.1088/0960-1317/20/4/045030/pdf

Efficient extraction of thin-film thermal parameters from numerical models via parametric model order reduction

The electrical characteristics of a normally off 4H-silicon carbide (SiC) bipolar-mode FET are investigated by means of a careful design activity and an intensive simulation study useful for a first-time-ever realization of this device in SiC Specific physical models and parameters strictly related to the presently available 4H-SiC technology are taken into account The device basically consists of a trench vertical JFET operating in the bipolar mode that takes full advantage of the superior material properties A drain-current density up to 500 A/cm2, a forced current gain on the order of 50, and a specific on-resistance as low as 13 mΩ·cm2 are calculated for a 13-kV blocking voltage device The turn-off delay is on the order of a few nanoseconds The presented analysis is supported by experimental results on the p-i-n diodes embedded in the device structure

Modeling of the Steady State and Switching Characteristics of a Normally Off 4H-SiC Trench Bipolar-Mode FET

This paper reports on a portable selective chemical sensor for hazardous vapors at trace levels, which combines a two-stage purge and trap vapor pre-concentration system, a Micro-Electro-Mechanical-System (MEMS) based fast gas-chromatographic (FAST-GC) separation column and a miniaturized quartz-enhanced photoacoustic spectroscopy (QEPAS) detector. The integrated sensing system provides two-dimensional selectivity combining GC retention time and QEPAS spectral information, and was specifically designed to be rugged and suitable to be deployed on unmanned robotic ground vehicles. This is the first demonstration of a miniaturized QEPAS device used as spectroscopic detector downstream of a FAST-GC separation column, enabling real-world analyses in dirty environments with response time of a few minutes. The main modules of the GC/QEPAS sensor device will be described in detail together with the system integration, and successful test results will be reported and discussed.

https://www.mdpi.com/1424-8220/20/1/120/pdf

A MEMS-Enabled Deployable Trace Chemical Sensor Based on Fast Gas-Chromatography and Quartz Enhanced Photoacousic Spectoscopy

Many investigations have been conducted on the growth conditions of SiO2 on SiC to improve the oxide quality and the properties of the silicon carbide-silicon dioxide interface. In this work a comparison between a wet oxidation and an oxidation in N2O ambient diluted in N2 is proposed. The interface state density Dit near the conduction-band edge of SiC has been evaluated by conventional C-V measurements obtaining results similar or better than the literature data. Furthermore, the slow trapping phenomena have been studied and preliminary results are reported.

Interfacial Properties of SiO2 Grown on 4H-SiC: Comparison between N2O and Wet O2 Oxidation Ambient

We propose a method to characterize the main figures of merit of IR thermopile detectors by means of electrical measurements performed at the wafer level. Finite element simulations are adopted in order to compare the results of wafer-level measurements with the actual device responsivity as expected by optical measurements. The employed finite-element model is validated by comparison with experimental data obtained on a micromachined thermal test structure

Wafer-level testing of thermopile IR detectors

A method suited to perform wafer-level measurements of thermal conductivity on thin films by exploiting micromachined test structures is proposed. To this purpose, a measurement procedure able to compensate for instrumental offsets and sensitivity limits typically existing in a standard wafer-level electrical instrumentation, and to eliminate the influence of heat exchange through air is applied. In order to validate the technique, measurements on different thin films of interest in thermal MEMS fabrication are presented (LPCVD polycrys-talline silicon, evaporated aluminum, LPCVD silicon oxide).

Wafer-Level Measurement of Thermal Conductivity on Thin Films

Aiming to minimize the interface state density, we fabricated MOS capacitors on n-type 4H-SiC by using wet oxidation of nitrogen implanted layers. We investigated a wide range of implantation dose, including a high dose able to amorphise a surface SiC layer with the intent to reduce the oxidation time. The oxide quality and the SiO2-SiC interface properties were characterized by capacitance-voltage measurements of the MOS capacitors. The proposed process, in which nitrogen is ion-implanted on SiC layer before a wet oxidation, is effective to reduce the density of interface states near the conduction band edge if a high concentration of nitrogen is introduced at the SiO2-SiC interface. We found that only the nitrogen implanted at the oxide-SiC interface reduces the interface states and we did not observe the generation of fixed positive charges in the oxide as a consequence of nitrogen implantation. Furthermore, the concentration of the slow traps evaluated from the Slow Trap Profiling technique was low and did not depend on the nitrogen implantation fluence.

Michele Sanmartin

Papers

A MEMS-Enabled Deployable Trace Chemical Sensor Based on Fast Gas-Chromatography and Quartz Enhanced Photoacousic Spectoscopy

Interfacial Properties of SiO2 Grown on 4H-SiC: Comparison between N2O and Wet O2 Oxidation Ambient

Wafer-level testing of thermopile IR detectors

Wafer-Level Measurement of Thermal Conductivity on Thin Films

Characterization of MOS capacitors fabricated on n-type 4H-SiC implanted with nitrogen at high dose