Yogesh K. Sharma

Enhanced field effect mobility on 4H-SiC by oxidation at 1500◦C

A novel 1500°C gate oxidation process has been demonstrated on Si face of 4H-SiC. Lateral channel metal-oxide-semiconductor-field-effect-transistors (MOSFETs) fabricated using this process have a maximum field effect mobility of approximately 40 cm\2 V-1 s-1 without post oxidation passivation. This is substantially higher than other reports of MOSFETs with thermally grown oxides (typically grown at the standard silicon temperature range of 1100-1200°C). This result shows the potential of a high temperature oxidation step for reducing the channel resistance (thus the overall conduction loss), in power 4H-SiC MOSFETs.

Impact of the Oxidation Temperature on the Interface Trap Density in 4H-SiC MOS Capacitors

Despite the material advantages of Silicon-Carbide (SiC), the on resistance of 4H-SiC metal-oxide-semiconductor transistors are severely degraded by high trap densities near the oxide/SiC interface (Dit). In this work, the effect of the oxidation ambient (oxygen flow rates of 0.05 l/min-2.5 l/min) and oxidation temperature (1200°C-1600°C) on the Dit is investigated. The Dit was reduced by up to an order of magnitude using a combination of a low oxygen flow rate and a high temperature. The Dit was extracted from capacitance-voltage measurements made on MOS capacitors.

Biofunctionalized AlGaN/GaN high electron mobility transistor for DNA hybridization detection

Label-free electrical detection of deoxyribonucleic acid (DNA) hybridization was demonstrated using an AlGaN/GaN high electron mobility transistor (HEMT) based transducer with a biofunctionalized gate. The HEMT DNA sensor employed the immobilization of amine-modified single strand DNA on the self-assembled monolayers of 11-mercaptoundecanoic acid. The sensor exhibited a substantial current drop upon introduction of complimentary DNA to the gate well, which is a clear indication of the hybridization. The application of 3 base-pair mismatched target DNA showed little change in output current characteristics of the transistor. Therefore, it can be concluded that our DNA sensor is highly specific to DNA sequences.

Growth of ZnO Nanorod Arrays on Flexible Substrates: Effect of Precursor Solution Concentration

We report a low-temperature aqueous solution growth of uniformly aligned ZnO nanorod arrays on flexible substrates. The substrate is Indium Tin Oxide (ITO) film coated on polyethylene terephthalate (PET). Solutions with five different concentrations of the precursors with equimolar Zinc Nitrate and Hexamethylenetetramine (HMT) in distilled water were prepared to systematically study the effect of precursor solution concentration on the structural and optical properties of ZnO nanorods. It was concluded that the precursor concentration have great influence on the morphology, crystal quality, and optical property of ZnO nanorods. The diameter, density, and orientation of the nanorods are dependent on the precursor solution concentration. X-ray diffraction and micro-Raman spectroscopy showed that the ZnO nanorods with the highest concentration of 50 mM were highly aligned and have the highest level of surface coverage. It was also found that the diameter and length of the nanorods increases upon increasing precursor solution concentration. This is the first systematic investigation of studying the effect of precursor solution concentration on the quality of ZnO nanorods grown on ITO/PET substrates by low-temperature solution method. We believe that our work will contribute to the realization of flexible organic-inorganic hybrid solar cell based on ZnO nanorods and conjugated polymer.

On the Ti3SiC2 metallic phase formation for robust p-type 4H-SiC ohmic contacts

This paper presents a detailed physical and electrical analysis of 4H-SiC ohmic contacts to p-type material, the main aim being to examine their ruggedness under high temperature conditions. XRD, FIB-TEM and SEM are techniques that have been utilized to examine the microstructure and interface properties respectively. A detailed physical study revealed the presence of a crystalline hexagonal Ti layer orientated in the same direction as the 4H-SiC epitaxial layer. This factor seems to be important in terms of electrical performance, having the lowest measured specific contact resistivity of 1x10-6 Ωcm2. We attribute this to the optimized formation of Ti3SiC2 at the metal/SiC interface. An initial high temperature study shows thermionic emission occurring across the metal/semiconductor junction.

Roughness of the SiC/SiO2 vicinal interface and atomic structure of the transition layers

The SiC/SiO2 interface is generally considered to be the cause for the reduced electron mobility of SiC power devices. Previous studies have shown a correlation between the mobility and the transition layer width at the SiC/SiO2 interface. The authors investigated this interface with atomic resolution Z-contrast imaging and electron energy-loss spectroscopy, and discovered that this transition region was due to the roughness of the vicinal interface. The roughness of a vicinal interface consisted of atomic steps and facets deviating from the ideal off-axis cut plane. The authors conclude that this roughness is limiting the mobility in the channels of SiC MOSFETs.

Improved performance of 4H-SiC PiN diodes using a novel combined high temperature oxidation and annealing process

In this paper, the application of a novel combined high temperature thermal oxidation and annealing process to mesa-isolated epitaxial-anode 4H-SiC PiN diodes with thick (110 μm) drift regions is presented, the aim of which was to increase the carrier lifetime in the 4H-SiC. Diodes were fabricated using 4H-SiC material having undergone this process, which consisted of a thermal oxidation in dry pure O2 at 1550°C followed by an argon anneal at the same temperature. Forward current-voltage characterization showed that the oxidised/annealed samples typically showed around 15% lower forward voltage drop and around 40% lower differential on-resistance (at 100 A/cm2 and 25°C) compared to control sample PiN diodes, whilst reverse recovery tests indicated a carrier lifetime increase also of around 40%. These findings illustrate that the use of this process is a highly effective and efficient way of improving the electrical characteristics of high voltage 4H-SiC bipolar devices.

4H-SiC Diode Avalanche Breakdown Voltage Estimation by Simulation and Junction Termination Extension Analysis

This paper presents and compares different avalanche breakdown voltage estimation methods in 4H-SiC (silicon carbide) using finite element simulation results on Schottky diode. 4H-SiC avalanche breakdown voltage and depletion width estimated with Baligas equations have shown to be higher than other estimation techniques and simulation results, especially for voltages higher than 5kV. This paper discusses the impact of choosing different junction termination extension (JTE) structures on two-dimensional junction curvature effects and electric field crowding for Schottky diodes Space-Modulated JTE (SMJTE) structure with optimum JTE dose and dimension could achieve up to 90% of the parallel plane breakdown voltage. For ultra high voltage devices (>15 kV) the SMJTE has significant improvement in terms of breakdown voltage. It also has a wider optimum JTE dose window. For 1 kV device there is not a significant difference in breakdown voltage between JTE and SMJTE structures.

Degradation and Reliability of Bare Dies Operated up to 300°C

In this paper, we demonstrate the degradation of commercially available 1.2kV SiC MOSFET bare dies subjected to long periods of isothermal heating at 300°C in air. Periodic electrical measurements indicated an increase in on-state resistance to different extents for three different vendor designs, and the discovery of a progressive rectifying type forward characteristic at low drain-source voltages. Subsequent investigations to determine the cause of the degraded electrical characteristics including sectioning and SEM/TEM analysis revealed some mechanical degradation within the device gate-source cross-sections and backside drain contact metal layers. While one vendor device was severely degraded after approximately 24 hours of heating, another vendor device was only just beginning to degrade after 100 hours, indicating that these devices may be used successfully in real applications at 300°C junction temperatures for relatively long periods.

Evaluation of commercially available SiC devices and packaging materials for operation up to 350°C

The characteristics of commercially available silicon carbide power devices and packaging technologies have been measured up to 350°C in order to obtain their reliability and suitability for use in a hybrid electric vehicle application. Electro-thermal simulations of representative power module packaging structures, using measured conduction losses, revealed the respective temperature profiles of the devices and packaging. By correlating lifetime data found from our passive thermal cycling of candidate packaging technologies, with the magnitude and number of thermal cycles extracted from simulated temperature profiles, the lifetime of high temperature power module packages has been predicted. It was found that the limiting factor for high temperature thermal cycled operation is the silicon nitride substrate material, followed closely by the pressure-less silver sinter die attach. In this case, no aluminum wirebond failures were observed.