Materials Science Forum Vols. 556-557

Abstract: This SiC diodes and switches provide greatly enhanced performance at higher operating voltage, but the base material provides a high barrier to adoption in commercial systems. Commercial adoption has not occurred by adopting SiC components in place of Si devices, but only when the characteristics of SiC components can successfully leverage system performance and cost. Doing so requires a close look at current roadblocks and examining topologies and techniques that can bring more optimal system performance (density, efficiency, cost) with SiC components. High density converters using conventional components are becoming limited by the volume and cost of passive components, particularly the EMI filters and bulk filter capacitors. To address this and demonstrate the performance capabilities of SiC in SMPS, a PWM isolation converter is proposed using the resonant reset forward converter in an interleaved configuration. This topology can extend energy recovery from the primary bulk bus capacitor used for hold up time, as well as minimizing filtering requirements for input and output capacitors. The interleaved converter is developed from cascode switches using experimental SiC JFETs, in a configuration derived from previous work with a single switch converter. Overall performance as well as design issues will be discussed.

Abstract: Recent work on the thermal and electrical challenges in realizing AlGaN/GaN microwave heterojunction field effect transistors grown on SiC substrates is discussed. Raman thermography has been used to directly measure the self-heating induced lattice temperature rise with dramatically improved resolution and accuracy compared to traditional infrared techniques. It is demonstrated that defects in the SiC substrate can influence the temperature distribution within the active device with potential consequences for reliability. Microwave devices require an insulating GaN substrate material for device isolation. It is shown that the net deep level acceptor concentration has to be accurately controlled to suppress short-channel effects and to achieve radio frequency power efficient operation.

Abstract: We report on investigation of the AC dielectrophoresis aligned assembly deposition (DAAD) of gallium nitride nanowires (GaN NWs) with both the variation of the electric field and the frequency. Our DAAD methods were used to align and manipulate GaN nanowires as well as to extract the electrical properties of semiconducting nanowires. We observed that the ability of the alignment strongly depends on the magnitude of the AC electric field and frequencies. For the higher AC peak-to-peak electric fields (up to 20 Vp-p), the GaN nanowires have a better alignment across the patterned Ti / Au electrodes with a high yield rate of ~ 90% over the entire arrays (in our case, 20 arrays) in the chip at the 20 kHz. From the transport measurements of our AC aligned GaN nanowires using conventional three-probe schemes in field-effect transistor structures, we found that the conductance of the GaN NWs increased for gating voltage greater than zero and decreased for gating voltage less than zero, indicating these GaN nanowires have n-type dopants.

Abstract: In this work, the structural and electrical properties of Ti/Al/Ni/Au contacts on n-type Gallium Nitride were studied. An ohmic behaviour was observed after annealing above 700°C. The structural analysis showed the formation of an interfacial TiN layer and different phases in the reacted layer (AlNi, AlAu4, Al2Au) upon annealing. The temperature dependence of the specific contact resistance demonstrated that the current transport occurs through thermoionic field emission in the contacts annealed at 600°C, and field emission after annealing at higher temperatures. By fitting the data with theoretical models, a reduction of the Schottky barrier from 1.21eV after annealing at 600°C to 0.81eV at 800°C was demonstrated, together with a strong increase of the carrier concentration at the interface. The reduction of the contact resistance upon annealing was discussed by correlating the structural and electrical characteristics of the contacts.

Abstract: In this report we present results on growth and characterization of AlN wires and thin films on SiC substrates. We have employed PVT technique in close space geometry for AlN deposition on SiC off oriented substrates, most of which were prepared to have scratch-free smooth as-grown surface by SiC sublimation epitaxy. By manipulating the surface kinetics we have been able to determine growth conditions yielding discontinuous or continuous morphologies corresponding to nanowires and thin films, respectively. A particular feature of the latter experiments is the fast temperature ramp up at the growth initiation. The AlN surface morphology was characterized by optical, AFM and XRD tools, which showed good crystal quality independent of the growth mode.

Abstract: The mechanism of drain current collapse in AlGaN/GaN high electron mobility transistors (HEMTs) was investigated. Current collapse was clearly observed for TiO2 passivated HEMTs. However, no evidence of current collapse was apparent for SiNx passivated HEMTs. This suggests that AlGaN surface traps play a major role in current collapse. The experimental results were compared with numerical device simulation results. The device simulations were performed taking into account hot electron generation and deep traps at the AlGaN surface. The simulated drain current transients were consistent with the degradation and recovery behavior of the experimental results. These results indicate that current collapse is caused by the trapping of hot electrons in deep levels at the AlGaN surface.

Abstract: The electrical properties of n+-GaN/p+-SiC heterojunction diodes have been investigated by varying the acceptor concentration of p+-SiC epilayers (Na) and polytype of SiC (4H- and 6H-SiC). The current-voltage (I-V) characteristics of diodes with Na ~ 1x1019 cm-3 were dominated by tunneling-assisted current. The diodes with Na ~ 1x1018 cm-3 exhibit excellent characteristics and 6H-SiC may be a better choice from a view point of electron injection into p-SiC base. Compared to previous investigations (Na<1016cm-3), we could obtain good rectification with p-SiC doped to two-order-of-magnitude higher acceptor concentration.

Abstract: AlGaN/GaN HFETs were fabricated around micropipes and on a domain boundary in a semi-insulating silicon carbide (SI-SiC) substrate and the DC characteristics of the fabricated devices were measured. Devices around micropipe showed no pinch-off or large gate leakage. The devices on the domain boundaries showed no degradation in the performances, even though an X-ray topographic analysis indicated that crystal imperfections, due to the defects, propagated to the GaN layer across the hetero interface. Based on these results, we concluded that micropipe degrades the DC characteristics and that the domain boundary does not affect the DC characteristics. From Raman analysis on the devices around the micropipes, these degradations could be attributed to the free carriers introduced into the GaN crystal by the micropipes.