Materials Science Forum Vols. 717-720

Abstract: APEI, Inc. designed, fabricated and tested a high gain AC coupled differential amplifier based on a custom-built silicon carbide (SiC) vertical junction field effect transistor (VJFET). This SiC differential amplifier is capable of high temperature operation up to 450 °C, at which a high differential voltage gain of more than 47 dB is maintained. This high gain AC coupled differential amplifier can be integrated with harsh environment sensors that deliver weak AC output signals to improve signal quality and noise immunity.

Abstract: An integrated bipolar OR-NOR gate based on emitter coupled logic (ECL) is demonstrated in 4H-SiC. Operated from 27 up to 300 °C on –15 V supply voltage the logic gate exhibits stable noise margins (NMs) of about 1 V in the entire temperature range.

Abstract: This paper pertains to development of high temperature capable digital integrated circuits in n-channel, enhancement-mode Silicon Carbide (SiC) MOS technology. Among the circuits developed in this work are data latch, flip flops, 4-bit shift register and ripple counter. All circuits are functional from room temperature up to 300C without any notable degradation in performance at elevated temperature. The 4-bit counter demonstrated stable behavior for over 500 hours of continuous operation at 300C.

Abstract: Silicon carbide based integrated circuits (ICs) have the potential to operate at temperatures exceeding that of conventional semiconductors such as silicon and silicon on insulator. Analog and digital silicon carbide (SiC) based circuits were fabricated and characterized at room temperature and 300°C. An operational amplifier and a ring oscillator were tested for prolonged period of time to evaluate their stability and reliability at 300°C. More than 1,000 hours was achieved with the operational amplifier without failures and the ring oscillator operated for almost 300 hours.

Abstract: This paper reports on direct frequency modulation of a RF Colpitts oscillator, realised from silicon carbide devices and proprietary components, capable of transmitting sensor data whilst operating at 300°C. Utilizing a reversed biased Schottky diode as a varactor in an LC oscillator, it is possible to modulate the frequency of an RF carrier by applying external voltage signals. These experiments have shown that a 10V bias will increase the frequency by as much as 10%, however signals as low as 10mV are easily detectable with standard silicon receivers.

Abstract: Carburization of silicon nanowires (NWs), with diameters of about 800 nm and lengths of about 10 µm, under methane at high temperature in order to obtain silicon carbide (SiC) nanostructures is reported here. The produced SiC nanostructures display a tubular shape and are polycrystalline. The as-prepared silicon carbide nanotubes (NTs) were characterized and studied by scanning electron microscopy (SEM), dual focused ion beam – scanning electron microscope (FIB-SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The formation of nanotubes can be explained by the out-diffusion of Si through the SiC during the carburization process.

Abstract: Growth of SiC nanowires (NWs) on monocrystalline 4H-SiC substrates was conducted to investigate a possibility of NW alignment and polytype control. The growth directions of the NWs on the top surfaces and the vertical sidewalls of 4H-SiC mesas having different crystallographic orientations were investigated. The majority of the NWs crystallize in the 3C polytype with the growth axis. Six orientations of the 3C NWs axis with respect to the substrate were obtained simultaneously when growing on the (0001) plane. In contrast, no more than two NW axis orientations coexisted when growing on a particular mesa sidewall. Growth on a particular {10-10} plane resulted in only one NW axis orientation, giving well-aligned NWs.

Abstract: This paper investigates the effect of silicon carbide (SiC) thin film as a stabilizing layer on porous silicon (PS) in metal–semiconductor–metal (MSM) photodiodes. Photo-assisted pulsed electrochemical etching method was used to produce the PS layers with large pore density and small crystallite size. As–prepared PS surface was modified with acetylene gas flow in a thermal process (750°C) in order to replace the hydrogen termination by Si–C bonds which is more stable. During the thermal carbonization, carbon atoms penetrated into the silicon lattice forming a thin (~4 nm) SiC layer. Because of high inertness of silicon carbide thin film, thermally carbonized porous silicon layer (TC-PS) was found to be more stable than the freshly prepared PS surfaces. A small reduction in specific surface area was found after carbonization which is due to the small size of acetylene molecules. The FTIR measurements confirmed the presence of SiC bonds in the TC-PS sample. The photocurrent of the fabricated photodiodes based on as-grown PS was lowered under prolonged green laser radiation (532nm, 5mW), but devices based on TC-PS showed more stable I-V characteristics under the same condition even for 120 min of laser exposure.

Abstract: Using the physical vapor transport (PVT) method, single crystal boules of AlN have been grown and wafers sliced from them have been characterized by synchrotron white beam X-ray topography (SWBXT) in conjunction with optical microscopy. X-ray topographs reveal that the wafers contain dislocations that are inhomogeneously distributed with densities varying from as low as 0 cm^-2 to as high as 10⁴ cm^-2. Two types of dislocations have been identified: basal plane dislocations and threading dislocations, both having Burgers vectors of type 1/3<112-0> indicating that their origin is likely due to post-growth deformation. In some cases, the dislocations are arranged in low angle grain boundaries. However, large areas of the wafers are nearly dislocation-free and section X-ray topographs of these regions reveal the high crystalline perfection.

Abstract: Seeded growth of gallium nitride (GaN) crystals on a spontaneously nucleated small GaN by the Na flux method was performed. In this study, we attempted to control the growth habit by changing the flux composition (Ga/Na) and by introducing a small amount of additives (Ca and Li). Our experiment clarified that a low Ga composition was preferred to grow high-crystallinity prismatic GaN crystals with a high growth rate. Furthermore, the transparent GaN single crystals with prism shape could be grown by the addition of Ca and Li.