We report on the fabrication of a boron-doped nanocrystalline diamond (NCD) 3 × 3 high-density microelectrode array (MEA) for amperometric measurements, with a single electrode area of 3 × 5 μm 2 and a separation in the μm scale. The NCD microelectrodes were grown by hot filament chemical vapor deposition (HFCVD) on a double-side polished sapphire wafer in order to preserve the diamond transparency. Bias enhanced nucleation (BEN) was performed to ensure a covalent adhesion of the films to the substrate. A current background noise of less than 5 pA peak to peak over a 1 kHz bandwidth resulted from an electrochemical investigation of the new device, using 100 mM KCl solutions and ferrocyanide red-ox couples. Cyclic voltammetry measurements in physiological buffer solution and in the presence of oxidizable biomolecules strengthened its suitability for bio-sensing. When compared to a 2 × 2 NCD microelectrode array prototype, already used for in vitro cell measurements, the signal to noise ratio of the amperometric response of the new 3 × 3 device proved twice as good. In addition, the optical transmittance of the boron-doped thin layers exceeded 40% in the visible wavelength range. The excellent electrochemical properties of NCD electrodes and the transparency in combination with the high spatial resolution make the new 3 × 3 NCD MEA a promising tool for electrochemical sensing in a variety of applications, ranging from medical to industrial, in neutral or harsh environments.
We report on InAlN/GaN high electron mobility transistors (HEMTs) grown by metal organic vapor phase epitaxy on sapphire with ultrathin buffers. Two dimensional electron gas (2DEG) exhibiting high mobility (1100 cm2/V s) and low sheet resistivity (356 Ω/□) is achieved at room temperature for a buffer thickness as low as ∼0.1 μm. It is shown that despite a huge dislocation density imposed by this thin buffer, surface roughness is the main factor which affects the transport properties. In addition, sapphire surface nitridation is found to drastically affect the properties of the InAlN/GaN 2DEG. Eventually, HEMTs are processed from these heterostructures. Maximum current densities of 0.35 A/mm and current on-off ratios higher than 109 are measured, which make them suitable for high performance GaN based sensing in harsh environments.
Lattice-matched InAlN/GaN high electron mobility transistors (HEMTs) have been prepared in a silicon-on-insulator (SOI)-like configuration. Here, this implies an ultrathin body 50 nm GaN channel/50 nm AlN nucleation layer material structure on sapphire with the active areas confined by mesa etching, resulting in semi-enhancement mode device characteristics. In contrast to conventional technologies, the device characteristics (maximum drain current, threshold voltage and 1 MHz large signal operation) change only within less than approx. 10% up to 600 • C compared to room temperature (RT). The current on/off ratio decreases from 10 10 at RT to 10 6 at 600 • C, due to residual defect activation. These first results of ultrathin body GaN-on-sapphire-based materials and device technology may indicate that essential improvements in the temperature-handling capability of electronic device structures beyond what is common at present may be possible with only limited sacrifice of device performance.
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