We have demonstrated a wireless hydrogen sensing system using commercially available wireless components and AlGaN/GaN High Electron Mobility Transistor (HEMTs) differential sensing diodes as the sensing devices. The active device in the differential pair is coated with 10 nm of Pt to enhance catalytic dissociation of molecular hydrogen, while the reference diode is coated with Ti/Au. Our sensors have a wide range of detection from ppm levels to ~30%, with the added advantages of a very rapid response time within a couple of seconds, and rapid recovery. The sensors have shown good stability for more than 18 months in an outdoor field test.
Indium-rich InGaN epitaxial layers with a p-i-n structure were grown pseudomorphically on a strain-relaxed InGaN template to reduce structural strain induced by lattice mismatch. We applied a nano-sculpting process to improve the crystal quality of the strain-relaxed InGaN template. The results show that the nano-sculpting process can suppress effectively the threading dislocation generation and improves significantly the I-V characteristic of the InGaN p-i-n structure. This InGaN template technique with nano-sculpting process shows great potential for future applications in indium-rich InGaN optic-electron devices.
In this SBIR Phase I project, Qrona Technologies in collaboration with subcontractor research groups, at UCF and UM, fabricated both p-n-junction and MIS metal-oxide-based photodetectors, by performing device modeling and simulation, thin-film epitaxy using MBE and MSE, and device processing and characterization.
We discuss the development of high-quality ZrTi metal layers that are epitaxially deposited by sputtering on sapphire and Si, and which are c-axis oriented and lattice matched for growth of low-defect hexagonal GaN films. We also explain and discuss the nucleation and growth of thick GaN layers by molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) on these metal layers. Finally, we present the results of thin-film measurements to demonstrate the high crystal qualities and very smooth surface morphologies of highly specular GaN layers grown on ZrTi buffers in this work.
Pt-gated AlGaN/GaN high electron mobility transistor based Schottky diodes were employed to detect methane. A detection sensitivity >100 was obtained for the diodes under reverse bias, and this was one order of magnitude higher than the sensitivity of the diodes operated under forward bias. A new method to extract the response time was demonstrated by taking the derivative of diode current, allowing a reduction in the sensor response time by 80%. Methane sensing experiments were conducted at different temperatures, and an Arrhenius plot of the data determined an activation energy of 57 kJ/mol for the sensing process.
GaN grown by molecular-beam epitaxy on Ga-polar GaN templates prepared by metal organic chemical vapor deposition shows a variety of morphologies that depend on defects and growth conditions. We measured the mean terrace widths of hexagonal growth spirals or hillocks versus ammonia and Ga fluxes and substrate temperature. The measurements were compared to a near equilibrium model of the growth. The results indicate that under excess Ga growth conditions, Ga-polar GaN(0001) has a mean step-edge energy of 0.27 eV/Å.
Homoepitaxial, GaN films on both c-plane surfaces of bulk GaN crystals were examined using reflection high-energy electron diffraction (RHEED). Differences in the RHEED pattern, time development of the RHEED intensity, and surface reconstructions were observed. The substrate surfaces were prepared either by mechanical polishing [GaN(0001)A] or by chemo-mechanically polishing [GaN(0001̄)B]. Then films were grown by molecular beam epitaxy; Ga was provide by a Knudsen cell and nitrogen from NH3. On the B surface, the Ga rich reconstructions reported by Smith and co-workers [Phys. Rev. Lett. 79, 3934 (1997)] were observed. On the A surface, a (2×2) reconstruction was observed. Both reconstructions were much sharper than those seen on GaN films grown on sapphire. RHEED measurements of the specular intensity vs time showed that two different surface terminations could be maintained on the B surface, one of which is a stable, gallided surface, while the other is a nitrided surface, which is unstable in vacuum. If the nitrided surface is heated in vacuum it changes to the gallided surface in several minutes at 800 °C. Only one termination was detected on the A surface. The results are complemented by desorption mass spectroscopy measurements, and the resulting surfaces were then investigated using atomic force microscopy and scanning tunneling microscopy. We were able to distinguish the two surface terminations on the B surface, and a unique annealing process under NH3 will be documented. Preliminary investigation of the A surface revealed decorated step edges. The results were compared to films grown on sapphire with different nucleation layers, which can be grown to yield either polarity.
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