Structural and optical properties of self-assembled AlN nanowires grown on SiO2/Si substrates by molecular beam epitaxy.

Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO2/Si (111) substrates. Using a combination of in situ reflective high energy electron diffraction and ex situ x-ray diffraction (XRD), we show that the NWs grow nearly strain-free, preferentially perpendicular to the amorphous SiO2 interlayer and without epitaxial relationship to Si(111) substrate, as expected. Scanning electron microscopy investigation reveals significant NWs coalescence, which results in their progressively increasing diameter and formation of columnar structures with non-hexagonal cross-section. Making use of scanning transmission electron microscopy (STEM), the NWs initial diameters are found in the 20-30 nm range. In addition, the formation of a thin (≈30 nm) polycrystalline AlN layer is observed on the substrate surface. Regarding the structural quality of the AlN NWs, STEM measurements reveal the formation of extended columnar regions, which grow with a virtually perfect metal-polarity wurtzite arrangement and with extended defects only sporadically observed. Combination of STEM and electron energy loss spectroscopy reveals the formation of continuous aluminum oxide (1-2 nm) on the NW surface. Low temperature photoluminescence measurements reveal a single near-band-edge (NBE) emission peak, positioned at 6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by XRD and in agreement with the values obtained on AlN bulk layers synthesized by other growth techniques. The significant full-width-at-half-maximum of NBE emission, found at ≈20 meV (at 2 K), suggests that free and bound excitons are mixed together within this single emission band. Finally, the optical properties of the hereby reported AlN NWs grown by PAMBE are comprehensively compared to optical properties of bulk, epitaxial and/or columnar AlN grown by various techniques such as: physical vapor transport, metal organic vapor phase epitaxy, metal organic chemical vapor deposition and molecular beam epitaxy.

Physisorption of functionalized gold nanoparticles on AlGaN/GaN high electron mobility transistors for sensing applications.

AlGaN/GaN high electron mobility transistors (HEMTs) were used to measure electrical characteristics of physisorbed gold nanoparticles (Au NPs) functionalized with alkanethiols with a terminal methyl, amine, or carboxyl functional group. Additional alkanethiol was physisorbed onto the NP treated devices to distinguish between the effects of the Au NPs and alkanethiols on HEMT operation. Scanning Kelvin probe microscopy and electrical measurements were used to characterize the treatment effects. The HEMTs were operated near threshold voltage due to the greatest sensitivity in this region. The Au NP/HEMT system electrically detected functional group differences on adsorbed NPs which is pertinent to biosensor applications.