AlN epilayers are grown directly on sapphire （0001） substrates each of which has a low temperature AlN nucleation layer. The effects of pretreatments of sapphire snbstrates, including exposures to NH3/H2 and to H2 only ambients at different temperatures, before the growth of AlN epilayers is investigated. In-plane misoriented crystals occur in N-polar AlN epilayers each with pretreatment in a H2 only ambient, and are characterized by six 60°-apart peaks with splits in each peak in （1012） phi scan and two sets of hexagonal diffraction patterns taken along the [0001] zone axis in electron diffraction. These misoriented crystals can be eliminated in AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient. AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient are Al-polar. Our results show the pretreatments and the nucleation layers are responsible for the polarities of the AlN epilayers. We ascribe these results to the different strain relaxation mechanisms induced by the lattice mismatch of AlN and sapphire.
In this work, we demonstrate an effective method to improve capacitive performance of NaV6O（15） intrinsically by annealing. NaV6O（15） nanorods（NRs） prepared by a simple annealing treatment exhibit significantly improved electrochemical performance compared with the untreated NaV6O（15） electrode, and yield a high specific capacitance（402.8 F/g at 300 mA/g）. Furthermore, the annealing treated nanorods show excellent rate capability and cycling stability（ca. 80% capacitance retention after 1000 cycles at a scan rate of100 mV/s）. Our results have confirmed that the annealing treatment has great influence on the capacitive performance of NaV6O（15）, which may be attributed to the intrinsic three dimensional（3D） tunneled structures of NaV6O（15）, and NR morphology. These findings may further broaden the application of NaV6O（15）-based materials for high performance supercapacitors（SCs）, aqueous rechargeable lithium batteries and Li-ion capacitors.
We report the fabrication and performance of solar-blind AlGaN Schottky avalanche photodiodes grown on sapphire substrates.An increased active donor density is found near the surface,leading to an enhanced electric field adjacent to the Schottky electrode.Multiplication gain over 2000 has been achieved in the fabricated devices with a mesa diameter of 200μm.The measured dark I–V curves at different temperatures show strong temperature dependence,suggesting that the gain mechanism in our devices is primarily due to impact ionization.Peak responsivity of 66.3 mA/W is obtained at 260 nm and at zero bias,corresponding to an external quantum efficiency of 31.6%.
