Growth Front Evolution of GaN Thin Films on Sapphire Substrate During HVPE

The growth front evolution of GaN thin films deposited on sapphire substrate by hydride vapor phase epitaxity has been studied with atomic force microscope. The evolution of the surface morphology presents four features of stage with the growth process. In initial growth stage, the surface is granular, and the typical grain diameter is about 250 nm for t =0.1 min. 3D growth plays a key role before the films come up to full coalescence, which causes a rough surface. After 0. 1 min the growth dimension decreases with the increase of lateral over growth, the surface roughness obviously decreases. From 0.4 min to 3 rain, the growth front roughness increases gradually, and the evolution of the surface roughness exhibits the characteristics of self-affined fractal. Beyond 3 min, the root-mean-square decreases gradually, which means the deposition behavior from hyper-2D growth gradually turns into layer growth mode with the increase of growth time.

Study of the morphology evolution of AlN grown on nanopatterned sapphire substrate

This study focused on the evolution of growth front about AlN growth on nano-patterned sapphire substrate by metal-organic chemical vapor deposition.The substrate with concave cones was fabricated by nano-imprint lithography and wet etching.Two samples with different epitaxy procedures were fabricated,manifesting as two-dimensional growth mode and three-dimensional growth mode,respectively.The results showed that growth temperature deeply influenced the growth modes and thus played a critical role in the coalescence of AlN.At a relatively high temperature,the AlN epilayer was progressively coalescence and the growth mode was two-dimensional.In this case,we found that the inclined semi-polar facets arising in the process of coalescence were{112^-1}type.But when decreasing the temperature,the{112^-2}semi-polar facets arose,leading to inverse pyramid morphology and obtaining the three-dimensional growth mode.The 3 D inverse pyramid AlN structure could be used for realizing 3 D semi-polar UV-LED or facet-controlled epitaxial lateral overgrowth of AlN.

Structure of the Potential Vorticity of an Explosive Cyclone over the Eastern Asian Region in Late November 2013

An explosive extratropical cyclone(EC)over the Eastern Asian region that caused two shipwrecks is analyzed using ERA-Interim reanalysis data from the European Center for Medium-Range Weather Forecasts.Analyses of the evolution of the EC reveal that the positive potential vorticity(PV)at the upper-tropospheric level displays a hook-shaped structure during the mature period of the cyclone.The PV distribution forms a vertically coherent PV structure called a PV tower.The vertical distribution of the PV can induce and strengthen cyclonic circulation from the lower-to upper-levels of troposphere,which is an important deepening mechanism of explosive cyclone.The PV tower occurs approximately ten hours prior to the development of surface occlusion in the cyclone.The evolution of surface fronts closely follows the development of the horizontal upper-tropospheric PV.This tandem development is largely attributed to the ability of the positive upper-tropospheric PV and the PV tower to induce cyclonic circulation simultaneously.The kinematic wrap-up process of cyclonic circulation also accelerates the formation of warm occlusion.A conceptual model of the distributions of positive PV and potential temperature combining the perspectives of dynamic tropopause folding,PV tower,and atmospheric stability,including westward tilting and baroclinicity,is proposed.This model can illustrate the explosive deepening mechanism of ECs.The regions of convective instability and rainfall determined by this model are consistent with those identified from the actual observation.