Ion-induced low-energy electron diffraction

H(+) and He(+) ions with an energy of 25 keV are scattered under a grazing angle of incidence from a clean and flat Cu(001) surface. For specific azimuthal orientations of the crystal surface with respect to low index directions in the surface plane we observe the ion induced emission of electrons with a conventional LEED (low energy electron diffraction) setup. By operating the instrument in an energy dispersive mode we find intensity distributions of emitted electrons which can unequivocally be ascribed to diffraction effects at the target surface

Practices on Cultural Construction in Experimental Chemistry Demonstration Center

根据国家级厦门大学化学实验教学示范中心的建设特点,从实验室环境文化、制度文化、管理文化和精神文化多个方面入手建设实验室文化的实践进行总结与分享。This paper summarized our practices on cultural construction in chemistry laboratory including laboratory environment, rules, management and spirit based on the characteristics of our center.国家基础科学人才培养基金(J1210014);; 2016年度教育部“基础学科拔尖学生培养试验计划”研究课题;; 福建省高等学校教学改革研究项目(JAS14634

Interface modification of the InGaN/GaN quantum wells: the strain pre-relief effect

National Natural Science Foundation of China [60876008]Interface modification by inserting an ultrathin low-temperature GaN layer prior to the growth of high-temperature GaN barriers followed by an annealing process was employed to improve the properties of the InGaN/GaN quantum wells. By detailed studies and comparisons of the surface morphology, photoluminescence and the surface compositions of the InGaN/GaN quantum wells at different growth stages with and without the interface modification, we find that with the interface modification the surface morphology was significantly improved with better smoothness, and smaller and shallower pits of lower density compared with that without interface modification; further, the indium aggregation and phase separation were suppressed. The physical phenomena are attributed to the 'strain pre-relief effect' by the formation of quasi-dots (similar to 20 nm in diameter) prior to temperature ramping and growth of high-temperature GaN barriers. Furthermore, the ultrathin low-temperature GaN layers have a good protection property as confirmed by PL and XPS measurements

Green Emission from a Strain-Modulated InGaN Active Layer

Strain-induced quantum dots (QDs) like island formations are demonstrated to effectively suppress pits/dislocation generation in high indium content (26.8%) InGaN active layers. In addition to the strain redistribution in the QD-like islands, strain modulation on the InGaN active layers by using the GaN island capping is employed to form an increased surface potential barrier around the dislocation cores, which inhibits the carrier transport to the surrounding dislocations. Cathodoluminescence shows distinct double-peak emissions at 503 nm and 444 nm, corresponding to the In-rich QD-like emission and the normal quantum well emission, respectively. The QD-like emission becomes dominated in photoluminescence due to the carrier localization effect of In-rich InGaN QDs at relatively low "carrier injection current". Accordingly, green emission may be enhanced by the following origins: (1) reduction in pits/dislocations density, (2) carrier localization and strain reduction in QDs, (3) strain modulation by GaN island capping, (4) enhanced light extraction with faceted GaN islands on the surface.National Natural Science Foundation of China [60876008, 61076091]; Program for New Century Excellent Talents in Fujian Province Universit

White emission by self-regulated growth of InGaN/GaN quantum wells on in situ self-organized faceted n-GaN islands

National Natural Science Foundation of China [60876008, 61076091]The in situ self-organization of three-dimensional n-GaN islands of distinct sidewall faceting was realized by initial low V/III ratio growth under high reactor pressure followed by variations of the V/III ratio and reactor pressure. The naturally formed faceted islands with top and sidewall facets of various specific polar angles may serve as an ideal template for self-regulated growth of the InGaN/GaN multiple quantum wells (MQWs), i.e. the growth behavior is specific polar angle dependent. Further, the growth behavior and luminescence properties of the InGaN/GaN MQWs on various facets of different specific polar angles are directly compared and discussed. Tetrachromatic white emissions (blue, cyan, green, and red) from single-chip phosphor-free InGaN/GaN MQWs are realized by color tuning through island shaping, shape variations, and self-regulated growth of the InGaN/GaN MQWs

Characterization of MOCVD-grown non-stoichiometric SiNx

The surface morphologies and X-ray photoelectron spectra of MOCVD-grown SiNx were investigated. Highly Si-rich SiNx nanoislands not fully covering the sapphire surface were observed for SiNx deposition at low temperature (545 degrees C) with NH3/SiH4 flow rate of 2500/40 sccm. The surface roughness decreased from 0.91 nm to 0.23 nm with the reduction of SiH4 flow rate from 40 sccm to 3 sccm. The reduction of the SiH4 flow rate did not cause a linear decrease of SiN ratio, which indicated that the SiH4 Supply Was saturated when the NH3 supply was 2500 sccm and deposition temperature was fixed at 545 degrees C. Relatively "thick" SiNx layers with stoichiometry close to 1 were formed for SiNx deposition at high temperature due to high decomposition rate of ammonia and high reaction rate between silane and ammonia. The SiNx layers almost fully covered the sapphire surface and showed surface structures of both nanoislands and nanoholes. By employing the same NH3/SiH4 flow rate of 2500/40 sccm the surface roughness of SiNx layers decreased from 0.91 nm to 0.17 nm with the increase of deposition temperature from 545 degrees C to 1035 degrees C. Saturated pre-nitridation would likely cause surface roughening. (C) 2008 Elsevier B.V. All rights reserved

Significant increase of light emission efficiency by in situ site-selective etching of InGaN quantum wells

Natural Science Foundation of China [60876008]An indium post-treatment of the InGaN epilayers was employed for InGaN-to-GaN interface modification. We find that the treatment could lead to selective etching of the InGaN epilayers around threading dislocations (TDs) due to preferential etching of the chemically active step-correlated TDs and formation of indium-rich InGaN nanostructures on the smooth InGaN surface. The intentionally formed V-shaped pits by site-selective etching of the InGaN epilayers resulted in an increased surface potential barrier at the pit sidewalls due to the relatively thin InGaN single quantum well. The increased energy bandgap of the InGaN active layers around the TDs cores caused the lateral carrier confinement away from nonradiative recombination at the defects and thus significantly enhanced the light emission efficiency. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3176931

Significant increase of crystalline quality and green emission by an interface modification of InGaN/GaN quantum wells

National Natural Science Foundation of China [60876008, 61076091]A GaN-to-InGaN interface modification by predeposition of an ultrathin In-rich InGaN incomplete layer followed by a thin triangular InGaN well layer was employed to overcome the negative effects of polarization field on light emission efficiency of InGaN/GaN quantum wells as well as to improve the crystalline quality by avoidance of a significant strain generation and enhanced surfactant effect. Further, the interface modification induced energy band structure engineering reduces the spatial separation of electrons and holes, and thus increases the carrier recombination rate. The improvement in crystalline quality, localized potential fluctuation, and energy band engineering contribute to the significant increase of green emission of the InGaN/GaN quantum wells

Self-organization of 3D triangular GaN nanoislands and the shape variation to hexagonal

We report on the self-organization of large-scale uniform aligned three-dimensional (3D) GaN islands with distinct triangular (0001) and smooth side facets and the shape variations of the (0001) facets from triangular to hexagonal during metalorganic vapor-phase epitaxy (MOVPE) growth of GaN films on Si-rich SiNx patterned sapphire substrates. The triangular island shaping during the recrystallization processes of GaN nucleation layers (NLs) can be attributed to the enhanced diffusion and regrowth anisotropy. The island shape transition from triangular to hexagonal in the early stages of high-temperature growth of GaN epilayers is due to the gas-phase transport dominating growth mechanism and the limited diffusion length of edge adatoms compared with the increased island size