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Stress dependence on N/Ga ratio in GaN epitaxial films grownon ZnO substrates
Tsutomu Minegishi,Takuma Suzuki,Chihiro Harada,Hiroki Goto,Meoung-Whan Cho,Takafumi Yao 한국물리학회 2004 Current Applied Physics Vol.4 No.6
GaN films were grown on ZnO substrate under various beam equivalent pressure ratios by plasma-assisted molecular beamepitaxy (P-MBE). We theoretically calculated the thermal stress caused by the dierence of thermal expansion coecients betweenGaN and ZnO. The changes of stress and critical thickness were evaluated by measurement of XRD for HT GaN and LT GaNbuer grown under Ga-rich and N-rich conditions. From this study, we observed that GaN grown under Ga-rich condition causesGaN lm to under compressive-stress, while GaN grown under N-rich condition was tensile-stressed. Consequently, interdiusionhas no eect on the variation of the critical thickness.
Lateral arrays of vertical ZnO nanowalls on a periodically polarity-inverted ZnO template
Lee, Sang Hyun,Minegishi, Tsutomu,Ha, Jun-Seok,Park, Jin-sub,Lee, Hyo-Jong,Lee, Hyun Jung,Shiku, Hitoshi,Matsue, Tomokazu,Hong, Soon-Ku,Jeon, Heonsu,Yao, Takafumi IOP Pub 2009 Nanotechnology Vol.20 No.23
<P>Well aligned ZnO nanowall arrays with submicron pitch were grown on a periodically polarity-inverted ZnO template using a carbothermal reduction process. Under the conditions of a highly dense Au catalyst for increasing nucleation sites, ZnO nanowalls with a thickness of 126 ± 10 nm, an average height of 3.4 µm, and a length of about 10 mm were formed on the template. The nanowalls were only grown on a Zn-polar surface due to a different growth mode with an O-polar surface. The results of x-ray diffraction and photoluminescence (PL) measurements revealed a single crystalline, vertical alignment on the template, and a large surface to volume ratio of the ZnO nanowalls.</P>
ZnO Nanorods and Periodicaly-Polarity-Inverted Structures for Photonic Devices
Sang Hyun Lee,Katsushi Fujii,Tsutomu Minegishi,Takafumi Yao 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.1
We control the morphology of ZnO nanorods and the crystal polarity of ZnO layers for applications to photonic devices. ZnO nanorods are grown through an aqueous reaction. The morphology of the ZnO nanorods is changed to porous by annealing folowed by wet chemical etching. The photoluminescence emision intensity from porous ZnO nanorods increases several timescomparedtothatfromas-grownnanorods. The enhancement of the emision intensity of the porous ZnO nanorods is explained in terms of an increase in the extraction eciency of emision. Periodicaly-polarity-inverted(PPI)ZnO structures are grown on paterned MgO buffers fabricated on c-Al2O3 substrates by using plasma-asisted molecular beam epitaxy, in which the paterned MgO consists of 8-nm-thick MgO and 1-nm-thick MgO. Zn-polar ZnO grows on the thick MgO buffer while O-polar ZnO on the thin MgO. Second harmonic generation for incident lightat 1060 nm under quasi-phase matching conditions is observed at 503 nm from the PPI structure, thus indicating the feasibility of using PPI ZnO structures for nonlinear optical devices.
Relation between interdiffusion and polarity for MBE growth of GaN epilayers on ZnO substrates
Takuma Suzuki,Chihiro Harada,Hiroki Goto,Tsutomu Minegishi,Agus Setiawan,고항주,Meoung-Whan Cho,T.Yao 한국물리학회 2004 Current Applied Physics Vol.4 No.6
We report on GaN growth on Zn-polar ZnO substrates using plasma-assisted molecular-beam epitaxy (P-MBE). Before GaNgrowth, ZnO substrate annealing conditions were optimized. Reection high-energy electron diraction (RHEED) patterns afterlow-temperature GaN buer layer annealing changed from streaky to spotty, suggesting that zinc and oxygen atoms interdiusefrom the ZnO substrate into the GaN epilayer. This interdiusion results in a mix-polar GaN epilayer.
Hiroki Goto,Hisao Makino,Agus Setiawan,Takuma Suzuki,Chihiro Harada,Tsutomu Minegishi,Meoung-Whan Cho,Takafumi Yao 한국물리학회 2004 Current Applied Physics Vol.4 No.6
Optical properties of ZnO thin lms with/without MgO-buer annealing were investigated by low and room temperature photoluminescence measurements. The ZnO lms were grown onc-sapphire substrates by plasma-assisted molecular-beam epitaxy employing a thin MgO-buer layer. Dislocation density of ZnO layer was reduced from 5.3· 109 to 1.9· 109 cm. 2 by annealingMgO-buer prior to the growth of ZnO. The intensity of free exciton emission from the sample with MgO-buer annealing wasalmost twice of that from the sample without annealing, while the deep level emission from the sample with MgO-buer annealingwas about 1/3 of that without annealing. The MgO-buer annealing improves optical quality of overgrown ZnO lms.
Anisotropic X-ray Rocking Curve Due to a Damaged Surface Layer in a Freestanding GaN Thick Film
seunghwan Park,jiho Chang,dongchoel Oh,G. Fujimoto,H. Goto,Hideyuki Suzuki,Takafumi Yao,Takashi Hanada,Tsutomu Minegishi,wookhyun Lee 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.3
The structural deformation of freestanding GaN film has been investigated. The specimen was grown on an Al2O3 substrate by using hydride vapor phase epitaxy (HVPE) and was self-detached during the growth. the atomic force microscopy (AFM) image showed much mechanical damages on the surface, which was presumably generated by the surface polishing process. After chemical etching, the near-band-edge to yellow-band (NBE/YB) luminescence intensity ratio was greatly increased. Also, using high resolution X-ray diraction (HRXRD), we found that the 2-! scan of (0002) reflection asymmetrically broadened at low diraction angles, which indicated that a damaged surface layer existed in the film with residual compressive strain. Moreover, a more detailed X-ray analysis showed that the surface had a concave curvature. We concluded that structural distortion of GaN could induce a serious deformation, which could aect not only the structural but also the optical and the electrical properties of the film.
Jang, Youn Jeong,Jang, Ji-Wook,Lee, Jaehyuk,Kim, Ju Hun,Kumagai, Hiromu,Lee, Jinwoo,Minegishi, Tsutomu,Kubota, Jun,Domen, Kazunari,Lee, Jae Sung The Royal Society of Chemistry 2015 ENERGY AND ENVIRONMENTAL SCIENCE Vol.8 No.12
<P>A gold-coupled ZnTe/ZnO-nanowire array is a new photocathode for selective CO<SUB>2</SUB> reduction to CO. At −0.7 V<SUB>RHE</SUB> under simulated 1 sun illumination, its photocurrent (−16.0 mA cm<SUP>−2</SUP>) and incident photon-to-current conversion efficiency (97%) represent the highest among reported ZnTe photocathodes for CO<SUB>2</SUB> reduction and dramatic enhancement from those of a bare electrode (−7.9 mA cm<SUP>−2</SUP>, 68%). In addition, the Au nanoparticles convert mainly-hydrogen-producing bare ZnTe/ZnO-nanowires into mainly-CO-producing photocathodes in photoelectrochemical CO<SUB>2</SUB> reduction. The remarkable effects of the Au co-catalyst originate from the formation of a Schottky junction with ZnTe to improve charge separation and to provide reaction centers for CO<SUB>2</SUB> reduction suppressing competing water reduction.</P> <P>Graphic Abstract</P><P>Au coupled ZnTe/ZnO-NW array is a new photocathode for selective CO production from CO<SUB>2</SUB>. The remarkable effects of an Au are to form of a Schottky junction with ZnTe to improve band bending and provide the reaction center for CO<SUB>2</SUB> reduction suppressing water reduction. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c5ee01445j'> </P>