Excitonic transitions and dynamics in front and back surfaces of ZnO films grown by plasma-assisted molecular beam epitaxy

이선균,고항주,Takafumi Yao,조용훈 한국진공학회 2010 한국진공학회 학술발표회초록집 Vol.38 No.-

TiO2(ZnS)/SiO2 one-dimensional photonic crystals and the proposal of vertical micro-cavity resonators

Hyun-Yong Lee,Takafumi Yao 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.44 No.2

Omnidirectional (omni-) re ectors and vertical-cavity resonators (VCRs) have been designed using TiO2(ZnS)/SiO2 one-dimensional photonic crystals (1D PCs). For a 1D PC re ector consisting of ten pairs of TiO2/SiO2 (thickness ratio = 38.8 nm/62.7 nm), an omni-photonic band gap (PBG) exists in the -range of approximately 340 380 nm (frequency-range of 0.267 0.299), except for large incident angles (i > 70) of the transverse magnetic-polarized light. We obtain the photonic band structures of TiO2/SiO2 1D PCs applicable for > 400 500 nm, where TiO2 and SiO2 appear to be nondispersive and nonabsorbing. VCRs can be fabricated by introducing a central cavity between two symmetric omni-PBG re ectors. Since the cavity resonant wavelength, cav, of the VCRs corresponds to a localized defect state in a PBG, a modulation of cav might be realized by controlling the structure parameters of the re ectors, not by changing cavity thickness. We also propose a cav-tunable VCR with TiO2/SiO21D PCs (as re ectors) and ZnCdSe three-quantum wells (as a cavity), in which cav-tuning can be obtained by controlling the structural parameters of the 1D PC reflectors.

Polarity control of ZnO layers grown by MBE and ZnO bulk crystals

고항주,한명수,박영식,홍순구,Takafumi Yao 한국진공학회 2004 한국진공학회 학술발표회초록집 Vol.- No.-

Plasma-Assisted Molecular-Beam Epitaxy of ZnO Films on (0001) Al2O3: Effects of the MgO Buffer Layer Thickness

이재욱,이정용,송정훈,Takafumi Yao,홍순구 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.1

ZnO films with MgO buffer layers were grown on (0001) Al2O3 by using plasma-assisted molecular beam epitaxy (PAMBE). We investigated the effect of the MgO buffer thickness on the growth of the ZnO films on (0001) Al2O3 by using PAMBE with neither intentional annealing of the MgO buffer nor low-temperature ZnO growth. The MgO buffer thickness strongly affected the crystal quality of the ZnO films. We believe that the crystal quality of the MgO buffer itself would be the most important factor affecting the crystal quality of the ZnO film on it.

(11-20)면 ZnO 기판의 열처리 시 표면형상과 구조적 특성의 상관관계에 관한 고찰

유진엽,최성국,조유진,장지호,이상태,이원재,Takafumi Yao 한국물리학회 2011 새물리 Vol.61 No.5

We have investigated the relation between the structural properties and the surface morphology of (11-20) ZnO substrates thermally treated in an O₂ambient. Surface steps in both the c- and the m-axis directions were observed from the sample annealed at 950℃. However, when we increased the annealing temperature to 1200℃, the $m$-axis direction step disappeared, and the c-axis direction step height became higher due to step bunching,which indicates that the annealing temperature was too high. The XRD results showed opposite tendency. With increasing annealing temperature, the full width at half maximum (FWHM) values of the omega and the omega-2theta scans were continually narrowed,indicating improved crystal quality. These results reveal that the damaged surface layer has a considerable thickness; hence, a pre-surface treatment to remove the damaged surface layer before the thermal treatment should be considered. (11-20)면 ZnO 기판의 표면을 결정 성장에 적합하도록 평탄화하기 위하여산소 분압 내에서 열처리를 실시하였고, 표면 형상의 변화에 따른 구조적특성의 변화를 고찰하였다. Atomic Force Microscopy(AFM) 결과로 950℃에서 열처리한 시료는 c축 방향과 m축 방향의 두 방향으로표면 스텝이 형성되어 있었고, 열처리 온도를 1200℃로증가시키면 스텝 번칭이 일어나 m축 방향 표면 스텝은 사라지고 c축방향 스텝의 높이가 증가함을 관찰하였다. 그러나 X-ray diffraction 결과는 열처리 온도가 1200℃까지 증가해도 지속적으로 omega 스캔과 omega-2theta 스캔의 반치폭이 줄어들고 비대칭성이 향상되어온도에 비례하여 결정성이 향상됨을 알 수 있었다. 이러한 결과로부터(11-20)면 ZnO 기판의 표면에는 상당한 두께의 표면 손상층이 존재하여,열처리만으로 이를 제거해 평탄하고 결정성이 양호한 표면을 얻는 것이곤란하며 별도의 선행 공정의 도입을 검토하는 것이 필요함을 알 수있었다.

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.

Waveguide lasing from V-shaped ZnO microstructure.

Lee, Sang Hyun,Goto, Takenari,Miyazaki, Hiroshi,Yao, Takafumi Optical Society of America 2013 Optics letters Vol.38 No.14

<P>A V-shaped optical resonance cavity was obtained from ZnO microstructures grown by thermal chemical vapor deposition. Strong laser emissions were observed in three regions--the tip of the two branches and the bottom facet of the V-shaped microstructures--under UV laser excitation at room temperature. In the region where the diameter of the branches was smaller than the wavelength of the laser light, light could not propagate into the tip due to the cutoff phenomenon, resulting in partial reflection. Quasi-Fabry-Perot resonance in the branch and light reflection at the bottom facet characterized the V-shaped microcavity.</P>

Temperature and Polarization Dependence of the Near-Band-Edge Photoluminescence in a Non-Polar ZnO Film Grown by Using Molecular Beam Epitaxy

남윤성,Sang Wook Lee,백경선,장수경,류지욱,송정훈,한석규,홍순구,Takafumi Yao 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.53 No.1

We investigated the temperature and the polarization dependence of the near-band-edge photoluminescence (PL) in non-polar (A-plane) ZnO films on R-plane sapphire grown by using plasma-assisted molecular beam epitaxy. We observed the band-edge emissions from the non-polar ZnO film at 3.449 and 3.386 eV, which were identified as a recombination of free excitons and as excitons bound to neutral donors (D0X) respectively. By analyzing the temperature dependent PL spectra, we attributed the transitions at 3.326 eV to the transition of free electrons in the conduction band to carriers bound to acceptors (FB). All the transitions in non-polar ZnO were blueshifted significantly by the anisotropic compressive strain. The polarized PL showed strong in-plane anisotropy of excitonic transitions even with the existence of strong strain. At low temperature, all the observed excitonic PL of non-polar ZnO was more than 90 % polarized perpendicular to c-axis which lay in the film's plane. We report that the D0X and the FB transitions show 93 % and 90 % polarizations at 12 K and 82 % and 80 % polarization at 130 K, respectively.

Molecular Beam Epitaxy로 성장한 ZnSe 박막의 결정성 및 계면 특성의 평가

정명훈,장지호,김광희,김홍승,박승환,안형수,양민,오동철,Takafumi Yao 한국물리학회 2006 새물리 Vol.52 No.3

Various properties of ZnSe thin films grown on GaAs substrates by using molecular beam epitaxy (MBE) have been investigated. Photoluminescence (PL) measurement show the high optical quality of the samples in terms of the narrow linewidth, the strong luminescence intensity of band edge emission, and the temperature dependence of the luminescence properties. The dislocation density is estimated as 9.485 $\times$ 10$^7$ dis/cm$^2$ by using XRD measurements. Serious degradation of the crystallinity is not observed in the PL and the XRD results. However, the photocurrent measurement shows various deep states related with defects and impurities located at the heterointerface, which strongly stresses the importance of this measurement for evaluating the quality of thin films. The growth of low temperature buffer to improve the quality further is discussed. GaAs 기판 위에 molecular beam epitaxy (MBE)법으로 성장된 ZnSe 박막의 광학적, 구조적, 전기적 특성을 조사하였다. Photoluminescence (PL) 측정에서 좁은 반치폭을 갖는 강한 밴드단 발광과 온도에 따른 발광 특성을 통해 우수한 광학적 특성을 볼 수 있었다. 또한 $X$-ray diffraction (XRD) 측정으로 구한 9.485 $\times$ 10$^7$ dis/cm$^2$의 전위 밀도에서도 결정성이 양호함을 알 수 있었다. 그러나 광전류 측정을 통하여 박막의 질을 평가하는데 있어 중요한 이종 계면에 기인하는 결함 및 불순물의 존재를 확인하였다. 이런 특성 평가를 통하여 고품질의 박막을 얻기 위해 저온 버퍼층을 도입한 성장 방법을 제안하였다.

Realization of a Low-Threshold-Voltage Field Emitter by Using High-Quality ZnO Nano-Tetrapods

Mina Jung,Dongchul Oh,고항주,Hyunchul Ko,Jiho Chang,Sunyeo Ha,Takafumi Yao,Wookhyun Lee 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.48 No.6

High-performance ZnO tetrapods-type field emitters with low turn-on voltage are realized. Systematic investigations of the formation conditions are performed. Tetrapod-shaped ZnO nanostructures are formed on Si substrates by using a vapor phase transportation method. The effects of two important growth parameters, the growth temperature and the VI/II ratio, are investigated. The growth temperature is controlled in the range from 600 C to 900 C while the VI/II ratio is adjusted by changing the flux of the carrier gas. ZnO tetrapods, formed at 800 C under a carrier gas flux of 0.5 cc/mm2min, show a uniform shape with 100-nm-thick and 1 1.5-μm- long legs. Also, the growth condition confirms a stoichiometric chemical composition (O/Zn 1) without any second phases. The excellent luminescence properties, a strong excitonic UV emission at 3.25 eV without deep-level emission, indicate that tetrapod structures with high crystallinity can be formed under optimized growth conditions.쵇