Browsing by Author "Feng, Zhe Chuan"

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Evolution of the local structure and crystal phase for thin ZnGaO films grown by metal organic chemical vapor deposition
(2019-08-15)
Lin, Xiwen
;
Chen, Daihua
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Niu, Wenlong
;
Huang, Chiung-Yi
;
Horng, Ray Hua
A systematic investigation is reported to comprehend the atomic and crystal structure of several thin ZnGaO films grown by metal organic chemical vapor deposition method using different flow rates of diethylzinc (DEZn) while keeping triethylgallium and oxygen flow rates constant. The x-ray absorption fine structure analysis has indicated that the local structure of Ga absorbing atom is close to that of beta-Ga2O3 at a lower DEZn flow rate of 30 sccm and transforming to ZnGa2O4 structure at higher 60 sccm. There is a certain proportion of Ga ions that occupies the tetrahedral sites. The ratios of tetrahedral and octahedral Ga species (Ga(t)/Ga(o)) are found to be within 5.5-8.8%. The x-ray diffraction patterns showed the crystal structure of ZnGaO films gradually evolving from monoclinic to cubic phase with an increase of DEZn flow rates from 30 sccm to 60 sccm. One film prepared at 40 sccm displays the best crystalline quality and the other grown at 50 sccm exhibits excellent smooth surface having the lowest root mean square value with a roughness of 0.42 nm. The correlation between atomic and crystal structure and the growth is helpful to understand the formation of single-crystalline ZnGa2O4 and its physical mechanism.
Green Light-emitting Diodes with InGaN/GaN Multiple Quantum Well Structures: Time-resolved Photoluminescence, Emission Dynamics and Related Studies
(2016)
Feng, Zhe Chuan
;
Jiang, Xiaodong
;
Lee, Yueh-Chien
;
Kuo, Hao-Chung
;
Wan, Lingyu
To explore the mechanism, breakthrough the current bottleneck and overcome the efficiency droop from green InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs). The photoluminescence properties and carrier dynamics of green InGaN/GaN MQW LEDs are investigated by temperature-dependent photoluminescence (PL) from 10 K to 300 K and time-resolved PL (TRPL) measurements at 10 K in our new-built lab. With increasing temperature, a blue shift of PL behavior is attributed to band-tail states formed in local potential minima resembling In-rich clusters. The energy-dependent TRPL experiments are measured at 10 K to study the carrier dynamics in the MQWs. The results show that the PL slow decays for the low-energy side are much slower than the high-energy side. The depth of carrier localization is obtained by fitting the reduced slow decay time with the emission energy increasing. All the results indicate that the PL peak is related to localized radiative recombination.
High quality 10.6 mu m AIN grown on pyramidal patterned sapphire substrate by MOCVD
(2019-01-28)
Long, Hanling
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Dai, Jiangnan
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Zhang, Yi
;
Wang, Shuai
;
Tan, Bo
In this letter, we demonstrate a crack and strain free AlN epilayer with a thickness of 10.6 mu m grown on a pyramidal patterned sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of the X-ray rocking curve was 165/185 arcsec for (002)/(102) planes, respectively. The total threading dislocation density was less than 3 x 10(8) cm(-2). The dislocation evolution and the coalescence process were probed by transmission electron microscopy and scanning electron microscopy. A dual coalescence of the AlN epilayer was observed, which can effectively relax strain during the heteroepitaxy process. Owing to the approximately entire strain relaxation demonstrated by reciprocal space mapping and Raman shift, the surface morphology was crack-free and atomically smooth with a root-mean-square roughness of 0.14 nm. Temperature dependent Raman spectra showed the Raman linewidth of 4.3 cm(-1) at 300 K which was comparable to that of bulk AlN; it also demonstrated good crystalline quality of the AlN epilayer. Published under license by AIP Publishing.
Modelling of microcavity effect in InGaN/GaN heterostructures for interfacial study
(2018-08-01)
Liu, Honghui
;
Lin, Tao
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Wan, Lingyu
;
Xu, Gu
;
Kuo, Hao-Chung
Photoluminescence (PL) spectrum provides the most conventional measurement for emission properties of GaN-based light-emitting diodes (LEDs), in which Fabry-Perot oscillations are often observed modulating the emission peaks. A fitting model for PL intensity accounting the microcavity between air/GaN and GaN/sapphire heterostructure was proposed to treat the PL spectra of different GaN-based LEDs, extracting key factors that implied the interfacial optical properties. This approach was successfully verified by measurements of spectroscopic ellipsometry, then was applied to quantitatively analyse the interfacial-defect-related distortion of dielectric properties. The extracted oscillation coefficient is sensitive to the change of interface qualities and reveals the optical properties of internal interfaces. The new method may also be applied to the other heterojunction LEDs.
Recombination Pathways in Green InGaN/GaN Multiple Quantum Wells
(2017-02-21)
Lin, Tao
;
Kuo, Hao Chung
;
Jiang, Xiao Dong
;
Feng, Zhe Chuan
;
光電工程學系
This paper reports the transient photoluminescence (PL) properties of an InGaN/GaN multiple quantum well (MQW) light-emitting diode (LED) with green emission. Recombination of localized excitons was proved to be the main microscopic mechanism of green emission in the sample. The PL dynamics were ascribed to two pathways of the exciton recombination, corresponding to the fast decay and the slow decay, respectively. The origins of slow decay and fast decay were assigned to local compositional fluctuations of indium and thickness variations of InGaN layers, respectively. Furthermore, the contributions of two decay pathways to the green PL were found to vary at different emission photon energy. The fraction of fast decay pathway decreased with decreasing photon energy. The slow radiative PL from deep localized exciton recombination suffered less suppression from non-radiative delocalization process, for the higher requested activation energy. All these results supported a clear microscopy mechanism of excitation-emission process of the green MQW LED structure.
Reflective second harmonic generation from ZnO thin films: A study on the Zn-O bonding
(2007-04-16)
Lo, Kuang Yao
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Huang, Yi Jen
;
Huang, Jung Y.
;
Feng, Zhe Chuan
;
光電工程學系
The structures of the Zn-O bonding in ZnO (0002) thin films prepared by metal organic chemical vapor deposition have been studied by reflective second harmonic generation (RSHG). The polar Zn-O bond on the top layer is not canceled out and presents 3 mm symmetrical structures on the well-grown ZnO (0002) surface. The average polar strength of the Zn-O bond is correlated with the quality of the ZnO (0002) thin film. The mirror symmetry is caused by the nonvanished polar of twin boundary due to the mismatch between the ZnO film and sapphire substrate and analyzed using s-polarized RSHG with s-polarized fundamental light irradiation. These results demonstrate that the Zn-O heteropolar bonds on the smooth ZnO surface contribute to the SHG intensity. (c) 2007 American Institute of Physics.