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Dive into the research topics where X. D. Chen is active.

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Featured researches published by X. D. Chen.


Applied Physics Letters | 2006

Current transport studies of ZnO∕p-Si heterostructures grown by plasma immersion ion implantation and deposition

X. D. Chen; C. C. Ling; S. Fung; C. D. Beling; Yongfeng Mei; Ricky K.Y. Fu; G. G. Siu; Paul K. Chu

Rectifying undoped and nitrogen-doped ZnO∕p-Si heterojunctions were fabricated by plasma immersion ion implantation and deposition. The undoped and nitrogen-doped ZnO films were n type (n∼1019cm−3) and highly resistive (resistivity ∼105Ωcm), respectively. While forward biasing the undoped-ZnO∕p-Si, the current follows Ohmic behavior if the applied bias Vforward is larger than ∼0.4V. However, for the nitrogen-doped-ZnO∕p-Si sample, the current is Ohmic for Vforward 2.5V. The transport properties of the undoped-ZnO∕p-Si and the N-doped-ZnO∕p-Si diodes were explained in terms of the Anderson model and the space charge limited current model, respectively.Rectifying undoped and nitrogen-doped ZnO∕p-Si heterojunctions were fabricated by plasma immersion ion implantation and deposition. The undoped and nitrogen-doped ZnO films were n type (n∼1019cm−3) and highly resistive (resistivity ∼105Ωcm), respectively. While forward biasing the undoped-ZnO∕p-Si, the current follows Ohmic behavior if the applied bias Vforward is larger than ∼0.4V. However, for the nitrogen-doped-ZnO∕p-Si sample, the current is Ohmic for Vforward 2.5V. The transport properties of the undoped-ZnO∕p-Si and the N-doped-ZnO∕p-Si diodes were explained in terms of the Anderson model and the space charge limited current model, respectively.


Applied Physics Letters | 2007

Hydrogen peroxide treatment induced rectifying behavior of Au/n-ZnO contact

Q. L. Gu; Chi Chung Ling; X. D. Chen; C. K. Cheng; Alan Man Ching Ng; C. D. Beling; S. Fung; Aleksandra B. Djurišić; L. W. Lu; G. Brauer; H. C. Ong

Conversion of the Au∕n-ZnO contact from Ohmic to rectifying with H2O2 pretreatment was studied systematically using I-V measurements, x-ray photoemission spectroscopy, positron annihilation spectroscopy, and deep level transient spectroscopy. H2O2 treatment did not affect the carbon surface contamination or the EC–0.31eV deep level, but it resulted in a significant decrease of the surface OH contamination and the formation of vacancy-type defects (Zn vacancy or vacancy cluster) close to the surface. The formation of a rectifying contact can be attributed to the reduced conductivity of the surface region due to the removal of OH and the formation of vacancy-type defects.


Journal of Applied Physics | 2008

Au/n-ZnO rectifying contact fabricated with hydrogen peroxide pretreatment

Q. L. Gu; C.K. Cheung; Chi Chung Ling; Alan Man-Ching Ng; Aleksandra B. Djurišić; Liwu Lu; X. D. Chen; S. Fung; C. D. Beling; H. C. Ong

Au contacts were deposited on n-type ZnO single crystals with and without hydrogen peroxide pretreatment for the ZnO substrate. The Au/ZnO contacts fabricated on substrates without H2O2 pretreatment were Ohmic and those with H2O2 pretreatment were rectifying. With an aim of fabricating a good quality Schottky contact, the rectifying property of the Au/ZnO contact was systemically investigated by varying the treatment temperature and duration. The best performing Schottky contact was found to have an ideality factor of 1.15 and a leakage current of 10 �7 Ac m �2 . A multispectroscopic study, including scanning electron microscopy, positron annihilation spectroscopy, deep level transient spectroscopy, x-ray photoelectron spectroscopy, and photoluminescence, showed that the H2O2 treatment removed the OH impurity and created Zn-vacancy related defects hence decreasing the conductivity of the ZnO surface layer, a condition favorable for forming good Schottky contact. However, the H2O2 treatment also resulted in a deterioration of the surface morphology, leading to an increase in the Schottky contact ideality factor and leakage current in the case of nonoptimal treatment time and temperature.


Applied Physics Letters | 1998

Formation of PIn defect in annealed liquid-encapsulated Czochralski InP

Yang Zhao; Xiaoliang Xu; M. Gong; S. Fung; C. D. Beling; X. D. Chen; N. F. Sun; T. N. Sun; S. L. Liu; G. Y. Yang; X. B. Guo; Y. Z. Sun; Lianhui Wang; Q. Y. Zheng; Zhen Zhou; J. Chen

Fourier transform infrared spectroscopy measurements have been carried out on liquid-encapsulated Czochralski-grown undoped InP wafers, which reproducibly become semi-insulating upon annealing in an ambient of phosphorus at 800–900 °C. The measurements reveal a high concentration of hydrogen complexes in the form VInH4 existing in the material before annealing in agreement with recent experimental studies. It is argued that the dominant and essential process producing the semi-insulating behavior is the compensation produced by an EL2-like deep donor phosphorus antisite defect, which is formed by the dissociation of the hydrogen complexes during the process of annealing. The deep donor compensates acceptors, the majority of which are shallow residual acceptor impurities and deep hydrogen associated VIn and isolated VIn levels, produced at the first stage of the dissociation of the VInH4 complex. The high concentration of indium vacancies produced by the dissociation are the precursor of the EL2-like phosp...


Journal of Applied Physics | 2011

ZnO nanorod/GaN light-emitting diodes: The origin of yellow and violet emission bands under reverse and forward bias

X. D. Chen; Alan Man Ching Ng; F. Fang; Yip Hang Ng; Aleksandra B. Djurišić; Hoi Lam Tam; Kok Wai Cheah; Shangjr Gwo; Wai Kin Chan; Patrick W. K. Fong; H. F. Lui; Charles Surya

ZnO nanorods have been prepared by electrodeposition under identical conditions on various p-GaN-based thin film structures. The devices exhibited lighting up under both forward and reverse biases, but the turn-on voltage and the emission color were strongly dependent on the p-GaN-based structure used. The origin of different luminescence peaks under forward and reverse bias has been studied by comparing the devices with and without ZnO and by photoluminescence and cathodoluminescence spectroscopy. We found that both yellow-orange emission under reverse bias and violet emission under forward bias, which are commonly attributed to ZnO, actually originate from the p-GaN substrate and/or surface/interface defects. While the absolute brightness of devices without InGaN multiple quantum wells was low, high brightness with luminance exceeding 10 000 cd/m2 and tunable emission (from orange at 2.1 V to blue at 2.7 V, with nearly white emission with Commission internationale de l’eclairage (CIE) coordinates (0.30,...


Journal of Applied Physics | 2003

Deep level transient spectroscopic study of neutron-irradiated n-type 6H–SiC

X. D. Chen; S. Fung; C. C. Ling; C. D. Beling; M. Gong

Deep level transient spectroscopy has been employed to study the deep level defects introduced in n-type 6H-SiC after neutron irradiation. Deep levels situated at E-C-0.23, E-C-0.36/0.44, E-C-0.50, and E-C-0.62/0.68 eV have been detected in the temperature range of 100-450 K, which have been identified with the previously reported deep levels ED1, E-1/E-2, E-i, and Z(1)/Z(2), respectively. Thermal annealing studies of these deep levels reveal that ED1 and E-i anneal at a temperature below 350degreesC, the Z(1)/Z(2) levels anneal out at 900degreesC, while the intensity of the E-1/E-2 peaks is increased with annealing temperature, reaching a maximum at about 500-750degreesC, and finally annealing out at 1400degreesC. The possible nature of the deep levels ED1, E-1/E-2, E-i, and Z(1)/Z(2) are discussed in the context of their annealing behavior. Upon further annealing at 1600degreesC, four deep levels labeled NE1 at E-C-0.44 eV, NE2 E-C-0.53 eV, NE3 E-C-0.64 eV, and NE4 E-C-0.68 eV are produced. Evidence is given that these levels are different in their origin to E-1/E-2 and Z(1)/Z(2)


Journal of Applied Physics | 1999

Compensation defects in annealed undoped liquid encapsulated Czochralski InP

S. Fung; Yang Zhao; Xiaoliang Xu; X. D. Chen; N. F. Sun; T. N. Sun; R. G. Zhang; S. L. Liu; G. Y. Yang; X. B. Guo; Y. Z. Sun; R. Y. Yan; Q. H. Hua

As-grown undoped n-type semiconducting and annealed undoped semi-insulating (SI) liquid encapsulated Czochralski (LEC) InP has been studied by temperature dependent Hall measurement, photoluminescence spectroscopy, infrared absorption, and photocurrent spectroscopy. P-type conduction SI InP can frequently be obtained by annealing undoped LEC InP. This is caused by a high concentration of thermally induced native acceptor defects. In some cases, it can be shown that the thermally induced n-type SI property of undoped LEC InP is caused by a midgap donor compensating for the net shallow acceptors. The midgap donor is proposed to be a phosphorus antisite related defect. Traps in annealed SI InP have been detected by photocurrent spectroscopy and have been compared with reported results. The mechanisms of defect formation are discussed.


Applied Physics Letters | 2004

Nature of the acceptor responsible for p-type conduction in liquid encapsulated Czochralski-grown undoped gallium antimonide

C. C. Ling; M. K. Lui; S. K. Ma; X. D. Chen; S. Fung; C. D. Beling

Acceptors in liquid encapsulated Czochralski-grown undoped gallium antimonide (GaSb) were studied by temperature dependent Hall measurement and positron lifetime spectroscopy (PLS). Because of its high concentration and low ionization energy, a level at EV+34meV is found to be the important acceptor responsible for the p-type conduction of the samples. Two different kinds of VGa-related defects (lifetimes of 280ps and 315ps, respectively) having different microstructures were characterized by PLS. By comparing their annealing behaviors and charge state occupancies, the EV+34meV level could not be related to the two VGa-related defects.


Applied Physics Letters | 1998

Compensation ratio-dependent concentration of a VInH4 complex in n-type liquid encapsulated Czochralski InP

S. Fung; Yang Zhao; C. D. Beling; Xiaoliang Xu; M. Gong; N. F. Sun; T. N. Sun; X. D. Chen; R. G. Zhang; S. L. Liu; G. Y. Yang; J. J. Qian; Muhua Sun; Xue-Yuan Liu

The concentration of hydroen-indium vacancy complex VInH4 in liquid encapsulated Czochralski undoped and Fe-doped n-type InP has been studied by low-temperature infrared absorption spectroscopy. The VInH4 complex is found to be a dominant intrinsic shallow donor defect with concentrations up to similar to 10(16) cm(-3) in as-grown liquid encapsulated Czochralski InP. The concentration of the VInH4 complex is found to increase with the compensation ratio in good agreement with the proposed defect formation model of Walukiewicz [W. Walukiewicz, Phys. Rev. B 37, 4760 (1998); Appl. Phys. Lett. 54, 2094 (1989)], which predicts a Fermi-level-dependent concentration of amphoteric defects


Journal of Applied Physics | 2003

Experimental study and modeling of the influence of screw dislocations on the performance of Au/n-GaN Schottky diodes

Yanyi Huang; X. D. Chen; S. Fung; C. D. Beling; C. C. Ling

Current–voltage (I–V) characteristics of macroscopic Schottky diodes fabricated on different GaN templates grown by metalogranic chemical vapor deposition on sapphire substrates were investigated. The number of dislocations under the Au Schottky contact was determined by atomic force microscopy combined with hot H3PO4 etching and the screw dislocations in the GaN films were found to have a strong influence on the reverse leakage current of the Au/n-GaN Schottky diodes. The leakage current is increased when high-density screw dislocations exist under the Au Schottky contact. A model based upon the presence of dislocations at the Au/GaN interface has been used to explain this behavior. It has been proposed that these dislocations result in the lowering of the barrier height in the localized regions, and thus significantly affect the reverse I–V characteristics of the Schottky diodes.

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S. Fung

University of Hong Kong

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C. D. Beling

University of Hong Kong

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C. C. Ling

University of Hong Kong

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W. Anwand

Helmholtz-Zentrum Dresden-Rossendorf

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W. Skorupa

Helmholtz-Zentrum Dresden-Rossendorf

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Alan Man Ching Ng

South University of Science and Technology of China

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Wai Kin Chan

University of Hong Kong

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