Browsing by Author "Zhang, Cheng"

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Mesoporous GaN for Photonic Engineering-Highly Reflective GaN Mirrors as an Example
(2015-07-01)
Zhang, Cheng
;
Park, Sung Hyun
;
Chen, Danti
;
Lin, Da-Wei
;
Xiong, Wen
A porous medium is a special type of material where voids are created in a solid medium. The introduction of pores into a bulk solid can profoundly affect its physical properties and enable interesting mechanisms. In this paper, we report the use of mesoporous GaN to address a long-standing challenge in GaN devices: tuning the optical index in epitaxial structures without compromising the structural and electrical properties. By controlling the doping and electrochemical etching bias, we are able to control the pore morphology from macro- to meso- and microporous. The meso- and microporous GaN can be considered a new form of GaN with unprecedented optical index tunability. We examine the scattering loss in a porous medium quantitatively using numerical, semiempirical, and experimental methods. It is established that the optical loss due to scattering is well within the acceptable range. While being perfectly lattice-matched to GaN, the porous GaN layers are found to be electrically highly conductive. As an example of optical engineering, we demonstrate record high reflectances (R > 99.5%) from epitaxial mesoporous GaN mirrors that can be controllably fabricated, a result that is bound to impact GaN opto and photonic technologies.
Significantly Improved Luminescence Properties of Nitrogen-Polar (000(1over bar>)over-bar) InGaN Multiple Quantum Wells Grown by Pulsed Metalorganic Chemical Vapor Deposition
(2015-01-14)
Song, Jie
;
Chang, Shih-Pang
;
Zhang, Cheng
;
Hsu, Ta-Cheng
;
Han, Jung
We have demonstrated nitrogen-polar (000 (1) over bar) (N-polar) InGaN multiple quantum wells (MQWs) with significantly improved luminescence properties prepared by pulsed metalorganic chemical vapor deposition. During the growth of InGaN quantum wells, Ga and N sources are alternately injected into the reactor to alter the surface stoichiometry. The influence of flow duration in pulsed growth mode on the luminescence properties has been studied. We find that use of pulsed-mode creates a high density of hexagonal mounds with an increased InGaN growth rate and enhanced In composition around screw-type dislocations, resulting in remarkably improved luminescence properties. The mechanism of enhanced luminescence caused by the hexagonal mounds is discussed. Luminescence properties of N-polar InGaN MQWs grown with short pulse durations have been significantly improved in comparison with a sample grown by a conventional continuous growth method.
Temperature-dependent electrical transport properties in graphene/Pb(Zr0.4Ti0.6)O-3 field effect transistors
(2015-11-01)
Zhang, Xiao-Wen
;
Xie, Dan
;
Xu, Jian-Long
;
Zhang, Cheng
;
Sun, Yi-Lin
We report the temperature and gate voltage dependent electrical properties of PZT gated graphene field effect transistors (PZT-GFETs) in the vacuum atmosphere. The PZT-GFETs exhibit p-type characteristics which are attributed to the chemical doping induced the Fermi level shifting below the Dirac point. Meanwhile, it also shows a large memory window. The temperature dependencies of the source-drain current in the range of 20-300 K indicate thermally activated hysteresis behaviors. The hysteresis in the transfer characteristics of PZT-GFETs shows a simultaneous enlargement with increasing temperature. The hysteresis appears to stem from the screening of charges that are transferred from graphene to traps at the interface of PZT and graphene. The magnitude of the charge neutrality point under opposite gate voltage sweep are enhanced with the increase of temperature and gate voltage can be ascribed to the common effects of the temperature and voltage magnitude dependent mechanisms such as interface charge trapping process and the polarization effects of PZT films. (C) 2015 Elsevier Ltd. All rights reserved.