4th U.S. Air Force - Taiwan Nanoscience Initiative Workshop :: Agenda

Agenda

Thursday, 8 February

07:30-08:10	Registration Athletics/Alumni Center
08:10-08:30	Welcoming remarks Dr. Harold Weinstock (AFRL/AFOSR), Prof. Paul Chu (University of Houston), Prof. M. K. Wu (Academia Sinica), Dr. Jim Chang (AFRL/AOARD), and Dr. Jack Agee (Rice University)
08:30-10:00	Session 1: Silicon Nanostructures
10:00-10:15	Break
10:15-11:30	Session 2: GaN and InN Nanostructures
11:30-13:30	Buffet Lunch
11:45-13:30	Poster Set-Up
13:30-14:45	Session 3: Electro-optical properties of Nanostructures
14:45-15:00	Break
15:00-16:15	Session 4: Biological Applications & Toxicity of Nanomaterials
16:30-18:00	Poster Session I
18:00-20:00	Dinner
20:00-21:30	Poster Session II

Friday, 9 February

08:30-09:30	Session 5: Carbon Nanotubes
09:30-09:45	Coffee/Tea Break
09:45-11:30	Session 6: Nanostuctured Sensors
11:30-13:15	Buffet Lunch
13:15-14:45	Session 7: Mechanical Properties of Nanostructures
14:45-15:00	Break
15:00	All Posters Taken Down
15:00-16:20	Session 8: Nanostructures for Computing & Sensing
16:20	Closing Remarks Dr. Harold Weinstock (AFRL/AFOSR) and Prof. M. K. Wu (Academia Sinica)
18:00	Optional Evening Event Click here for more information

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Session 2: GaN and InN Nanostructures

10:15–10:30
Enhanced Photoconduction and Surface Conduction in GaN and InN Nanowires
Kuei-Hsien Chen^1,2*, Reui-San Chen¹, Hsin-Yi Chen^1,3, Chien-Yao Lu¹, Li-Chyong Chen², Yang-Fang Chen^2,4, Ying-Jay Yang³
¹Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan,
²Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan,
³Graduate Institute of Electronic Engineering, National Taiwan University, Taipei 10617, Taiwan,
⁴Department of Physics, National Taiwan University, Taipei 10617, Taiwan

The advent of nanotechnology brings forth materials of reduced dimensionality and size, which renders novel transport phenomena such as two-dimensional electron gas, one-dimensional Luttinger liquid, zero-dimensional Coulomb blockade, etc. However, it remains unclear how the carrier transport processes evolve from conventional bulk behavior to surface-controlled nature in the semiconductor nanowires beyond quantum confinement regime. In this paper, we show for the first time that a significant increase in charge collection efficiency caused by spatial separation of carriers in nanowire with reduced diameter leads to a dramatic increase in its conductance, both under dark and upon light illumination. For instance, more than 100 times enhancement in the photoconduction responsivity and gain is reported when the size of GaN naniwires is reduced from 250 to 100 nm. A new transport mechanism in nanowires based on core-shell structure is proposed. By way of photoelectrochemical surface treatment, the contribution from the shell and the core can be distinguished. It is concluded that carrier separation occurs in the photoexcited GaN nanowires with electrons and holes accumulated at the surface and the core, respectively, resulting in ultra long lifetime of the carriers. Such semiconductor nanowires with efficient charge transport and collection show great promise as functional building blocks for solar energy harvesting, sensing, photocatalytic, and photochemical devices.
10:30–10:45
Optimized structure design for high extraction efficiency LED Growth on pattern substr
Jung-Min Hwang ^1,3, Yu-Zong Shu¹, Kei-May Lau² and Huey-Liang Hwang¹
¹Department of Electrical Engineering, National Tsing-Hua University, Hsin-Chu 300, TAIWAN
²Departments of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Hong Kong
³Devision of LED, IDTI.inc, Hsin-Chu 300, TAIWAN

Recently, high efficiency white LEDs have gained much interest because the replacement of fluorescent lamps could be realized in the near future. Nowadays the high values of internal extraction efficiency have already been obtained because of the mature growth of epitaxy technology. However, the light extraction efficiency is always small due to the total internal reflection effect. In order to enhance the light extraction efficiency of a high power operated flip-chip LED, we designed the different patterned structures between the sapphire layer and GaN layer. The patterned surface can reduce the total internal reflection and increase the probability of light escaping from the chip. In this work we designed different patterned surfaces to simulate the enhancement of light extraction efficiency of a GaN based flip-chip LED. The ray-tracing software employed the Monte-Carlo algorithm for ray-tracing. The optimized structure for high extraction efficiency LED growth on pattern substrate was designed. The basic model in the simulation is a GaN based flip-chip LED. The flip-chip consists of a sapphire layer and a GaN/n-GaN/InGaN MQW/p-GaN layers. Part of the InGaN MQW layer and the p-GaN layer are removed to expose the n-GaN layer, and the n-type contacts are formed on the region. Under the p-GaN layer, there is a thick p-type contact which acts as a reflective mirror. Lastly the chip is soldered to a silicon sub-mount.
The actual structure sizes and optical properties of each layer are specified. The chip area is 338x358 um2 and the peak wavelength of the light is 470nm. In addition, in the electron-hole pairs recombination process, the light rays were generated in the active layer of LED with a uniform random distribution. So we set the light source of the flip-chip model with a uniform distribution and site on the top surface and bottom surface of the active layer. Based on the model, we designed different rough structures between the sapphire layer and GaN layer in the simulation.
10:45–11:00
GaN Overgrowth on GaN Nanocolumns by Molecular Beam Epitaxy
J. D. Albrecht and R. Cortez
Air Force Research Laboratory, Sensors Directorate, WPAFB, OH
K. Averett, J. E. Van Nostrand, and J. Boeckl
Air Force Research Laboratory, Materials and Manufacturing Directorate, WPAFB, OH
C.C. Yang
Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan

GaN nanocolumns grown by plasma-assisted molecular beam epitaxy are explored as host structures for overgrowth of bulk-like GaN films. We investigate the layers of vertical GaN nanocolumns 90+/-10 nm in width which were grown on Al2O3(0001) and Si(111) substrates. We present photoluminescence, x-ray diffraction, and microscopy data that indicates that the columns are unstrained, low-defect wurtzite GaN nanostructures. The nanocolumns form discontinuous layers with areal densities controlled by grown conditions. The nanocolumn layers are subsequently overgrown with thick GaN films and compared with commercial GaN template materials used for device fabrication. The overgrowth of GaN is studied as a function Ga flux and growth temperature. We present photoluminescence and Hall effect characterization of the overgrown layers. The microstructure and morphology are probed by atomic force and transmission electron microscopy, as well as x-ray diffraction.
Cleared as AFRL-WS 05-2768.
11:00–11:15
Spin-splitting 2DEG in Al_xGa_1-xN/GaN and its application
Ikai Lo
Department of Physics, Center for Nanoscience and Nanotechnology,
National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C.

We observed the beating pattern of Shubnikov-de Haas (SdH) oscillations for 2DEG in AlGaN/GaN heterostructures (x = 0.25) with spin-splitting energy 9.0 meV [1]. The large spin-splitting energy of the 2DEG was attributed to the anti-crossing of conduction bands [2]. The beating pattern was reproduced on another sample with a different alloy composition (x = 0.22). By using a persistent photoconductivity [3], we have studied the spin-splitting pattern as a function of carrier concentration at different temperatures. We eliminated the possibility of mageto-intersubband scattering by temperature-dependent SdH measurement, and also eliminated the possibility of inhomogeneity effect [4] due to the absence of negative magneto-resistance, which resulted from a weak anti-localization effect produced by the inhomogeneity effect. Based on the results, we proposed a p-wave-enhanced InGaN/InAlN quantum well [3] for the application of spin-polarized field-effect transistor designed by Datta and Das [5].
[1] I. Lo, et al., Prys. Rev. B 65, 161306R, (2002) [Virtual Journal Nanoscale Science & Technology (4/29/2002)].
[2] I. Lo, et al. Appl. Phys. Lett. 88, 082108 (2006) [Virtual Journal Nanoscale Science & Technology (3/6/2006)].
[3] I. Lo, et al., Phys. Rev. B 74, 245325 (2006) [Virtual Journal Nanoscale Science & Technology (7/3/2007)].
[4] N. Thillosen, et al., Phys. Rev. B 73, 241311(R), (2006).
[5] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).
11:15–11:30
Nitride Semiconductor Nanostructures and Nano-photonics for Efficient Optoelectronics Devices
Cheng-Yen Chen, Chi-Feng Huang, Yung-Sheng Chen, Wen-Yu Shiao, Dong-Ming Yeh, Kun-Ching Shen, Tsung-Yi Tang, Chih-Feng Lu, Yen-Cheng Lu, Cheng-Hung Lin, Shao-Ying Ting, Jeng-Jie Huang, Ming-Hsiu Chang, Tzu-Chi Liu, and Chih-Chung (C. C.) Yang*
Graduate Institute of Electro-Optical Engineering, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei, Taiwan
* presenter and contact author
K. L. Averett and J. E. Van Nostrand
Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Ohio
J. D. Albrecht
Air Force Research Laboratory, Sensors Directorate, Wright-Patterson Air Force Base, Ohio

This presentation includes two parts. The first part is to discuss the growth of GaN nano-columns and their overgrowth for coalescence. This research represents a cooperation effort between US Air Force Research Laboratory (AFRL), Dayton, and National Taiwan University (NTU), Taipei. We will report the nano-material and optical analysis results of the GaN nano-columns grown with MBE at AFRL and the coalescence overgrowth samples with MOCVD at NTU. The growth of GaN nano-columns means to prepare threading dislocation-free GaN templates. The coalescence overgrowth means to implement flat GaN templates for optoelectronics and electronics device fabrications. The second part is related to the study of nano-photonics, particularly related to the evaluation of the feasibility of using the coupling process of surface plasmon (SP) with InGaN/GaN quantum wells (QWs) for enhancing the quantum efficeincy of such a light-emitting diode (LED). In such a device, a metal layer, such as silver, aluminum, and gold, of various morphologies is coated on the semiconductor surface close to the QWs. SP modes of various dispersion relations are generated at the interface between the semiconductor and the metal layer. The evanescent fields in the semiconductor extend into the QW layers such that the dipoles in the QWs can couple with the SP fields. Through the coupling process, the dipole energy can be transferred into the SP modes, similar to the process of stimulated emission. In this situation, if the SP modes can well phase-match with the emitted photons, radiation can occur through the SP coupling.