Congratulations! GIPO Professor Chi-Kuang Sun receives the 2008 Outstanding Electrical Engineer Award from the Chinese Institute of Electrical Engineering.

Congratulations! GIPO Professor Jian-Jang Huang receives the 2008 Excellent Youth Electrical Engineer Award from the Chinese Institute of Electrical Engineering. 

Congratulations! GIPO Professor Chi-Kuang Sun receives the title of “2009 IEEE Fellow”.

Ching-Fuh Lin, Professor of GIPO & President of IEEE LEOS' Taipei office, has published an introduction of the latest developments in Taiwan's optoelectronics research.  The article refers to several GIPO professors’ accomplishments and contributions in important fields. For the full article, please refer to the following link.

Congratulations! GIPO Professor Chi-Kuang Sun receives the title of “2009 SPIE Fellow”.

Highlights from the GIPO 2008 Christmas Luncheon

Time: December 24^th, 2008

Location: The atrium of Min Da Hall (3F), College of EECS, National Taiwan University

Highlights Compilation: Kuang-Chung Liu, President of the GIPO Student Association

2008 Christmas Luncheon, sponsored by the GIPO office and GIPO Student Association was held on December 24th, 2008 at the atrium of Min Da Hall, 3F. Three hundred students and professors attended, including GIPO Chairman Sheng-Lung Huang, Professor Lon A. Wang, Professor Chi-Kuang Sun, Professor Hoang-Yan Lin, Professor Jian-Jang Huang, Professor Yuh-Renn Wu, and Professor Jr Hau He. Scrumptious cuisine and sweet delicious drinks were served. In the midst of the fine dining and merry conversation, the year-end lottery began.

Nanostructures of Organic Light Emitting Materials

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

In the previous year, we have been working on the investigation of the arrangement and orientation of the organic thin films deposited on various surfaces. We have probed the organic molecular arrangement at the substrate in nano-scale with scanning tunneling microscope (STM). We deposited less than 0.1 mono-layer of NPB on silicon surface and investigate with STM. As shown in Figure 1, the NPB molecules can be seen clearly as the bright spots on the surface. The NPB molecules are evenly spread on the surface of the substrate. More amazingly, some of the molecules are form as dimers as shown in the Figure 2. The underlying interaction and physics of the formation of the dimers will be investigated this year. We also perform the photoelectron emission microscopy (PEEM) at National Synchrotron Radiation Research Center (NSRRC) to investigate the orientation of the organic electronic films on different substrates. From the spectra we can determine that CuPc molecules lie flatly on the Au surfaces, whereas the same molecules arrange perpendicular to the ITO surface with the same deposition condition. The result demonstrate that we can use PEEM to determine the stacking arrangement of the organic molecules at the surfaces with PEEM, which can be a good complement technology to STM and very useful to the nanotechnology projects.

Linearization of a Two-Axis MEMS Scanner Using a Differential Driving Scheme

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A driving scheme using a pair of differential voltages (V_x, V_y) over a bias voltage is proposed to linearize the dc characteristic (angle versus voltage) of a two-axis MEMS scanner. The micromirror has a gimbal-less structure and is driven by vertical comb-drive actuators in conjunction with a leverage mechanism (Figure 1). At an optimal bias voltage of 53 V, a linear optical scan range of ±3.2 deg. is achieved experimentally in both the x and y directions with the differential voltages ranging from −10 V to + 10 V (Figure 2, left).

For comparison, a non-differential driving scheme with two independent voltages, V_x and V_y, is implemented (Figure 2, right). In our experiment, V_x and V_y both vary from 0 to 16.5 V with an increment of 1.65 V. It can be seen that the pattern is profoundly distorted even with a small optical scan range of <±0.3 deg. The slight asymmetry is due to optical system misalignment and the non-uniformity among levers.

Figure 2. Scan patterns of differential (left) and non-differential (right) driving. © 2008 Institute of Physics (IOP) and IOP Publishing Ltd J. C. Tsai et al., “Linearization of a two-axis MEMS scanner driven by vertical comb-drive actuators,” J. Micromech. Microeng., Vol. 18, No. 1, 015015, Jan. 2008.   Temperature-Dependent Stability of On-Plastic a-Si:H Thin Film Transistors Professor I-Chun Cheng's group Graduate Institute of Photonics and Optoelectronics, National Taiwan University We investigated the temperature-dependent stability on the inverted-staggered back-channel-etched a-Si:H thin film transistors (TFTs) made at a process temperature of 150°C on plastic foil substrates. The shift of threshold voltage (Vt) increases with the stressing time and the stressing temperature. Different from TFTs made at temperatures of 300 °C or above, our low-temperature processed TFTs show an abnormal saturation of Vt at 50°C (323 K) in a constant gate-bias stress experiment. Around the same temperature, we observed abrupt increases in both the gate leakage current and the off current. Because of the low process temperature, the gate dielectric is less stable and more defective compared to that made at high process temperatures. A substantial amount of charges, trapped inside the dielectric during TFT fabrication and gate-bias stressing, was thermionically emitted into the channel by the Poole–Frenkel emission mechanism at a stressing temperature of 50°C, leading to the abnormal phenomena.

Analyzing the Optical Phase Conjugation Phenomenon via PSTD Simulations

Professor Snow H. Tseng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Turbidity Suppression via Optical Phase Conjugation is an optical phenomenon that uses the back propagation nature of optical phase conjugate light field to undo the effect of tissue scattering.  We use the computationally efficient and accurate pseudospectral time-domain (PSTD) simulation method to study this phenomenon; a key adaptation is the volumetric inversion of the optical wavefront E-field as a means for simulating a phase conjugate mirror.  Optical phase conjugation (OPC) phenomenon of a phase conjugate mirror may provide a means to disentangle the optical distortion caused by scattering of turbid media.

As shown in Fig. 1, PSTD simulation of light scattering through a macroscopic cluster of dielectric cylinders and reflected back by a PCM is shown.  After OPC, light back-traces and refocuses back to the original location where it first emerged.  Displacement effect (Dy) of the random media is further analyzed and shown in Fig. 2.  The refocused light pulse profile for random medium that is displaced by various Dy is shown in (a)-(f).  The ratio of the total refocused energy to the initial total energy for various Dy is shown on a semi-log scale in (g).  Notice that as Dy increases, the refocused light energy drops rapidly.

The PSTD simulation is a rigorous simulation technique capable of simulating light scattering phenomenon for large-scale problems.  Our simulation results provide important information to experiments to help understand the optical characteristics of OPC.      

Fig. 2. The displacement effect of the random medium on the OPC phenomenon.

Etching Depth Dependence of Emission Properties from InGaN/GaN Light Emitting

  Diodes with Nanohole Arrays: Analysis of Strain Relaxation and Surface States

Professor Yuh-Renn Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We apply our simulation models to analyze the etching depth dependence of emission characteristic of the InGaN/GaN quantum well. The effect of strain relaxation and surface states are discussed in this work. The device structures are shown in Fig. 1(a). Figure 1(b) and 1(c) show the strain tensors ε_xx with the hole depth equal to 16 nm and 23 nm, respectively.

Fig. 1 (a) A schematic of the nanohole structure. The hole depths are 16nm and 23nm. (b) and (c) show the calculated strain tensor εxx of the nanohole structure with 16nm hole depth and with 23nm-column hole depth, respectively. (d) shows the band structures and wavefunction of 16 nm nanohole structure at X=73.6nm and X=10.2nm, respectively.

As the strain data shown in Fig. 1, the nanohole structure with 23 nm hole depth might have a larger blue shift due to the strain relaxation. In 16 nm hole depth cases, because of the effect of surface states at the air/GaN interface, the fermi level is usually pinned at the surface states level (~ 0.9eV below conduction band for GaN). If the cap layer is thick enough, the effect of the surface states is negligible. However, the distance from the surface state to the quantum well region is only 2 nm in the cases. The pinning position of the surface state will strongly affect the band bending of the cap layer and the InGaN quantum well layer. When the top GaN layer is too thin as shown in Fig. 1 (d), the band bending at the GaN layer is not large enough so that potential at the left side of InGaN quantum well is lifted up, which reduces the QCSE significantly. Figure 2(a) shows the calculated emission spectrum at different position of the nanohole. Due to the surface states pinning effect, our results show that the emission rate is enhanced by 82 times and the maximum of 380 meV blue shift compared to the unetched region. The etched and unetched areas have a significant difference in the emission properties in Fig. 2(b). We can also find that for the hole depth larger than 11 nm (~ 7nm to the quantum well), the emission rates and emission peak start to have a significant change in Fig. 2(c). At X = 33.9 nm, the strain relaxation of quantum well and the emission rate reaches the maximum as shown in Fig. 2(d). Fig. 2(e) shows the emission peak and emission rates versus different positions X. The emission property of the nanohole changes significantly when the hole depth is close to or penetrates the quantum well. One is due to the effect of surface states and the other one is due to the strain relaxation.

Fig. 2 (a) shows the calculated emission spectrum. The emission spectrum at $X$ =10.2 nm and 32.6nm is multiplied by 10 times to make them clear. (b) shows the total emission rate and the emission peak energy versus different positions for the 16 nm nanohole structure. (c) The calculated emission strength and emission peak energy versus different hole depth. (d) shows the calculated emission spectrum of the 23 nm nanohole structure, and (e) shows the emission peak energy and the total emission rate versus different positions for the 23 nm depth nanohole structure.

In conclusions, we have analyzed the emission characteristic of InGaN/GaN LED with depth dependence of the nanohole structure. The emission property of the nanohole changes significantly when the hole depth is close to or penetrates the quantum well. One is due to the effect of surface states and the other one is due to the strain relaxation. Both effects lead to the blue shift of the spectrum and the increase of radiation recombination rates so that it is hard to be directly determined from experimental PL measurement. Our calculation provides useful information for analyzing the spectrum shift in the nanohole array and would be very important factors to be considered when making these similar structures such as nanocolumns and nanorods.

Electrical and Optoelectronic Characterization of a ZnO Nanowire Contacted by Focused-Ion-Beam-Deposited Pt

Professor J. H. He

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We report on the transport properties of single ZnO nanowires measured as a function of the length/square of radius ratio via transmission line method. The specific contact resistance of the FIB Pt contacts to the ZnO nanowires is determined as low as 1.1x10^-5 Ωcm². The resistivity of the ZnO nanowires is measured to be 2.2x10^-2 Ωcm. ZnO nanowire-based UV photodetectors contacted by the FIB-Pt with the photoconductive gain as high as ~10⁸ have been fabricated and characterized.

Figure 1.     (a) SEM image of the test structure of TLM measurement. (b) Total resistance as a function of the length over the square radius of ZnO NW. Figure 2.     (a) Photocurrent measurement as a function of excitation power intensity at applied bias of 0.5 volt. (b) Photoconductive gain as a function of excitation power intensity. (c) Time-dependent photocurrent rise and decay as obtained by sudden application (at 60 s) and removal (at 400 s) of UV light at the bias of 0.5 volt.