Professor Y. W. Kiang’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We report the theoretical and numerical study results of surface plasmon (SP)-dipole coupling based on a simple coupling model between a radiating dipole and the SP induced on a nearby Ag nanoparticle (NP). To include the dipole strength variation effect caused by the field distribution built in the coupling system (the feedback effect), the radiating dipole is represented by a saturable two-level system. The spectral and dipole-NP distance dependencies of dipole strength variation and total radiated power enhancement of the coupling system are demonstrated and interpreted. The results show that the dipole-SP coupling can enhance the total radiated power. The enhancement is particularly effective when the feedback effect is included and hence the dipole strength is increased. Figure 1 shows the geometry of the dipole-NP system, including a spherical Ag NP with radius R (=10 nm) centered at the coordinate origin and a radiating dipole located at (0, 0, a), which is represented by an arrow. Figure 2 (3) shows the dipole strength enhancement ratios, , of the x-oriented (z-oriented) dipole as functions of wavelength when the distance between the dipole and the Ag NP center, a, varies from 40 through 120 nm. Figure 4 (5) shows the total radiated power enhancement ratios as functions of wavelength at several a values for the x-oriented (z-oriented) dipole. The power ratio is defined as the total radiated power of the dipole-NP system over that in the case without the Ag NP.

Figure 1	Figure 2	Figure 3
Figure 4 Figure 5

 

Professor Hao-Hsiung Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Previous studies have shown that bonds in alloy resemble their original lengths in binaries. Quaternary InAsPSb contains two bonds, InP and InSb, that are with very large mismatch in length. This mismatch leads to strong bond distortion in the quaternary, which could affect the structural, electronic and optical properties. In this work, we investigate the structural properties of InAsPSb grown on GaAs substrate using reciprocal space mapping (RSM) and P K-edge and In K-edge extended X-ray absorption fine structure (EXAFS). Valence force field (VFF) is used to simulate the bond distortion in the quaternary. Fig. 1 shows the Fourier transformed In K-edge EXAFS signals. Two peaks representing InP and InSb bonds are clearly seen when As composition is low and gradually merge into a broad peak with the increasing As composition. Detailed analysis show that the bond lengths are close to their original values in binaries. Fig. 2 shows RSM results of a InAsPSb samples. Although the lattice mismatch between of these alloys and GaAs is ~7% and the layer thicknesses are 1000 times thicker than the critical thicknesses, strong residual strain exists in these layers. We used VFF model to calculate the bond distortion. Fig. 3 shows the distortion energy as a function of a_z/a_xy for each sample. As can be seen, the distortion energy is much larger than the strain energy and the residual strain, in term of a_z/a_xy ratio, is proportional to the distortion energy. This finding suggests that the randomly oriented bond distortion could hinder the generation of dislocation in the alloy, leading to the observed residual strain.

Fig. 1 Fourier transformed In K-edge EXAFS signal of InAsPSb.	Fig. 2 (004) and (-1-15) RSM measurements of InAs0.04P0.73Sb0.23 show a az/axy ratio of 1.017, resulting from the residual strain in the 1-mm-thick sample.

Fig. 3 Distortion energy as functions of az/axy for InAsPSb alloys. Symbols indicate the az/axy ratio obtained from RSM measurement for each samples.

 

Professor Ching-Fuh Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Producing ultra-thin Si wafer with low Si material waste is an important issue to achieve grid parity of Si solar cells. Although Si wafer with a thickness of 20μm can be fabricated by epitaxial techniques, these techniques need rigorous process environment, expensive equipment and high power consumption, limiting the potential of cost reduction. In this report, we explore a procedure involving very fast and low-cost process to fabricate crystalline Si thin foils of completely single domain from (100) Si bulk wafer.

The low-cost procedure is a multi-step metal-assisted etching (MAEtch). Figure 1 shows the procedure of forming Si thin foil using multi-step MAEtch. First of all, the Ag used as catalytic agent for MAEtch is deposited on the Si surface with photoresist (PR) pattern. The solution of electroless metal deposition consists of AgNO₃ and HF, which allows Ag⁺ to become silver dendrites on Si. Second, the multi-step MAEtch has two major steps including anisotropic and isotropic etching. The solution of the anisotropic etching (1^st step MAEtch) is composed of H₂O₂ and HF to form vertically-aligned Si micro-holes. In aqueous solution of H₂O₂ and HF, Ag dendrites will sink into Si substrate vertically. Then, at the other MAEtch step (2^nd step MAEtch), the Si micro-holes from the 1^st step MAEtch are undercut for lift-off of Si thin-foil. The solution of 2^nd step MAEtch contains a large H₂O₂/HF ratio to achieve isotropic etching, and thus Si micro-holes could be undercut from the bottom. Figures 2(a) and (b) are the SEM images of the micro-holes formed in 1^st step MAEtch, where vertical micro-holes can be observed. In Fig. 2(c) and (d), the micro-hole undercut with isotropic etching is also shown, and the thin foil can be lifted-off after 7-min of 2^nd step MAEtch. By multi-step metal-assisted etching, the entire procedure takes no more than 20 minutes to produce the Si thin foil with thickness of 15 μm.

The Si thin foil with micro-hole structure could induce strong light trapping effect, leading to high absorption. Figure 3 shows that the absorption of the Si thin foil is over 85% from 350 nm to 960 nm, exceeding the calculated absorption of planar Si thin foil with the same thickness. This is due to the fact that the back side of the thin foil provides excellent anti-reflection, and multiple internal light scattering is induced by the micro-hole structure. Finally, combined with solution-processed conducting polymer, the Si thin foil/PEDOT:PSS solar cell is demonstrated. The structure of the solar cell is depicted in inset of Fig.4. The interface between the PEDOT layer and n-type Si can form a good heterojunction for separation of electron-hole pairs, and it reveals rectifying characteristics as shown in Fig.4. In conclusion, the Si thin foil has potential for low-cost solar cell devices, and its light-weight and ultra-thin properties can enable wide applications.

Figure 1. (a) Deposition of silver on the PR -patterned Si surface. (b) 1st step MAEtch to form vertically-aligned Si micro-holes. (c) 2nd step MAEtch to undercut the Si micro-holes. (d) Lift-off of the Si thin foil from Si substrate.		Figure 2. (a) & (b) are side-view and top view of the Si micro-holes formed by 1st step MAEtch, respectively. (c) Side view of Si micro-holes after 2nd step MAEtch for 2min. (d) Side view of Si micro-holes after 2nd step MAEtch for 5min.
Figure 3. Absorption spectra of the Si thin foil.		Figure 4. J-V characteristics of Si thin foil/PEDOT solar cell under dark and light conditions. Inset picture: Device structure of the Si thin foil/PEDOT solar cell.

 

Professor Lung-Han Peng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

To explore the carrier transport properties in the short channel regime, we used the technique of electron-beam lithography to define transistors with drain-source distance L_DS of 1μm, and gate length of L_g= 500, 200, 100, and 50nm on the same wafer where the []-GaN/Ga₂O₃ NWs were grown. GaN/Ga₂O₃ nanowire metal-oxide-semiconductor field-effect-transistors (NW-MOSFETs) are shown to operate at average electron velocity of ~1.24×10⁷cm/sec and threshold-voltage roll-off of -0.2V as the transistor gate length L_g reduced from 500 to 50nm. Improvement of saturation current to 120μA and unity current/power-gain cut-off frequency to 150/180GHz are observed on L_g=50nm devices. This concurs with doubling of the device peak transconductance g_m value from 42μS of the long-gate value to 84μS of the short gate devices. Moreover, a drain induced barrier lowering (DIBL) factor (measured at a drain current of 10 mA/mm for V_DS =1 and 4V) below 35mV/V was observed in the GaN/Ga₂O₃ NW-MOSFETs compared with a DIBL value of ~50mV/V commonly observed in the planar-type devices. Our study reveals the advantages of using (i) polarization-induced positive charge and high-k dielectric at {}GaN/{002}Ga₂O₃ to provide carrier confinement and to shield the drain field, and (ii) polarization-induced negative charge at (0001)GaN/sapphire to form back-barrier to suppress leakage and improve the short-channel transport properties.

Ref: C.-W. Yu et al., “Short Channel Effects on Gallium Nitride/Gallium Oxide Nanowire Transistors,” Appl. Phys. Lett. 101, 183501 (2012).

 

 

 

Professor Sheng-Lung Huang's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The aim of cancer therapies is mainly to stop cell proliferation or induce cell death. Cell death regulation is important for normal development and homeostasis. Cancer cells escape from death signals and continue their abnormal proliferation. Therefore, the ability to induce death in cancer cells has been a crucial biomarker for the efficacy of chemotherapeutic agents. However, individual cancer cells, even from the same population, vary greatly in their response to cell death stimuli. Measuring the response at the single-cell levels provide further pharmacokinetic and pharmacodynamics information, which aids drug development and regimen design. Developing a microscopic technology with non-invasive, in situ, label-free, single-cell spatial resolution may serve this long-term need. We report the detection of cell death at the single-cell level using ultrahigh-resolution optical coherence tomography (UR-OCT). An improvement in OCT technology currently provides axial resolution to approximately 1 μm and lateral resolution to 2 μm. We demonstrated that UR-OCT not only provides three-dimensional in situ single-cell imaging but is also able to delineate subcellular structure (i.e., the nucleus). Dead cells cannot be differentiated from live cells based merely on size. Many parametric analytic methods have been used to address this issue, including speckle fluctuation in time-lapse images. It was confirmed that back-scattering signals are lower in apoptotic cells, which is most likely due to the perturbation of mitochondria morphology during apoptosis. Nuclear disintegration after chromatin condensation provides high-signal-intensity peaks that facilitate the identification of apoptotic cells.

With the collaboration between GIPO team at National Taiwan University (NTU) and Professor Jeng-Wei Tjiu’s team at the NTU Hospital, a homemade UR-OCT system was developed to image single-cell basal cell carcinoma (BCC) in three dimensions and differentiate between live and dead BCC cells by not only morphological recognition but also parametric analysis. The BCC cell line was used because BCC is the most common skin cancer, and we are familiar with it. An image analysis approach was also developed to automatically extract deterministic information of a single cell.

This result is featured at http://www.octnews.org in October 2012.

Fig. 1. Microscopic image of BCC cell line and UR-OCT/confocal microscopy images of live and dead BCC cells. BCC cell line was suspended in Matrigel and imaged by a bright-field microscope (a). Live and dead BCC cells were randomly selected from the sample and scanned by UR-OCT and confocal microscopy. Live cell were encoded in green (b) and dead cell which was were encoded in red (e), according to the calcein and propidium’s fluorescence spectrum. Two-dimensional cross-sectional imaging (c, f) were performed across the center of each cell. Three-dimensional imaging of the whole cell (d, g) were realized by combining several cross-sectional images whose covering range were slightly larger than the size of the cells.

Professor Chi-Kuang Sun’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Led by NTU GIPO, laser-based nanoultrasonic technology is one of the most innovative tools for the noninvasive sub-surface nano-imaging. For the present nano-fabrication processes (22/32/45nm), it provides a possible solution to nondestructively monitor the yielding rate for mass production. Thanks to the piezoelectric multilayer structures and the ultrafast optical coherent control, we successfully photo-generated and synthesized the acoustic waveform with <10 nm wavelength, which determined the longitudinal resolution of a nanoultrasonic image. However, the lateral resolution is still severely restricted by the optical diffraction limit. Besides the previously demonstrated saturation technology, one way to improve the lateral resolution is to use nanorods as nanoultrasonic sources and probes. In order to realize the application, our laboratory currently has devoted to the comprehensive investigation of nanoacoustic vibrations in nanorods, including multiple orders of extensional and radial breathing modes. Figure 1 shows the SEM picture of the GaN nanorod array. We successfully measured several quantized (confined) vibrational modes in the studied nanorods, of which the frequencies agree well with the predicted results by using a finite element method (FEM) simulation. Figure 2 shows the oscillation frequency of the fundamental radial breathing mode as a function of rod diameter. We discovered that the mechanical properties, e.g. elastic stiffness constant C₁₁, in piezoelectric GaN nanorods start to differ from those of bulk GaN when the rod diameter shrink down to the order of or less than 50nm. That is, the softening effect in nanorods was observed once the rod diameter was reduced to <50 nm. In brief, we have well characterized the various confined vibrational modes of nanorods. The explored information serves as a foundation for future nanorod-integrated nanoultrasonic technologies.

Figure 1 (a) SEM picture of nanorods array. The average length is 1150 nm and the average diameter is 100 nm. (b) Observed acoustic spectrum and FEM simulation of the mode shapes of the extensional modes.

 

Figure 2 Measured oscillatory frequency as a function of rod diameter. The horizontal error bars represent the diameter nonuniformity based on the SEM images and the vertical error bars represents FWHM of the amplitude in Fourier-transformed oscillatory spectra. The black curve shows the theoretical oscillation frequencies of the radial breathing modes.