Congratulations to GIPO Professor Yuh-Renn Wu for the honor of being awarded "The 2^nd Leap Fellowship of FAOS (Area of Expertise: Optoelectronics, Physics of Semiconductor Devices, Illuminance and Energy Technology)" from the Foundation for the Advancement of Outstanding Scholarship.

Performance Enhancement of P-type Tellurium Thin-Film Transistors via Ultralow-Temperature Deposition and Al₂O₃ Passivation

Professor I-Chun Cheng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Tellurium (Te), a group VI-A quasi-two-dimensional material, has recently garnered renewed interest as a promising p-type elemental semiconductor for transistor applications, owing to its relatively low processing temperature and excellent electronic properties. However, when Te is used as a transistor channel layer, rapid and uncontrollable nucleation during deposition often leads to the formation of small, randomly oriented crystals, which can degrade device performance. In this work, we demonstrate Te thin-film transistors (TFTs) with enhanced performance by depositing Te channel at ultralow temperature and introducing Al₂O₃ backchannel passivation. Fig. 1 shows the schematic diagram and optical micrograph of the inverted-staggered bottom-gate p-type Te TFT. The fabrication process began with the deposition of a 50 nm-thick Mo layer on a glass substrate, patterned via a lift-off technique to form the gate electrode. A 50 nm-thick HfO₂ layer was subsequently deposited by atomic layer deposition (ALD) to serve as the gate dielectric. The Te channel layer was then deposited by thermal evaporation at ultralow substrate temperatures, with its pattern defined using a lift-off process. A second 50 nm-thick Mo layer was deposited by DC magnetron sputtering and patterned to form the source and drain electrodes. Finally, a 10 nm-thick Al₂O₃ layer was deposited by ALD to serve as the back-channel passivation layer. Fig. 2(a) show the transfer characteristics of the Te TFTs with various channel thicknesses. The associated on/off current ratios and field-effect mobilities as functions of channel thickness are plotted in Fig. 2(b). A distinct trade-off is observed. As the channel thickness increases, the field-effect mobility improves, while the on/off current ratio decreases. The TFT with a 7 nm-thick channel exhibits a field-effect mobility of 18.1 cm²V^-1s^-1, on/off current ratio of 5.5 ´ 10³, and threshold voltage of 4.65 V. The grazing-incidence X-ray diffraction (GIXRD) analysis, shown in Fig. 2(c), confirms that the ultralow-temperature deposition promotes favorable crystallographic orientation (100), contributing to enhanced electronic transport properties. These results highlight the potential of Te TFTs for low-temperature processed CMOS applications, particularly in emerging fields such as flexible and wearable electronics.

Umbrella-like dual-axis mirror scanner fabricated with FDM 3D printing

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We present a low-cost mirror scanner design of umbrella-like architecture. The primary structures are 3D-printed using PLA (polylactic acid) as the material. A 400 mm² aluminum-deposited silicon chip is fixed on the mirror frame, which is part of the 3D-printed assembly, and serves as the reflective mirror. Miniature permanent magnets are embedded in the mirror frame so that the mirror can then be electromagnetically driven. Our latest-version scanner reaches optical scan angles of 7.8 and 7.4 degrees for the x- and y-scans at their resonant frequencies (37 Hz and 49 Hz), respectively. Under DC actuation, a full optical scan range of 10.1 degrees in the x- direction can be achieved when the voltage is tuned from +12 V (+0.089 A) to -12 V (-0.089 A). Additionally, we also demonstrate the integration of a mirror scanner with a smart phone camera to achieve a dynamic FOV (field of view).

© IEEE
Ming-Te Chiang, Yi-Wen Cheng, Wei-An Tsui, Pin-Hung Yeh, Ching-Kai Shen, and Jui-che Tsai, “Umbrella-Like Dual-Axis Mirror Scanner Fabricated With FDM 3D Printing and Its Application in Realizing a Dynamic FOV Camera,” IEEE Photonics Journal, VOL. 17, NO. 2, 6600506, Apr. 2025.

OLED Simulation Development: Carrier Transport and Exciton Dynamics in Randomly Doped Host–Guest Systems

Professor Yuh-Renn Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Organic light-emitting diodes (OLEDs) present unique challenges in device physics due to the interplay of carrier transport, exciton generation, and recombination in host-guest systems. Our recent work focuses not only on analyzing these mechanisms but also on the development of advanced simulation tools that capture the stochastic nature of dopant distributions and their impact on device performance. By integrating Poisson drift-diffusion equations, Gaussian density of states (DOS), and Poole-Frenkel field-dependent mobility into a two-dimensional random doping framework, we have created a versatile software platform capable of resolving local dopant concentration variations and their influence on charge mobility and recombination zones.

Coupled exciton diffusion equations extend the model to track singlet, triplet, and doublet excitons, incorporating Förster resonance energy transfer (FRET), Dexter transfer (DET), intersystem crossing (ISC), reverse ISC (RISC), and exciton–polaron quenching. This simulation framework reproduces experimental J-V characteristics and efficiency roll-off in CzDBA:TPA-PyBTM’ radical OLEDs, while identifying dominant loss channels such as triplet–triplet annihilation (TTA) and doublet–polaron quenching (DPQ). Beyond radical-based devices, the platform has been applied to hyperlayer triplet–triplet fusion (TTF) OLEDs, where exciton diffusion and suppression of triplet–polaron quenching are critical for achieving high internal quantum efficiency.

Our group has developed advanced OLED simulation technologies ranging from traditional one-dimensional device solvers to two-dimensional random-doping models that capture realistic material and device characteristics. By incorporating exciton-carrier coupling analysis, these methodologies provide deeper understanding of carrier transport, recombination, and efficiency-loss mechanisms in OLEDs. Based on these efforts, we established the DDCC software platform [1], a comprehensive simulation environment that combines drift-diffusion transport, Gaussian density-of-states modeling, Poole-Frenkel mobility, and exciton dynamics. DDCC is now available to the research community and serves as a powerful tool for the analysis and design of next-generation OLED technologies.

Sidewall Optical Leakage and Scaling Limits in Sub-20 µm AlGaInP Micro-Resonant Cavity LEDs

Professor Chao-Hsin Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We are delighted to present some of the latest developments that have been made by us in the field of advanced AR displays and VLC systems. Our research explores the optical properties of the AlGaInP micro-RCLEDs with dimensions below 20 µm and operating at 620 nm. [1]

Through our experiments, we showed that micro-RCLEDs with 13×20 µm² have strong spectral narrowing (FWHM < 10 nm) and excellent current-dependent wavelength stability. The reason why we observed unexpected high divergence in the far-field measurements turned out to be sidewall emission, which was proved by using hybrid optical model involving transfer matrix method combined with geometric optics. Indeed, as mesa size is smaller than the optical recycling length, the emitted light from lateral cavity modes cannot be absorbed any longer.

This demonstrates an inherent scaling compromise between vertical confinement of the cavity and directional emission of the beam. The essential requirement to overcome this problem is thus to implement appropriate sidewall emission control schemes for micro-RCLEDs of high-resolution applications. Our simulation results show that by using a sidewall mirror made of gold (Au), we can mitigate lateral escape and redirect photons forward, leading to a remarkable enhancement of light coupling into a ±20° acceptance cone AR waveguide, increasing the coupling efficiency (η_20°) from 8.4% to 20.0%.

Reference:

[1] N. Prechatavanich, M.-J. Wu, C.-W. Huang, C.-K. Yee, T. Sutheebanjerd, Y.-T. Tseng, Z.-A. Lin, Y.-H. Chen, K.-H. Lin, C.-C. Wu, and C.-H. Wu, "Sidewall Optical Leakage and Scaling Limits in Sub-20 µm AlGaInP Micro-Resonant Cavity LEDs," 2026.

A New Fast-Response VA-FFS LC Device Using Three-Electrode Design with Improved Transmission for VR Applications

Professor Wing-Kit Choi

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have proposed a new three-electrode VA-FFS liquid crystal device structure that improves optical transmission while maintaining the fast response required for VR/AR display applications.

Liquid crystal devices with very fast response time are important for applications such as virtual reality (VR) and augmented reality (AR) displays, where high-speed operation is required. In this work, we propose a new design to enhance the optical transmission of fast-response VA-FFS (Vertically Aligned Fringe-Field Switching) liquid crystal devices while maintaining their fast switching characteristics.

In conventional 3D VA-FFS structures, disclinations or “virtual walls” often appear near the center of the pixel, which can reduce the brightness of the switched state. To address this issue, we introduce a three-electrode design that generates additional fringing electric fields to help switch LC molecules in the central region of the pixel.

Simulation results show that the proposed design can effectively reduce the dark disclination region and improve the transmission of the device while maintaining the fast response characteristics of VA-FFS LC modes. These results suggest that the proposed structure provides a promising approach for high-speed liquid crystal devices used in VR/AR display systems.

Reference:

[1] W.K. Choi, B.K. Tseng, and C.H. Tung, “A New Fast-Response VA-FFS LC Device Using Three-Electrode Design with Improved Transmission for VR Applications,” SID Symposium Digest of Technical Papers, pp. 2111–2113, 2025.

Deep Learning for Retinal Lesion Detection in Multimodal Ophthalmic Imaging

Professor Hsiang-Chieh Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

This study introduces a high-performance deep learning framework for the automated detection and classification of retinal lesions, with a specific focus on myopic choroidal neovascularization (mCNV). Distinguishing between active and inactive mCNV is vital to preventing irreversible vision loss, yet traditional diagnostic methods such as fluorescein angiography are invasive and time-consuming. The proposed methodology leverages a multimodal approach by integrating Optical Coherence Tomography (OCT) and OCT Angiography (OCTA) images, providing both structural and functional retinal insights. To address the limitations of conventional Convolutional Neural Networks (CNNs) in capturing long-range dependencies, the framework adopts a Swin Transformer architecture featuring a hierarchical shifted-window design. Preliminary results using a VGG16 architecture on single-modality OCT data demonstrate a testing accuracy of 92%, a precision of 89%, and an AUC of 93%. Grad-CAM visualizations confirm the model's ability to accurately localize lesion-related regions. The research concludes that transitioning to a Swin Transformer-based multimodal system will significantly enhance diagnostic robustness and clinical utility. This work highlights the potential of integrated multimodal learning to support large-scale ophthalmic screening and clinical decision-making.