GIPO newsletter

Ultrahigh Responsivity and Superior Sensitivity Graphene-semiconducting Light Absorber Hybrid Phototransistors

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Graphene−semiconducting light absorber hybrid photodetectors have attracted increasing attention because of their ultrahigh photoconductive gain and superior sensitivity. However, most graphene based hybrid photodetectors reported previously have shown a relatively long response time caused by numerous long-lived traps in these hybrid systems, which greatly restricts device speed. In our studies, we found graphene/light-harvesting materials (such as thieno[3,4-b]thiophene/ benzodithiophene polymer (PTB7) and Bismuth(III) iodide (BiI₃)) hybrid photodetectors have high responsivity and short photocurrent response time. Figure 1 shows the graphene-PTB7 hybrid photodetectors fabricated on self-assembled-monolayer functionalized SiO₂ substrates with a maximum responsivity of ~ 1.8 ×10⁵ A/W and a relatively short photocurrent response time of ~ 7.8 ms.

Besides, the graphene/BiI₃ vertical heterostructure phototransistors also have a maximum responsivity of about 6×10⁶ A/W , EQEs of up to 10⁶ % in the visible light region, relatively short photocurrent response times of ~ 8 ms, and detectivity of 7×10¹⁴ Jones under the SNL condition. Compared to BiI₃ on bare SiO₂ substrates, the thermally evaporated BiI₃ films on graphene sheets possessed nearly flatter morphologies and remarkably better crystallinities because of the weak van der Waals interactions between graphene and BiI₃. In Figure 2, the photoemission spectra of graphene/BiI₃ heterojunctions also manifest the evidence that no chemical interactions occur between graphene and BiI₃, resulting in the van der Waals epitaxial growth. The measured band bending consistently illustrates that photo-induced charge transfer occurs at the graphene/BiI₃ interface. The high-quality crystal growth of the layered metal halide BiI₃ could make graphene/BiI₃ heterostructures a promising building block for high-performance optoelectronic applications.

Figure 1. (a) Schematic illustration of a graphene−PTB7 hybrid photodetector (b) Rise time and (c) fall time of the device

Figure 2. SEM images of 50 nm thermally evaporated BiI₃ film on (a) SiO₂ and (b) graphene/SiO₂. Photoemission spectra of graphene with incrementally increased BiI₃ film thickness. (c) XPS spectra of I_3d5/2 and Bi_4f. (d) UPS spectra for SEC (left) and VB (right) regions, respectively. (e) C_1s peak-fitting XPS spectra. (f) The onset values of the SEC spectra (black dot) and the corresponding WFs (blue dot) as a function of the BiI₃ thickness. The inset shows the VB maximum position of bulk BiI₃. (g) Individual electronic structures of pristine graphene and BiI₃. The energy level alignment and band bending at the graphene/BiI₃ heterojunction are depicted in the bottom panel.

Reference:

1. Chang P. H., Tsai Y.C., Shen S. W., Liu S.Y., Huang K. Y., Li C. S. Chang, H. P., and Wu C. I., Highly Sensitive Graphene−Semiconducting Polymer Hybrid Photodetectors with Millisecond Response Time, ACS Photonics 2017, 4, 2335−2344

2. Chang P. H., Li C. S., Fu F. Y, Huang K. Y, Chou A. S., and Wu C. I., Ultrasensitive Photoresponsive Devices Based on Graphene/BiI3 van der Waals Epitaxial Heterostructures, Advanced Functional Materials 2018, 28, 1800179

Powerful LED-based train headlight optimized for energy savings

Professor Guo-Dung J. Su's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A new LED-based train headlight that uses a tenth of the energy required for headlights using conventional light sources has been designed. If operated 8 hours every day, the electricity savings of the new design would reduce emissions of the greenhouse gas carbon dioxide by about 152 kilograms per year.

Train headlights not only illuminate the tracks ahead, they also play an important role in rail transportation. Because trains are difficult to stop, the headlights must be visible from a distance far enough away to give people or vehicles on the tracks ample time to move out of the way. Traditional train headlights, which use incandescent or halogen bulbs, are bright enough to meet safety regulations but are not very energy efficient because most of the energy powering the light is converted into heat rather than visible light.

We were approached by the engineering and design company Lab H2 Inc., to design locomotive headlights that use LEDs as a light source. In addition to requiring less energy, LEDs also last longer and are smaller and more rugged than traditional light sources.

Fig. 1 new train headlight design uses two half-circular parabolic, or cup-shaped, aluminized reflectors with high-efficiency LEDs placed in the plane where the two reflectors come together. Combining the strong beams from each reflector generates the light intensity necessary to meet safety guidelines. Image Credit: Wei-Lun Liang, National Taiwan University.

Transient carrier trapping effect in organic light-emitting diode

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Transient carrier trapping and de-trapping effect in an organic light-emitting diode (OLED) was studied under different voltage waveforms. As shown in Fig. 1 (a), with 1-kHz voltage pulse at 6 V during on-period of 30 μs and different reverse bias (0 V to -14 V) during the off-period (970 μs), the luminance increased with increasing reverse bias up to 36% due to the trapping effect of positive charge at the cathode and the electron transporting layer (ETL), which was proved by the shift of threshold voltage (V_th) in displacement current measurement with different driving waveforms, as shown in Fig. 1 (b).

Fig. 1. (a) Transient luminance responses with different reverse bias under the off-period. (b) Injection trace from -2 V to 4.5 V of displacement current measurement with different waveforms.

Investigation of Streptavidin-Ligand Biochemical Interactions by IGZO Thin Film Transistors Integrated with Microfluidic Channels

Professor Jian-Jang Huang's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

This is a joint research work between Prof. Y.-H. Lo from UCSD and Prof. J.-J. Huang from GIPO NTU. In this work, we demonstrate an IGZO TFT-based biosensor integrated with a microfluidic channel to monitor biochemical reactions between proteins and small molecules. The target biomaterials are applied to a Y-shaped microfluidic chamber while the reactant signal is readout by the TFT. Using Biotin-Streptavidin reaction as an example, the specific diffusion time of biotin, streptavidin, or streptavidin-biotin complex across the fluidic channel are benchmarked. By intentionally delaying the application of one specie to another, one can study properties of biochemical reactions, such as excess and exhaust conditions of the reactants, and the formation of the products while diffusing in the channel. The proposed TFT sensor provides an excellent platform in understanding the biochemical functions for the advance of biological science and drug discovery.

Ref: This work was published in “Sensors and Actuators B: Chemical. 262. 10.1016/j.snb.2018.01.177”

Figure 1. (a) The cross section of the IGZO-TFT (b) Schematic diagram of a reusable TFT biosensor. The TFT and microfluidic channel were fabricated separately and then connected together by a bond wire. The cross linkers were applied on the surface of ROI.

Figure 2. Scenarios of drain current responses when the biotin and streptavidin are applied to the fluidic channel. Using the transient response of drain currents, our system is able to detect the complete and incomplete bioreactions. The control measurement was also conducted to verify the reaction between ligands and proteins.

High-Performance Vertically-Stacked Complementary Inverter Composed of Oxide Thin-Film Transistors

Professor I-Chun Cheng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Complementary thin-film transistor (TFT) inverters are the fundamental building blocks for versatile thin-film logic circuits. They possess several desirable features, such as low static power consumption and high density circuit integration. Complementary inverters implemented in a vertically-stacked structure offer an additional advantage, small footprint, in comparison with their coplanar counterparts. Oxide semiconductors are promising candidate channel materials for TFT applications because they have relatively high carrier mobilities and can be processed at low temperatures. In this research, we report an on-glass high-performance vertically-stacked complementary inverter based on oxide TFTs. It is composed of a bottom-gate n-type ZnO TFT stacking on a top-gate p-type SnO TFT. Both active layers were deposited by a low-temperature rf-sputtering technique. Fig. 1 shows the transfer and output characteristics of both the ZnO and SnO TFTs. The ZnO TFT exhibits a threshold voltage of 5.1 V, field-effect mobility of 2.5 cm²/V-s, subthreshold swing of 1.8 V/decade, and on/off current ratio of 2.4х10⁷. The corresponding values for the SnO TFT are 6.2 V, 0.35 cm²/V-s, 0.9 V/decade, and 2.2х10⁴. Fig. 2 shows the static voltage transfer characteristics and gain curves of the vertically-stacked complementary oxide-TFT-based inverter with a geometric aspect ratio of 5 at supplied voltages ranging from 2 to 10 V. A high voltage gain of 56 V/V at a supply voltage of 10 V is achieved. The result demonstrates the feasibility of realizing oxide-TFT based complementary circuits with high-density integration.

Fig. 1 (a)(b) Transfer and (c)(d) output characteristics of the bottom-gate n-channel ZnO TFT and top-gate p-channel SnO TFT. The channel dimensions are W/L = 40 μm/ 20 μm.

Fig. 2 (a) Static voltage transfer characteristics and (b) gain curves of the vertically-stacked complementary TFT inverter with a geometric aspect ratio of 5 at supply voltages ranging from 2 V to 10 V.