Eric Kai Hsiang Yu

Porphyrin-Tape/C60 Organic Photodetectors with 6.5% External Quantum Efficiency in the Near Infrared

/1 0 4 While few examples have been demonstrated, near-infrared (NIR) organic photodetectors with response at wavelengths ( λ ) beyond the cutoff of Si (i.e., λ > 1100 nm) are interesting for use in imaging and other detection applications. [ 1 ] In previous work, polymer photodetectors with response at λ > 1000 nm have been demonstrated, but the optical sensitivity is generally due to a long absorption tail having an external quantum effi ciency (EQE) less than a few percent. [ 2 – 4 ] Organic materials systems with a large NIR photoresponse are rare for several reasons. A type-II (staggered) heterojunction must be formed between the donor and acceptor materials with a suffi cient energy offset to dissociate photogenerated excitons; as the energy gap is decreased, fi nding molecular combinations with suitable energy alignments becomes increasingly diffi cult. In addition, exciton lifetimes generally decrease with energy gap due to exciton–phononinduced recombination (i.e., internal conversion). [ 5 , 6 ] These diffi culties have motivated the development of hybrid organic– inorganic devices using polymeric and small-molecule materials in conjunction with II–VI quantum dots (with EQE <1% at λ > 1000 nm) [ 7 ] or single-walled carbon nanotubes (EQE ≈ 2% at λ = 1150 and 1300 nm). [ 8 ] Here, we demonstrate an NIR EQE = 6.5% at λ = 1350 nm using photodetectors based on triply linked porphyrin-tape dimers. These porphyrin tapes are representative of a promising new class of materials that can be modifi ed to exhibit even longer wavelength response by spatially extending the conjugation of the π-electron system. [ 5 ]

Density of states of amorphous In-Ga-Zn-O from electrical and optical characterization

We have developed a subgap density of states (DOS) model of amorphous In-Ga-Zn-O (a-IGZO) based on optical and electrical measurements. We study the optical absorption spectrum of the a-IGZO using UV-Vis spectroscopy. Together with the first-principles calculations and transient photoconductance spectroscopy from the literature, we determine that the valence band tail and deep-gap states are donors and can be described by exponential and Gaussian distributions, respectively. The conduction band tail and deep-gap states are examined using multi-frequency capacitance-voltage spectroscopy on a-IGZO thin-film transistors (TFTs). The extracted conduction band DOS are fitted to exponential (bandtail) and Gaussian (deep-gap) functions and their validity are supported by the activation energy vs. gate-source bias relationship of the a-IGZO TFT. The PL deep-level emission, which is almost identical to the conduction band deep-gap Gaussian, suggests that these states should be assigned as acceptors. The donor/acceptor assignments of subgap states are consistent with the 2D numerical TFT simulations.

Oxygen flow effects on electrical properties, stability, and density of states of amorphous In–Ga–Zn–O thin-film transistors

To investigate the origin of threshold voltage (Vth) shift of amorphous In?Ga?Zn?O (a-IGZO) thin-film transistors (TFTs), a combination of bias-temperature stress (BTS) and multi-frequency capacitance?voltage (C?V) measurements were used to evaluate the impact of oxygen partial pressure (PO2) during a-IGZO deposition on TFT electrical properties, electrical stability, and density of states (DOS). The extracted sub-gap DOS was decomposed into exponential bandtail states and Gaussian-like deep-gap states. The peak density of Gaussian-like states is larger for higher PO2. We conclude that the Gaussian-like states are excess/weakly-bonded oxygen in the form of O0 or O1? ions acting as acceptor-like states and are at the origin of TFT threshold voltage shift during positive BTS.

Dynamic Response of a-InGaZnO and Amorphous Silicon Thin-Film Transistors for Ultra-High Definition Active-Matrix Liquid Crystal Displays

The dynamic response of hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) and amorphous In-Ga-Zn-O (a-IGZO) T_FT are compared. We study the storage capacitor (Cst) charging characteristics by applying gate and data voltage waveforms corresponding to ultra-high definition (UHD) active-matrix liquid crystal displays (AM-LCDs). We show that the charging behavior of the a-Si:H T_FT is insufficient for UHD AM-LCDs and that the a-IGZO T_FT is capable of supporting at least 8 K ×4 K display resolution at 480 Hz. The impact of C_st and gate voltage falling edge (t_FE) on feedthrough voltage (ΔV_P) is investigated. Because of higher mobility of the a-IGZO T_FT, it is possible to reduce ΔV_P by mitigating channel charge redistribution with non-abrupt tFE. The a-IGZO TFT shows no drawbacks in terms of ΔV_P when compared to the a-Si:H T_FT. In addition, a larger C_st can be used in combination with the a-IGZO T_FT to reduce ΔV_P with minimal impact on its charging behavior. Gate overdrive operation is also evaluated for the a-IGZO T_FT, which may improve charging characteristics with no adverse effects on ΔV_P. Our results show that the a-IGZO T_FT is a suitable technology for UHD high-frame rate AM-LCDs.

AC Bias-Temperature Stability of a-InGaZnO Thin-Film Transistors With Metal Source/Drain Recessed Electrodes

In this paper, we fabricated metal source/drain recessed nearly self-aligned amorphous indium-gallium-zinc-oxide thin-film transistors (TFTs) that are highly stable under ac bias-temperature stress (BTS). For TFTs of the size W/L=60 μm/10 μm, the stress-induced threshold voltage shifts are all within -0.35 V. A comprehensive investigation of ac BTS stress polarity, frame time, and duty cycle dependence is presented in the context of high-resolution high-refresh rate active-matrix flat-panel displays. We find that higher frequency bipolar ac pulses increase the device instability. The threshold voltage instability may be reduced significantly by decreasing the duty cycle of the stress waveform.

High performance amorphous metal-oxide semiconductors thin-film passive and active pixel sensors

In this paper, for the first time, we report on high performance amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) based passive pixel sensor (PPS) and active pixel sensor (APS) circuits. Experimental results show that single-TFT PPS with a pitch length of 50μm can achieve a signal charge gain approaching to unity (Gain=0.93) under a fast readout time of 20μs and a dynamic range of 40dB. APS based on three a-IGZO TFTs, with a pitch length of ~100μm, established a high dynamic range of more than 60dB. 2-TFTs half active pixel sensor (H-APS) testing circuits are also designed to investigate the voltage gain (AV=ΔVOUT/ΔVG) properties for the APS circuit in this work. For the a-IGZO APS, AV is measured to be ~1.25, and through normalization of the pixel capacitance (CPIX) to a common value of 5pF, a large signal charge gain of 25 is obtained.

Dynamic response of amorphous In-Ga-Zn-O thin-film transistors for 8K×4K flat-panel display

In this paper, for the first time, we report on the dynamic characteristics of the bottom-gate a-IGZO TFTs. Experiment results obtained for a-IGZO TFT are compared to conventional hydrogenated amorphous silicon (a-Si:H) TFTs.

Photoluminescence Study of Amorphous InGaZnO Thin-Film Transistors

The defects within optical gap of the amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) before and after bias temperature stress (BTS) are studied using in situ photoluminescence (PL) spectroscopy. After applying positive BTS, the TFT threshold voltage (Vth) increased, and intensity of deep-level PL emission peak at 1.82 eV became stronger. The exact opposite is observed when negative BTS is applied. Both the 250-nm bulk a-IGZO film and 50-nm channel region TFT show the same PL peak locations of 3.35 and 1.82 eV. From the measured results and density of state model of a-IGZO, the radiative recombination between electrons trapped in oxygen-related acceptor-like states and holes in the valence band is responsible for the deep-level PL emission. This method of measuring the PL response within the TFT channel region could be used as a tool to evaluate the quality of a-IGZO TFT manufacturing processes.