Artificially Fabricated Subgap States for Visible-Light Absorption in Indium-Gallium-Zinc Oxide Phototransistor with Solution-Processed Oxide Absorption Layer

Low-Power Phototransistor with Enhanced Visible-Light Photoresponse and Electrical Performances Using an IGZO/IZO Heterostructure

In this study, we demonstrated the effective separation of charge carriers within the IGZO/IZO heterostructure by incorporating IZO. We have chosen IGZO for its high mobility and excellent on-off switching behavior in the front channel of our oxide-oxide heterostructure. Similarly, for an additional oxide layer, we have selected IZO due to its outstanding electrical properties. The optimized optoelectronic characteristics of the IGZO/IZO phototransistors were identified by adjusting the ratio of In:Zn in the IZO layer. As a result, the most remarkable traits were observed at the ratio of In:Zn = 8:2. Compared to the IGZO single-layer phototransistor, the IGZO/IZO(8:2) phototransistor showed improved photoresponse characteristics, with photosensitivity and photoresponsivity values of 1.00 × 107 and 89.1 AW-1, respectively, under visible light wavelength illumination. Moreover, the electrical characteristics of the IGZO/IZO(8:2) transistor, such as field effect mobility (μsat) and current on/off ratio (Ion/Ioff), were highly enhanced compared to the IGZO transistor. The μsat and Ion/Ioff were increased by about 2.1 times and 2.3 times, respectively, compared to the IGZO transistor. This work provides an approach for fabricating visible-light phototransistors with elevated optoelectronic properties and low power consumption based on an oxide-oxide heterostructure. The phototransistor with improved performance can be applied to applications such as color-selective visible-light image sensors and biometric sensors interacting with human-machine interfaces.

Solution-processed transparent p-type orthorhombic K doped SnO films and their application in a phototransistor

The exploitation of p-type oxide semiconductors with excellent optoelectrical properties as well as a simple preparation process is still challenging owing to the difficulty in producing hole carriers which results from strong hole localization in p-type oxide semiconductors. In this work, we succeeded in using ethylene glycol as a reductant to prepare orthorhombic structure SnO films using a sol-gel method and through K doping the optical and electrical properties of the films were improved. When the orthorhombic K doped SnO (K-SnO) films were applied in a phototransistor, it presented ultra-broadband photosensing from the ultraviolet to infrared region (300-1000 nm), demonstrating a photoresponsivity of 349 A W^-1 and a detectivity of 5.45 × 10¹² Jones at 900 nm under a light intensity of 0.00471 mW cm^-2. In particular, infrared photosensing was for the first time reported in the SnO based phototransistors. This work not only provides a simple method to fabricate high-performance and low-cost p-type K-SnO films and phototransistors, but may also suggest a new way to improve the p-type characteristics of other oxide semiconductors and devices.

Polyimide-Doped Indium-Gallium-Zinc Oxide-Based Transparent and Flexible Phototransistor for Visible Light Detection

We report a transparent and flexible polyimide (PI)-doped single-layer (PSL) phototransistor for the detection of visible light. The PSL was deposited on a SiO₂ gate insulator by a co-sputtering process using amorphous indium-gallium-zinc oxide (IGZO) and PI targets simultaneously. The PSL acted as both a channel layer and a visible-light absorption layer. PI is one of the few flexible organic materials that can be fabricated into sputtering targets. Compared with the IGZO phototransistor without PI doping, the PSL phototransistor exhibited improved optoelectronic characteristics under illumination with 635 nm red light of 1 mW/mm² intensity; the obtained photoresponsivity ranged from 15.00 to 575.00 A/W, the photosensitivity from 1.38 × 10¹ to 9.86 × 10⁶, and the specific detectivity from 1.35 × 10⁷ to 5.83 × 10¹¹ Jones. These improvements are attributed to subgap states induced by the PI doping, which formed decomposed organic molecules, oxygen vacancies, and metal hydroxides. Furthermore, a flexible PSL phototransistor was fabricated and showed stable optoelectronic characteristics even after 10,000 bending tests.

A Review of Phototransistors Using Metal Oxide Semiconductors: Research Progress and Future Directions

Metal oxide thin-film transistors have been continuously researched and mass-produced in the display industry. However, their phototransistors are still in their infancy. In particular, utilizing metal oxide semiconductors as phototransistors is difficult because of the limited light absorption wavelength range and persistent photocurrent (PPC) phenomenon. Numerous studies have attempted to improve the detectable light wavelength range and the PPC phenomenon. Here, recent studies on metal oxide phototransistors are reviewed, which have improved the range of light wavelengths and the PPC phenomenon by introducing an absorption layer of oxide or non-oxide hybrid structure. The materials of the absorption layer applied to absorb long-wavelength light are classified into oxides, chalcogenides, organic materials, perovskites, and nanodots. Finally, next-generation convergence studies combined with other research fields are introduced and future research directions are detailed.

Neuromorphic Active Pixel Image Sensor Array for Visual Memory

Neuromorphic engineering, a methodology for emulating synaptic functions or neural systems, has attracted tremendous attention for achieving next-generation artificial intelligence technologies in the field of electronics and photonics. However, to emulate human visual memory, an active pixel sensor array for neuromorphic photonics has yet to be demonstrated, even though it can implement an artificial neuron array in hardware because individual pixels can act as artificial neurons. Here, we present a neuromorphic active pixel image sensor array (NAPISA) chip based on an amorphous oxide semiconductor heterostructure, emulating the human visual memory. In the 8 × 8 NAPISA chip, each pixel with a select transistor and a neuromorphic phototransistor is based on a solution-processed indium zinc oxide back channel layer and sputtered indium gallium zinc oxide front channel layer. These materials are used as a triggering layer for persistent photoconductivity and a high-performance channel layer with outstanding uniformity. The phototransistors in the pixels exhibit both photonic potentiation and depression characteristics by a constant negative and positive gate bias due to charge trapping/detrapping. The visual memory and forgetting behaviors of the NAPISA can be successfully demonstrated by using the pulsed light stencil method without any software or simulation. This study provides valuable information to other neuromorphic devices and systems for next-generation artificial intelligence technologies.

Novel Method for Fabricating Visible-Light Phototransistors Based on a Homojunction-Porous IGZO Thin Film Using Mechano-Chemical Treatment

A homojunction-structured oxide phototransistor based on a mechano-chemically treated indium-gallium-zinc oxide (IGZO) absorption layer is reported. Through this novel and facile mechano-chemical treatment, mechanical removal of the cellophane adhesive tape induces reactive radicals and organic compounds on the sputtered IGZO film surface. Surface modification, following the mechano-chemical treatment, caused porous sites in the solution-processed IGZO film, which can give rise to a homojunction-porous IGZO (HPI) layer and generate sub-gap states from oxygen-related defects. These intentionally generated sub-gap states played a key role in photoelectron generation under illumination with relatively long-wavelength visible light despite the wide band gap of IGZO (>3.0 eV). Compared with conventional IGZO phototransistors, our HPI phototransistor displayed outstanding optoelectronic characteristics and sensitivity; we measured a threshold voltage (V_th) shift from 3.64 to -6.27 V and an on/off current ratio shift from 4.21 × 10¹⁰ to 4.92 × 10² under illumination with a 532 nm green light of 10 mW/mm² intensity and calculated a photosensitivity of 1.16 × 10⁸. The remarkable optoelectronic characteristics and high optical transparency suggest optical sensor applications.

A visible-light phototransistor based on the heterostructure of ZnO and TiO2 with trap-assisted photocurrent generation

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO₂). A thin layer of TiO₂ was deposited onto the spin-coated ZnO film via atomic layer deposition (ALD). The electrical characteristics of the TiO₂ layer were optimized by controlling the purge time of titanium isopropoxide (TTIP). The optimized TiO₂ layer could absorb the visible-light from the sub-gap states near the conduction band of TiO₂, which was confirmed via photoelectron spectroscopy measurements. Therefore, the heterostructure of TiO₂/ZnO can absorb and generate photocurrent under visible light illumination. The oxygen-related-states were investigated via X-ray photoelectron spectroscopy (XPS), and the interfacial band structure between TiO₂ and ZnO was evaluated via ultraviolet photoelectron spectroscopy (UPS). Oxygen-related states and subgap-states were observed, which could be used to generate photocurrent by absorbing visible light, even with TiO₂ and ZnO having a wide bandgap. The optimized TiO₂/ZnO visible-light phototransistor showed a photoresponsivity of 99.3 A W⁻¹ and photosensitivity of 1.5 × 10⁵ under the illumination of 520 nm wavelength light. This study provides a useful way to fabricate a visible-light phototransistor based on the heterostructure of wide bandgap oxide semiconductors.

Visible-light phototransistors have been fabricated based on the heterojunction of zinc oxide (ZnO) and titanium oxide (TiO₂).

Enhanced Electro-Optical Performance of Inorganic Perovskite/a-InGaZnO Phototransistors Enabled by Sn-Pb Binary Incorporation with a Selective Photonic Deactivation

Optoelectronic applications using perovskites have emerged as one of the most promising platforms such as phototransistors, photovoltaics, and photodetectors. However, high-performance and reliable perovskite photonic devices are often hindered by the limited spectral ranges of the perovskite system along with the lack of appropriate processing technologies for the implementation of reliable device architectures. Here, we explore a hybrid phototransistor with a heterojunction of a Sn-Pb binary mixed halide perovskite (CsSn_0.6Pb_0.4I_2.6Br_0.4) light absorber and an amorphous-In-Ga-Zn-O (a-IGZO) charge carrying layer. By incorporating Sn-Pb binary components with an all-inorganic base, broadening of light-absorbing spectral ranges with enhanced stability has been achieved, indicating inevitable highly increased conductivity, which triggers a high off-current of the devices. Accordingly, the selectively ultraviolet (UV)-irradiated electrical deactivation (SUED) process is carried out to suppress the high off-current with a reliable device structure. Particularly, it is noted that the selective UV irradiation can facilitate oxidation and distortion of the chemical structure in specific perovskite regions, providing enhanced gate bias modulation of the phototransistor with an increased on/off-current ratio from ∼10³ to ∼10⁶. Finally, the SUED-processed phototransistor exhibits an improvement in the photosensitivity by more than 3 orders of magnitude up to 8.0 × 10⁴ and detects in the spectral range from visible to near-infrared (NIR) light (∼860 nm) with good on/off switching behaviors.

High Photosensitive Indium-Gallium-Zinc Oxide Thin-Film Phototransistor with a Selenium Capping Layer for Visible-Light Detection

Visible light can be detected using an indium-gallium-zinc oxide (IGZO)-based phototransistor, with a selenium capping layer (SCL) that functions as a visible light absorption layer. Selenium (Se) exhibits photoconductive properties as its conductivity increases with illumination. We report an IGZO phototransistor with an SCL (SCL/IGZO phototransistor) that demonstrated optimal photoresponse characteristics when the SCL was 150 nm thick. The SCL/IGZO phototransistor exhibited a photoresponsivity of 1.39 × 10³ A/W, photosensitivity of 4.39 × 10⁹, detectivity of 3.44 × 10¹³ Jones, and external quantum efficiency of 3.52 × 10³% when illuminated by green light (532 nm). Ultraviolet-visible spectroscopy and ultraviolet photoelectron spectroscopy analysis showed that Se has a narrow energy band gap, in which visible light is absorbed and forms a p-n junction with IGZO so that photogenerated electron-hole pairs are easily separated, which makes recombination more challenging. We show that electrons generated in the SCL flow through the IGZO layer, which enables the phototransistor to detect visible light. Furthermore, the SCL/IGZO phototransistor exhibited excellent durability and reversibility owing to the constant light and dark current and the time-dependent photoresponse characteristics over 8000 s when a red light (635 nm) source was turned on and off at a frequency of 0.1 Hz.