Boosting Near-Infrared Detection in IGZO-Based Phototransistors via NaYF₄:Yb/Tm Core Upconversion Nanoparticles

In the era of the Internet of Things (IoT), various types of sensors are actively embedded not only into simple wearable devices but also into next-generation industrial applications such as healthcare, robotics, and aviation. Among several types of sensors, photosensors are drawing significant attention because they can easily connect objects and people using optical signals. While various semiconducting materials have been introduced for photosensor applications, amorphous oxide semiconductor (AOS), particularly indium-gallium-zinc-oxide (IGZO), are considered some of the most appropriate candidates. This is due to their high field-effect mobility, low leakage current, excellent uniformity, and high compatibility with industry standards. However, the large band gap of IGZO (~3.5 eV) limits the detection range of IGZO thin-film transistors (TFTs) to the visible to near-infrared (NIR) spectrum, posing a significant hurdle for applications in security systems, information technology, health monitoring, and neuromorphic image sensors.<br/>Herein, upconversion nanoparticle (UCNP) NaYF₄:Yb/Tm core was firstly introduced to detect NIR region of IGZO phototransistor. UCNP has emerged as a promising candidate for NIR sensing applications due to its absorption and converting characteristics of NIR light into visible or ultraviolet (UV) light. UCNP and IGZO are used as a NIR absorbing layer and UV & blue light absorbing channel layer, respectively. The optoelectronic characteristics under NIR region of UCNP on a-IGZO phototransistor are performed and compared with those of pristine IGZO phototransistor.<br/>To confirm that fabricated NaYF₄:Yb/Tm core UCNP performs upconversion characteristics, PL analysis was conducted. Under the illumination of 980 nm laser, NaYF₄:Yb/Tm core UCNP showed sharp emissions centered at 347, 362, 450, 483 nm, which is ultraviolet (UV) and blue light region. This indicates that the 980 nm light is converted to UV and visible light, showing intense upconversion luminescence. Throughout these analysis, it is shown that the fabricated NaYF₄:Yb/Tm core UCNP can convert NIR region wavelength to UV and blue light regions which can be absorbed by IGZO due to its proper bandgap.<br/>To analyze the improved performance of the light absorption characteristics of NaYF₄:Yb/Tm core UCNP on IGZO phototransistor, transfer curves under NIR illumination (808 nm ~ 1429 nm) were analyzed under the V_DS of 10.1 V. As a result, the NaYF₄:Yb/Tm core UCNP on IGZO phototransistor can detect a wide range of NIR region (808 nm – 1429 nm). Under the NIR illumination, as the effect of NaYF₄:Yb/Tm core UCNP, the photoresponsivity, photosensitivity, and detectivity were improved by 1.45 10²to 1.63 10³A/W, 9.02 10¹to 2.21 10⁶, 5.98 10⁷to 5.58 10¹⁰ Jones, respectively, under 808 nm illumination compared to pristine IGZO phototransistors.<br/>In summary, we fabricated NaYF₄/Tm core UCNP on IGZO phototransistor, analyzing its synthesis and characteristics using PL spectra. The upconversion characteristic of NaYF₄/Tm core UCNP under NIR illumination enhanced the photo-reactivity and optoelectronic properties of the IGZO phototransistor, demonstrating its effectiveness as the first AOS-based NIR phototransistor using UCNP material. Future work will focus on further analysis of the mechanism and additional experiments to highlight the advantages of our device.