Amorphous IGZO MIS Phototransistor With HfO₂ Gate Dielectrics for High Sensitivity Solar-Blind Ultraviolet Detection

Amorphous indium-gallium-zinc-oxide (a-IGZO) thin film was deposited by radio frequency sputtering in an oxygen-free atmosphere. The composition of the a-IGZO thin film was adjusted to be In0.2Ga1.8ZnO4 so that the bandgap of the a-IGZO thin film was 4.5 eV for solar-blind ultraviolet photodetectors. Then, the bottom-gate IGZO phototransistor was fabricated with a high-k gate dielectric layer of hafnium oxide (HfO2) to enhance the gate controllability. The phototransistor showed excellent performance with an on/off ratio of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{{9}}$ </tex-math></inline-formula> and field-effect mobility of 5.4 cm<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{2}}\cdot \text {V}^{-{1}}\cdot \text {s}^{-{1}}$ </tex-math></inline-formula>. Under the 255 nm illumination, the phototransistor presented a gate voltage-controlled ultraviolet response. With zero voltage gate bias, a responsivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.2\times 10^{{5}}$ </tex-math></inline-formula> AW<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula> and a detectivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.4\times 10^{{16}}$ </tex-math></inline-formula> Jones was obtained. As a phototransistor made from amorphous semiconductors, the response speed is very low. The rise time and decay time are estimated at about 12 s and over 1800 s, respectively. However, the response time was tremendously improved to be less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$25~\mu $ </tex-math></inline-formula>s by interleaving both positive and negative gate voltage pulses synchronized with the UV illumination. The synchronous gate voltage pulses can be easily realized in readout integrated circuits (ROICs). Therefore, a-IGZO thin-film phototransistor should be a good candidate for flexible solar-blind ultraviolet photodetector with excellent performance.