A systematic study of light dependency of persistent photoconductivity in a-InGaZnO thin-film transistors*

Abstract

Persistent photoconductivity (PPC) effect and its light-intensity dependence of both enhancement and depletion (E-/D-) mode amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) are systematically investigated. Density of oxygen vacancy (VO) defects of E-mode TFTs is relatively small, in which formation of the photo-induced metastable defects is thermally activated, and the activation energy (E a) decreases continuously with increasing light-intensity. Density of VO defects of D-mode TFTs is much larger, in which the formation of photo-induced metastable defects is found to be spontaneous instead of thermally activated. Furthermore, for the first time it is found that a threshold dose of light-exposure is required to form fully developed photo-induced metastable defects. Under low light-exposure below the threshold, only a low PPC barrier is formed and the PPC recovery is fast. With increasing the light-exposure to the threshold, the lattice relaxation of metal cations adjacent to the doubly ionized oxygen vacancies ( V O 2 + ) is fully developed, and the PPC barrier increases to ∼ 0.25 eV, which remains basically unchanged under higher light-exposure. Based on the density of VO defects in the channel and the condition of light illumination, a unified model of formation of photo-induced metastable defects in a-IGZO TFTs is proposed to explain the experimental observations.

Degradation and its fast recovery in a-IGZO thin-film transistors under negative gate bias stress*

“…[7][8][9][10] The negative shift of the transfer curve under NBIS is more evident, where the mechanism is similar to that under NBS, but the density of V 2+ o is larger with the help of illumination. [8,11] In this paper, positive shift of the transfer curve and severe degradation in the subthreshold characteristics were observed for a-IGZO TFTs under NBS and NBIS, and a gentle degradation is observed under PBS and PBIS. A fast recovery of the degradation in the threshold voltage (V TH ) and subthreshold swing (SS) is observed for the a-IGZO TFTs under NBS and NBIS, and no degradation can be observed when the transfer curves are measured with gate bias sweeping from positive gate bias to negative gate bias direction.…”

A review on realizing the modern optoelectronic applications through persistent photoconductivity

“…PPC is a light-induced mechanism that persists after light excitation is discontinued, meaning that the photo-response does not cease immediately but rather stays available for a period of time (ms to hours) after the light excitation has been terminated. PPC has been observed in variety of material systems ranging from binary metal oxides to complex alloys, doped semiconductors, superconductors, organic semiconductors and 2D materials like ZnO, Zn 0.3 Cd 0.7 Se, BiFeO 3 , a-IGZO, SrTiO 3 , 2H-MoSe 2 , InGaN, GaN, AlGaAs, In 2 O 3 films, Hf-In-Zn-O thin films, p-type 4H-SiC wafer, porous InP structures, 6H-SiC, p-type Si nano-membranes, Cu 2 ZnSnS 4 thin films, n-type GaN thin films, undoped ntype GaN, GaN NWs, MoS 2 , Graphene, inorganic perovskites etc [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. Despite the fact that the scientific community has been familiar of PPC for several years, only recently has it been examined as a promising technique for emerging applications.…”

Migration of weakly bonded oxygen atoms in a-IGZO thin films and the positive shift of threshold voltage in TFTs

“…A heat exchanger is used to adjust the temperature of the helium gas, and thus the sample temperature is controlled in the range of 20 K-300 K. All the electric measurements are done in the dark. [15,16] Transfer curves are measured by sweeping V g from −60 V to 60 V in steps of 0.5 V, with V d kept at 2 V. To evaluate the V th shift, a constant gate voltage (V g ) of 60 V and a drain voltage (V d ) of 2 V are biased for 400 s after each transfer curve measurement. For the recovery process, the a-IGZO TFTs are electrically disconnected for 400 s and the V g sweeping is stopped when I d is larger than 5 nA, taking the turn-on voltage (V on ) shift to represent the V th shift.…”