Trap-Assisted Enhanced Bias Illumination Stability of Oxide Thin Film Transistor by Praseodymium Doping

Addressing the Conflict between Mobility and Stability in Oxide Thin-film Transistors

Amorphous oxide semiconductor thin-film transistors (AOS TFTs) are ever-increasingly utilized in displays. However, to bring high mobility and excellent stability together is a daunting challenge. Here, the carrier transport/relaxation bilayer stacked AOS TFTs are investigated to solve the mobility-stability conflict. The charge transport layer (CTL) is made of amorphous In-rich InSnZnO, which favors big average effective coordination number for all cations and more edge-shared structures for better charge transport. Praseodymium-doped InSnZnO is used as the charge relaxation layer (CRL), which substantially shortens the photoelectron lifetime as revealed by femtosecond transient absorption spectroscopy. The CTL and CRL with the thickness suitable for industrial production respectively afford minute potential barrier fluctuation for charge transport and fast relaxation for photo-generated carriers, resulting in transistors with an ultrahigh mobility (75.5 cm² V^-1 s^-1 ) and small negative-bias-illumination-stress/positive-bias-temperature-stress voltage shifts (-1.64/0.76 V). The design concept provides a promising route to address the mobility-stability conflict for high-end displays.

The Mechanism of the Photostability Enhancement of Thin-Film Transistors Based on Solution-Processed Oxide Semiconductors Doped with Tetravalent Lanthanides

The applications of thin-film transistors (TFTs) based on oxide semiconductors are limited due to instability under negative bias illumination stress (NBIS). Here, we report TFTs based on solution-processed In₂O₃ semiconductors doped with Pr⁴⁺ or Tb⁴⁺, which can effectively improve the NBIS stability. The differences between the Pr⁴⁺-doped In₂O₃ (Pr:In₂O₃) and Tb⁴⁺-doped In₂O₃ (Tb:In₂O₃) are investigated in detail. The undoped In₂O₃ TFTs with different annealing temperatures exhibit poor NBIS stability with serious turn-on voltage shift (ΔV_on). After doping with Pr⁴⁺/Tb⁴⁺, the TFTs show greatly improved NBIS stability. As the annealing temperature increases, the Pr:In₂O₃ TFTs have poorer NBIS stability (ΔV_on are -3.2, -4.8, and -4.8 V for annealing temperature of 300, 350, and 400 °C, respectively), while the Tb:In₂O₃ TFTs have better NBIS stability (ΔV_on are -3.6, -3.6, and -1.2 V for annealing temperature of 300, 350, and 400 ℃, respectively). Further studies reveal that the improvement of the NBIS stability of the Pr⁴⁺/Tb⁴⁺:In₂O₃ TFTs is attributed to the absorption of the illuminated light by the Pr/Tb4fⁿ-O2p⁶ to Pr/Tb 4fⁿ⁺¹-O2p⁵ charge transfer (CT) transition and downconversion of the light to nonradiative transition with a relatively short relaxation time compared to the ionization process of the oxygen vacancies. The higher NBIS stability of Tb:In₂O₃ TFTs compared to Pr:In₂O₃ TFTs is ascribed to the smaller ion radius of Tb⁴⁺ and the lower energy level of Tb 4f⁷ with a isotropic half-full configuration compared to that of Pr 4f¹, which would make it easier for the Tb⁴⁺ to absorb the visible light than the Pr⁴⁺.

Effect of Sputtering Oxygen Partial Pressure on the Praseodymium-Doped InZnO Thin Film Transistor Using Microwave Photoconductivity Decay Method

The praseodymium-doped indium-zinc-oxide (PrIZO) thin film transistor (TFT) shows broad application prospects in the new generation of display technologies due to its high performance and high stability. However, traditional device performance evaluation methods need to be carried out after the end of the entire preparation process, which leads to the high-performance device preparation process that takes a lot of time and costs. Therefore, there is a lack of effective methods to optimize the device preparation process. In this paper, the effect of sputtering oxygen partial pressure on the properties of PrIZO thin film was studied, and the quality of PrIZO thin film was quickly evaluated by the microwave photoconductivity decay (µ-PCD) method. The μ-PCD results show that as the oxygen partial pressure increases, the peak first increases and then decreases, while the D value shows the opposite trend. The quality of PrIZO thin film prepared under 10% oxygen partial pressure is optimal due to its low localized defect states. The electric performance of PrIZO TFTs prepared under different oxygen partial pressures is consistent with the μ-PCD results. The optimal PrIZO TFT prepared under 10% oxygen partial pressure exhibits good electric performance with a threshold voltage (V_th) of 1.9 V, a mobility (µ_sat) of 24.4 cm²·V⁻¹·s⁻¹, an I_on/I_off ratio of 2.03 × 10⁷, and a subthreshold swing (SS) of 0.14 V·dec⁻¹.

Flexible Light-to-Frequency Conversion Circuits Built with Si-Based Frequency-to-Digital Converters via Complementary Photosensitive Ring Oscillators with p-Type SWNT and n-Type a-IGZO Thin Film Transistors

In this study, as system-level photodetectors, light-to-frequency conversion circuits (LFCs) are realized by i) photosensitive ring oscillators (ROs) composed of amorphous indium-gallium-zinc-oxide/single-walled carbon nanotube (a-IGZO/SWNT) thin film transistors (TFTs) and ii) phase-locked-loop Si circuits built with frequency-to-digital converters (PFDC). The 3-stage ROs and logic gates based on a-IGZO/SWNT TFTs successfully demonstrate its performance on flexible substrates. Herein, along with the advantage of scalability, a-IGZO films are used as photosensitive n-type TFTs and SWNTs are employed as photo-insensitive p-type TFTs for better photosensitivity in circuit level. Through the controlling a post-annealing condition of a-IGZO film, responsivities and detectivities of a-IGZO TFTs are obtained as 36 AW^-1 and 0.3 × 10¹² Jones for red, 93 AW^-1 and 3.1 × 10¹² Jones for green, and 194 AW^-1 and 11.7 × 10¹² Jones for blue. Furthermore, as an advanced demonstration for practical application of LFCs, a unique circuit (i.e., PFDC) is designed to analyze the generated oscillation frequency (f_osc ) from the LFC device and convert it to a digital code. As a result, the designed PFDC can exactly count the generated f_osc from the flexible a-IGZO/SWNT ROs under light illumination with an outstanding sensitivity and assign input frequencies to respective digital code.

Enabling high performance n-type metal oxide semiconductors at low temperatures for thin film transistors

The review highlights low temperature activation processes for high performance n-type metal oxide semiconductors for TFTs.