Suppression of Negative Gate Bias and Illumination Stress Degradation by Fluorine-Passivated In-Ga-Zn-O Thin-Film Transistors

Abstract

The negative gate bias and illumination stress (NBIS) stability and photo-response of In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) with either silicon oxide (SiO X ) or fluorinated silicon nitride (SiN X :F) passivation were investigated. NBIS degradation can be suppressed when the fluorine (F) in SiNx:F diffused into an IGZO channel during long annealing period. Photo-response analysis revealed that F passivated effectively electron traps in the IGZO channel existing at an energy level close to the valence band maximum (near-VBM state). The fluorine-passivated IGZO TFT is thus advantageous for achieving highly reliable IGZO TFT for next-generation displays. An amorphous In-Ga-Zn-O (a-IGZO) semiconductor is widely accepted as a promising channel material for TFT applications owing to its outstanding electrical properties.3 However, it is recognized that degradation under negative gate bias and illumination stress (NBIS) is a unique but serious issue for oxide TFTs.4 When a negative gate voltage and illumination stresses are simultaneously applied to oxide TFTs, the enhancements of negative threshold voltage (V th ) shift and hysteresis were observed due to photo-excitation of electrons from high-density electron traps existing in an IGZO at an energy level close to the valence band maximum (near-VBM state). [5][6][7][8][9] The origin of the near-VBM state has been actively investigated from both experimental and theoretical standpoints. Kamiya et al. reported the oxygen vacancy with void in oxygen deficient IGZO film is an origin of the near-VBM state.10 In addition, weakly-bonded oxygen, interstitial oxygen, undercoodinated oxygen, and peroxide have been reported as origins of the near-VBM state in an IGZO film.11-14 Moreover, the -OH groups in the film have also been reported to relate the density of the near-VBM state. [15][16][17] Although the origin of the near-VBM state is still debatable, passivation or reduction of the near-VBM state originating from oxygen-related defects is essential to improve NBIS reliability. Wet-O 2 annealing has been reported to improve the NBIS reliability of the IGZO TFT. 17,18 It has also been reported that formation of OH-bonds in an IGZO is an important factor to passivate the near-VBM state.17 A theoretical study also indicated that a hydrogen atom preferentially binds to undercoordinated oxygen, resulting in a passivation of the near-VBM state.14 Although many researches have made numerous efforts to improve the NBIS reliability of oxide TFTs, NBIS degradation has as yet not been suppressed.We reported that reliability of the IGZO TFT under positive gate bias and temperature stress (PBTS) was drastically improved by a fluorine-passivated IGZO TFT. 19 It was found that fluorine (F) effectively passivated electron traps and weakly bonded oxygen in an IGZO channel and at a gate insulator (GI)/channel interface when F diffused into an IGZO channel. However, both the F passivation effect of the near-VBM state and its influence on the NBIS stability of the IGZO TFT have not bee...

Top Interface Engineering of Flexible Oxide Thin‐Film Transistors by Splitting Active Layer

“…The F atoms in the side walls can also diffuse into the top interface during the postfabrication annealing, which was done at 370 °C in this work. It is reported high‐temperature, long‐time annealing enhanced F diffusion from SiO X –ES layer into an IGZO channel when F exists in the SiO X –ES layer …”

Rare-metal-free high-performance Ga-Sn-O thin film transistor

“…Zn stabilizes the amorphous structure in IGZO, however, Zn element is not needed in GTO for stabilizing the amorphous GTO TFT characteristics. In addition, improvement in the μ FE and stability of GTO TFTs can be expected by appropriate passivation or treatment of the back channel and optimization of the fabrication process applied for IGZO TFTs 25 43 44 .…”

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