Graded Channel Junctionless InGaZnO Thin-Film Transistors with Both High Transporting Properties and Good Bias Stress Stability

Abstract

InGaZnO (IGZO) is currently the most prominent oxide semiconductor complement to low-temperature polysilicon for thin-film transistor (TFT) applications in flat panel displays. However, the compromised transport performance and bias stress instability are critical issues inhibiting its application in ultrahigh-resolution optoelectronic displays. Here, we report the fabrication of graded channel junctionless IGZO:O|N TFTs with both high transporting properties and good bias stress stability by systematic manipulation of oxygen vacancy (V O ) defects through sequential O antidoping and O/N codoping of the continuous IGZO framework. The transporting properties and bias stress stability of the graded channel IGZO:O|N TFTs, which exhibited high field-effect mobilities close to 100 cm 2 V −1 s −1 , negligible performance degradations, and trivial threshold voltage shifts against gate bias stress and photobias stress, are simultaneously improved compared to those of the controlled single-channel uniformly doped IGZO:O TFTs, IGZO:N TFTs, and double-channel barrier-confined IGZO:O/IGZO:N TFTs. The synergistic improvements are attributed to the sequential mobility and stability enhancement effects of O antidoping and O/N codoping where triple saturation currents are induced by O antidoping of the front-channel regime while the trapped electrons and photoexcited holes in the back-channel bulk and surface regions are suppressed by O/N codoping. More importantly, fast accumulation and barrier-free full depletion are rationally realized by eliminating the junction interface within the graded channel layer. Our observation identifies that graded channel doping could be a powerful way to synergistically boost up the transport performance and bias stress stability of oxide TFTs for new-generation ultrahigh-definition display applications.

High Mobility and Photo‐Bias Stable Metal Oxide Thin‐Film Transistors Engineered by Gradient Doping

“…More significantly, to optimally tune the best balance between performance and stability, medium V O deficient IGZO:O II was created by solely O‐doping and built as channel bulk to bridge IGZO:O I and IGZO:N. The strategy used here is highly different from the as‐reported homogeneous single‐channel IGZO TFTs and double‐channel ITZO/IGZO TFTs, and our previous two‐level graded IGZO:O I |N (1.5 nm|98.5 nm) TFTs, where best balances between performance and stability are hardly to be engineered to an optimal level due to the incomplete or excessive suppression of the instability. [ 7,17,26 ]…”

“…The chemical composition and bonding states of the IGZO:O I , IGZO:O II , and IGZO:N were accurately characterized by X‐ray photoelectron spectroscopy (XPS) using monochromatic Ag L α (2984.2 eV) X‐ray source, where the commonly detected Ga LMM Auger peaks by conventional Al K α and Mg K α X‐ray sources that overlapped with N 1s peak are completely removed (Figure 1b,c and Figure S1, Supporting Information). [ 26,27 ] Experimentally, the front‐channel IGZO:O I was sputtering grown by pure argon with an optimized flow rate ≈10.00 sccm, which produced heavy O‐deficient IGZO with both high content of V O ($${N_{{{\rm{V}}_{\rm{O}}}}}$$≈26.7%, Figure 1d) and large Hall N e (≈5.2 × 10 19 cm –3 ) and μ (≈93.1 cm 2 V −1 s −1 ). Much higher I on and μ FE would be induced by artificially increasing N e in the space charge region.…”

“…The deteriorated transporting properties and improved stability is highly consistent with our previous IGZO:N TFTs built with high‐ k Al 2 O 3 /HfO 2 /Al 2 O 3 hybrid dielectrics, justifying the high reproducibility of N‐doping of IGZO. [ 26 ] However, the stability advantage of IGZO:N TFTs is compromised by the rapid loss of μ FE (6.61 versus 10.91 cm 2 V −1 s −1 ) compared to the IGZO:O II TFTs without N‐doping.…”

“…More significantly, to optimally tune the best balance between performance and stability, medium V O deficient IGZO:O II was created by solely O-doping and built as channel bulk to bridge IGZO:O I and IGZO:N. The strategy used here is highly different from the as-reported homogeneous single-channel IGZO TFTs and double-channel ITZO/ IGZO TFTs, and our previous two-level graded IGZO:O I |N (1.5 nm|98.5 nm) TFTs, where best balances between performance and stability are hardly to be engineered to an optimal level due to the incomplete or excessive suppression of the instability. [7,17,26] Bottom-gate top-contact IGZO:O I |O II |N TFTs with threelevel stepwise decreased V O defects were constructed, where IGZO:O I , IGZO:O II , and IGZO:N are functioning as mobility booster, balancer, and stability promoter, respectively (Figure 1a). The chemical composition and bonding states of the IGZO:O I , IGZO:O II , and IGZO:N were accurately characterized by X-ray photoelectron spectroscopy (XPS) using monochromatic Ag L α (2984.2 eV) X-ray source, where the commonly detected Ga LMM Auger peaks by conventional Al K α and Mg K α X-ray sources that overlapped with N 1s peak are completely removed (Figure 1b,c and Figure S1, Supporting Information).…”

“…Thus, backchannel defect engineering provides a feasible way to best tune the trade-off between performance and stability induced by homogeneous doping of the entire channel layer. [26] Similar gradient doping procedure was applied to grow the 100 nmthick IGZO:O I |O II |N channel layer (Figure S5, Supporting Information). Briefly, the front IGZO:O I (1.5 nm) was sputtered by pure argon that inlet from Channel 1 (10.00 sccm).…”