Role of Self-Assembled Monolayers on Improved Electrical Stability of Amorphous In-Ga-Zn-O Thin-Film Transistors

Abstract

Self-assembled monolayers (SAMs) have been used to improve both the positive and negative bias-stress stability of amorphous indium gallium zinc oxide (IGZO) bottom gate thin film transistors (TFTs). N-hexylphosphonic acid (HPA) and fluorinated hexylphosphonic acid (FPA) SAMs adsorbed on IGZO back channel surfaces were shown to significantly reduce biasstress turn-on voltage shifts compared to IGZO back channel surfaces with no SAMs. FPA was found to have a lower surface energy and lower packing density than HPA, as well as lower biasstress turn-on voltage shifts. The improved stability of IGZO TFTs with SAMs can be primarily attributed to a reduction in molecular adsorption of contaminants on the IGZO back channel surface and minimal trapping states present with phosphonic acid binding to the IGZO surface. 10-15Proposed mechanisms for bias stress threshold voltage (V th ) shifts include electron trapping at the semiconductor/dielectric interface, electron injection into the dielectric, or formation of sub-band gap states in the bulk of films. It has also been suggested that adsorbed species (e.g., O 2 and H 2 O) on the back channel of the IGZO-TFTs can be the dominant mechanism for device instability, by providing acceptor or donor states or through field induced adsorption or desorption of these species. [17][18][19][20] For example, molecular oxygen can form a depletion layer at the back channel surface through electron acceptor states, while absorbed water can form an accumulation layer at the back channel surface through electron donor states.A depletion layer results in a positive V th shift, whereas an accumulation layer results in a 4 negative V th shift. Recently it has been demonstrated that dense passivation layers 21 or selfassembled monolayers (SAMs) 22 on IGZO-TFT back channels can significantly minimize these threshold voltage shifts. A potential benefit of using SAMs for surface passivation is that one can obtain well controlled chemistries at the back channel interface, even when using polymeric dielectrics for flexible electronic applications or integrating polymeric sensing layers.Herein we investigate the use of n-hexylphosphonic acid (n-HPA) and (3,3,4,4,5,5,6,6,6- IGZO-TFT test structures were fabricated using a heavily p-doped Si substrate as the gate and thermally grown SiO 2 (100 nm) as the gate dielectric. Substrates were cleaned prior to deposition of IGZO with acetone, isopropyl alcohol, and DI water. Amorphous IGZO films (~50 nm thick) were deposited using RF magnetron sputter deposition with a 3 in. IGZO target (molar composition: In 2 O 3 :Ga 2 O 3 :ZnO), 100W RF power, ~4 mTorr chamber pressure, and 20 sccm flow rate with a 1:19 (O 2 :Ar) ratio. IGZO active layers were patterned using a shadow mask 5 during deposition, and the films were subsequently annealed in air to 300 °C. Source and drain electrodes were patterned using a shadow mask during thermal evaporation of Al (~500 nm thick) giving a W/L ratio of 1000 m/100 m. Back channel surface passivation using SAMs was ...

Amorphous In-Ga-Zn-O thin-film transistors fabricated by microcontact printing

“…30 Upon close inspection, even the ridges on the PDMS stamp were replicates of the grooves in the SU-8 master. Phosphonic acid has been used to tune the surface properties of metal oxide films; 18,20,[31][32][33] and for these studies, we used ODPA ink as the protective layers on top of the blanket ITO film. We found that the lCP ODPA transferred ink was continuous and provided excellent etch resistance to oxalic acid.…”

Low temperature annealed amorphous indium gallium zinc oxide (a-IGZO) as a pH sensitive layer for applications in field effect based sensors

“…core level spectra of the as-deposited film are fitted into three components (figure 6(a)) with their respective binding energies at 528.4, 530.0, and 531.4 eV. The low energy peak can be assigned to the lattice oxygen ions, the middle peak to the adsorbed hydroxyl ions, and the high energy peak to some contaminants as reported by Du et al 18 The area of the middle peak reduced (figure 6(b)) upon annealing at 400 o C in N 2 ambience, while it increased for the low and high energy peaks. A decrease in binding energy (0.14 eV) is also observed in middle peaks but there is no change in the low energy peak.…”

Investigations on the bias temperature stabilities of oxide thin film transistors using In–Ga–Zn–O channels prepared by atomic layer deposition

“…29,30 The obtained s and b values for both TFTs are summarized in Table 1. Because the DV TH values were in good agreement with the stretched exponential relationship, the charge-trapping mechanism could be concluded as the primary cause for the PBTS instabilities.…”