Metal Reaction-Induced Bulk-Doping Effect in Forming Conductive Source-Drain Regions of Self-Aligned Top-Gate Amorphous InGaZnO Thin-Film Transistors

In this paper, the aluminum (Al) treatment-induced doping effect on the formation of conductive source-drain (SD) regions of self-aligned top-gate (SATG) amorphous indium gallium zinc oxide (a-InGaZnO or a-IGZO) thin-film transistors (TFTs) is systematically investigated. Average carrier concentration over 1 × 1020 cm–3 and sheet resistance of around 500 Ω/sq result from the Al reaction doping. It is shown that the doping effect is of bulk despite the treatment at the surface. The doping process is disclosed to be a chemical oxidation–reduction reaction, that generates defects of oxygen vacancies and metal interstitials at the metal/a-IGZO interface. Both the generated oxygen vacancies and metal interstitials act as shallow donors, and the oxygen vacancies diffuse rapidly, leading to the bulk-doping effect. The fabricated SATG a-IGZO TFTs with the Al reaction-doped SD regions exhibit both high performance and excellent stability, featuring a low width-normalized SD resistance of about 10 Ω cm, a decent saturation mobility of 13 cm2/(V s), an off current below 1 × 10–13 A, a threshold voltage of 0.5 V, a slight hysteresis of −0.02 V, and a less than 0.1 V threshold voltage shift under 30 V gate bias stresses for 2000 s.