Amorphous indium-gallium-zinc oxide (a-IGZO) attracts much attention for next-generation flat-panel displays due to the large band gap, visible light transparency, printability, thickness-uniformity and low-temperature in film deposition. Using a-IGZO films, thin-film-transistors (TFTs) are realized, which show superior behavior compared with amorphous Si-based TFT. In order to further improve a-IGZO TFT performance via source-drain resistance reduction, plasma treatment, excimer laser annealing and ion implantation methods as the sheet resistance reduction techniques were investigated. Among them, the ion implantation methods have depth control ability through insulator layers and good verticality.
In our previous work we elucidated that a-IGZO sheet resistance reduction can be attributed to oxygen vacancy (Vo) generated by noble gas implantation. However, there is still much room for clarifying and improving the implantation method, since it is difficult to measure the gas itself and vacancies profiles in a-IGZO due to small ionization cross sections and small existence ratio. In addition many injectable ion species in a-IGZO are not investigated in detail.

In this work, we carried out one of the conventional ion boron (B⁺) or noble gas neon ion (Ne⁺) implantations and investigations in 50-nm-thick a-IGZO film on glass. Electron transport properties of B⁺ or Ne⁺ implanted a-IGZO films as a function of the a-IGZO depth were investigated by room temperature Hall measurements combined with wet etchings. In case of Ne+ implanted a-IGZO film, we estimated Vo relation density depth profile consistent with the electron concentration depth profile in a-IGZO and implanted Ne density depth profile, since the Ne itself cannot be a donor. In case of B⁺ implanted a-IGZO film, the Vo concentration profile is not agreement with electron concentration profile only, but also agreement the B profiles at the deep position in the a-IGZO. On the other hand, bonding analyses by X-ray photoelectron spectroscopy (XPS) were carried out. From the XPS results, we find that Vo increases after the Ne+ implantation in a-IGZO and Boron-Oxygen bonding is dominant for the implanted B⁺ in a-IGZO, which indicates also that boron itself contribute the a-IGZO resistance reduction. Based on the knowledges obtained by the Hall measurements and the XPS results, we carried out a-IGZO TFT processes with gate length L_g = 4-10 um and channel width 10 um via the appropriate ion energies and doses of B⁺ or Ne⁺ implantation. The TFTs show off current 10^-12 A/mm, on/off ratio ~10⁸ in the case of the both ions from relations between gate voltage and drain current. In addition, we investigated the device scalability. The electrical channel length (L_eff) is not the same as the L_g, and is determined by the carrier profile at the gate edges. L_eff = L_g - ΔL is obtained by the transmission line method (TLM). From TLM results, we find that the ΔL < 1 um in case of the B⁺ implantations and ΔL ~ 1 um in case of the Ne⁺ implantations. In summary, we elucidated that a-IGZO sheet resistance reduction can be attributed to Vo or boron itself.
In addition, a-IGZO TFT processes with the ion implantations were carried out, which indicate good on/off ratios and microfabrication possibilities for industrial application. We expect that the B⁺ or Ne⁺ implantations in a-IGZO films are useful as resistance control technique for a-IGZO device processing.

This work is supported by Beam and Plasma Technology Laboratories Research and Development Division NISSIN ELECTRIC CO., LTD. in the large number of sample preparations and analyses.