ZnO-based oxide semiconductors have attracted much attention as active layer for thin film transistors (TFTs) due to their special features such as low-temperature process compatibility, high transparency, and wide bandgap (~3.37 eV). However, the instability and difficulty in controlling the electrical conductivity of ZnO still remain as critical issues. To solve these problems, various metal atoms have been introduced in ZnO, such as In, Ga, Sn, Al, and Hf. Among them, Hf and Al can be acted as stability enhancer by suppressing oxygen vacancies formation and as conductivity modulator by adjusting doping concentration, respectively. To effectively exploit their advantages, the composition control is one of the important parameters to be carefully adjusted. Furthermore, the active channel layer should have homogenous and smooth surface for forming good interface between Hf-Al-Zn-O (HAZO) active and Al₂O₃ gate insulator layers. From these viewpoints, atomic layer deposition (ALD) can be an optimum method to deposit the HAZO thin films because of such benefits as excellent conformality and good controllability for composition and film thickness. Thus, in this work, we fabricated top-gate transparent oxide TFTs using newly presented HAZO active channels deposited by ALD method and investigated the effects of Hf and Al concentration on the device performance including basic characteristics such as carrier mobility, subthreshold swing (SS), and turn-on voltage (V_on), and reliability characteristics.

The ALD temperature of HAZO was fixed at 100 ^oC. Tetrakis(ethylmethylamino)hafnium (TEMAHf), trimethylaluminium (TMA), diethylzinc (DEZ), and H₂O were used as Hf, Al, Zn, and O₂ sources, respectively. In this work, the composition control of Hf and Al was selected as main strategy to modulate device property. The atomic composition of 20-nm-thick HAZO thin films was controlled by adding TEMAHf and TMA cycles among a total 100 cycles. Total ALD cycles were designed so that the HfO₂ and Al₂O₃ were located in the intervals of ZnO. For the HAZO thin film with 2-at%-Hf and 2-at%-Al, the position of two TEMAHf cycles were placed on 40^th and 80^th cycles, and two TMA cycles were placed on 20^th and 60^th cycles of total 100 cycles, separately. The incorporated amounts of Hf and Al were modulated into 2, 4, 6, 8, and 10 at% and 2 and 4 at%, respectively.

The data figure shows the comparisons in transfer characteristics for the fabricated top-gate TFTs using HAZO active layers with various amounts of incorporated Hf when the incorporated Al amount was fixed at 2 at%. The fabricated devices show distinct differences in device characteristics depending on the incorporated Hf and Al compositions. When the Hf amounts were varied from 2 to 10 at%, the V_on’s shifted to positive direction from negative side toward 0 V and SS values were improved from 1.12 to 0.37 V/dec. In contrast, the carrier mobility was slightly decreased from 2.97 to 1.24 cm²/Vs. The post-annealing effects were also investigated. The fabricated TFTs were thermally treated at the temperature range from 150 to 250 ^oC for 1 h in oxygen atmosphere. The V_on shifted toward positive direction and SS were improved as the increase in annealing temperature, although the on-current and carrier mobility showed decreasing trend.

In conclusion, we successfully confirmed that the new composition of oxide semiconductor, HAZO, can be utilized as an active layer for the oxide TFT, and that the electronic natures of the HAZO channel layers can be well controlled by adjusting the ALD conditions. The top-gate TFT using the HAZO channel with 10-at%-Hf and 2-at%-Al showed the best device characteristics such as low SS value of 370 mV/dec, low-voltage operation at around 0 V, and comparable carrier mobility of 1.24 cm²/Vs. It was also found that thermal treatment in oxygen atmosphere markedly influenced on the transfer characteristics including the location of V_on. Here, we believe that the device performance can be far improved by simultaneously optimizing the Hf and Al incorporating amounts and the post annealing process condition. Optimum ALD conditions for preparing the HAZO channel layers will also be important for better device performance for the HAZO TFTs. Furthermore, we have plans to investigate the device reliability, which is one of the most important factor in evaluating device characteristics, such as positive bias temperature stress stability (PBTS), and negative bias illumination stability (NBIS). As results, highly-stable TFTs will be implemented with practical value of carrier mobility by employing HAZO channel with appropriate incorporating amounts of Hf and Al at optimum ALD conditions.