Optimization of Ferroelectric and Memory Characteristics in HZO Gate Insulator FeTFTs Through ALD Cycle Control

Extended reality (XR) technology has gained significant attention in the display industry. Achieving high resolution is essential for XR displays, but storage capacitors occupying most of the backplane area hinder progress. To address this, ferroelectric materials have been introduced into the gate insulator of backplane thin-film transistors (TFTs). Since the discovery of ferroelectricity in Zr-doped HfO₂ (HZO) in 2011, extensive efforts have been made to enhance its properties for high-performance ferroelectric TFTs (FeTFTs) with multi-level conductance and a large memory window. In this study, we propose an atomic layer deposition (ALD) method to optimize HZO ferroelectric properties and demonstrate improved memory performance when applied as a gate insulator in FeTFTs.
Capacitors based on metal–ferroelectric–metal (MFM) structure were fabricated to demonstrate the ferroelectric properties of HZO film. A 10 nm-thick HZO film was deposited by thermal ALD. Two types of ALD deposition methods were attempted while fixing the total number of ALD cycles into 84. The first method consists of alternating 42 HfO₂ and 42 ZrO₂ cycles (conventional method). Another method for an enhanced ferroelectric property is composed of ZrO_{<span style="font-size:10.8333px">2</span>} placed in between HfO_{<span style="font-size:10.8333px">2</span>} cycles, while the proportion of ZrO_{<span style="font-size:10.8333px">2</span>} cycles was gradually increased starting from HfO_{<span style="font-size:10.8333px">2</span>}:ZrO_{<span style="font-size:10.8333px">2</span>} = 9:3 to 3:9 (gradually controlled cycle, GCC). The W top and bottom electrodes were deposited under the same conditions via radio frequency (RF) magnetron sputtering. Rapid thermal annealing (RTA) was performed at 500 °C for 1 minute for HZO crystallization. We confirmed the polarization-electric field (P-E) hysteresis curve of the MFM capacitor fabricated using two HZO deposition methods. In the pristine state, the remanent polarization value (2Pr) was 28.82 µC/cm² for the conventional method and 33.28 µC/cm² for the GCC method. After applying 10^₃ electric switching cycles for the wake-up process, the 2Pr value for the MFM capacitor fabricated using conventional method showed an approximately 20% increase compared to its pristine state, rising to 34.80 µC/cm². In contrast, the 2Pr value for the GCC method showed only an 8% increase, with a final value of 36.15 µC/cm². The mitigation of the wake-up effect and the improvement in remanent polarization can be attributed to the reduction of charge defects, such as oxygen vacancies, as well as the suppression of monoclinic phase formation by the ZrO₂ layers placed between the HfO₂ layers.
Also, FeTFTs were fabricated using HZO as the gate insulator to investigate the memory window characteristics. First, a W gate was deposited via RF sputtering. The HZO gate insulator was then formed using two different ALD methods. Next, an indium-gallium-zinc-oxide (IGZO) channel was deposited by RF magnetron sputtering, and then RTA was performed to crystallize the HZO. After the HZO crystallization, the channel was patterned and W for source and drain electrodes were deposited by RF magnetron sputtering. We confirmed the transfer characteristics with hysteresis of FeTFTs. The memory window of the FeTFT deposited by the conventional method was 1.96 V, but it was increased to 3.9 V when the GCC method was applied. These results suggested that the enhancement in memory characteristics can be attributed to the larger 2Pr value of HZO deposited using the GCC method. We expect that this novel ALD method for depositing HZO-based FeTFTs is expected to contribute to the advancement of next-generation display technology.