ALD/ALE 2024 Session AA3-WeA: Display Applications

The latest advancements in displays include micro-LEDs with chip sizes < 100 µm that are used as subpixels. To avoid color filters and bulky layers with separate color converter devices, the color converter is preferably deposited on-chip.

Their narrow emission spectrum and size-tunability make quantum dots (QDs) attractive as color converters in displays. The best alternative to Cd-based QDs are InP-based QDs, owing to their ability to emit the full spectrum of colors, their structural integrity and low toxicity¹. However, non-radiative pathways from defect-related traps or interactions with the environment (H₂O, O₂) can be detrimental to the luminescence efficiency.

State-of-the-art hybrid barriers are explored to safeguard the QD luminescence. Metal oxides grown by atomic layer deposition (ALD) have shown potential when paired with organic layers, such as those deposited by molecular layer deposition². The defect-free nature and conformality of the coatings is unique to ALD, making it an ideal technique for barrier applications.

This study investigates ALD for stable on-chip QD LEDs as follows: (a) Understanding the optical response of core/multi-shell InP/ZnSe/Zn(Se,S)/ZnS QDs to precursors (TMA, TDMAT, DEZ), reactants (H₂O, O₃), plasmas (Ar, H₂, O₂) and full ALD processes (Al₂O₃, TiO₂, ZnO) using a home-built in situ photoluminescence setup³. Results show that the QDs are highly susceptible to degradation during the ALD process (Fig. 1). (b) Characterizing ALD growth (Al₂O₃) on polymer substrates (Kraton) by spectroscopic ellipsometry and Fourier transform infrared spectroscopy. Island growth is identified as the dominant growth mode and is linked to the copolymeric structure of Kraton (Fig. 2). It is observed the stability of the QDs during ALD is vastly improved by polymer embedding. (c) Fabrication of pick-and-place, hybrid QD pockets using a digital light processing setup as a demonstrator for on-chip QD LEDs (Fig. 3)⁴. By combining the on-chip design of the QD pockets and seamless encapsulation of the QDs by a polymer (thiol:ene) with the barrier performance of ALD coatings, this work aims to contribute to the development of stable QD LEDs for displays. (d) Examining the long-term stability of (ALD coated) pristine QDs, polymer-embedded QDs, and QD pockets through accelerated aging tests using a humidity chamber. The hybrid QD pockets show superior stability demonstrating their potential for QD LEDs.

¹ACS Nano 2022, 16, 6, 9701–9712.

²Nanoscale Res. Lett. 2015, 10, 130.

³ACS Appl. Mater. Interfaces 2019, 11, 29, 26277–26287.

⁴ACS Appl. Mater. Interfaces 2023, 15, 7, 9629–9637.

Oxide semiconductors (OSs), known for their high mobility, large-area uniformity, low temperature processability, low off current, are garnering interest in display applications.¹ To apply in various fields, it is essential to enhance the electrical properties of OS thin-film transistors (TFTs). Atomic layer deposition (ALD) a-IGZO has been reported to exhibit improved electrical properties due to increased packing density compared to the conventional sputtering method.² Sputtering uses a target mixed with each element to form a homogeneous thin film, but supercycle-based ALD forms a relatively heterogeneous thin film. In this regard, it is crucial to consider the cation distribution resulting from the ALD supercycle configuration in multicomponent OSs.

In this study, we aimed to compare surface and bulk compositions based on cation distributions by reversing the supercycle sequence of the indium (In), gallium (Ga), zinc (Zn) elements in IGZO: A (In (14) -Ga (3) -In (14) -Zn (4)), A'(Zn (4) -In (14) -Ga (3) -In (14)). As a result, the surface composition (XPS) of channels A and A' was significantly higher for the element located in the last order of the supercycle sequence (A: Zn-rich surface, A': In-rich surface). However, the XRF analysis indicated a similar bulk composition for A and A', and notable distinctions in physical properties were not evident. Despite considerable differences in surface composition between A and A', the TFT electrical properties did not show substantial variation, but there was a significant distinction in reliability. The outcomes suggest that the bulk composition has a more pronounced influence on the electrical properties of the TFTs, and the composition ratio at the surface acting as the back channel has a significant impact on stability. The A' TFT, with an In-rich surface, experiences a hump phenomenon caused by the formation of a parasitic channel on the back channel, leading to performance degradation. Our research proposes that the cation distribution in ALD-based multicomponent heterogeneous channel TFTs can affect the electrical properties and reliability results, depending on the device structure.

References

1. Mativenga, Mallory, Sungjin An, and Jin Jang. Bulk accumulation a-IGZO TFT for high current and turn-on voltage uniformity. IEEE electron device letters 34.12 (2013): 1533-1535.

2. Cho, Min Hoe, et al. Comparative study on performance of IGZO transistors with sputtered and atomic layer deposited channel layer. IEEE Transactions on Electron Devices 66.4 (2019): 1783-1788.