ALD/ALE 2021 Session AA12: Display Applications: Thin Film Transistor, Diodes, Thin Film Encapsulation for OLEDs/QDs...

Two-dimensional electron gas (2DEG) has been realized with various fabrication process using diverse oxide heterostructure since epitaxial LaAlO₃/single-crystal SrTiO₃ (LAO/STO) heterostructure was reported as a typical 2DEG system, which shows high density of electrons (~10¹³-10¹⁴ cm^-2) confined at the oxide interface. The origin of 2DEG created at the heterointerface is still controversial mainly between the discontinuity of polarity at the atomic layers, inducing electron charge reconstruction and the presence of oxygen vacancies (V_o) at the interface, widely known as electron donors. Recently, we reported V_o generation mechanism-based 2DEG formation process using atomic-layer-deposited (ALD) ultrathin (~10 nm) binary metal oxide heterostructure: amorphous Al₂O₃/polycrystalline TiO₂, whose electrical property is comparable with typical LAO/STO epitaxial 2DEG system at room temperature (sheet carrier density, n_sh= ~10¹⁴ cm^-2, electron mobility, μ_n= ~4 cm²V^-1s^-1). To demonstrate V_o generation mechanism for 2DEG creation specifically, an in-situ resistance measurement was conducted to prove the presence and effect of V_o at the heterointerface. The resistance of the interface dropped significantly with the injection of trimethylaluminum (TMA) molecules, indicating that V_o were formed on the TiO₂ surface during the TMA pulse in the ALD of the Al₂O₃ film, such that they provide electron donor states to generate free electrons at the interface of the Al₂O₃/TiO₂ heterostructure, creating 2DEG. Being well-informed of this mechanism, ZnO as a different bottom material was applied for oxide heterostructure 2DEG system to improve electrical and structural property of TiO₂-based 2DEG due to its excellent intrinsic property. As expected, the Al₂O₃/ZnO heterostructure exhibited enhanced electrical properties (n_sh= ~10¹⁴ cm^-2, μ_n= ~15 cm²V^-1s^-1), even at the lower thickness of bottom layer (~ 5 nm) and lower deposition temperature (150^oC).

In this work, using ultrathin Al₂O₃/ZnO 2DEG layer as a channel, we succeeded to fabricate 2DEG field-effect transistors (FETs), achieving extremely low off-current (I_off ~10^-9 A/m), high on/off current ratio (I_on/I_off > ~10⁹), and low subthreshold swing (SS ~95 mV/dec.), which outperforms other oxide heterostructure-based FETs reported so far, including the previous work of TiO₂-based 2DEG system.Furthermore, due to facile film deposition with excellent thickness control of ALD, stacking 2DEG layers is possible to make three-dimensional multi-stacked 2DEG FETs, leading to great conductivity resulting from accumulated electron transport path. The detailed experimental results will be presented.

In the past decade, extensive research has been carried out on p-type oxide semiconductor materials for realisation of complementary metal oxide semiconductor (CMOS) circuits. In particular, SnO is of great interest due to its disperse valence band maximum due to hybridization between O 2p and Sn 5s orbitals, allowing a relatively high hole mobility [1]. While sputtering method has been widely used for SnO, atomic layer deposition (ALD) has not been widely reported [2]. ALD is a very attractive technique due to its precision on stoichiometry arising from the self-limiting growth, its repeatability and its conformality over a large substrate area [3].

In this report, SnO thin films were deposited using a novel Sn precursor and H₂O in a cross-flow wafer scale ALD reactor. The precursor bottle was heated at 100°C and depositions were carried out at temperatures between 170 and 210°C. To achieve high quality films suitable for channel layers in thin film transistors (TFTs), we investigated three deposition modes: single pulse (SP), multiple pulses (MP) and multiple pulses and exposure (MP+E). The SP mode is the standard ALD process comprising of the Sn and H₂O half cycles. The pulse/purge times of 1s/15s and 0.03s/10s are used for the Sn and H₂O cycles respectively. In the MP mode, three consecutive Sn pulses are applied with 5 s delay between the pulses which is then followed by the H₂O half cycle. The MP+E mode is a combination of the MP mode and stopping of the gas flow for 10s to increase the residence time in the chamber.

P-type SnO thin films were then incorporated as a channel layer in TFTs. A field effect mobilities of 0.5 and 2.5 cm²V^-1s^-1 were achieved for TFTs annealed at 250°C and 350°C respectively. We will discuss in detail the effect of different ALD deposition modes on the characteristic of the thin films and the performance of the TFTs.

[1] J. A. Caraveo-Frescas, P. K. Nayak, H. A. Al-Jawhari, D. B. Granato, U. Schwingenschlogl, and H. N. Alshareef, ACS Nano, vol. 7, no. 6, pp. 5160-5167, 2013.

[2] J. H. Han, Y. J. Chung, B. K. Park, S. K. Kim, H.-S. Kim, C. G. Kim, and T.-M. Chung, Chemistry of Materials, vol. 26, no. 21, pp. 6088-6091, 2014.

[3] S. George, Chem. Rev. 110, 111 (2010).

Recently, ALD oxide semiconductor has been attractive as TFT material that has the potential for high mobility and stability compared to conventional method due to precisely controlled thickness and metal composition. However, multi-component deposition using ALD is difficult to control without understating growth mechanism according to precursor and reactant, it is necessary to study and library the adsorption and reactivity of the surface depending on various precursor. In this study, InGaO (IGO) semiconductors were deposited by plasma enhanced atomic layer deposition (PEALD) using two sets of In and Ga precursors, which one set is In(CH₃)₃[CH₃OCH₂CH₂NHtBu] (TMION) and Ga(CH₃)₃[CH₃OCH₂CH₂NHtBu]) (TMGON) and the other is (CH₃)₂In(CH₂)₃N(CH₃)₂ (DADI), (CH₃)₃Ga (TMGa), as denoted as TM-IGO and DT-IGO, respectively. We changed the ratio of InO sub-cycles from 3 to 19 to control the chemical composition of ALD-processed films. The different growth properties are observed at different precursor set. This could be originated from the precursor structure and the density of adsorption sites. Despite this different growth behavior, it could set the IGO TFTs with the identical In/Ga ratio controlling each super-cycle. Interestingly, the both TFTs (TM-IGO and DT-IGO) showed different film properties and the associated TFT characteristics (TM-IGO: -5.5V, 36.7 cm²/Vs, DT-IGO: -9.7V, 27.7 cm²/Vs for the V_th and mobility respectively). This difference could be originated from not only the growth behavior but also the anion/cation ratio/binding states in the IGO thin films.

Nowadays, a novel deposition technique for thin film transistor (TFT) application using atomic layer deposition (ALD) such as semiconductor, gate insulator (GI), and encapsulation has been studied extensively. Herein, we developed unified-ALD (U-ALD), which deposits buffer, semiconductor and GI by ALD and named this structure as sandwich structure. In U-ALD IGZO TFTs, material forming interfaces with the channel layer exhibited a critical role in the electrical performance of IGZO TFTs because of hydrogen (H) diffusion, which has a Janus-faced effect in IGZO. Through measurement of hydrogen permeability of ALD insulators and Secondary Ion Mass Spectroscopy of each sandwich structure after annealing, we found a hydrogen accumulation effect in the ALD IGZO layer like a dam, which caused degradation of TFT properties. In contrast, TFTs with ALD SiO₂, which has proper hydrogen diffusivity, chosen as the buffer and GI had favorable electric properties of 28.17 cm²/Vs, 0.20 V/decade, 0.96 V, and 0.12 V for the mobility, V_th, SS, and hysteresis. In this regard, an optimized GI structure via the ALD SiO₂ and Al₂O₃ in situ process on the basis of excellent interface formation with the semiconductor and hydrogen barrier performance, respectively, was developed. This functional GI structure with SiO₂ and Al₂O₃ exhibited proper TFT characteristics (27.52 cm²/Vs, 0.24 V/decade, and 1.07 V for the mobility, SS, and V_th, respectively) and improved stability against hydrogen annealing, which was used to examine the resistance to external hydrogen.