Characterization of a 2D Electron Gas at the Interface of Atomic‐Layer Deposited Al₂O₃/ZnO Thin Films for a Field‐Effect Transistor

Abstract

The 2D electron gas (2DEG) phenomenon that occurs at the amorphous thin film hetero‐oxide interface attracts great attention since it can avoid the use of a single‐crystal oxide substrate. In this study, the analysis of 2DEG at the interface of amorphous‐Al2O3 (a‐AO)/ZnO is conducted using ZnO as the bottom substrate, where both the oxide films are grown by atomic layer deposition. Having used Al(CH3)3 as the Al‐precursor for the a‐AO film growth on the previously grown ZnO film, its strong reducing power induces the 2DEG formation at the interface. As a result of the Hall measurement, the 2DEG at the a‐AO/ZnO interface shows sheet resistance of 2.7 × 104 Ω ▫−1 and Hall mobility of 8.4 cm2 V−1 s−1. Using angle‐resolved X‐ray photoelectron spectroscopy, the thickness of the 2DEG layer is calculated as 0.62 nm, which is ≈120% of the c‐axis of the wurtzite ZnO unit cell. The field‐effect transistor fabricated exhibits a threshold voltage of −2.4 V, sub‐threshold swing of 0.33 V dec−1, and on/off ratio of 9.4 × 106, which significantly outperforms similar devices from previous works. The outstanding operation of 2DEG at the interface of AO/ZnO as a channel presents a possibility for application to a 2D‐based integrated circuit.

Threshold Voltage Modulation in a Transistor with a Two-Dimensional Electron Gas Channel at the Interface between Al2O3 and Sub-5 nm ZnO Films

“…Improvements in both I on and negative V th issues were reported by the authors’ group by adopting the ZnO thin film as a substitute for TO . The a -AO/ZnO thin film stack on amorphous SiO 2 exhibited a higher μ Hall of 8.4 cm 2 /(V s) and yielded a higher-performance FET with a low subthreshold swing (SS) of ∼0.33 V/dec and a high on/off ratio of ∼9.4 × 10 6 .…”

“…Application of 2DEG at the hetero-oxide superlattice, nanocomposite structure, semiconducting, or insulating hetero-oxide interface as FET channels has been extensively studied recently. Early reports on the emergence of the insulating hetero-oxide-based 2DEG between the hetero-epitaxial LaAlO 3 or amorphous-Al 2 O 3 ( a -AO) films on single-crystal SrTiO 3 have rendered such a Si-compatible device application impractical. − However, recent works on the fabrication of thin-film transistor (TFT) devices using the 2DEG generated at the a -AO/polycrystalline TiO 2 ( a- AO/TO) , or a -AO/polycrystalline ZnO ( a- AO/ZnO), which were deposited on SiO 2 /Si substrates, paved the way for the 2DEG to be used for Si-compatible FET applications. In these works, all the dielectric films could be deposited with atomic-scale precision using an industry-compatible atomic-layer deposition (ALD) method, which further strengthens the probability of their commercialization.…”

Harnessing Conductive Oxide Interfaces for Resistive Random-Access Memories

“…Later work has demonstrated that 2DEGs can be formed at more simple interfaces, between amorphous and single-crystalline oxides [30,31] with the benefit of room temperature preparation. Recently 2DEGs were shown to form even at amorphous/polycrystalline oxide interfaces [17,21,32,33] (Figure 1C). Furthermore, the oxide deposition temperatures were reduced from 650-900 °C to 25-300 °C, and the deposition techniques have been extended from PLD to the more scalable atomic layer deposition (ALD), which is widely used by the microelectronics industry.…”

“…These oxide interfaces provided a fertile ground for the discovery and manipulation of extraordinary physics, such as superconductivity [2][3][4][5], magnetism [6,7], magnetoelectric coupling [8,9], Rashba spinorbit coupling [10], persistent photoconductivity [11,12], and integer/fractional quantum Hall effect [13,14]. Over the last decade, leveraging these phenomena towards various devices, such as transistors [15][16][17][18][19], diodes [20], gas sensors [21], spintronic devices [22,23], and memory devices [24][25][26][27][28][29], has drawn considerable attention. In addition to the exotic phenomena listed above, the emergence of a high sheet density of electrons (typically 10 12 ∼10 15 cm −2 ) between two insulators is already attractive for some devices, such as in the role of channels or back electrodes.…”

“…The use of amorphous oxide on single crystal oxide substrates [16,24,30,31,[64][65][66][67][68] (Figure 1B) lowered the oxide deposition temperatures to <300 °C and expanded the deposition methods to ALD. The use of amorphous oxides deposited on polycrystalline oxide [17,21,25,32,33,69], which we term "All-ALD 2DEGs", provides a great advantage towards scalability and integration with existing and future technologies (Figure 1C). We note that significant progress has been demonstrated in MBE-based integration of epitaxial oxide 2DEGs systems with silicon [70][71][72] and other semiconductors [73,74].…”