Low‑Temperature ALD of In₂O₃ Nanofilms for High‑Performance Thin‑Film Transistors

Background

Indium oxide (In₂O₃) is a transparent metal‑oxide semiconductor with a wide band gap (~3.7 eV) at room temperature, high visible‑light transparency, and excellent chemical stability. It is therefore a key material for photovoltaic devices, electrochemical sensors, and flat‑panel displays. Conventional deposition techniques—sputtering, sol‑gel, and chemical vapor deposition (CVD)—often suffer from poor uniformity over large areas, inaccurate elemental composition, or high deposition temperatures (>300 °C). These limitations hinder the creation of uniformly thick, compositionally precise In₂O₃ films at low temperatures.

Atomic‑layer deposition (ALD) offers superior step coverage, atomic‑scale thickness control, excellent uniformity, and lower processing temperatures. Prior ALD studies of In₂O₃ have employed precursors such as InCl₃–H₂O, InCl₃–H₂O₂, InCp–O₃, InCp–O₂–H₂O, and (CH₃)₃In–H₂O. However, these processes typically require deposition temperatures of 200–500 °C, yield low growth rates (0.25–0.40 Å per cycle), and produce corrosive HCl by‑products that can damage equipment and the film. TMIn‑based precursors achieve higher growth rates (1.3–2 Å per cycle) but still necessitate temperatures above 200 °C.

In this work, we propose InCp and H₂O₂ as precursors for ALD of In₂O₃, enabling deposition at lower temperatures with satisfactory growth rates. The resulting films were characterized for structural, chemical, and optical properties, and integrated into TFTs with ALD Al₂O₃ gate dielectrics to assess electrical performance.

Experimental

Si (100) wafers were cleaned with a standard RCA process to serve as substrates. In₂O₃ films were deposited on the pre‑cleaned wafers using a Wuxi MNT Micro Nanotech ALD system at 150–210 °C. InCp (99.999 % purity) and 30 % aqueous H₂O₂ precursors were maintained at 130 °C and 50 °C, respectively, with nitrogen purging gas. To evaluate the functional film, In₂O₃‑channel TFTs were fabricated: a 38‑nm Al₂O₃ gate dielectric was grown at 200 °C on a p‑type Si (100) substrate (<0.0015 Ω·cm) via trimethylaluminium and H₂O ALD; a 20‑nm In₂O₃ channel was deposited at 160 °C; Ti/Au (30 nm/70 nm) source/drain contacts were defined by optical lithography, electron‑beam evaporation, and lift‑off. Devices were post‑annealed in air at 300 °C for varying times.

Characterization included X‑ray diffraction (Bruker D8 Discover), atomic force microscopy (Bruker Icon), X‑ray photoelectron spectroscopy (Kratos Axis Ultra DLD), UV‑VIS spectroscopy, and ellipsometry (Sopra GES‑SE). Electrical measurements employed an Agilent B1500A semiconductor parameter analyzer with a Cascade probe station at room temperature in ambient air.

Results and Discussion

Figure 1a illustrates the growth rate versus substrate temperature. A stable rate of ~1.46 Å per cycle is achieved between 160 and 200 °C, indicating a fast growth window for ALD In₂O₃. At 150 °C the rate rises due to InCp condensation, while at 210 °C it increases because of thermal decomposition of InCp. Figure 1b shows linear thickness growth with the number of ALD cycles at 160 °C, confirming uniform deposition.

a Growth rate of ALD In₂O₃ film on Si as a function of substrate temperature, and b dependence of film thickness on the number of ALD cycles at 160 °C.

X‑ray diffraction patterns (Fig. 2) reveal that films deposited at ≤160 °C are amorphous; at 170 °C early crystallinity appears, and at 210 °C pronounced peaks at 2θ = 30.3° and 35.4° indicate enhanced grain growth. Surface morphology (Fig. 3) shows increasing root‑mean‑square (RMS) roughness from 0.36 nm at 160 °C to 1.15 nm at 210 °C, correlating with crystallite size.

X‑ray diffraction patterns of In₂O₃ films deposited at different temperatures for 250 cycles.

AFM images of In₂O₃ films at 160 °C (a), 180 °C (b), 200 °C (c), and 210 °C (d), with 250 cycles each.

High‑resolution XPS spectra (Fig. 4) show a C 1s peak at 289.8 eV (C–O) at 160 °C, which diminishes at 180 °C and disappears at 200 °C, indicating reduced carbon contamination with higher temperatures. In 3d spectra display doublet peaks at 444.7 and 452.3 eV, confirming In₂O₃ bonding. O 1s spectra decompose into O¹ (529.8 eV), O² (531.0 eV), and O³ (532.0 eV) components, corresponding to lattice oxygen, oxygen vacancies, and –OH/CO species, respectively. As temperature increases, O¹ rises from 76 % to 92 %, while O² falls from 16 % to 4 %, reflecting a significant reduction in oxygen vacancies and hydroxyl groups.

High‑resolution a C 1s, b In 3d, and c O 1s spectra of In₂O₃ films deposited at 160, 180, and 200 °C. Samples were etched with in‑situ Ar ion bombardment for 6 min prior to measurement.

Optical absorption (Fig. 5) shows (αhν)² versus photon energy; the extrapolated band gaps increase from 3.42 to 3.75 eV as temperature rises from 150 to 200 °C. The widening band gap is attributed to reduced oxygen vacancies, lower carbon impurities, and the Burstein–Moss shift caused by increased carrier concentration in larger grains.

a (αhν)² vs photon energy for In₂O₃ films at different temperatures; b extracted band gap dependence on deposition temperature.

The fabricated TFTs initially displayed a conducting channel due to high oxygen‑vacancy density. Post‑annealing in air at 300 °C for 2 h introduced typical switching behavior. Extending annealing to 10 h further shifted the threshold voltage positively and improved the subthreshold swing to 0.32 V/dec. At 11 h, performance degraded, likely due to excessive crystallization changes or Ti electrode oxidation. A 10‑h annealed device achieved a field‑effect mobility of 7.8 cm²/V·s, V_th of –3.7 V, and an on/off ratio of 10⁷.

a Transfer characteristics of In₂O₃ TFTs annealed at 300 °C for various times; b Output characteristics after 10 h annealing.

XPS analysis of films annealed for 2–11 h (Table 3) shows the In:O ratio approaching the stoichiometric 1:1.5 value, confirming oxygen‑vacancy passivation. Excessive annealing, however, deteriorated device performance, underscoring the need for optimized annealing time.

Conclusions

ALD of In₂O₃ films using InCp and H₂O₂ precursors delivers a rapid, uniform growth rate of 1.46 Å per cycle at 160–200 °C. Higher deposition temperatures enhance crystallinity while markedly reducing oxygen vacancies and carbon contamination, thereby increasing the optical band gap. When incorporated as the channel layer in TFTs with ALD Al₂O₃ gate dielectrics, the In₂O₃ films yield devices that, after 10 h of air annealing at 300 °C, exhibit a field‑effect mobility of 7.8 cm²/V·s, a subthreshold swing of 0.32 V/dec, and an on/off ratio of 10⁷. These results demonstrate the viability of low‑temperature ALD In₂O₃ for high‑performance transparent electronics.