Abstract
Indium oxide (InxOy) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium and oxygen plasma in a low-temperature range of 80–200 °C. The optical properties, chemical composition, crystallographic structure, and electrical characteristics of these layers were investigated by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), as well as current-voltage and capacitance-voltage measurements. The SE results yielded a nearly constant growth rate of 0.56 Å per cycle and a thickness inhomogeneity of ≤1.2% across 4-in. substrates in the temperature range of 100–150 °C. The refractive index (at 632.8 nm) was found to be 2.07 for the films deposited at 150 °C. The PEALD-InxOy layers exhibit a direct (3.3 ± 0.2 eV) and an indirect (2.8 ± 0.1 eV) bandgap with an uptrend for both with increasing substrate temperature. Based on XPS characterization, all InxOy samples are free of carbon impurities and show a temperature-dependent off-stoichiometry indicating oxygen vacancies. XRD diffraction patterns demonstrate an onset of crystallization at 150 °C. Consistent with the optical, XPS, and XRD data, the films deposited at ≥150 °C possess higher electrical conductivity. Our findings prove that a low-temperature PEALD process of InxOy is feasible and promising for a high-quality thin-film deposition without chemical impurities on thermally fragile substrates.
Highlights
Indium oxide (In2O3), a transparent conducting oxide (TCO) material, is intensively investigated due to its promising material properties including a direct and an indirect bandgap of about 3.7 and 2.6 eV, respectively,[1,2] high optical transparency in the visible range,[3] excellent electrical properties,[4] and good chemical stability.[5]
It should be highlighted that the obtained growth rate of ∼0.56 Å/cycle using plasma-enhanced atomic layer deposition (PEALD) with TMIn and O2 plasma within a low-temperature range of 100–150 °C is higher than those of In2O3 films deposited by atomic layer deposition (ALD) with TMIn/ H2O and TMIn/O3 where growth rate per cycle (GPC) values of ∼0.4 and 0.46, respectively, have been achieved.[28,40]
This comparison of the electronic structure is not done on the same basis as in the mentioned electrical data of PEALD layers, we believe that the lower abundance of the excitonic states in the In2O3 single crystal[13] in comparison with the Ga2O3 single crystal[79] is somehow reflected in their amorphous counterparts
Summary
Indium oxide (In2O3), a transparent conducting oxide (TCO) material, is intensively investigated due to its promising material properties including a direct and an indirect bandgap of about 3.7 and 2.6 eV, respectively,[1,2] high optical transparency in the visible range,[3] excellent electrical properties,[4] and good chemical stability.[5]. A higher growth rate and a lower resistivity (
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