Abstract
Herein, we report an atomic layer deposition (ALD) process for Cu2O thin films using copper(II) acetate [Cu(OAc)2] and water vapor as precursors. This precursor combination enables the deposition of phase-pure, polycrystalline, and impurity-free Cu2O thin films at temperatures of 180–220 °C. The deposition of Cu(I) oxide films from a Cu(II) precursor without the use of a reducing agent is explained by the thermally induced reduction of Cu(OAc)2 to the volatile copper(I) acetate, CuOAc. In addition to the optimization of ALD process parameters and characterization of film properties, we studied the Cu2O films in the fabrication of photoconductor devices. Our proof-of-concept devices show that approximately 20 nm thick Cu2O films can be used for photodetection in the visible wavelength range and that the thin film photoconductors exhibit improved device characteristics in comparison to bulk Cu2O crystals.
Highlights
Copper(I) oxide (Cu2O) is a p-type semiconductor material that crystallizes in a simple cubic structure.[1]
These methods include solvothermal methods,[28] electrodeposition,[14,16,29] sol−gel synthesis,[30] thermal oxidation of copper,[4,5,31] sputtering,[2,7] chemical vapor deposition (CVD),[32−34] and atomic layer deposition (ALD).[23,35−39] From these methods, the solvothermal approach produces freestanding, one-dimensional Cu2O nanostructures, whereas the rest of the techniques are better suited for synthesizing Cu2O thin films
Because several thermal events occur before the source temperature is reached, Cu(OAc)2·H2O does not appear as an ideal ALD precursor
Summary
Copper(I) oxide (Cu2O) is a p-type semiconductor material that crystallizes in a simple cubic structure.[1] The p-type conductivity of Cu2O originates from intrinsic Cu vacancies in the crystal lattice, which facilitates the formation of acceptor levels approximately 0.25−0.45 eV above the valence band maximum.[2,3] Other properties of Cu2O related to its semiconducting character include an optical band gap of 2.1−2.2 eV for the bulk material,[4,5] a high absorption coefficient in the visible wavelength range,[4] hole mobility ranging from 40 to 120 cm2/(V s), and electron diffusion length reaching up to 5 μm.[6−11] Cu2O is nontoxic, low-cost, and consists of earth-abundant elements. Among the thin film synthesis methods, ALD stands out due to its unique ability to produce conformal, high-quality and pinhole-free films uniformly on large area substrates.[40,41]
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