Anomalous thickness-dependent optical energy gap of ALD-grown ultra-thin CuO films

Abstract

Usually an inverse square relation between the optical energy gap and the size of crystallites is observed for semiconducting materials due to the strong quantum localization effect. Coulomb attraction that may lead to a proportional dependence is often ignored or considered less important to the optical energy gap when the crystallite size or the thickness of a thin film changes. Here we report a proportional dependence between the optical energy gap and the thickness of ALD-grown CuO thin films due to a strong Coulomb attraction. The ultrathin films deposited in the thickness range of 9-81 nm show a p-type semiconducting behavior when analyzed by Seebeck coefficient and electrical resistivity measurements. The indirect optical energy gap nature of the films is verified from UV-vis spectrophotometric measurements. A progressive increase in the indirect optical energy gap from 1.06 to 1.24 eV is observed with the increase in the thickness of the films. The data are analyzed in the presence of Coulomb attractions using Brus model. The optical energy gap when plotted against the cubic root of the thickness of the films shows a linear dependence.

The Phase Evolution and Physical Properties of Binary Copper Oxide Thin Films Prepared by Reactive Magnetron Sputtering

“…Recently, Wang et al predicted that the indirect band gap of Cu 4 O 3 is 1.59 eV [4]. As for CuO, the type of band gap of CuO remains controversial; in some studies its band gap is suggested to be direct [16,17,18], but it is considered that its band gap is indirect in other studies [1,19,20], and its accurate band gap value is still a greater challenge for electronic structure calculations.…”

Composition Dependence of Optical Properties and Band Structures in p-Type Ni-Doped CuO Films: Spectroscopic Experiment and First-Principles Calculation

“…As an example, the absorption coefficient α of CuO-0.5Ni is plotted in Figure a based on the formula α = 4 πk /λ, where λ is the wavelength of the incident light. As can be seen, the maximum α value is ∼1.2 × 10 6 cm –1 , which is obviously larger (∼10 times) than that obtained from other experimental data. − That means a stronger ability to capture photons can be obtained, which is favorable for greater photocurrent in optoelectronic devices. The optical conductivity σ r at the photon energy of 3.3, 3.75, and 4 eV is plotted in Figure b.…”

Visible‐Light Absorbing TiO2 : Curcumin Thin Films with ALD/MLD

“…Recently, we demonstrated the possibility to tune the optical bandgap of different semiconducting metal oxide thin films through precise film thickness control (CuO), [22] or insertion of monomolecular organic layers within the metal oxide matrix (TiO 2 , ϵ‐Fe 2 O 3 ) [23,24] . These thin films were grown through ALD (atomic layer deposition) and MLD (molecular layer deposition) cycles, for the inorganic and organic layers, respectively.…”