Structural, Optical, and Electrical Properties of Copper Oxide Films Grown by the SILAR Method with Post-Annealing

Wen-Jen Lee,Xin-Jin Wang

doi:10.3390/coatings11070864

Abstract

Copper oxides are widely used in photocatalysts, sensors, batteries, optoelectronic, and electronic devices. In order to obtain different material properties to meet the requirements of different application fields, varied technologies and process conditions are used to prepare copper oxides. In this work, copper oxide films were grown on glass substrates by a successive ionic layer adsorption and reaction (SILAR) method with subsequent annealing under an atmospheric environment. The films were characterized by using an X-ray diffractometer, Raman spectrometer, Scanning electron microscope, UV-Visible-NIR spectrophotometer, and Hall Effect measurement. The results show that the as-deposited film has a Cu2O crystal structure, which begins to transform into Cu2O-CuO mixed crystal and CuO crystal structure after annealing at 300 °C for a period of time, resulting in the bandgap of being reduced from 1.90 to 1.34 eV. The results show that not only are the crystal structure and bandgap of the films affected by the post-annealing temperature and time, but also the resistivity, carrier concentration, and mobility of the films are varied with the annealing conditions. In addition, the film with a Cu2O-CuO mixed crystal shows a high carrier mobility of 93.7 cm2·V−1·s−1 and a low carrier concentration of 1.8 × 1012 cm−3 due to the formation of a Cu2O-CuO heterojuction.

Highlights

Copper oxide has two typical crystalline forms as cupric oxide and cuprous oxide that the CuO and Cu2O are well-known p-type semiconductors with a bandgap energy of about 1.3–2.1 and 2.1–2.6 eV, respectively [1]
In order to identify the crystalline structure of the copper oxide films, the films were analyzed by X-ray diffraction (XRD)
According to the XRD patterns, the as-deposited and 200 ◦C-annealed films have a polycrystalline structure with single-phase Cu2O, and the films annealed at 300 ◦C for 2 and 3 h have a polycrystalline structure with single-phase CuO

Summary

Introduction

Copper oxide has two typical crystalline forms as cupric oxide (tenorite monoclinic CuO) and cuprous oxide (cuprite cubic Cu2O) that the CuO and Cu2O are well-known p-type semiconductors with a bandgap energy of about 1.3–2.1 and 2.1–2.6 eV, respectively [1]. The SILAR method is a modified chemical bath deposition (CBD) developed by Nicolau in 1985 [31] It has the characteristics of being low cost, having a low processtemperature, precise film-thickness control, and being suitable for large-area deposition. This is different from the typical CBD process, where all chemicals are present in the reaction vessel at the same time for film deposition. The substrate is immersed in the anion precursor solution, and the anions will react with the chemically adsorbed cations to form a thin film of solid compound on the surface of the substrate. Since the film growth mechanism of SILAR is the layer-by-layer stacking of ions, SILAR is called solution atomic layer deposition (SALD) [32] or liquid atomic layer deposition (LALD) [33]

Methods

Results

Conclusion