Evaluation of the microstructural, electronic and optoelectronic properties of ƴ-CuCl thin films and their fabrication on Si substrates

Abstract

Cuprous chloride is a direct wide bandgap (Eg = ~ 3.4 eV) semiconductor with a large excitonic binding energy (~ 190 meV). In this study, CuCl has been deposited by the vacuum evaporation method on a variety of substrates (amorphous silica glass, indium tin oxide (ITO) coated on glass and Silicon (100)) substrates, encapsulated and characterized as a potential material for optoelectronic applications. Some of the samples were also oxygen plasma treated for durations of 1, 2 and 3 minutes, respectively. Room temperature x-ray diffraction (XRD) measurements show that CuCl grows preferentially in a (111) orientation irrespective of the underlying substrate. Microstructural properties of the films gave nearly the same values for untreated CuCl films deposited on glass, ITO and Si substrates (particle size, L = 9.6 nm ± 1 nm). On the other hand, the microstructural properties of the plasma treated films vary as a function of plasma treatment duration. At 10 K, the photoluminescence (PL) spectrum of the untreated CuCl/Si films using 244 nm excitation reveals four peaks: the Z₃ free exciton occurring at 3.203 ± 0.003 eV, the I₁ impurity bound exciton located at 3.181 ± 0.003 eV, the M free biexciton occurring at 3.160 ± 0.003 eV and N₁ impurity bound to bi-exciton located at 3.135 ± 0.003 eV. However, the 20 K PL spectra of the untreated CuCl films deposited on all three substrates (using a 325 nm excitation) revealed only the Z₃ free exciton, the I₁ impurity bound exciton and the N₁ impurity bound biexciton at 3.204 eV, 3.18 eV and 3.152 eV, respectively, irrespective of the underlying substrate. The room temperature PL spectra of the films were dominated by the Z₃ free exciton. The measured band gap increased as the temperature increases, which is opposite to most conventional semiconductors. This anomalous effect is believed to be related to electron-phonon renormalization or coupling of the electronic structure of CuCl. On the other hand the PL spectra of the O₂ plasma immersed film were all mainly dominated by the free Z₃ free exciton only. In addition, at low temperatures a broad band ascribed to an oxygen related emission process is observed at ~ 3 eV in all the plasma treated samples. The band gap of the O₂ plasma immersed films follow the anomalous temperature dependency in a similar manner to the untreated films; however the plasma treated films were less sensitive to temperature. Both steady state DC and AC impedance spectroscopy experiments suggested that the untreated CuCl is a mixed ionic-electronic semiconductor material. Room temperature steady state DC measurements using reversible electrodes (Cu) gave an Ohmic response while using irreversible electrodes (Au) gave an exponential I–V behaviour, both in conformance with Wagner’s defect chemistry analysis of a mixed ionicelectronic material. An electronic conductivity of the order of 2.3 × 10-7 S/cm was deduced to be in coexistence with Cu⁺ ionic conductivity using irreversible electrodes (Au), while a total conductivity of the order of 6.5 × 10-7 S/cm was obtained using reversible electrodes (Cu) at room temperature. The Arrhenius plot of the electrical characteristics of the untreated films reveal two distinct regimes corresponding to electronic conduction below ~ 270 K and a Cu⁺ extrinsic ionic conduction mechanism ix above that temperature. Due to the fact that at low temperatures, the thermal energy is inadequate for maintaining considerable ionic motion, it follows that the mode of conduction at lower temperatures is ascribed to electronic processes. On the other hand, the Arrhenius plot of the plasma treated films showed a single regime throughout most of the temperature range. This is interpreted to be an electronically dominant conduction mechanism. The large increase in the conductivity of the treated CuCl films (over 100 fold) is ascribed to effect of oxygen introducing an acceptor state in CuCl films. This is due to the fact that oxygen dissolves in cuprous halides on substitutional anionic sites. Cathodic deposition of Cu metal via electrolytic decomposition was observed when a steady state voltage greater than 5 V was applied to both the untreated and the plasma treated films. This poses a great challenge in utilizing this material to fabricate optoelectronic devices under the influence of steady state source. The untreated films were successfully encapsulated using organic polysilsesquioxane (PSSQ) and cyclo olefin copolymer (COC) dielectrics. However, both encapsulants failed to prevent the O2 plasma immersed films from oxidising, and this will also represent a future challenge for this technology