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

Abstract

CuCl is a I-VII semiconductor material with a direct band gap of ∼3.4 eV. It exhibits a zincblende structure (γ -phase) at low temperatures, up to ∼680 K. Unlike GaN, ZnO and related materials, CuCl has a relatively low lattice mismatch with Si (<0.4%) and a large excitonic binding energy (∼190 meV). This suggests the possibility of the fabrication of excitonic-based blue/UV optoelectronic devices on Si with relatively low threading dislocation densities. In this study, CuCl has been deposited and examined as a candidate material for the fabrication of these devices. X-ray diffraction (XRD) measurements confirmed that the deposited films were preferentially oriented in the (1 1 1) plane. Room temperature photoluminescence measurements reveal a strong Z 3 free exciton peak (3.232 eV). Both steady state dc and ac impedance spectroscopy experiments suggested that the deposited CuCl is a mixed ionic-electronic semiconductor material. An electronic conductivity of the order of 2.3 × 10 −7 S cm −1 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 −1 was obtained using reversible electrodes (Cu) at room temperature. Further to this, we have identified some of the challenges in fabricating an optoelectronic device based on a CuCl/Si hybrid platform and propose some possible solutions.

Structural, optical and electrical properties of Co‐evaporated CuCl/KCl films

“…Similar to the I 1 bound exciton, the most probable candidate for the impurity is considered to be a neutral acceptor [7]. The values of the excitonic peaks reported here are in excellent agreement with our previous results on undoped CuCl thin films [5,6]. Beyond 70 K, the PL spectra of all the films manifest the intense Z 3 excitonic peak even up to room temperature due to the large binding energy of the Z 3 exciton (~ 190 meV).…”

“…Based on our experience of deposition of polycrystalline vacuum evaporated CuCl films on glass substrates [5,6,25], CuCl grows preferentially in the (111) orientation, which is the case for the co-evaporated films reported in this study. The room temperature UV-VIS absorption spectra of all films were dominated by the contribution of both high and low excitonic bands, historically called the Z 12 and Z 3 excitons, respectively.…”

“…There have been several reports of high quality deposition of CuCl films by many authors, including our group [5,6] and by other workers [7]. However despite the large free excitonic binding energy of CuCl, the luminescence intensities appear to be weak when compared to mature optical emitters such as GaAs or GaN for which consistent high quality room temperature electroluminescence, spontaneous emission and lasing could be demonstrated.…”

CuBr blue light emitting electroluminescent thin film devices

“…This process of metallic cathodic deposition is known as electrolytic decomposition and occurs when a polar material is subjected to a steady state source greater than its decomposition threshold. According to [7,13], the threshold voltage is a function of the combination of the thermodynamic decomposition voltage, which could be deduced from the Gibbs free energy of formation of CuBr (2CuBr → Cu + CuCl 2 ; ~ 0.7 V at atmospheric pressure), over voltages for different processes, including Ohmic drop, charge transfer at the electrodes and the nucleation of the electrodeposits. On the other hand there was no indication of electrolytic decomposition in the samples subjected to AC voltage stresses even for durations up to 3 hours as shown on Fig.…”

“…Both CuCl and CuBr are known to be mixed ionic-electronic semiconducting materials [7,17], however CuBr has the advantage of superior electronic conductivity, better ambient and thermal stability than CuCl. Thus in this study, we report on the development of a blue thin film electroluminescent device (TFELD) based on physical vapour deposited CuBr thin films.…”