**1. Introduction**

Copper(I) chloride (CuCl) is a wide-bandgap, I–VII ionic semiconductor. γ-CuCl with zinc blende structure, which is stable below 407 ◦C, has a direct bandgap with *E*<sup>g</sup> = 3.395 eV at4K[1]. It has a high exciton binding energy of ~190 meV [2], which is higher than both GaN (25 meV) [3] or ZnO (60 meV) [4]. CuCl has been investigated for some time for its application to optoelectronic devices [5–8]. The high binding energy gives the possibility of stable, room temperature, UV emission which, together with high biexciton binding energies, enables optoelectronic effects such as bistability and four-wave mixing with the potential for new short wavelength devices [9,10].

In order to use CuCl in optoelectronic devices it has to be deposited in thin films or in arrays of nanoparticles. Thin films of CuCl have been deposited by thermal evaporation, molecular beam epitaxy, and magnetron sputtering [11–13]. Arrays of nanoparticles of Cu halides have been produced in a matrix of glass, silicon, or organic compounds by gas or liquid phase methods, typically in a three dimensional form [6,14–17] but the size of the crystallites has been difficult to control. Atomic layer deposition (ALD) has recently been shown to be able to produce two-dimensional nanocrystalline arrays of CuCl on substrates and the size and distribution of the crystallites could be controlled by the parameters of the deposition process [18,19]. These films have shown the crystal structure of γ-CuCl and demonstrated the luminescent and optical characteristics typical of CuCl. One important feature of CuCl is that it is sensitive to moisture in its environment. Films of CuCl will hydrolyse to oxy- or hydroxy-halides after a short time and they need hermetic protection for long-term stability. Successful encapsulation by spun-on organic materials such as polysilsesquioxanes and cycloolefin copolymers has proved successful while plasma-enhanced chemical vapour deposition of SiO2 has not provided adequate encapsulation [20]. These spun-on techniques required short-term exposure of the CuCl to the atmosphere so there is still the possibility of some degradation before encapsulation. In addition, the relative thickness and probable lack of uniformity of spun-on films will be a drawback in device construction.

ALD is a chemical vapour deposition technique characterised by its ability to controllably deposit ultrathin layers with extreme uniformity. In ALD, the substrate is exposed sequentially to pulses of reactant gases or vapours and each pulse forms an additional chemisorbed molecular layer on it. Between the reactant pulses, an inert gas is used as a purge gas for removing all the excess precursor molecules that have not chemisorbed or undergone exchange reactions with the surface groups, and removing the reaction byproducts [21]. A single sequence of precursor and purge pulses is known as an ALD cycle. Initially, the film formation may proceed by the nucleation and growth of individual nanocrystallites, whose size and density varies according to the number of ALD cycles used in the process [22]. The details of this nucleation process are dependent on the chemical interaction between the precursors and the initial surface chemical state of the substrate, which is affected by its pretreatment. This process of controlled nucleation provides a possible method of producing plasmonic structures in copper halides in a much more repeatable way. In the previous ALD work [18,19], the Cl precursor used, i.e., a solution of HCl in butanol, had limitations with respect to questionable stability of the vapour pressure and possible bi-reactions with butanol itself.

In this publication, we report on the sequentially pulsed chemical vapour deposition of CuCl using an alternative Cl precursor which circumvents the problems of the one used in previous ALD of CuCl. We study the change in the distribution of nanocrystallites as deposition progresses. We characterise the structural, optoelectronic and chemical properties of the film and explore the use of ultrathin diffusion barrier layers deposited in situ to provide environmental protection.
