Disorder creates new properties in compound semiconductors

An international research team has demonstrated that the intrinsic disorder of the compound semiconductor CuInSnS₄ can be exploited to influence its optical properties. While the atomic vibrations also sense the local disorder, their response is averaged over many different local environments and therefore appear isotropic, as expected for a cubic crystal. In contrast, the optical excitations, known as excitons, are much more sensitive to the local arrangement of atoms. Surprisingly, they show a direction-dependent optical response even though the average crystal structure is cubic. These findings shed new light on the relationship between disorder and material properties, opening up new options for targeted 'disorder engineering' in optoelectronic and photocatalytic devices.

Crystals are typically characterised by a periodic arrangement of atoms, in which each element occupies well-defined crystallographic sites throughout the structure. In compound semiconductors such as CuInSnS₄, a member of the adamantine chalcogenide family, the cations are ideally distributed over specific positions in the crystal structure. In practice, however, some of the tin sites (Sn) may be occupied by Indium-cations, which have roughly a similar size, and vice versa, resulting in so-called antisite disorder. Although this intrinsic disorder has only a minor effect on the lattice parameters, it can significantly influence the material’s optoelectronic properties. An international team led by Prof. Dr. Susan Schorr (HZB) and Dr. Mirjana Dimitrievska (EMPA) has now investigated these effects in detail.

Disentangling effects of disorder

As the distribution of In³⁺ and Sn⁴⁺ cations over their crystallographic sites is difficult to determine using conventional X-ray diffraction, the scientists combined vibrational spectroscopy techniques with photoluminescence measurements to disentangle the effects of disorder on lattice vibrations (phonons) and optical excitations (excitons). Some of these experiments were performed at the BESSY II synchrotron radiation source at HZB. The experimental data was interpreted based on theoretical models. 

‘It was the exceptional quality of the CuInSnS₄ single crystals which enabled us to clearly separate the effects of disorder on phonons and excitons. From thorough evaluation of experimental data, we can deduce that the disorder has hardly any effect on the lattice vibrations, but in contrast, it alters the optical properties’, says Susan Schorr.

Optical properties change

‘Our study shows that intrinsic disorder can localise excitons, meaning that the light-generated excitations become confined to specific local atomic environments,’ explains Mirjana Dimitrievska. ‘What is surprising is that these localised excitons do not respond equally in all directions. They acquire a preferred optical direction, causing the material to react differently to polarised light, even though the average crystal structure is cubic.’ Susan Schorr adds: ‘This class of materials opens up a new, exciting field of research: changes to the composition allow us to tune the band gap energy, while the degree of disorder provides an additional handle for tailoring optoelectronical properties.’

New applications

As a summary, Mirjana Dimitrievska points out: ‘Adamantine chalcogenides could be useful in optical technologies where the direction of light matters, such as polarisation-sensitive light emitters, photodetectors that distinguish the polarisation of incoming light, and exciton-based optical components for sensing or information processing. Their tunable optical response may also open new possibilities for light-driven catalysis.