DFG Research Unit 2857

Copper Iodide as Multifunctional Semiconductor

P07: Element-Specific Local Structure of Crystalline and Amorphous CuI-Based Alloys

Prof. Dr. Claudia S. Schnohr

Semiconductor technology permeates our society on nearly all levels of modern-day life. Most devices rely on both p- and n-type doped material. Additionally, transparent conductors or semiconductors are required for solar cells, flat panel displays and light emitting diodes. They further enable future applications such as smart windows or transparent displays. Suitable n-type transparent semiconductors, such as ZnObased materials, are already available. In contrast, a well-performing p-type transparent semiconductor is still lacking. Copper iodide (CuI) features high optical transparency and natural p-type conductivity. It can be easily grown on a variety of substrates and therefore constitutes a most promising candidate for the realization of high-performance fully transparent electronics. The electrical and optical properties of CuI can be specifically tailored by alloying with a third element such as Mn or Sn, resulting in crystalline or amorphous thin films, respectively. However, the local atomic arrangements in crystalline alloys often show a striking deviation from the long-range crystal structure. In amorphous materials, long-range order is lost while a significant level of short-range order is still retained. In both cases, the atomic structure on the subnanometer scale strongly impacts other material properties and thus the device performance. Therefore, it is the aim of this project to study the element-specific local structure of crystalline and amorphous CuI-based alloys using X-ray absorption spectroscopy. For crystalline alloys such as (Mn,Cu)I, this will yield element-specific bond lengths and atomic displacements as a function of the material composition. For elements such as Sn, insight can be gained into the crystalline-to-amorphous transition with increasing Sn concentration. Furthermore, the coordination and short-range structure within the amorphous phase will be determined and its thermal stability will be evaluated. This structural information provides important feedback for the optimization of the growth procedures, especially for the amorphous alloys, where standard diffraction techniques are no longer adequate. Moreover, the structural parameters determined in this project serve as benchmark for the ab initio calculations and allow the evaluation of different computational approaches. They will further help to interpret and understand the results from electrical and optical measurements of the material. In this way, the study of the element-specific local structure of crystalline and amorphous CuI-based alloys strongly contributes to understanding the correlation between preparation conditions, structure and important material properties. Together with the other proposals of the Research Unit, this project will thus significantly advance CuI and CuI-based materials as p-type transparent semiconductor for future transparent electronic devices.