Evolution of opto-electronic properties during film formation of complex semiconductors

M D Heinemann,H Rodriguez-Alvarez,D Greiner,C A Kaufmann,F Österle,R Mainz,T Unold

doi:10.1038/srep45463

M D Heinemann, H Rodriguez-Alvarez + Show 5 more

Open Access

https://doi.org/10.1038/srep45463

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Abstract

Optical and electrical properties of complex semiconducting alloys like Cu(In,Ga)Se2 (CIGS) are strongly influenced by the reaction pathways occurring during their deposition process. This makes it desirable to observe and control these properties in real-time during the deposition. Here we show for the first time the evolution of the band gap and the sub-band-gap defect absorption of CIGS thin film as well as surface roughness during a three-stage co-evaporation process by means of an optical analysis technique, based on white light reflectometry (WLR). By simultaneously recording structural information with in-situ energy dispersive X-ray diffraction and X-ray fluorescence we can directly correlate the evolution of opto-electronic material parameters with the structural properties of the film during growth. We find that the surface roughness and the sub-gap light absorption can be correlated with the phase evolution during the transformation from (In,Ga)2Se3 to Cu(In,Ga)Se2 by the incorporation of Cu into the film. Sub-bandgap light absorption is found to be influenced by the Cu-saturated growth phase and is lowered close to the points of stoichiometry, allowing for an advanced process design.

Highlights

Optical and electrical properties of complex semiconducting alloys like Cu(In,Ga)Se2 (CIGS) are strongly influenced by the reaction pathways occurring during their deposition process
Using in situ optical white light reflectometry (WLR) we show here for the first time how the band gap and sub-gap tail energy evolve during a multi-stage co-evaporation deposition process of CIGS
Due to the high absorption coefficient of CIGS of α> 5 · 104 cm−1 14,15, photons within this wavelength range are nearly completely absorbed within the film and the reflectance is only influenced by the film roughness, σRMS, and the refractive index at the surface[16]

Summary

Introduction

Optical and electrical properties of complex semiconducting alloys like Cu(In,Ga)Se2 (CIGS) are strongly influenced by the reaction pathways occurring during their deposition process. We show for the first time the evolution of the band gap and the sub-band-gap defect absorption of CIGS thin film as well as surface roughness during a three-stage co-evaporation process by means of an optical analysis technique, based on white light reflectometry (WLR). Using in situ optical white light reflectometry (WLR) we show here for the first time how the band gap and sub-gap tail energy evolve during a multi-stage co-evaporation deposition process of CIGS. In combination with in situ X-ray diffraction[11,12] we get access to structural, morphological and opto-electronic properties during the growth of CIGS This enables the design of deposition routines leading to reduced sub-gap defect densities, necessary for further improvement of high-efficiency CIGS solar cells, and smooth CIGS films for the application in CIGS/Perovskite tandem cells[13]. The application of this method is not limited to the growth of chalcopyrite thin-films but could be adapted to kesterite, perovskite or any other compound semiconductor thin-film growth

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