Chemical and Electrochemical Regeneration of InP after GD-OES/XPS Profiling Experiments
Authors: Bechu Solene , Badroudine Djoumana , Loubat Anais , Eypert Celine , Gaiaschi Sofia , Aureau Damien , Frégnaux Mathieu , Vigneron Jackie , Gonçalves Anne-Marie , Simon Nathalie , Chapon Patrick , Bouttemy Muriel , Etcheberry Arnaud 
Published: February 2020

The interest in III-V materials for photovoltaic applications has grown with the development of tandem or multi-junction solar cells. However, with the devices architecture getting more and more complex, the multiple processing steps have to be characterized by means of adapted characterization strategies. Indeed, the better efficiencies can be obtained once the entire solar cell is widely known and optimized. In this purpose, the best way would be a combination of electrical properties measurements, chemical and optical characterizations of all the layers and interfaces in the final cell stack.

The present study focuses on physico-chemical characterizations from the surface to the different interfaces of interest, to help in determining optimal experimental conditions during the whole cell fabrication. To this end, we developed and implemented a new and specific approach combining two analytical techniques. To access the different buried interfaces, a fine quantitative chemical profiling through each layer is performed thanks to a combination of Glow Discharge – Optical Emission Spectroscopy (GD-OES), with its new Differential Interferometry Profiling (DiP) module, and X-ray Photoelectron Spectroscopy (XPS). The advantage of coupling these techniques is to combine the high etching rate of GD-OES and the XPS surface chemical (composition and chemical environment) diagnostic. In particular, specific in-depth location can be precisely reached by live GD-OES intensity profile monitoring, while XPS analyses (400 µm spot size) can be directly performed inside the GD-OES crater (4mm diameter) in order to obtain the detailed chemical composition. The proof of concept has been established [1], however, previous studies have demonstrated that, stopping the GD-OES profiling, and so the dynamic of the plasma sputtering, leads to possible degradation of the material and re-deposition processes. Therefore, a dedicated study of the degradation nature and perturbed depth dimension has to be performed to adopt the best strategy to regenerate the initial chemical and structural information and ensure the reliability of the analyses implemented inside the crater. For our purpose, this point is critical to certify that surface XPS analyses inside the crater bring representative information about the chemistry. Different regeneration solutions can be employed to eliminate the damaged surface [2] and we specifically investigate the capabilities of surface wet chemical engineering, by soaking, or physical removal by Ar+ or Arn
+ sputtering.

This issue will be illustrated on the practical case of InP. First, the evaluation of the degradation of InP due to the GD-OES stopping for a better XPS exploitation of the GD crater will be presented. To obtain an overall characterization complementary analyses techniques were employed. Crater imaging and global chemical and microstructural analyses were achieved by scanning electron microscopy (SEM), Electron dispersive spectrometry (EDS) and electron BackScattered Diffraction (EBSD). Surface analyses were performed ex-situ by XPS and, at local scale (12 nm spot size) by nano-Auger) and in-situ by electrochemistry. Optical properties were obtained by ellipsometry (Fig. 1). From this work, a superficial In enrichment inside the GD-OES crater (Fig. 2), very close to the one obtained by the cathodic decomposition in aqueous solution, was shown. In addition, electrochemistry and XPS measurements displayed that this enrichment was attributed to the presence of metallic In.

Then, a comparative study of the InP GD-OES crater evolution after different regeneration options, including wet chemical nano-engineering, electrochemistry and physical treatments, will be detailed.

References:

[1]. D. Mercier, M. Bouttemy, J. Vigneron, P. Chapon and A. Etcheberry, "GD-OES and XPS coupling: A new way for the chemical profiling of photovoltaic absorbers," Applied Suface Science, vol. 347, pp 799-807, 2015.

[2]. Coupling GD-OES and XPS profiling to perform advanced physico-chemical characterizations of III-V layers for photovoltaic applications; A. Loubat, S. Béchu, M. Bouttemy, C. Eypert, S. Gaiaschi, M. Frégnaux, D. Aureau, J. Vigneron, N. Simon, P. Chapon, A.-M. Gonçalves, A. Etcheberry, 7th WCPEC - 45th IEEE PVSC Conference - Waikoloa, Etats-Unis, Proceeding 37, presented in Characterization Methods, 2018.

Figure 1