The assistance of photoemission in the development of nanostructured optical windows
Résumé
Optical windows are typically designed (i) to maximize transmission in a specific wavelength range, while minimizing reflection and absorption and (ii) to protect optical systems and electronic sensors from external environment. For industrial applications in the railway, aerospatial or military fields, additional properties such as antireflection or superhydrophobicity may be required. One possible way to obtain these desired properties is to develop a process for nanostructuring the window material, which can be glass for visible spectral range, silicon for midwave infrared or germanium for longwave infrared. To extend the multifunctionality of these optical surfaces and more specifically their robustness, hybrid optical windows based on synthetic diamond deposition are promising candidates because they will benefit from both the optical and the excellent mechanical properties of the diamond. In addition, this particular material represents a good candidate for subsequent surface chemical functionalization.
In this context, X-ray Photoelectron Spectroscopy (XPS) appears as a particularly suitable tool to provide physicochemical information on nanostructured surfaces. Indeed, when excited with soft X-rays, the emitted photoelectrons comes from the outermost 5–10 nm of the sample (inelastic mean free paths in the order of 1 nm). In addition to these elastic peaks providing informations on the chemical states and the associated concentrations, XPS spectra show a continuum intensity that tends to increase at lower kinetic energies. These are photoelectrons that were originally excited at the peak energies, but have lost energy on their way to the surface. They participate to the observed backgrounds which actually contains information about the elemental depth distribution in the analyzed volume of material1 (first nanometers).
In this presentation, we will specifically discuss the spectroscopic differences between flat and nanostructured germanium substrates. Relationships between germanium topology (spatial distribution) and obtained background by XPS will be established. From the spectroscopic point of view, germanium is interesting because photopeaks are detected at respectively high (Ge 2p) and low (Ge 3d) binding energy with Al Kα excitation source. These features will be exploited to highlight the peak intensity changes in both kind of architectures: planar and nanostructured ones.
The second part of the presentation will be dedicated to the study of the interface between germanium and synthetic diamond. XPS characterization performed in the early stage of the diamond growth on germanium will provide significant information on the chemical composition of the interface. The nature of the chemical bonds detected allowed us to make assumptions on the observed inefficient deposition of diamond on top of germanium and the necessity to add a silicon nitride interlayer to initiate it. The influence of plasma pretreatment before diamond deposition will also be discussed.
Finally, this work will aim to demonstrate the interest of photoemission and associated techniques for the development of pre-industrial processes and more specifically for the production of next-generation optical windows, even with nanostructuration.
Mots clés
instrumentation
diamond
optical window
transmission
reflection
absorption
sensor
antireflection
hydrophobicity
nanostructuring
multifunctionality
synthetic diamond
functionalization
X-ray Photoelectron Spectroscopy (XPS)
signal processing
germanium
spatial distribution
photopeaks
plasma pretreatment
diamond deposition