Raman spectroscopy investigation of the H content of heated hard amorphous carbon layers

TL;DR

This study demonstrates that Raman spectroscopy, specifically the HD/HG parameter, can reliably estimate hydrogen content in heated amorphous carbon layers, with validation against ion beam analysis and insights into spectral features.

Contribution

It establishes the quasi-linear relationship between the HD/HG ratio and H content, providing a new method for non-destructive hydrogen quantification in amorphous carbon.

Findings

HD/HG ratio correlates linearly with H content from 2% to 30%.

The 860 cm-1 band is linked to H bonded to C(sp2).

Photoluminescence background ratio m/HG is less reliable for H estimation.

Abstract

We revisit here how Raman spectroscopy can be used to estimate the H content in hard hydrogenated amorphous carbon layers. The H content was varied from 2 at.% to 30 at.%, using heat treatments of a a-C:H, from room temperature to 1300 K and was determined independently using ion beam analysis. We examine the correlation of various Raman parameters and the consistency of their thermal evolution with thermo-desorption results. We identify a weak band at 860 cm-1 attributed to H bonded to C(sp2). We show that the HD/HG parameter (Height ratio between the D and G bands) is quasi-linear in the full range of H content and can thus be used to estimate the H content. Conversely, we show that the m/HG parameter (ratio between the photoluminescence background, m, and the height of the G band), often used to estimate the H content, should be used with care, first because it is sensitive to various photoluminescence quenching processes and second because it is not sensitive to H bonded to C(sp2).