Influence of the hydrogen contained in amorphous silicon thin films on a pulsed radiofrequency argon glow discharge coupled to time of flight mass spectrometry. Comparison with the addition of hydrogen as discharge gas

Abstract

Thin film solar cells technology based on hydrogenated amorphous silicon (a-Si:H) has undergone a great expansion during recent years. Pulsed radiofrequency glow discharge time-of-flight mass spectrometry (rf-PGD-ToFMS) is able to perform depth profiling analysis of coated materials, providing an excellent tool for rapid and high sensitive chemical characterisation of photovoltaic devices. The hydrogen concentration on a-Si:H thin films is around 10%, which represents a challenge for quantitative depth profile analyses by using GD sources due to the so-called ''hydrogen effect''. It is well-known that when hydrogen is present in the Ar discharge, even in small quantities, significant changes can occur in the ion signal intensities and sputtering rates measured. Therefore, a critical comparison has been carried out by rf-PGD-ToFMS in terms of pulse profiles, spectral interferences and depth resolution for two modes of hydrogen introduction in the discharge, exogenous hydrogen in molecular gaseous form (using the mixture 0.2% H 2 + Ar as discharge gas) or endogenous hydrogen, sputtered as a sample constituent. For this purpose, non-hydrogenated materials (containing B, P and Si) and three types of a-Si:H thin films were investigated. Exogenous hydrogen was found to produce a noteworthy influence on the pulse profiles of the analytes, whereas the effect of the hydrogen sputtered from the samples could be considered less notorious. Moreover, the proper selection of the after-peak region was found to be critical to obtain optimum mass spectra (i.e. high analyte sensitivities free of interferences).

Depth Profile Analysis of Amorphous Silicon Thin Film Solar Cells by Pulsed Radiofrequency Glow Discharge Time of Flight Mass Spectrometry

“…The optimum conditions were 600 Pa and 90 W, similar to previous experiments [21]. As expected [7,22], higher analytical signals were obtained in the afterpeak region of the pulse profile, and so the afterpeak will be the only temporal region in the GD pulses shown throughout this Results and Discussion section.…”

“…On the other hand, although polyatomics attributable to recombination processes in the GD dynamic plasma are typically produced at the end of the afterglow region, the exact shape depends on the particular polyatomic. Finally, for silicon measurement similar results to those previously reported in experimental works carried out in our group were found [7,21]. Although several polyatomic interferences can occur for the three Si isotopes (e.g., Si + was 18.1, which corresponds to an 8% deviation from the natural isotope abundance ratios.…”

“…It has been reported that the addition of molecular hydrogen to Ar GD affects the intensity of the analytes and, therefore, a different trend for the two types of calibration standard could be expected [26]. However, as previously demonstrated, the endogenous hydrogen (a-Si:H layers) does not seem to exhibit such critical influence on ion signal intensities as exogenous hydrogen [21].…”

“…In this context, it should be stressed that the presence of hydrogen in a-Si:H thin films could represent a challenge for quantitative analysis because of the so-called "hydrogen effect": it has been previously reported that the addition of hydrogen to Ar GD could affect the intensity of the analytes and reduces the sputtering rate [18,19]. We have reported that molecular hydrogen added to the discharge gas seems to play a more critical role than the endogenous hydrogen present in the sample itself (e.g., for samples with hydrogen content in the order of 10% atomic) [20,21].…”

A path towards a better characterisation of silicon thin‐film solar cells: depth profile analysis by pulsed radiofrequency glow discharge optical emission spectrometry

“…However, to know the layer thickness and composition as well as possible concentration gradients, further investigations to develop proper quantification methods are needed. In this vein, it should be stated that the presence of hydrogen on a-Si:H thin films represents a challenge for quantitative analysis because of the so-called hydrogen effect [19], and possible hydrogen sources have been evaluated in our previous works by rf-PGD-OES and rf-PGD-MS in a-Si:H thin films [20,21]. By both analytical methods, we observed that molecular hydrogen added to the discharge gas plays a more critical role than the endogenous hydrogen present in the sample itself (samples with hydrogen content below 10% atomic).…”