Effect of hydrogenation on type II silicon clathrate films

In this study we investigate for the first time the effect of hydrogenation on the properties of type II silicon clathrate films (SiCL). These clathrates are an alternative form of silicon based on a cage structure. It can be either emptied or filled with sodium atoms, leading to a metallic or semiconducting behavior with a tunable direct bandgap of 1.6-1.8 eV. There are a wide range of potential applications for such materials such as in electronics, optoelectronics, photovoltaics, batteries, spintronics or hydrogen (H) storage. However, the role of H in such materials remains largely unexplored and is not well understood experimentally. In this work, we hydrogenate the clathrates films using a H plasma with a substrate temperature of 400 °C. We evaluate the H content in the films by Time-of-Flight Secondary Ion Mass Spectrometry and Elastic Recoil Detection Analysis. The latter indicates a SiH_0.006 molar concentration before and SiH_0.070 after hydrogenation. Such a H content within the SiCL films is too low for practical hydrogen storage applications. Nevertheless, the incorporated H plays the role of dopant, leading to a reduction in the work function by around 0.3 eV. This demonstrates that even a modest hydrogen uptake can significantly enhance the electronic properties of silicon clathrates.