Synthesis and characterization of type II silicon clathrate films with low Na concentration

A two-step process for the synthesis of the silicon clathrate film on a diamond silicon wafer is explored in detail. Key factors impacting the film quality are uncovered. We find that the optical properties of the films are strongly influenced by inhomogeneities and defect phases that dominate the top surface and grain boundaries of the material. For the first time, we systematically develop two approaches for minimizing the effects of defective structures and allow intrinsic properties of the clathrate material to be probed. One is separating the film surface from the Si substrate to expose the buried high-quality interface, and the other one is wet or dry etching of the clathrate film to remove the disordered material which is more heavily concentrated at the top surface. With high-quality clathrate surfaces and films produced, more reliable optical measurements are taken and interpreted. Techniques in this work provide a pathway for Si clathrate thin film toward an optically efficient alternative crystalline form of Si that can transform Si-based applications in optoelectronics.