Type II Ge clathrate (Na _x Ge ₁₃₆ ) films containing a small amount of Na were successfully synthesized on a sapphire substrate by thermal annealing of precursor NaGe composite films. A new technique for Na removal from Na _x Ge ₁₃₆ was developed by applying the electric field in an Ar environment at 275 °C. Rietveld refinement analysis of Na _x Ge ₁₃₆ films showed an almost guest free nature (Na contents x ∼ 0) using this new technique. In addition, absorbance spectra obtained from Na _x Ge ₁₃₆ films showed a decrease in the absorbance intensity in the lower photon energy (<0.5 eV) with annealing. It suggests the decrease in free carrier absorption by the removal of Na atoms in Na _x Ge ₁₃₆ .

Type II inorganic clathrates consist of cage-like structures with open frameworks, and they are considered promising materials due to their unique properties. However, the difficulty of synthesizing phase-pure and continuous films has hindered their application in practical devices. In this report, we demonstrate the synthesis of type II SiGe clathrate films through the thermal decomposition of a Na-deposited amorphous SiGe film on a sapphire substrate in a high vacuum. The as-prepared films of type II SiGe clathrates showed uniform growth and were evaluated for their structural and optical properties. Morphological studies conducted using a scanning electron microscope showed the presence of cracks on the film surface.

Type II Ge clathrate has been investigated by many researchers in powder and film forms; however, it lacks an extensive investigation of basic properties such as optical and electrical properties and application to practical devices. The main reason points to the difficulties in achieving a good quality film with minimal defects. In the present work, an experimental approach was investigated to establish the key synthesis parameters, actively affecting the type II Ge clathrate film quality. The main parameters which were investigated include temperature profile and duration, amount of guest atom (Na) evaporated during the synthesis steps, etc. As-prepared films were investigated by X-ray diffraction, Rietveld analysis, Raman spectroscopy for the structural characterization; scanning electron microscope for morphological studies, and UV–vis-NIR and FTIR spectroscopy measurements for optical properties. Optimization of the synthesis condition resulted in good-quality film in comparison to the previously reported results.

An attempt to prepare thin films of Al-doped type II Ge clathrates Na _x (Al _y Ge_1−y )₁₃₆ was made by using amorphous Ge films including 0–7 atomic percent Al as the starting materials. The preparations were conducted using our recently established method, which enables the deposition of Na on the starting material with simultaneous or subsequent annealing under a high vacuum. X-ray diffraction measurements confirmed the type II clathrate structure in the prepared film. Smaller lattice constants and an Al-Ge related Raman peak found in the Al-included samples suggested the partial substitution of Al for Ge in the clathrate structure. Optical transmission measurements indicated that the fundamental absorption edge is shifted to higher energies due to the inclusion of Al.