Synthesis and optical properties of Ge clathrate films with and without Al doping

Abstract

An attempt to prepare thin films of Al-doped type II Ge clathrates Nax(AlyGe1-y)136 was performed by using amorphous Ge films including 0 to 7 atomic percent Al as the starting materials. The preparations were conducted by 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.

Synthesis and Characterization of Type II Ge-Si Clathrate Films for Optoelectronic Applications

“…The refinement fitting parameters appeared unaffected by small changes in the Si molar ratio of the type II SiGe clathrate. The lattice constant (a) decreased slightly with the increase in the Si ratio, which is in good agreement with previously reported results [31,40]. The estimation of the Na contents (x) resulted in the empirical formulas of Na 1.00 (Si 0.05 Ge 0.95 ) 136 , Na 1.02 (Si 0.10 Ge 0.90 ) 136 , and Na 0.93 (Si 0.15 Ge 0.85 ) 136 for sample 1, sample 2, and sample 3, respectively.…”

“…For the type II Ge clathrate (0% Si), the absorption edge in the lower energy range was observed at 0.74 eV, which was attributed to the direct interband transitions at the L point, and the absorption edge observed at 1.04 eV was attributed to the direct interband transitions at the Γ point. A similar observation of two absorption edges was reported for the type II Ge clathrate and Al-doped type II Ge clathrate films [31,40]. For the type II SiGe clathrate films, the absorption edge in the lower energy region was estimated at 1.03 eV for sample 2 (10% Si) and 1.10 eV for sample 3 (15% Si), and in the higher energy region, it was 1.45 eV for sample 2 and 1.54 eV for sample 3.…”

“…The properties of the clathrates can also be modified flexibly by alloying with other elements such as group III elements, group V elements, etc. [31].…”

“…On the other hand, for the film form, a two-step thermal decomposition method, primarily derived from the powder synthesis method, was used predominantly [3,20,23,32,33]. Recently, our team has reported a single-step thermal decomposition method using a specially designed setup [31,40,41]. To control the guest concentration in the type II clathrate films, various techniques have been employed.…”