Growth of Ge clathrate on sapphire and optical properties

Abstract

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 .

Synthesis and characterization of silicon clathrates of type I Na8Si46 and type II NaxSi136 by thermal decomposition

“…4a reports the absorption coefficient obtained. For the SiCL-II films, the estimated values and behavior are in agreement with previously determined value obtained by UV-Visible measurement on thin film deposited on sapphire [21,47] or others methods [37]. The SiCL-I film is even more able to absorb light and the absorption coefficient is rising continuously as the photon energy increases to reach 6.7x10 5 cm -1 at 4.5 eV.…”

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

“…The unoccupied type II clathrates (guest-free clathrates) have also attracted attention due to the theoretically predicted direct band gap being wider than the diamond structure. [12][13][14] The guest-free type II clathrate structure has been experimentally prepared for silicon (Si 136 ), 7,[15][16][17][18] germanium (Ge 136 ), 19,20) and SiGe alloy. 21) The bandgap energies of Si 136 and Ge 136 have been experimentally reported as 1.8-1.9 15,22) and 0.6-0.7 eV, 19,20) respectively.…”

Synthesis of Type II Ge and Ge–Si Alloyed Clathrates Using Solid-State Electrochemical Oxidation of Zintl Phase Precursors

“…However, synthesis of the type II Ge clathrate phase has been more elusive; this is because the traditional synthetic approach for type II Si clathrates, thermal decomposition of the Na 4 Si 4 Zintl phase under vacuum, , is more difficult for the Ge analogue (Na 4 Ge 4 ) and generally results in the formation of many phases (Figure ). Recently, the selective growth of the type II Ge phase via thermal decomposition has been accomplished with specially designed reactors or via thin-film growth methods. − …”