Nanocomposites and nanomaterials

Urbach tails in the spectra of amorphous (GeTe)x(Sb2Te3)1-x alloys
Shportko K.V.
V. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, Nauki Av., 41, Kyiv-03028, Ukraine. 
E-mail: k.shportko@ukr.net

In recent years, pseudo-binary chalcogenide (GeTe)x(Sb2Te3)1-x alloys (GSTs) attract much attention from the fundamental and applied points of view. GSTs relate to the class of phase-change materials and possess unconventional  properties. GSTs being already employed in the optical data storages (CD, DVD, etc.) are among the most promising functional materials for new generations of multilevel data storage [1] and data visualization applications [2]. Stability of the amorphous phase in time, as well as possibility of tuning the properties of amorphous state in GSTs play one of the crucial roles in reliability of corresponding devices and therefore confirms the importance of the amorphous state of GSTs.
In this study we chose alloys along the pseudo-binary line between GeTe and Sb2Te3. To study their properties we applied FTIR spectroscopy in the range from 50 to 8000 cm-1 and in the temperature range from 5 to 400 K.
Amorphous materials do not have as sharp bandgap as crystalline materials have, as the lack of atomic long-range order is caused by disordering the bond angles and lengths. Therefore, their band structure including the valence and conduction bands has to be complemented by localized states at the band edges and additional defect states inside the bandgap that form co-called Urbach tails.
Obtained results exhibit compositional and temperature dependences of the dielectric constant, optical bandgap and absorption of defect states in studied materials. These dependences reveal possible ways to tune the properties of phase-change alloys in the amorphous state. 

1.	Wuttig M. Phase change materials: Towards a universal memory? // Nat. Mater.-2008.-4.-P. 265-266.
2.	Wuttig M., Bhaskaran H., Taubner T. Phase-change materials for non-volatile photonic applications // Nat. Photonics.-2017.-11, -P. 465-476.