Epitaxial chalcogenide Ge-Sb-Te thin films and superlattices by pulsed laser deposition

This thesis deals with the deposition of epitaxial chalcogenide (Ge2Sb2Te5 (GST225), GeTe and Sb2Te3) thin films and superlattice (SL) arrangement based on GeTe-Sb2Te3 using pulsed laser deposition (PLD) technique on (111)-oriented Si substrates. The thin films are characterized using in-situ RHEED, XRD, SEM, AFM and TEM. The epitaxial trigonal GST225 films with out-of-plane c-plane orientation were grown in 2D growth mode. For the first group of the films (substrate-target distance (dts) of ~7.5 cm), the epitaxial window was observed from 200 °C to 300 °C. By varying laser frequency, deposition rate as high as 42 nm/ min can be achieved. The deposition with a slight reduction of dts to ~6 cm (second group) at moderate Ts of 220 °C results in the epitaxial films with heterogeneous vacancy structures (coexisting metastable phases. i.e. with random and ordered vacancies, and stable trigonal phase). Thermal annealing (at 220 °C) leads to a phase transformation towards a pure trigonal phase. The epitaxial Sb2Te3 films with out-of-plane (0001) oriented trigonal structure were grown at Ts from 140 to 280 °C in 2D growth mode. The optimum Ts in terms of deposition rate and film quality was determined to be 240 °C. The epitaxial growth of Sb2Te3 thin films is initiated by the self-organized formation of a Sb/Te single-atomic passivation layer on the Si surface. The growth of GeTe was initialized by the formation of an ultra-thin amorphous layer. The films were predominantly grown in the mix of 2D and 3D growth modes. The deposited films possesses trigonal structure out-of-plane (0001)-orientated on Si(111). By employing a 2D-bonded Sb2Te3 as a seeding layer on Si(111), the epitaxial window of GeTe can be extended especially towards the lower temperature regime, up to 145 °C. Additionally, the surface topography can be significantly improved, indicating that the films are grown in 2D growth mode on the buffered substrate. The epitaxial SLs can be grown starting at Ts = 140 °C. Each layer of the SLs, i.e. Sb2Te3 and GeTe layer, was grown in 2D growth mode. An intermixing of GeTe and Sb2Te3 layers occurred at a higher temperature deposition. Studies on local structure of 140 °C-deposited SL showed that the SL consists of Ge-rich Ge(x+y)Sb(2–y)Tez and Sb2Te3 units intercalated by Van der Waals gaps with the inhomogeneity of layer thickness across the SL. The obtained results demonstrate the feasibility of PLD for deposition of good quality epitaxial chalcogenide thin films and SL structure on Si(111)