Molecular beam epitaxy of GeTe-Sb2Te3 phase change materials studied by X-ray diffraction

The integration of phase change materials into semiconductor heterostructures may lead to the development of a new generation of high density non-volatile phase change memories. Epitaxial phase change materials allow to study the detailed structural changes during the phase transition and to determine the scaling limits of the memory. This work is dedicated to the epitaxial growth of Ge-Sb-Te phase change alloys on GaSb(001). We deposit Ge-Sb-Te (GST) films on GaSb(001) substrates by means of molecular beam epitaxy (MBE). The film orientation and lattice constant evolution is determined in real time during growth using grazing incidence X-ray diffraction (GID). The nucleation stage of the growth is studied in situ using reflection high energy electron diffraction (RHEED). Four growth regimes of GST on GaSb(001) were observed: amorphous, polycrystalline, incubated epitaxial and direct epitaxial. Amorphous film grows for substrate temperatures below 100 ◦C. For substrate temperatures in the range 100–160 ◦C, the film grows in polycrystalline form. Incubated epitaxial growth is observed at temperatures from 180 to 210 ◦C. This growth regime is characterized by an initial 0.6 nm thick amorphous layer formation, which crystallizes epitaxially as the film thickness increases. The determined lattice constant of the films is 6.01Å, very close to that of the metastable GST phase. The films predominantly possess an epitaxial cube-on-cube relationship. At higher temperatures the films grow epitaxially, however the growth rate is rapidly decreasing with temperature. At temperatures above 270 ◦C the growth rate is zero. The composition of the grown films is close to 2:2:5 for Ge, Sb and Te, respectively. The determined crystal structure of the films is face centered cubic (FCC) with a rhombohedral distortion. The analysis of X-ray peak widths gives a value for the rhombohedral angle of 89.56 ◦. We observe two types of reflections in reciprocal space indicating two FCC sublattices in the structure. The analysis of X-ray structure factors give the same averaged electron density ratio in the two FCC sublattices as in the GeTe or the metastable Ge2Sb2Te5 with 20% vacancies. No tetrahedrally coordinated atoms are observed.