Employed for a long time in optical disks, Ge_2Sb _2Te_5 is nowadays considered the most promising material also for phase-change nonvolatile memories. In the current paper, Ge-Sb-Te phase-change material thin films with a nominal composition of Ge _2Sb_2Te_5 were grown by molecular beam epitaxy on slightly mismatched GaSb and InAs substrates with (001) and (111) orientations. In situ quadrupole mass spectrometry and reflection high-energy electron diffraction allowed tight control of the growth parameters, revealing that Ge_2Sb_2Te_5 grows in epitaxial fashion only within a narrow window of substrate temperatures around 200 °C. Smooth surfaces were achieved solely on (111)-oriented substrates. Rough surfaces and interfaces were observed by transmission electron microscopy for films grown on (001)-oriented substrates. Whereas films deposited on (001) substrates possess two different vertical epitaxial orientations, single-crystalline layers exclusively (111) oriented were achieved on (111) substrates, as shown by synchrotron radiation X-ray diffraction. All the results point to the superior crystalline quality and morphology of Ge_2Sb_2Te_5 layers grown on (111) surfaces, independent of the substrate material.
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