Structural Evolution and Phase Change Properties of C-Doped Ge_2Sb_2Te_5 Films During Heating in Air

We elucidate the importance of a capping layer on the structural evolution and phase change properties of carbon-doped Ge_2Sb_2Te_5 (C-GST) films during heating in air. Both the C-GST films without and with a thin SiO_2 capping layer (C-GST and C-GST/SiO_2) are deposited for comparison. Large differences are observed between C-GST and C-GST/SiO_2 films in resistance-temperature, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectra, data retention capability and optical band gap measurements. In the C-GST film, resistance-temperature measurement reveals an unusual smooth decrease in resistance above 110^\circ\!C during heating. X-ray diffraction result has excluded the possibility of phase change in the C-GST film below 170^\circ\!C. The x-ray photoelectron spectroscopy experimental result reveals the evolution of Te chemical valence because of the carbon oxidation during heating. Raman spectra further demonstrate that phase changes from an amorphous state to the hexagonal state occur directly during heating in the C-GST film. The quite smooth decrease in resistance is believed to be related with the formation of Te-rich GeTe_4-nGe_n (n=0, 1) units above 110^\circ\!C in the C-GST film. The oxidation of carbon is harmful to the C-GST phase change properties.