Vibrational properties and stabilization mechanism of the amorphous phase of doped GeTe

Doping chalcogenide phase change materials was shown to improve the stability of the amorphous phase at high temperature and to strongly increase the crystallization temperature. In this work, we use $ab$ initio molecular dynamics together with Fourier transform infrared spectroscopy to address the stabilization of GeTe doped with nitrogen and carbon in the amorphous phase. The comparison between the simulation and experimental results allows in-depth understanding of the mechanisms. The inclusion of C and N leads to an increase in high frequency vibrational modes and to a lowering of the boson peak intensity. The reduction of the density of floppy vibrational modes and the computed increase of the mechanical rigidity are responsible for the higher activation energy for crystallization. The mechanism described here could apply more generally to stabilize other Ge-Sb-Te phase change materials and ionocovalent glasses at high temperature.

Figure 1

Reflectivity curves for C (left panel) and N (right panel) -doped GeTe measured upon heating at a rate of $10{}^{\circ }$C/min. These curves show the optical contrast observed upon crystallization, as well as the increase of the crystallization temperature upon doping.

Figure 2

Experimental low frequency FTIR absorbance spectra of as-deposited undoped, C-doped, and N-doped GeTe thin films (upper panels). The corresponding calculated VDoS are plotted on the lower panels. The VDoS of undoped GeTe is null above 8 THz.

Figure 3

Experimental high frequency FTIR absorbance spectra of as-deposited undoped, C-doped, and N-doped GeTe thin films (upper panels: different scales are used for the left and right panels). The FTIR spectrum of undoped GeTe is null in the high frequency range. The corresponding calculated VDoS are plotted on the lower panels.

Figure 4

Total VDoS (normalized to unity) obtained from the AIMD trajectories (top panel). The total VDoSs are decomposed onto the different atomic species (lower panels): VDoS of GeTe, C-doped GeTe with 16 at. % C- and N-doped GeTe with 10 at. % N are plotted with plain black, plain blue, and dotted green lines, respectively.

Figure 5

Dopant site-projected vibrational densities of states for the AIMD simulated C-doped GeTe with 16 at. % C. Contributions have been summed up according to the local environment of the dopant atom (sketched on the graph). C, Ge, and Te atoms are drawn in orange, red, and blue, respectively. The dotted vertical lines separate the regions accessible to the different FTIR spectrometers.

Figure 6

Dopant site-projected vibrational densities of states for the AIMD simulated N-doped GeTe with 10 at. % N. N and Ge atoms are drawn in green and red, respectively. The arrows indicate the curves corresponding to the different kinds of atomic environments (molecular, pyramidal, and tetrahedral). The inset shows the contribution of N${}_2$ vibrations at high frequency.

Figure 7

VDoS of amorphous GeTe, C-doped GeTe with 16 at. % C and N-doped GeTe with 10 at. % N, together with the computed crystalline GeTe phonon DoS (from Ref. 24).