Structural study on amorphous and crystalline state of phase change material

Abstract

We report an inelastic (Raman) light scattering study on bulk crystalline GeTe (c-GeTe) and amorphous GeTe (a-GeTe) thin films and found to show pronounced similarities in local structure between the two states. In c-GeTe, the observed Raman modes represent the Ge atoms are in three different environments, namely, tetrahedral, distorted, and defective octahedral sites. On the other hand, in a-GeTe, Raman spectrum reveals Ge sites in tetrahedral and defective octahedral environment. We suggest that the structure of c-GeTe consists of highly distorted as well as defective Ge sites, which leads to the large concentration of intrinsic defects (vacancies). These random defects would act as topological disorder in the lattice and cause the bands to develop tails at the band edges, a continuum of localized levels appearing in the gap. The present study deepens the understanding of the local atomic structure, influence of defects and its close relation to the phase-change mechanism.

The role of atomic vacancies on phonon confinement in α-GeTe

“…However, the recent experimental study on bulk polycrystalline α-GeTe specimen reveals that the Raman modes of E and A 1 occur at 95 and 125 cm −1 along with additional bands with lesser intensity at high frequencies. 22 On the basis of ab initio results [17][18][19][20] and earlier Raman spectroscopy observations, 21,22 we confirm that the observed vibrational modes in 10µm crystal size are intrinsic part of bulk α-GeTe structure. Furthermore, the decrease in crystal size leads to downshift of phonon modes in addition to the peak broadening of the Raman spectra as shown in inset of Figure 3.…”

“…In addition to these bands, it is interesting to note that at lower crystal size, there are minor bands around 155 cm −1 and 230cm −1 corresponding to the vibrations of Ge atoms distinct from its distorted octahedral environmental sites. 22 More commonly, in bulk disordered and low dimensional semiconductors (quantum dot and nanowires) the optical phonon Raman lines found to show the asymmetrical broadening and downshift due to optical phonon confinement. 23 The consequences of phonon confinement are evident in the vibrational spectra of semiconductors with crystal sizes smaller than 20 lattice constants.…”

“…In addition to the redshift of Raman line, we also observe the development of new peaks at higher frequencies (156 cm −1 ) in lower crystal size in accordance with the recent reports. 22,28 Based on the recent reports, these new bands are assigned to defective octahedral sites of Ge atoms (mostly four coordinated sites). These random defects in the crystalline state of PCM based material would act as disorder and decide phase change properties at the nanoscale level.…”

Structural interpretation of the enthalpy relaxation kinetics of (GeTe4)y(GaTe3)1−y far-infrared glasses

“…Similarly, also the second most pronounced peak at approx. 154 cm −1 is known to correspond to symmetric stretching mode of edge-sharing GeTe 4 (or Ge-rich) tetrahedra, or to a contribution from short, amorphous, distorted Te chains [23,[25][26][27]. The subtle peak at 106 cm − 1 is then very close to the previously reported mode of corner-sharing GeTe 4 (n = 0) tetrahedra found at 108 cm −1 and vibrations of the defective octahedral environment in the GeTe 4 tetrahedra.…”

“…The subtle peak at 106 cm − 1 is then very close to the previously reported mode of corner-sharing GeTe 4 (n = 0) tetrahedra found at 108 cm −1 and vibrations of the defective octahedral environment in the GeTe 4 tetrahedra. [23,24,26] Lastly, the peak at 140 cm −1 was in [22,28,29] assigned to highly ordered Te structures.…”