Laser irradiation-induced modification of the amorphous phase in GeTe films: the role of intermediate Ge–Te bonding in the crystallization mechanism

Abstract

Modified amorphous GeTe, formed by the pulsed laser irradiation of as-grown GeTe, was analyzed in terms of variations in the local bonding structure using Raman spectroscopy and X-ray absorption fine structure in tandem with first-principles density functional theory. Amorphized GeTe (acquired from the crystalline phase) was compared with the modified amorphous GeTe to investigate the similarities and discrepancies between these two amorphous phases. Raman spectroscopy showed that these materials have a similar distribution of Ge-centered local structure in both phases, which is mainly composed of an octahedral-like structure. However, extended X-ray absorption fine structure results show the presence of a unique second type of Ge-Te bonding in the amorphized GeTe, which can effectively reduce the energy required for recrystallization. A computational study based on molecular dynamics simulations verified our experimental observations, including the existence of a second type of Ge-Te bonding in the amorphized phase. Moreover we distinguished the structural characteristics underlying the different amorphous phases, such as local atomic configurations and structural symmetries.

Quasicrystalline phase-change memory

“…The optical conductivity of QC and AC AMS can be fitted with Drude model. This is because the carrier concentration is higher and the mobility is lower in AC, compared with those in QC, and the electron plasma frequency ( ω p ) and electron scattering rate (γ 0 ) are ω p = 0.5 and 0.7 eV for QC and AC, respectively; γ 0 = 9 × 10 12 and 6 × 10 12 s −1 for QC and AC, respectively. The difference in the free carrier density is consistent with reports that the metallic-covalent bonding conversion is a characteristic feature of Al-based iQC systems 33 .…”

“…From a microscopic perspective, as phase-change processes can be driven by local environment evolution between first and second stable local environments, phase-change properties are determined by the energetic stability of the second stable local environment relative to that of the first stable local environment, as depicted in Fig. 1a [6][7][8][9][10][11][12][13] . Unfortunately, the development of new PCMs with low phase-change energy and high thermal stability has not yet met the requirements for widespread commercialization in the automobile industry 14 because the problem of trade-off between thermal stability and phasechange energy has not been solved yet in the amorphous-to-crystalline phase-change processes.…”

Nitrogen doping-induced local structure change in a Cr2Ge2Te6 inverse resistance phase-change material

“…18 Park et al also demonstrated the role of intermediate Ge-Te bonding in the crystallization mechanism by applying RS spectroscopy in laser irradiation-modified amorphous phase GeTe. 19 In this study, we show the possible CrGT phase transition evolution and N integration effect based on the results of RS analysis, combined with the results obtained from X-ray diffraction (XRD). Via RS spectroscopy, we revealed that the amorphous CrGT firstly crystallized to a metastable state at a lower temperature; in the metastable state, we observed a long-range order structure of CrGT, while some local bonding configuration that is similar to amorphous structures such as Cr-Te local pieces still remained.…”

“…Since the phonon lifetime of a disordered material is a random distribution, as opposed to a finite value in a crystalline material, the peak shape of the Raman mode can be described by a Gaussian line fitting procedure. 19 To obtain a good baseline without a larger error, we conducted a smoothing process for the raw data prior to the fitting procedure (Fig. 2).…”

“…17 However, Kolobov et al confirmed from extended X-ray absorption fine structure (EXAFS) study that the coordination number of the Ge atom is less than 4; therefore, octahedrallike 3(Ge):3(Te) and tetrahedral 4(Ge):2(Te) should coexist in as-deposited amorphous GeTe, and peaks B and C are associated with the vibrations of Ge atoms in a defective octahedral site with three shorter bonds. 19,25,26 Peak D can be assigned to a vibration mode of a Ge-richer GeTe 4Àn Ge n tetrahedral geometry. 27 In a previous EXAFS study of an as-deposited amorphous CrGT, the Ge-Te and Ge-Ge bond lengths were found to be 2.65 and 2.48 Å, respectively, which is similar to the cases of GeTe or GST, and suggests a covalent bonding nature of CrGT.…”