Understanding atomic structures of amorphous C-doped Ge2Sb2Te5 phase-change memory materials
Introduction
The aim to improve the information storage capability of phase-change memory materials is the cause of recent interest in doped Ge2Sb2Te5 (GST). So far, properties of several doped GST materials have been studied by either experimental methods or theoretical calculations, with the dopants including N [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], O [15], [17], [18], Si [1], [2], [19], SiO2 [20], In [21], [22], [23], [24], Sn [22], [23], [25], [26], Bi [22], [26], Sb [27], Ag [23], Cd [23] and H [28]. The reported changes in the properties as a function of doping mainly include increase in resistivity of the phases (which leads to smaller currents to write and erase information), change in crystallization speed and improved stability of the amorphous phase due to increase in the activation energy of the amorphous to crystalline phase transition.
Doped GST materials are good candidates for phase-change random access memory (PRAM) devices. However, to be able to replace widely used flash memory, such materials will need to satisfy several criteria, including high crystallization speed, high barrier to crystallization and high number of repeatable switching cycles without phase separation [29]. Novel doped GST materials allow the manufacture of PRAM chips that are now similar in storage density to existing flash memory chips, but with the added advantage of better reliability. These devices are expected to replace flash memory in technological applications in the coming years. The advantage of novel doped phase-change materials also means that development of high-speed PRAM chips with even higher storage density, featuring advanced PRAM cells, is also technologically within reach.
Detailed knowledge of the atomic structure of the amorphous phase is essential for understanding the relationship between the observed properties, the nature of the dopant and the atomic structure of the material, which is important for designing novel phase-change materials with improved properties. Recently we have described electron diffraction studies of atomic structures of amorphous phases of pure [30] and N-doped GST [31]. In this work we report an investigation into the atomic structure of thin film amorphous GST doped with 10 at.% carbon (Ge2Sb2Te5C; 10% C-GST) using experimental electron diffraction and theoretical calculations. For comparison, a material with the higher C doping level of 18 at.% (Ge2Sb2Te5C2) was also studied by theoretical simulations of a liquid quench. The atomic structures of such materials have never been studied before by either experimental or theoretical methods.
Section snippets
Diffraction data collection
A 300 nm thick film of amorphous Ge2Sb2Te5C (10% C-GST) was grown on an amorphous SiO2 film about 100 nm thick on a crystalline Si wafer by sputtering at room temperature from a target containing Ge2Sb2Te5 and C. The film’s stoichiometry was confirmed by nuclear reaction analysis. Electron diffraction data were collected to high scattering angles on a JEOL 3000F transmission electron microscope (TEM) operating at 300 kV. These data were converted to the reduced scattering intensity, ϕ(q), which is
Results
The peak positions in the RDF from the models obtained in the DFT MD simulations at 2000 and 300 K for 10% and 18% C-GST (Ge2Sb2Te5C and Ge2Sb2Te5C2, respectively) are summarized in Table 2, together with those from the experiment for 10% C-GST.
The RDF curves for the 10% and 18% C-GST from the MD simulations, averaged over the simulation time at 2000 K, are very similar in overall shape, with two distinct peaks below 7 Å (Fig. 1). With cooling to 300 K, a pronounced additional peak appears on the
Discussion
Comparing the experimental RDF for 10% C-GST with the MD (300 K) simulations, there is good agreement in overall shape (Fig. 1a), with the position of the first peak being relatively well reproduced, while the positions of the second and third peaks are at increasingly larger values in the simulations (Table 2).
According to the simulations, cooling of the melt results in shortening of the distances contributing to the first peak and lengthening of the distances from the third peak for 10% C-GST.
Conclusions
A model amorphous structure for 10% C-GST has been constructed that is consistent with both experimental diffraction data and the quenched model from MD simulations. This model structure suggests a predominant formation of Ge–Te, Sb–Te and Te–Te bonds, while preferred bonds for carbon are Ge–C and C–C, suggesting the formation of a germanium carbide phase and atomic scale carbon clusters. Visual analysis of the models and the bond angle distribution suggests that planar and distorted squares
Acknowledgments
K.B.B. and Y.C. thank Samsung for financial support. Support from the EPSRC Grant No. EP/F048009/1 to K.B.B. is also gratefully acknowledged.
References (44)
- et al.
Thin Solid Films
(2004) - et al.
Thin Solid Films
(2005) - et al.
Ultramicroscopy
(2008) - et al.
Appl Surf Sci
(2005) - et al.
Appl Surf Sci
(2006) - et al.
Solid State Commun
(2008) - et al.
Thin Solid Films
(2007) - et al.
Thin Solid Films
(2008) - et al.
J Non-Cryst Solids
(2009) - et al.
Chin Phys Lett
(2007)
Jpn J Appl Phys
Integr Ferroelectr
Appl Phys Lett
J Appl Phys
Appl Phys Lett
J Appl Phys
Appl Phys Lett
J Appl Phys
J Appl Phys
Appl Phys Lett
Jpn J Appl Phys
J Appl Phys
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